Liquid spreading in trickle-bed reactors: Experiments and numerical simulations using Eulerian--Eulerian two-fluid approach

Liquid spreading in gas-liquid concurrent trickle-bed reactors is simulated using an Eulerian twofluid CFD approach. In order to propose a model that describes exhaustively all interaction forces acting on each fluid phase with an emphasis on dispersion mechanisms, a discussion of closure laws available in the literature is proposed. Liquid dispersion is recognized to result from two main mechanisms: capillary and mechanical (Attou and Ferschneider, 2000; Lappalainen et al., 2009- The proposed model is then implemented in two trickle-bed configurations matching with two experimental set ups: In the first configuration, simulations on a 2D axisymmetric geometry are considered and the model is validated upon a new set of experimental data. Overall pressure drop and liquid distribution obtained from $\gamma$-ray tomography are provided for different geometrical and operating conditions. In the second configuration, a 3D simulation is considered and the model is compared to experimental liquid flux patterns at the bed outlet. A sensitivity analysis of liquid spreading to bed geometrical characteristics (void-fraction and particles diameter) as well as to gas and liquid flow rates is proposed. The model is shown to achieve very good agreement with experimental data and to predict, accurately, tendencies of liquid spreading for various geometrical bed characteristics and/or phases flow-rates

Heat transfer study in corrugated wall bubbling fluidized bed reactor: Experiments and CFD simulations

Gasification technology must ideally be bestowed with the following traits: auto/allothermal process, non-diluted biosyngas abolishing downstream N2 separation or upstream O2 enrichment, thermal coupling via micro-segmentation between endothermic and exothermic steps to improve heat exchanges and enhance thermal efficiency, and high yield and heating value of biosyngas. These attributes represent the foremost challenges next-generation biomass steam gasifiers must cope with. With the endeavor of approaching such ideal configuration, Iliuta et al. proposed a reactor concept of allothermal cyclic multicompartment bubbling fluidized beds. Gas solid fluidized bed reactors would be used to obtain the enhanced heat and mass transport and conversion performances as compared to packed beds. Corrugated walls were installed in narrow gas-solid bubbling fluidized bed (CWBFB) enclosures to decrease minimum bubbling velocity, reduce bubble sizes, improve gas distribution, offer stable operation and minimize the particles carryover or loss. Thorough analyses of wall-to-bed heat transfer coefficient in flat- (FWBFB) and corrugated- (CWBFB) wall bubbling fluidized beds were performed for a variety of wall declinations and operating conditions covering a range of corrugation angles, inter-wall clearances (C), initial rest bed heights (Hi) and ratios of gas superficial velocity to minimum bubbling velocity, Ug/Umb (1-1.55). Fast response self-adhesive heat flux probes and thermocouples were employed to simultaneously measure the wall-to-bed heat flux, surface temperature and bed temperature. These instruments were used to measure the heat transfer coefficient (HTC) at different (18) axial and lateral locations. For a given set of parameters, significant increase in HTC was observed at lower gas flow rate in CWBFB as compared to FWBFB. It showed that CWBFB inventory required lesser Umb (gas flow rate) and offered more economical gas solid fluidization phenomena as compared to FWBFB. Full 3-D transient Euler-Euler CFD simulations employing kinetic theory of granular flow were also carried out which corroborated with experimental findings

