US20080115532A1

US20080115532A1 - Method And Apparatus For Producing A Liquefied Natural Gas Stream

Info

Publication number: US20080115532A1
Application number: US11/792,450
Authority: US
Inventors: Marco Dick Jager
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2004-12-08
Filing date: 2005-12-07
Publication date: 2008-05-22
Also published as: BRPI0518464A2; JP5138381B2; AU2005313333B2; CN101072848A; JP2008523186A; KR20070091323A; KR101260693B1; BRPI0518464B1; RU2402592C2; EP1819800A1; RU2007125703A; PE20060989A1; CN101072848B; EG25612A; AU2009202409B2; MY146497A; WO2006061400A1; EP1819800B1; AU2005313333A1; AU2009202409A1

Abstract

Method and apparatus producing a LNG stream, wherein before liquefaction heavier hydrocarbon components are removed from a NG stream to be liquefied, the method at least comprising the steps of: providing a vaporous feed stream (1) of natural gas; feeding the feed stream (1) into a distillation column (10); drawing a bottom stream (17) and an overhead stream (16) from the distillation column (10); and liquefying at least a part of the overhead stream (16) thereby obtaining a LNG stream; wherein the step of feeding the feed stream (1) into the distillation column (10) comprises the sub steps of: splitting the feed stream (1) into first (3a) and second (3b) substreams; feeding the first substream (3a) into the distillation column (10) via a first feed point (7a), at a pressure not lower than the feed stream pressure minus a pressure drop brought about by the said splitting of the feed stream (1); cooling the second substream (3b); and feeding the cooled second substream (7) into the distillation column (10) at a second feed point (7b) overhead of the first feed point (7a).

Description

FIELD OF THE INVENTION

The invention relates to a method and apparatus for producing a liquefied natural gas (LNG) stream, the LNG stream primarily comprising methane (preferably >90 mol %).

BACKGROUND OF THE INVENTION

It is normal practice to liquefy natural gas so that it can be transported by a carrier, when other means of transport are not available or less attractive. Liquefaction of natural gas allows for a significant reduction in its volume, which makes transport much more efficient. In order to produce the liquefied natural gas (LNG), a liquefaction process is employed. The liquefaction process normally comprises a cryogenic zone containing one or more refrigeration cycles wherein the natural gas is cooled down in one or more stages from ambient temperature to the ambient boiling point of natural gas or somewhat lower. This boiling point is normally around minus 160° C.
The refrigeration cycle(s) generally make use of a refrigerant fluid, which can be formed of either a mixture or a pure constituent. The refrigerant is typically vaporised in one or more cryogenic heat exchangers, in which the natural gas is cooled. The vaporized refrigerant is subsequently compressed to a higher pressure level and temperature. In an ambient cooler heat from the refrigerant is rejected to a cooling medium, such as water or air, and subsequently cooled by expansion. It is very common in liquefaction processes with multiple cycles that consecutive refrigeration cycles are cooled by the first refrigeration cycle.
In current liquefaction processes it is also common to remove certain components from the natural gas before it is cooled in the cryogenic heat exchanger(s). Amongst the components that normally are to be removed are carbon dioxide, sulphur containing compounds, water and hydrocarbons with a higher molecular weight than that of butane. The latter are referred to in this specification by the term “heavier hydrocarbons”. These components must be removed from the natural gas as they could otherwise become solid at the cryogenic temperatures at which the liquefaction is carried out.
Normally, the raw natural gas stream is first decontaminated from water and acid gases for which numerous physical and/or chemical processes exist. The resulting stream of sweetened and dried natural gas mixture is then subjected to a step of removing the heavier hydrocarbons. The removal of heavier hydrocarbons is generally achieved by partial condensation of the natural gas mixture, followed by some separation between a vapour phase lean in heavier hydrocarbons and a liquid phase rich in heavier hydrocarbons. Most common is the use of a scrub column to achieve this separation. A scrub column is a type of distillation column comprising a series of separation stages between a bottom end and top end, whereby a mixture enriched with heavier hydrocarbons is discharged from the bottom end in the form of a bottom stream, and a lighter mixter of natural gas is discharged from the top end in the form of an overhead stream.

