Method and plant for liquefaction of pre-processed natural gas Technical Field

[0001] The present invention relates to improvements in methods and plants for liquefaction of natural gas to provide Liquefied Natural Gas (LNG) with enhanced efficiency and production rate, while at the same time providing improved economics, improved personnel safety and a reduction of the environmental impact. More specifically, the present invention relates to a method and plant for small scale LNG production environmentally suited to locations onshore, which can satisfy the requirements set with regard to increased safety and environmental protection with increased liquefaction efficiency and production rate.

Background Art

[0002] Natural gas is becoming more important as the world's energy demand increases as well as its concerns about air and water emissions increase. Natural gas is readily available, from gas reservoirs, from shale gas, and from stranded gas sources far from infrastructures such as gas pipelines. It is much cleaner-burning than oil and coal, and does not have the hazard or waste deposition problems associated with nuclear power. The emission of greenhouse gases is lower than for oil, and only about one third of such emissions resulting from combustion of coal. Numerous small gas deposits exist onshore, in locations without infrastructure such as gas pipelines. Utilization of these deposits will open vast environmentally friendly energy resources.

[0003] When gas pipelines are uneconomic, the best way to transport gas to markets is often in the form of Liquefied Natural Gas (LNG), which mainly comprises natural gas components lighter than pentane. These components are methane, ethane, propane, butane and traces of nitrogen. Methane concentration is typically above 85% on a molar basis, often above 90%, ethane may range from below 1 to about 10% on a molar basis, propane may be in the range from below 0.1 to about 3 mole%, while butane may be in the range from below 0.1 to 1 %. Nitrogen

concentration may be in the range from below 0.1 to 2 mole%.

[0004] LNG is produced using two major processing steps. The first step is gas pre- treatment to remove CO2, H2S and water which may become solids and plug pipes in the cryogenic liquefaction process. The first step also includes the removal of trace elements, such as e.g. mercury which can form amalgams - in particular with aluminium process components, and cause erosion / corrosion. Hydrocarbon fractions heavier than methane, including ethane and also components collectively referred to as Natural Gas Liquids (NGLs), such as propane, butane, pentane and heavier components, are removed from the gas to varying degrees.

[0005] Very heavy NGLs, hexane and heavier are removed to very low residual concentrations such as 50 to 100 ppm on a mole basis, since these may solidify in the liquefaction process. Ethane and other NGLs, propane, butanes and pentanes are removed to an extent determined by economic logic.

[0006] The NGLs may be removed from the gas in the first LNG processing step, the gas pre-processing, or as an integrated part of the second processing step, the gas liquefaction.

[0007] The second processing step is mainly liquefaction of the thus purified gas, which then comprises mainly methane. These first and second processing steps all take place at elevated pressures, typically in the range from 40 to 100 bar absolute (bara). A final processing step, downstream of the liquefaction process, includes pressure reduction to atmospheric pressure, and removal of any excessive amount of nitrogen, typically any amount that exceeds 1 mole%. This is done by flashing of the LNG at near atmospheric pressure. This produces the final LNG product, and a much smaller hydrocarbon gas stream enriched in nitrogen, typically used for fuel. The final LNG product is liquid at atmospheric pressure and about -163°C. It is stored in buffer storage tanks before being transported to destinations in for example LNG shuttle tankers or, for small onshore scale and local consumption, trucks. At the destination, the LNG is re-gasified and distributed to consumers.

[0008] LNG plant sizes range from less than 0.05 million tons annually (mtpa) for peak-shaving plants, via small scale LNG (SSLNG) plants in the range from 0.05 to about 1.0 mtpa, via conventional plants producing up to about 4.0 mtpa, to mega plants producing up to 7.8 mtpa.

[0009] Although small plants benefit less from any economics of scale, there has been a growing interest in SSLNG in recent years. Global SSLNG capacity is about 20 mtpa, expected to grow to 30 mtpa over a five year period. In comparison, global capacity for conventional LNG facilities is about 300 mtpa.

[0010] SSLNG is used in areas which have no gas pipeline infrastructure or grid, and there is natural gas available from for example local gas reservoirs or local oil production where gas is separated from the oil in oil stabilization processes. In many areas, gas from oil / gas separators is flared. SSLNG reduces such undesirable flaring. SSLNG enables the transport of gas by truck or small LNG carriers to end users. End use includes domestic heating and fuel for power plants, industries or transport such as trucking or marine vessels. Depending on application, the liquefied gas may be sold as such, or vaporized before delivery. In this way, SSLNG serves a wide range of end users.

[001 1] Drivers for SSLNG use include cost advantage over alternative fuels such as diesel, in areas without alternative gas transport such as pipeline grids. Drivers also include environmental advantages by for example reducing gas flaring and by replacing other fossil fuels such as oil or coal, which emit more CO2 and other pollutants. Some regions may have gas sources, and wish to develop these in order to reduce dependency on energy imports. If the production rate is too small for the development of pipeline infrastructure, SSLNG may be an attractive alternative.

