Electrically assisted membrane process

In many cases, membrane processes seem to be the most energy efficient separation processes available for industry. However, the costs related to cleaning or replacement of membranes, the need for skilled personnel, and the associated turn down costs determine their economical feasibility. Table 40.1 lists the various electrically assisted membrane processes with the type of effluents that could be treated most efficiently by each process. For most of the electrically assisted membrane processes (1 ), it is expected that the treatment of effluents would be most feasible when ion or particle concentration is high. These are also the effluents where alternative purification processes require a significant part of the process cost to be spent upon membrane... 

List of Electrically Assisted Membrane Processes and the Type of Effluents That Will Be Treated Most Efficiently... 

Since the electrical resistance of the effiuent and parasitic currents are minimal at high level of impurities, specihc interest in electrically assisted membrane processes could increase due to more strict laws and legislation around effluents. The depletion of freshwater resources and the necessity to process brackish or seawater to produce potable water could promote the use of electrically assisted membrane processes in the future. Electrodialysis will have to compete with pressure-driven membrane processes such as reverse osmosis. The growing awareness of the unique cleaning ability of electrically ionized water (EIW) [47], a byproduct of electrodialysis, may be a factor to consider in the choice between ED and RO systems. NMR relaxation measurements were used to determine the water cluster size of electrically ionized water EIW. It is known that the water cluster size of EIW is signihcantly smaller than that of tap water. The smaller water cluster size is believed to enhance the penetration and extractive properties of EIW. Recently, EIW has been produced and used in several cleaning processes [47] in industry. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Membrane technology may become essential if zero-discharge mills become a requirement or legislation on water use becomes very restrictive. The type of membrane fractionation required varies according to the use that is to be made of the treated water. This issue is addressed in Chapter 35, which describes the apphcation of membrane processes in the pulp and paper industry for treatment of the effluent generated. Chapter 36 focuses on the apphcation of membrane bioreactors in wastewater treatment. Chapter 37 describes the apphcations of hollow fiber contactors in membrane-assisted solvent extraction for the recovery of metallic pollutants. The apphcations of membrane contactors in the treatment of gaseous waste streams are presented in Chapter 38. Chapter 39 deals with an important development in the strip dispersion technique for actinide recovery/metal separation. Chapter 40 focuses on electrically enhanced membrane separation and catalysis. Chapter 41 contains important case studies on the treatment of effluent in the leather industry. The case studies cover the work carried out at pilot plant level with membrane bioreactors and reverse osmosis. Development in nanofiltration and a case study on the recovery of impurity-free sodium thiocyanate in the acrylic industry are described in Chapter 42. 

The mass transfer in dense membranes takes place by diffusion in the free volume between the polymer chains of the membrane material. The external driving force for this process is a difference of the chemical potential A/r, of the permeating species i on either side of the membrane. This difference can be expressed as a concentration difference Ac, a partial pressure difference Ap or an electrical potential difference A . The transport mechanism involves three distinctive steps, (a) selective adsorption of the feed components to the membrane, the feed components are dissolved in the membrane material (b) diffusion of the dissolved species through the membrane, and (c) desorption of the permeating species at the permeate-side of the membrane assisted by an applied sweep gas or vacuum. 

Figure 2.11 Comparison of the primary energy usage for ethanol/water separation using traditionai distillation/adsorption process (Fig. 2.9) and hybrid membrane-assisted vapor stripping (MAVS Fig. 2.10) process. Minimum energy (from minimum work calculation) shown as reference. Assumptions 37% and 85% efficient conversion of primary energy to electrical energy and thermal energy, respectively, 0.02 wt% ethanol in stripping column bottoms, and 99.5 wt% ethanol product (0.5 wt% water).

Following the conclusion of the authors, the best process designs are obtained using Hj-selective membranes, in configuration counter-current Nj sweep gas that operates warm for bulk Hj recovery, combined with low-temperature COj-selective membranes that assist CO2 purification and liquefaction they have the potential to restrict the increase of levelized cost of the electricity (LCOE) caused by 90% CO2 capture to about 15%. Of course, it is also expected that larger cost reductions are also possible if higher permeance, and especially higher H2/CO2 selectivity membranes, are developed in the next future. 

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