Process for the production of diglycerol

A process for preparing diglycerol in high concentrations and high yields by partially reacting glycerol in the presence of an alkaline catalyst to form a reaction mixture containing from 10 to 15% by weight of diglycerol and separating the unreacted glycerol from the reaction mixture in a wiped film or short path first distillation zone at a reduced pressure of 0.5 to 5 mbar and distilling a bottoms product from the first distillation zone in a second distillation zone which is a short path distillation zone at a pressure of 0.05 to 0.3 mbar to obtain a second bottom product containing at least 90% by weight diglycerol. Diglycerol of higher purity can be obtained by recovering diglycerol as a distillate from a third distillation zone.

This application is a 371 of PCT/EP94/00834, filed on Mar. 16, 1994, 
published as WO94/21582 Sep. 29, 1994. 
FIELD OF THE INVENTION 
This invention relates to a process for the production of diglycerol by 
condensation of glycerol in the presence of a basic catalyst, preferably 
sodium or potassium hydroxide, at reaction temperatures of 200.degree. to 
275.degree. C. and, more particularly, at reaction temperatures of 
220.degree. to 240.degree. C. and subsequent concentration of the 
diglycerol by distillation. 
RELATED ART 
Diglycerol and higher oligomers of glycerol, such as tri- and 
tetraglycerol, which are also known generally as polyglycerols, are 
normally obtained in the alkali-catalyzed condensation of glycerol at 
elevated temperature. These ethers have relative molecular weights of 166 
(6 carbon atoms) to 2,238 (90 carbon atoms) and contain 4 to 32 hydroxyl 
groups. The polyglycerols are prepared not only by the base-catalyzed 
dehydration of glycerol, but also by purification of the 
polyglycerol-containing distillation residue emanating from the synthetic 
production of glycerol via epichlorohydrin (DE 34 10 520 A1). Their 
purification is carried out in one or more cation exchangers and an anion 
exchanger by evaporation of water and vacuum distillation. 
For the industrial production of polyglycerol by condensation of glycerol, 
glycerol is heated together with an alkaline catalyst at a temperature of 
200.degree. to 275.degree. C. under normal pressure or reduced pressure. 
Sodium hydroxide or sodium acetate is normally used as the catalyst. In a 
typical process, approximately 0.3% of sodium hydroxide is added to 
glycerol and the reaction mixture is heated to a temperature of around 
230.degree. C. at which the water begins to distil off. The mixture is 
then slowly heated to 260.degree.-265.degree. C. and kept at that 
temperature until the calculated quantity of water has distilled off. The 
time required for removal of the water changes slightly with the reaction 
conditions, although a reaction time of 11 hours for the production of 
diglycerol is typical. The reaction product obtained contains a number of 
different polyglycerols and also unreacted glycerol. It is called 
diglycerol, triglycerol, etc. according to its average composition which 
is characterized by the hydroxyl value or the quantity of water distilled 
off (book: Miner and Dalton, Glycerol, Reinhold Publ. Corp. New York 1953, 
pages 366 to 368). 
It is also known that the polyglycerols present in the reaction mixture can 
be separated by acetylation and distillation of the acetates. Another 
method of separating the polyglycerols comprises distilling their allyl 
ethers or their isopropylidene derivatives. The polyglycerols are obtained 
by hydrolysis. It is also known that the lower polyglycerols can be 
directly separated by distillation providing a sufficiently low pressure 
(vacuum) is applied. 
Subsequent separation of the reaction mixture into the individual 
polyglycerols is not possible in the production of diglycerol by 
autocondensation of glycerol. Although there is no need for subsequent 
separation in certain known production processes (G. Jakobson, 
Fette-Seifen-Anstrichmittel 3(88) 1986, pages 101 to 106) these processes 
are attended by the disadvantage that either the starting substances are 
difficult to obtain or the synthesis involves several intermediate stages. 
In another known process for the production of polyglycerols by 
autocondensation of glycerol, glycerol is condensed in the presence of 
lithium hydroxide at temperatures of 200.degree. to 300.degree. C. and, 
more particularly, at 260.degree. C. The water of reaction is continuously 
distilled. Where a high glycerol content is required, the condensation 
reaction is terminated when the quantity of water theoretically required 
for the formation of diglycerol has been separated. The diglycerol may 
then be separated from the oligoglycerol mixture formed by distillation in 
a high vacuum (DE 41 24 665 A1). 
