Extended wet pressing at elevated temperature enables enhanced dewatering for tissue and linerboard

Abstract

The energy efficiency of papermaking critically depends on how much of the web moisture can be removed by wet pressing before the dryer section. Here, we explored an operation where the pressing was conducted over an extended period (70-540ms) at an elevated temperature (approaching 100°C). We found a large increase in the solids content for the tissue and board grades studied. The maximum measured solids content was 83 ± 2% for tissue and 62 ± 2% for linerboard, with approximately 5% pt. (tissue) or 1.5% pt. (linerboard) attributable to moisture evaporation after the pressure pulse based on our drying model. Both solids content levels were notably higher than those achieved with conventional pressure pulses. The results open up intriguing new directions for technological development towards a wet pressing concept that mimics the current metal-belt pilot lines but operates at higher web moisture levels. The high solids content achieved with extended wet pressing significantly reduces the amount of evaporated water. Thus, an ineffective Yankee + hood drying system with high temperature and energy loss would be no longer needed in tissue production. This would enable up to 90% drying energy savings and a corresponding CO2 emission reduction compared to existing pressing and drying technologies.