Specific features of solketal synthesis on KU-2-8 cation-exchange resin

Article abstract of DOI:10.1134/S1070427216010079

Technological specific features of the solketal synthesis on a KU-2-8 cation-exchange resin were improved. Data are presented on the mutual solubility of the components in the glycerol–acetone–water and glycerol–acetone–water–ethanol systems. It is shown that use of ethanol for homogenization of the starting reagents is more than twice more efficient than performing the process with delivery of an emulsion of the reagents or reaching a full solubility of glycerol in acetone due to the 20-fold excess of the latter at 35°C. It is shown that the delivery of a glycerol emulsion in acetone to an immobile bed of a heterogeneous catalyst working in the falling-film mode leads to flooding of the reactor at spraying densities exceeding 0.15 m³ m^–2 h^–1.

Full text of DOI:10.1134/S1070427216010079

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 1, pp. 45−50. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © G.S. Dmitriev, A.V. Terekhov, S.N. Khadzhiev, L.N. Zanaveskin, 2016, published in Zhurnal Prikladnoi Khimii, 2016, Vol. 89, No. 1, pp. 50−55.
VARIOUS TECHNOLOGCAL
PROCESSES
Specific Features of Solketal Synthesis
on KU-2-8 Cation-Exchange Resin
G. S. Dmitriev, A. V. Terekhov, S. N. Khadzhiev, and L. N. Zanaveskin
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences,
Leninskii pr. 29, Moscow, 119991 Russia
e-mail: dmitriev.gs@mail.ru
Received December 22, 2015
Abstract—Technological specific features of the solketal synthesis on a KU-2-8 cation-exchange resin were
improved. Data are presented on the mutual solubility of the components in the glycerol–acetone–water and glyc-
erol–acetone–water–ethanol systems. It is shown that use of ethanol for homogenization of the starting reagents
is more than twice more efficient than performing the process with delivery of an emulsion of the reagents or
reaching a full solubility of glycerol in acetone due to the 20-fold excess of the latter at 35°C. It is shown that
the delivery of a glycerol emulsion in acetone to an immobile bed of a heterogeneous catalyst working in the
falling-film mode leads to flooding of the reactor at spraying densities exceeding 0.15 m3 m–2 h–1.
DOI: 10.1134/S1070427216010079
Solketal (SK, mixture of the isomers 2,2-dimethyl-
4-hydroxymethyl-1,3-dioxolane and 2,2-dimethyl-
1,3-dioxan-5-ol) is a promising high-octane additive
improving the service properties of fuels for internal-
combustion engines [1]. Solketal is formed in the
condensation reaction of glycerol with acetone on acid-
type catalysts:
KU-2-8 cation-exchange resin is one of the most
promising catalysts, which exhibit a high activity [2] and
are readily available and inexpensive.
The starting reagents, glycerol and acetone, have a
very low mutual solubility and form emulsion under
intensive mixing. The starting reaction mixture can be
homogenized via addition of compounds well soluble in
both glycerol and acetone, e.g., alcohols [3].An advantage
of ethanol over other alcohols is in its inertness toward
the starting reagents and reaction products under the
solketal synthesis conditions and the boiling point close
45

46
DMITRIEV et al.
Glycerol
Acetone
chemically pure grade (Ekos-1 ZAO, Russia), rectified
ethanol, and distilled water.As the catalyst served KU-2-
8 cation-exchange resin preliminarily dried with acetone
on a water bath.
The mutual solubility of the components was studied
in a three-necked glass vessel equipped with stirrer, reflux
and a thermocouple cartridge, and electric heater. The
reactor temperature was controlled and measured with
a set of devices including a TRM-101 single-channel
microprocessor programmed temperature-measuring
and -controlling unit, solid-state relay, and laboratory
autotransformer.
To atmosphere
The vessel was charged with necessary amounts of
acetone and glycerol and the stirrer was switched on. The
reaction mixture was heated to a prescribed temperature.
Water and(or) ethanol were batched with a syringe in
an amount providing the disappearance of the emulsion
(homogenization of the system).
The laboratory installation for studying the process in
a reactor with fixed catalyst bed is shown schematically
in Fig. 1. The reactor had the following parameters: inner
diameter 2.6 cm; height of the catalyst bed, 47 cm; and
catalyst mass 205 g. The catalyst was placed on a layer of
glass packing constituted by rings in the form of a helix
with 2–3 spiral turns 3–4 mm in diameter and 1 cm thick.
The height of the packed bed was 3 cm.
To receiver
Fig. 1. Schematic of a laboratory installation for studying the
process in a reactor with fixed catalyst bed.
