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.
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 With advances in electronics, however, such methods have been largely superseded by meters which measure changes in the electrical conductivity of a material with moisture content and can be calibrated accordingly.
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.
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Relative humidity % Figure 21.42 Typical equilibrium moisture contents |
There are a number of different ways of removing moisture from a material:
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A) |
Compression (squeezing) |
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B) |
Centrifuging (spinning) |
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(these two methods are only possible down to a moderate m. c.) |
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C) |
Air movement (heated or ambient temperature) |
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D) |
Application of heat (air movement will be necessary to remove the moisture) |
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E) |
Vacuum drying |
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F) |
Freeze drying (often in a vacuum) |
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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.
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.
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.
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Material |
Critical moisture content |
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Pottery before firing |
14 to 16% |
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Rubber |
10% |
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Tea |
174% |
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Leather |
90 to 125% |
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Paper |
33 to 70% |
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Table 21.6 Critical moisture contents of various materials |
The whole process is shown diagrammatically in Figure 21.43.
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Rt W |
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Rrij — me ! m — m0 |
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Log, |
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M-me Initial moisture content final moisture content |
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Dt |
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Or |
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M, — me! m — mB |
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T=0 |
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= (171: — m |
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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" |
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Mi — me S^ K"’ |
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Equ 21.19 |
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Equ 21.20 |
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Dt |
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-k" |
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Dt, where: M = moisture content Me = equilibrium moisture content |
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-[loge(m — me) — log^m, — me)] = — j |
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K"t |
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K A A0 |
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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 |
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Temperature difference between the air and the surface |
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DML = KAAOdt or the rate of drying: dM kAA9 dt~_ L |
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Equ21.17 |
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Solution a) |
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F32]2 |
Ioge |
S — rrO Vm-meJ2 |
*2 |
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UJ |
Ioge |
(m ; — nO lvm-meJ1 |
Tl |
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F12V |
Ioge |
CM O O I I O Csl CM T- |
*2 |
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I e J |
Ioge |
|"20-10"| Ll4-10j1 |
20 |
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4 lo9 |
E5 |
_ *2 |
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Ioge |
2-5 |
20 |
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Ort2 |
= 80 ‘°9*5 loge25 = 80 0 699 0.3979 = 140 hours |
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Solution b) |
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Ioge|^ |
Mi — me"| m-meJ2 |
*2 |
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UJ |
Ioge[ |
Mi — nO m — me J, |
Ti |
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F12l |
I°ge[ |
20-10^| 12 -10 J |
__ ^2 |
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16 J |
Ioge[ |
20-10^| 14 10 j |
20 |
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2 log ioge |
Le5 2-5 |
20 |
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Or t2 |
= 40 0 699 0.3979 |
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= 70 hours |
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
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Figure 21.44 Psychrometry for direct system |
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Figure 21.45 Psychrometry for recirculated system |
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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
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.
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