NO20140358A1 - Coastalnear LNG production - Google Patents

Abstract

A process for liquefying a pre-processed natural gas to produce LNG, wherein pre-processed natural gas is introduced into an LNG heat exchanger, where the pre-processed natural gas is liquified by cooling against an evaporating primary refrigerant, where evaporated primary refrigerant is removed. from the LNG heat exchanger and liquefied again by compression in a series of compressor stages, where the compressed gas from each compression stage is cooled by means of heat exchangers, where an aqueous refrigerant cooled in an array of air coolers is used to cool the primary refrigerant.

Description

Description

Technical field

[0001] The present invention relates to improvements in a method and plant for the liquefaction of natural gas to produce liquefied natural gas (LNG) with improved economy and a reduction of the environmental burden, including the elimination of water intakes in the current liquid condensing plant. More specifically, the present invention relates to a method and facility for LNG production environmentally suitable for locations offshore, or for near coastlines with increased safety and efficiency by

liquefaction.

Background technology

[0002] Natural gas is becoming more important as the world's energy needs increase and its concerns about air and water emissions increase. Natural gas is readily available, especially with the new technologies for using shale gas. It burns much cleaner than oil and coal, and is not associated with the danger or problems of waste disposal associated with nuclear power. Emissions of greenhouse gases are lower than for oil, and only about a third of the emissions from coal.

[0003] There is considerable international trade in natural gas. However, the price differs significantly in different parts of the world. A large part of this trade is in the form of liquefied natural gas (LNG). LNG is produced using two main processing steps. The first step is gas pretreatment to remove components that can solidify when cooled to cryogenic temperatures, mainly acidic components and water. Trace elements, mainly mercury which can form amalgam - especially with aluminum process components - are also removed from the gas. Heavy hydrocarbon fractions or liquefied natural gas (NGL) can be removed from the gas in the first or second of the two LNG processing steps. The second treatment step is mainly condensation of the purified gas, which then mainly consists of methane. This methane, together with small amounts of heavier components, is liquid at atmospheric pressure and approx. -163 ° C. LNG is transported to the destination and re-gasified.

[0004] Processing of natural gas to produce LNG has traditionally been done in large land-based facilities that include the two steps of pretreatment and condensing at the same site. The latest developments in technology and markets have enabled the construction of LNG plants on floating structures, a development that has inspired the movement of a significant part of LNG processing plants offshore to floating plants for the liquefaction of natural gas (FLNG) to exploit large offshore gas reservoirs. The FLNs are usually designed to be located at a distance from a coast and are connected to natural gas reservoirs via subsea systems. The FLNGs are typically also designed to act as storage buffers and as terminals for loading LNG tankers used to transport LNG to the markets.

[0005] The later development towards FLNGs has made offshore natural gas resources more available on the market in relation to bringing the gas to land in pipes for liquefaction, and has resulted in a reduction in capital costs for establishing an LNG plant. Other important drivers include reducing land-based environmental impacts; reduction of land use problems for equipment and infrastructure; and reduced likelihood of resistance from local communities. The entire FLNG plant can be built at a shipyard, which is efficient and improves quality control, cost control and reduces construction time. FLNGs are also mobile and can be transferred to alternative locations if necessary.

[0006] Many studies of FLNG technologies have been carried out during the last couple of decades. Currently, several projects are underway worldwide. Actual construction has started for only three units, the Shell Prelude project, the Exmar / Pacific Rubicales barge project, and the Petronas FLNG 1 project.

[0007] In these and other projects, both gas pre-processing and condensation will typically be located on top of the FLNG. Space below deck is used for LNG storage and marine special equipment. The area available on FLNG decks is usually only about 20% of the area used for similar facilities on land. This reduced space for process layout presents safety issues, including proximity to residential quarters and limited space for safety barriers. Also of importance, it limits the size of processing facilities and the opportunities to exploit economies of scale.

