F r a u n h O F E r I n s t I t u t E F O r I n t E r F a C I a l E n G I n E E r I n G a n d B I O t E C h n O l O G y I G B

DRYING WITH SUPER-HEATED STEAM AT AMBIENT PRESSURE

SYSTEMS FOR ENERGY-EFFICIENT AND MILD PRODUCT DRYING

Dryingisanimportantandfrequentlyusedprocessinvarious

branchesofindustry.Intheproduction,treatmentandpro-

cessingofsolidmaterialsdryingisoftenanessentialprocess

step.Dryingwithheatedorcooledairisgenerallyused.In

manycasesalargepartoftheenergyrequiredinthewhole

processchainisusedfordrying.Furthermore,theseprocesses

oftenrequirealongresidencetimeandthereforeconsiderable

space.Usingsuper-heatedsteamasatmosphereinthedryer

offerssignificantadvantagestoimprovedryingprocessesin

termsofdwelltime,energyconsumptionandotherparameters.

the concept

Theproductmaterialtobedriedisexposedtoanatmosphere

ofsuper-heatedsteam.Theproductmaterialisheatedand

releasesitsmoistureassteam.Thelossofevaporationheat

energycoolsdownthesteam.Thevolumeofsteamatmo-

sphereiscontrolledandsurplussteamistakenoutofthepro-

cess.Thelostheatenergyisreturnedtothesteamatmosphere

tomaintaintheprocesstemperature.Theoperationaltemper-

atureisgenerally120°Cto180°Cinstandardapplications,

butcouldalsobehigher.Duetothedifferenceindensitybe-

tweenairandsteam,thesteamatmosphereformsastratifi-

cationlayerandsealsitself.Nosealinginstallationssuchas

sluicesarerequired.Therefore,anyconveyingsystem,suchas

belts,spiralsordrums,canbechosenthatisbestsuitedto

thetransporthandlingoftheproduct.

Effects and advantages

Intermsofdryingpropertiessuper-heatedsteamisdifferent

toair.Thevaluesforheatcapacityandheattransferaremuch

higher,whichresultsinasignificantlyfasterdryingprocess

withashorterdwelltime.Becauseofthepuresteamatmo-

sphereanditslowviscosityafastdiffusionandflowvelocityof

thegastoandintotheproductispossible;thishasapositive

impactontheshortprocesstime,butalsoresultsinamore

homogenousdrying.Duetotheseeffects,dryersbasedonthis

principlecanbedesignedtobemorecompactandwithlower

investmentcostscomparedtoair-baseddryers.Afurthercon-

sequenceofthelowprocesstimesisthatheatlossesarelower,

whichmeanslessenergyconsumptionandcosts.Basically,itis

feasibletousethesurpluspuresteamwithitsenergycontent

forotherpurposesinafacility.Insuchacasehighoverallener-

gyefficiencycanbeachievedinanindustrialfacility.Steam

compressorscanalsobeusedtoraisethespecificenthalpyand

thusthetemperatureonceagaintoaninterestinglevel.

Ontheotherhand,thesteamcanbecondensedandbe

usedasdemineralizedwater.Insomecases,othervolatile

compoundssuchasaromaticsubstancesorvolatileorganic

compounds(VOCs)areevaporatedtogetherwiththesteam.

Standardprocessespermitcondensingthemseparatelyand

recoveringthemasavaluableproductasforexamplefrom

variousagriculturalfruits.Duetotheinternalcirculation

ofthesteamasadryingmediumandthesealedhousing,no

wasteairtreatmentisrequired.Thepuresteamwithnooxy-

genpresentprovidesaninertprocessatmosphereinthedryer,

whichpreventsoxidationoftheproduct.Afurtheradvantage

2 31a 1B 1C 1d

oftheinertatmosphereisthereducedriskofexplosione.g.

bydust.Toenablesuper-heatingofthesteamatmospherethe

temperaturehastobeabout120°C.Thisleadstoahygieniza-

tionoftheproduct.Theshortdwelltimeduetothehighly

efficientdryingprocessresultsinalowrateofdegradationof

productcompounds,especiallyinfoodproducts.

reference data

Mineral raw materials

Whendryingabulkymaterialareductioninprocesstime

of30percentcomparedtotheexistinghot-airdryerwas

achieved.Besidesthereductioninheatlosses,thispermitted

areductionoftheoverallsizeofthedryerbyonethirdor

acorrespondingincreaseofthethroughputcapacity.

Drying of construction material

Itwaspossibletoreducetheprocesstimefrom4to6hours

downtolessthan3hoursinfirsttesttrialsonacontinuous

laboratorydryer.Thisledtoanincreaseinprocesscapacityof

25percentto50percent(withanadditionalsignificantsaving

ofspecificenergyconsumption).

Foodstuffs

Inthisfieldofapplicationdryingisakeyprocess.Significant

savingsontimeandenergyhavebeenachievedtoo.For

example,indryingofapplechipsdwelltimewasreduced

fromapprox.8hoursdownto50minuteswithoutanyloss

ofproductquality.Inthecaseofdryingapre-processed

foodproductbasedonpotatoesthedryingtimewascutby

90percentfromapprox7hoursto30minutes.

Fodder and pet food

Whendryingpetfoodandanimalfodder,thedwelltimehas

beenreducedfrom35to10minutes.Inthiscasethedrying

processwasoperatedat10°Clowerthanthecurrentlyem-

ployedhot-airdryingprocess.Accordingly,thesavingsinspe-

cificenergyconsumptionweresignificanttoo.

Hygienization

Basedonsystematictesttrialsithasbeenshownthatthe

processofsuper-heatedsteamdryingiswellsuitedtothe

hygienizationoffoodstuffs,forexample.Thespecificand

controlledcontaminationofmushroomsandpaprikawith

E. coli cellsandBacillusendosporeshasbeenreducedbya

logarithmfactorof7.

1 A-D: Examples of dried

products from the food-

stuffs, pet food and fodder,

and extractive industries.

2 Modelling of flow

velocity in a dryer.

2

Our services

Scientific characterization and specification, research

on tasks related to drying and heat transfer

Development of specific drying process concepts according

to the customer’s individual needs

Layout and specification of process by an interdisciplinary

team with background of process engineering, design,

chemistry, microbiology and electrical engineering

Laboratory plants for test trials

Product related analysis of the drying process using

a wide range of analytical equipment and expertise

Design specification of process unit and components, e.g.

by integrated combination of 3D CAD design and numeric

modeling of fluids, heat transfer etc. with latest software

Supporting our clients from first test trials to commissio-

ning of a plant. If partial or turn-key delivery of the

construction is required, we co-operate with a network

with qualified industrial suppliers.

Fraunhofer Institute for

Interfacial Engineering

and Biotechnology IGB

(Fraunhofer-Institut für

Grenzflächen- und

Bioverfahrenstechnik IGB)

Nobelstrasse 12

70569 Stuttgart

Germany

Phone +49 711 970-4401

Fax +49 711 970-4200

info@igb.fraunhofer.de

Director

Prof. Dr. Thomas Hirth

Phone +49 711 970-4400

thomas.hirth@igb.fraunhofer.de

www.igb.fraunhofer.de

ContactContact

Dipl.-Ing. Mike Blicker

Phone +49 711 970-3539

mike.blicker@igb.fraunhofer.de

Dipl.-Ing. Siegfried Egner

Head of Department Physical Process Technology

Phone +49 711 970-3643

siegfried.egner@igb.fraunhofer.de