Drying

Reference

Chapter 18 of Separation Process Principles,

3rd Ed., Seader, Henley, Roper, Wiley

Introduction

• Drying is the removal of moisture from solids, slurries, and pastes to give solid products.

• A process of mass-transfer

• Often refers to removing moisture from liquids and gases

• Industrial applications1. Chemical products: crystalline particles of inorganic salts,

organic compounds, films, coatings

2. Biological materials including food

3. Pharmaceuticals

4. Detergents

5. Lumber, paper, fibers

2

Introduction

• Pre-feed dewatering operations by mechanical means such as filtration and centrifugation can diminish the length of drying cycles.

• Common modes of heat transfer1. Convection from a hot gas in contact with the material

2. Conduction from a hot, solid surface in contact with the material

3. Radiation from a hot gas or surface

4. Heat generation within the material (microwave heating)

• Classification1. Batch (less than 500 lb/h) vs. continuous (more than 2000 lb/h)

2. Direct-heat (convection) vs. indirect-heat (conduction)

3. Stationary vs. agitated

3

Introduction

Hot metal

Hot air Hot air

moisture + air

- Indirect heat

- Stationary

- Direct heat

- Stationary

- Direct heat

- Agitated

(a) (b) (c)

Drying effectiveness

Sensitive products(temperature sensitive or dust-formed)

C > B > A

A > B > C4

Equipment: batch dryers

Tray dryers

• Oldest and simplest batch dryer

• Cross-circulation (solid tray bottom) or through-circulation (perforated tray

bottom) for solids with appreciable voids such as granules, noodles and pellets

• Mostly direct heating; Indirect heating is possible with hollow shelves carrying

hot steam under vacuum

• Useful when low production rates of multiple products are involved

5

Equipment: batch dryers

Agitated dryers

• Indirect heating with agitation; perhaps, under vacuum

• Can be used when

(1) material oxidizes or becomes explosive or dusty during drying;

(2) moisture is toxic, flammable, or explosive;

(3) material tends to agglomerate if not agitated;

(4) maximum product temperature is less than about 30 OC. 6

Equipment: continuous dryers

Band-conveyor dryer

• Circulation of heated gases upward and/or downward through a moving,

permeable, layered bed of wet material from 1 to 6 inches deep.

• Multiple sections, each with a fan and set of gas-heating coils, can be arranged

in series to provide a dryer, with a single belt as long as 150 ft with a 6-ft width,

giving drying times up to 2 h, with a belt speed of about 1 ft/minute.

• Granular or pelletized materials with appreciable voids should be used. 7

Equipment: continuous dryers

Direct-heat rotary dryer

• A popular dryer for evaporating water from free-flowing granular, crystalline, and flaked solids

of relatively small size, when breakage of solids can be tolerated

• Wet solids enter through a chute at the high end and dry solids discharge from the low end.

• Cylindrical shell is slightly inclined from the horizontal with a slope of less than 8 cm/m

• Hot gases flow counter currently or co-currently (may use cylinders not inclined) to the solids.

• Longitudinal lifting flights are mounted on the inside of the rotating shell causing the solids to

be lifted, then showered through the hot gas during each cylinder revolution.

• Typically the bulk solids occupy 8–18% of the cylinder volume, with residence times from 5

minutes to 2 h.8

Equipment: continuous dryers

Fluidized-bed dryer• Fluidized-bed dryers have become very popular in

recent years because they: (1) have no moving parts;

(2) provide rapid heat and mass transfer between gas

and particles; (3) provide intensive mixing of the

particles, leading to uniform conditions throughout the

bed; (4) provide ease of control; (5) can be designed for

hazardous solids and a wide range of temperatures,

pressures, residence times, and atmospheres; (6) can

operate on electricity, natural gas, fuel oil, thermal

fluids, steam, hot air, or hot water; (7) can process very

fine and/or low-density particles; and (8) provide very

efficient emissions control.

• Minimum fluidization velocity; where the pressure drop

is equal to the weight of the solids per unit cross-

sectional area of the bed normal to gas flow.

