Exergoeconomic Evaluation of LNG and NGL Co-production Process Based on the MFC Refrigeration Systems

Document Type: Research Paper

Authors

1 Department of Energy Systems Engineering, Petroleum University of Technology (PUT), Mahmoudabad, Iran

2 Renewable Energies and Environmental Department, Faculty of New Science and Technologies, University of Tehran, Iran

Abstract

In this paper, exergy and exergoeconomic analysis is performed on the recently proposed process for
the coproduction of liquefied natural gas (LNG) and natural gas liquids (NGL) based on the mixed
fluid cascade (MFC) refrigeration systems, as one of the most important and popular natural gas
liquefaction processes. To carry out this analysis, at first, the proposed process is simulated, and then
the exergy analysis of the process equipment is performed; finally, an economic model is used for the
exergoeconomic analysis. The results include cost of exergy destruction, exergoeconomic factor,
exergy destruction, and exergy efficiency. The results of the exergy analysis demonstrate that the
exergy efficiency of the proposed process is around 53.83%, and its total exergy destruction rate is
42617.5 kW at an LNG and NGL production rates of 68.99 kg/s and 27.41 kg/s respectively. The
results of exergoeconomic analysis indicate that the maximum exergoeconomic factor, which is
69.53%, is related to the second compressor in the liquefaction cycle and the minimum
exergoeconomic factor, which is 0.66%, is related to the fourth heat exchanger in the liquefaction
cycle. In this process, demethanizer tower holds the highest relative cost difference (100.78) and the
first air cooler in liquefaction cycle has the lowest relative cost difference (1.09). One of the most
important exergoeconomic parameters is the cost of exergy destruction rate. The second heat
exchanger has the highest exergy destruction cost (768.91 $/Gj) and the first air cooler in the
liquefaction cycle has the lowest exergy destruction cost (19.36 $/Gj). Due to the high value of fuel
cost rate (as defined in exergoeconomic analysis) in heat exchangers, their exergy destruction cost is
much higher than other devices.

Keywords


Liquefied natural gas (LNG) is regarded as one of the convenient energy carriers, especially for transportation to very long distances; it is produced directly from the natural gas by the relevant refrigeration processes. Natural gas liquids (NGL), which is used as a main feed in petrochemical processes (Elliot et al., 2005), is also produced by the relevant refrigeration system (Finn, 1999). LNG processes are classified by the refrigerant composition and the refrigeration system (Shukri, 2004). NGL and LNG production are performed in cryogenic processes in which the refrigeration system is a main part. Increasing the level of integration is a fundamental way for improving the efficiency and decreasing the operating and capital costs (Elliot et al., 2005). In this paper, the recently alternative integrated process for the cogeneration of LNG and NGL as proposed by (Mehrpooya et al., 2014) is investigated; specifically, an exergy analysis is performed to find the location, magnitude, and sources of inefficiencies in the proposed process.

The exergy analysis is used for the evaluation of LNG and NGL processes by many researchers. Exergy analysis was performed on four small-scale LNG processes and it was shown that the SMR process has the best exergy efficiency among four processes (Remeljeja and Hoadley, 2006). Vatani et al. (2014a) carried out energy and exergy analyses on five conventional LNG processes, and they concluded that the performance of the MFC process is considerable in terms of quality and quantity of energy consumption. Mehrpooya et al. (2006) performed exergy analysis on industrial refrigeration cycle used in NGL recovery units. Kanoglu (2002) presented the exergy analysis of cascade refrigeration cycle used for natural gas liquefaction. The exergetic efficiency of the multistage cascade refrigeration cycle is determined to be 38.5% indicating a great potential for improvements. Mafi (2009) carried out exergy analysis for multistage cascade low temperature refrigeration systems utilized in olefin plants. The exergy analysis results indicate that the major irreversibilities are due to losses within the compression system and driving forces across the heat exchangers. Morosuk et al. (2008) presented the exergy analysis of absorption refrigeration machines by a new method.

