Brazilian Journal of Chemical Engineering THE EFFECT OF SURFACTANT AND HIGH MOLECULAR WEIGHT POLYMER ADDITION ON PRESSURE DROP REDUCTION IN PIPE FLOW

The main aim of this paper is to present a possibility to enhance the drag reduction effect in straight pipe flow by the simultaneous addition to the transported liquid of a small amount of high molecular weight polymers and surfactants. Qualitative analysis of the polymer-micellar additive influence on the shape and character of flow resistance curves has been performed. Also multicomponent polymer-micellar solution flow resistance curves were compared with appropriate single additive polymer or surfactant solution flow resistance curves. The experimental data shows that, for polymer-micellar solutions, the stable transitional zone between the laminar and the turbulent flow regions is extended toward higher values of the critical Reynolds numbers. Occurrence of the phenomenon can be explained by the flow laminarization caused by polymer-micellar aggregates. Existence of the third extended drag reduction zone in the turbulent range of flow has also been observed for the first time.


INTRODUCTION
Abnormal flow drag reduction by surfactant or polymer additives is intensively examined and described in the subject literature (Matras, 1984;Borostow, 2008;White and Mungal, 2008;Wang et al., 2011;Shu-Peng, 2012;Tamano et al., 2010;).This phenomenon allows a significant increase of flow rate without increasing power demand, or vice versa -to reduce power demand while maintaining a constant flow rate.It provides large potential possibilities for the application of this effect in different industry branches, particularly in the oil industry or in heating, firefighting, transport of slurries, sludge and brines (Motier et al., 1996;Figueredo and Sabadini, 2003;Dujmovich and Gallegos, 2005;Wang et al., 2011).Causes of the described drag reduction have been perceived in the existence of a new internal solution structure which is formed in the moment when the special additives are introduced into the solution.Addition of high molecular weight polymer agents into the solvent results in macromolecule solution formation, which has a crucial influence on turbulence structure in the flow (Borostow, 2008;White and Mungal, 2008;Tamano et al., 2010).Although, there is still strong debate on whether a single polymer molecule or clusters of polymer molecules are responsible for the drag reduction effect, experimental results clearly prove that even a dozen ppm of polymer concentration in the solvent induces an Brazilian Journal of Chemical Engineering efficient drag reduction effect in the turbulent range of flow (White and Mungal, 2008;Wang et al., 2011;Zadrazil et al., 2012).
In case of application of surfactants as drag reducing additives, formation of micelle structures is observed (Tamano et al., 2010;Wang et al., 2011).In order to improve the micellarization process effectiveness in surfactant solutions, small amounts of electrolytes are applied (e.g.sodium salicylate or sodium bromide).At the no motion condition, the mentioned structures are chaotic.Only during fluid flow shearing do both macromolecules and micelles start to arrange in characteristic orientations, in accordance with the principle of the minimum resistance.In both surfactant and polymer induced experiments on the drag reduction effect, a hypothetical mechanism of the phenomenon that is widely accepted is the interaction between polymer or surfactant molecules and the flow turbulence structure.On the basis of advanced measurement techniques, such as Particle Image Velocimetry (PIV), it is observed that polymer additives can lead to reduction or elimination of the ejections of low-momentum fluid from the wall region to the outer velocity region (Warholic et al., 2001).It is also observed that the presence of polymers leads to a decrease in the frequency and the intensity of large-scale ejections when compared to a Newtonian solvent (Liberatore et al., 2004) and to the reduction of the magnitude and frequency of the small-scale eddies (Liberatore et al., 2004).Usually, elongational viscosity or elasticity of the polymer chain (White and Mungal, 2008) are proposed to explain the hypothetical polymer drag reduction mechanics.On the other hand, for a surfactant aqueous solution, which exhibit neither viscoelastic properties nor the presence of elongational viscosity (Wang et al., 2008), a local shearthickening hypothesis is proposed (Wang et al., 2008;Hadri et al., 2010).
A novel and poorly recognized effect is a phenomenon of fluid flow drag reduction by the simultaneous addition into the solvent of both high molecular weight polymer and surfactant with salt.In the few published works related to this subject (Minatti et al., 1996;Hou et al., 1999;Suksamranchit et al., 2006;Jung et al., 2011) the internal structure formation and chemical reaction process in polymermicellar solutions are mainly highlighted.Initial attempts at experimental examination of the drag reduction effect have been performed (Matras et al., 2008;Mohsenipour et al., 2013).The results confirmed that simultaneous addition into the solvent of these additives combines and intensifies the positive features of their purely polymer and micellar analogues, providing additional extension of the drag reduction zones.Moreover, the researchers indicate that this new effect requires a comprehensive experimental study to gain a deeper knowledge of this phenomenon.
The presence of polymer macromolecules in the surfactant solution enhance the micelle structure formation ability.It leads to the formation of micellar structure at a lower concentration.The newly formed macromolecule is called an aggregate (Matras et al., 2008;Mohsenipour et al., 2013).Adding a slight amount of salt (e.g.NaCl or NaSal) to the high molecular weight polymer and surfactant solution causes micelle size growth.The number of micelles linked with the polymer chains also increases.Furthermore, the addition of the salt can increase the solution viscosity.
The aim of this paper is to perform an analysis of the drag reduction efficiency induced by simultaneous addition to the solvent of both surfactants and high molecular weight polymer, comparing to the drag reduction effect obtained by addition of pure polymer or pure surfactant agents.

