Transport mechanism, membranes capillary condensation

Notwithstanding any particular structural model, water transport in PEMs, in general, should be considered a superposition of diffusion in gradients of activity or concentration and hydraulic permeation in gradients of liquid or capillary pressure. Hydraulic permeation is the predominant mechanism xmder conditions for which water uptake is controlled by capillary condensation, whereas diffusion contributes significantly if water strongly interacts with the polymeric host. The molar flux of liquid water in the membrane, N, is thus given by... 

Capillary condensation provides the possibility of blocking pores of a certain size with the liquid condensate simply by adjusting the vapor pressure. A permporometry lest usually begins at a relative pressure of 1, thus all pores filled and no unhindered gas transport. As the pressure is reduced, pores with a size corresponding to the vapor pressure applied become emptied and available for gas transport. The gas flow through the open mesopores is dominated by Knudsen diffusion as will be discussed in Section 4.3.2 under Transport Mechanisms of Porous Membranes. The flow rate of the noncondensable gas is measured as a function of the relative pressure of the vapor. Thus it is possible to express the membrane permeability as a function of the pore radius and construct the size distribution of the active pores. Although the adsorption procedure can be used instead of the above desorption procedure, the equilibrium of the adsorption process is not as easy to attain and therefore is not preferred. 

Figure 4.17 Transport mechanisms for gaseous mixtures through porous membranes (a) viscous How (b) Knudsen diffusion (c) surface diffusion (d) multi-layer diffusion (e) capillary condensation and (0 molecular sieving [Saracco and Specchia, 1994]...

There are, however, evidences that other more effective separating mechanisms such as surface diffusion and capillary condensation can occur in finer pore membranes of some materials under certain temperature and pressure conditions. Carbon dioxide is known to transport through porous media by surface diffusion or capillary condensation. It is likely that some porous inorganic membranes may be effective for preferentially carrying carbon dioxide through them under the limited conditions where either transport mechanism dominates. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

A membrane can be defined as a thin and selective barrier that enables the transport or the retention of compounds between two media. In the case of ceramic membranes, the usual driving force for transport is a pressure gradient between the feed and strip compartments (transmembrane pressure). The treated phases can be liquid or gas. For porous membranes, the pore size mainly manages the cutoff of the membrane. However, for retention of the smallest entities by the smallest pores, the transport mechanisms are more complex than simple sieving. Specific physical and chemical interactions (electrostatic repulsion, physisorp-tion, capillary condensation, etc.) become preponderant and determine the membrane selectivity. Table 25.1 summarizes the characteristics of the main processes in which ceramic membranes are involved. 

A final application that can be envisaged for permselective IMRs concerns the enhancement of reaction selectivity toward intermediate products of consecutive reaction pathways. Such a goal could be attained by developing a membrane capable of separating the intermediate product from the reaction mixture [39,40]. The most critical point in this regard is that intermediate product molecules (e.g., partially oxidized hydrocarbons) are often larger in size than the complete reaction products (e.g., CO2) or the reactants themselves (e.g., O2). This seriously complicates the separation process, limiting the number of selective transport mechanisms that can be utilized for the purpose of capillary condensation, surface diffusion, or multilayer diffusion (described later in this chapter). 

Pores, and espjecially mesopores and micropores, play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. The description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures. 

Porous membranes are characterized by high permeability, but low selectivity. The transport mechanisms in porous membranes can be viscous flow, Knudsen diffusion, surface diffusion, capillary condensation, and/ or molecular sieving, depending on the membrane pore size and its surface characteristics. The performance of porous MRs is very much dependent on the membrane structures. Close adherence to a rigid protocol is necessary to obtain membranes of consistent quality. 

The typical gas transport mechanisms in porous membranes are molecular diffusion and viscous flow, capillary condensation, Knudsen diffusion, surface diffusion, and configurational or micropore-activated diffusion. The contributions of these different mechanisms depend on the properties of both the membrane and the gas under the operating temperature and pressure. Figure 2.3 illustrates schematically the gas transport mechanisms in a single membrane pore. 

Among these mechanisms, viscous flow is non-selective while Knudsen diffusion is selective to smaller molecules. At high temperature, gas adsorption becomes weak and thus the surface diffusion and capillary condensation may be negligible. In fact, the perm-selectivity in micropo-rous membranes is a complex function of the temperature, pressure, and gas composition. Therefore, it is necessary to evaluate the perm-selectivity of the porous membranes using a gas mixture under similar operating conditions [3]. Table 2.2 gives an overview of the transport mechanisms in porous membranes. Note that the perm-selectivity is not always a key factor in MRs. 

The major part of the proton conductivity in PBI-based HT-PEMFCs is attributed to free acid in the membrane. It has been shown that redistribution of phosphoric acid in HT-PEMFC MEAs is a rather quick process either from the membrane to the catalyst layer or vice versa [32]. It is this unbound portion of the acid that is particularly susceptible to loss since it is not bonded to the PBl backbone. Loss of the doping acid from a PBl membrane can take place via a number of mechanisms including evaporation, diffusion, capillary transportation, MEA compression, or washing out by condensed water at low temperature. 

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