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Design and optimization of nitrogen expansion liquefaction processes integrated with ethane separation for high ethane-content natural gas
Applied Thermal Engineering 173 (2020) 115272
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Design and optimization of nitrogen expansion liquefaction processes integrated with ethane separation for high ethane-content natural gas Ting He, Wensheng Lin
T
⁎
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
HIGHLIGHTS
expansion LNG processes integrated with ethane separation are proposed. • Three refrigeration is supplied by N or N /CH expansion with or without precooling. • The of the distillation column heat load is achieved by heat integration. • Self-supply purity and recovery rate of liquid ethane products are both higher than 99.5%. • The • The specific power consumption decreases with ethane content increasing. 2
2
4
ARTICLE INFO
ABSTRACT
Keywords: Liquefied natural gas (LNG) Ethane Nitrogen expansion cycle Liquefaction process Genetic algorithm
Compared to ordinary natural gas, ethane has higher market value. Thus, recovery of ethane can increase the economic value of high ethane-content natural gas. This paper proposes a new liquefaction method for high ethane-content natural gas. Natural gas liquefaction and cryogenic distillation are combined to separate highpurity liquid ethane (no less than 99.95%) in the process of producing liquefied natural gas (LNG), which maximizes the economic benefits. And self-supply of the required heat of the distillation column is achieved by heat integration. For this new method, three nitrogen expansion processes are designed, and each process is simulated and optimized by HYSYS and genetic algorithm. Based on the optimization results, the specific power consumption and exergy efficiency of the three processes are analyzed. The results show that the specific power consumption of the three processes decrease with the increase of the ethane content. When the ethane content is 10% ~ 40%, the specific power consumption of the three processes is 0.5969 ~ 0.6060 kWh/Nm3 (NG), 0.5371 ~ 0.5592 kWh/Nm3 (NG), 0.5015 ~ 0.5403 kWh/Nm3 (NG), respectively, and the exergy efficiency of the proposed three processes is 33.3 ~ 35.4%, 37.1 ~ 38.3%, 39.7 ~ 39.9%, respectively.
1. Introduction Compared with coal and oil, natural gas is a cleaner primary energy source that produces almost no particulate matter and much less carbon dioxide. Using natural gas instead of coal and oil helps effectively reduce pollutants and greenhouse gas emissions. To ensure diversification of energy supply and improve energy consumption structure, increasing the proportion of natural gas in the energy consumption structure has become a strategic focus of energy development for many countries [1,2]. The development and utilization of shale gas has provided a new growth point for global natural gas production. The massive exploitation of shale gas began in the United States and has achieved great success, which has had a huge impact on the global gas supply market. Shale gas will play an important role in meeting future global energy ⁎
needs, and it will be an important part of many national energy policy portfolios [3,4]. In recent years, the role of liquefied natural gas (LNG) is becoming more and more important, and its trade share has significantly increased [5]. Changing shale gas into LNG products facilitates import and export trade, ocean shipping and peak shaving storage, and improves economic efficiency and market flexibility [6]. Although the liquefaction technology of conventional natural gas has matured, there are still problems about shale gas liquefaction. The ethane content in shale gas is generally higher than that of conventional natural gas. In US, ethane content of shale gas is as high as 12–35%. Considering the calorific value limit of pipeline natural gas, only a small amount of ethane can be left in natural gas for sale [7]. As we know, ethane has high industrial value. Compared to the
Corresponding author. E-mail address: [email protected] (W. Lin).
https://doi.org/10.1016/j.applthermaleng.2020.115272 Received 25 December 2019; Received in revised form 27 March 2020; Accepted 31 March 2020 Available online 02 April 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 173 (2020) 115272
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other methods of producing ethylene, ethane cracking has the advantages of low cost, high yield, low capital investment, low pollution and mature technology [8]. With the development of shale gas in North America, cheap ethane has contributed to the rapid development of ethylene production in the United States and Canada, and a large amount of ethane are exported from the United States. Meanwhile, US shale gas is going global, and countries are importing low-cost ethane from the United States for cracking to produce ethylene [9,10]. Yang et al. systematically compared the economic cost of ethylene produced by shale gas and naphtha. Their study shows that the production of ethylene from natural gas liquids (NGL) -rich shale gas is more economical than from naphtha [11]. Thus, the separation of high-purity ethane from shale gas is not only good for enhancing the market value of shale gas, but also of great significance for the benefits of the ethylene industry. Generally speaking, natural gas liquefaction processes are divided into three categories: cascade liquefaction process, mixed refrigerant liquefaction process, and liquefaction process with expander [12,13]. The expansion refrigeration liquefaction process is simple in process, small in equipment and space, compact in layout, flexible in adjustment, reliable in operation, quick in response to start-stop and convenient in maintenance of equipment. Small and medium-sized natural gas liquefaction plants may adopt a liquefaction process with an expander [14]. In the nitrogen expansion refrigeration process, the refrigerant is convenient to be replenished, stored and transported. Nitrogen is always in the gas phase, and it is a non-flammable working medium, so the process is safe and reliable. Therefore, for most of the scattered gas fields, unconventional gas fields, liquefaction devices and peaking devices under sea sloshing conditions, the nitrogen expansion process is a suitable choice. A large number of scholars have studied the design and optimization of the expansion process under different conditions. Xiong et al. [15] studied single expander pressurized liquefied natural gas (PLNG) processes for offshore natural gas which contains less than 0.5% CO2. Lin et al. [16] studied the nitrogen expansion process to produce liquefied natural gas and separate hydrogen from synthetic natural gas (SNG). Moein et al. [17] used genetic algorithm to optimize the nitrogen expansion cycle liquefaction process, effectively achieving energy saving. Pre-cooling of the natural gas feed stream can significantly improve the efficiency of the expansion refrigeration process [18]. Gao et al. [19] proposed and optimized a propane precooled nitrogen expansion liquefaction process for coalbed methane. The results showed that the liquefaction energy consumption is acceptable when the nitrogen content in feed gas did not exceed 70%. Yuan et al. [20] proposed a new small-scale liquefaction method of single nitrogen expansion process using carbon dioxide precooling. Ding et al. [21] used HYSYS and MATLAB to optimize the N2-CH4 expansion liquefaction process with pre-cooling, which reduced the unit energy consumption by 5.35%. Bukowski et al. [22] analyzed a variety of liquefaction processes for floating LNG production and storage device (FLNG), of which the AP-N process is the safest. In actual production, the Malaysian PFLNG project finally adopted the AP-N process with safety indicators account for. There are mainly two kinds of researches on the recovery of light hydrocarbons in natural gas. One is to recover light hydrocarbons before natural gas is liquefied [23], and the other is to recover light hydrocarbons from liquefied natural gas [24]. The latter technology is generally combined with the cold energy utilization of LNG [25]. Wang et al. [26] integrated the LNG regasification and shale gas NGL recovery process to achieve maximum energy savings and considered the uncertainty of shale gas feed rate changes. Park et al. [27] selected nine patented NGL recovery schemes and modified them by strengthening the suitability of heat exchange systems for offshore applications. A comprehensive analysis of each process was carried out while focusing on propane recovery. Ghorbani et al. [28] proposed a cascade refrigeration system in integrated cryogenic natural gas process (natural gas liquids (NGL), liquefied natural gas (LNG) and nitrogen rejection
unit (NRU)), which can recover 92% of ethane. Ghorbani et al. [29] also proposed an integrated LNG-NGL system which uses an absorption refrigeration cycle for pre-cooling and uses a cascaded mixing refrigeration cycle (MFC) for cooling and liquefaction, and the ethane recovery is 91.96%. Both processes proposed by Ghorbani were designed for natural gas with a C2+ content of 12.04%. He et al. designed a novel mixed refrigerant cycle integrated with NGL recovery process for smallscale LNG plant [30]. In this study, the ethane content of feed gas is only 6%. Vatani et al. [31] used two mixed refrigerant cycles to coproduct both NGL and LNG. The results showed that specific power consumption is 0.414 kWh/kg LNG) and the ethane recovery rate is higher than 93.3%. Mehrpooya et al. [32] selected C3-MR, DMR and MFC refrigeration systems to provide the required refrigeration for three integrated processes for LNG and NGL cogeneration. For a feed gas with an ethane content of 7%, the ethane recovery is higher than 90%. The prior studies that involved the separation of light hydrocarbons from the liquefaction process mainly concerned the recovery of C3+ heavy components, instead of separating and purifying ethane directly during the liquefaction process. The ethane content of the feed gas in these studies is less than 15%, but in fact, the ethane content of natural gas such as shale gas may be as high as 35%. Furthermore, the purity of the recovered ethane is not high enough to be directly used as an industrial raw material, and the separated ethane may only be used for fuel. Besides, the recovery rate of ethane in these processes is low. This study proposes a high ethane-containing natural gas liquefaction method combined with cryogenic distillation to obtain high-purity lowpressure liquid ethane and LNG products. The purity standard of industrial ethane can be directly achieved without secondary processing because the purity of the separated ethane is not less than 99.95%. What’s more, in this study, the recovery of ethane is above 99% when ethane content of the studied feed gas is in the range of 10% to 40%, which ensures maximum economic benefits of feed gas. 2. Process design 2.1 Assumptions To better investigate the influences of ethane content to the processes by avoiding the influence of other components, such as nitrogen and C3+, it is assumed that the feed gas is composed of only CH4 and C2H6 (ethane content in feed gas is 10% to 40%), and the pressure, temperature and molar flow of feed gas are 0.12 MPa, 40 °C, 1 kmol/h, respectively. The study is carried out by Aspen HYSYS, and the equation of state used in HYSYS simulation is the Peng-Robinson (P-R) equation. The following assumptions are given for simulation: (1) The gas at the outlet of the compressor is cooled by a water cooler to 40 °C; (2) The minimum temperature differences in heat exchangers are set as 3 °C; (3) The adiabatic efficiencies of compressors and expanders are set as 85% and 80%, respectively; (4) The storage pressure of LNG and liquid ethane is 0.12 MPa, and the liquefaction rate is 100%; (5) The purity of the liquid ethane product is not less than 99.5%, and recovery rate of ethane over 99%. (6) The pressure drop in each heat exchanger is ignored to simplify the simulation calculation process. 2.2 Process description Three different nitrogen expansion liquefaction separation processes are established, as shown in Fig. 1 (Process 1: two-stage nitrogen expansion process, Process 2: propane pre-cooling nitrogen expansion 2
Applied Thermal Engineering 173 (2020) 115272
T. He and W. Lin
(a) Process 1
(b) Process 2 Fig.1. Nitrogen expansion liquefaction processes integrated with ethane separation for high ethane-content natural gas C: compressor, E: expander, WC: water cooler, Q: heat flow, DT: distillation column, T: tank, V: valve, HEX: multi-flow heat exchanger, W: work; H: heat exchanger.
