Chemical and Biomolecular Engineering - Theses
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ItemSeparation of Hydrogen from Natural Gas Using Adsorption Technologies( 2023-05)There is growing global interest in hydrogen as an eco-friendly energy carrier due to the environmental concerns associated with global warming. One potential application is injecting hydrogen produced from various sources into the natural gas grid for efficient transportation and use. After co-transportation in the natural gas pipeline, either pure hydrogen or pure natural gas should be extracted from the mixture at desired locations for special end-users’ needs. This thesis deals with separation of hydrogen from natural gas mixtures to produce either high purity hydrogen or high purity natural gas for various applications. Natural gas is a complex mixture consisting of CH4, C2H6, CO2, N2, and trace amount of C3+ hydrocarbons. To produce high-purity hydrogen from a hydrogen + natural gas mixture, a separation technology is needed that not only operates at low capital and energy costs but also provides hydrogen of sufficient purity and recovery. A literature review on different gas separation technologies for deblending of low hydrogen concentration from natural gas mixture is presented. Adsorption technologies such as pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) have shown promising performance for gas separation due to design flexibility, high separation efficiency, low operational costs, and high-purity product. Accordingly, adsorption processes were selected for current separation. In this study, according to the natural gas network pressure levels in Australia, adsorption process performances were investigated over a range of feed pressures from 1 to 30 bar with different levels of feed hydrogen concentrations from 5 to 50%. First, to produce high purity hydrogen, a three-layered adsorption column was designed to capture specific groups of mixture components in each layer. Silica gel (to capture C3+), activated carbon (to capture CH4, C2H6, and CO2), and LiLSX zeolite (to capture N2) were chosen as the pre-layer, main-layer, and top-layer, respectively, based on the measured equilibrium isotherm data. Afterwards, a customized pressure swing adsorption (PSA) cycle was created and simulated using a six-bed PSA system with 12 steps. The PSA system achieved a high-purity hydrogen product (>99%) with a high recovery rate (>85%) for various hydrogen concentrations (5-30%) in a 30 bar feed stream. A comparison was made between the PSA system and an onsite electrolyzer for hydrogen production. The findings indicated that the PSA system was a viable alternative, even at low hydrogen recovery rates (40%), when the PSA plant is built at a pressure reduction station. Afterwards, in order to investigate the separation performance for low-pressure applications, vacuum swing adsorption (VSA) was investigated. The experiments were conducted using binary mixtures of H2 + CH4 in the lab. CH4 was the representative of natural gas in VSA experiments since it constitutes the major portion of natural gas. A four-bed VSA apparatus was filled with Norit RB4 activated carbon and achieved high purity hydrogen (>99%) through the VSA process for hydrogen feed concentrations of 30% and 50% at 102 kPa feed pressure. After validating the experimental results through simulations, a six-bed three-layered pressure vacuum swing adsorption (PVSA) system at a pilot scale was developed in Aspen Adsorption software to separate hydrogen from a representative natural gas mixture, and achieved high purity hydrogen (>99%) and high purity natural gas (>98%) simultaneously for different hydrogen concentrations (10 to 50%) in the feed at a feed pressure of 4 bar. An important outcome from these sets of experiments was that hydrogen could be replaced with helium in the low-pressure experiments showing similar adsorption behavior in vicinity of CH4, which is valuable for safer laboratory operations. The experimental and simulation results showed that as the H2 feed concentration and desorption vacuum pressure increased, the required power to achieve hydrogen purity above 99% decreased, and bed productivity increased with increasing H2 feed concentration. Since the H2 concentration in natural gas pipelines will be low at early implementation phases; in the final section, a double VSA system for industrial scale was developed and simulated using Aspen Adsorption to produce high hydrogen purity from a representative 10% hydrogen and natural gas mixture. The experiments were carried out in a laboratory using a four-bed VSA plant filled with activated carbon and a binary mixture of 10% H2 + CH4. After validating the simulation tool with the experimental results, Aspen Adsorption software was utilized to evaluate the performance of a double-stage VSA process at a small scale to achieve 99% hydrogen purity. The results revealed that upgrading H2 from 10% to 50% in the first stage and then to 99% H2 in the second stage leads to better separation performance and less energy consumption than a single-stage VSA for the same separation target. Subsequently, a five-bed double VSA system in industrial scale was developed to produce high hydrogen purity from a representative 10% hydrogen and natural gas mixture. The results showed that high purity hydrogen (>99%) and high recovery (>82%) were achieved at a feed pressure of 110 kPa by implementing higher vacuum (40 kPa) in the first stage and lower vacuum (10-20 kPa) in the second stage. These findings suggest that it is possible to achieve high purity hydrogen product from a low hydrogen feed concentration, 10% H2, using a double-stage VSA process with less energy consumption and higher recovery than a single-stage VSA. A range of future work was also recommended. A sensitivity analysis based on the level of different impurities available in natural gas like N2, CO2, CH4 and heavy hydrocarbons should be investigated. It is recommended to conduct high pressure PSA experiments at small scale using low H2 concentration in feed (10%-20%) to confirm the simulation results that achieving 99% H2 purity is possible using a single stage PSA. To determine the economic feasibility of implementing the technology on a large scale, a comprehensive economic analysis should be conducted. This analysis should consider both technical and financial factors. Once the evaluation is completed, an engineering map can be developed based on the findings. This map should indicate the areas where adsorption technologies such as PSA and VSA are economically viable. Finally, it is advisable to explore hybrid processes, such as combining membrane and PSA technologies, to produce ultra-pure hydrogen products as a potential solution to improve the overall efficiency of hydrogen production processes.
