Coalescence reaction

Almost all flows in chemical reactors are turbulent and traditionally turbulence is seen as random fluctuations in velocity. A better view is to recognize the structure of turbulence. The large turbulent eddies are about the size of the width of the impeller blades in a stirred tank reactor and about 1/10 of the pipe diameter in pipe flows. These large turbulent eddies have a lifetime of some tens of milliseconds. Use of averaged turbulent properties is only valid for linear processes while all nonlinear phenomena are sensitive to the details in the process. Mixing coupled with fast chemical reactions, coalescence and breakup of bubbles and drops, and nucleation in crystallization is a phenomenon that is affected by the turbulent structure. Either a resolution of the turbulent fluctuations or some measure of the distribution of the turbulent properties is required in order to obtain accurate predictions. 

This study could be extended to the synthesis of iron nanoparticles. Using Fe[N(SiMe3)2]2 as precursor and a mixture of HDA and oleic acid, spherical nanoparticles are initially formed as in the case of cobalt. However, a thermal treatment at 150 °C in the presence of H2 leads to coalescence of the particles into cubic particles of 7 nm side length. Furthermore, these particles self-organize into cubic super-structures (cubes of cubes Fig. ) [79]. The nanoparticles are very air-sensitive but consist of zerovalent iron as evidenced by Mossbauer spectroscopy. The fact that the spherical particles present at the early stage of the reaction coalesce into rods in the case of cobalt and cubes in the case of iron is attributed to the crystal structure of the metal particles hep for cobalt, bcc for iron. 

Rapid coaiescence will reduce the area available for mass transfer and may reduce the rate of reaction. Coalescence tends to be more severe in larger vessels. See Chapters 13 and 17. 

FIGURE 9.7 Time (in a.u.) profile of the absolute maximum eigenvalue of propagated matrix in the case of chemical reaction. Coalescence of caustics occurs in condensation region and appears as split peaks here. (Taken from Reference [47] with permission.)... 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

The in situ process is simpler because it requires less material handling (35) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80—100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubiUty to a higher molecular weight. If the reaction medium contains a protective coUoid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective coUoid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective coUoid serves to assist particle formation and stabUizes the final particles against coalescence. Some examples of protective coUoids are poly(vinyl alcohol), gum arabic, and hydroxyethjlceUulose. 

The long reaction time needed for this apparendy simple neutralization is on account of the phase inversion that takes place, namely, upon dilution, the soap Hquid crystals are dispersed as micelles. Neutralization of the sodium ions with sulfuric acid then reverses the micelles. The reverse micelles have a polar interior and a hydrophobic exterior. They coalesce into oil droplets. 

Types of Gas-in-Liquid Dispersions Two types of dispersions exist. In one, gas bubbles produce an unstable dispersion which separates readily under the influence of gravity once the mixture has been removed from the influence of the dispersing force. Gas-hquid contacting means such as bubble towers and gas-dispersing agitators are typical examples of equipment producing such dispersions. More difficulties may result in separation when the gas is dispersed in the form of bubbles only a few micrometers in size. An example is the evolution of gas from a hquid in which it has been dissolved or released through chemical reaction such as electrolysis. Coalescence of the dispersed phase can be helpful in such circumstances. 

Pipe Lines The principal interest here will be for flow in which one hquid is dispersed in another as they flow cocurrently through a pipe (stratified flow produces too little interfacial area for use in hquid extraction or chemical reaction between liquids). Drop size of dispersed phase, if initially very fine at high concentrations, increases as the distance downstream increases, owing to coalescence [see Holland, loc. cit. Ward and Knudsen, Am. In.st. Chem. Eng. J., 13, 356 (1967)] or if initially large, decreases by breakup in regions of high shear [Sleicher, ibid., 8, 471 (1962) Chem. Eng. ScL, 20, 57 (1965)]. The maximum drop size is given by (Sleicher, loc. cit.)... 

The second type of coalescence arises from the rupture of films between adjacent bubbles [Vrij and Overbeek, y. Am. Chem. Soc., 90, 3074 (1968)]. Its rate appears to follow first-order reaction kinetics with respect to the number of bubbles [New, Proc. 4th Int. Congr. Suif. Active Substances, Brussels, 1964, 2, 1167 (1967)] and to decrease with film thickness [Steiner, Hunkeler, and Hartland, Trans. In.st. Chem. Fng., 55, 153 (1977)]. Many factors are involved [Biker-man, Foams, Springer-Verlag, New York, 1973 and Akers (ed.). Foams, Academic, New York, 1976]. 

Coalescence Condensation Destruction Chemical reaction Incineration ... 

Villermaux, J. and Devillon, J.C., 1975. Representation de la coalescence et de la redispersion des domains de segregation dans un fluide par un modele d interaction phenomenologique. In Proceedings of the second international conference of chemical reaction engineering. Amsterdam, pp. Bl-13. 

---