计算流体力学在顺流喷雾干燥器模拟中的误差源研究

The paper is focused on identifying error sources in computational fluid dynamics(CFD) predictions of a spray drying process. Seven groups of drying and atomisation parameters were selected for analysis and 13 simulation trials were performed. The theoretical results were compared with experimental data and sensitivity of the simulation results to the analysed factors was determined. The following parameters affecting the accuracy of CFD spray modelling were found: gas turbulence model, particle dispersion, atomising air, initial parameters of atomisation and heat losses to the environment. A major difference in the errors committed during modelling of spray drying process for fine and coarse sprays was observed.     

Reduction of a model for single droplet drying and application to CFD of skim milk spray drying

In this work, a novel methodology for the development of a high-accuracy computational fluid dynamics (CFD) model for the spray-drying process is described. Starting point is an own spatially resolving model of droplet/particle drying, which was developed and validated on the basis of a series of single droplet drying (SDD) experiments. This sophisticated model is transformed to a much simpler version: the characteristic drying curve approach, after running the full SDD model in a wide range of operating conditions. Then, the obtained reduced model is implemented into the CFD solver. The CFD spray-drying model takes into account the hydrodynamics of the continuous phase, particle drying kinetics, changes in the particle diameter, and the heat loss from the drying chamber to the environment. Validation of the entire procedure is provided by data obtained from drying experiments performed in a co-current laboratory spray tower. High accuracy of the developed CFD model of skim milk spray drying has been found for both phases, for the mean outlet temperature of the continuous phase (air) and for the change in average particle moisture content along the spray tower (discrete phase).     

New perspectives on capturing particle agglomerates in CFD modeling of spray dryers

Abstract

Current computational fluid dynamic (CFD) models of spray dryers lack the capability to predict the structure of the agglomerates formed; loose or compact agglomerates. This is mainly due to the conventional simplistic approach in numerically “fusing” of the colliding particles forming the agglomerate. A new theoretical treatment is introduced in this work, suitable for implementation in CFD simulations, which numerically fuses the particles and yet retain information on the structure of the agglomerate. This new theoretical treatment is based on tracking the reduction of the agglomerate surface area as the agglomerate is progressively formed. Analysis revealed that the reduction in the agglomerate surface area exhibits a unified correlation with the degree of compactness of the agglomerate. Further analysis comparing this new approach to the conventional numerical fusing of the particles revealed inherent numerical discrepancies, which has not been noted in the literature before. Understanding these discrepancies will provide clarity to the interpretation of the modelling and simulation of spray drying particle agglomeration in CFD. Moreover, this work lays the groundwork for a more comprehensive CFD model for agglomeration which can be potentially utilized to predict final powder properties.     

Chemistry and radiation in oxy-fuel combustion: A computational fluid dynamics modeling study

In order to investigate the role of combustion chemistry and radiation heat transfer in oxy-fuel combustion modeling, a computational fluid dynamics (CFD) modeling study has been performed for two different oxy-fuel furnaces. One is a lab-scale 0.8 MW oxy-natural gas flame furnace whose detailed in-flame measurement data are available; the other is a conventional 609 MW utility boiler which is assumed to be operating under oxy-fuel combustion condition with dry flue gas recycle. A new model for gaseous radiative properties is developed, validated, and then implemented in the CFD simulations. The CFD results are compared to those based on the widely used model in literature, as well as the in-flame measurement data. The importance and advantage of the new model for gaseous radiative properties have been well demonstrated. Different combustion mechanisms are also implemented and compared in the CFD simulations, from which significant difference in the predicted flame temperature and species is observed. This difference is consistent with those expected from the equilibrium calculation results. As a conclusion, the appropriate combustion mechanisms applicable to oxy-fuel combustion modeling are identified. Among the key issues in combustion modeling, e.g., mixing, radiation and chemistry, this paper derives useful guidelines on radiation and chemistry implementation for reliable CFD analyses of oxy-fuel combustion, particularly for industrial applications.     

