Question 10.7: Use Xist to rate the kettle reboiler designed in Example 10....
Use Xist to rate the kettle reboiler designed in Example 10.2, and compare the results with those obtained previously by other methods.
Data from Example 10.2 are entered on the appropriate Xist input forms as indicated below. Parameters not listed are either left at their default settings or left unspecified to be calculated by the program.
(a) Input Summary.
| Case mode: Rating | Service type: Kettle reboiler |
(b) Geometry/Exchanger.
| TEMA type: BKU | Shell ID: 23.25 in. |
Note: The shell ID on this panel is actually the port diameter of the kettle.
(c) Geometry/Kettle Reboiler.
| Kettle diameter: 37 in. | Reboiler pressure location: At top of bundle |
Note: The kettle diameter on this panel is actually the shell ID.
(d) Geometry/Tubes.
| Tube OD: 1 in. | Tube passes: 2 |
| Average wall thickness: 0.083 in. | Straight tube length: 13 ft |
| Tube pitch: 1.25 in. | Tube count: 212 |
| Tube layout angle: 90º |
(e) Geometry/Baffles/Supports.
Support plates/baffle space: User set: 3
(f) Geometry/Nozzles.
| Shell side | Tube side |
| Inlet ID: 5.047 in. | Inlet ID: 6.065 in. |
| Number: 2 | Number: 1 |
| Outlet ID: 6.065 in. | Outlet ID: 3.068 in. |
| Number: 2 | Number: 1 |
| Liquid outlet ID: 4.026 in. |
(g) Control/Methods/Condensation
Pure component: Yes
(h) Process
| Hot fluid | Cold fluid | |
| Fluid name | Steam | Distillation Bottoms |
| Phase | Condensing | Boiling |
| Flow rate (1000 lb/h) | 5.645 | 96 |
| Inlet fraction vapor | 1 | 0 |
| Outlet fraction vapor | 0 | – |
| Operating pressure (psia) | 20 | 250 |
| Fouling resistance (h·ft²·ºF/Btu) | 0.0005 | 0.0005 |
(i) Hot fluid properties.
Physical property input option: Component by component
Heat release input method: Program calculated
Clicking on the Property Generator button opens the property generator as shown below. VMG Thermo is selected as the property package and Steam95 is selected from the list of thermodynamic methods for both the vapor and liquid phases. This method uses steam tables to obtain fluid properties
On the composition panel, water is selected from the list of components as shown below. Since it is the only component in the hot stream, its mole fraction is 1.0
On the conditions panel, property sets is selected as the temperature point method. Two pressure levels, 20 and 19 psia, are specified and the temperature range for fluid properties is set as shown below. The number of points in this range at which properties are to be generated is set at 20.
Clicking on the Generate Properties button produces the results shown below. The Transfer button is clicked to transfer the data to Xist. (Note that a maximum of 30 data points can be transferred.) Finally, clicking the Done button closes the property generator and returns control to the Xist input menu.
(j) Cold fluid properties.
Properties of the hydrocarbon stream are generated in the same manner as for steam. In this case, the Advanced Peng–Robinson thermodynamic method is chosen for both the vapor and liquid phases. On the conditions panel, property sets is selected as the temperature point method. Two pressure levels, 250 and 255 psia, are specified (to allow for the static pressure effect in the reboiler) with a temperature range of 195º F to 225º F. The number of data points is again set at 20.
The Xist Output Summary for this case is given below, from which it is found that the over-design for the unit is about 30%. Thus, according to Xist the reboiler is over-sized.
The following table compares the results from Xist with those obtained in previous examples using other methods. It can be seen that both the boiling and condensing heat-transfer coefficients from Xist are significantly higher than the corresponding values from HEXTRAN. Furthermore, the boiling-side coefficient from Xist is nearly double the value computed using Palen’s method in the hand calculation. This comparison again indicates that Palen’s method leads to very conservative results in the present application, with consequent over-sizing of the reboiler. The combined friction and acceleration loss in the kettle computed by Xist is an order of magnitude less than the value assumed (as an upper bound) in the hand calculations. This result reinforces the point made in Example 10.2 that these losses can generally be neglected in calculating the pressure balance for the kettle reboiler system.
| Item | Hand | HEXTRAN | Xist |
| h_{o} (Btu/h. ft².ºF) | 523 | 936^{a} | 1014 |
| [\left(D_ o/ D_i\right)\left(1 / h_i+R_{D i}\right)]^{-1} (Btu/h· ft²·ºF) | 1500 (assumed) | 857^{a} | 994 |
| U_{D} (Btu/h· ft²·ºF) | 297 | 335^{a} | 368 |
| Δ P_{i} (psi) | 0.3 | 0.43 | 0.355 |
| Δ P_{o} (psi), excluding nozzles | 0.2 (assumed) | – | 0.022 |
| Δ P_{o} (psi), nozzles only | 0.05 | 0.05 | 0.104 |
| Δ T_{m} (ºF) | 25.6 | 27.1^{a} | 25.7 |
| U_{D} Δ T_{m} (Btu/h· ft²) | 7603 | 9079^{a} | 9458 |
| \left(\hat{q} / \hat{q}_c\right)_{\max } | 0.11 | – | 0.095 |
^{a}Area-weighted average over first five zones; subcooled condensate zone not included
The tube layout generated by Xist is shown below. It contains 216 tubes (108 U-tubes), which agrees well with the number (212 tubes) obtained from the tube-count table.
Xist Output Summary for Example 10.7
