Ting He,Wensheng Lin

Institute of Refrigeration and Cryogenics,Shanghai Jiao Tong University,Shanghai 200240,China

Keywords:Natural gas liquefaction Ethane Dual mixed refrigerant cycle Cryogenic distillation Refrigerant selection

ABSTRACT Recovery and purification of ethane has a significant impact on economic benefit improvement of the high-ethane content natural gas.However,current LNG-NGL integrated processes mainly focus on conventional natural gas,which are not applicable to natural gas with high ethane content.To fill this gap,three dual mixed refrigerant processes are proposed for simulation study of high-ethane content natural gas liquefaction.The proposed processes are optimized by a combination method of sequence optimization and genetic algorithm.Comparatively analysis is conducted to evaluate the three processes from the energetic and exergetic points of view.The results show that the power consumption of Process 3 which compressing natural gas after distillation is the lowest.For safety or other considerations,some common compositions of the mixed refrigerant may need to be removed under certain circumstances.Considering this,case studies of mixed refrigerant involving six composition combinations are carried out to investigate the effects of refrigerant selection on the process performance.

1.Introduction

Since natural gas produces much less pollutants and greenhouse gases than coal and oil,it helps to improve air quality in many counties such as China [1].Natural gas continues to play an important role as economically and environmentally sustainable energy and thanks to natural gas liquefaction,the world natural gas trade continues to increase.In 2019,the global liquefied natural gas (LNG) trade is 354.7 Mt,an increase of 40.9 Mt from 2018[2].The growth of LNG trade has benefited from the increase in world natural gas production,and unconventional natural gas such as shale gas has driven the growth of natural gas production in recent years.For example,supported by abundant shale gas supply and increasing liquefaction capacity,LNG export of the United States has experienced rapid growth[3].In addition,Canada,Algeria,China and other countries are also vigorously developing shale gas[4].The composition of shale gas,more specifically,the content of ethane in shale gas is different from that of conventional natural gas,which makes the liquefaction process suitable for shale gas different from conventional processes.As reported,the content of ethane in shale gas is significantly higher than that of conventional natural gas,up to 42% of methane content [5].

Because natural gas needs to be cooled to -162°C at atmospheric pressure for liquefaction,it is necessary to remove heavy hydrocarbons before liquefaction to prevent the freeze-out of heavy hydrocarbons in heat exchangers.Moreover,the high heavy hydrocarbon content in natural gas will make the calorific value of natural gas exceed the upper limit of the pipeline or export specifications.Due to the high economic value,the recovered heavy hydrocarbons,called as natural gas liquids (NGL),is often processed as an additional product to improve the project profitability[6,7].Since both natural gas liquefaction and NGL recovery consume cold energy [8],integration of these two processes can reduce the energy consumption of the natural gas industry chain even if this increases the complexity of the liquefaction system.Vatani et al.[9]proposed mixed refrigeration cycle based process for NGL recovery and natural liquefaction integrated process.The results showed the liquefaction efficiency of the energy consumption of the process was 0.414 kWh?kg-1LNG with ethane recovery rate higher than 93%.Ghorbani et al.[10]conducted in-depth researches on the integrated NGL recovery,LNG production and nitrogen rejection processes and built propane pre-cooling mixed refrigerant (C3MR) cycle,dual mixed refrigerant (DMR) cycle [11]and cascade refrigeration systems [12]to provide required refrigeration.Mehrpooya et al.[13]proposed three processes of C3MR,DMR and mixed fluid cascade (MFC) for the production of NGL and LNG,and the exergy efficiency of processes reached 55%,56%and 59%,respectively.Jin et al.[14]conducted an economic evaluation of integrated NGL and LNG processing for lean feed gas and they pointed out that the capital cost can be reduced significantly by simplifying the NGL recovery section without reducing the liquefaction efficiency apparently.Ghorbani et al.[15]utilized absorption refrigeration system to precool natural gas in an LNGNGL recovery process to reduce required energy consumption and provide the possibility of using waste thermal energy.They found that using absorption refrigeration systems to replace compression refrigeration system can realize reduction in the specific power consumption and capital cost by 38.94% and 31.9% respectively[16].Ansarinasab et al.[17]carried out an advanced exergoeconomic analysis to study two novel LNG and NGL co-production processes.The result show that by using DMR and MFC cycles to provide needed refrigeration,the exergy efficiency of the proposed processes can reach 43.66% and 53.83% respectively.Integrated schemes for production of LNG and NGLs recovery are presented and DMR liquefaction cycle serves as a base process to which separation process of NGLs is integrated.Uwitonze et al.[18]split the demethanizer into rectifying and stripping sections,thus,the light components in the natural gas liquid stream flashed out in the entire phase separator are stripped by the stripping column.Khan et al.[19]studied the energy saving of integrated NGL/LNG process by feed splitting to provide reboiler duty of the methane scrubber.

