pinch technology

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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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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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.


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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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.


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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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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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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pinch technology

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