Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology

Accepted Manuscript Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology Xiaojun Xiong, Wensheng Lin, Anzhong Gu PII:

S1875-5100(16)30091-9

DOI:

10.1016/j.jngse.2016.02.046

Reference:

JNGSE 1296

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 22 June 2015 Revised Date:

16 February 2016

Accepted Date: 22 February 2016

Please cite this article as: Xiong, X., Lin, W., Gu, A., Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology, Journal of Natural Gas Science & Engineering (2016), doi: 10.1016/j.jngse.2016.02.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Design and optimization of offshore natural gas liquefaction processes adopting

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PLNG (pressurized liquefied natural gas) technology

3 Wensheng Lin*

Anzhong Gu

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Xiaojun Xiong

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

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Email address: [email protected] (Wensheng Lin)

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* Corresponding author. Tel.: +86 21 34206533

Abstract

Space limitation and energy consumption are two significant considerations for offshore natural gas liquefaction. This study designs three pressurized liquefied natural gas (PLNG) processes without CO2

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pretreatment facility for natural gas containing less than 0.5% CO2, in which natural gas is converted into LNG at an elevated pressure (2 MPa), associated with a raised temperature (165.7 K). The three proposed processes, the cascade, the single mixed refrigerant (SMR) and the single expander PLNG processes, are simulated with Aspen HYSYS and optimized by the genetic algorithm (GA) to achieve the minimum

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specific power consumption. The cascade, the SMR and the single expander PLNG processes show an energy saving of 46%, 50% and 63%, respectively, compared to representative conventional liquefied

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natural gas (LNG) process. Moreover, the cascade, the SMR and the single expander PLNG processes present a smaller heat transfer area than representative conventional LNG process by 42%, 25% and 4%, respectively. In conclusion, the proposed PLNG processes can achieve both low energy cost and small footprint, providing a promising option for offshore natural gas liquefaction.

Keywords: Pressurized liquefied natural gas (PLNG), Simulation, Optimization, Energy consumption.

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Introduction

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Thanks to scientific development and technological advancement, world’s proven natural gas reserves have risen from 179.5 TCM (trillion cubic meter) in 2004 to 187.1 TCM by 2014 according to BP’s

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statistics (BP, 2015). A significant portion of those reserves is proven to be located in offshore fields and remains unexploited (Yan and Gu, 2010; Zhu et al., 2010). As a result, the exploitation of offshore gas fields is attracting increasing interests (Khan et al., 2014). It is expensive to transport offshore gas by marine pipeline across long distances (Thomas and Dawe, 2003), so liquefied natural gas (LNG) is

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considered for transportation. LNG is the liquid form of natural gas, and it has a relatively small volume, about 1/600 of that of natural gas at standard state. The small volume enables large transportation capacity,

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which makes it an economical approach for delivery. The transport of offshore gas in the form of LNG suggests that offshore gas must be liquefied via proper liquefaction processes. Many different natural gas liquefaction processes have been brought forward in order to meet a number of challenges during the development of the LNG industry. The process first used in the world is the cascade process (Andress and Watkins, 2004) developed by Conoco Phillips in the 1960’s. The cascade process has three multi-stage Joule-Thompson (JT) refrigeration cycles and a considerable number of

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equipment units. To reduce the equipment unit number and simplify the process, Air Products and Chemicals Inc. put forward the single mixed refrigerant (SMR) process (Sun et al., 2012) that only has one multi-stage JT cycle. In order to further simplify the process, Black & Veatch developed a simpler SMR

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process called the PRICO process (Aspelund et al., 2010; Xu et al., 2013). Refrigeration in the PRICO process adopts a one-stage JT cycle, in which the mixed refrigerant (MR) only has one evaporation

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pressure level. Due to its simplicity, the SMR process is not energy efficient for large scale production. Consequently, in 1972, Air Product adopted a propane JT cycle for precooling at the hot end of the SMR process to achieve better efficiency, which is the well-known propane pre-cooled mixed refrigerant (C3MR) process (Mortazavi et al., 2014). Since the C3MR process has a liquefaction capacity up to 5 MTPA (million tons per annum), it is hard to meet the increasing requirement of production capacity. As a result, AP designed the AP-X process (Roberts et al., 2002) by adding a nitrogen Brayton cycle to the C3MR process at the cold end. Shell proposed the dual mixed refrigerant (DMR) process (Hwang et al., 2013) by replacing the propane pre-cooling cycle with a mixed refrigerant pre-cooling cycle at the hot end of the

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C3MR process. Finally, Linde brought forward the mixed fluid cascade (MFC) process by using mixed refrigerants instead of pure refrigerants in the cascade process. Aside from the cascade and the mixed refrigerant LNG processes, there is another type of natural gas liquefaction process using a Brayton

expander process and dual expander process (Zhu et al., 2010).

