Patterns froth flow

Figure 13. The pattern map for an upward vapor-liquid flow of R21 refrigerant through the assemblage with plain fins (8FPI). Here 1, 2, 3, 4 are the areas of annular flow, cell flow, froth flow, plug and bubble flows respectively. Solid lines indicate transition between flow modes and dashed lines indicate constant mass flux condition.

The mechanics and applications of multiphase flow has been an area of continuing interest to chemical, environmental, and civil engineers (23,77). The multiphase flow patterns may be classified as bubble flow, plug flow, stratified flow, wave flow, slug flow, annular flow, spray flow, and froth flow. Typical sketches of these various flow patterns are shown in Fig. 3. They are self-explanatory. In the field of absorptive bubble separation processes, only multiphase bubble flow and froth flow are of interest to the process engineer. 

Type of flow pattern(s) involved in an adsorptive bubble separation system depends on the type of process used. For example, bubble fractionation involves two-phase (gas-phase and liquid-phase) bubble flow, while solvent sublation involves multiphase bubble flow in their vertical bubble cells. Foam fractionation involves a two-phase bubble flow in the bottom bubble cell, and a two-phase froth flow in the top foam cell. However, all froth flotation processes (i.e., precipitate flotation, ion flotation, molecular flotation, ore flotation, microflotation, adsorption flotation, macroflotation, and adsorbing colloid flotation) involve multiphase bubble flow and multiphase froth flow. 

Bubble Flow. The gas flows in the form of bubbles along the upper surface of the liquid and the gas bubbles move at about the same velocity as that of the liquid. This pattern is found when there is a high liquid to gas ratio. If the bubbles are dispersed then it is called froth flow. 

Froth Flow. As the gas flow further increases, the slug flow pattern changes to froth. 

Martinelli and others observed that several types of flow patterns are present in two-phase or two-component flow. The different patterns visualized and described by Martinelli, Boelter, Taylor, Thomson, and Morrin [1] Bryan, Seigel [2] Baker [3] Isbin, Moen, and Mosher [4] were bubble flow, plug flow, separated or stratified flow, slug flow, annular flow and frothing flow. 

Pure oxygen, 20% in excess over the theoretical requirement operating pressure = 3.04 x 10 Pa, absolute liquid-side Reynolds number = 40,000 (to keep the two-phase flow patterns as froth) liquid-side mass transfer coefficient at the entrance (Uq = 0.127 m/s, Ui = 0.077 m/s) kifl = 0.12 s. ... 

The two-phase flow pattern (in this case, froth) does not change along the length of the pipeline. The liquid-side mass transfer coefficient varies linearly with the superficial gas velocity at a constant liquid flow rate. As indicated later, the pressure drop can be neglected. 

Figure 6.1 Flow patterns on trays, (a) Froth regime (liquid phase is continuous) (b) spray regime (gas phase is continuous). (Henry Z. Kister, excerpted by special permission from Chemical Engineering, September 8, 1980 copyright , by McGraw-Hill, Inc., New York, NY 10020.)...

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. 

Another novel concept is the Air-Sparged Hydrocyclone developed at the University of Utah. In this device, the slurry fed tangentially through the cyclone header into the porous cylinder to develop a swirl flow pattern intersects with air sparged through the jacketed porous cylinder. The froth product is discharged through the overflow stream. 

In principle, a colrrrrm tray can be operated even with very small liquid loads because the necessary height of the two-phase layer (froth) on the tray is provided by the exit weir. At extremely low liquid loads, however, the liquid will flow in an uneven pattern across the tray resulting in some degree of maldistribution of liquid. Accordingly, it is recommended to ensure a ttunimum liquid flow rate over the exit weir larger than Fl/ w 2 m /(m h). In small diameter columns, however, the liquid load can be considerably lower. 

A macro-mixed flow pattern contairrlng either a frothy or highly trrrbrrlent mixtrrre of gas and liquid, or two immiscible liquids tmder conditions in which neither is continuous. Such patterns are fotmd in flotation circuits m which froth is used to separate concentrate from gangue. 

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