Water-energy nexus approach for optimal design of hybrid cooling system in direct reduction of iron plant.

Direct reduction of iron process in steel industry has special production conditions. Low quality cooling water, low cold and high hot cooling water temperature, space limitation for new equipment installation, high value-added of product and severe effect of cooling water temperature on production rate are of these conditions. Considering technical and economic constraints and limitations, this situation makes this process an attractive case study for converting the existing wet cooling tower to hybrid cooling system. In this paper, based on integration of process, dry and wet cooling system and ambient conditions profiles, a new method for designing hybrid cooling system has been proposed. The calculation program has been developed as a MATLAB code and resulted: 1) Calculation of annual water saving and energy consumption with reasonable accuracy, 2) Control system of dry and wet cooling blocks to maximize water saving under various ambient conditions, 3) Closed loop simulation of cooling system considering thermal feedback of process to cooling system and 4) Obtaining optimum values for 16 design parameters of dry block cooling (consisting air cooled heat exchanger and plate heat exchanger). The optimization has been done via genetic algorithm, considering annual response of each design under various ambient temperature and relative humidity. Considering design standards for new equipment and operational and technical constraints, made the optimization model reliable as well as checking the design parameters with process equipment constructors. In optimum point, converting existing wet cooling tower to hybrid cooling system in direct reduction of iron plant of Mobarakeh Steel Complex located in Isfahan province in Iran, will decrease the annual water consumption by 70.15% and production loss by 98% with payback period of 1 years and 10 months. [ABSTRACT FROM AUTHOR]