Measurement of the Air Content of Pulp Suspension - Sonar Method

Gases have negative effects on production efficiency and product quality. Both entrained and dissolved gases can be found in virtually all pulp flows, but a fast and accurate inline measurement technique for gas content measurement has yet to be established. In these experiments a new sonar technique for gas content measurement was tested. Experiments were done in a pilot-scale pumping system where the pulp suspension was circulated with a centrifugal pump in a closed loop. Measurements were located in the vertical outlet pipe of the pump. The air content of the system could be adjusted using two different principles. Macro bubbles (∅>100 m) were fed to the system through a membrane tube and the volumetric air flow was controlled with rotameters. Micro bubbles (∅=20-100 m) were created by precipitating bubbles from dissolved form, and controlling the level of dissolved oxygen and volumetric flow of the supersaturated suspension. The measurement accuracy of the sonar method was tested with four different air content levels and two different bubble types. Furthermore, the effect of pulp grade, consistency, flow velocity and process pressure were tested. Microwave and radiometric methods suitable for air content measurement in a controlled environment were used as reference measurements. The results show that with the sonar method an accurate gas content result can be obtained independently of the pulp grade, process pressure, flow velocity or bubble size.