Comparative study of droplet drying models for CFD modelling

CFD simulation is used to study wall deposition and agglomeration phenomena commonly encountered in industrial spray dryers. This paper initially provides a comparison of two drying kinetics models: Characteristic Drying Curve (CDC) and Reaction Engineering Approach (REA). Comparisons are made with experimental data with application to carbohydrate droplet drying obtained from past workers. These models were then extrapolated to actual drying conditions to assess their performance. The REA model predicts the progressive reduction in drying rate better than the CDC model for the carbohydrate droplets. A modified CDC model incorporating a convex falling rate produced better agreement than the conventional linear falling rate model. Further analysis showed that the REA model can be extended to simulate the particle surface moisture which may affect the agglomeration process. The proposed concept was compared with reported simulation results from a diffusion model which showed reasonable fit with data.     

Structure-dependent re-dispersibility of graphene oxide powders prepared by fast spray drying

The graphene oxide powder (GOP) obtained from the spray drying process often exhibits poor re-dispersibility which is considered due to the partial reduction of GO sheets. The reduction of drying tem-perature can effectively increase the re-dispersibility of GOP, but result in a decreased drying efficiency. Herein, we found that the re-dispersibility of GOP is strongly affected by its microstructure, which is determined by the feed concentration. With the increase of feed concentration, the GO nanosheet assem-bly varies from the disordered stacking to relatively oriented assembly, making the morphology of the GOP transform from ball-like (the most crumpled one) to flake-like (the least crumpled one), and the 0.8 mg·ml-1 is the threshold concentration for the morphology, structure, and re-dispersibility change. Once the feed concentration reaches 0.8 mg·ml-1, the appearance of the nematic phase in droplet ensures the relatively oriented assembly of GO sheets to form the layered structure with a low crumpling degree, which greatly improves the polar parts surface tension of the solid GOP, making the GOP easier to form hydrogen bonding with water during the redispersion process, thus stabilizing dispersion. This work pro-vides useful information for understanding the relationships between the morphology, microstructure, and final re-dispersibility of GOPs.     

Simulation of fine particle formation by precipitation using computational fluid dynamics

The 4‐environment generalized micromixing (4‐EGM) model is applied to describe turbulent mixing and precipitation of barium sulfate in a tubular reactor. The model is implemented in the commercial computational fluid dynamics (CFD) software Fluent. The CFD code is first used to solve for the hydrodynamic fields (velocity, turbulence kinetic energy, turbulent energy dissipation). The species concentrations and moments of the crystal size distribution (CSD) are then computed using user‐defined transport equations. CFD simulations are performed for the tubular reactor used in an earlier experimental study of barium sulfate precipitation. The 4‐EGM CFD results are shown to compare favourably to CFD results found using the presumed beta PDF model. The latter has previously been shown to yield good agreement with experimental data for the mean crystal size at the outlet of the tubular reactor.     

Experimental study and computational fluid dynamics modeling of deposition of hydrate particles in a pipeline with turbulent water flow

The present paper describes a study of Freon R11 hydrate deposition in a turbulent flow of water. Eulerian–Eulerian CFD model was built in order to study the process numerically, the model was validated with experimental data from the multiphase flow loop. Different mechanisms for particulate stress were studied in the work in terms of their performance results, compared to experimental data. The model considered an expression for variable hydrate particle size which was validated in a series of population balance numerical experiments.     

