Analysis of Heat Transfer Irreversibility in Processes for Liquefaction of

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

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Analysis of Heat Transfer Irreversibility in Processes for Liquefaction of Coalbed Methane with Nitrogen Wensheng Lin,* Jingxuan Xu, Ting Gao, and Anzhong Gu Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: Liquefaction is considered as one of the best methods for using coalbed methane (CBM). Unlike conventional natural gas, CBM usually contains a high proportion of nitrogen, so the liquefaction system performance, especially the heat transfer characteristics, differs from that of conventional natural gas liquefaction processes and depends on the nitrogen content. Irreversibility is an important aspect of the heat transfer characteristics and reflects the potential for further optimization of the heat exchanger system. In this paper, four liquefaction processes for nitrogen/methane mixtures are evaluated. Heat exchanger irreversibilities in each of these four processes are analyzed and compared for CBM nitrogen contents ranging between 0 and 70% and a methane recovery rate of 95% on the basis of optimization results provided by previous studies. It is revealed that the irreversibility in the heat exchanger system decrease with the increase of nitrogen content, and it is higher for the mixed refrigerant system than for the nitrogen refrigerant system. The results provide a direction for improvement of the liquefaction system performance and a reference for the selection of liquefaction processes for CBMs with different nitrogen contents. and two open-loop expander processes. Shirazi et al.9 studied the exergy loss of components in the SMR process, and found that a low irreversibility is due to optimized values of key parameters in the LNG heat exchanger observed under a low temperature difference between hot and cold composite curves. Wang et al.10 performed a thermodynamic-analysis-based study of the minimization of the energy consumption of a typical natural gas liquefaction process. Rian et al.11 evaluated the first Arctic liquefied natural gas (LNG) plant using the exergy method and found the benefits of the cold climate compared to a similar plant in a tempered or a tropical climate. Khan et al.12,13 analyzed the mixed refrigerant systems and the nitrogen expander systems, respectively, and considered maximization of the heat exchanger exergy efficiency as the optimization objective to achieve an energy efficient design goal. Vatani et al.14 performed advanced exergetic analysis for five mixed refrigerant LNG processes and calculated four parts of irreversibility (avoidable/unavoidable and endogenous/exogenous) for the components with high inefficiencies. Khan et al.15 presented six case studies and evaluated the process efficiency of the propane precooling cascade cycle by adopting both energy and exergy analysis methods. Sun et al.16 conducted exergy analysis to calculate the lost work for main equipment

1. INTRODUCTION Coalbed methane (CBM), sometimes also called coal seam gas (CSG), is a valuable energy resource and its recovery is important not only for energy utilization but also for safety and environmental protection.1,2 As an efficient storage and transportation technology, liquefaction of CBM provides a convenient means for transport of CBM from producer to consumer.3 In recent years, liquefaction of CBM has drawn more and more attention, especially when Australia began to produce large amount of liquefied natural gas (LNG) from CBM.4−6 Unlike conventional natural gas, CBM usually contains a high level of nitrogen, which cannot be removed by the common purification technologies used to produce liquefied natural gas (LNG). Thus, nitrogen must be separated from the CBM to enrich its methane content. Such a separation could be achieved by distillation after liquefaction.6 In this way, nitrogen is liquefied together with methane so the liquefaction system performance differs from that of conventional natural gas liquefaction processes and is affected by the nitrogen content of the CBM feed gas. Exergy analysis has been widely used in LNG process performance analysis, reflecting the potential for further optimization of the liquefaction system. Kanoglu7 provided an exergy analysis of the multistage cascade refrigeration cycle and developed an expression for the minimum work requirement for the liquefaction of natural gas. Remeljej et al.8 performed a full exergy analysis of four processes for small-scale liquefied natural gas (LNG) production, including a single-stage mixed refrigerant (SMR), a two-stage expander nitrogen refrigerant © XXXX American Chemical Society

Special Issue: PSE Advances in Natural Gas Value Chain Received: September 30, 2017 Revised: February 19, 2018 Accepted: February 28, 2018

A

DOI: 10.1021/acs.iecr.7b04083 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. MRC liquefaction process for CBM.32

