US 2775551 A
Description (OCR text may contain errors)
Dec. 25, 1956 M F. NATHAN ETAL COAL CARBONI'ZATION SVSheets-Sheet l Filed June 25, 1955 ATTORNEYS Dec. 25, 1956 M. F. NATHAN ETAL 2,775,551
COAL CARBONIZATION Filed June 23, 1955 3 Sheets-Sheet 2 n: :i 1 *i o (I) our D d 3; Q
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[70 HOPPER MARVIN F. NATHAN WALTER E. LOBO BY GEORGE T. SKAPERDAS ATTORNEYS Dec. 25, 1956 MI F. NATHAN ETAL 2,775,551
COAL CARBONIZATION Filed June 25', 1955 3 Sheets-Sheet 3 FIG. 4 FIG. s
TO CHAR HOPPER TO CHAR HOPPER `cHAR cooLER INVENTORS MARVIN F. NATHAN WALTER E. LOBO BY GEORGE T. SKAPERDAS I ATTORNEYS COAL CARBoNIzATIoN Marvin F. Nathan, New York,` N. Y., Walter E', Lobo, Westfield, N. J., and George T. Skaperdas, Fresh Meadows, N. Y., assignorsto The M. W. Kellogg Company, Jersey City, N. J., a corporation of Delaware Application June 23, 1955, Serial No. 517,472
9 Claims. (Cl. 2in- 6) This invention relates to method and means for treating solid materials and more particularly to method and apparatus forthe liuidized treatment of carbonaceous materials such as coal, shale, lignite, oil sands, etc., at low temperature. Still more particularly, it relates to unitary `method and means for drying, preheating, pretreating and carbonizing fluidized carbonaceous materials at low temperatures.
The treatment of carbonaceous solids to form valuable liquid, gaseous and solid products is well known in the art. An example of oneprocess frequently employed entails the treatment of solids, such as coal at elevated temperatures whereby `volatile materials are released from the solids and a valuable solid residue is formed. It has been the practice in the past to carry out carboni- Zation in` both non-fluid and uid systems; however, the present invention is concerned with a carbonization process of the fluid type wherein the various steps are performed with a linely divided feed material which is maintained in a highly turbulent state of agitation by the passage therethrough of a iluidizing medium.
ln carrying out iiuidized carbonization of carbonaceous materials, it has been found that several process steps are necessary in order to provide a workable operation and assure a maximum yield of desirable vapor, liquid and solid products. More usually, the iirst step in the carbonization process concerns the proper preparation of the raw feed material. This involves not only proper selection and sizing of the carbonaceous solids to provide a readily fluiclizable` feed, but also includes drying the solids to a minimum moisture content prior to further processing. It has been found that certain finely divided `carbonaceous materials, though easily fluidized at low temperatures, when subjected to more elevated temperatures suffer a change in physical characteristics and become soft and gummy and the particles tend to agglomerato. For example, many coals pass through a so-called plastic state, usually at a temperature between about 700 F. and about 800 F. wherein the coal particles tend to stick together and form large particles which resist fiuidization. Several methods of combating this agglomerating tendency of coals and other carbonaceous Vmaterials have been suggested, one of the more successful of which comprises subjecting the finely divided solid particles to a mild oxidation treatment prior to further high temperature processing. It has been found that this method of treatment alters the physical charaoteristics of the carbonaceous material so as to minimize agglomeration of the solid particles. The changes which take place in the solids in a treatment of this type are not clearly understood; however, according to one Atheory the mild oxidation case hardens the particles thereby substantially nullifying their sticking tendencies when they pass `through the plastic state. When processing carbonaceous materials, therefore, the step after the drying operation preferably involves pretreating the dry solid particles at an elevated temperature in the presence of a limited amount of oxygen. During this process a portion of the vaporizable compounds present in the carbonaceous solids are released. The third and last step in the conventional solids carbonization pro-cess concerns the treatment of carbonaceous material at a still higher temperature whereby theremaining vaporizable compounds are released therefrom and a carbonaceous product residue is produced.) The va,
porizable portion of the coalwhich is commonly known as tar. comprises numerous organic compounds hav.
ing a wide range of boiling points. As used herein, the term tar includes any volatile organic compounds released from the coal, either liquid or vapor and either cracked or uncracked. The composition of the carbonaceous residue remaining after vaporization of the tar depends on the type of carbonaceous feed material used. For example, when carbonizing coal `the residue,
material is commonly called char. Although the description and discussion of the invention will be directed primarily to coal carbonization, the term char will be used hereinafter in a broader sense to designate any residue solids remaining after carbonization.
In addition to the three processing steps just described, a fourth operation is usually necessary. In carrying out the drying step it is usually preferred to heart the carbonaceous feed only to the minimum temperature necessary to .remove the surface water, which is, of course, the boiling point of water at the pressure conditions `maintained during this operation. The pretreating operation, however, is carried out at an elevated temperature, substantially above the temperature at which the :solids `leave the drying zone; Only a portion of the heat 4required in this operation is supplied by the oxygen consumed therein. The remaining heat required is furnished by what may be called a preheating step. It is possible to provide the required preheat in `conjunction with the drying operation or the pretreating operation, or it may be made an entirely separate and independent part of the carbonization process.
Each of the aforementioned processing steps, i. e., drying, preheating, pretreating and carbonization is customarily carried out in a separatevessel, which necessitates the use of numerous transfer lines and standpipes, and extensive supporting steel work. Such installations besides being mechanically complex have poor thermal eiiiciency and result in a process with high utility costs.
It is an object of this invention to provide an improved method and means for carrying out a solids carbonization process.
Another object of this invention is to provide unitary method and apparatus for carrying out the successive steps of drying, preheatng, pretreating and carbonizing of carbonaceous materials.
It is still another object of this invention to provide improved method and means for increasing product yield in the carbonization of carbonaceous materials.
These and other objects of the invention will become more apparent from the following detailed description and discussion. in the method of this invention, the aforementioned objects are achieved by providing a unitary carbonization system comprising` drying and preheating zones'superimposed upon a carbonizing Zone and a pretreating zone disposed within the carbonizing zone, said pretreating zone being in open communication with the carbon- 'izing zone and below the level of the solids present therein.
In carrying out the process of this invention, a finely subdivided solid carbonaceous feed is introduced into a drying zone where fthe solids are elevated in temperature and .retained in a fluidized state for a period of time suliicient for the removal of surface water. The solids are thereafter passed `downwardly as a confined stream Patented Dec. 25,
tion process. `conveniently conducted in auid system with both the low into a fluidized solids bed in a pretreating zone of higher` temperature wherein they are subjected 'to a mild oxidation and then passed upwardly and directly `into an adjacent and superposed iiuidized solids bed in a carbonizing zone of still higher ,temperature where volatile con stituents of the carbonaceous solids are removed, without the use of external transfer lines or standpipes.
It is within the scope of this invention to treat various carbonaceous materials in the manner described, including coals, shales, lignites, asphalts, oil sands, etc. The invention is particularly exemplified, however, by its application to the treatment of coal, and further discussion of the invention is directed to the use of this material. It is not intended, however, that this particular application should limit the scope of the invention in any way.
The first step to be considered in a proces for the carbonization of coal as previously mentioned is concerned with surface water present in the coal feed which may, unless removed, prevent iluidization of the coal. One of the problems encountered when handling car,- bonaceous materials such as coal in a fluidized system results from the tendency of the finely divided solids to agglomerato because of water condensed thereon. Most coals coming from a treating plant, for example, have a relatively high surface or free water content, usually between about 2 and about 1S percent by weight, or higher. Unless removed, this moisture causes the finely divided coal particles to stick together and resist liuidization. Even after uidization is achieved, moisture may cause packing or bridging in process equipment of restricted cross section, such as, for example in feed hoppers, standpipes, etc. It is not always economically feasible to remove all the moisture from the coil; however, it has been found that agglomeration and packing of coal particles due to the presence of water is minimized if between about 50 percent and about 90 percent of the water initially present is removed.
It has been suggested in the past to dry carbonaceous solids with air and other gases at high temperatures.
This method, although workable, suffers from several dev liciencies. Because of the low heat capacity of most gases, sufiicient heat for drying is not provided by this method unless the drying gas is present in large quantities and at elevated temperature. The use of a large amount of gas is expensive because of compression requirements, and it complicates the recovery of solids from the drying medium. The use of elevated temperatures is also undesirable since high temperature may cause a substantial part of the volatile material in the coal to vaporize and mix with the drying medium and thus further complicate the recovery problem. Furthermore, high gas temperatures may elevate the temperature of the coal to the plastic state and cause agglomeration of the coal particles thereby resulting in an inoperable condition.
Other methods of drying coal have also been suggested; however, all of the processes presently in use suffer from serious deiiciencies of one type or another. ln the drying process disclosed herein, drying problems are reduced to a minimum by using a two zone drying and preheating system and supplying the heat required for this operation through indirect heat exchange. In carrying out the drying operation, raw coal suitably subdivided for fluidization, that is, of a size vbetween about 1 0 and about 400 mesh is introduced into a first Zone wherein it is commingled with dry heated coal in suicient quantity to elevate the entire mass of coal to a temperature suitable to eiect the removal of water. The dry coal is then passed through a heater where it is further elevated in temperature by indirect heat exchange with a hot :duid and then into a second zone. The higher temperature coal in the second zone serves as the source of the coal commingled with the wet coal feed, and in addition, pro- .vides preheated coal for the next phase of the carboniza- The entire drying and preheating step is and high temperature zones containing a dense phase hed of liuidized coal. Adequate turbulence to maintain each dense phase bed is provided by maintaining a linear gas velocity therein between about 0.5 and about 5 feet per second, or more usually between about 0.75 and about 3 feet per second. Under normal operating conditions the density of the beds thus provided varies between about l0 and about 40 pounds per cubic foot. The temperatures in the two Zones may valfy, depending on the residence time of the coalin each zone and the moisture content of the raw coal feed. However, usually the first zone is operated at a temperature between about 220 F. and about 325 F., and the second zone is preferably maintained at a temperature of between about 350 F. and about 600 F. Fluidization of the solids in the low temperature zone is partially provided by moisture rcleased from the coal and may be augmented by the introduction into this zone of air or an inert gas such as, for example flue gas, steam, etc. The coal in the high temperature or preheating zone is maintained in a iiuid state by the introduction of a similar gasifying medium. It is necessary to circulate a suiiicient amount of coal from the low temperature zone through the heater to the high temperature zone and back to the low temperature zone to provide both the sensible heat acquired by the dry solids and the heat of vaporization of the water released therefrom. When operating in accordance with the zonal temperature ranges given, the amount of coal circulated relative to the raw coal feed rate is between about 2 and about 5 pounds per pound.
The heat transfer surface required for drying and preheating the coal is preferably provided by a conventional shell and tube heat exchanger with the solids being passed through the tubes in indirect heat exchange with a hot fluid passed through the exchanger shell. The heat required to dry the coal is provided by a uid heating medium which may be a petroleum oil or vapor, or mixtures thereof, or other liquid or vapor material which is easily transported and can withstand relatively high temperatures. In general, liquid heating fluids are more satisfactory than gases because of their high specific heats and low volume relative to gases. Examples of suitable heating fluids are residual petroleum oils, synthetic heat transfer liquids, inorganic salt mixtures, lead, mercury, etc. The temperature at which the heating medium is employed varies with the temperature maintained in the drying zone and with the heat transfer characteristics of the heating medium. Usually, it is preferred to introduce the heating medium at a temperature between about 350 F. and about l000 F. Temperatures greater than this are not desirable because of the danger of over heating coal particles in contact with the heat transfer surface.
The amount of heat exchange surface required to carry out the drying and preheating operations varies depending on several factors including the quantity of coal to be heated, the amount of moisture in the coal, heat transfer coefficients, etc. More usually a surface area between about 0.02 and about 0.30 square foot per pound of fresh coal feed per hour is suiiicient to provide the desired drying and preheat.
