(37b) Tracking Single Particle Temperature during Hot Melt Coating in the Free Stream

Coating
plays an important role in the pharmaceutical industry to protect agents or
release them in specific parts of the stomach or bowel, but as well in the food
industry to protect flavors. The coating is usually applied in a fluidized bed,
mainly in a Wurster coater. In this process the particles are transported and
dispersed by an air flow in the center of the device while being sprayed on with
the coating liquid. After coating in the center region of the Wurster coater,
particles are collected again in a part with lower air velocity. The coating
process itself, including spraying, hitting of the droplets on the particle,
spreading on the surface and solidification or drying influences the final
coating quality.

A
process of particular interest is coating with hot melt. Natural substances
like beeswax, carnauba wax and palm fat are interesting in the food industry,
as they do not need to be declared as food additives. Hot melt coating has
several advantages including a faster process, as significantly less energy needs
to be transferred. Additionally, substances like fats and waxes create a more
efficient barrier against vapor compared to water-soluble coatings. The coating
process is more difficult to control because of higher viscosity, which leads
to larger droplets and lower spreading of the material, and more complex crystallization
behavior. The process parameters and the suitability of the coating depend on
the crystallization temperature range, which needs to be passed to go from the
melt to the solidified coating. An additional challenge is the tendency of the
wax coatings to agglomerate.

It
is difficult to determine temperatures, drying characteristics and kinetics
during the process in the Wurster coater itself. Instead, the coating process
was reproduced by fluidizing a single particle in the free stream and spraying
on it similar to the work of Link [Link and Schlünder 1997]. The advantage is that
the complete information of the spraying process is hence accessible, including
the local air velocity.

A
test rig was set up to fluidize a single particle in the free stream above a
nozzle. Additionally, a second nozzle was installed above the fluidized
particle to spray coating liquid. The coating liquid was applied to investigate
the influence of isolated parameters on the coating quality. Spraying was done
by an ultrasonic nozzle to uncouple droplet size from the nozzle’s air volume
flow. The coating can hence be applied in a controlled way. In the free stream
the air velocity depends on the particle size and density. The particle fluidization
was calculated analytically by balancing drag force and gravity force. The
energy transport from the air to the sphere was calculated based on temperature
and differential velocity.

The
temperature of the particle was adjusted by setting the temperature in the fluidization
nozzle. Spraying coating dissolved in water at room temperature changes the
temperature on the surface of the sphere. The temperature evolution on the
surface of the sphere was monitored by an infrared camera (see Figure 1 left and middle). The
evaporation cools the particle until the liquid is completely evaporated,
allowing to monitor the coating process. It was hence possible to determine the
surface temperature and to monitor the process of drying of the coating on the
particle. This allowed to visualize zones with applied coating during the
process and to determine the duration of drying of the coating. It showed to be
in the range of up to 20 s depending on the sprayed coating volume, temperature
and on particle size and air flow. The coating yield in the free stream was low
due to the large spraying cone compared to the particle surface size, as well
as the fluidization air redirecting most of the liquid from the particle. Still
this reproduces the Wurster coater process well. It was possible to create
defined coating layers in the levitator. The single particle coating with hot
melts, which show a complex drying or crystallization behavior, is currently in
work.

The
hot melt process was reproduced in a Wurster coater. Taking the crystallization
temperature range into account, the process conditions can be realized
similarly to solvent-based coatings. While coatings like beeswax tend to
agglomerate, different melts like stearic acid could be processed well. The
produced coating was investigated in micro-computed tomography to evaluate for
a closed coating layer (see Figure 1 right). Hot melt coating
showed to be less homogeneous compared to solvent-based coating, with delamination
effects on the particle despite the low surface tension of waxes.

Figure 1:
Thermography image of fluidized particle (left); Temperature evolution during
spraying different volumes of solvent-based coating (middle); µCT image of a melt-coated
particle (right)

References

Link,
K. C. and E.-U. Schlünder (1997). "Fluidized bed spray granulation." Chemical
Engineering and Processing: Process Intensification 36(6): 443-457.