process; Process 3: propane pre-cooling nitrogen methane expansion process). In the three processes, the most critical equipment for efficient separation is the cryogenic distillation column. A full distillation column is selected, and the purity of liquid ethane is ensured through a reboiler at the bottom of the column, and the recovery rate of ethane is improved by condensing reflux at the top of the column. For Process 1, the liquefaction separation process consists of three
parts: (1) Nitrogen expansion refrigeration cycle In order to make full use of the expansion work of nitrogen, the expansion process is divided into two stages, and each turbo expander drives a coaxial compressor to recover expansion work. After multi-stage compression and water cooling, the low pressure 3
Applied Thermal Engineering 173 (2020) 115272
T. He and W. Lin
(c) Process 3 Fig.1. (continued)
consumption of the expansion refrigeration process. Process 3 is the nitrogen-methane expansion liquefaction separation process, and it is basically the same as the propane pre-cooling nitrogen expansion process. The only difference is that the material 111 behind the throttle valve V-101 is not completely liquefied, so there will be boil-off gas (BOG) in the LNG storage tank T-101. The BOG (113) enters the heat exchangers of each stage to recover the cold and then it is mixed into the feed gas for liquefaction. In order to ensure that the outlet of the expander does not yield liquid, the outlet temperature of the expander should be higher than the dew point of the corresponding pressure by 2 °C. The proposed processes mainly have the following advantages:
nitrogen (301) is compressed to high pressure nitrogen (309) with a pressure of 5 ~ 6 MPa and a temperature of 40 °C. Then the high pressure nitrogen enters heat exchangers HEX-101, HEX-102 in sequence, and is cooled to −65 ~ −45 °C (311). Then it expands to 0.26 ~ 0.3 MPa by a two-stage expander, and its temperature drops to −163.5 °C. The low temperature nitrogen (313) enters heat exchangers HEX-103, H-102 (representing the condenser of D-101), HEX-102, HEX-101 in sequence to provide cold energy and its temperature rises to 37 °C (301). It goes back to C-301, completing a cycle. (2) Ethane separation distillation module After a two-stage compression and water cooling, the feed gas (107) is cooled to −90 ~ −75 °C by heat exchangers HEX-101, H-101 (representing the reboiler of D-101) and HEX-102. The partially liquefied feed gas (108) then enters the distillation column to separate ethane. The natural gas (109) at the top of the column is further cooled by the condenser (H-102), and its ethane content is less than 1%. After re-boiling in the reboiler (H-101), the purity of liquid ethane (201) at the bottom of the column reached 99.5%. The liquid ethane is further cooled to −90 °C (203), and then throttled to 120 kPa (204) to be stored in the storage tank T-201. (3) Natural gas liquefaction module The methane-rich feed gas from the top of the distillation column (109) enters a heat exchanger HEX-103 is cooled to −160.5 °C (110), and is throttled to 120 kPa (111), finally enters the LNG tank (T-101) for storage.
(1) Integrating the cryogenic distillation with the natural gas liquefaction process, the produced LNG can meet the heating value of pipeline or combustion equipment, and the separated liquid ethane can be used in industrial applications to maximize economic benefits. (2) The heat required for the distillation column are provided by the process itself. (3) The two products obtained by the process are low pressure liquids, convenient for storage and transportation. 2.3 Process performance indicators The main indicators for evaluation the performance of natural gas liquefaction and separation processes are specific power consumption and ethane recovery rate. The specific power consumption (kWh/Nm3) of the process is defined as:
In Process 1, the required cold energy is completely provided by the expansion of nitrogen. Since the separated ethane does not require deep cooling, part of the cold in the high temperature section can be provided by the propane pre-cooling cycle. By this way the nitrogen flow rate will be reduced and the specific power consumption of the process will be reduced. Therefore, Process 2 is proposed, which is constructed by adding a propane precooling cycle to Process 1. In the pre-cooling cycle, the refrigerant propane is throttled after being compressed and water-cooled, and then supplied to heat exchanger HEX-101. The addition of methane to nitrogen can reduce the power
w=
WC WE MNG Vm
where: MNG—molar flow of natural gas, kmol/h; WC—total power consumption of the compressors, kW; WE—total output power of the expanders, kW; 4
(1)
Applied Thermal Engineering 173 (2020) 115272
T. He and W. Lin
W = WC
Create an initial population
where: h is specific enthalpy, kJ/kg; s is specific entropy, kJ/(kg·K); T0 is ambient temperature, T0 = 313.15 K; Tm is the temperature of the mixture; Subscript 1 and 2 represent before and after separation, respectively.
Pass the population to HYSYS via the interface
HYSYS performs simulation calculations and passes the results to MATLAB
New population
No
Restrictions
Mutation
Neglecting potential energy and kinetic energy, the exergy equilibrium equation of a steady-flow system and working fluid is defined as (10) and (11) respectively. 2
Punishment
1
End
No
Yes
Termination condition
MLC 2 MC 2
(2)
where: MC2—molar flow of ethane in feed gas, kmol/h; MLC2—molar flow of liquid ethane, kmol/h.
Wmin W
In this study the main purpose of optimization is reducing the power consumption of the process. So, the objective function is design as:
min f (x ) =
Theoretical minimum work includes theoretical minimum liquefaction work Wl and theoretical minimum separation work Ws: Theoretical minimum liquefaction work is expressed as follows:
Wl h 0, LC 2 )
T0 (sLC 2
s0, LNG )] + mLC 2
s0, LC 2 )]
(6)
Unit power consumption of the ideal separation process is:
ws, i = Tm (s1
s2)
(h1
h2)
(7)
Theoretical separation work of the binary mixtures of A and B is:
ws, i = ws, A mA / mm + ws, B mB / mm
WC WE MNG Vm
(3)
The processes proposed in this paper mainly consist of two parts: the refrigeration cycle to supply cold energy for liquefaction of natural gas, and the distillation column to separate methane and ethane. Consequently, the parameters to be optimized should include the main parameters of both the refrigeration and the distillation parts. The refrigeration cycle of the system consumes most of the power input, so the parameters such as flow, pressure and temperature of the refrigeration cycle should be optimized. The propane vapor compression refrigeration cycle is relatively simple. The condensing temperature before throttling is determined by the water cooler, which is 40 °C. The evaporating temperature is set as −38.25 °C, a little bit higher than the normal boiling point of propane. Therefore, this optimization of the refrigeration part focuses on the nitrogen expansion cycle. As for the distillation part, the temperature and pressure of distillation are also important factors influencing process power consumption. Among the parameters to be optimized, the distillation
(5)
Wmin = Wl + Ws
[(hLC 2
(11)
3.2 Optimization function and optimization variable selection
(4)
T0 (sLNG
T0 S0)
(1) The population size is 100; (2) Maximum generation number is 200; (3) Crossover rate and mutation rate is 0.6, and 0.05, respectively.
The exergy efficiency of the system is a key indicator to measure the energy utilization of the system. The exergy efficiency of a liquefaction system refers to the ratio of the ideal minimum liquefaction work Wmin of the system to the actual work W consumed by the system, expressed as follows (FOM refers Figure of Merit):
h 0, LNG )
(H0
(10)
Genetic algorithm is an intelligent optimization algorithm, which is suitable for large-scale and nonlinear optimization models. It has been widely used in chemical process simulation and optimization in recent years. The proposed process is connected with a GA (genetic algorithm) code in MATLB by using HYSYS COM server. In the optimization process, HYSYS plays the role of calculating the target function, and MATLB mainly provide the calculation process of the genetic algorithm. The genes of each generation generated by the genetic algorithm are converted into the parameters that HYSYS can recognize by decoding the code, and the result is calculated in HYSYS, and returned as the target function value to MATLB. The specific program framework is shown in Fig. 2 [29]. The parameters of the genetic algorithm are set as follows:
Ethane recovery rate is defined as:
= mLNG [(hLNG
T0 S )
EX1) + Wu + EX,loss
3.1 Optimization method
Output optimal population
Vm—molar volume of natural gas, taking 22.4Nm3/kmol.
FOM =
Q = (EX2
3. Process optimization
Fig. 2. HYSYS process optimization block diagram based on MATLAB genetic algorithm.
RC 2 =
T0 Tr
where, Tr is the temperature of heat source; Q is heat absorbed from a heat source; H is enthalpy; S is entropy; EX is exergy; Wu is useful work.
Objective function
Selection
1
EX = (H
Yes Crossover
(9)
WE
(8)
Actual work is expressed as follows: 5
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T. He and W. Lin
Optimization parameter
Lower bound
Upper bound
T311 (°C) P312 (kPa) P313 (kPa) P306 (kPa) P308 (kPa) MN2 (kmol/h) MC3 (kmol/h)
−65 1100 250 1600 4000 4 0
−45 1600 350 2100 7000 5.5 0.35
Specific power consumption (kW.h/Nm3)
Table 1 Optimization variables.
temperature and pressure affect the separation purity of the distillation column, and cannot be simply set by genetic algorithms like the parameters of the nitrogen cycle. Therefore, the two parameters are separately set in advance and then the other parameters are optimized by the genetic algorithm. The distillation temperature is set with reference to the bubble point and dew point temperature of the methane ethane mixture (ethane content: 10 ~ 40%). The upper and lower bounds of genetic algorithm optimization variables in Process 2 are shown in Table 1. Process 1 dos not include the propane flow, and Process 3 adds the methane flow in the refrigerant. The upper and lower limits of the parameters of the three processes are slightly different. In addition, the tray number in a distillation column has a considerate impact on the power consumption of the process. Since the number of distillation column trays cannot be optimized during the comprehensive optimization with genetic algorithm, a single factor optimization is adopted to optimize it before the comprehensive optimization. For the situation that the parameters of a stream entering a distillation column remain unchanged, a shortcut column in HYSYS is used to calculate a minimum theoretical tray number. And then, starting from the minimum theoretical tray number, the number of trays is gradually increased. By adjusting the reflux ratio of the distillation column and the flow rate of the refrigerant, the limiting conditions and separation requirement are satisfied. Observe the change of the specific power consumption of the system until the optimal tray number is found. Taking Process 2 as an example, when the ethane content is 10%, T108 = −85 °C, P108 = 2.5 MPa, the minimum theoretical tray number obtained from HYSYS through shortcut column is 4.8, which is 5 after rounding. After gradually increasing the tray number, the optimal tray number obtained is 10. In fact, since the minimum theoretical tray number itself obtained from HYSYS is based on scientific calculation, increasing the minimum theoretical tray number to the optimal one only has very limited effect of reducing power consumption. Considering this, the optimal tray number is good enough to be used in the following comprehensive process optimization.
