Technologies for Processing of Crude Glycerol from Biodiesel Production: Synthesis of Solketal and Its Hydrolysis to Obtain Pure Glycerol

Article abstract of DOI:10.1134/S1070427218090100

Information on the volume of production of biodiesel and crude glycerol is discussed. The possibility of using crude glycerol as a feedstock for preparing solketal is demonstrated. The specific features of the solketal synthesis from crude glycerol and of separation of the reaction products are described. A catalytic process is suggested for selective decomposition of solketal to glycerol to obtain purified glycerol of any required concentration up to 99.8 wt.%. A flowsheet is provided for processing of crude glycerol to obtain solketal and subsequently converting it to obtain pure glycerol.

Full text of DOI:10.1134/S1070427218090100

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2018, Vol. 91, No. 9, pp. 1478−1485. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © G.S. Dmitriev, L.N. Zanaveskin, A.V. Terekhov, V.O. Samoilov, I.A. Kozlovskii, A.L. Maksimov, 2018, published in Zhurnal Prikladnoi
Khimii, 2018, Vol. 91, No. 9, pp. 1314−1323.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Technologies for Processing of Crude Glycerol from Biodiesel
Production: Synthesis of Solketal and Its Hydrolysis
to Obtain Pure Glycerol
a,
a
a
G. S. Dmitriev *, L. N. Zanaveskin , A. V. Terekhov ,
a
b
a
V. O. Samoilov , I. A. Kozlovskii , and A. L. Maksimov
a
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
e-mail: dmitriev.gs@mail.ru
*
b
Mendeleev University of Chemical Technology of Russia, Moscow, 125047 Russia
Received June 5, 2018
Abstract—Information on the volume of production of biodiesel and crude glycerol from it is analyzed. The
possibility of using crude glycerol as a feedstock for preparing solketal is demonstrated. The specific features of
the solketal synthesis from crude glycerol and of separation of the reaction products are described. A catalytic
process is suggested for selective decomposition of solketal to glycerol to obtain purified glycerol of any required
concentration up to 99.8 wt %. A flowsheet is suggested for processing of crude glycerol to obtain solketal and
subsequently converting it to obtain pure glycerol.
Keywords: glycerol, solketal, biodiesel, fatty acids, sulfuric acid
DOI: 10.1134/S1070427218090100
Active studies are being performed today to search
for and bring into practice renewable power sources.
Biodiesel is considered as a viable alternative to fossil
fuels. Much interest in this product is largely due to its
use in production of renewable natural raw materials,
to its biodegradability, and to low level of pollutant
emissions into the atmosphere [1, 2].
Thus, utilization or processing of the by-product,
crude glycerol, becomes a serious problem at biodiesel
production plants. Purified glycerol can be the main
commercial product, but only a few plants in Asia and
Latin America have facilities for glycerol purification.
The purification is expensive, and the possible
applications of crude glycerol are very limited.
According to the estimates made by Abercade, in
016 the world volume of biodiesel production reached
2.9 bln L, exceeding by 12.7% the level of the previous
year. Such growth is due to a significant increase in the
biodiesel amount produced in Argentine (by 29%), the
United States (by 21%), and Indonesia (by a factor of
.6). The total cost of the biofuel produced in 2016 is
estimated at $24.8 bln, exceeding by 11.7% the level of
the year 2015.
Too large excess of crude glycerol from biodiesel
production, on the one hand, will affect the market of
refined glycerol and, on the other hand, can give rise
to environmental problems associated with glycerol
purification. The biodiesel production cost increases by
$0.021/L per $0.22/kg decrease in the glycerol sale price
[4]. Hence, commercial use of glycerol for producing
high-value-added chemicals is extremely important
for the financial efficiency of the biodiesel production.
In this case, crude glycerol can be considered as a
demanded by-product rather than production waste.
2
3
2
The major by-product in biodiesel production is
crude glycerol. About 1 kg of crude glycerol is formed
per 10 kg of biodiesel [3]. Thus, the total volume of
bioglycerol production in 2016 was almost 3 mln t.
