Membrane downstream face

Because a vacuum is applied for the removal of the solutes on the membrane downstream face, this side of the membrane is ideally dry in comparison to the more swollen (if polymeric membranes are employed) and hence more flexible membrane upstream face resulting from the solute uptake. This anisotropy of the membrane in the direction of the diffusion of the solute always exists for polymeric membranes and results in a non-uniform diffusivity of solute within the membrane. In other words, the diffusion coefficient of solute i in the membrane, will be position-dependent and not constant across the membrane. 

The solutes permeating through the membrane leave the membrane downstream face as a vapor owing to the vacuum which is established initially by the vacuum pump (4). The phase transition of the permeating solutes from the liquid to the vapor state involves heat consumption corresponding to the heat of vaporization of the solutes. This heat is taken up from the environment, namely the bulk feed, which consequently cools. In modules with a large membrane area this causes a temperature drop between the feed and the retentate and has to be compensated. 

Since the SF is a ratio of ratios, any measure of composition (mole fraction, mass fraction, concentration, etc.) can be used in Equation 7.1 as long as one consistently uses the same measure for both upstream and downstream phases in contact with the membrane. Locally within a module, the ratio of compositions leaving the downstream face of a membrane equals the ratio of the transmembrane fluxes of A vs. B. Local fluxes of each component are determined by relative transmembrane driving forces and resistances acting on each component. The ratio of the feed compositions in the denominator provides a measure of the ratio of the respective driving forces for the case of a negligible downstream pressure. This form normalizes the SF to provide a measure of efficiency that is ideally independent of the feed composition. 

Selective separation of hquids by pervaporation is a result of selective sorption and diffusion of a component through the membrane. PV process differs from other membrane processes in the fact that there is a phase change of the permeating molecules on the downstream face of the membrane. PV mechanism can be described by the solution-diffusion mechanism proposed by Binning et al. [3]. According to this model, selective sorption of the component of a hquid mixture takes place at the upstream face of the membrane followed by diffusion through the membrane and desorption on the permeate side. 

Graham, who was one of the first to consider the permeabilities of natural rubber films to a wide range of gases, found responses such as that seen in Fig. 2a. The description he formulated in 1866 of the so-called "solution-diffusion" mechanism still prevails today (30). He postulated that a penetrant leaves the external phase in contact with the membrane by dissolving in the upstream face of the film and then undergoes molecular diffusion to the downstream face where it evaporates into the external phase again. Mathematically, one can state the solution-diffusion model in terms of permeability, solubility and diffusivity coefficients, as shown in Eq(2). 

Fig. 3.6-11 The principle of concentration polarization (a) on the membrane upstream side in contact with the feed liquid and (b) on the membrane downstream side facing the vacuum. The straight lines indicating the concentration profiles are strongly oversimplified.

Desorption on downstream-side of the membrane described by the traditional models of sorption. The downstream pressure is very low and this step is fast. As the pressure decreases abruptly in the membrane to become equal to that of the downstream face, the pressure gradient through the film can be neglected. Usually, the transport can be considered as an isobar and an isotherm. 

Most membrane operations indicated in Table I are run as continuous steady state processes with a feed, permeate, and retentate stream (see Fig. 1). For example, in dialysis, a feed stream comprising blood with urea and other metabolic by-products passes across the upstream face of a membrane while an electrolyte solution without these by-products passes across the lower face of the membrane. A flux of by-products (A) occurs into the downstream where it is taken away as a permeate and the purified blood leaves as nonpermeate. 

Figure 13.4 Transmembrane receptors must bind their Ligand and propagate their signal across the plasma membrane, usually by way of a protein conformational change. On the cytosolic face of the plasma membrane, that change initiates an enzymic sequence (e.g. G-protein or tyrosine kinase), leading to downstream events.

In the PV mode the feed side is in contact with a liquid, which tends to sorb in the polymer and swell it. As the solute(s) diffuse across the membrane at the permeate side face of the membrane, the solutes are vaporized. Thus, the membrane exhibits different stmctures from the feed to the permeate side. The former is wet and swollen and hence yields high diffusion rates. The downstream part being fairly dry affords much lower diffusion coefficients than the swollen feed side. The assumption of a diffusion coefficient independent of the distance inside the membrane in Equation 2.38 is therefore not rigorously correct. 

---