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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to an industrially useful process for
preparing sucrose fatty acid esters. More particularly, the present
invention relates to a process for preparing sucrose fatty acid esters in
an aqueous system through the whole steps, including reaction step of
sucrose and fatty acid alkyl esters and purification step for the product,
without using an organic solvent, wherein the unreacted sucrose can be
recovered in high yield.
Sucrose fatty acid esters (sugar esters) useful as surface active agents
are prepared industrially at present by either a solvent process wherein
sucrose is reacted with a methyl ester of a higher fatty acid having 8 to
22 carbon atoms in the presence of a suitable catalyst in an organic
solvent such as dimethylformamide or dimethylsulfoxide, as disclosed in
Japanese Patent Publication Kokoku No. 35-13102; or an aqueous medium
process wherein sucrose is formed into a molten mixture with a fatty acid
salt (soap) using water without using an organic solvent, and is then
reacted with a higher fatty acid methyl ester in the presence of a
catalyst, as disclosed in Japanese Patent Publication Kokoku No. 51-14485.
However, even according to any of these processes, the obtained reaction
mixture contains impurities such as the unreacted sucrose, the unreacted
fatty acid methyl ester, residual catalyst, soap, free fatty acid and the
like in addition to the desired sucrose fatty acid ester. These
impurities, at least impurities whose contents exceed the specified
amounts must be removed prior to being put on the market. Particularly, in
case of sucrose fatty acid esters used as a food additive which requires a
high purity, removal of high boiling polar solvents such as DMF remaining
in the product produced by the solvent process is very important in view
of recent strict legal regulation, but it requires complicated procedures.
The aqueous medium process has no problem of contamination of the product
with the reaction solvent, but it still requires the purification
treatment since large amounts of impurities are included in the reaction
mixture.
In general, the conversion of sucrose is low. For example, in case of the
process using dimethylformamide as the reaction medium, the conversion is
at most 50%. Accordingly, recovery of the unreacted sucrose is also
important.
In order to remove the impurites and to recover the unreacted sucrose from
the reaction mixture (namely crude sucrose fatty acid esters), various
processes for the purification of crude product have hitherto been
proposed. These purification processes usually require a large amount of
organic solvents, e.g. butanol, toluene and methyl ethyl ketone. However,
in the production of sucrose fatty acid esters on an industrial scale, the
use of a large amount of organic solvents has the following disadvantages:
(1) risk of explosion and fire, (2) provision of explosion and fire
prevention means to electric devices, (3) application of closed system to
production equipment for explosion and fire prevention, (4) requirement of
fireproof construction for entire building by way of precaution against
explosion and fire, (5) rise in fixed cost due to the items (2), (3) and
(4), (6) rise in materials cost due to loss of solvent, (7) contamination
of the product with remaining solvent, and (8) adverse influence on health
of workers, and increase of cost resulting from increase in labor required
for the prevention therefor.
The disadvantages resulting from the use of organic solvents are an
obstacle particularly to the production of sucrose fatty acid esters on an
industrial scale. In view of these circumstances, it has been desired to
develop a purification technique capable of removing the unreacted sucrose
and other impurities from the crude reaction mixture without using organic
solvents.
Thus, purification processes using no organic solvent have hitherto been
proposed. For example, as representative methods, there have been known
(1) a method wherein a sucrose fatty acid ester is precipitated by
addition of an acidic aqueous solution to the reaction mixture, as
disclosed in British Pat. No. 809,815 and (2) a method wherein a sucrose
fatty acid ester is precipitated by addition of an aqueous solution of a
common neutral salt to the reaction mixture, as disclosed in Japanese
Patent Publication Tokkyo Kokoku No. 42-8850.
However, these methods have disadvantages. When an acidic aqueous solution,
for example, hydrochloric acid, is added to the reaction mixture as in the
method (1), the sucrose fatty acid ester immediately deposits, but the
unreacted sucrose is easily decomposed and converted into glucose and
fruit sugar. This cannot be avoided even if the addition is conducted at a
low temperature (e.g. 0.degree. to 5.degree. C.). Accordingly, the
recovery and reuse of the unreacted sucrose are difficult.