Operation of the drying portion of the carbonization process in the manner previously described provides a dry, easily fluidizable solid material which may be subjected to further processing without danger of agglomeration or equipment plugging due to Water. This method of operation is carried out without the disadvantages of previous drying methods and results in only slightly more than the minimum dust recovery problem. Operation in the manner described also has positive advantages in that the drying step is carried outwith a high degree of thermal efciency and a minimum amountof heat exchange equipment. lt has been v.found in the operation of conventional indirect heat exchangers wherein heat is transferredto amixture of entrained solids and gases that the rate of heat transfer is sensitive to the concentration of solids iii'thefluid Stream, with'st'rearns of high solids concentrations giving substantially higher heat transfer coeicients than gaseous mixtures containing only a few solids. vIt is important, therefore, when transferring heat in this manner `to prevent dilution of the gas solids stream with additional gases, for example water vapor. By operatingthe drying step in the aforedescribed manner, substantially all ofthe moisture removed from the coal is separated therefrom in the first low temperature zone. This assures a minimum amount of `vaporization of water from the coal in its passage through the exchanger and therefore a minimum dilution of this stream. The net result is a'process having a constanthigh heat transferrate. p
Another reason and advantagev in carrying out the drying processas described relates to the velocity of the solidgas stream owing through the heat exchanger. When using` a tubular solids heater it is necessary to pass the uidized solids therethrough at a rather high velocity,
l usually between about 10 and about A30 feet per second in order` to overcome the pressure drop in the exchanger tubesand maintain the solids in `a liuidized state. This is particularly true in an up-ow type of heater wherein the solids tend to settle in a direction opposite to the ow of theuidizing medium. If solids containing water are .passedrthrough the exchanger, being heated in the course thereof, the water is converted to steam which increases thevelocity of the gas-solids stream and may create a serious erosion problem.
The proposed method of operation provides still a further advantage by virtueof the removal of water from the coal prior to the heating step. Passage of coal through the heater requires the use of standpipes and transfer lines which of necessity employ-sharp bends and turns. In addition, conventional exchangers, more usually of the tube and shell type, also present ow paths of restricted Icross section; An attempt to pass a wet coal through such a system might very well lead to agglomeration of the coal particles and plugging, the very results which are sought to be prevented by the drying step. This operating hazard, as well as those previously mentioned, of course, is avoided by the drying method described herein.
After leaving the drying zone, the coal passes downwardly as a confined stream through a carbonization zone and into a pretreating zone enclosed within the carbonization zone. In the pretreating zone the coal is contacted with air or other oxygen containing gas and partially burned to provide the pretreating and case hardening effect previously discussed. The temperature at which this important `process step is carried out may-vary over a range between about 600 F. and about 825 F.; however, more usually it is preferred to pretreat the coal in a more narrow range of temperature, that is between about 650 F. and about 800 F. As in the previous operations, the coal pretreatment is carried out in a conventional dense phase fluidized bed, wherein the coal is maintained in a turbulent fluid state by passage therethrough of a gasiform medium. Adequate turbulence to maintain the dense ^phase bed is provided by maintaining a linear gas velocity therein between about 0.5 and about feet per second. `Under normal operating conditions, the density of the dense phase bed thus provided varies between about l and about 40 pounds, per cubic foot. Generally, a portion or all of the fluidizing medium is supplied in conjunction with the oxygen required for pretreating. This may be accomplished by diluting the oxygen with air, by using air alone or by diluting air or oxygen with steam or other inert gas. Thus, it is within the scope of the invention to supply the pretreatment oxygen to the pretreating zone in a gaseous stream of varying oxygen content. The amount of oxygen required for pretreating is usually between about 0.02 and about 0.08 pound per pound of dry coal feed. To provide sufcient time for the pretreating combustion reactionsto take place, the rate of introduction of coal to the pretreating zone is adjusted to allow an average particle the cross-sectional area of the pretreating zones.
4openings in the grid are usually circular in nature and 6. residence time therein of between aboutV 10 and about 60 minutes. Upon entering the pretreating zone, dry preheated coal at a relatively low temperature becomes intimately mixed with higher temperature pretreated coal and is swiftly elevated to the temperature level prevailing in this zone. As the temperature of the dry coal is increased, a portion of the lower boiling tar components presentpin the coal are vaporized and passed into the tiuidization and combustion gases. Since oxygen is relatively non-selective in `its action, this phase of the carbonization process may involve the consumption of a portion of the tar. For this reason, it is desirable to limit the introduction of oxygen to the pretreating zone to the minimum amount necessary to prevent agglomeration of the solids and maintain an operable system. Equally important to the operability of the pretreating system is the temperature of the solids bed maintained therein. Consideration of this important point is taken upin detail at a later point in the discussion.
As previously mentioned, the pretreating zone is disposed within the carbonizing zone. For reasons to be more fully considered later, the coal pretreatment step is carried out in a dense phase uidized bed of solids adjacent to and in upwardly open communication with a second dense solids bed when is contained within the carbonization zone. The preheated solids introduced into the pretreating zone completely ll this zone and pass upwardly therefrom into the carbonization solids bed. The carbonized solids or char on the other hand occupy only a portion of the carbonization zone, the remainder comprising a conventional dilute phase of relatively very low solids concentration super-posed above the dense phase char bed. By this arrangement of one zone within another, a common vapor space is provided which serves to accommodate the uidizing and combustion gases .from both zones. 4
It is necessary to the eicient' operation of the carbonization process that the pretreating and carbonization steps be kept separate and carried out at substantially different ternperatures. Thus, it is essential that solids from the higher temperature char bed be prevented from passing into the pretreating zone. In the method of this invention, pretreating and carbonization are maintained as separate operations by placing a grid or perforated plate between the two zones. The number and size of the openings in the grid or plate are fixed to provide sufficient pressure drop to prevent back mixing, that is passage of solids from the carbonization zone to the pretreating zone, but insufficient to prevent the flow of solids from the pretreating zone. The grid preferably encloses the entire top of the pretreating zone in order that the solids entering the carbonization zone may be uniformly distributed over a maximum area. Under vnormal operating conditions the cross-sectional area of ow through the grid is between about l percent and about l0 percent of The are of a sullcient size to allow passage of the largest coal particles. p More usually holes between about 1A and about 1 inch in diameter are adequate. The major factor in preventing back mixing through the grid is the high velocity of vapors and solids therethrough. It is contemplated sizing the area of flow through the grid to provide a pressure drop between about 1/2 and about 3 p. s. i. With this drop in pressure, velocities through the grid are -in the order of between about 50 and about 200 feet per second. By the aforedescribed means of dividing the two,
zones, it is possible to maintain them in open communica-` tion, yet at substantially different temperatures, and at the` same time introduce pretreated solids and vapors into thecarbonization zone in anevenly distributed manner.
'r As will becomeapparent from the subsequent discus--v sion, the physical location of the separating grid is im-r portant `in determining carbonization product yields... `More usually the pretreating zone openly communicates;
with the dense char bed in an upward direction. This is necessary irf the gases and pretreated solids are to be intrQdllced into the carbonization zone in an evenly distributed manner. On the other hand, the grid need not be limited to this location and other physical arrangements may be Used when desired.
In addition to the desirable process features which result therefrom, the pretreating and carbonization vessel arrangement presents a number of advantages of a mechanical nature. For example, the arrangement of the two zones in eiect eliminates one vessel, simplifies the transfer of solids from the preheater to the pretreater and from there to the carbonizer, decreases solids recovery costs by eliminating one set of cyclones, eliminates the transfer line which would be required if the pretreating and carbonization steps were carried out in separate vessels, etc., thereby providing a highly eicient process both thermally and mechanically.
Pretreated coal, iiuidizing and combustion gases, and volatile tar compounds released from the coal in the pretreating zone pass into the carbonization zone wherein the major portion of the volatile components in the coal are removed and a valuable residue char is formed. This, the major step of the process, as far as product formation is concerned, is also conveniently carried out in a dense phase lluidized bed similar to the drying, preheating and pretreating beads previously described. In order to effect removal of the volatile Coal components, a large amount of heat must be introduced to the combustion zone. Conventionally, this heat may be supplied from one or more of several sources, for example it may be provided in an inert gas such as a fuel gas heated to a high temperature, or it may be supplied from a combustible gas such as fuel gas mixed with oxygen or it may be furnished from the combustion of oxygen or an oxygen containing gas with a portion of the carbonaceous feed. This invention is concerned primarily with the method of supplying heat wherein a portion of the carbonaceous feed, viz. pretreated coal, is burned with oxygen or an oxygen containing gas. However, it is within the scope of the invention to Supply a portion of the heat by either of the other two methods mentioned. When using the aforementioned method of providing heat, the gasiform uidizing medium required to maintain the dense phase in the carbonization zone iS generally furnished by the combustion gases. If necessary, however, deficiencies in the quantity of uidizing medium may be made up by the introduction into the carbonization zone of a flue gas, stream or other extraneous inert gas.
The -carbonization of coal to remove distil'lable tars therefrom and produce a char residue product is conducted over a wide range of temperatures usually between about 700 F. and about 2400 F. The preferred thernaal range of operation is determined to a great extent by thev type of liquid product desired; for example, when it is preferred to` distill the coal tars with a minimum of cracking of volatile constituents, namely low temperature carboniz'ation, the temperature is held to a minimum of about 700 F. and not more than about l000 F. The type of coal is also of importance in establishing the operating temperature since some coals are more dificult to distill` than 4others. Contra to the pretreatment step, which is carried out entirely in the dense phase, the carbonization zone contains a dense phase bed superposed by a disperse or dilute phase which may have a solids concentration as low as 0.001 pound per cubic foot. Gases from both zones pass into this phase which provides a preliminary rough separation of vapors and solids. Further solids separation is provided by conventional means, such as, for example cyclones, filters, etc.
Substantially all of the desirable constituents of coal are removed at the aforementioned carbonization temperatures within a very short period of time, that is between about 0.25 and `about l minutes. As a further precaution to. prevent agglomeration of the coal particles in. the carbonizis zone, itis preferred to maintain a sub` stantial ratio of char to fresh feed therein. This serves to dilute the fresh ,pretreatedieoah which provides the desired beneficial effect; however, it also makes it necessary to substantially increase. the coal residence time. At the usual char to fresh feed ratios maintained in the carbonization zone, that is between about 5 pounds per pound and about 50 pounds per pound, the particle residence time therein is between about 2 minutes and about 200 minutes, more usually between about 20 minutes about minutes. v
Carbonization may be carried out over a. wide range of pressures; however, the pressure is usually maintained between atmospheric and 500 p. s. i. g., preferably between about atmospheric and about 100 p. s. i. g` Since a driving force is necessary for the passage of coal :from the yretreating zone into the carbonization zone the pretreating zone must operate at a pressure above the pressure in the carbonization zone; more usually the differential pressure between the two zones is between about 1/2 and about 2 p. s` i. By virtue of its physical location above the pretreating zone, the `drying zone may operate at a pressure either higher or lower than the pressure in the former zone. More usually, it is convenient to maintain the. pressure in the drying zone lower than the pressure in the pretreating zone and as a result the drying zone is ordinarily operated at between about 1 and about 20 p. s. i. less than the pretreating zone.
As previously mentioned, this invention is not limited in its scope to the treatment of coal, but encompasses the use of other carbonaceous feed materials, for example shales, asphalt, oil sands, etc. Similar processing considerations are important and similar operations are re.- quired when carbonizing these feed materials other than coal. The conditions appropriate for each specic feed material are well known to those skilled in the art and for this reason do not need repeating here.
The use of a unitary system for carrying out the carbonization 'of coal provides unexpected advantages and allows the use of several novel processing schemes. For example, in the conventional` coal carbonization unit wherein the coal pretreating step is carried out in `a separate vessel it is necessary to withdraw pretreated coal from this vessel and pass it through a transfer line to a carbonization zone. Since oxygen is used in the carbonization Zone as Well as in thepretreating zone, it has also been the practice to` introduce pretreated solids and oxygen together into the carbonization zone. More usually the oxygen in the form of air has been used for vtluidizing and transporting the pretreated solids between the two zones. This method of moving the pretreated solids is eiective; however, it has been found that it results in an excessive consumption of tar compounds lby burning, thereby reducing the `amount of tar produced in the process.- The reason or reasons for this are not clearly understood but are believed to be related to the time during which tar vapors and oxygen are in contact. In order to obtain an effective coal pretreatment, it is necessary to provide an average coal particle residence timel in the pretreating zone of several minutes. Since oxygen is introduced into. the pretreating zone continuously, the solids in thiszone, are, of necessity, in contact with oxygen for substantially this period of time. On the other hand, gases entering and released in the pretreating zone reside therein for only a few seconds, more usually between about 5 `and about 20 seconds and subsequently pass from this zone. Thus, tar vapors released from the coal in the pretreating zone are in contact with oxygen for a very short period of time compared to the time of oxygen-solids contact. When the pretreated coal and effluent gases from the pretreating zone, containing oxygen are passed to a carbonization zone through a transfer line, the total time of contact between the tar vapors and oxygen, is substantially increased, by as much as 100 percent or more depending on the length of the transfer line and the velocity of the Agases therein. The additional few yseconds of `contact time between thecoal particles and oxygen provided by use of the transfer line is, however, insignificant. `If it is assumed that oxygen shows equal preference for tar and coal it is obvious that the combustion of tar is increased by use of a processing method which includes the use of a transfer line with oxygen in the transferring medium.
