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Adsorption: Journal of the International Adsorption Society (v.9, #3)
Liquid Phase Counter-Diffusion Measurements of Aromatics in Silicalite Using the ZLC Method by P. Cherntongchai; S. Brandani (pp. 197-204).
We have developed a liquid phase Zero Length Column apparatus in order to study the counter-diffusion of aromatic compounds, benzene, toluene, m-, o- and p-xylene, in silicalite crystals. Our results show ZLC desorption curves that are consistent with the model of Brandani and Ruthven (1995). The apparent diffusivities fall in a narrow range of approximately 1 order of magnitude for all the investigated molecules. The results for o-xylene led us to investigate in detail possible mechanisms that can give erroneous results when applying the liquid ZLC technique. We have derived solutions to models for valve leakage and obtained an explanation to the experimental results.
Keywords: diffusion in zeolites; silicalite; xylenes; ZLC method
Diffusion of Paraffins in Dealuminated Y Mesoporous Molecular Sieve by Célio L. Cavalcante Jr.; Neuma M. Silva; Eduardo F. Souza-Aguiar; Eledir V. Sobrinho (pp. 205-212).
Mesoporous materials have been intensely studied recently, mainly as possible component for FCC (Fluid Catalytic Cracking) catalysts due to their large surface area and accessibility to large hydrocarbon molecules. It is thus of interest for the oil industry to understand the diffusion behavior of some standard molecules in these materials. Y Zeolites, usually employed in fluid catalytic cracking, can be modified by removal of aluminum atoms from the zeolitic framework to present a greater mesoporous contribution. Dealumination of Y zeolite framework is also known to improve the stability of the catalyst thus making it more suitable for the FCC operation. This study presents diffusion measurements performed with the ZLC (Zero-Length Column) method, developed in the late eighties by Eic and Ruthven (1988a, b). The ZLC method has been largely used for a number of systems, either in gas or in liquid phases. We have now applied the ZLC method for gas phase diffusion measurements of linear paraffins (C7–C10) in dealuminated Y zeolite (USY). Experimental data were obtained at different temperatures (150 to 240°C) and flow rates (40 to 120 ml/min) and correlated through a transient Fickian diffusion model.
Keywords: diffusion; mesoporous molecular sieve; Y zeolite; ZLC—Zero Length Column
Modeling, Simulation and Operation Performance of a Simulated Moving Bed for Enantioseparation of Fluoxetine on New β-Cyclodextrin Columns by H.W. Yu; C.B. Ching (pp. 213-223).
A Simulated Moving Bed (SMB) setup, including 8 new β-cyclodextrin columns, was used to achieve the enantioseparation of fluoxetine. The effects of feed concentration and feed flowrate on the operation performance parameters: purity, enrichment, recovery and productivity were studied. A simulation approach was applied to simulate the operation and performance of the simulated counter-current system. The model predicted the performance of the transient and cyclic steady state behaviour to a reasonably good extent. The adsorption isotherm and parameters determined by pulse experiments and moment analysis seem to be adequate in simulating both the transient and steady state behaviour. Generally, this model can provide guidance for designing operation condition of the SMB system.
Keywords: modeling; simulation; simulated moving bed; enantioseparation; fluoxetine
Transport Properties of Ethane, Butanes and Their Binary Mixtures in MFI-Type Zeolite and Zeolite-Membrane Samples by Ming Jiang; Mladen Eić (pp. 225-234).
Transport properties of ethane, butane and their binary mixtures in large crystals of silicalite-1, ZSM-5, and an MFI-zeolite membrane as well as agglomerates (pellets) of silicalite zeolites have been investigated by the Zero Length Column (ZLC) method. It was found that in the large crystals of silicalite-1 and ZSM-5, and in the membrane sample desorption of iso-butane was controlled by micropore diffusion, while in the case of pelleted silicalite sample it was controlled by macropore diffusion. The effective thickness of the zeolite membrane can be reasonably evaluated by comparing the diffusivity data obtained from the ZLC and gas permeation measurements. Desorption of ethane and n-butane in the large crystals of silicalite-1 and ZSM-5 and the membrane sample is attributed to both equilibrium effects and micropore diffusion. The diffusivity of ethane is significantly reduced in the presence of iso-butane giving rise to a micropore diffusion-controlled process. Furthermore, diffusion of iso-butane in the zeolite samples is affected by the counter flow of ethane.
Keywords: ethane; butane; ZLC method; diffusion; MFI-zeolites
A Surprising Drop of the Diffusivities of Benzene in a Mesoporous Material of Type MCM-41 at Medium Pore Filling Factors by C. Krause; F. Stallmach; D. Hönicke; S. Spange; J. Kärger (pp. 235-241).
Pulsed Field Gradient (PFG) NMR is applied to study molecular diffusion of benzene in a commercial sample of MCM-41. Over a very small concentration range at medium pore filling factors the diffusivity is found to sharply drop by up to one order of magnitude, while it remains essentially constant over the total remaining range from vanishing concentrations up to over-saturation. Similarly extreme deviations from monotonous concentration patterns have so far not been described in the literature. The observed effect may be rationalised as a consequence of the onset of capillary condensation in the transport pores in the hyper-structure of the MCM-41 particles under study.
