4 Process Integration for Efficient Use of Energy 5.pdf

Chen CL

1

Outline Smith: Chemical Process Design and Integration (Chapters 16-22) Kemp: Pinch Analy Kemp: Analysis sis and Proce Process ss Integrati Integration on (Chap (Chapter ter 9 9))

➢

Process Integration

Systematic Approach for Chemical Process Design How do we go about the design of a chemical process?

➢

for Efficient Use of Energy

What Is Process Integration? Onion model for process integration

Cheng-Liang Chen

PSE

➢

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

LABORATORY

Department of Chemical Engineering National TAIWAN University

Chen CL

2

Chen CL

The Problem Table and Grand Composite Curve

Utility Selection for

Individual Processes

3

Chen CL

4


Chen CL

Chen CL

5


6

Chen CL

7


The

Grand Composite

gives the hot and cold utility requirements of the process both in Enthalpy and Temperature

Chen CL

8

Chen CL

9

The Problem Table

A Flowsheet with Two-Hot-Two-Cold Streams T ∗

1

2

245

—

250

195

185

Stream

Type

1. Reactor 1 feed 2. Reactor 1 prod 3. Reactor 2 feed 4. Reactor 2 prod

Cold Hot Cold Hot

Supply Temp. T S (oC )

Target Temp. T T (oC )

∆H (MW )

Heat Capacity Rate ˙ p(MW/ oC ) mC

20

180

+32.0

0.20

250 140 200

40 230 80

−

31.5 +27.0 −30.0

0.15 0.30 0.25

Chen CL

Grand Composite Curve shows 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.

∆T int

—

—

——

————

——

10

+0.15

+1.5

——

————

——

40

−0.15

−6.0

75

—

230

⇓

⇑

200

190

200

——

————

——

⇓

⇑

⇓

10

+0.10

+1.0

180

190

180

190

——

————

——

⇑

⇓

⇑

⇓

40

−0.10

−4.0

150

140

150

——

————

——

⇓

70

+0.20

+14.

80

——

————

——

40

−0.05

−2.0

——

————

——

10

−0.20

−2.0

——

————

——

—

⇑

⇓

70

80

⇑

⇓

30

40

—

—

—

⇑ 25

20

CP

0.2

Chen CL

—

—

—

0.15

0.3

0.25

Surplus/ Defict

∆H int

2 40

145 140

35

10

The Grand Composite Curve

—

CP H −CP C

4

⇓ 235



3

—— Surplus

⇓

——

⇓ ∆H 2

——

⇓

——

⇓ ∆H 4

——

7.5

+1.5

9.0

−4.5

3.0

−3.5

4.0

⇓

−7.5

0

+6.5

14.

+4.5

12.

+2.5

10.

∆H 5

——

⇓

Defict

∆H 6

——

⇓

Defict

∆H 7

——

0

Add Hot Utility

∆H 3

Defict

Surplus

Cascade Surplus

∆H 1

Defict

Surplus

Hot Utility

⇓

CW

11

Chen CL

12

Grand Composite Curve with Alternative Utility Two levels of steams: 240oC, 180oC

Chen CL

Grand Composite Curve with Alternative Utility Hot oil with T S = 280oC , C p = 2.1 kJ·kg−1· K−1 Minimum flow rate: steepest slope and min. return temperature

Load of 175 − 145 = ×4 180oC 185 − 145 steam = 3.0 MW

Min. flow rate

Load of = 7.5 − 3 240oC steam

=

7.5 × 103 2.1(280 − 150)

= 27.5 kg·s−1

= 4.5 MW

Chen CL

13

14

Chen CL

15

Alternative Hot Utilities


Saturated Steams and Hot Oil

Flue Gas

Chen CL

16

Chen CL

17



Flue Gas with Increased Flame Temp

Flue Gas: Stack Temperature Limited by Acid Dew Point or Process Away from the Pinch

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

18

Flue Gas Matched Against the Grand Composite Curve of the Illustrative Process

Chen CL

Solution: ➢

Assigning ∆T min contributions to streams: The process streams are assigned a contribution of 5 oC and flue gas a contribution of 25 oC

➢

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

➢

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 160 oC. Ambient temperature is 10oC. Assume ∆T min = 10oC for process-to-process heat transfer but ∆T min = 30oC for flue-gas-to-process heat transfer. Calculate the fuel required, stack loss, and furnace efficiency.

