JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year INSTRUMETATION PRACTICES IN INDUSTRIES Previous Question Paper (September - 2021 - R16)

Semi-chemical pulping is a hybrid process that combines mild chemical treatment with mechanical refining to produce pulp from wood chips. It is primarily used for manufacturing corrugated medium (fluting) for cardboard boxes, as it retains a high yield of fibers (typically 65-85%) while providing sufficient strength.

The process involves the following key stages:

1. Chip Preparation: Wood logs are debarked and chipped into uniform sizes to ensure consistent chemical penetration.
2. Chemical Impregnation: The chips are fed into a digester where they are treated with mild chemicals, usually a neutral sodium sulfite solution or a mixture of sodium sulfite and sodium carbonate. The temperature is maintained between 150°C and 180°C. This stage softens the lignin, which acts as the binding agent between fibers, without fully dissolving it.
3. Mechanical Defibration: Once softened, the chips are passed through mechanical refiners (often disc refiners). The mechanical action separates the softened fibers from each other. Because the lignin is only partially removed, the fibers remain stiffer than those produced by fully chemical processes, which is ideal for the structural rigidity required in packaging materials.
4. Washing and Screening: The resulting pulp is washed to remove residual chemicals and dissolved organic matter, then screened to remove uncooked chips or large fiber bundles.

Diagrammatic representation of the process flow:

The primary advantage of this process is the high yield compared to chemical pulping (like the Kraft process), as a significant portion of the hemicellulose and lignin remains in the pulp, resulting in lower raw material costs and reduced chemical consumption.

The Fourdrinier machine is the primary industrial system used to convert a dilute pulp suspension into a continuous web of paper. The process involves several distinct stages that transform the slurry into a finished product.

1. Wet End (Formation): The process begins at the headbox, where a dilute pulp slurry (approximately 99% water) is discharged onto a moving, endless wire mesh screen. As the wire moves, water drains through the mesh by gravity and suction boxes, causing fibers to interlock and form a wet mat known as the sheet.

2. Press Section: The wet sheet is transferred from the wire to a felt conveyor. It passes through a series of heavy rollers that squeeze out additional water and consolidate the fiber web, increasing its density and strength.

3. Dryer Section: The sheet enters a series of steam-heated cylinders. As the paper passes over these cylinders, the remaining moisture is evaporated. The temperature and speed are carefully controlled to ensure uniform drying and prevent paper breakage.

4. Calendering and Winding: The dried paper passes through a stack of heavy steel rollers called a calender, which compresses the paper to achieve the desired thickness and surface smoothness. Finally, the finished paper is wound onto large reels.

Diagram Description: The process flow is linear: The Headbox feeds the slurry onto the Fourdrinier Wire (forming table). Below the wire, Suction Boxes and Foils facilitate drainage. The sheet then moves into the Press Section (nip rollers), followed by the Dryer Section (multiple steam cylinders), and finally the Calender Stack and Reel. The entire machine is synchronized to maintain constant tension and speed, ensuring a continuous manufacturing cycle.