Two polyethylenimine impregnated mesocellular foam silica sorbents (PEI_80a and PEI_80b) were evaluated for capturing CO2 from 420 ppm of CO2 in N2 under both dry and humid conditions. A fixed bed adsorption setup was used to evaluate the CO2 uptake under isothermal conditions between 33 and 81 °C with a gas flow of 200 mL/min. Under dry conditions, the highest capacity was observed at 46 and 52 °C for PEI_80a (1.29 mmol/g) and at 46 °C for PEI_80b (1.94 mmol/g). There was negligible uptake of CO2 at 81 °C, indicating possible regeneration at relatively low temperatures. For PEI_80b, the introduction of moisture (0.5 and 2% mol-H2O in feed gas) yielded up to 53% enhancement in capacity. Higher moisture levels (3% mol-H2O) appeared to be detrimental to the CO2 uptake. The highest capacity of 2.52 mmol/g was achieved at a temperature of 46 °C and a moisture level of 2% mol-H2O.

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A polyethylenimine impregnated silica sorbent was evaluated for steam-assisted temperature vacuum swing desorption (TVSD) of CO 2 , adsorbed from 420 ppm of CO 2 in N 2 . Results indicate that essentially all the CO 2 could be desorbed under mild vacuum levels (12−56 kPa abs) and temperatures (70−100 °C). The fastest average desorption rate (3.75 mmol/g/h) was observed at 12 kPa abs/100 °C at a steam flow rate of 6.2 g/h. In comparison, conventional TVSD under the same conditions only produced an average desorption rate of 0.23 mmol/g/h, confirming the superior kinetics of the steam-assisted process. When adsorption was carried out under humid conditions (1 and 2% mol-H 2 O), the co-adsorbed water on the sorbent was observed to slow down the desorption of CO 2 , due to the additional energy consumed for the desorption of water. The sorbent displayed excellent stability under the conditions studied, losing only 8% of its capacity after 50 cycles (>1500 h).

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Capturing CO2 directly from air is one of the options for mitigating the effects global climate change, and therefore determining its cost is of great interest. A process model was proposed and validated using laboratory results for adsorption/desorption of CO2, with a branched polyethyleneimine (PEI) loaded mesocellular foam (MCF) silica sorbent. The model was subjected to a Multi-Objective Optimization (MOO) to evaluate the technoeconomic feasibility of the process and to identify the operating conditions which yielded the lowest cost. The objectives of the MOO were to minimize the cost of CO2 capture based on a discounted cash flow analysis, while simultaneously maximizing the quantity of CO2 captured. This optimization identified the minimum cost of capture as 612 USD tonne−1 for dry air entering the process at 25 °C, and 657 USD tonne−1 for air at 22 °C and 39% relative humidity. The latter represents more realistic conditions which can be expected for subtropical climates. The cost of direct air capture could be reduced by ~42% if waste heat was utilized for the process, and by ~27% if the kinetics of the sorbent could be improved by a factor of two. A combination of both would allow cost reductions of ~54%.