Mixing performance comparison of milliscale continuous high‐shear mixers

Mixing performance of two continuous flow millilitre‐scale reactors (volumes 9.5 mL and 2.5 mL) equipped with rotor‐stator mixers was studied. Cumulative residence time distributions (RTD) were determined experimentally using a step response method. Distributions were measured for both reactors by varying impeller speed and feed flow rate. The mixing effect was determined by measured RTDs. Computational fluid dynamics (CFD) were used to verify that the residence time distribution in the measurement outlet agreed with the outlet flow. The mixing power of both reactors was determined using a calorimetric method. The reactor inlet flow rate was found to affect mixing performance at 1–13 s residence times but the effect of impeller speed could not be noted. Both milliscale reactors are close to an ideal continuous stirred‐tank reactor (CSTR) at the studied impeller speed and flow rate ranges. The specific interfacial area was found to depend on the reactor inlet flow rate at constant impeller speed for the case of copper solvent extraction.

     

Numerical Study on Thermal Safety of Emulsion Matrix in Emulsifier

The purpose of this study is to research the effect of high speed stirring by emulsifier blades on the thermal safety of emulsion matrix. In order to analyze thermal safety of emulsion matrix in emulsifying process, the heating effect influenced by rotor speed and emulsify temperature was combined with self‐heat properties of emulsion matrix in this study. Computational fluid dynamics (CFD) is used for analysis. The emulsifier model adopts simplified CYJ type emulsifier, which is a kind of vertical emulsifier and has a large stirring area. The self‐heat properties of emulsion matrix are described by Arrhenius equation. The simulation results showed that heat could not accumulate in emulsion matrix due to the strong turbulence formed by stirring. But, because of the shear stress and friction, a high temperature region will be formed around the blades of the rotor in a short time. If the stirring speed reaches 10300 rpm, the temperature in this region would be able to reach the critical ignition temperature of emulsion matrix.     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

Computational Fluid Dynamics Study of a Styrene Polymerization Reactor

The computational fluid dynamics (CFD) approach was adopted to simulate benzoyl peroxide (BPO)‐initiated styrene polymerization in a laboratory‐scale continuous stirred‐tank reactor (CSTR). The CFD results revealed the effects of non‐homogeneity and the short‐circuiting of the unreacted styrene and initiator on the reactor performance. The study also investigated the effects of the impeller speed and the residence time on the conversion and the flow behavior of the system. The CFD simulation showed that intense mixing remained confined to a small region near the impeller. With increasing impeller speed, it was found that the perfectly mixed region near the impeller expanded, thus reducing non‐homogeneity. Different contours were generated and exhibited the effect of the mixing parameters on the propagation rate and styrene conversion. The monomer and initiator conversions predicted with the CFD model were compared to those obtained with a CSTR model. The CFD model accounts for the non‐ideality behavior of the polymerization reactor, and hence conversion predictions are more realistic.     

Efficiencies of Sieve Tray Distillation Columns by CFD Simulation

A 3‐D two‐fluid CFD model in the Eulerian‐Eulerian framework was developed to predict the hydrodynamics and heat and mass transfer of sieve trays. Interaction between the two phases occurs via interphase momentum and heat and mass transfer. The tray geometries are based on the large rectangular tray of Dribika and Biddulph and FRI commercial‐scale sieve tray of Yanagi and Sakata. In this work a CFD simulation is developed to give predictions of the fluid flow patterns, hydraulics, and mass transfer efficiency of distillation sieve trays including a downcomer. The main objective has been to find the extent to which CFD can be used as a design and prediction tool for real behavior, concentration and temperature distributions, and efficiencies of industrial trays. Despite the use of simple correlations for closure models, the efficiencies obtained are very close to experimental data. The results show that values of point efficiency vary with position on the tray because of variation of affecting parameters, such as velocities, temperature and concentration gradients, and interfacial area. The simulation results show that CFD can be used as a powerful tool in tray design and analysis, and can be considered as a new approach for efficiency calculations and as a new tool for testing mixing models in both phases. CFD can be used as a “virtual experiment” to simulate tray behavior under operating conditions.     

Characteristics of Gas-Liquid Contact on Cross-Current Trays

The main outline of this investigation is demonstrating the behavior and details of gas-liquid contact on the cross-current trays using a 3D CFD model. The model has been applied to several operating conditions, and the effect of gas and liquid flow rates on the contact characteristics was studied, and in-depth picturing of local events was presented. Based on the picturing of details of flow features, an innovative division of flow region on the tray deck was presented. The tray is divided into four regions having different characteristics of flow phenomena, and these regions and their flow features are described as a novel outlook in this way. Although the extent of nonuniformities and interfacial area density are dependent on the operating conditions, but partial to total nonuniformities and weak phase mixing are present in all four regions.     

