Optimal design and operation of reverse osmosis desalination process with membrane fouling

This paper focuses on steady state performance predictions and optimization of the reverse osmosis (RO) based desalination process utilizing a given spiral wound type membrane modules. A set of implicit mathematical equations are generated by combining solution-diffusion model with film theory approach to model the RO process which is used for simulation and optimization. The simulation results for a three-stage RO process are compared with the results from literature and are found to be in good agreement having relative errors of 0.71% and 1.02%, in terms of water recovery and salt rejection, respectively. The sensitivity of different operating parameters (feed concentration and feed pressure) and design parameters (number of elements, spacer thickness, length of spacer filament) on the plant performance are also investigated. Finally, for the same type of spiral wound membrane, two optimization problems are formulated and solved. In the first one, a non-linear optimization problem is formulated for the same three-stage RO process (fixed configuration) to minimize the specific energy consumption at fixed product flow rate and quality while optimizing the operating and design parameters. The results showed a 20% savings in specific energy consumption compared to the base case. In the second one, a mixed-integer nonlinear programming (MINLP) problem based on a superstructure is formulated for fixed freshwater demand and quality to minimize the total annualized cost while optimizing the design and operation of the RO network. A variable fouling profile along the membrane stages is introduced to see how the network design and operation of the RO system are to be adjusted to minimize the total annualized cost. Outer-approximation algorithm has been used to solve the MINLP problem. The results show that the fouling distribution between stages affects significantly the optimal design and operation of RO process. (C) 2011 Elsevier B.V. All rights reserved.