In exergy analysis, however, there is no term that directly treats costs associated with exergy destruction, which can only be determined by exergoeconomic analysis. PRICO liquefaction process exergy-based analyses, i.e. exergetic, exergoeconomic, and exergoenvironmental analyses, were performed by Morosuk et al. (2015). Ghorbani et al. (2012) carried out exergy and exergoeconomic analyses on natural gas separation process. The results show that the percentage increases in the unit exergoeconomic costs of the compression and the demethanizer section are the highest. The exergoeconomic analysis of an industrial refrigeration cycle was investigated by Mehrpooya et al. (2009), in which the impact of component inefficiencies on the fuel plant consumption, intrinsic, and induced malfunctions were analyzed and quantified. Siddiqui et al. (2014) presented exergoeconomic analysis on the refrigeration cycle in which the components of the cycle are compared based on the initial capital and the irreversibilities costs. Moreira et al. (2005) carried out exergoeconomic analysis for a double effect absorption refrigeration system with the direct combustion of the natural gas in which fixed capital investment for each subsystem of the unit was considered. Garousi Farshi et al. (2013) performed exergoeconomic analyses for combined ejector double effect and flow double effect systems in series to compare the influence of different operating parameters on investment product cost flow rates and costs of the overall systems.

In this article, exergy and exergoeconomic analyses are applied to recently alternatives integrated processes for the cogeneration of LNG and NGL with reasonable energy consumption and high ethane recovery. After the determination of high consuming and inefficient elements, a relationship between economic costs and investment is also proposed and the components with the highest cost of exergy destruction are identified.

2. Process description

The process flow diagram (PFD) of the mixed fluid cascade (MFC) integrated process is shown in Figure 1. As seen, the NGL recovery and LNG production are carried out in an integrated process (Mehrpooya et al., 2014). A brief description is presented, and one may refer to the reference available for further details.

 Figure 1

Process flow diagram of MFC configuration (Mehrpooya et al., 2014).

2.1. NGL recovery section

Cleaned and pretreated natural gas feed, with composition shown in Table 1, enters the plant at 37 °C and 63.09 bar, and is cooled in two steps: in the E-1A heat exchanger to 3 °C, and further to -30 °C in the second heat exchanger, E-1B. 40% of D-2 gas product is directed to the exchanger E-2 and is sub-cooled to about -88 °C (stream 114). Next, stream 114 is directed to the top of the demethanizer tower via a J-T valve as cold reflux. Another portion of the outlet vapor from D-2 is expanded thought a turbo expander prior to entering the demethanizer, right below the top section of the tower. Also, the liquid bottom is spilled into two parts: stream 108 is introduced to the column for fractionation through passing a J-T valve, and another portion, stream 107, is sub-cooled via E-2 to -30 °C and enters the tower through a J-T valve. Demethanizer tower operates at about 25 bar and contains conventional trays used in demethanizer columns. This tower has three liquid draw trays to provide the required heat for stripping volatile component from the produced NGL. The required heat is supplied by two multi-stream heat exchangers, E-1A and E-1B. Side streams 1, 2, and 3 enter the heat exchangers at 17.7, -7.9, and -54 °C and return to the tower at 35, 0, and -15 °C respectively. In this configuration, no reboiler is needed, and ethane recovery is enhanced to 92% (Mehrpooya et al., 2014).

2.2. Liquefaction section

Lean gas stream that leaves the demethanizer tower at about -97 °C and 25 bar enters the LNG section. Stream 112 is pressurized to about 63 bar in compressor C-100, and it is then cooled in E-2 to about -85.2 °C. In this process, final cooling in LNG production is performed in the E-3 heat exchanger, and the stream 119 is cooled to about -162.5 °C and delivered to the D-1 flash drum by passing a J-T valve. The liquid product of D-1 is LNG at atmospheric pressure (Mehrpooya et al., 2014).

Table 1

MFC configuration main streams data.

