|Publication number||US3839186 A|
|Publication date||Oct 1, 1974|
|Filing date||Jul 2, 1973|
|Priority date||Jul 2, 1973|
|Publication number||US 3839186 A, US 3839186A, US-A-3839186, US3839186 A, US3839186A|
|Original Assignee||Universal Oil Prod Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (10), Referenced by (32), Classifications (9), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent Berger PROCESS FOR PRODUCING VOLATILE HYDROCARBON PRODUCTS FROM COAL AND HYDROGEN.
Primary ExaminerDelbert E. Gantz Assistant ExaminerJames W. Hellwege Attorney, Agent, or FirmJr. Hoatson; Thomas K. McBride; William B. Page, II
 Inventor: Charles V. Berger, Des Plaines, Ill.
 Assignee: Universal Oil Products Company, 57 ABSTRACT Des Plaines, lll. t A process for producmg volatile hydrocarbons, includ-  Filed: July 2, 1973 ing aromatics and olefins, from coal using two fluidized beds of devolatilized cell particles. Finely divided  Appl' 375997 coal is passed into a first fluidized bed of devolatilized coal particles in a reaction zone maintained at a tem-  US. Cl 208/8, 201/31, 201/38 perature of about 1,400F. to about 1,800F. hydro-  Int. Cl Cl0g l/00 gen, at a pressure of about 0.3 psia. to about 20 psia.,  Field of Search 208/8; 48/197 R, 202; is passed upwardly through the first fluidized bed; the 201/35, 36, 38, 31, 16 resulting hydrocarbon products are recovered from a mixture of hydrogen and hydrocarbons withdrawn  References Cited from the upper end of the reaction zone; heat is sup- UNITED STATES PATENTS plied to the reaction zone by continuously passing a 2,544,843 3/1951 Leffer 201/31 of the f? partlcle? from macho 2,582,710 1 1952 Martin 208/8 a Second fluldlzed P m a heatmg Zone 2,605,215 7/1952 gogmanm 201m, mamtamed at a temperature, higher than the tempera- 2,639,263 5/1953 Leffer 201/16 ,ture in the reaction zone, of about to about 2,709,675 5/1955 Phinney 201/31 2,000F., burning a portion of the carbon from the 2,840,462 6/1958 Gorin 208/8 devolatilized particles in the heating zone to heat the 3,047,472 7/1962 Gorin et a1 201/36 particles in the second fluidized bed, and continuously 5 passing a portion of the heated particles from the pac1 3,576,734 4 1971 Bennett 208/8 heatmg Zone back the macho Zone 6 Claims, 1 Drawing Figure r 5 Product Recovery V o/afifl/C 3 Zane Products "WOW/rob Zone tr l v /2 //3 9 /0 23 5\ Hopper Fue/ Gos ,5
7 Aromatic /22 i Products Redo/or l Ash i l 1-1 41 i z x PROCESS FOR PRODUCING VOLATILE HYDROCARBON PRODUCTS FROM COAL AND HYDROGEN BACKGROUND OF THE INVENTION This invention relates to a process for converting solid coal into normally liquid and normally gaseous hydrocarbon products.
This invention further relates to a process for producing aromatic hydrocarbons from coal.
This invention also relates to a process for producing unsaturated aliphatic hydrocarbons, such as olefins and diolefms, from coal.
A variety of methods are known in the art for converting solid coal into normally liquid and normally gaseous hydrocarbons. One of the simplest known methods is termed destructive distillation. Upon exposure of coal to high temperatures, volatile products such as monoand polycyclic aromatics and alkylaromatics are driven off from the coal as vapors. The remaining solids after this heating include primarily coke and non-volatile mineral matter. Destructive distillation of coal has been found to result in relatively low yields of the volatile products. Thus, use of destructive distillation of coal has been directed almost solely toward the maximum production of coke for use in metallurgy. Moreover, the volatile products produced by conventional destructive distillation often contain large amounts of undesirable, high molecular weight components. These heavy components, or tars, are generally unsuitable for use in organic chemical production without further costly refining where light hydrocarbons such as monocyclic aromatics or C -C1" aliphatics and cycloaliphatics are desired.
Methods for producing gases from coal are also well known. In these conventional gasification methods, typically. solid coal is contacted with a mixture of oxygen and steam at high temperatures and pressures. The oxygen reacts with carbon in the coal to form carbon monoxide and carbon dioxide with the simultaneous release of large amounts of heat. At the same time, steam reacts with the carbon in the coal to form hydrogen and carbon monoxide. The hydrogen formed in this reaction further reacts with carbon from the coal to form some methane. The gaseous product resulting from this type of gasification operation is a mixture containing carbon monoxide, carbon dioxide. hydrogen, methane and water. This gaseous mixture has a relatively low heating value, and is generally suitable only for industrial heat generation. In order to produce a gas suitable as a substitute for conventional natural gas. further upgrading of the conventional gasification product is necessary. Besides the expense of the upgrading required. such conventional gasification operations have several other inherent drawbacks. These gasification operations are necessarily performed at relatively severe operating conditions. including extremely high pressures. In addition. the only hydrocarbon product recovered from these gasification schemes in any substantial quantity is methane. Thus, the valuable aromatic and C2-C1" aliphatic hydrocarbons which are recoverable in low yields from destructive distillation operations, are generally not obtainable by conventional gasification operations.
