Jawaharlal Nehru Technological University Hyderabad B.Tech IV INSTRUMENTATION PRACTICES IN INDUSTRIES Previous Question Paper (October/November - 2020 - R16)

Thermo-Mechanical Pulping (TMP) is a process in the pulp and paper industry where wood chips are converted into pulp through the combined application of heat and mechanical energy. Unlike chemical pulping, which uses chemicals to dissolve lignin, TMP relies on mechanical refining to separate fibers, resulting in high yield and specific paper properties.

The process follows these key stages:

1. Chip Preparation: Wood logs are debarked and chipped into uniform sizes. These chips are washed to remove grit, sand, and other impurities that could damage the refiner plates.
2. Pre-steaming: The chips are fed into a pre-steaming vessel where they are treated with saturated steam at temperatures typically between 110°C and 130°C. This softens the lignin, which acts as the natural glue binding the wood fibers together, making the fibers easier to separate without excessive breakage.
3. Primary Refining: The softened chips are fed into a disc refiner, which consists of two rotating metal discs with patterned plates. The mechanical shear forces generated between these discs tear the chips apart into individual fibers. This stage is often performed under pressurized conditions to maintain the heat required for fiber separation.
4. Secondary Refining: The pulp may pass through a second stage of refining to further develop fiber properties, such as increasing the surface area for bonding (fines generation) and ensuring a uniform fiber length distribution.
5. Screening and Cleaning: The resulting pulp is diluted and passed through screens and centrifugal cleaners to remove shives (unrefined wood particles), knots, and any remaining contaminants.
6. Bleaching (Optional): Depending on the end-use requirements, the pulp may undergo a mild bleaching process, usually using hydrogen peroxide, to increase brightness without significantly reducing the yield.

The primary advantage of TMP is its high yield, often exceeding 90-95%, as most of the wood components are retained in the pulp. However, because the lignin is not removed, the resulting paper is prone to yellowing over time when exposed to light, making it most suitable for newsprint, catalogs, and directory paper.

Wet processing in the paper industry, often referred to as the stock preparation and wet end process, is the sequence of operations where wood pulp is converted into a fiber suspension and formed into a continuous sheet. The process begins with the pulper, where dry pulp sheets are mixed with water to create a slurry. This slurry is then passed through refiners, which mechanically treat the fibers to increase their surface area and bonding potential, a process known as beating.

Following refining, the stock is cleaned in centrifugal cleaners to remove heavy impurities and screened to remove oversized particles. The cleaned fiber suspension is then diluted in the machine chest and stuff box to achieve the precise consistency required for sheet formation. Chemical additives such as sizing agents, fillers, and dyes are introduced at this stage to impart specific properties to the final paper.

The prepared stock is delivered to the headbox, which is the critical component of the wet end. The headbox ensures a uniform distribution of fibers across the width of the machine. The slurry is discharged from the headbox onto a moving wire mesh (forming fabric). As the wire moves, water drains through the mesh by gravity and vacuum suction, causing the fibers to interlock and form a wet web. This process is supported by foil blades and suction boxes that control the rate of dewatering. The wet web, now containing approximately 80% water, is then transferred to the press section for further mechanical water removal before entering the dryer section.

Diagram description: The process flow starts from the Pulper, feeding into the Refiner, followed by the Cleaner and Screen units. The flow continues to the Headbox, which deposits the slurry onto the Fourdrinier wire table equipped with foils and suction boxes, culminating at the Couch Roll where the wet sheet is transferred to the press section.