Rapid Pressure Swing Adsorption Process Research Articles

Rapid pressure swing adsorption for small scale ammonia separation: A proof‐of‐concept

AbstractIn a typical Haber‐Bosch process, a gas stream containing 15–20 mol% ammonia is obtained from the reactor effluent, and ammonia is then partially separated in a phase‐changing condensation unit. When operating at lower pressure for distributed manufacturing, the single‐pass conversion drops to less than 10 mol%, which makes the condensation more cost‐intensive. A small adsorber is proposed for concentrating ammonia through rapid pressure swing adsorption (RPSA) that fits the small‐scale processing. A mathematical model is developed to evaluate the feasibility of the RPSA process for ammonia separation with high recovery. The ideal adsorbed solution theory, based on the Freundlich single‐component isotherm model, is proposed to predict binary isotherms for various commercially available adsorbents. The performance of the RPSA‐assisted adsorber is then studied at different process conditions for concentrating ammonia. The effect of various operating variables such as exhaust flow rate, cycle time, and feed pressure, is investigated. The proposed numerical model shows that nearly pure ammonia can be continuously produced at optimized conditions, with more than 95% recovery. This low‐pressure RPSA‐assisted adsorber can be used to design modular ammonia devices for distributed manufacturing. Our proposed technology can be further extended to concentrate other dilute gas mixtures, such as carbon dioxide.

Experimental design of a "Snap-on" and standalone single-bed oxygen concentrator for medical applications.

A novel single-bed, “Snap-on” and standalone, medical oxygen concentrator design based on a rapid pressure swing adsorption process was investigated for continuous oxygen supply. The Snap-on concentrator design is easy to hook up to an existing compressed air source, and the unit can then be readily used to produce oxygen for medical applications. It is easily transportable and compared to a traditional oxygen concentrator with its dedicated compressor, the Snap-on concentrator is particularly relevant for the oxygen therapy needs of a larger number of patients in situations such as COVID-19. A commercially available LiLSX zeolite was used for the separation of oxygen from compressed ambient air. The experiments were performed at different feed air pressures using a constant supply of house air in the lab. Further, the device performance was also analyzed using a standalone medium size air compressor. The minimum bed size factor obtained with compressed house air was 100 lb/tons per day contained (TPDc) O2 at a cycle time of 7 s, whereas the minimum bed size factor obtained with a medium size air compressor weighing about 12 lbs was 210 lb/TPDc O2 at a cycle time of 14.5 s under the same feed pressures of 3.1 bar at an oxygen product purity of 90%. The product oxygen flow rate was nearly double for the same amount of adsorbent when using house air for the Snap-on design. The primary reason for this significantly higher oxygen production was the substantially higher and stable air throughput capacity of a typical house air compressor that enabled rapid cycling of the process at near-constant feed pressure compared to a medium size compressor used in a medical oxygen concentrator. The oxygen recovery was approximately 34% for both cases. Thus, the Snap-on oxygen concentrator was found to be easier to build and it delivered more oxygen for medical use compared to standalone units in locations where a constant supply of compressed feed air is available. This is typically the case in facilities such as hospitals, military medical camps and cruise ships. Further, the Snap-on design offers other benefits such as ease of transportation, higher reliability and lower weight.

Effect of intermittent purge on O2 production with rapid pressure swing adsorption technology

The rapid pressure swing adsorption (RPSA) technology has been widely used for applications including portable oxygen generators due to high O2 productivities and low bed size factors (BSF, total amount of adsorbent per ton of O2 produced per day, kg/TPDO2). The improvement of RPSA process has been a key issue for efficient O2 production in terms of its generally lower O2 recovery as compared to other processes. In this work, a dual-column RPSA oxygen production system with an intermittent purge step (intermittently switching between purge and interruption) was designed to investigate the effects of interruption frequency (the number of interruptions during a purge step) and times on the O2 recovery, purity, productivity and BSF. Experimental results show the improvement on O2 production performance using the intermittent purge step in comparison to conventional continuous purge step. Under the optimized interruption frequency of 2 and time of 0.3 s, the O2 recovery, purity and productivity were increased from 36 to 39.2%, 89.5 to 93%, and 6.24 to 6.49 mmol/kg/s, respectively, while the BSF was decreased from 57.94 to 55.75 kg/TPDO2.

Multivariable model predictive control of a novel rapid pressure swing adsorption system

A multivariable model predictive control (MPC) algorithm is developed for the control and operation of a rapid pressure swing adsorption (RPSA)‐based medical oxygen concentrator. The novelty of the approach is the use of all four step durations in the RPSA cycle as independent manipulated variables in a truly multivariable context. The RPSA has a complex, cyclic, nonlinear multivariable operation that requires feedback control, and MPC provides a suitable framework for controlling such a multivariable system. The multivariable MPC presented here uses a quadratic optimization program with integral action and a linear model identified using subspace system identification techniques. The controller was designed and tested in simulation using a complex, highly coupled, nonlinear RPSA process model. The model was developed with the least restrictive assumptions compared to those reported in the literature, thereby providing a more realistic representation of the underlying physical phenomena. The resulting MPC effectively tracks set points, rejects realistic process disturbances and is shown to outperform conventional PID control. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1234–1245, 2018

Comments on the Reliability of Model Simulation of a Rapid Pressure Swing Adsorption Process for High-Purity Product

The uncertainty in the evaluation of the separation performance of a rapid pressure swing adsorption (RPSA) process for production of a high-purity product gas by using a numerical process model can be impractically high as a result of the theoretical or experimental uncertainty in the estimation of the high adsorbate mass transfer coefficient required by such a process. Actual experimental process performance data are required in order to judge the real value of the process.

