Latest Findings on Entrained Air and Dissolved Gases in Pulp Suspensions, 2000 Papermakers Conference Proceedings

Rainer Rauch, Reinhard Sangl--High air or gas contents in paper stocks constitute a serious problem area in papermaking. They may be responsible for poor drainage, pinholes, web breaks and many other problems. The levels of entrained and dissolved gases have to be measured online in the papermaking process in order to establish a relationship between these problems and high air or gas contents in pulp suspensions. In pilot trials, pulp suspensions of different paper grades were deaerated mechanically and chemically. Mechanical deaerators when operated at optimal pressure and flow conditions gave very good results in terms of removing both entrained and dissolved gases. Chemical deaeration by comparison primarily reduced the entrained gas levels. The results obtained in paper mills indicate the high relevance of dissolved gases in pulp suspensions for production and quality problems. The pressure drop at the wire and at the foils leads to dissolved gases being released. The gas bubbles thus formed may cause e.g. drainage problems and pinholes. It has been found that these problems fre-quently arise in systems with very low contents of entrained gases. Measurement systems that are incapable of de-termining the amount of dissolved gas in pulp suspensions therefore cannot detect the origins of these problems.