 

 

Feed gas

Cycle 200

Cycle 300

Cycle 400

NGL

LNG

Composition

methane

83.00

10.76

40.00

0.00

0.68

98.82

ethane

6.72

37.78

0.00

0.00

47.83

0.63

propane

5.18

19.84

0.00

73.74

39.58

0.06

n-butane

0.00

0.00

0.00

15.15

0.00

0.00

ethylene

0.00

31.62

42.00

11.11

0.00

0.00

nitrogen

0.50

0.00

18.00

0.00

0.00

0.41

Dioxide carbon

0.20

0.00

0.00

0.00

1.08

0.07

C4+

4.40

0.00

0.00

0.00

10.83

0.00

Operation Condition

Temperature (°C)

37.00

35.00

35.00

40.00

28.40

-162.74

Pressure (bar)

63.09

27.90

29.00

16.90

25.00

1.01

molar flow (kmol/hr)

18000

19000

12500

27400

2404

15315

2.3. Refrigeration system

Because of the advantages of the mixed refrigerant systems, including its high thermal efficiency and high flexibility, it was used in the proposed integrated process (Mehrpooya et al., 2014).

2.3.1. The hottest cycle (cycle 400)

The process flow diagram of this cycle is shown in red lines in Figure 1. The first mixed refrigerant cycle provides the required refrigeration for precooling of the feed; it is also a heat sink for cooler cycles, cycles 200, and 300. The cycle 400 refrigerant is a mixture of propane and ethane. The refrigerant compositions are presented in Table 1.

2.3.2. The middle cycle (cycle 200)

This cycle, shown in green lines in Figure 1, provides a portion of the required refrigeration for the liquefaction section and the main portion of the required refrigeration for NGL recovery unit. Also, the middle cycle is a heat sink for the coldest cycle, i.e. cycle 300. The refrigerant in this cycle is composed of methane, ethane, propane, and ethylene.

2.3.3. Liquefaction cycle (cycle 300)

The main function of this cycle, shown in blue lines in Figure 1, is supplying the required refrigeration for liquefaction and sub-cooling. The refrigerant in this cycle is composed of methane, ethylene, and nitrogen. The thermodynamic data of the streams are shown in Table 2.

3. Process simulation

The simulation of the proposed integrated processes is carried out by Aspen HYSYS software, employing the Peng–Robinson–Stryjek–Vera (PRSV) equation of state. The main equipment power consumption in this process, specific energy consumption (SEC), and the coefficient of performance (COP) are presented in Table 3.

4. Exergy analysis

Table 2

Thermodynamic data for MFC configuration material streams.

Stream

no.

Temperature

(°C)

Pressure

(bar)

Flow

(kmol/hr.)

Stream

no.

Temperature

(°C)

Pressure

(bar)

Flow

(kmol/hr.)

feed

37.00

63.09

18000

300

35.00

29.00

12500

101

3.00

63.09

18000

301

3.00

29.00

12500

102

-30.00

63.09

18000

302

-27.00

29.00

12500

103

-30.00

63.09

16550

303

-85.20

29.00

12500

104

-30.00

63.09

1449

304

-159.00

29.00

12500

105

-30.00

63.09

6620

305

-166.85

3.50

12500

106

-30.00

63.09

9930

306

-89.56

3.50

12500

107

-30.00

63.09

435

307

53.70

25.00

12500

108

-30.00

63.09

1015

308

35.00

25.00

12500

109

-65.13

26.00

9930

309

48.50

29.00

12500

110

-62.86

25.00

435

400

40.00

16.90

27400

112

-96.95

25.00

15595

401

8.80

16.90

27400

113

-36.72

63.00

15595

402

8.80

16.90

10910

114

-88.00

63.09

6620

403

-22.00

16.90

10910

115

-97.38

25.00

6620

404

-29.38

3.00

10910

116

-50.00

63.09

435

405

1.25

3.00

10910

117

-47.88

25.50

1015

406

8.80

16.90

16490

118

-85.20

63.00

15595

407

-0.34

6.70

16490

119

-162.50

63.00

15595

408

33.58

6.70

16490

120

-162.74

1.01

15595

409

35.34

6.70

27400

121

-162.74

1.01

280

410

37.99

6.70

10910

200

35.00

27.90

19000

411

81.86

16.90

27400

201

3.00

27.90

19000

side1

17.74

25.00

2300

202

-27.00

27.90

19000

side2

-7.88

25.00

2300

203

-81.50

27.90

19000

side3

-54.08

25.00

2300

204

-90.99

3.10

19000

side1R

35.00

25.00

2300

205

-29.72

3.10

19000

side2R

0.00

25.00

2300

206

67.63

15.00

19000

side3R

-15.00

25.00

2300

207

35.00

15.00

19000

NGL

28.40

25.00

2404

208

77.90

27.90

19000

LNG

-162.74

1.01

15315

In exergy analysis, the maximum useful work achievable by specific quantity of energy is calculated. Important parameters that are obtained from exergy analysis are exergy destruction, exergy efficiency, and exergy destruction ratio, as shown in the following expression (Bejan et al., 1996):