To provide a uniform distribution of the reagent flow,
the same packed bed with height of 5 cm was situated
above the catalyst. The reagents were preliminarily
mixed in a heated apparatus with a stirrer to form a stable
emulsion.
to that of acetone, which hardly complicates the scheme
of its evaporation from solketal and the organization of
recycle in the synthesis stage.
It should be noted, however, that there are no published
data on the influence of the ethanol concentration on
the homogenization process of a mixture of glycerol
and acetone at their various ratios and temperatures. In
addition, it should be taken into account that the reagents
are not absolutely dry and may contain up to 2 wt % water,
which can affect the homogenization process: water also
raises the mutual solubility of the starting reagents. A
study of the process of mutual solubility in the glycerol–
acetone–ethanol–water four-component system is a rather
topical task in development of an industrial technology
of solketal synthesis. In the end, the information obtained
will be used to select the type of a reactor for synthesizing
the oxygenate and the working conditions of the reactor.
The reaction products were analyzed on a Tsvet-800
gas chromatograph with ZB-WAX capillary column and
flame-ionization detector.
RESULTS AND DISCUSSION
It is known that the solketal synthesis process is
performed at a temperature of 35–55°C and glycerol-to-
acetone ratio of 1 : (5–20) [4]. Under these conditions, a
more or less stable emulsion of glycerol and acetone can
be only obtained with intense agitation of the components
of the mixture. Otherwise, the mixture of the reagents is
a two-component system, with a layer of acetone over
the lower layer of glycerol.
EXPERIMENTAL
Because a heterogeneous catalyst, KU-2-8 resin, is
used in the process, only two instrumentation variants
of the synthesis stage are possible: a bulk reactor with
We used the following reagents: glycerol of special-
purity grade (Kupavnareaktiv OOO, Russia), acetone of
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SPECIFIC FEATURES OF SOLKETAL SYNTHESIS
47
Water, wt %
high-intensity mechanical agitation of the reagents and
the catalyst or a column-type apparatus filled with a fixed
catalyst bed and sprayed with an emulsion of glycerol and
acetone. Only these two kinds of apparatus can provide
the maximum contact of the starting reagents with the
catalyst surface and, as a consequence, a high rate of the
condensation reaction.
The process in reactors with intense mechanical
agitation is not considered here because of leading
to disintegration (abrasion) of the catalyst and its
contamination with fine particles of reaction products,
which creates technological difficulties in the stage
of isolation and purification of solketal, i.e., it is
technologically unfeasible.
Homogeneous
system
The process in a reactor with a falling film of
preliminarily mixed reagents is devoid of the above
shortcomings. It was experimentally demonstrated that,
at a synthesis temperature of 35°C, the stratification
of the emulsion of glycerol and acetone in a thin
film at a spraying density of the catalyst bed of up to
0.15 m3 m–2 h–1 does not lead to a significant decrease
in the process rate. This is so because the insignificant
thickness of the film and its sufficiently high motion speed
provide an unobstructed contact of the reagents with the
catalyst surface.
Glycerol, wt %
Acetone, wt %
Fig. 2. Solubility diagram in the glycerol–acetone–water system
at 35°C.
high solubility of the reaction products in glycerol taken
into account, the rate of their diffusion from the catalyst
surface has in all likelihood no noticeable effect on the
solketal synthesis rate.
A solution to the appearing problems consists in
passing to use of the starting reagents in the form of
solutions.As noted above, introduction of certain amounts
of ethanol and(or) water into the emulsion of glycerol
and acetone makes it possible to obtain a homogeneous
mixture of the reagents.
Raising the density of catalyst praying by a mixture of
the starting reagents changes the course of hydrodynamic
and, as a consequence, kinetic processes on the catalyst.
Naturally, raising the flow rate of the emulsion leads to an
increase in the thickness of the falling film. At a spraying
density exceeding 0.15 m3 m–2 h–1, the stratification
results in that a thick glycerol layer is formed in the upper
part of the catalyst bed, which substantially diminished
the free-cross-section area of the reactor. As a result, the
flow of reagents cannot freely pass any more through the
catalyst bed and starts to fill the apparatus over the catalyst
bed. Thus, the process of solketal synthesis in a reactor
with a fixed catalyst bed and falling film of the starting
reagents becomes inoperative at a high spraying density.
Depending on the quality of raw materials used to
synthesize solketal, the starting mixture of the reagents
will contain a certain amount of water.
The data on the effect of water concentration on the
homogenization of the mixture of glycerol and acetone
at 35°C are presented in Fig. 2.
