laminar flow emulsification process to control petroleum art

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Laminar Flow Emulsification Process to Controlthe Viscosity Reduction of Heavy Crude Oils

S. Fournanty,1,2 Y. Le Guer,1 K. El Omari,1 and J.-P. Dejean2

1Laboratoire de Thermique Energetique et Procedes (LaTEP), Campus Universitaire, Universite dePau et des Pays de lAdour, Pau, France2IFP-Pau, Helioparc Pau Pyrenees, Pau, France

The formation of heavy crude oil in water (O/W) emulsion by a low energy laminar controlledflow has been investigated. The emulsion was prepared in an eccentric cylinder mixer. Its geome-try allows the existence of chaotic flows that are able to mix well highly viscous fluids. This newmixer design is used to produce high internal phase ratio emulsions for three oils: castor oil andtwo heavy crude oils of different initial viscosity (Zuata and Athabasca crude oils). The influenceof the stirring conditions, geometrical parameters, and water volume fraction on the rheologicalproperties of the resulting O/W emulsion is studied.

Keywords Emulsion, heavy crude oils, laminar emulsification process, viscosity reduction

INTRODUCTION

Due to the foreseeable exhaustion of the conventionalcrude oil reserves (i.e., the light crudes which are currentlyproduced, mainly in the Middle East), it is now reasonablefor petroleum companies to consider developing theproduction of heavy and extra-heavy crude oil. As anexample, large known reserves of heavy and extra-heavyoil in the Athabasca province (Canada) and in the Orinocobelt (Venezuela) are equivalent to those of Saudi Arabia.These potential resources are critical for providing worldenergy and political stability in the near future. These

crudes are characterized by densities close to or higherthan one. Moreover, they have very high viscosities. Asa consequence the difficulties in producing, processingand transporting these crude oils must be overcome. Itis thus necessary to decrease their viscosity. Currently,

different methods of viscoreduction are known to producecrude oils. Among them, the two main ones are dilutionby injection of lighter crude,[1] and secondly, thermalviscosity reduction by steam injection. These two techni-ques are very costly; the second one also produces largeamounts of CO2 emissions. An alternative is to create anoilinwateremulsion. This solution was studied in thepast for heavy oil surface transportation[2,3] and is beingreconsidered today.[47] Another cold method, referredto as core-annular flow, consists of a core oil flow lubri-cated by a film of water placed along the pipe. For this

solution, breaking the water film wall is a problem duringthe start-up.[8]

A heavy crude oil-in-water emulsion is a metastablesystem consisting of heavy crude oil dispersed in water.Due to its thermodynamical nonequilibrium state, themacroemulsion (550mm, generally) needs to be stabilizedby a surface-active agent (surfactant or particles) to avoidphase separation during transportation. Later, it will benecessary to destroy this macroemulsion to recover theoil. This crucial problem is not the objective of the presentwork. We will mainly focus on the study of the heavyoilin water laminar emulsification process.

As stated by Mabille et al.,[9] generally emulsion pro-

duction is based on empirical considerations where anuncontrolled turbulent flow is applied to a mixture of oiland water. On the contrary, in this study, we try todevelop the concept of the application of a controlled lami-nar flow (chaotic or not) for the production of the emulsionwith the desired properties. Until now emulsification stu-dies have been mainly done in the laboratory, under condi-tions very different from those of the oil well (take for

Received 16 October 2007; accepted 16 October 2007.We wish to thank E. Normandin, M. Rivaletto, and

D. Champier from the UPPA technical center InnovAdour fortheir assistance during the development of the prototypes. Wealso thank I. Henaut and J. F. Argillier from IFP Rueil, for theirhelp and fruitfull comments, and B. Grassl and J. Desbrieres from

EPCP/IPREM at UPPA for their aid during the rheological andsurface tension measurements. We are also grateful to F. Plantierfrom Fluid Complex Laboratory at UPPA carrying out thedensity measurements of the oils. This study was sponsored byIFP funds.

Address correspondence to Y. Le Guer, Laboratoire de Ther-mique Energetique et Procedes (LaTEP), Campus Universitaire,Universite de Pau et des Pays de lAdour, 64000 Pau, France.E-mail: [email protected]

Journal of Dispersion Science and Technology, 29:13551366, 2008

Copyright# Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932690701782871

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instance, the use of a high speed homogenizer). In highpressure homogenizers, 99.9% of the energy introducedin the system for the production of small droplets isdissipated as heat.[10] So, it is clear that the process is notoptimized and is not energy efficient.

