Froth packed column

It is worth emphasizing that Eqs. (13-61) to (13-68) hold regardless of the models used to calculate the interphase transport rates and EJ. With a mechanistic model of sufficient complexity it is possible, at least in principle, to account for mass transfer from bubbles in the froth on a tray as well as to entrained droplets in a spray, as well as transport between the phases flowing over and through the elements of packing in a packed column. However, a completely comprehensive model for estimating mass-transfer rates in all the possible flow regimes does not exist at present, and simpler approaches are used. 

It is traditional for chemical engineers to model packed columns through the concept of transfer units (in much the same way as we used transfer units in the treatment of transfer in the froth on a tray in Sections 12.1.2 and 12.2.1). For two component systems Eq. 12.3.12 simplifies to (cf Eq. 12.1.7)... 

For tray colnmns, liquid flow constrictions are normally found in the tray downcomers. Example causes are lack of adequate froth collapse time, excessive pressure drop for flow under the downcomer baffle, and foreign materials being left in the downcomer by construction people. For packed columns, clogged support plates, broken packings, or growth of fouling deposits can cause liquid flow restrictions. 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

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. 

During absorber cleaning, additional sulfur froth is formed in the packed column and is routed to the oxidizer. Sufficient buffer capacity in the sulfur make-up section, especially in the slurry tank, is required to handle the peak sulfur production experienced during absorber cleaning. 

For tray columns the net interfacial area is a = a hfA, where a is the interfacial area per unit volume of froth, hf is the froth height, and A j is the bubbling area. For tray columns the interfacial area is a hA, where a is the interfacial area per unit volume, h is the height of a section of packing, and A is the cross-sectional area of the column. 

High capacity in foaming system. Trayed columns use the continuous liquid phase to create a froth that is difficult to separate. Packed towers make the vapor phase continuous, and the liquid phase discontinuous. 

Mass transfer relationships in a distillation column are based on the basic interphase transfer model for a differential slice of the cross section, as shown in Figure 12.57. The slice is taken from the packed bed or from the tray froth. For component i,... 

The most common applications of this technique in distillation and absorption columns is for liquid level and liquid level interface detection, especially when normal level-measuring techniques suffer from plugging. Neutron backscatter techniques have also been used for froth height measurements on trays and downcomers, and for measuring the top and bottom of packed beds. One case history has been described (71) where downcomer froth height measurements using the neutron backscatter technique led to a detection of downcomer deposits which caused premature flooding of the column. The author is familiar with one case where this technique successfully detected overflow of a packed tower distributor. 

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