Drying

A drying system is often necessary in industry:

• to reduce the moisture within a material to improve it

• to make industrial processes more efficient

• to recover the moisture where this has value.

Moisture may be present in a solid material in various forms as:

• surface moisture

• absorbed moisture

• water of crystallisation

• liquid in which a solid is in suspension or solution.

Moisture content

The amount of moisture present may be mathematically pre­sented by the moisture content (m. c.), calculated in either of two ways:

• on a dry basis

Weight of moisture

M. c. =————————

Weight of dry stock

• on a wet basis

Weight of moisture

M. c. =——— ————————— —

Weight of moisture + dry stock

In the past, moisture content was determined by weighing be­fore and after drying in an oven:

A) at 100°C for about 5 hours

B) at 155°C for about % hour (Carter-Simon oven)

Depending on the ability of the material to withstand the particu­lar temperature. With advances in electronics, however, such methods have been largely superseded by meters which mea­sure changes in the electrical conductivity of a material with moisture content and can be calibrated accordingly.

Equilibrium moisture content

If a material is completely dried, it will regain moisture on con­tact with ambient air to an amount which will be dependent on the material, the air temperature and the air relative humidity. It will settle at some value, for this given set of conditions, known as the equilibrium moisture content (emc). It should also dry naturally in air to this value. Typical values may be obtained from Figure 21.42.

Drying

Relative humidity % Figure 21.42 Typical equilibrium moisture contents

Methods of removing moisture

There are a number of different ways of removing moisture from a material:

A)

Compression (squeezing)

B)

Centrifuging (spinning)

(these two methods are only possible down to a moderate m. c.)

C)

Air movement (heated or ambient temperature)

D)

Application of heat (air movement will be necessary to re­move the moisture)

E)

Vacuum drying

F)

Freeze drying (often in a vacuum)

G)

Electro osmosis

For the purposes of Fans & Ventilation we are interested in method c), which employs fans, and where appropriate, method d) which uses heat, assisted by fans.

The drying of solids in air

There are 3 parts to this process:

1. Drying from a surface saturated with moisture — drying is then at a constant rate.

2. Unsaturated surface drying — this is a fairly short period when dry patches appear and the drying rate falls uni­formly.

3. A second falling rate period in which the rate of drying is controlled by the rate of internal diffusion.

Critical moisture content

It should be observed that the critical moisture content is that at which unsaturated surface drying commences i. e., when the first dry spots appear. This varies from material to material as shown in Table 21.6.

Material

Critical moisture content

Pottery before firing

14 to 16%

Rubber

10%

Tea

174%

Leather

90 to 125%

Paper

33 to 70%

Table 21.6 Critical moisture contents of various materials

The whole process is shown diagrammatically in Figure 21.43.

Rt

W

Rrij — me

! m — m0

Log,

M-me

Initial moisture content final moisture content

Dt

Or

M, — me! m — mB

T=0

= (171: — m

Where: mi m

Since k" is a constant, drying time is proportional to (thick­ness)2.

Although values of k" are not readily available, this expression is useful for finding, from known conditions, the drying times for other conditions, but the same material.

Full-scale plant may therefore be designed from data obtained with pilot plants. It should be noted thatk" may vary if the stock temperature is changed. This expression holds for homoge­neous solids such as rubber, soap, gelatine, glue etc., but it is not so accurate for granular materials such as sand, paint pig­ments etc., probably due to the different way in which the mois­ture is released.

It has therefore been suggested that:

^dM^i _k"’fdM^ m-m„

“ s"

Mi — me

S^

K"’

Equ 21.19

Equ 21.20

Dt

-k"

Dt, where:

M = moisture content

Me = equilibrium moisture content

-[loge(m — me) — log^m, — me)] = — j

K"t

K

A

A0

21.5.7 Rate of drying

The rate of drying in each of the 3 parts of the drying process outlined in Section 21.5.5, may be calculated as follows (refer­ring also to Figure 21.43):

1. Constant rate

Heat required to evaporate dM of moisture = dM L where:

L = latent heat Heat supplied by the air = kA — A0 dt where:

= overall heat transfer coefficient = surface area

Temperature difference between the air and the surface

DML = KAAOdt or the rate of drying: dM kAA9 dt~_ L

Equ21.17

—Ifall = k’-d2M

 

Equ 21.18

 

Drying

Figure 21.43 Drying process

 

Dt

 

DS

 

Where:

M = mass of moisture

S = semi (half) thickness

K’ = constant depending on the rate of diffusion

For thin slabs and long drying times:

 

Drying
Drying

M — m

 

Drying
Drying

Or

 

Drying

For drying of solids in air with heat supplied by convection only, the surface of the material will be very nearly at the bulb temper­ature and A0 becomes the wet bulb depression, and then k = convective heat transfer coefficient.

When drying materials in trays, with the air flow parallel to the surface:

K-0.0128 G08 where:

G = mass velocity = pv When the airflow is perpendicular to the surface: k = 0/37 G°37

2. First falling rate

This is mostly controlled by the same process, since the sur­face drying is almost certainly well in excess of the diffusion rate. Calculations for this period are generally unnecessary in view of the relatively short time involved.

