The Role of Surfactants in Mechanical Degradation of Drag-Reducing

Dec 22, 2012 - Effect of salt on turbulent drag reduction of xanthan gum. Cheng Hai Hong , Hyoung Jin Choi , Ke Zhang , Frederic Renou , Michel Grisel...
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The Role of Surfactants in Mechanical Degradation of Drag-Reducing Polymers Ali Asghar Mohsenipour and Rajinder Pal* Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada ABSTRACT: Turbulent drag reduction behavior of mixed polymer−surfactant systems (anionic polymer/cationic surfactant; nonionic polymer/cationic surfactant; nonionic polymer/anionic surfactant) was studied in a pipeline flow loop to explore the role of surfactants in mechanical degradation of polymers. The polymers investigated were nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM). The surfactants studied were cationic octadecyltrimethylammonium chloride (OTAC) and anionic sodium dodecyl sulfate (SDS). The pipeline flow results obtained for PAM/OTAC mixtures support the idea that the coiling of polymer molecules does not protect the polymer molecules against shear degradation. The addition of oppositely charged cationic surfactant (OTAC) to anionic polymer (PAM) results in coiling of polymer molecules. The coiled polymer molecules undergo faster mechanical degradation than stretched polymer molecules. The addition of surfactant (cationic OTAC or anionic SDS) to nonionic polymer PEO increases the resistance of polymer molecules against shear degradation. This is reflected in the pipeline flow results. The effect is more significant for anionic surfactant (SDS) than for cationic surfactant (OTAC) especially at high concentrations of surfactant; the smaller size of the headgroup of anionic surfactant monomers allows them to have a greater influence on the polymer molecules. These results support the idea that extended polymer chains are more resistant to mechanical degradation as compared with coiled polymer molecules. The polymer chains undergo extension due to repulsion between the neighboring surfactant micelles attached to the backbone of the polymer chains.

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INTRODUCTION Turbulent drag reduction due to the addition of a small amount of dilute polymer was first observed by Toms.1 As a result of his pioneering contribution, drag reduction is often termed as Tom’s effect in the literature. Gyr and Bewersdorff2 and also White et al.3 have given a very detailed description of the drag reduction phenomenon. Numerous studies have been conducted in recent decades to explore drag reduction in various systems and to improve our understanding of drag reduction and its mechanisms. Among these studies, the early studies of Metzner and Park,4 Lumley,5,6 Virk,7 Zakin and Hunston.,8 Berman,9 and Tabor and Gennes10 and the recent contributions of Escudier et al.,11 Zakin and Ge,12 Zakin et al.,13 Sreenivasan et al.,14 Ptaskinski et al.,15 Kim,16 Vanapalli et al.,17 Shah and Zhou,18 and Tamano et al.19 should be mentioned. Several experimental studies have been carried out to obtain important information about the statistical properties of turbulence in the presence of a drag reducer.15,20−23 Variables that have been considered in the investigation of drag reduction are the type of drag reducing additive, additive concentration, molecular weight, additive structure, temperature, and solvent quality. Lumley6 suggested that there is a critical value of wall shear stress at which macromolecules become stretched due to the fluctuating strain rate. However, in the viscous sublayer close to the wall, polymer coils are not greatly deformed and viscosity does not increase greatly above the solvent viscosity. Lumley5 also concluded that the stretching of randomly coiled polymers due to strong turbulent flow is important for drag reduction (DR). Variation of turbulent structure in the buffer layer was discussed by Tiederman.24 On the basis of experimental evidence, Virk7 suggested that DR is limited by an asymptotic value (maximum DR). Warholic et al.25 performed experiments © 2012 American Chemical Society

with polymer solutions and concluded that the Reynolds shear stress becomes negligible near maximum DR. The DR by polymers is often explained in terms of viscoelastic effects resulting from the presence of polymer chains in the solution. Some authors believe that the high shear conditions of turbulent flow induce stretching of polymer chains, leading to an increase in the elongational viscosity.26,27 Due to the increase in the elongational viscosity in the buffer layer of turbulent flow, the buffer layer thickness increases, causing a reduction in wall friction.5 Also, the stream-wise and span-wise turbulent fluctuations are suppressed, the velocity profile is modified, and the shear in the boundary layer is redistributed. Tesauro et al.28 proposed that energy is transported by the velocity fluctuations to the polymer chain in the form of stretching of the polymer chain which in turn dissipates the energy into heat by relaxation of the polymer chain from its extended state to equilibrium state. Mysels29 was the first scientist who studied drag reduction behavior of surfactant solutions. However, the area of surfactant DR did not receive any attention for nearly a decade and was later revived by the work of Dodge and Metzner.30 Most of the drag-reducing surfactants can be classified as environmentally safe chemicals.31−33 Threadlike or wormlike micelles are believed to be necessary for surfactant solution to be a drag reducer.13,33,34 The morphology of surfactant solutions can be changed from spherical micelles to threadlike micelles by the addition of an oppositely charged surfactant, organic counterReceived: Revised: Accepted: Published: 1291

