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.