(72j) An Effective Synthesis Framework for Integrated Water and Membrane Networks

The scarcity of water and strict environmental regulations has made sustainable engineering a prime concern in the process and manufacturing industries. Water minimization involves the reduction of freshwater use and effluent discharge in chemical plants. This is achieved through water reuse, water recycle and water regeneration. Optimization of the water network superstructure (WNS) considers all possible interconnections between water sources, water sinks and regenerator units (membrane systems). In most published work, membrane systems have been represented using the “black box” approach, which uses a simplified linear model to represent the membrane systems. This approach does not give an accurate representation of the energy consumption and associated costs of the membrane systems.

This work therefore looks at the minimization of freshwater, wastewater and energy within a water and membrane network superstructure by incorporating a detailed synthesis of a reverse osmosis network (RON) superstructure within a WNS. The RON superstructure is synthesized using the “state space” approach that allows the consideration of multiple regenerators. The optimum synthesis of the RON includes design variables such as the optimum number of reverse osmosis units, pumps and energy recovery turbines. This integrated approach enables both water reuse/recycle and regeneration – reuse/recycle. The overall mathematical structure is a mixed-integer nonlinear programming model (MINLP).  The model was applied to a petroleum refinery case study and the results showed a reduction in freshwater usage, wastewater generation and a subsequent savings in total annualised cost of approximately 11 %.