Methane permeability selectivity

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

Porous alumina tube externally coated with a MgO/PbO dense film (in double pipe configuration), tube thickness 2.5 mm, outer diameter 4 mm, mean pore diameter 50 nm, active film-coated length 30 mm. Feed enters the reactor at shell side, oxygen at tube side. Oxidative methane coupling, PbO/MgO catalyst in thin film form (see previous column). r-750X,Pr ed 1 bar. Conversion of methane <2%. Selectivity to Cj products > 97%. Omata et al. (1989). The methane conversion is not given. Reported results are calculated from permeability data. 

Long term experiments were performed with the same membrane for hydrogen and methane permeation under the conditions, 30-50°C and 0.5-2.0 bar for 2 weeks. The current (5-15 mA) was applied for 2 h on average each day. After the series of the test, the crrrrent was switched off and the lydrogen and methane permeability tests were performed. Hydrogen permeability increased by 14% while the methane permeability increased 7%. In other words, both permeability and selectivity increased by applying the electric cnrrent. 

Figure 9. Pure gas propane permeability and propane/methane selectivity for a series of selected organic liquids (O), rubbery siloxane-based polymers ( ), and glassy polymers ( ). The glassy polymers include PI, a polyimide (79), PC, polycarbonate (80), PS, polystyrene (81), and PTMSP (82), Data for the siloxane-based rubber polymers are from Stem et al (83), The solubility of propane and methane in selected organic liquids (hexane, heptane, octane, acetone, benzene, methanol, and ethanol) is from the compilation by Fogg and Gerrard (72). Diffusion coefficients of propane and methane in these liquids were estimated using the Tyn and Calus correlation (46 48),...

The mixed-gas transport behavior of PMP is qualitatively similar to that of PTMSP. The data in Table II show that PMP is significantly more permeable to -butane than to methane. For a feed gas mixture of 2 mol% n-butane in methane at a feed pressure of 150 psig and atmospheric permeate pressure at 25°C, the mixed-gas -butane/methane selectivity of PMP is 14 and the -butane permeability is 7,500 x 10 ° cm (STP) cm/cm s cmHg. This result indicates that the -butane/methane selectivity in PMP is dominated by a high solubility selectivity, similar to the behavior of high-free-volume, glassy PTMSP 5JO). The methane permeability of PMP in the mixture was reduced 5-fold by co-permeation of -butane. 

The permeation properties of PMP were also determined as a function of feed gas composition using mixtures of 1 to 8 mol% w-butane in methane. The mixed-gas permeation conditions were the same as those described above. The i-butane and methane permeabilities of PMP as a function of the relative n-butane pressure are shown in Figure 2. The relative -butane pressure, p/psatj is the partial n-butane pressure in the mixture to the n-butane saturation pressure at 25 C (35.2 psia). As the relative n-butane pressure in the feed gas increased from 0 to about 0.1, the permeability of methane decreased about 5-foId, whereas the n-butane permeability was essentially constant. As a result, the n-butane/methane selectivity of PMP increased from 11 at a relative n-butane pressure of 0.05 to 16 at a relative n-butane pressure of 0.38, as shown in Figure 3. 

Figure 24.7 For a PDMS membrane (a) mixed-gas hydrogen, methane, ethane, propane, and n-butane permeability vs. inverse feed temperature, and (b) n-butane/hydrogen, propane/hydrogen, ethane/hydrogen, and methane/hydrogen selectivity vs. temperature (Pinnau and He, 2004).

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

ABSTRACT CO2-ECBM Micro-Pilots have been carried out in Canada and China to assess the response of low- and high-rank coal reservoirs to C02 injection. The selectivity of coals for C02, compared to methane, can vary from 10 to 1 depending on coal rank. Although the C02 is more efficiently stripped by coals of low rank, permeability impairment is greater and the amount of methane recovered is less than for high-rank coals, providing other reservoir properties are similar. If a pure C02 source is not available, N2-C02 mixtures may produce more favourable economics. 

In this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

Mixed matrix membranes have been prepared from ABS and activated carbons. The membranes are intended for gas separation. A random agglomeration of the carbon particles was observed. A close interfacial contact between the polymeric and filler phases was observed. This morphology between inorganic and organic phases is believed to arise from the partial compatibility of the styrene/butadi-ene chains of the ABS copolymer and the activated carbon structure. A good permeability and selectivity for mixtures of carbon dioxide and methane has been reported (91,92). 

The most extensive studies of plasma-polymerized membranes were performed in the 1970s and early 1980s by Yasuda, who tried to develop high-performance reverse osmosis membranes by depositing plasma films onto microporous poly-sulfone films [60,61]. More recently other workers have studied the gas permeability of plasma-polymerized films. For example, Stancell and Spencer [62] were able to obtain a gas separation plasma membrane with a hydrogen/methane selectivity of almost 300, and Kawakami et al. [63] have reported plasma membranes... 

This means that the enrichment can never exceed the pressure ratio of p,/Pe, no matter how selective the membrane. In the example above, the maximum water vapor enrichment across the membrane is 20 (1000 psia/50 psia) even though the membrane is 200 times more permeable to water than methane. 

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