Modelling and simulation of trickle‐bed reactors using computational fluid dynamics: A state‐of‐the‐art review

Trickle‐bed reactors (TBRs), which accommodate the flow of gas and liquid phases through packed beds of catalysts, host a variety of gas–liquid–solid catalytic reactions, particularly in the petroleum/petrochemical industry. The multiphase flow hydrodynamics in TBRs are complex and directly affect the overall reactor performance in terms of reactant conversion and product yield and selectivity. Non‐ideal flow behaviours, such as flow maldistribution, channelling or partial catalyst wetting may significantly reduce the effectiveness of the reactor. However, conventional TBR modelling approaches cannot properly account for these non‐ideal behaviours owing to the complex coupling between fluid dynamics and chemical kinetics. Recent advances in the application of computational fluid dynamics (CFD) to three‐phase TBR systems have shown promise of achieving a deeper understanding of the interactions between multiphase fluid dynamics and chemical reactions. This study is intended to give a state‐of‐the‐art overview of the progress achieved in the field of CFD simulation of TBRs over the past two decades. The fundamental modelling framework of multiphase flow in TBRs, advances in important constitutive models, and the application of CFD models are discussed in detail. Directions for future research are suggested.     

Volume‐of‐fluid‐based model for multiphase flow in high‐pressure trickle‐bed reactor: Optimization of numerical parameters

Aiming to understand the effect of various parameters such as liquid velocity, surface tension, and wetting phenomena, a Volume‐of‐Fluid (VOF) model was developed to simulate the multiphase flow in high‐pressure trickle‐bed reactor (TBR). As the accuracy of the simulation is largely dependent on mesh density, different mesh sizes were compared for the hydrodynamic validation of the multiphase flow model. Several model solution parameters comprising different time steps, convergence criteria and discretization schemes were examined to establish model parametric independency results. High‐order differencing schemes were found to agree better with the experimental data from the literature given that its formulation includes inherently the minimization of artificial numerical dissipation. The optimum values for the numerical solution parameters were then used to evaluate the hydrodynamic predictions at high‐pressure demonstrating the significant influence of the gas flow rate mainly on liquid holdup rather than on two‐phase pressure drop and exhibiting hysteresis in both hydrodynamic parameters. Afterwards, the VOF model was applied to evaluate successive radial planes of liquid volume fraction at different packed bed cross‐sections. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Catalytic porous ceramic prepared in‐situ by sol‐gelation for butane‐to‐syngas processing in microreactors

In this study, a novel flow‐based method is presented to place catalytic nanoparticles into a reactor by sol‐gelation of a porous ceramic consisting of Rh/ceria/zirconia nanoparticles, silica sand, ceramic binder, and a gelation agent. This method allows for the placement of a liquid precursor containing the catalyst into the final reactor geometry without the need of impregnating or coating of a substrate with the catalytic material. The so generated foam‐like porous ceramic shows properties highly appropriate for use as catalytic reactor material, e.g., reasonable pressure drop due to its porosity, high thermal and catalytic stability, and excellent catalytic behavior. To investigate the catalytic activity, microreactors containing this foam‐like ceramic are employed for the production of hydrogen and carbon monoxide‐rich syngas from butane. The effect of operating parameters such as the inlet flow rate on the hydrocarbon processing is analyzed and the limitation of the reactor by diffusion mass transport is investigated. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Flow regimes in trickle beds using magnetic emulation of micro/macrogravity

Strong inhomogeneous magnetic fields in atmospheric-bore superconducting solenoid magnets were used to investigate the hydrodynamics in bore-fitted trickle beds which undergo emulated earth-bound artificial micro/macrogravity. This environment was able to modify the apparent gravity for both diamagnetic and paramagnetic materials by means of magnetization body force densities. Body force vectors can be co-linear or antiparallel to the cocurrent two-phase downflow in trickle beds depending on material magnetic susceptibilities, magnetic field gradient and direction of magnetic field. Trickle-to-pulse flow transition was experimentally studied in microgravity, macrogravity and beyond-levitation conditions for the air-water and the phenylacetylene-kerosene/hydrogen systems. Magnetic fields were found to displace the transition boundary from trickle to pulse flow. This was rationalized in terms of an equivalent artificial gravity effect by formally commuting magnetization forces into an equivalent gravitational acceleration. A theoretical analysis, using a modified Grosser et al. [1988. Onset of pulsing in two-phase cocurrent downflow through a packed bed. A.I.Ch.E. Journal 34, 1850-1860] “artificial gravity” transition model, was carried out and model predictions were found to follow qualitatively the experimental findings.     

