Patent GB-833451-A (GB)
Title: Method of removing hydrogen sulphide alone or jointly with carbon dioxide and other acid gases from gaseous mixtures
IPC: C01B32/50
Filed: 1956-06-26
Abstract: In a method of removing hydrogen sulphide alone or together with other acid gases, e.g. carbon dioxide, the gases are scrubbed with alkaline arsenical solutions containing under operating conditions non-sulphuretted arsenic compounds, fundamentally arsenious oxide, which absorb hydrogen sulphide to form trivalent arsenic sulphide, the used solution being withdrawn and re-activated by converting the arsenic sulphide to arsenious oxide.  The arsenious oxide or non-sulphuretted arsenic <PICT:0833451/III/1> compounds may be dissolved in solutions of carbonates, phosphates, arsenates and borates of alkali metals and ammonium, phenates and ethanolamines. The absorbing solutions may be used hot and the regeneration of the used solutions may be effected by cooling the solutions to precipitate arsenic trisulphide, roasting the separated sulphide to obtain arsenious oxide and redissolving the arsenious oxide in the absorbing solution.  The alkalinity of the absorbing solution may be just sufficient to keep the arsenic tri-sulphide in solution and during regeneration the sulphide may be precipitated by acidifying the solution, e.g. by treatment with carbon dioxide or a carbon dioxide containing gas such as the gas to be purified, the precipitated sulphide being again roasted to obtain arsenious oxide which is redissolved in the absorbing solution. When the absorbing solution comprises ammonium salts the reduction in alkalinity necessary to precipitate the arsenic tri-sulphide may be effected by removal of ammonia.  Buffering agents may be added to the absorbing solutions to govern the alkalinity of the absorbing solutions.  In a further method of regenerating the used solution, the solution contains an excess of an alkaline arsenate and this is allowed to react with the arsenic tri-sulphide (or equivalent) formed by adsorption of hydrogen sulphide to produce arsenious oxide (or equivalent) and an alkaline oxytrioarsenate, the temperature being suitably maintained at 40-50 DEG  C. or up to 90 DEG  C. during the reaction.  The solution is then treated with carbon dioxide whereby sulphur is precipitated and arsenious oxide (or equivalent). Subsequently the solution thus obtained is withdrawn from the vessel in which sulphur is separated and is oxidized preferably by treatment with air to reform in the solution the quantity of alkaline arsenate necessary for the initial part of the regeneration.  The oxidation may be effected in the presence of a catalyst such as iron oxide and the use of air as the oxidizing agent removes carbon dioxide from the solution and restores the alkalinity of the solution to that necessary for the absorption.  Thiosulphate formed during the oxidation may be decomposed by heating the solution.  In the apparatus shown in Fig. 5 the crude gas under pressure is washed by regenerated solution in a tower 2 and gas free of hydrogen sulphide and carbon dioxide leaves by line 3.  Used solution is withdrawn for regeneration and passes in part directly through a heater 6 to a vessel 7 where the pressure is reduced and the reaction between arsenic tri-sulphide (or equivalent) and the alkaline arsenate proceeds to completion at a temperature between 50-90 DEG  C.  Carbon dioxide is recovered through line 8.  Some of the solution for regeneration passes to the vessel 7 via a vessel 23 in which the reaction between arsenic tri-sulphide (or equivalent) and thioarsenate takes place and a vessel 25 in which the solution is diluted with water and acidified with carbon dioxide to liberate sulphur which is recovered through line 26. The thus treated solution then passes through line 33 and the heater 6 to the vessel 7.  Carbon dioxide for the acidification may be supplied by line 27 from the crude gas line or by line 31 and a compressor 30 from the vessel 7. From the vessel 7 the solution passes to a tower 10 in which it is oxidized with air humidified with hot water in a tower 12.  Air and expelled carbon dioxide are washed in a tower 15 with water from and which is to be re-used in the tower 12.  Regenerated solution is returned under pressure to the tower 2 by a pump 21 and line 4 from the base of the tower 10.  In an example, the washing liquid employed contained 12 kg/m.3 tri-sodium phosphate, 56 kg/m.3 disodium phosphate and 50 kg/m.3 arsenious oxide (or equivalent) and the washing was effected at 55 DEG  C. and with a liquid/gas ratio of 1 : 400.  Arsenic tri-sulphide was precipitated from the used washing liquid by cooling the liquid to 28 DEG  C.  The arsenic trisulphide was separated and roasted to form sulphur dioxide and arsenious oxide, the latter being returned to the washing liquid. Specifications 286,633, 798,856 and 833,452 are referred to.