Air Fractionation by Adsorption

Abstract

Pressure swing adsorption (PSA) processes for air separation differ by the modes and conditions of operation of the adsorption, the desorption, and the complementary steps, as well as by the types of adsorbents used. Three commercial PSA processes for air separation are reviewed and compared. The first process uses a zeolitic adsorbent and produces only an oxygen-enriched product gas. The second process uses a carbon molecular sieve and produces only a nitrogen-enriched product gas. The third process uses a zeolite and simultaneously produces both oxygen-and nitrogen-enriched product gases. The performance and separation efficiency of the last process, called the ‘vacuum swing adsorption (VSA) process’, are reported to be superior to the others.     

Oxygen Production by Pressure Swing Adsorption

Abstract

The specific oxygen production capacity and the oxygen recovery of a pressure swing adsorption (PSA) process for the production of oxygen from ambient air by selective adsorption of nitrogen can be increased by operating the process at a superambient temperature. The higher temperature operation provides more efficient desorption of nitrogen from the adsorbent which more than off-sets the detrimental effects of the lower selectivity and capacity of adsorption of nitrogen from air at the elevated temperature. The concept is demonstrated by evaluating the performance of an eight-step PSA-oxygen process to produce a 90% oxygen product stream at different temperatures. It is shown that 10% higher oxygen production capacity and 14.5% larger oxygen recovery can be obtained by operating the PSA process at 60°C compared to its performance at 30°C. The PSA process and its performance data from a pilot plant are discussed.     

空分富氧沸石分子筛吸附剂的研究

介绍了国内外目前以PSA技术进行空气分离制备氧气所用沸石分子筛吸附剂的研究状况。从研究结果来看，N2吸附容量和N2／O2分离选择性的提高主要通过对沸石分子筛4A和13X进行离子交换，以对其表面进行改性，从而调整对N2、O2的吸附性能。另外，沸石分子筛制备过程中的硅铝比和成型条件等对N2和O2的吸附也有一定的影响。     

A Pressure Swing Adsorption Process for Production of 23–50% Oxygen-Enriched Air

Abstract

A simple pressure swing adsorption process for direct production of low to medium purity (23–50%) O₂-enriched gas from ambient air is described. The process provides a high O₂ production capacity per unit amount of the adsorbent and a high O₂ recovery combined with a very low energy requirement for the separation. The performance of the process using three different air separation adsorbents is described.     

Discovery of novel zeolites for natural gas purification through combined material screening and process optimization

An efficient computational screening approach is proposed to select the most cost‐effective materials and adsorption process conditions for CH₄/CO₂ separation. The method identifies eight novel zeolites for removing CO₂ from natural gas, coalbed methane, shale gas, enhanced oil recovery gas, biogas, and landfill gas sources. The separation cost is minimized through hierarchical material screening combined with rigorous process modeling and optimization. Minimum purity and recovery constraints of 97 and 95%, respectively, are introduced to meet natural gas pipeline specifications and minimize losses. The top zeolite, WEI, can recover methane as economically as $0.15/MMBTU from natural gas with 5% CO₂ to $1.44/MMBTU from natural gas with 50% CO₂, showing the potential for developing natural gas reservoirs with higher CO₂ content. The necessity of a combined material selection and process optimization approach is demonstrated by the lack of clear correlation between cost and material‐centric metrics such as adsorption selectivity. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1767–1785, 2014     

Nanocrystalline Zeolite Tetraethylammonium‐Beta Membrane for Preferential Sorption and Transport of CO2 over N2

CO₂/N₂ gas separation was performed over a nanocrystalline zeolite tetraethylammonium (TEA)‐beta membrane prepared on a stainless‐steel porous disc by repeated hydrothermal crystallization. Two to three consecutive hydrothermal syntheses were required to form a membrane comprised of a continuous and compact layer of zeolite beta nanocrystals on the support. The membrane TEA‐BEA3 obtained by three consecutive syntheses, in which the membrane from two consecutive syntheses was used as support, exhibited the highest structural order. When the separation experiment was performed over this membrane without applying any external applied pressure, 100 % selectivity of CO₂ over N₂ was observed. The separation was driven by differences in chemical potentials of the molecules generated only by the adsorption‐desorption behavior of the gases into the membrane. The novel zeolite TEA‐beta membrane provided promising results for the separation of small gas molecules due to the combined influence of diffusion and sorption selectivity.     

