Question 10.5: Use HEXTRAN to rate the kettle reboiler designed in Example ...
Use HEXTRAN to rate the kettle reboiler designed in Example 10.2, and compare the results with those obtained previously by hand.
Under Units of Measure, the English system of units is selected. Then, under Components and Thermodynamics, propane, i-butane, and n-butane are selected from the list of library components by double-clicking on each desired component. (Note that water is not required as a component for this problem.) The Peng–Robinson (PR) equation of state is selected as the principal thermodynamic method for the light hydrocarbon mixture. Thus, a New Method Slate called (arbitrarily) SET1 is defined on the Method tab and the options shown below are chosen from the pop-up lists obtained by right-clicking on the items in the thermodynamic data tree.
The API method for liquid density is chosen because it should be more reliable than the PR method for hydrocarbons. For transport properties, the Library method designates that property values are obtained from the program’s pure-component data-bank. No methods are required for entropy or inspection property data in this problem.
After setting up the flowsheet, the tube-side feed stream is defined as a Water/Steam stream by right-clicking on the stream and selecting Change Configuration from the pop-up menu. Double-clicking on the stream brings up the Specifications form, where the pressure is set to 20 psia, and the flow rate is specified as 5645 lb/h of steam. Saturated steam tables will automatically be used by the program to obtain property values for this stream.
The shell-side feed stream is defined as a compositional stream, i.e., a stream having a defined composition, which is the default category. On the Specifications form its thermal condition is set by entering the pressure (250 psia) for the first specification and selecting Bubble Point for the second specification. The total stream flow rate (96,000 lb/h) is also entered. The stream composition is specified by entering the mole percent of each component in place of the component (molar) flow rates. When these values sum to 100, they are automatically interpreted as percentages by the program.
Data for the exchanger are obtained from Example 10.2 and entered on the appropriate forms, with the exception of the shell ID, which is not specified. The reason is that when the correct value of 23.25 in. is entered, the program gives an error message and fails to generate a solution, apparently due to a bug in the software. When the shell ID is not specified, the program calculates the diameter based on the tube data supplied. In the present case it calculates a diameter of 23 in., which is essentially the correct result. In addition to the data from Example 10.2, a fouling factor of 0.0005 h · ft² · ºF/Btu is specified for steam. Fouling factors for both streams are entered on the Film Options form.
Finally, under Input/Calculation Options, the maximum number of iterations for the flowsheet is set to 100 because the default value of 30 proved to be insufficient for this problem.
The input file generated by the HEXTRAN GUI is given below, followed by a summary of results extracted from the HEXTRAN output file. From the latter it can be seen that the reboiler generates 48,571 lb/h of vapor, which is slightly more than the required rate of 48,000 lb/h. Thus, it appears that the unit is sized almost perfectly. In fact, however, the amount of vapor generated by the unit is limited by the amount of steam supplied, rather than by the available heat-transfer area. Referring to the zone analysis data given below, it is seen that all the steam condenses in the first five zones, leaving only condensate to be subcooled in the last zone. The area contained in the last zone is 117.2 ft², which is about 16% of the total surface area in the reboiler. If the steam flow rate is increased to 6850 lb/h, the subcooled condensate zone is eliminated and the amount of vapor generated increases to 58,349 lb/h. Taking the area of the first five zones (597 ft²) as the required area gives an over-design for the unit of about 20%, which is a reasonable margin for this application.
The following table compares results from HEXTRAN with those obtained by hand in Example 10.2. As expected, the boiling heat-transfer coefficient calculated by hand is considerably more conservative than the value computed by HEXTRAN. However, the effective coefficient for steam used in Example 10.2 is actually much higher than the value computed by HEXTRAN. This result is due to the fouling factor used for steam in the present example, without which the effective steam coefficient for HEXTRAN would be about 1760 Btu/h . ft² . ºF. The steam-side pressure drop found by HEXTRAN is comparable to the value estimated by hand. On the boiling side, the entire pressure drop calculated by HEXTRAN is due to the nozzles. The nozzle losses were not explicitly calculated in Example 10.2. They are obtained by using the appropriate equivalent lengths from Example 10.2; namely, the exit loss (28 ft of 5-in. pipe) for the inlet nozzles and the entrance loss (18 ft of 6-in. pipe) for the outlet nozzles. Finally, the mean temperature difference used in the hand calculations is quite close to the weighted average value from HEXTRAN.
