Thermal Drying

Drying

Introduction and Fundamental ConseptsProduct size: m ten of cmProduct porosity: 0 99%Drying times: 0.25sec (tissue paper) to 5months (hard woods)Production capacities: 0.10kg/h - 100t/hProduct speeds: 0 (stationary) - 2000m/min (tissue paper)Drying temperatures: < triple point - > liquid critical pointOperating pressures: < 1millibar - 25atmHeat supply: continuously, intermittently;convection, conduction, thermal andmicrowave radiationsPatents granted each year: 250 (US), 80 (European)Some Statistics and Factoids on Drying

Papermaking - massive dehydration operation - more energy-intensive than steel making!Industrially developed nations: 12-25% national industrial energy consumption -thermal dehydration Drying in some stages of all industrial opp. - controls the quality and market value or products marketabilityExcludingpetrochemical refining, drying is by far the most energy-intensiveImproper drying of the most expensive drugs may form polymorphs (notherapeutic value) mil$$$ of lossesImproper design & operation - can be a hazardous operation -fire/explosion damage (organic products)Most thermal energy comes from combustion of fossil fuels, a major environmental impactSome Statistics and Factoids on Drying

8% of total industrial energy consumption for drying (Low?)Approx. 27 million tons water removed / year in drying processesAn efficient dryer consumes about 1 ton of oil equivalent (TOE) to remove 8 tons of water (inefficient ones are as low as 1:3)Assuming average ratio of 1:6, 4.5 million TOE of fossil fuel energy is consumed annually in the U.K. for industrial drying emitting 13 million tons of CO2!Significance of Drying: Figure for the U.K. (1990)

230 x 1015 J/year used for drying17.1 million tons / year CO2 emissionCurrent efficiency levels 15-35% (EDRL)5% improvement in energy efficiency will decrease CO2 emission by 3 - 4 million tons / yearImproving existing dryers and developing new drying technologies have potential to reduce CO2 emission by 1.2 and 9 million tons / yearSignificance of Drying: Figures for Canada

World production ~ 2 billion tons35% worlds cereal crops need drying (25% to 15% water, w.b.)Post-Harvest Drying of Grains (source: FAO, 1996)

Pharmaceutical IndustryDrying / energy costs negligible component of market price of productsOver $190 billion worth pharmaceutical products are freeze dried around the worldComplexity of DryingLiquidSemi-solidSolidPhysical & thermal properties vary with moisture, temperatureEnormous varietyVarious modes of heat inputVarious flow configurations2-phase flows; transient, 3-D effectsDwell times vary widelyTemperaturePressureSteady, unsteadyAffect properties of productAffect micro-structureMaterial PropertiesEquipment CharacteristicsOperating ConditionsHeat and Mass Exchangers Complicating issuesBottom line: Total cost of drying per kg product !Complexity of Drying

Variety of energy sources (continuous, intermittent)Physical / chemical transformationsMulti-component transportTransient transport processesVarying moisture transport mechanismsProduct quality interactions6Complexity of DryingWhy is drying of solids so complex?Over 200 types of dryers in industrial useDiverse product physical, chemical properties vary widely for feeds and productsInvolves transient momentum, heat and mass transport through porous media, with phase change, with/without chemical/biochemical reactionsNo universal drying theory existsMinor changes of MC result in large changes in physical propertiesBasics of DryingDrying Removal of a liquid from a solid/semi-solid/liquid to produce solid product by thermal energy input causing phase change (liquid - vapor; solid - vapor)Needed for the purposes of preservation and storage, reduction in cost of transportation, etc.Most common and diverse operation with over 100 types of dryers in industrial useCompetes with distillation as the most energy-intensive operationDryingDrying is an operation in which a liquid usually water is removed from a wet solid in equipment termed dryers. When a wet solid is subjected to thermal drying,two processes occur simultaneously:Transfer of energy (mostly as heat) from the surrounding environment to evaporate the surface moistureTransfer of internal moisture to the surface of the solid and its subsequent evaporation due to process 1

