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Drying
Reference
Chapter 18 of Separation Process Principles,
3rd Ed., Seader, Henley, Roper, Wiley
Introduction
• Drying is the removal of moisture from solids, slurries, and pastes to give solid products.
• A process of mass-transfer
• Often refers to removing moisture from liquids and gases
• Industrial applications1. Chemical products: crystalline particles of inorganic salts,
organic compounds, films, coatings
2. Biological materials including food
3. Pharmaceuticals
4. Detergents
5. Lumber, paper, fibers
2
Introduction
• Pre-feed dewatering operations by mechanical means such as filtration and centrifugation can diminish the length of drying cycles.
• Common modes of heat transfer1. Convection from a hot gas in contact with the material
2. Conduction from a hot, solid surface in contact with the material
3. Radiation from a hot gas or surface
4. Heat generation within the material (microwave heating)
• Classification1. Batch (less than 500 lb/h) vs. continuous (more than 2000 lb/h)
2. Direct-heat (convection) vs. indirect-heat (conduction)
3. Stationary vs. agitated
3
Introduction
Hot metal
Hot air Hot air
moisture + air
- Indirect heat
- Stationary
- Direct heat
- Stationary
- Direct heat
- Agitated
(a) (b) (c)
Drying effectiveness
Sensitive products(temperature sensitive or dust-formed)
C > B > A
A > B > C4
Equipment: batch dryers
Tray dryers
• Oldest and simplest batch dryer
• Cross-circulation (solid tray bottom) or through-circulation (perforated tray
bottom) for solids with appreciable voids such as granules, noodles and pellets
• Mostly direct heating; Indirect heating is possible with hollow shelves carrying
hot steam under vacuum
• Useful when low production rates of multiple products are involved
5
Equipment: batch dryers
Agitated dryers
• Indirect heating with agitation; perhaps, under vacuum
• Can be used when
(1) material oxidizes or becomes explosive or dusty during drying;
(2) moisture is toxic, flammable, or explosive;
(3) material tends to agglomerate if not agitated;
(4) maximum product temperature is less than about 30 OC. 6
Equipment: continuous dryers
Band-conveyor dryer
• Circulation of heated gases upward and/or downward through a moving,
permeable, layered bed of wet material from 1 to 6 inches deep.
• Multiple sections, each with a fan and set of gas-heating coils, can be arranged
in series to provide a dryer, with a single belt as long as 150 ft with a 6-ft width,
giving drying times up to 2 h, with a belt speed of about 1 ft/minute.
• Granular or pelletized materials with appreciable voids should be used. 7
Equipment: continuous dryers
Direct-heat rotary dryer
• A popular dryer for evaporating water from free-flowing granular, crystalline, and flaked solids
of relatively small size, when breakage of solids can be tolerated
• Wet solids enter through a chute at the high end and dry solids discharge from the low end.
• Cylindrical shell is slightly inclined from the horizontal with a slope of less than 8 cm/m
• Hot gases flow counter currently or co-currently (may use cylinders not inclined) to the solids.
• Longitudinal lifting flights are mounted on the inside of the rotating shell causing the solids to
be lifted, then showered through the hot gas during each cylinder revolution.
• Typically the bulk solids occupy 8–18% of the cylinder volume, with residence times from 5
minutes to 2 h.8
Equipment: continuous dryers
Fluidized-bed dryer• Fluidized-bed dryers have become very popular in
recent years because they: (1) have no moving parts;
(2) provide rapid heat and mass transfer between gas
and particles; (3) provide intensive mixing of the
particles, leading to uniform conditions throughout the
bed; (4) provide ease of control; (5) can be designed for
hazardous solids and a wide range of temperatures,
pressures, residence times, and atmospheres; (6) can
operate on electricity, natural gas, fuel oil, thermal
fluids, steam, hot air, or hot water; (7) can process very
fine and/or low-density particles; and (8) provide very
efficient emissions control.
• Minimum fluidization velocity; where the pressure drop
is equal to the weight of the solids per unit cross-
sectional area of the bed normal to gas flow.
