GB901272A - Process of treating industrial combustible gases - Google Patents

Abstract

901,272. Purifying gas. METALLGESELLSCHAFT A.G. Oct. 14, 1959 [Oct. 14, 1958], No. 34875/59. Drawings to Specification. Class 55 (2). Combustible gases are purified by the removal of CO 2 , H 2 S, organic sulphur and nitrogen compounds and some hydrocarbons by washing under a pressure of at least 10 atm. gauge and at temperatures below 0‹ C. with liquid absorbents such as methanol and acetone in three successive stages after an initial washing with water at a temperature below +10‹ C., the absorbent in each stage being circulated through a regular atm. stage which in the first stage comprises cooling the absorbent until a separation into two layers is achieved, one layer containing the removed constituents and the other the absorbent, while in the second stage the absorbent is subjected to two successive distillations in the first of which the absorbent is recovered from the base of the distillation column and in the second at the top of the distillation column and in the third stage by subjecting the absorbent to distillation and/or decompression in a series of stages. Each washing stage may be carried out in a plurality of scrubbing towers operated in parallel or in series. In the treatment of gases containing 2-3% CO 2 , e.g. coke-oven gas the first scrubbing stage after a preliminary scrubbing with water to cool the gas, absorb ammonia and condense some hydrocarbons is carried with 50% aqueous methanol introduced into a scrubbing tower at -40‹ C. and wherein the gas is cooled to - 30‹ C. The absorbent from this stage is after leaving the scrubbing tower cooled by cold purified gas and a refrigerant to - 40‹ C. when it separates into an upper layer containing unsaturated hydrocarbons from C 4 upwards and a lower layer of aqueous methanol. The hydrocarbons are washed with water to recover any methanol and treated to remove organic sulphur and nitrogen compounds in solution, e.g. by hydrogenating decomposition. The separated aqueous methanol is returned to the scrubbing tower. The second scrubbing stage uses a small quantity of methanol introduced into the scrubbing tower at - 40‹ C. and serves to remove some residual C 4 hydrocarbons, gum-forming substances and organic sulphur and nitrogen compounds. The absorbent leaving this stage is regenerated distillation in two columns. In the first column there is obtained an overhead containing hydrocarbons and bottoms consisting mostly of methanol. The bottoms are distilled in the second distillation column and regenerated methanol is obtained as overhead and hydrocarbons and water constitute a residue which is withdrawn from the base of the column and passed together with the overhead from the first distillation column to the plant in which the hydrocarbons recovered in the first scrubbing stage are washed with water. The third scrubbing stage is carried out with a major quality of methanol introduced into the scrubbing tower at about - 60‹ C. and serves to remove remaining C 4 hydrocarbons as well as C 3 hydrocarbons, CO 2 and H 2 S. A temperature between -40‹ and - 60‹ C. is maintained in this stage by a refrigerating coil arranged in the scrubbing tower. The absorbent leaving the scrubbing tower is heated in a distillation column to drive off the hydrocarbons, CO 2 and H 2 S while methanol is recovered from the base of the column and returned after cooling to the scrubbing tower. The mixture of hydrocarbons, CO 2 and H 2 S is scrubbed with an alkaline solution to remove the CO 2 and H 2 S and a residual gas consisting mainly of propane is obtained from which pure propane can be recovered by re-compression. If, however, the propane content of the mixture of hydrocarbons, CO 2 and H 2 S is small the scrubbing with an alkaline solution may be conducted to effect selective absorption of the H 2 S and the residual gas containing hydrocarbons and CO 2 used as a fuel, the H 2 S absorbed being recovered during regeneration of the alkaline solution. Some methanol is also absorbed in the scrubbing with the alkaline solution and is washed out with water from the H 2 S liberated during regeneration of the alkaline solution. Aqueous extracts containing methanol from various stages of the process are combined and distilled to recover methanol. In the case of gases containing between 10 and 15% CO 2 the third scrubbing stage is divided into two sub-stages, the second substage serving to remove any residual CO 2 in the gas. In the first sub-stage the scrubber contains the coil of a refrigerating unit and the gas is washed with methanol regenerated by decompressing the combined absorbent from both sub-stages and in the second sub-stage the gas is washed with the regenerated methanol obtained from the absorbent leaving the second scrubbing stage. A portion of the methanol regenerated by decompression is supplied to the scrubbing tower of the second scrubbing stage. The absorbent from the first sub-stage is subjected to decompression in three stages. In the first stage the pressure is reduced to about 10 atm. gauge and small quantities of dissolved gases such as carbon monoxide, methane and hydrogen are liberated, recompressed and returned to the crude gas stream. In the second and third decompression stages the pressure is successively reduced to about 1.2 and 0.6 atm. gauge respectively, the evolved gases consisting principally of CO 2 , H 2 S and propane are combined and scrubbed with an alkaline solution to recover the H 2 S and the residual gas consisting of CO 2 and propane is used as a fuel. If the decompression of the methanol does not reduce its temperature to the desired figure some of the CO 2 evolved during the decompression may be added to the crude gas. In the case of gases, such as the gas obtained by the high pressure gasification of coal with oxygen and steam, containing between 20 and 40% CO 2 the third scrubbing stage is modified to obtain a stream of pure CO 2 . The third scrubbing stage is again divided into two sub-stages, each having an associated regenerating plant and the absorbent is cycled first through the scrubber of first sub-stage and then its regenerating plant and from there through the scrubber of the second sub-stage and its regenerating plant and back to the scrubber of the first sub-stage. The gases absorbed in the first sub-stage are principally H 2 S, some CO 2 and propane, together with some carbon monoxide, methane and hydrogen. The latter are removed in a first decompression stage and the former during further decompression to atmospheric pressure. The methanol from the final atmospheric decompression stage is blown with a cold inert gas, e.g. nitrogen, to complete its regeneration before being passed to the scrubber of the second sub-stage to absorb the remaining CO 2 . This sub-stage also contains the coil of a refrigerating unit to maintain the desired low temperature. The absorbent from the second sub-stage is regenerated by decompression in stages, the CO 2 stream being obtained from the final decompression stage and washed with water to remove methanol. The absorbent from the final decompression stage is returned to the scrubber of the first sub-stage. The residual gas after the third-scrubbing stage may comprise ethane, methane, carbon monoxde, hydrogen and nitrogen, depending on the composition of the original gas.