GB2112806A - Coal hydrogenation process using acid hydrolysis and precipitation of asphaltenes from recycle solvent - Google Patents

Abstract

The invention provides a coal hydrogenation process for producing clean hydrocarbon liquid and gas products, comprising: (a) slurrying coal 10 with a process- derived oil 15 and feeding the coal-oil slurry with hydrogen 17 to a catalytic hydrogenation reaction zone 22; (b) withdrawing effluent material 27 and phase-separating it to provide gaseous and liquid fractions; (c) adding acid 45 to the resulting separated bottoms liquid fraction 44 and precipitating asphaltenes and coal solids; (d) withdrawing from the acid precipitation step an overhead liquid stream 47/15 containing a reduced concentration of preasphaltenes and solids, and recycling said stream to the reaction zone; (e) withdrawing a heavy hydrocarbon bottom stream 48 containing precipitated preasphaltenes and coal solids for further processing; and (f) withdrawing a hydrocarbon liquid product stream 42/54 with low sulphur and ash contents. <IMAGE>

Description

SPECIFICATION Coal hydrogenation process using acid hydrolysis and precipitation of asphaltenes This invention relates to a coal hydrogenation process for producing hydrocarbon liquid and gas products, wherein unconverted coal and ash solids are removed from a coal-derived liquid fraction by acid precipitation so as to produce hydrocarbon liquid products with low sulphur and ash contents.

Conventional process steps for coal hydrogenation and liquefaction processes, such as the H-coal (Trade Mark) Process, usually include hydroclone devices for liquid-solid separation to produce a relatively low ash residual oil recycle stream used to slurry the coal for feeding it into the reactor.

However, the hydroclone devices present problems in that they must be very small to be effective for performing their solid-liquid separation function. In a commercial size coal liquefaction plant, undesirably large numbers of individual hydroclone units must be used together with complex associated piping systems. Furthermore, because of the hydroclones small size, high fluid velocities and the abrasive slurry being handled, they are subject to severe wear and require frequent replacement.

For coal hydrogenation processes such as in the H-Coal process boiler fuel product mode using higher space velocities, the process can also include an effective solid-liquid separation system using solvent precipitation to produce a low ash net product containing non-distillable oil, but low enough in sulphur and ash to be a legally burnable fuel for electric utilities. However, solid-liquid separation for coal liquids using such solvent precipitation to produce boiler fuels is a still more difficult problem. The solvent precipitation of coal solids is believed to function via precipitation of asphaltenes and preasphaltenes by the addition of a hydrocarbon liquid which is a poor solvent to a heavy coal-derived liquid.The asphaltenes coalesce around the solid particles of ash and unconverted coal and coprecipitate the solids, producing a highly viscous underflow material containing most of the solids and an overflow fraction containing most of the high molecular weight oils. Such solvent precipitation processes for coal-derived liquids using a solvent oil from an external source are disclosed in U.S. Patents Nos. 3,791,956 to Gorin, petal and 3,932,266 to Sze, et al. However, a problem with this approach is that, if an extraneous antisolvent liquid is used, i.e., one which is not self-generated in the coal hydrogenation process, a complete separation and recovery of the antisolvent liquid fraction is necessary, which is quite expensive.

The prior art also teaches treating petroleum distillate and coal-derived liquids with acid at near ambient temperatures to precipitate out heavy fractions. For example, U.S. Patent No. 2,068,847 to Davis mentions the coagulation of asphaltic material in oil by treatment with sulphuric acid and an aromatic material. U.S. Patent No. 2,209,123 to Koelbel discloses mixing coal tar oils with paraffin hydrocarbons and dilute acid to cause precipitation of sediments to make clean diesel fuels. Also, U.S.

