Drying Basics and Principles

1 Drying Basics and PrinciplesM Unit, CESD IRRI LB PhiliiIRRI Los Ba os, Drying process Moisture gradientTempering Tempering Heated air Drying and yglow temperature dryingDrying processyg I Preheating period Drying rate is slowly increasingincreasing only a very slight change in MC II Constant-rate period Drying rate is constant in time removal of surface water grain temperature is constantdtt energy used to evaporate water III Falling-rate period Drying rate declines over time-> temperatures for Drying dddrying rate declines over time transfer of internal moisture to the surface Drying rate declinespaddy Seeds Maximum of 43 C For milling grain heats up (danger) for paddy grain starts at 18%g During constant-rate period: > 100 C During falling-rate period: < 55 C(depends on Drying system) Drying rateyg Stated in percent moisture removed per hourpp Affected by: Temperature and relative humidity of the Drying air Seeds: max.

2 43 C Fist stage Drying : max. 120 C Second stage Drying : max. 55 C Air velocity Too low -> air is saturated before leaving the dryer Too high->faster Drying but wast of energyToo high faster Drying but wast of energy Low-temperature Drying : m/s Heated air Drying : m/s Fluidized bed Drying : 2 3 m/sFluidized bed Drying : m/sUniform dryingyg Moisture gradient grain at the air inlet dries faster moisture gradientunloadingdry grains unloading -dry grains are mixed with wet grains Re-wetting -> cracks in dry grains1520MC, % For even Drying sun Drying : stir every 30 minutesiiifidbd10051015 Drying time, h mix grain in fixed bed dryers use re-circulating batch dryersy use low temperature for more even dryingTemperingpg Temporary stopping of Drying (tempering) Temporary stopping of Drying (tempering) moisture will equalize inside the grains moisture between grains will equalize(reduces the moisture gradient)(reduces the moisture gradient) Re-start Drying Drying rate will be higherygg reduced energy requirement Applicationrecirculating batch dryers re-circulating batch dryers, grain goes through cycles of Drying followed by temperingtempering bins in continuous flow dryer plants tempering bins in continuous flow dryer plantsMechanical Drying MethodsHeated-air dryingLow-Temperature DryingDrying 15202530MC, % , % air temp.

3 : 43 CAir m/sygZone10051015 Drying time, hDrying ZoneDry Grains1004896144192240 Drying Time, hDrying air temperature: T=0-6 KAir m/sAirflow rate per t grain : > m /sPower grainLayer depth: < 40 cmDrying time: 6-12 hInitial MC:up to 30%+ Drying air temperature: T 06 K Air m/sAirflow rate per t grain :> m /sPower kW/t grain Layer depth: < 2 mDrying time: days to weeks Initial MC: 18% ( 28%)Initial MC: up to 30%+Advantages:Simple managementFast dryingAffordableLow level of integrationDi dtAdvantages:Very energy efficientBins can be filled at harvest rateMaintains grain quality optimallyDrying in storage structuresDi dDisadvantages:3-4% moisture gradient in final product, requires mixing or reduced layer depthReduction in milling yieldDanger of killing seeds Disadvantages:Increased risk with poor power suppliesRequires bulk handling system (high level of integration in postharvest system)Long Drying time Sun dryingyg AdvantagesHarvestingAdvantages Free energy Low capital HarvestingThreshingTransportDryinginvest ment DisadvantagesWeather riskDryingStorageMillingPackaging Weather risk Temperature control difficultPackagingMarketing High qualitative and quantitative losses High laborHigh labor requirementIRRI Flat-bed dryeryCapacity1 2 metric tons/batchPriceUS$ 1000-2000 Drying time6-8hGrain QualityMedium quality, uneven Drying of bottom and top layer HeaterDrying air temperature: 43 CHeaterDrying air temperature: 43CA.

