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