Process for making liquid and gaseous fuels from caking coals
United States Patent 3920418
An improvement in the process for converting coal to liquid and gaseous fuels wherein solvent extraction is used for the recovery of a liquid-rich fraction and a solids-rich fraction. The latter is converted by carbonization to an extremely finely divided char which is formed into agglomerates that are then further carbonized and finally gasified.
US Patent References:
Gasification of carbonaceous solids
Tarbox - September 1957 - 2807529
Method of making formcoke
Gorin et al. - January 1963 - 3073751
Process for producing high purity hydrogen from hydrocarbon gas and steam
Gorin - June 1965 - 3188179
Gasification of carbonaceous solids
Tarbox - September 1957 - 2807529
Method of making formcoke
Gorin et al. - January 1963 - 3073751
Process for producing high purity hydrogen from hydrocarbon gas and steam
Gorin - June 1965 - 3188179
Application Number:
05/395671
Publication Date:
11/18/1975
Filing Date:
09/10/1973
View Patent Images:
Export Citation:
Assignee:
Consolidation Coal Company (Pittsburgh, PA)
Primary Class:
Other Classes:
208/426, 208/415, 208/427, 48/197R, 48/206
International Classes:
C10G1/00; C10J3/16; C10J3/02; C10J3/16; C10G1/04; C10J3/00
Field of Search:
48/210,197R,202,206,203 208/8 201/5,6 423/648,650 252/372,373
Primary Examiner:
Scovronek, Joseph
Attorney, Agent or Firm:
Mickesell Jr. Jr., William Fowler Leigh A. D.
Parent Case Data:
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No. 215,160, filed Jan. 3, 1972, now abandoned.
Claims:
I claim
1. In a coal conversion process which includes extraction by means of a solvent of a finely divided caking coal, whereby a mixture of extract, solvent and undissolved carbonaceous residue is produced, the IMPROVEMENT which comprises:
2. In a coal conversion process which includes extraction by means of a solvent of a finely divided caking coal, whereby a mixture of extract, solvent and undissolved carbonaceous residue is produced, the IMPROVEMENT which comprises:
3. The process according to claim 2 wherein carbonization of said solids-rich fraction is conducted in a fluidized low temperature zone.
4. The process according to claim 3 wherein the hot char from the fluidized carbonization zone is further heated before admixture with the caking coal.
1. In a coal conversion process which includes extraction by means of a solvent of a finely divided caking coal, whereby a mixture of extract, solvent and undissolved carbonaceous residue is produced, the IMPROVEMENT which comprises:
2. In a coal conversion process which includes extraction by means of a solvent of a finely divided caking coal, whereby a mixture of extract, solvent and undissolved carbonaceous residue is produced, the IMPROVEMENT which comprises:
3. The process according to claim 2 wherein carbonization of said solids-rich fraction is conducted in a fluidized low temperature zone.
4. The process according to claim 3 wherein the hot char from the fluidized carbonization zone is further heated before admixture with the caking coal.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improvement in coal conversion processes, and, more particularly, to an improvement in those processes for making non-solid fuels from caking coals which involve partial extraction of the coal by solvent treatment.
2. Description of the Prior Art
The treatment of coal, and particularly caking coals, with a solvent at elevated temperatures, while not new, has received increased attention in recent years. Such treatment permits the ready extraction of that portion of the coal which has the highest hydrogen-to-carbon ratio. No attempt is made in such solvent treatment processes to extract the refractory portion of the coal, that is, that portion which requires excessive hydrogen (a costly commodity) to form a liquid fuel. The range of depths of extraction generally found to be most attractive economically is 50 to 80 weight percent of the moisture-and-ash-free (MAF) coal.
There are many patents illustrative of the so-called partial extraction approach to the conversion of coal to liquid fuels. Among them are the following: U.S. Pat. Nos. 3,018,241; 3,018,242, 3,117,921; 3,143,489; 3,162,594; 3,184,401; and 3,523,886.
