SCALE-UP OF SPRAY DRYERS WITH THE AID OF COMPUTATIONAL FLUID DYNAMICS

Abstract

The scale-up of spray dryer chambers is difficult because of the complexity of gas and spray flow patterns. The principal concerns in designing a spray drying chamber are to ensure sufficient residence time for drying and to avoid particle-wall collisions. Dimensionless groups are of limited use because it is practically impossible to achieve dynamic similarity between small and large chambers. In the past, empirical, rather than theoretically based, rules generated by experience with existing plant have been used in the design and scale-up of spray dryer chambers but these models, because of their empirical nature, are limited in their range of applicability. Computational fluid dynamics (CFD) is potentially a powerful tool to aid spray dryer design allowing much more flexibility in design but because of the difficulties of modelling such complex phenomena, especially the gas turbulence, its predictions cannot, at present, be considered absolutely reliable and experimental validation of the results is required. However, by considering the principles of similarity, it is shown that validations carried out on pilot scale equipment under the correct conditions will prove the accuracy of CFD applied to spray dryers of any size.     

A SCACE-UP STUDY OF NOZZLE TOWER SPRAY DRYERS

A quasi-one dimensional model taking into account hindered drying beyond the critical tmisture content and droplet size distribution is used to scale-up nozzle tower type of a spray dryer with uniformly distributed air entry, based on pilot plant data. A pilot plant with 8 m high and 1.7 m diameter spray dryer is used to collect the drying data. Equilibrium and critical mistwe contents of the particles are also experimentally msured. The index of drying rate in the hindered drying period is calculated frun the pilot plant data. This index is used in the M e 1 to determine the drying capacity and thereby the mixinnnn particle size that can be produced ina 3.5 m dimoeter 12 m high nozzle t- type of spray dryer. Experiments are carried out in this spray dryer to verify the calculated values of the spray drying capacity. It is s h m that the model when used along with the pilot plant data is adequate for scale-up calaculations for nozzle taer type of spray dryers.     

A SCACE-UP STUDY OF NOZZLE TOWER SPRAY DRYERS

A quasi-one dimensional model taking into account hindered drying beyond the critical tmisture content and droplet size distribution is used to scale-up nozzle tower type of a spray dryer with uniformly distributed air entry, based on pilot plant data. A pilot plant with 8 m high and 1.7 m diameter spray dryer is used to collect the drying data. Equilibrium and critical mistwe contents of the particles are also experimentally msured. The index of drying rate in the hindered drying period is calculated frun the pilot plant data. This index is used in the M e 1 to determine the drying capacity and thereby the mixinnnn particle size that can be produced ina 3.5 m dimoeter 12 m high nozzle t- type of spray dryer. Experiments are carried out in this spray dryer to verify the calculated values of the spray drying capacity. It is s h m that the model when used along with the pilot plant data is adequate for scale-up calaculations for nozzle taer type of spray dryers.     

SCALE-UP OF SPOUTED BED DRYERS: CRITERIA AND APPLICATIONS

The criteria required for reliable scale-up of the aerodynamics of spouted bed dryers of grains are stated and discussed. The dryer modeling role for effective scale-up, design and analyses of such dryers will be outlined. Suggestions are made on how small scale drying and aerodynamic data may be scaled up for spouted bed dryers. Appropriate empirical and semi-empirical correlations are listed. A flowchart is provided for design and analysis of a conventional and a two-dimensional spouted bed dryers.     

SCALE-UP OF SPOUTED BED DRYERS: CRITERIA AND APPLICATIONS

Abstract

The criteria required for reliable scale-up of the aerodynamics of spouted bed dryers of grains are stated and discussed. The dryer modeling role for effective scale-up, design and analyses of such dryers will be outlined. Suggestions are made on how small scale drying and aerodynamic data may be scaled up for spouted bed dryers. Appropriate empirical and semi-empirical correlations are listed. A flowchart is provided for design and analysis of a conventional and a two-dimensional spouted bed dryers.     