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conditions significantly contribute to overall interaction. It is believed that the nonionic and cationic surfactants do not interact with the polymer molecules in general. However, in the presence of some types of ions, a weak interaction can take place. The bulkiness of the cationic headgroup (compared to anionic surfactants) is one of the main reasons for such observations.63,64 Hydrophobicity is another factor which plays an important role in the interaction between nonionic polymers and cationic surfactant; polymers with a higher degree of hydrophobicity show a better interaction.65−67 Mya et al.68 have reported that cationic surfactant (hexadecyltrimethylammonium) and nonionic polymer PEO exhibit strong interactions when the temperature is greater than 25 °C. In this case, the hydrodynamic radius (Rh) of the polymer increases due to chain expansion. The expansion of the chain is caused by electrostatic repulsions between the bonded micelles on the backbone of the polymer chain. This observation is consistent with work of Hormnirun et al.69 The necklace model proposed by Nagarajan70 describes the interaction between nonionic polymer and ionic surfactants. In this model, the surfactant micelles are surrounded by polymer molecule and a complex is formed. At the same time, the polymer segments partially penetrate into the polar headgroup region of the micelles. Consequently, a reduction in the micelle core−water contact area takes places. On the basis of this model, formation of micelles on the backbone of polymer chains will cause those chains to be stretched. A thermodynamic model was also proposed by Ruckenstein et al.71 Their model was based on the adsorption of a polymer molecule on the micelle surface. They concluded that the headgroup size plays an important role in interaction of such systems. For a small headgroup (as in the case of an anionic surfactant), the interaction is more amplified compared with a large headgroup (as in the case of a cationic surfactant). While a considerable amount of research has been conducted on drag reduction by pure polymer and pure surfactant, only a limited number of studies have been reported in the literature on the influence of polymer−surfactant interactions on drag reduction. Among them, one can mention the studies carried out by Mohsenipour et al.,72 Mohsenipour and Pal,73,74 Patterson et al.,75 Suksamranchit et al.,76 Matras et al.,77 and Broniarz-Press et al.78 In our previous studies,72−74 the aim was to investigate the drag reduction capability of different surfactant/polymer combinations in pipeline flow and to compare the results for the mixed surfactant/polymer systems with the results for pure polymer and surfactant. Systematic studies were carried out on the interactions of polymer and surfactant at bench scale, and the bench scale experiments were linked to pipeline flow. In these studies, it was observed that, when the addition of surfactant caused a change in the polymer solution properties, the drag reduction behavior of polymer solution was also affected. Under certain conditions, a strong synergy was observed between polymer and surfactant in enhancing drag reduction. The objective of the present work is to investigate mechanical degradation of drag-reducing polymers under turbulent flow condition and to determine the role of surfactant in improving the resistance of polymer molecules against mechanical degradation. As mechanical degradation of polymer molecules can have a strong negative impact on the drag reduction ability of polymer, it is important from a practical point of view to explore ways to mitigate mechanical degradation of polymer molecules.