Patent No.3,084,118 to Overholt, et al, discloses a process for refining coal-derived liquids by adding a hydrocarbon precipitant oil and an acid coagulant such as sulphuric acid at relatively low temperature to provide a supernatant ash-free liquid product and form a soft sludge containing the acid and carbon residues. The present invention provides an improvement over the prior art for coal hydrogenation processes and petroleum refining in that an inorganic acid is used to cause precipitation of the preasphaltene components of the recycled coal liquid rather than using acid precipitation as a refining step on the product. The acid precipitation step replaces the hydroclone liquid-solids separation system in the usual H-Coal process arrangement, and the overflow liquid product is used as a recycle oil for slurrying the coal feed.

The present invention provides a coal hyrogenation process for producing clean hydrocarbon liquid and gas products, comprising: (a) slurrying coal with a process-derived oil and feeding the coal-oil slurry with hydrogen to a catalytic hydrogenation reaction zone maintained under elevated temperature and pressure conditions for hydrogenating the coal; (b) withdrawing a reacted hydrocarbon effluent material and phase-separating it to provide gaseous and liquid fractions; (c) adding acid to the resulting separated bottoms liquid fraction and precipitating asphaltenes and coal solids; (d) withdrawing from the acid precipitation step an overhead liquid stream containing a reduced concentration of preasphaltenes and solids, and recycling said stream to the reaction zone for further hydrogenation therein;; (e) withdrawing a heavy hydrocarbon bottom stream containing precipitated preasphaltenes and coal solids for further processing; and (f) withdrawing a hydrocarbon liquid product stream with low sulphur and ash contents.

This invention thus discloses a coal hydrogenation process for producing hydrocarbon liquid and gas products, wherein an inorganic acid is added to a coal-derived liquid fraction to cause precipitation of preasphaltenes, unconverted coal and ash solids, and to produce clean oil products. Because preasphaltenes contained in the heavy liquid fraction are salts of nitrogen bases and phenolic acids, it is found that these salts can be disassociated and the basic moiety precipitated by adding a strong inorganic acid, such as hydrochloric acid, to the heavy coalderived liquid. The supernatant overflow material is recycled to the catalytic reaction zone for further reaction, from which the precipitated material is withdrawn for further processing to increase liquid product yields.

More specifically, in a coal liquefaction process such as the H-Coal Process making use of this invention, the heavy coal-derived liquid fraction normally boiling above about 800"F (427"C) and containing oils, asphaltenes, preasphaltenes unreacted coal and ash solids is treated with an inorganic acid such as hydrochloric acid. The acid is added to the slurry liquid fraction of the coal liquefaction process at a temperature within the range of 400-600"F (204-316"C), and disassociation of the preasphaltenes occurs. The basic portion of the heavy molecules precipitates, leaving behind a coal tar acid liquid overhead stream having relatively low nitrogen. Destruction of the preasphaltenes reduces the viscosity of the remaining liquid.Precipitation of the basic constituent co-precipitates the solid ash and unconverted coal, similarly as for solvent precipitation. The overflow liquid stream, is low in solids, low in nitrogen due to removal of the nitrogen bases, and has low viscosity. This material provides a much more desirable recycle stream to the reaction zone than is produced by a hydroclone liquid-solids separation system, and produces more desirable product yields from the reactor. Thus, the usual hydroclone subsystem for solids removal from liquid recycled to the reactor is replaced by an acid treating and settling step.

The preferred process arrangement is to recycle this overflow liquid stream from the acid precipitation step for use in slurrying the coal feed. The composition of the recycle liquid stream can be adjusted by varying the pressure at which the feed to the acid precipitator is flashed to recycle greater or lesser amounts of distillate oil. The precipitator bottoms material is withdrawn and can be passed to a vacuum distillation step, coker or solid separation system provided downstream from the acid precipitation step to recover additional clean hydrocarbon liquid product. Also, if desired, the precipitator bottom material can be used either as feed to a hydrogen-producing plant, as feed to a gasifiation plant, or as boiler fuel with on-site stack gas scrubbing.

Acids useful in this invention must be strong inorganic acids and include both Bronstead acids such as hydrochloric (HCI), phosphoric (H3PO4) and sulphuric (H2SO4) acids, and Lewis acids such as boron trifluoride (BF3) and aluminium chloride (AICI3). Sufficient acid is added to the liquid to cause precipitation of the preasphaltene materials. The acid may be added either in liquid form, or as a gas bubbled up through the coal-derived liquid.

