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Smith: Chemical Process Design and Integration (Chapters 16-22)Kemp: Pinch Analysis and Process Integration (Chapter 9)
Process Integration
for Efficient Use of Energy
Cheng-Liang Chen
PSELABORATORYDepartment of Chemical Engineering
National TAIWAN University
Chen CL 1
Outline
Systematic Approach for Chemical Process Design
How do we go about the design of a chemical process?
What Is Process Integration?
Onion model for process integration
Pinch Analysis: Targeting Heat Recovery in Processes
Pinch Design Method for Heat Recovery Systems
A Pinch Study Performed on A Major Operating Plant
Utility Selection for Individual Processes
Heat Integration for Individual Processes
Putting It into Practice and Concluding Remarks
Chen CL 2
Utility Selectionfor
Individual Processes
Chen CL 3
The Problem Table and Grand Composite Curve
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Chen CL 4
The Problem Table and Grand Composite CurveChen CL 5
The Problem Table and Grand Composite Curve
Chen CL 6
The Problem Table and Grand Composite Curve
Chen CL 7
The Grand Compositegives the hot and cold utility requirements
of the process both in
Enthalpy and Temperature
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Chen CL 8
A Flowsheet with Two-Hot-Two-Cold Streams
Stream Type Supply Temp. Target Temp. H Heat Capacity Rate
TS(oC) TT(oC) (M W) mCp(M W/oC)
1. Reactor 1 feed Cold 20 180 +32.0 0.20
2. Reactor 1 prod Hot 250 40 31.5 0.15
3. Reactor 2 feed Cold 140 230 +27.0 0.30
4. Reactor 2 prod Hot 200 80 30.0 0.25
Chen CL 9
The Problem Table
T 1 2 3 4 Tint
CPH
CPCHint
Surplus/Defict
HotUtility
CascadeSurplus
Add HotUtility
245 250 0 7.5
10 +0.15 +1.5 Surplus H1
235 2 40 230 +1.5 9.0
40 0.15 6.0 Defict H2
195 200 190 200 4.5 3.0
10 +0.10 +1.0 Surplus H3
185 180 190 180 190 3.5 4.0
40 0.10 4.0 Defict H4
145 140 150 140 150 7.5 0
70 +0.20 +14. Surplus H5
75 70 80 80 +6.5 14.
40 0.05 2.0 Defict H635 30 40 +4.5 12.
10 0.20 2.0 Defict H7
25 20 +2.5 10.
CP 0.2 0.15 0.3 0.25 CW
Chen CL 10
The Grand Composite CurveGrand Composite Curveshows the utility requirements both in
enthalpy and temperature terms
interface between the process and the utility system
T Add Hot
Utility
245 7.5
235 9.0
195 3.0
185 4.0
145 0
75 14.
35 12.
25 10.
Chen CL 11
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Chen CL 12
Grand Composite Curve with Alternative Utility
Two levels ofsteams:
240oC, 180oC
Load of180oCsteam
= 175 145
185 145 4
= 3.0MW
Load of240oCsteam
= 7.5 3
= 4.5MW
Chen CL 13
Grand Composite Curve with Alternative Utility
Hot oilwith TS= 280oC,
Cp= 2.1 kJkg1K1
Minimum flow rate:
steepest slope and
min. return temperature
Min.flowrate
= 7.5 103
2.1(280 150)
= 27.5kgs1
Chen CL 14
Alternative Hot UtilitiesSaturated Steams and Hot Oil
Chen CL 15
Alternative Hot UtilitiesFlue Gas
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Chen CL 16
Alternative Hot UtilitiesFlue Gas with Increased Flame Temp
Increasing the theoretical flame temperature by reducing excess airor combustion air pre-heat reduces the stack loss
Chen CL 17
Alternative Hot UtilitiesFlue Gas: Stack Temperature Limited by Acid Dew Point or
Process Away from the Pinch
Chen CL 18
Flue Gas Matched Against the Grand CompositeCurve of the Illustrative Process
The process is to have its hot utility
supplied by a furnace. The theoretical
flame temperature for combustion is
1800oC, and the acid dew point for the flue gas is 160oC. Ambient temperature is
10oC. Assume Tmin= 10oC for process-to-process heat transfer but
Tmin= 30o
C for flue-gas-to-process heat transfer. Calculate the fuel required,stack loss, and furnace efficiency.