[0012] One of the main challenges with SSLNG is to provide technical solutions. The excellent safety record of the LNG industry needs to be maintained, while at the same time keeping costs low. SSLNG lacks economics of scale, and smaller plants usually do not have the same experienced personnel as larger units.

[0013] More efficient, cost effective and inherently safe small scale liquefaction processes are needed. They must be robust, easy to operate and suitable for remote locations.

[0014] WO 01/44735 A1 , describes a process for liquefaction of natural gas where compressed gas flow at a pressure above 1 1 ,000 kPa (approx. 735 bara) is split in two flows, a first flow that is expanded and used for cooling the second, and still compressed, gas flow. The thus cooled gas flow is then expanded to give a

Pressurized Liquefied Natural Gas (PLNG) at a temperature higher than -1 12 °C, and a pressure above 1 ,380 kPa (approx. 13.4 bara). PLNG is expensive to transport due to the combination of high pressure and low temperature, and is thus not suitable for solving the problems identified above.

[0015] In order to get high LNG production rates and in particular, high liquefaction efficiencies, ethane and NGL components have often been preferred as refrigerants. However, ethane and NGL component such as propane, butanes and pentanes are highly reactive and can be a safety hazard. In contrast, the natural gas to be liquefied contains mainly methane, which is much less reactive and therefore safer than NGL based refrigerants. Furthermore, purified NGL components which have to be mixed in precise ratios may be difficult to obtain in remote locations.

[0016] Much increased safety is possible with nitrogen refrigerant. However, such plants are less efficient. As an example, while hydrocarbon refrigerants may achieve specific power consumption of below 350 kWh per ton LNG, liquefaction based on nitrogen refrigerant may require 450 kWh per ton LNG or, for the very simplest plants, up to 600 to 800 kWh per ton LNG.

[0017] Several patents describe natural gas liquefaction systems which do not employ ethane or NGL refrigerants.

[0018] Two of these are US 8464551 B2 and US 8656733B2 to Air Products and Chemicals Inc. While systems for natural gas liquefaction processes employing nitrogen refrigerant normally utilize plate heat exchangers only, these patents describe a system where nitrogen streams are arranged such that natural gas pre- cooling and / or liquefaction and / or sub-cooling occur in separate wound-coil heat exchangers.

[0019] US 201 1/01 13825 A1 , to Statoil ASA, describes a method for natural gas liquefaction using nitrogen refrigerant. In essence, natural gas liquefaction is accomplished by using intermediate and low-pressure nitrogen suitable for liquefaction and sub-cooling, respectively.

[0020] US 7,386,996 to Den Norske Stats Oljeselskap AS, describes a process for natural gas liquefaction which uses a carbon dioxide pre-cooling circuit.

[0021] US 5,916,260 to BHP Petroleum Pty Ltd., describes a process for natural gas liquefaction using nitrogen refrigerant, arranged such that the warming curve of the refrigerant more closely matches the cooling curve of the product. [0022] US 6,446,465 to BHP Petroleum Pty Ltd. describes a process for natural gas liquefaction using nitrogen refrigerant, where the natural gas and the nitrogen refrigerant are pre-cooled using a separate refrigerant. The process uses a heat integrated stripper column for post treatment of the liquefied natural gas.

[0023] US 12/665,329, Kanfa Aragon AS, describes methods to integrate natural gas liquefaction and the extraction of natural gas liquids, using nitrogen refrigerant.

[0024] RU 2253809 describes single or multistage methods for liquefaction of natural gas by natural gas expansion, which provides cooling and partial liquefaction. The partial liquefaction creates large gas stream, which must be used for alternative purposes or recycled. If the liquefied natural gas is to be stable at atmospheric pressure, the recycles will also be at low pressure, requiring large pipes for transport and significant re-compression work.

[0025] WO 2013022529 describes a natural gas liquefaction process where a methane rich natural gas fraction is compressed to a pressure of at least 1000 psia (pound per square inch), or about 67 bara, cooled in a heat exchanger and then expanded to a pressure of 50 to 450 psia, or 3.4 to 30 bara, to give a liquid LNG phase and a gas phase. The liquid phase is withdrawn for export, whereas the cold gas fraction is heat exchanged against the incoming compressed methane rich fraction, and thereafter combined with the incoming methane rich fraction and recompressed. This is the first refrigeration loop. A secondary refrigeration loop is provided for additional cooling of the compressed methane rich gas in the heat exchanger. The secondary refrigeration loop provides extra refrigeration by heat exchange with natural gas inside the first refrigeration loop. The expanded pressure of 3.4 bara or higher, results in a liquid phase that is not stable under atmospheric pressure, i.e. a gas that will liberate significant additional volumes of methane gas if further expanded to atmospheric pressure. This is unacceptable in many cases, such as transport in large, non-pressurized containers.

