Reflux drums elevating

Reflux Drum Elevation Increase Promotes Subcooling... 

Therefore, the liquid level in the overhead condenser would have to be somewhere in the condenser s shell. But then, the liquid in the condenser would be below the reflux drum. How, then, does the liquid get from the lower elevation of the condenser to the higher elevation in the reflux drum We will have to explain this hydraulic problem later. But for now, we can say that most reflux drums are elevated 20 or 30 ft above grade to provide net positive suction head (NPSH) for the reflux pump. Also, most shell-and-tube condensers are located at grade, for easier maintenance during unit turnarounds. 

Figure 13.4 Elevation increase of reflux drum increases tower pressure.

Elevation increase promotes subcooling. Once upon a time many years ago, a tragic event occurred in Louisiana. A rat entered the condenser outlet pipe shown in Fig. 13.4. The condenser had been off line for cleaning. The rat, having crawled up the riser pipe to the reflux drum, got its head stuck in the drum s inlet nozzle. Your author, unaware of the rodent s predicament, put the exchanger back into service. The condensed butane now flowed across the rat. The rat died. Well, we all must come to that end eventually, although perhaps not quite that exact end. Such is the way of all flesh. 

Forget about Nat. Forget about the rat. Both stories are pure fiction. The truth is that the elevation of the reflux drum was 80 ft above the condenser. The specific gravity of the butane liquid was 0.59. This means that 80 ft of liquid exerted a head pressure of about 20 psig ... 

Common design error. Please refer back to Fig. 13.2. How can the liquid from the condenser rise to the higher elevation in the reflux drum, without being pumped Simple The pressure head of the liquid leaving the condenser is converted to elevation as the liquid flows up into the reflux drum. This works fine, as long as the liquid leaving the condenser is sufficiently subcooled. By sufficiently subcooled, I mean that when the lower-pressure liquid flows into the reflux drum, it has to be cold enough that it does not flash. 

The liquid leaving the condenser is subcooled. The liquid entering the reflux drum is saturated liquid at its bubble point. Of course, the temperature of the liquid is the same at both points. The subcooled liquid is subcooled in the sense that its pressure is above the bubble-point pressure, at the condenser outlet temperature. It is this extra pressure, above the bubble-point pressure, that may be converted to elevation. 

Incidentally, as I have shown in Figs. 13.6 and 13.7, the condenser may be located above or below the reflux drum. Both configurations require a subcooled liquid effluent from the condenser. But if the condenser is located below the reflux drum, additional subcooling to offset the elevation effect, described above, will be needed. 

Several variations exist for overhead reflux circuits. A condenser can be elevated above the reflux drum or the reflux drum can be elevated with the condenser at grade. These arrangements can be adjacent to or somewhat remote from the tower. The simplest overhead line is shown on Figure 7-9, sketch A. 

If gravity flow is required from the condenser outlet to a collecting drum, reflux drum or separator, exchangers must be elevated. Examples are shown in Figure 7-67 and 7-68. 

At the condensing-reflux system in Figure 7-67, the elevation of the condensers is influenced by several factors. First, the reflux drum must be elevated because of pump NPSH requirements, say 14 feet to the bottom of the drum (about 2-3 feet below this elevation, a platform is required). To this 14 feet must be added the drum diameter, estimated space for pipe lines, depth of structural steel members plus platform-to exchanger-centerline dimension for establishing the exchanger elevation above grade. 

Refinety Crude tower Column pressure and product gas rate sharply fluctuated during low-rate operation in this and other luiits. The condenser was a partial condenser located at ground level, with an elevated reflux drum. Problem was caused by slug flow in the riser from the condenser to the drum. Size risers to avoid sltig flow even during low-rate operations. 

The reflux drum is elevated above the condenser. The effluent from the condenser consists of a vapor-liquid mixture. The line between, the condenser and drum is called a riser. 

Devoting 10% of a condenser s surface area to subcooling is about right when the reflux drum is elevated above the condenser. If one finds 30% or more of the tubes in subcooling duty, something is wrong. 

FIGURE 18-6 Elevating the reflux drum above the condenser may cause slug flow in the riser. 

Reflux may flow by gravity to the tower, in which case the condenser and reflux drum (accumulator) must be elevated above the level of top tray of the tower. Alternatively, especially in order to obviate the need for elevated platforms and supports required for withdrawing the condenser tube bundle for cleaning, the assemblage may be placed at ground level and the reflux liquid pumped up to the top tray. Kern [28] describes the arrangements. 

I have another way to explain how the condensed liquid can flow uphill from the condenser into the elevated reflux drum. That is, the liquid leaving the condenser is pushed into the reflux drum by the higher pressure in the tower. This will work okay, as long as the liquid entering the reflux drum is at its saturated bubble-point temperature and pressure. If this is not the case, then the vapors flashing out in the reflux drum will have to be vented. If you refuse to open this vent, then condensate backup must increase in the condenser, with a consequent reduction in the condenser s capacity. 

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