During the constant-current batch electrolysis of L-cystine hydrochloride to produce L-cysteine hydrochloride in 2 mol dm–3 HCl at 298 K, a simple mathematical model can describe reactant conversion. Initially, the reduction is under charge-transfer control; as electrolysis proceeds, the process shows mixed control, then eventually pure mass-transport control (due to convective-diffusion of the reactant). A parametric study of the major process variables (the current density and the catholyte flow rate) has suggested favored electrosynthesis conditions at a range of cathodes. Selection of the applied current density is a compromise between achieving higher rates of reduction as the current density is raised, balanced against the lower current efficiencies that result due to the increased rates of hydrogen evolution. High hydrogen overpotential cathodes such as mercury-plated copper and lead are favored. At a practical current density (2 kA m–2) titanium cathodes show comparable performance to mercury-plated copper and lead. The relative performance of titanium is much poorer at low current densities. Prolonged electrolysis highlighted problems with electrode stability at mercury-plated copper, lead, and titanium cathodes. In contrast, carbon is inert; the use of reticulated vitreous carbon and carbon felt provides a high surface area and moves mass-transport control to lower reactant concentrations and higher fractional conversions during the batch electrolysis. This produces much more favorable figures of merit. Under mass-transport controlled conditions, high catholyte flow rates are favored at all cathodes, as is the inclusion of a plastic mesh turbulence promoter in the catholyte channel
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