Composite membranes PRISM membrane

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Membranes for vapor removal from air have a structure similar to the prism membrane, but they are prepared on a different principle.Aromatic PEI is used to produce a porous substrate membrane by the dry-wet phase inversion method. This polymer was chosen over PS/PES because of the higher durability of PEI to organic vapors. Unlike an asymmetric PS substrate for the prism membrane, the top layer of asymmetric PEI membrane has a large number of pores, the size of which is equivalent to those of UF membranes. When a layer of silicone rubber is coated on the top layer of the porous substrate membrane, the silicone rubber layer will govern the selectivity and the porous support will provide only mechanical strength to the composite membrane. Because the permeabilities of water and organic vapors through the silicone... 

Monsanto s Prism permeators for gas separation also employ composite membranes. Polyamide coatings are not used for the composite membrane in the Prism module. The Prism membrane consists of a coating of silicone rubber applied from an organic solvent on a porous polysulfone substrate. The Prism membrane is another good example of a composite membrane where Structure Level IV is used to obtain good membrane properties (22). 

Figure 6.3.35. (a) Schematic of a two-layer composite membrane, (b) Schematic of a composite PRISM-type membrane. 

Figure 19.6. Gas permeation equipment and performance, (a) Cutaway of a Monsanto Prism hollow fiber module for gas separation by permeation, (b) Flowsketch of a continuous column membrane gas separator, (c) Composition profiles of a mixture of C02 and Oz in a column 5 m long operated at total reflux [Thorman and Hwang in ( Turbak, Ed.), Synthetic Membranes II, American Chemical Society, Washington DC, 1981, pp. 259-279],...

A simple water scrubber is used as a preliminary stage to the Prism separators to remove all but traces of the methanol from the purge gas which might otherwise damage the membrane material. In order to avoid residual water condensation in the membrane system, the feed gas is then heated to some 10 °C above the water dew point and then fed to the Prism separators. The permeate gas, i.e. the gas that has permeated the membrane, consists of a mixture containing some 90% of the hydrogen in the feed gas and has the following composition ... 

Consider the composite "PRISM-type membrane illustrated in Example 6.3.14. For such a polysulfone membrane having a thin coating of silicone rubber, the values of the permeability coefficients through layers A and B for a H2-CO mixture are ... 

Non-porous polymeric membranes are usually employed for gas separation, although porous ones can also be used. Composite polymeric membranes developed in the 1970s made the separation of gas streams commercially feasible. The first large-scale gas separation modules were developed by DuPont in early 1970s, but the first successful commercial membrane gas separation processes (PRISM) were announced by Monsanto in late 1970s. 

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