(505d) Adsorption Studies of Carbon Dioxide, Methane, Ethane and Ethylene on 3A, 4A and 5A Zeolite Molecular Sieves

Light olefins from the petrochemical industry have a steady and growing demand because they are starting blocks for the synthesis of higher added value chemicals such as polymers, rubbers, plasticizers, gasoline additives, and others products. Olefins (e. g. ethene, propene) are generally obtained by steam or thermal cracking of light oil fractions in refineries. Taking into account the limited availability of light oils, the large reserves of natural gas have become a potential source for methane-derived chemicals such as olefins. In this direction, oxidative coupling of methane (OCM) has been developed to upgrade natural gas into valuable chemicals, and it is a promising alternative process to produce ethylene from methane.

In the OCM process, a fluidized bed reactor is used to react methane and oxygen at 750-850°C (exothermic reaction) using selective catalyst, producing a mixture of ethylene, ethane, carbon dioxide, and carbon monoxide. The conversion of reaction is ~30% and a large amount of methane is still present in the final product. Carbon dioxide is also present in the outlet stream (concentrations as high as 25%) and needs to be removed in order to reduce the gas flow for downstream processing (reducing costs and avoiding operational difficulties). This separation is generally accomplished by reactive absorption with amine solutions (MEA and MDEA). Once CO₂ is removed, the paraffin/olefin mixture separation should be addressed. This separation is very difficult because the C2s have similar physicochemical properties, and it is mostly accomplished by cryogenic distillation, which is very energy intensive and therefore expensive.

In order to reduce paraffin/olefin separation costs, pressure swing adsorption (PSA) has demonstrated to be a very efficient operation characterized by reduced energy consumption. For this reason research related with synthesis and evaluation of novel adsorbent materials, and the process modeling and simulation of PSA units have increased in recent years.

Taking into account that different zeolites have been used as adsorbent materials for different applications (ethanol dehydration, natural gas sweetening, N₂–O₂ separation, isomers separation, etc.), and that they are selective for olefin-paraffin separations, this work intends to evaluate the adsorption performance of zeolite 4A and 5A as adsorbent for the separation of OCM product gases.

In order to characterize the adsorbent material and its potential to accomplish the target separation, adsorption isotherms and dynamics experiments of carbon dioxide, methane, ethane and ethylene on 4A and 5A zeolites were done. Zeolites 4A and 5A were synthesized using sodium metasilicate, sodium aluminate and deionized water. The formed hydrogel was matured at room temperature and then crystallized in a stirred tank reactor. The product was filtered, washed and dried, and the final solid was characterized by X ray diffraction. Zeolites were agglomerated with attapulgite clay as binder, by using an rotary tilted pan to obtain 4mm mean diameter particles. Zeolite agglomerates were characterized by measuring crush strength, shape, and density (bulk and particle).

To carry out the adsorption isotherms experiments, a Belsorp-Mini equipment was employed and the measurement temperatures were 77, 298, 308 and 320K. From those experiments different isotherm models were fitted to determine the size pore distribution, surface area and parameters to obtain thermodynamic properties.

Further dynamical experiments were carried out in a fully controlled OCM miniplant. The experiments started with a purge stage of the adsorbent packing with helium at 300°C for 12 hours. After column conditioning, the columns was set to the operating pressure (1 or 5 barg) with nitrogen. Finally, and after stabilization, the column was fed with a mixture of nitrogen (80% v/v) and OCM gas (20% v/v) until the equilibrium condition was reached. The desorption process was done at 300°C and 1bar g. The outlet compositions of the outlet gas mixture were measured with gas chromatography and infrared gas analyzer. Results indicated that both zeolites 4A and 5A are suitable materials to be used as adsorbents in PSA columns for paraffin/olefin separations in a OCM process.