Theoretical Analysis and Economic Evaluation of Draw Solution Assisted Reverse Osmosis Process
Kiho Parka, Do Yeon Kimb and Dae Ryook Yanga,*
a Department of Chemical and Biological Engineering, Korea University, Seoul, Republic of Korea
b Department of Chemical Engineering, Imperial College London, London, United Kingdom
*Corresponding author: Tel. +82-2-3290-3298; Fax. +82-2-929-9613; E-mail: firstname.lastname@example.org
As human population has been increasing, water scarcity problem has been one of the most critical issues in the world. Since the portion of fresh water is just lower than 3% of all water in Earth, various efforts for utilizing seawater has been made in a worldwide. Seawater desalination technology could be classified into two groups, thermal based desalination and membrane based desalination. Among these technology, membrane based desalination has been favored due to its low energy consumption. 1 Reverse osmosis (RO) is a conventional and mature technology in membrane based desalination. However, due to its requirement of high pressure pump and high electric energy, numerous investigations for overcoming the problem of RO have been performed. 2 Forward osmosis (FO) is one of the notable candidates for future desalination technology. Even if there are so many researches for improving FO process, a breakthrough for substituting RO process has still not been found. 3
In RO process, high pressure more than 60 bar should be taken for obtaining water flux against seawater salinity gradient. 4 Whereas in FO process, there is no pressurizing equipment in feed stream because salinity of draw solution is much higher than seawater. It means that salinity gradient is a key parameter for determining how much hydraulic pressure has to be required. If salinity gradient could be reduced using draw solution concept, the required pressure also could be reduced.
In this present study, we suggest a novel seawater desalination concept called draw solution assisted reverse osmosis. It consists of two stages, seawater desalination stage and draw solution recycling stage. In seawater desalination stage, feed stream is pressurized and fresh water is permeated from feed stream to permeate stream. In permeate side, draw solution is located. The concentration of the draw solution is lower than the concentration of seawater for reducing the required pressure. In passing through the seawater desalination stage, the draw solution concentration is diluted by permeated fresh water. The permeate side stream of the seawater desalination stage becomes inlet stream of the draw solution recycling stage. In draw solution recycling stage, diluted draw solution is pressurized and fresh water could be produced in the permeate side. So the overall process consists of two reverse osmosis process in which the required pressure is lower than conventional one staged reverse osmosis.
Feasibility of the draw solution assisted reverse osmosis was investigated by theoretically and economically. The process modeling was performed with conservation equations and transport phenomena. Internal concentration polarization (ICP) and external concentration polarization (ECP) was also concerned for estimating effective osmotic pressure. 5,6 Based on the developed model, key parameters such as draw solution concentration, membrane module number, membrane characteristics and water recovery were screened and analyzed theoretically. And optimization problem was formulated and solved for obtaining most economic design of the process. The optimization problem is mixed integer and nonlinear programming (MINLP) due to membrane module arrangement. Specific water cost of the process is lower than $0.8/m3, so the process could be another option for low energy desalination process.
Keywords: modeling, reverse osmosis (RO), desalination, draw solution, optimization, economic evaluation
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