energy recovery by pinch technology

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Technical Note Energy recovery by pinch technology L. Matija seviæ * , H. Otma eiæ Faculty of Chemical Engineering and Technology, University of Zagreb, Savska c 16, 10000 Zagreb, Croatia Received 15 July 2001; accepted 17 September 2001 Abstract This paper shows how the application of pinch technology can lead towards great energy savings. The heat exchanger network of a nitric acid plant has been studied and it was found that it is possible to reduce requirements for cooling water and medium pressure steam. In order to enable these savings, three heat exchangers should be replaced with new ones. Energy consumption in steam power system increases slightly. However, the final result is a reduction of energy costs and a payback time of 14.5 months. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Energy recovery; Pinch technology; HEN design; Nitric acid plant 1. Introduction In this work the fundamental principles of the pinch for energy integration will be outlined and illustrated on the example of nitric acid production in the plant Petrochemical Industry––Kutina, located in Croatia. The plant produces 450 ton/day HNO 3 , w ¼ 5760%. The production technology dates back to 1982. 2. Case study––production of nitric acid 2.1. Analysis of energy in the process The process includes 17 heat exchangers, 2 turbines (steam and gases) and 2 compressors. The power of turbine is used for the compressors. Table 1 shows elementary data about exchangers, Applied Thermal Engineering 22 (2002) 477–484 www.elsevier.com/locate/apthermeng * Corresponding author. Tel.: +385-1-4597101/272318; fax: +385-1-4597133/4597260. E-mail address: [email protected] (L. Matija seviæ). 1359-4311/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S1359-4311(01)00098-9

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Page 1: Energy recovery by pinch technology

Technical Note

Energy recovery by pinch technology

L. Matija�sseviæ *, H. Otma�eeiæ

Faculty of Chemical Engineering and Technology, University of Zagreb, Savska c 16, 10000 Zagreb, Croatia

Received 15 July 2001; accepted 17 September 2001

Abstract

This paper shows how the application of pinch technology can lead towards great energy savings. Theheat exchanger network of a nitric acid plant has been studied and it was found that it is possible to reducerequirements for cooling water and medium pressure steam. In order to enable these savings, three heatexchangers should be replaced with new ones. Energy consumption in steam power system increasesslightly. However, the final result is a reduction of energy costs and a payback time of 14.5 months. � 2002Elsevier Science Ltd. All rights reserved.

Keywords: Energy recovery; Pinch technology; HEN design; Nitric acid plant

1. Introduction

In this work the fundamental principles of the pinch for energy integration will be outlined andillustrated on the example of nitric acid production in the plant Petrochemical Industry––Kutina,located in Croatia.

The plant produces 450 ton/day HNO3, w ¼ 57–60%. The production technology dates back to1982.

2. Case study––production of nitric acid

2.1. Analysis of energy in the process

The process includes 17 heat exchangers, 2 turbines (steam and gases) and 2 compressors. Thepower of turbine is used for the compressors. Table 1 shows elementary data about exchangers,

Applied Thermal Engineering 22 (2002) 477–484www.elsevier.com/locate/apthermeng

*Corresponding author. Tel.: +385-1-4597101/272318; fax: +385-1-4597133/4597260.

E-mail address: [email protected] (L. Matija�sseviæ).

1359-4311/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S1359-4311(01)00098-9

Page 2: Energy recovery by pinch technology

Table 2 shows the power of single units, and Table 3 shows production and consumption energyin the nitric acid plant. In case study analyses the energy consumption of the process. The HPsteam generated by heat transfer is utilized for other process in Petrochemical Industry. Thepower generated on turbine is used for compressor work. Fig. 1 shows the process flowsheet wherepinch methods are outlined.

The design employs 17 heat exchangers. The exchanger E103 for ammonia vaporization, andE115 air heater will not be considered. The energy MP steam (12.2 bara), LP steam (4.4 bara),cooling water and boiler feedwater is consumption for 16 streams. The data are given in Table 4.

Nomenclature

Symbolscp average specific heat capacity, kJ/kg �CCP heat capacity, kW/�CDH change in enthalpy, kWm mass flow rate, kg/sT temperature, �CDTmin minimum temperature difference, �C

IndicesT target stateS source stateh cold streamt hot stream

Table 1

Elementary data about exchangers

Tube-side temperature (�C) Shell-side temperature (�C) Heat load (W)

Ti To Ti To

E103 28 11.7 35 8.5 150 938

E112 210 187 8.5 85 153 600

E113 210 187 13.5 100 347 089

E102 194 120 50 70 360 461

E104 28 38 44 42.5 10 467 500

E105 258 400 850 767 2 581 050

E106 258 258 767 425 10 213 544

E107 427 337 235 350 2 695 319

E110 340 270 258 258 1 936 662

E119 255 50 28 38 175 928

E111 283 169 90 235 3 287 958

E116 25 90 210 187 1 489 704

E109 174 38 28 38 12 245 713

E108 232 140 105 200 2 631 366

E114 148 46 26.5 39.5 4 235 944

478 L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484

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Fig. 1. The process flowsheet of nitric acid plant.

