2.1 The
Drying Curve
For
each and every product, there is a representative curve that describes the
drying characteristics for that product at specific temperature, velocity and
pressure conditions. This curve is referred to as the drying curve for a
specific product. Fig 2.1 shows a typical drying curve. Variations in the curve
will occur principally in rate relative to carrier velocity and temperature.
Figure 2-1: Drying Curve
Drying
occurs in three different periods, or phases, which can be clearly defined.
The
first phase, or initial period, is where sensible heat is transferred to the
product and the contained moisture. This is the heating up of the product from
the inlet condition to the process condition, which enables the subsequent
processes to take place. The rate of evaporation increases dramatically during
this period with mostly free moisture being removed. In some instances,
pre-processing can reduce or eliminate this phase. For example, if the feed material
is coming from a reactor or if the feed is preheated by a source of waste
energy, the inlet condition of the material will already be at a raised
temperature.
The
second phase, or constant rate period, is when the free moisture persists on
the surfaces and the rate of evaporation alters very little as the moisture
content reduces. During this period, drying rates are high and higher inlet air
temperatures than in subsequent drying stages can be used without detrimental
effect to the product. There is a gradual and relatively small increase in the
product temperature during this period.
Interestingly,
a common occurrence is that the time scale of the constant rate period may determine
and affect the rate of drying in the next phase.
The
third phase, or falling rate period, is the phase during which migration of
moisture from the inner interstices of each particle to the outer surface
becomes the limiting factor that reduces the drying rate.
2.2
Moisture content
Measuring
moisture content allows control of the drying process such that drying is
carried out until a specific level of moisture content is achieved rather than
for a fixed time period.
Electrical
resistance type meters operate on the principle of electrical resistance, which
varies minutely in accordance with the moisture content of the item measured.
Most of these types of instruments are suitable for measuring moisture content
in grain, wood, food, textiles, pulp, paper, chemicals, mortar, soil, coffee,
jute, tobacco, rice, copra, and concrete. Resistance meters have an average
accuracy of + 1% MC over their operating range.
Dielectric
moisture meters rely on surface contact with a flat plate electrode that does
not penetrate the wood. Similar to resistance meters, the accuracy of
dielectric meters in measuring average MC is + 1% moisture content.
Modern
portable moisture balances are available with built in infrared heaters, which
directly measures the moisture content of the product and gives a profile of
moisture content variations with time. For measuring moisture content in paper
rolls or stacks of paper, advanced methods include the use of Radio Frequency
Capacitance method. The instrument measures the loss, or change, in RF
dielectric constant as affected by the presence of moisture.
Calculation
of the quantity of water to be evaporated is explained below with a sample calculation.
If
the throughput of the dryer is 60 kg of wet product per hour, drying it from
55% moisture to 10% moisture, the heat requirement is:
60
kg of wet product contains
60
x 0.55 kg water = 33 kg moisture
and
60 x (1 - 0.55) = 27 kg bone-dry product.
As
the final product contains 10% moisture, the moisture in the product is 27/9 =
3 kg and so moisture removed = (33 - 3) = 30 kg
Latent
heat of evaporation = 2257 kJ kg-1(at 100 °C so heat necessary to supply = 30 x
2257 = 6.8 x l04 kJ
2.3
Estimation of drying time
The
rate of drying is determined for a sample of substance by suspending it in a
cabinet or duct, in a stream of air from a balance. The weight of the drying
sample can then be measured as a function of time from wet product to bone dry
product. The curve of moisture content as a function of time, similar to fig
2.1, can be plotted. While different solids and different conditions of drying
often give rise to curves of very different shapes in the falling rate period,
the curve shown above occurs frequently.
During
the above measurements, the following conditions are to be followed.
1.
The sample should be subjected to similar conditions of radiant heat transfer
2.
Air should have the same temperature, humidity & velocity
Electronic
moisture balances with online data collection/plotting can be used to establish
drying curves of materials.
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