Electrolytic cell flow circulation

The electrodes were arranged as a stack of bipolar electrodes as shown in Figure 23. Aluminum formed at the lower electrodes in each individual cell flows concurrently together with the chlorine gas to the central vertical shaft, where the metal sinks to the bottom and the gas rises to the top. The gas movement promotes the necessary circulation of the electrolyte. Due to the compact bipolar arrangement and the short interpolar distance (10-20 mm) the electrical energy consumption was as low as 9.5 kWh/kg Al. 

The electrolysis of sucrose was performed in a filter press cell (micro-flow cell, electro-cell AB) (Figure 21.19). The working electrode was platinum deposited electrochemically on a titanium plate. The counter electrode was a plate of stainless steel. The two compartments of the cell were separated by an ion-exchange membrane (Nation 423). A part of this membrane immersed in a saturated potassium sulfate permitted to connect by capillarity the MSE. The electrolyte in the cell was circulated by an external peristaltic pump (1 cm3 min-1) and passed through a reservoir (100 cm3). 

The electrowinning feed electrolyte is pumped from the electrolyte circulation tank to the electrolytic cells. The electrolyte temperature is maintained at 45-50 °C. The change of the copper concentration in the electrolyte is about 2-3 g/1. The electrolyte from the cells flows by gravity back to the circulation tank. 

The interaction of forced and natural convective flow between cathodes and anodes may produce unusual circulation patterns whose description via deterministic flow equations may prove to be rather unwieldy, if possible at all. The Markovian approach would approximate the true flow pattern by subdividing the flow volume into several zones, and characterize flow in terms of transition probabilities from one zone to others. Under steady operating conditions, they are independent of stage n, and the evolution pattern is determined by the initial probability distribution. In a similar fashion, the travel of solid pieces of impurity in the cell can be monitored, provided that the size, shape and density of the solids allow the pieces to be swept freely by electrolyte flow. 

Figure 6.66 gives an example for a lead-acid secondary cell with electrolyte circulation. Compressed air enters the cell at the top, flows downstream to the bottom via a duct and, after leaving this duct, drags the electrolyte upstream from the bottom. On the opposite side of the cell, the electrolyte flows downstream. 

Circulation of the electrolyte through the parallel plate cell was provided by a magnetic pump (Iwaki MD 50 R) and the electrolyte flow rate was measured with a magneto hydrodynamic flow meter (Deltaflux). 

The simple batch reactor was considered in Chapter 4, A modified version in which the electrolyte is recirculated, however, is the preferred mode of operation in the electrochemical industry because it provides flexible batch volume and also enhances the mass transfer characteristics of the cell due to circulation. Further, with recirculation, the reactor can be operated either in the plug-flow or mixed-flow mode. We consider all three cases here along with a few other common modes of operation. 

Inner water circulation is much fancier, but also harder to realize. Under ordinary operating conditions, both in direct methanol fuel cells and in polymer electrolyte membrane fuel cells, a flow of water from the anode toward the cathode is observed. This flow has two origins water molecules are dragged along by hydrated hydrogen ions moving in the electric field from the anode to the cathode, and water diffuses under the influence of its own concentration gradient. 

---