Toward sustainable and eco-efficient novel catalytic distillation process for production of solketal using seepage catalytic packing internal

Article abstract of DOI:10.1016/j.cattod.2020.09.029

The initial aim of this paper is to dramatically improve the post-treatment stage of biodiesel production, which converts problematic glycerol to solketal (SK), by introduction of an eco-efficient integrated reactive and dividing wall distillation process. To overcome the chemical equilibrium limitations and the long post-treatment process with the high energy consumption and waste water emission, this study provides the potentially, sustainable and novel reactive distillation (RD) process for cleanly catalytic synthesis of SK, taking into account costs and environmental impact. The ketalization kinetics experiments were carried out to provide a basis for subsequent experiments and simulations. A reliable model was established for further RD technological design and validated by the pilot-scale experiments. Through sensitivity analysis, the effects of multiplex parameters in RD process were determined. The advanced intensification of SK production by reactive dividing wall column (RDWC) achieves reduction in energy, total annual cost (TAC) and CO₂ emissions of 13.9 %, 18.2 % and 16.4 % (as oil resources), respectively, compared to the optimal SK production by RD process. This proposed technology is also compared with the conventional industrial route of ketalization with 18.7 % energy saving.

Full text of DOI:10.1016/j.cattod.2020.09.029

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Toward sustainable and eco-efficient novel catalytic distillation process
for production of solketal using seepage catalytic packing internal
Xingang Li (Conceptualization) (Methodology) (Writing - review and
editing), Jianing Lai (Investigation) (Methodology) (Software)
(
(
(
Writing - original draft), Haifeng Cong (Investigation), Chang Shu
Investigation) (Resources), Runnan Zhao (Investigation)
Resources), Yiwei Wang (Resources), Hong Li (Project
administration) (Writing - review and editing), Xin Gao
Conceptualization) (Supervision) (Writing - original draft)
(
PII:
S0920-5861(20)30690-8
DOI:
https://doi.org/10.1016/j.cattod.2020.09.029
CATTOD 13170
Reference:
To appear in:
Catalysis Today
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Revised Date:
Accepted Date:
31 March 2020
12 September 2020
30 September 2020
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Toward sustainable and eco-efficient novel catalytic distillation
process for production of solketal using Seepage Catalytic
Packing Internal
Xingang Li, Jianing Lai, Haifeng Cong, Chang Shu, Runnan Zhao, Yiwei Wang, Hong Li, Xin
Gao*
School of Chemical Engineering and Technology, Tianjin University, National Engineering
Research Center of Distillation Technology, Collaborative Innovation Center of Chemical Science
and Engineering(Tianjin), Tianjin 300350, China
*
Corresponding author’s e-mail: gaoxin@tju.edu.cn
Graphical Abstract
Highlights

The ketalization kinetics using NKC-9 cation resin catalyst was investigated.




A rigorous model of RD was developed and validated by pilot-scale
experiments.
The effects of key parameters of RD column for SK synthesis were
determined.
The RDWC technology achieves reduction in energy, economics and CO
emissions.
2
ABSTRACT
The initial aim of this paper is to dramatically improve the post-treatment stage of
biodiesel production, which converts problematic glycerol to solketal (SK), by
introduction of an eco-efficient integrated reactive and dividing wall distillation
process. To overcome the chemical equilibrium limitations and the long
post-treatment process with the high energy consumption and waste water emission,
this study provides the potentially, sustainable and novel reactive distillation (RD)
process for cleanly catalytic synthesis of SK, taking into account costs and
environmental impact. The ketalization kinetics experiments were carried out to
provide a basis for subsequent experiments and simulations. A reliable model was
established for further RD technological design and validated by the pilot-scale
experiments. Through sensitivity analysis, the effects of multiplex parameters in RD
process were determined. The advanced intensification of SK production by reactive
dividing wall column (RDWC) achieves reduction in energy, total annual cost (TAC)
and CO emissions of 13.9%, 18.2% and 16.4% (as oil resources), respectively,
2
compared to the optimal SK production by RD process. This proposed technology is
also compared with the conventional industrial route of ketalization with 18.7%
energy saving.

Keywords: Solketal; Ketalization reaction; Catalytic distillation; Sustainable process;
Process intensification
1
. Introduction
At present, the decrease of oil reserves and global warming issues has prompted
countries to develop sustainable sources to substitute fossil fuels aiming to reduce
greenhouse gas carbon dioxide (CO ) emissions. Biodiesel, a long-chain fatty acid
methyl ester obtained through transesterification using animal and vegetable oils, is
2
[
1]
the most promising and viable new energy in the short term . It has the advantages
of environmental friendliness and regenerability that can be used as an alternative
source for petrochemical diesel [2-4]. It is estimated that the transesterification reaction
[
5]
may produce approximately 10 wt% by-product glycerol (G) . However, glycerol is
a low value-added chemical raw material, which market for existing has become
increasingly saturated. As the production of biodiesel increases continuously, if
improperly use of by-product glycerol, it will not only affect the economic feasibility
of biodiesel as a new energy source, but also cause serious environmental pollution,
[
6]
which is incompatible with sustainable development strategy . The issue of energy
utilization has continuously been concerned by the public and recognized as one of
important factors for industrial [7]. Each year about half investment of the chemical
industry is in separation field [8]. As a result, scholars around the world are actively
exploring more effective and feasible ways to use the excess substance glycerol [9,10]
.
Currently, a huge number of different methods have been studied for transforming it.

