3.2 Drying Processes

CHG 3111

Unit Operation

Chapter 9

Drying

3.2 Drying Processes

CHG 3111/B. Kruczek 2

Methods of Drying:

Batch – small scale production, long drying cycles

Continuous – large scale production, relatively short drying times, uniform product quality, less labor and floor space

Drying Equipment

Types of dryers

Batch process - tray dryers

Continuous process – i) conveyor dryers, ii) rotary dryers, iii) fluidized bed dryers, iv) pneumatic dryers, v) spray dryers, vi) rotary drum dryers

Factors for selecting dryers

Feed conditions: solid, liquid , paste, powder, crystals

Feed concentration, i.e., the initial liquid content

Product specification: dryness required, physical form

Throughput required

Heat sensitivity of the product

Nature of the vapor: toxicity, flammability

Nature of the solid: flammability (dust explosion hazard), toxicity


Tray dryers

Drying Equipment

Small quantity of solids, but wide range of materials

Material spread on metal trays or screens

Heated air is passed over and parallel to trays or through the permeable bed

High labor requirement, but close control over the drying conditions

Often used for drying valuable products


Vacuum-Shelf Indirect Dryer

Drying Equipment

Special type of tray dryer, very expensive

Indirect heated batch dryer

Operated under vacuum

Material is spread over the trays

Heat is conducted through the metal walls of

the shelf or by radiation from the shelf above.

Useful for temperature sensitive, easily

oxidized, toxic and dusty material.

Applications: pharmaceutical, food and

chemical products.


Continuous Drying – Conveyor Dryers

Drying Equipment

Solids are fed onto an endless, perforated conveyor belt through which hot air is

forced.

The belt is housed in a long, rectangular cabinet (tunnel)

The relative movement through the dryer of the solids and the drying air can be

parallel or, more usually, countercurrent

The cabinet (tunnel) can be divided into different zones, for example, air up, air

down, cooler, etc.


Continuous Drying – Rotary Dryers

Drying Equipment

Solids are fed into high end of a rotating, inclined cylinder and move downwards,

being directly in contact with has air or gases flowing through the cylinder.

Allow high throughput and have a high thermal efficiency and a relatively low

capital and labor cost

Non uniform residence time of solids (why?), possible dust generation, and high

noise level


Continuous Drying – Fluidized Bed Dryers

Drying Equipment

Drying gas is passed through the bed of solids at a velocity sufficient to keep the

bed in a fluidized state.

High heat transfer and drying rates

Suitable for granular and crystalline materials within the particle size range 1 to 3

mm (why?)

Rapid and uniform heat transfer (thus short drying times), low floor area

requirements, but compared to other dryers, high power requirement


Continuous Drying – Pneumatic Dryers

Drying Equipment

The product to be dried is dispersed into an

upward-flowing stream of a hot gas and is

conveyed by it to the top of the dryer duct

Short contact times, which limits the size of

the particles to be dried, which must be very

fine (smaller than in Fluidized Bed Dryers)

Suitable for heat sensitive materials, which

must be dried rapidly

Low thermal efficiency


Continuous Drying – Spray Dryers

Drying Equipment

Material to be dried must capability to be

handled by pumping, which is limited to

liquids and dilute slurries.

Feed is atomized in a nozzle or by a disc-

type atomized at the top of a vertical,

cylindrical vessel.

Hot air flows up (may also go down) the

vessel, conveys and dries the droplets.

Dried particles are removed in a cyclone

separator or bag filter.

Very short contact times – suitable for heat

sensitive materials

Good control of the product particle size,

bulk density and form (uniform product)

High heating requirements due to using

dilute liquid solutions and slurries


Continuous Drying – Rotary Drum Dryers

Drying Equipment

Revolving, internally heated drum or double drums.