Feed Purge Cycles in Pressure Swing Adsorption

Abstract

Three new pressure swing adsorption cycles for the production of oxygen-enriched air at very high recoveries are developed. The cycles involve a purge step using low-pressure feed gas. Local equilibrium theory is used to predict purities and recoveries for air separation with 5A zeolite. Novel methods for drying the inlet air are developed. For low product purities these processes are predicted to have very high recoveries and adsorbent productivities. A simple new method for determining the location of a curved shock wave is illustrated. Other possible applications of PSA feed purge cycles are briefly discussed.     

Computer simulation of the gas separation properties of zeolite Li-X

The criteria determining the effectiveness of a particular zeolite for gas separation are the physical pore size and the location, size, and charge of any cations present. To date the experimentalist has had to use a great deal of intuition when selecting a zeolite for a specific use. Computer modelling of such systems, using a Grand Canonical Monte Carlo method, has been successful in elucidating the behaviour of adsorbates in a wide range of systems. Successful predictions for adsorption isotherms for nitrogen, oxygen and argon have been previously reported by the authors for zeolites A, X and Y with calcium and sodium cations.The aim of the work reported in this paper is to investigate the air separation properties of a different, although similar system namely: zeolite X with lithium cations. The simulations performed using Cerius² molecular modelling software are able to predict adsorption isotherms for nitrogen and oxygen gases, both as single component, and as binary mixtures in Li-X. Further the predicted equilibrium separation factor is calculated to be in the range of 6 to 13 at room temperature, making this system ideal for the preferential adsorption of nitrogen and production of oxygen.     

Binary adsorption behaviour of methane and nitrogen gases

Separation of methane and nitrogen gases is critical in the upgrading of LFG (Landfill gas), natural gas and coal bed gas in order to have a commercial heating value for methane. From an environmental point of view, methane capture from landfill gas is essential to prevent greenhouse gas emissions. Adsorption could be a beneficial process to capture low purity methane from a landfill site that is nearing the end of its lifecycle and produce high purity methane. In this work, Ceca 13X zeolite and Alcan Activated Alumina AA 320-AP have been studied for their potential for this separation and compared with Silicalite in literature. Pure and mixture adsorption isotherms were determined at 40 and 100?°C for these adsorbents by constant volume method and concentration pulse chromatographic technique, respectively. Mixture adsorption isotherms for the binary system of methane and nitrogen gases at 40 and 100?°C and 1 atmosphere total pressure have been determined by VV?CCPM (Van der Vlist and Van der Meijden Concentration Pulse Method). The application of Extended Langmuir model for this binary system have also been discussed and compared to the experimental results. Results show that equilibrium separation factor for silicalite is larger than zeolite Ceca 13X and Alcan activated alumina AA320-AP. Both Silicalite and Ceca 13X find application in the bulk separation of methane from nitrogen when y _CH4?>?0.4, especially in LFG, coal bed gas and natural gas.     

Vacuum Sorption Pumping Studies of Argon and Oxygen on 4A Molecular Sieves

Abstract

Cryosorption pumping is a method of evacuating enclosed volumes by adsorbing gas on a deep bed of sorbent, such as Davison 4A zeolite, at cryogenic temperatures. Modeling the dynamic behavior of these systems for air pumping requires information on two major constituents of air: oxygen and argon. Particle size variation was the major variable in determining the mechanism of the process. The model comprises a fluid-phase mass balance representing the dynamics of gas in the bed and a spherical, one-dimensional diffusion equation describing adsorption in pellets of 4A zeolite. The present model calculates effective pore diffusivity taking into account Knudsen diffusion, ordinary diffusion, and Poiseuille flow. The primary diffusional resistance appears to occur in the macropores formed by the pelletizing clay binder, rather than in the bed interstices or microporous zeolite crystals.     

Sr2+ Exchanged Zeolite X as an Adsorbent Material for Chromatographic Separation of Argon-Oxygen Gaseous Mixture

The separation of argon and oxygen from their gaseous mixture is very difficult to accomplish by the adsorption process using zeolite adsorbents due to the closeness of their molecular properties and adsorption behavior in the zeolites. Strontium exchanged zeolite X (SrX) showed the adsorption selectivity for oxygen over argon at ambient temperature and demonstrated its potential as a column–packing material for the gas chromatographic analysis of argon-oxygen mixtures. The adsorption capacities and Henry's law constants for oxygen and argon increased in SrX compared to NaX at ambient temperature. Activation of the SrX adsorbent is shown to play a highly significant role in the separation of argon-oxygen mixture due to the low hydrothermal stability of SrX.     

