GB2156841A - Improvements in coal liquefaction - Google Patents

Abstract

Coal liquefaction processes using liquid hydrogen donor solvents too readily suffer from over- hydrogenation and saturation of the recycled solvent, causing precipitation of dissolved coal material and lowering of yields. Catalytic dehydrogenation of at least part of the solvent leads to a return to a balanced and active solvent.

Description

SPECIFICATION Improvements in coal liquefaction This invention concerns improvements in coal liquefaction and more particularly it concerns the treatment of coal liquefaction solvents.

It is well known that coals can be dissolved in oil-type solvents at high temperatures, and that improved yields can be obtained by the presence of hydrogen, under high to low pressures of hydrogen or through the use of so-called hydrogen donor solvents. Catalysts may be present.

Various process have been suggested for the liquefaction of coal using a so-called hydrogen donor solvent. A hydrogen donor solvent can be defined as an oil or a fraction of an oil, boiling in the range 200-500"C, which is essentially hydroaromatic in composition and can donate its chemically bound hydrogen to the depolymerising coal at high temperature, stabilising the coal extract produced by adding the hydrogen to the coal radicals, and thus preventing the radicals from forming coke.

A typical process, by way of example, for the liquefaction and destructive hydrogenation of coal, would consist of contacting crushed coal with a hydrogen donor solvent at high temperature in a first reactor to dissolve the coal, followed by filtration to remove ash and undissolved coal. In a second reactor the coal extract, together with the solvent or a fraction of the solvent, is contacted with a catalyst in a fixed bed together with hydrogen at high pressure and temperature. The coal extract is converted to distillable oils and the solvent is replenished with hydrogen donors. After fractionation of the products, the light oils can be further upgraded to gasoline, diesel and aviation fuels, and the hydrogen donor solvent can be recycled to the first reactor to dissolve more coal. Hence the process can be made continuous and independent of external sources of solvent.

A problem with the above described process is the high pressure of hydrogen required to convert adequately the coal extract in the second reactor and to prevent coking at the high reactor temprature. The high hydrogen pressure tends to give a recycle solvent which becomes saturated with hydrogen on multiple passes through the second reactor. Compounds such as alkyl decalins, perhydrophenanthrene and perhydropyrene are formed on repeated cycles. These compounds are paraffinic in nature and can cause precipitation of the dissolved coal extract leading to precipitates blocking process lines. Furthermore, the saturated compounds are poor hydrogen donors relative to the hydroaromatic compounds, which leads to lower extraction yields.

The saturates in the recycle solvent could, in theory, be removed by a number of methods, for instance liquid/liquid extraction or reaction of the saturates with elemental sulphur or selenium.

Liquid/liquid extraction is inconvenient and leads to a loss of solvent from the process. Reaction with sulphur requires large quantities of the element and produces a large quantity of hydrogen sulphide which is undesirable.

Another approach to the problem of over-hydrogenation of the recycle solvent is described in our Patent Application Number 82/03640. A solvent consisting of aromatic polycyclic hydrocarbons of three and/or four ring molecules and at least 25% of saturated naphthenes boiling in the range 180-300"C is employed. The aromatic portion of the solvent in the process is removed by distillation after extraction of the coal so that it does not pass through the hydrocracker and subsequently saturates are not allowed to increase on repeated recycle.

The objective of the present invention is to improve coal liquefaction processes employing hydrogen donor solvents by dehydrogenating the saturates contained in the recycle solvent or a fraction of it to hydroaromatics thus removing the chemical entities which cause precipitation of coal extract without losing solvent balance in the overall hydroliquefaction process. Dehydrogenation of the saturates to hydroaromatics enables the process to operate without the problem of precipitates in process lines and advantageously enables the hydrocracker to operate at high pressures, for instance 200 atmospheres, which are necessary to achieve high conversion of coal extract in the presence of the hydrogen donor solvent.

The present invention therefore provides a method of coal liquefaction in which coal is extracted using a liquid hydrogen donor solvent at elevated temperature and pressure, at least a fraction of the extract and at least a fraction of the solvent are hydrogenated together or separately and at least a portion of the hydrogenated solvent is recycled to the extraction, characterised in that part at least of the solvent is catalytically dehydrogenated to reduce the quantity of cyclic saturates. The part of the solvent which is catalytically dehydrogenated may be taken from any point of the cyclic liquefaction process, and the dehydrogenation may be carried out continuously or intermittently.

The part of the solvent which is dehydrogenated according to the invention may contain between 5 and 95% by weight of saturates, but preferably contains 10 to 20% of saturates, and it may contain 95 to 5% by weight of aromatics, but the aromatic content is preferably rather low, for example 5 to 25%. It is preferred to dehydrogenate saturates to hydroamatics, since it is thought that aromatics may inhibit the catalyst.

