|Publication number||US2933822 A|
|Publication date||Apr 26, 1960|
|Filing date||May 16, 1956|
|Priority date||May 16, 1956|
|Publication number||US 2933822 A, US 2933822A, US-A-2933822, US2933822 A, US2933822A|
|Inventors||Nathan Marvin F|
|Original Assignee||Kellogg M W Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Referenced by (5), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
April 26, 1960 M. F. NATHAN TREATMENT OF CARBONACEOUS souns Filed May 16, 1956 PUDDOEtl l IU 5:. H on.
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ATTORNEYS United States Patent ce TREATMENT OF CARBONACEOUS SOLIDS Marvin F. Nathan, New York, N.Y., assignor to The M. W. Kellogg Company, Jersey City, N.J., a corporation of Delaware Application May 16, 1956, Serial No. 585,354
5 Claims. (Cl. 34-10) This invention relates to method and means for treating solid materials and more particularly to method and apparatus for the fluidized treatment of carbonaceous materials such as coal, shale, lignite, oil sands, etc., at
- low temperatures. Still more particularly, the invention relates to unitary method and means for handling wet finely divided carbonaceous materials in a fluid system.
This application is a continuation-in-part of my copending application Serial No. 517,472, filed June 23, 1955, now U.S. Patent No. 2,775,551, dated December 25, 1956.
The treatment of carbonaceous solids to form valuable liquid, gaseous and solid products is well known in the art. An example of one process 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. This process is usually called carbonization. In another process, which comprises an extension of the carbonization process, the non-distillable portion of the carbonaceous solids is also converted to gas. This process is customarily referred to as gasification. It has been the practice in the past to carry out carbonization and gasiflcation in both non-fluid and fluid systems; however, the present invention is concerned with processes of the fluid type wherein the various steps are performed with a finely divided feed material which is maintained in a highly turbulent state of agitation by the passage therethrough of a fluidizingmedium.
. In'carrying out fluidized 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 first step in the carbonization process concerns the proper preparation of the raw feed material. This involves proper selection and sizing of the carbonaceous solids to provide a readily fluidizible feed and proper handling of the solids to main tain the fluid system.
One of the problems frequently encountered when handling carbonaceous materials such as coal in a fluidized system results from the tendency of the finely divided solids to agglomerate 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 15 percent by weight, or higher. This moisture may cause packing or bridging in process equipment of restricted cross section, such as, for example in feed hoppers, standpipes, etc.
It is an object of this invention to provide an improved method and means for handling wet solids in a fluidized system.
Another object of this invention is to provide improved method and apparatus for carrying out the gasification and carbonizing of carbonaceous materials.
It is still another object of this invention to provide improved method and means for drying wet finely divided solids in a fluidized system.
2,933,822 Pa ten ted a r. 26, race wet with a liquid material into a dense phase fluidized bed containing similar solids having a lower liquid content, said bed being maintained at a temperature below the boiling point of the wetting material. The solids of lower liquid content are provided by introducing to said bed dry solids fluidized with gaseous material condensable at the temperature in the dense phase bed. Fluidization of the dense phase bed is provided by the introduction thereto of a non-condensable gas. The amount of said gas is controlled to provide a solids bed of high density and low fluidizing gas velocity whereby entrainment of solids from said bed is held to a minimum.
In a narrower aspect of the invention the wettinginaterial is water and the solids of lower liquid content are provided by introducing to said bed dry solids fluidized with steam and a non-condensable gas, the quantity and composition of this fluidizing material being such that the fluid bed is maintained without entrainment of any substantial amount of solids therefrom.
In still another aspect of the'invention wet finely divided solids are passed downwardly from a non-fluid feed zone through a confined zone to a dense phase bed of dry solids. During said passage solids from the dense phase bed fluidized with gaseous wetting material and noncondensable gas are introduced to the confined zone in suflicient quantity to fluidize the descending wet solids and provide the desired moisture content thereof.
It iswithin the scope of this invention to treat various finely divided solid materials in the manner described including for example catalytic materials and solids normally used for contacting purposes, such as sand, pumice, Carborundum, clays, etc., also carbonaceous materials, such as coals, shales, lignites, asphalts, oil sands, etc. The invention is particularly exemplified; however, by its application to the treatment of the latter materials and specifically to the carbonization of coal.
The following discussion is directed to this type of operation; however, it is not intended that the particular application presented should limit the scope of the invention in any way.