CHARACTERISTIC OF POLYMER-MICELLAR SOLUTION INTERNAL STRUCTURE
Simultaneous addition to the solvent of small amounts of polymer and surfactant additives triggers initiation of the micellarization process at much lower concentration, compared to the critical micelle concentration (CMC) (Deo et al., 2007;Jönsson et al., 1998).This concentration at which micelle formation occurs in the presence of polymer macromolecules is called the critical aggregation concentration (CAC).The newly formed polymer-micelle macromolecules are called aggregates (Diamand and Andelman, 1999;Matras et al., 2008).From the experimental study of polymer-micellar aqueous solutions (Jönsson et al. 1998;Mohsenipour et al., 2013), the mechanism of aggregates formation process can be described.Initially polymer and surfactants molecules occur in the solution independently.The situation significantly changes when a small amount of salt is introduced into the solution.According to Minatti et al. (1996) and Matras et al. (2008)   A microprocessor frequency converter (7) was used to control the pump frequency and consequently volumetric flow rate.The main element supporting the action of the capillary-pipe rheometer is a multichannel data acquisition system SPIDER 8 (Hottinger Baldwin Messtechnik), arranged to measure the electrical signals from the different sensors (tension, force, pressure, displacement, acceleration and temperature sensors).
Additionally, large volume tanks (1 m 3 ) were used to eliminate the effect of foaming of micellar and polymer-micellar solutions and to minimize the influence of the unavoidable degradation of the polymer-micellar aggregates or macromolecular structures on the measurement results.
During the preparation of the polymer-surfactant solutions, the pH of the chosen drag reducing additives should be particularly considered.Incorrect selection of pH may result in an undesired chemical reaction.
Anionic surfactants cannot be used when aqueous solutions of certain polymers can have an acidic reaction.Cationic and anionic surfactants can be combined with non-ionic polymer solutions.
After preliminary study, the following drag reducers were used for experimental analysis: Poly(ethylene oxide) [CH 2 CH 2 O] n (PEO)non-ionic polymer with viscosity-based high molecular weight given by the manufacturer equal to 8•10 6 g/mol -1 , purchased from Sigma-Aldrich, Inc.
In order to lower the CAC value, the salt C 7 H 5 NaO 3 (NaSal) sodium salicylate has been used.Different compositions of polymer, surfactant and salt mass fraction in solvent were used in order to analyse the chemical additive concentration effect.Distilled water was used as solvent.A Polna, Inc. Electrical Distillatory Type DE20 was used to purify tap water.The conductivity of the solvent was of the order of 1 µS/cm.
After addition to the solvent of the appropriate drag reducers, solutions were mixed gently so as not to cause mechanical degradation of polymer chains.The first mixing was performed in cylindrical vessels, by the use of a roller mixer of our own design with very a low rotational speed equal to 1-5 rpm.Then the solution was diluted in the main tanks.Before measurements, mixtures were left to rest for 24 hours.
In order to conduct an analysis of drag reduction efficiency by simultaneous addition to the solvent of both surfactants and high molecular polymer, compared to the drag reduction effect obtained by the addition of pure polymer or pure surfactant agents, 15 solutions with different additive compositions and concentrations were investigated.Designation and the composition of the analysed mixtures are presented in Table 1.
Adiabatic steady flow of homogenous solutions was examined in 8 different straight pipes with diameters between 1.8 mm and 21 mm, all with a temperature of 27 °C.