Film of solids is deposited on the drum surface by either immersing part of the drum in

a trough of the liquid or by spraying, or splashing the feed on the drum surface

In the case of double drums, the feed is fed to the nip formed between the drums

Handle liquid solutions and dilute slurries , thus an alternative to spray dryers

Not suitable for heat sensitive materials, because of direct contact with a hot surface of

the drum


Analysis in batch dryers – general considerations

Batch Dryers

Direct drying

The main objective in the analysis of batch dryers is to determine the drying time

for the specified feed and product conditions

Direct drying: hot gas (typically air) flows over or through the wet solids – in both case the

mode of heat transfer is forced convection

Drying air may be in large excess and the removed moisture does not appreciably change

its properties – constant air condition

In practical dryers, the gas is not in a large excess and its properties change as it absorbs

water vapor from wet solids – variable air temperature

Indirect drying

Heat to wet solids is transferred by conduction and or radiation

Indirect drying may coexist with direct drying

Indirect drying results in “non adiabatic” temperature of solids


1. Constant-rate of drying period: R = RC:

2. Falling-rate drying period and

3. Falling-rate drying and

Calculation Method for Drying Period

Most important unknown: time, t, required to dry solids from a known initial moisture

content, X1, to a final content of X2.

Batch Dryers - Constant Air Conditions

SL dXR

A dt

2

1

XS

X

L dxt

A R

1 2S

C

Lt X X

AR

Special cases

1/R

X X2 X1

Regardless of the dependence of R on X, drying time can be evaluated by

graphical (numerical) integration:

R a X b d R a d X

R a X d R a d X

2 21

1 1 2

lnX R

S S S

X R

L L L RdX dRt t

A R A a R A a R


Estimation of Drying Rate in Constant-Rate Drying Period (RC)

Process is controlled by the rate of heat transfer to the evaporating surface (latent

heat of evaporation for the liquid).

Heat is transferred to the evaporating surface only by convection

At steady state: rate of mass transfer balances rate of heat transfer.


H2O

( 1 )C Wq R A

( )

WC y A ir W

W W

h T TqR k M H H

A

Heat effect associated with mass transfer

Heat Transfer Mass Transfer

( ) ( 2 )Wq h A T T

Heat effect associated with heat transfer

2

2

w h e r e : a n d A irC H O A A y W

H O

MR M N N k H H

M

Combining Eqs. (1) and (2) leads to:

Prediction of h [W/m2K]

Air flowing parallel to the drying surface:

0 80 0 2 0 4 .. h G 4 5 1 5 0 2 9 3 0 0o o

2

k gfo r : C C ; 2 ,4 5 0 ,

m hT G

Air flowing perpendicular to the surface:

0 3 71 1 7 .. h G 19 5002

k g fo r: 3 ,9 0 0 ,

m hG

Tw is the wet bulb temperature of the gas (T, H)


General Case for Estimation of (RC) in Constant-Rate Drying Period

Heat transfer to wet solids not only by convection but also by conduction and radiation


zS

zM

Hot radiating surface (TR)

Metal Tray

Solids

qR radiant heat Gas

T, H, y

Gas

T, H, y

qC convective heat

qk conduction heat

NH2O Drying surface

yS, TS, HS

Surface energy balance on the drying surface and: q = qC + qR + qK

c o n v e c t i v e h e a t t r a n s f e r : C C Sq A h T T

2 2r a d i a t i v e h e a t t r a n s f e r : w h e r e : = R R R S R R S R Sq A h T T h T T T T

1 1

1" c o n d u c t i v e " h e a t t r a n s f e r : w h e r e : = = K K S K

t o t c M M S S

q A U T T UR h z k z k

Question: What is the value of TS compared to TW?


General Case for Estimation of (RC) in Constant-Rate Drying Period

At steady state: rate of mass transfer balances rate of heat transfer.


