US 3867275 A
A process for producing liquid hydrocarbonaceous products from coal utilizing two steps of solvent extraction with different solvents. In the first stage, coal is contacted, at relatively high temperature and pressure, with a heavy hydrocarbon solvent containing a mixture of hydroaromatic hydrocarbons and saturated aliphatic hydrocarbons in a hydroaromatics/aliphatics weight ratio between about 1:2 and about 2:1. The solid materials remaining after the first liquefaction step are subsequently separated from the liquefied coal and the heavy solvent and the liquefied coal from the first extraction step is recovered as a product. The solid materials recovered from the first extraction step are solvent extracted, at relatively low temperature and pressure, with a monocyclic aromatic hydrocarbon solvent, and the resulting liquids are also recovered as a product.
Description (OCR text may contain errors)
United States Patent Gleim et al.
[ COAL LIQUEFACTION PROCESS  Inventors: William K. T. Gleim, Island Lake;
Mark J. OHara, Mt. Prospect, both of I11.
 Assignee: Universal Oil Products Company, Des Plaines, Ill.
 Filed: Apr. 9, 1973  Appl. No.: 349,344
 US. Cl. 208/8 51] Int. Cl .[Cidg 1704" 58 Field of Search, .2 2 08/81  References Cited "UNITED STATES PATENTS 3,583,900 6/1971 Gatsis 208/8 3,607,716 9/1971 Roach.... 208/8 3,607,717 9/1971 Roach 208/8 3,607,718 9/1971 Leadersetal. 2 s[ s i Primary Examiner-Veronica OKeefe m V I itfbfieyfAgeFr] bTFTfriz fanie s' RITloatson, J"'r.;' Thomas K McBride; William H. Page, ll
57] ABSTRACT A process for producing liquid hydrocarbonaceous products from coal utilizing two steps of solvent extraction with different solvents. ln the first stage, coal is contacted, at relatively high temperature and pressure, with a heavy hydrocarbon solvent containing a mixture of hydroaromatic hydrocarbons and saturated aliphatic hydrocarbons in a hydroaromatics/aliphatics weight ratio between about 1:2 and about 2:1. The solid materials remaining after the first liquefaction step are subsequently separated from the liquefied coal and the heavy solvent and the liquefied coal from the first extraction step is recovered as a product. The solid materials recovered from the first extraction step are solvent extracted, at relatively low temperature and pressure, with a monocyclic aromatic hydrocarbon solvent, and the resulting liquids are also recovered as a product.
6 Claims, N0 Drawings 1 COAL LIQUEFACTION PROCESS BACKGROUND OF THE INVENTION This invention concerns a process for converting solid carbonaceous materials such as coal into liquid hydrocarbonaceous products. More particularly, this invention relates to a process for solvent extracting coal, using two different solvents in two extraction steps, to provide hydrocarbonaceous liquid suitable for use as substitutes for petroleum liquid.
Solvent extraction of coal and similar carbonaceous materials is known as a method for producing hydrocarbonaceous liquids. The liquid product from such extraction operations may be substituted for petroleum fractions and can be refined to produce gasoline, etc., in a manner analogous to that used to refine petroleum liquids. Because of the abundance of coal reserves and decreasing petroleum resources, it is presently becoming increasingly important to develop practical methods for deriving petroleum substitutes from coal.
In a typical operation for solvent extraction of coal, the coal is pulverized and mixed with a hydrocarbonaceous solvent. The mixture of coal and solvent, typically with hydrogen gas included, is subjected to head and pressure. A fraction of the coal is dissolved and mixed with the solvent. The liquid mixture of solvent and dissolved coal is then separated from the coal ash and undissolved coal by settling, filtration, etc. Two problems in particular have created substantial difficulties in attempts to develop economical methods for solvent extracting coal. These difficulties can be characterized briefly as, first, inability to facilitate the transfer of sufficient hydrogen into the hydrogen-deficient coal during the extraction step and, second, lack of selectivity of the solvent used in extracting particular components of the coal. Since the coal to be extracted is relatively low in hydrogen content, it is necessary to add a significant amount of hydrogen to the coal during a solvent extraction operation. In order to form coal liquids which can be used as substitutes for petroleum fractions, the hydrogen content of the coal to be liquetied must be increased typically from about 5 weight percent of the coalup to about weight percent or more of the liquefied materials. Prior art has devised several methods for facilitating the necessary hydrogen addition. Of these, the most commonly suggested has been the use of extraneous catalysts and the use of hydrogen donor solvents.
