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.     

Simulation of a Spray Dryer Fitted with a Rotary Disk Atomizer Using a Three-Dimensional Computional Fluid Dynamic Model

Spray dryers fitted with a rotary disk atomizer are widely used in many industries requiring high throughputs to produce powders from liquid streams. The interaction between droplets or particles and the drying medium within the drying chamber is still not well understood and hence difficult to model reliably. In this article CFD results are presented to describe the behavior of the performance of a spray dryer fitted with a rotary disk atomizer in a cylinder-on-cone chamber geometry. Four different turbulence models, i.e., standard k - ε, RNG k - ε, Realizable k - ε, and Reynolds stress models were tested and compared to simulate the swirling two-phase flow with heat and mass transfer in the chamber. The results of this investigation can provide further insight into turbulent swirling flow modeling. The predicted results, such as, air flow patterns, air velocity and temperature, distributions, particle/droplet trajectories, drying performance etc., are obtained using the CFD code FLUENT6.1. Comparison with available limited experimental data shows that CFD results display reasonable agreement. Predicted results also show that the RNG k - ε model performs better in this specific case.     

Mathematical Modeling the Drying of Poplar Wood Particles in a Closed-Loop Triple Pass Rotary Dryer

Closed-loop drying systems are an attractive alternative to conventional drying systems because they provide a wide range of potential advantages. Consequently, type of drying process is attracting increased interest. Rotary drying of wood particles can be assumed as an incorporated process involving fluid–solid interactions and simultaneous heat and mass transfer within and between the particles. Understanding these mechanisms during rotary drying processes may result in determination of the optimum drying parameters and improved dryer design. In this study, due to the complexity and nonlinearity of the momentum, heat, and mass transfer equations, a computerized mathematical model of a closed-loop triple-pass concurrent rotary dryer was developed to simulate the drying behavior of poplar wood particles within the dryer drums. Wood particle moisture content and temperature, drying air temperature, and drying air humidity ratio along the drums lengths can be simulated using this model. The model presented in this work has been shown to successfully predict the steady-state behavior of a concurrent rotary dryer and can be used to analyze the effects of various drying process parameters on the performance of the closed-loop triple-pass rotary dryer to determine the optimum drying parameters. The model was also used to simulate the performance of industrial closed-loop rotary dryers under various operating conditions.     

Simulation of a Spray Dryer Fitted with a Rotary Disk Atomizer Using a Three-Dimensional Computional Fluid Dynamic Model

Abstract

Spray dryers fitted with a rotary disk atomizer are widely used in many industries requiring high throughputs to produce powders from liquid streams. The interaction between droplets or particles and the drying medium within the drying chamber is still not well understood and hence difficult to model reliably. In this article CFD results are presented to describe the behavior of the performance of a spray dryer fitted with a rotary disk atomizer in a cylinder-on-cone chamber geometry. Four different turbulence models, i.e., standard k ? ε, RNG k ? ε, Realizable k ? ε, and Reynolds stress models were tested and compared to simulate the swirling two-phase flow with heat and mass transfer in the chamber. The results of this investigation can provide further insight into turbulent swirling flow modeling. The predicted results, such as, air flow patterns, air velocity and temperature, distributions, particle/droplet trajectories, drying performance etc., are obtained using the CFD code FLUENT6.1. Comparison with available limited experimental data shows that CFD results display reasonable agreement. Predicted results also show that the RNG k ? ε model performs better in this specific case.     

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.     

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.     

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.     

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.     

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.     

A Parametric Study of the Gas Flow Patterns and Drying Performance of Co-current Spray Dryer: Results of a Computational Fluid Dynamics Study

A computational fluid dynamic study was carried out to investigate airflow pattern, temperature, and humidity profile at different levels in the drying chamber. Good agreement was obtained with published experimental data. The effects of operating pressure, heat loss from the chamber wall and inlet air conditions on the gas flow pattern, droplet trajectories, and overall dryer performance also were investigated. Results are presented and discussed in terms of the gas velocity, temperature, and humidity profiles within the chambers. The volumetric evaporation values, heat transfer intensity, and thermal energy consumption per unit evaporation rate were computed and compared for drying of a 42.5% solids solution in a spray chamber 2.215 m in diameter with a cylindrical top section 2.005 m high and a bottom cone 1.725 m high. Wall regions subject to formation of undesirable deposits are also identified.     

Combined Crystallization and Drying in a Pilot-Scale Spray Dryer

Combined crystallization and drying of lactose solutions was performed in a pilot-scale spray dryer over a wide range of operating conditions. The effect of different parameters, including temperature, moisture content, atomizing air flow rate, liquid feed rate, main drying air flow rate, and particle size, on the degree of crystallinity of the spray-dried powders was analyzed. Water-induced crystallization (WIC) and modulated differential scanning calorimetry (MDSC) were used to assess the effect of these parameters on the degree of crystallinity of the spray-dried powders. The particles were characterized in terms of the final moisture content using WIC and distinctive differences in the peak heights, which are indicative of the particle crystallinity, were found for spray-dried particles using different drying conditions, supporting the results from MDSC. MDSC showed that decreasing the inlet air temperature by 40°C increased the degree of crystallinity in the particles threefold from 22 to 72%. A decrease in the inlet air temperature may decrease the particle temperature, resulting in wetter particles, and a lower temperature meant a longer particle drying time and allowed the particles to rearrange themselves into a more crystalline form. Up to 72% crystallinity is achievable in a pilot-scale spray dryer by suitable adjustment of the operating conditions. The results suggest differences in the rate of crystallization and particle size between small and pilot-scale spray dryers.     

Validation of the Lagrangian Approach for Predicting Turbulent Dispersion and Evaporation of Droplets within a Spray

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.     

Validation of the Lagrangian Approach for Predicting Turbulent Dispersion and Evaporation of Droplets within a Spray

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.     

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.     

Investigation of Particle and Air Flows in a Mixed-Flow Dryer

Even though the mixed-flow dryer is well established on the commercial market for the drying of grain, maize, and rice, there further potential as well as a need to optimize the dryer apparatus and to improve product quality. Unfavorable designs can cause uneven mass flow and air flow distributions, resulting in locally different drying conditions and, hence, uneven grain drying. The aim of the present article is to evaluate traditional designs of mixed-flow dryers by numerical and experimental investigation of particle and air flows and to discover design deficits. For this purpose, the dryer geometry and different air duct arrangements (horizontal and diagonal) were studied using the discrete element method (DEM) and computational fluid dynamics (CFD). Drying experiments were performed to evaluate the grain moisture and temperature distributions. With regard to particle flow, a typical core flow was detected as in silos with a retarded particle flow at the dryer walls and a fast flow region in the center of the dryer. This was caused by the wall friction effect and the half air ducts fixed at the side walls. With regard to the air flow, dead zones were discovered for the diagonal air duct arrangement. Based on the design deficits identified for the traditional geometry, a new geometry for the mixed-flow dryer that is still under development is discussed.     

用颗粒热传递模型计算旋转列管干燥机的传热系数

采用基于移动加热板的颗粒热传递模型计算大型旋转列管干燥机的传热系数。在料床完全混合的简化假设下 ,按物料在干燥机内的实际干燥过程分段计算各段的传热系数及整机总传热系数 ,其结果与干燥机的实际运行情况基本相符。