The catalytic dehydrogenation may be carried out in a method analogous to the reforming of naphtha in petroleum oil refineries. It is not the practice, however, to reform fractions having the chemical composition of the hydrogenated solvent, nor does naphtha have similar cut points.

The catalyst must be capable of converting cyclic saturates to hydroaromatics, and will also thus convert hydroaromatics to aromatics although this latter reaction is less desirable. A careful selection by testing is, however, necessary since nickel/molybdenum on alumina converted hydroaromatics to aromatics but cyclic saturates were unconverted. Preferred catalysts include platinum and/or palladium on alumina, silica or active carbon at a loading of 0.1 to 10%, preferably 0.2 to 1 % by weight; these readily promote the dehydrogenation of saturates such as decalins to tetralins, perhydrophenanthrenes to octa-and tetra-hydrophenanthrenes and perhydrophyrenes to hexa- and di-hydropyrenes. Another preferred catalyst is chromia on alumina.

The catalyst may be used in a fixed of fluidised bed reactor.

The catalytic dehydrogenation is suitably carried out at pressures of from 1 to 40 bar, preferably 1 5 to 25 bar over a platinum catalyst and preferably 1 to 5 bar over a chormia catalyst, and suitable temperatures are from 400 to 550"C, preferably 460 to 480"C. Flowrates of hydrogenated solvent, measured as liquid hourly space velocity, are suitably 0.2 to 4.0 h-l, but tests over a platinum catalyst indicate that flow rates of 0.5 to 1.0 are most preferred.

Hydrogen may require to be fed to the process in order to achieve a high hydrogen partial pressure. Hydrogen to solvent molar ratios (H2: HC) are suitably 3 to 20, but are preferably 5 to 10.

Dehydrogenation catalyst are susceptible to poisoning, especially by sulphur, and it is desirable to ensure that the solvent steam being treated is low in catalyst poisons. If the solvent stream to be treated is not sufficiently free from catalyst poisons, then it is preferred to desulphurise the stream, for examp by hydrogenating over a Ni/Mo or Co/Mo catalyst; this is also effective to reduce the nitrogen content of the stream.

The invention will now be described by reference to the accompanying Fig. which is a flow diagram of a coal extraction plant utilising one embodiment of the present invention.

Finely divided bituminous or sub-bituminous coal of a particle size of below 200 um, is extracted in an extractor, A, with from 1 to 10:1, preferably 2:1 to 5:1, of its weight of a hydrogen rich high boiling oil. The temperatures and pressures used are preferably from 430 to 450"C and 10 to 1 5 bar respectively, and the solid and liquid residence times are preferably about 30 and 1 20 minutes respectively. The coal extract slurry product from extraction stage A is passed to a solids removal stage, B, in which ash and undissolved coal are removed by filtration, centrifugation, vacuum distillation, setting or otherwise.

The coal extract solution, substantially free of solids, is passed to a catalytic "hydrocracking" stage, C, in which the coal extract is destructively hydrogenated to distillable oils and the solvent is hydrogenated to replenish the hydrogen donor components, which have given up hydrogen to coal derived moieties in the extraction step. Hydrocracking is so named because both high molecular weight carbonaceous material is cracked to lower molecular weight oils and at the same time hydrogen is chemically bound to the oils and solvent, and may be carried out in a fixed or moving bed, eg. an ebullating bed, reactor. The catalyst used is suitably a sulphurresistant hydrogenation catalyst, molybdenum or tungsten sulphide, promoted with nickel or cobalt, supported on alumina, alumina- silicates, silica, active carbon, magnesia, carbon, magnesia, chromia, titania etc.According to the present invention, the operating conditions of the hydrocracker are not limited by the need to avoid saturation of the solvent oil. Pressures of 50 to 700 bar, preferably 200-250 bar, and temperatures of 410 to 480"C preferably 440 to 460"C, are therefore used to obtain optimum conversion of the heavy coal-derived material.

Liquid hourly space velocities of 0.2 to 2.0 h-l, preferably 0.4 to 1.0 h-l. depending upon the concentration of dissolved coal material, may be used.

Gases, including C1-C4 hydrocarbons, H2S and NH3, are separted, and the liquid product is passed to a fractionation stage, D, which may be atmospheric or vacuum distillation unit. A separation is made betwen light product oils boiling from C5 to 250"C or 300"C, pitch which is the non-distillable part of the products and a solvent fraction, suitably boiling between 250"C and 350"C. In this embodiment, a heavy solvent fraction boiling betwen 350 and 5000C is recycled without further treatment to the extraction step A.

