DISPLACEMENT DEWATERING TO MAINTAIN BULK

In conventional pressing operations, water removal is intimately linke d to sheet densification. If water removal could be partially decouple d from densification, the paper-maker could have more control over she et properties. This has been the objective of an ongoing study of disp lacement dewatering, in which a pressurized air or steam phase is used to expel liquid water from paper while the paper is simultaneously un der mechanical pressure. Displacement dewatering combines mild wet pre ssing with vapor-liquid drive to increase and maintain the hydraulic p ressure gradient for water removal. Displacement dewatering differs fr om through-drying, in which a gas phase blows through the paper to eva porate water. In displacement dewatering, a true displacement process is desired in which a gas-liquid interface is maintained. Recent exper imental results with bench-scale equipment are reported. The data sugg est that displacement dewatering can be a useful dewatering process fo r some grades of paper and for some specialty products. It has the pot ential to increase water removal without significantly increasing shee t densification. The greatest problem appears to be the long gas appli cation times required. Typically, the displacement dewatering process requires over 50 ms for effective water removal, which may be impracti cal for many processes. Improved strategies are being explored to more fully tap the potential of a displacement dewatering process.