High Recovery MSF Desalination Process

Marina Shmulevsky, chemical engineering, Technion, Haifa, Israel
Hilla Shemer, Chemical Engineering, Technion, Haifa, Israel
David Hasson, Chemical Engineering, Technion, Haifa, Israel
Raphael Semiat, Chemical Engineering, Technion, Haifa, Israel

Multiple Stage Flash (MSF) is a thermal seawater desalination process, widely used in Gulf countries for drinking water supply. The main advantages of MSF is that it is a robust simple process and that it is able to cope with poor quality feeds such as produced waters. Its unfavorable features are that the CaSO₄ scaling problem described below restricts its operation to a low fractional water recovery and to a low top evaporation temperature, resulting in a relatively high energy consumption.

Typical seawater contains 400 mg/L Ca²⁺ and 2600 mg/L SO₄^2-. Calcium sulfate solutions can precipitate this salt in three different polymorphs: gypsum (CaSO₄·2H₂O), anhydrite (CaSO₄) and hemihydrate (CaSO₄·½H₂O). To prevent the harmful deposition of calcium sulfate crystals on heat transfer surfaces, evaporation of seawater is limited to a concentration factor of less than two and the evaporation top temperature is maintained below 120°C.

The general objective of this research is to develop a high recovery – low energy demand MSF process by extending the CaSO₄ solubility limits. The basic approach for achieving this goal is partial electro-chemical removal of calcium ions from seawater by CaCO₃ precipitation. The solubility limits of the calcium sulfate are thus extended, enabling operation at increased concentration factors (i.e., higher fractional water recovery) and at increased evaporation top temperatures (i.e., lower specific energy consumptions).

Results of this ongoing research obtained with a single stage experimental MSF system show that removal of 80% of the calcium in seawater enables scale free operation at a concentration factor as high as four and top evaporation temperatures of 120-140°C. Preliminary economic evaluations indicate significant cost savings. It is hoped that the results of this project would advance the use of MSF systems for cost effective treatment of produced waters.