It has been suggested that coal may be converted into coke and volatile hyrocarbon products by the use of a fluidized bed of devolatilized coal which is used to supply heat in order to produce the volatile products and coke. The prior art has suggested that a hydrogencontaining gas may be utilized as a fluidizing medium in such an operation. Operating conditions suggested for use in such methods have included relatively low reactor temperatures in the range from about 800 to about 1,200F. and relatively high reactor pressures in the range from about 1,500 to about 3,000 psig. The volatile hydrocarbon products which are normally recovered from such fluidized bed operations consist chiefly of methane and ethane, with minor yields of other light saturated aliphatic hydrocarbons. These fluidized bed procedures have accordingly been suggested primarily for use in providing natural gas substitutes, as opposed to, for example, petrochemical or petroleum substitute hydrocarbons. However, in many cases, aromatic and olefinic hydrocarbons are more desirable products from an economic standpoint, since these hydrocarbons may be further utilized in petrochemical and petroleum product-producing operations. Prior art fluidized bed operations for recovering volatile products from coal have several advantages over conventional destructive distillation of coal and conventional gasification of coal using steam and an oxygen containing gas. However, prior art methods for converting coal using such fluidized bed technology and using a hydrogen-containing fluidizing gas also suffer from certain disadvantages. High pressure operations of the type conventionally employed require an unduly large investment in vessels and other equipment which must necessarily be capable of withstanding extremely high pressures while operating at very high temperatures. The combination of pressures and temperatures taught for conventional operation result, as noted above, in the production of large quantities of light saturated hydrocarbons. This necessitates the use of large amounts of hydrogen in order to produce the light saturates from the hydrogen-deficient coal. While high yields of saturates such as methane and ethane may be desirable in some circumstances, it is necessary to supply much more extraneously produced hydrogen in order to produce these products than would be required to produce, for example, aromatic hydrocarbons or olefins from coal. Thus, it is necessary either to utilize an undesirably large fraction of the coal supply, by reaction with steam and oxygen, to provide the necessary hydrogen, or in some manner to obtain an adequately large supply of hydrogen from other sources extraneous to the coal conversion operation. By concentrating on the production of olefins and aromatics from coal, rather than the production of saturates, it would be possible, to substantially reduce the requirements of hydrogen in order to provide volatile products from coal.
SUMMARY OF THE INVENTION It is an object of this invention to provide a process for converting coal into volatile hydrocarbon products.
Another object of this invention is to provide a fluidized bed coal conversion process employing low pressure hydrogen gas as a fluidizing medium.
Another object of the present invention is to provide a process for producing high yields of unsaturated aliphatic hydrocarbons, such as olefins and diolefins, from coal.
Another object of the present invention is to provide a process for producing high yields of monocyclic and dicyclic aromatic and alkylaromatic hydrocarbons from coal.
Another object of this invention is to provide a coal conversion process using a hydrogen-containing gas as a coal fluidizing medium in such a manner as to require a decreased hydrogen gas consumption, relative to prior art fluidized bed coal conversion operations.
In a broad embodiment, the present invention relates to a process for producing volatile hydrocarbon products from coal which comprises the steps of: continuously passing carboncontaining, devolatilized coal particles from a reaction zone containing a first fluidizied bed of said particle, maintained at a temperature of from about l,40()F. to about 1,800F., into a heating zone containing a second fluidized bed of said particles, maintained at a temperature, higher than the temperature in the reaction zone, of from about 1,700F. to
about 2,00()F.; continuously introducing finely divided coal into the first fluidized bed in the reaction zone, continuously introducing a hydrogen-containing gas into the lower end of the reaction zone and passing the hydrogen-containing gas upwardly through the reaction zone, at a hydrogen pressure of from about 0.3 pound per square inch absolute to about 20 pounds per square inch absolute, to produce hydrocarbon product vapors and carbon-containing, devolatilized coal particles from the finely divided coal, withdrawing a gaseous mixture of hydrogen and hydrocarbon product vapors from the upper end of the reaction zone, and recovering the volatile hydrocarbon products from the gaseous mixture; and, continuously contacting an oxygencontaining gas with the second fluidized bed of carboncontaining particles in the heating zone to heat the second bed of particles and to produce gaseous carbon oxides, removing a gaseous stream containing the carbon oxides from the heating zone, and continuously passing carbon-containing particles from the heating zone into the reaction zone.
