AU2008283102B2 - Method and system for producing LNG - Google Patents

Method and system for producing LNG Download PDF

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AU2008283102B2
AU2008283102B2 AU2008283102A AU2008283102A AU2008283102B2 AU 2008283102 B2 AU2008283102 B2 AU 2008283102B2 AU 2008283102 A AU2008283102 A AU 2008283102A AU 2008283102 A AU2008283102 A AU 2008283102A AU 2008283102 B2 AU2008283102 B2 AU 2008283102B2
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gas
cooling
heat exchanger
fractionation column
stream
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AU2008283102A1 (en
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Inge Sverre Lund Nilsen
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Aragon AS
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Aragon AS
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0285—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings
    • F25J1/0288—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
    • B01D3/14—Fractional distillation or use of a fractionation or rectification column
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L3/00—Gaseous fuels; Natural gas; Synthetic natural gas obtained by processes not covered by subclass C10G, C10K; Liquefied petroleum gas
    • C10L3/06—Natural gas; Synthetic natural gas obtained by processes not covered by C10G, C10K3/02 or C10K3/04
    • C10L3/08—Production of synthetic natural gas
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    • F25J1/0057—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream after expansion of the liquid refrigerant stream with extraction of work
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
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    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
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    • F25J1/0228—Coupling of the liquefaction unit to other units or processes, so-called integrated processes
    • F25J1/0235—Heat exchange integration
    • F25J1/0237—Heat exchange integration integrating refrigeration provided for liquefaction and purification/treatment of the gas to be liquefied, e.g. heavy hydrocarbon removal from natural gas
    • F25J1/0238—Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0228—Coupling of the liquefaction unit to other units or processes, so-called integrated processes
    • F25J1/0235—Heat exchange integration
    • F25J1/0237—Heat exchange integration integrating refrigeration provided for liquefaction and purification/treatment of the gas to be liquefied, e.g. heavy hydrocarbon removal from natural gas
    • F25J1/0239—Purification or treatment step being integrated between two refrigeration cycles of a refrigeration cascade, i.e. first cycle providing feed gas cooling and second cycle providing overhead gas cooling
    • F25J1/0241—Purification or treatment step being integrated between two refrigeration cycles of a refrigeration cascade, i.e. first cycle providing feed gas cooling and second cycle providing overhead gas cooling wherein the overhead cooling comprises providing reflux for a fractionation step
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0275—Construction and layout of liquefaction equipments, e.g. valves, machines adapted for special use of the liquefaction unit, e.g. portable or transportable devices
    • F25J1/0277—Offshore use, e.g. during shipping
    • F25J1/0278—Unit being stationary, e.g. on floating barge or fixed platform
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0294—Multiple compressor casings/strings in parallel, e.g. split arrangement
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/06—Splitting of the feed stream, e.g. for treating or cooling in different ways
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2220/00—Processes or apparatus involving steps for the removal of impurities
    • F25J2220/60—Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/64—Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/14—External refrigeration with work-producing gas expansion loop
    • F25J2270/16—External refrigeration with work-producing gas expansion loop with mutliple gas expansion loops of the same refrigerant
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/90—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration

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Abstract

A method is described for production of LNG from an incoming feed gas (1 ) on an onshore or offshore installation, and it is characterised by the following steps: 1 ) the feed gas is led through a fractionation column (150) where it is cooled and separated in an overhead fraction with a reduced content of pentane (C5) and heavier components, and a bottom fraction enriched with heavier hydrocarbons, 2) the overhead fraction from the fractionation column is fed to a heat exchanger system (110) and is subjected to a partial condensation to form a two-phase fluid, and the two-phase fluid is separated in a suitable separator (160) into 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 hydrocarbons and hydrocarbons heavier than C5 is exported for further processing in the heat exchanger system (110) for liquefaction to LNG with a maximum content of ethane and LPG 3) 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. A system for carrying out the method is also described.

