(613b) A Case Study in Miniplant Scale for the Carbon Dioxide Capture of the Oxidative Coupling of Methane Process

The Oxidative Coupling of Methane (OCM) to ethylene is a promising alternative for the oil based industry. Due to yield limitation is the process uneconomically and not applied in the industry yet. Several alternatives were purposed, but an efficient and economical process could not found in the literature. In this novel approach the whole process, including downstream separations is investigated simultaneously theoretical and experimental in a miniplant scale. The process is divided into three units: the reaction, the purification and the separation units1. This article focuses on the purification part particularly. The design task for the purification unit in this case study is the removal of the acid gas CO2 from 22 mol% down to 1 mol% at an operating pressure of 32 bar after drying. As possible process alternatives chemical absorption, membrane separation and a combination of both to form an amine-membrane separation process are discussed. For the chemical absorption the Monoethanolamine (MEA) is a widely used solvent and is the bases in this study. The regeneration of the loaded amine is the most energy consuming step in the process. The case study was performed using Aspen Plus® and Aspen Custom Modeller® (ACM). For the absorption unit an in-build rate based model with an electrolyte NRTL - model and chemical equilibrium reactions for the water-MEA-CO2 system was used. For a standard absorption Process with 95 % CO2 removal including solvent regeneration and heat integration the thermal energy demand in the stripper of 3.7 MJ/kgCO2 at 50% solvent regeneration of a 40 wt% MEA solution was found. For the membrane unit an ACM model was developed using the permeability-solubility model with the free-volume-theory for the gas permeation including the Joule-Thomson effect as well as the concentration polarisation². For the case study a rubbery Polyethylene oxide (PEO) and a glassy Cellulose Acetate (CA) membrane with experimental determined parameters by the GKSS3 was used. It was found, that the total CO2 removal with a single membrane unit causes product losses in the range of 30 % and is uneconomically for the OCM process. The use of a membrane unit for bulk removal of the CO2 from 22 mol% to 10 mol% causes 9 % ethylene losses. Therefore an integrated separation process was developed as a combination of a membrane and an absorption process for 95 % CO2 capture. In the first part a GKSS envelope-type CA membrane unit with a CO2/C2H4 selectivity of 12 is used for the bulk removal. In the second part the gas stream is purified from 10 mol% down to 1 mol% CO2 by an absorption process using 15 wt% MEA with thermal energy efficiency of 3.15 MJ/kgCO2. The combined membrane-absorption process achieves the design task of 95 % CO2 removal with a thermal energy demand of 1.6 MJ/kgCO2 with acceptable ethylene losses. Reducing these losses from 9% down to 2 % can be achieved with a two step membrane process for the bulk removal with an electrical energy demand of 2.75 MJ/kgCO2. In the next step the simulations of the amine process has to be extended to kinetic reactions and the results have to be validated experimentally for the miniplant scale.

References:

1. Stünkel et. al. Modeling and simulation of a hybrid separation process for the carbon dioxide removal of the oxidative coupling of methane process, Comp. Aided Chem. E., Vol. 26, 2009, p. 117-122 2. Brinkmann et al. in Ohlrogge, Membranen, Wiley-VCH, 2006, pp. 273 3. GKSS ? Research Centre Geesthach, Department of Process Engineering, Max-Planck-Straße 1, 21502 Geesthacht, Germany