Breakup of Oil Jets into Droplets in Seawater with Environmentally

Subscriber access provided by - Access paid by the | UCSF Library

Article

Breakup of Oil Jets into Droplets in Seawater with Environmentally Benign Nanoparticle and Surfactant Dispersants Guangzhe Yu, Jiannan Dong, Lynn M Foster, Athena E. Metaxas, Thomas M Truskett, and Keith P. Johnston Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503658h • Publication Date (Web): 19 Nov 2014 Downloaded from http://pubs.acs.org on December 12, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Industrial & Engineering Chemistry Research

Breakup of Oil Jets into Droplets in Seawater with Environmentally Benign Nanoparticle and Surfactant Dispersants Guangzhe Yu†, Jiannan Dong†, Lynn M. Foster, Athena E. Metaxas, Thomas M. Truskett and Keith P. Johnston* McKetta Chemical Engineering Department, University of Texas, Austin, Texas, USA †: equal contribution first authors; *: corresponding author

Abstract During deep sea oil leaks, dispersants may be used to break up the oil into droplets smaller than about 70 µm, which may then be bioremediated by bacteria before they reach the ocean surface. To investigate the mechanism of droplet formation, a flowing oleophilic stream containing amphiphiles was mixed with flowing dodecane and then atomized through a 0.25 mm circular nozzle as a function of dispersant type, concentration and jet velocity. The minimum droplet diameters were 2.2, 4.5, and 24 µm for only 5 w:v % amphiphile in the oil phase for Corexit 9500A, Tergitol 15-S-7 (C12H25CH(OCH2CH2)7OH) and a silica nanoparticle/Span 20 mixture, respectively. For Tergitol 15-S-7, the droplet size exhibited the expected scaling with Weber number (We) at low viscosity numbers (Vi 50), where inertial forces overcome viscous forces. However, in the case of the silica nanoparticle/Span 20 mixture, the magnitude of the exponent of We scaling was found to be smaller than -3/5. A better understanding of how low concentrations of dispersants (with relatively high

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

oil/water interfacial tensions) may be used to provide a sufficient We with high inertial forces (high Re) in jets to form small oil droplets is of interest for advancing environmental protection in the undesired event of a deep sea oil leak.

1. Introduction We appreciate all that Scott Fogler has taught us for 50 years at Michigan and in his papers about interfacial phenomena and emulsions, and even more so, about how to think about problem solving both in research and in the classroom. As we read his papers all the way back to the late 70’s energy crisis, we are more prepared for the current one. During deep-sea oil leaks, potential damage to the environment may be mitigated by dispersing the oil into small droplets which are then ingested by bacteria and decomposed. The emulsification of oil has been studied extensively at the relatively low shear rates commonly encountered in surface waves and ocean currents by employing swirling flasks,1, 2 baffled flasks,2, 3 and wave tanks.4-6 Gopolan and Katz have examined oil microthreads induced by turbulence of rising oil droplets from 300 to 1400 µm in diameter.7 For a deep sea blowout, it would be desirable to form droplets smaller than ~ 70 µm such that the rise velocity is low enough to enable effective natural bioremediation by bacteria before the droplets reach the ocean surface.8 To date, only one experimental study is available at large scale, the DeepSpill study, carried out at a depth of ~800 m in the Norwegian Sea.9 For the accidental Macondo well oil spill, the injection of dispersant had a profound effect on the amount of oil that reached the sea surface. However, the nature of the emergency made it difficult to characterize the mechanism of oil


Page 2 of 36

Page 3 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


break up and dispersion. To further prepare for the unwanted possibility of a future blow out, it would be advantageous to understand how various types of dispersants influence droplet formation in laboratory jets under a variety of flow conditions. The commercial chemical dispersant mixture Corexit 9500A has been shown to be effective for forming oil-in-water emulsions for a range of crude oils in seawater under low shear conditions.10 A primary component in Corexit 9500A, Tween 80, is resistant to desorption upon dilution and forms a robust interfacial film, which may favor droplet formation and stability.11 Recently a number of novel dispersants based on nanoparticles including silica,12 clay,13 iron oxide,14 and carbon black15 have been designed for oil-in-seawater emulsions on the basis of their interfacial properties. The nanoparticles provide remarkable stability against coalescence due to irreversible adsorption at the oil-water interface.16 These dispersants were highly efficient in the formation and stabilization of small oil droplets on the order of 10 to 100 µm in sea water at low concentrations for experiments with a high speed rotary homogenizer at high shear rates. These studies provide a useful basis for current studies of oil jets sprayed into seawater which exhibit characteristics encountered in deep-sea oil leaks. The breakup of oil in a turbulent jet is governed by inertial, viscous, and interfacial forces described by the Reynolds (Re, inertial/viscous forces), Weber (We, inertial/interfacial forces), and Ohnesorge (Oh, viscous/interfacial forces) numbers.17 At high values of Oh and Re, above a threshold in We, droplets break up by atomization, and at low values by Rayleigh capillary instabilities.18, 19 To reduce droplet size upon atomization, the We and Oh numbers may be increased by reducing the interfacial tension with the addition of surface active agents. In a turbulent jet where We number scaling is applicable, the reduced droplet size scales with the downstream distance and the exit energy



