GB2078252A - Hydrogenative Coal Liquefaction - Google Patents

Abstract

Non-liquefied solid components of the product of a liquefaction reactor 5 are concentrated in a hot separator 6 under approximately the same pressure and temperature conditions as in the reactor 5. Gaseous and vapour components are condensed in subsequent stages 9, 10, while the pumpable residue is withdrawn from the separator 6 and expanded. Hydrogen-containing recirculation gas from a gas cleaner 12 is fed through the hot separator 6 (directly at 17 and indirectly through the reactor 5) in contact with the outlet product of the reactor in a quantity of more than 5000 Nm<3> per tonne of (dry ash-free) coal. The oil for forming a coal slurry may be obtained in an intermediate separator 31 following the hot separator. <IMAGE>

Description

SPECIFICATION Hydrogenative Coal Liquefaction This invention relates to a process for liquefying coal by hydrogenation in the presence of oil and hydrogen-containing recirculation gas.

In known processes, coal liquefaction is carried out in pressure vessels to which the coal is fed together with an oil (sometimes referred to as grinding or slurrying oil) and hydrogen-containing gas, and in which the organic carbonaceous matter is decomposed into mainly fluid products at temperatures of 300 to 5500C and at pressures of more than 100 bars.

One more catalytically active chemical compounds may be present in the reaction chamber. In one process, known as the sump or liquid phase process, a suspended catalyst is continuously fed into the reaction chamber with the coal, and is removed together with the reaction prodkts. In other processes, pieces of the catalyst are kept in motion in the reaction chamber, substantially by producing a "seething" catalyst layer, or by means of devices which rotate in the reaction compartment and to which the catalyst is fixed.

The fluid products consist generally of light, middle, or heavy oils and range up to undistillable masses which are fluid only when in the molten state. These constituents are obtained in differing proportions according to the liquefaction conditions. In addition to the products formed by the chemical decomposition of the organic matter, which are substantially fluid under normal conditions, and the oil fed in with the coal, the product discharged from the liquefaction reactor also contains solid matter which are insoluble under the reaction conditions. The solid matter consists of carbonaceous matter which has not been liquefied, mineral constituents of the coal (ash) not liquefiable under the hydrogenative coal liquefaction conditions, and the non-liquefiable catalysts or catalytically acting additives, if these are continuously added to the feed stock.

Finally, the discharged products also include components that are gaseous under normal conditions. These are, on the other hand, the gases that are substantially inert, the context of hydrogenation, such as methane, ethane, and other hydrocarbons, together with carbon oxides, hydrogen sulphide, and ammonia, which are all formed as by-products of the liquefaction process, and, on the other hand, the hydrogen which has not been consumed in the coal liquefaction. For various reasons the hydrogen is fed in a generally relatively large excess to the coal liquefaction plant. Accordingly, it is generally recirculated. If necessary, the said substantially inert gases are removed in a special processing stage (recirculation gas cleaning) to the extent that a recirculation gas sufficiently rich in hydrogen can be reintroduced into the reaction chamber.

The separation of the solids, which can range from very finely divided to colloidal, from the viscous product oil represents a problem which is of critical importance for the economic operability of coal liquefaction.

The concentration of the solids generally takes place in so-called hot separators. The products drawn from the top of the hot separator at a temperature which is only slightly less than the reactor temperature and practically at the operating pressure of the liquefaction reactor, and under these conditions are of gaseous and vapour form. A tar-like fluid redsidue is withdrawn from the bottom of the hot separator. This contains all the solids and the major part of the asphalts. The liquid obtained from the vapours by means of subsequent cooling, the so-called stripping oil, is a distillate-like product generally free from solids.

Heavy oils are also contained in the stripping oil as these remain in vapour form under the conditions which exist in the hot separator. These heavy oils reach a so-called cold separator (i.e.

the cooling stage for the vapours leaving the hot separator), mainly because of their solubility in the compressed mixture of gases and vapours.

