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Journal of Dispersion Science and Technology, 29:1355–1366, 2008 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print=1532-2351 online DOI: 10.1080/01932690701782871
Laminar Flow Emulsification Process to Control the Viscosity Reduction of Heavy Crude Oils S. Fournanty,1,2 Y. Le Guer,1 K. El Omari,1 and J.-P. Dejean2 1
Laboratoire de Thermique Energe´tique et Proce´de´s (LaTEP), Campus Universitaire, Universite´ de Pau et des Pays de l’Adour, Pau, France 2 IFP-Pau, He´lioparc Pau Pyre´ne´es, Pau, France
The formation of heavy crude oil in water (O/W) emulsion by a low energy laminar controlled flow has been investigated. The emulsion was prepared in an eccentric cylinder mixer. Its geometry allows the existence of chaotic flows that are able to mix well highly viscous fluids. This new mixer design is used to produce high internal phase ratio emulsions for three oils: castor oil and two heavy crude oils of different initial viscosity (Zuata and Athabasca crude oils). The influence of the stirring conditions, geometrical parameters, and water volume fraction on the rheological properties 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 conventional crude oil reserves (i.e., the light crudes which are currently produced, mainly in the Middle East), it is now reasonable for petroleum companies to consider developing the production of heavy and extra-heavy crude oil. As an example, large known reserves of heavy and extra-heavy oil in the Athabasca province (Canada) and in the Orinoco belt (Venezuela) are equivalent to those of Saudi Arabia. These potential resources are critical for providing world energy and political stability in the near future. These crudes are characterized by densities close to or higher than one. Moreover, they have very high viscosities. As a consequence the difficulties in producing, processing and transporting these crude oils must be overcome. It is thus necessary to decrease their viscosity. Currently,
Received 16 October 2007; accepted 16 October 2007. We wish to thank E. Normandin, M. Rivaletto, and D. Champier from the UPPA technical center Innov’Adour for their assistance during the development of the prototypes. We also thank I. He´naut and J. F. Argillier from IFP Rueil, for their help and fruitfull comments, and B. Grassl and J. Desbrie`res from EPCP/IPREM at UPPA for their aid during the rheological and surface tension measurements. We are also grateful to F. Plantier from Fluid Complex Laboratory at UPPA carrying out the density measurements of the oils. This study was sponsored by IFP funds. Address correspondence to Y. Le Guer, Laboratoire de Thermique Energe´tique et Proce´de´s (LaTEP), Campus Universitaire, Universite´ de Pau et des Pays de l’Adour, 64000 Pau, France. E-mail:
[email protected]
different methods of viscoreduction are known to produce crude oils. Among them, the two main ones are dilution by injection of lighter crude,[1] and secondly, thermal viscosity reduction by steam injection. These two techniques are very costly; the second one also produces large amounts of CO2 emissions. An alternative is to create an oil–in–water–emulsion. This solution was studied in the past for heavy oil surface transportation[2,3] and is being reconsidered today.[4–7] Another cold method, referred to as core-annular flow, consists of a core oil flow lubricated by a film of water placed along the pipe. For this solution, breaking the water film wall is a problem during the start-up.[8] A heavy crude oil-in-water emulsion is a metastable system consisting of heavy crude oil dispersed in water. Due to its thermodynamical nonequilibrium state, the macroemulsion (5–50 mm, generally) needs to be stabilized by a surface-active agent (surfactant or particles) to avoid phase separation during transportation. Later, it will be necessary to destroy this macroemulsion to recover the oil. This crucial problem is not the objective of the present work. We will mainly focus on the study of the heavy–oil– in water laminar emulsification process. As stated by Mabille et al.,[9] ‘‘generally emulsion production is based on empirical considerations where an uncontrolled turbulent flow is applied to a mixture of oil and water.’’ On the contrary, in this study, we try to develop the concept of the application of a controlled laminar flow (chaotic or not) for the production of the emulsion with the desired properties. Until now emulsification studies have been mainly done in the laboratory, under conditions very different from those of the oil well (take for
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instance, the use of a high speed homogenizer). In high pressure homogenizers, 99.9% of the energy introduced in the system for the production of small droplets is dissipated as heat.