11_heat Integration
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CHAPTER NO. 11
Heat Integration
HEAT INTEGRATION 11.1 Introduction: While oil prices continue to climb, energy conservation remains the prime concern for many process industries. The challenge every process engineer is faced with is to seek answers to questions related to their process energy patterns. A few of the frequently asked questions are:
Are the existing processes as energy efficient as they should be?
Are the existing processes as energy efficient as they should be?
What changes can be made to increase the energy efficiency without incurring any cost?
What investments can be made to improve energy efficiency?
What is the most appropriate utility mix for the process?
How to put energy efficiency and other targets like reducing emissions, increasing plant capacities, improve product qualities etc, into a one coherent strategic plan for the overall site?
All of these questions and more can be answered understanding of Pinch Technology.
11.1.1 Pinch Technology: Pinch analysis is a methodology for minimizing energy consumption of chemical processes by calculating thermodynamically feasible energy targets and achieving them by optimizing heat recovery systems, energy supply methods and process operating conditions. It is also known as process integration, heat integration, energy integration or pinch technology. The term ‘Pinch Analysis’ is often used to represent the application of the tools and algorithms of Pinch Technology for studying industrial processes. Developments of rigorous software programs like PinchExpressTM, SuperTargetTM, and Aspen Pinch TM
195
CHAPTER NO. 11
Heat Integration
have proved to be very useful in pinch analysis of complex industrial processes with speed and efficiency. Pinch technology presents a simple methodology for systematically analyzing chemical processes and the surrounding utility systems with the help of the First and Second Laws of Thermodynamics. The First Law of Thermodynamics provides the energy equation for calculating the enthalpy changes (H) in the streams passing through a heat exchanger. The Second Law determines the direction of heat flow. That is, heat energy may only flow in the direction of hot to cold. This prohibits ‘temperature crossovers’ of the hot and cold stream profiles through the exchanger unit. In a heat exchanger unit neither a hot stream can be cooled below cold stream supply temperature nor can a cold stream be heated to a temperature more than the supply temperature of hot stream. In practice the hot stream can only be cooled to a temperature defined by the ‘temperature approach’ of the heat exchanger. The temperature approach is the minimum allowable temperature difference (DTmin) in the stream temperature profiles, for the heat exchanger unit. The temperature level at which DTmin is observed in the process is referred to as “pinch point” or “pinch condition”. The pinch defines the minimum driving force allowed in the exchanger unit.
11.1.2 Objectives of Pinch Analysis: Pinch Analysis is used to identify energy cost and heat exchanger network (HEN) capital cost targets for a process and recognizing the pinch point. The procedure first predicts, ahead of design, the minimum requirements of external energy, network area, and the number of units for a given process at the pinch point. Next a heat exchanger network design that satisfies these targets is synthesized. Finally the network is optimized by comparing energy cost and the capital cost of the network so that the total annual cost is minimized. Thus, the prime objective of pinch analysis is to achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads). The concept of process heat integration is illustrated in the example discussed below.