The addition of an aqueous solution of a neutral salt such as sodium
chloride or Glauber's salt, as in the method (2), causes sucrose fatty
acid esters to deposit rapidly. In this case, decomposition of unreacted
sucrose does not occur, but the monoester which is an effective component
in the product is dissolved in an aqueous phase. Consequently, not only
the dissolution results in a large loss of the product, but also it is a
hindrance particularly to production of sucrose fatty acid esrers having a
high HLB which are recently in great demand. Usually, the sucrose esters
have an HLB value of 1 to 20, and the larger the HLB value, the higher the
hydrophilic property.
In order to industrially realize the purification of crude sucrose fatty
acid esters using water, it is also important to give consideration to
recovery of the unreacted sucrose, and drying of wet product incident to
the use of water as a purification solvent.
Since the purification of the reaction mixture with the use of water is
based on difference in water solubility between a sucrose fatty acid ester
and unreacted sucrose, migration of a large amount of unreacted sucrose
into an aqueous phase cannot be avoided. The manufacture of sucrose fatty
acid esters cannot be industrially accepted unless such a dissolved
sucrose is recovered. Accordingly, it is very important to efficiently
recover the sucrose which has transferred into an aqueous phase upon
purification.
The water-containing sucrose fatty acid ester which has been separated from
the reaction mixture and to be dried, is usually in the form of an aqueous
solution when the water content is over 80% by weight, and is in the form
of a slurry when the water content is less than 80% by weight. In general,
an aqueous solution of a sucrose fatty acid ester shows a peculiar
viscosity behavior such that the viscosity rapidly increases from about
40.degree. C., reaches maximum at about 50.degree. C. and rapidly drops
over 50.degree. C. Some problems are encountered in removing water from
the sucrose fatty acid ester in the form of an aqueous solution or slurry.
The evaporation of water by heating under vaccum, for example, using a
usual agitated vacuum dryer, is practically difficult because of marked
foaming. In particular, due to the property of sucrose fatty acid ester
that the softing point or melting point is low (for example, sucrose
monostearate having a melting point of about 52.degree. C., and sucrose
distearate having a melting point of about 110.degree. C.), the sucrose
fatty acid ester itself tends to be hydrated at the final stage of
evaporation of water This makes the dehydration more difficult. Moreover,
when the evaporation is conducted at a high temperature and the contacting
time with a heating source is long, not only the sucrose fatty acid ester
is decomposed, resulting in marked coloration or caramel formation, but
also the acid value is raised by free fatty acid formed by decomposition,
as disclosed in Japanese Patent Publication Tokkyo Kokoku No. 37-9966. In
addition, it is also a cause which make the drying difficult that the
latent heat of evaporation of water is very high (more than 500 kcal/kg
H.sub.2 O) and the evaporation temperature is high.
Other usual drying methods are also not suitable for preparing dry sucrose
fatty acid esters. For example, in case of using a flash dryer wherein a
slurry is continuously heated, fed to a vacuum chamber and released
thereto, various difficulties are encountered when a sufficient drying is
desired because of a large latent heat of water. Even if these
difficulties are overcome, the sucrose ester dehydrated and dried under
vaccum is in the molten state and, therefore, it requires a pulverization
step after taking out of the drier and cooling to less than the melting
point to solidify, for instance, by blowing a cold air. In addition to
many steps being required, there is a risk of dust explosion in the final
pulverization step.
Accordingly, it is also important to solve the problems encountered by
drying in realizing the purification of sucrose fatty acid esters using
water as the purification solvent.
It is a primary object of the present invention to provide a process for
preparing a purified sucrose fatty acid ester without using organic
solvents in both the reaction step and the purification step, which is
suitable for the production of the sucrose ester on an industrial scale.
A further object of the invention is to provide a process for recovering a
sucrose fatty acid ester free from organic solvents from the crude
reaction mixture, with recovery of unreacted sucrose in high yield.
A still further object of the invention is to provide an industrially
useful process for purifying a sucrose fatty acid ester using water as the
purification solvent without substantial loss of the sucrose fatty acid
ester and sucrose.
Another object of the invention is to provide a process for preparing a dry
powder of a highly pure sucrose fatty acid ester having a high HLB with
ease and without deteriorating the quality in the drying step, while
recovering the unreacted sucrose.
Still another object of the invention is to provide a process for preparing
a dry powder of a highly pure sucrose fatty acid ester having a low HLB
with ease and without deteriorating the quality in the drying step.
These and other objects of the present invention will become apparent from
the description hereinafter.
