Pulping Process Flow Flashcards
What process steps occur during batch pulp cooking ( fibrous material)?
Pre steaming this is to eliminate air from the pores and open dried out areas
Impregnation: water and air are displaced, the whole surface is wetted, hemicellulose starts to swell
Cooking: lignin cleavage, hemicellulose degradation, partial cellulose degradation
Washing: removal of black liquor substitution of liquor to water in the pores.
Blowing: loosening of fibre structure by partial water evaporation
What are the main types of digesters?
Batch digestor:
Continuous digestor
- What are the process steps in batch cooking?
Screened chips charged into digestor
White liquor and black liquor are mixed in to reach the target effective alkali percentage. The digestor is then capped.
Heat exchanger brings contents to pulping temp, cooked and then blown.
- What are the process steps in continuous cooking?
Wood chips fed into the steaming vessel where a screw carries them. They are then flashed with steam which preheats the chips and removes air. They fall into the feeder where they mix with cooking liquid and are carried to the digestor. The first zone allows for impregnation, then a heating zone, the third zone is at maximum temp and the chips are retained. The reaction is then stopped by displacement of hot liquor with wash liquor, then the pulp is washed and then continuously removed.
What are the limitations of saw dust pulping?
Not possible to do chemical pulping of pure sawdust because you cannot get the lquor circulation.
Since sawdust has larger surface area it is much quicker to absorb cooking chemicals, leaving less for the rest of the pulp, additionally they need less cooking time.
What are the advantages of batch cooking?
Production relability
Operating flexibility: can change grades, ease of start up and shut down.
Less maintenance needed
What are the advantages of continuous cooking?
Lower steam/energy requirement.
More compact
More constant steam demand
Name the processing steps after cooking
Screening
Thickening
Storage
Defibering and deknotting
Centrifugal cleaning
Which factors affect the brown washing displacement efficiency?
The fibre characteristics. This includes the pulping process, stock hardness and freeness as well as the wood species
The shower characteristics, temperature and distribution.
Operating factors: amount of air in the stock. Stock temperature. Fabric characteristics.
Wash efficiency is a function of dilution factor and retention time.
Explain the diffusion washing principle!
Wash liquor is in the bottom of the tank, this wash liquor displaces the hot cooking liquor. Objective is to remove residual liquor that would contaminate further processing steps and recover the maximum amount of chemicals. Ideally no mixing should take place here.
Why is black liquor oxidized and what happens in that reaction?
Oxidised as an odour reduction step.
Black liquor oxidation converts sulfide into thiosulfste
What are the functions of the recovery boiler?
Evaporate residual moisture from liquor solids
Burn organic constituents.
Supply heat for steam generation
Reduces oxidised sulfur to sulfide
What happens in the re-causticizing process?
Function is to convert sodium carbonate into active sodium hydroxide and remove impurities.
Furnace smelt is dissolved in weak liquor to form green liquor which is then clarified and reacted with lime (CaO) to form white liquor. Which can then be reused.
Recaustiscising happens in 2 steps, first the lime reacts with water to form calcium hydroxyde, which then reacts with sodium carbonate to form sodium hydroxide.
