Process for the preparation of aqueous betaine solutions

A process for the preparation of aqueous solutions of betaines of the formula 1 ##STR4## in which:

• R.sup.1 is an alkyl radical having 6 to 22 carbon atoms or is a radical of the formula R'CONH(CH.sub.2).sub.z --, in which R' has the meaning of R.sup.1 and z is 2, 3 or 4, R.sup.2 is an alkyl radical having 1 to 4 carbon atoms or is a radical of the formula --(CH.sub.2).sub.m --OH, in which m is 1, 2 or 3, R.sup.3 is an alkyl radical having 1 to 4 carbon atoms or is a radical of said formula --(CH.sub.2).sub.m --OH and y is 1, 2 or 3,
• by reaction of a tertiary amine of the formula 2
• R.sup.1 --NR.sup.2 R.sup.3 ( 2)
• in which R.sup.1, R.sup.2 and R.sup.3 have the meanings given above,
• with an .omega.-halocarboxylic acid of the formula 3
• X--(CH.sub.2).sub.y --COOH (3)
• in which X is a halogen and y has the above-mentioned meaning,
• and with alkali metal hydroxide in the aqueous phase, which comprises carrying out the reaction continuously in two or three stirred tanks arranged in a cascade and proceeding in such a way that
• a) the first stirred tank is supplied simultaneously and continuously with the tertiary amine and the .omega.-halocarboxylic acid in an amount of 1 to 1.3 mol per mole of tertiary amine, the alkali metal hydroxide in an amount of only 0.80 to 0.98 mol per mole of .omega.-halocarboxylic acid and the water in an amount such that the betaine content of the finished aqueous betaine solution is 20 to 50% by weight, and in this tank at a temperature of 60 to 100.degree. C. a residence time is maintained such that the product leaving this tank and entering the second stirred tank still contains 2 to 15 mol-% of tertiary amine, based on tertiary amine used, and
• b) the second stirred tank is continuously supplied with a further amount of alkali metal hydroxide which is in the range from the remainder of the stoichiometric amount of alkali metal hydroxide remaining from the first tank up to an excess of 5 mol-%, and in this tank at a temperature of 60 to 100.degree. C. a residence time is maintained such that the aqueous betaine solution leaving the tank contains less than 2 mol-% of tertiary amine, based on tertiary amine used, where, when three stirred tanks are used, the third is used for a following reaction at a temperature of 70 to 100.degree. C. in order to obtain an aqueous betaine solution which, in comparison with the betaine solution from the second stirred tank, has a still lower content of tertiary starting amine.