(38e) Optimization of Complex Integrated Water and Membrane Network Systems

Optimization
of Complex Integrated Water and Membrane Network Systems

Musah
Abass and Thokozani Majozi*

School
of Chemical and Metallurgical Engineering, University of Witwatersrand, 1 Jan
Smuts Avenue, Braamfontein, Johannesburg, 2000, South Africa

*Corresponding
author: thokozani.majozi@wits.ac.za;
Tel: +27 11 717 7384; Fax: +27 82 456 1500 

Abstract

Water and energy are
key resources in the process industry. The increasing pressure on freshwater and
energy resources coupled with stringent environmental regulations on effluent
discharge limits have called for innovative designs for sustainable use of
water and energy.  This can be achieved
through process integration techniques that are environmentally benign and
economically feasible. Conventional methods for water minimisation through
water network synthesis often use the ?black box? approach to represent
regeneration. The degree of contaminant removal and cost of regeneration are
represented by linear functions. This approach may result in suboptimal
operating conditions of the regeneration units. Moreover, it does not provide
an accurate representation of the total annualised water network costs.

This work proposes a
robust water network superstructure optimisation approach for the synthesis of
a multi-regenerator network for water and energy minimisation. Two types of membrane
regenerators are considered for this work, namely, electrodialysis and reverse
osmosis. In each of the membrane regenerators, a detailed design model is
developed and incorporated into the water network model. As a result, the water
network is capable of direct reuse, recycle, regeneration reuse and regeneration
recycle. All necessary design variables and parameters are included in order to
accurately represent the operation and costs of the regenerators.

Logical constraints are
used to govern the existence of piping interconnections in the water
network.  The presence of continuous and
integer variables, as well as nonlinear constraints renders the problem a
MINLP. The developed model is applied to a pulp and paper case study to
demonstrate its applicability, assuming a single contaminant scenario. The
application of the model results in a 51.5% freshwater reduction, 59% decrease
in wastewater generation and 45% savings in total annualised cost compared to
the original case study.

Keywords: Sustainable Synthesis and Optimization, Reverse-osmosis,
Electrodialysis.