4 Process Integration for Efficient Use of Energy 5.pdf

8/11/2019 4 Process Integration for Efficient Use of Energy 5.pdf

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Smith: Chemical Process Design and Integration (Chapters 16-22)Kemp: Pinch Analysis and Process Integration (Chapter 9)

Process Integration

for Efficient Use of Energy

Cheng-Liang Chen

PSELABORATORYDepartment of Chemical Engineering

National TAIWAN University

Chen CL 1

Outline

Systematic Approach for Chemical Process Design

How do we go about the design of a chemical process?

What Is Process Integration?

Onion model for process integration

Pinch Analysis: Targeting Heat Recovery in Processes

Pinch Design Method for Heat Recovery Systems

A Pinch Study Performed on A Major Operating Plant

Utility Selection for Individual Processes

Heat Integration for Individual Processes

Putting It into Practice and Concluding Remarks

Chen CL 2

Utility Selectionfor

Individual Processes

Chen CL 3

The Problem Table and Grand Composite Curve


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Chen CL 4

The Problem Table and Grand Composite CurveChen CL 5


Chen CL 6


Chen CL 7

The Grand Compositegives the hot and cold utility requirements

of the process both in

Enthalpy and Temperature


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Chen CL 8

A Flowsheet with Two-Hot-Two-Cold Streams

Stream Type Supply Temp. Target Temp. H Heat Capacity Rate

TS(oC) TT(oC) (M W) mCp(M W/oC)

1. Reactor 1 feed Cold 20 180 +32.0 0.20

2. Reactor 1 prod Hot 250 40 31.5 0.15

3. Reactor 2 feed Cold 140 230 +27.0 0.30

4. Reactor 2 prod Hot 200 80 30.0 0.25

Chen CL 9

The Problem Table

T 1 2 3 4 Tint

CPH

CPCHint

Surplus/Defict

HotUtility

CascadeSurplus

Add HotUtility

245 250 0 7.5

10 +0.15 +1.5 Surplus H1

235 2 40 230 +1.5 9.0

40 0.15 6.0 Defict H2

195 200 190 200 4.5 3.0

10 +0.10 +1.0 Surplus H3

185 180 190 180 190 3.5 4.0

40 0.10 4.0 Defict H4

145 140 150 140 150 7.5 0

70 +0.20 +14. Surplus H5

75 70 80 80 +6.5 14.

40 0.05 2.0 Defict H635 30 40 +4.5 12.

10 0.20 2.0 Defict H7

25 20 +2.5 10.

CP 0.2 0.15 0.3 0.25 CW

Chen CL 10

The Grand Composite CurveGrand Composite Curveshows the utility requirements both in

enthalpy and temperature terms

interface between the process and the utility system

T Add Hot

Utility

245 7.5

235 9.0

195 3.0

185 4.0

145 0

75 14.

35 12.

25 10.

Chen CL 11


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Chen CL 12

Grand Composite Curve with Alternative Utility

Two levels ofsteams:

240oC, 180oC

Load of180oCsteam

= 175 145

185 145 4

= 3.0MW

Load of240oCsteam

= 7.5 3

= 4.5MW

Chen CL 13

Grand Composite Curve with Alternative Utility

Hot oilwith TS= 280oC,

Cp= 2.1 kJkg1K1

Minimum flow rate:

steepest slope and

min. return temperature

Min.flowrate

= 7.5 103

2.1(280 150)

= 27.5kgs1

Chen CL 14

Alternative Hot UtilitiesSaturated Steams and Hot Oil

Chen CL 15

Alternative Hot UtilitiesFlue Gas


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Chen CL 16

Alternative Hot UtilitiesFlue Gas with Increased Flame Temp

Increasing the theoretical flame temperature by reducing excess airor combustion air pre-heat reduces the stack loss

Chen CL 17

Alternative Hot UtilitiesFlue Gas: Stack Temperature Limited by Acid Dew Point or

Process Away from the Pinch

Chen CL 18

Flue Gas Matched Against the Grand CompositeCurve of the Illustrative Process

The process is to have its hot utility

supplied by a furnace. The theoretical

flame temperature for combustion is

1800oC, and the acid dew point for the flue gas is 160oC. Ambient temperature is

10oC. Assume Tmin= 10oC for process-to-process heat transfer but

Tmin= 30o

C for flue-gas-to-process heat transfer. Calculate the fuel required,stack loss, and furnace efficiency.

