Chemical and Biomolecular Engineering - Theses
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ItemDevelopment of new pressure swing adsorption (PSA) cycles( 2023-03)Pressure swing adsorption (PSA) is an adsorption-based process, which has been widely used and studied for gas mixture separation due to its low investment and operating cost and high automation. Numerous novel concepts based on basic features of traditional PSA cycles, such as simulated moving bed (SMB)-PSA, dual reflux (DR)-PSA and layered PSA, have been demonstrated for targeting specific separation requirements and obtaining better separation performance. Dual reflux pressure swing adsorption (DR-PSA), as a state-of-the-art process, uses a lateral feed inlet and both light and heavy reflux strategy while keeping the basic features of conventional PSA cycles, achieving the separation performance beyond the so-called separation limitations constrained by pressure ratio. However, there are still some typical problems of DR-PSA to be solved. The main objectives of this study are to develop new cycles to overcome some key problems of the DR-PSA process. This study is divided into three main sections, 1) parametric study of a dynamic-feed DR-PSA process for capturing dilute methane (CH4) from nitrogen (N2) gas; 2) development of a triple-reflux pressure swing adsorption (TR-PSA) for separating methane-nitrogen-helium ternary gas mixtures; 3) development of the dual reflux vacuum swing adsorption (DR-VSA) process for enrichment of low-grade CH4 which falls into the explosion range and demonstration of a new dual-objective optimization method. In the first section, it can be concluded that the dynamic-feed strategy can practically solve the mixing problem caused by the lateral feed inlet of the DR-PSA process and the performance of dynamic-feed DR-PSA elevates with the number of feed inlet positions along the column. Parametric studies are conducted based on three key operating parameters, heavy to feed flow ratio, light reflux flowrate and feed/purge step time, indicating that the dynamic-feed DR-PSA can always obtain both higher purity and recovery over the traditional DR-PSA process under same operating conditions; a typical comparison of separation results between two cycles is 53.5% vs. 47.5% for purity and 81.1% vs. 72.2% for recovery, respectively. In the second section, a triple-reflux (TR)-PSA process is demonstrated to separate a ternary gas mixture composing of 10% helium (He), 20% CH4 and 70% N2 via a single-stage process. The TR-PSA process can enrich 10% He up to 45.3 % with a recovery of 91.3% while achieving 60.0 % purity and 90.4% recovery for CH4 and 95.8 % purity and 68.9% recovery for N2 with a work duty of 49.6 kJ mol-1 (feed). The TR-PSA process can obtain slightly better product purity and recovery of He (45.3% purity with a recovery of 91.3 %) than the 2-stage DR-PSA process (purity is 44.8% and recovery is 89.7%) while leading to a mild decline in CH4 purity and recovery (1.4% and 1.7%, respectively) compared to the two-stage DR-PSA. TR-PSA process only requires a cycle work of 49.6 kJ/mol (feed), which is significantly (30%) lower than the specific work of the 2-stage DR-PSA process (64.3 kJ/mol (feed)) due to the use of one compressor in TR-PSA versus two in the 2-stage DR-PSA system. In the last section, vacuum swing adsorption (VSA) is integrated with the dual reflux strategy as the DR-VSA cycle which can be operated within a pressure range lower than the atmospheric pressure and avoid the usage of a compressor. Two types of DR-VSA cycles (A- or B-type cycle indicates that pressure reversal step is carried out through the heavy or light end, respectively) have been studied for enriching low-grade CH4 (CH4 molar fraction is between 5–20%) gas using activated carbon (AC) or ionic liquidic zeolites (ILZ). The optimal configuration is A-type cycle packing with ILZ adsorbents, which can enrich 20% CH4 to a purity of 80.2% with 95.5% recovery and a specific energy consumption of 180.8 kJ/(mol CH4 captured). The final optimal results achieve a good balance between purity and recovery by adopting the new optimization method which uses a dual convergence algorithm to iteratively vary operating conditions and a multiplicative score function to evaluate the separation performance of each case.