Monopolar membranes

Fig. 24. De Nora Technologies DD-Type monopolar membrane electroly2er.

MGC Monopolar Membrane Electrolycyer, OxyTech Systems, Inc., Chardon, Ohio, 1988. 

For the membrane cellroom of the same capacity there are two choices of technology type either monopolar or bipolar electrolysers. In the case of monopolar membrane electrolysers (Fig. 15.9), such as the ICI FM1500, one membrane electrolyser can replace one diaphragm cell. Since the membrane electrolyser has smaller dimensions there is an overall space saving. The monopolar membrane electrolysers may use the same pipework galleries and overhead crane from the... 

Fig. 15.12 Utilisation of rectifier capacity in monopolar membrane electrolysers.

Staged conversions may also be carried out whereby a number of diaphragm cells is replaced with the equivalent number of membrane electrolysers. For example, four diaphragm cells are removed, to be replaced by five monopolar membrane electrolysers. A special switch would be required of, say, 20 V capacity to enable the work to be carried out with minimal loss of production. A special manifold header enables the pipe connections of the five membrane electrolysers to be fed into the original four diaphragm cell flanges on the headers. 

Replacement of diaphragm cells with monopolar membrane cells is straightforward in that it can be a one-for-one replacement, ending up with the same electrical circuit but accomplished in a shorter new length (see Figs 15.15 and 15.16). 

Different versions of the above calculations, carried out for particular ionic contexts, form the basis of numerous studies of the ion-selective membrane transport, starting with classical papers by Teorell [7], and Meyer and Sievers [8]. Without attempting to give a full or merely fair account of all these studies, we shall mention here just a typical few Schlogl [5] (arbitrary number of ions in a monopolar membrane), Spiegler [9] (am-bipolar ionic transport in a unipolar membrane and solution layers adjacent to it — concentration polarization), Oren and Litan [10], Brady and Turner [11], Rubinstein [12] (multipolar transport in a unipolar membrane and the adjacent solution layers—effects of concentration polarization upon... 

With these efficient water splitting membranes and taking into account the resistances contributed by the electrolyte solutions and the monopolar membranes, AQUATECH Systems is considerably more power efficient than some conventional technologies. For example, the chlor-alkali industry would typically require some 2500 kwhr/MT NaOH produced. AQUATECH Systems can generate 1 MT NaOH aqueous plus an equivalent amount of the associated acid for just 1650-1800 kwhr. 

A two-stage ED process was also proposed to recover succinic acid [HOOC (CH2)2COOH] from sugar- and triptophane-based fermentation media (Glassner and Datta, 1992). The broth was previously concentrated via ED using monopolar membranes and then separated into sodium hydroxide-and free succinic acid-rich streams using bipolar membranes. Further removal of sodium cations and sulfate anions was achieved using weakly acid and -basic IER. 

The mobile cations are referred to as counterions and the mobile anions that carry the same electrical charge as the polymer membrane that are more or less completely excluded from the membrane are referred to as co ions. Due to the exclusion of the co ions, a cation-exchange membrane is more or less impermeable to anions. Anion-exchange membranes carry positive fixed charges and exclude cations. Thus, they are more or less impermeable to cations. To what extent the co ions are excluded from an ion-exchange membrane depends on membranes as well as on solution properties. Bipolar membranes enhance the dissociation of water molecules into H + and OH ions and are used in combination with monopolar membranes for the production of acids and bases from the corresponding salts [5],... 

In addition to the monopolar membrane described above a large number of special property membranes are used in various applications such as low-fouling anion-exchange membranes used in certain wastewater treatment applications or composite membranes with a thin layer of weakly dissociated carboxylic acid groups on the surface used in the chlorine-alkaline production, and bipolar membranes composed of a laminate of an anion- and a cation-exchange layer used in the production of protons and hydroxide ions to convert a salt in the corresponding acids and bases. The preparation techniques are described in detail in numerous publications [13-15]. 