[0026] Several patents describe natural gas liquefaction using refrigerants other than pure nitrogen gas, such as mixtures of nitrogen and methane, vaporizing refrigerant, systems integrated with processes for the extraction of natural gas liquids, systems for partial liquefaction only, systems for sub-cooling only, or systems with gas and / or nitrogen pre-cooling using an independent pre-cooling circuit and heat exchange. General liquefaction methods are also described in Mokhatab,S., Mak,J.Y., Valappil,J.V., Wood,D.A., "Handbook of Liquefied Natural as", Elsevier Inc. 2014.

[0027] Natural gas pre-cooling using lithium bromide or Freon based refrigeration systems are known to reduce the nitrogen liquefaction plant energy requirement. It also increases the liquefaction capacity. However, such pre-cooling equipment requires chemicals and associated storage, which is undesirable in remote areas for SSLNG.

[0028] An object of the present invention is to provide a method and a plant for LNG production environmentally suited to remote locations, for small scale LNG (SSLNG) plants, with high inherent safety and low specific energy requirement. It must satisfy any requirements set with regard to risk to personnel, a risk which shall in any case be as low as reasonable practicable. It shall satisfy the most stringent requirements set to environmental protection. The plant must be simple to operate and have low equipment count.

Summary of invention

[0029] According to a first aspect, the present invention relates to a method for liquefaction of a pre-processed natural gas, comprising pre-cooling, liquefaction and sub-cooling of the gas at an elevated pressure, expanding the compressed and cooled gas to a pressure of 1 to 1.3 bara to further cool the gas to produce a liquid and a gaseous phase, separating the phases, withdrawing the gaseous phase from the plant, withdrawing and exporting the liquid phase from the plant as LNG, where the cooling of the gas is performed in a pre-cooling part and a thereto serially connected liquefaction part, and where the thus cooled and liquefied natural gas is expanded to a pressure of 1 to 1.3 bara in an expansion and separation unit, wherein the pressure in the pre-cooling part is substantially equal to the pressure in the liquefaction part, and wherein one or more part gas streams is (are) withdrawn from the gas stream introduced into the liquefaction part, heated in liquefaction part, returned in conduit(s) to the pre-cooling part, expanded in expander(s) and used as cooling medium to cool the incoming gas stream in the pre-cooling part .

[0030] The method according to the present invention allows for building LNG plants having high energy efficiency, even for small scale LNG plants. By maintaining substantially the same pressure though the cooling stages for the natural gas, expanders between the cooling stages and other equipment making the construction of the plant expensive, are avoided. Accordingly, the present invention makes it possible to build small scale LNG plants at a reasonable cost.

[0031] According to a first embodiment, the liquefaction part is cooled in heat exchangers cooled by means of nitrogen as cooling medium.

[0032] According to a second embodiment, the pressure of the pre-processed natural gas in the gas stream to be cooled is from 70 to 1 10 bara, such as 75 to 100 bara, or about 85 bara.

[0033] According to a second aspect the present invention relates to a plant for liquefaction of a pre-processed natural gas to produce Liquefied Natural Gas (LNG) suitable for transport and / or storage, the plant comprising two cooling units, a pre- cooling unit, a thereto serially connected liquefaction unit comprising a closed cooling circuit with an circulating cooling fluid, and an expansion and separation unit , the precooling unit comprising a compressed gas conduit for introduction of pre- processed compressed natural gas into the plant, and one or more heat

exchanger(s) for pre-cooling of the pre-processed natural gas,

the liquefaction unit comprising one or more heat exchanger(s) for further cooling of the gas against a cooling medium, a gas conduit for transferring pre-cooled gas from the pre-cooling unit, and

the expansion and separation unit comprises hydraulic expander(s), a valve, a separator for separation of liquid and gas, a gas withdrawal conduit, and a LNG conduit,

the pre-cooling unit further comprising one or more side draw conduit(s) for withdrawal of part gas stream(s) of the gas introduced into the pre-cooling unit, turbo expander(s) for expanding and thus cooling of the part gas stream(s) in conduits, conduit(s) for piping the expanded and thus cooled gas from turbo expander(s) to the heat exchanger(s), gas return conduit(s) for piping the gas from the heat

exchanger(s) into one or more compressor(s) for compression of the cooling gas, and a gas return conduit for piping of the re-compressed gas and introducing the gas into conduit,

wherein the liquefaction unit comprises one or more conduits for withdrawing a part gas flow of the precooled natural gas introduced via conduit for introduction of said part gas flow into a heat exchanger for cooling of the circulating cooling fluid, where a conduit connected to an expander is arranged for withdrawing the thus heated part gas flow and introducing said part gas flow into said expander, and conduit(s) for introducing said part gas flow into heat exchanger.

[0034] According to one embodiment, the heat exchanger(s) of the liquefaction unit is (are) cooled by means of a nitrogen based heat pump system.

