Design and Optimization of Pressurized Liquefaction Processes for

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Design and Optimization of Pressurized Liquefaction Processes for Offshore Natural Gas Using Two-Stage Cascade Refrigeration Cycles Wensheng Lin,*,† Xiaojun Xiong,† Marco Spitoni,‡ and Anzhong Gu† †

Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China DIISM Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, via Brecce Bianche, 1 60131 Ancona, Italy

‡

ABSTRACT: Because of increasing interests in offshore natural gas and due to limited deck space, simpler liquefied natural gas processes are necessary. The aim of this paper is to develop three pressurized liquefaction processes using twostage cascade refrigeration cycles instead of the conventional three-stage refrigeration cycles. The proposed processes are CH4−C2H6, CH4−C2H4, and C2H4−C3H8 processes. Taking the specific energy consumption as the objective function, simulation based optimization is conducted for the three novel processes as well as two conventional cascade processes by a sequential search method. Optimization results show that the C2H4−C3H8 process is the most efficient one with a specific energy consumption of 0.2089 kWh/Nm3, 22% less than that of the conventional CH4−C2H4−C3H8 process. Moreover, a detailed thermodynamic analysis is carried out for the five processes. The thermodynamic analysis results confirm that the C2H4−C3H8 process presents the best composite curve match and the highest coefficient of performance. The CH4−C2H6 process requires the smallest heat transfer area, 70% less than that of the conventional CH4−C2H6−C3H8 process. The CH4−C2H4 process uses the least amount of key equipment, 35% less than that of the conventional CH4−C2H4−C3H8 process. In addition, an exergy analysis is performed for all processes and the results indicate that further improvements are requested the most in valves and heat exchangers, respectively, for the conventional and novel processes. following aspects: first, the cascade process employing pure refrigerants and separate refrigeration cycles is easy to control and simple to operate; second, the cascade process is applicable for a wide range of feed gas compositions and volumes; third, the cascade process has a moderate energy consumption among the three types. The main obstacle for the application of the cascade process is its complexity. A conventional cascade process typically consists of three stages of refrigeration cycle, presenting great process complexity. The complexity leads to not only high capital cost but also a large occupation space, which poses enormous challenges to offshore applications. Therefore, this study attempts to develop novel cascade processes with a focus on process simplicity. It is obvious that the primary cause of the complexity is the three-stage refrigeration cycle. Hence, process simplification should mainly concentrate on the refrigeration cycle. In a conventional cascade process, three refrigeration cycles are cascaded to reach 111 K to liquefy natural gas and each cycle is

1. INTRODUCTION With increasing concerns over energy efficiency and environment protection, natural gas as an efficient and clean fossil fuel is gaining increasing interests. It is predicted that the global natural gas consumption will grow up from 3.2 trillion cubic meters (tcm) in 2010 to 5.2 tcm in 2040,1 increasing by 63%. In order to meet the growing demand for natural gas, the world’s natural gas producers are keen to find new gas supplies. As offshore fields hold a significant amount of natural gas (140 tcm), accounting for about 45% of the total proven reserves,2 there is an increasing number of producers showing interest in deploying offshore fields and developing offshore technologies. Offshore liquefaction technology facilitates the transportation of natural gas from offshore fields to user markets due to the reduced volume. It particularly benefits those offshore fields where pipelines are technically infeasible or commercially unattractive. In the open literature, a number of natural gas liquefaction processes have been reported. They can be divided into three types: the cascade3,4 the mixed refrigerant (MR),5−7 and the expander.8−11 For offshore applications, some researchers recommended the mixed refrigerant process for energy consumption reasons; some preferred the expander process for its simplicity and quick startup and shutdown.8 Nevertheless, this study selected the cascade process due to the © XXXX American Chemical Society

Special Issue: PSE Advances in Natural Gas Value Chain Received: September 30, 2017 Revised: December 31, 2017 Accepted: January 12, 2018

A

DOI: 10.1021/acs.iecr.7b04080 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

develop a healthy and efficient process, the selection of refrigerants should follow the instructions below: • The normal boiling point temperature of the refrigerant used in the low temperature refrigeration cycle should be close to the liquefaction temperature of natural gas (165 K). If the boiling point temperature is too high, it will be impossible for the refrigerant to liquefy natural gas; if the boiling point temperature is too low, it will be too expensive to liquefy natural gas. • The normal boiling point temperature of the refrigerant used in the high temperature refrigeration cycle should be lower than the critical point temperature of the refrigerant in the low temperature cycle. If not, the refrigerant in the second stage would remain in the gaseous state, which will lead to unsteady operation of the Joule−Thompson (JT) liquid valve. • The refrigerant should be environmentally friendly. As known, freon has been forbidden for use all over the world, so it will not be used in this study. • Priority should be given to those refrigerants with low price. Since a large amount of refrigerant is required by the LNG process, a small price advantage will bring great economic benefits. Table 1 shows the properties of pure refrigerants. According to the above instructions, it can be found that CH4 and C2H4

