Carbon dioxide capture for the oxidative coupling of methane process – A case study in mini-plant scale
AbstractThe oxidative coupling of methane (OCM) to ethylene is a promising alternative for the oil based industry. In this process, beside the valuable product ethylene, unwanted by-products like CO2 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 was done for a design task of 90% CO2 capture from 25 vol% in the OCM product gas with an operation pressure of 32 × 105 Pa. Within the article is shown, how to resolve the lack of high separation cost for the purification and the development of an integrated, energy efficient CO2 capture process for the OCM refinery is described. Therefore a state of the art chemical absorption process using monoethanolamine (MEA) was developed and optimized for the base case. Therefore Aspen Plus® with the 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 based on the MEA-REA package was applied. In order to improve the energetic process performance, gas permeation with dense membranes was studied as process alternative. For this purpose a membrane unit was developed in Aspen Custom Modeler® (ACM). The solution-diffusion model with the free-volume-theory for gas permeation including Joule–Thomson effect as well as concentration polarization (Stünkel et al., 2009) was applied successfully. Furthermore several selective materials for a composite membrane with experimentally determined parameters were studied by this model and it was found, that a matrimide membrane provides the best selectivity performance for the OCM CO2 capture. Based on this material a membrane module was installed to form a hybrid separation process in combination with the amine based absorption process. The comparison of the state of the art process with the novel hybrid separation process shows an energy saving of more than 40% for the OCM CO2 capture. In the experimental study the stand alone performance of each unit, as well as the performance of the hybrid process were studied and the results are presented in this article.