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Spray drying technology: an overview

R.P.Patel,M.P.PatelandA.M.SutharDepartmentofPharmaceutics,S.K.PatelCollegeofPharmaceuticalEducationandResearch,GanpatUniversity,

Kherva,Mehsana,Gujarat-382711,India.Raka_77us@yahoo.com

Abstract: This systemic review covers the design andcritical elements of spray drying, types of spray drier,critical parameters of spray drying, innovations in spraydrying,anditsapplicationsinpharmaceuticalfield.Keywords :Spraydrying,atomization,pharmaceutical. Introduction

The development of spray drying equipment andtechniquesevolvedoveraperiodofseveraldecadesfromthe1870s through theearly 1900s. Spray drying comesof age during World War II, with the sudden need to

reducethetransportweightoffoodsandothermaterials.Thistechniqueenablesthetransformationoffeedfromafluidstateintodriedparticulateformbysprayingthefeedinto a hot drying medium. It is a continuous particle-processingdryingoperation.Thefeedcanbeasolution,suspension, dispersion or emulsion. The dried productcanbeintheformofpowders,granulesoragglomeratesdependinguponthephysicalandchemicalpropertiesofthe feed, the dryer design and final powder propertiesdesired(Michael,1993).

Spraydryingprocessmainlyinvolvesfivesteps:(i) Concentration: feedstock is normally concentrated

priortointroductionintothespraydryer.(ii)  Atomization: the atomization stage creates the

optimumconditionforevaporationtoadriedproducthavingthedesiredcharacteristics.

(iii)  Droplet-aircontact:inthechamber,atomizedliquidisbrought into contact with hot gas, resulting in theevaporation of 95%+ of the water contained in thedropletsinamatterofafewseconds.

iv) Droplet drying: moisture evaporation takes place intwostages-1)duringthefirststage,thereissufficientmoistureinthedroptoreplacetheliquidevaporatedat the surface and evaporation takes place at arelativelyconstantrate(Keey&Pham,1976),and2)the second stage begins when there is no longerenoughmoisturetomaintain

saturated conditions at thedroplet surface, causing adried shell to form at thesurface. Evaporation thendepends on the diffusion ofmoisture through the shell,which is increasing inthickness.

(v)Separation: cyclones, bagfilters, and electrostaticprecipitators may be used for

thefinalseparationstage.WetScrubbersareoftenuseto purify and cool the air so that it can be released tatmosphere.

Spray drying process (Fig.1) have advantages thacanbedesignedtovirtuallyanycapacityrequired.Feeratesrangefromafewpoundsperhourtoover100tonper hour. Operation is continuous and adaptable to fuautomatic control (Gharsallaouiet al ., 2007). It can beusedwithbothheat-resistantandheatsensitiveproductsNearly spherical particles can be produced. There arsome limitation that includes limited versatility iproducing particles or structures with the comple

morphologies, and rapid drug release rates ofteexhibitingabursteffect(Katta&Gauvin,1976). Design and critical elements of spray dryingAtomizers

The"heart"ofanyspraydryeristheatomizer,smainsize,biginimportance,installingtherightatomizerisessentialtospraydryingsuccess.

The atomizer must fulfill several important functionwhicharesummarizedbelow:i.  Itmust dispersethe feedmaterial intosmalldroplets

which should be well distributed within the dryeranmixedthoroughlywiththehotgas.

ii.  Thedropletsproducedmustnotbesolargethattheare incompletely dried, nor so small that producrecoveryis difficult.Smallparticlesmayalso overheaandbecomescorched.

iii.  The atomizer must also act as a metering devicecontrollingtherateatwhichthematerialisfedintothedryer:a)Airatomizationortwofluidnozzles,b)Airlessatomizationnozzles,c)Pressurenozzles,d)Rotaryodisknozzlesand,d)Ultrasonicnozzles.

