Maximum downcomer velocity

The Koch correlation (8) and Nutter correlation (9) are maximum residence time criteria (see below) which were converted into downcomer velocity criteria. The maximum downcomer velocity is calculated from... 

Figure .i3 Values of downcomer residence time (a to be used in the Koch correlation and in the Nutter correlation for maximum downcomer velocity [Eq. (6.26)]. Data based on Koch Engineering Co. Inc. Design Manual-Flexitray, Bulletin 960-1, Wichita, Kansas, 1962 and on Nutter Engineering Float Valve Design Manual, Tulsa, Oklahoma, 1976. 

Kister (1] surveyed tbe multitude of published criteria for maximum downcomer velocity. He pointed at tbe poor accuracy and inconsistency of these criteria, then incorporated them together with his own experience into the single set of guidelines shown in Table 6.5. The values in Table 6.5 are not conservative, and some may even be slightly optimistic. For a conservative design, a value from Table 6.5 can be multiplied by a safety factor of 0.75. 

Maximum downcomer velocity dehnes the liquid capacity limit and it can be predicted by several different correlations. According to Kister (1992), the Glitsch correlation (Glitsch, 1974) tends to predict the highest downcomer liquid load, the Koch correlation (Koch, 1982) predicts the lowest downcomer liquid load, while Nutter correlation (Nutter, 1981) provides the estimate in between. Let us focus on the Glitsch correlation next. 

FIGURE 12.13. Maximum downcomer velocity correlation based on Glitsch s Figure 4 (1974). (From Summers (2011), reprinted with permission by AICHE.)... 

Step 1 Assume Hs and VTl. Adjust them based on flood and weir loading criteria. Step 2 Calculated maximum downcomer velocity Vo,max based on equation (12.46), which determines the downcomer area Aj. 

Downcomer velocity. The most popular criteria for maximum velocity of clear liquid at the downcomer entrance are the Glitsch (7), Koch (8), and Nutter (9) correlations, Lockett (12) noted large differences between the predictions of these correlations, but made no recommendations on which correlation to use. Lockett s analysis shows that generally the Glitsch correlation tends to predict the highest downcomer velocities, while the Koch correlation tends to predict the lowest downcomer velocities, with the Nutter correlation giving intermediate values. The Glitsch correlation (7) is... 

When the liquid has foaming tendencies, additional residence time must be allowed for phase disengagement. System discount factors, recommended by Koch-Glitsch, Inc. on the basis of field experience, are shown in Table 12.7. As an example, the maximum allowable velocity of 0.1 m/s would be multiplied by the discount factor so as to provide adequate downcomer volume. 

Downcomer velocity. The maximum velocity of clear liquid in the downcomer needs to be low enough to prevent choking and to permit rise and satisfactory disengagement of vapor bubbles from the downcomer liquid. This is most restrictive in systems that have a high foaming tendency. 

It is possible for vapor to be entrained downward with the liquid, and this reverse entrainment has been studied by Hoek and Zuiderweg, using Fractionation Research, Inc. data taken at pressures of 20 and 27 atm arffl a system operating close to its critical point. The downward entrainment was found to affect significantly the overall efficiency of the column. A maximum superficial velocity in the downcomer should be about 0.12 m/s, based on clear liquid and the smallest cross section of the downcomer. 

For quick tray sizing purposes. Summers (2011) developed a simplihed version of the Glitsch correlation. First, data points on different curves in Figure 12.12 are extracted to generate a single data plot on VD.max (maximum downcomer entrance velocity) and Ap axis (Figure 12.13). Then a correlation is developed based on the best ht method to give... 

There are two capacity limits related to liquid loading, which are the downcomer baekup limit and downcomer veloeity limit. The downcomer backup limit is set at 80% of liquid settling height based on the froth level. The downcomer velocity limit is 75% of maximum velocity allowed to avoid downeomer choke. The number of tray passes is the most important parameter affeeting the downcomer loading and thus these two downcomer limits. 

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

Maintaining the downcomer or a standpipe under a moving bed condition allows a large pressure buildup along the downcomer. For a given quantity of particles in the downcomer, the pressure at the bottom of the downcomer is closely associated with the relative velocity between the gas and particles. The pressure drop rises with the relative velocity as the particles are in a moving packed state. The maximum pressure drop in the downcomer is established when particles are fluidized, a state which can be expressed in terms of the pressure drop under the minimum fluidization condition as... 

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