GB2065160A - Gas extraction of coal - Google Patents

Abstract

Coal can be extracted at a temperature of 420 to 500 DEG C at a pressure in excess of 70 bar, using a gaseous solvent which contains at least 25% of a hydrogen donor to maximise yield of extract and to minimise yield of carbonaceous residue (char.) When the char yield is below 50%, all the char can be used on site to provide process energy, hydrogen, etc.

Description

SPECIFICATION Gas extraction of coal This invention concerns a process for the treatment of coal, more especially the treatment of coal with a solvent which is above its critical temperature.

The techniques of gas extraction, also known as supercritical extraction, are known in the art. We found that coal substance can be extracted to form a gas phase solution when finely-divided coal is contacted with a substance which is supercritical, that is above its critical temperature and generally above its critical pressure. Although, in general, yields are appreciably lower than when coal is extracted with a liquid, gas extraction does permit a relatively easy separation of the extract from the solid residue (char) remaining and also permits relatively easy recovery of the extracted coal substance by precipitation from the gas phase solution by lowering the pressure or temperature of the solution.

Our British Patent No. 1,482,690 describes and claims a process for the hydrogenative extraction of coal involving the use of a gaseous solvent in the presence of a hydrogen donor. That patent was based on the discovery that the use of hydrogen donors as additives to supercriticai gases in the gas extraction of coal makes is possible to improve the yield significantly over the yields obtained using supercritical gas extraction without hydrogenation. Yields of up to 38% of dry ash free coal are disclosed when using an extractant of 10% tetralin in toluene.

We have now found that it is possible under certain selected conditions to achieve yields of the order of 50% or greater; which has considerable commercial significance which will be discussed later.

Accordingly, the present invention provides a process for the hydrogenative extraction of coal, which process comprises extracting coal at a temperature within the range 420 to 5000 C, with a gaseous solvent comprising at least 25% by weight of a hydrogen donor under a pressure in excess of 70 bar, separating the extract in the gas phase from the solids residue and thereafter separating the extracted products from the gas phase solution by temperature and/or pressure reduction.

The coal may be a lignite, brown coal, sub-bituminous or bituminous coal, but is preferably a high volatile bituminous coal, suitably of a volatile matter content exceeding 30% by weight of the dry ash free coal.

The solvent may be entirely a hydrogen donor or mixture of hydrogen donors or may be a mixture of a conventional solvent or solvents which would be above its or their critical temperature(s) at the reaction temperature stated together with a hydrogen donor. Many hydrogen donors are known in the field of liquid solvent extraction of coal and may be used in the present invention. However, especially suitable materials include tetralin and its alkyl derivatives, and also partly hydrogenated phenanthrenes and anthracenes and their alkyl derivatives. The hydrogen donor is used at a temperature above its critical temperature or may, when in admixture with another solvent or solvents, be in gas phase solution. Many supercritical solvents have been suggested in the art and these may be single compounds such as toluene, or mixtures, such as commercial petroleum-based solvents.

While is may be convenient in bench-scale experimental work to use as solvent a single hydrogen donor or a simple mixture of a hydrogen donor and a single compound, a large-scale plant would find it most suitable to use as a solvent a liquid produced in the process. The process yields partiallyhydrogenated coal substance which may be processed into chemical feedstocks or liquid fuels, suitably by further hydrogenation, for example hydrogenation under conventional conditions over a hydrocracking catalyst. It has been found that a fraction of hydrogenated extract is indeed a most satisfactory solvent for the process of the invention, as the hydrogenated extract contains material capable of acting as hydrogen donors.

Preliminary studies indicate that a solvent which-contains 25 to 35% by weight of hydrogen donor is capable of achieving an extract yield of 50 to 55% by weight. Each pass of solvent would reduce the quantity of hydrogen donor by a few percent as the hydrogen donor gives up its hydrogen to the coal substance, and therefore a mild hydrogenation to regenerate the solvent is necessary before recycle.

Such a hydrogenation may be carried out in a number of ways, and the skilled man could determine the most suitable by a straightforward experiment. Make-up solvent would be conveniently a hydrogenated extract fraction, preferably a medium or heavy fraction such as 1 700 to 2500C fraction.

The pressure used in the process of the invention is considerably higher than was proposed in early work on gas extraction, in which pressures were close to the critical pressure of the solvent chosen.

