-Paper dryer surface contamination can influence the severity of fiber picking, result in increased adhesion of the paper web to the surface, and impact the severity of cockle (out-of-plane deformation), a paper quality issue. The Web Adhesion and Drying Simulator (WADS) is a unique apparatus designed for investigation of paper sticking and fiber picking. Using the WADS, it has been found that the work of separation and amount of picking depend on roll surface material, contamination and temperature, and on the composition, moisture content and temperature of the web, and contact time. The WADS was also found to be convenient for studying the effect of drying conditions on severity of cockle. Cockle was found to depend on ingoing sheet solids content, surface temperature and degree of heat transfer non-uniformity.
High intensity paper drying is defined as any drying process in which the web is at or above the thermodynamic saturation temperature corresponding to the ambient pressure. Rapid generation of vapor under these circumstances causes the drying process to be driven by a gradient of total pressure and not by a gradient of partial vapor pressure. Therefore, the generated vapor leaves the web by a bulk (convective) flow mechanism rather than a slower diffusion mechanism. The vapor pressure build-up in the web also offers the opportunity for removal of moisture in liquid form, since the fast flowing vapor can displace and/or entrain liquid as it moves through the web. This can result in significantly lower energy usage relative to conventional drying, since only a fraction of the moisture has to be evaporated. The thesis objective is a mathematical model simple enough to be easily modified or expanded but comprehensive enough to be applicable to a wide variety of process conditions and sheet variables. Early experiments suggested that the high intensity drying process could be described effectively by a discrete "zone" model. The process is idealized by picturing the sheet as composed of different zones which contain various amounts of fiber, liquid water, and water vapor. The model is based on sets of equations which account for the heat and moisture transfer within and among the zones during three regimes: heatup, transition, and quasi-static. Once the hot surface temperature, boiling point temperature, basis weight, Canadian Standard Freeness, initial moisture ratio, and mechanical pressure pulse are specified, the equations may be solved to predict the moisture content as a function of time. Comparisons between experimental data and the model's predictions demonstrate that the model qualitatively and quantitatively describes high intensity drying
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