Description

1 METHOD AND SYSTEM FOR PRODUCING LNG. TECHNICAL FIELD The present invention relates to a method for optimal production of LNG on a 5 fixed or floating offshore installation, as can be seen in the ingress of the independent claim 1. The invention also relates to a system for carrying out the method comprising a fractionation column for feeding in of a feed gas, a heat exchanger system for 10 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 15 exchanger system for further cooling down and liquefaction to LNG. BACKGROUND: Liquefaction of natural gas can be carried out with the use of a gas expansion process, where a cooling medium goes through a processing circuit based on 20 compression, cooling, expansion and thereafter heat exchanging with the fluid that is to be cooled down. For example, for liquefaction of natural gas, one can use a compressed cooling medium in gas phase, normally nitrogen or methane, which is pre-cooled and thereafter expanded across an expansion valve or a turboexpander. The gas expansion means that very cold gas is generated, or a 25 mixture of gas and liquid, which is then used for liquefaction of natural gas and to pre-cool the compressed cooling gas. The gas expansion processes are relatively simple and therefore very well suited to offshore installation. However, the processes have a somewhat lower efficiency than the more advanced processes, such as, for example, mixed cooling medium processes, and thus require much 30 compression equipment and much energy.
2 To produce LNG it is normally required that the gas has a relatively high content of methane. However, most of the feed gases will also contain some heavier hydrocarbons such as ethane, propane, butane, pentane, etc. Some requirements are normally made with respect to the content of heavier 5 hydrocarbons in the liquid gas. The specific energy content per cubic meter of the liquid gas must normally not exceed given sales specifications. 10 The content of pentane (C5) and upwards, and also aromatic compounds of the liquid gas, must be kept below defined limits to avoid freezing out in the cooling down process. The simplest way to limit the content of heavier hydrocarbons in the liquid gas is 15 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 OC and -60 0 C. Separated condensate can be heated up again as a part of the cooling process to utilise the cooling potential. 20 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 25 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. However, for offshore LNG production it is undesirable to handle products other 30 than the liquid natural gas. Where oil or condensate is also produced one can however permit separation of condensate for stabilisation and export together with another oil and/or condensate. However, stabilised condensate will, in the 3 main, consist of C6+ with a relatively low content of pentane and lighter components. Hydrocarbons lighter than C6 can, on the whole, not be stored or transported safely without being cooled down or being under pressure. Some separated hydrocarbons or condensate can be used as fuel, but beyond that one 5 wishes to be able to retain these components in the LNG product. As a result of smaller LNG volumes and the possibility for later blending into large LNG volumes, it can be appropriate offshore to produce a liquid natural gas with a considerably higher, and preferably a maximum, content of heavier hydrocarbons. 10 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 15 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 20 installations where space is often a limiting factor. References to known technology and other publications, and comparisons with the present invention: 25 Initially reference is made to EP-1.715.267 which describes a method which includes natural gas being cooled and being led through a fractionation column where it is separated into an overhead fraction and a bottom fraction. The bottom fraction is enriched with heavier hydrocarbons and is exported out of the system. The overhead fraction is cooled and forms a two-phase fluid which is separated in 30 a separator. The liquid phase is re-circulated to the fractionation column whilst the gas phase is fed further to a heat exchanger system. Cooling of the overhead 4 fraction is carried out with a free standing cooler. The EP patent consequently describes a classical and well-known distillation process. Furthermore, the set-up is standard practice in so-called "base load" LNG 5 installations, where both cooler 5 and cooler 11 (see the figures in the EP patent) are parts of the pre-cooling installation of the plant, which is normally carried out as a multistep propane cooling installation. However, the set up in the EP patent does not integrate a fractionation column and a downstream LNG condensation process as one aims with the present invention. Integration is here meant that two 10 systems are tightly connected together and function as one system and that streams of material and/or streams of energy are flowing both ways between the systems. The cooling process which according to EP-1.715.267 cools the overhead fraction 15 and generates so-called reflux to the fractionation column, comes according to the description not from the same cooling circuit that carries out further cooling and condensation of the natural gas, but apparently from an external cooling process. 