Multiple Utilities Grand Composite Lec 22-24

8/6/2019 Multiple Utilities Grand Composite Lec 22-24

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Lecture 22Lecture 22 -- 2424 :: Multiple Utilities & Grand Composite CurvesMultiple Utilities & Grand Composite Curves

PROCESSFURNACE

Air Preheat

Fuel

Process normally used multiple level of utilities .

LP Steam

MP Steam

HP Steam

Refrigeration

W

Q + W

W

Fuel & Air

GAS TURBINEW

STEAM TURBINE

HEAT PUMP

W

Q + W

COOLING TOWER


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130

180

80

40

30

60

100

120

1080 4640 720

T

H

1600 3000 1000

Minimum temperature difference = 10 C

960 (Hot Utility)

120 (Cold Utility)

Composite curve method only tells us the amount of utility required to satisfy the process

requirement. But it does not tell us the mix utilities that can be used


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And so does the problem table algorithm

175 C

125 C

105 C

75 C

65 C

35 C

960

(H = - 1000

(H = - 480

1960

2440

(H = 1680

760

0

(H = 760

(H = -120

120

120

QH

QC

175 C

125 C

105 C

75 C

65 C

35 C

Stream Population (Tint 7 CpC - 7 CpH (Hint Surplus/Deficit

20

40 36

80

50 -20 -1000 Surplus

20 -24 -480 Surplus

30 56 1680 Deficit

10 76 760 Deficit

30 -4 -120 Deficit


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To enable designer to determine the various utilities mix that can be used, Grand Composite

Curve is useful. It is also a plot using T-H diagram.

HOT 1

HOT 2

COLD 1

COLD 2

180 C 80 C

130 C 40 C

30 C120 C

60 C100 C

Cp Q

20

40

36

80

2000

3600

3240

3200

Consider again the problem below

From the problem table algorithm, the heat cascade was derived


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175 C

125 C

105 C

75 C

65 C

35 C

0

(H = - 1000

(H = - 480

1000

1480

(H = 1680

-200

-960

(H = 760

(H = -120

-840

Adjust the heat

cascade

Highest ve value for

heat accumulated

175 C

125 C

105 C

75 C

65 C

35 C

960

(H = - 1000

(H = - 480

1960

2440

(H = 1680

760

0

(H = 760

(H = -120

120

120

QH

QC

The adjusted heat cascade is used to develop the Grand Composite Curve


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175 C

125 C

105 C

75 C

65 C

35 C

960

(H = - 1000

(H = - 480

1960

2440

(H = 1680

760

0

(H = 760

(H = -120

120

120

QH

QC

960

1960

2440

760

120

T

H

Grand Composite Curve

175 C

125 C

105 C

75 C

65 C

35 C


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960

1960

2440

760

120

T

H

175 C

125 C

105 C

75 C

65 C

35 C

How do we use them ?

Heat Recovery/

Process to Process Heat

Transfer

QH = 960

Lowest temperature

where hot utility couldbe supplied and still

satisfy the heating

requirement

We dont

necessarily have to

supply the heating

utility at this

temperature


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If we decide to use hot flue gas either from furnace or gas turbine exhaust

960

1960

2440

760

120

T

H

175 C

125 C

105 C

75 C

65 C

35 C

Heat Recovery/

Process to Process Heat

Transfer

QH = 960

Slope = mCp of flue gas

Flue gas source temperature i.e. Furnace flame

temp. (TFT) or Gas Turbine exhaust temp (TGT).

TTFT or TGT


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HOT 1

HOT 2

COLD 1

COLD 2

250 C 40 C

200 C 80 C

20 C180 C

60 C230 C

Cp Q

15

25

20

30

3150

3000

3200

2700

Lets try another example. But you will have to attempt it yourself

Over to younow ..


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245 C

235 C

195 C

185 C

145 C

75 C

35 C

25 C

0

(H = 600

(H = - 100

150

-450

(H = -1400

650

(H = 200

(H = 200

250

Adjust the heat

cascade

Highest ve value for

heat accumulated

(H = - 150

(H = 400

-350

-750

450

250

245 C

235 C

195 C

185 C

145 C

75 C

35 C

25 C

750

(H = 600

(H = - 100

900

300

(H = -1400

1400

(

H =200

(H = 200

1000

(H = - 150

(H = 400

400

-0

1200

1000


11/21

185

245

75

35

145

T

235

195

25

245 C

235 C

195 C

185 C

145 C

75 C

35 C

25 C

750

(H = 600

(H = - 100

900

300

(H = -1400

1400

(H = 200

(H = 200

1000

(H = - 150

(H = 400

400

-0

1200

1000

QH

QC

750

900

300

400

1400

1200

1000

Process to Process

Heat Transfer


12/21

185

245

75

35

145

T

235

195

25

245 C

235 C

195 C

185 C

145 C

75 C

35 C

25 C

750

(H = 600

(H = - 100

900

300

(H = -1400

1400

(H = 200

(H = 200

1000

(H = - 150

(H = 400

400

-0

1200

1000

QH

QC

750

900

300

400

1400

1200

1000

Process to Process

Heat Transfer

Identifying the various utilities mix possible and their respective amount.

