Selection of Dryers I- India

Copyright2006 - A.S. Mujumdar - SelectionCopyright2006 - A.S. Mujumdar - Selection of Dryers I of Dryers I

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Selection of DryersSelection of Dryers

Professor Arun S. MujumdarProfessor Arun S. Mujumdar

NUS, SingaporeNUS, Singapore

20062006

http://serve.me.nus.edu.sg/arunhttp://serve.me.nus.edu.sg/arun


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OutlineOutline

Classification-general ideasClassification-general ideas Why so many dryer types?Why so many dryer types? Key criteria for classificationKey criteria for classification Criteria for dryer selectionCriteria for dryer selection Dryers for pharma productsDryers for pharma products Some discussion of spray and fluid bed Some discussion of spray and fluid bed

dryers- classification and selectiondryers- classification and selection ClosureClosure


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Why so many dryer types?Why so many dryer types?

Over 500 reported in literature studies; Over 500 reported in literature studies; over 100 commercially availableover 100 commercially available

Over 50,000 materials are dried Over 50,000 materials are dried commercially at rates of a few kg/hr to 30 commercially at rates of a few kg/hr to 30 T/hr or moreT/hr or more

Temperature and pressure range from Temperature and pressure range from below triple point to super-criticalbelow triple point to super-critical

Wide range of feeds (liquid, solid, semi-Wide range of feeds (liquid, solid, semi-solid, particulate, pasty; sludge-like; sticky solid, particulate, pasty; sludge-like; sticky etc); wide specs on dried productetc); wide specs on dried product


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Why so many? More…Why so many? More…

Different sources of energy Different sources of energy input( conduction, convection, radiation, input( conduction, convection, radiation, MW,RF etc)MW,RF etc)

Energy input continuous or intermittentEnergy input continuous or intermittent Batch, continuous or semi-continuous Batch, continuous or semi-continuous

operationoperation Quality is key parameter for many Quality is key parameter for many

productsproducts Limited number used in pharma industryLimited number used in pharma industry


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……this is why..this is why.. Drying times (residence times within Drying times (residence times within

drying chamber) can range from 1/3 sec. drying chamber) can range from 1/3 sec. to 5 monthsto 5 months

Numerous constraints on Numerous constraints on physical/chemical properties of feed as physical/chemical properties of feed as well as dried product require a bewildering well as dried product require a bewildering array of dryer designsarray of dryer designs

Over 500 dryer types reported in Over 500 dryer types reported in literature; over 100 commercialized!literature; over 100 commercialized!

Again, a much smaller number used in the Again, a much smaller number used in the pharma industry!pharma industry!


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Criteria for selection of dryersCriteria for selection of dryers Numerous criteria , with different weightsNumerous criteria , with different weights Many dryers can typically meet specs; hence Many dryers can typically meet specs; hence

several dryers can do a given job in general.several dryers can do a given job in general. Choice depends on mode of operation, physical Choice depends on mode of operation, physical

form of feed and dried product desired; heat form of feed and dried product desired; heat sensitivity; quality requirements; production rate; sensitivity; quality requirements; production rate; whether non-aqueous solvents are present in whether non-aqueous solvents are present in feed; whether material is toxic/inflammable or feed; whether material is toxic/inflammable or friable etcfriable etc

Key criterion- dryer must be able to handle the Key criterion- dryer must be able to handle the product- move it from feed to exit! Other criteria product- move it from feed to exit! Other criteria follow.follow.

For pharma products -quality is NO 1 criterion!For pharma products -quality is NO 1 criterion!


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More on criteria for selectionMore on criteria for selection In view of tremendous diversity of dryers, buyer In view of tremendous diversity of dryers, buyer

must know more about dryers and dryingmust know more about dryers and drying Most vendors specialize in selected dryer types; Most vendors specialize in selected dryer types;

so buyer needs to make choiceso buyer needs to make choice Multiple choices are possible and can make Multiple choices are possible and can make

selection process complexselection process complex Expertise needed to make right choice!Expertise needed to make right choice! Energy, environment, safety and cost are Energy, environment, safety and cost are

important considerations in selection.important considerations in selection. Special care needed when handling nonaqueous Special care needed when handling nonaqueous

solvents in wet materialsolvents in wet material


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Why select dryer carefully?Why select dryer carefully? Can affect bottomline..Can affect bottomline.. Product quality , energy usage affected by choiceProduct quality , energy usage affected by choice Choose right drying system-not jut dryerChoose right drying system-not jut dryer Weakest link decides ultimate goodness of Weakest link decides ultimate goodness of

system choicesystem choice Survey of 10 largest pharma and chemical Survey of 10 largest pharma and chemical

companies in Europe in 1990’s identified dryer companies in Europe in 1990’s identified dryer selection as main problem facing industry!selection as main problem facing industry!

