Energy Conversion and Management 139 (2017) 245–259
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Energy Conversion and Management j o u r n a l h o m e p a g e : : w w w . e l s e v i e r . c o m / l o c a t e / e n c o n m a n
Analysis of an integrated cryogenic air separation unit, oxy-combustion carbon dioxide power cycle and liquefied natural gas regasification process by exergoeconomic method ⇑
Mehdi Mehrpooya , Masood Jalali Zonouz Renewable Energies and Environment Department, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history:
Received 9 October 2016 Received in revised form 12 February 2017 Accepted 17 February 2017
Keywords:
Air separation unit (ASU) Liquefied natural gas (LNG) Exergy Exergoeconomic Carbon dioxide cycle
a b s t r a c t
Exergoeconomic and sensitivity analyses are performed on the integrated cryogenic air separation unit, oxy-combustion Carbon dioxide power cycle and liquefied natural gas regasification process. Exergy destruction, exergy efficiency, cost rate of exergy destruction, cost rate of capital investment and operating and maintenance, exergoeconomic factor and relative cost difference have been calculated for the major components of the process. The exergy efficiency of the process is around 67.1% and after mixers, tees, tank and expansion valves the multi-stream heat exchanger H-3 have the best exergy efficiency among all process components. Total exergy destruction rate of the process is 1.93 107 kW. Results of exergoeconomic analysis demonstrates that maximum exergy destruction and capital investment operating and maintenance cost rate are related to the multi-stream heat exchanger H-1 and pump P1 with the values of 335,144 335,144 ($/h) and 12,838 12,838 ($/h), ($/h), respectively respectively.. In the sensitivity sensitivity analysis analysis section section the effects of the varying economic parameters, such as interest rate and plant life time are investigated on the trend of the capital investment operating and maintenance cost rate of the major components of the process process and in another cases the effect effect of the gas turbine turbine isentropic isentropic efficiency on the exergy and exergoeconomic parameters are studied. 2017 Elsevier Ltd. All rights reserved.
1. Introduction
LNG (liquefied natural gas) is a substance with very low temperature (163.15 C) that should be vaporized and brought to a desired temperature and pressure before entering the pipeline network. In the conventional LNG vaporization terminals cold energy of LNG is released and wasted into the water or air without any recovering recovering (about (about 0.2 kW kW h kg1). Cold Cold energy energy is an expres expressio sion n for describing cryogenic exergy of LNG material stream. The utilization of the cold energy of LNG during vaporization processes can improve the economic economic and environmen environmental tal aspects of these kinds of processes. Because of its very low temperature, LNG can be used used in differ different ent applic applicati ations ons such such as desali desalinat nation ion of seaseawater, deep freezing agro food industry facilities, space conditioning in the commercial commercial and residential residential sector, low temperature temperature power generation, manufacturing of dry ice, and rubber cryogenic grinding [1] grinding [1].. A novel integrated power plant using cold of LNG and solar energy is introduced and analyzed [2] [2].. Also an integrated oxy-fuel power cycle, high temperature solar system and LNG cold ⇑
Corresponding author. E-mail address:
[email protected] [email protected] (M. (M. Mehrpooya).
http://dx.doi.org/10.1016/j.enconman.2017.02.048 0196-8904/ 2017 Elsevier Ltd. All rights reserved.
recovery is introduced and analyzed. The results show that LNG flow rate is an important parameter which can affect the process efficiency. LNG cold energy is used in a combined chemical looping hydrogen production and power plant with carbon dioxide capture process [3] process [3].. In this study LNG cold energy is used as heat sink to improv improve e the electri electrical cal efficie efficiency ncy of the power power cycle. cycle. LNG cold cold energy is used in an oxy-fuel power cycle which a part of the required of it is supplied by a solar cycle [4] [4].. The results show that LNG flow rate can affect the exergy efficiency and net electrical power of the process significantly. significantly. One of these application applicationss is using cold energy of LNG as refrigeration source of cryogenic air separation processes [5] processes [5].. Operating temperature temperature of the air separation units (ASU) ( 173 C, 193 C) is lower than the LNG, hence LNG cold energy can be used with high cold recovery efficiency compared to other methods [6] methods [6].. The air separation units have high degree of power consumption and it is true fact that utilizing of LNG cold energy is leading to lower power consumption, but however ever integr integrati ating ng of air separat separation ion units units with with differ different ent types types of power generation cycles can be very efficient. A novel air separation process based on cold energy of LNG integrated with coal gasification, transcritical CO 2 power cycle is investigated [7] [7].. In these cycles cycles usually usually the pure oxygen product of ASU is utilizing utilizing instead
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Nomenclature
c C_ e E_ G h H i _ m N Q_ r s T _ W x Z Z_
cost per exergy unit ($/GJ) cost rate ($/h) specific exergy (kJ/kg) exergy rate (kW) Gibbs free energy (kJ) specific enthalpy (kJ/kg) annual working hour (h) interest rate (%) mass flow rate (kg/s) plant life time (year) heat duty (kW) relative cost difference (%) specific entropy (kJ/kg C) temperature (K) power (kW) mole fraction purchased equipment cost capital investment and operating and maintenance cost rate ($/h)
Greek letters g efficiency (%) e exergetic efficiency (%) u maintenance factor
D F In K Out P Ph Tot
destruction fuel input kth component output product physical total
Superscripts standard condition CL capital OM operating & maintenance
Abbreviations LNG liquefied natural gas ASU air separation unit IGCC integrated gasification combined cycle ORC organic Rankine cycle SPECO specific exergy costing SOFC solid oxide fuel cell CCHP combined cooling, heating and power NG natural gas LHV lower heating value CRF capital recovery factor
Subscripts
ch
dead state chemical
of air in combustion chamber component of power generation cycle and it is leading to very high efficient combustion process. These power generation cycles are known as ‘‘oxy-combustion” or ‘‘oxy-fuel” power plants. In recent years different configurations of air separation units integrated with oxy-combustion power plants are introduced. Fu et al. [8] proposed an advanced cryogenic air separation process integrated with oxy-combustion power cycle based on self-heat recuperation technology and in contrast with conventional two distillation columns air separation unit, they used one column type. The results of this process showed that power consumption would be reduced by 20.2% compared to other similar processes. In another work a comparative thermodynamic, economic and risk analysis of integrated cryogenic and hybrid air separation unit with oxy-combustion power plant was investigated [9] and similarly, another configurations of hybrid air separation units are introduced by Burdyny and Struchtrup [10] and compared with each other. The thermo-economic analysis of the oxy-type supercritical power plant integrated with the cryogenic air separation unit was proposed by Janusz-Szymanska and Dry jan´ska [11] and possibilities of improving were investigated. One of the most common integrated ASU-oxy-combustion units is the IGCC (integrated gasification combined cycle) plants. In this type of cycles the output pure oxygen of ASU section enters the component which name is ‘‘gasifier” and burning with coal, the output flue gases are leading to the gas turbine for power generation intention. Coal is one of the convenient fossil fuels in the matter of price but its environmental impacts always have been high. Fortunately integrating of these cycles with CO 2 capturing units have solved this problem [12]. According to the concept of integrating ASU cycles with oxy-combustion power plants, the application of transcritical CO 2 power cycles can be considered [13]. Carbon dioxide power cycles already have been used in different configurations of recent researches. An integrated molten carbonate fuel
cell-supercritical carbon dioxide is introduced and analyzed [14]. In this study a Brayton cycle is used to recover heat from the catalytic burner exhaust gas. A novel hybrid three reactor chemical looping, and fuel cell power plant cycle is introduced and analyzed [15]. Carbon dioxide is produced in one of these reactors. In one of these studies a transcritical CO2 power cycle which utilizes geothermal wells as its heat source and LNG cold energy as its heat sink, was studied [16]. In this very particular configuration chilled water and power generation in natural gas turbine, were the alternative products and for further examination also exergoeconomic and multiobjective optimization had been considered. Vélez et al. [17] exclusively investigated the low temperature heat sources and specially CO2 transcritical working fluid for power generation and their results showed that this type of power cycles, integrated or nonintegrated with other cycles would be one of the next generation of power production cycles. CO 2 power cycles also are integrated with solar energy sources [18]. In Al-Sulaiman and Atif [19] research study, a thermodynamic comparison of five supercritical carbon dioxide Brayton cycles integrated with a solar power tower was studied. In this work the heliostat solar field was optimized for better optical performance and then configured with the supercritical CO2 Brayton cycles. In Xia et al. [20] research study a solarpowered transcritical CO2 power cycle has been employed for reverse osmosis desalination of sea water and in this very particular work, LNG cold energy was used as heat sink of the power cycle process and in the following, LNG is vaporized and employed in natural gas turbine in order to producing power. Mahmoudi and Ghavimi [21] was proposed an integrated molten carbonate fuel cell (MCFC) – supercritical CO2 – organic Rankine cycle (ORC) and LNG cold energy as heat sink of the whole process and considered it in the aspects of thermoeconomic and multi-objective optimization.
