Abstract: A sulfonation apparatus comprising a plurality of vertical first-stage reaction tubes in which the sulfonatable material and an SO.sub.3 -containing gas are flowed upwardly from the lower ends thereof and a single vertical second-stage reaction tube in which the reaction products from the upper ends of all of the first-stage reaction tubes are combined and are flowed downwardly together with fresh SO.sub.3 -containing gas in order to complete the sulfonation reaction.

Abstract: Sulfonation of a sulfonatable organic compound is effected by means of a two-stage reaction with an SO.sub.3 -containing gas, in which the first-stage reaction is carried out by flowing, in parallel flow relation, said organic compound and said SO.sub.3 -containing gas upwardly into the lower ends of a plurality of vertically positioned cylinders or double-cylinders, and the second-stage reaction is carried out by the steps of collecting the reaction product of the first-stage reaction, adding fresh SO.sub.3 -containing gas to the reaction product thus collected and flowing in parallel flow arrangement the reaction mixture downwardly into the upper end of a vertically positioned cylinder or double-cylinder.