Mcavy

Figure 6.17 Sieve tray weeping mechanisms. [Reprinted with permission from M. J. Lockett and S. Banik, Ind. Eng. Ckem. Proc. Des. Dev., vol. 25, p. 561, Copyright (1986) American Chemical Society.]

Factors affecting weeping. Weeping tendency increases with

■ Taller weirs (28,48,63). Lockett and Banik (56) observed that taller weirs increased the weeping tendency, except (1) at high liquid rates and high weirs (2) at high vapor rates, low liquid rates, and low weirs. In both of these exceptions, weir height had little effect on weeping.

■ The effect of hole diameter varies, possibly due to the variation of the weeping mechanism with hole size (Fig. 6.17). Lemieux and Scotti (26) noted that the weep tendency is greater for large holes at low liquid rates, but for small holes at high liquid rates. Kreis and Raab (28) noted that the weeping tendency increases with hole diameter for holes smaller than 3/ie in at low liquid rates, and for holes smaller than V2 in at high liquid rates. For larger holes, they observed no effect of hole diameter on the weeping tendency. Lockett and Banik (56) observed that over a wide range of liquid flows, the weeping tendencies for Y&-in and Va-in holes were comparable and lower than the weeping tendency of Yi-in holes. On the basis of proprietary data, Hsieh and McNulty (63) concluded that an increase in hole diameter decreases weeping tendency. Others noted an increase in weeping tendency with an increase in hole diameter (4,39,49,61-63,66).

■ Lower surface tension (4,28,39,61,62,66). For hole sizes commonly used in commercial practice (> Ya in), however, experimental data (28,56,63) suggest a negligible surface tension effect.

■ Increasing plate thickness (48). Another source (62), however, states that decreasing plate thickness increases weeping tendency.

Weep point prediction. Until recently, there was no reliable means of predicting weep rates. Trays were designed (4,26,67) to operate above the weep point, which could be predicted with confidence. This practice forfeited the portion of area on the stability diagram (Fig. 6.6) between the "weep point" and "excessive weeping" curves. The recent appearance of reliable weep rate correlations (below) is likely to strip the weep point of most of its practical significance for tray design.

Most weep point correlations are based on a pressure balance between the static head of the clear liquid, and the tray pressure drop. Lockett (12) reviewed the performance of several weep point correlations, and noted that their success often depends on how the clear liquid height is estimated. Lockett (12) also presents Mayfield's (37),

Fair's (31), and Zuiderweg's (17) correlations as those that are generally more reliable. Other design publications (4,18,64) recommended the use of Fair's correlation (31). The author has also had favorable experience with it. According to Fair's model (31), the force balance defining the weep point is hd + h9 = hw + h„ (6.3 lo)

If the left-hand side is larger.than the right-hand side, the vapor will keep the liquid on the tray; if the converse occurs, liquid will weep. ha is given by (18)

PlVH

hd and how can be calculated from the equations in Sec. 6.3. Equation (6.31a) is the theoretical curve in Fig. 6.18. This equation gave poor agreement with experimental data, and Fair modified it empirically to give

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