Pressure swing adsorption processes to purify oxygen using a carbon molecular sieve

Four different pressure swing adsorption (PSA) cycles using CMS for oxygen purification were developed to produce high-purity oxygen of over 99% with a high level of productivity. The cyclic performances such as purity, recovery, and productivity of the four different PSA cycles were experimentally and theoretically compared under non-isothermal conditions. In addition, one binary (O₂/Ar; 95:5 vol%) and two ternary (O₂/Ar/N₂; 95:4:1 and 90:4:6 vol%) mixtures were used to study the effects of feed composition. The PSA cycles with two consecutive blowdown steps produced oxygen with 98.0-99.9% purity and 56-66% recovery. The PSA cycle with oxygen generation in the second blowdown step produced oxygen with a higher level of purity and productivity. Also, the cycle introducing a pressure equalization step instead of a pure step produced oxygen with about 99.8% purity and 78% recovery. The period for the cyclic steady state of the ternary feed with 1% N₂ was slightly longer than that of the binary feed, while the PSA performance of the ternary feed was similar to that of the binary feed without nitrogen. However, in the ternary feed with 6% N₂, the purity of the O₂ in the purification cycles decreased by up to 97.3%. Therefore, nitrogen played a key role in producing high-purity oxygen in the CMS PSA instead of argon.     

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.     

Production of argon free oxygen by adsorptive air separation on Ag‐ETS‐10

The purification of different components of air, such as oxygen, nitrogen, and argon, is an important industrial process. Pressure swing adsorption (PSA) is surpassing the traditional cryogenic distillation for many air separation applications, because of its lower energy consumption. Unfortunately, the oxygen product purity in an industrial PSA process is typically limited to 95% due to the presence of argon which always shows the same adsorption equilibrium properties as oxygen on most molecular sieves. Recent work investigating the adsorption of nitrogen, oxygen and argon on the surface of silver‐exchanged Engelhard Titanosilicate‐10 (ETS‐10), indicates that this molecular sieve is promising as an adsorbent capable of producing high‐purity oxygen. High‐purity oxygen (99.7+%) was generated using a bed of Ag‐ETS‐10 granules to separate air (78% N₂, 21% O₂, 1% Ar) at 25°C and 100 kPa, with an O₂ recovery rate greater than 30%. © 2012 American Institute of Chemical Engineers AIChE J, 59: 982–987, 2013     

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.     

13X-APG沸石真空变压变温耦合工艺吸附捕集烟道气中CO2

采用13X-APG沸石吸附捕集烟道气中CO2,并研发了五步循环真空变压变温(VTSA)耦合吸附捕集工艺. 实验测定了循环吸附/解吸过程中吸附剂再生率、烟道气中CO2回收率、产品气量及产品气中CO2纯度,并与传统的真空变压吸附工艺(VSA)和变温吸附工艺(TSA)比较. 由于VTSA在真空解吸的同时加热吸附剂,减少了真空泵的电耗,可在较温和的真空下(约3′103 Pa)操作,附加的吸附剂再生温度也不高,90~150℃下吸附剂再生率达97%以上,CO2回收率达98%以上. 吸附剂捕集CO2的量可提高到1.8 mol/kg,是VSA工艺产品气量的2倍,且产品气中CO2纯度提高到90%以上.     

Simulation and experiment of six-bed PSA process for air separation with rotating distribution valve

In this work, a six-bed pressure swing adsorption (PSA) process was investigated to produce medical oxygen from air, which uses the combination of six-way rotating distribution valve and PSA and has the main advantage of effectively saving space compared to the traditional two-bed or four-bed PSA process and can obtain greater productivity. The mathematical model of adsorption beds was developed based on the separation mechanism and the interaction among different equipment. Moreover, a pilot-scale device has been constructed to verify the accuracy of mathematical model by experiment. The oxygen product conformed to the medical standard (>93% (vol)) with a recovery of over 57%. Some related parameters were also discussed in detail, such as step time, ratio of length to the diameter, flow rate of product.     