• Materials that are successfully dried in fluidized-bed
dryers include coal, sand, limestone, iron ore, clay
granules, granular fertilizer, granular desiccant, sodium
perborate, polyvinylchloride (PVC), starch, sugar,
coffee, sunflower seeds, and salt.
9
Equipment: continuous dryers
Spray dryer
• When solutions, slurries, or pumpable pastes—
containing more than 50 wt% moisture, at rates greater
than 1,000 lb/h—are to be dried.
• Feed is pumped to the top center of the chamber,
where it is dispersed into droplets or particles from 2 to
2,000 µm by atomizers.
• Hot gas enters the chamber, causing moisture in the
atomized feed to rapidly evaporate. Gas flows co-
currently to the solids.
• Applications: detergents, milk, starch, yeast, zinc
sulfate, lignin, aluminum hydroxide, silica gel,
magnesium chloride, manganese sulfate, urea resin,
sodium sulfide, coffee extract, tanning extract, color
pigments, tea, tomato juice, polymer resins, and
ceramics
10
Equipment: continuous dryers
Drum dryer
• To process solutions, slurries, and pastes with indirect heat
• Applications: milk, detergents, brewer’s yeast, potatoes, skim milk, malted milk,
coffee, tanning extract, and vegetable glue.
11
Psychrometry (humidity chart)
• For air–water vapor mixtures at 1-
atm
• Calculations involving the properties
of moisture–gas mixtures for
application to drying are most
conveniently carried out with
psychrometric charts.
12
Use steam table to find:
- Vapor pressure (PAS)
- Enthalpy of vaporization (ΔHwvap)
Psychrometry
13
Psychrometry
14
Example: Air at 130 °F and 1 atm enters a direct-heat
dryer with a humidity of 0.03 lb H2O (A)/lb H2O-free air
(B). Determine by the psychrometric chart and the
relationships of Table 18.4: (a) relative humidity and (b)
humid volume.
Wet-bulb temperature
• In drying process, solids must be heated to a temperature at which its vapor pressure
exceeds the partial pressure of the moisture in the gas in contact with the wet solid.
• The wet bulb temperature, Tw, can be measured by covering a thermometer bulb with
a wick saturated with the liquid being evaporated and passing a partially saturated gas
past the wick.
• Tw is equal to the adiabatic saturation temperature for air-water system.
• In a typical drying process, the wet solid is heated to Tw.15
Equilibrium moisture content
Isotherm (Figure 18-23)
• At given T and P
• Equilibrium moisture content as a
function of relative humidity
• Provide useful information for direct-
heat drying processes
• Moisture content, X; wt% moisture
on a dry-solid basis
• W; wt% moisture on a wet-solid
basis
16
Equilibrium moisture content
Moisture adsorption is exothermic.
17
Equilibrium moisture content
18
Drying periodspreheating
until T=Tw
constant-rate
first falling-rate
second falling-rate
19
Constant-rate drying period
Drying-rate flux, R
A : mass-transfer area,
mv: mass of moisture evaporated
ms: mass of bone-dry solid
For constant-rate period
• The rate of mass transfer is determined by gas-phase boundary-layer or film
resistance at the wet surface of the solid.
• The wet solid is assumed to be at a uniform temperature, so the only
resistance to convective heat transfer is in the gas phase.
• The rate of moisture evaporation can then be based on convective heat
transfer or mass transfer
• i refers to the gas-solid interface
• Ti = Tw
20
Constant-rate drying period
Drying-rate flux for constant-rate drying period, Rc
Based on solid mass, (not wet-solid mass)
It is more common to use the heat-transfer relation of when air is the gas and water is the
moisture because of the wide availability of the psychrometric chart for that system and the
equality of wet-bulb and adiabatic-saturation temperatures.