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

dissipation rate, resulting in the relationship d/D = A We -3/5, where d is oil droplet size, D is nozzle inner diameter, and A is an empirical coefficient.20 In this regime, the jet break up requires sufficient inertia to overcome the Laplace pressure of an oil droplet (2σ/r), where σ is the interfacial tension. This model has been modified by Wang and Calabrese21 to include viscous stresses by introducing the viscosity number Vi = We/Re. Relatively few laboratory studies have investigated the breakup of oil in jets at high shear energy conditions which are relevant to subsea oil blowouts. Masutani et al. examined the evolution of underwater jets of crude oil and established three different breakup modes.22 Using a meso-scale test facility, Brandvik et al. studied the crude oil jet breakup with and without the dispersant Corexit 9500A.23 In addition, Johansen et al. developed a correlation for the droplet size in terms of the semi-empirical model of Wang and Calabrese, which utilizes a viscosity number Vi = We/Re to describe the effect of viscous stresses.24 The oil droplet size was governed by We at low Vi where inertial forces overcome interfacial forces, and by Re at high Vi where they overcome viscous forces.21 Our objective was to measure and analyze the size of oil droplets formed in jets in seawater at various We and Re for both molecular surfactants and particle-based amphiphiles. As the inertial forces increase upon increasing the flow velocity U, the same value of We = ρU2D/σ may be achieved with a higher σ, and thus a smaller amount of a dispersant amphiphile. Thus, it is likely that oil droplets much smaller than 100 µm may be formed in jets with low dispersant concentrations, well below the value of 20 wt% commonly used for surface spills and in the Macondo well leak. In each case a concentrated dispersant stream in an oleophilic phase was mixed in a tee with the primary oil stream, dodecane, and the mixture was atomized through a circular nozzle (D = 0.25 mm). Small oil droplets were formed with Corexit 9500A as in recent studies23, 24 and also with a nonionic surfactant Tergitol 15-S-7 (C12H25CH(OCH2CH2)7OH) even at surfactant concentrations down to 3 w:v%. Given this basis, we


Page 4 of 36

Page 5 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


investigated particle-based amphiphiles consisting of silica nanoparticles in isopropanol (IPA) to stabilize the oil droplets against coalescence augmented with very low concentrations of an environmentally benign low HLB (hydrophilic-lipophilic balance) food grade surfactant, Span 20 to lower σ. The oleophilic dispersant stream and the dodecane were mixed upstream of the jet nozzle to make the amphiphiles available to migrate rapidly to the oil/brine interface upon atomization. The silica and Span 20 acted synergistically in producing oil droplets as small as 24 µm at a total amphiphile concentration in the oil jet of 5.5 w:v%. For each dispersant system, the oil droplet size was correlated in terms of We or Re as a function of Vi for various values of σ. Ultimately, an understanding of the droplet size in laboratory scale jets may be utilized to guide the design of novel classes of highly efficient and nontoxic dispersants for dispersing oil in deep sea jets, in the event of an undesired future blow out.

2. Experimental 2.1. Materials Surface modified colloidal silica IPA-ST (30 wt% silica dispersed in isopropyl alcohol) was a gift from Nissan Chemicals and is characterized in the supplemental section by DLS to determine the hydrodynamic diameter (Figure S.1), FTIR spectroscopy (Figure S.2) and TGA (Figure S.3). The organic content on the surface was very small according to both FTIR spectroscopy and the weight loss via TGA. Finally the highly negative zeta potential measured in IPA indicated the presence of the charged silanol groups. Corexit 9500A was a gift from Nalco-Champion. Span 20 (Sorbitan



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

monolaurate) and Tergitol 15-S-7 were purchased from Sigma-Aldrich. Synthetic seawater (SSW) was purchased from Ricca Chemical Company. Dodecane was obtained from Acros Organics Company, and was passed through a basic alumina column to remove amphiphilic impurities prior to use. Deionized (DI) water was prepared with a Nanopure II water purification system from Barnstead Company. Given that Corexit 9500A is a complex mixture and that the silica particles have high molar masses, the dispersant concentrations are expressed in w:v%, rather than molar concentrations.