For petroleum applications two contrary objectivesmust be reached. Firstly, the mean droplet size of the emul-sion must not be too large. This is mainly to avoid coales-cence and phase separation. However, it must not be toosmall either, in order to keep the apparent viscosity ofthe emulsion at a value sufficiently low to allow its flood-ing. Consequently, the adequate viscoreduction and life-time of the emulsion will depend on multiple parameters(formulation, water dispersion, volume fraction, stirringprotocol, etc.)

In this study we investigate the formation of high inter-nal phase ratio (HIPR) oil in water macroemulsions in alaminar emulsification process. The emulsification methodstudied has potentialities for the production of heavy andextra-heavy crude oils, but it could also be used advanta-

geously for cosmetic or food applications.

PHYSICAL MECHANISMEMULSIFICATIONSCENARIO

The deformation and break-up of a droplet in a laminarflow is encountered in a broad range of engineering appli-cations. These include emulsification, liquid-liquid disper-sion, or extraction, encapsulation, mixing, and blendingof polymers and complex two-phase flows in chemical reac-tors. Contrary to the theory of emulsification in turbulentflow for laminar flows, no specific condition has beenestablished for the minimum droplet diameter as a function

of the input energy in the flow.[11]

The understanding of the emulsification phenomenon inlaminar flow generally starts with the study of the break-upmechanism of a single droplet in a well known steady shearor elongational flow. The droplet needs to reach a criticalstate of deformation to become unstable and break-up. [12]

If the deformation is not sufficient, the droplet relaxes toits original spherical shape. In the case of stretching andbreak-up of droplets in simple linear flows the dominantbreakage mechanism is capillary wave instabilities occur-ring on highly extended threads. Other modes of break-upoccur, such as necking (in sustained flows when Ca, thecapillary number is close to Cacrit), end-pinching (when a

droplet is deformed at Ca close to Cacrit and the flow isstopped abruptly) or tip-streaming.[13]

The break-up process depends on the local velocityfield experienced by the droplet. Even if the flow is lami-nar (regular or chaotic), a complex coupling betweenshear and elongation flows can be encountered. If the flowis linear (typically for a Stokes flow in highly viscous sys-tem), a two-dimensional flow can be represented by a flow

parameter a which characterizes mixed shear and elonga-tion flow fields:

a _eej _ccj j_eejThese two quantities are calculated in a reference framerelated to the flow direction. Let e be the strain tensorsuch as e

1

2 rV rVT

, and let ~ee1;~ee2 be the unitvectors in the flow direction, and in the direction perpendi-cular to it deduced from the velocity vector ~VV

u

v

as:

~ee1 uffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

u2 v2pvffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

u2 v2p

0B@

1CA and~ee2

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiu2 v2p

uffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiu2 v2

p0B@

1CA

The elongation and the shear are then:

_ee ~ee1e~ee1 and _cc ~ee1e~ee2The droplet deformation and break-up depends on the

value of the capillary number, which is defined as the rationof two time scales:

Ca srdsdd

lca

r1G

G lcar

Letting the characteristic length of the initial droplet beits radius, a, the droplet shape relaxation time is srd,whereas the time scale for droplet deformation sdd is asso-ciated with the sum of shear and elongation rates G:

G j _ccj j_eej

The capillary number compares the relative importance ofviscous to interfacial tension forces and the critical capil-lary number, which defines the droplet break-up condition,is a function of the type of flow and viscosity ratio. [12] Theviscosity ratio is defined as:

k ld=lcwhere lc and ld are, respectively, the dynamical viscositiesof the continuous (water) and dispersed (oil) phases. Onemust keep in mind that for a complex emulsification pro-cess, the shear and elongation phases depend on the velo-city field experienced by the droplet along its trajectory.

This situation is somewhat distant from the idealized lami-nar flows, which were used to establish the droplets criticalstability curves.[12,14] These stability curves are very differ-ent if we consider simple shear flows or simple elongationalflows. For very large viscosity ratios (typically thoseencountered in our study), only mixed or elongationalflows are very efficient if one hopes to obtain dropletbreak-up.

