Hydraulic pressure difference

LIF is used to filter any large molecules (e.g., proteins) present in a solution by using an appropriate membrane. Although the driving potential in UF is the hydraulic pressure difference, the mass transfer rates will often affect the rate of UF due to a phenomenon known as concentration polarization (this is discussed later in the chapter). 

The driving potential for UF - that is, the filtration of large molecules - is the hydraulic pressure difference. Because of the large molecular weights, and hence the low molar concentrations of solutes, the effect of osmotic pressure is usually minimal in UF this subject is discussed in Section 8.5. 

Equation 8.7 [6] was obtained to correlate the experimental data on membrane plasmapheresis, which is the MF of blood to separate the blood cells from the plasma. The filtrate flux is affected by the blood velocity along the membrane. Since, in plasmapheresis, all of the protein molecules and other solutes will pass into the filtrate, the concentration polarization of protein molecules is inconceivable. In fact, the hydraulic pressure difference in plasmapheresis is smaller than that in the UF of plasma. Thus, the concentration polarization of red blood cells was assumed in deriving Equation 8.7. The shape of the red blood cell is approximately discoid, with a concave area at the central portion, the cells being approximately 1-2.5 pm thick and 7-8.5 pm in diameter. Thus, a value of r (= 0.000257 cm), the radius of the sphere with a volume equal to that of a red blood cell, was used in Equation 8.7. 

In order to evaluate quantitatively the steady-state water profile in the membrane of a PEFC under given operating conditions, the membrane transport properties required can be seen to include the following (1) water uptake by the membrane as function of water activity in contact with the major membrane surfaces, X (aw), (2) the diffusion coefficient of water in the membrane as a function of membrane water content, D(X), (3) the electroosmotic drag coefficient as a function of membrane water content, (X), and (4) the membrane hydraulic permeability, y(j(A,), the latter being relevant when hydraulic pressure difference exists across the membrane. 

Hydrodynamic permeability, I, is defined by permeated water across the ion exchange membrane per unit time and unit membrane area (volume transfer per unit time and unit membrane area across the membrane, V) in the presence of an hydraulic pressure difference, AP,... 

When there is no hydraulic pressure difference across the membrane, the electroosmosis ( Jv/I)ap=o is... 

In hemodialysis, low molecular metabolic waste such as urea, creatinine, and other toxic substances (solutes up to 6000 mol wt) are removed from the blood of uremia patients by diffusive transport, which is driven by a concentration gradient of blood solutes being dialyzed against a physiological solution. A complimentary process is hemofiltration, in which solutes up to 20,000 mol wt are removed via an ultrafiltration membrane, the transport being caused by a convective transmembrane flux generated by mild hydraulic pressure differences across the membrane. 

AP is the hydraulic pressure difference across the manbrane C is the concentration at the membrane surface A)t(vp) is the corresponding osmotic pressure R is the manbrane resistance kC is the adsorbed layer resistance p is the fluid viscosity... 

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. 

Here, AP is the hydraulic pressure difference and An the osmotic pressure difference across the membrane. The vdue of An is determined by the concentration at the membrane surface c, and not by the bulk concentration Cj,. 

The GSTB textile barrage structure and material have to be strong enough in order to support the nearly 1 m (3.28 ft) water hydraulic pressure difference which remains constant along the height h. Fig. 81.1. The brittle fracture of the membrane would... 

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