Agglomeration in Spray Drying Installations (The EDECAD Project): Stickiness Measurements and Simulation Results

Spray drying is used for the manufacture of many consumer and industrial products such as instant dairy and food products, laundry detergents, pharmaceuticals, ceramics, and agrochemicals. During spray drying, agglomerates of powder particles are formed that determine the instant properties of the powder. Agglomeration during spray drying is considered to be a difficult process to control. The main cause of this is the complex interaction of the process variables: the atomization process, the mixing of spray and hot air, the drying of suspension droplets, and the collision of particles, which might lead to coalescence or agglomeration. As a consequence, agglomeration during spray drying is operated by trial and error. In an EC-sponsored project, named the EDECAD project and coordinated by NIZO food research, an industrially validated computer model, using CFD technology, to predict agglomeration processes in spray drying machines is developed. A Euler-Lagrange approach with appropriate elementary models for drying, collision, coalescence, and agglomeration of the dispersed phase is used. The main result of the EDECAD project is a so-called design tool, which establishes relations between the configuration of the drying installation (geometry, nozzle selection), process conditions, product composition, and final powder properties. The design tool has been validated on pilot plant scale and industrial scale. This article presents the setup and results of dynamic stickiness tests and some CFD simulation and validation results.     

Agglomeration in Spray Drying Installations (The EDECAD Project): Stickiness Measurements and Simulation Results

Spray drying is used for the manufacture of many consumer and industrial products such as instant dairy and food products, laundry detergents, pharmaceuticals, ceramics, and agrochemicals. During spray drying, agglomerates of powder particles are formed that determine the instant properties of the powder. Agglomeration during spray drying is considered to be a difficult process to control. The main cause of this is the complex interaction of the process variables: the atomization process, the mixing of spray and hot air, the drying of suspension droplets, and the collision of particles, which might lead to coalescence or agglomeration. As a consequence, agglomeration during spray drying is operated by trial and error. In an EC-sponsored project, named the EDECAD project and coordinated by NIZO food research, an industrially validated computer model, using CFD technology, to predict agglomeration processes in spray drying machines is developed. A Euler-Lagrange approach with appropriate elementary models for drying, collision, coalescence, and agglomeration of the dispersed phase is used. The main result of the EDECAD project is a so-called design tool, which establishes relations between the configuration of the drying installation (geometry, nozzle selection), process conditions, product composition, and final powder properties. The design tool has been validated on pilot plant scale and industrial scale. This article presents the setup and results of dynamic stickiness tests and some CFD simulation and validation results.     

Effect of Maltodextrin Addition during Spray Drying of Tomato Pulp in Dehumidified Air: I. Drying Kinetics and Product Recovery

This work investigates the effect of maltodextrin addition on the drying kinetics and the stickiness during spray drying of tomato pulp in dehumidified air. A pilot-scale spray dryer was employed for the spray-drying process. The modification made to the original design consisted in connecting the spray dryer inlet air intake to an absorption air dryer. Twenty-seven different experiments were conducted varying the dextrose equivalent (DE) of the maltodextrin, the ratio (tomato pulp solids)/(maltodextrin solids), and the inlet air temperature. Data for the residue remaining on the walls were gathered. Furthermore, the effect of maltodextrin addition on the drying kinetics and the stickiness of the product was investigated using a numerical simulation of the spray-drying process modeled with the computational fluid dynamics (CFD) code Fluent. The code was used to determine the droplet moisture content and temperature profiles during the spray-drying experiments conducted in this work. The stickiness was determined by comparing the droplet temperature with its surface layer glass transition temperature (T_g ). The T_g was determined using a weighted mean rule based on the moisture content profiles calculated by the CFD code and the experimental data of T_g , which were obtained for the different tomato pulp and maltodextrin samples and fitted to the Gordon and Taylor model.     

Challenges of Simulating Droplet Coalescence within a Spray

Abstract

This article reports various challenges that have been encountered in the process of developing validated Lagrangian and Eulerian models for simulating particle agglomeration within a spray dryer. These have included the challenges of accurately measuring droplet coalescence rates within a spray, and modeling properly the gas–droplet and droplet-droplet turbulence interactions. We have demonstrated the relative versatility and ease of implementation of the Lagrangian model compared with the Eulerian model, and the accuracy of both models for predicting turbulent dispersion of droplets and the turbulent flow-field within a simple jet system. The Lagrangian and Eulerian droplet coalescence predictions are consistent with each other, which implies that the numerical aspects of each simulation are handled properly, suggesting that either approach can be used with confidence for future spray modeling. However, it is clear that considerable research must be done in the area of particle turbulence modeling and accurate measurement of particle agglomeration rates before any Computational Fluid Dynamics tool can be employed to accurately predict particle agglomeration within a spray dryer.     