196
CHAPTER NO. 11
Heat Integration
197
Heat Integration
CHAPTER NO. 11
11.2 Heat Integration of the Process: Initial data for the process streams is provided in the table: Source Temperature (oC) Streams to be heater 2 30 4 117 17 34 28 105 Stream Number
Streams to be cooled 8 400 11 76.3 Salt Stream 360 32 136 36 202
Target Temperature (oC)
Enthalpy Change rate (kJ/sec)
110 380 95 161 Total = 6232
399 5270 310 253
76.3 60 340 30 30 Total = -16000
-7170 -450 -7667 -154 -558
Initial Temperature Interval table: Assuming an Enthalpy change rate of 11,000 kJ/sec at 30 oC for stream to be heated and 17,000kJ.sec at 400 oC for stream to be cooled as a baseline enthalpy value. Initial temperature Interval Table:
Source Temperature (oC) Streams to be Heated 2 30 2 &17 34 2 95 2 & 28 105 28 110 5 & 28 117 5 161 Stream Number
Streams to be cooled 8 400 8 & salt steam 360 8 340 8 & 36 202
Target Temperature (oC)
Enthalpy Change rate (kJ/sec)
Initial Enthalpy Selection
34 95 105 110 117 161 380 Total = 6232
5 397 310 35 50 1025 4410
Enthalpy 11000 11005 11402 11712 11747 11797 12822 17232
Temp 30 34 95 105 110 117 161 380
360 340 202 136
-894 -8118 -3020 -1790
17000 16106 7988 4968
400 360 340 202 198
Heat Integration
CHAPTER NO. 11
8, 36 & 32 11, 36 & 32 36 & 32
136 76 60
76 -1645 60 -480 30 -53 Total = -16000
3178 1533 1053 1000
136 76 60 30
199
Heat Integration
CHAPTER NO. 11
Composite Diagram: On the basis of previous initial temperature interval data composite diagram can be drawn as:
Initial Composite diagram
450 400 Temperature oC
350 300 250
Stream to be heated Stream to be cooled
200 150 100 50 0
0
5000
10000
15000
20000
Enthalpy Rate kJ/sec
Fig 11.1: composite diagram with 28oC approach temperature
Where, ΔTmin = 28 oC Closest vertical approach of two curves occurs at enthalpy change rate of 16800 kJ/sec that is the pinch point. Temperature of streams to be cooled at pinch point = 358 oC Temperature of streams to be heated at pinch point = 330 oC So,
ΔTmin at pinch point = 28 oC
Allowable ΔTmin for chemical processes = 3 - 10 oC To achieve a temperature approach of 10oC one of the curves must be moved horizontally to bring the two curves closer together. One way to do this is to move the curve representing the stream to be cooled to the right. In fig. 11.1 the slope of that portion of the curve at pinch point is obtained from the data. Slop of the curve at pinch point = 0.0475 Intercept of the curve = - 440 200
Heat Integration
CHAPTER NO. 11
Modified enthalpy change rate at 358 oC = 16800 kJ/sec This value of 16800 must be increased to 17000 to make the temperature approach at the pinch point to 10oC. Therefore 200 kJ/hr must be added to every enthalpy change rate value associated with the streams to be heated. Hence, Difference of baseline enthalpy change rates = 200 kJ/sec So, Revised baseline enthalpy change rate for streams to be cooled = 17200 kJ/sec Revised temperature interval table:
Source Stream Number Temperature (oC) Streams to be Heated 2 30 2 &17 34 2 95 2 & 28 105 28 110 5 & 28 117 5 161 Streams to be cooled 8 400 8 & salt steam 360 8 340 8 & 36 202 8, 36 & 32 136 11, 36 & 32 76 36 & 32 60
Target Temperature (oC)
Enthalpy Change rate (kJ/sec)
Initial Enthalpy Selection
34 95 105 110 117 161 380 Total = 6232
5 397 310 35 50 1025 4410
Enthalpy 11000 11005 11402 11712 11747 11797 12822 17232
Temp 30 34 95 105 110 117 161 380
360 340 202 136 76 60 30 Total = -16000
-894 -8118 -3020 -1790 -1645 -480 -53
17200 16306 8188 5168 3378 1733 1253 1200
400 360 340 202 136 76 60 30
201
Heat Integration
CHAPTER NO. 11
Revised Composite diagram
450 400 350 Temperature oC
300 250 Stream to be heated Stream to be cooled
200 150 100 50 0
0
20
00
40
0 0 0 0 0 0 00 00 00 00 00 00 10 12 1 4 1 6 18 2 0 Enthalpy Rate kJ/sec
00
60
00
80
00
Fig 11.2: Composite diagram with 10oC approach temperature
11.3 Conclusion: Revised Composite diagram 450 400
Temperature oC
350 300 250
Stream to be heated Stream to be cooled
200 150 100 50 0
0
20
00
40
00
60
00
80
00
0 10
00
0 12
00
0 14
00
0 16
00
0 18
00
0 20
00
Enthalpy Rate kJ/sec Overall heating utility required = 17232 – 17200 = 32 kJ/sec above 380 oC Overall cooling utility required = 11000 – 1200 = 9800 kJ/sec below 30 oC
202
Heat Integration
HEAT INTEGRATION 11.1 Introduction: While oil prices continue to climb, energy conservation remains the prime concern for many process industries. The challenge every process engineer is faced with is to seek answers to questions related to their process energy patterns. A few of the frequently asked questions are:
Are the existing processes as energy efficient as they should be?
