Transport mechanisms nonporous membranes

Transport through nonporous membranes follows the solution-diffusion mechanism, and separation is achieved either by differences in solubility or diffusivity. Therefore,... 

Solution-diffusion model is the generally accepted mechanism of mass transport through nonporous membranes (Figure 9.3). According to this mechanism, PV consists of three consecutive steps ... 

Nonporous membranes are used to perform separations on a molecular level. However, rather than molecular weight or molecular size, the chemical nature and morphology of the polymeric membrane and the extent of interaction between the polymer and the permeants are the important factors to consider. Transport through nonporous membranes occurs by a solution-diffusion mechanism and separation is achieved ei er by differences in solubility and/or diffusivity. Hence such membranes cannot be characterised by the methods described in the previous section, where the techniques involved mainly characterised the pore size and pore size distribution in the membranes. The determination of the physical properties related to the chemical structure is now more important and in this respect the following methods will be described ... 

The most important characteristic of nonporous membranes is that they are hydrophobic and contain no pores in the polymeric structure. This means that these membranes not only selectively act as a barrier to particles and polar species, but they also provide unique selectivity and specificity for the permeation and transport of a specific group of compounds that can readily solubilize and diffuse in the membrane material. The analyte extraction rate (permeability) in a nonporous membrane separation process is governed by the solution-diffusion mechanism, as commented on earlier. 

Synthetic membranes for molecular liquid separation can be classified according to their selective barrier, their structure and morphology and the membrane material. The selective barrier- porous, nonporous, charged or with special chemical affinity -dictates the mechanism of permeation and separation. In combination with the applied driving force for transport through the membrane, different types of membrane processes can be distinguished (Table 2.1). 

In the latter case (nonporous membrane), the space in which the transport occurs is not fixed in size and location. The free volume is the volume that is not occupied by the polymer molecules in the solid phase, and its size and location fluctuate with time at a given temperature. Accordingly, the transport through such a membrane is completely different from the transport through fixed pores, and can be expressed by the solution-diffusion mechanism. The permeant is first dissolved in the membrane phase, and the dissolved permeant diffuses through the membrane following the chemical potential gradient. 

The water transport mechanism changes from the flow mechanism in porous membrane to the diffusive transport in nonporous homogeneous membrane due to the deposition of a homogeneous LCVD layer that fills the pore, i.e., water transport changes from bulk flow to diffusive flow when pores are covered by LCVD film. 

Gas transport in nonporous polymer membranes typically proceeds by a solution-diffusion mechanism in which the permeability (P) is given by. xD, where S and D denote the solubility and diffusivity of the permeating species, respectively. The solubility provides a measure of interaction between the polymer matrix and penetrant molecules, whereas the diffusivity describes molecule mobility, which is normally governed by the size of the penetrant molecule as it winds its way through the permanent and transient voids afforded by the free volume of the membrane [42], Therefore gas transport has to be strongly dependent on the amount of free volume in the polymer matrix. 

An attractive feature of membranes relies on their ability to selectively control the flow of different molecules through their matrix. The transport mechanism is typically described through Pick s first law, and it results in a function of the penetrant s diffusion coefficient through the polymer matrix and the driving force applied across the two sides of the membrane, which depends on the boundary conditions at the membrane surfaces. Membranes can be generally classified as porous and nonporous. Depending on their nature, different mechanisms can be used to characterize the molecules transport process. 

Basically, the transport of a gas, vapour or liquid through a dense, nonporous membrane can be described in terms of a solution-diffusion mechanism, i.e. 

Gas separation is possible even w ith the two extreme types of membrane considered, i.e. porous and nonporous. The transport mechanisms through these two types of membrane, howet er, are completely different as discussed already in chapter V. 

The mechanism of separation by non-porous membranes is different from that by porous membranes. The transport through nonporous polymeric membranes is usually described by a solution-diffusion mechanism (Figure 9.12a). The most current commercial polymeric membranes operate according to the solution-diffusion mechanism. The solution-diffusion mechanism has three steps (1) the absorption or adsorption at the... 

Combinations of these mechanisms may be observed in any membrane system that has distinct fluid, amorphous, crystaUine, and functionalized regions, whether classified as porous or nonporous. Membranes maybe characterized with respect to these mechanistic events, as modeled, based on experimental transport measurements. The analysis tools used to interpret these results are briefly discussed later in the context of this example case study. 

Where A = A + A W are known to diffuse within nonporous or porous membranes according to various transport mechanisms. Table 16.3 illustrates the mechanism of transport depending on the size of pores. For very narrow pores, size sieving mechanism is realized that can be considered as a case of activated diffusion. This mechanism of diffusion is most common in the case of extensively studied nonporous polymeric membranes. For wider pores, the surface diffusion (also an activated diffusion process) and the Knudsen diffusion are observed [87-89]. 

Gas transport through nonporous inorganic membranes falls into two categories. It is known that the conventional solution-diffusion permeation mechanism is valid for nonporous membranes of silica, zeolite and inorganic salts. It is no longer so when the membrane is metallic in nature (Hwang and Kammermeyer, 1975). Diatomic gases such as O2, H2 and N2 dissolve atomically in the metallic membrane (see (3.3.67)). While a conventional flux expression is valid for atomic species i dissolved in the membrane, Le. 

The simplest practicable approach considers the membrane as a continuous, nonporous phase in which water of hydration is dissolved.In such a scenario, which is based on concentrated solution theory, the sole thermodynamic variable for specifying the local state of the membrane is the water activity the relevant mechanism of water back-transport is diffusion in an activity gradient. However, pure diffusion models provide an incomplete description of the membrane response to changing external operation conditions, as explained in Section 6.6.2. They cannot predict the net water flux across a saturated membrane that results from applying a difference in total gas pressures between cathodic and anodic gas compartments. 

Nonporous gel membranes - these membranes do not contain a porous structure and thus diffusion occurs through the space between the polymer chains (the mesh). Obviously in this case, molecular diffusion rather than convective transport is the dominant mechanism of diffusion in these membranes. 

The separation performance of membranes with nonporous barriers is - because of the transport via solution-diffusion (cf. Section 2.2) - predominantly influenced by the polymer material itself. Therefore, the material selection is directly related to the intrinsic (bulk) properties of the polymer, but - as for porous membranes - filmforming properties, mechanical and thermal stability form the basis of applicability (cf. Section 2.3.2.1). The following characteristics should be considered ... 

POLYMER MEMBRANES. The transport of gases through dense (nonporous) polymer membranes occurs by a solution-diffusion mechanism. The gas dissolves in the polymer at the high-pressure side of the membranes, diffuses through the polymer phase, and desorbs or evaporates at the low-pressure side. The rate of mass transfer depends on the concentration gradient in the membrane, which is proportional to the pressure gradient across the membrane if the solubility is proportional to the pressure. Typical gradients for a binary mixture are shown in Fig. 26.2. Henry s law is assumed to apply for each gas, and equilibrium is assumed... 

According to Cuperus and Nijhuis [5], the mechanisms in which NF membrane works are not completely clear. Possibly both size exclusion and solution-diffusion mechanisms play a role. This is in agreement with the work of Subramanian et al. [29]. These authors observed that solution-diffusion is the predontinant mechanism of the transport of vegetable oil constituents through nonporous (dense) membranes. The effect of viscosity (temperature) on permeation suggests that transport by convective flow exists in these membranes but the extent observed is not significant. 

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