Membrane process current efficiency

AGC has modified the carboxylic polymer and prepared one experimental membrane. This membrane is similar to F-8934 in the arrangement of the sub-structure, and it is almost the same as the F-8934 in both mechanical strength and ohmic resistance. Figure 19.13 illustrates the current efficiency trend of this experimental membrane in a laboratory test run at 8 kA irf2, compared with the F-8934 tested under the same conditions. The absolute value of the membrane s current efficiency is approximately 97.5%. No decline in current efficiency has been observed. AGC is now evaluating the stability and is optimising the carboxylic polymer feature and fabrication process for commercial production of this type of membrane. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

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. 

Commercially available membranes are usually reinforced with woven, synthetic fabrics to improve the mechanical properties. Several hundred thousand square meters of IX membranes are now produced aimuaHy, and the mechanical and electrochemical properties are varied by the manufacturers to suit the proposed appHcations. The electrochemical properties of most importance for ED are (/) the electrical resistance per unit area of membrane (2) the ion transport number, related to current efficiency (2) the electrical water transport, related to process efficiency and (4) the back-diffusion, also related to process efficiency. 

Major differences between ED and other processes are, first, the solute is transferred across the membrane against water in the other technologies discussed below, whereas only ionic species are removed by ED. As noted, two different membranes (anionic and cationic) are employed. Current consumption depends primarily on the TDS concentration. You should look at this very closely when comparing the operating cost benefits and tradeoffs of this technology to other options. Current efficiency can be calculated from the following formula ... 

In the brine system, sulphate ions are mixed with raw salt. These ions deposit on membranes in the electrolysis process and cause loss of current efficiency [3, 4]. 

As indicated in Fig. 17.2, the membrane process has long been characterised by substantial reductions in electric power consumption, through constant advances in membrane, electrolyser and electrode technologies. In the early years of its commercial establishment, some 25 years ago, it yielded a caustic soda concentration of 20% or lower, with less than 90% current efficiency. Today, the caustic soda concentration is 33%, the current efficiency is 97%, and the ohmic drop of the membrane has been lowered by approximately 1.0 V. During the same period, advances in electrolyser design have improved the uniformity of intracell electrolyte concentra-... 

The recovery of phenylalanine from an industrial process stream was carried out using a conventional ED stack (Grib et al., 2000). A preliminary soaking of such membranes in a bovine serum albumin solution for 2 hour allowed the reduction of phenylalanine loss to less than 5% and achievement of an average current efficiency of 98%. Such a process was also successful in removing 98% of Na2S04 and (NH4)2S04. 

This limiting current, /liin, is the maximum current that can be employed in an electrodialysis process. If the potential required to produce this current is exceeded, the extra current will be carried by other processes, first by transport of anions through the cationic membrane and, at higher potentials, by hydrogen and hydroxyl ions formed by dissociation of water. Both of these undesirable processes consume power without producing any separation. This decreases the current efficiency of the process, that is, the separation achieved per unit of power consumed. A more detailed discussion of the effect of the limiting current density on electrodialysis performance is given by Krol et al. [20],... 

The water temperature will also influence the electrocoagulation process. A1 anode dissolution was investigated in the water temperature range from 2 to 90°C. The A1 current efficiency increase rapidly when the water temperature increase from 2 to 30°C. The temperature increase will speed up the destructive reaction of oxide membrane and increase the current efficiency. However, when the temperature was over 60°C, the current efficiency began to decrease. In this case, the volume of colloid Al(OH)3 will decrease and pores produced on the A1 anode will be closed. The above factors will be responsible for the decreased current efficiency. 

However, and as for all the processes, EDBM presents limits [23]. The performances of a water splitter are controlled by the permselectivities of the individual component membranes and by diffusive transport. Figure 21.7 illustrates the various processes which contribute to the overall current efficiency. Zone 1 concerns the specific process which generates acid and base from salt and water. Competing with this are the undesirable processes, which reduce the current efficiency. Zone 2 represents the loss of permselectivity of each homopolar layers of the BPM. In the three-compartment configuration (Figure 21.5), this factor directly acts on the purity of the acid and base produced. Zone 3 represents the loss of permselectivity of the associated homopolar membranes. Zone 4 concerns the diffusional losses due to the concentration gradients, which significantly occur for poorly ionized small molecules (HF, NH3, SO2, etc.). 

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