磁补偿微重力环境实现及磁流体微重力内角流动研究

由于表面张力的作用，流体在微重力环境中沿一定夹角的内角壁面爬升过程与常重力状态不同。为了对微重力内角流动的物理过程进行研究，利用磁补偿方法搭建了常温磁流体微重力补偿实验台，实现了目标区域内纵向小于5%非均匀度的磁补偿微重力环境。并对不同重力条件下水基磁流体沿若干材料内角爬升过程进行了可视化实验研究，探究了微重力环境下流体与材料间的接触角以及内角角度对液体导流性能的影响以及毛细流动规律。结果表明，在满足Concus-Finn条件时，液面爬升高度和重力加速度近似呈反比关系。接触角和内角角度越小，流体输运能力越强，且重力水平越低，越为明显。当不满足Concus-Finn条件时，液面爬升高度和重力加速度近似呈线性关系，接触角和内角角度对流体输运能力的影响并不明显。     

Numerische Simulation disperser Gas/Flüssig‐Strömungen in Blasensäulen bei hohen Gasgehalten mit OpenFOAM® Teil 1 – Grundlagen der Modellierung

Various chemical products are synthesized in processes using gas/liquid reactors with bubbly flows. Hence, there is significant interest in a more efficient process design as well as in process intensification with a strong focus on this reactor class. However, the design of industrial gas/liquid reactors requires more detailed information about the flow structures and characteristics of two‐ or multiphase systems. The basic models for two‐fluid model simulations of dispersed gas/liquid flows in bubble columns at high gas fractions are presented..     

Use of pulsations to enhance the distribution of liquid injected into fluidized particles with commercial‐scale nozzles

Gas‐atomized liquid injections into fluidized beds are commonly used in industrial processes such as fluid coking, fluid catalytic cracking (FCC), and pharmaceutical granulation, where it is important to optimize the distribution of the injected liquid to prevent the formation of unwanted liquid–solid agglomerates and maximize product yields. This injection is typically a two‐phase, gas‐atomized liquid injection. One issue with such a liquid injection is that most of the liquid is deposited at the end of the jet tip within the reactor. This study proposes introducing artificial fluctuations into the injection flow, which causes the jet penetration to vary, allowing for a more even spread of liquid throughout a fluidized bed reactor. The effect of artificial pulsations in the atomized injection, of various frequency and amplitude, was investigated in this study. Using pulsations, greatly enhanced the liquid distribution on the bed particles. © 2012 American Institute of Chemical Engineers AIChE J, 59: 719–728, 2013     

Heterogeneous Catalytic Reactor Design with Non‐Uniform Catalyst Considering Shell‐Progressive Poisoning Behavior

A novel methodology has been developed to design an optimum heterogeneous catalytic reactor, by considering non‐uniform catalyst pellet under shell‐progressive catalyst deactivation. Various types of non‐uniform catalyst pellets are modelled in combination with reactor design. For example, typical non‐uniform catalyst pellets such as egg‐yolk, egg‐shell and middle‐peak distribution are developed as well as step‐type distribution. A progressive poisoning behavior is included to the model to produce correct effectiveness factor from non‐uniform catalyst pellet. As opposed to numerical experiment with limited type of kinetic application to the model in the past, this paper shows a new methodology to include any types of kinetic reactions for the modeling of the reactor with non‐uniform catalyst pellet and shell‐progressive poisoning. For an optimum reactor design, reactor and catalyst variables are considered at the same time. For example, active layer thickness and location inside pellet are optimised together with reactor temperature for the maximisation of the reactor performance. Furthermore, the temperature control strategy over the reactor operation period is added to the optimization, which extends the model to three dimensions. A computational burden has been a major concern for the optimization, and innovative methodology is adopted. Application of profile based synthesis with the combination of SA (Simulated Annealing) and SQP (Successive Quadratic Programming) allows more efficient computation not only at steady state but also in dynamic status over the catalyst lifetime. A Benzene hydrogenation reaction in an industry scale fixed‐bed reactor is used as a case study for illustration.     

