Turbulence modelling of multiphase flow in high-pressure trickle-bed reactors

Computational fluid dynamics (CFD) has been used as a successful tool for single-phase reactors. However, fixed-bed reactors design depends overly in empirical correlations for the prediction of heat and mass transfer phenomena. Therefore, the aim of this work is to present the application of CFD to the simulation of three-dimensional interstitial flow in a multiphase reactor. A case study comprising a high-pressure trickle-bed reactor (30 bar) was modelled by means of an Euler-Euler CFD model. The numerical simulations were evaluated quantitatively by experimental data from the literature. During grid optimization and validation, the effects of mesh size, time step and convergence criteria were evaluated plotting the hydrodynamic predictions as a function of liquid flow rate. Among the discretization methods for the momentum equation, a monotonic upwind scheme for conservation laws was found to give better computed results for either liquid holdup or two-phase pressure drop since it reduces effectively the numerical dispersion in convective terms of transport equation.After the parametric optimization of numerical solution parameters, four RANS multiphase turbulence models were investigated in the whole range of simulated gas and liquid flow rates. During RANS turbulence modelling, standard k-ε dispersed turbulence model gave the better compromise between computer expense and numerical accuracy in comparison with both realizable, renormalization group and Reynolds stress based models. Finally, several computational runs were performed at different temperatures for the evaluation of either axial averaged velocity and turbulent kinetic energy profiles for gas and liquid phases. Flow disequilibrium and strong heterogeneities detected along the packed bed demonstrated liquid distribution issues with slighter impact at high temperatures.     

Modeling bubble column reactor with the volume of fluid approach: Comparison of surface tension models

This work aims at comparing surface tension models in VOF (Volume of Fluid) modeling and investigating the effects of gas distributor and gas velocity. Hydrodynamics of a continuous chain of bubbles inside a bubble column reactor was simulated. The grid independence study was first conducted and a grid size of 1.0 mm was adopted in order to minimize the computing time without compromising the accuracy of the results. The predictions were validated by comparing the experimental studies reported in the literature. It was found that all surface tension models can describe the bubble rise and bubble plume in a column with slight deviations.     

Scale-up concept for modular microstructured reactors based on mixing, heat transfer, and reactor safety

Microstructured reactors are characterized by rapid mixing processes and excellent temperature control of chemical reactions. These properties allow the safe operation of hazardous chemistry in intensified processes. Problems occur during scale-up of these processes, where heat transfer becomes the limiting effect. With high flow rates and transitional or even turbulent flow regimes in small channels, rapid mixing and excellent heat transfer can be maintained up to high production rates. For exothermic reactions, limits for parametric sensitivity and safe operation are shown from literature and combined with convective heat transfer for consistent scale-up. Good knowledge of reaction kinetics, thermodynamics and heat transfer is essential to determine runaway regions for exothermic reactions. From these correlations, consistent channel design and continuous-flow reactor setup is shown.     

Flow field investigation in a photocatalytic reactor for air treatment (Photo-CREC-air)

The aerodynamic behavior of a photocatalytic reactor for air treatment, Photo-CREC-air, with demonstrated high quantum efficiency performance, is examined using CFX-5.7.1. Photo-CREC-air consists of a venturi section that features low pressure drop and uniform illumination of the photocatalyst, resulting in high oxidation quantum efficiencies. The numerical simulations allowed the identification of several design issues in the original Photo-CREC-air unit, which include extensive boundary layer separation close to the photocatalyst support and regions of flow recirculation that render ca. 77% of the support surface area inactive. The simulations reveal that this issue could be addressed by replacing the wire-mesh basket sidewalls with perforated plates. This modification causes an increase in the pressure drop downstream of the support and achieves significant uniformization of the mass flow and air-photocatalyst contact time distributions.A modified Photo-CREC-air design is also presented and studied using CFX-5.7.1. This modified design is envisaged with the objective of improving UV-irradiation uniformity, an issue that is not completely addressed in the original design due to the shape of the windows and divergent section. CFD simulations reveal that, although the flow field is uniform, mass flow and contact time distributions are not. Nonetheless, this problem is addressed by increasing the pressure drop downstream of the support through the addition of a region modeled as a perforated plate. The simulations reveal that the mass flow and contact time distributions are significantly uniformized once this modification is implemented.     

