Models of Hypochlorite production in electrochemical reactors with plate and porous anodes

Abstract

Pseudo two-dimensional finite element models were developed to predict the hypochlorite (chloric(I)) (HOCl + OCl À ) production by electrolysis of near-neutral aqueous sodium chloride solution, in reactors with (a) an anode and cathode in the form of plates, and (b) a lead dioxide-coated graphite felt anode and titanium plate cathode. The model was used to investigate the feasibility of using a porous anode to achieve high single pass conversions in oxidising chloride ions. For the model reactor with planar anode, the effects of diffusion, migration and convection on the mass transport of the reacting species were considered, whereas with the porous anode, a supporting electrolyte (Na 2 SO 4 ) was notionally present to eliminate the migrational contribution to reactant transport. For an electrolyte flow rate of 10 À6 m 3 s À1 (Re = 10 for plate electrodes, Re porous = 0.76 for porous anode), a cell voltage of 3.0 V and an inlet NaCl of 100 mol m À3 , the single-pass conversion of Cl À was predicted to increase from 0.45 for the reactor with a planar anode to 0.81 for the reactor with a porous anode. For the same operating conditions, the overall current efficiency was also predicted to increase from 0.71 to 0.77 by replacing the plate with the porous anode. Keywords Hypochlorite Á Chloric(I) Á Chloride Á Modelling Á Porous electrode Notation A Specific surface area of porous electrode (m 2 m À3 ) C i Concentration of species i (mol m À3 ) C i,in Inlet concentration of species i (mol m À3 ) D i Diffusion coefficient of species i (m 2 s À1 ) d Distance between the planar anode and cathode (m) d f Fibre diameter (m) d h Hydraulic diameter of felt fibre (m) E k Equilibrium electrode potential of reaction k versus reference electrode (SHE) (V) F Faraday constant (C mol À1 ) j k Current density of reaction k (A m À2 ) j L Limiting current density (A m À2 ) j 0 Exchange current density (A m À2 ) k Standard heterogeneous rate constant coefficient (m s À1 ) k m Mass transport rate coefficient (m s À1 ) K Equilibrium constant (mol À1 dm 3 ) N i Superficial flux of species i (mol m À2 s À1 ) n Number of electrons involved in a reaction (-) R Universal gas constant (J mol À1 K À1 ) Re Reynolds number for reactor with planar electrodes (vd/m) (-) Re porous Reynolds number for porous electrode (v eff d h /m) r k Rate of homogeneous reaction k (mol m 3Linear electrolyte velocity (m s À1 ) v eff Solution velocity in the empty cross-section area (m s À1 ) z i Number of charge on species i (-) iSubscript i refers to reacting species (-) kSubscript k refers to reactions (-)

Phenol removal from wastewaters by electrochemical oxidation using boron doped diamond (BDD) and Ti/Ti0.7Ru0.3O2 dsa® electrodes

“…When BDD is used, the • OH reacts directly with phenol molecules. The addition of Cl -when the BDD electrode is used leads to an increase of the COD depletion which can be attributed to the indirect oxidation mainly, due to chlorine formation in the very acid pH of the experiment (Cheng and Kelsall, 2007). It can also be seen that the operational variables flow velocity and current density have an influence on the velocity of COD depletion, especially when chloride is present in the medium.…”

A comparison of electrodeposited Ti/β-PbO2 and Ti-Pt/β-PbO2 anodes in the electrochemical degradation of the direct yellow 86 dye

“…As extensively discussed by Cheng and Kelsall, 22 when solutions containing Cl -ions are electrolyzed the generated chlorine gas is hydrolyzed and disproportionates, forming hypochlorous acid (HClO): For pH < 3.3, Cl 2 is the predominant species in solution, becoming the only species in solution for pH ≤ 1. For pH > 3, reaction 2 becomes irreversible, and HClO is the predominant species in the pH range 3.4 to 7.5.…”

“…6,14,18 When using active chlorine, it might be essential to have the solution pH under control in order to improve the pollutant removal efficiency, since pH dictates the predominant chlorine species in the electrolyzed solution. 22 Thus, different pH conditions were reported as the best ones for color and organic removals. 6,18,23 The possible formation of undesirable chloro-organic derivatives is a known drawback of electrooxidation processes in chloride-containing solutions, although very few papers have actually reported these compounds.…”

Electrochemical degradation of the reactive red 141 dye on a β-PbO2 anode assessed by the response surface methodology

“…9 Organics oxidation mediated by other electrogenerated oxidants, like Cl 2 , ClO -and HClO, is possible in the presence of chloride ions. 3 Concerning these electrogenerated oxidants, several works have been published on their reaction schemes, 3,10,11 speciation diagrams, 10 and parameters affecting their production [11][12][13][14] (e.g. anode material, temperature and pH), as well as their role on the dyestuff mineralization.…”