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Self-heat recuperation of cryogenic air separation processes has high degree of importance for reducing capital and operating and maintenance costs of additional hot and cold utilities [22]. For reaching to this purpose, this process has been integrated with respect to the ‘‘pinch” technology concept [23]. Exergy analysis is a tool which have been used widely for evaluation of the energy intensive processes [24]. An integrated molten carbonate fuel cell power plant and carbon dioxide capturing process is investigated by exergy analysis method [25]. The results show that exergy destruction of the combustion chamber is too high. Integrated liquefied natural gas and hydrocarbon recovery process were investigated by exergy analysis method [26]. The results show that compressors and multi stream heat exchangers have the greatest exergy distraction. Exergy analysis method is applied on a novel hydrocarbon recovery process configuration [27]. Based on the results distillation column and expansion valves have the highest exergy destruction. Helium extraction from natural process is investigated by advanced exergy analysis method [28]. Integrated electrochemical power plants and natural gas liquefaction process configurations were introduced and investigated [29]. Results of the exergy analysis show that absorption refrigeration subsystem has the highest exergy efficiency. A currently in operation hydrocarbon recovery was investigated by exergy and advanced exergy analysis methods [30]. Exergy efficiency of the used propane refrigeration cycle in the process was about 34%. Also based on the advanced exergy analysis results there is a good potential for improvement pf the components performance. A CCHP integrated fuel cell and ORC power plant was introduced and analyzed by exergy method [31]. Exergoeconomic analysis is a method that is considered for evaluation of the integrated ASU – CO 2 power cycle–LNG vaporization process. Exergoeconomic method is a combination of thermodynamic and economic concepts. The conventional and advanced approaches of this method were utilized in different processes. This method is employed for evaluating of single mixed refrigerant natural gas liquefaction processes and in particular popular SMRLinde and SMR-APCI [32]. The three main exergoeconomic parameters i.e. exergoeconomic factor, relative cost difference and cost of exergy destruction were calculated for each component of these cycles and the results are providing the possibility of comparison between process components in the sight of exergoeconomic evaluation. Advanced exergoeconomic analysis is applied for evaluation of the single mixed refrigerant LNG processes [33]. Based on the results costs of investment in most of the process components is endogenous. Meanwhile the advanced exergoeconomic analysis of the noted processes and multi stage mixed refrigerant ones, such as C3MRLinde, DMR-APCI and MFC-Linde have also been investigated [34]. Also integrated LNG and NGL process was evaluated by exergoeconomic analysis [35]. A trigeneration system which uses a diesel-gas engine was investigated by exergoeconomic analysis method [36]. The investigation of economic aspects of the air separation units has been conducted rarely. In authors’ knowledge no exergoeconomic evaluation for such a highly integrated layout i.e. ASU-CO 2 power cycle-LNG vaporization is available. The exergoeconomic evaluation mostly is being performed on the normal processes with ordinary layouts but some of the components of this process have been rarely investigated in exergoeconomic evaluation method. In one of these researches the energetic, exergetic and economic assessment of oxygen production from two columns cryogenic air separation unit were considered and some of the required equations of purchased equipment costs of the process were exploited from this research study [37]. In another study Ghorbani et al. [38] performed exergoeconomic analysis and multi-objective Pareto optimization on the C3MR natural gas
247
liquefaction process. In this work the total revenue requirement method is used for exergoeconomic analysis and a coded genetic algorithm from Matlab software which is linked to HYSYS simulation package is utilized for optimization of the process. Cavalcanti and Motta [39] utilized exergoeconomic analysis for the solar/fuel assisted Rankine cycle. They used SPECO (specific exergy costing) for their results and the parabolic trough collector was the heat source of their process. Ozbilen et al. [40] performed the exergoeconomic and exergoenvironmental analyses on the four-step CuACl cycle for hydrogen production purposes and one of the most important results of the analysis i.e. the total cost rate of the process was 165 ($/s). Exergoeconomic evaluation was investigated on a cogeneration plant consisting of a hydrogen-fed SOFC (solid oxide fuel cell), gas turbine and generator-absorber-heat exchange absorption refrigeration cycle [41]. The EES (engineering equation solver) software was used for the solving cost balances. The exergoeconomic factor, capital cost rate and the exergy destruction cost rate of the overall system were the 27.3%, 10.63 ($/h) and 28.3 ($/ h), respectively. A LNG process using a single effect absorption refrigeration cycle was investigated by advanced exergoeconomic method [42]. Based on the results process performance can be improved by considering compressors and air coolers investment costs. A hydrogen liquefaction process was investigated by advanced exergoeconomic analysis [43]. A power plant fueled by natural gas was evaluated by advanced exergoeconomic analysis method [44]. Based on the results, combustion chamber and high-pressure steam turbine have great potential for economic improvement. In this paper the novel integrated process is employed which is consisted of air separation unit (ASU), oxy-combustion Carbon dioxide power cycle and LNG vaporization section [13]. The results of exergy analysis are the basis of this evaluation which will be combined with economic concepts. The process is simulated with HYSYS simulator software package, exergy analysis is conducted with Matlab codes which is linked to Aspen HYSYS simulator and exergoeconomic evaluation is performed with EES (engineering equation solver) software and the method which is utilized for this purpose is the SPECO (specific exergy costing) method. With the results of exergoeconomic analysis there is this ability to see the thermo-economic situation of the process and compare the components with each other. This analysis reveals the weak-spots of the process and gives to engineers this opportunity to resolve and optimize it. 2. Process description
Fig. 1 shows a schematic diagram of the integrated one column cryogenic air separation unit (ASU) – oxy-combustion Carbon dioxide power cycle and LNG regasification process. In this layout the cold energy of LNG is utilized as refrigeration source in heat exchanger (H-1) for optimizing air separation section and in heat exchangers H-8, H-9 and H-10 for integrating power generation cycle. The whole process of air separation section is integrated with self-heat recuperation concept and this issue has been eliminated the requirement for additional heat sources. Pure liquid nitrogen, oxygen and Carbon dioxide, low and high purity of nitrogen gas, natural gas for transporting to pipe line network and finally power, are the all products of this complex integrated process. 2.1. Air separation unit section (ASU)
The heart of this process is ASU section. This section is consisted of two parts, heat circulation and air separation modules. Feed air in ambient conditions is passing through compressor
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Fig. 1. Schematic diagram of the integrated ASU-CO2 power cycle-LNG regasification process [13].
