COMMENTARIES
Pressure Swing Adsorption
Introduction
Pressure swing adsorption(PSA)is a very versatile technology for separation and purification of gas mix-tures.Some of the key industrial applications include (a)gas drying,(b)solvent vapor recovery,(c)fraction-ation of air,(d)production of hydrogen from steam-methane reformer(SMR)and petroleum refinery off-gases,(e)separation of carbon dioxide and methane from landfill gas,(f)carbon monoxide-hydrogen sepa-ration,(g)normal isoparaffin separation,and(h)alcohol dehydration.There are several hundred thousand PSA units operating around the world servicing these and other applications.In fact,PSA has become the state-of-the-art separation technology for application areas a-d listed above,and the sizes of these units range from very small(∼300SCFD units for the production of90% O2from air for personal medical use)to very large(∼100 MMSCFD units for the production of99.999+%hydro-gen from an SMR).Many of these processes are de-scribed in the published books and review papers on the subject.1-13
The growth in the research and development of PSA technology has been phenomenal since the first
U.S. patent on the subject,authored by C.W.Skarstrom,was granted in1960.14A recent survey showed that∼600 U.S.patents on PSA were issued in the application areas a,c,and d alone during1980-2000,while the number of published papers with PSA as the keyword exceeded800during the period of1970-2000.15
The concept of PSA for gas separation is relatively simple.Certain components of a gas mixture are selectively adsorbed on a microporous-mesoporous solid adsorbent at a relatively high pressure by contacting the gas with the solid in a packed column of the adsorbent in order to produce a gas stream enriched in the less strongly adsorbed components of the feed gas. The adsorbed components are then desorbed from the solid by lowering their superincumbent gas-phase par-tial pressures inside the column so that the adsorbent can be reused.The desorbed gases are enriched in the more strongly adsorbed components of the feed gas.No external heat is generally used for desorption.Many different nomenclatures are used to describe these concepts.A PSA process carries out the adsorption step at a superambient pressure,and the desorption is achieved at a near-ambient pressure level.A vacuum swing adsorption(VSA)process undergoes the adsorp-tion step at a near-ambient pressure level,and the desorption is achieved under vacuum.A pressure-vacuum swing adsorption(PVSA)process utilizes the benefits of both concepts.Although simple in concept, a practical PSA/VSA process can be fairly co
mplex because it involves a multicolumn design where the adsorbers operate under a cyclic steady state using a series of sequential nonisothermal,nonisobaric,and non-steady-state steps.These include adsorption,de-sorption,and a multitude of complementary steps which are designed to control the product gas purity and recovery and to optimize the overall separation perfor-mance.9The research trends are to(a)produce purer products at higher recovery,(b)lower adsorbent inven-tory and energy of separation,and(c)increase the scale of application at a lower overall cost.
Unique PSA cycles are also designed to simulta-neously produce two pure products from a multicompo-nent feed ,O2and N2from air,CO2and H2from SMR off-gas,and CO2and CH4from landfill gas)as well as to produce a product gas containing a component which is not initially present in the feed gas(ammonia synthesis gas from SMR off-gas).These examples dem-onstrate the wide flexibility of PSA process designs.10,16,17 The key reasons for such growth in this technology are as follows:18
(a)An extra degree of thermodynamic freedom for describing the adsorption process introduces immense flexibility in PSA process design as compared with other conventional separation tools such as distillation,ex-traction,or absorption.
(b)Numerous microporous-mesoporous families of adsorbents(new or modified)like activated carbons, zeolites,aluminas,silica gels,and polymeric sorbents exhibiting different adsorptive properties for separation of gas mixtures(equilibria,kinetics,and heats)are available.
(c)The optimum marriage between a material and a process in designing the PSA separation scheme pro-motes innovations.
(d)Many PSA process paths can be designed for the same separation objectives.
A good example of items a-d listed above can be found in the area of air fractionation by adsorption. Approximately390U.S.patents were issued on the subject during the last20years.15The processes are designed to(a)separately produce∼23-95%O2from air,(b)separately produce98-99.99+%N2from air,and (c)simultaneously produce∼90+%O2and99+%N2 from air.Many different zeolites having different ther-modynamic selectivities and capacities for adsorption of N2over O2are employed,and the processes are tailor-made to fit the zeolite properties in order to produce the specific product demands for cases a and c.11Many different carbon molecular sieves having different ki-netic selectivities and capacities for adsorption of O2 over N2are employed for case b with appropriate process designs for controlling the N2product purity and recovery.19,20These processes use many different de-signs of P
SA,VSA,or PVSA cycles and operating conditions in order to achieve the final goals.A key success in this area has been the lowering of the specific power(<10KWD/ton)of O2production(∼90+%)from air below that of the conventional cryogenic distillation route by the use of LiX zeolite as the N2selective
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adsorbent,21,22while retaining the traditional advantage of lower capital cost for a PSA/VSA process.This opens up the door for increasing the sizes of O2-PSA processes beyond today’s standards.Parallel developments can be cited in the other application areas.9,10,12,13It is expected that this trend will continue in the future.New PSA cycles using old or new adsorbents(preferably tailor-made for each other)will continue to be developed in the existing and new application areas.Good combina-tions of PSA processes and adsorbents are needed for bulk separation of N2and CH4at high pressures, separation of dilute O2from Ar,and bulk separation of propane-propylene mixtures.
