EP0056487B1 - Process for the hydrogenation of coal - Google Patents

Description

In the known process for the hydrogenation of coal, the economy largely depends on the utilization of the heat of reaction generated in the process. The heat content of the reaction vapors flowing out of the reactor is usually used to heat the coal slurry for the reaction in countercurrent in shell-and-tube heat exchangers. Large problems arise from encrustation of the heat exchanger surfaces and from difficulties in distributing the two-phase mixture on the slurry side, which become more significant the larger the dimensions of the apparatus.

Direct heat exchange between reaction vapors and coal pulp would offer advantages. The mixing section required for direct heat exchange is less sensitive to distribution problems of the two-phase mixture, and there is no risk of cracking on overheated heat exchanger surfaces. The use of hydrogen as an essential component of the heat transfer gas also guarantees that the coal pulp is always heated in the presence of hydrogen.

From DE-C-669660 a method for hydrogenating coal is known, wherein a direct heat exchange is carried out in one step or over a limited temperature range in a countercurrent apparatus. In both cases, some of the vaporous reaction products condense out in the pulp; this limits the application of the principle of direct heat exchange to a relatively high temperature level. Extensive heat recovery through direct exchange is not possible in this way.

It has now been found that better utilization of the heat of reaction can be achieved if at least some of the hot gases and vapors resulting from the hydrogenation are intimately mixed with the pulp in several stages and after the first mixing stage the gases are freed from the product and the product-free gases, in particular Hydrogen, used as a heat transfer medium.

In the hydrogenation process according to the invention, in a first mixing stage, the gases and vapors flowing from the reaction were cooled to a temperature not below 350 ° C., preferably to a temperature between 380 and 440 ° C., in particular 390 to 410 ° C. Then high-boiling oils are separated in an intermediate separator. In order to reach this temperature in the intermediate separator, a preheating section at lower temperatures is required, in which the coal slurry is preheated. With direct heat exchange, the reaction gas is cooled to temperatures lower than 350 ° C. When cooling below 350 ° C, however, an excessive amount of the reaction products would fail and be fed back to the reactor with the coal pulp. This accumulation of the reaction products in the coal slurry and in the reaction zone is prevented by the reaction products being previously removed from the reaction gases in a cold separator at about room temperature. The remaining gas, freed from the oil vapor, is then heated in countercurrent with the product-laden gas flowing to the cold separator in a heat exchanger, the gas flowing to the cold separator being cooled. This heated gas, freed from the product, can now serve as a heat transfer medium and supply the necessary heat to the coal pulp in direct heat exchange. If additional heat is required to cover a peak demand, the product-free gas can be heated with this external heat without the heat-transferring surfaces cracking or covering with carbon.

The advantage of the procedure according to the invention consists essentially in the fact that only clean gases enter the heat exchanger for the external heat supply, while the coal pulp is heated by direct heating in multiphase mixing sections which are considerably simpler in terms of equipment and therefore cheaper without the risk of cracking Laying individual pipes and overheating on the heat exchanger surfaces. The coal pulp is always heated in the presence of hydrogen. The effect of the countercurrent principle can largely be achieved by suitable selection of the number of stages of direct heating. The mixing stages, which are operated at temperatures below 400 ° C. and to which the heat is supplied by gas from which the product has already condensed, can also be designed as countercurrent apparatuses in order to achieve even better heat utilization.

In the indirect heat exchange between the gas from the reactor, which flows into the cold separator and contains the product, and the residual gas freed from the product, part of the usable heat content (e.g. the heat of condensation) is lost for the regeneration. If you want to use this heat in the process, the mass of the residual gas must be increased. This is easily possible if the exhaust gas cooled to room temperature from the coal preheater, which is to be used for gas cleaning, is divided, a partial stream is removed and this is mixed with the heat transfer gas (circuit circuit).

The advantage of the process according to the invention is that the coal pulp, which is difficult to treat, is brought to the reactor inlet temperature by direct mixing with gases from the process, the external heat supply, in contrast to a single-stage circuit, usually being completely eliminated or at least being kept considerably smaller.

The multi-stage direct mixing is made possible by separating the gas from the product of value after the first mixing section, so that no product can fail in the colder mixing stages. Only through the multi-stage direct mixing can the known advantages of direct heating compared to indirect heating in shell and tube heat exchangers be optimally used: With direct heating in a mixing section, the problem of distributing the pulp in the heat exchanger can be solved much better, especially with very large appliances, than with indirect appliances Heat exchange, in which dead zones can easily form, which are harmful to the product and occupy the exchange surfaces. This is the main advantage over indirect heat exchange in counterflow. Much simpler apparatuses than shell-and-tube heat exchangers can be used as mixing sections for direct heat exchange, especially at high pressure.