ions, or uncharged small compounds like alcohols. Bewersdorff35 reported that these additives give a much smaller interfacial charge density on the micelle, and cause the surfactant molecules to become more packed and come closer together in the micelles. Organic counterions, typically hydroxy- or halo-substituted benzoates, at equimolar or higher concentrations are the most effective counterions for cationic DR surfactant solutions.13 The exact mechanism of DR by surfactant solutions remains unknown; however, some researchers have proposed that viscoelastic effects in surfactant solution could be responsible for turbulent DR. Bewersdorff and Ohlendorf36 showed using micro and integral scales of turbulent axial velocity fluctuations that both of the scales increase substantially in the case of dragreducing surfactant solutions as compared with Newtonian solvent. This increase in the size of eddies is likely due to an increase in the local viscosity resulting from the formation of the shear induced structures. Also, the Reynolds stresses are found to be close to zero or significantly lower than the Newtonian solvent.37 Both polymers and surfactants are effective as drag reducing agents. However, they have certain advantages and disadvantages over each other. For example, polymers become effective in drag reduction at relatively low concentrations, whereas the surfactants need concentrations above the critical micelle concentration (CMC). The shear induced structures in surfactant solutions are regenerative, whereas the polymer macromolecules undergo permanent mechanical breakdown under high shear stress conditions. This mechanical degradation limits the use of polymeric additives to low or moderate shear stress conditions. The degradation of polymer molecules causes a reduction in the amount of drag reduction. In stagnant aqueous polymer solutions, oxidative/reductive degradation is the main mechanism of polymer degradation. This process involves a chain reaction caused by free radical mechanism. Under turbulent flow conditions, the mechanical energy can initiate shear degradation of polymer molecules. Mechanical degradation takes place when the polymer solution is subjected to high shear conditions. When subjected to high shear conditions, the polymer chains begin to break. This type of degradation is also called shear degradation. When polymer molecules fracture under high shear conditions, the average molecular weight decreases. At the same time, the molecular weight distribution is also changed.38,39 This shear degradation of polymers has been studied by a number of investigators.40−47 The factors affecting shear degradation include molecular weight, polymer concentration, and structure of the polymer. A good review of the factors affecting polymer degradation in turbulent pipe flow can be found in the work of Mousa and Tiu.48 The combination of polymers and surfactants has been used in a variety of applications such as drug delivery, oil recovery, and cosmetics industries.49−54 It is well-known that the selfassembly of surfactant monomers occurs at the critical micelle concentration (CMC), whereas the interaction between polymers and surfactants begins at a different surfactant concentration called the critical aggregation concentration (CAC). The CAC is usually lower than the CMC.55−57 The interaction between polymers and different types of surfactants has been the subject of interest among several researchers.58−62 Anionic surfactants are considered more effective in binding to nonionic polymers. Studies support the idea that the size of the anionic headgroup and the hydrophobic 1292

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Figure 1. (a) Schematic diagram of the experimental setup and (b) calibration curves.

at 25 ± 0.5 °C using a Cannon Ubbelhode Dilution Viscometer (E610-75). The low-shear relative viscosity was determined from the measurements of solution flow time (tp) and solvent (water) flow time (tw) as

The presence of surfactant can cause coiling or extension of polymer molecules depending upon the types of polymer and surfactant and their interactions. According to some researchers, the coiling of polymer macromolecules does not change the stability of the polymer chains against shear degradation. Some researchers support the idea that an extended model, in which the polymer molecules are fully stretched, is more stable under high stress conditions.45,79 However, little or no experimental work has been carried out to verify these ideas for dragreducing polymers in pipeline flow under turbulent conditions. In this work, two different types of polymers (anionic and nonionic) and two different types of surfactants (cationic and anionic) were selected. The following combinations of polymer and surfactant were studied: cationic surfactant (octadecyltrimethylammoniumchloride, OTAC)/anionic polymer (polyacrylamide, PAM), cationic surfactant (OTAC)/nonionic polymer (polyethylene oxide, PEO), and anionic surfactant (sodium dodecyl sulfate, SDS)/nonionic polymer (PEO). On the basis of our previous studies,72−74 it is expected that addition of cationic surfactant octadecyltrimethylammoniumchloride (OTAC) to anionic polymer polyacrylamide (PAM) will cause coiling of polymer chains, whereas the addition of surfactant (cationic OTAC or anionic SDS) to nonionic polymer polyethylene oxide (PEO) will result in stretching or expansion of polymer chains.

ηr =

tp tw

(1)

The specific viscosity was calculated as follows: ηs = ηr − 1 =

t p − tw tw

(2)

Pipeline Experiments. The pipeline experiments were carried out in a closed loop system shown in Figure 1a. The test fluid was prepared in a large jacketed mixing tank present in the flow rig. The temperature inside the tank was maintained at 25 ± 0.5 °C by passing cold or hot water through the tank jacket with the aid of a temperature controller. Three straight stainless steel pipe test sections with different diameters were installed horizontally. The pressure taps were made by drilling small holes through the tube walls. The pressure taps were located far enough from the pipe entrance to ensure fully developed flow in the test sections where pressure drop measurements were made. For the purpose of this study, only the largest diameter pipe (34.8 mm) was employed so that the experiments were close to real applications. The entrance and test-section lengths were 1.54 and 3.05 m, respectively. Four pressure transducers (Rosemount and Cole-Parmer: 0−0.1, 0−0.5, 0−5, 0−10 psi) were installed. The pressure transducers were configured in such a manner that a desired pressure transducer could be easily connected to any of the pressure taps in use. The flow rate was monitored using a coriolis flowmeter. The data from the pressure transducers and flowmeter were collected using a computer data acquisition system. The flow rig consisted of two centrifugal pumps (low and high capacities). Polymer was degraded by circulating the solution in a flow loop consisting of a small pump and a pipeline for a certain duration. Every effort was made to ensure that the same conditions applied to all the solutions. Figure 1b shows the baseline experimental results for water flowing through the tubes of different diameters. The experimental friction factors are compared with the values obtained from the well-known Blasius equation ( f = 0.079*Re−0.25) for turbulent flow of Newtonian fluids in smooth pipes. For laminar flow, a comparison of experimental data is made with the standard laminar flow equation (f = 16/ Re). As shown in Figure 1b, a good agreement is observed between the experimental data and the results obtained from