Figure 2. NEC liquefaction process for CBM.33

Queensland, Australia, which is the world’s first LNG project of this kind. Significant challenges for the project are discussed, such as reserves build, production build, well turn down and water management. Li et al.25 proposed the formulas of the methane explosion limit appropriate for low temperature conditions. Cui et al.26 proposed a new liquefaction and distillation process. To guarantee the security of the process, nitrogen as a diluent is added to the distillation column to reduce the oxygen content. It can be seen that there still have a lot of work to do for the in-depth analysis and optimization of the CBM liquefaction processes. Caution! Because there may be an explosion hazard, the authors of this paper do not recommend introducing oxygenbearing CBM into the liquefaction and distillation process. Instead, the authors tend to consider that oxygen has been removed in some ways before CBM is fed into the liquefaction process.

items used in the AP-X process designed by APCI. Exergy analysis was also used to analyze the irreversibility of processes that coproduce LNG and NGL.17−19 To take both exergy efficiency and equipment cost into consideration, exergoeconomic analysis, and even exergoenvironmental analysis, are also often adopted for LNG process design and optimization beside ordinary exergy analysis.20−22 Although CBM liquefaction has been realized on the industrial scale, not much literature has been published. Unsworth et al.23 addressed a number of the key technical challenges faced by developers of CSG-to-LNG projects. These challenges include the high demand reliability required to ensure the CSG wells are not shut-in (which leads to water flooding), the impact of feed gas composition on the facility configuration and the suitability of the resultant low heating value LNG product for the LNG market. Mitchley24 provided an overview of the Santos GLNG CSG to LNG project in B


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Figure 3. C3/MRC liquefaction process for CBM.31

Figure 4. C3/NEC liquefaction process for CBM.30

petrochemical industry.34 The Peng−Robinson (PR) equation of state is adopted for properties calculation. The four processes from our previous studies30−33 are directly referenced in this paper. First, a power consumption comparison of these processes will be made on an identical basis. More importantly, heat transfer irreversibilities of the four processes will be analyzed. The names and symbols of the equipment for the four processes are shown in Table 1. The MRC process is shown in Figure 1. CBM undergoes two stages of compression and cooling by water-coolers; then it is gradually cooled and liquefied with mixed refrigerant in four heat exchangers (HEX-101−HEX-104). Afterward, the pressure is reduced by throttling, and finally, the LNG product is separated out in the liquid−vapor separator (V-101). The flash gas from the separator is induced successively into these four heat exchangers to recover its cold energy. For the mixed refrigerant cycle, the mixed refrigerant (MR), which contains six compositions of nitrogen and alkanes from methane to pentane, is first two-stage compressed and cooled by a watercooler, and then goes through multistage flash and partial

In this paper, we focus on the heat transfer irreversibility analysis for CBM liquefaction processes. Four typical processes used for natural gas liquefaction are evaluated for the CBM liquefaction. These include a mixed refrigerant cycle (MRC), a two-stage nitrogen expansion cycle (NEC), a mixed refrigerant cycle with propane precooling (C3/MRC), and a two-stage nitrogen expansion cycle with propane precooling (C3/ NEC).27−29 Steady state simulations and full optimizations for CBMs with nitrogen contents ranging between 0 and 70% are carried out, with the methane recovery rate fixed at 95% as the restricted index to ensure that each process is compared on an identical basis. The heat transfer irreversibilities for the heat exchanger systems under different conditions are investigated and compared.

2. LIQUEFACTION PROCESSES Our previous studies 30−33 separately accomplished the optimization and analysis of the four processes as mentioned above, and their structures are shown in Figures 1−4. The simulation calculations and optimization of processes are completed with HYSYS software, which is widely used in C


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Industrial & Engineering Chemistry Research Table 1. Names and Symbols for the Equipment in the Processes symbol

name

C E HEX MIX V VLV WC

compressor expander heat exchanger mixer vapor−liquid separator throttling valve water-cooler

w=

condensation, providing cold energy at different temperature grade for these four heat exchangers step by step. For the NEC process shown in Figure 2, cold energy for liquefying CBM is provided by a two-stage nitrogen expansion cycle instead of the mixed refrigerant cycle: nitrogen first undergoes two stages of compression and cooling first by using water-coolers and then is precooled in HEX-1. Afterward, it undergoes the first stage of expansion to a median pressure and is further cooled in HEX-2; then it undergoes the second stage of expansion to a low pressure and produces cold energy. For the C3/MRC and C3/NEC processes shown in Figures 3 and 4, a propane precooling cycle is added, which is a vapor compression refrigeration cycle with regeneration. For the NEC and C3/NEC processes shown in Figures 2 and 4, the work recovered from the expanders is used to drive the compressors. For simplicity, this is not shown in the figures.

