Aqueous Solutions Of Xanthan Gum Research Articles

Thermal performance of a flat ohmic cell under non-fouling and whey protein fouling conditions

Temperature gradients, between electrode surfaces and bulk, in a continuous flat ohmic cell under whey protein fouling were studied. The temperature profiles in non-fouled cell were studied using two Newtonian fluids (water and an aqueous solution of sucrose at 55 g/100 g) and a pseudoplastic fluid (an aqueous solution of xanthan gum at 0.2 g/100 g). The temperature gradients were studied using two fouling fluids: an aqueous solution of β-lactoglobulin and an aqueous solution of β-lactoglobulin–xanthan gum mixture. Obtained result shows the existence of a temperature difference between electrode surfaces and the bulk when heating non-fouling fluids. The value and the shape of these gradients depend on the Reynolds number and the rheological behavior of the fluid. Under fouling conditions, the temperature gradient obtained at different Reynolds number exhibit a different trend. These differences could be explained by the effect of differential electrical conductivities between the bulk and the deposit, and the balance between heat generation by electrical power dissipation and thermal loss by convection (with the fluid) and conduction (with the electrode surfaces). Significance for the science community and food industry Food industry and in particularly the dairy industry, are faced with a severe problem due to equipment fouling during processing. Therefore, the development of alternative technologies for fouling limitation is of scientist and industrial relevance. Ohmic heating is one of these technologies, where the theoretical volume heating aspect should provide a considerable advantage to limit fouling phenomena. The present study evaluates the capability of a rectangular ohmic unit to provide a homogenous heat treatment of complexes dairy fluid (fluid rheology, flow rate and fouling presence).

Dynamic wetting of shear thinning fluids

The impact of non-Newtonian behavior on dynamic wetting is critical since many fluids exhibit such behavior somewhere in the high-shear environment inherent in the wedge flow near a moving contact line. This impact will be different for two broad categories of non-Newtonian behavior, shear thinning, and elasticity. In this paper, we discuss the steady-state wetting of a fluid, aqueous solutions of xanthan gum, dominated by shear thinning but with negligible elasticity. In the shear thinning fluid, viscous bending near the contact line is greatly reduced compared to a Newtonian fluid having the same zero-shear viscosity. Concomitant with this reduction in viscous bending, the effective dynamic contact angle has a much weaker dependence on capillary number, Ca, than is observed in, or predicted for, Newtonian fluids. A simple lubrication model using a constitutive relation with power-law shear thinning at high shear rates and a Newtonian plateau at low shear rates mimics the trends seen in our data and elucidates the origins of the reduced viscous bending.

AFM studies on gelation mechanism of xanthan gum hydrogels

The effect of annealing on xanthan gum molecules was investigated using atomic force microscopy (AFM). The values of height and width of xanthan gum molecules in AFM images are ca. 1 nm, which strongly indicates that xanthan gum molecules extended on the mica surface are in mono- or double layers. When xanthan gum aqueous solution was annealed, a network structure was observed. In contrast, a network structure was not observed for non-annealed solution. AFM images provide direct information concerning oscillational change of the network structure. It is concluded that xanthan gum molecular chains in aqueous solution aggregate and dissociate in an oscillational manner with increasing annealing time and that a homogeneous network structure was formed by annealing at 40 °C for 24 h.

Thermal and viscoelastic properties of xanthan gum/chitosan complexes in aqueous solutions

The viscoelasticity and thermal properties of aqueous solutions of blended xanthan gum (XA) with chitosan were investigated in order to study the electrostatic interaction established between two polysaccharides. Storage modulus G’, loss modulus G” and zero shear rate viscosity ηO attain a maximum at a chitosan concentration Cmax. The above results indicate that the junction between XA and chitosan is formed in a concentration range lower than Cmax and the viscoelasticity of systems increases with increasing concentration. In a concentration range higher than Cmax, junction formation may not occur effectively since the excess amount of chitosan completely screens anions of XA. The chain rigidity of XA decreases by the screening of the repulsive interaction between anions on XA chains. The ineffective junction formation and the decrease of XA chain rigidity may cause the decrease of viscoelasticity of systems with increasing concentration. The value of Cmax decreases with increasing molecular mass of chitosan. From melting enthalpy of the above system measured by DSC, the amount of non-freezing water (Wnf) was evaluated. Wnf shows a minimum at the concentration Cmax. This fact suggests that hydrophobic fields increased by junction structure formation through ion-complexation between XA and chitosan molecules.