Expert systems exist for selection. Different Expert systems exist for selection. Different expert systems give different selectionsexpert systems give different selections

Know product and process well before choosing Know product and process well before choosing drying system; imitation can cause problems!drying system; imitation can cause problems!


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Some notesSome notes

Do not copy dryer or dryer system used Do not copy dryer or dryer system used elsewhere without critical evaluation from elsewhere without critical evaluation from square 1! square 1!

Nickel ore concentrate is dried in different Nickel ore concentrate is dried in different places using spray, fluid bed, rotary and places using spray, fluid bed, rotary and flash dryers/ Which one do you COPY?flash dryers/ Which one do you COPY?

Local fuel availability and relative costs of Local fuel availability and relative costs of different energy sources, environmental different energy sources, environmental requirements as well as legislation can requirements as well as legislation can change selection of dryer for same change selection of dryer for same applicationapplication


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Tabulated Criteria for Classification Tabulated Criteria for Classification and Selectionand Selection

Following few slides list criteria used Following few slides list criteria used for classifying dryers and selecting for classifying dryers and selecting them for a given applicationthem for a given application

Please review these on your own Please review these on your own later (no time to discuss each later (no time to discuss each criterion in detail)criterion in detail)

Use these tables as a check-listUse these tables as a check-list Note several different dryers can be Note several different dryers can be

used for a given application.used for a given application.


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Table 1 Classification of dryersCriterionCriterion TypesTypes

Mode of operationMode of operation BatchBatchContinuous*Continuous*

Heat input-typeHeat input-type Convection*, conduction, radiation, Convection*, conduction, radiation, electromagnetic fields, combination of heat electromagnetic fields, combination of heat transfer modestransfer modesIntermittent or continuous*Intermittent or continuous*Adiabatic or non-adiabaticAdiabatic or non-adiabatic

State of material in dryerState of material in dryer StationaryStationaryMoving, agitated, dispersedMoving, agitated, dispersed

Operating pressureOperating pressure Vacuum*Vacuum*AtmosphericAtmospheric

Drying medium Drying medium (convection)(convection)

Air*Air*Superheated steamSuperheated steamFlue gasesFlue gases

Drying temperatureDrying temperature Below boiling temperature*Below boiling temperature*Above boiling temperatureAbove boiling temperatureBelow freezing pointBelow freezing point


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Relative motion Relative motion between drying between drying medium and drying medium and drying solidssolids

Co-currentCo-currentCounter-currentCounter-currentMixed flowMixed flow

CriterionCriterion TypesTypes

Number of stagesNumber of stages Single*Single*Multi-stageMulti-stage

Residence timeResidence time Short (< 1 minute)Short (< 1 minute)Medium (1 – 60 minutes)Medium (1 – 60 minutes)Long (> 60 minutes)Long (> 60 minutes)

* Most common in practice


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Table 2 Typical checklist for selection of industrial dryers

Physical form of feedPhysical form of feed Granular, particulate, sludge, Granular, particulate, sludge, crystalline, liquid, pasty, suspension, crystalline, liquid, pasty, suspension, solution, continuous sheets, planks, odd-solution, continuous sheets, planks, odd-shapes (small/large)shapes (small/large)Sticky, lumpySticky, lumpy

Average throughputAverage throughput kg/h (dry/wet); continuouskg/h (dry/wet); continuouskg per batch (dry/wet)kg per batch (dry/wet)

Expected variation in Expected variation in throughput throughput (turndown ratio)(turndown ratio)

Fuel choiceFuel choice OilOilGasGasElectricityElectricity

Pre- and post-drying Pre- and post-drying operations operations (if any)(if any)


