BRPI0518464B1 - METHOD AND APPARATUS FOR PRODUCING A LIQUID NATURAL GAS CURRENT - Google Patents

Abstract

method and apparatus for producing a stream of liquefied natural gas. a method and apparatus for producing a gnl stream, wherein prior to liquefaction heavier hydrocarbon components are removed from a gn stream to be liquefied, the method at least comprising the steps of: providing a gas vapor supply stream (1) Natural; feeding the feed stream (1) into a distillation column (10); removing a bottom stream (17) and a top stream (16) from the distillation column (10); liquefying at least a portion of the top stream (16) thereby obtaining a gnl stream; wherein the step of feeding the feed stream (1) to the distillation column (10) comprises the sub-steps of: dividing the feed stream (1) into first (3a) and second (3b) sub-streams; feed the first sub stream (3a) into the distillation column (10) through a first feed point (7a) at a pressure not less than the feed stream pressure minus a pressure drop brought about by said division of the feed stream. feeding (1); cool the second sub stream (3b); and feeding the second cooled sub stream (7) to the distillation column (10) at a second feed point (7b) above 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 comprising primarily methane (preferably> 90 ° C). mol%). Background of the Invention It is normal practice to liquefy natural gas so that it can be carried by a ship when other means of transport are unavailable or less attractive. Natural gas liquefaction allows a significant reduction in its volume, which makes transportation much more efficient. To produce liquefied natural gas (LNG) a liquefaction process is employed. The liquefaction process typically comprises a cryogenic zone containing one or more refrigeration cycles, in which natural gas is cooled in one or more stages from ambient temperature to the ambient boiling point of natural gas, or something lower. This boiling point is usually around minus 160 ° C.

Refrigeration cycles generally make use of a refrigerant which may be formed of a mixture or a pure constituent. Refrigerant is typically vaporized in one or more cryogenic heat exchangers, in which natural gas is cooled. The vaporized refrigerant is then compressed to a higher pressure level and temperature. In an ambient chiller, heat from the refrigerant is discarded into a cooling medium such as water or air and then cooled by expansion. It is very common in multi-cycle liquefaction processes that consecutive refrigeration cycles are cooled by the first refrigeration cycle.

In ordinary liquefaction processes, it is also common to remove certain components of natural gas before it is cooled in the cryogenic heat exchanger. Components that should normally be removed include carbon dioxide, sulfur and sulfur-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 natural gas as they could otherwise be taken up in solids at the cynogenic temperatures at which liquefaction is performed.

Normally the raw natural gas stream is first decontaminated from water and acid gases, for which there are numerous physical and / or chemical processes. The resulting stream of "sweetened" and dried natural gas mixture is then subjected to a step of removing the heavier hydrocarbons. Removal of heavier hydrocarbons is generally accomplished by partially condensing the natural gas mixture followed by some separation between a heavier hydrocarbon poor vapor phase and a heavier hydrocarbon rich liquid phase. The most common is to use a purification column to achieve this separation. A purification column is a type of distillation column that comprises a series of stages of separation between a bottom end and a top end, whereby, a heavier hydrocarbon enriched mixture is discharged from the bottom end in the form of a bottom stream, and a lighter mixture of natural gas is discharged from the top end in the form of a top stream.

Description of the Prior Art U.S. Patent 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.

Several embodiments of a purification method and column are known to pretreat a natural gas stream using a single purification column to remove freezable C5 + components and provide a LNG product are described in U.S. Patent 5,325,673.

In one embodiment, the natural gas feed stream is divided into several sub feed streams, cooled and fed into the purification column at different feed points near the top and middle of the purification column. A first part of the natural gas is expansion cooled in a Joule-Thompson valve and introduced into the purification column at reduced pressure. Second and third parts are first cooled in a cooling chiller, separated into a condensed liquid stream and a vapor stream before pressure reduction. The condensed liquid stream is fed to the purification column at a lower feed point than the steam stream.

