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Energy Targeting Procedure Module‐04 Lecture‐10

Module‐04 : Targeting

Lecture‐10 : Energy Targeting Procedure

Key words: Targeting, HEN, composite curve, Tmin, An important feature of Process Integration is the ability to identify Performance Targets before

the design step is started. Targeting procedure also helps in the evaluation of alternative HEN

designs. For heat recovery systems with a specified value for the minimum allowable approach

temperature (Tmin), targets can be established for:

1. Energy Target( Minimum Energy Consumption through external heating and cooling),

2. Fewest Number of Units (process/process heat exchangers, heaters and coolers) in the

HEN

3. Fewest number of shells in the heat exchanger network(HEN)

4. Minimum Total Heat Transfer Area of the HEN

5. Cost Targeting (Total annual cost of the HEN)

Results obtained from these targets lead the design in right direction and help to search for a

optimum topology.

Energy targeting can be done through Hot and cold composite curves, Grand composite curve and Problem Table Algorithm. The present lecture deals with energy targeting using hot and cold composite curves. To explain the energy targeting procedure using hot and cold composite curves a problem as given in Table 3.8 and reproduced below from lecture ‐07 is considered. The hot and cold composite curves placed together is a plot is taken from Fig.3.25 and reproduced below:

Table 3.8: Four stream problem for load integration and utility prediction for Tmin equal to

10C. Name of the stream Supply Temperature

Ts, C Target Temperature

Tt, C CP

kW/C H kW

Hot‐1 140 50 2 ‐180

Hot‐2 90 40 6 ‐300

Cold‐1 30 150 2 240

Cold‐2 70 125 3 165


Once Hot and cold composite curves are known, one can estimate with ease the minimum amount of external heating ( Hot utility) and external cooling(Cold utility) required for the process through energy targeting procedure. It should be noted that through this process one targets minimum amount of hot and cold utility that the process demands. However, higher amount of hot and cold utilities ( in comparison to minimum value) can be supplied to the process. A discussion in the later part of this lecture will show that one has to pay penalty in terms of energy and capital cost if he decides to use higher amount of utilities than that of minimum amount.

The overlap between the hot and cold composite curves represent the maximum amount of

heat that can be recovered within the process. The “overshoot” of the hot composite

represents the minimum amount of external cooling required in the process and the

“overshoot” of the cold composite represents the minimum amount of external heating

required in the process. This concept is based on vertical heat transfer in the internal heat

exchange area as well as at utility areas as shown in Fig.4.1. Energy targeting is a powerful

process integration concept.

Fig.3.25 Shows both composite curves put together for problem given in Table 3.8

70C

125C

150C 140C

90C

50C 40C 30C

T,C

H, kW

Tmin= 10C

Q HOT, MIN

Q COLD, MIN

Hot Pinch

Cold Pinch

Internal Heat Exchange

Source Section

80C

Sink Section

External Hot utility

External Cold utility

Hot Composite curve

Cold Composite curve

Below the Pinch Above the Pinch

Pinch Points

250 kW

175 kW 230 kW


Because of the kinked nature of the hot and cold composite curves, they approach most closely

at one point which is called the “Pinch”. Pinch is not a point unless both the curves touch each

other at a point. In fact, for heat to flow one has to maintain a value of T at pinch which is called Tmin. Thus at pinch there exists two points out of which one is called Hot pinch point and

the other cold pinch point as shown in Fig.3.25. From Fig.3.25 it can be seen that hot utility

demand is 175 kW , cold utility demand is 250 kW and internal heat exchange is 230 kW. The

hot pinch point is at 90C and cold pinch point is at 80C. This is for TminC equal to 10C. If one changes the Tmin value from 10C to a new value then the requirements of cold and hot

utility will change and so the internal heat exchange. To demonstrate the above fact Tmin value

is changed from 5C to 30 C in the steps of 5C and the results are reported in Table 4.1.

Table 4.1 Values of cold ,hot utility demand and internal heat exchange when Tmin is altered.

Tmin, C Cold Utility, kW Hot Utility, kW Internal Exchange, kW

Total Heat Exchange, kW

5 225 150 255 630

10 250 175 230 655

15 275 200 205 680

20 300 225 180 705

25 310 235 170 715

Fig. 4.1 Vertical heat transfer

70C

125C

150C 140C

50C 40C 30C

T,C

H, kW

Hot Composite curve

Cold Composite curve

Tmin

Hot Utility

10C Cold Utility


30 320 245 160 725

From Table 4.1 it is clear that as the value of Tmin increases, internal heat exchange decreases

and the value of cold and hot utility as well as value of total heat exchange increases. Due to the

increase in the value of external cold and hot utilities the operating cost of the HEN increases.

Further, the decrease in internal heat exchange decreases the scope of energy conservation in

the process. An increase in value of Tmin increases the value of T available to all the exchangers in the HEN.

It is known fact that increases in the value of Tmin decreases the heat transfer area as it

provides higher value of driving force as per the equation given below:

Q = U A T …(4.1)

Where

Q ‐load of exchanger A ‐ Heat transfer area U‐ overall heat transfer coefficient

T – temperature difference available for heat transfer

However, this conclusion is true only when Q and U remain constant when Tmin increases.

From Table 4.1 it is evident that with the increase in Tmin the total heat exchange value(Q) of

the HEN increases. Thus, the benefit of reduction of heat transfer area due to increase in Tmin will be nullify to some extent due to increase in the value of total heat exchange at the same

time. Thus Tmin is an important parameter for design as it affects operating as well as fixed costs of HEN.

