Moisture capillary flow

Critical moisture content is that obtained when the constant rate period ends and the falling rate periods begin. Second critical moisture content specifies that remaining in a porous material when capillary flow dominance is replaced by vapor diffusion. 

In porous and granular materials, Hquid movement occurs by capillarity and gravity, provided passages are continuous. Capillary flow depends on the hquid material s wetting property and surface tension. Capillarity appHes to Hquids that are not adsorbed on capillary walls, moisture content greater than fiber saturation in cellular materials, saturated Hquids in soluble materials, and all moisture in nonhygroscopic materials. 

Capillary Flow Moisture which is held in the interstices of solids, as liquid on the surface, or as free moisture in cell cavities, moves by gravity and capiUarity, provided that passageways for continuous flow are present. In diying, liquid flow resulting from capiUarity appUes to liquids not held in solution and to aU moisture above the fiber-saturation point, as in textiles, paper, and leather, and to all moisture above the equiUbrium moisture content at atmospheric saturations, as in fine powders and granular solids, such as paint pigments, minerals, clays, soU, and sand. 

The problems experienced in drying process calculations can be divided into two categories the boundary layer factors outside the material and humidity conditions, and the heat transfer problem inside the material. The latter are more difficult to solve mathematically, due mostly to the moving liquid by capillary flow. Capillary flow tends to balance the moisture differences inside the material during the drying process. The mathematical discussion of capillary flow requires consideration of the linear momentum equation for water and requires knowledge of the water pressure, its dependency on moisture content and temperature, and the flow resistance force between water and the material. Due to the complex nature of this, it is not considered here. 

Textile can take up a lot of moisture, but the paper tissue on the window sill can take up even more it still feels dry with four times its own weight of water. An estimate of the density of the roll tells you that over 90% of the volume consists of air. It is not easy to make a structure like that. The tissue takes up water rapidly, but the capillary rise is limited to about twelve centimetres as a simple experiment shows. You need to understand capillary flow if you want to improve such products. You do not see much under the microscope, but where two sheets have been torn apart you can see the separate cellulose fibres. They are about 10 am thick. 

Hougen, McCauley, and Marshall [Tram. Am. Imt. Chem. Eng., 36, 183 (1940)] discussed the conditions under which capillary and diffusional flow may be expected in a drying solid and analyzed the published experimental moisture-gradient data for the two cases. Their curves indicate that capillary flow is typified by a moisture gradient involving a double curvature and point of inflection (Fig. 12-40a) while diffusional flow is a smooth curve, concave downward (Fig. 12-40b), as would be predicted from the diffusion equations. They also showed that the liquid-diffusion coefficient is usually a function... 

An approximate equation for use for materials in which moisture movement is controlled by capillary flow is given as... 

Above fiber saturation, the effect of moisture content on D is even more complex because of the great variability in capillary flow... 

Capillary flow of moisture in porous media. The reduction of liquid pressure within small pores due to surface tension forces causes liquid to flow in porous media by capillary action. 

The mechanism of moisture flow in a droplet during spray drying is mainly diffusion supplemented by capillary flow. The drying characteristics of the droplet depend on the Dalance of bound and unbound as each category has distinct features. 

The presence of unbound moisture in the droplet means that the drying proceeds at a constant high rate as long as the moisture diffusion within the droplet is able to maintain saturated surface conditions. When the diffusional and capillary flows can no longer maintain these conditions, a critical point is reached and the drying rate will decline until equilibrium moisture content is reached. The evaporation of bound moisture is strongly dependent on the nature of the solid matter in the spray droplet. 

For solids with continuous pores, a surface tension driven flow (capillary flow) may occur as a result of capillary forces caused by the interfacial tension between the water and the solid particles. In the simplest model, a modified form of the Poiseuille flow can be used in conjunction with the capillary forces equation to estimate the rate of drying. Geankoplis (1993) has shown that such a model predicts the drying rate in the falling rate period to be proportional to the free moisture content in the solid. At low solid moisture contents, however, the diffusion model may be more appropriate. 

Several physical mechanisms contribute to moisture migration during the process. For a porous solid matrix, with free water, bound water, vapor, and air, moisture transport through the matrix can be in the form of either diffusion or capillary flow driven by individual or... 

First stage When both surface and core MC are greater than the F.S.P. Moisture movement is by capillary flow. Drying rate is evaporation controlled. 

Since many chemicals are processed wet and sold dry, one of the more common manufacturing steps is a drying operation (13) which involves removal of a liquid from a solid by vaporization of the liquid. Although the only basic requirement in drying is that the vapor pressure of the liquid to be evaporated be higher than its partial pressure in the gas stream, the design and operation of dryers represents a complex problem in heat transfer, fluid flow, and mass transfer. In addition to the effect of such external conditions as temperature, humidity, air flow, and state of subdivision on drying rate, the effect of internal conditions of liquid diffusion, capillary flow, equilibrium moisture content, and heat sensitivity must be considered. 

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