In carrying out a preferred embodiment of the invention, wet coal ground to a suitable size, usually between about 10 mesh and 5 microns, 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 fluidized 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 hopper. 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 v3 depending primarily on the water content and fluidization properties of the wet coal. More usually, however, an operable process is provided by employing a dry coal circulation rate between about 1 and about 3 pounds per pound of wet feed coal. It is preferred to 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. It 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 high density and low fluidizing gas velocity. The result is a minimum of solids entrainment in the effluent gases. To maintain the solids in the feed hopper in a fluidized state, a small amount of non-condensable gas, such as, .for example air or 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 foot. When operating under these conditions solids entrainment from the dense phase is negligible and it is not necessary to provide for solids recovery from the gases leaving the feed hopper. More usually the loss of solids in the fluidizing gas does not amount to more than about 0.02 percent by weight of the fresh feed and more usually between about 0.01 and about 0.001 percent thereof.
It is important that the temperature in 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 introducing 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 100 and about 200 F. The turbulent nature of the fluid bed in the feed hopper serves to quickly distribute the wet feed coal throughout the solids contained therein. As a result, only a very short solids residence time is provided in this vessel, usually between about /2 and about 5 minutes, and even though .a high circulation rate of dry coal is employed, 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 contained 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 of the recycled dry coal plus the fresh feed is removed from the hopper and passed upwardly into the drying zone in a fluidizing medium such as steam. Although an installation of this type is perhaps 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 coal recycle, it now becomes necessary to entrain coal from the drying zone in a fluidizing medium and pass the mixture upwardly from the drying zone into the feed hopper. The amount of fluidizing medium requrred to accomplish this, more usually between about 0.001 and about 0.05 pound per pound of dry coal circulated, is much greater 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 gascontaining primarily steam and only sufiicient air to maintain the partially dried sq ds asse in the feed hopper in a fluidized condition. The major portion of the steam in the fluidizing' gas immediately condenses upon entering the feed hopper solids bed and is distributed throughout the coal particles. The small amount of fluidizing air used and the uncondensed steam pass through the dense phase bed into a dilute phase and from there to the atmosphere. Since additional water is introduced into the feed hopper in this method of operation, it is necessary to circulate 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 fluidizing steam is still very small, usually less than about 10 percent of the water present in the wet feed coal. 7
Operation in the manner previously described provides a mixture of wet and dry coal which is readily fluidized and which may be subjected to further treatment without danger of agglomeration or equipment plugging. In a conventional system solids recovery equipment'is required to remove entrained solids from gases leaving the densev solids phase .and elaborate precautions are required to prevent leakage and lossof .solids from the. conveying system which delivers the raw feed material. Due to the low velocity of the fluidizing gases in the feed hopper no solids recovery system is required when operating in accordance with this invention and it is not necessary. to seal the conveying system which supplies wet feed coal to the feed hopper.
Following the predrying treatment, the coal is subjected to further processing which includes drying, preheating, pretreating and carbonization. In carrying out the drying operation, the mixture of wet and dry coal from the feed hopper is introduced to the drying zone as previously described wherein it is commingled with dry heated coal in sufficient quantity to elevate the entire mass of coal to a temperature suitable to effect 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 fluid 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, provides preheated coal for the next phase of the carbonization process. The entire drying and preheating step is conveniently conducted in a fluidvsystem with both the low and high temperature zones containing a dense phase bed of'fluidized 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 10 and about 40 pounds per cubic foot. IThe temperatures in the two zones will vary. 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 released 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 fluid state by the introduction of a similar gasifying medium. It is necessary to circulate a with cient 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 ofthe 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 passed through the tubes in indirect heat exchange with a hot fluid passed through the exchanger shell. The heat re quired to dry the coal is provided by a fluid 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 specificheats 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 1000 F. Temperatures greater than this are not desirable because of the danger of overheating 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 coefiicients, 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 suflicient to provide the desired drying and preheat.
After leaving the preheating zone, the coal is passed into a pretreating zone where it is contacted with air or other oxygen containing gas and partially burned. The purpose of this operation is to case harden the coal particles and thereby nullify their agglomerating tendency as they pass through the plastic state. The temperature at which this 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 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 prejtreating. 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. The amount of oxygen required for pretreating is usually between about 0.02 and about 0.08 pound per pound of dry coal feed. Io provide sufficient .time for the pretreating combustion reactions to take place, the rate of introduction of coal to the pretreating zone is adjusted to allow an average particle residence time therein of between about 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 present in the coal are vaporized and passed into the fluidization and combustion gases. Since oxygen is relativelynon-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.