To be commercially successful, a partial extraction process must provide satisfactorily for the use or disposition of the undissolved fraction of the coal which, as indicated above, may amount to as much as 50 weight percent of the MAF coal. And even further, if the separation of the undissolved fraction from the extract (in solvent) is, itself, only partial (as described in the above-cited U.S. Pat. No. 3,523,886 and U.S. Pat. No. 3,748,254), the undissolved fraction will contain some liquid consisting essentially of extract and solvent. Thus, the treatment of this solids-rich fraction becomes crucial to the economic success of the overall process. It has been suggested to subject this fraction to carbonization to make char which may be used as fuel or as feed to a gasifier for making, particularly, hydrogen or methane, or mixtures thereof. However, the size consist of the char so made is considerably finer than that required for the gasifier generally considered to be the most successful one available commercially today, namely the downwardly moving bed type. The latter requires a non-caking feed, having a minimum size of 1/8 inch. Since the size consist of the coal subjected to partial extraction is, for the most part, less than 1/8 inch, and coupled with the attrition that occurs in the extraction treatment, the char is unsatisfactory, albeit non-caking, as a feed to such gasifiers.
Accordingly, the primary object of the present invention is to provide an improvement in partial extraction coal conversion processes which will provide for converting the non-extracted portion of the coal to a suitable feedstock for a moving bed-type of gasifier.
SUMMARY OF THE INVENTION
The present invention is an improved process for partial conversion of coal by solvent extraction to both liquid and gaseous fuels which comprises, in its broadest aspects, the following essential steps:
1. subjecting a finely divided caking coal to treatment with a solvent at an elevated temperature, whereby between about 50 and 80 weight percent of the MAF coal is converted to extract soluble in the solvent at said elevated temperature,
2. at least partially separating the total extraction effluent to yield a liquid-rich fraction and a solids-rich fraction, the solids having a finer size consist than that of the coal treated in step (1),
3. subjecting the solids-rich fraction from step (2) to carbonization to yield a finely divided char having a size consist, for the greater part, less than 1/8 inch,
4. mixing caking coal having a size consist, for the greater part, less than 1/8 inch, with the char from step (3),
5. subjecting the mixture from step (4) to agglomeration above the temperature at which the caking coal in the mixture begins to soften, whereby agglomerates are formed, and
6. subjecting said agglomerates first to carbonization and then to gasification in a downwardly moving bed gasifier, whereby a gaseous fuel is produced.
For a better understanding of the present invention, its objects and advantages, reference should be had to the following description in which a preferred embodiment thereof is described, and to the accompanying drawing which schematically illustrates the preferred embodiment.
The preferred embodiment of the present invention comprises:
1. an Extraction Zone 10 wherein a caking coal is treated with a solvent at an elevated temperature;
2. a Separation Zone 20 wherein extract (in solvent) is at least partially separated from the undissolved coal residue, to yield a liquid-rich fraction and a solids-rich fraction;
3. an Extract and Solvent Recovery Zone 30 wherein the liquid-rich fraction from the Separation Zone is distilled to separately recover solvent from the extract for recycle;
4. a Carbonization Zone 40 wherein the solids-rich fraction from the Separation Zone is subjected to carbonization to yield a finely divided char;
5. an Agglomeration Zone 50 wherein agglomerates are formed from a mixture of the char and a sufficient amount of the caking coal used in the extraction process to serve as a binder when rendered plastic by heat from the char; and
6. a Gasifier 60 wherein the agglomerates from step (5) are first carbonized and then gasified by reaction with steam to yield hydrogen.
The unit operations of the preferred embodiment will now be described with reference to the drawing and with appropriate designation of the preferred equipment.
Partial Extraction of the Coal
Any caking bituminous coal may be used in the process of this invention. Preferably, it is one having a volatile matter content of at least 20 weight percent, for example, a Pittsburgh seam coal. A typical composition of a Pittsburgh seam coal suitable for use in the process is shown in Table I below.
TABLE I ______________________________________ Proximate Analysis: Wt. % MF 1 Coal Volatile matter 39.3 Fixed carbon 47.7 Ash 13.0 Ultimate Analysis: Wt. % MAF 2 Coal Hydrogen 5.5 Carbon 80.8 Nitrogen 1.4 Oxygen 7.5 Sulfur 4.8 100.0 ______________________________________ 1 MF means moisture-free. 2 MAF means moisture-and-ash-free.