supposed to drop the temperature of natural gas by 54 K on average. It is clear that if the liquefaction temperature of natural gas is increased by 54 K, one stage of the refrigeration cycle could be removed and the liquefaction process could be simplified. To increase the temperature, the operation pressure for natural gas liquefaction should also be raised. As a result, pressurized liquefaction is an alternative to liquefy natural gas at much lower energy consumption, with another advantage that more CO2 will be allowed in the process.12,13 In this paper, a few pressurized liquefaction processes using two-stage cascade refrigeration cycles are designed, and the liquefaction temperature and pressure of natural gas in the processes are determined to be 165 K and 2 MPa. For a natural gas liquefaction (LNG) process, efficiency and simplicity are always a pair of contradictions. Due to the simplicity, the efficiency of the process is not so good. Since the process is energy-intensive, it is necessary to improve the energy efficiency of the process. In published research, optimization is a common and effective method to improve the energy efficiency of LNG processes. Early in 2002, Lee et al.14 developed a systematic synthesis method using a nonlinear programming (NLP) technique, and they found that applying the method to a mixed refrigerant process can significantly save energy and improve efficiency. Later, Jensen and Skogestad15 suggested the optimal operation of a PRICO process using linear and nonlinear analysis of controlled variables. In recent years, numerous publications with regard to energy consumption minimization have been revealed. Alabdulkarem et al.,16 Li et al.,17 and Xu et al.18 performed optimization for different LNG processes using genetic algorithm (GA). Wang et al.19 conducted optimal design and operation of a C3MR system to minimize the total shaft work with application of a sequential quadratic programming (SQP) solver in Aspen Plus. Hwang et al.20 and Mortazavi et al.6 carried out optimization studies for MR processes to minimize the energy requirement using a combination of GA and SQP methods. Aspelund et al.21 combined Tabu Search (TS) and Nelder−Mead Downhill Simplex (NMDS) methods to optimize a PRICO process. Moreover, Morin et al.22 applied evolutionary search to PRICO and TEALARC processes for energy savings. Wang et al.23 adopted a mixed-integer nonlinear programming (MINLP) methodology to optimize a C3MR process with the objective function of energy consumption. Some researchers did not clearly present their methodologies24−27 concerning energy minimization of the LNG processes. In this paper, three novel cascade processes, namely, CH4− C2H6, CH4−C2H4, and C2H4−C3H8 cascade processes, are designed. In order to make comparisons, two conventional cascade processes, namely, CH4−C2H6−C3H8 and CH4− C2H4−C3H8 cascade processes, are also studied. First, the configurations of five processes are described in detail. Then, the optimization methodology using specific energy consumption as an objective function is introduced. Next, the optimum performance of the five processes are calculated and compared. Finally, an exergy analysis for each process is performed.

Table 1. Properties of Pure Refrigerants symbol

name

normal boiling point temperature (K)

R50 R14 R1150 R170 R32 R13B1 R125 R1270 R290

methane carbon tetrafluoride ethylene ethane methylene fluoride trifluoromonobromomethane pentafluoroethane propylene propane

111.65 145.25 169.45 184.35 221.95 215.4 224.7 225.45 231.08

critical temperature (K) 190.65 227.45 282.45 305.35 351.45 340.15 333.25 364.95 369.95

are two refrigerants applicable for the low temperature refrigeration cycle. When using CH4 refrigerant, C2H6 and C2H4 are two feasible refrigerants in the high temperature refrigeration cycle. When using C2H4 refrigerant, C3H8 and C3H6 are two feasible refrigerants in the high temperature refrigeration cycle. Considering the price advantage of C3H8 over C3H6, only C3H8 refrigerant is considered in this study. Consequently, the refrigerants selected for the novel two-stage cascade process are CH4−C2H6, CH4−C2H4, and C2H4−C3H8. 2.2. Process Description. Two conventional cascade processes, namely CH4−C2H6−C3H8 and CH4−C2H4−C3H8, and three novel cascade processes, namely CH4−C2H6, CH4− C2H4, and C2H4−C3H8, are presented for comparisons. The conventional processes use three refrigeration cycles while the novel processes utilize two refrigeration cycles. Each refrigeration cycle typically consists of four sections: compression, condensation, expansion, and evaporation. In this study, the stage of compression is carefully designed according to the pressure ratio. A proper pressure ratio for each stage of compression should be in the range of 1−5. In real applications, each refrigeration cycle adopts multiple pressure levels for evaporation so as to achieve high energy efficiency.28 In this

2. LIQUEFACTION PROCESSES 2.1. Refrigerant Selection. In conventional three-stage cascade processes, CH4−C2H6−C3H8 and CH4−C2H4−C3H8 are commonly used refrigerants.28 In the novel two-stage cascade process, the refrigerants should be carefully reselected because the process configurations have changed. In order to B


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Industrial & Engineering Chemistry Research study, only one pressure level is used for the sake of simplicity. To focus on the liquefaction processes, the pretreatment processes for removing impurities like Hg, H2O, etc., are omitted in this study. 2.2.1. CH4−C2H6−C3H8 Process. The flow sheet of CH4− C2H6−C3H8 cascade process is shown in Figure 1. This process

401) to complete the cycle. The main duty of this cycle is to offer cooling energy to the hot composites in the heat exchanger (HEX-101). 2.2.2. CH4−C2H4−C3H8 Process. Figure 2 shows the flow sheet of CH4−C2H4−C3H8 cascade process. This process is

Figure 1. CH4−C2H6−C3H8 cascade process.