Brief description of drawings

[0035]

Figure 1 shows a block diagram of the liquefaction process

Figure 2 shows composite curves for the first stage gas pre-cooling

Figure 3 shows composite curves for the second stage gas pre-cooling

Figure 4 shows composite curves for the nitrogen refrigerant pre-cooling

Figure 5 shows composite curves for gas liquefaction and / or sub-cooling

Detailed description of the invention

[0036] In the present description and claims, SSLNG is used as an abbreviation for Small Scale Liquefied Natural Gas. The term "natural gas" or "gas" is used for a gas comprising low molecular weight hydrocarbons, , which during cooling to produce LNG might be under sufficient pressure to provide supercritical state, where it remains a single phase, or at lower pressures where, depending on temperature, there may be gas only, mixtures of gas and liquid, or liquid only. The cooling process may include pre-cooling, which may be any degree of cooling down to about -100°C, main cooling which may be any degree of cooling in the temperature range from precooled temperature to about - 140°C, and sub-cooling which is cooling from main cooling temperature to LNG temperature, where the LNG is stable, or gives only very small amounts of gas, such as 1 to 2% on mass basis, when expanded to

atmospheric pressure. In some cases, the term "cooling" is used for both pre-cooling, main cooling and sub-cooling.

[0037] Natural gas is found in geological formations either together with oil, in gas fields, and in shale as shale gas. Dependent on the source, natural gas may differ in hydrocarbon composition but methane is almost always the predominant gas. The skilled person within this technical area will have good knowledge of the

abbreviations LNG and NGL, i.e., Liquefied Natural Gas, and Natural Gas Liquids, respectively. LNG consists of methane normally with a minor concentration of C2, C3 C ₄ and C5 hydrocarbons, and virtually no C6+ hydrocarbons. LNG is a liquid at atmospheric pressure at about -161 °C. NGL, at the other hand, is a collective term for mainly C3+ hydrocarbons, which exist in unprocessed natural gas. LPG is an abbreviation for liquefied petroleum gas and consists mainly of propane and butane.

[0038] The pressure is herein given in the unit "bara" is "bar absolute". Accordingly, 1.013 bara is the normal atmospheric pressure at sea level. In SI units, 1 bar corresponds to 100 kPa. As used herein, the expression "near atmospheric pressure" is intended to mean a pressure of 1 bara or slightly higher, such as typically a pressure of 1.1 to 1.3 bara, such as about 1.2 bara.

[0039] The expression "ambient temperature" as used herein may differ with the climate for operation of the plant according to the present invention. Normally, the ambient temperature for operation of the present plant is from about 0 to 40 °C, but the ambient temperature may also be from sub-zero levels to somewhat higher than 40°C, such as 50 °C, during some operating conditions.

[0040] The natural gas is pre-processed on a not shown gas pre-processing plant, which purifies natural gas from a pipeline network, from a gas field or from a combined oil and gas field. The pre-treatment normally comprises but is not limited to:

• Hg removal,

• Gas sweetening, i.e. removal of unwanted acid gases, such as CO2 and H2S from the natural gas,

• Dehydration, i.e. removal of water that may otherwise cause formation of hydrates from the gas,

• Full NGL extraction and processing, i.e. separation of the NGL from the gas, including separation of substantially all C6+ components. Optional fractionation of the NGL into saleable products, which depending on the NGL composition might include Liquefied Petroleum Gas (LPG). [0041] During the pre-treatment, all components that may solidify at liquefaction temperatures, i.e. about -161 °C, are removed or at least reduced to a concentration of less than 50 ppm. The upper limit for the concentration of solidifying components depends on the actual component, as e.g. water preferably is reduced to a maximum level of 1 ppb.

[0042] Figure 1 is a principle sketch of a system according to a preferred

embodiment of the present invention. The pre-processed natural gas is introduced through an incoming gas conduit 100, typically at a pre-processing pressure between 40 and 70 bara, such as about 60 bara. The incoming pre-processed natural gas is compressed in a compressor 102, operated by a driver 101 , to a pressure of typically 80 to 100 bara, such as about 85 bara. The pre-processed natural gas is then cooled at substantially the same pressure in two cooling parts, as will be described below. After serial cooling in a pre-cooling part 1 and a liquefaction part 2, the natural gas is expanded in part 3 to near atmospheric pressure, as defined above, and separated into a liquid phase (LNG), and a gas phase, as will be further described below, for storage and/or transport. The pressure drop from the compressor 102 until the final expansion in part 3 is the pressure drop caused by conduits, coolers, and heat exchangers, only. The skilled person will understand that compressor 102 and driver 101 may be omitted if the pressure of the incoming pre-processed natural gas is sufficiently high, i.e. as high as the preferred pressure for the gas in the pre-cooling 1 and the liquefaction part 2. In such a case, the pressure change from the incoming gas conduit 100 to the final expansion corresponds to the pressure drop caused by the conduits, coolers and heat exchangers, only.