DESCRIPTION OF THE PRIOR ART

U.S. Pat. No. 5,685,170 describes a system and process for recovering propane, butane and heavier hydrocarbon components from natural gas thereby also generating a gas stream consisting primarily of methane and ethane.
Various embodiments of a method and scrub column line-up for pretreating a natural gas stream using a single scrub column to remove freezable C₅+ components and provide an LNG product are described in U.S. Pat. No. 5,325,673.
In one of the embodiments, the natural gas feed stream is split into several feed substreams, cooled and introduced to the scrub column at different feed points near the top and middle of the scrub column. A first part of the natural gas is cooled by expansion in a Joule-Thompson valve and introduced in the scrub column at a lowered pressure. Second and third parts are first cooled in a refrigeration cooler, separated into a condensate liquid stream and a vapour stream, before lowering the pressure. The condensate liquid stream is fed into the scrub column at a feed point lower than the vapour stream.
A reboiler is provided to vaporize a fraction of the heavier hydrocarbon enriched bottom stream or liquid accumulated in the bottom end of the scrub column. A reboiler also serves to control the temperature in the bottom end of the scrub column so as to ensure that the bottom end does not become too cold thereby carrying the risk of accumulation of unwanted components such as carbon dioxide in the bottom stream.
The known embodiment has a number of drawbacks. Firstly, the letting down of pressure of the feed substreams prior to feeding into the scrub column reduces the efficiency of a later liquefaction step, as liquefaction of natural gas at lower pressure requires more energy.
A drawback of a reboiler is that it adds heat to the natural gas while by the very nature of the liquefaction process the natural gas should be cooled. The use of a reboiler adversely affects the overall efficiency of the liquefaction process.

SUMMARY OF THE INVENTION

It is an object of the present invention to minimize one or more of the above drawbacks.
It is a further object of the present invention to provide an alternative method of producing a liquefied natural gas stream, mainly comprising liquefied methane.
One or more of the above or other objects are achieved according to the present invention by providing a method of producing a liquefied natural gas stream, wherein before liquefaction heavier hydrocarbon components having a molecular weight higher than that of butane are removed from a natural gas stream to be liquefied, the method at least comprising the steps of:
providing a substantially vaporous feed stream of natural gas at a feed stream pressure and a feed stream temperature;
feeding the feed stream into a distillation column having two or more separation stages;
drawing a bottom stream from a lower part of the distillation column and an overhead stream from an upper part of the distillation column, the overhead stream containing a lower relative amount of the heavier hydrocarbon components than the bottom stream; and
liquefying at least a part of the overhead stream thereby obtaining a liquefied natural gas stream;
wherein the step of feeding the feed stream into the distillation column comprises the substeps of:
splitting the feed stream into first and second substreams at a selected split ratio;
feeding the first substream into the distillation column via a first feed point at the bottom of the distillation column, at a pressure not lower than the feed stream pressure minus a pressure drop brought about by the said splitting of the feed stream;
cooling the second substream in a heat exchanger to a lower temperature than the feed temperature;
feeding the cooled second substream into the distillation column at a second feed point overhead of the first feed point.
For the purpose of this specification, the split ratio is defined as the mass flow rate of the first substream divided by the mass flow rate of the second substream.
An advantage of the invention is that neither the pressure of the feed stream, nor that of the first and second substreams, is deliberately lowered in a dedicated pressure lowering device such as a (turbo-)expander or a Joule-Thompson valve.
Since the first substream is fed into the distillation column at a pressure essentially not lower than the feed stream pressure minus a pressure drop brought about by the splitting of the feed stream, the pressure at the second feed point does not need to be let down. Thus, the distillation is performed without significantly decreasing the pressure of the natural gas, which will be energetically beneficial in case that the overhead stream is to be liquefied.
Another consequence of deliberately not letting down the pressure in the first substream is that the temperature can be kept close to the feed stream temperature; preferably no warming of the first substream is provided. An advantage of this is that less additional heating power, normally provided for instance via a reboiler, needs be into the bottom end of the distillation column to avoid it becoming too cold.
By selecting the split ratio sufficiently high, no additional heating power even needs be added at all so that no reboiler needs to be provided for the purpose of controlling the bottom temperature.
It has been found that the split ratio can be selected such that the temperature in the bottom of the distillation column is maintained at -10 degrees Celsius or higher.
The temperature in the bottom end of the distillation column can be controlled by providing a selectable or controllably variable split ratio and selecting or controlling the split ratio.
The invention is also embodied in an apparatus for producing a liquefied natural gas stream, wherein before liquefaction heavier hydrocarbon components having a molecular weight higher than that of butane can be removed from a natural gas stream to be liquefied, the apparatus at least comprising:
a feed stream line for carrying a substantially vaporous feed stream of natural gas at a feed pressure and a feed temperature;
a distillation column having two or more separation stages for separating the heavier hydrocarbon components from the natural gas, a bottom stream discharge opening arranged in a lower part of the distillation column for discharging a bottom stream, and an overhead stream discharge opening arranged in an upper part of the scrub column for discharging an overhead stream containing a lower relative amount of the heavier hydrocarbon components than the bottom stream; and
a cryogenic zone in which at least a part of the overhead stream can be liquefied thereby obtaining a liquefied natural gas stream;
wherein the feed stream line comprises a feed stream junction fluidly connecting a main branch with first and second branches to split the feed stream into first and second substreams at a selected split ratio, wherein the first branch connects the feed stream junction with a first feed point at the bottom of the distillation column at a pressure not lower than the feed stream pressure drop brought about by the splitting of the feed stream, and wherein the second branch connects the feed stream junction with a second feed point in the distillation column, the second feed point being overhead relative to the first feed point, wherein the second branch is provided with a heat exchanger.
For the purpose of this specification, a heat exchanger is understood to include at least heat exchangers of the so-called spool-wound type.
In its broadest definition, the invention is applicable to any type of distillation column that is suitable for removing heavier hydrocarbon components having a molecular weight higher than that of butane from a hydrocarbon gas mixture. However, one or more preferred embodiments of the invention require a scrub stream to be fed into the column. In that case, the distillation column is by definition forms a so-called scrub column.
These and other features of the invention will be elucidated below by way of example and with reference to the accompanying non-limiting drawing.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing:

