Dual-reflux Pressure Swing Adsorption Research Articles

Direct helium recovery from natural gas by dual reflux pressure swing adsorption cascade

Low-cost helium recovery processes optimised for unconventional sources e.g. small-scale natural gas (NG) reservoirs, offer a new approach to meeting growing demand. We investigated a two-stage dual reflux pressure swing adsorption (DR PSA) cascade for the recovery and purification of dilute helium from various natural gas source analogues comprising of CH4 + N2 + He. First, adsorption isotherms of a binderless zeolite 13X with high methane and nitrogen uptake were measured at 283 to 323 K and pressures up to 10 bar. This material was then used in a laboratory-scale DR PSA apparatus to identify key parameters affecting the separation performance, namely feed step time and location, as well as heavy product and light reflux flow rate. The experiments agreed well with the results of a non-isothermal numerical DR PSA model with an average deviation of 1 mol% between simulated and experimental product purities. Then, the model was extended to consider a two-stage DR PSA cascade with waste recycle. Axial composition profiles within the beds of each stage were obtained numerically and analysed to explain the impact of key process parameters on the separation performance. Tuning these parameters accordingly led to a helium-rich product with over 99 mol% He purity, which could be produced from various natural gas feeds containing from 0.1 to 1 mol% He at 90 to 98% recovery, respectively. The separation performance was compared to other helium recovery processes based on cryogenic distillation, membranes, pressure swing adsorption, and combinations thereof. Compared to these other systems, the DR PSA cascade process delivered a 99 mol% purity product from a 0.1 mol% helium in natural gas feed with 8% higher helium recovery.

Effect of pressure ratio on the separation performances of different DRPSA configurations under equilibrium conditions

Dual reflux pressure swing adsorption is a peculiar application of pressure swing adsorption with relevant separation potential. In a proper range of operating parameters, high separation performances are achieved in many applications, including complete separation in the case of binary mixtures. In this work, a new representation of the design parameters suitable for complete separation based on the semi-analytical solution of the corresponding Equilibrium Theory model is proposed for the four basic process configurations. Namely, given feed position, feed composition, and adsorbent separation selectivity, the combinations of the remaining process parameters ensuring complete separation are identified in a 3D plot. Furthermore, the same model equations have been solved numerically to explore conditions of incomplete separation, where the semi-analytical solution is not available. In particular, the sensitivity of the separation performances of each configuration to the pressure ratio has been explored. More specifically, given the region of operating conditions suitable for complete separation, selected operating conditions outside this region have been explored aimed to recover/improve the separation quality. Even though the different configurations exhibit different behaviors, the general dependence of the product purity upon the pressure ratio is quite limited and only minor improvements can be obtained with a few exceptions.

Helium recovery and purification by dual reflux pressure swing adsorption

Global demand for helium has risen over the past few decades driven by a vast range of applications based on its unique properties. Helium is often produced industrially from the vent streams of nitrogen rejection units (NRU) in liquefied natural gas (LNG) plants, where it accumulates because of its low boiling point. Further deep cryogenic processing stages are then required to upgrade its concentration to saleable levels. We report here an experimental and simulation-based investigation of an alternative, non-cryogenic process based on dual reflux pressure swing adsorption (DR PSA) cycles to recover and purify helium from model binaries including a 1 mol% He + 99 mol% N2 mixture representative of NRU vent streams. Binderfree zeolite 13X was used as the adsorbent in an experimental campaign employing a laboratory-scale DR PSA apparatus. The experimental runs validated a non-isothermal numerical model with an average deviation of 1 mol% between the simulated and experimental product compositions. The single-stage DR PSA experiments produced helium product streams with purities ranging from (30 to 99) mol% from feeds with concentrations ranging from (1 to 50) mol% He. Two DR PSA stages were required to upgrade mixtures containing 1 mol% He to a target product purity of > 99 mol%. A cascade arrangement of two DR PSA systems with a waste recycle enabled a 99.999 mol% He purity product at 95 % recovery, which is competitive with conventional cryogenic systems. This cascade DR PSA process used either two or three compressors depending on the feed gas pressure, with associated duty costs of 1.5 MJ∙(mol He produced)–1. This is lower than several membrane processes described in the literature and could be more cost-effective for smaller-scale applications than cryogenic processes with similar levels of separation performance.

