Inaccessible pore spaces

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... 

Although we have measured and discussed effects of adsorption and inaccessible pore volume separately, we have no evidence that the two phenomena are actually unrelated physically. Indeed, it seems reasonable that much of the inaccessible pore space is in regions filled or blocked by adsorbed polymer molecules. 

Encouragingly, the adsorption-based m n coordination is inline with die mean number of pores that actually emanate from the pore junctions as determined by direct analysis of the pore space. However, analysis of where the SFs molecules diSuse during non-equilibrium MD simulations show that whilst not aU die pore throats are op i to the fluid, they do have access to all the pores this is refl ited in the dynamic coordination number for the pore network defined by SF at 2S6K of 3.6. Thus, the assumpticm that urtoerpms the method of Ldpez-Ramdn et al. [11] - i.e. that the differetice betwerai the PSIHt of die two molecules is due to some pores being inaccessible to the latter because of network - is not met here. 

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 ... 

Figure 7.10. Schematic two-region model of the pore space showing accessible and inaccessible regions for the polymer molecules.

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. 

The pore volume used in calculating M , in equation 17 includes the entire pore space in the slab. For equation 13 to be valid for porous slabs with inaccessible porosity, the definition of M, can be modified by substituting a new value for the slab porosity. The fraction of total pores which are accessible to the external environment is denoted < )a and is defined ... 

The total pore volume, Vp, sometimes called specific pore volume when referred to unit mass, is the total internal volume per unit mass of catalysts. Some of this pore volume may be completely enclosed and thus inaccessible to molecules participating in a catalytic reaction. The total accessible pore volume is often derived from the amount of vapour adsorbed at a relative pressure close to unity, by assuming that the pores are then filled with liquid adsorptive. The accessible pore volume may be different for molecules of different sizes. It may be useful to determine the dead space by means of a nonsorbable gas (normally helium) in conjuction with the determination of the bulk volume of the catalyst by means of a non-wetting liquid (mercury). 

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