The feed coal is ground to a finely divided state, typically minus 4 mesh Tyler Standard screen, preferably minus 14 mesh Tyler Standard screen, and is freed of substantially all extraneous water before introduction into the Extraction Zone 10.
The finely divided coal is introduced into the Extraction Zone 10 via a conduit 12. Recycle solvent is introduced into the Extraction Zone via a conduit 13; make-up solvent via a conduit 14. The solvent extraction process may be any of the processes commonly known to those skilled in the art, for example, continuous, batch, countercurrent or staged. It is generally conducted at a temperature in the range of 300° to 500°C., a pressure in the range of 1 to 6,500 psig, a residence time in the range of 1 to 120 minutes, a solvent-to-coal ratio of 1/1 to 4/1 and, if desired, in the presence of a catalyst and/or hydrogen.
Polycyclic, aromatic hydrocarbons which are liquid at the temperature and pressure of extraction are generally recognized to be suitable solvents for the coal in the extraction step. At least a portion of the aromatics may be partially or completely hydrogenated, whereby some hydrogen transfer from solvent to coal may occur to assist in the breakdown of the large coal molecules. Mixtures of the hydrocarbons are generally used and these may be derived from subsequent steps in the process of this invention. Other types of coal solvent, such as oxygenated aromatic compounds, may be added for special reasons, for example, to improve the solvent power, but the resulting mixture should be predominantly of the type mentioned.
The coal and the solvent are maintained in intimate contact at the elevated temperature until up to about 80 weight percent of the MAF feed coal has been converted, i.e., depolymerized, hydrogenated, dissolved, etc. The product, for want of a better term, is called "extract" even though more transpires in the conversion than simply dissolving the coal. Generally, in order to attain depths of extraction above 50 weight percent, hydrogen must be added to the coal during extraction. The hydrogen may be added by means of a hydrogen-transfer solvent of the type mentioned above, or simply as hydrogen gas.
Preferably, the solvent extraction proces is a noncatalytic, continuous, countercurrent process conducted in a vertical cylindrical vessel, at a temperature in the range of 300° to 500°C., a pressure in the range of 1 to 6,500 psig, a residence time in the range of 1 to 120 minutes and a solvent-to-coal ratio of 1/1 to 4/1. The preferred solvent is a polycyclic, aromatic hydrocarbon which is liquid under the temperature and pressure of extraction, and contains partially or completely hydrogenated aromatics. It is naturally derived from hydrogenation of the extract, and usually has a relatively wide distillation range with an initial atmospheric boiling point of about 225°C. and a final boiling point of 425°C. or even higher. The coal and the solvent are maintained in intimate contact within the Extraction Zone until the solvent has extracted 50 to 80 weight percent of the MAF feed coal.
The finely divided feed coal, in the process of being extracted, suffers further reduction in size consist. A typical size consist (Tyler Standard screen) of the solids in the total extraction effluent is presented in the following Table II.
TABLE II ______________________________________ Size Weight Percent ______________________________________ on 48 1.2 between 48 and 100 5.3 between 100 and 200 33.1 between 200 and 325 11.9 through 325 48.5 ______________________________________
Separation of the Extraction Product
Following extraction, the mixture of solvent, extract and residue is conducted rapidly, so as to avoid excessive cooling of the mixture, through a conduit 15 to the Separation Zone 20. The primary objective of this zone is to separate the extraction product into a liquid-rich fraction and a solids-rich fraction. The separation may be accomplished by filtration, centrifugation, sedimentation, or by hydrocyclones or by any other suitable means. Separation is effected at elevated temperatures at or close to the temperature maintained in the Extraction Zone.
Cooling of the extraction product may, and probably will, result in precipitation of higher molecular weight portions of the extract. At times, this is done deliberately to improve the ease of separation of the solids from the extract, as well as to improve the quality of the extract in solution. The foregoing precipitation process may be further intensified, if desired, by the addition of a saturated, i.e., paraffinic or naphthenic, solvent.
