Tray flooding separators

Prior to 1974, when fuel costs were low, distillation column trains used a strategy involving the substantial consumption of utilities such as steam and cooling water in order to maximize separation (i.e., product purity) for a given tower. However, the operation of any one tower involves certain limitations or constraints on the process, such as the condenser duty, tower tray flooding, or reboiler duty. 

Operators are usually more interested in making on-spec products than in saving energy. In the field one will often find pumparound rates cut back to 50% of their proper level. Try increasing pumparound circulation until the product separation is adversely affected. Perhaps an end-pont spec can be extended. Up to the point at which the trays flood, increasing pumparound flow will save furnace fuel. 

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. 

The tray may flood. Water and hydrocarbon mixing on the tray deck, stirred up by the flowing gas, creates an emulsion. The emulsion does not separate as readily as clear liquid from the gas. Premature downcomer backup, followed by tray deck flooding, result. 

The most conservative definition of vapor flooding capacity is the load for which tower pressure drop exhibits a sharp increase, signifying liquid buildup at some tray. It is, however, possible to operate the tower at somewhat higher load, perhaps 10% more. As the load is increased, reboiler pressure rises, allowing a semistable operation, albeit with reduced separating... 

Problem 12.6 deals with the dimensioning of a large ethylbenzene/styrene tray-type fractionator. Because high pressure drop in this separation leads to increased tar production, a packing (instead of trays) is to be considered. The conditions are the same as for Problem 12.6. For 80% flood, determine the required column diameter and associated pressure drop for a 50-mm Pall ring random packing. 

If the separation is satisfactory, the commercial column will require the same or fewer trays than the Oldershaw trays used. Comparisons are made at the same approach to flood for both columns. 

The column has 30 sieve trays, with a total condenser and a partial reboiler. The solvent enters tray 5 and the feed enters tray 15, from the top. The pressure in the condenser is 1.1 atm the pressure at the top tray is 1.2 atm, and the pressure at the bottom is 1.4 atm. The reflux ratio is 5 and the bottoms rate is 960 kmol/h. Use the nonequilibrium model of the ChemSep program to estimate the separation achieved. Assume that the vapor and the liquid are both well mixed and that the trays operate at 75% of flooding. In addition, determine from the tray-by-tray results the average Murphree tray efficiency for each component. 

A depropanizer is a distillation operation encountered in almost all oil refineries. Our task here is to design a sieve-tray column to separate 1000 mol/s of a mixture containing 100 mol/s of ethane, 300 mol/s of propane, 500 mol/s of n-butane, and 100 mol/s of n-pentane at 298 K and 15 atm. The distillate should contain no more than 3.5 mol/s of n-butane, and the bottoms should contain no more than 3.5 mol/s of propane. The trays should operate at about 70% of flooding (Taylor and Krishna, 1993). 

The downcomer in Fig. 4.12a is likely to lose its seal whenever its liquid height drops below tray level. When the seal is lost, vapor fium the tray ascends the downcomer, which may cause flooding, cycling, and/or poor separation. Downcomer unsealing by this mechanism is most likely to occur when the liquid drawn constitutes a laige portion of the downcomer liquid flow, when the quantity drawn tends to fluctuate, and/or when excessive leakage takes place due to tray weeping or draw pan leaks. 

Unlike the internals discussed in previous chapters, poor tray layout of one- or two-pass trays rarely causes spectacular column failures such as flooding at 50 percent of design rates or extremely poor separation. Ill effects resulting from poor tray layout seldom extend beyond suboptimum design or performance a moderate reduction in capacity, efficiency, or turndown and some increases in capital or... 

At high pressures, the difference between vapor and liquid density becomes smaller, and separation of vapor from liquid in the downcomer becomes difficult. Because of the more difficult separation, downcomer aeration increases, raising both downcomer frictional losses and froth backup in the downcomer. High liquid flow rates also increase tray pressure drop, tray liquid level, and frictional losses in the downcomer. For this reason, downcomer flooding is favored at high pressures and high liquid rates. 

Temperature gradients are an effective, low-cost method of determining the flood point, but the method s success depends on the existence of a sufficiently large temperature gradient under normal operating conditions. If the normal tray-to-tray temperature difference is small, as in close separations, the flooded temperature profile will not vary a great deal from the normal profile, and temperatiire profiles will be poor indicators of flooding. 

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