They are advantageously used as a compartment of the slurrying oil Because of the recycling of asphalts, which has hitherto been considered to be inevitable, mechanical separation of the solids from the residue has been necessary, and thishas had to be carried out by centrifuging the residue since filtration could not be satisfactorily used because of the asphalts. For centrifuging purposes, it is necessary to thin the residue, and this can be done because the clear off-liquid from the centrifuge is used as a component of the slurrying oil, and is therefore not distilled. However, because of the only moderately efficient solids separation by centrifuging the thinned residue, considerable quantities of solids arrive back in the reactors together with the asphalts undesirable for the liquefaction reaction, and the slurrying oil.

In addition, the concentrate from the centrifuge still contains much middle and heavy oil. Owing to the fact that it has not been possible to avoid their recovery, a technically and economically unsatisfactory low temperature carbonisation process has been necessary, in which some 1 5- 20% of oils based on the dry ash-free weight of coal, have been distilled under what from the hydrogenation aspect were unsatisfactory conditions. In this way, no chemically valuable distillates have been obtained. The major part of these heavy oils are fed to the residue as thinning oil before the centrifuging process.

A proposed technique for removing solids from the coal hydrogenation process, which differs from the above-described method of working is described in German Offenlegungsschrift 26 54 635. The proportion of middle and heavy oil contained in the residue is substantially withdrawn by expanding the hot residue under vacuum (flashing), so that a vacuum residue containing some 50% by weight of solids is obtained in the form of a pitch-like mass which can be melted and which contains by far the major part of the asphalts. This residue is all fed to a gasification stage; further process steps for separating the asphalt and solids are thus not required. By means of the gasification, the hydrogen necessary for hydrogenatively liquefying the coal is produced. In this process, the slurrying oil consists only of solids-free distillate.

The processing of the residue by distillation or flashing represent difficult process steps.

However, the production of the heaviest oils cannot be avoided-on the one hand for economic reasons, and on the other hand because these oils decisively influence the properties of the slurrying oil.

It would be desirable to be able to improve the efficiency and operability of the separation of solids-free and asphalt-free distillate-like oil, and thus to obviate a subsequent costly step involving oil production from the residue. The proportion of solids in the residue should ideally to lie in the range 40 to 60 wt.%, preferably 45 to 55 wt.%.

The term "solid" indicates a substance which is insoluble in pyridine.

The present invention provides a process for liquefying coal by hydrogenation, using slurrying or grinding oils and hydrogen-containing recirculation gas, in which the non-liquefied solid components are concentrated in a hot separator connected to the outlet of the actual liquefaction reactor, and under approximately the same pressure and temperature conditions as in the liquefaction reactor, and the gaseous and vapour outlet products of the liquefaction reactor are substantially condensed in the subsequent stages, whereas the separated products which are not in gaseous or vapour form and which are substantially free from distillable oils are withdrawn from the hot separator in a still pumpable state and expanded in an afterconnected vessel (degasifier), the hydrogencontaining recirculation gas being fed through the hot separator in contact with the outlet products from the liquefaction reactor in a quantity of more than 5000 m3 (at NTP) per tonne of dry ash-free feed stock coal.

By the measures according to the invention, the residue in the hot separator is freed from distillable oils to such an extent that subsequent distillation of the residue and centrifuging or flashing can be dispensed with, i.e. that at all events the raaction products are expanded down to normal pressure.

An acceptable lower limit for the solids concentration in the residue is 40%; at lower values, too many valuable liquid products may be lost. It is, however, preferable to set a lower limit of about 45%. An acceptable upper limit is 60%, because at greater values, even if the chemical composition of the fluid components of the residue is satisfactory, pumpability is no longer guaranteed. An upper limit of 55% is preferred, as in this case the residue from practically every coal hydrogenation product of the type with which the invention is concerned has proved to be still pumpable. In all cases, if the said limits are maintained, all undistillable asphalts are retained in the residue when using the process according to the invention.

