USE OF SULFONIC ACID FOR RECOVERING GLYCEROL RESULTING FROM THE
TRIGLYCERIDE TRANSESTERIFICATION REACTION
5 [0001] The present invention relates to the use of sulfonic acid for preparing
glycerol. More specifically, the present invention relates to a process for
purifying glycerol obtained as a by-product of triglyceride transesterification
during the preparation of fatty acids, of fatty esters and/or of fatty acid salts.
This invention thus makes it possible to exploit the glycerol derived from the
10 transesterification of triglycerides. The invention also relates to a combined
process for preparing fatty acid esters and glycerol from glycerides using at
least one sulfonic acid.
[0002] The triglycerides are predominantly triesters of glycerol and of fatty
acids (they can also contain monoglycerides and diglycerides) and are found in
15 abundance in nature, for example in oleaginous plants and animal fats, to
mention only the mdst significant sources of triglycerides. There are today a
very large number of industries carrying out the transesterification of these
natural products which are triglycerides.
[0003] Among these industries, mention may be made, by way of non-limiting
20 indication, of the industries for manufacturing cosmetic products, perfumes and
fragrances, organic solvents, biodiesel, soaps, and the like.
[0004] The general reaction for triglyceride transesterification, generally carried
out in a basic medium, in the presence of an alcohol, for example methanol, so
as to give the corresponding fatty acid methyl esters, corresponds to the
25 following scheme:
Triglycerides + MeOH
Basic
catalyst
____ ,_
(NaOH, KOH,
NaOMe or KOMe)
Fatty acid +
methyl esters
OH
OH
OH
Glycerol
[0005] This transesterification reaction, which can be carried out relatively
easily in the laboratory, exhibits, however, some difficulties from an industrial
point of view. This is because the starting triglycerides usually contain a variable
30 amount offree fatty acids (FFAs).
-2-
[0006] When this content of. free fatty acids in the triglycerides is relatively
high, typically greater than approximately 5% by weight, the FFAs can then
form, in the presence of the basic catalyst, fatty acid salts which act as
surfactants and cause foams that make it difficult to separate the glycerol from
5 the fatty acid methyl esters formed.
[0007] This problem has been solved, and it is now known practice to carry out
a pretreatment of the triglycerides, which consists of a first esterification of said
FFAs in the presence of methanol and of an acid catalyst, such as sulfuric acid
or methane sulfonic acid (d. FR2929621). The FFAs previously contained in the
10 triglycerides are then in the form of esters which will not be salified by the basic
catalyst during the transesterification reaction.
[0008] The FFAs present in the starting triglycerides may also be in lower
amounts, typically between 0.1% by weight and 5% by weight, and in this
case, basic washing may be sufficient to remove them in the form of· basic
15 salts.
[0009] After the transesterification reaction, the fatty acid esters, generally fatty
acid methyl esters, are present in the basic reaction medium with the glycerol.
This reaction medium can comprise larger or smaller amounts of water
depending on the conditions under which the transesterification reaction was
20 carried out The glycerol and also any water are not soluble in the fatty acid
esters and are separated from the latter by decanting or any other means for
phase separation.
[001 0] The demand for fatty acid esters is constantly increasing, if only for the
manufacture of biodiesel, and the industry today produces, and will even more
25 so produce in the future, ever increasing amounts of these esters. The
production of glycerol is consequently ever increasing and it would be entirely
advantageous to be able to more successfully exploit this "by-product", with the
objective cif achieving a very high degree of purity, for example of
pharmaceutical quality.
30 [0011] The prior art thus comprises a very large number of documents
concerning the preparation of fatty acid esters, in particular for the production of
biodiesel, via triglyceride transesterification reaction, as, for example, described
in patent EP-B-0 658 183. More recently, application US 2011/0245521, which
-3-
also relates to the production of biodiesel, only mentions very briefly the
recovery of glycerol by chemical purification and distillation for use as an
industrial or pharmaceutical 'raw material. No indication is provided with regard
to said chemical purification and the distillation of the glycerol considered to be
5 a "by-product".
[0012] Other prior art documents already relate to the possibility of exploiting
glycerol, for instance patent application CN101423456, which describes a
process for recovering and purifying glycerol as a by-product of biodiesel
production. This purification uses molecular distillation to obtain a medical-
10 quality glycerol. Such distillation of glycerol is carried out after the addition of
sulfuric acid to the transesterification reaction medium.
[0013] As another example, patent application CN101475444 also describes a
process for purifying glycerol resulting from the preparation of biodiesel which
consists in filtering the crude glycerol so as to remove the solid impurities, in
15 evaporating the filtered glycerol, and in introducing it into an ion exchange
column and then, finally, subjecting it to thin-film distillation. The glycerol is
obtained with a purity of more than 95%.
[0014] The prior art clearly teaches that the crude glycerol resulting from the
transesterification of triglycerides is present in a basic phase which contains a
20 more or less large amount of the surplus of alcohol used for the
transesterification reaction (generally methanol), which may equally contain
water, but also and especially residues of basic catalysts such as, for example,
sodium hydroxide, potassium hydroxide, sodium methoxide or potassium
methoxide, sodium or potassium salts of fatty acids, and also other impurities.
25 [0015] In addition, it is necessary to acidify this basic phase, in order to be able
to isolate a glycerol of suitable purity. This is because sodium and/or potassium
salts of fatty acids (often called "soaps") are in particular responsible for the
formation of more or less substantial foams, thus creating an unwanted
emulsion during the recovery of the alcohol (methanol) by evaporation, which
30 makes it difficult to carry out this operation without losses in terms of yield and
purity.