3. Second falling rate

The rate is controlled mainly by the rate of diffusion of moisture from the inside of the material to the outside surface. Air veloc­ity is of relatively little importance as it does not accelerate the rate of diffusion.

The application of heat is probably therefore of greater impor­tance, the air movement serving mainly to carry away the mois­ture reaching the surface. It has been suggested that the “soakage” equation can be used to indicate the processes in­volved:

 

I. e. drying time cc to thickness

Note: In calculations thickness maybe used in most cases to replace semi-thickness S.

21.5.7.1 Example

A material has an initial moisture content of 20% and is dried to 14% in 20 hours, the equilibrium moisture content of the mate­rial being 10% and the sample having a thickness of 6 mm.

How long would you expect it to take to dry a 12 mm thick mate­rial to 12% moisture content assuming:

A) a homogenous material?

B) a granular material?

 

Solution a)

F32]2

Ioge

S — rrO

Vm-meJ2

*2

UJ

Ioge

(m ; — nO lvm-meJ1

Tl

F12V

Ioge

CM

O O

I I O Csl CM T-

*2

I e J

Ioge

|"20-10"|

Ll4-10j1

20

4 lo9

E5

_ *2

Ioge

2-5

20

Ort2

= 80 ‘°9*5 loge25

= 80 0 699 0.3979

= 140 hours

Solution b)

Ioge|^

Mi — me"| m-meJ2

*2

UJ

Ioge[

Mi — nO m — me J,

Ti

F12l

I°ge[

20-10^| 12 -10 J

__ ^2

16 J

Ioge[

20-10^| 14 10 j

20

2 log ioge

Le5

2-5

20

Or t2

= 40 0 699 0.3979

= 70 hours

Elementary psychrometry

1. Air used directly and discharged

The psychrometry for such a system is illustrated in Figure 21.44.

In this case conditions A and B may be determined but point C cannot. The maximum amount of moisture which the air can ab­sorb is Wd — Wb. In practice only Wc — Wb is absorbed and

Drying

Figure 21.44 Psychrometry for direct system

Drying

Figure 21.45 Psychrometry for recirculated system

Drying

Figure 21.46 Fresh/recirculated air system W — Wt

—5—— may be between 0.1 and 0.7. Since it is desirable only

Wd — Wb

To supply latent heat of evaporation, discharge at condition C is generally wasteful.

2. Use of recirculated air

The psychrometry for such a system is illustrated in Figure 21.45 and a typical system is shown in Figure 21.46.

Here fresh air at A is mixed with recirculated air at condition F giving condition C. Condition F may be set by means of controls to a higher moisture content than without recirculation.

Thus, for a given volume of fresh air a greater amount of evapo­ration takes place. Moisture pick up = Wf — Wa.

Heat requirements

1. Heat given to the air = G Cair (W- tin)

2. Latent heat of evaporation = G (Wout~ Win) L

3. Heat given to solid = M CSOiid (tSOiid out — tSOiid in)

4. Heat given to moisture in solid = RM (tSOiidout— tSOiid in)

Tsolid out = wet bulb temperature of

Air leaving dryer

5. Heat loss from dryer

Practical drying systems

There are two main classification of dryers:

• Direct dryer — material is heated by direct contact with the heating medium e. g. hot air.

• Indirect dryer — material is indirectly heated by medium via conduction or radiation.

We are primarily concerned with direct dryers

• Batch dryers — Suitable for clay ware, powders and food­stuffs, see Figure 21.47.

• Continuous dryers — continuous dryers for sheet, Figures 21.48 and 21.49.

For drying of leather sheet 27 to 50 °C is typical, see Figure 21.50.

(Air velocity over trays 1 to 1.25 m/s)

 

Discharge

 

Figure 21.47 Batch dryer

 

Minimum

Possible

Opening

 

(G

 

, I

Xi 1/

 

Figure 21.48 Continuous dryer

 

Drying Drying

Material in

Xx

 

Air out

| | Fan impellers

 

/ /

 

.✓Heating surface

 

-Material falls through tray onto tray beneath

 

Figure 21.54 Spray dryer

 

Drying

Figure 21.55 Pneumatic or flash dryer

 

 

Material out dry

Figure 21.49 Turbo continuous dryer for sheet materials

 

/ . ! in

 

$Lа

 

Ц…W

 

— Velocty up to 20 m/s

 

Figure 21.50 Stentar drying method for woven material

Drying

 

In the “Shirley” accelerated dryer, which uses a rotating drum, air velocities of 15-20 m/s, ft/min are employed.

Other types of dryer are illustrated in Figures 21.51 to 21.55.

In Figure 21.53, instant drying is achieved because of very inti­mate contact between solution and heated air.

Rotary type atomisers probably give better control of particle size, dried egg, mild, detergents etc.

Posted in Fans Ventilation A Practical Guide


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