Process and apparatus for producing diglycerin

A process for producing diglycerin having a low content of cyclic components by reacting glycerin with epichlorohydrin by a continuous process in which the epichlorohydrin required for the reaction is introduced into at least one flow reactor at several separate spots or reaction zones mutually separated in space in the direction of flow; the reacted reaction mixture is substantially freed of excess glycerin in at least one evaporator; the remaining chlorohydrin/ether mixture is hydrolyzed in at least one first step with a 5 to 40 mole % excess alkali carbonate solution, calculated with respect to organically bound chlorine in the chlorohydrin ether, at a pH of 6.5 to 9.5, and hydrolysis is at least substantially completed in a second step with an alkali hydroxide at a pH above 10; the resulting hydrolyzed crude product solution is adjusted to a pH of 5.5 to 8 with a mineral acid, preferably hydrochloric acid; and the resulting solution is condensed, precipitating salts are removed, and the diglycerin solution containing less than 8% by weight residual inorganic salts is freed of cyclic diglycerin and residual glycerin in a flash evaporator. Apparatus for carrying out the foregoing method is also disclosed.

This application is a 371 of PCT/EP93/02261 filed Aug. 23, 1993. 
The present invention relates to a process for the production of 
diglycerin, which is low in cyclic constituents, by reacting glycerin with 
epichlorhydrin at temperatures of 
20.degree. to 120.degree. C., preferably 
50.degree. to 100.degree. C., 
in the presence of an acid catalyst and at a molar ratio of glycerin to 
epichlorhydrin of 20:1 to 1:1, preferably 12:1 to 5:1, in which case 
subsequently the excess glycerin is removed from the resulting reaction 
mixture, and the remaining chlorhydrin ether mixture or reaction mixture 
is reacted at a temperature of 
60.degree. to 120.degree. C., preferably 
80.degree. to 110.degree. C., 
(corresponding to the content of organically bound chlorine in the reaction 
mixture) with an alkaline reacting medium, preferably an alkaline reacting 
aqueous solution. 
Diglycerin (bis-(2,3-dihydroxypropyl)ether) is a polyol which is versatile 
in its use as a moisturizer and viscosity enhancing agent in cosmetic 
products and as a starting product for various derivatives, primarily 
surface-active compounds. In the known processes for the production of 
diglycerin, a mixture of diglycerin and higher polyglycerins is obtained 
as the reaction product, the product mixture also having a more or less 
high proportion of glycerin and cyclic polyglycerins. However, for the 
above-mentioned applications of diglycerin, it is necessary that the 
proportion of diglycerin in the polyglycerin mixture be as high as 
possible and the contents of glycerin and cyclic constituients be very 
low. 
From European Patent Document EP-A 0 374 699, a process already is known 
for producing polyglycerins which have a proportion of more than 50% by 
weight diglycerin and are low in cyclic constituents. The production takes 
place by the reaction of glycerin with epichlorhydrin at temperatures of 
from 20.degree. to 140.degree. C., preferably 60.degree. to 100.degree. 
C., in the presence of an acid catalyst and at a molar ratio of glycerin 
to epichlorhydrin of from 10:1 to 1:1, preferably 6:1 to 1.4:1, in which 
case subsequently the resulting reaction mixture, which optionally is 
freed of excess glycerin, is reacted at a temperature of from 50.degree. 
to 120.degree. C., preferably 60.degree. to 95.degree. C., corresponding 
to the content of organically bound chlorine, with an alkaline reacting 
medium, preferably an alkaline reacting aqueous solution. Subsequently, a 
further working up or purification of the resulting reaction mixture takes 
place, in which case, after the addition of water, the 
glycerin--polyglycerin mixture is deionized by means of one or more ion 
exchangers; is dehydrated by distillation; and is finally separated by 
fractional distillation into glycerin, diglycerin and higher 
polyglycerins. 
It was an object of the present invention to modify the known process such 
that, when the reaction is carried out continuously, a further increase is 
achieved of the diglyglycerin content in the polyglycerin mixture so that 
a product is obtained which preferably has a proportion of more than 80% 
by weight of diglycerin and is low in cyclic constituents. Furthermore, 
the increase of the diglycerin content should be realized in an economical 
process which, at the same time, permits a simplified working up of the 
reaction mixture and an advantageous reduction of undesired constituents 
so that the quality of the reaction product likewise can be improved. 
