(80d) The Effect of Hydration on the Adsorption of Carbon Dioxide with Tertiary Amidines on Activated Carbon

Green house gas emissions have come under increased scrutiny in recent years as political and scientific communities begin to understand the impact that human activity has on the environment. Carbon dioxide (CO2) stands at the forefront of this concern due to the copious amounts of CO2 produced anthropogenically. As more that 10 billion tons CO2 are emitted worldwide each year from fossil fuel combustion for energy creation [1], one option to reduce CO2 emissions is the separation, either before or after combustion, of the various gases in the combustion feed stream to facilitate subsequent storage and sequestration of CO2. Recent findings with carbon capture using gas phase adsorption experiments in a plug flow reactor highlight the benefits of this type of capture over membrane or absorption capture processes [2]. In this work, we evaluate the effectiveness of a bicyclic tertiary amidine impregnated on activated carbon in capturing carbon dioxide under post combustion flue gas conditions. Additionally, the effect of hydration on the sorbents was evaluated to help elucidate the mechanism of the interaction and to ascertain whether the presence of water is necessary for CO2 to associate with the tertiary amidine. Experimental Details A custom plug flow reactor (PFR) connected to a mass spectrometer was used for all experimental results. An activated carbon support was chosen, providing a high surface area for the impregnation of the amidines. The amidine, 1,5-Diaza-bicyclo[4.3.0]non-5-ene (DBN), was evaluated in order to determine its effectiveness in capturing CO2 under a range of operating conditions. Upon adsorption, subsequent changes in the operating temperature and partial pressure of CO2 provided the driving force for desorption of the CO2 from the amidine. Initial conditions were specified to closely emulate the conditions expected at the flue of a pulverized coal power plant (30°C - 40°C, 0.1 pCO2/p°, 5% water), while the final operating conditions consisted of an increase in temperature (80°C) and a reduction in the partial pressure of CO2 by switching to a He purge gas. A heated sparger was used to introduce a controlled amount of water in the system. Adsorption breakthroughs and desorption peak responses from a controlled temperature ramp were established from molar concentration measurements in the exit gas stream provided by the mass spectrometer. We establish both the capture efficiency (mol CO2/mol Amidine) and capture capacity (mol CO2/kg sorbent) as a quantitative indicator of the effectiveness of the sorbent. Results and Discussion Breakthrough and desorption plots for DBN are displayed in Figure 2. The deviation from the 10% CO2 feed stream recorded by the mass spectrometer during the adsorption phase is a result of the adsorption of the CO2 to the surface of the sorbent. We see in (2.b) that DBN has a close to negligible interaction with CO2 in the absence of water, however, when operating the sparger at room temperature (2.a, 40°C, 6.3% mol H2O), we see a much larger concentration change in both the adsorption and desorption phases. This result coincides with that found by Heldebrant et al [3], wherein it is believed that the amidine interacts with CO2 in the formation of a bicarbonate salt, which would require the presence of water for the interaction to commence. Besides the increased capture capacity obtained in our hydrated samples, we find from thermogravimetric measurements over long time scales, result in a continual mass gain in the presence of a hydrated feed stream most likely as a result of the hydrophilic activated carbon support. This might ultimately result in a long term reduction in capture efficiency unless this water can be removed in an additional process. We calculate that DBN has a overall capture efficiency of 37%, slightly less than the 40-56% attributed to the state-of-the-art liquid amine systems [4], however this efficiency is only at the temperatures and pressures mentioned previously and could be manipulated by adjusting the operating conditions of adsorption and desorption. Conclusion This work provides an insight on the interaction of the tertiary amidine, DBN, with CO2. We find that hydration is necessary for these tertiary amidines to interact, however the hydrophilic nature of the activated carbon support signifies that alternate, less hydrophilic supports might be better equipped to operate in the humidified flue streams of a coal power plant. Additionally, by changing the operating conditions of adsorption we can modify the capture efficiency of DBN for use in alternative CO2 separation processes. References 1. B. Metz, IPCC Special Report on Carbon Dioxide Capture and Storage (Cambridge University Press, 2005). 2. M. L. Gray et al., ?Capture of carbon dioxide by solid amine sorbents,? International Journal of Environmental Technology and Management 4, no. 1 (2004): 82-88. 3. D. J Heldebrant et al., ?The Reaction of 1,8 Diazabicyclo[5.4.0]undec7ene (DBU) with Carbon Dioxide,? The Journal of Organic Chemistry 70, no. 13 (2005): 53355338. 4. R. J. Hook, ?An investigation of some sterically hindered amines as potential carbon dioxide scrubbing compounds,? Ind. Eng. Chem. Res 36, no. 5 (1997): 1779. Figure 1. The tertiary amidine 1,5 Diazabicyclo [4.3.0] non-5-ene Figure 2. Adsorption and desorption profiles of 1,5 Diazabicyclo [4.3.0] non-5-ene. (a.) capture at 6.3% water content (b.) capture in dry gas conditions