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Exergetic Performance Analyses of Natural Gas Liquefaction Processes

A. S. Karakurt*, U. Gunes*, M. Arda*, Y. Ust*

*Yıldız Technical University, Turkey, asinan@yildiz.edu.tr, ugunes@yildiz.edu.tr,

marda1619@gmail.com, yust@yildiz.edu.tr

Abstract Liquefied Natural Gas (LNG) is the fastest growing energy carrier in the world because of its low

environmental impact, flexibility in the market and reserves capacity. In this study, the principal aim

is to investigate the theoretical performance of a natural gas liquefaction process which was modeled

and analyzed with EES (Engineering Equation Solver) based on the exergetic performance coefficient

(EPC). The results showed that maximum irreversibilities are occurred in propane cycle and

evaparator section.

Keywords: LNG, Liquefaction, Cascade Refrigeration Cycle

1. Introduction Natural gas is a fossil fuel formed when layers of buried plants and animals are exposed to intense heat

and pressure over thousands of years (EPA, 2014). It is increasingly used as a clean energy source for

industrial, residential, commercial and electric power sector applications. Natural gas (NG) importers

have to converted it to liquid form (the volume is reduced a factor 600 times compared to the standard

conditions) to storage or transfer it to oversea countries easily. Natural gas condenses minus 162 °C at

atmospheric pressure and named as Liquefied Natural Gas (LNG). LNG is the fastest growing energy

carrier in the world because of its low environmental impact, flexibility in the market and reserves

capacity, Figure 1.

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Fig. 1. 1973 and 2011 fuel shares of total primary energy supply (IEA, 2013)

Natural gas contains methane (%70-90-CH4), heavier hydrocarbons (Ethane-(C2H6), Propane-(C3H8),

and Butane-(C4H10)) and inert components which affect burner performance. For this reason pipeline

companies and LNG importers specify allowable ranges of components and heating values (KBR,

2014).

The first liquefaction process of gases was built by the Karl von Linde in 1895 who have benefit from

the Joule-Thomson effect. Using this effect, producing large amounts of liquid air for industrial

applications has become feasible and also pure liquid oxygen has successfully separated from the

liquid air. The first liquefaction of natural gases was held to separate ethane and propane from natural

gas for commercial applications in 1910. Until the 1950s, many projects and design studies were

conducted on to transport LNG to overseas and then liquefied methane has begun to transport to

overseas with Methane Pioneer, LNG Carrier, in 1959 (AVCI et al., 1995). Nowadays, approximately

30 countries have natural gas liquefaction plant in operation. A cycle from extraction to end use of

natural gas is shown in Figure 2.

Fig. 2. Natural Gas extraction to end use (Tsatsaronis, 2012)

Liquefied natural gas intended to be primarily selected foreign gas to be purified or bound to the

liquefaction process must be reduced to acceptable concentrations. After purification by one of the

known methods, the natural gas liquefaction process is subjected to the two stage cascade.

For natural gas dew point at 1 bar pressure and about -162 °C, the temperature for liquefaction of

natural gas must be reduced to at least that temperature. For this purpose the heat exchanger to be used

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and considering economic efficiency and productivity of the machine 60 to 90 °C above temperature

difference is not appropriate in the one-stage cooling. Therefore, it is necessary to make stage cooling.

In practice the natural gas liquefaction systems are cascade refrigeration systems, expansion and

Stirling cooling systems can be divided into three main groups (AVCI et al., 1995).

Cascade cooling process can be classified as classic, single fluid (Phillips optimized cascade process,

Linde mixed fluid cascade proses (MFC)) and mixed fluid (double mixed refrigerant (DMR), parallel

mixed refrigerant (PMR), propane precooled mixed refrigerant process (C3MR) and propane mixed

refrigerant and nitrogen (AP-X)) (AVCI et al., 1995).

Cascade refrigeration systems for cooling and the cooling fluid at each stage separately and separate

circuit is the system used. Each fluid works as a separate closed-loop single-stage or multiple-stage

operates at an appropriate temperature and pressure range. Refrigerant fluid group in the three stages

can be selected propane-ethylene-methane, ammonia-ethylene-methane or freon-methane,

respectively. The system is widely used today in many countries, but is more expensive than others

(AVCI et al., 1995).

