Experimental and numerical studies were carried out to compare the traditional vacuum freeze drying VFD, existing atmospheric freeze drying AFD and heat pump drying HPD methods with the
Trang 1CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS
8.1 CONCLUSIONS
A new atmospheric freeze drying (AFD) system was designed, fabricated and tested
for drying of heat sensitive materials A series of atmospheric freeze-drying
experiments with different food and biotechnological products were carried out under
different drying conditions Experimental and numerical studies were carried out to
compare the traditional vacuum freeze drying (VFD), existing atmospheric freeze
drying (AFD) and heat pump drying (HPD) methods with the AFD process The
experimental results are compared with simulation results Finally, a three-dimensional
CFD simulation of the vortex tube used to generate sub-zero temperature was carried
out Based on extensive experimental and analytical results, the main conclusions
drawn are as follows:
• A vortex tube can be used as a suitable means to achieve and maintain
desirable low temperature for an AFD drying process on a laboratory scale
• The multi-mode AFD process examined in this research project, using
conduction and radiation coupled with convection yields faster drying kinetics
for pieces of potato and carrot without compromising on dried product quality,
which compared favorably with vacuum freeze drying Experimental results
also showed that use of a vibro-fluidized bed with an adsorbent presents
significant improvement in terms of freeze drying kinetics when compared with
existing commercial AFD systems which use heat pump assisted fluidized beds
Trang 2• Experimental results revealed that osmotic treatment is not a suitable option in
case of AFD and VFD processes as it reduces the quality of the dried product
• On the basis of extensive parametric studies, it is concluded that the proposed
AFD system for drying pieces of heat-sensitive materials (Vibro-fluidized bed
dryer with a vortex tube, multimode heat input and mixed adsorbent) is an
attractive alternative to overcome some of the drawbacks of existing AFD as
well as VFD systems A technoeconomic feasibility study needs to be carried
out, however, after scale-up to a pilot scale
• A simplified one-dimensional simulation model for atmospheric freeze-drying
of foods in a fixed bed dryer was developed Simulation results compared well
with the experimental results The model gave a good prediction of the drying
kinetics This simple model can be used as a tool to optimize the process
parameters
suggests that the standard RNG k-є turbulence model leads to a better
agreement between the numerical and experimental data than the standard k-є,
k-omega and the swirl RNG k-є turbulence models This model captured well
the aerodynamic and thermal characteristics of the vortex tube Predicted
results showed that energy separation between the two opposite
counter-rotating vortices in the vortex tube occurs mainly due to transfer of loss of
angular momentum in the form of heat from the inner vortex to the outer
vortex Results also revealed that the magnitude of energy separation increases
as the inlet pressure increases They further confirm that the location of strip
inside the commercial vortex tube is the optimal position for maximum energy
Trang 3separation It is expected that such a model will be useful in future for design
and scale-up of vortex tubes
In summary, the key objectives of this research project were met successfully It is
noted that the AFD process can compete with the traditional VFD process in terms of
both capital and operating costs as it does not need vacuum operation
Recommendations for future work are summarized as follows:
• Further work on the experimental study using vortex tube Our results showed
the potential of using vortex tube to supply cryogenic air for atmospheric freeze
drying For better energy efficiency experiments with an AFD system where
heating and cooling outputs of vortex tubes are concurrently used are
recommended
• Optimize drying condition in AFD using vortex tube An extensive series of
experiments is recommended to identify the optimal drying conditions for two-
stage conditions: below and above sub-zero temperatures A large varity of
products, including those biological origins, should be tested to determine
viability of AFD If successful the capital and operating cost will be reduced
significantly
• Scale-up of vortex tubes in AFD systems for industrial application is suggested
through a careful CFD numerical simulation Finally, a technoeconomic
feasibility study should be carried out before commercialization can be
recommended