Drop breakage in a single-pass through vortex-based cavitation device: Experiments and modeling

Liquid–liquid emulsions are used in many sectors such as personal care, home care, and food products. There is an increasing need for developing compact and modular devices for producing emulsions with desired droplet size distribution (DSD). In this work, we have experimentally and computationally investigated an application of vortex-based hydrodynamic cavitation (HC) device for producing emulsions. The focus is on understanding drop breakage occurring in a single-pass through the considered HC device. The experiments were performed for generating oil-in-water emulsion containing 1%–20% rapeseed oil. The effect of pressure drop across the HC device in the range of 50–250 kPa on drop breakage was examined. DSD of emulsions produced through a single pass was measured using the focussed beam reflectance measurements. Comprehensive computational fluid dynamics (CFD) model based on the Eulerian approach was developed to simulate multiphase cavitating flow. Using the simulated flow, population balance model (PBM) with appropriate breakage kernels was solved to simulate droplet breakage in a vortex-based HC device. The device showed an excellent drop breakage efficiency (nearly 1% which is much higher than other commercial devices such as rotor–stators or sonolators) and was able to reduce mean drop size from 66 to ~15 μm in a single pass. The CFD and PBM models were able to simulate DSD. The presented models and results will be useful for researchers and engineers interested in developing compact devices for producing emulsions of desired DSD.     

CFD simulation of the hydrodynamics in an internal air-lift reactor with two different configurations

Computational fluid dynamics (CFD) was used to investigate the hydrodynamic parameters of two internal airlift bioreactors with different configurations. Both had a riser diameter of 0.1 m. The model was used to predict the effect of the reactor geometry on the reactor hydrodynamics. Water was utilized as the continuous phase and air in the form of bubbles was applied as the dispersed phase. A two-phase flow model provided by the bubbly flow application mode was employed in this project. In the liquid phase, the turbulence can be described using the k-? model. Simulated gas holdup and liquid circulation velocity results were compared with experimental data. The predictions of the simulation are in good agreement with the experimental data.     

Modeling of hydrodynamic cavitation reactors: a unified approach

An attempt has been made to present a unified theoretical model for the cavitating flow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase flow as the basis. This model has been used to describe the radial motion of bubble in the cavitating flow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an orifice plate. Simulations of the bubble dynamics in a venturi flow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity fluctuations the radial bubble motion in case of an orifice flow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation.     

Hydrodynamics and droplet size distribution of liquid–liquid flow in a packed bed reactor with orifice plates

A packed bed reactor with orifice plates (PBR@OP) was designed by adding orifice plates periodically in packed beds. Hydrodynamics and droplet size distribution in PBR@OP were experimentally investigated using fatty acid methyl esters (FAME)/water as the model liquid–liquid system. In PBR@OP, the flow pattern was close to plug flow. Droplets with Sauter mean diameter (d₃₂) of 150–550 μm were generated. The pressure drop of orifice, flow velocity and plate spacing were key parameters to control the droplet size. The reactor performance was evaluated by analyzing a FAME epoxidation process. At the same d₃₂ and residence time, the length and total pressure drop of PBR@OP were about 1/3 and 1/4 of those of PBR without orifice plates, respectively. Furthermore, a semi-empirical correlation describing the d₃₂ change in PBR@OP was developed, revealing a relative mean deviation of 8.64%. PBR@OP presents a cost-effective option for the intensification of liquid–liquid medium rate reactions.     

Comparison of two-phase upflow and downflow fixed bed reactors performance: Catalytic SO2 oxidation

A time- and space-dependent model based on the piston-dispersion-exchange model for liquid flow was developed to analyze the performance of two-phase upflow and downflow fixed bed reactors and was applied to the catalytic SO₂ oxidation. The hydrodynamic parameters were determined from residence time distribution measurements, using an imperfect pulse method for time-domain analysis of nonideal pulse tracer response. A transient diffusion model of the tracer in the porous particle coupled with the PDE model was used to interpret the obtained RTD curves. Gas-liquid mass transfer parameters were determined by a stationary method based on the least square fit of the calculated concentration profiles in gas phase to the experimental values. It is shown that two-phase downflow fixed bed reactor performs better at low liquid flow rates, while two-phase downflow fixed bed reactor performs better at low liquid flow rates, while two-phase upflow performs better at high liquid flow rates.     

