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Heat Integration. Sieder, et. al., Chapter 11 Terry Ring University of Utah. Lost Work = Lost Money. Transfer Heat from T 1 to T 2 ΔT [= T 1 -T 2 ] approach Temp. for Heat Exchanger T o = Temperature of Environment Use 1 st and 2 nd laws of Thermodynamics ΔT
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Heat Integration Sieder, et. al., Chapter 11 Terry Ring University of Utah
Lost Work = Lost Money • Transfer Heat from T1 toT2 • ΔT [= T1-T2] approach Temp. for Heat Exchanger • To= Temperature of Environment • Use 1st and 2nd laws of Thermodynamics • ΔT • LW=QTo(T1-T2)/(T1T2) • Q= UoAΔTlm =UoA(ΔT1-ΔT2)/ln(ΔT1/ΔT2) T1 Q T2
Costs • Heat Exchanger Purchase Cost (Inside BL) • Cp,i=K(Areai)0.6 • Area= Q/UoΔTlm • Annual Cost • CA=im[ΣCp,i+ ΣCP,A,j]+(s)Fs+(cw)Fcw • im=return on investment • Fs= Annual Flow of Steam, • $13.2/Tonne to $17.6/Tonne = s • Fcw=Annual Flow of Cold Water • $0.027/m3 =$0.027/Tonne = cw Auxiliary HX outside BL
Heat Integration • Make list of HX • Instead of using utilities can you use another stream to heat/cool any streams? • How much of this can you do without causing operational problems? • Can you use air to cool? • Air is a low cost coolant. • Less utilities = smaller cost of operations
Ultra-high purity Si plant design Si at 99.97% Powder H2 & HCl Separation Train Fluid Bed Reactor (400-900C) Si+7HCl SiHCl3 + SiCl4 +3H2 Si+ 2HCl SiH2Cl2 Flash HCl SiCl4 H2-HCl Separation HCl H2 SiCl4 Very Pure SiHCl3&SiH2Cl2 Fluid Bed Reactor(600C) Si+SiCl4+2HCl 2SiHCl3 Flash Reactor (1200C) SiHCl3+H2 Si+3HCl SiH2Cl2+1/2 H2 Si+3HCl Si H2 HCl Si at 99.999999999%
Terms • HEN=Heat Exchanger Network • MER=Maximum Energy Recovery • Minimum Number of Heat Exchangers • Threshold Approach Temperature • Optimum Approach Temperature
Minimize UtilitiesFor 4 Streams 470 480
Minimize UtilitiesFor 4 Streams 470 480
Pinch Analysis1) Adjust Hot Stream Temperatures to Give ΔTmin=10°F 2) Order T’s, 250, 240, 235, 180, 150, 120
Pinch Analysis1) Adjust Hot Stream Temperatures to Give ΔTmin Order T’s, 250, 240, 235, 180, 150, 120
Pinch AnalysisMinimum Utilities =ΔHi+50
Pinch AnalysisMinimum Utilities =ΔHi+50
Pinch Analysis Actual Endpoint Temperatures! ΔTapp MER values
How to combine hot with cold? • Big Exhangers 1st • 1st HX at Pinch (temp touching pinch) • Above Pinch Connect • Cc≥Ch • Below Pinch Connect • Ch≥Cc • 2nd Hx or not touching Pinch temp. • No requirement for Cc or Ch
Pinch Analysis Actual Endpoint Temperatures! Cc≥Ch 3*(260-190)=210 1.5*(250-190)=90 2*(235-180)=110 4*(240-180)=240 ΔTapp MER values
Pinch Analysis Actual Endpoint Temperatures! Cc≥Ch 3*(260-190)=210 1.5*(250-190)=90 2*(235-180)=110 90=2*(T-180) T=225 4*(240-180)=240 210=4*(T-180) T=232.5°F ΔTapp 20 30 MER values
How to combine hot with cold? • Big Exhangers 1st • 1st HX at Pinch (temp touching pinch) • Above Pinch Connect • Cc≥Ch • Below Pinch Connect • Ch≥Cc • 2nd Hx or not touching Pinch temp. • No requirement for Cc or Ch
Pinch Analysis Actual Endpoint Temperatures! Ch≥Cc 3*(190-160)=90 1.5*(190-130)=90 30=1.5*(190-T) T=170°F 2*(180-120)=120 90=2*(180-T) T=135°F 2*(135-120)=30 60 ΔTapp MER values
4 Heat ExchangerHEN for Min. Utilities Cc≥Ch Ch≥Cc CW MER Values Steam
Pinch AnalysisMinimum Utilities =ΔHi+50
Comparison Simple HEN HEN with Min. Utilities Saves CW 7.5e4 BTU/hr Steam 7.5e4 BTU/hr
Too Many Heat Exchangers • Sometimes fewer Heat exchangers and increased utilities leads to a lower annual cost • NHx,min= Ns + NU - NNW • s=No. streams • U=No. discrete Utilities • NW=No. independent Networks (1 above the pinch, 1 below the pinch) • Solution to Too Many Heat Exchangers • Break Heat Exchanger Loops • Stream Splitting • Attack small Heat Exchangers First for elimination 4+2-2=4
Stream Splitting 1 • Two streams created from one • one heat exchanger on each split of stream with couplings 1 1b 1a 1b 1a
Example CP=K(Area)0.6
Last Considerations • How will HEN behave during startup? • How will HEN behave during shutdown? • Does HEN lead to unstable or difficult to control plant operations?
Optimization of HEN • How does approach ΔT >ΔTmin effect the total cost of HEN? • Q= UA ΔT • Greater ΔT smaller A • Less capital cost • LW=QToΔT/(T1T2) • Greater ΔT more LW • More Utility cost
ΔTmin • S T(C) T(C) C Q(kW) • H1 300 200 1.5 150 • H2 300 250 2 100 • C1 30 200 1.2 204 ΔTapp=10C ΔTapp=105C LW=QToΔT/(T1T2)
Costs • Heat Exchanger Purchase Cost (Inside BL) • Cp,i=K(Areai)0.6 • Area= Q/UoΔTlm • Annual Cost • CA=im[ΣCp,i+ ΣCP,A,j]+sFs+(cw)Fcw • im=return on investment • Fs= Annual Flow of Steam, • $13.2/Tonne to $17.6/Tonne = s • Fcw=Annual Flow of Cold Water • $0.027/m3 = cw Auxiliary HX outside BL
Change ΔTmin Area=Q/(UF ΔTmin) CP=K(Area)0.6 More Lost Work LW=QToΔT/(T1T2)