Rheology of concentrated xanthan gum solutions: Steady shear flow behavior

Using a strain-controlled rheometer, the steady shear flow properties of aqueous xanthan gum solutions of different concentrations were measured over a wide range of shear rates. In this article, both the shear rate and concentration dependencies of steady shear flow behavior are reported from the experimentally obtained data. The viscous behavior is quantitatively discussed using a well-known power law type flow equation with a special emphasis on its importance in industrial processing and actual usage. In addition, several inelastic-viscoplastic flow models including a yield stress parameter are employed to make a quantitative evaluation of the steady shear flow behavior, and then the applicability of these models is also examined in detail. Finally, the elastic nature is explained with a brief comment on its practical significance. Main results obtained from this study can be summarized as follows: (1) Concentrated xanthan gum solutions exhibit a finite magnitude of yield stress. This may come from the fact that a large number of hydrogen bonds in the helix structure result in a stable configuration that can show a resistance to flow. (2) Concentrated xanthan gum solutions show a marked non-Newtonian shear-thinning behavior which is well described by a power law flow equation and may be interpreted in terms of the conformational status of the polymer molecules under the influence of shear flow. This rheological feature enhances sensory qualities in food, pharmaceutical, and cosmetic products and guarantees a high degree of mixability, pumpability, and pourability during their processing and/or actual use. (3) The Herschel-Bulkley, Mizrahi-Berk, and Heinz-Casson models are all applicable and have equivalent ability to describe the steady shear flow behavior of concentrated xanthan gum solutions, whereas both the Bingham and Casson models do not give a good applicability. (4) Concentrated xanthan gum solutions exhibit a quite important elastic flow behavior which acts as a significant factor for many industrial applications such as food, pharmaceutical, and cosmetic manufacturing processes.

Rheo-optical investigations and near-wall turbulence structure of polymer solutions in turbulent channel flow

In the present study, the turbulence behavior of polymer solutions is investigated using laser-Doppler velocimetry (LDV). At the same time, an attempt is made to determine a possibly occurring extension and orientation of the macromolecules by flow-induced birefringence (FIB). Flow-dynamic and rheo-optical experiments are presented which were performed in a smooth square channel in the area of the fully developed turbulent flow. The turbulent flow properties of aqueous solutions of PAAM and Xanthan Gum were studied. The Reynolds’ number was 30.000. The concentration of the polymer solutions was varied between 300 and 800 wppm for Praestol 2300 and between 30 and 120 wppm in the case of Xanthan Gum. The experimental data obtained indicate a significant dependence of the turbulent flow properties on the molecular structure of the polymers used. This could be clearly demonstrated by the values of the root mean square (rms) of the velocity fluctuation. Furthermore, the influence of the extension and orientation of the macromolecules on the turbulent flow properties was investigated.

The containment of oil spills in unconsolidated granular porous media using xanthan/Cr(III) and xanthan/Al(III) gels

The gelation in situ of aqueous solutions of the biopolymer xanthan gum may be a method for temporarily containing oil spills in soil whilst the remediation procedure is planned and accomplished. The gelling reaction has been carried out using as crosslinking agents either Cr(III) or Al(III) cations. By using Cr(III) the gelation time, which has been measured for a range of xanthan and/or Cr(III) concentrations, is of the order of the hour. On the contrary, the gelation by means of Al(III) cations takes place at quite low pH and is instantaneous. Therefore, depending on the crosslinker adopted, rather different techniques must be used for generating the gel structure in situ. The gels have proven to be suitable for the containment of water and of many hydrocarbons without loosing their strength even for a long period of time. The rheological properties of xanthan aqueous solutions evidence a shear-thinning behaviour that is most favourable for the application. Theoretical considerations have permitted the establishment of the mobility conditions of xanthan solutions in porous media before the gelation, and the estimation of a suitable injection pressure. The experiments for investigating the mobility of xanthan solutions in porous media, and for simulating the containment of an oil spill, have been done using packed beds of uniformly sized spherical glass beads, in glass columns. The glass assembly made it possible to visualize the evolution of the phenomena of interest.

