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Review on the design and optimization of natural gas liquefaction processes for onshore and offshore applications. Tianbiao He , Iftekhar A. Karimi , ...
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Design and Optimization of a Novel Mixed Refrigerant Cycle Integrated with NGL Recovery Process for Small-Scale LNG Plant Tianbiao He and Yonglin Ju* Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: In this study, a novel mixed refrigerant cycle (MRC) integrated with NGL recovery process for small-scale LNG plant is proposed and optimized. The proposed process can be used to produce LNG and NGL with low energy consumption. Genetic algorithm is chosen as the optimization method for the proposed MRC-NGL process. The unit energy consumption as an objective function is optimized with key parameters. The optimization results show that the unit energy consumption and the molar flow rate of the mixed refrigerants can be reduced by 9.64% and 11.68%, respectively, compared with that of the base case. The effects of several key parameters on the process performance are also investigated and discussed based on the optimization results. Furthermore, the exergy analyses of the main equipment are also presented and analyzed. The economic analysis also shows that the proposed process has a good profitability and a short payback period.

1. INTRODUCTION Global energy demand will rise by an average of 1.2% per year.1 Meanwhile, the worldwide requirement for reducing CO2 emissions is an important task for the governments. In order to fulfill the increasing energy demand and reduce CO2 emission, it is necessary to find an alternative energy to replace oil and coal.2 Natural gas is a clean fossil fuel which is widely used and can reduce the greenhouse emissions.3 The compositions of natural gas are mainly methane with a few of ethane, propane, butane, and other heavy hydrocarbons as well as water, nitrogen, and CO2. It is well-known that gas pipelines and liquefied natural gas (LNG) are the two main methods to transport natural gas. Natural gas can be liquefied when its temperature is below −161 °C at 101.325 kPa, and its volume reduces by a factor of more than 600.4 LNG is the cleanest form of natural gas and the preferred method for long distance transportation. Three main types of refrigeration cycle are used in LNG liquefaction process, namely cascade, mixed-refrigerant, and expander cycle.5 Mixed-refrigerant cycle liquefaction process is nowadays widely used due to its less required amount of equipment.6 The MRC process and its modified processes have been studied by many researchers during the past decade. Mortazavi et al.7 conducted a detailed study on the energy performance enhancement of propane precooled mixed refrigerant LNG plant. They utilized gas expanders, two phase expanders, and liquid turbines to replace the expansion valves. The results showed that the compressor power reduction, expansion work recovery, and LNG production increase can be achieved as much as 2.68 MW, 3.82 MW, and 1.24%, respectively. Hatcher et al.8 presented a systematic analysis of optimization formulation for mixed-refrigerant LNG process. Lee et al.9 proposed a novel method for the selection of refrigerant compositions based on combination of NLP and thermodynamic approach. A synthesis tool for the complete design of MR system was developed. Morin et al.10 used the evolutionary search to optimize the energy consumption in PRICO natural © 2014 American Chemical Society

gas liquefaction. The results showed that this method had a good match with the literatures. It also indicated that the method could be applied to any other gas liquefaction process. Wang et al.11 performed a thermodynamic-analysis-based study of the minimization of the energy consumption of C3MR liquefaction process. Xu et al.12 presented a potential solution to the correlation between mixed refrigerant composition and ambient conditions in the PRICO process by using genetic algorithm. Alabdulkarem et al.13 also selected genetic algorithm to reach a global optimum of propane precooled mixed refrigerant LNG plant. NGL recovery process can be integrated with LNG liquefaction process to improve the economic benefit of the LNG plant. Ghorbani et al.14 combined pinch and exergy analysis to optimize refrigeration cycle in NGL recovery plant. Mehrpooya et al.15 proposed a novel process configuration for recovery NGL from natural gas with a self-refrigeration system. Vatani et al.16 presented a novel process configuration for coproduction of NGL and LNG with low energy requirement. Although the paper introduced some good ideas in the integration of NGL and LNG processes, the process was quite complicated, and it would not be suitable for small-scale NGL and LNG plant. In summary, the previous published literatures have conducted many discussions to optimize the mixed refrigerant LNG plant and NGL recovery process. Only a few works focused on the process in the combination of the natural gas liquefaction process with NGL recovery process. In this paper, a novel mixed refrigerant liquefaction process integrated with NGL recovery process (MRC-NGL process) for small-scale LNG plant is proposed and designed. Unit energy consumption of the integrated process is selected as the objective function for Received: Revised: Accepted: Published: 5545

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Figure 1. Flow diagram of the novel MRC cycle integrated with NGL recovery process.

