Chapter 15 - Heat Exchanger Networks - I

Chapter 15 – Pinch TechnologyHeat Exchange Networks

Chemical Process Design

West Virginia University

Copyright - R. Turton and J. Shaeiwitz, 2012

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The Need to Heat and Cool Process Streams

• Feed usually enters a process from a storage vessel that is maintained at ambient temperature. If it is to be reacted at an elevated temperature, it must be heated.

• After the reaction has taken place, the reactor effluent stream must be purified, which usually requires cooling the stream, and possibly condensing it, prior to separating it.

• Thus energy must first be added, then removed.

Heat IntegrationHeat exchange networksIt saves money to match streams rather than pay to heat one and pay to cool another

••

• You have already done thisbasis in design projects

on ad hoc


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The Concept of Heat Integration

• The concept of Heat Integration, in its simplest form, is to find matches between heat additions and heat removals within the process.

• In this way, the total utilities that are used to perform these energy transfers can be minimized, or rather optimized.

• The number of heat exchangers needed to perform these energy transfers can also be minimised.

Outline•Heat Integration•Design Procedure for MUMNE (Minimum Utility, Minimum Number of Exchangers)- Temperature interval diagram- Cascade diagram- Temperature – Enthalphy diagram- Minimum number of heat exchangers- Design above and below pinch

Heat IntegrationThere is a rigorous methodology We will learn MUMNE (Minimum Utility, Minimum Number ofExchangers) method

••

• Not necessarily (andeconomic optimum

unlikely to be)


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Design ProcedureComplete energy balance on all streams to

1.determine allflows.

temperatures, m C p values, and heat

2. Choose minimum approach temperature. Typically,this is between 5°C and 20°C, but any positivenumber is valid.

3. Complete temperature interval diagram, Eachstream is drawn and labeled.interval is calculated.

The heat flow in each


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Design Procedure (continued 1)4. Complete the cascade diagram. The energy excess

or deficit is calculated for each interval on thetemperature interval diagram.

Find the minimum hot and cold utility requirements and identify the pinch temperature. Complete the composite temperature enthalpy

5.

6.diagram.process.

This is a T-Q diagram for the entire

Copyright - R. Turton and J. Shaeiwitz,

2012

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Design Procedure (continued 2) Determine the minimum number of heat7.exchangers required above and below the pinch.

8. Design the heat exchanger network.


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Example Problem Stream Properties for Example Problems


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Stream Tin Tout

m Cp

kW/°CQ

kW1 200 120 3 240

2 140 100 5 200

3 100 170 3 -210

4 110 190 2 -160

Net heat flow

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Example Problem (continued 1)The value of Q might not be given in above table,1.or Q is given and mC p is missing. These arecalculated from the energy balance. The signconvention is positive for heat available from astream and negative for heat needed by a stream.

2. Choose the minimum approachthis problem, it is 10°C.

temperature. For


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Example Problem (continued 2) Draw and label the temperature interval diagram.Label the intervals beginning with “A” for the

3.

highest temperature interval. The heat flow for m C p Teach interval is calculated from, Q , where

interval.the sum is over all streams existing in that


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Example Problem (continued 3)4. In this temperature interval diagram, each process

stream is represented by a vertical line with an arrow at the end indicating the direction of temperature change.

5. Horizontal lines are then drawn through the ends of the vertical lines to divide the diagram into temperature intervals.


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Example Problem (continued 4)6. Draw the cascade diagram. This represents the

cascade of heat flowing down from high to lowtemperatures. Add utilities where needed. Labelthe heat flows. The net utility flow should agreewith the net heat flow on the earlier table.


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Cascade

Diagram

2060Hot

Utility

Pinch Temp130-140C

60

80130

ColdUtility


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E50

D20

C60

B-80

A20

Example Problem (continued 5)

7. On the cascade diagram, there will be a location where the heat-flow cascade is not continuous. This represents the pinch temperature

Example Problem (continued 6)Construct the composite temperature enthalpy8.diagram. This provides useful information, but it isnot required to solve the problem.


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Example Problem

Hot

(continued 7) Cold


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interval T(°C)

Q(kW)

T(°C)

Q(kW)

E 100 90

D 110 50 100

C 120 100 110 30

B 140 260 130 130

A 180 380 170 330

200 440 190 370

Example Problem (continued 8)In the table, the temperature shown is from the lower end

summingto the

of the interval. The Q values are obtained byall m C p T existing on the interval and adding itprevious interval. The temperature difference is forthat interval. The m C p value is the sum

of all existing

streams on that interval.