In contrast to prior art teachings that inclusion of hydrogen gas in a coal devolatilization operation increases the yield of light saturate products, I have found that the use of low pressure hydrogen in combination with high temperatures in a fluidized bed coal conversion operation results in an increase in the production of olcfmically unsaturated hydrocarbons and of monocyclic and dicyclic aromatic hydrocarbons from the coal conversion operation. conventionally, the use of hydrogen gas as a fluidizing medium has been thought to result in an increased production of saturated hydrocarbons, such as methane and ethane, when used in the fluidized conversion of coal to provide volatile products. Production of saturates is not necessarily undesirable when the product is sought to be employed as a natural gas substitute. In such cases, only the relatively large amount of hydrogen which is consumed in the operation is particularly undesirable. The present process. by contrast, is directed particularly toward the production of olefins, diolefins and aromatic hydrocarbons, which are generally not available by prior art coal conversion methods using fluidized bed technology. By the method of the present invention, wherein very low hydrogen pressures are employed in conjunction with high temperatures and relatively short gas residence times in the fluidized reaction zone, a surprising result is the high yield of olefins, diolefins and aromatics which may be obtained in the process, even through hydrogen gas is employed as the fluidizing medium. Among the apparent advantages of the present process are the wide range of economically desirable hydrocarbon products which can be recovered, and the relatively low hydrogen gas consumption in the operation. The low pressures used in the conversion method of this invention also substantially reduce the capital and operating problems which have been associated with conventional high pressure coal gasification methods.
DESCRIPTION OF THE DRAWING The attached Drawing is a schematic representation of one preferred embodiment of the process of the present invention. The Drawing is intended solely as an illustration and the process of this invention is not limited to the exact mode of operation shown therein. Various other suitable embodiments of the present process will be apparent to those skilled in the art from the drawing and from the description of the invention provided hereinafter. 1
Referring to the drawing, a supply of finely divided coal is maintained in coal hopper l. The coal supply consists of ordinary bituminous coal ground to a particle size of about 50-100 microns average. A portion of the coal supply is continuously withdrawn from hopper 1 and charged through conduit 2 into reactor 3. Reactor 3 contains a fluidized bed of devolatilized coal particles, maintained at a temperature of about 1,600F. Finely divided coal from conduit 2 is passed into the fluidized bed in reactor 3 by the use of distribution means 4 in order to ensure adequately uniform distribution of the freshly introduced coal into the bed of devolatilized particles. Because of the turbulent state of the fluidized bed of devolatilized particles in reactor 3, the coal particles freshly introduced into reactor 3 through distribution means 4 are continuously admixed with and diluted by previously devolatilized particles in the fluidized bed. The freshly introduced particles are rapidly dispersed into the devolatilized particles in order to prevent undue agglomeration of the coal and to prevent the formation of zones of overly high concentration of fresh coal. Hydrogen gas is charged continuously to the bottom of reactor 3 through conduit 5. The average partial pressure of hydrogen in reactor3 is about 3 psia. Steam introduced into conduit 5 by means not shown may be used as needed in order to supplement the hydrogen charged to reactor 3 through conduit 5, to ensure turbulent fluidized bed conditions in reactor 3. Hydrogen is passed into reactor 3 from conduit 5 by way of distribution means 7 in order to provide a uniform fluidizing medium in reactor 3. Hydrogen attack on the fresh coal and thermal breakdown of the freshly introduced coal results in extremely rapid partial devolatilization of fresh coal introduced into reactor 3. The volatile materials which are thereby produced include, in particular, large fractions of aromatic hydrocarbons and olefins. The upwardly flowing hydrogen gas in reactor 3 sweeps the volatile product upwardly through reactor 3 and the mixture of hydrogen and volatile hydrocarbon products is passed into cyclone 8. Hydrogen gas and vapor phase hydrocarbon products are passed from cyclone 8 out of reactor 3 through conduit 9. Entrained solids from the fluidized bed in reactor 3 are removed from admixture with the gaseous products and hydrogen in cyclone 8 and are rejected back into the fluidized bed in reactor 3. The gaseous mixture of hydrogen and volatile hydrocarbon products is passed through conduit 9 into quench zone 10, where the temperature of the gaseous reactor effluent is rapidly lowered to about l,l00F. by admixing with the reactor effluent a suitable quench oil which is introduced into quench zone 10 through conduit 11. The cooled hydrogen-hydrocarbon mixture is removed from quench zone 10 and passed through conduit 12 into product recovery zone 13. In product recovery zone 13, hydrogen and various hydrocarbon products are separated by conventional flash vaporization, fractionation, etc. Hydrogen is removed from zone 13 via conduit 5 and recycled for further use in reactor 3 as described above. This recycle hydrogen stream need not be pure, and may contain substantial amounts of methane and carbon