Separation of He/N2/CH4 ternary mixtures by a triple‐reflux pressure swing adsorption process

AbstractConventional pressure swing adsorption (PSA) processes can only produce one high purity product in a single stage, whereas the state‐of‐art dual‐reflux PSA (DR‐PSA) can produce two high purity products simultaneously. However, multicomponent gas separation is often required in the industry, targeting at recovering several valued products at the same time. In this study, we propose a novel adsorption process, namely triple‐reflux PSA (TR‐PSA), to separate three components simultaneously. A middle product outlet and a middle reflux stream were introduced to the adsorption columns of a conventional DR‐PSA process to separate ternary mixtures of nitrogen, methane, and helium. Nonisothermal dynamic models were built to investigate the impacts of operating parameters particularly the location of the middle reflux/product stream and the middle reflux flow rates. Results showed that the TR‐PSA process successfully separated ternary mixtures obtaining three enriched products simultaneously in a single stage, yielding a separation performance comparable to that of the double‐stage DR‐PSA with significantly lower capital and energy cost.

An improved vacuum pressure swing adsorption process with the simulated moving bed operation mode for CH4/N2 separation to produce high-purity methane

Methane is a clean fossil fuel and an important greenhouse gas, so it is more significant to recover methane from massive resources of unconventional gas. Because of the similar physicochemical properties of methane and nitrogen, it is difficult to obtain the high-purity methane from the low-grade methane gas with nitrogen mixture by the conventional vacuum pressure swing adsorption (VPSA) process. In this work, this process was improved, where multicolumn in series operation was adopted at both adsorption and displacement steps to enhance the CH4/N2 separation, and one column went on blowdown, purge, and pressurization steps for the effective regeneration of the adsorbent. In order to run the improved VPSA process automaticly, the SMB operation mode was introduced, synchronously shifting the inlet and outlet stream ports from one column to other column along the gas flow direction, and a countercurrent contact between the gas phase and adsorbent phase in the multicolumn system was formed to enhance the CH4/N2 separation. To verify the feasibility of the improved VPSA process, the experimental device with eight-column unit had been set up in laboratory, and 30% ~ 50% CH4 gases were denitrified by the improved VPSA process using activated carbon beads. The experimental results demonstrated that the improved VPSA process was capable of producing a high-purity CH4 product gas (99.99% CH4) meeting pipeline grid specification and a high-concentration N2 by-product gas meeting emission standards from 30% ~ 50% CH4 feed gases. In the end, we compared the CH4/N2 separation performance among the conventional VPSA process, dual reflux-pressure swing adsorption process, improved VPSA process with SMB operation mode, CO2 displacement VPSA process and concentration temperature swing adsorption process.

Simulation and analysis of dual-reflux pressure swing adsorption using silica gel for blue coal gas initial separation

A dual-reflux pressure swing adsorption (DR-PSA) process was proposed and simulated to initially separate the blue coal gas, aiming to capture carbon dioxide (CO2) and enrich hydrogen (H2), simultaneously. With a feed flow rate of 7.290 slpm, a light product reflux flow rate of 0.505 slpm and the heavy product reflux flow rate of 3.68 slpm, the developed DR-PSA process could capture CO2 up to 64.01% with a recovery of 99.60% and enrich H2 up to 34.66% with a recovery of 97.63% from the blue coal gas (36.2% N2/28.5% H2/13.9% CO/12.7% CO2/8.7% CH4). In addition, in order to optimize the process, the effects of various operating parameters on the DR-PSA process performance in terms of product purity and recovery were discussed in detail, including the feed position, the light product reflux ratio and the heavy product reflux ratio. Moreover, the dynamic distribution behaviors of pressure, temperature and gas-solid concentration were presented to explain and evaluate the process separation performance in depth under different operating conditions.