The use of'extraneous catalysts has been found generally impractical. These catalysts rapidly become poisoned by the metals naturally present in the coal, and are quite difficult to separate from the solid residue remaining after the extraction operation. Hydrogen donor solvents have provided a partial solution to the problem of transferring hydrogen into the coal. These solvents donate hydrogen to the coal molecules and simultaneously revert from a partially saturated to an aromatic structure. However, the amount of hydrogen which such hydrogen donor solvents can normally transfer into the coal is quite limited, and a further hydrogenation treatment of the coal extract subsequent to an extraction operation is generally necessary when using hydrogen donor solvent to extract coal.
Lack of selectivity of coal solvents in extracting the various available components of coal is a problem related to the inability of solvents to transfer sufficient hydrogen into the coal as discussed above. Lack of selectivity is found generally in prior art solvents, but will be discussed hereinafter in terms of hydrogen donor solvents, since they have generally been employed to the exclusion of other solvents. Hydrogen donor solvents are capable of dissolving a large fraction of the solid coal, often as much as percent. Unfortunately, the coal liquids obtained by using hydrogen donor solvents invariably contain as much as 40 50 percent of undesirable components which are substantially refractory to further refining, are severely hydrogen sufficient and severly inhibit separation and refining of the more valuable components of the coal. For convenience, these undesirable components of coal extracts can be characterized as benzene-insoluble components and benzene-soluble but heptane-insoluble components. The benzene-insoluble components of coal liquids are generally thought to result primarily from polymerization of dissolved components of the coal subsequent to their extraction into the solvent. During the extraction operation, the molecules in the original solid coal are depolymerized and partially hydrogenated. Because of the lack of available hydrogen, a significant fraction of the liquefied materials remains in an easily polymerizable state and tends to repolymerize rapidly to form undesirable, high molecular weight compounds which are even more refractory than the original solid coal. Since prior art solvents, including the hydrogen donor type, are unable to supply sufficient hydrogen to saturate or crach these polymerizable compounds, they rapidly revert to undesirable, benzene-insoluble refractory compounds in prior art extraction operations. It is thus apparent that simply dissolving a large fraction of the original solid coal, without adequate hydrogenation, is not only futile, but actually undesirable. The polymerizable compounds which are produced by such operations cannot be adequately hydrogenated using prior art solvents and will rapidly form undesirable and untreatable benzeneinsoluble compounds. It has been found that a separate supply of hydrogen gas mixed with prior art solvents and coal is not sufficient to provide an adequate source of hydrogen which can be effectively utilized in the extraction operation, since uptake of the hydrogen gas into the solvent and transfer therefrom into the coal molecules is not rapid enough.
The benzene-soluble but heptane-insoluble fraction found in coal liquids is herein termed the asphaltenes" fraction. Generally, a hydrogen donor solvent dissolves a significantly large amount of asphaltenes, e.g., about 10 percent or more of the dissolved coal. Asphaltenes are high molecular weight, hydrogen-deficient compounds which are difficult to treat by conventional refining techniques. A particular drawback of asphaltenes is that they are undistillable and heat sensitive, and tend to form coke when exposed to distillation temperatures. This property of asphaltenes makes it extremely difficult to distill the coal liquids recovered in prior art extraction operations. Asphaltenes are difficult to treat by refining methods and extremely difficult to separate from more valuable components of the coal extract. They are, therefore, an undesirable fraction when found in comixture with distillable components of the coal, and reduction in the amount of asphaltenes contained in the liquids recovered by extraction of the coal or segregation of the asphaltenes components from the distillable fractions of the extract are desirable in coal liquefaction.
Because of the refractory and/or heat sensitive nature of a substantial fraction of coal extract produced in prior art operations, and because of the presence of readily polymerizable compounds in coal liquids after the extraction step, it has been found impractical to attempt to perform conventional refining operations, e.g., distillation and catalytic cracking, on coal liquids as they are recovered from extraction operations. A further hydrogen treatment of the coal liquids after solvent extraction has been found necessary in prior art in order to make the coal liquids suitable for catalytic cracking and distillation. The type of hydrogen utilized with these extracts is cumbersome and entails the use of expensive catalysts and high temperatures. The expense and difficulties of such post-extraction hydrogenation operations, when performed on the extract as a whole, has been significant barrier to the development of a successful process for solvent extracting coal to provide petroleum substitutes. As described above, the use of extraneous catalysts during the solvent extraction step itself, although successful in producing distillable coal extracts, is technically difficult and also prohibitively expensive, since such catalysts deactivate rapidly, requiring extensive regeneration facilities, and are also quite difficult to separate from solid coal residues. l
A drawback generally found in prior art liquefaction solvents and processes is the effect of the extraction operations on coal subjected to extraction which is not dissolved during the operation. Any of the coal which is not dissolved in a first extraction operation on the original coal has been found quite refractory in any further subsequent extraction. This is partly the result of the failure, discussed above, of prior art solvents to transfer sufficient quantities of hydrogen into the coal during the extraction step. Any of the coal not dissolved is thus quite hydrogen-deficient, and has, if anything, less hydrogen content than the original coal. Coal is thus degraded by prior art extraction operations into a relatively undesirable form, and is not further extractable in a practical manner. Since the object of solvent extraction operations is to convert coal into petroleum substitutes, it is obviously desirable to treat the coal by an extraction operation in a manner which converts the unliquefied, solid coal to a form more susceptible to conversion to a liquid rather than to degrade the unliquefied coal into a low value source. Such degradation of the unconverted coal has been a barrier to successful solvent extraction operations when selective conversion of only a fraction of the original coal has been attempted using, for example, hydrogen donor solvents.
