Drying tech/industrial dryers

Question 1

The different types of drying methods: (6)

Answer 1

Conventional Air 
Spray 
drum
vacuum
freeze
others

Question 2

What are the types of conventional air drying?

Answer 2

solar, cabinet, kiln, tunnel, belt, fluidized bed

Question 3

What factors differentiate the drying methods?

Answer 3

heat mode: direct vs indirect
operation mode: batch vs continuous
heating transfer: convection/conduction/radiation
moisture removal: sublimation/evaporation/sorption

Question 4

The simplest air dry technique is: ____. How does it work?

Answer 4

solar

sun heated air raises through bed with food, carry away moisture

Question 5

In kiln drying, air is heated by a ____, which is carried up to a ______ where food is held.
What products is this used for?

Answer 5

burner
slotted floor

particulates - cereals, seeds, wood

Question 6

How does cabinet drying work?

Answer 6

Air brought into cabinet by fans, heated by heaters -> remove moisture from product -> exhausted

Question 7

What types of air drying are continuous? How do they differ?

Answer 7

Tunnel - trucks carry loads of product through

Band (conveyor) - conveyor belt (perforated) carries product through

Question 8

the types of air currents/air flow that can be used:

Answer 8

co current
counter current
cross flow
through flow
fluidized bed

Question 9

In cross flow, the air moves in direction ____ to the product movement.

Answer 9

perpendicular

Question 10

compare co-current and counter current. What are the pros/cons of each?

Answer 10

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)

Question 11

How can you combine different air flows for better moisture removal?

Answer 11

use 2 step belt system: first dry rapidly, then slowly

or multi-pass: use several belts, alternating co and counter current

Question 12

In fluidized bed drying, ______ is used to achieve a ______ state. What products is this good for?

Answer 12

high velocity air; fluid like hydrodynamic state

low density products, particulates

Question 13

Particulate foods may be dried in a ___ drier, which provides ____.

Answer 13

rotary

agitation

Question 14

What is a good drying method for heat sensitive liquids or slurries? How does it work and what are the advantages?

Answer 14

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

Question 15

Spray driers have 2 essential parts:

What are their roles?

Answer 15

atomizer - reduce to invisible tiny droplets -> should fill entire chamber
cyclone separator - slows velocity so powder can settle and be collected

Question 16

What process involves controlled rehumidification? What is it for?

Answer 16

agglomeration - to make powder clumps (instant products)

humidify and re-dry

Question 17

What are the advantages of agglomerated instant powders? (4)

Answer 17

porous clumps - easily wettable
doesn’t float
dispersable
sinks slowly - doesn’t sediment

Question 18

What indirect drying process can be used for slurries? How can it be modified for more delicate products?

Answer 18

drum drying

vacuum drum drying

Question 19

How does drum drying work to make powder?

Answer 19

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

Question 20

Why would a vacuum chamber be used for drum drying - what are the advantages?

Answer 20

lower boil point of water - lower temp needed
more rapid drying due to vacuum

so good for delicate goods

Question 21

How might the product be applied to the drum dryer?

Answer 21

top fed, roller fed, dip fed, spray fed, pan fed

Question 22

How does structure of freeze-dried goods differ from regular dried goods? Why?

Answer 22

maintains structure volume, more delicate (normal dried will be collapsed, shrunken)
water is frozen to maintain structure of matrix, directly sublimated and removed

Question 23

When the 3 states of water can coexist, what is this known as? What conditions are required?

Answer 23

Triple point

0C, 4.58 Torr (pressure)

Question 24

At the triple point, what can occur?

Answer 24

direct conversion of vapor to ice, or ice to vapor; or between other states

Question 25

Describe the process of freeze-drying:

Answer 25

product frozen
pressure lowered (vacuum) below Torr 4.58
Now can cross directly from ice -> vapor states (either increase temp, or decrease pressure further)

Question 26

How is heat transfer accomplished in freeze drying?

Answer 26

conduction (heated plates)

radiation (heat radiates from plates through empty spaces)

Question 27

Between the heat plates and the product, there is a layer of:
Why?

Answer 27

expanded metal plate

account for changing volume of frozen product

Question 28

what machinery components are absolutely necessary for freeze drying?

Answer 28

vacuum pump - make vacuum 0.1-2mmHg

condensor - remove vapor

Question 29

overall, freeze drying is a very (fast/slow) process, and is very (expensive/economical).

Answer 29

slow; expensive

Question 30

A similar method to freeze drying:

What are the differences?

Answer 30

vacuum-shelf drier
no freezing
less vacuum

Question 31

Vacuum shelf drying is also known as: ____. What causes this?
What are advantages of this method?

Answer 31

puff drying - due to rapid air/liquid movement

faster moisture removal

Question 32

What method involves use of a foaming agent? Describe it.

Answer 32

foam mat drying
liquid concentrate + foaming agent is cooled, then beaten with gas to make foam (+ stabilizer)
dried -> porous solid -> flaked/screened into powder

Question 33

What is used as a foaming agent? What is used as a stabilizer for foams?

Answer 33

proteins

glycerol

Question 34

What are other novel drying methods?

Answer 34

air impingement
sorption drying
vibro-fluidized bed drying
spouted bed dryer
particulate drying with inert particles
osmo-MW drying

Question 35

4 main parts of MW osmotic drying:

Answer 35

1. gear pump continuously supply osmotic solution
2. temperature maintained by water bath, monitored by thermocouple
3. spray head disperse solution on sample
4. sample elevated in MW path

Question 36

what is inert particulate drying?

Answer 36

particulate product mixed with HOT inert particulates to transfer heat and dry product -> separated and collected (continuous process)

Question 37

What is a new technology similar to fluidized bed and spray drying?

Answer 37

spouted bed
funnel shaped, air blows up to lift liquid/slurry
mixed in with INERT particles to help transfer heat

Question 38

How is vibro-fluidized bed different from the original method?

Answer 38

includes mechanical vibrator/agitator to increase turbulence of particles

Question 39

What are some sorbents that can be used for drying?

Answer 39

silica gel, activated alumina, clay adsorbent

Question 40

What happens in impingement air drying?

Answer 40

multiple jets of high velocity hot air used to dry product (several nozzles)