Characterization of the fiber–water separation process through a suction box of a single-wire pilot paper machine

During paper forming on a single-wire paper machine, i.e. a Fourdrinier or a hybrid paper machine, the pulp suspension is dewatered on a moving wire mainly through vacuum assisted devices (suction boxes). The dewatering process is supposed to occur by dead-end filtration under constant pressure as long as the fiber mat (filter cake) remains fully saturated with water.A physical model was written based on the kinetics of deposition of a compressible fiber mat and a drainage model for the filtrate. After some mathematical arrangements, a model similar to Hermans and Bredée’s formulation [1] was obtained. It states that, at a constant suction pressure, the amount of filtrate per unit surface area is proportional to the square root of suction time.Our approach also yields an equation that is similar to the empirical drainage model used in the paper industry, which links the drained flow rate to a power law of the suction pressure and of the dwell time. By identification of the values of the coefficients, it is then possible to analyze the compressibility of the fibrous mat.For purpose of validation, experimental investigations were performed on one slotted suction box of a Fourdrinier pilot paper machine. The suction pressure was varied from 0.5 to 3.0 kPa and the dwell time was changed by successively increasing the number of slots from 1 to 6 slots. The flow rate of filtrate was measured in each case in order to determine the corresponding specific amount of filtrate. Result analysis confirmed that the specific volume of filtrate varies as the square root of the dwell time. In addition, we could calculate the basis weight of the mat deposited on the wire upstream of the suction box and on the suction box itself, the average specific filtration resistance which was 1010 m kg−1 for the studied case.The methodology developed in this study can be transposed to other dead-end filtration processes.

► Understanding the filtration law under constant pressure for a water-saturated medium.
► Identification of parameters in equations and giving them a physical meaning.
► Understanding the physical phenomena taking place on a vacuum assisted device.
► Proposal of a methodology which can be used for all dead-end filtration processes.