US3503866A - Process and system for producing synthetic crude from coal - Google Patents

Description

March 31, 1970 M, sKRlPx-:K ET AL FRocEss AND SYSTEM FOR RoDucING SYNTHETIC cRubE F'ROM coAL Filed April 24. 1968 Afro/PNE! United States Patent Office Patented Mar. 31, 1970 U.S. Cl. 208- 3 Claims ABSTRACT OF THE DISCLOSURE A process is disclosed for producing synthetic petroleum crude from'coal, for use in a petroleum refining system, which includes low temperature carbonization of dried pulverized coal, liquid recovery and recarbonization of solid and heavy volatile materials, hydrotreating of the recovered liquid, fractionation of the hydrotreated liquid, and recovery of naphtha fractions from the foregoing steps for recirculation into the` hydrotreating step.

CROSS REFERENCE TO RELATED APPLICATION This invention consitutes an improvement and modification of the process and the system described in our copending application Ser. No. 710,086, filed Mar. 4, 1968.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a process and apparatus for producing liquid hydrocarbon products from coal. More specifically, this process relates to methods for producing synthetic petroleum crude by carbonization of coal and hydrotreating of coal tar products.

DESCRIPTION OF THE PRIOR ART There has been a continuing interest in utilizing coal as a source of liquid hydrocarbon products since the advent of the internal combustion engine. Several processes have been developed which have attained technical importance to countries having limited access to petroleum reservoirs. A number of processes were developed in Germany to offset shortages in petroleum during World War II.

Of recent years, there has been a revival of interest in several processes for producing synthetic petroleum crude from coal, Researchers Disclose Cost of H-Coal Gasoline, Oil and Gas Journal, May 16, 1966, p. 155; Coal Knocks at Refinery Door, Chemical Week, Feb. 11, 1967, pp. 2.9-35. While numerous processes have been proposed for producing liquid hydrocarbon synthetic petroleum crude from coal and the technical feasibility of a substantial number of these processes has been proved, no commercially feasible process of this type has been put into operation. Accordingly, it is an object of this invention to provide a commercially and a technically feasible process for producing synthetic petroleum crude from coal.

Processes for producing synthetic crude from coal range from the so-called topping processes wherein only'a very minor fraction of the more volatile products produced by mild destructive distillation of coal is converted to liquid hydrocarbons, the remainder of the coal being burned in the normal manner to produce steam, and those wherein an effort is made to convert essentially all of the carbon content of coal to liquid hydrocarbons. Exemplary of the latter class of processes are those described by Clark et al., Hydrogenation of Coal in a Fluidized Bed, Industrial and Engineering Chemistry, May 1961, pp.

861-865; and Helwig et al., Make Liquid Fuels From Coal, Hydrocarbon Processing, vol. 45, pp. 165-169, May 196-6. In the former case, coal is carbonized in the dry state, the liquids and volatile components being carried out of a fluidized bed. In the latter case, coal is suspended in a liquid phase, which may consist of tar, along with a catalyst.

A number of processes have also been proposed wherein coal is subjected to a more severe carbonization or hydrogenation treatment to convert an intermediate fraction of the coal to liquid hydrocarbon material, leaving the more diiiicultly converted coal components for combustion in the normal manner, i.e., in coke-fired furnaces.

There is reason to believe that the most economical and commercially feasible process will be of this latter type wherein, by carefully balancing the energy requirements for converting coal fractions to liquid hydrocarbon materials against the value of the hydrocarbon materials produced, the value of the synthetic crude, plus the value of the less valuable combustible solid coal products and the byproducts will substantially exceed the cost of treating the coal.

Since the most valuable of the products of the coal treatment process is the synthetic petroleum crude it will be understood that the commercial feasibility of any such process is very largely dependent upon the quality, and hence the value, of the synthetic crude produced. For optimum value, the synthetic crude should be of such a quality that it can be processed in a conventional petroleum refinery, along with petroleum crude, without substantial alteration of operating conditions. It has been suggested, however, that certain superior grades of fuel may be produced from synthetic petroleum crude derived from low temperature carbonization of coal. Accordingly, it is an object of the present invention to provide a method for producing synthetic petroleum crude which is, in its refinery characteristics, substantially identical with high quality petroleum crude.

A large number of coal carbonizing processes are known; see e.g., Farr, Coal Carbonization New Methods and Objectives, Coal Age, Dec. 1966, pp. 88-96, and hydrocracking of liquid hydrocarbons, most particularly petroleum crude, and hydrotreating of liquid hydrocarbon materials are well known in the prior art. The present invention constitutes an improved combination of such known treating steps and includes novel cycle and recycle steps.

