US20160076808A1

US20160076808A1 - Method and system for treating and liquefying natural gas

Info

Publication number: US20160076808A1
Application number: US14/853,010
Authority: US
Inventors: W. Brent Mealey; Steven Shi
Original assignee: Propak Systems Ltd
Current assignee: Propak Systems Ltd
Priority date: 2014-09-15
Filing date: 2015-09-14
Publication date: 2016-03-17
Also published as: CA2904486A1

Abstract

A method for liquefying natural gas comprises: condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and separating the partially condensed stream into a first liquid fraction and a first vapour fraction; wherein the first liquid fraction comprises liquefied natural gas.

Description

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 52/050,447, filed Sep. 15, 2014, entitled “Method and System for Treating and Liquefying Natural Gas,” the entirety of which is hereby incorporated by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates natural gas. More specifically, the present invention relates to methods and systems for liquefying natural gas.

BACKGROUND

Small-scale liquid natural gas (LNG) plants are known and are often referred to as micro-LNG plants or peak shaving plants. Large scale LNG plants provide a means to transport large quantities of natural gas internationally and/or overseas, where pipelines are not available or practical. In contrast, micro-LNG plants are typically used to liquefy natural gas for the purpose of storing the natural gas or for the purpose of transporting it on and over shorter distances. Micro-LNG plants for natural gas storage, typically called peak shaving plants, liquefy natural gas and then store it in storage tanks during periods of low demand. Micro-LNG plants can also be used to liquefy stranded natural gas from the source, where no pipeline exists, so that it can then be transported in a liquid state by a tanker truck. The tanker truck can then deliver the LNG to another location. Furthermore, micro-LNG plants can be used to liquefy natural gas so that it can be stored in a vehicle fuel tank and used as a transportation fuel.
The industry demands, particularly for micro-LNG plants, high energy efficiency (low specific energy) relative to capital cost, ease of operability, and safety. The energy efficiency for methods of liquefying natural gas can be measured as the specific energy required to liquefy the natural gas to produce one metric ton of LNG (i.e., kW.d/ton) which can be compared relative to its capital cost. Some known methods of liquefying natural gas have a high specific energy requirement in the range of 25 to 40 kW.d/ton, generally due to high operating costs associated with a high refrigeration energy load, making the natural gas liquefaction process less economical. Other known methods achieve a relatively low specific energy requirement in the range of from 12.2 to 20.4 kW.d/ton, however have inherently higher capital costs, and/or have complex operational and safety disadvantages. In addition, known methods on not have the capability to integrally remove nitrogen components, meaning that this component must be removed in separate processing units adding cost to the LNG plant and thereby reducing its economic efficiency.
There is a need for methods and systems for liquefying natural gas that improve the efficiency of the liquefaction process.
There is also a need for methods and systems for integrally removing heavy hydrocarbon and/or nitrogen components from a natural gas feed stream.
There is also a need for methods and systems for improving the energy efficiency of refrigeration processes associated with liquefying natural gas.

SUMMARY

According to an aspect, there is provided a method for liquefying natural gas, the method comprising:

- condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and
- separating the partially condensed stream into a first liquid fraction and a first vapour fraction;
- wherein the first liquid fraction comprises liquefied natural gas.

In an aspect, condensing at least a portion of the natural gas feed stream comprises compressing and cooling the natural gas feed stream.
In an aspect, condensing the natural gas feed stream comprises partially flashing and cooling the partially condensed stream.
In an aspect, separating the partially condensed stream into the first liquid fraction and the first vapour fraction comprises directing the partially condensed stream into a first flash separator to concentrate nitrogen in the first vapour fraction and reduce the concentration of nitrogen in the first liquid fraction.
In an aspect, the method further comprises:

- condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction, and
- separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction;
- wherein the second liquid fraction comprises liquefied natural gas.

In an aspect, condensing at least a portion of the first vapour fraction comprises heating, compressing, and cooling the first vapour fraction.
In an aspect, condensing at least a portion of the first vapour fraction comprises partially flashing and cooling the partially condensed first vapour fraction.
In an aspect, separating the partially condensed first vapour fraction into the second liquid fraction and the second vapour fraction comprises directing the partially condensed first vapour fraction into a second flash separator to concentrate nitrogen in the second vapour fraction and reduce the concentration of nitrogen in the second liquid fraction.
In an aspect, the method further comprises compressing, cooling, and partially condensing the second vapour fraction to produce a partially condensed second vapour fraction, directing the partially condensed second vapour fraction into a third flash separator to concentrate nitrogen in a third vapour fraction and reduce the concentration of nitrogen in a third liquid fraction.
In an aspect, the method comprises repeating the method until a desired level of separation of nitrogen from the liquefied natural gas is achieved.
In an aspect, the method comprises repeating the method until the volume of vapour fraction is reduced to a desired level.
In an aspect, the method further comprises condensing heavy hydrocarbon components in the natural gas feed stream and separating the natural gas feed stream into a heavy hydrocarbon liquid fraction and a residual vapour fraction, wherein at least a portion of the residual vapour fraction is condensed and directed to the partially condensed stream.
In an aspect, the method further comprises partially flashing the heavy hydrocarbon liquid fraction and distilling heavy hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a residual fraction, wherein the residual fraction is directed to the natural gas feed stream for further processing.
In an aspect, the method further comprises directing at least a portion of the first vapour fraction to one or more refrigerant cycles for use as a refrigerant medium.
In an aspect, the one or more refrigerant cycles comprises a first methane refrigerant cycle that provides a portion of the duty required to cool and condense at least one of:

- the natural gas feed stream into the partially condensed stream;
- the first vapour fraction into the partially condensed first vapour fraction,
- the second vapour fraction into the partially condensed second vapour fraction; and
- the heavy hydrocarbon components in the natural gas feed stream.

In an aspect, the first methane refrigerant cycle comprises the following steps:

- compressing at least a portion of the first vapour fraction, which is a low pressure methane refrigerant vapour stream, to produce an outlet stream;
- compressing and pre-cooling the outlet stream to produces a pre-cooled high pressure methane refrigerant stream;
- expanding the pre-cooled high pressure methane refrigerant stream to produce expanded and cooled methane refrigerant stream;
- heating the expanded and cooled methane refrigerant stream to produce the low pressure methane refrigerant vapour stream, thus completing the first methane refrigerant cycle.

In an aspect, the one or more refrigerant cycles comprises a second methane refrigerant cycle that provides a portion of the duty required to pre-cool the first vapour fraction for use in the first methane refrigerant cycle.
In an aspect, the second methane refrigerant cycle further provides a portion of the duty required to cool and condense at least one of:

- the natural gas feed stream into the partially condensed stream;
- the first our fraction into the partially condensed first vapour fraction;
- the second vapour fraction into the partially condensed second vapour fraction; and
- the heavy hydrocarbon components in the natural gas feed stream.

