Current Status and Perspectives of Liquefied Natural Gas (LNG) Plant

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Current Status and Perspectives of Liquefied Natural Gas (LNG) Plant Design Wonsub Lim,† Kwangho Choi,‡ and Il Moon*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea ‡ GS E&C, GS Yeokjeon Tower, 537 Namdaemun-ro 5-ga, Joong-gu, Seoul 120-722, Republic of Korea ABSTRACT: Liquefied natural gas (LNG) is attracting great interest as a clean energy alternative to other fossil fuels, mainly due to its ease of transport and low carbon dioxide emissions, a primary factor in air pollution and global warming. It is expected that this trend in the use of LNG will lead to steady increases in demand over the next few decades. To meet the growing demand for LNG, natural gas liquefaction plants have been constructed across the globe. Furthermore, single train capacity has been increased to strengthen price competitiveness. To achieve greater capacity, more complex refrigeration cycle designs that combine two or more different conventional single refrigeration cycles are being developed to obtain synergistic effects in the liquefaction process. At the same time, a variety of recent studies have focused on designing suitable processes for offshore and small-scale plants to improve the profitability of stranded gas fields. LNG plants are known to be energy/cost-intensive, as they require a large amount of power for the processes of compression and refrigeration, and need special equipment such as cryogenic heat exchangers, compressors, and drivers. Therefore, one of the primary challenges in the LNG industry is to improve the efficiency of the current natural gas liquefaction processes in combination with cost savings. In this paper, we review recent developments in LNG processes, with an emphasis on commercially available refrigeration cycles. We also discuss recent research and suggest future directions for natural gas liquefaction processes. Up to this point, most studies have focused on operating cost. To achieve better results, future studies that investigate optimal design and operation of LNG technologies should consider both capital cost and operating cost.

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respectively.4 It is clear from this table that natural gas is an attractive option for reducing CO2 emissions. The geographical mismatch between gas reservoirs and customer demand is reflected in the continuous increase in the global natural gas trade. One of the dominating factors in the natural gas trade is the selection of an economically feasible transport method. Widely used methods to transfer natural gas trade are pipelines and liquefied natural gas (LNG). For transport over long distances over 3500 km, LNG is the preferred method for economical, technical, safety-related, and political reasons. A comparison of future projections of the global inter-regional natural gas trade through pipelines and LNG is presented in Figure 1.3,5 Pipeline and LNG trade is projected to increase continually in the future. However, the LNG trade is projected to have a higher growth rate than the pipeline trade. Therefore, LNG plays a major role in the global natural gas trade. To satisfy the projected demand, the natural gas supply will need to increase by almost 50% in the period designated in Figure 1. Several countries have commercialized natural gas resources and constructed LNG production facilities to meet this increasing demand. The global trade in natural gas is currently under rapid transition because several countries have been steadily increasing their LNG production capacity. In the

INTRODUCTION

Global energy consumption is projected to increase at an average rate of 0.9−1.6% per year.1−3 Fossil fuels are expected to remain the dominant energy sources into the foreseeable future. Natural gas is the fastest growing major energy source in the world; its consumption is expected to increase at an average rate of 1.4−1.6% per year from 2008 to 2035,2,3 due to its lower environmental impact than other fossil fuels. Combustion of natural gas results in lower emission of CO2 and other pollutants than combustion of coal and oil. Table 1 shows the amounts of CO2 emitted from the combustion of natural gas, coal, and oil, Table 1. Pounds of Air Pollutants Produced Per Billion BTU of Energya4 pollutant

natural gasb

oilc

coald

carbon dioxide carbon monoxide nitrogen oxides sulfur dioxide particulates formaldehyde mercury

117,000 40 92 0.6 7.0 0.750 0.000

164,000 33 448 1122 84 0.220 0.007

208,000 208 457 2591 2744 0.221 0.016

a

No post-combustion removal of pollutants. bNatural gas burned in uncontrolled residential gas burners. cNo. 6 fuel oil burned in an oilfired utility boiler. (Conversion factor: No. 6 fuel oil at 6.287 million BTU per barrel and 1.03% sulfur content.). dBituminous coal burned in a spreader stoker. (Conversion factor: Bituminous coal at 12 027 BTU per pound and 1.64% sulfur content.). © XXXX American Chemical Society

Special Issue: Process Engineering of Energy Systems Received: April 1, 2012 Revised: December 14, 2012 Accepted: December 14, 2012

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dx.doi.org/10.1021/ie302877g | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. Future projections of the global inter-regional natural gas trade.3,5.

Figure 3. Cost breakdown of the LNG value chain.7.

past 5 years, there has been a 40% increase in the global LNG production capacity. The liquefaction capacity is projected to more than double from 8 trillion cubic feet in 2008 to 19 trillion cubic feet in 2035.2 A LNG project is considered one of the most expensive energy projects and comprises a set of unique values in the supply chain (known as the LNG value chain), as schematized in Figure 2.6 The percentage of average capital costs for each component in the LNG value chain is presented in Figure 3;7 there are noticeable differences between the costs of each element in conventional natural gas fields. The cost of a liquefaction plant is the greatest in the value chain, accounting for more than 40% of the total cost. The costs of the other components account for nearly equal portions of the remaining total. Among the components, the costs of exploration and production vary greatly according to natural gas field. Accurate cost projection of a LNG project depends on a suite of factors including site conditions, safety, and traded volumes. From an economic perspective, the liquefaction plant is the most important component in the LNG value chain.

Algeria, the single mixed refrigerant (SMR) process was adopted to simplify the complex equipment configuration of the cascade liquefaction process using three pure refrigerant cycles. Due to the low thermodynamic efficiency of the SMR process, it was substituted with the propane precooled mixed refrigerant (C3MR) process comprising a propane precooling cycle and a mixed refrigerant cycle. This process was first used in the Lumut (Brunei) LNG plant in 1972.9 Since then, the C3MR process designed by Air Products and Chemicals, Inc. (APCI) has remained the dominant liquefaction process in the LNG plant market; over 60% of currently installed base load LNG plants use this process. Only three processesnamely Cascade, SMR, and C3MRwere applied in base load LNG plants from the 1970s to the 1990s. Steady progress has been made in liquefaction technology, with improvements in thermodynamic efficiency, a reduction in capital cost, and an expanded capacity per train. These continuous improvements have promoted stiff competition among liquefaction technology providers and created new challenges for developing liquefaction technology. The main purpose of the majority of early LNG projects was capacity expansion with the addition of production facilities to existing operating gas fields. However, to meet the rapid growth in demand for LNG, a number of new gas fields for use as LNG plant construction sites have been explored, and vendors are competing to develop new high-performance, low-cost technologies and processes. Due to innovations in

1. LIQUEFIED NATURAL GAS (LNG) PLANTS 1.1. Developments in LNG Plants. The world’s first commercial liquefaction plant was built in 1964 at Arzew, Algeria and was based on the cascade liquefaction process,8 which was then applied in the Kenai, Alaska LNG plant in 1969. In the early 1970s at Marsa El Brega, Libya and Skikda,

Figure 2. LNG value chain.6. B



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Figure 4. Conventional processes of a base load LNG plant.

its volume by a factor of more than 600.5 The liquefaction process requires a significant amount of energy due to the large and complicated refrigeration systems and the high capital cost of the equipment required such as compressors, heat exchangers, and other cryogenic equipment. Liquefaction is thus known as an energy intensive or capital intensive process. After liquefaction of natural gas, it can be stored in specially designed containers. Storage tanks and associated equipment should meet rigorous design standards due to the characteristics of LNG.

liquefaction technology and related equipment such as heat exchangers, gas turbines, compressors, and other utilities, LNG production costs per tonne have decreased substantially. For instance, liquefaction costs were around $560/tonne in 1995, but decreased to $222/tonne in 2004.10 However, recent liquefaction costs have increased to some extent due to an increase in engineering, procurement, and construction (EPC) costs. The liquefaction costs commissioned in 2005−2008 were around $430/tonne. The construction costs in 2009−2013 are expected to reach $830/tonne.11 The size of a single liquefaction train has been increasing to strengthen price competitiveness, with the goal of using fewer trains to achieve the same capacity. Furthermore, the profitability of stranded gas fields has increased greatly. Development of liquefaction process efficiency has focused on the replacement of expansion valves with liquid expanders or two-phase expanders, power reduction in the liquefaction process, or an increase in LNG production throughput at the same power. In the early days of the industry, the capacity of the majority of trains was approximately 1−2 million tonnes per annum (MTPA). Today, the largest train has a capacity of 7.8 MTPA.10 1.2. LNG Plant Processes. In general, a LNG plant mainly consists of pretreatment, liquefaction, and storage facilities. Figure 4 shows the processes performed in a conventional base load LNG plant. Natural gas is a mixture of hydrocarbons comprising methane, ethane, propane, and butane, among others. It also contains water, carbon dioxide, oxygen, nitrogen, and trace components of other gases. Before natural gas liquefaction, impurities such as water, carbon dioxide, oxygen, nitrogen, and hydrogen sulfide must be removed to prevent equipment damage caused by internal corrosion or solids formed during cooling. The natural gas pretreatment process consists of three main steps. The removal of acid gases, such as CO2 and H2S, before liquefaction is an important step in LNG production, because if the CO2 concentration exceeds a certain limit the gas will freeze in the pipelines. The next step is a dehydration process to remove water to avoid freezing. The last step is to remove mercury from the gases to protect the aluminum heat exchangers from corrosion. Following pretreatment, the natural gas undergoes a liquefaction process in which it is cooled to −161 °C at atmospheric pressure so that a phase transition takes place, allowing the natural gas to become liquid.12 Liquefying natural gas reduces

