LEAFLASH SPRAY DRYER: INVESTIGATION OF THE ATOMIZATION PROCESS

The atomization process of the LEAFLASH spray dryer has been investigated by laser diffractometry and video imaging. The LEAFLASH atomizer is a particular pneumatic nozzle where the pressurized gas is the hot drying air. The drying and the atomization processes are interrelated in the spray dryer. Consequently the mixing of the liquid droplets and the drying medium is very efficient, permitting to work with short drying duration and reduced chamber volume. The mean diameter and the drop size distribution are measured at the outlet of the nozzle by laser diffraction. We studied the coupling effects of the air temperature (150 to 300° C) and pressure (1.20 to 1.50 bars abs) and of the dry matter content of the liquid feed (aqueous maltodextrin solution, 20 to 55% w/ w) on the droplets/ size distribution and on the general pattern of the spray. Analyses of video images of the generated spray give the general pattern of the spray and an estimation of the liquid velocity     

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 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.     

Spray Drying Performance of a Laboratory Spray Dryer for Tomato Powder Preparation

Abstract

This work investigates the performance of a spray dryer for tomato powder preparation by spray drying of tomato pulp. Samples of tomato pulp with a 14% constant total solids concentration were used, and a pilot scale spray dryer (Buchi, B-191) with cocurrent operation and a two-fluid nozzle atomizer was employed for the spray drying process. Twenty-four different experiments were conducted keeping constant the feed rate, the feed temperature, and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying rate, and the air inlet temperature. In each experiment the air outlet temperature was recorded. Data for the residue remaining in the chamber and cyclone walls was gathered and two types of efficiencies were calculated as an indication of the spray dryer performance. Analysis of experimental data yielded correlations between residue accumulation and the variable operating conditions. The same operating parameters had a great influence on the air outlet temperature whereas temperature deviations were observed comparing measured air outlet temperatures with corresponding outlet adiabatic saturation temperatures.     

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.     

多相雾化喷嘴混合特性的研究

采用经典的KI-KIO_3平行竞争反应体系来研究多相雾化喷嘴的微观混合特性,考察了该多相雾化喷嘴在各种操作条件下的微观混合情况,找出了不同条件下微观混合效果的变化规律.实验结果表明,增大雾化气量或减小进液量,提高气液比,均可显著改善多相雾化喷嘴的微观混合效果,当气液质量比大于3.7时,微观混合良好.进料流量比a(V_A/V_B)对分割指数X,影响显著,当a小于0.35时,雾化效果对分割指数影响不大,在较低的气液质量比下就能达到较小的分割指数.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

Spray Drying Performance of a Laboratory Spray Dryer for Tomato Powder Preparation

This work investigates the performance of a spray dryer for tomato powder preparation by spray drying of tomato pulp. Samples of tomato pulp with a 14% constant total solids concentration were used, and a pilot scale spray dryer (Buchi, B-191) with cocurrent operation and a two-fluid nozzle atomizer was employed for the spray drying process. Twenty-four different experiments were conducted keeping constant the feed rate, the feed temperature, and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying rate, and the air inlet temperature. In each experiment the air outlet temperature was recorded. Data for the residue remaining in the chamber and cyclone walls was gathered and two types of efficiencies were calculated as an indication of the spray dryer performance. Analysis of experimental data yielded correlations between residue accumulation and the variable operating conditions. The same operating parameters had a great influence on the air outlet temperature whereas temperature deviations were observed comparing measured air outlet temperatures with corresponding outlet adiabatic saturation temperatures.     

Experimental investigation and model improvement on the atomization performance of single-hole Y-jet nozzle with high liquid flow rate

Y-jet nozzle, as an efficient multi-hole internal-mixing twin-fluid atomizer, has been widely used for liquid fuel spray in many industrial processes. However, single-hole Y-jet nozzle with high liquid flow rate is indispensable in some confined situations due to a small spray cone angle. In this paper, the atomization performance of single-hole Y-jet nozzles with high liquid mass flow rates ranging from 400 to 1500 kg/h for practical semidry flue gas desulfurization processes was investigated by the laser particle size analyzer, and the effects of spray water pressure, atomizing air pressure and air to liquid mass flow ratio on the liquid mass flow rate and the droplet size distribution were analyzed. Moreover, the secondary atomization model was modified on the basis of previous random atomization model of Y-jet nozzle. The predicted results agreed well with the experimental ones, and the improved atomization model of Y-jet nozzle was well validated to design the nozzle geometry and to predict the droplet size distributions for single-hole Y-jet nozzle with high liquid mass flow rate.     