Because the solketal formation reaction (1) is of
equilibrium nature, raising the humidity of the starting
raw material will lead to a decrease in the equilibrium
degree of glycerol conversion. Hence follows that glycerol
and acetone used in the synthesis process should be as
dry as possible. However, the cost of glycerol steeply
grows with its increasing concentration (purity), which
inevitably affects the technical and economic parameters
of the process. Fr example, the cost of glycerol of D-98
brand [GOST (State Standard) 6824–96] with 98%
main substance is 30–50% less expensive than glycerol
containing 99.3% main substance (GOST 6259–75).
In addition, it should e noted that the solketal
formation rate decreases with increasing spraying density
already below its value of 0.15 m3 m–2 s–1. It is clear that
the thickness of the glycerol film increases gradually,
and this film also gradually covers the increasingly large
surface area of catalyst particles. As a result, the access
of acetone to this catalyst surface is hindered and the rate
of the condensation process is now determined by the rate
of acetone diffusion across the glycerol layer. With the
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48
DMITRIEV et al.
The results of experiments on homogenization
of a glycerol–acetone–ethanol–water mixture in the
temperature range 20–35°C are presented in Fig. 3. The
four-component mixture is homogeneous in the region
lying above the curves, and heterophase in the lower
lying region.
The effect of the ethanol concentration on the
homogenization of the acetone–glycerol mixture can
be described by a linear function (e.g., at 35°C y =
16.18–0.96x) at ethanol concentrations of 2–18 mol %
and temperature in the range under study. The larger the
excess of acetone, the smaller the amount of ethanol
necessary for homogenizing the starting reagents. A
decrease in temperature by every 5° requires an increase
in the ethanol concentration by 0.7–1.1 mol %.
c, mol %
Fig. 3. Acetone : glycerol molar ratio at which, depending on
the ethanol concentration c, the glycerol–acetone–ethanol–water
heterophase mixture becomes a solution. T (°C): (1) 35, (2) 30,
(3) 25, (4) 20.
Table 1 lists data on how the concentration of water
in a mixture of the starting reagents changes when an
azeotrope of ethanol with water (96 wt % ethanol) is
used for homogenization with 98 wt % glycerol and
98.7 wt % acetone.
To provide the maximum conversion of glycerol,
reaction (1) is performed at 5–20-fold molar excess
of acetone [4]. The excess amount of acetone is
evaporated from the reaction products and recycled.
The concentration of water in acetone being evaporated
depends on the operation parameters of the rectification
column. Calculations and experiments demonstrated that,
when the reaction products formed in solketal synthesis
are separated, the average moisture content of acetone
is 1.3 wt %. This degree of acetone drying is acceptable
for performing solketal synthesis with sufficiently high
equilibrium degrees of glycerol conversion (≥90%).
Analysis of the data in Table 1 shows that addition of
the ethanol azeotrope to the starting mixture has only a
slight effect on the change in the concentration of water
and, consequently, on the equilibrium degree of glycerol
conversion. For example, when a mixture of acetone
and glycerol is homogenized at their molar ratio of 5 : 1,
addition of the ethanol azeotrope will lead to a decrease
in the equilibrium degree of glycerol conversion by only
0.8% (calculated from the data of [5]).
Thus, the homogenization of the starting mixture of
the reagents with ethanol is a rather effective way to
intensify the process, which affects the yield of solketal
only slightly.
Analysis of the data in Fig. 2 shows that, with 98%
glycerol and 98.7% acetone used as raw materials, the start-
ing mixture becomes homogeneous at a molar excess of
acetone of 20-fold and more. Use of more humid reagents
will make it possible to reduce the excess of acetone for
homogenizing the mixture, but this will be accompanied
by a decrease in the equilibrium yield of solketal.
Table 2 presents the results of a comparison of two
methods for homogenization of the starting mixture of
acetone and glycerol: with a 20-fold molar excess of
acetone and with ethanol. The calculations were made
for a temperature of 35°C with the use of published data
on equilibrium constants [5].
It is necessary to take into account that such a sig-
nificant excess of acetone will naturally lead to lower
working concentrations of glycerol and, consequently, to
a lower rate of the condensation reaction (1). In addition,
the energy expenditure for acetone recycling will become
several times higher.
It follows from a comparison of the data in Table 2
that use of ethanol for homogenization of the starting
mixture leads to a decrease in the equilibrium degree
of glycerol conversion in a single pass through the
reactor by 7.5%, but makes it possible to reduce by a
factor of 4.5 the expenditure for acetone recycling at
an insignificant increase (by 5.5%) in the amount of
glycerol in the recycle. At the same time, the product
of glycerol and acetone concentrations becomes 2.6
One of the most rational ways to homogenize the
glycerol–acetone mixture is by using an additional
solvent, e.g., ethanol. This makes it possible to obtain
a homogeneous mixture at a smaller excess of acetone.