An investigation of the droplet size distributions in a cyclone reactor for liquid-liquid heterogeneous reactions using FBRM and PVM

A multi-element process coupled cyclone reactor for the liquid-liquid heterogeneous reaction was proposed. This paper presents the investigation of dispersed phase droplet size distributions near the wall surface using FBRM and PVM. To translate a chord length distribution (CLD) measured by the FBRM instrument into its droplet size distribution (DSD), a CLD-DSD transformation model was proposed. Moreover, the effects of operational parameters on the droplet size distributions were studied. The results show that the total inlet flow plays a decisive role on the dispersed phase droplet size distributions, while the influence of feed ratio is mainly reflected in inhibiting the probability of droplet coalescence. In comparison, the influence of overflow ratio on the droplet size distributions can be neglected.     

Visual study on the characteristics of liquid and droplet in a novel rotor-stator reactor

Rotor–stator reactor (RSR), an efficient mass transfer enhancer, has been applied in many fields. However, the hydrodynamic characteristics of liquid flow in RSR are still a mystery despite they are fundamental for the mass transfer performance and processing capacity. In view of the above, this paper studies the liquid–liquid flow and liquid holdup in RSR under various conditions with a high-speed camera. The paper firstly demonstrates two flow patterns and liquid holdup patterns that we obtained from our experiment and then presents in succession a flow pattern and a liquid holdup criterion for the transition of film flow to filament flow and complete filling to incomplete filling. It is found that experimental parameters, including rotor–stator distance, rotational speed and volume flow rate exert great influence on the average droplet diameter and size distribution. Besides, by comparison and contrast, we also find that the experimental values match well with our previous predicted calculations of the average diameter, and the relation between the average diameter and the mean energy dissipation rate.     

气液两相流摆动特性实验的数值模拟

为了给鼓泡塔反应器设计提供依据,运用计算流体力学(CFD)软件模拟了鼓泡塔气液两相流动态行为。采用双欧拉法对鼓泡塔矩形反应器内不同曝气量下气液两相流的摆动特性进行了模拟考察,液相采用标准κ-ε紊流模型,气相采用分散相零方程模型,分析了网格尺寸、时间步长以及相间作用力对模拟结果的影响,模拟的曝气量为42.5~237 m L/s。结果表明,当相间作用力仅考虑阻力时,气液两相流呈现周期性摆动规律;随着气流量的增加,气泡羽流的摆动幅度和频率增大,同时液体的气含率也在增加;模拟的气液两相流摆动频率数据与实验值吻合较好,两者的相对误差为7.2%~12.9%。     

Experimental confirmation of thermal plasma CVD of diamond with liquid feedstock injection model

A three-dimensional model of diamond chemical vapor deposition in a thermal plasma system has been compared with experimental results to confirm the validity of the model in simulating reactor flow patterns and deposit characteristics. Model and experimental cases were tested with the same boundary and operating conditions. Several sets of operating conditions were analyzed to confirm the validity of the model. Trends in the diamond chemical vapor deposition system based on the effects of droplet size, injection probe to substrate offset, the addition of an inert carrier gas, and the differences associated with the use of liquid or gaseous precursor feedstock were investigated. To test the validity of flow patterns predicted by the model, a laser strobe video system was used to map droplet trajectories in the reactor. Experimental results were found to support the calculated droplet trajectories and flowlines in the reactor. Deposition characteristics such as the mass deposition rate and the area of deposit were examined in the model and experimental cases. General trends, with respect to deposition characteristics, produced by altering the operating conditions in the experiment, and respectively the boundary conditions in the model were found to be similar. Differences between model and experimental results are probably due to the use of an overly simplified surface chemistry model, which does not take into account graphite deposition. In addition, modeling of radial droplet injection does not take into account non-radial perturbations.     

