Energy Conservation in Process Industries

by:Dr.James Varghese

jamesvar@gmail.com

Mechanical Engineering,School of Engineering

Cochin University of Science and Technology

Outline

Process Process energy requirement PTA Heat exchanger network synthesis Network approach temperature optimization

Chemical process plantA chemical plant is an industrial process plant

that manufactures (or otherwise processes) chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials.

In a plant, each of the unit operations commonly occur in individual vessels or sections of the plant called units.

http://en.wikipedia.org/wiki/Chemical_process

Chemical process design

ProcessesReactions, separations, Mixing, pressure change ,heating, cooling, etc

Raw materials Products

Cold stream: Need to be heated (supply heat)

Hot stream: Need to be cooled (remove heat)

Energy, how much ?

Chemical reaction energy requirements

Reactor2

R1

R2

P1

20 155

Q

15

112

QQ

45

65

Q

175

125

Energy targeting

What is the minimum heat to be supplied (hot utility), heat to be removed (cold utility) ?Heat exchanger network design ?

10175

Mcp

40112

125

155 20

65

45

40

20

15

1300

2400

1080

2700

3700

3780

Q

Q

4565

125

0 1000 2000

Heat (kW)

Tem

pera

ture

(K) H1(10)

H2(40)

Hot composite curve (HCC)

0 1000 2000

Heat (kW)

H1(10)

H1(10)

H1+H2(10+40)

3000

175

Add MCp of streams in the same temperature range

4565

125

Heat (kW)

Tem

pera

ture

(K) H1(10)

H2(40)

Hot composite curve (HCC)

0

Heat (kW)

H1(10)

H1(10)

H1+H2(10+40)

175

0 1000 2000 0 1000 2000 3000

Cold composite curves

0 2000 4000

Heat (kW)

Tem

pera

ture

(K)

C3(20)

C4(15)

0 2000 4000

Heat (kW)

Tem

pera

ture

(K)

C3(20)

C3+C4(20+15)

C4(15)

4565

125

175

Grand composite curve (GCC)

250

350

450

550

0 1000 2000 3000 4000 5000 6000Heat (kW)

Tem

pera

ture

(K)

cold utility required

70K

HCC

CCC

30K

Hot utility required

Pinch

Minimum approach Tmin increases Heat load increases

Algorithm – Problem table algorithm (PTA)

Targeting

0

heat (kW)

Tem

pera

ture

(K)

cold utility required

HCC

CCC

30K

Hot utility required

Pinch

The composites can be moved laterally

4565

125

175

Process to process heat recovery

Minimum approach temperature increased

0

heat (kW)

Tem

pera

ture

(K)

cold utility required

HCC

CCC

70K

Hot utility required

Pinch

The hot utility and cold utility increased Process to process heat recovery reducedHeat exchanger cost reducedTrade –off between energy and capital to optimize the minimum approach temperature

4565

125

175

Energy targeting

10175

Mcp

40112

125

155 20

65

45

40

20

15

Problem Table Algorithm Assume ∆T, Ex: 20 º C Step 1: ∆T/2 is subtracted from

hot stream and ∆T/2 is added to cold stream temperature

Step 2: These shifted temperatures are sorted in descending order

Interval Tint ºC

0123456

175-10,155+10 =165112+10=122125-10=115

65-10=5540+10=5045-10=3520+10=30

Hot stream 175 45125 65

Cold stream 20 15540 112

Problem Table AlgorithmInterval Tint ºC

0123456

16512211555503530

Step4: Calculate net Mcp in each interval

Mcp,int= ƩMcp,c- ƩMcp,h Step 5: Calculate

enthalpy in each interval Qint= Mcp,int x Temp.diff

for that interval

Mcp,int Qint,KW

01025-15251020

0430175-900125150100

Hot stream 165 35 10115 55 40

Cold stream 30 165 2050 122 15

Problem Table Algorithm

Interval Tint ºC Mcp,int Qint,KW

0123456

16512211555503530

01025-15251020

0430175-900125150100

Step6: Calculate cascaded heat, subtract net enthalpy from the previous interval, Qcas

Step 7: Most negative Qcas in each interval is subtracted from each value in that column

R,cas = Q, cas – min(Qcas)

Q,cas, kW

R,cas, kW

0-430-60529517020-80

605175

0900775625525

Grand composite curve (GCC)

Useful to fix the utility levelsProblem Table Algorithm (PTA) method (Linnhoff and Flower, 1978)