Airflowa) Co-current flow: in a co-current dryer, the spray idirectedintothehotairenteringthedryerandbothpasthroughthechamberinthesamedirection.

b) Counter-current flow: in

thisdryerdesign,thespraand the air are introducedat opposite ends of thdryer, with the atomizepositioned at the top andthe air entering at thebottom.c)Mixedflow:dryersofthistype combine bothcocurrent and countecurrent flow. In a mixed

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flow dryer, the air enters atthe top and the atomizerislocatedatthebottom.Spraydryingchamber

Air within the chamber maintains a flow pattern,preventingdepositionofpartiallydriedproductonthewallor atomizer (Ronald, 1997). Air movement and

temperatureofinletairinfluencethetypeoffinalproduct.

 Types of spray drierSinglestagedryer(Fig.2)(1-Feedstock; 2-drying chamber; 3-dried product; 4-cyclone;5-wetscrubber;6-bagfilter)

Twostagedryer (Fig.3)Twostagedryersallowtheuseoflowertemperature

in the dryer, making the design a good choice foproducts that are particularly heat sensitive (Katta &Gauvin, 1976) (1-air; 2-feedstock; 3-dried product; 4drying chamber;5-cyclone; 6-stationaryfluid bed;7-flui

bedcyclone).Horizontaldryer(Fig.4)

The components are: 1-dryingair; 2-feedstock; 3-pneumaticonveyor; 4-drying chamber; 5powder conveyor; 6-filter bags; 7cyclone;8-dustreturn;9-exhausttoatmosphere;10-driedpowder.Verticaldryer

Itissuitedforbothnon-fatandfat-containing products, producinnon-agglomerated andagglomeratedfree-flowingpowders

Manufacturers of vertical spradryersincludeStork,NiroandAPVAnhydro. 

Fluidizedspraydrier(Fig.5)The Fluidized Spray Dryer combines spray drying

and fluid bed drying technologies and offer excellenproductflexibilityand excellentthermal efficiency. Stickyproducts can be dried successfully, and the concept iideal for drying heat sensitive products, and improvearoma retention is accomplished (Sommerfeld & Blei1992). Multistagedrier

The process produces non-dusty, free flowinagglomerated powders with high flavor retention.

operates with low outlet-temperatures, achieving higthermal efficiency. Thisdesign concept is successfulfodryinghighfats,hygroscopic,andstickyproductsthatardifficulttohandleinmoreconventionaldesigns.CompactSprayDryer

Atomizationiscreatedbyeitherarotaryatomizerospraynozzleatomizer.Thelocationofthefluidbedwithithedryingchamberachievesdryingatlowertemperaturelevels.Itresultsinhigherthermalefficienciesandcooleconditionsforpowderhandling.Integratedfilterdrier

IFD™IntegratedFilterDryer-Combinesanintegratefluid bed and filter arrangement. It is an adaptable an

flexible spray dryer for the foodingredients, food, dairychemical,andpharmaceuticalindustries. The Integrated Filter Dryer (IFD™): features and

benefits includes:  improves powder quality, no handlinof product outside drying chamber, reduced noise leveandlowerenergyconsumption.FILTERMAT®Dryer

TheFILTERMAT®SprayDryerisfrequentlyusedifood and dairy applications. It operates at a low outletemperature, achieving high thermal efficiency. It is th

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recommended system for drying high fat, sugar-based,hydrolyzed,andfermentedproducts. Critical parameters of spray dryinga)  Inlettemperatureofair:higherthetemperatureofinlet

air,fasteristhemoistureevaporationbutthepowder

issubjectedtohighertemperature,whichmaydistortthe chemical/physical properties of heat sensitiveproduct(Michael,1993).

b)  Outlet temperature of air: it govern the sizing ofpowderrecoveryequipments,higheristheoutletairtemperaturelargerwillbethesizeofpowderrecoveryequipmentandconveyingductsandplenums(Mauryet al ., 2005). Outlet air temperatures control finalmoisturecontentofpowder.

