PSA Cycle Research Articles

Equilibrium theory analysis of dual reflux PSA for separation of a binary mixture

AbstractA dual reflux (DR) PSA cycle that combines the features of a conventional (stripping reflux) PSA cycle with those of a new enriching reflux PSA cycle is analyzed to show its potential for separating gas mixtures. On the basis of isothermal equilibrium theory applied to linear isotherms, the ultimate separation is carried out where the binary feed is separated into two pure components with 100% recovery of each component. This very idealized analysis reveals that such a separation is possible over a wide range of conditions, even with pressure ratios as low as 1.1. This analysis also reveals that low throughputs and high heavy component recycle ratios are inherently associated with DR PSA cycles, both of which may be detrimental to the process economics. High throughputs and low heavy product recycle ratios are indeed achievable, but only when using low pressure ratios and less selective adsorbents, both counterintuitive results that make sense when considering the perfect separation is always being achieved. Although these trends may not carry over to actual practice, because the model developed here is overly simplified and invalid under certain conditions, this analysis shows that it may indeed be entirely feasible to separate a binary gas mixture into two relatively pure components with very high recoveries using a DR PSA cycle operating with a very low pressure ratio and, hence, expenditure of energy. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2418–2429, 2004

Landfill Gas: From Rubbish to Resource

The prospects of using landfill gas (LFG) as a high-grade fuel in the immediate future, in view of environmental regulations, the Kyoto Protocols, and energy prices, are discussed. Adsorption cycles suggested in the late 1980s by Sircar and co-workers for treating LFG are reviewed: one produced CO2-free methane and the other produced both CO2-free methane and methane-free CO2. Neither of those could be used to produce pipeline quality gas from LFG, due to contaminants such as nitrogen. Two new three-stage flowsheets are discussed as a means to separate pipeline-grade methane from LFG. One is a hybrid membrane—PSA system. The other is a TSA—PSA system. The third stage of both of these systems is crucial to obtaining pipeline quality gas, i.e., a PSA unit to extract the nitrogen and other light gases from methane. A novel PSA cycle is suggested and explained in terms of: a model by which the recovery, power requirements, and adsorbent bed size can be estimated.

Equilibrium theory analysis of rectifying PSA for heavy component production

AbstractAn isothermal equilibrium theory analysis, based on linear isotherms and a binary feed stream, was carried out to evaluate the feasibility of a rectifying PSA process for producing a pure heavy component at high recovery. Analytic expressions were derived to describe the performance of this process at the periodic state. The performance was also analyzed in terms of the different concentration and velocity profiles exhibited during various cycle steps that included the analysis of complex shock and simple wave interactions. Based on a parametric study, periodic behavior was established for a wide range of process conditions; and a design study with the PCB activated carbon–H2–CH4 system at 25°C further demonstrated the feasibility of a rectifying PSA cycle for producing a 100% CH4 stream from a dilute feed stream (y = 0.01) with a respectable recovery (80%), and reasonable process conditions. It also demonstrated the potential usefulness of an actual rectifying PSA process for bulk gas separation and purification.

CO2 recovery from flue gas by PSA process using activated carbon

An experimental study was performed for the recovery of CO2 from flue gas of the electric power plant by pressure swing adsorption process. Activated carbon was used as an adsorbent. The equilibrium adsorption isotherms of pure component and breakthrough curves of their mixture (CO2 : N2 : O2=17 : 79 : 4 vol%) were measured. Pressure equalization step and product purge step were added to basic 4-step PSA for the recovery of strong adsorbates. Through investigation of the effects of each step and total feed rate, highly concentrated CO2 could be obtained by increasing the adsorption time, product purge time, and evacuation time simultaneously with full pressure-equalization. Based on the basic results, the 3-bed, 8-step PSA cycle with the pressure equalization and product purge step was organized. Maximum product purity of CO2 was 99.8% and recovery was 34%.

Mixed cation zeolites: LixAgy‐X as a superior adsorbent for air separation

AbstractLi‐X zeolite (Si/Al = 1.0) is currently the best sorbent for use in the separation of air by adsorption processes. Silver is also known to strongly affect the adsorptive properties of zeolites and thermal vacuum dehydration of silver zeolites leads to the formation of silver clusters within the zeolite. In this work we synthesized type X zeolites containing varying mixtures of Li and Ag. Adding very small amounts of Ag at the proper dehydration conditions formed silver clusters and enhanced adsorptive characteristics and energetic heterogeneity, as compared to those of the near fully exchanged Li+‐zeolites. The performance for air separation by the best of these sorbents, containing, on average, only one Ag per unit cell, was compared to that of the near fully Li+‐exchanged zeolite using a standard PSA cycle by numerical simulation. The results show that the new sorbent provides a significantly higher (> 10%) product throughput at the same product purity and recovery when compared to that of the near fully Li+‐exchanged zeolite.

