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        Last update : August 05, 2003
 
                  Second Session : The yield issue                  
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  Logo EFPG     V - Reasons for yield losses during kraft cooking.
Possible strategies for yield increase

D. Lachenal (EFPG)
 

A - Origin of yield loss (slides 1-10)

 
1 - Chemical composition of plant materials
Plant Softwood Hardwood Monocotyledons
Cellulose 43% 45% 42%
Hemicellulose 28% 34% 40%
Glucomannans 16 5 0
Xylans 9 25 33
Lignin 29% 21% 17%
   
2 - Chemical pulping
Chemical pulping consists in removing most of the lignin in wood or annual plant, in order to get individual fibers of high flexibility and in most cases ready to be fully bleached.
Theoretical yield should be 70 to 80% if lignin removal were selective.
Yield figures are 45-48% for softwood and 47-50% for hardwood in the case of bleachable grades.
WHY is that ??
 
 
3 - Origin of yield loss
The origin of yield loss is carbohydrate (hemicellulose and cellulose) degradation during kraft cooking.
Two mechanisms affect carbohydrates during a kraft cook:
-- peeling, which starts at low temperature and goes very fast already at 100C
-- alkaline hydrolysis, which starts around 140C
   
4 - Cellulose molecule
Partial chemical structure of O-acetyl-galactoglucomannan from softwood
 
 
5 - Peeling mechanism (1)
 
 
6 - Peeling mechanism (2)
 
 
7 - Peeling mechanism (3)
 
 
8 - Peeling mechanism (4)
 
 
9 - Peeling mechanism (5)
 
 
10 - Peeling mechanism (6)
 
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