Patent Grant
9151537
The present invention relates to a method for optimal production of LNG.
As used herein, the term LNG shall refer to Liquefied Natural Gas, that is Natural gas that has been cooled down such that it condenses and becomes liquid.
As used herein, the term Natural Gas shall refer to a gaseous mixture of hydrocarbons where an essential part is methane.
As used herein, the term LPG shall refer to Liquid Petroleum Gas, that is a gaseous mixture of hydrocarbons comprising propane and butanes.
As used herein, the term “mixed refrigerant cycle” shall refer to a liquification process, known in the art, employing an optimized mixture of a plurality of refrigerants.
As used herein, the term “gas expansion process” or “gas expansion cycle” shall refer to a liquefaction process, known in the art, employing a gaseous cooling agent, wherein the gaseous cooling agent at a higher pressure is first fed to a heat exchanger system and cooled but such that the cooled cooling agent is a gas. Thereafter the cooled cooling agent is expanded in a gas expander to a lower pressure lower than said higher pressure to generate a cold gaseous cooling agent stream. The cold gaseous cooling agent is led back to the heat exchanger system where it cools down the cooling agent stream at the higher pressure and for heat exchange with the fluid that is to be cooled, such as a gas to be liquefied, and such that the said cold gaseous cooling agent becomes a heated cooling agent. The heated cooling agent is thereafter compressed for reuse.
As used herein, the term “warm composite curve”, as known in the art, shall refer to the heat flow versus temperature relationship for the sum of warm streams being cooled down in a heat exchanger or a system of heat exchangers.
As used herein, the term “cold composite curve”, as known in the art, shall refer to the heat flow versus temperature relationship for the sum of cold streams being heated in a heat exchanger or a system of heat exchangers.
A used herein, the term “warm end” of a liquefaction heat exchanger, as known in the art, shall refer to the area or range of the heat exchanger where the warmest streams involved in the heat exchange is entering or leaving the heat exchanger.
As used herein, the term “split gas expansion cycles” shall refer to a gas expansion cycle wherein the cooled refrigerant is split into a plurality of streams, the streams being utilized at different stages and at different temperatures in the cooling of the target fluid.
As used herein, the term “fractionation column” shall refer to an arrangement, known in the art, for distillation separation of a mixed hydrocarbon fluid, in particular a column that generates an overhead fraction and bottom fraction
It is known in the art to produce LNG from a feed gas comprising a mixture of hydrocarbons, wherein the feed gas first passes through a fractionation column and an overhead fraction subjected to the liquefaction process, for example the system disclosed in EP 1715267. Such systems are employed in large scale, so-called “base load” liquefaction systems. Such systems typically employ a mixed refrigerant cycle, due to the superior efficiency of the mixed refrigerant cycle compared to the gas expansion cycle. Because the mixed refrigerant mixture is optimized, the overhead fraction must be cooled by an external source prior to being fed into the liquefaction circuit. As it is the intention of such systems to achieve an LNG product with as high a relative content of methane as possible, these systems are further arranged such that the bottom fraction from the fractionation column comprises a relatively high content of hydrocarbons heavier than methane.
The simplest way to limit the content of heavier hydrocarbons in the liquid gas is to partially condense the gas and then separate the condensed liquid from the gas, which is further cooled to be liquefied. The separation is normally carried out as an integrated part of the cooling down process at typical temperatures of between 0° C. and −60° C. Separated condensate can be heated up again as a part of the cooling process to utilise the cooling potential.
In large land based LNG installations (so called “base load” installations) most of the propane and heavier hydrocarbons are normally removed and in many cases also a considerable part of ethane, before or as a part of, the liquefaction. This is done to meet the sale specifications and to be able to produce and sell the valuable ethane, LPG and condensate/naphtha. Elaborate processes are normally used with low temperature fractionation columns both as a part of the cooling down process and as separate units outside the cooling system.
Because of the complexity of large, “Base load” systems, the arrangements used therein are not suitable for many applications, for example offshore applications. In addition, it is undesirable to handle products other than the LNG, as hydrocarbons lighter than C5 can, on the whole, not be stored or transported safely without being cooled down or under pressure.
In such offshore applications it is known to utilize the gas expansion cycle for the liquefaction of natural gas. The gas expansion cycle is relatively simple, but is less efficient than the mixed refrigerant cycle. While the use of the “split gas expansion cycle” can improve efficiency there is nonetheless a need for greater efficiency, as even relatively small changes in efficiency can result in very large economic gains.
It is therefore an object of the invention to provide a more efficient liquification system employing the gas expansion cycle. It is also an object of the invention to provide a system in which the LNG product is enriched in ethane, propane, butane, and to a lesser degree pentane.