Electrodialysis is a well-proven technology with a multitude of systems operating worldwide. In Europe and Japan, electrodialysis dominates as a desalting process with total plant capacity exceeding that of reverse osmosis and distillation [3]. Electrodialysis with monopolar membranes is applied to different food systems, to demineralization of whey [5-8], organic acids [9], and sugar [10,11], separation of amino acids [12] and blood treatments [13], wine stabilization [14—16], fruit juice deacidification [17-19], and separation of proteins [20-22]. These applications use the sole property of dilution-concentration of monopolar lEMs in a stack of as many as 300 in an electrodialysis cell. 

Electrodialysis with bipolar membranes (EDBMs) [23] was applied recently to the production of mineral and organic acids [24-36], inhibition of enzymatic browning [37-39], and separation of protein [40-44]. These applications are based on water dissociation at the interface of a bipolar membrane (BPM) coupled with monopolar membrane action. A recent up-to-date overview gives the possibilities and the economic relevance of BPM technology [45]. 

In the domain of food industries, EED was used to reduce oxygen in fruit juice [46], to extract cytoplasmic proteins from alfalfa [47,48], to coagulate proteins [49], and to reduce disulfide bonds in proteins [50]. These applications are based on the electrode redox reactions coupled with monopolar membrane action. 

Alternatively, the bipolar membrane ED can be operated using a two-compartment configuration, where either the anionic (Figure 21.35a) or the cationic (Figure 21.35b) monopolar membranes are omitted. In this mode of operation, only cations or anions are removed from the feed compartment and replaced with either protons or hydroxide ions. A brine compartment is... 

The use of ED with a monopolar membrane for protein separation and acid caseinate production and in bioreactors for organic acid production is a well-proven technology with huge operating systems worldwide. Such ED is applied to different food systems, which include (Bazinet, 2004)... 

These applications are based on water dissociation at the interface of a bipolar membrane and are coupled with the action of the monopolar membrane action. Deacidification and acid production, however, entail conventional ED. In the recovery of organic acids from fermentation broths the elimination of cations has often been a major problem, as fermentation typically performs better in pHs significantly above the pfC, of the acid produced. Bipolar membranes offer a solution to the elimination... 

The Hooker-Uhde Monopolar Membrane electrolyzer is shown in Figure 19. The active electrode surface of the HUMM electrolyzer is 1.7 m per cell element. The cell elements consist of anode frames of titanium and cathode frames of steel. Anode-cathode gap is approximately 3 mm. A separate frame is provided for holding the membrane. The electrode block rests on a support structure that serves as the header system for electrolyte and electrolysis products. Peformance of this electrolyzer is reported to be 2750 KWH/M ton NaOH at 35% caustic soda and 95% current efficiency at 3 KA/M current density (65). 

FIGURE 22.34 Polarization curves for a monopolar membrane and for a bipolar membrane of hydrous ferric oxide in sodium chloride solution [69,72],... 

Brochure entitled MGC—Monopolar Membrane Electrolyzer—Data sheet, ECL-MGC-1 by Eltech Systems Corporation, 1983. 

S.5.3.2. Chlorine Engineers Electrolyzers. Chlorine Engineers Corporation (CEC), a subsidiary of Mitsui and Company, produces the filter-press type, monopolar membrane electrolyzer shown in Fig. 5.26 [94]. Uniform electrical current travels into each anode element through titanium-clad, copper-cored conductor rods and current distributors. The current distributor also serves as a downcomer, which promotes circulation... 

Shutdown of Individual Electrolyzers for Maintenance. In larger monopolar membrane cell rooms, individual electrolyzers may be removed by using an on-load shorting switch to bypass the current around the electrolyzer to be removed. In some small monopolar cell rooms, the entire circuit is shut down, an electrolyzer removed, and a replacement electrolyzer fitted. This practice is justified by the small loss that is sustained in production and the floor area and capital expense that are saved. Shutdowns for cell removal are infrequent, and the change can be made in about 4 hr. 

S. Ogawa, paper presented at Indian Inst. Chem. Eng., New Delhi, March 1980. Brochure entitled MGC— Monopolar Membrane Electrolyzer— Data sheet, ECL-MGC-1 by Eltech Systems Corporation, 1983. 

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