Interpretation of Catalyst Deactivation by Fouling from Interactions of Pore Structure and Foulant Deposit Geometries

Hughes, Chris; Mann, R.

doi:10.1021/bk-1978-0065.ch017

Cited by 22 publications

(14 citation statements)

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“…For small deposits, the assumption of a uniform coke profile ( x ag ≈ const) is reasonable; indeed this approximation is frequently adopted in continuum models. However, as the deposit becomes larger, neither the uniform profile nor the assumption of a wedge-shaped deposit of constant slope [4,8,9] is supported by our microscopic models. The wedge-shaped deposit of constant slope would correspond to a linear increase of x ag with N, in contrast to the decrease shown in Figs.…”

Section: Resultscontrasting

confidence: 57%

“…Similar results are found in some conditions of the molecular diffusion models with small bias. Consequently, the hypothesis of uniform coke profiles along the pores [6,7] and the hypothesis of a wedge-shaped deposit of constant slope [4,8,9], frequently adopted in continuous reaction-diffusion models during the whole deposition period, are not supported by our microscopic models.…”

Section: Resultsmentioning

confidence: 73%

“…Different values of the width W are considered and are expressed in lattice units. Such approximation was previously adopted in several models of diffusion, reaction and deactivation by coking [6,7,8,9].…”

Section: Modelsmentioning

confidence: 99%

“…In most works, uniform (flat) coke profiles along the pore are considered [6,7]. On the other hand, nearly 20 years ago, Mann and co-workers introduced models in which the coke deposits inside the pores grew with a wedge shape of constant slope, with a higher concentration at the pore entrance [4,8,9]. These models were motivated by observations of higher coke concentrations in the external surfaces of several catalysts [10,11], which is typical of systems with a parallel mechanism of coke formation [3].…”

Section: Introductionmentioning

confidence: 99%

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Statistical models of diffusion and aggregation for coke formation in a catalyst pore

Reis

2005

Physica A: Statistical Mechanics and its Applications

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We simulated models of diffusion and aggregation in long pores of small widths in order to represent the basic mechanisms of coke deposition in catalysts' pores. Coke precursors are represented by particles injected at the pore entrance. Knudsen diffusion, which is usually expected inside the pores, is modeled by ballistic motion of those particles. The regime of molecular diffusion is also analyzed via models of lattice random walks biased along the pores. The aggregation at the surface or near previously aggregated particles was modeled by different probabilistic rules, accounting for the possibilities of more compact or more ramified deposits. In the model of Knudsen diffusion and in some cases of molecular diffusion, there is an initial regime of uniform deposition along the pore, after which the deposits acquire an approximately wedge shape, with the pore plugging near its entrance. After the regime of uniform deposition and before that of critical pore plugging, the average aggregation position slowly decreases with the number N of deposited particles approximately as N −0.25 . The apparently universal features of deposits generated by microscopic models are compared with those currently adopted in continuum models.

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Section: Resultscontrasting

confidence: 57%

Section: Resultsmentioning

confidence: 73%

Section: Modelsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Statistical models of diffusion and aggregation for coke formation in a catalyst pore

Reis

2005

Physica A: Statistical Mechanics and its Applications

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“…Catalysts with larger pores have lower initial activity, but maintain it for a longer period of time. Hughes and Mann (1978) calculated catalyst activity assuming a wedge-shapped deposit. Ahn andSmith (1984) andShimura et al (1986), modelled metal deposition using single or parallel pore models.…”

Section: Introductionmentioning

confidence: 99%

Hydrodemetallation Catalyst Pellets With Nonuniform Radial Pore Size Distribution

Gavriilidis

Melis

Cao

1998

Chemical Engineering Communications

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Deactivation of hydrodesulfurization catalysts by metals deposition

Ahn¹,

Smith²

1984

AIChE Journal

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Primary causes of deactivation of hydrodesulfurization catalysts are partial poisoning of the interior pore surface and pore-mouth plugging by deposition of metals from organo-metallic compounds in the reactor feed. A model is developed to account for both causes of deactivation. It can be used to predict which occurs first: complete surface poisoning or pore-mouth plugging. Equations are formulated for catalyst activity as a function of time for demetallization and also for desulfurization. The results depend upon particle geometry and geometry of deposited species, Thiele moduli for demetallization and desulfurization, and relative rates of desulfurization on fresh and partially poisoned catalytic surfaces. The treatment is for a single, isothermal catalyst particle. BUM-JONG SCOPEThe irreversible deposition of metals, particularly vanadium and nickel, has been identified as a major cause of deactivation of hydrodesulfurization (HDS) catalysts. Tamm et al. (1981) has presented experimental data suggesting that partial surface poisoning by metals deposition is responsible for the initial rapid deactivation while pore-mouth plugging is predominant at longer times. Deactivation by pore plugging has been carefully analyzed mathematically by Rajagoplan and Luss (1979).They included the effect of restricted diffusivities in the pore when the metal deposit reduces the pore radius to the same magnitude as the size of the organo-metallic molecules.A similar quantitative treatment of the effects of combined partial surface poisoning and pore-mouth plugging on HDS catalyst activity does not appear to have been published. Our objective is to present such a combined model, which, amongst other aspects, will describe the conditions for which pore plugging will occur prior to complete surface poisoning. Since the large organo-metallic molecules have low diffusivities, the shell model of deactivation (Masamune and Smith, 1966) is employed to treat partial surface poisoning. The mathematical development is restricted to isothermal behavior of a single catalyst pellet with uniform pore-size distribution. First-order demetallization and desulfurization kinetics are assumed. CONCLUSIONS AND SIGNIFICANCEThe deactivation by metals deposition on HDS catalysts, due to the combination of partial surface poisoning and pore-mouth plugging, is described quantitatively in terms of two parameters. These are the Thiele modulus cp, for metals deposition and a partial poisoning parameter T which depends, among other quantities, on the porosity of the catalyst and the density of the metal deposits. For large values of cp, and T , it is shown that the pore mouth is physically blocked before the deposited metal reaches the center of the catalyst particle. When this occurs demetallization and HDS activity are reduced to zero even though only partially poisoned and fresh catalyst surface still exist within the particle. For small values of cp, and T all the pore surface becomes partially deactivated before pore plugging occurs.. Then some demetall...

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Interpretation of Catalyst Deactivation by Fouling from Interactions of Pore Structure and Foulant Deposit Geometries

Cited by 22 publications

References 0 publications

Statistical models of diffusion and aggregation for coke formation in a catalyst pore

Statistical models of diffusion and aggregation for coke formation in a catalyst pore

Hydrodemetallation Catalyst Pellets With Nonuniform Radial Pore Size Distribution

Deactivation of hydrodesulfurization catalysts by metals deposition

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