Adsorption of xanthan

The adsorption of xanthan is increased by calcium but to a less extent than that for HP AM. The increased adsorption seems to be due mainly to the effects of screening of polymer and surface charges by calcium, and the maximum adsorption density is equivalent to that induced by monovalent ions. 

Xanthates have been used for flotation of lead and copper. In these examples, it is the adsorption of xanthane that dominates the flotation. 

The influence of calcium on the adsorption of high molecular weight EOR polymers such as flexible polyacrylamides and semi-rigid xanthans on siliceous minerals and kaolinite has been studied in the presence of different sodium concentrations. Three mechanisms explain the increase in polyacrylamide adsorption upon addition of calcium (i) reduction in electrostatic repulsion by charge screening,... 

The effects of calcium on polymer-solvent and polymer-surface interactions are dependent on polymer ionicity a maximum intrinsic viscosity and a minimum adsorption density as a function of polymer ionicity are obtained. For xanthan, on the other hand, no influence of specific polymer-calcium interaction is detected either on solution or on adsorption properties, and the increase in adsorption due to calcium addition is mainly due to reduction in electrostatic repulsion. The maximum adsorption density of xanthan is also found to be independent of the nature of the adsorbent surface, and the value is close to that calculated for a closely-packed monolayer of aligned molecules. 

This study aims at determining the effects of calcium on the adsorption of polyacrylamides and xanthans on siliceous minerals and kaolinite. 

Adsorption on Siliceous Minerals. All adsorption studies of xanthan (XCPS) in the presence of calcium are conducted at pH 6.5 to avoid precipitation which has been reported at pH>7 for xanthan solutions containing calcium (25). 

The adsorption results of both xanthan samples on sand at pH 6.5 in 20g/i NaCl are shown in Figure 12. The presence of calcium is seen to increase the adsorption of both xanthan samples and a maximum in adsorption is reached at high calcium concentrations. These variations are very similar to those observed for XCPS adsorption in the presence of NaCl (26). 

Adsorption on Kaolinite. As for polyacrylamides, adsorption of XCPS on kaolinite is conducted as a function of S/L and the results extrapolated to S/L=0. However, the S/L dependence of XCPS adsorption on kaolinite is considerably less than that for HPAM. This is due to the flat conformation of the adsorbed molecules of semirigid xanthan (25) compared to the more extended conformation of flexible HPAM (27). The absence of loops and tails in the adsorbed XCPS layer thus diminishes the probability of flocculation of particles by polymer bridging. The slight dependence in adsorption on S/L may therefore be attributed to coagulation of particles induced by Ca. ... 

The maximum adsorption density of semi-rigid xanthan is not very sensitive to the nature of the adsorbent surface provided that the surface has a homogeneous adsorption site density. This maximum level is close to the value calculated for a closely-packed monolayer of xanthan molecules. 

Table 7.1 shows that rather similar results were also found by Makri et al. (2005) for samples of coarse emulsions containing thermodynamically incompatible mixtures of legume seed protein + xanthan gum. The protein surface load was found to be enhanced in the presence of xanthan gum, especially at elevated ionic strengths. That is, there was observed to be an increase in the adsorption of legume seed proteins at the surface of the emulsion droplets which could be attributed to an increase in the thermodynamic activity of the proteins in the system in the presence of the incompatible polysaccharide (see Table 7.1). Associated with the greater extent of protein adsorption, the authors reported an enhancement in the emulsion stability.

These experiments show that it is possible to achieve positive results using EOR after a thorough investigation of the nature of mineral rock constituents of the oil reservoir and the choice of the surfactant delivery method. The dynamic interfacial tension is crucial in EOR. Using a model acidic oil, alkali solutions and surfactants at an optimum ratio, ionised water and surfactant adsorb simultaneous onto the interface, resulting in low dynamic interfacial tension [229]. Combined adsorption of surfactant (alkyl propoxyethoxy sulphate) and polymer (xanthan) was studied in [230]. 