c)  Viscosity: high viscosity hinders correct drop forma-tion. As the viscosity is lowered, less energy orpressure is required to form a particular spraypattern.

d)  Solid content: care must be taken with high solidloadings(above30%)tomaintainproperatomizationtoensurecorrectdropletformation.

e)  Surface tension: addition of a small amount ofsurfactantcansignificantlylowerthesurfacetension.This can result in a wider spray pattern, smallerdropletsize,andhigherdropvelocity.

f)  Feedtemperature:asthetemperatureofasolutiontobe sprayed isincreased, thesolution may easilydryasitbringsmoreenergy to thesystem.

g)  Volatilityofsolvent:ahigh volatility isdesirable in anydrying process.Unfortunately,choices are limitedtoday. In manycases, these restrictthesolventchoicetowater.

h)  Nozzlematerial:mostpharmaceuticalapplicationsusestainless steel inserts. However, tungsten carbidenozzles are often available and have excellent

resistancetoabrasionandgoodcorrosionresistanceformostfeedstock. Innovations in spray dryingSterilespraydryingforstableinjectableliquidformulation

Solubleglassmicrospheresforminga monodispersesuspensioninanhydrousfluorocarbonliquidbecausethemicrospheres are solid, their density can be preciselycontrolledtomatchthatofthesurroundingliquid(Bucktonet al ., 2002; Roser, 2005). Such suspensions are

physicallystableandtheparticlesneithersettlenorfloaintheliquidphase.Foamspraydrying

Inthismethodliquidfoodisfoamed,suchasmilkocoffee,beforesprayingitintothedrier.Theresultisfastedrying rate from the expanded foamed droplet surfac

area,andlighterdensitydriedproduct.Thisisknownafoam-spraydrying(Hanrahan&Webb,1961).Spraydryingfortheproductionofcrystallineproducts

Spray drying is known to produce predominatelamorphous material due to the almost instantaneoutransition between liquid and solid phases. Howevespray drying can also be used to obtain crystallineproducts(Shoyele&Cawthorne,2006).Toachievesuca goal, the product is fed in a crystalline suspensioninsteadofa solution,tothedryingchamber.Feedingthcrystalsintherightformallowsspraydryingtofinetuncrystal size distribution and final content of residuasolvents(Jorge&Felipe,2004).

 Applications of spray drying in pharmaceutical fieldEffectofspraydryingonpowderproperties

Many spray drying operations produce sphericaparticles while others result in non-spherical particlesParticlesmay beholloworsolid.Pressurespraynozzlecan produce particles ranging in size from 20 to 60microns. Two-fluid nozzles generally produce particle

withsizesintherangfrom10to200micronand larger (Katta &Gauvin, 1976Sommerfeld & Blei2001). Rotary

atomizers producemore uniform particlesizes compared topressure atomizersCo-current dryersproduce powders withlower bulk densitiethan counter-currendryers.

GranulationIn general, a spray dried granulation has improve

flow,betterdistributionofdrug,colors,etc.andrequirelesslubricantthanwetmassedproducts(Michael,1993)

Spray drying results in a shell of concentrated binder athe surface of the granular material, providing strontabletsandmaximumuseofbinder.Bioavailability  

Withspraydryingonecan co-precipitateanAPIwitapolymerinastableamorphoussoliddispersion,therebgreatly improving the dissolution rate of many drusubstances, including tolbutamide, indomethacin anibuprofen (Gonnissen et al ., 2008). Complexes oparacetamol and diazepam have been prepared with pcyclodextrin.

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Encapsulation  With a cocurrent drier, heat exposure is minimized.

The product is usually recovered about 15°C belowtheoutlet temperature (Wan et al ., 1992). This has beenapplied to microencapsulation of products such asantibiotics,vaccines,peptidesandproteins.