In an aspect, the second methane refrigerant cycle comprises the following steps:

- compressing at least a portion of the first vapour fraction, which is a low pressure pre-cooling methane refrigerant vapour stream, to produce an outlet stream;
- compressing the outlet stream to produce a pre-cooled methane refrigerant stream;
- expanding the pre-pooled methane refrigerant stream to produce expanded and cooled pre-cooling methane refrigerant stream;

heating the expanded and cooled pre-cooling methane refrigerant stream to produce the low pressure pre-cooling methane refrigerant vapour stream, thus completing the second methane refrigerant cycle.
In an aspect, the second methane refrigerant cycle is used in place of propane refrigeration cycle.
In an aspect, at least a portion of the duty required to cool at least one of:

- the first vapour fraction for use in the first methane refrigerant cycle;
- the natural gas feed stream into the partially condensed stream;
- the first vapour fraction into the partially condensed first vapour fraction;
- the second vapour fraction into the partially condensed second vapour fraction; and
- the heavy hydrocarbon components in the natural gas feed stream is provided by a two-stage closed loop propane refrigeration cycle.

In an aspect, the propane refrigeration cycle comprises a high pressure cycle and a low pressure cycle, linked through a region of intermediate pressure.
In an aspect, the propane refrigeration cycle comprises the following steps:

- reducing the pressure on a high pressure saturated liquid phase propane refrigerant stream to produce an intermediate pressure stream;
- separating the intermediate pressure stream into an intermediate pressure propane refrigerant liquid fraction and an intermediate pressure propane vapour fraction;
- heating a first portion of the intermediate pressure propane refrigerant liquid fraction to produce a first vapour liquid stream;
- combining the first vapour-liquid stream with the intermediate pressure stream;
- reducing the pressure on a second portion of the intermediate pressure propane refrigerant liquid fraction to produce a low pressure stream;
- separating the low pressure stream into a low pressure propane refrigerant liquid fraction and a low pressure propane refrigerant vapour fraction;
- heating the low pressure propane refrigerant liquid fraction to produce a second vapour liquid stream;
- combining the second vapour-liquid stream with the low pressure stream;
- compressing the low pressure propane refrigerant vapour fraction to produce a stream that is combined with the intermediate pressure propane refrigerant vapour fraction to produce a combined intermediate pressure propane refrigerant vapour fraction;
- compressing the combined intermediate pressure propane refrigerant vapour fraction to produce a high pressure propane refrigerant vapour stream; and
- condensing the high pressure propane refrigerant vapour stream to produce the high pressure saturated liquid phase propane refrigerant stream, thus completing the propane refrigeration cycle.

According to another aspect, there is provided a method for producing a refrigerant medium from a natural gas feed stream for use in cooling the natural as feed stream, the method comprising:

- condensing at least a portion of the natural gas feed stream to produce a partially condensed stream;
- separating the partially condensed stream into a first liquid fraction and a first vapour fraction; and
- directing at least a portion of the first vapour fraction to a refrigerant cycle for use as a refrigerant medium for cooling the natural gas.

According to another aspect, there is provided a method for removing nitrogen from a nature; gas feed stream, The method comprising:

- partially condensing at /east a portion of a natural gas feed stream to produce a partially condensed stream; and
- separating the partially condensed stream into a first liquid fraction and a first vapour fraction in a flash separator to concentrate nitrogen in the first vapour fraction and reduce the concentration of nitrogen in the first liquid fraction.

In an aspect, the method further comprises:

- partially condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction; and
- separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction in a flash separator to concentrate nitrogen in the second vapour fraction and reduce the concentration of nitrogen in the second liquid fraction.

In an aspect, the method further comprises:

- partially condensing at least a portion of the second vapour fraction to produce a partially condensed second vapour fraction; and
- separating the partially condensed second vapour fraction into a third liquid fraction and a third vapour fraction in a flash separator to concentrate nitrogen in the third vapour fraction and reduce the concentration of nitrogen in the third liquid fraction.

According to another aspect, there is provided a system for liquefying natural gas, the system comprising:

- a first condensing system for condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and
- a first flash separator for separating the partially condensed stream into a first liquid fraction and a first vapour fraction;
- wherein the first liquid fraction comprises liquefied natural gas.

In an aspect, the first condensing system comprises at least one of an inlet gas compressor, an inlet gas compressor after-cooler, an indirect heat exchanger, and a valve.
In an aspect, the first flash separator concentrates nitrogen in the first vapour fraction and reduces the concentration of nitrogen in the first liquid fraction.
In an aspect, the system further comprises:

- a second condensing system for condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction; and
- a second flash separator for separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction;
- wherein the second liquid fraction comprises liquefied natural gas.

In an aspect, the second condensing system comprises at east one of an indirect heat exchanger, a flash gas compressor, a flash gas compressor after-cooler, and a valve.
In an aspect, the second flash separator concentrates nitrogen in the second vapour fraction and reduces the concentration of nitrogen in the second liquid fraction.
In an aspect, the system further comprises a third condensing system and a third flash separator for concentrating nitrogen in a third vapour fraction and reducing the concentration of nitrogen in a third liquid fraction.

- In an aspect, the system further comprises:
- a heavy hydrocarbon condensing system for condensing heavy hydrocarbon components in the natural gas feed stream; and
- a flash separator for separating the natural as feed stream into a heavy hydrocarbon liquid fraction and a residual vapour fraction and directing at least a portion of the residual vapour fraction to the partially condensed stream.

In an aspect, the heavy hydrocarbon condensing system comprises at least one of an inlet gas compressor, an inlet gas compressor after-cooler, and an indirect heat exchanger.
In an aspect, the system further comprises a heavy hydrocarbon distiller for distilling heavy hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a residual fraction, and directing the residual fraction to the natural gas feed stream for further processing.
In an aspect, the system further comprises a valve for directing at least a portion of the first vapour fraction to one or more refrigerant cycles for use as a refrigerant medium.
In an aspect, the one or more refrigerant cycles comprises a first methane refrigerant cycle that provides a portion of the duty required to cool and condense at least one of:

- the natural gas feed stream into the partially condensed stream;
- the first vapour fraction into the partially condensed first vapour fraction;
- the second vapour fraction into the partially condensed second vapour fraction; and
- the heavy hydrocarbon components in the natural gas feed stream,
- wherein the first methane refrigerant, cycle comprises at least one of an expander brake compressor, a first heat exchanger, a multi-stage methane refrigerant compressor, a second heat exchanger, an indirect heat exchanger, a methane refrigerant expander, and an expander brake compressor.

- the natural gas feed stream into the partially condensed stream;
- the first vapour fraction into the partially condensed first vapour fraction;
- the second vapour fraction into the partially condensed second vapour fraction; and
- the heavy hydrocarbon components in the. natural gas feed stream,
- wherein the second methane refrigerant cycle comprises at least one of a pre-cooling expander brake compressor, a first heat exchanger, a multi-stage pre-cooling methane refrigerant compressor, a second heat exchanger, a methane expander, and an indirect heat exchanger.