Numerical Study of Two-Stage Horizontal Spray Dryers Using Computational Fluid Dynamics

This article presents the findings of a numerical simulation model of the spray-drying process in a two-stage horizontal chamber design with the aid of a computational fluid dynamic (CFD) model. The model describes heat, mass, and momentum transfer between two phases; namely, a continuous gas phase and a discrete phase of droplets (or particles), using the finite volume method. In this study, a new two-dimensional horizontal spray dryer (HSD) geometry is considered as a pilot study into the spray-drying process in this novel chamber configuration. The tested model is able to predict some important features of the spray-drying process, such as air flow patterns indicating recirculation zones and particle trajectory plots. Some performance parameters for spray drying, such as the rate of evaporation, average volumetric heat and mass transfer rates, etc., are calculated and discussed. This two-stage drying process especially applicable for the horizontal spray dryer (HSD) model is investigated and modeled. The bottom wall of the HSD is assumed to be a shallow fluid bed used for second stage drying. In this article, the fluid bed drying conditions are changed and compared. The drying within the fluid bed itself is not modeled in this study, however. It is shown that the particle residence time is higher when the fluid bed is included. The drying performance of this two-stage horizontal spray dryer is expected to be better than that of a single-stage dryer.     

CFD Analysis of Dust Explosion Relief System in the Counter-Current Industrial Spray Drying Tower

The aim of the paper was to evaluate the performance of venting devices in an industrial counter-current spray tower during dust explosion using a CFD model and EN14491:2006 standard. Three-dimensional CFD simulations of dust explosion in the spray tower for six different configurations of safety vents located at the top and side walls of the dryer were carried out. In the calculations, in which dust was replaced with a flammable gas-air mixture, the following parameters of dust explosion were analyzed: reaction progress, maximum and average gas temperature, gas velocity, and pressure. Reduced pressures calculated according to EN 14991:2006 standard are close to maximum pressure obtained from CFD simulations for all analyzed spatial configurations of the safety vents, which proves that the CFD model can be used both to design explosion mitigation systems and to describe the mechanism of explosion propagation in the drying chamber. Results of the calculations allowed us to evaluate the performance of analyzed venting devices, to determine the spatial configuration and parameters of venting devices that should be installed to minimize damages of the dryer construction.     

Numerical Study of Two-Stage Horizontal Spray Dryers Using Computational Fluid Dynamics

This article presents the findings of a numerical simulation model of the spray-drying process in a two-stage horizontal chamber design with the aid of a computational fluid dynamic (CFD) model. The model describes heat, mass, and momentum transfer between two phases; namely, a continuous gas phase and a discrete phase of droplets (or particles), using the finite volume method. In this study, a new two-dimensional horizontal spray dryer (HSD) geometry is considered as a pilot study into the spray-drying process in this novel chamber configuration. The tested model is able to predict some important features of the spray-drying process, such as air flow patterns indicating recirculation zones and particle trajectory plots. Some performance parameters for spray drying, such as the rate of evaporation, average volumetric heat and mass transfer rates, etc., are calculated and discussed. This two-stage drying process especially applicable for the horizontal spray dryer (HSD) model is investigated and modeled. The bottom wall of the HSD is assumed to be a shallow fluid bed used for second stage drying. In this article, the fluid bed drying conditions are changed and compared. The drying within the fluid bed itself is not modeled in this study, however. It is shown that the particle residence time is higher when the fluid bed is included. The drying performance of this two-stage horizontal spray dryer is expected to be better than that of a single-stage dryer.     

