Computational studies of transients in packed tubular chemical reactors

Crider, John E.; Foss, Alan S.

doi:10.1002/aic.690120322

Cited by 67 publications

(51 citation statements)

References 7 publications

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“…The wrong-way behavior was predicted by Boreskov et al (1965), and Crider and Foss (1966). It was observed: by Hoiberg et al (1971) in a homogenous liquid-phase reaction in a packed-bed reactor; by Van Doesburg and DeJong (1976a,b) in the catalytic methanation of carbon monoxide, and by Sharma and Hughes ( I 979) during the oxidation of carbon monoxide.…”

Section: Introductionmentioning

confidence: 91%

Wrong‐way behavior of packed‐bed reactors: II. Impact of thermal dispersion

1988

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A sudden reduction in the feed temperature of a packed-bed reactor may lead to a transient temperature rise, referred to as a wrong-way behavior. As expected, the axial dispersion of heat decreases the magnitude of the temperature excursion and prolongs the transient shift to a new steady state. In addition, the thermal dispersion may enable the wrong-way behavior to ignite a low-temperature steady state leading to a disastrous runaway of the reactor. Moreover, it may create a transient high-temperature wave, which moves initially in the upstream direction. The axial dispersion of heat can lead to some behavioral features which are qualitatively different from those of a model which ignores it. The transient temperature excursion does not exceed a value, which can be estimated by a simple analytical expression.

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Section: Introductionmentioning

confidence: 91%

Wrong‐way behavior of packed‐bed reactors: II. Impact of thermal dispersion

1988

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“…The volumetric ratio oh heat capacities is the parameter determining if the accumulation term can be neglected in the energy balance. It is also the ratio of the velocity of theimal wave to fluid velocity (Crider and Foss, 1966). Tinkler and Lamb (1965) and Crider and Foss (1966) did not make this quasi-static approximation for homogeneous reactiobs in a liquid flowing past inert packings.…”

Section: Review Of Literaturementioning

confidence: 96%

“…It is also the ratio of the velocity of theimal wave to fluid velocity (Crider and Foss, 1966). Tinkler and Lamb (1965) and Crider and Foss (1966) did not make this quasi-static approximation for homogeneous reactiobs in a liquid flowing past inert packings. Both papers coppared the dynamic calculations to experiments and found that the thermal capacity of both the solid and liquig were important parameters in the calculations.…”

Section: Review Of Literaturementioning

confidence: 96%

Transient modeling of a catalytic converter to reduce nitric oxide in automobile exhaust

Ferguson

Finlayson

1974

AIChE Journal

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Mathematical models m e developed to study the catalytic reduction of nitric oxide contained in $utomobile exhaust in which the temperature, flow rate, and concentrations of various species vary widely with time. The quasi-static approximatio$ is compared to the fully dynamic model. In the quasi-static model all prQcesses are steady state except for the solid temperature and inlet condiGons. Suggestions are given for deciding a priori if the quasi-static model is appropriate. Suggestions are also given for integrating the quasi-sta6c equations in order to minimize errors compared to the dynamic model. The exhaust gas from an automobile contains nitric oxide which must be reduced to nitrogen in order to meet Federal pollution standards. The problem is compIicated because the temperaturg, flow rate, and concentrations of different species vary in time over wide ranges. We develop transient ma*ematical models for a catalytic muffler applicable to this situation. The mathematical model is then used to examine the performance of three different catalysts.The inIet conditions correspond to those encountered when an automobile is operated in the Federal Test Procedure. The automobili: begins cold, and as it operates the catalyst bed graduglly warms up. The one feature of interest is to compare aatalysts having different properties so that they warm up at different rates. Furthermore, as the automobile changes driving modes the exhaust properties (or their time rates of change) may change discontinuously.The mathematical npodels employ a mixing-cell model for the packed bed aqd consider the reduction of nitric oxide with two catalyFic reactions. The reaction rate of nitric oxide with carb+ monoxide is closely coupled with the reaction rate of qitric oxide with hydrogen because nitric oxide is reacted essentially completely inside the catalyst. Thus one new feature of this model is the inability to construct pl@ts of effectiveness factor vs. Thiele modulus a priori, due )to the coupling of the two reaction rates. Rather, the problem of diffusion and reaction inside the catalyst pellet must be solved at each time for each mixing cell, and the orthogonal collocation method is used to do this efficiently.Three models are developed. The main model employs the quasi-static approximation which recognizes that the time response of the system is governed by the thermal response of the packing. All other processes are assumed to occur so fast that they are essentially steady state. A dynamic model is also used to test the conditions under which the quasi-static model is inappropriate. In the dynamic model all processes are allowed to be transient. Due to the wide difference in time constants for the system, the dynamic model leads to stiff ordinary differential equations, which necessitate small integration time steps. The comparison of the two models is interesting because the quasi-static model is frequently used but seldom tested. An even simpler model results when, in the quasistatic model, the reaction rate expression is assu...

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“…In terms of time, the deactivation is of the order of months for the majority of reactions carried out in a fixed bed, where the transients reach the steady state in a few residence times of the order of seconds. Therefore, the problem associated with "wrong way" transients (e.g., Crider and Foss, 1966) can be avoided since by the time a control action is to be taken, the reactor reaches a new, stable steady state. Further, the effect of dead time, which is of the order of minutes, on deactivation can be neglected since the extent of deactivation changes little in minutes.…”

Section: An Illustrationmentioning

confidence: 98%

Fixed bed with catalyst deactivation: On‐line estimation of deactivation, control, and design

Hong

Lee

1985

AIChE Journal

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On-line measurements of temperature and concentration and the global rate for fresh catalyst can be used to estimate a measure of catalyst deactivation. A feedback control policy is obtained from this ineasure of catalyst deactivation for maintaining the reactor outlet conversion constant with time at a desired level. This feedback control enables one to maintain the desired conversion without any knowledge of deactivation kinetics. 'These results are applicable to any type of deactivation. The optimal control can easily be taken into consideration for reactor design. This combination of process design and control with due consideration of catalyst deactivation for both is shown to result in a substantial improvement of the reactor performance.

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Computational studies of transients in packed tubular chemical reactors

Cited by 67 publications

References 7 publications

Wrong‐way behavior of packed‐bed reactors: II. Impact of thermal dispersion

Wrong‐way behavior of packed‐bed reactors: II. Impact of thermal dispersion

Transient modeling of a catalytic converter to reduce nitric oxide in automobile exhaust

Fixed bed with catalyst deactivation: On‐line estimation of deactivation, control, and design

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