NO159474B - PROCEDURE FOR AA REMOVE CO2 AND / OR H2S FROM GASES CONTAINING THE SAME. - Google Patents

Abstract

1. A process for removing CO2 and/or H2 S from a gas which contains CO2 and/or H2 S, in which the gas containing CO2 and/or H2 S is treated, in an absorption stage at from 40 to 100 degrees C, with an aqueous absorption liquid containing from 20 to 70% by weight of methyldiethanolamine, the treated gas is taken off at the top of the absorption stage, the aqueous absorption liquid laden with CO2 and/or H2 S is taken off at the bottom of the absorption stage and then regenerated, and the regenerated absorption liquid is recycled to the absorption stage, characterized in that the regeneration is carried out exclusively in one or more flash stages, the flash gases being taken off at the top of the flash stage or flash stages, and, to compensate for the losses of water as a result of water present in the gas streams taken off at the top of the absorption stage and of the flash stage or flash stages, an amount of steam corresponding to the water loss is fed in at the bottom of the last flash stage or, if only one flash stage is used, at the bottom of this stage.

Description

The present invention relates to a method for removing C02 and/or H2S from gases by means of an aqueous absorption liquid.

It is known, e.g. from A.L. Kohl - F.C. Riesenfeld, Gas Purification, 3rd edition, 1979, to use aqueous solutions of monoethanolamine or diethanolamine or mixtures of cyclotetramethylene sulfone and an aqueous solution of diisopropanolamine as a solvent for removing CO2 and/or H2S from gases. With these methods, it is necessary to regenerate the C02- and possibly H2S-containing solvent in a stripping column by supplying steam, which is associated with significant energy costs. When removing C02 and possibly H2S from soil gases containing higher hydrocarbons using a mixture of cyclotetramethylene sulphone and an aqueous solution of diisopropanolamine, there is also the disadvantage that the higher hydrocarbons in this solvent exhibit a relatively high solubility, so that the acid gas that is removed above in the stripping column, exhibits a relatively high content of hydrocarbons which, if the sour gas contains H2S, can lead to difficulties in a downstream Claus plant. Primary or secondary alkanolamines, such as monoethanolamine or diethanolamine, can also only be used as aqueous solutions with a relatively low concentration of these alkanolamines, as the use of higher concentrations can cause major corrosion damage to plant parts.

Furthermore, a method is known from DE-A-2 551 717

for the removal of C02 and/or H2S from gases using piperazine-containing chemical or physical solvents. In connection with the chemical solvents, the use of methyldiethanolamine (MDEA) is also described. In the method, the enriched solvent obtained from the absorption step in a one-step washout is first regenerated by depressurization and then further regenerated in a second step in a stripping zone. This method shows a relatively high steam consumption.

Moreover, it is described in Chemie-Ingenieur-Technik, vol

53, No. 2, 1981, pp. 73-81 a method for removing C02 from gases, where a two-stage chemical leaching is used, and

where the enriched solvent is first de-stressed in two de-stressing stages and then further regenerated in a stripping device.

Furthermore, a two-stage gas scrubbing is known from DE-C-1 494 781 where the method must be carried out with two different solvents, with an aqueous solution of a trialkanolamine being used in a first absorption step and an aqueous solution of amino acid alkali salts being used in a second absorption step .

Finally, EP-A-107 046 (priority: October 2, 1982; filed September 22, 1983; published May 2, 1984) describes a process for removing CO2 and possibly H2S from gases by means of a methyldiethanolamine- containing aqueous absorption liquid in a one-stage washout, where the regeneration of the enriched absorption liquid is carried out exclusively in one or more relaxation stages.

There has therefore been a need for a method for removing C02 and/or H2S from gases by which the disadvantages of the known methods could be avoided.

