Electrochemical membrane separator

The theoretical minimum work for electrochemical membrane separation is precisely... 

Wimer, J.G. Williams, M.C. Archer, D.H. Osterle, J.F. Networking Electrochemical Membrane Separation Devices ... 

Alexander S.R. and Winnick J., Removal of hydrogen sulfide from natural gas through an electrochemical membrane separator, AIChE J. 40 613 (1994). 

The Electrochemical Membrane Separator (EMS) technology being developed is compared to a wet removal process with subsequent Claus Plant processing to elemental sulfur and SCOT Tail Gas treatment of flue gases. This wet process utilities aqueous Methyldiethanolamine (MDEA) as an absorbent in a scrubbing operation to bring the HjS level from 1.7% to 4 ppm. The flow rate to these processes is 50MM SCFD and the process pressure is 715 psi. The... 

The capital cost is more difiScult to estimate than the power consumption. In the MCFC, current densities greater than 160 mA/ca are routinely achieved. There are however, two major differences between the MCFC and the EMS (Electrochemical Membrane Separator). In the MCFC the gases are relatively rich, as compared with the dilute reactants treated in the EMS. Further, there is no competing reaction to dilute the current-carrying anion. Thus, gas-phase diffusion of H,S or sulfide migration in the membrane may limit the current density and define the needed active membrane area for a given duty. 

Proposed Electrochemical Membrane Separator Plant Layout. 

Removal of H S through an electrochemical membrane separator by S. Alexander 535... 

Electrochemical membrane processes. An electrochemical membrane process for gas separation can utilise the difference in electrochemical potentials set up across an appropriate membrane by the application of a potential gradient [61]. For example the purification of contaminated chlorine gas has been achieved using electrochemical membrane separation. The chlorine gas is cathodically reduced to chloride ion. The chloride ions are then transported across an aqueous HCl electrolyte. 

Burke A, Winnick J, Xia CR et al (2002) Removal of hydrogen sulfide from a fuel gas stream by electrochemical membrane separation. J Electrochem Soc 149 D160-D166... 

Robinson JS, Smith DS, Winnick J (1998) Electrochemical membrane separation of H2S from reducing gas streams. AlChE J 44 2168-2174. doi 10.1002/aic.690441006... 

Highly pure perchloric acid can also be produced by a patented electrochemical process ia which 22% by weight hypochlorous acid is oxidized to chloric acid ia a membrane-separated electrolyzer, and then additionally oxidized to perchloric acid (8,84). The desired electrochemical oxidation takes place ia two stages ... 

A newer technology for the manufacture of chromic acid uses ion-exchange (qv) membranes, similar to those used in the production of chlorine and caustic soda from brine (76) (see Alkali and cm ORiNE products Chemicals frombrine Mep rane technology). Sodium dichromate crystals obtained from the carbon dioxide option of Figure 2 are redissolved and sent to the anolyte compartment of the electrolytic ceU. Water is loaded into the catholyte compartment, and the ion-exchange membrane separates the catholyte from the anolyte (see Electrochemical processing). 

Although ED is more complex than other membrane separation processes, the characteristic performance of a cell is, in principle, possible to calculate from a knowledge of ED cell geometry and the electrochemical properties of the membranes and the electrolyte solution. 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

Fig. 5. Exploded view of an ion-exchange membrane electrochemical oxygen separator. Oxygen removal characteristics of the flow-through type oxygen removal system are shown. Air cathode area = 100 cm2, water temperature = 40 °C.

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new separators. Similarly, not much attention has been given to separators in publications reviewing batteries.A number of reviews on the on cell fabrication, their performance, and application in real life have appeared in recent years, but none have discussed separators in detail. Recently a few reviews have been published in both English and Japanese which discuss different types of separators for various batteries. A detailed review of lead-acid and lithium-ion (li-ion) battery separators was published by Boehnstedt and Spot-nitz, respectively, in the Handbook of Battery Materials. Earlier Kinoshita et al. had done a survey of different types of membranes/separators used in different electrochemical systems, including batteries."... 