Methanation in a fluidized bed reactor with high initial CO partial pressure: Part II— Modeling and sensitivity study

A model of a bench-scale methanation reactor was set-up by modifying the classical two-phase model approach and introducing an additional bulk flow from bubble to dense phase to consider the volume contraction of the methanation reaction. The model uses experimentally determined kinetics and hydrodynamic correlations from literature. It was satisfyingly validated by comparing the calculated gas concentration profiles with the experimental data, especially with respect to initial reaction rates and reactor exit concentrations.A sensitivity study with respect to different bubble size correlations, mass transfer rates and considering or neglecting the bulk flow (influence of volume contraction caused by the methanation reaction) was carried out. It showed that the bubble size correlation by Werther and the resulting gas concentration profiles fit the measured data better than the computed gas concentration profiles using the bubble size correlation by Rowe.Neither a variation of the mass transfer coefficient nor neglecting the bulk flow in the fluidized bed model did yield further improvement of the calculated concentration profiles.     

Glycerolysis of Fatty Acid Methyl Esters: 2. Simulation and Experiments in Continuous Reactors

The glycerolysis of methyl ester was investigated in flow reactors. This reaction represents a liquid two-phase reaction with changing reaction rates in a batch reactor. A semi-empirical model tested earlier with batch reactor data was used to simulate different continuous processes for this reaction. Among the processes simulated, a single continuous-flow stirred tank reactor (CSTR) without recycling was considered most appropriate for experimental implementation, although simulations showed that a faster reaction rate is possible with the application of a CSTR followed by a tubular reactor with certain associated residence times. The CSTR simulations were verified experimentally. A good agreement was found between the experimental data and simulation results.     

滴流床中持液量及流型转变的一维流体力学模型

针对滴流床中均匀球形填料,在颗粒尺度上分析气液两相在颗粒空隙中的流动.从基本的流体力学方程出发,建立了微观流动模型,求得滴流区液相在填料表面上的分布,进而计算液相总持液量,并通过对液膜波的稳定性分析,提出了滴流区向脉动区转变的判据.模型与实验结果及文献值作了比较,符合程度较好.     

分布器结构对环流反应器气含率分布的影响

采用κ-ε二方程模型和欧拉多相流模型,对一种单筒单级气升式气液环流反应器内的湍流气液两相流进行了全尺寸的数值模拟研究,考察了采用3种不同气体分布器时反应器内气含率和流速分布的细节.模拟结果表明不同结构的分布器对总体气含率和内筒中的两相速度分布有很大影响,因而对气含率分布和气液两相接触效果有较大影响,从而对反应过程产生影响.单环分布器产生的气液两相接触效果较差,对于反应过程很不利.对于大直径的环流反应器推荐使用多环分布器.计算所得的整体气含率与实测的整体气含率进行了对比,吻合较好.     

Experimental and theoretical analysis of axial dispersion in the liquid phase in external-loop airlift reactors

A model has been developed for the prediction of the axial dispersion coefficient in the riser of an external-loop airlift reactor. It takes into account both radial velocity and gas hold-up profiles as a function of flow regime, but also the two-phase turbulence. It is based on the mixing length model developed by [Vial et al., 2002. Chem. Eng. Sci. 57, 4745-4762] for the flow field prediction and uses a turbulent diffusion coefficient to estimate the influence of two-phase turbulence on mixing. The relations established using experimental data from an airlift reactor have been validated experimentally using a second reactor. A comparison with theoretical dispersion coefficients deduced from CFD calculations and correlations from the literature is also provided. The results show that in all the hydrodynamic regimes, dispersion stems from bubble-induced turbulence, despite the presence of a nearly parabolic liquid velocity profile in the homogeneous regime and marked liquid hold-up profiles in churn-turbulent flow.     

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.     