Many previous studies have done to describe loses and optimize the liquefaction systems and some of

them are summarized in this paragraph Tsatsaronis and Morosuk (2010) have analyzed a three-cascade

refrigeration system for liquefaction of natural gas based on conventional and advanced exergetic

analyses to determine the increasing potential of system and component efficiencies. Coşkun (2004)

has studied the thermodynamic analyses of classic cascade cooling cycle using in liquefaction of

natural gas. Castillo et al. (2013) have analyzed to determine the technical advantages and

disadvantages involved in the selection of the process for the precooling cycle. Analyses showed that

three stage propane precooled cycle was found to be the most energetically efficient among the studied

cases. Yoon et al. (2010) have offered basic information about performance characteristics of cascade

and C3MR processes through simulation with HYSYS software to secure competitiveness in the

industry of natural gas liquefaction plant. Sayyaadi and Babaelahi (2010) have performed a

thermodynamic model and made an efficiency optimization based on energy and exergy analyses for a

cryogenic refrigeration cycle using MATLAB genetic algorithm optimization toolbox. Yoon et al

(2012) have presented a new cascade liquefaction cycles using CO2-C2H6-N2 and CO2-N2 and have

analyzed exergetic performance of cycles using HYSYS software. The results were compared and

confirmed for LNG-FPSO ship. Chiu (1982) has criticized the applications of exergy analysis to

cryogenic process and equipment optimizations and discussed the iterative-comparative and the

analytical approaches in terms of their advantages and limitations. Yu et al. (2009) have made exergy

analyses of the ejector expansion Joule–Thomson cryogenic refrigeration cycle and also investigated

the effect of some main parameters on the exergy destruction and exergetic efficiency of the cycle.

Exergy analysis and the relations for the total exergy destruction and the exergetic efficiency of the

multistage cascade refrigeration cycle used for natural gas liquefaction have been done and an

expression was described to obtain the minimum work requirement by (Kanoglu, 2002). Kanoglu et al

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(2008) have presented a methodology for the first and second law analyses of the simple Linde–

Hampson cycle and have investigated the effects of some parameters on cycle performance. Kumar

and Misra (2014) have presented a detailed review for cryogenic systems and have done energy and

exergy analyses of the system for liquefaction of various gases. The effects of natural gas pressure,

temperature and composition on the performance of four processes for LNG-FPSO were investigated

with thermodynamic approach using HSYS software and a design and performance criteria was

established by (Pwaga, 2011). Wang et al. (2003) have established a simulation of liquefied natural

gas process of the mixed refrigerant cycle using Aspen Plus software and have studied on the exergy

loses of each unit. Marmolejo and Gundersen (2013) have studied on the exergy analysis of a mixed

refrigerant process (PRICO process) for LNG production and have discussed how to minimize loses to

get more efficient process. Beladjinea et al. (2013) have analyzed the Claude refrigeration cycle that

uses to reliquefied the LNG BOG installed onboard an LNG carrier. In this study EPC analysis of a

classical cascade cryogenic refrigeration system has done.

2. Thermodynamic Model Natural gas which is in cascade refrigeration cycle passes through three stages. In this cycle, propane,

ethylene, and methane are used as a refrigerant, respectively. Cascade cycle has 4 compressors, 3

evaporators, 4 throttling valves and 3 condensers. The classical cascade cycle process and T-s diagram

of the system are shown in Figs. 3-4.

Fig. 3. Classical cascade cycle

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Fig. 4. T-s diagram of the three stage cascade cycle

At 40 °C and 4 bar pressure natural gas comes from the , is separated from foreign substance, reaches

E1 heat exchanger after pre-cooling with 17 ºC sea water . Propane gas is cooled to -42 °C in E1

exchanger. Evaporated propane via heat rejection from E1 heat exchanger is compressed about 10 bar

in C1. After propane is cooled with water, it returns to the E1 heat exchanger after throttled to 1 bar

pressure and propane cycle is completed.