.iFollowing pre reating the coal is passedinto a carbon- '6 ization 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 fluidized bed similar tov the drying, preheating and pretreating beds previously described. In order to effect removal of the volatile coal components, a large amount of heat must be introduced to the carbonization 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 flue 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. When heat is provided by burning either fuel gas or coal, the gasiform fluidizing medium required to maintain the dense phase in the carbonization zone is generally furnished by the combustion gases. If necessary, however, deficiencies inthe quantity of fluidizing medium may be made up by the introduction into the carbonization zone of a flue gas, steam or other extraneous inert gas.
The carbonization of coal to remove distillable tars therefrom and produce a char residue product is conducted over a wide range of temperatures usually be-v tween about 700" F. and about 2400 F. The preferred thermal range of operation is determined to a great extent by the 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 carbonization, the temperature is held to a minimum of about 700 F. and not more than about 1000 F. The type of coal is also of importance in establishing the operating temperature since some coals are more difficult to distill than others. 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 the dense phase zone pass into the dilute phase, this 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 10 minutes. As a further precaution to prevent agglomeration of the coal particles in the carbonizing zone, it is preferred to maintain a substantial ratio of char to fresh feed therein. This serves to dilute the fresh pretreated coal, which provides the desired beneficial effect; however, it also makes it neces sary 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 and about minutes.
The predrying, drying, preheating, pretreating and 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.
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 required when carbonizing these feed materials other than coal. The conditions appropriate for each specific feed material are well known to those skilled in the 1 and for this reason do not need repeating here.
Hot char product from which the major portion'of'the a s-Baas volatile constituents of the'coaI -havebeen"removed is withdrawn from the lower portion of the carbonizer and is passed through acooler' wherein the temperatureof the -char-is lo were'd'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, comprisinga 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, fluids similar to those previously'disclosed for use in drying and preheating the coal are used. This operation is simplified and the c'ost'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'freshcoal feed. Inasmuch as the heat removed from the char in the cooling operation may not-be sufi'lcient to provide the heat required for drying an'd preheating the coal feed, or vice versa, it is desirable when using a common heat exchange fluid 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, whichever 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. The coolingfluid may be introduced to the cooler at any low temperature; however, when a common circulating stream is used the inlet temperature, of necessity, conforms to the temperature of the fluid 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 coeflicients 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 beremoved economically by indirect cooling, particularly when using a common'circulating heat exchange fluid. To further cool the char and provideo. more easily handled product, water is injected into the partially cooled fluidized 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 the char in the hopper in a fluidized state. The cooled product .is then conveniently removed from the hopper, defluidized by contact with additional water which condenses the fluidiaingsteam and is passed from the system by-means of .a' .conveyor or, other suitable. means.
7 As previously mentioned, the eflluent vapors from the carbonizer. comprise gaseous, products of combustion and yarions tar compoundsplus'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 carboniz'ation process. To efiect'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 effect necessary to condense liquid tars but also effects 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. With? in the tower are provided suitable baffles or plates whereby intimate contact between ascending gases and down? flowing liquid is effected. The pressure at which this operation is carried out is controlled by the pressure in the carbonizaion zone, being somewhat lower, usually between about 10 and about 2 p.s.i.g. It has been found that the major portion of the desirable liquid tar com-: pounds are condensed by cooling the carbonizer gases to between about 'F. and about 80 F. The remaining vaporous tar compounds and combustion products forma gas, which although low in heat content, may be usedas a fuel. If desired, of course, a further separation between the uncondensed 'tar compounds and combustion and flnidization gases may be eifected.
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 diagrammatic illustration in cross sec tion of process equipment used in carrying out a preferred embodiment of the invention comprising a unitary coal carbonization system which includes a feed hopper, 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 feed hopper, coal drierand preheater.