You can use a mix of HP

and MP steam to fulfill

the heating requirement

You can choose to generate LP steam instead of

discharging every thing to cooling water.

Steam vapourisation


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185

245

75

35

145

T

235

195

25

245 C

235 C

195 C

185 C

145 C

75 C

35 C

25 C

750

(H = 600

(H = - 100

900

300

(H = -1400

1400

(H = 200

(H = 200

1000

(H = - 150

(H = 400

400

-0

1200

1000

QH

QC

750

900

300

400

1400

1200

1000

Process to Process

Heat Transfer

Alternatively,

Cp (feedwater)

You can choose to use flue gas for fulfilling the hot utility

requirement especially dealing with very high temperature

process eg., reactor heating.

You can choose to generate superheated

LP steam instead of just saturated one.

Cp (flue gas)

Cp (steam vapour)

Allowable Stack Temp.


14/21

Temp.

Enthalpy (H)

QC

QH

(Tmin

COLD UTILITIES

HOT UTILITIES

PINCH

POINT

Composite Curve

T

(H

Hot Utility (QHMIN)

Cold Utility (QCMIN)

Grand Composite Curve

Further Insight !Further Insight !

Problem Table Algorithm

and Heat Cascade Diagram

Can help to identify the

possible level of utility

to be introduced to avoid

introduction of it at extreme point

!

Also, enable utility selection

and quantification if the options

of utilities are fixed.

CW

LP

MP

HP

alternatives

Introducing

INANUT SHELL


15/21

Hot Utility

Cold Utility

PINCH

T

(H

POCKETS OF

HEAT RECOVERY

POCKETS OF

HEAT RECOVERY

Furnace Heating

HP Steam level

LP Steam level

Cooling Water level

With the presence of multiple utilities, the selection for utilities will look more complex.

No scope for HP

Furnace Heating

MP Steam levelMP Steam Heating

Raising LowTemp Steam

Cooling Water

Grand Composite Curve facilitates the

selection of the multiple utilities


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T

(H

Hot Utility

Cold Utility

PINCH

But there is a systematic way of approaching the mix utility selection and quantification

HP

MP

LP

Targetingare done fromthelowestlevel utilitiesandmoving up to thehigher one.Therationalisto maximise thecheapest utilitiesasmuchas possiblebeforemovingto themoreexpensive one.

Targetingare done fromthehighertemperaturelevel

utilitiesand moving downto thelower one.Therationalisto maximisethegeneration ofhigherlevel utilitiesasmuchas possiblebeforemoving downto cooling waterand ifnecessary,into therefrigerationcooling.

LP

CoolingWater

Refrig.

Generate


17/21

Things to note on the use of hot flue gas as heating source .

(H

Hot Utility

Cold Utility

PINCH

Tdew

Potential recovery

inevitable

losses

T

flame temperature TTFT

Tpinch Case 1 : Limited by the Process Pinch

mCp


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T

(HCold Utility


Tdew

Potential recovery

inevitable

losses

Case 2 : Limited by the ProcessGrand Composite Curve

Flue gas lineFlue gas line

mCp


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inevitable

losses

T

Cold Utility

(H

Hot Utility


Case 3 : Limited by the Flue Gas StackTemperature

Tdew

mCp


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What about if hot oil cycle is used ?

Limited by the Process

ProcessHot Oil Heater

In addition, the hot oil cycle can also be limited by the process

pinch but not the flue stack temperature. Could you draw thegrand composite to reflect this?

Hot Utility

T

(H

Cold Utility

PINCH

Thot oil

Tpinch

Hot Oil return temperature

mCp


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So, in conclusion

1. Grand Composite curve provides the better insight for deciding multi level

utilities.

2. Satisfying hot utility requirement through steam, flue gas or hot oil heating

have different repercussion on the grand composite curve. Similarly for

cooling requirement where steam raising has different repercussion on thegrand composite curve compares to cooling water.

3. Selection of utility mix has to be made based on cost which is reflective also

on the level.

Documents

Multiple Utilities Grand Composite Lec 22-24