Comparison of the adsorption dynamics of air on zeolite 5A and carbon molecular sieve beds

Adsorption and desorption in zeolite 5A and CMS beds were compared by using a ternary mixture (N₂/ O₂/Ar; 78 : 21 : 1 vol%). Because the breakthrough curves for both beds show a tail by temperature variance, a non-isothermal mathematical model was applied to the simulation of adsorption dynamics. The LDF model with a constant rate parameter was enough to predict the experimental breakthrough and temperature curves of an equilibrium separation bed, while the modified LDF model with a concentration-dependent parameter should be applied to a kinetic separation bed. In the CMS bed initially saturated with He, Ar was the first breakthrough component with N₂ following after a short interval. Then, after a long interval, the breakthrough of O₂ occurred with a broad roll-up due to its fast diffusion rate and the relatively slow diffusion rate of N₂. In the CMS bed initially saturated with O₂, the breakthrough curves of O₂ and N₂ showed a very broad shape because of the slow diffusion of N₂ into CMS. In the zeolite 5A bed, the breakthrough time sequence was Ar, O₂, and N₂ at very close time intervals. After the sharp roll-ups of O2 and Ar, the variation of the breakthrough curves was negligible. The inflection of the temperature profile in the zeolite 5A bed was caused by the crossover of the O₂ and N₂MTZs, while in the CMS bed it was caused by the difference in the diffusion rates of O₂ and N₂.     

Effects of capillary condensation on adsorption and thermal desorption dynamics of water in zeolite 13X and layered beds

The effects of capillary condensation on the adsorption and thermal desorption dynamics of water in zeolite 13X beds and layered beds with zeolite 13X/silica gel or zeolite 13X/alumina were experimentally and theoretically studied. As the equilibrium isotherm of water on zeolite 13X pellet was found to be most favorable at a low relative humidity and indicated capillary condensation at a high relative humidity, it was possible to construct a non-isothermal model that included capillary condensation and that could successfully predict plateaus of temperature and concentration profiles in thermal regeneration. In adsorption breakthrough, by using a feed in the capillary condensation range of the isotherm on zeolite 13X, the breakthrough curve showed a shock wave in the low concentration and a proportionate pattern in the high concentration. In thermal desorption breakthrough, the desorbed water at the upper part of the bed was re-adsorbed at the lower part of the bed, and that re-adsorption mainly occurred in the capillary condensation range of the isotherm. Therefore, even though an adsorption was performed at a feed in the favorable range of the isotherm, and could be well predicted with type I isotherm, its desorption dynamics should be predicted by using the isotherm model with its consideration of capillary condensation. The layered bed with silica gel or alumina did not have any advantage over the zeolite 13X bed with respect to adsorption breakthrough performance. However, compared to the zeolite 13X bed, the complete regeneration time in the layered bed was drastically shortened due to a greater variation of the amount of equilibrium adsorption of water under temperature on both silica gel and alumina. In addition, since an increase in temperature led to a greater decrease of the amount of equilibrium adsorption of water on silica gel than on alumina, a layered bed with silica gel obviously could be regenerated more efficiently than a layered bed with alumina.     

Residual adsorption capacity of carbon beds

The retention time of a small (0.3 cm³) ethane pulse, to which various dry weights (1–3 g) of activated carbon were exposed, was studied as a nondestructive method of determining the residual adsorption capacity of the bed. Carbon beds, partially saturated with CCl₄ or water, adsorbed the ethane from and then desorbed the ethane into the nitrogen carrier gas stream. At a fixed flowrate and fractional carbon saturation the retention time in the bed varied linearly with carbon weight. A critical bed weight existed, below which the retention time of ethane in the bed was zero. The logarithm of a dimensionless time parameter, normalized with respect to the effective bed weight (total weight minus critical weight), was a linear function of the percentage carbon saturation (or the percentage residual adsorption capacity) of the bed. This approach constitutes a nondestructive, in situ method for determining the residual adsorption capacity of an activated carbon bed that is independent of bed weight.     

Modelling of adsorption separation of gas mixtures with allowance made for the thermal effects accompanying adsorption: III. Model equations for the non-isothermal kinetics and dynamics of adsorption of multicomponent mixtures

Model equations have been obtained for the non-isothermal kinetics and dynamics of adsorption of multicomponent mixtures allowing for the major features peculiar to the heat and mass exchange inside and outside the porous grains in a porous medium: the essential influence of the interdiffusion of the mixture components; the dependence of the diffusion and interdiffusion coefficients on the mixture component concentrations and temperature; the essential influence of the thermal diffusion and diffusion-thermal effects. It has been shown that the model equations are appropriate for describing the front behaviour of the non-isothermal dynamics of mixture adsorption in the adiabatic and non-adiabatic cases.     