According to one aspect of the invention is provided a method comprising a fractionation column for feeding in of a feed gas, a heat exchanger system for cooling down and partially condensing the overhead gas stream of the fractionation column, a separator to separate the two-phase stream from the heat exchanger system and an appliance for return of fluid from the separator to the fractionation column and feeding this fluid to the upper part of the column as reflux, and an appliance to feed the gas from the separator back to the heat exchanger system for further cooling down and liquefaction to LNG. The invention comprises a closed gas expansion process to liquefy the natural gas, wherein the gas is first fed through a fractionation column where the gas is cooled and separated into an overhead fraction with reduced content of hexane (C6) and heavier components, and a bottom fraction enriched with the heavier hydrocarbons (C6+), furthermore, in that the fractionation column reflux is generated as an integrated part of the system for liquefaction in that the overhead gas is partially condensed. By carrying out the liquefaction in accordance with the invention, production of liquid gas with maximum content of ethane, propane and butane (C2-C4) is achieved at the same time as the efficiency of the gas expansion process is increased and the by-production of unstable/volatile fluid with a high content of methane, ethane, LPG (propane+butane) is minimised.
In particular, the invention comprises a method and a system for liquefaction of natural gas or other hydrocarbon gas from a gas field or from a gas/oil field, where it is appropriate to liquefy the gas to make it possible to transport the gas from the source to the market. This is particularly relevant for oil/gas fields at sea.
The aim of the invention is to render liquefaction of gas energy efficient at the same time as the process is kept simple so that the equipment can be used offshore. In particular the invention is useful on floating installations since the by-production of condensate during the liquefaction is minimised and the efficiency is maximised (the need for fuel gas is minimised).
The method according to the invention is characterised by the following steps:
1) that the feed gas is led through a fractionation column (150) where it is cooled and separated into an overhead fraction with reduced content of C6 hydrocarbons and heavier components, and a bottom fraction enriched with heavier hydrocarbons,
2) that the overhead fraction from the fractionation column is fed into a heat exchanger system (110) and is subjected to a partial condensing to form a two-phase fluid, and the two-phase fluid is separated in a suitable separator (160) to a liquid (5) rich in LPG and pentane (C3-C5) which is re-circulated as cold reflux to the fractionation column (150), while the gas (6) containing lower amounts of C5 hydrocarbon and hydrocarbons heavier than C5, is led off for further treatment in the heat exchanger system (110) for liquefaction to LNG with maximum content of ethane and LPG, and
3) that the cooling circuit for liquefaction of gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step.
The system according to the invention is characterised in that the cooling system which is used for cooling down, condensing and liquefaction of the gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step. The system is preferably designed and configured to separate the feed gas so that the LNG product from the system will be enriched with most of the butane (C4) and hydrocarbons with a lower normal boiling point than butane, and the bottom product of the fractionation column will be enriched with most of C6 and components with a normal boiling point higher than C6.
The present invention represents a considerable optimisation for application offshore, and especially on a floating unit, in that a relatively simple and robust gas expansion process is used for liquefaction of natural gas, and in that the energy efficiency of this process is increased at the same time as the amount of liquid gas is maximised by maximising the content of ethane and LPG, at the same time as the amount of hydrocarbons heavier than methane which is separated out as bi-products in the liquefaction process is minimised.
An installation which comprises the system according to the invention can thereby simply be adapted and be installed, for example, on board floating offshore installations where space is often a limiting factor.
The invention will now be described in more detail with reference to the enclosed figures in which:
a shows a conventional, prior art, split flow closed gas expansion cooling cycle for pre-cooling, condensation and sub cooling of natural gas
b shows an example of a hot temperature curve and a cold temperature curve (composite curve) for a conventional closed split-flow gas expansion circuit as shown in
a shows a split flow closed gas expansion cooling cycle for pre-cooling, condensation and sub cooling of natural gas using the invention
b shows an example of a hot temperature curve and a cold temperature curve (composite curve) for a closed gas expansion circuit obtained by using the present invention.
With reference to
Incoming and cleaned feed-gas 1, for example, a methane rich hydrocarbon gas, is first fed to a fractionation column 150, where the gas is cooled down when it meets a colder reflux fluid. During the cooling down and counter current contact with the colder fluid, the feed gas is separated into an overhead fraction 2 with a reduced content of the hydrocarbons that have a molecular weight higher than pentane (C5), and a bottom fraction 3 enriched with C6 and hydrocarbons that have a higher molecular weight than C6. The overhead fraction 2 of the fractionation column is then led to the heat exchanger system 110, where the gas is cooled down and partially condensed so that the resulting two-phase fluid 4 can be separated in a suitable separator 160. A fluid 5 rich in LPG and pentanes (C3-C5), which is separated in the separator 160, is re-circulated as cold reflux to the fractionation column 160. As this fluid is generated by condensation by cooling down, the reflux fluid 5 will have a lower temperature than the feed gas 1. The gas 6 from the separator 160 has now further reduced its content of C5 hydrocarbons and hydrocarbons higher than C5. This gas is then led back to the heat exchanger system 110 for further cooling down, condensation and subcooling. The liquid gas 11 is alternatively led through a control valve 140 that controls the operating pressure and flow through the system.
