Process for the recovery of monoglycerides and diglycerides from a mixture containing monoglycerides, diglycerides, and triglycerides

Described is a process for the recovery of monoglycerides and diglycerides from a mixture containing monoglycerides, diglycerides and triglycerides by extraction with a supercritical extractant which comprises specific cosolvents. After the separation of the monoglycerides as bottom product in the separation column, the extract consisting of diglycerides and triglycerides is separated into diglycerides and triglycerides by fractionated pressure decrease and change of temperature in two consecutive regeneration columns. A portion of the product obtained in the first regeneration column is fed as reflux to the head of the separation column, and a portion of the product, obtained in the second regeneration column, is fed as reflux to the head of the first regeneration column for an improved separation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for the recovery of monoglycerides and 
diglycerides from a mixture containing monoglycerides, diglycerides, and 
triglycerides by extraction in countercurrent with a supercritical 
extractant. 
2. Brief Description of the Background of the Invention Including Prior Art 
Monoglycerides are partial esters of glycerol with higher molecular fatty 
acids. Commercially available monoglycerides consist of mixtures of 
monoesters and diesters with minor additions of triesters, and they may be 
obtained by transesterification of triglycerides with glycerol or by 
reaction of glycerol with fatty acids. 
Monoglycerides possess emulsifying, stabilizing, plasticizing and 
thickening properties. As the monoesters and diesters of glycerol are 
edible, they are used in various fields of the foodstuff industry, 
pharmaceutical industry, and in cosmetics. Normally, monoglycerides are 
water/oil emulsifiers, but by minor additions of, for example soaps, 
polyethylene oxide compounds, sulfated alcohols, they become 
"self-emulsifying" and are good oil/water emulsifiers under these 
conditions. Depending on the additive, they are able to form acid-stable 
and electrolyte-stable emulsions. The monoglycerides of higher fatty 
acids, such as, for example, glycerol monostearate, are used as lubricants 
in the processing of plastic materials. Monoglycerides obtained by 
molecular distillation and having a monoglyceride content of more than 90% 
are mainly used in the foodstuff industry, such as in farinaceous 
products, sweets and baking additives, margarine, ice cream. 
By the addition of monoglyceride, up to 5% of palmitic acid/stearic 
acid-monoglycerides with a purity of 90% or of 10% palmitic acid/stearic 
acid-mono/diglyceride, self-emulsifying properties of the shortenings 
intended for baking are achieved (superglycerolated shortenings). 
The term "shortening" expressis verbis means to shorten something and is 
derived from baking properties. Due to the specific structure, 
monoglycerides are able to change the plasticizing action of starch and 
gluten in the preparation of dough by entering finely divided into the 
homogenous plasticized material, thus breaking up the materials and 
interrupting them to make the dough more smoother, i.e. shorter. At the 
same time the incorporation of air is facilitated so that, altogether, 
bakery products are obtained with increased volume and improved 
"shortness". 
The essential components of margarine are edible fats and oils, drinking 
water, emulsifiers. As emulsifiers, lecithin, the egg yolk and /or 
monoglycerides and diglycerides of edible fatty acids may be used. 
Furthermore, the margarine may contain aroma finishing additives, sour 
milk, buttermilk, skim milk, fermented milk, cooking salt, starch sirup, 
citric acid and/or other edible acids, vitamins as well as permissible 
certified preservatives and certified dyestuffs (usually carotine or 
carotine-containing oils). Emulsifiers are obligatory auxiliary products 
in the preparation of margarine, because they enable the formation of 
water in oil emulsions. The most common ones are monoglycerides and plant 
lecithin assisting each other in the emulsifying action. Used in practice 
are products having about 40 and about 90%, respectively, of 
monoglycerides of C.sub.16 /C.sub.18 acids (palmitic acid, stearic acid, 
also in mixture with oleic acid) which, in addition, contain 60 and 10%, 
respectively, of diglycerides. Additions of up to 0.5 and 0.25%, 
respectively, corresponding to about 0.2% of monoglyceride, based on the 
fatty phase, are common. In the preparation of low-calorie margarine, 
higher proportions of emulsifier are usually necessary. 