We believe that the use of high pressure, preferably in the range 100 to 300 bar, is a major factor in the achievement of high yields and in the avoidance of difficulties. A very serious problem when using a solvent containing a hydrogen donor under conditions as previously suggested is that there is a tendency for the coal to soften and swell, and this may reach the extent that the reaction vessei and associated pipework is blocked. We have found that under the conditions of the invention, a higher proportion of hydrogen donor can be used without encountering coal swelling. It is thought that the increased pressure increases the density of the gaseous solvent and increased its solvent power; and liquids emerging from the coal particles are thus taken into solution rapidly rather than remaining on the coal and plasticising it.This suppression of undesirable effects is also seen by increasing temperature at a fixed pressure. For example, when extracting a high volatile bituminous coal from Daw Mill Colliery, England, up to 40% tetralin can be included in toluene at 4200C and 200 bar to avoid swelling, however if the temperature is raised to 450 C, there is no limit on the quantity of tetralin which can be included.

The quantity of solvent is suitable from 2 to 20:1, preferably 4 to 10:1 by weight, based on the dry ash free weight of coal to be extracted.

The coal is preferably in finely divided form, but the size is not critically determined by process requirements, rather it is dependent upon apparatus size and type, that is by the need to pass the coal through pipes, valves and pumps. A suitable size for commercial scale operation would be 8 in. (-0.32 cm), but in a 5 kg/hr continuous extractor, a suitable coal size is -72855(0.021 cm aperture size).

The coal may be extracted in a fixed bed or more preferably in a continuous contacting system such as a fluidised bed or a transport contractor.

As has been said above, there is considerable commercial significance attached to the achievement of a yield in excess of 50% in the process of the invention. Of course, any improvement of yield in a process is economically desirable, but in the process of the invention the achievement of a self-contained integrated process for the production of hydrocarbon liquids from coal is at last permitted. The residual char is a reactive char and it has already been suggested that this could be gasified. It is unlikely that the export of char falls below 45%, all the char can be consumed on site to provide process heat, power and hydrogen.

Thus the invention also provides a process for the production of liquid hydrocarbons from coal, in which coal is extracted in an extractor at a temperature in the range 420 to 5000 C, with a gaseous solvent comprising at least 25% by weight of a hydrogen donor, under a pressure in excess of 70 bar, separating the extract in the gas phase from the solids residue, hydrogenating the solvent after a pass through the extractor to regenerate it and recycling the solvent to the extractor, the extracted products being hydrogenated or hydrocracked while in gas phase solution or after recovery therefrom, to yield liquid hydrocarbons, the hydrogen used in the process being produced by gasifying the solids residue.

The gas phase solution may be separated into extracted products and recovered solvent, each being separately hydrogenated, the recovered solvent being regenerated for recyle and the extracted products being hydrogenated or, more prefereably, hydrocracked in manner known perse to yield valuable hydrocarbon oils. Alternatively, the gas phase solution, essentially at the high temperature and pressure used in the extraction, may be mixed with a suitable amount of hydrogen and passed through a catalytic hydrotreating reactor in manner known per se. In this way, the solvent will be rehydrogenated and the extract will receive a primary hydrogenation which will hydrofine it to some extent and lower its molecular weight thus making it easier to handle in subsequent treatments. It is thought that an improved catalyst life is achieved when the gas phase solution is hydrogenated.

The liquid hydrocarbons are suitable as such or after further hydrotreating and processing in manner known per se, as chemical feedstocks or liquid fuels.

The hydrogenation, hydrocracking and processing usable in this invention are analogous to that established in the field of petroleum refining, and any changes necessary because of the more aromatic nature of the feedstock may be made without difficulty by the competent chemical engineer.

The invention will now be illustrated by the following Examples.

