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… Read More »
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… Read More »
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… Read More »
This is perhaps an idea you remember from high school, but never quite understood. The phase rule corresponds to determining how many independent variables we can fix in a process before all the other variables become dependent variables. In a reflux drum, we can fix the temperature and composition of the liquid in the drum.… Read More »
As an operator reduces the tower pressure, three effects occur simultaneously: • Relative volatility increases. • Tray deck leakage decreases. • Entrainment, or spray height, increases. 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… Read More »
The process design engineer selects the tower design operating pressure as follows: 1. Determines the maximum cooling water or ambient air temperature that is typically expected on a hot summer day in the locale where the plant is to be built. 2. Calculates the condenser outlet, or reflux drum temperature, that would result from the… Read More »
Why are distillation towers designed with controls that fix the tower pressure? Naturally, we do not want to overpressure the tower and pop open the safety relief valve. Alternatively, if the tower pressure gets too low, we could not condense the reflux. Then the liquid level in the reflux drum would fall and the reflux… Read More »