S-2 is cooled by the HEX-1 and HEX-2, successively. Then, the vapor pressure is reduced by the throttling valve (V-2). The cold vapor refrigerant first provides the cold energy to the condenser of the demethanizer and then to the HEX-2. The liquid from the S-2 is cooled by the HEX-1 and reduces its pressure by the throttling valve (V-3). In the second mixer (MIX-2), the liquid refrigerant is mixed with the vapor refrigerant. The cold mixed refrigerant provides the cold energy to the condenser of the deethanizer and then to the HEX-1. 2.2. Process Parameters. The pressure PNG of the feed natural gas is set at 3 MPa, the temperature tNG is 35 °C, and the molar flow rate qNG is 92.95 kmol/h. The mole fraction of components of the natural gas and other given parameters are shown in Table 1. The Peng−Robinson equation is applied to calculate the thermophysical parameters in the process.

the process optimization. The compositions of mixed refrigerant, pressures of mixed refrigerant, inlet temperature of demethanizer, and inlet pressure of deethanizer are optimized simultaneously with genetic algorithm to reach the minimum unit energy consumption.

2. PROCESS DESIGN 2.1. MRC Liquefaction Process Integrated with NGL Recovery Process. The novel MRC-NGL process for smallscale LNG plant is illustrated in Figure 1. Natural gas is first cooled by the multistream heat exchanger (HEX-1) to approximately −60 °C. Then the cold natural gas goes into the demethanizer to separate methane from NGL. The methane-rich gas from the condenser of the demethanzier passes through the multistream heat exchanger (HEX-2) and is liquefied when its temperature decreases to −150 °C. The pressure of the liquefied natural gas is reduced to the LNG storage pressure by the throttling valve (V-2). After that the liquefied natural gas goes into the third vapor−liquid separator (S-3). BOG from S-3 is led into HEX-1 and HEX-2 to recover its cold energy. The liquid from the reboiler of the demethanizer first reduces its pressure and then goes into the deethanizer. NGL can be obtained from the reboiler of the deethanizer. Moreover, ethane can be also recovered from the condenser of the deethanizer. In the mixed refrigerant cycle, the mixed refrigerant is pressurized by the first-stage compressor (MRC-1). The hot pressurized mixed refrigerant flowes through the reboiler of the deethanizer to provide heat source. After that, it is cooled by water cooler (WC-1) and some components are condensed at the same time. In the first vapor−liquid separator (S-1), the vapor refrigerant, and the condensate are separated. The vapor refrigerant is pressurized by the second stage compressor (MRC-2) and provides hot energy for the reboiler of the demethanzier, then is cooled by the water cooler (WC-2). The condensate is first compressed by the pump (MRP-1) and then cooled by the water cooler (WC-3). In the first mixer (MIX-1), the vapor refrigerant and condensate are mixed together. As the mixed refrigerant passes through the second separator (S-2), it is separated into the vapor and liquid again. The vapor from the

3. PROCESS OPTIMIZATION 3.1. Genetic Algorithm. Genetic algorithm (GA) is a search heuristic that mimics the process of natural selection. This heuristic is routinely used to generate useful solutions to optimization problems.17 The genetic algorithm begins with a population which is generated by the initial function randomly. Then, it searches the global solution for the optimization objective function with the techniques such as mutation, selection, and crossover.18 The design of LNG plant is a highly nonlinear problem with many local optima. Genetic algorithm is a global optimization method which is applied in the optimization of LNG plant.19 In this study, the proposed process is simulated by Aspen HYSYS.20 Genetic algorithm in Matlab21 is selected as the optimization method for the process. Matlab can interface with Aspen HYSYS by using HYSYS COM server. Then, the Matlab code can read the HYSYS simulation variable and optimize them with GA. The framework of the process optimization with genetic algorithm is illustrated in Figure 2. 3.2. Optimization Objective Function. In the proposed MRC-NGL process, several key parameters play dominant roles in affecting the process performance. Hence, these key parameters should be optimized, such as the molar flow rate 5546

