Optimal cooling towers management for zero discharge

Abstract

In order to operate the cooling tower system as a zero discharge unit, the blowdown should be treated and recycled or used in other processes within the industrial plant limits. A mathematical model, applicable to wet cooling towers, was developed. The basic assumptions of the model were: • . Cooling tower blowdown and makeup might be treated in the same plant (Alternative I) or in different plants (Alternative II). • . Average removal efficiencies were used as basis for determining the feasibility of each one of the selected treatment techniques ((FILTRATION, SOFTENING, REVERSE OSMOSIS, ELECTRODIALYSIS, ION EXCHANGE, VAPOR COMPRESSION EVAPOPATION), to reduce the concentration of the limiting chemical compounds. • . Water Quality Guidelines for circulating cooling water, were uses as constraints to be satisfied in the cooling circuit. • . Three types of outflows: evaporation, drift and brine or sludge, were considered leaving the cooling system. • . The composition and temperature of the cooling water and drift streams were equal. • . The system was under steady-state conditions. • . All treatment plants had to be constructed . The Model's statement was: Objective Function: Minimize Total Cooling Tower Makeup and Blowdown Treatment Costs. Subject to: 1. (1) Flow constraints 2. (2) Water Quality constraints Costs functions were developed for the selected treatment techniques in order to be used as the objective functions of the Model. The optimization algorithm was based on the searching of a minimum cost objective function by varying the flow rates of the treated makeup and blowdown in an iterative way. Five computer programs were developed and the corresponding printouts show the feasibility region and the optimal basis. The Model was applied to three cooling towers scale levels (Equivalent reject heat loads: 20 MW, 150 MW, and 500 MW) and using two makeup sources of different quality.