JNTU B.Tech IV INSTRUMENTATION PRACTICES IN INDUSTRIES Previous Question Paper (February/March - 2022 - R16)

The Fourdrinier machine is the most widely used paper-making machine. It consists of a headbox that distributes a dilute pulp slurry onto a continuous, moving wire mesh belt. As the wire moves, water drains through the mesh, forming a wet paper web. The web then passes through a series of press rolls to remove more water, followed by steam-heated drying cylinders to reach the final moisture content. Finally, the paper is calendered and wound onto a reel.

The demerits of the Fourdrinier machine include:

• High capital investment and large space requirement.
• Significant energy consumption for water removal and drying.
• Two-sidedness of the paper, where the wire side and felt side have different textures.
• High maintenance costs due to the complexity of the wire and press sections.
• Limited speed for extremely heavy paper grades.

The sulfite pulping process is a chemical pulping method that uses salts of sulfurous acid to dissolve lignin from wood chips. The process involves cooking wood chips in a pressure vessel called a digester with a cooking liquor composed of bisulfite and sulfurous acid at elevated temperatures and pressures.

The working steps are:

1. Chip Preparation: Wood logs are debarked and chipped into uniform sizes.
2. Cooking: Chips are loaded into the digester, and the bisulfite cooking liquor is added. The mixture is heated to approximately 130-160°C.
3. Delignification: The chemicals react with lignin, making it soluble in the cooking liquor, thereby liberating the cellulose fibers.
4. Washing and Screening: The cooked pulp is discharged, washed to remove the spent liquor (black liquor), and screened to remove uncooked knots or shives.
5. Bleaching: The resulting pulp is often light in color and may undergo bleaching if high brightness is required.

The primary advantage of this process is the production of high-strength, bright, and easily bleachable pulp, though it is increasingly replaced by the Kraft process due to chemical recovery complexities.