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refrigeration cycle called the expander process, including the single expander process, propane pre-cooled

In addition, there are many other new processes specially designed for certain applications. Chang et al., 2011, designed a novel process that combines a nitrogen Brayton cycle with the ethylene JT cycle and

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propane JT cycle. He and Ju, 2014, proposed a new process for pipeline gas by utilizing the pipeline pressure energy. Yuan et al., 2014, developed a novel single expander process adopting carbon dioxide pre-

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cooling for small-scale liquefaction. Similarly, Khan et al., 2014, brought forward a single expander process with N2-CO2 refrigeration cycle for stranded offshore gas. It is obvious that there are numerous processes available for natural gas liquefaction. However, the above mentioned LNG processes commonly require a great deal of space due to a number of facilities especially those large pre-treatment facilities and large heat exchangers, which presents a great challenge for offshore liquefaction because of limited space (Bukowski et al., 2013; Hwang and Lee, 2014). In addition, those conventional processes traditionally

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convert natural gas into LNG at approximately atmospheric pressure (~0.1 MPa), corresponding to an extremely low temperature (112 K), which consumes a significant amount of energy. Therefore, this study attempts to design new liquefaction processes that are both space-saving and energy-saving.

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In this study, the new liquefaction processes adopt the pressurized LNG (PLNG) technology (Fairchild et al., 2005; Papka et al., 2005) to produce LNG. The PLNG process differs from the conventional LNG

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process in that natural gas is converted into LNG at a relatively higher pressure (1-2 MPa), associated with a higher temperature (153-173 K). As the raised temperature increases the solubility of CO2 in LNG (Shen et al., 2012), the PLNG process has a higher tolerance of CO2. Hence, CO2 pre-treatment facilities can be eliminated for a PLNG process, resulting in a lot of space-savings. Moreover, the raised temperature decreases the heat load that needs to be removed from natural gas, reducing the power consumed by a PLNG process. As a result, the PLNG process achieves a small footprint and a low energy cost. The idea of PLNG has for a long time remained as just a concept. Little information can be found about the PLNG process design and process performance analysis. Therefore, this paper attempts to study if

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and how the conventional natural gas liquefaction processes can be adopted for the PLNG application. Since natural gas liquefaction processes usually fall into three groups: the cascade, the MR and the expander, this study designs three types of PLNG processes: the cascade PLNG process, the SMR PLNG

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process and the single expander PLNG process. To minimize the specific power consumption of the three proposed processes, simulation and optimization are conducted for them. Moreover, the performance of three proposed processes are analysed and compared.

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2. Process description

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2.1. Cascade PLNG process

As the earliest used process, the cascade process is safe, simple to operate, easy to understand, and robust in reliability (Andress and Watkins, 2004). A conventional cascade process typically consists of three stages of refrigeration cycles, namely, propane cycle, ethylene (or ethane) cycle and methane cycle. The propane cycle is used to cool the feed gas and methane refrigerant, and to condense the ethylene

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refrigerant. The ethylene (or ethane) cycle is used to cool the feed gas and to condense the methane refrigerant. The main duty of methane cycle is to condense the feed gas. Different from conventional cascade processes, the cascade PLNG process proposed in this study is

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designed to consist of simpler refrigeration cycles due to the much higher liquefaction temperature. The proposed refrigeration cycle only consists of two stages; one is the high temperature refrigeration cycle and

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the other is the low temperature refrigeration cycle. Since the process configuration changes, the refrigerants should be re-selected so that the refrigeration cycle is able to provide the required cooling energy and low temperature. For the low temperature refrigeration cycle, ethylene and carbon tetrafluoride (CF4) are two possible refrigerant candidates because their boiling temperatures (169.4 K and 135.2 K) are close to the liquefaction temperature of PLNG product (153-173 K). However, CF4 is environmental unfriendly and is forbidden in use all over the world, so ethylene is the only proper choice. For the high temperature refrigeration cycle, both propane and propene can meet the requirements, but only propane is selected in this study because it can be obtained directly from natural gas.