Purified glycerol is widely used in chemical industry
as a feedstock for many processes including modern
1
478

TECHNOLOGIES FOR PROCESSING OF CRUDE GLYCEROL
1479
Table 1. Specification of the main glycerol grades, accepted in the United States, and the corresponding requirements of the
Russian state standards (GOST)
Glycerol
content
USA), wt %
GOST requirements
Glycerol grade
glycerol
Use of glycerol
(
content,
wt %
GOST, grade
Crude glycerol
70–90
99.5
78–86
88–94
≥94
6823–2000
Production of distilled glycerol and other
technical applications
Technical grade
6824–96, grades T-88, Production of distilled glycerol and
Т-94
other technical applications
Pharmacopoeia grade
99.5
6824–96, grade PK-94 For pharmacopoeia purposes and for
food and cosmetic industry
(USP)
Food grade (USP/
FCC)
99.7
≥98
6824–96, grade D-98
For producing explosives (GOST 6884–
96)
technologies for the production of epichlorohydrin
5] and oxygenates [6]. More than 2000 commercial
Crude glycerol contains impurities of fatty acids,
methanol, spent catalyst, and other chemicals; they can
negatively affect the human health and the environment.
Hence, glycerol, especially that intended for use in
pharmaceutics (see, e.g., United States Pharmacopoeia,
USP) and food industry (Food Chemical Codex, FCC),
should meet definite requirements and standards.
[
items including pharmacological, medical, and food
products are produced today using glycerol. Glycerol is
used in cosmetology, military industry, and agriculture,
including attempts of using it as forage for nonruminant
animals, etc. [7].
Various vegetable and animal fats are used for
biodiesel production. About 100 mln t of vegetable oils
are produced annually in the world, and they are mainly
used for food purposes. The production of vegetable oils
cannot yet grow at a rate corresponding to the growth
of the biodiesel production. Competition of biodiesel
with the food market can hardly be avoided. However,
today the main raw materials for biodiesel production
are rapeseed, soybean, and palm oils. The choice of
particular oil largely depends on the production region.
As a rule, to reduce the biodiesel production cost, the
oil that is the most readily available in the given region
is chosen. Rapeseed and sunflower oils are used in the
European Union, palm oil prevails in the biodiesel
production in tropical countries, and soybean oil and
animal fats are the main raw materials in the United
States.
The specification of the main grades of glycerol,
accepted in the United States, and the requirements
of the Russian state standards GOST 6823–2000
(Natural Crude Glycerol) and GOST 6824–96 (Distilled
Glycerol) is given in Table 1.
Crude glycerol obtained from various raw materials
will contain various impurities, and the purification
technology will be largely determined by the application
fields of the ready product. When glycerol is mainly
used for technical purposes, certain contaminants are
acceptable, whereas in most cases, e.g., when glycerol is
used in food and pharmaceutical industries, high degree
of purification is required.
The technologies developed for the purification
of glycerol obtained as a by-product from biodiesel
production are mainly based on adapted procedures
used at soap-making plants in glycerol recovery from
spent-soap lye. These processes include vacuum
distillation, ion exchange, adsorption treatment with
activated carbons, membrane separation, etc. The first
step involves neutralization and phase separation of
the reaction mixture to remove free fatty acids and
some salts. The next step is the solution concentration
Biodiesel as an alternative diesel fuel contains
a mixture of esters of higher fatty acids. The ester
interchange of triglycerides (e.g., of vegetable oil) with
methanol in the presence of such catalysts as NaOH
or NaOCH leads to the formation of the methyl ester
3
(biodiesel) and glycerol phase.
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DMITRIEV et al.