0.60 0.59 0.58 0.57 0.56 0
25
50
75
100
125
150
175
200
Generation number Fig. 3. GA optimization convergence curve.
The optimization results of the three processes at the four ethane contents are shown in Table 2. At the four ethane contents, the specific power consumption of Process 3 is the lowest, and that of Process 1 is the highest. Compared with Process 2 and 3, the pressure of stream 308 and the required flow rate of the nitrogen refrigerant of Process 1 are higher, which can explain why the power consumption of process 1 is the highest. Compared with Process 1 and 2, the specific power consumption of Process 3 decreases more with increasing ethane content. That means the advantage of Process 3 is more obvious in the case of high ethane content. In all cases, P313 is in a narrow range of 280–300 kPa and the minimum approach of heat exchange is very close to the set value of 3 °C. 4.2 Effect of ethane content on power consumption Fig. 4 shows the specific power consumption of the three processes at different ethane contents. As the ethane content increases, the specific power consumption of the three processes decreases gradually. However, the three processes differ in decreasing extent with the increase of ethane content. The specific power consumption of Process 1 decreases slightly, and that of Process 3 decreases obviously. As the ethane content increases, the amount of refrigeration required for liquefaction decreases but the work consumed by the distillation increases. However, the refrigeration required for liquefaction is reduced more, so the specific power consumption of the system is reduced. In terms of power consumption, Process 3 is best followed by Process 2, Process 1 is the worst. The specific power consumption of Process 2 and Process 3 is 8 ~ 10% and 10 ~ 16% lower than that of Process 1. 4.3 Analysis of distillation separation effect
3.3 Genetic algorithm optimization process The GA optimization convergence curve of Process 2 (10% ethane content) is shown in Fig. 3. It is obvious that the objective function value decreases quickly in the initial stage, because in the initial stage the genetic algorithm is easy to find a better next generation. When the optimization continues, the objective function will have a platform period. Since the objective function is already superior, it is difficult to find a better next-generation population on the platform, and it takes more time to find a better next generation. Finally, when the genetic algorithm is optimized to the 75th generation, the minimum value of the objective function is found, and the optimal solution is 0.5592 kWh/Nm3. The optimization iterative process of other working conditions also shows the same tendency.
For Process 2, the temperature distribution and component distribution in the distillation column with ethane content of 10% at a pressure of 2.5 MPa is shown in Fig. 5. The temperature from the top to the bottom of the tower gradually increased from −101.6 °C to 0.8 °C, and the separation effect of the distillation column is very good. The purity of methane in the top outlet gas is 99.95%, and the purity of ethane in the bottom outlet liquid is 99.51%. Under other distillation conditions, the purity of liquid ethane obtained by the three processes is higher than 99.5% with ethane recovery rate over 99.5%.
4. Optimization results and analysis
Fig. 6 shows the specific power consumption of Process 2 (10% ethane) at different distillation pressure and temperature. The distillation temperature has a greater impact on power consumption but there
4.4 Distillation pressure and temperature analysis
4.1 Optimization results 6
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Table 2 Optimization results. Parameter
Process 1
Specific power consumption (kWh/Nm3)
T311 (°C) P312 (kPa) P313 (kPa) P306 (kPa) P308 (kPa) MN2(kmol/h) MC3 (kmol/h) MC1 (kmol/h) ΔTHEX-101 (°C) ΔTHEX-102 (°C) ΔTHEX-103 (°C) W (kWh/Nm3)
Process 2
Process 3
10%
20%
30%
40%
10%
20%
30%
40%
10%
20%
30%
40%
−49.4 1562 298 1980 5332 5.222
−46.5 1425 298 1929 5631 5.162
−46.0 1489 298 2023 5678 5.238
−46.2 1412 297 1980 5652 5.316
−60.0 1245 300 1722 4394 4.721 0.185
−60.0 1208 298 1742 4362 4.732 0.224
−56.5 1289 295 1790 4622 4.62 0.258
−51.0 1280 295 1823 5128 4.384 0.288
3.0 3.2 3.0 0.606
3.0 3.0 3.3 0.603
3.0 3.1 3.0 0.599
3.0 3.3 3.1 0.597
3.0 3.0 3.1 0.559
3.0 3.0 3.1 0.551
3.0 3.0 3.0 0.540
3.0 3.0 3.0 0.537
−50.0 1123 288 1498 3892 1.658 0.090 3.300 3.0 3.1 3.0 0.545
−50.0 1212 288 1587 4045 1.748 0.21 2.718 3.0 3.0 3.1 0.528
−49.8 1223 284 1620 4124 1.843 0.262 2.481 3.0 3.0 3.2 0.511
−49.0 1220 278 1580 3964 1.498 0.298 2.803 3.0 3.0 3.1 0.502
In fact, the cooling energy is consumed in two aspects: 1) the cooling process of the gas; 2) the overhead condenser of the distillation tower. Increasing the liquefaction pressure, the amount of cooling required for gas liquefaction decreases, however the separation of methane and ethane becomes more difficult, which can be seen from the cooling load of which increases the cooling load of the condenser. Therefore, at low pressures, the specific power consumption required for liquefaction predominates. When the pressure is high, the specific power consumption of the separation column is dominant.
0.60 0.58
process 1 process 2 process 3
0.56 0.54 0.52
5. Exergy analysis
0.50 0.10
0.15
0.20
0.25
0.30
0.35
5.1 Heat transfer curve
0.40
Ethane content
LNG heat exchangers are very important in natural gas liquefaction process. Heat transfer performance in LNG heat exchangers can greatly influence the process power consumption. Thus, the analysis of heat transfer is necessary. The smaller gap between the hot and cold temperature curves will result in higher heat transfer efficiency in LNG heat exchangers, reducing the exergy destruction of the heat transfer process. Since the refrigeration in low temperature range consumes more power than that in high temperature range, it can save more energy when the gap between cold composite curve and hot composite curve in low temperature range is reduced. Moreover, reducing the gap in low temperature range is more effective in reducing the exergy loss. The heat transfer curves of the optimized processes are shown in Fig. 7. Since the feed gas is not pressurized to above the critical pressure, the temperature of the hot fluid remains unchanged during the liquefaction of the natural gas, causing the large temperature difference between the hot and cold fluids in this region. For Process 1, the heat exchange
Fig. 4. Effect of ethane content on power consumption.
0
1.0
-20
0.8 Molar fraction
Temperature (°C)
is no uniform influence regularity. In general, at a given pressure, too high or too low temperature of the distillation column feed flow will lead to an increase in distillation energy consumption. A temperature that makes the feed gas-liquid component reasonable will result in the lowest energy consumption of the system. As shown in Fig. 6(a), when the pressure is 2.5 MPa, the system consumes the lowest power at a feed temperature of −85 °C. However, at each distillation temperature, the specific power consumption exhibits the same regularity as the distillation pressure changes. As the liquefaction pressure increases, the specific power consumption approximately showing a trend of decreasing first and then increasing, and there is an optimum liquefaction pressure.
-40 -60 -80 -100
Methane Ethane
0.6 0.4 0.2 0.0
0
2
4
6
8
10
12
0
Tray
2
4
6
8
10
12
Tray
(a)Temperature distribution
(b) Component distribution
Fig.5. Parameters change of each layer of tray in the distillation column Tray 0 refers reboiler; tray 11 refers condenser. 7
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Condenser cooling consumption (kJ/h)
Specific power consumption (kWh/Nm3)
3500 0.64
T308 = -75°C T308 = -80°C T308 = -85°C T308 = -90°C T308 = -95°C
0.62 0.60 0.58
Wmin=0.5592
0.56 1.0
1.5
2.0
2.5
3.0
3.5
4.0
3000 2500 2000 1500 1000
1.0
1.5
2.0
2.5
3.0
3.5
Liquefaction pressure (MPa)
Liquefaction pressure (MPa)
(a) Specific power consumption
(b) Condenser cooling load
4.0
Fig.6. Optimization results of specific power consumption.
temperature difference of 10% ethane content in the heat exchangers HEX-101 and HEX-102 is smaller than that of 30% ethane content, so the heat transfer effect of 10% ethane content is better than that of 30% ethane content. The heat transfer curves of three processes with an ethane content of 30% show that the heat transfer temperature difference in Process 2 and Process 3 is significantly smaller than that of Process 1. In addition, processes 2 and Process 3 are pre-cooled by propane. In the propane evaporation section, the temperature of the cold fluid remains unchanged, and the temperature difference is relatively large.
5.2 Exergy efficiency Fig. 8 shows the exergy efficiency of the three processes. The exergy efficiency of Process 1 has a downward trend with the increase of ethane content. But, the exergy efficiency of Process 2 and Process 3 does not change significantly with the ethane content. Moreover, the exergy efficiency of Process 3 is significantly higher than that of Process 1. The exergy efficiency of Process 3 can be maintained at a higher level at different ethane levels due to the use of two mixed refrigerants, which fully reflects the advantages of Process 3.
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Fig. 7. Heat transfer curve of the optimized process. 8
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1/3 of the total, comes from the water coolers. When the hot compressed fluid is cooled to ambient temperature in the coolers, the exergy is dissipated into the environment. The lower the compressor efficiency, the lager the exergy destruction in the water coolers is. Unless waste heat utilization system is introduced, little can be done to reduce this part of exergy destruction. Expanders. Around 20% of exergy destruction comes from the expanders, since the adiabatic efficiency of expanders is 80%. A process engineer can do little about this. Distillation column and throttle valves. Exergy construction in these two kinds of devices is relatively small. It may be found that, as the ethane content increases, the proportion of exergy construction in the distillation column increases gradually. Heat exchangers. The temperature difference between the hot and cold fluids in the heat exchangers result in exergy destruction. In fact, the most important result that can be anticipated from a process optimization is to reduce the heat transfer difference so as to minimize exergy destruction of the process. In this point of view, the presented process optimization is successful, because the exergy destruction in heat exchangers is only a little bit more than 10% of the total.