Hot radiating surface

(TR)

Metal Tray

Solids

qR radiant heat Gas

T, H, y

Gas

T, H, y

zS

zM

qC convective heat

qk conduction heat

NH2O Drying

surface

yS, TS, HS

Based on rate equations for heat and mass transfer:

Unlike the case with just convection, TS > Tw, but the above equation must also intersect the

saturated humidity line i.e., TS and HS are on the saturated humidity line:

C K S R R SC y A ir S

S S

h U T T h T TqR k M H H

A

1

S S S SK RS R S

C C C y A ir S

H H H HU hT T T T

h h h k M c


Drying in Falling-Rate Period by Diffusion and Capillary Flow

Liquid diffusion of moisture in drying:

Staring from Fick’s 2nd law of diffusion, and assuming

constant liquid diffusivity (DL) in the material of thickness

2l dried from top and bottom, it can be shown

that single term approximate solution (Fo > 0.2) has the

following form:

Batch Drying - Constant Air Conditions

22 2

2 2 2 2

84

4 ln c s s L

L

L

X L L DX X l dXD t R X

t A dtz D X l A

Capillary of movement of moisture in drying

As water is evaporated, capillary forces are set up by the interfacial

surface tension between the water and solid.

These forces provide the driving force for moving the water through the pores to the drying surface

It can be shown that in this case the drying is inversely proportional to the thickness of the

material, which leads to RC = a X.



Example 2: Constant air conditions

A wet, granular material is being dried in the constant-rate period in a pan 0.61 m x 0.61 m

and the depth of material is 25.4 mm.. The pan has a metal bottom having a thermal

conductivity of kM = 43.3 W/m.K and a thickness of 1.59 mm. The thermal conductivity of the

solid is kS = 1.125 W/m.k. The air flows parallel to the top exposed surface and the bottom

metal at a velocity of 3.05 m/s. The temperature and the humidity of the drying air are 65.6˚C

and 0.010 kg H2O/kg dry solid, respectively. Direct radiation heat from steam pipes having a

surface temperature of 104.4 ˚C falls on the exposed top surface, whose emissivity is 0.94.

a) Calculate the surface temperature and the drying rate for the constant-rate period.

b) If the pan contains 11.34 kg of dry solid having a free moisture content of 0.35 kg H2O/kg dry

solid, calculate the time required to reduce the moisture content to 0.05 kg H2O/kg dry solid.

c) Repeat your calculations by considering only heat transfer by convection to the drying

surface of the wet material

Please note that the critical moisture content is Xc = 0.20 kg H2O/kg dry solid


Batch Dryers - Variable Air Conditions

Drying with constant versus variable air conditions

Up to this point we assumed that the properties of drying gas (air), i.e., T and H, do

not change as the gas flows over wet solids

H2O This is only possible when there

is large excess of drying gas

In the actual dryers, the conditions of the drying gas change as it passes through

the dryer, for example in (a) through circulation packed bed, and (b) try dryer

T1, H1

T2, H2

z

Wet

solids

(a)

b T1, H1 T2, H2

x1

Wet solids

Lt

Wet solids

(b)



Analogy with heat exchangers

The previously derived equation for the drying time are no longer applicable because the

driving force for drying varies along the dryer

In in the case of adiabatic dying, the wet solids at the wet bulb temperature of entering air

(Tw) as long as moisture content is greater than Xc – single stream heat exchanger

When the moisture content is less than Xc, the solid temperature will be greater than Tw

and will depend on the position within the dryer– two stream heat exchanger

b T1, H1 T2, H2

x1

Wet solids

Lt

Wet solids

Example: variation of gas temperature in a batch dryer in which solids

are at constant temperature equal to Tw

Tgas

Tw T2

T1



Derivation of the expression for the average driving force for drying

Consider through-circulation drying in packed bed – the analysis for tray dryer is the same.