20 International patent application WO-2005/071333 describes a well-known double gas expansion which is used to liquefy boil off gas from storage tanks for LNG. In practice, such boil off gases contain only methane and nitrogen. In the patent publications US2006/0260355 Al and US 6,662,589 systems are 25 described which apparently are similar to the present invention, but which in reality are considerably different from the present invention. The systems in the referred publications comprise processes for simultaneous liquefaction of natural gas and recovery / separation of components heavier than methane, i.e. ethane and heavier components, where ethane, LPG and heavier components are 30 fractionated into sales products and where the liquid gas has a considerably reduced content of ethane and heavier components. This is achieved by leading the feed gas to a fractionation column where it comes into contact with a reflux 5 rich in ethane so that the fractionation column separates the feed into an overhead gas fraction with a considerably reduced content of components heavier than methane and a liquid stream from the bottom considerably enriched with components heavier than methane. The reflux rich in ethane is generated in that 5 the gas from the fractionation column is partially condensed, and also by cooling down and condensing a stream rich in ethane which is re-circulated from a fractionation train for fractionation of the bottom fraction from the fractionation column. 10 In patent publications US 6,401,486, US 6,742,358 and W02006/115597 A2 systems are described for simultaneous liquefaction of natural gas and recovery / separation of components heavier than methane, i.e. ethane and heavier components. The processes themselves are also considerably different from and more complex than the present invention in that the feed gas is firstly cooled 15 down in, amongst others, the heat exchanger(s) for liquefaction of gas and also by heat exchange with a flash expanded separated liquid and with fluid from the bottom of the column. Furthermore, the whole or part of the feed gas stream is expanded through a turboexpander or a Joule-Thompson expansion valve before it is led to the fractionation column. 20 The patent publications US 2006/0260355 Al, US patent 6,662,589, US patent 6,401,486 and also US patent 6,742,358 consequently relate to processes to minimise the content of ethane, LPG and also the heavier hydrocarbons in the liquid gas, whilst the present invention comprises a system and a method to 25 maximise the content of methane, ethane and LPG in the liquid gas. But none of the US patent application 2006/0260355 Al, US patent 6,662,589, US patent 6,401,486 or US patent 6,742,358 describe the increase in energy efficiency which can be achieved for a gas expansion process with the integrated separation column that receives a reflux rich in C3-C5 from the liquefaction heat 30 exchanger(s) for production of LNG.
6 A process is described in DE patent 10205366 for simultaneous liquefaction of natural gas and recovery / separation of components heavier than ethane, and where separated LPG and heavier components are fractionated to sales products. This is achieved by first partially cooling down the feed gas in the 5 condensation installation for liquefaction of natural gas and then by leading the cooled down feed gas to a fractionation column where it comes into contact with a reflux rich in ethane so that the fractionation column separates the feed into an overheard gas fraction with a considerably reduced content of components heavier than ethane, and a liquid stream from the bottom considerably enriched 10 with components heavier than ethane. The reflux rich in ethane is generated in that the gas from the fractionation column is partially condensed and thereafter brought into contact with a C4/C5 stream in a second fractionation column, and where the C4/C5 fraction is re-circulated from a fractionation train for fractionation of the bottom product from the first fractionation column. DE patent 10.205.366 15 comprises, in other words, a process to minimise the content of LPG of the liquid gas, and also the heavier hydrocarbons, while the present invention comprises a system and a method to maximise the content of LPG in the liquid gas. The publication DE 10.205.366 does not describe an increase in energy efficiency which can be achieved in a gas expansion process with the integrated separation 20 column which receives a reflux rich in C3-C5 from the liquefaction heat exchanger(s) for production of LNG. In US patent 4,690,702 an LNG process is described where the feed gas is firstly pre-cooled in the cooling installation for LNG production, thereafter to be fed to a 25 first fractionation column where it is brought into contact with a cooled down ethane rich reflux that is re-circulated from a second fractionation column for fractionation of the bottom stream from the first column. The publication does not encompass a system where a reflux rich in C3-C5 for a fractionation column is achieved by partially condensing the overhead gas product from the fractionation 30 column as an integrated part of an LNG process.
