Elsevier

Applied Thermal Engineering

Volume 115, 25 March 2017, Pages 885-898
Applied Thermal Engineering

Research Paper
Evaluation of novel process configurations for coproduction of LNG and NGL using advanced exergoeconomic analysis

https://doi.org/10.1016/j.applthermaleng.2017.01.019Get rights and content

Highlights

  • Advanced exergoeconomic analysis is applied on two processes for co-production of LNG/NGL.

  • Cost of investment is divided into avoidable/unavoidable and endogenous/exogenous.

  • Results show that interactions between the components is not considerable.

Abstract

Advanced exergoeconomic analysis is applied on two novel process configurations for co-production of LNG and NGL. Dual mixed refrigerant (DMR) and mixed fluid cascade (MFC) refrigeration systems are used for supplying the required refrigeration. Based on the avoidable cost of exergy destruction in DMR process configuration, C-300 compressor with 504.43 ($/h) and in MFC process configuration, C-200A compressor with 251.05 ($/h) should be modified first. Based on the avoidable endogenous/exogenous part, three strategies are proposed for reducing exergy destruction cost in the process components. Cost of exergy destruction and investment in most of the process components are endogenous. So interactions among the components in these processes is not strong. Investment cost of turbo expander and compressors are unavoidable due to technological and economic limits while air coolers and heat exchangers have potential for improvement. Cost of exergy destruction of the air coolers and heat exchangers are unavoidable while turbo expander and compressors are avoidable.

Introduction

Liquefied natural gas (LNG) operating condition is −161 °C at 101 kPa [1]. But there are impurities in natural gas that should be separated. Natural gas liquids (NGL) are extracted for added value and as a main feed in the petrochemical processes [2]. LNG production and NGL recovery are cryogenic processes so refrigeration system is the most important part in both of them. Increasing the level of integration is logical to increase the efficiency and to decrease the capital and operating costs [3]. The produced residue gas in NGL recovery plants leaves the demethanizer tower at about −100 °C so is can be used for supplying a part of the required refrigeration in the process before flowing to the pipeline. The main idea of integrated NGL and LNG process configurations is using outlet low temperature gas from the demethanizer tower straightly to the liquefaction unit as inlet feed. With integration of NGL recovery and LNG processes total cost decreases and efficiency increases [4]. Required refrigeration in nonintegrated processes is provided by separated heat exchangers and cycles, while in integrated processes, refrigeration is supplied from the shared refrigeration cycles and devices, In other words it is obvious that NGL and LNG processes are series plants. Integrated hydrocarbon recovery processes are investigated by stochastic optimization methods [5]. In this study operating condition and process configuration are considered as continuous and discrete decision variables respectively. Benefits of integration have been attracted companies to design integrated systems. In another paper a currently in operation ethane recovery plant is optimized by shuffled frog leaping optimization algorithm [6]. The results show that by adjusting the operating variables, ethane recovery can be increased up to 18%.

Fluor Technologies indicates that by integration of LNG and NGL processes 10% of the required energy is saved [7]. LNG production capacity of ConocoPhillips integrated process is almost 7% more, while it requires same power [2]. Four integrated LNG and NGL process configuration is proposed which are different in the NGL recovery section [8]. A novel integrated process configuration for NGL/LNG production is introduced. This process uses two mixed refrigerant cycles to provide the required refrigeration [9]. LNG and NGL processes are costly and consume high amount of energy. This will show the necessity of a techno-economic evaluation for such processes. Many researchers has utilized conventional exergy and exergoeconmic analysis methods on the refrigeration cycles to determine the improvement potential. Energy and exergy optimization is performed for cogeneration plant consisting of an Otto cycle [10], Brayton cycle [11] and irreversible Carnot heat engine [12]. Energy and exergy analysis of internal combustion engine fueled with diesel and natural gas are presented [13]. Thermodynamic and exergy analysis are carried out for heat engine and combined cogeneration systems with steam and gas turbines [14], [15]. A hybrid power cycle using solar energy and cold energy of LNG is evaluated by exergy analysis [16]. The results show that the most exergy lost is related to the air preheater, combustion chamber and boiler. An integrated combined cooling heating and power process is investigated by exergy analysis method [17]. A currently in operation is investigated by advanced exergy analysis method [18]. The results show that the exergy destruction is related to the air coolers. Cryogenic helium extraction from natural gas processes are evaluated by advanced exergy analysis method [19]. The results show that the compressors have the lowest exergy destruction. Cryogenic air separation processes are investigated by exergy analysis method. In [20] one column configuration and in [21] two column configuration are considered. In both of them cold energy of LNG is used as the cold sources in the process. A novel hydrocarbon recovery process with auto-refrigeration system is analyzed by exergy method [22]. Exergy analysis is applied to refrigeration cycles in ethylene and propylene production processes. Rate of exergy destruction in the process can be decreased by adjusting the operating conditions of the cycles [23]. A new parameter is introduced for evaluation of the integration level in the hydrocarbon recovery processes. This parameter is defined based on the exergy concept [24]. Exergy analysis of the cascade refrigeration cycles used for natural gas liquefaction is investigated [25]. Exergy analysis is applied to multistage cascade low temperature refrigeration systems utilized in olefin plants [26]. Exergy analysis of absorption refrigeration machines is presented by a new method [27]. The exergy analysis of a hydrogen liquefaction system is considered [28].

