Separation Process Principles- 2n - Seader & Henley - Solutions Manual

Given feed of 100 mols of 20 mol ethanol and 80 mol

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Unformatted text preview: t data, using the FUG results, with N = 2 x Nmin. A guess of R is made: Pressures as stated above. Number of stages = 12 (includes the total condenser and the partial reboiler) Feed stage for recycle, D2 = 4 from the top Entry for feed, F = 6 from top Reflux ratio = 0.5 Bottoms mole flow rate = 55.1 mol/s Estimated distillate rate = 275.76 mol/s Estimated reflux rate = 138 mol/s Estimated temperatures: Stage 1, 35oC Stage 12, 40oC Stage 2, 36oC Stage 11, 39oC Exercise 11.13 (continued) Analysis: Column 1 (continued) A converged result, which gave a 99.8 mol% purity of ethanol in the bottoms, was obtained in 10 iterations. A purity of only 99 mol% was needed. This indicated that the reflux could be reduced. Accordingly, using the profiles from the first rigorous run, a second run was made with a column top specification of D = 173.75 mol/s in place of the reflux specification. A converged result, with the desired 99 mol% ethanol purity in B1, was obtained at a reflux ratio of 0.279 in 24 iterations. The material balance was in agreement with the above table. Although the feed locations could probably be improved, no pinch region is evident, as shown in the liquid-phase composition profile on the next page. Also included on the next page are the column profiles for temperature, liquid flow, and vapor flow. Column 1 was sized with Chemcad for sieve trays, with: 24-inch tray spacing 12% downcomer area 10% hole area 1/4-inch holes 80% of flooding The resulting column inside diameter was determined to be 12 feet. The condenser duty was computed to be 12.91 MW, with a reboiler duty of 11.58 MW. For the 10 theoretical plates, the total pressure drop was computed to be 8.2 kPa. For a final design, the computations should be repeated taking into account tray pressure drop and tray efficiency. Column 2: Using the above material balance table, the specifications for the ShortCut Column model are 101.60/102.05 = 0.9956 for the recovery of ethanol to the distillate, and 129.30/173.75 = 0.7442 for the recovery to the distillate of benzene. Thus, it is again expected that little rectification is necessary. The feed to Column 2 is the distillate from Column 1 after being pumped to a pressure of 120 kPa for entry into Column 2 and heating to the bubble point at that pressure. The results are: Minimum number of equilibrium stages = 5.0 Minimum reflux ratio = very small and probably in error. Therefore, the Gilliland correlation was not reliable. The SCDS Column model of the Chemcad program experienced convergence difficulties and was replaced with the TOWER model, which also experienced difficulty until the following specifications were made with a damping factor of 0.7. Pressures as stated above. Number of stages = 6 (includes the total condenser and the partial reboiler) Entry for feed = 2 from top (giving in effect a reboiled stripper with some reflux) Distillate flow rate of ethanol = 101.60 mol/s Bottoms mole flow rate = 44.9 mol/s Estimated distillate rate = 230.90 mo...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.

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