Low liquid rate

For larger diameter columns, and for low liquid rates, the distributor must be almost exactly level (e.g., within 6 mm for a 3-m diameter) or all pour points will not function. On the other hand, the rises must be high enough to accommodate the backup caused by high liquid rates. The needed head can be estimated from the orifice equation, with a discharge coefficient of 0.5. In some cases the orinces discharge directly into tubes that extend to the packed bed (the Tubed drip-pan distributor ). 

The Flexipac structural packing have better efficiency than available random packing, particularly at low liquid rates, per Reference 101. 

Equations 8.57 and 8.58 are satisfactory except at low liquid rates when the frictional pressure drop is a very small proportion of the total pressure drop. Frictional effects can then even be negative, because the liquid may then flow downwards at the walls, with the gas passing upwards in slugs. 

The lower limit of the vapour flow is set by the condition of weeping. Weeping occurs when the vapour flow is insufficient to maintain a level of liquid on the plate. Coning occurs at low liquid rates, and is the term given to the condition where the vapour pushes the liquid back from the holes and jets upward, with poor liquid contact. 

To ensure an even flow of liquid along the weir, the crest should be at least 10 mm at the lowest liquid rate. Serrated weirs are sometimes used for very low liquid rates. 

Packed columns are not suitable for very low liquid rates. 

If very low liquid rates have to be used, outside the range of FLV given in Figure 11.44, the packing wetting rate should be checked to make sure it is above the minimum recommended by the packing manufacturer. 

At low liquid rates, the onset of instability occurs at a constant value of the total superficial velocity, and is predictable from holdup and flooding data for wetted wall columns. As liquid flow rates increase, Nicklin and Davidson predict that unstable flow begins at lower values of the gas flow rate. For high liquid flow rates, however, the slug length becomes important, and the unstable flow will begin at higher values of gas flow rate. Therefore, a definite liquid flow rate exists at which an unstable flow pattern appears with a minimum gas flow rate. 

In upward or climbing-film flow, waves or ripples are always present. At low gas rates and low liquid rates, films are thin, wave amplitude and entrainment may be relatively small, and straightforward hydrodynamic... 

This flow-pattern occurs only in horizontal flow, in larger pipes and channels, at low liquid rates. Few studies have been made of this type of flow in confined channels, as distinct from wave studies on a more extended liquid surface. The initial work was by Bergelin and Gazley... 

Here, issues in relation to the trickle flow regime—isothermal operation and plug flow for the gas phase—will be dealt with. Also, it is assumed that the flowing liquid completely covers the outer surface particles (/w = 1 or aLS = au) so that the reaction can take place solely by the mass transfer of the reactant through the liquid-particle interface. Generally, the assumption of isothermal conditions and complete liquid coverage in trickle-bed processes is fully justified with the exception of very low liquid rates. Capillary forces normally draw the liquid into the pores of the particles. Therefore, the use of liquid-phase diffusivities is adequate in the evaluation of intraparticle mass transfer effects (effectiveness factors) (Smith, 1981). 

Low liquid rates. With the aid of serrated weirs, splash baffles, reverse-flow trays, and bubble-cap trays, low liquid rates can be handled better in trays. Random packings suffer from liquid dewetting and maldistribution sensitivity at low liquid rates. 

The turndown of valve trays is much better than sieve trays, but not as good as bubble-cap trays. Bubble-cap trays are the moBt suitable to handle extremely low liquid rate applications (less than 2 gpm per foot of average flow width (10)]. 

Turndown About 2 1. Not generally suitable for operation under variable loads About 4-5 1. Some special designs achieve (or claim) 10 1 or more Excellent, better than valve trays. Good at extremely low liquid rates Low, even lower than sieve trays (10). Unsuitable for variable load operation... 

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrainment (jet) flooding limit is normally reached when vapor flow is raised, while the downcomer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached. 

Figure 6.12a shows the structure of the fluid mixture in a downcomer operated at low liquid rates with a nonfoaming mixture. At the upper (froth) zone of the downcomer, the vapor fraction is high and of the same order as in the tray froth. As the mixture travels downward, much of the vapor is disengaged. The froth zone transforms into an aerated liquid zone where vapor bubbles rise through a liquid pooL Upon further vapor disengagement, the aerated liquid zone transforms into a clear liquid zone. 

Effect of liquid rate. At low liquid rates, entrainment diminishes with higher liquid loads, while at high liquid rates entrainment increases with liquid loads (22,24,26,33,40,53-55 Fig. 6,15), When most of the dispersion is in the form of a spray, entrainment diminishes with higher liquid loads (22,24,27). The point at which the trend reverses, and entrainment begins to increase with liquid rate, has been interpreted either as the point where the dispersion changes from partially developed spray to froth (40,53), or where the dispersion changes from the spray to froth regime (22-24,45,55). 

Koziol and Mackowiak (55a) found the Kister and Haas correlation to give good agreement with experimental data. They developed a new dimensionless correlation [thus overcoming the need for a dimensional exponent in Eq. (6.27)] for spray regime entrainment at very low liquid rates (0.1-1.5 gpm/in). Their correlation, however, postulates that entrainment rises with tower diameter at the same steep rate at which it rises with hole diameter. This postulate conflicts with the industiy s experience that entrainment does not increase upon tower diameter scale-up. 

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. 

Tests by Banik (72) and Zhang et al. (70) show that weeping from valve trays is nonuniform. In Banik s 4 ft x 2 ft rectangular simulator, most of the weep issued from the inlet half of the tray at low liquid rates (< 3 gpm/in of outlet weir) and from the outlet half of the tray ax high liquid rates (>10 gpm/in of outlet weir). The nonuniformity appeared to escalate as weir height increased. This pattern of nonuniformity is similar to that observed by Banik and Lockett (56) on sieve trays. In Zhang et al. s (70) 5 ft x 1 ft rectangular simulator. 

Spray (Flge. 6.25d, 6.260, and 6.276 sometimes referred to as the drop" regime). As vapor load is increased at relatively low liquid rates, the spray regime is reached. While in the previous three regimes (and also... 

At low liquid rates ( 1 gpm/in of weir), the tray liquid flow profile strongly depends on the liquid distribution at the tray inlet (e.g., inlet weir) but is practically independent even of msyor disturbances at the outlet weir (163). The importance of Liquid distribution at tray inlet was also noted by others (159,160,164,165). 

---