Pressure Equalization Step Research Articles

Hydrogen production from low pressure coke oven gas by vacuum pressure swing adsorption

Hydrogen is a major component of coke oven gas (COG) and can be used in gas-based direct reduction iron processes to help the steel industry achieve the carbon peaking and carbon neutrality goals. However, the high compression power consumption is the main drawback of the current PSA cycle since the COG needs to be compressed from atmospheric pressure to ∼18 bar to achieve a hydrogen production with purity >99.9% and recovery < 85%. Here in this study, we experimentally demonstrate by using a 4-bed 12-step lab-scale VPSA rig filled with activated carbon RB3 and can obtain a hydrogen product with a purity of 99.67% and a recovery of 91.97% at a feed pressure of only 5.17 bar. We then constructed a non-isothermal non-adiabatic VPSA model using our in-house code and validated the model against experimental breakthrough and VPSA cycles. Parametric optimization produced a H2 product of 99.995% purity and 92.29% recovery at a slightly increased adsorption pressure of 6 bar and a vacuum pressure of 0.02 bar with 10.21 mol.H2/kg.ads/hr productivity and 7.09 kJ/mole.H2 specific power. We also confirmed that light product purge at vacuum pressure can effectively improve the H2 purity and the gas flow direction of the pressure equalization step has a significant effect on the mass transfer front. Our research presents the success of low-pressure hydrogen production from COG, which will greatly accelerate the transformation of COG utilization and promote the decarbonization of the iron and steel industry.

Experimental performance analysis of vacuum pressure swing adsorption air separation process under plateau special conditions

ABSTRACT A small scale (~270 Nm3·h−1) vacuum pressure swing adsorption (VPSA) air separation system was established in Qinghai Province (in China) with an altitude of 3800 m, which was used to supply oxygen for relieving the hypoxia. The effects of adsorption and desorption time, purge and pressure equalization step and feed temperature on the performance of the process were investigated by performing experiments under plateau special conditions. Results showed that pressure equalization and intermittent purge steps could effectively improve the performance of the VPSA process. The system could produce 270 Nm3·h−1 of 90% O2 with 62.6% oxygen recovery from compressed air. The maximum productivity and minimum energy consumption with pressure ratio of 160:42 for ~90% O2 were 121.7 L·h−1·kg−1 and 0.55 kWh·Nm−3, respectively.

Design of an industrial multi-bed (V)PSA unit for argon concentration

Pressure swing adsorption systems have been devised to concentrate argon from a binary gas mixture of oxygen and argon (O2:Ar = 95:5 mol%) that an industrial oxygen generation vacuum swing adsorption (VSA) unit produces from air. The kinetically-driven adsorption processes investigated in this study contain a self-purging step and up to two double-ended pressure equalization steps. Three adsorption cycle configurations, each of which has one, two and three beds, were simulated. The effects of pressure ranges of the adsorption cycle, either pressure swing adsorption (PSA, 1–3 bar) or vacuum swing adsorption (VSA, 0.1–1 bar), as well as adsorption step time were extensively assessed with respect to the following separation key performance indicators (KPIs): argon purity, argon recovery, bed productivity and specific energy consumption. It turned out that argon purity and recovery could be significantly improved in the VSA cycles at the expense of bed productivity and energy consumption. The single VSA unit could not concentrate argon up to a purity of 98+% which is typically required for certain applications such as steel production and inert gas welding. Thus, a second VSA unit was added to increase further the argon purity and it was found that the integrated two-stage VSA system is capable of achieving the following overall performances: argon purity of 98.1%, argon recovery of 20.3%, bed productivity of 0.011 molAr kgads−1h−1 and specific energy consumption of 53.2 MJ kgAr−1. Considering real efficiencies of turbomachinery the energy consumption of the proposed VSA unit resulted 75% higher than that of a conventional stand-alone cryogenic distillation system designed to achieve the same separation. However, the VSA technology is expected to be a more attractive option than the cryogenic process in terms of CAPEX.