a : external SA of solid/mass of solid (A/ms)
ms: mass of solid
Drying rate per solid mass
(Tg=Td)
constant-rate drying period
21
𝑑𝑚𝑣
𝑑𝑡=
ℎ𝑎 𝑇𝑔 − 𝑇𝑤 𝑚𝑠
∆𝐻𝑤𝑣𝑎𝑝
𝑅′ = 𝑅𝐴
𝑚𝑠𝑡𝑐 =
𝑚𝑣
𝑅𝑐𝐴=𝑚𝑠(𝑋 − 𝑋𝑐)
𝑅𝑐𝐴=𝑋 − 𝑋𝑐𝑅𝑐′
Constant rate drying time
Constant-rate drying period
where in (1), G is the mass velocity of air in
the flow channel that passes over the wet
surface. In (2), G is the mass velocity of the
air impinging on the wet surface. In (3) to (8),
dp is the particle diameter and G is the
superficial mass Velocity.
uavg = avg. velocity of gas (m/s)
ρ = density of humid gas (kg/m3)
υ= humid volume (wet gas basis) (m3/kg)(volume of moisture-gas/mass of moisture-gas)
22
𝑣 =𝑣ℎ
1 + ℎ
Constant-rate drying period
23
Constant-rate drying period
24
Falling-rate drying period
- When moisture travels from the interior of a wet solid to the surface, a
moisture profile develops in the wet solid. The profile’s shape depends on the
nature of the moisture movement.
- When the moisture is free moisture in the interstices of particles like soil and
sand, or is moisture above the fiber-saturation point in paper and wood,
moisture movement occurs by capillary action.
- When the internal moisture is bound moisture, as in the last stages of drying
of paper and wood, or soluble moisture, as in soap and gelatin, moisture
migrates to the surface by liquid diffusion.
25
Falling-rate drying period (empirical approach)
- During the falling-rate period, estimation of drying rate using capillary flow and
diffusion theory is often challenging due to non-idealities. Alternatively,
estimates could be made by a strictly empirical approach that ignores the
mechanism of moisture movement, but relies on experimental determination of
drying rate as a function of average moisture content for a particular set of
conditions.
26
For constant-rate period,
Fig. 18-31
General equation for drying time calculation
Falling-rate drying period (empirical approach)
Rate of drying for falling-rate period
Drying time in the falling-rate period
Total drying time
Curve fitting using a parabolic function
Drying time in the falling-rate period
27X must be the free-moisture content!!
Fig. 18-31
(18-43)
(18-44)
Falling-rate drying period (empirical approach)
28
Falling-rate drying period (empirical approach)
29
Falling-rate drying period (liquid diffusion theory)
For slow-drying materials for which the rate of drying is controlled by internal diffusion of moisture
to the exposed surface: during the evaporation of a surface liquid film in a constant-rate drying
period controlled by gas-phase mass transfer, no moisture diffuses to the surface, and after
completion of evaporation of that film, resistance to mass transfer is due to internal diffusion in a
falling-rate period.
Fick’s second law
BCs
X=Xo at t=0 for -a < z < a
X=X* at z=±a for t ≥ 0
unaccomplished free-moisture change
Fourier number of diffusion, NFoMposition ratio
30X is the total moisture content!!
Falling-rate drying period (liquid diffusion theory)
The rate of mass transfer from one surface
The average moisture content
The equation can be used to determine the moisture diffusivity, DAB, from
experimental data, and then that value can be used to estimate drying rates for
other conditions.
When NFoM > 0.1 , only the first term in the infinite series is significant.
31
Falling-rate drying period (liquid diffusion theory)
32
Falling-rate drying period (liquid diffusion theory)
33
Roughly estimate DAB using the simplified eqn,
y = -0.0858x - 0.2135R² = 0.9935
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0 2 4 6 8 10
ln(Eavg) vs time
Falling-rate drying period (liquid diffusion theory)
34
r
For sphere
The average moisture content
When NFoM > 0.1 , only the first term in the infinite series is significant.
1
2
22
22*
0
*
)exp(16
n
ABavg
avgr
tDn
nXX
XXE
2r
tDN AB
FoM
)exp(6
2
2
2*
0
*
r
tD
XX
XXABavg