旋流式压力雾化器的研究应用

介绍了旋流组合式压力雾化器的结构原理，并通过对旋流式单喷嘴压力雾化特性的理论和试验研究，得出了组合式喷嘴雾化的特征参数。采用计算流体力学(CFD)技术，对喷雾干燥塔内气、固、液3相流场加以分析，优化了干燥系统的结构及工艺配置参数。以碱式硫酸铬为干燥对象，经工业化应用证明：采用该雾化装置的压力喷雾干燥系统在保证产品质量的前提下，可替代离心喷雾干燥装置，设备投资省，体积蒸发强度高，节能效果好。     

FACTORS INFLUENCING FLOW PATTRENS, TEMPERATURE FIELDS AND CONSEQUENT DRYING RATES IN SPRAY DRYING

The PSI-Cell model is used to predict flow patterns and drying rates for laboratory-scale spray dryers. The liquid feed is water, with atomization producing a spectrum of droplet sizes. An analysis of the cause-and-effect relationships determining droplet trajectories, temperature fields, the location and magnitude of the backflow recirculation effect, and drying rates provides insight into the factors governing spray-air contacting and dryer performance. Independent effects are assessed for spray angle, dryer column diameter, initial droplet size distribution and droplet velocity (separately and as linked through atomizer pressure), air/water flow ratio, and liquid volatility. Inferences are made for effective dryer design, selection of operating conditions, and logic for scale-up.     

SCALE-UP OF CONTACT DRYERS

The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer.

This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions.

The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-hygroscopic in the moisture range investigated.     

Narrow tube spray drying

Spray drying conventionally necessitates relatively large or elongated drying chambers. The present study examined the possibility in shrinking the spray drying chamber into narrow tube-like geometry. The key was in utilizing fine droplets which had low transport response time. A narrow copper/steel tube spray dryer (internal diameters between 12.7 and 48.0 mm) was constructed and was fitted with a two-fluid atomizer producing droplets in the size range smaller than 10 µm. Maltodextrin, lactose, and sucrose were spray-dried. The narrow tube approach allowed direct manipulation of the drying conditions via heating or cooling along the wall of the tube. This form of manipulation in the drying conditions, surprisingly, resulted in very distinctly crystalline spray-dried sucrose particles. The tube spray dryer was further modified with a long coiled-up tube, extending the particle residence time with minimal space requirements. Endoscopic analysis revealed that particle deposition within the tube resembled loosely attached particle and granules. The tube spray drying concept can potentially be used to provide precise control of the particle drying history along the length of the drying chamber, not limited to the control of the drying conditions at the inlet or outlet of a spray dryer.     

A Sensitivity Study on CFD Modeling of Cocurrent Spray-Drying Process

This article presents CFD modeling of drying process in a cocurrent spray dryer. Initial parameters of discrete and continuous phase were determined experimentally and used in the model. The theoretical results were compared with experimental data and sensitivity of the simulation results to the selected parameters was determined. Results show that the applied gas turbulence model, drying kinetics, effect of atomizing air, and turbulent particle dispersion are crucial parameters that control accuracy of the CFD modeling.     

A Three-Dimensional Simulation of a Spray Dryer Fitted with a Rotary Atomizer

Spray dryers fitted with rotary atomizers are commonly used in diverse industries to produce engineered powders on a large scale. Scale-up of such units is still largely empirical and based on prior experience and know-how. In the present study, a three-dimensional spray dryer with rotary atomizer is investigated numerically with a commercial CFD code. Continuous-phase, i.e., air, conservation equations are formulated in the Eulerian model while the droplet or particle equations are set up in the Lagrangian model. Two-way coupling between the continuous and dispersed phases is taken into account in the governing equations. The stochastic approach is used to predict the particle trajectories. The RNG k - ε turbulence model was used. Typical results, viz. air velocity, temperature, humidity profiles, and particle trajectories are presented and discussed. Compared with the pressure nozzle spray dryer, more volume of drying chamber is used effectively by the rotating disc type spray dryer. It is found that evaporation and drying take place mainly in the region and in the vicinity of first contact between air and spray. A parametric study is presented and, where appropriate, comparison is made with experimental data obtained with the simulated spray dryer.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

SCALE-UP OF CONTACT DRYERS

ABSTRACT

The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer.

This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions.

The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-hygroscopic in the moisture range investigated.     

