Triamine-grafted pore-expanded mesoporous silica for carbon dioxide adsorption in closed-circuit breathing systems

Title: Triamine-grafted pore-expanded mesoporous silica for carbon dioxide adsorption in closed-circuit breathing systems
Authors: Serna Guerrero, Rodrigo Ivan
Date: 2011
Abstract: In the search for efficient, less energy-intensive alternatives for CO2 capture, and inspired by the scrubbing processes using liquid amine solutions, amine groups have been incorporated onto porous supports. Our research group tailored mesoporous materials type MCM-41 by enlarging their pore size to incorporate a high content of readily available amine groups. In earlier studies, triamine-grafted pore-expanded mesoporous silica (TRI-PE-MCM-41) presented large CO2 removal capacity combined with a fast rate of adsorption. This thesis focused on evaluating TRI-PE-MCM-41 as CO 2 adsorbent by determining a variety of adsorptive properties with the aim of gaining a deep understanding of its behavior and to outline its advantages and limitations. The CO2-amine chemistry in gas-solid processes was investigated under humid and dry conditions using aminopropyl-grafted pore-expanded MCM-41 silica (MONO-PE-MCM- 41). To draw accurate conclusions regarding CO2 adsorption in the presence of water vapor, an analytical method coupling thermogravimetric analysis (TGA) and mass spectrometry (MS) was developed. The CO2/N molar ratio in dry streams was close to 0.5, consistent with the formation of carbamate and, as the relative humidity in the feed increased, the CO2/N ratio increased up to 0.88, in line with the gradual formation of bicarbonate. With respect to the effect of moisture on TRI-PE-MCM-41, it was found that its capacity for CO2 in dry streams was enhanced by ca. 17% in streams with 74% relative humidity. In addition, breakthrough curve analysis using a packed-bed column was performed for CO2 mixtures in dry and humid air. The results showed a practically infinite selectivity of TRI-PE-MCM-41 towards CO2 over N2 and O2. While water vapor was also removed by TRI-PE-MCM-41, it did not adsorb competitively with CO 2. With the aim of predicting the behavior of fixed-bed columns packed with amine-functionalized adsorbents, we developed equilibrium and kinetic models capable of describing CO2 adsorption on TRI-PE-MCM-41 under various operating conditions. A new semi-empirical equilibrium model was developed based on the assumption that adsorption of CO2 occurs via two independent mechanisms: (i) chemical adsorption on the amine functional groups, and (ii) physisorption on the surface of the adsorbent. To investigate the adsorption kinetics of CO2 on TRI-PE-MCM-41, experimental data of CO 2 uptake as a function of time at temperatures between 25 and 70°C were fit to a series of kinetic models, namely Lagergen's pseudo-first and pseudo-second order and Avrami's kinetic models. The best fit was obtained using Avrami's model, as it provided a fractional reaction order (ca. 1.4), which has been associated with the occurrence of multiple adsorption pathways. Using the aforementioned equilibrium and kinetic models, a series of simulations of CO2 adsorption in a column packed with amine-grafted mesoporous silica was carried out to predict breakthrough curves. The simulation results were compared with experimental data produced at various flow rates of a stream of 5% CO2/N2. In all cases, the predicted breakthrough time and the corresponding CO2 uptake were in close agreement with the experimental data. To further support the potential of TRI-PE-MCM-4l for commercial scale applications, it was necessary to demonstrate its stability throughout intensive cycling. The behavior of the adsorbent was evaluated in batch experiments, via adsorption-desorption cycling using various regeneration conditions. Using a 23 factorial design of experiments, the impact on the performance of the adsorbent of different levels of temperature, pressure and flow rate of purge gas during desorption was determined. It was found that all the investigated parameters have a statistically significant influence on the working adsorption capacity, but only temperature is influential with respect to desorption rate. If vacuum is applied, regeneration can be achieved at a temperature as low as 70°C with only a 13 % penalty in terms of working adsorption capacity. It was also demonstrated that under the proper regeneration conditions, TRI-PE-MCM-4l is stable over 100 adsorption-desorption cycles. The final chapter of this thesis dealt with the potential application of TRI-PE-MCM-41 as CO2 scrubber in closed-circuit breathing systems, including a first evaluation in a commercial-scale setup. A comparative study between currently used soda lime and TRI-PE-MCM- 4l showed that although the former has a significantly higher CO2 uptake, it was negatively affected by high flow rates, while the former suffered a lower impact, attributed to fast adsorption kinetics. The performance ofTRI-PE-MCM-4l in an anesthesia delivery unit under real operating conditions is presented. Under the current configuration, lower adsorption capacities than those at laboratory-scale testing were measured. Discussion of this low performance and recommendations for future work are included.
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
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