[0043] As mentioned above, the present liquefaction process comprises three parts, a pre-cooling part 1 which is gas pre-cooling based on gas compression and expansion, a gas liquefaction or main cooling part 2 which is gas liquefaction and / or sub-cooling based on gas heat exchange with nitrogen refrigerant, and an expansion part 3 where the gas pressure is reduced to near atmospheric pressure. The liquefaction part 2 is located in series with, and down-stream of the pre-cooling part 1. The expansion part 3 is located in series with, and down-stream of the liquefaction part. The gas to be cooled and liquefied maintains substantially the same high pressure throughout the complete cooling process, parts 1 and 2. However, the skilled person will understand that the pressure will fall through the path of the gas being cooled due to pressure drop in conduits, heat exchangers, coolers etc. A special arrangement enables the pre-cooling part 1 to substantially improve the performance of the liquefaction part 2.

[0044] In the pre-cooling part 1 of the process, pre-processed natural gas is delivered from the not shown pre-processing plant at a pre-processing pressure, typically from 40 to 70 bara, such as about 60 bara, via a conduit 100. The pre- processed natural gas entering through conduit 100 is compressed in a compressor 102, with a driver 101 , from pre-processing pressure, which is typically between 40 and 70 bara, such as 60 bara, to about 70 to 1 10 bara, such as 85 bara, and is led to a gas cooler 1 10 in a compressed gas line 104. The skilled person will understand that the compressor 102 may be omitted if the pre-processed natural gas in conduit 100 already is at a pressure suitable for further processing / cooling, such as the mentioned span from about 70 to about 1 10 bara.

[0045] A re-cycle gas, as will be described further below, is introduced into the compressed gas conduit 104 from a discharge conduit 103, before the gas is introduced into the cooler 1 10. The gas cooler 1 10 is preferably an air cooler and is provided for cooling the gas that has been heated due to compression. The gas cooler 1 10, which conveniently comprises several gas coolers, comprises the first part of the pre-cooling part 1.

[0046] The gas from conduit 104 is, after cooling in the gas cooler 1 10, led in conduit 1 1 1 to a pre-cooling gas / gas heat exchanger 135. The gas / gas exchanger 135 cools the gas from near ambient temperature, such as 30°C, typically to about - 30 to -45°C. Downstream of gas / gas exchanger 135, the cooled gas flows in conduit 134 to a second gas / gas heat exchanger 133, where it is further cooled to about -75 to -95°C. This completes the pre-cooling of the gas, which is then led in a conduit 132 to the main cooling or liquefaction part 2.

[0047] Cooling for the gas / gas exchangers 133, 135 is provided by withdrawing a part of the compressed and cooled natural gas from the main stream for the natural gas as side draws, expanding this side draw gas in turbo expanders, and using the thus cooled gas as cooling medium in said gas / gas heat exchangers 133, 135, as described in more detail below. In other words, one or more part stream(s) of the pre- processed, optionally compressed, and partly cooled natural gas to be liquefied is withdrawn from the main gas stream through the pre-cooling part 1 and is used as cooling medium for the heat exchangers in the pre-cooling part 1 of the plant .

[0048] A side draw from conduit 1 1 1 , conveniently slightly less than 50%, such as about 48% of the gas in conduit 1 1 1 , is led in conduit 126 via valve 121 to a turbo expander 1 15. Some additional gas, a recycle gas part from the liquefaction part 2, is led in a conduit 136, and is mixed with the gas in conduit 126. The added recycle gas introduced via conduit 136 typically about 2.0 to 2.5% of the gas flow in conduit 1 1 1. A valve 121 is provided in line 126 for reducing the pressure therein to the pressure in conduit 136 to allow the gas from conduit 136 to flow freely into line 126. Typically, the reduction of pressure over valve 121 is 1 to 3 bar. As an example, the pressure in conduit 126 is typically reduced by 1 to 4 bar, such as typically about 2 bar, e.g. from about 85 bara to about 83 bara over the valve 121 , to compensate for the reduced pressure in conduit 136 caused by the pressure drop in the path of the recycle gas through the path of the gas stream withdrawn in conduit 136. The combined gas from conduit 136 and 126, the last expanded over valve 121 , is introduced into a turbo expander 1 15 arranged to expand the combined gas stream to result in cooling of the gas to give a temperature of the gas that is slightly lower, such as 2 to 5 °C colder, such as about 3 °C colder, than the desired gas temperature in conduit 134. As an example, if desired temperature in conduit 134 is -39°C, then, if the pressure is reduced from about 83 bara to a pressure of about 23 bara in turbo expander 1 15, the expansion cools the gas to a temperature of about -42°C, being about 3°C lower than the desired temperature in conduit 134. This cooled gas is led in a conduit 1 16 to a conduit 129 and mixed with gas in conduit 129. This mixed gas in conduit 129 may, depending on gas composition, contain small amounts of liquid, such as for example 3 to 4% on a molar basis. The fluid in conduit 129 is separated into a gas and a liquid phase in a separator 123, in order to ensure proper distribution of gas and liquid in gas / gas heat exchanger 135. The resulting gas phase from the separator 123 is led in a conduit 122, and the liquid phase is led in a conduit 128, via pump 127, to the gas / gas heat exchanger 135 where the two flows are introduced such that both fluids are evenly distributed. Persons skilled in the art will know the detailed design of such arrangements. This is therefore not shown in detail in Figure 1.