Adsorption and Desorption of Carbon Dioxide and Nitrogen on Zeolite 5A

The adsorption equilibrium data of CO₂ and N₂ at (303, 333, 363, 393, 423) K ranging 0-1 bar on zeolite 5A is reported. The pressure and temperature range covers the operating pressure in adsorption units for CO₂ capture from power plants. Experimental data were fitted by the multi-site Langmuir model. The adsorbent is much more selective to CO₂: loading at 303 K and 100 kPa is 3.38 mol/kg while loading of N₂ at the same pressure is 0.22 mol/kg. The Clausius-Clapeyron equation was employed to calculate the isosteric enthalpy of adsorption. The fixed-bed adsorption and desorption of carbon dioxide and nitrogen on zeolite 5A pellets has been studied. A model based on the bi-LDF approximation for the mass transfer, taking into account the energy and momentum balances, had been used to describe the adsorption kinetics of carbon dioxide and nitrogen. The model predicted satisfactorily the breakthrough curves obtained with carbon dioxide–nitrogen mixtures. Desorption process (consisting of depressurization, blowdown, and purge) was also performed. Following the feasibility of concentration and capture of carbon dioxide from flue gases by Pressure Swing Adsorption (PSA) process was simulated. A CO₂ recovery of 91.0% with 53.9% purity was obtained using a five-step Skarstrom-type PSA cycle.     

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。     

变压吸附空分制氧的技术进展

介绍了近年来变压吸附空分制氧的技术进展情况,分别从空分制氧的工艺和吸附剂的改进状况进行了详细论述,并简单的描述了空分制氧的发展前景。     

煤层气在活性炭和炭分子筛上变压吸附分离

变压吸附分离是有效的气体分离提纯方法,采用合适的吸附剂可对煤层气(CH4/N2混合气体)进行高效分离,节约能耗。在单床吸附装置上测量了CH4/N2混合气体在3种活性炭和4种炭分子筛吸附柱上的穿透曲线,并进行实验研究再生条件对吸附剂分离性能的影响。实验结果表明,7种吸附剂均对CH4/N2混合气具有一定程度的分离能力,且高温真空再生后吸附效果更好;但仍需开发出更有效的吸附剂。     

Performance Comparison of Different Separation Systems for H2 Recovery from Catalytic Reforming Unit Off‐Gas Streams

The performance of pressure swing adsorption (PSA), membrane separation, and gas absorption systems for H₂ recovery from refinery off‐gas stream was studied by simulation‐based data. The PSA process was simulated using adsorbents of silica gel and activated carbon for removing heavy and light hydrocarbons. The mole fraction profiles of all components and the relationship between hydrogen purity and recovery as a function of feed pressure were examined. The solution‐diffusion model was applied for modeling and simulation of a one‐stage membrane process. The gas absorption process with a tower tray was simulated at sub‐zero temperature and the correlation between hydrogen purity and recovery as a function of tower pressure and temperature was evaluated at different solvent flow rates.     

Kinetic separation of carbon dioxide from hydrocarbons using carbon molecular sieve

Experimental results are reported on a pressure swing adsorption (PSA) process using carbon molecular sieve (CMS) for the separation of a gas mixture containing carbon dioxide, hydrocarbons (methane, ethane, propane, etc.) and nitrogen. This PSA process has direct applications in carbon dioxide removal or purification from landfill gas, natural gas processing plants and tertiary oil recovery effluent streams. The CMS-based PSA process separates the carbon dioxide in a single stage by using the differences in component diffusivities. This approach, therefore, provides a significant advantage compared to conventional equilibrium adsorption processes which require one separation stage for removing components such as ethane and propane that are more strongly adsorbed than carbon dioxide and another separation stage for removing components such as methane and nitrogen that are less strongly adsorbed than carbon dioxide. The CMS-based PSA process operates between a feed pressure of 20 to 40 bars and a regeneration pressure of 1.5 bars at ambient temperature and produces a 98+% carbon dioxide product. The PSA process can be integrated with a liquid carbon dioxide plant to produce food grade product.     

