Immiscible liquid segregation

Immiscible liquid phases are formed because of chemical effects, namely, the mutual solubilities of the two phases. The design for liquid-liquid separations is affected therefore by changes in temperature, pressure, presence of contaminants such as surfactants, and stream mixing effects. In this section, however, we will not consider any solubility factors, only the effect of physical forces. 

The presence of two liquid phases may be the result of an extraction, washing, or reaction operation. The two phases may have been contacted in pipes, static mixers, agitated vessels, or at the impeller of a pump. The type of contacting, the level of shear forces applied, the concentration of the phases, and the time of contacting all influence the difficulty of the separation process. 

To define the iramiscible-liquid-segregation problem, the drop size range of the dispersed phase must be known or approximated. Weinstein and Middleman present drop correlations for pipeline contacting and static mixers while Coulaloglou and Tavlarides present correlations of liquid in liquid drop sizes for agitated vessels. All use the Sauter mean drop size definitions and involve use of the Weber number, a 

Part of the drop size prediction requires knowii which phase is dispersed and which is continuous. Selker and Sleichef provide a useftil correlation to predict which is the diqiersed phase based on phase volume ratios and density and viscosi of each ptase. This information may be approximated by Eq. (3.3 ), 

0 Phase invetnon possible design for the worst case 

A wide variety of designs are used for gravity segregation in immiscible-liquid separators horizontal or vertical vessels, troughs (API separators), and vessels with various internet confignrauons or parallel plates. 

FIGURE 3.3-1 Drop size correlation for pipes and static mixes. Reprinted with permission from S. Middlentun. IEC Chem Process Des Dev.. 13, 78 (1974), American Chemical Society. 

The vessels are often referred 10 as decanters and mey operate as a belch or a continuous operation. The emphasis here will be on continuoas designs, The basic design approach is as follows  

Pick a preliminary size besed on overflow rate. 

Drop settling velocity is eslimeted from Stokes Law using Newton s basic drag equation  

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A. Segregation Immiscible liquid segregation and accumulation Sudbury Cu—Ni Insi2wa, South Africa... 

Membrane separators are us for immiscible liquid segregation. These devices are extensively covered elsewhere in this book. For actual segregation, ultrafiltration or microfiltntion membranes are another variation of an impaction or interception mechanism. Ultrafiltration membranes have discrete pores about 0.001-0.02 (im in diameter which allow the continuous phase to pass while trapping the much huger individual drops, which are usually greater than 0.1 pm. Row velocity across the membrane surface resuspends trapped dispersed-phase material to prevent blinding of the pores. Pressure drop requirements... 

A microemulsion is a thermodynamically stable, optically clear dispersion of two immiscible liquids such as water and oil, stabilized by the presence of a surfactant and, in some cases, a co-surfac-tant.i i7i,265-267 synthesis of nanoparticles by microemulsions has two main advantages. On the one hand, particle size can be controlled by adjusting the size of the micelle containing the metal precursors. Therefore, thermal treatments for particle size control can be avoided. On the other hand, since the micelles have the same composition, i.e. metal precursors are distributed homogeneously the nucleation of metallic particles renders particles of the same composition. This latter feature is very important for the synthesis of bimetallic (or ternary) catalysts. The main drawback of the microemulsion, or any other approach using surfactants, is surfactant removal. Severe thermal treatments are required in order to achieve complete removal of the surfactant which may result in particle aggregation and/or surface enrichment, or complete phase segregation of the components of the bimetallic samples. ... 

The ability of particles to mix and their tendency to segregate depend on differences in their size, density, shape, elastic properties, surface characteristics, and magnitude of interparticle forces. The difference in particle size is probably the most important factor. Unlike immiscible liquid systems, the density differences play a relatively minor role in de-mixing or segregation of particulate mixtures. The large body of literature available on fluid mixing therefore cannot be used to predict or evaluate solids mixing applications. 

Attempts to form alloys with compositions under the two-liquid dome will result in an almost completely segregated s) tem because of the density differences between the two immiscible liquid phases. Even attempts to form such alloys under microgravity conditions led to similar results, but for a different reason. Instead of buoyancy effects, the phases became separated because of their relative interfadal energy differences with the container walls (see Cahn, J.W., /. Chem. Phys.). 

Liquid-liquid phase segregation has been accomplished using two immiscible solvents (i.e., phase transfer DCC) by several laboratories. For example, the Morrow group has reported on imine [73] and acylhydrazone [74] DCLs targeting extraction of metal ions from aqueous to halogenated solutions. As discussed above in the context of Pd-mediated transesterification, the Miller group has also contributed to this area. 

A compound that has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule (as mentioned earlier). Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences, depending on the volume balances between the hydrophilic part and the hydrophobic part. These structures are formed through the microphase segregation of two incompatible components on a nanometer scale. Hand soap is an everyday example of a lyotropic liquid crystal (80% soap + 20% water). 

The separatory funnel is the classical liquid-liquid apparatus used to segregate immiscible phases. The pear-shaped funnel developed by E. R. Squibb in the 1880s is still the most commonly used by chemists. Other separatory funnel designs which have higher overall efficiency have been designed but have not become popular. 

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