分子蒸馏过程的计算流体力学模拟

采用计算流体力学(CFD)方法,分析了蒸发器上液体负荷、刮膜器旋转速度对蒸发器上液膜厚度变化及液膜中速度矢量的影响。将CFD模拟结果与文献报道的计算值和实验值进行比较,发现具有较好的吻合性,同时可以用模拟计算结果解释一些实验现象。采用脉冲注射示踪剂法测定了蒸发器上流体的平均停留时间分布,与CFD模拟的在刮膜器作用下蒸发器上流体平均停留时间分布数据进行比较,二者的偏差在±8%以内,验证了CFD模拟的准确性。     

Using computational fluid dynamics modeling to study the mixing of pseudoplastic fluids with a Scaba 6SRGT impeller

The 3D flow field generated by a Scaba 6SRGT impeller in the agitation of xanthan gum, a pseudoplastic fluid with yield stress, was simulated using the commercial CFD package. The flow was modeled as laminar and a multiple reference frame (MRF) approach was used to solve the discretized equations of motion. The velocity profiles predicted by the simulation agreed well with those measured using ultrasonic Doppler velocimetry, a non-invasive fluid flow measurement technique for opaque systems. Using computed velocity profiles across the impeller, the effect of fluid rheology on the impeller flow number was investigated. The validated CFD model provided useful information regarding the formation of cavern around the impeller in the mixing of yield stress fluids and the size of cavern predicted by the CFD model was in good agreement with that calculated using Elson's model.     

Shaping technology of coal gas sorbents by using a spray dryer

The goal of this study was to develop HTHP sorbents for adaptation in fluidized bed desulfurizers or Transport Reactors. For this purpose, we shaped zinc-based sorbents (ZAC series) by using a spray dryer with a pressurized nozzle atomizer. To prepare the slurry for spraying, raw materials in submicron units and organic additives were used. We characterized the sorbents by SEM, XRD, Mean Particle Size Analysis, BET Surface Area and Attrition Ratio. The mean particle size was about 112 μm, the attrition resistance was very good and the TGA sulfur capacity of the fresh sample was calculated at about 23–24 wt%.     

Workflow for computational fluid dynamics modeling of fixed-bed reactors packed with metal foam pellets: Hydrodynamics

In recent years, the catalyst pellets made of open-cell metallic foams have been identified as a promising alternative in fixed-bed reactors. A reliable modeling tool is necessary to investigate the suitability of different foam properties and the shapes of foam pellets. In this article, a workflow for a detailed computational fluid dynamics (CFD) model is presented, which aims to study the flow characteristics in the slender packed beds made of metal foam pellets. The CFD model accounts for the actual random packing structure and the fluid flow throughout the interstitial regions is fully resolved, whereas flow through the porous foam pellets is represented by the closure equations for the porous media model. The bed structure is generated using rigid body dynamics (RBD) and the influence of the catalyst loading method is also considered. The mean bed voidage and the pressure drop predicted by the simulations show good agreement with the experimental data.     

计算流体力学在化学工程中的应用

计算流体力学(CFD)用于求解固定几何形状设备内的流体的动量、热量和质量方程以及相关的其他方程,已成为研究化工领域中流体流动和传质的重要工具。本文概述了CFD的基本原理以及CFD在化学工程领域方面的应用,重点介绍了CFD在搅拌槽、换热器、蒸馏塔、薄膜蒸发器、燃烧等方面的应用。     

Effects of skim milk concentrate dry matter and spray drying air temperature on formation of capsules with varying particle size and the survival microbial cultures in a microcapsule matrix

The impact of total solid (TS) content in combination with the feed rate and air inlet temperature on the survival of Lactobacillus paracasei ssp. paracasei F19 after spray drying in a skim milk matrix has been investigated and correlated with the capsule size. Depending on the experimental conditions, the survival rates ranged from 64 to 0.2%. The higher the air inlet temperature, the lower was the survival rate and an inversely correlation between the TS content and particle size has been determined. These results clearly indicate that process stress analyses and product-related characteristics must not be regarded separately.     