(CR-1) and after pressurizing is cooled via (H-1 & H-2) heat exchangers in heat circulation module section. This feed air with (181 C) temperature and a mixture of gas and liquid phases is entering to the distillation column for separating to its components. In distillation column the high purity nitrogen gas (5) is divided to two streams (8 & 9). Stream (8) is cooled via two heat exchangers (H-4 & H-3) and with ( 187 C) temperature and liquid phase is refluxing to the distillation column as stream (13). The other stream i.e. (9) is utilizing for self-heat recuperation purposes and after cooling stream (22) in heat exchanger (H-5) is returning to the heat circulation module. After passing and further cooling via heat exchanger (H-2) the stream (24) is entering to the cascade compression-intercooling procedure which is consisted of multi stream heat exchanger (H-1) and four compressors (CR-4, CR-5, CR-6 & CR-7). The output stream of (CR-7) i.e. (33) is entering to the main multi stream heat exchanger (H-1) and is being cooled until ( 180 C). This temperature is reaching to (194 C) via two components (H-6 & V-3). The high purity liquid nitrogen with ( 194 C) temperature is ready to be stored in storage tanks. From the middle of distillation column, the low purity nitrogen gas (6) is utilized for self-heat recuperation purposes in (H-3) and then passes through heat exchangers (H-2 & H-1) as refrigeration source. This stream is reached to ambient temperature and becomes one of the desired products i.e. low purity nitrogen gas. The pure liquid oxygen from the bottom of column (7) is exploited. This stream is divided into two streams (18 & 19). The stream (19) is one of the main products, but stream (18) similarly to nitrogen streams is employed in self-heat recuperation section via (H-4 & H-5) and is refluxing to the middle of distillation column. Only one of the air separation products has been remained which name is high purity nitrogen gas. It is simple, the vaporized nitrogen contents in tank is extracted as stream (38), this stream with (194 C) temperature is excellent source of refrigeration. After releasing its cold energy via heat exchangers (H-6 & H-1) it becomes gaseous and as stream (40) is stored in high purity nitrogen gas capsules.
2.2. Oxy-combustion carbon dioxide power cycle
CO2 is the main working fluid of the power generation cycle. This cycle is the combination of classic Rankine and Brayton power cycles. The stream (41) after condensation in heat exchanger (H-8) as the condenser of Rankine part is passing through pump (P-2) and pressurizing to 3 MPa. The stream (42) with (66.6 C) temperature and high cryogenic exergy is entering to the heat exchanger (H-1) which is playing the role of the boiler in Rankine layout. This stream (42) is utilized as refrigeration source in this heat exchanger and improving the heat circulation module of the ASU section. The stream (43) is mixing with the prepared pure oxygen gas from the distillation column. From this point the role of steam turbine of the classic Rankine cycle is eliminatedand Brayton cycle is showing itself, which means the mixed stream (44) after passing through recuperation heat exchanger (H-7) and absorbing high degree of heat from gas turbine exhaust, is burning with vaporized natural gas of LNG regasification section, as the fuel of combustion process. The outlet flue gases of combustion chamber component with the temperature of (900 C) are entering to the gas turbine for power generation purposes. The stream (47) after releasing high degree of heat in recuperation heat exchanger (H-7), is more cooled in heat exchanger (H-9) via LNG cold energy and the existing water of it, is sequestrated in separator (S-1). The pure Carbon dioxide stream (50) is then compressed to the condenser pressure with (CR-9) and with this point the combined oxy-fuel CO2 power cycle, is completed. In order to supplying liquid Carbon dioxide for the power cycle the gaseous CO 2 with (34 C) temperature and ambient pressure is passing through compressor (CR-10) and then is cooled by the extracted LNG from (T-3) in heat exchanger (H-10). The liquid CO2 with proper temperature and pressure is the output of this section and after a little pressurizing can be injected in stream (41). 2.3. LNG regasification section
Liquefied natural gas (LNG) after transportation to the usage terminals should be vaporized and leaded to the pipe line network.
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This substance has high cryogenic exergy that it would be a disaster if be wasted. Different processes with different configurations have been proposed for LNG regasification and this process is one of them. In this process LNG is passing through pump (P-1) and in first step approaching to the pipe line network pressure. Remember if the process, pressurize natural gas after vaporization, the required power of this procedure would be 20 times higher [6]. The stream (57) is entering to (H-1) as one of the refrigeration sources of this 9 sides plate-fin multi stream heat exchanger and releasing about half of its cryogenic exergy. It then as stream (58) is divided into three different streams (59, 60 & 61). The stream (60) is going to (H-8) as one of the refrigeration sources of Rankine part power generation cycle condenser. Stream (61) is going to heat exchanger (H-10) for liquefaction of Carbon dioxide. The streams (59, 62 & 63) are joining together in (M-1) and reaching to (32 C). Stream (64) is helping to cooling CO 2-water stream in heat exchanger (H-9) and after reaching to (8 C) temperature a fraction of it, is going to pipe line network and the rest is being utilized in combustion chamber as the fuel of the combustion process.
Table 2
Simulation results of the power components.
Component
Pressure ratio
Polytropic efficiency (%)
Power (kW)
P-1 P-2 CR-1 CR-2 CR-3 CR-4 CR-5 CR-6 CR-7 CR-8 CR-9 CR-10 GT E-1 E-2
66.79 4.29 2.97 1.67 2.00 15.57 1.92 1.49 1.33 12.61 6.89 4.80 26.70 2.26 2.27
76.24 79.16 78.38 76.88 77.47 82.75 77.35 76.07 75.43 81.42 79.19 78.50 69.40 74.13 74.12
6.40 105 9.51 103 5.87 106 3.30 105 3.24 105 1.44 106 2.39 105 9.90 104 5.43 104 1.87 103 6.16 105 1.39 104 1.62 106 1.47 105 3.53 105
Carbon dioxide power cycle is the combination of Rankine and Brayton layouts with one important difference. This configuration uses the pure oxygen product of ASU section instead of air in combustion chamber and this issue increases the efficiency of the cycle. In Rankine and Brayton parts of the power cycle, H-1 and H-7 heat exchangers have the role of boiler and regenerator, respectively.
3. Process analysis
In simulation of the process heat loss is ignored and Aspen HYSYS software package is utilized for simulation of the process [45]. Comprehensive thermodynamic data bank is one of the advantages of Aspen HYSYS software. Peng-Robinson equation of state [46] is selected for calculation of thermodynamic properties of air, LNG and Carbon dioxide streams. Tables 1–3 present the simulation results and specification of the process equipment.
_ in ¼ Q _ 68 ¼ Q _ NG Q
ð4Þ
_ net CO2 ¼ W _ GT W _ CR8 W _ CR9 W _ P 2 W
ð5Þ
;
_ net W Q in _ _ O2 ðh55 h54 Þ m _ CO2 ðh52 h50 Þ m _ CO2 ðh42 h41 Þ mCO2 ðh46 h47 Þ m ¼ _ NG LHV m ð6Þ
gCO cycle ¼ 2
3.1. Energy analysis
The total power consumption of the process is related to the ten compressors and two pumps as follows:
The overall energy efficiency of the whole process is the ratio of the net generated power and the net output work rate of the LNG expander to the rate of the required fuel for combustion process and recovering of LNG cold energy.
