Membrane deactivation

Generally, the hydrogen deficiency in the reaction zone due to the membrane separation also results in an increased coke formation, leading to catalyst and membrane deactivation. This suggests that more effort needs to be deployed on the development of new dehydrogenation catalysts with high activity per unit volume and high resistance to deactivation. 

Furthermore, the application of the SOD membrane in a FT reaction has been investigated. The advantages of water removal in a FT reaction are threefold (i) reduction of H20-promoted catalyst deactivation, (ii) increased reactor productivity, and (iii) displaced water gas shift (WGS) equilibrium to enhance the conversion of CO2 to hydrocarbons [53]. Khajavi etal. report a mixture of H2O/H2 separation factors 10000 and water fluxes of 2.3 kg m h under... 

Figure 2.9 Hyperpolarisation-activated cation current 4 and its role in pacemaking in a guinea-pig thalamic relay neuron. (Adapted from Figs 2 and 14 in McCormick, DA and Pape, H-C (1990) J. Physiol. 431 291-318. Reproduced by permission of the Physiological Society.) (a) Records showing the time-dependent activation of the h-current by hyperpolarisation and its deactivation on repolarising, (b) Interpretation of rhythmic activity in a thalamic relay neuron. (1) The inter-spike hyperpolarisation activates 7h to produce a slowly rising pacemaker depolarisation. (2) This opens T-type Ca " channels to give a more rapid depolarisation, leading to (3) a burst of Na" spikes (see Fig. 2.8). At (4) the depolarisation has closed (deactivated) the h-channels and has inactivated the T-channels. The membrane now hyperpolarises, assisted by outward K+ current (5). This hyperpolarisation now removes T-channel in-activation and activates 7h (6), to produce another pacemaker potential...

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

After reaching its maximum productivity (after ca. 8 hours.) the [Gl]-Nii2 showed a fast deactivation when applied in continuous catalysis performed in a membrane reactor (Figure 4.12). The fast loss of activity cannot be due to a lack of retention of the catalyst. Due to the high retention measured, this process should be much slower. A model study revealed that this deactivation process probably takes place by the formation of insoluble Ni(III) species (see Section 4.5 for further details). 

After an induction period of ca. 9 hours, the maximum productivity was reached. This was followed by a decrease in activity, which cannot solely be explained by the lack of retention. Calculations showed that at least 20% of the catalyst should still remain in the reactor after 80 h. Additional to this wash-out effect, a deactivation process took place, visible by precipitation of palladium black on the surface of the membrane. Although the catalytic system suffered from deactivation, its selectivity towards 3-phenylbut-l-ene was excellent, being 98% and 85% for the G0- and Gr catalysts respectively. 

The use of such an oxazaborolidine system in a continuously operated membrane reactor was demonstrated by Kragl et /. 58] Various oxazaborolidine catalysts were prepared with polystyrene-based soluble supports. The catalysts were tested in a deadend setup (paragraph 4.2.1) for the reduction of ketones. These experiments showed higher ee s than batch experiments in which the ketone was added in one portion. The ee s vary from 84% for the reduction of propiophenone to up to >99% for the reduction of L-tetralone. The catalyst showed only a slight deactivation under the reaction conditions. The TTON could be increased from 10 for the monomeric system to 560 for the polymer-bound catalyst. 

Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions (e.g. fuel cells). In the instance of aqueous HC1 electrolysis, the basic drawback is corrosion or deactivation of the catalyst during cell shutdown, owing to chemical attack from HC1 and chlorine that diffuse across the membrane. 

These selection and evaluation criteria were applied systematically to four technological fields, three of which contribute to new energy-efficient solutions. Passive houses, for example, with their major components of insulation solutions, window systems, ventilation and control techniques are close to market diffusion within the next ten years. Fuel cells for mobile uses in vehicles, however, are still a long way from market introduction, for instance, because of unresolved problems regarding the deactivation of the membrane electrode assembly (MEA) and the need for cost reductions by about one order of magnitude. Other types of fuel cells for stationary uses may be closer to market introduction, owing to less severe technical bottlenecks and better economic competitiveness. 

Stable performance was demonstrated to 4,000 hours with Nafion membrane cells having 0.13 mg Pt/cm and cell conditions of 2.4/5.1 atmospheres, H2/air, and 80°C (4000 hour performance was 0.5 V at 600 mA/cm ). These results mean that the previous problem of water management is not severe, particularly after thinner membranes of somewhat lower equivalent weight have become available. Some losses may be caused by slow anode catalyst deactivation, but it has been concluded that the platinum catalyst "ripening" phenomenon does not contribute significantly to the long-term performance losses observed in PEFCs (5). 

The other major issue in reactor design concerns catalyst deactivation and membrane fouling. Both contribute to loss of reactor productivity. Development of commercially viable processes using inorganic membrane reactors will only be possible if such barriers are overcome. These subjects will receive greater attention as current R D efforts expand beyond laboratory scale evaluations into field demonstrations. 

Most ultrasonic experiments are carried out in temperature controlled systems to ensure that isothermal conditions are maintained. Even a small general increase in microbial temperature can influence both the active and passive transport systems of the cell membrane/wall and this in turn may lead to an increased uptake of compounds. If the temperature is not controlled then sonication could result in a large temperature increase which will lead to the denaturation (deactivation) of enzymes, proteins and other cellular components present within the microorganism [7]. 

The Bis A-PSF can be sulfonated on the Bis A residue, but the Bis S-PSF will not sulfonate due to the deactivating effect of -SO2- on electrophilic aromatic substitution. Therefore, such a block copolymer would allow the study of sequence length effects on membrane performance. 

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