Download drying - UniMAP Portal

ERT 356 UNIT OPERATIONS
DRYING
PREPARED BY:
AIMI ATHIRAH BINTI AZNAN
LECTURER
SCHOOL OF BIOPROCESS ENGINEERING, UNIMAP
Topic 2: Drying
• In
this topic, you will learn on:
 Basic drying theory
 Heat and mass transfer in drying
 Equilibrium moisture content
 Air drying
 Conduction drying
 Drying equipment
• Lab
1: Moisture content in different system
INTRODUCTION
One of the
oldest
methods of
preserving
food
Drying meat &
fish under the sun
Drying
Stored for
long periods
Still
important in
food
preservation
WHY DRIED FOOD CAN BE
STORED FOR LONG PERIOD?
Microorganisms
Cause: Food spoilage & decay
 Unable to grow & multiply
In the absence of sufficient water
Cause: Promote undesired changes in the
chemical composition of the food
 Malfunction without water
INTRODUCTION cont.
Drying is accomplished by vaporizing the water in the food by
supplying the latent heat of vaporization.
 Process-controlling factors that enter into the unit operation of drying:
a) Transfer of heat to provide the necessary latent heat of vaporization.
b) Movement of water vapor through the food and then away from it to separate the
water from food.
INTRODUCTION cont.
3 categories of drying processes.
Sublimed: Change directly into vapor when heated
BASIC DRYING THEORY
THREE STATES OF WATER
•
3 state of pure water (Solid, liquid,
vapor)
•
Each state at any time depends on
temperature and pressure condition.
•
What can you describe on:
 Point O The Triple Point
 Line A – A’
 Line B – B’
0.0098ºC,
0.64kPa
•
Line OP:
 Liquid & vapor coexist in equilibrium
 Line = vapor pressure/temperature
line.
 Vapor pressure = the measurement of
the tendency of molecules to escape as
a gas from liquid.
THREE STATES OF
WATER
THREE STATES OF WATER
cont.
•
Boiling occurs when:
 water vapor pressure = total pressure on the
water surface
•
Boiling point at atmospheric pressure?
•
At pressures above or below atm pressure,
water boils at corresponding temperatures
abv or below 100ºC.
1 atm = 101.325 kPa
HEAT REQUIREMENTS FOR
VAPORIZATION
•
Energy supplied to vaporized the water at any temperature depends on
the required temperature.
•
If from liquid = LATENT HEAT OF VAPORIZATION; The quantity of
energy required per kg of water.
•
If from solid = LATENT HEAT OF SUBLIMATION.
•
The heat energy required can be calculated from latent heats value
given in the steam table in Appendix 8.
•
http://www.nzifst.org.nz/unitoperations/appendix8.htm
EXAMPLE 1: Heat Energy in Air Drying
A food containing 80% water is to be dried at 100°C down to moisture
content of 10%. If the initial temperature of the food is 21°C, calculate:
• The quantity of heat energy required per unit weight of the original
material for drying under atm pressure
• The energy requirement/kg water removed
Latent heat of vaporization of water at 100°C at standard atm pressure =
2257kJ/kg
Specific heat capacity of the food = 3.8kJ/kg.°C
Specific heat capacity of water= 4.186kJ/kg.°C
HEAT REQUIREMENTS FOR
VAPORIZATION cont.
Heat Energy in Vacuum
Drying
Using the same material as in
previous example, if vacuum drying
is to be carried out at 60°C under
the
corresponding
saturation
pressure of 20kPa abs.
Calculate
the
heat
energy
required to remove the moisture
per unit weight of raw material.
HEAT TRANSFER IN DRYING
•
The rates of drying are generally determined by the rates at which heat energy can be
transferred to the water or ice.
•
All of the 3 mechanisms of heat transfer (conduction, radiation and convention) may enter
into drying.
•
Rate of heat transfer in air drying (conduction) is:
q = hsA(Ta – Ts)
q = Heat transfer rate (Js-1)
hs = Surface heat transfer coefficient (Jm -2s-1ºC)
A = Area through which the heat flow (m2)
Ta = Air temperature (ºC)
Ts = Drying surface temperature (ºC)
HEAT TRANSFER IN DRYING
•
Rate of heat transfer in roller dryer (conduction) is:
q = UA(Td – Ts)
q = Heat transfer rate
U = Overall heat transfer coefficient
A = Area of drying surface on the drum
Td = Drum temperature
Ts = Surface temperature of the food
Moist material is spread over the
surface of the heated drum
HEAT TRANSFER IN DRYING
•
By radiation, the surface temperature of the food may be higher than the air
temperature.
