Reflux drum liquid equilibrium

Vapor and liquid products Dy and are taken off the reflux drum and are in equilibrium. Dynamics of the vapor space in the reflux drum and throughout the column are negligible. 

The backup of butane liquid in the condenser would continue until the butane, leaving the condenser, was cold enough so that it would not flash as it flowed across the rat—that is, until equilibrium conditions had been reestablished in the reflux drum. 

Hot-vapor bypass pressure control. A more modern way of controlling a tower s pressure is shown in Fig. 13.6. This is the hot-vapor bypass method. When the control valve on the vapor bypass line opens, hot vapors flow directly into the reflux drum. These vapors are now bypassing the condenser. The hot vapors must condense in the reflux drum. This is because there are no vapors vented from the reflux drum. So, at equilibrium, the hot vapors must condense to a liquid on entering the reflux drum. They have no other place to go. 

But the liquid in the reflux drum is in equilibrium with a vapor space. This liquid is then at its bubble, or boiling, point. If the liquid draining from the condenser is colder than this bubble point liquid, then it must be subcooled. But how can a vapor condense directly into a subcooled liquid Well, it cannot. 

Analysis of Partial Condenser Data. Gunness (Ref. 1) reports experimental data obtained on the partial condenser oi a rectifying column stabilizing absorption naphtha. The vapor from the column was passed downward through the partial condenser, and the uncondensed vapor and condensate from the bottom of the condenser passed together to the reflux drum where they were separated. Owing to the concurrent flow of the vapor and the liquid, it would be expected that this system might approximate equilibrium partial condensation. The data for a test... 

Let s look at the reflux drum of that same distillation tower that we used for the Bubble Point calculation (see Fig. 50.4). Again the vapor coming off the reflux drum is in equilibrium with the liquid in the reflux drum. Using our Cox vapor pressure chart (Fig. 50.3), we find the following. 

In this case we do not have the total pressure (P., ) in the vapor space above the liquid in the reflux drum, but we still want to find the composition of the liquid in the reflux drum that is in equilibrium with that vapor. 

However, in reactive distillation, pressure affects both chemical kinetics and vapor-liquid equilibrium. Therefore, the optimum pressure may not correspond to the minimum attainable while stiU using cooling water in the condenser. For example, the optimum pressure for the base case is 8 bar, as we will demonstrate. The corresponding reflux-drum temperature is 353 K (80 °C, 176 °F), which is well above the temperature that could be achieved using 305 K (32 °C, 90 °F) cooling water. 

The vapor-liquid equilibrium is assumed ideal. The column pressures are set using the vapor pressures of pure components and liquid compositions in the reflux drum xd at 320 K (so that cooling water can be used in the condenser). Temperature T, and vapor composition on tray i can be calculated with given pressure P and tray liquid... 

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