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During the emulsification process, as the dropletsradius a decreases, the capillary number decreases. AtCa 1, interfacial tension is forced to become the sameorder as the viscous stresses, and the extended oil filamentscan break into many smaller droplets. On the other hand,large drops, corresponding to Ca>>1, stretch and break,while the smallest droplets at Ca< 1 may collide with eachother and coalesce into larger droplets, the latter at Ca> 1may in turn break again. This classical approach does nottake into account the history of the flow field, whichinduces the progressive deformation of the droplet. In thereal process, memory effects exist and play a role in thedynamics of droplet rupturing.[15]

In this study, the emulsification process required to gen-erate a droplet size between 1 and 50 mm was developed inthe eccentic cylinders mixer (ECM) by using a HIPR step,followed by a dilution step during the final concentrationof the dispersed phase. This was done in order to stronglydecrease the apparent viscosity and to the avoid coales-cence phenomena. An HIPR emulsion is characterized by

nonspherical droplets shaped separated by thin films. Thisis due to the low ratio between the continuous- anddispersed-phase concentrations.[16]

In our process, the mechanism involved during the firstphase of the emulsification is probably the stepwise incor-poration of large oil blobs (of a few millimeters) in thevolume of water introduced (14 mL at 9% water dilution)in the ECM. Secondly the oil blobs are broken up intosmall droplets (


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In order to find the best experimental conditions to pro-duce the emulsion, it is useful to know the flow propertiesin the ECM, such as velocity and the shear and elonga-tional fields. An in-house code (Tamaris) allows fluid flowcalculations by the use of an unstructured finite volumeformulation. To solve the unsteady incompressibleNavier-Stokes monophasic equations in two dimensions,this code uses a collocated variables arrangement in con-

junction with the Rhie and Chow interpolation. Convec-

tive fluxes at cell faces are approximated by the upwindbiased blending scheme of Peric, while a central differen-tiating scheme is applied to diffusive fluxes. The twoapproximations are second order accurate in space. Thepressure-velocity coupling is assured by the SIMPLEalgorithm. The solution is advanced in time by the secondorder accurate Gears scheme that uses three time steps.

While we focus here on the distribution of quantities suchas strain and shear stress, which are calculated by usingvelocity gradients, we use a third order accurate formula-tion of the least square method to calculate the gradients.This method uses secondary gradients and an extendedstencil of neighboring cells to give better resolution ofthe gradients. Since the mixer geometry is rather simple,the mesh is only constructed with quadrilateral shapedcells, because they are more suitable for flow and gradi-ents calculations. A fine resolution mesh containing12,544 cells was adopted after a study of sensitivity ofresults to grid size for both flow and gradients. A smalltime step (dt 0.01second) was fixed. The calculationstarted at resting position and was conducted for 100 seconds,even if the steady state solution was reached in less than10 seconds.

Figure 2a shows a sketch of the geometrical configura-tion for which the simulations were undertaken. Thestreamlines of the flow (for X1X2 90rpm ande 7 mm) are given in Figure 2b. A secondary flow mate-rialized by a large island with a stagnation point is clearlyhighlighted. In Figure 2c a plot of the a parameter isreported, the distribution of a values indicates that shearflow is almost encountered, whereas elongational flow isconfined to a small area inside the secondary flow island.In Figure 2d we plotted the capillary number, constructedwith the above mentioned definition for the viscosity of a

FIG. 2. (a) Geometrical configuration of the eccentric cylinders mixer (top view) and fluid flow simulation results for X1X2 90 rpm, and eccen-tricity e18 mm, (b) streamlines, (c) a evolution and (d) capillary number evolution for r 6.3mN.m1 and considering a drop radius a of 1mm.

TABLE 1Geometrical and dynamical parameters

Tank radius R2 75 mmCylinder radius R1 50 mmEccentricity 0< e< 12.5 mmBottom gap 1< d< 3 mmRotational speed X1

100


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continuous oil phase equal to 2.5 Pa.s. There was an oil=water surface tension of 6.3 mN m1 and an initial dropletsize a of 1 mm. We have made the assumption that the flowfield is not disturbed by the presence of the small individualdroplet (the flow is also governed by viscous forces,Re 0.007). A broad range in capillary number is observed,from 0.1 to 24.6. The maximum values of Ca are obtainedalong the inner rotating cylinder in the strongly shearedregion where droplet break-up is favored. During theemulsification process the droplet diameter and apparentviscosity of the emulsion will decrease but, at the sametime, the deformation rate G will increase. Finally, this willallow the capillary number to remain large.

