Industrial Spray Dryer Research Articles

Sedimentation and rheological behavior of reactive and non-reactive magnesium hydroxide pulps for industrial spray dryer processing

Two different reactive concentrated magnesium hydroxide (HM) pulps, produced in a lab reactor mill, and a synthetic and non-reactive HM-pulp, for comparative analyses, have been prepared and studied. Sodium polyacrylate was selected as a dispersant for improving the HM pulp fluidity, with concentrations varying from 0.42% to 2.0% on a dry basis. The two reactive pulps differed from each other mainly by the impurity level and for both, a residual MgO amount was detected, implying that the hydration reaction could proceed. Results (with and without dispersant) confirmed the formation of particle clusters as HM concentration rose until a critical value, above which the pulp behaved as a solid. Reactive HM-pulps with high impurities may represent a limiting factor in the drying operation. Among all HM-pulps studied, the purest reactive pulp with 1% dispersant presented the best flow behavior making this reactive pulp the most suitable to be tested in a pilot spray dryer.

Parametric analysis of industrial spray dryer for performance enhancement applicable in effluent management

In this work, the industrial case study of effluent drying with a direct and hybrid heat recovery configuration is investigated for the purpose of improving heat recovery potential and understanding the influence of various design parameters on the performance indicators in the spray drying process. Initially, data collection and performance analysis of industrial spray dryer of effluent management is performed. Then, the effect of design variables such as recirculation ratios, mass flow rate of dry hot air, water vapor, temperature of the mixing chamber, and heat input on performance parameters (thermal efficiency, energy savings, and the sustainability index) are evaluated for several configurations of the spray drying unit used in effluent management. As an outcome, substantial improvement in energy efficiency (83%) and energy-saving (26%) was achieved for a spray drying system with a 70% recirculation ratio of exhaust air for a hybrid heat recovery configuration, which is approximately 2.5% higher than the direct heat recovery configuration and it is significantly higher than that without an energy recovery system. In addition, a significant reduction in the blower power and heat input is noted in the direct and hybrid heat recovery system. The sustainability index is higher for the hybrid heat recovery configuration, indicating it being the most environment friendly configuration. The absolute humidity in the exhaust air is kept constant during entire investigation for uniformity in analysis.

CFD Modelling of the Evaporation of Biphasic (Solid-Liquid) Individual Droplets in a Spray Dryer

An analysis of the literature shows the capability of Computational Fluid Dynamics to represent the hydrodynamics and heat transfer within spray dryers, as well as the prediction of droplet collision, agglomeration and wall deposition. The literature also reveals that no model published so far describes the drying of individual droplets, the study of which is restricted to experimental methods such as ultrasonic levitation. This article presents an Euler-Lagrange three-phase numerical set-up to simulate the drying of individual droplets within a spray dryer. The defining parameters of both phases, i.e. air and the biphasic (solid-liquid) droplets are taken from a barbotine slurry used for ceramic tile production. A single droplet size has been used—the characteristic Rosin-Rammler droplet size from the distribution generated by an actual spray injector published previously in the literature. The model predicts the theoretical linear evolution of the square diameter according to the D2-law, and the temperature and mass exchange with the environment. This unique numerical set-up allows calculating the Stokes number associated with a given trajectory path as well, providing designers with an accurate degree of control over the fate of individual droplets. The proposed model is therefore a powerful tool intended to support the design and optimization of industrial spray dryers.

Numerical and experimental characterization of the hydrodynamics and drying kinetics of a barbotine slurry spray

Spray drying is a basic unit operation in several process industries such as food, pharmaceutical, ceramic, and others. In this work, a Eulerian-Lagrangian three-phase simulation is presented to study the drying process of barbotine slurry droplets for the production of ceramic tiles. To this end, the simulated velocity field produced by a spray nozzle located at the Institute of Ceramic Technology in Castelló (Spain) is benchmarked against measurements obtained by means of laser Doppler anemometry in order to validate the numerical model. Also, the droplet size distribution generated by the nozzle is obtained at operating conditions by means of laser diffraction and the data obtained are compared qualitatively to those found in the literature. The characteristic Rosin-Rammler droplet size from the distribution is introduced thereafter in the three-phase simulation to analyse the drying kinetics of individual droplets. The model predicts the theoretical linear evolution of the square diameter (D2-law), and the temperature and mass exchange with the environment. The proposed model is intended to support the design and optimization of industrial spray dryers.

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

ABSTRACTThe 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.

