3.2 drying processes
TRANSCRIPT
CHG 3111
Unit Operation
Chapter 9
Drying
3.2 Drying Processes
CHG 3111/B. Kruczek 2
Methods of Drying:
Batch – small scale production, long drying cycles
Continuous – large scale production, relatively short drying times, uniform product quality, less labor and floor space
Drying Equipment
Types of dryers
Batch process - tray dryers
Continuous process – i) conveyor dryers, ii) rotary dryers, iii) fluidized bed dryers, iv) pneumatic dryers, v) spray dryers, vi) rotary drum dryers
Factors for selecting dryers
Feed conditions: solid, liquid , paste, powder, crystals
Feed concentration, i.e., the initial liquid content
Product specification: dryness required, physical form
Throughput required
Heat sensitivity of the product
Nature of the vapor: toxicity, flammability
Nature of the solid: flammability (dust explosion hazard), toxicity
CHG 3111/B. Kruczek 3
Tray dryers
Drying Equipment
Small quantity of solids, but wide range of materials
Material spread on metal trays or screens
Heated air is passed over and parallel to trays or through the permeable bed
High labor requirement, but close control over the drying conditions
Often used for drying valuable products
CHG 3111/B. Kruczek 4
Vacuum-Shelf Indirect Dryer
Drying Equipment
Special type of tray dryer, very expensive
Indirect heated batch dryer
Operated under vacuum
Material is spread over the trays
Heat is conducted through the metal walls of
the shelf or by radiation from the shelf above.
Useful for temperature sensitive, easily
oxidized, toxic and dusty material.
Applications: pharmaceutical, food and
chemical products.
CHG 3111/B. Kruczek 5
Continuous Drying – Conveyor Dryers
Drying Equipment
Solids are fed onto an endless, perforated conveyor belt through which hot air is
forced.
The belt is housed in a long, rectangular cabinet (tunnel)
The relative movement through the dryer of the solids and the drying air can be
parallel or, more usually, countercurrent
The cabinet (tunnel) can be divided into different zones, for example, air up, air
down, cooler, etc.
CHG 3111/B. Kruczek 6
Continuous Drying – Rotary Dryers
Drying Equipment
Solids are fed into high end of a rotating, inclined cylinder and move downwards,
being directly in contact with has air or gases flowing through the cylinder.
Allow high throughput and have a high thermal efficiency and a relatively low
capital and labor cost
Non uniform residence time of solids (why?), possible dust generation, and high
noise level
CHG 3111/B. Kruczek 7
Continuous Drying – Fluidized Bed Dryers
Drying Equipment
Drying gas is passed through the bed of solids at a velocity sufficient to keep the
bed in a fluidized state.
High heat transfer and drying rates
Suitable for granular and crystalline materials within the particle size range 1 to 3
mm (why?)
Rapid and uniform heat transfer (thus short drying times), low floor area
requirements, but compared to other dryers, high power requirement
CHG 3111/B. Kruczek 8
Continuous Drying – Pneumatic Dryers
Drying Equipment
The product to be dried is dispersed into an
upward-flowing stream of a hot gas and is
conveyed by it to the top of the dryer duct
Short contact times, which limits the size of
the particles to be dried, which must be very
fine (smaller than in Fluidized Bed Dryers)
Suitable for heat sensitive materials, which
must be dried rapidly
Low thermal efficiency
CHG 3111/B. Kruczek 9
Continuous Drying – Spray Dryers
Drying Equipment
Material to be dried must capability to be
handled by pumping, which is limited to
liquids and dilute slurries.
Feed is atomized in a nozzle or by a disc-
type atomized at the top of a vertical,
cylindrical vessel.
Hot air flows up (may also go down) the
vessel, conveys and dries the droplets.
Dried particles are removed in a cyclone
separator or bag filter.
Very short contact times – suitable for heat
sensitive materials
Good control of the product particle size,
bulk density and form (uniform product)
High heating requirements due to using
dilute liquid solutions and slurries
CHG 3111/B. Kruczek 10
Continuous Drying – Rotary Drum Dryers
Drying Equipment
Revolving, internally heated drum or double drums.
