CHBE 401 - Papermaking Drying
Pulp and Paper Centre, Department of Mechanical Engineering,
University of British Columbia
Introduction
Types of water in web
Free water
• Water held in pores, in between fibres, and in lumen
• Held in place by capillary forces
• 35-70% consistency is free water
Imbibed water
• Water held in the swollen cell walls
• Makes up the “fibre saturation point”
• Web consistencies between 70% and 90% are imbibed
Chemically bound water
• Water bound to cellulose. Zero vapour pressure
Fibre Bonding
Water removal induces strong
surface tension forces
Leads to
hydrogen bonding
Fibre straightening
Lumen collapse
Fibre bonding
As water is removed
strength increases due to
improved bonding.
Drying process
Heat transfer to web
Mass transfer of vapour from the web
Drying process
Stage 1 Warm up stage
Stage 2 Constant drying rate stage
Heat and mass transfer are not rate controlling steps
Stage 3 Falling rate stage
Not sufficient water to completely fill the web
Water in contact with roll evaporates and diffuses
outward … complex heat and mass trnasfer
Corresponds with removal of free to imbibed water
Drying process
Dryer configuration
Dryer configuration
Reverse section to reduce 2-sidedness of paper
Other types of dryers
Yankee dryer:
8mm diameter roll
jets of hot air blow on paper
used for tissue.
Other types of dryers
Flakt dryer
Hot air blown as paper passes through a number of tiers
Unconstrained drying
Lower temperature drying: inhibits darkening,
embrittlement of paper
Heat transfer
Heat transfer
Condensate shape
inside cylinder changes
significantly as a
function of amount and
cylinder speed
Some estimates of the
thickness have been
made
Rimming Speed, VR
Condensate Thickness, δ
( )31 1
2 2 2
0.0260.2RV R(1) 9.1
gg
− υ = δ δ ⋅ δ
12
020
V 2gR(2) 1 sinV V
− δ
= = − ϕ δ
V condensate velocityV0 cylinder velocityδ local condensate thicknessδ average condensate thickness
Heat transfer
Heat transfer
Correlations for mass transfer coefficient
Heat transfer calculations
Tappi TIS 0404-07
Assumes constant
drying rate
Gets drying rate form
experimental
measurements