[0008] In addition to safety, the liquefaction process involves environmental issues. The liquefaction process generates large amounts of heat which must

transferred to the environment. Large quantities of seawater are needed for

cooling purposes on board the FLNG, water which is later discharged at a higher temperature. The mechanical stresses in seawater pipes, pumps and fixtures and increased water temperature are harmful to marine life. In addition, the use of toxic chemicals to prevent fouling which leads to reduced cooling effect and eventually clogging is harmful to marine life and will probably be banned in many coastal waters, such as in the state of Louisiana in the near future. Submerged cooling coils are not preferred as the performance of cooling coils is difficult, if not impossible, to predict due to varying operating conditions as a result of variations in voltage, seawater temperature and fouling.

[0009] A new adaptation of FLNG is the Kystnær liquefaction, storage and offloading (CLSO) Liquefaction, Storage and Offloading (CLSO) plant. The CLSO adaptation addresses FLNG safety, environmental impact and issues related to processing capacity. The first treatment step, gas pre-processing, is mainly carried out onshore, at separate terminals or on dedicated floating systems, instead of taking up valuable space on the FLNG. This includes extraction of heavier hydrocarbons, NGL, from the gas. In contrast to some known systems in particular that extract NGL at the same location of the LNG plant, by using low temperatures in the condensation plant for this purpose, as described e.g. in WO9801335, in the name of Den norske states oljeselskap as (now Statoil ASA), there is usually no extraction of NGLs at a CLSO.

[0010] Fully pre-processed gas is piped to one or more floating CLSOs, which now have much more deck space available. Extra deck space on CLSO, freed up by removing pre-processing, can be used for extra security. There are also opportunities for greater LNG capacity, which will provide additional economic benefits. The extra deck space also opens up the possibility of using air instead of seawater cooling, and thereby problems in connection with seawater intake and associated environmental problems.

[0011] Air coolers are less efficient and require much larger space compared to seawater cooling. This presents a design challenge even with the extra deck space available on a CLSO. Furthermore, air-cooled heat exchangers typically require an air temperature 10 to 15<0>C below the temperature of the fluid to be cooled. In normal cases, it will be desirable or necessary to cool / condense the LNG cooler to approx. 30 to 40 °C before the refrigerant is led to LNG exchangers. However, in temperate regions, the design ambient air temperature may be relatively high, for example 32<0>C (90<0>F) or higher, and it is expected that the approach temperature for the air-cooled heat exchangers should be at least 10 °C, preferably 15 °C or more. In addition, deck space on a CLSO is severely limited. Therefore, the footprint of the air-cooled chillers must be minimized.

[0012] This problem can be solved by operating compressor inter-stage coolers at higher temperatures, and compressing refrigerants, especially a refrigerant that must condense before returning to the LNG exchangers, to a higher pressure than normal. All cooling and condensation therefore occurs at higher temperatures, enabling efficient air cooling and higher logarithmic mean temperature difference (LMTD) and higher air cooler approach temperatures. However, all this significantly reduces the condensing efficiency and increases the energy demand and thus increases the cooling effect, which partially defeats the purpose of using air cooling as such, as indicated in Table 1.

[0013] The extra space required for air cooling can be minimized if the temperature difference between the process flow and the surrounding air is large. This is illustrated in Figure 1, which shows a typical air-cooled heat exchanger footprint for 100 MW cooling of hydrocarbons, with an ambient temperature of 40 °C. For the same effect, higher hydrocarbon outlet temperature significantly reduces the desired footprint area. Correspondingly, higher hydrocarbon inlet temperature also reduces the footprint. Cooling to temperatures below the ambient temperature of 40 °C is not feasible in this example. In practice, cooling to low temperatures is almost always more difficult with air cooling than with water cooling.

[0014] Liquefaction processes are driven by compressors with inter-coolers and after-coolers, which are shown very simplified in Figure 2. Low-pressure cooling medium enters the first compression stage, is compressed and cooled in an inter-cooler. The coolant is then further compressed in a next compression stage, and cooled in an aftercooler. The refrigerant, now at high pressure and low enthalpy, is returned to the liquefaction process. With air cooling, the coolers will have higher output temperatures. This increases compressor work according to at least three mechanisms.

