Disperson and Flocculation in a Dry Mat-Forming Process
Doctoral thesis, 2004
The absorption cores in disposable nappies, incontinence pads and female hygiene products are manufactured by a dry mat-forming process. Cellulose fibers and super-absorbing particles are suspended in air and blown onto a forming wheel on which the cores are formed. It is vital for the quality of the product that the fibres and particles are evenly distributed in the suspension. The purpose of this project has been to establish a platform of scientific knowledge from which the fluid mechanics aspect of the manufacturing process can be understood.
To better understand the problems relevant to the process, air/particle flows and air/fiber flows have been studied separately. A two-fluid model with a modified κ-ε model for the gas phase has been used to simulate a horizontal air/particle flow. An optical probe and a calibration procedure for measuring the local particle volume fraction have been developed and used to verify the models. The probe uses two optical fibres, to emit infrared light and detect light reflected by the particles. The calibration procedure is based on the assumption that light paths including reflections on more than one particle do not make a significant contribution to the probe signal. Measurements of a horizontal air/particle flow have been made to verify the simulations. The agreement between simulations and experiments is good.
For the air/fiber flow, it is suggested that a two-fluid model be used together with a turbulence model for the gas phase and a transport equation for a floc characterizing scalar. This scalar may be either flocculation intensity or floc size. A technique for characterizing the flow using optical probes has been developed. Measurements of floc velocity and size distributions have been made in a production-scale facility for process research. The shape of the measured floc size distribution indicates that the mechanism for aggregating fibres into flocs is strong compared to the mechanism for breaking up flocs.