The preferred separation system is sedimentation (i.e., settling). Settling is conducted at or about 300°C. While settling may be conducted so as to effect substantially complete separation of liquid and solids, it is preferred to provide an underflow from the Separation Zone which is a flowable slurry, that is, one having about 45 to 55 weight percent solids.
Extract Recovery Zone
The liquid-rich fraction produced in the Separation Zone is conducted to the Extract and Solvent Recovery Zone 30 through a conduit 22. The liquid-rich fraction consists of a low-solids-containing liquid, being principally extract and solvent, with generally less than five weight percent solids, the amount of solids being a function of the particular separation system employed. The mixture is fractionally distilled to recover at least solvent and extract. The solvent, which boils in the range of about 225° to 425°C., is withdrawn through a conduit 24 for reuse in the Extraction Zone 10. The solvent may first be subjected to suitable hydrogenation (not shown) in conventional fashion to make it effective as a hydrogen-transfer solvent if the desired depth of extraction demands it.
Extract, usually associated with a relatively small amount of solvent, is conducted through a conduit 26 to storage or to further treatment, such as hydrocracking, to make distillate fuels since, as is, extract is substantially non-distillable without decomposition. The conversion of extract to distillate fuels is described in many patents, including those previously cited herein.
Carbonization Zone
The solids-rich fraction from the Separation Zone is introduced via a conduit 28 into the Carbonization Zone 40. The Carbonization Zone is maintained at a temperature in the range of 400° to 750°C. Preferably, the zone is a low temperature zone, i.e., 425° to 500°C., and also fluidized. However, if desired, other conventional devolatilization zones may be used. A liquid distillate and gas are withdrawn through a conduit 32 to a Fractional Condensation Zone 34 wherein solvent is separately recovered for recycle through a conduit 36 and conduit 13 to the Extraction Zone, or in part, to the Separation Zone for washing and dilution (not shown). Tar and gas are also recovered, as shown schematically, by conduit 38.
A finely divided hydrocarbonaceous solid, i.e., char, is withdrawn from the Carbonization Zone through a conduit 42. If the Carbonization Zone is a fluidized carbonization zone, as is preferred, some agglomeration of solids occurs in the fluidized bed, but not so much as to impair the fluidization of the bed. The average particle size of the char withdrawn from such a fluidized bed is typically 150 to 200 Tyler Standard mesh.
Agglomeration Zone
The primary objective of the Agglomeration Zone 50 is to form agglomerates of proper size and strength for use as feedstock to the Moving Bed Gasifier 60. Such agglomerates may be formed from a mixture of the hot char from the Carbonization Zone and finely divided caking coal introduced by conduit 44. Preferably, for reasons of convenience, the caking coal is the same as that fed to the Extraction Zone. The agglomeration process may be any conventioanl briquetting or extrusion proces, or a hot pelletizing process such as described in U.S. Pat. Nos. 3,073,751 and 3,401,089 which are hereby incorporated herein by reference. Some of these agglomeration processes may require that the temperature of the mixture of caking coal and char be higher than that provided by the heat of the char as received (without intentional cooling) from the Carbonization Zone. Accordingly, either the char or the coal may be heated in Char Heater 46 and Coal Heater 48, respectively, to achieve the desired temperature. The amount of caking coal in the mixture is such as to provide adequate binder for the formation of the agglomerates, depending upon the cakiness of the coal and the particular agglomeration process employed. In any case, the temperature of the mixture in the Agglomeration Zone should be above the temperature at which the coal begins to soften. Some tar and gas will be produced in the Agglomeration Zone by the partial devolatilization of the coal. A conduit 52 serves to remove such. The agglomerates are withdrawn through a conduit 54, and those in excess of 1/8-inch size are conducted to the Gasifier 60.
