Dispersed coalescers

FIGURE 9.39 Dispersion, coalescence, and suspension. (From Church et al., 1961.)... 

The emulsions were prepared on an ultrasonic disperser, coalescence stability was determined from the volume of the separated oil phase after 24 hours. As it is seen from Fig. 6.8, a complete coalescence stability of the emulsions is achieved at surfactant concentrations beyond the CMC. At the same time, the concentration at which the maximum coalescence stability is achieved is not sufficient to provide sedimentation stability of the emulsions. 

L Lobo, I Ivanov, DT Wasan. Dispersion coalescence Kinetic stability of creamed dispersions. AIChE J 39 322, 1993. 

A complication is that also in stirred liquid/liquid dispersions coalescence and redispersion may occur. These phenomena are not only governed by the interfacial tension, but also by concentration gradients. So the surface area may be influenced by the mass transfer process itself. This applies also to the mass transfer coefficients. It has been shown that reversal of e direction of mass transfer may increase or reduce the mass transfer rate by a factor of 3. Consequently, general emprical relations for mass transfer in stirred liquid/liquid dispersions are not very reliable. In the practice of reactor development this is often not a serious problem, since in most situations mass transfer is not a limiting factor in liquidAiquid processes. If mass transfer does limit the reaction rate, one can increase the mass transfer rate by making the dispersion finer. 

Turbulent Dispersion Coalescence. After the dispersed phase leaves the motionless mixer, it will tend to coalesce to an equilibrium drop or bubble size characteristic of the shear field in the downstream piece of pipe. This coalescence is not just a phenomenon of the downstream tailpipe but is a process happening in parallel with dispersion. It is not as well understood. We do know that just like dispersion, coalescence is affected by volume concentration and is promoted by turbulence. Coalescence is strongly affected by surface chemistry effects. The role of many chemicals added to stabilize dispersions is to slow down the coalescence rate. 

An agitated liquid-liquid process involves many simultaneous, interdependent phenomena, such as dispersion, coalescence, suspension, heat and mass transfer, and chemical reaction. Previously described nitration requires control of the interfacial area rather than specific drop size, but some processes require precise control of drop size. For example, equipment for suspension polymerization processes must be capable of producing uniform beads of specified size range as well as providing for heat transfer and drop suspension. 

For fine aerosol particles, X 1.0 and the agglomeration rate is the collision rate. However, for hqnid-hqnid systems, the coalescence efficiency is often small and rate limiting. Therefore, classical agglomeration theory (e.g., Smolnchowski eqnation) cannot be directly applied to liquid-liquid dispersions. Coalescence is known as a second-order process ( n ) since the coalescence rate is proportional to F(d, d0n(d)n(d0, where n(d) and n(d ) represent an appropriate measure of the number of drops of size d and d, respectively. 

Section 12-6 provides a discussion of drop suspension, dispersion formation, and the interrelationships between dispersion, coalescence, and suspension. Additional... 

Church and Shinnar (1961) described the interrelationship between suspension, dispersion, and coalescence. Figure 12-25 shows drop size as a function of agitator speed in a turbulent process vessel. A stable region exists in the center area bounded by three lines representing dispersion, coalescence, and suspension phenomena. Consider constant impeller speed. If a large drop exists above the upper dispersion line, it will continue to break up until the dispersion line is reached. Breakage can result in some drops whose size lies below the lower coalescence line. These drops will continue to coalesce until the coalescence line is reached. Inside the bounded region, equilibrium is established between dispersion and coalescence. 

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