In an aspect, the second methane refrigerant cycle is used in place of a propane refrigeration cycle.
In an aspect, at least a portion of the duty required to cool at least one of:

- the first vapour fraction for use in the first methane refrigerant cycle;
- the natural as feed stream into the partially condensed stream;
- the first vapour fraction into the partially condensed first vapour fraction;
- the second vapour fraction into the partially condensed second vapour fraction; and
- the heavy hydrocarbon components in the natural gas feed stream is provided by a two-stage closed loop propane refrigeration cycle.

In an aspect, the propane refrigeration cycle comprises a high pressure cycle and a low pressure cycle, linked through a region of intermediate pressure.
In an aspect, the propane refrigeration cycle comprises at least one of a first valve, an intermediate pressure propane refrigerant drum, an indirect heat exchanger, a second valve, a low pressure propane refrigerant drum, a low pressure compressor, a high pressure compressor, and a heat exchanger.
According to another aspect, there is provided a system for producing a refrigerant medium from a natural pas feed stream for use in cooling the natural gas feed stream, the system comprising:

- a first condensing system for condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and
- a first flash separator for separating the partially condensed stream into a first liquid fraction and a first vapour fraction and for directing at least a portion of the first vapour fraction to a refrigerant cycle for use as a refrigerant medium for cooling the natural gas.

According to another aspect, there is provided a system for removing nitrogen from a natural gas feed stream, the system comprising:

- a first condensing system for condensing at least as portion of a natural gas feed stream to produce a partially condensed stream; and
- a first flash separator for separating the partially condensed stream into a first liquid fraction and a first vapour fraction and for concentrating the nitrogen in the first vapour fraction and reducing the concentration of nitrogen in the first liquid fraction.

In an aspect, the system further comprises:

- a second condensing system for condensing at least a portion of first vapour fraction to produce a partially condensed first vapour fraction; and
- a second flash separator for separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction and for concentrating the nitrogen in the second vapour fraction and reducing the concentration of nitrogen in the second liquid fraction.

According to another aspect, there is provided a system for liquefying natural gas, wherein the system comprises a flash separator for integrally removing nitrogen from the natural gas.
According to another aspect, there is provided a system for liquefying natural gas, wherein the system comprises a heavy hydrocarbon separator for integrally removing heavy hydrocarbon components from the natural gas.
According to another aspect, there is provided a system for liquefying natural gas, wherein the system comprises at least one methane refrigerant cycle, derived at least in part from a vapour fraction of the natural gas feed stream.
In an aspect, the system further comprises a second methane refrigerant cycle, derived at least in part from a vapour fraction of the natural gas feed stream.
In an aspect, the system does not comprise a propane refrigerant cycle.
In an aspect, the system has a specific energy requirement of about 14.5 kW.d/ton or less.
In an aspect, the system has a specific energy requirement of about 12.5 kW.d/ton or less.
According to another aspect, there is provided a methane refrigerant system for use in liquefying natural gas, the system comprising a methane gas source derived at least in part from a vapour fraction of the natural gas feed stream.
According to another aspect, there is provided a micro-LNG plant comprising the system described herein.
According to another aspect, there is provided a peak shaving plant comprising the system of described herein.
Other features and advantages of the present invention will become apparent from the following detailed description, it should be understood, however, that the detailed description and the specific examples, while indicating aspects of the invention, are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will now be described, by way of example only, with reference to the attached figures, in which:

FIG. 1 shows a schematic diagram of a system for treating and liquefying natural gas utilizing two flash stages with integral nitrogen removal capability;

FIG. 2 shows a schematic diagram of a system for treating and liquefying natural gas utilizing three flash stages with integral nitrogen removal capability;

FIG. 3 shows a schematic diagram of an additional subsystem for treating and liquefying natural gas that incorporates an integral system for removing heavier hydrocarbons from a natural gas stream for use in combination with the system of FIG. 1 or 2;

FIG. 4 shows a schematic diagram for a closed loop methane refrigeration cycle for use in combination with the system of FIG. 1, 2, or 3;

FIG. 5 shows a schematic diagram for a pre-cooling dosed loop two-stage propane refrigeration cycle for use in combination with the system of FIG. 1, 2, 3, or 4 and

FIG. 6 shows a schematic diagram for a pre-cooling dosed loop methane refrigeration cycle for use in combination with the system of FIG. 1, 2, 3, or 4 as an alternate to the system of FIG. 5.

