Study of methane and natural gas films on carbonaceous adsorbents in static and dynamic systems
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Adsorbent materials, such as activated carbons, zeolites and metal organic frameworks, for natural gas storage have been of interest since the early 1970's as alternatives to compressed and liquified natural gas. These adsorbent materials are comprised of a complex series of pores in which van der Waals forces create extremely dense natural gas films on the surface of the adsorbent. Though these materials have been studied for nearly 50 years, very little experimental work has been done to see how these films form in non-equilibrium conditions or how they are affected by the non-methane components of natural gas. To answer these questions, the University of Missouri created a 40 L adsorbed natural gas tank, containing 20.5 kg of monolithic carbon adsorbent. Using this system, methane loading and unloading experiments were conducted under different conditions to study how these adsorbed films evolve. Two natural gas cycling experiments, one with and one without active thermal control, consisting of 20 cycles each, were also performed to study the effect of non-methane components on adsorbed films, as well as, the effect thermal control has on the cyclability of these materials. The methane loading experiments showed that the adsorbed films first form in the smaller (< 10 [subscript]) micropores. The films in these pores fill quickly during loading and saturate at either approximately 350 or 260 g/L depending on the chosen method for determining the film volume. These values were found to be consistent under both the 35 and 40 bar loading experiments, showing that small changes in the loading procedure does not affect the density evolution in small micropores. Large micropores showed a strong dependence on the filling procedure. Depending on the chosen film volume, densities in these pores ranged from 205-227 or 101-110 g/L. Loading procedures with higher inlet pressures resulted in lower large micropore film densities. This suggests that the film volume is not constant during a loading procedure, and in the case of the monoliths, was found to vary linearly with increasing system temperature. Unloading experiments showed that the increase in the film volume below room temperature and slow desorption in small micropores can cause inefficient gas delivery. Natural gas cycling showed that hydrocarbons larger than methane are selectively adsorbed by the carbon, resulting in some of these components being retained in the adsorbed film upon unloading. This results in an increase in the film density. Since most of these retained components can be assumed to be in the small micropores, this increase was found to possibly be upward of 250 g/L but can be reduced to 150 g/L with active thermal control. These densities are comparable to that of liquid propane in similar conditions, suggesting that a liquid layer of larger hydrocarbons may be forming in small micropores, clogging pore space, and reducing the storage capacity of the system. The storage reduction was 16 g/L when not using active thermal control and 8 g/L when using thermal control. Pore clogging slowed near the end of the cycling process due to the ejection of previously retained ethane in the adsorbed film by C+ hydrocarbons. This suggests that competition between the non-methane hydrocarbons occurs in certain micropores before larger pores are clogged.
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