Thin-Gap Single-Pass High-Conversion Reactor for Organic Electrosynthesis I. Model Development

Design calculations are presented for a thin-gap, single-pass, high-conversion electrochemical cell suitable for process intensification in electro-organic synthesis. The interest of a microstructured design in electro-organic synthesis is highlighted, and it is shown that the specific productivity of the electrochemical cell is inversely proportional to the interelectrode gap. A dimensionless reactor model is developed for the case of an electro-organic synthesis characterized by three consecutive oxidation steps, coupled with possible mass-transfer limitations. For a given range of kinetic parameters, the reactor performance depends on only three independent dimensionless parameters: a Wagner-like number, a number of transfer units, and a dimensionless current. Model simulations are performed for the case of methoxylation of 4-methoxyanisole and provide identification of optimal operating conditions for process performance: a Wagner number higher than 0.5, a number of transfer units between 6 and 12, and a dimensionless current close to unity. In practice, the number of transfer units and the dimensionless current are easily adjusted, whereas it is much more difficult to adjust the Wagner number. For the thin-gap cell geometry, low Wagner numbers lead to uniform current distributions which favor undesired reactions at the reactor outlet, where the reagent concentrations are low. For industrial application, high-pressure operation can be advantageous. If the system is operated at a sufficiently high pressure (P > 10 bar), the drawback of hydrogen evolution at the counter electrode can be drastically reduced and the single-pass, high-conversion cell is feasible even at high levels of reagent concentrations.