The monoglycerides and diglycerides are obtainable by esterification of 
glycerol with fatty acids. Another route is the transesterification of 
triglycerides with glycerol. Furthermore, an enzymatic cleavage of 
triglycerides has lately found acceptance in technology. All methods 
result in a blend of monoglycerides, diglycerides, and triglycerides. 
During the esterification, an equilibrium mixture of about 60% of 
monoglycerides, 35% of diglycerides, and 5% of triglycerides is finally 
generated. The mixture is separated by molecular distillation. A 
disproportionation occurs to a minor extent at high temperatures in the 
film evaporator so that the monoglycerides or diglycerides contain small 
amounts of the other two esters and minor amounts of free fatty acids. 
Because of the disproportionation at the temperatures prevailing in the 
film evaporator, monoglycerides having contents of more than 95% may not 
be obtained by molecular distillation at ambient pressure in an 
economically feasible manner. However, there exists a great interest in 
monoglycerides possessing a purity of 99% and more. 
This is surprisingly achieved by the process according to this invention by 
means of the extraction with a supercritical extractant consisting of a 
highly volatile hydrocarbon having 2 to 6 carbon atoms and e.g. carbon 
dioxide and/or N.sub.2 O. 
It is known that monoglycerides may be removed from a mixture of 
monoglycerides, diglycerides, and triglycerides by means of dense carbon 
dioxide. However, pressures of more than 350 at. at temperatures of 
40.degree. C. are necessary for this. Furthermore, the loading of dense 
carbon dioxide even at pressures of 350 at is still so small (less than 
1%) that an economical recovery of monoglycerides of high purity is not 
possible. 
Also suggested was the use of, for example, acetone as entrainer (DE-OS 
2,340,566.5). By this the monoglycerides arrive at the head product during 
the countercurrent extraction as they are the more soluble components. 
However, the separation factors are relatively low, so that the recovery 
of pure monoglycerides is economically not interesting, all the more so, 
as the achieved loadings of 1 to 1.5% are small. Furthermore, the removal 
of the entrainer acetone from the product is cumbersome. 
SUMMARY OF THE INVENTION 
1. Purposes of the Invention 
It is an object of the invention to provide a process for an economical 
recovery of monoglycerides and diglycerides. 
It is a further object of the invention to provide an economical process 
for the recovery of monoglycerides of high purity. 
These and other objects and advantages of the present invention will become 
evident from the description which follows. 
2. Brief Description of the Invention 
The present invention for a process for the recovery of monoglycerides and 
diglycerides from a mixture containing monoglycerides, diglycerides and 
triglycerides includes the following steps: A mixture containing 
monoglycerides, diglycerides, and triglycerides is extracted in a 
countercurrent system with a supercritical extractant, where the 
supercritical extractant contains as cosolvent a hydrocarbon in an amount 
of 30 to 90 weight-percent. The extractant is maintained in a 
supercritical state during the extraction process. 
A hydrocarbon in an amount of from about 40 to 80 weight-percent can be 
employed as a cosolvent. 
The supercritical extractant can be furnished by a member selected from the 
group consisting of carbon dioxide, nitrogen monoxide N.sub.2 O, 
sulfurhexafluoride, trifluoromethane, tetrafluoromethane, and mixtures 
thereof. 
Hydrocarbons employed can contain from about 2 to 6 carbon atoms. 
Carbon dioxide CO.sub.2 can be used as an extractant and propane or butane 
can be used as cosolvent. 
The extraction can be performed at a temperature in the range of from about 
10.degree. to 150.degree. C. and preferably in the range of from about 
20.degree. to 80.degree. C. 
The extraction can be performed at pressures of from about 60 to 200 at., 
and preferably of from about 80 to 150 at. 