Figure 2. CH4−C2H4−C3H8 cascade process.

consists of four parts: natural gas side (number 1xx), CH4 cycle (number 2xx), C2H6 cycle (number 3xx), and C3H8 cycle (number 4xx). The detailed description of each part is as follows: • Natural gas side. The treated natural gas (101) first flows into the heat exchangers (HEX-101, HEX-102, and HEX-103) in which it is cooled and then goes across the valve (VLV-101) for expansion, by which natural gas decreases its pressure to atmospheric pressure and drops its temperature to around 111 K to become liquid. Finally, the liquefied natural gas (105) is sent into the separator (S-101) for end-flash, removing the volatiles (107) and producing the LNG product (106). The required cooling energy for natural gas liquefaction is provided by CH4 cycle, C2H6 cycle, and C3H8 cycle. • CH4 cycle. The gaseous CH4 refrigerant (201) first passes through a three-stage compression with intercooling and then gets precooled in the heat exchangers (HEX-101 and HEX-102). Afterward, it (209) expands across the valve (VLV-201) to become liquid and evaporates in the heat exchangers (HEX-103, HEX-102, and HEX-101) to release its cooling energy. Lastly, it (201) goes back to the compressor (C-201) to complete the cycle. • C2H6 cycle. The C2H6 refrigerant (301) is first compressed by two stages of compressors (C-301 and C-302) and then cooled by water-coolers (WC-301 and WC-302). Further, it is cooled in the heat exchanger (HEX-101). Next, it (306) goes across the valve (VLV301) for expansion and flows through the heat exchanger (HEX-102 and HEA-101) for evaporation. Eventually, it (301) returns to the compressor (C-301) to make a cycle. • C3H8 cycle. The C3H8 refrigerant (401) first boosts its pressure through a two-stage compression with intercooling and then condensates into liquid in the watercooler (WC-402). Afterward it expands through the valve (VLV-401) and evaporates in the heat exchanger (HEX101). In the end, it flows back to the compressor (C-

similar to the CH4−C2H6−C3H8 cascade process but uses a C2H4 cycle instead of the C2H6 cycle. Because C2H4 refrigerant has a lower boiling point temperature than C2H6 refrigerant, the C2H4 cycle in the CH4−C2H6−C3H8 process shares more of the cooling burden than does the C2H6 cycle in the CH4− C2H4−C3H8 process. As a result, the CH4 cycle in the CH4− C2H6−C3H8 process shares less of the cooling burden than that in the CH4−C2H4−C3H8 process, resulting in one less stage of compression in the CH4 cycle of the CH4−C2H6−C3H8 process. 2.2.3. CH4−C2H6 Process. The flow sheet of CH4−C2H6 cascade process is represented in Figure 3. This process

Figure 3. CH4−C2H6 cascade process.

includes three parts, namely, the natural gas side (number 1xx), CH4 cycle (number 2xx), and C2H6 cycle (number 3xx). Each part is described as follows: • Natural gas side. The treated natural gas (101) is gradually cooled in the heat exchangers (HEX-101, HEX102 and HEX-103). Then it expands across the valve (VLV-101) to turn into liquid, dropping its pressure to 2 MPa and decreasing its temperature to 165 K. It can be noted that the main difference of natural gas side between conventional processes and novel processes lies in the operation pressure and temperature. Finally, the pressurized liquefied natural gas (105) goes into the C


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Industrial & Engineering Chemistry Research separator (S-101), through which the volatiles (107) are removed from the PLNG product (106). • CH4 cycle. The CH4 refrigerant (201) is first compressed by the compressor (C-201) and then cooled by the water-cooler (W-201). After this, it is precooled in the heat exchangers (HEX-101 and HEX-102), thus it expands across the valve (VLV-201). Next, the CH4 refrigerant (206) flows into the heat exchangers (HEX103, HEX-102, and HEX-101) to release its cooling energy. In the end, it (201) returns to the compressor (C-201) to start a new cycle. The main duty of this cycle is to provide cooling energy for the heat exchangers (HEX-101, HEX-102 and HEX-103). • C2H6 cycle. The C2H6 refrigerant (301) boosts its pressure to a high level though a three-stage compression with intercooling, and then it (307) is precooled in the heat exchanger (HEX-101), next it (308) expands across the valve (VLV-301) to reach a low temperature. Subsequently, it (309) passes through the heat exchangers (HEX-102 and HEX-101) to release its cooling energy. Finally, it goes back to the compressor (C-301) to complete a cycle. The main duty of this cycle is to offer cooling energy to the heat exchangers (HEX-101 and HEX-102). 2.2.4. CH4−C2H4 Process. Figure 4 illustrates the flow sheet of CH4−C2H4 cascade process. This process is similar to the

Figure 5. C2H4−C3H8 cascade process.

101) to decrease its pressure to 2 MPa and drop its temperature to 165 K, and finally flows into the separator (S-101) for end-flash. The pressurized liquid (106) is the final product. • C2H4 cycle. The gaseous C2H4 refrigerant (201) first passes through a two-stage compression with intercooling and then it is precooled in the heat exchanger (HEX101). After that, it (206) expands through the valve (VLV-201) to become liquid. Then, the liquid C2H4 refrigerant (207) evaporates in the heat exchangers (HEX-102 and HEX-101) to offer cooling energy to hot flows. Finally, it (201) flows back to the compressor (C201) to begin a new cycle. The main duty of this cycle is to offer cooling energy to the heat exchangers (HEX-101 and HEX-102). • C3H8 cycle. The C3H8 refrigerant (301) subsequently undergoes a two-stage compression with intercooling, condensation, expansion, and evaporation to complete a cycle. The main duty of this cycle is to provide cooling energy for the heat exchanger (HEX-101).