The present invention relates to a method for removing C02 and/or H2S from C02- and/or H2S-containing gases, in which the C02- and/or H2S-containing gases are treated in an absorption step at temperatures of 40-100° C with an aqueous absorption liquid containing 20-70% by weight methyldiethanolamine, withdraws the treated gases upstream of the absorption stage, withdraws the C02- and/or H2S-enriched aqueous absorption liquid at the bottom of the absorption stage and then regenerates it and returns the regenerated absorption liquid to the absorption stage, characterized by carrying out the regeneration exclusively in one or more relaxation stages and thereby removing the relaxation gases from above in the relaxation stage or relaxation stages, and adding water vapor at the bottom of the last relaxation stage, or if only one relaxation stage is used, at the bottom of this one relaxation stage, in an amount which corresponds to and thus compensates: for the water loss due to water content one in the gas streams which are withdrawn from the top of the absorption stage and the relaxation stage or stages.

According to the new method, the C02- and/or H2S-enriched solvent is regenerated without the use of a stripping column only by depressurization in one or more depressurization stages, so that a significant reduction is achieved both in investment costs and in energy consumption costs. Furthermore, with the new method, relatively high concentrations of methyldiethanolamine can be used in the absorption liquid without corrosion damage occurring in the gas scrubbing system. A further advantage of the method is that water loss that occurs in the gas scrubbers due to the water content of the gas streams that are withdrawn from the top of the absorption column and the depressurization vessel is compensated for by adding an amount of water vapor corresponding to the water loss at the bottom of the last depressurization stage. In this way of working, in addition to regulating the gas washing plant's water management, the gas washing plant's heating management can be regulated at the same time, so that a heat exchanger that is normally used in the gas washing plant for regulating the vanae household can be of a smaller design or possibly be omitted altogether.

As gases which are treated according to the method according to the invention, for example coal gasification gases, synthesis gases, coke oven gases and preferably soil gases come into consideration. The method is advantageously used for the removal of C02 and/or H2S from soil gases which, in addition to methane, also contain higher hydrocarbons. These higher hydrocarbons are generally C2-C30 hydrocarbons, preferably C2-C20 hydrocarbons, especially C2-C12 hydrocarbons, which are usually aliphatic, e.g. ethane, propane, isobutane, n-butane, isopentane, n-pentane, the hexanes, heptanes, octanes, nonanes, decanes and the higher homologues. In addition to the aliphatic hydrocarbons, the higher hydrocarbons may also contain aromatic hydrocarbons such as benzene. In general, the content of higher hydrocarbons in the soil gases amounts to 0.1-40 mol%, preferably 0.5-30 mol%, especially 1-

20 mol%.

The gases generally have a C02 content of 1-

90 mol%, preferably 2-90 mol%, especially 5-60 mol%. In addition to C02, the gases may contain H2S as an additional acid gas or they may contain H2S alone, e.g. in amounts from a few mol-ppm, or 1 mol-ppm, to 50 mol%.

An aqueous absorption liquid containing 20-70% by weight, preferably 30-65% by weight, especially 40-60% by weight of methyldiethanolamine is used as solvent in the method according to the invention. Appropriately, an aqueous methyldiethanolamine solution is used, e.g. an aqueous solution of methyldiethanolamine of technical purity. In a preferred embodiment according to the invention, an aqueous methyldiethanolamine solution is used which additionally contains 0.1-1 mol/l, preferably 0.2-

0.8 mol/l, especially 0.25-0.6 mol/l of a secondary amine or alkanolamine, preferably methylmonoethanolamine, very particularly preferred piperazine. The method according to the invention is carried out in such a way that the C02- and/or H2S-containing gas is first treated in an absorption step with the methyldiethanolamine-containing absorption liquid. Here, temperatures of 40-100°C, preferably 50-90°C, especially 60-80°C, are maintained in the absorption step for the absorption liquid. In general, a pressure of 10-110 bar, preferably 20-100 bar, especially 30-90 bar, is used in the absorption step. An absorption column is suitably used as an absorption step, generally a packed column or a column equipped with ledges. Appropriately, the gas to be treated is supplied,

at the bottom, and the absorption liquid above in the absorption column, whereby the acidic gases C02 and/or H2S are washed out in countercurrent. While any H2S that may occur is expediently washed out extensively, generally to an H2S content in the treated gas of no more than 120 mol-ppm, preferably no more than 10 mol-ppm, especially no more than 3 mol-ppm, it can be advantageous that C02 in the gas is washed out so much that the C02 content in the treated gas amounts to approx. 0.5-6, preferably 0.5-5, especially 1-4 mol%. The treated gas is suitably withdrawn from the top of the absorption stage, suitably at a point above the supply of the absorption liquid. The absorption liquid, which contains the acid gases C02 and/or H2S, is conveniently withdrawn at the bottom of the absorption zone.