(1)

where W com is the sum of each compressor’s power consumption, Wexp is the sum of each expander’s power generation, and q̇nLNG is the molar flow rate of LNG product. Because it is unusual to relate specific power to the molar flow rate of LNG product in engineering practice, the final specific power is converted to be related to volume flow at standard condition. It is clear that the unit power consumption rises sharply with the nitrogen content of CBM feed gas and the order of energy efficiency for the processes was C3/MRC > MRC > C3/NEC > NEC for most cases, whereas the energy efficiency of the NEC process was higher than that of the C3/NEC process when the nitrogen content of CBM was high.

4. HEAT TRANSFER IRREVERSIBILITY ANALYSIS The ability to match the hot and cold composite curves demonstrates the degree of optimization available within each process, whereas the temperature difference is the cause of process irreversibility. However, the Carnot factor for refrigeration Ω = T/(T0 − T) could be incorporated into the composite curve analysis by plotting the composite curves on the basis of Carnot factor rather than temperature against the heat flow rate to demonstrate the irreversibility in heat exchanger systems. Exergy analysis is another direct approach for evaluating heat exchange irreversibility for each process. Therefore, in this section, heat transfer irreversibility is analyzed and compared from three viewpoints: hot and cold composite curves plotted as temperature versus heat flow rate, hot and cold composite curves plotted as Carnot factor versus heat flow rate, and the exergy loss rate calculation. 4.1. Composite Curves on the Temperature Basis. For heat flow processes, it is possible to estimate the minimum shaftwork by comparing the composite hot streams and cold streams, because the area between these two curves equates to the minimum driving force for heat exchange and also the lost work in the heat exchange network. The hot and cold composite curves for the whole heat exchange network (exclude the propane precooling heat exchanger) plotted as temperature versus heat flow rate for different processes, when taking the nitrogen content as 30% for instance, are shown in Figure 6, and the curves for different nitrogen content using the C3/MRC process as an example are shown in Figure 7.

Table 2. Assumed Parameters for the Four Processes parameter

qnLNG ̇

Figure 5. Unit power consumption for various processes with different nitrogen contents.

3. PROCESS OPTIMIZATION RESULTS The optimization results for the four processes from our previous studies30−33 are directly referenced in this paper on an identical basis. A number of parameters were assumed and were identical across the four processes, as listed in Table 2. Other parameters were optimized to minimize the unit power consumption, calculated using eq 1, and the unit power consumption of the optimized processes under different conditions is shown in Figure 5.

temperature after water cooling compressor adiabatic efficiency expander adiabatic efficiency pressure drop of heat exchanger minimum temperature difference of heat exchanger compression ratios and expansion ratios feed temperature feed pressure feed flow rate LNG pressure CBM pressure after the twostage compression propane evaporation temperature methane recovery rate

Wcom − Wexp

value 35 °C 85% 80% 0 kPa 5 °C for liquid−gas or gas−gas heat transfer or 3 °C for liquid−liquid heat transfer same ratio for each stage 35 °C 101.325 kPa 1 kmol/h 110 kPa 4.5 MPa −40 °C 95% D


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Figure 8. LMTD in different locations and the average value for the various process when the nitrogen content is 30%.

Figure 6. Hot and cold composite curves plotted as temperature versus heat flow rate for different processes when the nitrogen content is 30%.

Figure 9. LMTD in different locations and the average value for the C3/MRC process at various nitrogen contents.

Figure 7. Hot and cold composite curves plotted as temperature versus heat flow rate for the C3/MRC process at various nitrogen contents.

comparison of the processes are not meaningful due to different heat exchangers adopted. Only the average LMTD is quantitatively comparable. From Figure 8 it can be seen that the NEC process has a smaller temperature difference than the MRC process in higher temperature locations (locations 1 and 2) because the nitrogen does not change phase. However, the temperature difference of the NEC process is extremely high in the cryogenic temperature region (location 3), because, as a gaseous refrigerant, the lower temperature of nitrogen is needed to supply sufficient cold energy to natural gas. On the one hand, the temperature difference for the C3/NEC process is higher than that of the NEC process because of the propane phase change occurring in the propane precooling cycle. On the other hand, the propane precooling process makes up for the deficiency of the mixed compositions refrigeration efficiency in the high temperature location and leads to a smaller temperature difference for the C3/MRC process than for the MRC process. The order of the average temperature difference for the four processes is consistent with the order of the unit power consumption, as shown in Figure 5: a larger temperature difference causes a higher unit power consumption. Figure 9 reveals that the temperature difference in the cryogenic temperature location (location 4) increases markedly with nitrogen content. This increase occurs because the liquefaction temperature of the CBM falls significantly as its nitrogen content increases, resulting in greater difficulty for the refrigerant to cool the CBM. As a result, the average temperature difference and the unit power consumption increase with the nitrogen content of the CBM feed gas.

