Process for the production of light-colored .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes

In the production of .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes by reaction of fatty acid alkyl esters with gaseous SO.sub.3, subsequent after-reaction in liquid phase and neutralization with aqueous alkali metal hydroxide solutions, the crude sulfonation product is fed to an at least two-stage cascade of stirred tanks equipped with heating and cooling systems in which it is subjected with mechanical agitation to a temperature-controlled after-reaction until a degree of sulfonation of at least 90% is reached, after which the aged sulfonation product is further processed in known manner to .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for the production of light-colored 
.alpha.-sulfofatty acid alkyl ester alkali metal salt pastes, in which the 
crude sulfonation product is subjected immediately after the sulfonation 
reaction to a temperature-controlled after-reaction in a substantially 
ideally mixed liquid phase until a degree of sulfonation of at least 90% 
is reached. 
2. Statement of Related Art 
.alpha.-Sulfofatty acid alkyl ester alkali metal salts are acquiring 
increasing significance as surfactants for detergents and cleaning 
preparations based on renewable natural raw materials. In known processes, 
the .alpha.-sulfofatty acid alkyl ester alkali metal salts are obtained in 
the form of aqueous solutions or pastes by neutralization of 
.alpha.-sulfofatty acid alkyl esters which may be synthesized by reaction 
of lower fatty acid alkyl esters with gaseous SO.sub.3. In the final 
analysis, the basis for the production of the .alpha.-sulfofatty acid 
alkyl ester alkali metal salts are fats and oils of natural origin from 
which the lower fatty acid alkyl esters are obtained by lipolysis and 
subsequent esterification of the free fatty acids with lower alkanols or 
by transesterification of the natural triglycerides with lower alkanols. 
In both reactions, methanol is preferably used as the lower alkanol. The 
lower fatty acid alkyl esters are mixtures in which C.sub.6-22 fatty acid 
residues occur, the chain length distribution being dependent on the 
origin of the natural fats or oils. In many cases, these fatty acid ester 
mixtures are not used for the synthesis as such, but rather in the form of 
certain fractions. Sulfonation of the fatty acid ester mixtures with 
gaseous SO.sub.3 gives acidic .alpha.-sulfofatty acid alkyl esters which 
are converted into aqueous pastes of .alpha.-sulfofatty acid alkyl ester 
alkali metal salts by neutralization to a pH value of 6 to 8. The crude 
.alpha.-sulfofatty acid alkyl esters and their alkali metal salts are more 
or less colored products which generally have to be treated with typical 
bleaches before and/or after neutralization. 
The sulfonation of the fatty acid alkyl esters is normally carried out with 
gaseous SO.sub.3 at temperatures in the range from 30.degree. to 
100.degree. C., the molar ratio of fatty acid ester to SO.sub.3 being from 
1:1.2 to 1:1.8. The reaction of the fatty acid alkyl esters with SO.sub.3 
takes place in two steps. In a first, rapid step, 2 molecules SO.sub.3 
react with 1 molecule fatty acid alkyl ester to form a mixed anhydride of 
.alpha.-sulfofatty acid and alkyl sulfuric acid. In a slow, second step, 
the mixed anhydride acts as sulfonating agent for unreacted fatty acid 
alkyl ester. The first step takes place in the particular typical 
sulfonation reactor used, for example in a falling-film reactor or in a 
sulfonation cascade. Very little information is available to show how the 
second reaction step is carried out in known processes, generally being 
confined to the reaction temperature and, at most, to the reaction time. 
Thus, according to DE-OS 31 23 681, the sulfonation product is said to be 
aged at 30.degree. to 100.degree. C. According to DE-OS 33 34 517, the 
sulfonation product is aged for 10 to 20 minutes at 80.degree. to 
100.degree. C. 
During the work culminating in the present invention, it was found that the 
conduct of the second step of the sulfonation reaction is crucially 
important to the degree of sulfonation and to the quality of the end 
product. It has been found that product quality depends to a large extent 
on exact temperature control and on a narrow holding time distribution in 
the after-reaction. Short holding times lead to readily bleachable, but 
inadequately sulfonated .alpha.-sulfofatty acid alkyl ester alkali metal 
salt pastes. Long holding times lead to high degrees of sulfonation, but 
also to strongly colored products which cannot be bleached to the 
necessary color values after neutralization. 
Known holding-time installations are unable to satisfy the dual requirement 
of a narrow holding time distribution and exact temperature control. 