[0049] A second side draw is drawn though a conduit 1 17 from conduit 134.

Conveniently slightly less than 60%, such as 57%, of the gas flow in conduit 134 is withdrawn in a conduit 1 17 to a turbo expander 1 18. The gas is expanded in the turbo expander 1 18 to a pressure slightly higher than the pressure in conduit 1 16, such as 0.5 bar higher, to result in cooling of the gas and mixing of gas from conduit 1 16 with the gas in conduit 129. As an example, the gas may be expanded to 23.5° C, which depending on gas properties will give a temperature in conduit 120 of -90° C. This cooled gas may, depending on gas composition, contain some amounts of liquid, such as for example 10 to 20% on a molar basis. In order to ensure proper distribution of gas and liquid in gas / gas heat exchanger 133, the fluid in conduit 120 is separated into a gas and a liquid phase in separator 125. The resulting gas phase is led in conduit 124, and the liquid phase in conduit 131 , via pump 130, to gas / gas heat exchanger 133 where the two flows are introduced such that both gas and liquid phase are evenly distributed. Persons skilled in the art will know the detailed design of such arrangements. This is therefore not shown in detail in Figure 1. Depending on the size of heat exchanger 123 and flow rate in conduit 1 17, the gas in conduit 132 is cooled to a temperature slightly above the temperature in conduit 120, such as -87° C.

[0050] Expanded gas from gas / gas exchangers 133 and 135 is led in conduit 1 12 to compressor 107, powered by expanders 1 15 and 1 18 via shafts 1 13 and 1 14 and a not shown gear arrangement. The compressed gas is led in conduit 108 via gas cooler 109 to a compressor 105 operated by a driver 106, in which compressor the gas is further compressed and led via conduit 103 to conduit 104, thus completing the gas pre-cooling loops. The skilled person will understand that the gas in conduit 108 is compressed in compressor 105 to a pressure equal to the pressure in conduit 104 as described above.

[0051] Persons skilled in the art will know that there are various ways to arrange compressors 102, 105 and 107, depending on system pressures and compressor capabilities. For example, compressors 102 and 105 may be on a common shaft, powered by a common driver. [0052] A small side draw from conduit 1 12, such as 0.5%, is led in conduit 1 19 to heat exchanger 141 where it is cooled and mixed with fluid in conduit 146, as will be described further below.

[0053] In the gas liquefaction part 2 of the process, wherein the gas is liquefied and / or sub-cooled, pre-cooled gas is received from the pre-cooling part 1 through the conduit 132. Additional interfaces between process parts are gas return from part 2 to part 1 in conduit 136, and natural gas supply from part 1 to part 3, using gas which has been expanded and heated, in conduit 1 19.

[0054] Pre-cooled natural gas in conduit 132 may contain up to 15 to 20% liquids on a molar basis. In order to ensure proper distribution of gas and liquid in heat exchangers 153 and 139, the fluid in conduit 132 is separated into a gas and a liquid phase in a separator 152. The resulting gas phase is led in a conduit 138 to a heat exchanger 139. A part of the gas in conduit 138, conveniently about 10% on mass basis, is led in a side draw conduit 137 to a heat exchanger 153.

[0055] The liquid phase from separator 152 is led in a conduit 151 via a pump 150 to a heat exchanger 153. A large part of the liquid in conduit 151 , conveniently about 90% on mass basis, is led in a side draw conduit 149 to the heat exchanger 139. The remaining part of the liquid in conduit 151 is combined with the flow in conduit 137 and introduced into heat exchanger 153 as described above. Arrangements in the inlet of heat exchangers 153 and 139 ensure that liquid and gas are evenly

distributed. Persons skilled in the art will know the detailed design of such

arrangements. This is therefore not shown in detail in Figure 1.

[0056] Pre-cooled gas and liquid led to heat exchanger 153 improve the operation of heat exchanger 153, by providing better pre-cooling of compressed nitrogen introduced into the heat exchanger from conduit 165. In this process, the liquid is vaporized and the hydrocarbon mixture is heated to a temperature about 3°C below the temperature of the nitrogen in conduit 165, after cooling in cooler 157. The heated hydrocarbons are led in conduit 136 to line 126 for expansion in turbo expander 1 15.

[0057] Pre-cooled gas and liquid led to heat exchanger 139 is further cooled by heat exchange with nitrogen from conduit 170. The thus liquefied and sub-cooled hydrocarbons are led in conduit 147 to part 3 of the process, hydraulic turbine 148, where the pressure is reduced, conveniently from between 75 and 85 bara to about 4 bara. The resulting low pressure, cold hydrocarbons are then led in conduit 146 via valve 143 to gas liquid separator 144. Liquid from separator 144, led in conduit 145 to not shown storage, is the final LNG product.