_ tot used ¼ W _ CR1 þ W _ CR2 þ W _ CR3 þ W _ CR4 þ W _ CR5 þ W _ CR6 W _ CR7 þ W _ CR8 þ W _ CR9 þ W _ CR10 þ W _ P 1 þ W _ P 2 þ W ;
ð 1Þ
gI ov erall ¼
The total produced power of the cycle is associated to the gas turbine of the power generation cycle and two expanders of the air separation section: _ tot produced ¼ W _ GT þ W _ E 1 þ W _ E 2 W
;
The amount of LNG cold energy which is absorbed in H-1, H-8, H-9 and H-10 heat exchangers is obtained from Eq. (3): _ LNG ¼ m _ LNG DhLNG Q
ð7Þ
_ _ is the mass where W net is the net generated power of the process, m flow rate of the natural gas fuel of the combustion chamber and input LNG stream, LHV is the lower heating value parameter of natural gas and q LNG is the specific heat of LNG. The thermodynamic data of the process streams are available in Table 4.
ð 2Þ
;
_ net þ W _ LNG W _ NG LHV þ m _ LNG qLNG m
ð 3Þ
3.2. Exergy analysis
The overall LNG streams duty of these heat exchangers should have supplied from external sources if there was not this possibility to utilizing LNG as refrigeration source.
Exergy analysis is an engineering tool which is used for thermodynamic analysis of a process and to determine maximum useful
Table 1
Specifications of the feed and product streams.
Stream name Flow (kmol/h) Temperature (C) Pressure (kPa) Components (mol%) CH4 C2H6 C3H8 i-C4H10 N2 O2 H2O CO2
Air (1) 5.01 10 25 101.3 0 0 0 0 79.06 20.94 0 0
LNG (56) 6
6.50 10 162
NG (67) 6
6
O2 (l) (19)
N2 (g) (40)
N2 (l) (37)
5
5
6
4.34 10 172
104.8
6.50 10 8 6590
93.50 4.22 0.98 0.84 0.46 0 0 0
93.50 4.22 0.98 0.84 0.46 0 0 0
LP N2 (g) (26)
1.20 10 194
276.3
1.97 10 25 130
130
3.17 10 25 111.3
0 0 0 0 0 100 0 0
0 0 0 0 99.87 0.13 0 0
0 0 0 0 99.55 0.45 0 0
0 0 0 0 81.07 18.93 0 0
6
CO2 (l) (71) 8.18 103 57
466.5 0 0 0 0 0 0 6.11 93.89
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Table 3
Specifications of the process heat exchangers.
HEX name Duty (kW) LMTD (C) Min approach (C) Number of sides
H-1
H-2 7
1.73 10 0.714 0.276 9
15.00 10 10.502 0.348 3
H-3 3
H-4 4
3.19 10 4.643 4.239 2
H-5 5
1.65 10 1.238 1.080 2
H-6
2.32 10 15.020 15.015 2
5
H-7 4
2.13 10 3.111 0.234 2
1.63 10 60.051 45.001 2
H-8 6
H-9
4.37 10 35.920 8.002 4
6
H-10
1.02 10 19.081 8.003 2
5
9.95 106 427.1 8.001 2
Table 4
Thermodynamic data of the process streams.
Stream no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
T (C) 25 168 165 181 187 183 172 187 187 168 181 186 187 195 187 191 191 172 172 182 174 143 172 191 191 25 54 160 130 147 134 142 137 180 181 194
P (kPa) 101 301 301 301 251 253 276 251 251 420 429 271 251 111 111 111 111 276 276 111 111 222 276 111 111 111 1733 1733 3336 3336 4978 4978 6600 6600 6600 130
Flow rate (kmol/h) 6
5.01 10 5.01 106 5.01 106 5.01 106 3.84 106 3.17 106 1.85 106 2.45 106 1.40 106 2.45 106 2.45 106 2.45 106 2.45 106 1.40 106 1.40 106 3.17 106 3.17 106 1.42 106 4.34 105 1.42 106 1.42 106 1.42 106 1.41 106 1.40 106 3.17 106 3.17 106 1.40 106 1.40 106 1.40 106 1.40 106 1.40 106 1.40 106 1.40 106 1.40 106 1.40 106 1.40 106
Stream no.
T (C)
P (kPa)
Flow rate (kmol/h)
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
194 194 180
130 130 130 130 700 3000 2940 2940 2884 2884 108 106 103 103 103 710 276 238 3000 105 7000 6800 6800 6800 6800 6680 6660 6660 6590 6590 6590 6590 101 487 467
1.20 106 1.97 105 1.97 105 1.97 105 3.08 105 3.08 105 3.08 105 3.09 105 3.09 105 3.09 105 3.09 105 3.09 105 3.09 105 3.09 105 3.40 102 3.09 105 5.55 102 5.55 102 5.55 102 6.50 106 6.50 106 6.50 106 1.30 106 2.60 106 2.60 106 2.60 106 2.60 106 6.50 106 6.50 106 6.50 106 6.50 106 1.64 102 8.18 103 8.18 103 8.18 103
25 68 67
8 8 429 900 548 70 40 0 0 181 172 25 417 162 158 76 76 76 76 176 17 32 33 8 8 8 34 177 57
work achievable by a certain amount of input energy. By combining the first and second laws of thermodynamics, exergy analysis has become one of the most powerful tools for conducting qualitative and quantitative investigations of energy consumption in processes. Two important parameters are introduced in exergy analysis, exergy destruction rate and exergy efficiency [47]. Exergy rate of the process streams consists of four different terms: physical, chemical, kinetic and potential exergy. The kinetic and potential terms are usually being ignored in exergy calculations. The relations have been given in the following [48]:
of each process streams have been presented in Table 5. Exergy destruction and exergy efficiency of the process components are the most important parameters of the exergy analysis which are obtained from exergy balance of each component. The exergy destruction term is used in exergoeconomic analysis for obtaining cost rate of exergy destruction. These terms are shown in detail in Table 6.
e ¼ e ph þ ech
ð8Þ
gII ¼
e ph ¼ ðh h0 Þ T 0 ðs s0 Þ
ð9Þ
Likewise energy analysis section, the overall exergy efficiency of the plant can be calculated as follows [47]:
ech ¼
X
xi e0i þ G
X
xi Gi
_ D ¼ E _ F _E P E
ð10Þ
where ‘‘h” and ‘‘s” are the specific enthalpy and entropy respectively, subscript ‘‘0” is related to the thermodynamic dead state i.e. 25 C and 101.325 kPa. The term xi is the mole fraction of stream components, e 0i is the standard chemical exergy of the stream components which is attainable from different references [49] and G is the Gibbs free energy. The physical, chemical and total exergy rate
_ P _D E E ¼ 1 _ F _ F E E
ð12Þ
P_ _ D tot E k E D k ¼ 1 P erall ¼ 1 _ _ used þ m _ LNG eLNG E F W
gII ov ;
ð11Þ
;
;
ð13Þ
In this equation the E_ D;tot is the summation of exergy destruc_ tion of all process components, W used is the overall power con_ is the mass flow rate of sumption of pumps and compressors, m input LNG stream and ‘‘e” is the specific exergy of LNG stream.
P
251
M. Mehrpooya, M.J. Zonouz / Energy Conversion and Management 139 (2017) 245–259 Table 5
Exergy results of the process streams.
Stream no.
E_ ph ð kWÞ
E_ ch ðkWÞ
E_ tot ðkWÞ
Stream no.
