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A practical CFD modeling approach to estimate outlet boundary conditions of industrial multistage spray dryers: Inert particle flow field investigation
Abstract
Industrial multistage spray drying systems often have limited in situ process measurements to provide sufficient information for computational fluid dynamics (CFD) simulations of the primary drying chamber. In this case study on the spray dryer at Davis Dairy Plant (South Dakota State University), uncertainties were encountered in specifying the outlet boundary conditions of the spray drying chamber with two outlets: the side outlet and the bottom outlet leading to the second stage external vibrating bed. Using the available data on the vacuum pressure of the chamber, a numerical framework was introduced to approximate suitable outlet boundary conditions for the drying chamber. The procedure involved analyzing the ratio of the airflow rate between the two outlets and using a pseudo-tracer inert particle injection analysis. The goal of this approach was to determine a suitable range of outlet vacuum pressure that will lead to realistic particle movement behaviors during the actual plant operation. The protocol developed here will be a useful tool for CFD modeling of large scale multistage spray drying systems.
Abbreviations: ARC: Australian Research Council; CFD: Computational Fluid Dynamics; FFT: Fast Fourier Transform; MCC: Micellar Casein Concentrate; PRESTO: Pressure Staggering Option; SDSU: South Dakota State University; SIMPLE: Semi − Impilicit Method for Pressure Linked Equations; WPC: Whey Protein Concentrate
... From the manufacturer's blower performance curve and the measured average pressure at the chamber inlet, the air mass flow rate was determined to be 1.1 kg/s. airflow and particles trajectories ratios between two outlets revealed that the -250 Pa pressure would be a reliable prediction for the bottom outlet [6]. ...
... [7]. The details of mesh dependency is described in [6]. Boundary condition for air inlet was mass flow inlet that was entered according to the manufacturer's blower air flow specifications (mass flow rate=1.1 kg/s). ...
... This approach allowed numerical capturing of the negative pressure within the drying chamber. The pseudo tracer analysis and how to predict the bottom outlet pressure is explained in [6]. Convergence criteria was 1x10 -3 for all scaled residuals. ...
Most of the CFD simulations of spray dryers reported in the literature utilizes a fixed air inlet temperature numerical framework. In this paper, a numerical framework was introduced to model spray drying as an outlet air temperature controlled process. A P-controller numerical framework was introduced which allows the inlet temperature to be automatically adjusted based on the required outlet temperature set point. This numerical framework was evaluated with a simulation of a two-stage pilot scale spray drying system at the Davis Dairy Plant (South Dakota State University) which is used for commercial contract spray drying operation.
... Afshar et al., 2019) Density (kg/m 3 ) − 10 − 10 Τ 4 +2 × 10 − 7 Τ 3 − 10 − 4 Τ 2 +0.0206Τ Specific heat capacity (J/Kg•K) − 4 × 10 − 8 Τ 4 +6 × 10 − 5 Τ 3 − 0.036Τ 2 +9.94Τ ...
In this paper, three kinds of wall conditions are numerically simulated to investigate deposition, powder recovery and energy aspects of spray dryers. A co-current dryer with a pressure nozzle is chosen as a base dryer and two dryers with fully insulated, and cooled walls are compared with the base one. Governing equations of a transient flow field are solved through the Eulerian approach, while Lagrangian particle tracking predicts particles’ motion. The sticky point curve is employed as a criterion of bouncing or stickiness to model the deposition pattern of skim milk particles on the surfaces. Results show that the spray dryer with cooled walls has better deposition characteristics, while the dryer with insulated surfaces works with higher drying efficiency. Overall, it is observed that the wall conditions can be changed to improve the drying efficiency and wall deposition. However, changing the thermal boundary conditions does not seem to be effective in improving the powder recovery.
... 'Flipping' of the flow field is akin to having changing snapshots of the flow field at successive steady state iterations. Afshar et al. [102] reported a similar or same experience simulating a predominantly transient large-scale spray dryer using the steady state approach. Depending on the degree of 'flipping', such significant changes in the flow field may not even be detected by observing the numerical residuals from the iterations, leading to pseudo-converged simulations. ...