Challenges of Simulating Droplet Coalescence within a Spray

This article reports various challenges that have been encountered in the process of developing validated Lagrangian and Eulerian models for simulating particle agglomeration within a spray dryer. These have included the challenges of accurately measuring droplet coalescence rates within a spray, and modeling properly the gas-droplet and droplet-droplet turbulence interactions. We have demonstrated the relative versatility and ease of implementation of the Lagrangian model compared with the Eulerian model, and the accuracy of both models for predicting turbulent dispersion of droplets and the turbulent flow-field within a simple jet system. The Lagrangian and Eulerian droplet coalescence predictions are consistent with each other, which implies that the numerical aspects of each simulation are handled properly, suggesting that either approach can be used with confidence for future spray modeling. However, it is clear that considerable research must be done in the area of particle turbulence modeling and accurate measurement of particle agglomeration rates before any Computational Fluid Dynamics tool can be employed to accurately predict particle agglomeration within a spray dryer.     

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.     

CFD Modeling of Air Flow on Wall Deposition in Different Spray Dryer Geometries

Wall deposition is one of the most conventional problems in the spray drying process. The operation of a spray dryer is affected by the wall deposition fluxes inside the equipment. In this study, computational fluid dynamic (CFD) simulation was used to investigate the effect of spray dryer geometry on wall deposition. A CFD model was developed for different geometries of spray dryer with a conical (case A) or a parabolic (cases B and C) bottom. The results implied that the parabolic geometry resulted in a lower deposition rate on the spray dryer walls. A comparison of results using the P-values (F-test) of the air velocity, in the conical and parabolic geometries, showed that there was a significant difference in air stability between them. The flow field in conical geometry case A was significantly more unstable, and parabolic geometry case C produced the most uniform airflow patterns. Moreover, the higher wall shear stress in case C, with lower values of the vorticity, would result in less wall deposition.     

Simulation of Agglomeration in Spray Drying Installations: The EDECAD Project

Spray drying is used for the manufacture of many consumer and industrial products such as instant dairy and food products, laundry detergents, pharmaceuticals, ceramics, and agrochemicals. During spray drying, agglomerates of powder particles are formed which determine the instant properties of the powder. Agglomeration during spray drying is considered to be a difficult process to control. The main cause of this is the complex interaction of the process variables: the atomization process, the mixing of spray and hot air, the drying of suspension droplets and the collision of particles which might lead to coalescence or agglomeration. As a consequence, agglomeration during spray drying is operated by trial-and-error. In an EC-sponsored project, named the EDECAD projects, an industrially validated computer model, using CFD technology, to predict agglomeration processes in spray drying machines is developed. An Euler-Lagrange approach with appropriate elementary models for drying, collision, coalescence and agglomeration of the dispersed phase is used. The main result of the EDECAD project is a so-called “Design Tool,” which establishes relations between the configuration of the drying installation (geometry, nozzle selection), process conditions, product composition and final powder properties. The Design Tool is being validated on pilot-plant scale and industrial scale. It will provide an advanced tool for improved design and optimization of spray drying and agglomeration equipment, to improve the quality of products and to increase the productivity of such equipment. This article introduces the background and approach of the project and some preliminary results.     

Numerical Study of the Drying Process of Different Sized Particles in an Industrial-Scale Spray Dryer

Development of high-performance spray dryers that are more energy efficient and are able to produce high-quality milk powders is very important for the future of the dairy powder industry. Understanding and optimization of the exiting dryers are also of great value. Computational fluid dynamics is a powerful tool to simulate and help understanding the characteristics of spray drying and to introduce potentially improved designs. The present study has concentrated on the multiphase flow in an industrial-scale spray dryer using the CFD package FLUENT. A Eulerian-Lagrangian approach is used in the simulations. A new drying model, REA model, for milk particles has been implemented for the first time in a CFD application. The numerical results match well with the plant data. It is argued that the “reflecting wall” boundary condition produced more physically correct results for normal dryer operation than the “escaping wall” boundary condition. The second one is an oversimplification. The influences of the particle size and particle size distribution, residence time, kinetic energy, and maximum temperature have been analyzed. The initial droplet size range was set to be from 100 to 500 µm with Rosin-Rammler distribution function. One significant result of this study is that rather dense particle clouds of medium-size particles (224–285 µm) are found near the side walls. They are transported upwards along the side wall (conical wall surface) and then are dispersed. It is found the particles with initial diameters of between 225 and 270 µm have the largest residence times.     