Standard reactors operated solely with a heating and cooling circuit lead 
to far too broad a holding time spectrum on account of the constant 
back-mixing. Following temperature-controlled pipe coils always have to 
operated under full turbulent flow conditions, i.e. under plug-flow 
conditions, and accordingly cannot be operated independently of the output 
of the sulfonation reactor. 
Accordingly, the problem addressed by the present invention was to provide 
a process which would enable the crude sulfonation product in the 
production of light-colored .alpha.-sulfofatty acid alkyl ester alkali 
metal salts by reaction of fatty acid alkyl esters with gaseous SO.sub.3 
and neutralization with aqueous alkali metal hydroxide solutions to be 
subjected to an after-reaction under exact temperature control and, at the 
same time, a narrow holding time distribution before it was neutralized 
and bleached in known manner. 
DESCRIPTION OF THE INVENTION 
It has been found that a conventional cascade of stirred tanks following 
the sulfonation reactor and equipped with heating and cooling systems 
ideally satisfies the conditions which the second reaction step of the 
fatty acid alkyl ester sulfonation requires in regard to temperature 
control and holding time behavior. The cascade may consist of any number 
of stirred tanks arranged one behind the other and preferably consists of 
four stirred tanks. If the cascade of stirred tanks consists of four or 
more units, its holding time behavior is equivalent to that of a 
turbulent-flow tube. The average holding time for the second sulfonation 
step may readily be adjusted through the liquid level in the individual 
stirred tanks. By means of several ideally mixed stirred tanks arranged 
one behind the other, it is possible to establish both isothermal reaction 
conditions and also any other desired temperature profile. 
The present invention relates to a process for the production of 
light-colored .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes 
by reaction of fatty acid alkyl esters with gaseous SO.sub.3, subsequent 
after-reaction in a substantially ideally mixed liquid phase and 
neutralization with aqueous alkali metal hydroxide solutions, 
characterized in that 
a) fatty acid alkyl esters are reacted with an at least 10% molar excess of 
SO.sub.3 in a typical sulfonation reactor, 
b) the crude sulfonation product is fed to an at least two-stage cascade of 
stirred tanks with heating and cooling systems in which it is subjected 
with mechanical agitation to a temperature-controlled after-reaction until 
a degree of sulfonation of at least 90% is reached and 
c) the sulfonation product aged in this way is further processed in known 
manner to .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes. 
The fatty acid esters are reacted with gaseous SO.sub.3 as the sulfonation 
reagent at temperatures in the range from 30.degree. to 100.degree. C. The 
SO.sub.3 is reacted with the fatty acid esters after dilution with air or 
nitrogen, preferably in the form of a gas mixture containing 1 to 10% by 
volume SO.sub.3. The quantity in which the SO.sub.3 is used is preferably 
gauged in such a way that the molar ratio of fatty acid ester to SO.sub.3 
is from 1:1.2 to 1:1.8. This reaction may be carried out in typical 
reactors suitable for the sulfonation of organic compounds, such as fatty 
alcohols, alkyl benzenes or olefins, more particularly in falling-film 
reactors or multistage cascades of stirred tanks. 
The temperature-controlled after-reaction is carried out in a conventional 
cascade of stirred tanks equipped with heating and cooling systems. The 
cascade may comprise from 3 to 6 stirred tanks, four-stage cascades being 
preferred. The heating and cooling systems of the cascade of stirred tanks 
preferably consist of pipe coils or heat-transfer jackets. 
During the after-reaction, the exothermically reacting reaction mixture is 
kept at temperatures of 60.degree. to 100.degree. C. in all the stirred 
tanks of the cascade, temperatures in the range from 80.degree. to 
90.degree. C. being preferred. 
During the temperature-controlled after-reaction, the sulfonation product 
should be present in substantially ideally mixed liquid phase. The 
mechanical agitation of the sulfonation product required for this purpose 
in the cascade of stirred tanks is obtained by stirring, by introduction 
of the product under pressure or by installed chicane-like baffles. 
The crude sulfonation product is left in the cascade of stirred tanks until 
a degree of sulfonation of at least 90% and preferably 94 to 98% is 
reached. To this end, the crude sulfonation product is left in the cascade 
of stirred tanks for 10 to 60 minutes and preferably for 20 to 40 minutes. 
In the production of light-colored .alpha.-sulfofatty acid alkyl ester 
alkali metal salt pastes in accordance with the invention, the aged 
sulfonation product is neutralized and bleached in known manner after the 
temperature-controlled after-reaction. 
The aged sulfonation product is neutralized with aqueous alkali metal 
hydroxide solutions, preferably with aqueous sodium hydroxide solutions. 
