Process Modeling and Analysis of an Industrial Fresh Water Desalination System

The desalted water station is widely installed in the refinery, petrochemical, and coal chemical enterprises to remove calcium and magnesium ions in fresh water to produce desalted water for boilers, hot water stations, and process units. Increasing the water production ratio of the fresh water desalination system is favorable to improving the utilization ratio of fresh water resources and reducing wastewater discharge. Few studies have been conducted on the modeling of fresh water desalination systems and analyzed the operation on its water production ratio. In this paper, an industrial fresh water desalination system is modeled and analyzed. It mainly includes ultrafiltration (UF), reverse osmosis (RO), cation-anion exchange resin bed (the mixed bed), and water storage tanks. The flow rate balance and contaminant mass balance for batch operation units (i.e., UF, RO, mixed bed) and storage tanks are established, and the objective is to maximize the water production ratio. It is formulated as a nonlinear programming model and then transformed into a mixed integer linear programming problem (MILP) using a linearized technique. The calculated water production ratio using the proposed mathematical model is 82.86%, which agrees with the actual ratio (81.67%) of a fresh water desalination system in a refinery plant in northern China. Next, the sensitivity analysis is conducted to analyze the impacts of variables (i.e., demand for desalted water, bed-changing time, and operation time of the mixed bed) on the peak liquid level of RO storage tank s4 and water production ratio of the whole system. Results show that the peak liquid level of RO storage tank s4 is positively correlated with desalted water demand. The longer time of bed-changing leads to the increasing peak liquid level of RO storage tank s4. The bed operation time has little influence on the peak liquid level of RO storage tank s4. The proposed model can guide grassroots design and operation analysis of industrial fresh water desalination systems.