Online characterization of wood pulp : foundations for a photoacoustic sensing technique

Abstract: Today, there is an increasing demand in the paper industry for better utilisation of the raw material and improved energy efficiency. Also, the increasing use of recycled paper in the manufacturing process leads to a situation where the knowledge of pulp properties needs to be improved and controlled. These demands calls for improved control strategies and this in turn requires new measurement techniques that can make a better characterisation of the pulp. One important process parameter to be controlled is pulp consistency. Poor control of pulp consistency leads to an unstable process that compromises the production, quality and energy efficiency in the pulp mill. Hence, accurate consistency control is fundamental to achieve process stability. Another important parameter is pulp composition, such as the share of long fibres and short fibres (fines). Using only a single measurement technique, it is difficult to accurately determine the pulp properties such as total consistency and the share of fibres and fines in a pulp suspension. The overall objective of this work is to investigate whether a sensing strategy based on the photoacoustic technique can provide wood pulp property information online.This thesis shows that fibres are the predominant source of attenuation of ultrasonic waves. It also shows that fines are the predominant source of optical scattering. Hence, additional information about the examined media can be extracted by using a combination of the measurement techniques. The combination enables an estimation of the total consistency and, in addition, identification of the two different length fractions, fibres and fines in a given pulp suspension. A photoacoustic sensor has been developed that is able to relate the optical extinction of a laser light pulse propagating through a turbid medium to a photoacoustic signal. Simultaneously, a signal related to the ultrasonic attenuation in the medium is also obtained. The sensor is fast and has no moving parts, leading to a low manufacturing cost, low maintenance requirements and greater robustness. Because the sensor simultaneously measures optical and acoustic properties, the sensor can extract more information about the pulp suspension compared to a single technique and is therefore applicable in the pulp and paper industry. Hence, better process control is achievable and improvements in production, quality and energy efficiency in the paper making process are obtainable.