It has also been found that a temperature of 440 to 4800C in the hot separator is particularly advantageous. At lower temperatures, in many cases, the required 40% solids proportion in the residue can no longer be attained, even by using excessively large recirculation gas quantities. At higher temperatures, the coking tendency of the hydrogenation products in the hot separator has proved in many cases to be unacceptably high.

In the case of recirculation gas quantities of less than 5000 Nm3 per tonne of coal (dry ashfree weight), even when using the highest possible temperature in the hot separator, the desired minimum solids proportion of 40% in the residue is generally no longer attainable. This lower limit relates to the recirculation gas alone, i.e. it excludes fresh hydrogen which is continuously fed to the process in order to compensate for the hydrogen consumed. The hydrogen concentration in the recirculation gas is normally about 80%; however, different concentrations are possible. The upper limit for the quantity of recirculation gas fed through the hot separator is determined by the maximum allowable solids proportion in the residue. As stated, this is about 60%, having regard to the need for maintaining pumpability.

It has been shown that, as the temperature in the hot separator increases, less recirculation gas has to be used for a given solids proportion in the residue, although the said lower limit must not be passed. Conversely, for a given temperature in the hot separator, higher solids proportions in the residue require larger recirculation gas quantities.

For optimum results, the contact between the outlet products from the liquefaction reactor and the hydrogen-containing recirculation gas in the hot separator should be as intimate as possible.

It is not necessary for all of the recirculation oas quantity to be fed through the liquefaction reactor before it enters the hot separator. Part of the recirculation gas to be fed through the hot separator can be branched off before the liquefaction reactor and fed directly into the hot separator. This can be particularly favourable to the effectiveness of the process.

The branched-off recirculation gas quantity which is fed directly into the hot separator can be fed thereto either by mixing it with the outlet products from the liquefaction reactor or by feeding it into the sump of the hot separator. In any case, feeding part of the recirculation gas directly into the hot separator is of particular advantage, because the liquefaction reactor can under certain circumstances be incompletely utilised or even dry out if it is fed with too much gas. If the recirculation gas quantity which is fed directly to the hot separator is preheated to a temperature of 300 to 500 , this has inter alia the advantage that on the one hand the pumpability of the residue is influenced in the required manner, and on the other hand the temperature in the hot separator can be controlled.

After being expanded to normal pressure in the degasifier, the residue is, as already stated, free from oil products to such an extent that any further oil recovery is largely unnecessary, even from the point of view of economy. However, in the course of thermal decomposition, such as low-temperature carbonisation or coking, it is possible to extract further hydrocarbon products from the residue, or to obtain by gasification the fresh hydrogen necessary for the process.

In the process of the invention, it is no longer necessary (as it was hitherto) to extract from the residue the oil necessary for slurrying the feed coal, and instead this can be done in a particularly economical and attractive manner in the condensation stage following the hot separator, it being particularly advantageous to produce the slurrying oil is an intermediate separator provided directly after the hot separator exclusively and especially for this purpose.The best result from the industrial processing aspect can be obtained in this manner, namely that the total residue from the liquefaction reaction is collected in the hot separator-to a large extent still usable in thermal decomposition or gasification, that the total slurrying oil necessary for the process (not no more than this) is produced in the intermediate separator by known choice of the temperature and pressure conditions, and finally that the total solids-free product oil, and only this, is collected in a cold separator.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a coal liquefaction plant; and Figure 2 is a similar reprnsentarion of an alternative plant.

With regard to the feed p'osition 14 for fresh hydrogen and the feed position 1 7 for that part of the recirculation gas quantity fed to the hot separator, various points are possible for these in both plants, as shown for example in Figure 1 by broken lines.