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EXPERIMENTAL WORK Materials. Polyacrylamide (PAM), supplied by Hychem Inc., USA, is a copolymer of acrylamide and sodium acrylate with an average molecular weight of (10−12) × 106 g/mol and a charge density of 30%. Polyethylene oxide (PEO) (trade name Polyox WSR 303) with a molecular weight of 7 × 106 g/ mol was supplied by DOW Company, USA, in powder form. The anionic surfactant sodium dodecyl sulfate (SDS) with a purity of 98.6% was supplied by Duda Diesel LLC, USA. The cationic surfactant OTAC having a purity of 98.6% was supplied by Molekula Ltd. UK. The counterion used with the cationic surfactant was 99.5% pure sodium salicylate (NaSal) available in crystalline powder form and supplied by Wintersun Chemical, USA. Deionized water (electrical conductivity of 2.0−4.0 μS/cm) was used as a solvent. Test Facilities. To perform the conductivity measurements, a Thermo Scientific Conductivity meter (Orion 3 star) was used. The conductivity was measured at 25 ± 0.5 °C. A Fann coaxial cylindrical viscometer with a gap width of 0.117 cm was used for the measurement of shear viscosity at moderate to high shear rates. The viscosity measurements were carried out at 24 ± 1 °C. The relative viscosities at low shear rate were measured 1293

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Figure 2. Specific viscosity vs PEO concentration.

be below the polymer overlap concentration and one concentration (2000 ppm) above the overlap concentration (see Figure 2). For PAM experiments, three levels of OTAC concentration (300, 500, and 800 ppm) with no salt were used. For PEO/ OTAC mixtures, two different levels of OTAC concentration (1000 and 2500 ppm) were used with salt (NaSal); the mol ratio of salt/surfactant was kept equal to 2. The critical micelle concentration (CMC) of OTAC in the presence of salt (NaSal, molar ratio of 2) is around 700 ppm. Thus, the OTAC concentrations (1000 and 2500 ppm) were higher than the CMC value. For PEO experiments, the anionic surfactant (SDS) was also used at three different levels of concentration (1000, 3000, and 5000 ppm). Coiling of Polymer Chains. Effect of Cationic Surfactant on Mechanical Degradation of Anionic Polymer PAM. Figure 3 shows friction factor versus Reynolds number data for pure PAM solution (500 ppm) and a mixed PAM (500 ppm)/ OTAC (500 ppm) system, with and without mechanical degradation. For the degradation experiments, the solution is subjected to 30 h of shear degradation by circulation in the flow loop. The following points should be noted from Figure 3: (a) Pure 500 ppm PAM solution (solution without OTAC) exhibits very good resistance against shear degradation. The polymer solution retains its drag reduction ability even after 30 h of shearing, (b) the addition of surfactant OTAC to polymer solution does not have any significant effect on the drag reduction ability of the polymer when the solutions are fresh and there is no mechanical degradation, (c) when subjected to 30 h of shear, pure polymer solution and mixed polymer/ surfactant solution exhibit very different behaviors. Pure polymer solution retains its drag reduction ability, whereas mixed polymer/surfactant solution loses its drag reduction ability by a large amount (see Figure 3 and Table 1). The shear degradation behavior of the PAM/OTAC system for a different set of concentrations (250 ppm PAM/300 ppm OTAC) is depicted in Figure 4. The pure polymer and mixed polymer/surfactant systems are subjected to 3 h of shear degradation. Once again, the mixed polymer/surfactant solution behaves differently from pure polymer solution. While the pure polymer solution retains its drag reduction ability to a large extent (especially at high Reynolds number),

the standard friction factor relationships. This indicates that the experimental setup and methodology are satisfactory. The following equations are used to calculate the friction factor: τ f= 1 w2 ρV (3) 2

τw =

ΔP·D 4L

(4)

where τw is the shear stress at the wall, ρ is the density of the solution, V is the mean velocity, ΔP is the pressure drop, D is the pipe diameter, and L is the length of the pipe test section. For non-Newtonian power-law solutions, the generalized Reynolds number, defined below, is used: ReG =

