Processes 2021 , 9 , 1707
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2.2. Description of the Papermaking Process Pulp and paper can be separated into two clearly differentiated and interconnected subsectors: pulp production and papermaking, which can be in a single location or separate. The first, pulp production, uses mainly wood as a raw material or recycled paper to transform into pulp. The pulp or furnish is screened to remove impurities. The papermaking plant can also be divided into four main phases: the sheet section, press section, drying section, and finishing section. As seen in Figure 1, the pulp is converted into paper. In the first section (forming section), a sheet is formed by feeding the pulp in a fourdrinier machine, with a moving belt of fine mesh screening to remove water initially by gravity and later by applying a progressive vacuum, reaching a fiber content of 25%. Subsequently, progressive mechanical pressing begins (press section), eliminating the water until a dryness of approximately 52% is reached. A final dryness of 6% is reached in the drying section, which is composed of a series of steam-heated drying cylinders, and the water is removed by thermal means. The process can have a section of smoothing and/or calendering; later, the paper is cut to the measure requested by the client and packed for later issue.
Figure1. Process flow.
This study is focused on the drying section, in which the water remaining in the incoming paper sheet is eliminated by thermal means; the initial moisture coming into this section is close to 52%, which the drying process reduces to approximately 6%, depending on the type of paper and the customer’s requirements. The drying section of the studied facility has 38 drying cylinders internally heated by steam, which is supplied by only one steam generator. The steam energy is transferred to the sheet when the paper contacts the drying cylinders. The energy transferred to the sheet causes water evaporation and consequently dries the sheet. A ventilation hood system is used to remove this evaporated water from the sheet; this system controls the main variables associated with air circulation inside the hood. The air extracted from the hood passes through a heat recovery system to heat the air blown into the hood (which compensates for the air previously extracted, maintaining a slight reduction in air). This system ensures energy recovery and directly
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