Multiphase Transport Model For Freeze-drying Of Hygroscopic Porous Media

Freeze-drying is a technology that plays an irreplaceable role in many drying technologies due to its producing high-quality dried products.The main problem facing the current freeze-drying technology is the low drying rate,long process time and expensive cost.Freeze-drying is a complex process involving conjugate mass and heat transfer.Traditional liquid frozen materials have no initial internal pore,migration resistance of water vapor is large.That is the reason why it's long and energy-intensive.In order to improve the economic benefit of freeze-drying process,a novel idea,i.e.,enhance freeze-drying process by porous media with prefabricated porosity,was proposed by the author's research team accordingly.A multiphase transport model for freeze-drying of hygroscopic porous media based on the local mass non-equilibrium assumption was constructed to verify the effect of initially unsaturated frozen material with prebuilt porosity on freeze-drying of liquid in this paper.Gauss error function was constructed in the present work to describe the hygroscopic effect of moist porous media,and then compared with power function,fractional polynomial and exponential function forms of the adsorption-desorption equilibrium relationship used by the author's research team previously.The governing equations were solved numerically on the platform of COMSOL Multiphysics based on the finite element method.Numerical results show excellent agreements with the experimentally measured drying curves for the case of the local mass non-equilibrium compared with the case of the local mass equilibrium.It is impressive to find that the four types of the equilibrium relations have excellent agreements.The ratio of vapor pressures between the adsorption-desorption equilibrium and the phase equilibrium can be a function of ice saturation or moisture content.Different forms of the proposed equilibrium relations can be mathematically unified into a polynomial form through Taylor extension.Freeze-drying of liquid can be significantly enhanced by frozen material with prebuilt porosity.The smaller the initial saturation of liquid frozen material,the greater the rate of freeze-drying process.The entire cumulative radiation energy absorbed by the samples are almost the same.However,the unsaturated material absorbs higher radiation power than the saturated one.Volumetric sublimation/desorption is achieved numerically through the analyses of saturation,temperature and mass source profiles.Investigation on variations of effective mass diffusivity and effective heat conductivity shows that the predominant freeze-drying rate-controlling factor is mass transfer for conventional saturated material,while heat transfer for material with prebuilt porosity.The constructed model also provides satisfactory prediction capabilities of ambient temperature effects on freeze-drying.Raise the operating temperature and lower the pressure of drying chamber appropriately is beneficial to the freeze-drying process.The reinforcement of this technique is influenced by the material geometry.Freeze-drying can be enhanced by frozen material with prebuilt porosity,until the mass of two samples with same diameter reaches a certain value.Fill depth of the samples to be dried has therefore to be considered carefully.