Overall energy balance:

2 1 1 2Based on temperature change of the gas: Eq.(1)S S W Wq GAc T T GAc T T T T

Energy balance in a differential element of thickness dz:

Based on temperature change of the gas: Eq.(3)Sdq GAc dT

B a s e d o n t h e r a t e o f h e a t t r a n s f e r : E q . ( 4 )Wd q h a A d z T T

B a s e d o n h e a t t r a n s f e r f r o m t h e g a s : E q . ( 2 )W L Mq h a Z A T T

where: (T – Tw)LM is the average driving force in the dryer

Equating Eq.(3) and Eq.(4) and rearranging:

2 21

1 1 2 1 2

lnln

z TW

Sz TS W S W W W

T Tha dT ha haZdz Z Gc

Gc T T Gc T T T T T T

Substituting expression for GcS into Eq.(1) and comparing it with Eq.(2):

1 2

1 2

ln

W W

W LMW W

haAZ T T T Tq haZA T T

T T T T

1 2

1 2

ln

W W

W LMLMW W

T T T TT T T

T T T T

dz

H

T+dT H+dH

G, T1, H1

T2, H2

Z

T

0



Expressions for drying time in through-circulation packed bed

Constant-rate drying period, i.e., R = Rc when X > Xc

NB: In packed beds:

1 2 an dS

C

t X XaR

( )

WC y A ir W

W

h T TR k M H H

1 2 1 2

( )

W S S

W y A ir W

X X X Xt

ah T T ak M H H

Replacing the local difference with the log-mean-difference, i.e. the

average driving force:

1 2W S

W LM

X Xt

ah T T

2 3w h e r e : m m i s t h e s p e c i f i c s u r f a c e a r e a o f t h e b e d a n d i s t h e d e n s i t y o f d r y s o l i d s i n t h e b e dSa

m a s s o f b e d, T h u s c o r r e s p o n d s to

s u r fa c e a r e a o f b e dS S S SA L L

A L a a a A

1 2

1 2

w here: ln

W W

W LMW W

T T T TT T

T T T T

If T2 required for (T – TW)LM is unknown, it can be evaluated from:

12 1

2

ln expWW W S

S W

T ThaZ T T T T haZ Gc

Gc T T

dz

H

T+dT H+dH

G, T1, H1

T2, H2

Z

T

0



Expressions for drying time in through-circulation packed bed

Falling-rate drying period (X < Xc) and R is directly proportional to X

If solids temperature were at Tw, which is not the case (why?):

ln ln

( )

W S C C S C C

W y A ir W LMLM

X X X X X Xt

ah T T ak M H H

Prediction of heat transfer coefficient in packed beds for adiabatic

evaporation 0 59

0 410 151 350

.

R e.. fo r: t P t

P

G D Gh N

D

0 49

0 510 214 350

.

R e.. fo r t P t

P

G D Gh N

D

2

2

Wwhere : is in while in ,

m K

kg kg in , and in

m hh m

p

t

h D m

G

Specific surface area of the bed

6 1F o r sp h e rica l p a rtic le s : w h e re : is th e v o id fra ctio n in th e b e d

P

aD

4 1 0 5.F o r cy lin d rica l p a rtic le s o f d ia m e te r a n d le n g th :

C Cc c

C C

H DD H a

H D

NB: Equivalent diameter (Dp) is the diameter of a sphere having the same surface area as the particles

dz

H

T+dT H+dH

G, T1, H1

T2, H2

Z

T

0

What would the actual t compared to the value

calculated assuming that solids are at Tw?



