Pinch advance 2

8-Heat & Power IntegrationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin1

054402 Design and Analysis II

LECTURE 8: HEAT & POWER INTEGRATION

Daniel R. Lewin

Department of Chemical Engineering

Technion, Haifa, Israel


• Unit 6. Data Extraction– Class Exercise 8

• Unit 7. Heat Integration in Design– Grand Composite Curves (GCCs)

– Heat-integrated Distillation

– Heat Engines

– Heat Pumps

Schedule – Heat and Power Integration


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


UNIT 6: 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.



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




k 1 k

k

k 1 k

h hC

T T

Cooling of vapor

Condensation



k 1 k

k

k 1 k

h hC

T T




(b) MER Targeting:


Class Exercise 8 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 100

o

40o

50o

125o

140o

150o 30

o


Class Exercise 8 - Solution W

C

H

HC

H = 100

H = 100

CP = 0.6

CP = 0.4

CP = 1.0

130o 100

o

40o

50o

125o

140o

150o 30

o

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 8 - 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 = 150

oC QHQH

H = 0

Q1

H = 4

Q2

H = 36

Q3

H = 8

H = 12

Q4

Q5

Assume

QH = 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 = 20oC

66 166

H = -100

H = +100

This defines:Cold pinch temperature = 140oCQHmin = 100 kWQCmin = 166 kW



Feed

130o 100

o

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



Feed130

o 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

Feed130

o 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

Retrofitted flowsheet – one additional match for MER

90o


UNIT 7: 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



Alternative heating and cooling utilities can be used, to reduce operating costs:



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?



Returning to the GCC:



Possible designs using CW, BFW, LPS and HPS:



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



Option B:

2-effect distillation: (a) Tower and heat exchanger configuration;

(b) T-Q diagram.


Option C: Distillation configurations involving compression:

(a) heat pumping

(b) vapor recompression

(c) reboiler flashing


(b) vapor recompression

(a) heat pumping

(c) reboiler flashing


Option C: Distillation configurations involving compression:


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

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



Heat Engines and Heat Pumps– When processes have significant power demands, normally

sound practice to operate at or near minimum utilities. Let us consider alternative placement of heat engines and heat pumps.

Heat Engine Heat Pump


Heat Engines and Heat Pumps (Cont’d)

Alternatives for the placement of heat engines:

(a) Above Tpinch; (b) Across Tpinch; (c) Below Tpinch



Alternatives for the placement of heat pumps:

(a) Above Tpinch; (b) Below Tpinch ;(c) Across Tpinch



These results lead to the following general recommendations for the placement of heat engines and heat pumps relatively to the pinch:

Townsend and Linnhoff heuristics:

– To reduce the total utilities, place heat engines entirely above or below the pinch

– To reduce the total utilities, place heat pumps across the pinch


Heat and Power Integration - Summary

• Unit 6. Data Extraction

– Getting data for HEN synthesis from material and energy balances (i.e., from simulator)

• Unit 7. Heat Integration in Design

– Use of Grand Composite Curves for selection of utilities

– Options for heat-integrated distillation

– Optimal positioning of heat engines and heat pumps

Pinch advance 2

Documents

Transcript of Pinch advance 2