When utilizing the conventional method of transferring solids from a pretreating vessel to a separate carbonization Vessel, it has further been found that substantial amounts of coke are deposited in the transfer line. The result is a restriction in the ow between the two vessels which may eventually cause a shutdown. `This phenomenon also apparently is related to the residence time of the tar vapors in the transfer line. At the temperatures maintained in the pretreating zone it is not dicult to Visualize some thermal cracking of thetar compounds. Any appreciable deposition of coke due `to cracking would of course immediately become apparent in such a zone of relatively small cross-section. It is also possible that the coking is due either partially or entirely to the combustion of tar in the transfer line rather than by cracking. Whichever the cause, however, the occurrence of coke as described presents a problem which can seriously affect the operability of the carbonization process.
In the method of this invention these problems are avoided by passing pretreated solids directly from the pretreating zone into the carbonizing zone without the use of a transfer line, thus minimizing contact between the tar vapors and oxygen and reducing the time during which these vapors are maintained at the pretreating temperature. Oxygen required in the carbonization zone is introduced into this zone separately from the pretreated solids. The direct passage of solids between the two zones is provided by using contiguous openly communicating zones disposed within a single vesselin the manner previously described.
Solid particles leaving the pretreating zone pass through the grid or perforated plate into the carbonization zone where they are immediately commingled with high temperature char solids. rlfhe heat transfer characteristics of the dense highly turbulent char bed are such that the pretreated coal is rapidly heated to carbonization ternperature. During this process, `the remaining and major portion of the volatile constituents of the` coal are vaporized and pass upwardly through the char` bed. Thus the solids region adjacent to the grid separating the two zones is particulraly rich in volatile materials. The separate introduction of pretreated coal and oxygen into the carbonization zone is eiective in reducing the consumption of tar already vaporized; however, unless the pretreated coal is maintained free from contact with oxygen in this zone for a period of time sufficient for the remaining tars to be distilled, a substantial portion of these volatile components may also be consumedf Thus, in addition to passing the pretreated solids directly from the pretreating zone to the carboniz-ation zone, it is further desirable to introduce these solids into a` region of the latter zone which is free or relatively free of oxygen, whereby remaining tar materials are distilled therefrom and removed from the carbonization zone before the pretreated coal enters the region of combustion. In the method of this invention, this is accomplished by introducing combustion oxygen into a dense phase bed of char in the carbonization zone in the lower portion thereof whereby the oxygen is substantially consumed before the lluidizing and combustion gases pass into the upper portion of the bed. The pretreated coal is introduced into the top portion ofthe same dense phase bed below the surface thereof and is heated by contact with the hot char in a relatively oxygen-free atmosphere. Both during and 'after the solids heating process, the char is in contact with `ascending fluidization and combustion gases. zThese vapors exert a stripping t tar components. Thus the favorable results ofthis operagree of solids turbulence therein.
tion may be attributed to a combination of heating and stripping although it is probable that the primary separating effect is provided by the heat transferred to .the
pretreated coal. The total gases from both zones after leaving the solids bed enter the dilute phase thereabove and are passed from the system.
In addition to the advantages already mentioned,` the proposed method of operation eliminated another defect of previous carbonization processes. Because of the relatively low temperatures used in the pretreating operation, it is diicult to provide for complete consumption of the oxygen introduced into the pretreating zone. As a result, the effluent gases from this zone usually contain some free oxygen. In normal operations, for example the amount of oxygen break through may be as high as 10 to l5 percent of the total introduced. When pretreating is carried out in a conventional manner in a separate vessel in a conventional dense phase bed, unconsumed oxygen passes into the dilute phase of the pretreating zone and reacts therein with tar vapors released from the coal. In the method of this invention, there is no dilute solids phase in the pretreating zone and Oxygen which is not consumed in this zone passes into :a dense phase bed of char in the carbonization zone along with the combustion gases and pretreated coal. Here the oxygen has at least an equal chance to react with solids rather than tar, 'thus effectively increasing the tar yield.
The depth of solids bed required in the carbonization zone to successfully carry out the invention depends on several factors, including the velocity of the iiuidizing medium therein, the degree of turbulence in the uidized bed, the temperature at which carbonization is carried out and the diameter of the bed. In general, it has been found that a bed of depth normally maintained in commercial catalyst regeneration processes is adequate although deeper beds may be used to assure the complete absence of tar-oxygen contact.` The depth of coal maintained in the pretreating zone is less critical; however, here too the degree' of oxygen consumption is an important factor in determining bed depth. Usually in either zone a bed of between about l0 feet and about 40 feet in depth is maintained, although, if desired, more shallow beds and beds up to 70 feet in depth may be used. It is not necessary that the beds be of equal depth and either may be greater or lesser in depth than the other.
Both the pretreating and carbonization steps are carried out in conventional fluid beds which are maintained by passing a fluidizing medium through finely subdivided particles of solids. The amount of vapor` and solids introduced into these beds per unit of time is important in determining both the volume of the beds and the de- It is desirable in carrying out these processing steps, to maintain solids beds of relatively constant size having a suicient ow of uidizing medium therethrough to provide adequate turbulence of the uidized solids. Therefore, control of vapor and solid flow rates to the uil beds is of utmost importance. In carrying out the pretreating of finely subdivided coal particles, it has been found that the rate of combination of oxygen with coal at a given temperature is dependent on the size distribution of the coal particles, that is on the amount of coal surface presented to the oxygen. If the coal particles increase in size the coal surface is decreased and the amount of oxygen consumed in unit time is also decreased. On the other hand, when the coal particles decrease in size the reverse occurs. Conventionally, coal particle size distribution on a commercial scale is provided by mechanical crushing and grinding. Any variation in the operation of the equipment employed for this purpose, which is` not uncommon, usually means a change in the size distribution of the coal produced. As previous 1y niemeer-.dy if the @al particle size Suddenly increases the amount Of'orvgen @Inward in the pretreatr decreases and the temperature therein also decreases. If this occurs, the obvious solution which springs to mind is to introduce more oxygen into the pretreating zone. This has the effect of increasing the concentration of oxygen in the pretreating zone whereby the combustion reaction rate is accelerated and the temperature may be brought baci:v to its former level. Unfortunately, however,introduction of more oxygen into this zone requires decreasing the quantity of airintroduced into the carbonization zone unless the temperature in the latter zone is also increased. Obviously, an increase in carbonization temperature is undesirable from the viewpoint of uniformity of operation and product yields. Furthermore, withdrawal of oxygen from the caraonization zone decreases the vapor velocity in this zone and affects the degree of turbulence and size of the dense phase bed maintained therein, both of which are equally undesirable. Still another disadvantage of increasing the amount of oxygen entering the pretreating zone lies in the fact that only a portion of the additional oxygen is consumed in the pretreatment and the remainder along with the unreacted portion of theoriginal oxygen4 passes from the pretreating zone into the carbonization zone. Here it is consumed at least in part by reactions which involve tar rather than the non-volatile portion of the coal.
In the method of this invention it has been found that the problem of temperature decrease due to changes in the size distribution of the fresh coal feed is substantially eliminated and effective temperature control obtained in the pretreating zone by recycling a small, variable quantity of hot char from the carbonization zone to the pretreating zone. Not only is this method of temperature control simple in operation, but it is without the defects attendantwith attempts to, control the temperaure by varying the oxygen to carbon ratio. The amount of lchar required for effective temperature control may vary at any instant from as low as zero to about 5 pounds per hour per pound of coal present in the pretreating zone; however, more usually the quantity of char required to compensate for temperature upsets is relatively low, that is between 0.01 and about 2 pounds per hour per pound of coal present in the pretreating zone. The recycle char flow rate may be controlled manually or more usually by the installation of a temperature controller in the recycle line. it is contemplated that a small amount of char will be recycled continuously Vwhen using this method of temperature control in order to prevent plugging of the recycle line. The amount of heat introduced into the pretreating zone in this continuous char stream, however, is very small compared` to the total heat required in the pretreating zone, usually not more than about 5 percent thereof.
During normal operation a major portion, more usually at least 90 percent of the oxygen introduced into the pretreating zone is consumed therein. Because of this, the problem of increasing pretreating temperature due to a decrease in the average particle size of the coal is not nearly as critical as temperature movement in the opposite direction. The unconsumed oxygen even if totally reacted can raise the preti-eating temperature only a few degrees. The major reason for temperature control is to prevent excessive drops in temperature which may affect the operability of the process. On the other hand, increases in pre'treating temperature will rarely, if ever, have any detrimental effect on operability, although increased ternperature may lower the yield of tar products. Other operating changes besides particle size may affect the temperature in the pretreating zone; for example, there may be a temporary variation in the rate of introduction of fresh coal into this zone. It is within the sc ope of this invention to provide a degreeof teriugerature4 control by the aforedescribed` method irni 12 material Q f the muses of temperature variation. however, the Proposed mode Qt Operation is directed primarily to eliminating the problem resulting from recurrent variations in feed coal particle size,
Hot char product from which the major portion of the volatile constituents of the coal have been removed is withdrawn from the lower portion of the carbonizer and is passed through a cooler wherein the temperature of the char is lowered by indirect heat exchange with a fluid cooling medium. When operating in accordance with the ranges of process variables previously enumerated the amount of this material varies between about 0.6 and about 0.9 pound per pound of wet `feed coal. The remainder of the raw material delivered to the process is now in a vapor state, comprising a mixture of steam, combustion gases and tar vapors. The apparatus used in conjunction with the char cooling preferably comprises one or more conventional tubular heat exchangers similar to those previously described in conjunction with drying and preheating the coal feed. The type and quantity of cooling fluid passed through the exchanger may be varied to meet the particular needs of the process. In general, uids similar to those previously disclosed for use in drying and preheating the coal are used. This operation is simplified and the cost is substantially reduced, if a common fluid medium is used for both coal drying and preheating, and for cooling the product char. When operating with this type of system, a continuous circulating fluid stream is provided, which extracts heat from the hot char product and transfers it to the fresh coal feed. inasmuch as the heat removed from the char in the cooling operation may not be sufficient to provide the heat required for drying and preheating thev coal feed, or vice versa, it is desirable when using a common heat exchange Huid to provide an additional heat source, such as for example a conventional tubular heater, or an additional source of cooling, such as for example a water cooler, which ever is required.
In this preliminary cooling step, the char temperature is usually reduced to between about 700 F. and about 400 F., although it may be brought to a still lower temperature if desired.v The cooling fluid may be introduced to the cooler at any low temperature; however, when a common circulating stream is used the inlet temperature, o f necessity, conforms to the temperature of the uid leaving the heaters which serve the drying and preheating stages of the carbonization process, i. e. between about 650 F. and about 350 F. The size of the cooler required varies with the amount and temperature of the char product, the heat transfer coeicients of the flowing streams and other operating variables; however, more usually a surface area between about 0.01 and about 0.10 square foot per pound of char product per hour is adequate to provide the desired cooling.
Normally, only a portion of the heat contained in the product char can be removed economically by indirect cooling, particularly when using a common circulating heat exchange fluid. To further cool the char and provide a more easily handled product, water is injected into the partially cooled uidized char which is then passed into a receiver or char hopper. The quantity of water used for this purpose may vary; however, usually it is preferred to limit it to not more than the amount necessary to cool the char to the dew point of water at the pressure existing in the receiver, thus converting the entire quantity of cooling water to steam. By operating in this manner, advantage is taken of the high vaporization heat of water to provide maximum cooling with a minimum of water consumption and at the same time provide additional vapors to maintain thev char in the hopper in a` fluidized state. The cooled product is then conveniently removed from the hopper, deuidized bycontact with. additional water which condenses the ceiver substantially above the dew point of water.
system as a slurry in the scrubbing water. `is conveniently mixed with char removed from the char product.
. 13 y iluidizing steam and is passed from the system by means of a `conveyor or other suitable means.
The water used to cool the hot char may be introduced thereto prior to entry of this material into the char` hopper, or after the char enters the hopper, or a portion may be admitted at both localities. It is preferredthat the` char be cooled to as low a temperature as possible; however, if a suitable use for higher temperature steam exists, the amount `of cooling water may be controlled to provide a char temperature in the 1re- A so, although it is preferred to maintain the charin the hopper in a fluidized state, the defluidization of this material may be accomplished therein by increasing the amount of cooling water introduced into the char to the point where liquid water is present in the hopper. The char is then removed from this vessel as a slurry rather than as a fluidized mass.
The amount of water required to accomplish the second stage of the char cooling process varies with the initial temperatures of both the char product and the water and the inal temperature of `the char. More usually the water is introduced at a low temperature, i. e. between about 60 F. and about` 100 F. The pressure in the char hopper or receiver is also desirably maintained at a low level, usually less than the pressure in the carbonizer, viz. between about and about 5 p. s. i. g. Setting the pressure establishes the dew point temperature and accordingly the amount of water required as quench, which is usually between about 0.05 and about 0.15 pound per pound of char product.