[0016] One of the advantages of this acidification operation is the conversion
of these soaps into non-surfactant, free fatty acids which can then be separated
--4-
much more easily from the glycerol phase, thus making it possible to
subsequently exploit the latter under good conditions of purity and yield.
[0017] This acidification operation is generally carried out by adding at least
one strong acid, generally in the form ?fan aqueous solution. The strong acids
5 normally used are sulfuric acid, hydrochloric acid and phosphoric acid, which
generate,· ·respectively, sodium or potassium sulfates, chlorides and
phosphates. However, each of these acids has drawbacks, in terms of
corrosion, or of generation of effluents that are harmful to the environment, to
mention only some of these drawbacks.
10 [0018] What is more, the presence of these salts (sulfates, chlorides and
phosphates) in reality proves to be a great hindrance in several respects during
the purification of the glycerol, quite particularly during its subsequent
distillation. Indeed, it has been possible to observe that the sulfates, chlorides
and phosphates have low solubility in glycerol and glycerol/water mixtures. This
15 observed low solubility can be detrimental to the conducting of the distillation
and the recovery of purified glycerol under acceptable yield and purity
conditions.
[0019] The presence of insoluble compounds in the streams of an industrial
distillation apparatus is highly prejudicial in that these insoluble compounds can
20 cause disruption of the streams, in particular in the distillation column itself and
can, consequently, lead to pressure drops, or even risks of clogging, deposits,
etc. Furthermore, any pressure drop requires a greater energy consumption, in
particular requires operating at a higher temperature, which results in
degradation and decomposition of the product, thus leading to a loss of quality
25 of the purified glycerol and overall distillation yield losses.
[0020] Application EP 1 889 899 A1 discloses a process for producing
biodiesel also comprising a glycerol recovery step, in which the crude glycerol is
acidified wlth an· acid, preferably a weak organic acid, such as acetic acid,
formic acid or propionic acid. The use of an organic acid does not result,
30 according to said document, in the formation of insoluble salts. It is indicated
that the crude glycerol must be acidified to a pH of less than 8, preferably
between 6.5 and approximately 7.
-· 5-
[0021] Similarly, application US 2012/0245371 proposes a process for
purifying crude alkaline glycerol obtained as a by-product during the
manufacture of biodiesel from the transesterification reaction of vegetable oils
and animal fats. This process comprises the acidification of the crude glycerol to
5 a pH value of from approximately 4 to approximately 6 with an organic alkyl
carboxylic acid, in the presence of water, acetic acid being the only
representative exemplified. It is indicated in said application that the acidification
with acetic acid does not cause the formation of solids above the glycerol
phase.
w [0022] The two documents previously mentioned thus teach that one of the
problems encountered during the purification of an alkaline glycerol crude is
the presence of insoluble salts which form during the neutralization or the
acidification of the crude glyceroL This problem appears to be solved by using
a weak acid, acetic, formic or propionic acid and in particular acetic acid,
15 rather than a strong inorganic acid, such as hydrochloric, sulfuric or
phosphoric acid.
[0023] However, the use of these weak acids (PKa greater than 3.5) does not
allow sufficient acidification of the species present in the crude glycerol, in other
words does not make it possible to reach a sufficiently low pH value to enable
20 effective removal of soaps (in particular alkali metal salts of fatty acids) by
converting them into free fatty acids, and sufficient solubilization of the other
possible salts and impurities present in the crude glycerol.
[0024] In addition, the prior art shows that the use of these weak acids 1s
accompanied by the addition of more or less large amounts of water, thus
25 requiring the management of large stream volumes and the subsequent
treatment of large amounts of effluents.
[0025] There remains therefore a need to improve the process for recovering
crude glycerol obtained during transesterification reactions of glycerides, in
particular natural glycerides, and more particularly glycerides used in the
30 preparation of biodiesel.
[0026] The inventors have now discovered that the technical problems set out
above can be totally, or at least partially, solved by virtue of the present
invention. Thus, a first objective of the present invention consists in providing an
-· 6-
improved process for recovering purified glycerol originating from a reaction
crude resulting from the transesterification of glycerides, in particular natural
glycerides, and more particularly glycerides used for the preparation of
biodiesel.
5 [0027] Another objective consists in providing an improved process for
recovering glycerol originating from a reaction crude resulting from the
transesterification of glycerides, in which the operation of intermediate
distillation of the methanol and/or water and final distillation of the glycerol is not
disrupted by significant foaming in the distillation reboiler, and in which the
10 stream and effluent volumes are entirely acceptable for industrial production of
highly pure glycerol. Other further objectives will emerge in the disclosure of the
present invention which follows.
[0028] The inventors have discovered, entirely surprisingly, that these
objectives can be achieved, totally, or at least partially, by virtue of the process
15 of the invention. This. process comprises a step of acidification with at least one
sulfonic acid which has the advantage of allying the properties of strong acids
and of weak acids which are required for this process, i.e. allowing sufficient
acidification to eliminate the risk of foam formation, and allowing the formation
of soluble salts in order to eliminate the risks of clogging during the distillation
20 operation. The use of at least one sulfonic acid represents an entirely
unexpected and advantageous compromise for the industrial exploitation of
glycerol resulting from triglyceride transesterification.