SUMMARY OF THE INVENTION
The present inventors have made experiments about salting out in the
purification of crude product using water as the purification medium in
order to achieve the following purposes: namely (1) minimizing the amount
of sucrose fatty acid esters dissolved in an aqueous phase, (2) preventing
decomposition of unreacted sucrose, (3) purifying the precipitated sucrose
fatty acid esters and forming a dry powder thereof, and (4) efficiently
recovering the unreacted sucrose from the filtrate (or supernatant)
obtained by removing the above-mentioned precipitate.
It has been found that when a neutral salt is dissolved in an aqueous
solution of the reaction mixture obtained by the reaction of sucrose and a
fatty acid alkyl ester in an aqueous system, a large portion of the
sucrose fatty acid esters is precipitated under a proper combination of
pH, temperature, concentration of neutral salt and amount of water, and
moreover, a salt derived from the reaction catalyst is included in the
aqueous phase with the unreacted sucrose. Thus, on the basis of this
discovery, it has now been found that the unreacted sucrose and the
catalyst-derived salt can be separated from the sucrose fatty acid esters,
without substantial loss of sucrose fatty acid esters, by repeating the
salting out procedure, namely by dissolving the precipitated sucrose fatty
acid esters again in water and repeating the precipitation procedure by
the addition of an aqueous solution of the neutral salt, and that the
unreacted sucrose can be efficiently recovered from the residual liquid
after removal of the above precipitate by contacting it with an adequate
reverse osmosis membrane
It has been further found that sucrose fatty acid esters having a high HLB
included in the precipitate is transferred into an aqueous phase by
washing the precipitate with an acidic water having an appropriate pH,
while leaving sucrose fatty acid esters having a low HLB as the solid, and
that the recovery of sucrose fatty acid esters having a high HLB
transferred into the aqueous phase, which has not been achieved by
conventional processes, can be made on an industrial scale by means of
ultrafiltration to give an aqueous solution of purified sucrose esters and
from which a powder can be obtained without deterioration of the quality
by spray drying, while the sucrose fatty acid esters having a low HLB can
be obtained in the form of a dry powder from the solid remaining after the
washing treatment with an acidic water by spray drying the solid in the
form of a slurry.
In one of the aspects of the present invention, there is provided a process
for preparing a powder of sucrose fatty acid esters having a high HLB,
which comprises reacting sucrose with a fatty acid alkyl ester in an
aqueous reaction system containing a catalyst, adjusting the resulting
reaction mixture to a neutral pH region, adding water and a neutral salt
to the reaction mixture to precipitate the sucrose fatty acid ester
product, separating the resulting precipitate, washing the precipitate
with an acidic water, subjecting the washing liquid to ultrafiltration and
spray drying the resulting concentrate in the form of an aqueous solution.
In another aspect of the present invention, there is provided a process for
preparing a powder of sucrose fatty acid esters having a low HLB, which
comprises reacting sucrose with a fatty acid alkyl ester in an aqueous
reaction system containing a catalyst, adjusting the resulting reaction
mixture to a neutral pH region, adding water and a neutral salt to the
reaction mixture to precipitate the sucrose fatty acid ester product,
separating the resulting precipitate from the aqueous phase, washing the
precipitate with an acidic water, neutralizing the washed precipitate and
spray-drying it.
In still another aspect, from the aqueous phase obtained by separation of
the precipitate after salting out, sucrose is recovered by subjecting the
aqueous phase to reverse osmosis.
Thus, according to the present invention, from the reaction mixture, it is
now possible (1) to remove the impurities, (2) to recover unreacted
sucrose, (3) to obtain a powder of purified sucrose fatty acid esters
having a high HLB and (4) to separate SE product into sucrose fatty acid
esters having a high HLB and those having a low HLB, without using organic
solvents on an industrial scale.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a triangular graph showing the liquid-cake equilibrium
relationship between the distribution of sucrose fatty acid esters (having
a high HLB) being dissolved in aqueous phase and (having a low HLB) being
in the precipitated cake.
DETAILED DESCRIPTION
(Synthesis of sucrose fatty acid esters by aqueous medium process)
In the present invention, sucrose fatty acid esters (hereinafter referred
to as "SE") are prepared from sucrose and a fatty acid alkyl ester by a
known aqueous medium process. For example, sucrose is formed into a molten
mixture with a fatty acid soap in the presence of water and is reacted
with a higher fatty acid alkyl ester such as methyl ester in the presence
of a catalyst such as potassium carbonate, as disclosed in Japanese Patent
Publication Tokkyo Kokoku No. 51-14485. The production of SE by the
aqueous medium process is advantageous in that the reaction mixture does
not contain an organic solvent as used in the reaction according to the
solvent process, though the reaction mixture contains a larger amount of
the soap as compared with the solvent process.
