DRYING AND
HEAT TRANSFER
Yusron Sugiarto, STP, MP, MSc
DRYING PROCESSES
 Drying
is perhaps the oldest, most common
and most diverse of chemical engineering unit
operations.
 Over
four hundred types of dryers have been
reported in the literature while over one
hundred distinct types are commonly available.
 Energy
consumption in drying ranges from a
low value of under five percent for the chemical
process industries to thirty five percent for the
papermaking operations.
DRYING PROCESSES
 Objective
- removal of solvents
 Contact solids with fluid not saturated with
solvent
 Economics
 Recover
solvent
 Avoid shipping solvent
 May avoid spoilage
DRYING CURVE
DRYING EQUIPMENT
 CATEGORIES


BATCH OR CONTINUOUS
DRYING MECHANISM
 THERMAL
http://www.grecobrothers.com/centrifugaldryers/K94.jpg
 VACUUM
 FREEZING
 MECHANICAL
http://www.arrowhead-dryers.com/steam-tubedryers.html
VACUUM DRYERS
 SHELF
ROTARY
http://www.mcgillairpressure.com/index.html
CONICAL
SHELF DRYERS


Batch units
Circulate air past trays of fluids





Over (cross-circulation)
And/or perpendicular to (through-circulation)
Can operate under vacuum
Long batch cycles (4 - 48 hours) are common
Primary uses





Plastics
Metals
Chemicals
Pharmaceuticals
Foods
http://www.bocedwards.com/ind
ex.cfm?ProcessVacuum/pharmac
eutical-upgrades.cfm~content
TUNNEL DRYERS





Move material on belt or conveyor through drying zone
Used for a wide range of free-flowing particulates (granular, flake or
fibrous)
Used for pastes and filter cakes with even application to belt
Drying times approximately 5 - 120 minutes
Large capacity is practical with these units
http://www.dryer.com/Columbia%20Flyer%2005%20.pdf
ROTARY DRYERS




Drop solids through counter current flowing hot gases
Can be lined with refractory to allow very high temperature
operation
High volume with wide stable operating range
Residence times typically measured in minutes
http://www.siko.co.id/images/kiln.jpg
DRUM AND WIPED-FILM DRYERS

Drum dryers



Thin film dryers with indirect heating
Slurry applied to drum and dried solid removed (see fig. 9.2-4)
Wiped film dryers



Inverse of drum dryer with internal wiper to apply film to vertical surface
Material leaving dryer must be free flowing
High thermal efficiency
FEED
WIPED FILM DRYER
FINISH DRYER
http://www.atlascoffee.com/imgz/1br/1br07.jpg
FREEZE DRYERS
 USED
FOR BIOLOGICALS
 USE SUBLIMATION
http://www.niroinc.com/html/ch
emical/freezedryers.html
http://www.pharmaceuticaltechnology.com/contractors/process_automa
tion/telstar/telstar1.html
FLASH OR SPRAY DRYERS

