Monolithic Stirrer Reactors for the Sustainable Production of Dihydroxybenzenes over 3D Printed Fe/γ-Al2O3 Monoliths: Kinetic Modeling and CFD Simulation

Pablo López,Irene Díaz-Herrezuelo,Alma D Salazar-Aguilar,Jose A Casas,Sofía M Vega-Díaz,Asunción Quintanilla,Manuel Belmonte

doi:10.3390/catal12020112

Abstract

The aim of this work is to evaluate the performance of the stirring 3D Fe/Al2O3 monolithic reactor in batch operation applied to the liquid-phase hydroxylation of phenol by hydrogen peroxide (H2O2). An experimental and numerical investigation was carried out at the following operating conditions: CPHENOL,0 = 0.33 M, CH2O2,0 = 0.33 M, T = 75–95 °C, P = 1 atm, ω = 200–500 rpm and WCAT ~ 1.1 g. The kinetic model described the consumption of the H2O2 by a zero-order power-law equation, while the phenol hydroxylation and catechol and hydroquinone production by Eley–Rideal model; the rate determining step was the reaction between the adsorbed H2O2, phenol in solution with two active sites involved. The 3D CFD model, coupling the conservation of mass, momentum and species together with the reaction kinetic equations, was experimentally validated. It demonstrated a laminar flow characterized by the presence of an annular zone located inside and surrounding the monoliths (u = 40–80 mm s−1) and a central vortex with very low velocities (u = 3.5–8 mm s−1). The simulation study showed the increasing phenol selectivity to dihydroxybenzenes by the reaction temperature, while the initial H2O2 concentration mainly affects the phenol conversion.

Highlights

Rotating reactors aim to increase the mixing efficiency and, in this way, enable the reduction of the reactor volumes, estimated by a factor of 10–100 [1], which is of interest for the intensification of the chemical processes [2]
These studies demonstrate the potential of the catalytic stirrer reactors over the conventional slurry reactors due to the obvious operational advantages such as no need of catalyst recovery, prevent attrition of the catalyst, lower pressure drops and intrinsically safer operations, and because Monolithic stirrer reactors (MSR) improves the conversions and selectivity as a result of the faster gas-liquid mass transfer rates induced by the stirring [5,8] and the larger geometrical surface areas available for the catalysis
This paper aims to provide insight into the implementation of MSR by the integration of the 3D printed technology for the monolithic catalyst manufacturing and computational fluid dynamic (CFD) simulation for a better understanding of the flow and species mass transport in the stirrer reactor

Summary

Introduction

Rotating reactors aim to increase the mixing efficiency and, in this way, enable the reduction of the reactor volumes, estimated by a factor of 10–100 [1], which is of interest for the intensification of the chemical processes [2]. The concept was demonstrated by an aqueous phase reaction, the decomposition of hydrogen peroxide (H2O2) over manganese oxide catalyst [3] and has since been extended to multiphase processes (gas-liquid phase reactions) such as the selective hydrogenation of alkyne [3,4,5] and more viscous systems such as the hydrogenation of edible oil over palladium catalysts [6] and the enzyme-catalyzed reactions in organic media [7] These studies demonstrate the potential of the catalytic stirrer reactors over the conventional slurry reactors due to the obvious operational advantages such as no need of catalyst recovery, prevent attrition of the catalyst, lower pressure drops and intrinsically safer operations, and because MSR improves the conversions and selectivity as a result of the faster gas-liquid mass transfer rates induced by the stirring [5,8] and the larger geometrical surface areas available for the catalysis. MSRs have been demonstrated to be a suitable alternative for the intensification of selective oxidation of hydrocarbons, i.e., dihydroxybenzene production over Fe/SiC by phenol hydroxylation with H2O2 [10] and lactobionic acid production over gold catalyst by lactose oxidation by O2 [11], and for liquid-liquid catalytic reactions such as the alcoholysis of urea to propylene carbonate over different mixed-metal oxides [12]

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