In a preferred embodiment the gas feed stream 1 is cooled down in advance by a suitable external cooling agent such as available air, water, seawater or a separate suitable cooling installation/pre-cooling system. For the latter external cooling method, a separate closed, mechanical cooling system with propane, ammonia or other appropriate cooling means is often used.
In a preferred embodiment the fractionation column 150 and the separator 160 are operated at pressures and temperatures that lead to the complete system (the fractionation column 150 and reflux separator 160) generating a component split/separation point in the normal boiling point area (NBP) between −12° C. and 60 C. This can, for example, correspond to the light key component for the separation being butane (C4) with a normal boiling point between −12° C. and 0° C., and the heavy key component being a C6 component with a boiling point between 50° C. and 70° C. The overhead gas stream 6 of the system will then be enriched with most of the butane (C4) and hydrocarbons with a lower normal boiling point than butane. The bottom product 3 from the fractionation column will be enriched with most of C6 and components with a normal boiling point higher than C6, while pentane (C5, NBP=28−36° C.) is a transitional component which is distributed in the gas product of the system and the bottom product from the fractionation column.
Cooling down and condensing of the feed gas in the heat exchanger system 110 is provided by a closed or open gas expansion process. The cooling process starts in that a gaseous cooling agent 21 encompassing a gas or a mixture of gases (such as pure nitrogen, methane, a hydrocarbon mixture, or a mixture of nitrogen and hydrocarbons), at a higher pressure, preferably between 3 and 10 MPa, is fed to the heat exchanger system 110 and cooled to a temperature between 0° C. and −120° C., but such that the cooling agent stream is mainly a gas at the prevailing pressure and temperature 31. The pre-cooled gaseous cooling agent 31 is then led into a gas expander 121 where the gas is expanded to a lower pressure between 5%-40% of the inlet pressure, but preferably to between 10% and 30% of the inlet pressure, and such that the cooling agent mainly is in the gas phase. The gas expander is normally an expansion turbine, also called turboexpander, but other types of expansion equipment for gas can be used, such as a valve. The flow of pre-cooled gaseous cooling agent is expanded in the gas expander 121 at a high isentropic efficiency, such that the temperature drops considerably. In certain embodiments of the invention, some liquid can be separated out in this expansion, but this is not necessary for the process. The cold stream of cooling agent 32 is then led back to the heat exchangers 110 where it is used for cooling down and possibly condensing of the other incoming hot cooling agent streams and the gas that shall be cooled down is condensed and subcooled.
After the streams 32 of cold cooling agent have been heated in the heat exchanger system 110, the cooling agent will exist as the gas stream 51, which in a closed loop embodiment is recompressed in an appropriate way for reuse and is cooled with an external cooling agent, such as air, water, seawater or an appropriate cooling unit.
Alternatively, the cooling system in an open embodiment will use a cooling agent 21 consisting of a gas or a mixture of gases at a higher pressure produced by an appropriate source, for example, from the feed gas that is to be treated and cooled down. Furthermore, the open embodiment will encompass a low pressure cooling agent flow 51 used for other purposes or, in an appropriate way, be recompressed to be mixed with the feed gas that is to be treated and cooled down.
In a preferred embodiment, the returning cooling agent stream 51 is led from the heat exchanger 110 to a separate compressor 101 driven by the expansion turbine 121. In this way, the expansion work is utilised, and the energy efficiency of the process is improved. After the compressor 101, the cooling agent is cooled further in a heat exchanger 131, before the stream is further compressed in the circulation compressors 100. The circulation compressors 100 can be one or more units, possibly one or more steps per unit. The circulation compressor can also be equipped with intermediate cooling 132 between the compressor steps. The compressed cooling agent 20 is then cooled by heat exchange in an aftercooler 130 with the help of an appropriate external cooling medium, such as air, water, seawater or a suitable separate cooling circuit, to be reused as a compressed cooling medium 21 in a closed loop.
In a preferred embodiment, the system of heat exchangers 110 is a heat exchanger which comprises many different “hot” and “cold” streams in the same unit (a so-called multi-stream heat exchanger).
The same goes for the heating of the returned cold cooling agent streams 32, 42. The embodiment is otherwise in accordance with
It is emphasised that the embodiment of the invention is not limited to the cooling processes described above only, but can be used with any gas expansion cooling process for liquefaction of natural gas or other hydrocarbon gas, where the cooling down is mainly achieved by using one or more expanding gas streams.
