Air in pulp and papermaking processes

Abstract A pulp suspension consists of water, fibres, fines, fillers and chemicals, but air or other gases are also present in practically all pulping processes either in dissolved form or as bubbles. Dissolved gases seldom disturb the processes, but they are readily converted to gaseous form when conditions change. The gas bubbles affect the properties of the pulp suspension, reduce the accuracy of certain measurements, interfere with the runability of the papermachine and detract from the quality of the end-product. Gases are removed from the process by either mechanical or chemical means, resulting in increased investments and operational costs. The aim of this work was to study the behaviour of gas in pulp and papermaking processes with laboratory, pilot-scale and mill-scale experiments. Five main areas of the research can be identified: 1. Occurrence of gases in pulp and paper mill processes, 2. Dissolution, precipitation and hold-up of gases in the pulp suspension and mill water, 3. Effects of gases on certain consistency measurements, centrifugal pumping and operation of the hydrocyclone. 4. Measurement of the gas content of the pulp suspension by compression, radiometric, microwave and sonar methods and 5. Removal of gases with a centrifugal pump equipped with vacuum pump or hydrocyclone equipped light reject removal. The results show that the dissolution and precipitation of gas is strongly dependent on the pulp and water properties. Dissolved and colloidal material reduces the solubility potential of gas, but also accelerates the precipitation of dissolved gases in gaseous form. The hold-up of precipitated gas bubbles was found to be much more pronounced in hydrophobic mechanical pulps than in lignin-free chemical pulps. The accuracy of consistency measurements was affected by free gas in the pulp suspension, requiring special attention when assessing the results. The operation of pressure screens and hydrocyclones was affected only at high volumes of free gas in the feed suspension. According to the experiments, a reliable gas content measurement can be achieved by in-line radiometric, microwave or sonar methods, and also by the off-line compression method if a representative sample is obtained. A centrifugal pump equipped with a gas removing unit is designed mainly to ensure undisturbed pumping, whereas its gas removal efficiency remains quite low, especially with small bubbles and at a low gas content. The gas removal efficiency of a hydrocyclone equipped with light reject removal is good, but decreases with small precipitated bubbles. These results offer new information of the behaviour of the gas in pulp suspensions and white water and underline the importance of the bubble generation mechanism in this context