Drying/ Pengeringan

Drying process can generally be defined as the process of removing an amount of water (in small amounts) contained in a material. While evaporation can be defined as the process of removing an amount of water (in considerable numbers) contained in a material. In the process of evaporation, the water is removed from the material in the form of steam by the time the material reaches its boiling point. While in the process of drying, the water is usually removed in vapor form with the help of hot gas.

pict 1 (Rotary Dryer)

(Source :http://image.made-in-china.com)

            

The air that enters the dryer rarely are in a state completely dry, but always contains wetness and have a certain relative humidity. For the air that have a certain humidity, wetness content in the solids out of the dryer can not be less than wetness balance relating to air humidity sign. Part of water contained in the wet solids which can not be removed with the incoming air, because the incoming air contains moisture Similarly, the so-called wetting balance (equilibrium moisture). So although it has undergone the process of drying, the material can not be fully free of water content. Water can be removed only to the limits contentnya equilibrium moisture. The water content of the products that have undergone a process of drying varies depending on the type of product. For example dried salt contains approximately 0.5% water, coal contains about 4% water, and most food products contain approximately 5% water.

            Methods and drying process can be classified into several different ways, including batch process and continuous process. Drying process is classified as a batch process, when material is inserted into the drying apparatus and processed at a given time. Whereas in a continuous process, material is inserted continuously into the drying apparatus and the material is dried moved constantly as well. Drying process can also be categorized according to their physical condition when adding heat and remove moisture, namely:

1. In the first category, the heat added by direct contact with heated air at atmospheric pressure and water vapor that is formed is removed by air.
2. In the vacuum drying, evaporation of water works well at low pressures, and heat is added indirectly by contact with a steel wall or with radiation
3. In freeze drying, water undergoes the process of sublimation of the frozen material. 

(Geankoplis, 1997)

Source :

Geankoplis, C. J, 1997, �Transport Process and Unit Operations�,3^rd ed, pp 521 �547, University of Minnesota: New Delhi.

KEYWORD :

DRYING, Drying, KATEGORI DRYING, CLASSIFICATON DRYING