Modeling of food drying processes in industrial spray dryers

The shift from a trial-and-error approach on food product development to a quality-by-design paradigm requires tools that support the scientist in making decisions for the design of research and development activities. Presently, many of these tools require high investments in software or time for implementation. Consequently, the present work had the objective to develop a simplified model of the spray drying process in an industrial spray dryer to support activities of food product development and that can be easily implemented in any software. The model was verified and validated using an industrial spray dryer in food drying processes covering a wide range of operating conditions and food products. The model was further extended so that the scientist could estimate not only key operating parameters such as feed flow rate or drying gas outlet temperature but also final particle size and drying kinetics for a better understanding of the drying phenomenon by spray drying. From the comparison of the experimental and the estimated results it is concluded that the model successfully describes all spray drying operations independently of the product nature. Raw material and laboratory time can be reduced by replacing the traditional trial and error methodologies by using the developed tool. Estimates of wet bulb temperature, dew point and glass transition temperature increase process knowledge preventing process errors. Scale-up is also facilitated by the use of non-dimensional estimated parameters such as outlet relative humidity.

Modeling and Scaling Up of Industrial Spray Dryers: A Review

Modeling and Scaling Up of Industrial Spray Dryers: A Review

Modeling of Dust Explosion in the Industrial Spray Dryer

Standard ANSYS FLUENT software was used to model a dust explosion inside a counter current spray-drying tower by replacing the dust with a flammable gas–air mixture. Two sets of computational fluid dynamics (CFD) calculations for a dust explosion in the tower were carried out at a uniform gas concentration and variable concentration in the dryer, which reflect the distribution of wet and dry powder zones in the spray dryer. The CFD calculation showed that a dust explosion developed from the ignition point mostly to the upper part of the tower. The temperature, velocity, and reaction profiles followed the same upward pattern. Results of the calculations allowed us to determine the position and parameters of venting devices that should be installed to minimize the risk of damage to the dryer construction.

Modeling of Air Flow in an Industrial Countercurrent Spray-Drying Tower

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.

Particle shrinkage and morphology of milk powder made with a monodisperse spray dryer

The drying of monodisperse droplets can produce uniform powders where every particle has experienced similar, predictable air conditions. For the first time, a single-stream dryer was used with monodisperse milk droplets having solids contents as high as those used in industrial spray dryers (over 40wt% total solids). With size measurement of droplets and powders, the particle shrinkage was precisely determined. Changes in shrinkage and morphology were observed when varying feed solids contents and drying temperatures. The majority of particles had an inwardly buckled morphology and hollow, inflated particles were produced at higher temperatures due to boiling. Drying was modelled with a numerical simulation using the Reaction Engineering Approach (REA), which is a semi-empirical model of moisture removal rate. This model accurately predicted the moisture contents of the powders collected. Several sub-models were used for particle shrinkage. The most accurate shrinkage assumption was found to be isotropic shrinkage from the removal of water (known as perfect shrinkage). This may suggest that the surface crust (initially spherical) is viscoelastic and deforms in response to drying stresses.

Entropy production during the drying process of milk droplets in an industrial spray dryer

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.

A Fundamental Model of Particle Deposition Incorporated in CFD Simulations of an Industrial Milk Spray Dryer

Particle deposition is an important, undesirable problem during the operation of spray dryers. A new mathematical model of particle deposition has been developed and implemented in the computational fluid dynamics (CFD) software FLUENT. The pseudo Ohnesorge number Oh* and Weber number We* have been used to formulate the model to the determine possibility of particle deposition. The reentrainment of the particles back into the dryer chamber was determined according to the wall shear stress. The transient multiphase flow process in an industrial spray dryer that produces around 9 tonnes of milk powder per hour was simulated to validate the deposition model. The numerical results show that reentrainment of particles into the dryer chamber is significant at the chamber walls. The deposition intensity has been found to be higher near the adjunctions of the ceiling wall, the vertical wall, and the conical wall because of smaller wall shear stress. The distribution of wall temperature, Oh* and We*, which has important influence on wall deposition, has been obtained from the numerical results. More experimental and computational work is required for future validations.

Air flow patterns in an industrial milk powder spray dryer

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.

Erratum to: “Hydrodynamic stability of the two-phase nonisothermal flow in a countercurrent spray dryer”

Experimental data on the fluid dynamics, temperature fields, and mass transfer in industrial spray dryers for synthetic detergents are reported. The distribution of the components of the flow velocity in the radial cross-section of a turbulent swirl flow in the active drying zone is obtained on the basis of balance equations and experimental data. The radial hydrodynamic stability interval of 0.2 < r/R < 0.9 is calculated using the Howard hydrodynamic stability criterion. The results are used to optimize the operation of an industrial dryer with a dry-product rate of 15 t/h.

