Permeance defined

Eq. 3 correctly predicts that, for given values of the upstream and downstream partial pressures (pn and Pl) the flux [and therefore the permeance defined as J/(Ph-Pl)] will pass through a maximum with temperature, as commonly observed. Note that at low loadings (bp 1.0) Eq. 3 reduces to Eq. 1. 

The flux, and hence the permeance and permeability, can be defined on the basis of volume, mass or molar flowrates. The accurate prediction of permeabilities is generally not possible and experimental values must be used. Permeability generally increases with increasing temperature. Taking a ratio of two permeabilities defines an ideal separation factor or selectivity awhich is defined as ... 

The permeability of gases through membranes is most commonly measured in Barrer, defined as 10-10 cm3(STP)/cm2 s cmHg and named after R.M. Barrer, a pioneer in gas permeability measurements. The term ji/ pio — pit), best called the pressure-normalized flux or permeance, is often measured in terms of gas permeation units (gpu), where 1 gpu is defined as 10 6 cm3(STP)/cm2 s cmHg. Occasional academic purists insist on writing permeability in terms of mol m/m2 s Pa (1 Barrer = 0.33 x 10-15 mol m/m2 s Pa), but fortunately this has not caught on. 

The performance of a specific already produced membrane characterized by a defined (known or unknown) thickness depends on permeance, which can be evaluated by means of a permeation measurement. 

The ratio of the permeability (or permeance) of two species (ith and jth) defines the selectivity... 

It is common practice to define the permeance, and the permeability, Pm as follows ... 

Pmp Pr and Pp are considered fixed or determined by conditions external to the model. The remaining variables are 0> Pi, and A, totaling 2C + 2 variables. Hence, one variable must be specified in order to define the process. This could be the fraction permeated, the membrane area, or one of the components mole fraction in the permeate or residue. For existing equipment the membrane area is known and the products flow rates and compositions can be calculated. In a design situation the membrane area can be calculated to satisfy a performance specihcation such as the fraction permeated or a component mole fraction in one of the products. In this discussion the permeances are assumed to be known, which implies given permeabilities and membrane thickness. 

Fig. 39. Demagnetization curves of a sintered SmCo5 production magnet measured at four temperatures. (After Mildrum et al. 1981.) Note The scale markers on top indicate the end points of permeance lines that may be drawn from the origin. Their intercepts with the B versus H-curve define operating points with the indicated unit permeance values, p = — Bd/Hd = 1,..., 10.

Define and apply the concepts permeability, permeance, permselectivity, separation factor, and cut. 

The ideal dense membrane has a high permeance for the penetrant molecules and a high selectivity between the components to be separated. The selectivity is expressed in terms of the separation factor, aAB, defined similarly to the relative volatility in distillation ... 

Pm, is the permeance, which is defined as the ratio of permeability Pm, to membrane thickness ... 

The lUPAC defines a membrane reactor as a device for simultaneously carrying out a reaction and membrane based separation in the same physical enclosure. In fact, designing a cost-effective membrane reactor requires that heat transfer, mass transfer, reaction rate, and membrane permeance are well matched in the reactor design. 

As anticipated in the previous section, an inhibition coefficient is necessary in order to evaluate conveniently the inhibition effect only without the polarization one. Analogously to what was done for CPC, an inhibition eoefficient, IC, was defined according to eqn (14.14), where the superscript clean and on permeance indicates that membrane is not affected by inhibition ... 

The quantity fco is the permeance (as defined in the diode discussion) of the triode, and its value is specific to a particular triode type. The plate characteristic curves generated by Eq. (5.4), for a triode in which fco = 0.0012, are illustrated in Fig. 5.8. As the plate characteristic curves indicate, the triode is approximately a voltage-controUed voltage source. 

The overall ability of a core to carry flux also depends on its size and shape, and its cross-sectional area. This is described by a quantity called permeance. The basic relationship of permeance to permeability in a core is defined in Eq. (10.2), where P is the permeance, p is the permeability of the material, A is the cross-sectional area of the core, and Z is the mean length of the flux path in the core. This equation assumes uniform flux distribution in the core and constant permeability inside the core. It does not take into account the variations in the length of the flux path from the inside of the core to the outside. The reciprocal of permeance is reluctance... 

Helium permeance experiments on M2 sample at 273 K and 308 K indicate that the increase in permeability, in cm s" units (Table 2), is not proportional to the square root of the temperatinre, as defined by molecular flow (Knudsen regime). The higher value of the perm bility ratio, against the temperature square root ratio, suggests that helium flux caimot be described by the Knudsen approximation, but by the (xnresponding of activated diffusion. In this case, the presence of ultra micropores and the constrictions in the pores, hinder the molecular motion. As the temperature rises, the kinetic raiergy of the molecules increases and they can overcome the energy barrier of the diffusion. The phenomenon of activated diffusion is very common on micropore system and it can be expressed by an Arrhenius relation ... 

Local flow rate inside the membrane separator G General driving force property H Henry s constant K Equilibrium distribution coefficient k Mass transfer coefficient L Membrane thickness M Molecular weight M Factor defined by Equation 18.26 N Flux P Total pressure Pm Permeability p Partial pressure Pm Permeance... 

Oxygen Permeance (P 02) may be defined as the ratio of the oxygen transmission rate to the differential partial pressure of oxygen on the two sides of the film. Oxygen Permeabihty Coefficient (PO2) is the product of the permeance and the thickness of the him as shown by Eqn (8.19). 

The rate of water vapor transmission (WVT) is defined as the time rate of water vapor flow of a body between two specified parallel surfaces normal to the surfaces, under steady conditions through unit area, under conditions of test. Water vapor permeance is defined as the ratio of the WVT of a body between two specified parallel surfaces to the vapor pressure difference between the two surfaces. An accepted unit of permeance is perm. Water vapor permeability is defined as the product of permeance and thickness. The unit for permeability is perm-in. 

The permeability (permeance) P is defined as the gas flow rate multiplied by the thickness of the membrane, divided by the area and by the pressure difference across the membrane (Equation 1). The permeability is given in the unit barrer or gas permeation unit (GPU) (Equations 2 and 3). 

The development of membranes with improved permeance is more important than the development of more selective materials. For polymeric materials with a physical separation process, the best performances reported up to now can be defined as a CO2/N2 selectivity of 50-70 and a permeance of 1000 GPU (gas permeation unit). 

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