The researches mainly focus on the development of some high value-added products
such as 1, 3-propanediol, propylene oxide, cyclic acetal and ketals, etc. [11-14]
.
Solketal (SK) is an important organic synthesis intermediate, which can be used
in the synthesis of antihypertensive drugs propranolol, drug carrier and as an
inexpensive inducing compound for chiral synthesis [15-17]. On the other hand, it can
be used as a polyhydroxy protecting group to synthesize high-purity monoglyceride,
participated in preparing glycerol phosphate [18,19].The existence of a branched
oxygen-rich compound makes it possible to be added into diesel oil, which can
effectively enhance the low temperature performance of diesel and reduce its
viscosity [20], decrease the particulate emission [21]
.
Solketal is generally obtained through the ketalization of glycerol and acetone
with catalyst such as strong liquid acid H SO
and HCl [22]. Although the catalytic
2
4
effect is obvious, the homogeneous process has the disadvantages of corrosion
equipment and environmental issues. With the strengthening of people's
environmental awareness and the development of green chemistry, more studies have
targeted on heterogeneous catalysts such as Amberlyst-36 [11], mesoporous amorphous
AlPO4 [23], Zeolite [24] etc., which can overcome the shortcomings of traditional liquid
acid catalysts.
The condensation product of glycerol and acetone has both five-membered ring
(4-hydroxymethyl-2,2-dimethyl-1,3-dioxolane,solketal) and its isomer six-membered
ring (5-hydroxy-2,2-dimethyl-1,3-dioxane) [25], however, the solketal is more
favorable with selectivity of reaching 98% or more, which are commonly both

considered as final product in industry. The basic properties of this ketalization
system are listed in Table 1 and the reaction scheme for ketalization of glycerol with
acetone is shown in Scheme1.
After reaction, the catalyst was neutralized, it was recovered by filtration,
distillation, 80~83 ° C/1.33 k Pa product was collected to be Solketal (SK). However,
low single-pass conversion owing to its thermodynamic limitation results in excessive
energy consumption [26].Therefore, the production route has the weakness of
cumbersome post-processing, large-scale equipment occupation and low conversion
with energy consumption, which result in high cost.
Solketal synthesis is also needed to consistent with the principles of green
chemistry and engineering which can be achieved by process intensification (PI) [7]
.
By utilizing of RD technology, the product can be separated in real time while the
reaction is carried out, thereby not only improving the conversion rate and product
yield, but also energy consumption can be saved effectively [27,28]. Because of
conversion improvement, energy saving and effective CO emissions reduction, RD
2
has advantages in terms of economy, environmental friendliness, sustainability and
safety [29], which makes it an attractive alternative to conventional procedure.
At present, researches for solketal mainly focuses on catalysts and kinetics by
other scholars. Now, several synthetic devices for solketal have also been investigated
actively, such as semi-batch reactor [22], fixed bed reactor [30], microwave assisted [31]
and membrane reactor [32]. But they still have some drawbacks, such as complicated
post-processing and the clogging of the reactor with the catalysts, hence raising

environmental and economic concerns. Clarkson et al [33] has focused on the study of
solketal formation by a continuous reaction column, which computes based on the
theoretical model using heterogeneous catalyst. Domínguez-Barroso et al [34] has built
a continuous flow reactor system in laboratory scale which connected to distillation
device for synthesizing solketal in the presence of catalyst Brönsted acid, but none
process coupling was involved by them. Al-Saadi et al [35] has investigated a novel
process for combining solketal synthesis during the triglyceride transesterification to
minimize the crude glycerol generation, although the parameters correlation were
studied by DoE, this established kinetic model hasn’t applied in further research for
any integrated process. However, we have not seen any detailed reports about SK's
production by RD technology on experimental scale. Therefore, the RD process will
be proposed to realize continuous production of SK from Acetone (Ac) and Glycerol
(G) using solid acid as catalysts in this paper.
This research focused on the basic study of kinetics with cation resin catalyst
NKC-9, green and sustainable process feasibility of RD and RDWC and optimization
for SK production. Firstly, the kinetics studies were performed under different
conditions catalyzed by NKC-9. Further, based on the data from experiments and
thermodynamic method, the possibility of using RD was discussed by simulation
through a rigorous RD mathematical model established and validated by the results
obtained from pilot-scale experiments, providing theoretical basis for SK's process
plant design. And then, the preliminary concept design of RD was systematically
studied by using this model to implement optimization of this process. Finally, the

