Sulphate Pulping

Question 1

What chemicals used in kraft process?

Answer 1

Sodium hydroxide and sodium sulfide at pH above 12

Question 2

What is H-Factor

Answer 2

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

Question 3

What are the advantages and disadvantages of kraft cooking process

Answer 3

+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

Question 4

Draw the kraft liquor cycle

Answer 4

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

Question 5

What is black liquor?

Answer 5

Waste liquor from cooking process. Contains lignin, sulphonation products etc
Every 1 ton pulp you get 7 tons black liquor (15% solids)

Question 6

What is white liquor

Answer 6

these are your chemicals you need to cook. such as NaOH and Na2S

Question 7

Why can you use woods with a high resin content in the kraft process but not in the sulphite process

Answer 7

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

Question 8

Draw the kraft liquor cycle!!

Answer 8

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

Question 9

define total alkali

Answer 9

All sodium alkali compounds

Question 10

define sulfidity

Answer 10

the ratio of Na2S to NaOH + Na2S
This tells us the proportion of sodium sulfide in the total alkali present

Question 11

define total titratable alkali

Answer 11

NaOH + Na2S + Na2CO3

Question 12

define active alkali

Answer 12

This is the sum of all active cooking chemicals
NaOH + Na2S

Question 13

why is the kraft process also called the sulphate process?

Answer 13

because the addition of sodium sulphate (Na2SO4) in chemical recovery plant

Question 14

What are the important lignin reactions in kraft pulping

Answer 14

cleavage of ether bonds in phenolic units
Condensation reactions which form carbon to carbon bonds

Question 15

Explain the carbohydrate reactions in kraft process

Answer 15

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

Question 16

How has the kraft process improved

Answer 16

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

Question 17

what is the purpose of pre hydrolysis

Answer 17

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

Question 18

Can you give typical compositon of kraft pulp and that of black liquor

Answer 18

Pulp: 5% lignin, 10% hemicellulose 70% cellulose
Black liquor: lignin 35% organic acids 50%, extractives 5%

Question 19

Can you tell me some by products and their commercial use of the kraft process

Answer 19

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

Question 20

Recovery of kraft cooking chemicals

Answer 20

combustion of organic substances
Causticising using CaO
Reduction with sodium sulphate Na2SO4

Question 21

How does Na2S (sodium sulfide) react with lignin under kraft pulping conditions?

Answer 21

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.