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medium pressure of mixed refrigerant P203, the high pressure of mixed refrigerant P207, the inlet temperature of demethanizer t101, and the inlet pressure of deethanizer P104. Unit energy consumption w (kWh/Nm3) is the rate of total energy consumption in the process to the volume flow rate of the productions. It is a major index for the evaluation of different liquefaction process. As a result, it is usually regarded as the objective function of the process optimization. It can be defined as the following equations:

Table 1. Mole Fraction of Components of the Feed Natural Gas and Other Parameters in the Process mole fraction components CH4 C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 C6∼C9 N2 total param. temp. pressure flow rate pressure drop in heat changer pressure drop in water cooler LNG storage pressure temperature after water cooler ambient temperature adiabatic efficiency of compressor adiabatic efficiency of pump tray number of demethanizer tray number of deethanizer

0.8187 0.0611 0.0650 0.0085 0.0196 0.0042 0.0048 0.0051 0.0130 1 value

Wcom =w + q105 + q106

(1)

X = [fC1 , fC 2 , fC 3 , fC 5 , fN 2 , P219 , P203 , P207 , t101 , P104]

(2)

f (X )min =

qln G

where Wcom is the sum of compressors’ and pump’s power, which also contains the power consumption of condensers in demethanizer and deethanizer. qLNG, q105, and q106 are the standard volume flow rate of LNG, 105 and 106 respectively. 3.3. Constraints and Penalty Function. During the optimization of the MRC-NGL process, there are several constraints to make the process operate safely and stably. The constraints can be described as follows: (1). The suction temperature of the first stage mixed refrigerant compressor (MRC-1) should be superheated at least 2 °C.

35 °C 3 MPa 92.95 kmol/h 0 kPa 0 kPa 200 kPa 40 °C 20 °C 80% 75% 20 20

t 202 − tdew,202 > 2

(3)

(2). The minimum temperature approaches in the HEX-1 and HEX-2 should be larger than 2 °C. Δtmin > 2

(4)

(3). The temperatures of the cold mixed refrigerant should be lower at least 5 °C than the temperatures of the condensers of the demethanizer and deethanizer. t 219 − tdeme‐con > 5

(5)

t 223 − tdeet‐con > 5

(6)

(4). The temperatures of the hot mixed refrigerant should be higher at least 5 °C than the temperatures of the reboilers of the demethanizer and deethanizer. t 207 − tdeme‐reb > 5

(7)

t 203 − tdeet‐reb > 5

(8)

Penalty function is used when the constraints are not satisfied. The penalty function can be shown as follows: p(X ) = f (X ) e[1 + g(X )]

(9)

g (X ) = max[(2 + tdew,202 − t 202), (2 − Δtmin), (5 + tdeme‐con − t 219), (5 + tdeet‐con − t 223), (5 + tdeme‐reb − t 207), (5 + tdeet‐reb − t 203)]

Figure 2. Framework of the process optimization with genetic algorithm.

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3.4. Optimization Results. The number of design variables for the MRC-NGL process is 10. The genetic algorithm search convergence curve is shown in Figure 3. It is obvious that the best solution can be found at the about 50th generation. It means that genetic algorithm can get the optimal results in a quite short time.

of the components of mixed refrigerant (methane for f C1, ethylene for f C2, propane for f C3, isopentane for f C5 and nitrogen for f N2), the low pressure of mixed refrigerant P219, the 5547

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Table 4. Process Performances of the Optimized Case and the Base Case

Figure 3. Genetic algorithm convergence curve.