Chen CL 19
Solution:
AssigningTmincontributions to streams:The process streams are assigned a contribution of5oCand flue gas a contribution of25oC
Starting point of flue gas:1800oC 1775oC on grand composite curve
The flue gas can be cooled to pinch temperature (T = 145oC) before venting
to atmosphereactual stack temperature = 145 + 25 = 170> 160oC
QHmin = 7.5MW
CPflue gas = 7.5
1775 145 = 0.0046MW/oC
Fuel req. = 0.0046(1800 10) = 8.23MW
Stack loss = 0.0046(170 10) = 0.74MW
Furnace Eff. =
QHmin
Fuel req.
100 =
7.5
8.23
100 = 91%
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Chen CL 20
Combined Heat and Power (Cogeneration)
Heat engine exhaust can be integrated eitheracrossor not across the pinch
The process still requires QHmin and theheat engine performs NO better than operated stand-alone
Chen CL 21
Combined Heat and Power (Cogeneration)
Heat engine exhaust can be integrated either across ornot acrossthe pinch
Net effect is the import of extra energy Wfrom heat source to produce W power
Chen CL 22
Steam Turbine Expansion
Turbine Isentropic Efficiency T =actual work
ideal work =
H1 H
2
H1 H2
Chen CL 23
Steam Turbine Integration
QFUEL= QHP+ QLP+ W+ QLOSS
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Chen CL 24
Gas Turbine IntegrationChen CL 25
Combined Heat and Power Schemes:Example
The stream data for a heat recovery problem are given below. A problem table
analysis for Tmin= 20
o
C is also given. The process also has a requirement for 7MW of power. Two alternative combined heat and power schemes are to becompared economically.
Stream TS TT Heat Cap.
No. Type (oC) (oC) Rate (MW/oC)
1 Hot 450 50 0.25
2 Hot 50 40 1.50
3 Cold 30 400 0.22
4 Cold 30 400 0.025 Cold 120 121 22.0
T (oC) Casc. heat flow (MW)
440 21.90
410 29.40
131 23.82
130 1.80
40 030 15
Chen CL 26
1. A steam turbine with its exhaust saturated at 150oC used for process heating.Superheated steam is generated in the central boilerhouse at 41 bar with atemperature of 300oC. This superheated steam can be expanded in a single-stage turbine with an isentropic efficiency of 85%. Calculate the maximumgeneration of shaftwork possible by matching the exhaust steam against theprocess.
2. A second possible scheme uses a gas turbine with a flow rate of air of97kgs1
which has an exhaust temperature of400oC. Calculate the shaftwork generation
if the turbine has an efficiency of30%. Ambient temperature is 10oC.
3. The cost of heat from fuel for the gas turbine is $4.5GW1. The cost ofimported electricity is $19.2GW1. Electricity can be exported with a value of$14.4GW1. The cost for fuel for steam generation is $3.2GW1. The overallefficiency of steam generation and distribution is 60%. Which scheme is mostcost-effective, the steam turbine or the gas turbine ?
Chen CL 27
Steam Turbine:Heat flow required fromthe turbine exhaust= 21.9MW
Use Steam TableT1 = 300oC, P1 = 41 bar :h1 = 2959kJkg
1,s1 = 6.349kJkg
1K1
T2 = 150oC, P2 = 4.77bar :
s2 = 6.349 kJkg1K1; s= 1.842, sv= 6.838kJkg
1 (saturated entropies)h= 632, hv = 2747 kJkg
1K1 (saturated enthalpies)
s2 = xs+ (1 x)sv 6.349 = 1.842x + 6.838(1 x) x = 0.098
h2 = xh+ (1 x)hv = (0.098)632 + (1 0.098)2747 = 2540 kJ kg1
h
2 = h1 T(h1 h2) = 2959 0.85(2959 2540) = 2603kJ kg1 (isen. eff=85
h
2 = xh+ (1 x)hv 2603 = 632x + 2747(1 x) x = 0.068
Steam flowto process =
21.9103
2747632= 10.35 kg/s Steam flow
thr. turbine = 10.35
10.068 = 11.13kg/s
W = 11.13(2959 2603) 103 = 3.96 MW (Shaftwork generated)
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Chen CL 28
Gas Turbine:CP of exhause CP of airflow
Cp for air= 1.03kJ/kg/K
CPEXHAUST = 97 1.03 = 100 kW/K
QEXHAUST = (400 10) 100 103 = 39 MW
QFUEL = 39
0.7 = 55.71MW
W = 55.71 39 = 16.71MW
Chen CL 29
Steam Turbine Economics:
Cost offuel = (21.9 + 3.96)
3.2103
0.6
= $0.14 s1
Cost ofimported
electricity= (7 3.96) 19.2 103
= $0.06 s1
Net cost = $0.20s1
Gas Turbine Economics:
Cost offuel = 55.71 4.5 10
3
= $0.25s1
Electri-city
credit= (16.71 7) 14.4 103
= $0.14s1
Net cost = $0.11 s1
Chen CL 30
Combined Heat and Power Schemes:Example
The problem table cascade fo a process is given forTmin = 10oC. It is proposed to provide processcooling by steam generation from boiler feedwaterwith a temperature of100oC.