Recent initiatives covered in this review can be divided into three broad categories. The first category concerns mathematically describing the spray drying process using a plug-flow model which allows quick what-if evaluations of the process. Intricacies associated with capturing the drying of a range of droplet sizes are discussed. Challenges in adapting the plug-flow simulation approach to counter-current spray drying, which is common in the production of non-heat sensitive detergent powder, was further elaborated. The second broad category features modeling the spray drying process using the Computational Fluid Dynamics technique. A critical review was provided to guide readers on a common dilemma plaguing this area of spray drying modeling. Recent developments in agglomeration modeling in spray dryers are discussed as well. The final part of this review touches on modeling techniques which can be used to predict the development of specific powder qualities. This includes modeling the degree of protein denaturation during spray drying, predicting the characteristics of crystalline type spray dried powder, and determining the properties of the powder surface at the molecular scale.
... 20) In addition, the effect of drying condition and the drying tower geometry on the fluid behavior and droplet trajectory have been investigated. [21][22][23][24][25] Recently, the CFD studies that considered the collision between droplets (or particles) and deposition on a spray dryer wall have increased, and the importance of the boundary condition between a particle and wall has been discussed. [26][27][28][29][30] Previous studies have primarily focused on a single droplet drying or the overall fluid and droplet drying behaviors inside a spray dryer. ...
Spray drying process is widely used to produce particulate materials in the pharmaceutical industries, such as porous materials for direct compression, solid dispersion for improvement of drug dissolution properties, micro encapsulation to stabilize active compounds, taste masking, preparation of dry powder for inhalation. However, as many factors affect the physical properties of dried particles and the spray drying processes have complex behaviors in which heat and mass transfer occur simultaneously, the detailed mechanisms of dry particle generation have yet to be sufficiently elucidated. In this study, computational fluid dynamics was used to simulate water droplet evaporation in a spray dryer, and the evaporation kinetics of “individual droplets” in the droplet aggregate (group) were analyzed. The numerical simulation revealed that each droplet had different evaporation rates owing to the following two reasons. First, the driving force of evaporation, ΔT, changed every moment as the droplets traveled through different temperature fields in the drying tower. Second, it was calculated the driving force for droplet evaporation differed from the ideal system because the evaporation of other droplets changed the fluid characteristics around the droplets. The obtained results are important findings that lead to the understanding the spray drying process to design and manufacture the pharmaceutical products.
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... By combining drying models/theories with CFD simulations, many studies have been conducted to evaluate the behaviors of both, fluids (velocity, humidity, and temperature) and droplets (particle size distribution and moisture content) (Langrish et al., 2004;Mezhericher et al., 2014;Sebastia-Saez et al., 2019). The hot air and droplet drying behaviors in an industrial-scale spray dryer have been analyzed using CFD simulations (Afshar et al., 2018;Wawrzyniak et al., 2017). Recently, CFD studies have been conducted considering depositions on a spray dryer wall and collisions between droplets (Ali et al., 2017(Ali et al., , 2015Jaskulski et al., 2018;Jubaer et al., 2017;Mezhericher et al., 2010), and the importance of the boundary conditions between a droplet and the wall has been discussed. ...
Spray drying is a fundamental process in several industries. As many factors affect the particulate product property in complex manners, optimal operating conditions have been determined through trial and error. In a spray dryer, numerous sprayed droplets form a tear-shaped droplet aggregate (droplet group), in which each droplet evaporates at a different rate. Owing to the complicated evaporation behavior of a droplet group, a detailed mechanism of dried particle generation has yet to be sufficiently elucidated. The objective of this study is to investigate the effects of the operating conditions of spray drying processes on the evaporation rate of a droplet group. Numerical simulations were performed under various conditions to calculate the evaporation rates of a droplet group in a spray drying process. A method for estimating the evaporation rate of a droplet group based on the operating conditions, using the thermal efficiency and Nusselt number, is proposed.
... Several other works (e.g. Afshar et al., 2018b;Jaskulski et al., 2017;Mezhericher et al., 2015;Tran et al., 2017;Woo et al., 2008Woo et al., , 2009bWoo et al., , 2011Yang et al., 2015) can be found in the literature that utilized various forms of the k − ε based two equation closures in order to investigate several different aspects of spray drying. ...