Film of solids is deposited on the drum surface by either immersing part of the drum in
a trough of the liquid or by spraying, or splashing the feed on the drum surface
In the case of double drums, the feed is fed to the nip formed between the drums
Handle liquid solutions and dilute slurries , thus an alternative to spray dryers
Not suitable for heat sensitive materials, because of direct contact with a hot surface of
the drum
CHG 3111/B. Kruczek 11
Analysis in batch dryers – general considerations
Batch Dryers
Direct drying
The main objective in the analysis of batch dryers is to determine the drying time
for the specified feed and product conditions
Direct drying: hot gas (typically air) flows over or through the wet solids – in both case the
mode of heat transfer is forced convection
Drying air may be in large excess and the removed moisture does not appreciably change
its properties – constant air condition
In practical dryers, the gas is not in a large excess and its properties change as it absorbs
water vapor from wet solids – variable air temperature
Indirect drying
Heat to wet solids is transferred by conduction and or radiation
Indirect drying may coexist with direct drying
Indirect drying results in “non adiabatic” temperature of solids
CHG 3111/B. Kruczek 12
1. Constant-rate of drying period: R = RC:
2. Falling-rate drying period and
3. Falling-rate drying and
Calculation Method for Drying Period
Most important unknown: time, t, required to dry solids from a known initial moisture
content, X1, to a final content of X2.
Batch Dryers - Constant Air Conditions
SL dXR
A dt
2
1
XS
X
L dxt
A R
1 2S
C
Lt X X
AR
Special cases
1/R
X X2 X1
Regardless of the dependence of R on X, drying time can be evaluated by
graphical (numerical) integration:
R a X b d R a d X
R a X d R a d X
2 21
1 1 2
lnX R
S S S
X R
L L L RdX dRt t
A R A a R A a R
CHG 3111/B. Kruczek 13
Estimation of Drying Rate in Constant-Rate Drying Period (RC)
Process is controlled by the rate of heat transfer to the evaporating surface (latent
heat of evaporation for the liquid).
Heat is transferred to the evaporating surface only by convection
At steady state: rate of mass transfer balances rate of heat transfer.
Batch Dryers - Constant Air Conditions
H2O
( 1 )C Wq R A
( )
WC y A ir W
W W
h T TqR k M H H
A
Heat effect associated with mass transfer
Heat Transfer Mass Transfer
( ) ( 2 )Wq h A T T
Heat effect associated with heat transfer
2
2
w h e r e : a n d A irC H O A A y W
H O
MR M N N k H H
M
Combining Eqs. (1) and (2) leads to:
Prediction of h [W/m2K]
Air flowing parallel to the drying surface:
0 80 0 2 0 4 .. h G 4 5 1 5 0 2 9 3 0 0o o
2
k gfo r : C C ; 2 ,4 5 0 ,
m hT G
Air flowing perpendicular to the surface:
0 3 71 1 7 .. h G 19 5002
k g fo r: 3 ,9 0 0 ,
m hG
Tw is the wet bulb temperature of the gas (T, H)
CHG 3111/B. Kruczek 14
General Case for Estimation of (RC) in Constant-Rate Drying Period
Heat transfer to wet solids not only by convection but also by conduction and radiation
Batch Dryers - Constant Air Conditions
zS
zM
Hot radiating surface (TR)
Metal Tray
Solids
qR radiant heat Gas
T, H, y
Gas
T, H, y
qC convective heat
qk conduction heat
NH2O Drying surface
yS, TS, HS
Surface energy balance on the drying surface and: q = qC + qR + qK
c o n v e c t i v e h e a t t r a n s f e r : C C Sq A h T T
2 2r a d i a t i v e h e a t t r a n s f e r : w h e r e : = R R R S R R S R Sq A h T T h T T T T
1 1
1" c o n d u c t i v e " h e a t t r a n s f e r : w h e r e : = = K K S K
t o t c M M S S
q A U T T UR h z k z k
Question: What is the value of TS compared to TW?
CHG 3111/B. Kruczek 15
General Case for Estimation of (RC) in Constant-Rate Drying Period
At steady state: rate of mass transfer balances rate of heat transfer.
