Liquid-phase permeability, pressure

Table 2.2 Pressure-Independence for Liquid-Phase Permeability and Selectivity...

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

By definition osmotic pressure equals the pressure that has to be applied onto a mobile semi-permeable membrane (filter) separating foam and liquid in order to prevent liquid phase from entering the foam. This pressure can be calculated as the difference between pressure of liquid column and pressure of foam column, both having the same height [84]... 

In this connection, a series of steady state displacement tests were conducted in a 170-200 mesh bead pack with a porosity of 0.4 and an absolute permeability of 7000 md. The experimental apparatus is shown in Figure 7. Two types of gas injection were employed - constant pressure injection and constant rate injection air was used in the former, nitrogen in the latter. The liquid phase was water plus surfactant (0.25% by volume Amphosal CA, Stepan Chemical Company) and was always injected under constant... 

If the foam phase is thought of as a pseudo continuous fluid with an apparent viscosity Vapp = it follows that Papp is greater than that of the aqueous liquid phase. (For the tests here, values of Uapp were on the order of 1 to 50 times that of water). Because of this, when foam and liquid move through a porous medium under an applied pressure drop, the foam, being the most viscous phase, must occupy a larger region of the pore space. Consequently, as observed, the gas saturation is increased over that of non-dispersed phase flow and the liquid permeability is correspondingly decreased. 

The first one is the Katz and Thompson s model (1986) which interprets transports within pore solids in terms of these percolation ideas [2]. From that, the authors introduced a fractal percolation model to predict the permeability of a disordered porous media. In invasion percolation, a non-wetting fluid can have access to the first connection from one face of the sample to the other only when the driving pressure is sufficient to penetrate the smallest pore-throat of radius rc in the most efficient conducting pathway. So, the permeability of rocks saturated with a single liquid phase is given from the following relationship ... 

Miscible Liquid Behavior. Further concerning osmotic behavior, there is the behavior of, say, two miscible liquids, or miscible liquid components, but with each separated from the other by a semipermeable membrane, more permeable to one liquid than the other. Movement of the more permeable liquid phase to the less permeable liquid phase will occur, building up a pressure difference. However, there will likely be membrane leakage from the latter phase to the former, in the reverse direction, eventually producing the same equilibrium composition on both sides of the membrane. This brings up the relative permeation of two miscible liquids from a zone of composition on the one side of a membrane... 

The permeability of hardwoods (such as birch treated here) is 10 to 10 times higher in the axial direction than in the radial and tangential directions [14]. Therefore the axial flow was included in the conservation equations (Equation 2, 3 and 4). A discussion on the anisotropy of wood, the inclusion of axial convective terms and the omission of axial diffusive terms are presented in [7], In the conservation of gas species and of liquid phase the axial flow was estimated by Equation 7 and 8 assuming, for simplicity, a linear pressure gradient in the axial direction of the sample. For the energy equation (Equation 4) two cases were studied, q H) and an axial heat loss according to ... 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

Composite Materials. Many solid foods can be considered as solid matrixes, interspersed with a continuous liquid phase. Transport through such a material may be greatly hindered. Flow of the liquid through the matrix under the influence of a pressure gradient is proportional to a material constant called permeability, which is about proportional to pore diameter squared and pore volume fraction. 

Accurate description of barrier films and complex barrier structures, of course, requires information about the composition and partial pressure dependence of penetrant permeabilities in each of the constituent materials in the barrier structure. As illustrated in Fig. 2 (a-d), depending upon the penetrant and polymer considered, the permeability may be a function of the partial pressure of the penetrant in contact with the barrier layer (15). For gases at low and intermediate pressures, behaviors shown in Fig. 2a-c are most common. The constant permeability in Fig.2a is seen for many fixed gases in rubbery polymers, while the response in Fig. 2b is typical of a simple plasticizing response for a more soluble penetrant in a rubbery polymer. Polyethylene and polypropylene containers are expected to show upwardly inflecting permeability responses like that in Fig. 2b as the penetrant activity in a vapor or liquid phase increases for strongly interacting flavor or aroma components such as d-limonene which are present in fruit juices. 

Production history for a foam generated in a displacement of surfactant solution by gas at fixed pressures, and then subjected to a series of stepwise AP increases, is shown in Figure 7. An initial trickle of gas appears some time after the foam front has reached the outlet, and reaches a constant flow rate at a time that coincides with the overall water saturation, 5W, stabilizing at 0.05 as production of liquid phase ceases. The vertical portions of the dotted line occur at the time of each differential pressure increase. In this case, the gas rate stabilizes at five successive gas-blocking states. The sequence of gas-blocking states defines a AP-dependent permeability such as that shown in Figure 8 for three repeat experiments. The rather broad error band is ascribed to the random nature... 

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