Drying Ii
Drying Ii
Drying Ii
• To determine the size of equipment and to set up the drying schedules, it is necessary to know
the time required to dry a substance from one moisture content to another under specified
conditions.
• The time required for drying of a moist solid to final moisture content can be determined from
the knowledge of rate of drying under a given set of conditions.
• The drying rate of a solid is a function of temperature, humidity, flow rate and transport
properties of the drying gas.
Drying tests:
• The rate of drying can be determined experimentally for a sample of a 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.
• The laboratory tests are generally done under ‘constant drying conditions’, i.e. exposure of
sample to air of constant temperature, humidity, and velocity.
A = drying area
= wet surface over which gas blows and through which evaporation takes place (cross-circulation
drying)
= cross-section area of the bed measured at right angles to the direction of gas flow (through-
circulation drying)
Two major parts to the rate curve
• Constant rate
• Falling rate
If a solid is initially very wet, covered with a thin film of liquid, assuming its entirely unbound moisture.
When it is exposed to relatively dry air, evaporation will take place from the surface.
For cross-circulation drying, rate at which moisture evaporates:
𝑁𝐶 = 𝑘𝑌 (𝑌𝑠 − 𝑌)
𝑘𝑌 : gas mass-transfer coefficient
𝑌𝑠 : humidity of the gas at liquid surface
• 𝑘𝑌 will remain constant as long as the speed and direction of gas flow past the surface do not
change.
• 𝑡𝑠 is the equilibrium temperature and remains constant because the rate of heat flow from the
surroundings to the surface equals the rate of heat absorption (latent heat).
• The capillaries and interstices of the solid, filled with liquid, can deliver liquid to the surface as
rapidly as it evaporates there.
The initial period is usually so short that it is ordinarily ignored in subsequent analysis of the drying
time.
First falling rate period (Unsaturated surface drying)
• Once critical moisture content Xc is reached, most of the surface film of moisture is evaporated.
• However this period may take much longer than constant rate period because the drying is slow.
Time of Drying
Recall, rate of drying
−𝑆𝑆 𝑑𝑋
𝑁= (1)
𝐴 𝑑𝜃
Rearranging and integrating,
𝜃
𝑆𝑠 𝑋1 𝑑𝑋
𝜃 = ∫ 𝑑𝜃 = ∫ (2)
0 𝐴 𝑋2 𝑁
a) General case: For any shape of falling-rate curve whatsoever, Eq. (2) can be integrated
graphically by determining the area under a curve of 1/N as ordinate, X as abscissa, the data
for which can be obtained from the rate-of-drying curve.
𝑁 = 𝑚𝑋 + 𝑏
where, m is the slope of the linear portion of the curve and b is a constant.
𝑆𝑠 𝑋1 𝑑𝑋 𝑆𝑠 𝑚𝑋1 + 𝑏
𝜃= ∫ = 𝑙𝑛
𝐴 𝑋2 𝑚𝑋 + 𝑏 𝑚𝐴 𝑚𝑋2 + 𝑏
For simplification (or lack of data), entire falling-rate curve can be taken as a straight line between points
C & E.
𝑁𝑐 (𝑋 − 𝑋 ∗ )
𝑁 = 𝑚(𝑋 − 𝑋 ∗ ) =
𝑋𝑐 − 𝑋 ∗
𝑆𝑠 (𝑋𝑐 − 𝑋 ∗ ) 𝑋1 − 𝑋 ∗
𝜃= 𝑙𝑛
𝑁𝑐 𝐴 𝑋2 − 𝑋 ∗
Drying Equipment
Tray Dryers
Freeze Dryers
Drum Dryer
• Consists of one or more internally-heated metal rolls on the outside of which a thin layer of
liquid is evaporated to dryness.
• The dried product is scraped from the surface of the drum as they slowly revolve.
• The drying rate can be controlled by adjusting rotation speed and temperature of drum.
• This type of dryer mainly handles the materials that are too thick for a spray dryer and too thin
for a rotary dryer.
Spray Dryer
• Provides a large area for heat and mass transfer by atomizing the liquid
to small droplets.
Rotary Dryer
• Consists of a revolving cylindrical shell, horizontal or slightly inclined toward the outlet.
• Wet feed enters one end of the cylinder and dry material discharges from the other.
• As the shell rotates, internal flights lift the solids and shower them down through the interior of
the shell.
• They are heated by direct contact of gas with the solids, by hot gas passing through an external
jacket, or by steam condensing in a set of longitudinal tubes mounted on the inner surface of
the shell.
• These types of dryers are widely used for continuous drying of powders and granules on a large
scale.
Screw-conveyor Dryers
• The solid to be dried is fed on to endless, perforated, conveyor belt through which hot air is
forced.
• The belt is housed in a long rectangular drying chamber or tunnel, which is divided into series of
separate sections, each with its own fan and air heater.
• Air may be recirculated through, and vented from each section separately or passed from one
section to another counter current to the solid movement.
Selection of Batch Dryer
Selection of Continuous Dryer