Chemical Products and Physical Properties

Emulsions were prepared with a nonionic surfactant,distilled water and three types of viscous oil. The primaryoil used is a blown castor oil from Seatons (Ricin Oil). Thisnatural oil consists of a triglyceride of fatty acids. It waschosen mainly for its high density and high viscosity. The

two others, which are more viscous, are mineral heavycrude oils hereafter referred to as A and B. The densityand viscosity of the oils are given in the Table 2. InTable 3, the initial viscosity ratio (defined as kld=lc withthe subscripts d for the dispersed phase, and c for thecontinuous phase) is given for the three oil=water couples.

The surfactant used to stabilize the emulsion is anoctylphenoxy polyethoxy ethanol non ionic surfactant pur-chased by Dow: Triton X405. It has a high HLB value of17.9, which favours the formation of oil in water emulsion.The surfactant viscosity at 25C is 0.49 Pa.s.

Density measurements were made for the castor oil witha U tube Coriolis effect Anton-Paar densimeter with an

accuracy of1103

kg m3

.

Optical Measurements

The microscopic observations during emulsion formula-tion were carried out through photomicroscopy, with aMotic numeric microscope equipped with a CCD numericcamera. droplet size distribution (DSD) measurements werecarried out by using a Malvern Mastersizer 2000 MU

granulometer based on a light diffraction technique. Thisequipment delivers frequency and cumulative data involumes from 0.02 to 2000 mm.

Rheological Measurements

The rheological measurements were carried out on aBolhin C-VOR digital rheometer, which allows strain,stress, or shear rate controlled measurement. Strain con-trolled tests were conducted with a cone-plate configura-tion with a diameter of 60mm and a cone angle of2 degrees. The sample size for this cone-plate configuration

was 2.53mL. The gap between the cone and the plate isadjusted to the value of 70 mm. All the rheological measure-ments were undertaken at a temperature of 25C.

Interfacial Tension Measurements

The equilibrium interfacial tension (IFT) was measuredfor an oil-surfactant aqueous solution at 25 at the concen-tration 1.7 wt.%. A drop tensiometer (Tracker, IT Concept)was used to measure the interfacial tension by analysingthe axial symmetric shape (Laplacian profile) of a risingsurfactant-aqueous drop in castor oil. A mean value of6.3mN m1 is measured for a castor oil=surfactantaqueous solution. At the time of the study the heavy

crude oil=surfactant aqueous solution was not evaluatedbecause of experimental difficulties (viscoelastic behavior,oil opacity).

EXPERIMENTAL RESULTS AND DISCUSSION

Emulsification Protocol

Every emulsion was prepared with a 150 mL volume cas-tor oil or heavy crude oil. This amount of oil was firstintroduced in the stirred vessel, so each experiment beganwith the oil in the continuous phase. This situation, inwhich surfactant aqueous solution is dispersed into oil, isrequired for petroleum applications and so it involves a

laminar emulsification process. Experiments were carriedout at 1.7wt% Triton X405 surfactant concentration.Despite the high HLB value of the water soluble tensioac-tive agent, it was injected into the oil phase while the innercylinder was rotating, in order to obtain a liquid premixedwith oil. Secondly, large blobs of water were graduallyadded into the vessel with a low volume ratio, typicallyfrom 9 to 18% in weight. The stirring time allowed for

TABLE 2Fluid properties at 25C. The oil viscosities are given at a

low shear rate value (10 Pa.s)

FluidDensity

q (kgm3) API

Viscosity lat 25C

(Pa.s)

Blown castor oil 0.974 13.77 2.5Zuata crude oil 0.957 16.35 5.6Athabasca crude oil 1.011 8.46 41water 0.997 8.90104

TABLE 3Oil=water initial viscosity ratios

Fluids Viscosity ratio k at 25C

Blown castor oil oil=water 2809Zuata crude oil=water 6292Athabasca crude oil=water 46067

LAMINAR FLOW EMULSIFICATION PROCESS 1359


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the HIPR emulsion formation depends on the formulationparameters and also on the desired final droplet size distri-bution (typically ranging from 2 to 10 mn). Before stoppingstirring when the HIPR emulsion was created, water couldbe added to obtain the required dilution rate. The why ofthis procedure will be explained further on. Moreover,different water volume fractions were considered. Indeed,high water ratios reduce physical instabilities likeflocculation and creaming or coalescence. All emulsifica-tion formulations were carried out at isothermal ambienttemperature (25C). However, for blown castor oil theemulsion formation can be observed by a change in colour(from translucent to a yellowish milky hue). The O=Wemulsion for heavy crude oil keeps its dark chestnut color.