A versatile flow visualisation technique for quantifying mixing in a binary system: application to continuous supercritical water hydrothermal synthesis (SWHS)

A versatile image analysis system which can be used to quantify the mixing pattern of a two-fluid/phase system has been developed. This visualisation technique, light absorption imaging (LAI), is based on the absorption of light by dyed fluids. In the context of this paper, the technique is applied to supercritical water hydrothermal synthesis (SWHS), which is used in the production of nano-particulate metal oxides. This case study was carried out in an attempt to highlight potential process pitfalls and to optimise overall reactor design. However, the versatility of this LAI technique makes the scope of possible applications much wider. The SWHS process for the production of metal oxide nano-particulates is a relatively new and promising development. A key aspect that requires fundamental research is the phase interaction or ‘mixing mechanism’ within these continuous reactors. The manner in which the supercritical water stream interacts with the cold metal salt stream is of paramount importance since it will inevitably affect the quality and quantity of product as well as highlighting any potential fundamental flaws in a design of the reactor.     

On the residence time distribution in reactors with non-uniform velocity profiles: The horizontal stirred bed reactor for polypropylene production

Horizontal stirred bed reactors for polypropylene production have a typical axial powder mixing pattern which is unique for polyolefin gas-phase reactors. The polymer, although adhering to the catalyst particles, has a shorter mean residence time and a broader residence time distribution than the catalyst. Both polymer and catalyst residence time distributions, strongly depend on the temporal catalyst activity profile. The powder mixing pattern in a horizontal stirred bed reactor results from two transport effects: the continuously increasing powder net flow in the downstream direction caused by the particle growth, and the simultaneous stirring flows with equal intensity in the up- and downstream directions. Both together lead to a residence time distribution of the reacting catalyst particles between fully backmixed and plug flow. A modelling approach is proposed which describes the axial flow contributions of stirring and polymerisation separately, and which is more accurate and flexible than simple cells-in-series models used in most papers so far. For large-scale reactors, residence time distributions are predicted from experimental pulse-response curves obtained in a miniaturised mixing model without reaction. Residence time distributions equivalent to “three to five” well-mixed reactors in a series reported in the literature are experimentally accessible only with difficulties under polymerising conditions. With the modelling approach proposed in this paper, the “reacting” RTD can be calculated on the basis of the cold-flow experimental data. We compare the proposed modelling concept with simple cells-in-series models by simulating catalyst yields, particle size distributions and catalyst transitions using two model catalysts with different activity profiles. The selection of the right modelling approach has indeed a significant influence on the simulation results, especially if the throughput or the axial velocity profile is changing and the catalyst is not deactivating.     

Model validation for low and high superficial gas velocity bubble column flows

In the present work, gas-liquid flow dynamics in a bubble column are simulated with CFDLib using an Eulerian-Eulerian ensemble-averaging method in a two-dimensional Cartesian system. The two-phase flow simulations are compared to experimental measurements of a rectangular bubble column performed by Mudde et al. [1997. Role of coherent structures on Reynolds stresses in a 2-D bubble column. A.I.Ch.E. Journal 43, 913-926] and a cylindrical bubble column performed by Rampure et al. [2003. Modeling of gas-liquid/gas-liquid-solid flows in bubble columns: experiments and CFD simulations. The Canadian Journal of Chemical Engineering 81, 692-706] for low and high superficial gas velocities, respectively. The objectives are to obtain grid-independent numerical solutions using CFDLib to reconcile unphysical results observed using FLUENT with increasing grid resolutions [Law, D., Battaglia, F., Heindel, T.J., 2006. Numerical simulations of gas-liquid flow dynamics in bubble columns. In: Proceedings of the ASME Fluids Engineering Division, IMECE2006-13544, Chicago, IL], and to validate computational fluid dynamics (CFD) simulations with experimental data to demonstrate the use of numerical simulations as a viable design tool for gas-liquid bubble column flows. Numerical predictions are presented for the local time-averaged liquid velocity and gas fraction at various axial heights as a function of horizontal or radial position. The effects of grid resolution, bubble pressure (BP) model, and drag coefficient models on the numerical predictions are examined. The BP model is hypothesized to account for bubble stability, thus providing physical solutions.     