The absorption liquid containing acid gases is then regenerated in one or more relaxation stages. In this way, in the last relaxation stage, or if only one relaxation stage is used, in this only relaxation stage, it is suitably relaxed to a pressure of approx. 1-3 bar, preferably 1-1.8 bar, especially 1-1.5 bar. It may also be appropriate for the last relaxation stage, or the only relaxation stage, to be operated at reduced pressure in relation to atmospheric pressure, e.g. at a pressure of from 0.5 to approx. 1 bar, preferably from 0.8 to approx. 1 bar. It can be advantageous to use at least two relaxation steps, e.g. 2-5, preferably 2 or 3, especially 2 relaxation stages, for regeneration of the acid gas-enriched absorption liquid. In this way, the acid gas-enriched absorbent is preferably de-stressed in the first de-stressing step after the absorption step to a pressure of at least 5 bar. It can then be advantageous that the pressure to which it is relaxed in the first relaxation step after the absorption step is also at least the sum of the partial pressures of CO2 and/or H2S in the gas which is supplied in the absorption step and which contains C02 and/or H2S. With this method of working, the evaporation of water and the associated energy loss as well as the loss of hydrocarbons can be kept particularly low. For the relaxation, suitable relaxation containers such as e.g. can also take the form of columns. These stress relief containers can be free of special devices. However, columns equipped with devices can also be used, e.g. with fillers.

Generally, heat is added during the process to compensate for process-related heat losses, which are caused, for example, by the stress-relief evaporation. This conveniently takes place in a heat exchanger zone connected before the last relaxation stage, or if only one relaxation stage is used, in front of this only relaxation stage, in which the acid gas-containing absorption liquid is heated before the relaxation in the last, or the only, relaxation stage. Usually, the absorption liquid in the exchanger zone is heated by a maximum of 20°C, whereby the absorption liquid is generally heated to a temperature of at most 90°C, usually at most 85°C. Heat exchangers are generally used for the heat exchanger zone, e.g. a tube heat exchanger.

As compensation for the water loss that occurs when gas streams containing water are withdrawn from above in the absorption stage and the relaxation stage respectively the relaxation stages, water vapor is supplied at the bottom of the last relaxation stage, or if only one relaxation stage is used, at the bottom of this one relaxation stage, in an amount corresponding to water loss. Usually, water contained in the extracted gas streams is withdrawn, essentially as water vapour. At the bottom of the relaxation stage, low-pressure, medium-pressure or high-pressure steam can be supplied, i.e. e.g. steam with pressure 1-100 bar. Steam is preferably used in the lower pressure range, e.g. steam with a pressure of 1.5-10 bar, advantageously 1.5-5 bar, because this low-pressure steam is generally cheap and available. In the case of the steam supply to the relaxation step according to the invention, in addition to regulating the water household, the heating household in the gas washing plant can be regulated at the same time. In a preferred embodiment of the method, as compensation for process-related heat losses according to the invention, either simultaneous heat is supplied in the heat exchanger zone which is connected before the last relaxation stage, or if only one relaxation stage is used, before this one relaxation stage, as well as by means of the steam supplied at the bottom of the last relaxation stage or the only relaxation stage, or heat alone by means of the steam supplied at the bottom of the last relaxation stage or the only relaxation stage.

The acid gases CO2 and/or H2S are suitably extracted from above in the last relaxation step. If the extracted acid gas contains H2S, it is suitably processed by oxidation of H2S, e.g. in a Claus facility. The regenerated absorption liquid which is withdrawn in the last relaxation step is fed into the absorption zone.