2. NATURAL GAS LIQUEFACTION PROCESS 2.1. Fundamental Principles. Liquefaction of natural gas requires the removal of sensible and latent heat over a wide range of temperatures using one or more refrigerants,13 and thus a complicated refrigeration systemeither compression refrigeration or absorption refrigerationis required. In LNG plants, a compression refrigeration cycle is generally used. The ideal compression refrigeration cycle is represented in Figure 5a.14 The cycle works as follows. In the first step (1→2), a refrigerant absorbs heat at constant pressure and temperature in an evaporator; in the second step (2→3), pressure is increased at constant entropy in the compressor. In the third step (3→4), desuperheating and condensation, in which enthalpy decreases at a constant pressure, are performed in a condenser. In the last step (3→4), the refrigerant returns to its original pressure at constant entropy in an expander. Improvements in refrigeration cycle efficiency can be achieved by modifying the cycle configuration. Vaporization of the liquid in the evaporator provides refrigeration. Thus, compression work can be reduced by adding an intermediate level. This intermediate level is added between two temperature levels, with the vapor formed after the first pressure reduction separated from the liquid and fed directly into the high-pressure compressor, as shown in Figure 6a and b.14 The intermediate level allows the insertion of an intercooler to reduce the amount of superheat in the compressors. However, it is not feasible to use an intercooler in a lower temperature cycle. In this case, an economizer (phase separator) can be substituted with a presaturator (vapor cooler) to reduce the superheat to zero, as shown in Figure 6c and d.14 The presaturator can reduce the inlet temperature to the next compression level by direct contact with the liquid refrigerant. However, presaturation requires a C



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Figure 5. Compression refrigeration cycles.14

higher vapor refrigerant flow rate. Two or more cycles with different refrigerants may be operated at the same heat exchanger as shown in Figure 7.14 The cascade refrigeration cycle is employed to provide very low-temperature refrigeration in cases where a single refrigerant cannot be used for operation due a wide range of operating temperatures. The liquefaction process is completed when the natural gas is cooled to a temperature in the two-phase region. A simple Linde liquefaction process, which depends solely on throttling expansion, is shown is Figure 8.15 Feed gas is mixed with the uncondensed portion of the gas from the previous cycle, and the mixture is compressed by a multistage compressor. The compressed gas is precooled to ambient temperature and can be cooled even further by refrigeration. The high-pressure

gas is cooled by a return gas stream in a heat exchanger and then expanded through a throttle valve, and the outlet stream exists in an equilibrium state of liquid and vapor phases. The outlet stream is flashed in the separator, producing liquefied product at its bottom. Gas from the separator is used to cool the high-pressure gas stream in the heat exchanger above. The reverse-Brayton cycle, as shown in Figure 9,15 uses gas phase refrigerant. This cycle is widely used for cryogenic liquefaction, and it forms the basis of the expander process in natural gas liquefaction. The fundamental principles of a variety of gas liquefaction processes and how these processes work efficiently have been reviewed by Barron,16 Walker,17 and Timmerhaus and Flynn.18 Natural gas liquefaction processes have been developed by combining characteristics of different refrigeration cycles. D



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Figure 6. Multistage compression refrigeration cycles.14

in the world’s first LNG plant in CAMEL. This process comprises three separate refrigeration cycles as shown in Figure 10.13 The three refrigerant cycles are typically operated at three evaporation temperature levels with multistage compression. High-pressure propane is condensed by ambient air or cooling water in the multistage compression step. In the propane cycle, the propane is used to cool the natural gas and the other two refrigerants to −30 °C. The ethylene cycle then cools the natural gas and methane to about −100 °C. Finally, the methane is used to produce LNG at −160 °C.19 A kettle-type heat exchanger is employed in the propane cycle, and a coil-wound heat exchanger (CWHE) is used in the ethylene and methane cycles. Steam turbines contribute to drive compressors for each refrigerant cycle and condensers uses cooling water as a coolant. The three-train capacity is a mere 1.1 MTPA. Phillips Cascade.19 The LNG plant in Kenai applied an early version of the Phillips cascade process similar to that used in the CAMEL plant. This process also uses propane, ethylene, and methane cycles, but the single train capacity is 50% greater than that of the three trains at CAMEL. This process is considered to be the first that employed gas turbine/compressor sets and a plate-fin heat exchanger (PFHE) in each refrigeration cycle.19 Phillips’ Optimized Cascade (POC).20−23 A new version of the cascade process, known as Phillips’ Optimized Cascade (POC), was developed and applied to a Trinidad LNG plant in 1999 by ConocoPhillips as shown in Figure 11.21 This process also uses three pure refrigerants (propane, ethylene, and methane), and each cycle is operated separately at multiple pressure levels.

Figure 7. Cascade refrigeration cycle.14

2.2. Commercial Natural Gas Liquefaction Processes. Many liquefaction processes have been developed and applied in LNG plants over the last few decades. The processes can be classified into three general categories based on the type of refrigeration cycle and equipment used: a cascade process using pure refrigerants, a mixed refrigerant process using refrigerant mixtures, and an expander process using expanders instead of Joule−Thomson (J−T) valves. Each category is described in detail in the next three subsections. Additional classification is possible based on whether or not a precooling process is applied. 2.2.1. Cascade. Technip/Air Liquide Cascade.19 The first cascade process, designed by Technip/Air Liquide, was applied E



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Figure 8. Simple Linde liquefaction process.15.

relatively low. Therefore, this process is suitable for large capacity trains. The current capacity per train is 5.2 MTPA. 2.2.2. Mixed Refrigerant. APCI Single Mixed Refrigerant (SMR).19 The first single mixed refrigerant (SMR) process was developed by APCI and applied in the Marsa el-Brega plant in 1970. This process employs a single cycle with a mixture of nitrogen and hydrocarbons (methane, ethane, propane, etc.) as a refrigerant instead of several pure refrigerants as in the cascade process. The condensation and evaporation steps are carried out in a single cycle over a wide range of temperatures to cool the natural gas to about −160 °C.19 The cycle uses CWHE as the main cryogenic heat exchanger (MCHE). This process was developed to decrease the amount of equipment required compared to the cascade process; however, a relatively greater amount of power is required for this process than the cascade process due to the larger refrigerant flows in former process. Technip/Air Liquide TEALARC.24 The TEALARC twopressure single mixed refrigerant process was developed by Technip/Air Liquide and applied in the Skikda plant. This process uses a single mixed refrigerant cycle with a 33% increase in capacity per train. This process consists of two refrigeration cycles, precooling and liquefaction, as shown in Figure 12.24 In the precooling cycle, the refrigerant used in the liquefaction cycle is precooled and partially condensed by a mixed refrigerant composed mainly of ethane and propane. The liquefaction cycle cools the natural gas by a mixed refrigerant comprising mainly methane and ethane. Black & Veatch Pritchard PRICO.25−27 The PRICO process, one of the well-known SMR processes, was developed by Black & Veatch Pritchard.25 This process was used in the Skikda (Algeria) plant in the 1970s. This process uses a single mixed refrigerant composed of a set of nitrogen and hydrocarbons such as methane, ethane, propane, butane, and pentane.22,26 As shown in Figure 13,25 cooling and liquefaction steps are performed at several pressure levels. The mixed refrigerant is compressed by a compressor and then condensed in the main heat exchanger. The refrigerant is expanded through a Joule− Thomson (J−T) valve and then evaporated as it returns through the main heat exchanger. This process uses PFHEs in cold boxes. The axial compressors are driven by steam turbines.22 APCI Propane Precooled Mixed Refrigerant (C3MR).28−32 The propane precooled mixed refrigerant (C3MR) process developed by APCI is dominant in the LNG plant market. A basic schematic of the C3MR process, which consists of two main stages, namely propane precooling and mixed refrigerant (MR) stages, is shown in Figure 14.28 The precooling cycle cools the natural gas to around −40 °C at three or four different pressure levels using a pure propane refrigerant. This cycle may

Figure 9. Reverse-Brayton cycle.15

Figure 10. Classic cascade process.13

The process has evolved from the original cascade process, in that the methane cycle is now an open cycle or a feed-flash system, rather than a closed cycle. This improvement enables a separate fuel gas compressor to be eliminated, as well as allowing stored vapors and vapors from tanker loading to be used for reliquefaction rather than being routed directly to fuel or flare, thereby increasing LNG production.20 The parallel arrangement of gas turbine/compressor sets in each refrigerant cycle increases availability and allows easier operation. Further, the process configuration enables the same amount of power to be used in each cycle.21 Refrigeration and liquefaction are achieved in a series of PFHEs arranged in vertical cold boxes. Precooling can be carried out in a core-in-kettle type exchanger. The refrigerants are compressed by centrifugal compressors driven by gas turbines.22 The latest POC process uses highly efficient aero-derivative gas turbines for the liquefaction process.23 The cascade process requires relatively high capital and maintenance costs due to the amount of equipment needed for the refrigerant cycle, even though the power requirements are F



