Industrial experience in using cyclic distillation for the purification of ethanol food grade

Abstract

Cyclic distillation is an intensified fluid separation that uses a different way of contacting the liquid and vapor phases, which avoids mixing of liquids of different compositions. This can be achieved with specific internals and a periodic operation mode. The cyclic mode of operation leads to advantages such as: increased column throughput, reduced energy requirements, and better separation performance. This study provides more details about the industrial introduction of cyclic distillation in the production of ethanol, which confirmed all the theoretical predictions of increasing separation efficiency with excellent results. The 15 cyclic distillation trays were superior in separation capacity and performance to the 50 traditional bubble cap trays. At the same time, the quality of ethanol (food grade) increased due to the purification of the most polluted streams in an additional column. All rights reserved by the authors as per DA2022 copyright notice.

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The 12th international conference Distillation & Absorption 2022
Toulouse – France 18-21 September 2022
1
Industrial experience in using cyclic distillation for the purification of ethanol food grade
Olesja Bedryk,
a
Alexander Shevchenko,
b
Vladimir N. Maleta,
a
Anton A. Kiss
c,d*
a
Maleta Cyclic Distillation LLC OÜ, Parnu mnt 130-38, 11317 Tallinn, Estonia
b
National University of Food Technologies, Vladimirskaya st. 68, Kyiv-33, 01601, Ukraine
c
Department of Chemical Engineering, Delft University of Technology, 2629 HZ, Delft, The Netherlands
d
Centre for Process Integration, Department of Chemical Engineering, The University of Manchester, M13 9PL,
Manchester, United Kingdom
Abstract. Cyclic distillation is an intensified fluid separation that uses a different way of contacting the liquid and
vapor phases, which avoids mixing of liquids of different compositions. This can be achieved with specific internals
and a periodic operation mode. The cyclic mode of operation leads to advantages such as: increased column
throughput, reduced energy requirements, and better separation performance. This study provides more details about
the industrial introduction of cyclic distillation in the production of ethanol, which confirmed all the theoretical
predictions of increasing separation efficiency with excellent results. The 15 cyclic distillation trays were superior in
separation capacity and performance to the 50 traditional bubble cap trays. At the same time, the quality of ethanol
(food grade) increased due to the purification of the most polluted streams in an additional column.
All rights reserved by the authors as per DA2022 copyright notice.
Keywords Process intensification, Cyclic distillation, Fluid separations, Industrial process
Introduction
Mass transfer processes are widely used in many industries, and the number of production processes (which includes
rectification processes) is growing every year. Among the advanced fluid separations technologies, cyclic distillation
stands out as a new contender due to a different way of contacting the liquid and vapor phases (Maleta et al., 2011).
Cyclic distillation uses separate phase movement (SPM) that can be achieved with specific internals and a periodic
operation mode (Kiss and Bildea, 2015). One operating cycle consists of two parts: a vapor flow period (when the
thrust of rising vapor prevents liquid down flow) followed by a liquid flow period (when the liquid flows down the
column, dropping by gravity, first to a lock chamber and then moving to the tray below). This leads to several key
advantages, such as increased column throughput, reduced energy requirements, and better separation performance.
The selection and design of fluid separation processes is very important in all chemical processes (Blahušiak et al.,
2018). To ensure high and stable distillation quality, the typical scheme of ethanol fermentation-distillation plants
includes 6-7 columns required for the separation and purification of alcohol (Maleta et al., 2019). It should be noted
that the main columns are the fermentation column, hydro-selection column, and distillation column. Experimental
studies in the field of rectification have shown that the process of separation of the binary mixture into fractions by
the traditional method does not fully reveal the potential of such installations (Maleta et al., 2015). A review of ways
to increase the efficiency of mass transfer equipment in recent years has shown that the use of cyclic distillation with
separate phase movement is successful at industrial scale and it is gaining momentum (Bildea et al., 2016).
Problem statement
The alcohol industry aims to improve the purification of ethanol. Its purification with the production of high-quality
food alcohol involves the sequential distillation of the water-alcohol mixture in the fermentation-distillation unit,
which ensures the separation of impurities (more than 70 species) from the zones of their maximum concentration.
Their concentration and removal as a by-product occur in the hydro-selection columns. In this respect, it is necessary
to increase the water supply for hydro-selection in these columns, which reduces the concentrations. This, in turn,
disrupts the distribution of intermediate impurities in the distillation column, complicating their separation.
To solve this problem, this research focuses on determining the optimal conditions for the removal of impurities due
to the uniform distribution of ethanol over the entire height of the column for a fixed value of the concentration of
the bottom fluid. The maximum effect of the exit of the main amount of impurities from the column is achieved at a
low concentration of ethanol on the trays. This is facilitated by hydroselection. However, the distribution of water in
the column depends on where the water is fed into the column. The aim is to determine the optimal conditions for the
removal of impurities in the column in view of the uniform distribution of ethanol concentration over the column.