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The hot and cold stream lines are plotted, as shown on the following figure. Clearly, there is a temperature cross, so the cold stream line is shifted to the right until the minimum approach temperature of 10°C exists at one point. (It could exist at more than one point by coincidence.) For this problem, all Q values for the cold stream must be increased by 130 kW, as shown in the figure. Note how the hot and cold utility requirements are apparent from the diagram.


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T(°

C)

QH=60

180

160

140

10°C - minimum approach temp.

120

100Q

c=130

0 100 200 300 400

500 600

Q(kW)hot stream cold streamcold stream adjusted

Composite T-Q Diagram 19


7. The easiest way to determine the minimum number of heat exchangers is to draw boxes to represent the heat available in each stream and from the utilities both above and below the pinch. This identifies the minimum number, but not necessarily the correct stream matches. The correct number of heat exchangers is the number of process streams + the number of utility streams – 1.

above

HU60

pinch - 3 exchangers1

180

below pinch

160

- 4 exchangers

2200

60 60 120 60 30 13040

4120

3120

440

390

CU130

Each arrow identifies one heat exchanger,Total number of arrows is total number of heat exchangers,but not necessarily the correct stream matches.

Top number in a box is stream number.Bottom number in a box i s energy in the stream.Number on arrow is energy transferred in a heat exchanger.

HU : Hot Utility CU : Cold Utility


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Note that if a “direct match” is found, i.e., where sets of two streams match heat flows

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exactly, one fewer exchanger may appear tobe possible. However, be careful, theminimumviolated.

approach temperature may be


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Design the heat exchange network.9. Theremay not be unique streams here. Thedesign is started at the pinch and you workaway from the pinch. Above the pinch, foranycan

streams that exist at the pinch,only be matched such that

streams

m CpH m

CpC


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When dealing with streams away from the pinch, this criterion is no longer needed. Any streams can be matched as long as the

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temperatures are valid. If the criterion atthe pinch appears impossible to satisfy,streams can be split to satisfy the criterion.


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Example Problem (continued 13)11. From the previous diagram, stream 1 can

be matched with stream 3 or 4. Note that, for this step, only streams that are present at the pinch are considered.

12. The next step is to transfer heat from hot to cold stream by placing a heat exchanger (1) in the temperature diagram.


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Example Problem (continued 14)13. Heat exchanger (1) transfers Q1= 120 from

stream 1 to stream 3. This fulfils requirement of stream 3.

14. There is still remaining Q2 = 60 in stream 1 which can be transferred to stream 4 by adding another heat exchanger (2) as in the following diagram.


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15. Next, heat exchanger (3) is used to transfer Q3 = 60 from Hot Utilities to Stream 3.


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The same procedure is done below the pinch, except that the criterion is

mC p H mC pC

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17. Streams are matched and heat exchangers areadded until all required heat transfer isaccomplished. The entire network, both aboveand below the pinch, can then be represented onone diagram.


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In-class Example ProblemStream Properties for In-class Example Problem


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Stream Tin Tout m

kg/sCp

kJ/kg°C

1 250 100 1 1

2 280 120 2 2

3 100 200 1 2

4 120 230 1 5

In-class Example Problem

Determine (minimum approach T = 20°C)

a.

b.c.

minimum hot and cold utility consumption

pinch temperaturesminimum number of heat exchangersrequired above and below the pinch

d. design of heat exchange network abovebelow the pinch

and


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Cascade

Diagram

120

120

40HotUtility Pinch

Temp120-140C

6080 Cold

Utility


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E20

D60

C-160

B0

A120


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above pinch - 4 exchangers

1 2 HU110 560 40

110 10 550 40

3 4160 550

below pinch - 2 exchangers, if possible

1 2

40 80

3 CU40 80

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4

sp lit

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Summary

•••

Heat Exchange Networks

Well-established procedureNot necessarily (and unlikely to be) economic optimum but a very good starting point

• Straight forward, but must be carefulmatching streams at pinch

Different correct answers possible

when

•


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Documents

Chapter 15 - Heat Exchanger Networks - I