•
Estimation of surface temperature can be made using the relationships developed for
radiant heat transfer.
•
In freeze drying, energy must be transferred to the surface at which sublimation
occurs.
•
However, the energy must be supplied at such rate as not to increase the temperature
at the drying surface above the freezing point.
•
In most freeze drying application, the heat is transferred mainly by conduction.
•
As drying proceeds, the character of the heat transfer changes.
•
Dry material begins to occupy the surface layers and conduction must take place
through these dry layers which are poor heat conductors so that heat is transferred to
the drying region progressively more slowly.
MASS TRANSFER IN DRYING
•
Mass is transferred under the driving force provided by a partial pressure or
concentration difference.
•
The rate of mass transfer is proportional to the potential pressure or concentration
difference and to the properties of the transfer system characterized by a mass
transfer coefficient.
𝑑𝑤
𝑑𝑡
= kg’ A ∆Y
dw/dt = mass (moisture) being transferred (kg/s) in time
A = Area through which the transfer is taking place
kg’ = mass transfer coefficient (kgm-2s-1)
ΔY = humidity difference (kg/kg)
PSYCHROMETRY
The study of moist air or
mixture of dry air & water
vapor
•
Humidity (Y): Measure water content of the air.
•
Absolute humidity or Humidity Ratio: Mass of water vapor per unit mass of dry air
(kg/kg)
•
Saturated air: Air is saturated with water vapor at given T
and P if it achieved maximum humidity.
If add >water; it appear as liquid water
(mist/droplets)
•
At saturation; Partial pressure of water vapour in the air =
saturation vapor pressure of water at that T
•
Total Pressure: Sum of the pressure of its constituents
(Partial Pressures) such as air & water vapor
PSYCHROMETRY cont
•
Relative humidity (RH): Ration of the partial pressure of the water
vapor in the air (p) to the partial pressure of saturated water vapor at
the same T (ps).
%RH = p/ps
•
Humidity (Y): Measurement of water content in the air.
•
It is related to water partial pressure (p w) in air vapor by:
Y = 18 pw / [29(P-pw)]
P: Total pressure
•
Dew point: The T at which moisture starts to condense out of the air.
•
Also known as saturation T.
WET BULB TEMPERATURE
Wet-bulb temperature: Temperature recorded by
thermometer when the bulb is enveloped by cotton
wick saturated with water.
Psychometric Chart
3. Read RH
2. Dry Bulb (25°C)
4. Read Enthalpy
1. Wet Bulb (20°C)
EXAMPLE:
5. Read Sp Vol
If the wet-bulb temperature in a particular room is measured and found to be 20°C in
air whose dry-bulb temperature is 25°C (that is the wet-bulb depression is 5°C)
estimate the relative humidity, the enthalpy and the specific volume of the air in
the room.
EXAMPLE
• If
the air in previous example is then to be heated to a dry
blub temperature of 40°C, calculate the rate of heat supply
needed for a flow of 1000m3/hr of this hot air for a dryer, and
the RH.
MOISTURE CONTENT &
EQUILIBRIUM MOISTURE
CONTENT
Moisture Content of a Material
•
Wet basis (MCwb)
Mass of water/Mass of wet product (Undried)
•
Dry basis (MCdb)
Mass of water/Mass of dry product
Conversion of Moisture Content
•
MC are often expressed as percent (multiplying by 100)
EQUILIBRIUM
MOISTURE
CONTENT
Under a given vapor
pressure of water in the
surrounding air, a food
attains a MC in equilibrium
with its surroundings
WHEN there is no exchange
of water between the food
and its surroundings.
Example:
 For potato, at temp 20°C of RH 30%, the equilibrium MC is _______.
•
It would not possible to dry potatoes below 10% using an air dryer with air at 20°C
and RH 30%
•
In equilibrium MC graph, mostly the curse is presented by sigmiod (S-shape)
•
MC at low humidity is high for food of dry solids contained high protein, starch or
other high molecular weight polymers.