Numerical Simulation of Absorbing CO2 with Ionic Liquids

Although separating CO₂ from flue gas with ionic liquids has been regarded as a new and effective method, the mass transfer properties of CO₂ absorption in these solvents have not been researched. In this paper, a coupled computational fluid dynamic (CFD) model and population balance model (PBM) was applied to study the mass transfer properties for capturing CO₂ with ionic liquids solvents. The numerical simulation was performed using the Fluent code. Considering the unique properties of ionic liquids, the Eulerian‐Eulerian two‐flow model with a new drag coefficient correlation was employed for the gas‐liquid fluid dynamic simulation. The gas holdup, interfacial area, and bubble size distribution in the bubble column reactor were predicted. The mass transfer coefficients were estimated with Higbie's penetration model. Furthermore, the velocity field and pressure field in the reactor were also predicted in this paper.     

Simulation of Barium Sulfate Precipitation using CFD and FM‐PDF Modeling in a Continuous Stirred Tank

A mixing‐precipitation model combining computational fluid dynamics (CFD), finite‐mode PDF (probability density function) model, population balance and kinetic modeling has been proposed to simulate the barium sulfate precipitation process in a continuous stirred tank agitated by a Rushton turbine. The effect of various operating conditions such as impeller speed, feed concentration, feed position and mean residence time on the barium sulfate precipitation process is clearly demonstrated. It is shown that the mean crystal size increases by increasing the impeller speed and mean residence time. However, when the feed concentration is increased, the mean crystal size decreases. The predictions are in reasonable agreement with the experimental data in the literature.     

Study on gas-liquid flow characteristics in stirred tank with dual-impeller based on CFD-PBM coupled model

Study on gas-liquid flow in stirred tank with two combinations of dual-impeller (six-bent-bladed turbine(6BT)+six-inclined-blade down-pumping turbine (6ITD),the six-bent-bladed turbine (6BT)+six-inclined-blade up-pumping turbine (6ITU)) was conducted using computational fluid dynamics (CFD) and popula-tion balance model (PBM) (CFD-PBM) coupled model.The local bubble size was captured by particle image velocimetry (PIV) measurement.The gas holdup,bubble size distribution and gas-liquid interfacial area were explored at different conditions through numerical simulation.The results showed that the 4 mm bubbles accounted for the largest proportion of 33％ at the gas flow rates Q =0.76 m3·h-1 and 22％ at Q =1.52 m3·h-1 for combined impeller of 6BT + 6ITU,while the bubbles of 4.7 mm and 5.5 mm were the largest proportion for 6BT + 6ITD combination,i.e.25％ at Q =0.76 m3·h-1 and 22％ at Q =1.52 m3·h-1,respectively,which indicated that 6BT + 6ITU could reduce bubble size effectively and promote gas dispersion.In addition,the gas holdup around impellers was increased obviously with the speed compared with gas flow rate.So it was concluded that 6ITU impeller could be more conductive to the bubble dispersion with more uniform bubble size,which embodied the advantages of 6BT + 6ITU combination in gas-liquid mixing.     

CFD modelling and scale-up of Confined Impinging Jet Reactors

Confined Impinging Jet Reactors (CIJRs) are appealing devices for precipitation of nanoparticles because of their high mixing efficiency. In fact, since precipitation processes are generally very fast, mixing plays a crucial role and it is of great importance to operate under very fast mixing conditions. In this work mixing and reaction in CIJRs are studied by means of Computational Fluid Dynamics (CFD). Mixing at the molecular level is modelled with a presumed Probability Density Function (PDF) approach: the Direct Quadrature Method of Moments coupled with the Interaction by Exchange with the Mean (DQMOM-IEM) model. The influence of operating conditions and reactor geometry on mixing is also evaluated and a scale-up criterion for CIJRs is developed, showing that scaling up by means of CFD is a practicable path, worth of further investigation.     

Numerical simulations of partially‐filled rubber mixing in a 2‐wing rotor‐equipped chamber

Partially filled internal batch mixers are used for mixing of rubber compounds in the polymer industry. The use of mixing in such mixers equipped with a rotor is critical to the process itself, and hence, understanding of mixing is important in terms of evaluating how various operating parameters such as rpm, fill factor, and ram pressure affect distribution and dispersion of materials. The objective of the current study is to gain valuable insights on the influence of fill factor, which is the volume of the material relative to the volume of the chamber. Two‐dimensional (2D) computational fluid dynamics (CFD) simulations of rubber mixing in a 2‐wing rotor‐equipped chamber are presented here, for the first time, for fully‐filled/100% and partially‐filled/75% chambers. The volume‐of‐fluid (VOF) technique is employed to capture the interface between the rubber and air in partially filled isothermal simulations. Flow patterns are visualized to analyze the material movement. Massless particles are injected and various statistics are calculated from their positions in order to compare dispersive and distributive mixing characteristics between the fully‐filled and partially‐filled cases. Specifically, quantities such as mixing index and the maximum shear stress distribution history of particles are analyzed to obtain information about dispersive mixing, while length of stretch and cluster distribution index, also calculated from particles, are presented to investigate distributive mixing capabilities. All the results consistently demonstrated the superior effectiveness of partially‐filled mixing chambers in terms of their dispersive and distributive mixing characteristics in comparison to fully‐filled chambers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44250.     

