Optimal process design for forward osmosis desalination on the basis of draw solute searching method

Abstract

Optimal draw solute for forward osmosis desalination is found, and the desalination process is designed using the draw solute. At first, a systematic approach is proposed to find proper compounds for draw solute. The proposed approach has following three steps.

1)Property database is established. The database contains osmotic pressure, boiling point, solubility and toxicity information for a total of 4,058 compounds. Osmotic pressures are gathered from OLI stream analyzerTM and boiling point and solubility are calculated by elecNRTL and NRTL models. Toxicity information is gathered from literature. 2)Screening method is applied. Screening criteria include osmotic pressure, solubility, boiling point, and toxicity. Finally seven candidates are selected by the method. 3)Water production cost for each candidate is compared. Operation and capital cost model for the FO membrane process and draw solute recovery process are established, and the cost minimization problem is solved for various conditions.

After the systematic approach is applied, separation processes for ammonium bicarbonate, ammonium hydroxide, and ethanol are designed using Aspen Custom Modeler®. Design factors for the distillation column, heat exchanger, and absorber are determined, and flow configuration is optimized. CO2:NH3 ratio is added as a design factor since enrichment of ammonium helps to dissolve carbon dioxide. An optimal separation process is designed which achieves complete redissolving of decomposed gas and minimum water production cost.