Adsorption of pollutants on to activated carbon in fixed beds

The adsorption of certain pollutants, namely phenol, p-chlorophenol, sodium dodecyl sulphate and mercuric ions, on to activated carbon has been studied using fixed bed systems. There are three main methods of contacting in solid/liquid adsorption systems, namely batch, fixed bed and fluidized bed systems. In fixed bed adsorption the adsorption rate is determined on the basis of adsorption equilibrium (unfavourable, linear, favourable or completely irreversible) and the controlling mechanism (external film mass transport, internal pore diffusion, internal solid phase diffusion or longitudinal diffusion). One or more of the previous transport mechanisms may be rate controlling depending on the solute-adsorbent system. For an adsorbent like activated carbon which is highly porous both external transport and pore diffusion will be very important. An adsorption model, based on external mass transport and internal pore diffusion, has been applied to the systems to predict theoretical breakthrough curves. These curves have then been compared with experimental data and using a ‘best fit’ technique, an effective pore diffusion coefficient can be determined for each sorbate–carbon system.     

Competitive adsorption of two dissolved organics onto activated carbon—III: Adsorption kinetics in fixed beds

Adsorption breakthrough curves for bisolute systems of dissolved organics on activated carbon are measured in fixed beds.Results for strongly adsorbable species indicate that at low liquid concentrations (X<0.1 mmol/l.) only external mass transfer resistance is rate determining.However, at higher liquid concentrations internal mass transfer becomes increasingly significant. Breakthrough behaviour is predicted using alternatively three different models with different assumptions about diffusion in the liquid filled pores and diffusion on the surface in series with external film diffusion.Multi-solute adsorption equilibria are predicted from single-solute data using the ideal adsorbed solution theory developed by Myers and Prausnitz, while the single-solute equilibria are represented by Freundlich isotherms. The external mass transfer coefficient for each component is calculated by a general correlation for heat and mass transfer in fixed beds. The internal diffusion coefficient for each component is determined in batch reactor tests with the single-solute system.Systematic deviations between measured breakthrough curves and those calculated from different models using only single-solute data are observed in all experiments with mixed solutes if there is significant internal diffusional resistance and marked displacement of one component inside the carbon particles. The deviations may be due to mutual interference of diffusing molecules. A better agreement between calculated and observed breakthrough curves can be obtained using an extended model in which mixture data are required.     

Hydrate phase equilibria for gas mixtures containing carbon dioxide: A proof-of-concept to carbon dioxide recovery from multicomponent gas stream

Three-phase equilibrium conditions (aqueous liquid-hydrate-vapor) of CO₂-N₂ binary mixtures in the temperature range of 271.75 K to 284.25 K and the pressure range of 12 to 235 bar. In addition, three-phase (aqueous liquid-hydrate-vapor) behavior for CO₂-CH₄ mixture were measured in the temperature range of 272 to 284 K at the constant pressures of 15, 20, 26, 35 and 50 bar. In high concentration of CO₂, upper quadruple points were also measured. The obtained data indicates that three-phase equilibrium temperatures become higher with increasing concentration of CO₂. For the prediction of three-phase equilibrium, the vapor and liquid phases were treated by employing the Soave-Redlich-Kwong equation of state (SRK-EOS) with the second order modified Huron-Vidal (MHV2) mixing rule and the hydrate phase with the van der Waals-Platteeuw model. The calculated results showed good agreement with experimental data. The concentration of vapor and hydrate phases was also determined experimentally. This work can be used as the basic data for selective separation process by hydrate formation. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

The choice of distributor to bed pressure drop ratio in gas fluidised beds

The choice of the gas distributor to bed pressure drop ratio for the stable operation of gas fluidised beds is determined by the type of gas distributor and by the characteristics of the bed material. A critical analysis regarding the choice of the ratio has been made and reasons for diverse views expressed in the literature have been offered. An equation to determine U_M, the superficial gas velocity at which all the orifices in a distributor become operative in a uniformly fluidised bed has been suggested, i.e.

where U_mf and U_t are the minimum fluidisation velocity and the terminal velocity respectively. The distributor to bed pressure drop ratio (ΔP_d/ΔP_b) can be calculated from U_mf and U_M using the equation

where the constant C equals 2.     

Liquid-phase thiophene adsorption on MCM-22 zeolite and activated carbon

The liquid-phase adsorption of thiophene from thiophene/iso-octane and toluene-rich thiophene/toluene/iso-octane solutions has been investigated in batch conditions at room temperature and atmospheric pressure on MCM-22 zeolite and activated carbon. The adsorption of toluene from toluene/iso-octane solutions has also been investigated on both the adsorbents. These were characterised by X-ray diffraction, nitrogen physisorption, scanning electron microscopy and adsorption microcalorimetry of ammonia. Both MCM-22 and activated carbon were able to adsorb thiophene and toluene from the corresponding single-adsorptive iso-octane solutions. MCM-22 appeared superior to activated carbon for the adsorption of thiophene from toluene-rich thiophene/toluene/iso-octane mixtures. The adsorption data for MCM-22 were correlated with the acid sites and silanol groups concentrations, as determined by microcalorimetric experiments. No such correlation was found for activated carbon. Apparently, on MCM-22 thiophene adsorption occurs first on the acid sites until they are saturated and then continues on the silanol groups. Toluene adsorption also occurs on the acid sites of MCM-22 until saturation, but no further adsorption by interaction of toluene with the silanol groups takes place. GC/MS investigation of the nature of the organosulphur material retained by the adsorbent revealed that the tiophene adsorbed on MCM-22 undergoes transformation into heavy products. Besides thiophene trimers and tetramers, these include other compounds formed by two and three thiophenic rings linked by alkyl groups, as well as a rather high molecular weight product where a trimer is linked to thiophenic ring by a cyclohexadienic ring. Reaction pathways leading to these products have been tentatively outlined.     

