Sulphate Pulping Flashcards
What chemicals used in kraft process?
Sodium hydroxide and sodium sulfide at pH above 12
What is H-Factor
This is a term that combines both cooking temperature and time into a single variable. Can be done because when you increase temperature you decrease time, we assume delignification is one single reaction
What are the advantages and disadvantages of kraft cooking process
+highes strength pulp
+efficient reocvery of chemicals
+ can handle variety of wood species with high resin content and bark is tolerable (sulphite cannot)
+relatively short cooking time
- smell
- low unbleached brightness and difficulty brightening
-high investment costs
Draw the kraft liquor cycle
1.Cooking and washing (in chips. Out pulp)
black liquor line
2. Evaporation and burning (increase solids content)
Green liqour line
3. Causticising (addition of CaO)
white liquor line
What is black liquor?
Waste liquor from cooking process. Contains lignin, sulphonation products etc
Every 1 ton pulp you get 7 tons black liquor (15% solids)
What is white liquor
these are your chemicals you need to cook. such as NaOH and Na2S
Why can you use woods with a high resin content in the kraft process but not in the sulphite process
because in alkaline conditions carboxylic groups of the resins will dissociate and release a proton, this leaves a negativelly charged COO- sodium can then combine with this and will make it possible for the resin to be go into solution and be removed
Whereas in the sulphite process we have acidic solutions with a high concentration of H+ ions, the carboxylic acid will not dissociate and the resins will not go into solution. This is saponification reaction
Draw the kraft liquor cycle!!
1.Cooking and washing (in chips. Out pulp)
black liquor line
2. Evaporation and burning (increase solids content)
Green liqour line
3. Causticising (addition of CaO)
white liquor line
define total alkali
All sodium alkali compounds
define sulfidity
the ratio of Na2S to NaOH + Na2S
This tells us the proportion of sodium sulfide in the total alkali present
define total titratable alkali
NaOH + Na2S + Na2CO3
define active alkali
This is the sum of all active cooking chemicals
NaOH + Na2S
why is the kraft process also called the sulphate process?
because the addition of sodium sulphate (Na2SO4) in chemical recovery plant
What are the important lignin reactions in kraft pulping
cleavage of ether bonds in phenolic units
Condensation reactions which form carbon to carbon bonds
Explain the carbohydrate reactions in kraft process
Peeling reactions occur. This is progressive removal of sugar units from the reducing ends of polysaccharide chains.. Hydrolysis reactions can cleave glycosidic bonds in the middle of chains and form new reducing ends where peeling reactions can occur.
This can be stopped when the reducing end is converted into a stable carboxylic acid group
How has the kraft process improved
Includes a pre hydrolysis step (this obtains pure alpha cellulose)
Addition of catalysts (help oxidise end groups in polysaccharides helping with peeling)
Improved emissions
Possibility of elemental chlorine free bleaching
what is the purpose of pre hydrolysis
This helps to remove hemicellulose content, Hemi is much more susceptable to hydrolysis reactions than cellulose because of amporphous structure. helps us obtain a very pure cellulose pulp
Can you give typical compositon of kraft pulp and that of black liquor
Pulp: 5% lignin, 10% hemicellulose 70% cellulose
Black liquor: lignin 35% organic acids 50%, extractives 5%
Can you tell me some by products and their commercial use of the kraft process
hexoses and pentoses can be distilled into ethanol chemicals which can be used in chemical and fuel industries.
Can form turpentine solvents
Tall oil from lignin resins and oils
Recovery of kraft cooking chemicals
combustion of organic substances
Causticising using CaO
Reduction with sodium sulphate Na2SO4
How does Na2S (sodium sulfide) react with lignin under kraft pulping conditions?
Sodium sulfide helps to cleave ether bonds in lignin, this produces smaller lignin fragments, it also reduces phenolic units. these fragments are soluble in alkaline solution and can therefore be removed.