Double-Slit Model for Partially Wetted Trickle Flow Hydrodynamics

A Double-Slit Model Developed Can Predict the Frictional Two-Phase Pressure Drop, External Liquid Holdup, Pellet-Scale External Wetting Efficiency, and Gas - Liquid Interfacial Area in Cocurrent Downflow Trickle-Bed Reactors Operated under Partially Wetted Conditions in the Trickle Flow Regime. the Model, an Extension of the Holub Et Al. (1992, 1993) Mechanistic Pore-Scale Phenomenological Approach, Was Designed to Mimic the Actual Bed Void by Two Inclined and Interconnected Slits: Wet and Dry Slit. the External Wetting Efficiency is Linked to Both the Pressure Drop and External Liquid Holdup. the Model Also Predicts Gas - Liquid Interfacial Areas in Partially Wetted Conditions. an Extensive Trickle-Flow Regime Database Including over 1,200 Measurements of Two-Phase Pressure Drop, Liquid Holdup, Gas - Liquid Interfacial Area and Wetting Efficiency, Published in 1974-1998 on the Partial-Wetted Conditions, Was Used to Validate the Modeling Approach. Two New Improved Slip-Factor Functions Were Also Developed using Dimensional Analysis and Artificial Neural Networks. High-Pressure and -Temperature Wetting Efficiency, Liquid Holdup, Pressure Drop, and Gas - Liquid Interfacial Area Data from the Literature on the Trickle-Flow Regime using Conventional Monosized Beds and Catalyst Bed-Dilution Conditions Were Successfully Forecasted by the Model

High-Pressure Trickle-Bed Reactors: A Review

A Concise Review of Relevant Experimental Observations and Modeling of High-Pressure Trickle-Bed Reactors, based on Recent Studies, is Presented. the Following Topics Are Considered: Flow Regime Transitions, Pressure Drop, Liquid Holdup, Gas-Liquid Interfacial Area and Mass-Transfer Coefficient, Catalyst Wetting Efficiency, Catalyst Dilution with Inert Fines, and Evaluation of Trickle-Bed Models for Liquid-Limited and Gas-Limited Reactions. the Effects of High-Pressure Operation, Which is of Industrial Relevance, on the Physicochemical and Fluid Dynamic Parameters Are Discussed. Empirical and Theoretical Models Developed to Account for the Effect of High Pressure on the Various Parameters and Phenomena Pertinent to the Topics Discussed Are Briefly Described

Discriminating Trickle-Flow Hydrodynamic Models: Some Recommendations

The Forecasting Ability of Five One-Dimensional (1-D) Two-Fluid Phenomenological Models for Liquid Holdup and Two-Phase Pressure Drop in Trickle-Flow Reactors Was Evaluated using the Most Comprehensive Trickle-Flow Regime Database. All of These Models, Namely, the Permeability Model, the Slit Model, the Extended Slit Model, the 1-D CFD Model, and the Double-Slit Model Can Be Used to Predict Liquid Holdup. among Them, the Permeability and the Slit Models, Because of a Much Simpler Structure, Are Recommended. the Extended Slit Model based on Iliuta Et Al. (Ind. Eng. Chem. Res. 1998, 37, 4542) Shear and Slip Constitutive Relationships Can Be Employed for Two-Phase Pressure Drop Predictions. When the Knowledge of Wetting Efficiency Becomes Essential at Very Low Liquid Flow Rates, the Double-Slit Model is Recommended

Pore-network modeling of trickle bed reactors: Pressure drop analysis

A pore network model (PNM) has been developed to simulate gas–liquid trickle flows inside fixed beds of spherical particles. The geometry has been previously built from X-ray micro-tomography experiments, and the flow in the throats between pores is modeled as a pure viscous Poiseuille two-phase flow. The flow distribution between pores and throats is obtained by solving mass and momentum balance equations. As a first application of this simple but powerful meso-scale model, a focus is proposed on the ability of PNM to estimate pressure drop and liquid saturation in co-current gas–liquid flows. PNM results are compared to the classical 1D pressure drop models of Attou et al. (1999), Holub et al. (1992) and Larachi et al. (1991). Agreement and discrepancies are discussed, and, finally, it has been found that the actual PNM approach produces realistic pressure drops as far as inertial contributions to friction are negligible. Concerning liquid saturation, the PNM only estimates its value in the throats between pores. As a consequence, liquid saturations are overestimated, but they can be easily corrected by an ad hoc empirical model