Batch Dryers - Constant Air Conditions
Hot radiating surface
(TR)
Metal Tray
Solids
qR radiant heat Gas
T, H, y
Gas
T, H, y
zS
zM
qC convective heat
qk conduction heat
NH2O Drying
surface
yS, TS, HS
Based on rate equations for heat and mass transfer:
Unlike the case with just convection, TS > Tw, but the above equation must also intersect the
saturated humidity line i.e., TS and HS are on the saturated humidity line:
C K S R R SC y A ir S
S S
h U T T h T TqR k M H H
A
1
S S S SK RS R S
C C C y A ir S
H H H HU hT T T T
h h h k M c
CHG 3111/B. Kruczek 16
Drying in Falling-Rate Period by Diffusion and Capillary Flow
Liquid diffusion of moisture in drying:
Staring from Fick’s 2nd law of diffusion, and assuming
constant liquid diffusivity (DL) in the material of thickness
2l dried from top and bottom, it can be shown
that single term approximate solution (Fo > 0.2) has the
following form:
Batch Drying - Constant Air Conditions
22 2
2 2 2 2
84
4 ln c s s L
L
L
X L L DX X l dXD t R X
t A dtz D X l A
Capillary of movement of moisture in drying
As water is evaporated, capillary forces are set up by the interfacial
surface tension between the water and solid.
These forces provide the driving force for moving the water through the pores to the drying surface
It can be shown that in this case the drying is inversely proportional to the thickness of the
material, which leads to RC = a X.
CHG 3111/B. Kruczek 17
Batch Dryers - Constant Air Conditions
Example 2: Constant air conditions
A wet, granular material is being dried in the constant-rate period in a pan 0.61 m x 0.61 m
and the depth of material is 25.4 mm.. The pan has a metal bottom having a thermal
conductivity of kM = 43.3 W/m.K and a thickness of 1.59 mm. The thermal conductivity of the
solid is kS = 1.125 W/m.k. The air flows parallel to the top exposed surface and the bottom
metal at a velocity of 3.05 m/s. The temperature and the humidity of the drying air are 65.6˚C
and 0.010 kg H2O/kg dry solid, respectively. Direct radiation heat from steam pipes having a
surface temperature of 104.4 ˚C falls on the exposed top surface, whose emissivity is 0.94.
a) Calculate the surface temperature and the drying rate for the constant-rate period.
b) If the pan contains 11.34 kg of dry solid having a free moisture content of 0.35 kg H2O/kg dry
solid, calculate the time required to reduce the moisture content to 0.05 kg H2O/kg dry solid.
c) Repeat your calculations by considering only heat transfer by convection to the drying
surface of the wet material
Please note that the critical moisture content is Xc = 0.20 kg H2O/kg dry solid
CHG 3111/B. Kruczek 18
Batch Dryers - Variable Air Conditions
Drying with constant versus variable air conditions
Up to this point we assumed that the properties of drying gas (air), i.e., T and H, do
not change as the gas flows over wet solids
H2O This is only possible when there
is large excess of drying gas
In the actual dryers, the conditions of the drying gas change as it passes through
the dryer, for example in (a) through circulation packed bed, and (b) try dryer
T1, H1
T2, H2
z
Wet
solids
(a)
b T1, H1 T2, H2
x1
Wet solids
Lt
Wet solids
(b)
CHG 3111/B. Kruczek 19
Batch Dryers - Variable Air Conditions
Analogy with heat exchangers
The previously derived equation for the drying time are no longer applicable because the
driving force for drying varies along the dryer
In in the case of adiabatic dying, the wet solids at the wet bulb temperature of entering air
(Tw) as long as moisture content is greater than Xc – single stream heat exchanger
When the moisture content is less than Xc, the solid temperature will be greater than Tw
and will depend on the position within the dryer– two stream heat exchanger
b T1, H1 T2, H2
x1
Wet solids
Lt
Wet solids
Example: variation of gas temperature in a batch dryer in which solids
are at constant temperature equal to Tw
Tgas
Tw T2
T1
CHG 3111/B. Kruczek 20
Batch Dryers - Variable Air Conditions
Derivation of the expression for the average driving force for drying
Consider through-circulation drying in packed bed – the analysis for tray dryer is the same.