The polyglycerols, including in particular diglycerol, are used as starting 
materials for esterification and transesterification reactions with fatty 
acid and fatty acid esters. The esters of polyglycerols and of diglycerol 
offer an even greater range of variation in their properties than the 
simple glycerides and, accordingly, are used for a number of applications 
of which only two are to be mentioned. They are suitable as raw materials 
for the production of polymers, being incorporated for example in alkyd 
resins by condensation via the hydroxyl groups and representing 
polycondensation units of the type used in particular for the development 
of polyurethane foams (DE 41 24 665 A1). By virtue of its lipophilic and 
hydrophilic properties, esters of diglycerol are also widely used as an 
emulsifier in the cosmetics and food industry. 
The problem addressed by the present invention was to enable diglycerol to 
be economically produced in concentrations of more than 90% without 
significant losses of starting material, glycerol condensed in the 
presence of a basic catalyst being used as the starting material. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the invention, the solution to this problem is characterized 
in that the reaction is only carried out to a partial conversion of 10 to 
15% by weight of diglycerol in the reaction mixture and is terminated by 
cooling the reaction mixture to temperatures below 200.degree. C. In a 
first distillation stage, the reaction mixture is distilled in a 
wiped-film evaporator or short-path evaporator under a pressure of 0.5 to 
5 mbar, more particularly under a pressure of 1 to 2 mbar, and at a bottom 
temperature of 125.degree. to 170.degree. C., more particularly at a 
bottom temperature of 130.degree. to 140.degree. C. Wiped-film evaporators 
are evaporators in which a highly viscous, high-boiling mixture is applied 
to a heated wall on which it is mechanically distributed by rotating 
wiping elements. Thin continuous liquid layers or rather liquid films are 
thus formed. The film surfaces are continuously renewed so that local 
overheating is avoided. The vapors formed flow against the flow of product 
film and leave the evaporator in the externally arranged condenser. 
Wiped-film evaporators are generally operated under pressures of only a 
few mbar and the residence time for the product is only a few seconds. 
The short-path evaporator mentioned, which is also known as a molecular 
evaporator, is suitable for the distillation of even more highly boiling 
and temperature-sensitive products. Through a condenser built into the 
evaporator, it provides for operating or boiling pressures in the fine and 
high vacuum range (1 to 10.sup.-3 mbar or 10.sup.-3 to 10.sup.-5 mbar). 
After passing through the first distillation stage, the sump from the first 
distillation stage is distilled in a second, following distillation stage 
comprising a short path evaporator under a pressure of 0.05 to 0.3 mbar 
and, more particularly, under a pressure of 0.1 to 0.2 mbar and at a 
bottom temperature of 140.degree. to 170.degree. C. and, more 
particularly, at a bottom temperature of 145.degree. to 155.degree. C. so 
that a bottom product containing more than 90% by weight of diglycerol is 
obtained in this second distillation stage. 
Accordingly, the problem addressed by the present invention is solved by a 
coordinated combination of reaction management and product separation. The 
production of higher oligomers of glycerol, namely triglycerol, 
tetraglycerol, etc., is avoided by the special control of the reaction. It 
is consciously accepted that the reaction mixture has a relatively low 
content of diglycerol by comparison with known processes. This 
disadvantage is obviated by the following two distillation stages so that 
a diglycerol with a content of more than 90% is obtained as the end 
product. The problems of known processes, namely that both glycerol and 
higher oligomers of glycerol have to be removed to increase the diglycerol 
content and that, in addition, the formation of these higher oligomers 
affects the economy of the process, do not arise in the process according 
to the invention because the formation of the higher oligomers is avoided. 
The process may be carried out both discontinuously and continuously. In 
one preferred embodiment of the invention, the reaction is carried out 
continuously in a tube reactor. The catalyst (for example NaOH) is 
introduced in the form of a solution.

DETAIL DESCRIPTION OF THE INVENTION 
In the continuous embodiment, it is of advantage to ensure that no back 
mixing occurs in the reactor. Accordingly, it is proposed that the tube 
reactor should contain inert packings to suppress circulation flows, 
particularly where it is of relatively large diameter, for example more 
than 0.3 m in diameter. Alternatively to the homogeneously dissolved 
catalyst, it is also possible to use a catalyst-containing fixed bed. The 
fixed-bed catalyst may be, for example, an NaX zeolite, more particularly 
Wessalith P (a product of Henkel KGaA). In addition, it is of advantage in 
the continuous embodiment to dry the reaction mixture formed after the 
reaction, i.e. to remove the water of reaction, before the reaction 
mixture is subjected to the first distillation stage. In the discontinuous 
embodiment, a large part of the water of reaction is distilled off during 
the actual reaction. 
The process according to the invention may also be carried out 
discontinuously by replacing the tube reactor with a stirred tank reactor. 