The cool char which accumulates in the char hopper forms a dense fluidized solids bed above which there exists a conventional dilute phase Zone of low solids concentration. The solids density in the dense 'phase bed is usually between labout 15 and aboutv 25 pounds per cubic foot; whereas, the concentration of solids in the dilute phase is very small, often less than 0.1 pound per cubic foot. Vapors and solids leaving the hopper Vdilute phase pass through conventional separation means, for example cyclones, for the removal of a major portion of the solids and thence to a secondary solids recovery system. In order to minimize the facilities required for separating entrained solids, the overhead gases from the feed coal drier and preheater, which `contain entrained coal particles, are also introduced into the secondary solids recovery system.
In one embodiment, this system comprises a vertical elongated scrubbing tower, with baflles suitably disyis condensed. Preferably the scrubbing liquid is supplied `by recycling a warm solids-water slurry from the bottom of the scrubber and combining with this stream necessary makeup water from an outside source. Since the solids-water slurry is at the dew-point temperature of the steam in the scrubber, usually between about 212 F. and about 240 F., this method of operation provides a relatively high temperature scrubbing stream and a minimum of steam in condensed in the process. In addition, by recycling, it is possible to closely control the scrubbing operation for maximum solids removal.
The recovered solids are removed from the scrubbing This slurry hopper in .order to lower its temperature and to reduce the dust problem associatedwith the inely divided solids As a result of this, the product char solids contain a mixture of coal and char; however, the amount 1,'4 of coal recovered in thisoperation is insignificant when compared to the char, being only between about 0.1 and about 0.8 percent thereof Iby weight, and when combined with the char is insufficient in quantity to alter its prop'- 'erties or characteristics. y It is apparent that the aforedescribed method of solids recovery olers several important advantages. The combination treatment of gases from the char hopper and the drying and preheating zones substantially reduces the number of cyclones or other solids recovery equipment required. In addition, controlling the scrubbing operation to prevent condensation of the iluidizing steam pro- 'vides an important heat economy and reduces the amount 'of scrubbing water required'for the operation. Furthermore, introducing recovered coal into the char product not only provides a lconvenient method of disposing of this material, but also increases the char yield without affecting the properties of the char.
As previously mentioned, the efuent vapors from the carbonizer comprise gaseous products of combustion and various tar compounds plus a small amount of entrained char. The major portion of the tar materials in the gases condense to liquids at ordinary temperatures and form a valuable product of the `carbonization process. To effect the separation of the normally liquid tar, the carbonizer gas stream is passed to a quench tower where the vapors are contacted with a low temperature liquid tar. This material not only provides the cooling eiect necessary to condense liquid tars but also eiects the removal of entrained solids from the gases. The scrubbing and condensing liquid is preferably obtained by circulating tar condensed in the quench tower through a cooler and recycling it to the upper portion of the tower. Within the tower are provided suitable baffles or plates whereby intimate contact between ascending gases and downflowing liquid is effected. The pressure` at which this operation is carried out is controlled by the pressure in the carbonization zone, being somewhat lower, usually be- Atween about 10 and about 2 p. s. i. g. It has been found that the major portion of the desirable liquid tar compounds are condensed by cooling the carbonizer gases to between about F. and about 80 F. The remaining vaporous tar compounds and combustion products form a gas, which although low in heat content, may be used as a fuel. If desired, of course, a further separation between the uncondensed tar compounds and combustion and lluidization gases may be effected.
In the past, diiculty has been encountered in physically separating all of the condensed tar constituents from the uncondensed tar vapors and combustion gases. Experience has shown that when tar vapors are quenched in the manner described, a portion of the tar condenses at very small droplets which form a dispersion or fog in the uncondensed gases. The dispersed tar is unaffected by subsequent after-cooling of the gases and is separated therefrom only with diiculty, usually by passing the gases through a special separating means, such as, for example, a Cottrell precipitator. It has been found that a major portion of this entrained liquid tar may be successfully removed in the quench tower by passing the gas stream through a mixt extractor, thereby effecting substantially complete separation of liquid tar from vapors.
As previously mentioned, the material used to condense tar from the carbonizer vapors also serves as a scrubbing medium to effect removal of entrained char from these gases. The amount of solids concerned usually is quite small, being between about l and about Vl0 percent of the char product. If it is contemplated using the tar product as a fuel the presence of this minor quantity of solids usually is not detrimental; however, for other uses of the tar, it may require further treatment to remove the char solids.
In order to more clearly'describe the invention and provide a better understanding thereof, reference is had to the following drawings of which:
Figure 1 is a diagrammatioillustration in cross section of process equipment used in carrying out a preferred embodiment of the invention comprising a unitary coal carbonization system which includes a dryer, preheater, pretreater, carbonizer, char hopper, solids recovery system, tar quench tower and associated lines and heat exchange equipment.
Figure 2 is a diagrammatic illustration in cross section of another embodiment of a coal drier, preheater, and associated heat exchange equipment.
Figure 3 is a diagrammatic illustration in cross section of still another embodiment of a drier, preheater and associated heat exchange equipment and includes in addition, feed hopper means for reducing the average moisture content of the coal. Feed hopper permits wet coal to be fed prior to drying.
Figure 4 is a diagrammatic illustration in cross section of a process arrangement for cooling product char produced in the carbonization process.
Figure 5 is a diagrammatic illustration in cross section of a method of introducing wet solids into the drier from a `feed hopper, somewhat similar to that disclosed in Figure 3.
Referring now to Figure l, a finely subdivided coal at a temperature of about 60 F. having a particle size distribution between about l0 mesh and about 10 microns is delivered through conduit 2 into feed standpipe 4 wherein it is maintained in a dense turbulent state by passage therethrough of a fluidizing medium. Entering the system the coal contains about 8% of surface water by weight. From standpipe 4 the uidized coal passes downwardly through conduit 6 where it is entrained in additional gases, in this instance steam, and passed upwardly through conduit 8 to a drier and preheater vessel 10 which is at a substantially higher elevation. Within the drier vessel there is maintained a dense highly turbulent bed 12 lof dry coal particles at a ternperature of about 270 F. The upper portion of this bed occupies the entire cross section of the drier vessel 10; however, in the lower portion thereof, the dry coal is conned within an annular space lying between the walls of the drier and a cylindricalelongated conduit extending upwardly through the bottom of the drier. Within this conduit lies a preheating zone 14 in which there is maintained a higher temperature dense bed of coal particles which overow continuously into the lower temperature dry solids bed 12. Above the dense beds of dry and preheated coal is a dilute phase 16 of low solids concentration. Water vapors released from the coal pass upwardly through this space into a cyclone 18 from which separated solids are returned to the dense phase of dry coal, and from which the vapors leave the drier through conduit 20.
To provide the sensible heat required to heat the wet coal and the latent heat of vaporization of the water present therein, a stream of dry coal is removed from the annular drying zone 12 through conduit 22, entrained in tluidizing steam and passed upwardly through conduit 26 and coal heater 28 wherein the temperature of the coal is increased to about 480 F. From the heater the het coal is passed into conduit 14 from which it eventually overflows to the drying zone. In order to maintain the desired temperature in the drying zone, it is necessary to overflow about 2 pounds of solids from conduit 14. per pound of wet coal introduced into the unit. Thus the solids circulation rate through the coal heater 2S is about 3 pounds of coal per pound of wet feed. The heat required in the combined drying andpreheating operation is supplied by passing the circulating solids stream in indirect heat exchange with a cat cracker decanted oil having an APl gravity of about l5. This material is introduced to heater 2S through conduit 30 at a temperature of about 680 F. and exists therefrom through conduit 32 at a temperature of about 400 F. The foregoing method of drying the coal is simple in application and in addition to effecting removal of moisture from the coal, it provides a ready means of adding to the coal the amount of preheat'require'd before pretreating the coal, the next step in the process.
Although the hot coal leaving heater-28 enters zone 14 in a tiuidized condition, it may be desirable to introduce additional gases, such as for example steam through conduit 13. Generally, the water vaporized in the drying zone is adequate to provide the desired turbulence in the dry solids bed; however, if necessary, an additional quantity of fluidizing gases may also be introduced to zone 12. The amount of fluidizing gases passed through each zone is controlled to provide a velocity. therein of about 2 feet per second, thereby maintaining a solids density in each bed of about 25 pounds per cubic foot. As previously mentioned, the effluent gases from both zones pass through a conventional cyclone 18 for the separation of entrained solids which are returned to zone 12. in spite of this, some solids, in quantity equal to about 0.2 percent by weight of the wet feed are retained in the gases and leave the system through conduit 20.
The combined drying and preheating vessel 10 forms a part of a single unitary vessel structure being superposed above a carbonization vessel 36 which contains within its lower portion a pretreating zone 42. Passage of solids from the preheating zone 14 to the pretreating zone 42 is effected by flowing them downwardly through a standpipe 44 enclosed within the carbonizer vessel. inasmuch as the standpipe passes through the carbonizer before it reaches the pretreating zone, it is exposed to the high temperatures in the former zone and it may be desirable to provide some form of insulation to protect this conduit. The rate of ilow of solids from the preheating zone 14 to the pretreating zone 42 is controlled to maintain a more or less constant level in vessel 10 by a conventional plug valve 58 in contact with the bottom terminus of the standpipe 44. The pretreating zone 42 is separated in part from the carbonization zone 40 by a vertical baffle 66 attached at the bottom and sides to' the inner wall of the carbonizer vessel 36. The bottom portion of the treating zone contains a distribution grid 60 for distributing fluidizing gases throughout the pretreated coal.. The pretreating zone opens upwardly into the carbonizing zone 40 and is separated therefrom by a grid 54 through which pretreated solids and vapors pass from the former to the latter zone.
The pretreating operation involves contacting the coal particles with a controlled amount of oxygen, viz., about 0.04 pounds per pound of preheated coal, whereby the coal particles are partially oxidized. In this manner, the physical characteristics of the particles are altered so as to nullify their tendency to adhere to each other as they are elevated in temperature and pass through the so-called plastic stage. The effectiveness of the pretreating step is dependent not only on the extent to which the coal particles are oxidized, but is also a function of the pretreatment temperature, which is substantially increased over the preheating temperature, that is to about 725 F. The heat required to elevate the coal to this temperature is in normal operation supplied entirely from the heat of combustion of the coal.` In carrying out the pretreating step, oxygen is introduced through conduity 64 and is distributed in the lower portion of the pretreating zone through grid 60. The oxygen may be supplied in a relatively pure state; however, more usually, it is preferred to use air, not only from the viewpoint of cost, but also to supply the additional gases necessary to maintain the Solids in the pretreating zone in a fluidized state. VAlthough the air admitted to the system normally suffices for this purpose, additional gases such as, for example, steam, ue gas, etc., may be introduced through conduit 64 for uidization purposes.
Coal entering the pretreating zone commingles with the solids contained therein and is partially oxidized and rapidly increased lin temperature to that of the dense phase bed.' In this process about 4 percent by weight of the preheated coal is reacted with the oxygen and converted to combustion products. The resulting mixture of pretreated coal and combustion gases, along withk any portion of unconsumed oxygen, passes upwardly through the pretreating zone and through grid 54 into the carbonization zone 43. Within this zone there is -maintained a dense phase turbulent bed of solid char particles at a substantially higher temperature, that is about 950 F. Inasmuch as the pretreating zone is entirely beneath the top level of the solids in the carbonization zone, the grid 54 serves the dual purpose of distributing the solids and gases leaving the pretreating zone and at the same time prevents passage of solids from the carbonization zone to the pretreating zone. By use of this separating means, it is possible to maintain two contiguous, yet distinct and separate dense phase beds of solids at quite `different temperatures.
. The preheated coal from zone 14 contains a large nurnber of organic tar compounds varying widely in molecular structure and boiling point. The increase in temperature in this zone releases a portion of the lower boiling of these volatile compounds which pass upwardly into the carbonization zone 43 along with the pretreated solids and other gases. Upon entering the latter zone, the pretreated solids are quickly elevated to the temperature prevailing therein and large additional amounts of volatile components are released from the coal. The total time required in the two zones to carry out the process of tar removal is of short duration; however, in order to prevent solids from agglomerating and thereby assure an operable iluid process, it is desirable to maintain a large excess of pretreated solids in the pretreating zone and a similar excess` of carbonized solids or char in the carbonization zone. This is effectively provided by sizing the pretreating and carbonization zones to allow an average particle residence therein of about 25 minutes and about 60 minutes, respectively. The pretreated solids bed is maintained in a highly turbulent state by controlling the flow of vapors therethrough to provide a gas velocity of about 1.2 feet per second and a solids density of about 25 pounds per cubic foot. Usually, this is effected by varying the oxygen rate through conduit 64; however, if necessary, an extraneous gas (not shown) is admitted to zone 42. The degree of turbulence and density of the solids in zone 43 is regulated in a similar manner.
