GB2077757A - Hydrogenative Coal Liquefaction - Google Patents

Abstract

4000 to 8000 Nm<3> of hydrogen- containing recirculation gas per tonne of (dry ash-free) coal is passed together with comminuted coal, oil (recycled), and fresh hydrogen through the whole liquefaction reactor 9, which is at a pressure above 100 bars and a temperature of 300 to 550 DEG C. <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 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 (preferably 400 to 5000C) and at pressures of more than 100 bars.

One or 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 together with the coal, and is removed together with the reaction products. In other processes, pieces of the catalyst are kept in motion in the reaction chamber, for example by producing a "seething" catalyst layer, or by means of devices which rotate in the reaction chamber and to which the catalyst is fixed.

The fluid products generally consist 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 is insoluble under the reaction conditions. This solid matter consists of carbonaceous material that 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 feedstock.

Finally, the discharged products also include components that are gaseous under normal conditions. These are, on the one hand, the gases that are substantially inert, in the context of hydrogenation, such as methane, ethane, and other hydrocarbons, as well as carbon oxides, hydrogen sulphide, ammonia, and small amounts of other gases, which are all formed as by-products of the liquefaction, and, on the other hand, the hydrogen that 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, the hydrogen is in general recirculated (recirculation gas). If necessary, the substantially inert gases are removed in a special processing stage (recirculation gas cleaning) to such an extent that a recirculation gas sufficiently enriched in hydrogen can be reintroduced into the reaction chamber.

A hydrogen surplus in the recirculation gas has principally been considered necessary hitherto since otherwise such a low hydrogen partial pressure would prevail at the discharge end of the liquefaction reactor (or in the hot separator usually connected thereto) that coking of high molecular weight products, especially asphalts, could occur. Normal concentrations of hydrogen in the circulation gas have therefore been more than 80% by volume. However, lower concentrations have also been possible if the hydrogen partial pressure has been maintained sufficiently high by means of a suitably chosen total pressure. The hydrogen consumed in the hydrogenation reaction and by losses is added at any convenient point to the recirculation gas as "fresh hydrogen".

A further reason for the excess of hydrogen-containing recirculation gas in the hitherto known coal liquefaction processes has been the desire to control the reaction temperatures in the liquefaction reactor, which on account of the strong heating effect of the hydrogenation reaction tend to "run away". To this end, a proportion of the amount of recirculation gas introduced into the liquefaction reactor-in fact at a lower temperature than the reaction temperature-is blown into the liquefaction reactor at various points in order to exert a damping action (firstly by virtue of the cooling effect and secondly on account of the fairly thorough intimate mixing at critical regions) on any "hot spots" that tend to arise.

The amounts of recirculation gas used have generally amounted to about 4000 Nm3 (m3 at NTP) of gas per tonne (t) of dry, ash-free coal, but have in many cases been far below this value. About 2060% of this total amount is required for temperature regulation purposes and thus does not flow through the whole liquefaction reactor but is introduced at various points in the reactor. The hydrogen concentration has as a rule been more than 70%. In such processes the slurrying oils have only been partially freed from asphalts, and for this reason the liquefaction has had to take place at pressures of about 700 bars.

It has also been proposed to avoid recycling asphalts in the slurrying oil. This has been achieved by removing the asphalts, together with the solid and insoluble reaction products, as a distillation residue from the hydrogenation process and adding them to a subsequent gasification stage to produce hydrogen. By omitting the asphalt recirculation, it is possible to reduce the pressure in the liquefaction reactor to, for example, 300 bars.

In this way, in a coal liquefaction process according to the prior art about 40% of the feedstock coal is obtained as distillate oil. What is desired is a process in which the yield of distillable oils is particularly high.

The present invention provides a hydrogenative coal liquefaction process using slurry or grinding oils and hydrogen-containing recirculation gas as well as supplementary fresh hydrogen at pressures above about 100 bars and temperatures between about 300 and 5500C in a liquefaction reactor, in which an amount of recirculation gas of 4000 to 8000 m3 (at NTP) per tonne of feedstock coal is passed together with the starting mixture of comminuted coal, slurrying or grinding oil and supplementary fresh hydrogen through the whole liquefaction reactor.