Stable oxygen-selective sorbents for air separation

Commercial sorption-based air separation is usually done using nitrogen selective zeolites in pressure swing adsorption (PSA) systems. Separation of air by adsorption of less abundant oxygen is more desirable. In this study we have developed some stable oxygen selective sorbents with silver and cerium salts. AgCl, AgBr, AgI and CeCl₃ all showed stable adsorption characteristics with pure component selectivity of O₂/N₂∼2.0-3.0 at 1 atm. For these salts heat of adsorption of oxygen was found to be slightly higher than that of nitrogen, which was also predicted by ab initio molecular orbital calculations by Chen and Yang (Ind. Eng. Chem. Res. 35 (1996) 4020). The adsorption capacity of these salts was increased by thermal dispersion on high surface area SiO₂ support. AgBr thermally spread on SiO₂ is the best sorbent obtained in this study. AgBr/SiO₂ (1.0 g/g) showed a pure component oxygen selectivity of ∼3 at 1 atm and ∼5 at 7 atm. PSA simulations were used to show the feasibility of nitrogen production using AgBr/SiO₂.     

Effect of Drying Method on Gas Adsorption Characteristics of Carbon Gel Microspheres

Abstract

The effect of drying method used in the preparation of carbon gel microspheres was studied by comparing the porous properties of carbon cryogel microspheres (CCM) and carbon xerogel microspheres (CXM), which were respectively obtained using freeze drying and hot air drying. CCM were found to possess higher mesoporosity than CXM because freeze drying was effective to suppress the shrinkage of the mesopores during drying. On the other hand, the microporosity of the carbon gel microspheres was hardly influenced by not only the drying method but also the synthesis condition. Although the amounts of nitrogen and oxygen adsorbed were almost the same, the adsorption rate of nitrogen on both CCM and CXM possessing ultramicroporous surfaces was much larger than that of oxygen, which indicated the applicability of the carbon gel microspheres to adsorbents for pressure swing adsorption (PSA) of air. The relations between the temperature and the amount of oxygen adsorbed showed the adsorption characteristics of CCM and CXM as adsorbents for temperature swing adsorption (TSA) were almost the same.     

Comparative simulation study of PSA,VSA, and TSA processes for purification of methane from CO2 via SAPO-34 core-shell adsorbent

Cyclic adsorption processes of PSA, VSA, and TSA were modeled and numerically simulated using SAPO-34 core-shell adsorbent. The results were compared with ordinary SAPO-34 to achieve a more efficient process for CO₂–CH₄ separation. OCM coupled with method of lines was used for numerical solution of the mechanistic model. The simulation results revealed higher efficiency of core-shell adsorbent with less usage of SAPO rather than the ordinary adsorbent to achieve the same degree of purification and recovery. VSA and TSA processes against PSA resulted in CH₄ purification capability more than 99% with more than 73% recovery. However, VSA process has revealed higher productivity rather than TSA.     

π型向心径向流吸附器变质量流动特性研究

对径向流吸附器内变压吸附(PSA)制氧的变质量流动规律进行研究,有助于准确掌握吸附过程及床层内的变量因素对制氧性能的影响。对π型向心径向流吸附器建立气固耦合的两相吸附模型,并对其PSA制氧过程进行了数值模拟研究,得到了床层内氧气浓度分布、温度分布以及产品气浓度的变化规律。结果表明：首次循环结束时床层内氧气最高摩尔分数可达66.02%,回收率29.2%。非稳定循环期间,氧气摩尔分数从66.02%升高至 97.5%,回收率从29.2%提高至38.5%。循环达到稳定后,床层内氧气摩尔分数最高可达98.6%,回收率38.9%左右,且达到稳定状态后床层内气固两相温差减小,逐渐达到热平衡。获得了吸附器内部气体与吸附剂两相间的传质、传热过程,为π型向心径向流吸附器用于PSA制氧提供技术支持。     

