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Unit Operations in Food Processing5. Drying of solids
Dr. JIA Yuanyuan
[email protected]
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Learning on Internet
Food Processing
Handbookhttp://rapidshare.com/files/103853443/Food_Processing_Handbook.pdf
Unit operations in food
processinghttp://www.nzifst.org.nz/unitoperations
Website of GEA Process Engineering Inc. ---World Leaders in
DryingTechnology and Equipment
http://www.niroinc.com/about/about_Niro_Inc.asp
Dairy Processing Handbook, Chapter
17http://www.ales2.ualberta.ca/afns/courses/nufs403/PDFs/chapter17.pdf
Animations provided by Prof.
Singhhttp://rpaulsingh.com/animated%20figures/animationlisttopic.htm
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General principles
Removing relatively small amount of water in
solid foods to an acceptably low value byheat .
Mechanical separations Cheaper. To reduce water as
much as possible before dryingDrying Energy consumptive
Preservation in
conjunctionwith otherprocessing
First
Next
The earliest few techniques forpreservation foods in human
history
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Humidity of air
Wet air = dry air + humidity 1. Steam partial pressure in
air
P=pa+pWFor saturated air, pW=ps
2. Humidity
3. Relative humidityFor saturated air:
For unsaturated air:
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A combination of heat transfer and
mass transfer
Q
pw, H
N
t
Moisture
tw
ps, Hw
Heat transfer byconvection, conduction
or irradiation
Driving force: t-twQ=A(t-tw)
Mass transferDriving force:ps-pw or Hw-HN=kHA(HW-H)
Oppositedirection
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Phase equilibrium for moist solids in air
Equilibrium
moisture
Water
contentona
drybasisX,p
ercent
Relative humidity H, percent
Freemoisture
Boundwater
Unbound water
Equili
brium
moisture
This curve is nearly independent oftemperature.X: mass of water
per unit mass of
bone-dry solid.w: mass of water per unit mass ofwet solid.
X=w/(100-w)
X*: the content ofwater cannot be
removed by the air
has a definiterelative humidity
The differencebetween the totalwater content ofthe solid and
X*
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Watercontent
onadrybasisX,percent
Relative humidity H, percent
Freem
oisture
Boundwater
Unbound water
Equilibrium
moisture
Bound water exerts a
vapor pressure less thanthat of liquid water at thesame
temperature.Water in fine capillaries;in cell or fiber walls.
Unbound water exerts its fullvapor pressure and largely heldin
the voids of the solid.
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Continuous air dryer
Blower Preheater
Air
Wet
solid
Dry
product
Exhaustair
Steam
Condensate
Hot air
Convective drying
Cross-circulation
Dryer
Carrier ofheat andmoisture
One-through. It canalso be circulatedpartially to save
energy
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Drying rates under constant drying
conditions
Critical moisturecontent is not only aproperty of material.It
also varies with
the thickness anddrying rate.
Equilibriummoisture
The temperature,
humidity, velocity,and direction of flowof the air across
thedrying surface areconstant.
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Drying time under constant drying
conditions
21 +=Drying time inconstant rate period
Drying time in
falling rate period
Governed by the vaporizationof surface moisture.
Therefore,higher temperature, faster airvelocity and lower
humidity
lead to higher drying rate.
Governed by diffusion of
internal moisture to thesurface.
Decreasing thethickness of thematerial gives a longercritical
moisture content.
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Drying equipment
Spray dryers---for slurries
Click
Nozzle atomizesthe liquid into tinydrops. The heartof the
process
Very short dryingtime; Permits dryinghighly
heat-sensitivematerials.Example: milk
powder,
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Rotating disc for atomizing milk
Stationary nozzles
Counterflow Nozzle discharging in thedirection of the air
flow
Spray dryers oftencombine with
cyclones to recoverthe powder in
exhaust airNEXT PAGE
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Milk powder manufacture
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Fluid-bed dryers: for powders, granules, agglomerates,and
pellets between 50 microns and 5 mm.
Click Cyclone if fine particlesare present
Click
http://www.youtube.com/watch?v=vQmfmsJpp9I&feature=related
Click
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Vibrating Fluid-bed dryer
Click
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The VIBRO-FLUIDIZER from GEA Niro
http://www.niro.com/niro/cmsdoc.nsf/webdoc/webb7n3c3l
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Fluid-bed dryer
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Tray-dryers
Click Useful when the
production rate is small.Expensive to operatebecause of labor
cost.Maybe operated undervacuum.
Many smaller
try dryers inour labs.
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Disc Type Drier
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Series Flow Through RotationDrier
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Drum-dryers: conductive heatingCentral
feedKnife
Steamheated
drum
Conveyor
Effective with dilute,concentratedsolutions andmoderately
heavy
slurries, such ashigh heat milkpowder or babyfoods.
More details:
http://www.gmfgouda.com/drum-dryer/info-drum-dryer.html#top
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A 100% closed system
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pneumatic conveying dryer
----10~20m
----
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10~20/s0.5
1m
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60
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Freeze-drying or lyophilization:
Principles
1Torr1mmHg133 Pa
Moisture removedfrom ice directly tothe vapor state by
sublimation
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Advantages: maintaining the structure of the food; flavor
retention; higherquality.
Disadvantages: long operation time; small throughout; high
operation
cost; for high valued foods.
Examples: instant coffee; backpacker meals; active enzymes;
VitaminsSpecial for space foods
http://videos.howstuffworks.com/nasa/2205-how-space-food-processing-works-video.htm
Schematic operation of freeze dryer
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Volume
1,200,000 ml100Pa
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Industrial Freeze
dryer
Freeze dried fruit andvegetable
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Student activities
1. Watch the video. Try to tell where is the spray dryerand
where are cyclones and blowers.
http://www.youtube.com/watch?v=Pt25GuAf7cI&feature=related
2. Tour labs and pilot plant of Food Science and
Engineering Department to see how many varietiesof driers and
understand their functions.
Click
