New applications for carbonaceous adsorbents

The most important adsorption properties of carbonaceous adsorbents are based on the microporous structure which can be determined by the mechanism of pore filling at equilibrium and by the diffusion behaviour of adsorbent molecules from the external surface into the grain. New processes of application of granular activated carbon in the gaseous phase besides the solvent recovery have been developed for the adsorption of sulfur containing and radioactive gases and in the aqueous phase for the purification of drinking and wastewater. New carbon molecular sieves are the base for gas separation processes, e.g. O₂- or N₂-recovery from air or H₂-recovery from H₂ containing gases.     

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。     

碳分子筛变压吸附提纯沼气的性能

选择碳分子筛,以CH₄和CO₂为原料气,对变压吸附法提纯沼气中生物甲烷的分离性能进行了研究。采用高精度智能重量分析仪IGA-100测定了25℃下CH₄、CO₂和N₂纯组分气体在碳分子筛上的吸附平衡等温线,计算了3种气体在碳分子筛内的扩散速率CO₂>N₂>CH₄。使用单塔变压吸附装置测量了动态吸附穿透曲线,考察了吸附压力、气体流量和少量氮气等因素对吸附分离的影响,并对吸附机理做了初步探讨。实验结果表明,在吸附压力为0.4MPa、气体流量为200mL/min时,在碳分子筛上CO₂穿透吸附量为35.9mL/g,CH₄穿透吸附量为5.4mL/g,CO₂/CH₄分离系数高达12.6,可直接从吸附塔顶富集纯净的CH₄,而且碳分子筛可以通过抽真空完全再生,是一种理想的吸附材料;在有少量氮气存在的实验条件下,由于碳分子筛对CH₄和N₂具有动力学分离效应,仍能在塔顶富集高浓度的CH₄。     

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.     

Kinetische Untersuchungen der adsorptiven Luftzerlegung an Kohlenstoffmolekularsieben

In recent years, the adsorptive air separation was established as one of the most important thermal separation processes for the generation of nitrogen and oxygen from air. Both the effects of equilibrium and kinetics can be applied for the separation of nitrogen and oxygen. Nitrogen can be enriched by an equilibrium effect in the adsorbed phase of zeolites, whereas oxygen remains in the gas phase. In contrast to zeolites, oxygen can diffuse much faster into the pores of carbon molecular sieves resulting in a gas phase enriched with nitrogen. In order to exploit this kinetic separation effect in industrial applications, the properties of carbon molecular sieves, e.g., pore aperture, overall pore volume, or release of heat of adsorption are important. Furthermore, research activities are focused on theoretical approaches to describe the kinetic behavior.     

Modelling of a pressure swing adsorption process for oxygen enrichment with carbon molecular sieve

Adsorptive separation of oxygen from nitrogen and argon is carried out during the desorption steps of a pressure swing adsorption (PSA) process which uses carbon molecular sieves developed by Bergbau-Forschung GmbH. The adsorption isotherms of the three main components of air are very similar. On account of the pore size distribution of CMSN2, the diffusion coefficient of oxygen is more than eight times those of nitrogen and argon so that air separation occurs by adsorption kinetics. Experimental results for the individual steps and cyclic operation of the PSA process are presented and compared with the predictions of an isothermal plug-flow model. Adsorption rate is represented by a linear driving force equation. If the diffusion coefficients are adapted separately to every step, a good agreement is observed between the model calculations and experimental results.     

气体分离用变压吸附剂的研究进展

介绍了在应用变压吸附生产N2、O2和CO2的生产过程中,沸石分子筛和碳分子筛的吸附分离机理及它们的改性研究,并讨论了国内外目前使用的沸石分子筛和碳分子筛的优缺点,评述了现阶段国内外变压吸附剂的研究现状,进而展望了变压吸附剂的研究和发展趋势。     

气体分离用碳分子筛吸附剂的制备研究进展

介绍了作为气体分离用途的碳分子筛（CMS）的典型的和非常规的制备方法及应用。讨论了制备工艺条件对CMS分离气体性能的影响；分析了目前CMS制备工艺中存在的不足，并展望了CMS制备的发展趋势。     

低浓度煤层气提纯的研究现状

煤矿开采过程中排放出大量低浓度煤层气,提纯利用这部分煤层气对我国能源开发利用和环境保护意义重大,其难点是经济高效地分离CH4和N2.本文从CH4/N2分离技术、变压吸附分离CH4/N2吸附剂和制备新型炭分子筛3个方面逐层对低浓度煤层气提纯进行了综述和展望.介绍了CH4/N2分离技术研究进展,对其在低浓度煤层气提纯中的应用前景进行了对比.概述了常用变压吸附剂分离CH4/N2的研究现状,分析了它们在低浓度煤层气提纯应用中的优缺点,并提出了制取适合低浓度煤层气提纯用的新型炭分子筛的方法.     