Low rotational speeds rotational speeds were consideredfor stirring: from 40 to 90 rpm with counter rotations of thecylinder and tank. This corresponds respectively to aReynolds number ranging from 4.4 104 to 0.007 forthe flow of heavy crude oils. The fact that the hydrosolublesurfactant mixed primarily with oil gave better emulsifica-

tion results than its direct dissolution into water couldprobably be explained by the use of a low volume fractionof water that does not facilitate a rapid surfactant access tothe whole oil volume.

Parametric Study

A comprehensive parametric study was carried out inorder to identify the emulsification processs key parameterswith the ECM. The parameters studied were water volumefraction (9 or 18%), stirring conditions (inner and outercylinders rotational speed, 60 and 90 rpm) and geometricalparameters (cylinders eccentricity e 7 or 11mm and thebottom gap d

1 or 3 mm). For this parametric study, the

time required to create a 20 mm droplet emulsion was chosenas the specific objective, in order to characterize the influ-ence of each parameter for process efficiency. For eachparameter two test values were selected: a low value and ahigh one. Finally 16 experiments were conducted. Theconclusions from this study follow in order of importance:

1. The water volume fraction greatly influences the result(a lower volume fraction facilitates the emulsification).

2. The efficiency increases significantly when the rotationalspeed of the cylinders increases.

3. The efficiency is found to increase with the increase ofcylinder eccentricity.

4. The efficiency is found to increase with the decrease ofthe bottom gap.5. Parameters have nonsignificative effects on each other.

This study was very useful in highlighting the benefits offormulating emulsions with low aqueous phase content(i.e., 9%). For the best result (90 rpm, e 7mm, d 1mm,and 9% water fraction), the desired emulsion is obtainedin 3 minutes. According to experimental conditions, a

maximum time of 130 minutes can be obtained for a parti-cular set of parameters.

Photomicroscopy Analysis

The primary analysis of emulsion was carried out byusing a digital video camera connected to an optical micro-scope. It allows the determination of a rough mean diameterof the droplet size distribution at different times during theemulsification process. Photomicrograhs of the differentemulsions obtained are shown in Figure 3a for the castoroil and Figure 3c for the Zuata crude oil. One will pointout that the Zuata crude oil originally contains small quan-tities of water (see Figure 3b). For some operating condi-tions, waterinoilinwater (W=O=W) multiple emulsionsare observed. There are emulsion systems where small waterdroplets are entrapped within larger oil droplets, which inturn are dispersed into a continuous water phase.

Effect of Stirring

The effect of stirring in emulsification has been little

studied contrary to the influence of formulation variables.This is mainly due to the fact that the majority of the knownstudies consider turbulent flows for the emulsificationprocesses. For turbulent flows, high shear is generally encoun-tered in rotors of very high rotational speed (for which thestirring is not a key parameter.[10] However, the effect oftwo different turbulent flow regimes on emulsification, whichby pass the fluid in a narrow-gap homogenizer, has beenrecently pointed out by Vankova et al.[19] They have com-pared the behavior of emulsification for viscous and inertialturbulent regimes, in regard to several factors concerningthe mean and maximum droplet size. One of their main con-clusions is that the inertial regime is inappropriate for the

emulsification of viscous oil (for viscosity higher than0.5 Pa.s) if micrometer size drops are desired. In our studywe have investigated the influence of two factors (the watervolume fraction and the stirring speed) on the evolution ofthe mean diameter during the laminar emulsification process.The mean diameter is evaluated by sampling the emulsionand by photomicroscopy analysis.

Effect of Water Volume Fraction

Figure 4 illustrates the change in mean droplet diameterduring the emulsification process for castor oil for three dif-ferent water fractions (23, 21, and 9%). The stirring condi-tions are the same for the three cases (X1

X2

90 rpm).

