(446h) Experimental Evaluation and Computer Simulation of the Adsorption Separation Process of Oxidative Coupling of Methane Effluent Gases over 5A Zeolite Molecular Sieves

Ethylene is a major industrial olefin that is used as building block for the synthesis of a large variety of chemicals. Generally, it is obtained in oil refineries by steam or thermal cracking of light oil fractions. In last few decades, an alternative process for the synthesis of ethylene has been proposed by using methane from natural gas (NG). Because of the large NG reserves worldwide, and the possibility to produce methane from bio-digestion processes, methane turns out to be a feedstock with huge potential for commodities production in the near future.

Production of ethylene from NG involves an oxidative coupling of methane (OCM) where it reacts with oxygen in an exothermic process at 750-850°C using a selective catalyst. Major products of reactions are ethylene, ethane, carbon dioxide and carbon monoxide. The process is carried out in a fluidized bed reactor reaching conversions of around 30%. As the complete oxidation of methane is thermodynamically more favorable, the product stream contains large amounts of CO₂, sometimes larger than 25%. Separation of ethylene from this complex mixture involves energy intensive operations. Initially, CO₂is removed by absorption with amine solutions (MEA and MDEA). Afterwards, the paraffin/olefin mixture separation is accomplished by cryogenic distillation.

Among others, pressure swing adsorption (PSA) process appears as a promising alternative for the cost-effective separation of OCM effluent gases. In this process, an adsorbent material is used for the selective separation by using cycling operation of high and low pressures. Among different materials, zeolites have been used as industrial adsorbents in different applications (ethanol dehydration, natural gas sweetening, N₂â??O₂separation, isomers separation, etc.) and also in the selective separation of olefin-paraffin mixtures.

In this work, the experimental evaluation of a zeolite 5A molecular sieve as adsorbent material for the separation of OCM effluent gases was carried out, and the computer simulation of the process was accomplished. Synthesized 5A molecular sieve was characterized by measuring shape, size, crush strength, water adsorption capacity, adsorption isotherms at -196, 25, 35 and 50°C, surface area, and pore size distribution.

Subsequently, the molecular sieve was packed in the adsorption unit of an OCM Miniplant (TU Berlin), and the breakthrough curves using OCM effluent gases were experimentally evaluated. Nitrogen was used as carrier gas due to its negligible adsorptive capacity. Experiments were carried out with single or mixed gases with molar percentage up to 20% at 30, 60 and 90°C and 2 and 5bar. Additionally, two model multicomponent mixtures were prepared and processed taking into account the conditions of OCM effluent gases. In the first model, a mixture similar to the OCM reactor effluent was used. The second corresponded to a mixture close to the obtained after carbon dioxide removal. 5A molecular sieves demonstrated to be selective for OCM gases separation, mainly for CO₂ free effluent gases.

Adsorption modelling was carried out in the Aspen Adsorption® software, using convection and dispersion models for mass transfer, and the Ergun equation for the momentum balance. A linear driving force was employed to model the mass transfer, an the equilibrium condition was modeled with a Langmuir type isotherm. Energy balance considered gas conduction and heat transfer to environment. 20 nodes were set to solve the differential partial equations of the mass and heat balance using the numerical Method of Lines (MOL). For each simulation, the model was validated by comparing the experimental molar fraction in outlet stream and the gas temperature inside the column. Mass and heat transfer coefficients were used as the adjustable parameters. The results show good agreement of the model with experimental observations, so it can be used for process up-scaling and further economic evaluation.