Separation of ethanol and water by extractive distillation with salt and solvent as entrainer: process simulation
Brazilian Journal
of Chemical
Engineering
ISSN 0104-6632
Printed in Brazil
www.abeq.org.br/bjche
Vol. 25, No. 01, pp. 207 - 215, January - March, 2008
SEPARATION OF ETHANOL AND WATER BY
EXTRACTIVE DISTILLATION WITH SALT AND
SOLVENT AS ENTRAINER: PROCESS
SIMULATION
I. D. Gil*, A. M. Uyazán, J. L. Aguilar, G. Rodríguez and L. A. Caicedo
Departamento de Ingeniería Química, Universidad Nacional de Colombia
Ciudad Universitaria, Phone: +(57) (1) 3165672, Fax: +(57) (1)-3165617,
Carrera 30 45-03, Bogotá D.C., Colombia
E-mail:
(Received: February 1, 2007 ; Accepted : November 6, 2007)
Abstract - The aim of this work is to simulate and analyze an extractive distillation process for azeotropic
ethanol dehydration with ethylene glycol and calcium chloride mixture as entrainer. The work was developed
with Aspen Plus® simulator version 11.1. Calculation of the activity coefficients employed to describe vapor
liquid equilibrium of ethanol – water – ethylene glycol – calcium chloride system was done with the NRTL-E
equation and they were validated with experimental data. The dehydration process used two columns: the
main extractive column and the recovery column. The solvent to feed molar ratio S/F=0.3, molar reflux ratio
RR=0.35, number of theoretical stages Ns=18, feed stage Sf=12, feed solvent stage SS=3, and feed solvent
temperature TS=80 ºC, were determined to obtain a distillate with at least 99.5 % mole of ethanol. A
substantial reduction in the energy consumption, compared with the conventional processes, was predicted by
using ethylene glycol and calcium chloride as entrainer.
Keywords: Simulation; Extractive distillation; Ethylene glycol; Calcium chloride.
INTRODUCTION
Anhydrous ethanol is widely used in chemical
industry as powerful solvent and as raw material or
intermediate in chemical synthesis of esters, organic
and cyclic compound chains, detergents, paints,
cosmetics, aerosols, perfumes, medicine and food,
among others. Besides, ethanol and gasoline
mixtures can be used as fuels reducing
environmental contamination and anhydrous ethanol
addition improves octane index (Barba et al., 1985;
Black, 1980; Chianese and Zinnamosca, 1990;
Meirelles et.al., 1992). Several processes for ethanol
dehydration are used such as heterogeneous
azeotropic distillation, which uses different solvents
such as benzene, pentane and cyclohexane;
*To whom correspondence should be addressed
extractive distillation with solvents and salts as
separating agents (Fu, 2004a,b); adsorption with
molecular sieves and processes that include the use
of pervaporation membranes (Black, 1980; Lynn and
Hanson, 1988; Jacques et al., 1999; Pinto et al.,
2000; Ulrich and Pavel, 1988). All these processes
have had industrial application but some are no
longer in use due to the high operating costs,
operative problems and high energy consumption. In
the case of gasoline as separating agent it reverses
ethanol-water volatility, causing water to be removed
as the top product and ethanol, mixed with solvent,
to be withdrawn as the bottom product. Water is
completely absent in the bottom product, it is
withdrawn in the top mixed with some traces of
ethanol and lighter hydrocarbons. For this reason, the
208
I. D. Gil, A. M. Uyazán, J. L. Aguilar, G. Rodríguez and L. A. Caicedo
ethanol produced by such a process can be used only
as gasohol and for this particular process the amount
of solvent is high and can only be applied with a
petroleum refinery (Chianese and Zinnamosca,
1990).
Extractive distillation is a partial vaporization
process, in the presence of a non-volatile and high
boiling point separating mass agent that it is usually
called entrainer or separating agent, which is added
to the azeotropic mixture to alter the relative
volatility of the key component without additional
azeotrope formation (Perry et al., 1992; Black and
Distler, 1972). Generally, entrainer is fed in the
upper part of the column, above the feed stream and
it remains in a considerable concentration in the
liquid phase along the column. It is withdrawn as a
bottom product with one of the components that are
being separated and it is sent to a second
regeneration column operated under vacuum. The
most common solvents used in extractive distillation
are glycols (Perry et al., 1992; Meirelles et al., 1992),
glycerol (Lee and Pahl, 1985), gasoline (Chianese
and Zinnamosca, 1990) and for the case of saline
extractive distillation, acetate and inorganic salts
such as: CaCl2, AlCl3, KNO3, (CuNO3)23H2O,
Al(NO)39H2O, K2CO3 (Barba et al., 1985; Furter,
1992; Ligero and Ravagnani, 2003; Llano and
Aguilar, 2003; Pinto et al. 2000; Schmit and
Vogelpohl, 1983).
Extractive distillation with salts and solvents as
separating agents turns up as a new possibility to
obtain high purity products. This process combines
the traditional extractive distillation with the “salt
effect” principle. With this combined method, it is
possible to solve several eventual problems of
transport, dissolution, corrosion and obstruction
found when only salt is used as separating agent. In
addition, it has the following characteristics (Rongqi
and Zhanting, 1999):
Allows continuous operation because of the high
efficiency and the low waste of solvent.
A high purity product can be obtained.
The relative volatility of the ethanol-water system
is increased, compared to the effect produced by
each of independent agents.
Improves the solvent performance. Compared
with normal extractive distillation, the quantity of the
solvent to recycle is reduced to its fourth or fifth, the
number of theoretical stages required can be reduced
to its third, as well as energy consumption.
There is one industrial packing column reported
by Fu (2004b) used to separate ethanol-water
mixtures, which was simulated and the results were
in good agreement with industrial data. Also, some
laboratory studies have been done to separate nonpolar systems, such as C4 hydrocarbon mixtures and
polar systems, such as ethanol-water, from which it
is possible to reach to some conclusions about the
efficiency of separating agents in the processes
(Zhigang et al., 2002). Analysis of ethanol-water
system was focused on obtaining experimental data
of the vapor-liquid equilibrium for the system
ethanol-water-ethylene glycol-calcium chloride, as
well as on performing experimental runs in the
laboratory that have allowed to establish some
operating parameters, reported by Rongqi and
Zhanting (1999). One of the results to enhance from
this work is that the separating agent (salt-solvent
mixture) was recovered in a later stage of the
separation and recycled to the process, not being
necessary to separate the salt and the solvent.
The aim of this work is to study and establish
industrial operating conditions for the extractive
distillation of ethanol using ethylene glycol-calcium
chloride mixture as entrainer.
METHODOLOGY
Problem Definition
All the work was developed using Aspen Plus
Simulator, version 11.1 (...truncated)