Gasifier
The Gasifier 60 is the so-called moving bed-type of gasifier. Such a gasifier requires a non-caking or weakly caking carbonaceous feed of at least 1/8-inch size for satisfactory commercial operation. In such a gasifier, a bed of the solids (in this instance, the agglomerates) which are relatively stationary with respect to each other, moves progressively downwardly, first through a carbonization zone 62 wherein the agglomerates are further devolatilized and, at the same time, hardened to withstand the burden of the bed in the gasification zone 64 through which they pass downwardly. Steam and air (or oxygen instead of air) are introduced into the gasification zone 64 through conduits 65 and 66, respectively, and are circulated upwardly through the downwardly moving bed. The temperatures in the carbonization zone 62 are maintained within the range 375° to 550°C. by the hot gases issuing from the gasification zone. The gasification zone is maintained at a temperature in the range of 750° to 1,100°C. The pressure is 100 to 500 psig. The incoming agglomerates are carbonized in the carbonization zone 62, yielding tar vapors which are withdrawn with the effluent gas via conduit 68. The carbonized agglomerates move downwardly in reactive contact with the upflowing steam and oxygen to form CO 2 , CH 4 , H 2 and CO. Unreacted ash is withdrawn through conduit 69. The product gases pass through the carbonization zone 62 and into conduit 68. The effluent gas, including the tar vapors, is passed into a condenser 70 in which the tar vapors are condensed and removed through conduit 72. The tar-free gas is conducted by a conduit 74 to suitable hydrogen and methane recovery or treatment systems. A hydrogen-enriched gas may be recovered by conventional methods and used, if desired, to hydrogenate the extract recovered through conduit 26 to make distillate fuels. Or, if desired, the gas may be treated and recovered for use as fuel gas.
According to the provisions of the patent statutes, I have explained the principle, preferred construction and mode of operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated.
1. Field of the Invention
This invention relates to an improvement in coal conversion processes, and, more particularly, to an improvement in those processes for making non-solid fuels from caking coals which involve partial extraction of the coal by solvent treatment.
2. Description of the Prior Art
The treatment of coal, and particularly caking coals, with a solvent at elevated temperatures, while not new, has received increased attention in recent years. Such treatment permits the ready extraction of that portion of the coal which has the highest hydrogen-to-carbon ratio. No attempt is made in such solvent treatment processes to extract the refractory portion of the coal, that is, that portion which requires excessive hydrogen (a costly commodity) to form a liquid fuel. The range of depths of extraction generally found to be most attractive economically is 50 to 80 weight percent of the moisture-and-ash-free (MAF) coal.
There are many patents illustrative of the so-called partial extraction approach to the conversion of coal to liquid fuels. Among them are the following: U.S. Pat. Nos. 3,018,241; 3,018,242, 3,117,921; 3,143,489; 3,162,594; 3,184,401; and 3,523,886.
To be commercially successful, a partial extraction process must provide satisfactorily for the use or disposition of the undissolved fraction of the coal which, as indicated above, may amount to as much as 50 weight percent of the MAF coal. And even further, if the separation of the undissolved fraction from the extract (in solvent) is, itself, only partial (as described in the above-cited U.S. Pat. No. 3,523,886 and U.S. Pat. No. 3,748,254), the undissolved fraction will contain some liquid consisting essentially of extract and solvent. Thus, the treatment of this solids-rich fraction becomes crucial to the economic success of the overall process. It has been suggested to subject this fraction to carbonization to make char which may be used as fuel or as feed to a gasifier for making, particularly, hydrogen or methane, or mixtures thereof. However, the size consist of the char so made is considerably finer than that required for the gasifier generally considered to be the most successful one available commercially today, namely the downwardly moving bed type. The latter requires a non-caking feed, having a minimum size of 1/8 inch. Since the size consist of the coal subjected to partial extraction is, for the most part, less than 1/8 inch, and coupled with the attrition that occurs in the extraction treatment, the char is unsatisfactory, albeit non-caking, as a feed to such gasifiers.
Accordingly, the primary object of the present invention is to provide an improvement in partial extraction coal conversion processes which will provide for converting the non-extracted portion of the coal to a suitable feedstock for a moving bed-type of gasifier.