3. OPTIMIZATION METHODOLOGY To evaluate the performance of the processes mentioned in section 2, the steady-state simulation based optimization is conducted in this study. The process models are built in Aspen HYSYS simulator by connecting different operation units with material and energy streams. The Peng−Robinson equation of state is used as the fluid package to calculate the state of each material stream. To optimize the processes, a sequential search method,12 which is a simple and reliable method, is used in this study. The central concept of a sequential search method is consecutively checking every value in a data list until the desired value is found. To illustrate the optimization methodology, the objective function, the key parameters, and the constraints are introduced in the following subsections. 3.1. Objective Function. The objective of optimization is to minimize the energy consumption of the processes. The energy consumption of the processes mainly comes from compressors. Other devices only require a small amount of power, which could be ignored if compared to the compressor power. Therefore, the energy consumption of the compressors represents the total energy consumption of the process. In this study, the compressor isentropic efficiency is set to be 85%.29 Regarding the production yield, the specific energy consumption is used as the objective function, which is calculated by eq 1.

Figure 4. CH4−C2H4 cascade process.

CH4−C2H6 cascade process but uses a C2H4 cycle instead of a C2H6 cycle. The C2H4 cycle differs from the C2H6 cycle in that the C2H4 cycle employs a two-stage compression while the C2H6 cycle adopts a three-stage compression. That difference is justified by the pressure ratio. In this study, the stage of compression in each refrigeration cycle is carefully designed according to the pressure ratio. If the pressure ratio for each stage of compression exceeds 5, one more stage of compression is required in order to reduce the energy consumption. Since the pressure ratio is determined by the optimized result of refrigerant high pressure and low pressure, the stage of compression is also an optimized result. 2.2.5. C2H4−C3H8 Process. The flow sheet of C2H4−C3H8 cascade process is represented in Figure 5. This process also consists of three parts, namely, natural gas side (number 1xx), C2H4 cycle (number 2xx), and C3H8 cycle (number 3xx). For each part of the cycle, information is given as detailed below: • Natural gas side. The treated natural gas (101) passes through the heat exchangers (HEX-101 and HEX-102) to be cooled down, then goes across the valve (VLVD


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Industrial & Engineering Chemistry Research w = Wtotal /qLNG

ratio exceeds the upper limit, multistage compression should be employed. To make the energy consumption as low as possible, the pressure ratio of each stage of compression is set to be equal.

(1)

where w represents the specific work consumption, and Wtotal is the total work consumed by all compressors, and qLNG is the volume flow rate of the LNG product. The equations of specific work consumption for different processes are listed in Table 2.

4. RESULTS AND ANALYSIS 4.1. Optimization Results. To perform theoretical analysis in a simplified way, the natural gas flow rate is assumed to be 1 kmol/h. The conventional and the novel processes are optimized with the application of sequential search method. The optimization results of different processes are summarized in Table 4. As shown, the optimized refrigerant high pressure of

Table 2. Equations of Specific Work Consumption for Different Processes liquefaction process conventional process

novel process

CH4−C2H6− C3H8 CH4−C2H4− C3H8 CH4−C2H6 CH4−C2H4 C2H4−C3H8

equation of specific work consumption w = (W201 + W202 + W203 + W301 + W302 + W401 + W402)/q106 w = (W201 + W202 + W301 + W302 + W401 + W402)/q106 w = (W201 + W301 + W302 + W303)/q106 w = (W201 + W301 + W302)/q106 w = (W201 + W202 + W301 + W302)/q105

Table 4. Optimization Results of Different Processes conventional process

parameter

3.2. Key Parameters for Optimization. During the simulation, many parameters in the processes can be varied. Among those parameters, there are several parameters having significant influence on the energy consumption, such as the refrigerant flow rate, refrigerant high pressure and low pressure. As a result, these parameters are regarded as the key parameters that need to be optimized. The key parameters for optimization in different processes are shown in Table 3.

natural gas flow rate (kmol/h) natural gas feed pressure (kPa) natural gas feed temperature (K) total refrigerant flow rate (kmol/ h) refrigerant high pressure (kPa)

Table 3. Key Parameters for Optimization in Different Processes liquefaction process conventional process

novel process

CH4−C2H6−C3H8 process CH4−C2H4−C3H8 process CH4−C2H6 process CH4−C2H4 process C2H4−C3H8 process

key parameters for optimization q201, p201, p206, q301, p301, p304, q401, p401, p404 q201, p201, p204, q301, p301, p304, q401, p401, p404 q201, p201, p202, q301, p301, p306 q201, p201, p202, q301, p301, p304 q201, p201, p204, q301, p301, p304

refrigerant low pressure (kPa)

LNG product pressure (kPa) LNG product temperature (K) specific work consumption (kWh/Nm3)

3.3. Constraints. To make the process smooth in operation, the following constraints should be considered during the optimization: 3.3.1. Heat Transfer Constraints. The second law of thermodynamics tells that heat transfer naturally proceeds in a certain direction, and that is from high temperature to low temperature. Based on that, the temperature of hot composite in heat exchangers must be always higher than that of cold composite, therefore no temperature crosses should occur in any heat exchanger. Moreover, the temperature difference between the hot and cold composites should also be carefully considered because it has a positive effect on energy efficiency and a negative effect on heat transfer area. As a compromise result between those two aspects, 3 K is adopted as the minimum temperature difference in this study. 3.3.2. Compressor Constraints. Liquid occurrence at the entrance of the compressor is not allowed because liquid will cause damage to the blade. This constraint is always active in Aspen HYSYS simulator. In addition, the pressure ratio of one stage compression should be neither too high nor too low. If the pressure ratio is too high, the energy consumption will be too large. If the pressure ratio is too low, an extra compressor will be needed. As a result, the pressure ratio of one stage compression should be within the range of 1−5. If the pressure