SUMMARY OF THE INVENTION An object of the present invention is to provide a process for the liquefaction of coal and similar carbonaceous solids to form hydrocarbonaceous 'liquids which can replace petroleum fractions and products.
Another object of the present invention is to provide a coal liquefaction process in which components of solid coal are selectively extracted in an operation using two stages of extraction of solid coal.
Another object of the present invention is to provide a process for solvent extraction of coal using two extraction steps which provides a hydrocarbonaceous liquid product substantially free from non-distillable components.
Another object of the present invention is to provide a process for the extraction of coal in which the transfer of hydrogen into the coal during the liquefaciton of the coal is enhanced.
Another object of the present invention is to provide a coal liquefaction process which produces a liquefaction product which can be subjected directly to conventional catalytic cracking without intermediate hydrogenation treatment.
Another object of the present invention is to provide a process for liquefying coal which produces a liquefaction product substantially free from polymerizable, high molecular weight components.
Another object of the present invention is to provide a process for liquefying coal using two steps for extracting the coal solids, in which the first extraction step enhances the hydrogen content and solubility of the coal not liquefied during the first extraction step.
In an embodiment, the present invention relates to a process for producing liquid hydrocarbonaceous products from a solid carbonaceous material which comprises the steps of contacting the solid carbonaceous material, at liquefaction conditions, with a liquefaction solvent comprising a combination of a hydroaromatic hydrocarbon component and a saturated aliphatic hydrocarbon component with a hydroaromatic com ponent/saturated aliphatic component weight ratio of about 1:2 to about 2: l; separating the resulting mixture into a solid phase and a liquid phase; recovering a first hydrocarbonaceous product from the liquid phase; contacting the solid phase with a monocyclic aromatic hydrocarbon at solvent extractant conditions; and, recovering a second hydrocarbonaceous product from the resulting mixture.
We have found that when a coal liquefaction solvent containing a mixture of hydroaromatic hydrocarbons and saturated aliphatic hydrocarbons, at a weight ratio of about 1: 2 to about 2:1, is employed in a first liquefaction step, the coal liquids recovered are substantially free from polymerizable and coke forming components. Even more important, the coal solids which are not liquefied during the first extraction step are modified in-a very' beneficial manner. The unliquefied portion of the coal treated in a first liquefaction step using the aforesaid solvent mixture is rendered soluble in a monocyclic aromatic solvent such as benzene at extraordinarily mild extraction conditions which are utilized in a second extraction step in the present process.