For the two higher water fractions, the mean droplet diameterevolves linearly with stirring time. This evolution in time isvery slow; it takes, respectively, from 46 to 66 minutes toreach a mean droplet diameter of 10 microns in size. This lin-ear behavior is not observed at the lower water fraction (9%),for which an exponential decrease toward a minimum dia-meter close to 10 mm is obtained in 12 minutes. For this waterfraction, the maximum packing concentration / 74% (if



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one assumes the oil droplets to be rigid spheres) isexceeded. This was possible because for HIPR emulsion, sincethe oil droplets are deformable, higher concentration canbe obtained. We get the formation of planar films betweendroplets like for foam. The proximity of the droplets may

facilitate stress transmission, and so break-up phenomenaare faster than in higher water fractions. These results implythat for our application, the preparation of the emulsions willhave to be carried out in two stages: a first stage for the crea-tion of a mother emulsion at low water fraction (typicallyaround 10%) and a second one for the dilution of the motheremulsion (up to 30% for example) in order to reach the

desired apparent viscosity of the emulsion.

Effect of Stirring Speed

As for the castor oil and the water fraction of 9%, whichleads to creating a HIPR emulsion, we have studied theinfluence of the stirring speeds (from 40 to 90 rpm). Evolu-tion of the mean droplet diameter over time is reported inFigure 5. For the lower stirring speeds (40, 50, and 60 rpm),the mean droplet diameter begins to decrease exponentiallyover time, and then when the mean droplet diameterapproaches 20 mm, its decrease is greatly slowed down. At40 rpm the time to obtain a mean droplet diameter of

10mm is about half an hour. For higher stirring speeds,the behavior is different. It only exponentially decreases.This is because the mean droplet diameter and the coeffi-cients of the exponential evolution are almost the same.This remarkable property is very interesting for obtainingan emulsion with a mean diameter chosen in advance. Aswe know, it is mainly the diameter of the droplets whichcontrols the apparent viscosity of the emulsion. For a given

FIG. 3. Photomicrographs: (a) castor O=W emulsion, (b) Zuata crude, and (c) Zuata O=W emulsion.

FIG. 4. Time evolution of the mean drop diameter for the blown cas-tor oil and for three different volume fractions of the water dispersedphase (9, 21, and 23%).



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stirring speed (e.g., 90 rpm) we must only stop the laminaremulsification process at a precise time to obtain a pre-scribed mean diameter.

Rheological Behavior

The rheological behavior of emulsions is controlled bymany parameters (the nature and concentration of thesurfactant, water-oil fraction, type of oil, temperature,etc.). Among them, one of the main parameters is oildroplet size distribution. If droplet sizes are too large, a

coalescence phenomenon will occur. On the other hand,if droplet sizes are too small, molecular interactions willincrease emulsion viscosity.[20] We have shown that ourECM is able to produce emulsions of different mean dro-plet size diameters.

The castor oil used in this study has a viscosity of2.5 Pa.s and the two heavy crude oils (here, called Zuataand Athabasca in reference to their origin) have viscositiesof approximately 5.6 and 41 Pa.s at 25C. At 5.6 Pa.s pipe-line transportation of the crude oil is very difficult, and at41 Pa.s, it is impossible.[3] The rheograms of the viscous oilsshow different behaviors. The blown castor oil is a per-fectly Newtonian (cf. Figure 6) fluid, whereas the Zuata

and Athabasca crude oils present a shear-thinning behavior(cf. Figure 7). This rheological behavior is known for heavycrude oil[21,22] and is often attributed to the arrangement ofoverlapped asphaltenes under shear rate.

Effect of Water Volume Fraction

Castor oil emulsions of different volume fractions wereprepared in order to study the effect of water volume

fraction on emulsion viscosity. Following the lessons fromthe parametric study, the rotational speeds of the ECMcylinders chosen for the formulation of the mother emul-sion are X1X2 90 rpm. This mother emulsion, witha mean droplet diameter of 10 mm, was then diluted toobtain the different water=oil ratios (WOR) for thesteady-state viscosity measurements. The apparent viscos-ity of the formulated emulsions was measured at 25Cfor shear rates ranging between 0 and 400 s1. The evolu-tion of the apparent viscosity of the O=W emulsions is

FIG. 6. Apparent viscosity as function of the shear rate for four dif-ferent initial mean drop diameter of the emulsion: 10, 20, 25, and50mm. The initial continuous phase is the blown castor oil.