Adsorption of carbon dioxide, ethane, and methane on titanosilicate type molecular sieves

The separation of carbon dioxide from light hydrocarbons is a vital step in multiple industrial processes that could be achieved by pressure swing adsorption (PSA), if appropriate adsorbents could be identified. To compare candidate PSA adsorbents, carbon dioxide, methane, and ethane adsorption isotherms were measured for cation exchanged forms of the titanosilicate molecular sieves ETS-10, ETS-4, and RPZ. Mixed cation forms, such as Ba/H-ETS-10, may offer appropriate stability, selectivity, and swing capacity to be utilized as adsorbents in CO₂/CH₄ PSA processes. Certain cation exchanged forms of ETS-4 were found to partially or completely exclude ethane by size, and equivalent RPZ materials were observed to exclude both methane and ethane, while allowing carbon dioxide to be substantially adsorbed. Adsorbents such as Ca/H-ETS-4 and Ca/H-RPZ are strong candidates for use in PSA separation processes for both CO₂/C₂H₆ and CO₂/CH₄, potentially replacing current amine scrubber systems.     

沼气变压吸附剂吸附性能比较

研究了工程上常用的3种吸附剂CO2吸附剂硅胶、13X分子筛和碳分子筛在高压(0.1~1 MPa)下对CO2和CH4的吸附容量、稳定性和选择性,以确定其在沼气变压吸附分离中应用的可行性. 结果表明,硅胶的吸附稳定性非常好,0.1 MPa时吸附选择性系数为8~10,能有效分离CH4?CO2;13X分子筛对CH4和CO2吸附容量最高、吸附选择性最好,重复使用5次CO2的吸附容量略有下降;以CH4的损失率作为评价标准时,13X分子筛吸附CH4的损失率最低. 降压不能使被碳分子筛吸附的CH4完全解吸,吸附位不能充分释放,不适用于CH4与CO2的分离.     

Structured carbon adsorbents from clay, zeolite and mesoporous aluminosilicate templates

Porous carbons templated from inorganic materials such as zeolites and mesoporous molecular sieves received considerable attention in the last decade. In this context, we discuss the effects of different templating structures on the carbons yielded. We describe templating using a commercial cationic clay (Wyoming bentonite), a commercial zeolite (Tosoh H-Beta) and a synthetic mesoporous aluminosilicate (Al-MCM-48), as exemplars of the categories of inorganic templates we have studied. The main carbon precursor used was furfuryl alcohol, complemented in some materials by an additional treatment using propylene carbon vapour deposition. The structures of the templates and carbons were compared using electron microscopy and powder X-ray diffraction. Carbons were further characterised with elemental analysis and nitrogen adsorption. The templating process is more complex than previously reported; the combination of these different techniques enhances the understanding of its mechanisms. A careful study of the synthesised carbons’ pore size distributions using DFT with various pore geometries was carried out and comparison with two commercial carbon adsorbents made, in order to assess the potential of such templated carbons for gas separation and gas storage.     

Separation of Gases by Pressure Swin

Abstract

The recent status of pressure swing adsorption (PSA) as a process for separating multicomponent gas mixtures is reviewed. The applications of a new generation of adsorbents, such as zeolites, carbon molecular sieves, and, more recently, pore engineered molecular sieves, are described in detail. The more important theories of adsorption from gas mixtures as well as those of the PSA process are described briefly. The commercial applications of PSA the process-present and potential-are discussed at length.     

Effect of regeneration pressure level in kinetically controlled pressure swing adsorption

The kinetic separation of air for the production of nitrogen by pressure swing adsorption was studied by using a highly selective carbon molecular sieve. The recovery generally increases as the specific purge decreases, and the pressure ratio increases. However, for certain ranges of purities and cycle times, a maximum in the performance is found experimentally at intermediate pressure ratios. This phenomenon can be explained with an existing model of adsorption kinetics on carbon molecular sieves: the slit-potential model. The theory is used to analyse the effect of different process variables on the pressure swing adsorption performance, and a good agreement between the experimental results and the predictions is found.     

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.     

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.     

Bindemittelfreie zeolithische Molekularsiebe der Typen LTA und FAU

Zeolite molecular sieves belong to the most important adsorbents. For dynamic, technical adsorption processes the use of shaped bodies is required. In conventional molecular sieves usually clay type minerals are used as binder. In this paper novel binderless zeolite molecular sieves and their advantages in comparison to binder containing molecular sieves are shown.     

Separation of Binary Liquid Systems by Sorption—A Comparison with Pervaporation

Abstract

Studies were carried out on the separation of the ethanol-water and the methanol-acetone systems by sorption using different commercially available molecular sieves (m.s.) as adsorbents. The separation factor (α) is found to be around 100 for m.s. 3A/ethanol-water system and is around 85 for m.s. 4A/ methanol-acetone system at respective azeotropic compositions. These separation factors appear to be better than the results reported by many workers using membrane process “pervaporation.” A comparison is made between the selectivity of pervaporation and sorption with respect to the systems studied.     

Carbon molecular sieves for gas separation processes

The development and commercialization of carbon molecular sieves (CMS) are closely connected with the development of pressure swing adsorption (PSA) processes for the separation of gases. It was already known in the 1960s that certain carbonaceous materials have a molecular sieving effect similar to that of the well known zeolitic molecular sieves. The effect was observed during basic research on anthracite and bituminous coal which are both known to be porous. However, the separation effect, e.g. for oxygen/nitrogen, was very small. It was not until the 1970s that large-scale production of uniform quality CMS suitable for commercial application in PSA processes was established. Nowadays, different types of CMS are successfully used in PSA plants, e.g. for the generation of nitrogen from air, for the production of methane from biogases, and for the recovery of hydrogen from coke oven and steam reforming gases^1–4.