[10] So, it is clear that the process is not optimized and is not energy efficient. For petroleum applications two contrary objectives must be reached. Firstly, the mean droplet size of the emulsion must not be too large. This is mainly to avoid coalescence and phase separation. However, it must not be too small either, in order to keep the apparent viscosity of the emulsion at a value sufficiently low to allow its flooding. Consequently, the adequate viscoreduction and lifetime of the emulsion will depend on multiple parameters (formulation, water dispersion, volume fraction, stirring protocol, etc.) In this study we investigate the formation of high internal phase ratio (HIPR) oil in water macroemulsions in a laminar emulsification process. The emulsification method studied has potentialities for the production of heavy and extra-heavy crude oils, but it could also be used advantageously for cosmetic or food applications. PHYSICAL MECHANISM–EMULSIFICATION SCENARIO The deformation and break-up of a droplet in a laminar flow is encountered in a broad range of engineering applications. These include emulsification, liquid-liquid dispersion, or extraction, encapsulation, mixing, and blending of polymers and complex two-phase flows in chemical reactors. Contrary to the theory of emulsification in turbulent flow for laminar flows, no specific condition has been established for the minimum droplet diameter as a function of the input energy in the flow.[11] The understanding of the emulsification phenomenon in laminar flow generally starts with the study of the break-up mechanism of a single droplet in a well known steady shear or elongational flow. The droplet needs to reach a critical state of deformation to become unstable and break-up.[12] If the deformation is not sufficient, the droplet relaxes to its original spherical shape. In the case of stretching and break-up of droplets in simple linear flows the dominant breakage mechanism is capillary wave instabilities occurring on highly extended threads. Other modes of break-up occur, such as necking (in sustained flows when Ca, the capillary number is close to Cacrit), end-pinching (when a droplet is deformed at Ca close to Cacrit and the flow is stopped abruptly) or tip-streaming.[13] The break-up process depends on the local velocity field experienced by the droplet. Even if the flow is laminar (regular or chaotic), a complex coupling between shear and elongation flows can be encountered. If the flow is linear (typically for a Stokes flow in highly viscous system), a two-dimensional flow can be represented by a flow
parameter a which characterizes mixed shear and elongation flow fields: a¼
e_ j_cj þ j_ej
These two quantities are calculated in a reference frame related to the flow direction. Let ½e� be the strain tensor such as ½e� ¼ 12 ðrV þ ðrV ÞT Þ, and let ð~ e1 ;~ e2 Þ be the unit vectors in the flow direction, and in the direction � perpendi� ~ u as: cular to it deduced from the velocity vector V v 0 1 0 1 u �v pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi B B 2 2 C 2 2 C ~ e2 ¼ @ u uþ v A e1 ¼ @ u vþ v A and ~ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2 þ v 2 u2 þ v 2
The elongation and the shear are then:
e1 and c_ ¼ ~ e1 ½e�~ e2 e_ ¼ ~ e1 ½e�~ The droplet deformation and break-up depends on the value of the capillary number, which is defined as the ration of two time scales: Ca ¼
srd ¼ sdd
lc a r 1 G
¼
G lc a r
Letting the characteristic length of the initial droplet be its radius, a, the droplet shape relaxation time is srd, whereas the time scale for droplet deformation sdd is associated with the sum of shear and elongation rates G: G ¼ j_cj þ j_ej The capillary number compares the relative importance of viscous to interfacial tension forces and the critical capillary number, which defines the droplet break-up condition, is a function of the type of flow and viscosity ratio.[12] The viscosity ratio is defined as: k ¼ ld =lc where lc and ld are, respectively, the dynamical viscosities of the continuous (water) and dispersed (oil) phases. One must keep in mind that for a complex emulsification process, the shear and elongation phases depend on the velocity field experienced by the droplet along its trajectory. This situation is somewhat distant from the idealized laminar flows, which were used to establish the droplet’s critical stability curves.[12,14] These stability curves are very different if we consider simple shear flows or simple elongational flows. For very large viscosity ratios (typically those encountered in our study), only mixed or elongational flows are very efficient if one hopes to obtain droplet break-up.
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During the emulsification process, as the droplet’s radius a decreases, the capillary number decreases. At Ca ¼ 1, interfacial tension is forced to become the same order as the viscous stresses, and the extended oil filaments can 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 each other and coalesce into larger droplets, the latter at Ca > 1 may in turn break again. This classical approach does not take into account the history of the flow field, which induces the progressive deformation of the droplet. In the real process, memory effects exist and play a role in the dynamics of droplet rupturing.[15] In this study, the emulsification process required to generate a droplet size between 1 and 50 mm was developed in the eccentic cylinders mixer (ECM) by using a HIPR step, followed by a dilution step during the final concentration of the dispersed phase. This was done in order to strongly decrease the apparent viscosity and to the avoid coalescence phenomena. An HIPR emulsion is characterized by nonspherical droplets shaped separated by thin films. This is due to the low ratio between the continuous- and dispersed-phase concentrations.[16] In our process, the mechanism involved during the first phase of the emulsification is probably the stepwise incorporation of large oil blobs (of a few millimeters) in the volume of water introduced (14 mL at 9% water dilution) in the ECM. Secondly the oil blobs are broken up into small droplets (
