heat exchanger network synthesis, part iii

8 - Heat & Power Integration1

Heat Exchanger Network

Synthesis, Part III

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


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


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).


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.


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.


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


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


Example – Dealing with Real Systemsk 1 k

k

k 1 k

h hC T T

Equivalent, piece-wise flowing heat capacity:


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


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


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


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



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



Feed130o 100o

150o

140o

150o

30o

50o

150o

CP

1.0

0.6

0.4

40o


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


Botts

Cond

Recy

Reb

40o

H

C

C

C

100

30

36

100

36

Retrofi tted flowsheet – one additional match f or MER

90o


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


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


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

reduce operating costs:


The Grand Composite Curve (Cont’d) Example:

GCC:


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?


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


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

HPS:


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


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


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

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


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).


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


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


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

heat exchanger network synthesis, part iii

Documents

class exercise

solution tmin

stream data

extract data

cold streams

real systemstoluene

existing network

extract process data