Emulsion flow Fields

Consider a bubble rising in a fluidized bed. It is assumed that the bubble is solids-free, is spherical, and has a constant internal pressure. Moreover, the emulsion phase is assumed to be a pseudocontinuum, incompressible, and inviscid single fluid with an apparent density of pp(l — amf) + pamf. It should be noted that the assumption of incompressibility of the mixture is not strictly valid as voidage in the vicinity of the bubble is higher than that in the emulsion phase [Jackson, 1963 Yates et al., 1994]. With these assumptions, the velocity and pressure distributions of the fluid in a uniform potential flow field around a bubble, as portrayed by Fig. 9.10, can be given as [Davidson and Harrison, 1963]... 

Flow around a droplet induces a shear force onto the droplet. When this force is sufficiently large, the droplet can break up into smaller droplets. Generally, the more intense a flow field is, the smaller the droplets become. Therefore, most methods are designed to generate a flow field that is very strong at a very small volume, through which the emulsion passes. 

How do the techniques described above work First, a coarse premix emulsion is made. Then, when the premix is subjected to an intense flow field, the droplets are broken apart by the forces exerted by the flow around the droplets. If one applies a force that is bigger than the forces that keep a droplet together, the droplet will be disrupted. The ratio of the externally applied stress and the internal, coherent tension is called the Weber number. For many situations this is defined as ... 

Emulsions can be made in different ways. The most common class of processes applies strong flow fields, usually a combination of simple shear flow and of exten-sional flow. Extensional flow is more effective than shear flow in breaking up droplets into smaller ones. Industrial equipment usually operates with turbulent flow, in which inertial forces can be important as well. In addition, for some applications ultrasound is used, which creates, through cavitation, strong local turbulence. 

Industrial processes are at this moment predominantly based on the first class (using intense flow fields). The second class based on membranes or micro-channels seems to have large potential for making more complex products for example, monodisperse emulsions or double emulsions. The third class is used in the production of foods, but also in other industries where emulsions need to be made in which the dispersed phase has a high viscosity compared to the continuous phase. 

While mechanistic simulators, based on the population balance and other methods, are being developed, it is appropriate to test the abilities of conventional simulators to match data from laboratory mobility control experiments. The chapter by Claridge, Lescure, and Wang describes mobility control experiments (which use atmospheric pressure emulsions scaled to match miscible-C02 field conditions) and attempts to match the data with a widely used field simulator that does not contain specific mechanisms for surfactant-based mobility control. Chapter 21, by French, also describes experiments on emulsion flow, including experiments at elevated temperatures. 

The volume fraction of the dispersed phase is the most important factor that affects the viscosity of emulsions. When particles are introduced into a given flow field, the flow field becomes distorted, and consequently the rate of energy dissipation increases, in turn leading to an increase in the viscosity of the system. Einstein (24, 25) showed that the increase in the viscosity of the system due to addition of particles is a function of the volume fraction of the dispersed particles. As the volume fraction of the particles increases, the viscosity of the system increases. Several viscosity equations have been proposed in the literature relating viscosity to volume fraction of the dispersed phase. We discuss these equations in a later section. 

Unlike a solid-in-liquid suspension, the viscosity of an emulsion may depend upon the viscosity of the dispersed phase. This dependence is especially true when internal circulation occurs within the dispersed droplets. The presence of internal circulation reduces the distortion of the flow field around the droplets (26), and consequently the overall viscosity of an emulsion is lower than that of a suspension at the same volume fraction. With the... 