The monoglycerides can be separated in the separation column for 
withdrawing the monoglycerides as a bottom product. An extract comprising 
diglycerides and triglycerides can be separated into diglycerides and 
triglycerides by fractionated pressure decrease and change of temperature 
in two regeneration columns. 
A portion of the product, obtained in the first regeneration column, can be 
fed as reflux to the head of the separation column for an improved 
separation. A portion of the product, obtained in the second regeneration 
column, can be fed as reflux to the head of the first regeneration column. 
The novel features which are considered as characteristic for the invention 
are set forth in the appended claims. The invention itself, however, both 
as to its method of operation, its products and physical requirements, 
together with additional objects and advantages thereof, will be best 
understood from the following description of specific embodiments and 
examples. 
DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENT 
In accordance with the present invention, there is provided a process for 
the recovery of monoglycerides and diglycerides from a mixture containing 
monoglycerides, diglycerides and triglycerides by extraction in 
countercurrent direction with a supercritical extractant. The 
supercritical extractant contains as cosolvent a hydrocarbon in an amount 
of 30 to 90 weight-percent. The extractant remains in the supercritical 
state during the cyclic process. 
Carbon dioxide N.sub.2 O, sulfurhexafluoride, trifluoromethane, 
tetrafluoromethane, or their mixture can be used as supercritical 
extractant. 
Hydrocarbons having 2 to 6 carbon atoms, or their mixtures can be used as 
cosolvent. 
Carbon dioxide CO.sub.2 can be used as extractant, and propane or butane 
can be used as cosolvent. 
A hydrocarbon in an amount of 40 to 60 weight-percent can be employed as a 
cosolvent. The extraction can be carried out in a temperature range of 
from 10.degree. to 150.degree. C., and preferably of from 20.degree. to 
80.degree. C., and at pressures of from 60 to 200 at., and preferably of 
from 80 to 150 at. 
After the separation of the monoglycerides as bottom product in the 
separation column, the extract consisting of diglycerides and 
triglycerides can be separated into diglycerides and triglycerides by 
fractionated pressure decrease and change of temperature in two 
consecutive regeneration columns. 
For an improved separation, a portion of the product obtained in the first 
separator, can be fed as reflux to the head of the separation column, and 
a portion of the product, obtained in the second regeneration column, can 
be fed as reflux to the head of the first regeneration column. 
The present invention discloses a process for the recovery of 
monoglycerides and diglycerides from a mixture containing monoglycerides, 
diglycerides, and triglycerides by extraction in countercurrent by a 
supercritical extractant, which process is characterized in that a 
supercritical extractant contains a hydrocarbon in an amount of 30 to 90 
weight-percent, preferably 40 to 80 weight-percent, as a cosolvent, and 
wherein the extractant remains in the supercritical stage during the 
cyclical process. 
Contrary to the process described in DE-OS 2,340,566.5, in the process of 
this invention, the monoglycerides do not pass to form a head product but 
they appear as bottom product of a separation column. The extractant at 
the head of the column contains the diglycerides and triglycerides as 
extracted material which may subsequently be separated from the extractant 
by fractionated separation. Two regeneration columns may be used for the 
fractionated separation. If there is no fractionated separation of the 
diglycerides and triglycerides, one regeneration column is sufficient. 
By using for example carbon dioxide and/or N.sub.2 O as supercritical 
extraction medium and by using low molecular hydrocarbons, such as ethane, 
propane, butane, as cosolvents, high separation factors may be achieved 
according to the process of the invention together with a high loading of 
the extractant. In this context, substances are termed to be in the 
supercritical state, the temperature of which at the respective pressures 
is higher than the critical temperature and, respectively, the pressure of 
which at the respective temperature is higher than the critical pressure. 