 

(1)

  or 

(2)

 

(3)

where, Ė shows exergy rate, and subscripts F, P, D, and k denote the fuel, product, destruction, and component respectively. Also, the exergy data of the streams are shown in Table 4.

Table 3

Main equipment power consumption, specific energy consumption, and coefficient of performance.

   

Component Name

Power(kW)*

Compressors

C-100

6064.86

 

 

C-200A

24107.69

 

 

C-200B

10203.72

 

 

C-300A

16807.21

 

 

C-300B

1664.71

 

 

C-400A

7123.01

 

 

C-400B

21791.78

Turbo expander

 

TE-100

2319.30

Air coolers

AC-200A

92.60

   

AC-200B

1393.89

 

 

AC-300A

299.75

 

 

AC-300B

232.81

   

AC-400

4851.01

Specific energy (kWh/kg LNG)

 

0.3572

COP

 

2.218

* Mechanical efficiency =0.75

 

 

               

5. Exergoeconomic analysis

Exergoeconomic combines exergy analysis with the principles of economy to provide the information for designing a system, not possible through conventional energy analysis and economic evaluations.

5.1. Economic model

In the course of energy systems, economic analysis and optimization, annual investment, fuel cost, and systems maintenance cost must be calculated. In this research, total revenue requirement (TRR) method, as developed by the Electric Power Research Institute (EPRI, 1993), is used for the economic analysis of the system. In this method, all the costs, including return on investment, are also calculated; based on the hypotheses given in Table 5, equipment and fuel purchase prices, and the total revenue requirement are calculated annually. Finally, all the costs including maintenance and fuel costs during system operation would be balanced.

Table 4

Exergy and unit exergy cost for each stream of MFC configuration.

Stream

no.

ĖPH

(kW)

ĖCH

(kW)

ĖTOT

(kW)

Ċ

($/hr)

c

($/Gj)

Stream

no.

ĖPH

(kW)

ĖCH

(kW)

ĖTOT

(kW)

Ċ

($/hr)

c

($/Gj)

