Abstract
The purpose of this work was to develop a novel semi-empirical method for calculation of limiting currents in electrodialysis modules. Experiments were carried out at pilot-scale module with five electrolytes. Current-voltage curves were measured and limiting currents were determined. Subsequently, dependence of the mass transfer coefficient on linear flow velocity was evaluated and its parameters a, b were estimated. The main advance of this work is that comparison of electrolytes showed the clear relationship between parameter a, electrolyte diffusion coefficient and the difference of cation transport numbers in membrane and solution. The semi-empirical relationship between the mass transfer coefficient and linear flow velocity was set up, and the limiting current calculation run was proposed. At the same time, dependence of the diffusion layer thickness on linear flow velocity was examined and the way of its calculation was established. The proposed method is unique thanks to its applicability in a broad range of experimental conditions; its use is not limited neither by electrolyte type nor electrodialysis module size.
Highlights
1.1 Electrodialysis Electrodialysis is an electrically-driven membrane separation process using ion exchange membranes to separate ions from the solution
When the electric potential difference is applied, cations moving towards cathode can pass only through cation exchange membranes (CM) and anions moving towards anode can pass only through anion exchange membranes (AM), assuming absolute selectivity for counter-ions
The current-voltage curves were measured in a broad range of experimental electrodialysis conditions in order to determine the limiting currents using pilot-scale ED module
Summary
1.1 Electrodialysis Electrodialysis is an electrically-driven membrane separation process using ion exchange membranes to separate ions from the solution. The membrane stack consists of alternating cation and anion exchange membranes separated by spacers to form individual cells. When the electric potential difference is applied, cations moving towards cathode can pass only through cation exchange membranes (CM) and anions moving towards anode can pass only through anion exchange membranes (AM), assuming absolute selectivity for counter-ions. That means the alternating arrangement of membranes enables the ion accumulation in the space defined by CM on the anode side and AM on the cathode side of the stack (concentrate cell). The electrodialysis performance can be affected by several process variables: membrane type, number of cell pairs, solution path length in the stack, applied voltage, flow rate and concentration of feed solution, temperature etc. The electrodialysis performance can be affected by several process variables: membrane type, number of cell pairs, solution path length in the stack, applied voltage, flow rate and concentration of feed solution, temperature etc. [8, 9]
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