An integrated natural gas liquids (NGL) and liquefied natural gas (LNG) processes is investigated by exergy and exergoeconomic analysis methods [29]. Exergoeconomic analysis is performed on the process for the coproduction of liquefied natural gas (LNG) and natural gas liquids (NGL) based on the mixed fluid cascade (MFC) refrigeration systems, as one of the most important and popular natural gas liquefaction processes [30]. Exergoeconomic analysis is carried out for single mixed refrigerant natural gas liquefaction processes [31]. PRICO liquefaction process is analyzed by exergy-based methods [32]. Exergy and exergoeconomic analyses on product recovery and separation systems of natural gas plant is performed [33]. A large industrial propane refrigeration cycle is analyses by thermoeconomic analysis method [34]. Conventional exergy and exergoeconomic analysis does not useful to determine the origin of irreversibilities and compute improvement potential. Advanced exergy and advanced exergoeconomic analysis are important tools to recognize sources of irreversibility and determine interactions among the process components. Advanced exergy analysis is applied on a hybrid fuel cell power plant [35]. The results show that the most portion of the exergy destruction is avoidable (more than 65%). An industrial propane refrigeration cycle is investigated by the advanced exergy analysis method [18]. The results shows that 59.61% of the exergy is lost in the unavoidable form in all components. Advanced exergy analysis on five conventional LNG processes is done [36]. Power plant system and a vaporization liquefied natural gas are analyzed by advanced exergy method [37]. Advanced exergy analysis on a cascade refrigeration system for liquefaction of natural gas is performed [38]. Advanced exergoenvironmental and advanced exergy methods are studied on a natural-gas regasification plant [39]. A tri-generation cycle is optimized using advanced exergoeconomic concepts [40]. By optimization, the total costs rate is reduced about 26%. A two stage hybrid absorption compression refrigeration system is evaluated by exergoeconomic analysis [41]. Advanced exergoeconomic analysis is carried out for three multi stage mixed refrigerant liquefaction processes. They are propane precooled mixed refrigerant (C3MR), dual mixed refrigerant (DMR) and mixed fluid cascade (MFC). Results show that interactions between the process components is not considerable [42]. Advanced exergoeconomic analysis is studied on two single mixed refrigerant (SMR) processes. The results show that investment costs and exergy destruction costs in most of the process components are endogenous [43]. Advanced exergoeconomic analysis is performed on a building heating system [44]. The results show that cost of exergy destruction in the distribution and generation stages is exogenous and in the emission stage is endogenous. Conventional and advanced exergoeconomic analyses is used for a geothermal district heating systems [45]. The results show that cost rate related to the components is avoidable. Advanced exergoeconomic analysis is studied on a novel process for production of LNG by using a single effect absorption refrigeration cycle [46].

Advanced exergoeconomic analysis gives more information for investment and exergy destruction cost with more sense about the processes. In this study, advanced exergoeconomic analysis is performed on two novel integrated processes for coproduction of LNG and NGL. For providing the required refrigeration in the processes, double mixed refrigerant (DMR) and mixed fluid cascade (MFC) processes are used. These process configurations have not been investigated by conventional and advanced exergoeconomic analysis so the results are novel and would be valuable. Also their performance are investigated by economic parameters.

Section snippets

Dual mixed refrigerant (DMR) process configuration

Process flow diagram (PFD) of DMR process configuration is presented in Fig. 1. As can be seen, Liquefaction system and NGL recovery are done in one process. Operating conditions of DMR configuration stream are presented in Table 1.

Numerical implementation

In order to carry out process simulation and the thermodynamic calculations Peng-Robinson (PR) and Peng-Robinson-Stryjek-Vera (PRSV) equations of state can be obtained for each part the process (LNG liquefaction and NGL recovery). In this study PRSV is used for simulation in Aspen HYSYS software. The original Peng-Robinson equation of state provides accurate vapor pressure prediction for hydrocarbons in the 6–10 carbon number range. It is defined as follows:P=RTυ-b-aυ-(υ+b)+b(υ-b)wherea=0.457235

Conventional exergy and exergoeconomic analyses

We can called the exergy as the maximum work potential that can be obtained from a system that its final state is in equilibrium with the environment [50]. An exergy analysis permits us to determine the irreversibilities in a system.

Exergy efficiency, exergy destruction and the ratio of exergy destruction are three variables which are calculated from exergy analysis. Exergy destruction is calculated based on the difference between the fuel exergy rate and the product exergy rate (relation 1).

Conventional exergy and exergoeconomic analysis

Results of conventional exergy and exergoeconomic analysis of DMR and MFC processes are presented in Table 8. For both processes, the highest irreversibility is related to compressors C-301 (11,141 kW) and C-201 (5423 kW), respectively. Exergy efficiency and total exergy destruction rate of the DMR process are 43.66% 26,788 kW respectively. Exergy efficiency and total exergy destruction rate of the MFC process are 53.83% and 22,272 kW respectively.

Results of the exergoeconomic analysis for DMR

Conclusions

In this study, advanced exergoeconomic analysis was performed on two novel integrated processes for coproduction of LNG and NGL. For providing the required refrigeration in the processes, double mixed refrigerant (DMR) and mixed fluid cascade (MFC) process are used. Result of exergy analysis show that exergy efficiency of the DMR process is 43.66% and its total exergy destruction rate is 26,788 kW. Also, exergy efficiency of the MFC process is 53.83% and its total exergy destruction rate is

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