This pressure and temperature region corresponds to the hatched area of 
FIG. 1),, i.e. the area wherein at least either the pressure is higher 
than the critical pressure or the temperature is higher than the critical 
temperature in the phase diagram outside of the solid state area. While 
only insignificant effects have been observed when adding 10 
weight-percent of the cosolvent, the separation factors surprisingly 
passes over a maximum value in the concentration range of 30 to 90 
weight-percent of the cosolvent. In this range, the concentration of 
glycerides of 2 to 12 weight-percent, obtainable in the extractant, is so 
high that an economical operation is possible. A loading of the extractant 
of 6 weight-percent at 40.degree. C. is obtained even at the low pressures 
of 120 at. With pure hydrocarbons, the separation factors are again very 
low, i.e. almost 1. 
The extractant, for example consisting of CO.sub.2 and the cosolvent, for 
example, propane, flows from bottom to top in the separation column. The 
mixture to be separated is fed about in the middle or at the head of the 
separation column, and the liquid phase flows downward in a 
countercurrent. On the way down, the latter becomes depleted of 
diglycerides and triglycerides until, finally, a bottom product, having a 
monoglyceride content of more than 99%, is obtained. The extractant, which 
leaves the head of said column, contains, apart from a minor residue of 
monoglycerides, the portions of diglycerides and triglycerides present in 
the feed. By a stepwise pressure decrease in two consecutive regeneration 
columns, the diglycerides may be preferably removed in the first 
regeneration column and, in the second regeneration column, the 
triglycerides may preferably be separated from the recycled extractant. A 
portion of the glycerides separated in the first regeneration column is 
introduced into the separation column as reflux, and the remainder is 
withdrawn as product. The glycerides which are optionally separated in the 
second regeneration column are partially returned to the head of the first 
regeneration column as reflux, and the remainder is withdrawn as further 
product which essentially contains only triglycerides. The product 
withdrawn from the first regeneration column contains the diglycerides. 
According to the process of this invention it is thus possible to separate 
in one operational step the mixture of monoglycerides, diglycerides, and 
triglycerides into three fractions which contain the monoglycerides, the 
diglycerides, and the triglycerides, respectively, in high purity. 
With the help of the schematic illustration in FIG. 2, an embodiment of the 
process of the present invention is shown in detail: The device consists 
of three columns of which one, i.e. the separation column, serves to 
separate the monoglycerides from a glyceride mixture. The monoglycerides 
are obtained as bottom product (point A). The regeneration columns I and 
II serve to remove the extracted components from the cycle gas. 
The glyceride mixture is introduced into the separation column at the 
intermediate part (point F). The circulating extractant is preferably 
loaded with diglycerides and triglycerides in the separation column. At 
the same time, the extractant dissolves in the liquid phase which gets 
enriched in monoglycerides and flows downward. 
The extractant loaded with the more soluble components diglycerides and 
triglycerides leaves the rectyfying section of separation column at the 
head (point B). The loaded extractant is pressure-released via a 
pressure-reducing valve 1 to arrive at the regeneration column I. Pressure 
and temperature in regeneration column I are chosen so that preferably 
diglycerides and the small remainder of monoglycerides are separated. The 
condensed phase is withdrawn from bottom (C) of regeneration column I and 
divided into product P.sub.RI and reflux R.sub.RI. The recycled stream is 
introduced at the head of separation column after having passed the heat 
exchanger WT1. The extractant stream, loaded with triglycerides from 
regeneration column I, is pressure-released via the pressure-reducing 
valve 2 to arrive at the regeneration column II. In the regeneration 
column II, pressure and temperature are chosen so that the extractant 
leaving the column is free of low volatile components. The condensed phase 
is withdrawn from the bottom (D) of regeneration column Il and divided 
into product P.sub.RII and reflux R.sub.RII. After having passed the heat 
exchanger WT2, the reflux from regeneration column II is introduced at the 
head of regeneration column I. The recycle system enables the 
fractionation of diglycerides and triglycerides. From regeneration column 
I, a diglyceride-rich bottom product is obtained, while from regeneration 
column II, a triglyceride-rich bottom product is withdrawn. 