Analysis of mixing during melt-melt blending in twin screw extruders using reactive polymer tracers

Mixing during melt-melt blending of segregated polypropylene melt streams in a co-rotating twin screw extruder was experimentally investigated. The mixing limited reaction between two polymer reactive tracers, which are terminally functionalized polyolefin oligomers, was used to determine the mixing performance of a kneading block section. The selected functional groups were succinic anhydride and a primary amine, and Fourier-Transform Infrared Spectrometry (FT-IR) was used to determine the anhydride conversion. In the absence of interfacial tension, the reaction conversion was directly related to the amount of interfacial area generated. Experiments were completed to study the effects of operating conditions, kneading block design, and polymer material properties. The screw speed effect was observed to be non-linear because of competing contributions from shear rate, residence time, channel fill, and viscous heating. The mixing performance of kneading blocks backed by a reverse conveying element was observed to follow the trend of: forward > reverse > neutral. For each kneading block design, the mixing performance decreased with an increase in polymer viscosity.     

Liquid Flow on Structured Packing: CFD Simulation and Experimental Study

A CFD model of the two‐phase countercurrent flow in the geometry of the plate‐type structured packing Mellapak 250.Y was built, tested and verified. The model was applied to determine the effect of liquid and gas flow rates and physicochemical properties of the flowing liquids on the interfacial area formed on structured packing. The CFD model allowed us to determine the minimum liquid flow rate at which an unbroken liquid film was observed on the packing surface. The simulations confirmed that with an increase of the wetting rate the surface of the packing covered with a liquid film increased until the surface was totally covered up, while further slight changes of an interfacial area were the result of wave formation. The effect of gas load (F factor) on the film surface was in the range of a calculation error. Results of the CFD simulation allow us to predict the stages of film formation during liquid flow, to follow local velocity oscillations, film thickness and velocity profiles of phases.     

Effectiveness of a backward mixing screw element for glass fiber dispersion in a twin‐screw extruder

In the kneading of glass–fiber‐reinforced plastics by twin‐screw extrusion, the use of a backward‐mixing screw (BMS) element for melt mixing has been found to be effective in dispersing glass–fiber bundles. In this study, we use computational fluid dynamics (CFD) to investigate the mechanism for the effectiveness of BMS for glass fiber dispersion. CFD of BMS melt mixing revealed that there is high uniformity of transport in the direction of extrusion and efficient transportation occurs from low‐stress to high‐stress regions. These findings demonstrate that BMS melt mixing is highly effective at imparting stress to the overall resin passing through. In addition, there is a correlation between the incidence of nondispersion of glass–fiber bundles measured experimentally and the stress history minimum value. On the basis of the above factors, we propose a method for predicting the operating conditions in which the nondispersion of glass–fiber bundles is controlled. The operating conditions for controlling glass–fiber nondispersion can be determined for various different mixing elements and the possible production rate can be predicted. Predictions for the operating conditions were applied to BMS and a forward kneading disk element (FKD). The effectiveness of BMS for controlling glass fiber nondispersion is characterized for a broad range of operating conditions. POLYM. ENG. SCI., 54:2005–2012, 2014. © 2013 Society of Plastics Engineers     

Effect of the processing parameters on the surface resistivity of acrylonitrile–butadiene rubber/multiwalled carbon nanotube nanocomposites

In this study, acrylonitrile–butadiene rubber (NBR) was melt‐mixed with multiwalled carbon nanotubes (MWCNTs). Because the electrical conductivity and mechanical properties of composites are strongly influenced by the filler's state of dispersion and the extent of filler breakage during processing, the processing conditions are very important parameters. The effects of the mixing time, rotor speed, cooling rate, and sulfur concentration on the surface resistivity were investigated. Increasing the rotor speed from 20 to 60 rpm at mixing times of 15 and 30 min led to an increase in the surface resistivity from around 10⁴ to 10¹¹ Ω/square. However, at a mixing time of 7 min, the surface resistivity slightly decreased with increasing rotor speed. When slow cooling was applied, a surface resistivity of 10⁴ Ω/square was obtained at around 2‐phr MWCNTs. However, when the fast cooling was applied, a surface resistivity of 10⁶ Ω/square was obtained at 5‐phr MWCNTs. The tensile strength and tensile modulus at 300% elongation were improved with the addition of MWCNTs into NBR. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Evaluation of the mixing performance for one novel twin screw kneader with particle tracking