Figure 6 shows that the C3/MRC process has the best ability to match the hot and cold composite curves, the MRC process has poorer matching, and the C3/NEC process is the worst. Figure 7 shows that it is not clear if higher nitrogen content in the CBM feed gas causes poorer hot and cold composite curves matches. This may be observed more clearly in the latter analysis of the logarithmic mean temperature difference (LMTD). Figure 6 also shows that the heat load is higher for the mixed refrigerant than for the nitrogen refrigerant and is higher for a process without propane precooling than with propane precooling. The former behavior occurs because the heat exchange process is more complex and more refrigerant requires self-cooling in the mixed refrigerant cycle than in the nitrogen refrigerant cycle. The latter behavior occurs because the propane refrigeration cycle is more efficient in the high temperature region and the total heat load is reduced for a process with propane precooling. For a clearer demonstration and analysis, Figures 8 and 9 show the logarithmic mean temperature difference (LMTD) for different positions (each heat exchanger in the process) and the average value when the different processes and nitrogen contents are considered, respectively. To be more clear, Area 1 in Figure 8 refers to HEX-101 in Figures 1−4, Area 2 refers to HEX-102 in Figures 1−4, Area 3 refers to HEX-103 in Figures 1−3, and Area 4 refers to HEX-104 in Figures 1 and 3. It should be pointed out that in locations 1−4, quantitative E


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Industrial & Engineering Chemistry Research 4.2. Composite Curves Using the Carnot Factor Basis. The hot and cold composite curves plotted as Carnot factor versus heat flow rate are further studied to identify the irreversibility existing within the LNG heat exchanger system. The Carnot factor for refrigeration can be expressed as eq 2: Ω=

T T0 − T

and refrigerant streams within the process, additional to the feed gas-cooling duty. Figure 10 shows the heat load comparison for each process more clearly: the heat load is greater for the mixed refrigerant than for the nitrogen refrigerant and is greater for a process without propane precooling than a process with propane precooling. The cold composite curve, as shown in Figure 11, reflects the irreversibility existing within the LNG heat exchanger system and indicates the exergy loss in each process. It is clear that a process using mixed refrigerant has more exergy loss than a process using nitrogen refrigerant. This occurs because, in the MRC and C3/MRC processes, the cold refrigerant stream must remove all of the heat through the heat exchange network, whereas for the NEC and C3/NEC processes, the expanders remove a substantial amount of energy below ambient temperature. 4.3. Exergy Loss Comparison. For more accurate comparison, the exergy loss rate for the heat exchanger system of each process at different nitrogen contents was further calculated and is shown in Figure 12. It should be emphasized again that the exergy losses here are strictly related to heat transfer irreversibilities, not the whole process.

(2)

where subscript “0” can normally be considered as the state of environment, for which 40 °C and 101.325 kPa are adopted here. Taking the case where the nitrogen content is 30% as an example, the hot and cold composite curves plotted as Carnot factor versus heat flow rate for the various processes are shown in Figures 10 and 11.

Figure 10. Hot composite curves for all processes and the feed gas plotted as Carnot factor versus heat flow rate.

Figure 12. Exergy loss rate for the heat exchanger system of each process at different nitrogen contents.

The exergy loss rate, Ė x,l, can be expressed as ̇ = Ex,in ̇ − Ex,out ̇ Ex,l where ̇ + Ex,Q ̇ + Eẇ Eẋ = Ex,H

(3)

(4)

̇ = q ̇ [h − h0 − T0(s − s0)] Ex,H n

(5)

̇ = Q̇ − q ̇ T0(s1 − s2) Ex,Q n

(6)

Eẇ = W

(7)

where molar flow rate, q̇n (kmol/s), molar enthalpy, h (kJ/ kmol), molar entropy, s (kJ/(kmol·°C)), heat flow rate, Q̇ (kJ/ s), and power consumption, W (kW), can all be obtained from the HYSYS program. Figure 12 shows that the total exergy loss in the heat exchanger systems reduces with increasing nitrogen content. This result is consistent with the comparison of heat load discussed above. However, the exergy loss in the heat exchanger systems per unit LNG product, which is converted to be related

Figure 11. Cold composite curves for all processes and the feed gas plotted as Carnot factor versus heat flow rate.