Optimization of pressure swing adsorption for hydrogen purification based on Box-Behnken design method

Pressure swing adsorption (PSA) is an important technology for mixture gas separation and purification. In this work, a dynamic model for a layered adsorption bed packed with activated carbon and zeolite 5A was developed and validated to study the PSA process. The model was validated by calculating breakthrough curves of a five-component gas mixture (H2/CH4/CO/N2/CO2 = 56.4/26.6/8.4/5.5/3.1 mol%) and comparing the results with available experimental data. The purification performance of six-step layered bed PSA cycle was studied using the model. In order to optimize the cycle, the Box-Behnken design (BBD) method was used, as implemented in Design Expert™. The parametric study showed that, for adsorption step durations ranging from 160 to 200 s, as the adsorption time increased, the purity decreased, whereas the recovery and productivity increased. During the pressure equalization step, the purity increased as the pressure equalization time increased, but the recovery and productivity decreased for step durations ranging from 10 to 30 s. As the P/F ratio (hydrogen used in purge step to hydrogen fed in adsorption step) increased from 0.05 to 0.125, the purity increased, whereas the recovery and productivity decreased. The optimization of the layered bed PSA process by the BBD method was then performed. In addition to the adsorption time, the pressure equalization time and the P/F ratio were considered as independent optimization parameters. Quadratic regression equations were then obtained for three responses of the system, namely purity, recovery, and productivity. When purity is set as the main performance indicator, the following values were obtained for the optimization parameters: an adsorption time of 168 s, a pressure equalization time of 14 s, and a P/F ratio of 0.11. Under those conditions, the system achieved a purity of 99.99%, a recovery of 57.76%, and a productivity of 6.41 mol/(kg·h).

Parallel and series multi-bed pressure swing adsorption processes for H2 recovery from a lean hydrogen mixture

The demand for clean energy sources has made H2 recovery from various lean hydrogen mixtures increasingly attractive. In this study, parallel and series pressure swing adsorption (PSA) processes were investigated experimentally and theoretically, and > 99% pure H2 was produced from a lean hydrogen mixture (H2:CO:N2:CO2 = 19.9:0.1:44.6:35.4 mol%) at 10 bar. A mathematical model for a PSA process using activated carbon and zeolite 13X was simultaneously validated with results from breakthrough experiments and a parallel two-bed PSA process. The parallel two-bed PSA process using a layered bed (lower bed: activated carbon, upper bed: zeolite 13X) experimentally produced H2 with a purity of 94.6–98.3% and a recovery of 33.5–63.2%; CO was not detected in the H2 product. In the parallel four-bed PSA process, the H2 recovery was drastically increased to 77.3% due to an additional pressure equalization step, but the increase in H2 purity was minute. The series PSA process, which was divided into the bulk separator and the purifier, was theoretically studied under various operating conditions. The series three- and four-bed PSA processes could produce H2 with over > 99% purity and a recovery of 62.478% and 82.643%, respectively, due to the additional pressure equalization step and the utilization of blowdown gas. The parallel four-bed PSA process showed the highest H2 productivity (33.58 molH2 kgads−1 day−1), while the series four-bed PSA process achieved an H2 productivity of 23.96 molH2 kgads−1 day−1 with > 99% H2 purity.

Enrichment of low grade CH4 from N2/CH4 mixtures using vacuum swing adsorption with activated carbon

Methane (CH4) is a cleaner fuel resource with relatively low carbon emissions and negligible particulate matters, heavy metals, and sulphur oxides, compared with those emission intensive fuels like coal. However, large amounts of CH4 emitted to the atmosphere drive significant greenhouse gas (GHG) effects due to the low value of its low concentration sources and technical difficulty of CH4/N2 separation. Therefore, it becomes crucial to advance the technology for separating low concentration CH4 from N2. In this study, a commercial activated carbon was used to separate CH4 from N2 with a 4-column vacuum swing adsorption (VSA) facility. Both light and heavy streams were firstly collected and stored in gas tanks, and then used for light/heavy purge. Four process modes involving A) 2-column and 8 steps, B) 3-column and 9 steps, C) 4-column and 16 steps, and D) 4-column and 20 steps were employed to investigate the effects of process design factors on their separation performance. Results showed that both pressure equalization and heavy product purge steps have significant impacts on product purity, recovery and productivity. In the case of 11.7 and 17.0% (CH4) feed gases, the methane products reached a purity around/above 30% and these gases can be compressed and transported for industrial utilization. Furthermore, the recovery of these low concentration methane gases will bring extra incentives with GHG mitigation benefits.