monoxide, such as might be expected from operation of a demethanizing column in an ethylene recovery unit. In order to provide a continuous, adequate supply of hydrogen for reactor 3, a hydrogen-containing gas is introduced into conduit 5 from conduit 6 from an outside source, as needed. Referring again to product recovery zone 13, olefinic products, including ethylene and propylene with minor amounts of butenes, but significant amounts of butadiene, along with some saturates such as methane, ethane, etc., and gaseous sulfur and nitrogen compounds, are withdrawn from zone 13 through conduit 14 and passed to further purification, separation and refining operations not shown. Aromatic products, including benzene, alkylbenzenes, naphthalene, alkylnaphthalenes, and related ring compounds such as phenol, thiophene. pyridine. etc., along with minor amounts of heavy saturated and unsaturated aliphatic hydrocarbons. are withdrawn from product recovery zone 13 through conduit 15 and are passed out of the process to further separation and refining operations not shown. A portion of this aromatic fraction, withdrawn through conduit 15, may be used as a quench oil directed into quench zone 10, if desired. Any carbonaceous solids which escape from reactor 3 in admixture with the gaseous products into conduit 9 may be recovered in slurry form in product recovery zone 13 by well known means and returned to reactor 3 by way of conduit 16 as a slurry for further conversion. Referring again to reactor 3, a portion of the devolatilized coal particles in the fluidized bed in reactor 3 is continu' ously withdrawn from the lower end of reactor 3 through conduit 17. Air (and steam. if desired) is introduced via conduit 18 and is utilized to convey the particles which have been removed from reactor 3 through conduit 17 into and through conduit 19 and into heater 20. Heater 20 contains a turbulent fluidized bed of devolatilized coal particles which are maintained at a temperature of l.90()F. In order to achieve this relatively high temperature in heater 20, air or another oxygemcontaining gas is necessarily used as the fluidizing and combustion medium in heater 20. In heater 20, oxygen from the air charged through conduit 19 reacts with carbon from the devolatilized particles which make up the fluidized bed to form carbon oxide and to produce heat. Thus. the relatively low-temperature devolatilized particles which are charged to heater 20 from conduit 19 are heated while the temperature in heater 20 is maintained at the high 1,900F. level. If steam is charged to heater 20, it will react with carbon from the devolatilized particles and also will react with carbon monoxide formed by reaction of oxygen with the carbon in the particles. Thus, if desired, some hydrogen gas may be produced in heater 20. When air is employed, as in the embodiment depicted in the drawing, the amount of hydrogen which can be produced in heater 20 is quite limited, because the overall reaction in heater 20 must be sufficiently exothermic to supply the basic heat requirements in reactor 3. Alternatively, if oxygen is employed diluted with steam, a synthesis gas can be produced in heater 20 which, after a combination of shift and CO extractions, can be used as a hydrogen source for reactor 3. A portion of the relatively hot devolatilized particles in the fluidized bed in heater 20 is continuously withdrawn from heater 20 through conduit 21 and passed into the middle or upper end of reactor 3. Since the devolatilized particles passsed into reactor 3 through conduit 21 are at a significantly higher temperature than are the other particles and gases in reactor 3, the devolatilized particles charged into reactor 3 through conduit 21 act as the heat source to maintain the temperature in reactor 3 at the desired level of I,600F. The gaseous mixture resulting from reaction of the oxygen with carbon and carbon monoxide in heater 20 is passed into cyclone 22. In cyclone 22 the gaseous reaction products and any inert diluent gases such as nitrogen are separated from devolatilized coal particles and ash. The gaseous mixture is withdrawn from heater 20 and cyclone 22 through conduit 23. The solid particles removed from the gaseous stream in cyclone 22 are rejected back into heater 20. The gaseous mixture passed out of heater 20 through conduit 26 has some fuel value, although the hydrogen content is quite low. If steam is used in combination with oxygen in heater 20, the fuel value is greater but the energy available for heating reactor 3 may be less. A portion of the devolatilized coal particles in the fluidized bed in heater 20, is continuously removed from heater 20 through conduit 24 in order to prevent excess build up of coal ash in the overall heater-reactor system. The solid particles removed from heater 20 through conduit 24 are charged into cyclone 25, wherein the solid particles are separated from any gaseous components removed from heater 20 in conduit 24. Solid, devolatilized coal particles having a high ash content are removed continuously from cyclone 25 through conduit 26. These high-ash particles may be recovered and utilized as a conventional solid fuel source. Alternatively, the solid particles removed through conduit 26 may be discarded, since their carbon content is normally less than percent. The gases separated from solids in cyclone 25 are charged back into heater 20 through conduit 27. Various conventionalitems of equipment such as pumps, compressors, heat exchangers, etc., have not been shown in the drawing, and have not been described in the foregoing, since the placement and use of such conventional equipment will be apparent to those skilled in the art from the foregoing description.