Influence of the main operating parameters on the DRPSA process design based on the equilibrium theory

Among the adsorption-based separation processes for gaseous mixtures, those exploiting pressure variations, so-called Pressure Swing Adsorption (PSA) processes, are the most popular. In this work, we focus on the specific PSA configuration known as Dual Reflux-Pressure Swing Adsorption (DR-PSA) given its ability to achieve sharp separations. In the case of binary mixtures, an analytical approach based on Equilibrium Theory has been proposed to identify the operating conditions for complete separation under the assumption of linear isotherms. This same approach is not available when the separation is not complete. Accordingly, in this work we study the features of non-complete separations by solving numerically a general DR-PSA model with parameter values suitable to approach equilibrium conditions (no mass transport resistances, no axial mixing, isothermal conditions and no pressure drop), thus reproducing the analytical solution when complete separations are examined. Even for non-complete separations, triangularly shaped regions at constant purity can be identified on a plane whose axes correspond to suitable design parameters. Moreover, we found a general indication on how to select the lateral feed injection position to limit the loss in product purities when complete separation is not established, whatever is the composition of the feeding mixture. Finally, a sensitivity analysis with respect to pressure ratio, light reflux ratio and heavy product flowrate is proposed in order to assess how to recover product purities according to the specific degrees of freedom of a DR-PSA apparatus.

Rejecting N2 from natural gas by dual reflux pressure swing adsorption with activated carbon

Nitrogen is a component that often needs to be removed from natural gas. A dual-reflux pressure swing adsorption (DR PSA) technology with activated carbon as the adsorbent was explored for N2 rejection from a binary mixture of 75 mol% CH4 and 25 mol% N2 via non-isothermal numerical modelling. The performance of the four DR PSA configurations (PL-A, PH-A, PL-B and PH-B) was studied by varying the following process parameters: heavy product/feed flow rate ratio (H/F), light reflux/feed flowrate ratio (RL/F), and feed step time duration. The simulation results showed that A-cycles were generally better than B-cycles with respect to the achievable heavy product methane purity and recovery. The underlying reasons for the different performances of A-cylces and B-cylces for this methane-rich feed were investigated via the comparison of the methane profiles in the pressure reversal step. The best performing cycle was the PL-A configuration which obtained a methane product purity and recovery of 93%, satisfying regional pipeline gas specifications and recovery target. The impact of the feed location on the process performance was also studied and the results showed that for A-cycles the feed location as a fraction of the bed length should be maintained at a value lower than the feed methane mole fraction to achieve optimal heavy product purity and recovery. For the PH-B cycle, the feed location should be as close as possible to the heavy product end to maximise product purity and recovery. For PL-B cycles, the effect of feed location is negligible.

Upgrading sub-quality natural gas by dual reflux-pressure swing adsorption using activated carbon and ionic liquidic zeolite

• Sub-quality natural gas was upgraded to pipeline specification by DR PSA. • Minimal greenhouse gas emissions with 99 % CH 4 recovery for sub-quality natural gas. • General trends of the process performance for different adsorption cycles presented. The upgrading of a model sub-quality natural gas mixture containing 40 mol% CH 4 and 60 mol% N 2 by dual reflux-pressure swing adsorption (DR PSA) was studied numerically. A separation target of >93 mol% CH 4 in the heavy product and <1 mol% CH 4 in the light product was adopted for the purpose of evaluating the viability of the upgrading process. Two different CH 4 -selective adsorbents were considered: (i) a commercial activated carbon (AC1) and (ii) a novel ionic liquidic zeolite (ILZ), which has twice the selectivity of activated carbon. Key process parameters such as the heavy product to feed ratio, the feed step time, and the light reflux ratio were varied iteratively to maximize the separation performance and minimize the energy consumption in each of the four DR PSA cycle configurations (PL-A, PH-A, PL-B and PH-B). The simulation results showed that the separation performance of A cycles was superior to that of B cycles for both of the adsorbents studied. The ILZ adsorbent was able to achieve the separation target and in general performed substantially better under the conditions investigated than AC1, which did not meet the product specifications for any of the cycles tested. The best-performing DR PSA cycle (PL-A) with ILZ achieved 94 mol% CH 4 in the heavy product and 1 mol% CH 4 in the light product with a compressor duty of 19.1 kJ mol −1 . The corresponding cycle with AC1 as the adsorbent only delivered 87 mol% CH 4 in the heavy product and 6.1 mol% CH 4 in the light product. This result shows that a DR PSA process using ILZ can effectively upgrade sub-quality natural gas while keeping associated methane emissions low.