CH4− C2H6− C3H8

novel process

CH4− C2H4− C3H8

CH4− C2H6

CH4− C2H4

C2H4− C3H8

1

1

1

1

1

5000

5000

5000

5000

5000

308.15

308.15

308.15

308.15

308.15

2.664

2.529

2.885

4.885

1.404

5000 (CH4) 1018 (C2H6) 1220 (C3H8) 120 (CH4) 110 (C2H6) 140 (C3H8) 110

2818 (CH4) 1861 (C2H4) 1220 (C3H8) 130 (CH4) 120 (C2H4) 140 (C3H8) 110

5000 (CH4) 5000 (C2H6)

5000 (CH4) 5000 (C2H4)

1674 (C2H4) 1220 (C3H8)

1899 (CH4) 130 (C2H6)

1899 (CH4) 290 (C2H4)

75 (C2H4) 120 (C3H8)

2000

2000

2000

112.57

112.57

165.70

165.70

165.70

0.2994

0.2667

0.3028

0.4637

0.2089

CH4 in CH4−C2H6−C3H8 process (5000 kPa) is almost twice that in CH4−C2H4−C3H8 process (2818 kPa), thus one more stage of compression is required in CH4−C2H6−C3H8 process, as illustrated in Figure 1. Similarly, the optimized refrigerant low pressure of C2H6 in CH4−C2H6 process (130 kPa) is almost half of the optimized refrigerant low pressure of C2H4 in CH4−C2H4 process (290 kPa), consequently, one more stage of compression is employed in CH4−C2H6 process, as presented in Figure 3. It should be noted that the optimized refrigerant low pressure of C2H4 in the C2H4−C3H8 process (75 kPa) is subatmospheric. This is mainly because the boiling point temperature of C2H4 (169 K) refrigerant is slightly higher than the pressurized liquefaction temperature of natural gas (165 K). In order to liquefy natural gas, C2H4 refrigerant must evaporate at subatmospheric pressure. From Table 4, it can be seen that the LNG product pressure (2000 kPa) in novel processes is much higher than that (110 kPa) in conventional processes; correspondingly, the LNG product temperature (165 K) in novel processes is much higher E


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0.27, and 0.64, respectively. Comparing the novel cascade processes with their representative conventional one, it can be found that the COP of the CH4−C2H6 process is 27% less than that of the CH4−C2H6−C3H8 process and the COP of the CH4−C2H4 process is 58% less than that of the CH4−C2H4− C3H8 process. Differently, the COP of the C2H4−C3H8 process is 36% more than that of the CH4−C2H4−C3H8 process, which once again demonstrates that the C2H4−C3H8 process is the most energy efficient process among the three novel processes. 4.3. Composite Curves. Composite curves are the temperature profiles of hot and cold flow while passing through the LNG heat exchanger. Many researchers analyzed the composite curves of heat exchangers in LNG processes.7,4,17 It has been demonstrated that the gap between the hot and cold composite curves has a close relationship with the energy efficiency. Generally, a small gap indicates a high energy efficiency. The composite curves in heat exchangers for different processes are shown in Figure 6. As can be seen the

than that in conventional processes (112 K), increased by 53 K. That brings both advantages and disadvantages. The advantage is that high temperature increases the solubility of CO2 in LNG, thus eliminating the CO2 removal unit for natural gas with low CO2 content. Moreover, high temperature reduces one stage of the refrigeration cycle, bringing great simplicity in the process configuration. The disadvantage is that high pressure storage of LNG may result in higher capital cost in the LNG storage system. According to the optimization results, the specific energy consumption of CH4−C2H6−C3H8 and CH4−C2H4−C3H8 processes are 0.2994 and 0.2667 kWh/Nm3, respectively. By comparison, CH4−C2H4−C3H8 process consumes 11% less energy than the CH4−C2H6−C3H8 process, which suggests C2H4 refrigerant has lower operating cost than the C2H6 refrigerant. Nevertheless, the C2H6 refrigerant has lower capital cost than the C2H4 refrigerant because C2H6 can be directly extracted from natural gas but C2H4 cannot. Therefore, when deciding which conventional process should be used for natural gas liquefaction, a trade-off should be carefully made between the operating cost and the capital cost. As shown in Table 4, the specific energy consumption of CH4−C2H6, CH4−C2H4, and C2H4−C3H8 processes are 0.3028, 0.4637, and 0.2089 kWh/Nm3, respectively. Compared with the CH4−C2H4−C3H8 process, the C2H4−C3H8 process presents an energy saving of 22%. The CH4−C2H6 process presents an energy consumption close to the CH4−C2H6− C3H8 process with only a slight increase of 1%. The CH4−C2H4 cascade process consumes 74% more energy than the CH4− C2H4−C3H8 process. As a result, from the perspective of energy consumption, the C2H4−C3H8 process should be the best choice for natural gas liquefaction among the three novel processes. 4.2. Coefficient of Performance. The coefficient of performance (COP) is commonly used to evaluate the efficiency of a refrigeration system, which is defined by the heat being removed from natural gas divided by the total energy consumed by compressors. The equations of COP for different processes are given in Table 5. As shown, the COP of CH4− Table 5. Coefficient of Performance (COP) for Different Processes COP liquefaction process conventional process

novel process

CH4− C2H6− C3H8 CH4− C2H4− C3H8 CH4− C2H6 CH4− C2H4 C2H4− C3H8

equation

value

COP = (h101 − h106)/(W201 + W202 + W203 + W301 + W302 + W401 + W402) COP = (h101 − h106)/(W201 + W202 + W301 + W302 + W401 + W402) COP = (h101 − h106)/(W201 + W301 + W302 + W303) COP = (h101 − h106)/(W201 + W301 + W302) COP = (h101 − h105)/(W201 + W202 + W301 + W302)

0.56

changes

Figure 6. Composite curves of heat exchangers for different processes. (a) CH4−C2H6−C3H8 process. (b) CH4−C2H4−C3H8 process. (c) CH4−C2H6 process. (d) CH4−C2H4 process. (e) C2H4−C3H8 process.