1. Determine how much steam can be generated at asaturation temperature of230oC.
2. Determine how much steam can be generated at asaturation temperature of 230oC and superheatedto the maximum temperature possible against theprocess.
3. Calculate how much power can be generated fromthe superheated steam from Part (2), assuming a
single-stage condensing steam turbine is to be usedwith an isentropic efficiency of85%. Cooling wateris available at 20oC and is returned to the coolingtower at 30oC.
Interval T Heat flow(oC) (MW)
495 3.6
455 9.2415 10.8
305 4.2
285 0.0
215 16.8
195 17.6
185 16.6
125 16.6
95 21.1
85 18.1
Chen CL 31
Solution (1):Heat available fr steamgeneration at 235oC intervaltemperature is 12.0MWLatent heat of water a sat. temp.of235oC is 1812 kJ kg1K1
SteamProd. =
12.0 103
1812 = 6.62 kg
s1
Taking the heat capacity of water to be 4.3 kJ kg1K1, heat duty on boilerfeedwater preheating
= 6.62 4.3 103(230 100) = 3.70 MW
The process can support both boiler feedwater preheat and steam generation.
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Chen CL 32
Solution (2,3):Maximum superheat temp.:
= 285oC interval= 280oC actual
Heat available for steamgeneration at 235oC interval
temperature is 12.0 MW.From steam table, enthalpy ofsuperheated steam at 280oC and28bar is 2947kJ kg1,
and enthalpy of saturated water at 230oC and28 bar is 991 kJ kg1
SteamProd. =
12.0 103
2947 991= 6.13kg s1
The lowest condensing temp is cooling water temp plus Tmin= 30 + 10 = 40oC.From steam table, inlet condition at T1= 280oC and P1= 28 bar are:
H1= 2947 kJ kg1; S1= 6.488kJ kg
1K1
Chen CL 33
Turbine outlet conditions for isentropic expansion to 40oC from steam tables are:P2= 0.074 bar. For S2= 6.488kJ kg
1K1, the wetness fraction (X) andoutlet enthalpy H2 can be calculated (?)
X= 0.23, H2 = 2020kJ kg1
For a single-stage expansion with isentropic efficiency of85%:
H2 = 2947 0.85(2947 2020) = 2159kJ kg1
The power generation (W) is given by
W = 6.13(2947 2159) 103 = 4.8 MW
The wetness fraction for the real expansion is given by
H2= 2159 = XH+ (1 X)Hv
= 167.5X + 2574(1 X)
X = 0.17
Chen CL 34
Heat Pump and Power Cycle
Chen CL 35
Integration of Heat PumpsSchematic of a simple vapor compression heat pump
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Chen CL 36
Integration of Heat Pumps
Schematic of a simple vapor compression heat pump (again)
Chen CL 37
Integration of Heat Pumps
Schematic of a simple vapor compression heat pump (again)
Chen CL 38
Integration of Heat PumpsSchematic of a simple vapor compression heat pump (again)
Chen CL 39
Integration of Heat PumpsSchematic of a simple vapor compression heat pump (again)
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Chen CL 40
Integration of Heat Pumps
Schematic of a simple vapor compression heat pump (again)
Chen CL 41
Integration of Heat Pumps
Integration of a heat pump abovethe pinch
The system converts power into heat !!
Chen CL 42
Integration of Heat PumpsIntegration of a heat pump belowthe pinch
Power is turned into waste heat !!
Chen CL 43
Integration of Heat PumpsIntegration of a heat pumpacrossthe pinch
Heat is pumped from a heat source part to a heat sink part
Coefficientof
PerformanceCOPHP =
QHP+ W
W
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Chen CL 44
Integration of Heat Pumps
The grand composite curve
Chen CL 45
Integration of Heat Pumps
The grand composite curve allows heat pump cycles to be sized
A temperature lift greater than 25oC is rarely economic
Chen CL 46
Integration of Heat PumpsLittle scope for heat pumpingacross process pinch
Chen CL 47
Integration of Heat PumpsHeat pump placedacross a utility pinch
Ch CL 48 Ch CL 49
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Chen CL 48
The Grand Composite Curveallows selection of utility mix
for individual processes
Chen CL 49
Thank You for Your Attention
Questions Are Welcome