Over the years, great attention has been paid to the environment and to the rationalization of water consumption in tissue paper production. In the diagram in the page 80, we want to show the quantities of water associated with a unit quantity of fiber for each phase of the production process. The fiber, with a fixed quantity throughout the process, is represented by the small orange rectangles at the bottom, while the rectangles in two different shades of blue identify the quantities of water associated with each single phase. Specifically, the water deriving from the previous phase is represented in light blue, while in dark blue we have the variation in the amount of water that occurs in the phase itself.
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The use of water in the tissue plant begins in the stock preparation phase, where the proportion between fiber and water, called consistency, is about 5% (20 parts of water are required for one part of cellulose). After pulping, the stock is transferred to temporary storage tanks (dump chests) which have the task of “amortizing” the discontinuities of the plant. Downstream of the dump chests, the stock goes through a cleaning phase by HD Cleaners. For the correct functioning of this process, the stock must be further diluted to reach a consistency of about 4%, which means adding another 4 parts of water for each part of processed fiber. At this point the stock is ready for refining, after which it is transferred to the machine chest. Upstream, the stock can be mixed via a static mixer, with fractions deriving from other processes (second preparation line or broke line). From the machine chest, the stock is sucked in by the stock pump and injected into the inlet of the fan pump.
The stock ends up in a large conduit through which the fan pump sucks in a large quantity of white water from the so-called silo, where the waste waters coming from the forming section of the paper machine are collected. In the channel and in the silo, white waters release the air they have trapped during drainage, then they are sucked back by the fan pump, mixed with the fraction of fresh stock and sent back to the machine. This process, repeated several times every hour, forms the white waters short circuit, where white waters transport the fibers to the paper machine at a consistency of around 0.3%. At this point the fan pump pressurizes the stock, an operation necessary to ensure it comes out of the headbox at a speed equal to that of the machine (which can reach over 2000 mpm).
The transfer and pressurization of these large masses of stock in this phase represent one of the most energy-intensive processes of the entire plant, where small variations in the required consistency can lead to significant variations in the energy consumption of the entire plant. Using solutions such as latest generation headboxes and Crescent Formers capable of producing high quality sheets of paper by operating at high consistencies can result in significant energy savings for the entire plant. The excess of white water in the silo is collected in the white water chest, from which the flows necessary for the functions external to the short circuit are withdrawn. A part of the white water is instead withdrawn from the chest to be clarified in a flotation unit and collected downstream of this in a clarified water chest. In turn, part of the clarified water is withdrawn and filtered to obtain substantially pure water for more delicate uses, such as felts and wires conditioning. Another part of water, coming from the cooling systems of the hydraulic and lubrication units, is instead conveniently used for the dilution of the chemical additives necessary for the process and for the lubrication of the suction press rolls seals. Coming back to the paper machine, the newly formed sheet still contains about 15% of “dry weight”. In the next machine section another fraction of water is eliminated by means of the action exerted by the vacuum in the suction roll and by forced contact pressing (nip) between a press roll and a Yankee Dryer. After this, the proportion between parts of water and parts of fiber is almost equal. The vacuum through the felt conditioning suction boxes is also necessary to recover the water contained in the felt of the press downstream of the nip.
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The energy associated with the vacuum generation plant is an important fraction of the factory’s total consumption. Having high-efficiency machines for vacuum generation, such as turbine machines (turbo blower vacuum pumps), can lead to considerable and continuous savings on the energy consumption. At this point, the amount of water remaining in the sheet will be eliminated through physical processes (evaporation) by means of the Yankee Dryer and the active hood that surrounds it, arriving at a water content of only 5-6%. The evaporation process involves a considerable waste of thermal energy, which is supplied by means of steam and hot air generation systems. Also in this case, small percentage variations in dryness after the press section can correspond to large variations in energy consumption.
Wastewater treatment
As we have seen, most of the water is recycled continuously. Since the water is progressively enriched with chemicals and electrolytes that tend to “poison it”, a portion of water must be renewed at each cycle to keep the characteristics of the total within the acceptable limits. Otherwise, it is necessary to withdraw a certain amount of fresh water from the environment and give back an approximately equivalent amount in the form of waste water. The progressive improvement of Waste Water Treatment plants (MBBR biological treatment, micro and ultrafiltration) has made it possible to increasingly safeguard the environment and to reuse ever larger fractions of water within the plant, “closing” the water cycle more and more. In some plants, more than 90% of closure has already been reached, which in practice means zeroing the discharges and withdrawing from the environment only the amount of water evaporated during the drying process.
However, closing the cycle is not free, neither from an energy nor from an investment point of view, and therefore must be carefully evaluated case by case in terms of cost-benefit ratio. Finally, the following diagrams show, in a simplified way, the hydraulic balances of plants with respectively “open” and “closed” water cycles through a Waste Water Treatment plant.
Fig. 1: “Open” water cycle
Fig. 2: “Closed” water cycle