DETAILED DESCRIPTION OF THE INVENTION The solid carbonaceous materials which may be solvent extracted utilizing the process of the present invention includes in general, coal, lignite, peat, oil shale, tar sand, and like naturally occurring carbonaceous materials. The present invention is particularly applicable to the conversion of bituminous coal. A typical preferred bituminous coal is an Illinois bituminous stoker coal having a volatile content of about 5 percent or higher in the moisture and ash free (MAF) coal. Although the following description is given with reference only to the preferred bituminous coal, the discussion is equally applicable to the other solid carbonaceous materials noted above, unless otherwise stated. In general, better results are obtained in the presentprocess when the coal to be extracted is pulverized to fairly small size particles, e.g., mesh particle size or smaller. Howsaturated aliphatic hydrocarbon components. It is essential to the present process that the first stage liquefaction solvent contain compounds from both of these two groups. Other hydrocarbons or extraneous materials may or may not be present in varying amounts up to about weight percent of the total liquefaction solvent without any particular adverse effect on the results obtained in the first liquefaction step. Preferably, compounds not fitting into either of the two categories of essential components in the first stage liquefaction solvent are not present in the liquefaction solvent at more that about 10 weight percent of the solvent as a whole. The weight ratio of the two essential types of components in the first stage liquefaction solvent is an important aspect of effective operation in both the first and second stages of extraction in the present process. The best result in the first stage can only be obtained by maintaining the weight ratio of the liquefaction solvent within a specified range. This weight ratio also has a strong effect on the ability of the second stage extraction solvent to convert a large fraction of the coal at mild conditions. Good results are obtained when the weight ratio of hydroaromatic compounds to saturated aliphatic compound is maintained between about 1:2 and about 2:1. A preferred weight ratio range of hydroaromatic to satuated aliphatic components is from about 2:3 to about 3:2. The hydroaromatic component of the liquefaction solvent employed in the first stage liquefaction step may be selected from a broad range of compounds which are characterized as partially hydrogenated condensed aromatic rings with from 1 to 4 (adjacent) CH groups in which the carbon atom of the CH group forms part of the condensed ring structure. An example of a hydroaromatic compound having nonadjacent CH groups is 9,1 O-dihydroanthracene. An example of a hydroaromatic having two adjacent CH groups is 9,10-dihydrophenanthrene. An example of a hydroaromatic having 3 adjacent CH groups is indane. An example of a hydroaromatic having 4 adjacent CH groups tetrahydronaphthalene. The hydroaromatics suitable for use in the present process have normal boiling points above about 400F. and preferably above about 500F. It is generally preferred that the hydroaromatic fraction of the solvent utilized in the first stage liquefaction step in the present invention is a mixture of different partially hydrogenated polycyclic aromatic compounds, since pure hydroaromatic compounds, although giving equivalent results, are no more suitable than a mixture and are expensive and difficult to obtain. While the partially saturated naphthalenic hydrocarbons, i.e., dihydronaphthalene and tetrahydronaphthalene, are operative as hydroaromatic hydrocarbon compounds in the first stage liquefaction solvent, the preferred hydroaromatic compounds are higher boiling, partially saturated condensed ring aromatics with structures containing three or more condensed benzene nuclei. Examples of suitable compounds include anthracene, and phenanthrene derivatives, etc. Other suitable compounds, by way of example, include the partially saturated derivative of naphthacene, benzoanthracene, chrysene, benzophenanthrene, triphenolene, pyrene, pentacene, benzonaphthacene, pentaphene, benzochrysene, debenzophenanthrene, dibenzoanthracene, picene, naphthanthracene, benzopyrene, perylene, hexaphene, benzopentacene, hexacene, debenzonaphthacene, debenzochrysene, benzopentacene, anthroanthracene, tribenzoanthrancene, naphthochrysene, benzopicene, phenanthrophenanthrene, naphthonaphthacene, benzoperylenes, dibenzopyrenes, heptacene, benzohexacenes, debenzopentacenes, benzohexaphene, heptaphene, anthranaphthacene, naphthopentacene, benzonaphthochrysenes, debenzopicene, debenzopentaphene, trinaphtholene, trinaphtholene, tetrabenzeanthracene, benzonaphthonaphthacene, dibenzopyrolene, naphthopyrolene, debenzopicene, naphthopicene, benzonaphthochrysene, coronene, etc. Particularly preferred hydroaromatic compounds include dihydrophenanthrene, dihydroanthracene, tetrahydroanthracene, dihydropyrene, tetrahydropyrene, and hexahydro coronene, partially saturated condensed ring compounds having one or more relatively short alkyl groups (e.g., 1-5 carbon atoms) in place of one or more of the hydrogen atoms in the condensed ring structure as well as phenyl group substituents are also suitable. It will be apparent to those skilled in the art that any mixture of partially hydrogenated polycyclic compounds which is derived from petroleum or coal liquids by, for example, partial hydrogenation of a particular fraction of the petroleum or coal oil, will generally contain at least a small fraction of many or most of the above listed compounds, their alkyl substituted or phenyl group substituted analogs, etc., in various states of partial saturation. As noted above, the compounds which are included in the term hydroaromatic hydrocarbon as used herein are those in which the condensed range structure is at least partially aromatically unsaturated and at least partially saturated, with the saturated carbon atoms being bonded to hydrogen atoms. If this condition is satisfied, the presence or absence of short chain alkyl group substituents, phenyl group substituent, etc., is not of critical importance. These short alkyl chains and phenyl groups which replace some hydrogen atoms bonded to the saturated carbon atoms in at least some of the condensed ring compounds are not believed to play any part in the first stage liquefaction solvent used in the present process.
