pinch technology
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Module 1 : Classical Thermodynamics
A method of using the overall thermal balance of a process
Theoretically predicts minimum energy consumption
Predicts the construction costs of the plant using a heat recovery system
First put into practical use in 1984, at Linhoff March Co.
Pinch point technology, or process integration, is the name given for a technique
developed by Prof. Linnhof and co-workers (1978) at Leeds University, UK to optimize
the heat recovery in large complex plants with several hot and cold streams of fluids.
To illustrate the basic principle take a case of a plant with two hot and two cold
streams, as shown in Table 1.9.
Table 1.9 Data for 4 (four) fluid streams
The hot streams can be combined into an equivalent composite stream as follows:
From Table 1.9, it is clear that both stream 1 and 2 are having common temperature
drop between 170°C to 70°C. For the common processes, we go for process
integration (heat recovery) by considering composite thermal capacity.
The hot composite curve will consists of the following
1. Stream1 from 200°C to170°C with heat capacity 1.98 kW/K,
2. Stream (1+2) between temperature 170°C to 70°C, a combined stream of thermal
capacity rate (2.2+3.9) = 6.1 kW/K
3. Stream2 between temperature 70°C to 50°C, stream 1 with heat capacity rate 1.98
kW/K.
To plot the composite heating curve the calculations can be estimated as shown in
Table 1.10.
Table 1.10 Composite Heating Curve
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Module 1 : Classical Thermodynamics
Lecture 11 : Pinch Point Technology
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Similarly, the cold streams can be combined.
The cold composite curve will consists of
1. Stream3 from 40°C to 100°C with heat capacity 2.8 kW/K,
2. Stream (3+4) between temperature 100°C to 150°C, a combined stream of thermal
capacity rate (2.8+5.12) = 7.92 kW/K
3. Stream4 between temperatures 70°C to 50°C, stream 1 with heat capacity rate 1.98
kW/K.
To plot the composite cooling curve the calculations can be estimated as shown in
Table 1.11.
Table 1.11 Composite Cooling Curve
The two composite streams are then plotted on a temperature heat load graph. The
temperature and rate of change of enthalpy for cold stream and hot stream are
estimated in Table 1.12.
At temp 50°C the rate of change of enthalpy = Heat capacity rate × Δt
= 2.2 × 50
= 110 Kw
Similarly, at temp 40°C the rate of change of enthalpy = Heat capacity rate × Δt
= 2.8 × 40
= 112 Kw
Similar calculations are done for the selected data.
Table 1.12 Estimation of hot and cold steam
The two composite streams are then plotted on a temperature heat load graph as
shown in Fig. 1.34.
Fig 1.34 Temperature verses rate of change enthalpy change for composite
hot and cold streams
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Module 1 : Classical Thermodynamics
Lecture 11 : Pinch Point Technology
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The pinch point is defined as the point where temperature difference
between the two composite curves is minimum.
The temperature difference at the pinch point depends on the design of the heat
exchanger. Smaller the temperature difference, the more expensive is the heat
exchanger. A high value of pinch point indicates high thermal losses due to external
irreversibility.
Example 1: Pinch point temperature of 7°C
Pinch point temperature of 7°C, then the cold stream (composite) can be moved from
left to right on the diagram horizontally, keeping the hot composite curve fixed, until
the temperature difference at pinch point is 7°C. It is then seen from Fig 1.35 that the
external heating load of 30 kW and external cooling load of 102kW are required for
the system, all other energy changes can be achieved by the heat exchangers
between the various streams.
Fig 1.35 Temperature verses rate of change enthalpy change for composite
hot and cold streams for 7°C Pinch
Mathematically to obtain the pinch point at 7°C the values of cold steams are
increased by 100KW, keeping the values of hot streams constant. Required estimation
of hot and cold streams for 7°C pinch point is given in Table. 1.13.
Table 1.13 Estimation of hot and cold streams for 7°C pinch point
The mathematically the cooling and heating above and below 7°C Pinch point is
mentioned in Table 1.14.
