Optimization of the operating conditions of azeotropic distillation columns with pervaporation membranes
P.M. Hoch, M.C. Daviou, A.M. Eliceche
Planta Piloto de Ingenieria Quimica - PLAPIQUI (UNS-CONICET)
Camino La Carrindanga km 7 - 8000 Bahía Blanca - Argentina
e-mail: phoch@plapiqui.edu.ar
Abstract - The main objective of this work is the optimization of the operating conditions of azeotropic distillation columns in hybrid distillation/pervaporation systems. In the configuration analyzed, a liquid side stream extracted from the azeotropic distillation column with the distributing non-key component is treated in a pervaporation membrane while the retentate is recycled to the column. The pervaporation membrane separates the pure distributing non-key component from the mixture, thus helping to improve the purity in the top and/or bottom products of the distillation column. The case study shows the hybrid process of a distillation column combined with pervaporation membranes, compared to the classical two-column process for Methyl tert-Butyl Ether production. In both cases, the operating conditions of the distillation columns such as reflux ratio, product and side draw flowrates are selected optimally. The numerical results show a significant reduction in operating cost obtained by using a pervaporation membrane instead of the second column in the original process.
Keywords - Azeotropic Distillation. Pervaporation. Optimization. MTBE.
I. INTRODUCTION
Hybrid membrane systems can be successfully applied to debottlenecking and
azeotrope breaking problems. Pervaporation is an interesting membrane separation
alternative, because it is generally less energy consuming than distillation.
It is not influenced by the equilibrium between components, making azeotrope
breaking easier than using a sequence of distillation columns. The separation
is based on a selective transport through a dense layer associated with the
evaporation of the permeants. This phase changing is usually obtained by lowering
the partial pressure of the permeants at the downstream side of the membranes
to vacuum pressure.
Recent patents propose hybrid distillation/pervaporation technologies for azeotrope
breaking processes involving the separation of alcohols and ethers (Chen et
al., 1988, Chen et al., 1989) applied to the MTBE process, replacing
the Hüls process. The pervaporation membrane used shows high flux and
high selectivity to the permeation of methanol, effectively breaking the azeotrope
methanol-MTBE. The process called "Total Recovery Improvement for MTBE" or
TRIM™ is a combination of an organophilic pervaporation membrane and
distillation, using two different layouts. The integration of the TRIM™ process
to an existing one would be attractive if the production could be increased
by 5%.
Lipnizki et al. (1999) presented an extensive review of pervaporation-based
hybrid processes, focusing on industrial applications and pointing out the
need of optimization of some of the processes analyzed. One of the fields where
there is need of optimization is the distillation/pervaporation hybrid process.
Hömmerich and Rautenbach (1998) studied the integration of pervaporation
and vapor permeation into the Hüls process, analyzing the influence of
the operating conditions in a hybrid distillation-pervaporation-vapor permeation
system for the Methyl tert-Butyl Ether (MTBE) production.
González and Ortiz (2001a) carried out experimental work to find the
parameters for modeling the pervaporation membrane to separate methanol and
MTBE, and an approach to define a hybrid process distillation/pervaporation
based on the Hüls process. González and Ortiz (2001b) simulated
the hybrid distillation/pervaporation process using gPROMS (PSEnterprises,
2000). Different operating conditions were explored by simulation performing
a cost analysis.
However, the formal optimization of the debutanizer column with a pervaporation
membrane to treat the side stream has not been attempted previously. In this
work, the optimum operating conditions such as reflux ratio and product flow
rates are calculated solving an optimization problem to minimize the operating
cost.
II. Hybrid distillation/pervaporation process
This work was motivated by the possibility of revamping the Methyl tert-butyl
ether (MTBE) sector of a refinery. The production process of Methyl tert-butyl
ether (Figure 1), which is used as a high octane fuel additive, consists of
a reaction sector where i-C4H10 is combined with methanol
to form the ether, and a separation sector where all the MTBE must be separated
from non-reacted methanol and C4s. Non-reacted methanol forms azeotropic
mixtures with MTBE and butanes. A sequence of azeotropic distillation columns
is used to break the azeotropes, thus recovering MTBE as the final product
and methanol to be recycled to the reactor. A hybrid distillation-pervaporation
process, as shown in Figure 2, seems to be an attractive alternative, as it
combines the advantages of both methods. The use of hybrid systems can improve
the cost of the traditional Hüls separation sequence (Hömmerich
and Rautenbach, 1998).
