Simultaneous axial dispersion and adsorption in a packed bed

Chao, Raúl E.; Hoelscher, H. E.

doi:10.1002/aic.690120213

Cited by 47 publications

(12 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…According to several investigators (Masamune and Smith, 1964;Chao and Hoelscher, 1966;Nemeth and Stuart, 1970Costa et al 1971, 1975, if each adsorbent particle is considered as a heterogeneous system formed by a porous solid phase and a gaseous phase filling the void fraction of the solid, the internal diffusion coefficient D1 can be expressed in terms of two other diffusion coefficients that characterize the two possible simultaneous mechanisms of diffusion: (a) A molecular or Knudsen diffusion coefficient for the adsorbate molecules diffusing through the gas filling the pores, Dg, and (b) a surface migration coefficient for the adsorbed molecules moving along the solid surface, Ds. Thus, for linear adsorption isotherms, the total flux of adsorbate into the solid adsorbent could be expressed by Fick's law of diffusion using a global internal diffusion coefficient represented by an equation similar to that used by Masamune and Smith (1964):…”

Section: Theorymentioning

confidence: 99%

Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

1985

View full text Add to dashboard Cite

The internal diffusion coefficients, Di, of pure methane, ethane and ethylene as well as some of their binary and ternary mixtures, have been calculated at 20°C for solid particles of a commercial activated carbon. It has been observed that the contribution of the surface migration mechanism to the global mass transfer process inside the adsorbent particles can be as much as 7040%. Values for the surface migration coefficient Ds have also been calculated from the relation Di = Dg + KD,, where K is a dimensionless mean slope factor. Values found for both coefficients are of the same order of magnitude as those reported in the literature for similar systems.All the values for the internal diffusion coefficients of these pure components and their mixtures fit into a single correlation curve, the characteristic kinetic curve of the adsorbent. SCOPEThe increasing importance of adsorption processes for the separation of gaseous mixtures is due to the high selectivity and adsorption capacity of solid adsorbents, the less extreme operation conditions needed, and the small energy consumption required. Adsorption is becoming a competitive operation that can advantageously substitute for other separation operations such as distillation or liquid-liquid extraction. The advantages are especially attractive when the problem is the separation of light gases such as methane, ethane, ethylene and other typical hydrocarbons of gaseous refinery streams, since their separation by distillation requires expensive high pressure units.Although there are some commercial processes for the separation of hydrocarbons using gas adsorption (Milton, 1963; Priegnitz, 1973; Yatsurugi, 1974;Broughton, 1977), the extension of this method to other cases of practical interest is generally limited by the lack of reliable fundamental data for estimating the feasibility of a given separation and for reducing the empiricism of the present design of adsorbers. Among these fundamental data, the equilibrium adsorption isotherms and the internal diffusion coefficients of individual species of the mixture should be emphasized. Although substantial attention has been paid in the literature to the adsorp tion isotherms of pure compounds and their mixtures, even to the point of predicting multicomponent adsorption equilibria (Lewis et al., 1950 shown that the adsorption rates of pure propane, n-butane, ibutane, l-butene and other light gases into molecular sieves were lower when these compounds were present in binary mixtures. Feng et al. (1973Feng et al. ( ,1974 have extensively studied the diffusion of gaseous mixtures inside the pores of solid adsorbents, using several diffusion models. Their results, which were achieved by assuming a single pore size for the adsorbent and a reasonable tortuosity factor, were erroneous for not considering surface migration. We present here new values for the internal diffusion coefficients of methane, ethane, and ethylene, both as pure components and as part of the mixtures of methane-ethylene, ethaneethylene, and methane...

show abstract

Section: Theorymentioning

confidence: 99%

Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

1985

View full text Add to dashboard Cite

show abstract

“…Present results(3,8 ) indicate that detectors providing approximately the same voidage as the bed are less disruptive of flow patterns than void detectors. Present results(3,8 ) indicate that detectors providing approximately the same voidage as the bed are less disruptive of flow patterns than void detectors.…”

mentioning

confidence: 61%

Detector effects in packed bed measurements

Schmalzer¹,

Hoelscher²

1971

AIChE Journal

Self Cite

View full text Add to dashboard Cite

“…Such procedures depend on the Schumann model which makes the assumption amongst others that axial conduction or dispersion in the fluid phase is of negligible importance. The addition of a term for fluid phase axial dispersion (5,7 ) to the Schumann equation greatly complicates the mathematical solution of the equations and raises the question of whether it is necessary to devise a completely new set of design procedures.…”

