Example 10-2:

DO NOT copy other old answers, and Solve correctly.

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D5. Repeat Example 10-2 except calculate the diameter at the bottom of the column at a pressure of 700.0 kPa. The surface tension of pure n-heptane at 20C is 20.14 dynes/cm (0.0214 N/m), and the temperature coefficient is -0.0980 dynes/cm/K(www .surface-tension.de). Use the DePriester chart to estimate bottoms temperature. EXAMPLE 10-2. Diameter calculation for tray column Determine the required diameter at the column top for the distillation column in Example 10-1 Solution We can use Eq. (10-16) with 75% of flooding. Since distillate is almost pure hexane, we can approximate properties as pure n-hexane at 69.0c Physical properties: P = 0.659 g/ml (at 20"); viscosity = 0.22 CP: MW = 86.17 (Perry and Green, 1984). Surface tension d = 13.2 dynes/cm (Reid et al. 1977. p. 610). The ideal gas law is used to estimate vapor density 1b (latm)(86.17 p(MW) 1bmol 3 RT atmft -= 0.19171b/ft (1.314 (342 K K lbmol P. = MW, MW, and A = (0.659)(62.4) = 41.12 ib/ft" (will vary but not a lot). At the top of the column W L MW L WV MW The flow parameter is w (Px 30.5 = 0.0546 F W, PL -=0.8 V Ordinate from Figure 10-16 for 24-in. tray spacing. Cu = 0.36 while Ch = 0.38 from Eq. (10-10e). Then Ufood =K K=C)2 = 0.366 13.2 302 = 0.331 20 K=0.35 if Eq. (10-10e) is used. The lower value is used for a conservative design. From Eq. (10-8) PL-PO 41.12-0.1917 = (0.331) 39.5 = 4.836 PM 0.1917 We will estimate n as 0.90. V = L + D. From external mass balances, D=500.0. Since L = V(L/V) and L/V +0.8. D 500 V= = 2500 lbmol/h 1-L/V 0.2 Diameter Eq. (10-16) becomes 4(2500)(1.314)(342) Dia = = 11.03 ft (0.90)(3600)(1)(075)(4.836) Since Fair's diameter calculation procedure is conservative, an 11.0-ft diam Cher column would probably be used. If n = 0.95, Dia = 10.74 ft , thus, the value of n is not critical. This is a large-diameter column for this feed rate. The reflux rate is quite high, and thus, V is high, which leads to a larger diameter. The effect of location on the diameter calculation can be explored 395 Column Diameter Calculations