Membrane cells/processes current efficiency

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... 

For a profitable electrochemical process some general factors for success might be Hsted as high product yield and selectivity current efficiency >50%, electrolysis energy <8 kWh/kg product electrode, and membrane ia divided cells, lifetime >1000 hours simple recycle of electrolyte having >10% concentration of product simple isolation of end product and the product should be a key material and/or the company should be comfortable with the electroorganic method. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

For the application of these membranes to the electrolytic production of chlorine-caustic, other performance characteristics in addition to membrane conductivity are of interest. The sodium ion transport number, in moles Na+ per Faraday of passed current, establishes the cathode current efficiency of the membrane cell. Also the water transport number, expressed as moles of water transported to the NaOH catholyte per Faraday, affects the concentration of caustic produced in the cell. Sodium ion and water transport numbers have been simultaneously determined for several Nafion membranes in concentrated NaCl and NaOH solution environments and elevated temperatures (30-32). Experiments were conducted at high membrane current densities (2-4 kA m 2) to duplicate industrial conditions. Results of some of these experiments are shown in Figure 8, in which sodium ion transport number is plotted vs NaOH catholyte concentration for 1100 EW, 1150 EW, and Nafion 295 membranes (30,31). For the first two membranes, tjja+ decreases with increasing NaOH concentration, as would be expected due to increasing electrolyte sorption into the polymer, it has been found that uptake of NaOH into these membranes does occur, but the relative amount of sorption remains relatively constant as solution concentration increases (23,33) Membrane water sorption decreases significantly over the same concentration range however, and so the ratio of sodium ion to water steadily increases. Mauritz and co-workers propose that a tunneling process of the form... 

The hydrogen ion is reduced to H2 with a rate of 69 pL h-1 cm-2. Kim et al also studied the nafion layer-enhanced photocatalytic conversion of CO2 [181]. The main role of nafion membrane is to enhance the proton activity. It also prevents the reoxidation of the CO2 reduction products. The current efficiency of solar fuel cells restricts this process from real-life implementation. 

The current efficiency of acid/base generation and the purity of the acid and base made with bipolar membranes drops off as concentrations increase, because Donnan exclusion diminishes with increasing solution concentrations. Further, the production rate is limited by the rate of diffusion of water into the bipolar membrane. Nevertheless, there are substantial advantages to the process. Since there are no gases evolved at the bipolar membranes, the energy associated with gas evolution is saved, and the power consumption is about half that of electrolytic cells. Compared to the electrodes used in conventional electrolytic cells, the bipolar membranes are inexpensive. Where dilute (e.g., 1 N) acids or bases are needed, bipolar membranes offer the prospect of low cost and minimum unwanted by-products. 

Thus, various chlorinated aliphatic and aromatic compounds were dechlorinated in a flow-through electrochemical cell with a graphite fibre cathode, a Nafion (cation-permeable) membrane and a Pt gauze anode. The concentration of pentachlorophenol decreased from 50 to about 1 mg per litre after 20 min of electrolysis at a current efficiency of about 1 %, and the product was phenol. Similar results were obtained with other chlorode-rivatives. The expected total costs of the process are of the order of 10 DM per 1 m of waste water, which is comparable with the cost of adsorption on active carbon [42]. 

A. General. Preferential transport of selected species is the primary characteristic property of membranes. In a chlor-alkali cell, for example, one equivalent of cation will pass across the cation-exchange membrane for each Faraday of electricity if the selectivity is perfect. In practice, some OH passes through the membrane in the opposite direction, resulting in current inefficiency. The membrane selectivity, therefore, directly determines the caustic current efficiency of the process. 

Figure 6.10 shows the anolyte balance. The sulfate flow into and out of the electrolyzers is an arbitrary but reasonable number chosen to suit a typical membrane supplier s specification. The balance ignores any flow of sulfate out of the anolyte. Sulfate in some form is known to penetrate the membrane (therefore the need to limit its concentration). The rate of penetration is so small, however, that the assumption of zero flux is the basis for one method of estimating current efficiency [3]. The chlorate flows are in the same category as the sulfate flows. There is a small difference between the flows out and in, representing the rate of chlorate production in the cells. This amount would be removed by purging or by deliberate destruction in the brine process. 

Analysis of the chlorine flows shows that the anode current efficiency is different from the cathode current efficiency. The current efficiency commonly used in discussing membrane cells is the caustic current efficiency, determined by the amount of hydroxide ion lost by leakage through the membranes into the anolyte. The hydrogen current efficiency, on the other hand, is nearly 100%. In other words, the electrode process is nearly quantitative, but the membranes allow some of the product of electrolysis to escape. 

In practice, the anode current efficiency of a membrane cell can be manipulated and may be either higher or lower than the cathode current efficiency. Leaving aside the question of recovery of some of the chlorine value after the anolyte leaves the cell, the manipulation usually takes the form of acid addition along with the feed brine. This combines with some of the hydroxide ion and reduces its ability to degrade the anode process. Discussion of the techniques, limitations, and practicality of acid addition is not part of this chapter. 

Early membrane cells operated at relatively low current efficiencies and were able to tolerate correspondingly higher concentrations of impurities. As membranes were improved and better results became possible, the requirements for brine purity became stricter. For a time, the addition of phosphate to the brine to sequester the hardness ions and prevent them from entering the membranes mitigated some of the effects of hardness. Finally, it became necessary to devise a process to increase the purity of the brine well beyond that obtained by chemical treatment, and ion exchange is now the standard technique. Several general reviews of the brine ion-exchange process itself are available [119-121]. 

In a balanced plant, all the chemically treated brine flows through ion exchange, not just that equivalent to the membrane-cell production. This adds a cost to the process, because in a segregated operation only that brine corresponding to the membrane-cell production would be so treated. At the same time, it improves the quality of the brine fed to the diaphragm cells. This in turn improves the current efficiency of those cells, but little information is available in the literature, and it is not possible to quantify the phenomenon here. 

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