Transcript
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Classification and Selection of
Industrial Drying Systems
Dr. B. N. Thorat
Reader in Chemical Engineering
UICT
Refresher Course in Chemical Engineering for Plant Personnel
9th and 10th December, 2005
DEFINITION OF DRYING
Converts liquids/ solid/ paste into a solid product by evaporation of
liquid into vapor phase via application of heat. (Sometimes converts
solid moisture into vapor by sublimation eg. Freeze drying with
application of heat.)
Note: Mechanical dewatering (filtration, sedimentation, Centrifugation
etc.) is much cheaper (upto 100 times cheaper than drying)
DRYING AS A
THERMAL
PROCESS
Multicomponent
Moisture transportChange of physical
structure
Transient
Change in
quality
Input
Continuous/
intermittent
Coupled with
mass
transfer
Shrinkage
Phase changeChemical/
biochemical
reactions
Drying of solids is a complex process involving several rate processes
occurring co-currently and or sequentially.
MATERIAL BEING
DRIED
Moisture
Heat input
Liquid diffusion
Vapor diffusion
Capillary flow (Permeability)
Knudsen diffusion (Mean free path < pore dia.)
Surface diffusion
Poiseuille flow
Combination of above
Conduction
Convection
Radiation
Dielectric
Combined mode
Various modes of moisture and heat transport
Over 200 types of dryers in industrial use.
Diverse products- physical, chemical properties vary widely for feed
and products.
Involves transient energy/ mass and momentum transport through
porous media, with phase change, with/ without chemical/
biochemical reactions.
No universal drying theory exists.
Little opportunity for generalization.
Minor changes of moisture content result in large changes in physical
properties (eg. Fluidization of dry vs. wet particles).
Why is drying of solids so complex?
Thousands of different products dried in industry, often new products,
new processes, high production rates etc. need new dryers.
Various fuels (gas, oil, electricity, flue gases, waste heat etc.).
Environmental regulations.
Need to reduce costs.
Need to consider drying system rather than dryer, ie. Pre- and post-
drying stages are important and often cost more than dryer.
Why so many dryers?
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MAIN DRYER TYPES
I. Direct (Convective)
DirectDryer
Hot gas
Wet product Dry product
Humid gas
Drying medium directly contacts material to be dried and carries
evaporated moisture.
II. Indirect (Contact, Conduction)
Gas flow (low)
Wet product Dry product
Vacuum or low gas flow
Heat supplied
by heat exchanger
(through metal wall)
III. Radiant
Wet feed Dry product
Vacuum or low gas flow to carry evaporated moisture away.
IV. Microwave or RF
Electromagnetic energy absorbed selectively by water (volumetric heating).
Heater (radiant)
Typically less than 50% of total heat supplied in most direct dryers
is used for evaporation. Water is the most common solvent removed
in dryers.
Top 10 Reasons Why
Drying R&D has been Ignored
1. Very old hence well-understood operation.
2. Conceptually simple (so design must be simple!).
3. Misleading coverage in standard texts and handbooks.
4. Too easy (? ).
5. Too difficult (for academics conversant in transport phenomena!).
6. Vendors should do R&D.
7. Excessively long lifetime of dryers.
8. Low capital costs.
9. Low energy costs as it was thought but not now!!!
10. Lack of legislatives support.
Top 10 Reasons for
Innovation in Drying
1. Better product quality.
2. Improved productivity.
3. Environmentally-friendly process.
4. Improved economics.
5. Reduced energy consumption.
6. Better control .
7. Flexibility (high turn-down ratios).
8. Multi-processing capacity.
9. Safer operation.
10. New products / processes.
Classification of Dryers
Solid Exposure to Heat Conditions
Typical residence time
within dryer
Dryers
0- 10
sec
10- 30
sec
5- 10
min
10- 60
min
1- 6
hr
Convection
Belt conveyor dryer X
Flash dryer X
Fluid bed dryer X
Rotary dryer XSpray dryer X
Tray dryer (batch) X
Tray dryer (continuous) X
Conduction
Drum dryer X
Steam jacket rotary dryer X
Steam tube rotary dryer X
Tray dryer (batch) X
Tray dryer (continuous) X
Granular material - 0.05 to 5 mm
Pastelike materials - 0.1 to 50m S olutions or suspensions - 1 0- 5 0 m- F ines
- 0.1-10 m- Ultrafines
-
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Classification of Granular Material
Group Pore size (nm) Drying time insuspended state
Types of dryersrecommended
I > 100 0.5 - 3.0 sec. Cyclone dryers
Flash dryersTwo- stage flash dryers
II 100 - 6 3 - 30 sec. Two- stage flash dryersFast spouted bed
III 6 - 4
4 - 2
0.5 - 2 min.
