Coalescence measurement

Figure 15.2 Bubble coalescence measured in a stirred tank reactor at 1000 Hz with a single 10-bit monochrome camera (From [8]).

Earlier, the first studies by Cockbain and McRoberts (C7) indicated that the time necessary for coalescing, measured from the point that the drop apparently has come to rest on the flat interface till the moment the first coalescence sets in, is spread statistically around an average value. Both the standard deviation and the mean value depend on the phase system and decrease with increasing temperature and with decreasing drop size, while surface active agents and small impurities that collect at the interface have a strong retarding effect. 

At the same time, rotation about the formally single bond between N-l and C-2 in these compounds is more restricted than the drawing of a single bond implies, just as it was with amides. The two A-methyl groups in both enamines 2.63 and 2.82 have different chemical shifts and coalescence measurements show that the free energy of activation for rotation is 56 kJ mol 1 (13 kcal mol-1) for the former and 69 kJ mol-1 (16.5 kcal mol-1) for the latter. Decreasing the stabilisation of the anionic centre in the transition structure with a less powerful acceptor than a nitro group, as in the ester 2.83 reduces the barrier to rotation about the N—C bond to 58 kJ mol-1 (14 kcal mol-1). 

Figure 14 shows a very interesting and an important correlation between the rate of coalescence in macroemulsions and the apparent viscosity in the flow through porous media. It was observed that a minimum in apparent viscosity for the flow of macroemulsions in porous media coincides with a minimum in phase separation time at the optimal salinity. This correlation between the phenomena occurring in the porous medium and outside the porous medium allows us to use coalescence measurements as a screening criterion for many oil recovery formulations for their possible behavior in porous media. It is. very likely that a rapidly coalescing macroemulsion may give a lower apparent viscosity for the flow in porous media (53). 

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

The prediction of drop sizes in liquid-liquid systems is difficult. Most of the studies have used very pure fluids as two of the immiscible liquids, and in industrial practice there almost always are other chemicals that are surface-active to some degree and make the pre-dic tion of absolute drop sizes veiy difficult. In addition, techniques to measure drop sizes in experimental studies have all types of experimental and interpretation variations and difficulties so that many of the equations and correlations in the literature give contradictoiy results under similar conditions. Experimental difficulties include dispersion and coalescence effects, difficulty of measuring ac tual drop size, the effect of visual or photographic studies on where in the tank you can make these obseiwations, and the difficulty of using probes that measure bubble size or bubble area by hght or other sample transmission techniques which are veiy sensitive to the concentration of the dispersed phase and often are used in veiy dilute solutions. 

Finding F Either Eq. (22-45) or Eq. (22-46) can be used to find the surface excess indirectly from experimental measurements. To assure a close approach to operation as a single theoretical stage, coalescence in the rising foam should be minimized by maintaining a proper gas rate and a low foam height [Brunner and Lemhch, Ind. Eng. Chem. Fundam. 2, 297 (1963)]. These precautions apply particularly with Eq. (22-45). 

---