A later study [66] focused on the nonequilibrium adsorption of C9-Ph-(E0)e-S03Na, 88 mol% sulfonate and 12 mol% unconverted nonionic surfactant, with a polymer, xanthan, onto oil-containing sandstone cores from the North Sea. Addition of the polymer reduced the surfactant adsorption by 80% relative to adsorption without xanthan, yet there was no complex formation between the surfactant and the xanthan. This study reflects one of the current trends of using systems containing surfactant-polymer mixtures and emphasizes the need for system specific adsorption studies in EOR applications. 

A more recent study [70] examined the effects of the polymer on surfactant adsorption in a low tension polymer water flood (LTPWF). The surfactant was alkylpropoxyethoxy sulfate, Ci2-i5-(PO)4-(EO)2-0S03 Na, and the polymers were xanthan and a copolymer of acrylamide and sodium 2-acrylamido-2-methylpropane sulfonate (AN 125 from Floerger). The solid materials were sandstone cores from a North Sea oil reservoir, Berea, and Bentheim cores. For these systems the xanthan caused a 20% reduction in the adsorption of the surfactant. It was also observed that surfactant adsorption appeared to increase as the water... 

In the following two subsections, some of the main features of the adsorption of HPAM and xanthan in porous media will be outlined. It is convenient to separate the discussion of these two polymer types because of their structural differences, as explained earlier in this work. HPAM is a flexible coil polyelectrolyte and, for the reasons discussed previously, shows much more sensitivity to solution conditions, pH, salinity, etc., than xanthan, which has a more rigid molecular structure. 

Most of the experimental data on polymer adsorption that have been published in the oil literature refer to HPAM, although a smaller amount of data is available on xanthan adsorption (e.g. Dawson and Lantz, 1972 Maerker, 1973 Sandvik and Maerker, 1977 Willhite and Dominguez, 1977 Teew and Hesselink, 1980 Lecourtier and Chauveteau, 1985 Zaitoun and Kohler, 1987a Chauveteau and Lecourtier, 1988 Kolodziej, 1988). Broadly, the finding is that xanthan adsorption in porous media is rather less than that of HPAM and also tends to show less sensitivity to the salinity/hardness conditions of the solvent. However, many of the early reported values of xanthan adsorption (possible up to the mid or late 1970s) may be somewhat suspect because of the quality of the powder biopolymer products that were available at that time. For this reason, this section will concentrate on more recent studies of xanthan adsorption in porous media. 

In a study of the transport of xanthan in porous media, Kolodziej (1988) measured xanthan adsorption in dynamic core floods both at 100% water saturation and at residual oil in Berea cores. The isotherm which he derived is shown in Figure 5.20 (Kolodziej, 1988) and indicates adsorption levels of 751b/AF at 100% brine saturation and 381b/AF at residual oil. These... 

Studies of adsorption of polyacrylamides and xanthans onto kaolinite have appeared recently (Rahbari etal, 1990 Pefferkorn etal, 1990). The adsorption of PAM is high on lateral faces of the kaolinite ( 3500/ig/m ) which is attributed to hydrogen bonding between the carboxyl groups on the polymer and the surface aluminols. However, adsorption of PAM is low on the hydroxide aluminium basal face ( 500/xg/m ) and zero on the siloxan basal face. In contrast, these workers have found that xanthan adsorbs only on... 

The behavior is attributed to the filtration of gel particles that formed during the gelation reaction. Adsorption of polymer and reaction between adsorbed polymer andCr(II])/polymer in the flowing stream also contributed to the development of the resistance. The location of the point of buildup of resistance has been shown to be a fimetion of flow rate and shear in the porous medium. Similar behavior was observed for xanthan/Cr(III) gels. dlS... 

Littmann, W., Kleinitz, W., Christensen, B.E., Stokke, B.T., and Haugvallstad, T. 1992. Late Results of a Polymer Pilot Test Performance, Simulation Adsorption, and Xanthan Stability in the Reservoir. Paper SPE 24120 presented in the SPE/DPE Enhanced Pil Recovery Symposium, Tulsa, 22-24 April. DPI 10.2118/24120-MS. 

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