(550f) Zeolite X Molecular Sieves As Active Materials in the Separation of Effluent Gases from the OCM Process

Methane from natural or shale gas, is considered the preferred transition raw material for the chemical industry of the coming future. However, despite its larger and more evenly distributed reserves worldwide, methane is still a fossil resource which exploitation can contribute negatively with the global climate change. In comparison, biobased methane, obtained from the digestion of biobased residues (molasses, manure, biomass, etc.) or as a residue in landfills, has turned into a potential renewable building block. Nowadays, Biogas is mainly used as fuel for energy generation, internal combustion fuel, or for heating purposes. However this can be used transform into a variety of chemicals through different processes. Among these, the conversion of the methane into ethylene via Oxidative Coupling (OCM) is imperative, as the olefin is a key building block for the chemical and polymer industry. In addition to ethylene, the OCM process also generates large amount of CO2, water, ethane and other minor components.

While significant efforts have been put into the development of new catalysts and reactor configurations to enhance OCM conversion and selectivity, there is still challenges to overcome in the separation of the effluent gases. Currently, the industrial separation methods implemented for the OCM process involve high pressure and cryogenic distillation trains, which are highly energy intensive, with large operating costs. Then, the development of novel separation alternatives are of major importance to guarantee a sustainable implementation of the OCM process. Among the different alternatives, selective adsorption and pressure swing (PS) can be used for the efficient recovery of ethylene from the OCM gas mixture.

Despite the PS adsorption process is well known, the development of selective materials for suitable industrial operation is necessary. Most research efforts in the development of materials for the selective olefin/paraffin separation have been focused on the use of activated carbons and crystalline silicates (zeolites). In particular, both, zeolite A and X, have demonstrated good separation capabilities for the OCM gases at the laboratory scale. In this direction, a Ca-X zeolite type molecular sieve was synthesized via hydrothermal treatment, and used as adsorbent material in a pilot scale PSA system for OCM gases.

The synthesis of the zeolite was carried out in batch reactors at different gel composition (Varying Na2O/Al2O3, H2O/Al2O3 and SiO2/Al2O3 Ratios) and temperatures. The experimental design during the synthesis followed a response surface methodology, using as objective goals the crystallinity of the material (characterized by XRD) and the yield of the process. Once the optimal X zeolite synthesis conditions were obtained, the process was up-scaled to the pilot level. The obtained crystals were subjected to further ion exchange to obtain the required Calcium form of the X zeolite. Then, the CaX zeolite was processed through an agglomeration process to obtain granulated molecular sieves (ca. 3 mm diameter) as packing material for a pilot scale PSA unit. The obtained particles were characterized by measuring surface area, pore distribution, adsorption capacity, adsorption isotherms and breakthrough curves with OCM gases. Results indicate that the material is suitable to be used as adsorbent in pressure swing separation systems for OCM gases, showing good efficiency for the olefin/paraffin separation.