(303l) Process Integration Strategies for Optimization of Biocide Usage and Discharge for Seawater Cooling

With shortages of fresh water supplies, the use of seawater for industrial cooling is receiving addition attention. An inherent problem in the use of seawater for industrial cooling is the biological growth on heat exchangers and other surfaces contacting seawater, biocides (e.g., chlorine products) are used. Notwithstanding their effectiveness, these biocides constitute a major environmental problem to receiving water. Seawater cooling is typically a once-through system. The discharged seawater contains residual quantities of the biocide as well as byproducts of the depleted biocide. These residuals and byproducts are toxic to aquatic lives and present a major ecological problem. As a result of the growing awareness of the hazardous consequences of the biocide discharge, there has been an increasing attention and more stringent environmental regulations governing the discharge of biocides and their byproducts.

The objective of this work is to develop optimum and systematic strategies for using the proper dosage and frequency of biocides and for reducing the negative environmental consequences of biocide discharge. Process integration is used as the overarching framework for design and operation. Mass and energy integration techniques are used to identify performance benchmarks for the process and to screen alternative biocide-reduction techniques. The paper addresses steady-state systems as well as dynamic systems. The use of dynamic systems is needed when unsteady-state dosage of biocide is used. Examples of this include pulse and shock dosage. The problem is posed as a mixed-integer nonlinear programming optimization formulation that seeks to minimize the cost of the biocide-reduction solutions while meeting technical and environmental requirements. A case study is used to illustrate the applicability of the developed approach.