Conceptual design of adsorptive recovery of ethane from residue gas stream using Na-ETS-10

This work provides the conceptual design of a pressure/vacuum swing adsorption cycle for recovering ethane from the residue gas stream (a mostly methane stream) using Na-ETS-10 as the adsorbent. The conceptual design for treating a stream with 2.4% ethane (and the rest methane) evolved from a simple 4-step cycle to more complex configurations which were able to simultaneously improve the ethane purity and recovery. Feed temperature played a key role in the cycle performance in terms of purity and recovery. The minimization of the recompression work of the light product (methane) was achieved by proposing a cycle design which includes a heavy reflux step at high pressure and a further pressure equalization step. Purity values up to 88% ethane with recoveries of 95% and productivities higher than 0.4 mol ethane/m3(ads)s were quantified through the optimization of the operating conditions for the reduced energy-cost adsorption cycle suggesting an alternative for ethane recovery in gas processing plants.

A parametric study of process design and cycle configurations for pre-combustion PSA applied to NGCC power plants

Natural gas combined cycle (NGCC) power plants show favourable conditions for the implementation of pressure swing adsorption (PSA) for the capture of carbon dioxide. These plants also show the advantage of a hydrogen co-production system. The challenge when implementing PSA in these plants is to achieve reference configurations that can obtain both products (hydrogen and carbon dioxide) at high purity levels, maximizing the recovery of hydrogen as a valuable product. This study presents the scale-up of a previously reported laboratory-based, four-bed, seven-step PSA model and a parametric study of the scaled-up PSA variables to maximize the product performance parameters. The capacity of the PSA model is based on the flow rate requirements of a GE-10 gas turbine, which can operate with up to 95% hydrogen purity. A parametric study using global system analysis (GSA) showed the effect of the bed diameter, length-to-diameter ratio and purge-to-feed flow rate ratio upon the product performance parameters. A purity of carbon dioxide of 95.37% and a hydrogen recovery of 92.27% was obtained with a purge-to-feed flow rate ratio of 0.22. The purity of hydrogen stayed close to 99.99%, with maximum deviations around 0.0001% for all case studies. The purity of the carbon dioxide and the recovery of hydrogen were further improved by considering additional PSA configurations. The addition of an assisted purge step and three pressure equalization steps improved these performance parameters by two percentage points. Overall, the model with one pressure equalization step, assisted purge step and rinse step after the feed step showed promising results with a purity of carbon dioxide of 98.28% and hydrogen recovery of 95.48%. Lower capitals costs are expected for this configuration, compared to adding pressure equalization steps using more than four fixed-bed units.

Cycle design and optimization of pressure swing adsorption cycles for pre-combustion CO2 capture

Novel pressure-swing adsorption (PSA) cycles were developed based on patented TDA AMS-19 (activated carbon) adsorbent for pre-combustion CO2 capture in integrated gasification combined cycle (IGCC) power plants. A variety of cycles comprising of counter-current blowdown, pressure equalization, steam purge and light product pressurization steps were designed and simulated using an in-house one dimensional detailed model. Full process optimization studies were performed for all cycles to evaluate their feasibility for pre-combustion CO2 capture. The CO2 purity and recovery Pareto fronts obtained using the multi-objective optimization were used to assess their ability to simultaneously achieve high CO2 purity (>95%) and recovery (>90%). The cycles that achieved the purity-recovery (95–90%) requirements were subjected to energy-productivity optimizations under the constraints of CO2 purity and recovery. Three cycle designs were ranked in terms of lowest energy consumption at 95% CO2 purities and 90% CO2 recoveries. It was found that a 10-step cycle with three pressure equalization steps achieved a minimum energy consumption of 95.7 kWhe/tonne of CO2 captured at a productivity of 3.3 mol CO2 captured/m3 adsorbent/s.

