Mercury porosimetry method development

Lindstrom and Boersma (1971) pioneered the prediction of breakthrough curves from equivalent cylindrical pore size distributions, determined by either the water retention or mercury porosimetry methods. The model developed by these authors includes the effects of bothintra- and interpore dispersion. In general, dispersion due to differences in tube size has a much greater influence on the shape and position of the breakthrough curve than mixing within tubes due to microscopic velocity profiles (Rao et al., 1976). For completeness, however, it is preferable to include both effects. Lindstrom and Boersma (1971) defined a CDE for each tube, so that C/C0 for the bundle as a whole is given by ... 

The technique of mercury porosimetry consists essentially in measuring the extent of mercury penetration into an evacuated solid as a function of the applied hydrostatic pressure. The full scope of the method first became apparent in 1945 when Ritter and Drake developed a technique for ... 

The incorporation of the new material without any increase in the overall length of the book has been achieved in part by extensive re-writing, with the compression of earlier material, and in part by restricting the scope to the physical adsorption of gases (apart from a section on mercury porosimetry). The topics of chemisorption and adsorption from solution, both of which were dealt with in some detail in the first edition, have been omitted chemisorption processes are obviously dependent on the chemical nature of the surface and therefore cannot be relied upon for the determination of the total surface area and methods based on adsorption from solution have not been developed, as was once hoped, into routine procedures for surface area determination. Likewise omitted, on grounds of... 

Surface Area and Permeability or Porosity. Gas or solute adsorption is typicaUy used to evaluate surface area (74,75), and mercury porosimetry is used, ia coajuactioa with at least oae other particle-size analysis, eg, electron microscopy, to assess permeabUity (76). Experimental techniques and theoretical models have been developed to elucidate the nature and quantity of pores (74,77). These iaclude the kinetic approach to gas adsorptioa of Bmaauer, Emmett, and TeUer (78), known as the BET method and which is based on Langmuir s adsorption model (79), the potential theory of Polanyi (25,80) for gas adsorption, the experimental aspects of solute adsorption (25,81), and the principles of mercury porosimetry, based on the Young-Duprn expression (24,25). 

We found the latter factor-voids to be important. Experimental results showed that when green coke was calcined under the new methods, and the derived calcined coke was observed by scanning electron microscopy (Figure 2) and its pore size distribution was measured by mercury porosimetry (Figure 3), microcracks of significant sizes (1 to 60 microns) were developed. This was an important contribution to the reduction of the thermal expansion coefficients of the calcined coke processed under the new method. 

The method of mercury porosimetry requires evacuation of the sample and subsequent pressurization to force mercury into the pores 49). This technique was originally developed to enable pore sizes to be determined in the macropore range, where the gas adsorption method breaks down for practical reasons (6). Application of mercury porosimetry is based on the Washburn equation 62,63),... 

While mercury porosimetry still ranks among the most commonly used methods for the determination of pore structure, volume, size, and related data, concerns associated with the presence of elemental mercury, although well contained, triggered the development of alternative equipment. For example, Fig. 11.11 shows different... 

A low melting point alloy (LMPA) intrusion technique, exactly similar in principle to the well-established mercury porosimetry technique, has been developed as the basis for a new method of visualised characterisation of catalyst pore structure. 

In recent years it has been recognized that a more accurate method of pore analysis should consist of an appropriate combination of techniques of which mercury porosimetry is but one of the components (refs. 18,30,31). First, serial sectioning analysis of pore casts (refs. 32-35) can be used to determine the chamber-size distribution, the correlation between the sizes of adjacent chambers, and information pertaining to the interconnectivity of the network (e.g. specific genus and coordination number). Then, the capillary pressure curves can be used to determine the throat-size distribution, and the correlation between the sizes of contiguous throats and chambers. In order to deconvolve these curves a reliable simulator of intrusion and retraction of mercury in evacuated chamber-and-throat networks must be developed. 

As new membranes are developed, methods for characterization of these new materials are needed. Sarada et al. (34) describe techniques for measuring the thickness of and characterizing the structure of thin microporous polypropylene films commonly used as liquid membrane supports. Methods for measuring pore size distribution, porosity, and pore shape were reviewed. The authors employed transmission and scanning electron microscopy to map the three-dimensional pore structure of polypropylene films produced by stretching extended polypropylene. Although Sarada et al. discuss only the application of these characterization techniques to polypropylene membranes, the methods could be extended to other microporous polymer films. Chaiko and Osseo-Asare (25) describe the measurement of pore size distributions for microporous polypropylene liquid membrane supports using mercury intrusion porosimetry. 

Out of other physical methods used for porosity analysis, mercury intrusion porosimetry is worth mentioning. This method can be tried at all levels of electrode development, i.e. from catalyst to matrix stage. Testing of a matrix is done at low pressures (upto 400 kPa) so that the soft structure of the matrix is not deformed. Fig. 11 shows a typical result of PAFC cathode developed by the author s laboratory. The matrix is also characterized by other simple tests, like water loading, rate of water migration when dry matrix is dipped in one end etc. (Caires et al., 1997). 

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