0.64 0.41

−27%

0.27

−58%

0.88

+36%

composite curves of cascade processes are not as smooth as that of mixed refrigerant processes. This is because mixed refrigerant has continuously varying evaporating temperature but pure refrigerant only has a constant evaporating temperature. Figure 6a and b illustrate the composite curves of heat exchanger for CH 4−C2 H6 −C 3H 8 and CH4 −C 2H 4−C3 H8 cascade processes, respectively. It is clear that the gap between the composite curves is very small, which suggests that conventional cascade processes have very high energy efficiency. Figure 6c−e present the composite curves of the heat exchanger for CH4−C2H6, CH4−C2H4, and C2H4−C3H8 cascade processes, respectively. Obviously, the composite curves in Figure 6e have a better match than those in Figure 6c and d. This implies that the C2H4−C3H8 cascade process has

C2H6−C3H8 and CH4−C2H4−C3H8 processes are 0.56 and 0.64, respectively. It is clear that the COP of the CH4−C2H4− C3H8 process is higher than that of the CH4−C2H6−C3H8 process, thus the CH4−C2H4−C3H8 process is more energy efficient than the CH4−C2H6−C3H8 process, as mentioned above. From Table 5, it can be seen that the COP of CH4− C2H6, CH4−C2H4, and CH4−C2H4−C3H8 processes are 0.41, F


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all simpler than conventional processes. Also, they have lower capital cost and smaller occupation space than conventional processes. Among the three novel processes, it can be found that the CH4−C2H4 process uses the smallest total amount of equipment. More importantly, it has the least compressors which are much more expensive than other equipment, such as water-coolers and heat exchangers. Therefore, the CH4−C2H4 process should be the one with the lowest capital cost.

higher energy efficiency than the other two novel processes, as demonstrated in Table 4. 4.4. Heat Transfer Area. Heat exchanger is one of the main equipment in a LNG plant and heat transfer area is directly related with the heat exchanger size. Usually, the heat transfer area A and the heat transfer coefficient U are discussed as a whole. Table 6 gives the UA of heat exchangers for Table 6. UA of Heat Exchanger for Different Processes

5. EXERGY ANALYSIS Exergy analysis has been proven to be a powerful tool in the performance evaluation of complex thermodynamic systems.29−31 Because it points out the directions for potential improvement of a thermodynamic system, it is frequently used by LNG process researchers.32,33 Generally, exergy analysis of a thermodynamic system can be performed in two ways: one is analyzing the components of the system separately and the other one is analyzing the system as a whole, using the exergy efficiency index. In this work, exergy analysis is conducted in both ways. 5.1. Theory. Exergy is the maximum potential work that can be obtained from the specified initial state to the state of its environment, by means of a reversible process. The exergy of a steady flow stream is expressed as follow:

UA (kJ/h K) liquefaction process conventional process

novel process

CH4− C2H6− C3H8 CH4− C2H4− C3H8 CH4− C2H6 CH4− C2H4 C2H4− C3H8

HEX101

HEX102

HEX103

total

5029

1407

281

6717

3879

1717

224

5820

718

1013

270

2001

70%

1503

1027

277

2807

52%

3355

42%

2922

432.6

reduction

different processes, derived from the HYSYS simulator. It is easy to find that the novel processes have much smaller UA than conventional three-stage cascade processes. Compared with the CH4−C2H6−C3H8 process, the CH4−C2H6 process reduces UA by 70%. Compared with the CH4−C2H4−C3H8 process, the CH4−C2H4 and C2H4−C3H8 processes save UA by 52% and 42%, respectively. Assuming that the novel processes have the same U as the conventional processes, it is calculated that the CH4−C2H6, CH4−C2H4, and C2H4−C3H8 processes can reduce the heat transfer area by 70%, 52%, and 42%, respectively. That is a significant reduction in heat exchanger size and, therefore, a great progress in space savings. 4.5. Amount of Key Equipment. In a cascade LNG process, compressor, water cooler, heat exchanger, JT valve, and tank are key equipment. The amount of key equipment not only represents the complexity of the process but also indicates the capital cost and the occupation space. Table 7 lists the number of pieces of key equipment in different processes. As shown, the CH4−C2H6−C3H8 cascade process uses more key equipment than the CH4−C2H4−C3H8 cascade process because of having one more stage of compression in the CH4 cycle. Moreover, it can be found that the novel processes use much less key equipment than conventional processes. Specifically, CH 4 −C 2 H 4 , C 2 H 4 −C 3 H 8 , and C 2 H 4 −C 3 H 8 cascade processes respectively have a reduction of 32%, 35%, and 30% in amount of key equipment compared to their representative conventional one. That means that the three novel processes are similar in process complexity and they are

Ex = H − H0 + T0(S0 − S)

