Xanthan shear thinning

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

The technological importance of xanthan gum rests principally on its unusual and distinctive properties25 28 29 49,116,251,257-260 in aqueous solution. Some of these properties are (1) remarkable emulsion-stabilizing and particle-suspension ability, (2) low concentrations yield high viscosities, (3) recoverable shear-thinning (extremely large shear dependence of viscosity), (4) little variation in viscosity with temperature under normal conditions of industrial utilization, and (5) gel formation when mixed with certain other, nongelling polysaccharides. 

The effective oral shear rates at which the objective viscosity was equal to the equivalent Newtonian viscosity, calculated from the above equation were in good agreement with the results of Shama and Sherman (1973) (Figure 7-7) (Cutler et al., 1983) the only exceptions to this observation were the highly shear-thinning xanthan gum dispersions that deviated slightly from the Shama and Sherman s results. 

Xanthan forms the most pseudoplastic (instantaneous, reversible shear thinning) solutions of all the gums. This property is due to the stiffness of its molecules and/or intermolecular associations of two or more molecules. In plots of viscosity vs. concentration, there is a Newtonian (non-pseudoplastic) plateau at very low shear rates, which at least, makes its solutions appear to have a yield value (a yield value being the force required to initiate flow). As a result, xanthan is an excellent stabilizer for suspensions and emulsions. 

Xanthan is a high-molecular-weight polysaccharide of widespread utility as a viscosity-increasing additive for water. Aqueous xanthan solutions are markedly shear-thinning (non-Newtonian) the viscosity of a 1 mg/ml solution changes from 700 cP to less than 10 cP as the shear stress varies from 0.01 to 20 dyne cm"2. 

Laboratory testing shows that visual examination and viscosity measurements are not sufficient to fully define polymer solvation. In this work, the solvation of hydroxyethyl cellulose (HEC) and xanthan has been studied. These polymers are both widely used in various petroleum applications. HEC is used in many workover and completion applications, while xanthan has its most wide spread uses in drilling and enhanced oil recovery (EOR) applications. Solublization of both polymers results in fluids with pseudoplastic (or shear thinning properties). Even though the polymers both exhibit pseudoplastic behivior, the polymers vary considerably as to their molecular size and physical properties. 

A cursory inspection of Table 1 shows that in most instances polymer solutions (carboxymethyl cellulose, polyacrylamide, xanthan) have been used to mimic the non-Newtonian features of biological systems, encompassing wide ranges of shear thinning conditions (though viscoelastic effect have been studied only scantily) in bubble columns up to as large as 760mm in diameter. 

Fig. 5.10. Intrinsic viscosity [q] determined at high shear rates Y with a capillary viscosimeter and at lower shear rates with a Zimm-Crothers viscosimeter for different xanthan gums in 0.1 mol/l sodium chloride (NaCI) solution at 25 C. Data from [93]. For strongly shear thinning polymer solutions, only low shear viscosimeters reach the shear rate independent viscosity region...

The sedimentation of particles in non-Newtonian fluids, such as aqueous solutions containing high molecular weight compounds (e.g. hydroxyethyl cellulose or xanthan gum), is not simple since these non-Newtonian solutions are shear thinning with the viscosity decreasing with increase in shear rate. As discussed above, these solutions show a Newtonian region at low shear rates or shear stresses, usually referred to as the residual or zero shear viscosity /(0). 

It was noted earlier in this chapter that the flow of both polyacrylamide and xanthan solutions in a capillary is similar in character, i.e. it is shear thinning... 

The discussion in this section is confined to the behaviour of inelastic, shear thinning fluids in porous media. Xanthan biopolymer is taken as the main example, and virtually all of the papers on the flow of this polymer through porous media are related to its importance in oil recovery. Work on the in-situ rheology of xanthan has been reported for flow through sandstone cores, sandpacks, bead packs and other unconsolidated material. 

Whilst the fluid rheology of xanthan changes from Newtonian to shear thinning with increasing flow rates, polyacrylamide additionally exhibits elastic properties at high flow rates. This has been shown schematically by Heemskerk et al (1984), as illustrated in Figure 6.8. Chapter 3 discussed the fact that flexible coil polymers, such as HPAM, show viscoelastic and extensional viscosity effects, and it is the intention of this section to review the behaviour observed in porous media flow as a result of these properties. 

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