Carbon molecular sieves: production from coal and application in gas separation

Carbon molecular sieves are adsorbents for gas separation processes of increasing importance. A variety of carbonaceous adsorbents, activated carbons and improved carbon molecular sieves, all with different pore size distributions, can be produced, systematically, by using different steps in the manufacturing process. The two types of carbon molecular sieves, CMSN2 and CMSH2, carefully developed by Bergbau-Forschung GmbH, can be distinguished by different separation mechanisms. The procedure of manufacturing the carbon molecular sieves type CMSN2 and type CMSH2 and their application in an air separating process for nitrogen recovery and hydrogen or nobel gas recovery processes, respectively, using different feed gases, is discussed.     

Die Adsorptionstechnik – ein Gebiet mit Zukunft

Adsorption technology – an area with potential for the future . Adsorption offers considerable industrial potential for separation of individual components or mixtures of components from gases and liquids. Since the separation process occurs at ambient temperatures or only slightly raised temperatures it offers the advantage of energetically favourable conditions. Adsorption is also particularly good for isolation of components present in very low concentrations. It therefore plays a significant role in the development of trace technology. Future possibilities depend on both the development of high performance adsorbents and on the development of adsorption devices of high specific performance. Solution of physicochemical and of engineering problems are of comparable importance. The present article describes general fundamentals of adsorption, the principal properties of adsorbents, and some technical adsorption processes. Contemporary adsorber designs are presented and some developmental trends addressed.     

Improved ethylene and LPG recovery through dephlegmator technology

High efficiency cryogenic processes using dephlegmators have been developed for the recovery of ethylene and heavier components from gases produced in refinery and petrochemical operations. These are characterized by high hydrocarbon recovery, low energy consumption, simplicity of operation, and modest capital investment. This paper discusses principles, arrangement, and benefits of some important dephlegmator cycles. Economics for one application are presented.Dephlegmators are specially designed brazed aluminium core heat exchangers in which liquid condensed from the rising feed vapour flows back as reflux. Superior vapour and liquid product purities and recoveries follow from the action of the reflux on the vapour. The use of dephlegmators in commercial cryogenic plants has become possible through advances in understanding of the interacting heat, mass and momentum transfer processes which govern their performance.     

回收FCC干气中乙烯的工艺技术与吸附剂

概述了回收干气中乙烯的几种工艺技术和高效吸附剂。在我国现有ＦＣＣ规模和分布的情况下，利用变压吸附法回收ＦＣＣ干气中的乙烯是行之有效的工艺技术路线，且有较好的经济效益。     

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.     

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.     

我国变压吸附制氧吸附剂及工艺研究进展

新型吸附剂和变压吸附工艺开发是提高变压吸附装置性能的主要途径。本文综述了我国变压吸附制氧吸附剂改性及变压吸附制氧工艺研究进展,并指出目前存在问题:LiLSX型及Li⁺和其他离子的混合型吸附剂具有良好氧氮分离性能,但仍存在离子交换利用率低、成本高等问题;其他离子改性的吸附剂制备过程相对简单,但也有分离系数低或成本高等缺点;基于循环步骤改进的变压吸附制氧工艺优化研究大多以实验室规模制氧装置为主,未深入研究优化工艺的过程性能,难以有效地指导工业级制氧装置工艺优化;快速和双回流变压吸附制氧新工艺具有广阔发展前景,但相关研究欠深入;而多级和耦合变压吸附制氧工艺具有广阔应用前景,但存在流程复杂、能耗较高等缺点。指出未来制氧吸附剂和制氧工艺研究应进一步研究固相离子交换方法,提高LSX吸附剂Li⁺交换利用率,降低成本;开展优化工艺的过程性能研究,指导工业级制氧装置优化;加强快速变压吸附和双回流变压吸附制氧工艺研究,推动工程化应用。     

Strategies for CO2 capture from different CO2 emission sources by vacuum swing adsorption technology☆