PERFORMANCE OF SPRAY DRYER WITH INTEGRATED FILTER AND FLUID BED

Spray dryers featuring a fluid bed integrated into the base of a spray drying chamber have proved one of the most significant developments introduced into industry during the last decade, producing dust-free particulates under low product temperature conditions. The latest design development involves all particulate collection and exhaust air cleaning within the drying chamber with the use of integrated metallic filter elements (with CIP capability). This eliminates handling of fines outside the drying chamber and simplifies the exhaust air system contributing to lower pressure drop losses and lower overall energy consumption. This paper describes the performance of a pilot plant sized spray dryer featuring a drying chamber with both integrated particulate filters and fluid bed. Various products were tested. The results showed that the placing of particulate filters inside the drying chamber does not adversely affect the agglomeration process and that the powder quality compared with that achieved in a standard Fluidized Spray Dryer can be reproduced in this new design concept, with every possibility for improved quality due to no powder handling outside the drying chamber. The work also showed that by containing the powder within the drying chamber, notable operational advantages are apparent and that scale-up of the design concept represents no apparent difficulties.     

A Three-Dimensional Simulation of a Spray Dryer Fitted with a Rotary Atomizer

Abstract

Spray dryers fitted with rotary atomizers are commonly used in diverse industries to produce engineered powders on a large scale. Scale-up of such units is still largely empirical and based on prior experience and know-how. In the present study, a three-dimensional spray dryer with rotary atomizer is investigated numerically with a commercial CFD code. Continuous-phase, i.e., air, conservation equations are formulated in the Eulerian model while the droplet or particle equations are set up in the Lagrangian model. Two-way coupling between the continuous and dispersed phases is taken into account in the governing equations. The stochastic approach is used to predict the particle trajectories. The RNG k ? ? turbulence model was used. Typical results, viz. air velocity, temperature, humidity profiles, and particle trajectories are presented and discussed. Compared with the pressure nozzle spray dryer, more volume of drying chamber is used effectively by the rotating disc type spray dryer. It is found that evaporation and drying take place mainly in the region and in the vicinity of first contact between air and spray. A parametric study is presented and, where appropriate, comparison is made with experimental data obtained with the simulated spray dryer.     

Pro-oxidative effects of spray drying orange oil in a bench top dryer

The influence of potential heat exposure during spray drying on the oxidative stability of spray dried orange oil was studied. The design of some of the table top spray driers expose dried product to dryer exit air temperatures in the powder collection chamber or on the walls of the dryer if there is an accumulation of material on the drying chamber walls. This heat exposure may accelerate oxidation of the product in subsequent storage. To determine the potential for heat damage to affect oxidation of the powders produced, an orange oil infeed emulsion (carrier material - modified starch) was prepared and spray dried using the sample collection chamber supplied by the manufacturer as standard equipment. The spray dryer was then modified to extend the collection chamber inlet such that the product remained cooler than in the standard collection chamber. In this study, the spray dryer was operated for 1?h (inlet air temperature; 180°C and an exit air temperature; 100°C). Thus, the spray dry product could have been exposed to as much as 1?h of heating in the collection chamber (potentially at temperatures as high as the exit air temperature). In the case of spray drying with a collection chamber extension, the collected product was maintained at ca. room temperature. This would approximately mimic the heat exposure powders receive in an industrial spray dryer. Powders produced using both equipment designs were taken from both the collection and drying chambers, adjusted in water activity (0.33) under a nitrogen environment, and then put into storage in an incubator maintained at 35°C for 4 weeks (exposed to air). The ratio of limonene oxide to limonene was used to monitor oxidation using gas chromatography. This study showed a substantial increase in rate of oxidation of the spray dried powder from the table top spray dryer with the standard commercial collection chamber and much less in case of an extended collection chamber. The powder from the respective drying chamber also showed a higher rate of oxidation in comparison to its collection chamber. Thus, we urge researchers studying heat damage (e.g., oxidation) of powders produced on the table top dryers to be conscious of overestimating heat damage during drying.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

ABSTRACT

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.