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Development and testing of an optimisation model at selected Eskom sites for an integrated water solution
Expanded Title:Volume 1 in this two-volume series focuses on “Integrated Water and Membrane Network Systems”, whilst Volume 2, “Cooling Tower Model Development” is dedicated to cooling water system design that is characterized by multiple cooling towers. A cooling water system, in the context of this investigation, refers to a cooling tower with its associated set of heat exchangers. In Volume 1, the developed model was validated using Eskom Kriel Power Station. Results have shown potential savings of more than 12% in freshwater use. This facility operates on a zero liquid effluent discharge philosophy. Consequently, no mention is made of wastewater savings. As part of knowledge transfer, a workshop was conducted at Eskom, demonstrating the applications of process integration in water minimization. Graphical and mathematical optimization techniques were presented in detail. In particular, the attendees were taken through the entire thinking process that is necessary for identification of relevant streams for optimization, as well as characterization of streams into sources and sinks. The developed model has been successfully tested and applied to Kriel Power Station. Various scenarios were explored and analyzed as potential sources for the final design. The most outstanding among the scenarios involved the reuse of blowdown from one cooling tower to the other and yielded 12% savings in freshwater use. This scenario was discussed in detail with the plant personnel and proved to be feasible. This is currently happening in Lethabo. Volume 2 is premised on the observation that cooling water systems are generally designed with a set of heat exchangers arranged in parallel. This arrangement results in higher cooling water flow rate and low cooling water return temperature thus reducing cooling tower efficiency. Previous research on cooling water systems has focused mainly on heat exchanger network thus excluding the interaction between heat exchanger network and the cooling towers. This report presents a technique for grassroots design of cooling water system for wastewater minimization which incorporates the performances of the cooling towers involved. The study focuses mainly on cooling systems consisting of multiple cooling towers that supply a common set of heat exchangers. The heat exchanger network is synthesized using the mathematical optimization technique. This technique is based on superstructure in which all opportunities for cooling water reuse are explored. The cooling tower model is used to predict the thermal performance of the cooling towers. Two case studies are presented: The first case resulted in nonlinear programming (NLP) formulation and the second case yield mixed integer nonlinear programming (MINLP). The nonlinearity in both cases is due to the bilinear terms present in the energy balance constraints. In both cases the cooling towers operating capacity were debottlenecked without compromising the heat duties.
Date Published:01/07/2016
Document Type:Research Report
Document Subjects:R & D - Research capacity
Document Keywords:Water Quality
Document Format:Report
Document File Type:pdf
Research Report Type:Technical
WRC Report No:TT 672/16
ISBN No:978-1-4312-0819-7
Authors:Majozi T; Buabeng-Baidoo E; Gololo V
Project No:K5/2289//3
Originator:WRC
Organizations:University of Pretoria; University of the Witwatersrand
Document Size:6 221 KB
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