Gs Yablonsky

Thin-zone TAP-reactor – theory and application

Abstract A new reactor model for TAP pulse response experiments called a ‘thin-zone reactor’ is developed and applied to irreversible adsorption/reaction and reversible adsorption. In a thin-zone reactor, concentration gradients across the catalyst bed can be neglected, and diffusion and chemical reaction can be separated. The expressions for kinetic parameters for a thin-zone reactor can be calculated using moments, and are much simpler than the expressions for a one-zone or a three-zone reactor. The thin-zone model is particularly useful for investigating fast chemical reactions, since the extent of reaction can be controlled by the thickness and position of the catalyst zone. The expression for conversion in the case of irreversible adsorption/reaction in a thin-zone reactor is governed by a relationship that is analogous to the expression for first-order reaction in a CSTR. Moment based ‘fingerprints’ are defined for irreversible adsorption, reversible adsorption, and diffusion. The thin-zone model is used to determine kinetic parameters for the oxidation of propene over a VPO catalyst.

Multi-zone TAP-reactors theory and application : I. The global transfer matrix equation

Abstract A general theory of single-pulse state-defining experiments for a multi-zone TAP (temporal analysis of products) reactor, is developed using the Laplace transform formalism; the theory gives explicit expressions for the moments of the outlet flux, series expansions for the transient values of this flux, and offers an efficient means to compute the actual profiles of gas concentration in the reactor and the values of the outlet flux numerically, using e.g. Fast Fourier Transform. The central concept of the theory is the global transfer matrix equation, which determines completely the dynamic behavior of the reactor. Using efficient computer algebra methods, the theory generates previous theoretical results reported in the literature for all the known TAP–reactor configurations, and yields new results related to the reversible adsorption/reaction–diffusion case and the thin-zone case. It can be used for further theoretical studies in the area of diffusion/reaction dynamics.

Uniformity in a thin-zone multi-pulse TAP experiment: numerical analysis

Abstract The thin-zone TAP reactor (TZTR) model of a multi-pulse experiment is computationally validated based on a more general three-zone reactor model. The analysis is focused on the uniformity of gaseous and surface concentrations in the catalyst zone, which is a key property of TZTR model. It is shown that if the TZTR model is valid for the first pulse in a multi-pulse experiment then it is valid for all subsequent pulses. For a typical reactor packing (the ratio of the thin-zone thickness to the length of reactor is 1/30) and with the first pulse conversion up to 97%, the gaseous and surface concentration profiles can be considered uniform and characterized by their spatial average values only. The reaction rate in the catalyst zone may also be characterized by its spatial average value and directly related to the spatial average gaseous and surface concentrations, in the same way as an elementary rate is related to concentrations. As a result of these unique characteristics, the TZTR may be considered a “perfectly-mixed” reactor even at high conversion.

General expression for primary characterization of catalyst activity using TAP pulse response experiment

A general expression for primary catalyst characterization using TAP pulse response data has been obtained for porous and non-porous catalysts, and for one- and two-step irreversible catalytic reactions. Using this expression or the corresponding nomogram, the apparent kinetic parameter can be obtained.

Multi-zone TAP-reactors theory and application: II. The three-dimensional theory

The rigorous three-dimensional theory for a TAP (Temporal Analysis of Products) Knudsen pulse response experiment is developed for the combined diffusion and reaction cases for multi-zone packing, in order to determine the domain of validity of the commonly used one-dimensional model. The analysis is based on a specific modification of the transfer matrix formalism previously introduced for one-dimensional TAP-reactor models. The outlet flux can be written as a sum of three independent terms: one corresponding to the one-dimensional solution, a term accounting for axially symmetric radial nonuniformity, and a term needed for a fully three-dimensional model. The theory provides a method for estimating the accuracy of the one-dimensional model and for finding the domain of its validity. The theory is illustrated by the diffusion-only case for a one-zone reactor. It is shown that the one-dimensional model is valid for aspect ratios L/R>3.5.

Effect of surface nonuniformity on the kinetics of simultaneous adsorption of SO2-NOx over Na-γ-Al2O3 sorbent: a coverage-dependent stoichiometry

Abstract The effect of nonuniformity of the surface of Na–γ-Al2O3 on the kinetics of surface reactions is investigated for the simultaneous adsorption of NO, O2 and SO2. A nonuniform kinetic model is developed by considering the variations in the rate coefficients of important surface reactions with species coverages. Experimental data from a transient fixed bed microreactor and a steady-state riser reactor are used for the estimation of model parameters. The model explains the large variation in the NO removal in the above two reactors by the difference in the coverage of the sulfite species SO 2 ∗ and accounting for the nonuniformity in the adsorption and surface reactions. The SO 2 ∗ species is produced from the primary adsorption of gas phase SO2 on a free site. The adsorption of NO occurs first on a sulfate species SO 2 ∗∗ , resulting from the interaction of SO 2 ∗ with a free site. Subsequently, a complex formation occurs in multiple series-parallel steps involving many SO 2 ∗ and O2 per mole of NO. In the nonuniform model, the rate of the series-parallel steps and hence the average stoichiometry of the complexes depend strongly on the degree of SO 2 ∗ coverage. The calculated average number of SO 2 ∗ entities in the complex is much smaller at the riser (2–3 moles/mole of NO) than at the fixed bed conditions (9 moles/mole of NO). A lower consumption of SO 2 ∗ in the complex allows a higher rate of formation of SO 2 ∗∗ , leading to a higher NO removal as observed in the riser. There is an optimum inlet SO2/NO ratio of 2.5 at which maximum NO removal in the riser is achieved.

Interrogative kinetic characterization of active catalyst sites using TAP pulse response experiment

The approach for interrogative characterization of catalyst active sites using a combination of TAP pulse experiment is presented. This approach includes two principal steps: a state-defining experiment is used to determine the apparent kinetic parameters related to a given catalyst state, and a state-altering multi-pulse experiment is used to determine the number of active sites related to the same catalyst state. Using moment-based analysis, analytical expressions that correspond to both steps are obtained. The thin-zone TAP reactor that minimizes the influence of concentration gradient on observed kinetic characteristics and can be used to obtain information about very fast catalytic reactions is presented.