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.