A non-isothermal pore network drying model: influence of gravity
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media (T2-7P)
Keywords: capillary porous media, mono-modal pore size distribution, invasion percolation, phase distributions, temperature field
Drying of porous media is a major operation in solids processing and one of the most energy consuming processes in industry; at the same time it is a problem of significant scientific interest due to the complexity of involved transport mechanisms and pore geometry, as well as the use of different drying methods. Pore scale models, which represent void space as a network of pores and throats and are based on statistical physics and percolation theory, are suited to overcome drawbacks of traditional continuous modeling.
The concept of immiscible displacement as an invasion percolation process (IP) driven by heat and mass transfer is used in a pore network model. The interplay between capillary and gravitational forces is described in the presence of temperature gradients as occurring in convective drying. Quasi-steady diffusive vapor transport is assumed, viscous forces are neglected in both liquid and gas, and heat transfer is only by conduction. The coupling between heat and mass transfer occurs at the liquid-gas interface: local evaporation (or condensation) acts as heat sink (or source), vapor diffusion is controlled by equilibrium vapor pressures at the menisci, and capillary flow is influenced by surface tension gradients (due to temperature gradients).
Drying simulations are carried out on a square 51x51 pore network with mono-modal (40 +/- 2 µm) throat radius distribution, throat length 500 µm and solid properties as for glass. Dry air at 80°C is flowing parallel to the top edge of the network; the remaining sides are impermeable to heat and mass fluxes. IP patterns, i.e. phase distributions, as well as drying rates are studied with and without gravity.
In the absence of gravity, capillary forces alone determine the order of throat invasion; they are given by random throat radius distribution and current local temperatures. If temperature gradients are low, then the gas phase rapidly invades the depth of the pore network creating many liquid clusters in the whole network. These disconnected liquid regions then evaporate one by one starting from the network surface. For week disorder (i.e. small standard deviation) of throat radius, temperature gradients play a more important role and invasion order is modified via the temperature dependency of surface tension. In convective drying, the porous medium heats up from the surface; this leads to favorable invasion of near surface throats and hence to a stabilized drying front.
For the given geometry, gravity counteracts capillary forces. Throat invasion order is then determined by an invasion potential which combines both effects. Capillary pumping is limited to a finite drying front width in which small clusters form. This front recedes into the network without major changes of its width. The gravitational stabilization of the drying front is superimposed by thermal stabilization. This is investigated by a comparison with isothermal drying simulations.
See the full pdf manuscript of the abstract.
Presented Monday 17, 13:30 to 15:00, in session Transport Phenomena in Porous/Granular Media (T2-7P).