feed

48837

4947926

4996763

355201

19.74

300

28136

3146299

3174436

852985

74.64

101

49019

4947926

4996945

355361

19.75

301

28239

3146299

3174538

853076

74.65

102

50898

4947926

4998823

356224

19.79

302

28894

3146299

3175193

853376

74.66

103

46736

4262763

4309499

307106

19.79

303

39759

3146299

3186058

856681

74.69

104

3599

685725

689324

49123

19.79

304

59385

3146299

3205684

862276

74.72

105

18694

1705105

1723800

122842

19.79

305

58069

3146299

3204369

862276

74.75

106

28041

2557658

2585699

184263

19.79

306

14336

3146299

3160635

850508

74.75

107

1080

205717

206797

14737

19.79

307

27132

3146299

3173431

852669

74.64

108

2519

480008

482527

34386

19.79

308

26966

3146299

3173265

852704

74.64

109

24605

2557658

2582263

184019

19.79

309

28243

3146299

3174542

852955

74.64

110

1083

205718

206801

14755

19.82

400

40799

16532598

16573397

6225893

104.35

112

40443

3621054

3661497

262010

19.88

401

40519

16532598

16573117

6225647

104.35

113

44571

3621054

3665625

262834

19.92

402

16134

6582870

6599004

2478898

104.35

114

23729

1705105

1728834

124374

19.98

403

17366

6582870

6600236

2479464

104.35

115

23114

1705105

1728219

124374

19.99

404

16918

6582870

6599788

2479464

104.36

116

1140

205717

206857

14756

19.81

405

8128

6582870

6590999

2476162

104.36

117

2346

480008

482354

34386

19.80

406

24385

9949728

9974113

3746749

104.35

118

54931

3621054

3675985

265985

20.10

407

23850

9949728

9973578

3746749

104.35

119

76598

3621054

3697653

272163

20.45

408

20385

9949728

9970113

3745447

104.35

120

74191

3621054

3695246

272163

20.46

409

33905

16532598

16566503

6222568

104.34

121

281

58726

59006

4346

20.46

410

13525

6582870

6596395

2477121

104.31

200

40366

8003347

8043713

2264895

78.21

411

51073

16532598

16583671

6225338

104.27

201

41870

8003347

8045217

2266218

78.25

side1

4308

1184086

1188394

446441

104.35

202

46445

8003347

8049792

2268319

78.27

side2

4683

1133586

1138270

427635

104.36

203

55031

8003347

8058378

2270931

78.28

side3

5863

1031562

1037425

389749

104.36

204

53551

8003347

8056898

2270931

78.29

side1R

4331

1184020

1188352

446425

104.35

205

15917

8003347

8019264

2260323

78.29

side2R

4587

1133253

1137840

427473

104.36

206

34600

8003347

8037947

2263374

78.22

side3R

4970

1031491

1036461

389386

104.36

207

33767

8003347

8037115

2263389

78.23

NGL

4158

1330829

1334987

95529

19.88

208

41782

8003347

8045129

2264734

78.19

LNG

73870

3562369

3636239

267820

20.46

5.2. Levelized costs

With an increase in the system operating years, investment related costs decreases, while fuel costs increase. Therefore, the balanced amount of annual total revenue requirement (TRRL)is calculated by the capital return factor (CRF) and a decrease of money value as given in Equation 4 (Bejan et al., 1996):

 

(4)

where, TRRj is the total revenue requirement in jth year of system operation, and BL denotes economic life cycle of the system as measured in years; ieff stands for the average annual rate of effective devaluation. Capital recovery factor (CRF) is calculated as follows:

 

(5)

In this study, the value for TRRj is the sum of four annual values, including minimum return on investment (ROI), total capital recovery (TCR), operation and maintenance costs (OMC), and fuel costs (FC).

Table 5

Economic constants and assumptions.

Economic parameters

Value

Average annual rate of the cost of money (ieff)

10%

Average nominal escalation rate for the operating and maintenance cost (rOMC)

5%

Average nominal escalation rate for fuel (rFC)

5%

Plant economic life (book life)

25 years

Total annual operating hours of the system operation at full load

7300

More details on the general procedure, definitions, and calculations are presented elsewhere (Bejan et al., 1996):

 

(6)

Fuel cost for this system (as defined in exergoeconomic analysis) is the electricity cost which is calculated for the first year of operation as follows:

 

(7)

where, τ is operating hours of the system per year (7300 hrs) and is the compressor power (kW); cw is a constant related to the electricity cost. Then, the levelized value of FCL for the series is calculated by multiplying the fuel cost at the beginning of the first year of operation by the constant-escalation levelization factor (CELF):

 

(8)

where,

   

    (9)

where, rFC and CRF are annual escalation rate of fuel cost and capital recovery factor respectively. Similarly, the levelized annual operation and maintenance cost (OMCL) is obtained by the following equation:

 

(10)

where,

   

(11)

where, rOMC is the nominal escalation rate of operation and maintenance costs. Finally, the levelized carrying charge (CCL) is obtained by the following relation:

 

(12)

where, Ż shows the cost rate associated with the capital investment and operating and maintenance costs:

 

(13)

where, τ and PECk denote the total annual time (hours) of system operation at full load and the purchased-equipment cost of the kth system component respectively. The equations for estimating cost of the equipment is shown in Table 6 and the results calculated are shown in and Table 7.

Table 6

Equation of cost of the process unit.