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Figure 11. POC process.21

Figure 12. TEALARC process.24

and is flashed across a J−T valve on the shell-side of the MCHE. This stream flows downward to provide cooling duty for the cold bundle. Then, the vapor and liquid output MR streams are merged to provide partial cooling duty for the lower bundle. The overall vaporized MR is compressed up to 45−48 bar. It is cooled and partially liquefied, first by ambient air and cooling water, and then by the propane in the precooling cycle. Precooling is achieved in a kettle-type heat exchanger. The MR cycle uses a CWHE as the MCHE. Propane compression is carried out by a centrifugal compressor. In earlier plants, only centrifugal compressors with steam turbine drivers were used for MR compression. However, recently constructed plants use axial compressors for the low-pressure (LP) stage and centrifugal compressors for the high-pressure (HP) stage, together with gas turbine drivers. In the most recently constructed plants, the SplitMR technology, in which a driver is associated with a set of

also be used to cool and partially liquefy the mixed refrigerant. To employ propane for cooling the natural gas, the propane is compressed to a high pressure at which it can be condensed by ambient air or cooling water. In the MR cycle, a mixed refrigerant comprising nitrogen, methane, ethane, propane, and sometimes butane is used in a single MCHE to liquefy and subcool the natural gas from typically −35 °C to between −150 and −162 °C.29,30 The refrigerant that has been processed in the precooling cycle is separated in a high-pressure separator. The liquid and vapor MR streams pass through separate circuits in the MCHE. The liquid MR stream participates in cooling in the warm bundle of the MCHE that cools the natural gas, and is flashed across a J−T valve on the shell-side of the MCHE. The liquid MR evaporates and flows downward to provide cooling duty for the lower bundle. The vapor MR stream is used to liquefy and subcool the natural gas stream to −162 °C in the cold bundle, G



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Figure 15. DMR process.33

Figure 13. PRICO process.25

This process uses two CWHEs supplied by Linde or APCI, and different amounts of power are required for each cycle. This process was selected for the Sakhalin LNG project, in which each train was designed for 4.8 MTPA LNG production. Parallel Mixed Refrigerant (PMR) Process.37 Shell developed the parallel mixed refrigerant (PMR) process, which consists of precooling and liquefaction cycles, as shown in Figure 16.37 Either propane or MR can be used as a refrigerant

Figure 14. C3MR process.28

propane and MR compressors, is used in the C3MR process. This configuration allows full utilization of the gas turbine power, thereby increasing the train capacity for the same number of compressors and drivers.30 Since the first LNG plant using the C3MR process was commissioned in 1972 in Brunei, the capacity of a single train has increased from less than 0.5 MTPA to about 5 MTPA. Shell and APCI Dual Mixed Refrigerants (DMR).33−36 Shell and APCI developed the dual mixed refrigerant (DMR) process to overcome the inherent limitations of compressor size in using a pure propane refrigerant for the C3MR process. As shown in Figure 15,33 this process has a configuration similar to the C3MR process, comprising two separate cycles, precooling and liquefaction.33 Use of a mixed refrigerant (composed mainly of ethane and propane) instead of pure propane in the precooling cycle allows for more flexible design while maintaining the compressor configuration. Natural gas is cooled to about −50 °C in the precooling cycle and then liquefied and subcooled to about −153 °C in the liquefaction cycle using a mixture of nitrogen, methane, ethane, and propane. LNG is produced using a liquid expander and end-flash vessel at its atmospheric boiling temperature of about −161 °C.35

Figure 16. PMR process.37

for the precooling cycle. The main feature of the PMR process is that two MR cycles for liquefaction are configured in parallel, which reduces the pressure drop in the system and improves the reliability of the plant, thereby improving process efficiency; the train capacity can reach 8 MTPA using existing compressors.37 IFP/Axens Liquefin.33,34 IFP/Axens developed the Liquefin process with two mixed refrigerant cycles, precooling and liquefaction cycles, as shown in Figure 17.33 The two refrigerants are composed of methane, ethane, propane, butane, and nitrogen, but the compositions differ in each cycle. The heavy MR precooling cycle is used to cool the natural gas, and precool and liquefy the other mixed refrigerant at three different pressure levels. The light MR liquefaction cycle is used to liquefy and subcool the natural gas. Both cycles are carried out in PFHE H



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liquefy and subcool the natural gas. Similar to the single cycle processes, precooling, liquefaction, and subcooling are carried out in the same heat exchanger. APCI AP-X.39,40 The AP-X process was developed from the C3MR process by APCI. As shown in Figure 19,40 this process comprises three cycles: a propane precooling cycle, a mixed refrigerant cycle, and a nitrogen subcooling cycle. A unique feature of this process is that the LNG is subcooled using a nitrogen expander cycle rather than a mixed refrigerant cycle.39 Natural gas precooled to about −30 °C in the propane cycle using kettle-type heat exchangers is cooled and liquefied to about −120 °C in the MCHE with a mixed refrigerant. The LNG is subcooled using cold gaseous nitrogen from the nitrogen expander. In the nitrogen cycle, nitrogen is compressed to a high pressure and then cooled to near ambient temperature. The highpressure nitrogen is cooled in a nitrogen PFHE-type economizer with low-pressure nitrogen returning to the compressor. The high-pressure nitrogen passing through the nitrogen economizer is expanded to a low pressure to further reduce its temperature in the expander. Compared to the C3MR process, the nitrogen expander subcooling cycle allows the flow of both propane and mixed refrigerant to be reduced without affecting production, enabling much higher capacities (approximately 8 MTPA) using existing equipment.40 CWHEs are used for the MR and nitrogen subcooling cycles. Statoil-Linde Mixed Fluid Cascade (MFC).41,42 The mixed fluid cascade (MFC) process was developed by Linde in collaboration with Statoil and was applied in the Snohvit LNG project. The capacity of a single train that uses this process is 4 MTPA. As shown in Figure 20,41 this process is similar to the cascade process and also consists of the three cycles of precooling, liquefaction, and subcooling.41 Compared to the cascade process, the MFC process has higher efficiency as it uses three mixed refrigerants instead of three pure refrigerants. The mixed refrigerants are composed of methane, ethane, propane, and nitrogen, but the compositions differ in each cycle. Another feature is that the power requirements for each cycle are not the same, unlike the POC process. The precooling cycle uses PFHE, while the liquefaction and subcooling cycles use CWHE.22 2.2.3. Expander. Single Nitrogen Expander.43,44 Nitrogen expander processes, which are based on reverse-Brayton and

Figure 17. Liquefin process.33

arranged in a cold box. Each cycle is designed to use the same amount of power so that the same set of drivers can be used for the compressor across different cycles, which translates into significant cost savings. In addition, the relatively lower flow rate of the mixed refrigerant permits a much greater train capacity to be achieved with existing axial compressors.34 Gaz de France Integral Incorporated Cascade (CII).38 A new single cycle process named the Integral Incorporated Cascade (CII) process that was developed by Gaz de France is shown in Figure 18.38 This process uses a mixture of nitrogen and hydrocarbons (from methane to propane) as a refrigerant and consists of three subunits: a compression line, a cycle fluid fractionation unit, and a heat-exchange line. The cycle fluid fractionation unit separates the mixed refrigerant into two fluid types: heavy fluid (pentane and butane) and light fluid (nitrogen, methane, and ethane). The heavy fluid is used to precool the natural gas, while the light fluid is used to liquefy and subcool the natural gas. A single heat-exchange line consists of two large cores of PFHEs and comprises two sections: an upper section to precool the natural gas, and a lower section to

Figure 18. CII process.38 I



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Figure 19. AP-X process.40

Figure 20. MFC process.41

Figure 21. Single expander process.44

Claude cycles, are used mostly in offshore and small-scale liquefaction plants. Nitrogen expander processes have been widely used for cryogenic liquefaction, including LNG peakshaving, and in industrial gas liquefiers.43 A simple single nitrogen expander cycle is illustrated in Figure 21.44 Refrigeration is carried out through compression and work-expansion using nitrogen as the refrigerant. High-pressure nitrogen is cooled in the heat exchangers, with low-pressure

refrigerant returning to the compressor. The high-pressure nitrogen is then work-expanded in the expander to reduce its temperature. The expander generates simultaneously useful work, which is usually supplied to the booster compressor. The lowpressure nitrogen from the expander liquefies natural gas and cools the high-pressure nitrogen in heat exchangers. Nitrogen passing through the heat exchangers is compressed by the main cycle compressor and booster-compressor. The process efficiency J



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Figure 22. Double expander process.45

