Drying Flashcards
Convective drying
Drying of Wet Solids
Conductive drying
Drying of Wet Solids
Radiation drying
Drying of Wet Solids
Fixed (or static) bed convective drying
Convective drying of wet solids
Dynamic convective driers
Convective drying of wet solids
Factors affecting drying in this manner can be illustrated by reference to the construction and use if a simple form of drier – the tray (shelf or compartment) drier
Fixed (or static) bed convective drying
Fluidized bed drier
Dynamic convective driers
An excellent method of obtaining good contact between the warm drying air and wet particles is found in fluidized bed drier
Dynamic convective driers
General principle of Fluidized bed drier
fluidization
In this process the wet solid is in thermal contact with a hot surface and the bulk of heat transfer occurs by conduction
Conductive Drying of Wet Solids
Vacuum oven and vacuum tumbling drier
Conductive Drying of Wet Solids
Consists of jacketed vessel sufficiently stout in construction to withstand vacuum within the oven and steam pressure in the jacket
Vacuum Oven
Main advantage – is that drying takes place at a low temperature, and as there is little air present there is minimum risk of oxidation. The temperature of the drying solid will rise to the steam or water temperature at the end of the drying, but this is not usually harmful
Vacuum Oven
One design of tumbler drier resembles a large Y cone mixer
Vacuum Tumbling Drier
The vessel is steam jacketed and is connected to a vacuum.
Vacuum Tumbling Drier
It can be used for drying tablet granules, which tumble over the heated surface as the vessel slowly revolves.
Vacuum Tumbling Drier
Heat transfer rates in this equipment are much higher than can be attained in a convectional vacuum oven, where the material is static
Vacuum Tumbling Drier
Radiant heat transmission
Radiation Drying of Wet Solids
use of microwave radiation
Radiation Drying of Wet Solids
Differs from heat transfer by conduction or convection in that no transfer medium (solid, liquid, or gaseous) need be present
Radiant Heat Transmission
Heat energy in the form of radiation can cross empty space or travel through the atmosphere virtually without loss. If it falls on a body capable of absorbing it then it appears as heat, although a proportion maybe reflected or transmitted
Radiant Heat Transmission
________ radiation is seldom used as heat source because it is absorbed very quickly and does not penetrate far into the wet mass. The surface layers dry quickly and the absorption of further energy then raises the temperature of the dry material to a high value, which is often detrimental to the product
Infrared
______ radiation in the wavelength range 10 nm to 1 m penetrates much better than IR radiation
Microwave
Microwaves are produced by an electronic device known as
magnetron
To avoid interference with radio and tv microwave is permitted to operated only at certain frequencies, which normally ____________
960 and 2450 MHz
rapid drying at fairly low temperature
Advantages of Microwave Drying
Thermal efficiency is high, as the drier casing and
the air remain cool
Advantages of Microwave Drying
Batch size of Microwave Drying in commercial production microwave driers is _____ than those available for fluidized bed driers
smaller
objective of these driers is to spread the liquid to a large surface area for heat and mass transfer and to provide an effective means of collecting the dry solids
Driers for Dilute Solutions and
Suspensions
spreading the liquid to a thin film on to a drum
Drum driers
dispersing the liquid to a spray of small droplets
spray driers
Drum driers and spray driers
Driers for Dilute Solutions and
Suspensions
Can handle a variety of materials, either as solutions or as suspensions
Drum Driers
substances that
are dried by this method include starch products, ferrous salts and suspensions of kaolin and zinc oxide
Drum Driers
Method gives rapid drying, the thin film spread over a large area resulting in rapid heat and mass transfer
Advantages of Drum Driers
equipment is compact, occupying much less space than the spray drier
Advantages of Drum Driers
Heating time is short, being only a few seconds
Advantages of Drum Driers
drum can be enclosed in vacuum jacket,
enabling the temperature of drying to be reduced
Advantages of Drum Driers
product is obtained in flake form, which is convenient for many purposes
Advantages of Drum Driers
Operating conditions are critical and it is necessary to impose careful control on feed rate, film thickness, speed of drum rotation and drum temperature
Disadvantage of Drum Drier
Provides a large surface area for heat and mass transfer by atomizing the liquid to small droplets
Spray Drier
These are sprayed into a stream of hot air, so that each droplet dries to an individual solid particle
Spray Drier
The equipment is very bulky, and with the ancillary equipment is expensive. In a large installation the drying chamber alone may be as much as 15 m in height and 6 m in diameter
Disadvantages of Spray Drying Process
overall thermal efficiency is rather low, as the air must still be hot enough when it leaves the drier to avoid condensation of moisture
Disadvantages of Spray Drying Process
include citric acid, sodium phosphate gelatin, starch, barium sulfate, calc
Spray Drying Process
