Paraffin separation 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. 

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. 

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. 

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. 

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. 

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 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. 

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... 

There are several industrial processes for succinic acid production (1) oxidation of paraffins forms a mixture of different carboxylic acids, followed by separation of succinic acid (2) catalytic hydrogenation of maleic acid or trani-maleic acid (3) electrochemical synthesis from maleic anhydride in a bipolar membrane or nonmembrane cell (4) production from acetylene, carbon monoxide, and water catalyzed by [Cq(CO) ] in an acid medium under a pressure of 2.94-49.03 MPa at 80-250°C. Among them, electrochemical synthesis is a generally applied process for succinic acid, which has the advantage of high yield, low cost, high purity of... 

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