(177h) A Systems Approach for the Development of Osn Membrane Cascades | AIChE

(177h) A Systems Approach for the Development of Osn Membrane Cascades

Authors 

Chachuat, B. - Presenter, Imperial College London
Livingston, A. G., Imperial College London
Adi, V. S. K., National Cheng Kung University
Peeva, L. G., Imperial College London
Cook, M., Imperial College London

The processes involved in separating the
components of chemical mixtures into pure or purer forms, such as
distillation, account for 10-15% of the worldâ??s energy consumption.
Methods to purify chemicals that are more energy efficient could, if
applied to the U.S. petroleum, chemical and paper manufacturing
sectors alone, save 100 million tonnes of CO
2
emissions and US$4 billion in energy costs annually [1].
Membrane-based separation methods, or other non-thermal ones, can be
an order of magnitude more energy efficient than heat-driven
separations that use distillation, but these methods are
underdeveloped [2].

Given the recent advances in organic
solvent nanofiltration (OSN) membranes [3], the focus of this
presentation is on OSN membrane cascades for separating
organic/hydrocarbon mixtures. As a case study, we investigate the
design of a lab-scale membrane cascade for purifying a binary mixture
of heptane and hexadecane, with a 75-25 weight fraction. The main
objective is to maximize heptane purity in the permeate stream by
using a maximum of 10 identical membrane units, each having an
effective area of 14 cm.

In a first step, we use a superstructure
optimization approach based on mixed-integer nonlinear programming
(MINLP) to determine the most promising cascade configurations, in
terms of the number of stages in the cascade, the number of membrane
units in each stage, and the interconnections thereof, along with
certain operating parameters [4,5]. Constraints are imposed for
limiting the transmembrane pressure drop and the stage-cut in each
unit. The permeate and retentate flux and composition for each
membrane unit are predicted by a classical solution-diffusion model,
where the permeability coefficients for heptane and hexadecane are
estimated based on dedicated experiments for the membranes at hand,
and the activity coefficients of heptane and hexadecane are
approximated using the UNIFAC method for different mixture
compositions. An optimal 3-stage cascade obtained with this approach
is shown on the figure below, with GAMS 24.5.6 and the global
optimizer BARON 15.9 used to solve the MINLP problem.

In a second step, we implement the
optimised OSN membrane cascades experimentally in order to verify the
predictions. For the 3-stage cascade shown on the figure, the
mismatch between the model predictions and the measurements remains
small, within a few percents, thereby validating the model-based
optimization approach.

References:

1. U.S.
Department of Energy, Advanced Manufacturing Office, "Bandwidth
Study on Energy Use and Potential Energy Saving Opportunities in U.S.
Petroleum Refining", June 2015.

2. D.S. Sholl,
R.P. Lively, "Seven chemical separations to change the
world," Nature 532:435-43, 28 April 2016. U.S. Petroleum
Refining (US Dept. Energy, 2015).

3. P. Marchetti,
M.F.J. Solomon, G. Szekely, A.G. Livingston,
"Molecular separation with organic solvent nanofiltration: A
critical review," Chemical Reviews 114:10735-10806, 2014.

4. R.H. Qi,
M.A. Henson, "Membrane system design for multicomponent gas
mixtures via mixed-integer nonlinear programming," Computers &
Chemical Engineering 24:2719-2737, 2000.

5. V.S.K. Adi,
M. Cook, L.G. Peeva, A.G. Livingston, B. Chachuat,
"Optimization of OSN membrane cascades for separating organic
mixtures," Proc. 26th European Symposium on Computer Aided
Process Engineering (ESCAPE), 12-15 June 2016, Portorož, Slovenia.

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