2.2. T-mixer Circular Nozzle Jet System A schematic of the jet system apparatus is presented in Figure 1, along with photographs of the oil jet with a conical shape indicating intense atomization. A T-fitting with 0.030 inch inner diameter (I.D.) tubing was used to combine the oil stream and the dispersant stream upstream from the oil jet exit. Pure dodecane (oil stream) and dispersant stock solution (dispersant stream) were pumped and mixed in situ through the T-fitting before the combined oil jet was sprayed out from the nozzle with an I.D. of 0.01 inch (i.e. 0.25 mm) into the receiving synthetic seawater (SSW) phase in a glass vial. The oil-based streams were pumped with peristaltic pumps purchased from Cole-Parmer through #14 viton tubing. The energy of the jet atomized the oil phase into small droplets. The flow rate was calibrated by flowing liquid for a certain amount of time, and the variation was within 3%. In the current studies, the oil jet / aqueous phase (in the beaker/vial) volume was kept constant at 2ml:14ml. The oil droplet size in the emulsion was then measured with static light scattering (Malvern Mastersizer S). Typically, 100μL of emulsion sample was added into the Malvern batch cell containing ~14ml SSW, and an obscuration of 10%-30% was obtained. The volume-averaged drop size d[4,3] was reported as the average value of triplicate measurement. All the measurements were done at atmospheric pressure and room temperature.


Page 6 of 36

Page 7 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


2.3. Interfacial Tension Measurement The oil-water interfacial tension (IFT) was measured with the pendant drop shape analysis method. Equilibrated IFT was ensured by keeping a pendant SSW drop in the oil phase containing a known concentration of dispersant. The drop shape profile was recorded and fitted to the YoungLaplace equation using a commercial drop shape analyzer (CAM200, KSV Ltd., Finland). Ten measurements were taken with 10-second intervals to obtain an average value. For Corexit 9500A in dodecane with SSW, the IFT was measured with a spinning-drop tensiometer (Kruss, SITE 100) at 25˚C with a spinning rate of 4000 rpm.

2.4. Partitioning of silica nanoparticles between SSW and dodecane In order to study the partitioning of silica nanoparticles between dodecane and SSW, a dispersion of 1 w:v% silica particles in dodecane without isopropyl alcohol was prepared. The IPA-ST was diluted from a 30 wt% stock solutions to 1 w:v% with purified dodecane, and then the IPA was removed under vacuum at room temperature. The amount of solvent lost was compensated by adding dodecane and/or surfactant (Span 20) stock solutions to produce the desired surfactant concentrations. The mixtures were allowed to stand for 1 hour to allow surfactant to adsorb onto the silica nanoparticles. Then 1 ml of the mixtures was shaken to ensure homogeneity, and gently dropped on top of 1 ml of SSW in 5 ml vials. The vials were equilibrated for 1 hour to allow the nanoparticles to partition between the phases. Photographs were taken to qualitatively visualize the partitioning behavior of the nanoparticles.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

3. Results and Discussion 3.1. Partitioning of silica nanoparticles modified with Span 20 Before discussing droplet breakup, it is instructive to examine the phase behavior of the amphiphiles. The relatively hydrophobic food-grade sugar-based commercial surfactant (Span 20) was combined with the silica NPs (surface area=350 m2/ml assuming a spherical shape, calculated from the hydrodynamic diameter (8.6 nm) from DLS described in Figure S.2 in the supplemental material). The surfactant was used to reduce the surface tension, and to modify the HLB of the otherwise highly hydrophilic silica nanoparticles. The partitioning behavior of Span 20 modified silica NPs between dodecane and SSW was tested to determine the HLB of the modified solid dispersants. The partitioning experiments utilized 1 w:v% silica NPs in dodecane, upon dilution from the 30 w:v% silica IPA dispersion. After dilution, Span 20 was added at levels of 0, 0.001, 0.01 and 0.1 w:v% . These mixtures are termed as unmodified, 1:1000, 1:100, and 1:10 modified silica NPs initially in dodecane. 1 ml of the silica NP dispersion in oil was carefully dropped on top of 1 ml of SSW. As presented in Figure 2, after 1 hour of equilibration, silica NPs without Span 20 and with 0.001 w:v% Span 20 underwent complete transport into the aqueous phase. However, particles were seen at the oil/water interface in the vials with 0.01 w:v% and 0.1 w:v% Span 20 modified silica, as a consequence of the hydrophobicity imparted by the adsorbed surfactant. Furthermore, most of the particles resided on the oil/water interface in the vial with 0.1 w:v% Span 20. In this case with the greatest modification