As summarized above,many scholars have studied and optimized the LNG-NGL integration processes.However,the above researches mainly consider the optimization of the refrigeration cycle when optimizing the process,and few researchers pay attention to the optimization of the distillation column.In fact,in the LNG-NGL process,the energy consumption of the distillation column has a considerable influence on the performance of the system.Ghorbani et al.[20]pointed out that demethanizer column has small exergoeconomic factor and big exergy destruction.Therefore,the operation temperature,pressure and tray number of the distillation column need to be optimized.Luo et al.[21]concluded that heat integration of distillation system can reduce the energy cost of the separation system.This paper will simultaneously consider the optimization of the parameters of the distillation column and the refrigeration cycle and achieve the reduction of the overall energy consumption of the system through energy integration.In addition,the existing processes are basically targeted at conventional natural gas,the ethane content in the feed gas involved and the recovery rate of ethane is generally low.Focusing on high ethane-containing natural gas,He et al.[22,23]proposed a new method that combines cryogenic distillation and natural gas liquefaction to co-produce LNG and high-purity ethane.The nitrogen expansion process and C3MR process are used to provide the required refrigeration.Both processes greatly improved the recovery rate of ethane compared with the existing processes.This article will continue to conduct research on the liquefaction system for natural gas with high ethane content,and consider compressing the natural gas after rectification again to explore the possibility of further reducing the energy consumption of the system.

Most LNG in the world is produced by a single refrigerant vapor compression refrigeration cycle,a mixed refrigerant vapor compression refrigeration cycle [24,25],a gas expansion refrigeration cycle [26],or a liquefaction process in which these cycles are cascaded in different ways[27,28].Among those processes,the C3MR process is currently the most widely used,occupying about 60%of the operational LNG capacity globally [2].However,for technical perfectionists,the zigzag temperature curve in the propane precooling part[29]is unacceptable.To solve this problem,companies such as APCI and Shell have proposed the DMR process [30].The pre-cooling section of C3MR process can only reach about-35°C,while that of DMR process can reach about -50°C.Moreover,under extreme cold conditions,the pre-cooling section of DMR can reach a lower temperature by adjusting the MR composition,so that the pre-cooling section has a greater significance for the entire process even when the ambient temperature is very low.In addition,compared with the C3MR process,the DMR process does not have a propane precooling cycle,so the amount of propane used is greatly reduced.For safety reasons,DMR is basically the only option for large floating LNG (FLNG) production and storage plant with capacity above 3 Mt per year.Although DMR process is mainly used in large installations,with the increasing emphasis on energy efficiency in the LNG industry,it has already been used in small scale LNG plants.For example,the DMR process developed by CNPC has been successfully applied in the projects of Shaanxi Ansai LNG (0.5 Mt?a-1) and Taian LNG(0.6 Mt?a-1) [2].In order to make the process highly energy efficient,this study will use the DMR process to provide cold energy for natural gas liquefaction and ethane recovery.

