Fluidized bed dryers(Chris Tremblay)

BY: Chris Tremblay
Piece of equipment used to remove moisture
from a wet solid by bringing the moisture
into a gaseous state.
A drying medium (usually air) is utilized to
fluidize and dry moist particles.
May be carried out in a batch or continuous
On average 12% of energy used on drying for
a manufacturing process. Which could be 6070% of the total cost.
Chemical industries
The results show that exergy efficiencies are
less than energy efficiencies due to
irreversibilities which are not taken into
consideration in energy analysis, and that
both energy and exergy efficiencies decrease
with increasing drying time
Drying kinetics is another significant aspect
in drying processes. The drying rate, N.
The exergy balance is derived by applying
mass, energy and entropy balances to the
drying column in batch fluidization
The energy efficiencies decrease
exponentially as the surface moisture
evaporates until the end of the drying
• Exergy leaving: Outlet
• Exergy Destruction(3
irreversibilities): heat
transfer, mixing, and
• Exergy losses: exergy
loss to drying air
leaving the column.
Heat transfer across
the boundary
• Exergy of material:
Increase of exergy of
the material
Both energy and exergy efficiencies decrease
sharply with decreasing moisture content of
the material.
The effects of parameters such as:
1. inlet gas velocity
2. inlet gas temperature
3. particle size diameter
on the energy, exergy efficiencies and the
availability of gas
When the inlet gas temperature is increased from 70
C to 100 C with the inlet gas velocity of 4 m/s,
energy efficiency increases up to 25% and the exergy
efficiency up to 21%.
However there is practical limitation due to damage
of the material.
Article does not explain well, but larger particles
have lower efficiencies due to larger distance
moisture must travel to reach the outer edge of
the particle and smaller surface area
The availability of gas to do work decreases
with time due to the decreasing difference is
Exergy is consumed or destroyed, due to
irreversibilities in any real process. The
exergy consumption during a process is
proportional to the entropy created due to
irreversibilities associated with the process.
Lower exergy efficiency leads in general to
higher environmental impact.
Cost of energy, the availability of fuel, and an
impact on the environment make efficiency of
exergy important in the drying process.
The moisture diffusion coefficient of wheat is
mainly dependent on temperature
The moisture diffusion coefficient of corn is
dependent upon both the temperature and
the moisture content of particles
Thermodynamic efficiencies were obtained to be
higher at the beginning of the process than at
the final stage
inlet air temperature effect:
◦ wheat particles where the diffusion coefficient is only a
function of the temperature, the increase of the drying
air temperature increases the efficiency though not in a
linear way
◦ corn particles, where the diffusion coefficient depends
on the temperature and the moisture content of
particles, the increase of the drying air temperature does
not increase the efficiency automatically
Gas velocity effect:
◦ For wheat particles, energy and exergy efficiencies increase
for a reduction of about 15% in the air velocity
◦ For corn particles, both energy and exergy efficiencies do
not show any difference at the end of the drying process
Particle size, bed depth, and inlet drying air
temperature were evaluated to asses the
effects on exergy and energy efficiency.
Exergy loss through outlet air and dryer
frame account for large percentage, so it
important to reduce these and have more
exergy loss into the product being dried.
Exergy loss
increased with
drying air
and bed depth
Exergy loss
decreased with
cube size
A comparison of three types of dryers were
conducted using exergy destruction rates and
improvement potential rates.
Tray dryer, fluid bed dryer, and a heat pump
Exergy destruction and improvement
potential rates were lowest in the fluidized
bed dryer
The exergy analysis of a fluidized bed dryer is
a much more accurate way to look at the
efficiency of the dryer than looking at the
energy analysis.

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