The advantages of this acid precipitation separation step are that it is a potentially simple and inexpensive process step having low utilities re quirementsthat replaces both hydroclones and solvent precipitation solid-liquid separation equip ment. Also, it provides a hydrocarbon liquid product having reduced viscosity and low sulphur, and which is a more desirable fuel product.

Reference is now made to the accompanying drawing, which is a schematic drawing illustrating a coal hydrogenation process utilizing an acid precipitation step to remove preasphaltenes and coal solids according to an embodiment of the present invention.

This invention will be described as used in a coal hydrogenation process having an ebullated catalyst bed type reactor as illustrated in the drawing. A bituminous or semi-bituminous coal is provided at 10, and is first passed through a preparation unit generally indicated at 12. In such unit the coal is dried of substantially all surface moisture, ground to a desired size range, and screened for uniformity.

For our purposes, it is preferable that the coal have a particle size of about 50 to 375 mesh (U.S. Sieve Series).

The coal fines are removed through conduit 13 and pass to a slurry tank 14, where the coal is blended with a slurrying oil at 15 which is made in the process. To provide an effectively transportable coal-oil slurry, the ground coal should be mixed with at least about an equal weight of the slurrying oil.

The resulting coal-oil slurry is pressurized by a pump 16 to superatmosperhic pressure, such as 500-5000 psi (34-345 bar), and then passed through a heater 18 for heating the slurry to a temperature in the range of 600"F to 8000F (316-427"C). The heated coal-oil slurry along with recycle hydrogen at 17, is then passed into a reactor feed line 19, where it is supplied with fresh makeup hydrogen as needed at 33a.

The entire mixture of coal-oil slurry and hydrogen then enters a reactor 20 containing a catalyst bed 22, passing uniformly upwardly from the bottom through a flow distributor 21 at a flow rate and at a temperataure and pressure to accomplish the desired hydrogenation. The catalyst in the bed 22 is from the class of cobalt, iron, molybdenum, nickel, tin or other hydrocarbon hydrogenation catalyst metals known in the art, deposited on a base selected from the class of alumina, magnesia, silica, or similar materials. In addition, a particulate hydrogenation catalyst may be added to the reactor 20 at a connection 23 in the ratio of about 0.1 to 2.0 pounds (0.045 to 0.907 kg) of catalyst per ton (1.016 tones) of coal processed.

By concurrently flowing liquid and gasiform materials upwardly through the reactor containing a bed of solid particles of a contact material, which may be a specific catalyst as indicated above, and expanding the bed of solid particles by at least 10% and usually by 20 - 100% over its stationary volume, the solid particles are placed in random ebullated motion within the reactor by the upflowing streams. The characteristics of the ebullated bed at a particular degree of volume expansion can be such that finer, lighter particulate solids will pass upwardly through the catalyst bed, so that the contact particles constituting the ebullated bed are retained in the reactor and the finer, lighter material may pass from the reactor. The catalyst bed upper level 22a, above which few if any contact particles ascend, is the upper level ofebullation.

In general, the gross density of the stationary mass of contact material will be about 25 to 200 pounds per cubic foot (401 to 3204 kbm-3), the flow rate of the liquid will be about 5 to 120 gallons per minute per square foot (20.4 to 489 cm3m-1cm-2) of horizontal cross-section area of the reactor, and the expanded volume of the ebullated bed usually will be not more than double the volume of the settled mass. To maintain the desired superficial upward liquid velocity in the reactor, a portion of the liquid slurry is usually recycled to the reactor, such as a liquid stream 24 which is removed from above the upper level of ebullation 22a and recycled via downcomer conduit 24 and pump 25 to the bottom of the reactor 20, and upwardly through distributor 21. Alternatively, this recycle liquid stream and pump may be located external to the reactor.Spent catalyst may be removed by drawoff at connection 26 to maintain the desired catalytic activity within the reaction zone.