Khan et al.[31]used Box methodology and controlled elitist genetic algorithm to optimize the specific energy consumption and overall heat transfer coefficient.They pointed out that the direct intermixing of non-equilibrium streams in DMR process will cause thermodynamic irreversibility and efficiency reduction.Vikse et al.[32]used a nonsmooth flowsheeting methodology to optimize DMR process,which reduced the compression power consumption by 14.4% compared to the initially designed process.Study on evaluation of liquefaction processes applicable to offshore units is increasing with the requirement for offshore platforms.Jin et al.[33]investigated the efficiency and economic performance of N2expansion,SMR,DMR processes offshore LNG units.The processes are optimized by genetic algorithm (GA) and the results show that DMR process is the best option for long plant operation periods.Hwang et al.[34]used a hybrid optimization method that combine the GA and sequential quadratic programming (SQP) to determine the optimal operating conditions for the DMR process for the LNG FPSO topside liquefaction process.He et al.[35]designed a dynamic model to investigate the dynamic responses of DMR process for small-scale natural gas liquefaction plant.They pointed out that DMR process has good operation flexibility in handling some disturbances.Qyyum et al.[36]investigated the uncertainty levels of the total energy consumption and minimum internal temperature approach inside the heat exchangers when the operational variables of the DMR process vary.They found that the energy consumption was much more affected by the variable changes in the cold mixed refrigerant cycle than those in the warm mixed refrigerant cycle.

As reviewed,many researches have been conducted to optimize the energy consumption of DMR liquefaction processes.However,little attention has been paid to the selection of mixed refrigerant components and the comparative analysis of the DMR process structure.In the DMR process,components C1–C5 plus N2are generally used,but in some cases a certain component needs to be removed for safety or equipment simplicity.For example,the use of propane is artificially reduced or even eliminated for the offshore LNG process[37]and the removal of C5 to avoid configuring a gas–liquid separator in front of the secondary compressor.Since mixed refrigerant selection has a significant influence on the energy and exergy performance of the SMR process [38],it is necessary to carry out a process optimization study in the cases of different refrigerant combinations.In addition,the existing DMR processes are equipped with or without a gas–liquid separator in the cold mixed refrigerant cycle [39].As for the processes of NGL recovery,some consider compression after distillation,and some do not.There are few comparative studies on these processes with different structures.This paper will propose three DMR liquefaction processes suitable for high ethane-containing natural gas to coproduce LNG and high-purity ethane and optimize the processes using a combination of sequence optimization and GA.Substantial measures are adopted to improve the energy efficiency of the process,including compression of the methane from the distillation column,and the self-supply of the heat load in the reboiler of the distillation column is achieved by energy integration.In addition,the proposed processes are compared in energy and exergy performance to analyze the impact of the refrigerant separation in cold mixed refrigerant cycle and the recompression of natural gas after distillation mentioned above.

2.Process Simulation and Optimization

2.1.Simulation assumptions

The equation of state used in this study is the Peng-Robinson(PR) equation.It is assumed that the pressure of the feed gas is slightly higher than atmospheric pressure,which is 120 kPa.In order to facilitate our research on the characteristics of high ethane content,we assume that the feed gas contains only two components of methane and ethane and the content of ethane is 10%–40%.

To simplify the simulation process properly,set the assumptions as follows:

1.The fluids after compression are cooled to 40°C at the watercoolers.

2.The adiabatic efficiencies of the compressors and pumps are 85% and 75%,respectively.

3.The pressure drops in heat exchangers are ignored.

2.2.Process description

As demonstrated in Fig.1,the dual mixed refrigerant liquefaction processes for high-ethane content natural gas consist of three parts:the natural gas liquefaction-separation system,the mixed refrigerant precooling cycle and the mixed refrigerant deep cooling cycle.In Process 1,the details of the three parts are as follows:

Natural gas liquefaction-separation system:The pressurized feed natural gas is firstly cooled by heat exchangers HEX-101,H-101 and HEX-102 to become a gas–liquid mixture.Then it enters the distillation column to be separated as methane with purity of 99.8% and ethane of 99.5%.Next,the natural gas from the top of the distillation column enters heat exchanger HEX-103 for further cooling.Finally,the subcooled LNG is throttled by V-101 to 120 kPa and stored in LNG tank T-101.For the requirement of low pressure storage,the separated high purity ethane returns to exchangers HEX-101 and HEX-102 again to be subcooled.Similarly,it is throttled by V-201 to 120 kPa and stored in liquid ethane tank T-201.

Mixed refrigerant precooling cycle:After the first compression in compressor C-301,the mixed refrigerant 302 is cooled in WC-301.To ensure there is no liquid at the inlet of the second stage compressor C-302,the refrigerant from the water cooler first passes through a gas–liquid separator.The vapor and liquid from separator S-301 is pressurized by C-302 and P-301,respectively.Afterward,the mixed refrigerant 308 passes through water cooler WC-302 and heat exchanger HEX-101 to be cooled down.Finally,refrigerant 310 is throttled in valve V-301 to realize temperature reduction and then after providing cold energy in heat exchanger HEX-101 it returns to compressor C-301.