7 US patent 7,010,937 shows a system for simultaneous liquefaction of natural gas and recovery / separation of components heavier than methane. According to this publication the gas feed is pre-cooled and partially condensed so that a liquid stream can be separated in a separator and where this liquid stream is 5 fractionated in a first fractionation column to generate an overhead gas which is cooled down to produce a reflux for a second fractionation column. The gas flow from the separator is expanded across a gas expander and fed to the second fractionation column. Therefore this US patent has little in common with the present invention as it is defined in the subsequent claims. 10 The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. 15 SUMMARY OF THE INVENTION The invention aims to use a closed gas expansion process to liquefy the natural gas, and in that the gas is first fed through a fractionation column where the gas is 20 cooled and separated into an overhead fraction with reduced content of pentane (C5) and heavier components, and a bottom fraction enriched with the heavier hydrocarbons, 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 25 invention, production of liquid gas with maximum content of ethane and LPG (liquid petroleum gas) 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 ethane, LPG and pentane is minimised. 30 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, 8 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. Natural gas here means a mixture of hydrocarbons where an essential part 5 consists of methane. Natural gas is normally liquefied by the gas being considerably cooled down such that it condenses and becomes a liquid. With LPG is meant liquid petroleum gas that encompasses propane and butanes (C4, C4 components). 10 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, and then especially on floating installations by-production of condensate during the liquefaction is minimised the efficiency is maximised (the need for fuel gas is minimised). 15 The present invention provides a Method for production of LNG from an incoming feed gas, wherein the method comprises: a) leading the feed gas through a fractionation column where it is cooled and separated into an overhead fraction, 20 b) leading the cooled overhead fraction of the fractionation column into a heat exchanger system where it is subjected to a partial condensation to form a two phase fluid, c) separating the two phase fluid in a separator into a liquid component and a gas component, 25 d) leading the liquid component to the fractionation column as a cold reflux, and e) leading the gas component to the heat exchanger system for further cooling, condensation and sub-cooling, wherein f) the cooling and condensation of the feed gas in the heat exchanger system 30 is provided by an open or closed gas expansion process with at least one gas expansion step, and further wherein: 9 g) the fractionation column and separator are operated at temperatures and pressures such that an overall component split/separation point is achieved in the normal boiling point range (NBP) between -12*C and 60*C. 5 The present invention further provides a System for carrying out the method comprising a fractionation column for receiving a feed gas and arranged to cool and separate the feed gas into an overhead gas stream and a bottom liquid fraction, a liquification circuit comprising a heat exchanger system for cooling down and partially condensing the overhead gas stream of the fractionation 10 column into a two-phase stream, a separator to separate the two-phase stream from the heat exchanger system into a liquid component and a gas component, the system being further arranged to recycle the liquid component from the separator to the fractionation column as a cold reflux, and further arranged to lead the gas component from the separator back to the heat exchanger system for 15 further cooling down and liquefaction to LNG, wherein the system is operated at temperatures and pressures such that an overall component split/separation point is achieved in the normal boiling point range (NBP) between_-12*C and 60"C, and further in that the cooling system which is used for cooling down, condensing and liquefying of gas in the heat exchanger system comprises an open or closed gas 20 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. 25 The system is preferably designed and configured to separate the feed gas so that the overhead gas stream of 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 30 and components with a normal boiling point higher than C6.
10 BRIEF DESCRIPTION OF THE DRAWINGS: The invention will now be described in more detail with reference to the enclosed figures in which: Figure 1 shows a principal embodiment with main components and main method 5 of action. Figure 2 shows the invention with an alternative embodiment. Figure 3 shows the invention with an alternative embodiment that includes further 10 stabilisation of the heavier hydrocarbons that are separated out (condensate). Figure 4 shows the invention in detail carried out by using a double gas expansion process. 15 Figure 5 shows the invention carried out by using a hybrid cooling circuit with a gas expansion loop and a liquid expansion loop. Figure 6 shows an example of a hot temperature curve and a cold temperature curve (composite curve) for a conventional nitrogen expansion circuit. 20 Figure 7 shows an example of a hot temperature curve and a cold temperature curve (composite curve) for a nitrogen expansion circuit obtained by using the present invention. 25 Figure 8 shows a comparison of the curves shown in the figures 6 and 7. DESCRIPTION OF EMBODIMENTS With reference to figure 1 the system for optimised liquefaction of gas comprises, as a minimum, the following principle components: 30 - an incoming gas stream 1 which shall be cooled down and liquefied, 11 - a fractionation column 150 in which the incoming gas is cooled and is separated into an overhead fraction 2 with a reduced content of pentane and heavier components, - a bottom fraction 3 enriched with the heavier hydrocarbon components, 5 - a system of heat exchangers 110, in which the incoming gas is cooled down and partially condensed for separation of heavier hydrocarbons for subsequent cooling down and liquefaction, - a product stream 11 that encompasses a cooled down and liquefied gas, - a product stream 3 which, in the main, encompasses pentane and heavier 10 hydrocarbons, and - a cooling system for cooling down and liquefying the gas comprising a gas cooling agent stream 20, at least one circulation compressor 100, at least one aftercooler 130, at least one gas expander 120. 15 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 20 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 pentane (C3 25 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 30 heat exchanger system 110 for further cooling down, condensation and undercooling. The liquid gas 11 is alternatively led through a control valve 140 that controls the operating pressure and flow through the system.