In general, the reaction mixture obtained by the aqueous medium process has
approximately the following composition.
______________________________________
Ingredients % by weight
______________________________________
SE 15 to 74
Unreacted sucrose 1.0 to 80
Unreacted fatty acid methyl ester
0.5 to 10
Salt derived from K.sub.2 CO.sub.3
0.05 to 7
Soap 10 to 50
Fatty acid 0.5 to 10
______________________________________
In that case, the proportion of the monoester in the SE is from 10 to 75%
by weight and the proportion of di- and higher esters is from 90 to 25% by
weight.
Also, the acid radical mainly included in each of the fatty acid methyl
ester, soap and fatty acid is usually a saturated acid radical having 16
to 22 carbon atoms common to them.
(Addition of water and neutralization)
To the reaction mixture which has the above-mentioned composition, water is
added in a water/reaction mixture ratio of 5:1 to 40:1 by weight,
preferably 20:1 by weight, while the pH is adjusted to 6.2 to 8.2,
preferably 7.5.
When the ratio of water to the reaction mixture is less than 5, the
viscosity of the obtained aqueous solution is high and the following
procedures become difficult. Also, when water is added in excess to the
reaction mixture such that the weight ratio of water to the reaction
mixture exceeds 40, the viscosity of the obtained aqueous solution is low
and accordingly the following procedures become easy, but a large energy
cost is required in removing water upon recovery of unreacted sucrose,
etc., thus the economy is impaired.
In order to prevent decomposition of the desired SE, it is preferable to
adjust the aqueous solution of the reaction mixture to pH of 6.2 to 8.2.
When the pH is more than 8.2, there is a possibility that SE is
quantitatively hydrolyzed by an alkali. On the other hand, even in a weak
acidic region of less than pH 6.2, there is a fear of acid hydrolysis of
SE, for example, when it is exposed to a high temperature over 90.degree.
C.
(Salting out)
A neutral salt is added to the thus pH-adjusted aqueous solution of the
reaction mixture, preferably with keeping at a temperature of 50.degree.
to 80.degree. C. in order to salt out SE rapidly.
Any of neutral salts can be used so long as they are soluble in water and
nontoxic. Representative examples of the neutral salt are, for instance,
sodium chloride, Glauber's salt (Na.sub.2 SO.sub.4.10H.sub.2 O), a lactic
acid salt such as potassium lactate and an acetic acid salt such as
potassium acetate.
It has been found that when the concentration of the neutral salt is
maintained at not less than 5.5% by weight, preferably not less than 6% by
weight, and when the aqueous solution containing SE precipitate formed by
adding the neutral salt is heated to a temperature of 50.degree. to
80.degree. C., the greater part of SE is precipitated regardless of the
kinds of neutral salts to be added. This is a peculiar phenomenon and is
of important value in connection with the objects of the present
invention. By utilizing this phenomenon, SE can be separated in the form
of a slurry or cake from the unreacted sucrose, the salt derived from the
catalyst and the neutral salt which have transferred to the aqueous phase.
Since the aqueous phase is not acidic, sucrose is not decomposed and,
therefore, it can be recovered and reused, as occasion demands.
Table 1 shows this phenomenon more cleary. When Y (g) is the weight of SE
dissolved in an aqueous phase and X (g) is the weight of SE precipitated,
the weight percentage (.phi.%) of SE dissolved in the aqueous phase based
on the whole SE (X+Y) is shown by the following equation:
##EQU1##
Experiment was made under the following conditions by using a crude SE.
Conditions
Temperature=75.degree. C.
pH=7.8
Water/crude SE=20/1 by weight
Fatty acid radical: stearic acid
______________________________________
Composition of crude SE (% by weight)
SE 94%
Unreacted fatty acid methyl ester
2%
Soap 2%
Fatty acid 1%
Others 1%
Composition of SE (% by weight)
Monoester 73%
Di- and higher esters 27%
______________________________________
TABLE 1
______________________________________
Weight ratio of .PHI. (weight % of SE
water/neutral salt added
dissolved in water)
______________________________________
99.8/0.2 99.0
98.5/1.5 3.5
97.5/2.5 2.6
95.0/5.0 1.9
94.0/6.0 1.2
92.5/7.5 1.0
90.6/9.4 1.3
87.5/12.5 1.2
______________________________________
By determining the amount of neutral salt to be dissolved in the aqueous
solution of the reaction mixture and adding it to the aqueous solution,
approximately the whole amount of SE can be precipitated, and by
separating the precipitate from the aqueous phase, for example, by means
of filtration or centrifugation, neutral salt and sucrose dissolved in the
aqueous phase can be easily removed.
