Reflux Pressure Swing Adsorption Research Articles

A numerical comparison of heavy‐purge and dual‐reflux strategies in pressure swing adsorption for methane enrichment

AbstractDual reflux pressure swing adsorption (DR‐PSA) has been regarded as a state‐of‐the‐art adsorption‐based process which can simultaneously obtain two streams of pure product gases with a narrow pressure window. However, the DR‐PSA has not yet been reported in industrial applications. Herein, a DR‐PSA and a heavy‐purge pressure vacuum swing adsorption (HP‐PVSA) were numerically investigated for the enrichment of 1%, 8% and 15% CH4 from N2 gas mixtures in pilot‐scale. Key separation indicators such as purity, recovery and energy cost of the two cycles were compared to analyze the limitations of the DR‐PSA process while scaling‐up. This study reveals the impact of heavy to feed (H/F) ratios on purity and recovery for both cycles and analyses the energy consumption of each process. For feed gas with 15% CH4, while DR‐PSA can achieve a slightly better purity and recovery (88.3% and 88.3%, respectively) compared to HP‐PVSA (87.5% and 80.3%, respectively), it also involves an order of magnitude higher energy consumption (181.6 versus 24.6 kJ/mol CH4 captured). DR‐PSA shows significantly superior performance than HP‐PVSA when the CH4 content in the raw feed gas is low. Under the investigated operating conditions, HP‐PVSA can only enrich 1% CH4 to 10% with 78.7% recovery while DR‐PSA can obtain 75.3% purity and 77.3% recovery. Results indicate that DR‐PSA exhibits superiority in enrichment of dilute gas, however, its high energy consumption, high capital expenditures and limitations in processing high throughput are the chief reasons hindering its industrial application.

Separation of Methane and Nitrogen Using Heavy Reflux Pressure Swing Adsorption: Experiments and Modeling

Separation of Methane and Nitrogen Using Heavy Reflux Pressure Swing Adsorption: Experiments and Modeling

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.

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.

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.

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.

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.

Non-isothermal numerical simulations of dual reflux pressure swing adsorption cycles for separating N2 + CH4

A non-isothermal model of dual reflux pressure swing adsorption (DR-PSA) was developed using a commercially available software package to numerically solve the dynamics of the unit operation. Importantly the model includes a full energy balance, which is a feature not reported previously in the literature for simulations of DR-PSA cycles even though bed temperature swings of 10–20K have been observed in experimental studies. The simulation allowed solution of the pressure-flow network for cycles in the PL-A configuration, where feed gas enters the middle of the low pressure column and where pressure inversion is conducted by transferring gas rich in the more adsorbed component between beds. At cyclic steady state the simulations contained material balance errors comparable in magnitude to those reported previously for isothermal DR-PSA simulations; however, these were accounted for using a robust correction scheme. Predictions of the pressure, flow and temperature profiles within the beds for a range N2+CH4 mixtures being separated using activated carbon were in good agreement with the corresponding results of 24 DR-PSA experiments recently reported by Saleman et al. (2015). The root mean square deviation of the predicted methane mole fractions from the experimental values were 0.003 and 0.024 for the light (N2-rich) and heavy (CH4-rich) product streams, respectively. Parametric studies conducted with the model show how the cycle design can be optimised with respect to reflux flow and the feed-purge step duration, and also illustrate the need for reliable values of the sorption mass transfer coefficient.

Modelling and evaluation of dual reflux pressure swing adsorption cycles: Part II. Productivity and energy consumption

In this second part of our dual reflux PSA study, we have examined the power and productivity tradeoffs of all of the dual reflux cycles developed in Part I. It was found that the concentration of heavy component in the feed is the primary variable that influences the choice of dual-reflux configuration. Feed to the high-pressure bed and equalisation, pressurisation and blowdown with the strongly adsorbed component is favoured at low concentrations of heavy component in the feed gas. Feed to the low-pressure bed and equalisation, pressurisation and blowdown with weakly adsorbed component is favoured for high concentrations of heavy component in the feed gas. For highly selective adsorbents, the differences between the work-productivity tradeoff curves for the four configurations are quite distinct, becoming less distinct as the ease of separation decreases.

Simulation of an Enriching Reflux PSA Process with Parallel Equalization for Concentrating a Trace Component in Air

The utility of a simple numerical model, based on the LDF approximation and the frozen solid phase assumption, was developed to predict the performance of an enriching reflux (ER) pressure swing adsorption (PSA) cycle with parallel equalization (PEQ) for concentrating a trace component. The utility of this model was successfully demonstrated with the enrichment of Xe from air using 13X zeolite and a two-bed, 6-step, ER PSA cycle with PEQ. With the LDF mass transfer coefficient (Ka) ranging from 2.0 to 3.0 kg m-3 s-1, the model predictions of the enrichment factor (YHP/YF) compared well with the experimental results over a wide range of conditions. Both the modeling and experimental results showed that ambient Xe in air could be enriched up to 80 times by the ER PSA cycle with PEQ. The enrichment factor that could be obtained from the more conventional stripping reflux PSA cycle was theoretically limited to 12.5, i.e., the high to low pressure ratio, with the actual value usually being much less than half ...

Solvent Recovery by PSA, an Experimental and a Simulation Study

Solvent recovery by pressure swing adsorption (PSA) with resin adsorbent or high silica zeolite was tried and was related with characteristics of each adsorbent, i.e., the adsorption isotherms and the rates of adsorption and desorption. CH2Cl2 vapor was chosen as adsorbate. The performance by each adsorbent, shown in terms of the yield against purity of the product gas, was discussed. The high silica zeolite gave better performance than the resin adsorbent in a small-scale experiment, as expected from their isotherms (Chihara et al., 2004). Computer calculations were carried out to simulate the experimental results using the Stop-Go method to show the calculated results coincide well with experimental results. The method is useful to predict the performance of a solvent recovery system operated by PSA. The performance of Dual Reflux PSA which is the new type of PSA method (Wakasugi et al., 2002) was also simulated by this method.

High Enrichment and Recovery of Dilute Hydrocarbons by Dual-Reflux Pressure-Swing Adsorption

A twin-bed, dual (stripping and enriching)-reflux pressure-swing adsorption (DR-PSA) system utilizing an intermediate feed position in the columns was designed, constructed, and operated to study the separation and concentration of a dilute feed of ethane in nitrogen using WestVaco BAX-1500 activated carbon. Four runs were performed, changing only the stripping or enriching reflux ratios. In all the runs, the ethane was enriched between 50 and 90 times with a pressure ratio of only 8. In addition, high heavy-component recoveries were obtained, with the purity and recovery of nitrogen also being very high. The best case enriched a 0.75 vol % ethane feed to 68.4 vol % with an ethane recovery of 99.6%. The nitrogen purity was also very high containing only 30 ppm of ethane in the light product stream. This separation was markedly better than that achievable with a conventional stripping reflux PSA process, and it suggests that it should be entirely feasible to produce two very pure products from a dilute fee...