Pressure Swing Adsorption Process for the Separation of Hydrogen, Methane and Ethane With the MOF

The direct conversion of methane to hydrogen and/or higher hydrocarbons by plasma reaction is a recent field of interest. Indeed, a significant number of reports were published recently on this topic. Plasma reaction field can be generated by various discharge methods or equipment under atmospheric pressure. Yet, the non-thermal equilibrium plasma is one with highest potential, due its unique property to generate electrons with high temperature (high energy) at low gas temperatures. The available literature is mainly focused on product distribution, energy efficiency and reaction improvement strategies. As examples of improvement strategies we can find the integration of plasma field with catalysis (plasma assisted catalytic reaction), the study of different plasma generator configurations, and the influence of co-feed gases such as He, Ar, or even H₂. However, studies on downstream processes, such as the separation of unreacted methane are still insufficient from the engineering point of view, neglecting a possible improvement of the plasma reactor efficiency. The conversion of methane is strongly dependent on reaction conditions, such as discharge method, input power, composition of feed gas, structure of discharge equipment, among others. Usually, methane conversion of only 10% is obtained, when pure methane is fed to the plasma reaction field under ambient temperature and pressure. Therefore, downstream separation of the product gas mixture (H₂, CH₄, C₂H₆ …) is very important in order to recycle the unreacted methane to the feed stream. This separation has similarities with the natural gas purification. Several different processes have been developed to produce pure methane from natural gas, by removing the carbon dioxide, nitrogen, hydrogen sulfide and a minor amount of hydrocarbons from natural source. Particularly, adsorption based processes, like Pressure Swing Adsorption (PSA), have been intensively studied, due to its high energy efficiency. Within those studies, different adsorbents have been proposed, such as molecular sieves, activated carbons, and more recently Metal Organic Frameworks (MOFs). In this work, a MIL-100(Fe) sample, synthesized and shaped into granulates by the Korea Research Institute of Chemical Technology (KRICT) is evaluated for methane purification, from the exhaust gas of the plasma reactor, by PSA. Firstly, the equilibrium of adsorption of hydrogen, methane and ethane was studied by means of a gravimetric method, at three different temperatures (303, 313, and 333 K). MIL-100(Fe) is ethane selective, presenting an ideal selectivity for C₂H₆/CH₄ of about 4, at 303 K and 5 bar. The calculated isosteric heats of adsorption of methane and ethane from this work, and the ones presented by Plaza et al.[1] for propane and isobutane, when plotted in function of the carbon atoms number present a linear behavior, with a value of 6.44 kJ per –(CH₂)-. Single component, binary, and ternary fixed-bed adsorption experiments were carried out, and these were used to validate the mathematical model by performing their simulation. From the breakthrough experiments it was confirmed the ethane selective behavior of the adsorbent, as predicted from the pure component isotherms. Finally, a PSA cycle was simulated, and experiments were carried out to validate the separation.

[1] Plaza, M.G., Ribeiro, A.M., Ferreira, A., Santos, J.C., Hwang, Y.K., Seo, Y.K., Lee, U.H., Chang, J.S., Loureiro, J.M., Rodrigues, A.E.: Separation of C3/C4 hydrocarbon mixtures by adsorption using a mesoporous iron MOF: MIL-100(Fe). Microporous and Mesoporous Materials 153(0), 178-190 (2012)