Application of the minimum transport velocity model for drag-reducing polymers

The objective of this work is to acquire further insight into the procedure for predicting the minimum fluid velocity, which is required to transport cuttings from a directional well. The minimum transport velocity model (MTV) was used for drag-reducing polymers such as xanthan gum (XG). A series of experiments to evaluate the predictions of the model were conducted using water, aqueous solutions of polyanionic celluose (PAC) and xanthan gum (XG). The experiments conducted in a 4-m-long and 8-cm-diameter test section with recirculation facilities. The tests were carried out by measuring critical velocities, which are required to initiate the motion of sand bed particles. Four sand beds with different particle size ranges were used in the experiments. The model predictions were compared to the experimental results. The model predictions of critical velocity show satisfactory agreement with the measured data.

Friction losses in valves and fittings for power-law fluids

Data on pressure drop were obtained in stainless steel, sanitary fittings and valves during laminar and turbulent flow of aqueous solutions of sucrose and xanthan gum, which were selected as model fluids. The rheological properties of these solutions were determined and the power-law model provided the best fit for experimental data. Friction losses were measured in fully and partially open butterfly and plug valves, bends and unions. Values of loss coefficients (kf) were calculated and correlated as a function of the generalized Reynolds number by the two-k method. The model adjustment was satisfactory and was better in the laminar flow range (0.976 < r2 < 0.999) than in the turbulent flow range (0.774 < r2 < 0.989). In order to test the adequacy of the results for predicting loss coefficients during flow of real fluids, experiments were conducted with coffee extract. Comparison between experimental and predicted loss coefficients showed very good agreement.

Some effects of rheology on the spatial distribution of gas hold-up in a mechanically agitated vessel

The effects of rheology on gas hold-up in non-Newtonian circulation flow has been rarely discussed in the literature, especially so far as local gas distribution in mechanically agitated vessels is concerned. This study analyses experimental gas dispersion behaviour for a wide range of complex-rheology conditions. Non-Newtonian aqueous solutions of xanthan gum with concentrations from 0.5 to 5 kg/m 3 and Newtonian glycerol solutions of 450 and 800 kg/m 3 containing also 5% electrolyte are studied. Hold-up is measured conductometrically at 36 spatial points within the stirred vessel mainly around and above the impeller for a conventional geometry (Rushton standard configuration with T=0.2 m and D/ T=0.33). In addition to increasing viscosity, three aspects of rheology impact on gas hold-up and different dominances of component effects upon the overall gas dispersion mechanism are revealed. The results are presented as local and global gas hold-up vs. consistency, pseudoplasticity, and position. By comparing data obtained for Newtonian and non-Newtonian flow conditions and data obtained for the same polymer solution at constant plasticity and variable consistency, an attempt to reveal the net effects of consistency and pseudoplasticity is made. The boundaries of regions with large differences of mixing intensity reflected in terms of local gas availability are quantified.

The effect of the expansion ratio on a turbulent non-Newtonian recirculating flow

Measurements of the mean and turbulent flow characteristics of shear-thinning moderately elastic 0.1% and 0.2% xanthan gum aqueous solutions were carried out in a sudden expansion having a diameter ratio of 2. The inlet flow was turbulent and fully developed, and the results were compared with data for water in the same geometry and with previous published Newtonian and non-Newtonian data in a smaller expansion of diameter ratio equal to 1.538. An increase in expansion ratio led to an increase in the recirculation length and in the axial normal Reynolds stress at identical normalised locations, but the difference between Newtonian and non-Newtonian characteristics was less intense than in the smaller expansion. An extensive comparison of mean and turbulent flow characteristics was carried out in order to understand the variation of flow features.