SUMMARY OF THE INVENTION
The present invention is an improved process for partial conversion of coal by solvent extraction to both liquid and gaseous fuels which comprises, in its broadest aspects, the following essential steps:
1. subjecting a finely divided caking coal to treatment with a solvent at an elevated temperature, whereby between about 50 and 80 weight percent of the MAF coal is converted to extract soluble in the solvent at said elevated temperature,
2. at least partially separating the total extraction effluent to yield a liquid-rich fraction and a solids-rich fraction, the solids having a finer size consist than that of the coal treated in step (1),
3. subjecting the solids-rich fraction from step (2) to carbonization to yield a finely divided char having a size consist, for the greater part, less than 1/8 inch,
4. mixing caking coal having a size consist, for the greater part, less than 1/8 inch, with the char from step (3),
5. subjecting the mixture from step (4) to agglomeration above the temperature at which the caking coal in the mixture begins to soften, whereby agglomerates are formed, and
6. subjecting said agglomerates first to carbonization and then to gasification in a downwardly moving bed gasifier, whereby a gaseous fuel is produced.
For a better understanding of the present invention, its objects and advantages, reference should be had to the following description in which a preferred embodiment thereof is described, and to the accompanying drawing which schematically illustrates the preferred embodiment.
The preferred embodiment of the present invention comprises:
1. an Extraction Zone 10 wherein a caking coal is treated with a solvent at an elevated temperature;
2. a Separation Zone 20 wherein extract (in solvent) is at least partially separated from the undissolved coal residue, to yield a liquid-rich fraction and a solids-rich fraction;
3. an Extract and Solvent Recovery Zone 30 wherein the liquid-rich fraction from the Separation Zone is distilled to separately recover solvent from the extract for recycle;
4. a Carbonization Zone 40 wherein the solids-rich fraction from the Separation Zone is subjected to carbonization to yield a finely divided char;
5. an Agglomeration Zone 50 wherein agglomerates are formed from a mixture of the char and a sufficient amount of the caking coal used in the extraction process to serve as a binder when rendered plastic by heat from the char; and
6. a Gasifier 60 wherein the agglomerates from step (5) are first carbonized and then gasified by reaction with steam to yield hydrogen.
The unit operations of the preferred embodiment will now be described with reference to the drawing and with appropriate designation of the preferred equipment.
Partial Extraction of the Coal
Any caking bituminous coal may be used in the process of this invention. Preferably, it is one having a volatile matter content of at least 20 weight percent, for example, a Pittsburgh seam coal. A typical composition of a Pittsburgh seam coal suitable for use in the process is shown in Table I below.
TABLE I ______________________________________ Proximate Analysis: Wt. % MF 1 Coal Volatile matter 39.3 Fixed carbon 47.7 Ash 13.0 Ultimate Analysis: Wt. % MAF 2 Coal Hydrogen 5.5 Carbon 80.8 Nitrogen 1.4 Oxygen 7.5 Sulfur 4.8 100.0 ______________________________________ 1 MF means moisture-free. 2 MAF means moisture-and-ash-free.
The feed coal is ground to a finely divided state, typically minus 4 mesh Tyler Standard screen, preferably minus 14 mesh Tyler Standard screen, and is freed of substantially all extraneous water before introduction into the Extraction Zone 10.
The finely divided coal is introduced into the Extraction Zone 10 via a conduit 12. Recycle solvent is introduced into the Extraction Zone via a conduit 13; make-up solvent via a conduit 14. The solvent extraction process may be any of the processes commonly known to those skilled in the art, for example, continuous, batch, countercurrent or staged. It is generally conducted at a temperature in the range of 300° to 500°C., a pressure in the range of 1 to 6,500 psig, a residence time in the range of 1 to 120 minutes, a solvent-to-coal ratio of 1/1 to 4/1 and, if desired, in the presence of a catalyst and/or hydrogen.
Polycyclic, aromatic hydrocarbons which are liquid at the temperature and pressure of extraction are generally recognized to be suitable solvents for the coal in the extraction step. At least a portion of the aromatics may be partially or completely hydrogenated, whereby some hydrogen transfer from solvent to coal may occur to assist in the breakdown of the large coal molecules. Mixtures of the hydrocarbons are generally used and these may be derived from subsequent steps in the process of this invention. Other types of coal solvent, such as oxygenated aromatic compounds, may be added for special reasons, for example, to improve the solvent power, but the resulting mixture should be predominantly of the type mentioned.
