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Unformatted text preview: 13-1 CHAPTER 13 SOLUTIONS 13.1 First, use the results of Example 13.7 to estimate the heat exchanger area for a minimum ∆T approach of 10oF. For Example 13.7: Log-mean ∆T = 46.23oF Heat duty = 9,472,000 Btu/hr Toluene exit temperature = 268.7oF Closest approach is 31.3oF at the styrene feed end For the conditions of this exercise: Toluene exit temperature for a 10oF approach (at the styrene feed end) = 290oF. Approximate heat duty = 9,472,000(290-100)/(268.7-100) = 10,668,000 Btu/hr Approximate temperature decrease of styrene= (300-165.2)(10,668,000)/(9,472,000) = 151.8oF Therefore, temperature of styrene out = 300-151.8 = 148.2oF. Temperature approach at toluene feed end = 148.2-100 = 48.2oF Log-mean ∆T= (48.2 - 10)/ln(48.2/10) = 24.3oF In Example 13.7, Uo= 69.5 Btu/hr-ft2-oF and F= 0.916. Assume these are the same for this exercise. Then, for Q=UoAoF∆TLM, Ao= 10,668,000/(69.5)(24.3)(0.916) = 6,896 ft2 This is more than twice that area in Example 13.7. Trial Design 1:As a first guess at the design, split the toluene and styrene streams into three fractions each and feed each pair into an exchanger of the design of that in Example 13.7. Thus, we will have 3 exchangers in parallel. Using the ASPEN PLUS HEATX model, the result is a minimum ∆Tapproach of 17.23oF, which is too high. Trial Design 2:Increase tube length from 16 ft to 20 ft. The result from HEATX is a minimum ∆Tapproach of 11.76oF, which is close to the desired value of 10oF. Trial Design 3:Use four exchangers in parallel, each having the 20 ft tube length. The result from HEATX is a minimum ∆Tapproach of 9.96oF, which meets the specification. The ASPEN PLUS input and result are as follows: 13-2 ASPEN PLUS Program TITLE 'Exercise 13.1 - SSL' IN-UNITS ENG DEF-STREAMS CONVEN ALL DESCRIPTION "General Simulation with English Units : F, psi, lb/hr, lbmol/hr, Btu/hr, cuft/hr. Property Method: None Flow basis for input: Mole Stream report composition: Mole flow " DATABANKS PURE10 / AQUEOUS / SOLIDS / INORGANIC / & NOASPENPCD PROP-SOURCES PURE10 / AQUEOUS / SOLIDS / INORGANIC COMPONENTS STY C8H8 STY / TOL C7H8 TOL FLOWSHEET BLOCK B1 IN=12 13 OUT=15 14 PROPERTIES RK-SOAVE STREAM 12 SUBSTREAM MIXED TEMP=300. PRES=50. MASS-FLOW STY 37500. STREAM 13 SUBSTREAM MIXED TEMP=100. PRES=90. MASS-FLOW TOL 31250. BLOCK B1 HEATX PARAM P-UPDATE=YES U-OPTION=FILM-COEF F-OPTION=GEOMETRY FEEDS HOT=12 COLD=13 PRODUCTS HOT=15 COLD=14 EQUIP-SPECS TUBE-NPASS=8 TEMA-TYPE=F SHELL-DIAM=39. <in> TUBES TOTAL-NUMBER=1024 PATTERN=SQUARE LENGTH=20. & INSIDE-DIAM=0.584 <in> OUTSIDE-DIAM=0.75 <in> PITCH=1. <in> & TCOND=26. NOZZLES SNOZ-INDIAM=2.469 <in> SNOZ-OUTDIAM=2.469 <in> & TNOZ-INDIAM=4.026 <in> TNOZ-OUTDIAM=4.026 <in> SEGB-SHELL NBAFFLE=38 BAFFLE-CUT=0.25 HOT-SIDE H-OPTION=GEOMETRY FOUL-FACTOR=0.002 SHELL-TUBE=SHELL & DP-OPTION=GEOMETRY COLD-SIDE H-OPTION=GEOMETRY FOUL-FACTOR=0.002 SHELL-TUBE=TUBE & DP-OPTION=GEOMETRY Stream Variables 12 13 14 15 ----------- STREAM ID 12 13 14 15 FROM : ---- ---- B1 B1 TO...
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- Spring '08
- Heat Transfer, Log mean temperature difference, T= P= V=, PSI PSI PSI PSI PSI PSI, FT FT FT