Nitrogen rejection from natural gas by dual reflux-pressure swing adsorption using activated carbon and ionic liquidic zeolite

The separation of a natural gas mixture containing 85 mol% CH4 and 15 mol% N2 by dual reflux pressure swing adsorption (DR PSA) was investigated using dynamic non-isothermal numerical simulations previously benchmarked against experiments. Two different adsorbents were employed: a commercially available activated carbon (AC1) and the recently developed ionic liquidic zeolite (ILZ), which has about twice the selectivity for methane over nitrogen than the activated carbon. Four DR PSA cycle configurations (PL-A, PH-A, PL-B and PH-B) were studied and iteratively optimized in terms of separation performance and energy consumption. The dynamic non-isothermal simulations, implemented in the software platform Aspen Adsorption, included a polytropic compressor model with temperature dependent heat capacities for more realistic predictions of the compressor duty. The heavy product to feed ratio, the feed step time and the light reflux ratio were selected as key optimization parameters for all four cycles at fixed bed pressure ratio, feed location and feed flow rate. The simulations showed that A-cycles were superior to B-cycles in terms of their separation performance for both adsorbent materials. Overall, the separation performance achieved by AC1 was lower than that by ILZ. At optimized conditions for AC1, PL-A and PH-A cycles achieved approximately 92 mol% CH4 in the heavy product and 4.5 mol% CH4 in the light product with corresponding energy requirement at 9.9 kJ mol−1 (feed) and 8.7 kJ mol−1 (feed), while using the ILZ adsorbent, both PL-A and PH-A cycles were able to deliver 0.9 mol% CH4 in the light product and 93.5 mol% CH4 in the heavy product. The cycle work of the PL-A and PH-A configuration using ILZ was 8.8 and 7.7 kJ mol−1 (feed), respectively. These simulation results show that DR PSA processes using ionic liquidic zeolites can meet strict natural gas separation requirements and that such processes can efficiently upgrade sub-quality natural gas.

Nitrogen rejection from methane using dual-reflux pressure swing adsorption with a kinetically-selective adsorbent

Dual-reflux pressure swing adsorption (DR PSA) processes using a kinetically nitrogen selective molecular sieve carbon, MSC-3K 172, were investigated for the removal of nitrogen from a gas mixture consisting of 75 mol% CH4 + 25 mol% N2. The four DR PSA configurations (PL-A, PH-A, PL-B and PH-B) were experimentally demonstrated and compared in terms of their nitrogen rejection and methane recovery performance. The experimental results indicated the PH-B configuration in general performed the best in terms of the methane product purity and recovery using the kinetic MSC-3K 172 adsorbent. A non-isothermal numerical model was constructed for the kinetic DR PSA process to simulate and optimise the performance of the PH-B configuration. Model predicted gas loadings on the adsorbent indicated that selectively removing nitrogen from methane was feasible when a feed step time close to the characteristic adsorption time of nitrogen was selected. The effective usage of the adsorption bed in the PH-B configuration was also numerically studied via the capacity ratio C, with the feed step time and light reflux flow as key operational parameters, at fixed heavy product-to-feed ratio, bed pressure ratio and feed location. The numerical model for the PH-B configuration was also employed to study the effect of pressure in the purge step beyond those conditions accessible in the experiment. The simulation results revealed that the PH-B configuration could achieve a separation performance of 90 mol% CH4 in the light product with 90% CH4 recovery when the pressure in the light purge column was lower than 40 kPa while the adsorption pressure was maintained at 500 kPa. This result opens new opportunities for efficiently upgrading sub-quality natural gas reserves.

Integrated VPSA Processes for Air Separation Based on Dual Reflux Configuration

A six-step, two-bed dual-reflux pressure swing adsorption process for air separation using LiLSX zeolite was first developed and studied systematically. Then two original integrated vacuum pressure...

Optimal Design Procedure of Dual-Reflux Pressure Swing Adsorption Units for Nonlinear Separations

In the frame of dual-reflux pressure swing adsorption processes, design strategies applicable to the complete separation of binary gas mixtures are available only in the case of linear adsorption isotherms. Therefore, in this work we propose a simple and efficient design procedure which enables the selection of suitable operating conditions when sharp separation is required and Langmuir adsorption isotherms are involved. Starting from the available strategy for the linear adsorption case, we adapt it to the nonlinear case tuning properly the pressure ratio. Then, the resulting design strategy is validated by application to selected study cases involving different values of key parameters such as different levels of nonlinearity of the isotherms, but also different values of the pressure ratio, feed composition, and selectivity.