The earliest concerted effort in the research and development of Nafion perfluorosulfonate ionomers was directed toward their use as a permselective membrane separator in electrochemical cells used in the large scale industrial production of NaOH, KOH, and CI2. In short, the membrane in this application, in addition to keeping CI2 and H2 gases separated, prevents the unfavorable back migration of hydrated OH ions from the catholyte (concentrated aqueous NaOH or KOH) chamber, while allowing for the transport of hydrated Na+ ions from the anolyte chamber in which is aqueous NaCl. 

In physics, an elastic two-dimensional plate is tenned a membrane (Latin membram = parchment) but in chemistry the tenn denotes a body, usually thin, which serves as a phase separating two other bulk phases. If this body is penneable to the same degree for all components of the adjacent phases and does not affect their mobility, then its only function is to prevent rapid mixing of the two phases. This is then tenned a diaphragm. A real membrane must exhibit a certain selectivity, based on different penneability for the components of the two phases, and is then tenned a semipenneable membrane. Membranes separating two electrolytes that are not penneable to the same degree for all ions are called electrochemical membranes. It is with these that we are concerned here. 

Consider an electrochemical membrane formed of poorly soluble salt JA, separating two solutions with different concentrations of anion A", Cj and Ci. 

A large number of techniques have been described in the literature, for example, dyestulf adsorption, oxidative and reductive treatments, electrochemical oxidation or reduction methods, electrochemical treatment with flocculation, membrane separation processes, and biological methods [37-55]. Each of these techniques offers special advantages, but they can also be understood as a source of coupled problems, for example, consumption of chemicals, increased COD, AOX, increased chemical load in the wastewater, and formation of sludge that has to be disposed. 

The experimentally determined dependence of the concentration of the various entities as a function of time is shown in Fig. 15.26. The catholyte and anolyte concentrate NaOH and HN03, respectively, using a membrane separator. A plan for the electrochemical treatment of low-level nuclear wastes is shown in Fig. 15.27. The considerable electricity costs of such processes could be compensated by the sale of HN03 and NaOH. The Ru might be commercially valuable for some purposes, but its use may be compromised by residual radioactivity. 

The electrochemical 2-chlorophenol and 2,6-dichlorophenol removal from aqueous solutions using porous carbon felt (Polcaro and Palmas 1997) or a fixed bed of carbon pellets (Polcaro et al. 2000) as three-dimensional electrodes was investigated by Polcaro s group. The group s experimental setup consisted of a two-compartment electrochemical cell separated by an anionic membrane where the carbon felt or pellets could be lodged and the solution was recirculated by peristaltic pumps. Both carbon-based anodes effectively removed the chlorophenols as well as their reaction... 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Thiele, W. and Foerster, H.-J. (2006) Progress in electrochemical ozone generation and disinfection of ultra-pure water using new electrochemical cell with polymer membrane separators (in German). Proceedings of the Annual GDCh Meeting, Bayreuth 2006. 

A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electricity. The cell consists of three main parts the fuel compartment, the oxidant compartment, and an electrolyte membrane separating the fuel and oxidant. At the fuel side, the fuel is oxidized and electrons are released. At the oxidant side, the oxidant is reduced by accepting the electrons released from the fuel side. The electrons flowing through the fuel side to the oxidizer side can be harnessed, producing electric power. For an H2/air fuel cell, the reactions are ... 

In one procedure, as mentioned above, Mn02 was employed as the oxidant to reoxidize the hydroquinone to benzoquinone. In another study it was demonstrated that the hydroquinone can be recycled electrochemically by anodic oxidation [61]. The reaction is performed in acetic acid with LiC104 as electrolyte with catalytic amounts of both Pd(OAc)2 and p-benzoquinone in a membrane-separated cell. 

As is clear solid oxide electrolytes are not useful for applications as oxygen separation membrane, unless operated with external circuitry (oxygen pump) or as a constituent phase of a dual-phase membrane. Both modes of operation, classified in this paper as electrochemical oxygen separation, are briefly discussed in Section 10.4.3. But we first start with a discussion of the models that have been developed to describe the oxygen semi-permeability of solid oxide... 

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