Direct mixing makes it possible to use coal that has not been pre-dried. The expelled water vapor and other low boilers are not passed through the reactor, but are discharged directly via a cold separator. Even when predried, the coal still contains a significant proportion of water, which increases the reaction pressure as steam in the reactor. In the case of direct heating, this amount of water is also removed together with the volatile constituents of the coal and of the grinding oil in front of the reactor, so that the reaction pressure can be lower by the partial pressures of these low boilers.

• Die aus dem Hydrierreaktor (9) abströmenden Brüden (10) werden, nachdem sie im Heißabscheider (13) vom flüssigen Abschlamm getrennt worden sind, in der Mischstrecke (8) mit dem Kohlebrei innig vermischt, so daß sich eine Mischungstemperatur von etwa 400 °C einstellt. Sodann werden Kohlebrei und Gas im Zwischenabscheider (7) getrennt.

In the following, a brief description of the process is given based on the illustration using the example of two-stage direct heating of the coal pulp in the process for hydrogenating coal:

• The vapors (10) flowing out of the hydrogenation reactor (9), after they have been separated from the liquid sludge in the hot separator (13), are intimately mixed with the coal slurry in the mixing section (8), so that a mixing temperature of about 400 ° C. sets. Then coal pulp and gas are separated in the intermediate separator (7).

The slurry is fed into the reactor with the pump (6). At the same time as the coal pulp, the hydrogen required for the hydrogenation, namely fresh hydrogen and the process-related circulating hydrogen, enters the reactor (21). This gas stream is heated with heat recovery in the heat exchangers (19) and (15). The heat exchanger (12) is additionally provided in order to be able to supply any external heat that may be required.

The reactor gas separated in the intermediate separator (7) is cooled in the heat exchangers (14) and (15) in a heat network and brought close to room temperature; the cold separator oil condenses out and can be separated in the cold separator (16) (stream (17)).

The product-free gas, which is heated in the heat exchanger (14) with heat recovery and - if there is a need for external heat - in the heat exchanger (11), reaches the mixing section (2), where it is mixed with the coal pulp entering the system (1). The mash is heated.

For example, the coal pulp (1) is preheated in stage (2) from 100 ° C to 230 ° C, in the other mixing stage (8) it is then heated to the necessary temperature of 400 ° C. The need for external heat can be brought to zero through consistent use of the heat network, which requires corresponding exchange areas.

When the coal pulp is heated in the preheating stage (2), low boilers, e.g. B. water, free, these are separated from the mash together with the heating gas in the separator (3). While the pulp is pumped to the preheating section (8) with the aid of the slurry pump (5), the gas stream (4) goes to the separator (22) while cooling in the heat exchanger (19). The condensed water is separated off here together with other low boilers below room temperature.

If, for better heat recovery, the internal circulating gas flow passed through the exchange stage (2) is to be increased, a partial flow (20) can be branched off from the gas flow that is to be led to gas purification behind the separator (22) and with the help of the circulating gas compressor (18) be pumped via (14) and (11) to stage (2).

Claims (6)

1. A process for hydrogenating coal which has been slurried with oils in particular, by bringing the coal slurry to the reaction pressure by pumping, heating the slurry and catalytically hydrogenating it with hydrogen, some or all of the hot gases and vapors which form during the hydrogenation being intimately re-mixed with the coal slurry, wherein the mixing of the hot gases and vapors with the coal slurry takes place in a plurality of stages and, after the first mixing stage, the gases are freed from the product and the product-free gases, especially hydrogen, are used as a thermal medium.

2. A process as claimed in claim 1, wherein, after the first mixing stage, heat exchange between the gas and the coal slurry is effected in countercurrent.

3. A process as claimed in claims 1 and 2, wherein the first mixing stage is operated at not less than 350 °C.

4. A process as claimed in claims 1 and 2, wherein the first mixing stage is operated at from 380 to 440 °C.

5. A process as claimed in claims 1 and 2, wherein the first mixing stage is operated at from 390 to 410 °C.

6. A process as claimed in claims 1 to 5, wherein the amount of gas, consisting essentially of hydrogen, which serves as the thermal medium is increased by recycling a part-stream so as to permit better utilization of the heat of reaction and to minimize any additionally required external heat.

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