reactive dividing-wall column (RDWC) structure is designed, by contrast, in the
aspects of the energy consumption, economic analysis and CO emissions of
2
environmental impact were conducted between the conventional production routes,
new RDC and the RDWC ketalization process.
2
. Experiments Section
2
.1. Materials: Chemicals and Catalyst
Table 2 listed the information of chemicals which was used in this study.
The catalyst used in kinetic experiments and reactive distillation experiments,
NKC-9, is purchased from the Chemical Plant of Nankai University, China. The
specific catalyst properties are shown in supplementary information Table S1and Fig.
S1.
2
2
.2. Experimental setup and procedure
.2.1. Kinetics
The kinetics experiments were conducted in a 100 ml three-necked flask made
by Tianjin Pengxiang Glass Experimental Apparatus Co., Ltd, sampling needles and
thermometer were placed separately on left and right bottle sides (Fig. 1). The system
was heated by a constant temperature water bath, temperature control accuracy is
±
1 °C.
The composition of reactant was 36.836 g glycerol, 22.232 g acetone in the
presence of 0.737g NKC-9 catalyst, whose volume was 59.74 mL totally. Put the
weighed amount of acetone and catalyst into flask and heat to the set temperature
while preheating glycerol, then transferring the glycerol into reactor what time was
used as the time zero. After certain intervals as 2 min in the first 10 minutes and then

take samples every fifteen minutes until equilibrium, about 0.1 mL liquid samples
were taken out for further composition analysis. Several experiments until the
equilibrium state were performed in turn to determine kinetic data and study the
different influence of parameter changes on the kinetics. Notably, the experiment
under the condition of temperature of 343.15K was conducted in autoclave
AUTOCHEM100 with temperature and pressure monitored.
2
.2.2. Reactive distillation experiments
The experiments of synthesizing solketal studied in this paper were carried out in
continuous operation mode. The diagrammatic sketch for the RD column of
pilot-scale experiments was depicted in Fig. 2.
The RD column is manufactured of stainless steel, total height of 4 m, 50mm
diameter (inner) divided into 4 units, which were recorded as 1-4 sections from
bottom to the top, with each one fixing by flange connection. The reactive zone was
in the middle about 3m filled with SCPI packing internals. The rectification section
and the stripping section were respectively equipped with θ-ring packing (Φ5*5) for
0
.5 m height. The number of theoretical stages per meter (NTSM) is 11-14/m packed
with θ-ring packing in nonreactive zone, and 7-9/m for SCPI filled with NKC-9
catalyst in reaction zone [36, 37]. The condenser uses circulating cooling water to cool
the rising vapor. The reboiler was heated by externally circulated oil bath; the
temperature was adjusted by the temperature and flow of the circulating oil. The
amount of the rising stream is controlled by the thermal conductive oil. The bottom
has a sampling port for timely analysis, which outlet is connected to a product tank to
store product. The mass flow of the bottom stream can be obtained by recording the

change of the storage tank quality a fixed time. During the experiment, the mass flow
rate of the distillate can be obtained by the weighing the quality of collected distillate
in a period of time to calculate the variation. The reflux ratio is adjusted by the
electromagnetic controller.
The column was provided with pressure measurement points. Thermocouples
were installed in each section to measure the temperature in column, at the same time,
the temperature of the kettle and the top reflux liquid is also monitored and recorded.
During the reaction, the pressure and temperature change of each section were
continuously monitored, which data were displayed on the control panel and
transmitted to the computer for recording in real time. In order to prevent heat
dissipation and reduce the impact of ambient temperature, the column outside is
totally wrapped by insulation mineral wool. The cooled condensate of acetone and
water from the top condenser were taken from the column according to the reflux
ratio. The produced SK was in the bottom stream and was withdrawn periodically.
The temperature and pressure measurement program were set up to automatically
record data every 10 minutes. The detailed experimental procedure of RD process was
shown in supplementary information SI 1-2.
In order to compare the influence of different fed type on glycerol conversion,
for the Exp2 the feed positions of G and Ac are respectively placed at 3.5 m and 0.5 m
rd
nd
height metered from the column bottom, which called 3 section (Feed 1) and 2
zone (Feed 2) drawn in Fig. 2. For the Exp5, we set G position constant (Feed 1),
rd
added Ac from the top of 3 section (Feed 3), which can make them co-fed.

The study investigated the influence of reflux ratio, operating pressure, feed type
and feed molar ratio on the performance of RD process for SK continuously synthesis.
2
.3. Analysis
Chromatography: PE Auto System 690; detector: hydrogen flame ionization
detector (FID); column: Agilent 30-m DB-Wax non-polar column, 2.5mm; split ratio:
0:1; carrier gas: nitrogen; carrier gas, air and hydrogen flow rate: 1.00 mL / min, 450
4
mL / min, 45 mL / min; injector and detector chamber temperature 493.15 K; oven
temperature 503.15 K kept 6.0 minutes. The samples were detected repeatedly more
than 3 times whose error is less than 0.2% was feasible.
The concentration of water was measured on a Karl Fischer Titration Meter of
Trace Moisture Determination (ZDJ-3S), which supplied by Beijing Pioneer Weifeng
Technology Development Co., Ltd. A suitable amount of the sample to be tested was
taken out using a micro syringe, after examination detection two sets of data with an
error of 0.2% were accepted.
V×t
w= m ×100%
（1）
V means the Karl Fischer reagent volume that consumed (mL)；t represents the titer of
Karl Fischer, mg/mL；m is the mass weight of liquid sample (g).
The acid value was determined by acid-base titration. It is the amount of
milligrams of KOH required to neutralize 1 g of sample. The determination was
described in supplementary information SI 1-3.
Acid value =56.11×KOH concentration (0.1mol/L) ×KOH volume (mL) ÷sample
weight/g, mgKOH/g.
. Kinetics Experimental Results
（2）
3