A referrer is provided for vaporizing a fraction of the enriched bottom stream of heavier hydrocarbons or accumulated liquid at the bottom end of the purifying column. A referrer also serves to control the temperature at the bottom end of the purification column to ensure that the bottom end does not become too cold, thereby carrying the risk of unwanted components such as carbon dioxide accumulating in the bottom stream. . The known configuration has numerous disadvantages. Primarily reducing the pressure of the feed sub streams before feeding to the purification column reduces the throughput of a later liquefaction step, since lower pressure natural gas liquefaction requires more energy.

A disadvantage of a referrer is that it adds heat to natural gas although by the very nature of the liquefaction process natural gas should be cooled. Using a referrer adversely affects the overall performance of the liquefaction process.

Summary of the Invention It is an object of the present invention to minimize one or more of the above disadvantages. It is another object of the present invention to provide an alternative method of producing a liquefied natural gas stream comprising mainly liquefied methane.

One or more of the above or other objects are achieved according to the present invention by providing a method for producing a liquefied natural gas stream in which prior to liquefaction heavier hydrocarbon components having a higher molecular weight 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 natural gas vapor vapor feed stream at a feed stream pressure and a feed stream temperature. ; feeding the feed stream to a distillation column that has two or more stages of separation; withdrawing a bottom stream from a bottom of the distillation column and a top stream from a top of the distillation column, the top stream containing a relatively lower amount of the heavier hydrocarbon components than the bottom; and liquefying at least a portion of the top stream, thereby obtaining a liquefied natural gas stream, wherein the step of feeding the feed stream to the distillation column comprises the sub-steps of: dividing the feed stream first and second sub currents in a selected division ratio; feeding the first sub stream to the distillation column through a first feed point at the bottom of the distillation column at a pressure not less than the supply stream pressure minus a pressure drop brought about by said division of the supply stream; cool the second subcurrent in a heat exchanger to a temperature lower than the supply temperature; feed the second cooled subcurrent to the distillation column at a second feed point above the first feed point.

For the purpose of this specification the division ratio is defined as the mass flow of the first sub current divided by the mass flow of the second sub current.

An advantage of the invention is that neither the pressure of the supply stream nor that of the first and second sub currents is deliberately reduced in a dedicated pressure reducing device, such as an expansion tube or a Joule-Thompson valve.

Since the first sub-current is fed to the distillation column at a pressure essentially no less than the supply current pressure minus a pressure drop brought about by the division of the supply current, the pressure at the second supply point need not be decreased. . Thus, distillation is performed without significantly reducing the pressure of natural gas, which will be energetically beneficial in case the top stream is to be liquefied.

Another consequence of not deliberately reducing the pressure in the first subcurrent is that the temperature can be kept close to the supply current temperature; preferably no heating of the first sub current is provided. An advantage of this is that less additional heating energy, usually provided for example by means of a referrer, must be at the bottom end of the distillation column to prevent it from becoming too cold.

By selecting the sufficiently high division ratio no additional heating energy even needs to be added at all, so that no referrer needs to be provided for the purpose of controlling the bottom temperature.

It has been found that the division ratio can be selected such that the temperature at the bottom of the distillation column is kept at 10 ° C or higher. The temperature at the bottom end of the distillation column can be controlled by providing a selectable or variably controllable split ratio and selecting or controlling the split ratio. The invention is also embodied in an apparatus for producing a liquefied natural gas stream in which prior to liquefaction heavier hydrocarbon components having a higher molecular weight than that of butane may be removed from a natural gas stream to be liquefied, the apparatus comprising at least: a feed stream line for charging a substantially natural gas vapor feed stream at a feed pressure and feed temperature; a distillation column having two or more stages of separation for separating the heavier hydrocarbon components from natural gas, a bottom stream discharge opening arranged in a lower part of the distillation column to discharge a bottom stream, and an opening top stream discharge arranged on an upper part of the purification column to discharge a top stream containing a relatively lower amount of the heavier hydrocarbon components than the bottom stream; and a cryogenic zone in which at least a portion of the top stream may be liquefied, thereby obtaining a stream of liquefied natural gas; wherein the supply current line comprises a supply current junction that directly connects a main branch with first and second branches to divide the supply current into first and second sub currents in a selected division relationship, wherein the The first branch connects the junction of the feed stream with a first feed point at the bottom of the distillation column at a pressure no lower than the drop in pressure of the feed stream brought about by the feed current split, and in which the second The branch connects the junction of the feed stream with a second feed point in the distillation column, the second feed point being above the first feed point, at which 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 reel type.

