Removal of CO2 and/or H2S from gas made by thermal cracking of hydrocarbons comprises treatment in a absorption zone with an aq. soln. of an alkali salt of a N,N-dialkyl alpha-amino monocraboxylic acid, to which is added an alkali hydroxide and/or alkali salt of an acid with pK value of =5. The absorbent is regenerated as usual by heating. Esp. suitable for treating ethylene-contg. gases made by thermal cracking of hydrocarbons (e.g. petrol, diesel oil or gas oil) in heated tube furnaces in presence of H2, with outlet temp. 760 degrees C (800-870 degrees C), esp. 810-860 degrees C. Life of the absorbent is extended by addn. of the alkali hydroxide or salt, which is cheaper and easier to use than known additive e.g. hydrazine or phenylhydrazine.
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