After natural gas about 4 bar is cooled to -37.5 °C E1 heat exchanger, it comes to E2 heat exchanger

and cooled to -100 °C in ethylene cycle. Ethylene is compressed up to 10 bar pressure after it is

evaporated via heat rejected. It comes to E1 heat exchanger and after pre-cooling with propane, it

returns to the E2 heat exchanger after throttled to 1 bar pressure and ethylene cycle is completed.

Natural gas comes to E3 heat exchanger after cooled to -100 °C in E2 heat exchanger. Here, methane

vaporize at -162 °C and 1 bar pressure by rejected heat from natural gas, comes to E2 heat exchanger

which is in propane cycle after compressed to 15 bar pressure in C3 compressor which is in methane

cycle. Then natural gas comes to ethylene cycle passing through precooling in the last stage. Here, it is

cooled about -100 °C after throttled to 1 bar pressure, it comes to E3 heat exchanger then the methane

cycle is completed. Then natural gas passes through E3 heat exchanger after throttled to 1 bar pressure

then it comes to storage tank. In storage tank, there is some natural gas, remaining from not liquefy

and vaporize again due to heat addition from the surroundings to the natural gas, send to entry of LNG

system or power plants to use it.

The heat transfer rate from the heat source to the evaporator is LQ , and from the condenser to the heat

sink is HQ may be written respectively as:

L,i i in,1 out ,1 iQ m h h (1)

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1 1H ,i i in, out , iQ m h h (2)

comp,i i in out iW m h h (3)

here i indicates the propane, methane and ethylene cycles, m indicates the mass flow rate of

refrigerants at three cycles and h denotes the enthalpy. The power inputs for the propane, methane and

ethylene compressor are given as:

In a classical performance analysis of refrigeration systems, the coefficient of performance (COP) is

used as a major performance criterion. The coefficient of performance gives information about the

necessary electrical power input required in order to produce a certain amount of cooling load. From

the first law of thermodynamics, the coefficient of performance is defined as the ratio of cooling load

to the power input for cascade refrigeration cycle and is given as below:

L,Pr ophane L,Methane L,Ethylene

comp,Pr ophane comp,Methane comp,Ethylene

Q Q QCOP

W W W

(4)

An exergy analysis of a process is a supplement to an energy analysis and is used to assess the work

potential of input and output material and heat streams as well as to determine the location and

magnitude of irreversibility losses. The exergetic analysis provides important information about the

total irreversibility distribution of the system among the components, thereby determining which

component carries more weight on the overall system inefficiency (Aprea, et al, 2004). Since the

variation of kinetic, potential and chemical exergy is considered negligible in this study, the physical

exergy is only taken into account in the system itself. The physical exergy translation is obtained

through the thermal and mechanical processes. These equations may be expressed on the basis of their

definitions as follows (Kotas, 1985):

0 0 0PH

X i i iE m h h T s s (5)

here ��𝐸𝑥𝑥𝑃𝑃𝑃𝑃 is the physical exergy, ��𝑚 is the mass flow rate, s is the specific entropy at the condition

specified for the species and the subscript 0 represents environment conditions. The general exergy

balance may be expressed in the rate form as (Kabul et al, 2008; Morosuk et al, 2013):

X , in X , out XDĖ Ė Ė . (6)

here ��𝐸𝑋𝑋,𝑖𝑖𝑖𝑖 is the exergy input, ��𝐸𝑋𝑋,𝑜𝑜𝑜𝑜𝑜𝑜 is the exergy output and ��𝐸𝑋𝑋𝑋𝑋 is the exergy destruction.

For control volume of any component undergoing a steady state process, a general equation for the

exergy destruction rate derived from the exergy balance is:

0 0XD,i X Xin out in out

in out

T TE Q 1 Q 1 W W E ET T

(7)

here EXD,i represents the rate of exergy destruction existing in the process components. The first and

second terms of the right-hand side of the above equation represent exergy rate transfers by heat and

by work, respectively. T0 represents the environment temperature of the system’s surroundings and

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Q represents the heat transfer rate across the boundary of the system at a constant temperature of T. EX

is the exergy rate transfer by mass of each of the substance flows crossing the system boundary.