Air separation by pressure swing adsorption

The original pressure swing air separation process, developed almost simultaneously by Exxon and Air Liquide, uses a nitrogen selective zeolite adsorbent to produce a high purity oxygen product. The same basic process is still widely used in small scale units although, for larger scale units, many modifications to the cycle have been introduced in order to reduce power consumption. Although nitrogen can in principle be recovered from the blowdown stream of such systems, if high purity nitrogen is the required product, it is more economic to use an oxygen selective adsorbent. Most adsorbents show either no selectivity or preferential adsorption of nitrogen. However, in small pore carbon molecular sieves or 4A zeolite there is a substantial difference in diffusion rates so that an efficient kinetic separation is possible. Somewhat different cycles are generally used in such processes. Progress in modelling the dynamic behaviour of both types of PSA system is reviewed and comparisons between experimental performance and the model predictions are shown. A simple linear driving force model provides a good overall prediction of the effects of process variables but the computationally more cumbersome diffusion model gives better quantitative agreement with experiment. Comparisons are drawn between the performance achieved (in nitrogen production) with two different kinetically selective adsorbents; RS-10 (a modified 4A zeolite) and Bergbau Forschung carbon molecular sieve.     

Sorption of nitrogen,oxygen, and argon in Cd (II) exchanged zeolite X: volumetric equilibrium adsorption and grand canonical Monte Carlo study

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) cations exchanged zeolite X at 288.2 and 303.2 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite X was showed that increased adsorption capacity for nitrogen, oxygen, and argon with increase in Cd (II)-exchange levels, indicating as charge density increases adsorption capacity also increase. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Cd (II)-exchanged zeolites than argon molecules. The selectivity of N₂/O₂ of cadmium-exchanged zeolite X is better than only sodium containing zeolite-X. Heats of adsorption and adsorption isotherms were also calculated using GCMC simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.     

Use of Clinoptilolite in Ethanol Dehydration

Abstract

Clinoptilolite-type natural zeolite, which exists in various regions of Turkey, has been experimentally studied. For the ethanol-water-local clinoptilolite system, uptake and breakthrough curves were determined under a nitrogen gas atmosphere. In adsorption kinetics and adsorption equilibrium studies, the effects of particle size, temperature and, amount of zeolite on the uptake rate have been investigated. The breakthrough curves for four different flow rates of ethanol and three different bed heights were determined in dynamic column studies. The results of the experiments show that intraparticle diffusion is the main resistance. The local clinoptilolite is a promising adsorbent for water adsorption from aqueous ethanol.     

Separation of Methane and Nitrogen by Adsorption on Carbon Molecular Sieve

Abstract

Adsorption equilibrium of methane and nitrogen on CMS 3K from Takeda Corp. were gravimetrically measured at 298, 308, and 323 K and at pressures up to 2000 kPa. The most adsorbed gas is methane followed by nitrogen. The adsorption loading at 550 kPa and 308 K is 1.73 mol/kg for methane and 0.91 mol/kg for nitrogen. Experimental data were fitted with the multisite Langmuir model. Single component uptake of these gases at low pressures was used to determine the adsorption kinetics. Adsorption of nitrogen is much faster than methane, although this gas is preferentially adsorbed. The adsorption rate of both gases was controlled by a surface barrier resistance at the mouth of the micropore combined with micropore diffusion. Breakthrough curves of pure gases and their binary mixtures were measured at ambient temperature. A bi‐LDF (Linear Driving Force) model was used to predict the fixed‐bed behavior. Large differences in the adsorption kinetics were observed: at 308 K the LDF constant ratio was K_μ,N2 /K_μ,CH4 =133, although because of much higher adsorption of methane, the overall kinetic selectivity was 1.9 at 308 K. The data obtained in this work can be used for adsorption separation processes modeling for methane purification from nitrogen‐contaminated streams.     

干气中烷烃、烯烃新型分离吸附剂的研究进展

综述了二十年来烯烃烷烃新型分离吸附剂的研究进展,与传统的π络合吸附剂相比,优先吸附烯烃的吸附剂包括金属有机材料（MOF）、Engelhard Titanosilicate （ETS）、高硅分子筛和介孔分子筛,主要是利用氢键作用、孔径大小、分子扩散速率差异或π络合作用,将烷烃烯烃分离;而优先吸附烷烃的吸附剂包括AlMePO-α、ZIF-7和凹凸棒黏土,主要利用吸附剂上甲基与烷烃的相互作用。MOF对烯烃吸附量大,但选择性较差;高硅分子筛对烯烃的动力学分离效果良好;介孔分子筛经过渡金属改性后,对烯烃有优良的选择性;ETS类对烯烃有较高的吸附量和选择性,且稳定性强,有很好的应用前景。     