Such high amounts of recirculation gas have hitherto been avoided on the grounds that the recirculation gas surplus should, as far as possible, not be too high, i.e. should be just high enough that the objectives pursued, as mentioned above, would be achieved. It was assumed that an increased proportion of gas in the liquefaction reactor and the resultant reduction in the residence time of the solid and liquid contents of the reactor would lead to a reduction in yield. In "Hydrogenation of coal and tar", Bureau of Mines, Report No. 633 (1963), pages 21 to 22, it is reported that a change in the amount of hydrogen recirculation gas, at least in the boiling point range up to 3300C, has no significant effect on the yield of distillable products and on their composition.It is therefore all the more surprising that the measures according to the invention result in a substantially better yield of these valuable liquid commodities.

The reaction products obtained with the aid of the measures according to the invention are, after having been removed from the liquefaction reactor, worked up in a manner known per se with the object of obtaining hydrocarbon oils. This means that the reaction products are resolved into distillable hydrocarbon oils, solids-containing heavy oils and asphaltenes, water, and gaseous substances, and that the amount of oil required for preparing the coal slurry is separated from the oils. Especially advantageous is the use, known per se, of asphalt-free and solids-free distillate oils obtained in this process, as slurrying oil.

The sole Figure of the accompanying drawing shows by way of example a diagrammatic representation of a coal liquefaction plant with which the process according to the invention can be carried out.

Coal 1 passes via a line 2 to the mixer 3 and is mixed there together with slurrying oil from line 34, and is then passed through a line 4, a slurry pump 5, a line 6 and a preheater 7 to a liquefaction reactor 9. All the reaction products are conveyed from the liquefaction reactor 9 via a line 10 to a hot separator 11, where solids (consisting of mineral constituents of the coal as well as undecomposed coal, and catalyst, insofar as it is added to the coal slurry), as well as asphalts and to a lesser extent heavy oils, are separated at the operating pressure and temperature of the coal liquefaction reactor, i.e.

at a pressure above about 100 bars and at temperatures between about 3000C and about 5500C, preferably 400 to 5000C. The above-mentioned separated substances pass from the hot separator 11 to a degasifier 21, in which they are flashed. The remaining, very viscous mixture (sludge) is discharged through a line 23 and is separated by distillation at 24 into distillate oils and a distillation residue 26, which latter may for example be passed to a gasification plant to obtain hydrogen, while the distillate oils 25 are used for grinding the coal to form the coal slurry.

The vaporous or gaseous products leaving the hot separate 11 through a line 12 are condensed in a cold separate 13 at temperatures of down to about 30 to 400C. The condensate is withdrawn through a line 27, flashed in a degasifier 28, and then dehydrated and distilled in a distillation unit 31.

The lighter distillate oils thus obtained at 32 are the desired coal liquefaction product. Higher boiling point oils obtained at 33 are used as a mixing component of the slurrying (grinding) oil.

The gas withdrawn through a line 14 from the top of the cold separator 1 3 under a high pressure is brought by a recirculation gas cleaning unit 1 5 to a hydrogen concentration of preferably more than 70% by volume, is passed via a line 1 6 to a compressor 17, and is recycled from there in an amount of 4000 to 8000 Nm3 (preferably about 5500 to about 7000 Nm3) per tonne of feedstock coal (dry ashfree weight), to the hydrogenation stage. Consumed hydrogen is replaced by adding fresh hydrogen from a line 1 9 to the recirculation gas, so that the hydrogenation gas added to the coal slurry (preferably between the slurry pump 5 and the preheater 7, via a line 18, as shown) has a further increased concentration of hydrogen. The total amount of hydrogenation gas is about 10 to 30% higher than the actual amount of recirculation gas.

The addition of the recirculation gas from the gas cleaning unit 1 5 as well as the addition of the subsequently supplied fresh hydrogen to the coal slurry may in this connection be effected jointly (as shown) or separately, and before (as shown) and/or after heating the coal slurry.