Component

Purchased equipment cost functions

Compressor

CC=7.90(HP)0.62  [43]

CC= Cost of Compressor (k$)

Heat exchanger

C=1.218*fdfmfpCb

Cb=exp[8.821-0.30863(lnA)+0.0681(lnA)2], 150<A<12000,

fd=exp(-1.1156+0.0906*(ln(A))),  fM= Material Factor,  fP=Pressure Factor

Separator

C=1.218[a+bW], K$  5<W

a= 42,  b=1.63

Air cooler

CAC=1.218fmfPexp[a+blnQ+c(lnQ)2],  Q in KSCFM [43]

CAC= Cost of Air cooler (k$)

fm=Material Factor,  fP=Pressure Factor, a=0.4692, b=0.1203, c=0.0931

Turbo Expander

CTE=0.378(HP)0.81  [43]

CTE= Cost of Turbo Expander (k$)

Absorption

CT=1.218[f1Cb+Nf2f3f4Ct +Cp1]

Ct =457.7 exp (0.1739D),  2<D

Cb=1.218 exp [6.629+0.1826(lnW)+0.02297(lnW)2], 4250<W<980,000 lb shell

Cp1=300D0.7396 L0.7068, 3<D<21, 27<L

f1= Material Factor,  f2= 1.189+0.0577D, f3=Tray Types Factor, f4=2.25/(1.0414)N

5.3. Cost balance equations

To calculate the unit cost of exergy for each stream, exergy balance equations are written for each component as follows:

 

(14)

For some of the equipment with more than one output flow, there is more than one unknown parameter, so auxiliary equations based on the laws of P and F (Lazzaretto and Tsatsaronis, 1997) for these equipment are defined. Tables 8 and 9 respectively show the cost balance and auxiliary equations for a system. Considering that, for some of the components, equations cannot be solved independently; there is a set of linear dependent equations that should be solved simultaneously. A computer program is developed in the MATLAB for solving the cost balance and auxiliary equations to obtain the unit exergy cost for each flow, as presented in Table 4.

Table 7

Purchased equipment and investment costs for MFC process components.

 

PEC ($)

ŻCI ($/hr)

ŻOM ($/hr)

Ż ($/hr)

E-1A

123171.36

8.01

0.20

8.21

E-1B

79843.06

5.19

0.13

5.32

E-2

127701.63

8.30

0.21

8.51

E-3

65857.68

4.28

0.11

4.39

AC-200A

56774.85

3.69

0.09

3.78

AC-200B

56774.85

3.69

0.09

3.78

AC-300A

24819.81

1.61

0.04

1.65

AC-300B

56774.85

3.69

0.09

3.78

AC-400

103729.52

6.74

0.17

6.91

C-100

2098886.04

136.44

3.46

139.90

C-200A

4938254.48

321.01

8.14

329.15

C-200B

2897793.17

188.37

4.78

193.15

C-300A

3948609.45

256.68

6.51

263.19

C-300B

941603.69

61.21

1.55

62.76

C-400A

2318935.48

150.74

3.82

154.56

C-400B

4654001.65

302.53

7.67

310.20

TE-100

255051.40

16.58

0.42

17.00

D-1

55387.15

3.60

0.09

3.69

D-2

74983.07

4.87

0.12

4.99

T-101

77848.47

5.06

0.13

5.19

Table 8

Main equations for MFC process.

Equipment

Main Equation

E-1A

 

E-1B

 

E-2

 

E-3

 

V-1

 

V-2

 

V-3

 

V-4

 

V-100

 

V-101

 

V-200

 

V-300

 

C-100

 

C-200A

 

C-200B

 

C-300A

 

C-300B

 

C-400A

 

C-400B

 

TE-100

 

AC-200A

 

AC-200B

 

AC-300A

 

AC-300B

 

AC-400

 

D-1

 

D-2

 

TEE-100

 

TEE-101

 

TEE-102

 

MIX-1

 

T-101

 
         

Table 9

Auxiliary equations for MFC process.

Equipment

Auxiliary Equation

E-1A

 

E-1B

 

E-2

 

E-3

 

TE-100

 

D-1

 

D-2

 

TEE-100

 

TEE-101

 

TEE-102

 

T-101

 

5.4. Exergoeconomic variables

As we define one fuel and production for every system components in exergy analysis, cost flow rate related to fuel (ĊF) and production (ĊP) of a component can be obtained similar to exergy flow rate of ĖF and ĖP. Average fuel cost for kth element of a system (cF,k) shows the average cost by which unit fuel exergy for the kth element is supplied:

 

(15)

Unit product cost (cp,k) is defined as the average cost by which unit exergy for the product of the kth element is provided:

 

(16)

Cost of exergy destruction for the kth element in the system, which is related to exergy destruction (ĖD,k), is considered a hidden cost which can only be revealed by an exergoeconomic analysis:

 

(17)

Relative cost difference between the average cost per exergy unit of product and fuel is given by:

 

(18)

Exergoeconomic factor indicates the ratio of investments cost to investment and exergy destruction costs as follows:

 

(19)

6. Results and discussion

6.1. Exergy analysis

In this paper, exergy and exergoeconomic analyses are applied to the recently alternatives integrated processes for the cogeneration of LNG and NGL. The exergy analysis of this process results are presented in Table 10. In this process, the highest irreversibility is related to the compressor C-200A, with a value of 5423 kW, and the second highest value was related to units C-400B, C-300A, and E-2 respectively. Exergy efficiency for expansion valves was lower than the other units. The exergy efficiency of this process is around 53.83%, and its total exergy destruction rate is equal to 42617.5 kW.

Table 10

Exergy efficiency and exergy destruction of MFC process.

Component

ĖD (kW)

ε (%)

Component

ĖD (kW)

ε (%)

C-100

1937.11

68.06

E-1A

1934.46

97.77

C-200A

5423.14

77.50

E-1B

1438.37

97.73

C-200B

2188.88

78.55

E-2

2727.97

96.84

C-300A

4010.81

76.14

E-3

2439.32

94.31

C-300B

387.46

76.72

V-1

615.25

54.00

C-400A

1726.58

75.76

V-2

172.76

53.00

C-400B

4623.88

78.78

V-3

56.16

69.00

TE-100

1117.28

67.49

V-4

2406.98

31.00

AC-200A

693.85

98.00

V-100

534.65

30.00

AC-200B

835.19

98.00

V-101

447.51

40.00

AC-300A

272.65

99.00

V-200

1480.35

90.00

AC-300B

283.69

99.00

V-300

1315.85

86.00

AC-400

1825.99

96.00

MIX-1

4.77

100.00

T-101

1716.58

50.01

 

 

 

6.2. Exergoeconomic analysis

By exergoeconomic analysis, a rational relationship between initial investment and current costs, due to failures, can be established, which enables us to determine whether a system works economically or not. In this method, the investment costs are estimated first, as shown in Table 7, and then, using total revenue requirement method and writing cost balance equation, exergy unit cost for every stream is determined (Table 4). Finally, the exergoeconomic factor and relative cost difference are determined by exergoeconomic analysis and the results are shown in Tables 11. The exergoeconomic factor value shows information about the investment cost and exergy efficiency of a system: large values indicates that in order to decrease the system cost, elements costs must be reduced, while small values indicates that in order to reduce the system cost, the performance and efficiency of a system must be improved.

Table 11

The results of exergy and exergoeconomic analysis of MFC process.

Component

ĖD

(kW)

CF

($/Gj)

CP

($/Gj)

ĊD

($/hr)

Ż

($/hr)

ε

(%)

YD

(%)

r

(%)

f

(%)