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For particulate feed productsFor particulate feed products Mean particle sizeMean particle sizeSize distributionSize distributionParticle densityParticle densityBulk densityBulk densityRehydration propertiesRehydration properties

Inlet/outlet moisture contentInlet/outlet moisture content Dry basisDry basisWet basisWet basis

Chemical/biochemical/microbiolo-Chemical/biochemical/microbiolo-gical activitygical activity

Heat sensitivityHeat sensitivity Melting pointMelting pointGlass transition temperatureGlass transition temperature

Sorption isotherms (equilibrium Sorption isotherms (equilibrium moisture content)moisture content)

Drying timeDrying time Drying curvesDrying curvesEffect of process variablesEffect of process variables


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Drying timeDrying time Drying curvesDrying curvesEffect of process variablesEffect of process variables

Special requirementsSpecial requirements Material of constructionMaterial of constructionCorrosion Corrosion ToxicityToxicityNon-aqueous solutionNon-aqueous solutionFlammability limitsFlammability limitsFire hazardFire hazardColor/texture/aroma requirements (if Color/texture/aroma requirements (if any)any)

Foot print of drying systemFoot print of drying system Space availability for dryer and Space availability for dryer and ancillaries ancillaries


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Table 3 Spray drying of emulsion-PVC. Effect of selection of atomizer on spray dryer performance: A Comparison between different atomizers

ParameterParameter Rotary diskRotary disk Two-fluidTwo-fluid(sonic)(sonic)

Two-fluid Two-fluid (standard)(standard)

Dryer geometryDryer geometry Conical/Conical/cylindricalcylindrical H/D≈H/D≈1.2-1.51.2-1.5

Tall-formTall-form CylindricalCylindrical H/D≈H/D≈44

Tall-formTall-form CylindricalCylindrical H/D≈H/D≈55

EvaporationEvaporation capacity (water)capacity (water)

1600 kg/h1600 kg/h 1600 kg/h1600 kg/h 1600 kg/h1600 kg/h

Chamber (Chamber (D D HH)) 6.5 m 6.5 m 8 m8 m 3.5 m 3.5 m 15 m 15 m 3 m 3 m 18 m18 m

Number of nozzlesNumber of nozzles 1, 175-mm disk1, 175-mm disk 15,000 rpm15,000 rpm

16 nozzles16 nozzles 4 bar pressure4 bar pressure

18 nozzles18 nozzles 4 bar pressure4 bar pressure

Power for atomizerPower for atomizer 25 W/kg slurry25 W/kg slurry 20 W/kg slurry20 W/kg slurry 80 W/kg slurry80 W/kg slurry

Capital costCapital cost HighHigh MediumMedium MediumMedium

Operating costOperating cost MediumMedium LowLow HighHigh


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Table 4 Conventional versus innovative drying techniques

FeedFeed typetype DryerDryer typetype New techniques*New techniques*

Liquid Liquid SuspensionSuspension

DrumDrumSpraySpray

Fluid/spouted beds of inert Fluid/spouted beds of inert particlesparticlesSpray/fluid bed combinationSpray/fluid bed combinationVacuum belt dryerVacuum belt dryerPulse combustion dryersPulse combustion dryers

Paste/sludgePaste/sludge SpraySprayDrumDrumPaddlePaddle

Spouted bed of inertsSpouted bed of inertsFluid bed (with solid Fluid bed (with solid backmixing)backmixing)Superheated steam dryersSuperheated steam dryers

ParticlesParticles RotaryRotaryFlashFlashFluidized bed (hot air Fluidized bed (hot air or combustion gas)or combustion gas)

Superheated steam FBDSuperheated steam FBDVibrated bedVibrated bedRing dryerRing dryerPulsated fluid bedPulsated fluid bedJet-zone dryerJet-zone dryerYamato rotary dryerYamato rotary dryer


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FeedFeed typetype DryerDryer typetype New techniques*New techniques*

Continuous Continuous sheets (coated sheets (coated paper, paper, paper, paper, textiles)textiles)

Multi-cylinder contact Multi-cylinder contact dryersdryersImpingement (air)Impingement (air)

Combined Combined impingement/radiation dryersimpingement/radiation dryersCombined impingement and Combined impingement and through dryers (textiles, low through dryers (textiles, low basis weight paper)basis weight paper)Impingement and MW or RFImpingement and MW or RF

*New dryers do not necessarily offer better techno-economic performance for all products

Final selection also depends on overall cost and the entire drying system used, including pre-drying and post-drying stages.