RHEOLOGICAL CHARACTERISTICS AND FLOW RESISTANCE MEASUREMENT RESULTS
In order to identify the rheological characteristics of the analysed solutions, each of the experimental/ pipe flow curves was represented in the form of the functional relationship described by the Equation ( 1 Interpretation of the experimental results presented in the form of function (1) indicates that the solutions can be successfully approximated with the Ostwald de Waele power-law fluid model.Rheological parameters K (fluid consistency constant) and n (flow behavior index) for each of the analysed solutions are summarized in Table 1.Representative rheological characteristics in the form of experimental/pipe flow curves are illustrated in Figure 2.
Additionally, representative shear viscosity curves vs. shear rate for the analysed solutions of Figure 2 are presented in Figure 3.
The mass fractions of individual additives in the rheologically complex polymer-micellar solution affect the value of the n flow index.This parameter characterizes non-Newtonian properties of a fluid.There is no way to predict a priori, its value for a solution prepared arbitrarily and having different mass fractions of particular drag reducers.It was only observed that the increase of both CTAB and NaSal concentration in the examined solution, at a Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 933 -943, October -December, 2016 constant polymer concentration, leads to the intensification of non-Newtonian properties of the fluid, i.e., to the increase of the value of the fluid consistency constant K and the decrease of the n flow index.The interpretation of experimental data and the assessment of the respective solution additives' influence on the increase or reduction of the flow resistance and the shape and location of resistance curves depend significantly on the coordinate system adopted for data presentation.(2) and additionally in the form of drag reduction coefficient DR defined as a function of the Reynolds number (2), and described in percentage terms: Figure 4 presents the flow resistance curves of polymer, surfactant and polymer-surfactant water solutions, defined in the system of dimensionless numbers (2) and (3).Analyses of flow resistance curves reveal that, in any of the analysed flow ranges, the measurement points do not correspond to the theoretical functions which describe Newtonian fluid flow.The simultaneous addition of even small amounts of high molecular weight polymers and Brazilian Journal of Chemical Engineering surfactants causes an increase of flow resistance in the laminar range of flow.Due to the difficulty in unambiguous determination of the critical value of the Reynolds number Re s , the DR values presented in Figure 5 are calculated in such a way that it was assumed that c fs =16/Re s in the range of the Reynolds number Re s <2100, whereas the formula c fs =0.079/ Re s -0.25 was used in the range of Reynolds number Re s ≥2100.Therefore, a sharp increase of the DR value observed in the transition zone, particularly in the polymer solution (see in Figure 5), does not reflect the actual degree of drag reduction in this range of flow.In the case of the polymer solution flow, in the initial stage of the turbulent flow, no noticeable reduction of flow resistance is observed.Only after exceeding a certain characteristic Reynolds number Re s ≈ 1.5×10 4 , the onset of the drag reduction effect occurs and the phenomenon increases with the increase in the value of the Reynolds number.A similar increase in the reduction of the shear resistance for the turbulent flow range, induced with PEO additive, was observed in the drag reduction effect obtained by the use of a rotating-disk apparatus (Kim et al., 2001).
Furthermore, the value of the critical Reynolds number for which the transition from the laminar flow to the turbulent flow is observed takes various values that depend on pipes diameter, type and concentration of chemical additives introduced into the solvent.
A better interpretation of the simultaneous addition of the polymer and surfactant with salt effect on drag reduction, in comparison with adequate addition of pure polymer or pure surfactant with salt, can be achieved by presentation of the same measurement date in a modified system of "pseudorheostable" numbers [Re M , c fM ].The modified system of dimensionless numbers used in the analysis is described by formulas ( 5) and ( 6): ( ) As one knows (Matras, 1986), in such a defined dimensionless number system, flow resistance curves of rheostable (purely viscous) non-Newtonian fluids are transformed to a single curve -in the whole range of modified Reynolds number (5) -identical to the classical Newtonian curve described in the laminar range by the Fanning equation and in the turbulent flow by the Blasius formula.Selection of such a coordinate system was dictated additionally by the fact that it facilitates identification and description of the characteristic drag reduction flow zones.In this Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 933 -943, October -December, 2016 modified system of pseudorheostable dimensionless numbers [Re M , c fM ] each deviation of the experimental flow resistance curve, which indicates abnormal drag reduction from the pseudorheostable Blasius curve, allows for the identification of the influence of specific additives (polymers or/and surfactants with salt) on the range of the drag reduction effect.Figure 6 presents a comparison of flow resistance curves in the modified number system (5) and ( 6) for 3 types of solutions with different internal structures.