• Materials that are successfully dried in fluidized-bed

dryers include coal, sand, limestone, iron ore, clay

granules, granular fertilizer, granular desiccant, sodium

perborate, polyvinylchloride (PVC), starch, sugar,

coffee, sunflower seeds, and salt.

9

Equipment: continuous dryers

Spray dryer

• When solutions, slurries, or pumpable pastes—

containing more than 50 wt% moisture, at rates greater

than 1,000 lb/h—are to be dried.

• Feed is pumped to the top center of the chamber,

where it is dispersed into droplets or particles from 2 to

2,000 µm by atomizers.

• Hot gas enters the chamber, causing moisture in the

atomized feed to rapidly evaporate. Gas flows co-

currently to the solids.

• Applications: detergents, milk, starch, yeast, zinc

sulfate, lignin, aluminum hydroxide, silica gel,

magnesium chloride, manganese sulfate, urea resin,

sodium sulfide, coffee extract, tanning extract, color

pigments, tea, tomato juice, polymer resins, and

ceramics

10

Equipment: continuous dryers

Drum dryer

• To process solutions, slurries, and pastes with indirect heat

• Applications: milk, detergents, brewer’s yeast, potatoes, skim milk, malted milk,

coffee, tanning extract, and vegetable glue.

11

Psychrometry (humidity chart)

• For air–water vapor mixtures at 1-

atm

• Calculations involving the properties

of moisture–gas mixtures for

application to drying are most

conveniently carried out with

psychrometric charts.

12

Use steam table to find:

- Vapor pressure (PAS)

- Enthalpy of vaporization (ΔHwvap)

Psychrometry

13

Psychrometry

14

Example: Air at 130 °F and 1 atm enters a direct-heat

dryer with a humidity of 0.03 lb H2O (A)/lb H2O-free air

(B). Determine by the psychrometric chart and the

relationships of Table 18.4: (a) relative humidity and (b)

humid volume.

Wet-bulb temperature

• In drying process, solids must be heated to a temperature at which its vapor pressure

exceeds the partial pressure of the moisture in the gas in contact with the wet solid.

• The wet bulb temperature, Tw, can be measured by covering a thermometer bulb with

a wick saturated with the liquid being evaporated and passing a partially saturated gas

past the wick.

• Tw is equal to the adiabatic saturation temperature for air-water system.

• In a typical drying process, the wet solid is heated to Tw.15

Equilibrium moisture content

Isotherm (Figure 18-23)

• At given T and P

• Equilibrium moisture content as a

function of relative humidity

• Provide useful information for direct-

heat drying processes

• Moisture content, X; wt% moisture

on a dry-solid basis

• W; wt% moisture on a wet-solid

basis

16

Equilibrium moisture content

Moisture adsorption is exothermic.

17

Equilibrium moisture content

18

Drying periodspreheating

until T=Tw

constant-rate

first falling-rate

second falling-rate

19

Constant-rate drying period

Drying-rate flux, R

A : mass-transfer area,

mv: mass of moisture evaporated

ms: mass of bone-dry solid

For constant-rate period

• The rate of mass transfer is determined by gas-phase boundary-layer or film

resistance at the wet surface of the solid.

• The wet solid is assumed to be at a uniform temperature, so the only

resistance to convective heat transfer is in the gas phase.

• The rate of moisture evaporation can then be based on convective heat

transfer or mass transfer

• i refers to the gas-solid interface

• Ti = Tw

20

Constant-rate drying period

Drying-rate flux for constant-rate drying period, Rc

Based on solid mass, (not wet-solid mass)

It is more common to use the heat-transfer relation of when air is the gas and water is the

moisture because of the wide availability of the psychrometric chart for that system and the

equality of wet-bulb and adiabatic-saturation temperatures.