Further ,it is important to carry out lab and pilot tests before final selection since , even after all the criteria are met, some products may not dry well in the dryer selected.Note that , any dryer selected must be operated at optimal conditions for it to deliver best performance. A good dryer operated poorly will a poor dryer!


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Notes on dryers for pharmaceutical Notes on dryers for pharmaceutical productsproducts

These feature highly heat-sensitive products of very high These feature highly heat-sensitive products of very high value; cost is no consideration but quality isvalue; cost is no consideration but quality is

Characterized typically (not always) by batch processing Characterized typically (not always) by batch processing and smaller capacity requirementsand smaller capacity requirements

Freeze drying, vacuum drying with indirect heating are Freeze drying, vacuum drying with indirect heating are often technologies of choiceoften technologies of choice

Convective drying poses problems of contamination, Convective drying poses problems of contamination, product emission/collection etcproduct emission/collection etc

Presence of organic solvents also causes safety issues, low Presence of organic solvents also causes safety issues, low ppm residual in product etcppm residual in product etc

Filtration/drying often combined to eliminate potential for Filtration/drying often combined to eliminate potential for contamination as product as transferred from one unit to contamination as product as transferred from one unit to another- filter dryersanother- filter dryers

Microwave dryers have found some niche applications- Microwave dryers have found some niche applications- often proprietary in natureoften proprietary in nature


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Pharma drying…contdPharma drying…contd

Each dryer type as pros and consEach dryer type as pros and cons Shelf/tray dryers were most common in Shelf/tray dryers were most common in

early days-using convection; now vacuum early days-using convection; now vacuum dryers with conductive heating of trays are dryers with conductive heating of trays are usedused

Slow drying, crust formation, lumping due Slow drying, crust formation, lumping due to lack of mixing, cumbersome to lack of mixing, cumbersome loading/unloading, tedious cleaning loading/unloading, tedious cleaning procedures are some limitations- good for procedures are some limitations- good for 1 kg scale batches1 kg scale batches


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Double cone vacuum dryers use Double cone vacuum dryers use contact drying under vacuum with contact drying under vacuum with mixing induced by slow rotation mixing induced by slow rotation around a vertical axisaround a vertical axis

Easy to clean, load/unload, operate Easy to clean, load/unload, operate in closed cycle etcin closed cycle etc

Danger of lumping due to rotation Danger of lumping due to rotation ( snow-balling effect), slow drying , 8-( snow-balling effect), slow drying , 8-10 hrs per batch10 hrs per batch


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Paddle dryer-indirect dryer with a Paddle dryer-indirect dryer with a large paddle rotating ( 10 large paddle rotating ( 10 rpm)around a horizontal axis in rpm)around a horizontal axis in vacuumvacuum

Shorter batch times, good mixing Shorter batch times, good mixing and de-lumping due to stirringand de-lumping due to stirring

Higher maintenance costsHigher maintenance costs New development- a spherical New development- a spherical

paddle dryer (Siegfried, Switzerland)paddle dryer (Siegfried, Switzerland)


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Spherical paddle dryer- stirrer at 25 Spherical paddle dryer- stirrer at 25 rpm, heated, under vacuumrpm, heated, under vacuum

Can be washed with hot solventCan be washed with hot solvent Drying times 2-6 hrsDrying times 2-6 hrs Vertical loading/unloading using Vertical loading/unloading using

HEPA filtered air to minimize HEPA filtered air to minimize environmental contaminationenvironmental contamination

New design –no reported dataNew design –no reported data


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Active pharmaceutical ingredients (API): Active pharmaceutical ingredients (API): API drying with filter/dryerAPI drying with filter/dryer

Product isolation (separation form mother Product isolation (separation form mother liquor) and drying in one unit simplifies liquor) and drying in one unit simplifies operation, cleaning, maintaining, operation, cleaning, maintaining, loading/unloading, reducing risk of loading/unloading, reducing risk of contamination etccontamination etc

