Mercury intrusion porosimetry porous structure

We now consider application of percolation theory to describing mercury intrusion into porous solids. First we briefly recall the main physical principles of mercury porosimetry (in particular, the Washburn equation). These principles are treated in detail in many textbooks [e.g., Lowell and Shields 49)]. The following discussions (Sections IV,B and IV,C) introduce general equations describing mercury penetration and demonstrate the effect of various factors characterizing the pore structure on this process. Mercury extrusion from porous solids is briefly discussed in Section IV,D. 

The support is a microporous PVC-silica sheet having a porosity in the 70-80% range. The pore size as determined by mercury intrusion porosimetry is in the 0.2 u to 2.0 urn range. The support is extremely hydrophilic, has a negative charge, and a surface area of 80 m /g. The material is non-compressible under normal conditions, is steam sterilizable, and has a low dry density of 0.45 g/cm. The microporous support has received FDA approval for direct food contact. The tortuosity of the pore structure requires that the substrate make intimate contact with the active enzyme as it passes through the support material. The active sites are attributed to the silica contained within the porous matrix which allows the addition of organic functionality. 

Pore size and pore size distribution. Particularly two methods have been used to determine the pore size in adsorbing materials of both organic and inorganic nature, namely, the gas adsorption technique and the mercury intrusion porosimetry. On the basis of information provided by these methods, a number of serious conclusions have been drawn on the porous structure of macroporous styrene-DVB copolymers. This necessitates a more critical analysis of the possible errors in the interpretation of the results of measuring adsorption isotherms and mercury intrusion. 

Figure 9.7. In mercury intrusion porosimetry, the volume of mercury entering the pores of a body at various applied pressures can be interpreted generally as data on the size distribution of the pores within a porous body. However the presence of large pores with small access throats can lead to misinterpretation of the true structure of the porous body.

Few techniques are able to characterise the complex pore structure of hydrated cementitious materials. One of the most used techniques is mercury intrusion porosimetry (MIP). This technique is based on the intrusion of a nonwetting fluid (mercury) into porous structures under increasing pressure. This simple principle often makes users forget about the underlying assumptions and the limitations of the MIP technique. 

The pressures involved in porosimetry are so high (e.g. 1000 atm = 6-6 ton in" ) that the question as to whether the pore structure is damaged by mercury intrusion naturally arises. This possibility was recognized by Drake, but as a result of several intrusion-extrusion runs at pressures up to 4000 atm on a number of porous catalysts Drake concluded that any deformation caused by compression was elastic and therefore not permanent. 

Several of the more popular models for deactivation involve "pore mouth plugging" wherein the transport limiting constrictions within the pore network are selectively reduced in dimension. If one realizes that intrusion mercury porosimetry and desorption measurements specifically characterize the constriction ("throat") dimensions then decreases in these dimensions would be greater than the changes found in the retraction porosimetry or in the desorption (which measure the opening dimensions). To understand the changes in network structure on the deactivation process it seems necessary to measure and analyze each aspect of the porous structure. 

Mercury intrusion. Mercury porosimetry represents another popular method for the characterization of porous structure. Its use is fully justified for rigid inorganic adsorbents, but may lead to artifacts with relatively soft... 

The best known method for investigating the porous structure of different materials is the method of mercury porosimetry (MMP) (Drake, 1949). This method is based on intrusion of mercury into samples of porous material under high pressure. When an external pressure P is applied, all pores with radii r > rmin become filled with mercury. The value of rmin corresponds to the condition that the mercury capillary pressure in the pore is equal to the applied pressure P. The capillary pressure is determined by the thermodynamic Laplace equation ... 

Many commercially important processes involve the transport of fluids through porous media and the displacement of one fluid, already in the media, by another. The role play by pore stnicture is of fundamental importance, and its size distribution determination necess. in order to obtain an understanding of the processes. The quality of powder compacts is also affected by the void size distribution between the constituent particles. For these reasons mercury porosimetry has long been used as an experimental technique for the characterization of pore and void structure. Although quantitative information is contained in mercury intrusion - extrusion curves it can only be elucidated fiilly by the use of a theoretical model for pore structure. 

Kloubek [S3] considers the concept of pore dimension to be erroneous because of the above errors and recommended that the results be presented using the actual values of p instead of calculated radii. He suggested that the dependence of net re-intrusion and retention volumes on mercury pressure should be evaluated [54], In this way pores can be separate into two groups, one in which mercury is retained reversibly and the other where retention is irreversible. This method of mercury porosimetry evaluation offers a valuable contribution to understating porous structures and their properties. 

Being able to identify the necessary properties to define a porous medium in no way implies that there are analytical methods to evaluate them. Using porosimetry and sorption, it is possible to measure 6 properties of the porous medium structure (refe. 5-7). It is not clear that these measures are independent. These properties are obtained by analyzing both mercury intrusion and retraction profiles (i.e., the nature of the hysteresis commonly found with most porous materials). These properties are enumerated below ... 

The use of LMPA intrusion along with visual analysis techniques, such as serial sectioning or microtomography, can provide a clear insight into the structural characteristics of porous media such as catalyst supports. Without the use of these visual analysis techniques, relying solely on mercury porosimetry and gas adsorption to derive a psd and subsequent diffiision-reaction models, a major error could be made, if the structure of the media is too spatially complex and non-uniformly variable. 

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