Hydrodynamic characteristics of a horizontal flow ejector in a rectangular chamber

The effects of the volumetric flow rate of primary motive water, water height, and the geometric parameters of the hydrodynamiccharacteristics of the gas suction rate and gas phase holdup were investigated in a rectangular chamber (0.22×0.26×1.2 m-high) with a horizontal flow ejector. Gas suction rate increased with increasing volumetric flow rate of the primary motive water, mixing tube length and diffuser length, but it decreased with increasing water height and nozzle diameter. The gas phase holdup was directly proportional to gas suction rate, indicating its corresponding increase with the volumetric flow rate of the primary motive water. Conversely, it decreased with increasing water height and nozzle diameter. However, the mixing tube length affected the gas phase holdup minimally compared to other operating parameters. Both the gas suction rate and gas phase holdup correlated with the dimensionless equations of operating parameters.     

Development of an axisymmetric population balance model for spray drying and validation against experimental data and CFD simulations

An incremental model for spray drying, including a full droplet size distribution, has been implemented in a flowsheeting package incorporating tracking of distributed particle properties. Results were compared with expected trends based on standard theory and with results from a laboratory-scale spray dryer with a two-fluid nozzle for atomization. Predicted trends were as expected, with larger droplets giving substantially longer drying times and higher final moisture content. Predicted final moisture content was lower than measured values, as the very short residence times for fine particles were inadequately represented by first-order falling-rate drying kinetics. Dryer gas flow patterns were simulated by computational fluid dynamics. Calculated droplet residence times were much lower than for a plug-flow or fully mixed gas flow, because a high-velocity gas flow zone from the two-fluid atomizer persists down a substantial part of the dryer.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

Powder Properties and System Behavior during Spray Drying of Bauhinia forficata Link Extract

This article presents a study of the effects of the spray-drying conditions on product properties and dryer performance during manufacture of dried extracts of Bauhinia forficata. The product properties (loss on drying of the dried extract, flavonoids degradation ratio, product size distribution, bulk and loose densities, powder morphology) and the equipment performance were determined as a function of the input parameters (dryer inlet temperature, ratio between the feed flow rate of the extract to the dryer evaporation capacity, and feed flow rate of the drying gas). Regression equations correlating powder characteristics and dryer behavior to input process parameters were obtained. The results demonstrate significant impact of the processing conditions on product properties and dryer performance. The loss on drying is a key property, since low values are demanded for the acceptance of the dried extract. In general, due to strict product specifications, the drying conditions that give an end product with the required quality are not associated with the optimal drying performance.     

Pressure drop and mixing in single phase microreactors: Simplified designs of micromixers

Continuous flow microreactors can greatly improve the safety and product yields of processes in the pharmaceutical and fine chemical industry by overcoming many of the drawbacks of traditional batch and semi-batch stirred reactors. This study compares on a common scale the pressure drop and mixing performance of different size commercial microreactor plates composed of a tangential, SZ-shaped or caterpillar mixer followed by a rectangular serpentine main channel. The pressure drop was fitted to a friction factor model, which suggests that the mixing zone had significant chaotic secondary flow patterns, whereas the main channel did not. Moreover, the mixing zone was the main contributor to the overall pressure drop. Mixing performance was then characterized using competitive parallel reactions. Upon the formation of chaotic secondary flows, typically due to the interactions of artificially induced vortices, the mixer performance was found to be independent of geometry for a given energy dissipation rate. However, the mixer geometry will affect the critical Reynolds number that induces chaotic advection and changes the mixing time scale.     