DETAILED DESCRIPTION OF THE INVENTION The types of coal which may be converted using the method of this invention include bituminous and subbituminous coals in general. Particularly preferred as coal feedstocks in the present process are bituminous coals having a high volatile content, e.g., 20 percent or more of the moisture and ash free (MAF) coal. It is necessary to pulverize the coal to be converted to a relatively small particle size. Generally a particle size of about 60 to about 200 mesh or finer is suitable, although more coarse particle sizes may be employed in some cases with adequate results. Preferably, coal particles between about 100 mesh and about 200 mesh are used for the coal feed to the reaction zone in this process. Coal milling and pulverization techniques are well known in the art, and any suitable method for producing the desired size coal particles may be utilized.
An essential feature of the present process is the use of a hydrogen-containing gas at a low hydrogen pressure in order to provide a fluidizing medium in the reaction zone, to devolatilize the coal introduced into the reaction zone, and to react a fraction of the carbonaceous molecules in the coal with hydrogen. The use of hydrogen in the fluidizing gas in the reaction zone results in a more rapid devolatilization and a higher yield of volatile products than would be produced by, for example, an inert, hot fluidizing gas such as nitrogen. Furthermore, the products of reaction in the present process would not be the same as those which result if, for example, carbon oxides or steam were employed as the fluidizing gas in the reaction zone. The use of inert fluidizing gases, without hydrogen present, provides only a simple coking of coal similar to simple destructive distillation, and results in a reduced recovery of valuable volatile hydrocarbons from the coal conversion operation. It is also to be noted that the use of an oxygen compound such as carbon dioxide or water profoundly alters the nature of the effluent gaseous products by providing substantially decreased yields of light olefins and aromatics, which are the desired products in the case of the process of this invention. When hydrogen is present in the fluidizing and devolatilizing gas in the reaction zone, the hydrogen rapidly attacked the coal molecules, particularly at the sites occupied by oxygen, sulfur, and nitrogen atoms, and prevents undue polymerization and coking of the desired volatile hydrocarbon products prior to the time they can be removed from the reaction and quenched to a suitable low temperature. When hydrogen is used at high pressures, however, large amounts of low molecular weight, saturated hydrocarbons, such as methane and ethane, are produced by the conversion operation, and excessive amounts of hydrogen are thereby consumed in the reaction zone. The present invention contemplates the use of low pressure hydrogen, within a relatively narrow range of pressures, in order to provide a fluidizing gas capable of converting coal into volatile hydrocarbon products of primarily unsaturated and aromatic natures, which have not generally been available in substantial yields from previously known coal hydroconversion operations. In the present process, any hydrogen containing gas may be utilized as the fluidizing gas in the reaction zone, as long as the oxygen level. either combined or elemental, is less than a few percent. Thus, the hydrogen may be suitably diluted with higher concentration of certain gases such as nitrogen, methane, ethane, etc.. which are either inert in character or are potential hydrogen donor gases. Such diluent gases may be required in some cases in order to provide a gas flow rate through the reaction zone sufficiently high to support the fluidized bed of devolatilized coal particles in the reaction zone. The average hydrogen pressure in the reaction zone is maintained between about 0.3 pound per square inch absolute (psia) and 20 psia. A preferred range of hydrogen pressures within the reaction zone is between about 1 psia., and about 10 psia.
The reaction zone employed in the present process may be any vertically elongated vessel capable of containing a fluidized bed of devolatilized coal particles at relatively high temperatures. Preferably, the reaction zone is a relatively narrow diameter vessel, so that fluidized bed conditions may be maintained therein using a relatively low gas flow rate through the reaction zone. In general, fluidized coking reaction vessels, a large number of which are known in the art, may suitably be employed as the reaction zone in the present process. In practice of the present process, finely divided coal is continuously passed into the reaction zone, preferably near the upper end of the reaction zone. It is desirable to utilize means for charging the coal to the reactor which will distribute the freshly introduced coal particles rapidly and evenly into dilution in the fluidized bed of previously devolatilized particles in the reactor. Thus, introduction of fresh coal at several points near the upper end of the reaction zone may be desirable. Although it is not essential to introduce the fresh coal near the upper end of the reaction zone, it is preferred to do so in order to hold the residence time of volatile hydrocarbon products in the reaction zone to a minimum. A longer residence time in the reaction zone for the volatile hydrocarbon products results in excess coking and polymerization of the hydrocarbon products. This reduces the product yield from the process. Rapid removal of the volatile hydrocarbon products from the reaction zone is facilitated by maintaining a short residence time in the reaction zone for the hydrogen containing fluidizing gas employed in the reaction zone. It is preferred to employ a gas flow rate upwardly through the reaction zone at a residence time of less than about 20 seconds in order to sweep the volatile hydrocarbon products effectively out of the reactor almost immediately after they are formed.