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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.
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ItemElectric field-controlled admission of guest molecules in microporous materials( 2022)Microporous materials (MPMs) are sponge-like solids with pore diameters less than 20 angstrom. They have been used in various industries and technologies, such as clean energy, healthcare, and environmental protection. Gas adsorption is one of the most important applications of MPMs and is the basis of gas separation, storage, and detection. Active regulation of pore accessibility in MPMs by external stimuli has aroused great attention in recent years. Numerous MPMs undergo structural changes in response to physical or chemical stimulation, such as guest accommodation, temperature change, light absorption, etc., which enables selective molecular adsorption. Compared to other stimuli, the electric field (E-field) can be a faster and more energy-efficient alternative. However, to date, the feasibility of using an E-field to regulate molecule adsorption in MPMs has been mainly limited to computational studies, while the experimental demonstration is still very rare. Here, for the first time, we demonstrate that the adsorption of small gas molecules in dielectric MPMs can be regulated by an external E-field. The adsorption capacity of CO2 in the metal-organic framework (MOF) MIL-53 was significantly reduced by applying an external direct current E-field gradient at adsorption. Gas separation selectivities of zeolite molecular sieves were also improved by pre-activating the zeolites with an external E-field. Our findings demonstrate the feasibility of regulating molecular adsorption in MPMs using E-fields. It proves that E-fields can be exploited to sharpen the molecular sieving capability and opens up new avenues for regulating pore accessibility in porous materials. This thesis comprehensively describes the E-field-controlled gas adsorptions in various MPMs: Chapter 1 is an extensive literature review on the gating effect of gas adsorption, which reflects existing approaches to regulate guest admission in MPMs. Chapter 2 describes the E-field-controlled CO2 adsorption in the flexible MOF MIL-53 (Al). The adsorption capacity of CO2 in MIL-53 (Al) was significantly reduced while that of NH2-MIL-53 (Al) changed insignificantly under a direct current E-field at the intensity of 286 V/mm. The Ab initio computational calculations revealed that the E-field decreased the charge transfer between the CO2 molecule and the adsorption site in the MIL-53 framework, which resulted in reduced binding energy and consequently lowered CO2 adsorption capacity. This effect was only observed in the narrow pore state MIL-53 (Al) but not in its large pore configuration. This work demonstrated the feasibility of regulating the adsorption of molecules in microporous materials using moderate E-fields. Chapter 3 demonstrates pre-activating zeolites in an external E-field can change the gas adsorption capacities and improve the gas separation selectivities. After the E-field activation, the potassium chabazite showed a higher adsorption capacity for CO2, but a lower one for CH4 and N2. The separation selectivities of CO2/CH4 and CO2/N2 were greatly improved by at least 25% after the E-field activation. Ab initio computational studies revealed the possible cation relocation in chabazites caused by the E-field, which led to the expansion of zeolite frameworks and created more favorable adsorption sites for CO2 molecules. The change of gas adsorption capacity after E-field activations was also demonstrated in zeolites TMA-Y and ZSM-25. These findings prove E-field can be exploited to sharpen the molecular sieving capability. Chapter 4 studies the feasibility to use E-field activation to improve the N2/CH4 selectivity in ZSM-25 zeolites. The E-field pre-activation of ZSM-25-K led to a gate-opening effect which significantly increased the CH4 adsorption capacity at low temperatures. It was attributed to the relocation of trapdoor potassium cations induced by the E-field. In contrast, the CH4 adsorption capacity of ZSM-25-Na was decreased after the E-field activation due to the framework expansion and the decreased heat of adsorption. The N2 adsorptions in both sodium and potassium types of ZSM-25 were remarkably improved, which partially resulted from the increase of N2 adsorption sites at the eight-membered rings. Consequently, the changes in the adsorption capacities after the E-field activation led to a higher CH4/N2 selectivity in ZSM-25-Na at the temperature range of 252-294 K and in ZSM-25-K at 294 K.