Methane permeability

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. 

The permeation of hydrogen and methane through the membrane was investigated, revealing increased permeability for hydrogen on raising the temperature, while the methane permeability remained at a low level. Between 100 and 525 °C, the separation factor (see Section 2.6.3) increased from 7.5 to 31. The hydrothermal stability of the membrane was verified at 525 °C for 8 h for a feed composed of 18% hydrogen, 18% methane and 74% steam. It revealed a decrease of the separation factor from 31 to 26 [51]. 

Plasma polymer films from cyanogen bromide and benzonitrile were also deposited on PPO films. The composite films gave hydrogen-to-methane permeability ratios of 297 and 68 for cyanogen bromide and benzonitrile plasma-polymerized composites, respectively. The PPO substrate film itself has a higher permeability ratio (23.5) than the SC substrate (0.87), although lower hydrogen (6.42 X 10 ) and methane (2.73-10 ) permeability than the SC substrate. 

Upstream pressure (psia) Polyimide Methane permeability (cB) Mylar Oxygen permeability (cB)... 

The Barrer model provides a better prediction for mixed-gas carbon dioxide permeability data, while methane permeability is predicted more accurately by the simpler model of Koros and Paul. Thus none of the models emerge as having a clear advantage over the other two in predicting PPO/CO2/CH4 permeability. However, several factors can be suggested as possible sources of error inherent in all of the models. 

Membrane Methane permeability (ml/mV24h/atm) Time to fill room to 5% v/v methane through intact membrane (yr) Time to fill room to 5% v/v methane through punctured membrane 1 mm diameter puncture (h)... 

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 3. Methane diffusivity and permeability in a series of glassy, aromatic polyimides at 35°C (78), (a) Methane diffusion coefficients as a function of reciprocal free volume, (b) Correlation between methane diffusivity and methane permeability.

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 4. Mixed gas -butane and methane permeability of poly(4-methyl-2-pentyne) as a function of temperature. Feed gas 2 mol% w-butane in methane feed pressure 150 psig permeate pressure atmospheric (0 psig).

Table 4 The effect of Rigid Polymer Chain Structure on Hydrogen and Methane Permeability ...

Figure 3.9 Influence of exposure to CO2 during physical aging on the aging response in 6FDA-DAM as tracked by methane permeability. Throughout the aging process, the film was periodically exposed to CO2 for short periods ( 10 min) at 2 atm pressure. Reproduced with permission of Elsevier. ...

Table 4. Water and CH4 permeability and water/methane permeability ratio of PPO and PPOBr membranes...

Membrane Methane Permeability Barrer Water permeability Barrer Permeability ratio (H2O/CH4)... 

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