An open-source population balance modeling framework for the simulation of polydisperse multiphase flows

Polydispersity is a challenging feature of many industrial and environmental multiphase flows, influencing all related transfer and transport processes. Besides their size, the fluid or solid particles may be distributed with respect to other properties such as their velocity or shape. Here, a population balance model based on the method of classes is combined with a multifluid solver within the open source computational fluid dynamics library OpenFOAM. The model allows for tracking the evolution of one or more size-conditioned secondary properties. It is applied to two different problems, the first being bubbly flow of air and water in a vertical pipe, where considering the velocity as a secondary property allows to resolve the size-dependent radial segregation. The second application is the gas phase synthesis of titania powder, where non-spherical particle aggregates appear whose shape is modeled through a collision diameter, leading to an improved prediction of the size distribution.     

CFD simulation of mixing and reaction: the relevance of the micro-mixing model

The aim of this work is to understand the role of the micro-mixing model in computational fluid dynamics (CFD) simulations of fast reactions. Using CFD, in fact, the reactor is modelled through a computational grid and the governing equations are discretised using numerical methods. However, the mixing phenomena that occur at scales that are smaller than the grid size remain unresolved. This means that the probability density function (PDF) of all scalars is assumed to be a delta function centred at the mean value. In order to take into account micro-mixing effects a model must be added. In this work the finite-mode PDF approach is used to predict the selectivity of a parallel reaction in a Taylor-Couette reactor working in semi-batch conditions. Experimental data are compared with model predictions in order to investigate the relevance of the micro-mixing model. The case of precipitation is also discussed.     

A semi-implicit immersed boundary method and its application to viscous mixing

Computational fluid dynamics (CFD) simulations in the context of single-phase mixing remain challenging notably due the presence of a complex rotating geometry within the domain. In this work, we develop a parallel semi-implicit immersed boundary method based on Open∇FOAM, which is applicable to unstructured meshes. This method is first verified on academic test cases before it is applied to single phase mixing. It is then applied to baffled and unbaffled stirred tanks equipped with a pitched blade impeller. The results obtained are compared to experimental data and those predicted with the single rotating frame and sliding mesh techniques. The proposed method is found to be of comparable accuracy in predicting the flow patterns and the torque values while being straightforwardly applicable to complex systems with multiples impellers for which the swept volumes overlap.     

Polymer reaction engineering: From reaction kinetics to polymer reactor control

Polymer reaction engineering is a relatively “young”, very broad, multidisciplinary, rapidly developing field. It is the combination of polymer science, chemistry and technology with process engineering principles. The outcome of this high degree of synergism has evolved over the last fifteen or so years towards an area that includes any or all of the following: polymerization and post-polymerization (chemical modification) reaction kinetics; mathematical modelling and process simulation; polymer reactor design and scale-up; sensor development and process monitoring; and polymer reactor optimization, state estimation and computer control. This article will attempt to give an overview of the results obtained in our laboratory over the last seven years from systematic studies of polymer reaction engineering and polymer production technology problems. These problems cover all aspects of polymer reaction engineering mentioned above. Going from fundamentals to practice, the basic premise of the article is that only by adopting a holistic approach can one devise effective strategies in order to achieve the final objective of more efficient polymer reactor design and control, and hence improved production systems of polymeric materials.     

