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Dynamics >>22
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).
Fig
4: SEM-photograph of spray dried and agglomerated
powder
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.
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.
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