(Recover of unreacted sucrose by reverse osmosis)
It is also important to selectively separate and recover unreacted sucrose
from the thus treated aqueous solution from which the precipitated SE has
been removed, namely the aqueous phase containing unreacted sucrose, a
salt derived from catalyst (K.sub.2 CO.sub.3) and the neutral salt added
for salting out. The present inventors have found that utilization of a
reverse osmosis membrane is particularly effective for this purpose. After
separating the precipitated SE in a usual manner, for example, by
filtration, the filtrate is subjected to reverse osmosis.
It is expected that if a fractionation molecular weight ranging from 130 to
200 is selected as that of the reverse osmosis membrane, the unreacted
sucrose (molecular weight: 342) and the SE (molecular weight: more than
600) which has incidentally leaked into the aqueous phase in the prior
salting out step, would be filtered off without any problem by the reverse
osmosis treatment. On the other hand, substances having a molecular weight
less than the fractionation molecular weight of 130 to 200, namely the
salt derived from catalyst such as potassium lactate (molecular weight:
128) and the neutral salt added, would pass through fine pores of the
reverse osmosis membrane without any problem.
As a result of conducting a large number of experiments on the basis of the
above presumption, it has been found that when an aqueous solution
containing sucrose, the salt derived from catalyst, the neutral salt added
in the salting out step, and sometimes further a slight amount of SE, is
brought into contact with a reverse osmosis membrane having a
fractionation molecular weight of about 150 to about 200 at a temperature
of 40.degree. to 60.degree. C. under a pressure, the salt derived from
catalyst and the neutral salt easily pass with water through fine pores of
the membrane. By this reverse osmosis procedure, low molecular weight
substances such as the catalyst-derived salt and the neutral salt added
for salting out and water are removed from the impure aqueous sucrose
solution (which may contain a slight amount of SE) to thereby form the
concentrated aqueous solution of crude sucrose. An aqueous sucrose
solution having a higher purity can be obtained by dissolving the
concentrate in fresh water again and subjecting the solution to the
reverse osmosis treatment in the same manner, and if necessary, further
repeating these procedures.
The temperature of the aqueous solution to be fed to the reverse osmosis is
important for obtaining a good result. If the temperature is lower than
40.degree. C., the treating ability is remarkably lowered. Accordingly, it
is desirable to select a temperature over 40.degree. C. from a practical
point of view. On the other hand, it is advisable to conduct the treatment
at a temperature below 60.degree. C., from the viewpoint of the heat
resistance of the reverse osmosis. The pH of the aqueous solution to be
treated is also important, and the pH ranging from 6.2 to 8.2 is preferred
because a fear of influence on the quality of sucrose is minimized.
Recently, various reverse osmosis membranes have been put on the market
from various companies. Among them, for instance, reverse osmosis
membranes of polyamide, crosslinked polyamide or polyether have excellent
properties such as durability, heat resistance, acid resistance, alkali
resistance, fungus resistance and pressure resistance. Such membranes are
commercially available, for example, under a trade mark "SU-200" from
Toray Engineering Kabushiki Kaisha, which has a fractionation molecular
weight of about 200 and is suitable to attain the objects of the
invention.
In the case of using the reverse osmosis membrane with the fractionation
molecular weight of about 200, the treatment of the aqueous solution can
be achieved with an industrially acceptable capacity by adjusting the
upper limit of the concentration of the solute in the aqueous solution to
be supplied to the membrane to about 20% by weight, preferably about 15%
by weight.
When the solute concentration is more than 20% by weight, it is difficult
to pass water and the salt derived from the catalyst through fine pores of
the membrane, and accordingly it is obliged to increase the pressure to be
applied as the actuation force for reverse osmosis, thus resulting in
increase of the area of the reverse osmosis membrane This is also very
uneconomical because of necessity of great electric power. On the other
hand, when the aqueous solution contains the solute in a concentration of
about 8-15% by weight, it is sufficiently possible to industrially isolate
sucrose.