Keywords: self-diffusion; mesoporous materials; MCM-41; benzene; pulsed field gradient NMR
Trace Impurity Removal from Air by R. Kumar; M. Huggahalli; S. Deng; M. Andrecovich (pp. 243-250).
In addition to Nitrogen, Oxygen and Argon, ambient air contains many trace impurities. These impurities include moisture, Carbon Dioxide, Oxides of Nitrogen and light Hydrocarbons. Prior to cryogenic distillation of air to produce Nitrogen, Oxygen and Argon these trace impurities have to be removed since, some of these constitute a safety hazard in the cryogenic plant. A significant amount of information is available in the literature for the removal of Water and Carbon Dioxide from air. However, only limited information is available for the removal of other trace impurities. We discuss the results of an experimental study on the removal of these trace impurities from air.These days, air pre purification is carried out primarily by adsorption based technologies. There are two main choices: Thermal Swing Adsorption (TSA) processes, and Pressure Swing Adsorption (PSA) processes. Main differences between these two approaches and the trace impurity removal results are discussed.
Keywords: adsorption; trace removal; nitrous oxide; pre-purification units; PPU; air separation; ASUs
Simulation of a Combined Isomerization Reactor & Pressure Swing Adsorption Unit by Abdulhadi A. Al-Juhani; Kevin F. Loughlin (pp. 251-264).
The Total Isomerization Process developed by Union Carbide in 1970 (Gary, 1987) for the conversion of normal paraffin's to their isomers consists of a reactor followed by a PSA unit each operating at similar pressures and temperatures. The combination of these two operations in one unit in a Pressure Swing Adsorption Reactor (PSAR) process may provide an increased throughput and a significant cost saving in ancillary equipment.The simulation of a mathematical model linking the catalyst packed-bed and the adsorbent packed-bed is reported. The catalyst is a Pd/Y-zeolite and the adsorbent is 5A zeolite. The simulated feed consists of 17% each of n- and isopentane with the remainder being hydrogen. The mathematical model assumes dispersed plug-flow in both sections, constant velocity in the reactor section but varying in the adsorber, with mass transfer in the adsorber section due to external fluid film resistance and macropore diffusion in series. The fraction of the total column length occupied by the catalyst (denoted by ω) is accounted for in the model by solving numerically using orthogonal collocation on finite elements. Parameters varied are the ratio of catalyst/column length (ω), temperature range (506–533 K), high pressure (15–20 bars), with the low pressure held constant at 2 bars. The catalyst/column ratio has a strong effect at low temperatures. The optimum catalyst/column length ratio appears to be controlled by the low pressure step and occurs at ω = 0.7 for the assumptions used in this work.
Keywords: isomerization; pressure swing adsorption reactor; simulation; n-pentane; isopentane
An Experimental and Theoretical Study of the Performance of a Continuous Counter-Current Adsorber by Diana S.C. Phelps; Douglas M. Ruthven (pp. 265-274).
The performance of a simple counter-current adsorber based on the endless belt concept has been studied for vapor phase operation, using as a model system the adsorption of CO2 from N2 on a silicalite adsorbent. For this system the isotherm is essentially linear. The steady-state performance can be represented either by the Kremser equation or on a McCabe-Thiele operating diagram. The system is equivalent to 3–8 theoretical stages, depending on to the flow conditions, and both the steady-state and transient behavior are reasonably well predicted by this model. Under properly selected operating conditions it is possible to obtain a reduction in the CO2 concentration by a factor of 100.
Keywords: counter-current adsorption; CO2-N2 separation; silicalite; moving bed
Use of Concentration Pulse Chromatography for Determining Binary Isotherms: Comparison with Statically Determined Binary Isotherms by P.J.E. Harlick; F.H. Tezel (pp. 275-286).
The characterization of adsorption properties of binary gas mixtures is an important factor in the design of cyclic periodic adsorption processes. Presently, there are a few methods for determining the binary adsorption behaviour. However, most of these methods are very time demanding (static approaches). One dynamic approach for determining the binary adsorption behaviour is by employing the concentration pulse method. Although this experimental method has been shown to reliably reproduce the data obtained with static approaches, the application has been limited to systems where the adsorption capacities are similar. In our previous work, a novel method (HT-CPM) was shown to be capable of handling systems where the adsorption capacity of the heavy component is much greater than the light component.In order to further validate the novel method, HT-CPM, previously published statically determined binary adsorption data have been used to supply the data required for the concentration pulse method (CPM) data reduction techniques. From this data, the HT-CPM was then applied and the resulting data compared. The two other CPM approaches were also applied and these results are shown.Four binary systems were chosen so that a range of heavy/light adsorption capacities was examined. The results show that the HT-CPM was able to produce results that were consistent with the static data taken from the literature. The other data reduction techniques used with the CPM could only produce meaningful data when the adsorption capacities of the heavy and light components were similar.
Keywords: adsorption; equilibrium; binary isotherms; multi-component; HT-CPM; concentration pulse method