Press Room - Spray drying in the food industry

Spray drying in the food industry

M. Verschueren, R.E.M Verdurmen, M. Gunsing, J. Straatsma, NIZO food research, Ede, the Netherlands
S. Blei, M. Sommerfeld, Department of Engineering Science, Martin Luther University Halle-Wittenberg, Germany

Spray drying is an essential unit operation for the manufacture of many products with specific powder properties. It is characterized by atomization of a solution or suspension into droplets, followed by subsequent drying of these droplets by evaporation of water or other solvents. Spray drying is used for the manufacture of many consumer and industrial products such as instant food products, laundry detergents, pharmaceuticals, ceramics and agrochemicals. The best known example of an instant food product is milk powder, but instant beverages (e.g. coffee) can also be produced by spray drying.

Spray drying equipment

Figure 1 schematically shows an example of an industrial spray dryer for the production of agglomerated powder. In the spray chamber the incoming product is atomized by rotary wheel atomizers or pressure nozzles and dried by the hot air introduced at the top. The powder particles leave the spray dryer at the bottom into the fluid bed, where further drying takes place. Most of the air leaves the spray chamber through the air outlet. Powder particles in the outlet air (small dry particles) are separated by a cyclone and can be reintroduced into the spray chamber (fines return) or into the fluid bed to enhance the agglomeration process.

Fig 1: An industrial 2-stage spray dryer with fines return (courtesy of Anhydro)

Drying and fouling

The primary goal of spray drying is removing water. Under normal operating conditions the powder particles are enough dry before they hit the walls of the spray dryer, such that they do not stick to the walls. The drying behavior strongly depends on the spray characteristics and the feed composition. Stickiness is related to the drying state of the particles. Incorrect operating conditions, which do not match the drying behavior of the particles, can therefore lead to fouling.
The airflow field, the particle trajectories (figure 2) and the local temperature and humidity (figure 3) inside the spray dryer can be computed by using Euler-Lagrange CFD techniques, taking into account the coupling for mass, momentum and energy. The difference from other spray calculations (e.g. diesel sprays) mainly concerns the drying part: stickiness primarily depends on the drying state of the outer layer of the particles. Additional sub-models for moisture diffusion inside the particles and for the relation between the drying state and stickiness are therefore required to be able to compute the drying and fouling behavior of spray drying systems.


	Fig 2: particle trajectories		Fig 3: humidity

To enhance the accessibility of CFD knowledge, CD-adapcoand NIZO food research have cooperated to develop an Expert System for predicting the drying and fouling behavior of spray dryers: es-spraydry. An easy-to-use GUI guides the user through defining the spray drying system, defining the product characteristics, setting up the processing conditions, mesh generation, running the calculations and post-processing the results. The es-spraydry tool can be used to design spray dryers, to check whether a specific dryer is suitable for a specific product or to investigate the effect of changes in processing conditions on the drying and fouling behavior of dryers.

Agglomeration

Some powder properties (e.g. solubility) can be related to the moisture content and the temperature-time history of the particles. For these properties the modeling techniques described above can be used. Many powder quality properties, however, are related to the degree of agglomeration. Agglomeration is a size enlargement process of powders, where small particles combine to form large relatively permanent structures, in which the original particles are still identifiable (see figure 4).

Agglomeration facilitates the handling of powders (free flowing powders without dust formation) and enhances the instant properties of powders (a quick dissolution and dispersion without the formation of lumps). The improved instant properties are a result of the open porous structure of the agglomerates, allowing water to penetrate and disperse throughout to its original constituting particle, forcing the particle to sink. In a spray dryer agglomeration can take place within the spray of an atomizer, between sprays of various atomizers and between sprays and dry material being introduced into the drying chamber (fines return, see figure 1). The latter technique is the most effective way to achieve and control agglomeration in spray dryers.

The EDECAD project

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 2001 an EC-sponsored project started, coordinated by NIZO food research, entitled EDECAD (Efficient DEsign and Control of Agglomeration in spray Drying machines, www.edecad.com).
The aim of the project is to develop an industrially validated CFD model to predict agglomeration processes in spray drying machines.
The modeling technique used is an extension of the Euler-Lagrange model for the drying and fouling behavior of spray dryers described above. Besides the drying model, additional models for collision and agglomeration are required. The initial spray conditions were measured and the sub-models for drying, collision and agglomeration were developed and validated by the academic partners in the project.

Fig 4: SEM-photograph of spray dried and agglomerated powder

A stochastic collision model is used to predict the collision probability and impact details. When a collision occurs, the drying state of the particles and the impact details determine whether the particles rebound, coalesce or agglomerate. Agglomeration occurs when particles are sticky. For many food products stickiness is strongly related to glass transition. The particle composition and the moisture content of the outer layer determine the glass transition temperature and thereby the stickiness of the particle, which influences the agglomeration process.

Figure 5 shows the result of a test calculation in a cubic geometry, initially containing a binary mixture of dry and viscous primary particles. The probability density function of the relative penetration depth is shown for various viscosities. There is a significant influence of viscosity on agglomeration and the structure of agglomerates. With increasing viscosity the mass fraction of agglomerates increases, as given the label of figure 5. Moreover, penetration depths are reduced and the agglomerate size distribution becomes narrower, resulting in a larger, more homogeneous agglomerate population with improved powder properties.

Fig 5: results agglomeration model (S. Blei, MLU-Halle)

Fig 6: Air flow

With the help of CD-adapco, NIZO food research integrated all sub-models in STAR-CD. Pilot plant validation trials and industrial scale experiments were carried out by the industrial partners in the project to produce validation data for the CFD model. The final validation work is currently in progress.

References

• M. Sommerfeld, ‘Validation of a stochastic Lagrangian modeling approach for inter-particle collisions in homogeneous isotropic turbulence’, Int. J. Multiphase Flow 2001, Vol. 27, 1829-1858.
• P. Menn, G. Schulte, K. Bauckhage, ‘Experimental Investigation of High Pressure Spray Drying Nozzle Performance at Industrial Operating Conditions’, Proc. 9th International Conference on Liquid Atomization and Spray Systems, July 13-17, Sorrento, Italy.
• S. Blei, M. Sommerfeld, ‘Lagrangian Modeling of Agglomeration During Spray Drying Processes’, Proc. 9th International Conference on Liquid Atomization and Spray Systems, July 13-17, Sorrento, Italy.
• R.E.M. Verdurmen, M. Verschueren, J. Straatsma, M. Gunsing, ‘Simulation of Agglomeration in Spray Drying Installations: the EDECAD Project’, Proc. 9th International Conference on Liquid Atomization and Spray Systems, July 13-17, Sorrento, Italy.
• R.E.M Verdurmen et al., ‘Simulation of Agglomeration in Spray Drying Installations: the EDECAD Project’, Drying Technology 2004, to be published.