For example, when passing an aqueous solution having the composition shown
in Table 2 through the reverse osmosis membrane "SU-200" with an effective
area of 8 m.sup.2 per unit at 50.degree. C. and pH 7.5 and under a
pressure applied as the actuation force for reverse osmosis of 56.0
kg/cm.sup.2 G, the sucrose isolation velocity of 7.3 kg/hour is achieved.
Other reverse osmosis membranes similar to "SU-200", commercially
available from companies other than Toray Engineering Kabushiki Kaisha,
also gave similar results.
TABLE 2
______________________________________
Ingredients Weight (kg)
______________________________________
Sucrose fatty acid ester (stearate)
0.4
Sucrose 39.0
Potassium lactate 9.0
Soap and fatty acid 5.1
Subtotal 53.5
Water 481.0
Total 534.5
______________________________________
Like this, by repeating the reverse osmosis membrane treatment, both the
salt derived from the catalyst and the added neutral salt are sufficiently
removed from the aqueous solution. The thus obtained aqueous solution
containing sucrose can keep a sucrose concentration of about 15 to 20% by
weight. It is economically disadvantageous as well as technical difficulty
to obtain the aqueous solution of sucrose with a concentration of more
than 20% by weight by the reverse osmosis means. Accordingly, when it is
desired to obtain the aqueous solution of sucrose having a sucrose
concentration of more than 20% by weight, the solution is concentrated by
using a usual concentration apparatus such as a multiple effect evaporator
to the desired concentration such as not less than 50% by weight. Thus
recovered sucrose can be reused to the synthesis of SE as a raw material
or used for other purposes.
(Washing of the precipitate)
The SE precipitated and separated in the salting out step is in the form of
a slurry. It still contains a slight amount of impurities such as salts
and sucrose. It has been found that these impurities can be easily removed
by treating the slurry with an acidic water.
An aqueous slurry or cake of crude SE obtained in the salting out step is
washed with an aqueous solution of an acid having a pH of 3.0 to 5.5. The
acid is not particularly limited. Preferable examples of the acid are, for
instance, a mineral acid such as hydrochloric acid or sulfuric acid, and
an organic acid such as acetic acid or lactic acid. Preferably, the acid
solution is kept at a temperature of 10.degree. to 40.degree. C. By this
treatment, the impurities such as sucrose, neutral salt and
catalyst-derived salt can be transferred into the acidic water. When the
temperature of the acidic water is higher than 40.degree. C., the
viscosity rises, as previously described, to hinder the operation in
addition to a fear of acid decomposition of SE if the operation for a long
period of time, for example, over several months, is required. On the
other hand, it is uneconomical to keep the acidic water at a low
temperature lower than 10.degree. C., because a cooling device is required
therefor. Accordingly, the acid solution treatment is effected usually at
a temperature of 10.degree. to 40.degree. C., preferably at ordinary
temperature.
The above-mentioned three components included as impurities in the cake or
slurry, namely unreacted, sucrose, neutral salt and salt formed by
neutralization of the catalyst, should be removed as much as possible from
the cake or slurry by the acid treatment. Accordingly, it is desirable
that the SE cake or slurry is in the form of particles as small as
possible so that the impurities are easily released or eluted into water
upon washing with acidic water. This can be efficiently attained by
conducting the washing in a device having an ability to break into small
particles, for example, a mixer (such as "homomixer" made by Tokushu Kiki
Kogyo Kabushiki Kisha), a homogenizer, or a colloid mill, whereby
substantially the whole amounts of the above-mentioned impurities included
in the SE cake or slurry can be transferred into the acidic water.
In the washing of the precipitate with an acidic water, there is observed a
noticeable phenomenon that SE having a high HLB (hereinafter referred to
as "high HLB-SE") included in the precipitate begins to dissolve into the
acidic water. The solubility of high HLB-SE varies depending on the
temperature, pH, etc. of the system. For example, when the temperature and
pH of the system are ordinary temperature and 3.5, respectively, the
equilibrium relationship between the distribution of SE having a high HLB
in aqueous phase and SE having a low HLB in the cake is as shown in FIG.
1.
The high HLB-SE has a high solubility in water, and here it is referred to
as "water-soluble SE" and assigned with mark "Y". Since Y has a high HLB
and accordingly a high water solubility, it does not precipitate even in
the acidic aqueous solution and is present therein in a dissolved state.
