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Research Papers: Fundamental Issues and Canonical Flows

Characterization of Ion Transport and -Sorption in a Carbon Based Porous Electrode for Desalination Purposes

[+] Author and Article Information
Carlos Hidrovo

Assistant Professor
e-mail: hidrovo@mail.utexas.edu
Department of Mechanical Engineering,
The University of Texas at Austin,
Austin, TX 78712

1Corresponding author.

Manuscript received September 3, 2012; final manuscript received December 14, 2012; published online February 22, 2013. Assoc. Editor: Prashanta Dutta.

J. Fluids Eng 135(4), 041201 (Feb 22, 2013) (8 pages) Paper No: FE-12-1430; doi: 10.1115/1.4023294 History: Received September 03, 2012; Revised December 14, 2012

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although the focus has been placed on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses an electric field to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization and electrodialysis reversal where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow rates. Voltage and electrical current measurements are conducted during the desalination and electrode regeneration processes and used to evaluate the energy recovery ratio. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption efficiency. Two analytical species transport models are developed to estimate the net ionic adsorption rates in a steady-state and nonsaturated porous electrode scenario. Finally, the convective and electrokinetic transport times are compared and their effect on desalination performance is presented. Steady test results for outlet to inlet concentration ratio show a strong dependence on flow rate and concentration independence for dilute solutions. In addition, transient test results indicate that the net electrical energy requirement is dependent on the number of carbon electrode regeneration cycles, which is thought to be due to imperfect regeneration. The energy requirements and adsorption/desorption rate analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the reprocessing of lubricants or spent nuclear fuels in a near future.

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Figures

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Fig. 1

(a) Overview of desalination cycle. Voltage difference applied across the electrodes induce opposite electrode charges to attract and adsorb the ions. (b) Overview of regeneration cycle. Ion concentration on the electrode surfaces decrease together with the electrode charge, due to current flow across the resistor.

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Fig. 2

(a) Actual experimental setup, and (b) schematic of the experimental setup

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Fig. 3

Schematic of CDI cell

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Fig. 4

(a) Outlet conductivity as a function of time for a transient test, and (b) circuit current as a function of time for a transient test

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Fig. 5

Control volume analysis for salt adsorption

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Fig. 6

Ion movement in CDI cell. The paths 1, 2 and 3 correspond to the ionic paths when the convective time is larger, equal, and smaller than the electrokinetic time, respectively.

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Fig. 7

Energy recovery based on cycle number

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Fig. 8

Steady tests at varying solution concentrations at a constant flow rate of 0.5 cm3·min−1

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Fig. 9

Steady tests at varying flow rates at a constant solution concentration of 0.05 mg·cm−3

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Fig. 10

Characteristic conductivity and adsorption efficiency for steady tests

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Fig. 11

(a) Constant aerogel concentration model results for constant concentration steady tests, and (b) constant aerogel adsorption model results for constant concentration steady tests

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