The 12th international conference Distillation & Absorption 2022
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Results and discussion
The object of this work is a hydro-selection column, which is associated with a wide range of demonstrations of its
capabilities of the proposed method and equipment. Different impurities are removed under different conditions. By
changing the feed location and the amount of water for hydro-selection, one can influence the composition of the
removed impurities. The hydro-selection column operates in cyclic distillation mode. Such columns are already used
in industry and provide the greatest efficiency of separation of components. Based on a mathematical model for
cyclic distillation processes Maleta Cyclic Distillation has created a program to calculate the distribution of alcohol
concentration on the plates of the column. This model was used to simulate this dependence (Maleta et al., 2011), but
other models could be also used as described in the literature (Patrut et al., 2014; Nielsen et al., 2017).
The simulation of the column was performed on the example of a plant capacity of 30,000 L/day on 15 plates. The
feed supply was on 8th stage, with heating turned on. The input simulation parameters are shown in Table 1.
Table 1. Parameters used in the simulation of the cyclic distillation column.
Description Unit Value
Consumption of steam kg / h 200
Feed flowrate L / h 200
Water flowrate (for hydro-selection) L / h 820
Power supply % vol. 90,00
Bottom product concentration % vol. 15,00
Three main options for water supply for hydro-selection were considered:
• Water and reflux are fed to the first plate of the top section of the column (Fig. 1a);
• Water is fed to the first plate from the top, while reflux is fed to one of the lower plates (Fig. 1b);
• Water is fed to the feed tray, while the reflux is fed to the first tray at the top (Fig. 1c).
The scheme of flow distribution in the column is shown in Fig. 1, while the results of the calculations of the hydro-
selection column are illustrated in Fig. 2.
Fig. 1 Scheme of flows in the hydro-selection column: a) water and reflux are both fed on the first plate from the top; b) water is
fed on the first tray from the top, while reflux is fed on the lower trays; c) water is fed on the same stage as the feed (diluting the
feed), while reflux is fed on the first stage at the top. Notation: 1 - column; 2 - boiler; 3 - reflux condenser; 4 – heat exchanger; 5 –
reflux drum; C - main fraction concentrate; F - feed supply; R - reflux; B - bottom product; V - steam; W - water.

The 12th international conference Distillation & Absorption 2022
Toulouse – France 18-21 September 2022
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Fig. 2 shows that the location of the water feed significantly affects the distribution of the concentration of alcohol
along the height of the column and accordingly, the conditions for the removal of impurities. It can be observed the
presence of concentration barriers on the plates of the column. The concentration barrier is a new terminology that
we have proposed to describe the process in the hydro-selection column. This can be defined as an increase in the
concentration of ethanol in any part of the column, preventing the removal of impurities.
Fig. 2 Column profiles showing the distribution of alcohol concentration on the trays of the hydro-selection column: a) water and
reflux are served on the first plate from the top; b) water is fed on the first plate from the top, and reflux is fed on the lower plates;
c) water is fed on the feed stage, and reflux on the first plate at the top.

The 12th international conference Distillation & Absorption 2022
Toulouse – France 18-21 September 2022
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In Fig. 2a and Fig. 2b, the concentration barrier is located in the stripping part of the column. In Fig. 2c, the
concentration barrier is located in the rectification part of the column. This distribution of hydro-selection water
leads to different conditions for the removal of impurities (different concentrations of ethanol) for different parts of
the column, which negatively affects the overall separation process. Yet, equalization of ethanol concentrations along
the height of the column is possible in the case of redistribution of water between the feed and the first plate above.
Two modes of operation of the column were simulated, as these modes allow controlling the concentration of ethanol
along the entire height of the column:
• Mode I: water supply on 15th tray and dilution of feed on 8th stage, with reflux supply on 15th tray (at top).
• Mode II: water feed on 15th tray and dilution of feed on 8th stage, with reflux supplied to 12th tray.
The input parameters of the calculation of the hydro-selection column do not change compared to the previous cases.
Fig. 3 illustrates the scheme of distribution of flows in the simulated cyclic distillation column. The hydro selection
column has a diameter of 560 mm, a height of 5500 mm. The raw material used is a mixture of streams from all
columns of the distillation system containing the largest amount of impurities (e.g. head fraction, extracted impurities
from various columns). The feed stream is pre-heated by the bottom product stream.
Fig. 3 Flow distribution scheme in the hydro-selection column of the cyclic mode: a) Mode I – water is fed on 15th tray and also
in on 8th stage to dilute the feed, while the reflux is fed on 15th tray (at the top); b) Mode II – similar to mode I but the reflux is
fed on 12th tray (i.e. 3 stages below the top tray). Notation: 1 - column; 2 - reboiler; 3 - reflux condenser; 4 – heat exchanger; 5 –
reflux drum; C - main faction concentrate; F - feed supply; R - reflux; B - bottom product; V - steam; W - water.
The distribution of ethanol concentration in mode I of the hydro-selection column is shown in Fig. 4a, Fig. 5a, and
Fig. 6a. The supply of water on the 15th stage and the dilution of the feed on 8th stage with reflux fed to the 12th