Table 2. Physicochemical parameters of crude glycerol from biodiesel production at OAO Mogilevkhimvolokno
Parameter
Value
External appearance
Dark brown liquid
Weight fraction of glycerol
Weight fraction of ash
Weight fraction of water
75.6
3.8
17.8
1.28
1.30
0.2
Weight fraction of soap (in terms of sodium oleate)
Weight fraction of fatty acids (in terms of oleic acid)
Weight fraction of methanol
by evaporation to remove methanol from the glycerol
stream. The final step is a combination of different
purification procedures including vacuum distillation,
ion exchange, membrane separation, and adsorption.
construction of epichlorohydrin production plants. To
compare, by 2022 only in Europe the annual glycerol
consumption is expected to exceed 1190 thousand tons,
and the global glycerol market will exceed $4 bln (data
of Greenea: Market Watch, January 2017).
As noted above, the technology for purification of
crude glycerol is complicated and power-consuming,
which does not allow many small and medium biodiesel
producers to use it in the practice.
Among production of epichlorohydrin, large amount
of glycerol can be demanded for the production of a new
commercial product, solketal (SK), a promising octane
booster improving the operation properties of fuels for
internal combustion engines [8].
On the other hand, the price of purified glycerol
in Europe in June 2017 was €850 per ton of 99.7%
glycerol and €650 per ton of 99.5% glycerol. In the
beginning of 2018, the price decreased to €550 and 500,
respectively. The price of purified glycerol in Asia is
today $390–400 per ton CFR (delivery terms: Cost and
Freight) (in China, $420–460 per ton CFR). The price
of 80% glycerol in Europe fluctuates between €150 and
It should be noted that specifically crude, and not
distilled, glycerol is an environmentally advantageous
feedstock for producing high-value-added organic
compounds.
This study deals with the synthesis of solketal from
glycerol formed as a by-product of biodiesel production
and with the subsequent hydrolysis of the isolated
solketal as an alternative procedure for purification of
crude glycerol.
2
75 (data of the Greenea: Market Watch consulting and
broker company, April–March 2018).
The complexity of the technology for purification
of crude glycerol and, as a consequence, the large
difference in the product price depending on purity
make it topical to develop new technologies for using
crude glycerol for the production of high-value-added
commercially demanded products. The use of crude
glycerol as a raw material is particularly attractive for
the Russian industry, because the biodiesel production
in Russia makes no sense owing to large crude oil
resources, and the available facilities for production of
highly concentrated glycerol are extremely limited (the
possible annual output does not exceed 5–6 thousand
tons). Therefore, Russia imports today more than
EXPERIMENTAL
As starting chemicals we used acetone (chemically
pure grade), sulfuric acid (chemically pure grade),
sodium hydroxide (analytically pure grade), distilled
water, Amberlyst 35Wet resin, and crude glycerol from
biodiesel production at OAO Mogilevkhimvolokno
(Belarus) (for composition, see Table 2).
Fatty acids were removed from crude glycerol by
adding sulfuric acid, vigorously stirring, and settling
in a separation funnel. The lower glycerol layer was
separated from the fatty acid layer and was subsequently
used for the synthesis of solketal.
6
0 thousand tons of glycerol annually, and this volume
can increase by a factor of more than 2 with the planned
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TECHNOLOGIES FOR PROCESSING OF CRUDE GLYCEROL
1481
Scheme.
Experiments on the solketal synthesis from crude
glycerol were performed in a 500-mL three-necked flask
equipped with a thermometer, a reflux condenser, and a
power-driven controllable-rate stirrer. The experiments
were performed at 20–25°С; the stirrer rotation rate was
vaporizer temperature 220°С, detector temperature
200°С, programmable column temperature 120–180°С.
The water concentration was determined by gas
chromatography on the same device using Polysorb-1
packing.
1
500 rpm. Acetone was added in a 3–6.5-fold molar
The concentration of fatty acids and soaps in the
initial glycerol and reaction products was determined
by conductometric titration. The analysis results were
calculated in terms of oleic acid (as the major fatty acid
component of rapeseed oil from which the biodiesel is
produced at OAO Mogilevkhimvolokno) and sodium
oleate.
excess relative to glycerol.