A homogeneous chemical reactor analysis and design laboratory: The reaction kinetics of dye and bleach

An experimental module for senior-level reaction engineering/reactor design students is described. The module is used to characterize the kinetics of dye (food coloring) neutralization by household bleach, and the reactor system is configurable for use in either batch reactor or continuous-stirred tank reactor (CSTR) modes. The reactor temperature, volume, reactant feed rates, and reactant concentrations may be adjusted to enable students to obtain a wide range of kinetic data. Dye concentrations in the reactor are monitored by absorbance spectroscopy, and the kinetic rate law is determined directly from the batch reactor performance data. Students use the completed kinetic rate law to compare experimental steady-state CSTR performance data to the mathematical models derived from reactor design equations. Finally, the students use the kinetic behavior of the system to design a hypothetical plug-flow reactor for the same chemical reaction and a set of stated operational goals.     

CFD simulations of bubble column reactors: 1D, 2D and 3D approach

CFD simulations have been carried out for the predictions of flow pattern in bubble column reactors using 1D, 2D and 3D k-ε models. An attempt has been made to develop a complete correspondence between the operation of a real column and the simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. All the models showed good agreement with the experimental data for axial liquid velocity and the fractional gas hold-up profiles. However, as regards to eddy diffusivity, only the 3D model predictions agree closely with the experimental data.The CFD model has been extended for the estimation of an axial dispersion coefficient (D_L) using 1D, 2D and 3D models. Excellent agreement was found only between the experimental values and the 3D predictions. The 1D and 2D simulations, however, yielded D_L values, which were lower by 25-50%. For this, a mechanistic explanation has been provided.     

Investigation of Fischer-Tropsch synthesis performance and its intrinsic reaction behavior in a bench scale slurry bubble column reactor

A bench scale slurry bubble column reactor (SBCR) with active-Fe based catalyst was developed for the Fischer-Tropsch synthesis (FTS) reaction. Considering the highly exothermic reaction heat generated in the bench scale SBCR, an effective cooling system was devised consisting of a U-type dip tube submerged in the reactor. Also, the physical and chemical properties of the catalyst were controlled so as to achieve high activity for the CO conversion and liquid oil (C₅₊) production. Firstly, the FTS performance of the FeCuK/SiO₂ catalyst in the SBCR under reaction conditions of 265 °C, 2.5 MPa, and H₂/CO = 1 was investigated. The CO conversion and liquid oil (C₅₊) productivity in the reaction were 88.6% and 0.226 g/g_cat-h, respectively, corresponding to a liquid oil (C₅₊) production rate of 0.03 bbl/day. To investigate the FTS reaction behavior in the bench scale SBCR, the effects of the space velocity and superficial velocity of the synthesis gas and reaction temperature were also studied. The liquid oil production rate increased up to 0.057 bbl/day with increasing space velocity from 2.61 to 3.92 SL/h-g_Fe and it was confirmed that the SBCR bench system developed in this research precisely simulated the FTS reaction behavior reported in the small scale slurry reactor.     

CFD modeling of tapered circulating fluidized bed reactor risers: Hydrodynamic descriptions and chemical reaction responses

A comprehensive two-dimensional transient Eulerian model combined with the kinetic theory of granular flow was developed to obtain the hydrodynamic and chemical reaction behaviors in tapered circulating fluidized bed reactor risers. In this study, the focus was on the chemical reactions and its behaviors inside three different riser geometries. The model was verified by using an experimental dataset from the literature, and was then used for both predicting the hydrodynamic behaviors and computing the system turbulent properties. The tapered-out riser improves the system turbulence or mixing which can be explained by the dispersion coefficients. On the other hand, the tapered-in riser increases the solid particle residence time and gives a more uniform temperature distribution, because it does not have sufficient force to support the weight of the particles. The same riser geometries but with the addition of the chemical reaction were then used for evaluating the previously proposed criteria that the riser geometry should be chosen with respect to the characteristics of the reactions. Reactions with a medium reaction rate were best suited to the typical riser, whilst reactions with a fast and slow reaction rate best fitted the tapered-out and tapered-in risers, respectively.     