Surface‐center temperature differences within milk droplets during convective drying and drying‐based Biot number analysis

AbstractAn assumption of uniform temperature is frequently used when evaluating average temperature‐time and average moisture content‐time profiles for the convective drying of small droplets or thin‐layer materials. In most studies, the assumption of uniform temperature was justified by estimating the heat‐transfer Biot number at the beginning and end of the drying process. However, the conventional Biot number analysis performed in the literature does not reflect the evaporative effect. In this article, we have examined the temperature uniformity and the heat‐transfer Biot number during the drying of skim milk droplets under laboratory drying conditions following the entire drying process. Surface‐centre temperature differences and conventional and drying‐based Biot numbers are calculated during the drying of skim milk droplets. A simple procedure is outlined to estimate the extent of temperature nonuniformity within the droplet. Results demonstrated that temperature nonuniformity within the skim milk droplets under drying conditions examined is very small, thus the uniform temperature assumption is more likely to be a reliable approach to model heat and mass transfer processes in industrial spray dryers. The analyses provided in this study help in understanding a few assumptions used in literature, and offer a framework that may be used in the future. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Comparative study of droplet drying models for CFD modelling

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

Simulation of an Industrial Spray Dryer and Prediction of Off-Design Performance

A computational fluid dynamic model along with a new drying kinetics model was employed to simulate the spray-drying performance of an industrial-scale co-current spray dryer for a relatively heat-sensitive product. Predictions are compared with experimental measurements made on a full-scale spray dryer. Satisfactory agreement was achieved. Predictions of zones where deposits are likely to occur are close to those in operational experiments. Off-design performance predictions were also made to show the effects of various operating parameters on the dryer performance, which can be used to improve the dryer efficiency and minimize wall deposits.

Energy Consumption of Industrial Spray Dryers

: In 2000, the U.K. government's Energy Efficiency Best Practice Programme commissioned a survey to determine the energy consumption of spray dryers within the chemicals, foods, and ceramics industries. The results of this survey, which included dryers having evaporation rates ranging from 0.1 to 12 t/h, revealed values of the specific energy consumption E s varying from around 3 to 20 GJ/t water evaporated. The average for all dryers included in the survey was 4.87 GJ/t. The fuel-to-electricity consumption ratio averaged around 27. The data obtained in the survey were interpreted with the aid of a newly developed model that enabled the performance of a particular dryer to be compared with that of its ideal adiabatic counterpart. Using the model, it was estimated that around 29% of the energy supplied to the dryers included in the survey was being wasted.

A Study of the Limitations to Spray Dryer Outlet Performance

The concept of the product moisture locus was tested in this work using a pilot-scale modified Niro spray dryer (diameter 0.8 m, height 2 m), where the residence time of the particles inside this spray dryer is lower compared with larger industrial spray dryers. The moisture contents of skim milk powder produced from spray drying skim milk (solids content 8.8% w/v) at different operating conditions, namely different swirl vane angles (0°, 25°, 30°), inlet air temperatures (170°C, 200°C, 230°C) and process fluid flowrates (1.4 kg h−1, 1.6 kg h−1, 1.8 kg h−1), were compared with the predicted equilibrium moisture contents. In addition, the residence time of the particles was also increased in the spray dryer by decreasing the inlet air mass flowrate from 0.016 to 0.013 kg s−1. The outlet moisture contents of the skim milk powder for all the 23 runs carried out in this work were within 0.4% of the equilibrium values. Thus, the skim milk powder particles were in close equilibrium with the gas inside the drying chamber. These equilibrium limitations are confirmed by other literature data (Boonyai, P. Comparative Evaluation of Soymilk Drying in a Spray Dryer and Spouted Bed of Inert Particles. M.Sc. Thesis. Asian Institute of Technology: Bangkok, Thailand, 2000; 90 pp; Harvie, D.J.E.; Langrish, T.A.G.; Fletcher, D.F. A computational fluid dynamics study of a tall-form spray dryer. Trans IChemE 2002, in press). The use of this finding to predict spray dryer performance is demonstrated by mass and energy balance calculations.

Experimental Measurement and Numerical Simulation of the Effect of Swirl on Flow Stability in Spray Dryers

A 0.29 m, 0.5 m tall, hydraulic model of an industrial spray dryer, constructed from perspex, has been used to study the precession of the central jet of air inside this equipment, where this central jet may be connected to the occurrence of wall deposits inside this equipment,. Experiments showed that the flow was strongly time-dependent, with two characteristic frequencies that depended on the angle of the inlet swirl vanes over a range of angles from 0° to 40°. The main frequency of precession of the central jet occurred over a time scale of a few seconds, and image analysis allowed the characteristic frequencies of the precession to be quantitatively assessed. Numerical simulations of the flow, conducted with CFX5, a CFD package, predicted Strouhal numbers for the jet precession that were within experimental errors of those observed for no swirl and 40° swirl, and only slightly outside the error bounds for 25° swirl. This agreement suggests that the k – ɛ turbulence model is suitable for estimating the frequencies of precession in spray dryers. This is of industrial significance, since the precession may be linked to the occurrence of wall deposits.