Exergy efficiency
0.40 0.38 0.36
process 1 process 2 process 3
0.34 0.32
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5.3. Equipment exergy destruction Based on Eqs. (10) and (11) the exergy destruction of each device is calculated. Fig. 9 shows the ratio of the exergy destruction to the total exergy destruction for each device in Process 2. Some discussions about the equipment exergy destruction are as follows. Compressors. A large proportion of exergy destruction, about 30% of the total exergy destruction, comes from the compressors. This may be explained in two aspects. Firstly, the adiabatic efficiency of compressors is 85%, and there must be large exergy destruction caused by the irreversibility in the compressors. Secondly, the refrigeration in a nitrogen expansion process is provided by the sensible heat of nitrogen, resulting in a large flow of nitrogen and a large power consumption for compression. It seems that a compressor engineer has much to do to enhance the compressor efficiency, while at the same time, a process engineer has little to do unless another process is adopted or more compressors are used to substitute each compressor in the process. Water coolers. The largest proportion of exergy destruction, about
Compressor 29.91%
12.03% Heat exchanger 1.41% Throttle valve
6. Conclusions The exploitation, storage, transportation and trade of shale gas have been an important research topic in recent years, and have important strategic significance in energy structure optimization. In order to maximize economic benefits, this paper propose three nitrogen expansion refrigeration processes to numerically simulate the liquefaction separation process of high-ethane-containing shale gas, and optimizes the process through genetic algorithm. The optimization results of the proposed three processes under different ethane contents are analyzed, and the main conclusions are as follows: (1) The purity of the liquid ethane product obtained by the cryogenic distillation separation is not less than 99.5%, and the ethane recovery rate is higher than 99.5%. (2) The specific power consumption of the three processes decreases Compressor 29.89%
Water cooler 32.67%
11.26% Heat exchanger 2.03% Throttle valve
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(a) 10% ethane content Compressor 29.01%
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Water cooler 33.03%
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(c) 30% ethane content
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(d) 40% ethane content
Fig. 9. Exergy destruction of Process 2 at different ethane contents. 9
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with the increase of the ethane content. When the ethane content is 10% ~ 40%, the specific power consumption of the three processes is 0.5969 ~ 0.6060 kWh/Nm3, 0.5371 ~ 0.5592 kWh/Nm3, 0.5015 ~ 0.5403 kWh/Nm3, respectively. Compared to Process 1, the specific power consumption for Process 3 is more affected by the ethane content. (3) The exergy efficiency analysis shows that the exergy efficiency of Process 1 is the lowest, and that of Process 3 is the highest. When the ethane content is 10% ~ 40%, the exergy efficiency of the proposed three processes is 33.3 ~ 35.4%, 37.1 ~ 38.3%, 39.7 ~ 39.9%, respectively. (4) Of the three proposed processes, Process 3 has the lowest specific power consumption and best adaptability to the ethane content in the feed gas. Among the parameters to be optimized, the distillation pressure is the critical factor affecting the specific power consumption of the process.
[12] [13] [14] [15] [16] [17] [18] [19]
Declaration of Competing Interest
[20]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[21] [22]
Appendix A. Supplementary data [23]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2020.115272.
[24]
References
[25]
[1] Q. Wang, China should aim for a total cap on emissions, Nature 512 (2014) 115. [2] S.L. Burns, M. Krott, H. Sayadyan, L. Giessen, The world bank improving environmental and natural resource policies: power, deregulation, and privatization in (post-soviet) Armenia, World Develop. 92 (2017) 215–224. [3] Q. Wang, X. Chen, A.N. Jha, H. Rogers, Natural gas from shale formation – the evolution, evidences and challenges of shale gas revolution in United States, Renew. Sustain. Energy. Rev. 30 (2014) 1–28. [4] W.J. Lee, S.Y. Sohn, Patent analysis to identify shale gas development in China and the United States, Energy Policy 74 (2014) 111–115. [5] S. Kumar, H.T. Kwon, K.H. Choi, W. Lim, J.H. Cho, K. Tak, LNG: An eco-friendly cryogenic fuel for sustainable development, Appl. Energy 88 (2011) 4264–4273. [6] A. Dutta, I.A. Karimi, S. Farooq, Heating value reduction of LNG (liquefied natural gas) by recovering heavy hydrocarbons: technoeconomic analyses using simulationbased optimization, Ind. Eng. Chem. Res. 57 (2018) 5924–5932. [7] J.F. Goellner, Expanding the shale gas infrastructure, Chem. Eng. Prog. 108 (2012) 49-52, 59. [8] M.B. Yang, X.Y. Tian, F.Q. You, Manufacturing ethylene from wet shale gas and biomass: comparative techno-economic analysis and environmental life cycle assessment, Ind. Eng. Chem. Res. 57 (2018) 5980–5998. [9] M. Mccoy, U.S. shale gas heads overseas, Chem. Eng. News 92 (2015) 3. [10] D. Ding, Y.Y. Zhang, W. Wu, D.C. Chen, M.L. Liu, T. He, A novel low-thermal-budget approach for co-production of ethylene and hydrogen via electrochemical nonoxidative deprotonation of ethane, Energy Environ. Sci. 11 (2018) 1710–1716. [11] M.B. Yang, F.Q. You, Comparative techno-economic and environmental analysis of
[26] [27] [28]
[29] [30] [31] [32]
10
ethylene and propylene manufacturing from wet shale gas and naphtha, Ind. Eng. Chem. Res. 56 (2017) 4038–4051. W.S. Lin, N. Zhang, A.Z. Gu, LNG (liquefied natural gas): a necessary part in China's future energy infrastructure, Energy 35 (2010) 4383–4391. W.S. Lin, X.J. Xiong, M. Spitoni, A.Z. Gu, Design and optimization of pressurized liquefaction processes for offshore natural gas using two-stage cascade refrigeration cycles, Ind. Eng. Chem. Res. 57 (2018) 5858–5867. T.B. He, Y.L. Ju, Performance improvement of nitrogen expansion liquefaction process for small-scale LNG plant, Cryogenics 61 (2014) 111–119. X.J. Xiong, W.S. Lin, A.Z. Gu, Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology, J. Nat. Gas. Sci. Eng. 30 (2016) 379–387. W.S. Lin, J.X. Xu, L. Zhang, A.Z. Gu, Synthetic natural gas (SNG) liquefaction processes with hydrogen separation, Int. J. Hydrogen Energy 42 (2017) 18417–18424. P. Moein, M. Sarmad, M. Khakpour, H. Delaram, Methane addition effect on a dual nitrogen expander refrigeration cycle for LNG production, J. Nat. Gas. Sci. Eng. 33 (2016) 1–7. N.M. Shah, A.F.A. Hoadley, G.P. Rangaiah, Inherent safety analysis of a propane precooled gas-phase liquefied natural gas process, Ind. Eng. Chem. Res. 48 (2009) 4917–4927. T. Gao, W.S. Lin, A.Z. Gu, M. Gu, Coalbed methane liquefaction adopting a nitrogen expansion process with propane pre-cooling, Appl. Energy 87 (2010) 2142–2147. Z.M. Yuan, M.M. Cui, Y. Xie, C.L. Li, Design and analysis of a small-scale natural gas liquefaction process adopting single nitrogen expansion with carbon dioxide precooling, Appl. Therm. Eng. 64 (2014) 139–146. H. Ding, H. Sun, M. He, Optimisation of expansion liquefaction processes using mixed refrigerant N2–CH4, Appl. Therm. Eng. 93 (2016) 1053–1060. J.D. Bukowski, Y.N. Liu, M.R. Pillarella, S. Boccella, W.A. Kennington, Natural gas liquefaction technology for floating LNG facilities, Proceedings of the 17th International Conference & Exhibition on Liquefied Natural Gas (LNG17), Houston, USA, (2013). C. Olsen, T.A. Kozman, J. Lee, K. Yuvamitra, A comparative study of natural gas liquids recovery methods, Distrib. Gener. Alternative Energy J. 27 (2012) 42–55. T. Gao, W.S. Lin, A.Z. Gu, Improved processes of light hydrocarbon separation from LNG with its cryogenic energy utilized, Energ. Convers. Manage. 52 (2011) 2401–2404. Y.J. Li, H. Luo, Integration of light hydrocarbons cryogenic separation process in refinery based on LNG cold energy utilization, Chem. Eng. Res. Des. 93 (2015) 632–639. M.Q. Wang, Q. Xu, Optimal design and operation for simultaneous shale gas NGL recovery and LNG re-gasification under uncertainties, Chem. Eng. Sci. 112 (2014) 130–142. J.H. Park, M.S. Khan, R. Andika, M. Getu, A. Bahadori, M. Lee, Techno-economic evaluation of a novel NGL recovery scheme with nine patented schemes for offshore applications, J. Nat. Gas Sci. Eng. 27 (2015) 2–17. B. Ghorbani, M.H. Hamedi, M. Amidpour, M. Mehrpooya, Cascade refrigeration systems in integrated cryogenic natural gas process (natural gas liquids (NGL), liquefied natural gas (LNG) and nitrogen rejection unit (NRU)), Energy 115 (2016) 88–106. B. Ghorbani, R. Shirmohammadi, M. Mehrpooya, A novel energy efficient LNG/NGL recovery process using absorption and mixed refrigerant refrigeration cycles – economic and exergy analyses, Appl. Therm. Eng. 132 (2018) 283–295. T.B. He, Y.L. Ju, Design and optimization of a novel mixed refrigerant cycle integrated with NGL recovery process for small-scale LNG plant, Ind. Eng. Chem. Res. 53 (2014) 5545–5553. A. Vatani, M. Mehrpooya, B. Tirandazi, A novel process configuration for coproduction of NGL and LNG with low energy requirement, Chem. Eng. Process. Process Intensif. 63 (2013) 16–24. M. Mehrpooya, M. Hossieni, A. Vatani, Novel LNG-based integrated process configuration alternatives for coproduction of LNG and NGL, Ind. Eng. Chem. Res. 53 (2014) 17705–17721.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Design and optimization of nitrogen expansion liquefaction processes integrated with ethane separation for high ethane-content natural gas Ting He, Wensheng Lin
T
⁎
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
HIGHLIGHTS
expansion LNG processes integrated with ethane separation are proposed. • Three refrigeration is supplied by N or N /CH expansion with or without precooling. • The of the distillation column heat load is achieved by heat integration. • Self-supply purity and recovery rate of liquid ethane products are both higher than 99.5%. • The • The specific power consumption decreases with ethane content increasing. 2
2
4
ARTICLE INFO
ABSTRACT
Keywords: Liquefied natural gas (LNG) Ethane Nitrogen expansion cycle Liquefaction process Genetic algorithm
Compared to ordinary natural gas, ethane has higher market value. Thus, recovery of ethane can increase the economic value of high ethane-content natural gas. This paper proposes a new liquefaction method for high ethane-content natural gas. Natural gas liquefaction and cryogenic distillation are combined to separate highpurity liquid ethane (no less than 99.95%) in the process of producing liquefied natural gas (LNG), which maximizes the economic benefits. And self-supply of the required heat of the distillation column is achieved by heat integration. For this new method, three nitrogen expansion processes are designed, and each process is simulated and optimized by HYSYS and genetic algorithm. Based on the optimization results, the specific power consumption and exergy efficiency of the three processes are analyzed. The results show that the specific power consumption of the three processes decrease with the increase of the ethane content. When the ethane content is 10% ~ 40%, the specific power consumption of the three processes is 0.5969 ~ 0.6060 kWh/Nm3 (NG), 0.5371 ~ 0.5592 kWh/Nm3 (NG), 0.5015 ~ 0.5403 kWh/Nm3 (NG), respectively, and the exergy efficiency of the proposed three processes is 33.3 ~ 35.4%, 37.1 ~ 38.3%, 39.7 ~ 39.9%, respectively.
