An Innovative Approach for Natural Gas Liquefaction Using a Mixed-Multi-Component-Refrigerant

The natural gas liquefaction is the most expensive and energy-intensive phase in the natural gas-to-liquefied natural gas-to-natural gas chain. In addition, this region has the biggest development potential. As a result, numerous LNG production methods have been developed and are deployed at export facilities around the world. The goal of this study is to describe and assess an innovative approach for mixed-refrigerant (MR) LNG method. The authors have dubbed this technique the MR-X approach. The MR-X process was developed based on the globally proven liquefaction technology C3MR and its large-scale successor AP-X™ (which offers many benefits and flexibility), but with a novel precooling phase construct. In pre-cooling and liquefaction phases, the refrigerant is a combination of methane, nitrogen, propane, ethane, butane, and isobutane. The paper investigates the creation of the MR-X technology, as well as its modelling, energy, and exergy investigations.


Introduction
In the 1960s, the industry of liquefied natural gas (LNG) was established.In its more than 50-year history, this market has undergone tremendous growth, and future increase is projected [1]- [3].
Four blocks make up the total sequence "natural gas LNG -natural gas" as shown in Figure 1.
The LNG production in one AP-X TM train ranges from 5 to 10 MTA (million tonnes per year), while it fluctuates between 1 and 7 MTA in one C3MR or DMR train and a maximum of 2 MTA for other processes (million tonnes per annum).Using relative values, it is possible to demonstrate the LNG production per unit of supplied power: If a maximum known value of 100% is used, then the AP-X, C3MR, and DMR processes are all close to 100%, the Optimized Cascade processes are between 80 and 90%, and the remaining processes are approximately 70% of this value.The advantages and disadvantages of each of these processes vary based on operating conditions, environmental temperature, and plant capacity.

The creation of an innovative MR-X method
This research discusses a unique LNG process utilising mixed refrigerants.The authors dubbed this procedure the MR-X procedure.In order to improve comprehension of the new process, we would like to discuss briefly the four existing LNG technologies, as depicted in Figure 2 [9].
C3-MR/Split MR (C3MR modified with phase separators) and C3MR (without phase separators) are combinations for the phases of pre-cooling and liquefying.The refrigerant on the liquefying step is a mixed-multi component working fluid, whereas the refrigerant on the precooling step is of propane.AP-X TM method is an enhanced system of the C3-MR/Split MR technology, which adds a subcooling procedure that use nitrogen as the operating refrigerant.
The Dual Mixed Refrigerant (DMR) technique consists of liquefaction and precooling blocks.This approach was created for arctic working environments, which include a low yearly normal temperature and a reasonably wide annual temperature range.Throughout the year, the precooling load for such operating conditions fluctuates dramatically.Propane has the following disadvantages for arctic climatic conditions: (a) limited applicability only to minus 35 ⁰C (to avoid vacuum formation within the components of the pre-cooling block), (b) processes with pure working fluid are not adaptable to continuously changing operation conditions, and (c) significant effect on the overall efficiency of the entire process due to variation of the operation conditions within the pre-cooling block.All these negative effects are neutralised by replacing propane with a mixed refrigerant, although the intricacy of the process increases.Note that the constituents of the mixed multi component operating fluids used in the liquefaction and pre-cooling phases are distinct.

Natural gas (NG) Mixed refrigerant (MR) MR (gas phase) MR (liquid phase) LNG Operating fluid on pre-cooling phase on sub-cooling phase
The pioneering MR-X technology comprises of a few steps: pre-refrigerating or cooling, liquefaction, and sub-cryogenic cooling.On the steps of pre-refrigerating and liquefying, mixed-multicomponent working fluids are employed, followed by nitrogen on the sub-refrigerating phase.The benefits of the MR-X method include an increase in overall efficiency and plant economy.This article discusses the simulation and thermodynamic evaluation of the initial version of the MR-X process.
In the subject of LNG processes, exergetic analysis is covered in few papers.The authors of references [10], [11] conducted a literature review on the modelling, evaluation followed by optimization of LNG processes, and applying exergy analysis to these systems.Only the current state of four LNG processes will be discussed here: C3-MR/Split MR, DMR, C3MR, and AP-X.Remeljej and Hoadley [12] stated that they analysed the small-scale LNG process exergy.However, merely relative estimates from a comparative evaluation of a single MR cycle, two distinct open-loop processes (GCL and New LNG), as well as a nitrogen cycle (cLNG process), are presented.In their study, Dolatshahi and Amidpour [13] conducted an analysis on the exergetics of a C3-MR process.Once again, the simulated data pertaining to the system remains inaccessible, hence impeding the determination of the total exergetic efficiency.This study only presents the efficiency and destruction of exergy values on each individual part.The study was conducted with respect to the relationship between exergy output and energy intake.The study conducted by Alabdulkarem et al. [14] included an investigation of a C3-MR LNG facility, specifically focusing on modeling and optimization of energy use.The determinants considered in this research pertain to the "pinch temperatures" seen among heat exchangers and whatnot.The overall efficiency of the process' basic cycle is 45.43%, which increases to 49.97% after undergoing optimization.The definition of efficiency is not supplied in any of these articles.In their study, Bin Omar et al. [15] provided a detailed description of the modelling and exergy assessment of the AP-X TM LNG cycle.Additionally, they introduced the idea of "exergy of product/exergy of fuel" and its use in the study.
Vatani et al. [16] performed a comprehensive exergy study on five liquefied natural gas (LNG) facilities, including the aforementioned operational processes.The outcomes are only presented for a subset of the components within each process, whereas information pertaining to the full system is lacking.The authors included the concepts of "outlet exergy/inlet exergy" and "exergy of product/exergy of fuel" into their definitions of efficiency.
The exergetic assessment of the MR-X process discussed in this study only employs the model of "exergy of product/exergy of fuel."Hence, it may not be inherently useful to draw comparisons between the outcomes derived from the MR-X process and those reported by other researchers in relation to the AP-X TM , C3MR, as well as DMR procedures.

