chapter 3 heat integration

8/17/2019 Chapter 3 Heat Integration

http://slidepdf.com/reader/full/chapter-3-heat-integration 1/18

3-1

CHAPTER 3

PROCESS INTEGRATION

3.1 Introduction

Process integration can lead to a substantial reduction in the energy requirements of

a process. In recent years much work has been done on developing methods for

investigating energy integration and the efficient design of heat exchanger networks;

Gundersen and Naess (1988). Pinch technology is one of the most successful and

generally useful techniques that developed by Bodo Linnhoff and others workers.

The term derives from the fact that in a plot of the system temperatures

versus the heat transferred, a pinch usually occurs between the hot stream and cold

stream curves. It has been shown that the pinch represents a distinct

thermodynamic break in the system and that, for minimum energy requirements,

heat should not be transferred across the pinch, Linnhoff and Townsend (1982).

In this section the fundamental principles of the pinch technology method for

energy integration will be outlined and illustrated with reference to a simple problem.

The method and its applications are described fully in a guide published by the

Institution of Chemical Engineers, ICheme (1994). (Sinnott, 2006)

3.2 Pinch Technology

The development and application of the method can be illustrated by considering the

problem of integrating the utilization of energy between hot and cold stream. Each

stream starts from a source temperature Ts and is to be heated or cooled to a target

temperature Tt. The heat capacity of each stream is shown as CP (Sinnott, 2006).

For stream where the specific heat capacity can be taken as constant and there is

no phase change, CP will be given by:

= ṁ



3-2

ṁ = mass flow rate (kg/s)

Cp = average specific heat between stream supply and target

Table 3.1: Stream data for heat integration problem (Hysys)

T actual

streamstream

typesupply T (°C) target T (°C)

Heat

capacity

flowrate

(kW/°C)

Heat Load,

∆H, kW

Product hot 117.2000 20.0000 8.4529 821.62107

DC 2 hot 76.9118 30.6300 1.4672 67.9043414

Condenser hot 83.4500 40.0000 1.7262 75.0019296

DC 1 cold 108.7000 120.2000 10.8761 125.074732

Reactor 2 cold 110.0000 120.0000 11.6552 116.552111

Reactor 1 cold 89.5300 110.0000 15.4710 316.69137

Figure 3.1: Hot streams plotted separately



3-3

Figure 3.2: The composite hot stream

Figure 3.3: Cold stream plotted separately



3-4

Figure 3.4: Composite cold stream

Figure 3.5: The hot and cold composite curve plotted together at ∆Tmin =10°C

Minimum cooling

requirement

Minimum heating requirement



3-5

Figure 3.6: The grand composite curve

Minimum temperature difference is the driving force for heat transfer. ∆Tmin must be

determined for the energy recovery system of plant. The optimum value of ∆T min isvery significant because ∆Tmin will determine the size of the heat exchanger in a

network.

Table 3.2: Values of ∆Tmin for the respective industries (Cheresource.com, 2011)

Industries Optimum, ∆Tmin

Oil refining 20-40°C

Petrochemical and Chemical 10-20°C

Low temperature processes 3-5°C

2-EHA plant is categorised under chemical industries. Therefore, in our plant, we

assume ∆Tmin = 10°C.

3.3 The Problem Table Method

Linnhoff and Flower give the name as problem table to a numerical method for

determining the pinch temperatures and the minimum utility requirements. (Sinnott,

2006)

The procedure is as follows:

1) By converting the actual stream temperature Tact into interval temperature

Tint by subtracting half the minimum temperature differences from the hot

stream temperatures, and by adding half to the cold stream temperatures:

ℎ = − ∆

2

= + ∆2

Tpinch=94.53°C



3-6

The use of the interval temperature rather than the actual temperatures

allows the minimum temperatures differences to be taken into account ∆Tmin = 10°C

for the problem being considered.

Table 3.3: Interval temperature

T actual T interval

Streamstream

typesupply T (°C) target T (°C)

Supply T

(°C)

target T

(°C)

Product Hot 117.20 20.00 112.20 15.00

DC 2 Hot 76.91 30.63 71.91 25.63

Condenser Hot 83.45 40.00 78.45 35.00

DC 1 Cold 108.70 120.20 113.70 125.20

Reactor 2 Cold 110.00 120.00 115.00 125.00

Reactor 1 Cold 89.53 110.00 94.53 115.00

2) Note any duplicate interval temperatures.

3) Rank the interval temperature in order of magnitude, showing the duplicated

temperature only once in the order.



3-7

Figure 3.7: Rank the interval temperature in order of magnitude

4) Carry out a heat balance for the stream falling within each temperature

interval:

For the nth interval:

∆ = − ℎ ∆ Where:

∆ = ℎ ℎ ℎ = ℎ ℎ ℎ

ℎ=

ℎ

ℎ

ℎ

ℎ

∆ = = −1 −



3-8

Figure 3.8: Rank the interval temperature with heat balance

Table 3.4: Problem table

Rank

interval

temperature,

°C

∆Tn, °C

− ℎ ,

/°

∆H, kW Surplus

or deficit

Heat

cascade

Correction

125.2 Nil Nil 0 0 408.9556

125 0.2 10.8761 2.1752 D -2.1752 406.7804

115 10 22.5313 225.3127 D -227.4880 181.4677

113.7 1.3 26.3471 34.2512 D -261.7391 147.2165

112.2 1.5 15.4710 23.2065 D -284.9456 124.0100



3-9

94.53 17.67 7.0181 124.0100 D -408.9556 0.0000

76.45 16.08 -8.4529 -135.9225 S -273.0331 135.9225

71.9118 6.5382 -10.1791 -66.5525 S -206.4806 202.4750

35 36.9118 -11.6463 -429.8843 S 223.4037 632.3593

25.63 9.37 -9.9201 -92.9512 S 316.3549 725.3105

15 10.63 -8.4529 -89.8542 S 406.2091 815.1647

5) “Cascade” the heat surplus from one interval to the next down the column

of interval temperatures.