Contact flow with concurrent flow of hot
air
 Solids may be entrained
 Solids may fall through air
 May incorporate cyclone
 May incorporate sprayer to produce
slurry droplets
 May be included on tall tower (prilling)
operation
http://www.ocsd.co.jp/english/exampleusage/
index01.html
FLUID BED DRYERS
 Suspend
solids in hot air stream
 Gentle processing – no degradation
 Uniformity of process conditions
 Fed slurry from a centrifuge
 Recover fines with either cylcone, filter or esp and re-slurry
http://www.niroinc.com
/html/drying/fluidbed.h
tml
FLUID BED DRYER EXAMPLE
http://www.barr-rosin.com/english/products/fluid-bed-dryer.htm
Heat Transfer
 Heat always moves from a warmer place to a
cooler place.
 Hot objects in a cooler room will cool to room
temperature.
 Cold objects in a warmer room will heat up to room
temperature.
Question
If a cup of coffee and a red popsickle were
left on the table in this room what would
happen to them? Why?
The cup of coffee will cool until it reaches room
temperature. The popsickle will melt and then
the liquid will warm to room temperature.
Basics to heat transfer
(1) Heat (Q) = a form of energy [ J or Btu ]
(2) Rate of heat transfer (q) = amount of heat (J, Btu)
unit time (s ,hr)
*** (J/s = Watts)
(3) Nature of heat flow
“Net heat flow is always
in the direction of
temperature decrease”
(4) Heat flux
= rate of heat flow per unit area
=
q/A
=
Q
t
X
[J/s m2]
A
(5) Temperature gradient = changes of temperature
with distance, i.e. for x
direction = dT/dx [°C/m]
Heat Transfer Methods
Heat transfers in three ways:
Conduction
Convection
Radiation
Conduction
When you heat a metal strip at one end, the heat
travels to the other end.
As you heat the metal, the particles vibrate, these
vibrations make the adjacent particles vibrate, and so on
and so on, the vibrations are passed along the metal and
so is the heat. We call this? Conduction
Metals are different
The outer e______
lectrons of metal atoms
drift, and are free to move.
When the metal is heated, this
‘sea of electrons’ gain k_____
energy and transfer it
inetic
throughout the metal.
Insulators, such as w___ and
ood
p____, do not
lastic
have this ‘sea of
electrons’ which is why they do not conduct heat as well as
metals.
Why does metal feel colder than wood, if they
are both at the same temperature?
Metal is a conductor, wood is an insulator. Metal
conducts the heat away from your hands. Wood
does not conduct the heat away from your hands as
well as the metal, so the wood feels warmer than
the metal.
* Heat Transfer by Conduction
- If temperature gradient exists in a continuous
substance (solid, fluid and gas), heat can flow without
observable motion of matter.
- Heat flux is oppositely proportional to the
temperature gradient (Fourier’s law)
dq
dT
 k
dA
dx
where,
q
=
A =
T =
k
=
……………… ( I )
rate of heat flow in direction normal to surface
surface area
temperature
x = distance normal to surface
proportionality constant or thermal conductivity
Convection
What happens to the particles in a liquid or a
gas when you heat them?
The particles spread out and
become less dense.
What
A liquid
is afluid
fluid?
or gas.movement.
This effects
Fluid movement
Cooler, more d____,
ense fluids
armer
sink through w_____,
less
dense fluids.
In effect, warmer liquids and gases r___
up.
ise
Cooler liquids and gases s___.
ink
Water movement
Cools at the
surface
Cooler
water sinks
Convection
current
Hot water
rises
Why is it windy at the seaside?
Cold air sinks
Where is the
freezer
compartment
put in a fridge?
It is put at the top,
because cool air sinks,
so it cools the food on
the way down.
Freezer
compartment
It is warmer at the
bottom, so this
warmer air rises
and a convection
current is set up.
* Heat Transfer by Convection
- Flow of heat associated with the movement of
fluid
wall
Cold fluid
q
High
Temperature
- Convective flux  T (Newton’s law of cooling)
q
 h (Ts Tf )
A
……………….. ( II )
where,
Ts = surface temperature
Tf = bulk temperature of fluid, far from surface
h = heat-transfer coefficient
* Heat Transfer by Convection
(next)
**unlike k, h depends not only on thermal properties of fluid but
also flow patterns**
้ อ
convection แบ่งออกเป็ น 2 แบบ ดังนี คื
- Force convection
- Natural convection
 Temperature gradient in fluid
buoyancy forces
flow

The third method of heat transfer
How does heat energy get from
the Sun to the Earth?
?
There are no particles between the
Sun and the Earth so it CANNOT
travel by conduction or by
convection.
RADIATION
Radiation
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False
Absorption experiment
Four containers were placed equidistant from a heater. Which
container would have the warmest water after ten minutes?
Dull metal
Shiny metal
Shiny black
Dull black
dull black container would be the warmest after ten minutes
The __________
shiny metal
because its surface absorbs heat radiation
_______ the best. The _________
container would be the coolest because it is the poorest at
absorbing heat radiation.
__________
*Heat Transfer by Radiation
Transfer of energy through space by
electromagnetic waves
Through empty space, energy not transformed nor
diverted
Through matters, transmitted, reflected, or
absorbed
Absorbed energy is in form of heat
Energy emitted by a black body (absolute temp.)4
W  εσT
b
4
Wb  T
4
……….. (III)
 where,
Wb = rate of radiant energy emission per unit /area
 = Stefan-Boltzmann constant
T = absolute temperature
 = emissivity