A numerical Study of Single Column Adsorber for Optimal Design of Pressure Swing Adsorption

Four different configurations of pressure swing adsorption (PSA) were designed numerically using Matlab software. The PSA system was applied to separate oxygen from air. The software was used first to validate previous experimental results of single bed adsorber. The results showed that the breakthrough time with product pressurizing is about 1000 s and mass transfer zone (MTZ) is steeper, while with the air pressurizing happened within 60s and MTZ was very wide. High purity of oxygen (>90%) was observed with the first and fourth configuration. Purity of about 88% was achieved with the third configuration, and low purity (75%) with the second configuration. In addition, high power consumption was indicated by the first, and second configurations. However, the third and fourth configurations consumed low power due to the pressure equalization step. Based on the results, the more economic and effective design is the fourth configuration.

Improved method to converge pressure equalization steps when simulating a cyclic adsorption process

Two new techniques for single-vessel Pressure or Vacuum Swing Adsorption (P-VSA) mathematical models are presented that improve convergence stability to Cyclic Steady State (CSS) when Pressure Equalization (PE) steps are involved.The first method, the Reflected Equalization Pressure (REP) method uses simplifying assumptions to derive a simple and efficient analytical solution for the unknown pressure profile boundary condition (BC) to use during PE steps on the first simulated cycle of a P-VSA simulation without the need for user-defined functions or additional ODEs.The second method, the Weighted Store-and-Retrieve (WSR) method is a variation on the Traditional Store-and-Retrieve (TSR) method for simulating PE steps where gases flow from one vessel to another in a single-vessel mathematical model. The WSR method retains history across several cycles, passing this back into the mathematical model via weighting factors to average the data and dampen the oscillatory response of a mathematical model to PE steps.Several examples of this technique are applied to the O2 VSA and H2 PSA systems.

Evaluation of simplified pressure swing adsorption cycles for bio-methane production

Pressure swing adsorption (PSA) is a mature technique for biogas upgrading. However, it constitutes the most expensive step to obtain fuel-quality bio-methane, particularly in small-scale units. To reduce the cost of upgrading in small-scale plants, we have evaluated different PSA cycles with two and three columns (less than commercial units). An equalization tank was used to perform one asynchronous pressure equalization step and keep the process fed continuous even in the case of using two columns. The effect of the purge step was also evaluated. Carbon molecular sieve was used as adsorbent. The feed composition was 40% CO2/60% CH4 and pressure swinging between 5 and 0.1 bar in the adsorption and blowdown steps, respectively. Four performance indicators (methane purity and recovery, productivity and energy consumption) were used to evaluate the PSA cycles. The mathematical model could predict the experimental PSA performance. Bio-methane with purity higher than 97.5% (specification) and recovery higher than 90% was obtained experimentally using a PSA with two columns and an equalization tank. When a third column is used (implementing an additional pressure equalization), the recovery increases in approx. 4% showing the importance of pressure equalization to reduce the methane slip.

Performance analysis of an eight-layered bed PSA process for H2 recovery from IGCC with pre-combustion carbon capture

Integrated gasification combined cycle (IGCC) plants that are also capable of CO2 capture have received significant attention as the next generation of coal-based power plants for the co-production of H2 and electrical power. Accordingly, for a cost-effective and environmentally friendly poly-generation IGCC process, efficient techniques need to be developed to recover H2 from the syngas of an IGCC plant with carbon capture.In this study, an eight-layered bed pressure swing adsorption (PSA) process using activated carbon and zeolite simultaneously was developed to produce high purity H2 from the H2-rich syngas of an IGCC plant. As a first step, the separation performance was compared between four-and eight-layered bed PSA processes. The eight-layered bed PSA process led to higher recovery of H2 at the condition of a similar H2 purity owing to a greater number of pressure equalization steps in the operational step configuration. It is noteworthy that the productivity could be greatly improved as the purge gas was replaced from the product gas to residual gas in a bed. When the adsorption bed was purged by using the residual gas after the first pressure equalization step, recovery improved by about 3 ∼ 6% at the condition of 99.99+mol% H2 purity in comparison with the product-purge PSA configuration. On the other hand, the highest H2 recovery that could be obtained for the requirement of 99.99mol% H2 purity, was ∼ 89.7% from the eight-layered bed H2 PSA process when the purge gas was provided from the residual gas of the adsorption bed after the last depressurization pressure equalization step. However, as the concentration of CO in the desired H2 product was higher in the PSA configuration when using residual gas compared to the product gas, the operational configuration of PSA needed to be decided by the desired H2 purity and impurity constraints for application. Furthermore, the tail gas from the PSA contained 45–66mol% of H2 and CO, depending on the applied PSA configurations, and could be used to drive a gas turbine without any loss of the syngas, even though the recompression energy loss required evaluation.