[0029] It has thus been discovered, surprisingly, that sulfonates, in particular
alkane sulfonates, and more particularly methane sulfonates of alkali metals
25 and alkaline-earth metals are more soluble in glycerol and glycerol/water
mixtures than the other alkali metal and alkaline-earth metal salts formed from
other strong acids, and in particular sulfuric acid, hydrochloric acid and
phosphoric acid.
[0030] Thus, and according to a first aspect, the present invention relates to
30 the use of at least one sulfonic acid, preferably at least one alkane sulfonic acid,
for the recovery of glycerol resulting from a reaction crude from
transesterification of glycerides, in particular of glycerides of vegetable and/or
animal origin.
•
-7-
[0031] In the present invention, the term "sulfonic acid" is intended to mean the
acids of general formula R-S03H, where R represents an alkyl or aryl radical,
preferably an alkyl radical, and in the latter case the term alkane sulfonic acids
is used. The alkane sulfonic acids preferred for the needs of the present
5 invention are the acids of formula R-S03H, where R represents a linear or
branched, saturated hydrocarbon-based chain containing from 1 to 4 carbon
atoms.
[0032] The alkane sulfonic acids usable in the context of the present invention
are particularly chosen from methane sulfonic acid, ethane sulfonic acid,
10 n-propane sulfonic acid, iso-propane sulfonic acid, n-butane sulfonic acid, isobutane
sulfonic acid, sec-butane sulfonic acid, tert-butane sulfonic acid, and
mixtures of two or more of them in any proportions.
[0033] The pKa values of the sulfonic acids, in general, and alkane sulfonic
acids in particular, are all less than zero, whereas the pKa values of the
15 alkylcarboxylic acids. are all greater than 3.5. These acids, with pKa values
greater than 3.5, are not sufficiently strong to ensure total acidification of all the
species present in the medium.
[0034] According to one most particularly preferred embodiment, the alkane
sulfonic acid used in the context of the present invention is methane sulfonic
20 acid or ethane sulfonic acid; entirely preferably, the acid used is methane
sulfonic acid of formula CH3S03H.
[0035] Thus, the use according to the present invention employs at least one
alkane sulfonic acid chosen from linear-chain or branched-chain alkane sulfonic
acids containing from 1 to 4 carbon atoms, and preferably at least methane
25 sulfonic acid (MSA).
[0036] Any type of formulation comprising at least one sulfonic acid, preferably
alkane sulfonic acid, may be suitable. It is for instance thus possible to use at
least one sulfonic aeid in anhydrous form or in the form of an aqueous solution.
As a general rule, the formulation comprises from 1% to 10% by weight of
30 sulfonic acid(s), more generally from 5% to 90% by weight, in particular from
10% to 80% by weight of sulfonic acids, and more particularly from 15% to 75%
by weight, the rest to 100% generally consisting of water. It goes without saying
that, when the formulation comprises 100% by weight of sulfonic acid(s), this
-8-
means that the sulfonic acid(s) is (are) used pure, more specifically used alone,
without the addition of other formulation constituents.
[0037] The formulation is, for example, an aqueous formulation which can be
prepared in the form of a concentrated mixture which is diluted by the final user.
5 As a variant, the formulation can also be a ready-to-use formulation, i.e. a
formulation which does not need to be diluted. Use may, for example, be made
of methane sulfonic acid in an aqueous solution, sold by the company Arkema,
for example an aqueous solution of methane sulfonic acid at 70% by weight in
water, or else anhydrous methane sulfonic acid or AM SA.
10 [0038] The inventors have discovered that the sulfonates of alkali metals and
alkaline-earth metals, in particular sodium and potassium methane sulfonates,
are more soluble in glycerol or glycerol/water mixtures than the sulfate, chloride
or phosphate salts of these same cations in this same medium.
[0039] According to one preferred embodiment, the present invention relates to
15 the use, for recovering glycerol resulting from a glyceride transesterification
reaction, of methane sulfonic acid (MSA) in any possible concentrations,
ranging from AMSA (anhydrous MSA) to concentrations of about 5% by weight
of MSA in water, and in particular the aqueous solutions of MSA at 70% by
weight in water, sold by the company Arkema.
20 [0040] Another advantage associated with the use of a strong acid, and in
particular a sulfonic acid, preferably alkane sulfonic acid, more preferably
methane sulfonic acid, lies in the fact that the addition of such an acid makes it
possible to reduce the viscosity of the glycerol-rich medium and to facilitate pH
measurements. This is because, in the presence of water, strong acids, unlike
25 "weak" organic acids, more readily confer a hydrophilic nature on the medium,
consequently allowing better dissociation of the salts, thus facilitating pH
measurements. In addition, reducing the viscosity of the medium, glycerol being
a product with a relatively high viscosity, makes it possible to very substantially
improve the separation, on the one hand, of the aqueous phase and, on the
30 other hand, of the glycerol phase, for example through distillation, when
necessary.
[0041] The term "reaction crude" comprising the glycerol that it is desired to
recover is intended to mean the basic medium which constitutes the phase
/ .i
--9-
containing the glycerol after separation of the phase containing the fatty acid
esters, the whole of the two phases resulting from the glyceride
transesterification reaction,' as described, for example, in applications
EP 1 889 899 and US 2010/0186289.