Mixed refrigerant deep cooling cycle:After two stage compression,the mixed refrigerant 405 passes through three heat exchangers (HEX-101-HEX-103) to be cooled down.Then,the cold refrigerant 409 from valve V-401 undergoes four heat exchangers(HEX-103,H-102,HEX-102,HEX-101) to release cold energy and finally returns to the compressors for recycling.

In Process 2,the mixed refrigerant deep cooling cycle is a bit different to that in Process 1.The refrigerant 406 from the outlet of heat exchanger HEX-101 is separated by gas–liquid separator S-401.After further cooling and throttling,the gas part and the liquid part provide two-stage cooling capacity.In Process 3,the mixed refrigerant cycles are the same as in Process 1.The only difference is that the natural gas from the distillation column is further compressed by C-103 to a higher pressure before it is liquefied.

2.3.Process optimization

In this study,the minimum specific power consumption (w) is adopted as objective function to optimize the parameters of the process,which is defined as follows:

Obviously,the composition and flow of refrigerant and the pressure and temperature of each node in the refrigeration cycle have a significant impact on the power consumption of the system.Since multiple parameters are involved,this study uses genetic algorithms to optimize the parameters of the refrigeration cycle.The parameters to be optimized and their upper and lower bound are shown in Table 1.And,for natural gas separation system,the main parameters that affect the energy consumption of the system are the distillation temperature,distillation pressure and the tray number of the distillation column.Considering that these three parameters will affect the convergence process of the distillation column,it is not appropriate to use genetic algorithms to optimize them.Therefore,they are optimized by the method of sequential optimization.The steps of the optimization process are shown in Fig.2.

In addition,in order to achieve the corresponding separation effect and heat exchange requirements,the optimization process should also meet the following conditions:

1.The LNG and liquid ethane is 100% liquefied.

2.Both the purity and the recovery rate of ethane are higher than 99.5%.

3.The minimum approach of each heat exchanger is not less than 3°C.

3.Results Analysis

3.1.Simulation results of process parameters

When ethane content is 20%,the optimized parameters of the liquefaction separation system is shown in Tables 2 and 3.The specific refrigerant combinations of precooling refrigeration cycle(P-c) and deep cooling refrigeration cycle (D-c) are shown in Table 4.In terms of natural gas liquefaction part,the parameters of Process 1 and Process 2 are exactly the same,but those of Process 3 are quite different from the previous two processes.In the pre-cooling cycles,the flow,pressure and other parameters of the three processes are not apparently different,but the refrigerant composition varies a lot.However,the parameters in the deep cooling cycles of the three processes show significant differences.

3.2.Effect of ethane content on power consumption

Fig.1.The diagram of the dual mixed refrigerant natural gas liquefaction integrated with ethane separation processes.C:compressor,D:distillation column,H:heat exchanger,HEX:multi-flow heat exchanger,P:pump,Q:heat flow,S:separator,T:tank,V:valve,W:work;WC:water cooler.

3.3.Heat transfer

Fig.1 (continued)

Table 1 Optimization variables

As we know,the heat transfer efficiency is one of the most vital factors that directly influence the power consumption of the natural gas liquefaction process.The larger the temperature difference between the cold and hot fluids,the more energy the refrigerant cycle consume.What’s more,the lower the temperature,the more energy is consumed for obtaining the same amount of refrigeration.So,if we can improve the heat transfer,especially in the low-temperature section,the energy efficiency of the system will be improved.Fig.4 illustrates the heat transfer curve of the whole temperature range.In the high temperature section,the heat exchange temperature difference of the three processes show similar trend.However,the temperature difference of the three processes vary in the low-temperature section.Since the natural gas from the top of the distillation column is directly liquefied in Process 1 and Process 2,there exists phase change in the natural gas liquefaction process.While liquefying,the temperature of the natural gas remains unchanged,which makes the temperature difference between the hot composite and the cold composite large.In Process 2,the maximum temperature difference of HEX-103 is greater than 15°C,and the heat transfer efficiency is obviously worse than that of Process 1 where the maximum heat exchange temperature difference of HEX-103 does not exceed 10°C.In Process 3,the phase change of natural gas is avoided by compressing the natural gas above the critical pressure of methane and the maximum temperature difference in HEX-103 does not exceed 5°C,making Process 3 the most energy efficient one.