Different configurations for the hybrid distillation/pervaporation process
can be used, locating the pervaporation membrane to treat the feed, products
or side stream. In this work, the pervaporation membrane is located to treat
the side stream and remove the distributing component as permeate, thus helping
to improve the top and bottom purity.
A. Pervaporation membrane modeling
A scheme of the pervaporation membrane is shown in Figure 3. It is assumed
that only two components permeate through the membrane, while the rest of
the components remain at the retentate side and do not permeate.
The equation system representing mass and energy transport for a binary mixture
can be found in González and Ortiz (2001a). The system methanol-MTBE
was used to test the model. The membrane used favored the permeation of methanol
over MTBE. Experimental data were available for the system being studied.
It was assumed that the feed to the membrane is a pseudo-binary mixture of
methanol-MTBE calculated as the composition of the multi-component mixture
in C4s-free basis. As it is assumed that C4s do not permeate,
a simple split of the C4s is performed before entering the pervaporation
unit and the C4s are mixed with the retentate stream once it leaves
the membrane module to be recycled to the column. This simplification allows
to pose the differential algebraic system as if it was for a binary mixture,
then the calculation of the compositions of the retentate and permeate streams
is simpler than for a multi-component mixture. This assumption holds for the
system being studied, as shown in Chen et al. (1988, 1989). Mass and
energy transport equations are solved. A mathematical model based on the generalized
Fick´s law and the assumption that transport through the membrane is the rate-limiting
step was developed in order to describe the pervaporation flux of both components
(González and Ortiz, 2001a). The prediction of the flux of methanol
needed a concentration-dependent diffusion coefficient whereas a simple model
with concentration-independent diffusivity was sufficient for the description
of the MTBE flux.
The energy requirements to perform the pervaporation and condense the permeate
are calculated as the interstage heating between modules and the cooling needed
for condensation. The pervaporation membrane can be modeled in terms of a differential/algebraic
system involving lumped and distributed parameters. This allows the discretization
of the differential equations along the membrane thickness (zP)
and the use of a package to solve Initial Value Problems (IVPs) to integrate
the equations along the membrane length zA (Fig. 3).
The rigorous model gives as a result the dimensions of the membrane needed
to perform the separation. This is necessary in order to calculate the membrane
replacement costs, as shown in Appendix A. The membrane has a lifetime of two
years, and because of the short replacement cycle this cost was considered
an operating cost.
III. Selection of the operating conditions in the separation of MTBE from a mixture of butanes and methanol.
The example deals with the separation of a mixture of i-butane, n-butane,
i-butene, 1-butene, 2-butene trans and 2-butene cis, methanol and MTBE. The
separation can be performed either using the two-column approach (Separation
sector of Fig. 1) or replacing the second column (Methanol-MTBE separation)
with a pervaporation membrane (Fig. 2).
The composition, temperature and pressure of the fresh feed are shown in Table
1, where C4’s stands for a mixture of i-butane, n-butane, i-butene,
1-butene, 2-butene trans and 2-butene cis.
Table 1: Feed to the debutanizer column
For the separation of MTBE, the classical two-column process and the hybrid
process were studied, in order to be able to compare the operating costs of
each one and to quantify the savings that can be obtained by replacing the
second column of the conventional process with a pervaporation membrane.
The optimization problem is formulated as:
![]() |
(P1) |
where the optimization variables x are the reflux ratios (r) and product flow
rates (B) of each column in the two-column scheme, or the reflux ratio (r),
product flow rate (B) and the side draw flow rate (E) of the distillation
column for the hybrid process. bL and bU are lower and
upper bounds on the optimization variables x.
g(x) are constraints on required product compositions and recovery. The maximum
rate of liquid sidedraw was considered an additional constraint for the hybrid
process (Hömmerich and Rautenbach, 1998) to avoid column entrainment.
IV. Numerical Results
A. Two-column scheme:
The separation of pure MTBE as a bottom product of an azeotropic distillation
column from a mixture of C4s, methanol and MTBE is performed by
means of two distillation columns.
The process being studied is shown in Figure 1.
The azeotrope formation of methanol with both MTBE and C4 limits
the purity of the products. The azeotrope C4-Methanol is obtained
as top product in the debutanizer column, while the bottom product consisting
mainly of methanol and MTBE in azeotropic composition at 6 bar is fed to a
second column in order to obtain high-purity MTBE. This second column must
operate at a higher pressure (12 bar) in order to be able to obtain pure MTBE
as the bottom product. The process requires a composition of MTBE of 98 %w
and 95% recovery.
