Gas permeation and micropore structure of carbon molecular sieving membranes modified by oxidation

Shape-Selective Ultramicroporous Carbon Membranes for Sub-0.1 nm Organic Liquid Separation

Liquid-phase chemical separations from complex mixtures of hydrocarbon molecules into singular components are large-scale and energy-intensive processes. Membranes with molecular specificity that efficiently separate molecules of similar size and shape can avoid phase changes, thereby reducing the energy intensity of the process. Here, forward osmosis molecular differentiation of hexane isomers through a combination of size- and shape-based separation of molecules is demonstrated. An ultramicroporous carbon membrane produced with 6FDA-polyimides realized the separation of isomers for different shapes of di-branched, mono-branched, and linear molecules. The draw solvents provide the driving force for fractionation of hexane isomers with a sub-0.1 nm size difference at room temperature without liquid-phase pressurization. Such membranes could perform bulk chemical separations of organic liquids to achieve major reductions in the energy intensity of the separation processes.

A Review on Polymer Precursors of Carbon Molecular Sieve Membranes for Olefin/Paraffin Separation

Carbon molecular sieve (CMS) membranes have been developed to replace or support energy-intensive cryogenic distillation for olefin/paraffin separation. Olefin and paraffin have similar molecular properties, but can be separated effectively by a CMS membrane with a rigid, slit-like pore structure. A variety of polymer precursors can give rise to different outcomes in terms of the structure and performance of CMS membranes. Herein, for olefin/paraffin separation, the CMS membranes derived from a number of polymer precursors (such as polyimides, phenolic resin, and polymers of intrinsic microporosity, PIM) are introduced, and olefin/paraffin separation properties of those membranes are summarized. The effects from incorporation of inorganic materials into polymer precursors and from a pyrolysis process on the properties of CMS membranes are also reviewed. Finally, the prospects and future directions of CMS membranes for olefin/paraffin separation and aging issues are discussed.

Cellulose-Based Carbon Molecular Sieve Membranes for Gas Separation: A Review

In the field of gas separation and purification, membrane technologies compete with conventional purification processes on the basis of technical, economic and environmental factors. In this context, there is a growing interest in the development of carbon molecular sieve membranes (CMSM) due to their higher permeability and selectivity and higher stability in corrosive and high temperature environments. However, the industrial use of CMSM has been thus far hindered mostly by their relative instability in the presence of water vapor, present in a large number of process streams, as well as by the high cost of polymeric precursors such as polyimide. In this context, cellulosic precursors appear as very promising alternatives, especially targeting the production of CMSM for the separation of O₂/N₂ and CO₂/CH₄. For these two gas separations, cellulose-based CMSM have demonstrated performances well above the Robeson upper bound and above the performance of CMSM based on other polymeric precursors. Furthermore, cellulose is an inexpensive bio-renewable feed-stock highly abundant on Earth. This article reviews the major fabrication aspects of cellulose-based CMSM. Additionally, this article suggests a new tool to characterize the membrane performance, the Robeson Index. The Robeson Index, θ, is the ratio between the actual selectivity at the Robeson plot and the corresponding selectivity—for the same permeability—of the Robeson upper bound; the Robeson Index measures how far the actual point is from the upper bound.

Carbon molecular sieve membranes: a promising alternative for selected industrial applications

Carbon molecular sieve (CMS) membranes (hollow fibers) have been studied for application as possible separation units for selected industrial gas streams. Gas streams at petrochemical plants (polypropene and polyethene) and upgrading of biogas to fuel specifications have been in focus. Gases present in biogas (N(2), CO(2), H(2)O(vap), and CH(4)) and gas streams at polyolefin plants (C(2)H(4), C(3)H(6), and C(3)H(8)) have been measured; both as pure gases and in mixtures. Aging of the CMS-membranes as a function of humidity and pore blocking is discussed; likewise, possible regeneration methods when flux decrease is experienced. Transport mechanisms depending on pore size and molecular properties are also discussed. Excellent separation properties were documented for these applications, but also the need for frequent regeneration of the membrane in order to maintain permeability flux. The mixed gas experiments documented clearly the need for careful pore tailoring in order to optimize selectivity when the membranes were used for alkane-alkene separation.