EXAMPLE 1 Semi-Continuous Extraction A bed of coal of Coal Rank Code 902 (according to "The Coal Classification System used by the National Coal Board" (Revision of 1964), published by National Coal Board Scientific Control, London), having the following properties: moisture 6.6%, ash 13.7%, volatile matter 39.4% of dry ash free coal, (79.3%, H 5.1%, 0 11.7%, N 1.3% and 52.0% (daf), of size range -10 + 72 BSS (-0.1 68 cm + 0.021 cm) was confined in a tubular autoclave with an amount of solvent mixture equal in weight to the weight of coal. The autoclave was rapidly heated at a rate of 1000C per minute to a temperature of 420"C in a fluidised said heater, and on reaching this temperature a flow of the solvent mixture was introduced at a rate equal to four times the coal weight in twenty minutes. The pressure was maintained at 207 bar. The coal bed was maintained at 4200C for 20 minutes while extractable material was removed in the supercritical gas phase. The gas phase was passed through a condensation train at atmospheric temperature and pressure and the extracted coal substance was subsequently recovered by distilling off the solvent mixture at 2500C and 66.66 millibar. GLC analysis of the recovered distillate allowed the quantity of tetralin converted to naphthalene to be determined. The results of this extraction are set out below.

A.

Solvent mixture extract yield char. residue % tetralin % tetralin in % daf coal. % daf coal. converted to toluene/tetralin naphthalene.

mix.

0 29 64 10 40 54 19 25 44 46 13 40 53 41 10 50 Coal swelled, run not completed.

B. The above procedure was followed, except that the temperature was raised to, and maintained at, 4500 C.

solvent mixture extract yield char residue % tetralin in % daf coal. % daf coal.

toluene/tetralin mix.

0 32 63 25 45 43 50 61 29 75 69 21 100 76 18 At this temperature and pressure, swelling and softening was not a problem, in contrast to earlier attempts to use 1 00% tetralin at pressures close to the critical pressure of tetralin and in the range 40 to 50 bar.

EXAMPLE 2 Continuous Extraction The same coal as used in Example 1 was crushed so that 99% passed a 120 BS sieve (0.012 cm aperture). It was then dispersed in eight times its own weight of a solvent mixture consisting of 25 parts by wt of tetralin and 75 parts by wt of a commercially available aromatic solvent with a boiling range of 1 85-21 40C. The resulting slurry was pumped at 207 bar through a heater tube, where its temperature was raised to 4200 C, into a fluidised bed contactor in which the average solids residence time was ten minutes. In the contactor, the extractable coal substance dissolved in the supercritical gas solvent mixture. The gas solution and solids were continuously transferred to a separating vessel where most of the solids were removed on a filter from the solution, and the solution was continuously depressurised to atmospheric pressure into a condensing system from which the extracted material was recovered by distilling off the solvent mixture. The yield of extract free from solvent was measured as 56% of the dry ash free coal weight. The weight of daf char residue was 38% of the daf coal.

Claims (9)

1. A process for the hydrogenative extraction of coal, comprising extracting coal at a temperature within the range 420 to 5000 C, with a gaseous solvent comprising at least 25% by weight of a hydrogen donor under a pressure in excess of 70 bar, separating the extract in the gas phase from the solids residue and thereafter separating the extracted products from the gas phase solution by temperature and/or pressure reduction.

2. A process according to claim 1, wherein the coal is a bituminous coal of volatile matter in excess of 30% by weight of the dry ash free coal.

3. A process according to claim 1 or 2, wherein the hydrogen donor is selected from or comprises tetralin and its alkyl derivatives and partly hydrogenated phenanthrenes and anthracenes and their alkyl derivatives.

4. A process according to any one of the preceding claims, wherein the proportion of hydrogen donor is 25 to 35% by weight of the total solvent.

5. A process according to any one of the preceding claims, wherein the extraction is in the range 100 to 300 bar.

6. A process according to any one of the preceding claims, wherein the quantity of solvent used is from 4 to 10:1 by weight based on the dry ash free weight of coal to be extracted.

7. A process according to claim 1, substantially as hereinbefore described.

8. A process for the production of liquid hydrocarbons from coal, in which coal is extracted in an extractor at a temperature in the range 420 to 500 C, with a gaseous solvent comprising at least 25% by weight of a hydrogen donor, under a pressure in excess of 70 bar, separating the extract in the gas phase from the solids residue, hydrogenating the solvent after a pass through the extractor to rregenerate it and recycling the solvent to the extractor, the extracted products being hydrogenated or hydrocracked while in gas phase solution of after recovery therefrom, to yield liquid hydrocarbons, the hydrogen used in the process being produced by gasifying the solids residue.

9. A process according to claim 8, substantially as hereinbefore described.