[0058] In some cases, the fluid in conduit 146 contains small amounts of nitrogen originating from pre-treated natural gas in conduit 100. The amount of such nitrogen may be above the allowable amount in the LNG product. Gas from separator 144, conduit 142, is enriched in nitrogen, thus reducing the amount of residual nitrogen in the LNG product. Acceptable levels of residual nitrogen in the LNG product are thus obtained, such as about 1 % on a molar basis.

[0059] In order to preserve valuable refrigeration capacity in the gas from separator 144, a side draw of low pressure gas from conduit 1 12 is led in conduit 1 19, via heat exchanger 141 and conduit 140, to be mixed with fluid in conduit 146. In heat exchanger 141 gas from conduit 142 is thus heated by heat exchange with the side draw gas in conduit 1 19, and the coldness remains within the system in the form of cold gas in conduit 140.

[0060] The amount of gas separated in separator 144 is determined by the amount of nitrogen that must be removed from the fluid in conduit 146. Adjustment of the enthalpy of the fluid in conduit 146, or the temperature of this fluid, ensures adherence to the required amount of gas from separator 144. The temperature in conduit 146 may typically be about -160°C, and the amount of gas in conduit 142 may typically be about 1 to 3% of the fluid entering separator 144, on a mass basis. The skilled person will understand that the gas flow in conduit 1 19 is balanced towards the gas flow in conduit 142 so that the heat capacity of the gas flows are balanced to maintain the cold temperature of the gas in conduit 142 in the system to reduce the energy demand of the plant.

[0061] Heat exchangers 139, 153 are cooled by means of a heat pump described below, preferably using nitrogen as heat medium. Heated, low pressure nitrogen refrigerant, typically near ambient temperature and at a pressure conveniently in the range 5 to 15 bara, is led from heat exchanger 153 to a first stage compressor 159. Compressed gas from compressor 159 is led to a second stage compressor in conduit 160, via cooler 161. Further compressed gas from compressor 159 ^' is led via cooler 16Γ to a final compressor stage 166, from which it is returned to heat exchanger 153 in conduit 165, via cooler 157. Nitrogen pressure in conduit 165 may be in the range 45 to 65 bara. The temperature, after cooling in cooler 157, may be about 5 to 15°C above the temperature of the available cooling medium, such as in the range from for example 15 to 40°C.

[0062] Compressors 159, 159 ^' are driven by a separate drive such as a diesel engine or a gas turbine. Compressor 166 is driven by turbo expander 169, via shaft 167. People skilled in the art will know that the sequence of compressors 159, 159 ^' and 166 may be different from what is shown in Figure 1. As an example, compressor 166 may be at the low pressure end, as a low pressure stage, instead of, as shown, a high pressure stage.

[0063] Compressed and cooled gas from conduit 165 is further cooled in heat exchanger 153, conveniently to a temperature close to the temperature of the pre- cooled gas in conduit 132, such as for example -80 to -90°C. The nitrogen is then led in conduit 156 to turbo expander 169, where the pressure is reduced to between 5 and 15 bara, such as about 10 bara. This reduces the temperature to a few degrees below the temperature in conduit 147, such as about 3°C lower which will typically be about -163°C.

[0064] Nitrogen from turbo expander 169 is led to heat exchanger 139 in conduit 170. In heat exchanger 139 the nitrogen is heated by heat exchange with gas to be liquefied and / or sub-cooled. The thus heated nitrogen is then led to heat exchanger 153 in conduit 155. In heat exchanger 153, the nitrogen is further heated by heat exchange with high pressure nitrogen from conduit 165. The nitrogen then exits heat exchanger 153 in conduit 154, completing the nitrogen refrigerant loop.

Example

[0065] In order to liquefy pre-processed gas containing 1 mole% nitrogen, 90 mole% methane, 5 mole% ethane, 2 mole% propane, 1 mole% i-butane and 1 mole% n-butane, a process with the following design basis is employed:

Process parameter Parameter Comment

numerical value

Annual LNG production 0.1 MTPA Measured at plant outlet

Operational days per year 345

Table 1

Design basis for the example

[0066] Pre-processed gas, 12.40 tons per hour at 30°C and 50 bara is compressed to 85 bara in compressor 102. In the example, compressor 102 has two stages with intercooler. For clarity, only one stage is shown in Figure 1. The compressed gas is mixed with 48.97 tons per hour gas from compressor 105. The total gas flow, 61.37 tons per hour, is cooled to 30°C in cooler 1 10. The cooled gas, conduit 1 1 1 , flows to heat exchanger 134 after side draw of 29.72 tons gas per hour in conduit 126.

Downstream of heat exchanger 135, the gas has been cooled to -38.4°C. After side draw of 18.10 tons gas per hour in conduit 1 17, the remaining gas, 13.55 tons per hour, is further cooled to -86.8°C in heat exchanger 133. This pre-cooled gas flows to process section 2 in conduit 132.

