Membrane feed nature

Plate-and-frame modules were one of the earliest types of membrane system. A plate-and-frame design proposed by Stem [110] for early Union Carbide plants to recovery helium from natural gas is shown in Figure 3.38. Membrane, feed spacers, and product spacers are layered together between two end plates. The feed mixture is forced across the surface of the membrane. A portion passes through the membrane, enters the permeate channel, and makes its way to a central permeate collection manifold. 

D8. A perfectly mixed membrane module is used to concentrate helium in the retentate. The feed is 5 vol % helium and 95 vol % hydrogen at 5 atm. The membrane is natural rubber of the type used by Nakagawa (Table 17-2) and operation is at 25°C. Active portion of membrane is 1.0 micron thick. Permeate pressure is 1.0 atm. Assume gases are ideal. 

Carrier Membrane configuration Nature of feed Strip composition... 

Salt flux across a membrane is due to effects coupled to water transport, usually negligible, and diffusion across the membrane. Eq. (22-60) describes the basic diffusion equation for solute passage. It is independent of pressure, so as AP — AH 0, rejection 0. This important factor is due to the kinetic nature of the separation. Salt passage through the membrane is concentration dependent. Water passage is dependent on P — H. Therefore, when the membrane is operating near the osmotic pressure of the feed, the salt passage is not diluted by much permeate water. 

Carbon Dioxide-Methane Much of the natural gas produced in the world is coproduced with an acid gas, most commonly CO9 and/or H9S. While there are many successful processes for separating the gases, membrane separation is a commercially successfufcompetitor, especially for small instaUations. The economics work best for feeds with very high or veiy low CH4 content. Methane is a slow gas CO9, H9S, and H9O are fast gases. 

Gas separation Hollow-fibre for high-volume applications with low-flux, low-selectivity membranes in which concentration polarisation is easily controlled (nitrogen from air) Spiral-wound when fluxes are higher, feed gases more contaminated, and concentration polarisation a problem (natural gas separations, vapour permeation). 

When the feed solution is 1 mM in KCl and 0.5 mM in the cationic dye methylene blue and the receiver solution is 1 mM KCl, the initially colorless receiver solution turns blue due to transport of the cationic dye across the membrane [71]. In contrast, when the feed solution is 1 mM in KCl and 5 mM in KMn04 (Mn04 is red) and the receiver solution is 1 mM KCl, the receiver solution remains colorless [71]. These experiments provide simple visual evidence that this membrane transports a large cation but does not transport a much smaller anion. We have used potentiometric measurements to explore the nature of this cation permselectivity. 

A C02-CH4 methane process gas stream, similar to a typical high CO2 natural gas has been under test by SEPAREX for CO2 removal in a 2-in. diameter element pilot plant since September 1981. The feed gas contains 30% CO2 and is delivered to the membrane test unit at 250-450 psig under ambient temperature conditions. The objective of the system is to reduce the CO2 level of the methane to less than 3.5%. The membrane system consists of 5 pressure tubes in series, each tube containing three 40-in. long elements. The gas is conditioned to maintain it at a minimum of 20°F above the dewpoint. The system was operated at a variety of flow rates, pressures, recoveries and temperatures. Selected data are presented in Figures 6 through 8. 

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