Coupled membrane processes

The idea of coupling membrane separation with bioreaction is not a new one. A lot of works published in the literature bear witness to this fact. But most of 

Supercritical fluid extraction is used to recover small organic solutes with molecular weight below 1500 daltons. In a state of continuity between vapour and liquid, supercritical fluids exhibit intermediate transport properties with lower viscosities than liquid and higher diffusivities than gcises. Because of its 

12 — TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 

Cross-flow filtration with mobile turbulence promoters 

It is well known that pumping of the fluid has a major effect on flux in the mass tTcinsfer controlled region for UF/MF process. Indeed agitation and mixing of the fluid near the membrane surface sweep away the accumulated solutes, thus reducing the thickness of boundary layers. This is the simplest and most effective method of controlling the effect of concentration polarization. 

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In the production of bioethanol, sugar is fermented, yielding low concentrated alcohol solutions that are recovered from the broth by ultrafiltration or pervaporation membranes. Ultrafiltration and pervaporation of bioethanol from fermentation broth are IG biofuel processes. As in all bioreactor-coupled membrane processes, membrane fouling and drop in permeate fluxes during continuous operation are the main concerns. 

G-Protein Coupling. The heterotrimeric guanosine triphosphate (GTP) binding proteins, known as G-proteins, are a principal family of proteins serving to couple membrane receptors of the G-protein family to ionic and biochemical processes. This topic is reviewed in References 63—67. 

Enzymatic Reactions in RMs Coupled with Membrane Processes. . 164... 

G-Protein Coupling. The heterotrimenc guanosine triphosphate (GTP) binding proteins, known as G-proteins, are a principal family of proteins serving to couple membrane receptors of the G-protein family to ionic and biochemical processes. The G-proteins are heterotrimers made of three families of subunits, o,/3. and y, which can interact specifically with discrete regions on G-protein-coupied receptors. This includes most receptors for neurotransmitters and polypeptide hormones (see Neuroregulators), G-protein-eoupled receptors also embrace the odorant receptor family and the rhodopsin-linked visual cascade. 

Carrier facilitated transport processes often achieve spectacular separations between closely related species because of the selectivity of the carriers. However, no coupled transport process has advanced to the commercial stage despite a steady stream of papers in the academic literature. The instability of the membranes is a major technical hurdle, but another issue has been the marginal improvements in economics offered by coupled transport processes over conventional technology such as solvent extraction or ion exchange. Major breakthroughs in performance are required to make coupled transport technology commercially competitive. 

An alternative approach is to make the simplification that the rate of chemical reaction is fast compared to the rate of diffusion that is, the membrane diffusion is rate controlling. This approximation is a good one for most coupled transport processes and can be easily verified by showing that flux is inversely proportional to membrane thickness. If interfacial reaction rates were rate controlling, the flux would be constant and independent of membrane thickness. Making the assumption that chemical equilibrium is reached at the membrane interfaces allows the coupled transport process to be modeled easily [9], The process is... 

A form of liquid membrane that received a great deal of attention in the 1970s and 1980s was the bubble or emulsion membrane, first developed by Li at Exxon [11-13], Figure 11.14 is a schematic illustration of an emulsion liquid membrane process, which comprises four main operations. First, fresh product solution is emulsified in the liquid organic membrane phase. This water/oil emulsion then enters a large mixer vessel, where it is again emulsified to form a water/oil/water emulsion. Metal ions in the feed solution permeate by coupled... 

Coupling a membrane process to this technology it is possible to obtain the separation of the clarified solution or the reaction product and also the recovery and the reuse of the catalyst. In fact, the choice of an appropriate membrane allows a selective separation of the product to be performed and to maintain the catalyst in the reaction environment in a continuous process that increases the efficiency of the whole system. 

A very promising method to solve this problem is coupling the photocatalysis with membrane techniques, obtaining a very powerful process with great innovation in water treatment. In fact, membrane processes, thanks to the selective property of the membranes, have been shown to be competitive with the other separation technologies for what concerns material recovery, energy costs, reduction of the environmental impact and selective or total removal of the components [77]. 