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The proposed novel cascade PLNG process is illustrated in Fig. 1. In order to focus on the liquefaction process, the upstream processes, like water removal, mercury removal and gas sweetening processes, are omitted here. In real applications, the cascade process usually has two, three or even more evaporation

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pressure levels in each stage of refrigeration cycle so that the temperature difference in heat exchangers can be reduced. In this study, for simplicity, each stage of refrigeration cycle only adopts one evaporation

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pressure level.

Fig.1. Cascade PLNG process

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Part 1 is the natural gas side, feed gas 101 is assumed to have undergone a pre-treatment process (CO2 pre-treatment process is not included), removing Hg, H2O, etc., prior to the liquefaction process. The

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treated gas firstly enters heat exchanger HEX-101 and it is cooled down, and then it goes into heat exchanger HEX-102 and condenses into liquid. Finally, it expands through JT valve VLV-101 and is separated into gas phase and liquid phase by separator S-101. The liquid phase is the LNG product at the pressurized state.

Part 2 is the ethylene cycle, namely the low temperature refrigeration cycle. Firstly, the ethylene refrigerant undergoes a two-stage compression with intercooling, which reduces the irreversibilities as well as the compressor work. Then ethylene enters heat exchanger HEX-101 and is cooled down. By expanding across JT valve VLV-201, it continues to lower its temperature. With a low enough temperature, it flows

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into heat exchangers HEX-102 and HEX-101 and evaporates into gas. Finally, it goes back to compressor C-201. Part 3 is the propane cycle, namely the high temperature refrigeration cycle, typically consisting of four

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sections: compression, condensation, JT expansion and evaporation.

2.2. SMR PLNG process

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Unlike the cascade process, which uses pure refrigerants, a MR process normally employs a mixture of several refrigerants, like methane, ethane, propane, butane, pentane and nitrogen. The composition and

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flow rate of the MR are specified so the liquid refrigerant evaporates over a temperature curve similar to that of the natural gas being liquefied, and that makes the MR process energy efficient. Among various MR processes, the SMR process has the simplest configuration that best satisfies the requirement of small footprint for offshore liquefaction. Therefore, a SMR PLNG process is designed in this study. The flow sheet of the SMR PLNG process is shown in Fig. 2. Usually, more stages of heat exchange lead to less power consumption due to the better match of hot and cold composites (Finn et al., 1999).

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However, more stages means more equipment and larger space, thus there must be a compromise between the stages and the space. Considering the limited space of offshore liquefaction, two stages of heat

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exchange are selected in this study.

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Fig. 2 SMR PLNG process

Part 1 is the natural gas side, where the treated feed gas passes through heat exchangers HEX-101 and HEX-102, JT valve VLV-101, and is finally separated into gas phase and liquid phase through separator S-

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101. The liquid phase is the PLNG product.

Part 2 is the MR refrigeration cycle. The MR splits into liquid and gaseous states after compression and water cooling. The liquid part flows into heat exchanger HEX-101 and JT valve VLV-201 for further temperature drop; the vapour part enters heat exchangers HEX-101 and HEX-102 and JT valve VLV-203

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for further cooling down and condensation, then it returns back to heat exchanger HEX-102 to be heated up

and goes back to compressor C-201.

2.3. Single expander PLNG process

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and mixed with the former liquid MR in mixer MIX-201. Afterwards, the MR evaporates into gaseous state

In a single expander process, high pressure refrigerant expands in a near isentropic manner through an

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expander, reducing its temperature to the required level, and then returns through various stages of heat exchanger, cooling and liquefying natural gas. Among various expander processes, the single expander process has the fewest pieces of equipment and requires the least space, so it is a good choice for offshore liquefaction especially in consideration of space limitations. Therefore, a single expander PLNG process is

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proposed in this study.

The flow sheet of the expander PLNG process is presented in Fig. 3. In the LNG industry, nitrogen,

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methane and their mixtures are favourable refrigerants for expander processes. In this study, nitrogen is selected as the single component refrigerant, because its inherent safety and nonflammability would reduce the distance between process equipment (Mokhatab et al., 2008), and that is an important aspect considering the space limitations in offshore processing.

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Fig. 3. Single expander PLNG process

In part 1 (the natural gas side), the treated natural gas goes through heat exchangers HEX-101 and HEX-102, JT valve VLV-101 in sequence and finally becomes gas and liquid in separator S-101. The pressurized liquid is the PLNG product.

