The Heat of Sorption in Paper Drying – An Investigation of Measurement Methods and Influence of Pulp Parameters

When the dry content during pulp drying reaches a level above 75% to 80% the free water, i.e. unbound water, has been evaporated. The remaining water is bonded to the surface due to physisorption, additional energy is necessary to overcome these bonding effects. This additional energy is called heat of sorption. At 80oC and a dry content of 95% for unbleached softwood kraft pulp the evaporation energy increases up to 2800 kJ per kg water compared to the latent heat of water of about 2300 kJ/kg at the same temperature.

Different measuring methods to determine the heat of sorption HS are described in the literature, the reported values for HS of pulp show large differences. The first aim of this work is to compare the results of different measurement methods using the same sample pulp. We investigated calculation of HS from sorption isotherms collected with a conventional climate chamber (ESC) and differential vapor sorption (DVS) analysis. Furthermore we applied direct measurement of the heat flux generated by sorption using differential scanning calorimetry (DSC), differential scanning calorimetry combined with thermogravimetric analysis (DSC/TGA) and reaction calorimetry (RC). All but one measurement method delivered consistent results in the range of HS=40 kJ/kg to 70 kJ/kg additional energy due to surface sorption. The advantages and disadvantages of the different measurement techniques are discussed.

In the second part of this work the impact of pulping and pulp treatment on the heat of sorption has been investigated. Refining and bleaching seems to have no impact. Pre drying reduces the overall heat of sorption by 15 %. The addition of inorganic fillers to paper reduces its heat of sorption due to a negligible HS of the filler.

In conclusion the total additional energy caused by sorption effects is less than 2% of the overall energy necessary to fully dry the pulp. Additionally the heat of sorption is nearly unaffected by pulping, bleaching, refining or pre-drying of the pulp. Therefor the heat of sorption HS is playing only a minor role for industrial applications of paper drying.