The saturated aliphatic components of the liquefaction solvent utilized in the first stage liquefaction step includes all types of alkanes and cycloalkanes having boiling points above about 400F. and preferably above about 500F. Thus, aliphatic saturates contained in a petroleum fraction which boils above about 400F. (which can be characterized broadly as the kerosene fraction and heavier) may suitably be used to provide saturated aliphatic components for the first stage liquefaction solvents. A mixture of paraffinic and naphthenic saturates such as is typically found in commerical sources of petroleum fractions, tar sand fractions, etc., is preferred over any particular saturated com pound, because best results are obtained using a mixture of paraffinic hydrocarbons and naphthenic hydrocarbons. Examples of suitable saturated aliphatic compounds which may be used, preferably in admixture, include C and heavier normal paraffins and isoparaffins with at least C or higher molecular weight preferred, C and longer chain phenyl substituted alkanes, alkyl naphthenes, fused ring naphthenic structures, etc.
A convenient and suitable liquefaction solvent for use in the first liquefaction stage of the present process may be derived from the bottoms product remaining after vacuum distillation of crude petroleum. Essential to achievement of good results by the use of a particular petroleum bottoms fraction is the requirement that the particular bottoms fraction must contain condensed ring aromatic compounds and aliphatic compounds at an aromatic/ aliphatic weight ratio of about l:2 to about 2:1, and preferably about 2:3 to about 3:2. The vacuum bottoms recovered from petroleum distillation are not, per se, usable as a solvent in the first stage liquefaction step in the present process and must be treated to increase the hydrogen content to an adequate level before adequate results can be obtained in either the first stage liquefaction step or the second stage extraction step. Various suitable methods for hydrogen treating a vacuum bottoms petroleum fraction are known to those skilled in the art. By way of example, one method which is suitable for hydrogenation of such a petroleum or tar sand faction, and which is a preferred method for deriving the first stage solvent used in the present process, includes passing a vacuum bottoms fraction over a fixed bed of a suitable catalyst at a temperature of about 700 to about 800F. and a hydrogen pressure of about 170 atmospheres or more. A suitable catalyst may, for example, be a combination of a Group VI and a Group VIII metal on a refractory inorganic support. One preferred catalyst for such a hydrogenation operation comprises a combination of nickel and molybdenum on an aluminasilica spherical support. Using such a catalyst, a suitable liquid hourly space velocity (total volume of charge per hour divided by the volume of catalysts) of about 0.5 to about 1 is generally maintained. Hydrogen is typically recycled in such an operation at a rate of about 5,000 to about 15,000 cubic feet per barrel of hydrocarbon charged. After the vacuum bottoms have been processed in this or any other suitable manner in order to provide sufficient fraction of the hydroaromatic compounds and saturated aliphatic compounds essential to the liquefaction solvent used in the first stage liquefaction step, the light end (materials boiling below about 400F. and preferably all materials boiling below about 500F.) are removed by distillation. The bottoms product remaining after removal of these light ends is a preferred solvent for use in the first liquefaction step in the present process. It is essential to the use of any petroleum fraction as a liquefaction solvent in the first, high temperature extraction step in the present process that such a fraction contain both hydroaroniatic compounds and saturated aliphatic compounds in particular relative amounts. Unless aparticular hydrocarbon fraction contains both types of compounds in sufficient quantity, it is unsuitable. For example, catalytic cracker slurry oils have been proposed as coal solvents but, unlike the suitable hydrogenated petroleum and tar sand vacuum bottoms, slurry oils are unsuitable for use as the first stage liquefaction solvent in the present process even when subjected to hydrogen treatment by the method described above. This is believed to be the result of the composition of slurry oil, which is overly high in aromatics content and deficient in aliphatics content. Thus, unless a mixture of pure hydro-aromatic compounds and saturated aliphatic compounds is especially prepared for use as the first stage liquefaction solvent, particularly hydrocarbon fractions sought to be employed as a solvent in the present process, must be analyzed to determine their composition and if the composition is either too high in aromatics content or in aliphatics content, these solvents are not suitable for use in the first stage of the present process. Those skilled in the art will recognize that the relative amount of aromatics and aliphatics in particular, petroleum and tar sand fractions, varies over a wide range depending on the source of the petroleum or tar sand, so that not all properly hydrogenated vacuum bottoms fractions are necessarily suitable. By analysis of particular petroleums, tar sands, or other available sources of a heavy hydrocarbon fraction, and from the description provided herein, it will be apparent to those skilled in the art which hydrocarbon fractions may suitably be employed in the first stage of the present process.
