Foam formation, fractionating column

At the bottom of the vessel there is a perforated barrier with a hermetically sealed filter paper on it. The foam column height is 50 mm. After foam formation a reduced pressure is created in the space below the barrier which is by 3 kPa less than the atmospheric (the absolute pressure is 97 kPa). The foam is dried for 10 min. Then the upper foam layer is brought into contact with another filter over which a solution of the NaDoBS and water-soluble fraction of thymol blue and NaCI are placed. The pressure in the space above this filter is 99.3 kPa to ensure a 1.3 kPa total pressure drop in the foam. Under the pressure drop the mixture enters the foam and through the Plateau borders advances from top to bottom. The liquid outflow is collected in a microtrap and samples of ca. 0.2 ml volume are taken to analyse the concentrations of the sulphonate and the dye. 

The higher the surface tension the more likely it is to make foam during distillation and this can result in filling the column with a stable froth which will prevent fractionation. A tray column, that creates mass transfer by bubbling vapour through liquid, is much more vulnerable to foam formation than a packed column which does not rely upon bubbling. 

A common example of foam formation in the bottom of a fractionator inducing flooding occurs in a crude preflash tower. In this case, stable foam accumulates in the bottom of the column as a consequence of the "flow improver" chemicals added to crude oil. These chemicals reduce pressure drop in the crude pipelines. Once the foam level rises to the feed inlet nozzle, the trays flood and black distillate is produced. Please see Chapter 18 (Preflash Towers). 

The formation of micelles in the foam breaker does not affect the limiting equations because of the theoretically unhmited opportunity in a sufficiently tall column for their transfer from the reflux to the ascending stream [Lemhch, Principles of Foam Fractionation, in Perry (ed.), Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1]. 

The biomarker analyses also revealed that nonpolar, relatively insoluble compounds exist in the water column (e.g. steroids, phospholipids) indicating that interactive processes may be at work to enhance the ability of these compounds to remain in solution, such as the formation of an aggregate. The existence of a foam fraction that will not pass through a 0.45 micron filter yet maintains similar solubility characteristics to the foam fraction that did pass through the filter is evidence for the existence of aggregates which are simply too large in size to pass through the 0.45 micron filter. 

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