1. Introduction Compared with coal and oil, natural gas is a cleaner primary energy source that produces almost no particulate matter and much less carbon dioxide. Using natural gas instead of coal and oil helps effectively reduce pollutants and greenhouse gas emissions. To ensure diversification of energy supply and improve energy consumption structure, increasing the proportion of natural gas in the energy consumption structure has become a strategic focus of energy development for many countries [1,2]. The development and utilization of shale gas has provided a new growth point for global natural gas production. The massive exploitation of shale gas began in the United States and has achieved great success, which has had a huge impact on the global gas supply market. Shale gas will play an important role in meeting future global energy ⁎
needs, and it will be an important part of many national energy policy portfolios [3,4]. In recent years, the role of liquefied natural gas (LNG) is becoming more and more important, and its trade share has significantly increased [5]. Changing shale gas into LNG products facilitates import and export trade, ocean shipping and peak shaving storage, and improves economic efficiency and market flexibility [6]. Although the liquefaction technology of conventional natural gas has matured, there are still problems about shale gas liquefaction. The ethane content in shale gas is generally higher than that of conventional natural gas. In US, ethane content of shale gas is as high as 12–35%. Considering the calorific value limit of pipeline natural gas, only a small amount of ethane can be left in natural gas for sale [7]. As we know, ethane has high industrial value. Compared to the
Corresponding author. E-mail address: [email protected] (W. Lin).
https://doi.org/10.1016/j.applthermaleng.2020.115272 Received 25 December 2019; Received in revised form 27 March 2020; Accepted 31 March 2020 Available online 02 April 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
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other methods of producing ethylene, ethane cracking has the advantages of low cost, high yield, low capital investment, low pollution and mature technology [8]. With the development of shale gas in North America, cheap ethane has contributed to the rapid development of ethylene production in the United States and Canada, and a large amount of ethane are exported from the United States. Meanwhile, US shale gas is going global, and countries are importing low-cost ethane from the United States for cracking to produce ethylene [9,10]. Yang et al. systematically compared the economic cost of ethylene produced by shale gas and naphtha. Their study shows that the production of ethylene from natural gas liquids (NGL) -rich shale gas is more economical than from naphtha [11]. Thus, the separation of high-purity ethane from shale gas is not only good for enhancing the market value of shale gas, but also of great significance for the benefits of the ethylene industry. Generally speaking, natural gas liquefaction processes are divided into three categories: cascade liquefaction process, mixed refrigerant liquefaction process, and liquefaction process with expander [12,13]. The expansion refrigeration liquefaction process is simple in process, small in equipment and space, compact in layout, flexible in adjustment, reliable in operation, quick in response to start-stop and convenient in maintenance of equipment. Small and medium-sized natural gas liquefaction plants may adopt a liquefaction process with an expander [14]. In the nitrogen expansion refrigeration process, the refrigerant is convenient to be replenished, stored and transported. Nitrogen is always in the gas phase, and it is a non-flammable working medium, so the process is safe and reliable. Therefore, for most of the scattered gas fields, unconventional gas fields, liquefaction devices and peaking devices under sea sloshing conditions, the nitrogen expansion process is a suitable choice. A large number of scholars have studied the design and optimization of the expansion process under different conditions. Xiong et al. [15] studied single expander pressurized liquefied natural gas (PLNG) processes for offshore natural gas which contains less than 0.5% CO2. Lin et al. [16] studied the nitrogen expansion process to produce liquefied natural gas and separate hydrogen from synthetic natural gas (SNG). Moein et al. [17] used genetic algorithm to optimize the nitrogen expansion cycle liquefaction process, effectively achieving energy saving. Pre-cooling of the natural gas feed stream can significantly improve the efficiency of the expansion refrigeration process [18]. Gao et al. [19] proposed and optimized a propane precooled nitrogen expansion liquefaction process for coalbed methane. The results showed that the liquefaction energy consumption is acceptable when the nitrogen content in feed gas did not exceed 70%. Yuan et al. [20] proposed a new small-scale liquefaction method of single nitrogen expansion process using carbon dioxide precooling. Ding et al. [21] used HYSYS and MATLAB to optimize the N2-CH4 expansion liquefaction process with pre-cooling, which reduced the unit energy consumption by 5.35%. Bukowski et al. [22] analyzed a variety of liquefaction processes for floating LNG production and storage device (FLNG), of which the AP-N process is the safest. In actual production, the Malaysian PFLNG project finally adopted the AP-N process with safety indicators account for. There are mainly two kinds of researches on the recovery of light hydrocarbons in natural gas. One is to recover light hydrocarbons before natural gas is liquefied [23], and the other is to recover light hydrocarbons from liquefied natural gas [24]. The latter technology is generally combined with the cold energy utilization of LNG [25]. Wang et al. [26] integrated the LNG regasification and shale gas NGL recovery process to achieve maximum energy savings and considered the uncertainty of shale gas feed rate changes. Park et al. [27] selected nine patented NGL recovery schemes and modified them by strengthening the suitability of heat exchange systems for offshore applications. A comprehensive analysis of each process was carried out while focusing on propane recovery. Ghorbani et al. [28] proposed a cascade refrigeration system in integrated cryogenic natural gas process (natural gas liquids (NGL), liquefied natural gas (LNG) and nitrogen rejection
unit (NRU)), which can recover 92% of ethane. Ghorbani et al. [29] also proposed an integrated LNG-NGL system which uses an absorption refrigeration cycle for pre-cooling and uses a cascaded mixing refrigeration cycle (MFC) for cooling and liquefaction, and the ethane recovery is 91.96%. Both processes proposed by Ghorbani were designed for natural gas with a C2+ content of 12.04%. He et al. designed a novel mixed refrigerant cycle integrated with NGL recovery process for smallscale LNG plant [30]. In this study, the ethane content of feed gas is only 6%. Vatani et al. [31] used two mixed refrigerant cycles to coproduct both NGL and LNG. The results showed that specific power consumption is 0.414 kWh/kg LNG) and the ethane recovery rate is higher than 93.3%. Mehrpooya et al. [32] selected C3-MR, DMR and MFC refrigeration systems to provide the required refrigeration for three integrated processes for LNG and NGL cogeneration. For a feed gas with an ethane content of 7%, the ethane recovery is higher than 90%. The prior studies that involved the separation of light hydrocarbons from the liquefaction process mainly concerned the recovery of C3+ heavy components, instead of separating and purifying ethane directly during the liquefaction process. The ethane content of the feed gas in these studies is less than 15%, but in fact, the ethane content of natural gas such as shale gas may be as high as 35%. Furthermore, the purity of the recovered ethane is not high enough to be directly used as an industrial raw material, and the separated ethane may only be used for fuel. Besides, the recovery rate of ethane in these processes is low. This study proposes a high ethane-containing natural gas liquefaction method combined with cryogenic distillation to obtain high-purity lowpressure liquid ethane and LNG products. The purity standard of industrial ethane can be directly achieved without secondary processing because the purity of the separated ethane is not less than 99.95%. What’s more, in this study, the recovery of ethane is above 99% when ethane content of the studied feed gas is in the range of 10% to 40%, which ensures maximum economic benefits of feed gas. 2. Process design 2.1 Assumptions To better investigate the influences of ethane content to the processes by avoiding the influence of other components, such as nitrogen and C3+, it is assumed that the feed gas is composed of only CH4 and C2H6 (ethane content in feed gas is 10% to 40%), and the pressure, temperature and molar flow of feed gas are 0.12 MPa, 40 °C, 1 kmol/h, respectively. The study is carried out by Aspen HYSYS, and the equation of state used in HYSYS simulation is the Peng-Robinson (P-R) equation. The following assumptions are given for simulation: (1) The gas at the outlet of the compressor is cooled by a water cooler to 40 °C; (2) The minimum temperature differences in heat exchangers are set as 3 °C; (3) The adiabatic efficiencies of compressors and expanders are set as 85% and 80%, respectively; (4) The storage pressure of LNG and liquid ethane is 0.12 MPa, and the liquefaction rate is 100%; (5) The purity of the liquid ethane product is not less than 99.5%, and recovery rate of ethane over 99%. (6) The pressure drop in each heat exchanger is ignored to simplify the simulation calculation process. 2.2 Process description Three different nitrogen expansion liquefaction separation processes are established, as shown in Fig. 1 (Process 1: two-stage nitrogen expansion process, Process 2: propane pre-cooling nitrogen expansion 2
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(a) Process 1
(b) Process 2 Fig.1. Nitrogen expansion liquefaction processes integrated with ethane separation for high ethane-content natural gas C: compressor, E: expander, WC: water cooler, Q: heat flow, DT: distillation column, T: tank, V: valve, HEX: multi-flow heat exchanger, W: work; H: heat exchanger.
