Electrochemical membrane reactors

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Tatapudi, P. and Fenton, J. M. (1993) Synthesis of ozone in a proton exchange membrane electrochemical reactor. J. Electrochem. Soc. 140, 3527-3530. 

S. Stucki, H. Baumann, H. J. Christen and R. Kotz, Performance of a pressurized electrochemical ozone generator, J. Appl. Electrochem., 1987, 17, 773-778 P. Tatapudi and J.M. Fenton, Synthesis of ozone a proton exchange membrane electrochemical reactor, J. Electrochem. Soc., 1993, 140, 3527-3530. 

Tatapudi, P. and J. M. Fenton, "Synthesis of Hydrogen Peroxide in a Proton Exchange Membrane Electrochemical Reactor", submitted for publication in Journal of the Electrochemical Society, July 1992. 

Mazanec, T., Cable, T., Frye, J., et al. (1994). Solid multi-component membranes, electrochemical reactor components, electrochemical reactors and use of membranes, reactor components, and reactor for oxidation reactions, US Patent 5,306,411. 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

This type of electrochemical reactor is composed of two bodies by mechanical manufacturing [66, 67]. It contains a two-compartment cell with an anodic and cathodic chamber separated by a membrane as diaphragm. The anodic chamber is equipped with a carbon felt anode made of carbon fibers a platinum wire is inserted in the cathodic chamber (Figure 4.30). 

The coefficient s shonld be low for separators in electrochemical reactors. It has valnes between 1.1 and 1.6 for simple separators, but for porous diaphragms and swollen membranes it has valnes between 2 and 10. The total porosity shonld be at least 50%, and the separator s pore space shonld be impregnated completely and sufficiently rapidly with the liqnid electrolyte. 

Figure 8 (Top) Electrochemical flow cell for the oxidation of phenol and aniline (a) Pb anode feeder (b) packed bed of 1-mm lead pellets (c) stainless steel cathode plate (d) Nation membrane (e) stainless steel screen (f) Luggin capillary (g) glass beads (h) gasket (i) reactor inlet (j) reactor outlet. (Bottom) Schematic of apparatus (a) electrochemical reactor (b) peristaltic pump (c) water bath (d) heater (e) anolyte reservoir (t) gas sparging tube (g) C02 adsorbers. (From Ref. 39.)... 

There have been severe criticisms about the extended use of chlorine gas in industry, owing to concern primarily derived from its ability to form toxic chlorinated organic compounds. In order to avoid its co-production during the electrolytic production of sodium hydroxide, a process has been developed in which a sodium carbonate (soda ash) solution is used as the anolyte in an electrochemical reactor divided by an ion-exchange membrane. Hydrogen gas is produced at the cathode and sent to a gas diffusion anode. Assuming no by-products in the liquid phase and only one by-product in the gas phase ... 

Fig. 6. An electrochemical reactor used for aniline oxidation on a lead dioxide packed-bed anode (A) lead anode (B) 1 mm lead pellets (C) stainless steel cathode (D) Na-fion 427 membrane (E) stainless steel retaining screen (F) glass beads (G) gasket (H) inlet (I) outlet (J) cathode gas vent. (Adapted from (27]).

There are many applications in chemical engineering where diffusion of charged species is involved. Examples include ion exchange, metals extraction, electrochemical reactors, and membrane separations. There is an excellent textbook in this area (Newman, 1991). Here we will be content to show that the treatment of electrolyte diffusion follows naturally from the generalized treatment of diffusion given in Section 2.3. 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

Propene oxidation was carried out by using an electrochemical reactor constructed from a Sm doped ceria electrolyte coated with YSZ (YSZ 1 SDC) as a membrane. In a blank test where nitrogen gas alone was passed over the Au anode instead of the reaction gas at 450 C, it was confirmed that the oxygen pumping was well controlled by the applied current, i.e., the amount of oxygen gas evolved at the anode coincided well the value calculated from the electric current by using Faraday s law. 

While the combination of the apphed current and current efficiency in an electrochemical reactor is a measure of the overall rate of product output, it is the product of the current and cell voltage that will determine the reactor s electrical power consumption, as indicated by Equation (26.103). The overall voltage in an electrochemical reactor is composed of the following components (1) thermodynamic cell potential, (2) anode kinetic and mass transfer overpotentials, (3) anolyte IR drop, (4) diaphragm/membrane IR drop, (5) catholyte IR drop, and (6) cathode kinetic and mass transfer overpotentials. For more information on each of these terms, the reader should refer to Section 26.1. 

To minimize the cell voltage in an electrochemical reactor, the anode and cathode electrodes are placed as close as possible to minimize the resistance (IR) overpotential. Such voltage minimization is achieved in zero-gap and capillary cells [31, 32], by placing the electrodes directly adjacent to the membrane/diaphragm that separates the anode and cathode compartments. Figure... 

A counter electrode reaction is needed in the organic electrochemical reactor. The reactant and product(s) of this reaction must not interfere with the primary organic electrode reaction. Hence, a membrane separator is often used to divide the anode and cathode compartments of the reactor the membrane adds to the cost of the reactor and usually increases its operating voltage. 

---