D R Y I N G

DRYING generally refers to the removal of small amounts of volatile liquid, usually water, from solids by evaporation into gas stream.

TERMS/DEFINITIONS

MOISTURE CONTENT– the amount of water in solid; usually expressed (on a dry basis) as kg water per kg of moisture-free (bone-dry) solid

EQUILIBRIUM MOISTURE CONTENT (X*) – the definite amount of moisture attained (or remaining) in a solid after long exposure to an excess amount of air after which no further drying (or removal of water) will take place.

•X* depends on humidity and temperature of the air the solid is in contact with

BOUND WATER – the moisture content of a solid material that is in equilibrium with air having 100% RH (as indicated in Fig. 9.4-1)

• Bound water exerts a vapor pressure less than that of liquid water at the same temperature.

•Substances containing bound water are often called hygroscopic materials.

UNBOUND WATER. If a material contains more water than indicated by intersection with the 100%RH-line, the water in excess is called unbound moisture, which can exert pressure as high as the its vapor pressure at the same temperature.

FREE MOISTURE CONTENT (X) – is the moisture content of a solid material above the equilibrium moisture content (X*)

TOTAL MOISTURE CONTENT (Xt) – total amount of moisture in the solid (bound plus unbound water or free moisture plus equilibrium moisture content)

Note: bound water is not equal to equilibrium moisture in the same manner as free moisture is not equal to unbound water

soliddrykg

OHtotalkg

W

WWX

s

st

2

*XXX t

Rate of Drying for Constant-Drying Conditions

dt

dX

A

LR s

Ls = weight of dry solid

A = area of exposed surface

t = time

X

t

Typical drying curve for constant drying conditions

Fig 9.5-1a

R

XXc

FRP CRP

CRP – Constant-rate period

FRP – Falling-rate period

Rate of drying curve versus free moisture content

Fig 9.5-1b

Drying at CONSTANT-RATE PERIOD

dt

dX

A

LR s

1

20

X

X

st

R

dX

A

Ldt

21 XXAR

Lt s

Since R = Rc and X2 = Xc

cc

s XXAR

Lt 1

The equations for predicting constant-rate drying are given: 9.6-7 to 9.6-11

Drying at FALLING-RATE PERIOD

1

20

X

X

st

R

dX

A

Ldtt Can be solved by graphical

integration

1/R

X

SPECIAL CASES OF FALLING-RATE REGION

1. Rate is a linear function of X: R = aX +b

adXdR

2

1

R

R

s

R

dR

aA

Lt

21

21

XX

RRa

2

1

21

21 lnR

R

RR

XX

A

Lt s

2

1lnR

R

aA

Lt s

2. Rate is a linear function of X thru origin: R = aX

adXdR

2

1

R

R

s

R

dR

aA

Lt

2

1lnR

R

aA

Lt s

c

c

X

R

X

R

XX

RRa

1

1

21

21

2

lnR

R

AR

XLt c

c

cs

2

lnX

X

AR

XLt c

c

cs

Total time for drying:

FRPCRPtotal ttt