Page 8 of 36

Page 9 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


to add hydrophobicity, the aqueous phase was completely transparent indicating a very low concentration of NPs. Similarly, Dong et al. observed that montmorillonite clay particles settled from a dodecane phase onto the dodecane/SSW interface upon surface modification with a cationic surfactant, Ethomeen O/12 LC for a surfactant to clay ratio above 1:50 (w:w).13 Furthermore, Dong et al. correlated the change in partitioning behavior of the particles with the contact angle for a SSW droplet on mica (model for clay) in the presence of dodecane. The contact angle of the modified particle became more hydrophobic with an increase in the concentration of the surfactant.13 The behavior is similar for the silica NPs. The silica NPs without added surfactant are too hydrophilic (high HLB), and favor SSW markedly over dodecane. In contrast, Span 20 alone is too hydrophobic (low HLB) to stabilize oil-inwater emulsions. Thus, the partitioning experiments provide useful guidelines for modifying the silica to achieve an HLB whereby the particles favor the oil/water interface, which ultimately favors emulsion stabilization. The relatively low HLB of 8.6 for Span 20 and the high partitioning to the oil phase are beneficial for avoiding loss of surfactant to the aqueous phase during dispersion of oil. Likewise the low loss of silica NPs to the aqueous phase will be small with the 1:10 of [Span 20]:[silica] w:w ratio, where the surface modification is largest. The design of amphiphiles with low loss to the vast oceanic aqueous phase is an important goal. Additionally, the low loss to the aqueous phase will maintain a higher concentration at the interface needed for emulsion stabilization.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

3.2. Interfacial tension reduction by dispersants The dynamics of IFT reduction by surfactants influence atomization in transient jets. Given the challenges in measuring and characterizing the dynamic behavior we measured instead the simpler equilibrium IFT with a pendant droplet. The IFT was used to determine We as a function of surfactant composition and initial concentration in the oil phase. Table 1 shows that 5% w:v% Corexit 9500A in dodecane lowers the IFT to 0.03 mN/m, from an initial value of 51 mN/m without surfactant. Such a low IFT not only aids oil jet drop breakup, but also increases stabilization of the newly formed oil droplets against coalescence and Ostwald ripening. 25

The environmentally benign surfactant Tergitol

15-S-7, an ethoxylated alcohol with an HLB of 12.1, reduced the IFT between dodecane and SSW from 1.6 to 0.13 mN/m with increasing concentrations from 0.1 to 5 w:v%, respectively. The lowest concentration is already well above the critical micelle concentration of 0.0038 wt.% and produces a very low IFT. The interfacial tension was measured for a series of samples with the silica NPs in IPA with and without added surfactant. The silica NP dispersion in IPA was mixed with dodecane to give final concentrations of 5 w:v% silica and 11 w:v% IPA. As a control, the IFT of 11 w:v% of IPA in dodecane versus SSW was measured without silica and found to be 7.4 mN/m, indicating modest interfacial activity of the very small amphiphile, IPA. The effect of the silica NPs on IFT was not measured as silica NPs coated with low molecular weight ligands on the surface have been found to have little effect on the IFT.16 Within an hour the above mixture of silica NPs in dodecane settled in this relatively nonpolar mixture. Because of the settling, the IFT of the supernatant was measured against SSW and found to range from 1.9 to 3.9 mN/m, lower than the case for IPA alone (7.4 mN/m) as shown


Page 10 of 36

Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


in Table 1. Furthermore, σ decreased with an increase in the Span 20 concentration. Part of the surfactant may have been removed from the system with the NPs during settling. These values of σ were used to approximate We for the oil jets. In the case of the actual jets, the silica NPs do not have sufficient time to settle during the time of transit from the mixing tee to the jet nozzle.