Emulsion Pipeline Operations. Prediction of pipeline pressure gradients is required for operation of any pipeline system. Pressure gradients for a transport emulsion flowing in commercial-size pipelines may be estimated via standard techniques because chemically stabilized emulsions exhibit rheological behavior that is nearly Newtonian. The emulsion viscosity must be known to implement these methods. The best way to determine emulsion viscosity for an application is to prepare an emulsion batch conforming to planned specifications and directly measure the pipe viscosity in a pipe loop of at least 1-in. inside diameter. Care must be taken to use the same brine composition, surfactant concentration, droplet size distribution, brine-crude-oil ratio, and temperature as are expected in the field application. In practice, a pilot-plant run may not be feasible, or there may be some disparity between pipe-loop test conditions and anticipated commercial pipeline conditions. In these cases, adjustments may be applied to the best available viscosity data using adjustment factors described later to compensate for disparities in operating parameters between the measurement conditions and the pipeline conditions. 

The importance of FIPI is twofold. It can be used to promote phase inversion without changing the thermodynamics of the system to obtain a higher entropy state, or it is possible to delay phase inversion while reducing the system entropy. The characteristics of the microstructure formed (such as emulsion droplet size) are dependent on the type of microstructure and deformation (shear, extension, or combined), as well as the deformation rate. To maximize the fluid micro-structure/flow field interactions, the flow fleld must be uniform, which requires the application of the flow field over a small processing volume, which can be achieved by using MFCS mixers or CDDMs. 

Fig. 9 Process intensification in water-in-crude oil separation under electric field with or without PHP variation of percentage of separation with electric field strength applied over a distance of 10 cm when the emulsion flow rate is kept constant at 60ml/min. Percentage separation into oil-water layers is carried out either immediately (within 10 min) or after 1 hr of emulsion passing through the electric field. (From Ref. " l) (View this art in color at www.dekker. com.)...

The templates for the oxygen and hydrogen flow fields/electrodes in this book (see page 248) are intended to be printed on plastic transfer film and then transferred to copper circuit board for etching. This method is commonly used to transfer printed circuit board designs to the copper surface of the boards. We used Press-n-Peel , which is a plastic film that has an emulsion on one side and comes in sheets of 8V2"x 11" size. The plastic sheet is loaded into the printer just as any other 8 /2"x 11" paper would be. 

If particles are known to be spherical in shape and nondeformable in the relatively weak flow fields associated with Brownian motion (this may be expected in the case of synthetic latex particles, many proteins, and viruses and probably also holds for certain emulsion particles with rigid ordered interfaces, the Stokes radius will closely correspond to the hard sphere radius R, related to Rg through Rg = 3/5 R and may also be similar to that observed in the electron microscope Rem. The value of Rg should, however, on detailed inspection be greater than the radii measured by the latter methods because it includes bound solvent molecules. The discrepancy can be used to estimate the degree of solvation 81 grams solvent/gram of the particle through the relation ... 

O Brien s theoretical analysis (8,10) is for a suspension of solid particles, but the evidence to date indicates that emulsion droplets behave in the same way as solid particles at the frequencies involved in the ESA effect. This is understandable on a number of counts. First, it is usually observed that surfactant-stabilized emulsion droplets in a flow field do not behave as though they were liquid. The presence of the stabilizing layer at the interface restricts the transfer of momentum across the phase boundary so that there is little or no internal motion in the drop. Also, the motions which are involved are extremely small (involving displacements of the order of fractions of a nanometer) so the perturbations are small compared to the size of the drop. Finally, O Brien has shown in some unpublished calculations that if the surface is unsaturated, so that the surfactant groups can move under the influence of the electric field, then the effect on the electroacoustic signal would depend on the quantity dy/dT, where y is the surface tension and T is the surface excess of the surfactant. We have not been able to find any evidence for such an effect, if it exists, so we will assume that the analysis for a solid particle holds also for emulsions. 

The study of emulsion rheology was pioneered by Geoffr Taylor (1,2), who not only experimentally identified e dimensionless groups (capillary number and viscosity ratio) that control droplet deformation in an emulsion in simple shear and hyperboUc flow fields, but also proposed a linear theory for droplet deformation in flow. The droplet Cs illary number is defined as Ca f -... 