A series of experiments were carried out to investigate the impacts of molar ratio,
temperature on the glycerol conversion (X ) of the ketalization reaction. That X is
G
G
defined as the conversion of glycerol and YSK represents the yield of solketal,
computed by Eq. (3) and Eq. (4), respectively.
[
Glycerol] [Glycerol]
0- t
Glycerol conversion (X
G
): X
G
(%) =
×100%
（3）
（4）
[
Glycerol]0
[
Solketal]t
Yield of Solketal (YSK): YSK (%) =[
×100%
Glycerol]0
3
.1. Effect of speed of agitation
According to the literature [38-39], for the resin catalysts such as NKC-9 used in
this paper, the particle size of the catalyst is generally controlled as 0.5 to 0.8 mm in
the production process to eliminate internal diffusion. The study conducted three sets
of comparative experiments under different particle size catalytic conditions to
evaluate the effect of internal diffusion as it shown in in SI 1-4 Figure.S2.
In order to eliminate the effect of external diffusion, four comparative
experiments were performed under the stirring speed of 200 to 500 rpm. The other
conditions were maintained (323 K, the molar ratio of (Ac/G) was 1:1, and the
catalyst loading was 2wt% of glycerol).The equilibrium conversion rate and
production rate of glycerol were obversed，as it described in Fig. 3, when the stirring
speed reaches 400 rpm, the slope of the generated rate curve varies smoothly , and the
ultimate equilibrium conversion rate is nearly identical at 67%, indicating that the
influence of external diffusion has been eradicated. It is considered that the side-effect
of diffusion resistance at speeds > 400 rpm could be negligible. However,
continuously increasing the speed will have negative impact on the stability of the

device and the catalyst. In summary, other experiments were performed under the
condition of stirring speed of 400 rpm.
3
.2. Effect of mole ratio of Ac/G
The effect of different molar ratio of acetone to glycerol (Ac/G) on glycerol
conversion was also examined through four experiments. The investigation is
performed to obtain appropriate reaction kinetics expressions. Acetone is a polar
solvent and the increase of Ac/G will lead to increase in solketal yield and conversion
[41].
be used as both a reactant to drive the forward reaction and excess miscible acetone to
remove unfavorable by-product water as a stripping agent [41]
An excess of acetone was selected, because of the cheap price of acetone, it can
.
Comparisons were detected separately which the ratio of the acetone to glycerol
were set 1:1, 1:2, 1:4, 1:6. It can be obviously seen from Fig. 4 the conversion rate
reached a maximum of 80.4% that close to the literature [41] when the ratio of G/Ac
achieved at 1:6. The formation rate was relatively fast for the initial 10 minutes, and
basically reached equilibrium by 30 minutes, which was consistent with Nanda et al
[
26]
and Rossa et al [41] reported already. The molar ratio of Ac/G would influence the
equilibrium conversion because it will increase the concentration of reactant acetone
to promote the forward reaction. As it obviously shown in Fig. 4, under different
molar ratio conditions, a higher Ac/G molar ratio will lead to conversion of glycerol
increasing, which is consistent with Agirre et al [12] described before.
3
.3. Effect of reaction temperature

The effects of temperature on glycerol conversion were investigated from
93.15K to 343.15K. The difference rates of change were clearly illustrated in Fig. 5.
2
As temperature higher, the reaction rate was accelerated which can be seen from the
gradually steep slope of conversion curve. At the same time, the equilibrium
conversion decreased from 62.9% to 59.4% because of the exothermic reactions if the
temperature goes up will promote reverse reaction. For reversible reactions, the
reaction rate and conversion were impacted directly by temperature because chemical
equilibrium will be in the direction of weakening this trend as Le Chatelier’s principle
referred [41]
.
3
.4. Comparison of previous studies
As mentioned above, researchers have reported on ketalization kinetics with
acetone and glycerol using diverse catalysts to investigate the reaction conversion and
kinetics parameters. As comparison, a detailed literature review is given in Table S2
[42-45].
As traditional homogenous catalyst, Suriyaprapadilok et al [42] has used PTSA on
ketalization, they have studied the conversion of glycerol on different Ac/G molar
ratio for over 12hours, obtained the conversion from 60% under the condition of Ac/G
of 2:1 to 82.7% of 6:1. Menezes et al [43] have investigated typical Lewis acid, such as
SnCl
2
, SnF
2
, Sn (OAc) 2, they find Sncl own the most excellent catalytic efficiency
2
with the glycerol conversion of 77%, however it will cause some corrosion.
Amberlyst-15 is one of the most promising activators for ketalization, with 95%
glycerol conversion and the solketal selectivity of 95%. A reversible kinetic model

