Mobile carrier model

The main differences between mobile and fixed types of carriers (see refs. 33, 37, 53, 60-63) have been well summarised in the context of electron and hydrogen transport through the respiratory chain by Chance et They compared the current flow and fluid flow models of Holton and Lunde-gardh with the normal kinetic oxido-reduction model, and pointed out that the latter consists of a series of bimolecular reactions, while the former is equivalent to a unimolecular process. Hence, in the fixed carrier model there is effectively a single chemical channel, whereas in the bimolecular or circulating mobile carrier model there are two chemical channels. 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

In the present study, a new solid facilitated transport membrane has been prepared by incorporating both fixed and mobile carriers in cross-linked PVA. Based on the membrane transport properties, we have also developed a mathematical model to study the performance of the C02-selective WGS membrane reactor. 

Reports are also available on CO2 selective membrane reactors for WGS reaction. Zou et al. [40] first time synthesized polymeric C02-selective membrane by incorporating fixed and mobile carriers in cross-linked poly vinyl alcohol. Micro-porous Teflon was used as support. They used Cu0/Zn0/Al203 catalyst for low temperature WGS reaction. They investigated the effect of water content on the CO2 selectivity and CO2/H2 selectivity. As the water concentration in the sweep gas increased, both CO2 permeability and CO2/H2 selectivity increased significantly. Figure 6.18 shows the influence of temperature on CO2 permeability and CO2/H2 selectivity. Both CO2 permeability and CO2/ H2 selectivity decrease with increasing reactimi temperature. After the catalyst activation, the synthesis gas feed containing 1% CO, 17% CO2, 45% H2 and 37% N2 was pumped into the membrane reactor. They are able to achieve almost 100% CO conversion. They also developed a one-dimensional non-isothermal model to simulate the simultaneous reaction and transport process and verified the model experimentally under an isothermal condition. 

Kandwal, P, Dixit, S., Mukhopadhyay, S., Mohapatra, P.K., Mass transport modeling of Cs(I) through hollow fiber supported liquid membrane containing calix-[4]-bis(2,3-naptho)-crown-6 as mobile carrier, Chem. Eng. J. 174, 110, 2011. 

From a number of studies at thin films of unsubstituted phthalocyanines which have been reviewed earlier " ", it has become clear that the films behave as semiconductors. The photoconductivity generally observed for films of phthalocyanines clearly showed the possibility to dissociate excitons into mobile carriers. Under most conditions films of unsubstituted phthalocyanines were found to be partly oxidized and hence showed p-type characteristics. This was seen from measurements of the electrical conduction, the thermopower established by a temperature gradient across a sample, and by the rectifying characteristics injunctions. A generally quite low mobility of about lO -lO" cm s was found for the electron vacancies in phthalocyanines, barely allowing the applicability of a band model which dwells upon the assumption of delocalized... 

Fletcher. The classical transmission line is illustrated in Fig. 1.13(a). Note that the distributed capacitance connects a resistive line to a wire of zero resistance. This circuit element is appropriate when there are two types of charge carrier, one of which is very much more mobile than the other. In this model, the more mobile carrier is modeled by the wire of zero resistance, whereas the resistive line describes the less mobile carrier. However from a more general viewpoint, it is preferable to... 

The simplest description of the dielectric function of the mobile carriers in a metal is the Drude model, which assumes that carriers relax in response to an electric field via elastic scattering with a rate where Vp is the Fermi... 

Nevertheless, it is a simple model which seems to reconcile a lot of experimental data. In contrast to this is the two carrier model in which two types of carriers are present and the electron hops between trapping sites. The recent Hall measurements by Munuo and Holroyd (1988) find no evidence for two carriers, at least, in medium and fluids where the electrons have medium to high mobilities. 

---