Ionic Transport Processes: In Electrochemistry and Membrane Science

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The lack of mechanical stability is especially critical for liquid membranes. As the solution is maintained in the support pores by capillary forces, when the pressure exceeds a given critical value, the liquid phase is pushed out of the pores. Evaporation of the solvent also affects both flux and selectivity of the membrane. As the solvent evaporates, solution viscosity increases, reducing the diffusivity through the membrane. If evaporation occurs, complete loss of solvent will cause the gas to leak to the product side of the membrane by convective flow.

To enhance membrane stability, several procedures have been adopted. Air saturation with solvent vapor, for instance, decreases evaporation rates, extending the membrane lifetime Jonhson et al. In the same work, Johnson et al. This procedure ensured the creation of a stagnant layer of solvent vapor adjacent to the liquid membrane, minimizing its evaporation. Another method to increase the stability of liquid membranes is gelation. It consists in creating either a gel network inside the support pores or a thin dense layer on the feed side of the membrane Neplenbroek, The presence of common compounds like CO 2 and water in the air has a deleterious effect on the stability of oxygen-enriching membranes.

Even in usual concentration of ppm, CO 2 can irreversibly poison the carrier molecule Johnson et al. It is known that protic solvents such as water increase the carrier oxidation rate in a way not totally understood. Oxidation of the metal center responsible for oxygen binding has been reported in several works Kawakami et al. In our work Ferraz, we used myoglobin, a naturally occurring protein, as oxygen carrier in facilitated transport membranes. We demonstrated that the preparation of cobalt-substituted proteins Co-Myoglobins and recombinant proteins can markedly increase myoglobin stability.

Together the two strategies should result in a myoglobin twenty times more stable than a native one. Some relevant results found in our work using facilitated transport membranes for oxygen separation from air will be discussed in the following section.

Ionic Transport Processes: In Electrochemistry and Membrane Science

Facilitated Transport Membranes Containing Myoglobin. Liquid membranes were prepared using a nylon microporous membrane support, impregnated by immersion in a myoglobin aqueous solution.

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Liquid phase is kept in the pores by capillary forces. The chart in Figure 5 shows the effect of the protein concentration on membrane permeability. This behavior is explained by the dual-mode transport mechanism, which attributes the total oxygen flux through the membrane to two terms. One contribution is due to the ordinary diffusion of oxygen through the liquid membrane, well described as a Henry-type sorption. Thus, at very low myoglobin concentrations, the second term becomes less significant. For high pressures, all carrier molecules are saturated with oxygen and facilitated transport is less significant than diffusive transport.

Conversely, at low pressures, almost all transport will be performed by the carrier. Nitrogen permeability, however, is not affected by the carrier at all, remaining constant at about Barrer. Myoglobin was also immobilized in a poly vinylalcohol membrane. Immobilization did not jeopardize protein functionality, as seen by the appearance of the membrane, which preserved the protein color. The transport properties of the fixed carrier membrane can be verified in Figure 7.

An increase in permeability with a decrease in pressure, which characterizes the facilitated transport, was observed. The separation of olefins from paraffins is a challenge for membrane processes.

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  • Propylene is largely used in the petrochemical industry as feedstock for the synthesis of polymers and chemicals. This separation is currently carried out by energy-intensive cryogenic distillations. As propylene and propane have about the same boiling point, large towers and high reflux ratios are required to achieve good separation, which requires a large capital investment and entails high operation costs. Therefore, more economical processes to separate propane and propylene have been investigated.

    Polymer membranes can hardly distinguish between propane and propylene because these compounds have similar physical properties and about the same molecular size. Thus, the difference in solubility and in diffusivity shown by propane and propylene through the polymer matrix sorption-diffusion mechanism is not significant and separation is not generally efficient. The effectiveness of the polymer membranes can be increased by adding fixed carriers, which interact specifically and reversibly with the propylene molecules, to the polymer matrix.

    Transition metal cations are the most commonly used carriers for propylene facilitated transport given their ability to form reversible complexes with molecules that have double bounds. The interaction that can occur between transition metal cations and the olefin double bonds has been known since the XIX century.

    The low stability allows the propylene to be released more easily on the permeate side. This mechanism is represented schematically in Figure 8 adapted from Cotton et al. The arrows indicate the direction of electron transfer.

    Mathematical Model of Electrodiffusion

    The same mechanism is used to explain the complexation between the other transition metals and olefins, and it is cited by different authors Coates et al. Although these membranes show high performance for the separation, the carrier can be washed out of the membrane, which limits membrane lifetime.

    The forces related to these interactions are important and significantly change the olefin facilitated transport through the membrane. Thus, the separation performance will depend on the material that constitutes the membrane, on the silver salt used, and on the presence of a swelling agent Yoon et al. The hydrophilic polymer membranes containing fixed carrier were produced as dense films, which were deposited on a porous support, and permeability values were normalized for a superficial dense layer thickness equal to 1 mm. Analysis of the results reveals that in the polymer membranes without carriers there is an inverse relationship between permeability and selectivity.

    One way of circumventing the trade-off between permeability and selectivity is by using facilitated transport membranes. In this investigation, a methodology was developed to analyze the effect of adding transition metal salts to the PU matrix. Interactions between the ions and the polymer backbone and cation avaliability for the facilitated transport of propylene were evaluated.

    The membranes were produced as dense films, to which different silver salts were added. The main advantage of fixed carrier membranes over liquid membranes is their increased mechanical stability, since there is no loss of carrier during operation. However, improvement in the chemical stability of the carrier still needs to be proven.

    A thermoplastic PU, which contains polyester as the flexible segment, was used as polymer matrix. The films were characterized structurally and the intensity of the interaction between the cations and the electron donor group in the polymer chains was evaluated by measurement of ionic conductivity under argon and propylene.

    Propylene and propane permeability was also measured at different pressures. The results showed that the membrane structure changes according to the kind of salt used and this depends not only on the cation, but also on the anion type.

    The conductivity results also evidenced that the different salts interact in different ways with the PU chains. As can be seen, the values for conductivity in propylene are higher than the ones obtained in argon.

    And in experiments carried out in propylene, conductivity increases with silver concentration. Figure 11 shows the propane and propylene permeability values as a function of the concentration of the same silver salt as that in PU films, as previously discussed. The permeability results show that the addition of the silver salt to the PU matrix produces two opposite effects in the propylene transport through the membranes. The negative effect is due to the decrease in chain mobility produced by the coordination between the PU chains and the ions that act as transient crosslinks. The lower chain mobility makes the movement of the propane and propylene molecules through the chains more difficult and their permeability values decrease.

    Besides, at high concentrations, crystalline regions are formed as detected by X-ray , which also contributes to the decrease in permeability values, given that the crystalline phases are impermeable.

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    This effect does not allow the propylene permeability to decrease significantly, as observed for the propane. These results evidence the occurrence of facilitated transport of the propylene molecules.

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    Sugar separation is often a relatively difficult and costly task. Chromatography is the method most frequently used for commercial sugar separation.