Drying tech/industrial dryers Flashcards
The different types of drying methods: (6)
Conventional Air Spray drum vacuum freeze others
What are the types of conventional air drying?
solar, cabinet, kiln, tunnel, belt, fluidized bed
What factors differentiate the drying methods?
heat mode: direct vs indirect
operation mode: batch vs continuous
heating transfer: convection/conduction/radiation
moisture removal: sublimation/evaporation/sorption
The simplest air dry technique is: ____. How does it work?
solar
sun heated air raises through bed with food, carry away moisture
In kiln drying, air is heated by a ____, which is carried up to a ______ where food is held.
What products is this used for?
burner
slotted floor
particulates - cereals, seeds, wood
How does cabinet drying work?
Air brought into cabinet by fans, heated by heaters -> remove moisture from product -> exhausted
What types of air drying are continuous? How do they differ?
Tunnel - trucks carry loads of product through
Band (conveyor) - conveyor belt (perforated) carries product through
the types of air currents/air flow that can be used:
co current counter current cross flow through flow fluidized bed
In cross flow, the air moves in direction ____ to the product movement.
perpendicular
compare co-current and counter current. What are the pros/cons of each?
co-current: air and product move in same direction
counter: move in opposite directions.
co-current is SLOWER, but BETTER QUALITY
counter is FASTER, but may cause excess drying (dry air reaches final product first)
How can you combine different air flows for better moisture removal?
use 2 step belt system: first dry rapidly, then slowly
or multi-pass: use several belts, alternating co and counter current
In fluidized bed drying, ______ is used to achieve a ______ state. What products is this good for?
high velocity air; fluid like hydrodynamic state
low density products, particulates
Particulate foods may be dried in a ___ drier, which provides ____.
rotary
agitation
What is a good drying method for heat sensitive liquids or slurries? How does it work and what are the advantages?
spray drying
atomize to fine droplets -> use hot air to dry
cyclone separator - separate air from dry powder
advantages: continuous (large through-put), large surface area -> very rapid, short contact time, keeps product cool
Spray driers have 2 essential parts:
What are their roles?
atomizer - reduce to invisible tiny droplets -> should fill entire chamber
cyclone separator - slows velocity so powder can settle and be collected
What process involves controlled rehumidification? What is it for?
agglomeration - to make powder clumps (instant products)
humidify and re-dry
What are the advantages of agglomerated instant powders? (4)
porous clumps - easily wettable
doesn’t float
dispersable
sinks slowly - doesn’t sediment
What indirect drying process can be used for slurries? How can it be modified for more delicate products?
drum drying
vacuum drum drying
How does drum drying work to make powder?
product applied in thin layer on rotating drum
drum heated by hot steam -> conduction heating
dried in 1 turn (usually) -> scraped off by blade at end
crush dry flakes to make powder
Why would a vacuum chamber be used for drum drying - what are the advantages?
lower boil point of water - lower temp needed
more rapid drying due to vacuum
so good for delicate goods
How might the product be applied to the drum dryer?
top fed, roller fed, dip fed, spray fed, pan fed
How does structure of freeze-dried goods differ from regular dried goods? Why?
maintains structure volume, more delicate (normal dried will be collapsed, shrunken)
water is frozen to maintain structure of matrix, directly sublimated and removed
When the 3 states of water can coexist, what is this known as? What conditions are required?
Triple point
0C, 4.58 Torr (pressure)
At the triple point, what can occur?
direct conversion of vapor to ice, or ice to vapor; or between other states
Describe the process of freeze-drying:
product frozen
pressure lowered (vacuum) below Torr 4.58
Now can cross directly from ice -> vapor states (either increase temp, or decrease pressure further)
How is heat transfer accomplished in freeze drying?
conduction (heated plates)
radiation (heat radiates from plates through empty spaces)
Between the heat plates and the product, there is a layer of:
Why?
expanded metal plate
account for changing volume of frozen product
what machinery components are absolutely necessary for freeze drying?
vacuum pump - make vacuum 0.1-2mmHg
condensor - remove vapor
overall, freeze drying is a very (fast/slow) process, and is very (expensive/economical).
slow; expensive
A similar method to freeze drying:
What are the differences?
vacuum-shelf drier
no freezing
less vacuum
Vacuum shelf drying is also known as: ____. What causes this?
What are advantages of this method?
puff drying - due to rapid air/liquid movement
faster moisture removal
What method involves use of a foaming agent? Describe it.
foam mat drying
liquid concentrate + foaming agent is cooled, then beaten with gas to make foam (+ stabilizer)
dried -> porous solid -> flaked/screened into powder
What is used as a foaming agent? What is used as a stabilizer for foams?
proteins
glycerol
What are other novel drying methods?
air impingement sorption drying vibro-fluidized bed drying spouted bed dryer particulate drying with inert particles osmo-MW drying
4 main parts of MW osmotic drying:
- gear pump continuously supply osmotic solution
- temperature maintained by water bath, monitored by thermocouple
- spray head disperse solution on sample
- sample elevated in MW path
what is inert particulate drying?
particulate product mixed with HOT inert particulates to transfer heat and dry product -> separated and collected (continuous process)
What is a new technology similar to fluidized bed and spray drying?
spouted bed
funnel shaped, air blows up to lift liquid/slurry
mixed in with INERT particles to help transfer heat
How is vibro-fluidized bed different from the original method?
includes mechanical vibrator/agitator to increase turbulence of particles
What are some sorbents that can be used for drying?
silica gel, activated alumina, clay adsorbent
What happens in impingement air drying?
multiple jets of high velocity hot air used to dry product (several nozzles)