3.3. Effect of dispersant concentration and flow rate on droplet size In the deep-sea Macondo well oil spill, a highly concentrated dispersant in an oleophilic phase was injected directly into the oil jet. To simplify the injection in laboratory experiments, a highly concentrated 20 w:v% dispersant stock solution (diluted from as received conditions) was mixed with the pure dodecane model oil stream at various ratios. One of the advantages of the 8.6 nm silica NPs in IPA is the high colloidal stability despite the very high silica concentration. The flow rate of the mixed jet was set at 8 and 70 ml/min, and nozzles with two different I.D.s (0.25 or 0.84 mm) were used to measure the effect of shear energy on the jet breakup as described by the volume average droplet size (d[4,3]). Figure 3a shows oil-jet breakup with 6 different dispersants at a 70 ml/min flow rate. Corexit 9500A is a widely used dispersant26 to remediate surface oil spills typically with ~1:20 dispersant to oil ratio (DOR).27 It is used in this study as a reference formulation. Silica nanoparticles mixed with Span 20 were studied for the same ratios as in the partitioning behavior study. The drops formed in the oil jet with dodecane alone without an amphiphile were not measurable, because they immediately coalesced and creamed into a continuous oil phase. As presented in Figure 3a, with increasing concentrations of a given dispersant, the oil droplet size decreased rapidly and then dropped off much more slowly at the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

highest concentrations. For example, Span 20 lowered the droplet size in the oil jet to a minimum of 68±6 μm with a 3 w:v% concentration in dodecane. Corexit 9500A was the most efficient dispersant among those tested as the jet atomized into oil drops as small as 10±2 μm with a concentration of only 1 w:v% in the jet. Neither Span 20 nor silica NPs alone led to drops smaller than ~60 μm. However, with 1:10 or 1:100 modified silica NPs, the droplets were as small as ~30 μm with ~5 w:v% amphiphile in the oil jets. In particular, the 1:10 mixture was efficient enough to make ~30 μm oil droplets with 3 w:v % total amphiphile in the oil jet. As presented in Figure 3b, the synergistic effect between the silica NPs and Span 20 on the emulsion droplet size was observed for three shear energy conditions including high-energy long-time (ultra turrax homogenizer 13500rpm, tmixing = 2 min), high-energy short-time (oil jet, I.D. = 0.25mm, flow rate = 70 ml/min, tjetting ~ 1 sec), and low-energy (oil jet, I.D. = 0.25mm, flow rate = 8 ml/min, tjetting ~ 10 sec). The ultra turrax homogenization over the long time period breaks up the oil into the smallest drops, but those produced at a jet flow rate of 70 ml/min were almost as small. The breakup of the oiljet at 70 ml/min is very efficient in such a short time scale (~seconds). For all three shear energies, smaller droplets were always formed with silica NP-Span 20 mixtures than with either of the two dispersants alone for the same concentrations, when the concentrations of silica NPs only were held constant. The synergy between the silica NPs and Span 20 is similar to that observed by Dong et al., for montmorillonite clay particles and a cationic surfactant.13 Dong argued that the surfactant both reduced the IFT and modified the HLB of the clay particle to stabilize the emulsion drops. In this study, Span 20 with a low HLB of 8.6 does not stabilize oil-in-water emulsions as effectively as would a higher HLB surfactant, although it reduces the IFT to 0.8 mN/m at 5 w:v%. According to the Bancroft rule,28 the


Page 12 of 36

Page 13 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


phase preferred by the surfactant is the continuous phase, and this HLB is too low for the surfactant to favor water. Therefore, the drops created with Span 20 only were relatively large. In contrast, in the silica NP-Span 20 mixtures the oil droplets are stabilized by the modified silica NPs leading to smaller droplets. Only Corexit 9500A led to breakup of oil drops to smaller than 70 μm by the energy from both the 8 ml/min and the 70 ml/min oil jets. However, with 70 ml/min flow rate, the mixture of an oilsoluble surfactant (Span 20) and the relatively hydrophilic silica NPs enabled breakup of the oil jet into droplets as small as ~30 μm, which would be sufficiently small for deep sea oil spills. In addition to the average droplet sizes, we further compared the droplet size distributions in the oil-jet with 5 w:v% of various dispersants (See Figure 3c). Here Corexit 9500A was the most efficient dispersant, which broke the oil into drops on the order of a few µm and smaller than 30 µm. Drops from oil jets by IPA-ST silica nanoparticles or Span 20 have distributions ranging up to ~250 µm. The 1:10 modified silica NP aided the oil jet to break up into drops smaller than 100 µm, and most of the drops (> 98 vol%) were smaller than 70µm, which meets the droplet-size criterion for permanent oil dispersion in sea water.29 The droplet size distribution confirmed the efficacy of 1:10 modified silica NP dispersants at a 70 ml/min flow rate. The reason we chose relatively high concentrations of surfactant in the oil phase was to provide data relevant to the droplet formation process for the jets, where the surfactant starts out in the oil phase and then migrates to the oil/water interface. The final surfactant concentration in the total volume, which is primarily water, becomes very low in some case below 0.1 wt%. The dispersant concentrations in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 14 of 36