In part 2 (the nitrogen cycle), the nitrogen firstly undergoes a two-stage compression with intercooling, then enters heat exchanger HEX-101 and expands through expander E-201, where the useful work is

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directly recovered by the second-stage compressor so as to save power consumption (Mortazavi et al., 2012). In engineering, the compressor can be constructed together with the expander, which is referred to as a compander. After that, the nitrogen flows back through heat exchangers HEX-102 and HEX-101,

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where the nitrogen is heated up as the natural gas is cooled down. Eventually, the nitrogen flows back to

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compressor C-201 to complete the Brayton cycle.

3. Simulation and optimization method

3.1. Simulation method

Aspen Plus (Alabdulkarem et al., 2011; Xu et al., 2013) and Aspen HYSYS (Aspelund et al., 2010; Yuan et al., 2014; Park et al., 2014) are two preferred software packages commonly used for LNG process simulation. They both have an abundant database containing a wide range of substances with their

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thermodynamic and physical properties. The process models built in these two simulators are both based on blocks, such as compressor, expander, heat exchanger, valve and so on. By connecting the different blocks with material streams and energy streams, a process will be constructed. According to a previous study,

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simulation results of a C3MR process in Aspen Plus and Aspen HYSYS show a perfect agreement with an average discrepancy of 1.27% (Alabdulkarem et al., 2011). Because HYSYS is easy to connect with Microsoft Excel software for writing and reading data, this study chose Aspen HYSYS as the simulator.

Accurate simulation is based on an adequate equation of state, which can predict the properties of

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accuracy in predicting the properties of both liquid and gas.

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working fluids precisely. Peng-Robinson (PR) equation of state was used in this study since it has a good

3.2. Optimization method

The optimization of a natural gas liquefaction process is a very complex nonlinear problem, which has several variables as well as many constraints (Jacobsen and Skogestad, 2013). Although this type of problem is hard or even impossible to solve by conventional methods, it can be solved by the genetic

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algorithm (GA) (Mokarizadeh Haghighi Shirazi and Mowla, 2010; Alabdulkarem et al., 2011). Basically, there are four elements in the GA, genome, mutation, crossover and fitness. At the beginning of optimization, the values of initial genomes are given based on experience. All genomes constitute the first

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individual as well as the first generation of the population. Through mutation, changing the values of genomes by a fixed step size, more new versions of genomes are generated. By crossover, different

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genomes recombine with each other and generate many new individuals, which are the second generation of the population. According to the natural selection principle, those ones who have the best fitness for their environment will survive.

3.1.1Genomes

In this study, the genomes are those ones that have direct influence on the fitness of the process. Since LNG processes are energy intensive, the power consumption is considered as an important index of the

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fitness. Therefore, the genomes are those variables closely related to the power consumption of the process, which can be calculated from Eq. (1).

Where

 n −1        pout  n   × − 1       pin   

(1)

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 pin  n  W =qmass   CF   n −1  ρin

qmass and ρin are the mass flow rate and density of the inlet stream; n is the volume exponent; CF

is the correlation factor; pin and pout represent the pressure of inlet and outlet streams. According to Eq. (1), it can be found that the refrigerant flow rate, the refrigerant composition, the refrigerant high pressure

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and low pressure in the process play significant roles in affecting power consumption. Therefore, the above

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four variables are four genomes for optimization.

3.1.2 Mutation and crossover

The mutation of genomes was realized by changing the values of genomes with a fixed step size. As long as the step size is small enough, using the GA can guarantee to find a global optimal solution. However, a too small step size will lead to a very long computational time. It is therefore important to

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choose a proper step size. In this study, a relatively large step size was used to find several local optimum results in the global space and a relatively small step size was used to find the global optimum result in the local space. Moreover, tight bounds of genomes will also help reduce the searching space as well as the computational time. The step sizes and bounds of genomes are shown in Table 1. Mutation generates many

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new versions of genomes, which are recombined with each other through crossover. By crossover, many

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new processes with different parameters are generated.

Table 1

The step sizes and bounds of genomes

Genome

Bounds

Step sizes Small step size

Lower bound

Upper bound

Large step size

Refrigerant high pressure

Refrigerant low pressure

5000 kPa

100 kPa

10 kPa

Refrigerant low pressure

50 kPa

Refrigerant high pressure

100 kPa

10 kPa

Refrigerant component flow rate

0 kmol/h

10 kmol/h

0.1 kmol/h

0.001 kmol/h

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3.1.3 Fitness For LNG processes, the fitness represents the constraints and the objective function of optimization.