DETAILED DESCRIPTION

Natural gas supply in North America and worldwide has become increasingly abundant, due to new shale gas discoveries and associated recovery technologies. Due to a widening of the price spread between crude oil and natural gas, fuel usage in these areas is expected to switch from crude oil-derived fuels (gasoline & diesel) to natural gas, thus increasing its demand. it is desirable to convert the natural as to LNG, in order to make it more transportable. Where natural gas pipelines do not exist, LNG provides a means by which the natural gas fuel can reach natural gas demands in remote areas. LNG can be used to supply more economical natural gas fuel to remote towns, industries, mines, etc. Furthermore, LNG can be stored and utilized as a fuel onboard vehicles that are used in industrial, mining, and transportation sectors; as a substitute to crude of based fuels or compressed natural gas (CNG).
Using LNG as a natural gas fuel, in lieu to CNG, is advantageous, since LNG is 1.7 times more dense than CNG. Thus, more LNG can be stored on a vehicle relative to CNG. In addition, LNG can be loaded in the vehicle's fuel tank faster than CNG, from a practical stand point. Another advantage is LNG fuel engine technology that achieves comparable engine specific powers close to their diesel counterparts, which are 40% greater than CNG.
Described herein are methods and systems that relate to various aspects of liquefying natural gas, cooling natural gas, removing various components from a natural gas feed stream, and to the storage and transportation of natural gas in the form of liquefied natural gas (LNG); as well as the usage of natural gas as LNG as a transportable fuel. Natural gas will convert to a liquid state when coded to cryogenic temperatures. The method and system described herein has the ability to cryogenically cod and convert gaseous natural gas to LNG, and finds particular use in smaller scale “micro-LNG plants” with a typical capacity in the range of from about 20 to about 500 metric ton/D (about 7,300 to about 182,500 metric ton/annual).
A process and system to liquefy natural gas is described herein, in particular for use in smaller scale micro-LNG plants, with a capacity typically in the range of 20 to 500 metric ton/D. in aspects, the process and system described herein realize improved efficiency of natural gas liquefaction in relation to associated capital cost and energy consumption, provide a design that can effectively be packaged as a modular skid mounted micro-LNG plant, improved operability, and/or improvements to the operation safety. Further improvements, in aspects, include the ability to integrally remove heavy hydrocarbon components (HHC; some HHC can solidify in the cryogenic cooling process), and integrally remove a portion of the nitrogen component (in order to meet the LNG product specification limits for nitrogen content). Further, the process in aspects can produce its own refrigerant medium and the cooling duty is typically provided by two closed loop refrigeration cycles.
As mentioned above, many conventional micro-LNG plants are inefficient both in terms of energy consumption and economically. Using the method and system described herein, as competitive capital cost plant can be designed, having a specific energy requirement of about 14.5 kW.d/ton or less (subject to inlet gas and environment conditions). in another aspect, the method and system described herein can be designed to have a specific energy requirement of about 12.5 kW.d/ton or less (subject to inlet gas and environment conditions), in aspects, the method and system described herein improves the operability and safety, and/or energy efficiency over comparable prior art. As will be understood, the balance between a lower capital cost plant and higher energy efficiency will be different for each particular application. However, it will also be understood that the economics of the method and system described herein are particularly advantageous in the event nitrogen removal is required, since the nitrogen removal does not require expensive additional equipment.
In an aspect, the system described herein, which incorporates a main refrigerant medium as a single gaseous phase, inherently has less hydrocarbon liquid inventory within the plant than prior art mixed refrigerant type process technologies, which have comparable high energy efficiency. Subsequently, the system described herein is both safer and provides enhanced operability, than prior art mixed refrigerant type process technologies.
The present design also provides a means to self-generate the main refrigerant medium, used to cool and condense the natural gas into LNG, from the natural gas supply. This advantageously does not require the transporting, handling, and storage of an externally sourced refrigerant medium. This reduces the capital cost of the LNG plant, as well as improves its operability and safety.
In another aspect, the system described herein provides a means to self generate the main refrigerant as well as the pre-cooling refrigerant medium; used for pre-cooling as well as for use to cool and condense the natural gas into LNG. This further enhances its operability and safety. This aspect would be comparable in operability and safety to an open area nitrogen cycle process technology, however the system described herein would have an improvement in energy efficiency to the nitrogen cycle process.
Additionally, the system and method described herein is modular, which is advantageous because it greatly reduces the field construction and overall cost of the facility.
The process and system described herein finds use in, for example, peak shaving plants, such as in the case when natural gas is being transported into a utility system via a pipeline. When there is lower demand for the natural gas in the utility system. the gas could be converted to LNG and then stored in a storage tank. Then, during peak natural gas demand in the utility system, the LNG is converted back to gaseous natural gas state, to meet a demand that can be in excess of the pipeline capacity.
The process and system described herein also finds use in capturing stranded natural gas from the source. One example of a stranded natural gas source is associated natural gas produced with oil production. If a natural gas pipeline does not exist at the source where the oil is produced, typically the associated gas is disposed of by means of flaring. The method and system described herein provides a means to liquefy the natural gas and produce an LNG product. that could then be transported in a liquid state to a market. Liquefaction of stranded natural gas is typically in the capacity range of micro-LNG plants, since larger quantities of natural gas should warrant this installation of a natural gas pipeline.
The process and system described herein finds particular use in micro-LNG plants, to produce LNG for industrial, mining, and transportation fuel usage. Applications for liquefaction of natural gas, to produce LNG as a transportable fuel, are typically in the capacity range of micro-LNG plants, since there is a practical limit to the maximum distance that LNG can be transported over land.
As will be explained, the method and system described herein involve integration of flash separation and an open loop refrigeration stream using the flash vapour with a base refrigeration load method (a closed loop refrigeration method). The combination of this effectively helps achieve the required heating/cooling curves, provides part of the refrigeration duty, and partially separates the nitrogen component out of the LNG product.
Furthermore, in the process and system described herein, the methane refrigeration cycle (loop) is typically the main refrigeration load used for LNG liquefaction and is derived from the inlet gas, in contrast, conventional processes and systems use closed loop refrigeration utilizing ethane, ethylene, nitrogen, or a mixed refrigerant.
In a typical aspect, the process and system described herein for cooling and liquefying natural gas to produce LNG comprises a main methane refrigeration cycle, with a supplemental propane refrigeration cycle, and includes the integration of flash separation and an open loop refrigeration using the flash vapour. This complete arrangement provides, in aspects, a cost effective, energy efficient process solution that has other features as noted, such as operability, safety, integral nitrogen removal, etc.
“Natural gas” is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, The actual composition of any given natural gas feed stream varies depending upon its source. For example, the gas source may be a natural gas well, a natural gas gathering system or pipeline transmission system, or flare gas, A natural gas feed stream may include other hydrocarbons and gases in various concentrations, including hydrogen, helium, carbon dioxide, and nitrogen. Furthermore, the natural gas feed stream may or may not contain contaminants such as hydrogen sulfide or mercury. The natural gas feed stream may or may not contain water.
Before natural gas can be used as a fuel, it typically undergoes processing to clean the gas and remove impurities, including water, to meet the specifications of marketable natural gas and to protect equipment in the LNG plant, For example, water is removed from the feed natural gas feed stream in order to reduce hydrate formation and freezing in the LNG plant, and in order to meet product specifications. Carbon dioxide is removed from the natural gas feed stream in order to reduce solid formation and free it in the plant, and also in order to meet product specifications.
Additionally, processing of the natural gas, upstream of an LNG plant, may include removal of heavier hydrocarbon components, such as, for example, ethane, propane, butane, pentane, and higher molecular weight hydrocarbons. Essentially all pentane and heavier hydrocarbons are generally required to be removed from the gas prior to entering the LNG plant in order to meet the LNG product specifications as well as to prevent solidification of cyclo-hexane components in the natural gas. The required extent of removal of ethane, propane, and butane will depend on the LNG product specifications as well as the LNG heating value specification depending on its usage and fuel specifications. Thus, the by-products of natural gas processing may include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulfide, carbon dioxide, water vapour, helium, and nitrogen.
“Liquefied natural gas (LNG)” is produced for the purpose of storage and transportation of natural gas, because LNG has a volume that is nearly 600 times smaller than that of natural gas. LNG is becoming the preferred state for storage as fuel on a transport vehicle, as opposed to compressed natural gas (CNG), since a greater amount of fuel can be stored on board the vehicle. Liquefied natural gas is one end-product of the method described herein.
The insulation in storage containers, as efficient as it is, will not keep LNG cold enough by itself. Inevitably, heat leakage will warm and vaporise the LNG. Industry practice is to store LNG as a boiling cryogen. That is, the liquid is stored at its boiling point for the pressure at which it is stored (atmospheric pressure). As the vapour boils off, heat for the phase change cools the remaining liquid. Because the insulation is very efficient, only a relatively small amount of boil off is necessary to maintain temperature. This phenomenon is also called auto-refrigeration.
The term “cryogenic” relates to low temperatures. A “cryogenic as” is a gas that has been cooled to a liquid state below about 150 Kelvin.
“Heavy hydrocarbons (HHC)” are hydrocarbons that include propane and any hydrocarbons heavier than propane, such as cyclo-hexane.
Terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.
When introducing elements disclosed herein, the articles “a”, “an”, “the”, and “said” are intended to mean that there may be one or more of the elements.
In understanding the scope of the present application, the term “comprising” and its derivatives, as used herein, are intended to he open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers end/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. It will be understood that any aspects described as “comprising” certain components may also “consist of” or “consist essentially of” wherein “consisting of” has a closed-ended or restrictive meaning and “consisting essentially of” means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1% by weight of non-specified components.
It will be understood that any component or method step defined herein as being included may be explicitly excluded from the claimed invention by way of proviso or negative limitation.
A liquefied natural gas (LNG) plant 10 is shown schematically in FIGS. 1 through 3. Although specific temperatures and pressures will be referred to below for exemplary purposes, it will be understood that the operating temperatures and pressures at various locations within the process will depend upon the natural gas feed steam composition, the plant inlet and outlet conditions (including temperature and pressure), and whether or not nitrogen and/or heavy hydrocarbon removal is required. Optimizing the temperatures and pressures described below to meet the requirements of any particular system requirement is within the ability of a skilled person.
Referring to FIG. 1, an inlet natural has feed stream 12 enters the LNG plant 10, The inlet natural gas feed stream 12 has been pre-treated and processed upstream of the LNG plant 10 to remove water, carbon dioxide and heavier hydrocarbons and typically has a composition similar to that shown in Table 1.

