Mechanical entrapment of polymer

If this physical picture of polymer mechanical entrapment is correct then there are several consequences that might be expected. Firstly, the concentration in the effluent of the core would either fail to reach full input concentration or would only do so after many pore volumes (pv) of polymer fluid throughput. The latter situation would be the case where there were a small number of entrapment sites which had been fully blocked, thus causing 

Dominguez and Willhite (1977) studied the retention and flow characteristics of an HPAM sample (Pusher 700, Dow Chemical Co.) in an 86 mD core of compacted Teflon powder. Static measurements indicated that the level of polymer adsorption on the Teflon was negligible ( 1 jUg/g limited 

Cohen and Christ (unpublished) presented a new experimental technique for determining mobility reduction resulting from polymer retentions in porous media. Their method was designed to separate the contributions of adsorption and non-adsorptive retention to be measured in their flow experiments using HPAM. This was done by using a silane treatment of the silica in their sandpacks, which changed the surface such that it no longer adsorbs HPAM. 

Dominguez and Willhite (1977) also noted a flow rate dependence of polymer retention in their floods, but this effect is normally associated with the phenomenon of hydrodynamic retention, which is discussed further in the following section. 

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There are two mechanisms which cause the retention of some fraction of a PHPA polymer injected into the porous medium (1) the adsorption of polymer molecules on the pore walls, and (2) the mechanical entrapment of polymer molecules (see Figure 2). 

The purpose of this study was to develop answers for the above questions. It will be shown that in low permeability silica sands the mechanical entrapment of polymer molecules is more dominant than physical adsorption. Physical adsorption is more important far from the injection face, particularly during the time when the porous rock is in contact with a polymer solution. 

The greater retention values at residual oil saturation show a greater amount of mechanical entrapment of polymer molecules. At residual oil saturation, oil globules... 

Table 5.3 summarizes some polymer retention data from displacement experiments for partially hydrolyzed polyacrylamides. Retention varies from 35 to about 1,000 Ibm/acre-ft over a wide range of fluid and rock properties. Information on converting retention values from pounds per acre-foot to micrograms per gram of rock is given as a footnote in Table 5.3. Several trends are present in the limited amount of retention data in the literature. Fig. 5.22 shows the variation of polymer retention with brine permeability at ROS. The retention at low permeabilities is large and is probably a result of excessive mechanical entrapment of polymer molecules in small pores. Another possible explanation is high clay content. Polymer concentration appears to have little effect on retention for the data shown in Fig. 5.22. The weak concentration dependence in Fig. 5.22 is reinforced by data from Shah for the retention of partially hydrolyzed polyacrylamide on Berea core material shown in Fig. 5.23. Retention at 50 ppm polymer concentration is 77% of the retention at 1,070 ppm.

C. Instantaneous and irreversible adsorption (including mechanical entrapment) of polymer. 

Polymers are retained in the porous media also by the entrapment of the macromolecules mechanically at the small pores and pore constrictions. As the first polymer molecule enters the pore, it does not necessarily plug the pore, but it can be adsorbed on the walls of a pore constriction. Another newly arrived polymer molecule may entangle with the previously adsorbed or entrapped polymer molecule. Other incoming new polymer molecules may also accumulate on or around the previous ones. Eventually, there will be enough polymer accumulation to plug the pore, and there will not be any outflow of polymer molecules from this pore. At this moment, the mechanical entrapment of the polymer molecules is completed and the flow is diverted into other available flow paths. ... 

An attempt will be made to clearly separate the two basic retention phenomena mechanical entrapment of molecules and physical adsorptioa Factors influencing these retention mechanisms will be discussed in detail The resultant distribution of retained polymer in porous media will be given. 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

Sometimes much greater amounts of solvent can be immobilised by mechanical entrapment within particle aggregates. This occurs when voluminous flocculent hydroxide precipitates are formed. In solutions of long thread-like molecules the polymer chains may cross-link, chemically or physically, and/or become mechanically entangled to such an extent that a continuous three-dimensional network is formed. If all of the solvent becomes mechanically trapped and immobilised within this network, the system as a whole takes on a solid appearance and is called a gel. 

These correspond, respectively, to polymer or electrolyte entrapped within surface features, the polymer film, and the solution. The first of these is a minor effect when using polished crystals the surface mechanical impedance of this contribution is Z, = wp where j = V-l, o> = 2nf0, and p is the areal mass density of the entrapped material. For finite and semiinfinite viscoelastic layers, the surface mechanical impedance is given by Z, = (GPf)m and Zv = (Gpf)1/2 tanh(y/i/), respectively, where prf and hf are the film density and thickness and y = /w(p/G)l/2. For the solution, Zs = (tapsT]J2)m (1 + j), where p, and tj, are the density and viscosity of the solution. When rigid mass, finite viscoelastic film and semi-infinite liquid loadings are all present, as in the experiment of Fig. 13.7, one can show that [42] ... 

Phase Composition and Simultaneous Polymerization. Theoretically the phase composition of the SIN s should not be determined by the true solubility of one polymer in the other. Even though the true solubility of one polymer in the other is low because the components of the SIN s are incompatible, simultaneous polymerization and gelation are expected to cause entrapment of one component in the other. The degree of entrapment presumably will be controlled by the relative rates of the two reactions and their degree of simultaneity. The phase composition is reflected in the glass transition behavior of the material. Thus a close look at the dynamic mechanical spectra of the SIN s is necessary to determine the effect of simultaneous polymerization on phase composition. 

Mechanical whipping of gases (frothing) into a polymer system (melt, solution or suspension) which hardens, either by catalytic action or heat, or both, thus entrapping the gas bubbles in the polymer matrix. 

For the preceding three mechanisms of polymer retention, mechanical entrapment can be avoided by prefiltering or preshearing the polymer or by applying the polymer in a high permeability formation. Hydrodynamic retention is probably not a large contributor in the total retention and can be neglected... 

Mechanical properties and control of particle size are the main drawbacks of CLEAs. Particles are compressible and shear sensitive and size is usually small so that recovery of the biocatalyst may pose a problem for conventional reactor configurations (see Chapter 5). To solve that problem, basket-type bioreactors can be used or else the biocatalyst can be modified. An interesting approach is the encapsulation of CLEAs within polymer gels, as shown in Fig. 4.4c. Entrapment of CLEAs within polyvinyl alcohol lens-shaped gel particles (LentiKats) produced very robust biocatalysts of a convenient size to be easily recovered (Wilson et al. 2004c). 

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