Schematic Diagram of Various Heat and Mass Transfer Modes and Moisture Transport Mechanisms in DryingEnergy input by:ConductionConvectionRadiationMW / RFCombination of the above concurrently or sequentiallyMoisture out by:Internal diffusion (liquid or vapor)CapillaritySurface diffusionKnudsen diffusionCombination of aboveDrying ParticleINTERNAL CONDITIONS: MOISTURE CONTENT EXTERNAL CONDITIONS: PSYCHROMETRICMECHANISM OF DRYINGCLASSIFICATION AND SELECTIONENERGY COSTS, SAFETY, AND ENVIRONMENTALDESIGN OF DRYERSDRYER SELECTIONDryingDrying Rate Heat and Mass Transfer RateExternal ControlInternal ControlEnergy SupplyConvectionConductionRadiationDielectricAffected byPressureTemperatureHumidityGas flowDryer ConfigurationGas flow patternResidence timeMass Transfer (Aroma/Shelf life*)Liquid / vapor diffusionEquilibrium MCEnergy Transfer (Chemical/Thermal damage*)Temperature distributionsThermal/chemical degradationMomentum Transfer (Delamination/Cracking/Puffing*)Single phaseTwo phaseMechanical Effect (Cracking*)DeformationStrengthStressesINTERNAL CONDITIONS: MOISTURE CONTENT

Nonhygroscopic capillary-porous media.Hygroscopic-porous media. Colloidal (nonporous) media.MOISTURE CONTENT: Wet basis

Dry basis

Water of hydration: integral part of the material Bound water: water which is in some way bound to material so that it exerts a vapour pressure less than that of pure waterFree water: Water which is bound by such minute forces, that its vapour pressure is equal to the vapour pressure of pure waterINTERNAL CONDITIONS :Types of Water in Material

Water Activity versus moisture content for different types of foodINTERNAL CONDITIONS :Various types of moisture content

INTERNAL CONDITIONS :INTERNAL CONDITIONS :

(1) asbestos fiber, (2) PVC (508C), (3) wood charcoal, (4) Kraft paper, (5) jute, (6) wheat, (7) potatoes.Moisture IsothermsINTERNAL CONDITIONS :SorptionDesorption HysteresisA typical isotherm

Typical sorption isotherms

INTERNAL CONDITIONS :INTERNAL CONDITIONS :Temperature Variations

INTERNAL CONDITIONS :

EXTERNAL CONDITIONS: Gas & Water Vapor

Vapor in Air:

The ClausiusClapeyron EquationReference Substance:EXTERNAL CONDITIONS: Gas & Water Vapor

Specific Humidity:

EXTERNAL CONDITIONS: Gas & Water Vapor

Dry Air & Water Vapor:

26

Relative Humidity:

EXTERNAL CONDITIONS: Gas & Water Vapor

Dry air & Water Vapor

Mole Fraction:

Dry Air & Water Vapor:

27

EXTERNAL CONDITIONS: Gas & Water Vapor

28EXTERNAL CONDITIONS: Gas & Water Vapor

Humid Volume: Dry Air & Water Vapor:

29EXTERNAL CONDITIONS: Gas & Water Vapor

Above a humidity of 0.05 kg/kgda:

Enthalpy30EXTERNAL CONDITIONS: Gas & Water Vapor

EnthalpyHumidity Charts

31EXTERNAL CONDITIONS: Gas & Water Vapor

Adiabatic Saturation:

Mass Balance:Energy Balance:

32EXTERNAL CONDITIONS: Gas & Water Vapor

Wet Bulb Temperature

33EXTERNAL CONDITIONS: Gas & Water Vapor

Wet Bulb Temperature

34EXTERNAL CONDITIONS: Gas & Water Vapor

35Pendahuluan, konsep dasar, ruang lingkup.

Mekanisme, klasifikasi & pemilihan pengering.

Fenomena Perpindahan (1).

Fenomena Perpindahan (2).

Fenomena Perpindahan (3).

Teknik Eksperimen dalam Pengering.

Perhitungan dalam Pengering.

Perancangan Sistem Pengering.

Pengering Heat PumpRuang LingkupPengering Fluidized Bed & Spouted Bed

Pengering Drum

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Pengering Vacum dan Pengering Freeze

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