was established by Rossa et al [41], they have calculated equilibrium constants for a
range of 40-80°C and provide the accurate comparison of conversion between the
experimental and calculated value, statistical analyze the ways for the kinetics
properties improving.
In this work, effect of different conditions (such as particle size, stirring speed,
temperature and the feed molar ratio of Ac/G) on kinetics (which reflected on
conversion vs. time curve) were also studied, with the glycerol conversion range from
6
0.4% to 80.4%. The reaction rate constants were determined experimentally in the
temperature range of 293-343 K, with the activation energy (Ea) of forward and
reverse reaction were obtained, respectively.
It can be concluded from Table S2 that the conversion of glycerol under different
conditions are similar with our study; the activation energy has some difference due to
the presence of the internal diffusion, the effects of it were shown in SI 1-4, but the
0
value of enthalpy ∆H , difference between forward and reverse activation energy,
still on the same order of magnitude. The application of the kinetic data could better
match the situation in the practical industrial process.
4
. Model Description
4
.1. Kinetics
In the simulation, due to the presence of the ketalization, the interactivity
between the components is affected by both the thermodynamic equilibrium and the
chemical reaction equilibrium. Therefore, it is necessary to add a reaction kinetics
equation in the simulation. According to the literature, the reaction is a two-step

reversible reaction, that rate expression for ketalization can be described by
Langmuir−Hinshelwood−Hougen−Watson model [26,45]. Based on the assumption of
the surface reaction of the absorbed water generation process considering as the rate
controlled step [45,47], the ketalization reaction rate of glycerol and acetone can be
expressed by the Eq. (5):
k1[C ][CAc]- k [C ][C ]
dc
G
-1 SK
W
Sk
r =
SK
=mcat
mol/(L·min)
(5)
(1+K [퐶푊])2
dt
w
Where k
i
represents the reaction rate constant of the forward and reverse reactions, K
W
means the adsorption equilibrium constant of water.
MATLAB was used to fit each substance concentration in order to determine the
apparent rate constant, which was calculated by Levenberg–Marquardt algorithm,
above mentioned differential equation (5) was numerical integrated through the
method of fourth-order Runge– Kutta [48]. The experiments and fitted results are
shown in Fig. 6 and Fig. 7. Through established the ode45 function, the reaction rate
constants at different temperatures were found in Table 3. The calculation of the rate
constants are based on the catalyst per unit mass.
According to the Arrhenius equation:
−
퐸푎,푖/푅푇
푘 = 퐴 푒
(6)
푖
푖
The logarithm of the both sides of Eq. (6), a straight line was obtained by
drawing fit of ln k vs. 1/T.
퐸ꢁ,i
ln퐾 = ꢀ
+ ln퐴푖
푖
푅푇
(7)
1
Ea,ꢂ
R
Ln푘 - Graph which was shown in Fig. 7, the slope is equal to −
, the
푖
T
intercept represents the value of ln A .
i

Calculate activation energy (Ea) based on the slope of the fitted straight line, and
compute the pre-exponential factor (A ) by the straight line intercept. Find the
i
corresponding value of Ea and pre-exponential factor. The conclusion is that the
activation energy value of the forward reaction using NKC-9 catalyst was determined
as 44.312 kJ/mol and 47.23 kJ/mol for the reverse reaction; the pre-exponential factor
6
−1
7
−1
was determined as 7.336× 10 L·(mol·min·gcat) and 1.065× 10 L·(mol·min·gcat) ,
respectively.
Get the reaction kinetic equation:
3
3
10
10
7 -47.23×
.336×106e-44.312× RT CAcCG- 1.065×10 e
RT CSK
C
H O
2
7
r =m {
}
mol/(L·min)
(8)
i
cat
3
1
0
ꢃꢄ1.9ꢅ03×10ꢀ7ꢆ40.7266×ꢇꢈ 퐶푊)2
(
The quantity of catalyst (about 2/3 volume of the catalyst containers in SCPI)
was filled in the RD column and it was calculated as 70.67g. The residence time could
be controlled by the volume of catalyst filled in the SCPI as 20s on a stage according
to the liquid flow behavior of SCPI. The mass of the catalyst is converted into the
a
a
corresponding pre-exponential factor A (A represents the pre-exponential factors
used in Aspen). Then we obtained the reaction rate equation 9 based on the catalyst
quantity and modified the reaction rate expression for reactive distillation simulation
in Aspen.
3
3
10
10
7 -47.23×
ꢅ -44.312×
8
.64×10 e
RT CAcCG- 1.ꢉ54×10 e
RT CSK
C
H O
2
3
ri=
kmol/(m ·s)
(9)
3
1
0
ꢃꢄꢉ.309×10ꢀ7ꢆ40.7266×ꢇꢈ 퐶푊)2
(
4
.2. Thermodynamic
The accuracy of the Aspen simulation results is highly dependent on the physical
data and thermodynamic equations. As mentioned above, there are four compounds of
acetone, glycerol, solketal and water. The solution of the reaction system exhibits

strong non-ideality, so it is necessary to find a physical method suitable for it. At
present, the description of this strong non-ideal vapor-liquid equilibrium relationship
uses NRTL, WILSON and UNIQUAC three physical methods.
Currently, as it referred in the literature, the case of the NRTL equation was
successfully used in this system [49]. In this simulation, the UNIFAC group
contribution method was employed to evaluate the absent parameters, as listed in
Table 4. The physical property, such as Antoine equation parameters (vapor pressure)
and the enthalpy of vaporization (298.15 K) are provided in Table 5.
4
.3. Reactive Distillation Modeling
By establishing a rigorous simulation the effect of all the parameters in RD
process can be evaluated systematically. Simultaneously, the validity of the RD model
could generally be verified on the basis of the experiments results, which can be
useful foundation for the further amplification of the industrial process for SK
production.
The RD process involved in this article was based on Aspen V8.4，established by
columns unit called Radfrac and equilibrium stage model(EQ model). Because of the
interaction among the parameters, the optimization of the RDC is complicated.
Therefore, follow the process below to optimize parameters. Basic variables are listed
in Table 6 and detail introduced in SI 1-6.
5
. Reactive distillation experiment
A total of eight RD experiments were successively conducted to explore the
effects of different conditions: feed Ac/G molar ratio, reflux ratio, operating pressure,