By carrying out the liquefaction of the natural gas in accordance with the invention, a product of liquid gas is produced which has a maximum content of methane, ethane and LPG, but which, at the same time, does not contain more than the permitted level of pentanes (C5) and heavier hydrocarbons with a normal boiling point above 50-60° C. At the same time, the content of volatile methane, ethane, propane and butane in the by-produced liquid (condensate/NGL) is considerable minimised or eliminated, At the same time more liquid natural gas will also be produced with lower energy consumption than for corresponding cooling circuits configured without the fractionation column which receives cold reflux enriched with C3-C5 from the cooling down process.
In addition to optimising the split between light components, which are wanted in the LNG product, and heavy hydrocarbons, which are wanted in the condensate by product, the invention significantly reduces the energy (gas compression power) required for liquefaction, when a gas expansion cooling cycle is used.
The main reason for the performance improvement when using gas expansion cooling is related to the fact that gas expansion cycles are characterised by relatively linear heat flow vs. temperature relations in the heat exchanger system (100). The exception is an area/range when significant hydrocarbon condensation (liquefaction) occurs but this is limited to a section of the entire cooling range. Due to the linear heat vs temperature relation, the performance of such cooling processes is normally limited by temperature pinch points. Most optimised gas expansion cycles have one pinch point in the warm end and one pinch point in the cold end, and in addition normally one or more temperature pinches in the hydrocarbon condensation area, as shown in
For the energy consumption required for cooling, liquefaction and subcooling a methane rich hydrocarbon gas, particularly the warm end pinch is an important limitation for reduced compression power, as it sets the lower limit for refrigerant gas mass flow. This can be seen in
The difference between a conventional gas expander process and the gas expander process according to the invention shall now be explained closer with reference to
a shows a conventional dual (split flow) closed gas expansion process for cooling, condensing and sub cooling an incoming gas stream (1). Heavy hydrocarbons are first removed conventionally by precooling the incoming feed gas (1) to an intermediate temperature 2-phase stream (4) in a heat exchanger system (100) by means of the dual gas expansion cooling system, separation of said stream in a separator (160), leading the overhead gas (6) from the separator back to the heat exchanger system for further cooling, condensation and sub cooling, and leading the heavy liquid stream (3) out of the system. With reference to
a shows a dual (split flow) closed gas expansion process according to the invention, for cooling, condensing and sub cooling an incoming gas stream (1). Heavy hydrocarbons are first removed from an incoming gas stream (1) in the column (150) by counter current contact with a cold reflux liquid (5). This contacting separates C6+ hydrocarbons and reduces the gas temperature of the overhead gas stream (2). The overhead gas stream (2) can therefore be introduced in the heat exchanger system (100) at a lower temperature than without the column. The overhead gas stream is precooled to an intermediate temperature 2-phase stream (4) in the heat exchanger system by means of the dual gas expansion cooling system, separation of said stream in a separator (160), leading the overhead gas (6) from the separator back to the heat exchanger system for further cooling, condensation and sub cooling, and leading the heavy liquid stream (5) back to the column as cold reflux. With reference to
While reducing feed gas (2) temperature by the use of external pre-cooling (as in base load systems) may also effect pinch point, the effect is negligible in such systems since external pre-cooling will require additional refrigeration work, since it is assumed that all ambient cooling possible is already used. With the present invention a surprising increase in efficiency is realized as this additional cooling work is achieved integral with the process as the cooling work is provided by the cold reflux liquid (5) exchanging heat in counter current contact with the feed gas in the column (150). No external refrigeration work is needed to achieve a temperature lower than the heat exchanger (100) warm end temperature,
An additional effect achieved with the present invention is that the heavier hydrocarbons, which are preferably separated out to prevent freezing during the liquefaction, will be condensed and be separated out at considerably higher temperatures than in conventional methods, in that much of the condensing takes place in the fractionation column and not in the heat exchanger at a lower temperature. This reduces the required cooling work at that said lower temperature, hence reduced energy loss in the cooling process in that a cooling duty is moved to a higher temperature range.
Preliminary analyses and comparisons show that necessary compressor work per kg liquid natural gas which is produced can be reduced by 5-15% for a gas expansion circuit carried out in accordance with the invention compared to conventional methods.
The example below shows natural gas with 90.4% methane by volume which is to be liquefied, where the invention is used to maximise the amount of liquid gas and at the same time minimise the by-production of unstable hydrocarbon liquid with a high content of ethane, propane and butane. The stream data refer to
The examples below shows example of the percentage of feed gas pr. component in some of the key streams with the present invention, for different methane content in feed gas.
This application claims the benefit under 35 USC §119(e) of U.S. provisional application 61/138,793 filed 19 Dec. 2008.
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| Number | Date | Country | |
|---|---|---|---|
| 20100154470 A1 | Jun 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61138973 | Dec 2008 | US |