[0015] First, a higher inter-stage temperature results in a larger suction volume in the next compressor stage and thus an increased compressor load, even if the refrigerant flow and the pressure increase are constant.

[0016] Secondly, especially in cases where the refrigerant is condensed in compressor coolers, higher pressure is necessary to achieve condensation at a higher temperature, as shown in Figure 3. Figure 3 illustrates the effect of increasing the pressure from e.g. 35-52 bara on the condensation temperature of the gas. At 35 bara, cooling to below 40 °C is necessary for condensation, as shown by a line), while condensation is complete at 70 °C at a pressure of 52 bara, as shown by line b).

[0017] Third, when the refrigerant is recycled to the higher temperature liquefaction process, greater flow is required to give it the same condensing capability. Generally, all these effects increase the work of the compressor by about 20% when air cooling is used instead of water cooling.

[0018] Numerous liquefaction processes have been developed and are known by one

specialist in the field. All use refrigerant compressors and chillers. Liquefaction efficiency is measured as compressor work per kg of liquefied gas. Typically, the efficiency of large baseload liquefaction processes with water cooling can be 0.3 kWh / kg LNG. The efficiency of smaller top-shaving liquefaction processes can be 0.5 kWh/kg LNG. The efficiency depends on the process and on the refrigerant(s).

[0019] Table 1 shows a comparison of work and cooling effect for two liquefaction processes with water and air cooling. Liquefaction rate is 400 metric tons per hour, in the feed gas at 60 bar and 25 °C, and consists of 98 mol% methane, 1.5 mol% ethane and 0.5 mol% propane;

[0020] Air cooling increases the overall cooling duty by 10 to 15%. Air cooler footprint increases in roughly the same proportion. In addition, air coolers require electricity to drive fans, typically about 2 to 3 MW for the examples in Table 1.

[0021] In addition to increased compressor work and reduced liquefaction efficiency, which results in increased cooling effect when air coolers are used, there are two other and potentially more serious problems.

[0022] First, the increased cooling temperature from the compressor aftercoolers limits the capability of the air coolers and the temperature of the ambient air, and significantly increases the amount of heat that must be transferred in LNG heat exchangers where pre-processed natural gas is liquefied. This is because more heat must be transferred from the incoming, pressurized refrigerant in the LNG exchanger to the expanded, cold refrigerant in order to obtain sufficient precooling of the pressurized refrigerant before expansion. The work for this pre-cooling is therefore much greater, which results in a larger LNG heat exchanger. There is an upper limit to the size of LNG exchangers. Therefore, several units may be required for the same liquefaction work, which requires additional space, or the condensing capacity is reduced, which has serious economic consequences.

[0023] Secondly, there is a safety problem with air cooling, if flammable refrigerant is distributed, in large pipe networks and over very large areas, to a significant number of air coolers. It would be better to keep flammable and volatile refrigerants in a small, secure area, as is the case with water cooling, for example.

[0024] One purpose of the present invention is to provide a method and a system for the generation of LNG from natural gas on coastal liquefaction, storage and offloading (CLSO) devices that enable the least possible LNG heat exchanger work, and thus maximum production using air coolers, while minimizing any safety issues. Other purposes will become clear to a person skilled in the art by reading the present description and claims. Consequently, an efficient base load LNG system should be used, with safe air cooling where flammable refrigerants are limited to a small, safe area, where air cooling does not increase the work of the LNG heat exchanger compared to corresponding water-cooled systems, and with all gas pre-processing located on separate platforms or

floats, or on land.

