טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYu Qianli
SubjectModeling of reverse osmosis membrane
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Emeritus Raphael Semiat
Full Thesis textFull thesis text - English Version


Abstract

One of the most popular desalination techniques to provide water for various uses is the Reverse Osmosis (RO), in which seawater or brackish water is fed under pressure through a semipermeable membrane. Water molecules pass through the membrane pore channels and are accumulated as product on the other side. Operational parameters affecting the flow rate of permeate flow include velocity, pressure, concentration profile and permeability coefficient of RO membrane, among which the database for permeability coefficients is not well established. Therefore, there is a need to obtain full description of permeability profile along the membrane system. Permeability coefficients may change according to the working conditions, such as pressure and concentration. Examining membrane behavior by analytically or numerically modeling the important parameters or the membrane itself is a method widely implemented. Particularly, some models focus on single aspect of the reverse osmosis system, such as the effect of feed concentration and permeate concentration or permeate flow for different applied pressure. Some include everything for feed domain in flat sheet membrane with single or several inlet points. Yet, there is no such a model that integrates flow conditions in feed, membrane (including support layer), and permeate domains for spiral wound membrane system.

This thesis is aimed to create such a computerized 2D cross section model for RO membrane system in COMSOL Multiphysics to calculate the flow rate, pressure and concentration information for single reverse osmosis treatment unit. The established model contains 3 different domains: feed domain, membrane domain, and permeate domain. Navier-Stokes and Brinkman equation were used for calculating the flow in different domains. Convection and diffusion equation is applied to calculate the distribution of salt and concentration profiles. Concentration, velocity and pressure profile distribution are part of the result of this model, as well as the permeability coefficients for water and salt. In order to validate the developed model, the permeability coefficients calculated by the COMSOL model were compared with the result given by IMSDesign (a software published by the membrane manufactures: Hydranautics). The differences between these two methods are attributed to the inlet assumptions. Extremely high flow rate condition was also tested to check if the model is flexible enough for different flows.

The established model is able to present flow details and calculate the average permeability coefficient inside a reverse osmosis treatment unit, which changes according to operational conditions. Further work can include adding membrane microstructure to the model, combining several units in series to build a larger model and solving the problem of the discrepancy caused by some calculation assumptions. Experiments for smaller scale membrane are also recommended, so the permeability coefficients can be calculated more precisely for each different local concentration rather than average values for the whole unit.