C-100

1937.11

19.72

38.39

137.52

139.90

68.06

2.10

94.67

50.43

C-200A

5423.14

19.72

30.34

384.99

329.15

77.50

5.87

53.84

46.09

C-200B

2188.88

19.72

31.80

155.39

193.15

78.55

2.37

61.26

55.42

C-300A

4010.81

19.72

31.61

284.74

263.19

76.14

4.34

60.31

48.03

C-300B

387.46

19.72

39.35

27.50

62.76

76.72

0.42

99.55

69.53

C-400A

1726.58

19.72

33.99

122.57

154.56

75.76

1.87

72.34

55.77

C-400B

4623.88

19.72

30.05

328.26

310.20

78.78

5.01

52.38

48.58

TE-100

1117.28

11.93

19.72

48.00

17.00

67.49

1.21

65.23

26.15

AC-200A

693.85

19.72

20.15

49.26

3.78

98.00

0.75

2.20

7.13

AC-200B

835.19

19.72

20.15

59.29

3.78

98.00

0.90

2.17

5.99

AC-300A

272.65

19.72

19.94

19.36

1.65

99.00

0.29

1.09

7.85

AC-300B

283.69

19.72

19.96

20.14

3.78

99.00

0.31

1.20

15.80

AC-400

1825.99

19.72

20.58

129.63

6.91

96.00

1.98

4.39

5.06

E-1A

1934.46

104.35

106.75

726.71

8.21

97.77

2.09

2.30

1.12

E-1B

1438.37

104.36

106.81

540.38

5.32

97.73

1.56

2.35

0.97

E-2

2727.97

78.29

80.88

768.91

8.51

96.84

2.96

3.30

1.09

E-3

2439.32

74.75

79.29

656.41

4.39

94.31

2.64

6.07

0.66

T-101

1716.58

104.35

209.51

644.85

5.19

50.01

1.86

100.78

0.80

The results for this process demonstrate that C-300B compressor has the highest exergoeconomic factor (69.53) and the smallest exergoeconomic factor is related to E-3 heat exchanger (0.66). The relative cost difference is an indication of relative increase in the exergy cost of product with respect to the exergy cost of fuel in an element which plays a significant role in the evaluation and optimization of the system. In this process, T-101 demethanizer tower holds the highest relative cost difference (100.78) and AC-300A air cooler has the lowest relative cost difference (1.09).

The magnitudes of exergy cost of fuel and product determines the cost of exergy rate in an element. The maximum fuel cost rate is related to E-1B heat exchanger with a value of 104.36 $/Gj, while the maximum product cost rate is related to T-101 demethanizer tower with a value of 209.51 $/Gj. One of the most important exergoeconomic parameters is the cost of exergy destruction rate; E-2 heat exchanger has the highest exergy destruction cost (768.91 $/Gj) and AC-300A air cooler has the lowest exergy destruction cost (19.36 $/Gj).

8. Conclusions

In this paper, exergy and exergoeconomic analyses are applied to the recently alternative integrated processes for the cogeneration of LNG and NGL .After the determination of high consuming and inefficient elements, a relationship between economic costs and investment is also proposed and the components with the highest cost of exergy destruction have been identified.

The results of the exergy analysis show that the exergy efficiency of this process is around 53.83%, and its total exergy destruction rate is equal to 42617.5 kW. The results obtained from exergoeconomic analysis in the form of exergy destruction cost and exergoeconomic factor can be summarized as follows:

  1. The most important elements in exergy destruction cost are heat exchangers due to the high value of fuel cost rate (as defined in exergoeconomic analysis) in these devices;
  2. The exergoeconomic factor in compressors and turbo expander is higher than the other elements, and, hence, to reduce the total system cost, these elements costs must be minimized;
  3. The exergoeconomic factor in heat exchangers and demethanizer tower compared to the other elements of the system is negligible, and, hence, to reduce the total system cost, the performance and efficiency of these elements must be maximized.

According to the above conclusions, the most important elements in the exergy destruction cost are heat exchangers. Therefore, the researchers should consider more effort on this element for improving this integrated process in the future studies. To decrease the costs, attention should be focused on decreasing the costs of compressor and turbo-expander by academic and industry experts.

Nomenclature

f

: Exergoeconomic factor (%)

FC

: Fuel cost ($/s)

ieff

: Average annual discount rate (cost of money)

j

: jth year of operation

OMC

: Operating and maintenance cost

PEC

: Purchase equipment cost ($)

r

: Relative cost difference (%)

rFC

: Annual escalation rate for the fuel cost

ROI

: Return on investment

rOM

: Annual escalation rate for the operating and maintenance cost

TCR

: Total capital recovery

TRR

: Total revenue requirement

: Power (kW)

Y

: Exergy destruction ratio

Ż

: Total cost rate of component (capital investment & operating-maintenance cost)

Greek Symbols

τ

: Annual operating hours (hr)

ε

: Exergy efficiency

Superscripts

CI

: Capital investment

OM

: Operating and maintenance

Subscripts

D

: Destruction

F

: Fuel

k

: kth component

L

: Levelized

P

: Production

Abbreviations

AC

: Air cooler

C

: Compressor

D

: Flash drum

E

: Multi stream heat exchanger

Mix

: Mixer

V

: Expansion valve

TE

: Turbo expander

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