Abstract
In this project, the concept of employing liquefied gas compounds as draw agent has been investigated among 137 gaseous compounds by determining their high solubility in water, the resulting osmotic pressure and their re-generation through thermal-depression methods. The screening process resulted in an organic liquefied gas draw solution suitable for Forward Osmosis desalination process. This is a polar, non-ideal with partially miscibility under 4 bars external pressure generates an osmotic pressure at maximum solubility (34% weight percentage) of 220 bars which is seven times more than seawater osmotic pressure. In addition, there is a significant reduction in solubility of the liqefied gas in water when the external pressure on draw solution is reduced from 4 bars to atmospheric pressure. This suggests that the liquefied gas draw agent could be separated from the solution by depression – thermal processes such as gas striping or atmospheric-vacuume flash methods. The performance of FO process using the novel liquefied gas draw solution was simulated using Excel software to achieve optimum operating conditions including operating temperature, cross flow rate and draw solution concentration. The results showed that the draw solution side should be kept under a pressure of maximum 10 bars. This depends on the operating temperature to dissolved the liquefied gas in water as much as possible. However, the operating pressure of the feed side could vary to cover a range 1 bar to 10 bar. Furthermore, the feasibility of the integrated Forward Osmosis process and depression - compression methods for seawater desalination was investigated in terms of estimating the specific energy consumption (SEC) using HYSYS 7.2 simulation software. The specific energy consumption (SEC) was predicted at optimum operating conditions resulting from FO process simulation based on the production of 1m3/h of potable water from seawater at a recovery rate of 50%. The electrical energy requirement of the process was calculated and the result of simulation was compared to the energy requirement of current desalination technologies. Energy saving of the novel FO desalination process is projected to range from 30% to 60%. The estimated SEC of the present FO desalination process was 2.7kWh/m3 and could be decreased to 0.5kWh/m3 when a heat recovery process is used. The results presented in this project demonstrate that the proposed novel forward osmosis desalination process with thermal-depression regeneration using the liquefied gas draw solution is a feasible and cost-effective desalination method. The novel draw agent produces high osmotic pressure and can be easily separated from the product clean water by using low-pressure steam with temperature input less than 150°C. While the feed water recovery in the FO process is higher than other desalination methods, the specific energy consumption of this novel FO desalination process is significantly low. The future works should focus on experimental tests to measure the osmotic pressure, permeated water flux, reverse draw agent flux and energy consumption in a bench scale or a pilot unit studies. A patent application, based on the present process, has recently been filled at the UK patent Office and the application number is GB1321711.2 (Adel Sharif and Maryam Aryafar, A novel Forward Osmosis Desalination process, GB1321711.2, 2013).