Figure 6:
The flow resistance curves of polymer, surfactant and polymer-surfactant water solutions, defined in the system of cardinal numbers (5) and ( 6).
The results of experimental data analysis indicate that polymer-surfactant additives cause significant drag reduction in a wider range of flow in comparison with pure polymer and pure surfactant solutions.Surfactant and salt additives (micellar solution) induce the appearance of the stable transitional zone, in which a spectacular reduction of flow resistance is observed -usually greater when compared to the same effect achieved with polymer additives.This zone was not observed for the polymer-surfactant solution without the salt additives.Figure 7 demonstrates the influence of NaSal additive on the drag reduction effect.It is clearly evidenced by the flow resistance curves that, with the addition of a small amount of electrolyte (e.g., salts or alcohols), a reformation of spherical micelles into threadlike micelles must proceed.It leads to significant extension of the transitional zone.An efficient drag reduction effect is observed within this zone of flow.
In the stable transitional zone, loss of stability of the laminar flow increases gradually when the Reynolds number values grow.In this range the relative drag reduction is the greatest.Beyond a certain second critical value of the Reynolds number the occurrence of an unstable transitional zone is observed.In this range of flow a rapid loss of the drag reduction effect occurs.
Surfactant and salt concentration effects were observed within this zone as illustrated in Figure 8.In comparison with a purely micellar solution, the collapse of drag reduction is normally observed in this region.It should also be noted that addition to the micellar solution of even small amounts of high molecular weight polymer (about 10ppm) causes a reduction of the non-Newtonian properties of the solution.
The results of drag reduction measurements indicate the effect of pipe diameter on drag reduction efficiency.Figure 10 illustrates the pipe diameter effect on the flow resistance curves of polymersurfactant water solutions.Increasing the pipe diameter d results in a clear extension of the stable transitional zone towards higher values of the Reynolds number.Moreover, decreasing the pipe diameter value d results in an increase of the drag reduction effect in the third additional turbulent range of flow.
The experimental results reveal that polymermicellar solutions can be characterized by a lower susceptibility to mechanical degradation during flow or that the degradation can be almost invisible in the analysed range of flow.Figure 11 illustrates a shift of the collapse of the drag reduction effect toward greater values of Reynolds number caused by polymer-surfactant additives.
Pure PEO solution degrades very fast under high shearing conditions.Collapse of drag reduction is gradually observed in such a case.Experimental results show a considerable increase of the Reynolds number value at which the mechanical degradation of polymer and the collapse of the DR effect are observed in polymer-micellar solution.

