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

Comparison and discussion of results analysis the

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Unformatted text preview: using the SRK method for Kvalues and enthalpies. (a) It was determined for the equilibrium stage method that the average K-value for propane was about 3.0. Therefore, the approximate absorption factor is A = L/KV = 11/[(3)(11)] = 0.33 For this small an absorption factor, using the Kremser plot in Fig. 5.9, the fraction of propane absorbed is independent of the number of equilibrium stages above about 3. However, the effect of stages on n-butane and n-pentane is significant. Therefore, one possible specification was 20 trays with a 75% overall efficiency or the equivalent of 15 equilibrium stages. This gave almost 40% absorption of propane, 94% absorption of n-butane, and 96% absorption of n-pentane. The splits were as follows: Component Methane Ethane Propane n-Butane n-Pentane n-Dodecane Total Flow rate, kmol/s: Rich gas Absorber oil 0.00 3.146 0.00 1.727 0.00 2.640 0.22 1.859 0.55 1.628 10.23 0.000 11.00 11.000 Lean gas 3.050 1.503 1.618 0.121 0.084 0.001 6.377 Rich oil 0.096 0.224 1.022 1.958 2.094 10.229 15.623 (b) Difficulty was experienced in converging the rate-based method for this exercise, probably because of the tray design calculations for a bubble-cap column, which led to a very large diameter column with many liquid passes. Accordingly, the following procedure was employed. The number of actual trays was fixed at 20. The profiles of temperature, vapor and liquid flow rates, and vapor and liquid compositions from the equilibrium-stage calculations in part (a) were saved and used as "old results" to initialize the rate-based calculations. Default physical properties were chosen, along with the AIChE method for mass-transfer coefficients and interfacial area. Mixed flow was assumed for both vapor and liquid phases and 85% of flooding Exercise 12.17 (continued) Analysis: (continued) was chosen, along with a system factor for foaming of 0.75. The calculations for a bubble-cap column terminated with an error before completing even one iteration, probably because of difficulty in determining an initial estimate of column diameter. A sieve-tray column was then specified and the system factor was reduced to 0.1. The calculations converged. The system factor was then increased to 0.2 and then to 0.3, with convergence both times using profiles from the previous run. A column diameter of about 13 m was then computed by taking the diameter calculated for a system factor of 0.3 and scaling it to a system factor, FF, of 0.75, using Eqs. (6-42), (6-40), and (6-44). A bubble-cap column was then attempted with a specified column diameter of 13 m. This time the calculations converged, with the following splits: Component Methane Ethane Propane n-Butane n-Pentane n-Dodecane Total Flow rate, kmol/s: Absorber oil Rich gas 0.00 3.146 0.00 1.727 0.00 2.640 0.22 1.859 0.55 1.628 10.23 0.000 11.00 11.000 Lean gas 3.050 1.501 1.605 0.120 0.081 0.001 6.358 Rich oil 0.096 0.226 1.035 1.959 2.097 10.229 15.642 These results are almost identical to tho...
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