Figure 1: MTBE process with two-column configuration separation sector.
An optimization of this process was performed using Hysys. The "Mixed" optimization
option was used, which begins the iterations with Box method with loose tolerance,
and once the solution is conveniently bounded, uses an SQP method to find
the solution within the desired tolerance.
Two rigorous distillation columns were used, using for both the azeotropic
option in Hysys. Wilson equation with binary interaction coefficients taken
from Espinosa et al. (1995) was used for the thermodynamic property
prediction.
Table 2: Optimization results for the two-column scheme.
The objective is to minimize the operating cost of both columns. Cooling water is used to condense the distillate of the columns. Low pressure steam is used to heat the bottoms of the debutanizer column, and medium pressure steam is used to heat the bottoms of the methanol-MTBE column. Details of the cost function are given in Appendix A. The bottom flow rate and reflux ratio of each column were posed as optimization variables. The debutanizer column has 22 theoretical stages plus condenser and reboiler, and the feed is introduced at stage 10. The methanol-MTBE column has 30 stages plus condenser and reboiler, and the feed is introduced at stage 20. The optimization results are shown in Table 2. There is a cost reduction of 13% comparing the initial point with the optimum point given by Hysys.
B. Hybrid distillation/pervaporation system:
The separation of pure MTBE as a bottom product of an azeotropic distillation column from a mixture of C4s, methanol and MTBE is performed by means of a combined distillation column and pervaporation membrane process. The process being studied is shown in Figure 2.
Figure 2: Schematic flowsheet for the distillation/pervaporation process.
The azeotrope formation of methanol with both MTBE and C4 limits
the purity of the products. A high purity of MTBE is required in the bottom
product (B) of the column that separates a multicomponent mixture (F1) including
MTBE, C4’s and methanol. A sidestream (E) of the column, rich in
methanol, is processed through the membrane. The membrane selectivity allows
the methanol in the column sidestream to be permeated and then condensed and
recycled to the reactor (Permeate liquid stream), thus helping to improve
the MTBE bottom product purity. The retentate, consisting of a mixture of
C4 and MTBE with a lower composition of methanol, is then recycled
to the column (F2).
The pervaporation takes place when a vacuum of 15 torr is maintained at the
permeate side. For the start-up a vacuum pump is needed; but, when steady state
is reached, vacuum is maintained by the permeate condensation as shown in Fig.
3.
The process was also simulated with the commercial package Hysys with the azeotropic
distillation option. The distillation column has 22 theoretical stages plus
a reboiler and a condenser. The objective function is the operating cost of
the process. It is assumed that cooling water is used to condense the distillate
of the column and low pressure steam is used to heat the bottoms of the column.
A refrigerant is needed to maintain the vacuum at the permeate side by condensation.
Figure 3: Scheme of the pervaporation membrane
The cost is a function of the heat withdrew from the condenser of the column,
the heat added to the reboiler of the column, the interstage heating between
pervaporation modules and the heat withdrew in the pervaporation unit to condense
the methanol. Details of the cost functions are given in Appendix A.
For this configuration, the Hysys selected optimization option was
also the Mixed method. The desired MTBE weight composition and MTBE recovery
in the bottom product are the same as in the two-column process. The optimization
results are shown in Table 3. A significant reduction in the operating cost
of 19 % can be observed from the initial to the solution point, showing the
importance of optimization with rigorous simulation in hybrid distillation/pervaporation
processes.
Table 3: Optimization
results for the hybrid process.
A significant cost reduction of 24 % is achieved by changing from the two-column
scheme to the hybrid process, optimizing the operating variables of the debutanizer
column. Side stream E is withdrew from the plate were the liquid composition
of methanol reaches its maximum value.
For the hybrid distillation/pervaporation process, it was also possible to
obtain a bottom product with an MTBE recovery of 98 %, which was not possible
to obtain with the two-column scheme. The cost of the hybrid system with an
MTBE recovery of 98% is 131.6 $/h, which is also cheaper than the operating
cost of the two-column process.
There are important decisions to be made regarding the location of the fresh
feed, side stream and recycle of the retentate stream. The systematic selection
of the number of plates requires the incorporation of integer optimization
variables. The procedure developed by Hoch and Eliceche (1991) treats the number
of stages as continuous optimization variables, allowing the simultaneous selection
of feed and side stream locations with the flowrates in a nonlinear programming
problem formulation. The implementation of this option at the conceptual design
stage is currently being studied.
