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WEEK 2 spray Drying and Dehydration Lecture 2_2021.pdf -...

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Spray drying and dryingof proteinsSpray drying systemsDrying of proteins (lactoferrin asexample)
SPRAY DRYING SYSTEMS
Spray Dryingliquid (solution/suspension) drying methodliquid phase- pumpable- containing solublesolids, and/or insoluble insuspensions/slurryAtomisation into droplets- large totalsurface area of droplets (increased rate ofheat and mass transfer)nearinstantaneous evaporation
Spherical droplet surface versus droplet sizeTotalVolume(m3)DiameterofdropletsNo. ofdropletsSurfacearea perdropletTotal surfaceof droplets(m2)Surface arearatio0.5241 m13.14 m23.1410.5241 cm11063.14 cm2314.161000.5241 mm11093.14 mm23141.5910000.524100m110120.0314mm231415.93100000.5241m110183.14m23141592.651000000
Advantagesof spray dryingVery short drying time (maximum moisture evaporationwithin a second)Low particle temperature due to evaporativecoolingEffective control of product properties and quality (almostuniform spherical particles)Drying of heat sensitive foods, biological products, andpharmaceuticals at atmospheric pressure and relatively lowtemperaturesInert (N2)atmosphere can also be employed and non-watersolvent can be usedHigh tonnage production in continuous operationVery high sanitation (closed cycle and aseptic processing possible)
DisadvantagesFails to obtain high bulk density productsNot flexible (in general)High initial investment costProduct recovery and dust collection increase thecost of dryingRelatively low energy efficiency compared toother water removal processes
Heat consumption (kJ/kg) for water evaporation(Filkova and Mujumdar, 1995)Membrane processes140Evaporator 1 stage2600Evaporator 2 stages1300Evaporator 6 stages430Evaporator 6 withthermocompression370Evaporator 6, with mechanicalcompression220Spray drying processUp to 6000
Stages of spray drying process1.Atomisation2.Spray-air mixing3.Moisture evaporation(drying)4.Powder recovery
Atomisationmost important operationinfluences:energy requirement to form the spray (type ofatomiser)size and size distribution of the atomised dropletstrajectory and speed of dropletschamber design
Atomiser TypesRotary wheel (centrifugal)- mostcommonPressure nozzlePneumatic or twin fluid nozzle(Sonic..)Atomised Drop Size and Size Distribution:Single most important factor affecting quality of dry powder
Droplet size and atomisers(low viscosity, Newtonian fluids- Masters, 1991)Rotary atomiser(m/s)Droplet size (m)75-125 (m/s)150-275125-150 (”)75-150150-180 (”)30-75Pressure Nozzle15-25 (atm)150-350 (m)25-50(”)50-150>100(”)15-30Twin fluid nozzle1.5:1 (air:liquid) ratio50-2002.5:1 (”)30-505:1(”)5-20
Wheel AtomiserRotating wheeluses centrifugal energy for atomisation-disintegrates the solution sheet at the wheel edge intodropletsSpray angle180obroad cloud, horizontal trajectoryrequires large diameter chambersAngular speed 10000 to 50000 rpm (linear peripheral speed100-200m/s)Radial or curved vanesCurved vane wheels used in dairy industries to produce highbulk density powder (about 15% higher)- removal of air

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Term
Fall
Professor
NoProfessor
Tags
Thermodynamics, Spray drying

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