The resulting bottom products are continuously withdrawn and 
pressure-released and collected in containers. The amounts of gaseous 
solvents escaping from the solution are returned into the cycle via a 
compressor K. The regenerated cycle gas, having left the regeneration 
column I at point E and having passed the heat exchanger WT3, is again fed 
into separation column by means of a compressor K. 
In the fractionated separation of components dissolved in the extractant 
when operating isothermally, the pressure in the first regeneration column 
is decreased by about 10 to 60 bar, preferably 20 to 50 bar, compared to 
the pressure in the separation column. 
The fractioned separation in the first regeneration column, however, can 
also be achieved by isobar operation. In this case, the temperature in the 
regeneration column is increased by 10.degree. to 80.degree. C., 
preferably by 20.degree. to 50.degree. C., compared to the temperature in 
the separation column. A combination of pressure decrease and temperature 
increase is also possible. 
The conditions of separation in the second regeneration column correspond 
to the conditions of the regeneration of the extractant without any 
fractionation. The regeneration of the extractant is effected either by 
pressure release alone or by pressure release together with simultaneous 
temperature increase. The pressure during the regeneration is 30 to 80 bar 
in a temperature range of 40.degree. to 120.degree. C.

EXAMPLE 1 
A glyceride mixture consisting of 60.9 weight-percent of monoglycerides, 
35.3 weight-percent of diglycerides, 3.4 weight-percent of triglycerides 
of oleic acid and 0.4 weight-percent of free acids was pumped into the 
middle of a separation column at a temperature of 40.degree. C. and a 
pressure of 120 bar and subjected to extraction. As extractant a mixture 
of 42 weight-percent of carbon dioxide and 58 weight-percent of propane 
was used, which mixture is supercritical under the operation conditions, 
i.e. exists and forms in one single phase. The extractant was loaded with 
3.1 weight-percent of glycerides. The column was 4 m high and contained a 
Sulzer wire-net packing CY. The extractant stream leaving the head of the 
separation column was transferred into the regeneration column I. The 
operational conditions in the regeneration column I were 60.5 bar and 
104.degree. C. By simultaneous heating to 104.degree. C. and pressure 
decrease to 60.5 bar, the loading of the extractant decreased to 0.08 
weight-percent. The thus regenerated extractant was cooled to 40.degree. 
C. and returned to the bottom of the separation column by means of a cycle 
compressor. A portion of the product separated in the regeneration column 
I was fed to the head of the separation column as reflux. The obtained 
bottom product contained 98.4 weight-percent of monoglycerides, 1.3 
weight-percent of diglycerides and 0.3 weight-percent of free fatty acids. 
The concentration of triglycerides was below the detection limit of 0.005 
weight-percent. The head product obtained in the regeneration column I 
contained 31.4 weight-percent of mono-glycerides, 59.8 weight-percent of 
diglycerides, 8.9 weight-percent of triglycerides and 0.4 weight-percent 
of free fatty acids. There was used only one regeneration column, because 
there was no fractionated separation of diglycerides and triglycerides. 
EXAMPLE 2 
The same starting product as in example 1 was employed and extracted in a 
separation column at 20.degree. C. and 120 bar by the same extractant as 
in example 1. The separation column, however, was only 2 m high. The 
regeneration of the extractant was effected at 98.degree. C. and 57 bar. 
The loading of the extractant in the separation column was 3 
weight-percent. With the high solvent ratio of 30, the following products 
were obtained: bottom product consisting of 99.5 weight-percent of 
monoglycerides, 0.3 weight-percent of diglycerides and 0.2 weight-percent 
of free fatty acids; head product consisting of 39.1 weight-percent of 
monoglycerides, 52.1 weight-percent of diglycerides, 8.4 weight-percent of 
triglycerides and 0.4 weight-percent of free fatty acids. 