Liquefaction conditions utilized in the first stage extraction step in the present process include a temperatur of 513613 600 1 to about 850 F. and a pressure of about atmospheres to about 350 atmospheres or more. A temperature of about 675F. to about 800F. and a pressure of about atmospheres to about 250 atmospheres are preferred. The first stage liquefaction solvent and coal are admixed and charged to a liquefaction zone at solvent/ coal weight ration of about 1:2 to about 10:1. A solvent/coal weight ratio of about 1:1 to about 3:1 is preferred. The first extraction step of the present process may be embodied in either a batch type operation or a continuous type operation with good results. In a batch type operation, a quantity of the liquefaction solvent and coal are admixed and placed in a suitable liquefaction reactor, such as an autoclave, subjected therin to liquefaction conditions, and then withdrawn after a suitable residence time. A residence time of about /2 hour to about 24 hours may be employed with good results in such batch type embodiments, with a residence time of about 2 hours to about 6 hours being particularly preferred. In a continuous type operation, the liquefaction solvent and coal are continuously admixed, charged to a suitable liquefaction reactor, maintained therein at liquefaction conditions for a suitable time, and continuously withdrawn. A suitable liquid hourly spaced velocity (volume of reactants, solvent and hydrogen charged per hour divided by the volume of the reactor employed) of about 0.25 to about 4 may be employed. A liquid hourly space velocity of about 0.5 to about 1 is prefered. Any batch type or continuous type reactors employed in prior art coal liquefaction are suitable for use in the first stage liquefaction step in the present process.
After the first stage liquefaction operation, the resulting mixture of the liquefaction solvent, liquefied coal, solid coal, ash and light gases is withdrawn from the liquefaction reactor and the liquefied coal is recovered as a product of the process. The coal liquids and solvent are separated from the solid materials at this point by settling, filtration, centrifugation, hydroclone, extraction with light solvents, or othersimilar methods well known in the prior art. Any suitable method for separating the ash and other solids from the liquid materials may be utilized in the present process. After separation of the solid materials, the first stage coal liquid and solvent may be fractionated directly without any further treatment (by hydrogenation) as has been found necessary in prior art non-catalytic coal liquefaction operations. A fraction of the liquids recovered from the first stage liquefaction zone may by recycled, in a continuous type operation, for use as a part of the first stage liquefaction solvent or in some cases the whole solvent in such a fraction conforms to the composition of the first stage liquefaction solvent described above as necessary to adequate operation of the present process. The fraction of the original solid coal which is converted to liquid hydrocarbonaceous products in the first stage liquefaction step of the present process is generally maintained between about 20 weight percent and about 70 weight percent of the original coal. Preferably, the amount of liquefaction in the first liquefaction step is between about 45 weight percent and about 60 weight percent of the original coal.
The aromatic hydrocarbons which may be employed as the extraction solvent in the second extraction step in the present process include benzene and alkylbenzenes. Suitable alkylbenzenes include, for example, toluene, ethylbenzene, isopropylbenzene, butylbenzenes, xylenes, diethylbenzenes, methylethylbezenes, polymethylbenzenes, polyethylbenzenes, etc. The lower boiling alkyl benzenes, such as toluene, xylenes, ethylbenzenes, cumene, etc., are preferred, and benzene is especially preferred.
Solvent extraction conditions employed in the second stage extraction step in conjunction with the monocyclic aromatic solvent include a temperature in the range from about 50F. to about 300F. and a pressure sufficient to maintain the solvent employed at least partially in the liquid phase. Generally, adequate pressures range from about 1 atmosphere to about atmospheres or more. Extraction can be accomplished in the second stage more rapidly at higher temperatures and pressures, but lower temperatures and pressures, e.g., about 75F. to about 200F., and atmospheric pressure to about 5 atmospheres, are generally preferred because they are more economical. The second extraction step can be performed in either a batch type operation or a continuous type operation. In a batch type operation a quantity of the solid residue remaining after the first liquefaction step and a quantity of monocyclic aromatic solvent are placed in a suitable batch extraction zone, such as, for example, a Soxhlet extractor, and contacted at solvent extraction conditions therein for about V2 hour to about 24 hours. When benzene is employed as the extraction solvent, 21 contact time of about 1 hour to about 6 hours is generally preferred. In a continuous type operation, solid residue from the first stage liquefaction step and the monocyclic aromatic solvent are continuously admixed an charged to a suitable continuous reactor and contacted therein at appropriate conditions of temperature and pressure for a resident time of about 0.5 to about 25 hours. The monocyclic aromatic solvent and the undissolved materials recovered from the first liquefaction operation may be contacted in either cocurrent or countercurrent fashion in continuous type operation in the second extraction stage.