Because of the hydrogen present in the reaction zone and because of the high temperatures employed therein, the freshly introduced coal is converted almost immediately upon being charged to the reaction zone. Volatile hydrocarbon products are rapidly produced and removed from the freshly introduced coal, leaving behind solid, carbon-containing particles which become an indistinguishable part of the fluidized bed in the reactor. However, the average carbon content of the particles making up the fluidizing bed in the reaction zone will be somewhat less than the carbon content of the freshly devolatilized particles, since some of the carbon in the particles is consumed in the heating zone in providing heat energy for the reaction zone, as described hereinafter. The attached drawing illustrates one mode of operation of the present process, in which air is used as the oxygen donor in the heating zone. The flue gas which results from the heating operation is high in carbon monoxide, and can be used as a low grade fuel. However, if a mixture of oxygen and steam is employed in the heating zone, the flue gas will be suitable, after shift and carbon monoxide absorption operations, for use as the hydrogen supply to the reaction zone. The temperature maintained in the reaction zone is between about l,400F. and about l,800F. The preferred operating temperature in the reaction zone is between about l,500F. and about 1,700F. The temperature of operation employed in the reaction zone will depend, in part, on the particular temperature maintained in the heating zone, since there must be a significant temperature gradient between the heating zone and the reaction zone. The reaction zone is maintained at a total pressure about 1 psi. to about 15 psi. higher than the pressure in the heating zone. This is done in order to facilitate the fluidized transfer of devolatilized particles from the reaction zone into the heating zone. The flow rate of oxygen-containing gas upwardly through the reaction zone is maintained at a sufficient level to provide very turbulent fluidized bed conditions within the reaction zone. Thus, the gas flow rate will depend,
in part, on the average diameter of the devolatilized coal particles in the reaction zone. The gas flow rate upwardly through the reaction zone should be sufficient to fluidize the devolatilized coal particles, but not so high as to drive the devolatilized particles upwardly through the reaction zone to provide an overly dense bed of particles at the upper end of the reaction zone and create an overly dilute fluidized bed at the lower end of the reaction zone.
The volatile hydrocarbon products produced by the action of hydrogen and heat on the freshly charged coal are commingled with hydrogen-containing gas and are rapidly passed upwardly through the reaction zone and almost immediately removed from the reaction zone. Some means for separating the gaseous mixture of hydrocarbon product vapors and fluidizing gas from the fluidized solids should be employed so that devolatilized particles do not escape overhead from the reaction zone and contaminate the hydrocarbon products. This may suitably be a conventional cyclone. Any solids which are entrained in the gaseous overhead from the reactor may be returned to the reaction zone in a manner described below. After the gaseous mixture of hydrogen-containing gas and hydrocarbon product vapors has been removed from the upper end of the reaction zone, the temperature of this gaseous mixture is preferably lowered rapidly by quenching to a level at which little or no cracking, coking or polymerization of the volatile hydrocarbon products will occur. The hydrogen containing gas is separated from the volatile hydrocarbon products by, for example, compressing and cooling the gaseous mixture recovered from the reaction zone so that the volatile hydrocarbons are condensed, while the hydrogen-containing gas remains in gaseous form. The hydrogen containing gas may then be separated from the hydrogen products by conventional gas-liquid separation techniques. Preferably, at least part of the hydrogen-containing gas thus recovered is recycled to the reaction zone for further use subsequent to any necessary purification. The various compounds and fractions which make up the volatile hydrocarbon products may suitably be further separated, fractioned and refined, as desired, to provide particular petrochemical materials and/or fuels. The volatile hydrocarbon products include significant fractions of C -C1; olefins and dioleflns. benzene, and alkylaromatics. Generally, the hydrocarbon products comprise primarily ethylene, propylene, butadiene, benzene, toluene and xylene. Some saturates, particularly methane and ethane, are also generally produced in relatively small amounts as well as some acetylene. The extraneous elements present in the coal, such as oxygen, nitrogen, sulfur. etc., are significantly converted by hydrogenation to reduced forms and are removed from the reaction zone as water, ammonia, hydrogen sulfide, etc. Some organic molecules including these elements may be present, contaminating the hydrocarbon products and may be removed by conventional purification techniques. A variety of separation and purification techniques known in the art, such as fractional distillation, flash distillation, etc., may be used to separate the various hydrocarbon products in any desired manner.
In order to maintain the reaction zone at an elevated temperature, a continuous circulation of relatively cooler devolatilized coal particles from the reaction zone to a heating zone, and a circulation of hotter particles from the heating zone to the reaction zone is maintained. Devolatilized particles are continuously withdrawn from the reaction zone, preferably from the lower end thereof, and passed to the heating zone, within which is maintained a fluidized bed of devolatilized coal particles at a temperature of about 1,700F. to about 2,000F. One suitable method for transferring the devolatilized particles from the reaction zone to the heating zone is by the use of the reacting gas medium used in the heater. Alternatively, other conventional solid transferring means such as a screw conveyor may be employed. Simple gravitational settling of particles through a conduit equipped with the necessary pressure locks may also be used. Since the pressure of the 7 reaction zone is generally maintained somewhat higher than the pressure of the heating zone, provisions are made for ensuring that the fluidizing gases used in the heating zone are not directly passed into the reaction zone. Similar solids transferring means may be employed in passing the relatively hotter devolatilized particles from the heating zone into the reaction zone. Gravitational settling of particles through a conduit equipped with the necessary pressure locks is the preferred method of transferring the hotter particles from the heating zone into the reaction zone.