Referring to Figure 1, coal at a temperature of about 60 F., having a particle size distribution between about 10 mesh and about 5 microns and containing about v8 percent water is introduced from a feed means through conduit 162 into feed hopper 166 in a non-fluidized condition; Within this vessel, which is at atmospheric pressure, there is maintained a conventional dense 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 very low solids concentration through which the gases leaving the dense bed pass prior to release through conduit 164. The wet solids entering the dense phase bed 172 are-quickly raised in temperature to the level of the solidscontained 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. Within this vessel there is maintained a dense highly turbulent bed 12 of dry coal particles at a temperature 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 confined within an annular space lying between the walls of the drier and a cylindrical elongated conduit extending upwardly through the bottom of the drier. Within this conduit lies a preheating zone 14 in which there is maintained a higher tem' perature dense bed of coal particles which overflow-continuously into the lower temperature dry solids bed 12. Above the dense beds of dry and preheated walls 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 lower solids water content required in bed 172, dry coal 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, the major portion of the fluidizing steam is almost immediately condensed and distributed throughout the solids bed 172. To provide solids turbulence in bed 172 and maintain the coal therein in' a fluid state, a small amount of non-condensable gas in this specific illustration air, is introduced either through conduit 171 or through conduit 178 or both. Taking into account the amount of water introduced to the feed hopper as fluidizing steam, a solids circulation rate of about 2 pounds per pound of wet coal feed is required to provide the desired temperature and solids water content. The flow of solids between beds 12 and 172 takes place through the standpipe 174 which is controlled by a slide valve 175 or other conventional means.
The fluidizing mixture of air and uncondens'ed steam passes through the dense phase bed at a very low velocity, about 0.2 foot per second, providing thereby a bed density of about 35 pounds per cubic foot. Although the velocity of the fluidizing gas is sufficient to maintain solids turbulence, it is not great enough to entrain any substantial amount of solids from the dense phase. As a result, it becomes unnecessary to provide for solids recovery from said gases.
To obtain the heat required in the drying zone, a stream of dry coal is removed therefrom through conduit 22, entrained in fluidizing 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 hot 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 28 is about 3 pounds of coal per pound of wet feed. The heat required in the combined drying and preheating operatoin is supplied by passing the circulating solids stream in indirect heat exchange with a cat cracker decanted oil having an API gravity of about 15. This material is introduced to heater 28 through conduit 30 at a temperature of about 680 F. and exits therefrom through conduit 32 at a temperature of about 400 F. Theforegoing method of drying the coal is simple in application and in addition to efiecting the removal of moisture from the coal, it provides a ready means of adding to the coal the amount of preheat required before pretreating the coal, the next step in the process.
Although the hot coal leaving 28 enters zone 14 in a fluidized condition, it may be desirable .to introduce ad ditional 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 fiuidizing gases may also be introduced to zone 12. The amount of fluidized gases passed through each zone is controlled to provide a velocity therein of about 1.2 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 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 flow 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 thestandpipe 44. The pretreating zone 42 is separated in part from the carbonization zone 43 by a vertical bafifle 66 attached at the bottom and sides to the inner wall of the carbonizer vessel 36. The bottom portion of the pretreating zone contains a distribution grid 60 for distributing fluidizing gases throughout the pretreating zone. The pretreating zone opens upwardly into the carbonizing zone 43 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 pound 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 socalled 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 conduit 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. Although the air admitted to the system normally suffices for this purpose, additional gases such as, for example, steam, flue gas, etc., may be introduced through conduit 64 for fiuidization purposes.
Coal entering the pretreating zone commingles with the solids contained therein and is partially oxidized and rapidly increased in 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 with any portion of unconsumed oxygen, passes upwardly through the pretreating zone and through grid 54 into the carbonization zone 43. Within this zone 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 number of organic tar compounds varying widely in molecular structure and boiling point. The increase in temperature in the pretreating zone 42 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 latterzone, the pretreated solids are quickly elevated tothe 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; how- 7 ever, in order to prevent solids from agglomerating and thereby assure an operable fluid 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 zone42. Thedegree of turbulence and density of the solids in zone .43 is regulated in a similar manner.
The heat required for 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 air 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 sis introduced into the bottom of the carbonization zone through conduit 70 which is at a point remote from the area of introduction of pretreatedcoal into the samezone. .Oxidation and combustion of the carbonized coal particles proceeds rapidly and is substantially completed before the carbonizer fluidizing and 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 also proceeds at a fast rate and, by the time the pretreated solids reach the zone of combustion, they are substantially free of volatile tars.
,By reason of the location of withdrawal conduit 62, char product from the main upper portion of the carbonization solids bed is forced to flow downwardly through the space provided between baffle ,66 and the wall of the carbonizer vessel 36. Hot combustion and fiuidizing gases flow upwardly through the 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 carbonization 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 countercurrent to ascending combustion and fluidizing 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.
