Drying Flashcards
Almost all drying processes involve…
…the removal of water by vaporization and thus require the addition of heat
What are the methods of drying?
- By passing hot gases over its surface (tray and spray dryers)
- Indirect heating of the substance containing water (drum and rotary dryers)
The final choice of dryer depends on….
…the amount and kind of material to be dried, the type of final product, available facilities and the economics of the process
Explain how a tray dryer works
A tray dryer contains a number of trays in which the material to be dried is placed. Hot air passes over these trays and carries away the evolved water vapour. When the material becomes sufficiently dry, the trays and the contents are removed.
Explain how a drum dryer works
A drum dryer consists essentially of a revolving drum with heat supplied to the inside of the drum. The liquid will cling to the outside of the revolving surface in a thin film and be dried in one revolution. The dried material is then removed by a knife blade
Explain how a rotary dryer works
Rotary dryers consist of a horizontal cylindrical shell rotating around its horizontal axis. The wet feed material enters one end of the cylindrical shell and hot gases are forced into the other end. The rotating action lifts the wet material and permits it to drop through the hot gas, thus bringing about a rapid rate of drying. The one end of the cylinder is slightly lower than the other end to permit the feed to move through the drier by gravity.
Explain how a spray dryer works
Liquid mixtures may be dried very effectively by spraying the mixture into a stream of hot gases. The small droplets fall through the hot gases and the large amount of heat-transfer area exposed permits a rapid rate of drying.
What are the advantages to using vacuum dryers
It is possible to obtain high rates of water removal at low temperatures. Heat sensitive materials are often dried this way.
If a material is exposed to air at a given temperature and humidity….
….the material will either lose or gain water until an equilibrium condition is established
Moisture may be present in either or both of what forms….?
- Bound moisture
- Free moisture
Describe bound moisture
Water retained in such a way that it exerts a vapour pressure less than that if free water at the same temperature.
It is retained in small capillaries, absorbed on surface or as a solution in cell walls
Describe free moisture
Water which is in excess of the equilibrium moisture content (F = X - XE)
What is constant drying state
When drying is carried our under conditions of airflow which is so high that the increase in air humidity and the decrease in temperature of the air is so small that it can be ignored
Drying is carried out for what reasons?
- To reduce the cost of transport
- To make a material more suitable for handling
- To provide definite properties (free flowing salt)
- To avoid the presence of moisture which may lead to corrosion