HYPOTHETICAL MECHANISM OF DRAG REDUCTION CAUSED BY POLYMER-MICELLAR SURFACTANT ADDITIVES
Most of experimental studies have shown that simultaneous addition of small amounts of polymer and surfactant agents to the solvent causes initiation of the micellarization process at lower concentration, compared to the CMC (Hou et al., 1999;Matras et al., 2008).This concentration is called the critical aggregation concentration (CAC).
In the case of an ionic surfactant mixed with a non-ionic or oppositely-charged polyelectrolyte, only a small part of the polyelectrolyte is absorbed by the surfaces of the micelles.Furthermore, the CAC has a lower magnitude than the original CMC due to the following facts: electrostatic interaction occurs between the polyelectrolyte and the surface of the micelles, hydrophobic interaction occurs between the nonionic polymer and ionic micelles, surfactant counter-ions on the micelles surface are replaced by the electrolyte or polyelectrolyte, highly charged electrolytes can provide a certain amount of concentrated counter-ions that combine with the micelles.
As a result of electrostatic or hydrophobic interaction, the micelles combine with the polymer chains by coiling around them.The final state of the mixture has single threadlike micelles with a part of the polymer macromolecule chain coiled around rigid micelles.According to Hou et al. (1999), Matras et al. (2008) andJung et al. (2011), these form the so-called aggregates.
Such newly created polymer-micellar solutions can be characterized by a lower susceptibility to mechanical degradation during flow or the degradation can be almost invisible in the analysed range of flow.
Aggregates, schematically illustrated in Figure 12(a), subjected to the shear stress, take an orientation consistent with the aforementioned principle of minimum resistance -Figure 12(b).With the increasing value of the Reynolds number, internal friction forces stretch and extend the aggregates, leading to the laminarization of the initial phase of the turbulent flow.
Therefore, it can be hypothesized that the rigid rodlike micelles, which create the core of the aggregates, are responsible for reducing the flow resistance in the extended transitional zone between the laminar and the turbulent flow.The aggregates and micelles are responsible for transmission of internal friction within the liquid.The value of the critical Reynolds number for which the transition to the turbulent zone is observed is greater for polymer-micellar solutions.This means that the stable transition zone is extended.The reason for such behaviour can be the partial disintegration of aggregates to their original forms, i.e., micelles (formed from the surfactant) and macromolecules (formed from the polymer) due to a significant increase of the shear rate.From this moment, both micelles and macromolecules interact separately in the transported solution, causing a further drag reduction effect.Passing further into the turbulent range of flow, micelles lose their orientation and no longer have a major impact on the drag reduction.A key role is played in this zone by the polymer.Not having undergone an earlier degradation, the polymer macromolecules still cause the flow reduction.
In drag reduction caused by the use of polymersurfactant solution, one cannot talk about the socalled collapse of the drag reduction.It occurs over a wide range of Reynolds numbers.In the turbulent zone, polymer macromolecules undergo a certain mechanical degradation.Decreasing the shear rate leads to the reconstruction of the internal structure of the solution.As a result of electrostatic or hydrophobic interaction, the recreated micelles are combined with the polymer chains by coiling around them.These chains are much shorter and such newly created aggregates do not have the same rheological properties as the original ones.This results in a slight increase of the flow resistance in comparison with a freshly prepared solution.

CONCLUSIONS
A comparative analysis of the drag reduction efficiency produced by pure high molecular polymers, pure surfactants and surfactants with high molecular weight polymer additives indicates that the simultaneous addition of surfactants together with high molecular polymers causes an increase of flow resistance in the laminar range of flow compared to the analogous flow of pure solvent.In the case of the turbulent flow, simultaneous application of the examined chemical additives produces a drag reduction effect.It produces, however, a significant extension of the stable transitional zone between the laminar flow and the turbulent flow.The surfactant with salt additives has the major influence on efficiency of drag reduction in this zone.
shear rate (value of the shear rate on pipe wall).

Figure 2 :
Figure 2: Representative experimental flow curves for polymer, surfactant and polymer-surfactant solutions.

Figure 3 :
Figure 3: Representative shear viscosity curves as a function of shear rate for polymer, surfactant and polymer-surfactant solutions.
Firstly, experimental results of flow resistance are presented in the classical system of dimensionless numbers [Re s , c f ] described by formulas (2) and (3

Figure 4 :
Figure 4: The flow resistance curves of polymer, surfactant and polymer-micellar water solutions, defined in the system of cardinal numbers (2) and (3).

Figure 7 :
Figure 7: The effect of NaSal additive on the polymer-surfactant flow resistance curve.

Figure 8 :
Figure 8: Surfactant and salt concentration effects on the flow resistance curves of polymer-micellar aqueous solutions.An increase of surfactant and salt concentration leads to a further extension of the transitional zone towards greater values of the Reynolds number.Beyond a certain third critical value of the Reynolds number the fluid starts to behave like a classical

Figure 9 :
Figure 9: Polymer concentration effect on the flow resistance curves of polymer-surfactant aqueous solutions.

Figure 10 :
Figure 10: Pipe diameter effect on the flow resistance curves of polymer-surfactant water solutions.

Figure 11 :
Figure 11: Shift of the collapse of drag reduction effect toward greater values of Reynolds number caused by polymer-surfactant additives.

Figure 12 :
Figure 12: Polymer-micellar aggregate: a) in the shape of spherical structures, b) elongated during shear stress action.