(Invited) Current Density Distribution In a Pilot-Scale Electrodialysis Unit - 3D Mathematical Modeling and Experimental Investigation

Abstract

Electrodialysis (ED) represents a matured and well-established desalination and membrane separation technology. In order to satisfy application demands, there is a trend for increasing dimensions of not only the industrial ED, but also of the other electrochemical membrane units, like electrolyzers, fuel cells etc.), capable of high process intensity and efficiency. These activities are driven mainly by cost reduction. This is also the case of reverse electrodialysis as an allied process, currently meeting rapidly growing popularity as a promising green energy source. However, experiences in a development of larger scale membrane electrochemical devices document scale-up from laboratory or pilot system to industrial ones not to be straightforward. The scale-up based on an direct increase of smaller devises dimensions, like e.g. increasing membrane active area or number of membranes without appropriate modifications of entire unit design, often leads to pronounced nonuniformity of local current density, mass and heat distribution inside the system. Consequently, the performance, reliability and lifetime of industrial scale apparatuses is significantly exacerbated. A uniformity of the above-mentioned fluxes distribution strongly depends on numerous aspects, particularly on the geometry of electrodes, manifolds and fluid distributors. Due to these facts, there is an urgent need of fundamental understanding of the relations between the system geometry and uniformity of the fluxes, especially on an industrial scale.The presented work focuses at investigation on local current density distribution in the pilot-scale ED unit consisting of 200 vertically orientated membrane pairs with working area of 0.64 × 0.32 m2 per membrane. Planar-plate electrodes are horizontally segmented into the 6 identical and electrically insulated segments allowing determination of current distribution along main direction of the solutions flow. The current distribution is investigated at different operating conditions (salt concentration, applied current, solution flow rate, electrode compartment solutions properties etc.). In parallel, a macrohomogeneous (volume-averaged) 3D mathematical model is validated and employed to calculate and visualize the local fields of electric potential, current density and salt concentrations. A nonuniformity in the current distribution mainly due to the by-pass current through the manifold is observed. The qualitative relationships between the nonuniformity of current density distribution and process performance and reliability observed for pilot- or industrial-scale can be generalized to be valid also for other similar electromembrane systems with plate-and-frame design.