DnV 2 − nρ k 8

n

( 6n n+ 2 )

(5)

where k and n are power-law constants. The percent drag reduction (%DR) is defined as follows: %DR =

f0 − f f0

× 100 (6)

where f is the friction factor of the drag reducing fluid and f 0 is the friction factor of the corresponding non-drag-reducing fluid at the same Reynolds number. For non-Newtonian power-law fluids, the following Dodge−Metzner30 equation is used to calculate f0:

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1 4 0.4 = 0.75 log(ReGf01 − (n /2) ) − 1.2 n n f0

(7)

RESULTS AND DISCUSSION Three different concentrations of anionic polymer PAM (100, 250, and 500 ppm) and three different concentrations of nonionic polymer PEO (500, 1000, and 2000 ppm) were selected for mechanical degradation study. These concentrations were selected on the basis of the specific viscosity plots and previous work of the authors.72 For example, two concentrations of PEO (500 and 1000 ppm) were chosen to 1294

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Figure 3. Friction factor and %DR vs ReG for fresh and mechanically degraded 500 ppm PAM/OTAC systems.

the mixed system loses a large portion of its drag reduction ability when subjected to shear degradation. Figure 5 shows the interaction between anionic PAM and cationic OTAC schematically. The polymer molecules collapse upon interaction with OTAC monomers. A high ratio of OTAC/PAM is expected to give even more accessibility of polymer anionic sites to OTAC molecules, leading to a greater degree of electrostatic neutralization for polymer molecules.72 Figure 6 shows the plots of power-law parameters for various freshly prepared PAM/OTAC mixtures in deionized (DI) water. The flow behavior index n increases and the consistency

Table 1. Percent Drag Reduction for Fresh and Mechanically Degraded 500 ppm PAM/OTAC Systems

ReG

%DR, 500 ppm PAM/ 500 ppm OTAC (no degradation)

%DR, 500 ppm PAM/ 500 ppm OTAC (30 h degradation)

percent loss in drag reduction

10 000 15 000 20 000 25 000 30 000

56.5 64.1 69.6 73.8 77.3

12.5 20.8 26.7 31.2 34.9

77.8 67.6 61.7 57.7 54.8 1295

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Figure 4. Friction factor and %DR vs ReG for fresh and mechanically degraded 250 ppm PAM/OTAC systems.

Figure 5. Schematic representation of interaction between anionic PAM and cationic OTAC.

index k decreases with the increase in OTAC concentration for any given PAM concentration. Pure polymer solution behaves as a shear thinning fluid. When OTAC monomers are added to the polymer solution, they neutralize the anionic charges on the backbone of the PAM polymer, resulting in the collapse of the macromolecule chains. Consequently, the consistency index k decreases and the flow-behavior index n approaches unity, indicating a change in the solution flow behavior from nonNewtonian shear-thinning to Newtonian. Figure 7 shows the variation of power-law parameters for 500 ppm PAM/OTAC mixtures with the duration of mechanical

Figure 6. Power-law parameters for different PAM/OTAC mixtures in DI water.

degradation. The consistency index k decreases and the flow behavior index increases with an increase in the duration of shear. This is consistent with the pipeline results that the mixed polymer/surfactant systems lose their drag reduction ability when subjected to shear degradation. 1296

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Figure 7. Variation of power-law parameters with duration of mechanical shear for different mixtures of PAM/OTAC at a fixed PAM concentration of 500 ppm.

Expansion of Polymer Chains. Polyethylene oxide (PEO), a nonionic polymer, is considered to be a good drag reducer. By increasing the PEO concentration, the %DR increases initially and then levels off after a certain concentration (see Figure 8).

Figure 9. Effect of mechanical degradation on friction factor and %DR of 2000 ppm PEO solution.

impact was more pronounced at low PEO concentrations and high surfactant concentrations.73 Figure 10 shows the effect of mechanical degradation on % DR for the (500 ppm PEO)/(1000 ppm OTAC + NaSal (mol

Figure 8. Variation of %DR with PEO concentration at Re = 60 000 (D = 34.8 mm).