Overall energy balance:
2 1 1 2Based on temperature change of the gas: Eq.(1)S S W Wq GAc T T GAc T T T T
Energy balance in a differential element of thickness dz:
Based on temperature change of the gas: Eq.(3)Sdq GAc dT
B a s e d o n t h e r a t e o f h e a t t r a n s f e r : E q . ( 4 )Wd q h a A d z T T
B a s e d o n h e a t t r a n s f e r f r o m t h e g a s : E q . ( 2 )W L Mq h a Z A T T
where: (T – Tw)LM is the average driving force in the dryer
Equating Eq.(3) and Eq.(4) and rearranging:
2 21
1 1 2 1 2
lnln
z TW
Sz TS W S W W W
T Tha dT ha haZdz Z Gc
Gc T T Gc T T T T T T
Substituting expression for GcS into Eq.(1) and comparing it with Eq.(2):
1 2
1 2
ln
W W
W LMW W
haAZ T T T Tq haZA T T
T T T T
1 2
1 2
ln
W W
W LMLMW W
T T T TT T T
T T T T
dz
H
T+dT H+dH
G, T1, H1
T2, H2
Z
T
0
CHG 3111/B. Kruczek 21
Batch Dryers - Variable Air Conditions
Expressions for drying time in through-circulation packed bed
Constant-rate drying period, i.e., R = Rc when X > Xc
NB: In packed beds:
1 2 an dS
C
t X XaR
( )
WC y A ir W
W
h T TR k M H H
1 2 1 2
( )
W S S
W y A ir W
X X X Xt
ah T T ak M H H
Replacing the local difference with the log-mean-difference, i.e. the
average driving force:
1 2W S
W LM
X Xt
ah T T
2 3w h e r e : m m i s t h e s p e c i f i c s u r f a c e a r e a o f t h e b e d a n d i s t h e d e n s i t y o f d r y s o l i d s i n t h e b e dSa
m a s s o f b e d, T h u s c o r r e s p o n d s to
s u r fa c e a r e a o f b e dS S S SA L L
A L a a a A
1 2
1 2
w here: ln
W W
W LMW W
T T T TT T
T T T T
If T2 required for (T – TW)LM is unknown, it can be evaluated from:
12 1
2
ln expWW W S
S W
T ThaZ T T T T haZ Gc
Gc T T
dz
H
T+dT H+dH
G, T1, H1
T2, H2
Z
T
0
CHG 3111/B. Kruczek 22
Batch Dryers - Variable Air Conditions
Expressions for drying time in through-circulation packed bed
Falling-rate drying period (X < Xc) and R is directly proportional to X
If solids temperature were at Tw, which is not the case (why?):
ln ln
( )
W S C C S C C
W y A ir W LMLM
X X X X X Xt
ah T T ak M H H
Prediction of heat transfer coefficient in packed beds for adiabatic
evaporation 0 59
0 410 151 350
.
R e.. fo r: t P t
P
G D Gh N
D
0 49
0 510 214 350
.
R e.. fo r t P t
P
G D Gh N
D
2
2
Wwhere : is in while in ,
m K
kg kg in , and in
m hh m
p
t
h D m
G
Specific surface area of the bed
6 1F o r sp h e rica l p a rtic le s : w h e re : is th e v o id fra ctio n in th e b e d
P
aD
4 1 0 5.F o r cy lin d rica l p a rtic le s o f d ia m e te r a n d le n g th :
C Cc c
C C
H DD H a
H D
NB: Equivalent diameter (Dp) is the diameter of a sphere having the same surface area as the particles
dz
H
T+dT H+dH
G, T1, H1
T2, H2
Z
T
0
What would the actual t compared to the value
calculated assuming that solids are at Tw?