A structured catalytic bubble column reactor: hydrodynamics and mixing studies

This paper reports the results of a comprehensive experimental study of the hydrodynamics and mixing in two bubble column reactors of 0.1 and 0.24 m in diameter with KATAPAK-S^® as packing material. Total gas hold up and axial dispersion coefficients were measured in the structured bubble columns and the values were compared with experimental results obtained in the same work with empty bubble columns. The results reveal that the gas hold up in structured bubble columns is practically the same as in empty bubble columns when compared at the same superficial gas velocity based on open area available for gas–liquid dispersion. The presence of the structured elements in the bubble column reactor reduces the liquid phase backmixing by one order of magnitude.     

A comparative study for nanoparticle production with passive mixers via solvent-displacement: Use of CFD models for optimization and design

Active and passive mixers, including a considerable variety of micro-devices, are nowadays widely used for the production of nanoparticles. Polymer nanoparticles for controlled drug delivery applications are investigated in this work with two specific objectives. The first one is to experimentally quantify the efficiency of confined impinging jets reactors and Tee-mixers in the production of nanoparticles constituted by two polymers: poly-?-caprolactone and poly(methoxypolyethyleneglycolcyanoacrylate-co-hexadecylcyanoacrylate). The second objective is the development of a simple and reliable mathematical model to be used for the design, optimization and scale up of mixers for polymer nanoparticle production. Although the behaviour of the polymers investigated is quite different, it is possible to conclude that confined impinging jets reactors are more efficient than Tee-mixers, in converting the pressure drop into turbulent kinetic energy and as a consequence in producing smaller particles. The very simple modelling approach proposed here (based on the evaluation of the mixing time) seems to be able to correlate well experimental data obtained under different operating conditions, independently on the type of device used. Moreover, in the case of poly-?-caprolactone it was also possible to successfully quantify the particle formation time with a simple power law, further exploiting the model.     

Optimising mixing and nutrient removal in membrane bioreactors: CFD modelling and experimental validation

Membrane Bioreactors (MBRs) have been used successfully in biological wastewater treatment to solve the perennial problem of effective solids-liquid separation. The optimisation of MBRs requires knowledge of the membrane fouling, mixing and biokinetics. MBRs are designed mainly based on the biokinetic and membrane fouling considerations even though the hydrodynamics within an MBR system is of critical importance to the performance of the system. Current methods of design for a desired flow regime within the MBR are largely based on empirical techniques (e.g. specific mixing energy). However, it is difficult to predict how vessel design in large scale installations (e.g. size and position of inlets, baffles or membrane orientation) affects hydrodynamics, hence overall performance. Computational Fluid Dynamics (CFD) provides a method for prediction of how vessel features and mixing energy usage affect the hydrodynamics and pollutant removal and subsequently allowing optimisation of MBR design and performance. In this study, a CFD model was developed which accounts for aeration and biological nutrient removal. The modelling results are compared against experimental results of two full scale MBRs for the hydrodynamics and against a modelling benchmark for the biological nutrient removal component of the model.     

New Engineered Particles from Spray Dryers: Research Needs in Spray Drying

This article reviews developments in the simulations of spray dryer behavior, including the challenges in modeling the complex flow patterns inside the equipment, which are often highly transient and three-dimensional in nature. There appears to be considerable scope for using CFD simulations for investigating methods to reduce the rates of wall deposition and of thermal degradation for particles by modifying the air flow patterns in the chamber through small changes in the air inlet geometry. Challenges include building particle drying kinetics and reaction processes, as well as agglomeration behavior, into these simulations. The numerical simulations should be valuable supplements to pilot-scale testing, enabling more extensive and accurate optimization to be carried out than hitherto possible. New understanding of reaction processes and materials science, in combination with recent knowledge of the application of CFD to these problems, may enable new engineered powder products to be developed from the one-step spray-drying process.