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 VACUUM FREEZING MECHANICAL

http://www.grecobrothers.com/centrifugal-dryers/K94.jpg

http://www.arrowhead-dryers.com/steam-tube-dryers.html

VACUUM DRYERS

SHELF ROTARY CONICAL

http://www.mcgillairpressure.com/index.html

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/index.cfm?ProcessVacuum/pharmaceutical-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

WIPED FILM DRYER

FINISH DRYER

FEED

http://www.atlascoffee.com/imgz/1br/1br07.jpg

FREEZE DRYERS

USED FOR BIOLOGICALS

USE SUBLIMATION

http://www.pharmaceutical-technology.com/contractors/process_automation/telstar/telstar1.html

http://www.niroinc.com/html/chemical/freezedryers.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.oc-sd.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.html

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 [J/s m2]

t X 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______ of metal atoms drift, and are free to move.

When the metal is heated, this ‘sea of electrons’ gain k_____ energy and transfer it throughout the metal.

Insulators, such as w___ and p____, do not have this ‘sea of electrons’ which is why they do not conduct heat as well as metals.

lectrons

inetic

ood lastic

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.

- 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)

where,

q = rate of heat flow in direction normal to surface

A = surface area

T = temperature x = distance normal to surface

k = proportionality constant or thermal conductivity

* Heat Transfer by Conduction

……………… ( I )dx

dTk

dAdq

Convection

What happens to the particles in a liquid or a gas when you heat them?

The particles spread out and become less dense.

This effects fluid movement.What is a fluid?A liquid or gas.

Fluid movement

Cooler, more d____, fluids sink through w_____, less dense fluids.

In effect, warmer liquids and gases r___ up.

Cooler liquids and gases s___.

ensearmer

ise

ink

Water movement

Hot water rises

Cooler water sinks

Convection current

Cools at the surface

Why is it windy at the seaside?

Cold air sinks

Where is the freezer

compartment put in a fridge?

Freezer compartment

It is put at the top, because cool air sinks, so it cools the food on the

way down.

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

- Convective flux T (Newton’s law of cooling)

where,Ts = surface temperatureTf = bulk temperature of fluid, far from surfaceh = heat-transfer coefficient

)( fs TThAq

……………….. ( II )

Cold fluidwall

qHigh

Temperature

**unlike k, h depends not only on thermal properties of fluid but also flow patterns**

convection แบ่�งออกเป็น 2 แบ่บ่ ดั งน��คื�อ- Force convection- Natural convection

Temperature gradient in fluid

buoyancy forces flow

* Heat Transfer by Convection (next)

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?

The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation.

dull black

radiation shiny metal

absorbing

Shiny metal

Dull metal

Dull black

Shiny black

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 heatEnergy emitted by a black body (absolute temp.)4

where,Wb = rate of radiant energy emission per unit /area = Stefan-Boltzmann constantT = absolute temperature = emissivity

*Heat Transfer by Radiation

4TεσWb ……….. (III)

4TWb