Flashcards in Drying lecture 20 Deck (16):
Why is drying important in quality tests, in what concepts?
Define moisture content:
the amount of water contained in a material
% moisture by volume: the molecules of water per unit volume by the total number of molecules per unit volume
Define relative humidity:
the amount of moisture in the air as a % of the max amount of moisture that the air can hold
What is the relation between temp, RH and water vapour?
the higher the temp, the more water vapour that the air can hold and so there is a decrease in RH
Define dew point:
just as increasing temp decreases RH, decreasing temp will increase RH and when RH reaches 100%, water vapour will condense to form dew
During drying, what 2 factors can change?
1. moisture content (water can evaporate)
2. temperature (can decrease to reduce heat-exchange with the material)
Define hygroscopic substances:
attract and hold water molecules from the surrounding environment e.g. paper (if less moisture in the air, paper will release water) and some plastics
Total moisture = ...
FREE (unbound) + EQUILIBRIUM (bound)
How can equilibrium moisture be varied?
by varying external conditions such as adding a desiccant like silica gel or phosphorous pentoxide to reduce RH
What are the 2 types of solute migration?
1. inter-granular migration = solute moves between granules towards the surface (differences in solute composition between granules)
2. intra-granular migration = granules are separated during the process (even distribution of solute within a granule)
How can you reduce solute migration (to ensure homogeneity)?
a) Initial moisture content
b) Slow convective drying
c) Drying by microwave radiation
d) Dynamic drying methods
What are the 3 methods to dry wet solids?
1. fluidised bed dryer (convection)
2. microwave (radiation)
3. vacuum oven (conduction)
What are the 2 methods to dry solutions and suspensions?
1. spray dryer
Why is solvent removal useful?
to prevent chemical and microbial degradation