Archive for January, 2011

Circulating vs. Once-Through Thermosyphon Reboilers

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We said before that it was wrong to return the effluent from a oncethrough reboiler with a vertical baffle to the cold side of the tower’s bottom. Doing so would actually make the once-through thermosyphon reboiler work more like a circulating reboiler. But if this is bad, then the once-through reboiler must be better than …

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Circulating Thermosyphon Reboilers

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The important differences between a once-through thermosyphon reboiler and a circulating thermosyphon reboiler is critical. Figure 7.4 shows a circulating reboiler. In this reboiler • The reboiler outlet temperature is always higher than the tower-bottom temperature. • Some of the liquid from the reboiler outlet will always recirculate back into the reboiler feed. • Some …

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Loss of Once-Through Thermosyphon Circulation

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There are several common causes of loss of circulation. The common symptoms of this problem are • Inability to achieve normal reboiler duty. • Low reflux drum level, accompanied by low tower pressure, even at a low reflux rate. • Bottoms product too light. • Reboiler outlet temperature hotter than the tower-bottom temperature. • Opening …

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Once-Through Thermosyphon Reboilers

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Figure 7.2 shows a once-through thermosyphon reboiler. The driving force to promote flow through this reboiler is the density difference between the reboiler feed line and the froth filled reboiler return line. For example: • The specific gravity of the liquid in the reboiler feed line is 0.600. • The height of liquid above the …

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How Reboilers Work

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Four types of reboilers are : • Once-through thermosyphon reboilers • Circulating thermosyphon reboilers • Forced-circulation reboilers • Kettle or gravity-fed reboilers There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the type of reboiler used, the following statement is correct: Almost as many towers flood …

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Internal Reflux Evaporation

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The tray temperatures in our preflash tower, shown in Fig. 6.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows …

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Conversion of Sensible Heat to Latent Heat

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When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors more of the heavier or higher-molecular-weight components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top …

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Effect of Feed Preheat

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Up to this point, we have suggested that the weight flow of vapor up the tower is a function of the reboiler duty only. Certainly, this cannot be completely true. If we look at Fig. 6.2, it certainly seems that increasing the heat duty on the feed preheater will reduce the reboiler duty. Let us …

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Heat-Balance Calculations

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If you have read this far, and understood what you have read, you will readily understand the following calculation. It is simply a repetition, with numbers, of the discussion previously presented. However, you will require the following values to perform the calculations: • Latent heat of condensation of alcohol vapors = 400 Btu/lb • Latent …

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The Reboiler

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All machines have drivers. A distillation column is also a machine, driven by a reboiler. It is the heat duty of the reboiler, supplemented by the heat content (enthalpy) of the feed, that provides the energy to make a split between light and heavy components. A useful example of the importance of the reboiler in …

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