[0067] The pressure of the side draw gas in conduit 126 is reduced to 82.9 bara in valve 121 and the mixed with 1.35 tons per hour gas from conduit 136. This total flow, 31.07 tons per hour, is expanded to 23 bara in turbo expander 1 15. This reduces the gas temperature to -41.7°C, and turbo expander 1 15 produces a shaft power of 927 kW. The cooled gas from expander 1 15 is mixed with gas from heat exchanger 133, conduit 129, and used as cooling medium in heat exchanger 135.

[0068] Side draw gas in conduit 1 17 is expanded to 23.5 bara in turbo expander 1 18. This reduces the temperature from -38.4°C to -89.7°C. Turbo expander 1 18 produces a shaft power of 275 kW. Cold gas from turbo expander 1 18, conduit 120, is used as cooling medium in heat exchanger 133.

[0069] Used cooling medium from exchangers 135 and 133, conduit 122 at 22.5 bara, is compressed in compressor 107 after side draw of 0.2 tons per hour in conduit 1 19. Compression ratio in compressor 107 is determined by shaft powers 1 13 and 1 14 from expanders 1 15 and 1 18 respectively. The total power, 1203 kW, gives compression ratio about 1.69, and the outlet pressure from compressor 107, conduit 108, is about 38 bara. This gas is cooled to 30°C in cooler 109, and further compressed to 85 bara in compressor 105. In the example, compressor 105 has two stages with intercooler. For clarity, only one stage is shown in Figure 1.

[0070] Pre-cooled gas in conduit 132, 13.54 tons per hour, is split into two streams. A smaller stream, 1.35 tons per hour, is led to heat exchanger 153 as cooling medium. This gas flow enters heat exchanger 153 at the same temperature as the temperature in conduit 132, or -86.8°C, and exits the heat exchanger in conduit 136 at 27°C. A larger stream, 12.20 tons per hour, is led to heat exchanger 139 where it is cooled from -86.8°C to -159.4°C by counter current heat exchange with nitrogen refrigerant. The sub-cooled gas at liquefied temperature exits the heat exchanger in conduit 147. The pressure is reduced from 82.9 bara to 4.0 bara in hydraulic turbine 148. After mixing with 0.2 tons per hour gas from conduit 140, which was cooled to - 89.6°C in heat exchanger 141 , the pressure is further reduced to 1.2 bara in valve143. Separation in separator 144 gives 0.3 tons per hour gas and the final product LNG, 12.1 tons per hour or about 0.1 million tons per annum (MTPA) when the annual production time is 345 days.

[0071] Nitrogen cooling medium, 36.2 tons per hour at 9.4 bara and -162.2°C, enters heat exchanger 139 from conduit 170. In heat exchanger 139 it is heated to - 89.8°C, and then flows in conduit 155 to heat exchanger 153 where it is further heated to 25.7°C. The resulting flow exits heat exchanger 153 in conduit 154, is compressed in compressors 159, 159 ^' and 166 to 62.0 bara, cooled to 30°C in cooler 57, and returned to heat exchanger 53 for pre-cooling. After pre-cooling to -83.7° C, the nitrogen is led in conduit 156 to expander 169, where the pressure is reduced to 9.4 bara and the temperature is reduced to -162.2°C. The gas exits expander 169 in conduit 170, completing the nitrogen refrigeration loop. [0072] Composite cooling curves, or overall temperature versus duty for cold and hot side in heat exchangers 135, 133, 153 and 139 are shown in Figures 2, 3, 4 and 5, respectively.

[0073] Overall energy balance is shown in Table 2.

Table 2

Energy balance for the example

[0074] Total production rate 12.1 tons LNG per hour and total power 3903.6 kW gives specific power requirement of 323 kWh/ton LNG. This is a very large reduction from conventional dual expander nitrogen systems, known to require between 420 and 450 kWh/ton LNG. Furthermore, hydrocarbon refrigerants are eliminated, greatly improving the safety of the system relative to liquefaction based on mixed hydrocarbon refrigerants. The need for make-up refrigerants are eliminated, since nitrogen refrigerant is easily made on site. The simple operation and quick start-up of conventional nitrogen expander systems is preserved. The system therefore provides an extremely good alternative for SSLNG liquefaction processes.

[0075] The pre-cooling system shown in Figure 1 , process section 1 , essentially comprises two pre-cooling stages. The first stage is comprised of turbo expander 1 15 and heat exchanger 135. The second stage is comprised of turbo expander 1 18 and heat exchanger 133. People skilled in the art will know that elimination of one of these stages, for example the stage comprising turbo expander 1 18 and heat exchanger 133, is possible. This gives a simplified pre-cooling system with reduced pre-cooling capability. Flow from process part 1 , pre-cooling, to process part 2, liquefaction / sub-cooling, would then be via conduit 134 instead of conduit 132.

[0076] People skilled in the art will also understand that instead of mixing fluids from conduit 1 16 into conduit 129, constraining the pressures in these conduits, it would be possible to route conduit 1 16 to a separate pass in heat exchanger 135, and rearranging compressors 107 and 105 to receive the two cold streams now exiting from heat exchanger 135.