Figure 23. Dual independent expander process.48

methane increases the need to ensure adequate spacing between equipment to prevent jet fires and blast pressure damage.48 BHP Compact LNG (cLNG).49 BHP and Linde developed a compact LNG (cLNG) process similar to the SMR process, except that pure nitrogen refrigerant is used. As shown in Figure 24,49 this process is operated under two pressure levels of nitrogen expansion to improve thermodynamic efficiency. To cool the nitrogen to a low-enough temperature to liquefy natural gas, the cLNG process uses both self-cooling and turbo expanders.49 The power generated from the turbo expander is recovered and used to recompress the refrigerant. Other Expander Processes.50−54 Existing expander processes can be modified to increase their efficiency.50 For example, propane precooling can reduce power consumption by approximately 20% in conventional expander processes. Many expander processes have been developed by various vendors, e.g. EXP by Kryopak,51 MiniLNG by Hamworthy,52 Optimised Expander Cycle by Kanfa Aragon,53 and LNG Smart Liquefaction Technologies (OCX-Angle, OCX-R, OCX-2 and NDX-1) by Mustang Engineering.54 2.3. Criteria for Process Selection. We have described a variety of different commercial natural gas liquefaction processes that are currently used in LNG plants or that have been proposed for LNG projects. The success of a LNG project is crucially dependent on the processes selected for natural gas

is relatively low because a pure gas refrigerant is used over a wide temperature range. Therefore, the single nitrogen expander process is only suitable for plants with small capacities. Double Nitrogen Expander.45,46 The double nitrogen expander process is a modification of the single nitrogen expander process. This process has been widely used to liquefy nitrogen and oxygen for the last few decades. This process comprises two expander cycles: warm and cold expander cycles, as shown in Figure 22.45 Both expander cycles enable natural gas to be liquefied and subcooled at small temperature differences, reducing the specific power requirements but increasing the size of the heat exchanger required. Nitrogen refrigerant can be substituted with methane refrigerant in the existing process. The methane refrigerant may reduce the specific power for liquefaction, but this is outweighed by the safety implications of using a hydrocarbon refrigerant rather than inert nitrogen.46 Dual Independent Expander.47,48 As shown in Figure 23,48 the dual independent expander process employs two separate refrigeration cycles, methane and nitrogen cycles, to improve process efficiency by reducing the temperature difference between liquefaction and subcooling. While this process requires a larger heat exchanger than a single expander process, the required specific power can be reduced compared to that required for a single expander process. However, the process safety decreases because of the use of a hydrocarbon refrigerant. The use of K



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exchangers. Some processes use proprietary equipment. For example, CWHE used in C3MR, AP-X, and MFC is proprietary to both APCI and Linde. In addition, some equipment causes bottleneck problems in terms of the capacity of a LNG plant. In a propane cycle, the capacity of propane compressors is limited by the Mach number at the tip of the blades.21 The pros and cons of equipment used in natural gas liquefaction processes are listed in Table 3.22,56,57 Other aspects that should be considered when selecting appropriate processes are reliability, site conditions, safety, process requirements (NGL or LPG recovery), train capacity, availability of equipment, space, and total costs (including capital and operating costs). However, the priority of each item in this list will differ depending on the characteristics of the LNG project. In offshore plant projects, energy efficiency may be less important than safety, operability, and compactness compared to onshore projects. The evaluation criteria for successful offshore LNG projects are listed in Table 4.44−48 The liquefaction processes used for LNG plants in operation are summarized in Figure 26. Based on existing LNG plants and other portfolios provided by APCI and Shell,9,58,59 a portfolio for the selection of a liquefaction process is proposed in Figure 27.

Figure 24. cLNG process.49

liquefaction. We therefore propose a set of criteria for selecting appropriate processes, taking technical and economic aspects into consideration. One key aspect is process efficiency. This is not only related to the thermodynamic efficiency of the process, but also to the efficiency of the equipment. Improving thermodynamic efficiency leads to a reduction in energy consumption and capital costs. A higher thermodynamic efficiency can be achieved by applying composite curves. Typical warming and cooling curves in liquefaction processes are presented in Figure 25.

3. PATENTING ACTIVITIES FOR NATURAL GAS LIQUEFACTION PROCESSES There are a vast numbers of patents related to natural gas liquefaction technologies. While associated patent publication activity began before the 1970s, patent activity only started increasing rapidly in the mid-1970s. Figure 28 shows trends in patenting activities regarding natural gas liquefaction technologies in the U.S. for the period from 1975 to 2007.60 Patenting activity remained stagnant in the 1990s; however, there has been a sharp increase in activity in the 2000s due to the steep increase in demand for natural gas liquefaction plants. A list of representative patents with a high citation index and/or of great influence are summarized in Table 5;32,36,61−78 the dominance of leading companies such as APCI, ConocoPhillips, Exxon Mobile, and Shell is apparent, and the majority of the patents are focused on process developments and associated equipment improvements. Two APCI patents granted in the U.S. (US 4,755,200 and US 4,277,949), which are among the most influential patents filed in the field of natural gas liquefaction technologies, are concerned with a mixed refrigerant system combined with closed-loop pure refrigerant cycle. In general, the profitability of natural gas liquefaction plants is highly dependent on the liquefaction process design and configuration. Process technologies have evolved from simple cycles to complex cycles to enhance process efficiency in association with plant capacity, natural gas composition, project objectives, and site conditions. While natural gas liquefaction was mainly performed by partial condensation in the early patents, since the 1990s, the focus has shifted to liquefaction or condensation of gas or gas mixtures. Recent patent publications are concerned with complex cycles that have a higher efficiency due to the combination of two or more different conventional single cycles that have synergistic effects. The technical trends in patents for natural gas liquefaction processes are shown in the upper portion of Figure 29.60 Equipment technologies for the natural gas liquefaction have focused mainly on improvements in heat exchangers and expansion equipment. Early patents were primarily concerned with cryogenic plate-fin heat exchangers and compressors

Figure 25. Typical composite warming and cooling curves.

Mixed refrigerant processes have smaller mean temperature differences between the curves in the heat exchanger than pure refrigerant processes, reflecting better heat exchange performance in the mixed refrigerant processes. However, this leads to require relatively larger heat exchangers than those in pure refrigerant process. According to Ransbarger,55 the performance of liquefaction process can be improved by changing the number of refrigeration stages. This means that it is necessary to optimize the number of stages to increase efficiency. Configurations of natural gas liquefaction processes are summarized in Table 2. Each process has its characteristic refrigeration systems and uses different refrigerants and heat L



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Table 2. Comparisons of Refrigeration Cycle Configurations refrigerant

heat exchanger

process

precooling

liquefaction

subcooling

precooling

liquefaction

POC PRICO C3MR DMR Liquefin PMR CII AP-X MFC

propane MR propane MR MR propane or MR MR propane MR

ethylene

methane

PFHE or core-in-kettle cold box (PFHEs) core-in-kettle CWHE PFHE

PFHE

MR MR MR parallel MR MR MR

heat exchanger

type coil-wound plate-fin

compressor

centrifugal

axial

pros robust/high operability competitive vendors low pressure drop low temperature differences robust/light/simple design low manufacturing cost high efficiency high compression ratio

driver

steam turbine

gas turbine

aero-derivate turbine

electric motor

a

several vendors high reliability and availability cost effective/high efficiency small space/ease of installation suitable for high flow rates

high efficiency high reliability and availability ease of installation/ small space shorter maintenance period high efficiency/no emissions high operability and availability flexible maintenance simple layout

CWHE and PFHE CWHE

Table 4. Evaluation Criteria for Offshore LNG Projects44−48

Table 3. Pros and Cons of Available Equipment for Use in LNG Plants22,56,57 equipment

PFHE

CWHE CWHE PFHE

heat-exchange line (two PFHEs) core-in-kettle CWHE PFHE CWHE

N2 MR

subcooling

criteria

cons

thermal efficiency complexity equipment count hydrocarbon refrigerant storage overall space requirement compactness and lightness simplicity of operation ease of start-up/shutdown flexibility sensitivity to vessel motion capital investment

proprietary/more expensive careful design vulnerable to upsets

impossible at high flow rates low efficiency and compression ratio suitable only at high flow rates more expensive vulnerable to FODa large space requirements more expensive

cascade

mixed refrigerant

expander

high high high large

medium−high medium low−medium medium−large

low low low none

high low medium medium high medium high

medium medium medium low medium medium low−medium

low medium−high high high high low low

turbines. The technical trends in patents dealing with equipment enhancements are shown in the lower section of Figure 29.60

4. RESEARCH ON THE LIQUEFACTION PROCESSES A number of studies have investigated how to reduce the cost and improve the efficiency of liquefaction processes. These studies can be roughly divided into those that evaluated the performance of liquefaction processes, those that investigated the optimal design and operation of liquefaction processes, and those that improved the performance of entire LNG plants. 4.1. Evaluation on the Performance of Liquefaction Processes. The majority of studies that have evaluated liquefaction processes have focused on thermodynamic efficiency. Several of these studies used exergy analysis to evaluate thermodynamic efficiency. Exergy analysis is well-known as a useful method in studies on the effectiveness of an energy system, and its fundamental principles and methodology can be found in literatures.79,80 Kanoglu et al.81 developed relations that can be used in first/second-laws analyses of a simple Linde−Hampsone cycle used in gas liquefaction systems. They developed a set of expressions for the minimum work requirement and performed numerical calculations to obtain the performance parameters. They found that the minimum work depends only on the properties of the incoming and outgoing gas streams. Kanoglu82 performed an exergy analysis of the multistage cascade refrigeration cycle used for natural gas liquefaction. He developed a set of equations for exergy destruction and exergetic efficiency of the main equipment, and determined the relations between the total exergy destruction in the cycle and the cycle exergetic efficiency. He reported that the exergetic efficiency of a multistage cascade refrigeration cycle is only

low thermal efficiency and reliability intensive maintenance high CO2 emissions fixed size and optimum speed high NOx emissions intensive maintenance fixed size and optimum speed

large power plant starting problems with large motors

FOD: Foreign object damage.

equipped with relief valves. Since the 1990, patents related to expansion equipment combined with compressors or pumps, and coil-wound heat exchangers have appeared. The focus of recent patents has been alternatives to gas turbines in the refrigeration cycle and compressors driven by single-shaft gas M



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Figure 26. Current status of a LNG plant in operation.