Inferring Liquid Chaotic Dynamics in Bubble Columns

Experiments Carried Out to Study the Liquid Displacements in Bubble Columns Via the Computer Automated Radioactive Particle Tracking Technique Are Analyzed by Means of Lagrangian and Qualitative Dynamics Tools. the Lagrangian Approach Yields the Detailed Motion Sequences of the Tracer as Entrained by the Fast Ascending Bubbles or by the Liquid Flow Alongside the Column Walls. the Qualitative Dynamics Tools, on the Other Hand, Provide Prima Facie Corroboration of Chaos in Liquid Motion based on an Analysis of the Volume-Averaged Kolmogorov Entropy and the Mutual Information Function. Other Features of the Chaotic Motion, the Reconstructed Attractors and the Radial and Axial Distributions of Lyapunov Exponents, Are Noted. Variations in the Liquid Hydrodynamics Due to Changes in Column Diameter and Operating Pressure Are Inspected. by Increasing Pressure the Attractor\u27s Correlation Dimension and the Information Loss Rate Decrease, Whereas the Liquid Flow Path is Dramatically Affected. © 2001 Published by Elsevier Science Ltd

Stabilization of Basic Oxygen Furnace Slag by Hot-stage Carbonation Treatment.

Treatment and disposal of Basic Oxygen Furnace (BOF) slag, a residue of the steel production process characterized by high basicity and propensity for heavy metal leaching, is a costly burden on metallurgical plants; a sustainable valorization route is desired. The stabilization of BOF slag utilizing hot-stage carbonation treatment was investigated; this approach envisions carbonation during the hot-to-cold pathway followed by the material after the molten slag is poured and solidified. Three experimental methodologies were employed: (i) in-situ thermogravimetric analyzer (TGA) carbonation was used to assess carbonation reaction kinetics and thermodynamic equilibrium at high temperatures; (ii) pressurized basket reaction carbonation was used to assess the effects of pressurization, steam addition and slag particle size; and (iii) atmospheric furnace carbonation was used to assess the effect of carbonation on the mineralogy, basicity and heavy metal leaching properties of the slag. Free lime was found to be the primary mineral participating in direct carbonation of BOF slag. Initial carbonation kinetics were comparable at temperatures ranging from 500 to 800 oC, but higher temperatures aided in solid state diffusion of CO2 into the unreacted particle core, thus increasing overall CO2 uptake. The optimum carbonation temperature of both BOF slag and pure lime lies just below the transition temperature between carbonation stability and carbonate decomposition: 830-850 oC and 750-770 oC at 1 atm and 0.2 atm CO2 partial pressures, respectively. Pressurization and steam addition contribute marginally to CO2 uptake. CO2 uptake progressively decreases with increasing particle size, but basicity reduction is similar independent of particle size. The solubility of some heavy metals reduced after carbonation (barium, cobalt and nickel), but vanadium and chromium leaching increased

Modelling and simulations of a Monolith Reactor for three-phase hydrogenation reactions – Rules and recommendations for mass transfer analysis

A strategy for the scale-up of a monolith reactor dedicated to gas-liquid catalytic reactions is worked out; focus is made on the crucial step of gas-liquid mass transfer modelling via a steady-state numerical study based on a single channel and single unit cell representation, using a frame moving with the bubble and solving the liquid phase only. The relevance of this simplified approach is assessed through a specific case (given bubble shape, channel diameter and fluid flow rates), and hydrodynamics as well as mass transfer results are successfully compared to previously published numerical, semi-analytical and experimental works. Influence of unit cell length and of catalytic surface reaction rate is thoroughly investigated. Inferred overall mass transfer coefficients are found to increase with bubble frequency and resulting higher interfacial area in unit cell and intensified recirculation in slug. Film contribution to mass transfer is proved dominant in the case of short bubbles with reactive wall, and hardly varies with reaction rate. However, this contribution is strongly linked to bubble frequency, and a reliable evaluation of local mass transfer by correlations demands accurate knowledge on the precise dimensions of bubble, slug and film entities