Expressions for drying time in tray dryer

Schematic representation of tray dryer

1 lnS W C C

W LM

x X X Xt

h T T

Variation of temperature and humidity of air in the dryer

Drying time in constant-rate period:

b G, T1, H1 G, T2, H2

x1

Wet solids

Lt

Wet solids

z 0

Adiabatic drying so that the temperature of solids is

constant and equal to wet bulb temperature of air (Tw)

Using the same analysis as for the through circulation bed dryer it can be shown that:

1

1 2 1ln e x p , th u s : e x p( )

WW W S W W t S

S W

T ThzT T T T hz G c b T T T T hL G c b

G c b T T

1 1S W C

W LM

x X Xt

h T T

11 S S

S

L Axx

A A

1 2

1 2

ln

W W

W LMW W

T T T TT T

T T T T

Drying time in falling-rate period*:

NB: In tray dryers: Similarly to flow-through packed beds, the expression

for t in the falling-rate period underestimates the actual t


Example 3: Variable air conditions

Spherical wet catalyst pellets having a diameter of 12.7 mm are being dried in a through-

circulation dryer. The pellets are in a bed of 63.5 mm thick on a screen. The solids are being

dried by air entering with a superficial velocity of 0.914 m/s at 82.2˚C and having a humidity

H = 0.01 kg H2O/kg dry air. The dry solid density is determined as 1522 kg/m3, and the bulk

density of the dry solid in the bed is 982.3 kg/m3. The initial free moisture content is 0.90 kg

H2O/kg solid and the solids are to be dried to a free moisture content of 0.45, which is above

the critical free moisture content.

Calculate the time of drying.



Continuous Dryers

Comparison of continuous and batch dryers

Tray Dryer (batch drying)

Wet material is loaded uniformly on removable trays; loading is done outside an oven.

Once the trays are loaded and placed inside the oven, the drying is initiated by circulating hot, dry air over the trays for a specific time.

Mass flow rate of drying air and its properties determine the drying time for a given design of trays

Continuous Tunnel Dryer

Trays or trucks containing the material move continuously through a tunnel of hot gasses.

Mass flow rate of dry air, its properties, loading rate of wet material, velocity of the belt for a given length of the tunnel determine the dryness of the product

Analysis of continuous dryers:

1. Overall mass and energy balances

2. Design of dryer and operation parameters


Continuous Dryers Overall Mass and Energy Balance – dryer is treated as “black box”

Drying gas can be in parallel, counter, cross, or any combination of these three flows

relative to the solids flow.

Typically, counter-flow configuration is assumed along with constant solid

temperature equal to wet bulb temperature of drying gas TW

Material balance on moisture:

GH2 + Ls X1 = GH1 + LSX2

Enthalpy of the gas:

H'G = cs (TG – To) + H λo [kJ/kg dry air]

Enthalpy of wet solids:

H'S = cps (TS – To) + XcpH2O (TS – To) [kJ/kg dry solid] (enthalpy of solid + enthalpy of free moisture)

Overall heat balance on dryer:

GH'G2 + LsH's1 = GH'G1 + LsH'S2 + QLoss

Dryer Solid

LS, TS1, X1 TS2, X2

Gas

G, TG2, H2 TG1, H1

Q

(Q is heat loss in the dryer (kJ/h). Adiabatic process, Q = 0,

If heat added to the system, Q is negative.)

G: kg dry air/h

Ls: kg dry solid/h

H: kg H2O/kg dry air

X: kg H2O/kg dry solid

cPS: kJ/kg dry solid K

cPH2O: kJ/kg H2O K

cS = 1.005 + 1.88 H [kJ/kg dry air ˚C]

To = 0 ˚C

Λo = 2501 kJ/kg H2O


Continuous Dryers Heat Recovery by Air Recirculation

Air recirculation helps to reduce steam consumption in dryers and to control the

conditions at which drying occurs.

Dryer Heater

Q

Dry Solid

Ls, TS2, X2

Wet Solid

Ls, TS1, X1

Recirculated air, G6, TG2, H6

(6)

Fresh Air

(1)

G1, TG1, H1

TG4

(4)

TG2, H2

(2)

Moist Air, H5

(5) (3)

TG3, H3

H4=H3

Water balance on heater: G1H1 + G6H6 = (G1 + G6)H4 (H6 = H5 = H2)

Water balance on dryer: (G1 + G6)H4 + LSX1 = (G1 +G6)H2 + LSX2

Heat balance on heater: G1H'G1 + G6H'G6 + Q = (G1 + G6)H'G4

Heat balance on dryer: (G1 + G6)H'G4 + H'S1LS = (G1 +G6)H'G2 + LSH'S2


Continuous Dryers Design of Continuous Dryers

Analysis of temperature profile in counter-current flow dryer

Dryer Solid

LS, TS1, X1 TS2, X2

Gas

G, TG2, H2 TG1, H1

Preheat Zone: Solids heats up to wet bulb temperature, little evaporation occurs.