5 [0042] Thus, the reaction crude, or crude reaction medium, is a basic mixture,
with a pH generally between 1 0 and 14, and which typically comprises, for
example:
glycerol,
- the alcohol(s) used for the transesterification reaction,
10 optionally water, or traces of water,
- the basic catalyst(s) (used for the transesterification reaction), possibly in
trace form,
- fatty acid salt(s), possibly in trace form,
- optionally one or more fatty acid ester(s), possibly in trace form,
15 - optionally traces of mono-, di- and/or triglycerides,
- optionally traces of organic residues other than glycerol,
- optionally traces of metals.
[0043] The term "traces" is intended to mean amounts generally between a few
ppm (weight) and 5% by weight relative to the total weight of the reaction crude,
20 preferably between a few ppm (weight) and 2% by weight relative to the total
weight of the reaction crude, entirely preferably between a few ppm (weight)
and 1% by weight relative to the total weight of the reaction crude.
[0044] Among the basic catalysts which are present, possibly in the form of
traces in the reaction crude, mention may be made of all the catalysts that can
25 be used for glyceride transesterification reactions and in particular triglyceride
transesterifications. Preferably, these basic catalysts are chosen from oxides,
hydrides, hydroxides, carbonates, hydrogen carbonates, acetates and other
alkoxides of alkali metals and alkaline-earth metals, the alkoxides originating
from alcohols preferably containing from 1 to 5 carbon atoms. Among the basic
30 catalysts, sodium hydroxide, potassium hydroxide, sodium alkoxides and
potassium alkoxides are preferred. Entirely preferably, the basic catalysts are
chosen from sodium hydroxide, potassium hydroxide, sodium methoxide and
potassium methoxide, the latter two alkoxides being quite particularly preferred.
-10-
[0045] The expression "alcohol used for the transesterification reaction" is
intended to mean, in general and by way of indicative but non-limiting
examples, alcohols comprising from 1 to 10 carbon atoms, and preferably those
chosen from methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-
5 butanol, 3-methyl-1-butanol, neo-pentyl alcohol, pentanol and its isomers,
hexanol and its isomers, heptanol and its isomers, octanol and its isomers,
nonanol and its isomers, decanol and its isomers, and also mixtures of two or
more of them in any proportions. Among the alcohols mentioned above,
preference is. given to methanol and ethanol, and particularly preferably to
10 methanol.
[0046] The term "fatty acid salts" is intended to mean the alkali metal or
alkaline-earth metal salts, preferably sodium salts and/or potassium salts, of the
fatty acids resulting from the glyceride transesterification reaction and which can
be found in trace amounts in the glycerol phase after the step of separating, on
15 the one hand, the glycerol phase and, on the other hand, the fatty acid ester
phase.
[0047] Likewise, traces of mono-, di- and triglycerides may be present in the
glycerol phase, in particular when the transesterification reaction was carried
out partially.
20 [0048] Likewise, one or more fatty acid ester(s) may be present in the basic
reaction crude containing predominantly glycerol. These esters may be present
in the glycerol phase for example because of their partial solubility in this phase,
because of insufficient decanting, because of poor separation of the two
phases, and the like.
25 [0049] The basic glycerol crude may also contain traces of organic residues
originating from the starting oils before transesterification or else obtained as
by-products of degradation during said transesterification reaction. The starting
oils subjected to transesterification may also contain traces of metals, it being
possible for the latter to also be introduced by the catalysts employed, the water
30 or else the alcohol used.
[0050] Finally, water may be present, in trace amounts or in larger amounts.
Traces of water may in fact be present in the triglycerides used in the
transesterification reaction, but also and especially the water may be introduced
--11-
in particular by the basic catalyst, for example when the latter is sodium
hydroxide or potassium hydroxide in the form of an aqueous solution.
[0051] The glycerol phase thus comprises, in addition to the glycerol of interest
that it is desired to recover, numerous other impurities, some of which may be
5 present in the solid state. According to one preferred embodiment, these solid
impurities are separated according to any separation means known to those
skilled in the art, for example physical separation, such as centrifugation and/or
filtration.
[0052] As previously indicated, this glycerol phase is basic, owing to the basic
10 catalyst(s) used during the transesterification reaction, and it is advisable to
acidify this basic phase, in order to bring its pH to a value strictly below 4,
preferably below 3, for example to a pH value close to 2.
[0053] To this effect, and according to the use of the present invention, at least
one sulfonic acid, and preferably methane sulfonic acid, is added to said
15 reaction crude before carrying out a distillation reaction aimed at recovering the
glycerol contained in said reaction crude, from which solid elements present
therein have optionally, but preferably, been removed as previously indicated.
[0054] The amount of sulfonic acid(s) introduced into the reaction crude can
therefore vary in large proportions according to the various basic elements
20 present in said reaction crude. Those skilled in the art will know how to adjust
the amount of sulfonic acid(s) to be added to the reaction crude according also
to the concentration of said acid(s). According to one embodiment, the addition
of acid is carried out with stirring and the pH of the solution is continuously
checked. The addition of acid(s) is then stopped when the pH of the solution
25 reaches a value strictly below 4, preferably below 3, for example close to 2, the
pH value which allows total, or even virtually total but sufficient, acidification of
the soaps and other salts present in the medium and which might result in
foaming during the subsequent distillation.
[0055] After this acidification step, the formation of an upper phase is generally
30 observed, which upper phase mainly comprises free fatty acids, as well as
traces of fatty acid esters. The medium can then be advantageously and
preferably subjected, although this step is not obligatory, to a phase separation
step (for example decantation, centrifugation, aspiration, withdrawal, or any
-12-
other technique known by the skilled in the art) in order to separate the esters
possibly present, and also the free fatty acids originating from the fatty acid
salts after acidification.
