Pulp & Paper - Paper Mill
Stock Preparation: Stock Blending
Grade change time
A typical grade of paper will include several pulp furnishes; “virgin” hardwood and softwood for strength; groundwood to lower the cost, broke from the paper machine, and multiple additives to give the paper the right physical properties, like opacity, smoothness, sizing, color, brightness, etc. Some additives that can aid in bonding by changing the charge of the fibers are also added – often times closer to the head box. The flow rate of each of these flows must be very carefully measured to provide the right ratio of each fiber and additive. The demand signal from the tank level is ratioed to each flow controller. Control is either a “wet basis”, or volumetric, or a “dry basis”, which is preferred by many. That requires a good consistency measurement, or at the very least good consistency control coming out of the high density towers. The consistency of pulp is “lbs fiber / lb fiber+water”, and combined with a flow measurement of fiber+water, you can get the actual fiber flow. This is dry basis blending and is the best, as the additives can be ratioed to the actual lbs of fiber.
Each flow uses a mag meter; some additives that are very expensive, like TiO2 (Titanium Dioxide) have used a Micro Motion meter to be able to reduce their additive usage with more accurate metering. These furnish (pulp) flows are around 3-3.5%, and the standard mag should be fine. Tank levels are important here, and there are typically multiple flush-mount pressure transmitters on the stock lines.
Once the furnish and additives are blended, they are sent to a last refiner and then to the machine chest that feeds a particular paper machine (the blending area can feed 2-3 machines). This level measurement is critical to be sure the paper machine has enough feed.