Unformatted text preview: using the SRK method for Kvalues and enthalpies.
(a) It was determined for the equilibrium stage method that the average Kvalue 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 nbutane and npentane 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 nbutane, and 96%
absorption of npentane. The splits were as follows:
Component
Methane
Ethane
Propane
nButane
nPentane
nDodecane
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 ratebased method for this exercise,
probably because of the tray design calculations for a bubblecap 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 equilibriumstage calculations in
part (a) were saved and used as "old results" to initialize the ratebased calculations. Default
physical properties were chosen, along with the AIChE method for masstransfer 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 bubblecap
column terminated with an error before completing even one iteration, probably because of
difficulty in determining an initial estimate of column diameter. A sievetray 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.
(642), (640), and (644). A bubblecap 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
nButane
nPentane
nDodecane
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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 Spring '11
 Levicky
 The Land

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