The heat required tor carbonizing .the coal feed -is also supplied by burning `a portion of this material. For .this purpose, about 0.03 pound of oxygen per pound of pretreated coal feed is introduced into the carbonization zone 43. In this `operation .also the oxygen is introduced in the form :of lair rather than in a pure state, for the `reasons previously given. Since one of the important features in optimizing liquid product yield is minimum contact between oxygen and volatile .tar constituents, .the oxygen required for carbonization is introduced int-o the bottom of the carbonizaltion zone through conduit 70 which is art .a point remote from the area of introduction of pretreated coal into the same zone. Oxidation and combustion of the carbonized coal particles proceeds rapidly :and is substantially completed before the carbonizer ilu-idizing land combustion gases reach the elevation .at which the pretreated coal is present in quantity. The 'heat released by the combustion reactions is quickly transmitted throughout .the dense char bed providing .a hot turbulent mass into which lower temperature pretreated solids are introduced. The transfer of heat from the char particles to .the pretreated solids in turn is equally swift and 'these solids reach .the general char bed temperature level within a very short period of time. The process of devolatilization aiso proceeds at a fast raite and, by .the time .the pretreated solids reach the zone of combustion, they are substantially tree of volatile tats.
By reason of the location of withdrawal conduit 62,
char product from .the main upper portion of the carbonizaticn solids bed is forced to ilcw downwardly through the Isp-ace provided between baie 66 and the wall of the carbonizer vessel 36. Hot combustion and iluidizi-ng gases ow upwardly through fthe same space countercurrent to .the descending char and provide a stripping action which .assists .in the removal of tar compounds from the char. The removal of volatile components from the coal in the carbonizaftion zone, therefore, is effected in two ways, i. e., by elevating the pretreated coal particles to .the carbonization temperature and by passing these particles downwardly countercurrenr .to ascending combustion and luidizing gases before withdrawing them from the carbonization zone. Without a doubt, increased temperature is the major factor in effecting tar removal; however, .the stripping action of the combustion gases contributes to the total tar yield by removing some residual volatile materials.
As mentioned in the previous discussion, the size distribution of the coal particles introduced to the carbonization unit -is .subject to variation, particularly in commercial operations Where the crushing and grinding equipment used must handle large quantities of coal. The effects of size distribution `on oxygen consumption and .the temperature in .the pretreating zone have also previously been discussed in detail. In orderto compensate for variations in .temperature in this zone, provision is marde -t-o pass hot char trom the carboniziastion zone to |the pretreating zone .through conduit 48 and :standpipe 50. The quantity of this stream .at any given `instant may vrary over a wide range, depending on the rapidity with which fthe :size distribution of Ithe coal particles changes. `I-t is ldesirable Ito pass .a small :amount of char, -namely about 0.01 pound per pound of coal feed, continuously into .the pretreating zone through the char recycle system. The continuous char recycle allows more complete temperature control since slight increases in lthe pretreating .temperature can be compensated for by decreasing or entirely halting the flow of char. The motive force required to .transfer the char between the two zones is supplied by superhealted steam admitted to recycle conduit 48 .through conduit 68.
The final products of the carbonization process comprise .a mixture of tar vapor-s, steam .and combustion gases, and carbonaceous char solids. Distribution of .these products, based .on .the wet coal feed, is approximately 8 percent steam, 14 percent tar compounds and 76 percent char. The remainder of .the coal is converted .to combustion products to supply the process heat requirements. The Igaseous products pass trom the dense phase bed 'of char 43upwardly into a dilute phase 47 and from :there .through ya cyclone separator 46 and conduit 52. Solids recovered in the cyclone are returned to the `dense char bed below the surface Ithereof. Chrar solids product .are removed from .the bottom of the carbonzizer 36 through conduit 62, .are picked up by a stre-am of fluidizing 4steam rand .are passed through conduit 72 upwardly through `a char cooler 74 for preliminary cooling. The fluidized char solids enter the cooler at a temperature substantially the same .as that maintained in the .carbonization zone, i. e., about 925 F., and exit from fthe cooler .at la temperature of about 500 F. To extract the heat trom fthe char, .a cat cracker decanted oil of about 15 API gravity is introduced into .the cooler .through conduit 32 at sa temperature of lab-out400 F. This material flows through the cooler counterourrent fto the char and exits- .therefrom through conduit 30, being heated in .its passage .through the cooler to about 600 F. To provide .a process of maximum thermal eiciency,
a continuous circulating lluid system (not shown) is used` in which la common hydrocarbon lfluid accomplishes botli lfd system, it is necessary to supply an. additional amount of heat to the oil prior to its` introduction into the coal heater 28. This may be done in .any conventional manner, such las, for example, by passing fthe decanted oil through a conventional fired heater (not shown) or other conventional heating means.
The lower tempenature char leaving cooler 74 is passed into `a char pot 98 trom which it ows downwardly through conduit 80 into a char hopper 84 where it accumulates in a conventional dense phase duidized bed 83, superposed by :a .dilute phase 86. Although a substantial amount of heat is removed from `the char in the cooler, this material lis still much too hot to be yielded as produc-t. It i-s preferable, for convenience in handling the char, that i-t be cooled yto a much Lower temperature and, if possible, by .a more efoient method than indirect heat exchange. The large :amount of additional cooling required is conveniently and economically furnished by introducing water into the cbm through conduit 82 prior to passage of the char into the char hopper 84. The water is immediately converted to steam, thus providing, in addition to the cooling effect, .additional .tluidizing med-ium suitable for maintaining the solids in conduit 80 in a turbulent strate. The amount of water combined with the chair is controlled `to provide a temperature in the char hopper at or slightly above the dew point of water at the pressure existing therein. ln this' speoic illustra- Ition, the hopper pressure is about 3.5 p. s. i. g. and the temperature of the dense char bed 88 is about 230 F. These conditions 'aire maintained by coo-lling the char with labout 0.07 pound of 80 F. water per pound of char. Operating in this manner prevents liquid water iirorn pas-sing into the hopper, and the ysolids contained therein are readily maintained in a lfluid state.
Steam which results from the char cooling disengages from the solids in bed S8, passes upwardly through dilute phase 86 and a conventional cyclone separator 90 for the removal of entrained solids, and thence -through conduit 108 into a secondary solids recovery tower 98, Before entering this tower, the char hopper gases lare joined through conduit by the gaseous effluent from the drier and preheater 10. Within the solids recovery tower 98 which contains a number of baffles 106, the combined gases containing both char and coal solids are yscrubbed with water introduced through con-duit 100, and spray ring 104. The resulting solids-water slurry is withdrawn from the bottom of the recovery tower through conduit 110, is diluted with additional water from conduit 94 and then combined with char removed from the bottom of the char hopper through conduit 92. The slurry water serves to condense any steam remaining in the char released from the hopper, thereby deiluidizing this material. The total solids product comprising char admixed with a small amount of coal is then removed from the unit by a conveyor or by other suitable means (not shown).
The temperature in the solids recovery tower is about 216 F., which, at the pressure existing therein, that is about 2 p. s. i. g., is equal to the dew point of water. It is preferred in carrying out the solids recovery process that a minimum amount of the steam introduced to tower 98 be condensed. In order to assure this result, the temperature of the scrubbing water is maintained at substantially the same level as the temperature within the tower. This is conveniently accomplished by heating the water prior to its introduction to the recovery tower, `or more preferably by recycling hot slurry from conduit 110 to the top of the recovery tower (not shown). Even when using recycle slurry for scrubbing, however, it is necessary to introduce extraneous warm make-up water through conduit 100 to compensate for water in the slurry combined with the char product. The scrubbed gases, consisting of essentially solids-free steam, accumulate in the upper portion of tower 98 and are removed therefrom through conduit 102. This gas, although low in pressure and temperature, contains a large amount of latent heat 20 and may be used in any conventional service where low pressure steam is of value.
Tar vapors formed in the pretreating and carbonization zones, together with the gaseous produc-ts of combustion, pass from the carbonizer 36 through conduit 52 and are introduced into a tar quench tower 112. A substantial portion of the tar in these gases consists of compounds which are liquid under normal atmospheric conditions. These compounds are readily condensed in the quench tower by contacting the hot gases with a quantity of cool liquid tar. The liquid tar also serves as a scrubbing medium and operates to remove char solids entrained in the hot gases. In carrying out this step, the vapors are introduced into the bottom of the tar quench tower and pass upwardly around ballles 114 countercurrent to liquid tar introduced into the tower through conduit 142. The cooler vapors subsequently pass through a number of perforated trays 116, through a mist extractor 118 to remove entrained liquid droplets and exit from the quench tower through con-duit 120. The liquids and solids removed from the vapors by the scrubbing tar are transferred from the bottom of the quench tower through pump 136 and are passed through conduit 138 and cooler 140. A portion of the cooled material is returned to the quench tower through conduit 142 and the remainder is yielded as product through conduit 138. The temperature of the gases leaving the top of the quench tower is `about 160 F This is still substantially above atmospheric temperature and in order to lower the temperature of the gases still further they are passed through a water cooler 122, where additional tars are condensed, and then into an accumulator 124 where a further separation of gas and liquid takes place. This final cooling step reduces the temperature of the gases to about 100 F. The gases are released from the accumulator through conduit 126 and pass into a Cottrell precipitator 128. Liquid is removed from the precipitator through conduit 132, is combined with accumulator liquid from pump and conduit 134, and this combined stream is in turn added to the tar product passing through conduit 138. The final vapor product comprising primarily combustion gases and steam leaves the precipitator and the unit through conduit 130.
The preceding discussion has been directed to a preferred embodiment of the invention as specifically illustrated in Figure l. It is not intended that the material presented be construed in any limiting sense, but that other equipment, process conditions, flows, etc. are also used within the scope of the invention. For example, to limit the possibility of equipment plugging prior to the drying operation it may be desirable to introduce wet coal directly from a mechanical conveyor, such as a bucket elevator, into the dryer vessel 1i). Again, to provide a more flexible process, less dependent, or even separate, drying and preheating facilities may be provided. An alternate arrangement of the pretreating and carbonization zones is also contemplated, in which pretreating is carried out in a cylindrical Zone centrally disposed within the carbonization Zone, sealed therefrom and separated at the top in a manner similar to the pretreater of Figure 1. ln this apparatus arrangement, combustion in the carbonization zone would be carried out in an annular space surrounding the pretreating zone. Although internal circulation of char from the carbonization zone to the pretreating zone for temperature control is preferred for a number of reasons, it is also within the scope of the invention to carry out this step through external means. These and other -modes of operation are explained in more detail in the subsequent discussion of certain other embodiments of the invention.
The following data are presented to illustrate a typical commercial carbonization operation based on the processing arrangement of Figure' 1.
`Example Flows.' t p,
Wetcoal: f p to 400 mesh lb./hr 450,000 Water content lb./hr 35,000 Dry coal lb./hr n 415,000 Char product 1b./hr 345,000 Volatile product lb./hr 60,000 Solids content nlb./hr 1,000 Pretreater air lb./hr 80,000 Carbonizer air lb./hr- 50,000 Char recycle (for pretreater temperature control) a ..lb./hr 25,000 Feed coal heater:
Coal circulation rate lb./hr 1,250,000 Heating fluid- API hydrocarbon oil lb./hr 1,550,000 Product char cooler-cooling fluid-15 API hydrocarbon oil lb./hr 1,550,000 Cooling water injected into char product i lb./hr-` 28,000 Water to solids recovery toWer lb./hr 96,000 Tar quench tower reflux lb./hr 580,000 Temperatures:
Wet coal F-- 60 Drying zone F 270 Preheating zone F 480 Preti-eating zone F-- 725 Carbonization zone F-.. 950 Char hopper F-- 230 Solids recovery tower F-- 216 Tar quench overhead F 165 Feed coal heater:
Coal in F` 270 Coal out 3' F 480 Heating uid in F-- `500 Heating fluid out F-- 400 Product char cooler:
Char in F-.. 950 Char out F-- 500 Cooling fluid in F 400 Cooling fluid out F-- 470 Cottrell precipitator f F 150 Product char F-- 190 Tar product F 350 Pressures:
Drying zone p. s. 1. g-- 3.8 Preheating Zone p. s. i. g 6.0 P-retreating zone (top) p. s. i. g 11.0 Carbonization zone (disperse phase) p. s. i. g-- 8.0 Char hopper p. s. i. g-- 3.5 Solids recovery tower ..-p. s.i g-- 2.0 Tar quench tower p. s. 1. g-- 6.0 Average residence time of coal in:
Pretreating zone minutes 60 Carbonization zone do 30 Gas velocity in:
Drying zone -..fh/sec-- 2.0 Preheating zone ft/sec-- 2.5 Pretreating zone ft./ sec-- 1.0 Carbonization zone ft/sec-- 1.5 Density of solids in:
Drying zone 1b./cu.ft 25 Preheating zone lb./cu.ft Pretreating zone lb./cu. ft-- 22 Carbonization zone lb./cu. ft-- 18 As previously noted, the amount of oxygen consumed in the carbonization process and the conditions under which it is consumed a-re very important in determining the type and quantity of the gaseous, liquid and solid products derived from the carbonization process. In order to obtain maximum product yields, it is desirable that the proportion of the total oxygen consumed in the pretreating step be limited to the minimum required to particle agglomeration is allowed to reach the maximum possible without resulting in an inoperable condition. This point is important, for if an excessive amount of oxygen is used in the pretreating zone, valuable tar components are consumed therein and the tar yield is ycorrespondingly reduced. Also, the introduction of an excessive amount of oxygen produces more than the required amount of` case hardening which retards devolatilization of the coal and encourages cracking, particularly in the carbonization zone. On the other hand, if insuicient pretreatment oxygen is used, there is a danger that the carbonaceous particles may adhere together and that the process may become inoperable. In addition to the quantity of oxygen used, the pretreatment temperature is also important in determining the operability of the lluidized carbonaceous solids system and it is preferred to carry out the pretreating step at an elevated temperature. At temperatures normally used in this operation, it is impossible to supply all of the heat required in the pretreating zone by burning carbonaceous material therein and at the same time provide a degree of pretreating consistent with optimum product yields. For this reason, it is necessary to provide an additional large Vquantity of heat fromanother source, usually by preheating the carbonaceous solids in a conventional manner, such as, for
example by indirectheat exchange with a hot fluid. The further amount of heat required to raise the carbonaceous material from pretreating temperature to the carbonizing temperature and to carry out the carbonization processv preparing the coal for pretreatment, although it possesses many advantages, is relatively inexible in operation. For reasons of heat transfer eiciency, the temperature in the drying zone is preferably maintained at the minimum necessary to drive off surface water introduced with the wet coal. Similarly the preheater temperature, once established, is more or less fixed and cannot be varied without seriously affecting the subsequent pretreating and carbonization steps. In an operating unit the range of variation of these temperatures serves to limit the amount of coal solids circulated through the coal heater. This is important since the heat transfer coefficient of the gassolids stream passing through the heater is directly proportional to the quantity of solids in said stream and, therefore, it maybe desirable to vary the composition of this stream. i
A more llexible drying and preheating process arrangement wherein the advantages of a variable composition gas-solids stream lare realized is possible if the two steps are made somewhat less interdependent. In one embodiment of such an arrangement, the dry and preheated solids are again maintained in dense phase lluidized beds similar to those illustrated in Figure 1; however, in this instance the beds are entirelyseparated and solids do not flow from the preheating zone into the drying zone. In carrying out the drying operation, wet coal is introduced into the drying zone solids bed wherein it is quickly elevated in temperature with a concurrent release of surface water as steam. A stream of dry coal is passed from this bed through a first indirect tubular heater and is then returned to the drying zone to supply the heat required therein. The range of temperature through which drying may be effected is substantially theisame as for the drying process previously discussed. It is necessary to provide sufficient residence time in the drier for the coal'to become heated to the drying temperature and for the Vwater vapors released therein to escape from the solids bed; more usually a residence time of between about 1 and .about l5 minutes is zsuficient. The amount ,of
solids .circulated Athrough the =heater1varies with zthetmoisi ture content of the coal and the .temperature :maintained in the drying zone; more usually, the solids circulation rate is maintained between about 2 and about l5 pounds of solids per ,pound of wet coal feed. With circulation rates .of this order of magnitude, it is necessary in border to provide the heat lrequired for drying to increase the temperature of thecoa'l during passage through ithe heater to between about 250 and about 500 F. Heat :is transmitted to the dry coal solids from a hot fluid stream, preferably selected from the group of iiuids previously listed. This material is admittedto the heater in suficientquantity and at an adequate temperature `to provide the necessary :transfer of heat. -As stated before, lit is usually preferred to introduce this fiuid into the heater at a temperature between about 350 F. and l000 F.
vIn addition to the solids circulated through the drying zone a further amount ofsolids vis passed from this zone through the first heater, then through a vsecond heater and into the preheating zone. -In the latter heater, additional thermal energy is Vtransferred to the coal Vto provide `the necessarydegree of preheat needed Afor the pretreating voperation. Since the -preheated coal lis the only -source of feed -to the pretreating zone, the ysecond solids stream must contain at least an amount of coal equivalent -to the wet coal feed on a dry basis. This quantity of solids, however, is not sufficient to provide the solids densityV in the second heater necessary for 'good heat transfer. To remedy this, an additional streamvof solids is recycled from the preheater to the inlet of the second heater. More usuali it is preferred to circulate coal solids through the preheating zone at a rate between about 2 and about nl pounds per pound of wet coal feed. The variables and operating conditions contemplated for use in this portion of the process, in general, conform in magnitude to those previously given in the general discussion of coal preheating. For example, the temperature in the preheating zone is usually maintained between about 350 and about 650 F. A similar heating fluid is used to supply heat to the solids in the second heater and the quantity and temperature of this material is suitable adjusted to give the desired results.
The aforedescribed method of drying and preheating provides a number of advantages over the method illustrated in Figure l. 'For example, it provides a more flexible process in which coal circulation rates through the heaters and the temperatures in the drying and preheating zones can be varied more or less independently of each other. rIhis increased flexibility of operation is particularly important from the viewpoint of obtaining maximum heat transfer rates in the heaters and reducing the heating surfaces to a minimum. As previously mentioned, the heat transfer coefficient is directly proV portional to the -density of solids in the gases passing through the tubular heater. That is, when the concentration of solids in the gas is increased the kheat transfer coefiicient increases, and when the solids density decreases the heat transfer coeflicient is lowered. The density of the gas-solids mixture in turn is ydetermined by two factors, the velocity of the fiuidizing gases and the solids circulation rate. When the solids circulation rate is relatively fixed, as in the drying and preheating method of Figure 1, the only way in which solids density can be changed is by varying the velocity of the fiuidiz ing gas. Thus, if it is desired to increase solids density, the velocity of the fiuidizing gas must be decreased, and conversely a decrease in solids density is brought about by increasing the velocity of the fluidizing medium. Unfortunately, this variable can only be manipulated within certain limits. For example, at a velocity below about 0.25 foot per second solids do not remain in suspension and at high velocities erosion of 'the heat exchange equipment may become la problem. With the 224 system of preheating and .drying just described, it is possible to vary both the` solids circulation rate and the :fluidizing gas velocity thereby providing a more fiexible operation.
Another important advantage vof this method of operating lies in the fact that the drying and preheating steps are carried .out in entirely separate heaters `rather than in one heater. The efficiency of heat transfer in an exchanger -is a function not only of heat transfer coefficients but also =of temperature difference lbetween the heat source .and the heat absorbing stream. The greater the averagel difference 'between these temperatures, the more -eiciently is the heat exchanged. The use of a separate heater for drying the coal provides a substantially higher temperature differential in this heater than exists in the heater of -Figure l. Since the major portion of the heat required in the two heating steps, usually between about 55 and about 75 percent, is transferred in the first heater, a more thermally' efficient process results from this method of operation. This is true even though the temperature difference in the preheat exchanger is less favorable than the temperature difference in the heater of Figure -1. The thermal advantage of the two-heater system is further enhanced by passing a substantial part of the solids to be preheated through the first exchanger, wherein a portion of the required preheat'isprovided. The net result is a more efficient drying and preheat-ing process -which substantially decreases heat exchange equipment requirements. Still another important -ad-vantage -resu-lts Yfrom the use of the two-ex changer system. It -often happens in commercial installations that heat exchange equipment must be limited in physical dimensions, particularly in height. With the use of vertical type heaters as contemplated herein the restriction of exchanger Ylength becomes an important problem. 'In a system where the rate of solids circulation, and thus the solids concentration or density, can be varied -it is possible to obtain similar results to those which are produced in the apparatus of Figure l with an exchanger of lower surface area. This, of course, means a shorter exchanger which simplifies the supporting structure and allows a greater'variety of equipment location.
Although in the `aforedescribed operation there is no passage of solids from the preheating Zone to the drying zone, -as Lin the process illustrated by Figure l, this method of treat-ing the coal maybe carried out in similar` apparatus with only a slight modification, that is by extendingthe'vessel which encloses the preheating zone above the level of the dense phase bed of vsolids in the drying zone. Thus this system also provides the advantages of carrying out both drying and preheating in a single vessel.
In order to more clearly illustrate this aspect of the invention, reference is had to Figure 2. Referring to Figure 2, drying and preheating of the coal are carried out in a vertical cylindrical vessel l0 somewhat similar to that illustra-ted in Figure l. Internally vessel 10 is divided into concentric zones 12 and M by a cylindrical member which terminates below the inside top lof vessel 10 and extends downwardly outside and below vessel l0, being supported -on -t-he carbonizer vessel 36. The drying portion of the process is carried vout in the annular space which forms zone i2 in adense phasc turbulent bed of dry Ycoal particles, the upper level of which lies below the .top terminus of the cylindrical member. Preheatet coal is accumulated in zone 14 and is maintained therein in a similar dense phase bed, also having an upper level `below the top of the cylindrical member whereby the two beds are kept entirely separate.
Vapors released within the two zones enter a common dilutephase 16 above the solids bed, pass through a conventional cyclone 18 for the removalof entrained solids, and exit from the system through conduit 20.` Wet coal solids are introduced to the drier through conduit 8 and enter the dense turbulent bed 12 where they are quickly heated to the drying temperature, that is to about 270 F. At the pressure maintainedlon the system, namely about 4 p. s. i. g., this is considerably above the boiling point of Water. As a-result, the water introduced with the coal is quickly converted to steam which escapes to the dilute phase 16 and from the`drying and preheating system as previously described. To provide the large amount of heat required for this operation, a steam of solids is removed from the dense phase bed 12, passed downwardly through conduit 22, entrained in steam and passed upwardly through conduit 26 and feed coal heater 144. An amount of heated solids equal in quantity to the wet coal feed rate, that is about percent of the total solids passed through the heater, is further passed through the dry coal heater 152 and returned to the preheating zone 14. The remainder of the dry heated coal is passed through conduit 150 to the drying zone 12. The total quantity of coal circulated through the feed coal heater 144 is about 10 pounds per pound of wet coal feed and in its passage through lthis heater the coal is elevated in temperature from about 270 F. to about 325 F. Passage of the aforementioned solids stream through heater 152 does not provide a suicient concentration of solids in the heat to assure good heat transfer. To increase the solids density in this stream to a more desirable level, a second stream of solids is circulated from the preheating zone 14 also through heater 152, and is returned to zone 14. The circulated solids are removed from the aforesaid zone through conduit 157, entrained in uidizing steam introduced through conduit 155 and the mixture is combined with solids leaving the feed coal heater 144 through conduit 158. The total solids then pass through the heater 152 and return to the preheating zone through conduit 34. Tovmaintain the preheater temperature at about 480 F., it is necessary to provide a rate of solids withdrawal from bed 14 for circulation through heater 152 of about 5 pounds per pound of wet coal feed. This quantity of solids combined with the solids from the feed coal heater 144 produces a temperature of about 450 F. entering heater 152. In its passage through the latter heater the coal is further elevated in temperature to about 480 F.
