07-11-2016, 03:19 PM
1466318355-ArunMujumdar2.pdf (Size: 814.92 KB / Downloads: 126)
SUPERHEATED STEAM DRYING (SSD)
• Proposed over 100 years ago; received serious attention
only during the past 20-30 years
• Uses superheated steam instead of hot air or
combustion/flue gases in a direct (convection) dryer
• More complex equipment than hot-air drying system;leakproofing
needed; feed/discharge can be difficult
• Lower net energy consumption (if exhausted steam can
be used elsewhere in the process and not charged to
dryer)
• Better product quality (in most cases);Safe operation, no
fire/explosion hazard; no oxidation
If steam pressure is kept constant and more energy
is added, its temperature increases and saturated
steam becomes superheated steam (SHS)
• If extra heat can be transferred to an available heat
sink, SHS returns to saturated conditions
• Any convection dryer can be made into SHSD e.g.
fluidized bed, flash, rotary, conveyor type, spray,
impinging jet; opposing jets (impinging streams) etc.
Additional heat sources e.g. radiation, conduction,
MW etc can also be added
• Low, near-atmopsheric or high pressure operation
possible
SOME ADVANTAGES OF SSD
• Dryer exhaust is steam so it is possible to recover all
latent heat supplied to SSD- some cost involved in
equipment and operation
• No oxidative reactions possible due to lack of O2 Color
and some nutrients are better preserved in foods- even
silk!
• Higher drying rates possible in both CRP and FRP
depending on steam temperature (above the so-called
inversion temperature SSD is faster than air drying)-
not a decisive factor for selection!
SOME ADVANTAGES OF SSD
• Casehardened skin is unlikely to form in SSDmechanism
not known- product dependent
• SSD yields higher product porosity due to evolution
of steam within the product ; boiling in interior opens
up elastic wet solid- bulk density is thus lower and
rehydration behavior is better; also affects color
• Sterilization, deodorization or other heat treatments
(e.g. blanching, boiling, cooking) can be performed
simultaneously with drying
SOME DISADVANTAGES OF SSD
• SSD system is more complex than its hot-air
counterpart
• Initial condensation is inevitable - more time may
be required to dry the product –if feed comes below
saturation temperature
• Products that may melt, undergo glass transition or
be damaged at saturation temperature of steam
cannot be dried in SSD e.g silk cacoon (hence can
be dried in a low pressure SHSD)
• Limited industrial experience with SSD
BASIC PRINCIPLES OF SSD
• SHS has superior thermal properties to air at the
same temperature: h is higher!
• In hot-air drying
∆T is higher at low drying
temperatures
• These counter-acting effects lead to phenomenon
of inversion; beyond inversion temperature SSD
is faster than hot-air drying
• Over-rated in literature; of fundamental interest as
it is a function of geometry and flow configuration
as well
Inversion temperature depends on flow
configurations and flow regimes
• Inversion temperatures of many systems are in the
range of 160-200 C ( simulations for superheated
organic vapors and vaopr-noncondensible gas
mixtures show similar behaviour)
• Inversion temperature is defined only for CRP
drying - some researchers did not adhere
to this limitation!.
FRP drying rate of SSD is sometimes higher than
that of hot air - mechanisms responsible are
different, however! Vapor transfer is faster than
liquid diffusion
• FRP drying rate of SSD is sometimes higher than
air drying rate since product temperature is higher.
Casehardening is unlikely to form and product is
likely to be more porous as well
SSD OF FOOD PRODUCTS
• Received serious attention during the past 10 years
• Possesses several advantages that are of special
interest to food processors e.g. lack of oxidative
reactions, ability to maintain color, nutrients, yields
product of higher porosity
• Ability to inactivate microorganisms; enzymes etc
• Many heat treatments can be performed
simultaneously with drying
HIGH-PRESSURE SSD OF FOODS
• Drying of pressed beet pulp after extraction of sugar
• Operates at pressure ~ 5 bar
• Consumes 50% less energy than conventional air
dryer
• Product quality i.e. appearance, texture, digestability
by cattle is better than air drying
• Pilot tests with spent grain from brewery, alfalfa, fish
meal, pulp from citrus, etc.
NEAR-ATM PRESSURE SSD OF FOODS
• Most SHSDs operate near-atmospheric condition
as equipment cost is lower
• Wide variety of products dried successfully e.g.
potato chips, tortilla chip, shrimp, paddy, soybean,
noodles etc-
• Better product quality (in some cases) than in air
drying
SOME PRODUCTS DRIED IN SSD
• Potato chips, tortilla chips
• Shrimp, pork, chicken, fermented fish
• Sugar beet pulp, spent grain from brewery, okara
• Paddy, soybean, sunflower seed, cacao bean
• Asian noodles
• Vegetables, fruits, herbs - problems here! Must
lower pressure
Products that may melt, undergo glass transition
or be damaged at saturation temperature of
steam cannot be dried in SSD
• First reported study and application was by Chen
& Mujumdar (1985) to dry silk caccoons in low
pressure SHS;enhanced brightness and strength
of silk fibre produced (China)
• Vacuum SHSD kiln for wood is now popular
technology for wood drying.
LOW-PRESSURE SSD (LPSSD)
• Combines the ability to dry product at low
temperature with some advantages of SSD
• Dryer is operated at reduced pressure (5-10 kPa)
• Steam becomes saturated (and superheated) at
lower temperature
• Suitable for highly heat-sensitive products e.g.
herbs, fruits and vegetables and other bio-active
materials