After the solketal synthesis, a 5 wt % NaOH solution
was added to the reaction mixture to neutralize the
excess acidity to pH 7–7.5. Mineral salts and soaps
precipitate in the course of the solketal synthesis and
neutralization of the reaction products. After separating
the precipitate on a blue ribbon paper filter, the reaction
products were distilled on a glass column 20 mm in
diameter and 500 mm high, packed with glass Raschig
rings 4 mm in diameter and equipped with a unit for
vapor condensation, for reflux with the possibility
of controlling the reflux amount, and for condensate
collection in a receiver at atmospheric pressure or in a
vacuum.
The ash residue was determined according to GOST
(State Standard) 7482–96.
RESULTS AND DISCUSSION
Solketal is formed by condensation of glycerol with
acetone on acid catalysts. The reaction is shown in the
scheme.
The solketal decomposition into glycerol was
performed in a 250-mL three-necked glass flask in the
presence of sulfuric acid or Amberlyst 35Wet resin
under the conditions of straight distillation of the acetone
formed in the process or of fractional distillation on a
column described above.
Hereinafter, the term solketal is applied to a mixture
of these two isomers.
The initial crude glycerol has weakly acidic reaction
(pH 5–6) due to fatty acid impurities. When crude
glycerol is mixed with acetone, two liquid phases are
formed because of limited mutual solubility of the
components [9]. Vigorous stirring of this mixture for 24
h led to no changes in the reaction system, and solketal
was not formed even in trace amount. That is, weak fatty
acids cannot catalyze reaction (1).
The organic reaction products were analyzed with
a Tsvet-800 chromatograph equipped with a thermal
conductivity detector under the following conditions:
2
-m packed column, stationary phase 20% poly(ethylene
glycol succinate) and 3% di(2-ethylhexyl) sebacate
on Chromaton N-AW (0.25–0.315 mm fraction),
It is known that sulfuric acid is an active catalyst of the
solketal formation [10]. When using distilled glycerol,
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DMITRIEV et al.
after vigorous stirring for 2.5 h at 20–25°С, with more
than 50% glycerol conversion. Homogenization of the
reaction mixture considerably accelerates the reaction,
which was limited in the emulsion by the component
diffusion rate. The solketal concentration in the reaction
mixture reached 25.1 wt % in 10 min after the formation
of the homogeneous solution and did not change further
with time; i.e., the equilibrium was attained. The
equilibrium conversion of glycerol was 78%.
The dependence of the equilibrium conversion
of glycerol on its molar ratio with acetone is shown
in Fig. 2. As can be seen, when synthesizing solketal
from crude glycerol, an increase in the molar excess of
acetone over 5 influences the equilibrium concentration
of glycerol insignificantly. On the other hand, larger
excess of acetone leads to increased cost of its separation
by fractional distillation. Therefore, approximately
fivefold molar excess of acetone should be considered
as optimum.
Fig. 1. Content of fatty acids in crude glycerol (in terms of
oleic acid) as a function of pH.
catalytic concentrations of H SO (about 0.01 wt %)
2
4
are sufficient to ensure the acceptable reaction rate.
When using crude glycerol, the required amount of
sulfuric acid increases considerably, which is caused
by its neutralization with soaps present in glycerol with
the formation of fatty acids and sulfates. The solketal
synthesis can be performed directly from crude glycerol
after acidification of the mixture with sulfuric acid;
however, in this case all the fatty acids remain in the
reaction mixture, and they should be neutralized prior to
distillation of the reaction products, which increases the
NaOH consumption.
With the progress of the solketal synthesis and glyc-
erol consumption, the volume of the glycerol phase de-
creases, and the mineral salts that were present in the
initial crude glycerol in the dissolved form partially
precipitate because of high acetone concentration in the
second phase. By the moment of the equilibrium attain-
ment in the experiment with a 4–5-fold molar excess of
acetone, approximately 80% of the mineral compounds
initially present in crude glycerol precipitated.