Different VSA(Vacuum Swing Adsorption) cycles and process schemes have been evaluated to find suitable process configurations for effectively separating CO2 from flue gases from different industrial sectors. The cycles were studied using an adsorption simulator developed in our research group, which has been successfully used to predict experimental results over several years. Commercial zeolite APGIII and granular activated carbon were used as the adsorbents. Three-bed VSA cycles with- and without-product purge and 2-stage VSA systems have been investigated. It was found that for a feed gas containing 15% CO2(representing flue gas from power plants), high CO2 purities and recoveries could be obtained using a three-bed zeolite APGIII VSA unit for one stage capture, but with more stringent conditions such as deeper vacuum pressures of 1–3 k Pa. 2-stage VSA process operated in series allowed us to use simple process steps and operate at more realistic vacuum pressures. With a vacuum pressure of 10 k Pa, final CO2 purity of 95.3% with a recovery of 98.2% were obtained at specific power consumption of 0.55 MJ·(kg CO2)-1from feed gas containing15% CO2. These numbers compare very well with those obtained from a single stage process operating at1 k Pa vacuum pressure. The feed CO2 concentration was very influential in determining the desorption pressure necessary to achieve high separation efficiency. For feed gases containing N 30% CO2, a singlestage VSA capture process operating at moderate vacuum pressure and without a product purge, can achieve very high product purities and recoveries.     

Numerical simulation of low-concentration CO2 adsorption on fixed bed using finite element analysis

Accurately predicting distributions of concentration and temperature field in fixed-bed column is essential for designing adsorption processes.In this study,a two-dimensional (2D),axisymmetric,nonisothermal,dynamic adsorption model was established by coupling equations of mass,momentum and energy balance,and solved by finite element analysis.The simulation breakthrough curves fit well with the low-concentration CO2 adsorp-tion experimental data,indicating the reliability of the established model.The distributions of concentration and temperature field in the column for CO2 adsorption and separation from CO2/N2 were obtained.The sensitivity analysis of the adsorption conditions shows that the operation parameters such as feed flow rate,feed concentra-tion,pellet size,and column height-to-diameter ratio produce a significant effect on the dynamic adsorption performance.The multi-physics coupled 2D axisymmetric model could provide a theoretical foundation and guidance for designing CO2 fixed-bed adsorption and separation processes,which could be extended to other mixed gases as well.     

Influence of noncondesaables on modern thermal desalination

Today in thermal desalination it is general practice to counter the adverse effects of noncondensable gases by eliminating dissolved gases from the feed brine and by providing condensers with ejectors. This paper points out that though an ejector maintains the condenser pressure level, it does not keep the condenser free from noncondensables but keeps their concentration low. Even at low concentrations noncondensables impair heat transfer, especially in thermal desalination processes with augmented heat transfer. Consequently vents or ejecters are not the complete solution to the problem of noncondensables but are only palliatives. The development of leak resistant systems is thus shown to be a necessary step towards improving the economy of thermal desalination units.     

Sorption of Acetaldehyde and Hexanal in Trace Concentrations on Carbon‐Based Adsorbents

Adsorption processes are frequently applied to separate traces of hazardous and toxic substances from gas streams. Hence, knowledge of sorption characteristics of these substances on standard adsorbents is essential. Sorption of hexanal and acetaldehyde from a nitrogen gas stream in trace concentrations on activated carbon and ordered mesoporous carbon‐based adsorbents (CMK) is studied. A magnetic suspension balance and an attached gaschromatograph‐mass spectrometer were used to analyze the sorption process both gravimetrically and spectrometrically. Both types of adsorbents show a higher capacity for hexanal than acetaldehyde. The activated carbon exhibits considerable differences in regard to desorption of hexanal compared to the mesoporous CMK.     

Thermally robust chelating adsorbents for the capture of gaseous mercury: Fixed-bed behavior

Thermally robust chelating adsorbents for the capture of vapor-phase mercuric chloride (HgCl₂) have been developed, to address the issue of mercury removal from flue gases from coal-fired power plants. The adsorbents are mesoporous silica substrates functionalized with a chelating agent and coated with an ionizing surface nano-layer. This architecture enables selective, multi-dentate adsorption of mercury directly from the gas phase with high capacity. The capture efficiency of the adsorbents was evaluated in the fixed-bed mode for oxidized mercury at 160 °C. Two chelating adsorbents, one functionalized with 3-mercaptopropyltrimethoxysilane (MPTS) and the other with 2-mercaptobenzothialzole (MBT), were studied. For both adsorbents a high mercury uptake capacity was observed, several times higher than that of commercial activated carbon. The mechanism for mercury uptake in the two adsorbents is different. The effect of pore size on uptake was also evaluated. It was found that pore size does not have a significant effect on the mercury adsorption, and mercury diffusion through the ionic coating is believed to be the rate-limiting step for capture.     

