Energy, Equipment, and Cost Savings by Application of Membranes in Amine-Based Absorption-Desorption Processes for CO2 Removal (original) (raw)

This contribution is concerned with the experimental and simulative examination of a hybrid process consisting of a membrane stage for gas separation and a chemical absorption unit for the removal of CO 2 from gas streams. The feed stream to the separation process is the product of the oxidative coupling of methane (OCM) and contains 26 vol% CO 2 in addition to methane, ethylene, and ethane. For the investigation of the process a mini-plant was built at TU Berlin, operating at feed flow rates of up to 25 Nm 3 /h and pressures up to 32 bar. The mini-plant closely mimics industrial operating scenarios. The potential for the reduction of energy, equipment, and installation cost by using membranes is assessed and evaluated experimentally in comparison with state of the art separation techniques. The benchmark in CO 2 gas purification for high purity and high selectivity consists of a chemical absorption process using an amine-based absorbent. The mini-plant shows an energetic optimum at 5 MJ/kg CO2 for the absorption using 30 wt% monoethanolamine (MEA) to achieve a CO 2 removal from the feed gas of 90%. In addition, 37 wt% Nmethyldiethanolamine (MDEA) with 3 wt% piperazine as an activator are examined. The ternary amine is an example for a high performance absorbent showing energetic advantages in the regeneration step. Experimental studies in the mini-plant resulted in a specific energy demand of 3.47 MJ/kg CO2 . Installing a HZG 1 gas permeation module upstream to the absorption unit leads to a reduction of the specific energy demand per kilogram of captured CO 2 of 40%. The module is equipped with MATRIMID ® membranes and is found to reduce the CO 2 content of the feed gas to 17 vol%. Hence, equipment cost reductions in the absorption process by requiring fewer theoretical plates in both columns, and in general smaller utility equipment, i.e. heat exchangers and pumps, can also be achieved. Furthermore, a 1 HZG: Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research, formerly known as GKSS, http://www.hzg.de/index.html.en decrease of the flow rate of the absorbent by 20% is attained. The described hybrid process configurations shows a stable separation performance during 500 operating hours with more than 50 start-ups and shutdowns at the investigated feed pressures of 5, 10, and 32 bar respectively. In order to address the somewhat lower product recovery of the hybrid process, two stage membrane arrangements will be assessed. These arrangements allow for the combination of different CO 2 -selective membrane materials in order to minimize the additional gas compression costs of such an installation. Two sequentially connected membranes, in comparison to a single stage, provide more options for optimization. Based on the simulation for the mini-plant level, ethylene recovery could be increased while allowing for the same CO 2 removal rate as in a one-stage system.