Accurate modeling of the flow field by means of capturing turbulence is crucial in CFD simulations. However, choosing the appropriate turbulence model remains quite challenging for simulating spray drying applications. Only a few studies have touched on this issue, although experimentally validated comparisons throughout the dryer are rare. This work aims to provide an assessment of five different turbulence models (RNG k − ε standard, BSL and SST k − ω as well as transition SST) in terms of the predicted flow field throughout a lab-scale counter-current spray dryer. None of the tested models could initially provide a satisfactory match with locally measured temperatures within the chamber. The popular choice RNG k − ε model led to highest discrepancies, while the k − ω variants performed only slightly better. All these models under-predicted the dissipation of the central hot air jet. Modification to the k − ω variant's characteristic constant to allow increased production of turbulence led to satisfactory agreement between the measurements and simulation results. Extended analysis revealed that different turbulence models produced significantly different drying histories. Only the k − ω SST variant with modified constant could provide predictions close to measured outlet particle moisture content and air conditions. The RNG model proved unsuitable due to unrealistic results with particle injection as well. The differences in predictions with injection among the models were attributed to different transient self-sustained air fluctuation behavior predicted within the chamber. This work will be useful in the selection of turbulence models which is fundamental to accurate CFD modeling of spray dryers.
Spray drying, as a crucial operation in industrial production, converts solution to fine particle. The spray moiety directly affects the final particle morphology, size and distribution. Compared with the experimental method, computational fluid dynamics (CFD) modeling is a powerful and convenient tool for simulating the spray process. Based on the verified CFD model, different materials of atomizer were simulated to investigate the effect on droplet size and distribution in this work. The modeling result proved that the droplet size and distribution were influenced by the resistance coefficient of materials, wherein the Reynolds number could change the effect of roughness along with the change of mass flow rate on spray process. The results in this work have implication for controlling droplet size through developing new spray nozzle with different materials or surface coating.
IntroductionThe Euler–Lagrange Approach: an Extended Model for Spray-dryer CalculationsDroplet-drying ModelsCollisions of ParticlesExample of a Spray-dryer CalculationPrediction of Product PropertiesSummaryAdditional Notation used in Chapter 5
A food property is a particular measure of the food’s behavior as a matter or its behavior with respect to energy, or its interaction with the human senses, or its efficacy in promoting human health and well-being. An understanding of food properties is essential for scientists and engineers who have to solve the problems in food preservation, processing, storage, marketing, consumption, and even after consumption. Current methods of food processing and preservation require accurate data on food properties; simple, accurate, and low-cost measurement techniques; prediction models based on fundamentals; and links between different properties. The first edition was well received,
secured bestseller from the publisher, and received an award. Appreciation from scientists, academics, and industry professionals around the globe encouraged me to produce an updated version. This edition has been expanded with the addition of some new chapters and by updating the contents of the first edition. The seven chapters in the first edition have now been expanded to 24 chapters. In this edition, the definition of the terminology and measurement techniques are clearly presented. The theory behind the measurement techniques is described with the applications and limitations of the methods. Also, the sources of errors in measurement techniques are compiled. A compilation of the experimental data from the literature is presented in graphical or tabular form, which would be very useful for food engineers and scientists. Models can reduce the numbers of experiments, thereby reducing time and expenses of measurements. The empirical and theoretical prediction models are compiled for different foods with processing conditions. The applications of the properties are also
described, mentioning where and how to use the data and models in food processing.
Chapter 1 provides an overview of food properties including its definition, classification, and predictions. Chapters 2 through 4 present water activity and sorption isotherm including its terminology, measurement techniques, data for different foods, and its prediction models. Chapters 5 through 12 present thermodynamic and structural characteristics including freezing point, glass transition, gelatinization, crystallization, collapse, stickiness, ice content, and state diagram. Chapters 13 through
15 discuss the density, porosity, shrinkage, size, and shape of foods. Chapters 15 through 23 present the thermophysical properties including specific heat, enthalpy, thermal conductivity, thermal diffusivity, and heat transfer coefficient. Chapter 24 provides the acoustic properties of foods.
This second edition will be an invaluable resource for practicing and research food technologists, engineers, and scientists, and a valuable text for upper-level undergraduate and graduate students in food, agriculture and biological science, and engineering. Writing such a book is a challenge, and any comments to assist in future compilations will be appreciated. Any errors that remain are entirely mine. I am confident that this edition will prove to be interesting, informative, and enlightening.
The paper presents a unique methodology of CFD modeling of heat and mass transfer during a counter-current spray-drying process based on the concept of a negative heat source reflecting the energy necessary for evaporation of water. The volumetric internal heat source term in Navier-Stokes equations describing heat transfer was used to determine energy transfer from explicit to latent heat. Total power consumption during the drying process was estimated on the basis of extensive industrial measurements of air temperature distribution along the axis and radius during regular tower operation. The probability function to control the distribution of density of power consumption as a function of the distance from the atomizers was determined by the trial and error method to fit experimental temperature measurements in the industrial tower. This approach allowed us to carry out 3D CFD accurate calculations of the counter-current spray-drying process analyzing continuous phase only. General methodology applied in the developed CFD model of heat and mass transfer is universal and can be applied for scaling up of the countercurrent spray-drying process.