Two‐phase production of cyclohexene and cyclooctene oxides in water as an approach to pollution prevention

Liquid–liquid two‐phase epoxidation from cyclohexene and cyclooctene in aqueous potassium peroxymonosulfate (commercially available as Oxone^®) solution was studied as an application in pollution prevention. To avoid potential emissions of volatile organic compounds an aqueous solution was employed to replace the usual chlorinated solvents used in epoxide production. A droplet column reactor and stirred tank reactor were used to investigate two‐phase synthesis of epoxide. An aqueous Oxone^® solution was used to oxidize a dispersion of alkene droplets and form epoxide. The study of aqueous epoxidation in both reactors showed that the epoxidation of alkenes can be represented as a first order reaction with respect to alkene. The salting out effect of Oxone^® concentration was studied in both reactors and found to be very similar at optimal conditions. In comparing the two reactors, it was found that the droplet column reactor produces larger quantities of product per unit reactor volume for the same reaction time. The objective of this study is to provide an alternative reactor design and synthesis route that can meet pollution prevention goals. Copyright © 2004 Society of Chemical Industry     

PORE SCALE MODEL FOR FLOW REGIME TRANSITION IN A TRICKLE BED REACTOR

The hydrodynamic modeling of gas-liquid cocurrent downflow through a trickle bed reactor based on pore scale is presented. The pore scale hydrodynamic is related to the macroscopic scale flow parameters. The two-phase flow at pore level channel is modeled similar to annular two-phase flow in tubes. Using the film instability due to shear by the gas at the pore level channel, the transition criterion for the trickle flow to pulse flow regime is obtained. The model predictions agree with the published data. A model for transition from partial wetting to complete wetting in trickling flow is presented. The model predictions are in agreement with previous models. The model indicates better wetting of the smaller size packing for the same liquid mass flux.     

PORE SCALE MODEL FOR FLOW REGIME TRANSITION IN A TRICKLE BED REACTOR

The hydrodynamic modeling of gas-liquid cocurrent downflow through a trickle bed reactor based on pore scale is presented. The pore scale hydrodynamic is related to the macroscopic scale flow parameters. The two-phase flow at pore level channel is modeled similar to annular two-phase flow in tubes. Using the film instability due to shear by the gas at the pore level channel, the transition criterion for the trickle flow to pulse flow regime is obtained. The model predictions agree with the published data. A model for transition from partial wetting to complete wetting in trickling flow is presented. The model predictions are in agreement with previous models. The model indicates better wetting of the smaller size packing for the same liquid mass flux.     

气液喷射反应器内液滴粒径分布PLIF研究

气液喷射反应器是一种高强度反应器,反应器内部液滴粒径大小和分布对反应收率和选择性具有决定性影响。本文建立了气液喷射反应器内液滴粒径分布测量实验装置,并利用面激光诱导荧光（PLIF）技术对气液喷射反应器内液滴粒径分布进行了研究,得到不同气液流率情况下的液滴粒径的分布规律,结果显示：液相流率不变时,随气相流率的增大,反应器内液滴平均粒径逐渐减小,分布范围变小; 气相流率不变时,随液相流率的增大,液滴平均粒径逐渐减小,粒径分布趋于集中。     

Computational fluid dynamics simulation of a novel bioreactor for sophorolipid production

This paper describes three-dimensional computational fluid dynamics(CFD) simulations of gas–liquid flow in a novel laboratory-scale bioreactor contained dual ventilation-pipe and double sieve-plate bioreactor(DVDSB)used for sophorolipid(SL) production. To evaluate the role of hydrodynamics in reactor design, the comparisons between conventional fed-batch fermenter and DVDSB on the hydrodynamic behavior are predicted by the CFD methods. Important hydrodynamic parameters of the gas–liquid two-phase system such as the liquid phase velocity field, turbulent kinetic energy and volume-averaged overall and time-averaged local gas holdups were simulated and analyzed in detail. The numerical results were also validated by experimental measurements of overall gas holdups. The yield of sophorolipids was significantly improved to 484 g·L~(-1)with a 320 h fermentation period in the new reactor.