The final products of the carbonization process comprise a mixture of tar vapors, steam and combustion gases, and carbonaceous char solids. Distribution of these products, based on the unconverted wetcoal feed,
is approximately 8 percent steam, 14 percent tar compounds and 78-percent char. The remainder of the coal is converted t'o combustion products to supply the process' heat requirements. The gaseous products pass from the densephase bed of char 43 upwardly into a dilute phase 47 'and from there through a cyclone separator 46 and conduit 52. Solids recovered in the cyclone are returned tothe dense char bed below the surface thereof. Char solids product are removed from the bottom of the carbonizer 36 through conduit 62, are picked up by a stream of fluidizing steam and 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 the cooler at a temperature of about 500 F. To extract the heat from the char, a cat cracker decanted oil of about 15 API gravity is introduced into the cooler through conduit 32 at a tempera ture of about 400 F. This material flows through the cooler countercurrent to the char and exits therefrom through conduit 30, being heated in its passage through the cooler toabout 600 F. To provide a process of maximum thermal efficiency, a continuous circulating fiuid system (not shown) is used in which a common hydrocarbon fluid accomplishes both char cooling and the drying and preheating of the coal feed. Substantially more heatis required in the drying and preheating operation than is obtained by the cooling char. Therefore, in order-to thermally balance the 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 as, for example, by passing the decanted oil through a conventional fired heater (not shown) or other conventional heating means. i
The lower temperature char leaving cooler 74 is passed into "a char pot 78 from which it flows downwardly through conduit 80 into a char hopper 84 where it accumulates in 'a conventional dense phase fluidized bed 88, superposed by a dilute phase 86. Although a substantial amountof heat is removed from the char in the cooler, this material is still much too hot to be yielded as product. It is preferable, for convenience in handling the char, that it be cooled to a much lower temperature and, if possible, by a more efiicient method than indirect heat exchange. The large amount of additional cooling required is conveniently and economically furnished by introducing water into the char 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 efiect, additional 'fluidizing medium suitable for maintaining the-solids in conduit 80 in a turbulent state. The amount of water combined with the char is controlled to provide a temperature in the char hopper at or slightly above the dew point of water at the pressure existing therein. In this specific illustration, the hopper pressure is about 37.5 p.s.i.g. and the temperature of the dense char bed 88 is about 230 F. These conditions are maintained by cooling the char with about 0.07 pound of 80 F. water per pound of char. Operating in this manner prevents liquid water from passing into the hop per, and the solids contained therein are readily maintained in a fluid state.
Steam which results'from the char cooling disengages from the solids-in bed 88, passes'upwardly through dilute phase 86 and a conventional cyclone separator 90 for the removal of entrained solids, and thence through conduit into a secondary solids recovery tower 98; Withinthe tower 98 which contains a number of baffies 106, the gases containing char solids are scrubbed with water introduced through conduit 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 fluidizing this material. The total solids product 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 R, 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 and may be used in any conventional service where low pressure steam is of value.
Tar vapors formedin the pretreating and carbonization zones, together with the gaseous products 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 baflies 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 conduit 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 portiohof the cooled material is returned to the quench tower through conduit 142 and the remainder is yielded asproduct through conduit 138. The temperatures of the gases leaving the top of the quench tower is about 160 11B. 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 tin-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 130 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 illus-.
trated'in, Figure 1. It is not intended that the material presented be construed in any lirnitiug sense, but that 14 other equipment, process conditions, flows, etc. are also used within the scope 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: Lb./Hr.
