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Separation by Drying

This document discusses drying of solids and provides information on various dryer equipment. It describes drying as involving two simultaneous processes: heat transfer to evaporate liquid and mass transfer of liquid and vapor within the solid and from its surface. Factors governing these heat and mass transfer rates determine the drying rate. Key considerations for selecting suitable dryer equipment include properties of the material to be dried, its drying characteristics, required flow quantities and conditions, product quality, and potential dust/solvent recovery. Equations are provided for calculating drying rates and residence times in different dryer types based on parameters like temperature, thickness of solid bed, and drying rate constant. Design procedures involve selecting initial assumptions based on reference tables and iteratively adjusting drying medium

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Arfel Marie Fuentes
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172 views30 pages

Separation by Drying

This document discusses drying of solids and provides information on various dryer equipment. It describes drying as involving two simultaneous processes: heat transfer to evaporate liquid and mass transfer of liquid and vapor within the solid and from its surface. Factors governing these heat and mass transfer rates determine the drying rate. Key considerations for selecting suitable dryer equipment include properties of the material to be dried, its drying characteristics, required flow quantities and conditions, product quality, and potential dust/solvent recovery. Equations are provided for calculating drying rates and residence times in different dryer types based on parameters like temperature, thickness of solid bed, and drying rate constant. Design procedures involve selecting initial assumptions based on reference tables and iteratively adjusting drying medium

Uploaded by

Arfel Marie Fuentes
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Separation

by drying
CHE 61 A
HRCascon, PhD
Drying of solids
Encompasses two fundamental and simultaneous processes
involving:

(2) transfer of liquid and vapor


(1) the transfer of heat to
within the solid and vapor from
evaporate the liquid and
the surface

Factors governing the rates of (1) and (2) govern the drying rate
Heat transfer methods utilized in
dryers: convection, conduction,
radiation or a combination of
these

Drying of Mass transfer occur within the


solid because of concentration
solids gradient

Drying process evaluation is


generally based on the effects of
external conditions of T, P,
humidity, gas flow rate, state of
solid subdivision, etc
 Keyfactors to
consider:
1. Properties of material
Equipment to be dried
selection 2. Drying characteristics
of the material
for drying 3. Flow quantities and
conditions
solids 4. Product quality
5. Possible dust and
solvent recovery
(continuation)
Visual
familiarization
of some dryer
equipment
Tunnel dryer
Rotary
dryer
Vacuum dryer
Tower contactor dryer

wet
Drum dryer

The drums are heated to


dry the coating material
prior to scraping off
Screw conveyor dryer
Fluid bed dryer
Spouted bed dryer
Pneumatic conveyor dryer
Spray dryer
Liquid feed
Consult Table 15-15 to decide on the best dryer type to use

Feed input/output parameters are usually set as process requirements


while parameters of the drying medium are to be assumed

General
Begin assumptive calculations based on Table 15-16, using theory-based
design design equations

procedure Final design decisions are usually based on dimensions or dimensional


ratio normally encountered in actual applications

for dryers
Calculations may be iterative, requiring adjustments in the drying
medium parameters, to arrive on recommended dimensional ratio(s)

Change of dryer type maybe required if design parameter


recommendation(s) is/are difficult to obtain
General design procedures
Drying calculations

𝑑𝑋 𝑘𝑑′ 𝑋(𝑇𝐺 − 𝑇𝑤𝑏 )


=
𝑑𝜃𝑟 𝑡𝑠 For tunnel, tower contactors, drum dryers…
X = weight ratio of liquid to dry solid
TG = dry-bulb temp of gas
ts = thickness of solid bed
𝑋𝑜 Kd’ = drying rate constant
𝑡𝑠 ln( )
𝑋𝑖
𝜃𝑟 =
𝑘𝑑′ (𝑇𝐺 − 𝑇𝑤𝑏 )
Drying calculations
 For dryers that provide agitated beds or intimate gas-solid
contact, approximate residence times can be evaluated by
energy balance on the solids stream

𝑞 = 𝑈𝐴∆𝑇𝑙𝑜𝑔𝑚𝑒𝑎𝑛

𝑞 = 𝑈′𝑣𝑑 𝐴∆𝑇𝑙𝑜𝑔𝑚𝑒𝑎𝑛

• Where U’ is the overall HT coefficient based on dryer volume


• ∆T log mean is based on the differences between dry bulb gas
temp and wet bulb temperatures at the inlet an exit of the
dryer. For drying of hygroscopic materials, the wet bulb temp
should be replaced by the saturation temp
Drying calculations

 Once the area or volume of the dryer has been


stablished, the residence time can be evaluated
from:
𝑣𝑑 𝑓𝑠 𝜌𝑠
𝜃𝑟 =
𝑚ሶ 𝑠
Where
• vd is the dryer volume
• fs = fraction of the dryer occupied by the solid
Given: you should draw the schematic diagram
of the solid to the countercurrent airflow, it can
heat transfer rather than by mass transfer. The
approximated from Table 15-16 to be
the internal surface area of the dryer. Thus,

1 - 0.125

Log mean

3.0 kg/s·m2 (midway


Then from
dry air
and outlet
respec-

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