262837 Nanoclay-Based Solid-Amine Sorbents for Carbon Dioxide Capture
Nanoclay-Based Solid-Amine Sorbents for Carbon Dioxide Capture
The objective of the research project is to develop an efficient, low cost, regenerable solid sorbent for carbon dioxide adsorption from large point sources, which can be regenerated with more realistic recycling schemes. The main regeneration scheme that has been studied in the literature for solid adsorbents has been temperature swing desorption while using nitrogen as a sweep gas. This technique, while very useful in the lab to show regeneration is possible, is not realistic as one again ends up with a mixture of CO2 and N2 as an outlet stream. In this work we show that a low cost nanoclay-based solid sorbent can be regenerated with more industrially relevant regeneration methods. We show that it is possible to use pressure-swing vacuum desorption to regenerate the sorbent. We also show that CO2 containing moisture itself can be used as a sweep gas with temperature swing to regenerate the sorbent.
The sorbent developed here is composed of a montmorillonite nanoclay, commonly used in the production of polymer nanocomposites, grafted with commercially available amines. Aminopropyltrimethoxysilane (APTMS) was chemically grafted on to the edge hydroxyl groups of the clay, and polyethylenimine (PEI) was then attached to the surface of the clay by electrostatic interactions. FTIR analysis was used to confirm the amine grafting. The amount of amine loaded onto the support was determined by TGA techniques. The treated clay was initially analyzed for CO2 adsorption in a pure CO2 stream in a TGA. At atmospheric pressure, the optimized adsorption temperature was determined to be between 75°C and 85°C. The maximum CO2 adsorption capacity for clay treated with APTMS at 85°C in pure CO2 at atmospheric pressure was 6.7 wt% CO2, but with the additional treatment of attaching PEI to the surface of the clay the maximum CO2 adsorption capacity increased to 9.7 wt% CO2. In a more realistic simulated flue gas of 10% CO2 and 90% N2, the adsorbents had essentially the same overall maximum CO2 adsorption capacity of 9.6% for clay treated with APTMS and PEI.
Adsorption studies conducted by us in pure CO2 at room temperature and under pressures from 275-2070 kPa showed that the average adsorption capacity of the adsorbents did not change significantly under the pressures examined, indicating that the uptake of CO2 was due mainly to chemical reaction and not due to the physical absorption of CO2. The average CO2 adsorption capacity at 2070 kPa for clay treated with APTMS was 7.57 wt% CO2. The combination of APTMS and PEI treatment on the clay increased the average adsorption capacity to 11.39 wt% CO2. While these results are comparable to those available in the literature for other solid sorbents, the cost of the sorbents developed here is expected to be, as much as an order of magnitude, lower than that of competing sorbents.
Initial desorption studies were conducted using pure N2 at 100°C as a sweep gas, and the process was successful in completely regenerating the adsorbent. Multiple cycles of adsorption and desorption can be carried out with very little loss in CO2 capture capacity. Other regeneration schemes that we have studied that are more commercially realistic are vacuum regeneration and regeneration using CO2 with a small amount of water at 155°C. Vacuum regeneration was studied by first adsorbing CO2 on the sample using a TGA and then placing the sample in a vacuum oven at 65°C-85°C at 93 kPa vacuum for 1 hour. The sample was then returned to the TGA were CO2 was adsorbed onto the sample. The amount of CO2 adsorbed was compared and showed that vacuum desorption is successful at regenerating the adsorbent. Regeneration using CO2 and water was also successful in regenerating the adsorbent at the temperature of 155°C. The CO2 and water studies were conducted in the TGA and water was incorporated into the reaction gas by bubbling the CO2 through water. Although the desorption temperature using CO2 and water was higher than using nitrogen, this scheme is a much more realistic way to regenerate the adsorbent on a large scale since water can easily be condensed out of the exit stream forming an almost pure CO2 stream for sequestration. This work shows that a nanoclay based solid adsorbent can be regenerated using more industrially relevant regenerations schemes that have not been studied much in the literature.