FUNDAMENTALS OF DRYING

By: Poonam Shah | Views: 4894 | Date: 25-Apr-2012

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.

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.

Fig 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.

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

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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Anil Chaudhari  |  01-Sep-2016 17:09:31 IST
What is Delta-T in case of Counter and Co-current flow?
Difference between which two components?
Please explain.
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