E_ ph ðkWÞ
E_ ch ðkWÞ
E_ tot ð kWÞ
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
0 4.83 106 8.30 106 9.71 106 7.36 106 5.82 106 9.57 106 4.68 106 2.68 106 4.77 106 1.29 107 1.29 107 1.29 107 2.34 106 1.85 106 5.03 106 5.03 106 7.33 106 2.24 106 7.29 106 1.51 106 1.65 106 2.37 106 1.97 106 4.44 106 2.05 105 2.88 106 4.16 106 4.27 106 4.97 106 5.02 106 5.62 106 5.64 106 7.54 106 7.59 106 7.23 106
1.46 105 1.46 105 1.46 105 1.46 105 6.87 105 9.49 104 2.04 106 4.37 105 2.50 105 4.37 105 4.33 105 4.33 105 4.33 105 2.50 105 2.48 105 9.49 104 9.51 104 1.56 106 4.78 105 1.56 106 1.56 106 1.56 106 1.55 106 2.48 105 9.51 104 9.51 104 2.48 105 2.48 105 2.48 105 2.48 105 2.48 105 2.48 105 2.48 105 2.48 105 2.48 105 2.48 105
1.46 105 4.98 106 8.45 106 9.86 106 8.05 106 5.91 106 1.16 107 5.12 106 2.93 106 5.21 106 1.33 107 1.33 107 1.33 107 2.59 106 2.10 106 5.13 106 5.13 106 8.89 106 2.72 106 8.85 106 3.08 106 3.21 106 3.92 106 2.22 106 4.53 106 3.00 105 3.13 106 4.41 106 4.52 106 5.22 106 5.27 106 5.86 106 5.89 106 7.79 106 7.84 106 7.48 106
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
6.92 106 3.10 105 2.58 105 3.37 104 9.12 105 9.18 105 6.82 105 6.83 105 1.28 106 2.73 106 8.96 105 2.01 104 4.68 103 7.07 103 8.12 5.17 105 9.36 102 3.26 102 1.98 103 3.13 107 3.13 107 2.22 107 4.45 106 8.89 106 8.89 106 8.18 106 7.33 106 1.80 107 1.80 107 1.80 107 1.80 107 4.55 102 6.17 101 1.15 104 2.19 104
2.11 105 3.66 104 3.66 104 3.66 104 1.69 106 1.69 106 1.69 106 1.69 106 1.69 106 1.69 106 1.69 106 1.69 106 1.69 106 1.69 106 2.96 102 1.69 106 6.12 102 6.12 102 6.12 102 1.61 109 1.61 109 1.61 109 3.21 108 6.43 108 6.43 108 6.43 108 6.43 108 1.61 109 1.61 109 1.61 109 1.61 109 4.05 104 4.32 104 4.32 104 4.32 104
7.13 106 3.47 105 2.94 105 7.03 104 2.60 106 2.61 106 2.38 106 2.38 106 2.97 106 4.42 106 2.59 106 1.71 106 1.70 106 1.70 106 3.04 102 2.21 106 1.55 103 9.37 102 2.59 103 1.64 109 1.64 109 1.63 109 3.26 108 6.51 108 6.51 108 6.51 108 6.50 108 1.62 109 1.62 109 1.62 109 1.62 109 4.10 104 4.33 104 5.48 104 6.51 104
Table 6
Definitions of the exergetic efficiency of process components [47].
Component
Exergy destruction
Pumps
P E þ W P E E ¼ P E þ W P E E ¼ P E P E W E ¼ P E P E W E ¼ P E P E E ¼ P E P E E ¼ P E P E E ¼ P E P E E ¼ P E P E E ¼ P E , n = number of devices E ¼
Compressors Gas turbine Expanders Heat exchangers Combustion chamber Distillation column Separators/mixers Valves Cycle
_ _ _ _ _ _ _ _ _ _
D D D D D
D D D D
Dtot
_ _ _ _ _ _ _ _ _
in in in in in
in in in in
_
Exergetic efficiency _
_
_
_ _ _ _ _ _ _
out out
P P ¼ ¼ P ¼ P
e ¼
out
e
_
e
_
e
out
e
e ¼
out
_ E out
_ W
_ E out
_ E in
_ W _ W _ E in
_ E out
_ W
_ E in
n
out
P e DP eD in e ¼ Dout T einT eD out
in
Exergoeconomic is the branch of engineering that combines exergy analysis and economic principles. In this method a cost balance is written around each component of the process. With this evaluation the cost rate of the process streams are determined and these results are being utilized for optimization of the whole process. This typeof analysis is very crucial for designand performance of expensive systems [50]. Theequationof cost balanceis as follows:
_ E in
m
P
1
_ W C out þ C
c
_ E out
PP
e ¼ 1
X_
_ E
_ E NG
e ¼
_ Q þ Z _ ¼ C in þ C
h
1 m _ Q 1
_ þE _ þE _ þE _ E 5 7 6 53 _ þE _ þE _ E 4 13 23
out
X_
m
_ E
_ _ E flue E O2 þCO2
e ¼
3.3. Exergoeconomic analysis
_ E out
1 n _ Q 1
out
n _ k¼1 D;k
_ E in
P P P P P P P P ¼ 1
out
_ E D;tot _ _ W consumedþE LNG
ð14Þ
out
_ ¼ c E _ C
ð15Þ
_ Q ¼ c E _ Q C
ð16Þ
_ W ¼ c W _ C
ð17Þ
252
M. Mehrpooya, M.J. Zonouz/ Energy Conversion and Management 139 (2017) 245–259
situations the relations are solved with auxiliary equations. The auxiliary equations are obtained with respect to the fuel and product concept of exergy streams. For a control volume fuel is the source of exergy which is consumed to generate the product and is different with actual fuel, such as natural gas, gasoline, diesel, etc. Also the product is the desired result generated by using the fuel. Table 9 shows the cost balances and auxiliary equations of the process components. For the component ‘‘k” as a control volume always there are the fuel and product streams and the average cost per exergy unit of these streams is one of the most important parameters which are resulted in exergoeconomic evaluation.
Table 7
Parameters involved in the simulation of the integrated process.
Parameter
Value
Ambient temperature (C) Ambient pressure (kPa) Interest rate (%) Plant life time (year) Maintenance factor () Plant annual working hour (h) Cost per exergy unit of LNG ($/GJ) [32] Cost per exergy unit of electricity ($/GJ) [51] LNG inlet temperature (C) LNG pressure (kPa) LNG mass flow rate (kg s1) Isentropic efficiency of pumps (%) Isentropic efficiency of compressors (%) Isentropic efficiency of gas turbine (%)
25 101.3 10 20 1.06 7446 13.69 25 162 104.8 31,269 75 75 75
c F k ¼ ;
c P k ¼
where denotes a cost rate associated with an exergy stream: stream of matter, power or heat transfer while the variable Z_ represents all remaining costs. Also c is the cost per unit of exergy, E_ is the exergy of material streams, E_ Q is the exergy of heat transfer _ is the generated or consumed power of process components and W _ is considering the capital which is the pure exergy. The term Z investment and operating and maintenance costs of the process components. Eq. (19) shows the terms which are utilizing for calcu_ variable. The term Z is the purchased equipment cost of lating Z each component, CRF is the capital recovery factor which is related to the capital investment costs, u is the maintenance factor which is related to the operating and maintenance costs and finally H is the annual working hour of the plant. In order to calculating CRF term, i is the interest rate and N is the life time of the integrated plant. The required economic parameters and other assumptions which are utilizing in the analysis are presented in Table 7.
CRF ¼
;
_ P k C _ P k E
ð22Þ
;
;
Relative cost difference of component k ð rk Þ is defined based on the average cost per exergy unit of fuel and product streams and is a useful variable for evaluating and optimizing of a system component. r k ¼
c P k c F k c F k ;
ð23Þ
;
;
This parameter represents the difference between the average cost of products and fuels which is due to the destruction and the investment cost. As mentioned before exergy destruction is one of the most important indicators of the exergy analysis and definitely the cost rate of this term has the greatest impact in the analysis, because this term determines the waste dollars of the components and whole process and gives us this scope to improving the weakest components and finally improving the overall plant.