Zone I:

Surface moisture evaporates, temperature of solid stays constant, rate of drying stays constant,

gas temperature decreases where as gas humidity increase, solid moisture content reaches the

critical value of XC.

Zone II:

Solid dries to a final value of X2, gas temperature further decreases, gas humidity increases to HC.

NB: The outlet temperature of the drying gas (TG1) must be greater than the inlet temperature of wet solids,

which could be less than Tw. In practice TG1 should be several degrees greater than Tw. Why?


Continuous Dryers Categories of Design Analysis in Continuous Dryers

1. Determination of the length of dryer in zone I

Given: (i) desired rate of processing of the wet

solids, including the inlet and outlet moisture

content, (ii) packing of the wet material (i.e., Ls/A

or equivalent), (iii) the mass flow rate of the drying

gas and its properties including T and H, (iv)

velocity at which wet solids move the dryer.

Assumptions: (i) adiabatic drying, constant heat

and mass transfer coefficients

Analysis: (i) Overall mass and energy balances to determine inlet and outlet conditions of

drying in each zone, (ii) determination of the heat transfer or mass transfer coefficient (iii)

determination of the residence time in each zone using the equation developed for batch drying

with variable dying gas condition

Bottom line: The inlet and outlet conditions of solids and air must be known, or they can be

easily calculated

Question: How do we evaluate the required residence time in the preheat zone?

Answer: Neglecting evaporation of water, the analysis in the preheat zone become identical to the

analysis of a two-stream heat exchanger.


Continuous Dryers Categories of Design Analysis in Continuous Dryers

2. Determination of the outlet conditions of wet solids and heating air when drying in zone I

Given: (i) Dryer parameters such as the total length,

surface area for dying, the height of the gas channel,

(ii) flow rate of wet solids and the inlet moisture content,

(iii) the mass flow rate of the drying gas and its

properties including T and H, (iv) velocity at which wet

solids move the dryer, or the total residence time.

Assumptions: (i) adiabatic drying, constant transfer

coefficient

Analysis:

Starting from energy balance on differential element of wet solids of length dz it can

be shown that the moisture content X2 at the outlet of the dryer (z = L) will be:

2 1T h e o u t le t a ir te m p e r a tu r e r e q u ir e d fo r is d e te r m ie d fr o m : e x pW W W t SLMT T T T T T h L G c b

2 1 2 1

( ) substituting for the average , i.e. in term s of ( ) :c W W LM

C W LM

S pW W S pW W

R WL h T T AX X R T T X X

L c T L c T


Drying-Fundamentals

Example 4: Air recirculation in a continuous dryer

The wet feed material to a continuous dryer contains 50 wt% water on a wet basis and is dried to

27 wt% by countercurrent air flow. The dried product leaves at the rate of 907.2 kg/h. Fresh air to

the system is at 25.6oC and has humidity of H = 0.007 kg H2O/kg dry air. The moist air leaves the

dryer at 37.8oC and H = 0.020 kg H2O/kg dry air and part of it is recirculated and mixed with the

fresh air before entering a heater. The heated mixed air enter the dryer at 65.6oC and H = 0.010.

The solid enters at 26.7oC and leaves at the same temperature. Calculate:

a) The fresh air flow,

b) The percent of air leaving the dryer that is recycled

c) The heat added in the heater

d) The heat loss from the dryer

3.2 Drying Processes

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