2.1. Exergy destruction rate in the system components

By taking Eq. (6) into consideration, the exergy destruction rate in each component of the cascade

refrigeration system is ascertained by using the equations given in Table 1. The subscripts used in the

exergy balance equations comp, con, exva and evap indicate the compressor, condenser, expansion

valve and evaporator, respectively.

2.2. Exergetic efficiency

The exergetic efficiency, ε, of the cascade system is defined as (Morosuk et al, 2013; Bejan et al,

1996):

X ,out X ,D

X ,in X ,in

E E1

E E (8)

For the cascade refrigeration system, the exergy input is equal to the total electrical power input for the

compressors,

X ,in ,tot comp ,ii

E W (9)

and the exergy output of the cascade system is the total exergy rate of the heat transferred to the

evaporator from the cooled space at the temperature is given in Table 1 as ∑ X ,outE .

2.3. Exergetic performance coefficient

In order to obtain information about the exergy destructions of the cascade refrigeration system,

another performance criterion must be added. As such, a performance criterion has been defined as

“exergetic performance coefficient” (EPC) and which has been previously introduced by Ust et al

(2011). The EPC objective function for a cascade refrigeration system is defined as the ratio of exergy

output to the total exergy destruction (or loss rate of availability), i.e.:

X ,out X ,out X ,in

XD,tot XD,tot0 g

E E EEPC 1

E ET S (10)

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Table 1. Exergy destruction rate, EPC and Exergy eficiency equations for each components.

D,Comp in out in

out

Comp

in out in

out

Ex,Comp

in in

Ex =Ex -Ex +W

ExEPC =

Ex -Ex +W

Ex=

Ex +W

Evap in out

out

Evap

Evap

out

Evap

in

ExD = Ex - Ex

ExEPC =

ExD

Ex=

Ex

Expv in out

out

Expv

in out

5

Expv

6

ExD = Ex -ExEx

EPC =Ex -Ex

Ex=

Ex

con 0

Con H

con

Con in,1 out,1 Con

1 Con

Con

Con

out,1 Con

Con

in,1

T -TEx = Q

T

ExD =Ex -Ex -Ex

Ex ExEPC =

ExD

Ex +Ex=

Ex

3. Results and Discussion

In order to observe the results of the EPC analysis for a classical cascade refrigeration system, a

selection of numerical examples have been given and discussed. This analysis was carried out using

the refrigerants propane, ethylene and methane. Condensation temperature of gasses which is used in

classic cascade refrigeration system are important. In addition to this, environmental effects, toxicity

and flammability characteristics are also taken into account other factors. Thermo-physical properties

of refrigerants are given in Table 2. The thermodynamic analysis of the system was performed based

on the following assumptions:

The chemical, kinetic and potential energy and exergy of the components are to be neglected,

Pressure drops in the pipe line are neglected,

Heat transfers from/to the compressor and expansion valve are to be neglected,

The content of natural gas is 100% CH4,

Mass flow rate of LNG is constant (10 kg/s).

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Table 2. Thermo-physical properties of refrigerants (ASHRAE, 2009)

Chemical

Name

Atmospheric

Lifetime

(years)

Semi-

Empirical

ODP

Net

GWP

100-yr

OEL/PEL ppm

(v/v) &

ASHRAE 34

Safety Group

Normal

Boiling

Point(s)

°C

Critical

Temp.

°C

Critical

Pressure

(absolute)

kPa

Methane 12 ± 3 <0(smog) 25 1000 A3 -162±2 -82.3 4640

Ethylene 12 ± 3 <0(smog) 3.7 200 A3 -103.7 9.19 5040

Propane 12 ± 3 <0(smog) 3.3 1000 A3 -42.1±.2 96.7 4248

The calculated thermodynamic properties data for each node and the base case exergetic performance

results of the classical cascade refrigeration cycle along with its components are given in Table 3 for

the refrigerations, respectively.