feeding type on glycerol conversion and reboiler duty. The experiments operating
condition and results are listed in Table 7 and the detail of molar flow rate was in
supplementary information SI 1-7.
5
.1. Model Validation
The validity of RD model was evaluated by a series of RD experimental work.
The experimental results were compared with the simulation to verify the reliability of
the established RD mathematics model.
Fig. 8 shows the composition and temperature of different conditions obtained
from RD experiments (single points) with the data calculated by simulation used
Aspen Plus (filled curves). It can be seen the results in Fig. 8 a favorable consistent
between them. It can be observed from the picture that differences put together less
than ± 5% deviation. Some experimental deviations are unavoidable, the main reasons
are: (1) the simplified model assumes that the reaction occurs under adiabatic
conditions, but in actual operation, there will inevitably be a little heat loss at the
junction of the sections. (2) During the experiment, in EQ model it is reckoned to
achieve steady-state operation. However, there are some fluctuations in the actual
flow conditions and heat transfer conditions in the column. (3) There are experimental
errors in the analysis during temperature measurement by thermocouples. (4) The
pressure drop was not considered in the simulation, which may cause the
experimental temperature is slightly higher than the simulated value as the closer to
the bottom. As a result, some deviations between the experimental and simulated

values are still reasonable. In temperature profile (Fig. 8 a.b.d.), the local temperature
change is caused by the exothermic reaction.
The experimental data and simulation values agree reasonably well, and the
reliability of the RD Radfrac model is verified. Lately, this method will be used to
optimize this process. After that, sensitivity analysis was used to analyze the impact
of parameters.
5
.2. Effect of reflux ratio
As it shown in Table 7 Exp1, Exp2and Exp3 presented the reflux ratio’s
influence on glycerol conversion and the purity of solketal in bottom stream. While
the other experimental conditions remained, it can be seen that as the reflux ratio
increases gradually from 1.5 to 5 in Exp1, Exp2and Exp3, the conversion of G firstly
rises from 62.4% to 67.5% gradually and then decreases slightly to 65.4%. The results
describes that larger reflux ratio have an advantage on conversion, however, excess
than a value, slightly decrease.
5
.3. Effect of operating pressure
The influence of operating pressure was illustrated between Exp2 and Exp4, with
the pressure decreasing from 993 hPa to 775 hPa, the conversion dropped from 67.5%
to 55.9%, the mass purity of SK was also reduced from 68.91% to 60.43% after the
pressure lowered. It indicates that higher pressure in column would promote reaction
to conduct expectedly. However, temperature rises as the pressure increasing, if the
temperature is too high, the performance of the catalyst will be reduced and the
service life of it will be affected. Therefore, the pressure setting is not suitable for as

high as possible. The trend of the reboiler duty and the conversion of glycerol with the
pressure are shown in supplementary information Fig.S3. The optimal operating
pressure that can achieve the desired conversion rate without affecting the use of the
catalyst is 101.325kPa according to our study, which was described in detail in SI 1-8.
5
.4. Effect of feed molar ratio
The effect of the molar feed ratio can be illustrated by comparing the results of
Exp2, Exp6, and Exp7. With Ac/G molar ratio increasing from 2:1 (Exp2) to
:1(Exp6), the conversion of G (glycerol) changed significantly from 67.5% to 85.9%.
3
The experimental results indicate that the increased molar ratio of Ac/G was
beneficial to the reaction to proceed toward the direction in which the forward
reaction generates SK. However, the excess acetone will cause the acetone
concentration in the column overhead stream to rise which will lead to consume more
energy in the later separation.
5
.5. Effect of feeding type
The influence of feeding type of glycerol and acetone was investigated by
comparing Exp2 with Exp5. The glycerol with acetone is fed separately and contacted
in countercurrent mode in Exp2 and co-fed in Exp5. In Exp5, because of delamination
after mixing of acetone and glycerol, for Exp5, the feed position of glycerol is
unchanged (Feed1), the reactant acetone is fed at the 3rd reaction section, allowing
them to be mixed (Fig. 2).When the feed type changed from Exp2 to Exp5, the
conversion of glycerol was decreased from 67.5% to 61.7%. The experimental results
show that the downward shift of the feed position, that is separate feeding mode,