The reactor operating conditions are maintained in the ranges of 700-930 F (371-499"C) temperature and 1000-5000 psi (69-345 bar) partial pressure of hydrogen and preferably 750-900"F (399-482"C) and 1000-4000 psi (69-276 bar) hydrogen partial pressure. The coal throughput or space velocity is at the rate of 15to 150 pounds coal per hour per cubicfoot of reactor volume (240 to 2403 kgh-'m-3), so that the yield of unconverted coal as char is between about 5 and 15W % of the moisture and ash-free coal feed.The relative size of the coal and catalyst particles and the conditions of eblullation are such that catalyst is retained in the reactor, while the ash and unreacted char particles are carried out with the liquid reaction products.

From the reaction zone 20, an effluent stream 27, which is virtually free of solid particles of contact material or catalyst, is withdrawn, cooled at 28, and then passed to a phase separator 30. From the separator 30, a light gas stream is removed at 31 and passed go a hydrogen purification step 32. A medium-purity hydrogen stream 33 is recovered from the purification step 32, warmed at a heat exchanger in combination with cooling means 28, and recycled through the heater 18 to the reactor 20 to provide most of the hydrogen requirements therein as hydrogen stream 17.

From the separator 30 a liquid stream 34 is withdrawn, pressure-reduced at 35 and is passed to a phase separator 38. This separator operates at near atmospheric pressure and 500-650 F (260-343"C) temperature, and permits removal of a light liquid stream at 39 and a heavy hydrocarbon liquid stream at 44. The stream 39 contains naphtha and light distillate fractions and is passed to a fractionation step 40, from which hydrocarbon gas products are withdrawn at 41 and a light distillate product a 42.A hydrogenated coal liquid fraction, usually having a normal boiling range of 500 to 1050"F (260-566"C) containing oil asphaltenes, preasphaltenes, unconverted coal and ash solids, is withdrawn at 44 and is mixed with an acid at 45, and passed to an acid precipitation step 46. If desired, acid stream 45 can be added directly into precipitation step 46. It has been found that the ratio of the acid stream 45 to the asphaltenes in stream 44 should be from 3 to 5W % of the preasphaltenes therein. The stream 44 will usually contain 12 to 40 W % preasphaltenes.The resulting mixed stream is maintained at a temperature of 400-600"F (204-316"C) and at pressure condi tions sufficient to avoid vaporization of the precipi tating acid, generally about 50 psig (3.4 bar gauge) and usually not exceeding about 200 psig (13.8 bar gauge). An overflow liquid stream containing a reduced concentration of solids is removed at 47, and is recycled usually as a coal slurrying oil 15 to the reactor 20 to help control the solids concentra tion in the reactor and to achieve further conversion and increased yields of low-boiling hydrocarbon products. An underflow liquid stream, containing acid increased concentration of preasphaltenes and coal solids is withdrawn at 48.

To facilitate the withdrawal of the underflow liquid stream 48 from the acid precipitation step 46, a rotary device 49 or equivalent mixing means can be used to provide sufficient mixing and continuous agitation to prevent premature solidifiation of the precipitated solids component. Because of the effectiveness of the combination of the acid and coalderived liquids to produce precipitation of solids, the residence time in the settler to achieve significant solids settling is usually less than about 45 minutes and is preferably 15-30 minutes.

From the settler 46, the overflow liquid 47 is continuously withdrawn and contains less than about 2.0 W % solids comprising fine particles of unconverted coal and ash. This overflow liquid 47 contains only a minor amount of acid, and is recycled to the reactor 20, preferably as coal slurrying oil 15 for further reaction therein. The underflow liquid stream 48, containing an increased solids concentration, is removed from the settler 46 by pumping and with the aid of an internal rotary rake 49, can be reduced in pressure by passage to a flash drum 50 which is maintained at a pressure of at least about 5 psig (0.34 bar gauge) and a temperature of 550 F (288"C) for vaporization of acid, and passed to vacuum distillation at 52. The resulting overhead liquid 53 from the vacuum still may be joined with the stream 43 to provide a heavy distillate product stream 54. The heavy bottoms streams 55 from the vacuum still 52 containing some asphaltenes and unconverted coal and ash solids may be further processed by coking to recover oil products or by gasifiation to produce the makeup hydrogen needed in the process.