2 - 20 min.
Vortex dryersBatch dryersFluid bedVibrated fluid bedBatch dryers
IV Ultra-micropores, particlesize 1 - 2 mm
Particle size > 2 mm
10 - 60 min
40 - 90 min.
> 90 min.
Vibrated fluid bedMultistage fluid bedBatch dryersBatch dryers
Suspended state dryersnot recommended
Always think DRYING SYSTEM
not just DRYING
DRYING SYSTEM
Pre-drying
StagesDrying
Post-drying
Stages
Feeders
Pre-forming
(extrusion,
pelletizing)
Backmixing
Metering
Blending
Mechanical
dewatering
Solar collector /
storage
May involve
chemical reactions. Cooling
Agglomeration
Solvent recovery
(if applicable)
Grinding
Gas cleaning
(cyclone, filters,
scrubbers, etc.)
Product
collection
packaging
Continuous---Good15Spray
ContinuousFairGoodGood-185Spin Flash
ContinuousFairGoodPoor-33Rotary
(indirect)
ContinuousFairGoodFair-750Flash
Continuous--FairGood22Film Drum
ContinuousGood-Fair-30Band
ContinuousGoodGood--130FBD
BatchPoorFairPoor-10Double Cone
BatchGood---7.5Forced
Convection
(through flow)
OperationGranules,
pellets
PowdersPastesFluid, liquid
suspension
Evap. Rate
(kg/m2/hr)
Dryers
Product Classification and Dryers Types
as an aid to SelectionFalling rate Constant rate
Critical moisture, XcEquilibrium moisture, Xe
Zone- BZone- C Zone- A
Idealized Drying Rate Curve
Rate of
drying
0X = moisture content, kg moisture/ kg BDS (bone dry solids)
DRYER CHOICE BASED ON SOLIDS DRYING CHARACTERISTICS
Zone A(Constant Rate)
Zone B(Falling Rate)
Zone C(Internal diffusion)
Turbo- tray Turbo- tray Turbo- trayRotary Rotary R otary
Belt Belt Belt--------- Fluid Bed (Spray) Fluid BedSpray Spray - -------
Flash Flash - -------
dX
dt
STEP 1- BASIC CHOICES
Batch Or Continuous
Batch dryers favored by:
Low throughput (under 50 Kg/H)
Long residence time (I.E. Mainly falling-rate drying)
Batch equipment upstream and downstream
Requirement for batch integrity
Continuous dryers favoured by opposite conditions
EXAMPLE - Fluidized Bed Dryers
Sub types:
Batch: Well-Mixed (WM)
Continuous: Well- Mixed, Plug- Flow (PF), Multi- Stage (MS)
Options: Internal Heating Coils, Split Distributor, Vibration, Feed End Rake,
Expanded Freeboard, Special Distributor, etc.
Flowsheet options:
Gas Recycle, Backmixing, Direct Firing, Self- Inerting
Cost criteria:
Simple types cheapest, most options add to cost.
Vibro fluidized beds
Have high capital cost/ low energy costs
Payback periods (depend on if dryer is new or replacing an existing
operating unit)
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STEP 3- SUBTYPES AND REFINEMENTS
Choice Between Subtypes
Feed difficult to fluidized-Use WM, MS (WM before PF),
If PF used need ; rotating Rake, Backmixing, Vibrated Feed Region
Narrow X0 specification - Prefer in order PF= MS= WM
X0 much less than X1- prefer PF and MS to WM.
Wide size distribution- Use Vibration, Special Distributor, Expanded
Freeboard, Fines Collections
Small particles, low gas velocity- consider internal coils
WM: WELL- MIXED; PF= PLUG FLOW
Small Scale Lab Tests
Small- scale tests give valuable information:
Drying kinetics- drying rates (parametric effects)
Equilibrium moisture content- effect of t,h (humidity)
Microscopic examination- surface, agglomeration
Lab-scale rotary evaporator- overheating, balling, adhesion
Rotating drum tester- attrition, dustiness
Cohesion and adhesion- handling, sticky point
Vital to have a representative sample of final material
Not necessary to carry out all of above tests in all cases
Selection: Proper selection is crucial. Best designed wrong dryer is still a poor choice.