The optimization results of the design variables are listed in Table 2. The mixed refrigerant compositions based on the

variable

lower bound

upper bound

optimal value

base value

1 2 3 4 5 6 7 8 9 10

f C1 (kmol/h) f C2 (kmol/h) f C3 (kmol/h) f C5 (kmol/h) f N2 (kmol/h) P219 (kPa) P203 (kPa) P207 (kPa) t101 (°C) P104 (kPa)

75.00 70.00 40.00 45.00 45.00 900.00 2700.00 200.00 −63.00 1650.00

95.00 90.00 60.00 65.00 65.00 1000.00 3200.00 300.00 −57.00 1850.00

84.22 77.84 51.23 58.66 54.34 980.90 3024.00 253.40 −60.20 1780.00

90.45 79.94 59.39 78.35 56.78 961.30 3000.00 242.80 −58.00 1820.00

optimized case

base case

0.3710 2.173 2.015 3.909 7.426 424.1 319.0 0.9453 59.68 76.24 36.64 46.96 96.32 1655.81 124.45 225.12

0.4106 2.637 2.328 5.909 7.932 479.2 342.0 2.609 62.97 74.65 36.67 47.63 96.32 1655.81 124.45 225.12

are a little smaller than those in the base case. Log mean temperature difference (LMTD) in the multistream heat exchangers are 3.909 and 7.426 °C, respectively, for the optimized case, while those are 5.909 and 7.932 °C for base case. The hot and cold composite curves in the multistream heat exchangers are shown in Figure 4. From Figure 4 and LMTD values, we can see that the cold composite curve is

Table 2. Optimization Results and Its Lower and Upper Bound no.

name unit energy consumption (kWh/Nm3) min. temp. approach in HEX-1 (°C) min. temp. approach in HEX-2 (°C) LMTD in HEX-1 (°C) LMTD in HEX-2 (°C) power of MRC-1 (kW) power of MRC-2 (kW) power of MRP-1 (kW) power of demethanizercondenser (kW) power of demethanizerreboiler (kW) power of deethanizercondenser (kW) power of deethanizerreboiler (kW) liquefaction rate (%) qLNG (Nm3/h) q105 (Nm3/h) q106 (Nm3/h)

Table 3. Mixed Refrigerant Compositions and the Flow Rate of the Optimized Case and the Base Case name

optimized case

base case

flow rate (kmol/h) methane (mol %) ethylene (mol %) propane (mol %) isopentane (mol %) nitrogen (mol %)

326.29 25.81 23.85 15.70 17.98 16.65

364.91 24.79 21.91 16.28 21.47 15.56

optimization results and the base case are shown in Table 3. The optimized MRC-NGL process has a mixed refrigerant flow rate of 326.29 kmol/h, which is 11.68% less than that in base case. The mole fractions of the methane, ethylene, and nitrogen increase while those of the propane and isopentane decrease, as given in Table 3. The process performances of the optimized case and the base case are illustrated in Table 4. The unit energy consumption of the optimized case is 0.371 kWh/Nm3, which is 9.64% lower than that of the base case. It means that genetic algorithm is an efficient optimization method for the MRC-NGL process. The minimum temperature approaches of the HEX-1 and HEX-2 in the optimized case are 2.173 and 2.105 °C, respectively, which

Figure 4. Hot and cold composite curves in the multistream heat exchangers (a) optimized case, (b) base case. 5548

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The effect of the ethylene molar flow rate on the process performance is shown in Figure 6. With the increase of the

much closer to the hot composite curve in the optimized case, which results in less exergy losses in heat exchangers. The heat transfer areas in optimized case will be larger than that in base case due to reduced approach temperature. The heat transfer area in optimized case is 2781.61 m2 compared with 2453.67 m2 in base case due to reduced approach temperatures in heat exchangers. Thus, the capital investment of heat exchangers in optimized case is 13.36% higher than that in base case. However, the capital investments of compressors and other equipment in optimized case are less than that in base case due to reduced mixed refrigerant flow rate. If the operation cost and capital investment are considered simultaneously, the optimized case is more economic than the base case.

4. EFFECTS OF KEY PARAMETERS ON THE PROCESS PERFORMANCE 4.1. Effect of Mixed Refrigerant Flow Rate on the Process Performance. The flow rate of the mixed refrigerant has a great influence on the process performance in the MRCNGL process. In this section, the effects of the flow rate for five different refrigerants on the process performance are investigated based on the optimal values, as shown in Figure 5−9. Only one refrigerant flow rate is changed once a time and the flow rates for other refrigerants stay at optimal values.

Figure 6. Effect of ethylene molar flow rate on the process performance.