ð18Þ
_ D k ¼ c F k _E D k C
_ ¼ Z CRF u Z H ið1 þ iÞ
ð21Þ
;
;
C_
_ ¼ Z _ CL þ Z _ OM Z
_ F k C _ F k E
;
ð19Þ
ð20Þ
N
ð24Þ
;
The exergoeconomic factor is the final indicator which is determines the ratio of the contribution of the capital investment and operating and maintenance costs and cost rate of exergy destruction of the process components. The high value of this parameter shows that the share of capital investment and operating and maintenance costs is higher and the low value shows that the exergy destruction has the higher impact in cost rates of process components.
N
ð1 þ iÞ 1
;
The cost balance of each component is a linear equation that should be solved for obtaining desired parameters, but in some of these equations there is more than one unknown term. In these
Table 8
Purchased equipment cost functions of the process components.
Component
Purchased equipment cost functions
Pumps
_ Þ þ 0:1538ðlogðW _ ÞÞ log Z ¼ 3:3892 þ 0:0536logðW _ in :1m Z ¼ 071 r ðr Þ :92gis p ln p
Compressors Gas turbine & expanders
[16]
2
_ 479 :34m Z ¼ 0:92g g ln is
[52]
ð1 þ expð0:036T 54:41ÞÞ P in P out
in
[52]
Multi-stream heat exchangers
Z ¼ FC þ ð A UC C 1Þ FC = Fixed cost of MHEX, A = Total heat transfer area , UC = Unit cost of the MHEX material , C1 = 0.055
Combustion chamber
Z ¼
Distillation column
Z ¼ Z essel þ Z tray 0:87 1:23 2 Z essel ¼ 1780ðlÞ ðdÞ ½2:86 þ 1:694F M ð10:01 7:408ln P þ 1:395ðln P Þ Þ 2 Z tray ¼ N act ½193:04 þ 22:72ðdÞ þ 60:38ðdÞ l = Length of the column , d = Diameter of the column, F M = Material factor , P = Column mean pressure, N act = Actual number of trays Z ¼ F M C b þ C a 2 C b ¼ 1:218exp½9:1 0:2889 ln W þ 0:04576ðln W Þ 0:7396 0:7066 C a ¼ 300D L
_ air 46:08m 0:995P out =P in
ð1 þ expð0:018T out 26:4ÞÞ
[53] [52]
v
[37]
v
Separator
Tank
2 Z ¼ 1:218F M exp½11:662 0:6104ln V þ 0:04536ðln V Þ V = Volume of tank
[54]
[54]
253
M. Mehrpooya, M.J. Zonouz / Energy Conversion and Management 139 (2017) 245–259 Table 9
Cost balances and related auxiliary equations of the process components.
Component
Cost balance
P-1 P-2 CR-1 CR-2 CR-3 CR-4 CR-5 CR-6 CR-7 CR-8 CR-9 CR-10 GT E-1 E-2 H-1
_ þ C _ þ Z _ _ C 56 W P 1 ¼ C 57 _ C
_ C
_ þ Z
41 þ W P 2 ¼ 42 _ þ C _ þ Z _ _ C W 1 CR1 ¼ C 2
_ þ C _ þ Z _ _ C W CR2 ¼ C 10 8
_ þ C _ þ Z _ _ C 24 W CR4 ¼ C 27 _ þ C _ þ Z _ _ C W 28 CR5 ¼ C 29 _ þ _C þ Z _ _ C 30 W CR6 ¼ C 31 _ þ C _ þ Z _ _ C W 32 CR7 ¼ C 33 _ þ C _ þ Z _ _ C W CR8 ¼ C 55 54 _ þ _C þ Z _ _ C W CR9 ¼ C 52 50 _ þ C _ þ Z _ _ C 69 W CR10 ¼ C 70 _ þ Z _ ¼ C _ þ _C C W 46 47 GT
c 46 ¼ c 47
_ þ Z _ _ _ C 9 E 1 ¼ C 14 þ C W
c 9 ¼ c 14
_ C
6
16 þ
_ C
þ _C
31 þ
_ C
E 2 ¼
27 þ
H-3 H-4 H-5 H-6 H-7 H-8
_ C
_ þ Z
_ C
H-2
None None c 1 ¼ 0 None None None None None None None None None
_ þ C _ þ Z _ _ C 21 W CR3 ¼ C 22
¼
_ C
29
_ C
28 þ
_ C
30 þ
_ C
32
c 6 ¼ c 16
W
33 þ
_ C
_ C
2 þ
þ _C
34 þ
25 þ
_ C
3 þ
_ C
26
_ C
39 þ
_ C
42 þ
þ _C
40 þ
_ C
57
_ C
43 þ
_ þ Z
H 1
_ C
c 17 ¼ c 25 ,
_ þ C _ þ Z _ _ _ C 12 16 H 3 ¼ C 13 þ C 17
c 16 ¼ c 17
_ þ _C þ Z _ _ _ C 10 20 H 4 ¼ C 11 þ C 21
c 20 ¼ c 21
_ þ C _ þ Z _ _ _ C 14 22 H 5 ¼ C 15 þ C 23
c 14 ¼ c 15
_ C
_ C
_ þ Z
_ C
_ C
c 38 ¼ c 39
_ C
_ C
_ þ Z
_ C
_ C
_ C
_ C
þ _C
44 þ 52 þ
38 47 53
H 6 ¼ H 7 ¼
60
35 þ 45 þ
þ _C
65
39
H 8 ¼
_ C
41 þ
_ C
54 þ
_ C
63 þ
_ C
_ þ Z
4 þ 13 þ 23 DC ¼ _ þ C _ þ _C ¼ C _ C 59 62 63 64
_ C
_ C
5 þ
_ C
6 þ
_ C
7 þ
53
44
c 67 ¼ c 68
_ C
_ þ Z
_ C
_ C
_ C
_ þ Z
_ C
_ C
_ C
_ C
_ C
_ C
18 ¼
12 20
_ ¼ C _ C 35 36
_k Z
50 þ 37 þ
;
;
_
4
3
_
_
65
_
41
52
c 37 ¼ c 38
38
None None None
For evaluation the cost rate of the whole process Eq. (26) is being employed, in this equation C_ L ;tot is the cost rate of exergy losses which are related to the heat transfer or material stream from the plant to the environment. _ P tot ¼ C _ F tot _C L tot þ Z _ tot C
_
c 51 ¼ 0
51
ð25Þ
_Dk _ k þ C Z
27
c 8 ¼ c 9 c 59 ¼ c 60 , c 60 ¼ c 61
11 ¼
_
28
None None
_ ¼ C _ þ C _ C 66 67 68 Tank ¼
_
29
c 4 ¼ c 13 , c 13 ¼ c 23 , c 23 ¼ c 53
c 18 ¼ c 19
36
_
30
None
_ C
_ ¼ C _ þ C _ þ C _ C 58 59 60 61 S 1 ¼
_
31
c 61 ¼ c 62
_ ¼ C _ þ _C C 7 18 19
49
_
32
c 48 ¼ c 49
_ C
43 þ 55 ¼ _ ¼ C _ þ C _ C 5 8 9
_
33
4 C 3 ¼ C _ E _ E
_
_ C
_ C _ C 24 15 _ E _ E 24 15
66
_ þ C _ þ Z _ ¼ C _ C 45 68 46 CC _ C
_
34
C 65 C 52 c 53 ¼ c 54 , c 60 ¼ c 63 C E _ 66 ¼ C E _ 41 _ _ E E
66
_ þ C _ þ Z _ _ _ C 61 70 H 10 ¼ C 62 þ C 71 _ C
_
2
c 47 ¼ c 48
48
_ þ Z
_ þ C _ þ Z _ _ _ C 48 64 H 9 ¼ C 49 þ C 65
_ C
_
3
58
;
;
_
C 2 C 34 C 33 C 31 C 29 C 27 c 57 ¼ c 58 , c 42 ¼ c 43 , c 39 ¼ c 40 , c 25 ¼ c 26 C E _ 3 ¼ C E _ 32 ¼ C E _ 30 ¼ C E _ 28 _ ¼ E _ E _ _ _ _ E E E E
_ þ C _ þ C _ þ Z _ _ _ _ C 3 15 17 H 2 ¼ C 4 þ C 24 þ C 25
34 þ
H-9 H-10 CC DC M-1 M-2 T-1 T-2 T-3 T-4 S-1 Tank V-1 V-2 V-3
f k ¼
_ C
Auxiliary equation (fuel rule)
ð26Þ
4. Results and discussion
In this section the exergy, exergoeconomic and sensitivity analyses results of the process are being discussed and after that the general evaluation of the process components is achievable. 4.1. Exergy analysis
Exergy efficiency and exergy destruction are the most important parameters of the exergy analysis. The results of exergy efficiency are presented in Fig. 2 and as you can see after mixers, tees, tank and expansion valves with highest values the multi-
