Wet porous material

Ilic, M. and Turner, I.W. Convective drying of a consolidated slab of wet porous material, Int. JHeat Mass Transfer, 32 (12),[1989). 

For larger pores, say in the mesoporous range, much larger molecules are needed to characterize the pore size and the Hterature is scarce in this field. Polymen can be used (e.g., dextran) to evaluate the pore size of membranes. One then assesses a molar mass cutoff rather than a real pore size. The solute exclusion technique was also proposed to assess a pore size distribution [71]. It is well suited for wet porous materials [72]. 

Miscellaneous techniques. Recently the rate of evaporation of water or other liquids from a wetted porous material imder diffusion-controlled conditions was shown to depend on the size of pores of the material [193]. This fact was interpreted in terms of the dependence of the equilibrium vapor pressure above the sample on the radius of the meniscus of the liquid in the pores. This technique is sensitive and relatively simple. However, estimation of the pore radius from the rate of evaporation is again based on the use of the Kelvin equation in a wide range of relative pressures and, therefore, inherits all the uncertainties of the technique and incorporates some additional assumptions. This technique reveals two maxima in the pore size distribution of the water-wetted hypercrosslinked material prepared from a styrene-0.3%... 

FIGURE 16.2 Variation of drying rate and temperature of a wet porous material during drying. 

FIGURE 9.21. (a) Meniscus alongside a wet porous material (b) drop resting on a wet porous surface. 

Mass transfer in the drying of a wet porous material will depend on two mechanisms movement of moisture within the porous material which will be a function of the internal physical nature of the solid and its moisture content and the movement of water vapor from the material surface as a result of water vapor from the material surface as a result of external conditions of temperature, air humidity and flow, area of exposed surface and supernatant pressure. 

Both Wei et al and Perkin l have attempted to model the situation arising from the use of microwaves. Wei and colleagues first developed a mathematical model to explain the heat and mass transfer phenomena occurring in convectionally heated wet, porous materials water-filled sandstone... 

Wetting is an absolute condition for detergency. However, wetting plays an important role in other applications as well. A special case is the penetration of fluids in porous material. That may be a bundle of fibers in the dying process or the stone matrix in enhanced oil recovery. One of the steps of lubrication is wetting of surfaces by lubricant liquids. Because other conditions must also be considered, the use of phosphorus-containing surfactants is beneficial. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

The effect of an inert material on M was observed in a series of tests on the burning rates of aqueous ethyl alcohol solutions from a wetted porous sphere (62). As is to be... 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

In many applications, powders come into contact with a liquid and we would like to quantify their wetting behavior. The usual way to do this is by the capillary rise method [233,234], In a capillary rise measurement the powder is pressed into a tube of typically 1 cm diameter (Fig. 7.7). This porous material is then treated as a bundle of thin capillaries with a certain effective radius [235-237], In order to measure this effective radius, first a completely wetting liquid is used. Either the speed of the liquid rise is measured (this technique is sometimes referred to as the capillary penetration technique [238]) or the pressure required to keep the liquid out of the porous material, is determined. This backpressure is equal to the... 

Figure 7.7 Capillary rise method to quantify the wetting properties of powders or porous materials.

Porous materials are often analyzed with a mercury porosimeter. With a mercury porosi-meter we can measure the pore distribution of a solid. Thus, we can determine the specific surface area. Mercury is used because of its high surface tension (0.48 N/m) it does not wet... 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be heated to about 290°C (555°F) for several minutes to remove the surfactant. Lower temperatures and longer times are used if the substrate cannot tolerate such a high temperature. In some cases, the impregnated material is calendered or compressed in a mold to compact the PTFE resin and to hold it in place. 

Detailed studies on the effect of surface-area and heat of wetting are not available. However, it is known that the heat of wetting of silica gel, which is a highly porous material, is approximately 30 cal per g (approximately 200 times that of coarse quartz as shown in Table 46). 

The total specific volume, Vtoi, is determined pycnometrically by imbibition of the porous material in mercury at 0.1 MPa pressure. Note that most materials are not wetted by mercury. Hence, mercury will not penetrate into the pores at 0.1 MPa pressure. 

In reality things are not so clear cut. The solid surface is heterogeneous and dirty, and it is usually rough on a molecular level. As a result air can get trapped when the stmcture fills up and liquid can be retained when it is drained. Nevertheless it is usually quite clear whether a porous material wets (such as with kitchen tissue) or does not (such as with a Goretex raincoat). 

At present, many authors I20-I26 follow another concept From the plot of pj p [M /t) versus the surface tension of the liquids, the geometric factor K is calculated for those liquids that should wet the solid completely. By inserting this K value and [t]/p- ] M2/t for these liquids into Eq. (13). their contact angles 0 are calculated and used for the interpretation of the solid-vapor surface tension of the porous material. This procedure is dubious, because it can be expected that the contact angles, calculated from the Washburn equation, are affected by roughness and porosity. If we apply this procedure to the PTFE powder for hexadecane, a contact tingle 0 = 88 would be obtained. However, it is well known that the contact angle of hexadecane on a flat and smooth... 

Carman s Gas Permeability Method. A gas or a wetting liquid is made to flow through the porous material in a tube by applying vacuum or pressure. The pressure drop or flow rate is measured. For pigments, a modified procedure is used in which mainly nonlaminar flow takes place [1.16]. For standards, see Table 1.1 ( Specific surface Permeability techniques ). 

Applications of ultrasonic techniques to solid-gas systems rely on the fact that velocity and attenuation of US-waves in porous materials is closely related to pore size, porosity, tortuosity, permeability and flux resistivity. Thus, the flux resistivity of acoustic absorbents oan be related to US attenuation [118,119], while the velocity of slow longitudinal US is related to pore tortuosity and diffusion, and transport properties, of other porous materials [120]. Ultrasound attenuation is very sensitive to the presence of an external agent suoh as moisture in the pore space [121] and has been used to monitor wetting and drying prooesses [122] on the other hand, US velocity has been used to measure the elastic coefficients of different types of paper and correlate them with properties such as tensile breaking strength, compressive strength, etc. [123]. 

This quantity is best defined in terms of how it is measured. A porous material like cloth or cotton is soaked in water and wrapped around the bulb of a thermometer to form a wick. and the thermometer is placed in a stream of flowing air, as in the figure shown below. Evaporation of water from the wick into the flowing air is accompanied by a transfer of heat from the bulb, which in turn causes a drop in the bulb temperature and hence in the thermometer reading.Provided that the wick remains moist, the bulb temperature falls to a certain value and remains there. The final temperature reading is the wet-bulb temperature of the air flowing past the wick. 

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