Media inaccessible pore volume

When polymer molecular sizes are larger than some pores in a porous medium, the polymer molecules cannot flow through those pores. The volume of those pores that cannot be accessed by polymer molecules is called inaccessible pore volume (IPV). In an aqueous polymer solution with tracer, the polymer molecules will run faster than the tracer because they flow only through the pores that are larger than their sizes. This results in earlier polymer breakthrough in the effluent end. On the other hand, because of polymer retention, the polymer breakthrough is delayed. In other words, if only polymer retention is considered, the polymer will arrive in the effluent later than the tracer. 

Before casting Equation 7.1 in dimensionless form, the inclusion of terms to describe adsorption and velocity enhancement of the transported species will be considered. These phenomena are, of course, more appropriate to polymer transport than tracer transport but the form of the equation is very similar. The velocity enhancement referred to concerns the effect of the excluded volume or inaccessible pore volume effect which the polymer shows (Chauveteau, 1982, Dawson and Lantz, 1972) and which is discussed in more detail below. However, the physical observation on polymer transport is that there appears to be a fraction of the pore space—either the very small pores (Dawson and Lantz, 1972) or regions close to the wall of the porous medium (Chauveteau, 1982)—which is inaccessible to polymer transport. This leads to an enhancement of the average velocity of the polymer through the porous medium. When there is both adsorption of transported polymer onto the rock matrix and a fraction of the pore volume is apparently inaccessible to the polymer, Equation 7.1 must be extended to ... 

A more qualitative explanation for the acceleration of the polymer front by inaccessible pore volume may be given as follows. Consider for a moment the movement of the polymer front with respect to the accessible pore volume only. Besides this accessible polymer pore volume, there is a certain volume of water in the porous medium that we have called the inaccessible pore volume and that communicates with the accessible pore volume by diffusion (and perhaps by convection as well). The movement of polymer concentration fronts through the accessible pore volume appears perfectly normal these fronts emerge from the core after the injection of one accessible pore volume (except for effects of polymer adsorption). However, salt fronts are delayed by transfer of salt into the water located in the inaccessible pore spaces. This transfer, even if by... 

The above discussion shows how the presence of inaccessible pore volume causes salt peaks to move through a porous medium more slowly than polymer peaks. Only one final point remains to be made — the connected pore volume of a core, measured by saturation starting from an evacuated condition, is not just the polymer pore volume it is the total pore volume occupied by water, including that fraction inaccessible to polymer. With this as the pore volume, then the salt front velocity is the true interstitial velocity and the polymer moves faster its velocity is greater because it does not enter the inaccessible pore volume. 

Dawson and Lantz discovered that solutions of partially hydrolyzed polyacrylamide do not flow through all the pore volume in a porous medium and introduced the concept of the inaccessible pore volume. Inaccessible pore volume may consist of pores that are too small to permit entry of polymer molecules and pores plugged by polymer molecules, as well as the hydrodynamic volume occupied by polymer adsorped on the surface of the porous medium or retained in the pore space. A relationship between polymer retention and inaccessible pore volume has not been established. 

All these studies have shown that high-raolecular-weight, partially hydrolyzed polyacrylamides are retained during flow through porous media by combined mechanisms of adsorption and mechanical entrapment. The contribution of each mechanism to the resistance factor, residual resistance factor, and inaccessible pore volume of the polymer solution has not been clearly demonstrated since all experimental investigations were carried out in porous media with finite adsorption characteristics. The purpose of this study was to characterize the entrapment and flow properties of partially hydrolyzed polyacrylamide in a porous medium on which adsorption was considered to be negligible. 

In catalytic systems morphological changes of the pore structure, brought upon by the reaction and sorption processes, typically result in a reduction of the available pore volume. In some instances the internal pore structure is eventually blocked and becomes completely inaccessible to transport and/or reaction. In the field of noncatalytic fluid-solid reactions and acid rock dissolution, on the other hand, the chemical reaction consumes the solid matrix of the porous medium leading eventually to fragmentation and... 

Density data were used to develop a new concept of inaccessible pores which were to include closed pores and pores accessible to helium at 303 K but not to rtitrogen at 77 K. Surface areas as determined by SAXS were higher than surface areas determined from the nitrogen isotherm. This latter result is to be anticipated as SAXS measures the areas of interfaces between volume elements of high and low electron density, that is all porosity without relation to accessibility of any adsorbate or density medium which is used. The density data are used to create the description of porosity as shown in Figure 3.7(b) and which includes a volume (V ) of solid carbon. 

Figure 7.1 Migration across a stationary phase packing. Left, illustrative description of separation in SEC by a porous packing according to the size of the pores. The non porous part of the bead, called the backbone, is inaccessible to the sample molecules. Right, a chromatogram displaying the separation of three species (1, 2, 3) of different sizes. The large molecules (excluded) 1 are the first to arrive followed hy medium sised molecules (partial access) 2, and finally by the smallest (full access) 3. The elution volumes are located between Vj for ATsec = 0 and Vm for K ec = ...

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