Cascading the heat from one interval to the next implies that the temperature

difference is such that the heat can be transferred between the hot and cold

streams. The presence value in the column indicates that the temperature gradient

is in the wrong direction and that the exchange is not thermodynamically possible.

This difficulty can be overcome if heat is introduced into the top of the cascade.

Below is the figure of cascade which Cascade 1 on the right hand side and for

Cascade 2 on the left hand side.



3-10

Figure 3.9: Heat cascade before and after

6) Introduce just enough heat to the top of the cascade to eliminate all

the negative values.



3-11

Figure 3.10: Flow of heat cascade

∴Therefore, the pinch temperature is 89.53°C and 99.53°C.

3.4 The Heat Exchanger Network

3.4.1 Network design for maximum energy recovery



3-12

The analysis carried out in figure 3.10 has shown that the minimum utility

requirements for the problem set out in table 3.4 are 408.3556 kW of the hot and

730.6358 kW of the cold utility and the pinch occurs where the cold streams are at

89.53°C and the hot 99.53°C.

PINCH

Above pinch Below pinch

117.2°C99.53 °C 89.53 °C

20 °C

30.83 °C

40 °C

76.91 °C

83.45 °C

89.53 °C

H (kW)

149.3626 kW 672.2585 kW

67.9043 kW

75.0019 kW

108.27°C120.2 °C

129.7514 kW

110 °C316.6914 kW

120 °C110 °C

116.5524 kW

Figure 3.11: Grid for streams problem

3.4.1.1 The network design above the pinch

ℎ ≤

Stream 1:

∆ = − = 8.4529 117.2 − 99.53 = 149.3626

Stream 2:

∆ = − = 10.8761 120.2 − 108.27 = 129.7514

Stream 3:

∆ = − = 11.6552 120 − 110 = 116.5521

Stream 4:

∆ = − = 15.4710 110 − 89.53 = 316.6914

3.4.1.2 The network design below the pinch

ℎ ≥

Stream 5:

∆ = − = 8.4529 89.53 − 20 = 672.2585

Stream 6:

∆ = − = 1.4672 76.9118 − 30.63 = 67.9043

Stream 7:

∆=

−

= 1.7262 83.45

−40 = 75.0019



3-13

3.5 Minimum Number of Exchanger

The network shown in figure 9.11 was designed to give the maximum heat recovery,

and therefore give the minimum consumption and cost of the hot and cold utilities.

Heat would cross the pinch and the consumption of the utilities would be increased.

(Sinnott, 2006)

Whether the revised network would be better, more economic, would depend

on the relatives cost of capital and utilities. For any network there will be an optimum

design that gives the least annual cost; capital charges plus utility and other

operating costs.

After heat integration, according to the figure 9.11 there are 2 heat

exchangers, 3 coolers and 2 heaters.

PINCH=94.53°C

Above pinch Below pinch

117.2°C 99.53 °C 89.53 °C

20 °C

30.83 °C

40 °C

76.91 °C

83.45 °C

672.2585 kW

67.9043 kW

75.0019 kW

120.2 °C

129.7514 kW

110 °C

116.5524 kW120 °C

E-6

E-7

E-8

heater

heater

cooler

cooler

cooler

E-11

316.6914 kW

149.3626 kW

E-1

E-2

DC 1

R2

R1

CONDENSER

PRODUCT

DC 2

Figure 3.12: Available stream heat



3-14

Figure 3.13: Heat Exchanger Network design

3.6 Difference Between Before and After Heat Integration.

Table 3.5: Difference between Before and After Heat Integration

before heat integration after heat integration

number of heat exchanger 0 1

number of heater 3 2

number of cooler 3 3

hot utility 2373.6 kW 409.1 kW

cold utility 1259.3 kW 815.2 kW

total utility 3632.9 kW 1224.3 kW

Therefore, percentage of energy saving after heat integration:

=3632.9 − 1224.3

3632.9 × 100% = 66.3 %



3-15

3.7 Conclusion

There are energy saving by 66.3%, but it is decided not to proceed with our

plant since it is difficult to integrate by using the product stream since the product

will tend to polymerize along the tube which will result in pressure drop. Shut down

procedure of the plant also need to be conducted frequently due to the

polymerization problem. It is also required to add-up more inhibitor in order to

prevent polymerization to occur in which will result in the add-up to cost. The

amount of increment should not be wasted; it could be use and spend for

maintenance cost.



3-16

To

Incinerator

R-101

T-102 T-103

59

12

11

7

4

16

17

18 19

21

13

T-101

R-102

S-101

S-102E-103

3

8

10

15

20

14

1 2

6

E-101

M-101

E-102

E-105

Figure 9.12: PFD before heat integration



3-17

P-5

P-4

ToIncinerator

R-101

T-102 T-103

59

12

11

7

4

16

17

18 19

21

13

T-101

R-102

S-101

S-102E-103

8

10

15

20

14

1

6

M-101

E-102

E-105

E-1

P-2

3

P-3

2

Figure 9.13: PFD after heat integration



3-18

chapter 3 heat integration

Documents