Viscoelastic properties of xanthan gum hydrogels annealed in the sol state

Viscoelastic properties of xanthan gum aqueous solutions and hydrogels were investigated using a cone-plate type rheometer. The change of viscoelasticity during annealing the solution and cooling to gelation temperature was examined as functions of annealing time, temperature and frequencies. In the annealing process, the storage modulus G′ increases with increasing annealing time. In the subsequent cooling process, G′ of the annealed solution increased, whereas the G′ of non-annealed solution remained almost constant. G′ of hydrogels increased with the increase of annealing temperature and concentration. Based on the experimental results obtained, the structural change of the solution in the annealing process and the structure of gels were investigated.

Gas Holdup in Circulating Bubble Columns with Pseudoplastic Liquids

The contributions of pressure drop due to wall frictional losses to the total gas holdup of two-phase viscous non-Newtonian systems were experimentally investigated using a 150 dm3 circulating bubble column. The column had a downcomer-to-riser cross-sectional area ratio of 0.54 and a dispersion height of 2.5 m. Aqueous solutions of xanthan gum and carboxymethyl cellulose were used to simulate a wide range of rheological properties. The average wall shear stress was estimated from Al-Masry's (1999) correlation for the average wall shear rate in external loop airlift reactors. Pressure drop due to wall shear stress was found significantly contributed by 10–70 % to the total gas holdup. This contribution has always been ignored in the data presented in the literature due to the absence of reliable and simple correlations for the average shear rate and shear stress. Corrections to gas holdup were found necessary for non-Newtonian solutions with concentrations of ≥ 0.5 wt/wt.-%.

Turbulent characteristics of shear-thinning fluids in recirculating flows

A miniaturised fibre optic Laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1–0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present downstream turbulence field was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers.

A versatile thermostatted glass tube MRI rheometer

A glass tube rheometer optimized for use with a magnetic resonance imaging (MRI) scanner has been developed. Mounted on a trolley, its `plug-and-play' design allows flow- and temperature-equilibria to be attained before the rheometer is inserted in the magnet without disruption of the flow of fluids. Design principles, construction details and rheological test results for water, and aqueous solutions of sucrose (50% w/w) and xanthan gum (0.5% w/w) are presented. Results for water and aqueous sucrose in the temperature range 10-60 °C, which showed that measured shear viscosity was independent of the radial position, demonstrate that the temperature control is reliable. The good agreement of MRI measured viscosities with those produced by classical rheometry indicates the accuracy of the MRI rheometer. Results for 0.5% w/w aqueous xanthan gum reveal an initial time dependency before the flow reached a steady state. The initial time dependency was predominant for the fluid flowing in the central region of the tube; in contrast, the flow in the region near the wall showed the time-independent characteristic of power law fluid. Comparisons with data from cone-and-plate rheometry demonstrate the complementary power of MRI for studies of rheologically complex fluids; importantly, the MRI method can be used to measure the effects of `shear history' on the flow rheology.

Drop breakage in stirred tanks with Newtonian and non-Newtonian fluid systems

Drop size distributions were measured in agitated non-Newtonian fluid systems using a 0.09 m diameter mechanically stirred tank. The dispersion process was carried out in the absence of coalescence by keeping the dispersed phase volume fraction at less than 0.005. Aqueous solutions of carboxymethyl cellulose and xanthan gum were used as the continuous phase with palm oil forming the dispersed phase. Additionally, agar solutions were used as the dispersed phase with salad oil as the continuous phase, which is weakly non-Newtonian. It was experimentally found that the non-Newtonian characteristics of the continuous phase caused an increase in the maximum drop size, particularly at low impeller speeds and wide drop size distributions. The Sauter drop diameter was proportional to the maximum drop diameter in non-Newtonian and Newtonian fluid systems. Models for drop breakage in a stirred tank have been developed to account for the effect of non-Newtonian flow behaviour. The boundary-layer shear force concept was applied to discuss the influence of non-Newtonian flow behaviour on the shear stress acting on the drop and drop break-up in a stirred tank. It was found that the experimental data correspond to the boundary-layer shear force models in non-coalescing systems.

