Question

If the ethanol water system is considered an ideal system, an equilibrium curve can be plotted using only the ‘combined Raoult’s Law’, as shown. (This curve is not completely accurate as activity coefficients are not considered.) Along the x axis is the liquid mole fraction of ethanol, the light key, with the vapor mole fraction of ethanol along the y axis. For this problem, assume that you can use this curve to estimate the number of necessary distillation stages for a separation of ethanol and water. Assume that the reflux ratio is 5 and the boil up rate is 5. You are to distill the mixture with the distillate at a concentration of 95% ethanol and the bottoms at a concentration of 5% ethanol. The feed concentration is 50% ethanol, a saturated liquid.

(a) Plot the rectifying section and stripping section lines.

(b) Plot the q line.

(c) Estimate the optimal number of stages and show these stages, along with the lines in a

and b. Indicate where the optimal feed stage would be.

(d) Second and third identical graphs are provided. Estimate the number of stages need for

total reflux and show these stages on one graph. (Will be graded as a separate problem)

(e) Estimate the minimum reflux ratio and make the necessary plot on the last graph (also

showing the q line). (Will be graded as a separate problem)

Answer 1