Thermomechanics of Drying Processes

This book presents a systematic thermomechanical theory of drying. The book has an interdisciplinary character and combines the knowledge of Chemical Engineering and Mechanics of Continua. It contents a unified approach to the heat and mass transfer in drying processes as well as a precise analysis of all local drying effects. Methods for the improvement of the quality of the dried products and optimization of drying processes with respect to the strength of materials and the drying time are presented. The book also presents the theory for fully coupled multiphase transport in deformable capillary porous media as well as computer simulations of drying processes. Finally the acoustic emission method is described as an experimental way of identification of material fractures during drying.

Texte du rabat

This book is interdisciplinary in character and combines the knowledge of me­ chanics and chemical engineering with the aim of presenting a more exhaustive analysis ofthe phenomena occurring in wet materials during drying. Traditionally, the subject of drying has been an almost exclusive domain of chemical engineers. The drying curricula have mostly included only the courses of heat and mass transfer or diffusion. The mechanical phenomena that accompany drying, as for example, warping or deformation of dried materials, or the drying induced stresses and fissures of the material, were ignored or considered in a rather obscure way. This book broadens the scope of drying theory, bringing into the curriculum the tools enabling the study of both heat and mass transport processes and the me­ chanical phenomena that occur in wet materials under drying. There is little available literature that brings together heat and mass transport processes and mechanical phenomena in a unified approach to drying processes.

Contenu

Nomenclature.- 1 Properties of dried materials.- 1.1 Classification of wet materials.- 1.2 Characterization of moisture bounding a solid skeleton.- 1.3 The equilibrium humidity.- 1.4 Mechanisms of moisture movement.- 2 Characterization of drying processes.- 2.1 Drying technique.- 2.2 Kinetics of drying processes.- 2.3 Mechanisms of heat and mass transfer.- 3 The equations of balance.- 3.1 Preliminaries.- 3.2 Balance equations: control volume approach.- 3.3 Second law of thermodynamics.- 4 Thermodynamic foundation of dried materials in elastic range.- 4.1 Postulate of local state.- 4.2 Equations of state.- 4.3 Phenomenological rate equations of heat and mass transfer.- 4.4 Physical relations for elastic materials under drying.- 5 Thermodynamics of viscoelastic materials under drying.- 5.1 Rheological properties of dried materials.- 5.2 Constitutive equations for a viscoelastic body.- 5.3 Analogy between viscoelasticity and elasticity.- 6 Plasticity in drying.- 6.1 Elastoplastic behavior of dried materials.- 6.2 Thermodynamical restrictions.- 6.3 Theory of plastic flow.- 6.4 Identification of the coefficients ? and L in the plastic potential.- 7 Destruction of materials by drying.- 7.1 Preliminary remarks.- 7.2 Cohesion forces in drying processes.- 7.3 Theoretical strength of dried materials.- 7.4 Fracture of brittle dried materials.- 7.5 Fracture of partly ductile dried materials.- 7.6 A measure of damage.- 8 Conditions of stress generation and boundary conditions.- 8.1 Conditions of stress generation.- 8.2 Balance equations for a body with a discontinuity surface.- 8.3 Boundary conditions in the first and second period of drying.- 8.4 Receding of the evaporation zone.- 9 Approach to numerical analysis in drying.- 9.1 Set of governing equations.- 9.2 Galerkin's formulation of the numerical problem.- 9.3 Time integration.- 9.4 Numerical solution for two-dimensional problem.- 10 One-dimensional initial-boundary value problem.- 10.1 Distribution of temperature and moisture content in wet porous plates under drying.- 10.2 Drying induced stresses in an elastic saturated plate.- 10.3 Drying induced stresses in a viscoelastic saturated plate.- 11 Drying induced stresses in products of cylindrical and spherical symmetry.- 11.1 The phenomenon of stress reverse in a cylindrical sample.- 11.2 Determination of shrinkage factor on a spherical sample.- 12 Mechanical effects in dried materials examined on the basis of two-dimensional boundary value problems.- 12.1 Deformations and stresses dependent on material shape.- 12.3 Thermal effects in dried materials.- 12.4 Analysis of dried body response on drying conditions the response of dried body to drying conditions.- 12.5 Significance of the wet bulb temperature in drying.- 12.6 Control of drying processes.- 12.7 Mechanical properties dependent on moisture content.- 12.8 Concentration of stresses around grooves.- 12.9 Phase transition inside the dried body.- 13 Dried materials with anisotropic structure.- 13.1 Constitutive equations for anisotropic materials with variable moisture content.- 13.2 Experimental identification of mechano-sorptive strain in wood.- 13.3 Deformation and stresses in dried wood.- 13.4 Identification of wood fracture during drying.- 14 Experimental studies in drying.- 14.1 Determination of coefficients in physical relations.- 14.2 Determination of coefficients in rate equations.- 14.3 Application of acoustic emission in studies of dried materials.- References.