Chapter 9 S,S&L T&S Section 3.5 Terry Ring University of Utah Heat Integration Chapter 9 S,S&L T&S Section 3.5 Terry Ring University of Utah

Lost Work = Lost Money Transfer Heat from T1 to T2 ΔT approach Temp. for Heat Exchanger To= Temperature of Environment Use 1st and 2nd laws of Thermodynamics LW=QToΔT/(T1T2) ΔT=T1-T2 To= Environment Temperature Q= UAΔTlm=UA (ΔT1-ΔT2)/ln(ΔT1/ΔT2) T1 Q T2

Simple Heat Exchange Network (HEN) Use another stream for HX instead of a utility. What happens when delta T in Exchanger is lowered? To Zero?

Costs Heat Exchanger Purchase Cost Annual Cost CP=K(Area)0.6 CA=im[ΣCp,i+ ΣCP,A,j]+sFs+(cw)Fcw im=return on investment Fs= Annual Flow of Steam, $5.5/ston to $12.1/ston = s Fcw=Annual Flow of Cold Water $0.013/ston = cw CpA is HX in auxiliary networks

Capital and Operating Cost Optimization Capital cost goes down when A is less. This is caused by delta T being larger for Q to remain the same.

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

Terms HEN=Heat Exchanger Network MER=Maximum Energy Recovery Minimum Number of Heat Exchangers Threshold Approach Temperature Optimum Approach Temperature

Process

Minimize Utilities For 4 Streams

Simple HEN

Pinch Analysis 1) Adjust Hot Stream Temperatures to Give ΔTmin Put T’s in order Max to Min Order T’s, 250, 240, 235, 180, 150, 120

Interval Heat Loads

Enthalpy Differences for Temperature Intervals

Pinch Analysis Minimum Utilities =ΔHi+50 Pinch Analysis Minimum Utilities R’s = residulas

Pinch Analysis ΔTapp MER values

Process

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

4 Heat Exchanger HEN for Min. Utilities Cc≥Ch Ch≥Cc MER Values

Pinch Analysis Minimum Utilities =ΔHi+50 Pinch Analysis Minimum Utilities R’s = residulas

Minimum Utilities HEN

Simple HEN

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

Stream Splitting Two streams created from one 1 Two streams created from one one heat exchanger on each split of stream with couplings 1 1b 1a 1b 1a

Break Heat Exchanger Loops

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 plant operation?

Optimization of HEN How does approach ΔT (ΔTmin) effect the total cost of HEN? Q= UA ΔT LW=QToΔT/(T1T2) More Utility cost

ΔTmin ΔTapp=10C ΔTapp=105C LW=QToΔT/(T1T2) 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 Annual Cost CP=K(Area)0.6 CA=im[ΣCp,i+ ΣCP,A,j]+sFs+(cw)Fcw im=return on investment Fs= Annual Flow of Steam, $5.5/ston to $12.1/ston Fcw=Annual Flow of Cold Water $0.013/ston

Change ΔTmin CP=K(Area)0.6 Area=Q/(UF ΔTmin) LW=QToΔT/(T1T2) More Lost Work ΔT =UA/Q Utilities increase due to Lost work since it increases as ΔT increases LW=QToΔT/(T1T2)

Capital and Operating Cost Optimization Capital cost goes down when A is less. This is caused by delta T being larger for Q to remain the same. ΔTthres

Distillation Columns

Heuristic “Position a Distillation Column Between Composite Heating and Cooling Curves”

Heat Integration for Indirect Distillation Sequence

Multi-effect Distillation Adjust Pressure in C2 for ΔTmin

Heat Pumps in Distillation

Heat Pumps How do they work? Carnot Efficiency ηmax= 1-Tc/Th Endoreversible η =1-√(Tc/Th) Same as Air Conditioner Convert low temperature heat to high temperature heat. Must add work as heat can not go up hill.

Heat Pumps/Heat Engines Heurisitcs When positioning heat engines, to reduce the cold utilities, place them entirely above or below the pinch When positioning heat pumps, to reduce the total utilities, place them across the pinch.

Heat Pumps Where can they be used? Heuristic When positioning heat pumps, to reduce the total utilities, place them across the pinch.

Heat Engines Where can they be used? Tp Heuristic When positioning heat engines, to reduce the cold utilities, place them entirely above or below the pinch