(2)

where Ex represents the exergy of a stream, H and S are the enthalpy and entropy respectively, and T is the temperature. The subscript 0 represents the environmental state (101 kPa and 300 K). 5.2. Exergy Destruction of Equipment. In a real process, the exergy of a stream will be partly destroyed due to irreversibility as it flows through the equipment, such as compressors, water coolers, heat exchangers, and valves. Therefore, it is important to calculate the exergy destruction of each piece of equipment to better understand where improvements are most needed. Table 8 gives the equations of Table 8. Equations of Exergy Destruction in the Equipment equipment

exergy destruction equation

compressor water cooler/heat exchanger/JT valve

Ex,d = ∑Ex,in − ∑Ex,out + ∑W Ex,d = ∑Ex,in − ∑Ex,out

exergy destruction depending on the equipment. Where, Ex,d indicates the exergy destruction inside the equipment, Ex,in represents the exergy of a stream at the equipment inlet, Ex,out is the exergy of a stream at the equipment outlet, and W is the input work of the compressor. Based on the optimization results, the exergy destruction of equipment for different processes is shown in Table 9. By comparing the two conventional processes, it can be found that

Table 7. Amount of Key Equipment for Different Processes pieces of equipment liquefaction process conventional process novel process

CH4−C2H6−C3H8 CH4−C2H4−C3H8 CH4−C2H6 CH4−C2H4 C2H4−C3H8

compressors

water-coolers

heat exchangers

JT valves

tanks

total

reduction

7 6 4 3 4

7 6 4 3 4

3 3 3 3 2

4 4 3 3 3

1 1 1 1 1

22 20 15 13 14

32% 35% 30%

G


Article

Industrial & Engineering Chemistry Research Table 9. Exergy Destruction of Equipment for Different Processes exergy destruction of equipment (kJ/h) liquefaction process conventional process novel process

CH4−C2H6−C3H8 CH4−C2H4−C3H8 CH4−C2H6 CH4−C2H4 C2H4−C3H8

compressor

water-cooer

heat exchanger

JT valve

total

2836 2477 3062 4422 1439

3884 3427 4445 7853 1999

3409 3184 10823 17969 2362

6796 4869 3217 5090 1977

16925 13957 21547 35335 7776

the CH4−C2H6−C3H8 process destroys more exergy than the CH4−C2H4−C3H8 process in each piece of equipment, resulting in a larger amount of total exergy destruction. That clearly explains why the CH4−C2H6−C3H8 process has higher specific energy consumption than the CH4−C2H4−C3H8 process. A comparison result of the three novel processes shows that the CH4−C2H4 process has the largest amount of total exergy destruction while the C2H4−C3H8 process has the smallest one. That agrees well with the conclusion that the CH4−C2H4 process is the most energy costly process while the C2H4−C3H8 process is the most energy efficient process. In order to better illustrate the exergy destruction for different processes, Figure 7 shows the exergy destruction of

30% for the C2H4−C3H8 process. The main distinction between the conventional processes and the novel processes is due to the fact that the conventional processes have more valves and better matched composite curves than the novel processes. According to the above analysis, it can be deduced that further improvements for higher efficiency are most needed at valves for the conventional processes and at heat exchangers for the novel processes. 5.3. Exergy Efficiency. For a natural gas liquefaction process, exergy efficiency is defined as the ratio of the minimum work input to the actual work input.34 The minimum work input is the minimum amount of work needed by a stream for undergoing a process from the specified initial state to the final state, which equals the exergy difference between the initial and final states. The actual work input represents the total energy consumed by compressors. Generally, the exergy efficiency for natural gas liquefaction process is far less than 100% due to some irreversibility factors, such as friction, nonquasiequilibrium compression, and heat transfer through a finite temperature difference. Table 10 presents the exergy efficiency for different processes. As can be seen, the exergy efficiency of CH4− C2H6−C3H8 and CH4−C2H4−C3H8 processes are 0.3009 and 0.3430, respectively. It is obvious that the CH4−C2H4−C3H8 process presents higher exergy efficiency. That is because the CH4−C2H4−C3H8 process destroys less exergy in each piece of equipment than the CH4−C2H6−C3H8 process, as shown in Table 9. Among the three novel processes, it can be found that only the C2H4−C3H8 process has a comparative exergy efficiency with conventional ones, which is 0.3400. The exergy efficiency of the CH4−C2H6 process is 0.1610, less than 46% if compared with the CH4−C2H6−C3H8 process, and the exergy efficiency of the CH4−C2H4 process is 0.1048, 69% less than that of the CH4−C2H4−C3H8 process. The sharp decrease in exergy efficiency for CH4−C2H6 and CH4−C2H4 processes is due to the large heat transfer temperature difference. To improve their exergy efficiency, selecting mixed refrigerants with varying evaporating temperature should be an effective method.

Figure 7. Exergy destruction of equipment for different processes. (a) CH4−C2H6−C3H8 process. (b) CH4−C2H4−C3H8 process. (c) CH4− C2H6 process. (d) CH4−C2H4 process. (e) C2H4−C3H8 process.

6. CONCLUSIONS Three novel natural gas liquefaction processes, focused on process simplicity, are proposed in this paper. The novel processes convert natural gas into LNG at a higher pressure (2 MPa) and temperature (165 K), eliminating CO2 pretreatment facilities and avoiding one stage of the refrigeration cycle. Taking the specific energy consumption as an objective function, key parameters in the processes are optimized by means of a sequential search method, considering heat transfer and compressor constraints. The optimization results show that C2H4−C3H8 cascade process with the best match of composite curves and highest COP achieves the lowest specific work consumption of 0.2089 kWh/Nm3, 22% less if compared with

the equipment for different processes. From Figure 7a and b, it can be seen that the exergy destruction for the two conventional processes is similar. Regarding the conventional processes, major exergy destruction comes from the JT valve, accounting for 40% for the CH4−C2H6−C3H8 process and 35% for the CH4−C2H4−C3H8 process. Moreover, Figure 7c−e shows that the largest exergy destruction for the novel processes depends by the heat exchanger, which is 50% for the CH4−C2H6 process, 51% for the CH4−C2H4 process, and H