In our previous 2D axisymmetric study of the spray-drying process, it was shown that the introduction of comprehensive formulation of drying kinetics had a substantial influence on both predicted gas and particle flow patterns in a spray dryer. On the other hand, it was demonstrated that the concept of 2D axisymmetric modeling is suitable for fast and low-resource consumptive numerical calculations of the spray-drying process and that the predicted velocity, temperature, and vapor mass fraction are of reasonable accuracy. However, due to their restrictions, 2D axisymmetric simulations fail to predict asymmetry of flow patterns and the presence of the transversal air flow and cannot provide actual D representation of particle trajectories inside the spray chamber. Therefore, in the present study a novel 3D theoretical model of multiscale multiphase transport phenomena in a steady-state spray-drying process is proposed for predicting more realistic particles paths, residence times, temperatures, and moisture contents. A drying process of silica slurry in a short-form pilot-scale spray dryer fitted with a pressure nozzle atomizer is investigated. The simulated 3D drying behavior of the dispersed phase and flow patterns of air velocity, temperature, and humidity are compared with the previously published results of 2D axisymmetric modeling. A significant influence of both drying kinetics and number of utilized dimensions on the predicted particle trajectories and transport phenomena in the drying chamber is observed. Hence, a proper formulation of the droplet drying kinetics and realistic 3D flow modeling is crucial for accurate numerical representation of the actual spray dryer performance.
This article presents both theoretical and experimental determination of the hydrodynamics of drying air in an industrial countercurrent spray dryer. Air velocity measurements were performed in selected areas of the tower to determine the flow pattern in the dryer and to collect data to validate the computational fluid dynamics (CFD) modeling and verify the results. For no-swirl operation mode, 3D CFD calculations showed high instability of the air flow in the dryer. A bent, pillow-shaped flow above the drying air inlet, which promotes deposition of particles in this area of the wall, was detected. CFD calculations also proved that when drying air was introduced to the tower tangentially; from 20° to 30° in relation to the initial air flow configuration in the spray-drying tower, the air flow was stabilized and air velocity near the wall increased, which might reduce wall deposition of particles. Comparison of experimental and theoretical results showed that the CFD model of countercurrent spray-drying process developed in this study can be used for a reliable estimation of tower performance.
The use of Computational Fluid Dynamics (CFD) packages in the area of spray drying has been reviewed by Bahu (Bahu, 1992, Proc. Eighth International Drying Symposium, IDS '92, Montreal, Canada, pp. 74–91) and Reay (Reay, 1988, Proc. Sixth International Drying Symposium, IDS '88, Versailles, France, KL-1–KL-8), and the movement towards the use of CFD by dryer manufacturers has been outlined by Masters (Masters, 1994, Drying Technology, 12 (1 & 2), 235–237, 1994). Applications include tighter design of spray dryers and reducing operational problems, such as wall deposition. There is still considerable scope for the application of this approach, and here possible future directions for application are reviewed. Particular issues in the use of spray drying for food ingredients are identified and discussed, namely thermal degradation, aroma loss and particle stickiness.
One-dimensional (1-D) simulation is a useful technique for the evaluation of dryer operating parameters and product properties before conducting real spray drying trials. The main advantage of a 1-D simulation tool is its ability to perform fast calculations with significant simplicity. Mathematical models can be formulated using heat, mass and momentum balances at the droplet level to estimate time-dependent gas and droplet parameters. One of the purposes of this paper is to summarize key mathematical models that may be used to perform 1-D simulation for spray drying processes, predict essential product-drying gas parameters, assess the accuracy of prediction using pilot-scale spray drying data and perhaps most importantly address the main benefits and limitations of the 1-D simulation technique in relation to industrial spray drying operations. The results of a recent international collaborative study on the development of spray drying process optimization software for skim milk manufacture are presented as an example of the application of 1-D simulation in milk processing.