oil phase in Figure 3 are comparable to the values used in Brandvik at al. Furthermore, the surfactant concentrations in oil were small compared to the level of 20 wt.% for the treatment in the Macondo well oil spill.

3.4. Effects of inertial, interfacial, and viscous forces on oil jet breakup and droplet size Table 1 presents the droplet sizes of the aforementioned oil-jet breakup tests, along with the values of the Re, We and Oh given by: Re =

ρUD μ

ρU D We = σ

Oh =

√We = Re

(1) (2) (3)

where D is the diameter of the circular jet orifice, µ and ρ are the viscosity and density of the oil phase, respectively, U is the linear flow velocity, and σ is the interfacial tension (IFT) between the two immiscible phases. In a turbulent jet where We scaling is applicable, the energy dissipation rate scales with the downstream distance whereby the exit energy dissipation rate (ε0~U3/D). Hinze’s model20 describes the volume average droplet diameter


Page 15 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


d[4,3] / D = A ∙ We

(4)

In this regime, the jet break up is limited by the inertial forces relative to the interfacial tension. This model may be modified to include viscous stresses by introducing the viscosity number Vi = We/Re, which is given in a semi-empirical model by Wang and Calabrese:30 (/

d[4,3] d[4,3] / 1 + BVi & D = AWe D'

)

/

(5)

where A, B are empirical factors. This model reduces to the Hinze model in Eq 4 (We scaling) when Vi is small (Vi→0). In the low Vi regime, high values of inertial forces overcome the interfacial forces, and hence low interfacial energy is not required, for example in the case of relatively low Re in surface ocean waves. For high Vi (Vi→∞), i.e. when We is high, for example when the dispersant lowers σ to a low level, then this model reduces to Re scaling: d[4,3] /+ D = C ∙ Re

(6)

where overcoming the viscous forces becomes key as described by Re. For the calculations of the dimensionless numbers, the viscosity was chosen as that of the dispersed oil phase, which was approximated as that of pure dodecane following a recent study.24 For the experiments in Table 1, the linear flow velocity (U) ranged from 0.24 to 24 m/s, versus an estimated



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

value of 2 m/s in the BP deep-sea blowout, as calculated from the published reports.31, 32 Re ranged from 110 to 3500, and We from 101 to 106. The ranges in Re and We investigated in this study are similar to those of Johansen et al.,24 despite the differences in flow rates and nozzle diameters. The Rayleigh and atomization jet breakup regimes may be delineated on a plot of Oh versus Re, for a constant value of We. The criterion for the atomization regime in an oil-in-water jet was reported to be We>324, and for the Rayleigh regime We1.3 0.63 0.66 0.26

6.7

1.6

5

63

0.24

0.25

75

61

24.7

0.79

2.3×10

0.25

14

166

4.6

1.6

5.3×102 2.5×103

4.7

0.65

0.25

14

123

4.6

0.81

4.9×102 5.0×103

10

0.49

42

0.14

0.25

16

36

5.3

0.22

2

5.6×10

1.5×10

4

2.4×10


Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


mM) 3% Tergitol 0.25 34 7 11.2 3% Tergitol 0.25 66 5.8 21.7 5% Tergitol (98 0.25 18 17 5.9 mM) 5% Tergitol 0.25 40 5.5 13.2 5% Tergitol 0.25 75 4.5 24.7 5% Corexit 9500 0.25 75 2.2 24.7 *: data from manual injection from syringes with 18G needles

0.22 0.22

1.2×103 1.1×105 2.3×103 4.1×105

90 175

0.028 0.023

0.13

5.5×102 5.1×104

93

0.066

207 387 1502

0.022 0.018 0.0094

0.13 0.13 0.033

3

5

1.2×10 2.5×10 2.3×103 8.9×105 2.6×103 3.9×106



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 36 of 36

Breakup of Oil Jets into Droplets in Seawater with Environmentally

Recommend Documents