Trays cross-flow pattern

Fig. 10.9 Cell model to represent cross-flow pattern on distillation tray...

Fig. 10.10 Cell model to represent cross-flow pattern on distillation tray. Ethyiene glycoi synthesis top row), methyl formate synthesis (bottom row). Well-mixed liquid (left) and liquid plug flow with dispersion for four cells (right)... 

Figure 11.21. Liquid flow patterns on cross-flow trays, (a) Single pass (b) Reverse flow (c) Double pass...

Liquid Flow Patterns on Large Trays The most popular theoretical models (below) postulate that liquid crosses the tray in plug flow with superimposed backmixing, and that the vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. 

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 horizontal liquid flow pattern is very complicated due to the mixing by vapor, dispersion, and the round cross section of the column. On single-pass trays, the latter results in the flow path, which first expands and then contracts. A rigorous modeling of this flow pattern is very difficult, and usually the situation is simplified by assuming that the liquid flow is unidirectional and the major deviation from the plug flow is the turbulent mixing or eddy diffusion. In [80], two different models, the eddy-diffusion model and the mixed pool model were developed and tested in the context of the rate-based approach for RD trays. The details of these models can be found in [81]. 

Figure 10-3. Flow patterns on trays (A) cross-flow, (B) double-pass...

On trays with controlled vapor and liquid flow (reflux), the basic flow pattern is cross flow. In columns without controlled liquid guidance, the flow pattern is counterflow. 

The flow pattern on column trays is usually controlled. Due to the design arrangement of the entry and exit, including an exit weir, the reflux liquid flows across the tray while the vapor flows upward (Fig. 2-59). Vapor and liquid therefore flow in a cross current-counter-flow manner through the column. 

When straight or serrated segmental weirs are used in a column of circiilar cross secdion, a correction may be needed for the distorted pattern of flow at the ends of the weirs, depending on liquid flow rate. The correction factor F from Fig. 14-33 is used direcdly in Eq. (14-112) or Eq. (14-119). Even when circular downcomers are utilized, they are often fed by the overflow from a segmental weir. When the weir crest over a straight segmental weir is less than 6 mm V in), it is desirable to use a serrated (notched) weir to provide good liquid distribution. Inasmuch as fabrication standards permit the tray to be 3 mm Vh in) out of level, weir crests less than 6 mm V in) can result in maldistribution of hquid flow. 

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