Lecture 16: Drying Technology and Industrial Dryers

Question 1

What are factors that differntiate drying methods?

Answer 1

1. heat transfer: direct or indirect
2. heating mode: convection, conduction, radiation
3. operations: batch or continuous
4. moisture removal: evaporation, sublimination, sorption

Question 2

what are spray driers used for?

what happens in this process?

Answer 2

• slurries and liquids

- they are atomized to form small droplets

Question 3

what are benefits of spray dryers?

Answer 3

• provides large surface area
• fast drying rates and short contact time
• suitable for heat sensitive foods
• rapid drying keeps product cool

Question 4

what are the main 2 parts of the spray dryer?

Answer 4

atomizer: dispenses product in a fine spray

cyclone separator: separates air from dry powder

Question 5

describe the heat transfer in spray dryer:

Answer 5

• higher velocity hot air or forced convection

- direct contact

Question 6

what is fluidization? how is it achieved?

Answer 6

fluid-like hydrodynamic state of particles

- achieved by high velocity air flow

Question 7

describe agglomerization

what are the steps?

Answer 7

process of making clumps of powdery materials

1. controlled humidification of powders to form small clumps
2. Re-drying of clumps

Question 8

what is agglomerization used to make?

Answer 8

instant powders

Question 9

why is agglomerization often used to make instant powders?

Answer 9

• desireable wettable surfaces
• sinkability
• dispensability
• resistant to sedimentation

Question 10

what type of contact do roller/drum driers have?

Answer 10

indirect contact

Question 11

what are the steps of drum driers?

Answer 11

1. the hot rotating roller/drum is kept hot by steam flow inside
2. heat is transferred by conduction to the product
3. product is applied as a thin film
4. product is scraped by a contact blade at the other end of drum

Question 12

what are the benefits of drum driers?

what products is it suitable for?

Answer 12

• fast and efficient (since product is applied as a thin film, it is dried fast)
• suitable for slurries

Question 13

in drum drying, how is heat transferred?

Answer 13

by conduction

Question 14

what is the difference of vacuum roller drum drier and the regular drum drier?

what is the significance of this?

Answer 14

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

Question 15

describe the drying process in freeze drying

Answer 15

drying by sublimination (ice to vapour w/o entering liquid phase)

Question 16

what is the significance of drying by sublimation in freeze drying?

Answer 16

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

Question 17

describe the product of freeze drying

Answer 17

has lower bulk density and will be more fragile

Question 18

what is the triple point of water?

Answer 18

4.58mmHg and 32degF

Question 19

how is heat transferred during freeze drying?

Answer 19

conduction and radiation from hot surface

- extremely rapid sublimination rate due to high vacuum

Question 20

what are requirements needed for freeze drying?

Answer 20

• very efficient dryer (to be able to create high vacuum)
• expensive (mostly used for delicate items)
• very slow process

Question 21

what is vacuum shelf drying similar to? how is it different?

Answer 21

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

Question 22

what causes puff dryig?

Answer 22

when the rapid movement of water causes foaming or puffing in vacuum shelf dryers

Question 23

describe the mode of het transfer in vacuum shelf drier

Answer 23

radiation and conduction

Question 24

in osmotic drying, does H2O, sugar and solutes move into product or solution?

Answer 24

H2O: product to solution

sugar: solution to product
solutes: product to solution