The heating zone may suitably be any vertically extended vessel which will adequately contain a fluidized bed of devolatilized coal particles at temperatures up to about 2,()O()F. The types of heaters used in such conventional operations as fluidized coking may suitably be employed in the present process as the heating zone. It is preferred that the diameter of the vessel used as the heating zone in the present process be small enough to provide a sufflciently rapid superficial velocity of the upwardly flowing gases through the heating zone to create conventional fluidized bed conditions in the heating zone. The fluidizing gas employed in the heating zone is an oxygen-containing gas, i.e., the fluidizing gas contains at least some free oxygen capable of combining exothermically with carbon contained in the devolatilized coal particles in the heating zone, in order to provide the heat requirements for the operation of the process. The oxygen-containing gas may be pure oxygen, or may be oxygen diluted with other relatively inert gases such as nitrogen, carbon dioxide, etc. Generally, air is a suitable oxygen-containing gas for use in the present process. The oxygen-containing gas utilized is charged to the lower end of the heating zone, and is usually also used in transporting the solid feed to the heater. It is charged at a rate sufflcient to provide turbulent fluidized bed conditions in the heating zone and to provide sufflcient oxygen to sustain combustion in the heating zone at a level such'that the temperature in the heating zone is maintained at the desired high level. Diluent gases such as steam may be utilized to regulate the temperature in the heating zone by substitution for a part of the oxygen-containing gas, without loss of fluidizing conditions in the heating zone.-As stated above, although it is not essential to the operation of the pro-' cess of this invention, steam may be charged to the heating zone in combination with the oxygencontaining gas in order to produce a fuel gas which contains carbon monoxide and carbon dioxide. By charging the mixture of steam and oxygen to the heating zone as the fluidizing gas therein, the hydrogen requirement for the fluidizing gas used in the reaction zone may be provided by the operation of the process itself. Hydrogen may be provided from the flue gas resulting from the heating operation after appropriate shift and carbon dioxide removal operations. In cases where little or no steam is charged to the heating zone, the oxygen introduced into the heating zone reacts with carbon contained in the devolatilized coal particles to provide almost solely carbon monoxide and carbon dioxide. These gases, along with any diluent gases such as nitrogen, are then withdrawn from the upper end of the heating zone and removed from the operation of the process. The gases withdrawn from the heating zone may be freed from devolatilized coal particles by the use of a conventional cyclone. Additional gas purification operations such as conventional scrubbing may also be necessary in order to remove some very fine particles from the flue gas. The heat generated in burning a portion of the carbon from the devolatilized coal particles in the heating zone is primarily absorbed by the particles which make up the fluidized bed in the heating zone. The relatively hot particles which result are continuously withdrawn from the heating zone and passed into the reaction zone. The hotter particles thus withdrawn from the heating zone are preferably withdrawn from the heating zone near the upper end thereof and passed into the reaction zone at or near the section of the reaction zone into which the fresh coal particles are introduced as described previously.
A second stream of solid particles is removed from the heating zone intermittently or continuously and recovered from the process in order to prevent a build up of noncarbonaceous mineral matter, or ash, in the heat ing zone and reaction zone. The devolatilized particles at the upper end of the fluidized bed in the heating zone generally contain the lowest average amount of carbon within the heater-reactor system, so that the stream of particles removed to prevent ash build up is preferably removed from this section of the heating zone. The ashcontaining stream thus removed may be used as a low grade fuel similar to coal, or may be simply discarded. The carbon content of the devolatilized coal particles in this discarded stream is maintained at an average of less than about 50 weight percent. Similarly, the average carbon content of the particles which are returned to the reaction zone from the heating zone in order to provide the heat requirements for the reaction zone, is also less than about 50 weight percent. The amount of devolatilized particles which is removed from the heating zone and discarded is simply the amount required to maintain a material balance within the heaterreactor system.