4 Kerosene burner, l kerosene/hB.) Rice hull furnace, 3-4 kg rice hull/hFanAxial flow fan, 2200 rpm 1800 cfm at 20mm H2O1800 cfm at 20mm H2OA.) Electric motor, kW B.) Gasoline engine, gasoline/hDimensionsWeightL: ; W: ; H: 1m; bin: 220 kg; furnace: 290kgWeightbin: 220 kg; furnace: 290kgAdvantageSimple design, can be locally made, can handle very wet cropDisadvantageMoisture gradient, mixing necessary, dust when unloading kerosene smelldust when unloading, kerosene smellKerosene burnerRice hull furnaceVietnam: Reversible airflow flat bed 4t Flat Bed Dryer in Btt bC bdiBattambang, CambodiaVietnamese type Flat Bed Dryer in yMyanmar4-6t capacityDrying rate: 1%/hDrying rate: 1%/hCost: US$ 3,000 Rice husk furnaceVietnamese Low-cost dryerCapacity1 t/batchPriceUS$ 150-200 Drying time2-4 daysGrain QualityGood, low temperature Drying principleHeaterDrying air temperature: 6 C above ambient, if weather is badA.

5 Coal stove, kg/hB.) Electric heater, 1 kW),FanAxial flow fan, 1750 rpmblowers from automotive coolers Electric motor, kWAdvantageVery cheap, simple design, can be locally made, can utilize traditional storage structures, highly mobileDisadvantaSmall moisture gradient, longDisadvantageSmall moisture gradient, long Drying timeLow-cost seed dryeryCapacity100-250 kgPriceUS$ 150-200 Drying time6-9h (initial MC of 22%) grain QualityGood seed quality HeaterDrying air temperature: 43 CRice hull stove, kg rice hull/hFanCentrifugal fan, 3200 rpm g, m /sElectric motor, 220 WAdvantageSimple design, can be locally made,affordable mobileaffordable, mobileDisadvantageMoisture gradientRe-circulating batch dryergyCapacity10 metric tons/batchPriceUS$ 10 000 PriceUS$ 10,000 Drying time6-8 h, 55 min circulation periodGrain QualityBest quality, continuous mixing, tempering Drying cyclestempering Drying cyclesHeaterDrying air temperature: adjustable,Kerosene burner, l/hFan2 axial flow fans 1730 rpmAxial-flow blowerFan2 axial flow fans, 1730 rpm 1800 cfm at 20mm H2OA.

6 Electric motor, kW B.) Gasoline engine, gasoline/hKerosene burnerDimensionsL: m; W: m; H: m; 2,110kgElectricity kW for blower, conveyors, electronic controlKerosene burnerCAdvantageAutomatic operation, excellent quality,Automatic safety featuresDisadvantageProblems with very wet crop (>30%)Control panelAgridry seed t/batch (1m )PriceUS$ 1,350 Drying hours from to rate: QualitySee quality analysis HeaterLPG gas, 20 MJ/hFanCentrifugal fanElectric motor 1 5 kWElectric motor, kWAdvantageMobile, convenient operation, automatic temperature control, high Drying rategygDisadvantageUneven grain bed thicknessBin needs to be totally filledInconvenient unloadingComponents of a flat bed dryerAirFlue gasHeated airFurnaceFanDrying binHarvestingThreshingTransportDryingSto rageMillingPackagingMarketingComponents of a dryer and their function Main components Drying bin-holds the grainDrying bin holds the grain Air distribution system provide Drying air.

7 Remove waterFtdifl Fan create pressure and air flow Air heater reduce relative humidity of air Optional Conveyors loading and unloading Accessories Moisture meter monitor Drying processDttlhti Dust separator clean exhaust airDrying bin and plenum chamberCircular Drying bin AdvantagesSelf supporting Self supporting structure Simple plenum chamber Short air delivery pathwayspathways DisadvantageUneven air velocity Uneven air velocity Air velocity highest at the inletReversible air flow dryer Reversal of air flow Reduced moisture gradientgradient No mixing neededneeded Increased capacitycapacityAir distribution systemRe-circulating batch dryersRecirculating batch dryers Cross flow grain moves downwards, air across Less hindrance of grain flow works better with wetflow, works better with wet paddy with high amount of straw and chaff Mixed flowMixed flow Air flows from inlet to outlet ducts Ducts are shaped so that thi thithey mix the grain Better mixing leads to better qualityFansFan typeAxial flowCentrifugal, forwards curvedCentrifugal, backwards curvedCostCheapmore expensivemost expensiveCharacteristics1non-overloading overloadingNon-overloadingPressure creation10-15 cm water0-15 cm water0-30 cm waterPressure creation1015 cm water015 cm water030 cm waterUnstable region of operationAt high pressureNoneNoneConstructionSturdyLightS turdyNoise levelHighLowMediumTypical useAeration, recirculation batch dryer, batch dryerIn-store dryersAir distribution systemType of dryersPressure systemSuction systemFixed bed batchDryers with moving grain recirculatinggrain.