Article

Industrial & Engineering Chemistry Research Table 10. Exergy Efficiency for Different Processes exergy efficiency liquefaction process conventional process

novel process

equation

η=

CH4−C2H4−C3H8

η=

(E106 + E107 − E101) (W201 + W202 + W301 + W302 + W401 + W402)

0.3430

CH4−C2H6

η=

(E106 − E101) (W201 + W301 + W302 + W303)

0.1610

−46%

CH4−C2H4

η=

(E106 − E101) (W201 + W301 + W302)

0.1048

−69%

C2H4−C3H8

η=

(E105 − E101) (W201 + W202 + W301 + W302)

0.3400

−1%

0.3009

(8) Li, Q. Y.; Ju, Y. L. Design and analysis of liquefaction process for offshore associated gas resources. Appl. Therm. Eng. 2010, 30, 2518− 2525. (9) Yuan, Z.; Cui, M.; Xie, Y.; Li, C. Design and analysis of a smallscale natural gas liquefaction process adopting single nitrogen expansion with carbon dioxide pre-cooling. Appl. Therm. Eng. 2014, 64, 139−146. (10) Khan, M. S.; Lee, S.; Hasan, M.; Lee, M. Process knowledge based opportunistic optimization of the N2−CO2 expander cycle for the economic development of stranded offshore fields. J. Nat. Gas Sci. Eng. 2014, 18, 263−273. (11) Khan, M. S.; Lee, S.; Getu, M.; Lee, M. Knowledge inspired investigation of selected parameters on energy consumption in nitrogen single and dual expander processes of natural gas liquefaction. J. Nat. Gas Sci. Eng. 2015, 23, 324−337. (12) Xiong, X. J.; Lin, W. S.; Gu, A. Z. Integration of CO2 cryogenic removal with a natural gas pressurized liquefaction process using gas expansion refrigeration. Energy 2015, 93, 1−9. (13) Xiong, X. J.; Lin, W. S.; Gu, A. Z. Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology. J. Nat. Gas Sci. Eng. 2016, 30, 379−387. (14) Lee, G. C.; Smith, R.; Zhu, X. X. Optimal synthesis of mixedrefrigerant systems for low-temperature processes. Ind. Eng. Chem. Res. 2002, 41, 5016−5028. (15) Jensen, J. B.; Skogestad, S. Optimal operation of a simple LNG process. International Symposium on Advanced Control of Chemical Processes, Gramado, Brazil, 2006; pp 241−246. (16) Alabdulkarem, A.; Mortazavi, A.; Hwang, Y.; Radermacher, R.; Rogers, P. Optimization of propane pre-cooled mixed refrigerant LNG plant. Appl. Therm. Eng. 2011, 31, 1091−1098. (17) Li, Y.; Wang, X.; Ding, Y. An optimal design methodology for large-scale gas liquefaction. Appl. Energy 2012, 99, 484−490. (18) Xu, X.; Liu, J.; Jiang, C.; Cao, L. The correlation between mixed refrigerant composition and ambient conditions in the PRICO LNG process. Appl. Energy 2013, 102, 1127−1136. (19) Wang, M.; Zhang, J.; Xu, Q.; Li, K. Thermodynamic-analysisbased energy consumption minimization for natural gas liquefaction. Ind. Eng. Chem. Res. 2011, 50, 12630−12640. (20) Hwang, J.; Roh, M.; Lee, K. Determination of the optimal operating conditions of the dual mixed refrigerant cycle for the LNG FPSO topside liquefaction process. Comput. Chem. Eng. 2013, 49, 25− 36. (21) Aspelund, A.; Gundersen, T.; Myklebust, J.; Nowak, M. P.; Tomasgard, A. An optimization-simulation model for a simple LNG process. Comput. Chem. Eng. 2010, 34, 1606−1617. (22) Morin, A.; Wahl, P. E.; Mølnvik, M. Using evolutionary search to optimize the energy consumption for natural gas liquefaction. Chem. Eng. Res. Des. 2011, 89, 2428−2441. (23) Wang, M.; Zhang, J.; Xu, Q. Optimal design and operation of a C3MR refrigeration system for natural gas liquefaction. Comput. Chem. Eng. 2012, 39, 84−95.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.L.). ORCID

Wensheng Lin: 0000-0003-0921-9810 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for funding by National Natural Science Foundation of China for the Project (No. 51076098).

■

changes

CH4−C2H6−C3H8

the conventional CH4−C2H4−C3H8 process. The CH4−C2H6 process requires the smallest heat transfer area, 70% less than that of the conventional CH4−C2H6−C3H8 process. Finally, the CH4−C2H4 process needs the least amount of key equipment, 13 pieces, which is 35% less than that of the conventional CH4−C2H4−C3H8 process. The exergy analytical results indicate that the exergy efficiency of CH4−C2H6, CH4−C2H4 and C2H4−C3H8 processes are 0.1610, 0.1048 and 0.3400, decreasing by 46%, 69%, and 1%, by comparison with their representative conventional ones, respectively. The major exergy destruction for novel processes comes from the heat exchanger. For further improvement, selecting mixed refrigerants with varying evaporating temperatures should be an effective method.

■

value

(E106 + E107 − E101) (W201 + W202 + W203 + W301 + W302 + W401 + W402)

REFERENCES

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Design and Optimization of Pressurized Liquefaction Processes for

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