[0056] Likewise, in cases where insoluble salts are present, a filtration step
5 can be envisaged although this does not represent a preferred variant of the
process of tlie Invention:
[0057] The acidified solution comprises an amount of glycerol of generally
between 30% and 99% by weight relative to the total weight of the acidified
reaction crude, more frequently between 50% and 95% by weight of glycerol
10 relative to the total weight of the acidified reaction crude. In addition, the
acidified solution may contain methanol, in more or less large amounts, and
water, also in more or less large amounts.
[0058] In addition, and as indicated above, the glycerol phase generally
contains greater or lesser amounts of fatty acid salts, commonly referred to as
15 "soaps". Acidification of this glycerol phase also allows for the transformation of
these soaps into free fatty acids (FFA). Now it has been found, and this is still
another advantage linked with the use of at least one sulfonic acid, preferably at
least one alkane sulfonic acid, preferably methane sulfonic acid, that the acid
number (lA) of the glycerol phase, after the acidification step with at least one
20 sulfonic acid according to the invention to eliminate these and FFA, is weaker
than when another acid is used.
[0059] The resulting benefit is quite significant, considering that this acidified
glycerol phase, after removal of the FFA, must be neutralized at neutral pH
(about pH = 7) before distillation. This neutralization may be carried out
25 according to any means known per se, for example using a base, preferably a
strong base, preferably sodium hydroxide or potassium hydroxide. A lower acid
number, thanks to prior acidification with at least one sulfonic acid, thus leads to
a smaller amoutit of base which is necessary for the neutralization and
therefore the formation of a smaller amount of salts.
30 [0060] Thus, this glycerol-rich acidified phase is then neutralized (at a pH of
approximately 7) by means of at least one base, preferably at least one strong
base, such as sodium hydroxide or potassium hydroxide. This neutralized
--13-
solution can then be used in the glycerol distillation step, after distillation of the
methanol and water, as indicated below.
[0061] The neutralization of the acid phase has consequences on the nature of
the medium which is intended to be distilled. Indeed, during this neutralization
5 step, the acid species are neutralized in the form of salts in a glycerol-rich
medium.
[0062] The inventors have discovered, surprisingly, that the various alkali
metal and/or alkaline-earth metal salts present in this phase thus neutralized
and intended to be distilled are much more soluble in glycerol when the
10 neutralization has been carried out on a medium acidified beforehand using at
least one sulfonic acid, in particular at least one alkane sulfonic acid, more
particularly methane sulfonic acid, whereas the same salts are much less
soluble when the acidification has been carried out with other acids, in particular
strong inorganic acids commonly used in the field, such as sulfuric- acid,
15 hydrochloric acid or phosphoric acid.
[0063] In addition, the abovementioned salts in the form of sultanates,
preferably alkane sultanates, more preferably methane sultanates, have been
found to be much more soluble than the sulfates, chlorides and other
phosphates, not only in glycerol, but also in glycerol/water mixtures, whatever
20 the glycerol/water ratio and whatever the temperature, in particular whatever the
temperature included in the temperature range at which the distillation column
operates.
[0064] This is all the more notable since distillation operations are very
sensitive to the solid impurities present in distillation equipment and in particular
25 in the distillation reboiler (or bottom), but also in the distillation column. As it
happens, the temperature and glycerol/water concentration gradients vary along
the distillation column.
[0065] The aCidification with at least one sulfonic acid, preferably at least one
alkane sulfonic acid, and more preferably with methane sulfonic acid, offers the
30 advantage of better solubility of the salts, in particular of the sodium and/or
potassium salts present in the glycerol, whatever the glycerol/water gradient,
which means that the solubility of the salts is higher not only in the reboiler, but
also throughout the height of the column.
-14-
[0066] This advantage of sulfonic acid compared with the other strong acids
commonly used thus makes it possible to avoid the formation of solid deposits
which can cause disruptions of the streams, in particular in the distillation
column itself, and consequently lead to pressure drops, or even risks of
s clogging, deposits, etc.
[0067] In addition, this greater solubility of the sulfonic acid salts in the phase
comprising glycerol, and in particular in the reboiler, at the bottom of the
column, makes it possible to continue the distillation operation to a more
advanced degree, and thus to further improve the distillation yield. Another
10 advantage associated with better solubility of the salts in glycerol is the
reduction of the risk of clogging in the bottom of the column, where the
glycerol/water mixtures are the most concentrated in terms of glycerol. The
overall yield of the distillation is thus greatly improved.
[0068] The greater solubility of the salts in the medium to be distilled can also
1s make it possible to envisage a substantial decrease in the number of theoretical
plates of the column and, consequently, the physical height of the column, just
as it can make it possible to substantially reduce the amount of energy used for
the total distillation of the glycerol.
[0069] Yet another advantage, associated with the acidification with at least
20 one sulfonic acid, of the reaction crude containing the glycerol lies in the fact
that there are fewer solid deposits and, consequently, the periods of interruption
for cleaning the distillation equipment are further apart in time.
[0070] Yet another advantage is that the sulfonates, and in particular the
alkane sulfonates and more particularly the methane sulfonates, are highly
25 soluble in an aqueous medium and are biodegradable. The equipment is
consequently easier to clean and, as a result, requires much smaller volumes of
water, and the cleaning effluents are more environmentally friendly.