Figure 27. Available portfolio for the selection of a liquefaction process.9,58,59

38.5%, indicating great potential for improvement. Remeljej and Hoadley45 performed an analysis of four small-scale LNG production processes: SMR, cLNG, and two open-loop processes. These authors made more general and diversified analyses and comparisons through exergy analysis, and concluded that there was still room for improvement of the process efficiency within each of the schemes. Venkatarathnam83 evaluated the performance of the mixed refrigerant LNG process through exergy analysis. A number of mixed refrigerant processes were discussed: a single-stage mixed refrigerant LNG process without phase separators, a precooled LNG process without phase separators, a single-stage mixed refrigerant LNG process with a phase separator, a precooled LNG process with a phase separator, a propane precooled phase separator (C3MR) process, a mixed refrigerant precooled phase separator (DMR) process, a LNG process with multiple phase separators (Kleemenko process), a cascade liquefaction process operating with mixtures, and a LNG process with turbines. The exergy efficiencies of liquefaction processes employed precooling cycle were nearly the same. Further, precooled liquefaction

processes were preferable for large liquefaction systems. On the other hand, processes with phase separators such as the Kleemenko process or the PRICO process may be more suitable for small LNG plants because of their simplicity. Exergy analysis is a powerful tool with which to obtain useful information about the maximum performance of an energy system, the pattern of lost work, and direction for potential improvements. Furthermore, exergy analysis can be performed by analyzing the components of a system separately.81 Other studies have compared efficiency of processes by calculating the shaft power requirements of compressors. If the condition of the feeds and the products and the design of the liquefaction process are specified in advance, the shaft work is the main determinant of the operating cost. Specific work is determined by power consumption per unit mass of LNG and can be relatively easily calculated. Table 6 summarizes the specific work values of processes, which were obtained from various references.13,32,38,39,46 Different specific works were reported by different studies, as shown in Table 5. Each study claimed that the different process was the most efficient. N



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Figure 28. Trend of U.S. patenting activities for natural gas liquefaction technologies.60

Table 5. Major Cited Patents and Influencing Patents32,36,61−78 U.S. patent

year

assignee

3,957,47361 4,445,91662 4,525,18536 4,755,20032 5,791,16063 6,250,10564 6,347,53265 6,658,89066 6,691,53167 6,722,15768 6,725,68869 6,789,39470 7,100,39971 7,234,32272 4,265,30273 4,566,88574 4,880,05575 6,640,58676 7,069,73377 7,266,97678

1976 1984 1985 1988 1998 2001 2002 2003 2004 2004 2006

Exxon Research Engineering APCI APCI APCI APCI Exxon Mobil Upstream Research APCI ConocoPhillips ConocoPhillips ConocoPhillips Shell Oil

process process process process process process process process process process process

objective

mixed refrigerant cycle with multiple heat exchanger separation of heavier hydrocarbons in a scrub column dual mixed refrigerant cycle with staged compression precooling cycle with pure or mixed refrigerants enhanced control system for mixed refrigerant cycle Dual multicomponent refrigeration cycle Partial condensation of mixed refrigerant Open methane cycle employed for natural gas liquefaction Optimum configuration of drivers and compressors nonvolatile refrigerant employed in the natural gas liquefaction system advanced control based on model predictive control

contents

2006 2007 1981 1986 1989 2003 2006 2007

ConocoPhillips ConocoPhillips Rosenthal Technik AG Shell Oil Sundstrand ConocoPhillips APCI ConocoPhillips

process process equipment equipment equipment equipment equipment equipment

enhanced operation with refluxed heavies removal column nitrogen removal from a relatively warm natural gas stream heat exchanger with a ceramic body compressor system for gas liquefaction light and compact impingement plate-type heat exchanger electric motors employed as compressor drivers compressor driven by single-shaft gas turbines vertical heat exchanger comprising one or more core-in-kettle heat exchangers

varied depending on the plant capacity and NGL recovery. Yin et al.85 performed an economic analysis of a single mixed refrigerant cycle and nitrogen expander cycle. They concluded that the SMR process was a more suitable process for smallscale liquefaction because it is comparatively simple and consumes less energy than the other processes they evaluated. 4.2. Optimal Design and Operation. There are a number of studies related to the design, simulation, and optimization of the liquefaction process. Studies that focused on the design of the liquefaction process in a LNG plant using simulation are discussed in the next paragraph. Kikkawa86 studied the design of a precooling mixed refrigerant cycle and expander cycle by simulation using CHEMCAD.

One possible reason for the different results is that comparisons were made under different conditions. In addition, different levels of optimization and the use of different equipment and efficiencies in each process could also explain the discrepancies among studies. To accurately compare various processes, evaluation criteria should include not only operating cost, but also capital cost. However, cost estimation of liquefaction processes is difficult because of the absence of detailed information. Only a few studies have incorporated capital cost into economic analyses. Kotzot et al.84 evaluated the economics of a LNG project based on the specific plant cost, which they expressed as $/tonne of LNG production. They reported that the cost of a LNG plant O



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Figure 29. Technical trends in patents for natural gas liquefaction processes and related equipment.60.

Table 6. Efficiency Comparisons of the Liquefaction Process Based on Specific Work13,32,38,39,46 relative specific work (specific work, kJ/kg) 13

process cascade SMR C3-MR DMR MFC single N2 expander C3 precooled single N2 expander double N2 expander a

Dam et al.

32

32a

Finn et al. (relative to cascade)

Dam et al. (relative to DMR)

Foerg38 (relative to MFC)

Vink et al.39 (relative to C3-MR)

Barclay et al.46 (relative to C3-MR)

1.00 (1188) 1.25 (1485) 1.15 (1366)

1.39 (1382)

1.155 1.142 1.033

1.156 (1218) 1.189 (1253) 1.000 (1054)

1.000 (1054)

1.06 (1054) −1.09 (1083) 1.00 (994)

1.025 (1080) 1.000

2.00 (2376)

3.10 (3266) −3.32 (3499)

1.70 (2020) 1.70 (2020)

1.35 (1426)

only reported relative values, not detailed values.

Terry87 performed simulation and optimization of a typical liquefaction process in a peak shaving plant using HYSYS commercial software. Li et al.88 designed small-scale LNG systems that used a nitrogen expander cycle and mixed refrigerant cycle based on numerical simulations and system optimization. Cao et al.89 designed and simulated two typical types of small-scale liquefaction processes in a skid-mounted package. They reported that the N2-CH4 expander cycle preceded the mixed refrigerant cycle on the premise of lacking propane precooling. In addition, it was found that large temperature differences and high heat exchange loads were the primary reasons for exergy loss in heat exchangers. Jensen and Skogestad90 studied design optimization of the PRICO process. The process was modeled and optimized using gPROMS, and Multiflash was used for thermodynamic calculations. The objective function included design parameters (e.g., heat transfer area) and operating parameters (e.g., flow rate, pressure, splits) with respect to thermodynamics and cost. Further, a set of key constraints concerning compressor feasibility was discussed. Li and Ju91 conducted research on the design of three different liquefaction processes including SMR, C3MR, and N2 expander for special offshore gases in the South China Sea. A systematic analysis and comparison of these

processes was performed to select a process suitable for use in an offshore plant. It was concluded that the N2 expander process was the most suitable for LNG Floating Production Storage Offloading (FPSO) due to its inherent safety, ease of operation, and simple and compact design. The results of exergy analysis revealed that compression equipment and aftercoolers, expansion devices, and LNG heat exchangers were the main equipment contributing to the total exergy losses. Combined multistage Brayton−JT cycles were proposed by Chang et al.92 They applied thermodynamic optimization theory for three refrigeration systems: a nitrogen Brayton and ethylene JT (N2−C2) cycle, a nitrogen Brayton and propane precooled ethylene JT (N2−C2/C3) cycle, and a nitrogen Brayton and ethylene propane JT (N2−C2−C3) cycle. They found that the N2−C2−C3 cycle not only had a higher efficiency than the other cycles, but also the potential for higher capacity than the other cycles. Al-Sobhi et al.93 simulated and optimized a typical LNG plant by using ASPEN Plus to determine flows, temperatures, and heat duties for various equipment and streams. In addition, they performed thermal pinch analysis to reduce heating and cooling utilities. In a case study, the result showed a 15% possible reduction in heating utilities and a 29% reduction in cooling utilities. P



