Carbon Dioxide Capture for the Oxidative Coupling of Methane Process – a Case Study in Mini-plant Scale

The Oxidative Coupling of Methane (OCM) to ethylene is a promising alternative for the oil based industry. In this process, beside the valuable product ethylene, other by-products are produced. Hence the gas stream has to be refined further. The process is not applied in the industry yet, because of high separation costs. This article focuses particular on the CO2 purification of the OCM product stream. Therefore a case study were done for a design task of the reaction product gas cleaning from 22 mol% CO2 to 1 mol% on an operating pressure of 32 bar. A state of the art chemical absorption process using Monoethanolamine (MEA) was developed and optimized for the base case. Therefore Aspen Plus with build-in rate based model for the mass transfer with an electrolyte NRTL – approach and chemical equilibrium reactions for the water-MEA-CO2 system as well as kinetic reactions was applied. In order to improve the energetic performance, gas permeation with dense membranes was studied as alternative process. Therefore a membrane unit was developed in Aspen Custom Modeler (ACM). The solutiondiffusion model with the free-volume-theory for gas permeation including JouleThomson effect as well as concentration polarisation2 was applied successfully. Furthermore a rubbery polyethylene oxide (PEO) and a glassy cellulose acetate (CA) composite membrane with experimentally determined parameters were used in this model. The standard absorption process was developed with 97% CO2 removal including solvent regeneration and heat integration. Whereas the thermal energy demand results to 2.8 MJ/kgCO2 at 53% solvent regeneration in the desorption column for 30 wt% MEA. It was observed, that substitution of the absorption process only by a membrane process causes high product losses, so that the process became uneconomical. Hence the design task can be fulfilled only by a hybrid process, a combination of a membrane unit with an absorption process. This reduces the energy demand down to 1.6 MJ/kgCO2 with moderate product loss of 9 %. This article describes the process, as well as the simulation study. Experimental investigations of the process are carried out and results will be given in the presentation. Keyword: Carbon Dioxide Capture, Membrane, Absorption, Oxidative Coupling of Methane, Hybrid Process