The solketal is isolated from the reaction products by
fractional distillation. Because reaction (1) of solketal
formation is reversible, prior to distillation of the
reaction products the catalyst (sulfuric acid) should be
removed from the system by neutralization. Otherwise,
heating of the reaction mixture in the stripping section
of the distillation column will lead to decomposition of
solketal to acetone and glycerol.
Preliminary treatment of crude glycerol with sulfuric
acid, followed by separation of the fatty acids, which
float up, is more appropriate. This not only reduces
the alkali consumption for neutralization of reaction
products, but also allows separation of the concentrated
phase of fatty acids, which can become a commercial
product after additional purification.
Figure 1 shows how the concentration of fatty acids
in terms of oleic acid) in crude glycerol depends on pH
after the acidification with sulfuric acid and settling of
the reaction mixture.
The crude solketal is neutralized with a sodium
hydroxide solution to pH 7–7.5. In the process, an
additional amount of sodium sulfate precipitates, and
small amounts of fatty acids present in the reaction
mixture transform into soaps. The soaps formed are
virtually fully adsorbed by crystals of mineral salts,
which are separated by common methods.
(
The lowest content of fatty acids in the glycerol layer
was reached upon acidification of crude glycerol with
sulfuric acid to pH 1–2. This pH value is recommended
for the first step of the process.
The reaction products after neutralization and sep-
aration of the precipitate are subjected to fractional
distillation. Excess acetone is distilled off first; it can
be used again in the solketal synthesis. Then, water is
The separated acidic solution of crude glycerol, when
mixed with a 4–5-fold molar excess of acetone (relative
to glycerol), forms an emulsion, which transforms
into a homogeneous solution of reaction products
distilled off, and solketal is distilled in a vacuum (Т =
b
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TECHNOLOGIES FOR PROCESSING OF CRUDE GLYCEROL
1483
9
9
0°С at 10 mmHg). The solketal purity is no less than
9.5 wt %.
Thus, the available and relatively cheap crude
glycerol does not require any preliminary purification
or concentration and can be used directly for preparing
solketal using sulfuric acid as a catalyst.
On the other hand, the reversibility of the solketal
synthesis and relatively mild conditions of this reaction
allow this process to be used as an alternative procedure
for purification of crude glycerol formed as a by-product
from biodiesel production.
Acetone : glycerol molar ratio
Fig. 2. Equilibrium conversion of crude glycerol (OAO
Mogilevkhimvolokno) as a function of acetone : glycerol
molar ratio.
The solketal prepared by the above procedure should
be diluted with water, an acid catalyst should be added,
and the equilibrium of reaction (1) should be shifted to
the left by distilling off the low-boiling acetone formed
by hydrolysis of solketal.
bottoms (190°С), required for ensuring the vapor flow
throughout the column height.
In hydrolysis of solketal in the mode with straight
distillation of acetone, heating to such high temperatures
is not required, and the acetone distillation is complete
at temperatures of up to 150°С. The acrolein formation
can thus be avoided. However, in this case the distilled
acetone is moist, and it should be additionally dried
prior to the solketal synthesis.
The conditions of our experiments on hydrolysis of
solketal are given in Table 3. In the first experiment, we
distilled the acetone off using a distillation column. As
expected, the condensate obtained at 60°С was acetone
with 0.5 wt % moisture content. In the process, the
temperature of the bottoms increased to 190°С. Then,
the temperature of the bottoms started to gradually
decrease, and at 95°С the condensation of a water–
acrolein mixture started.
The results of experiments in the presence of
heterogeneous (Amberlyst 35Wet) and homogeneous
(
H SO ) catalysts show that sulfuric acid is more active
2 4
The acrolein formation was practically avoided in
the experiment with straight distillation in the presence
of Amberlyst 35Wet resin. After the completion of the
acetone distillation, as indicated by a sharp increase in
the condensation temperature to 100°С, the bottoms was
a pale yellow liquid consisting of glycerol with impuri-
ties of water (5 wt %), solketal (0.2 wt %), and acro-
lein (0.05 wt %). Because the process was performed
in the straight distillation mode, the moisture content
of the distilled acetone was 6–15 wt %; no impurities
of acrolein and other compounds were detected in the
condensate.
in the side reaction of the acrolein formation, which is
apparently due to its stronger acid properties.