One of the main objectives in the use of a membrane process coupled to a photocatalytic reaction is the possibility of recovering and reusing the catalyst. Moreover, when the process is used for the degradation of organic pollutants, the membrane must be able to reject the compounds and their intermediate products, while if the photocatalysis is applied to a synthesis, often the membrane have to separate the product(s) from the environment reaction. Therefore, in a PMR the choice of a suitable membrane is essential to obtain an efficient system. 

In membrane reactors, the reaction and separation processes take place simultaneously. This coupling of processes can result in the conversion enhancement of the thermodynamically-limited reactions because one or more of the product species is/are continuously removed. The performance of such reactors depends strongly on the membrane selectivity as well as on the general operahng conditions which influence the membrane permeability. 

Heterodisulfide (CoM-S-S-HTP) reduction - coupled to primary translocation. The reaction in which ATP is synthesized during methanol reduction to CH4 could be identified with energetically competent membrane vesicles of the methanogenic strain G61. These vesicles, which are orientated more than 90% inside-out, catalyzed CH4 formation from CH3-S-C0M by reduction with H2 (Reactions 7,8) and coupled this process with the synthesis of ATP [112]. CH3-S-C0M reduction generated a ApH (inside acidic) as monitored by acridine dye quenching protonophores and ATP synthase inhibitors exerted their effects in accordance with a chemiosmotic type of ATP synthesis [113],... 

Membrane processes are versatile and flexible they can be combined with other methods in hybrid processes. Adapted to actual needs they can treat various process streams of different compositions and concentrations. Membrane distillation coupled with evaporation or reverse osmosis may improve the purification efficiency and increase decontamination factors. The flow chart of such hybrid processes is presented in Figure 30.21. In Figure 30.21a, the combination of MD unit with evaporator is shown. 

Application of Donnan Membrane Process for Recovery of Coagulants from Water Treatment Residuals which describes the coupled interdiffusion of A and B in terms of one diffusion coefficient ... 

Prakash, P. and SenGupta, A.K. Coupled transport of trivalent-monovalent ion pair (Al and H ) during Donnan membrane process for coagulant recovery. AIChE J., 51(1), 333-344, 2005. 

The Pe number is an important parameter which has an influence on the performance of the membrane process. Permeation and surface area cire coupled via the Pe number. In the equation Qh2 is the permeation of the fastest permeating component (usually H2 in this study). In membrane gas separation processes Pe is usually between 0.1 and 1.0. For new applications Pe = 0.5 can be taken as a first guess. The actual performance of the systems depends on many more parameters than the Pe number only, i.e. membrane selectivity, pressure drop, sweep gas flow to feed gas flow ratio, composition of the feed. 

The alteration of fermentation conditions, such as pH, drastically affects product concentrations. Research with C. ljungdahlii has shown that at high pH values (5.5-6), acetate was the dominant product, while at a lower pH (4-4.5), there was a drastic shift towards the production of ethanol. " Inhibition by end products or intermediates is the principal factor that limits metabolic rates and final product concentrations in many fermentation processes. Product inhibition can greatly affect the economics of commercialization. With regards to ethanol inhibition, growth of B. methylotrophicum was inhibited at alcohol concentrations of 5g/L. " However, a recently isolated clostridial strain was shown to tolerate ethanol concentrations up to 78g/L. Efforts have been made to eliminate the drawbacks of inhibition by improvement of bacterial strains to tolerate higher product concentrations and/or by use of novel separation coupled fermentation processes such as pervaporation, extraction, and membrane separation. 

Brown PR, Hallman JL, Whaley LW, Desai DH, Pugia MJ, Bartsch RA, Competitive, proton-coupled, alkah metal cation transport across polymer-supported hquid membranes containing sym-(decyl)-dibenzo-16-crown-5-oxyacetic acid Variation of the alkyl 2-mtrophenyl ether membrane solvent. J. Membr. Sci. 1991 56, 195-206. Michaels AS, Membranes, membrane processes, and their apphcations Needs, unsolved problems, and challenges of the 1990s. Desahnation, 1990 77, 5-34. 

Apart from ammonia, some other inorganic species extracted by hquid membranes are strong acids like nitric acid and thiocyanate ions from aqueous solutions using carrier-mediated coupled transport process. 

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

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