CHG 3111/B. Kruczek 23
Batch Dryers - Variable Air Conditions
Expressions for drying time in tray dryer
Schematic representation of tray dryer
1 lnS W C C
W LM
x X X Xt
h T T
Variation of temperature and humidity of air in the dryer
Drying time in constant-rate period:
b G, T1, H1 G, T2, H2
x1
Wet solids
Lt
Wet solids
z 0
Adiabatic drying so that the temperature of solids is
constant and equal to wet bulb temperature of air (Tw)
Using the same analysis as for the through circulation bed dryer it can be shown that:
1
1 2 1ln e x p , th u s : e x p( )
WW W S W W t S
S W
T ThzT T T T hz G c b T T T T hL G c b
G c b T T
1 1S W C
W LM
x X Xt
h T T
11 S S
S
L Axx
A A
1 2
1 2
ln
W W
W LMW W
T T T TT T
T T T T
Drying time in falling-rate period*:
NB: In tray dryers: Similarly to flow-through packed beds, the expression
for t in the falling-rate period underestimates the actual t
CHG 3111/B. Kruczek 24
Example 3: Variable air conditions
Spherical wet catalyst pellets having a diameter of 12.7 mm are being dried in a through-
circulation dryer. The pellets are in a bed of 63.5 mm thick on a screen. The solids are being
dried by air entering with a superficial velocity of 0.914 m/s at 82.2˚C and having a humidity
H = 0.01 kg H2O/kg dry air. The dry solid density is determined as 1522 kg/m3, and the bulk
density of the dry solid in the bed is 982.3 kg/m3. The initial free moisture content is 0.90 kg
H2O/kg solid and the solids are to be dried to a free moisture content of 0.45, which is above
the critical free moisture content.
Calculate the time of drying.
Batch Dryers - Variable Air Conditions
CHG 3111/B. Kruczek 25
Continuous Dryers
Comparison of continuous and batch dryers
Tray Dryer (batch drying)
Wet material is loaded uniformly on removable trays; loading is done outside an oven.
Once the trays are loaded and placed inside the oven, the drying is initiated by circulating hot, dry air over the trays for a specific time.
Mass flow rate of drying air and its properties determine the drying time for a given design of trays
Continuous Tunnel Dryer
Trays or trucks containing the material move continuously through a tunnel of hot gasses.
Mass flow rate of dry air, its properties, loading rate of wet material, velocity of the belt for a given length of the tunnel determine the dryness of the product
Analysis of continuous dryers:
1. Overall mass and energy balances
2. Design of dryer and operation parameters
CHG 3111/B. Kruczek 26
Continuous Dryers Overall Mass and Energy Balance – dryer is treated as “black box”
Drying gas can be in parallel, counter, cross, or any combination of these three flows
relative to the solids flow.
Typically, counter-flow configuration is assumed along with constant solid
temperature equal to wet bulb temperature of drying gas TW
Material balance on moisture:
GH2 + Ls X1 = GH1 + LSX2
Enthalpy of the gas:
H'G = cs (TG – To) + H λo [kJ/kg dry air]
Enthalpy of wet solids:
H'S = cps (TS – To) + XcpH2O (TS – To) [kJ/kg dry solid] (enthalpy of solid + enthalpy of free moisture)
Overall heat balance on dryer:
GH'G2 + LsH's1 = GH'G1 + LsH'S2 + QLoss
Dryer Solid
LS, TS1, X1 TS2, X2
Gas
G, TG2, H2 TG1, H1
Q
(Q is heat loss in the dryer (kJ/h). Adiabatic process, Q = 0,
If heat added to the system, Q is negative.)
G: kg dry air/h
Ls: kg dry solid/h
H: kg H2O/kg dry air
X: kg H2O/kg dry solid
cPS: kJ/kg dry solid K
cPH2O: kJ/kg H2O K
cS = 1.005 + 1.88 H [kJ/kg dry air ˚C]
To = 0 ˚C
Λo = 2501 kJ/kg H2O
CHG 3111/B. Kruczek 27
Continuous Dryers Heat Recovery by Air Recirculation
Air recirculation helps to reduce steam consumption in dryers and to control the
conditions at which drying occurs.