Modeling and Simulation of Pressure Equalization Step Between a Packed Bed and an Empty Tank in Pressure Swing Adsorption Cycles

Background: It is of paramount importance to pay a great attention when modeling pressure equalization step of pressure swing adsorption cycles for its notable effect on the accurate prediction of the whole cycle performances. Studies devoted to pressure equalization between an adsorption bed and a tank have been lacking in the literature. Many factors could affect the accuracy of the dynamic simulation of pressure equalization between a bed and an empty tank. Methods: The method used for the equilibrium pressure evaluation has a significant impact on simulation results. The exact equilibrium pressure (Peq) obtained when connecting an adsorption column and an empty tank could only be obtained by numerical simulation given the complexity of the set of partial differential equations. Results: It has been shown that, with some simplifying assumptions, one can analytically determine Peq with satisfactory precision. The analytical solution proposed permits to assess rapidly the equilibrium pressure and the equilibrium mole fraction of the adsorbable species in the tank (Yeq) and without the need to resort to a cubersome modeling. Conclusion: Based on the comparisons presented, one can conclude that the agreement between the experimental and numerical results relative to Peq and Yeq is very satisfactory.

A New Multi-bed Vacuum Swing Adsorption Cycle for CO2 Capture from Flue Gas Streams

Adsorption Processes for CO2 capture have been extensively researched within the last decade and have been piloted at many field installations and in research laboratories. Many reported VSA cycle designs for CO2 capture are based on single- and dual- adsorbent beds. The performance of such cycles is typically poor due to the lack of room to incorporate more relevant auxiliary intermediate process steps such as pressure equalization, co-current purge, etc. Very low vacuum pressures (< 3kPa) are therefore necessary to achieve higher CO2 purity. This is difficult and expensive to provide at large scale. To address this concern, a new 4-bed PVSA cycle has been developed in our laboratory and tested experimentally and through simulation. The cycle configuration provides a means to internally recycle parts of the product and CO2-lean gases to enhance the separation efficiency and also to minimize energy consumption. A dry gas mixture of 15%CO2 in N2 was used as the feed gas and zeolite 13x as the adsorbent. A product purity of 92.4% was obtained at 64% recovery at 8 kPa desorption, while 5 kPa desorption produced >75% recovery with a purity of > 95%CO2. The calculated specific power consumption is approximately 0.33 MJ/kgCO2. Including two pressure equalization steps after a product rinse step contributed to the relatively lower specific power consumption achieved.

Cycle Development and Process Optimization of High-Purity Oxygen Production Using Silver-Exchanged Titanosilicates

Producing oxygen with purity higher than 95.0% from atmospheric air (78.0% N2, 21.0% O2, and 1.0% Ar) is challenging because of the similar physical properties of oxygen and argon. Silver-exchanged titanosilicates have shown the potential to separate these gases based on their thermodynamic affinities. In this study, various vacuum swing adsorption (VSA) cycle configurations including the simple Skarstrom cycle and more complicated 6-step VSA cycles were simulated using mathematical models to maximize O2 purity and recovery. The simulations were verified by conducting simple 3-step and Skarstrom VSA cycle experiments. A mixture of 95.0%/5.0% O2/Ar feed was considered in the simulations, and a rigorous multiobjective optimization was conducted to maximize O2 purity and recovery. The simulations predicted 27.3% recovery for a product with 99.5% purity for a 6-step cycle with pressure equalization and light product pressurization steps. The recovery for the same level of purity was improved significantly to ...