[0071] The glycerol present in the acidified and optionally neutralized solution
is then separated from the water and from the residual alcohol used during the
30 transesterification reaction. This separation can be carried out according to any
method known to those skilled in the art, and preferably by distillation. During
this distillation operation, the alcohol, generally methanol, is first of all distilled,
followed by the water and, finally, the glycerol. The distillation of the glycerol is
\ --_
-15-
generally carried out under reduced pressure, in particular in order to minimize
the risks of decomposition of said glycerol at high temperature, for example
approximately 10 mbar (approximately 1 kPa), the boiling point of the glycerol
then being approximately 160°C, or else between 15 mbar and 150 mbar
5 (between 1.5 kPa and 15 kPa), the boiling point of the glycerol then being
between approximately 160oc and approximately 280°C.
[0072] According to another aspect, the present invention relates to an
improved process for recovering glycerol from a reaction crude resulting from
the triglyceride transesterification reaction, comprising at least the following
10 steps:
a) provision of a basic reaction crude which is rich in glycerol and also
contains water and an alcohol, for example methanol;
b) optional separation of the solid impurities;
c) acidification of the reaction crude resulting from step a), with at least one
15 sulfonic acid, to a pH value less than or equal to 4, preferably less than or
equal to 3, for example of about 2;
d) optional decanting and separation of the phase rich in glycerol and of the
phase rich in free fatty acids and in fatty acid esters;
e) neutralization to a pH of approximately 7, using a base, for example a
20 strong inorganic base, preferably chosen from sodium hydroxide and
potassium hydroxide;
f) distillation of the alcohol and of the water;
g) distillation of the glycerol, preferably under reduced pressure;
h) recovery of the glycerol distilled.
25 [0073] According to one variant of the improved process for recovering
glycerol, step f) of distillation of the methanol and of the water can be carried
out before the neutralization step e). It is also possible to carry out the
distillation cif the methanol, then to perform the neutralization step, and to
conduct the step of distillation of the water and then of the glycerol.
30 [0074] According to yet another aspect, the present invention relates to a
combined process for preparing, on the one hand, fatty acid esters or mixtures
of fatty acid esters and, on the other hand, glycerol, comprising at least the
following steps:
-- v
--16-
1) provision of triglycerides of vegetable and/or animal origin,
2) transesterification, in a basic medium, of said glycerides, in the presence
of at least one alcohol 'comprising from 1 to 10 carbon atoms, preferably
from 1 to 5 carbon atoms, preferably in the presence of methanol, so as to
5 obtain, on the one hand, a phase rich in fatty acid esters or mixtures of
fatty acid esters and, on the other hand, a phase rich in glycerol,
3) separation of the phases rich in esters and rich in glycerol, forexample by
decanting,
4a) recovery. of the fatty acid esters and mixtures of fatty acid esters, after
10 acidification, with at least one sulfonic acid, of the phase comprising said
fatty acid esters and mixtures of fatty acid esters, and
4b) recovery of the glycerol according to the process described above, step a)
to step h), comprising the acidification of the phase comprising the glycerol
with at least one sulfonic acid.
15 [0075] This process enables the combined production of both, on the one
hand, highly pure glycerol and, on the other hand, fatty acid esters, from oils, in
particular from vegetable or animal oils which contain large amounts of
triglycerides.
[0076] This process is characterized in that it uses at least one sulfonic acid,
20 preferably at least one alkane sulfonic acid and more preferably methane
sulfonic acid, both for the acidification of the phase rich in fatty acid esters and
for the acidification of the phase rich in glycerol. The use of the same acid for
the acidification of the abovementioned two phases has, inter alia, the
advantage of greater simplicity of implementation of the industrial operation, the
25 advantage of having a non-corrosive, biodegradable and environmentally
friendly acid, and the advantage of solubilizing the salts present in the phase
rich in glycerol, making it possible to conduct the final distillation of glycerol
under more economical conditions, as has been explained above in the
description.
30 [0077] The highly pure distilled glycerol thus obtained can, for example, be
used as it is as a solvent, as a component of sanitary or cleaning formulations,
or else as a synthesis precursor or intermediate in numerous fields of
-17-
application, for instance the preparation of pharmaceutical and veterinary
products, of cosmetic products, of phytosanitary products, and the like.
[0078] The fatty acid esters and mixtures of fatty acid esters may have, for
their part, numerous applications, and for example can be used as solvents,
5 surfactants, monomer precursors, fatty alcohol precursors, or lubricants, or else
as constituents of biodiesel, i.e. for the manufacture of diesel-type fuel
comprising a more or less large amount of compounds of renewable origin,
such as diesel fuel corresponding to the European standard.
[0079] The following examples illustrate the present invention without,
10 however, limiting the scope thereof defined by the claims which follow.
Example 1: Solubilities of the sodium salts in glycerol
[0080] 30 g of glycerol/water mixture are placed in a 50 ml three-necked flask
equipped with a magnetic stirrer, with a condenser and with a temperature
15 probe. The salt to be tested is then added in portions of approximately 0.1 g
until undissolved crystals appear (saturation of the medium). The percentage of
dissolved salt is calculated according to the following formula:
%dissolved salt= w I (W + w)
where W represents the weight of glycerol/water mixture, and w represents the
20 weight of salt introduced.
