2. Drying
curve
• Drying curve is the plot of the drying rate Φ or N
versus the remaining water content X. Water
content X is expressed as kg of water per kg of
dry matter. Drying process of a material can be
described as a series of steps in which drying
rate plays a key role.
• Following figure shows typical drying rate curve
for a constant drying conditions.
3.
4. • Point Brepresentsequilibrium temperature conditions of
the productsurface.
• Section Bto Cof the curve, known asthe constant rate
periods, representsremoval of unbound water from the
product.
• The water acts asif the solid is not present.The surface of
the product is very wet and water.And the water activity
is equal toone.
• The constant rate period continues as long asthe amount
Constant rate
period
5. In the constant-rate period, the water is being evaporated from
what is effectively afree water surface. The rate of removal of
water canthen be related to the rate of heat transfer, if there is
no change in the temperature of the material and therefore all
heat energy transferred to it must result in evaporation of
water.The rate of removal of the water is also the rate of mass
transfer, from the solid to the ambient air. Thesetwo - mass
and heat transfer - must predict the same rate of drying for a
given set ofcircumstances.
6. • The falling rate period is reached when the drying rate starts
to decrease, and the surface water activity falls to less than
one.
• The rate of drying is governed by the internal flow of liquid
or vapor.This point is represented byCin the figure.
• At this point there is not enough water on the surface to
maintain awater activity value of one.
Falling rateperiod
7. • Thefalling rate period canbe divided
into twosteps.
• First falling dryingrate
• Second falling dryingrate
8.
9. A first falling drying rate occurs
when wetted spots in the surface
continually diminish until the
surface is dried (PointD).
First falling dryingr
10. • Second falling rate period begins at point D
when the surfaceis completely dry.The plane of
evaporation recedes from the surface.
• Heat required for moisture removal is
transferred through
the solid to the vaporization of moisture in the
solid and the vapor moves through the solid into
Second fallingdrying
rate
11. • Theamount water removed in this period
can be relatively small compared to the
constant rate and first falling rate period.
• However this period may take much
longer than constant rate period because
the drying isslow.
12. Drying rate experiments canbe simply done
by measuring the weight change during
drying.Thematerial to be dried is placed on
the tray. The tray is suspended from a
balance and exposed to air flow in drying
tunnel.
Drying rateexperimen
18. Drying rates, oncedetermined experimentally or
predicted from theory, can then be used to
calculate drying times so that drying equipment
and operations can be designed. In the most
general cases,the drying rates vary throughout
the dryer with time asdrying proceeds, and with
the changingmoisture content of the material.
Sothe situationis complicated.
Calculation of DryingTimes
19. However, in many casesasimplified approachcanprovide
useful results. One simplification is to assumethat the
temperature and RHof the drying air areconstant.
In this case,for the constant-rate period, the time neededto
remove the quantity of water which will reduce the food
material to the critical moisture content Xc (that
corresponding to the end of the constant-rate period and
below which the drying rate falls) can be calculated by
dividing this quantity of moisture by the rate.
20. In this case,for the constant-rate period, the time
needed to remove the quantity of water which will
reducethe food material to the critical moisture content
Xc(that corresponding to the end of the constant-rate
period and below which the drying rate falls) can be
calculated by dividing this quantity of moisture by the
rate.
21. andXois the initial moisture content andXcthe final
moisture content (the critical moisture content in this
case)both on adry basis, w is the amount of dry
material in the food and(dw/dt) constisthe constant-
drying rate.
22. Where the drying rate is reduced by a factor f then this can
be incorporated to give:
Dt = w (DX)/ f(dw /dt)const.
and this has to be integrated piecemeal down to X = Xf
where subscript f denotes the final water content, and f
expresses the ratio of the actual drying rate to the maximum
drying rate corresponding to the free surface-moisture
situation.