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The constraints for LNG processes are given as follows:


The minimum approach temperature for hot and cold composites should be as small as possible so as to reduce the exergy loss and achieve a high efficiency. However, the smaller the minimum approach temperature is, the larger the heat exchanger area is. In addition, a very low minimum approach

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temperature will lead to oversensitivity to process disturbances. So the minimum approach temperature for gas to gas heat exchanger was set to be 5 K, for gas to liquid and liquid to liquid heat

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exchanger was 3 K.


The compressors commonly used in LNG plants are sensitive to liquid, therefore, no liquid should be allowed at the entrance of compressors, which can be supervised by Aspen HYSYS.




The refrigerant is easy to partly change into liquid at the outlet of an expander due to too low temperature. However, most expanders in use today are ordinary turbines, unable to handle liquid. So no liquid should be allowed at the exit of expanders, which also can be supervised by Aspen HYSYS.

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In industry, optimization efforts of LNG processes are usually conducted by considering the process efficiency, machinery fit and plant compactness. In academia, many researchers concentrated on energy minimization while optimizing LNG processes (Aspelund et al, 2010; Alabdulkarem, et al, 2011; Hatcher

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et al, 2012) because a considerable amount of power is consumed by the processes. Therefore, this study chose the specific power consumption as the objective function of optimization, which is calculated by Eq.

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(2). w = W qmass

(2)

Where w represents the specific power consumption of LNG, W is the total work of all compressors, and qmass is the mass flow rate of the LNG product.

4. Results and analysis

4.1. Simulation results and analysis

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Natural gas is a complicated gas mixture mainly consisting of various amounts of hydrocarbons as well as small amounts of impurities such as nitrogen and carbon dioxide. For the sake of simplicity, it is usually

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assumed to only consist of a few constituents. In this study, the feed gas was assumed to consist of methane and carbon dioxide because methane is the largest constituent of natural gas and the PLNG processes are particularly beneficial to carbon dioxide constituent. Considering the upper limit of CO2 content that PLNG process can handle (Xiong et al, 2015), the feed gas composition was set as 99.5% CH4 + 0.5% CO2. For

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simplicity, the pressure drop in water coolers, heat exchangers, separators and mixers was assumed to be zero. The adiabatic efficiency of compressor and expander were set to be 85% and 80% (Lin et al., 2010),

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respectively.

In the PLNG concept, the operating pressure of natural gas liquefaction varies from 1 MPa to 2 MPa. For 99.5% CH4 + 0.5% CO2, the highest triple point pressure is about 1.8 MPa. The operating pressure must be higher than that, otherwise solid CO2 would form by desublimation in the liquefaction process. As a result, the operating pressure should vary in the range 1.8-2 MPa. Since the power consumption decreases with the increase in operating pressure, 2 MPa was chosen as the operating pressure.

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Using HYSYS to simulate the three PLNG processes, the parameters of each stream were obtained. The parameters of each stream in the cascade PLNG process, the SMR PLNG process and the single expander PLNG process are given in Tables 2, 3 and 4, respectively. The temperature-pressure diagrams of the three

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Table 2

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PLNG processes are illustrated in Fig. 4.

Parameters of each stream in the cascade PLNG process Stream 101 102 103 104 105 106 201 202 203 204

Temperature K 308.2 237.9 167.2 165.7 165.7 165.7 299.3 405.5 308.2 417.4

Pressure kPa 5000 5000 5000 2000 2000 2000 75 354.3 354.3 1674

Molar flow kmol/h 1.000 1.000 1.000 1.000 1.000 0.000 0.702 0.702 0.702 0.702

Composition (mol/mol) CH4 C2H4 C3H8 CO2 0.995 0.005 0.995 0.005 0.995 0.005 0.995 0.005 0.995 0.005 0.999 0.001 1.000 1.000 1.000 1.000 -

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205 206 207 208 301 302 303 304 305 306

308.2 237.9 164.1 234.9 299.2 346.1 308.2 358.5 308.2 234.9

1674 1674 75 75 120 382.6 382.6 1220 1220 120

0.702 0.702 0.702 0.702 0.702 0.702 0.702 0.702 0.702 0.702

-

1.000 1.000 1.000 1.000 -

1.000 1.000 1.000 1.000 1.000 1.000

-

Table 3

Molar flow

Composition (mol/mol)