Millisecond mixing of liquids using a novel jet nozzle

A millisecond mixing process for liquids was implemented using a new mixer design, i.e., a jet nozzle connected with a trumpet-shaped module. The jet nozzle can facilitate two or three liquid channels, performing an initial impingement mixing of liquid sheets in the thickness at millimeters. Then, the joint liquids sheet out of the jet nozzle was stretched thinner and thinner on the expanded solid surface of the trumpet-shaped module, which significantly intensified the liquid mixing process. Accordingly, dual controls on the liquid mixing can be accomplished flexibly by optimizing the operating conditions and the module configuration. Experiments were carried out to investigate the influencing factors on the mixing performance, where the planar laser induced fluorescence (PLIF) technique was used to measure the mass transport of fluorescent dye between the liquids. The intensity of segregation (IOS) and 95% mixing time (τ₉₅) were employed to characterize the mixing performance. The results showed that the module with a greater curvature surface possessed a better mixing performance owing to the rapid reduction of the liquid sheet thickness, which strengthened the mixing process in the lateral direction along the flow development. The mixing behaviors are greatly influenced by the flow rate ratio between the liquids. An optimum mixing state could be achieved when Q_S1/Q_S2 is 1:1. An increase of Q_T under the same flow rate ratio does not affect the mixing pattern in space, but the corresponding τ₉₅ is almost linearly shortened. By splitting one liquid stream into two surrounding streams, the so called Sandwich operation brought further improved mixing performance compared with the two liquids mixing process. Using the novel jet nozzle design, millisecond(s) mixing of liquids can be easily achieved with flexible control.     

Modeling the mixing of a gas-liquid spray jet injected in a gas-solid fluidized bed: The effect of the draft tube

In several important industrial processes such as fluid coking, fluid catalytic cracking and gas-phase polymerization, a gas-liquid spray jet is injected into a fluidized bed. Recent findings [Chan et al., 2004. US Patent, US 2004065590] have shown that for the fluid coking operation, locating a cylindrical draft tube near the exit of the injector can enhance the contacting efficiency of the droplets and the particles.The main objective of this paper was to model the liquid-solids mixing in the draft tube. A two-dimensional model that characterizes the radial mixing by means of an eddy diffusivity coefficient is presented. The model predictions are validated by experimentally measuring the liquid-solids mixing by a technique based on temperature measurements in the spray region. The simulation results for a commercial fluid coking nozzle confirm the potential benefits of the mixing chamber. Moreover, the model allows ascertaining the effect of some important parameters such as nozzle size, gas and liquid properties, and the air-to-liquid mass ratio to the nozzle.     

Numerical simulation on mixing kinetics of slender particles in a rotary dryer

The mixing processes of slender particles in a rotary dryer fitted with lifters were simulated in three dimensions. Particle motion was modeled by the Discrete Element Method (DEM) and a three dimensional collision model for slender particles was developed. Contact force, friction force and gravitational force acting on an individual slender particle were considered when establishing mathematics models. The influences of rotational velocity on the mixing of slender particles were discussed and compared with those of spherical particles under identical operating conditions. It was found that the mixing characteristics of slender particles and spherical particles all followed a constant rate until a completely mixed state was encountered. But there were still certain differences between these two kinds of particles. The influences of the lifters with different shapes were further discussed for slender particles. Selected stimulation results were obtained and would provide consults for the further study of slender particles.     

Eulerian–Lagrangian Simulation and Experimental Validation of Pneumatic Conveying Dryer

This paper explores numerical and experimental studies on the performance of a pneumatic conveying dryer. The four-way coupling Eulerian–Lagrangian approach is utilized in the numerical study and the experimental study is carried out in a pilot-scale vertical pneumatic conveying dryer of diameter 8.1 cm and 4.5 m length. The effects of Reynolds number, particle size, solid mass flow rate, and inlet gas temperature on the dryer performance are investigated. It is found that the present model predictions agree well with the experimental data. Generally, it is concluded that the drying rate increases as the Reynolds number increases, while increasing the particle size or the solid mass flow rate decreases the drying rate.