The coal and the solvent are maintained in intimate contact at the elevated temperature until up to about 80 weight percent of the MAF feed coal has been converted, i.e., depolymerized, hydrogenated, dissolved, etc. The product, for want of a better term, is called "extract" even though more transpires in the conversion than simply dissolving the coal. Generally, in order to attain depths of extraction above 50 weight percent, hydrogen must be added to the coal during extraction. The hydrogen may be added by means of a hydrogen-transfer solvent of the type mentioned above, or simply as hydrogen gas.
Preferably, the solvent extraction proces is a noncatalytic, continuous, countercurrent process conducted in a vertical cylindrical vessel, at a temperature in the range of 300° to 500°C., a pressure in the range of 1 to 6,500 psig, a residence time in the range of 1 to 120 minutes and a solvent-to-coal ratio of 1/1 to 4/1. The preferred solvent is a polycyclic, aromatic hydrocarbon which is liquid under the temperature and pressure of extraction, and contains partially or completely hydrogenated aromatics. It is naturally derived from hydrogenation of the extract, and usually has a relatively wide distillation range with an initial atmospheric boiling point of about 225°C. and a final boiling point of 425°C. or even higher. The coal and the solvent are maintained in intimate contact within the Extraction Zone until the solvent has extracted 50 to 80 weight percent of the MAF feed coal.
The finely divided feed coal, in the process of being extracted, suffers further reduction in size consist. A typical size consist (Tyler Standard screen) of the solids in the total extraction effluent is presented in the following Table II.
TABLE II ______________________________________ Size Weight Percent ______________________________________ on 48 1.2 between 48 and 100 5.3 between 100 and 200 33.1 between 200 and 325 11.9 through 325 48.5 ______________________________________
Separation of the Extraction Product
Following extraction, the mixture of solvent, extract and residue is conducted rapidly, so as to avoid excessive cooling of the mixture, through a conduit 15 to the Separation Zone 20. The primary objective of this zone is to separate the extraction product into a liquid-rich fraction and a solids-rich fraction. The separation may be accomplished by filtration, centrifugation, sedimentation, or by hydrocyclones or by any other suitable means. Separation is effected at elevated temperatures at or close to the temperature maintained in the Extraction Zone.
Cooling of the extraction product may, and probably will, result in precipitation of higher molecular weight portions of the extract. At times, this is done deliberately to improve the ease of separation of the solids from the extract, as well as to improve the quality of the extract in solution. The foregoing precipitation process may be further intensified, if desired, by the addition of a saturated, i.e., paraffinic or naphthenic, solvent.
The preferred separation system is sedimentation (i.e., settling). Settling is conducted at or about 300°C. While settling may be conducted so as to effect substantially complete separation of liquid and solids, it is preferred to provide an underflow from the Separation Zone which is a flowable slurry, that is, one having about 45 to 55 weight percent solids.
Extract Recovery Zone
The liquid-rich fraction produced in the Separation Zone is conducted to the Extract and Solvent Recovery Zone 30 through a conduit 22. The liquid-rich fraction consists of a low-solids-containing liquid, being principally extract and solvent, with generally less than five weight percent solids, the amount of solids being a function of the particular separation system employed. The mixture is fractionally distilled to recover at least solvent and extract. The solvent, which boils in the range of about 225° to 425°C., is withdrawn through a conduit 24 for reuse in the Extraction Zone 10. The solvent may first be subjected to suitable hydrogenation (not shown) in conventional fashion to make it effective as a hydrogen-transfer solvent if the desired depth of extraction demands it.
Extract, usually associated with a relatively small amount of solvent, is conducted through a conduit 26 to storage or to further treatment, such as hydrocracking, to make distillate fuels since, as is, extract is substantially non-distillable without decomposition. The conversion of extract to distillate fuels is described in many patents, including those previously cited herein.