The heat required to carry out the drying and preheating step is supplied by using a hot hydrocarbon oil similar to that mentioned in the discussion of Figure 1. A separate stream of oil may be used in each of the two heaters; however, it is preferred to use a common heating uid for both services with the oil being passed through the heater countercurrent to the fluidized coal, the sequench of passage through the heaters being opposite to the coal stream circulated from the drying zone. The How of oil through the heaters is maintained at a sufcient rate to provide an inlet temperature to the dry coal heater 152 of about 675 F. At the coal circulation rates illustrated, the oil leaves the dry coal heater at about 580 F. and the feed coal heater 144 at about As previously described in the discussion of Figure l, preheated coal passes from zone 14 downwardly through standpipe 44- into a pretreating zone 42 contained within the carbonization zone 36. Fluidization of the solids `Within zone 14 is maintained by introducing a small 26 Example Flows: l Wet coal: A
10 to 400mesh lb./hr. 450,000 Water content lb./hr 35,000 Dry coal lb./hr 415,000 Feed coal heater:
Coal circulation rate 1b./hr 4,500,000 Heating Huid-15 API hydrocarbon oil lb/hr-- 1,550,000 Dry coal heater: v
Coal circulation rate lb./hr 2,250,000 Heating fluid-15 API hydrocarbon oil flb./hr 1,550,000
Temperatures: v l
Wet coal F 60 Drying zone F..- `270 Preheating zone t F-- 480 Feed coal heater: q
Coal in F-- 270 Coal out F 325 Heating fluid in F-.. 465 Heating fluid out F ,400 Dry coal heater: v
Coal in F..- 450 Coal out F-- 480 Heating fluid in F-- 500 Heating uid out F-.. 465 Gas velocity in: r l
Drying zone ft/sec-- 2.0 Preheating zone ft./sec 2.5 Density of solids in:
Drying zone 1 lb./cu. ft-- 25 Preheating zone lb./cu. ft-- 25 The problem of water in the coal feed has been previo usly discussed in 4detail in conjunction with thedrying phase of the carbonization process. The problem of operability in the predrying portion of the unit, however, has not been considered. As shown in Figure l, before entering the unit, wet coal is introduced into a feed standpipe 4 from which it is further passed into atransfer line 8 for introduction into a drier and preheater. There is Valways a possibility that this portion of the system may become inoperable when handling coals of high water content. This is particularly true if the flow in any part of the system is momentarily interrupted, such as by an equipment failure or a temporary loss of iluidizing medium. As an answer to this problem, another method of operation has been conceived in which the danger of solids plugging or packing due to water in the coal is eliminated. In carrying out this embodiment of the invention, wet coal ground to a suitable size is introduced into an elevated feed hopper in a non-fluidized condition. Elevation of the non-fluid'solids to the hopper level is accomplished through the use of a conventional solids conveying apparatus, such as for example a bucket elevator. Within the hopper there is maintained a dense phase fluidized bed of coal having a lower average moisture content and a higher temperature than the wet feed coal. Wet coal entering the hopper commingles with the solids in this bed and because of the excellent mixing characteristics thereof, is quickly elevated in temperature and distributed throughout the dry solids. The result is a mixture of wet and dry coal which is readily maintained in a lluidized state. In a separate drier and preheater vessel there is provided a second dense phase bed of dry coal at a temperature substantially higher than the solids in the hopepr. The average moisture content of the coal in the feed hopper is maintained below the level of the wet coal feed by circulating dry solids from the second dense phase bed to the hopper and by returning an equal amount of solids plus the fresh feed from the hopper to the dry coal bed. The amount of solids circulated varies depending primarily on the water content and uidization properties of the wet coal. More usually, however, an operable process is provided by employing a dry coal circulation rate between about l and about 3 pounds per pound of wet feed coal.
It is preferred yto carry out the mixing of wet and dry solids in as small a vessel as possible, both for reasons of economy and to limit supporting superstructure, since the feed hopper .is normally located near the drier and preheater vessel. tl is also preferred to perform this operation without .the installation of cyclones .or other expensive solids recovery apparatus. In the method of this invention, a system is provided in which the wet and dry solids .are mixed in a turbulent solids bed characterized by its `4-highdensity .and low iiuidizing gas velocity. The result is a minimum of solids entrainment in the eluent gases. To maintain the solids in the feed hopper in a iiuidized state, a small amount of inert gas, such as, :for example air `of flue .gas is introduced into this vessel. Ordinarily, the amount of such gas is regulated to provide a velocity in the solids bed of between about 0.25 and about `0.5 foot per second, which provides a solids density therein between about 40 and about 30 pounds per cubic feet. -It is important that the temperature lin the feed hopper be prevented from exceeding the boiling point of water since any substantial -amount of vaporization in the feed hopper would result in increased gas velocities and excessive entrainment of solids from the bed. Due to the method -of-int-roducing the wet solids into the system, it is preferred to operate the hopper at essentially atmospheric pressure. Accordingly, the temperature therein is suitably maintained between about V100 and about 200 F. The turbulent nature of the tluid bed in the feed hopper serves to quickly distribute the wet feed coal throughout -the solids conta-ined therein. As a result, only a very short solids residence time is provided in this vessel, and even though a high circulation rate of dry coal is em- .ployed this stage ,of the coal carbonization process is easily carried out in a vessel small in size relative to the drier and preheater.
The feed hopper may be physically located adjacent to the drier and preheater vessel in any conventional manner. For example, the hopper may be placed along side the drier but below the level of the dense phase bed con- -tained within the latter vessel. With such an installation, dry coal is conveniently passed downwardly into the solids bed, the hopper and a stream -of solids of low water :content consisting yof the recycled dry coal plus the fresh feed is removed from the hopper and `passed upwardly Vinto the drying zone in a fluidizing medium such as steam. Although an installation of this type is prehaps preferable from an operating standpoint, it is much more desirable structurally ,to place the feed hopper above and support it on the drier and preheater. This presents an additional problem, however. To provide the required dry 4coal recycle, it now becomes necessary -to entrain coal from the drying zone in a lluidizing medium and pass the mixture upwardly .from the drying zone into the feed hopper. The amount `of iiuidizing medium required to accomplish this, more usually between about 0.001 and about 0.05 pound per pound of dry coal circulated is lmuch lgreater than the quantity of vapor which can be handled .in the feed hopper without excessive solids .entrainment. One method of overcoming this difficulty is to use a mixed fluidizing gas containing primarily steam and only suicient air to maintain the partially .dried solids in vthe feed hopper in a fiuidized condition. The major portion of the steam in the tluidizing gas immediately condenses upon entering the feed hopper solids bed and is distributed throughout the coal particles. The small amount of iluidizing air used and the uncondensed steam pass through the dense phase -bed into a dilute phase and from -there `through `a `conventional solids separation means. Since additional water is introduced into the feed hopper in this method of operation, it -is 223 necessary Ito Lcirculate .more dry coal to this vessel when it is located above rather .than below the drier and preheater vessel. However, the amount of water introduced as -lluidizing steam is very small, usually less than about l0 percent ofthe water present in the wet feed coal.
A somewhat 4different drying and preheating arrangement is provided in this embodiment of the invention. Among other things, the heat required for drying and pretreating is furnished by the use of entirely separate heaters. That is, all of the solids .circulated to supply heat to the drying zone are passed through oneheater and the total solids circulated through the preheating zone are passed through a second heater. This method of operating provides a process arrangement which is more flexible than either of the -systems previously discussed. For reasons of economy, the drying and preheating steps are carried out in a single vessel. To simplify construction of this vessel and provide easier laccess to the ypreheating zone, the vtwo zones are separated by a transverse batiie through which solids are allowed to flow from the drying to preheating Zone. The drying zone is again maintained within a temperature range between about 215 and about 300 F. and above the dew point of Water at the pressure existing therein. The pressure maintained in the drying step Lis `also the same, being between -about l and about 30 p. s. i. g. The dryer solids circulation rate is somewhat lower in the .system illustrated by Figure 2, since none of the -preheat is supplied from these solids. In normal operation, coal is circulated -through the dryer at between about v2 vand about l5 pounds .per pound of wet feed coal. Conditions for -preheating the dry coal also are similar to those discussed during consideration of the several other embodiments of the invention. These include a preheating temperature between about 350 F. and about 650 F., a pressure inthe preheating zone between about 1 and about 30 p. s'. i. g. and a solids circulation rate through the dry coal heater 'between about 2 and about l0 pounds per pound of wet coal feed. In carrying out this embodiment of the invention, it is again preferred to use the same heating iluid in both heaters selected from among the group previously considered. The temperature of the heating iluid and -the quantity of this material required is of the same order of magnitude ns described for the systems of Figures -l and 2. The conditions of solids density and velocity under which heat is transferred to the dry and preheated solids are maintained at an optimum through the `flexibility of the process.
in order to more clearly describe this aspect of the invention and provide a better understanding thereof, reference is had to Figure 3. Referring to this figure, wet finely subdivided coal containing about 8 percent water is introduced from a feed means through conduit 162 into feed hopper 166 in a non-uidized condition. Within this vessel which is at atmospheric pressure there is maintained a Conventional Adense phase fluid bed of coal particles 172 having a temperature of about 195 F. and containing on the average about 3.3 percent water. Above the dense phase is a dilute phase 170 of low solids concentration. Gases leaving the dense bed pass through this Zone and are released from the hopper through conduit 164. The wet solids entering the dense phase bed '.1172 are quickly increased in temperature to the level of the solids contained therein and due to the turbulent nature of the bed are mixed and dispersed throughout the hopper. As a result, a highly operable process is `provided and the possibility of solids agglomeration and equipment plugging is eliminated.
Support for the `feed hopper is provided by a subjacent drier and -preheater vessel 10 containing two :dense phase turbulent beds of coal 12 and 14 separated by a halide c rs1. Ab@ the beds is *a 29 common dilute phase 16 containing a conventionalcyclone 18 through which vapors pass for the removal of entrained solids. The'solids in bed 12, which comprises the dryingzone, are supplied from the vdense bed 172` through standpipe 174. The flow ofsolids between the two beds is controlled by a slide valve 175 or other conventional means. In order toprovide the lower solids water content required in bed 172, drycoal from bed 12 is` passed upwardly through conduit 176 into the feed hopper 166. The motive force necessary to transfer this solids stream is supplied by steam introduced into the bottom of riser 176 through conduit 178. Upon entering the feed hopper, most of the fluidizing steam is almost immediately condensed and distributed throughout the solids bed 172. `v To provide solids turbulence in bed 172 and maintain the coal therein inY a fluid state, a small amount of non-condensable gas,
account the amount of water introduced to the feed hopper as fluidizing steam,1a solids circulation rate of about 2 pounds per pound of wet ycoal feed is required to provide the desired temperature and solids water content. The temperature in the drying zone is maintained at about 270 F. by circulating a quantity of the solids from this zone through a feed coal heater 144. Suflcient heat is provided by this operation, not only to heat the solids in the drying zone but also to vaporize the water introduced with the coal from the feed hopper. In carrying out this operation, a circulating solids stream at a rate of about 8 i pounds of coal per pound of wet feed coal is removed l lfrom through conduit r148 at about 400 F.
In this specific illustration, dry coal in an amount equal to the coal in the wet feed is introducedto `the preheater solids bed 14 through an opening 180 in baille 181. These solids are quickly preheated from the temperature maintained in bed 12 to the preheating temperature, that is about 4809 F. 'I'helheat required in this stage of the carbonization process is supplied byy passing a solids stream from the preheating zone through a `dry coal heater in a manner similar to that just described. Preheated solids in an amount equal to about 6 pounds per pound of dry coal is passed'downwardly through conduit 160, is entrained in steam fromconduit 158 and is passed upwardly through heater 152 and returned to the preheating zone through conduit 34. The heat transferred to the `coal in heater 152 -is supplied from the same heating fluid used in heater 144 but prior to the passage of this c fluid through heater 152 is much lower, namely about 35 F. thus requiring an inlet heating fluid temperature of about 500 F. i
Transfer of solids from the preheating Zone to the pretreating zone (not shown) is eected in a similar manner to that previously discussed in the consideration of Figures l and 2, that is, by passing the solids downwardly` through standpipe 44 which extends within the carbonization vessel 36.
A typical application of this embodiment of the inven- `tion on a commercial scale is illustrated by the following data. i
10 to 400 mesh lb./hr 450,000 Water content lb./hr 35,000 Dry coal lb./l11 V 415,000 Coal circulation through feed hopper lb./hr 1,250,000 Feed coal heater:
Coal circulation rate lb./hr 3,600,000 Heating fluid-15 API hydrocarbon oil lb./,hr 1,550,000 Dry coal heater:
Coal circulation rate 1b./hr 2,900,000 Heating fluid-15 API hydrocarbon oil lb./hr 1,550,000
Wet coal F 60 Feed hopper F 195 Drying zone F 270 Preheating zone F 480 Feed 'coal Heater:
Coal in F 270 Coal out F 325 Heating fluid in F 465 Heating fluid out F 400 Dry coal heater:
Coal in F 480 Coal. out F 510 Heating fluid in F 500 Heating fluid out F 465 Pressures:
Feed hopper p. s. i. g-- 0 Drying and preheating zones p. s. i. g 4 Gas velocity in:
Feed hopper ft./sec 0.3 Drying zone ft./sec 1.8 Preheating zone ft./sec 1.8 Density of solids in:
Feed hopper lb./cu. ft-- 36.0 Drying zone lb./cu. ft 27.0 Preheating zone lb./cu.,ft 27.0
l Another method of introducing wet coal into the coal carbonization system whereby plugging of equipment and lines is avoided is illustrated in Figure 5. In carrying out this embodiment of the invention, wet nely subdivided coal at a temperature of about 60 F. and again having a moisture content of about 8 percent by weight is introduced into ya feed hopper 166 in a non-uidized state. This material may form a level in the feed hopper,
however, more generally, the feed rate to this vessel is sufficient to maintain it completely full of solids. From the hopper the wet solids pass downwardly through a conduit 176 into a uidized stream of hot dry solids passing substantially in a parallel direction. The two streams are commingled and the wet coal is raised in temperature and distributed throughout the dry solids. The entire mass is readily maintained in a uidized condition within the descending portion of conduit 193 by the introduction of a small amount of air or other inertgas through conduit 194. Below the feed hopper is a drier Vand preheater vvessel 10 similar to that shown in Figure l. The mixture of solids of lower moisture content passes downward through conduit 193 and enters a dense phase bed 12 of dry solids where the moisture contained in this material is converted to steam.` Above the dense bed 12 is a dilute phase 16 into which the vaporized water passes. From the dilute phase, the water vapor is introduced into a conventional cyclone 18 and after the removal of solids is released from the drier and preheater 10 through conduit 20. The dry solids circulating stream into which wet coal is introduced from the feed hopper 162 is supplied lfrom the dense phase bed 12, being entrained in `lluidizing steam through conduit 190 and passed upwardly through the initial part of conduit 192. This stream reverses its direction bypassing in a curved path and contacts the wet solids while tlowing in a downward direction. As in the previously described operation, illustrated by Figure 3, the major portion Nof the fluidizing steam condenses during the mixing .of Ithe two solids streams. Similarly in this embodiment of ,the invention, it is desirable to maintain `the ,pressure at the point Where the wet solids are introduced into conduit 192 at substantially atmospheric pressure.