TABLE 1

Example feed stream composition.

	Component	Mole %

	Nitrogen	1.50
	Carbon dioxide	<50 ppm
	Methane	92.10
	Ethane	5.15
	Propane	1.25
	Iso-butane	Trace
	n-Butane	Trace
	Iso-pentane	Trace
	n-Pentane	Trace
	Hexane +	<50 ppm
	Water	<0.01 ppm

The inlet natural gas feed stream 12 is fed into an inlet gas compressor 14 and an inlet gas compressor after-cooler 16, producing feed stream 18. Inlet gas compressor 14 compresses the inlet natural gas feed stream 12 followed by after-cooling the compressed inlet gas in the inlet gas compressor after-cooler 16, to a suitable pressure for the liquefaction process to take place to produce feed stream 18. The gaseous state natural gas feed stream 18 has, for example, a pressure of 7240 kPa absolute and a temperature of 43.3° C.
The feed stream 18 is fed into section 20 of an indirect heat exchanger 22, where the feed stream 18 is cooled to −131.7° C., thereby condensing at least a portion of feed stream 18 to produce partially condensed stream 24.
Partially condensed stream 24 is passed through valve 26 and pressure is reduced. The drop in pressure across valve 26 causes partially condensed stream 2$ to partially flash and cool further due to the Joule-Thompson (JT) effect, producing stream 28. Stream 28 has a pressure of 152 kPa absolute and a temperature of −156.3° C. and contains both liquid and gas phases.
Stream 28 is fed into a first flash separator 30, which separates stream 28 into a first liquid fraction 32 and a first, vapour fraction 3$. The first liquid fraction 32 is fed out of the LNG plant 10 and exits the LNG plant 10 as the first portion of the LNG product. The nitrogen components in the gas tend to concentrate in the first vapour fraction 34, thus lowering the nitrogen content in the first liquid fraction 32 and the resulting LNG. product
A first portion of the first vapour fraction 34 is directed to valve 115, where it is directed to stream 116 in FIG. 4 (described below), A second portion of the first vapour fraction 34 is directed to valve 191, where it is directed to stream 192 in FIG. 6 (described below). A third portion of the first vapour fraction 34 is directed to section 36 the indirect heat exchanger 22, where it is heated to 36.1° C. to produce heated first vapour fraction 38. The heated first vapour fraction 38 is compressed to a pressure of 2137 kPa absolute by flash gas compressor 40 to produce stream 42, which is then cooled by flash gas compressor after-cooler 44, thereby producing compressed first vapour fraction 46.
Compressed first vapour fraction 46, at a pressure of 2117 kPa absolute and a temperature of 38° C., is fed into section 48 of the indirect heat exchanger 22. This cools compressed first vapour fraction 46 to a temperature of −142.6° C., thereby condensing at least a portion of compressed first vapour fraction 46 to produce partially condensed first vapour fraction 50.
Partially condensed first vapour fraction 50 is passed through valve 52 and pressure is reduced. The drop in pressure across valve 52 causes partially condensed first vapour fraction 50 to partially flash and cool further due to the Joule-Thompson (JT) effect, producing stream 54. Stream 54 has a pressure of 152 kPa absolute and a temperature of −161.2° C. and contains both liquid and gas phases.
Stream 54 is fed into a second, flash separator 56, which separates stream 54 into a second liquid fraction 58 and a second vapour fraction 60. The second liquid fraction 58 is fed out of the LNG plant 10 and exits the LNG plant 10 as the second portion of the LNG product. The nitrogen components in the gas tend to concentrate in the second vapour fraction 60, thus lowering the nitrogen content in the second liquid fraction 58 and the resulting LNG product. Second vapour fraction 60 is heated in exchanger 62 and is then used as fuel gas 64 for utility consumption.
Turning now to FIG. 2, an additional LNG flash separation stage is incorporated into the LNG plant 10 in order to increase separation of the nitrogen from the LNG product and thereby decrease the amount of methane gas in the separated nitrogen product stream. As well, this method improves the energy efficiency by means of additional indirect heat exchange in section 66 of the indirect heat exchanger 22, between he flash as and cooling and condensing inlet gas, thus reducing the refrigerant duty. This provides a second flash separation step that reduces the methane component loss to stream 94 (as an alternate to stream 64), as will be described below, thus increasing the liquefied natural gas (LNG) product yield.
In this additional LNG flash separation stage, instead of being directed to exchanger 62 for use as fuel as 64, the second vapour fraction 60 is directed to section 66 of the indirect heat exchanger 22. In section 66 of the indirect heat exchanger 22, the second vapour fraction 60 is heated from −162.5° C. to 36.1° C. to produce a heated second vapour fraction 68. The heated second vapour fraction 68 is compressed to a pressure of 2068 kPa absolute by flash gas compressor 70 to produce stream 72, which a then cooled by flash gas compressor after-cooler 74, thereby producing compressed second vapour fraction 76.
Compressed second vapour fraction 76, at a pressure of 2034 kPa absolute and a temperature of 38° C., is fed into section 78 of the indirect heat exchanger 22. This cools compressed second vapour fraction 76 to a temperature of −156.7° C., thereby condensing at least a portion of compressed second vapour fraction 76 to produce partially condensed second vapour fraction 80.
Partially condensed second vapour fraction 80 is passed through valve 82 and pressure is reduced. The drop in pressure across valve 82 causes partially condensed second vapour fraction 80 to partially flash and cool further due to the, Joule-Thompson (ST) effect, producing stream 84. Stream 84 has a pressure of 138 kPa absolute and a temperature of −179.0° C. and contains both liquid and gas phases.
Stream 84 is fed into a third flash separator 86, which separates stream 84 into a third liquid fraction 88 and a third vapour fraction 90. The third liquid fraction 88 is fed out of the LNG plant 10 and exits the LNG plant 10 as the third portion of the LNG product. The nitrogen components in the gas tend to concentrate in the third vapour fraction 90, thus lowering the nitrogen content in the third liquid fraction 88 and the resulting LNG product. Third vapour fraction 80 is heated in exchanger 92 and is then used as fuel gas 94 for utility consumption.
Turning now to FIG. 3. if the natural gas feed stream 12 contains propane and heavier hydrocarbons, including cyclohexanes, removal of these hydrocarbons may be required to either meet INC product specifications or to mitigate their solidification during; the cryogenic LNG process. In this case, the natural gas feed stream 12 enters the LNG plant 10 ands fed into the inlet gas compressor 14 and the inlet gas compressor after-cooler 16, producing feed stream 18, which is at a pressure of 7240 kPa absolute and a temperature of 43.3° C.
The feed stream 18 is fed into section 96 of the indirect heat exchanger 22, where the feed stream 18 is cooled to −31° C., thereby condensing the propane and heavier hydrocarbons in the feed stream 18 to produce partially condensed stream 98.
Partially condensed stream 98 is fed into heavy hydrocarbon (HHC) separator 100, which separates partially condensed stream 98 into a heavy hydrocarbon liquid fraction 101 and a residual vapour fraction 104. The heavy hydrocarbon liquid fraction 101 is dropped in pressure across valve 102 to produce a partially flashed heavy hydrocarbon liquid fraction 103. The partially flashed heavy hydrocarbon liquid fraction 103 is then fed to a heavy hydrocarbon distiller 106, which distils heavy hydrocarbons 108 from the partially flashed heavy hydrocarbon liquid fraction 103 and recycles the residual stream 110 to the natural gas teed stream 12 for further processing.
Residual vapour fraction 104 is fed into section 112 of the indirect heat exchanger 22, where the residual vapour fraction 104 is cooled to −131.7° C., thereby condensing at least a portion of vapour fraction 104 to produce partially condensed stream 24. Partially condensed stream 24 is then processed as described above.
Turning now to FIG. 4, a methane closed loop refrigeration cycle 114 is shown. The closed loop methane refrigeration cycle 114 provides a portion of the duty required to cool and condense the natural gas feed stream 18 in sections 20 (or 96 and 112 in the case of FIG. 3) and the compressed first vapour fraction 46 in section 48 of the indirect heat exchanger 22 (and the compressed second vapour fraction 76 in section 78 of the indirect heat exchanger 22, in the case of FIG. 2).
A source of low pressure methane refrigerant vapour stream 116, at 36.1° C. and 641 kPa absolute, is compressed by an expander brake compressor 118, producing discharge stream 120. The discharge stream 120 has a temperature of 94° C., and a pressure of 1189 kPa absolute and is after-cooled in heat exchanger 122 to produce outlet stream 124.
