Paraffin separation facilitated transport

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. 

Another type of gas exchange process, developed to the pilot plant stage, is separation of gaseous olefin/paraffin mixtures by absorption of the olefin into silver nitrate solution. This process is related to the separation of olefin/paraffin mixtures by facilitated transport membranes described in Chapter 11. A membrane contactor provides a gas-liquid interface for gas absorption to take place a flow schematic of the process is shown in Figure 13.11 [28,29], The olefin/paraffin gas mixture is circulated on the outside of a hollow fiber membrane contactor, while a 1-5 M silver nitrate solution is circulated countercurrently down the fiber bores. Hydrophilic hollow fiber membranes, which are wetted by the aqueous silver nitrate solution, are used. 

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. 

Huang JF, Luo H, Liang C et al (2008) Advanced hquid membranes based on novel ionic liquids for selective separation of olefin/paraffin via olefin-facilitated transport. Ind Eng Chem Res 47 881-888... 

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... 

The separation of olefin/paraffin gas mixtures is one of the most energy-intensive processes in the petrochemicals industry, because it is mainly performed by cryogenic distillations. Membrane processes using the concept of facilitated transport have been considered as an intriguing alternative to cryogenic distillation, as they can simultaneously improve both permeability and selectivity. Silver ions incorporated in liquid membranes act as olefin... 

Kang, S.W., Char, K. and Kang, Y.S. 2008a. Novel apphcation of partially positively charged silver nanoparticles for facilitated transport in olefin/paraffin separation membranes. 

The results presented here demonstrate several points. Although the facilitated transport of alkenes in a pervaporative mode is not quite as productive as in a condensed phase mode, the separation is still highly effective for both alkene-alkene separations and alkene-paraffin separations. The requirement, however, is that the membrane must be swept with a water-saturated gas. This requirement at first may seem cumbersome due to consumption of a sweep gas and the presence of water with the collected permeate. However, the separation of water from hexenes is trivial and one can easily envision a system which recirculates a non-condensing sweep gas such as heliiun or nitrogen. The alkene-saturated water condensed from the sweep stream could also be recycled after thawing and separating from the organics. 

The chapters in this book by Langsam, Xu et aL, Hirayama et aL, Fritsch, and Maier et al focus on polymer structure modification to improve the performance of gas separation membranes relative to the upper bound tradeoff relations. Mahajan et al, describe characteristics of hybrid inorganic/organic membranes as a route to break the simple rules that result in equations 8 and 9, possibly resulting in materials with properties which are above and beyond the upper bound lines. Koval et al and Eriksen et al, describe facilitated transport membranes. They seek to strongly enhance solubility selectivity for penetrant pairs i,e, olefin/paraffin) where... 

Table 6.10 Olefin/paraffin separation performances of facilitated transport membranes...

Facilitated transport membranes have been studied for a long time. The earliest report was on the facilitated transport of oxygen through the hemoglobin solutions (Scholander, 1960). Since then, new membranes and new applications have emerged in many areas. Today, the potential applications of facilitated transport membranes include metal recovery, acid gas removal, bioseparations, olefin/paraffin separation, and O2/N2 separation. 

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