Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year INSTRUMETATION PRACTICES IN INDUSTRIES Previous Question Paper (March - 2021 - R16)

Kraft pulping, also known as the sulfate process, is the dominant chemical pulping method used to convert wood chips into wood pulp. The process involves treating wood chips with a cooking liquor called white liquor, which consists of sodium hydroxide (NaOH) and sodium sulfide (Na₂S).

The process steps are as follows:

1. Wood chips are fed into a large pressure vessel called a digester.
2. White liquor is added, and the mixture is heated to temperatures between 160°C and 180°C under pressure.
3. The chemicals dissolve the lignin that binds the cellulose fibers together, effectively separating the fibers.
4. Once the reaction is complete, the pulp is discharged from the digester.
5. The spent cooking liquor, now called black liquor, is separated from the pulp through washing stages.

Diagram description: The process flow begins with wood chips entering a digester, where white liquor is introduced. The digester outputs a mixture of pulp and black liquor. The pulp is sent to a washing system to remove residual chemicals, while the black liquor is sent to a recovery boiler to regenerate the cooking chemicals.

Thermo-mechanical pulping (TMP) is a mechanical pulping process where wood chips are softened by steam heating before being mechanically refined. Unlike chemical pulping, TMP relies primarily on mechanical energy to separate fibers.

The process involves:

1. Pre-steaming: Wood chips are washed and then pre-heated with steam at elevated temperatures and pressures to soften the lignin.
2. Refining: The softened chips are fed into a refiner, which consists of two rotating metal discs with patterned surfaces. The mechanical shear forces between the discs break the chips apart into individual fibers.
3. Screening and Cleaning: The resulting pulp is screened to remove shives (fiber bundles) and cleaned to remove impurities.

Because the lignin is not removed but only softened, TMP results in a higher yield compared to chemical pulping, though the fibers are shorter and weaker due to the mechanical damage sustained during refining.