After an appropriate contact time between the monocyclic aromatic hydrocarbon solvent and the coal solids in the second stage extraction step, remaining solid residue is separated from the monocyclic aromatic solvent and coal liquids dissolved therein by any suitable conventional means. The aromatic solvent and liquefied coal may be separated from ash and any other insoluble solids by filtration, centrifugation, etc. It has been found that about 20 weight percent to about 40 weight percent of the coal undissolved after the first stage extraction is dissolved when the coal is subjected to the second stage extraction step in the present pro cess, so that the solids remaining after the second extraction operation are composed primarily of fusain and inorganic ash. Solid residue from the second extraction step is therfore discarded. The coal liquids recovered in the second extraction step are similar to the liquids recovered by prior art coal liquefaction methods. That is, the second stage extraction product is not directly distillable (in contrast to the coal liquids recovered in the first stage liquefaction step), and generally the second stage coal liquids contain less hydrogen and more asphaltenes than the product of the first liquefaction step. It may thus be desirable to hydrogen treat the coal liquids recovered in the present process from the second extraction step. The monocyclic aromatic solvent used in the second extraction step may be conveniently flash distilled off to separate it from the heavier coal liquids recovered in the second extraction step, and the aromatic solvent can then be further utilized in subsequent operation of the second stage extraction step.
The following examples are presented to illustrate the operation of the process of the present invention and to indicate some preferred modes of carrying out the present process. The examples are intended solely as illustrations and are not considered to indicate limitations on the generally broad scope of the present process as hereinbefore described.
EXAMPLE I In this run, a hydrogenated vacuum bottoms fraction from an American petroleum source was used as the liquefaction solvent in the first stage liquefaction step and benzene was employed as the monocyclic aromatic hydrocarbon solvent in the second stage extraction step. The liquefaction solvent had been prepared by processing petroleum vacuum bottoms at 760F. and 200 atmospheres hydrogen pressure in a continuous hydrogenation operation using a conventional catalyst at a liquid hour space velocity (defined as volume of hydrocarbons processed per hour per volume of catalyst) of about 0.5. The hydrocarbons resulting form this treatment were fractionated and the bottoms product boiling above 850F. was recovered as the liquefaction solvent for use in this run. Analysis of this solvent indicated it contained 50 weight percent hydroaromatics, 40 weight percent saturated aliphatics, and 10 weight percent condensed ring aromatics. It had an API gravity of 20.0 and contained 1 weight percent heptane insoluble components. In the first liquefaction step in this run, a 200 gram sample of this liquefaction solvent and 200 grams of an Illinois Belleville District Coal, pulverized to mesh and finer particles, were placed in an 1,800 cc. rocking autoclave. The hydrogen content of the original coal in this run was found to be 5.2 weight percent The autoclave was sealed and sufficienthydrogen was introduced to provide 100 atmospheres hydrogen pressure. The contents of the autoclave were heated to 750F. and agitated at that temperature for 4 hours. The autoclave was then cooled and excess pressure was released. The mixture of solids and liquids remaining in the autoclave was removed and the solids were separated form the liquids by centrifuging the mixture. The liquids were recovered as the product of the liquefaction step and analyzed. It was found that 47.0 weight percent of the original coal had been con verted to distillable liquid products in the first liquefaction step. The first stage product had an API gravity of 20.1 and contained only 1.1 weight percent heptane insoluble components. The first stage product was thus found to be readily distillable and substantially free from asphaltenes. The solids recovered from the first liquefaction step were analyzed and found to contain 5.2 weight percent hydrogen, indicating that the coal not converted to liquid products in the first stage liquefaction step had not been degraded during the first liquefaction operation. In the second stage extraction step, the pentane-insoluble solid residue from the first stage liquefaction was placed in a conventional Soxhlet extractor and extracted with benzene at 175F. and at mospheric pressure. After 4 hours of extraction at these conditions, which are notably mild extraction conditions for use in extraction of coal, the extraction operation was discontinued. The solid residue resulting was separated from the benzene solvent and liquefied second stage product. The product of the second stage extraction was then separated from the benzene solvent by flash distillation of the benzene. It was found by analysis of the product that 35.5 weight percent of the original coal had been recovered as benzene-soluble liquids in the second stage extraction operation. The combined conversion of the original coal to hydrocarbonaceous products in the first and second stage extraction operation was thus 82.5 weight percent.