PEO readily undergoes mechanical degradation when subjected to turbulent conditions in pipeline flow. Figure 9 shows the effect of mechanical degradation on drag reduction behavior of 2000 ppm PEO solution. The polymer loses its drag reduction ability when it flows through a pipe even for a short period of time under turbulent conditions. For example, when the polymer solution is subjected to 1.5 h of shear under turbulent conditions at Re = 40 000, %DR decreases from 70 to 35, which means almost a 50% drop in DR ability. The %DR falls below 10% after 27 h of shear. A series of experimental runs were conducted to find out the effect of expansion of PEO chains on resistance against mechanical degradation. The stretching of PEO chains was caused by the addition of surfactant. The addition of a cationic surfactant OTAC (with salt NaSal) or an anionic surfactant SDS to PEO is known to cause expansion of PEO chains. Mechanical Degradation of Mixture of PEO and Cationic Surfactant OTAC. It was found in our earlier work that a significantly higher degree of drag reduction (lower friction factors) is achieved when cationic surfactant OTAC is added to a PEO solution. The addition of surfactant to the polymer always increased the extent of drag reduction; however, the

Figure 10. Effect of mechanical degradation on %DR for pure PEO solution and for a mixed PEO/OTAC system.

ratio = 2)) system. For comparison purposes, the degradation behavior of pure 500 ppm PEO is also shown. The following points should be noted from Figure 10: (a) For freshly prepared solutions (no mechanical degradation), the addition of surfactant OTAC to 500 ppm PEO solution enhances drag reduction (%DR increases). The increase in %DR is due to expansion of polymer chains. The OTAC micelles become attached to the backbone of polymer chains, and the repulsion between neighboring micelles causes the expansion of the 1297

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ability. When subjected to 24 h of shear degradation, the mixture loses only about 33% of its original %DR. Figure 12 shows the effect of shear degradation on friction factor for 1000 ppm PEO combined with 1000 ppm OTAC/

chains. (b) Upon shearing for 3 h, the pure polymer solution loses its drag reduction ability to a large extent, whereas the mixed system retains its drag reduction ability to a significant extent. Thus, the addition of surfactant improves the resistance of polymer molecules against mechanical degradation. When the surfactant (OTAC) concentration is increased to 2500 ppm, PEO becomes even more resistant to shear degradation (see Figure 11). An interesting point to note is that the degraded mixtures of PEO and OTAC exhibit more drag reduction than the freshly prepared (no degradation) pure PEO solution.

Figure 12. Effect of mechanical degradation on friction factor for pure 1000 ppm PEO solution and for the mixed 1000 ppm PEO/1000 ppm OTAC system.

NaSal (MR = 2). For comparison purposes, the data for 1000 ppm pure PEO solution are also shown. This combination of PEO and OTAC concentrations is an exception in that: (a) no enhancement of drag reduction is observed for fresh solutions (without shear degradation) when surfactant is added to the polymer solution (friction factors are the same for pure polymer and mixed polymer−surfactant systems) and (b) no improvement in resistance against mechanical degradation is observed upon the addition of surfactant to PEO solution (the mixture and pure polymer both lose their drag reduction ability when subjected to 3 h of shear). These observations indicate that, for the given combination of polymer and surfactant concentrations, there occurs negligible interaction between surfactant and polymer and hence negligible expansion of polymer chains; the reason for negligible interaction between the polymer and surfactant at this combination of concentrations is not clear at present. At a higher PEO concentration of 2000 ppm, the resistance against mechanical degradation improved once again upon the addition of 1000 ppm surfactant OTAC to the system (see Figure 13). Mechanical Degradation of Mixture of PEO and Anionic Surfactant SDS. To investigate the role of anionic surfactant SDS on mechanical degradation of polymer PEO, three concentration levels of PEO (500, 1000, and 2000 ppm) and three concentration levels of SDS (1000, 3000, and 5000 ppm) were selected to perform a series of experiments. In the first phase, the synergistic effects of surfactant and polymer on drag reduction were determined for freshly prepared solutions without mechanical degradation. In the second phase, the effect of surfactant on mechanical degradation of polymer was investigated. The interaction points (e.g., CAC and PSP) between surfactant SDS and polymer PEO were determined using electrical conductivity measurement. The critical aggregation concentration (CAC) is the concentration of surfactant where the interaction between the polymer and the surfactant begins. The polymer saturation point (PSP) is the surfactant concentration where the polymer molecules become saturated with surfactant. The interaction points are helpful in the

Figure 11. Effect of mechanical degradation on friction factor and % DR for pure PEO solution and for a mixture of PEO and OTAC.