In its broadest definition, the invention is applied to any type of distillation column that is suitable for removing heavier hydrocarbon components having higher molecular weight than that of butane from a hydrocarbon gas mixture. However, one or more preferred embodiments of the invention require a purification stream to be fed to the column. In this case the distillation column is by definition in the form of a so-called purification column.

These and other aspects of the invention will be elucidated below by way of example and with reference to the accompanying non-limiting design. Brief Description of the Drawings In the accompanying drawing: Figure 1 shows schematically a process gram flow scheme according to a first embodiment of the invention; Figure 2 shows schematically a process flowchart according to a second embodiment of the invention Figure 3 shows schematically a process flowchart according to a third embodiment of the invention; Figure 4 shows schematically an alternative process flowchart according to the third embodiment of the invention; Figure 5 shows schematically an alternative process flowchart according to the second embodiment of the invention; Figure 6 shows schematically a process flowchart according to a fourth embodiment of the invention; Figure 7 shows schematically a process flowchart according to a fifth embodiment of the invention; and Figure 8 shows schematically a process flowchart in which the supply current is not divided.

For the purpose of this description, a single reference numeral will be assigned to a line as well as a current loaded on that line. The same reference numerals refer to similar components.

Detailed Description of the Configurations Figure 1 schematically shows a process flowchart involving a system for removing heavier hydrocarbon components having a higher molecular weight than that of butane or a hydrocarbon gas mixture as part of an apparatus for produce a LNG stream that primarily comprises methane. For the purpose of this specification, the hydrocarbon gas mixture will be admitted to be formed from a previously treated natural gas mixture to remove water, CO2, and sulfur by methods well known in the art. Generally, the pretreated natural gas mixture will contain lighter components that include methane and steam ethane that include C3 and C4, and heavier C5 + components that are potentially freezable during methane liquefaction. The apparatus of Figure 1 includes a natural gas feed line that is arranged to receive and carry a hydrocarbon gas feed stream from which the heavier hydrocarbons must be removed. The feed line is divided into a main branch 1 and first and second subcurrent branches 3a and 3b, respectively. A feed stream junction 2 is provided for dividing the feed stream in main branch 1 into first and second sub currents which may respectively flow through first and second sub stream branches 3a and 3b. The feed current junction 2 is arranged to divide the feed current according to a specified split ratio, which is defined as the mass flow of the first sub current divided by the mass flow of the second sub current. One skilled in the art will understand that the supply current junction 2 is not a phase separator but instead divides the main supply current into two or more sub currents.