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energy consumption. Their methodology is based on a mixedinteger nonlinear programming (MINLP) formulation. To reduce the MINLP model complexity, thermodynamic calculations were carried out using a simplified model based on the regression of rigorous simulation results. The optimization problem was solved using the LINDOGloal solver in GAMS. The optimization results were validated through rigorous simulation using Aspen Plus. A case study of the C3MR process showed that the proposed methodology reduced the energy consumption by 13%. Aspelund et al.103 developed a methodology for process synthesis by extending traditional pinch analysis with exergy analysis, that is, Extended Pinch Analysis and Design (ExPAnD). ExPAnD showed great potential for improving subambient processes. Energy requirements (total shaft work) were minimized by optimizing the compression and expansion work of process streams, as well as the work required to generate necessary cooling utilities. The efficacy of the proposed methodology was verified by applying it to design of LNG process: exergy efficiency increased from 49.7% to 85.7% compared to standard pinch analysis. Shah and Hoadley104 proposed a shaft work targeting method for multistage gas-phase auto refrigeration systems. This targeting method demonstrated the relationship between the expansion/compression ratio and the heat exchange design parameter delta Tmin. In the case of natural gas liquefaction, a dual gas-phase system can achieve the net shaft work requirements with values only slightly higher than those of single mixed refrigerant systems. Jensen and Skogestad105 warned that the method using specified delta Tmin when designing heat exchangers could lead to wrong decisions. The minimum temperature approach favors a more constant temperature difference profile; however, in the case of optimal operation, it was found that the temperature difference at one end was small. These authors proposed a simplified total annualized cost (TAC) method that considers both operating and capital costs. In addition, the results showed that there is a trade-off relationship between heat exchanger capital cost and compressor power. More recent studies have discussed mixed refrigerant systems with a focus on mitigating the energy requirements for refrigerant compression through optimizing operating conditions such as pressure, flow rate, and the composition of mixed refrigerants. Cammarata et al.106 presented a genetic algorithm (GA)based optimization methodology for liquefaction/refrigeration systems. Nogal et al.100 also used a GA-based approach for optimal design of mixed refrigerant cycles. Shirazi and Mowla107 attempted to minimize energy consumption in the PRICO process designed for a peak shaving plant using a GA implemented in MATLAB. The set of design variables included pressure, flow rate, and composition of the mixed refrigerant. The Peng−Robinson (PR) equation of state (EOS) was used for thermodynamic calculations of process streams. The optimization results showed that the specific work of the process could be reduced by 3% or 6.5% as compared to the results of Lee et al.99 Taleshbahrami and Saffari108 performed thermodynamic simulation and optimization of the C3MR process using a GA approach. Modeling and simulation of the C3MR process was carried out using MATLAB. The thermodynamic properties were calculated by PR EOS. Minimization of total compressor power was defined as the objective function. The optimal power consumption was reduced by 23% as compared to the base case. Furthermore, the hot and cold composite

In academia, the majority of publications have focused on determining optimal design and operating conditions. Several studies have used a systematic approach to design refrigeration and liquefaction systems. Barnes and King94 focused on the synthesis of pure refrigerant cascade systems. They derived and evaluated numerous design rules, and identified the minimum-cost refrigeration system configuration using a dynamic programming method. While the approaches they used could handle detailed equipment cost correlations and thermodynamic property models, only a limited number of refrigerant levels and operating temperature ranges were determined by using a heuristically developed procedure. Cheng and Mah95 developed an interactive computational strategy for designing cascade refrigeration systems. Refrigerants were chosen on the basis of the desired operating temperature range and temperature of the process streams to be cooled. Vaidyaraman and Maranas96 proposed a systematic methodology to simultaneously select a set of pure refrigerants for use at each stage and to design the topology of the refrigeration system. They used mixed-integer linear programming to minimize the weighted sum of investment and operating costs. Vaidyaraman and Maranas97 also addressed the synthesis of mixed refrigerant cascade cycles within an optimization framework. Both the model equations for configuring the mixed refrigerant cascade cycle and the design objective of minimizing the total work input taking refrigerant compositions, pressure levels, and vaporization fractions in the flash tanks into account were incorporated into an optimization model that was formulated as a nonconvex nonlinear program (NLP). Kim et al.98 proposed a synthesis method for designing and retrofitting industrial refrigeration systems for subambient temperature cooling, together with exergy analysis. Their proposed method was based on optimization of the composition of the mixed refrigerant with the objective of maximal power savings. They reported that the shaft work requirement could be reduced by 19.5%, compared to the commercial PRICO process. Lee et al.99 developed a systematic synthesis method to determine the optimal operating conditions of a mixed refrigerant system by using nonlinear programming (NLP) techniques with a set of key variables such as pressure level, refrigerant flow rate, and refrigerant composition. Three different types of objective functionsminimization of crossover, the sum of crossovers, and shaft work requirementwere employed. They concluded that approximately 25% of the shaft work requirement could be reduced in the PRICO process. The limitation of their proposed method is that it is to unable to simultaneously take pressure levels and refrigerant flow rates into account during optimization of the refrigerant composition. Nogal et al.100 proposed a genetic algorithm (GA)-based approach to optimize mixed refrigerant cycles in terms of multistage refrigerant compression, full enforcement of the minimum temperature difference in heat exchangers, and simultaneous optimization of variables and capital costs. The objective function was minimum compressor power as well as minimum capital cost. Nogal et al.101 also proposed a design methodology for a cascade mixed refrigerant system with multistage heat exchangers. Optimization was performed with a set of key decision variables, namely economic trade-offs, partition temperature, refrigerant compositions, operating conditions, and refrigerant flow rate. Wang et al.102 suggested a methodology for synthesizing natural gas liquefaction systems with the target of minimizing Q



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minimization of Net Present Value (NPV) was optimal. However, for cases with limited area available, such as offshore production plants, minimization of the objective function (sum of compressor power across all compressors plus sum of UA values across all heat exchangers) was more appropriate. Hassan et al.116,117 derived optimal compressor operations in C3MR and AP-X processes to minimize total power consumption. However, one flaw of their model was their failure to account for uncertainties in operating conditions such as natural gas feed composition, flow rate, pressure, temperature, and seasonal variations. There are few publications related to the control of LNG plants. Singh and Hovd118 studied the effect of simplification of the heat exchanger model on systematic control structure design in the PRICO process in order to develop a dynamic model. The simplified model (heat transfer through conduction along the longitudinal direction of the metal wall was neglected, enthalpy was assumed to be a conserved property rather than internal energy, and all streams were assumed to exchange heat via a common wall) did not affect the control structure design and the fundamental limitation of bandwidth in the PRICO process. Jensen and Skogestad119,120 focused on the operation of simple refrigeration cycles. They discussed degrees of freedom, optimality of subcooling, and selection of controlled variables. Further, Jensen and Skogestad121 proposed a control structure to minimize the cost during operation of the PRICO process. Jensen and Skogestad122 focused on determining the degrees of freedom for optimal operation and plant-wide control. The steady-state degrees of freedom available for optimization are very important, because they determine the number of free variables available to solve the optimization problem as well as how many steady-state controlled variables must be selected to operate the process. Methods to determine the potential degrees of freedom and the actual degrees of freedom for vapor compression cycles were presented. The C3MR and MFC processes were studied with respect to operational degrees of freedom to solve the optimization problem related to operation and control. The three studies above illustrate the effect of operating strategy (maximum production and given production) on the number of unconstrained degrees of freedom. Michelsen et al.123 developed systematic methods for integrated process and control structure design and applied them to the TEALARC process design. The proposed methods could improve both the design and operation of plant processes, which could translate into potential economic benefits. In particular, these authors showed how a design problem (e.g., determination of compressor size) could influence the control structure design. Michelsen et al.124 investigated how controlled variables of the regulatory control layer in the TEALARC process could be chosen as linear combinations of measurements using self-optimizing control principles. Self-optimizing control can reduce the need for online reoptimization and may be used in the process design phase to place measurements by reducing the maximum candidate set of measurements to the best possible subset of measurements with acceptable loss. They proposed a relatively simple method for Successive Selection (SS) of measurements and compared it to the more comprehensive Partial Bidirectional Branch-and-Bound (PBB) method for selecting measurements. Michelsen et al.24 developed a dynamic, control relevant, and mechanistic model for operability analysis of the TEALARC process. Although the thermodynamics of this model are simplified, it has sufficient