Our experiments show that heterogeneous acid cata-
lysts are preferable for the for the solketal hydrolysis.
The use of such catalyst, as compared to a homogeneous
catalyst (e.g., H SO ), does not require the subsequent
2
4
neutralization of the reaction mixture; the catalyst is not
consumed in noticeable amounts and exhibits high se-
lectivity. Virtually no acrolein, the major and extremely
undesirable by-product, is formed on Amberlyst resin.
Acrolein is a colorless low-boiling liquid with strong
order and strong lacrimating effect. This impurity can
impart characteristic unpleasant odor to pure glycerol,
which is inadmissible.
Replacement of Amberlyst resin by sulfuric acid in
the straight distillation mode led to the formation of
1
.4 wt % acrolein in the bottoms.
The concentrated glycerol formed after the solketal
hydrolysis can be used for technical purposes without
additional treatment or can be subjected to vacuum
distillation to obtain a high-quality commercial product
suitable for use in pharmaceutical and food industry.
It is known that acrolein is produced from glycerol
in a liquid-phase process at atmospheric pressure at Т >
50°С in the presence of homogeneous or heterogeneous
acid catalysts (e.g., H SO , KHSO , initial and acid-
1
2
4
4
promoted zeolites, γ-Al O , etc.) [11, 12].
2
3
Based on the data obtained, we suggested a flowsheet
for the synthesis of solketal from crude glycerol as a by-
product from biodiesel production and a procedure for
Intense formation of acrolein in the course of acetone
distillation in a column is due to high temperature of the
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1484
DMITRIEV et al.
Table 3. Conditions of experiments on solketal hydrolysis
Reactants loaded
Run
no.
Mode of acetone
distillation
Tbottoms, °C Tcondensate,
(max)
°C
catalyst
m , g
m , g
mH2O, g
cat
SK
1
2
3
Fractional distillation Amberlyst 35Wet
2.5
164
152
152
25
30
30
190
150
136
60 and 95
70–80
Straight distillation
Straight distillation
Amberlyst 35Wet
H SO
0.1
0.02
70–80
2
4
glycerol purification via solketal (Fig. 3). The procedure
that we developed allows preparing from crude glycerol
a number of commercial products: solketal, glycerol,
and fatty acid concentrate, which, if necessary, can be
separated into individual acids or their mixtures.
require any pretreatment or complication of synthesis
conditions as compared to the process with distilled
glycerol. The process occurs at room temperature in
the presence of sulfuric acid as a catalyst. The sulfuric
acid consumption is mainly determined by the content
of soaps, which transform into fatty acids and sulfates
under the action of the acid. The fatty acids themselves
do not catalyze the solketal synthesis because of their
low strength. The optimum molar ratio of acetone
to glycerol, required for preparing solketal, is 5 : 1.
CONCLUSIONS
Crude glycerol from biodiesel production can
serve as a feedstock for producing solketal. It does not
Bottoms
Crude
glycerol
Removal of fatty
acids from crude
glycerol
Acetone
Solketal synthesis
Fatty acids
Acetone + methanol
Separation of reaction
products
Separation of mineral
salts
NaOH
solution
Solketal
Water
Sodium sulfate
Acetone
Solketal
decomposition
Glycerol
separation
Glycerol
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TECHNOLOGIES FOR PROCESSING OF CRUDE GLYCEROL
1485
The equilibrium conversion of glycerol in this case is
0% at 20–25°С. The major fraction of mineral salts
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8
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main substance content of no less than 99.5 wt % can
be isolated from the neutralized reaction products by
fractional distillation after separating the precipitate.
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1
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ACKNOWLEDGMENTS
1
1. Patent JP 2006290815, Publ. 2006.
The study was performed within the framework of
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 91 No. 9 2018