FIG. 1 schematically shows a process flow scheme according to a first embodiment of the invention;

FIG. 2 schematically shows a process flow scheme according to a second embodiment of the invention;

FIG. 3 schematically shows a process flow scheme according to a third embodiment of the invention;

FIG. 4 schematically shows an alternative process flow scheme according to the third embodiment of the invention;

FIG. 5 schematically shows an alternative process flow scheme according to the second embodiment of the invention;

FIG. 6 schematically shows a process flow scheme according to a fourth embodiment of the invention;

FIG. 7 schematically shows a process flow scheme according to a fifth embodiment of the invention; and

FIG. 8 schematically shows a process flow scheme wherein the feed stream is not split.

For the purpose of this description a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 schematically shows a process flow scheme involving a system for removing heavier hydrocarbon components having a molecular weight higher than that of butane from a hydrocarbon gas mixture, as part of an apparatus for producing an LNG stream primarily comprising methane. For the purpose of this specification, the hydrocarbon gas mixture will be assumed to be formed of a natural gas mixture, previously treated to remove water, CO₂, and sulphur by means and methods well known in the art. Generally, the pretreated natural gas mixture will contain lighter components including vaporous methane and ethane and including C₃and C₄, and heavier components C₅+ that are potentially freezable during liquefaction of methane.
The apparatus of FIG. 1 includes a natural gas feed line that is arranged to receive and carry a feed stream of the hydrocarbon gas mixture from which the heavier hydrocarbons are to be removed. The feed streamline is divided in a main branch 1, and first and second substream branches 3 a and 3 b, respectively. A feed stream junction 2 is provided to divide the feed stream in the main branch 1 into first and second substreams that can respectively flow through first and second substream branches 3 a and 3 b. The feed stream junction 2 is arranged to divide the feed stream in accordance with a specified split ratio, which is defined as the mass flow rate of the first substream divided by the mass flow rate of the second substream. The person skilled in the art will understand that the feed stream junction 2 is not a phase separator, but rather divides the main feed stream in two or more substreams.
Both first and second substream branches 3 a,3 b are in fluid communication with a scrub column 10. The scrub column 10 in the present embodiment is a distillation column provided with a number of trays 11 to allow separation of lighter hydrocarbon components from heavier hydrocarbon components in a plurality of separation stages. The temperature in the scrub column typically varies, becoming cooler with each higher stage. The scrub column 10 further comprises a bottom stream discharge opening 8 in a lower part of the scrub column 10 for discharging a bottom stream enriched in the heavier hydrocarbon components for instance via discharge line 17, and an overhead stream discharge opening 12 arranged in an upper part of the scrub column 10 for discharging, for instance via discharge line 16, an overhead stream enriched in lighter hydrocarbon components. The overhead stream 16 is connected with a cryogenic zone (not shown) for producing LNG.
The first branch 3 a connects the feed stream junction 2 with a first feed point 7 a in the scrub column 10. The first feed point 7 a is relatively close to the bottom of the scrub column 10 to feed the first substream 3 a at one of the lower trays 11. Preferably the substream 3 a feeds below the lowest phase separating tray 11. The first branch 3 a is essentially free of pressure-lowering devices so that the feed stream junction 2 is fluidly connected to the first feed point 7 a at essentially no pressure loss. Preferably, the connection is dimensioned such that the pressure loss does not exceed 5 bar, more preferably it does not exceed 2 bar under normal operating conditions. Further the first substream 3 a is not warmed
The second branch 3 b connects the feed stream junction 2 with a second feed point 7 b in the scrub column 10. The second feed point 7 b is situated overhead relative to the first feed point 7 a to feed the first substream at one of the trays above the lower trays.
The second branch 3 b is provided with a heat exchanger 6 dividing the second branch 3 b into a warm part 3 b and a cool part 7. The heat exchanger 6 is arranged to cool the second substream 3 b essentially without deliberately letting off pressure. The heat exchanger 6 can be any suitable type of heat exchanger, such as a so-called spool-wound heat exchanger.