Analysis of Equilibrium PSA Performance with an Analytical Solution

Five-step PSA cycles consisting of pressurization with product, adsorption, co-current depressurization, blowdown, and purge steps have been analyzed with equilibrium model assuming uncoupled linear isotherms and isothermal condition. Unlike the previous models, the proposed model is not restricted to the operating conditions that ensure a complete shock transition of concentration profile at the end of the high pressure adsorption step. The operating conditions could have two classifications: one is utilizing the column completely before blowdown, and the other is not. As the selectivity increases, it is more difficult to utilize the column completely before the blowdown step. There is an optimum co-current depressurization pressure which maximizes the recovery at the given extent of purge. The optimum co-current depressurization pressure decreases as the purge quantity decreases. On the less selective adsorbent, the recovery at the optimum co-current depressurization pressure increases with the decrease of purge quantity without much sacrifice of the throughput. But, on the highly selective adsorbent, there is an extent of purge and corresponding value of cocurrent depressurization pressure below which the recovery is not greatly improved while the throughput decreases rapidly, which limits the number of pressure equalization steps can be included.

Multicomponent pressure swing adsorption. Part II. Experimental verification of the model

The mathematical model of nonisothermal multicomponent pressure swing adsorption, developed in Part I of this study (K. Warmuziński, M. Tańczyk, Chem. Eng. Proc., 36 (1997) 89–99) is validated based on experimental data concerning the separation of CH 4 and H 2 on activated carbon and the production of hydrogen on zeolite 5A. For a wide range of the various operating parameters, the measured values of the purity and recovery of the products, together with the concentration and temperature profiles during a PSA cycle are compared with those predicted by the model. In all cases a satisfactory agreement is found between theory and experiment.

Developing and testing changes in delivery of care.

Improving the daily practice of medicine requires making changes in processes of care.In many circumstances, the most powerful way to make such changes is to conduct small, local tests-Plan-Do-Study-Act (PSA) cycles-in which one learns from taking action. Learning in these cycles has much in common with learning from prudent clinical work, in which therapies are initiated under close observation and adjustments are made as data and experience accumulate. For many system improvements, PSA cycles are more appropriate and informative than either formal studies with experimental designs (such as randomized trials) or the mere implementation of changes without reflection or evaluative measurement. Physicians can encourage systemic improvement by endorsing and participating in prudent, local tests of change in their own offices and in the health care organizations in which they work. To do this, they must understand the scientific value and integrity of such small-scale tests.

Production of High-Purity Nitrogen from Air by Pressure Swing Adsorption on Zeolite X

Abstract A PSA air separation process for the production of high purity nitrogen is studied experimentally and theoretically. The experimental apparatus is composed of three columns filled with zeolite X. The PSA cycle consists of six steps: pressurization with air, adsorption, recovery, null, high pressure purge with product nitrogen, and desorption. The nitrogen purity analyzed between 95.0 and 99.992%. The effects of adsorption pressure and reflux ratio on the PSA performance are studied experimentally. The productivity is 2.8 NL/(kg·min) and the recovery is 55% at a nitrogen purity of 99.99% and an adsorption pressure of 800 mmHg. The experimental results are compared with the theoretical model, and the effects of the recovery step and the operating variables on the performance are studied theoretically. The recovery step makes the PSA performance favorable, especially in the high product purity region above 99%.

Experimental study on a four‐bed PSA air separation process

AbstractA 200‐s PSA cycle involving both pressure equalization and product backfill steps has been experimentally studied on a four‐bed system, where LINDE 5 A zeolites were used as the adsorbent to separate oxygen from air. This cycle is operated under a pressure ratio of 4.3. During the experiment, the pressure history and flow rates, as well as the concentration of the product stream have been continuously monitored. This is the first time detailed experimental data on a four‐bed system are presented. Under favorable conditions, this system produces better than 90% oxygen at a recovery of 17%. For the low‐pressure ratio, such a recovery could not have been achieved without the pressure equalization step and the reduced purge operation. Recovery and throughput, however, are not as high as one would expect from a linear local equilibrium model. The self‐broadening effect of the purge wave has been identified as the major cause of underperformance.

Front analysis and cycle policy in PSA operations

Because the concentration distributions in a PSA process represent an intermediate observable level of information between the microscopic scale and the practical performance indices of a plant, they offer a convenient and efficient approach to analysing PSA cycles. A classical model is used to illustrate qualitative behaviour.

Pressure swing adsorption. Part III: Numerical simulation of a kinetically controlled bulk gas separation

AbstractA numerical simulation has been developed for a simple two‐bed PSA system in which kinetic effects and changes in flow rate due to adsorption are significant. The model is therefore more general than most previous models for the PSA cycle. The model equations are solved by the method of double collocation. When applied to air separation on a carbon molecular sieve, using independently measured kinetic and equilibrium parameters, the predicted performance appears consistent with available performance data for a commercial unit.