Wet coal 10 to 5 micron 450,000 Water content 35,000 Dry coal 415,000 Coal circulation through feed hopper 1,250,000 Char product. 345,000 Carbonizer Volatile product 60,000 Solids content 1,000 Gases leaving feed hopper- Steam 350 Air 200 Solids 5 Pretreater air 80,000 Carbonizer air 50,000 Char recycle (for pretreater temperature control) 25,000 Feed coal heater- Coal circulation rate 1,250,000
Heating fluid-15 API hydrocarbon oil 1,550,000 Product char cooler--cooling fluid15 API hydrocarbon oil 1,550,000 Cooling water injected into char product- 28,000 Water to solids recovery tower 96,000 Tar quench tower reflux 580,000
Temperatures: F. Wet coal 60 Feed lhopper 195 Drying zone 270 Preheating zone 480 Pretreating 70ne 725 Carbonization zone 950 Char hopper 230 Solids recovery tower 216 Tar quench overhead Feed coal heater- Coal in 270 Coal out 480 Heating fluid in 500 Heating fluid out 400 Product char cooler- Char in 950 Char out 500 Cooling fluid in 400' Cooling fluid out 470 Cottrell precipitator; 150 Product char .Tar product 350 Pressures: P.s.i.g. Feed hopper 0 Drying zone 3.8 Preheating zone 6.0 Pretreating zone (top) 11.0 Carbonization zone (disperse phase) 8.0 I Char hopper 3.5 Solids recovery tower 2.0 Tar quench tower 6.0
Average residence time of coal in: Minutes Feed hopper 0 Pretreating zone 60 carbonization zone 30 G'as velocity in: I Ft./sec Feed hopper- 7 0.3 Drying zone 2.0 Preheating zone 215 Pretreating zone -1'.0 Carbonization zone 1.5
Density of solids in: Lb./cu. ft.
Feed hopper ..'36i' Drying zon 25 Preheating 7nne 25 Pretreating zone 22 Carbonization mne 18 Another method of introducing wet coal into the: coal carbonization system whereby plugging-of equipment'and lines is avoided is illustrated in Figure" 2. In carrying out this embodiment of the invention, wet. finely subdivided coalat a temperature-of about60f F. and again having a moisture contentof about 8 percent'by weight is introduced into a feed hopper166'in a non-fluidized state. Generally, the feed-ratetothiswessel issufiicient to maintainit-completely full ofsolids;v From-the hopper the wet solids pass downwardlythrough a conduit 176 into a fluidized streamof'hotdry solids passingLsubstantially 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 fluidized condition within the descending portion of conduit 193 by the introduction of a small amount of air or'other non-condensable gas through conduit 194. Below the feed hopper is a drier and preheater vessel 10 similar to that shown in Figure 1. 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 con tained in this material is converted to steam. Above the I 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 18and after the removal of solids is released from the drierand'preheater 10 through conduit 20. The dry solids circulating stream into which wet coal is introducedfrorn the feed'hopper 166 is supplied from the dense phase bed 12, being entrained in fluidizing steam supplied to theconduit 192 through conduit 190 and passed upwardly through the initial part of conduit 192. This stream reverses its direction by passing in a curved path and contacts the specific example thereof, it is understood'thatno' undue limitations or restrictions are to be imposed by reasonv thereof, but that the scope of the invention'is defined vention are similar to those previously described in the discussion of Figure 1. The method and -rnean's for supplying the heat required for drying and preheating is also similar to the system of Figure 1. One important difference between this method of wet feed introduction and. that previously described lies in the reduction of fiuid beds from 3 to 2, since in this embodiment; the material in the feed hopper is maintained in a non-fluid condition. Actually, the feed hopper amounts only to a wide extension of conduit 176, and maybe substituted for by a similar conduit of appropriatedimensions;
The illustration shows commingling of'wet" and dry solids in conduit 193 in substantially parallel flow. This is not intended, however, in a limiting senseand combination of the two streams at an angle up'to and in cluding the perpendicular is contemplated although flows approaching parallel are preferred'andprovide greater operability; I
The'remainder'of'the coal carboniz ation process, at
though not shown, conforms generally to the process" illustrated in Figure 1.
The preceding illustration, Figures '1' and 2', exemplify;
preferred embodiments of the invention; however, it"is' flow tubular exchangers may be used rather than the upflow types. In certain of its aspects this invention is" much broader in its scope than in other aspects. For
example, the pretreating and carbonizaiton phases of the process are limited to the treatment in a more or'less This is not true, however, of the predrying solids treatment'and the drying step. Because of their broader applicability, it is con:
specific manner of carbonaceous solids.
templated that'the' processing methods relating to these operations may be used in other processes and in the treatment of solids other than carbonaceous solids, suchji as, for example catalytic materials and solids normally used for contacting purposes, such as pumice, 'carborundum, sand, etc.
o t-finely subdivided solids and removal'ot such liquids is also contemplated by the appropriate use of the afore- When solids wet with 9.
described operating procedures. I I I I liquid other than water are being treated in accordance with the invention, it is contemplated that the preferred fluidization medium for transferring dry s olids .to'thefemdf v hopper will be vaporized wetting; material. I However, if: desired, other condensable fluidizing gases may be used within the scope of the invention. The temperature range given for the predrying solids treatment and dryingstep are'primarily applicable to a low pressure carboni'zation system and might not be appropriate in a process operating under either reduced pressures or elevated pressures, or where a different use of the dry solids is It is, therefore, intended that the invention include within its scope operating conditions suitable.
to the particular process in which the aforedescribed solids treatment is required. Although uses of this method;
described herein. I
Having thus described my invention by reference to a by the appended claims.