In accordance with the invention, it has been found that this object is 
achieved by means of a process in which the production of diglycerin, 
which is low in cyclic constituents, takes place by the reaction of 
glycerin with epichlorhydrin at temperatures of 
20.degree. to 120.degree. C., preferably 
50.degree. to 100.degree. C., 
in the presence of an acid catalyst and at a molar ratio of glycerin to 
epichlorhydrin of 20:1 to 1:1, preferably 12:1 to 5:1, in which case, in a 
continuous implementation of the reaction, the epichlorhydrin required for 
the reaction is introduced into at least one flow reactor at several 
points or reaction zones which are spatially separate from one another in 
the flow direction. The reacted reaction mixture thereafter is largely 
freed of or separated from excess glycerin in at least one evaporating 
device, and the remaining chlorhydrin ether mixture is hydrolyzed at a 
temperature of 
60.degree. to 120.degree. C., preferably 
80.degree. to 110.degree. C., 
in a first stage with an aqueous alkali carbonate solution at an excess of 
5 to 40 molar %, preferably 
15 to 25 molar %, 
calculated with respect to the organically bound chlorine of the 
chlorhydrin ether, and in a pH-range of 
6.5 to 9.5, preferably 
7 to 8.5, 
and in a second stage, the hydrolysis is carried out to completion by means 
of alkali hydroxide, preferably an aqueous alkali hydroxide solution, at a 
pH-value of above 10. After the completion or substantial completion of 
the hydrolysis, the crude product solution is adjusted by addition of a 
mineral acid, preferably hydrochloric acid, to a pH-value of 
5.5 to 8, preferably 
6 to 7, 
and the resulting solution is evaporated, whereby the precipitating salts 
are separated, and subsequently the diglycerin solution which contains an 
under 8% by weight residual content of inorganic salts is freed from 
cyclic diglycerin and the residual proportions of glycerin by distillation 
in at least one flash evaporator. 
The process according to the invention makes it possible to continuously 
carry out the reaction of epichlorhydrin with glycerin and to produce a 
diglycerin which is not only low in cyclic constituents but which, even as 
a crude product before the final distillative treatment, has a very low 
content of glycerin, whereby the proportion of open-chained diglycerin in 
the raw product already amounts to over 80% by weight. 
The advantageous improvement with respect to the yield of diglycerin is 
achieved particularly as the result of two measures according to the 
invention: firstly, the epichlorhydrin required for the reaction is 
introduced into at least one flow reactor at several points or reaction 
zones which are spatially separated from one another in the flow direction 
so that the epichlorhydrin stream is divided into several partial streams, 
whereby, corresponding to the number of introduced partial streams, a 
plurality of reaction zones or reaction centers are formed in the flow 
reactor at which the reaction of glycerin and epichlorhydrin to diglycerin 
as the main product of the synthesis takes place; and secondly the 
unreacted glycerin is separated in a distillation device immediately after 
the reaction mixture emerges from the flow reactor. The substantial 
removal of the glycerin by means of relatively simple distillative methods 
is possible only in this stage of the process according to the invention. 
In addition, in this manner the volume of the stream which is treated in a 
subsequent hydrolysis by means of alkaline media can be significantly 
reduced. 
According to one advantageous embodiment, the content of excess glycerin 
used in the reaction mixture is reduced in at least one evaporating 
device, preferably a continuously operated evaporating device, to below 
20% by weight (relative to the reaction mixture), preferably to below 10% 
by weight, and particularly to below 3% by weight. 
Thin-film evaporators and/or flash evaporators, preferably a combination of 
these two types of evaporators, are particularly suitable for use as the 
evaporating devices. The glycerin obtained as a head product has a mass 
content of &gt;98% and may be recycled to the process. 
The two-stage hydrolysis according to the invention of the chlorhydrin 
ether mixture, which follows the above-mentioned process step, enables a 
complete substitution of each of the organically bound chlorines by 
respective OH-groups so that the corresponding polyglycerin compounds are 
obtained. 