Under normal operating condition, the pressure drop in the second substream is less than 6 bar, preferably less than 3 bar. The heat exchanger 6 has at least one supply of refrigerant 4 and one removal of spent or vaporized refrigerant 5. The heat exchanger 6 may be a dedicated heat exchanger or an integrated heat exchanger that also provides cooling for other duties. Preferably the heat exchanger 6 uses an external refrigerant, making the heat exchanger 6 a dedicated heat exchanger.
Although not required by the invention, which relates more to the splitting of the feed stream into the first and second substreams 3 a,3 b, in the embodiment of FIG. 1 a third feed point 7 c is advantageously provided in the scrub column 10. The third feed point 7 c is situated near the top of the scrub column 10 overhead relative to the second feed point 7 b. An optional scrub stream line 18 connects the third feed point 7 c with a scrub stream source. The scrub stream source serves to supply another liquid or multi-phase stream capable of scrubbing heavier hydrocarbons and promote downward transport of those in the scrub column 10. The scrub stream can contain one or more of the group consisting of further cooled natural gas, condensate from an overhead condenser, LNG, chilled LPG, chilled condensate, mixtures hereof or any other stream with the appropriate properties to promote the removal of heavier hydrocarbons from the natural gas.
In operation, the apparatus of FIG. 1 works as follows. A feed stream of the pre-treated hydrocarbon gas mixture is provided via line 1 at a feed stream pressure and a feed stream temperature. The feed stream pressure generally lies between 20 and 80 bar, and more typically between 40 and 65 bar. The feed stream temperature is generally between 0 and 50 degrees Celsius, typically between 15 and 25 degrees Celsius, more typically between 15 and 20 degrees Celsius.
The feed stream is split into first and second substreams 3 a,3 b in feed stream junction 2, preferably in the form of minor and major substreams. The minor substream 3 a is fed into the scrub column 10 via the first feed point 7 a, at a pressure that is not lower than the feed stream pressure minus a pressure drop brought about by the splitting of the feed stream 1 in the feed stream junction 2. In practice, it means that the pressure in the minor substream 3 a is not deliberately let down.
The second substream 3 b, usually the major substream, is cooled in the heat exchanger 6 to a lower temperature than the feed temperature. Broadly, the major substream 3 b is cooled to a temperature not lower than −50 degrees Celsius, preferably not lower than −20 degrees Celsius. Preferably, the major substream 3 b is cooled to a temperature of −10 degrees Celsius or lower.
The cooled major substream is fed, via the second feed point 7 b and the cold part 7 of the second sub branch 3 b, into the scrub column 10 at a location overhead of where the minor substream 3 a is fed into the scrub column 10.
The optional scrub stream in scrub line 18 has a temperature lower or equal to that of the second substream entering via the second feed point 7 b.
The relatively cool major substream 7, together with the relatively warm minor substream 3 a, contributes to maintaining the desired temperature gradient inside the scrub column 10.
It is possible to control the split ratio between the major and minor substreams. Herewith the temperature gradient and/or the temperature in the bottom of the scrub column 10 can be controlled. It has been found that the split ratio is preferably chosen smaller than ⅕ in order to assure that the temperature in the scrub column is sufficiently low to achieve an efficient separation of heavier hydrocarbon components from the mixture. More preferably the split ratio is chosen smaller than 1/10.
It has been found that the split ratio is preferably chosen higher than 1/100, in order to achieve a beneficial effect in lowering the demand for external heat required to maintain the temperature in the bottom of the scrub column higher than −10 degrees Celsius. Preferably, the split ratio is chosen higher than 1/50, so that the need for a reboiler can be removed entirely. Thus, in a preferred embodiment no reboiler is present, as a result of which no reboiling takes place between the overhead stream discharge opening 12 and the third feed point 7 c.
Preferably, the scrub stream 18 is fed to the scrub column 10 via the third feed point 7 c overhead of the second feed point 7 b. The temperature of the scrub stream 18 is typically lower than that of the cooled minor substream and usually between −70 and −10 degrees Celsius. This further helps in maintaining the desired temperature gradient inside the scrub column 10.