A new model of whey protein thermal inactivation has been combined with a CFD model developed for skim milk spray drying. Extensive evaporation and particle formation models were used to calculate particle moisture contents, temperatures and residence times. Calculated parameters were then used as input data for an experimentally developed quality model based on Williams-Landel-Ferry (WLF) equations for inactivation kinetics. The developed quality model was implemented into the CFD code and calculated in parallel to simulations of skim milk droplets evaporation based on the characteristic drying curve approach. The quality model and the simulation procedure were validated by comparison of protein activity levels obtained from the CFD with data obtained from differential scanning calorimetry (DSC) of milk powder samples collected during skim milk spray drying experiments. The simulations for different feed rates fit well with measurement results and show that the loss of whey protein activity is lower at higher feed rates, due to lower temperature fields in this case.
Shrinkage behavior or the change of droplet diameter in the course of drying is quite diverse and has been the focus of many studies due to its crucial importance in accurate modelling of spray drying. However, the specific impacts of capturing it while performing CFD simulations have not yet been investigated comprehensively. Therefore, this work aimed to directly compare predictions obtained by the perfect shrinkage and linear shrinkage models. As compared to linear shrinkage, the assumption of perfect shrinkage led to a decrease in the surface area as well as an increase in the transport coefficients, resulting in an overall decrease in drying rate. Furthermore, the predicted particle size distribution was significantly affected by the implemented shrinkage model, while contrary to the expectation commonly expressed in literature the residence time of the particles was similar for the investigated pilot scale dryer. Considerable difference ascertained in predicted drying histories, particularly for larger droplets, led to the conclusion that the assumption of perfect shrinkage leads to overprediction of particle stickiness and underestimation of drying rates. This difference is particularly important in modelling phenomena like agglomeration and particle-wall deposition.
The spray drying of orange peel extracts was scaled up from a laboratory-scale spray dryer to a pilot-scale spray dryer using two methods to predict the parameters of the spray-drying process. The mass and energy balance model predicted the outlet gas temperature, absolute humidity and the final moisture content of the particles with 5%, 2% and 74% errors, respectively whereas, the plug-flow model successfully predicted these parameters with < 1.5%, 1% and 15% error, respectively. The total phenolic content and the antioxidant capacity of the powders were retained to the extent of 88 ± 7% and 95 ± 8%, respectively, after the scale-up process. Therefore the plug-flow model was found to be a useful method in the scale-up process as a rapid estimation method for predicting the key parameters in spray drying with good accuracy, in order to keep the quality of the products within the required range. Industrial relevance Peels and seeds of citrus fruits, such as oranges, contain bioactive phenolic compounds that have demonstrated cancer-inhibition properties. However, due to their bitterness, they are not consumed and are considered to be waste material. The compounds can be extracted and turned into concentrated powder supplements. In powder form, these compounds can be easily incorporated in controlled and concentrated dosages into food formulae in order to create functional foods while masking the unpleasant taste. However, due to the heat sensitivity of these compounds, the minimum amount of heat treatment is required in the process. Spray drying, which is one of the fastest drying techniques available, can be used to convert these extracts into powders. However, spray drying these extracts is very challenging due to the presence of large amounts of sugars that cause stickiness and product loss during the process. In general, adding significant amounts of carriers to sugar-rich foods to overcome their stickiness during spray drying has been used in industry, compromising the purity of the final products. Moreover, in order to mass produce the powders, the process needs to be scaled-up. However, spray drying is extremely difficult to scale up using dimensional analysis (Oakley, 1994; Zlokarnik, 2003b). Computational fluid dynamic (CFD) simulations have been suggested as a powerful tool in this scale up of spray drying, but they are very time consuming. Therefore, if simpler models that do not require CFD simulations can adequately predict the process parameters, they can be used as fast estimation techniques for scale up. The results from this study guide the use of a plug-flow model for scaling up the process of spray drying orange-peel extracts while keeping an acceptable quality for the final product.
Many food ingredients are supplied in powdered form, as reducing water content increases shelf life and aids ease of storage, handling and transport. Powder technology is therefore of great importance to the food industry. The Handbook of food powders explores a variety of processes that are involved in the production of food powders, the further processing of these powders and their functional properties. Part one introduces processing and handling technologies for food powders and includes chapters on spray, freeze and drum drying, powder mixing in the production of food powders and safety issues around food powder production processes. Part two focusses on powder properties including surface composition, rehydration and techniques to analyse the particle size of food powders. Finally, part three highlights speciality food powders and includes chapters on dairy powders, fruit and vegetable powders and coating foods with powders. The Handbook of food powders is a standard reference for professionals in the food powder production and handling industries, development and quality control professionals in the food industry using powders in foods, and researchers, scientists and academics interested in the field. Explores the processing and handling technologies in the production of food powders. Examines powder properties, including surface composition, shelf life, and techniques used to examine particle size. Focusses on speciality powders such as dairy, infant formulas, powdered egg, fruit and vegetable, and culinary and speciality products.