Figure 5. Effect of the methane molar flow rate on the process performance.

ethylene molar flow rate, both the unit energy consumption and the minimum temperature approach in the HEX-1 increase. However, the minimum temperature approach in the HEX-2 increases first and then decreases to the lowest value. After that, it increases and then decreases at last. The reason is that the ethylene provides cold energy for the HEX-2. As the phenomenon shown in Figure 5, the increase of the ethylene molar flow rate will decrease the mole fractions of the methane and nitrogen in the mixed refrigerant. When the molar fractions of the methane and nitrogen are out of the requirement ranges, the cold energy in the HEX-2 is not enough to cool down the hot streams. Thus, the ethylene molar flow rate should be avoided from 75 to 76.5 kmol/h when the process is designed. The effect of the propane molar flow rate on the process performance is given in Figure 7. The unit energy consumption increases with the increase of the propane molar flow rate. The minimum temperature approach in the HEX-1 increases first and then keeps at constant when the propane molar flow rate is larger than 54 kmol/h. However, the minimum temperature approach in the HEX-2 decreases with a small gradient with the increase of propane molar flow rate. The reason is that the

The effect of the methane molar flow rate on the process performance is illustrated in Figure 5. The unit energy consumption increases with the increase of the methane molar flow rate. Moreover, the minimum temperature approach in the HEX-1 increases with the increase of the methane molar flow rate. However, the minimum temperature approach in the HEX-2 decreases first and then increases with the increase of the methane molar flow rate. When the methane molar flow rate is approximately 80 kmol/h, the minimum temperature approach in the HEX-1 is −0.5 °C. The reason is that only the increase of the methane molar flow rate can increase the mole fraction of the methane in the mixed refrigerant. However, it also decreases the mole fraction of other refrigerants in the mixed refrigerant, especially the nitrogen and ethylene. Since the methane, ethylene, and nitrogen provide refrigeration for the HEX-2, the molar fractions of the mixed refrigerant can lead to insufficient cold energy in the HEX-2, which results in a minus minimum temperature approach in the HEX-2.

Figure 7. Effect of the propane molar flow rate on the process performance. 5549

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molar flow rate. With the increase of nitrogen molar flow rate, the minimum temperature approach in the HEX-1 decreases with a small gradient. However, the minimum temperature approach in the HEX-2 increases first and then keeps at constant. The reason is that the nitrogen has the lowest boiling temperature in the mixed refrigerant. It mainly provides cold energy for HEX-2. As a result, the nitrogen molar flow rate has great effect on the minimum temperature approach in the HEX-2, while it has very small effect on the minimum temperature approach in the HEX-1. 4.2. Effect of the Inlet Temperature of the Demethanizer t101 on the Process Performance. The inlet temperature of the demethanizer t101 will affect the powers of the reboiler and condenser in the demethanizer. As shown in Figure 10, the power of the reboiler decreases with the increase

propane mainly provides the refrigeration for the HEX-1 and has little influence on the HEX-2. The effect of the isopentane molar flow rate on the process performance is presented in Figure 8. The unit energy

Figure 8. Effect of the isopentane molar flow rate on the process performance.

consumption increases with the increase of the isopentane molar flow rate. The minimum temperature approach in the HEX-1 increases first and then decreases with the increase of the isopentane molar flow rate. What’s more, the minimum temperature approach in the HEX-2 keeps at constant first and then decreases with the increase of the isopentane molar flow rate. The reason is that the isopentane has the highest boiling temperature in the mixed refrigerant. Thus, the isopentane molar flow rate has a great effect on the HEX-1. The total molar flow rate of the mixed refrigerant increases with the increase of the isopentane molar flow rate. Consequently, the minimum temperature approach in the HEX-1 increases first. However, as the isopentane molar flow rate increases too much, the mole fraction of the other refrigerants decreases quickly, which results in a low refrigeration capacity at low temperature range. As a result, the minimum temperature approaches in both the HEX-1 and HEX-2 decrease. The effect of the nitrogen molar flow rate on the process performance is illustrated in Figure 9. The unit energy consumption increases with the increase of the nitrogen

Figure 10. Effect of the inlet temperature of the demethanizer t101 on the process performance.