In contrast, SE having a low HLB (hereinafter referred to as "low HLBSE")
is low in water solubility, and in general it tends to deposit in an
acidic water having a certain acidity. Here, the low HLB-SE is referred to
as "depositable SE" and assigned with mark "X". Since X has a low HLB, it
is apt to deposit from an acidic aqueous solution thereof
FIG. 1 shows a part of a triangular graph wherein the total of monoester,
diester and triester is 100%. In FIG. 1, point M indicates the composition
of original sample SE, point X indicates the composition of the
depositable SE which has a low HLB, and point Y indicates the composition
of the water-soluble SE which has a high HLB. Also, suffixes 1, 2 and 3
attached to M, X and Y show SE having a different proportions of sucrose
esters (different ester distribution).
For example, in FIG. 1, when an aqueous acid solution of pH 3.5 is added to
a SE sample M.sub.2 consisting of 73% by weight of monoester, 22% by
weight of diester and 5% by weight of triester so that the concentration
of SE is 3% by weight, the SE is divided into a depositable SE (X.sub.2)
consisting of 68% by weight of the monoester, 25% by weight of the diester
and 7% by weight of the triester, and the water-soluble SE (Y.sub.2)
consisting of 84% by weight of the monoester, 13% by weight of the diester
and 3% by weight of the triester.
The weights WX.sub.2 of X.sub.2 and WY.sub.2 of Y.sub.2 divided from
M.sub.2 are obtained by solving the following equations (a) and (b):
##EQU2##
wherein Y.sub.2 M.sub.2 is the distance between point M.sub.2 and point
Y.sub.2, X.sub.2 M.sub.2 is the distance between point X.sub.2 and point
M.sub.2, WM.sub.2 is the weight of M.sub.2, WX.sub.2 is the weight of
X.sub.2, and WY.sub.2 is the weight of Y.sub.2, provided that the weights
are those of the dried matters.
Like this, SE having a relatively high monoester content (namely SE having
a high HLB) is easy to dissolve into the acidic water, whereas SE having a
relatively low monoester content (namely SE having a low HLB) is easy to
present on the precipitate side. By utilizing this property, the SE
included in the reaction mixture can be quantitatively divided into a high
HLB-SE and a low HLB-SE. There has also been found a tendency that in
general, the higher the monoester content in SE, the more increased the
amount of SE (Y) dissolved in water be, and in the reverse case, the
amount of SE (Y) dissolved in water is decreased.
The solid component remaining after the washing with the aqueous acid
solution is then separated and dried. The purity can be further increased
by repeating the washing procedure prior to the drying, thus the low
HLB-SE having a high purity is obtained.
Since the aqueous acid solution obtained in the washing step contains a
relatively large amount of high HLB-SE, it is separated from the solid SE
composed mainly of low HLB-SE in a usual manner such as filtration or
centrifugation. The obtained filtrate or supernatant contains, in addition
to the high HLB-SE, small amounts of salts and sucrose and, therefore, it
is necessary to further purify the SE.
By many experiments we have found that ultrafiltration is suitable for
removing these impurities from high HLB-SE in the filtrate or supernatant.
(Ultrafiltration)
It seems that sucrose fatty acid ester molecules aggregate with each other
to form apparent high molecular weight micelles under certain conditions
in the aqueous solution.
Sucrose monoester, diester and triester are compounds wherein 1, 2 or 3
fatty acid residues are attached to any of oxygen atoms of the 3 primary
hydroxyl groups of sucrose molecule, respectively. As well known, since
the monoester is low in ability to form micelles in water while having a
larger hydrophilic property than diester and triester, it forms a
relatively low weight micelle (in other words, a micelle having a small
diameter). In contrast, the diester and triester have a very large micelle
forming ability while being relatively low in hydrophilic property and,
therefore, they form micelles of large apparent molecular weight (namely
large micellar diameter).
