CHEE 440 1

DRYINGDRYING

we are primarily concerned with drying wet porous solids (granules)

important in ensuring proper moisture content low enough to prevent product degradation and ensure

good powder flow, but high enough to provide good tablet properties

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General processheated air

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drying is a result of simultaneous heat and mass transfer

mass is transferred as a result of diffusion capillary flow

drying rate is determined by T, gas flow rate, humidity of gas

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definitionsdefinitions psychrometry

determination of the vapor concentration and carrying capacity of drying gas

wet bulb T

dry bulb T

relative humidity (RH)

moisture content

%RH=Pwater in air

Pw,sat T( )×100

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Psychrometric ChartPsychrometric Chart

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ExampleExample

using psychrometric chart, determine the relative humidity and moisture content in the air if the wet bulb T is 35 C and the dry bulb T is 50 C.

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Types of DryersTypes of Dryers

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Tray/Truck DryersTray/Truck Dryers

Static - Bed common

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Drying Time profileDrying Time profile

drying behavior for static bed

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Fluidized BedFluidized Bed

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Spray DryerSpray Dryer

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Drying EquationsDrying Equations

Tray/Truck Dryers constant rate drying stage only

parallel flow over a static bed

perpendicular flow through a bed

dmdt

=hcAΔTλ

hc =8.8G0.8

D0.2

Nu=1.17Re0.585Pr0.333

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Drying EquationsDrying Equations

Fluidized beds and spray driers use correlation for flow past an individual spherical

particle

Nu=2.0+0.61Re0.5Pr0.333

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ExampleExample

Wet granules (density = 1.5 g/cm3) are spread onto a screen at an amount of 10 kg/m2 of screen. The bed porosity is 45% and the wet granules have an average diameter of 300 m. Air at 60C(dry bulb) and a wet bulb temp of 25 C is passed through the bed at a velocity of 0.15 m/s. How long will it take to dry the granules from 20 to 10 % moisture? The critical moisture content of the granules is 9 %.