The process is characterized in that, for the most part, only diglycerol, 
water of reaction and unreacted glycerol are formed. It is of advantage to 
return the distillate of the first distillation stage, which consists of 
substantially pure glycerol, to the reaction stage. However, this 
distillate may also be used as a starting substance in other processes. 
However, recycling to the reaction stage is particularly advantageous 
because, in this case, it is mainly only water of reaction and pure 
diglycerol that are formed over the process as a whole. Product losses are 
minimal. 
In order further to reduce product losses, it is of advantage to mix the 
distillate of the second distillation stage, which contains glycerol and 
diglycerol, with the feed of the first distillation stage and, in this 
way, to return it to the first distillation stage. 
The bottom product of the second distillation stage has a diglycerol 
content of more than 90% and a negligible glycerol content. In another 
advantageous embodiment of the process according to the invention, the 
purity of the product can be increased by distilling the bottom product of 
the second distillation stage in a short-path evaporator under a pressure 
of at most 0.05 mbar and at a temperature of 185.degree. to 215.degree. C. 
in a third distillation stage to obtain a distillate containing more than 
95% by weight of diglycerol. This fine distillation not only increases the 
diglycerol content, it also removes salts from the product and decolors 
the product. 
Embodiments of the invention are described in the following. 
The invention with its key features and most important advantageous 
embodiments is illustrated in the form of a flow chart in FIG. 1. After 
addition of the catalyst (sodium hydroxide) and glycerol, the condensation 
reaction is carried out to a partial conversion of 10 to 15% by weight. 
The reaction is terminated by cooling and, in the case of the continuous 
process, the reaction mixture is dried. The distillate 1 consisting almost 
solely of glycerol obtained in the following first distillation stage is 
returned to the reaction stage. The bottom product 1 obtained is separated 
into a distillate 2 and a bottom product 2 in a second distillation stage. 
The distillate 2 is returned to the first distillation stage. The bottom 
product 2 contains more than 90% of diglycerol and, in many cases, is the 
required end product. In other cases where an even higher diglycerol 
content is required, the bottom product 2 is separated into a distillate 
containing more than 95% of diglycerol and a residue (bottom product 3) in 
a third distillation stage. 
EXAMPLE 1 
(Discontinuous Reaction) 
Glycerol was reacted with the catalyst NaOH (used in a quantity of 0.5% by 
weight, based on the glycerol) in a stirred reactor at a temperature of 
230.degree. C. 
After a reaction time of 8 hours, the diglycerol content is 12.5% by 
weight. The triglycerol content is 1%. The percentage content of higher 
oligoglycerols is below the detection limit of 0.1% by weight. 
EXAMPLE 2 
(Continuous Reaction) 
Glycerol was reacted with the catalyst NaOH (used in a quantity of 0.35% by 
weight, based on glycerol) at 240.degree. C. in a tube reactor which was 
filled with inert packings to suppress back-mixing effects. 
A diglycerol content of 12% by weight was obtained; the triglycerol content 
was 0.8% by weight. Higher oligoglycerols could not be detected (&lt;0.1% by 
weight). 
EXAMPLE 3 
(Continuous Reaction, Heterogeneous Catalysis) 
Glycerol was reacted at 240.degree. C. in a fixed-bed reactor filled with 
an NaX catalyst (zeolite of the Wessalith P type). A diglycerol content of 
12.5% and a content of higher glycerols of 0.9% were obtained. 
EXAMPLE 4 
(Two-Stage Distillation) 
The reaction mixture of Example 1 was distilled in two stages. 
The first distillation stage was carried out in a short-path evaporator 
under a pressure of 1.2 mbar and at a temperature of 140.degree. C. The 
second distillation stage was also carried out in a short-path evaporator, 
but under a pressure of 0.1 mbar and at a temperature of 155.degree. C. 
The distillate of the second stage was continuously returned to the first 
distillation stage. The following concentrations were obtained: 
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Distillate Bottom product 
______________________________________ 
1st Stage &lt;0.5% by weight 
30-40% by weight 
diglycerol diglycerol 
2nd stage 15% by weight 90-91.5% by weight 
diglycerol diglycerol 
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EXAMPLE 5 
(3rd Distillation Stage) 
The distillation product (bottom product of the 2nd stage) of Example 4 was 
distilled in a short-path evaporator at a temperature of 205.degree. C. 
and under a pressure of 10.sup.-2 mbar. The distillate was 98% salt-free 
diglycerol. 
The loss (bottom product of the 3rd stage) in the form of residue (salt and 
polyglycerol) amounted to 25% by weight.