Outlet stream 124 has a temperature of 43° C. and is fed into a multi-stage methane refrigerant compressor 126, producing stream 128. Stream 128 is after-cooled heat exchanger 130, producing stream 132. Stream 132 has a pressure of 5516 kPa absolute and a temperature of 43° C. Stream 132 is fed into section 134 of the indirect heat exchanger 22, where it is pre-cooled and exits as pre-cooled high pressure methane refrigerant stream 136.
Pre-cooled high pressure methane refrigerant stream 136 has a temperature of −34.4° C. and a pressure of 5500 kPa absolute and is expanded to a discharge pressure of 655 kPa absolute in a methane refrigerant expander 138. Methane expander 138 extracts work, which is used to mechanically drives the expander brake compressor 118. This lowers the enthalpy of the pre-cooled high pressure methane refrigerant stream 136, so that it exits the methane expander 138 as expanded and cooled methane refrigerant stream 140, having a temperature of −133° C.
Expanded and cooled methane refrigerant stream 140 is fed into section 142 of the indirect heat exchanger 22, where it provides cooling duty to the system and is therefore heated from −133° C. to 36.1° C. Expanded and cooed methane refrigerant stream 140 exits section 142 of the indirect heat exchanger 22 as low pressure methane refrigerant vapour stream 116, thereby completing the methane refrigeration cycle 114.
Turning now to FIG. 5, a two-stage closed-loop propane refrigeration cycle 144 shown. The propane refrigeration cycle 144 provides a portion of the pre-cooling duty required to cool and condense the natural gas feed stream 18 in sections 20 (or 96 and 112 in the case of FIG. 3) and the compressed first vapour fraction 46 in section 48 of the indirect heat exchanger 22 (and the compressed second vapour fraction 76 in section 78 of the indirect heat exchanger 22, in the case of FIG. 2), and also pre-cools the pre-cooled high pressure methane refrigerant stream 136 in section 134 of the indirect heat exchanger 22.
The propane refrigeration cycle 144 contains discrete regions; a high pressure HP cycle and low pressure LP cycle, indicated by the dashed lines. The high pressure HP cycle and low pressure LP cycle are linked through a region of intermediate pressure.
A high pressure saturated liquid phase propane refrigerant stream 146 at a temperature of 43° C. and a pressure of 1500 kPa absolute is passed through valve 148 and pressure is reduced. The drop in pressure across valve 148 causes the high pressure saturated liquid phase propane refrigerant stream 146 to partially flash and cool further due to the Joule-Thompson (JT) effect, producing intermediate pressure stream 150. Intermediate pressure stream 150 has a pressure of 386 kPa absolute and a temperature of −6.8° C. and contains both liquid and gas phases. intermediate pressure stream 150 is fed into intermediate pressure propane refrigerant drum 162, which separates intermediate pressure stream 150 into an intermediate pressure propane refrigerant liquid fraction 154 and an intermediate pressure propane refrigerant vapour fraction 156.
A first portion 158 of the intermediate pressure propane refrigerant liquid fraction 154 is thermo-syphoned into section 160 of the indirect heat exchanger 22, where it is heated and vaporized at a temperature of −6.8° C. to produce vapour-liquid stream 162. Vapour-liquid stream 162 is fed back into intermediate pressure propane refrigerant drum 152, where it combines with intermediate pressure stream 150.
A second portion 164 of the intermediate pressure propane refrigerant fraction 154 is passed through valve 166 and pressure is reduced. The drop in pressure across valve 166 causes the second portion 164 of the intermediate pressure propane refrigerant liquid fraction 154 to partially flash and cool further due to the Joule-Thompson (JT) effect, producing low pressure stream 168. Low pressure stream 168 has a pressure of 107 kPa absolute and a temperature of −40° C. and contains both liquid and gas phases. Low pressure stream 168 is fed into a low pressure propane refrigerant drum 170, which separates low pressure stream 168 into a low pressure propane refrigerant liquid fraction 72 and a low pressure propane refrigerant vapour fraction 174.
The low pressure propane refrigerant liquid fraction 172 is thermo-syphoned into section 176 of the indirect heat exchanger 22, where it is heated and vaporized at a temperature of −40° C. to produce vapour-liquid stream 178, Vapour liquid stream 178 is fed back into low pressure propane refrigerant drum 170, where it combines with low pressure stream 168.
Low pressure propane refrigerant vapour fraction 174 is compressed to a pressure of 386 kPa absolute in a low pressure compressor 180 and exits as stream 182. Stream 182 combines with intermediate propane refrigerant vapour fraction 156 resulting in a combined intermediate pressure propane refrigerant vapour fraction 157.
The combined intermediate pressure propane refrigerant vapour fraction 157, having a pressure of 386 kPa absolute, is then fed into a high pressure compressor 184 where it is compressed to high pressure propane refrigerant vapour stream 186, which has a pressure of 1530 kPa absolute. High pressure propane refrigerant vapour stream 186 is then fed into heat exchanger 188, where it is cooled and condensed to produce the high pressure saturated liquid phase propane refrigerant stream 146, which has a temperature of 43° C. and a pressure of 1500 kPa absolute, thus completing the two-stage closed-loop propane refrigeration cycle 114.
Turning now to FIG. 6, an alternate pre-cooling methane refrigeration cycle 190 is shown. The pre-cooling closed loop methane refrigeration cycle 190 is an alternative to the pre-cooling propane refrigeration cycle 144 that is suitable for use in areas where fire hazard risks are a particular concern, such as in off-shore applications. For example, the pre-cooling methane refrigeration cycle 190 finds use on an off-shore platform, a floating production storage and offloading (FPSO) vessel, or an LNG transport ship. Fire hazards risks are reduced through use of the pre cooling methane refrigeration cycle 190 in place of the propane refrigeration cycle 144, as the pre-cooling methane refrigeration cycle 190 uses refrigerants that do not require liquid inventory within the LNG plant 10.
Like the pre-cooling propane refrigeration cycle 144, the alternate pre-cooling closed loop methane refrigeration cycle provides a portion of the pre-cooling duty required to cool and condense the natural gas feed stream 18 in sections 20 (or 96 and 112 in the case of FIG. 3) and the compressed first vapour traction 46 in section 48 of the indirect heat exchanger 22 (and the compressed second vapour fraction 76 in section 78 of the indirect heat exchanger 22, in the case of FIG. 2), and also pre-cools the pre-cooled high pressure methane refrigerant stream 136 in section 134 of the indirect heat exchanger 22.
A source of low pressure pre-cooling methane refrigerant vapour stream 192, at 21° C. and 600 kPa absolute, is compressed by a pre-cooling expander brake compressor 194, producing discharge stream 196. The discharge stream 196 has a temperature of 107.4° C. and a pressure of 1543 kPa absolute and is after-cooled in heat exchanger 198 to produce outlet stream 200.
Outlet stream 200 has a temperature of 43° C. and is fed into a multi-stage pre-coding methane refrigerant compressor 202, producing stream 204. Stream 204 is after-cooled in heat exchanger 206, producing stream 208. Stream 208 has a pressure of 4482 kPa absolute and a temperature of 43° C.
Stream 208 is expanded to a discharge pressure of 620 kPa absolute in a pre cooling methane expander 210, Methane expander 210 extracts work, which is used to mechanically drives there-cooling expander brake compressor 194. This lowers the enthalpy of stream 208, so that it exits the pre-cooling methane expander 210 as expanded and cooled pre-cooling methane refrigerant stream 212, having a temperature of −70° C.
Expanded and cooled pre-cooling methane refrigerant stream 212 is fed into section 214 of the indirect heat exchanger 22, where it provides cooling duty to the system and is therefore heated from −70° C. to 21° C. Expanded and cooled pre-cooling methane refrigerant stream 212 exits section 214 of the indirect heat exchanger 22 as low pressure pre-cooling methane refrigerant vapour stream 192, thereby completing the pre-cooling methane refrigeration cycle 190.
The refrigerant medium for the closed loop methane refrigeration cycle 114 (shown in FIG. 4) and the pre-cooling closed loop methane refrigeration cycle 190 (shown in FIG. 6), is sourced from the first vapour fraction 34. As required for make-up, a portion of the first vapour fraction 34 is directed via valves 115 and 191 to stream 116 (shown in FIG. 4) and stream 192 (shown in FIG. 6), respectively.
The above disclosure generally describes the present invention. Changes in form and substitution of equivalents are contemplated as circumstances may suggest or render expedient. Although specific terms have been employed herein, such terms are intended in a descriptive sense and not for purposes of limitation.
All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety.
Although exemplary aspects of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims

