Moisture Transport within Solids

From the above it appears clear that the moisture transport takes jdace at the surface of the fiber (solid-liquid interface). The capadty of water retenticm, on the other hand, appears to reside primarily in macrovcMds within the fiber. Actually, acrylic fibers with high water retention can be (Stained, accordii to a recent patent by spinning a fiber from a blend of an acrylic copt maer and a hqjdy soluble con ronent (e. g., glycerine), which later is washed out in the further treatment, leaving behind voids and capillaries which provide the water retention. 

Chipping, the process of reducing a log to chips of about 2.5 x 2.5 x 0.5 cm thick is a common preliminary to all pulping methods except stone groundwood. Wood chips are more convenient and uniform for solids transport within the mill complex by conveyor belt or pneumatic delivery systems than are whole logs. Also a chip format is more amenable to direct physical conversion to fibers because of the much easier and more uniform penetration of heat and moisture through the thin sections of wood. 

This study proposes a simple approach to modelling the dynamics of solids transport within a flighted rotary dryer. The approach taken was to model the system in a series-parallel formulation of well-mixed tanks. The concept of active and passive solids is important, since it will lend itself well to the addition of mass and energy balance relations. This model formulation predicts the RTD of the system. Industrial RTD data was obtained from a 100 tonne per hour dryer and compared with the model predictions. gPROMS parameter estimation has delivered overall transport coefficients for this system. The transport coefficients are not independent, nor completely physically meaningful. However, they produce a very simple model formulation, which forms the basis for more detailed rotary dryer models incorporating mass and energy balances. Future work will see the development of a full dryer model based on the proposed solids transport model. Refinements will be made to the model to incorporate the effects of solids moisture and interaction with the counter current air stream. 

In the initial constant rate drying period, a knowledge of the heat diffusivity and heat conductivity coefficients of the wet material is necessary because they are the controlling factors for heat transport within the material. In this section of drying it is presumed that the pores of the wet material are saturated by moisture. Consequently, the above-mentioned characteristics of a heterogeneous material consisting of a solid skeleton and, within this, a capillary system (filled... 

Evaporation of moisture within a solid and gas transport out of the solid by diffusion and/or convection. Evaporation can occur within a solid if it is boiling or porous. Subsequently vapor must move out of the sample. 

Internal transport of moisture within the solid may occur by any one or more of the following mechanisms of mass transfer ... 

---