EXAMPLE ll In this run, a hydrogenated vacuum bottom faction from a mideastern petroleum source was used as the first stage liquefaction solvent and benzene was used as the second stage monocyclic aromatic extraction solvent. The first stage liquefaction solvent in this run have been derived by hydrogen treating petroleum vacuum bottoms at 750F. and 200 atmospheres hydrogen pressure using a conventional hydrogenation catalyst at a 0.5 LHSV. The resulting hydrocarbons were hot flashed and the bottoms product was employed as the liquefaction solvent in this run. This liquefaction solvent had an API gravity of l8.8, and an intitial boiling point of 475F. It contained 1.1 weight percent heptane-insoluble material. Analysis of this liquefaction solvent showed that it contained 49 weight percent bydroaromatic hydrocarbons, 46 weight percent saturated aliphatic hydrocarbons and 4 weight percent polycyclic aromatics. A 200 gram sample of this liquefaction solvent and 200 grams of the same coal, pulverized to 100 mesh particle size, used in Example [were placed in the same 1,800 cc. rocking autoclave used in Example I. The autoclave was sealed and sufficient hydrogen was charged to provide 100 atmospheres hydrogen pressure. The content of the autoclave were heated to 750F. and agitated for 4 hours at that temperature. The autoclave was then cooled and excess pressure was released. The mixture of liquids and solids in the autoclave was removed and the liquids were separated from the solids by centrifugation and recovered as the first stage product. It was found that 58.1 weight percent of the original coal had been converted to liquid hydrocarbonaceous products in the first stage liquefaction step. The liquids recovered from the first stage lique-' faction step were analyzed and found to have an API gravity of about 205 and a heptane-insoluble content of only 1.2 weight percent. The solids recovered from the first liquefaction step were also analyzed and found to contain 5.95 weight percent hydrogen as compared to only 5.2 weight percent hydrogen in the original coal before the first stage liquefaction operation. The first stage liquefaction operation thus increased the hydrogen content of the undissolved, solid coal recovered after the first stage operation. In the second stage solvent extraction step in this run, the pentane-insoluble solids recovered from the first liquefaction step were placed in the Soxhlet extractor used in Example I and extracted with benzene at a temperature of 175F. and atmospheric pressure. After a period of 6 hours, the extraction operation was discontinued and the benzene solvent and coal liquids were separated from the remaining solid residue by filtration. After analysis of the liquids and solids recovered in the second stage, it was found that 18.6 weight percent of the original coal had been recovered as benzenesoluble liquids in the sec- 0nd stage extraction operation. The two steps of extraction in this run, according to the present invention, thus produced a total conversion of solid coal to hydrocarbonaceous liquid product of 76.7 weight percent of the original coal.
From the foregoing description and examples, it is apparent that the process of the present invention, employing two stages of extraction with two different solvents, provides a novel and superior method for recovering hydrocarbonaceous liquid products from coal. The products recovered in the first liquefaction step in the present process, using a heavy hydrocarbonaceous solvent containing a mixture of hydroaromatic and saturated aliphatic hydrocarbons, provides a first stage product which is readily distillable and substantially free from heptane-insoluble components. The second stage operation may be performed at extremely mild operating conditions and enables almost complete conversion of carbonaceous materials on the original coal to liquid hydrocarbonaceous products with a desirably small investment in utilities and equipment because of the mild conditions employed.
We claim as our invention:
1. A process for the liquefaction of coal which comprises the steps of:
a. contacting the coal, at a temperature of from about 600F. to about 850F., a pressure of at least about atmospheres and a time period of from about /2 to about 24 hours, with a liquefaction solvent comprising a partially hydrogenated condensed ring aromatic hydrocarbon boiling above about 400F. and having from 1 to 4 adjacent CH groups in which the carbon atom of the CH group forms part of the condensed ring structure, said solvent further comprising an alkane or cycloalkane C and heavier having a boiling point above about 400F., the weight ratio of the first-mentioned solvent component to the second-mentioned solvent component being from about 1:2 to about 2:1; separating the resultant mixture into a solids phase and a liquids phase;
recovering a first hydrocarbonaceous product from said liquids phase;
contacting said solids phase with a benzene or alkylbenzene solvent, the alkylbenzene having an alkyl group of from 1 to 4 carbon atoms, at a temperature of from about 50F. to about 300F., a
4. The process of claim 1 wherein said partially condensed ring aromatic hydrocarbon contains at least 3 benzene nuclei.
5. The process of claim 1 wherein the solvent employed in step (d) is benzene.
6. The process of claim 1 wherein said secondmentioned solvent component of said liquefaction solvent is a mixture of paraffmic and naphthenic hydrocarbons.