Flow Regimes on Trays

Flow Regimes on Trays 6.4.1 The common flow regimes... 

The classic hydraulic model (Sec. 6.2,1) oversimplifies tray action. There are five main flow regimes on distillation trays (12,99). ThesB regimes (Figs. 6.25 to 6.28) may all occur on the same tray under different liquid and vapor flow rates (Fig. 6.29). An excellent overview of the fundamentals and modeling of these flow regimes is presented by Lockett (12). 

Flgurs S.25 Common flow regimes on distillation trays. 

Figure 6.29 suggests guidelines for the occurrence of each flow regime on commercial distillation trays. As stated earlier, the bubble and cel-... 

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.)...

Fig. 7.3 Choice of hydrodynamic flow regimes on reactive and non-reactive trays...

Ffgur 6.2 Common flow regimes on di Ilation trays. 

Sketch and label the parts of a distillation system explain the operation of each part and the flow regime on the trays... 

The value of o is a function of the flow regime on the tray. The tray is said to operate in the spray regime when the liquid is dispetsed almost completely into small droplets by the action of the gas jets. This occurs at high gas velocities and low liquid loads. In the spray regime... 

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

FIG. 14-19 The flow regime likely to exist on a distillation tray as a function of vapor and liquid loads. (From H. Z. Kister, Distillation Design, copyright 1992 hy McGraw-Hill reprinted hy permission.)... 

Spray entrainment flooding (Fig. 6.7a). At low liquid flow rates, trays operate in the spray regime, where most of the liquid on the tray is in the form of liquid drops (Figs, 6,25c and 6.276), As vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above. The liquid accumulates on the tray above instead of flowing to the tray below,... 

The flow regime likely to exist on Industrial trays... 

The flow regime likely to exist on a diet illation tray, as a function of vapor and liquid loads,... 

Flow across the tray. The classical hydraulic model implies that liquid flows across the tray by building up a liquid head on the tray, and when this head exceeds the weir height it overflows it. This mechanism is valid in principle when liquid is the continuous phase, but in the spray regime, vapor is the continuous phase and the liquid is present as drops in the vapor space (Figs, 6,25d, 6.26c, and 6.276). In... 

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. 

In the development of the above series of equations, Zuiderweg has used the work of many prior investigators and relied henvjly on the data recently released by Fractionation Research, Inc, (FRl), as reported by Sakata and YanagP 26 on the performance of two types or commercial perforated tray. Zui-derweg also presents correlations that define the flow regime transitions, pressure drop, entrainment, columa capacity, aad other operating parameters of sieve trays. 

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