Batch process; complex equipmentBatch process; complex equipment Centrifugal filter/dryer-recent developmentCentrifugal filter/dryer-recent development


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Centrifugal filter/dryerCentrifugal filter/dryer Employs sintered metal-no cloth-as filter medium, Employs sintered metal-no cloth-as filter medium,

can be made of Hastelloy C22 (corrosion-can be made of Hastelloy C22 (corrosion-resistant) for centrifuge; drying in hot nitrogen resistant) for centrifuge; drying in hot nitrogen stream in fluidized mode at 5.5 bar pressure at stream in fluidized mode at 5.5 bar pressure at 25 C to 150 deg.C.25 C to 150 deg.C.

Fully automated operationFully automated operation Basically a horizontal centrifuge with 1.3 m Basically a horizontal centrifuge with 1.3 m

basket (Siegfried)basket (Siegfried) Product down to 10 micron, batches up to 4oo kg/ Product down to 10 micron, batches up to 4oo kg/

800 litres800 litres Shortest batch time claimed in Centrifuge Dryer Shortest batch time claimed in Centrifuge Dryer

( 6hrs) compared with centrifuge/paddle (17 hrs) ( 6hrs) compared with centrifuge/paddle (17 hrs) and centrifuge/spherical dryer (11 hrs)and centrifuge/spherical dryer (11 hrs)


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Nonaqueous solventsNonaqueous solvents See Pakowski and Mujumdar ( Handbook of See Pakowski and Mujumdar ( Handbook of

Industrial Drying for a chapter on the subjectIndustrial Drying for a chapter on the subject Many products require removal of organic Many products require removal of organic

solvents to low ppm rangesolvents to low ppm range Single or multi-component solvents need to be Single or multi-component solvents need to be

removed and recoveredremoved and recovered Closed cycle operation is necessaryClosed cycle operation is necessary Traces of solvents in exhaust stream maybe Traces of solvents in exhaust stream maybe

incineratedincinerated Asceptic conditions must be maintained in dryerAsceptic conditions must be maintained in dryer


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Other dryer types also used..Other dryer types also used.. Spray with closed cycle operationSpray with closed cycle operation Fluidized beds ,including low pressureFluidized beds ,including low pressure Conical dryer with screw mixer (indirect)Conical dryer with screw mixer (indirect) Fluid bedsFluid beds Pulsed fluid bedsPulsed fluid beds Vibrated deck bedsVibrated deck beds Spouted beds (tablets coating)Spouted beds (tablets coating) Flash dryersFlash dryers Spin-flashSpin-flash Turbo dryer (continuous tray)Turbo dryer (continuous tray) Vortex/Cyclone dryersVortex/Cyclone dryers


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Selection of Novel DryersSelection of Novel Dryers Many are not available in most parts of the Many are not available in most parts of the

worldworld More R&D needed to make the new More R&D needed to make the new

technologies “mature”technologies “mature” No “disruptive drying technology” on No “disruptive drying technology” on

horizon yethorizon yet Superheated steam, heat pump-assisted Superheated steam, heat pump-assisted

dryers and pulse combustion dryers may dryers and pulse combustion dryers may become popular in certain industriesbecome popular in certain industries

Intelligent combinations of existing drying Intelligent combinations of existing drying techniques and optimized operation are techniques and optimized operation are still in vogue.still in vogue.


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Pulse Combustion DryingPulse Combustion DryingAn ExampleAn Example

Pulsed combustion in valved or valveless Pulsed combustion in valved or valveless combustors is thermally efficient and eco-combustors is thermally efficient and eco-friendly ; tailpipe gives high temperature, highly friendly ; tailpipe gives high temperature, highly turbulent shear flow in which a liquid stream turbulent shear flow in which a liquid stream atomizes and dries in a fraction of a second.atomizes and dries in a fraction of a second.

High combustion efficiency; no need for atomizer High combustion efficiency; no need for atomizer or compressor –unlike spray dryer. Can dry heat-or compressor –unlike spray dryer. Can dry heat-sensitive materials as well.sensitive materials as well.

Noise ,scale-up etc are problem areas due to lack Noise ,scale-up etc are problem areas due to lack of adequate R&D.of adequate R&D.