The MR-X Process Simulation and Energy Analysis
The procedural flowchart for the MR-X method is shown in Figure 3. Two pre-cooling heat exchangers are used to decrease the temperature of purified and processed natural gas (Stream 1) to -33 °C.In addition to natural gas, the primary mixed-refrigerant (Stream 111) is introduced to provide a refrigeration effect.In the meantime, the pre-cooling phase incorporates a compression of two-stage procedure that involves refrigerant interstage cooling.The process of liquefaction takes place inside the simulated HEX0 and HEX1 cryogenic heat exchanger.The NG emerges from the liquefaction process at a temperature of -111 degrees Celsius.The liquefaction module comprises a compression procedure at two-stage that incorporates interstage cooling via the use of environmental streams.The procedure of LNG subcooling inside HEX 2 involves reducing the temperature down to 166 ⁰C.The nitrogen cycle used in the sub-cooling block is characterized by its intricate nature, including a compression of threestage procedure, cooling on interstage using ambient streams, an expansion process, and the utilization of HEX 3 for both the liquefying and sub-cooling steps.The LNG is subjected to a pressure somewhat higher than the surrounding atmospheric pressure in order to facilitate its storage and transportation.The MR-X technology was simulated using the AspenPlus programme (applying the equation of state: Soave-Redlich-Kwong).The surrounding conditions are: The temperature is supposed to be 43 °C, and the pressure is expected to be 1.013 bar.The MR-X system is projected to have a liquefaction capacity of 7.8 mmtons/per year, which equates to a natural gas mass flow rate of 224.22 kg/s.Table 1 displays the composition and concentration of all employed working fluids.The thermodynamic values are shown in Table 2.

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Figure 4 depicts the global cooling curve for the MR-X process, which is comprised of three components (processes inside three blocks): precooling, liquefaction, and subcooling.The correlation between the liquefaction and subcooling curves and the data published by other authors is excellent (for example, [14], [17], [18]).The disparity in curves throughout the pre-cooling cycle demonstrates the advantages of using a mixed operating fluid instead of using propane as a refrigerant with a single component.

The Analysis of Exergy
The exergy concept serves as a valuable addition to an energetic analysis as it enables the calculation of actual thermodynamic inefficiencies, referred to as exergy destructions, within a system.It also allows for the determination of the authentic thermodynamic significance of all energy transporters, as well as the identification of variables that accurately characterize the thermodynamic performance of a system or its individual components [19][20].The physical exergy on any line is separated into mechanical and thermal components due to the fact that all operations in this context take place either below or above the ambient temperature [19].The exergy of fuel and the exergy of product descriptions for the MR-X process are formulated in accordance with the principles described in [19]- [22], taking into account the fact that the processes occur at temperatures below the ambient temperature, and are presented in Table 4 for selected components.Coolers and mixers are classified as dissipative components [10], [13], [23]- [25].
Table 3. Definitions of the exergy of product and exergy of fuel for chosen productive components of the MR-X process Compressor LPN2 Compressor HPN2

E ˙F
,HPN 2 Valve V0 Valve PV1    E ˙D,tot 231 MW.The complete exergetic efficiency is correspond to 32%.In assumption it is worth to mention that for the energy and exergy calculations, the EXP1 produces no electricity but rather powers one of the subcooling (nitrogen) block's compressor(s).

Discussion and Recommendations
The results of the analysis of the exergy indicate that the components with the highest exergy destruction values are (Figure 4) HEXO with y=10% and LPMR with y=12% throughout the liquefying cycle, and LPN2 (y=6%), MPN2 (y=9%), as well as EX1 (y=6%) for the sub-cooling phase.Nevertheless, all MR-X process parts have relatively high exergetic efficiencies (Figure 5).The sub-cooling block offers the greatest potential for MR-X technology improvement.This can be accomplished, for instance, by substituting the single-component refrigerant (nitrogen) with a mixture.

Conclusions
In this study, an innovative MR-X technology for LNG liquefaction is presented.The MR-X method combines the benefits and overcomes the constraints of the C3MR, AP-X, and DMR methods to the greatest extent possible.The outcomes of the energetic and exergetic assessments are provided.The values of the COP and exergetic efficiency are 0.58 and 32%, respectively.These reasonably high figures (remember that we are dealing with an extremely low-temperature refrigeration system) indicate that this novel process is thermodynamically efficient.In the future, advanced exergy-based methodologies will be utilised in order to gain more specific information regarding the thermodynamic and economic enhancement prospects of this process.

Figure 4 .
Figure 4.The calculated general cooling curve for the MR-X method.

Figure 5 .Figure 6 .
Figure 5.The ratio of exergy destruction in (%) and in (MW) for the MR-X technology components.

Figure 5 and
Figure 5 and Figure 6 illustrate the results obtained from the exergetic analysis conducted on the selected components of the MR-X process.The destruction ratios for exergy are determined as y E ˙ D,k .In

Table 1 .
Working fluid composition and concentration

Table 2 .
Data on the thermodynamics of the material streams (operating in real conditions)