The lighter solvent fraction is passed to a catalytic dehydrogenation stage, E. In this lighter fraction, the major proportion of the saturates in the solvent are found. Also, the dehydrogenation of a lighter fraction, rather than the whole of the solvent, enables the dehydrogenation to be carried out more efficiently in the gaseous phase instead of liquid phase. Suitable conditions etc for the catalytic dehydrogenation have been described above. Hydrogen is removed as a gaseous product and the treated solvent fraction is recycled to the extraction stage together with the untreated but hydrocracked heavy fraction.

The invention may be illustrated by the following three Examples, in which varying coal liquefaction plant streams were treated in a laboratory reactor packed with a platinum on alumina catalyst. The reactor was maintained at a temperature of 460"C and a pressure of 25 bar, and the feedstock was passed at a LHSV of 1.0 h-1. The hydrogen to hydrocarbon molar ratio was maintained at 8:1.

Example 1 A fraction boiling in the range 250 to 400"C of a mixture of recycle solvents from a coal liquefaction pilot plant run, in which the saturates contents had increased on recycling up to fourteen times, and problems with blocked lines had occurred, was dehydrogenated over the platinum on alumina catalyst. The feed contained 9.6% hydrogen and 22.3% saturates and after dehydrogenation the product oil contained 7.4% hydrogen and 2.0% saturates. Feed and product analyses are shown in the Table hereinafter.

Example 2 An anthracene oil fraction boiling in the range 200-350"C was hydrogenated to prepare a start-up synthetic coal liquefaction solvent containing 10.6% hydrogen and 29.3% saturates.

After dehydrogenation the hydrogen content was 7.4% and the saturates content was 3.7% (see example 2 in the Table).

Example 3 A coal extract solution which has been filtered to remove the ash was distilled to remove a 200-350" coal solution distillate. This distillate did not pass through the hydrocracker stage as illustrated in Fig. 1. The hydrogen content of the solvent was reduced from 10.0% to 8.2% and the saturates content from 14.0% to Table. This Example was carried out to show that the removal of saturates can be effected on a solvent fraction from any part of the cyclic coal liquefaction process.

TABLE Example number 1 2 3 Feed Type Recycle HAO coal soln distillate Feed quality Distillation (wt Z) IBP-2500C 6.9 25.6 28.9 250-300 C 41.8 37.6 42.1 300-350 C 38.5 35.5 22.4 350-400*C 10.9 1.3 5.5 4000c 2.0 0.1 1.2 SG 0.991 0.962 0.960 Hydrogen wt Z 9.6 10.6 10.0 Aromatic hydrogen (Xof H) 24.0 22.0 24.0 Saturates wt Z 22.3 29.3 14.0 Hydroaromatics wt Z - 62.1 60.7 Aromatics wt Z - 6.0 24.0 Product quality Distillation (wt %) IBP-2500C 9.2 19.7 27.0 250-300 C 34.8 30.0 41.5 300-350 C 37.5 36.5 25.4 350-400 C 17.1 13.7 4.2 4000c 1.4 0.1 2.0 SG 1.038 1.038 0.983 Hydrogen wt X 7.4 7.4 8.2 Aromatic hydrogen (Z of H) 50.2 55.2 37.1 Saturates 2.0 3.7 6.1 Hydroaromatics - 23.9 47.0 Aromatics - 72.4 46.0 Product yields (wt X) Liquid 96.6 95.1 97.9 C14 gas 1.4 1.4 0.7 Hydrogen 2.0 3.6 1.5

Claims (10)

1. A method of coal liquefaction in which coal is extracted using a liquid hydrogen donor solvent at elevated temperature and pressure, at least a fraction of the extract and at least a fraction of the solvent are hydrogenated together or separately and at least a portion of the hydrogenated solvent is recycled to the extraction, characterised in that part at least of the solvent is catalytically dehydrogenated to reduce the quantity of cyclic saturates.

2. A method accounting to claim 1, wherein said part of the solvent contains 10 to 20% of cyclic saturates and 5 to 25% of aromatics, by weight.

3. A method according to claim 1 or 2, wherein the catalyst used is dehydrogenation is platinum and/or palladium on alumina, silica or active carbon, or chromia on alumina.

4. A method according to claim 3, wherein the catalyst loading on the support is in the range 0.1 to 10% by weight.

5. A method according to any one of the preceeding claims, wherein the dehydrogenation is carried out at a temperature of 400 to 550"C.

6. A method according to claim 3, wherein the temperature is 460 to 480"C.

7. A method according to any one of the preceeding claims, wherein the dehydrogenation is carried out at a pressure of from 1 to 40 bar.

8. A method according to claim 7, wherein the dehydrogenation is carried out under a pressure of 1 5 to 25 bar over a platinum catalyst or at 1 to 5 bar over a chromia catalyst.

9. A method according to any of the preceding claims, wherein the dehydrogenation is carried out in the gaseous phase.

10. A method according to Claim 1, substantially as hereinbefore described.