Implications of the emulsion regime for design and operation

The difference in the behavior of the aerated mass between the emulsion and froth regimes is far less apparent than the difference between the froth and spray regime.

Flow across the tray. In the froth regime the liquid is thrown over as drops and slugs, and therefore the flow does not obey the Francis weir formula (85,104). In the emulsion regime a "homogeneous" dispersion flows over the weir. While in the froth regime the bubbles are of different sizes, in the emulsion regime most of the vapor is vigorously emulsified into small bubbles (Figs. 6.25e and 6.27c),

Flooding and entrainment. Entrainment represents very little problem in the emulsion regime, and is usually negligible (17,45,105). On the other hand, the quantity of vapor that enters the downcomer is much greater than in the froth regime (17,45,104). Further, emulsion flow tends to coincide with conditions where separation of liquid from vapor in the downcomer is difficult. For these reasons, the most likely flooding mechanisms in the emulsion regime are downcomer choking, downcomer backup, or both.

Vapor recycle. Because of the large quantity of vapor that enters the downcomer and the difficulty of separation in the downcomer, vapor recycle is a major consideration in the emulsion regime (Sec. 6.2.8) and can lead to reduction of both capacity and efficiency, and to an increase in pressure drop (17).

Hoek and Zuiderweg (44) described the effect of vapor entrainment on tray efficiency in terms of a theoretical model. The tray is divided into several mixing zones. Ascending vapor is split in parallel among the zones, while liquid passes in series from one zone to another. Liquid entering and leaving each zone contains the same amount of entrained vapor per mole of liquid as the downcomer underflow. In each zone, liquid is perfectly mixed while the ascending vapor is in plug flow. The model showed that the effects of vapor entrainment are most severe when liquid-phase resistance to mass transfer is controlling. When vapor-phase resistance to mass transfer controls, the effects are less severe, because the recycled vapor enhances the stripping action of the more volatile component, thus counteracting the entrainment effect.

Foaming. The emulsion flow regime is particularly sensitive to foaming (17,85). The presence of foaming impurities stabilizes the emulsion and therefore brings about a premature capacity limitation.

Mass transfer. Tray efficiency increases with pressure in the froth regime (118,119), but decreases with increased pressure in the emulsion regime (44,104,105). The efficiency decrease in the emulsion regime is caused by the greater vapor recycle (44,104,105). In general, there is otherwise little difference between mass transfer in the froth and emulsion regimes.

6.5 Column Sizing 6.5.1 General considerations

The sizing procedure is a trial-and-error calculation. A preliminary design is set, and then refined by checking against the performance correlations until an adequate design is achieved. The sizing calculations are performed at the points where column loading is expected to be highest and lowest for each section, i.e.,

■ Above every feed, product drawoff, or point of heat addition or removal

■ Below every feed, product drawoff, or point of heat addition or removal

* The bottom tray

■ At any point in the column where the calculated vapor or liquid loading peaks

For a single-feed two-product column, there is generally a need to carry out the sizing calculation for the top tray, bottom tray, tray just above the feed, and tray just below the feed. The column is then designed for the more severe conditions.

6.5.2 Tray sizing example and initial steps

Example 6.1 Size the depropanizer from Examples 2.4 (Sec. 2.3.1) arid 3.4 {Sec. 3.2.5). The feed rate is 2000 lb-mole/h of mixture. Minimum anticipated load is 60 percent of design.

solution The material balance based on 100 lb-mole/h feed was shown in Table 2.7. Accordingly, vapor and liquid flow rates were calculated (Sec. 2.3.4). These are prorated to 2000 lb-mole/h feed. Table 6.10 shows the column profile, loadings, and physical properties at a feed rate of 2000 lb-mole/h. The data shown in Table 6.10 are typical of data generated by commercial computer simulations.

maximum loads Table 6.10 shows that both the volumetric and mass vapor and liquid flow rates in the top section peak at stage 3. Stage 3 is therefore the appropriate basis for the maximum throughput calculation for the top section.

table 6.10 Column Loading and Physical Properties: Depropanizer example 6.1 (Basis: 20,000 lb-mole/h teed)

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