Chen CL 19

Solution:

AssigningTmincontributions to streams:The process streams are assigned a contribution of5oCand flue gas a contribution of25oC

Starting point of flue gas:1800oC 1775oC on grand composite curve

The flue gas can be cooled to pinch temperature (T = 145oC) before venting

to atmosphereactual stack temperature = 145 + 25 = 170> 160oC

QHmin = 7.5MW

CPflue gas = 7.5

1775 145 = 0.0046MW/oC

Fuel req. = 0.0046(1800 10) = 8.23MW

Stack loss = 0.0046(170 10) = 0.74MW

Furnace Eff. =

QHmin

Fuel req.

100 =

7.5

8.23

100 = 91%


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Chen CL 20

Combined Heat and Power (Cogeneration)

Heat engine exhaust can be integrated eitheracrossor not across the pinch

The process still requires QHmin and theheat engine performs NO better than operated stand-alone

Chen CL 21

Combined Heat and Power (Cogeneration)

Heat engine exhaust can be integrated either across ornot acrossthe pinch

Net effect is the import of extra energy Wfrom heat source to produce W power

Chen CL 22

Steam Turbine Expansion

Turbine Isentropic Efficiency T =actual work

ideal work =

H1 H

2

H1 H2

Chen CL 23

Steam Turbine Integration

QFUEL= QHP+ QLP+ W+ QLOSS


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Chen CL 24

Gas Turbine IntegrationChen CL 25

Combined Heat and Power Schemes:Example

The stream data for a heat recovery problem are given below. A problem table

analysis for Tmin= 20

o

C is also given. The process also has a requirement for 7MW of power. Two alternative combined heat and power schemes are to becompared economically.

Stream TS TT Heat Cap.

No. Type (oC) (oC) Rate (MW/oC)

1 Hot 450 50 0.25

2 Hot 50 40 1.50

3 Cold 30 400 0.22

4 Cold 30 400 0.025 Cold 120 121 22.0

T (oC) Casc. heat flow (MW)

440 21.90

410 29.40

131 23.82

130 1.80

40 030 15

Chen CL 26

1. A steam turbine with its exhaust saturated at 150oC used for process heating.Superheated steam is generated in the central boilerhouse at 41 bar with atemperature of 300oC. This superheated steam can be expanded in a single-stage turbine with an isentropic efficiency of 85%. Calculate the maximumgeneration of shaftwork possible by matching the exhaust steam against theprocess.

2. A second possible scheme uses a gas turbine with a flow rate of air of97kgs1

which has an exhaust temperature of400oC. Calculate the shaftwork generation

if the turbine has an efficiency of30%. Ambient temperature is 10oC.

3. The cost of heat from fuel for the gas turbine is $4.5GW1. The cost ofimported electricity is $19.2GW1. Electricity can be exported with a value of$14.4GW1. The cost for fuel for steam generation is $3.2GW1. The overallefficiency of steam generation and distribution is 60%. Which scheme is mostcost-effective, the steam turbine or the gas turbine ?

Chen CL 27

Steam Turbine:Heat flow required fromthe turbine exhaust= 21.9MW

Use Steam TableT1 = 300oC, P1 = 41 bar :h1 = 2959kJkg

1,s1 = 6.349kJkg

1K1

T2 = 150oC, P2 = 4.77bar :

s2 = 6.349 kJkg1K1; s= 1.842, sv= 6.838kJkg

1 (saturated entropies)h= 632, hv = 2747 kJkg

1K1 (saturated enthalpies)

s2 = xs+ (1 x)sv 6.349 = 1.842x + 6.838(1 x) x = 0.098

h2 = xh+ (1 x)hv = (0.098)632 + (1 0.098)2747 = 2540 kJ kg1

h

2 = h1 T(h1 h2) = 2959 0.85(2959 2540) = 2603kJ kg1 (isen. eff=85

h

2 = xh+ (1 x)hv 2603 = 632x + 2747(1 x) x = 0.068

Steam flowto process =

21.9103

2747632= 10.35 kg/s Steam flow

thr. turbine = 10.35

10.068 = 11.13kg/s

W = 11.13(2959 2603) 103 = 3.96 MW (Shaftwork generated)


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Chen CL 28

Gas Turbine:CP of exhause CP of airflow

Cp for air= 1.03kJ/kg/K

CPEXHAUST = 97 1.03 = 100 kW/K

QEXHAUST = (400 10) 100 103 = 39 MW

QFUEL = 39

0.7 = 55.71MW

W = 55.71 39 = 16.71MW

Chen CL 29

Steam Turbine Economics:

Cost offuel = (21.9 + 3.96)

3.2103

0.6

= $0.14 s1

Cost ofimported

electricity= (7 3.96) 19.2 103

= $0.06 s1

Net cost = $0.20s1

Gas Turbine Economics:

Cost offuel = 55.71 4.5 10

3

= $0.25s1

Electri-city

credit= (16.71 7) 14.4 103

= $0.14s1

Net cost = $0.11 s1

Chen CL 30

Combined Heat and Power Schemes:Example

The problem table cascade fo a process is given forTmin = 10oC. It is proposed to provide processcooling by steam generation from boiler feedwaterwith a temperature of100oC.

