heat exchanger network synthesis, part iii

8 - Heat & Power Integration1

Heat Exchanger Network

Synthesis, Part III

Ref: Seider, Seader and Lewin (2004), Chapter 10

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Instructional Objectives • This Unit on HEN synthesis serves to expand

on what was covered in the last two weeks to more advanced topics.

• Instructional Objectives - You should be able to:– Extract process data (from a flowsheet

simulator) for HEN synthesis– Understand how to use the GCC for the

optimal selection of utilities– Have an appreciation for how HEN impacts

on design

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Data Extraction

Process analysis begins with the extraction of “hot” and “cold” streams from a process flowsheet

Required: The definition of the

“hot” and “cold” streams and their corresponding TS and TT

CP for each stream is either approximately constant or H=f(T).

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What is considered to be a stream ?

In general: Ignore existing heat exchangers

Mixing: Consider as two separate streams through to target temperature.

Splitting: Assume a split point wherever convenient.

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Example – Dealing with Real Systems

o Toluene is manufactured by dehydrogenating n-heptane.o Furnace E-100 heats S1 to S2, from 65 oF to 800 oF. o Reactor effluent, S3, is cooled from 800 oF to 65 oF. o Install a heat exchanger to heat S1 using S3, and thus

reduce the required duty of E-100. a) Generate stream data using piece-wise linear

approximations for the heating and cooling curves for the reactor feed and effluent streams.

b) Using the stream data, compute the MER targets for Tmin = 10 oF.

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Example – Dealing with Real Systems Equivalent, piece-wise flowing heat

capacity:k 1 k

k

k 1 k

h hC T T

Evaporation of n-heptane

Heating of vapor

Heating of liquid

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Example – Dealing with Real Systems

Equivalent, piece-wise flowing heat

capacity:k 1 k

k

k 1 k

h hC T T

Cooling of vapor

Condensation

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Example – Dealing with Real Systemsk 1 k

k

k 1 k

h hC T T

Equivalent, piece-wise flowing heat capacity:

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Example – Dealing with Real Systems (b) MER Targeting:

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Class Exercise 7 a) Extract data for hot and cold streams from the

flowsheet below.b) Assuming Tmin = 10o, compute the pinch temperatures, QHmin and QCmin.c) Retrofit the existing

network to meet MER. W

C

H

HC

H = 100

H = 100

CP = 0.6

CP = 0.4

CP = 1.0

130o 100o

40o

50o

125o

140o

150o 30o

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Class Exercise 7 - Solution W

C

H

HC

H = 100

H = 100

CP = 0.6

CP = 0.4

CP = 1.0

130o 100o

40o

50o

125o

140o

150o 30o

Stream TS

(oC) TT

(oC) H

(kW) CP

(kW/oC) Feed

Bottoms Cond Recyc Reb

Stream TS

(oC) TT

(oC) H

(kW) CP

(kW/oC) Feed 130 100 30 1.0

Bottoms 150 30 72 0.6 Cond 40 40 100 Recyc 50 140 36 0.4 Reb 150 150 100

Tmin = 10 oC

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Class Exercise 7 - Solution (Cont’d)

Stream TS

(oC) TT

(oC) H

(kW) CP

(kW/oC) Feed 130 100 30 1.0

Bottoms 150 30 72 0.6 Cond 40 40 100 Recyc 50 140 36 0.4 Reb 150 150 100

Tmin = 10 oCT1 = 150oC QHQH

H = 0

Q1

H = 4

Q2

H = 36

Q3

H = 8

H = 12

Q4

Q5

AssumeQH = 0

-100

-96

-60

-52

60

Eliminate infeasible(negative) heat transfer

QH = 100

0

4

40

48

160

T2 = 140oC

T3 = 120oC

T4 = 90oC

T5 = 50oC

T6 = 30oC

H = 6

QC

T7 = 20oC66 166

H = -100

H = +100

This defines:Cold pinch temperature = 140oCQHmin = 100 kW

QCmin = 166 kW

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Class Exercise 7 - Solution (Cont’d)