Available commercially in USA, Japan, Canada Available commercially in USA, Japan, Canada etc.etc.

R&D also in Poland, Singapore, Canada, Japan R&D also in Poland, Singapore, Canada, Japan etc.etc.


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Pulse combustion dryerPulse combustion dryer

Not common technologyNot common technology Lab/pilot tests show suitability to dry Lab/pilot tests show suitability to dry

highly heat-sensitive products like highly heat-sensitive products like yeast, vitamins, foodstuffs etc.yeast, vitamins, foodstuffs etc.

Following slides provide some Following slides provide some relevant data and comparison with relevant data and comparison with conventional spray dryerconventional spray dryer


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Pulse Combustion Drying -Compare with Pulse Combustion Drying -Compare with conventional Spray Dryingconventional Spray Drying


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Pulse Combustion-Pulse Combustion-dried powder: SEMdried powder: SEM


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Spray-dried powder SEMSpray-dried powder SEMStructure is different in PCDStructure is different in PCD


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Pulse Combustion DryersPulse Combustion DryersNote the high heat transfer rates Note the high heat transfer rates

possiblepossible


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Comparison – Pulse Combustion Dryer vs. Comparison – Pulse Combustion Dryer vs. Spray DryerSpray Dryer

Dryer Dryer CharacteristicsCharacteristics

PCS Spray PCS Spray DryerDryer

Typical Nozzle-Type Typical Nozzle-Type Spray DryerSpray Dryer

Typical Rotary – Typical Rotary – Disk Spray DryerDisk Spray Dryer

AtomizationAtomization Gas Dynamic Gas Dynamic – No shear– No shear

High Pressure Orifice High Pressure Orifice – High shear– High shear

Moderate shearModerate shear

Feed PumpFeed Pump Low Pressure Low Pressure (1 psi)(1 psi)

High PressureHigh Pressure Low PressureLow Pressure

Feed Delivery to Feed Delivery to DryerDryer

Open PipeOpen Pipe Precision OrificePrecision Orifice Various Wheel Various Wheel TypesTypes

Evaporative Evaporative HeatHeat

At Point of At Point of atomizationatomization

Remote, ducted to Remote, ducted to drying chamberdrying chamber

Remote, ducted to Remote, ducted to drying chamberdrying chamber

Mixing of heat & Mixing of heat & dropletsdroplets

Rapid and Rapid and completecomplete

SlowSlow SlowSlow

Heat transfer Heat transfer raterate

HighHigh Relatively lowRelatively low Feed pump, rotary Feed pump, rotary drive unitdrive unit


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Some final words…Some final words… Know product well; list all specs carefully- do not Know product well; list all specs carefully- do not

ignore even minor detail and potential change in ignore even minor detail and potential change in formulation etcformulation etc

Minor changes can affect drying characteristics Minor changes can affect drying characteristics significantly e.g. whole vs skim milk dryingsignificantly e.g. whole vs skim milk drying

Presence of nonaqueous solvents must be Presence of nonaqueous solvents must be carefully consideredcarefully considered

Very low moisture content or solvent content in Very low moisture content or solvent content in dried product must be considered carefullydried product must be considered carefully

Extremely heat-sensitive materials are freeze-Extremely heat-sensitive materials are freeze-dried; some can be vacuum-dried or dried in dried; some can be vacuum-dried or dried in dehumidified airdehumidified air


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ClosureClosure Make a coarse selection based on general Make a coarse selection based on general

understanding of drying principles and understanding of drying principles and dryer classification/selectiondryer classification/selection

Consult appropriate vendors to carry out Consult appropriate vendors to carry out lab scale testslab scale tests

Drying times can be scaled well but not Drying times can be scaled well but not product quality; watch quality and see if it product quality; watch quality and see if it can be scaled. Pilot testing often required can be scaled. Pilot testing often required for new products.for new products.

Consult Handbook of Industrial Drying and Consult Handbook of Industrial Drying and other tests/references for further other tests/references for further guidance.guidance.


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THANK YOUTHANK YOU

The EndThe End

Questions??Questions??

Visit : Visit : http://serve.me.nus.edu.sghttp://serve.me.nus.edu.sg or or Google search Arun MujumdarGoogle search Arun Mujumdar

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Selection of Dryers I- India