Membrane process cell voltage

The process operates at current densities of about 1 kA/m2 and unit cell voltage of 1.5 V. The specific energy consumption is about 2 kWh/kg NaOH. Under the influence of the electric gradient the H + and OH ions emerge on opposite faces of the membrane. Bipolar membrane electrodialysis is being developed by several companies, e.g. WSI Technologies Inc. [270] and Aquatech Systems [129,275,276], Typical product specification ranges for the ICI electrodialysis process is summarized in Table 19. 

To date, as indicated in Fig. 17.3, the effort to reduce cell voltage has been focused on the membrane and the electrodes. In current commercial operations at 4 kA m-2 with a 2 mm electrode gap, the voltage loss of the membrane, anode and cathode has been reduced to approximately 350 mV, 50 mV and 100 mV, respectively, and thus a total of approximately 500 mV for these cell components, or less than one-third the voltage loss of these components in the early years of the commercial membrane process. 

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

The cell voltage required depends on the product of current and voltage drop in the dilute streams, particularly toward the end of the process when the total ion concentration is very low. Hence, the cell must be designed to minimize this term, i.e., the gap between the membranes must be as small as possible. 

Asea Brown Boveri of Switzerland has commercialized the MEMBREL process for the electrolytic generation of ozone. The synthesis takes place in a cell whose anodic section is made of titanium and the cathodic section of stainless steel. A Nafion membrane (Du Pont) acts not only as the electrolyte for the system, but also as the separator. This membrane is sandwiched between the anode (lead dioxide) and the cathode (platinum). Electrolysis takes place at a total cell voltage of 3-5 V with a current density of 0.5 - 2.0 A/cml The corresponding cathodic reaction is hydrogen evolution. 

Accurate cost figures for processes early in development are impossible to project. However, it is possible to roughly estimate the power and capital requirements to assess viability. The power consumption is overwhelmingly due to tile cell current, which is near stoidtiometric. Cell voltage, as shown earlier, can be estimated with reasonable accuracy. Capital costs can be estimated by analogy with MCFC stacks, whose design these membrane cells will mimic. 

Brine and soft water of ultrapure quality is essential for smooth and efficient operation of membrane cell process since Ca2+ and Mg2+ ions can harm the performance of ion-exchange membranes in the following ways. The precipitated Ca(OH)2 and Mg(OH)2 offer increased electrical resistance across the membrane thereby increasing the cell voltage. Furthermore, the anolyte diffusion layer characteristics are affected which would alter the optimum current density. More seriously, the membrane performance is affected... 

The two Monsanto processes are compared in Table 6.1. First, it must be recognized that the undivided cell stack is very much cheaper than the complex plate-and-frame divided cell (in fact, the cost is less than 10%). This reduces the capital cost of the plant to such an extent that when both the capital and energy costs are taken into account, the optimum current density is lower for the undivided cell (although this obviously implies that more cells must be built). Secondly, it is clear from the table that the absence of a membrane, the reduction in the interelectrode gap and the increase in the electrolyte conductivity combine to have a dramatic effect on the cell voltage. The change from —11.65 V to —3.84 V reduced energy consumption by almost two-thirds. Moreover the continuous extraction of the product in the cell greatly simplifies the adiponitrile isolation procedures. 

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