Inhalation Highly specialized spray drying nozzles that give

increasedparticleengineeringcapabilities,evenonlarge-scale- making it possible to accurately manipulateaerodynamic particle size and properties. Spray dryingtechnologies make it easier than ever to efficientlyproducetherapiesintheformoffree-flowingparticlesthatareideallysuitedforinhalation(Sevilleetal .,2007).Controlreleaseproducts

Creating a shell-like structure around the granularallows spray drying to be used for the manufacture ofcontrolled-releaseproducts. 

Future impactsSpray drying is presently one of the most excitingtechnologies for the pharmaceutical industry, being anideal process where the end-product must comply withprecise quality standards regarding particle sizedistribution, residual moisture content, bulk density andmorphology.Theproductionofparticlesfromtheprocessofsprayinghasgainedmuchattentioninrecentyears.

Multistage processes, new spray techniques, andtemperature-gradient systems hold promise for futurepharmaceutical application (Masters, 1991). ClassicequipmentdesignsarebeingusedmoreandmoreintheUnited States as a means for preparing pharmaceuticalproductsofvarioustypes.Theirversatileoutputcapacity,

continuous operation and controllability are desirablefeatures. References1.  BucktonG,ChidavaenziOandKooshaF(2002)The

effect of spray drying feed temperature andsubsequentcrystallizationconditions on thephysicalformoflactose.AAPSPharm.Sci.Tech .3(4),17.

2.  Gharsallaoui A, Roudaut G, Chambin O, Voilley Aand Saurel R (2007) Applications of spray-drying inmicroencapsulationoffoodingredients:anoverview.FoodRes.Intl .40,1107–1121.

3.  Gonnissen Y, Verhoeven E, Peeters E, Remon JP

andVervaetC (2008)Coprocessingviaspraydryingas a formulation platform to improve thecompactability of various drugs.Eur. J.Pharmac. &Biopharma .69,320–334.

4.  HanrahanFPandWebbBH.(1961)USDADevelopsfoam-spraydrying.FoodEng .33(8),37.

5.  Jorge MCP and Filipe G (2004) Spray dryingtechnology for better API crystals. Process

Development .38-39.www.sp2.uk.com6.  Katta S and Gauvin W (1976) Basic concepts of

spraydryerdesign.AICHEJ .22(4),713–724.

7.  Keey RB and Pham QT (1976) Behavior of spradrierswithnozzleatomizers. Chem.Engg .311,516521.

8.  MastersK(1991)Spraydryinghandbook,JohnWile&Sons,NY,5

thedition.

9.  Maury M, Murphy K, Kumar S, Shi L and Lee G

(2005) Effects of process variables on the powdeyield of spray-dried trehalose on a laboratoryspraydryer.Eur.J.Pharmac.&Biopharma,59,565–573.

10.  MichaelJK(1993)Spraydryingandspraycongealinof pharmaceuticals. In: Encyclopedia opharmaceuticaltechnology. Marcel Dekker INC, NY14,207-221.

11.  RonaldCD(1997)Spraydryinginnovativeuseofaoldprocess.DesignElements .7, 97–113.

12.  Roser B (2005) Sterile spraydrying for stable liqui21

st century pharmaceuticals. In: Innovations i

pharmaceuticaltechnology.pp:50-54.13.  SevillePC,LiHyandLearoydTP(2007)Spraydried

powdersforpulmonarydrugdelivery.Crit.Rev.TherDrugCarrierSyst .24(4),307-360.

14.  Shoyele SA and Cawthorne S (2006) Particlengineering techniques for inhalebiopharmaceuticals. Adv. Drug Delivery Rev . 581009–1029.

15.  Sommerfeld M and Blei S (1992) Lagrangiamodeling of agglomeration during spray dryinprocesses. Encyclopedia of Pharma. Technol . 6171-173.

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17.  Wan LS, Heng PW, Chia CG and Cececilia GH

(1992) Spray drying as a process for encapsulatioand the effect of different coating polymers. DrugDev.Ind.Pharm.18(9),997-1011.