According to the investigation of the present inventors, since high HLB-SE
having a high monoester content, for instance, as high as 70%, forms
micelles of a lower apparent molecular weight in comparison to low HLB-SE
having a monoester content as low as 50%, the microscopic diameter of
micelle is small as much and it has a tendency to pass through a
ultrafiltration membrane having a specified pore diameter in comparison
with low HLB-SE having a monoester content of 50%. Therefore, high HLB-SE
having a high monoester content has an undesirable tendency to pass
through the membrane together with the unreacted sucrose, a salt formed by
neutralization of a reaction catalyst with an acid, and the like. Such a
problem can be easily eliminated by selecting the ultrafiltration
membrane, and it is necessary for recovery of high HLB-SE to select the
membrane having a relatively low fractionation molecular weight (namely
small pore diameter) when desired to remove sucrose and the salts from
high HLB-SE having a high monoester content, and it is necessary for
recovery of low HLB-SE having a low monoester content to select the
membrane having a relatively large fractionation molecular weight (namely
large pore diameter).
It is confirmed by the present inventors that it is practically impossible
to separate from SE the unreacted fatty acid methyl ester, soap and fatty
acid included in the reaction mixture by a filtration means because they
are present in the state of being included in the micelles of SE. In other
words, impurities permeable together with water to a filtration membrane
having an appropriate fractionation molecular weight by a given pressure
as actuating force are the unreacted sucrose, the salt derived from the
catalyst and the neutral salt added, while the unreacted fatty acid ester,
soap and free fatty acid are entrapped in the sucrose ester micelles and
they are not impermeable to the filtration membrane.
Thus, in this ultrafiltration step, by skillfully utilizing these facts and
by selecting a filtration membrane having an appropriate fractionation
molecular weight, the unreacted sucrose and the salts are removed together
with water from other components, namely SE, unreacted fatty acid ester,
soap and fatty acid.
In order to select a ultrafiltration membrane having an adequate
fractionating molecular weight, it is necessary to previously know
approximate molecular weights of the subject substances. The molecular
weights of typical single compounds involved in the present invention are
defined approximately as follows:
(1) Sucrose=342
(2) Unreacted fatty acid methyl ester
(main) Methyl stearate=290
(3) Salt produced by neutralization of catalyst (K.sub.2 CO.sub.3)
In case of lactic acid: potassium lactate 128
In caser of acetic acid: potassium acetate=98
(4) Neutral salt
NaCl=58.5
(5) Sucrose fatty acid ester (single compound not forming micelle)
(main) Sucrose monostearate=600
(main) Sucrose distearate=858
(main) Sucrose tristearate=1116
Other sucrose fatty acid esters such as myristate, palmitate, arachate and
behenate have also similar molecualr weights to the above molecular
weights.
(6) Soap
(main) Sodium stearate=298
(main) Potassium stearate=314
(7) Fatty acid
(main) Stearic acid=276
(8) Water=18
On the supposition that the apparent molecular weight of a sucrose fatty
acid ester micelle may be approximately estimated as follows, if 10
molecules associate per micelle, the apparent molecular weight of the
micelle is:
molecular weight of monoester (600).times.10=6,000 (regarded as 100%
monoester),
molecular weight of diester (858).times.10=8,580 (regarded as 100%
diester), and
molecular weight of triester (1,116).times.10=11,160 (regarded as 100%
triester).
Since the actual SE is a mixture composed mainly of mono-, di- and
triesters, the apparent molecular weight of a SE micelle is defined as the
average value thereof.
The selection of a membrane for ultrafiltration adequate for the purposes
of the present invention is conducted as follows:
In case of a ultrafiltration membrane having a fractionation melecular
weight of 200, even if it is attempted to remove the unreacted sucrose and
the salts such as K.sub.2 CO.sub.3 while feeding the washing liquid
obtained in the previous step with applying a pressure, the components
separable by such a membrane are only water and the salts which have lower
molecular weights than the fractionation molecular weight 200 of the
membrane. Since sucrose which has a molecular weight of 342 larger than
the fractionation molecular weight 200, is impermeable to the membrane, it
cannot be separated and removed from SE.
In case of a ultrafiltration membrane having a fractionating molecular
weight of 5,000, sucrose and the salts can easily pass throught fine pores
of the membrane, since they have a molecular weight less than 5,000. SE
forms micelles as mentioned above, and accordingly it is estimated to have
an apparent molecular weight of 6,000 or more on the assumption that the
number of sucrose ester molecules associated may be 10 or more. Therefore,
the micelles would not be permeable to the membrane having a fractionation
molecular weight of 5,000. Since the apparent molecular weight of the
micelle would be in fact more than 6,000, a membrane having a
fractionation molecular weight of more than 5,000 can be used and this is
experimentally confirmed by the present inventors.
Investigation has been made als | | |