Dehydration of Ethanol by PSA Process with Pressure Equalization Step Added

AbstractIn this work, the ethanol dehydration production process is carried out using the Mathematical Modeling Pressure Adsorption Process. A new model is suggested, it has two equalization steps, and is compared with the Industrial Pressure Swing Process operating cycle. An analysis of the effects of introducing the pressure equalization step is performed on four main response variables: purity, production, recovery and energy consumption and it is compared with the current cycle configuration operating in the industry. We used Aspen Adsorption for the valuation and simulation of the cyclic PSA process. We analyzed and processed the simulation results in Statgraphics Centurion to obtain optimum operating conditions for the process. This evaluation shows that purity decreases slightly, whereas recovery and production increase. The most important thing is that the energy consumption is reduced. These results clearly show that by modifying the operating cycle schema, optimum operating conditions also change. The optimization of the new cycle was executed considering as variables bed pressure, adsorption time and purging flow. We found that a smaller column is more productive for the equalization cycle than that of a 14m bed, which is optimal in the industrial cycle with a consequent reduction in adsorbent material.

CO2 capture by vacuum swing adsorption: role of multiple pressure equalization steps

The performance of a 4-bed/16-step vacuum swing adsorption cycle containing three pressure equalization (PE) steps has been analysed in order to understand the role played by the multiple PE steps in the process performance. The cycle was designed for CO2 capture from a feed gas mixture of 15 %CO2/85 %N2, with zeolite ×13 adsorbent from UOP (PSO2HP). Simulations were performed with the help of the commercial Aspen Adsorption simulator to help interpret the experimental results. It was found that CO2 loading decreased only slightly, but N2 loading decreased significantly and uniformly across the bed after each PE step. Thus, while CO2 working capacity remained almost constant, working selectivity and CO2 product purity increased with the number of PE steps. An experimental purity of 91.3 mol% CO2 could be obtained at a recovery of 77 % at 3 kPa desorption pressure, with a cycle containing 3 pressure equalisation steps. Specific energy consumption (calculated with a constant pump efficiency of 70 %) was calculated as 0.3 MJ/kg CO2, which was lower than the 1 and 2 pressure equalisation cycles. We evaluated 2-bed and 3-bed cycles containing one and two pressure equalisation steps respectively, by means of simulation in order to compare their performance with the base 4-bed 3PE cycle. For a constant recovery of 75–77 %, CO2 product purities increase by 7.4 and 4.2 % (relative) in going from 1PE, to 2PE and 3PE cycles respectively, at an evacuation pressure of 3 kPa. Specific energy consumption also decreased with the number of PE steps, owing to the lowering of the starting pressure for desorption and some savings in repressurization energy with the number of PE steps. The specific energy dropped by 13 % in going from 1PE to 2PE and 3PE steps. However, the extra beds and extra cycle time required for the 3PE steps led to a reduction in productivity by almost 33 % in going from the 2PE to 3PE cycles. The choice for including additional PE steps therefore relies on the tradeoff of capital and operating costs which is strongly location and project specific.

The effect of number of pressure equalization steps on the performance of pressure swing adsorption process

Cycle sequence has an important effect on the performance of pressure swing adsorption (PSA) processes. Pressure equalization steps influence significantly the purity and recovery of product, and therefore, may be properly designed to improve the performance of PSA processes. Open literature lacks of a systematic study on the effect of cycle sequence design on the performance of a specific PSA process as a controlling parameter. In this work, the results of recent studies on different cycle schedule design strategies have been used as a basis for comparing various cycle schedules (proposed by the authors of this work) on the performance of a six-bed PSA process for hydrogen purification. Three different cycle sequences have been designed, the pressure equalization and idle steps consisting the main controlling parameters. Simulation results showed that designs with more pressure equalization steps result in higher product recovery and those with less pressure equalization steps result in higher product purity. The proper performance of a PSA process is a tradeoff between product recovery and product purity. In this view, a target function has been developed that enables us to lump the latter performance parameters into one function for comparing the performance of the different cycles employed.

The experimental study of a new pressure equalization step in the pressure swing adsorption cycle of a portable oxygen concentrator

For portable oxygen concentrator by pressure swing adsorption (PSA) method, its volume, mass, power, oxygen flux and oxygen saving efficiency are the most important parameters which are affected strongly by the PSA cycle. In this paper, we propose a new pressure equalization step to optimize the PSA cycle. According to the experimental results, when the product ends of two beds are connected and the feed gas is switched from the high pressure bed to the low pressure bed during the pressure equalization step, the system has a larger oxygen flux, a less energy consumption and a more simple structure.