In Figure 1, coal 1 is fed to a mixing tank 2, and slurrying oil from a distillation stage 1 8 is also fed thereto through a line 1 9. The mixture formed from these components is fed by slurry pumps 3 through a preheater 4 to a liquefaction reactor 5.

The reaction products flow from here into the hot separator 6, in which they are separated into residue and gaseous and vapour products. The residue is fed to a degasifier 7 from which concentrated residue 7a and expanded gases 21 are withdrawn.

The gaseous and vapour products from the hot separator 6 are fed through a line 8 to a cold separator 9, where they condense to a large extent. The condensate is expanded at 10 and degasified (expansion gas 22) and is fed from here into the distillation stage 18, from which the slurrying oil (line 19) and product oil 20 are drawn off.

The gas which does not condence in the cold separator 9 is fed through a line 11 to a possibly necessary gas cleaning stage 12, in which the gas components undesirable for the further process are drawn off at 23. The gas which is thus cleaned and concentrated to the required hydrogen content is fed back through a compressor 13 as recirculation gas to the liquefaction process. A quantity sufficient for the liquefaction process reaches the liquefaction reactor 5 by way of a line 1 5 and the preheater 4, whereas the remaining part is fed directly into the hot separator 6 by way of a recirculation-gas heater 16 and a line 17.

Figure 2 differs from Figure 1 in that an intermediate separator 31 expressly for producing the slurrying oil is connected between the hot separator 6 and the cold separator 9. This intermediate separator 31 operates at a temperature below that prevailing in the hot separator 6. The slurrying oil produced herein expands in a degasifier 32 and flows through the line 19 (as in Figure 1) back to the mixing tank 2, whereas the gases separated in the degasifier 32 are drawn off at 33.

The use of the intermediate separator 31 makes the costly distillation stage 1 8 for the product components which condense in the cold separator 9 superfluous.

The process illustrated in Figure 2 is inter alia also particularly advantageous because the product oil 20 which collects after the cold separator 9 is known to be more valuable than comparable boiling fractions from the intermediate separator 31, because it contains oils which are richer in hydrogen. The advantage of oils richer in hydrogen derives in particular from their easier processability in catalytic processes.

Claims (10)

Claims

1. A process for liquefying coal by hydrogenation at elevated pressure and temperature in the presence of slurrying oil in a liquefaction reactor, in which the non-liquefied solid components of the product of the reactor are concentrated to form a residue in a hot separator connected to the outlet of the reactor, under approximately the same pressure and temperature conditions as in the reactor, and the gaseous and vapour components of the product of the reactor are substantially condensed in subsequent stages, whereas the residue-not in gaseous or vapour form and substantially free from distillabie oils-is withdrawn fron the hot separator in a pumpable state and expanded, hydrogen-containing recirculation gas being fed through the hot separator in contact with the outlet product from the liquefaction reactor in a quantity of more than 5000 Nm3 per tonne of coal (dry ash-free weight).

2. A process as claimed in claim 1, in which the temperature in the hot separator is 440 to 480"C.

3. A process as claimed in claim 1 or 2, in which the part of the recirculation gas fed through the hot separator is fed to the hot separator without passing through the liquefaction reactor.

4. A process as claimed in claim 3, in which the said part is fed to the hot separator together with the outlet product from the liquefaction reactor.

5. A process as claimed in claim 3, in which the said part is fed into the sump of the hot separator.

6. A process as claimed in any of claims 3 to 5, in which the said part is preheated to a temperture of 300 to 5500C.

7. A process as claimed in any of claims 1 to 6, in which the slurrying oil is substantially obtained from the condensation stages following the hot separator.

8. A process as claimed in claim 7, in which the slurrying oil is produced exclusively in an intermediate separation stage which directly follows the hot separator.

9. A process as claimed in any of claims 1 to 8, in which the expanded residue is thermally decomposed to produce hydrocarbons.

10. A process as claimed in claim 1, substantially as described herein with reference to Figure 1 or Figure 2 of the accompanying drawings.