SUMMARY OF THE INVENTION Without limiting the scope or applicability of the invention, the present process may be described, in its principal steps, as including the low temperature carbonization of coal f-or converting more valuable carbonaceous components to liquid hydrocarbon materials, feeding the liquid hydrocarbon materials to' a liquid re covery system which may be a scrubbing or distillation system for separating out the very low boiling point and the very high boiling point materials and the entrained solid materials from the usable middle oil and tar fractions. The middle oil and tar fractions are hydrotreated and the hydrogenated liquid material is fractionated to produce a stream of synthetic crude for use in a renery. Naphtha is recovered from the carbonization, liquid recovery and hydrotreating steps and is recirculated to the hydrotreater. Hydrogen is generated from the low molecular weight hydrocarbons for use in the hydrotreater. It is, accordingly, a principal object of the invention to provide an improved method for producing synthetic crude of a quality comparable to petroleum crude by low temperature carbonization of coal to produce a liquid fraction and hydrotreating of the liquid fraction.

A more specific object of the invention includes a process and system for adding naphtha components with middle oil and tar in the hydrotreating of the liquid carbonization products.

A further specic object of the present invention includes the combination of phenols recovered from the carbonization step in the hydrotreater for increasing the yield of synthetic crude.

An additional object of the invention is to provide a system for carrying out a process of the type described.

A more specific object of the invention is to provide a novel combination carbonization and hydrogenation system for extracting liquid hydrocarbon fractions from coal.

The specific process and system disclosed constitutes further specific objects of the invention.

Other objects will be apparent from the specification which follows and from the drawing to which reference is now made.

BRIEF DESCRIPTION OF THE DRAWING The figure is an overall schematic diagram showing the system for carrying out the inventive process and illustrative process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the figure, the system comprises four major components and process steps. The system further comprises important secondary process steps and components for handling auxiliary streams of solids and liquids. The major components are the low temperature carbonizer 10, the liquid recovery system 20, the hydrotreater 30, and the fractionator 40. The important auxiliary components include the naphtha recovery system 50, the acid gas removal system 60, the hydrogen generator 70, the char pulverizer 80, the phenol extractor 90 and the ammonia stripper 100. The general flow of the liquid components is from left to right as illustrated in the drawing. The components of the system are, individually, known in the prior art and have been described.

The low temperature carbonizer may be of the type constructed by Lurgi Gesellschaft, Frankfurt am Main and described in the Lurgi Manual (1961). This process is also described by Peters, W. and Bertling, H., American Chemical Society Division of Fuel Chemistry Preprints, vol. 8, No. 3, pp. 77-88; (148th National American Chemical Society meeting, 1964). The carbonization unit consists of three main sections, carbonization, heat generation and volatiles collection, and utilizes hot char recycle to supply heat for the carbonization. Dried coal is fed by a double screw mixer., where it is mixed with hot recycle char, into the carbonization chamber. As the solids leave the mixer, they fall into the devolatilization vessel of the carbonizer at a temperature of from 800 to 1250 F., preferably about l100 F. The devolatilization vessel is substantially free of air. Here slow devolatilization reactions are allowed to proceed, giving maximum yields of gas and liquids from the coal. This overall technique, which is termed flash carbonization, provides the very rapid solid-solid heat transfer which is essential for maximum devolatilization of coal. This process gives higher liquid yields than gas-solid heat transfer yields. Char from the carbonization zone flows by gravity through a gas seal at the bottom of a lift pipe and the heat generation zone of the carbonizer and is conveyed upwardly by a high velocity mixture of flue gas and air where part of the char is consumed with the air to produce heat. Char at 1250 F. is collected in a bunker and allowed to flow by gravity for recycling into the inlet of the double screw mixer. Excess char is removed.

It should here be pointed out that the type of carbonizing unit just described has been used and found to be successful in the overall process; however, other carbonizers have been used as well with varying degrees of success and the description herein is simply exemplary of the preferred embodiment of the invention and the process is not limited to the use of the Lurgi-Ruhrgas (L-R) type carbonizer.

The L-R type carbonizer is modified for the present invention to the extent that all of the liquid components and the entrained solid components are removed to the separate liquid recovery system 20.

