Question 10.8: Use Xist to rate the initial configuration for the vertical ...
Use Xist to rate the initial configuration for the vertical thermosyphon reboiler of Example 10.4 and compare the results with those obtained previously by hand.
Data from Example 10.4 are entered on the 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: Thermosiphon reboiler |
| Hot fluid: Shell side |
(b) Geometry/Exchanger
| TEMA type: AEL | Shell Orientation: Vertical |
| Shell ID: 15.25 in. |
(c) Geometry/Tubes.
| Tube OD: 1 in. | Tube passes: 1 |
| Average wall thickness: 0.083 in. | Tube length: 8 ft |
| Tube pitch: 1.25 in. | Tube count: 106 |
| Tube layout angle: 30° |
(d) Geometry/Baffles.
| Baffle cut: 35% | Central baffle spacing: 6.1 in. |
(e) Geometry/Tube Layout/Bundle Clearances.
Pairs of sealing strips: None
(f) Geometry/Nozzles.
| Shell side | Tube side |
| Inlet ID: 4.026 in. | Inlet ID: 6.065 in. |
| Number: 1 | Number: 1 |
| Outlet ID: 2.067 in. | Outlet ID: 10.02 in. |
| Number: 1 | Number: 1 |
(g) Geometry/Thermosyphon Reboiler.
Reboiler pressure location: At column bottom liquid surface
(h) Geometry/Thermosiphon Reboiler/Piping.
The detailed piping forms are used here to illustrate the procedure. They are invoked by checking the box for detailed piping on the main piping panel. The inlet piping panel is shown below:
The piping elements are selected from a list box that appears when a blank field in the first column is clicked. In this case there are only three elements because the straight pipe equivalent length is assumed to account for all entrance, exit, and fitting losses. Height changes are negative in the downward direction and positive in the upward direction. Height changes of individual elements are arbitrary here as long as they total to negative 8 ft. This puts the lower tubesheet a vertical distance of 8 ft below the liquid surface in the column sump. The outlet piping form is similar and is shown below.
The outlet header extends from the upper tubesheet to the return pipe. Since the upper tubesheet is at the same elevation as the liquid surface in the column sump, the specified height change puts the return pipe a vertical distance of 2 ft above the surface of the liquid in the sump. The value assumed for this distance has a relatively small effect on the calculations. In practice, however, the bottom of the return line should be at least 6 in. above the highest liquid level expected in the column sump.
(i) Control/Methods/Condensation.
Pure component: Yes
(j) Process
| Hot fluid | Cold fluid | |
| Fluid name | Steam | Cyclohexane |
| Phase | Condensing | Boiling |
| Flow rate (1000 lb/h) | 2.397 | 113.814 |
| Inlet fraction vapor | 1 | 0 |
| Outlet fraction vapor | 0 | – |
| Operating pressure (psia) | 18 | 16 |
| Fouling resistance (h · ft²·ºF/Btu) | 0 | 0.0005 |
(k) Hot fluid properties.
VMGThermo and Steam95 are selected for the property package. On the conditions panel, property sets is selected as the temperature point method. Pressure levels of 20, 18, and 16 psia are specified with a temperature range of 200º F to 230º F, and the number of data points is set at 20.
(l) Cold fluid properties.
VMGThermo and the Advanced Peng–Robinson method are selected for the cyclohexane stream. Selecting property sets for the temperature point method, pressure levels of 20, 18, and 16 psia are specified, with a temperature range of 180º F to 220º F. The number of data points is again set at 20.
The Xist Output Summary for this case is shown below, from which the unit is seen to be under-designed by about 20%, in agreement with the hand calculation. Data from the Output Summary and detailed output files were used to prepare the results comparison shown in the table below. The heat-transfer coefficients and pressure drops calculated by Xist agree fairly well with the hand calculations despite the fact that the circulation rates computed by the two methods differ by a large amount.
| Item | Hand | Xist |
| Circulation rate (lb/h) | 113,814 | 70,919 |
| h_{i} (Btu/h· ft²·ºF) | 565^{a} | 513 |
| h_{o} (Btu/h · ft²·ºF) | 1500 (assumed) | 1379 |
| U_{D} (Btu/h· ft²·ºF) | 243^{a} | 257 |
| Δ P_{i} (psi)^{c} | 0.862 | 0.863 |
| Δ P_{o} (psi) | – | 0.56 |
| Δ T_{m} (ºF) | 34.7^{b} | 33.4 |
| \left(\hat{q} / \hat{q}_c\right)_{\max } | 0.48 | 0.29 |
| Under-design (%) | 20 | 19.8 |
^{a} Area-weighted average of values for sensible heating and boiling zones.
^{b} Value for boiling zone.
^{c} Friction and acceleration only; excluding entrance, exit, and static head losses.
The tube-side pressure drop was not explicitly calculated in Example 10.4, although all parameters needed for the calculation were evaluated. For completeness, the friction and acceleration losses are computed here.
\Delta P_{a c c}=\frac{G_t^2 \gamma}{3.75 \times 10^{12} s_L}=\frac{(283,029)^2 \times 10.77}{3.75 \times 10^{12} \times 0.7208}=0.3192 psi
For the sensible heating zone, the friction loss is:
\Delta P_{f, B C}=\frac{f_t L_{B C} G_t^2}{7.50 \times 10^{12} D_t s_L}=\frac{0.0319 \times 2.9(283,029)^2}{7.50 \times 10^{12} \times 0.0695 \times 0.7208}=0.0197 psi
For the boiling zone, the friction loss is:
\Delta P_{f, C D}=\frac{f_t L_{C D} G_t^2 \bar{\phi}_{L O}^2}{7.50 \times 10^{12} D_t s_L}=\frac{0.0319 \times 5.1(283,029)^2 \times 15.08}{7.50 \times 10^{12} \times 0.0695 \times 0.7208}=0.5231 psi
The total friction loss is:
\Delta P_f=\Delta P_{f, B C}+\Delta P_{f, C D}=0.0197+0.5231=0.5428 psi
Therefore,
\Delta P_{a c c}+\Delta P_f=0.3192+0.5428=0.862 psi
Xist Output Summary for Example 10.8
