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

Lost Work = Lost Money Transfer Heat from T 1 to T 2 ΔT approach Temp. for Heat Exchanger T o = Temperature of Environment Use 1 st and 2 nd laws of Thermodynamics LW=QT o ΔT/(T 1 T 2 ) –ΔT=T 1 -T 2 –T o = Environment Temperature Q= UAΔT lm =UA (ΔT 1 -ΔT 2 )/ln(ΔT 1 /ΔT 2 ) T1T1 T2T2 Q

Simple Heat Exchange Network (HEN)

Costs Heat Exchanger Purchase Cost – C P =K(Area) 0.6 Annual Cost –C A =i m [ΣC p,i + ΣC P,A,j ]+sF s +(cw)F cw i m =return on investment F s = Annual Flow of Steam, –$5.5/ston to $12.1/ston = s F cw =Annual Flow of Cold Water –$0.013/ston = cw

Capital and Operating Cost Optimization

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

Minimize Utilities For 4 Streams

Pinch Analysis 1) Adjust Hot Stream Temperatures to Give ΔT min Order T’s, 250, 240, 235, 180, 150, 120

Interval Heat Loads

Pinch Analysis 1) Adjust Hot Stream Temperatures to Give ΔT min Order T’s, 250, 240, 235, 180, 150, 120

Enthalpy Differences for Temperature Intervals

Pinch Analysis Minimum Utilities =ΔH i +50

Pinch Analysis Minimum Utilities =ΔH i +50

Pinch Analysis ΔT app MER values Actual Endpoint Temperatures!

Process

How to combine hot with cold? Big Exhangers 1 st 1 st HX at Pinch (temp touching pinch) –Above Pinch Connect C c ≥C h –Below Pinch Connect C h ≥C c 2 nd Hx or not touching Pinch temp. –No requirement for C c or C h

Pinch Analysis ΔT app MER values Actual Endpoint Temperatures! 3*( )= *( )=90 2*( )=110 4*( )=240 C c ≥C h

Pinch Analysis ΔT app MER values Actual Endpoint Temperatures! 3*( )= *( )=90 2*( )=110 90=2*(T-180) T=225 4*( )= =4*(T-180) T=232.5°F C c ≥C h

How to combine hot with cold? Big Exhangers 1 st 1 st HX at Pinch (temp touching pinch) –Above Pinch Connect C c ≥C h –Below Pinch Connect C h ≥C c 2 nd Hx or not touching Pinch temp. –No requirement for C c or C h

Pinch Analysis ΔT app MER values Actual Endpoint Temperatures! 3*( )=90 1.5*( )=90 30=1.5*(190-T) T=170°F 2*( )=120 90=2*(180-T) T=135°F 2*( )= C h ≥C c

4 Heat Exchanger HEN for Min. Utilities C c ≥C h C h ≥C c MER Values Steam CW

Pinch Analysis Minimum Utilities =ΔH i +50

Minimum Utilities HEN

Simple HEN

Comparison Simple HENHEN 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 N Hx,min = N s + N U - N NW –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 4+2-2=4

Break Heat Exchanger Loops

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

Example C P =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 >ΔT min effect the total cost of HEN? Q= UA ΔT –Less capital cost LW=QT o ΔT/(T 1 T 2 ) –More Utility cost

ΔT min ST(C)T(C)CQ(kW) H H C LW=QToΔT/(T1T2) ΔT app =10C ΔT app =105C

Costs Heat Exchanger Purchase Cost – C P =K(Area) 0.6 Annual Cost –C A =i m [ΣC p,i + ΣC P,A,j ]+sF s +(cw)F cw i m =return on investment F s = Annual Flow of Steam, –$5.5/ston to $12.1/ston F cw =Annual Flow of Cold Water –$0.013/ston

Change ΔT min C P =K(Area) 0.6 Area=Q/(UF ΔT min ) More Lost Work LW=QToΔT/(T1T2)

Capital and Operating Cost Optimization ΔT thres

Distillation Columns

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

Heat Integration for Direct Distillation Sequence

Multi-effect Distillation Adjust Pressure in C2 for ΔT min

Heat Pumps in Distillation

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

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? Heuristic When positioning heat engines, to reduce the cold utilities, place them entirely above or below the pinch Tp