EXAMPLE 3 
A glyceride mixture consisting of 53 weight-percent of monoglycerides, 31 
weight-percent of diglycerides, and 13 weight-percent of triglycerides of 
stearic acid, and 3 weight-percent of free fatty acids was fed to the 
middle of a separation column at 140 bar and 50.degree. C. A portion of 
the product separated in the regeneration column I was fed to the head of 
the separation column as reflux. A mixture of 70 weight-percent of propane 
and 30 weight-percent of carbon dioxide was used as extractant. The 
stearic acid glyceride mixture, which is present as a solid under normal 
conditions, is liquid under the mentioned operational conditions, the 
extractant mixture is supercritical, i.e. there is one phase. The height 
of the column was 8 m. Sulzer packings, type CY, were used. The 
regeneration was effected at 105.degree. C. and 55 bar. The loading of the 
extractant with glycerides was 4 weight-percent in the separation column. 
With a solvent ratio of 30, the following products were obtained: bottom 
product consisting of 99.0 weight-percent of monoglycerides, 0.4 
weight-percent of diglycerides, and 0.6 weight-percent of free fatty 
acids; head product consisting of about 5 weight-percent of 
monoglycerides, 70 weight-percent of diglycerides, 20 weight-percent of 
triglycerides and 5 weight-percent of free fatty acids. 
EXAMPLE 4 
A glyceride mixture of 57.5 weight-percent of monoglycerides, 36 
weight-percent of diglycerides, 6.15 weight-percent of triglycerides, and 
0.35 weight-percent of free fatty acids, the glycerol esters being mainly 
derived from fatty acids with 18 carbon atoms (content of C.sub.14 acids 
4% of C.sub.16 acids 8%), was introduced into the middle of a separation 
column at 120 bar and 40.degree. C. As extractant, a mixture of 57 
weight-percent of propane and 43 weight-percent of carbon dioxide was 
used. The column was 16 m high; Sulzer packings, type CY, were also used. 
The extractant, with a head loading of difficultly volatile or low 
volatile components of about 10 weight-percent was partially 
pressure-released to 80 bar at 40.degree. C. in the first regeneration 
column. By this, the diglycerides and the minor remainder of 
monoglycerides were separated from the solution with preference, while the 
triglycerides remained dissolved. The extractant, which was loaded mainly 
with triglycerides, was subjected to a further pressure release to 50 bar 
in a second regeneration column, the temperature being raised to 
120.degree. C. At these conditions, the extractant loses its solvent 
capacity for difficultly volatile products almost completely. As bottom 
product a triglyceride-rich mixture was obtained. The regenerated 
extractant was again fed to the bottom of the separation column. To 
increase the diglyceride concentration in the bottom of the first 
regeneration column and to minimize losses of monoglyceride, a portion of 
the condensate from the first regeneration column was fed to the head of 
the separation column as reflux. In analogous manner, a portion of the 
condensate from the second regeneration column, was fed to the head of the 
first regeneration column as reflux in order to increase the diglyceride 
concentration in the bottom of the first regeneration column. With a 
solvent ratio of about 30, the following products were obtained: 
Separation column: Bottom product consisting of 99.2 weight-percent of 
monoglyceride, 0.4 weight-percent of diglyceride, and 0.4 weight-percent 
of free fatty acids. First regeneration column: Bottom product consisting 
of 95 weight-percent of diglyceride, 1 weight-percent of monoglyceride, 
3.7 weight-percent of triglyceride, and 0.3 weight-percent of free fatty 
acids. Second regeneration column: Bottom product consisting of about 90 
weight-percent of triglyceride, g weight-percent of diglyceride, 0.8 
weight-percent of monoglyceride, and 0.2 weight-percent of free fatty 
acids. 
It will be understood that each of the steps, conditions and reagents 
described above, or two or more together, may also find a useful 
application in other types of reactions, recovery procedures and products 
differing from the types described above. 
While the invention has been illustrated and described as embodied in the 
context of a process for the recovery of monoglycerides and diglycerides 
from a mixture containing monoglycerides, diglycerides, and triglycerides, 
it is not intended to be limited to the details shown, since various 
modifications and structural changes may be made without departing in any 
way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.