lesson about drying

7/24/2019 Lesson about Drying

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DRYING

October 28, 2015



Drying

Transfer of liquid from a wet solid into an

unsaturated gas phase



Moisture Movement

Two types:

Capillary Mechanism

Diffusion Mechanism or Molecular Diffusion

Granular or crystalline Amorphous, fibrous, or

gel-like

Crushed rock Eggs

Titanium dioxide Detergents

Zinc sulfate monohydrate Starch

Sodium phosphates Soybean extract



Capillary Mechanism

For beds of particulate solids or crystalline

solids

Movement of liquid within the solid

Results from a net force arising from differences

in hydrostatic head and in surface tension effects

Surface tension causes pressure under a curved liquid

surface to be different from that of a flat surface. For a

sphere of radius r,

r P

2



Molecular Diffusion

For relatively homogeneous solids such as

fibrous organics, gel-like substances or porous

cakes

Fick’s Law:

Simplest case: constant DL*Drying would occur from one face of a slab, with

the bottom and sides which are insulated

2

2

*

dx

X d

dt

X d L



General Drying Behavior

Typical drying curve for constant drying

conditions, moisture content as a function of

time

Time, t

M o i s t u r e c o n t e n t , X ’




Typical drying rate curve for constant drying

conditions, drying rate as a function of

moisture content

Moisture content, X’

D r y i n g

r a t e ,

R

A

B

C

D

E




Typical drying rate curve for constant drying

conditions, drying rate as a function of time

Time, t

D r y i n g

r a t e ,

R

A

B

C

D

E




Unsteady-state period

(A-B)

Solid temperature

adjusts until it reachesa steady-state

In some cases, the

extent of drying in this

period could beignored


D

r y i n g r a t e ,

R

A

BC

D

E




Constant Rate Period(B-C)

T = Twet-bulb

Rate of liquid diffusionfrom within the solid tothe external surface =rate of evaporationfrom surface External surface is

completely covered witha thin film of liquid, andmaintained throughoutthe period


D r y i n g r a t e ,

R

A

B

C

D

E




Constant Rate Period

(B-C)

Rate does not vary

with moisture contentDepends on solid

structure of solid

capillary:

Diffusion: shortconstant-rate period



R

A

B

C

D

E





(B-C)

Ends at the critical

moisture contentDiffusion of liquid to

the external surface

becomes insufficient to

replace the liquidbeing evaporated

′ = critical moisture

contentMoisture content, X’


R

A

B

C

D

E




First Falling Rate

Period (C-D)

Liquid on surface

starts todeplete



R

A

B

C

D

E

′ = second critical

moisture content




Second Falling Rate Period

(D-E)

No more liquid is present on

the surface

Drying occurs from within thesolid



R

A

B

C

D

E

′ = equilibrium

moisture content




At the end of drying, when R=0, any moisture

content in the solid is defined as the

equilibrium moisture content, XE’

Some materials can be completely dried such thatXE’=0

For others, the solid will contain significant

moisture content no matter how long the drying is

continued



Calculation of Drying Time

Drying rate:

where

R: drying rate, lb liquid evaporated per hr-ft2 ofsolid surface

WS: mass of dry solid, lb ′: bulk moisture content of solid, lb liquid/ lb dry

solid

dt

X d

A

W R

S



Calculation of Drying Time

Drying time:

where

1′ : initial moisture content (time = 0)

2′ : final moisture content

2

1

X

X

S

R

X d

A

W t




Constant rate (Rc) depends upon the heat andmass transfer coefficients from the dryingmedium to the solid surface

In most cases, heat is transferred primarily byconvection

iV

V

aV iY c

T T h

M Y Y k R




For air drying, experimental results give the ff.

correlation:

where

hV : Btu/hr-ft2-oF

GV : gas mass velocity, lb/hr-ft

2

8.00128.0

V V

Gh




Total time for constant rate period is

1

X X

AR

W t

C

C

S

C



Critical Moisture Content

Depends on the pore structure of the solids

Has weak dependence on drying rate



Falling Rate Period

In many cases, the drying curve during the

falling rate period approaches a line from ′

to ′

Thus, the rate at any time during the fallingrate period is:

C

E C

E

E C

C

E

R X X

X X R

X X

R

X X

R



Falling Rate Period

Total time to reach moisture content 2′ during

the falling rate period is

When the equilibrium moisture content is

negligible,

E C

E

C

E C S

C

X

X E C

E C S t

t

X X

X X

AR

X X W t t

X X

X d

AR

X X W dt

C C

2ln

2

C C

C S

C

X

X

AR

X W t t

2

ln

C X



Equilibrium

Moisture Content

Depends on:

Structure of solid

Temperature of gas

Moisture content of gas



Solid Types

Crystalline Amorphous

ExamplesInorganic solids, particulate

beds

Organic solids, fibrous

materials, gel-like substances

Solid structure Unaffected by moistureStructure affected by moisture

removal

Initial moisture

content

Moisture found in the

interstices between particles, or

in open pores

Moisture is an integral part of

the solid structure, trapped

within fibers or fine pores

Liquid diffusion

Unhindered, primarily by

capillary mechanism

Slow, primarily by molecular

diffusion

Drying curve

characteristic

Bulk of drying is under

Constant Rate Period (CRP)

Very short CRP, high XC’, bulk

of drying is under 2nd Falling

Rate Period

Equilibrium moisture

content Nearly zero Significant



Types of Moisture

Unbound moisture Moisture in excess of the

equilibrium moisturecontent corresponding tosaturation humidity

Bound moisture

Liquid which exerts avapor pressure less thanthat of the pure liquid

Free moisture content Liquid which is removable

at a given temperatureand humidity; may includeboth bound and unbound

moisture



Example 18.1of Foust

Raw cotton of 0.7 g/cm3 density when dry is to bedried in a batch tray drier from a moisture contentof 1 g H2O/g dry solids to 0.1 g H2O/g dry solids.Trays are 60 cm square and 1 cm thick and are

arranged so that drying occurs from the topsurface only with the bottom surface insulated. Air at 750C with a 500C wet-bulb temperature (Y =0.122 mol H2O/mol dry air, %sat = 20%) circulatesacross the pan surface at a mass flowrate of 2500

kg/hr-m2

. Previous experience under similar drying conditions indicates that the criticalmoisture content will be 0.4 g H2O/g of dry solids,and that the drying rate during the falling rateperiod will be proportional to the free moisture

content. Determine the drying time required.



Drying Equipment

Tray Dryers

Conveyor and Tunnel Dryers

Particulate Dryers

Slurry and Paste Dryers

Spray Dryers



Tray Dryers

Simplest type

For batch drying – small-capacity operation

Suitable for laboratory operations

Trays may be solid-bottomed pans or they

may have a screen base



Conveyor and Tunnel Dryers

Material in the form of sheets

Conveyors to make it continuous



Particulate Dryers

For free-flowing particulate material

Rotary Dryer

Plate Dryer

Flash Dryer



Slurry and Paste Dryers

Rotary VacuumDryer

Usually used forbatch drying

Drum Dryer

Usually used forbatch drying



Spray Dryers

To produce fine powders for dilute solutions,

slurries, gel, or emulsions

lesson about drying

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