12 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 5 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 10 fractionation column 150 and reflux separator 160) generating a component split/separation point in the normal boiling point area (NBP) between -120 0 C and 60C. 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 00C, and the heavy key component being a C6 component with a boiling point between 15 50 0 C and 700C. 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 - 360C) is a transitional component which is distributed in 20 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 25 starts in that a 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 0C and 1200C, but such that the cooling agent stream is mainly a gas at the prevailing 30 pressure and temperature 31. The pre-cooled 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 13 pressure, and such that the cooling agent, in the main, 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 cooling agent is expanded in the gas expander 121 at a high 5 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 10 and the gas that shall be cooled down is condensed and undercooled. 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 15 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 20 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. 25 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 30 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 14 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 5 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). 10 Figure 2 shows an alternative embodiment where several multi-stream heat exchangers are connected together in such a way that the necessary heat transfer between the cold and hot streams can be brought about. Figure 2 shows a heat exchanger system 110 comprising of several heat exchangers in series. 15 However, the invention is not related to a specific type of heat exchanger or number of exchangers, but can be carried out in several different types of heat exchanger systems that can handle the necessary number of hot and cold process streams. 20 Figure 3 shows an alternative embodiment where the fractionation column 150 is fitted with a reboiler 135 to further improve the separation (a sharper split between light and heavy components), and also to reduce the volatility of the bottom fraction in the column. This can be used to directly produce condensate which is stable at ambient temperature and atmospheric pressure. 25 Figure 4 shows the invention in detail carried out in a more advanced embodiment where a double gas expansion process is used. In this embodiment, the compressed cooling agent stream 21 is first cooled down to an intermediate temperature. At this temperature, the cooling agent stream is divided into two 30 parts, where the one part 31 is taken out of the heat exchanger and is expanded in the gas expander 121 to a low pressure gas stream 32. The other part 41 is pre-cooled further to be expanded in the gas expander 122 to a pressure 15 essentially equal to the pressure in stream 32. The expanded cold cooling agent streams 32, 42 are returned to different inlet locations on the heat exchanger system 110 and are combined to one stream in this exchanger. Heated cooling agent 51 is then returned to recompression. In an alternative embodiment to the 5 system in Figure 3, the compressed cooling agent stream 20 in the double gas expansion circuit can be split into two streams before the heat exchanger 110 to be cooled down to different temperatures in separate flow channels in the heat exchanger 110. 10 The same goes for the heating of the returned cold cooling agent streams 32, 42. The embodiment is otherwise in accordance with Figure 3. Figure 5 shows in detail the invention carried out with the use of a hybrid cooling loop where one and the same cooling agent is used both in a pure gas phase and 15 in a pure liquid phase. In this embodiment a closed cooling loop provides the cooling down of the feed gas in the heat exchanger system 110. Said cooling loop starts by methane or a mixture of methane and nitrogen, where methane makes up at least 50 % of the volume, being compressed and aftercooled to a compressed cooling agent stream 21, and where this cooling agent stream is pre 20 cooled, and at least a part 31 of the cooling agent stream is used in the gas phase in that it is expanded across a gas expander 121 and that at least a part 41 of the cooling agent stream is condensed to liquid and is expanded across a valve or liquid expander 141. 25 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. 30 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 16 than the permitted level of pentane and heavier hydrocarbons with a normal boiling point above 50 - 60 0 C. At the same time, the by-production of volatile hydrocarbons with a considerable content of ethane, propane and butane is minimised or eliminated, which will be difficult to handle on an offshore installation 5 for LNG production. 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 LPG-rich reflux from the cooling down process. 10 The reason for the energy consumption for the gas expansion processes for liquefaction of the natural gas being reduced with the use of the invention compared to a corresponding cooling process without the integrated separation column has several connections: 15 The heavier hydrocarbons which are essential to separate 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. This reduces the exergy loss in the cooling process in that a cooling down load is moved to a higher 20 temperature area. The heat exchanger system 100 of the cooling down process receives the gas which is to be liquefied as stream 2 (the overhead gas stream in the fractionation column), which has a reduced temperature with respect to the actual gas feed 25 stream 1. A gas expansion process is characterised in that the hot and cold cooling curves are dominated by the large amount of gas which is used as cooling agent. These gas streams form linear cooling curves. The reduced feed temperature into the heat exchanger results in a "break point" on the hot cooling curve (the sum of the streams which are being cooled), so that it is possible to 30 obtain a general reduction of the distance between the hot and cold cooling curves. This provides a better temperature adaption, reduced exergy loss and thus a reduced energy consumption to drive the cooling process.
