Module 5: Drying Flashcards
preservation of an agricultural crop that involves the removal of moisture to a level considered safe for storage
drying
process of removing moisture from a material until the desired level is reached and equilibrium with the storage environment is achieved
drying
drying is most concerned with what property of the air?
vapor pressure
in drying, grain is exposed to ambient air with low relative humidity or to heated air, this will evaporate the moisture from the grain
true or false
true
in drying, the _______ remove the moisture from the grain bulk
drying air
rice is a hydroscopic material
true or false
false; hygroscopic yon haha
if dry rice is exposed to air with high RH, what will happen to the grains?
it will absorb water from the air (re-wetting)
when wet rice is exposed to air with _____(low/high) RH, the rice grains will release water to the air
low RH; this is drying
drying permits the farmer to secure a greater economic return for the ff reasons
- production operations are facilitated for products like cotton and corn.
- higher economic value products, such as tobacco and dried fruits and vegetables, are produced.
- waste products can be converted into useful products, like livestock feed from fruit pulp and almond hulls.
- early harvest at high moisture content minimizes field damage, shatter loss, and facilitates tillage operations.
- long-term storage without product deterioration is possible.
- seed viability is maintained over long periods.
during drying, these two processes are occurring at the same time
- heat transfer
- mass transfer
during drying, heat from the heated air transfers to the solid, providing energy for __________ ____________
moisture evaporation
in drying, moisture (liquid or vapor form) moves from within the grain to ___ _______ and then evaporates into the _________ ___.
its surface; drying air
in paddy grain, moisture is present at two places:
- at the surface of the grain
- inside the kernel
moisture in paddy grain that readily evaporate when grain is exposed to heated air
surface moisture
internal moisture has to move first from the kernel to the outside surface for it to evaporate
true or false
true
evaporation of surface moisture is slower than internal moisture
true or false
false; it is faster
since surface moisture and internal moisture evaporate at a different rate, the difference in results leads to different ‘________ ____’ for different period of drying
drying rates
four main stages of drying
- initial induction period
- constant rate period
- 1st falling rate period
- 2nd falling rate period
in the first stage of drying, drying rate is slowly increasing and there is a very slight change in MC
true or false
true
after the grain’s contact with the drying medium, its temp temperature increases to a ______ state while the air temperature remains ________.
steady; constant
after adjustment, the drying rate and grain _______________ (temperature/moisture content) remain constant, while the ______________ (temperature/moisture content) continuously decreases over time.
temperature; moisture content
at constant rate period, heat is transferred from the drying air to the grain, providing the energy needed for moisture evaporation
true or false
true
at constant rate period, mass transfer occurs how?
moisture moves from the grain surface to the surrounding air
how is surface water removed in constant rate period?
the entire surface of the grain is saturated with water during this phase –> the continuous heat transfer and mass transfer facilitate the removal of that surface moisture
this refers to the difficulty in converting liquid moisture at the surface of the solid into vapor
resistance to evaporation
refers to the difficulty of moisture moving from the interior of the solid to its surface
internal resistance to diffusion
if the resistance to evaporation is much greater than the internal resistance to diffusion, what does it mean?
it means that the primary bottleneck for drying is the evaporation process at the surface rather than the movement of moisture from within the solid to the surface
factors affecting drying
- surface area
- drying air temp
- drying air RH
- air velocity
- rate of removal of water vapor from the surface
in convective drying by hot air, heat is transferred from the hot air to the _______ ____ _________ through a boundary layer
the wet material ________ this heat, inducing evaporation
the resulting water _______ is then transferred from the surface to the air by convection
cooler wet surface; absorbs; vapor
stage
in the ____________________ of grain drying, moisture moves from within the material to the surface and is then removed from the surface
first falling rate period
in first falling rate period, when the MC reaches CMC, the grain surface temperature _______ (decreases/increases), and the drying rate _________ (decreases/increases) rapidly
increases; decreases
constant rate period takes shorter than first falling rate
true or false
true
in first falling rate period, the internal resistance to moisture movement is significantly higher than external resistance to evaporation
true or false
true
in the first falling rate period, it is important to note that the structure of agricultural materials complicates moisture movement due to
- their internal and external cavities
- capillaries
- cell walls
- spaces
mechanisms of moisture mo
- diffusion (liquid water and/or vapor)
- capillary action
- shrinkage
- vapor pressure gradients
- gravity
- vaporization
in the second falling rate, drying rate rate further decreases until it becomes zero and MC reachers?