The recovery system 20 may be of any of the types of systems conventionally used for liquid separation in a petroleum refinery. The higher boiling point fraction (900 R+) of the tar is rapidly quenched and scrub-bed out along with the entrained solids and returned from the liquid recovery system 20 to the carbonizer. The remaining liquid components (900 F.-) are fractionated into streams according to composition and boiling point. Apparatus of the type contemplated are described in Chemical Technology of Petroleum, Gruse, W. A. and Stevens, D. R., McGraw-Hill, New York, 1960; Distillation Principles and Design Procedures, lHengstebeck, R. J., Reinhold, New York, 1961; Distillation and Rectiication, Kirschbaum, E., Chemical Publishing Company, 1948; and Distillation in Practice, Nielson, C. H. (Ed), Reinhold, New York, 1956. Other publications and patents also describe apparatus of this type. Since equipment of this type is conventional and well known in the prior art no detailed description of the stripping and distillation apparatus is included herein except by reference to the prior publications, the reference cited therein and the practices of the art generally.

The 900 F.- fractions of the volatile carbonization products, as recovered by the methods and apparatus discussed, are essentially free of solids and have signicantly higher hydrogen content than the average hydrogen content of the input to the liquid recovery system. These I' liquid fractions can be upgraded by hydrogenation in a conventional fixed bed reactor such as hydrotreater 30. Since xed bed hydrogenation reactors are described in the previously cited publications and are well known in the prior art no detailed description is undertaken.

ln addition to the conventional fixed bed hydrotreater, ebullated bed hydrotreaters of the type described by Helwig, et al., Hydrocarbon Processing, supra; Chervenak, M. C., et al., H-Oil Process Treats -Wide Range of Oils, yPetroleu-m Rener, vol. 39, No. 10, pp. 15l-156 (October 1960); Rapp, L. M., and Van Driesen, R. P., H-Oil Process Gives Product Flexibility, Hydrocarbon Processing/Petroleum Rener, vol. 44, No. 12, pp. 103-108 (December 1965); and Prescott, J. H., Novel Route Upgrades The Bottom Iof The Crude Barrel, Chemical Engineering, vol. 72, No. 15, pp. 142-144 (July 1S, 1966). Since the conventional xed bed hydrotreater can, in general, -be operated more economically, however, it is preferred to the ebullated bed hydrotreaters of the type described. Further details of this process and other processes are given in our aforementioned copending application, the disclosure of which is incorporated herein by reference.

The output of the hydrotreater 30 is fed to the fractionator 40 from which the synthetic crude for handling in a conventional petroleum rening process is derived directly. The fractionator -40 is, of course, of the conventional type known in the prior art and commonly used in refineries.

The auxiliary components and process steps are adequately described -in the aforementioned copending ap- .plication and reference is made thereto for the necessary descriptions. All of these units are conventional and known in the art and are described in prior publications and patents to which reference is made for the operating details.

Referring yagain to the drawing, the process is described in detail as follows. Dried coal which has been previously pulverized to a mesh size of one-fourth inch is fed through line `9 to the low temperature carbonizer 10. All of the liquid components and the solid material entrained therein is transferred through line 11 to the liquid recovery systern 20. Waste and liuc gases are returned through line A12 for use in drying the coal. The char is removed from the bottom of the carbonizer through line 13 to char pulverizer 80. The main liquid streams, the middle oil and the tar, are passed respectively through lines 21 and 22 to the hydrotreater 30. These liquid components may be combined to form a light tar if desired prior to hydrotreating, however. The gases and the light hydrocarbons flownthrough line 23 to the naphtha recovery system as will be described. The phenols lare carried through line 24 with the waste water to the phenol extractor `90. The heavy liquid fractions, boiling at 900 F. `and higher, and entrained solids are removed from the liquid recovery scrubber and returned through line 25 for recoking in the low temperature carbonizer.

The hydrogenated liquid product flows from the hydrotreater 30 through line 31 to the fractionator 40. Fixed gases and light hydrocarbons exit through line 32 to the naphtha recovery system. Ammonia-rich Waste Water from the hydrotreater is transferred through line 33 to the lammonia stripper 100 as will be described. The liquid product, the syncrude, flows from the system through line 41 to storage or directly to the refinery input. The low molecular -weight hydrocarbons, C4 minus, exit from the top of the fractionator 40 through line 42 and are combined with the similar streams flowing in line 32 from the hydrotreater and from line 23 from the liquid lrecovery system |(and indirectly from the carbonizer) to the naphtha recovery unit 450. Naphtha is recycled through line 51 to the hydrotreater. The naphtha may be fed independently into the hydrotreater or may be mixed with the Imiddle oil fraction or with the combined middle oil and tar fraction. The gases from the naphtha recovery unit are introduced to the acid-gas removal unit 60 through line 52. The hydrocarbon gases are carried through line 61 to the hydrogen generator 70, with any excess hydrocarbon gas being returned to the system as fuel gas for providing auxiliary heat. The acid gases, primarily HZS, are sent to the sulfur recovery unit through line 62. Hydrogen from the'generator 70 is fed through lines 71 to the hydrotreater 30 and the byproduct carbon dioxide is recovered through line 72 from the hydrogen generator 70. The other major byproduct of the process, pulverized char, is removed through line 81.