17 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 5 conventional methods. Figure 6 shows hot and cold cooling curves (hot and cold composite curves, i.e. the sum of all hot streams that are to be cooled down and the sum of all cold streams that are to be heated up, respectively) for the heat exchanger system 10 110 carried out in accordance with the present invention, and with a double nitrogen expansion process as a cooling system. Figure 7 shows corresponding hot and cold cooling curves for a corresponding cooling process with the same feed, but carried out in a conventional way without the fractionation column. The curves appear to look alike, but by considering Figure 8, which shows a section 15 and both the systems are the same curve, the "break point" and the better adaption can clearly be seen. Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood 20 to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
18 Example 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 5 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 Figures 1, 2, 3, 4 or 5. Stream No. 1 2 3 4 5 6 11 Gas fraction 1.00 1.00 0.00 0.95 0.00 1.00 0.00 Temperature 40.0 19.2 35.9 -20.0 -20.0 -20.0 -155.0 (0C) Pressure 2740 2738 2745 2725 2730 2723 2655 (kPa abs) Mole flow 4232 4422 44 4422 235 4185 4185 (kmol/h) Mass flow 78980 87539 3410 87539 11969 75541 75541 (kg/h) Mole fraction (%) Nitrogen 0.51 % 0.49 % 0.02 % 0.49 % 0.03 % 0.52 % 0.52 % Methane 90.4% 87.4% 11.8% 87.4% 19.5% 91.3% 91.3% Ethane 4.38 % 4.53 % 2.58 % 4.53 % 6.84 % 4.40 % 4.40 % Propane 2.29 % 2.95 4.17 % 2.95 % 15.04 % 2.27 % 2.27 % i-Butane 0.68 % 1.25 % 2.80 % 1.25 % 11.92 % 0.65 % 0.65 % n-Butane 0.66% 1.52 % 3.79 % 1.52 % 17.30 % 0.62 % 0.62 % i-Pentane 0.17 % 0.70 % 2.52 % 0.70 % 10.57 % 0.14 % 0.14 % n-Pentane 0.17% 0.79 % 3.61 % 0.79 % 12.49 % 0.12% 0.12 % n-Hexane 0.44 % 0.32 % 43.62 % 0.32 % 6.25 % 0.02 % 0.02 % n-Heptane 0.19 % 0.00 % 18.29 % 0.00 % 0.02 % 0.00 % 0.00 % n-Octane 0.055 % 0.000 % 5.187 % 0.000 % 0.000 % 0.000 % 0.000 % n-Nonane 0.014% 0.000% 1.339% 0.000% 0.000% 0.000% 0.000% n-Decane+ 0.002 % 0.000 % 0.214 % 0.000 % 0.000 % 0.000 % 0.000 % 10

Claims (20)

1. A Method for production of LNG from an incoming feed gas, wherein the method comprises: 5 a) leading the feed gas through a fractionation column where it is cooled and separated into an overhead fraction, b) leading the cooled overhead fraction of the fractionation column into a heat exchanger system where it is subjected to a partial condensation to form a two phase fluid, 10 c) separating the two phase fluid in a separator into a liquid component and a gas component, d) leading the liquid component to the fractionation column as a cold reflux, and e) leading the gas component to the heat exchanger system for further 15 cooling, condensation and sub-cooling, wherein f) the cooling and condensation of the feed gas in the heat exchanger system is provided by an open or closed gas expansion process with at least one gas expansion step, and further wherein: g) the fractionation column and separator are operated at temperatures and 20 pressures such that an overall component split/separation point is achieved in the normal boiling point range (NBP) between -120C and 60 0 C.