a : external SA of solid/mass of solid (A/ms)

ms: mass of solid

Drying rate per solid mass

(Tg=Td)

constant-rate drying period

21

𝑑𝑚𝑣

𝑑𝑡=

ℎ𝑎 𝑇𝑔 − 𝑇𝑤 𝑚𝑠

∆𝐻𝑤𝑣𝑎𝑝

𝑅′ = 𝑅𝐴

𝑚𝑠𝑡𝑐 =

𝑚𝑣

𝑅𝑐𝐴=𝑚𝑠(𝑋 − 𝑋𝑐)

𝑅𝑐𝐴=𝑋 − 𝑋𝑐𝑅𝑐′

Constant rate drying time

Constant-rate drying period

where in (1), G is the mass velocity of air in

the flow channel that passes over the wet

surface. In (2), G is the mass velocity of the

air impinging on the wet surface. In (3) to (8),

dp is the particle diameter and G is the

superficial mass Velocity.

uavg = avg. velocity of gas (m/s)

ρ = density of humid gas (kg/m3)

υ= humid volume (wet gas basis) (m3/kg)(volume of moisture-gas/mass of moisture-gas)

22

𝑣 =𝑣ℎ

1 + ℎ

Constant-rate drying period

23

Constant-rate drying period

24

Falling-rate drying period

- When moisture travels from the interior of a wet solid to the surface, a

moisture profile develops in the wet solid. The profile’s shape depends on the

nature of the moisture movement.

- When the moisture is free moisture in the interstices of particles like soil and

sand, or is moisture above the fiber-saturation point in paper and wood,

moisture movement occurs by capillary action.

- When the internal moisture is bound moisture, as in the last stages of drying

of paper and wood, or soluble moisture, as in soap and gelatin, moisture

migrates to the surface by liquid diffusion.

25

Falling-rate drying period (empirical approach)

- During the falling-rate period, estimation of drying rate using capillary flow and

diffusion theory is often challenging due to non-idealities. Alternatively,

estimates could be made by a strictly empirical approach that ignores the

mechanism of moisture movement, but relies on experimental determination of

drying rate as a function of average moisture content for a particular set of

conditions.

26

For constant-rate period,

Fig. 18-31

General equation for drying time calculation

Falling-rate drying period (empirical approach)

Rate of drying for falling-rate period

Drying time in the falling-rate period

Total drying time

Curve fitting using a parabolic function

Drying time in the falling-rate period

27X must be the free-moisture content!!

Fig. 18-31

(18-43)

(18-44)

Falling-rate drying period (empirical approach)

28

Falling-rate drying period (empirical approach)

29

Falling-rate drying period (liquid diffusion theory)

For slow-drying materials for which the rate of drying is controlled by internal diffusion of moisture

to the exposed surface: during the evaporation of a surface liquid film in a constant-rate drying

period controlled by gas-phase mass transfer, no moisture diffuses to the surface, and after

completion of evaporation of that film, resistance to mass transfer is due to internal diffusion in a

falling-rate period.

Fick’s second law

BCs

X=Xo at t=0 for -a < z < a

X=X* at z=±a for t ≥ 0

unaccomplished free-moisture change

Fourier number of diffusion, NFoMposition ratio

30X is the total moisture content!!

Falling-rate drying period (liquid diffusion theory)

The rate of mass transfer from one surface

The average moisture content

The equation can be used to determine the moisture diffusivity, DAB, from

experimental data, and then that value can be used to estimate drying rates for

other conditions.

When NFoM > 0.1 , only the first term in the infinite series is significant.

31

Falling-rate drying period (liquid diffusion theory)

32

Falling-rate drying period (liquid diffusion theory)

33

Roughly estimate DAB using the simplified eqn,

y = -0.0858x - 0.2135R² = 0.9935

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 2 4 6 8 10

ln(Eavg) vs time

Falling-rate drying period (liquid diffusion theory)

34

r

For sphere

The average moisture content

When NFoM > 0.1 , only the first term in the infinite series is significant.

1

2

22

22*

0

*

)exp(16

n

ABavg

avgr

tDn

nXX

XXE

2r

tDN AB

FoM

)exp(6

2

2

2*

0

*

r

tD

XX

XXABavg