In the case of the known processes, it was found that complete reaction of 
the chlorhydrin ethers could not be achieved in a single hydrolysis stage 
by means of an alkali carbonate solution, so that in this case up to 10% 
of the chlorhydrin ethers remained in the reaction solution and a 
diglycerin is obtained which still contains organically bound chlorine. It 
is not possible to separate the chlorhydrin ether from the glycerin by 
means of a thin-film or flash evaporator. 
In accordance with the invention, the advantageous complete reaction of the 
organically bound chlorine takes place particularly in the second 
hydrolysis stage by means of an aftertreatment of the residual chlorhydrin 
ether with alkali hydroxide at a pH-value of above 10. According to the 
invention, it has been found in this case that the adjustment of the 
pH-value &gt;10 is critical for the effectiveness of the second treatment 
stage. 
In order to achieve, following the hydrolysis, a substantial desalination 
of the product mixture and thus a decrease in the viscosity of the crude 
product solution after the evaporation, the alkaline crude product 
solution is adjusted by means of a mineral acid, preferably hydrochloric 
acid, to a pH-range of 6.5 to 9.5, preferably 7 to 8.5. In this manner, 
easily filtrable salts are obtained after the evaporation of the solution. 
According to a preferred embodiment of the process of the invention, the 
crude product solution, which has been treated with mineral acid, is 
concentrated in a flash evaporator and is subsequently freed of 
precipitated salts by means of filtration or centrifugation. 
The resulting crude diglycerin solution is desalted by this process step to 
such an extent that by means of a subsequent, simple distillation in at 
least one flash evaporator, cyclic diglycerin and residual proportions of 
glycerin can be removed and finally diglycerin is obtained in high yield 
and purity. 
According to another advantageous embodiment of the process according to 
the invention, the proportion of diglycerin in the crude product may be 
increased by introducing the epichlorhydrin in two or more partial streams 
in at least one flow reactor which is subdivided by constrictions in its 
cross-section into two or more individual chambers or reaction zones, in 
which case at least one partial stream is preferably introduced into each 
individual chamber or reaction zone. 
The increase of the diglycerin content in the crude product according to 
the invention may also be boosted by dividing the total flow of the 
epichlorhydrin required for the reaction into two partial streams, and 
introducing a first partial stream into a first flow reactor at several 
points or reaction zones which are spatially separated from one another in 
the flow direction, and subsequently continuously transferring the 
reaction mixture to a second flow reactor where the second partial flow of 
the epichlorhydrin is fed in in a metered fashion, preferably at several 
points or reaction zones which are spatially separated from one another in 
the flow direction. 
According to another advantageous embodiment of the process of the 
invention, the introduction of the epichlorhydrin into the flow reactor 
takes place in such a way that the ratio of glycerin to epichlorhydrin in 
the individual reaction zones, preferably in the area and/or in the 
proximity of the inlet opening or inlet point for the epichlorhydrin, is 
greater than the preselected overall ratio of glycerin to epichlorhydrin 
of 20:1 to 1:1, preferably 12:1 to 5:1. 
According to an advantageous embodiment, in the first hydrolysis stage of 
the process according to the invention, a higher than 1.5-molar, 
preferably a higher than 2-molar, soda solution is utilized in order to 
already achieve a high rate of conversion of the organically bound 
chlorine in this first hydrolysis stage. 
In the second hydrolysis stage, preferably a soda lye of more than 10% by 
weight is used, particularly a soda lye of more than 35% by weight. 
According to a preferred embodiment of the process of the invention, the 
residence time of the reaction mixture in the first stage of the 
hydrolysis is set at a time period of more than 1 hour, preferably at more 
than 1.5 hours, and the residence time in the second hydrolysis stage is 
set at more than 0.5 hours, preferably at more than 0.75 hours. 
In order to effect complete desalination, the product solution, which has 
been freed of glycerin and cyclic diglycerin by distillation, can be 
treated by means of ion exchangers, preferably a combination of several 
basic cation exchangers and at least one acidic anion exchanger, whereby 
the diglycerin/polyglycerin solution is first diluted with water. 
After the product solution has been passed through the ion exchangers, a 
water can again be removed by evaporation and subsequently a distillative 
separation may be carried out into diglycerin and higher polyglycerins. 