In Figures 10 and 11, the area between the horizontal axis and the feed curve represents the absolute minimum reversible work required to cool the feed stream to the product LNG. In Figure 10, different slopes of the hot composite curves, relative to the feed curve, represent the different heat capacity flow rates associated with the self-cooling duty of the recycle F


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Industrial & Engineering Chemistry Research to standard volume flow, should increase with the nitrogen content, because of the significant decline in LNG production, as shown in Figure 13.

sibility is greater for a process without propane precooling than one with propane precooling. It is also found that unit power consumption increases markedly with nitrogen content, mainly because of the low heat exchange efficiency caused by the extremely low liquefaction temperature when the nitrogen content is high. However, the irreversibility in the heat exchanger systems decreases with nitrogen content because of the lower specific heat capacity and the lower latent heat of vaporization for nitrogen than for methane. This investigation into the heat exchange irreversibility of CBM liquefaction processes provides a direction for improvement of liquefaction system performance and a reference for the selection of liquefaction processes for CBMs with different nitrogen content.

■

Figure 13. Unit exergy loss rate for the heat exchanger system of each process at different nitrogen contents.

AUTHOR INFORMATION

Corresponding Author

*Wensheng Lin. E-mail: [email protected].

Furthermore, this indicates that although processes using mixed compositions as the refrigerant have higher energy efficiency than those where nitrogen is used and also have higher exergy efficiency for the whole system because of lower exergy loss in the compressor, the exergy efficiency for the heat exchanger system is lower. As a result, mixed refrigerant processes may have more room for further improvement in the heat exchanger systems. However, adding propane cooling in the high temperature region can efficiently reduce exergy loss in the heat exchanger system and improve the energy efficiency to some degree. However, for the nitrogen expansion process, propane precooling is less efficient when the nitrogen content is higher because of the large temperature difference needed for the propane phase change process.

ORCID

Wensheng Lin: 0000-0003-0921-9810 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the support of China’s National High Technology Research and Development Program (863 Program) (No. 2006AA06Z234).

■

5. CONCLUSIONS Four liquefaction processes suitable for a CBM liquefaction plant are analyzed in this paper. These include a mixed refrigerant cycle (MRC), a two-stage nitrogen expansion cycle (NEC), a mixed refrigerant cycle with propane precooling (C3/ MRC), and a two-stage nitrogen expansion cycle with propane precooling (C3/NEC). The effects of nitrogen content on liquefaction process energy efficiency and the performance of the heat exchanger systems are evaluated. The energy efficiencies are compared for the different processes with different CBM nitrogen contents. The matching of the hot and cold composite curves plotted as temperature versus heat flow rate and plotted as Carnot factor versus heat flow rate are each investigated, and the exergy loss rates of the heat exchanger systems are calculated and compared. It is revealed that energy efficiency is higher for processes using mixed refrigerant than those using nitrogen and is higher for processes with propane precooling than processes without propane precooling. The exception is the NEC process, which has a higher energy efficiency than that of the C3/NEC process when the nitrogen content of the CBM is high. That means, from the energy point of view, mixed refrigerant and precooling are usually preferable for CBM liquefaction. Conversely, the irreversibility in the heat exchanger system is higher for the mixed refrigerant than for the nitrogen refrigerant, because of the higher heat load of the MRC processes. That means, for the preferable MRC processes, further optimization of heat exchanger design is of great significance for reducing exergy loss. However, the irrever-

NOMENCLATURE Ex = exergy rate, kW or kJ h−1 Ex,I = exergy loss rate, kW or kJ h−1 Ex,H = enthalpy exergy rate, kW or kJ h−1 Ex,Q = heat exergy rate, kW or kJ h−1 Ew = work exergy rate, kW or kJ h−1 h = molar enthalpy, kJ kmol−1 p = pressure, kPa or MPa Q = heat flow rate, kW or kJ h−1 qn = molar flow rate, kmol h−1 s = molar entropy, kJ kmol−1 °C1− t = temperature, °C W = power consumption, kW w = unit product liquefaction power consumption, kWh Nm3−

Greek Symbols

Ω = Carnot factor for refrigeration

Subscripts

■

0 = state of environment com = compressor exp = expander

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Analysis of Heat Transfer Irreversibility in Processes for Liquefaction of

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