Modeling dual reflux-pressure swing adsorption processes: Numerical solution based on the finite volume method

Even if the suitability of Finite Volume discretization methods for Pressure Swing Adsorption modelling is well-established, when simulating the peculiar process called Dual Reflux-Pressure Swing Adsorption, simulations relying only on Finite Difference discretization methods are available in literature. Therefore, a detailed single-bed model of the Dual Reflux-Pressure Swing Adsorption process, the solution of which is based on the Finite Volume discretization method, is proposed in this work and validated against experimental data. Details of the system representation, the numerical schemes and the implementation of the boundary conditions are given. Moreover, a comparison between the performances and limitations of the Finite Volume and Finite Difference discretization methods when simulating a Dual Reflux-Pressure Swing Adsorption process are discussed through two case studies, leading to conclude that Finite Volume Method is the most effective approach for simulating this specific separation process.

Equilibrium Theory-Based Assessment of Dual-Reflux Pressure Swing Adsorption Cycles That Utilize Light Gas for Pressure Swing

Binary gas mixtures can be completely separated via dual-reflux pressure swing adsorption (DR-PSA) process, where pressure of the column to which the feed gas is supplied and the kind of gas employ...

Dual-reflux pressure swing adsorption process for carbon dioxide capture from dry flue gas

This work presents a detailed experimental and simulation study for capturing and recovering high purity carbon dioxide from a dry flue gas (15%CO2/85%N2) via a laboratory-scale dual-reflux pressure swing adsorption apparatus using silica gel as an adsorbent. To achieve a favorable carbon capture performance, parameter study was implemented to obtain a series of optimized settings of process operating variables by investigating the effects of flowrate of light product reflux, flowrate of heavy product and feed inlet position on the separation performance in terms of product purity, product recovery, and adsorbent productivity, as well as specific power-consumption. Under the optimal manipulation parameter configuration, a high-purity CO2 product with a concentration of 95.55% and a higher CO2 product recovery of 96.81% with a competitive specific power consumption of 110.58kJmol−1CO2 can be obtained. This finding demonstrated an efficient and economical method in the field of carbon capture and storage. In addition, a good consistency between simulation results and experimental data confirmed the reliability of the mathematical model of dual-reflux pressure swing adsorption process, and the dynamic distribution behaviors of pressure, temperature and gas-solid concentration were obtained from the simulation results to illuminate and validate the process separation performance in depth under different operating conditions.

Advanced non-isothermal dynamic simulations of dual reflux pressure swing adsorption cycles

An advanced non-isothermal dynamic model for the simulation of Dual Reflux Pressure Swing Adsorption (DR-PSA) cycles was constructed using Aspen Adsorption®. A new column pressure control method was developed to describe cycles that either continuously change pressure during the feed/purge steps or are held at constant pressure during these steps. This control method uses a dual convergence algorithm to adjust both the inlet volumetric flow of the compressor and the light product flow rate to achieve the desired pressure profile in the simulation. In addition, a choked flow valve after the compressor was employed to enable a high-fidelity simulation of the flows during the pressure reversal step in contrast to previous simulation approaches. To validate the model, predictions were benchmarked against the most detailed two sets experimental DR-PSA data available in the literature, which involved the separation of C2H6+N2 mixtures where bed pressures changed continuously during the feed/purge steps and the separation of CH4+N2 mixtures where bed pressures were constant during these steps. At cyclic steady state, the standard deviations of the model’s predictions from the experimental data involving C2H6+N2 mixtures separations were 0.003mol fraction in a rich product stream with an average ethane composition of 0.63mol fraction; and for the separation of CH4+N2 mixtures, the standard deviation of the predicted product mole fractions from the experimental values were 0.002 and 0.040 for the N2 in the light (N2-rich) product and CH4 in the heavy (CH4-rich) product streams, respectively.

