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

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

Authors 

Roizard, D., Nancy Université
Favre, E., Laboratoire Réaction et Génie des Procédés LRGP- CNRS, Université de Lorraine



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.

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