Both the first and second subcurrent branches 3a and 3b are in direct communication with a purification column 10. Purification column 10 in the present configuration is a distillation column provided with a number of trays 11 to allow separation of hydrocarbon components. heavier hydrocarbon components in a plurality of separation stages. The temperature in the purification column typically varies, becoming colder with each higher stage. The purification column 10 further comprises a bottom stream discharge opening 8 in a lower part of the purification column 10 for discharging an enriched background stream into the heavier hydrocarbon components, for example through the discharge line 17, and a top stream discharge opening 12 arranged on an upper part of the purification column 10 to discharge for example through discharge line 16 a top stream enriched with lighter hydrocarbon components. The top stream 16 is connected with a zone (not shown) to produce LNG. The first branch 3a connects the junction of the feed stream 2 to a first feed point 7a in the purification column 10. The first feed point 7a is relatively close to the bottom of the purification column 10 to feed the first sub current 3a into a from lower trays 11. Preferably the sub stream 3 is olimAntci Α * λ míiiç mfpTinr 11 Δ UlilllWll LLA. UUUUVU UU L'UJ.AU.VJU VtV See i.uuv u Ai-AWi iii-J-VA A va j. The first branch 3a is essentially free of pressure reducing devices, so that the supply current junction 2 is directly connected to the first supply point 7a essentially without pressure loss. Preferably the connection is sized such that the pressure loss does not exceed 5 bar, more preferably should not exceed 2 bar under normal operating conditions. In addition, the first subcurrent 3a is not heated. The second branch 3b connects the junction of the power supply 2 with a second power point 7b in the purification column 10. The second power point 7b is situated above the first power point 7a to feed the first sub current into a above the lower trays. The second branch 3b is provided with a heat exchanger 6 which divides the second branch 3b into a hot part 3b and a cold part 7. The heat exchanger 6 is arranged to cool the second sub stream 3b essentially without deliberately reducing pressure. The heat exchanger 6 may be any suitable type of heat exchanger such as a so-called reel heat exchanger.

Under normal operating condition the pressure drop in the second subcurrent is less than 6 bar, preferably less than 3 bar. Heat exchanger 6 has at least one refrigerant supply 4 and a spent or vaporized refrigerant removal 5. Heat exchanger 6 may be a dedicated heat exchanger or an integrated heat exchanger, which also provides cooling for other services. Preferably the heat exchanger 6 utilizes an external coolant with the heat exchanger 6 having a dedicated heat exchanger.

Although not required by the invention, which relates more to the division of the feed stream into the first and second sub streams 3a, 3b, in the embodiment of Figure 1 a third feed point 7c is advantageously provided in the purification column 10. The third Feed point 7c is situated near the top-end darxokmarrie purification 10 above the second feed point 7b. An optional purification current line 18 connects the third power point 7c with a purification current source. The purification stream source serves to supply another liquid or multiphase stream capable of purifying heavier hydrocarbons and to transport below those in purification column 10. The purification stream may contain one or more of the group consisting of natural gas. cooled, condensate from a top condenser, LNG, cooled LPG, cooled condensate, mixtures thereof or any other stream having the appropriate properties to promote the removal of heavier hydrocarbons from natural gas.

In operation, the apparatus of Figure 1 operates as follows. A feed stream of the pretreated hydrocarbon gas mixture is supplied across line 1 at a feed stream pressure and a feed stream temperature. The supply current pressure is generally between 20 and 80 bar and more typically between 40 and 65 bar. The temperature of the feed stream is generally between 0 and 50 ° C, typically between 15 and 25 ° C, more typically between 15 and 20 ° C. The feed current is divided into first and second sub currents 3a, 3b at the feed current junction 2, preferably in the form of smaller and larger sub currents. The minor subcurrent 3a is fed to the purification column 10 through the first supply point 7a at a pressure that is not less than the supply chain pressure minus a pressure drop brought by the division of the supply chain 1 at the chain junction. 2. In practice this means that the pressure in the minor subcurrent 3 a is not deliberately reduced. The second subcurrent 3b, usually the largest current, is cooled at the heat exchanger-6 stop-a temperature lower than the supply temperature. Broadly the larger subcurrent 3b is cooled to a temperature of not less than 50 ° C, preferably not less than 20 ° C. Preferably the larger sub current 3b is cooled to a temperature of minus 10 ° C or lower. The cooled larger sub stream is fed through the second feed point 7b to the cold part 7 of the second sub branch 3b for the purification column 10 at a location above which the smaller sub current 3a is fed to the purification column 10. A The optional purification stream in purification line 18 has a temperature less than or equal to that of the second sub stream that enters through the second feed point 7b. The relatively cold larger under-current 7 together with the relatively warm under-current 3a contribute to maintaining the desired temperature gradient within the purification column 10. It is possible to control the division ratio between the largest and smallest under-currents. Thus, the temperature gradient and / or the temperature at the bottom of the purification column 10 can be controlled. It has been found that the split ratio is preferably chosen less than 1/5 to ensure that the temperature in the purification column is sufficiently low to achieve efficient separation of heavier hydrocarbon components from the mixture. More preferably, the chosen split ratio is less than 1/10.

