Ethane, ethylene separation using membranes

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and separation. 

A new type of configuration, the flowing liquid membrane (FLM) was studied by Teramoto et al. [20]. In this case, the membrane liquid phase is in motion as the feed and strip phase. In this type of system a plate-and-frame and spiral-wound configuration with flat membrane was used. The scheme of the FLM configuration is drawn in Fig. 7.3A. The hquid phase flows (FLM) between two hydrophobic microporous membranes. The two membranes separate the hquid membrane phase from feed and strip phases. In Fig. 7.3B, it is reported the classical plate-and-frame module employed for the separation of ethylene from ethane [20]. The liquid membrane convection increased the membrane transport coefficient in gas separation. However, the membrane surface packing density (membrane surface area/ equipment volume) is much lower in spiral-wound system than in hollow fiber. 

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. 

Figure 13.12 Flow schematic of process using two membrane contactors for the separation of ethylene/ethane mixtures proposed by Bessarabov et al. [30]...

Teramoto M, Matsuyama H, Yamashiro T, and Okamoto S. Separation of ethylene from ethane by a flowing liquid membrane using silver nitrate as a carrier. J Mem Sci, 1989 45(1-2) 115-136. 

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]. 

The same authors proposed a flowing hquid membrane [20], for the separation of ethylene over ethane, in which a liquid membrane solution flowed in a thin channel between two microporous membranes. Also in this case a silver nitrate was used as carrier of ethylene. The selectivity for ethylene over ethane was about 460 at a higher concentration of 4 X 10 inol/in This type of membrane was stable for more than 11 days. 

Sometimes reaction rates can be enhanced by using multifunctional reactors, i.e., reactors in which more than one function (or operation) can be performed. Examples of reactors with such multifunctional capability, or combo reactors, are distillation column reactors in which one of the products of a reversible reaction is continuously removed by distillation thus driving the reaction forward extractive reaction biphasing membrane reactors in which separation is accomplished by using a reactor with membrane walls and simulated moving-bed (SMB) reactors in which reaction is combined with adsorption. Typical industrial applications of multifunctional reactors are esterification of acetic acid to methyl acetate in a distillation column reactor, synthesis of methyl-fer-butyl ether (MTBE) in a similar reactor, vitamin K synthesis in a membrane reactor, oxidative coupling of methane to produce ethane and ethylene in a similar reactor, and esterification of acetic acid to ethyl acetate in an SMB reactor. These specialized reactors are increasingly used in industry, mainly because of the obvious reduction in the number of equipment. These reactors are considered by Eair in Chapter 12. 

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... 

Ploegmakers, J., Jelsma, A.R.T., Ham, A.G.J.v.d. and Nijmeijer, K. (2013) Economic evaluation of membrane potential for ethylene/ethane separation in a retrofitted hybrid membrane-distillation plant using unisim design. Industrial Engineering Chemistry Research, 52 (19), 6524—6539. 

Separation of Ethylene from Ethane Using Perfluorosulfonic Acid Ion-Exchange Membranes... 

The concept of using Ag+ in liquid membranes to promote facilitated transport of simple gaseous alkenes, specifically ethylene/ethane separations, began with papers by LaBlanc et al. (6) and Teramoto et al. (7,8). Interest in this process waned somewhat when it was discovered that Ag+ formed explosive side products with acetylene which was present in the feed stocks. Despite this potential problem, researchers at BP America developed a Ag- --based separation process for propene/propane separation (9). 

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