Static coalescers

A coalescer works in the same way as a demister, except that it is used to accelerate the removal of droplets of a heavier liquid from a flowing lighter liquid. An ordinary coalescer is shown in Fig. 39.5. 

This coalescer was used to remove entrained caustic from a flowing isobutane stream. The liquid isobutane would impact the coalescer pad at a velocity of 1 to 2 ft/min. The droplets of caustic, which have a higher surface tension than isobutane, would adhere to the surface of the coalescer fibers. As the caustic droplets grew bigger and heavier, they would drain down the fibers of the pad and into the boot. 

This particular coalescer pad was made out of ordinary straw. It would gradually plug and lose its effectiveness every few days. To restore its efficiency, we would open the intermittent water-wash valve, shown in Fig. 39.5, for 15 minutes to wash off the offending deposits. 

one day (this was in 1974), I decided that the use of straw in a modern industrial process plant was inappropriate. I ordered a new coalescer pad constructed from state-of-the-art synthetic fibers. We replaced the sorry-looking straw pad with the really attractive, modern fiber pad— which did not coalesce nearly as well as the straw. The moral of this story is, of course, If it ain t broke, don t fix it.  

Some coalescers consist of sets of parallel, slightly sloping, horizontal plates. The idea is to reduce the settling height and hence speed settling. My experience with this sort of coalescer is quite positive. 

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An issue as interesting as it is contentious is that of electrolyte inhibition of bubble coalescence. Recently, a number of studies have reported the ion-specific nature of electrolyte inhibition of bubble coalescence, albeit in static (non-acoustic) fields [43 -9]. Some electrolytes appear to be highly efficacious whereas others almost completely ineffectual in inhibiting coalescence and ion combination rules have been devised to predict the behavior of various ion pairs. Various explanations have been proposed, most implying a gas-liquid interfacial mechanism. Christenson and Yaminsky [44] have reported a correlation between the inverse Marangoni factor, (dy/ dc]) 2, and coalescence inhibition ability for several different... 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

From their crystal structures, (l,3-diene)CrL4 complexes46 are found to be approximately octahedral coordinate. The low temperature (—90 °C) 13C NMR spectrum of (butadiene)Cr(CO)4, which consists of 3 M-CO signals (1 2 1 ratio), is consistent with this static structure. At higher temperature, these coalesce into a single signal20. The chiral complex (2-ethyl-l,3-butadiene)Cr(CO)4 (8) shows similar behavior, however... 

A static settling time of less than 10 minutes was required to achieve residual water cuts between 0.1 and 1.0T (F1g. 2). This is better than that obtained in the laboratory due to the somewhat "softer 1 field emulsion and perhaps some pipe coalescence. The required chemical dosage was 400 ppmv, which is twice that required in the laboratory. In general, the reverse would normally be expected the hiqher chemical dosage reouired during the on-site test might be due to inadequate reaction time, since the demulsifier was injected in the cold stream (circa 25°C) only some 20 m upstream of the sampling point. 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, i.e., there is no thin-film persistence. It is the adsorption of surfactant, at the gas-liquid interface, that promotes thin-film stability between the bubbles and lends a certain persistence to the foam structure. Here, when two bubbles of gas approach, the liquid film thins down to a persistent lamella instead of rupturing at the point of closest approach. In carefully controlled environments, it has been possible to make surfactant-stabilized, static, bubbles, and films with lifetimes on the order of months to years [45],... 

This is a serious misnomer as these inert constituents of pitch are certainly not inert during the carbonization processes. It is well-established that the size of the optical texture of a coke can be reduced by the presence within the pitch of primary QI material (102-105). The QI material within the pitch becomes adsorbed on the surfaces of the growth units of mesophase. This thereby prohibits coalescence of these growth units into the larger sized optical textures. When this process is viewed by hot-stage optical microscopy (106) this lack of coalescence is seen to reduce markedly the flow characteristics of the mesophase - it becomes almost static. 

The growth and coalescence of the interfacial cavities under the influence of static and cyclic tensile loads results in extensive microcracking ahead of the main crack tip. Figure 7.8a shows an example of the formation of a diffuse microcrack zone ahead of a main crack in the air environment at 1500°C. Figure 7.8b is an example of microcracking damage at 1400°C. 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

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