Experiment and simulation study of a dual-reflux pressure swing adsorption process for separating N2/O2

Abstract A dual-reflux pressure swing adsorption (DR-PSA) process using LiLSX zeolite to produce both high purity and recovery of N 2 and O 2 from mimical air (21%O 2 /79%N 2 ) was developed in the commercial software, Aspen Adsorption. In this work, adsorption equilibrium isotherms of pure oxygen and nitrogen on LiLSX zeolite were firstly measured and fitted with the extended Langmuir model in order to describe the behavior of the static adsorption. A total of sixteen experiments were carried out on a laboratory scale DR-PSA apparatus for verifying the reliability of the mathematical model. Finally, an optimal operating condition was obtained by comparing the purity and recovery of products under different experimental conditions. Results revealed that feed or light reflux time (t FE/LR ), heavy reflux flow rate (Q HR ), vacuum pressure (P L ) and feed flow rate (Q F ) all had significant effects on the process performance. O 2 in light product could be enriched to 99.1% with recovery of 98.0% while N 2 in heavy product is 99.0% in concentration with recovery of 99.3% under the optimal experimental condition. All in all, the DR-PSA process could be capable of producing both heavy and light products simultaneously with high purity and recovery.

Demonstration and optimisation of the four Dual-Reflux Pressure Swing Adsorption configurations

Abstract The four base configurations of Dual-Reflux Pressure Swing Adsorption (DR PSA) cycles, namely feed to the low (PL) or high (PH) pressure bed, with blowdown and pressurisation using either the heavy (A) or light adsorbate (B), were demonstrated and compared experimentally in terms of their respective performance separating N 2 and CH 4 using activated carbon. Non-isothermal numerical simulations of these four cycles were validated against the experiments and used to help develop a means of optimising DR PSA cycles. While previous theoretical treatments and experimental studies have largely focussed on situations where the heavy product-to-feed ratio, H / F , is near to the ideal value necessary to produce two pure products, we demonstrate how H / F can be used to bias the separation towards either improved stripping or enrichment; this can be of practical importance in physical cycles where perfect separation is not possible. Consequently, H / F can be considered together with bed’s capacity ratio, C , and the (heavy) reflux-to-feed ratio R / F as key operational parameters available for optimising cycle performance in terms of product purity, cycle work and productivity when adsorbent selectivity, bed pressure ratio and feed location are fixed. For N 2 -CH 4 separations biased towards stripping (larger H / F values), PL-A cycles achieved the best separation performance while PH-B cycles were best for separations biased towards enriching (smaller H / F values). In terms of cycle work per mole of feed treated, for a particular set of experiments with the same light reflux flow (2 SLPM) B configurations required about 15% less work than A configurations. For these experiments PH cycles required about 25% less work than the PL cycles because the feed was at a lower pressure than either of the products, which were both delivered at high pressure.

Optimal design of dual-reflux pressure swing adsorption units via equilibrium theory: Process configurations employing heavy gas for pressure swing

Dual-reflux pressure swing adsorption process is theoretically capable of completely separating binary feed gas mixtures into two pure species. The pressure of bed to which the binary gas mixture is fed and the type of gas utilized for pressure swing, results in different process cycle configurations, even if the majority of the previous studies of DR-PSA are restricted to two cycle configurations: that employ heavy gas for pressure swing and deliver feed to the bed operated at either high or low pressure. However, the comparative assessment and the optimal operating pressure ratio of these two process cycle configurations are not well-established. We previously reported an optimal design strategy (that identified a triangular operating zone, inside which, complete separation of binary gas mixtures can be achieved) for one such DR-PSA process cycle configuration. In this work, we report an optimal design strategy for another DR-PSA process cycle configuration: feed to low pressure bed and pressure swing using heavy gas. With respect to previous literature, the equilibrium theory based comprehensive tracking of the characteristic curves and shock transitions during constant and non-constant pressure steps of this specific cyclic process revealed distinct constraints, design parameter values and boundary conditions of the triangular operating zone. Additionally, an in-depth comparative assessment of the impact of process variables (adsorbent selectivity, feed gas composition and, operating pressure ratio) on the design parameters (optimal feed injection position and ratio of pure light reflux to feed rate) and a novel selection criterion is discussed for both of these cycle configurations in order to (i) facilitate the choice of appropriate cycle configuration and (ii) identify the optimal high to low operating pressure ratio range.