Experimental and computational fluid dynamics modeling of mixing by Visco-jet impellers

This paper reports experimental and computational frluid dynamics (CFD) studies on an impeller called Visco-jet with the aim of finding the effect of two side diameters ratio of its blade, which has a semi-conical shape, on drawdown process of floating polymericparticles into high viscosity glycerin solution. Nine different geometries were examined experimentally, and there were significant differences in their performance. The results reveal that when diameter of smaller side of semi-cone impeller is half of the other side, mixing was performed in a more efficient way. The CFD-predicted results have been used for explaining the experimental observation. The CFD-predicted hydrodynamics parameters confirm superiority of this geometry compared with the other ones.     

Effect of maltodextrin concentration and inlet temperature during spray drying on physicochemical and antioxidant properties of amla (Emblica officinalis) juice powder

The effects of inlet temperatures of 125, 150, 175 and 200 °C and maltodextrin levels at 3, 5, 7 and 9% on the physicochemical properties, total phenolic content (TPC) and 2,2-diphenyl picryl hydrazile (DPPH) scavenging activity of spray dried amla juice powder were studied. Moisture content and hygroscopicity of powder were significantly affected by inlet temperature and maltodextrin level. However, an increase in the level of maltodextrin did not significantly affect the bulk density and water solubility index (WSI). An increase in drying temperature and maltodextrin concentration decreased the free radical scavenging activity of the powder. Morphological study revealed that at higher inlet temperatures the spray dried powder had small sized particles that were densely packed. Spray dried amla juice powder made with 7% maltodextrin and processed at 175 °C inlet temperature had less hygroscopicity, acceptable color and potent free radical scavenging activity.     

Firing characteristics of size-controlled silver-glass composite powders prepared by spray pyrolysis

Size-controlled spherical silver-glass composite powders were directly prepared by using spray pyrolysis. The mean sizes of the composite powders changed from 0.34 to 0.78 μm when the concentration of the spray solution was changed from 0.05 to 2 M. The firing characteristics of composite powders formed from the spray solutions with a glass content equal to 3 wt.% of the silver component were affected by the mean sizes of the powders. Silver-conducting films formed from large-sized composite powders had a denser structure than those formed from small-sized composite powders. Further, silver-conducting films formed from composite powders with a mean size of 0.78 μm had specific resistances of 3 and 2 μΩ cm at firing temperatures of 450 and 500 °C. However, silver-conducting films formed from composite powders with a mean size of 0.34 μm had specific resistances of 8.2 and 6.9 μΩ cm at firing temperatures of 450 and 500 °C.     

溶胶-凝胶-喷雾干燥法制备超细Al_2O_3-SiO_2二元粉体材料

以硝酸铝(Al(NO3)3.9H2O)和正硅酸乙酯(TEOS)为原料,采用溶胶-凝胶(Sol-Gel)技术并结合喷雾干燥(SPD)技术制得超细Al2O3-SiO2二元复合粉体材料,并分别于400℃、800℃、1000℃、1150℃和1200℃煅烧2h;采用全自动比表面积与孔隙率分析仪、TEM、TG-DSC及XRD等仪器研究了热处理温度、pH值(分别为5.5、7和8)以及干燥方法对粉体材料的表面性能、显微形貌、物相组成及Al2O3-SiO2二元系晶体转变过程的影响。结果表明:由Sol-Gel-SPD制备的超细Al2O3-SiO2二元粉体材料的比表面积>448m2.g-1,而经1200℃煅烧2h后所得的超细莫来石的比表面积为34.05m2.g-1;TG-DSC分析表明:采用Sol-Gel-SPD制得的Al2O3-SiO2二元粉体材料的质量损失主要发生在500℃之前;XRD分析表明:粉体试样的开始莫来石化温度为1000℃,铝硅尖晶石(6Al2O3.SiO2)与非晶态SiO2在1150~1200℃完全转化为莫来石;比较不同pH值试样经1200℃煅烧后的TEM照片发现,当pH=7时,得到的超细莫来石粉体粒径最小,为50nm。     

CFD modeling of a pilot-scale countercurrent spray drying tower for the manufacture of detergent powder

A steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.