Section: Applicability Of Single Parameter Modelsmentioning

confidence: 99%

“…Assuming for the present that the first two contributors to breakthrough curve dispersion are negligible, the Babcock formu1;i for Pea becomes The similarity between Equations (8) and (11) arises because for simplified boundary conditions ( 5 ) K2' = u'2 = 2/Pea so that the variances of the equivalent conductivity and Rosen models are identical. The use of the single parameter Pea to describe the complex processes which occur in a packed bed is thus similar to the use of u* or Y , as a parameter in the' Schumann model.…”

Section: The Equivalent Conductivity Modelmentioning

confidence: 99%

“…De is a dispersion coefficient which accounts for the experimentally observable fact that when an impulsive or chromatographic change in concentration is applied to the fluid flowing through a bed of nonadsorptive or inert packing, axial spreading of the pulse occurs (5,7). In heat transfer work, the phenomenum cannot be observed independently of fluid-solid transfer processes.…”

Section: Predicted Effects Of Axial Fluids Dispersionmentioning

confidence: 99%

See 1 more Smart Citation

Prediction of breakthrough curves in packed beds: I. Applicability of single parameter models

Jeffreson

1972

AIChE Journal

View full text Add to dashboard Cite

Applicability of Single Parameter ModelsConditions under which simplified, single parameter models may be used to predict breakthrough curves in linear, packed bed adsorbers and thermal regenerators are examined with reference, in particular, to earlier conditions proposed by Handley and Heggs and by Babcock and co-workers. I t is found that for high values of heat capacity ratio the Schumann model provides a satisfactory description of packed bed dynamics where fluids axial dispersion and solids conduction effects are lumped into a single parameter the curve spread parameter u*.For low values of heat capocity ratio, the equivalent conductivity model of Babcock and co-workers should provide a more accurate description. The value of the Biot number Bi was found to provide the sole criterion for estimating the relative importance of the internal solids conduction. Empirical correlations are presented which allow rapid estimates of breakthrough curves to be mode from the curve spread parameter and which facilitate analysis of experimental data from breakthrough curve experiments.The prediction of breakthrough curves in linear, packedbed adsorbers and thermal regenerators has been the subject of many theoretical and experimental investigations since Schumann's work (19) in 1929. This interest arises fundamentally because of the economic importance of fixed bed-fluid transfer processes and because of the resultant need to rapidly and accurately design related equipment.Design procedures for well insulated thermal regenerators or adsorbers in which particle internal conductivities may be regarded as infinite have been well developed and were summarized by McAdams (21 ) . Such procedures depend on the Schumann model which makes the assumption amongst others that axial conduction or dispersion in the fluid phase is of negligible importance. The addition of a term for fluid phase axial dispersion (5, 7 ) to the Schumann equation greatly complicates the mathematical solution of the equations and raises the question of whether it is necessary to devise a completely new set of design procedures.In this paper the conditions under which the Schumann solution may be extended to cover the cases of finite particle conductivity and axial fluids dispersion are considered. It has also been necessary to examine critically the conditions proposed earlier by Handley and Heggs (9) and by Babcock and co-workers (2, 3 ) .It is shown that for most cases of practical importance the well-established design procedures referred to above may continue to be used, subject to the satisfaction of a number of simple criteria. At the same time, one of the objects of this paper is to make the task of the design engineer easier by presenting empirical correlations of points on a breakthrough curve against nondimensional groups commonly used in chemical engineering. The use of these groups simplifies the tasks of design and analysis and facilitates translation of the results from heat to mass transfer.The application of complex mathematical "models" to...

show abstract

Simultaneous axial dispersion and adsorption in a packed bed

Abstract: = volumetric expansion coefficient = dimensionless bulk mean temperature = dimensionless temperature function for horizontal = dimensionless temperature function for vertical LITERATURE CITED 1. Berman, A. S., . I .

Cited by 47 publications

References 19 publications

Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

Detector effects in packed bed measurements

Prediction of breakthrough curves in packed beds: I. Applicability of single parameter models

Contact Info

Product

Resources

About