We claim:

1. A method for liquefying natural gas, the method comprising:

condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and

separating the partially condensed stream into a first liquid fraction and a first vapour fraction;

wherein the first liquid fraction comprises liquefied natural gas.

2. The method of claim 1, wherein separating the partially condensed stream into the first liquid fraction and the first vapour fraction comprises directing the partially condensed stream into a first flash separator to concentrate nitrogen in the first vapour fraction and reduce the concentration of nitrogen in the first liquid fraction.

3. The method of claim 1, further comprising:

condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction; and

separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction;

wherein the second liquid fraction comprises liquefied natural gas.

4. The method of claim 3, wherein condensing at least a portion of the first vapour fraction comprises heating the first vapour fraction in a first indirect heat exchanger with the natural gas feed stream, thereby providing a portion of the cooling and condensing duty and reducing the refrigeration duty, compressing, after-cooling in a second indirect heat exchanger, and then cooling and partially condensing the first vapour fraction in the first indirect heat exchanger.

5. The method of claim 3, wherein separating the partially condensed first vapour fraction into the second liquid fraction and the second vapour fraction comprises directing the partially condensed first vapour fraction into a second flash separator to concentrate nitrogen in the second vapour fraction and reduce the concentration of nitrogen in the second liquid fraction, thereby reducing methane component loss to the fuel gas product stream and increasing the liquefied natural gas (LNG) product yield.

6. The method of claim 5, further comprising compressing, after-cooling in a separate exchanger, and then cooling and partially condensing the second vapour fraction to produce a partially condensed second vapour fraction, directing the partially condensed second vapour fraction into a third flash separator to concentrate nitrogen in a third vapour fraction and reduce the concentration of nitrogen in a third liquid fraction, thereby further reducing methane component loss to the fuel gas product stream and further increasing the liquefied natural gas (LNG) product yield.

7. The method of claim 1, further comprising condensing heavy hydrocarbon components in the natural gas feed stream and separating the natural gas feed stream into a heavy hydrocarbon liquid fraction and a residual vapour fraction, wherein at least a portion of the residual vapour fraction is condensed and directed to the partially condensed stream.

8. The method of claim 7, further comprising partially flashing the heavy hydrocarbon liquid fraction and distilling heavy hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a residual fraction, wherein the residual fraction is directed to the natural gas feed stream for further processing.

9. The method of claim 1, further comprising directing at least a portion of the first vapour fraction to one or more refrigerant cycles for use as a refrigerant medium.

10. The method of claim 9, wherein the one or more refrigerant cycles comprises a first methane refrigerant cycle that provides a portion of the duty required to cool and condense at least one of:

the natural gas feed stream into the partially condensed stream;

the first vapour fraction into the partially condensed first vapour fraction;

the second vapour fraction into the partially condensed second vapour fraction; and

the heavy hydrocarbon components in the natural gas feed stream; and/or to pre-cool the methane refrigerant stream.

11. The method of claim 10, wherein the first methane refrigerant cycle comprises the following steps:

compressing at least a portion of the first vapour fraction, which is a low pressure methane refrigerant vapour stream, to produce an outlet stream;

compressing and pre-cooling the outlet stream to produce a pre-cooled high pressure methane refrigerant stream;

expanding the pre-cooled high pressure methane refrigerant stream to produce expanded and cooled methane refrigerant stream;

heating the expanded and cooled methane refrigerant stream to produce the low pressure methane refrigerant vapour stream, thus completing the first methane refrigerant cycle.