气体组成对活性炭吸附放射性气体影响研究

建立了可模拟一定相对湿度气流条件下的活性炭性能评价系统。研究湿度、CO_2及Xe浓度对活性炭吸附放射性气体性能影响。结果表明,一定条件下气流相对湿度升高不利于活性炭吸附放射性气体;对于放射性氪氙,随着CO_2浓度升高,活性炭吸附性能逐渐降低,气流中稳定性氙浓度影响可以忽略。     

Acid gas adsorption on zeolite SSZ-13: Equilibrium and dynamic behavior for natural gas applications

In developing new adsorption separation processes, it is necessary to study both the equilibrium and dynamic adsorption properties of potential materials. Experimental determination of isotherms and dynamic breakthrough properties aid in the development of modeling new adsorption systems toward process development. Here, the equilibrium adsorption properties of a small-pore zeolite, Na-SSZ-13, are studied for its natural gas separation potential. Using volumetric, gravimetric, and dynamic column breakthrough adsorption techniques, the adsorption properties of CO₂, CH₄, C₂H₆, H₂S, and H₂O are determined. High-pressure breakthrough experiments demonstrate the mixed gas separation performance of Na-SSZ-13 in mixtures containing CO₂, CH₄, C₂H₆, and H₂S. Simulations of these breakthrough experiments show that ideal adsorbed solution theory adequately describes the mixed gas adsorption modeling for this zeolite. In gas mixtures containing both CO₂ and H₂S, there is an observed acid gas reaction that results in elution of carbonyl sulfide, COS.     

Adsorption dynamics measured by permeation and batch adsorption methods

We have measured the adsorption equilibrium and kinetics of carbon dioxide on a commercially available activated carbon by two methods; permeation and batch adsorption. The two methods are compared and found to yield consistent results. All experiments are performed at low pressure (<5 torr) and in this range the isotherm was found to be reversible and non-linear. Because of the observed non-linearity, the batch adsorption experiments were conducted differentially in order to obtain the adsorbed phase diffusivity at local conditions. The diffusion process was described by gas phase diffusion, adsorption and adsorbed phase diffusion which was modelled using the Darken relation. Results of adsorption equilibria and kinetics will be discussed in detail in this investigation.     

Prediction of multicomponent adsorption equilibrium of gas mixtures including supercritical components

A new model to predict the adsorption equilibrium of mixtures containing supercritical components was presented. The difference in the adsorption mechanism between the supercritical components and the condensable components was accounted for in the new model. Lateral interaction in the adsorbed phase was also considered in the local isotherm. The new model was verified by the experimental data of the mixture CH₄/CO₂/C₂H₆ on activated carbon JX101 collected for 283-313 K and pressures up to . It was also verified with the experimental data published previously in literature. Considerable improvement in the prediction precision was achieved in using the new model to predict the adsorption of mixtures on activated carbon.     

Adsorption dynamics of carbon dioxide in molecular sieving carbon

The sorption equilibrium isotherm of carbon dioxide at 20 °C on a commercially manufactured carbon molecular sieve has been measured with a variable volume (vacuum to high pressure) volumetric adsorption apparatus. Measurement was taken over the pressure range <10-2000 Torr and the isotherm is characterized by Dubinin-Radushkevich analysis which provides the micropore size distribution. The equilibrium information is subsequently employed to characterize the dynamics of adsorption and it is shown that the uptake of carbon dioxide is Fickian with some deviation from Fickian behavior noted at lower pressures. The derived mobility parameter agrees reasonably well with that predicted by the Darken relation over more than a 200-fold change in pressure.     

Adsorption of gas mixture on activated carbon

Equilibrium adsorptions of H₂ and CH₄ from their mixture on three commercial activated carbons are measured by analyzing the breakthrough curves of the more strongly adsorbed CH₄ at 25 and 204°C and pressures up to 1000 psia. Three methods of predicting adsorption of mixture based on single-gas isotherms are used and compared with experimental data: ideal adsorbed solution theory, modified Polanyi-Dubinin potential theory and loading ratio correlation. None of the methods seems to have a distinct advantage over the others. However the loading ratio correlation is the only non-iterative method.     

煤矸石制备沸石-活性炭复合材料的吸附性能研究

利用煤炭生产的废弃物煤矸石为原料,在不同的条件下,制得A型和X型沸石-活性炭新型复合材料,通过X射线粉末衍射、低温N2(77 K)吸附、水和正己烷的吸附等表征:该复合材料具有中孔和微孔双重孔结构特征,同时具有亲水性和亲油性以及离子交换性质。对其应用性能的研究表明:复合材料对水和正己烷有高的吸附容量,并可一次性脱除废水中的重金属Cr3+和有机酚等有毒污染物。