The 12th international conference Distillation & Absorption 2022
Toulouse – France 18-21 September 2022
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stage in mode II is shown in Fig. 4b, Fig. 5b, and Fig. 6b. Hydro selective water dilutes the feed to a concentration of
ethanol on feed stage of 30%, 35% and 33% vol., respectively. Note that at a concentration of 30% the concentration
barrier will be at the top of the column, while at a concentration of 35% ethanol on the feed stage the concentration
barrier will be at the bottom of the column. However, at a concentration of 33% ethanol on the feed stage, the
concentration barrier disappears (see Fig. 6). Fig. 4, Fig. 5, and Fig. 6 show that the location of the water supply for
hydro-selection and its distribution between the entry points significantly affects the concentration of alcohol.
Fig. 4 Distribution of ethanol concentration on the plates of the
column (Ethanol concentration on the feed tray 30% vol.)
Fig. 5 Distribution of ethanol concentration on the plates of the
column (Ethanol concentration on the feed tray 35% vol.)
Fig. 6 Distribution of ethanol concentration on the plates of the column (Ethanol concentration on the feed tray 33% vol.)

The 12th international conference Distillation & Absorption 2022
Toulouse – France 18-21 September 2022
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With the help of a hydro selective column, impurities of the head (such as acetaldehyde, ethyl acetate, acetone, and
others) and fusel impurities (such as propanol, butanol, amylol, and others) are removed. Simulation results have
been confirmed under industrial conditions by chromatographic analysis of the bottom and top products. The degree
of purification from impurities was determined by the amount of impurities in the bottom product. The concentration
of the bottom product may be different, depending on the type of impurities that need to be removed from the
ethanol. Note that the column concentrating the impurities removes three types of impurities: head (volatile)
impurities, intermediate impurities and end (heavy) impurities. Accordingly, the conditions for their separation are
somewhat different. For head impurities, the concentration of ethanol at the bottom of the column should be 15-20%
(Fig. 6a). For intermediate impurities, the concentration of ethanol at the bottom of the column should be 5-10%
(Fig. 6b). For end impurities, the column configuration shown in Fig. 1c should be used. At the same time, the
amount of water for hydro selection should be minimized to reduce heating costs for the subsequent processing of
the bottom product. Overall, the most effective option was the one shown in Fig. 6b, since the ethanol concentration
at the exit from the top of the column was minimal.
Conclusions
The introduction of new technology is an integral part of innovative development in distillation processes. The
hydro-selection technology proposed in this work makes it possible to create optimal conditions for the removal of
various head impurities and fusel impurities by equalizing the concentration of ethanol throughout the height of the
column. The use of cyclic distillation technology makes it possible to reduce water consumption for hydro selection
by increasing the efficiency of separation of components, as well as to increase the degree of purification of ethanol
from accompanying impurities. Moreover, in terms of separation efficiency, the cyclic distillation with only 15 plates
manages to outperform the traditional distillation column with 50 bubble cap trays, thus proving additional yield of
alcohol (up to 3-4%), improving the physico-chemical and organoleptic characteristics of alcohol, and reducing the
energy requirements per unit of ethanol product (by up to 30%).
Considering that the relative volatility of the bulk of the impurities increases with decreasing ethanol concentration,
mode II is preferred for practical operation as the concentration on the plates of the upper and lower parts of the
column is lower than on the middle stages. For the studied conditions, the optimal ethanol concentration on the feed
tray is 33% vol. Other concentration ranges around the optimal point were used as an example. Range extensions on
both sides only aggravate the separation conditions (increase the concentration barrier) so they are not considered.
The optimal distribution of water for hydro-selection is in the proportion: feed tray - 42% and 15th plate - 58%.
However, when changing any of the input parameters (such as feed flowrate, feed composition, steam flowrate), the
optimal value must be recalculated accordingly.
Acknowledgments
AAK thankfully acknowledges the Royal Society Wolfson Research Merit Award (No. WM170003).
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