process; Process 3: propane pre-cooling nitrogen methane expansion process). In the three processes, the most critical equipment for efficient separation is the cryogenic distillation column. A full distillation column is selected, and the purity of liquid ethane is ensured through a reboiler at the bottom of the column, and the recovery rate of ethane is improved by condensing reflux at the top of the column. For Process 1, the liquefaction separation process consists of three
parts: (1) Nitrogen expansion refrigeration cycle In order to make full use of the expansion work of nitrogen, the expansion process is divided into two stages, and each turbo expander drives a coaxial compressor to recover expansion work. After multi-stage compression and water cooling, the low pressure 3
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(c) Process 3 Fig.1. (continued)
consumption of the expansion refrigeration process. Process 3 is the nitrogen-methane expansion liquefaction separation process, and it is basically the same as the propane pre-cooling nitrogen expansion process. The only difference is that the material 111 behind the throttle valve V-101 is not completely liquefied, so there will be boil-off gas (BOG) in the LNG storage tank T-101. The BOG (113) enters the heat exchangers of each stage to recover the cold and then it is mixed into the feed gas for liquefaction. In order to ensure that the outlet of the expander does not yield liquid, the outlet temperature of the expander should be higher than the dew point of the corresponding pressure by 2 °C. The proposed processes mainly have the following advantages:
nitrogen (301) is compressed to high pressure nitrogen (309) with a pressure of 5 ~ 6 MPa and a temperature of 40 °C. Then the high pressure nitrogen enters heat exchangers HEX-101, HEX-102 in sequence, and is cooled to −65 ~ −45 °C (311). Then it expands to 0.26 ~ 0.3 MPa by a two-stage expander, and its temperature drops to −163.5 °C. The low temperature nitrogen (313) enters heat exchangers HEX-103, H-102 (representing the condenser of D-101), HEX-102, HEX-101 in sequence to provide cold energy and its temperature rises to 37 °C (301). It goes back to C-301, completing a cycle. (2) Ethane separation distillation module After a two-stage compression and water cooling, the feed gas (107) is cooled to −90 ~ −75 °C by heat exchangers HEX-101, H-101 (representing the reboiler of D-101) and HEX-102. The partially liquefied feed gas (108) then enters the distillation column to separate ethane. The natural gas (109) at the top of the column is further cooled by the condenser (H-102), and its ethane content is less than 1%. After re-boiling in the reboiler (H-101), the purity of liquid ethane (201) at the bottom of the column reached 99.5%. The liquid ethane is further cooled to −90 °C (203), and then throttled to 120 kPa (204) to be stored in the storage tank T-201. (3) Natural gas liquefaction module The methane-rich feed gas from the top of the distillation column (109) enters a heat exchanger HEX-103 is cooled to −160.5 °C (110), and is throttled to 120 kPa (111), finally enters the LNG tank (T-101) for storage.
(1) Integrating the cryogenic distillation with the natural gas liquefaction process, the produced LNG can meet the heating value of pipeline or combustion equipment, and the separated liquid ethane can be used in industrial applications to maximize economic benefits. (2) The heat required for the distillation column are provided by the process itself. (3) The two products obtained by the process are low pressure liquids, convenient for storage and transportation. 2.3 Process performance indicators The main indicators for evaluation the performance of natural gas liquefaction and separation processes are specific power consumption and ethane recovery rate. The specific power consumption (kWh/Nm3) of the process is defined as:
In Process 1, the required cold energy is completely provided by the expansion of nitrogen. Since the separated ethane does not require deep cooling, part of the cold in the high temperature section can be provided by the propane pre-cooling cycle. By this way the nitrogen flow rate will be reduced and the specific power consumption of the process will be reduced. Therefore, Process 2 is proposed, which is constructed by adding a propane precooling cycle to Process 1. In the pre-cooling cycle, the refrigerant propane is throttled after being compressed and water-cooled, and then supplied to heat exchanger HEX-101. The addition of methane to nitrogen can reduce the power
w=
WC WE MNG Vm
where: MNG—molar flow of natural gas, kmol/h; WC—total power consumption of the compressors, kW; WE—total output power of the expanders, kW; 4
(1)
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W = WC
Create an initial population
where: h is specific enthalpy, kJ/kg; s is specific entropy, kJ/(kg·K); T0 is ambient temperature, T0 = 313.15 K; Tm is the temperature of the mixture; Subscript 1 and 2 represent before and after separation, respectively.
Pass the population to HYSYS via the interface
HYSYS performs simulation calculations and passes the results to MATLAB
New population
No
Restrictions
Mutation
Neglecting potential energy and kinetic energy, the exergy equilibrium equation of a steady-flow system and working fluid is defined as (10) and (11) respectively. 2
Punishment
1
End
No
Yes
Termination condition
MLC 2 MC 2
(2)
where: MC2—molar flow of ethane in feed gas, kmol/h; MLC2—molar flow of liquid ethane, kmol/h.
Wmin W
In this study the main purpose of optimization is reducing the power consumption of the process. So, the objective function is design as:
min f (x ) =
Theoretical minimum work includes theoretical minimum liquefaction work Wl and theoretical minimum separation work Ws: Theoretical minimum liquefaction work is expressed as follows:
Wl h 0, LC 2 )
T0 (sLC 2
s0, LNG )] + mLC 2
s0, LC 2 )]
(6)
Unit power consumption of the ideal separation process is:
ws, i = Tm (s1
s2)
(h1
h2)
(7)
Theoretical separation work of the binary mixtures of A and B is:
ws, i = ws, A mA / mm + ws, B mB / mm
WC WE MNG Vm
(3)
The processes proposed in this paper mainly consist of two parts: the refrigeration cycle to supply cold energy for liquefaction of natural gas, and the distillation column to separate methane and ethane. Consequently, the parameters to be optimized should include the main parameters of both the refrigeration and the distillation parts. The refrigeration cycle of the system consumes most of the power input, so the parameters such as flow, pressure and temperature of the refrigeration cycle should be optimized. The propane vapor compression refrigeration cycle is relatively simple. The condensing temperature before throttling is determined by the water cooler, which is 40 °C. The evaporating temperature is set as −38.25 °C, a little bit higher than the normal boiling point of propane. Therefore, this optimization of the refrigeration part focuses on the nitrogen expansion cycle. As for the distillation part, the temperature and pressure of distillation are also important factors influencing process power consumption. Among the parameters to be optimized, the distillation
(5)
Wmin = Wl + Ws
[(hLC 2
(11)
3.2 Optimization function and optimization variable selection
(4)
T0 (sLNG
T0 S0)
(1) The population size is 100; (2) Maximum generation number is 200; (3) Crossover rate and mutation rate is 0.6, and 0.05, respectively.
The exergy efficiency of the system is a key indicator to measure the energy utilization of the system. The exergy efficiency of a liquefaction system refers to the ratio of the ideal minimum liquefaction work Wmin of the system to the actual work W consumed by the system, expressed as follows (FOM refers Figure of Merit):
h 0, LNG )
(H0
(10)
Genetic algorithm is an intelligent optimization algorithm, which is suitable for large-scale and nonlinear optimization models. It has been widely used in chemical process simulation and optimization in recent years. The proposed process is connected with a GA (genetic algorithm) code in MATLB by using HYSYS COM server. In the optimization process, HYSYS plays the role of calculating the target function, and MATLB mainly provide the calculation process of the genetic algorithm. The genes of each generation generated by the genetic algorithm are converted into the parameters that HYSYS can recognize by decoding the code, and the result is calculated in HYSYS, and returned as the target function value to MATLB. The specific program framework is shown in Fig. 2 [29]. The parameters of the genetic algorithm are set as follows:
Ethane recovery rate is defined as:
= mLNG [(hLNG
T0 S )
EX1) + Wu + EX,loss
3.1 Optimization method
Output optimal population
Vm—molar volume of natural gas, taking 22.4Nm3/kmol.
FOM =
Q = (EX2
3. Process optimization
Fig. 2. HYSYS process optimization block diagram based on MATLAB genetic algorithm.
RC 2 =
T0 Tr
where, Tr is the temperature of heat source; Q is heat absorbed from a heat source; H is enthalpy; S is entropy; EX is exergy; Wu is useful work.
Objective function
Selection
1
EX = (H
Yes Crossover
(9)
WE
(8)
Actual work is expressed as follows: 5
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Optimization parameter
Lower bound
Upper bound
T311 (°C) P312 (kPa) P313 (kPa) P306 (kPa) P308 (kPa) MN2 (kmol/h) MC3 (kmol/h)
−65 1100 250 1600 4000 4 0
−45 1600 350 2100 7000 5.5 0.35
Specific power consumption (kW.h/Nm3)
Table 1 Optimization variables.
temperature and pressure affect the separation purity of the distillation column, and cannot be simply set by genetic algorithms like the parameters of the nitrogen cycle. Therefore, the two parameters are separately set in advance and then the other parameters are optimized by the genetic algorithm. The distillation temperature is set with reference to the bubble point and dew point temperature of the methane ethane mixture (ethane content: 10 ~ 40%). The upper and lower bounds of genetic algorithm optimization variables in Process 2 are shown in Table 1. Process 1 dos not include the propane flow, and Process 3 adds the methane flow in the refrigerant. The upper and lower limits of the parameters of the three processes are slightly different. In addition, the tray number in a distillation column has a considerate impact on the power consumption of the process. Since the number of distillation column trays cannot be optimized during the comprehensive optimization with genetic algorithm, a single factor optimization is adopted to optimize it before the comprehensive optimization. For the situation that the parameters of a stream entering a distillation column remain unchanged, a shortcut column in HYSYS is used to calculate a minimum theoretical tray number. And then, starting from the minimum theoretical tray number, the number of trays is gradually increased. By adjusting the reflux ratio of the distillation column and the flow rate of the refrigerant, the limiting conditions and separation requirement are satisfied. Observe the change of the specific power consumption of the system until the optimal tray number is found. Taking Process 2 as an example, when the ethane content is 10%, T108 = −85 °C, P108 = 2.5 MPa, the minimum theoretical tray number obtained from HYSYS through shortcut column is 4.8, which is 5 after rounding. After gradually increasing the tray number, the optimal tray number obtained is 10. In fact, since the minimum theoretical tray number itself obtained from HYSYS is based on scientific calculation, increasing the minimum theoretical tray number to the optimal one only has very limited effect of reducing power consumption. Considering this, the optimal tray number is good enough to be used in the following comprehensive process optimization.
0.60 0.59 0.58 0.57 0.56 0
25
50
75
100
125
150
175
200
Generation number Fig. 3. GA optimization convergence curve.