低浓度煤层气吸附浓缩技术研究与发展

我国是一个多煤少气贫油的国家,煤层气储量约30万亿立方米,由于缺乏先进实用的低浓度煤层气甲烷分离浓缩技术,当前抽采煤层气利用率仅为50%左右。因此,对低浓度煤层气甲烷富集浓缩过程开展研究,可在开发能源的同时减少温室气体的排放,具有重大的应用价值和战略意义。简要介绍了我国煤层气资源开发利用情况,综述了近年来低浓度煤层气吸附浓缩技术研究进展,包括新型吸附材料及先进吸附工艺。对于低浓度煤层气中CH₄/N₂分离,目前文献报道吸附材料的吸附容量及分离系数仍然处于较低水平;受吸附材料的分离性能较差影响,传统变压吸附工艺对低浓度煤层气中CH₄浓缩效果并不理想。最后指出,高吸附容量、高选择性吸附材料及多种方法结合的新型吸附工艺是未来低浓度煤层气吸附浓缩技术的发展方向。     

A New Composite Sorbent for Methane-Nitrogen Separation by Adsorption

Abstract

Composite sorbents were prepared by depositing molybdenum dioxide on Super A activated carbon. The adsorption isotherms of CH₄ and N₂ were measured. The adsorption capacities and BET surface areas of these adsorbents decreased with an increase in the amount of MoO₂; however, the equilibrium selectivity ratio (CH₄/N₂) increased significantly. Therefore, an optimum quantity of MoO₂ should be used to produce a sorbent with high adsorption capacity and selectivity ratio (CH₄/N₂). An adsorbent containing 18.2 wt% MoO₂ was found to be optimum which showed a CH₄/N₂ selectivity ratio of 4.25. The selectivity ratio decreased with pressure, and leveled off to about 3 at 12 atm. The separation of a CH₄/N₂ (50/50) mixture by pressure swing adsorption (PSA) was studied by using an equilibrium model. A new PSA cycle is described by which pipeline quality methane (>90% CH₄ purity) could be produced using the composite sorbent at a recovery of 73% and a throughput of 200 L STP/h/kg.     

Efficient pressure swing adsorption for improving H<Subscript>2</Subscript> recovery in precombustion CO<Subscript>2</Subscript> capture

An efficient design for pressure swing adsorption (PSA) operations is introduced for CO₂ capture in the pre-combustion process to improve H₂ recovery and CO₂ purity at a low energy consumption. The proposed PSA sequence increases the H₂ recovery by introducing a purge step which uses a recycle of CO₂-rich stream and a pressure equalizing step. The H₂ recovery from the syngas can be increased over 98% by providing a sufficient purge flow of 48.8% of the initial syngas feeding rate. The bed size (375m³/(kmol CO₂/s)) and the energy consumption for the compression of recycled CO₂-rich gas (6 kW/(mol CO₂/s)) are much smaller than those of other PSA processes that have a CO₂ compression system to increase the product purity and recovery.     

The Pore Mouth Tailoring of Coal and Coconut Char Through Acid Treatment Followed by Coke Deposition

Carbon Molecular Sieves (CMS) obtained by coke deposition through deep cracking of hydrocarbons on the wide pore mouths of coal and coconut char are important adsorbents for separation of, difficult to separate, gaseous as well as liquid mixtures. The adsorption studies on these CMS show a high selectivity towards the adsorption of one or the other component from its mixture. In this work, CMS is prepared from pre treated raw materials like bituminous coal and coconut shell. The product samples are characterized in terms of kinetic adsorption and equilibrium adsorption of various gas adsorbents. It is observed that, all these samples are very good for CO₂ removal from mixtures containing CH₄ or H₂ in it. The CMS prepared from coconut shell showed an uptake ratio 4, for adsorption of O₂ and N₂, indicating that separation of nitrogen from air is viable by choosing suitable conditions in Pressure Swing Adsorption (PSA) Technique.