In this article, a novel continuous twin‐screw kneader was proposed. The end‐cross section of the screw rotor consists of convex arcs and cycloidal curves and the rotors profiles were presented. The mixing performance of the novel twin screw kneader was simulated using finite element method (FEM) combined with mesh superimposition technique (MST). Statistical analysis was carried out for flow field using particle tracking technique to research the effect of geometry parameters and working parameters on the mixing performance. To study the dispersive mixing performance, specifically the maximum shear rate, maximum shear stress, maximum mixing index, residence time distribution (RTD) and RTD density function of tracer particles, and dispersive mixing is evaluated using the mixing index in combination with the shear stress. The results show that the changes of centre distance between female and male rotor have little influence on dispersive mixing performance, the lead of rotor has little effect on maximum shear stress and maximum shear rate, while it has an obvious effect on mixing index, cumulative RTD, and RTD density function. The rotor speed has obvious influence on mixing performance, and average residence time of material decreases greatly and the mixing ability is weakened, while the self‐cleaning performance of rotor improved obviously with the increasing of rotor speed. POLYM. ENG. SCI., 54:2407–2419, 2014. © 2013 Society of Plastics Engineers     

有机工质向心透平变工况特性的数值模拟与极差分析

以R245fa为工质设计向心透平,采用CFD方法对向心透平性能进行全流域三维模拟研究,考察了入口温度、转子转速和膨胀比(进出口压力比)对向心透平工况特性的影响,对主要影响因素进行极差分析。结果表明,向心透平工作转速为设计转速的80%~100%时,输出功率和等熵效率波动较小,工作转速高于设计值时透平性能迅速下降。随入口温度升高,透平输出功率与等熵效率增大;随膨胀比增大,透平输出功率线性增加。透平存在最佳膨胀比使等熵效率最大,且实际运行压比大于最佳膨胀比时,透平等熵效率变化较小。出口压力对向心透平输出功率影响最大,温度的影响最小;转子转速对等熵效率影响最大,入口压力的影响最小。     

Open-Source CFD Simulations of Liquid–Liquid Flow in the Annular Centrifugal Contactor

Simulation of the fluid dynamics of solvent extraction in centrifugal contactors requires advanced models to account for complex physical phenomena including turbulent free-surface flow and liquid-liquid dispersion physics. The use of an open-source computational fluid dynamics (CFD) framework allows for implementation of advanced models not feasible in commercial CFD applications. The open-source CFD package OpenFOAM has been used to simulate turbulent, multiphase flow in the annular centrifugal contactor, including simulations of the mixing zone (annular region), and of the coupled operation of the mixing and separation (rotor interior) zones. These simulations are based on the Volume of Fluid (VOF) methodology along with Large Eddy Simulation (LES) for turbulence. The results from these simulations compare favorably with previous simulations using a commercial CFD tool and with available experimental data. They also give insight into the requirements for more advanced multiphase models needed to accurately capture flows in these devices.     

Optimization of gas-liquid reactor using computational fluid dynamics

The relationship between the geometry and the operating conditions, hydrodynamics and performance of an industrial gas-liquid stirred reactor has been studied with the help of CFD modeling and gamma ray tomography. It is seen that the interfacial area distribution is very wide. This in some areas leads to mass transfer limitations. Strategies for retrofitting the reactor were then developed (e.g. change in the operating speed, change of impeller type, change in the feed introduction, etc.) and tested by CFD for improving the reactor performance. The benefits of the implementation were in terms of the improvement in the product quality, reduction in the by-product formation, increase in the reactor throughput.     

The mastication characteristics of powdered carbon black filled natural rubber during internal mixing

Powdered carbon black filled natural rubber, P(NR/HAF), is a premixture of natural rubber and carbon black in powdered form with good carbon black dispersion throughout the rubber matrix. In this study, the mastication properties of P(NR/HAF) were observed under a wide temperature range (50–110°C) and rotor speed (30–100 rpm) range, using a mixing head attached to Brabender Plasticorder. It was found that P(NR/HAF) showed different mastication characteristic, compared to the traditional internal mastication theories of natural rubber; poor masticating properties of P(NR/HAF) are observed with low rotor speed and high temperature and a ‘‘stable zone’’ with middle rotor speed and lower temperature. The Mooney viscosity of rubber batch under different rotor speed and temperature was almost the same, and mastication properties were unsatisfactory. It was considered that the higher temperature build‐up of the mixing batch of P(NR/HAF) during the early stage of internal mixing results in the special mastication properties. A quadratic mastication model of P(NR/HAF), based on the multivariate regression analysis and stepwise regression analysis, was used to predict the mastication characters of P(NR/HAF) in internal mixer under varied temperature and rotor speed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers