Pilot plant scale test on a fluidized bed dryer
TO: Mr. Point, Chief Engineer, Black Forest Sawmills.
FROM: Susyn Kelly
DATE: 22nd March 2011
Summary
An experiment was used to characterize the drying system in a fluidized drying bed before
reliable predictions are made. The main objective of the experiment was to obtain data to
calculate the critical moisture content and the rate of evaporation. This was then used to
check the accuracy of the Kothari equation in determining the rate of evaporation and its
correlation to the observed rate in the constant rate period of drying. Recommendations on
the type of dryer most suited to the drying of sawdust can then be made.
The data was obtain by measuring the relative humidity of in and out flowing air through a
wet bed of sawdust as well as the air flow rate. The pressure drop across the bed was also
recorded using a manometer. These sets of data allowed the plot of a a pressure drop vs.
airflow rate to be plotted. This showed a minimum fluidization velocity of 0.5m/s and a
pressure drop across the fixed bed region of 220Pa. Another plot of the dependence of
drying rate on moisture content was also produced. This gave the critical moisture content
and showed that capillary forces dominated in the falling rate period (due to its linear
relationship). The evaporation rate per unit surface area was calculated using a mass
balance (0.0001kg/m2s) and the value calculated using the heat correlation gave a value
of 0.000077kg/m2s.
This experiment showed that there is accuracy in the equations, as they gave similar
results to those observed in the plotted data.
Recommendations to Mr.Point on the selection of dryer would be that the fluidized bed
dryer would be most suited to the drying of sawdust as it reduces drying time in
comparison to a tray dryer.
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Introduction
The performance of a pilot scaled perforated tray drier has previously been investigated,
after reviewing the findings it has been suggested that a fluidized bed dryer may prove
more attractive.
When a fluid is flowing through a bed of solid particles (eg: saw dust) a force is exerted on
the particles. This force, on the particles, is equal to the frictional pressure drop multiplied
by the area of the bed in contact with the flowing fluid (the cross-sectional area of the bed).
At low velocities, flow is laminar and the frictional pressure drop across the bed is low and
the bed is fixed. The fixed bed porosity can be estimated from the slope of the graph.
As the velocity is increased (v) , the pressure drop increases (ΔP) until a force greater than
the opposing force of the particles is reached.
ΔP = 150(1-ε)2 μvL
ε3 d p2
+
laminar flow
1.75 (1-ε) ρv 2 L
ε3 d p
transitional flow
The force is now sufficient to lift the particles in the bed, suspending them in the flowing
fluid. Now the bed of particles has been fluidized, the lowest velocity at which this will
occur is called the ‘minimum fluidization velocity.’ The velocity of the fluid through the
emulsion stay relatively constant once this point is reached. The porosity at this point can
be determined from the measured expansion of the bed;
L fixed bed ( 1 - ε fixed bed ) = L fluid bed ( 1 - ε fluid bed )
Further increases in the velocity causes bubbles of fluid to rise up through the fluidized
particle emulsion.
After fluidization the pressure drop remains relatively constant with increasing velocities,
this pressure drop is the support required to hold the weight of the bed.
ΔP = mws g
Acs
The rate of drying is a function of the moisture content of the bed. At high moisture
contents, the rate of drying is constant as the surface of the particles are wet. As these
surfaces begin to dry, the moisture from the inside has to travel to the surface of the
particles determining the drying rate. Below the critical moisture content the rate of drying
declines (with decreasing moisture contents) until the particles are dry (rate of drying = 0).
The rate of evaporation can be calculated using a suitable correlation for the heat transfer
coefficient; The Kothari equation.
Nu = h dp = 0.03 Re p 1.3
λ
where Re = ρ v dp
μ
2
The rate of evaporation in the constant rate period is obtained using:
Rc = h( θa - θs )lm
h fg
As the product is wet during this period, the surface temperature remains constant and is
equal to the wet bulb temperature.
A pilot plant scale test was carried out on the fluidized bed dryer; the benefits of using this
method of drying were observed. Experimental data is to be produced for the critical
moisture content and the rate of evaporation, this will be used to determine if the heat
transfer correlation of the Kothari equation is useful for predicting the rate of evaporation.
Experimental
Fluidisation Hydrodynamics
Recordings of the initial manometer levels and angle were recorded, this was to provide a
baseline for subsequent manometer readings.
Pressure readings were then take for the empty bed over a full range of flowrates, this was
to determine if any error was present in the pressure drop readings. As expected the
pressure drop was zero with the empty bed (directly proportional relationship between
pressure drop and the distributor plate).
Next the saw dust was weighed out (approximately 100g). This was added to the empty
bed and the heigh of the sawdust bed was recorded (L).
The flowrate of the air was increased in 20l/min increments and the pressure drop was
recorded. The onset of fluidisation was observed. The maximum flow rate reached before
sawdust began to leave the bed was 200l/min.
Determining Dry Solid Density and Initial Moisture Content
A sample of wet sawdust was weighed, this gave us the initial mass of the wet sawdust.
Water was added to cover the wet sawdust and it was re-weighed. It was then placed in an
over at 150℃ overnight. The following day the now dry sawdust was re-weighted, this gave
us the mass of dry solids added in the initial sample of wet solids.
Fluidised Bed Drying
With the dry bed already in place, set the flowrate to 200l/min (this is greater than the
minimum fluidisation velocity to allow for the addition of wet sawdust without defluidising
the bed) and the inlet temperature to 50℃ (turn the heater on). Add a measure amount of
wet saw dust to the dry bed (approximately 100g). At 2 minute time intervals measure the
pressure drop across the bed and distributor plate, the inlet and outlet temperature and the
inlet and outlet air humidity. As the sawdust begins to dry the air flowrate will need to be
reduced so the sawdust doesn’t get blown out the top of the column. When the outlet
temperature is almost equal to the inlet temperature the bed is dry and the experiment
complete.
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Results and Discussion
Pressure Drop
Minimum
Fluidization
Velocity
Fluidisation Study
In determining the fluidisation hydrodynamics the air flow rate required to fluidize the
material was calculated. Minimum fluidisation velocity occurred at around 0.5m/s air flow
rate and the pressure drop suggested by the graph is 210Pa. Precautions to keep in mind
when using the fluidised bed dryer to prevent fluidisation problems are the thickness of the
bed. The spread of the bed should be of a reasonable depth to prevent the air from
escaping freely through it. Overloading the dryer requires higher drying temperature and
longer period of drying. In general, finer material should be spread thinner. Bigger material
requires slightly longer period of drying and should be spread thicker.
The pressure drop observed here is similar to the one calculated using the measure bed
weight;
ΔP = mws g
Aws
This produced a value of 222Pa. However the pressure drop calculated from using the
observed minimum fluidisation velocity was not as accurate, a value of 472Pa was
calculated, assuming laminar flow as the bed of particle is fixed, using the equation:
ΔP = 150(1-ε)2 μvL
ε3 d p2
The validity of the first equation to predict the pressure drop was relatively accurate.
However the second equation produced a more deviated prediction. Perhaps the
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assumption of only laminar flow is incorrect and the influence of transitional flow needs to
be taken into account.
Constant Rate Period
Xc critical moisture content
Drying Study
Drying usually occurs in two stages, a constant rate period where the drying rate (R c)
doesn't vary with changing moisture contents and a falling rate period where the drying
rate gets smaller as the moisture content approaches zero. The point where the drying
rate changes from a constant to a falling rate is the critical moisture content, Xc.
Data was collected for the drying of the fluidisation bed, this produced a plot of the rate of
drying as a function of moisture content as shown below:
The mechanism for drying in the falling rate period is determined from the relationship
shown in the graph above. It is roughly linear and thus capillary forces dominate. The
drying rate is observed to decline linearly with decreasing moisture content. In the constant
rate region, a fully wetted surface will achieve a dynamic equilibrium where the rate of heat
transferred from the air equals the rate of heat lost from the surface by evaporation. Water
is transferred from the interior to the surface as fast as it is evaporating. The surface
temperature is constant in this region and governed by the heat inputs.
Once the water on the surface of the particles has evaporated the mechanism for
transporting water to the surface is through capillary action. Before the falling rate period
the critical moisture content is reached, according to the graph this alludes to a value of
1.0 kg of water per kg of dry solid.
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The evaporation rate calculated for the constant rate period using heat correlation gave a
somewhat accurate rate. The calculated rate was 0.000077 kg/m 2s (refer to Appendix),
and the graph shows a value of around 0.0001 kg/m2s. Kothari equation is useful for
predicting the rate of evaporation.
To check if the graph was correct a calculation of td was performed to see if this calculated
time match the actual drying time recorded in the experiment. A calculated time of 53min
(for details refer to the appendix) and an actual time of 58min showed a relatively good
agreement and added confidence that the critical moisture content obtained from the
graph can be used with relatively little error (as also proven by the good agreement
between drying rates above).
At the start of the experiment the pressure change was recorded across an empty bed.
This would give us an error we would need to incorporate into later manometer readings.
There was no pressure drop (change in manometer readings) when measured across an
empty bed, so the accuracy of the temperature and humidity measurements can be
assumed accurate and constant as the same equipment was used for each reading.
Conclusion
The plot produced from the experimental data for the moisture content overtime showed
that in the constant rate period the drying rate remained relatively constant as the moisture
content decreases. This was due to the evaporation occurring at only the wetted surface of
the particles. Once all this water had evaporated the critical moisture content was reached
and after this point the falling rate period was observed. It was deemed that capillary
forces dominated this stage of evaporation due to the linear relationship between drying
and moisture content. During the falling rate period the drying rate was determined by the
rate at which the water moved to the surface, using capillary action.
As water is evaporated from the product and transferred to the air, the weight of the
product decreases. The Kothari equation predicted a relatively accurate drying rate in the
constant rate period using heat transfer correlations. The value was slightly smaller than
the value observed on the graph. This error may have been because of the assumption
that the surface of the particles was saturated.
Recommendations
The main benefit of using the fluidised bed dryer observed in the experiment was the time
taken to dry the sawdust, to reach a rate of drying = 0.
When comparing the fluidisation dryer and a tray dryer there are a few point principals
which make the fluidisation dryer stand out as the most suited for drying sawdust.
Fluidisation dryers provide an effective means of drying free flowing particles within a
narrow particle size range. In a fluidisation bed dryer a gas passes through the product
perpendicular to the surface of the bed of wet material. The key principal used in a
fluidized bed dryer, is the vertical suspension of the material against gravity with an
upward-flowing air stream. The heat is transferred from the air to the material mainly
through convection. A horizontal air flow may be introduced in a continuous process to
convey the material through the dryer.
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In Tray Dryers the wet material is usually required to be spread out quiet thinly. Heating for
tray dryers is usually by vertical air currents sweeping across the trays by conduction from
heated shelves of by radiation from heated surfaces. Using air as a heat source also
removes moisture. Dry air is brought into the dryer while the wet air is exhausted out,
dehumidisation.
Fluidisation bed dryers are mainly used to powders, with average particle dimeters
between 50 and 5000 mirometers. For finer feeds vibration can be introduced to make the
drying process more successful.
Fluid bed drying offers advantages over other methods, such as tray drying of drying
particulate materials.Particle fluidization after the minimum fluidisation velocity is reached,
gives: easy material transport, high rates of heat exchange at high thermal efficiency while
preventing overheating of individual particles. Using tray dryers requires much longer
drying times, usually in the order of 10 to 20hours.
A series of factors influence dryer design. Some of theses influences include;
- The method of operation, the nature of the production schedule. In large scale production
continuos flow dryers are required. The dryer must be designed to cope with continuous
flow of the material into and out of the dryer. For smaller scaled production batch
operations are usually desired.
- The physical properties of the material is probably one of the most important factors
when considering dryer designs. As the properties of the feed greatly influence the
design of the dryer, dryers with similar feed have similar design characteristics. For
example, dryers with a powdered feed usually can accommodate a thicker layer of feed.
- The physical state of the feed determines the method of conveyance of the material
through the dryer. This is also a key design feature as it can limit the methods of heat
transfer. Dryers which use conduction as a heat transfer method use direct or indirect
contact with a heated medium while direct (adiabatic) dryers use heat supplied via
convection, usually exposing them to a hot gas.
For Black Forest Sawmill a fluidised bed dryer would be suggested over a tray dryer as the
fluidised dryer reduces drying time and ensure the quality of the saw dust after drying, the
risks over overheating each particle is reduced.
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References
Bronlund, B., McCarthy, O., Jones, J., Edwards, J (2011). 280.391 Process Operations
and Modelling Laboratory Manual. Institute of Food, Nutrition and Human Health, Massey
University, Palmerston North.
Goyal, A., (2009). Dryers. Sourced on the 17th March 2011, from:
http://www.scribd.com/doc/29308196/Dryers
IPS (1997), Engineering Standard for Process Design of Dryers. Iranian Ministry of
Petroleum, Iran. Sourced obn the 17th March 2011, from:http://217.174.18.60/ips/pr/e-pr905.pdf
Love, R., Hindmarsh, J.P., Paterson, A.H.J., Edwards, J., Carr, A (2010). Laboratory
Manual. 141.294 Engineering Principles. Institute of Food, Nutrition and Human Health,
Massey University, Palmerston North.
White, S., Jones, J (2010). Mass Transfer and Drying 2010. College of Sciences, Massey
University, Palmerston North.
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