Consistency measurement of pulp suspension containing entrained air

Advanced process control technology used in the pulp and paper industry requires accurate and reliable consistency measurements. Microwave measurement is suitable for this purpose if it is installed in a position where the pulp contains only a minor amount of gaseous components. However, a considerable amount of gaseous components are often present in process positions where good consistency measurement would be necessary. This study aimed to clarify the effects of gaseous air on the microwave consistency measurement of a wood pulp suspension, and additionally attempted to find a method which would diminish or remove the accuracy problems deriving from the gas. A microwave consistency analyzer (Valmet MCA) was used as the measuring instrument, and the effects of air were studied both with bleached kraft pulp and ground wood. The results indicated that the air content of the pulp has a significant effect on the readout of the microwave consistency measurement. The value of the consistency measurement always increased when the air content of the pulp was increased The results also indicated that the deviation of the microwave signal reflects the specific volume of air in the pulp. This makes it possible to compensate for the error deriving from air. The compensation will improve the accuracy of the measurement and make it possible to use the microwave consistency measurement in several new applications where entrained air has disturbed the measurement until now.