(702b) Major Interest of Hollow Fiber Membrane Contactors for An Ammonia-Based CO2 Capture Process: Detailed Analysis of An Innovative Process

Up to now, the reference technology for CO₂ capture in postcombustion makes use of monoethanolamine (MEA) as chemical solvent in a packed column. Despite high capture efficiencies, MEA induces too high energy penalties. Consequently, an abundant literature is discussing the use of aqueous ammonia for flue gas scrubbing. Indeed ammonia is cheaper than MEA, shows a higher absorption capacity and, most of all, its energy regeneration demand is much lower. Nevertheless, the high ammonia volatility is a considerable drawback which causes a high NH₃ slip within the flue gas. Specific operatic conditions are required to limit NH₃ losses and a post absorber scrubbing step is therefore needed.

Amongst growing technologies, membrane contactors appear as a promising technology able to allow significant process intensification when compared to conventional gas/liquid contactors. So far, no study dealing with the use of membrane contactors for the CO₂ capture in aqueous ammonia has been disclosed. For the first time, a detailed analysis of an ammonia based CO₂ capture process using hollow fiber membrane contactors will be given during this presentation. Particularly, membrane contactors ability to both improve CO₂ mass transfer performances and mitigate NH₃ volatilization compare to conventional gas/liquid contactors will be discussed.

First of all, it will be shown that CO₂ absorption in ammonia can not be performed using conventional microporous hollow fiber membrane contactors and composite membranes made of a thin polymeric layer facing the liquid phase must be used. In that sense, composite hollow fiber membrane contactors have been specifically designed, prepared and tested to favor CO₂ absorption and, in the same time, to limit the ammonia slip in the flue gas.

This involved precisely selecting the dense layer and it will be shown how an inadequate material selection could lead to its significant degradation. The chosen dense skin must be thin, highly permeable to CO₂ and selective for CO₂ over NH₃. This should allow reaching high CO₂ mass transfer coefficient while reducing drastically the ammonia slip in the flue gas. This was undoubtedly one of the main challenges. Indeed, it has to be recalled that a faster permeation of CO₂ compared to NH₃ in dense polymers was totally unexpected and to our knowledge unexplored. For the first time, 4 chemically resistant polymers showing a reverse selectivity CO₂/NH₃ were identified. These meaningful results obtained thanks to time lag experiments will be presented in this presentation.

In a second step, CO₂ absorption experiments in aqueous ammonia using lab scale composite hollow fiber membrane contactors (CHFMC) have been successfully carried out. The effect of the main operating conditions namely ammonia concentration, temperature, gas and liquid flow rates, nature of the dense layer, on the CO₂ absorption performances and the ammonia loss has been studied. The overall volumetric CO₂ absorption capacity in CHFMC has been compared to the one obtained in packed columns. It has been concluded that membrane contactors strongly enhance the CO₂ absorption performances in comparison to conventional contactor. Moreover, a sharp decrease of the ammonia slip has been observed. The above results will be detailed in the presentation and the performances and advantages of this new CO₂ absorption process will be discussed further.