of t101, while the power of the condenser in the demethanizer increases with the increase of t 101 . The unit energy consumption decreases first and then increases with the increase of t101. The energy requirement in the HEX-1 will decrease with the increase of t101. It can reduce the unit energy consumption of the MRC-NGL process. However, the increase of the power of the condenser leads to the increase of the unit energy consumption of the MRC-NGL process. As a result, it should be an optimal value of t101 to achieve the lowest unit energy consumption. It means that the inlet temperature of the demethanizer has a great influence on the process performance. 4.3. Effect of the Inlet Pressure of the Deethanizer P104 on the Process Performance. The inlet pressure of the deethanzier P104 will influence the powers of the reboiler and condenser in the deethanizer. The effect of the inlet pressure of the deethanizer P104 on the process performance is illustrated in Figure 11. The power of the reboiler increases by a high gradient with the increase of P104, while the power of the condenser increases by a small value with the increase of P104. The unit energy consumption almost keeps at constant with the increase of P104. The reason is that the inlet temperature of the deethanzier will increase by a small value with the increase of P104, which can result in the small increasing value of the power of the condenser. Consequently, the unit energy consumption almost keep at constant. In conclusion, the inlet pressure of the deethanizer has a small effect on the process performance.

Figure 9. Effect of the nitrogen molar flow rate on the process performance. 5550

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19.66% to the total exergy losses. It can be reduced by matching the hot and cold composite curves better. What’s more, the exergy losses of the demethanizer and the deethanizer are 20.92% and 4.12%, respectively. The exergy losses of the demethanizer is much bigger than that of the deethanizer. The reason is that the demethanizer operates at higher pressure and lower temperature, which are far from the ambient conditions. The exergy losses in compressing system (including water coolers and compressors) could be reduced by decreasing the outlet temperature of the compressor. so it needs to improve the adiabatic efficiency of the compressors. With the increase of adiabatic efficiency of the compressors, the outlet temperature of the compressors will decrease and the shaft work will also be reduced. Moreover, the heat load in water coolers can be reduced, which can result in less exergy losses. On the other hand, if we can use the heat of high temperature gas coming out from the compressors, the exergy losses of water coolers will also be reduced. Thus, heat exchange network integration should be applied in LNG process in future researches. It is necessary to keep hot streams or cold streams at the same temperature in multistream heat exchangers to reduce the exergy losses. As a result, it should make effort to reduce exergy losses both on parameter optimization and equipment improvement.

Figure 11. Effect of the inlet pressure of the deethanizer P104 on the process performance.

5. EXERGY ANALYSIS Exergy is the maximum theoretical work obtainable from an overall system consisting of a system and the environment as the system comes into equilibrium with the environment.22 Exergy analysis is a useful method in the design, optimization, and performance evaluation of energy systems and processes.23 The equipment in the MRC-NGL process can generate exergy losses, including compressors, pump, water coolers, multistream heat exchangers, valves, demethanizer, and deethanizer. Exergy analysis is applied to these equipment to evaluate their exergy losses. The exergy losses can be defined as following equation:24 ΔEx = (H − T0S)state2 − (H − T0S)state1

6. EOCNOMIC ANALYSIS The proposed MRC-NGL process can produce LNG, NGL, and ethane simultaneously by using single refrigeration cycle. The process has the advantages of simplicity, low energy consumption, and good economic benefit. The economic analysis is adopted to the MRC-NGL process to demonstrate the initial investment and the payback period. The results of the economic analyses for MRC-NGL and MRC processes are shown in Table 5.The operation time is assumed as 330 days per year. In general, the building period for these systems is 1 year without any profit. For MRC-NGL process, the production capacities of the LNG, NGL and ethane are 9464.4 ton/a, 4159.6 ton/a, and 1323.4 ton/a, respectively. It is shown that the initial investment is about 39.60 million CNY and the net profit is around 16.72 million CNY. For MRC process, the production capacity of the LNG is 14811 ton/a. The initial investment is about 36.30 million CNY and the net profit is around 13.83 million CNY. Moreover, discounted cash flow method is applied to calculate the payback period for MRC-NGL process and MRC process. The payback periods of MRC-NGL process and MRC process are 3.84 years and 4.19 years, respectively. From the analyses above, the payback period of MRC-NGL process is shorter than that of MRC process. Moreover, the net profit of MRC-NGL process is also higher than that of MRC process. In conclusion, it means that the proposed MRC-NGL process demonstrates a good profitability and is considerable for real engineering project (actually, the MRC-NGL process is planning to be applied to a small-scale LNG plant in China).

