The determination of drying kinetics and equilibrium characterisation at low moisture contents
Keech, Andrew M.
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Recently there has been focus on determining the drying behaviour of particulate material at low moisture contents (< 1 %), as a result of increased energy costs and purity requirements for dried products. As little is known about. the drying behaviour of particulate material at these low moisture levels, driers have been oversized by including large safety factors in the sizing calculations, thus incurring unnecessary capital expenditure. A research programme was formulated in an attempt to provide a greater understanding of the moisture movement processes occurring at these low moisture levels. The results of this work indicate the significant effects that influence the drying behaviour of particulate material, which may be implemented into drier sizing calculations. Most particulate materials require a significant amount of energy to remove residual amounts of moisture, particularly from microporous media or from strongly hydrophilic surfaces. Quantifying the energy required to remove this bound moisture is possible from a knowledge of heats of wetting as a function of moisture content. These can be calculated from isotherms under controlled conditions using the Clausius-Claperyon equation. As part of a research programme to look at this type of drying behaviour, a novel apparatus was designed and constructed. The function of this apparatus was to record drying profiles of particulate material under various drying conditions and sample sizes. The sensitivity and reproducibility of the drying profiles were the overiding parameters in the design of such an apparatus. Moisture movement at low moisture levels in thin layers and single particulate arrangements can be described by two mechanisms. Solute bound to hydrophilic and/or active pore surfaces requires a normally distributed activation energy to be overcome, for surface diffusion to occur. Solute held in "water clusters" by hydrophobic media, such as polymeric, some plastic and generally swelling solids, can be described by Fickian movement similar to the familiar transport processes known to occur at higher moisture contents. Thus, it is possible to extrapolate the drying kinetics at higher moisture contents to lower-moisture contents in this instance, but not for the surface diffusion process which takes place only at lower moisture levels. For the drying of thicker layers and beds of particles, accurate overall diffusion coefficients need to include a gas phase diffusion term for gaseous movement through the particulate bed. The studies here have extended the understanding of the drying kinetics dissimilarities between drying single isolated particles, single particle agglomerates, thin layers and beds of particulate material. In sizing calculations of drying units, preliminary kinetics tests on the material to be dried need to be performed under similar bed geometries to that in the full-scale drying unit. For example, thin-layer experiments appear to provide accurate drying kinetic data for the design of cascade rotary driers. The thickness of the thin layer in the drying kinetic tests is determined by the observed thickness of the falling film of material during operation of the cascade rotary drier. Higher gas velocities in a fluidised bed, or flash driers, may be best modelled by drying kinetics studies on single particles or single particle agglomerates. However, at lower gas velocities, thin-layer tests may be more suitable because the drying material in the fluidised bed may dry in a bubbling motion. The final section of this work looked at selective drying of various isopropyl alcohol/water mixtures at low moisture levels. Selective drying at higher moisture contents is generally well understood, by applying azeotropic principles to describe liquid-phase moisture removal from porous matrices. Two-component bound moisture in a porous particulate resides in a solid phase. It is not possible for a binary mixture bound directly to the solid to form an azeotropic mixture. Modelling the removal of this binary bound moisture was shown to fit a surface diffusion model, in which each of the two solvents was treated as a separate entity. In short, selective drying is prevalent at low moisture contents but can be described by two distinct surface diffusion models working independently on each binary species.