The crude sulfonation products and their alkali metal salts are more or 
less colored substances. For this reason, they have to be bleached by 
known methods before and/or after neutralization, aqueous hydrogen 
peroxide and/or hypochlorite solutions preferably being used as bleaches. 
Bleaching before neutralization with hydrogen peroxide is described in 
DE-PS 11 79 931. According to DE-AS 12 34 709, the acidic 
.alpha.-sulfofatty acid alkyl ester is treated with aqueous hydrogen 
peroxide solution in a first bleaching step. The partly bleached 
sulfonation product is then neutralized before being treated with more 
hydrogen peroxide solution or aqueous hypochlorite solution in a second 
bleaching step. According to DE-OS 33 19 591, the partly neutralized 
sulfonation product is initially bleached with aqueous hypochlorite 
solution at pH values of 7 to 11. Aqueous hydrogen peroxide solution is 
then added at pH values of .ltoreq.7 to stabilize the color values 
reached. 
In the context of the invention, fatty acid alkyl esters are understood to 
be lower alkyl esters of saturated fatty acids, more particularly esters 
of fatty acids containing 10 to 18 carbon atoms and saturated aliphatic 
alcohols containing 1 to 4 carbon atoms. Basically, individual fatty acid 
alkyl esters may be used as starting material. In general, however, ester 
mixtures of the type obtainable from fats and oils of natural origin 
either by ester cleavage and subsequent esterification with lower alkanols 
or by transesterification with lower alkanols by known methods are used as 
the starting material, the corresponding fatty acid methyl ester mixtures 
being preferred. If the fatty acid ester mixtures obtained in this way 
have relatively large percentage contents of esters of fatty acids 
containing less than 10 carbon atoms, these "head-fractionated fatty acid 
esters" are generally removed by distillation. Apart from the CH.sub.2 
group in the .alpha.-position to the ester group, the fatty acid esters 
should not contain any sulfatable or sulfonatable groups. For this reason, 
hydroxyfatty acid esters or mixtures containing hydroxy-fatty acid esters 
are not suitable as starting materials. Fatty acid ester mixtures 
containing non-negligible quantities of esters of unsaturated fatty acids, 
more particularly esters having an iodine value above 5, are only suitable 
as starting materials after saturation of the double bonds in the course 
of hardening by hydrogenation using known methods. During the 
hydrogenation, the iodine values of the ester mixtures are preferably 
reduced to values of 0.2 and lower.

EXAMPLES 
Examples 1 to 6 
The starting material used was a technical palmitic/stearic acid methyl 
ester (in % by weight according to chain length in the fatty acid part: 
0.2 C.sub.12 ; 1.2 C.sub.14 ; 61.4 C.sub.16 ; 0.9 C.sub.17 ; 35.9 C.sub.18 
; 0.4 C.sub.20 ; average molecular weight 281.5; acid value 1.1; iodine 
value 0.1; saponification value 202.1). The fatty acid methyl ester was 
continuously sulfonated with an SO.sub.3 /air mixture (5% by volume 
SO.sub.3) in a molar ratio of 1:1.25 in a standard falling-film reactor at 
a temperature of 80.degree. C. Individual batches of the resulting 
reaction mixture were subjected to the after-reaction in a holding-time 
cascade of four stirred tanks with holding times of 10, 30 and 60 minutes 
at 80.degree. and 90.degree. C. 
Following the after-reaction, the aged batches were neutralized with 
aqueous sodium hydroxide solution. Aqueous hydrogen peroxide solution was 
introduced into the reaction mixture with the neutralization base, 
hydrogen peroxide being added in a quantity of 1.5% by weight, expressed 
as 100% substance and based on WAS. After neutralization, all the batches 
were stirred for 20 hours at a temperature of 80.degree. C. The Klett 
color values were then measured on solutions containing 5% by weight 
washing-active substance at pH 7.5 in a 5 cm cuvette using a blue filter 
(420 nm). 
The after-reaction temperature and the holding time and also the degree of 
sulfonation reached and the associated Klett color value are shown for 
each Example in the following Table. 
TABLE 
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Degree of sulfonation and Klett color value in dependence 
upon the after-reaction temperature and holding time 
Degree of 
Temperature 
Holding time 
sulfonation 
Klett color 
Example 
(.degree.C.) 
(mins.) (%) value 
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1 80 10 90 100 
2 80 30 94 200 
3 80 60 97 250 
4 90 10 92 150 
5 90 30 96 200 
6 90 60 97 400 
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