0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

120

140

160

180

Hot Utility required

Cold Utility required

Grand composit curve

Pinch

Heat exchanger network meeting the energy target

Important Pinch Design Criteria Number Criterion

No utility cooling above the pinch Every hot stream must be brought to its pinch temperature by cold stream Nh ≤ Nc above the pinch Nh ≥ Nc below the pinch If the above condition is not satisfied then stream splitting is necessary

MCp Criterion MCp, h ≥ MCp, c above the pinch MCp, h ≤ MCp, c below the pinch If the above condition is not satisfied then stream splitting is necessary

Energy targeting

10175

Mcp

40112

125

155 20

65

45

40

20

15

125

105

Divides the problem into two – Above the pinch and Below the pinch

Energy targeting above pinch

10175

Mcp

40112

125

20

65

45

40

20

15

125

105

105

105

2.1

7.9

605 395

124.75155

H

Satisfies number criterion – not MCp criterion – so split the streams

Energy targeting

10175

Mcp

40

112

125

155 20

65

45

40

20

15

125

105Satisfies number criterion – MCp criterion not satisfied – so split the streams

975

Mc-23.75

Mc-16.25

1425

Mc-16.77

105

Mc-3.23275

97.5

525

Network for 20ºC minimum T

• Heat exchanger network synthesized based on pinch principles meeting the target set.• Further simplifications possible using the network evolution principles.

3780

370010

175Mcp

40

112

125

155 20

65

45

40

20

15

125

105 975

Mc-23.75

Mc-16.25

1425

Mc-16.77

105

Mc-3.23275

97.5

525

605

H

7.9

124.75

105

2.1

395

Total Annualised Cost

Cost of heat exchanger

Operating cost

Capital recovery factor

TAC = CRF x Chx + Copr

111

nn

i)+(i)+i(=CRF

Optimum temperature differenceC

ost $

/Yea

r

Tmin

TAC

Energy

Capital

References The pinch design method for heat exchanger networks, Linnhoff, B.

and E. Hindmarsh, Chem. Eng. Sci., 38(5), 745–763 (1983) Understanding heat exchanger networks, Linnhoff, B., E. Mason, and I.

Wardle, Comp.Chem. Eng., 3, 295–302 (1979). Heat Exchanger Network Synthesis: Process Optimization by Energy

and Resource Analysis, Uday V. Shenoy, Gulf Professional Publishing, 1995

J. Varghese and S. Bandyopadhyay, Fired Heater Integration into Total Site and Multiple Fired Heater Targeting, Applied Thermal Engineering, 42, 111-118, 2012.

 J. Varghese and S. Bandyopadhyay, Improved area—energy targeting for

fired heater integrated heat exchanger networks, Chemical Engineering Research and Design, 90, 213–219, 2012.

 Korobeinikov, A. and McCarthy, J. and Melnik, A. and Mooney, E. and

Rojas, J. and Semkov, K. and Varghese, J. and Zhelev, T, Model based methodology development for energy recovery in ash heat exchange systems Mathematics in Industry, http://www.maths-in-industry.org/miis/567/ , 2012.

 

References… contd.S. Bandyopadhyay ,J. Varghese and V Bansal, Targeting for cogeneration potential

through total site integration, Applied Thermal Engineering, 30 , 6–14, 2010. J. Varghese and S. Bandyopadhyay, Energy integration of fired heaters into overall

processes,International Journal of Environment and Sustainable Development (IJESD) Vol. 8, No. 1, 36-59, 2009.

 J. Varghese and S. Bandyopadhyay, Targeting and Integration of Total Site, 11th Conference on Process Integration, Modeling and Optimisation for Energy Saving and Pollution Reduction (PRES 2008) in conjunction with 18th International Congress of Chemical and Process Engineering (CHISA), Praha, Czech Republic, August 24-28, 2008.

 J. Varghese and S. Bandyopadhyay, Integration of multiple fired heaters into the process and network synthesis, Ind. Eng. Chem. Res. 46, 5631-5644, 2007.

 J. Varghese and S. Bandyopadhyay, Integration of fired heaters into total site, 19th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2006),Crete, Greece,Vol 2,715-722, 12-14 July, 2006.

 J. Varghese and S. Bandyopadhyay, Targeting for energy integration of multiple fired heaters, 19th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2006),Crete, Greece,Vol 2, 723-730, 12-14 July, 2006.

  J. Varghese and S. Bandyopadhyay, Energy integration of fired heater, Proceedings of International Mechanical Engineering Conference (IMEC-2004), Kuwait, Book 2, 30-46, December 5-8, 2004.