19

The flue gas can be cooled to pinch temperature (T ∗ = 145oC) before venting to atmosphere o ⇒ actual stack temperature = 145 + 25 = 170 > 160 C QH min = 7.5 MW 7.5 CP flue gas = = 0.0046 MW/oC 1775 − 145 Fuel req. = 0.0046(1800 − 10) = 8.23 MW Stack loss = 0.0046(170 − 10) = 0.74 MW QH min 7.5 Furnace Eff. = × 100 = × 100 = 91% Fuel req. 8.23

Chen CL

20

Chen CL

21

Combined Heat and Power (Cogeneration)

Combined Heat and Power (Cogeneration)

Heat engine exhaust can be integrated either across or not across the pinch

Heat engine exhaust can be integrated either across or not across the pinch

The process still requires QH min and the heat engine performs NO better than operated stand-alone

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

Chen CL

22

Steam Turbine Expansion

Chen CL

23

Steam Turbine Integration 

Turbine Isentropic Efficiency η T =

actual work H 1 − H 2 = ideal work H 1 − H 2

QFUEL = Q HP + QLP + W + QLOSS

Chen CL

24

Chen CL

Gas Turbine Integration

25

Combined Heat and Power Schemes: Example The stream data for a heat recovery problem are given below. A problem table analysis for ∆T min = 20oC is also given. The process also has a requirement for 7 MW of power. Two alternative combined heat and power schemes are to be compared economically. Stream No.

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 a temperature of 300oC. This superheated steam can be expanded in a singlestage turbine with an isentropic efficiency of 85%. Calculate the maximum generation of shaftwork possible by matching the exhaust steam against the process. 2. A second possible scheme uses a gas turbine with a flow rate of air of 97kgs−1 which has an exhaust temperature of 400 oC. Calculate the shaftwork generation if the turbine has an efficiency of 30%. Ambient temperature is 10oC. 3. The cost of heat from fuel for the gas turbine is $4.5GW−1. The cost of imported electricity is $19.2GW−1. Electricity can be exported with a value of $14.4GW−1. The cost for fuel for steam generation is $3.2GW−1. The overall efficiency of steam generation and distribution is 60%. Which scheme is most cost-effective, the steam turbine or the gas turbine ?

T S o

T T o

Heat Cap.

T ∗ (oC)

o

Casc. heat flow (MW)

Type

( C)

( C)

Rate (MW/ C )

440

21.90

1 Hot 2 Hot 3 Cold

450 50 30

50 40 400

0.25 1.50 0.22

410 131 130

29.40 23.82 1.80

4 Cold 5 Cold

30 120

400 121

0.02 22.0

40 30

0 15

Chen CL

27

Steam Turbine: Heat flow required from the turbine exhaust = 21.9 MW Use Steam Table T 1 = 300oC, P 1 = 41 bar : h1 = 2959 kJkg−1 , s1 = 6.349 kJkg−1 K−1 T 2 = 150oC, P 2 = 4.77 bar : s2 = 6.349 kJkg−1K−1; s = 1.842, sv = 6.838 kJkg−1 (saturated entropies) h = 632, hv = 2747 kJkg−1 K−1 (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 kg−1  h2 = h1 − ηT (h1 − h2) = 2959 − 0.85(2959 − 2540) = 2603 kJ kg−1 (isen. eff=85  h2 = xh + (1 − x)hv ⇒ 2603 = 632x + 2747(1 − x) ⇒ x = 0.068 Steam flow = 21.9×103 = 10.35 kg/s Steam flow = 10.35 = 11.13 kg/s 2747−632 1−0.068 to process thr. turbine W = 11.13(2959 − 2603) × 10−3 = 3.96 MW (Shaftwork generated)

Chen CL

28

Chen CL

29

Steam Turbine Economics: Gas Turbine: CP of exhause CP of airflow

Cost of = fuel = Cost of imported = electricity =

≈

C p for air = 1.03 kJ/kg/K

Net cost =

Gas Turbine Economics:

$0.06 s −1

Cost of = fuel = Electri -city = credit =

$0.20 s −1

Net cost =

−3

10 (21.9 + 3.96) × 3.2× 0.6

$0.14 s −1 (7 − 3.96) × 19.2 × 10−3

55.71 × 4.5 × 10−3 $0.25 s −1 (16.71 − 7) × 14.4 × 10−3 $0.14 s −1 $0.11 s −1

CPEXHAUST = 97 × 1.03 = 100 kW/K QEXHAUST = (400 − 10) × 100 × 10−3 = 39 MW QFUEL = 039 = 55.71 MW .7 W = 55.71 − 39 = 16.71 MW ⇒

Chen CL

30

Combined Heat and Power Schemes: Example The problem table cascade fo a process is given for ∆T min = 10oC. It is proposed to provide process cooling by steam generation from boiler feedwater with a temperature of 100oC. 1. Determine how much steam can be generated at a saturation temperature of 230 oC. 2. Determine how much steam can be generated at a saturation temperature of 230oC and superheated to the maximum temperature possible against the process. 3. Calculate how much power can be generated from the superheated steam from Part (2), assuming a single-stage condensing steam turbine is to be used with an isentropic efficiency of 85%. Cooling water is available at 20oC and is returned to the cooling tower at 30 oC.

Interval T (oC)

Heat flow (MW)

495

3.6

455 415 305

9.2 10.8 4.2

285 215 195

0.0 16.8 17.6

185 125 95

16.6 16.6 21.1

85

18.1

Chen CL

31

Solution (1): Heat available fr steam generation at 235oC interval temperature is 12.0 MW Latent heat of water a sat. temp. of 235 oC is 1812 kJ · kg−1K−1 3

Steam = 12.0 × 10 = 6.62 kg · s−1 Prod. 1812 Taking the heat capacity of water to be 4.3 kJ · kg−1K−1, heat duty on boiler feedwater preheating = 6.62 × 4.3 × 10−3(230 − 100) = 3.70 MW The process can support both boiler feedwater preheat and steam generation.

Chen CL

32

Chen CL

33

Turbine outlet conditions for isentropic expansion to 40 oC from steam tables are: P 2 = 0.074 bar. For S 2 = 6.488 kJ · kg−1 K−1, the wetness fraction (X ) and outlet enthalpy H 2 can be calculated (?)

Solution (2,3): Maximum superheat temp.: = 285oC interval = 280oC actual Heat available for steam generation at 235oC interval temperature is 12.0 MW. From steam table, enthalpy of superheated steam at 280oC and 28 bar is 2947 kJ · kg−1 ,

X = 0.23,

H 2 = 2020kJ · kg−1

For a single-stage expansion with isentropic efficiency of 85%: H 2 = 2947 − 0.85(2947 − 2020) = 2159kJ · kg−1 The power generation (W ) is given by

and enthalpy of saturated water at 230 oC and 28 bar is 991 kJ · kg−1 W = 6.13(2947 − 2159) × 10−3 = 4.8 MW

3 Steam = 12.0 × 10 = 6.13 kg · s−1 Prod. 2947 − 991

The wetness fraction for the real expansion is given by

The lowest condensing temp is cooling water temp plus ∆T min = 30 + 10 = 40oC. From steam table, inlet condition at T 1 = 280oC and P 1 = 28 bar are: H 1 = 2947 kJ · kg−1 ;

H 2 = 2159 = X H  + (1 − X )H v = 167.5X  + 2574(1 − X  )  = 0.17 ⇒ X

S 1 = 6.488 kJ · kg−1K−1

Chen CL

34

Heat Pump and Power Cycle

Chen CL

35

Integration of Heat Pumps Schematic of a simple vapor compression heat pump

Chen CL

36

Chen CL

37

Integration of Heat Pumps


Schematic of a simple vapor compression heat pump (again)


Chen CL

38

Chen CL

39





Chen CL

40

Chen CL

41




Integration of a heat pump above the pinch

The system converts power into heat !!

Chen CL

42

Chen CL

43



Integration of a heat pump below the pinch

Integration of a heat pump across the pinch

Power is turned into waste heat !!

Heat is pumped from a heat source part to a heat sink part Coefficient QHP + W COPHP = of W Performance

Chen CL

44

Chen CL


45


The grand composite curve

The grand composite curve allows heat pump cycles to be sized

A temperature lift greater than 25oC is rarely economic

Chen CL

46

Chen CL

47



Little scope for heat pumping across process pinch

Heat pump placed across a utility pinch

Chen CL

48

The

Grand Composite Curve allows 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

Recommend Documents