Oldroy s model was extended by Palieme (1990) to emulsions with polydisperse spherical drops. The model considered viscoelastic liquids, the concentration range was extended up to that at which drop-drop interactions start complicating the flow field. However, the drops must be spherical, undergoing small deformation, and the interfacial tension coefficient was considered constant, independent of stress and the interfacial area. The following relation was derived for the complex modulus ... 

A linear viscoelastic constitutive model of dilute emulsion viscoelastic properties was proposed by Oldroyd [111, 112]. The model considered low deformation of monodispersed drops of one Newtonian liquid in another, with an interphase. Choi and Schowalter [113] extended their cell model to dilute emulsions with Newtonian matrix and viscoelastic drops under infinitesimally small oscillatory deformation. Oldroyd s model was modified by Palierne [126, 127] for dilute viscoelastic hquids emulsions with polydispersed spherical drops (thus, subject to small deformations) with constant interfacial tension coefficient, Vu, at concentrations below that where the drop-drop interactions start complicating the flow field, that is, < 0.1 ... 

Emulsion elasticity expressed by Eq. (2.18) as the first normal stress difference, Ni, originates from the deformability of the interphase thus it is present even in Newtonian liquid blends [113]. The relation predicts that Ni increases with vdthout bound. Since drops do not deform at high viscosity ratio, A > 4, as well as when the interfadal tension coefficient is high, the elasticity should decrease as the dispersed liquid viscosity or the interfacial tension coefficient became large. Similarly, G in Eq. (2.23) and its homologues depends on the R/V12 ratio [126], but here the prediction for both limiting values of V12 is the same. As in the case of viscosity, these two direct measures of elasticity are expected to differ due to different strains imposed in the steady-state and dynamic flow fields. 

Asymmetrical flow field-flow fractionation was used for size determinations of SLN in comparison to an emulsion and oil-loaded SLN. The differences found in the sizes and elution profiles were attributed to differences in the particle shapes. Due to their anisometric, platelet-like shape it is likely that SLN are more retained by the cross flow applied compared to spherical emulsion droplets. This method appears very promising as additional size determination method particularly with regard to separation and detection of different colloidal structures. 

Forthis reason, several experimental and simulation techniques have been outlined. Optical techniques, such as PIV or DIA, can acquire whole-field data on the emulsion phase and bubble phase behavior simultaneously. DPMs and TFMs can be used to accurately simulate fluidized beds at small scales. These techniques have been used to investigate the effect of extraction or addition of gas via membranes into an FBMR. When the membranes were mounted in the vertical walls, it was observed and quantified that flow field... 

Rheology is a part of continuum mechanics that assumes continuity, homogeneity and isotropy. In multiphase systems, there is a discontinuity of material properties across the interface, a concentration gradient, and inter-dependence between the flow field and morphology. The flow behavior of blends is complex, caused by viscoelasticity of the phases, the viscosity ratio, A (that varies over a wide range), as well as diverse and variable morphology. To understand the flow behavior of polymer blends, it is beneficial to refer to simpler models — for miscible blends to solutions and mixtures of fractions, while for immiscible systems to emulsions, block copolymers, and suspensions [1,24]. 

Capital Costs. Based upon the results from the field tests, cost evaluations were developed that are related to the potential use of the LEM technique for removing and recovering copper from waste waters and dilute process solutions. Figure 2 shows the effect of the copper level in the feed on the capital cost of a 4,000 GPM plant. The predicted cost of the plant would decrease as the level of copper in the feed decreased. As the copper concentration in the feed decreases, the ratio between emulsion flow rate and feed flow rate also decreases. This has the effect of resulting in lower capital costs. The cost of the LEM plant is greatly influenced by the amount of emulsion used. 

Many types of emulsification equipment are widely appUed in industry, such as high pressure homogenizers and rotor-stator systems. In these machines the premix droplets are deformed and disrupted in the flow field of the emulsification device [1]. In addition to these techniques, alternative methods for the production of emulsions using microporous devices have been developed since the early 1990s. 

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