Table 3. Base case simulation results at each node of the cascade cycle

Node h[i] T[i] s[i] P[i] Ex[i] i kJ/kg K kJ/kg.K Bar kW 1 270.9 300.1 1.244 10 6473 2 270.9 230.8 1.346 1 4803 3 525.5 230.8 2.449 1 745 4 652.5 324.5 2.509 10 6722 5 -529.2 221.3 -2.955 10 8784 6 -529.2 169.1 -2.844 1 7962 7 -178.8 169.1 -0.771 1 1315 8 -6,783 303.9 -0.684 10 4949 9 -732.5 158.5 -5.378 15 16258

10 -732.5 111.5 -5.077 1 14589 11 -400.4 111.5 -2.098 1 4234 12 -149.3 239.5 -1.935 15 8009 13 -19.09 290 -0.057 1 2,427 14 -147 233 -0.530 1 133.8 15 -22.18 290 -0.783 4 2135 16 -141 235.8 -1.237 4 2298 17 -274.4 173.1 -1.893 4 2919 18 -893.6 116.5 -6.531 4 10549 19 -22.18 290 -0.783 4 2135 20 -133.5 235.8 -1.167 10 5875 21 -181.1 235.8 -2.072 15 8175 22 -336.2 173.1 -2.879 15 11012

Exergy destruction rates of each companent of cascade cycle are shown with percentage in Figure 5

and base case exergetic performance results of the cascade cycle components are given in Table 4. In

these figure and table, it is observed that the highest exergy destruction rate in the system occurs in the

evaparator sections which are due to the heat losses from the evaporators.

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Fig. 5 Exergy destruction rates of each companent

Table 4. Base case exergetic performance results of the cascade cycle components

EPC ExD (kW) ε Compressor Ethylene 7.655 647 0.885 Compressor Methane 8.855 904 0.899 Compressor Propane 6.921 971 0.874 Condenser Ethylene 14.790 372 0.937 Condenser Methane 9.871 1013 0.908 Condenser Propane 4.580 1205 0.821

Evaparator Ethylene 4.782 3188 0.827 Evaparator Methane 5.424 2725 0.844 Evaparator Propane 6.098 2803 0.859 Exp. Valve Ethylene 9.681 822 0.906 Exp. Valve Methane 8.741 1669 0.897 Exp. Valve Propane 2.876 1670 0.742

Total exergy destruction rates of prophane, ethylene and methane refregirant cycles of cascade system

are given in Figure 6 and thermodynamic and exergetic performance parameters of the system are

given in Table 5. Here, we observe from these figure and table that both the maximum total exergy

destruction rate and the maximum exergetic efficiency are in the propane cycle. In order to improve

the individual EPC performance and reduce the exergy destruction rate in the evaparator section, a

great deal of interest should be paid to the heat losses from the evaparators for cascade refrigeration

cycle.

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Fig. 6 Exergy destruction rates of each cycle

Table 5: Thermodynamics and exergetic performance results of the cascade cycle

EPC COP Power Qcon Qevap ExD ε kW kW kW kW

Ethylene Cycle 3.032 2.038 4280 9849 8722 5029 0.2807 Methane Cycle 2.342 1.323 4679 7388 6192 6312 0.3165 Propane Cycle 2.571 2.005 6948 20876 13928 6649 0.4065

Total 1.096 1.813 15907 38113 28842 17990 0.8843

4. Conclusion In this study, a measurement of exergy output per unit exergy destruction rate for each individual

component as well as the system as a whole may be provided. This means that it is possible to find the

best exergetic performance conditions at the minimal loss rate of availability. In order to carry out this

aim, a cascade refrigeration system must first be thermodynamically modeled based on mass, energy,

and exergy balance equations. This thermodynamic analysis has been carried out using the prophane,

ethylene and methane refrigerants. The sources of irreversibilities are determined from these analyses

and it’s suggesting that to improve more efficient systems to minimize the irreversibilities.

28%

35%

37%

Exergy Destruction RatesExD_SM_ethylene ExD_SM_Methane ExD_SM_propane

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