12. The method of claim 10, wherein the one or more refrigerant cycles comprises a second methane refrigerant cycle that provides a portion of the duty required to pre-cool the first vapour fraction for use in the first methane refrigerant cycle.

13. The method of claim 12, wherein the second methane refrigerant cycle further provides a portion of the duty required to cool and condense at least one of:

the natural gas feed stream into the partially condensed stream;

the first vapour fraction into the partially condensed first vapour fraction;

the heavy hydrocarbon components in the natural gas feed stream.

14. The method of claim 12, wherein the second methane refrigerant cycle comprises the following steps:

compressing at least a portion of the first vapour fraction, which is a low pressure pre-cooling methane refrigerant vapour stream, to produce an outlet stream;

compressing the outlet stream to produce a pre-cooled methane refrigerant stream;

expanding the pre-cooled methane refrigerant stream to produce expanded and cooled pre-cooling methane refrigerant stream;

heating the expanded and cooled pre--cooling methane refrigerant stream to produce the low pressure pre-cooling methane refrigerant vapour stream, thus completing the second methane refrigerant cycle.

15. The method of claim 12, wherein the second methane refrigerant cycle is used in place of a propane refrigeration cycle.

16. The method of claim 1, Wherein at least a portion of the duty required to cool at least one of:

the first vapour fraction for use in the first methane refrigerant cycle;

the natural gas feed stream into the partially condensed stream;

the first vapour fraction into the partially condensed first vapour fraction;

the heavy hydrocarbon components in the natural gas feed stream is provided by a two-stage closed loop propane refrigeration cycle.

17. The method of claim 16, wherein the propane refrigeration cycle comprises a high pressure cycle and a low pressure cycle, linked through a region of intermediate pressure.

18. The method of claim 17, wherein the propane refrigeration cycle comprises the following steps:

reducing the pressure on a high pressure saturated liquid phase propane refrigerant stream to produce an intermediate pressure stream;

separating the intermediate pressure stream into an intermediate pressure propane refrigerant liquid fraction and an intermediate pressure propane vapour fraction;

heating a first portion of the intermediate pressure propane refrigerant liquid fraction to produce a first vapour-liquid stream;

combining the first vapour-liquid stream with the intermediate pressure stream;

reducing the pressure on a second portion of the intermediate pressure propane refrigerant liquid fraction to produce a low pressure stream;

separating the low pressure stream into a low pressure propane refrigerant liquid fraction and a low pressure propane refrigerant vapour fraction;

heating the low pressure propane refrigerant liquid fraction to produce a second vapour-liquid stream;

combining the second vapour-liquid stream with the low pressure stream,

compressing the low pressure propane refrigerant vapour fraction to produce a stream that is combined with the intermediate pressure propane refrigerant vapour fraction to produce a combined intermediate pressure propane refrigerant vapour fraction;

compressing the combined intermediate pressure propane refrigerant vapour fraction to produce a high pressure propane refrigerant vapour stream; and

condensing the high pressure propane refrigerant vapour stream to produce the high pressure saturated liquid phase propane refrigerant stream, thus completing the propane refrigeration cycle.

19. A system for liquefying natural gas, The system comprising:

a first condensing system for condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and

a first flash separator for separating the partially condensed stream into a first liquid fraction and a first vapour fraction;

wherein the first liquid fraction comprises liquefied natural gas.

20. The system of claim 19, wherein the first flash separator concentrates nitrogen in the first vapour fraction and reduces the concentration of nitrogen in the first liquid fraction.

21. The system of claim 19, further comprising:

a second condensing system for condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction; and

a second flash separator for separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction;

wherein the second liquid fraction comprises liquefied natural gas.

22. The system of claim 21, wherein the second flash separator concentrates nitrogen in the second vapour fraction and reduces the concentration of nitrogen in the second liquid fraction.

23. The system of claim 22, further comprising a third condensing system and a third flash separator for concentrating nitrogen in a third vapour fraction and reducing the concentration of nitrogen in a third liquid fraction.

24. The system of claim 19, further comprising:

a heavy hydrocarbon condensing system for condensing heavy hydrocarbon components in the natural gas feed stream; and

a flash separator for separating the natural gas feed stream into a heavy hydrocarbon liquid fraction and a residual vapour fraction and directing at least a portion of the residual vapour fraction to the partially condensed stream.

25. The system of claim 24, wherein the heavy hydrocarbon condensing system comprises at least one of an inlet gas compressor, an inlet gas compressor after-cooler, and an indirect heat exchanger.

26. The system of claim 24, further comprising a heavy hydrocarbon distiller for distilling, heavy hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a residual fraction, and directing the residual fraction to the natural gas feed stream for further processing.

27. The system of claim 19, further comprising a valve for directing at least a portion of the first vapour fraction to one or more refrigerant cycles for use as a refrigerant medium.

28. The system of claim 27, wherein the one or more refrigerant cycles comprises a first methane refrigerant cycle that provides a portion of the duty required to cool and condense at least one of:

the natural gas feed stream into the partially condensed stream;

the first vapour fraction into the partially condensed first vapour fraction;

the heavy hydrocarbon components in the natural gas feed stream; and/or to pre-cool the methane refrigerant stream,

wherein the first methane refrigerant cycle comprises at least one of an expander brake compressor, a first heat exchanger, a multi-stage methane refrigerant compressor, a second heat exchanger, an indirect heat exchanger, a methane refrigerant expander, and an expander brake compressor.

29. The system of claim 27, wherein the one or more refrigerant cycles comprises a second methane refrigerant cycle that provides a portion of the duty required to pre-cool the first vapour fraction for use in the first methane refrigerant cycle.

30. The system of claim 29, wherein the second methane refrigerant cycle further provides a portion of the duty required to cool and condense at least one of:

the natural gas feed stream into the partially condensed stream;

the first vapour fraction into the partially condensed first vapour fraction;

the heavy hydrocarbon components in the natural gas feed stream,

wherein the second methane refrigerant cycle comprises at least one of a pre cooling expander brake compressor, a first heat exchanger, a multi-stage pre-cooling methane refrigerant compressor, a second heat exchanger, a methane expander, and an indirect heat exchanger.

31. The system of claim 27, wherein the second methane refrigerant cycle is used in place of a propane refrigeration cycle.

32. The system of claim 19, wherein at least a portion of the duty required to cool at least one of:

the first vapour fraction for use in the first methane refrigerant cycle;

the natural gas feed stream into the partially condensed stream;

the first vapour fraction into the partially condensed first vapour fraction;

33. The system of claim 32, wherein the propane refrigeration cycle comprises a high pressure cycle and a low pressure cycle, linked through a region of intermediate pressure.

34. A methane refrigerant system for use in liquefying natural gas, the system comprising a methane gas source derived at least in part from a vapour fraction of the natural gas feed stream.