'In order to provide increased syncrude yields, the phenols are returned from the phenol extractor 90 through line 91 to the hydrotreater where, following hydrogenation treatment, they ow through line 31 to the fractionator 40. The water fro-m the phenol extractor 90 is carried through yline 92 and combined with the water from the hydrotreater llowing in line 33 and fed to the ammonia stripper 100. Ammonia is separated and sent to the recovery plant through -line 101 and waste water from the process is forwarded to the water treatment plant through line 102 for chemical and biological treatment and purification.

The importance of the naphtha recycle step in the process, wherein naphtha is fed through line 51 from the naphtha `recovery system to the hydrotreater, is of particular note. The addition of naphtha to the hydrotreater permits hydrogenation of the liquid stream to proceed in theV normal manner in conventional catalytic hydrogenation equipment.

In addition, the use of the liquid recovery system following the carbonizer to remove the 900 IP. and higher liquid components permits economically feasible operation of the system and of the process. It has been found that when the liquid output of the carbonizer remains at high temperatures for any appreciable period of time or is placed in storage prior to further refining for any significant length of time, complex polymerization type reactions occur to produce a substantially solid mass having a high pitch content which cannot be refined in a conventional petroleum refining process. "It has been found,

6 however, that the components -of the carbonizer output liquid which create the diiculty constitute only a very minor fraction of the total liquid stream and can be removed to produce a stream of liquid products which can be hydrogenated and refined without difficulty.

By careful control of the Operating conditions of the system, especially the low temperature carbonizer, the hydrotreater, the naphtha recycle system and the hydrogen generation and recycle system, the process may be balanced so that the maximum quantity and quality of synthetic petroleum crude is produced with a minimum consumption of heat. Optimum operating conditions are achieved when the value of the synthetic crude components extracted exceeds the marketable heat value of the char, giving consideration to the market value of the byproducts. Deeper extraction of hydrocarbon values from the coal may result in a relative net loss.

Careful consideration of the economic factors which control the relative values of the products of the described process will suggest the very significant advantage the present process has over the processes of the prior art. If the heat value of char or coke and the value of synthetic crude, as well as the values of the minor byproducts Iwere fixed, a single set of process conditions could be determined which would result in maximum economic benefit to the operator and lower costs to the public. As is well known, however, the market values for each of the products of the process described fiuctuate according, e.g., to seasonal and industrial demands, the political forces in operation at a given time and general economic conditions. It is, therefore, highly desirable to provide a system and a process which can be varied to produce maximum economic value as the values of the individual products of the process shift. In the present process, the only products which will have a substantial effect upon the process variables are, of course, the relative values of char as a fuel and of the synthetic crude. As previously pointed out, however, the value of the synthetic crude is very highly dependent upon its quality.

Taking into consideration current economic and technioal conditions the following operating conditions are given as exemplary of a preferred embodiment of the invention, on a small pilot plant scale.

Coal from the Last Chance Creek outcrop of coal beds in southern Utah, known as Kaiparowits coal, was crushed and screened to pass through a 1A inch mesh screen. This grade of coal contains about 6.4 percent moisture, 40 percent volatile matter, 7.3 percent ash, and 46.3 percent fixed carbon. Of course, other sub-bituminous coals, such as those known as Black Mesa coals from northern Arizona, or any roughly equivalent grade of coal may be used. The coal was dried to 3 weight percent moisture by countercurrent contact with hot ue gases from the carbonizer. These flue gases are byproducts of the char of combustion in the heat generation portion of the carbonizer. Any type of dryer may be used but the dryers found effective in this embodiment were vertical columns for dilute phase contact between hot gases and the wet coal, electrostatic precipitators being used to remove solids from the water-saturated waste gases.

The crushed, dried coal was fed to the double screw mixer of the carbonizer Where it was mixed with hot recycle char. Contact time in the mixer is only a few seconds but a large percentage of the devolatilization of the coal occurs during this interval. The coal-char mixture Was conveyed into the carbonization vessel of the previously described carbonizer. The average temperature of the coal in the carbonizer vessel was 1100 F.

