Moisture Contents and Its Types

Moisture :

moisture is the amount of the water that is present in an compound . these water content in the substance  present in the three forms , which are as follows  :

Total Moisture Content :

this is the total amount of the liquid associated with a wet solid . the easily removable water is known as free moisture content  . and the moisture in which is more difficult to remove is the equilibrium moisture content .

UnBound Water :

it is the amount of the water held by material that exerts an equilibrium vapour pressure equal to that of pure water at same temperature . during a drying process this water is easily lost but the resulting solid is not completely free from water molecules  .

Bound Water :

it is the minimum water held by the material that exerts equilibrium vapour pressure less then the pure water at the same temperature  . such moisture is described as "bound" and is more difficult to remove then unbound water .

Notes  : -

1. hygroscopic material water in : - fine capillaries  , cells and fibre walls and in physical interaction . it contains bound water .  condition : -  vapour presure of wet solids is less then the vapour pressure of pure water .

2. Non-hygroscopic materials contain water in void spaces  . it contains unbound water . Conditions : -  Vapour Pressure of wet solids is equal to vapour pressure of pure water .

 

3. heat must be transferred to the material to be dried in order to supply the latent heat required for the vaporization of the moisture  . water difusses through the material to the surface and subsequently evaporates into the air stream . Thus Drying involves both heat transfer and mass transfer  . operations simentaniously .

 

4. Theory of drying can be described by "Equilibium Relationship" and "Rate Relationship["