Summary of the invention

[0025] The present invention relates to a method for liquefaction of a pre-processed natural gas to produce LNG, where pre-processed natural gas is introduced into an LNG heat exchanger, where the pre-processed natural gas is liquefied by cooling against an evaporating primary cooling medium, where vaporized primary coolant is taken from the LNG heat exchanger and liquefied again by compression in a series of compressor stages, where the compressed gas from each compression stage is cooled using heat exchangers, which are characterized by an aqueous coolant cooled in an array of air coolers, used for cooling of the primary refrigerant

[0026] According to a first embodiment, evaporated primary coolant is compressed to a pressure of 45-55 bar and condensed at said pressure. Operation at said pressure makes it possible to carry out the condensation at temperatures from 60 °C, to approximately 70 °C, or higher, which makes it possible to use the air coolers even in hot climates, where the temperature can exceed 40 °C, temperatures that makes it impossible to use air coolers in conventional plants at pressures of e.g. 35 bara, where the temperature is lower than 40 °C is necessary for condensation. See Figure 3.

[0027] According to another embodiment, the compressed and cooled refrigerant is further cooled by means of a secondary refrigerant in coolers 16, 16'. Further cooling after compression and cooling using an aqueous coolant reduces a duty for the LNG heat exchanger, or makes it possible to produce / liquefy more LNG at the same LNG heat exchanger output.

[0028] According to a further embodiment, the secondary refrigerant cools the primary refrigerant by evaporation in the coolers, and the secondary refrigerant is compressed, and then cooled in an array of air coolers. The secondary refrigerant operates in a heat pump circuit where additional cooling effect from air coolers is obtained to further cool the primary refrigerant to reduce duty of the LNG heat exchanger by creating cooling effect that would otherwise be possible by

using water cooling or air cooling in cold climates.

Brief description of drawings

[0029] Figure 1 is a plot of footprints for air-cooled heat exchangers to achieve a fixed hydrocarbon outlet temperature after cooling at different hydrocarbon inlet temperatures Figure 2 is an illustration of a compressor train consisting of two compression stages with air-cooled inter-cooler and after-coolers, Figure 3 is an illustration of condensing temperature and pressure for an example refrigerant, Figure 4 is an illustration of a natural gas condensing plant with air cooling using circulating water as the refrigerant for high temperature cooling and non-flammable, non-toxic and non-ozone depleting refrigerant as the refrigerant for cooling at lower temperatures, and Figure 5 is an illustration of a natural gas condensing plant with air cooling using circulating water as a coolant for cooling at high temperatures and non-flammable, non-toxic and non-ozone-depleting coolant as a coolant for cooling at lower temperatures, including pre-prose cooling ssert natural gas.

Detailed description of the invention

[0030] LNG plants according to the present invention will be placed on offshore floats. Floaters will receive pre-treated natural gas from an external location, which can be a pre-treatment plant at an offshore terminal, a barge or other floater, or land-based facilities.

[0031] The full pre-treatment of natural gas at the external site normally includes, but is not limited to:

1. Hg removal,

2. gas neutralization, i.e. removal of unwanted acid gases from

natural gas,

3. dehydration, i.e. removal of water that could otherwise lead to the formation of

hydrates from gas,

4. full or partial NGL extraction and processing, i.e. separation of NGL

from the gas, and possibly fractionation of NGL into salable products, which, depending on the NGL composition, can be liquid petroleum gas (LPG) consisting mainly of propane and butane, and a heavier C5 + fraction.

[0032] On board the floater, the pre-treated gas is liquefied by cooling to approx. -163 ° C. The condensing plants as such will be based on known technology, preferably efficient base load systems, but less efficient top-shaving systems can also be used. Known refrigerants for LNG, such as hydrocarbons or nitrogen, will circulate in a circuit consisting of compressors, compressor inter-coolers and after-coolers, and LNG exchangers. Depending on the cooling system, refrigerants may or may not condense in the compressor coolers before being routed to the LNG exchangers.

[0033] Figure 4 illustrates an embodiment of the present invention. Simple mixed refrigeration (SMR) liquefaction process is assumed. Pretreated natural gas comes to the floating element and is introduced into the LNG plant on board the float in a gas pipe 1. The gas in line 1 is introduced into an LNG heat exchanger 2. In the LNG heat exchanger 2, the gas is cooled, liquefied and subcooled by heat exchange with a cold refrigerant , as will be described in more detail below. Liquid and supercooled gas leaves the LNG heat exchanger in pipe 3, where it has an enthalpy close to the enthalpy of LNG at atmospheric pressure. The pressure is then reduced to near atmospheric in a valve 4. Those skilled in the art will understand that the resulting LNG in pipe 5 will be post-processed, stored in intermediate storage tanks and finally exported in shuttle tankers.