重力对垂直相分离冷凝管内流型调控的影响

随着重力的降低,冷凝换热性能急剧恶化,特别在微重力条件下,流型与传热极不协同,所需的冷凝长度要比地面常规重力情况大一个数量级。基于非能动相分离概念的适用于不同重力条件的新型相分离冷凝管,促进了流型与传热的协同,极大强化了冷凝换热。在地面常规重力、小重力和微重力情况下,数值研究了新型冷凝管内垂直上升的空气-水两相流型的调控过程。通过研究得出:在三种不同重力条件下,均呈现“气在壁面,液在中心”的全新分布模式,特别是在微重力情况下,环隙区域内完全被气体占据,液体完全在核心区域内流动;重力越小,调控后液膜厚度减小幅度越大,特别在微重力条件下,减薄到1/32;重力越小,调控后薄液膜主导的冷凝换热量提高幅度越大,特别在微重力条件下,调控后提高到57.4倍,极大地强化了冷凝换热。总体来说,重力越小,流型调控过程越有利于强化冷凝换热。     

Importance of proper hydrodynamics modelling in fixed‐bed reactors: Fischer‐Tropsch synthesis study case

The importance of hydrodynamics, particularly gas density, superficial gas velocity, and total pressure in axial and radial directions, was analyzed for the modelling of a catalytic reactor using a non‐isothermal pseudo‐homogeneous approach. The modelling of a fixed‐bed reactor in one and two stages for CO conversion by Fischer‐Tropsch synthesis was taken as a study case. For the validation of the proposed model, the results of the simulations for the CO conversion and temperature profiles were compared with experimental data reported in the literature. Simulations for CO conversion and reactor temperature profiles confirmed the model's ability to predict the selectivity of the liquid products in the Fischer‐Tropsch synthesis reactor in one and two stages. The proposed model predicts more suitable profiles of CO conversion and temperature along the reactor, which makes it a more robust and efficient tool for design, optimization, and control purposes.     

On the flow‐phase diagram for nematic liquid crystalline polymer under magnetic field

The effect of magnetic fields on molecular configuration of liquid crystalline polymers under shear flows are numerically analyzed using the extended Doi theory in which a molecular shape parameter is admitted. The evolution equation for the probability density function of the LCP molecules is directly solved without any closure approximations. One case is considered that the magnetic field makes 45° with respect to the flow direction. We can find that the magnetic fields strongly affect on the transition among flow‐orientation modes, such as tumbling, wagging, and aligning modes. And a new aligning flow‐orientation mode emerges at low shear rate, which is macroscopically same as the ordinary aligning mode, but is microscopically quite different from the ordinary one. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Multiphase catalytic reactors: a perspective on current knowledge and future trends

ABSTRACT

Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas–liquid downflow, trickle-beds with countercurrent gas–liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors.

Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.     

Kinetics of homogeneous 5‐hydroxymethylfurfural oxidation to 2,5‐furandicarboxylic acid with Co/Mn/Br catalyst

2,5‐furandicarboxylic acid (FDCA) is a potential non‐phthalate based bio‐renewable substitute for terephthalic acid‐based plastics. Herein, we present an investigation of the oxidation rate of 5‐hydroxymethylfurfural (HMF) to FDCA in acetic acid medium using Co/Mn/Br catalyst. Transient concentration profiles of the reactant (HMF), intermediates [2,5‐diformylfuran (DFF), 5‐formyl‐2‐furancarboxylic acid (FFCA)], and the desired product (FDCA) were obtained for this relatively fast reaction in a stirred semi‐batch reactor using rapid in‐line sampling. Comparison of the effective rate constants for the series oxidation steps with predicted gas–liquid mass transfer coefficients reveals that except for the FFCA → FDCA step, the first two oxidation steps are subject to gas–liquid mass transfer limitations even at high stirrer speeds. Novel reactor configurations, such as a reactor in which the reaction mixture is dispersed as fine droplets into a gas phase containing oxygen, are required to overcome oxygen starvation in the liquid phase and further intensify FDCA production. © 2016 American Institute of Chemical Engineers AIChE J, 63: 162–171, 2017     

Numerical Simulation of Dispersed Gas/Liquid Flows in Bubble Columns at High Phase Fractions using OpenFOAM®. Part I – Modeling Basics

Various chemical products are synthesized in processes using gas/liquid reactors with bubbly flows. Hence, there is a significant interest in a more efficient process design as well as in process intensification with a strong focus on this reactor class. However, the design of industrial gas/liquid reactors requires more detailed information about the flow structures and characteristics of two‐ or multiphase systems. The basic models for two‐fluid model simulations of dispersed gas/liquid flows in bubble columns at high gas fractions are presented.     