Droplet-Droplet Interactions in Spray Drying by Using 2D Computational Fluid Dynamics

A 2D axisymmetric model of the spray drying process is presented. The two-phase flow theoretical model is based on a combined Eulerian-Lagrangian approach and takes binary interactions (coalescence or bouncing) between spray droplets into consideration. Validation of the model (incorporated in FLUENT 6.3.26) demonstrated good agreement and consistency with the literature data. The results of transient simulations showed that droplet-droplet interactions displace the region of heat and mass transfer from the central core toward the periphery of the drying chamber. It was also found that insulation of the spray dryer can substantially affect temperature and humidity patterns, whereas its influence on the velocity flow field is less marked.     

Experimental Study and CFD Modeling of Wall Deposition in a Spray Dryer

The wall deposition phenomenon in a pilot-scale spray dryer was investigated based on mathematical modeling and experimental trials. For this purpose, the governing equations were obtained and solved numerically by applying a mathematical modeling technique and an open-source computational fluid dynamics (CFD) software. The wall deposition, velocity distribution of the existing phases, and droplet trajectory in the drying chamber were determined. The effect of the operating parameters including the feed flow rate, inlet concentration of dissolved solid, and initial droplet diameter on the air flow pattern, droplet trajectory, and wall deposition was investigated. Through the experiments, the wall deposition of powder product in different positions of the drying chamber was measured. In modeling part of this study, we attempted to determine the effect of particle diameter on the percentage of wall deposition and the position where it occurred.

The model results obtained for wall deposition were compared with collected experimental data and good agreement was observed.     

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.     

Spray Drying of Tomato Pulp: Effect of Feed Concentration

The effect of feed concentration on spray drying of tomato pulp preconcentrated to 78, 82, and 86% wet basis is investigated in two spray drying systems: a pilot scale spray dryer (Buchi, B-191) with cocurrent regime and a two-fluid nozzle atomizer, and the same connected with an absorption air dryer (Ultrapac 2000). Data for the residue on the chamber and cyclone walls were gathered and two types of efficiencies were calculated as an indication of the spray dryer performance. Tomato powders were analyzed for moisture, particle size, and bulk density. In both spray drying systems, with increases in tomato pulp concentration overall thermal efficiency, evaporative efficiency, material loss in the cyclone, powder moisture content, and bulk density decreased, whereas powder particle size increased. On the contrary, the effect of feed solids content on residue formation and product recovery was dependent on the drying medium. In the standard dryer, the higher the feed concentration, the higher was the residue accumulation, and the lower the product recovery, whereas in the modified system increases in pulp concentration resulted in lower residue formations and higher product yields.     

A hybrid CFD—reaction engineering framework for multiphase reactor modelling: basic concept and application to bubble column reactors

The coupling of turbulent mixing and chemical phenomena lies at the heart of multiphase reaction engineering, but direct CFD approaches are usually confronted with excessive computational demands. In this hybrid approach, the quantification of mixing is accomplished through averaging the flow and concentration profiles resulting from a CFD flow field calculation and a computational (“virtual”) tracer experiment. Based on these results, we construct a mapping of the CFD grid into a generalised compartmental model where the chemistry calculations can be efficiently carried out. In contrast to the empirical models used in the residence time distribution (RTD) approach, the compartmental model in this methodology, owning to its CFD origins, retains the essential features of the equipment geometry and flow field. A procedure for extracting the mixing information from k-ε based CFD codes is outlined, but the main concept of the approach is not restricted to any particular type of turbulence modelling, and will therefore benefit from future developments. A phenomenological model of mass transfer and chemical reaction, based on the penetration theory, is employed to simulate the interfacial phenomena in gas-liquid reactors, and a study of CO₂ absorption into alkali solution is presented to demonstrate the method.     

Spray Dryer Modeling in Theory and Practice

This article considers the modeling of spray dryers at various levels and the selection of the most appropriate level of detail for practical situations. The following model levels are described: (1) Heat and mass balances; (2) Equilibrium based models; (3) Rate based models; (4) Computational fluid dynamic (CFD) models. The value of each is discussed in relation to some typical problem scenarios. These include preliminary process design; process improvement; and troubleshooting operational and product quality problems. One particular focus of this article is finding realistic models of the performance characteristics of spray dryers which can be included in process flow sheet simulations while not imposing excessive run times and complexity.     

Modeling of the Spray Zone for Particle Wetting in a Fluidized Bed

In a spray agglomeration process the particle wetting influences the agglomerate growth and particle dynamics in the granulator. The mass of binder liquid that is deposited on single particles affects the amount of energy dissipation during particle contacts. For the agglomeration of colliding particles the whole impact energy has to be dissipated due to viscous and capillary adhesion forces in the liquid film and plastic deformation of the material. Therefore, a detailed knowledge of the particle wetting is necessary to model the agglomeration process. This contribution uses a coupled DEM‐CFD approach to describe the spray zone of a two‐fluid nozzle in a fluidized bed agglomerator. Droplets modeled as discrete elements showed the formation of a spray zone with a conical shape. Simulations of the spray zone and the wetting of single particles are in good agreement with experimental results.     