Experimental Study of a Novel Rapid Pressure-Swing Adsorption Based Medical Oxygen Concentrator: Effect of the Adsorbent Selectivity of N2 over O2

It was experimentally demonstrated that the performance of a medical oxygen concentrator based on rapid pressure swing adsorption (RPSA) process using a LiLSX zeolite was improved [lower bed size factor (BSF) and higher oxygen recovery (RO)] when the adsorbent exhibited higher selectivity of adsorption of N2 over O2. The effects of N2 selectivity on BSF and RO as functions of the RPSA process cycle times were found to be complex and nonintuitive. The key properties of a LiLSX zeolite sample for adsorption of N2 and O2 (pure and binary gas isotherms, binary selectivity, pure gas isosteric heats, pure gas mass-transfer characteristics, and degrees of adsorbent heterogeneity), which exhibited a relatively higher selectivity of adsorption for N2 over O2 compared to a previously reported sample, were measured and used in this study.

Performance of a medical oxygen concentrator using rapid pressure swing adsorption process: Effect of feed air pressure

The effects of feed air pressure on the steady‐state performance of a medical oxygen concentrator (MOC) were experimentally evaluated using a novel design of a MOC unit which produced a continuous stream of ∼90% O2 employing a rapid pressure swing adsorption (RPSA) process scheme. Dry, CO2 free air containing ∼1% Ar at different feed gas pressures was used in the tests in conjunction with a commercial sample of LiLSX zeolite as the N2 selective adsorbent in the process. The bed size factor (BSF) can be systematically reduced by increasing the feed air pressure for any given total cycle time. The effect of feed air pressure on the oxygen recovery (R) is, however, more complex; it increases with increasing feed pressure only at longer cycle times while the effect is marginal at shorter cycle times. The BSF cannot be indefinitely reduced by lowering total process cycle time at any pressure—a minimum is exhibited in the BSF‐cycle time plot. The minimum value of the BSF decreases as the feed pressure is increased. The cycle time for the minimum BSF is, however, not significantly altered by the feed pressure in the data range of this work. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1212–1215, 2016

New momentum and energy balance equations considering kinetic energy effect for mathematical modelling of a fixed bed adsorption column

It was aimed to derive rigorous momentum and energy balance equations where the change of kinetic energy in both spatial and temporal domains of a fixed-bed adsorption column was newly taken into account. While the effect of kinetic energy on adsorption column dynamics is negligible in most cases, it can become more and more influential with an adsorption column experiencing a huge pressure drop or with the gas velocity changing abruptly with time and along the column. The rigorous momentum and energy balance equations derived in this study have been validated with two limiting cases: (1) an inert gas flow through a packed column with a very high pressure drop and (2) blowdown of an adiabatic empty column. The new energy balance including the kinetic energy effect paves a way for simulating with an improved accuracy a Rapid Pressure Swing Adsorption process that inherently involves a very high pressure drop along the column and requires very high pressure change rates for column blowdown and pressurisation.

Anatomy of a rapid pressure swing adsorption process performance

A detailed numerical study of the individual and cumulative effects of various mass, heat, and momentum transfer resistances, which are generally present inside a practical adiabatic adsorber, on the overall separation performance of a rapid pressure swing adsorption (RPSA) process is performed for production of nearly pure helium gas from an equimolar binary (N2 +He) gas mixture using 5 A zeolite. Column bed size factor (BSF) and helium recovery (R) from the feed gas are used to characterize the separation performances. All practical impediments like column pressure drop, finite gas‐solid mass and heat transfer resistances, mass and heat axial dispersions in the gas phase, and heats of ad(de)sorption causing nonisothermal operation have detrimental impacts on the overall process performance, which are significantly accentuated when the total cycle time of a RPSA process is small and the product gas helium purity is high. These impediments also prohibit indefinite lowering of BSF (desired performance) by decreasing process cycle time alone. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2008–2015, 2015

Numerical simulation of rapid pressurization and depressurization of a zeolite column using nitrogen

The required durations of pressurization and depressurization steps of a rapid pressure swing adsorption process are primarily governed by adsorbent particle size, adsorption kinetics, column pressure drop, column length to diameter ratio, and the valve constant of the gas inlet and outlet control valve attached to the adsorbent column. A numerical model study of the influence of these variables for an adiabatic LiX zeolite column is presented using pure N2 as an adsorbate gas. An adsorbent particle size range of 200–350 μm was found to minimize (<1 s) the times required for the pressurization and depressurization steps.

Production of oxy-rich air by RPSA for combustion use

The capital and energy costs of production of oxygen enriched air by a rapid pressure swing adsorption (RPSA) process can be reduced by decoupling the air drying and the air separation duties of the process. Integration of the oxygen-RPSA process with an enhanced combustion application system allows thermal swing adsorption drying of air feed to the RPSA process. The air separation process then can be run using an ad(de)sorption pressure envelope of 2:1 atmospheres, which significantly reduces the cost and energy of operation of the air compressor.