Claims (14)

1. Acoal hydrogenation process for producing clean hydrocarbon liquid and gas products, comprising: (a) slurrying coal with a process-derived oil and feeding the coal-oil slurry with hydrogen to a cataytic hydrogenation reaction zone maintained under elevated temperature and pressure conditions for hydrogenating the coal; (b) withdrawing a reacted hydrocarbon effluent material and phase-separating it to provide gaseous and liquid fractions; (c) adding acid to the resulting separated bottoms liquid fraction and precipitating asphaltenes and coal solids; (d) withdrawing from the acid precipitation step an overhead liquid stream containing a reduced concentration of preasphaltenes and solids, and recycl ing said stream to the reaction zone for further hydrogenation therein;; (e) withdrawing a heavy hydrocarbon bottom stream containing precipitated preasphaltenes and coal solids for further processing; and (f) withdrawing a hydrocarbon liquid product stream with low sulphur and ash contents.

2. A process as claimed in claim 1, wherein the hydrogenation reaction conditions are within the ranges of 700-930"F (371-499 C) temperature and 1000-5000 psig (69-345 bar gauge) hydrogen partial pressure.

3. A process as claimed in claim 1 or 2, wherein the acid precipitation step temperature is from 400 to 600"F (204-316"C).

4. A process as claimed in any of claims 1 to 3, wherein the acid precipitation step pressure ranges from 50 to 500 psig (3.4 to 34 bar gauge).

5. A process as claimed in any of claims 1 to 4, wherein the acid added to the coal-derived liquid fraction is from 3 to 5W % of the preasphaltene material contained in the liquid fraction.

6. A process as claimed in any of claims 1 to 5, wherein the acid added to the liquid fraction is HCI, H2SO4, H3PO4, BF3 or AICI3.

7. A process as claimed in claim 6, wherein the acid added to the liquid fraction is HCI.

8. A process as claimed in claim 6, wherein the acid added to the liquid fraction is AICI3.

9. A process as claimed in any of claims 1 to 8, wherein the precipitated bottoms material from the acid precipitation step is vacuum-distilled to recover additional clean hydrocarbon liquid product.

10. A process as claimed in any of claims 1 to 9, wherein the overhead material from the phase separation step is fractionated to provide gas and distillate liquid products.

11. A process as claimed in any of claims 1 to 10, wherein the acid added to the solids precipitation step is recovered for reuse.

12. Acoal hydrogenation process for producing clean hydrocarbon liquid and gas products, comprising: (a) slurrying coal with a process-derived oil and feeding the coal-oil slurry with hydrogen to a catalytic hydrogenation reaction zone maintained at 700-930"F (371-499'C)temperature and 1000-5000 psig (69-345 bar gauge) hydrogen partial pressure conditions for hydrogenating the coal; (b) withdrawing a reacted hydrocarbon effluent material and phase-separating it to provide gaseous and liquid fractions; (c) adding acid to the resulting separated bottoms liquid fraction at a temperature of from 400 to 600"F (204-316 C) and precipitating asphaltenes and coal solids; ; (d) withdrawing from the acid precipitation step an overhead liquid stream containing reduced preasphaltenes and solids, and recycling said stream to the reaction zone for further hydrogenation; (e) withdrawing a heavy hydrocarbon bottom stream containing precipitated preasphaltenes and coal solids for further processing; and (f) withdrawing a hydrocarbon liquid product stream containing low sulphur and ash.

13. A coal hydrogenation process substantially as hereinbefore described with reference to the accompanying drawing.

14. Hydrocarbon liquid or gaseous products when produced by a process as claimed in any of claims 1 to 13.