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The exergy losses results of the MRC-NGL process are presented in Figure 12. The exergy losses of the water coolers

Figure 12. Exergy losses of main equipment in MRC-NGL process.

contribute around 24.73% exergy losses, which are the most significant contributor to the total exergy losses. The exergy losses of the compressors and pump are about 19.83%, which can be reduced by using more efficient compressor and pump. Besides, the exergy losses of the valves are much bigger than the normal MRC liquefaction processes. The reason is that there are four valves in the proposed MRC-NGL process. The exergy losses of the multistream heat exchangers also contribute about

7. CONCLUSIONS In this paper, a novel mixed refrigerant cycle (MRC) for LNG integrated with NGL recovery process is proposed and designed. Genetic algorithm is selected as the optimization method for the proposed MRC-NGL process. The unit energy consumption as an objective function is optimized with the molar flow rate of the mixed refrigerant, the pressures of the 5551

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Table 5. Economic Analysis Results of the MRC-NGL Process and MRC Process MRC-NGL Investment (million CNY)a equipment 36.00 others 3.60 total 39.60 operation cost (million CNY) natural gas (1 CNY/Nm3)b 16.50 electricity (0.8 CNY/kWh)c 4.89 payment for workersd 0.5 maintenance 0.20 others 3.20 total 25.29 Sales Income (million CNY/a) LNG (4000 CNY/ton)e 37.86 NGL (5000 CNY/ton)e 20.79 ethane (4500 CNY/ton)e 5.96 added-value tax (20% of the income) 12.92 income tax (25% of the profit) 9.68 net profit 16.72 payback period (years) 3.84

MRC 33.00 3.30 36.30 16.50 4.56 0.5 0.20 3.20 25.00 59.24 0.00 0.00 11.85 8.56 13.83 4.19

The initial equipment investment is estimated with the project which is in progress in China by using MRC-NGL process. bThe price is the average price of natural gas in China. The price of natural gas usually is lower the 1 CNY/Nm3. c The industrial electricity price is approximately 0.8 CNY/kWh in China. dThe salary of each workers is 50 000 CNY/a. The plant always needs 10 workers to work for it. e The prices of LNG, NGL, and ethane are the average prices in Chinese market.

■

ΔEx = exergy losses (kJ) f = flow rate (kmol/h) H = enthalpy (kJ) n = molar flow (kmol/h) P = pressure (kPa) q = volume flow rate (Nm3/h) S = entropy (kJ/K) T = temperature (K) t = temperature (°C) tdeet‑con = temperature of condenser in deethanzier (°C) tdeme‑con = temperature of condenser in demethanzier (°C) tdeet‑reb = temperature of reboiler in deethanizer (°C) tdeme‑reb = temperature of reboiler in demethanizer (°C) Δtmin = minimum temperature approach (°C) tdew,202 = dew point temperature of 202 (°C)

REFERENCES

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mixed refrigerant, the inlet temperature of the demethanizer, and the inlet pressure of the deethanizer. The optimization results show that the unit energy consumption can be reduced by approximately 9.64% compared with the base case. In addition, the effects of several key parameters on the process performance are also investigated and discussed according to the optimization results. Furthermore, the exergy analyses of the main equipment are also presented and analyzed. The economic analyses show that the proposed process demonstrates a good profitability and a short payback period. In conclusion, the proposed MRC-NGL process can be used to coproduce LNG, NGL, and ethane with low energy consumption.

ASSOCIATED CONTENT

S Supporting Information *

Comparison between Peng−Robinson equation and GERG2008, the tuning parameters of genetic algorithm, and the products of the optimized case and base case. This material is available free of charge via the Internet at http://pubs.acs.org.

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ABBREVIATION a = annual CNY = China Yuan HEX = heat exchanger LMTD = logarithmic mean temperature difference LNG = liquefied natural gas LPG = liquefied petroleum gas MIX = mixer MRC = mixed refrigerant cycle NG = natural gas NGL = natural gas liquid WC = water cooler

Nomenclature

a

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 21 34206532. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is partly supported by SJTU-ENN LNG Engineering Center. 5552

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