A knowledge of the trajectories of atomized droplets in both the nozzle zone (where the droplets are rapidly decelerating from their initial high velocity) and in the free-entrainment zone (where the droplets are conveyed by the drying gas) is required for the design of spray dryers, since it governs the evaporative capacity and thermal efficiency of the chamber, while affecting the moisture content and general quality of the product through the control of the drying time. The trajectories of droplets in three-dimensional motion were determined theoretically in both zones. In the case of two-fluid pneumatic atomizers, the characteristics of the jet of atomizing fluid were found to be important in both the zones. Predictions of droplet trajectories were tested in an experimental circular cocurrent spray-drying chamber with a conical bottom, in which the drying air was introduced tangentially near the top. Water was used as the feed material. A study was made of the effects of liquid feed rate and temperature, drying air flowrate and temperature, and of nozzle position on the thermal efficiency and evaporative capacity of the chamber. The results were interpreted in the light of the droplet trajectories predicted.
This paper is concerned with the numerical simulation of unsteady turbulent flows behind sudden expansions without inlet swirl. Time dependent simulations have been carried out using the VLES approach with the standard k - epsilon model. The expansion ratio investigated is in the range from 1.96-6.0. The simulations show that the flows in axisymmetric sudden expansions are inherently unstable when the expansion ratio is above a critical value. The processing phenomenon, which features self sustained precession of the global flowfield around the expansion centerline, is predicted successfully using CFD, with simulated oscillation frequencies that are in general agreement with reported data. For the case of expansion ratios from 3.5-6.0, a combination of a precession motion and a flapping motion in a rotating frame of reference is predicted in terms of the jet movment. Largescale structures are identified in the downstream flowfield. Other important phenomena, such as the transition of the oscillation patterns, have also been predicted.
Computational fluid dynamics (CFD) modeling of spray dryers requires a simple but sufficiently realistic drying model. This work evaluates two such models that are currently in discussion; reaction engineering approach (REA) and characteristic drying curve (CDC). Two versions of the CDC, linear and convex, drop in drying rate were included. Simulation results were compared to the overall outlet conditions obtained from our pilot-scale experiments. The REA and CDC with a linear drop in drying rate predicted the outlet conditions reasonably well. This is contrary to the kinetics determined previously. Analysis shows that the models exhibit different responses to changes in the initial feed moisture content. Utilizing different models did not result in significantly different particle trajectories. This is due to the low relaxation time of the particles. Despite the slight differences in the drying curves, both models predicted similar particle rigidity depositing the wall. For the first time in a CFD simulation, the REA model was extended to calculate the particle surface moisture, which showed promising results for wet particles. Room for improvement was identified when applying this concept for relatively dry particles.
In a co-current pilot plant spray dryer measurements were done of the airflow pattern (no spray) and the temperature and humidity pattern (water spray). These patterns were simulated with a computational fluid dynamics package (FLOW3D). The measured air velocities showed large fluctuations. The measured and predicted flow pattern showed good agreement qualitatively, but the measured profiles showed less variation than the predicted ones. The measured temperatures and humidities showed good agreement in large areas of the dryer, but the agreement in the zone near the central axis leaves room for improvement.
A one-dimensional numerical model for a detergent slurry drying process in a counter-current spray drying tower is developed for the prediction of the gas and droplet/particle temperature profiles within the tower. The model accommodates droplets/particles over a range of sizes. A semi-empirical slurry droplet drying model is integrated with a counter-current tower simulation based on mass, energy and particulate phase momentum balances in order to calculate the drying rate and the particle residence time within the tower. The coupled first order ordinary differential equations for the two phases are solved numerically using the iterative shooting method in an algorithm developed within MATLAB. The predictions of the numerical model are compared with industrial pilot plant data. The results are found to vary significantly with the specified size distribution of the droplets. Despite the simplicity of the model in ignoring the coalescence, agglomeration, wall deposition and re-entrainment, the model gives reasonable agreement with the experimental data.
