ANALYSIS OF THE CSTR APPROXIMATION UNDER TRANSIENT OPERATION

Abstract

A CSTR approximation used to describe the behavior of a homogeneous reactor and a heterogeneous catalyst bed is analyzed under transient conditions. When uniformity in reactant concentration exists, the volume of the heterogeneous CSTR is defined as the equivalent volume in the gas phase of intermediates that are adsorbed on the active catalytic surface at the reactor operating condition. Several reaction models are tested to determine the conditions under which a first-order response can be used directly to predict kinetic information useful for modeling.

The relaxation time constant is obtained from the transient response of the reaction product subsequent to a step change of the reactant by using a first-order assumption. This result is used to examine the proposal that reaction intermediate concentrations can be determined from such transient data. We calculate the reaction intermediate concentration, referred to as the postulated value, based on the aforementioned proposal. Solution of the mass balance equations yields the actual intermediate concentration which we refer to as the predicted value.

The validity of a first-order relaxation assumption to determine intermediate concentrations is analyzed by an operating line which is the locus of points along which the predicted and postulated value for the intermediate concentration are equal. It is found that the first-order relaxation analysis to obtain intermediate concentrations is valid only under a limited range of experimental conditions. For homogeneous systems the intermediate concentration can be an overestimate or an underestimate to the actual value. For heterogeneous systems the intermediate concentration is overestimated. In both cases, the first-order approximation is only valid when the dead volume is a negligible part of the total volume and the ratio of the volume to the flow rate is small. For the heterogeneous case, this implies that the perturbation in the feed stream should be made to a reactant which occupies a small fraction of the total surface. This study establishes experimental criteria for evaluating other proposals to determine kinetic information directly from transient techniques.

KEYWORDS:

• CSTR
• First-order approximation
• Relaxation time constant
• Intermediate concentration
• Homogeneous CSTR
• Heterogeneous CSTR

Notes

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