Permeate gas

Gasoline reformate Gasoline, reformulated Gasolines Gas permeation Gas-phase adsorptu... 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

FIG. 22-78 Influence of feed purity on total membrane area when the permeate gas at fixed purity is the product. Feed-gas volume is constant, H2/CH4 cellulose-acetate membrane, (X = 45, CouHesy VI/ R. Grace. )... 

For gases, both permeation and diffusion data are best measured by permeation tests, many different types been described elsewhere. The same sheet membrane permeation test can quantify permeation coefficient Q, diffusion coefficient D, solubility coefficient s, and concentration c. The membrane, of known area and thickness, must be completely sealed to separate the high-pressure (initial) region from that containing the permeated gas it may need an open-grid support to withstand the pressure. The permeant must be suitably detected and quantified (e.g., by pressure or volume buildup, infrared (IR) spectroscopy, ultraviolet (UV), gas chromatography, etc.). 

Gas separations also show non-Knudsen behaviors. In the case of the H2/n-butane mixture, the temperature has a drastic effect on the main permeating gas, at low temperature almost only butane, the heavier component, permeates (Figure 9). 

When the membrane performs only a separation function and has no catalytic activity, two membrane properties arc of importance, the permeability and the selectivity which is given by the separation factor. In combination with a given reaction, two process parameters are of importance, the ratio of the permeation rate to the reaction rate for the faster permeating component (c.g. a reaction product such as hydrogen in a dehydrogenation reaction) and the separation factors (permselectivities) of all the other components (in particular those of the reactants) relative to the faster permeating gas. These permselectivities can be expressed as the ratios of the permeation rates of... 

For single-component gas permeation through a microporous membrane, the flux (J) can be described by Eq. (10.1), where p is the density of the membrane, ris the thermodynamic correction factor which describes the equilibrium relationship between the concentration in the membrane and partial pressure of the permeating gas (adsorption isotherm), q is the concentration of the permeating species in zeolite and x is the position in the permeating direction in the membrane. Dc is the diffusivity corrected for the interaction between the transporting species and the membrane and is described by Eq. (10.2), where Ed is the diffusion activation energy, R is the ideal gas constant and T is the absolute temperature. 

Hence, the addition of inorganic impermeable nanoplatelets improves the barrier properties of polymers. This is attributed mostly to the lengthening of the diffusion path of the permeating gas molecules due to the increase of the tortuosity. Increasing the aspect ratio of the platelets and their volume fraction improves these... 

Spiral-wound elements, as shown in Figure 2, consist primarily of one or more membrane "leaves, each leaf containing two membrane layers separated by a rigid, porous, fluid-conductive material known as the "permeate channel spacer." The permeate channel spacer facilitates the flow of the "permeate", an end product of the separation. Another channel spacer known as the "high pressure channel spacer" separates one membrane leaf from another and facilitates the flow of the high pressure stream through the element. The membrane leaves are wound around a perforated hollow tube, known as the "permeate tube", through which the permeate is removed. The membrane leaves are sealed with an adhesive on three sides to separate the feed gas from the permeate gas, while the fourth side is open to the permeate tube. 

This relationship can be simplifed when the gases do not chemically associate with each other and when the gases are sparingly soluble in the membrane material. In such cases, the diffusivity of the permeating gas is constant through the film and the solubility of the gas at the membrane surface is essentially directly proportional to its partial pressure in the gas phase adjacent to that surface, i.e., Henry s Law applies ... 

It can be shown that the composition of the permeate gas, P 11, for permeation of a binary gas mixture is given by the following quadratic equation ... 

The separation efficiency for a given membrane with a particular binary gas mixture will be dependent mainly upon three factors gas composition, the pressure ratio between feed and permeate gas, and the sepration factor for the two components. A higher separation factor gives a more selective membrane, resulting in a greater separation efficiency. This parameter is a function of the membrane material and is determined by the individual gas permeation rates. 

In gas separation, a gas mixture at a pressure p0 is applied to the feed side of the membrane, while the permeate gas at a lower pressure (pt) is removed from the downstream side of the membrane. As before, the starting point for the derivation of the gas separation transport equation is to equate the chemical potentials on either side of the gas/membrane interface. This time, however, the chemical potential for the gas phase is given by Equation (2.8) for a compressible fluid, whereas Equation (2.7) for an incompressible medium is applied to the membrane phase. Substitution of these equations into Equation (2.20) at the gas/membrane feed interface yields3... 

At the permeate gas/membrane interface, the pressure drops from pa in the membrane to pi in the permeate vapor. The equivalent expression for the chemical potentials in each phase is then... 

Figure 4.14 (a) Flow schematic of permeation using a permeate-side sweep gas sometimes used in laboratory gas separation and pervaporation experiments, (b) The concentration gradients that form on the permeate side of the membrane depend on the volume of sweep gas used. In laboratory experiments a large sweep-gas-to-permeate-gas flow ratio is used, so the concentration of permeate at the membrane surface is very low... 

The drawback of using an external permeate-side sweep gas to lower the partial pressure on the permeate side of the membrane for an industrial process is that the sweep gas and permeating component must subsequently be separated. In some cases this may not be difficult some processes that have been suggested but rarely used are shown in Figure 4.15. In these examples, the separation of the sweep gas and the permeating component is achieved by condensation. If the permeating gas is itself easily condensed, an inert gas such as nitrogen can be used as the sweep [18], An alternative is a condensable vapor such as steam [19-21],... 

In the case of the counter-flow/sweep membrane module illustrated in Figure 4.18(c) a portion of the dried residue gas stream is expanded across a valve and used as the permeate-side sweep gas. The separation obtained depends on how much gas is used as a sweep. In the calculation illustrated, 5 % of the residue gas is used as a sweep even so the result is dramatic. The concentration of water vapor in the permeate gas is 13 000 ppm, almost the same as the perfect counter-flow module shown in Figure 4.18(b), but the membrane area required to perform the separation is one-third of the counter-flow case. Mixing separated residue gas with the permeate gas improves the separation The cause of this paradoxical result is illustrated in Figure 4.19 and discussed in a number of papers by Cussler et al. [16]. 

Figure 4.19(b) shows an equivalent figure for a counter-flow module in which 5 % of the residue gas containing 100 ppm water vapor is expanded to 50 psia and introduced as a sweep gas. The water vapor concentration in the permeate gas at the end of the membrane then falls from 1900 ppm to 100 ppm, producing a dramatic increase in water vapor permeation through the membrane at the residue end of the module. The result is a two-thirds reduction in the size of the module. 

Figure 8.31 Flow scheme of one-stage and two-stage membrane separation plants to remove carbon dioxide from natural gas. Because the one-stage design has no moving parts, it is very competitive with other technologies especially if there is a use for the low-pressure permeate gas. Two-stage processes are more expensive because a large compressor is required to compress the permeate gas. However, the loss of methane with the fuel gas is much reduced...

Figure 8.32 A typical membrane/amine plant for the treatment of associated natural gas produced in carbon dioxide/enhanced oil projects. The membrane permeate gas is often used as a fuel for the amine absorption plant...

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