Table 1.14 Cooling and heating above and below 7°C pinch point
From Table 1.14, and Fig 1.35 we can infer that for a pinch point of 7°C, we require
external heating load of 29.7kW and cooling load of 101.7kW above and below the
pinch point respectively.
The process integration is now as follows:
Above pinch point, stream 1 and 3 exchanges 204.6 kW heat, stream 2 and stream 4
exchanges 245.7 kW. Below pinch point, stream 2 and stream 3 exchange 168 kW
heat. Stream 3 and 4 are to be externally heated with (-224+204.6 =-19.4 kW) and (-
256+245.7 =-10.3 kW) respectively to meet the deficit /demand of 29.7 kW. Similarly,
stream 1 has to be cooled externally with 125.4 kW and 3 has to be heated externally
with (144.3-168 =-23.7 kW) heat exchangers respectively to meet the demand of
101.7 kW.
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Module 1 : Classical Thermodynamics
Lecture 11 : Pinch Point Technology
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Possible processes are shown in Fig. 1.36.
Fig 1.36 Possible plant to heat and cool four fluid streams for a minimum 7°C
temperature difference.
Example 2: Pinch point temperature of 23.5°C
Similar to example1 the cold stream (composite) is moved from left to right on the
diagram (Fig 1.37) horizontally, keeping the hot composite curve fixed, until the
temperature difference at pinch point is 23.5°C. It is then seen that the external
heating load of 130 kW and external cooling load of 202kW are required for the
system, all other energy changes can be achieved by the heat exchangers between
the various streams.
Fig 1.37 Temperature verses rate of change enthalpy change for composite
hot and cold streams for Pinch point temperature of 23.5°C
Mathematically to obtain the pinch point at 23.5°C the values of cold steams are
increased by 200KW, keeping the values of hot streams constant. Required estimation
of hot and cold streams for 23.5°C pinch point is given in Table. 1.15.
Table 1.15 Estimation of hot and cold streams for 23.5°C pinch point
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Module 1 : Classical Thermodynamics
Lecture 11 : Pinch Point Technology
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Different heat loads are shown in Table 1.16
Table 1.16 Cooling and heating above and below 23.5°C pinch point.
From Table 1.16, mathematically with reference to the Fig 1.37 we can infer that for a
pinch point of 23.5°C, we require external heating load of 130.35kW and cooling load
of 202.35kW above and below the pinch point respectively.
The process integration is now as follows:
Above pinch point, stream 1 and 3 exchanges 168.3 kW heat, stream 2 and stream 4
exchanges 181.35kw. Below pinch point, stream 2 and stream 3 exchange 168 kW
heat. Stream 3 and 4 are to be externally heated with (224-168.3 = 55.7 kW) and
(256-181.35 = 74.65 kW) respectively to meet the deficit /demand of 130.35 kW.
Similarly, stream 1 and 3 are to be cooled externally with 161.7 kW and (208.65-168
= 40.65 kW) heat exchangers respectively to meet the demand of 202.35 kW.
Possible processes are shown in Fig. 1.38.
Fig 1.38 Possible plant to heat and cool four fluid streams for a minimum
23.5°C temperature difference.
The following rules should be followed in process integration
1. Do not transfer heat from one fluid to another across the pinch point
2. No external heating below pinch point
3. No external cooling above the pinch point
4. A heat exchanger should operate on one side of the pinch, either taking a heat supply
from below the pinch, or rejecting heat to a fluid above the pinch
5. A heat pump should operate across the pinch from a cold stream below the pinch to a
hot stream above the pinch.
Summary:
1. Exergy is the maximum work potential of a system
2. Exergy transfer with heat, work and mass
3. For an isolated system exergy always decreases
4. Exergy remains constant in a reversible process
Anything that generate entropy is responsible for decrease of exergy
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Module 1 : Classical Thermodynamics
Lecture 11 : Pinch Point Technology
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Quiz 11
1. What is a pinch point?
2. What happens if the pinch point is small?
3. How do you draw a composite curve?
4. What are the rules applied for process integration?
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