Numerical simulations of the air flow patterns within a small scale tall-form countercurrent spray dryer have been performed. The simulations were performed using CFX 4.3, a finite volume based, computational fluid dynamics package. This study represents the first application of the Very Large Eddy Simulation (VLES) approach to the simulation of spray dryers. They have been performed in order to gain a more detailed understanding of the flow patterns and their stability in this design of dryer, which is commonly used in countercurrent drying applications, such as the drying of detergents. Limited validation of the simulations was achieved through comparison against qualitative experimental flow pattern information. It was found that by altering the angle of the inlet air streams into the dryer, the nature of the flow within the dryer could be significantly altered. In the majority of the cases simulated, large transients developed in the flow, the nature of these transients being critically dependent on the inlet conditions. The existence of such transients would be detrimental to actual spray dryer performance, however the flow patterns can be stabilised by introducing a large amount of swirl into the chamber.
This work presents a simulation study of the spray dryer operation using the whole milk suspension as the emulsion to be dried. Two approaches are used to obtain a general description of this operation. The first approach comprises a population balance model, in which drops and particles make up the discrete phase and are distributed into temporal compartments following their residence time in the dryer. Air is the continuous and well-mixed phase. Mass and energy balances are developed for each phase, taking into account their interactions. Constitutive model equations to describe the drop swelling and drying mechanisms as well as the heat and mass transfer between particles and hot air are also analyzed. The set of algebraic-differential equations obtained in this approach is solved by DASSL numerical code. The second approach involves a three-dimensional model solved by a computational fluid dynamic (CFD) code. The continuous air phase follows the time-averaged Navier-Stokes equations coupled with the RNG turbulence model while particle equations are set up in the Lagrangian model using the stochastic method to predict the particle trajectories. Experiments carried out in a pilot spray dryer generate input data for both model approaches. Results are compared to lead to a better understanding of the spray dryer operation.
It is now well-established that flows occurring when a jet enters a large vessel are highly transient. Such is the case in spray dryers, where the inlet gas flow can generate transient flow patterns that affect the droplet trajectories and wall deposition behaviour in the dryer significantly. This paper applies the relatively recent scale-adaptive simulation (SAS) approach to the simulation of flow in a pilot-scale dryer. It is shown that this approach produces a much more realistic flowfield than use of URANS equations that have not been adapted to resolve problems associated with their inability to predict turbulence length-scale distributions correctly. A study of the effect of the inlet swirl angle is used to illustrate some features of the transient results. This work makes it clear that transient, 3D simulations of spray dryers can now be made that capture the large-scale turbulence behaviour and that the questionable practice of using unconverged steady-state simulations to provide information is no longer justified.
This paper provides simple analytical correlations for selected thermodynamic and fluid transport properties for the mixture dry air and water vapor. These correlations are derived from theory as well as from numerical fitting procedures and give expressions for density &Vrgr;, viscosity μ, thermal conductivity k, specific heat cp, and Prandtl number Pr at a working pressure of p=1 bar and for a temperature range from 100 °C to 200 °C. The main purpose is to present a comparatively simple set of equations, as the correlations do not reflect in every case the underlying physical background. Since experimental data are scarce for the properties under investigation, it was in some cases necessary to extrapolate the available correlations to temperatures or water vapor contents where no experimental data could be found. The derived equations are compared with the pure component values for dry air and water vapor and, as far as possible, also for air-water vapor mixtures.
The air flow patterns in an industrial milk powder spray dryer have been investigated. Isothermal three-dimensional transient simulations in the absence of atomised liquid droplets have been carried out using the commercial CFD code (CFX10.0) in which the transient Navier–Stokes equations are solved. The shear stress transport (SST) turbulence model was implemented to model the effects of turbulence.These simulations are possibly the first to include the following approximations to industrial spray dryer practice: (a) introduction of an internal fluid bed, (b) outlet ducts located near the top rather than the bottom of the dryer and (c) placing the constant atmospheric pressure condition downstream of the dust collector rather than at the dryer exit.As expected, the simulations showed that the main air jet oscillated and precessed about the central axis with no apparent distinct frequency. In turn, the recirculation zones between the main jet and the chamber walls fluctuated in size. Good agreement was found between the movements of the main jet via simulations and from telltale tufts installed in the plant dryer. This supported other indications that the simulations were an accurate representation of the actual flows. The different outlet boundary condition (including a flow resistance representing the baghouse) appeared to have little influence on the overall flow field. In the gas-only simulations, different fluid bed flows within the range used industrially had only a local influence by reducing the length of the main jet. This may have an effect on the particle capture by the fluid bed.