The optimization results of the three processes at the four ethane contents are shown in Table 2. At the four ethane contents, the specific power consumption of Process 3 is the lowest, and that of Process 1 is the highest. Compared with Process 2 and 3, the pressure of stream 308 and the required flow rate of the nitrogen refrigerant of Process 1 are higher, which can explain why the power consumption of process 1 is the highest. Compared with Process 1 and 2, the specific power consumption of Process 3 decreases more with increasing ethane content. That means the advantage of Process 3 is more obvious in the case of high ethane content. In all cases, P313 is in a narrow range of 280–300 kPa and the minimum approach of heat exchange is very close to the set value of 3 °C. 4.2 Effect of ethane content on power consumption Fig. 4 shows the specific power consumption of the three processes at different ethane contents. As the ethane content increases, the specific power consumption of the three processes decreases gradually. However, the three processes differ in decreasing extent with the increase of ethane content. The specific power consumption of Process 1 decreases slightly, and that of Process 3 decreases obviously. As the ethane content increases, the amount of refrigeration required for liquefaction decreases but the work consumed by the distillation increases. However, the refrigeration required for liquefaction is reduced more, so the specific power consumption of the system is reduced. In terms of power consumption, Process 3 is best followed by Process 2, Process 1 is the worst. The specific power consumption of Process 2 and Process 3 is 8 ~ 10% and 10 ~ 16% lower than that of Process 1. 4.3 Analysis of distillation separation effect
3.3 Genetic algorithm optimization process The GA optimization convergence curve of Process 2 (10% ethane content) is shown in Fig. 3. It is obvious that the objective function value decreases quickly in the initial stage, because in the initial stage the genetic algorithm is easy to find a better next generation. When the optimization continues, the objective function will have a platform period. Since the objective function is already superior, it is difficult to find a better next-generation population on the platform, and it takes more time to find a better next generation. Finally, when the genetic algorithm is optimized to the 75th generation, the minimum value of the objective function is found, and the optimal solution is 0.5592 kWh/Nm3. The optimization iterative process of other working conditions also shows the same tendency.
For Process 2, the temperature distribution and component distribution in the distillation column with ethane content of 10% at a pressure of 2.5 MPa is shown in Fig. 5. The temperature from the top to the bottom of the tower gradually increased from −101.6 °C to 0.8 °C, and the separation effect of the distillation column is very good. The purity of methane in the top outlet gas is 99.95%, and the purity of ethane in the bottom outlet liquid is 99.51%. Under other distillation conditions, the purity of liquid ethane obtained by the three processes is higher than 99.5% with ethane recovery rate over 99.5%.
4. Optimization results and analysis
Fig. 6 shows the specific power consumption of Process 2 (10% ethane) at different distillation pressure and temperature. The distillation temperature has a greater impact on power consumption but there
4.4 Distillation pressure and temperature analysis
4.1 Optimization results 6
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Table 2 Optimization results. Parameter
Process 1
Specific power consumption (kWh/Nm3)
T311 (°C) P312 (kPa) P313 (kPa) P306 (kPa) P308 (kPa) MN2(kmol/h) MC3 (kmol/h) MC1 (kmol/h) ΔTHEX-101 (°C) ΔTHEX-102 (°C) ΔTHEX-103 (°C) W (kWh/Nm3)
Process 2
Process 3
10%
20%
30%
40%
10%
20%
30%
40%
10%
20%
30%
40%
−49.4 1562 298 1980 5332 5.222
−46.5 1425 298 1929 5631 5.162
−46.0 1489 298 2023 5678 5.238
−46.2 1412 297 1980 5652 5.316
−60.0 1245 300 1722 4394 4.721 0.185
−60.0 1208 298 1742 4362 4.732 0.224
−56.5 1289 295 1790 4622 4.62 0.258
−51.0 1280 295 1823 5128 4.384 0.288
3.0 3.2 3.0 0.606
3.0 3.0 3.3 0.603
3.0 3.1 3.0 0.599
3.0 3.3 3.1 0.597
3.0 3.0 3.1 0.559
3.0 3.0 3.1 0.551
3.0 3.0 3.0 0.540
3.0 3.0 3.0 0.537
−50.0 1123 288 1498 3892 1.658 0.090 3.300 3.0 3.1 3.0 0.545
−50.0 1212 288 1587 4045 1.748 0.21 2.718 3.0 3.0 3.1 0.528
−49.8 1223 284 1620 4124 1.843 0.262 2.481 3.0 3.0 3.2 0.511
−49.0 1220 278 1580 3964 1.498 0.298 2.803 3.0 3.0 3.1 0.502
In fact, the cooling energy is consumed in two aspects: 1) the cooling process of the gas; 2) the overhead condenser of the distillation tower. Increasing the liquefaction pressure, the amount of cooling required for gas liquefaction decreases, however the separation of methane and ethane becomes more difficult, which can be seen from the cooling load of which increases the cooling load of the condenser. Therefore, at low pressures, the specific power consumption required for liquefaction predominates. When the pressure is high, the specific power consumption of the separation column is dominant.
0.60 0.58
process 1 process 2 process 3
0.56 0.54 0.52
5. Exergy analysis
0.50 0.10
0.15
0.20
0.25
0.30
0.35
5.1 Heat transfer curve
0.40
Ethane content
LNG heat exchangers are very important in natural gas liquefaction process. Heat transfer performance in LNG heat exchangers can greatly influence the process power consumption. Thus, the analysis of heat transfer is necessary. The smaller gap between the hot and cold temperature curves will result in higher heat transfer efficiency in LNG heat exchangers, reducing the exergy destruction of the heat transfer process. Since the refrigeration in low temperature range consumes more power than that in high temperature range, it can save more energy when the gap between cold composite curve and hot composite curve in low temperature range is reduced. Moreover, reducing the gap in low temperature range is more effective in reducing the exergy loss. The heat transfer curves of the optimized processes are shown in Fig. 7. Since the feed gas is not pressurized to above the critical pressure, the temperature of the hot fluid remains unchanged during the liquefaction of the natural gas, causing the large temperature difference between the hot and cold fluids in this region. For Process 1, the heat exchange
Fig. 4. Effect of ethane content on power consumption.
0
1.0
-20
0.8 Molar fraction
Temperature (°C)
is no uniform influence regularity. In general, at a given pressure, too high or too low temperature of the distillation column feed flow will lead to an increase in distillation energy consumption. A temperature that makes the feed gas-liquid component reasonable will result in the lowest energy consumption of the system. As shown in Fig. 6(a), when the pressure is 2.5 MPa, the system consumes the lowest power at a feed temperature of −85 °C. However, at each distillation temperature, the specific power consumption exhibits the same regularity as the distillation pressure changes. As the liquefaction pressure increases, the specific power consumption approximately showing a trend of decreasing first and then increasing, and there is an optimum liquefaction pressure.
-40 -60 -80 -100
Methane Ethane
0.6 0.4 0.2 0.0
0
2
4
6
8
10
12
0
Tray
2
4
6
8
10
12
Tray
(a)Temperature distribution
(b) Component distribution
Fig.5. Parameters change of each layer of tray in the distillation column Tray 0 refers reboiler; tray 11 refers condenser. 7
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Condenser cooling consumption (kJ/h)
Specific power consumption (kWh/Nm3)
3500 0.64
T308 = -75°C T308 = -80°C T308 = -85°C T308 = -90°C T308 = -95°C
0.62 0.60 0.58
Wmin=0.5592
0.56 1.0
1.5
2.0
2.5
3.0
3.5
4.0
3000 2500 2000 1500 1000
1.0
1.5
2.0
2.5
3.0
3.5
Liquefaction pressure (MPa)
Liquefaction pressure (MPa)
(a) Specific power consumption
(b) Condenser cooling load
4.0
Fig.6. Optimization results of specific power consumption.
temperature difference of 10% ethane content in the heat exchangers HEX-101 and HEX-102 is smaller than that of 30% ethane content, so the heat transfer effect of 10% ethane content is better than that of 30% ethane content. The heat transfer curves of three processes with an ethane content of 30% show that the heat transfer temperature difference in Process 2 and Process 3 is significantly smaller than that of Process 1. In addition, processes 2 and Process 3 are pre-cooled by propane. In the propane evaporation section, the temperature of the cold fluid remains unchanged, and the temperature difference is relatively large.
5.2 Exergy efficiency Fig. 8 shows the exergy efficiency of the three processes. The exergy efficiency of Process 1 has a downward trend with the increase of ethane content. But, the exergy efficiency of Process 2 and Process 3 does not change significantly with the ethane content. Moreover, the exergy efficiency of Process 3 is significantly higher than that of Process 1. The exergy efficiency of Process 3 can be maintained at a higher level at different ethane levels due to the use of two mixed refrigerants, which fully reflects the advantages of Process 3.
40
40
20
20
0
HEX-102
-20
-20 -40
Temperature (°C)
Temperature (°C)
0
HEX-103
-60 -80 -100
cold composite hot composite
HEX-102
-120
HEX-101
-40
HEX-103
-60
cold composite hot composite
-80 -100
HEX-101
-120 -140
-140
-160
-160 0
5000
10000
15000
20000
25000
0
30000
5000
10000
20
20
0
0
-20
-20
HEX-103
cold composite hot composite
-80 -100
HEX-102 HEX-101
-120
25000
30000
(b)Process 1: 30% ethane 40
Temperature (°C)
Temperature (°C)
(a) Process 1: 10% ethane 40
-60
20000
Heat flow (kJ/h)
Heat flow (kJ/h)
-40
15000
-140
-40
HEX-103
-60
cold composite hot composite
-80 -100
HEX-102 HEX-101
-120 -140
-160
-160 0
5000
10000
15000
20000
25000
30000
0
5000
Heat flow (kJ/h)
10000
15000
20000
Heat flow (kJ/h)
(c) Process 2: 30% ethane
(d)Process 3: 30% ethane
Fig. 7. Heat transfer curve of the optimized process. 8
25000
30000
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0.42
1/3 of the total, comes from the water coolers. When the hot compressed fluid is cooled to ambient temperature in the coolers, the exergy is dissipated into the environment. The lower the compressor efficiency, the lager the exergy destruction in the water coolers is. Unless waste heat utilization system is introduced, little can be done to reduce this part of exergy destruction. Expanders. Around 20% of exergy destruction comes from the expanders, since the adiabatic efficiency of expanders is 80%. A process engineer can do little about this. Distillation column and throttle valves. Exergy construction in these two kinds of devices is relatively small. It may be found that, as the ethane content increases, the proportion of exergy construction in the distillation column increases gradually. Heat exchangers. The temperature difference between the hot and cold fluids in the heat exchangers result in exergy destruction. In fact, the most important result that can be anticipated from a process optimization is to reduce the heat transfer difference so as to minimize exergy destruction of the process. In this point of view, the presented process optimization is successful, because the exergy destruction in heat exchangers is only a little bit more than 10% of the total.