For granular solids, for example, numerous choices exist.
Type Advantages/ limitations
Rotary- Convective (direct only) Flexible/ high operating cost; large volume; high
carryover of fines
Rotary- Convection/ Conduction (steamtube)
Flexible/ better efficiency/ low fines carryover/ expensive/large volume
Fluid bed (Convective) Compact/ more efficient/ less flexible
Fluid bed (Combined mode) Compact/ efficient
Vibrated Fluid Bed More efficient, flexible, for friable materials, polydisperse
solids
Vibrated Bed with Immersed
Exchangers
More efficient, less carryover, low blower power
Spouted Bed Dryer Compact, limited range of operation, high efficiecy, high
blower power, limited capacity
Centrifugal Fluid Bed Very high transfer rates, expensive
Rotary Tray (Turbo Dryer) Gentle handling, flexible
Spiral Dryer (Flash with Indirect
heating)
High drying rates, more expensive
Flash/ Pneumatic Dryer Flash + Fluid
Bed
For surface moisture removal only
Jet- Zone Dryer (layers of particlesfluidized by impinging jets)
High drying rates, flexible
Through Dryers (hot air through beds ofparticles)
Expensive for fine particles, non-uniform drying
Vacuum Dryer For heat- sensitive solids, expensive
Combination Dryers
Definitions
R.H. 50%
EMC
Bound moisture
Free moisture
Content
X Moisture Content(dry basis)X*
Unbound moisture
100%
T= Constant
Ls= Dry mass
A= Area
C.R.P.
F.R.P.
X
XcritX*
R =
-Lsd X
A dt
Drying rate curve
0
CLASS I: Glass beads sand, clay, mixtures of sand and clay, calcium carbonate, silica gel, paper pulp, leather, pig
manure. CLASS II: glass beads, ceramic tiles, clay, silica gel. CLASS III: organic liquid in glass beads, for
example,(a) benzene and n-propanol, (b) n-pentanol. CLASS IV: glass beads, polystyrene beads. CLASS V: sand,
plastic-clay mix, silica-brick mix, whiting slab, ceramic plate, leather, lactose granulation. CLASS VI: special case
of CLASS I: Schlunder reports that for molecular sieve. CLASS VII: CLASS I with different curvature during bthe
period of decreasing drying rate, aluminium silicate particles as a function of air temperature, and sand and paper
pulp as a function of thickness of the sample. CLASS VIII: (a) fir wood; (b) cypress wood. CLASS IX: (a) paper,
wool, aluminiumstearate dough; (b) potatoes, tapioca tuber, and rice flour.CLASS X: (a) rye bread, yeast; (b) butter
and margarine. CLASS XI: (a) wheat corns; (b) and (c) the same for lower X normalized tothe initial drying rate for
(a). CLASS XII: limestone granules saturated with (a) water, (b) 0.05-M NaCl. Similar behavior, due to crust
formation, has been observed for p[lastertiles and for clay.
XII
XIXIX
VIIIVIIVIV
IVIIIIII
a
b
a
b
a
b
a
b
a b
c
c
a
b
Examples Of Normalized Drying- Rate Curves For Different Types Of Media
Turbo Tray Dryers
Suitable for granular feeds, operate with rotating shelves and force
convection of air above the shelves.
The Dryer can have 30+ trays and provide large residence time.
Hermetic sealing is possible for solvent recovery.
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Rotary Dryer
Combined cascade motion
with heat & mass transfer.
Large capital & operating cost.
Used in fertilizers,
pharmaceutical, lead & zinc
concentrate for smelting,
cement.
Size 0.3 to 5 m diameter &
2 to 90 m length.
Steam Tube Rotary Dryer
Pneumatic Conveying (Flash) Dryers Tunnel Dryers
FBD Dryers - VariationsRotocone Dryers (Batch)
Agitated Dryers
Drying of fine & moderately
wet materials such as
gypsum, pigments, and
dyestuffs.