curves were closer together after optimization. Alabdulkarem et al.109 optimized the C3MR process to reduce power consumption using Aspen HYSYS for thermodynamic calculations and Matlab’s GA optimizer to find the global optimum. The propane precooling and mixed refrigerant cycles were optimized separately. The optimization variables included not only pressures, refrigerant composition, and refrigerant flow rate, but also propane flow rate, pressure levels, and the split ratio. In addition, optimization was carried out while changing the pinch temperature in heat exchangers to investigate the effect of pinch temperature on LNG plant power consumption. Power consumption was significantly reduced at a pinch temperature of 1 K as compared to pinch temperatures of 3 or 5 K. However, a low pinch temperature (0.01 K) resulted in little improvement as compared to 1 K. The power consumptions of C3MR process were decreased by 6.98% and 13.6% compared to previous publications. Aspelund et al.110 optimized the PRICO process using an optimization-simulation model that combined the Tabu Search (TS) and the Nelder-Mead Downhill Simplex (NMDS) methods. The local optimal solution obtained from the TS was fine-tuned with NMDS to reduce the number of simulations. The optimization model was connected to Aspen HYSYS through Microsoft Excel Visual Basic for Applications (VBA). The objective function was to minimize energy requirements using a set of decision variables consisting of refrigerant flow rate, refrigerant composition, and suction and condenser pressures. Morin et al.111 minimized energy consumption for two types of single mixed refrigerant processes, PRICO and TEALARC, using an evolutionary search. They demonstrated that an evolutionary search is a worthy alternative to a sequential quadratic programming (SQP) when the process is complicated. Paradowski et al.112 carried out a parametric study of the C3MR process, focusing on pressures, temperatures, and compressor speeds in the precooling cycle, as well as the composition of mixed refrigerant. Venkatarathnam83 also optimized mixed refrigerant processes using the Sequential Quadratic Programming (SQP) implemented in APSEN Plus software with a set of optimization variables, including compression ratio and mixed refrigerant composition, and the objective function of maximizing cycle exergy efficiency. Tak et al.113 optimized the SMR process using a NLP model to minimize the power consumption of the compressors. Their optimization model included the variables of the compression ratio of the refrigeration cycle, the mixed refrigerant flow rate, and mixed refrigerant composition. Although they reported a decrease in power consumption, their optimization model did not consider feasibility of heat exchangers. Wang et al.114 combined thermodynamic analysis, rigorous simulation, and optimization to minimize energy consumption. They reported energy savings as high as 13.7%. Hatcher et al.115 performed a systematic analysis of optimization formulations for natural gas liquefaction processes. They tested eight objective functions and compared them with each other to identify the most appropriate formulation. Four objective functions focused on operational aspects, while the other four functions concentrated on design aspects. They found that the most effective objective function for operation optimization was to minimize the major operating cost of compressor power. However, the most effective objective function for design optimization was dependent on the requirements of those designing and constructing the process. For the case of no restriction on the area available for LNG plant construction, R



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complexity for both steady-state and dynamic operability analyses. 4.3. LNG Plant Performance. Commercial LNG plants have evolved along with cost-saving improvements in equipment. Equipment enhancements are also important in the design of the liquefaction process for operation of an actual plant. The following studies focused on replacing and configuring equipment to improve the liquefaction process. Kanoglu125 investigated various aspects of cryogenic turbines intended to produce power by replacing the JT valve used in the LNG expansion. Barclay126 investigated the potential for improvement in the SMR and DMR processes using expansion equipment. When the Joule−Thomson (J−T) valve was replaced with a liquid expander or a 2-phase expander, the process efficiencies were improved by 8.5−8.8%. Mortazavi et al.127 suggested several options to enhance the energy efficiency of the C3MR process. Four expansion loss recovery options were created by replacing conventional expansion processes with expanders. The C3MR process enhanced by these options was modeled and simulated using Aspen Plus. Simulation results showed that by replacing the J−T valve with a gas expander, a 2-phase expander, or a liquid turbine, the amount of compressor power required was reduced, expansion work was recovered, and there was an increase in LNG production. By implementing all of these improvements, the energy consumption per unit mass of LNG was reduced by 7.07% by deducting the recovered power from the total required power, and 3.68% without this deduction. Kalinowski et al.128 analyzed the effect of replacing propane chillers with absorption refrigeration systems powered by waste heat from the power-generating gas turbine. They demonstrated that recovering waste heat from a 9 MW electricity generating process could provide 5.2 MW of waste heat for additional cooling to the LNG plant and could save 1.9 MW of electricity. Furthermore, Mortazavi et al.129 investigated enhancements of the C3MR process using an absorption chiller powered by gas turbine waste heat. The absorption chillers both reduced compressor power and gas turbine fuel consumption by 21.32%, and the capital cost of the gas turbine driver was also reduced due to reduced capacity. However, this improvement should recover at least 97% of the gas turbine waste heat. Only a few studies have focused on performance improvements of entire plants and value chains. Hudson et al.130 investigated the effects of integrating the hydrocarbon removal step into the LNG liquefaction process. This integration could potentially produce both LNG and a separate heavier hydrocarbon liquid product using significantly less energy than a process comprising separate liquids recovery and liquefaction processes. Aspelund and Gundersen131,132 proposed a transport chain for utilization of stranded natural gas for power production with CO2 capture and storage. In this transport chain, natural gas was liquefied in an offshore process by utilizing the cold exergy in Liquid Carbon Dioxide (LCO2) and Liquid Inert Nitrogen (LIN). An exergy efficiency of 87% was achieved in the optimized offshore process. With regard to performance and cost, the multistream heat exchanger (MSHE) is very important in LNG plants. Therefore, some researchers have focused on developing a rigorous MSHE model. Hasan et al.133 demonstrated and predicted MSHE behavior using operational data in a LNG plant by modeling. A superstructure of heat exchanger networks and a piecewise quadratic function for calculating the enthalpies of the various phases were employed. Further, Hasan et al.134

discussed extending the traditional heat exchanger network (HEN) synthesis to accommodate nonisothermal phase changes. The limitation of their methodology is that it only generates a feasible solution when the underlying model is nonconvex and highly nonlinear. Wechsung et al.135 developed an optimization formulation for the synthesis of heat exchanger networks. Their model combines pinch analysis, exergy analysis, and mathematical programming. Kamath et al.136 developed an equationoriented (EO) approach based on cubic EOS to focus on MSHE modeling in the NLP formulation, with an emphasis on phase change by piecewise constant heat capacity flow rates. They divided the mainstream into a set of substreams depending on state to optimize the SMR process. The composite curves and dew point and bubble point of each stream changed when temperature was added as a decision variable.

5. FUTURE RESEARCH DIRECTIONS AND CONCLUSIONS Potential improvements in the natural gas liquefaction process used in LNG plants can be realized by taking the following into consideration. • Development of advanced liquefaction processes in terms of the capacities of LNG plants and the characteristics of natural gas fields: The optimal single train capacity and the number of trains should be determined based on consideration of technical and economic facets to achieve economical LNG production. Furthermore, the process should be developed and modified according to the characteristics of natural gas fields. • Single large train/multiple small trains • Large-scale LNG plant/small- or midscale LNG plant • Conventional onshore LNG plant/offshore LNG plant (LNG FPSO) • Improvement of liquefaction processes intended for use in large-scale LNG plants in terms of economy of scale: Because alternative equipment has been developed and the efficiency of existing equipment has been improved, the liquefaction process should be designed on the basis of equipment availability. For optimal design of a liquefaction process for a large-scale LNG plant, the methodology for process design must be integrated or considered simultaneously together with driver selection. • A variety of recent studies have focused on improving the profitability of stranded gas fields. Equipment and environment restrictions affect the design of the liquefaction process in small- and midscale LNG plants. Future studies should focus not only on improving efficiency, but also on reducing the capital cost associated with the liquefaction process. • Design of the liquefaction process for offshore plants depends on various additional factors that are different from those for onshore plants because of the completely different environment on the topside of ships. Prior to considering process efficiency, applicable technology criteria for the liquefaction process design must be taken into account. The criteria for offshore plants include compact size, low weight, simple operation, easy startup and shutdown, inherent safety, sensitivity to vessel motion, robustness to the marine environment, and flexibility to changes in process conditions. Very few commercial applications of pretreatment or liquefaction for LNG S



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Table 7. Comparison of Optimization Methodologies83,89,90,93−104,107−111,113,114 publication

objective function

a

optimization formulation

equations of state (EOS)

process

Venkatarathnam83 exergy efficiency

SQP (Aspen plus)

Cao et al.89

shaftwork requirement

Aspen Hysys optimizer (original mode)b

PR or LKP (Aspen Hysys)

Jensen and Skogestad90 Al-Sobhi et al.93

capital and operating costs

gPROMS with Multiflashc

SRK (Multiflash)

heating and cooling utilities

pinch analysis with heat integration


dynamic programming with heuristics

PR and ELECNTRL (Aspen Plus), KEMDEA (property package) SRK

Cheng and Mah95 capital and operating costs

dynamic programming with heuristics

Vaidyaraman and Maranas96 Vaidyaraman and Maranas97 Kim et al.98 Lee et al.99


MILP [GAMS(CPLEX)]

total work input to the system

nonconvex NLP (MINOS)

SRK

shaftwork requirement (i) crossover (ii) sum of crossover (iii) shaftwork requirement capital cost and shaftwork requirement shaftwork requirement

pinch analysis and NLP pinch analysis and NLP

PR

GA

PR (WORK)

Wang et al. Aspelund et al.103

shaftwork requirement shaftwork requirement

PR SRK

Shah and Hoadley104 Shirazi and Mowla107 Taleshbahrami and Saffari108 Alabdulkarem et al.109 Aspelund et al.110


MINLP [GAMS (LINDOGlobal)] ExPAnD (pinch analysis with exergy analysis) and heuristics targeting method


GA (MATLAB)

PR

cascade nitrogen expander PRICO


GA

PR

C3MR


GA (MATLAB) linked with Aspen Hysys PR (Aspen Hysys)

C3MR


PRICO

Morin et al.111 Tak et al.113 Wang et al.114

shaftwork requirement shaftwork requirement shaftwork requirement

TS with NMDS (Aspen Hysys linked with Microsoft VBA) evolutionary search NLP (gPROMS with Multiflashc) SRK (Multiflash) SQP (Aspen plus) PR (Aspen Plus)