CMC
in the ____________________, all evaporation occurs from the interior of the solid (transfer of internal moisture to the surface)
second falling rate period
in the last stage of drying, grain temp increases until it reaches the temperature of _________ ___
drying air
rate at which grain moisture content declines during the drying process
drying rate
_________ ____ is normally expressed in percent moisture removed per hour
drying rate
typical drying rate of rice dryers are in the 0.5-1%/hr range
true or false
true
stirring, turning, and circulating grains in various drying methods improve drying consistency and maintain grain quality by preventing re-wetting
this process is referred as
uniform drying
process of stopping intermittently
tempering
resuming the drying process after tempering results in a higher drying rate compared to continuous drying
true or false
true
when drying is paused, the moisture inside the grain redistributes evenly due to __________
diffusion
in modern re-circulating grain dryers, grains undergo cycles of drying followed by tempering, this cycling have benefits which are
- improves drying rates
- enhances grain quality
- reduces energy costs
factors in selecting a dryer
- farm size and output
- crop
- future development
- local atmospheric conditions
- marketing
- labor
- capital
factors affecting the selection of dryer type
- raw material characteristics
- operation costs
types of dryer
- cabinet dryer
- tunnel dryer
- continuous conveyor dryer
- spray dryer
- drum dryer
- fluidized bed dryer
- freeze dryer
types of dryer
uses air-convection tray drying intended for relatively small-scale operations
cabinet dryer
types of dryer
applications of cabinet dryer
- air dried foods in piece form
- preservation of variety of fruits and veggies
types of dryer
a continuous type of dryer where wet mats are loaded in a thin, uniform layer on the drying trays at one end of the dryer
tunnel dryer
types of dryer
in a tunnel dryer, dried products are removed at the other end
true or false
true
types of dryer
food mats are dried in warm circulating air
continuous conveyor dryer
types of dryer
in a continuous conveyor dryer, wet mats supplied to the __________ _____ at the left is loaded evenly on the surface of a slowly moving conveyor belt
spreader device
types of dryer
in a continuous conveyor dryer, hot air flows through the _______ or ____________ in the conveyor
meshes or perforations
types of dryer
mats are dried and dispersed into a stream of heated air
spray dryer
types of dryer
in a spray dryer, moisture evaporates into the?
air stream
in a sprayer dryer, the dryer particles are separated from the air and collected
true or false
true
the mat to be dried in a spray dryer must have at least ____% soluble solids
20%
types of dryer
comprises of a roll or drum with heating medium circulating through the drums
drum dryer
in a drum dryer, viscous material is fed from the top to the _______ surface
hot drum
in a drum dryer, product dries on the drums and is scraped off by the ________ ____ fixed along the surface of the drum
stationary blades
milk and juices powder are produced using drum dryer
true or false
false; using spray dryer
3 types of drum dryer
- dip-feed drum dryer
- twin-cylinder drum dryer
- single-cylinder drum dryer
types of dryer
product is fed in at one bed to lie on a porous plate
fluidized bed dryer
in a fluidized bed dryer, product is agitated and moved along towards the exit at the other end by ___ ___ which is blown up through the product
hot air
soup mixes are produced by fluidized bed dryer
true or false
true
type of dryer
it is also known as a dehydrator
freeze dryer
in freeze drying, water is removed by ________ __________ from the frozen state to the vapor state, without water passing through an intermediate liquid state
direct sublimation
freeze drying temp for most products is at -21.7 degree Celsius
true or false
true
freeze drying temp for fruit juices is?
-32 to -34 degree Celsius
driver of heat transfer
difference in temp
driver and agent of vapor transport
difference in vapor pressure
process of removing moisture from a material until the desired level is reached and equilibrium with the storage environment is achieved
drying
accomplished by vaporizing the water that is contained in the food
drying
a process of simultaneous heat and moisture transfer
grain drying
categories of drying process
- air and contact drying
- vacuum drying
- freeze drying
categories of drying process
a drying process where heat is transferred through the foodstuff either from heated air or from heated surfaces
air and contact drying
categories of drying process
evaporation of water occurs more readily at lower pressures than at higher ones
vacuum drying
categories of drying process
the water vapor is sublimed off frozen food
freeze drying
heated air used in drying has two functions
- supply heat for moisture evaporation
- carry away the evaporated moisture
a drying system must have the following:
- method of heating the drying air
- blower to force the heated air through the grain mass
- device to hold the grain being dried
- system for loading, moving, and unloading the grain on the holding device
- central system for programming the drying process for prevention of damage to the quality of the
grain - safety control network
predominant method of drying in the Philippines because of economic considerations
sundrying
drying methods
- sundrying
- mechanical drying
- solar drying
drying methods
a blower forces the heated air through the mass of grains
mechanical drying