ILLUSTRATIVE EMBODIMENT A reactor-heater system identical to the one depicted in the attached drawing is employed in the illustration. One hundred pounds per hour of bituminous coal ground to 200 mesh particle size is continuously charged to a reactor maintained at a temperature of 1,600F. The coal utilized contains 81 weight percent carbon, 5 weight percent hydrogen, 6 weight percent oxygen, 1 weight percent sulfur, 1 weight percent nitrogen and 6 weight percent coal ash. The reactor contains a turbulent fluidized bed of devolatilized coal particles at a total pressure of about 20 psig. Fluidized bed conditions in the reactor are maintained by passing a hydrogen-containing gas upwardly through the reactor at the rate of about 2 pounds per hour of hydrogen. Hydrogen pressure in the reactor is about 5 psia. The total residence time for the fluidizing gas in the reactor is from about 2 to about 20 seconds. Diluent gases such carbon dioxide, methane, steam, etc., are employed in the hydrogen-containing gas in any amounts necessary to provide fluidized bed conditions in the reactor. Hydrogen-containing gas and hydrocarbon product vapors are continuously removed overhead from the reactor. The composition of the overhead is, to some extent, variable, and depends, for example, on the grade of coal, amount of contaminants in the coal in the feed to the reactor, etc., and also depends on the exact composition of the hydrogen-containing gas used to maintain fluidized bed conditions in the reactor. Typically, the gaseous overhead recovered from the reactor comprises about 40 pounds per hour of hydrocarbon products, about 6 pounds per hour of oxygen compounds, mostly water, about 1 pound per hour of nitrogen compounds calculated as ammonia, about 1 pound per hour of sulfur compounds calculated as hydrogen sulfide and about 0.6 pounds per hour of hydrogen. The hydrocarbon vapor products in the overhead from the reactor contain approximately 40 mole percent ethylene, about 25 mole percent miscellaneous aromatic and unsaturated aliphatic hydrocarbons such as alkylaromatics, propylene, phenols, thiophenes, pyridines, etc., about 20 mole percent methane, about 10 mole percent butadiene and about 5 mole percent benzene. The types and fractions of the various hydrocarbon products varies to some extent, depending on the type of coal, composition of the fluidizing gas in the reactor, etc. About 1,750 pounds per hour of solids is withdrawn from the lower end of the reactor and passed into the heater using about 237 pounds per hour of air in order to pass the particles into the heater in a fluidized state in the stream of air. Gas residence time in the heater is from about 5 to about 30 seconds. The heater is maintained at a temperature of about 1,900F. Oxygen from the air charge and carbon from the devolatilized coal particles are reacted in the heater to produce heat, carbon monoxide, and carbon dioxide. Gaseous materials are removed from the heater at the rate of about 14 pounds per hour of carbon dioxide, pounds per hour of carbon monoxide and 182 pounds per hour of combined inert gases such as nitrogen and argon. Hot, devolatilized particles are removed from the heater and returned to the reactor at the rate of about 1,700 pounds per hour in order to provide heat energy for the reactor. About 12 pounds per hour of devolatilized particles is removed from the heater and withdrawn from the process. This particle stream may be used as a low grade fuel source or may be discarded. It comprises about 50 weight percent carbon and about 50 weight percent coal ash.
I claim as my invention: 1. A process for producing volatile hydrocarbo products from coal which comprises the steps of:
a. continuously passing carbon-containing, devolatilized coal particles from a reaction zone containing a first fluidized bed of said particles, maintained at a temperature of from about 1,400F. to about 1,800F., into a heating zone containing a second fluidized bed of said particles, maintained at a higher temperature than said first fluidized bed and in the range of from about l,700F. to about 2,000F.;
b. continuously introducing finely divided coal into said first fluidized bed in said reaction zone, continuously introducing a hydrogen-containing gas into the lower end of said reaction zone and passing said hydrogen-containing gas upwardly through said reaction zone at a hydrogen pressure of from about 0.3 pound per square inch absolute to about pounds per square inch absolute to produce hydrocarbon product vapors and carbon-containing, devolatilized coal particles from said finely-divided coal, withdrawing a gaseous mixture of hydrogencontaining gas and hydrocarbon product vapors from the upper end of said reaction zone, and recovering said volatile hydrocarbon products from said gaseous mixture; and,
. continuously contacting an oxygen-containing gas with said second fluidized bed of carboncontaining particles in said heating zone to heat said second bed of particles and to produce gaseously introduced into said heating zone in admixture with said oxygen-containing gas and said gaseous stream removed from said heating zone contains hydrogen, methane and said carbon oxides.
3. The process of claim 1 wherein the temperature in said reaction zone is maintained at from about 1,500F. to about 1,700F.
4. The process of claim 1 wherein the temperature in said heating zone is maintained at from about 1,800F. to about 2,000F.
5. The process of claim 1 wherein the particles in said first portion of particles removed from said heating zone contain less than about 50 weight percent carbon.
6. The process of claim 1 wherein the residence time of said hydrogen-containing gas in said reaction zone is less than about 20 seconds.
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|U.S. Classification||208/408, 201/31, 201/38, 208/414, 208/427|
|International Classification||C10G1/06, C10G1/00|
|Apr 27, 1989||AS||Assignment|
Owner name: UOP, A GENERAL PARTNERSHIP OF NY, ILLINOIS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:UOP INC.;REEL/FRAME:005077/0005
Effective date: 19880822
|Sep 21, 1988||AS||Assignment|
Owner name: UOP, DES PLAINES, IL, A NY GENERAL PARTNERSHIP
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:KATALISTIKS INTERNATIONAL, INC., A CORP. OF MD;REEL/FRAME:005006/0782
Effective date: 19880916