Partial pressure driven processes

In a partial pressure-driven vapor permeation process, the NC-dimensional vector of mass fluxes through the porous membrane can be expressed as [33] ... 

Membrane pressure-driven processes, namely, MF, UF, NF, and RO are normally carried out in the liquid phase. Although water permeates through the membrane, other species are partially or completely rejected. According to Fane et al.. The MF-UF range can be considered as a continuum [11] both processes involve porous membranes. MF is carried out using symmetric membranes, with pore size ranging from 0.05 to 10 pm. UF, instead, requires asymmetric membranes, with pore size from 1 to 100 nm. The NF and RO spectrum is also considered as a continuum [11]. NF/RO membranes are usually thin-film composite (TFC) structures with nonpo-rous skin. The most important features of pressure-driven membrane processes are resumed in Table 1.3. 

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

The distinction is best made clear by means of an example Take the case of the chemical reaction which occurs between water and sulphuric acid Let us think of an apparatus similar to that indicated in Fig s In one vessel, A, there is a quantity of liquid water, and m contact with it some saturated vapour at pressure p0 The vapour fills the space on the left-hand side of the tap C In the vessel B there is some concentrated sulphuric acid, that is acid containing a little water, and above this acid is some vapour in equilibrium with the water in this sulphuric acid mixture The partial pressure of the water vapour is here p, where p is much less than pa This water vapour at low pressure (along with some sulphuric acid vapour which does not come into the calculation) occupies the space on the right of the tap C If we simply open the tap, water vapour would stream from left to right, that is from the region of high pressure p0 to that of low pressure p If a piston were placed in the tube it would be driven at a speed not by any means infinitely slowly, and the pressure difference on the two sides of the piston would be finite, 1 e (p0 - p ) This process, which is the spontaneous one, is an irreversible one, since the piston is not made to move infinitely slowly with infinitely small pressure difference on the two sides... 

Pervaporation membranes In the pervaporation separation process, a liquid mixture is brought in direct contact with the feed side of the membrane, and the permeate is removed as vapor from the other side of the membrane. The mass flux is driven by maintaining the downstream partial pressure below the saturation pressure of the liquid feed solution. The transport of liquids through the membranes differs from other membrane processes such as gas separation because the permeants in pervaporation usually show high solubility in polymeric membranes. 

Due to the water vapour partial pressure gradient, in both MD and osmotic membrane distillation (OMD), water vapour is transferred through the pores from one side of the membrane to the other. Both MD and OMD differ from other membrane techniques as the driving force for the process is the difference in total water pressure across the membrane itself. As such, in both MD and OMD the driving force is quite different from other well-known membrane separation processes using hydrophilic membranes, such as RO, driven by hydraulic pressure difference, dialysis (DA), driven by concentration difference, and ED, driven by electric potential difference. 

As in other pressure-driven membrane processes, control of membrane fouling is a key factor in the operation of an MBR system. Membrane fouling manifests as a decline in flux with time of operation, reducing productivity and shortening membrane life (Nilsson, 1990). It is a complex phenomenon, depending on specific membrane-solution interactions and caused by the build-up of particles in the form of a cake layer onto the membrane surface which introduces additional resistance to permeate flow or by a complete or partial blocking of membrane s pores which changes the effective pore size distribution (Field et al, 1995). 

We start by noting that gas permeahon is driven by a concentration or partial pressure difference across the membrane, i.e., it is a process based on diffusion, and not on D Arcy-type bulk flow. For diffusion to take place, the feed partial pressure must be higher than that on the permeate side i.e., we must have... 

Figure 12.1 is a simplified representation of the cavitation process. Figure 12. L4 represents a vessel containing a liquid. The vessel is closed by an air-tight plunger. When the plunger is withdrawn (B), a partial vacuum is created above the liquid, causing vapor bubbles to form and grow within the liquid. In essence, the liquid boils without a temperature increase. If the plunger is then driven toward the surface of the liquid (C), the pressure in the liquid increases and the bubbles...

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