The middle oil and tar streams, individually or mixed, along with the naphtha stream which may be mixed with the middle oil or light tar stream, is hydrotreated, in an exemplary embodiment, over Cyanamid HDS-3A Ni-Mo catalyst at about 1750 p.s.i.g. hydrogen, 750 F., and a liquid hourly space velocity of about 1.0. The reactor the middle oil and tar fractions in or before the hydrotreater provides highly desirable lluid characteristics which permit normal operation of conventional hydrotreating equipment. This eliminates the need for a distillation column to regenerate a lean oil stream.

The high pressure gases from which the naphtha has been removed are fed to the acid gas removal unit, which may be a Rectisol extraction tower, described in the Lurgi Manual, where all of the HZS and most of the CO2 are removed by extraction with methanol. This unit also in` cludes equipment for solvent (methanol) regeneration.

The scrubbed gases from the Rectisol plant are then sent to the hydrogen plant as required to produce hydro gen for the process.

The other auxiliary units operate under conventional process conditions which have been described.

Complete data on the composition of the syncrude resulting from the above process are not yet available; however, the product appears to be compatible with petroleum crude in all respects and may be processed in a conventional petroleum refining process. There is considerable evidence that the relative energy content of the Syncrude produced by this process is substantially higher than the energy content of a comparable quantity of conventional petroleum crude.

Review of pilot plant operations indicates that improved tar quality may result from the carbonization process utilized but optimum values were not determined. It is to be emphasized, however, that any type of low temperature carbonization process may be utilized as an individual step in the inventive process Without departing from the scope and spirit of the invention which is defined in the claims.

It will be understood, also, that the embodiment given is merely exemplary of the invention and that the con ditions given are not necessarily limiting but rather indicate a workable operating condition. With these considerations in mind there are several areas in which it is known and/or expected that certain operating conditions may be varied to improve either the quality of the product or to improve the overall efliciency of the process. For example, while complete data are not available to confirm these findings, there is good reason to believe, based on preliminary data, that as the fraction of the coal which is liquefied to produce synthetic crude is increased the overall quality of the crude tends to go down. 'Ih'at is, the hydrogen content and the energy content as Well as the compatibility of the syncrude with petroleum crude, are reduced. Thus, the severity of the coking operation is, from an economic standpoint, dependent in part on the quality of the syncrude which is desired.

From the foregoing specifi-cation it will be apparent that a highly useful and commercially valuable process has been disclosed and that while a particular embodiment of the invention has been disclosed in detail .departures may be lmade from the exemplary embodiment without departing from the scope and the spirit of the invention as defined in the following claims.

We claim: i

1. The process for producing synthetic petroleum crude from coal which comprises the steps of:

carbonizing coal at a relatively low temperature to produce a stream of volatile hydrocarbonaceous liquids;

separating the liquids from the carbonization step to remove entrained solids and liquids having a boiling' point higher than about 900 F. for recycle to the carbonizer and highly volatile naphthas and xed gases to produce at least one stream of normally liquid hydrocarbons; hydrotreating said normally liquid hydrocarbons from the liquid recovery step to produce a stream of improved synthetic crude and a stream of highly volatile uid products including naphtha; fractionating lthe improved synthetic crude to produce a stream of synthetic petroleum crude having rening properties substantially equivalent to natural petroleum crude; combining the naphtha containing streams; stripping the naphtha from said combined streams; and recycling at least a portion of the naphtha for combination with the normally liquid hydrocarbon stream. 2. The process of claim 1 wherein the liquid recovery 'step produces a normally liquid tar stream and a middle oil stream, at least a portion of each of said streams being separately fed to the hydrotreating step.

3. The process of claim 1 wherein a phenol containing stream is separated in the liquid recovery step and further comprising the steps of extracting the phenols from said phenol containing stream; and

feeding said phenols to the hydrotreating step along with the normally liquid hydrocarbon from the liquid recovery step.

References Cited UNITED STATES PATENTS 2,654,695 10/1953 Gilbert et al. 20S-10 2,738,311 3/1956 Frese et al. 208-8 2,913,388 1l/1959 Howell et al. 208-8 2,913,397 11/1959 Murray et al 208-107 3,018,242 1/1962 Gorin 208-10 3,107,985 10/ 1963 Huntington 208-10 '3,117,921 1/1964 Gorin 20S-8 3,157,589 11/1964 Scott et al. 208-108 3,231,486 1/1966 Perry 208-8 3,321,393 5/1967 Schuman et al 208-10 DELBERT E. GANTZ, Primary Examiner V. OKEEFE, Assistant Examiner

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