K

kPa

kmol/h

CH4

C2H6

C3H8

CO2

308.2 238.2 167.2 165.7 165.7 165.7 305.1 404.2 308.2 308.2 238.2 227.9 308.2 238.2 167.2 163.8 235.2 234.0

5000 5000 5000 2000 2000 2000 1020 5000 5000 5000 5000 1020 5000 5000 5000 1020 1020 1020

1.000 1.000 1.000 1.000 1.000 0.000 2.662 2.662 2.662 0.578 0.578 0.578 2.084 2.084 2.084 2.084 2.084 2.662

0.995 0.995 0.995 0.995 0.995 0.999 0.465 0.465 0.465 0.206 0.206 0.206 0.537 0.537 0.537 0.537 0.537 0.465

0.128 0.128 0.128 0.127 0.127 0.127 0.128 0.128 0.128 0.128 0.128 0.128

0.407 0.407 0.407 0.667 0.667 0.667 0.335 0.335 0.335 0.335 0.335 0.407

0.005 0.005 0.005 0.005 0.005 0.001 -

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Pressure

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101 102 103 104 105 106 201 202 203 204 205 206 207 208 209 210 211 212

Temperature

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Stream

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Parameters of each stream in the SMR PLNG process

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Table 4

Parameters of each stream in the expander PLNG process Stream 101 102 103 104 105 106 201

Temperature

Pressure

Molar flow

Composition (mol/mol)

K

kPa

kmol/h

CH4

N2

CO2

308.2 238.2 167.2 165.7 165.7 165.7 303.2

5000 5000 5000 2000 2000 2000 1086

1.000 1.000 1.000 1.000 1.000 0.000 5.594

0.995 0.995 0.995 0.995 0.995 0.999 -

1.000

0.005 0.005 0.005 0.005 0.005 0.001 -

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2882 2882 5000 5000 5000 1086 1086

5.594 5.594 5.594 5.594 5.594 5.594 5.594

-

1.000 1.000 1.000 1.000 1.000 1.000 1.000

-

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416.8 308.2 369.8 308.2 238.2 164.2 208.9

(a) Cascade PLNG process

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202 203 204 205 206 207 208

(b) SMR PLNG process

(c) Expander PLNG process

Fig.4. Temperature-pressure diagrams of three PLNG processes

4.2.1 Optimum variables

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4.2. Optimal results and analysis

By applying the GA to the three PLNG processes, the optimum value for genomes in the three

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processes were found, as shown in Table 5. It should be noted that the optimum refrigerant low pressure in the cascade PLNG process is at a vacuum pressure level. That is because the boiling point temperature of

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C2H4 (169.4 K) refrigerant is slightly higher than the pressurized liquefaction temperature of natural gas (165.7 K). In order to liquefy natural gas, C2H4 (169.4 K) refrigerant must evaporate at a subatmospheric pressure. From Table 5, it can be seen that the expander PLNG process has the largest refrigerant flow rate among the three PLNG processes. That can be explained by the fact that the expander process uses the sensible heat of refrigerant which is much smaller than the latent heat, thus more refrigerant is needed. Also, it can be found from Table 5 that the optimum mixed refrigerant composition does not include the frequently used nitrogen component. This is because the PLNG process has a relatively high liquefaction temperature, making nitrogen unnecessary for liquefying natural gas.

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Table 5

Refrigerant high pressure (kPa) Refrigerant low pressure (kPa) Refrigerant flow rate (kmol/h) CH4 C2H4

5000 1020 2.662 0.465

5000 1080 5.594

1

C2H6 C3H8

Expander

0.128 1

N2

0.407 0

4.2.2 Specific power consumption

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Refrigerant composition (mole/mole)

SMR

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Cascade Part 3 Part 2 1220 1674 120 75 0.702 0.702

Optimum variable

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Optimum variables for three PLNG processes

For comparison, three conventional LNG processes, which are the cascade LNG process, the SMR LNG process and the single expander LNG process, were also studied in this work. The cascade LNG process is similar to the cascade PLNG process in configuration but with the addition of a methane refrigeration cycle and a heat exchanger. The SMR LNG process and the single expander LNG process are

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identical in layout to their pressurized ones. In order to make comparisons on an identical basis, the conventional LNG processes were simulated and optimized under the same conditions with PLNG processes, for instance, the same ambient temperature, the same feed gas pressure, temperature and