Drying of pharmaceuticals -
tableting formulation
Maximum capacity 10 m3.
Evap. rate 2-7 kg/hr.m2
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Microwave Dryers Used in ceramics industries,
foods & pharmaceuticals to
drive of last traces of moisture.
Paddle Dryers
Provides drying time
upto several hours.
Suitable for pastelike
& granular material.
Evap. rate upto
10 kg/hr.m2
Vacuum Dryers Heat Sensitive MaterialsScrew Conveyer Dryer
Freeze Dryer
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BN
A
V/
P
ReO
O+=
221
Two phase theory for fluidization Thorat and Joshi, 2001
I & ECFixedbed
Incipient
fluidization
Aggregative
fluidization
V< VONo particle
mixing
V = VOModerate particle
mixing
V> VOV
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INTRODUCTION
Drying of Foods To increase the shelf life, to reducepackaging cost
Acceptable final moisture content 5 to 12%
Quality Changes occurs during drying of FOODS
Factors affecting structural Properties
1. Drying Method
2. Drying Conditions
3. Moisture Content
Mostly Used Drying Techniques in
Food Industry
Conventional Air Dryer
Osmotic Dehydration
Freeze Dryer
Microwave Dryer
Spray Dryer
Fluid Bed Dryer
Heat Pump Dryer
Infrared Dryer
Case study : Shrimp drying
Shrimp (Prawn) is one of the most popularseafood products
Sun Drying a traditional way, results inpoor product quality
High local and export market
Superior quality and energy minimizationcould result in value addition to product
Different combinations of drying can be
used
Combinatorial Drying for Shrimps
Osmoticdehydration
15% w/v NaCl30 min450C
4 to 2 kg/kg db
Heat pumpdrying
17% RH40-440C
X=2 to 0.5 kg/kg db
Infra-reddrying
Max. 20 min600C
X=0.2 db
What is Heat Pump
Why heat pump + Drying
Heat savings
Product Quality
Recovery of solvent
How it works ?
Heat pump drying
Evaporator
Condenser
Dryer Compressor
Simple Scheme
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Refrigerant: R134A (Dichloro Difluoro Ethane)
Enhancement of performance using bypass coil
Dehumidified air
17 % RH
40-440C
Shrimps taken out after Heat Pump drying
693.7542.7830.40.2890.608Rice powder5.
75055.7830.750.2650.798Suji4.
7905230.80.2260.655Rice rawa
(Semolin)
3.
688.745530.50.230.65Chickpea
(=0.945
mm)
2.
712.4149.9630.30.240.67
Chickpea (mix)
1.
Temp(C)DriedRawMaterials
Bulk Density
(kg/m3)
Angle of ReposeWater ActivityNo.
Water Activity, Angle of repose& Bulk densities after drying of Materials @ 60 C
3 MotorGearBox
N2
Wetmaterial
N2
Hot water
Hot
water
Screw
Dry
Product
Screw
Dryer developed at UICT:
Screw Conveyor Dryer
Motor
Gear box
Jacketed dryer
Material outlet
Material inlet
Bearing support
Gas inlet
Actual SCD set-up
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Dryer Throughput
Where,
C = Screw conveyor throughput, m3/h
= degree of fullness
Dsc = screw diameter, m
Dsh = shaft diameter, m
r = radial clearance, m
P = screw pitch, m
t = flight thickness, m
N = screw speed, rpm
Note: The allowable loading or degree of fullness and screw
speed are limited by the material characteristics
( )( ) 60NtPD2r)(D4
C2
sh
2
sc +=
Degree of fullness or dryer loading
Furnace slag, dry
sand,
alumina etc.
Abrasive and poor
flowability
15 %Class IV
Dry ash, cement,
salt,
charcoal,
crushed
chalk etc.
Similar in size and
flowability to Class
II solids, but more
abrasive
30 %Class III
Baking powder,
pulverized
coal, corn
grits etc.
Non-abrasive, less free
flowing than Class I,
small lumps mixed
with fines
30 %Class II
Wheat, flour,
graphite etc.
Light, free-flowing,
non-abrasive
45 %Class I
ExamplesMaterial
characteristics
Degree of
fullness
Material
type
Note: Class I materials fill the trough deeply, permitting a higherrotating speed than heavier and more abrasive materials
Thank You
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