Understanding the performance of membrane for direct air capture of CO2

Direct air capture (DAC) of CO₂ is becoming increasingly important for reducing greenhouse gas concentrations in the atmosphere. However, the cost and energy requirements associated with DAC make it less economically feasible than carbon capture from flue gases. While various methods like solid sorbents and gas–liquid absorption have been explored for DAC, membrane processes have only recently been investigated. The objective of this study is to examine the separation performance of a membrane unit for capturing CO₂ from ambient air. The performance of a membrane depends on several factors, including the composition of the feed gas, pressure ratio, material selectivity, and membrane area. The single-stage separation process with the co-current flow and constant permeability flux model is evaluated using a commercial module integrated with a process simulator to separate a binary mixture of carbon dioxide and nitrogen to assess the sensitivity of selectivity on purity and recovery of CO₂ in permeate, and power requirement. Additionally, three levels of CO₂ reduction from the feed stream to the retentate stream (25%, 50%, and 75%) are studied. A trade-off between purity and recovery factor is observed, and achieving high purity in permeate requires high concentration in the retentate.     

Propane/Propylene Separation by Simulated Moving Bed II. Measurement and Prediction of Binary Adsorption Equilibria of Propane,Propylene, Isobutane,and 1-Butene on 13X Zeolite

Abstract

The design of a simulated moving bed (SMB) process relies on valid thermodynamic predictions of multicomponent adsorption built up from accurate binary adsorption equilibrium data. Experimental adsorption equilibria of binary mixtures constituted by propane, propylene, isobutane and 1-butene on 13X zeolite were determined using breakthrough experiments at 373 K and 150 kPa. In addition, these binary adsorption experiments allow to confirm the choice of isobutane as an interesting desorbent for the separation of propane-propylene by SMB, since it has an intermediate selectivity between the two species to separate. Various prediction models are available in the literature but only a few of them have both physical and thermodynamical consistency. The ideal adsorbed solution theory (IAST), the thermodynamically consistent extended Toth model (TCET), and the physically-consistent extended Toth isotherm (PCET) were used to predict binary adsorption equilibria from pure component adsorption isotherms parameters. The PCET model was found suitable for representing the adsorption equilibrium of the different hydrocarbon mixtures with a reasonably good accuracy.     

Desorption Kinetics of A N,N-Dimethylformamide Vapor from Granular Activated Carbon and Hydrophobic Zeolite

Abstract

Such thermodynamic properties as enthalpy, free energy, and entropy of adsorption have been computed for N,N-dimethylformamide (DMF) vapor on two commercial adsorbents: coconut shell Type PCB of activated carbon and Type DAY of hydrophobic zeolite. The computation is based on the Langmuir adsorption isotherms obtained at 293, 303, and 313 K as reported by Tsai et al. The laden adsorbents were regenerated with hot inert nitrogen gas and studied by thermal gravimetric analysis at three different heating rates. The apparent activation energies (E _des) of thermal desorption were determined by using the Friedman method. The zeolite DAY has an adsorption potential higher than that of activated carbon PCB as indicated by the more negative value of the adsorption enthalpy of DMF vapor. The average value of E _des of zeolite DAY is larger than that of activated carbon PCB.     

Performance of iron-incorporated A-type zeolites for O2/N2 separation from air by pressure swing adsorption

The performance of various metal ion-exchanged A-type zeolites and metal-incorporated A-type zeolites for O₂ and N₂ adsorption was studied to provide a pressure swing adsorption (PSA) process. Metal-incorporated A-type zeolites adsorbed N₂ in larger quantities than metal ion-exchanged A-type zeolites. Compared with Na-A zeolite, both Cu-incorporated and Ni-incorporated A-type zeolites adsorbed larger quantities of N₂. The incorporation of Cu or Ni enlarged the pore size of the zeolite, while Fe incorporation reduced it. However, the adsorption volume ratio of O₂ to N₂ could be increased to as high as 4.2 on Fe-incorporated A-type zeolite calcined at 750°C, which was the highest value among the samples tested. The amount of adsorption of O₂ was 38.0 ml g⁻¹, which was comparable with ordinary Na-A zeolite. The Fe incorporation markedly improved the performance of ordinary Na-A zeolite in O₂/N₂ separation. Therefore, Fe-incorporated A-type zeolite has a high potential as a good adsorbent for pressure swing adsorption in O₂/N₂ separation from air.