The top product 16 is drawn from the scrub column 10 via overhead stream discharge opening 12, which is the natural gas from which the heavier hydrocarbons have been removed to a sufficient extent. Stream 17 is the bottom product enriched in heavier hydrocarbons that is discharged via discharge opening 8. In particular, the top product 16 is a natural gas vapour stream lean in heavier hydrocarbons, meeting the requirements to avoid formation of solids during further cooling of the natural gas vapour stream ultimately into liquefaction in a cryogenic zone (not shown). As the person skilled in the art will readily understand how to liquefy the top product 16 in a cryogenic zone (e.g. using heat exchangers), this is not further discussed here. The bottom product 17 may find any application, one of which is further processing it to form liquefied petroleum gas (LPG).
FIGS. 2 to 5 schematically depict alternative process flow schemes involving alternative apparatuses. In these Figures, parts already described above with reference to FIG. 1 will carry identical reference numerals and will not be again described here. Also their function and operation will be in accordance with the description above.
FIGS. 2 to 5 show embodiments wherein the scrub stream 18 is at least in part drawn from the feed stream 1.
Starting with FIG. 2, a main difference with the embodiment of FIG. 1 is reflected by the presence of a second feed stream junction 20 provided in the second branch 7 upstream of the second feed point 7 b and downstream of the heat exchanger 6. The second branch 7 continues downstream of the feed stream junction 20 and a third branch 22 is formed to carry a third substream of the feed stream 1. The third branch 22 is provided with a second heat exchanger 26, the down-stream side thereof being connected to scrub stream line 18.
The second heat exchanger 26 is arranged to further cool the third substream 22 to a temperature lower than that of the second substream, essentially without deliberately letting off pressure. Under normal operating condition, the pressure drop in the third substream 22 is less than 6 bar, preferably less than 3 bar. As shown in FIG. 2 at least one supply of refrigerant 24 is provided to feed to the second heat exchanger 26, wherein the removal of spent or vaporized refrigerant 25 can form the supply of refrigerant 4 to feed the first mentioned heat exchanger 6.
Alternatively, first and second heat exchangers are each provided independently with at least one supply and removal of refrigerants. The second heat exchanger 26 may be a dedicated heat exchanger or an integrated heat exchanger that also provides cooling for other duties.
Referring now to FIG. 3, an alternative to FIG. 2 is schematically shown wherein the second feed stream junction 20 is provided in the second branch 3 b upstream of the first heat exchanger 6. The second heat exchanger 26 is provided in a parallel relationship with the first heat exchanger 6 instead of the serial arrangement of FIG. 2. The second branch 3 b continues downstream of the second feed stream junction 20 and third branch 22 is formed to carry the third substream of the feed stream 1. As before in FIG. 2, the down-stream side of the second heat exchanger is connected to scrub stream line 18.
First and second heat exchangers 6,26 each have individually at least one supply of refrigerant 4,24 and removal 5,25 of spent refrigerant.
The first and second heat exchangers 6,26 can be combined in one housing, whereby the refrigerant can be operative at one pressure level.
Referring now to FIG. 4, there is schematically shown an example based on parallel cooling of the second and third substreams, whereby the first and second heat exchangers are integrated into one housing each represented by a flow path. FIG. 5 shows an example of an integrated heat exchanger embodying serial cooling of the embodiment of FIG. 2. In this example, the second feed stream junction 20 is located outside the heat exchanger housing whereby the second and third branches can be led out and into the heat exchanger housing. Alternatively, although currently considered less practical, the feed stream junction 20 can be located inside the heat exchanger housing.
Thus, in the embodiments of FIGS. 2 to 5, the scrub stream source, which is connected to the scrub column 10 via the third feed point 7 c overhead of the second feed point 7 b, comprises the second feed stream junction 20 and the second heat exchanger 26.
In operation, the apparatuses of FIGS. 2 to 5 work similar to that of FIG. 1. However, the scrub stream in line 18 is obtained by drawing a fraction from the second substream 3 b to form a third substream. The residue carries on as the second substream 3 b. The third substream is cooled in the second heat exchanger 26 downstream the second feed stream junction 20, to a temperature that is lower than that of the second substream as it has been cooled by the first heat exchanger 6.