stream heat exchanger (H-3) and distillation column (DC) components with 95.1% and 94.4% values have the best performance respectively. On the other hand the lowest values of exergy efficiency of the process components are related to multi-stream heat exchanger (H-10) and pump (P-1). Fig. 3 shows the percentage contribution of exergy destruction of the components type, proportional to overall exergy destruction of the process. Heat exchangers and compressors with 59.82% and 11.97% respectively have the highest share of the overall exergy destruction of the process. The significant point is related to combustion chamber component which is supposed to have the high exergy destruction rate but oxy-fuel concept has improved the performance of the equipment. The exergy destruction rate of CC is 158,344 kW which is very low in relation with other components of the process. Exergy efficiency for expansion valves is lower than it should be, however they also have lower exergy destruction rate compared to other units. This demonstrates that in order to examine performance and efficiency of an element of a system, both exergy efficiency and exergy destruction rate must be taken into consideration. Finally the overall exergy efficiency of the integrated ASU-CO 2 power cycle-LNG regasification process is 67.1% which is the excellent value compared to similar processes.
254
M. Mehrpooya, M.J. Zonouz/ Energy Conversion and Management 139 (2017) 245–259
Fig. 2. Exergy efficiency of the integrated ASU-CO2 power cycle-LNG regasification components.
Fig. 3. Percentage distribution of exergy destruction.
4.2. Exergoeconomic analysis
Exergoeconomic analysis combines thermodynamic and economic concepts simultaneously. In this method a cost balance is written for each component of the process and solved simultaneously with the help of auxiliary equations (Table 9). The cost rate of capital investment and operating and maintenance is one of the input cost rates of the cost equations (see Fig. 4). After solving cost equations, cost per exergy unit and cost rate of each stream are calculated as shown in Table 10. Finally exergoeconomic factor and relative cost difference are the most important parameters which are evaluating process in the sight of exergoeconomic analysis (Table 11). Exergoeconomic factor determines the contribution of capital investment and operating and maintenance cost rate against exergy destruction cost rate for each component of the process. Large exergoeconomic factor indicates that the share of capital investment and operating and maintenance cost is higher than
exergy destruction cost and for reducing the system cost, cost of elements must be reduced. Small exergoeconomic factor determines this fact that for decreasing system cost, performance and efficiency of the system components must be improved. Results for integrated ASU-CO2 power cycle-LNG regasification process demonstrate that combustion chamber has the highest exergoeconomic factor (36.14%) and after that pump P-1 with the value of 20.18% therefore for reducing total system cost, costs of combustion chamber (CC) and P-1 should be minimized. Also the smallest exergoeconomic factors are related to the distillation column (0.027%) and H-5 multi-stream heat exchanger (0.224%) and for reducing the process cost, the performance and efficiency of these components should be maximized. The relative cost difference is a description of relative increase in exergy cost of product with respect to exergy cost of fuel in an element which plays significant role in evaluating and optimizing of the system. After evaluation the highest relative cost difference is related to H-10 heat exchanger (6769) and the lowest value is related to the H-3 heat exchanger (0.066). Magnitudes of exergy cost of fuel and product determine cost of exergy rate in an element. Maximum cost per exergy unit of fuel streams is related to expander E-1 and tee T-1 (38.28 $/GJ) while the maximum product cost per exergy unit is related to H-10 multi-stream heat exchanger (941.1 $/GJ) and pump P-1 (229.7 $/GJ). Exergy destruction cost rate determines the current performance of the process elements. In this process heat exchanger H-1 with 335,144 ($/h) has the highest value and compressor CR-8 with 19.53 ($/h) has the lowest exergy destruction cost rate among all the process components. 4.3. Sensitivity analysis
One of the most important methods for process optimization is adjusting constant parameters and operating conditions of different units of a process. Sensitivity analysis of the important parameters in an energy system can determine the effectiveness of the decision variables on the objective function. Fig. 5 shows the variation of capital investment and operating and maintenance cost rates of the major components of the process with respect to the variation of interest rate as one of the most important economic parameters across the world countries. In this case the plant life time of the process has been considered constant. As you can see pump P-1 and multi-stream heat exchanger H-1 have
255
M. Mehrpooya, M.J. Zonouz / Energy Conversion and Management 139 (2017) 245–259
Fig. 4. Cost rate distributions among investment-O&M and exergy destruction for major components of the process.
Table 10
Cost rate and unit exergy cost of the process streams.
Stream no.
_ $ 103 C h
c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
0 529.0 877.8 917.1 1109 405.4 1006 705.5 403.2 735.3 1238 1238 1238 357.3 289.7 351.8 351.8 769.9 235.7 770.0 267.6 296.9 364.5 292.9 310.9 20.6 425.4 554.4 575.9 646.4 655.5 715.2 720.2 911.7 918.4 918.3
0 29.51 28.87 25.84 38.28 19.05 24.05 38.28 38.28 39.20 25.76 25.83 25.84 38.28 38.28 19.05 19.05 24.05 24.05 24.16 24.16 25.69 25.84 36.70 19.05 19.05 37.79 34.93 35.38 34.38 34.56 33.88 33.97 32.50 32.54 34.10
$
GJ
Stream no.
_ $ 103 C h
c
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
875.8 42.6 36.1 8.6 273.4 274.2 249.5 249.8 325.3 380.4 222.8 147.4 146.1 146.5 0 202.3 0.1 0.1 0.3 80,714 80,777 80,328 16,066 32,131 32,131 32,096 32,054 80,238 80,236 80,238 80,236 2.0 0 1.3 36.4
34.10 34.10 34.10 34.10 29.15 29.18 29.18 29.21 30.44 23.90 23.90 23.90 23.90 23.93 0 25.42 25.84 25.84 27.52 13.69 13.70 13.70 13.70 13.70 13.70 13.70 13.70 13.72 13.72 13.72 13.72 13.72 0 6.35 155.30
$
GJ
256
M. Mehrpooya, M.J. Zonouz/ Energy Conversion and Management 139 (2017) 245–259
Table 11
Exergy and exergoeconomic results of the integrated process.