facilitates the more conversion of G to the target product SK. In order to increase the
concentration of the Ac, we have also considered feeding the acetone at a lower point
on the stripping stage in the RD column. But the final experimental result is not very
ideal, as a result, the conversion rate of glycerol is 59.90% lower than 67.4% before
and the concentration of acetone is increased.
6
. Reaction distillation process analysis
For the industrial process feasibility and economical, it is desire to increase
conversion rate and reduce energy consumption (i.e., reboiler duty), which is limited
by only experimental research. Aspen plus, generic software developed by Aspen
Tech, USA is used for steady state simulation, process design and optimization. The
model analysis, sensitivity analysis, was applied to study the operating parameters of
the RDC and to investigate the impact of each variable change on process. Then, the
optimal results will provide further guidance for design and construction of the
industrial device.
The sensitivity analysis, each parameter changed once, was successively applied
for discussing the corresponding effects on conversion of G (X ), and RD energy
G
consumption per mass g of SK (QSK) to obtain optimal condition for the SK synthesis
by RD technology.
[
Glycerol] [Glycerol]
0-
t
X
G
(%)=
×100%
(10)
(11)
[
Glycerol]0
Reboiler duty
mole flow rate of SK in stream
Q
SK
=
W/g
6
.1. Effect of feed mole ratio

The excess of one reactant can promote the chemical reaction, which requires
more attention to the feed molar ratio of Ac/G in the operation. As the price of
acetone was cheaper with lower investment, the excess acetone was usually used in
industrial production to improve the conversion of glycerol. However, in turn, too
much excessive acetone would lead to separation difficulty and huge energy
consumption. In this part, Fig. 9 depicts the trend of Ac/G feed mole ratio on glycerol
conversion and the reboiler duty when the total feed quality and distillate amount
remains constant, raised molar ratio of Ac: G from 1:1 to 6:1.
When the molar ratio is more than 1:1 (the stoichiometric ratio of Ac to G),
glycerol conversion enlarges rapidly with the feed ratio increasing. When it is set
from 1:1 to 6:1, the conversion rate of G increased from 59.6% to 83.3%.As it can be
observed form Table 8, with Ac/G molar ratio reaching 20:1, conversion of G
achieved 93.2%. With the molar ratio of Ac/G increasing from 1:1 to 20:1, the energy
consumption per unit mass of SK increases from 0.2438w/g to 1.9496w/g. It can be
obviously found that although the conversion is significantly increased, due to the
relatively less amount of glycerol fed, the quantity of the target product SK obtained
was reduced. From the perspective of energy consumption, although the excess of
reactants will promote the increase of conversion rate, it will also require larger
amount of energy consumption.
Despite the molar feed ratio facilitates the movement of the equilibrium towards
the direction of formation, which also greatly added energy consumption. However,
when the molar ratio of feed exceeds 3, the slope of the purity of SK varies gradually

slightly; the reboiler duty expands almost linearly. As it reported in research [50]
,
energy consumption is one of the key indicators for evaluating equipment’s capability.
By calculation, when feed ratio reached 3, continue to increase the amount of feed to
obtain a unit quality product, the energy consumption gradually increases.
6
.2. Effect of reflux ratio
The influence of reflux ratio on RD process includes that it not only influences
efficiency of separation and progress of the reversible ketalization, but also affects the
energy consumption, thus the operation cost of the RDC. The influence of reflux ratio
was investigated in the interval of 0.8-4in this part.
It can be observed from Fig. 10 that as the reflux ratio increases, the trend of the
change in G conversion is gradually rising firstly and then slightly down.
Simultaneously, the reboiler duty was rapidly increased. This means no improvement
in G conversion while operation cost still inflated. The relationship between reflux
ratio and the performance of RD process is complex because separate and reactive
coupled in one unit.
When the reflux ratio is less than 2.4, that is, the curve appears an inflection
point; because of mainly limiting by chemical equilibrium, if the effective separation
between water and acetone is enhanced, the forward movement of the reaction is
promoted. If the separation in the system is more favorable, the reaction happened
more adequately, the conversion rate increases correspondingly. After 2.4 that as the
reflux ratio increase, the reaction rate becomes the main factor restricting the

conversion rate. The liquid residence time on the stage decreases with reflux ratio
increasing. At this time, further increase of the reflux ratio will reduce the conversion
rate due to the decrease of the reaction rate.
The results indicate that when the reflux ratio is large, the impact on increasing
for the conversion is not obvious, even decrease, however, the reboiler duty was
always increased linearly.
6
.3. Number of reaction stages
In RD process, the reaction section stages have effect on the efficiency of the
performance of reaction and separation. By appropriately increasing reaction
theoretical stages can better combine reaction process and the distillation process to
achieve better results. For the purpose of ensure that it has sufficient time to match
separation and reaction unit to increase reaction equilibrium conversion, it’s necessary
to increase the number of theoretical stages reasonably. More stages in reactive
section will lead to more glycerol convert into target product solketal. However, when
the stages of this section are excess, the investment will be increased unnecessarily.
As it can be seen from the Fig. 11, when the reaction section theoretical stages
number was increased from 3 to11, the conversion of glycerol increase rapidly; but
too much stages in reaction zone will result in waste if it is no positive effect on
reaction conversion. When set 9 stages from 5 stages the feed position of Ac to 14th
the position of G, with 9 stages to reaction is enough. Fig. 12 shows the results of the
change in the theoretical stages on concentration of reactants and products in column.
It can also be observed that excessive stages will bring negative effect, because
th