| Item | Hand | HEXTRAN |
| h_{0} (Btu/h. ft².ºF) | 523 | 936^a |
| \left\{\left(D_i / D_i\right)\left(1 / h_i+R_{D i}\right)\right\}^{-1} (Btu/h. ft².ºF)
U_{D} (Btu/h. ft².ºF) |
1,500 (assumed)
297 |
857^a
335^a |
| Δ P_{i} (psi) | 0.3 | 0.43 |
| Δ P_{0} (psi)^b | 0.2 (assumed) | – |
| Δ P_{0} (psi), nozzles only | 0.05 | 0.05 |
| Δ T_{m} (ºF) | 25.6 | 27. l^a |
^aArea-weighted average over first five zones; subcooled condensate zone not included.
^bFriction and acceleration, excluding nozzle losses.
HEXTRAN Input File for Example 10.5
| $ GENERATED FROM HEXTRAN KEYWORD EXPORTER $ $ General Data Section $ TITLE PROJECT=Example 10-5, PROBLEM=Kettle 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 $ LIBID 1, PROPANE /* 2, IBUTANE /* 3, BUTANE $ $ Thermodynamic Data Section $ THERMODYNAMIC DATA $ METHODS SET=SET1, KVALUE=PR, ENTHALPY(L)=PR, ENTHALPY(V)=PR, * DENSITY(L)=API, DENSITY(V)=PR, VISCOS(L)=LIBRARY, * VISCOS(V)=LIBRARY, CONDUCT(L)=LIBRARY, CONDUCT(V)=LIBRARY, * SURFACE=LIBRARY $ WATER DECANT=ON, SOLUBILITY = Simsci, PROP = Saturated $ $Stream Data Section $ STREAM DATA $ PROP STRM=PROD, NAME=PROD$ PROP STRM=CONDENSATE, NAME=CONDENSATE $ PROP STRM=STEAM, NAME=STEAM, PRES=20.000, STEAM=5645.000 $ PROP STRM=FEED, NAME=FEED, PRES=250.000, PHASE=L, * RATE(W)=96000.000, * COMP(M)= 1, 15 / * 2, 25 / 3, 60 $ $ Calculation Type Section $ SIMULATION $ TOLERANCE TTRIAL=0.01 $ LIMITS AREA=200.00, 6000.00, SERIES=1, 10, PDAMP=0.00, * TTRIAL=100 $ CALC TWOPHASE=New, DPSMETHOD=Stream, MINFT=0.80 $ PRINT UNITS, ECONOMICS, STREAM, STANDARD, * EXTENDED, ZONES $ ECONOMICS DAYS=350, EXCHANGERATE=1.00, CURRENCY=USDOLLAR $ UTCOST OIL=3.50, GAS=3.50, ELECTRICITY=0.10, * WATER=0.03, HPSTEAM=4.10, MPSTEAM=3.90, * LPSTEAM=3.60, REFRIGERANT=0.00, HEATINGMEDIUM=0.00 $ HXCOST BSIZE=1000.00, BCOST=0.00, LINEAR=50.00, * EXPONENT=0.60, CONSTANT=0.00, UNIT $ $ Unit Operations Data $ UNIT OPERATIONS $ STE UID=KETTLE TYPE Old, TEMA=BKU, HOTSIDE=Tubeside, ORIENTATION=Horizontal, * FLOW=Countercurrent, * UESTIMATE=50.00, USCALER=1.00 TUBE FEED=STEAM, PRODUCT=CONDENSATE, * LENGTH=13.00, OD=1.000, * BWG=14, NUMBER=212, PASS=2, PATTERN=90, * PITCH=1.2500, MATERIAL=1, * FOUL=0.0005, LAYER=0, * DPSCALER=1.00 $ SHELL FEED=FEED, PRODUCT=PROD, * SERIES=1, PARALLEL=1, * MATERIAL=1, * FOUL=0.0005, LAYER=0, * DPSCALER=1.00 $ BAFF NONE $ TNOZZ TYPE=Conventional, ID=6.065, 3.068, NUMB=1, 1 $ SNOZZ TYPE=Conventional , ID=5.047, 6.065, NUMB=2, 2 $ |
HEXTRAN Output Data for Example 10.5
