Reactant permeability

Figure 11.29 Conversion and separation index of a membrane reactor as a function of permeation to reaction rate ratio when reactant permeability is smaller than product permeabilities [Mohand and Govind, 1988a]...

The amount of polyelectrolyte binder used in CLs is not as large as that in membranes [18] however, the amount is important because it is closely related to CL performance, catalyst utilization, and MEA durability [23, 24]. In current PEMFCs, PFSA ionomers are employed in the CL as binders and in the proton conducting electrolyte to extend the formation of the electrochemical three-phase interface [3]. The latter is important for obtaining desirable catalyst utilization and, thus, high performance of MEA. Since the reactant must be transported through the proton conducting electrolyte before it arrives at the reaction sites to carry out reactions, the binder in the CL must be reactant-permeable to avoid reactant mass transport limitations [25]. The reactant-permeable property of the binder is... 

Reactant permeability is an important quantity in the context of durability, since interdiffusing and in the PEFC will lead to the formation of aggressive radical species in the catalyst layers. In styrene grafted ETFE (50 jm) based membranes,... 

Membrane reactors are known on the macro scale for combining reaction and separation, with additional profits for the whole process as compared with the same separate functions. Microstructured reactors with permeable membranes are used in the same way, e.g. to increase conversion above the equilibrium limit of sole reaction [8, 10, 11, 83]. One way to achieve this is by preparing thin membranes over the pores of a mesh, e.g. by thin-fihn deposition techniques, separating reactant and product streams [11]. 

Both processes - referring to the non-substituted and substituted methanol reactant- utilize elemental silver catalyst by means of oxidative dehydrogenation. Production is carried out in a pan-like reactor with a 2 cm thick catalyst layer placed on a gas-permeable plate. A selectivity of 95% is obtained at nearly complete conversion. This performance is achieved independent of the size of the reactor, so both at laboratory and production scale, with diameters of 5 cm and 7 m respectively. 

The predominant RO membranes used in water applications include cellulose polymers, thin film oomposites (TFCs) consisting of aromatic polyamides, and crosslinked polyetherurea. Cellulosic membranes are formed by immersion casting of 30 to 40 percent polymer lacquers on a web immersed in water. These lacquers include cellulose acetate, triacetate, and acetate-butyrate. TFCs are formed by interfacial polymerization that involves coating a microporous membrane substrate with an aqueous prepolymer solution and immersing in a water-immiscible solvent containing a reactant [Petersen, J. Memhr. Sol., 83, 81 (1993)]. The Dow FilmTec FT-30 membrane developed by Cadotte uses 1-3 diaminobenzene prepolymer crosslinked with 1-3 and 1-4 benzenedicarboxylic acid chlorides. These membranes have NaCl retention and water permeability claims. 

Figure 5.12 Hydrolysis of methylparathion to p-nitrophenol. The cross-linked phospholipid-based nanocapsules are permeable to reactant and product, while allowing for the retention of enzyme activity. Reproduced with permission from [92].

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

For a flat-plate porous particle of diffusion-path length L (and infinite extent in other directions), and with only one face permeable to diffusing reactant gas A, obtain an expression for tj, the particle effectiveness factor defined by equation 8.5-5, based on the following... 

The particle shape is illustrated in Figure 8.10(a), with reactant A entering the particle through the permeable face on the left. 

One of the main parameters that would improve the overall performance of a fuel cell is better mass transport of reactants through the diffusion layer toward the active catalyst zones. In order to quantify and characterize how well the gas mass transport is in a specific DL material and design, it is important to measure the in-plane and through-plane permeabilities. Most of the published permeability results report the viscous permeability... 

When the membrane performs only a separation function and has no catalytic activity, two membrane properties arc of importance, the permeability and the selectivity which is given by the separation factor. In combination with a given reaction, two process parameters are of importance, the ratio of the permeation rate to the reaction rate for the faster permeating component (c.g. a reaction product such as hydrogen in a dehydrogenation reaction) and the separation factors (permselectivities) of all the other components (in particular those of the reactants) relative to the faster permeating gas. These permselectivities can be expressed as the ratios of the permeation rates of... 

Benefits of the technology include avoiding risks to public health and worker safety associated with excavation, surface treatment, transportation, and disposal and gaseous reactants increase permeability of soils to gases thereby allowing gaseous mixtures to invade smaller soil pores to react with soil contaminants. All information has been supplied by the developer and has not been independently verified. 

The Damkdhler-Peclet product also had an impact on performance the optimal value ranged from is 1.0 x 10 at 773K, to 1.0 x 10 at 873K. Little or no improvement was observed when the pressure in the tube was larger than the pressure in the shell, and no improvement was seen when the shell pressure exceeded the tube pressure. When the inert gas sweep rate was increased, the membrane reactor improved until the amount of sweep gas to reactant gas was approximately one hundred as seen in Figure 3. Once again there was an asymptotic limit to the amount of enhancement seen. There was no improvement when the permeabilities of any other component were increased over the permeability of methane. 

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