2. The Method as claimed in claim 1, wherein the light key component for the separation is butane (C4) with a normal boiling point between -12 0 C and 0 0 C. 25
3. The method as claimed in claim 1, wherein the heavy key component is a C6 component with a boiling point between 50 0 C and 70'C.
4. The method as claimed in any one of claims 1 to 3, wherein the 30 fractionation column and the separator are operated so that pentane (C5, NBP= 28 - 36 0 C) is a transitional component that is distributed both in the overhead gas stream of the system and the reject stream of the system. 20
5. The method as claimed in any one of the preceding claims, wherein the temperature of the feed gas is reduced through the fractionation column so that the temperature of the gas when it is fed into the heat exchanger system is lower 5 than the temperature of the gaseous cooling agent streams at the hot end of the heat exchanger system (hot pinch point temperature).
6. The method as claimed in any one of the preceding claims, wherein a reboiler is connected to the fractionation column to reduce the vapour pressure of 10 the bottom product.
7. The method as claimed in any one of the preceding claims, wherein the heat exchanger for liquefaction (LNG production) comprises one or more multi stream heat exchangers. 15
8. The method as claimed in any one of the preceding claims, wherein it is carried out with a closed gas expansion process with at least one nitrogen expander. 20
9. A System for carrying out the method as claimed in any one of claims 1 to 8 comprising a fractionation column for receiving a feed gas and arranged to cool and separate the feed gas into an overhead gas stream and a bottom liquid fraction, a liquification circuit comprising a heat exchanger system for cooling down and partially condensing the overhead gas stream of the fractionation 25 column into a two-phase stream, a separator to separate the two-phase stream from the heat exchanger system into a liquid component and a gas component, the system being further arranged to recycle the liquid component from the separator to the fractionation column as a cold reflux, and further arranged to lead the gas component from the separator back to the heat exchanger system for 30 further cooling down and liquefaction to LNG, wherein the system is operated at temperatures and pressures such that an overall component split/separation point is achieved in the normal boiling point range (NBP) between_-12*C and 60 0 C, and 21 further in that the cooling system which is used for cooling down, condensing and liquefying of gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step. 5
10. The system as claimed in claim 9, wherein the gas expansion process is a closed circuit.
11. The System as claimed in claim 9 or 10, wherein the system is operated at temperatures and pressures designed and configured to separate the feed gas 10 such that the overhead gas stream of 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 in the fractionation column will be enriched with most of the C6 and components with a normal boiling point higher than C6.
12. The system as claimed in any one of claims 9 to 11, wherein the 15 liquefaction circuit comprises the gaseous refrigerant at an inlet pressure of 3 to 10 MPa being fed to the heat exchanger or system of heat exchangers and cooled to a temperature between 0 and -120 0C, and further wherein the cooled gaseous refrigerant is expanded to a pressure between 5% and 40% of the inlet pressure, and then being led back to the heat exchanger or system of heat 20 exchangers to provide cooling.
13. The system as claimed in any one of claims 9 to 12, wherein the liquefaction circuit comprises two expansion stages, wherein the gaseous refrigerant is split in two parts either before or after pre-cooling, and where the parts are pre-cooled to different temperatures before expansion to essentially the 25 same lower pressures and led back to the heat exchanger or system of heat exchangers to provide cooling.
14. The system as claimed in any one of claims 9 to 13, wherein cooling in the fractionation column is essentially provided by the reflux liquid from the separator. 22
15. The system as claimed in any one of claims 9 to 14, wherein a reboiler is connected to the fractionation column to reduce the vapour pressure of the bottom product.
16. The system as claimed in any one of claims 9 to 15, wherein the heat 5 exchange system comprises one or more multi-stream heat exchangers configured in series or parallel, or both.
17. The system as claimed in any one of claims 9 to 16, wherein the liquefaction circuit comprises one heat exchanger comprising a plurality of warm and cold streams in the same unit. 10
18. The system as claimed in any one of claims 9 to 17, wherein the gas expander essentially isentropically cools the refrigerant.
19. The system as claimed in any one of claims 9 to 18, wherein the liquefaction circuit comprises a closed gas expansion process with two or more gas expansion stages for cooling the refrigerant by gas expansion, and where the 15 refrigerant inlet temperature for the second gas expander stage is lower than the refrigerant inlet temperature for the first gas expander stage.
20. The system as claimed in any one of claim 9 to 19, wherein the gaseous refrigerant comprises nitrogen gas.
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