Membrane ethane

In a facilitated transport membrane process, carriers react or coordinate reversibly with a solute which is transported through the membrane. The principles for such a membrane process for separation of ethene from ethane are shown in Figure 1. On the feed side of the membrane, ethane and ethene are dissolved in the membrane surface. Only ethene can form a complex with the silver ions within the membrane. Diffusion of the ethene-silver ion complex across the membrane to the permeate side takes place according to a concentration gradient. On the permeate side the partial pressure of ethene must be low so that a decomplexation reaction occurs and ethene is released from the membrane surface. Ethane can only diffuse through the membrane according to the concentration gradient across the membrane. 

Although ethylene is produced by various methods as follows, only a few are commercially proven thermal cracking of hydrocarbons, catalytic pyrolysis, membrane dehydrogenation of ethane, oxydehydrogenation of ethane, oxidative coupling of methane, methanol to ethylene, dehydration of ethanol, ethylene from coal, disproportionation of propylene, and ethylene as a by-product. 

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. 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Fig. 7. Equihbrium conversion of ethane versus temperature at 210 kPa in a membrane reactor. The effect of hydrogen removal on ethane conversion is...

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

Using a polymer electrolyte membrane cell in which flowed through the anode chamber. The major intermediate chlorinated products from tetrachloroethene or tet-rachloromethane were trichloroethene or trichloromethane, and these were finally reduced to a mixture of ethane and ethene, or methane (Liu et al. 2001). 

Dehydrogenation of Ethane to Ethylene Porous AI2O3 membranes Nonporous Pd/Ag membranes Fumeaux, Davidson and Ball (1987) Pfefferie (1966)... 

Hydrogenation of Ethylene to Ethane Porous AI2O3 membranes... 

Carbon monoxide oxidation, ethane dehydrogenation, ethane hydrogenolysis, ethene hydrogenation. Pt, Mg, Zn catalysts placed either in the pores of the membrane or at the entrance of the membrane pores. 

Natural gas is usually produced from the well and transported to the gas processing plant at high pressure, in the range 500-1500 psi. To minimize recompression costs, the membrane process must remove impurities from the gas into the permeate stream, leaving the methane, ethane, and other hydrocarbons in the high-pressure residue gas. This requirement determines the type of membranes that can be used for this separation. Figure 8.30 is a graphical representation of the factors of molecular size and condensability that affect selection of membranes for natural gas separations. 

Figure 11.21 Long-term performance of a composite solid polymer electrolyte membrane consisting of 80 wt% AgBF4 dissolved in a propylene oxide copolymer matrix. Feed gas, 70 vol% ethylene/30 vol% ethane at 50 psig permeate pressure, atmospheric [33,61]...

Figure 11.22 Mixed-gas ethylene/ethane selectivity of a solid polymer electrolyte membrane as a function of AgBF4 concentration in the polyamide-polyether matrix [62]...

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 13.11 Flow schematic of the membrane contactor process developed by British Petroleum to separate ethylene/ethane mixtures by absorption into silver nitrate solution [28,29]...

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

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