The most appropriate value of Tmin or in other words the relative positions of the hot and cold composite curves is determined by an economic trade‐off between energy and capital. Once

the correct economic value of Tmin is known then the energy targets in terms of the values of

hot and cold utilities are automatically fixed. The Tmin in general appears at one location between hot and cold composite curves called “heat recovery pinch”. As pinch point is related

to Tmin , it has special significance in the design.

The principle of Pinch

Fig.4.2 (a) shows the heat exchange system separated at pinch . The section above the pinch

works as a heat sink as it accepts heat from external heating sources ( heat utility). The amount

of heat required is exactly equal to Q HOT MIN. The required heat which is necessary to strike heat

balance of this section after hot composite curve transfers the heat to cold composite curve is

Q HOT MIN. As this section takes heat from outside it is termed as heat sink. With the external

heating equal to Q HOT MIN the section is in heat balance, i.e. heat required by the cold stream is


satisfied by heat transferred from the hot composite stream and the Q HOT MIN from external hot

utility.

Similarly Fig.4.2(a) also shows that the section below the pinch works as Heat source as it

rejects heat to the external cold utility. The amount of heat rejected to cold utility is Q COLD MIN .

In this section, hot composite curve has excess heat (Q COLD MIN) available with it even after

transferring heat to cold composite curve. Once Q COLD MIN is transferred to external cold utility

this section is also in heat balance.

Thus both sections i.e. above the pinch and below the pinch sections are in heat balance and no

heat flows through the pinch section. This is true, only if we consider vertical heat transfer in

the whole section of the heat exchange process as given in Fig.4.1.

However, as shown in Fig.4.2(b), if an additional amount of heat, , over and above Q HOT MIN is

transferred to the above pinch section the total external heat given to this section becomes Q

HOT MIN + and then the additional amount of heat ,, flows through pinch ( as both sections above & below pinch are under heat balance) and increases the cold utility to Q COLD MIN + . Thus adding more heat than required in the above pinch section is not fruitful instead it is

harmful. This fact is explained below.

When additional amount of heat, , than required amount of heat Q HOT MIN , is introduces in

above pinch section then one has to supply appropriate heat transfer area to push it. Further,

the external hot utility cost proportional to is also increased. Thus, total cost in the above pinch section increases due to increase in the cost of heat exchange area( proportional to ) and also the increased in external hot utility cost which is also proportional to . The same is true

for below pinch section where the cold utility cost increases proportional to as well as the cost of heat transfer area also increases proportional to . Thus for every unit of excess heat one has to provide the required heat transfer area twice once in hot utility side and other in

cold utility side. Thus the penalty is twice for passing additional heat through the system than

required. Thus, B. Linn off et. al have coined the phrase “ More in, More out”. This vital insight,

many a times, helps us to decrease both utility and fixed cost of a heat transferring system by

eliminating the above discussed error.

An inefficient process always requires more hot utility than the minimum hot utility required

and as a consequence will consume more cold utility than required. Thus inefficient systems

are screened rapidly once energy targets (Q HOT MIN and Q HOT MIN) are determined.

Further, let us examine what happens when hot utility is used below pinch section and cold

utility is used in the above pinch section. Fig. 4.3 (a) is drawn show the effect. If the extra cold

utility of amount is used in the above pinch area which is under heat balance, it will cool the hot stream or hot utility by amount and will disturb the heat balance. To bring it to heat


balance extra hot utility of amount has to be added to Q HOT MIN and thus total hot utility

required to bring the section in to heat balance will be now Q HOT MIN + .

Fig.4.2 Source and Sink sections of a heat exchange system

Heat Source

(a)

T,C

H, kW

Tmin= 10C

Q HOT, MIN

Q COLD, MIN

Hot Pinch

ColdPinch



Below the Pinch

Above the Pinch

Heat Sink

Heat Source

(b)

T,C

H, kW

Tmin= 10C

Q HOT, MIN +

Hot Pinch

Cold Pinch



Below the Pinch

Above the Pinch

Heat Sink

Q COLD, MIN +

Q COLD, MIN +


Heat Source

(b)

T,C

H, kW

Q HOT, MIN

Hot Pinch

Cold Pinch



Below the Pinch

Above the Pinch

Heat Sink

Q COLD, MIN+

Fig.4.3 Effect of inappropriate use of utilities

Heat Source

(a)

T,C

H, kW

Q HOT, MIN +

Hot Pinch

Cold Pinch



Below the Pinch

Above the Pinch

Heat Sink

Q COLD, MIN


This will increases the total hot utility cost as well as fixed cost of heat exchanger which will now

transfer Q HOT MIN + heat in place of Q HOT MIN . As below the pinch section is under heat

balance the cold utility requirement will be Q COLD MIN .

Further, if hot utility is used in the below pinch section by an amount it will disturb the heat balance of this section. Now additional heat amounting to is available in this section to be cooled. This will increase the cold utility requirement to Q COLD MIN + to bring to heat balance again. However, the above pinch section which is under heat balance already will only require

external heating of Q HOT MIN .

From the above analysis following conclusions are made.

1. Do not transfer heat across the pinch as the penalty is twice.

2. Do not use cold utility in the above pinch section

3. Do not use hot utility in below pinch section

The above faults are generally committed in old designs which were carried out without using

pinch analysis and hence provide opportunity to correct these designs and save fixed as well as

utility cost.

References

1. Linnhoff March, “Introduction to Pinch Technology” Targeting House, Gadbrook

Park, Northwich, Cheshire, CW9 7UZ, England

2. Chemical Process Design and Integration, Robin Smith, John Wiley & Sons Ltd.

3. Ian C Kemp, Pinch Analysis and process integration, a user guide on process

integration for effective use of energy, IChemE, Elsevier Limited, 2007.

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