V. Conclusions
Optimization results are reported for the conventional process and the hybrid configuration for MTBE production. An important reduction in the operating cost of the separation process to obtain pure MTBE with the distillation/pervaporation process has been achieved. Significant improvements are obtained by optimizing the operating conditions of the debutanizer column, as shown in Table 3. Similar results can be expected in new applications of hybrid distillation/pervaporation systems. Future work on this process contemplates the inclusion of the number of stages of the column as optimization variables at the conceptual design stage, expecting even greater savings than considering the operating variables alone.
Acknowledgments
We greatly acknowledge the financial support from ANPCyT (Agencia Nacional de Promoción Científica y Tecnológica) under grant PICT 14-04065 BID 1201 OC/AR. We also thank CONICET (Consejo Nacional de Investigaciones Científicas y Tecnológicas).
APPENDIX A: Operating cost evaluation
Distillation column cost
The operating cost of the column i is calculated as the sum of the operating costs for the condenser and reboiler:
(A.1)
where
(A.2)
is the operating cost of the condenser, and
(A.3)
is the operating cost of the reboiler.
Low pressure steam is used to heat the bottoms of the debutanizer column for both the conventional and hybrid process; while medium pressure steam is required to heat the bottoms of the second column. Cooling water is used for the condensers of both columns.
Pervaporation cost
The operating cost of the pervaporation process can be expressed as a sum of the interstage heating cost, the condensing cost and the membrane replacement cost:
(A.4)
where
(A.5)
![]() |
(A.6) |
![]() |
(A.7) |
(A.5) represents the cost of condensing to maintain vacuum at the permeate side. A refrigerant is needed because of the low temperature required.
(A.6) is used to find the interstage heating cost.
(A.7) allows to calculate the membrane replacement cost. n represents the
number of pervaporation modules; Cm,r, the replacement cost of
each module and a the annuity, calculated considering a lifetime of two years
and an inflation rate of 10% a year. It was considered that the process operates
8000 h/year.
Cost coefficients for both processes can be found in Seider et al. (1999).
Objective function
For the conventional two-column scheme, the operating cost is obtained as
(A.8)
For the hybrid process,
(A.9)
List of symbols
References
Chen, M.S., Eng R.M., Glazer J.L. and C.G. Wensley, "Pervaporation
process for separating alcohols from ethers", U.S. Patent 4774365 (1988).
Chen, M.S., Markiewickz G.S. and K.G. Venugopal, "Development of membrane
pervaporation TRIM™ process for methanol recovery from CH3OH/MTBE/C4 mixtures", AIChE
Symp. Ser. 85, 82-88 (1989).
Espinosa, H.J., P. Aguirre and G. Pérez, "Product composition region
of single-feed reactive distillation columns: mistures containing inerts", Ind. & Eng.
Chem. Res., 34, 853-861, (1995).
González B. and I. Ortiz. Mathematical modelling of the pervaporative
separation of Methanol - Methyl tert-Butyl Ether mixtures, Ind. & Eng.
Chem. Res, 40, 1720-1731, (2001a).
González, B. And I. Ortiz, Modelling and simulation of a hybrid process
(pervaporation-distillation) for the separation of azeotropic mixtures of alcohol-ether, J.
Chem. Technol. Biotechnol. 76, 1-14 (2001b).
González B. Aplicación de la pervaporación a la separación
de mezclas azeotrópicas alcohol/éter, PhD. Thesis, Universidad
de Cantabria, (2000).
Hoch P.M. and A.M. Eliceche, "Optimal Design of non-conventional distillation
columns", Process Technology Proceedings 10, Computer Oriented
Process Engineering, 369-374, (1991).
Hömmerich U. and R. Rautenbach. Design and optimization of combined pervaporation/distillation
processes for the production of MTBE, J. Membr. Sci., 146, 53-64,
(1998).
Hyprotech, HYSYS user manual, 1999.
Lipnizki, F., Field R.W. and P.K. Ten, Pervaporation-based hybrid process:
A review of process design, applications and economics, J. Membr. Science, 153,
(1999), pp. 183-210.
PSEnterprises, gPROMS Users manual, 2000.
Seider W.D., Seader J.D. and Lewin D.R. Process Design Principles, Chapter
10: Profitability Analysis, John Wiley and Sons (ed.), (1999).
Received: September 16, 2001.
Accepted for publication: July 26, 2002.
Recommended by Guest Editors: J. Cerdá, S. Díaz and A. Bandoni.