Yet another embodiment will now be described with reference to FIG. 6. Again, parts already described above with reference to FIG. 1 will carry identical reference numerals and will not be again described here. Also their function and operation will be in accordance with the description above.
In the embodiment of FIG. 6, an overhead condenser is provided in discharge line 16, in the form of an overhead heat exchanger 14. The heat exchanger 14 has at least one supply of refrigerant 30 and one removal of spent or vaporized refrigerant 31. The heat exchanger 14 may be a dedicated heat exchanger or an integrated heat exchanger that also provides cooling for other duties. Discharge line 16 fluidly connects a downstream outlet of heat exchanger 14 to a separator 27. Separator 27 has a condensate outlet 35 discharging into line 15 and a vapour outlet 33 discharging into line 13. Line 15 can be directly connected to the scrub column 10 via third feed point 7 c and line 18. In FIG. 6 an optional reflux pump 19 is provided between line 15 and line 18.
The overhead condenser 14 and separator 27 may also be integrated into one housing or into one piece of equipment wherein the functions are combined.
In operation, the embodiment of FIG. 6 works as follows. The top product overhead stream that is being discharged from the scrub column 10 through line 16 is led to the overhead condenser 14 where it is partially condensed using a refrigerant. The partially condensed forms a mixed phase stream of vapour and condensate, which is led to the separator 27. The vapour that is discharged from the separator 27 via line 13 is the natural gas from which heavier hydrocarbons have been sufficiently removed and which is to be liquefied to obtain LNG. The condensate in the form of condensed liquid is drawn from the mixed phase stream to obtain the scrub stream 18, or to add to another scrub stream, that is supplied to the scrub column 10. The reflux pump 19 may be employed to bring the liquid to a desired pressure level.
An advantage of the embodiment of FIG. 6 is that it allows freedom in choosing the temperature of the second substream 3 b because the tray number (corresponding to a height in the distillation column 10) at which the second substream 3 b is fed into the distillation column 10 can be chosen. Thus, without constraint, the temperature of the second substream in line 7 can be chosen to optimise the refrigeration cycle. The temperature profile in the bottom part of the scrub column 10 and the temperature of the bottom product discharged via outlet 8 and line 17 can be optimally controlled by selecting or controlling the split ratio. An advantage of the embodiments of FIGS. 2 to 5 is that these avoid the use of the reboiler in the form of overhead separator 27 and/or the reflux pump 19.
It will be understood that the embodiment of FIG. 6 can be combined with one of FIGS. 2 to 5.
In all the embodiments described above, the third substream forms a major fraction of the second substream or more than half of the original second substream as split in feed stream junction 2. The third substream is typically cooled to a temperature lower than −10 degrees Celsius and not lower than −100 degrees Celsius. Preferably the third substream is cooled to a temperature lower than −30 degrees Celsius. Preferably the third substream is cooled to a temperature not lower than −60 degrees Celsius. This third substream is then entered into the scrub column 10 at the third feed point 7 c.
In FIG. 7 still another embodiment of the invention is schematically depicted. In comparison with the embodiment of FIG. 1, even fewer equipment items are required because the function of the third feed point 7 c is now taken over by the second feed point 7 b. To this end, the second feed point 7 b is provided in the vicinity of the top of the scrub column 10 where normally would be the scrub stream inlet. Thus no specific reflux equipment is required. The heat exchanger in the second branch is here depicted by a plurality of heat exchangers 6 and 6′ operating in series of each other. It will be understood that the heat exchanger can be provided in the form of a single piece of equipment.
Before the second substream in second branch 3 b is fed into line 7, it is cooled to a temperature low enough to form a liquid/vapour mixture. The temperature is typically lower than −10 degrees Celsius and not lower than −60 degrees Celsius. Preferably the second substream is cooled to a temperature lower than −30 degrees Celsius. Preferably the third substream is cooled to a temperature not lower than −60 degrees Celsius.
An advantage of the embodiments of FIGS. 2 to 5 and of the embodiment of FIG. 7 over the embodiment of FIG. 6 is that the flow rate through the second heat exchanger 26 or the second part 6′ of the heat exchanger is lower than the flow rate through overhead condenser 14, because part of the natural gas is sent to the scrub column without passing the second heat exchanger 26 or the second part 6′.