In the following, further details of the invention will be elucidated with the help of an embodiment example, the process sequence of which is shown schematically in the figure.

A C02- and/or t^S-containing gas, e.g. a soil gas containing higher hydrocarbons, e.g. aliphatic C2~ c10~ hydrocarbons, are introduced through line 1 under pressure to the bottom of absorption column 2. At the same time, 20-70% by weight aqueous methyldiethanolamine solution is introduced as absorption liquid upstream of the absorption column via line 3. The absorption liquid, which is fed countercurrently to the gas, absorbs the acid gases C02 and/or H2S, and the acid gas-containing absorption liquid is withdrawn through line 4 from the bottom of the absorption column.

The washed gas is withdrawn through line 13 at the top of the absorption column. The acid gas-containing absorption liquid stream 4 is then de-stressed in a de-stressing evaporation container 6, suitably to a pressure of at least 5 bar, for example through a valve or preferably through a de-stressing turbine 5. This releases an intermediate de-stressing gas containing hydrocarbons and C02 from the absorption liquid, which is withdrawn through line 7 above the vent in the de-stressing container 6 At the bottom of the de-stressing container 6, the partially de-stressed absorption liquid is withdrawn through line 8, heated in a heat exchanger 9, e.g. with 1-15°C, and the heated absorption liquid is de-stressed in a second de-stressing container 10, e.g. to a pressure of 1-2 bar. This releases a C02-rich relaxation gas with e.g. a C02 concentration of 98 mol%, which is withdrawn through line 11 at the top of the de-stressing container 10. In the bottom of the de-stressing container 10, steam is introduced, e.g. low-pressure steam with a pressure of 2.5 bar, through line 14 to equalize the water loss in the system. The regenerated absorption liquid withdrawn from the bottom of the relaxation container 10 is returned upwards in the absorption column 2 by means of a circulation pump 12.

The following Example illustrates the invention.

Example

In an absorption column, 3.15 kmol/h of a C02-containing soil gas is washed with a 50% by weight aqueous methyldiethanolamine solution as absorption liquid at a pressure of 75 bar. The gas to be purified has the following composition:

The temperature of the absorbent in the feed stream to the absorption column is 70°C. The C02 content in the washed gas amounts to 2.0 mol%. The acid gas-containing detergent that exits the absorption column is de-stressed in a first de-stressing container to a pressure of 20 bar. In this way, 0.04 kmol/1 of a hydrocarbon-rich intermediate relaxation gas with a C02 concentration of 34.3 mol% is released from the solution and taken out overhead in the first relaxation container. The partially relaxed absorption liquid is then heated in a heat exchanger. The heated absorption liquid is de-stressed in a second de-stressing container to 1.3 bar. This releases 0.241 kmol/h of a C02-rich relaxation gas with a C02 concentration of 97.55 mol%, a methane concentration of 1.68 mol% and a concentration of higher hydrocarbons of 0.77 mol%, which is withdrawn into the other relaxation tank. Low-pressure steam (pressure 2.5 bar) is introduced into the bottom of the second expansion tank to equalize the water loss in the system. The absorption liquid that is taken out at the bottom of the relaxation container is pumped back up into the absorption column by means of a circulation pump.

Claims (1)

1. Process for removing C02 and/or H2S from C02- and/or H2S-containing gases, in which the C02- and/or H2S-containing gases are treated in an absorption step at temperatures of 40-100°C with an aqueous absorption liquid containing 20-70% by weight methyldiethanolamine, withdraws the treated gases upstream in the absorption step, withdraws the C02- and/or H2S-enriched aqueous absorption liquid at the bottom of the absorption step and then regenerates it and returns the regenerated absorption liquid to the absorption step, characterized by carrying out the regeneration exclusively in one or more relaxation stages and thereby withdraws the relaxation gases from above in the relaxation stage or relaxation stages, and adds water vapor at the bottom of the last relaxation stage, or if only one relaxation stage is used, at the bottom of this one relaxation stage, in an amount that corresponds and thus compensates for the water loss due to the water content in the gas streams taken from t up of the absorption step and the relaxation step or steps.