[0034] Compressed, cooled and fully or partially condensed refrigerant in a cold refrigerant line 17 is introduced for the LNG exchanger 2, where it is further cooled and, if necessary, completely condensed. After cooling, the refrigerant is expanded in the valve 7, which further reduces the temperature and, most importantly, enables boiling at low temperatures. The cold and expanded refrigerant flows via pipe 8 through the LNG exchanger 2.1 during this process, the refrigerant is gasified and heated.

[0035] The heated and gasified refrigerant is withdrawn from the LNG heat exchanger in the cooling pipe 9, which routes the refrigerant to serially arranged main compressors 10, 10' and 10" in which the refrigerant is compressed in a series of compressions. Due to the heating caused by compression, heat exchangers 11 , 11 'and 11", arranged after each compressor to cool the compressed refrigerant to a temperature of e.g. approximately 70°C. The heat exchangers are normally located in an area consisting of equipment containing hydrocarbons, i.e. LNG and/or pre-treated natural gas for LNG production, an area which is classified as a hazardous area. Accordingly, a non-flammable, non-toxic high heat capacity liquid such as water is preferably used as a cooling medium for heat exchangers 11, 11' and 11".

[0036] This coolant is taken out from the heat exchangers 11, 11', 11" in a coolant recycling pipe 27 and fed into a matrix of air coolers 14 which are arranged in a non-explosive area. In air coolers 14, the coolant is cooled to the lowest possible temperature allowed by the space available for air coolers 14, the efficiency of the air coolers and air temperature. The coolant from the air cooler 14 is pumped in the pump 15 to overcome the frictional pressure losses, and re-distributed to each of the heat exchangers 11, 11' and 11" in the coolant recirculation pipe 27'.

[0037] Liquid separator drums 12, 12' are arranged on the downstream side of the coolers 11, 11", 11" in the flow of the gasified refrigerant to remove liquid from the gas phase to avoid two-phase flow in compressors 10', 10" and 10 '". The liquid removed from the gas phase in the liquid separator drums is pumped in pumps 13 and 13', and is then mixed with the refrigerant coming out of the last compressor stage 10" via a liquid bypass pipe 28.

[0038] The effect of the LNG heat exchanger 2 is largely determined by the temperature of the compressed refrigerant in pipeline 17. A two-stage cooling system, which cools the compressed refrigerant in heat exchangers 16 and 16' is arranged in the path of the refrigerant downstream of coolers 11". Chillers 16 and 16' use a non-flammable, non-toxic and non-ozone depleting and low greenhouse effect refrigerant as the permanent refrigerant used in buildings. This refrigerant is called "secondary refrigerant" in the following, to distinguish it from the primary refrigerant which is used in the LNG heat exchanger. The secondary refrigerant will operate at approximately the same pressure and temperature range as residential air conditioning systems. Examples of refrigerants that can be used as a secondary refrigerant are R-410A and R-407C and others under development, replacing the well-known ozone-depleting R-22.

[0039] The secondary refrigerant is compressed, as will be described below, and cooled in an arrangement of air coolers 20 where cooling, condensation and subcooling take place. These air coolers can be located outside safe areas since the secondary cooler is non-flammable or nearly non-flammable. Most of the heat is removed from the secondary refrigerant in the condensation process, which takes place at an almost constant and relatively high temperature, such as approx. 70 ° C. This gives a relatively high air cooler LMTD, even in hot climates, which enables efficient cooling.

[0040] The condensed and subcooled secondary refrigerant is cooled in coolers 29, is led to the LNG system's safe area in a secondary refrigerant line 31, and is expanded in valve 21, for example to a pressure of approx. 14 only. At this pressure, the boiling point of the secondary refrigerant is much lower, for example 35 ° C. The secondary refrigerant is then used as a refrigerant in the heat exchanger 16, where the primary (LNG) refrigerant is cooled to intermediate temperatures, such as 45 ° C. In this process, the secondary refrigerant is only partly evaporated, such as approx. 50%.