To further study the heat transfer effect,Table 5 shows the temperature differences of the optimized processes.From the perspective of temperature difference,ΔTminof each heat exchanger in the three processes reaches 3°C or slightly higher than 3°C.But this does not mean that the heat transfer efficiency of the three processes is comparable,because the LTMD of the heat exchanger is the index reflecting the heat transfer effect.It can be seen that the LTMD of Process 3 is always small,especially in heat exchanger HEX-103,which shows that the heat transfer effect of Process 3 is the best.In addition,the LTMD of heat exchange HEX-102 in Process 1 and HEX-103 in Process 2 is large,indicating that the heat exchange effect is poor.

3.4.Exergy analysis

3.4.1.Exergy efficiency

In addition to specific power consumption,another important index to evaluate the performance of the liquefaction system is exergy efficiency,which is defined as the ratio of the ideal minimum work (including minimum liquefaction work and minimum separation work)to the net input power of the system.The specific form of the formula has been explained in the previous study[22],and according to the formula,the system efficiency of the three processes at different ethane contents is calculated and shown in Fig.5.The exergy efficiency of the system changes little with the increase of ethane content,and the exergy efficiency of Process 3 is the highest,basically maintained at about 55%.The efficiency of Process 2 is between 50.5% and 51%,and not surprisingly it is the lowest among the three processes.The difference in exergy efficiency between Process 3 and Process 2 is about 4.5%–5%.As an energy-intensive industry,even if the efficiency can be increased by only 1%,it means that the natural gas liquefaction plant can save megawatts of energy.Therefore,we can say that process 3 has obvious advantage in energy consumption compared to the other two processes.

Fig.2.Flow chart of process optimization.N represents the number of distillation pressure and temperature combinations.

Table 2 Optimized parameters of the natural gas side

3.4.2.Exergy destruction

The actual processes such as compression and heat transfer in the process are irreversible,which will inevitably lead to exergy destruction in the equipment.Analyzing the exergy destruction of each equipment helps guide the process optimization.When neglecting the potential and kinetic energies,the exergy of a stream is obtained as equation (2),and the exergy equilibrium equation for a steady-flow system is shown in equation (3).

Based on the above equations,the exergy destruction of the main equipment in the processes is calculated according to the characteristics of each physical process.In general,the exergydestruction of Process 2 in each device except the throttle valve is the largest of the three processes,while the exergy destruction of Process 3 in each device except the throttle valve is the smallest.This is because in Process 3,the pressure of natural gas before throttling is higher than that in Process 1 and Process 2.As shown in Fig.6,about 1/3 of the total of exergy destruction comes from the water coolers,which accounts for the largest proportion followed by the compressors.When the hot compressed fluid is cooled to near the ambient temperature in the water coolers,the exergy is dissipated into the cooling water.The lower the compressor efficiency,the larger the power energy converted into internal energy of the fluid,resulting in more energy being dissipated into the environment.Limited by the adiabatic efficiency of the compressor and the temperature difference between the compressed fluid and the cooling water,it is difficult to reduce the exergy destruction of these two parts through the optimization of the process parameters.The exergy destruction of heat exchangers in Process 3 is significantly larger than the other two processes,accounting for about 20% of the total,indicating that the heat transfer effect of the Process 3 is relatively poor.What’s more,with the increase of ethane,the exergy destruction in compressors and water coolers decreases,while that in distillation column increases,which also shows that the higher the ethane content,the more difficult the separation.

Table 3 Optimized parameters of the mixed refrigerant cycles

Table 4 Key parameters of the mixed refrigerant cycles

Fig.3.Specific power consumption of the optimized processes.