The invention will be described in more detail below with the aid of the following examples. In these examples long-flame coal (open burning coal) was hydrogenatively liquefied in a coal liquefaction plant corresponding to that illustrated.

In the liquefaction reactor the following operating conditions were established in all the tests: Pressure 300 bars Reaction temperature 4750C Throughput of coal slurry 1.36 kg per hour per litre of reaction chamber volume Ratio of coal to mixing oil 1:1.42 kg/kg Catalyst 1.2 wt.% iron oxide and 0.3 wt.% sodium sulphide, referred to dry ash-free coal Amount of fresh hydrogen 1.0 m3/kg coal Properties of the Raw Materials: Type of coal Long-flame coal Water content 2.34 wit.%.

Ash 4.1 wt% of the dry coal Volatile constituents 36.7 wt.% of the dry coal Mixing oil approx. 40% middle oil (200 to 3250C boiling point range) approx. 60% heavy oil (boiling down to 3250C) (obtained as recycle oil from the continuous hydrogenation of coal) All gas amounts are referred to NTP.

The sludge removed from the hot separator was worked up in a vacuum distillation device so as to obtain a vacuum residue having a softening point of 1 60 to 1800C.

The liquid removed from the cold separator was, after separating an aqueous phase by distillation, resolved into a lighter boiling fraction, product oil, and into such an amount of a higher boiling oil that the latter together with the vacuum distillate from the sludge corresponded to the amount of slurrying (grinding) oil required.

Example 1 Under the above-mentioned experimental conditions and with the recirculation gas passed through the whole liquefaction reactor in an amount of 4.25 m3/kg of coal (dry ash-free weight) and with a hydrogen concentration of the recirculation gas of 81%, an hourly product oil amount of 2.40 kg was obtained, which corresponds to a yield of distillate oil of 47% of the (dry ash-free) feedstock coal.

Example 2 Under the same experimental conditions as in Example 1, but with a recirculation gas amount of 5.6 m3/kg of (dry ash-free) coal and with a hydrogen concentration in the recirculation gas of 80.5%, a yield of distillate oil of 49.5% of the (dry ash-free) coal was obtained, corresponding to an hourly amount of 2.54 kg of oil.

Example 3 Under the same experimental conditions as in Example 2, but with a recirculation gas amount of 6.9 m3/kg of (dry ash-free) coal and with a hydrogen concentration in the recirculation gas of 81%, an amount of 2.68 kg of distillate oil per hour was obtained, corresponding to a yield of 52.5% of the (dry ash-free) coal.

With recirculation gas amounts of more than about 8 m3/kg the temperature in the reaction space could no longer be maintained sufficiently constant under conditions otherwise identical to those in Example 1. The increase in the amount of recirculation gas thus reaches a limit if it is no longer possible to carry out the reaction in a smooth manner, e.g. because the reactor contents have too strongly dried out.

For comparison purposes, an experiment was carried out with recirculation gas amounts according to the state of the art: under experimental conditions otherwise identical to those in Example 1 , the recirculation gas amount was 2.4 m3/kg of (dry ash-free) coal, with a hydrogen concentration of 86% by volume. The amount of oil obtained as product oil was in this case 2.05 kg/h. Only 40% of the (dry ash-free) feedstock coal was thus recovered as distillate oil, i.e. considerably less than according to the process of the invention. It is clear from these examples that a substantial increase in the yield of distillable coal-liquefaction oils can be achieved by the process according to the invention.

Claims (4)

Claims

1. A process for liquefying coal by hydrogenation at a pressure above 100 bars and a temperature of 300 to 5500C in a liquefaction reactor, in which 4000 to 8000 Nm3 of hydrogen-containing recirculation gas per tonne of coal (dry ash-free weight) is passed together with comminuted coal, oil, and supplementary fresh hydrogen through the whole liquefaction reactor.

2. A process as claimed in claim 1, in which the amount of recirculation gas is 5500 to 7000 Nm3/t.

3. A process as claimed in claim 1, substantially as described with reference to the accompanying drawing.

4. A process as claimed in claim 3, substantially as described in any of Examples 1 to 3.