Fractionation trays, flooding

A conservative design would be to take 90% of the Glitsch value as the flooding load with Sin. bubble caps, and 70% of the valve-tray flooding load for a tray wifh 6 in. caps (Fractionation Research, Tulsa, Oklahoma, U.S.A., private communication). 

The vapor flood velocity calculation is one step in the tray design, or rating of existing columns. The column should be designed or operated such that the actual vapor velocity where /is the fraction of flood velocity, and typically should be between 0.70 and 0.85. Since is based on A - Aj, the actual vapor volumetric flow rate is... 

Equation 14.3 is rearranged to calculate the tray diameter when designing a column, or to calculate the fraction of flood velocity to check for the possibility of flooding in an existing column ... 

The calculations include fraction of flood velocity, pressure drop per tray, downcomer backup, tray liquid holdup, and check for weeping. 

Sieve trays with 10% hole area and 0.5 cm diameter holes will be used. Trays are available in standard diameters of 0.25 m increments (0.25,0.50, 0.75, 1, 1.25, 1.50,. .., m). Based on the top tray conditions, determine the required tray diameter rounded up to the nearest larger standard size. Assume a tray spacing of 0.5 m, a foaming factor of 0.80, and a fraction of flood of 0.80. The liquid density is given as 730 kg/m and the vapor density may be estimated based on the ideal gas equation. The liquid surface tension is 27 dynes/cm. 

The column has 3 m diameter sieve trays with 0.5 cm diameter holes and 10% hole area. The tray spacing is 45 cm. Assuming a foaming factor of 0.85, calculate the vapor flood velocity at the top tray. Check if the column diameter is acceptable. The fraction of flood velocity should be within a 60-85% range. 

Tray type Weir height Weir length Downcomer clearance Tray spacing Foaming factor Fraction of flood Surface tension Liquid viscosity... 

Sieve trays will be used with 60 cm spacing, 6 cm weir height, 0.6 cm hole diameter, 0.25 cm tray thickness, 5 cm downcomer clearance, and hole area 10% of the total tray area. The foaming factor is 0.80 and the froth density in the downcomer is 0.5. The target fraction of flood velocity is 0.70. 

Efficiencies can be scaled up from laboratory data taken with an Oldershaw column (a laboratory-scale sieve-tray column) tFair et al.. 1983 Kister. 1990T The overall efficiency measured in the Oldershaw column is often very close to the point efficiency measured in the large commercial column. This is illustrated in Figure 10-15. where the vapor velocity has been normalized with respect to the fraction of flooding IFair et al 19831. The point efficiency can be converted to Murphree and overall efficiencies once a model for the flow pattern on the tray has been adopted (see section 16.6T... 

Accounting for a loss in fractionation is a common troubleshooting assignment. For example, crude unit operators find that they can no longer meet furnace oil end-point specs unless they sacrifice furnace oil yield. On one unit, furnace oil production had dropped from 7,000 B/SD to 4,000 B/SD. Possible explanations for this type of problem are tray flooding, improper heat balance, and tray damage. 

A common example of foam formation in the bottom of a fractionator inducing flooding occurs in a crude preflash tower. In this case, stable foam accumulates in the bottom of the column as a consequence of the "flow improver" chemicals added to crude oil. These chemicals reduce pressure drop in the crude pipelines. Once the foam level rises to the feed inlet nozzle, the trays flood and black distillate is produced. Please see Chapter 18 (Preflash Towers). 

When the liquid level in the bottom of a column rises to the reboiler return nozzle, the liquid in the bottom of the column is forcibly lifted by the reboiler vapors. The entrained liquid is blown against the underside of the bottom tray. Since flooding will progress up a lower, a liquid level covering the reboiler return nozzle will cause the entire fractionator to flood. 

The fractionator tower becomes overloaded and the trays flood. 

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... 

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. 

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. 

The heavy naphtha-Ught gas oil fractionation zone of a crude tower has to be revamped to handle 25% more capacity. Because trays would be working at high percent flooding, Gempak structured packing is condensed (Figures 9-56A-D). 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

Reducing the pumparound duty increases the tray loadings on trays 1 through 7. But in so doing, the trays operate closer to their incipient flood point. This is fine. The incipient flood point corresponds to the optimum tray performance. But if we cross over the incipient flood point, and trays 5, 6, and 7 actually start to flood, their fractionation efficiency will be adversely affected. Then, as we decrease the pumparound heat-removal duty, the mutual contamination of diesel and gas oil will increase. 

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