Towards a new sensing strategy for measuring paper pulp properties a combination of ultrasonic and optical measurement techniques

Abstract: In the manufacturing process of paper the mass fraction and material properties of the fibres in the pulp suspension are important for the strength and quality of the finished product. The properties of the fibres in the pulp suspension that enters the paper machine is not well understood and therefore an adequate prediction of the strength and quality of the finished paper is not possible. Also with the demand for a more environmentally friendly process the amount of recycled paper that enters the process has increased. The properties of the recycled fibre are unknown and also vary. Thus, the pulp suspension contains fibres of different sizes, mechanical properties and from fibres with different origin. The pulp and paper industry is a highly energy consuming industry. Therefore to be competitive on the world market the energy consumption needs to be reduced. Altogether, there is an increasing demand for methods of on-line characterisation of the pulp suspension as well as the fibres in the pulp suspension. If the knowledge of the pulp suspension is increased it will give potential to make an adequate prediction of the strength and quality of the finished paper. Also with the increased knowledge of the pulp suspension improvement of the process control will be feasible. Hence, a reduction of energy consumption is possible. The aim of this thesis is to describe and investigate a sensing strategy that consist of a combination of three existing measurement techniques, ultrasound, optical and photoacoustical. The combination of these techniques is believed to extract more information from the paper pulp suspension than a single technique by itself. The long-term goal is to find a measurement technique for the pulp and paper industry that are on-line, fast and increases the knowledge of the properties of the paper pulp suspension. And at the end the finished paper strength and quality. If the fibre properties are determined on-line it is believed that an improved process control is achievable. Three studies has been conducted, the first using only ultrasound technique, the second using a combination of ultrasound and optical technique and in the last study a combination of ultrasound and photoacoustical technique. Two studies were conducted on pulp suspensions. The mass fraction range corresponds to the ranges used in the paper forming process. In the third study properties of Nylon 66 suspended in water was investigated. The result shows that the sensing strategy of using a combination of existing measurement techniques gives additional information about the examined media. This shows that the sensing strategy have a potential in better process control within the pulp and paper industry. It is noted that the investigated measurement techniques do not require any special set-up. Thus, they can all be used together in the same measurement cell and in an on-line set-up. It is also observed that all investigated measurement techniques are fast compared to sampling and evaluation. However, the overall knowledge of the fibre in a pulp suspension has not increased to the extent that it can measure all needed requirements (physical, chemical and mechanical). Hence an optimisation of the process control in the preparation of a pulp suspension is not achievable in this state. The thesis also describes a parametric estimator for estimation of observable properties in an ultrasonic measurement system. It is shown that the proposed estimator enables accurate estimation of the observable properties with considerably lower variance compared to standard Fourier analysis methods.