COMPARATIVE EXAMPLES

FIG. 8 represents a comparative example wherein the feed stream in feed streamline 1 is not split into substreams, but optionally cooled in heat exchanger 6 prior to feeding into the scrub column 10 via feed point 7 d. Feed point 7 d can be in or near the bottom of the scrub column, or somewhat higher than feed point 7 a.
Mass and energy balance calculations were performed in relation to the process flow schemes shown in FIGS. 6, 7, and 8, for a typical feed gas and typical ambient conditions.
In the process of FIG. 8, a relative power (including end-flash power over production) of 13.1 kW/tpd is calculated to result in a content of C₅+ in the stream in line 13 of 0.03 mol. %.
In the process of FIG. 7, the split ratio was set to 8% so that the major part of the feed stream was led through the heat exchangers 6 and 6′. The temperature of the second substream in line 7 was lowered to about −20 degrees Celsius. The calculated relative power (including end-flash power over production) is 13.1 kW/tpd, whereby the content of C₅+ in the stream in line 16 is 0.06 mol. %.
Thus, the splitting of the feed stream gives the option of getting rid of the components for generating a reflux stream, such as the overhead separator 27 and/or the reflux pump 19, at the cost of only a slightly worse separation. At the same time, an improved control over the temperature gradient in the scrub column 10 is achieved, and the material flow in the bottom of the scrub column 10 is strongly reduced so that it can be made slimmer.
In the process of FIG. 6, the split ratio was chosen 6%. The same separation was achieved as in the process of FIG. 7 (C₅+ content in line 13 of 0.06 mol. %), but using a relative power of 12.9 kW/tpd which represents a reduction of 1.5%. In view of the large amounts of product to be processed, a reduction in power consumption of 1.5% is a significant improvement. This reduction in power consumption offsets the additional cost of providing the reflux equipment. Comparing to the process of FIG. 8, improved control over the temperature gradient in the scrub column 10 is achieved, and the material flow in the bottom of the scrub column 10 is strongly reduced so that it can be made slimmer.

Claims

1. A method of producing a liquefied natural gas stream, wherein before liquefaction heavier hydrocarbon components having a molecular weight higher than that of butane are removed from a natural gas stream to be liquefied, the method at least comprising the steps of:

providing a substantially vaporous feed stream of natural gas at a feed stream pressure and a feed stream temperature;

feeding the feed stream into a distillation column having two or more separation stages;

drawing a bottom stream from a lower part of the distillation column and an overhead stream from an upper part of the distillation column, the overhead stream containing a lower relative amount of the heavier hydrocarbon components than the bottom stream; and

liquefying at least a part of the overhead stream thereby obtaining a liquefied natural gas stream;

wherein the step of feeding the feed stream into the distillation column comprises the sub steps of:

splitting the feed stream into first and second substreams at a selected split ratio;

feeding the first substream into the distillation column via a first feed point at the bottom of the distillation column below the lowest separation stage thereof, at a pressure not lower than the feed stream pressure minus a pressure drop brought about by the said splitting of the feed stream, wherein the first substream is not warmed between splitting the feed stream and feeding of the first substream at the first feed point into the distillation column;

cooling the second substream in a heat exchanger to a lower temperature than the feed temperature;

feeding the cooled second substream into the distillation column at a second feed point overhead of the first feed point;

wherein the pressure of neither the first substream nor the second substream is lowered in a dedicated pressure lowering device.