[0041] The partially vaporized secondary coolant from the heat exchanger 16 is taken out through a secondary coolant return line 31' and introduced into a liquid separator 19. Gaseous coolant is separated in liquid separator 19, taken out through a gaseous coolant line 32, and then compressed, as will be further described below.

[0042] Liquid from liquid separator 19 is drawn out through a liquid secondary refrigerant line 30 and is expanded in the valve 22 to lower pressure, for example to approximately 5 bar and is fed into a heat exchanger 16' for further cooling of the primary refrigerant. At this pressure, the boiling point of the secondary refrigerant can be approx. 0 ° C, which is sufficient to cool the primary (LNG) refrigerant to approximately 15 ° C. Vaporized secondary refrigerant from the heat exchanger 16 'is taken out through a refrigerant line 30', and is optionally introduced into a liquid separator 19 '. Normally there will be no liquid from liquid separator 19'. The gaseous refrigerant in line 30' is introduced into a compressor 18', either directly from line 30' or via liquid separator 19', and is typically compressed to a pressure of approx. 14 only.

[0043] [0003] The gaseous absorbent is compressed in the compressor 18 'drawn out through a line 32', mixed with the gaseous refrigerant in line 32, and the mixed refrigerant in gaseous form is fed into a compressor 18, to be compressed into a press approx. 30 only. The compressed secondary refrigerant leaving the compressor 18 is then introduced into the above-mentioned series of air coolers 20, to complete the secondary refrigeration circuit.

[0044] Table 2 shows examples of the first embodiment of the present invention. It refers to Figure 4, and the purpose is to illustrate how a duty for the LNG heat exchanger is reduced when the temperature of the main refrigerant, line 17 in Figure 4, is reduced. The examples are based on the liquefaction of pre-processed natural gas with a composition of 98.0 mol% methane, 1.5 mol% ethane and 0.5 mol% propane. The gas flow is 400 metric tons per hour. The temperature of the ambient air is 32°C in all cases, and the pre-processed gas temperature is 25°C. This air temperature would normally enable cooling of the main refrigerant (line 17, fig 4) to about 45 °C, but not lower.

[0045] The first column in table 2 shows equipment reference numbers that correspond to reference numbers in Figure 4. The second column describes the equipment. The third and fourth columns show variables and units respectively.

[0046] The results are shown for four cases. The first case shows the main compressor, point 10 in Figure 4, aftercooler outlet temperature of 45°C. This is a reference case achieved either with the present invention, or with direct cooling of the LNG refrigerant in air coolers. The next three cases show results with successive reduction of the main compressor aftercooler outlet temperature of 30, 15 and 0 °C respectively. At an ambient temperature of 32 °C, none of these temperatures will be achieved by direct cooling of the LNG refrigerant in air coolers.

[0061] With the main compressor outlet temperature 45 ° C, the LNG heat exchanger total power is 346.3 MW. Reducing this temperature to 30, 15 and 0<0>C reduces this power to 302.4, 264.0 and 233.1 MW respectively. This lower power can be used to increase the condensing capacity, bringing the power back up to the maximum value of 346.3 MW. The increased liquefaction rate has a significant and positive economic effect.

[0062] As shown in Table 2, this benefit is achieved without an increase in the air cooler's total footprint or an increase in the total compressor power. At the same time, the safety of the liquid system is significantly improved by ensuring that no flammable liquid is carried out of the LNG plant's safe area. Instead, non-flammable liquids are distributed in the air cooler's piping network.

[0063] Another embodiment of the present invention is shown in figure 5. The second embodiment differs from the first embodiment in that part of the coolant in line 30 downstream of the valve 22 is taken out via a gas cooler line 25 and led into a gas cooler 23 arranged on gas pipe 1 to cool the incoming gas. Gasified refrigerant is taken out from the gas cooler 23 in a refrigerant return line 25 and is combined with the gas in the line 30' and compressed in compressor 18', as described above, possibly after liquid separation in liquid separator 19'.