FIG. 7. Apparent viscosity as function of the shear rate for threedifferent final water contents (18, 25, and 30%) for the Zuata crude oil.

FIG. 5. Time evolution of the mean drop diameter for the blown cas-tor oil and different rotational speeds: 40, 50, 60, 70, and 90 rpm. The

initial continuous phase is the blown castor oil. The W=O ration is 9=91.



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represented in Figure 8 for different water volume fractionsbetween 30% and 50%. For all the WOR, a non Newtonianshear-thinning behavior is observed for the diluted emul-sions as the shear rate increases. The shear-thinning beha-vior is the result of the reversible structural breakdown ofthe emulsion under shear, which causes the deformationof the droplets. When the shear rate increases, the dropletsare more elongated, and their major axis is aligned gradu-

ally in the direction of the flow.[23]

The flow curves show adecrease of the apparent viscosity of the emulsion as thewater volume fraction is increased. All the flow curves tendtoward the same Newtonian plateau at 0.2 Pa.s, but moreshear rates are needed to reach this plateau for a lowWOR. A maximum viscosity reduction factor of 12.5 isquantified at 400 s1.

Effect of Mean Droplet Size

Figure 6 shows the evolutions, with a shear rate, of theapparent viscosity of the castor O=W emulsion for motheremulsions of different mean droplet diameters (from 10 to50mm). All the mother emulsions were diluted to obtain a

30% water volume fraction. The flow curves all show ashear-thinning behavior and tend toward a Newtonian pla-teau for the high shear rate. The Newtonian plateau isalmost reached at a low shear rate (50s1) for the 50 mmmother emulsion (for the 10 mm emulsion this plateau isnot reached). Due to the excess of surfactant, which is pre-sent in the emulsion, the stability is still ensured for a meandroplet of 50mm. The main conclusion is that the viscosity

reduction of the O=W emulsion increases when the meandroplet diameter increases.

Effect of Oil Phase Viscosity

For the Zuata crude oil the mother emulsion was cre-ated with the ECM for an initial water volume fractionof 18%. The mean diameter obtained for this emulsion isabout 15mm for a stirring time of 10 minutes at 90 rpm.

The flow curves for the Zuata crude oil are reported inFigure 7 for the mother emulsion and two diluted emulsions(25 and 30% water volume fraction). As for the castor oil, ashear-thinning behavior is observed, but the Newtonianplateau is reached for lower shear rates (typically for50 s1). A high viscosity reduction is obtained, even thoughthe mother emulsion and the effect of dilution is not verysignificant. As for the Athabasca crude oil, only roughemulsions could be obtained for the same experimentalconditions as those used to produced the Zuata crudeO=W emulsion. They are not very stable because theirmean droplet diameter is about 100mm. Also slippageoccurs during the flow of Athabasca heavy crude oil emul-

sion in the rheometer.[24] In Table 4 we give the maximumviscosity reduction ratios obtained for the three oils andfor a water volume fraction of 30%. One notices that theviscosity ratio is all the more large as the initial viscosityof the crude oil is high (for the same apparent viscosity ofthe created O=W emulsion ranging between 0.1 and0.2 Pa.s). So, for the laminar emulsification process studied,the apparent viscosity of the generated emulsion is

FIG. 8. Apparent viscosity as function of the shear rate for four different W=O ratios: 30=7036=6440=60 and 50=50. The initial continuous phaseis the blown castor oil.



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independent of the viscosity of the initial crude oil. Thisproperty is not found for a viscosity reduction process bydilution.