2. The method according to claim 1, wherein the substantially vaporous feed stream comprises >90 vol. % vapour.

3. The method according to claim 1, wherein the pressure drop in the first substream between the splitting of the feed stream and the first feed point is <6 bar.

4. The method according to claim 1, wherein the selected split ratio is kept smaller than 1, wherein the split ratio is defined as the mass flow rate of the first substream divided by the mass flow rate of the second substream.

5. The method according to claim 4, wherein the selected split ratio is kept higher than 1/100.

6. The method according to claim 1, wherein the second substream is cooled in the heat exchanger against an external refrigerant.

7. The method according to claim 1, wherein the distillation column is provided in the form of a scrub column wherein a scrub stream is fed to the scrub column via a third feed point overhead of the second feed point, at a temperature that is lower than that of the cooled second substream.

8. The method according to claim 7, wherein the scrub stream is substantially liquid.

9. The method according to claim 7, wherein the scrub stream is obtained by:

drawing a fraction from the second substream to form a third substream, wherein the residue carries on as the second substream;

cooling the third substream in a second heat exchanger to form the scrub stream.

10. The method according to claim 7, wherein the scrub stream is obtained by:

partly condensing the overhead stream to form a mixed phase stream of vapour and condensate, drawing the condensate from the mixed phase stream to obtain the scrub stream.

11. The method according to claim 10, wherein the part of the overhead stream that is used as the scrub stream is not expanded between the scrub column and the third feed point.

12. The method according to claim 7, wherein the temperature of the scrub stream is sufficiently low whereby a condensate of heavier hydrocarbons is formed.

13. An apparatus for producing a liquefied natural gas stream, wherein before liquefaction heavier hydrocarbon components having a molecular weight higher than that of butane can be removed from a natural gas stream to be liquefied, the apparatus at least comprising:

a feed stream line for carrying a substantially vaporous feed stream of natural gas at a feed pressure and a feed temperature;

a distillation column having two or more separation stages for separating the heavier hydrocarbon components from the natural gas, a bottom stream discharge opening arranged in a lower part of the distillation column for discharging a bottom stream, and an overhead stream discharge opening arranged in an upper part of the scrub column for discharging an overhead stream containing a lower relative amount of the heavier hydrocarbon components than the bottom stream; and

a cryogenic zone in which at least a part of the overhead stream can be liquefied thereby obtaining a liquefied natural gas stream;

wherein the feed stream line comprises a feed stream junction fluidly connecting a main branch with first and second branches to split the feed stream into first and second substreams at a selected split ratio, wherein the first substream connects the feed stream junction with a first feed point at the bottom of the distillation column at a pressure not lower than the feed stream pressure drop brought about by the splitting of the feed stream, and wherein the second substream connects the feed stream junction with a second feed point in the distillation column, the second feed point being overhead relative to the first feed point, wherein the second substream is provided with a heat exchanger;

wherein the first feed point is situated below the lowest separation stage of the distillation column;

wherein the first substream is not provided with a heat exchanger for heating the first substream; and

wherein no dedicated pressure lowering device is present for lowering the pressure of the first substream and the second substream.

14. An apparatus according to claim 13, wherein the distillation column is in the form of a scrub column, the apparatus further comprising a scrub stream source connected to the scrub column via a third feed point in the scrub column overhead of the second feed point.

15. An apparatus according to claim 14, wherein the scrub stream source comprises a second junction provided in the second substream upstream of the second feed point to feed a third substream out of the second substream, wherein the third substream is provided with a second heat exchanger.

16. An apparatus according to claim 14, wherein the scrub stream source comprises a condenser provided to receive the overhead stream downstream the scrub column in cooperation with a separator having a condensate outlet and a vapour outlet, wherein the condensate outlet is connected to the scrub column via the third feed point.

17. An apparatus according to claim 14, wherein between the overhead stream discharge opening and the third feed point no expander is present.