4.Analysis of Mixed Refrigerant Composition

4.1.Refrigerant selection

The key to achieve low energy consumption in the mixed refrigerant liquefaction process is a reasonable mixed refrigerant composition.In some specific cases,a certain component of mixed refrigeration is not welcome for safety or investment considerations.For example,it is encouraged to remove propane for safety consideration of a much more compact offshore conditions,and it is sometimes better to remove C5 to avoid configuring a gas–liquid separator in front of the secondary compressor.However,the energy consumptions with different refrigerant composition selections are different.Take Process 1 as an example,a case study of mixed refrigerants with six composition combinations is carried out to investigate the effects of compositions selection.By process optimization,the optimal operation parameters and the fraction of each component are obtained.Take the ethane content of 20%as an example,the specific refrigerant combinations in each case are shown in Table 6(‘‘—”represents that this component is not used).Case 0 is taken directly from the previous results of Process1,and it is used as the base case for energy consumption comparison without components adjustment.

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4.2.Comparison of specific power consumption and exergy efficiency

Figs.7 and 8 show the changes in the specific power consumption and exergy efficiency of the system with ethane content in each case.It is obvious that by optimizing the composition,the specific power consumptions of the system with various refrigerant combinations show the same trend with the ethane contentincrease.Compared with the basic process,the specific power consumption of each case increases slightly,but the increase is less than 2%.In particular,the elimination of propane in both the precooling cycle and the deep cooling one of Case 1 does not cause a significant increase in energy consumption,indicating this is a good alternative for offshore LNG plants.In addition,the exergy efficiency can be maintained at a high level and does not differ by more than 1% in all cases.This shows not only that the various refrigerant combinations can achieve efficient liquefaction,but also that the proposed DMR process has good adaptability.

Fig.4.Heat transfer curves of the processes.

Table 5 Temperature differences of the optimized processes

Fig.5.Exergy efficiencies of the optimized processes.

4.3.Analysis of temperature difference

Fig.6.Exergy destructions of the optimized processes.

Table 6 Mixed refrigerant component selection of the cases

Table 7 shows the temperature difference of the heat exchangers in each case.Clearly,the LTMD of the heat exchanger HEX-102 in case 0–4 is quite large,and only in case 5 it is relatively small.In case 5,replacing ethane with ethylene can make the heat transfer curve of the cold and hot fluid match even better,because the boiling point of ethylene is lower than that of ethane,which is closer to the middle of the boiling point temperature of methane and propane.It is worth mentioning that only reducing any one of C3,C4,and C5 has little effect on the LTMD of the heat exchange HEX-102.

5.Conclusions

Three dual mixed refrigerant processes for the liquefaction of high-ethane content natural gas are proposed in this paper,which produce LNG and high purity ethane simultaneously.The optimized processes are compared and analyzed from the aspects of energy consumption and exergy destruction.What’s more,innovative case study of refrigerant combinations is carried out to explore the effect of refrigerant selection on process performance.The main conclusions of this study are as follows:

Fig.7.Specific power consumption (w) and increase in w at different ethane content.

Fig.8.Exergy efficiency.

(1) The proposed processes achieve ethane recovery and purity of not less than 99.5% by rationally optimizing the parameters of the distillation column.And through energy integration,the energy consumption of the systems is effectively reduced.

(2) With the increase of ethane content,the specific power consumption of the processes decreases approximately linearly.By adding a low-temperature compressor after the distillation column,Process 3 can significantly improve the heat transfer performance of the low-temperature section,making the specific power consumption the lowest among the three processes.However,Process 2 with separation in deep cooling refrigeration cycle has the highest specific power consumption among the three processes.

(3) The case study results show that refrigerant combinations different from the basic process involved in this article has a very small negative impact on the process performance.By process optimization,the exergy efficiency can be maintained at a high level and does not differ by more than 1%in all cases.The elimilation of propane provides a good alternative for the application of DMR process in offshore palnts.

Nomenclature

D-c deep cooling cycle

DMR dual mixed refrigerant

GA genetic algorithm

LMTD Logarithmic mean temperature difference,°C

LNG liquefied natural gas

M molar flow,kmol?h-1

MR mixed refrigerant

NG natural gas

P-c precooling cycle

p Pressure,kPa

T temperature,°C

VMnominal molar volume of the gas,m3?kmol-1

WCpower consumption of compressors,kW

WPpower consumption of pump,kW

Wmintheoretical minimum work,kW

W total power consumption of the system,kW

w specific power consumption,kW?h?m-3

x an array of all parameters to beoptimized

η exergy efficiency

ΔTminminimum approach of the heat exchangers,°C

Table 7 Temperature differences of the cases