Design of an ammonia-carbon dioxide forward osmosis desalination process

Abstract

A commercial scale continuous desalination process for water production from seawater using forward osmosis (FO) was developed. The FO desalination process consists of membrane unit which uses ammonia and carbon dioxide as draw solutes to produce the fresh water and stripping columns to recover the draw solute. The operating conditions of the membrane unit and the stripping columns were determined by simulation studies based on ElecNRTL and OLI model which represent the property models for electrolyte system. The membrane unit was designed to use a plate-and-frame module with water flux model which includes the concentration polarization phenomenon occurring at the membrane surface. The draw solution recovery process consists of energy integrated parallel stripping columns for the optimal use of the condensing heat from the top side product of the first column to the reboiler of the second column. To reduce brine discharges and increase the productivity of the desalination process, about 50% of the brine from the membrane unit is recycled and mixed with the fresh feed to the membrane unit in the FO desalination process. Furthermore, to reduce the number of membranes in the membrane module and the heat duty of the reboilers in the stripping columns, the feed stream was pre-heated using the waste heat from a power plant. A standard design of FO desalination process and two improved designs of FO desalination processes were proposed and compared in this research.