Carbonization Zone
The solids-rich fraction from the Separation Zone is introduced via a conduit 28 into the Carbonization Zone 40. The Carbonization Zone is maintained at a temperature in the range of 400° to 750°C. Preferably, the zone is a low temperature zone, i.e., 425° to 500°C., and also fluidized. However, if desired, other conventional devolatilization zones may be used. A liquid distillate and gas are withdrawn through a conduit 32 to a Fractional Condensation Zone 34 wherein solvent is separately recovered for recycle through a conduit 36 and conduit 13 to the Extraction Zone, or in part, to the Separation Zone for washing and dilution (not shown). Tar and gas are also recovered, as shown schematically, by conduit 38.
A finely divided hydrocarbonaceous solid, i.e., char, is withdrawn from the Carbonization Zone through a conduit 42. If the Carbonization Zone is a fluidized carbonization zone, as is preferred, some agglomeration of solids occurs in the fluidized bed, but not so much as to impair the fluidization of the bed. The average particle size of the char withdrawn from such a fluidized bed is typically 150 to 200 Tyler Standard mesh.
Agglomeration Zone
The primary objective of the Agglomeration Zone 50 is to form agglomerates of proper size and strength for use as feedstock to the Moving Bed Gasifier 60. Such agglomerates may be formed from a mixture of the hot char from the Carbonization Zone and finely divided caking coal introduced by conduit 44. Preferably, for reasons of convenience, the caking coal is the same as that fed to the Extraction Zone. The agglomeration process may be any conventioanl briquetting or extrusion proces, or a hot pelletizing process such as described in U.S. Pat. Nos. 3,073,751 and 3,401,089 which are hereby incorporated herein by reference. Some of these agglomeration processes may require that the temperature of the mixture of caking coal and char be higher than that provided by the heat of the char as received (without intentional cooling) from the Carbonization Zone. Accordingly, either the char or the coal may be heated in Char Heater 46 and Coal Heater 48, respectively, to achieve the desired temperature. The amount of caking coal in the mixture is such as to provide adequate binder for the formation of the agglomerates, depending upon the cakiness of the coal and the particular agglomeration process employed. In any case, the temperature of the mixture in the Agglomeration Zone should be above the temperature at which the coal begins to soften. Some tar and gas will be produced in the Agglomeration Zone by the partial devolatilization of the coal. A conduit 52 serves to remove such. The agglomerates are withdrawn through a conduit 54, and those in excess of 1/8-inch size are conducted to the Gasifier 60.
Gasifier
The Gasifier 60 is the so-called moving bed-type of gasifier. Such a gasifier requires a non-caking or weakly caking carbonaceous feed of at least 1/8-inch size for satisfactory commercial operation. In such a gasifier, a bed of the solids (in this instance, the agglomerates) which are relatively stationary with respect to each other, moves progressively downwardly, first through a carbonization zone 62 wherein the agglomerates are further devolatilized and, at the same time, hardened to withstand the burden of the bed in the gasification zone 64 through which they pass downwardly. Steam and air (or oxygen instead of air) are introduced into the gasification zone 64 through conduits 65 and 66, respectively, and are circulated upwardly through the downwardly moving bed. The temperatures in the carbonization zone 62 are maintained within the range 375° to 550°C. by the hot gases issuing from the gasification zone. The gasification zone is maintained at a temperature in the range of 750° to 1,100°C. The pressure is 100 to 500 psig. The incoming agglomerates are carbonized in the carbonization zone 62, yielding tar vapors which are withdrawn with the effluent gas via conduit 68. The carbonized agglomerates move downwardly in reactive contact with the upflowing steam and oxygen to form CO 2 , CH 4 , H 2 and CO. Unreacted ash is withdrawn through conduit 69. The product gases pass through the carbonization zone 62 and into conduit 68. The effluent gas, including the tar vapors, is passed into a condenser 70 in which the tar vapors are condensed and removed through conduit 72. The tar-free gas is conducted by a conduit 74 to suitable hydrogen and methane recovery or treatment systems. A hydrogen-enriched gas may be recovered by conventional methods and used, if desired, to hydrogenate the extract recovered through conduit 26 to make distillate fuels. Or, if desired, the gas may be treated and recovered for use as fuel gas.
According to the provisions of the patent statutes, I have explained the principle, preferred construction and mode of operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated.