Flood trays

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

Capps [188] examines sieve and valve tray capacity performance and Figure 8-151 [188] is offered for preliminary colunm sizing or for determining whether a debottlenecking study is justified. The correlation for flooding, tray rating, and design of a tray are all based on the capacity factor, Cx, equation (Souders and Brown [68] by Capps [188]). 

The column cross-sectional area, and hence the tray diameter, is sized such as to prevent flooding. Tray sizing is initially based on preventing entrainment flooding, but is subsequently checked for downcomer flooding and other considerations. Above a certain vapor velocity, the vapor flood velocity, liquid droplets are entrained with the vapor, causing flooding. The tray should be sized such that the actual vapor velocity is below the vapor flood velocity. 

Flooding is characterized by the accumulation of liquid in the column. This accumulation propagates from the first flooded tray (or from the... 

In the ebb-and-flow method, nutrient solution contacts roots in cycles after flooding trays containing roots and substrates then draining and returning to a tank to store for additional dehvery. Timers control pump mechanisms, which move nutrient solutions. Aquaponics systems transport water from ponds or greenhouses where fish tanks are kept to greenhouses where plants are grown so that wastes from the fish can provide nutrients for plants. 

Trays can flood even below design loads because of fouling of the tray decks. Flooded trays lose fractionating efficiency (see Chapter 19). Check the pressure drop across the suspect trays. If the pressure drop per... 

Flooding was being initiated from the bottom of the tower. I knew this because 79 in. of water-pressure drop represented a AP per tray of about 7 in. of water. Therefore, each of the trays below the LCO product draw tray was flooding. Put another way, the apparent fluid specific gravity on a flooded tray deck in hydrocarbon service is typically 0.3. For trays spaced 24 in. apart, the observed pressure drop corresponding to a fully flooded tray is then ... 

Entrainment occurs when spray or froth formed on one tray enters the gas passages in the tray above. In moderate amounts, entrainment will impair the countercurrent action and hence drastically decrease the efficiency. If it happens in excessive amounts, the condition is called priming and will eventually flood the downcomers. 

The trays may be fitted with rims this is particularly useful for flooding the trays in washing operations. Scavenger leaves are often used. FUtration areas up to 50 m are avaUable. Like aU horizontal leaf filters, horizontal vessel, horizontal leaf filters are particularly suitable when thorough washing is needed. 

Fig. 18. Flooding correlation for crossflow trays (sieve, valve, bubble-cap) where the numbers represent tray spacing in mm. Also shown are approximate...

Fair s empirical correlation for sieve and bubble-cap trays shown in Fig. 14-26 is similar. Note that Fig. 14-26 incorporates a velocity dependence (velocity) above 90 percent of flood for high-density systems. The correlation implicitly considers the tray design factors such as the open area, tray spacing, and hole diameter through the impact of these factors on percent of flood. 

TABLE 14-13 Dependency of Distillation Flood Velocity on Physical Properties and Tray Open Area... 

Lieberman gives two rules of thumb for troubleshooting fractionators that could also be used as checks on a design. First, the pressure drops across a section of trays must not exceed 22% of the space between the tray decks, to avoid incipient flood. Mathematical , hold... 

Percentage of Flood for Existing Ballast Tray Columns... 

ADM = Minimum downcomer area, fT ATM = Minimum column cross-sectional area, fr CAF = Vapor capacity factor CAFo = Flood capacity factor at zero liquid load CFS = Vapor rate, actual ftVsec DT = Tower diameter, ft DTA = Approximate tower diameter, ft FF == Flood factor or design percent of flood, fractional FPL = Tray flow path length, in. 

The preceding discussion on reflux assumes that the condenser is not limiting when the reflux is raised. For a severely limited condenser, an evaluation must first be made of the condenser heat transfer before analyzing the effect of a reflux increase with Smith-Brinkley. Likewise, a limiting reboiler or trays close to flood would have to be evaluated prior to Smith-Brinkley calculations. 

Trays from one section installed in wrong section. Limited capacity. High pressure drop. Flooding in restricted section. Installation error. 

Chimneys (risers) blocking flow to draw sump forcing liquid to overflow prematurely. Flooding of trayed section below pump around. Lack of response to pump around flow changes. Design error. 

Improper tray spacing at feed location. Premature flooding. Design error. 

Leaving tray support rings in towers revamped with packed beds. Poor separation. Premature flooding. Installation error. There are too many examples of this for us to continue to make this one. If you are not confident enough to remove the tray support rings, don t pack the tower. 

Jet Flood. Flooding generally occurs by jet flood or downcomer backup. Reference 15 gives Equations 1, 2, and 3 for Jet flood, using Ballast trays. 

Downcomer Backup Flood. For downcomer backup. Equation 4 can be used. Reference 15 states that if the downcomer backup for valve trays exceeds 40% of tray spacing for high vapor density systems I3.01bs/ft-), 50% for medium vapor densities, and 60% for vapor densities... 

This problem, as with flooding, also impairs product quality. No fractionation occurs in the dry section, so the temperature difference decreases. However, unlike flooding, the pressure drop decreases and stays very steady at the ultimate minimum value. This problem is usually easier to handle than flooding. The problem is caused by either insufficient liquid entering the section or too much liquid boiling away. The problem is solved by reversing the action that caused the dry trays. 

If a tower does become flooded in the bottom section, a common operator error is to try to pump the level out too quickly. This can easily damage trays by imposing a downward acting differential pressure produced by a large weight of liquid on top of the tray and a vapor space immediately below the tray. To eliminate the flooding, it is better to lower feed rate and heat to the reboiler. It is important to be patient and avoid sudden changes. 

Here, we refer to small amounts of water rather than large slugs that could damage the trays. Often the water will boil overhead and be drawn off in the overhead accumulator bootleg (water drawoff pot). However, if the column top temperature is too low, the water is prevented from coming overhead. This plus too hot a bottom temperature for water to remain a liquid will trap and accumulate water within the column. The water can often make the tower appear to be in flood. 

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