An equimolar mixture of hexane and heptane is continuously produced in an oil refinery at a rate of 390lbmol/h. The alkane mixture is to be separated via continuous distillation with a design specification of 2% impurity allowable in each stream. The feed is a 2-phase mixture (75 mol% liquid and 25 mol% vapor). Assume that the liquid is ideal and that the columnâs operating pressure is 1 atm. Assume a full condenser and a partial reboiler are used.
Do the following. You will find L08C and L08D to be helpful.
1.Use http://app.knovel.com/web/toc.v/cid:kpYHACVPEH to find the relative volatility a when the temperature is such as to give about 1 atm vapor pressure for hexane, and another value at the temperature giving about 1 atm vapor pressure for heptane.
2.Use the average of the relative volatilities found in 1 to calculate y versus x, choosing the component giving y > x. (Hint: see slide 18 in L05B.) Plot this using whatever method you like. Include the 45o x=y line. Youâll need 4 copies of this.
3.Graphically determine the minimum number of theoretical trays Nmin in the column that would meet the specifications.
4.Graphically determine the minimum reflux ratio Rmin .
5.Graphically determine the minimum boilup ratio RB,min . (This corresponds to the minimum reflux ratio determined in 4, but using the operating line for the stripping section of the column.)
6.Let us consider the economics of distillation. There are two costs â construction and operating. The operating cost is primarily for coolant for the condenser and heating fluid for the reboiler. At total reflux, we get the lowest construction cost and a low operating cost, but no product. Thus, the cost per unit of product is infinite. At minimum reflux, the operating cost is a minimum but the column is infinitely high, and so again the cost per unit of product is infinite. The economic optimum condition is somewhere in between.
Estimate the economic optimum reflux ratio using the Gilliland Correlation (Figure 9.20 on
p 306 of the text) by selecting (R - Rmin)/(R+1) = 0.4. Calculate R using your Rmin, construct a McCabe-Thiele diagram using this R, and graphically determine the number of theoretical trays in the column. Compare this with that predicted by the Gilliland Correlation. What is the optimal feed tray, counting from the top?
Submit all of your calculations and plots.