Table 2 shows the loss in percent drag reduction when the polymer/surfactant system is subjected to shear degradation. Table 2. Loss in %DR with Shear Degradation of Mixture of 500 ppm PEO and 2500 ppm OTAC + NaSal (MR = 2) ReG

loss in %DR after 3 h

loss in %DR after 24 h

30 000 35 000 40 000 45 000 50 000 55 000 60 000 average loss

21.0 19.4 18.0 16.9 16.0 15.1 14.4 17.3

41.1 37.5 34.6 32.2 30.1 28.3 26.7 32.9

While pure 500 ppm PEO solution loses almost all of its drag reduction ability (i.e., %DR is negligible) after 3 h of shear degradation, the mixture containing 500 ppm PEO and 2500 ppm OTAC/NaSal (MR = 2) loses only about 17% of its DR 1298

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Figure 13. Effect of shear degradation on friction factor and %DR for 2000 ppm PEO/1000 ppm OTAC (MR = 2) system.

interpretation of pipeline drag reduction data. The values of CAC and PSP are given in Table 3. The CAC value decreases and the PSP value increases as the concentration of PEO is increased. Table 3. CAC and PSP Values for the PEO/SDS System Obtained from the Conductivity Method solution

CAC (ppm)

pure SDS in DI water 500 ppm PEO/SDS 1000 ppm PEO/SDS 2000 ppm PEO/SDS

2310 (same as CMC) 1770 1750 1460

PSP (ppm)

Figure 14. Friction factor vs ReG for freshly prepared (without mechanical degradation) PEO/SDS mixtures.

2780 2950 3280

1000 ppm PEO solution is around 1700 ppm SDS, it is not surprising that the decrease in friction factor is negligible at 1000 ppm SDS. For the 2000 ppm PEO system, the addition of 1000 ppm SDS results in a significant decrease in friction factor. The CAC value for 2000 ppm PEO solution is about 1460 ppm SDS. It appears that, under turbulent flow conditions, the interaction between the surfactant molecules and the polymer chains occurs even below the CAC value (obtained under quiescent conditions). Upon increasing the SDS concentration from 3000 to 5000 ppm, the decrease in friction factor is small as the polymer molecules are saturated with surfactant. The PSP value of 2000 ppm PEO solution is about 3280 ppm SDS. In order to explore the effect of surfactant (SDS) addition to PEO solutions on mechanical degradation, different mixtures of PEO and SDS were prepared and degraded under the same process conditions (in 34.8 mm pipe). Figure 15 shows the effect of shear degradation on friction factor and %DR for 500 ppm PEO/SDS mixtures. The results show that the pure 500 ppm PEO solution undergoes

Figure 14 shows friction factor and %DR versus ReG for freshly prepared PEO/SDS mixtures containing 500, 1000, and 2000 ppm of PEO and different SDS concentrations. Increasing the SDS concentration at any given PEO concentration reduces the friction factor, resulting in an increase in the value of %DR. For the 500 ppm PEO system, the friction factor decreases considerably by increasing the SDS concentration until 3000 ppm. Above 3000 ppm, the decrease in friction factor with further increase in SDS concentration is negligible. It should be noted that the PSP for 500 ppm PEO solution is about 3000 ppm SDS (see Table 3). Once the polymer molecule is saturated with the surfactant molecules, no further change in the conformation of polymer molecules is possible, and therefore, no further enhancement in drag reduction (or decrease in friction factor) occurs. For the 1000 ppm PEO system, the addition of 1000 ppm SDS has a little effect on friction factor. As the CAC value for 1299

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PEO/SDS system even when the amount of SDS added is lower than the CAC value. When polymer is subjected to high shear stress conditions, the micelles present on the backbone of the polymer chains help them to resist mechanical breakdown. It appears that the addition of 1000 ppm SDS is not enough to form the required number of micelles on the backbone of polymer molecules to improve the resistance of polymer chains against mechanical degradation. Figure 16 shows the influence of shear degradation on friction factor and %DR for 1000 ppm PEO/SDS mixtures.

Figure 15. Effect of mechanical degradation on friction factor and % DR for 500 ppm PEO solutions containing different amounts of SDS.

mechanical degradation much faster compared with PEO/SDS mixtures (see Table 4). While the pure 500 ppm PEO solution Table 4. Average Percent Loss in DR for 500 ppm PEO/SDS Mixtures Subjected to 3 and 24 h of Shear Degradation time (h) after 3h after 24 h

percent loss in DR percent loss in DR for percent loss in DR for for pure 500 ppm 500 ppm PEO/3000 500 ppm PEO/5000 PEO solution ppm SDS solution ppm SDS solution 70.8

41.5

22.4

90.2

67.4

51.3

Figure 16. Effect of mechanical degradation on friction factor and % DR for 1000 ppm PEO solutions containing different amounts of SDS.