| ============================================================================== SHELL AND TUBE EXCHANGER DATA SHEET I—————————————————————————-I I EXCHANGER NAME UNIT ID KETTLE I I SIZE 23x 156 TYPE BKU, HORIZONTAL CONNECTED 1 PARALLEL 1 SERIES I I AREA/UNIT 715. FT2 ( 714. FT2 REQUIRED) AREA/SHELL 715. FT2 I I—————————————————————————-I I PERFORMANCE OF ONE UNIT SHELL-SIDE TUBE-SIDE I I—————————————————————————-I I FEED STREAM NUMBER FEED STEAM I I FEED STREAM NAME FEED STEAM I I TOTAL FLUID LB /HR 96000. 5645. I I VAPOR (IN/OUT) LB /HR 0./ 48571. 0./ 0. I I LIQUID LB /HR 96000./ 47429. 0./ 0. I I STEAM LB /HR 0./ 0. 5645./ 0. I I WATER LB /HR 0./ 0. 0./ 5645. I I NON CONDENSIBLE LB /HR 0. 0. I I TEMPERATURE (IN/OUT) DEG F 197.6 / 202.4 228.3 / 217.2 I I PRESSURE (IN/OUT) PSIA 250.00 / 249.95 20.00 / 19.57 I I—————————————————————————-I I SP. GR., LIQ (60F / 60F H2O) 0.569 / 0.571 0.000 / 1.000 I I VAP (60F / 60F AIR) 0.000 / 1.916 0.631 / 0.000 I I DENSITY, LIQUID LB/FT3 28.406 / 28.369 0.000 / 59.738 I I VAPOR LB/FT3 0.000 / 2.758 0.049 / 0.000 I I VISCOSITY, LIQUID CP 0.074 / 0.074 0.000 / 0.275 I I VAPOR CP 0.000 / 0.009 0.012 / 0.000 I I THRML COND,LIQ BTU/HR-FT-F 0.0462 / 0.0459 0.0000 / 0.3942 I I VAP BTU/HR-FT-F 0.0000 / 0.0141 0.0147 / 0.0000 I I SPEC.HEAT,LIQUID BTU /LB F 0.8054 / 0.8106 0.0000 / 1.0080 I I VAPOR BTU /LB F 0.0000 / 0.5763 0.5049 / 0.0000 I I LATENT HEAT BTU /LB 105.64 0.00 I I VELOCITY FT/SEC 0.30 0.13 I I DP/SHELL(DES/CALC) PSI 0.00 / 0.05 0.00 / 0.43 I I FOULING RESIST FT2-HR-F/BTU 0.00050 (0.00050 REQD) 0.00050 I I—————————————————————————-I I TRANSFER RATE BTU/HR-FT2-F SERVICE 282.91 ( 282.62 REQD), CLEAN 410.66 I I HEAT EXCHANGED MMBTU /HR 5.479, MTD(CORRECTED) 27.1, FT 0.982 I I—————————————————————————-I I CONSTRUCTION OF ONE SHELL SHELL-SIDE TUBE-SIDE I I—————————————————————————-I I DESIGN PRESSURE/TEMP PSIA /F 325./ 300. 75./ 300. I I NUMBER OF PASSES 1 2 I I MATERIAL CARB STL CARB STL I I INLET NOZZLE ID/NO IN 5.0/ 2 6.1/ 1 I I VAPOR NOZZLE ID/NO IN 6.1/ 2 3.1/ 1 I I INTERM NOZZLE ID/NO IN 0.0/ 0 I I—————————————————————————-I I TUBE: NUMBER 212, OD 1.000 IN , BWG 14 , LENGTH 13.0 FT I I TYPE BARE, PITCH 1.2500 IN, PATTERN 90 DEGREES I I SHELL: ID 23.00 IN, BUNDLE DIAMETER(DOTL) 22.50 IN I I RHO-V2: INLET NOZZLE 1297.0 LB/FT-SEC2 I I TOTAL WEIGHT/SHELL,LB 6685.2 FULL OF WATER 0.138E+05 BUNDLE 4024.7 I I—————————————————————————-I ============================================================================== ZONE ANALYSIS FOR EXCHANGER KETTLE |
