Question 10.6: Use HEXTRAN to rate the horizontal thermosyphon reboiler of ...
Use HEXTRAN to rate the horizontal thermosyphon reboiler of Example 10.3 and compare the results with those obtained previously by hand. Assay data (ASTM D86 distillation at atmospheric pressure) for the petroleum fraction fed to the reboiler are given in the following table. The feed stream has an average API gravity of 60º.
| Volume Percent Distilled | Temperature (ºF) |
| 0 | 158.8^a |
| 10 | 210 |
| 30 | 240 |
| 50 | 260 |
| 70 | 275 |
| 90 | 290 |
| 100 | 309^b |
^aInitial boiling point.
^bEnd point.
For this problem, the tube-side feed (Therminol) is defined as a bulk property stream and the values of C_P, k, μ and ρ (55.063 lbm/ ft³) given in Example 10.3 are entered as average liquid properties on the appropriate form. Note that the density, not the specific gravity, must be entered. Additional data required for this stream are the flow rate (425,000 lb/h), temperature (420ºF) and pressure. Since the stream pressure was not specified in Example 10.3, a value of 40 psia is (arbitrarily) assumed.
The shell-side feed (petroleum fraction) is defined as an assay stream, and its flow rate (300,000 lb/h) and pressure (35 psia) are entered on the Specifications form. To complete the thermal specification of the stream, Bubble Point is selected from the list of available specifications. Next, under Components and Thermodynamics, the Assay tab is selected and a new assay name (A1) is entered. Clicking on the Add button activates the data entry tree that includes the listings Distillation and Gravity, as shown below. Clicking on each of these items in turn brings up the panels where the ASTM distillation data and average API gravity are entered. HEXTRAN uses the assay data to determine a set of pseudo components that represent the composition of the stream. The assay name, A1, is also entered on the Specifications form for the feed stream in order to link the assay with the stream to which it applies.
A set of thermodynamic procedures is also required for the assay stream. The PR EOS is selected as the method for equilibrium, enthalpy, and vapor density calculations; the API method is chosen for calculating liquid density. The petroleum method is selected for all transport properties (viscosity, thermal conductivity, and surface tension).
Data for the heat exchanger are entered as given in Example 10.3, including the number of tubes (290) and the shell ID (23.25 in). A type A front head and no baffles are assumed. Tubesheet thickness and shell-side nozzles are left unspecified; HEXTRAN will determine suitable values for these items, which were not specified in Example 10.3. Fouling factors from Example 10.3 are entered on the Film Options form.
The input file generated by the HEXTRAN GUI is given below, followed by a summary of results extracted from the HEXTRAN output file. It is seen that the reboiler generates 82,390 lb/h of vapor, about 37% more than the 60,000 lb/h
required. The tube-side pressure drop is 9.34 psi, which is less than the maximum of 10 psi specified for the unit. HEXTRAN does not compute a critical heat flux, so this check must be done by hand. In the present case, the heat flux is approximately 13,000 Btu/h . ft², well below the critical value of 36,365 Btu/h. ft² calculated by hand in Example 10.3. (In actual operation, the heat flux would be about 37% lower.) Therefore, the reboiler is suitable for the service, in agreement with the result obtained in Example 10.3.
Results from HEXTRAN are compared with those calculated by hand in the following table. The shell-side (boiling) heat-transfer coefficient calculated by hand is very conservative compared with the value given by HEXTRAN, but the overall coefficients differ by less than 20%. The mean temperature difference used in the hand calculations is slightly higher than the value calculated by HEXTRAN. However, the heat flux ( U_D \Delta T_m) calculated by hand is on the safe side, about 10% below the HEXTRAN value. Notice
that virtually all of the shell-side pressure drop occurs in the nozzles. If two pairs of nozzles (6-in. inlet, 10-in. outlet) are assumed instead of the single pair used by HEXTRAN, the shell-side pressure drop is reduced to 0.33 psi.
| Item | Hand | HEXTRAN |
| h_{i} (Btu/h. ft².ºF) | 346 | 346.2 |
| h_{0} (Btu/h. ft².ºF) | 278 | 555^a |
| U_{D} (Btu/h. ft².ºF) | 113 | 132^a |
| Δ P_{i} (psi) | 8.4 | 9.34 |
| Δ P_{0} (psi) | – | 1.39 |
| Δ T_{m} (ºF) | 104.2 | 98.5^a |
| U_{D} Δ T_{m} (Btu/h ∙ ft²) | 11,775 | 13,002 |
^aArea-weighted average from zone analysis.
