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earlier2 for laminar flow in the monolith channels and De is effective diffusivity. After estimation of the mass transfer coefficient k900, it was then employed in the simulations to model the breakthrough front of the ideal 900 cpsi adsorbent, see Figure 10 (b). Process cycle time in semi-batch type processes such as PSA and VSA is an important parameter determining the adsorbent volume and total size of the adsorber. Shorter cycles on one hand give rise to higher throughput for the same amount of adsorbent; on the other hand, too short cycles may lead to insufficient time for adsorption due to mass transfer limitations. To investigate the impact of adsorbent geometry on VSA performance at different cycle times, a series of simulations were conducted for a feed temperature of 27.7 °C and a feed pressure of 1.2 bar for a 3-step cycle (Figure 1 (a)). The product purity is calculated as the ratio of CO2 to total flow obtained in the desorption gas during step 2 of the cycle. Figure 11 shows that the product purity obtained with the packed beds is higher than for the 900 and 400 cpsi structured adsorbents. This can be attributed to the higher voidage and lower zeolite loadings in the monolith structures. The channel voids in the structured adsorbents lead to significant contamination of the product with N2. Figure 11 also shows that for all cases, a plateau value of product purity is reached by increased cycle times. This is due to the reduction in gas velocity resulting in a sharper mass transfer front and hence improved CO2 purity. The product purity for all adsorbents approach feed purity (10%) as the cycle time is decreased. When cycle time is decreased below 120 s, the CO2 purity undergoes a gradual decline in the case of monoliths whereas a much larger decrease is experienced for the packed bed. This reduction is due to pressure drop in the packed bed which leads to broadening of the mass transfer front and hence contamination of the top of the bed. Product recovery is defined as the total amount of CO2 recovered during step 2 (evacuation step) to the total CO2 amount fed to the system. Figure 12 shows that the 125PDF Image | Structured Zeolite Adsorbents for PSA Applications
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