Component
_ (kW) E F
_ (kW) E P
_ (kW) E D
c F
P-1 P-2 CR-1 CR-2 CR-3 CR-4 CR-5 CR-6 CR-7 CR-8 CR-9 CR-10 GT E-1 E-2 H-1 H-2 H-3 H-4 H-5 H-6 H-7 H-8 H-9 H-10 CC DC S-1
6.40 105 9.51 103 5.87 106 3.30 105 3.24 105 1.44 106 2.39 105 9.90 104 5.43 104 1.87 103 6.16 105 1.39 104 1.83 106 3.33 105 7.82 105 1.38 107 1.53 106 8.19 106 8.14 106 7.08 105 5.24 104 8.76 105 1.56 106 3.84 104 7.11 105 1.61 106 2.71 107 1.71 106
7.58 104 5.55 103 4.83 106 9.09 104 1.33 105 9.10 105 1.13 105 4.59 104 2.45 104 1.66 103 5.10 105 1.15 104 1.62 106 1.47 105 3.53 105 7.95 106 5.96 105 7.78 106 5.78 106 4.91 105 4.72 104 5.94 105 4.29 105 1.54 104 1.04 104 1.45 106 2.56 107 1.70 106
5.64 105 3.96 103 1.03 106 2.39 105 1.91 105 5.27 105 1.26 105 5.31 104 2.97 104 2.17 102 1.06 105 2.40 103 2.13 105 1.87 105 4.29 105 5.84 106 9.31 105 8.33 103 2.36 106 2.17 105 5.25 103 2.83 105 1.13 106 2.30 104 7.00 105 1.58 105 1.51 106 9.64 103
25 25 25 25 25 25 25 25 25 25 25 25 23.9 38.3 19.1 15.9 7.7 30.1 17.2 26.5 34.1 23.9 13.7 13.7 13.7 9.0 25.8 23.8
$
GJ
c P
$
GJ
229.7 43.3 30.4 91.0 61.1 40.5 53.0 55.0 55.5 28.5 30.4 30.3 27.1 87.7 43.0 27.9 19.1 30.1 24.2 38.3 39.1 35.4 47.1 23.9 941.1 10.5 27.4 23.9
_ C D
$
h
5.08 104 3.56 102 9.31 104 2.15 104 1.72 104 4.74 104 1.13 104 4.78 103 2.68 103 1.95 101 9.54 103 2.16 102 1.83 104 2.57 104 2.94 104 3.35 105 2.59 104 9.02 102 1.46 105 2.08 104 6.44 102 2.43 104 5.59 104 1.14 103 3.45 104 5.14 103 1.40 105 8.26 102
_ Z
e ð %Þ
r ð%Þ
f ð%Þ
12838 18.2 909.4 114.2 121.5 3254 95.8 45.4 28.5 1.1 349.2 5.1 581.4 418.0 979.9 8370 1788 7.3 425.9 46.7 62.9 78.0 227.7 25.7 107.7 2907 37.6 3.0
11.9 58.4 82.4 27.6 41.1 63.4 47.2 46.4 45.2 88.4 82.8 82.7 88.4 44.0 45.1 57.6 39.0 95.1 71.0 69.3 90.0 67.8 27.5 40.1 1.5 90.2 94.4 99.4
818.6 73.0 21.6 264 144.3 61.8 111.9 120.2 121.9 13.9 21.6 21.4 13.6 129.1 125.6 75.3 146.8 0.1 40.9 44.3 14.8 47.9 243.8 74.2 6769 16.9 5.9 0.6
20.21 4.86 0.97 0.53 0.70 6.43 0.84 0.94 1.05 5.33 3.53 2.30 3.08 1.60 3.22 2.44 6.46 0.80 0.29 0.22 8.89 0.32 0.41 2.21 0.31 36.12 0.03 0.36
$
h
Fig. 5. Effect of the different interest rates on the capital investment and operating and maintenance cost rate of some important components.
the steeper slope and the increasing of interest rate has the more impact on the associated cost rate. On the other hand compressor CR-4 and combustion chamber have the gentle slope and consequently with increasing interest rate, the capital investment and operating and maintenance cost rates are changing smoothly. In another economic case the plant life time of the process is varying from 15 years to 30 years and as it has been expected the increasing of this term is leading to the reduction of capital investment and operating and maintenance cost rate. Likewise previous case, pump P-1 and heat exchanger H-1 are more sensitive in relation with plant life time variation, against this trend the curves of compressor CR-4 and combustion chamber are similar to horizontal straight lines and have the less sensitivity. The curves of second economic case are presented in Fig. 6. In order to considering sensitivity analysis in the process elements, the gas
turbine component has been chosen. This equipment produces power for the process and has a key role in the plant. Fig. 7 shows the variation of exergoeconomic factor and exergy efficiency by changing the isentropic efficiency of the gas turbine. The exergoeconomic factor first has a gentle trend but suddenly the slope of the curve becomes steeper which means the exergy destruction cost rate has been decreased significantly. The exergy efficiency of the component has the normal trend and differing almost linearly by changing the isentropic efficiency. In the final case of sensitivity analysis, with respect to Fig. 8 the variation of exergy destruction and capital investment and operating and maintenance cost rate, have been investigated. It is obvious that by increasing isentropic efficiency the exergy destruction cost rate will be reduced, but the interesting point is the linear trend of this variation. Table 8 shows the purchased equipment cost of
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Fig. 6. Effect of the different plant life time economic term on the capital investment and operating and maintenance cost rate of some important components.
Fig. 7. Effect of the gas turbine isentropic efficiency on its exergoeconomic factor and exergy efficiency.
Fig. 8. Effect of the gas turbine isentropic efficiency on its exergy destruction and capital investment and operating and maintenance cost rate.
the gas turbine and as you can see the isentropic efficiency is in the denominator of the equation which means with increasing the isentropic efficiency the denominator becoming smaller and
the value of overall equation is becoming larger. The curve at the beginning has the smooth trend but gradually the slope of the curve becomes steeper.
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5. Conclusions
Exergy, exergoeconomic and sensitivity analyses are performed on the integrated cryogenic air separation unit, Carbon dioxide oxy-combustion power cycle and LNG regasification process. Results of exergy analysis shows that after mixers, tees, tank and expansion valves with highest exergy efficiency, the multistream heat exchanger H-3 with 95.1% and distillation column with 94.4% have the best exergy performance. The contribution of the multi-stream heat exchangers from the overall exergy destruction of the process with the value of 59.82% is the highest percentage. The overall exergy efficiency of the integrated process is 67.1% which is a brilliant efficiency for such a complicated process like this. The results of exergoeconomic analysis are as follows: 1. The multi-stream heat exchanger H-1 which has 9 sides is the worst component in the sight of exergy destruction cost rate with the value of 335,144 ($/h) and after that heat exchanger H-4 with 146,230 ($/h) and distillation column with 140,252 ($/h) have the second and third places. 2. The lowest exergy destruction cost rate of the process components is related to compressor CR-8 with 19.6 ($/h), compressor CR-10 with 216 ($/h) and pump P-2 with 356 ($/h), respectively. 3. The highest capital investment and operating-maintenance cost rate of the process components is related to pump P-1 with 12,838 ($/h), heat exchanger H-1 with 8370 ($/h) and compressor CR-4 with 3254 ($/h), respectively. 4. Exergoeconomic factor in combustion chamber and pump P-1 with 36.1% and 20.2% values respectively, is higher than other elements and in order to reducing total system cost, cost of these elements must be minimized. 5. Exergoeconomic factor in distillation column, compressors and heat exchangers compared to other elements of the system, except a few exceptions is negligible and in order to reducing total system cost, performance and efficiency of these elements must be maximized.
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