[0081] The salts tested for their solubility in glycerol/water mixtures are the
sodium salts of sulfuric (pK8 = -9), hydrochloric (pK8 = -6.3), acetic (PKa = +4. 75)
and methane sulfonic (PKa = -1.9) acids.
[0082] It could thus be observed that the maximum amount, at 20oc and at
25 100oc respectively, in a 90/10 glycerol/water mixture by weight, of solubilized
salt is:
2.5% (at 20°C and at 1 oooc) in the case of the salt formed with sulfuric acid
(sodium sulfate);
8% and 9.4% in the case of the salt formed with hydrochloric acid (sodium
30 chloride);
15% and 30% 1n the case of the salt formed with acetic acid (sodium
acetate);
-18-
15% and 22% in the case ofthe salt formed with methane sulfonic acid
(sodium methane sulfonate).
[0083] The maximum solubility of sodium methane sulfonate reaches the value
of 22% in a 90/10 glycerol/water mixture by weight, at 1 00°C.
5 [0084] The above results show that the sodium salts of methane sulfonic acid
are 2 to 10 times more soluble in glycerol/water mixtures than, respectively,
sodium sulfate and sodium chloride. A solubility of the same order of magnitude
is also observed for the salts of methane sulfonic acid and the salts of acetic
acid, which is a weak organic acid.
10 [0085] A similar test of glycerol solubility is carried out, however at a
temperature of 180oC (without water). At this temperature of 180°C, it is
observed that the maximum quantity of salt solubilized in glycerol is:
- 0,6% for the sodium salt formed with sulfuric acid;
- 7,5% for the sodium salt formed with hydrochloric acid; and
15 - 26 %for the sodium salt formed with methane-sulfonic acid.
Example 2 : Comparative study of various acids with a crude glycerol phase
[0086] For this study, a crude glycerol phase is used. This crude glycerol
phase is obtained after trans-esterification with sodium hydroxide of a soybean
20 oil and separation of the phase containing methyl esters (biodiesel).
[0087] A quantity of 40 g of this crude glycerol phase is mixed with 4 g of
water, at room temperature. Each of the acids to be tested is added over 15
minutes, until a pH value of about 4 is obtained. The reaction mixture is mechanically
stirred and allowed to decant for a period of about 4 hours. The upper
25 phase mainly contains free fatty acids (FFA) and traces of esters of fatty acids,
whereas the lower phase mainly contains glycerol (glycerol enriched phase).
[0088] The following acids are comparatively tested:
• methane sulfonic aCid (MSA) at 70% by weight in water (Arkema);
• phosphoric acid (H3P04) at 75% by weight in water;
30 • sulfuric acid (H2S04) at 95% by v..(eight in water;
• citric acid (ACi) at 48% by weight in water;
• pure acetic acid (AAc);
• hydrochloric acid (HCI) at 37% by weight in water.
-19-
[0089] For each test, the following data are measured:
• the added acid quantity, expressed in weight % relative to the mass of crude
glycerol phase (40 g);
• the increase of the temperature of the reaction mixture (exotherm, ll T in oc);
5 • the quantity of insoluble compounds, expressed in weight % relative to the
mass of crude glycerol phase (40 g);
• the quantity of obtained glycerol phase, expressed in weight % relative to the
total mass of the two phases; and
• the acid number of the glycerol enriched phase.
10 [0090] The acid number (lA) is assessed by neutralization of the acidity of the
reaction medium with a strong base (for example potassium hydroxide or
sodium hydroxide). The acid number is defined as the quantity (in mg) of KOH
necessary to neutralize one (1) gram of sample.
[0091] The implemented method is as follows: in a beaker equipped With a
15 stirrer and a pH-measuring electrode (Mettler DG111 for aqueous medium), a
sample of exactly about 1 g (mass m) of glycerol enriched phase is introduced,
to which are added approximately 50 ml of deionized water. An aqueous
sodium hydroxide solution 0.1 moi/L is then added dropwise, with stirring, until
pH 12. The equivalence is indicated by a pH jump that gives the equivalent
20 volume v expressed in ml. The acid number (lA), expressed as mg of KOH, is
calculated by the following formula:
lA= vx0,1x56
m
where v is in ml, and m in grams.
[0092] The results are presented in the following Table 1:
~ -- h~1 --
acid MSA H3P04 H2S04
added guantit'l. (weight %/ 11.5 11.3 4.5
1!. T (OC/ 6 6 12 -- .
insoluble comeounds 0 3.4 Q (weight%/ •.
/A (mg of KOH!g/ 1 61 2
·--- ___ -__
gl'{.cerol ehase (weight %/ 68 63 32
ACi AAc HCI
19.5 15.0 9.3
8 2 8
-·
Q 0 Q
37 92 2
-=-·-· .
65 64 67
-· 20-
[0093] The amount of added acid is substantially identical for MSA, phosphoric
acid and hydrochloric acid. This amount is approximately two times higher for
the weak organic acids (citric acid), and two times lower for sulfuric acid.
However, with sulfuric acid, the formation of a gel which is detrimental to a good
5 phase separation is observed, and results in a twice lesser amount of recovered
glycerol.
[0094] In addition, the strong exotherm observed in the test with sulfuric acid
can be troublesome at the industrial level, requiring precautions during
acidification, including special facilities, e.g. a cooling module.
10 [0095] The amount of insoluble compounds produced during the test with
phosphoric acid makes this an unsuitable acid, the insoluble compounds being
particularly troublesome, especially during the subsequent glycerol purification
step by distillation, as described above.