reverse reaction between water and solketal may happen, which is adverse for solketal
formed.
6
.4. Number of rectification stages
When the number of rectification section stages is changed during reactive
distillation, the concentrating of the light components will change accordingly, and
then the progress of the chemical reaction would be affected. However, more
theoretical stages will increase the equipment cost and the operation expense of the
reboiler. Therefore, it’s very important to select adequate value of rectification section
stages.
The number of rectification section was studied in this part varied from one stage
to 20stages. The conversion of G and reboiler duty over different rectification sections’
number was shown in Fig. 13. As the number of the stages in rectifying section
increasing, the conversion of G increases accordingly. This is due to the increase the
theoretical stages, which improves the concentration of liquid acetone in the reaction
zone, furthermore promotes the equilibrium removes forward. The directional
movement helps to improve glycerol conversion and solketal production quantity.
When the number is increased to 5, the conversion rate change is no longer obvious.
6
.5. Number of stripping stages
In the RD process, the increase of theoretical stages in stripping section can raise
the heavy components purity in column, causing separation be promoted and the
improvement of conversion rate. Fig. 14 depicts the effect of theoretical stages in
stripping section on the glycerol conversion and reboiler duty. When the number of

stripping sections increased from 2 to 10stages, glycerol conversion rate is gradually
increasing from 55.8% to 57.5%. Due to the separation efficiency of the stripping
section enhancing, promotes the progress of the forward reaction and glycerol
conversion will increase. The substances in this system own considerable difference
in boiling points (As shown in Table 1) resulting in relatively easy to separate them,
so only a few number of theoretical stages demanded for implementing effective
separation. When theoretical stages in this section exceed 6, the conversion rate
remains almost constant. At this time, if the number of the stripping section is
increased, the conversion rate hardly increases, but the reboiler duty increases, the
equipment cost goes up.
7
. Process intensification by RDWC
Because of non-ideal boiling ranking, the conventional RDC can achieve the task,
but since the unconverted lowest boiling point reactants Acetone and highest Glycerol
gradually tend to move through the stage for withdrawing the intermediate product
and bring consequently separation difficulties. In this research, the novel concept of
RDWC is introduced into synthetic process of SK.
Fig. 15 illustrates the equivalent process of RDWC sequence comprising a RD
column accompanied with another separation and purification column, which linked
mutually through two internal streams, vapor streams (V
2
) and liquid streams (L ), to
1
simulate the internal mass transfer process, specific process are illustrated in SI 1-9.
The entire column is divided into four segments, we called reaction zone region I,

product purification section region II, public distillation section and public stripping
section.
7
.1. Process design
The RDWC is designed with a conventional RDC range as foundation, requiring
both feed, product purity, and operating pressure under the same conditions, the
design process can be divided into concept design and optimization.
In this paper, the simulation is performed on the Aspen Plus; the Radfrac unit is
selected for both columns. There are many variables in RDWC, which are directly
interacted to each other. It is difficult to comprehensively analyze all variables. In
order to simplify the effects between them, univariate analysis was applied to provide
an initial value for optimal analysis.
The optimization process is shown in Fig. 16.
7
.2. Process optimization
The optimal design parameters of RDWC were obtained through economic
analysis, as calculated by equation in SI 1-10. The objective function annual total cost
TAC) is utilized to select optimal parameters [51,52] and TAC based on the economic
(
optimization of global was obtained [53]. After maintain product purity requirements
by adjusting the two manipulated variables, reflux ratio (RR) and vapor split volume
(Vr), the value of other parameters are investigated.
Due to energy integration in the system, where combustion fuel was used, not
only improve energy efficiency, but also reduce CO
2
emissions. The rule of
calculation [54]
Environmental Protection Agency “E9-5711” was applied for CO
2
.

[
CO2
]
Emission reduction = (Q Fuel/NHV)*α*44/12
(12)
Q Fuel= Qs/ Ƞ
(13)
Note: α represents CO
2
emission factor; Qs is energy saving value in the process; Q Fuel is the
energy saved from fuel combustion; NHV is the net heating value; Ƞ represents the thermal
efficiency coefficient.
7
7
.3. Optimization results
.3.1. Vapor phase distribution ratio
The objective function of the investigation is the purity of SK product and
reboiler duty. The vapor split volume (Vr) is an important variable. The vapor
distribution ratio and reflux ratio (RR) determine minimum energy consumption of
the column. The distillate amount and the side extraction of the column are used to
ensure the quality requirements of product SK, the other two degrees of freedom are
changed to minimize the energy consumption, effect of the reflux ratio of the main
column and the gas phase distribution on reboiler duty is depicted in Fig. 17.
In Fig. 17, reboiler duty monotonically decreases with it value increasing, it is
because as the amount of gas entering the region II (Fig. 15) has decreased, the
separation ability of the corresponding rectifying section reduced, so the purity of the
product solketal decreases. The reaction would be insufficient if the volume of gas
phase enters into the region I which came from the stripping section is reduced, it will
lead to the amount of solketal decrease. The value for vapor is 100g/hr; RR is 2,
whose reboiler duty is lowest.
7
.3.2. Liquid phase distribution ratio
Other conditions remained, the liquid distribution flow rate was changed, and the
effect on the experimental results was examined. It can be seen from the Fig. 18 that