The invention also relates to an improved apparatus for carrying out the 
process according to the invention. This apparatus contains or comprises 
a reactor or reaction vessel, preferably a vertically arranged flow reactor 
with a cylindrical reactor housing, having at least one inlet for the 
glycerin and the catalyst at the lower end and having at least one outlet 
for the reaction mixture at the upper end, having at least one stirring 
shaft which is coaxially and centrally arranged in the reactor or reaction 
vessel and on which at least one stirring device is attached, and having a 
supply conduit for the epichlorhydrin which is arranged parallel to the 
stirring shaft and opens in several conduit ends or conduit openings at 
different levels into the reactor space, 
at least one evaporating device, preferably a thin-film and/or flash 
evaporator, 
at least one reaction vessel for carrying out the hydrolysis, 
at least one reaction vessel for carrying out the subsequent hydrolysis, 
and 
at least one additional flash evaporator, 
whereby the reaction space is subdivided by constrictions in its 
cross-section into at least two, preferably more than three reaction zones 
or reaction chambers, the supply conduit for the epichlorhydrin is 
arranged within the reactor space separated a distance from the outer 
circumference of the stirring device or devices, and at least one conduit 
end or conduit opening of the supply conduit for the epichlorhydrin 
opening into each reaction zone or reaction chamber and at least one 
stirring device being attached to the stirring shaft in each reaction zone 
or reaction chamber.

The reactor or reaction vessel 1, which is preferably constructed as a 
vertically arranged flow reactor or as a reaction column with a 
cylindrical reactor housing, has on its lower end, preferably in the 
reactor bottom, an inlet or a supply conduit through which the glycerin 
reaction educt 9 with the added catalyst is introduced into the reactor; 
at the upper end of the reactor, preferably above the uppermost or highest 
cross-sectional constriction, there is an outlet or a discharge line 
through which the formed reaction mixture 3 is discharged from the 
reactor. The introduction of the educt stream and/or the discharge of the 
product stream may be aided by the use of pumps. 
Furthermore, a coaxial stirring shaft 2 is arranged centrally in the 
reactor space which is continuously rotatable and on which at least one 
stirring device 10 is attached. Such a stirring device may, for example, 
consist of a disk stirrer. Parallel to the axis of the stirring shaft, the 
apparatus according to the invention has a tubular conduit 7 through which 
the epichlorhydrin 6 is conveyed into the reactor space. In accordance 
with the invention, the supply conduit 7 is arranged inside the reactor 
space separated a distance from the outer circumference of the stirring 
device or devices. It opens in several conduit ends or conduit openings 8 
at different levels into the reactor space, in which case the latter is 
subdivided by means of constrictions in its cross-section into at least 
two, preferably more than three, reaction zones or reaction chambers 4, 
and at least one conduit end or conduit opening 8 of the supply conduit 
for the epichlorhydrin and at least one stirring device 10 fastened on the 
stirring shaft are provided in each reaction zone or reaction chamber. 
As a result of the division of the reaction space into several reaction 
zones, into each of which at least one partial stream of the 
epichlorhydrin is introduced into the mixture of glycerin and the catalyst 
through a respective one conduit end or one conduit opening of the supply 
conduit 7, a number of reaction centers are formed, corresponding to the 
number of partial streams which are introduced, in which the reaction of 
glycerin and epichlorhydrin takes place with a surprisingly higher yield 
of diglycerin than in the case of the known processes. 
Although, in accordance with the invention, this higher yield of diglycerin 
is obtained simply by introducing the epichlorhydrin into the flow reactor 
at several points which are spatially separated from one another in the 
flow direction, as a result of the formation of individual reaction zones 
or reaction chambers, by means of the introduction of constrictions in the 
cross-section and as a result of the continuous mixing of the reaction 
mixture in each chamber by means of at least one stirring device, a very 
favorable ratio of glycerin to epichlorhydrin is produced so that the 
yield can be further improved as a result of the construction of the flow 
reactor in accordance with the invention. 
This improvement of the yield is also promoted by the fact than, according 
to a preferred embodiment of the apparatus according to the invention, the 
constrictions in cross-section are constructed in a particular manner. 
Accordingly, they are made of intermediate base plates, deflecting plates 
or separator plates 5 which have a plurality of perforations or 
through-openings for the reaction mixture. In this way, an unimpaired 
mixing of the product solutions formed in the individual reaction centers 
and a free-flowing upward-directed flow of the reaction mixture in the 
reactor space are assured. 