8 Re-circulating batch and continuous flowAiFi d b t hbMihil tAir tightness of binFixed batch can be made airtight easily, large outletMoving mechanical parts make sealing difficultHtBffBfdiltHeaterBefore fanBefore dryer inletFanHigh temperature resistance needed, Lower temperature resistance,sometimes exposed to flamesDustStays mainly in fixedSucked out with theDustStays mainly in fixed bed, set free when unloadingSucked out with the Drying airAir distribution systemOptions for batch dryersOptions for batch dryersStAi dtP f tdflAiflSystemAir ductsPerforated false floorAir-sweep floorCostLowMediumHigh (grill shaped metal plus fan)metal plus fan)Air distributionUnevenOptimalOptimalRequirem entsSealed floorStable supportSupport structureRequirementsSealed floorAdditional plenum chamber neededStable support structure needs withstand walking on itSupport structureStrong fan for conveyingConstraintsManual unloadingUneven Drying at high MCManual unloadingDust creationAir distribution systemsAir ducts for batch dryersAir ducts for batch dryersAir distribution systemsAir sweep floorsHeating systemsgyHeating SystemsKerosene / Diesel burnerSmall Rice Hull FurnaceAutomated Rice Hull FurnaceLPG burnerElectricitySolar energyAlitiHi hMdiLFNNA pplicationHighMediumLowFewNoneNoneCapita l costLowMediumHighMediumLowHighOperatingM ediumLowLowMedium-HighHighNonecostggAdva ntages Easy fuel handlingAutomatic Cheap fuel CO2 neutral Cheapfuel CO2 Easyhandlingoffuel Convenient Easy to control CO2neutralmatic operation High energy

9 ContentCO2neutralfuel Automaticoperation Clean flamecontrol CleanConstraints Smell Labor High Availability Expensive Low heat intensive Material difficult to convey Bulky fuelgcapital cost Wear of componentsy Cost of fuelp Limitedpower load Highestenergy formgeneration Bulky fuelnts. Bulky fuelAccessories: Pre-cleaner Fines in rice create dust during the loading and Drying process and reduce airflow through the riceairflow through the rice grain . Pre-cleaners usually consist of scalper that lets through the grain but retains strawg a smaller second screen that removes small stones and other impuritiesp air aspirator for sucking out dust and light empty grains Accessories: Elevators and Conveyors For horizontal and vertical transport ofvertical transport of grains: Loading Circulation Discharge Need to be matched toNeed to be matched to the capacity of the dryer.

10 A properly designed b cket ele ator for a rebucket elevator for a re-circulating batch dryer can easily reach capacities of 10t/h10t/h. Accessories: Moisture meter Monitoring grain ittidmoisture to avoid over Drying incomplete dryingOdi Over Drying Unnecessary weight loss Reduced milling yields Iltdi Incomplete Drying causes qualitative and quantitative losses from fungal growth fungal growth insect activity respirationAccessories: Dust collection systemG i hdliill grain handling will create dust, making working around aworking around a grain Drying hazardous. Need for dust collection systems Cyclone Cyclone Need to be properly sized depending on th dthe dryer specifications. Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution NonCommons Attribution Non Commercial License 2013 The International Rice Research instituteYllibili fdi l li biliid i hResearch instituteYou assume all responsibility for use and potential liability associated with any use of the course material.