(788c) Hybrid Membrane Cryogenic Process for Post-Combustion CO2 Capture

In Post-Combustion Carbon Capture and Storage (CCS) technology, the main challenge is the reduction of the energy requirement for the capture step.

Different capture processes have been investigated in the literature, such as absorption, adsorption, cryogenic and membrane separation. Among these processes, cryogenic separation is known to be energy efficient once a concentrated CO₂ flue gas is treated (approximately > 40%). Moreover, membrane separation has been shown to offer attractive performances when a moderate purity in the permeate is desired. This suggests that a synergy between the two processes could occur leading to a minimal overall energy requirement.

The aim of this work is to evaluate, through a simulation study, the potential for a hybrid process combining membrane and cryogenic separation to achieve efficient Carbon capture. The hybrid process combines a first step CO₂ pre-concentration with a membrane unit and a second step CO₂ cryogenic concentration.

Because large range of CO₂ content in the flue gas can be found in post-combustion capture, the impact of CO₂ feed content (in the range 5-30%) has been analysed in this study. Moreover, in order to explore the potential interest of improved membrane materials on the energy requirement of the hybrid process, a hypothetical membrane showing a CO₂/N₂ selectivity of 100 and available membrane selectivity of 50 has been simulated for this purpose. Furthermore, two compression strategies:  (i) Vacuum Pumping, and (ii) Feed Compression with Energy Recovery System (ERS) have been investigated and the corresponding process performance in term of energy requirement and membrane surface area are evaluated and compared.

It is shown that for CO₂ feed concentration ranging between 15 and 30% (corresponding to large proportion of emission sources), the hybrid process shows a reduced energy requirement compared to the reference technology, namely, chemical absorption in MEA. Compared to standalone cryogenic approach, the hybrid process effectively improves the energy efficiency when diluted (i.e. < 30%) CO₂ inlet concentrations have to be treated. The percentage energy decrease increases significantly for lower inlet CO₂ mole fractions.

From the membrane material point of view, an improved selectivity (100 in place of 50) does not offer significant benefit in term of energy requirement while the membrane surface area increases. Thus, in this hybrid process, available membranes of selectivity of 50 seem to be appropriate. Moreover, the use of feed compression with Energy Recovery System (ERS) in the membrane module shows close energy requirement comparing to vacuum pumping option, while the membrane surface area is significantly decreased.