Lecture 16: Drying Technology and Industrial Dryers Flashcards
What are factors that differntiate drying methods?
- heat transfer: direct or indirect
- heating mode: convection, conduction, radiation
- operations: batch or continuous
- moisture removal: evaporation, sublimination, sorption
what are spray driers used for?
what happens in this process?
- slurries and liquids
- they are atomized to form small droplets
what are benefits of spray dryers?
- provides large surface area
- fast drying rates and short contact time
- suitable for heat sensitive foods
- rapid drying keeps product cool
what are the main 2 parts of the spray dryer?
atomizer: dispenses product in a fine spray
cyclone separator: separates air from dry powder
describe the heat transfer in spray dryer:
- higher velocity hot air or forced convection
- direct contact
what is fluidization? how is it achieved?
fluid-like hydrodynamic state of particles
- achieved by high velocity air flow
describe agglomerization
what are the steps?
process of making clumps of powdery materials
- controlled humidification of powders to form small clumps
- Re-drying of clumps
what is agglomerization used to make?
instant powders
why is agglomerization often used to make instant powders?
- desireable wettable surfaces
- sinkability
- dispensability
- resistant to sedimentation
what type of contact do roller/drum driers have?
indirect contact
what are the steps of drum driers?
- the hot rotating roller/drum is kept hot by steam flow inside
- heat is transferred by conduction to the product
- product is applied as a thin film
- product is scraped by a contact blade at the other end of drum
what are the benefits of drum driers?
what products is it suitable for?
- fast and efficient (since product is applied as a thin film, it is dried fast)
- suitable for slurries
in drum drying, how is heat transferred?
by conduction
what is the difference of vacuum roller drum drier and the regular drum drier?
what is the significance of this?
vacuum roller drum drier is has drum locate in a vacuum chamber
the vacuum creates a lower boiling point of water, which results in faster drying. Thus, the drying can be applied at a lower temp
useful for delicate foods
describe the drying process in freeze drying
drying by sublimination (ice to vapour w/o entering liquid phase)
what is the significance of drying by sublimation in freeze drying?
in conventional drying, H2O evaporation collapses the tissue due to loss in cell turgidity
in freeze drying, there is no movement of liquid water due to sublimination. Thus the original structure and volume is preserved
describe the product of freeze drying
has lower bulk density and will be more fragile
what is the triple point of water?
4.58mmHg and 32degF
how is heat transferred during freeze drying?
conduction and radiation from hot surface
- extremely rapid sublimination rate due to high vacuum
what are requirements needed for freeze drying?
- very efficient dryer (to be able to create high vacuum)
- expensive (mostly used for delicate items)
- very slow process
what is vacuum shelf drying similar to? how is it different?
similar to freeze drying
difference:
- the product is not frozen
- vacuum is not as high, but still high enough to cause fast evaporation of liquid water
what causes puff dryig?
when the rapid movement of water causes foaming or puffing in vacuum shelf dryers
describe the mode of het transfer in vacuum shelf drier
radiation and conduction
in osmotic drying, does H2O, sugar and solutes move into product or solution?
H2O: product to solution
sugar: solution to product
solutes: product to solution