1. A process for fluidizing finely divided solids wet with a liquid material which comprises, supp ying 'fineIyI' divided solids wet with a liquid material tov a feed-zone,
providing in a second zone a dense phase bed" of dry: finely divided solid material fiuidizedby'a conde 'lsable gaseous material, supplying heat to saidbedin said sec i' end zone to maintain the temperature therein. above the."
boiling point of said liquid material, introducing fluid-' fluidizing material condensable at the feed zone tempe ture introducing solids from said feed zone densephase"-- The presence of volatile surface liquids: other than water may also affect the fluidizing properties 17 bed to said second zone, controlling the amount of noncondensable gas introduced to said feed zone to prevent substantial entrainment of finely divided solids from 'said feed zone dense phase bed, yielding finely divided splids from said second zone, and venting fluidizing gas" from said feed zone.
2. A process for fluidizing finely divided coal wet with water which comprises maintaining in a feed zone at a temperature below the boiling point of water a dense phase fluidized bed of coal having a lower;m'oisture content than the wet coal, maintaining a dense'phase bed of dry coal in a second zone, supplying heat to said bed in said second zone to maintain the temperature thereof above the boiling point of water, introducing wet finely divided coal to the feed zone, passing coal from the feed zone dense phase bed to the second zone, introducing to said feed zone'dry coal from the second zone fluidized in steam, maintaining fluidization in the feed zone dense phase bed by introducing thereto fluidizing air, controlling the amount of said air to substantially .prevent entrainment of coal from the dense phase bed, yielding dry coal from the second zone and venting fluidizing gas from the drying zone.
3. A process for fluidizing finely divided solids wet with a liquid material which comprises introducing wet finely divided solids in a non-fluidized state to a feed zone, passing said solids downwardly from the feed zone to a confined elongated zone to a drying zone containing a dense phase fluidized bed of dry solids, supplying heat to said bed in said drying zone to maintain said bed in said drying zone at a temperature above the boiling point of the liquid wetting material, maintaining the temperature of the wet solids while in the confined elongated zone below the boiling point of the wetting material, combining dry solids from the drying zone fluidized in a gaseous material which is condensable at the temperature in the confined elongated zone with the down flowing wet solids whereby a mixture of wet and dry solids is produced prior to the introduction of the wet solids to the said drying zone, introducing to the elongated confined zone a non-condensable gas, controlling the amount of said gas to provide fluidization of the flowing solids mixture but insufficient to produce entrainment of solids from the feed zone, yielding solids from the drying zone and venting fluidizing gases from the feed zone.
4. The process of claim 3 in which the dry solids introduced to the confined elongated zone are fluidized in gaseous material of the same composition as the liquid material which wets the said solids.
5. A process for fluidizing finely divided coal wet with water which comprises introducing finely divided wet coal to a feed zone, passing said coal downwardly from the feed zone through a confined elongated zone to a dry zone containing a dense phase fluidized bed of dry coal, supplying heat to said bed in said drying zone to maintain said bed in said drying zone at a temperature above the boiling point of water, maintaining the temperature of the wet coal while in the confined elongated zone below the boiling point of water, combining dry coal from the drying zone fluidized in steam with the down flowing wet coal whereby a mixture of wet and dry coal is produced in said elongated zone, introducing fluidizing air to the elongated confined zone, controlling the amount of said air to provide fluidization of the flowing coal mixture but insuflicient to produce entrainment of coal from the feed zone, yielding coal from the drying zone and venting fluidizing gas from the feed zone.
References Cited in the file of this patent UNITED STATES PATENTS 2,480,670 Peck Aug. 30, 1949 2,534,051 Nelson Dec. 12, 1950 2,582,712 Howard Jan. 15, 1952 2,677,604 Nelson May 4, 1954
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|U.S. Classification||34/364, 208/409, 208/400|
|International Classification||B01J8/24, B01J8/28|