Abstract
The forward-osmosis (FO) processes have received much attention in past years as an energy saving desalination process. A typical FO process should inclu de a draw solute recovery step which contributes to the main operation costs of the process. Therefore, investigating the energy consumption is very important for the development and employment of the forward osmosis process. In this work, NH3-CO2, Na2SO4, propylene glycol mono-butyl ether, and dipropylamine were selected as draw solutes. The FO processes of different draw solute recovery approaches were simulated by Aspen PlusTM with a customized FO unit model. The electrolyte Non-Random Two-Liquid (Electrolyte-NRTL) and Universal Quasi Chemical (UNIQUAC) models were employed to calculate the thermodynamic properties of the feed and draw solutions. The simulation results indicated that the FO performance decreased under high feed concentration, while the energy consumption was improved at high draw solution concentration. The FO process using Na2SO4 showed the lowest energy consumption, followed by NH3-CO2, and dipropylamine. The propylene glycol mono-butyl ether process exhibited the highest energy consumption due to its low solubility in water. Finally, in order to compare the equivalent work of the FO processes, the thermal energy requirements were converted to electrical work.
Highlights
The shortage of freshwater has become a main challenge in the 21st century as a consequence of world population growth, increasing industrial consumption and pollution, development of agriculture, climate change, etc. [1,2]
The following equation was applied to estimate the theoretical minimum energy consumption for an FO process: πdraw = π f eed + πmin where πdraw represents the osmotic pressure of the draw solution side, π f eed is the osmotic pressure of feed side. πmin is the minimum osmotic pressure difference above which noticeable flow rate could be observed
ENRTL can be used to calculate the thermodynamic properties of organic solution, we found that the Universal Quasi Chemical (UNIQUAC) model based on group contribution theory was more convenient and provided better results for calculating the osmotic pressure of water in organic solutions
Summary
The shortage of freshwater has become a main challenge in the 21st century as a consequence of world population growth, increasing industrial consumption and pollution, development of agriculture, climate change, etc. [1,2]. The commercialized desalination technologies such as reverse osmosis (RO), multi-effect distillation (MED), and multi-stage flash (MSF) have been widely used, which still have the common problem of high energy consumption [3,4,5]. Osmosis (FO) has received much attention as a low energy cost desalination technology when compared with the above traditional processes [6,7,8]. In FO, the water molecules from the feed solution move to the draw side through a semipermeable membrane as a result of an osmotic pressure difference across the membrane. The recovery of draw solutes in an FO process contributes to the major the energy consumption of the FO process. The selection of draw solutes significantly affects the performance of an FO process
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