Dryer Heater
Q
Dry Solid
Ls, TS2, X2
Wet Solid
Ls, TS1, X1
Recirculated air, G6, TG2, H6
(6)
Fresh Air
(1)
G1, TG1, H1
TG4
(4)
TG2, H2
(2)
Moist Air, H5
(5) (3)
TG3, H3
H4=H3
Water balance on heater: G1H1 + G6H6 = (G1 + G6)H4 (H6 = H5 = H2)
Water balance on dryer: (G1 + G6)H4 + LSX1 = (G1 +G6)H2 + LSX2
Heat balance on heater: G1H'G1 + G6H'G6 + Q = (G1 + G6)H'G4
Heat balance on dryer: (G1 + G6)H'G4 + H'S1LS = (G1 +G6)H'G2 + LSH'S2
CHG 3111/B. Kruczek 28
Continuous Dryers Design of Continuous Dryers
Analysis of temperature profile in counter-current flow dryer
Dryer Solid
LS, TS1, X1 TS2, X2
Gas
G, TG2, H2 TG1, H1
Preheat Zone: Solids heats up to wet bulb temperature, little evaporation occurs.
Zone I:
Surface moisture evaporates, temperature of solid stays constant, rate of drying stays constant,
gas temperature decreases where as gas humidity increase, solid moisture content reaches the
critical value of XC.
Zone II:
Solid dries to a final value of X2, gas temperature further decreases, gas humidity increases to HC.
NB: The outlet temperature of the drying gas (TG1) must be greater than the inlet temperature of wet solids,
which could be less than Tw. In practice TG1 should be several degrees greater than Tw. Why?
CHG 3111/B. Kruczek 29
Continuous Dryers Categories of Design Analysis in Continuous Dryers
1. Determination of the length of dryer in zone I
Given: (i) desired rate of processing of the wet
solids, including the inlet and outlet moisture
content, (ii) packing of the wet material (i.e., Ls/A
or equivalent), (iii) the mass flow rate of the drying
gas and its properties including T and H, (iv)
velocity at which wet solids move the dryer.
Assumptions: (i) adiabatic drying, constant heat
and mass transfer coefficients
Analysis: (i) Overall mass and energy balances to determine inlet and outlet conditions of
drying in each zone, (ii) determination of the heat transfer or mass transfer coefficient (iii)
determination of the residence time in each zone using the equation developed for batch drying
with variable dying gas condition
Bottom line: The inlet and outlet conditions of solids and air must be known, or they can be
easily calculated
Question: How do we evaluate the required residence time in the preheat zone?
Answer: Neglecting evaporation of water, the analysis in the preheat zone become identical to the
analysis of a two-stream heat exchanger.
CHG 3111/B. Kruczek 30
Continuous Dryers Categories of Design Analysis in Continuous Dryers
2. Determination of the outlet conditions of wet solids and heating air when drying in zone I
Given: (i) Dryer parameters such as the total length,
surface area for dying, the height of the gas channel,
(ii) flow rate of wet solids and the inlet moisture content,
(iii) the mass flow rate of the drying gas and its
properties including T and H, (iv) velocity at which wet
solids move the dryer, or the total residence time.
Assumptions: (i) adiabatic drying, constant transfer
coefficient
Analysis:
Starting from energy balance on differential element of wet solids of length dz it can
be shown that the moisture content X2 at the outlet of the dryer (z = L) will be:
2 1T h e o u t le t a ir te m p e r a tu r e r e q u ir e d fo r is d e te r m ie d fr o m : e x pW W W t SLMT T T T T T h L G c b
2 1 2 1
( ) substituting for the average , i.e. in term s of ( ) :c W W LM
C W LM
S pW W S pW W
R WL h T T AX X R T T X X
L c T L c T
CHG 3111/B. Kruczek 31
Drying-Fundamentals
Example 4: Air recirculation in a continuous dryer
The wet feed material to a continuous dryer contains 50 wt% water on a wet basis and is dried to
27 wt% by countercurrent air flow. The dried product leaves at the rate of 907.2 kg/h. Fresh air to
the system is at 25.6oC and has humidity of H = 0.007 kg H2O/kg dry air. The moist air leaves the
dryer at 37.8oC and H = 0.020 kg H2O/kg dry air and part of it is recirculated and mixed with the
fresh air before entering a heater. The heated mixed air enter the dryer at 65.6oC and H = 0.010.
The solid enters at 26.7oC and leaves at the same temperature. Calculate:
a) The fresh air flow,
b) The percent of air leaving the dryer that is recycled
c) The heat added in the heater
d) The heat loss from the dryer