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flowrate, the same minimum approach temperature and the same compressor efficiency and expander efficiency. Because conventional LNG processes can only handle CO2 up to 50 ppm (Berstad et al., 2012;

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Rufford et al., 2012), the feed gas composition used in the conventional processes was set as 99.995% CH4 + 0.005% CO2, which is only slightly different from that used in the PLNG processes (99.5% CH4 + 0.5% CO2). The specific power consumption of three PLNG processes and conventional LNG processes are given in Table 6. Compared with conventional processes, the specific power consumption of the cascade PLNG process, the SMR PLNG process and the expander PLNG process are reduced by 46%, 50% and 63%, respectively. The large reduction in energy consumption is mainly due to two aspects: the raised liquefaction temperature in PLNG processes reduces the heat load that needs to be removed from natural

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gas, and also the raised liquefaction temperature enables the refrigeration cycle to achieve a higher efficiency.

Specific power consumption of different liquefaction processes

Cascade SMR Single expander

Specific power consumption (kWh/kg) Conventional PLNG 0.3871 0.2088 0.4460 0.2202 0.8651 0.3203

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Liquefaction process

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Table 6

4.2.3 Composite curves

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Composite curves are very important for LNG processes because it directly reflects the level of energy efficiency. Several publications have focused on the analysis of composite curves in heat exchangers (Anantharaman et al., 2006; Aspelund et al., 2010; Alabdulkarem et al., 2011). Generally, the smaller the gap between hot and cold composite curves, the higher the heat transfer efficiency. For a LNG process, high heat transfer efficiency infers high energy efficiency. Figure 5 shows the composite curves in heat

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exchangers for the three PLNG processes. It is obvious that a significant part of hot and cold composite curves are very close to each other in Figs. 5(a) and (b), while the hot and cold composite curves in Fig. 5(c) are relatively far away from each other. Therefore, the cascade and MR PLNG processes are more energy

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efficient than the expander PLNG process (See Table 6).

(a) Cascade PLNG process

(b) SMR PLNG process

(c) Expander PLNG process

Fig.5. Composite curves in heat exchangers for three PLNG processes

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Composite curves are important for LNG processes also because they point out where the improvement should be made in a LNG process for better energy efficiency. As Fig. 5(a) shows, the gap between hot and cold composite curves at the cold end is obviously larger than that at the hot end. Consequently, the

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improvement should be made mainly at the cold end for the cascade PLNG process. Similarly, the composite curves in Figs. (b) and (c) indicate that the improvement should be made at the hot end for the SMR PLNG process, like adopting propane precooling, and at the cold end for the single expander PLNG

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process, for instance, using another expander.

4.2.4 Heat transfer area

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Heat exchanger area plays a significant role in determining the heat exchanger size. Usually, heat transfer area A and heat transfer coefficient U are calculated as a whole UA. Table 7 compares the calculated UA of the heat exchanger between the PLNG processes and the conventional processes. The results were derived from HYSYS. As shown in Table 7, the UA of PLNG processes are obviously smaller than that of conventional processes. Assuming a fixed heat transfer coefficient, it can be calculated that the heat transfer area of the cascade PLNG process, the SMR PLNG process and the single expander PLNG

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process are 42%, 25% and 4% smaller than that of conventional processes. The results indicate that PLNG processes also save space in heat exchangers. Moreover, the reduction in UA contributes to the savings in capital cost. It is reported that capital cost of heat exchangers is proportional to the UA with a proportional

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coefficient of 0.0871 (Wang et al., 2014). By calculation, we can deduce that the cascade PLNG process, the SMR PLNG process and the single expander PLNG process can save the capital cost of heat exchangers

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by 3.7%, 2.2% and 0.3%, respectively.

Table 7

UA of heat exchangers in different liquefaction processes Liquefaction process Cascade SMR Single expander

UA (kJ/hK) Conventional PLNG 5819.9 3354.6 9003.0 6722.0 2171.6 2082.2

4.2.5 Applicability

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The raised temperature in the PLNG process enables a higher tolerance of CO2; thereby CO2 pretreatment facility can be eliminated under certain feed gas conditions. Therefore, the applicability of the three PLNG process without CO2 pre-treatment facility mainly depends on the feed gas conditions,

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especially the CO2 content in feed gas. If the feed gas contains a too high level of CO2, the three PLNG processes will surely encounter the CO2 freezing problem. As a result, only when the CO2 content in the feed gas is lower than the solubility of CO2 in LNG, the three PLNG processes are feasible because all CO2 can be dissolved. Fig. 6 shows the solubility of CO2 in liquid CH4 (Shen et al., 2012). The solubility of CO2

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under PLNG condition (153-173 K) is about 1-3%, which indicates that the three PLNG processes are suitable for natural gas containing less than 1% CO2. Considering some safety margin, the three PLNG

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processes are applicable for natural gas with less than 0.5% CO2.