According to another advantageous embodiment of the apparatus according to 
the invention, the intermediate base plates, the deflecting plates or the 
separator plates have a circular construction and are arranged 
horizontally at different levels in the reactor space, whereby the 
intermediate base plates, deflecting plates or separator plates each have 
a central opening through which the stirring shaft extends and an opening 
through which the supply conduit for the epichlorhydrin passes. 
The free guidance and the mixing of the product streams and/or of the 
reaction constituents may be further improved in that an unsealed gap is 
formed between the reactor wall and the outer margin of the intermediate 
base plates, deflecting plates or separator plates. 
Moreover, it has been found to be advantageous for the intermediate bottom 
plates, deflecting plates or separator disks to have approximately the 
same spacing from one another so that reaction zones or reaction chambers 
of approximately equal size are formed. 
According to another preferred embodiment of the apparatus of the 
invention, the mixing of the reaction constituents is increased by 
constructing the conduit ends or conduit openings of the supply conduit 
for the epichlorhydrin as Venturi nozzles. 
A single reactor or a single reaction column may be used in the apparatus 
according to the invention. However, two or more reactors of the type 
according to the invention may also be used which are connected in series, 
whereby the overall epichlorhydrin stream is once again separated into a 
higher number of partial streams. 
Explanation of the Drawing 
FIG. 1: 
Schematic representation of a reaction vessel for explaining the apparatus 
according to the invention and the process according to the invention; the 
meaning of reference numbers 1 through 10 is found in the foregoing 
description. 
The following working embodiment has the purpose of explaining the 
invention without being limited to it. 
Working Embodiment 
Within one hour 5.526 kg (.apprxeq.60 mole) of glycerin mixed with a 
catalytic quantity of sulfuric acid are introduced into a flow reactor 
(1-liter double-wall reactor made of glass with a stirrer and two conduits 
which are spatially separated from one another in the flow direction for 
introducing controlled amounts of the epichlorhydrin), and 0.695 kg 
(.apprxeq.7.5 mole) of epichlorhydrin in two partial streams are metered 
into the flow reactor, while a reaction temperature of from 60.degree. to 
70.degree. C. is maintained by cooling via the double wall. 
The discharge of the reactor is conveyed to a combination of a thin-film 
evaporator and flash evaporator and the excess glycerin used is removed 
from the reaction mixture by distillation (residual content of glycerin in 
the chlorhydrin ether mixture .ltoreq.3%). The sump discharge of the 
evaporator device is conducted to a buffer vessel and subsequently the 
chlorhydrin ether mixture is converted in portions through a 2-stage 
hydrolysis, 
1st Stage: Addition of 2.5-molar soda solution (20 mole-% excess of sodium 
carbonate, reaction time approx. 2 hours, reaction temperature approx. 
95.degree. C., pH-value 7-8.5) 
2nd Stage: Addition of 50% soda lye (approx. 10 to 15%, relative to the 
quantity of 2.5-molar soda solution used, reaction time approx. 1 hour, 
pH-value 10.5), to a crude diglycerin/polyglycerin mixture. 
The resulting aqueous diglycerin/polyglycerin solution is adjusted to a 
pH-value of from 6 to 7 with concentrated hydrochloric acid and is 
subsequently concentrated by evaporation under vacuum, while precipitated 
portions of salt are separated by filtration. 
The product solution, which still contains salts (residual salt content 
below 8% by weight) is freed of glycerin and cyclic diglycerin by 
distillation in a thin-film evaporator, and the sump discharge of the 
evaporator is completely desalted by means of a combination of acidic and 
basic ion exchangers (after prior addition of water); is then concentrated 
by evaporation, and is separated by distillation into diglycerin and 
higher polyglycerins. 
A GC-analysis of the desalted and dehydrated crude diglycerin/polyglycerin 
solution after the two-stage hydrolysis yields in the following 
composition (numerical values each relate to g/kg): 
glycerin 20 
cycl. diglycerin 30 
diglycerin 830 
cycl. triglycerin 20 
triglycerin 80 
cycl. tetraglycerin 5 
tetraglycerin 10 
pentaglycerin 5