Computational screening of porous metal‐organic frameworks and zeolites for the removal of SO2 and NOx from flue gases

Sulfur oxides (SO₂) and nitrogen oxides (NO_x) are principal pollutants in the atmosphere due to their harmful impact on human health and environment. We use molecular simulations to study different adsorbents to remove SO₂ and NO_x from flue gases. Twelve representative porous materials were selected as possible candidates, including metal‐organic frameworks, zeolitic imidazolate frameworks, and all‐silica zeolites. Grand canonical Monte Carlo simulations were performed to predict the (mixture) adsorption isotherms to evaluate these selected materials. Both Cu‐BTC and MIL‐47 were identified to perform best for the removal of SO₂ from the flue gases mixture. For the removal of NO_x, Cu‐BTC was shown to be the best adsorbent. Additionally, concerning the simultaneous removal of SO₂, NO_x, and CO₂, Mg‐MOF‐74 gave the best performance. The results and insights obtained may be helpful to the adsorbents selection in the separation of SO₂ and NO_x and carbon capture. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2314–2323, 2014     

Nickel(Ⅱ) removal from water using silica-based hybrid adsorbents:Fabrication and adsorption kinetics

A series of novel silica-based hybrid adsorbents were prepared by the crosslinking reaction of N-[3-(trimethoxysilyl)propyl] ethylene diamine(TMSPEDA) with epichlorohydrin(ECH) via a sol–gel process.Fourier transform infrared(FTIR) spectra confirmed that the reaction occurred.TGA curves showed that the thermal stability of these hybrid adsorbents reached as high as 180 °C.As a typical example,the adsorption performance of nickel(II) ions onto an adsorbent(the volume ratio of TMSPEDA and ECH was 4:1) was explored.It was found that the adsorption of nickel(II) ions onto this adsorbent followed the Lagergren pseudo-second-order kinetic model.The investigation of the adsorption mechanism demonstrated that nickel(II) adsorption was chiefly controlled by diffusion–chemisorption,suggesting that more diffusion processes were involved in the adsorption of nickel(II) ions onto this type of adsorbents.Desorption experiment indicates that these hybrid adsorbents can be regenerated.These findings reveal that this type of silica-based hybrid adsorbent is promising in the separation and recovery of nickel(II) ions from Ni-containing wastewater or contaminated water.     

Separation of ethylene/ethane mixtures by adsorption on small-pored titanosilicate molecular sieves

Adsorptive separation of ethylene and ethane may represent a less energy-intensive alternative to current sub-ambient distillation methods. In this approach, ethylene and ethane are separated by inverse-phase gas chromatographic and gravimetric isotherm techniques on ion-exchanged forms of ETS-4 and RPZ titanosilicate molecular sieves. Gas chromatography reveals that kinetic separation of ethylene from ethane is possible using ETS-4 and RPZ adsorbents exchanged with Zn and mixed Ca/H. Gas chromatographic selectivities of 4–40 for the adsorptive separation of ethylene and ethane were measured for these adsorbents, and the limiting selectivity calculated from gravimetric adsorption isotherms was between 4 and 12. In accordance with the ideal adsorbed solution theory, ethylene/ethane selectivity increases with pressure in all cases studied, especially for strontium- and barium-exchanged ETS-4. Due to the molecular sieving character of the adsorbents, steric effects were observed for the adsorption of ethylene and ethane, and contact time between the adsorbent and the gas mixture was the key factor in the separation of the gases. ETS-4 and RPZ materials exchanged with Zn or Ca/H are excellent candidates for the commercial adsorptive separation of ethylene and ethane.     

Die Adsorptionsmittel als Helfer für den Umweltschutz und technische Testmethoden

Adsorbents as Aid in Environmental Protection and Technical Test Methods Practically all impurities from air and water can be removed by adsorbents upto a concentration which is no longer harmful. A special adsorbent can be employed for each field of application. Adsorbents are classified into hydrophobic (activated charcoal) and hydrophilic (silica gel, alumina gel, molecular sieves, active bentonites) types. Activated charcoal is primarily used for the removal of substances which are insoluble or sparingly soluble in water, whereas the inorganic adsorbents are employed for the removal of water-soluble substances and gases. Combinations of the two are often suitable. The adsorbed substances can be recovered or destroyed in concentrated form by steaming the adsorbents or by regenerating the latter with a hot gas (cooling during circulation).     

