Transport mechanism performance

An alternative approach to solving stability problems with ILMs is presented by Bhave and Sirkar (114). Aqueous solutions are immobilized in the pore structure of hydophoblc, polypropylene hollow fibers by a solvent exchange procedure. Gas permeation studies are reported at pressures up to 733 kPa with the high pressure feed both on the shell and lumen sides of the laboratory scale hollow fiber permeator. No deformation of the hollow fibers is observed. Immobilizing a 30 weight % KjCO, solution in the hollow fibers greatly improved the separation factor, a(C02/Na). from 35.78 with pure water to 150.9 by a facilitated transport mechanism. Performance comparisons with commercial COj separation membranes are made. 

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

The ability of any experimental method to produce accurate and reproducible results and provide the sensitivity needed to discern differences between transport mechanisms depends on minimizing variability intrinsic to the method. However, formal error analysis is rarely undertaken, even for commonly used methods. Fawcett and Caton [45] performed an error analysis of the capillary method for determining diffusion coefficients more than 25 years after the method was introduced. The value of the analysis is that it reveals which factors contribute the greatest variability to the dependent variable of interest. In the case of transport studies, the dependent variable of primary interest is diffusant concentration, C(t), where... 

The enthusiasm for using Caco-2 cells and other epithelial cell cultures in studies of drug transport processes has been explained by the ease with which new information can be derived from these fairly simple in vitro models [7]. For instance, drug transport studies in Caco-2 cells grown on permeable supports are easy to perform under controlled conditions. This makes it possible to extract information about specific transport processes that would be difficult to obtain in more complex models such as those based on whole tissues from experimental animals. Much of our knowledge about active and passive transport mechanisms in epithelia has therefore been obtained from Caco-2 cells and other epithelial cell cultures [10-15]. This has been possible since Caco-2 cells are unusually well differentiated. In many respects they are therefore functionally similar to the human small intestinal enterocyte, despite the fact that they originate from a human colorectal carcinoma [16, 17]. 

Similar conclusions regarding how high PTFE content decreases the gas permeability and porosity of CFPs (TGP-H-060 and TGP-H-090) were presented by Park et al. [102]. It was also observed and concluded that the main transport mechanism of the water through the DL was shear force or evaporation, instead of capillary forces (main force in CLs). They also showed that a CFP (TGP-H-060) with 15 wt% PTFE had the best performance with relatively dry conditions compared to a thicker paper with the same hydro-phobic content. 

As stated earlier, CEP and CC are the most common materials used in the PEM and direct liquid fuel cell due fo fheir nature, it is critical to understand how their porosity, pore size distribution, and capillary flow (and pressures) affecf fhe cell s overall performance. In addition to these properties, pressure drop measurements between the inlet and outlet streams of fuel cells are widely used as an indication of the liquid and gas transport within different diffusion layers. In fhis section, we will discuss the main methods used to measure and determine these properties that play such an important role in the improvement of bofh gas and liquid transport mechanisms. 

Accumulation of water inside the DLs and CLs may cause serious failure modes that can significantly deteriorate the performance and lifetime of a fuel cell. To ensure appropriate water removal, it is vital to understand the water transport mechanism inside a fuel cell, especially in the DLs. Because CFP and CC contain complex structures and porosities, many researchers have developed methods that could facilitate the characterization and design of optimal diffusion layers with proper water removal capabilities. A lot of work has also been performed on mathematical models that attempt to analyze the water flooding and transport inside DLs. A comprehensive review describing these models can be found in Sinha, Mukherjee, and Wang [222]. This section will discuss only examples of the experimental techniques. 

Rica et al. [46] evaluated the various parameters affecting the performance of the transport mechanism. Snook and Dean [47] have further extended this research. Both groups consider that the volume of the bell jar is important in the cell design. 

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

Coupled transport was the first carrier facilitated process developed, originating in early biological experiments involving natural carriers contained in cell walls. As early as 1890, Pfeffer postulated that the transport in these membranes involved carriers. Perhaps the first coupled transport experiment was performed by Osterhout, who studied the transport of ammonia across algae cell walls [1], A biological explanation of the coupled transport mechanism in liquid membranes is shown in Figure 11.2 [2],... 

The discussion in Section 4.4.1.3 on transport mechanisms in SLM has manifestly demonstrated another facet of tuning analyte-selective extraction. For example, Figure 4.5 clearly demonstrates the selective extraction of a basic compound—all that is required here is a simple adjustment of the pH on either side of the membrane. Also, Figure 4.6 neatly illustrates the possibility of performing such selective extraction of anionic and cationic species in another transport mechanism that employs selective carriers. Thus, by fine-tuning the chemistry/composition of the sample, membrane liquid, and acceptor phases, analyte-selective extraction can be tailor-made. 

Pervaporation with ceramic membranes is less well understood in terms of transport mechanisms. Consequently, modeling of ceramic pervaporation is still less mature, although the performance of the process was reported to be good [89]. Nomura et al. [90] studied the transport mechanism of ethanol/water through silicalite membranes in... 

Problems to be solved are related to membrane stability (of polymeric membranes, but also the development of hydrophobic ceramic nanofiltration membranes and pervaporation membranes resistant to extreme conditions), to a lack of fundamental knowledge on transport mechanisms and models, and to the need for simulation tools to be able to predict the performance of solvent-resistant nanofiltration and pervaporation in a process environment. This will require an investment in basic and applied research, but will generate a breakthrough in important societal issues such as energy consumption, global warming and the development of a sustainable chemical industry. 

Fundamental aspects of chemical membrane reactors (MRs) were introduced and discussed focusing on the peculiarity of MRs. Removal by membrane permeation is the novel term in the mass balance of these reactors. The permeation through the membrane is responsible for the improved performance of an MR in fact, higher (net) reaction rates, residence times, and hence improved conversions and selectivity versus the desired product are realized in these advanced systems. The permeation depends on the membranes and the related separation mechanism thus, some transport mechanisms were recalled in their principal aspects and no deep analysis of these mechanisms was proposed. 

The performance of the membrane used was investigated in detail [33]. However, as it acts in this study simply as a distributor, and controlled convection is the essential transport mechanism, no specific knowledge is required to predict fluxes. A membrane suitable for the purpose described does not need to possess a special permselectivity. 

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