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SPECIFIC FEATURES OF SOLKETAL SYNTHESIS
49
Table 1. Change in the concentration of water in the starting reaction mixture at various molar ratios between acetone and glycerol
in relation to the concentration of the introduced ethanol azeotrope
Mass, g
Content in the mixture,
wt %
Acetone : glycerol molar
ratio
Content of water without
addition of ethanol, wt %
mixture of
glycerol and
acetone
ethanol
ethanol
water
15 : 1
10 : 1
5 : 1
100
100
100
100
1.3
4.8
1.23
4.40
1.40
1.52
1.69
1.88
1.37
1.40
1.47
1.61
9.7
8.49
2 : 1
12.9
10.97
Table 2. Comparison of methods for homogenization of the glycerol–acetone mixture
Amount in the recycle,
Equilibrium
Product of the reagent
concentrations cac·cgl at the
initial instant of time, mol mol
Homogeneous mixture of
kg per 1 kg of solketal
degree of glycerol
the starting reagents
conversion, %
ethanol
acetone
glycerol
GL : AC molar ration
of 1 : 20
91.25
83.70
9.96
2.18
–
0.128
0.135
8.85
With ethanol, GL : AC
molar ratio of 1 : 5
0.33
23.11
Table 3. Results of experiments in an apparatus with fixed catalyst bed. GL : AC molar ratio 1 : 5
With ethanol, delivery of a
homogeneous mixture
Parameter
Delivery of an emulsion
Flow rate of glycerol, g h–1
Spraying density, m3 m–2 h–1
Volumetric flow rate, h–1
15.7
0.12
0.25
62
33.5
0.33
0.70
65
Conversion of glycerol, %
Yield of the product per unit mass of the
–1
–1
catalyst, g
g
h
0.067
4.06
0.15
1.45
SK
cat
Contact duration (per volume occupied by the
catalyst), h
larger and, accordingly, the reaction rate grows because
of the substantial increase in the content of glycerol at
a GL : AC molar ratio of 1 : 5. The increase in the rate
of the condensation process by at least a factor of 2.6
makes it possible to proportionally reduce the volume of
the reaction unit or, accordingly, raise its output capacity.
The efficiency of homogenization of the mixture of
the starting reagents with ethanol was experimentally
confirmed in a column-type apparatus with a fixed bed
of KU-2-8 ion-exchange resin in the falling-film mode.
Table 3 lists the results of experiments with delivery
of the emulsion of the starting reagents with nearly the
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50
DMITRIEV et al.
maximum spraying density, or with that of a mixture
homogenized with ethanol. In both cases, the GL : AC
molar ratio was 1 : 5, and the temperature, 35°C.
its output capacity. At a spraying density exceeding
0.15 m3 m–2 h–1, the adsorbed glycerol film becomes so
thick that the free volume along the apparatus diameter
becomes significantly smaller, which, in the end, causes
flooding of the catalyst.
If a homogenized mixture of the reagents is used,
there are no limitations on the spraying density. At a
higher spraying density and 2.8 times shorter contact
duration, the homogenization of the starting mixture of
the reagents enables a higher conversion of glycerol all
other conditions being the sample. The yield of solketal
per unit mass of the catalyst became 2.2 times higher.
The homogenization of the starting mixture of the
reagents helps to lift these restrictions. Data are presented
on how the concentration of water and ethanol affects the
solubility of glycerol in acetone. It was shown that use
of ethanol for homogenization of the starting reagents
is more than twice more efficient than performing the
process with delivery of an emulsion of the reagents or
with a 20-fold excess of acetone.
Thus, when an emulsion of the reagents is delivered,
the glycerol phase blocks about half of the active centers
of the catalyst, thereby reducing its output capacity.
Despite the comparatively low conversion of glycerol
in the reactor for solketal synthesis, recycling the starting
reagent in the industrial technique will in the end provide
a yield of the target product of no less than 97%.
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CONCLUSIONS
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Ser. B, 2015, vol. 77, no. 2, pp. 1454–2331.
The study demonstrated that the limited solubility
of glycerol and acetone leads to stratification of the
emulsion of the starting reagents in a film falling along
the surface of KU-2-8 cation-exchange resin. This leads
to blocking of active centers of the catalyst and impairs
4. RF Patent 2 522 764 (publ. 2014).
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