The temperatures, pressures, solid circulation rates, gas flow rates,etc., employed in thisembodiment of the invention are similar to those previously described in the discussion of Figure 3. The methodand means for supplying .the heat required `for drying .and preheating are also similar to :those ypreviously discussed and may conform `to the systems of Figures l, 2, or 3 `or combinations thereof. ()ne important difference .between this methodof wet feed introduction and that previously described flies in the reduction of tluid beds from 3 to 2, since inthis embodiment, the material in the feed hopper .ismaintained in a non-huid condition. Actually, the feed hopper amounts only to a wide extension of conduit 176, and may :be substituted for by a similar conduit of appropriate dimensions.
The illustration shows commingling of wet .and dry solids in conduit 193 in substantially parallel how. This is not intended, however, in a limiting sense and combination of the two streams at an angle up toland including .the perpendicular is contemplated although flows approaching uparallel are preferred yand provide greater Operability- The remainder of the coal carbonization process, although not shown, conforms generally to the process il- ,lustrated in Figure l.
Another important aspect of the coal carbonization 32 `order to `pr-ovideva clearer understanding of this embodiment-ofthe invention. Referring to the drawing, hot char Iat a Itemperature of about 950 F. is removed from the carbonization Vessel through conduit `62, entrained in ffluidizing steam and passed upwardly Ithrough conduit 72 and into char cooler 74. Prior 4to entering cooler this stream is joined by a circulating stream of cooler char from conduit 18'2, ythe latter material having a temperature of about 600 F. The combined stream is cooled in the char cooler by indirect heat exchange with a petroleum oil introduced .through conduit 32 from about 720 F. Ito about 600 F. This oil, in turn, is heated in the cooler from about 400 F. to about 470 F. and exits therefrom through conduit 30. The total cooled solids .pass upwardly from the exit of the char cooler through conduit 76 into a dense phase bed 188 of cha-r ymaintained within a char pot 9'8. `Fluidizing gases containing entrained solids escape from this bed pass through a conventional cyclone l186, `and :the solids free gases are Iremoved throughconduit .184. The major portion of the solids introduced into bed `188 yare returned to the inlet of .process relates to the method of removing heat from lthe product char to `provide a more easily handled material. The method of char cooling illustrated Yin Figure l wherein the char is passed through a tubular cooler reffectively reduces the temperature of this material; however, because of its relative inexibility, it is necessarily restricted. to operation-within rather narrow limits for maximum thermal efficiency. To provide a more thermally efficient system `an expedient similar to `that used in the drying and preheating steps, namely circulation of solids through the heat transfer apparatus, is also resorted to for cooling the char. The yamount-of solids circulated vthrough the char cooler in this operation varies depending on -the inlet temperature of the char andthe amount of cooling desired. More usually a solids circulation rate of between about l and about l() pounds per pound of product char is provided. The proportion ofthe total char cooling obtained in this operation may, of course,
be varied by changing the cooler surface larea and the temperatureof the cooling fluid. For reasons 'of economy, it is usually preferred to use as a cooling medium .the tiuid stream provided to dry and preheat the coal. However, When a common stream is used it is necessary to Weigh the desirability of maximum char coolingas against the necessity of a high temperature heating medium for drying and preheating. Actually, when carbonizing coal under the conditions previously described a balanced system is not possible and heat is added to the uid leaving the char cooling Istep from -an outside source. More usually the cooling tiuid leaves the char cooler at a temperature between about 400 F. and about 900 F.
With Ithe above opera-ting conditions, the char which passes from the carbonizer `at la -t-emperature between about 700 F; and about 1000 F. is reduced in temperature in the cooler to between Iabout 400 F. `and about 700 This material is again accumulated in a charpot; however, the use lof recirculation requires the maintenance of a conventional huid bed in this vessel, which was not necessary in the ysystem of Figure l.
rI Ilhe accompanying drawing, Figure 4, is referred to in the charcooler 74 through conduit '182. The remainder, equal in quantity to the net .char yield is Withdrawn through conduit and passed to a char hopper (not shown). zone through the char .cooler is `determined according to ythe operating conditions required in the process. In this specic illustration, the char Vrecycle rate is about 4 pounds Vper pound of hot char. The advantages of the increased :flexibility provided `by solids recirculation ,plus the eect of this expedient on heat transfer surface required and the .thermal ,efficiency of this type of heat exchange system have been discussed previously and need not be repeated here. It may be noted, however, `that the use Iof solids circulation is particularly advantageous when .it Lis desirable to 'restrict the length of `the cooler.
The preceding illustration, 'Figures l through 5 exemplify preferred embodiments iof the invention; however, it is not intended that they be construed in a limiting sense. Thus other processing schemes and Variations and modiiications well known to those sleille-d in the art are also within the scope of this invention. For example, the drying and preheating phases of `the .carbonization process `may be vcarried out in separate vessels rather than in contiguous zones in a single vvessel :as shown. Also `downtlow tubular exchangers may be used rather than vthe upftow type; however, the former, being much more etlicient, are preferred. In certain Aof its aspects this invention is much broader in its scope than in other aspects. For example, the preheating and carbonization phases of the` process are limited to the treatment in a more or less specific manner, of carbonaceous solids. This is not true, however, of the drying and preheating steps, nor is it true of the char cooling method or the use of 'a common heat exchange fluid. Because of their broader applicability, Iit is contemplated that the processing methods Irelating to these operations may be used in other processes and in the treatment of solids other than carbonaceous solids, such as, for example catalytic materials vand solids normally used for contacting purposes, such as pumice, carborundum, sand, ctc. The presence of volatile surface liquids other :than Water may also affect the uidizing properties of inely subdivided solids and removal of such liquids is also contemplated by the ap- The amount `of solids circulated from the char i example in coal gasification, preparation of powdered solid fuels, catalytic cracking processes, etc., is contemplated it should be understood that equivalent results are not to be expected in all uses and the preferred embodiments are those illustrated and described herein.
Having thus described the invention by referenceto a specific example thereof, it is understood that no undue limitations or restrictions are to be imposed by reason thereof, but that the scope of the invention is defined by the appended claims.
l. A unitary process for the treatment of carbonaceous solids to remove therefrom volatile constituents which comprises heating and drying the solids in a finely subdivided state in a drying zone in a dense phase iluidized bed, further heating the dry solids in a preheating zone in a second dense phase bed, passing dry heated solids from the preheating zone downwardly as a confined stream through a su'bjacent carbonization zone to a pretreating zone wherein there is maintained at an elevated temperature above the temperature in the heating zone a third dense phase bed of finely subdivided pretreated solids, said pretreating zone being enclosed within the carbonization zone and openly communicating therewith through a grid, introducing oxygen int-o the pretreated solids bed and partially burning heated solids introduced thereto, maintaining within the carbonization zone a dense phase bed of finely subdivided char at a temperature above the temperature in the pretreating zonej passing pretreated solids and combustion gases through the grid directly into the `enclosing carbonization zone beneath the level of said dense phase bed of char, vaporizing from the pretreated solids volatile tar constituents whereby said carbonaceous solids are converted to char, introducing additional oxygen into the char solids bed to burn a portion of the devolatilized char and provide the heat vrequired Ito elevate the pretreated soli-ds to the carbonization temperature and vaporizing volatile components therefrom, recovering combustion gases and volatile tar components from a dilute phase above the dense char bed and removing product char from said` bed.
2. A unitary process for the treatment of wet agglomerative carbonaceous solids to remove therefrom volatile constituents which comprises heating and drying the solids in a finely subdivided state in a drying zone in a dense phase iiuidized bed, further heating the dry solids 1n a preheating zone in a second dense phase bed, passing dry heated solids from the preheating Zone downwardly as a confined stream through a carbonization zone subjacent to the drying and preheating zones to a pretreating zone wherein there is maintained at an elevated temperature above the temperature in the heating zone a third dense phase bed of finely subdivided pretreated solids, said pretreating zone lbeing enclosed within the carbonization zone and separated therefrom except :by an upwardly communicating grid, introducing oxygen into the pretreated solids bed and partially burning heated solids Iintroduced thereto whereby the agglomerating tendency of the solids is reduced, maintaining within the carbonization zone a dense phase bed of finely subdivided char at a temperature above the temperature in the pretreating zone, passing pretreated solids and combustion gases upwardly through the grid directly into the enclosing carbonization zone beneath the level of said char bed, vaporizing from the pretreated solids volatile tar constituents whereby said carbonaceous solids are converted to char, introducing additional oxygen into the char solids =bed to burn a portion of the devolatilized char and provide the heatrequired to elevate the pretreated solids to the carbonization temperature and vaporizing volatile components therefrom, recovering combustion gases and volatile tar components from a dilute phase above the dense char fbed and removing product char from said bed.
3. A unitary process for the treatment of wet agglomerative carbonaceous solids to remove therefrom volatile constituents which comprises heating and drying the solids in a finely subdivided state in a drying zone in a dense phase iluidized bed, further heating the dry solids in a preheating zone in a second dense phase bed, passing dry heated solids from the preheating zone downwardly as a confined stream through a carbonization zone subjacent to the drying and preheating zones to a pretreating zone wherein there is maintained at an elevated temperature -above the temperature in the heating zone a third dense phase bed-of finely subdivided pretreated solids, said pretreating zone being enclosed within the carbonization zone and openly communicating therewith through a grid, introducing oxygen into the pretreated solids `bed and partially burning heated solids introduced thereto whereby the agglomerating tendency of the solids is reduced, maintaining within the carbonization zone a dense phase lbed of finely subdivided char at a temperature above the temperature in the pretreating Zone, passing pretreated solids and combustion gases through the grid directly into the enclosing carbonization zone beneath the level of said dense phase bed of char, vaporizing from the pretreated solids volatile tar constituents whereby said carbonaceous solids are converted to char, introducing additional oxygen into the char solids bed to burn a portion of the devolatilized char and provide the heat required to elevate the pretreated solids to the carbonization temperature and vaporizing volatile components therefrom, recovering combustion gases and volatile tar components from a dilute phase above the dense char bed and removing product char from said bed.
4. A unitary `process for the treatment of wet coal vdownwardly as a confined stream through a carbonization zone subjacent to the drying and preheating zone to a pretreating zone wherein there is maintained at an elen vated temperature above the temperature in the heating Hzone a third dense ,phase bed of finely subdivided pretreated solids, said pretreating zone being enclosed within the carbonization zone and openly communicating therewith through a grid, introducing oxygen into the pretreated solids bed and partially burning heated solids introduced thereto, maintaining within the carbonization zone a dense phase bed of finely subdivided char at a temperature above the temperature in the pretreating zone, passing pretreated solids and combustion gases through the grid directly into the enclosing carbonization zone =be neath the level of said dense phase bed of char, vaporizing :from the. pretreated solids volatile tar constituents whereby said carbonaceous solids are converted to char, introducing -additional oxygen into the char solids bed to burn a portion of the devolatilized char and provide the heat required to elevate the pretreated solids to the carbonization temperature and vaporizing volatile cornponents therefrom, recovering combustion gases and volatile tar components from a dilute phase above the dense char bed and removing product char from said bed.
5. A process as in claim l in which the carbonaceous solids are coal'.
6. A unitary apparatus for the treatment of carbonaceous solids to remove therefrom volatile constituents which comprises in combination drying means, preheating means, supporting carbonization means subjacent thereto; pretreating means disposed within said carbonization means, each of said means being adapted to contain a uidized bed of carbonaceous solids, means for introducing wet solids into the drying means, means adapted for the passage of dry solids from the drying means