Emerging Concepts
Several very interesting PSA process developments are emerging.They include(a)rapid PSA cycles,(b) novel PSA adsorber designs,(c)sorption-enhanced reac-tion processes(SERP),and(d)high-temperature PSA (HTPSA)cycles.
(a)The rapid PSA processes are designed to increase the productivity of the processes by an order of magni-tude by using total process cycle times of seconds instead of minutes as in the case of the conventional PSA cycles.This is achieved by simply running a conventional cycle faster using novel , rotary valves)23or by changing both the adsorber and the process cycle designs.11However,separation ef-ficiency and performance may be compromised(lower product purity or recovery)because of time limitations in incorporating the complementary cyclic steps and limitations caused by adsorption kinetics.24
(b)Faster PSA cycles are often limited by the hydro-dynamic constraints(gas maldistribution,adsorbent fluidization,column pressure drop,etc.),which limit gas flow rates through packed beds.Radial bed adsorbers where the adsorbent is placed between two concentric cylinders and gas flows radially through the packed section may alleviate many of these problems.These adsorbers,however,are more
expensive.Many such designs have been proposed and developed which allow faster cycles,higher gas throughputs at lower pressure drops,and complete absence of fluidization.25,26
An interesting possibility will be the development of a PSA cycle using a rotary bed adsorber which has been successfully employed for practicing thermal swing adsorption concepts.27
(c)The SERP is a hybrid concept where an equilib-rium-controlled reaction is carried out in the presence of an adsorbent which selectively removes one of the undesired reaction products from the reaction zone,thus increasing the yield and the rate of formation of another desired product by Le Chatelier’s principle.The adsor-bent is then regenerated periodically using the prin-ciples of PSA.The concept has also been called pressure swing reactor.28Very recently,a novel process called SERP-H2was demonstrated in a pilot-scale unit for production of CO-and CO2-free(<40ppm)H2by SMR using a CO2selective chemisorbent and a SMR catalyst inside the reactor.29,30The primary impurity in the90+ mol%H2product(dry basis)was unreacted CH4.A high conversion of CH4to H2at a much lower reaction temperature than the conventional SMR process could be achieved.The concept may be useful for direct production of CO-free H2in a fuel cell application. (d)A HTPSA(∼200°C)process was developed for sequestering and recovering CO2from a hot,wet waste gas.31This process also used a chemisorbent for CO2 (∼10-20mol%in feed),and it recovered∼80%of feed CO2as an essentiall
y pure CO2product(dry basis).The concept may be very attractive for controlling green-house effects by removing CO2from a hot and wet flue gas without cooling the gas and removing the water. PSA Process Design
Despite such growth in the practical applications of this technology,the design and optimization of a PSA system still largely remains an experimental effort.A priori design of a practical PSA system that can guarantee the commercial specifications without the use of supporting data from a bench-or pilot-scale process rig may not yet be feasible for the following two reasons. First,most of the practical PSA processes are fairly complex,involving a number of sequential but interact-ing unsteady-state cycle steps.It may be possible to formulate a rigorous mathematical framework(model) to describe such processes,but it is usually expensive and time-consuming to solve such models with the accuracy and reliability needed for industrial design.It requires repeated numerical solutions of a set of coupled nonlinear partial differential equations in the time and space domain with different initial and boundary condi-tions defining the steps of the process for any given cycle until a cyclic steady-state solution is achieved.The computation time for such calculations is often prohibi-tive,and model simplification becomes imperative.10 Second,fundamental understanding of the multicom-ponent gas-solid interactions(thermodynamic and kinetic)that govern the performance of a PSA adsorber is very limited
because of their complexity.An accurate description of these interactions must be available before the mathematical process model can be solved.10 It is essential that these interactions are known under all conditions of temperature,pressure,and composition prevailing in a PSA adsorber during all steps of the process.Because these conditions may vary widely in a practical PSA process,the measurement and correlation of such a massive volume of data are often impractical. One needs to predict these interactions from a minimum source of experimental data for the system of interest. Although much progress has been made in this area, the state of the art is often not adequate for satisfying the general design needs.