Are the existing processes as energy efficient as they should be?
What changes can be made to increase the energy efficiency without incurring any cost?
What investments can be made to improve energy efficiency?
What is the most appropriate utility mix for the process?
How to put energy efficiency and other targets like reducing emissions, increasing plant capacities, improve product qualities etc, into a one coherent strategic plan for the overall site?
All of these questions and more can be answered understanding of Pinch Technology.
11.1.1 Pinch Technology: Pinch analysis is a methodology for minimizing energy consumption of chemical processes by calculating thermodynamically feasible energy targets and achieving them by optimizing heat recovery systems, energy supply methods and process operating conditions. It is also known as process integration, heat integration, energy integration or pinch technology. The term ‘Pinch Analysis’ is often used to represent the application of the tools and algorithms of Pinch Technology for studying industrial processes. Developments of rigorous software programs like PinchExpressTM, SuperTargetTM, and Aspen Pinch TM
195
CHAPTER NO. 11
Heat Integration
have proved to be very useful in pinch analysis of complex industrial processes with speed and efficiency. Pinch technology presents a simple methodology for systematically analyzing chemical processes and the surrounding utility systems with the help of the First and Second Laws of Thermodynamics. The First Law of Thermodynamics provides the energy equation for calculating the enthalpy changes (H) in the streams passing through a heat exchanger. The Second Law determines the direction of heat flow. That is, heat energy may only flow in the direction of hot to cold. This prohibits ‘temperature crossovers’ of the hot and cold stream profiles through the exchanger unit. In a heat exchanger unit neither a hot stream can be cooled below cold stream supply temperature nor can a cold stream be heated to a temperature more than the supply temperature of hot stream. In practice the hot stream can only be cooled to a temperature defined by the ‘temperature approach’ of the heat exchanger. The temperature approach is the minimum allowable temperature difference (DTmin) in the stream temperature profiles, for the heat exchanger unit. The temperature level at which DTmin is observed in the process is referred to as “pinch point” or “pinch condition”. The pinch defines the minimum driving force allowed in the exchanger unit.
11.1.2 Objectives of Pinch Analysis: Pinch Analysis is used to identify energy cost and heat exchanger network (HEN) capital cost targets for a process and recognizing the pinch point. The procedure first predicts, ahead of design, the minimum requirements of external energy, network area, and the number of units for a given process at the pinch point. Next a heat exchanger network design that satisfies these targets is synthesized. Finally the network is optimized by comparing energy cost and the capital cost of the network so that the total annual cost is minimized. Thus, the prime objective of pinch analysis is to achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads). The concept of process heat integration is illustrated in the example discussed below.