1. Determine how much steam can be generated at asaturation temperature of230oC.

2. Determine how much steam can be generated at asaturation temperature of 230oC and superheatedto the maximum temperature possible against theprocess.

3. Calculate how much power can be generated fromthe superheated steam from Part (2), assuming a

single-stage condensing steam turbine is to be usedwith an isentropic efficiency of85%. Cooling wateris available at 20oC and is returned to the coolingtower at 30oC.

Interval T Heat flow(oC) (MW)

495 3.6

455 9.2415 10.8

305 4.2

285 0.0

215 16.8

195 17.6

185 16.6

125 16.6

95 21.1

85 18.1

Chen CL 31

Solution (1):Heat available fr steamgeneration at 235oC intervaltemperature is 12.0MWLatent heat of water a sat. temp.of235oC is 1812 kJ kg1K1

SteamProd. =

12.0 103

1812 = 6.62 kg

s1

Taking the heat capacity of water to be 4.3 kJ kg1K1, heat duty on boilerfeedwater preheating

= 6.62 4.3 103(230 100) = 3.70 MW

The process can support both boiler feedwater preheat and steam generation.


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Chen CL 32

Solution (2,3):Maximum superheat temp.:

= 285oC interval= 280oC actual

Heat available for steamgeneration at 235oC interval

temperature is 12.0 MW.From steam table, enthalpy ofsuperheated steam at 280oC and28bar is 2947kJ kg1,

and enthalpy of saturated water at 230oC and28 bar is 991 kJ kg1

SteamProd. =

12.0 103

2947 991= 6.13kg s1

The lowest condensing temp is cooling water temp plus Tmin= 30 + 10 = 40oC.From steam table, inlet condition at T1= 280oC and P1= 28 bar are:

H1= 2947 kJ kg1; S1= 6.488kJ kg

1K1

Chen CL 33

Turbine outlet conditions for isentropic expansion to 40oC from steam tables are:P2= 0.074 bar. For S2= 6.488kJ kg

1K1, the wetness fraction (X) andoutlet enthalpy H2 can be calculated (?)

X= 0.23, H2 = 2020kJ kg1

For a single-stage expansion with isentropic efficiency of85%:

H2 = 2947 0.85(2947 2020) = 2159kJ kg1

The power generation (W) is given by

W = 6.13(2947 2159) 103 = 4.8 MW

The wetness fraction for the real expansion is given by

H2= 2159 = XH+ (1 X)Hv

= 167.5X + 2574(1 X)

X = 0.17

Chen CL 34

Heat Pump and Power Cycle

Chen CL 35

Integration of Heat PumpsSchematic of a simple vapor compression heat pump


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Chen CL 36

Integration of Heat Pumps

Schematic of a simple vapor compression heat pump (again)

Chen CL 37



Chen CL 38

Integration of Heat PumpsSchematic of a simple vapor compression heat pump (again)

Chen CL 39

Integration of Heat PumpsSchematic of a simple vapor compression heat pump (again)


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Chen CL 40



Chen CL 41


Integration of a heat pump abovethe pinch

The system converts power into heat !!

Chen CL 42

Integration of Heat PumpsIntegration of a heat pump belowthe pinch

Power is turned into waste heat !!

Chen CL 43

Integration of Heat PumpsIntegration of a heat pumpacrossthe pinch

Heat is pumped from a heat source part to a heat sink part

Coefficientof

PerformanceCOPHP =

QHP+ W

W


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Chen CL 44


The grand composite curve

Chen CL 45


The grand composite curve allows heat pump cycles to be sized

A temperature lift greater than 25oC is rarely economic

Chen CL 46

Integration of Heat PumpsLittle scope for heat pumpingacross process pinch

Chen CL 47

Integration of Heat PumpsHeat pump placedacross a utility pinch

Ch CL 48 Ch CL 49


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Chen CL 48

The Grand Composite Curveallows selection of utility mix

for individual processes

Chen CL 49

Thank You for Your Attention

Questions Are Welcome

4 Process Integration for Efficient Use of Energy 5.pdf

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