Addition of SDS improves the resistance against mechanical degradation. The average percent loss in drag reduction (DR) for 1000 ppm PEO/SDS solutions is presented in Table 5. For

loses almost 70% of its DR ability, the same polymer solution loses only 40 and 22% of its DR ability when the surfactant SDS is added at concentrations of 3000 and 5000 ppm, respectively. The SDS micelles present on the backbone of PEO chains can cause the polymer chains to expand. As the chains extend, the %DR and resistance to shear degradation improve. It is interesting to note the difference in the behaviors of polymer/surfactant solutions containing different SDS concentrations of 3000 and 5000 ppm. These two solutions show almost the same %DR when fresh (without degradation); however, the 3000 ppm SDS solution loses its drag reduction ability much more rapidly than the 5000 ppm SDS solution when subjected to shear degradation. With the addition of 1000 ppm SDS to 500 ppm PEO, drag reduction was enhanced (see Figure 14). However, no improvement against mechanical degradation was observed. As pointed out earlier, under turbulent flow conditions, the interaction of polymer and surfactant starts earlier than under quiescent conditions. This results in an improvement in the drag reduction ability of the

Table 5. Average Percent Loss in DR for 1000 ppm PEO/ SDS Mixtures Subjected to 3 and 24 h of Shear Degradation time (h) after 3h after 24 h

percent loss in DR for pure 1000 ppm PEO solution

percent loss in DR for 1000 ppm PEO/3000 ppm SDS solution

percent loss in DR for 1000 ppm PEO/ 5000 ppm solution

68.2

40.2

17.1

80.2

71.4

37.6

the 5000 ppm SDS solution, the loss in the DR ability is about 17% after 3 h of shear and about 37% after 24 h of shear degradation. This is a great improvement in resistance against mechanical degradation as compared with pure 1000 ppm PEO solution which shows about 68% reduction in the DR ability after 3 h of shear. For the 3000 ppm SDS solution, the loss in the DR ability was about 40% after 3 h and about 70% after 24 1300

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ACKNOWLEDGMENTS The financial assistance provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada is appreciated.

h of shear. While this solution also shows a significant improvement in degradation resistance as compared with pure PEO solution, the improvement is not as prominent as that shown by 5000 ppm SDS solution.

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CONCLUSIONS

D f0 f k L n Q Re ReG tp tw V

New aspects of polymer degradation in turbulent pipeline flow are reported. The mechanical degradation of drag-reducing polymers is investigated experimentally in the presence of surfactants in turbulent pipeline flow of water. The following combinations of polymer and surfactant are studied: anionic polymer/cationic surfactant, nonionic polymer/cationic surfactant, and nonionic polymer/anionic surfactant. The polymers studied are: PAM (anionic polyacrylamide) and PEO (nonionic polyethylene oxide). The surfactants used are OTAC (cationic) and SDS (anionic sodium dodecyl sulfate). On the basis of the experimental results, the following conclusions can be drawn: • The drag reduction capability of anionic polymer PAM is reduced upon the addition of oppositely charged cationic surfactant (OTAC) molecules due to charge neutralization and hence coiling of PAM chains. Furthermore, the presence of surfactant accelerates mechanical degradation of polymer chains as reflected in the pipeline drag reduction data. Thus, it can be concluded that coiling of polymer chains (due to charge neutralization by surfactant molecules) makes them more susceptible to mechanical degradation. • The addition of cationic surfactant OTAC to nonionic polymer PEO enhances drag reduction and improves the resistance of polymer chains against shear degradation. The improvement is more pronounced in solutions with low polymer concentration and high surfactant concentration. • The addition of anionic surfactant SDS to nonionic polymer PEO always improves the extent of drag reduction up to a surfactant concentration corresponding to PSP (polymer saturation point); however, the impact is more at low PEO concentration (e.g., 500 ppm) and high surfactant concentration. • The resistance of PEO chains against mechanical degradation improves significantly upon the addition of surfactant SDS to the solution. The improvement in resistance against degradation is more at high SDS concentrations (e.g., 5000 ppm). • Although an improvement in resistance against shear degradation is observed when either OTAC or SDS is added to PEO, the effect is more prominent in the case of anionic surfactant SDS. As the addition of surfactant (OTAC or SDS) to polymer PEO causes stretching of polymer chains, it can be concluded that the expansion of polymer chains makes them less susceptible to mechanical degradation.

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Article

NOMENCLATURE Pipe diameter Friction factor of non-drag-reducing fluid Solution friction factor Flow consistency index Pipe length Flow behavior index Flow rate Reynolds number Generalized Reynolds number Polymer flow time Water flow time Mean velocity

Greek Letters

γ̇ ρ τ ηr ηs

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Shear rate Density Shear stress Relative viscosity Specific viscosity

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1301

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