Magnetically responsive porous materials for efficient adsorption and desorption processes

Magnetically responsive porous materials possess unique properties in adsorption processes such as magneticinduced separation and heat generation in alternating magnetic fields, which greatly facilitates recycling procedures, favors long-term operation, and improves desorption rate, making conventional adsorption processes highly efficient. With increasing interest in magnetic adsorbents, great progress has been made in designing and understanding of magnetically responsive porous materials varying from monoliths to nanoscale particles used for adsorption including oil uptake, removal of hazardous substances from water, deep desulfurization of fuels, and CO_2 capture in the past few years. Therefore, a review summarizing the advanced strategies of synthesizing these magnetically responsive adsorbents and the utilization of their magnetism in practical applications is highly desired. In this review, we give a comprehensive overview of this emerging field, highlighting the strategies of exquisitely incorporating magnetism to porous materials and subtly exploiting their magnetic responsiveness. Further innovations for fabricating or utilizing magnetic adsorbents are expected to be fueled. The potential opportunities and challenges are also discussed.     

Anion-pillared microporous material incorporated mixed-matrix fiber adsorbents for removal of trace propyne from propylene

Separation and purification of light hydrocarbons have been the critical processes for producing basic chemicals and polymers in petrochemical industry. Adsorbents with high performance are highly demanded for the separation of light hydrocarbons. However, the adsorbents filling in the packed bed when evaluated will inevitably induce reduced performance and increased pressure drop that considerably impede their application. Herein, mixed-matrix fiber adsorbents with anion-pillared hybrid microporous materials (SIFSIX-2-Cu-i) embedded within poly (ether sulfone) (PES) are constructed to enable an energy-efficient propylene/propyne separation. Owing to the sufficient binding sites and abundant channels for selective gas transfer, the fiber adsorbents achieve high C₃H₄ uptake capacity of 0.963 mmol g⁻¹ in the propylene/propyne (99/1) mixture breakthrough experiments. Moreover, fiber adsorbents exhibit good tolerance toward high gas velocity and show distinctively low pressure drop, which will be advantageous in industrial applications. This work provides a strategy for fabricating high-performance mixed-matrix fiber adsorbents.     

An isotherm model for adsorption of gases and vapours on heterogeneous adsorbents

The adsorption of gases and vapours on heterogeneous adsorbents is generally described by assuming that the surface of an adsorbent consists of several energetic sites or patches. The adsorption on a site or a patch can be described by a local isotherm, such as the Langmuir, BET or Jovanovich equations, whereas the heterogeneity or the energy distribution on the adsorbent is expressed by a probability density function such as normal distribution, gamma distribution and exponential distribution. The overall adsorption isotherm is then derived by summing the contributions of all sites. Although the parameters of the energy distribution are generally related to the heat of adsorption, the overall adsorption isotherm is often insensitive to the parameters of the probability density functions, which are assumed on statistical grounds. In the present work, a new isotherm model is derived for the adsorption of gases and vapours on heterogeneous adsorbents, which combines the Jovanovich local isotherm and an exponential-six type energy potential function. The new model can correlate both Type I and Type II isotherms and has been tested successfully with the literature data for the adsorption of various gases and vapours on several different heterogeneous adsorbents. The absolute average error for most of the systems is below 3%. A comparison of the present model with other models for heterogeneous surfaces is presented.     

A novel process configuration for co-production of NGL and LNG with low energy requirement

In this study a novel integrated process configuration for NGL/LNG production is introduced and analyzed. This configuration uses two mixed refrigerant cycles in order to supply the required refrigeration for production of both NGL and LNG. The results showed that not only liquefaction efficiency of the process is considerable (0.414 kWh/kg LNG) but also it can recover the ethane (from a rich typical feed gas (methane 75%, and heavier hydrocarbons 23%)) higher than 93.3%. Four multi stream heat exchangers were utilized, the composite curves of them shows that they have been designed optimally. These heat exchangers perform their role for both targets simultaneously due to the integration of the NGL and LNG processes. Even though this point increases their size, but the total capital costs of the plant decrease. This process can be used for large LNG plants in the natural gas refineries. It can also be said that the overall efficiency will be higher for the leaner feed gases.