Development of high energy efficiency spray dryers is of great importance for the future of the dairy powder industry. The present research studied energy efficiency in an industrial scale spray dryer with the second law of thermodynamics. The entropy production rate is computed from the CFD modeling results of the transient multi-phase flow in the spray dryer. Influences of particle size, inlet air flowrate and inlet air temperature on the entropy production rate are analyzed. The results show that large particles produce more entropy per unit mass on dry basis. To decrease the moisture content of product powders, it will produce more entropy by increasing inlet mass flowrate than by increasing the air temperature. The study shows that the second law of thermodynamics is a potential tool for optimizing dryer operation and design.
This investigation describes and demonstrates the effectiveness of computational fluid dynamics, CFD, method to predict the moisture changes during drying process of salt solution in co-current spray dryer. In order to prove the validity of predicted profiles, the obtained results were compared by reported experimental data. The moisture profile calculated by two dimensions model confirmed the sharp change in moisture pattern close to the center of drying chamber. It is proven that the moisture reaches to constant value as radial distance increases up to 20%. The model was also employed to investigate the influence of operating parameters such as, salt concentration, feed rate, hot air flow rate and temperature, inlet air moisture, and outlet suction pressure on moisture distribution. It was found that the feed rate and suction pressure significantly affect the moisture profile in radial and axial directions. The obtain results can be used in industrial practice.
The air flow pattern in a co-current pilot plant spray dryer (diameter 2.2 metres) was modelled and measured for the case without spray. The swirl angle was zero and the modelling was done with a computational fluid dynamics package (FLOW3D by CFDS). The boundary conditions for the CFD-model (velocity and turbulence quantities at the inlet) were derived from measurements with a hot-wire probe. To validate the CFD model, air velocity magnitudes were measured at numerous locations in the spray drying chamber. To interpret the data, a novel approach was developed based on the interpretation of velocity distributions rather than time-averaging the signals. This was necessary because of flow reversals and large fluctuations in the air velocity. The measurements were compared with the CFD model results and the agreement between model and measurements was reasonable.
An existing ''discrete droplet'' model of liquid sprays has been extended to include a stochastic representation of turbulent dispersion effects. Applications to simple test cases, including the dispersion of single particles, produce reasonable agreement. However, two further applications involving volatile and combusting sprays show that the turbulent dispersion effects are small in comparison to those due to uncertainties about the initial conditions of the spray.
Commercial success of a new spray dryer investment depends upon the dryer meeting its specified performance in all respects, and this puts great importance on the scale-up procedures used in the projecting of the new spray dryer. Scale-up still relies heavily on the experience of the designer. However, as the applications and specifications become more and more complex, so does the need for improved test work in pilot plants, and computational fluid dynamics simulations become more important, so that better scale-up tools are available to minimize the possibility for personal errors that may lead to the new dryer investment becoming an unsuccessful commercial venture.
This paper numerically investigated the effect of chamber aspect ratio and operating conditions on flow stability within a short form spray dryer without the inclusion of droplet injection. Extensive analysis using different mesh configurations led to a new finding on the effect of expansion ratio on flow stability. A larger expansion ratio produces a more stable flow, which is due to the limitation of the jet fluctuation by the outlet geometry constriction. However, the flow might not be completely steady as fluctuations are magnified at higher inlet velocities, which can be explained by the confined jet feed-back mechanism. Fast Fourier Transform (FFT) analysis of the computed instantaneous velocity at selected spatial locations in the chamber revealed the effect of the chamber expansion ratio on periodicity of the flow structure. It is observed that changes in the expansion ratio primarily affect the amplitude of the 'noise' region in the frequency spectrum.
Mean and fluctuating velocities of both phases as well as particle mass fluxes were measured in turbulent, dilute, monodisperse, particle-laden jets injected into a still environment. The new measurements were used to evaluate a stochastic separated flow model of the process that treated effects of interphase slip and turbulent dispersion using random-walk computations for particle motion. The continuous phase was treated using a modified kappa - epsilon model allowing for direct contributions of interphase transport of both mean and turbulence properties. The model performed reasonably well over the new data base, with all empirical parameters fixed from earlier work. In contrast, simplified models ignoring either interphase slip or turbulent dispersion yielded poor agreement with the measurements.
Thermal Science, Essentials of Thermodynamics Fluid Dynamics and Heat Transfer
- E A Baskharone