CFD and experimental studies of reactive pulsing flow in environmentally-based trickle-bed reactors

Abstract

Pulsing flow in trickle-bed reactors (TBRs) has been claimed to promote the averaged heat and mass transfer rates, thereby enhancing the overall conversion and productivity. In this work, we focused on the mineralization of organic matter by catalytic wet oxidation at different liquid flow modulations. The convective nature of the disturbances that lead to pulsing was simulated by an Eulerian Computational Fluid Dynamics (CFD) model and validated with experimental data. In order to evaluate the predicted effects of pulsing on reaction outcome, first the multiphase flow governing equations were detailed with the computational methodology used in the simulation procedure. Prominent numerical parameters were optimized in terms of mesh aperture and time step. Second, to enable objective assessments of the merits of the different cyclic strategies, several computational runs were performed addressing the effect of nominal gas and liquid flow rates as well as the oxidation temperature. Here, we found that the concentration profile computed by the CFD model for pulsing flow conditions demonstrated superior oxidation performance over the trickling flow regime, which has been further corroborated by experimental evidences. Afterwards, the normalized concentration series close to the top of the TBR exhibited sharp fronts and gradually become less intense as they travel downward that reflected the nonisolated nature of the traveling liquid pulsations. Finally, these computational and experimental findings enabled us to intensify the detoxification of high-strength wastewaters and can be further exploited due to advantageous dispersive and convective heat/mass transfer phenomena under pulsing flow conditions.

Introduction

Two-phase flow in TBRs has been intensively studied both in the realm of fluid dynamics and in the domain of chemical reaction engineering. The gas–liquid cocurrent flow in these packed-bed reactors found common applications in the petrochemical industry, where hydrotreating reactions of the various petroleum fractions play a major role (Al-Dahhan et al., 1997). More recently, the unprecedented number of publications devoted to biochemical processes as well as advanced oxidation processes have envisaged TBRs for the purification of wastewaters and gases polluted with toxic organic compounds, using a bed of immobilized bacteria or the chemical catalyst, respectively (Bhargava et al., 2006). As trickle-beds are governed by different hydrodynamic states, the literature has primarily reported on trickle and pulse flow regimes in view of the fact that these contacting patterns are conventionally encountered in commercial-scale trickle-beds. Our case-study follows up the long-standing interest on the environmental applications of trickle-beds in catalytic wet oxidation to realize theoretically and experimentally the advantages of the convective nature of liquid disturbances that grow into pulses in TBRs (Ayude et al., 2007, Guo and Al-Dahhan, 2005).

The comprehensive understanding on the nature and characteristics of the hydrodynamics in those flow regimes and the transition between them is ultimately recognized to push forward the oxidation conversions. In order to accomplish such a task, precise tools are required for the identification of the flow pattern and the reaction behavior. Important emphasis is being put nowadays on advanced Computational Fluid Dynamics codes to deal with an accurate prediction of the trickle-to-pulse flow transition, which is a key feature of major importance in the characterization of flow pattern apart from the quantification of phase holdups, two-phase frictional pressure drop, and mass fluxes. Here, we focus on the development of a phenomenological framework that enables to assess how catalytic wet oxidation can be promoted when the trickle flow state evolve to forced pulse flow regimes. The pulsing hydrodynamic regime is attained at severe flow conditions, where sharp, structured, and traveling disturbances of high liquid payloads substitute the low interaction regime achieved under trickling flow conditions. As long as pulsing flows affect the heat and mass transfer rates, and thus reaction rates, hence it is necessary to grasp how superior the detoxification of phenol-like pollutants can be advanced, and this may be cited as the motivation cause for the current work.