Drying and heat transfer
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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 .

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DRYING AND HEAT TRANSFER

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Drying and heat transfer

DRYING AND HEAT TRANSFER

Yusron Sugiarto, STP, MP, MSc


Drying processes

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 processes1

DRYING PROCESSES

  • Objective - removal of solvents

  • Contact solids with fluid not saturated with solvent

  • Economics

    • Recover solvent

    • Avoid shipping solvent

    • May avoid spoilage


Drying and heat transfer

DRYING CURVE


Drying equipment

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

VACUUM DRYERS

  • SHELFROTARY CONICAL

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


Shelf dryers

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

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

ROTARY DRYERS

  • Drop solids through countercurrent 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 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

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


Freeze dryers

FREEZE DRYERS

  • USED FOR BIOLOGICALS

  • USE SUBLIMATION

http://www.niroinc.com/html/chemical/freezedryers.html

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


Flash or spray dryers

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

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

FLUID BED DRYER EXAMPLE

http://www.barr-rosin.com/english/products/fluid-bed-dryer.htm


Heat transfer

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

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.


Drying and heat transfer

Basics to heat transfer

  • 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”


Drying and heat transfer

(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 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.

Conduction

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

Metals are different

The outer e______ of metal atoms drift, and are free to move.

lectrons

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

inetic

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.

ood

lastic


Why does metal feel colder than wood if they are both at the same temperature

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.


Drying and heat transfer

……………… ( I )

* 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)

    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


Convection

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

Fluid movement

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

ense

armer

In effect, warmer liquids and gases r___ up.

ise

Cooler liquids and gases s___.

ink


Water movement

Water movement

Cools at the surface

Convection current

Hot water rises

Cooler water sinks


Why is it windy at the seaside

Why is it windy at the seaside?


Cold air sinks

Cold air sinks

Where is the freezer compartment put in a fridge?

Freezer compartment

It is warmer at the bottom, so this warmer air rises and a convection current is set up.

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


Drying and heat transfer

wall

Cold fluid

q

High

Temperature

* Heat Transfer by Convection

  • - Flow of heat associated with the movement of fluid

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

  • where,

  • Ts= surface temperature

  • Tf= bulk temperature of fluid, far from surface

  • h= heat-transfer coefficient

……………….. ( II )


Drying and heat transfer

* Heat Transfer by Convection (next)

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

convection แบ่งออกเป็น 2 แบบ ดังนี้คือ

- Force convection

- Natural convection

  • Temperature gradient in fluid 

    buoyancy forces flow


The third method of heat transfer

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

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

Dull metal

Shiny black

Shiny metal

Dull black

Absorption experiment

Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?

dull black

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.

radiation

shiny metal

absorbing


Drying and heat transfer

*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

  • where,

    Wb= rate of radiant energy emission per unit /area

    = Stefan-Boltzmann constant

    T= absolute temperature  = emissivity

……….. (III)


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