(239c) Comparison of Different Intensified Alternatives for the Downstream Separation in the OCM Process

The oxidative coupling of methane (OCM) has been investigated for over 40 years, owing to this process is an alternative method for ethylene production using natural gas and biogas as feedstocks. Despite its economic potential, and the recent implementation of an OCM demonstration plant by Siluria technologies and Braskem, there are many technical challenges to overcome for a sustainable implementation. Major limitations for the OCM industrialization are the low conversion of methane into ethylene, and the high energy consumption in the downstream processing.

In addition to ethylene and the unconverted methane, the reactor effluents contain a variety of byproducts such as water, ethane, CO₂, N₂, and other minor impurities, which must be removed. The current separation operations involve CO₂ removal through amine absorption, followed by high pressure cryogenic distillation to split the light hydrocarbons and the remaining impurities. However, according to some reports, only the demethanizer unit accounts for nearly 60% of the total energy consumption of the process, and the distillation trains consume 97% of the total energy use in the plant [1]. In this regard, the downstream purification of the OCM products requires the development of enhanced and intensified separations processes to achieve a cost-effective implementation.

In this direction, this work developed a comparison between the current separation technology with two different intensified processes: Dividing wall column (DWC) under cryogenic conditions, and a pressure swing adsorption (PSA) system using zeolite molecular sieves. The process was modeled based upon corroborated thermodynamic models, and validated absorption data. The models were implemented, solved and optimized in Aspen Plus and Aspen Adsorption. According to results, CO₂ removal is a key step in the separation process, allowing to reduce distillation heat duties, and permitting a high selectivity during the PSA separation. After CO₂ removal, the dividing wall column configuration can reduce energy consumption up to 30% with respect to the conventional process, allowing to obtain polymer grade ethylene. Comparatively, and despite it involves high level of complexity for the dynamic operation, the PSA system reduces energy consumption up to 70% with respect to the traditional process. High purity methane, ethane and ethylene can be obtained as major products of the PSA system. The obtained and validated models can be used for further process up-scaling and preliminary economic evaluations.

References

[1] X. S. Nghiem, “Ethylene Production by Oxidative Coupling of Methane: New Process Flow Diagram Based on Adsorptive Separation,” Technische Universität Berlin, 2014.