[0096] These results also show that the acid number (lA) of the acidified
15 glycerol phase is lower in the case of MSA, H2S04 and HCI. This is a
particularly interesting advantage, considering the subsequent neutralization of
this glycerol phase with a strong base until pH 7, before distillation.
[0097] The use of hydrochloric acid is not required on an industrial scale
because of corrosion problems associated with this acid. Moreover, at 37% in
20 water, which represents the maximum attainable concentration, hydrochloric
acid contains a very large amount of water, which is weakly interesting on an
economical point of view, during the subsequent glycerol distillation.
[0098] These results show the great advantage associated with the use of
MSA, relative to other acids in industrial processes for production of glycerol
25 from triglycerides, but also in industrial processes for the combined production
of biodiesel and glycerol from triglycerides:
• MSA is a weakly corrosive and environmental friendly acid;
• salts formed with this acid are much more soluble in glycerol;
• the amount of MSA necessary to acidify the crude glycerol phase and the
30 exotherm due to acidification are. quite comparable with those observed with
phosphoric acid, however without generating insoluble compounds;
--21-
• the acid number of the enriched glycerol phase is very low, leading to a lower
amount of base necessary for the subsequent neutralization and accordingly
a smaller amount of neutralization salts, and finally
• a quite important amount of recovered enriched glycerol phase.
s [0099] MSA thus represents a particularly advantageous alternative, 1n
particular to phosphoric, sulfuric or hydrochloric acids, for the production of
glycerol from triglycerides.
5
-22-
CLAIMS
1. Use of at least one sulfonic acid, preferably at least one alkane sulfonic
acid, for recovering glycerol resulting from a reaction crude from
transesterification of glycerides, in particular of triglycerides of vegetable and/or
animal origin.
10 2. Use according to claim 1, in which said at least one sulfonic acid
corresponds to the general formula R-S03H, where R represents an alkyl or aryl
radical, preferably an alkyl radical.
3. Use according to either one of the preceding claims, in which said at
15 least one sulfonic acid corresponds to the general formula R-S03H, where R
represents a linear or.branched, saturated hydrocarbon-based chain containing
from 1 to 4 carbon atoms.
4. Use according to any one of the preceding claims, in which said at least
20 one sulfonic acid is methane sulfonic acid (CH3S03H).
5. Use according to any one of the preceding claims, in which said at least
one sulfonic acid is in anhydrous form or in the form of an aqueous solution
comprising from 5% to 90% by weight, in particular from 10% to 80% by weight
25 of sulfonic acid, and more particularly from 15% to 75% by weight, the rest to
1 00% consisting of water.
6. Use according to any one of the preceding claims, in which said at least
one sulfonic acid is used for the acidification of a reaction crude from
3o trarisesterification of glycerides, comprising:
- · glycerol,
- the alcohol(s) used for the transesterification reaction,
- optionally water, or traces of water,
-23-
- the basic catalyst(s) (used for the transesterification reaction), possibly in
trace form,
- fatty acid salt(s), possibly in trace form,
- optionally one or more fatty acid ester(s), possibly in trace form,
5 - optionally traces of mono-, di- and/or triglycerides,
- optionally traces of organic ~esidues other than glycerol,
- optionally traces of metals.
7. Process for recovering glycerol from a reaction crude resulting from the
10 triglyceride transesterification reaction, comprising at least the following steps:
a) provision of a basic reaction crude which is rich in glycerol and also
contains water and an alcohol, for example methanol;
b) optional separation of the solid impurities;
c) acidification of the reaction crude resulting from step a), with at least one
15 sulfonic acid, to a pH value less than 4, preferably less than 3, for example
close to 2;
d) optional decanting and separation of the phase rich in glycerol and of the
phase rich in free fatty acids and in fatty acid esters;
e) neutralization to a pH of approximately 7, using a base, for example a
20 strong inorganic base, preferably chosen from sodium hydroxide and
potassium hydroxide;
f) distillation of the alcohol and of the water;
g) distillation of the glycerol, preferably under reduced pressure;
h) recovery of the glycerol distilled,
25 it being possible for stepf) to be carried out before step e).
8. Process according to claim 7, in which the at least one sulfonic acid is
methane sulfonic add.
3o 9. Combined process for preparing, on the one hand, fatty acid esters or
mixtures of fatty acid esters and, on the other hand, glycerol, comprising at least
the following steps:
1) provision of triglycerides of vegetable arid/or animal origin,
-. 24-
2) transesterification, in a basic medium, of said triglycerides, in the presence
of at least one alcohol comprising from 1 to 10 carbon atoms, preferably in
the presence of metharol, so as to obtain, on the one hand, a phase rich
in fatty acid esters or mixtures of fatty acid esters and, on the other hand,
5 a phase rich in glycerol,
3) separation of the phases which are, on the one hand, rich in esters and,
on the other hand, rich in glycerol, for example by decanting,
4a) recovery of the fatty acid esters and mixtures of fatty acid esters, after
acidification, with at least one sulfonic acid, of the phase comprising said
10 fatty acid esters and mixtures of fatty acid esters, and
15
4b) recovery of the glycerol according to the process of claim 7, comprising
the acidification of the phase comprising the glycerol with at least one
sulfonic acid.
10. Process according to claim 9, in which said at least one sulfonic acid of
step 4b is methane sulfonic acid.
11. Process according to claim 9 or claim 10, in which said at least one
sulfonic acid of step 4a is methane sulfonic ackL