•
MC at low humidity is low for foods with high soluble solids. (Ex: Fats and
crystalline salts and sugar)
DRYING CURVE
http://moisturecontrol.weebly.com/drying-curve.html
Typical Drying Curve
•
Drying curve usually plots the drying rate
versus drying time or moisture contents.
•
Three major stages of drying can be observed
in the drying curve.
1.
Transient early stage, during which the
product is heating up (transient period)
2.
Constant rate period, in which moisture is
comparatively easy to remove
3.
Falling rate period, in which moisture is
bound or held within the solid matrix
Typical Drying Curve
•
Critical moisture content: The moisture content at the point when the drying
period changes from a constant to a falling rate.
1
2
The drying
behaviours of food
materials depend on
the porosity,
homogeneity, and
hygroscopic properties.
Hygroscopic food
materials enter into the
falling rate
faster compared to nonhygroscopic food
materials.
Air Drying
AIR DRYING
Air
Condition
Rate of
Water
Removal
Type of
Dryer
Food
properties
AIR DRYING
•
Water is held by forces, whose intensity ranges from very weak forces to a very
strong chemical bonds.
•
In drying, loosely bonding liquid can be removed easily.
•
Thus, drying rate decrease as MC decreases, with the remaining water bonded are
stronger.
•
In many cases, a substantial part of the water is loosely bound such as the free water
on the surface of a food.
•
Most cases; a substantial part of
water is loosely bound.
•
Free water at the surface
•
Sand > free water
•
Meat > bound water
•
Comparison between sand and meat drying curve shows the effect of water bond on
drying rates.
•
Water at a free surface
Constant drying rate
Calculation of Constant Drying Rate
• Water evaporated at free water surface during drying rate.
• Rate of removal of water can be relate to rate of heat transfer (If no change in T of
the material)
All heat energy transferred is used to evaporate water
Rate of removal of water is also the rate of mass transfer
• Mass transfer based on drying force of the diff of water partial pressure between
food & air.
• Alternatively, driving force maybe be expressed in term of humidity driving forces.
 Numerical values of the mass transfer coeff are linked to partial
pressure/humidity relationship as Eq 7.4 & Eq 7.5.
Food
MASS TRANSFER
Air
Calculation of Constant Drying Rate
Psychrometric table with humid air properties:
http://www.engineeringtoolbox.com/moist-air-properties-d_1256.html
Equation 7.5
•
Use Lewis Relationship to find mass transfer coefficient (k g’) & heat
transfer coefficient (hc)
kg’ = hc/1000
Or
hc= 1000 x kg’
kgm-2s-1 ≈ Jm-2s-1 ˚C
Calculation of Drying Time
Drying Rate
Drying Time
Drying
Equipment +
Operations
Experiment/
Theory
Assume T and RH of the drying air are constant
Drying Time at Constant Rate
t = w (Xo – Xc) / (dw/dt)constant
Where
Mass
Transfer
(dw/dt)constant = kg’A(Ys – Ya)
Xo : Initial MCdb
Xc : Final MCdb (Critical MC)
w : Amount of dry material in food
(dw/dt)constant : Constant drying rate
DRYING EQUIPMENT
• The
principles of drying may be applied to any type of dryer
that are available in the industry.
• Major
problem in calculation of real dryers:
Condition changes as the drying air and the drying solids move
along the dryer in a continuous dryer or change with time in the
batch dryer.
EXAMPLE OF DRYING EQUIPMENT
• Food is spread out in thin layer on the trays.
• Heating maybe by air current across trays,
conduction from heated tray or shelves or radiation
from heated surface.
• Food is spread over the surface of heated drum.
• Drum rotate with food on the surface.
• Food remain on the drum surface and then being
scraped off.
• Video URL: https://youtu.be/JGb77K_K55M
• Liquid/fine solid material in a slurry is sprayed in
fine droplets into a current of heated air.
• Dryer body usually large enough for the dried
particles to drop down without sticking on the walls.
• Commercial dryers are large in size; may be in 10 m
(diameter) x 20m (high).
• Video URL: https://youtu.be/KL4-SpP-Ghk
• Foodstuff is heated by the air flow through the
horizontal inclined cylinder or by heat conduction
from the walls.
• Video URL: https://youtu.be/RSjNRwiR1iU What about
other types
of drying
equipment?
Summary
Summary