Table 3

Production and consumption energy in actual nitric acid plant

Energy production

HP steam (44 bar) 7119 kg/h

Energy consumption

MP steam (12.2 bar) 4800 (summer)/9800 kg/h (winter)

LP steam (4.4 bar) 703 kg/h

Cooling water 2702 m3/h

Boiler feedwater 3431 kg/h

Table 2

Power of recovery and consumption units

Recovery units Consumption units

Steam turbine 3415 kW 40.2% Air compressor 4710 kW 56.0%

Gases turbine 5070 kW 59.8% Nitrous compressor 3775 kW 44.0%

L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484 479

Page 4: Energy recovery by pinch technology

Choosing the correct value of DTmin is crucial. The DTmin is easily identified using the pinchdesign method. Increasing DTmin will decrease the amount of heat exchanged into the system.

In our case study with DTmin ¼ 38 �C, the amount of heat exchanged is the same as DTmin ¼ 10�C, 25 133.7 kW. Capital cost of cold utilities decreases. In the case considered, the pinch tem-perature is 277 �C, with minimum transferred energy 3.8 kW.

2.2. The network design [1]

The network above pinch, features one hot stream and three cold streams (Fig. 2). There arethree heat exchangers (shown as linked circles on the relevant streams). It is suggested to linkstreams 3 and 13 because term CPh 6CPc is fulfilled.

Table 4

Area and cost of new exchangers

Heat exchanger A (m2) C ($)

Ammonia vaporizer 13.8 9024

Preheater NH3 50.8 33 022

Waste heat boiler 172.2 110 112

Total 152 058

Fig. 2. Network design above pinch: (a) existing arrangement of nitric acid plant and (b) retrofit existing network.

480 L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484

Page 5: Energy recovery by pinch technology

Network design below pinch, features one hot stream and one cold stream. The match betweenstreams 3 and 9 is feasible because the CP of the hot stream is greater than of the cold stream. Thenetwork design of the process–production of nitric acid is shown on Fig. 3.

The grid representation of heat exchangers in the actual nitric acid plant show that the streams10 and 14 are connected by exchanger E102, streams 6 and 15 are connected by exchanger E108.The stream 16 has a higher temperature, but has not enough heat for transferring. A good so-lution is to connect stream 6 with streams 1 and 2. If the heat capacities of streams are such, that isnot possible to make a match, then the heat capacity can be altered by splitting a stream. Dividingthe stream will reduce the mass flow rates in each leg and hence the heat capacities. This is il-lustrated in Fig. 4 with stream 2 which is split.

Fig. 4 shows the proposed heat exchanger network after the synthesis above and below thepinch. The network features nine hot streams, 3–8, 10, 12 and 16 (running from left to right at thetop) and seven cold streams, 1, 2, 9, 11 and 13–15 (running from right to left at the bottom). Aloop exists between exchanger 3 and 5 but with different heat transferred. Exchanger E111 is not

Fig. 3. Network design below pinch.

L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484 481

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satisfactory, exchangers E109 (streams 3, 4 and 5) and E114 (streams 6, 7 and 8) needs less area.Exchanger E116 is cancelled.

3. Results

New heat exchangers should be added using the nitrous gas for vaporization of ammonia andoverheating (exchanger 9þ 10 on the Fig. 4) for preheating of ammonia (exchanger 8) and finallyfor preheating waste gases (part of exchanger 5). Capital cost estimated for heat exchangers isbased on the purchase cost. Table 4 shows areas and the price of heat exchangers. The purchasecost and capital cost are estimated using correlations by the authors Sinnot and Ulrich:

Capital cost is 152058� 2:84 ¼ 431845 $:

Fig. 4. The proposed heat exchanger network, DTmin ¼ 38 �C.

482 L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484

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The results of changes will influence streams of flash vessel D101. Table 5 shows streams data ofD101 before and after the changes. There will be a decrease in the usage of cooling water si-multaneously with the changes in the process. Table 6 shows the final results of the changes.Summary of annual cost:

Exchangers amortized over ten years: 431845=10 ¼ 43184:5 $=year:Utilities: 357120 $=year:

4. Conclusion

This paper shows how the application of pinch technology makes it possible to reduce thedemand for cooling water and medium pressure steam. With the problem table algorithm, datawere quickly extracted from the flowsheet and were analyzed for energy saving.

From the thermodynamic point of view the process requires only cooling utilities and does notneed any heating utilities. This case study corresponds to that of a threshold case where only coldutility is needed. The nitrous gas stream is used for heat transfer with ammonia without anyadditional heating stream.

Table 6

Utilities cost after changing

Cost

Increase consumption energy

LP steam 1380� 703 ¼ 677 kg/h �7.0 $/hBoiler feedwater 6350� 3431 ¼ 2919 kg/h �1.9 $/h

Decrease consumption energy

MP steam 4800� 0 ¼ 4800 kg/h �52.2 $/hCooling water 2247� 2075 ¼ 172 m3/h �3.2 $/hDifference: (52:2þ 3:2)� (7:0þ 1:9)¼ 46.5 $/h � 7680 h/y¼ 357 120 $/y

Table 5

The inlet and outlet streams of vessel D101

Streams T (�C) Mass flow (kg/h)

Before exchange After exchange

In

LP steam 156 703 1380

Boiler feedwater 95 3431 6350

Condensate MP steam 187 3500 –

Turbine condensate 70 15 361 15 361

Out

Feedwater 105 22 845 22 835

Saturated steam 105 150 256

L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484 483

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The total number of exchangers is one less than in the original case and three of them had to beredesigned. The final result is reduction of energy costs with the payback time of 14.5 months.

Reference

[1] B. Linnhoff et al., Users Guide on Process Integration for the Efficient Use of Energy, IChemE, London, 1994.

484 L. Matija�sseviæ, H. Otma�eeiæ / Applied Thermal Engineering 22 (2002) 477–484