Granulometry

For the castor oil, the influence on three different formu-lation parameters was studied for the resulting DSD of fourHIPR emulsion samples. The three parameters are the rota-tional speeds of the cylinders, the water volume fraction,and the bottom gap. For this granulometric study, contraryto the parametric study evoked above, a stirring time of 6minutes was imposed and the mean droplet diameter wasevaluated from DSD histograms. The eccentricity was fixedat e 7 mm. We know that with the use of the ECM, themean droplet diameter is directly related to the stirring

time, so the DSD should confirm the results given by theparametric study. This is the case. Typical droplet volumesand number distribution histograms are shown in Figure 9for example 2 (see Table 5 for experimental conditions). Thevolume histogram shows a quasi uni-modal distributioncentred on 3mm. This result is also confirmed for the othersamples (data not provided). The monodispersed characterof the O=W emulsion could be explained by the capillarybreak-up mechanism of the oil cylinder threads describedabove in the part 2. The second DSD histogram (represen-tation in numbers of droplets) shows that a high number ofvery few droplets (which not very representative in volume)are present in the emulsion. This slightly polydispersedeffect could contribute to the improvement of the viscosityreduction of the emulsion.[25] Indeed, it is known that with

FIG. 9. Droplet size distribution histograms for the sample 2 (see Table 5).

TABLE 4Viscosity reduction ratios obtained for the three studied

oils. All the mother emulsions were diluted to a watervolume fraction of 30%

loil=lemulsion

Shearrate

Castoroil

Zuatacrude oil

Athabasca

crude oilRough emulsion

100s1 210200s1 12.5 70

TABLE 5Granulometry results for three varying parameters, X, thewater volume fraction, and d. The eccentricity is e 18mm

and the stirring time is 6 minutes

SampleX

(rpm)Water

(%)d

(mm)Mean drop

diameter (lm)

1 60 9 1 252 90 9 1 33 90 9 3 154 90 12 1 40



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highly dispersed volume fraction bimodal emulsions, thedroplet break-up mechanisms efficiency is enhanced. Smalldroplet fraction may facilitate stress transmission, and thusthe break-up phenomena. Table 5 gives the mean diametersobtained from the four samples. The diameters aredistributed from 3 to 40 mm, which confirms the importantchoice of the right experimental parameters for formulationand consequently for the apparent viscosity of the generatedemulsion.

Emulsion StabilityThe stability of an emulsion is mainly related to coales-

cence and aging (Ostwald ripening) phenomena. Here, asufficiently high surfactant concentration is used to avoidthe interdroplet coalescence. Figure 10 gives the apparentviscosity for the fresh O=W Zuata emulsion after four days,for three different water contents. It is notable that the gen-eral evolution of the apparent viscosity with the shear rateis the same after four days. However, the major differenceis a shift of each curve toward the higher viscosities. Thisincrease in viscosity cannot be explained by the evapora-tion of water since the coalescence of droplets will also pro-duce the reverse effect on viscosity. This phenomenon is

probably due to the absorption and rearrangement overtime of the asphaltenes present in the crude oil at the oil=water interface[26,27].

CONCLUDING REMARKS

We have shown that the production of a highly stableviscous crude oil in water emulsion is possible in a mixer

apparatus of very smooth geometry and without consum-ing a considerable amount of energy. Neither the premixstep nor phase preheating is needed to obtain a HIPRO=W emulsion with a closely monodispersed dropletsize distribution. In comparison, the surface Orimulsionprocess[5] requires three mixers for the different steps ofmixing to achieve the proper emulsion.

The process of emulsification described here can be usedin petroleum production and during the transportation ofheavy and extra heavy oils. The laminar flow regime

generated in the ECM allows one to control the viscoreduc-tion via the close monitoring of the droplet size distributionof the created O=W emulsion over a relatively short time. Itis then possible to produce droplets of a choosen prescribedsize. The latter is an interesting property for petroleumapplications whenever a compromise in droplet size isrequired for the stability of the emulsion. A new designof the batch mixer, which still gives better results evenfor the Athabasca crude oil, is actually being tested. Theresults will be presented in a subsequent paper.

NOMENCLATURE

Ca capillary number

Re Reynolds numbera flow parametera droplet radius (m)G deformation rate (s1)(u,v) Components of the velocity field (m.s1)V velocity field or tangential velocity (m.s1)e eccentricity (mm)[e] strain tensor

FIG. 10. Evolution in the time of the apparent viscosity of the Zuata crude emulsion for three different final water contents: 4 days stability.



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d bottom gap (mm)~ee1;~ee2 unit vectorsc elongation rate (s1)c shear rate (s1)k viscosity ratiol liquid viscosity (Pa.s)X rotational speed (rpm or rd.s1)

q liquid density (kg.m3

)r interfacial tension (N.m1)srd droplet shape relaxation time (s)sdd droplet deformation time (s)

Subscripts

c continuous phased dispersed phase

REFERENCES

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