Single and Multiphase Catalytic Oxidation of Benzyl Alcohol by Tetrapropylammonium Perruthenate in a Mobile Microreactor System

A mobile microreactor system, with flow and temperature control for organic synthesis, is described. The system can be used anywhere a venting outlet is available. The microreactors can be operated in different flow patterns (continuous flow, stop‐flow, or programmed‐flow) providing reaction times from a few minutes to a few hours. The system was tested for the catalytic oxidation of benzyl alcohol to benzaldehyde by tetrapropylammonium perruthenate (TPAP) with N‐methyl‐morpholine‐N‐oxide in the liquid phase under stop‐flow mode and on supported TPAP with oxygen under continuous flow mode. The conversion of benzyl alcohol in the microreactor was close to that of a small batch reactor for the liquid phase reaction. For the multiphase reaction, a conversion of 30–40 % was obtained with residence times below 1 min.     

Multiphase catalytic reactors: a perspective on current knowledge and future trends

Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas-liquid downflow, trickle-beds with countercurrent gas-liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors.

Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.     

Characteristics‐Based Model Predictive Control of a Catalytic Flow Reversal Reactor

This paper describes the formulation and tuning of a model‐based controller for a catalytic flow reversal reactor (CFRR). A plug flow non‐linear pseudo‐homogeneous mathematical representation of the process is used to model the mass and energy transport phenomena for the model‐based controller. A combination of the method of characteristics and model predictive control (MPC) technology is used to formulate the controller (Shang et al., Ind. Eng. Chem. Res. 43 (9) 2140–2149 (2004)). Mass extraction from the midsection of the reactor is used as the manipulated variable. Numerical simulations are used to show the performance of the formulated controller. The performance of the controller is evaluated on a simulated catalytic flow reversal reactor unit for combustion of lean methane streams for reduction of greenhouse gases emissions.     

Advantages of Forced Non‐steady Operated Trickle‐Bed Reactors

Trickle‐bed reactors are usually operated in the steady state trickle flow regime. Uneven liquid distribution and the formation of hot spots are the most serious problems experienced during trickle flow operation. In this paper, we advocate the use of non‐steady state operation of trickle‐bed reactors. Based on a square‐wave cycled liquid feed, several operation modes are developed that involve the artificial induction of natural pulses and control of the catalyst wetting efficiency over longer times. The operation modes aim at increasing the mass transfer rate of the limiting reactant and simultaneous prevention of flow maldistribution and hot spot formation. The operation modes are distinguished by a relatively fast and slow cycling of the liquid feed. The potential advantages of the developed feed strategies on reactor performance are evaluated.     

Reactive distillation: Non-ideal flow behaviour of the liquid phase in structured catalytic packings

Reactive distillation, the combination of chemical reaction and multistage distillation, is one of the most important industrial applications of the multifunctional reactor concept. The most promising column internals for reactive distillation are the so-called structured catalytic packings that combine favourable characteristics of traditional structured packings and heterogeneous catalysts. The non-ideal flow behaviour of the gas and the liquid phase is a fundamental aspect in multiphase reactor design since it has a strong influence on the reactor performance. In this study, liquid phase residence time distributions for the catalytic structured packing MULTIPAK^® were measured by means of conductivity measurements under different liquid and gas flow rates and evaluated with differential models.     

Aqueous‐Phase Reforming in a Microreactor: The Role of Surface Bubbles

In heterogeneous catalysis, the creation of gaseous products as bubbles in a liquid phase on the catalytic surface is associated with slip phenomena. In a microreactor, the slip length at the gas‐liquid interface is in the same order of magnitude as the reactor dimensions, which can affect fluid dynamics and transport phenomena. Here, the interplay of momentum, heat and mass transfer in a microreactor, when bubbles form on the catalytic surface, was investigated using two‐dimensional simulations. The effect of bubbles on the endothermic process of aqueous‐phase reforming of a glycerol solution was evaluated in terms of conversion and conversion and temperature in the reactor. Altogether, this study highlights the impact of bubbles, not only on the transport phenomena but also on the reactor performance.