Feed 130o 100o

150o

140o

150o

30o

150o

CP

1.0

0.6

0.4

40o

QHmin = 100 QCmin = 166

Botts

Cond

Recy

Reb

40o

HEN Representation of existing flowsheet

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Class Exercise 7 - Solution (Cont’d)

Feed130o 100o

150o

140o

150o

30o

50o

150o

CP

1.0

0.6

0.4

40o

QHmin = 100 QCmin = 166

Botts

Cond

Recy

Reb

40o

125o

H

H

C

C

100

6 30

100

72

Tmin violation

HEN Representation of existing flowsheet

Feed130o 100o

150o

140o

150o

30o

50o

150o

CP

1.0

0.6

0.4

40o

QHmin = 100 QCmin = 166

Botts

Cond

Recy

Reb

40o

H

C

C

C

100

30

36

100

36

Retrofi tted flowsheet – one additional match f or MER

90o

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Heat Integration in Design The Grand Composite Curve

An enthalpy cascade for a process is shown on the right.

Note that QHmin = QCmin = 1,000 kW

Also, TC,pinch = 190 oC

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The Grand Composite Curve (Cont’d) The Grand Composite Curve presents the same

enthalpy residuals, as follows:

Internal heat exchange

Internal heat exchange

TC,pinch

Minimum external heating, at 310 oC

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The Grand Composite Curve (Cont’d) Alternative heating and cooling utilities can be used, to

reduce operating costs:

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The Grand Composite Curve (Cont’d) Example:

GCC:

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GCC Example (Cont’d) Possible designs using CW and HPS:

Umin = 4 + 2 – 1 = 5

How many loops?

Does this design meet Umin ? If not, what is the simplest change you can make to fix it?

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GCC Example (Cont’d) Returning to the GCC:

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GCC Example (Cont’d) Possible designs using CW, BFW, LPS and

HPS:

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Heat Integration in Design Heat-integrated Distillation

Distillation is highly energy intensive, having a low thermodynamic efficiency (as little as 10% for a difficult separation), but is widely used for the separation of organic chemicals in large-scale processes.

Thermal integration of columns can be done by manipulation of operating pressure.

Note: Qreb Qcond for columns with saturated liquid products.

Need to position column

carefully on composite

curve

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Heat-integrated Distillation (Cont’d) Option A: Position distillation

column between hot and cold composite curves:

(a) Exchange between hot and cold streams

(b) Exchange with cold streams

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Heat-integrated Distillation (Cont’d) Option B: 2-effect distillation:

(a) Tower and heat exchanger configuration; (b) T-Q diagram.

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Heat-integrated Distillation (Cont’d) Option B: Variations on two-effect distillation: (a) Feed Splitting (FS) (b) Light Split/forward heat integration (LSF) (c) Light Split/Reverse heat integration (LSR).

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Option C: Distillation configurations involving compression:

(a) heat pumping (b) vapor

recompression (c) reboiler flashing

Heat-integrated Distillation (Cont’d)

(b) vapor recompression (a) heat pumping

(c) reboiler flashing

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Option C: Distillation configurations involving compression:

Heat-integrated Distillation (Cont’d)

All 3 configurations involve the expensive compression of a vapor stream.

May not be cost-effective except where pressure changes required are small. Example: separation of close-boiling mixtures

For further reading:

Smith, R., “Chemical Process Design and Integration”, Wiley, 2005, Chapter 11.

(a) heat pumping (b) vapor recompression (c) reboiler flashing

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Heat Integration - Summary

• Data Extraction– Getting data for HEN synthesis from

material and energy balances (i.e., from simulator)

• Heat Integration in Design– Use of Grand Composite Curves for

selection of utilities– Options for heat-integrated distillation