Minimum liquid fluidization velocity in two- and three-phase fluidized beds with non-Newtonian fluids

Abstract Measurements have been taken of the relationship between flow rate and pressure drop for non-Newtonian fluids in two-and three-phase fluidized beds and the minimum fluidization velocities have been determined. Aqueous solutions of carboxymethyl cellulose and xanthan gum have been used as the non-Newtonian liquids. The measurementscovered Reynolds numbers at minimum fluidization from 0.0377 to 540 and flow indices in a power-law model from 0.489 to 1. The minimum fluidization velocity decreased with an increase in the gas velocity and the magnitude of this decrease became smaller as particle size decreased. The increase of the viscous non-Newtonian flow behaviour or pseudoplasticity led to a decrease of the minimum fluidization velocity. The result for the minimum liquid fluidization velocity for two-phase systems have been examined using the correlation based on the free-surface cell model. The predicted minimum fluidization velocities were in reasonable agreement with the experimental data for non-Newtonian liquid-solid two-phase systems over the whole Reynolds number region.

The effect of parallel combined steady and oscillatory shear flows on blood and polymer solutions

Human blood at physiological volume concentration exhibits non-Newtonian and thixotropic properties. The blood flow in the microcirculation is pulsatile, initiated from the heart pulse and can be considered as superposition of two partial flows: a) a steady shear, and b) an oscillatory shear. Until now steady and viscoelastic behavior were separately investigated. Here we present the response to the combination of steady and oscillatory shear for human blood, a high molecular weight aqueous polymer solution (polyacrylamide AP 273E) and an aqueous xanthan gum solution. The polyacrylamide and xanthan solutions are fluids that model the rheological properties of human blood. In general, parameters describing blood viscoelasticity became less pronounced as superimposed steady shear increased, especially at low shear region and by elasticity, associated with reduction in RBC aggregation. The response of polymer solutions to superposition shows qualitative similarities with blood by elasticity, but their quantitative response differed from that of blood. By viscosity another behavior was observed. The superposition effect on viscous component was described by a modified Carreau equation and for the elastic component by an exponential equation.

Nuclear magnetic resonance imaging of apparent slip effects in xanthan solutions

Nuclear magnetic resonance imaging has been used to investigate the flow of 0.2% aqueous solutions of xanthan gum. Apparent slip was observed in solutions made from the material supplied by UNAM but not in that supplied by Aldrich or Kelco. The apparent slip velocity was a constant fraction of the maximal velocity for a given contraction ratio. The apparent slip velocity also appeared to be independent of L/D but was strongly influenced by the wall stress, the observed apparent slip velocities being comparable with those determined using the Mooney analysis on capillary flows. After exposure to the action of a syringe pump, which reduced the mean molecular weight of the xanthan, the UNAM xanthan solution no longer exhibited apparent slip. Slip appears to be a function of molecular weight, possibly through sensitivity to the aspect ratio of the molecule.

Gas Hold-Up and Oxygen Transfer in Three-Phase External-Loop Airlift Bioreactors: Non-Newtonian Fermentation Broths

The effects of solids loading on gas hold-up and oxygen transfer in external-loop airlift bioreactors with non-Newtonian fermentation media are discussed. Experiments were performed in two model external-loop airlift bioreactors with aqueous solutions of carboxymethyl cellulose (CMC) and xanthan gum representing non-Newtonian flows. Low-density plastic particles of 1030 and 1300 kg m -3 were used and the solids loading was varied in the range 0-20% (v/v). For the inelastic non-Newtonian CMC aqueous solutions, the presence of low-density solid particles slightly increased the riser gas hold-up, Φ gr , but decreased the volumetric mass transfer coefficient, k L a. On the other hand, Φ gr decreased but k L a increased with solids loading in the viscoelastic non-Newtonian xanthan gum aqueous solution. The extent of these effects depended on non-Newtonian flow behavior. Theoretical models of riser gas hold-up and volumetric mass transfer coefficient have been developed. The capability of the proposed models was examined using the present experimental data obtained in the model external-loop airlift bioreactors and the available data in the literature. The data were successfully correlated by the proposed correlations except the results for k L a coefficient in the xanthan gum solution.