196
CHAPTER NO. 11
Heat Integration
197
Heat Integration
CHAPTER NO. 11
11.2 Heat Integration of the Process: Initial data for the process streams is provided in the table: Source Temperature (oC) Streams to be heater 2 30 4 117 17 34 28 105 Stream Number
Streams to be cooled 8 400 11 76.3 Salt Stream 360 32 136 36 202
Target Temperature (oC)
Enthalpy Change rate (kJ/sec)
110 380 95 161 Total = 6232
399 5270 310 253
76.3 60 340 30 30 Total = -16000
-7170 -450 -7667 -154 -558
Initial Temperature Interval table: Assuming an Enthalpy change rate of 11,000 kJ/sec at 30 oC for stream to be heated and 17,000kJ.sec at 400 oC for stream to be cooled as a baseline enthalpy value. Initial temperature Interval Table:
Source Temperature (oC) Streams to be Heated 2 30 2 &17 34 2 95 2 & 28 105 28 110 5 & 28 117 5 161 Stream Number
Streams to be cooled 8 400 8 & salt steam 360 8 340 8 & 36 202
Target Temperature (oC)
Enthalpy Change rate (kJ/sec)
Initial Enthalpy Selection
34 95 105 110 117 161 380 Total = 6232
5 397 310 35 50 1025 4410
Enthalpy 11000 11005 11402 11712 11747 11797 12822 17232
Temp 30 34 95 105 110 117 161 380
360 340 202 136
-894 -8118 -3020 -1790
17000 16106 7988 4968
400 360 340 202 198
Heat Integration
CHAPTER NO. 11
8, 36 & 32 11, 36 & 32 36 & 32
136 76 60
76 -1645 60 -480 30 -53 Total = -16000
3178 1533 1053 1000
136 76 60 30
199
Heat Integration
CHAPTER NO. 11
Composite Diagram: On the basis of previous initial temperature interval data composite diagram can be drawn as:
Initial Composite diagram
450 400 Temperature oC
350 300 250
Stream to be heated Stream to be cooled
200 150 100 50 0
0
5000
10000
15000
20000
Enthalpy Rate kJ/sec
Fig 11.1: composite diagram with 28oC approach temperature
Where, ΔTmin = 28 oC Closest vertical approach of two curves occurs at enthalpy change rate of 16800 kJ/sec that is the pinch point. Temperature of streams to be cooled at pinch point = 358 oC Temperature of streams to be heated at pinch point = 330 oC So,
ΔTmin at pinch point = 28 oC
Allowable ΔTmin for chemical processes = 3 - 10 oC To achieve a temperature approach of 10oC one of the curves must be moved horizontally to bring the two curves closer together. One way to do this is to move the curve representing the stream to be cooled to the right. In fig. 11.1 the slope of that portion of the curve at pinch point is obtained from the data. Slop of the curve at pinch point = 0.0475 Intercept of the curve = - 440 200
Heat Integration
CHAPTER NO. 11
Modified enthalpy change rate at 358 oC = 16800 kJ/sec This value of 16800 must be increased to 17000 to make the temperature approach at the pinch point to 10oC. Therefore 200 kJ/hr must be added to every enthalpy change rate value associated with the streams to be heated. Hence, Difference of baseline enthalpy change rates = 200 kJ/sec So, Revised baseline enthalpy change rate for streams to be cooled = 17200 kJ/sec Revised temperature interval table:
Source Stream Number Temperature (oC) Streams to be Heated 2 30 2 &17 34 2 95 2 & 28 105 28 110 5 & 28 117 5 161 Streams to be cooled 8 400 8 & salt steam 360 8 340 8 & 36 202 8, 36 & 32 136 11, 36 & 32 76 36 & 32 60
Target Temperature (oC)
Enthalpy Change rate (kJ/sec)
Initial Enthalpy Selection
34 95 105 110 117 161 380 Total = 6232
5 397 310 35 50 1025 4410
Enthalpy 11000 11005 11402 11712 11747 11797 12822 17232
Temp 30 34 95 105 110 117 161 380
360 340 202 136 76 60 30 Total = -16000
-894 -8118 -3020 -1790 -1645 -480 -53
17200 16306 8188 5168 3378 1733 1253 1200
400 360 340 202 136 76 60 30
201
Heat Integration
CHAPTER NO. 11
Revised Composite diagram
450 400 350 Temperature oC
300 250 Stream to be heated Stream to be cooled
200 150 100 50 0
0
20
00
40
0 0 0 0 0 0 00 00 00 00 00 00 10 12 1 4 1 6 18 2 0 Enthalpy Rate kJ/sec
00
60
00
80
00
Fig 11.2: Composite diagram with 10oC approach temperature
11.3 Conclusion: Revised Composite diagram 450 400
Temperature oC
350 300 250
Stream to be heated Stream to be cooled
200 150 100 50 0
0
20
00
40
00
60
00
80
00
0 10
00
0 12
00
0 14
00
0 16
00
0 18
00
0 20
00
Enthalpy Rate kJ/sec Overall heating utility required = 17232 – 17200 = 32 kJ/sec above 380 oC Overall cooling utility required = 11000 – 1200 = 9800 kJ/sec below 30 oC
202
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