Gas separation by permeation Part II: Effect of permeate pressure drop and choice of permeate pressure

128

The permeation behavior of the high-flux asymmetric membrane differs from that of the conventional symmetric membrane. A calculation method for predicting the gas separation performance of a permeator with asymmetric membrane is presented. The model takes into account the permeate pressure drop and is applicable to both hollow-fiber and spiral-wound modules. The effect of permeate-feed flow pattern on module performance is analyzed. It is shown that for the high-flux asymmetric membrane, the countercurrent flow pattern is not necessarily always the preferred operating mode. The mathematical model is verified by large-scale field pilot-plant experiments for helium recovery from natural gas using large hollow-fiber modules (220 m2/unit). C. Y. PANAlberta Research Council Edmonton, Alberta, Canada SCOPEThe conventional symmetric membrane has a homogeneous structure with uniform permeation properties across its thickness. This type of membrane has not been widely used for gas separation mainly due to low rates of permeation imposed by the membrane thickness required for maintaining membrane integrity and strength. Recent development of asymmetric membranes, however, has made membrane permeation an important unit operation for gas separation. The membrane consists of an ultrathin skin and a porous supporting layer with negligible resistance to gas flow. The skin, which acts as the separation barrier, is highly permeable due to its thinness. This permits the use of highly selective polymers with inherently poor permeability for specific gas separation. The presence of the porous supporting layer, howeve:, renders the permeatiop behavior of the asymmetric membrane somewhat different from that of the familiar symmetric membrane. A calculation method for predicting the performance of a permeator with the high-flux asymmetric membrane is presented. Both hollow-fiber and spiral-wound modules are considered. The effect of flow pattern on the performance of the asymmetric membrane is found to be significantly different from that of the symmetric membrane. Laboratory and pilot-plant data are presented to substantiate the mathematical model. CONCLUSIONS AND SIGNIFICANCEThe porous supporting layer of the asymmetric membrane prevents the mixing of permeate fluxes of varying compositions on the membrane skin surface. Consequently, the asymmetric membrane always gives rise to cross-flow type of permeation regardless of the flow pattern and direction of the bulk permeate stream flowing outside the porous layer. It is this characteristic that sets the permeation behavior of the asymmetric membrane apart from the conventional symmetric membrane.It is shown that, for the asymmetric membrane with narrow permeate flow path such as hollow fibers, the permeate pressure build-up is strongly dependent on the feed-permeate (the bulk stream) flow pattern. The countercurrent mode has the lowest permeate pressure build-up but the feed flow is in the undesirable direction in relation to the permeate pressure build-up. The cocurrent pattern, on th...

Section: Mathematical Model On Asymmetric-membrane Permeationmentioning

confidence: 97%

Gas separation by permeators with high‐flux asymmetric membranes

Pan¹

1983

128

“…For the binary system, Eq. 15 may be used in conjunction with the composition and stage cut calculated by the Weller and Steiner (1950a) solution, or the following equations (Pan and Habgood, 1978a):…”

Section: Permeation Systems With Constant Permeate Pressurementioning

confidence: 99%

Gas separation by high‐flux, asymmetric hollow‐fiber membrane

Pan¹

1986

185

131

A mathematical model of multicomponent permeation systems with high-flux, asymmetric hollow-fiber membranes is presented. The model takes into account the permeate pressure variation inside the fiber. In the special case of negligible permeate pressure drop, the model yields a simple analytical solution for membrane area calculation that eliminates the numerical integration step required in existing methods. Laboratory multicomponent permeation experiments have verified the mathematical model and have demonstrated the technical feasibility of using the high-flux asymmetric cellulose acetate hollow fiber for H, , CO,, and H, S separation. It is shown that the selectivity of the cellulose acetate membrane is ideally suited to the recovery of hydrogen from the purge gas of reactor recycle loops. For the separation of high-concentration CO, or H,S, the test data show that the permeabilities of the individual components in mixed gas permeation are significantly different from those of pure gases.

“…Methods of computing the compositions of the product streams and the membrane area have been discussed by Oishi et al (1961), Stern and Walawender (1969), Walawender and Stern (1972), Hwang and Kammermeyer (1975), and Pan andHabgood (1974, 1978a,b).…”

Section: General Considerationsmentioning

confidence: 99%

Modeling of permeators with two different types of polymer membranes

Perrin

Stern

1985

Mathematical models have been developed for the separation of binary gas mixtures in permeator modules housing two different types of membranes simultaneously. The membranes are selected so as to exhibit reverse selectivities toward the components of a mixture, i.e., so that one membrane is more permeable to one of the components while the second membrane is more permeable to the other component. The mathematical models describe the membrane separation process for three kinds of flow patterns of the permeated (low pressure) and unpermeated (high pressure) gas streams in the permeator, namely, "perfect mixing," countercurrent flow, and cocurrent flow. Numerical solutions of the models indicate that the extent of separation achievable in a two-membrane permeator can be much higher than in a conventional single-membrane permeator. Also, for given product compositions, the membrane area requirements of the former permeator can be lower than those of the latter. Countercurrent flow is generally the most efficient flow pattern in a two-membrane permeator, and "perfect mixing" is the least efficient one, but the opposite is true under special operating conditions. SCOPEThe separation of gas mixtures by selective permeation through polymer membranes is currently under active investigation by many laboratories due to the fact that this separation technique is potentially energy-efficient. Additionally, the required process equipment is simple, modular, and relatively easy to control.Significant advances have been made in membrane technology for gas separation in recent years. These advances may be placed into two categories: membrane materials and process design concepts. The most noteworthy advance in materials has been the development of asymmetric hollow fiber and composite membranes that exhibit high gas permeabilities as well as high selectivities for specific components of gas mixtures. In the area of process design, one of the most interesting innovations has been the simultaneous use of two or more different types of membranes for a given separation process (Ohno et al., 1973-78; Kimura et al., 1973). The membranes are chosen so as to exhibit special selectivities for different components of a gas mixture to be separated, Thus, according to this concept, a binary gas mixture is best separated by means of two different membranes, one of which is more permeable to one of the components of the mixture while the other membrane is more permeable to the second component.The advantages of using two or more different types of membranes for a given separation process, rather than a single type of membrane, are twofold: The extent of separation is increased, and the extent of recovery of a desired component of the feed mixture is also increased. The use of more than one type of membrane, where possible, should therefore lower the energy and capital investment costs of membrane separation processes. An additional advantage of multimembrane systems lies in their ability to produce more than two product streams. Hence, such sy...