Effect of Maltodextrin Addition during Spray Drying of Tomato Pulp in Dehumidified Air: II. Powder Properties

This work investigates the effect of maltodextrin addition on the main powder properties during spray drying of tomato pulp in dehumidified air. A pilot-scale spray dryer was employed for the spray-drying process. The modification made to the original design consisted in connecting the spray dryer inlet air intake to an absorption air dryer. 21 DE, 12 DE, and 6 DE maltodextrins were used as drying agents. Tomato pulp was spray dried at inlet air temperatures of 130, 140, and 150°C and (tomato pulp solids)/(maltodextrin solids) ratios of 4.00, 1.00, and 0.25. The tomato powders were analyzed for rheological properties, moisture content, bulk density, solubility, hygroscopicity, and degree of caking. It was found that maltodextrin addition improved powder hygroscopicity, caking, and solubility, whereas it deteriorated slightly its moisture content and density. In addition, analysis of experimental data yielded correlations between powder properties and the above-mentioned variable operating conditions. Regression analysis was used to fit a full second-order polynomial, reduced second-order polynomials, and linear models to the data of each of the properties evaluated. F values for all reduced and linear models with an R ² ≥ 0.70 were calculated to determine if the models could be used in place of full second-order polynomials.     

Hygrothermal and Quality Properties Applicable to Drying. Data Sources and Measurement Techniques

ABSTRACT

The efficient dryer design and the optimization of existing drying processes require both effective drying models and hygrothermal properties. Following extensive research during the previous decades, today's existing models for the drying process are mostly adequate. Thus.most of the requirements for appropriate process design and operation depend on the existence of data for the various properties. The correct estimation of properties is of fundamental importance for the design.optimization and control of the process. The existing knowledge is adequate regarding the hygrothermal properties of the drying medium (air). Most of the efforts should address the problem of finding and standardizing appropriate measuring techniques and selecting and compiling the existing data, concerning hygrothermal and quality properties of the product being dried. The article identifies those areas, where adequate data not exist and future research is needed or data banks should be created.     

Flow field investigation in a photocatalytic reactor for air treatment (Photo-CREC-air)

The aerodynamic behavior of a photocatalytic reactor for air treatment, Photo-CREC-air, with demonstrated high quantum efficiency performance, is examined using CFX-5.7.1. Photo-CREC-air consists of a venturi section that features low pressure drop and uniform illumination of the photocatalyst, resulting in high oxidation quantum efficiencies. The numerical simulations allowed the identification of several design issues in the original Photo-CREC-air unit, which include extensive boundary layer separation close to the photocatalyst support and regions of flow recirculation that render ca. 77% of the support surface area inactive. The simulations reveal that this issue could be addressed by replacing the wire-mesh basket sidewalls with perforated plates. This modification causes an increase in the pressure drop downstream of the support and achieves significant uniformization of the mass flow and air-photocatalyst contact time distributions.A modified Photo-CREC-air design is also presented and studied using CFX-5.7.1. This modified design is envisaged with the objective of improving UV-irradiation uniformity, an issue that is not completely addressed in the original design due to the shape of the windows and divergent section. CFD simulations reveal that, although the flow field is uniform, mass flow and contact time distributions are not. Nonetheless, this problem is addressed by increasing the pressure drop downstream of the support through the addition of a region modeled as a perforated plate. The simulations reveal that the mass flow and contact time distributions are significantly uniformized once this modification is implemented.     

Wall-to-particle heat transfer in steam reformer tubes: CFD comparison of catalyst particles

Computational fluid dynamics (CFD) was used to simulate non-reacting heat transfer in a steam reforming packed reactor tube of tube-to-particle diameter ratio (N) equal to 4, with cylindrical multi-hole catalyst particles. These simulations extend those of our previous study [Nijemeisland, M., Dixon, A.G., Stitt, E.H., 2004. Catalyst design by CFD for heat transfer and reaction in steam reforming. Chemical Engineering Science 59, 5185-5191] to provide accurate tube wall temperatures, runs at constant pressure drop in addition to those at constant mass flow rate and simulations of particles with different sizes of holes. At constant pressure drop, particles with higher void fractions allowed higher mass flow rates, resulting in tube wall temperatures and radial temperature profiles in order: solid cylinders>one-hole particles>multi-hole particles. Little difference was seen between three-hole and four-hole particles. The particles with multiple holes gave a substantial reduction in tube wall temperature, with only a small decrease in core tube heat transfer. The effect of hole size was small, for the cases investigated in this study.