94

Barnes and King

Nogal et al.100 Nogal et al.101 102

GA

PR (Aspen Hysys)

SMR, C3MR, DMR, Kleemenko, etc. SMR, N2-CH4 expander PRICO LNG plant cascade pure refrigerant cascade pure refrigerant cascade pure refrigerant cascade mixed refrigerant PRICO PRICO cascade mixed refrigerant cascade mixed refrigerant C3MR new expander131,132

PRICO, TEALARC PRICO C3MR

a

NLP: Nonlinear Programming; MILP: Mixed-Integer Linear Programming; MINLP: Mixed-Integer Nonlinear Programming; SQP: Sequential Quadratic Programming; GA: Genetic Algorithm; TS: Tabu Search; NMDS: Nelder−Mead Downhill Simplex; PR: Peng−Robinson; LKP: Lee− Kesler−Ploecker; SRK: Soave−Redlich−Kwong. bAspen HYSYS Optimizer (original mode) includes the following types of algorithms: Function Setup, Box Method, SQP Method, Mixed Method, Fletcher Reeves Method, and Quasi-Newton Method. cgPROMS is used for process modeling, simulation, and optimization, and Multiflash is used for physical and thermodynamic properties calculations.

major equipment such as MSHEs, drivers, and expanders would be worthwhile. • In most studies, processes were compared by calculating the power consumption of compressors, because these contribute the most to the operating cost. However, this could lead to wrong decisions as capital cost was not taken into account. Therefore, processes should be compared based on both capital cost and operating cost. • The minimum temperature approach is commonly used for heat exchangers in the design and optimization of the liquefaction process. However, this approach may lead to wrong decisions as it does not consider the heat exchange area. For optimal design of heat exchangers, research must focus on equipment size and costs as well as thermodynamic efficiency. • Optimization of liquefaction processes has been mostly performed using either deterministic or stochastic methods with a set of decision variables including mixed refrigerant composition, pressure levels, and mixed refrigerant flow rate to minimize power consumption in the mixed refrigerant processes. The optimization methodology is

FPSO have been reported. Additional studies in this area are required to realize effective offshore LNG production. • Use of a dynamic model for effective control design is highly recommended. If the design constraints of a process are changed, the entire control design structure may be affected. Therefore, control structure design should be considered when process design is changed. This can yield economic benefits through optimal process and control design. Additional dynamic simulation and optimization studies to improve plant-wide control are needed. • Few studies have focused on NGL fractionation, material recovery, or energy recovery in the liquefaction process. Findings made in these research areas will help maximize profit and improve the efficiency of entire LNG plants and projects. • Recent studies have been concerned with developing new LNG processes or optimizing existing LNG processes. The efficiency of commercial LNG plants has been enhanced due to improvements of equipment as well as of process. Therefore, further research on T



Review

summarized in Table 7.83,89,90,93−104,107−111,113,114 According to analysis of Table 7, it is expected that stochastic methods will become of more interest when liquefaction processes become more complex. • Few studies have focused on modeling or improving the equipment itself. In particular, a rigorous model for a MSHE is required to determine optimal solutions. The other rigorous models should be developed in association with equipment innovations, such as the use of a 2-phase expander, liquid turbine, or absorption chiller. New process configurations and efficient cycles coupled with improved equipment will yield alternative processes for new LNG projects. • To increase the accuracy of simulation and optimization, suitable equations of state (EOS) should be selected to calculate thermodynamic properties. Peng−Robinson (PR) and Soave−Redlich−Kwong (SRK) EOSs are widely used in LNG processes (Table 7). However, Kunz et al.137 recently proposed GERG-2004, which was specifically designed to calculate the thermodynamic properties of LNG constituents. The deviations of cubic equations for liquid densities are low compared to those used in the PR and SRK equations.138 To date, GERG2004 is the most suitable EOS to calculate thermodynamic properties in LNG processes. Over the past decades, various natural gas liquefaction processes have been proposed. Although only a few processes have been successfully applied in commercial LNG plants, various studies on ways to improve the liquefaction processes have contributed greatly to technical progress in the past decade. Most studies have focused on increasing process efficiency without evaluating capital costs, which could lead to wrong decisions. Further research studies focused on maximizing profit in combination with process efficiency are required to achieve more economical LNG plants.

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(6) Liquefied Natural Gas: Understanding the Basic Facts; DOE NETL, 2005. (7) The Global Liquefied Natural Gas Market: Status & Outlook; U.S. Energy Information Administration (EIA), December 2003. (8) Investors’ Handbook. http://www.investorshandbook.shell.com/ 2010/servicepages/downloads/files/download.php?file=all_shell_ ih10.pdf (accessed September 3, 2012). (9) LNG Technology. http://www.airproducts.com/∼/media/Files/ PDF/industries/energy-lng-brochure-0408.ashx (accessed September 3, 2012). (10) Castle, W. F. Cryogenics and Gas Separation Processes. Bull. IIR 2004, 5−15. (11) World Energy Outlook 2009; International Energy Agency (IEA), November 2009. (12) Kirillov, N. G. Analysis of Modern Natural Gas Liquefaction Technologies. Chem. Pet. Eng. 2004, 40 (7−8), 401−406. (13) Finn, A. J.; Johnson, G. L.; Tomlinson, T. R. Developments in natural gas liquefaction. Hydrocarb. Process. 1999, 78 (4), 47−59. (14) Smith, R. Chemical Process Design and Integration; Wiley: Chichester, West Sussex, England/Hoboken, NJ, 2005; p xxiii, 687 pp. (15) Smith, J. M.; Van Ness, H. C.; Abbott, M. M. Introduction to Chemical Engineering Thermodynamics, 7th ed.; McGraw-Hill: Boston, MA, 2005; p xviii, 817 pp. (16) Barron, R. F. Cryogenic Systems, 2nd ed.; Oxford University Press/Clarendon Press: New York/Oxford, Oxfordshire, 1985; p xvii, 507 pp. (17) Walker, G. Cryocoolers; Plenum Press: New York, 1983. (18) Timmerhaus, K. D.; Flynn, T. M. Cryogenic Process Engineering; Plenum Press: New York, 1989; p viii, 612 pp. (19) Bosma, P.; Nagelvoort, R. K. In Liquefaction Technology; Developments through History, 1st Annual Gas Processing Symposium, Doha, Qatar, January 10−12, 2009. (20) Andress, D. L. The Phillips Optimized Cascade LNG Process: a Quarter Century of Improvements; Phillips Petroleum Company: Bartlesville, OK, 1996 (21) Martin, P.-Y.; Fischer, B. In Natural Gas Liquefaction Processes Comparison, 14th International Conference and Exhibition on Liquefied Natural Gas (LNG-14), Doha, Qatar, March 21−24, 2004. (22) Shukri, T. LNG Technology Selection. Hydrocarbon Eng. February 2004. (23) ConocoPhillips Optimized Cascade Process. http:// lnglicensing.conocophillips.com/EN/Documents/ ConocoPhillipsLNG_Brochure.pdf (accessed September 3, 2012). (24) Michelsen, F. A.; Halvorsen, I. J.; Lund, B. F.; Wahl, P. E. Modeling and Simulation for Control of the TEALARC Liquified Natural Gas Process. Ind. Eng. Chem. Res. 2010, 49 (16), 7389−7397. (25) Stebbing, R.; O’Brien, J. An Updated Report on the PRICO Process for LNG Plants. In Gastech 75: LNG & LPG Technology Congress, Paris, France, September 30−October 3, 1975. (26) Singh, A.; Hovd, M. Dynamic Modeling and Control of the PRICO LNG process. In 2006 AIChE Annual Meeting, San Francisco, CA, November 12−17, 2006. (27) Swenson, L. K. Single mixed refrigerant, closed loop process for liquefying natural gas. U.S. Patent 4,033,735, Jul. 5, 1977. (28) Schmidt, W.; Kennington, B. Air Products meets requirements of full range of Floating LNG concepts. LNG J. March 10, 2011. (29) McKeever, J.; Pillarella, M.; Bower, R. An ever evolving technology. LNG Industry Spring, 2008. (30) Pillarella, M.; Liu, Y.-N.; Petrowski, J.; Bower, R. The C3MR liquefaction Cycle: Versatility for a Fast Growing, Ever Changing LNG Industry. In 15th International Conference on LNG (LNG-15), Barcelona, Spain, April 24−27, 2007. (31) Longsworth, R. C. Cryostat with serviceable refrigerator. U.S. Patent 4,277,949, Jul. 14, 1981. (32) Liu, Y.-N.; Newton, C. L. Feed gas drier precooling in mixed refrigerant natural gas liquefaction processes. U.S. Patent 4,755,200, Jul. 5, 1988.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-363-9375. Fax: +82-2-312-6401. E-mail: ilmoon@ yonsei.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a grant from the GAS Plant R&D Center funded by the Ministry of Land, Transportation and Maritime Affairs (MLTM) of the Korean government and also respectfully supported by BK 21 Program funded by the Ministry of Education (MOE) of Korea.

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Current Status and Perspectives of Liquefied Natural Gas (LNG) Plant

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