[0064] Table 3 shows examples of the second embodiment of the present invention. It refers to figure 5, and the purpose is to illustrate how the effect for the LNG heat exchanger is reduced when, in addition to reducing the temperature of the main refrigerant, the temperature of the pre-processed natural gas feedstock is also reduced. Like the examples for the first embodiment of the present invention, the examples are based on the liquefaction of pre-processed natural gas with a composition of 98.0 mol% methane, 1.5 mol% ethane and 0.5 mol% propane. The gas flow is 400 metric tons per hour. Ambient air temperature is 32 °C in all cases.

[0065] The temperature of the natural gas feed to the LNG heat exchanger, line 1, has now been reduced from the previous 25 ° C. It has now been set to the same numerical value as for the primary refrigerant in line 17, which is 15 and 0 ° C, respectively.

[0066] The first column in table 3 shows the element's reference number which corresponds to the reference number in figure 5. The second column describes the element. The third and fourth columns show variables and units, respectively.

[0067] The results are shown for two cases. The first case shows the main compressor, element 10 in Figure 5, aftercooler outlet temperature of 15 °C. The next case shows results when the aftercooler 16 outlet temperature is reduced to 0 °C.

[0047] With both the main compressor discharge temperature, line 17, and the natural gas supply temperature, line 1, equal to 15<0>C, the LNG heat exchanger's total output is 257.2 MW. Reducing these temperatures to 0 °C reduces this power to 191.2 MW. This lower power can be used to increase the condensing capacity, bringing the power back up to the maximum value of 346.3 MW (table 2). The increased liquefaction rate has a significant and positive economic effect.

[0048] As shown in Table 3, this benefit is achieved without an increase in the air cooler's total footprint or an increase in the total compressor power. At the same time, the safety of the liquid system is significantly improved by ensuring that no flammable liquids are carried out of the LNG plant's safe area. Instead, non-flammable liquids are distributed in the air coolers' piping network.

[0049] For a professional in the field, and depending on permits and environmental conditions, it will be possible to optimize the system by partial use of seawater for cooling, for example by means of a submerged pipe in which hot water is introduced, flows and is cooled by pipe of heat to the surrounding seawater, exits and is returned to the process for reuse as a refrigerant, by locating air coolers in a remote area, such as an offshore platform, a barge or on land, using more than two stages of secondary refrigerant cooling, or using secondary refrigerants for compressor inter-stage cooling in addition to after-cooling, by taking the gas turbine inlet air cooling as part of the secondary refrigerant circuit, by having a secondary refrigerant circuit serve more than one gas condensing plant, by using alternative liquefaction processes such as N2 refrigerant for smaller installations and C3MR or DMR for base load systems, and in the case of a water shower at the air intake to selected air coolers that will reduce the temperature to wet bulb temperature. In addition, power production for CLSO can be partly done at the terminal and the power is transferred to CLSO via submarine cable.

Claims (4)

1. Method for liquefaction of a pre-processed natural gas to produce LNG, where pre-processed natural gas is introduced into an LNG heat exchanger, where the pre-processed natural gas is liquefied by cooling against an evaporating primary coolant, where vaporized primary coolant is taken out of the LNG heat exchanger and liquefied again by compression in a series of compressor stages, where the compressed gas from each compression stage is cooled using heat exchangers, characterized in that an aqueous coolant cooled in an array of air coolers is used to cool the primary coolant. 2. Method according to claim 1, characterized in that the evaporated primary refrigerant is compressed to a pressure of 45 to 65 bar and condensed at said pressure. 3. Method according to claim 1 or 2, characterized in that the compressed and cooled refrigerant is further cooled by means of a secondary refrigerant in coolers 16, 16'. 4. Method according to claim 3, characterized in that the secondary coolant cools the primary coolant by evaporation in the coolers 16, 16', and the secondary coolant is compressed, and then cooled in an array of air coolers.