Fig.6. The solubility of CO2 in liquid CH4

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4.2.6 Advantages and disadvantages

Compared with conventional processes, the proposed PLNG processes present both advantages and

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disadvantages. The elimination of CO2 pre-treatment facilities and the reduction of heat exchanger size make the proposed PLNG processes being space saving. Also, the raised liquefaction temperature makes the proposed PLNG processes being energy saving. However, the increased pressure of PLNG processes leads to high pressure storage of LNG. Currently available LNG tanks are atmospheric tanks with a large volume. To bear a high pressure, the tanks must be manufactured with a much smaller volume, which will lead to a reduction in space utilization efficiency. Considering the limited deck space, a decreasing space utilization efficiency will surely result in an increasing transportation cost. Moreover, special attentions

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must be paid to ensure the pressure of the PLNG product at any conditions not being lower than the initial pressure, otherwise solid CO2 may form.

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5. Conclusions

In order to meet the requirement of offshore liquefaction, this study designs three novel processes, the cascade PLNG process consisting of two stages of refrigeration cycles, the SMR PLNG process employing

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mixed refrigerant without nitrogen composition and the single expander PLNG process utilizing the output work of the expander to drive the compressor. The three PLNG processes converting natural gas into LNG

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at an elevated pressure (2 MPa), corresponding to a raised temperature (165.7 K), enable the elimination of CO2 pre-treatment facilities for natural gas with less than 0.5% CO2. To minimize the specific power consumption of the proposed processes, simulation based optimization are conducted for the proposed processes using the GA. Optimization results show that the proposed PLNG processes are more energy saving and space saving than conventional LNG processes. By comparison, it is found that the specific power consumptions of the cascade PLNG process, the SMR PLNG process and the single expander PLNG

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process are lower than that of conventional processes by 46%, 50% and 63%, respectively. Moreover, the heat transfer areas of three PLNG processes are smaller than that of conventional processes by 42%, 25% and 4%, respectively. However, it should also be noted that the PLNG processes face the problem of high-

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pressure storage, which leads to a high manufacturing cost and a low space utilization efficiency. As a result, a fair evaluation of the PLNG processes should take all the above factors into account.

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The present study mainly focuses on the investigation of the liquefaction process and rarely involves the upstream and downstream parts. Therefore, future work with regard to gas pretreatment and PLNG storage can also be integrated with this study. Moreover, the cost analysis including the operation cost analysis and the capital cost analysis for the whole processes can also be conducted in the future to better help the decision makers in the LNG field.

Acknowledgments

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The authors are grateful for funding by National Natural Science Foundation of China for the Project (No. 51076098).

A

heat transfer area [m2]

CF

correlation factor

n

volume exponent

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Nomenclature

inlet pressure [kPa]

pout

outlet pressure [kPa]

qmass

mass flow rate [kg/h]

U

heat transfer coefficient [kW/Km2]

w

specific power consumption [kWh/kg]

W

total power consumption [kW]

ρin

density of inlet stream [kg/m3]

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Abbreviations

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pin

compressor

C3MR

propane pre-cooled mixed refrigerant

DMR

dual mixed refrigerant

GA

genetic algorithm

HEX

heat exchanger

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JT

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C

Joule-Thompson

LNG

liquefied natural gas

MFC

mixed fluid cascade

MIX

mixer

MR

mixed refrigerant

MTPA

million tons per annum

PLNG

pressurized liquefied natural gas

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Peng-Robinson

S

separator

SMR

single mixed refrigerant

TCM

trillion cubic meter

VLV

valve

WC

water cooler

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PR

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ACCEPTED MANUSCRIPT Highlights Three pressurized liquefaction processes for offshore natural gas are proposed. The proposed processes are simulated and optimized by the genetic algorithm.

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The cascade PLNG process saves energy by 46% and reduces heat transfer area by 42%. The SMR PLNG process saves energy by 50% and reduces heat transfer area by 25%.

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The expander PLNG process saves energy by 63% and reduces heat transfer area by 4%.