טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentDor Elad
SubjectComputational Analysis of a Capacitive Deionization System
DepartmentDepartment of Chemical Engineering
Supervisor Professor Simon Brandon
Full Thesis textFull thesis text - English Version


Abstract

Capacitive Deionization (CDI) is a water purification technique based on electro adsorption. In this technique a solution of brackish water is placed between two electrodes which are connected to a power source. This leads to the creation of an electric field between them. The electric field causes the ions in the salt solution to migrate and adsorb on the electrodes which purified the solution. The next step is to replace the purified solution with an initial-concentration-solution. A short circuit between the electrodes is then induced, thereby nullifying the electric field. The adsorbed ions are thus released from the electrodes into the solution, yielding a relatively high-concentration salt solution. The last step is to replace the high concentration solution with an initial concentration solution thereby completing one purification cycle.

In this thesis we discuss a number of mathematical models for the mass transport in CDI process. Based on these models, simulators for two types of CDI systems were generated. In order to calibrate our models, we worked in collaboration with a research group from Bar Ilan University who experimentally tested the two types of CDI systems. The first one is based on a "Flow-By" operation mode (FBs) and the second one relies on a "Flow-Through" operation mode (FTs). In the FBs the solution flows parallel to the electrodes while in the FTs the solution flow is perpendicular to the electrodes (the solution goes through the porous electrodes).

As a starting point, the FBs simulator was based on the commercial COMSOL multi physics software which was used to simulate the desorption part of the CDI process; this simulation accounts for transient three-dimensional mass transfer in the system.

A significant discrepancy between results obtained from this calculation and experiments was revealed. After Taylor dispersion and dispersion due to pressure inlet effects were ruled out as reasons for this discrepancy, it was discovered that the difference in results is likely to be due to dispersion caused by non-trivial inaccuracies in the internal structure of the system.

The Flow-Through simulator is based on a transient one-dimensional model for mass transport, enabling detailed simulation via the finite differences method. The solutal model accounts for effects caused by diffusion, convection and migration. The results obtained from these simulations are consistent with those reported in experiments and serve to demonstrate the sensitivity of the system to operating parameters as well as suggest optimal operational conditions.