Olefins carbon membranes

Okamoto, K.,S. Kawamura, M. Yoshino, H. Kita, Y. Hirayama, N. Tanihara, and Y. Kusuki, Olefin/ paraffin separation through carbonized membranes derived from an asymmetric polyimide hollow fiber membrane, Ind. Eng. Chem. Res., 38, 4424,1999. 

A semi-interpenetrating network based on PEEK can be used as a precursor for carbon membranes. In addition to PEEK, a photosensitive crosslinking agent, 2,6-bis(4-azidobenzylidene)-4-methyl-cyclohexanone, is used [94]. The pyrolysis at 450-650 °C of this combination can produce carbon membranes with an excellent separation performance for olefins and paraffins. 

A recent study estimated that about 10,000 BTU of energy is used armually for olefin-paraffin distillation. The distillation process is used commercially in this separation process. However, membrane separation with low energy consttmption and with relative ease in operation, can be significantly competitive with the distillation process [6]. Therefore, carbon membranes can contribute greatly to the petrochemical industry. 

Several post-treatments have been screened in order to enhance furthermore the olefin/paraffin selectivity of carbon membranes, such as (i) the addition of a silicone coating over the selective layer of polyimide-derived carbon membranes seems to fill surface defects, which leads to an enhancement of the propylene/propane selectivity while lowering the overall propylene permeability of those materials and (ii) air oxidation of phenolic resin-based carbon membranes leads to a decrease of the olefin/paraffin selectivity paired with an increase of the olefin permeability, except if chemical vapor deposition with trichloroethane is applied to the membrane. ... 

Later, the CMRs were also used in an attempt to carry out homogeneous catalytic reactions for example, hydration of propene. Lapkin et al prepared a carbon membrane from a macroporous phenohc resin and constructed a CMR for the hydration reaction. In this gas phase continuous catalytic membrane reactor, the flat carbon membrane was used as a contactor for carrying out reactions at high temperature and pressure. In particular, the hydration of propene, catalyzed by an aqueous solution of phosphoric acid, was selected as a suitable model reaction. Olefin and water were fed separately in order to have the additional benefit of an increased alcohol concentration in the product stream because of the absence of steam in the propene feed. 

Significant progress has been made in alleviating the first two physical causes of membrane instability. The magnitude of the long-term chemical stability problem depends on the process. It is a major issue for carriers used to transport oxygen and olefins, but for carriers used to transport carbon dioxide, chemical stability is a lesser problem. 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

In membrane separation of the olefin/paraffin mixture, the predominant selective separation of the olefin is evident. First, the olefin molecule is smaller in size compared to the respective paraffin. Specifically, C—C distance in paraffins is 0.1534 nm, whereas the C=C distance in olefins is 0.1337 nm. Atoms of carbon in paraffins feature sp hybridization and free rotation around C—C bonds. Atoms of olefins feature sp hybridization. The rigid C=C bond impedes internal rotation in the olefin molecule and makes it flat. It is therefore clear why olefin molecules are smaller in size compared to paraffin and why the diffusion coefficients of olefins in polymers would be higher than those of paraffins. Second, the presence of unsaturated bonds in olefin molecules makes them capable of specific interactions with the membrane matrix. Efforts to take advantage of these capabilities resulted in the development of an important field of research facilitated transport. 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

Gas separation membrane technologies are extensively used in industry. Typical applications include carbon dioxide separation from various gas streams, production of oxygen enriched air, hydrogen recovery from a variety of refinery and petrochemical streams, olefin separation such as ethylene-ethane or propylene-propane mixtures. However, membrane separation methods often do not allow reaching needed levels of performance and selectivity. Polymeric membrane materials with relatively high selectivities used so far show generally low permeabilities, which is referred to as trade-off or upper bound relationship for specific gas pairs [1]. 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

Xu, L., Rungta, M., Brayden, M.K. et al. (2012) Olefins-selective asymmetric carbon molecular sieve hollow fiber membranes for hybrid membrane-distillation processes for olefin/paraffin separations. Journal of Membrane Science, 423 24, 314—323. 

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