Processes using CMS membranes upgrade the value of gas streams by efficiently separating components from various feed sources. Examples of such applications include removing carbon dioxide (CO2) and hydrogen sulfide (H2S) from natural gas streams; separation of propylene (C3H6) from propane (C3H8) and ethylene (C2H4) from ethane (C2H6) in hydrocarbon mixtures; and separation of oxygen (O2) from air. In these examples one or more valuable products can be separated from a less valuable feed stream in an energy efficient manner. Asymmetric multilayer CMS hollow fiber membranes are preferred for large scale high pressure applications due to their ability to be formed into compact modules with high surface-to-module volume properties.
It is known that dense flat polymer films can be used as precursors for forming CMS membranes, but the productivity of these membranes tends to be low. To increase the surface to volume packing, precursor asymmetric polymer fibers used to form CMS membranes can be formed in a so-called dry-jet/wet-quench spinning process that is well-known in the membrane art. These precursors are also known to be useful for forming CMS membranes. Important functional properties of CMS hollow fiber membranes include permeance and selectivity. Permeance measures the pressure-normalized flux of a given penetrant and provides a measure of membrane productivity. Selectivity measures the comparative ability of different gases to permeate through a membrane and provides a measure of separation efficiency. These properties, and the methods by which asymmetric multilayer CMS hollow fiber membranes may be tested to determine these properties, are described in more detail in, for example, U.S. Pat. Nos. 6,565,631 and 8,486,179. Pyrolysis of an appropriate precursor fibers at temperatures above the glass transition temperature (Tg) of the polymer creates a CMS fiber. Unfortunately, since the pyrolysis occurs above the polymer Tg, partial or even total collapse of the porous core layer typically occurs. This collapse creates a separation layer that is much thicker and that has a much lower permeance, and is therefore much less productive, than would be expected if the collapse could be avoided. Substructure morphology collapse occurs when high temperatures during pyrolysis relax the polymer chains in the porous core layer. The movement of the polymer segments allows collapse of the substructure, thereby undermining the productivity advantage provided by the asymmetric fiber.
U.S. Patent Application No. US201301522793A1, and International Patent Application No. WO2013095775A1 describe a method for post-treating precursor fibers in order to limit substructure collapse during pyrolysis. By soaking precursor fibers in a chemical modifying agent, such as vinyl trimethoxy silane (VTMS), before pyrolysis, asymmetric multilayer CMS hollow fibers having an increased permeance are formed. The chemical modifying agent stabilizes the precursor fiber prior to pyrolysis to prevent collapse of the substructure morpohology between the polymer Tg and point of actual carbon formation. The above approach, although workable, requires an additional post-treatment step, thereby adding cost and complexity.
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