HEXTRAN Input File for Example 10.6
| $ GENERATED FROM HEXTRAN KEYWORD EXPORTER $ $ General Data Section $ TITLE PROJECT=Example 10-6, PROBLEM=Horizontal Thermosyphon Reboiler, SITE= $ DIME English, AREA=FT2, CONDUCTIVITY=BTUH, DENSITY=LB/FT3, * ENERGY=BTU, FILM=BTUH, LIQVOLUME=FT3, POWER=HP, * PRESSURE=PSIA, SURFACE=DYNE, TIME=HR, TEMPERATURE=F, * UVALUE=BTUH, VAPVOLUME=FT3, VISCOSITY=CP, WT=LB, * XDENSITY=API, STDVAPOR=379.490 $ PRINT ALL, * RATE=M $ CALC PGEN=New, WATER=Saturated $ $ Component Data Section $ COMPONENT DATA $ $ ASSAY FIT= SPLINE, CHARACTERIZE=SIMSCI, MW= SIMSCI, * CONVERSION= API87, GRAVITY= WATSONK, TBPIP=1, TBPEP=98 $ TBPCUTS 100.00, 800.00, 28 /* 1200.00, 8 /* 1600.00, 4 $ $ Thermodynamic Data Section $ THERMODYNAMIC DATA $ METHODS SET=SET1, KVALUE=PR, ENTHALPY(L)=PR, ENTHALPY(V)=PR, * DENSITY(L)=API, DENSITY(V)=PR, VISCOS(L)=PETRO, * VISCOS(V)=PETRO, CONDUCT(L)=PETRO, CONDUCT(V)=PETRO, * SURFACE=PETRO $ WATER DECANT=ON, SOLUBILITY = Simsci, PROP = Saturated $ $Stream Data Section $ STREAM DATA $ PROP STRM=THERM_COLD, NAME=THERM_COLD $ PROP STRM=PROD, NAME=PROD $ PROP STRM=FEED, NAME=FEED, PRES=35.000, PHASE=L, * RATE(W)=300000.000, ASSAY=LV, BLEND D86 STRM=FEED, * DATA= 0.0, 158.80 / 10.0, 210.00 / 30.0, 240.00 / 50.0, 260.00 / * 70.0, 275.00 / 90.0, 290.00 / 100.0, 309.00 API STRM=FEED, AVG=60.000 $ |
HEXTRAN Output Data for Example 10.6
| SHELL AND TUBE EXCHANGER DATA SHEET I—————————————————————————-I I EXCHANGER NAME UNIT ID REBOILER I I SIZE 23x 192 TYPE AXU, HORIZONTAL CONNECTED 1 PARALLEL 1 SERIES I I AREA/UNIT 904. FT2 ( 904. FT2 REQUIRED) AREA/SHELL 904. FT2 I I—————————————————————————-I I PERFORMANCE OF ONE UNIT SHELL-SIDE TUBE-SIDE I I—————————————————————————-I I FEED STREAM NUMBER FEED THERMINOL I I FEED STREAM NAME FEED THERMINOL I I TOTAL FLUID LB /HR 300000. 425000. I I VAPOR (IN/OUT) LB /HR 0./ 82390. 0./ 0. I I LIQUID LB /HR 300000./ 217610. 425000./ 425000. I I STEAM LB /HR 0./ 0. 0./ 0. I I WATER LB /HR 0./ 0. 0./ 0. I I NON CONDENSIBLE LB /HR 0. 0. I I TEMPERATURE (IN/OUT) DEG F 288.9 / 298.3 420.0 / 368.1 I I PRESSURE (IN/OUT) PSIA 35.00 / 33.61 40.00 / 30.66 I I—————————————————————————-I I SP. GR., LIQ (60F / 60F H2O) 0.739 / 0.742 0.883 / 0.883 I I VAP (60F / 60F AIR) 0.000 / 3.577 0.000 / 0.000 I I DENSITY, LIQUID LB/FT3 39.063 / 39.027 55.063 / 55.063 I I VAPOR LB/FT3 0.000 / 0.463 0.000 / 0.000 I I VISCOSITY, LIQUID CP 0.179 / 0.179 0.840 / 0.840 I I VAPOR CP 0.000 / 0.009 0.000 / 0.000 I I THRML COND,LIQ BTU/HR-FT-F 0.0547 / 0.0541 0.0613 / 0.0613 I I VAP BTU/HR-FT-F 0.0000 / 0.0136 0.0000 / 0.0000 I I SPEC.HEAT,LIQUID BTU /LB F 0.6013 / 0.6051 0.5340 / 0.5340 I I VAPOR BTU /LB F 0.0000 / 0.4936 0.0000 / 0.0000 I I LATENT HEAT BTU /LB 122.02 0.00 I I VELOCITY FT/SEC 0.51 7.95 I I DP/SHELL(DES/CALC) PSI 0.00 / 1.39 0.00 / 9.34 I I FOULING RESIST FT2-HR-F/BTU 0.00050 (0.00050 REQD) 0.00100 I I—————————————————————————-I I TRANSFER RATE BTU/HR-FT2-F SERVICE 132.17 ( 132.10 REQD), CLEAN 172.96 I I HEAT EXCHANGED MMBTU /HR 11.772, MTD(CORRECTED) 98.6, FT 0.998 I I—————————————————————————-I I CONSTRUCTION OF ONE SHELL SHELL-SIDE TUBE-SIDE I I—————————————————————————-I I DESIGN PRESSURE/TEMP PSIA /F 100./ 500. 100./ 500. I I NUMBER OF PASSES 1 2 I I MATERIAL CARB STL CARB STL I I INLET NOZZLE ID/NO IN 6.1/ 1 6.1/ 1 I I OUTLET NOZZLE ID/NO IN 10.0/ 1 6.1/ 1 I I—————————————————————————-I I TUBE: NUMBER 290, OD 0.750 IN , BWG 14 , LENGTH 16.0 FT I I TYPE BARE, PITCH 1.0000 IN, PATTERN 90 DEGREES I I SHELL: ID 23.25 IN, SEALING STRIPS 0 PAIRS I I RHO-V2: INLET NOZZLE 4416.7 LB/FT-SEC2 I I TOTAL WEIGHT/SHELL,LB 3701.3 FULL OF WATER 0.113E+05 BUNDLE 5000.4 I I—————————————————————————-I SHELL AND TUBE EXTENDED DATA SHEET ZONE ANALYSIS FOR EXCHANGER REBOILER |