A common practice is to develop a simplified and specific model for the PSA process of interest and use simplistic descriptions(models or empirical)of the gas-solid interactions for the relevant system,to evaluate approximately the effects of the design variables on the performance of the PSA process and finally obtain a crude optimum design.The effort is always closely tied to experimental verification and fine-tuning by measur-ing actual process data from a pilot plant.Such models have often proved to be very useful for process optimiza-tion and screening new ideas.10
The partial differential equations to be solved are the coupled mass,momentum,and heat(both gas and adsorbed phases)conservation equations describing the state of the adsorber for each step of the PSA
process. The key input data for solving these equations are(i) the multicomponent adsorption equilibria,(ii)the multicomponent adsorptive/desorptive mass-transfer characteristics,and(iii)the multicomponent isosteric heats of adsorption for the system of interest.
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There lies the crux of the problem.The design engineer must be able to predict these input data from a limited source of experimental data because the range of conditions(pressure,temperature,and gas composi-tion)encountered within the adsorber of a practical PSA process can be immense.
Although numerous efforts have been made in the past30years to develop techniques to predict multi-component adsorption characteristics(thermodynamic and kinetic)from pure gas adsorption characteristics of the components,these models have met with only limited success.10,32The state of the art does not allow a priori selection of these methods for the system of interest without extensive experimental testing of the method.This obviously defeats the purpose.Currently, an extensive experimental database is needed(equilib-ria and kinetic)for the design of each separation system of interest.
The quality of simulation of a PSA process perfor-mance using a process model largely depends on th
e detailedness of the model and the accuracy of the input data.33The results can be extremely sensitive to small errors(say(10-15%)in the input data for many process designs because these models often act as amplifiers of errors.On the other hand,many industrial PSA processes require very stringent product specifica-tions.For example,a PSA process for production of H2 from SMR off-gas must have a product H2purity of 99.999+mol%containing less than10ppm of CO x impurities.Furthermore,a difference of(2%in the estimation of the product H2recovery from the feed gas can make or break the economics of a process design for a medium-sized unit(∼50MMSCFD H2).These critical industrial demands are not often appreciated, and a large volume of published literature on PSA models fails to seriously evaluate the quality of the model and the input data by comparing the model calculations with actual experimental process data only qualitatively.
Nature of the Problem
The problem at hand is by no means simple.The adsorbents of practical use,such as activated carbons, aluminas,silica gels,and zeolites,can be very hetero-geneous microporous solids.The first three types of adsorbent contain intricate networks of interconnected micro-and mesopores of various shapes and sizes,which give rise to a nonuniform distribution of gas-solid and gas-gas interaction fields within the adsorbent mass. Heterogeneity in these adsorbents is also caused by differences in th
e chemical nature of the surfaces at different parts of the adsorbent mass.The pore struc-tures of the zeolites may be ideally well-defined and uniform,but energetic heterogeneity for adsorption is introduced by lattice defects,the presence of hydrated and nonhydrated ion species of one or more kinds at different locations within the framework,the presence of trace moisture within the pore structure,nonuniform hydrolysis of the zeolite framework during regeneration, the existence of distributed Si/Al ordering of the frame-work,etc.34
Another problem is introduced by the dissimilar nature of the adsorbate molecules in a gas mixture (sizes,shapes,polarities,etc.).This can severely com-plicate their interactions with the heterogeneous porous solid surfaces and with each other inside the pores of the adsorbent.
It is apparent that realistic and accurate estimation of single-component or multicomponent gas-solid or gas-gas interactions within such distributed energy fields inside the adsorbent pores(which often remains unknown)will be difficult,if not impossible.Even the current experimental methods to characterize the pore structures of real heterogeneous microporous solids are very crude and full of uncertainty.
The challenge,therefore,is to develop methods for quantitative characterization of adsorbent heterogen
eity and to accurately predict multicomponent gas adsorp-tion characteristics(equilibria,kinetics,and heats) using a limited data source.The models,however,must also take into account dissimilar adsorbate properties and lateral interactions between the adsorbates at higher coverages.The models must also be analytical in order to be practically useful.This area should be a subject of serious research by adsorption scientists for many years to come because the problem is complex and success will not be easy.Unfortunately,the topic is being ignored in the recent years.
There is also a desperate need to generate and compile a multicomponent gas adsorption database.A large volume of pure gas and some binary gas adsorp-tion equilibrium data are currently available in the literature,35but multicomponent gas equilibrium data are rare.The situation is identical for gas adsorption kinetics data.36The data for heats of adsorption are only emerging.37-39These data will be needed to test the reliability of the theoretical models.
In conclusion,it can be said that the development of practical PSA processes for gas separation has been extremely successful despite the fact that an“in-depth”understanding of the complex physicochemical phenom-enon governing this technology may be lacking.Much more basic research is needed before a priori design of PSA processes can be made with acceptable accuracy and confidence.A judicious balance between experi-mental process development and theoretical designs u
sing process models will continue to be the state of the art in the near future.
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Shivaji Sircar
Air Products and Chemicals,Inc.,
Allentown,Pennsylvania18195
IE0109758
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