purposes of the plenum in a mechanical dryer
it equalizes the air pressure in the drying bin, which holds the grains
types of mechanical dryer
a. burners/furnace
- direct-fired burner
- indirect-fired burner
b. blowers
- propeller type
- centrifugal type
c. drying bins
- shallow-bed batch type bin
- deep-bed batch type bin
- continuous-flow non-mixing type bin
- continuous-flow mixing type bin
drying methods
burned fuel is forced through the mass of grains
direct-fired burner
drying methods
makes efficient use of heat, which makes operation less expensive
direct-fired burner
disadvantage of using a direct-fired burner
possibility of damage or undesirable odor, color, and taste imparted on the grains because of smoke and unburned fuel comes in contact with the grains being dried
drying methods
burned fuel is directed to a heat exchanger and discharged through a smokestack
indirect-fired burner
drying methods
_______-_____ _____ is safer but less efficient in heat utilization and more expensive to acquire and operate
indirect-fired burner
drying methods
used for large air volume at low static pressure, air moves parallel to the fan axis and at a right angle to the field of rotation of the blades
propeller type blower
drying methods
used for high static pressure at low air volume, air enters the housing parallel to the axis and is discharged perpendicular to the direction in which it enters the fan
centrifugal type blower
drying methods
drying bins can be classified into two
- batch-type
- continuous
drying methods
type is widely used by small farmers because of their relatively low cost and ease of operation
shallow-bed batch type bin
drying methods
a deep-bed batch type bin has perforated floor with a flat or sometimes inclined bed to facilitate unloading
true or false
false; it’s shallow-bed batch type bin
drying methods
type is usually used for in-storage drying using slightly heated air to prevent overdrying at the bottom and low air flow rate
deep-bed batch type bin
drying methods
shallow-bed batch type bin
optimum drying parameters are __ inches depth, ___ °F air temperature and 30 – 35 cfm of grain air flow
12 inches; 110 degree Fahrenheit
drying method
consist of two parallel screen columns up to 18 inches apart
continuous-flow non-mixing type bin
drying method
mixing of grains occurs as the grains flow down the system, consist of a bin with layers of air channels shaped like inverted “V”s
continuous-flow mixing type bin
drying methods
solar radiation would be allowed to pass through a transparent sheet where heat absorbent
solar drying
solar drying systems are categorized as passive or active
- passive system
- active system
solar drying
depends on natural convention, conduction, or radiation to transfer thermal energy from the point of collection to the storage component and to the point of use
passive system
solar drying
has a mechanical mean such as pump or fan to drive a heat-transfer fluid from one part of the system to another
active system
solar drying
active system has many agri applications such as greenhouses, field drying of crops use passive solar systems
true or false
false; active system
efficient utilization of solar energy for drying grains and crops depends on
- geographical location
- crop type
- size of operation
- government policy
factors to consider in selecting a dryer
- farm size and output
- type of crop
- future and development
- local atmospheric conditions
- labor
- capital
type of dryer
- cabinet dryer
- tunnel dryer
- continuous conveyor dryer
- spray dryer
- drum dryer
- fluidized bed dryer
- freeze dryer
factors to consider in selecting a type of dryer
- raw material specification
- operating costs
drying techniques
- continuous-flow intermittent drying
- flash drying
drying techniques
grain heating time is __ minutes per exposure
15 minutes
drying techniques
heated air temp at 140-170 degree Fahrenheit
continuous-flow intermittent drying
continuous-flow intermittent drying
after heating, it is followed by a tempering time of __-__ hours
3-4 hours
continuous-flow intermittent drying
blower remains running during the ________ period
tempering
drying technique
the grain moisture is reduced to an intermediate level of 18% MCwb which is a safe level to store grains up to three weeks while waiting for sundrying
flash drying
in flash drying, grains are subjected to high air temp of ___ degree Fahrenheit for a ___ min and then discharged
170 degree Fahrenheit; 15 mins
drying efficiencies
each time unit is the time of one-half response of rain fully exposed to the air entering the drying systems
drying time
drying efficiencies
moisture is expressed as moisture content ratio
grain moisture
drying efficiencies
a depth factor is describe as “containing enough grain that if all the theoretical available heat could be used it would all dry to equilibrium” in a period of time equal to the time of one half response
grain depth
drying efficiencies
drying air fulfills two functions in a mechanical grain drying system: (1) to carry the necessary energy to the grain to evaporate the moisture, and (2) to carry the evaporated water out of the grain
air movement
efficiency of the drying operation is an important factor in the assessment and selection of the optimum dryer for a particular task
drying efficiency
three groups of factors affecting drying efficiency
- related to the environment, in particular, ambient air conditions;
- those specific to the crop; and
- those specific to the design and operation of the dryer
REVIEW QUESTIONS (no answer in the flashcard)
- Define each type of dryer.
EXPLAIN