Exergy efficiency
0.40 0.38 0.36
process 1 process 2 process 3
0.34 0.32
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Ethane content Fig. 8. Exergy efficiency of the system.
5.3. Equipment exergy destruction Based on Eqs. (10) and (11) the exergy destruction of each device is calculated. Fig. 9 shows the ratio of the exergy destruction to the total exergy destruction for each device in Process 2. Some discussions about the equipment exergy destruction are as follows. Compressors. A large proportion of exergy destruction, about 30% of the total exergy destruction, comes from the compressors. This may be explained in two aspects. Firstly, the adiabatic efficiency of compressors is 85%, and there must be large exergy destruction caused by the irreversibility in the compressors. Secondly, the refrigeration in a nitrogen expansion process is provided by the sensible heat of nitrogen, resulting in a large flow of nitrogen and a large power consumption for compression. It seems that a compressor engineer has much to do to enhance the compressor efficiency, while at the same time, a process engineer has little to do unless another process is adopted or more compressors are used to substitute each compressor in the process. Water coolers. The largest proportion of exergy destruction, about
Compressor 29.91%
12.03% Heat exchanger 1.41% Throttle valve
6. Conclusions The exploitation, storage, transportation and trade of shale gas have been an important research topic in recent years, and have important strategic significance in energy structure optimization. In order to maximize economic benefits, this paper propose three nitrogen expansion refrigeration processes to numerically simulate the liquefaction separation process of high-ethane-containing shale gas, and optimizes the process through genetic algorithm. The optimization results of the proposed three processes under different ethane contents are analyzed, and the main conclusions are as follows: (1) The purity of the liquid ethane product obtained by the cryogenic distillation separation is not less than 99.5%, and the ethane recovery rate is higher than 99.5%. (2) The specific power consumption of the three processes decreases Compressor 29.89%
Water cooler 32.67%
11.26% Heat exchanger 2.03% Throttle valve
2.72% 21.26% Expander
Distillation column
(a) 10% ethane content Compressor 29.01%
Compressor
Throttle valve
Water cooler
29.23%
32.25%
11.53%
3.5% 20.34% Expander
20.96% Expander
2.82% Distillation column
(b) 20% ethane content
Water cooler 33.83%
11.11% Heat exchanger 2.21%
Water cooler 33.03%
Heat exchanger 2.44%
Distillation column
Throttle valve
(c) 30% ethane content
4.09% 20.47% Expander
(d) 40% ethane content
Fig. 9. Exergy destruction of Process 2 at different ethane contents. 9
Distillation column
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T. He and W. Lin
with the increase of the ethane content. When the ethane content is 10% ~ 40%, the specific power consumption of the three processes is 0.5969 ~ 0.6060 kWh/Nm3, 0.5371 ~ 0.5592 kWh/Nm3, 0.5015 ~ 0.5403 kWh/Nm3, respectively. Compared to Process 1, the specific power consumption for Process 3 is more affected by the ethane content. (3) The exergy efficiency analysis shows that the exergy efficiency of Process 1 is the lowest, and that of Process 3 is the highest. When the ethane content is 10% ~ 40%, the exergy efficiency of the proposed three processes is 33.3 ~ 35.4%, 37.1 ~ 38.3%, 39.7 ~ 39.9%, respectively. (4) Of the three proposed processes, Process 3 has the lowest specific power consumption and best adaptability to the ethane content in the feed gas. Among the parameters to be optimized, the distillation pressure is the critical factor affecting the specific power consumption of the process.
[12] [13] [14] [15] [16] [17] [18] [19]
Declaration of Competing Interest
[20]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[21] [22]
Appendix A. Supplementary data [23]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2020.115272.
[24]
References
[25]
[1] Q. Wang, China should aim for a total cap on emissions, Nature 512 (2014) 115. [2] S.L. Burns, M. Krott, H. Sayadyan, L. Giessen, The world bank improving environmental and natural resource policies: power, deregulation, and privatization in (post-soviet) Armenia, World Develop. 92 (2017) 215–224. [3] Q. Wang, X. Chen, A.N. Jha, H. Rogers, Natural gas from shale formation – the evolution, evidences and challenges of shale gas revolution in United States, Renew. Sustain. Energy. Rev. 30 (2014) 1–28. [4] W.J. Lee, S.Y. Sohn, Patent analysis to identify shale gas development in China and the United States, Energy Policy 74 (2014) 111–115. [5] S. Kumar, H.T. Kwon, K.H. Choi, W. Lim, J.H. Cho, K. Tak, LNG: An eco-friendly cryogenic fuel for sustainable development, Appl. Energy 88 (2011) 4264–4273. [6] A. Dutta, I.A. Karimi, S. Farooq, Heating value reduction of LNG (liquefied natural gas) by recovering heavy hydrocarbons: technoeconomic analyses using simulationbased optimization, Ind. Eng. Chem. Res. 57 (2018) 5924–5932. [7] J.F. Goellner, Expanding the shale gas infrastructure, Chem. Eng. Prog. 108 (2012) 49-52, 59. [8] M.B. Yang, X.Y. Tian, F.Q. You, Manufacturing ethylene from wet shale gas and biomass: comparative techno-economic analysis and environmental life cycle assessment, Ind. Eng. Chem. Res. 57 (2018) 5980–5998. [9] M. Mccoy, U.S. shale gas heads overseas, Chem. Eng. News 92 (2015) 3. [10] D. Ding, Y.Y. Zhang, W. Wu, D.C. Chen, M.L. Liu, T. He, A novel low-thermal-budget approach for co-production of ethylene and hydrogen via electrochemical nonoxidative deprotonation of ethane, Energy Environ. Sci. 11 (2018) 1710–1716. [11] M.B. Yang, F.Q. You, Comparative techno-economic and environmental analysis of
[26] [27] [28]
[29] [30] [31] [32]
10
ethylene and propylene manufacturing from wet shale gas and naphtha, Ind. Eng. Chem. Res. 56 (2017) 4038–4051. W.S. Lin, N. Zhang, A.Z. Gu, LNG (liquefied natural gas): a necessary part in China's future energy infrastructure, Energy 35 (2010) 4383–4391. W.S. Lin, X.J. Xiong, M. Spitoni, A.Z. Gu, Design and optimization of pressurized liquefaction processes for offshore natural gas using two-stage cascade refrigeration cycles, Ind. Eng. Chem. Res. 57 (2018) 5858–5867. T.B. He, Y.L. Ju, Performance improvement of nitrogen expansion liquefaction process for small-scale LNG plant, Cryogenics 61 (2014) 111–119. X.J. Xiong, W.S. Lin, A.Z. Gu, Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology, J. Nat. Gas. Sci. Eng. 30 (2016) 379–387. W.S. Lin, J.X. Xu, L. Zhang, A.Z. Gu, Synthetic natural gas (SNG) liquefaction processes with hydrogen separation, Int. J. Hydrogen Energy 42 (2017) 18417–18424. P. Moein, M. Sarmad, M. Khakpour, H. Delaram, Methane addition effect on a dual nitrogen expander refrigeration cycle for LNG production, J. Nat. Gas. Sci. Eng. 33 (2016) 1–7. N.M. Shah, A.F.A. Hoadley, G.P. Rangaiah, Inherent safety analysis of a propane precooled gas-phase liquefied natural gas process, Ind. Eng. Chem. Res. 48 (2009) 4917–4927. T. Gao, W.S. Lin, A.Z. Gu, M. Gu, Coalbed methane liquefaction adopting a nitrogen expansion process with propane pre-cooling, Appl. Energy 87 (2010) 2142–2147. Z.M. Yuan, M.M. Cui, Y. Xie, C.L. Li, Design and analysis of a small-scale natural gas liquefaction process adopting single nitrogen expansion with carbon dioxide precooling, Appl. Therm. Eng. 64 (2014) 139–146. H. Ding, H. Sun, M. He, Optimisation of expansion liquefaction processes using mixed refrigerant N2–CH4, Appl. Therm. Eng. 93 (2016) 1053–1060. J.D. Bukowski, Y.N. Liu, M.R. Pillarella, S. Boccella, W.A. Kennington, Natural gas liquefaction technology for floating LNG facilities, Proceedings of the 17th International Conference & Exhibition on Liquefied Natural Gas (LNG17), Houston, USA, (2013). C. Olsen, T.A. Kozman, J. Lee, K. Yuvamitra, A comparative study of natural gas liquids recovery methods, Distrib. Gener. Alternative Energy J. 27 (2012) 42–55. T. Gao, W.S. Lin, A.Z. Gu, Improved processes of light hydrocarbon separation from LNG with its cryogenic energy utilized, Energ. Convers. Manage. 52 (2011) 2401–2404. Y.J. Li, H. Luo, Integration of light hydrocarbons cryogenic separation process in refinery based on LNG cold energy utilization, Chem. Eng. Res. Des. 93 (2015) 632–639. M.Q. Wang, Q. Xu, Optimal design and operation for simultaneous shale gas NGL recovery and LNG re-gasification under uncertainties, Chem. Eng. Sci. 112 (2014) 130–142. J.H. Park, M.S. Khan, R. Andika, M. Getu, A. Bahadori, M. Lee, Techno-economic evaluation of a novel NGL recovery scheme with nine patented schemes for offshore applications, J. Nat. Gas Sci. Eng. 27 (2015) 2–17. B. Ghorbani, M.H. Hamedi, M. Amidpour, M. Mehrpooya, Cascade refrigeration systems in integrated cryogenic natural gas process (natural gas liquids (NGL), liquefied natural gas (LNG) and nitrogen rejection unit (NRU)), Energy 115 (2016) 88–106. B. Ghorbani, R. Shirmohammadi, M. Mehrpooya, A novel energy efficient LNG/NGL recovery process using absorption and mixed refrigerant refrigeration cycles – economic and exergy analyses, Appl. Therm. Eng. 132 (2018) 283–295. T.B. He, Y.L. Ju, Design and optimization of a novel mixed refrigerant cycle integrated with NGL recovery process for small-scale LNG plant, Ind. Eng. Chem. Res. 53 (2014) 5545–5553. A. Vatani, M. Mehrpooya, B. Tirandazi, A novel process configuration for coproduction of NGL and LNG with low energy requirement, Chem. Eng. Process. Process Intensif. 63 (2013) 16–24. M. Mehrpooya, M. Hossieni, A. Vatani, Novel LNG-based integrated process configuration alternatives for coproduction of LNG and NGL, Ind. Eng. Chem. Res. 53 (2014) 17705–17721.