It has been found that the division ratio is preferably chosen higher than 1/100 to achieve a beneficial effect in reducing the external heat demand required to maintain the bottom temperature of the purification column higher than minus 10 ° C. Preferably the division ratio is chosen higher than 1/50, so that the need for a referrer can be entirely offset. Thus, in a preferred embodiment there is present, as a result of which, no further boiling takes place between the top stream discharge opening 12 and the third feed point 7c.

Preferably the purification stream 18 is fed to the purification column 10 through the third feed point 7c above the second feed point 7b. The temperature of the purification stream 18 is typically lower than that of the smaller cooled substream and usually between minus 70 and minus 10 ° C. This further helps to maintain the desired temperature gradient within the purification column 10. The top product 16 is taken from the purification column 10 through the discharge opening of the top stream 12, which is the natural gas from which hydrocarbons Heavier weights have been removed to a sufficient extent. Stream 17 is a heavier hydrocarbon-enriched bottom product than that which is discharged through the discharge port 8. In particular, top product 16 is a low-grade natural gas vapor stream. heavier hydrocarbons, which meets the requirements to prevent solids from forming during additional cooling of the natural gas vapor stream sparingly for liquefaction in a cynogenic zone (not shown). As one of ordinary skill in the art will readily understand how to liquefy a top end product in a cynogenic zone, for example using heat exchangers, this is no longer discussed here. Background 17 may find any application, one of which is further processing it to form liquefied petroleum gas (LPG).

Figures 2 to 5 schematically outline alternative process flowcharts involving alternative apparatuses. In these Figures, parts already described above with reference to Figure 1 will carry identical reference numerals and will not be described again here. Also its function and operation will be accordingly. With the description.

Figures 2 to 5 show configurations in which purification stream 18 is at least withdrawn from feed stream 1.

Starting with Figure 2, a major difference with the configuration of Figure 1 is reflected by the presence of a second supply chain junction 20 provided in the second branch 7 upstream of the second supply point 7b and downstream of the heat exchanger 6. Second branch 7 continues downstream of the feed stream junction 20 and a third branch 22 is formed to carry a third feed stream sub-stream 1. The third branch 22 is provided with a second heat exchanger 26, whose side of downstream is connected to the purification stream line 18. The second heat exchanger 26 is arranged to further cool the third sub stream 22 to a lower temperature than that of the second sub stream essentially without deliberately reducing pressure. Under normal operating condition the pressure drop in the third subcurrent 22 is less than 6 bar, preferably less than 3 bar. As shown in Figure 2, at least one refrigerant supply 24 is provided to feed to the second heat exchanger 26, wherein removal of spent or vaporized refrigerant 26 may form the refrigerant supply 4 to feed the first mentioned heat exchanger. 6

Alternatively, first and second heat exchangers are each supplied independently, with at least one refrigerant supply and removal. The second heat exchanger 26 may be a dedicated heat exchanger or an integrated heat exchanger that also provides cooling for other services.

Referring now to Figure 3, an alternative to Figure 2 is shown schematically, in which the second junction of the feed stream 20 is provided at the second branch 3b upstream of the first heat exchanger 6. The second heat exchanger 26 is provided. in parallel relationship with the first heat exchanger 6 instead of the series arrangement of Figure 2. The second branch 3b continues downstream of the second junction of the feed stream 20 and the third branch 22 is formed to carry the third sub stream 1. As before in Figure 2, the downstream side of the second heat exchanger is connected to the purification current line 18.

First and second heat exchangers 6, 26 each have at least one supply of refrigerant 4, 24 and removal of spent refrigerant 5, 25 individually.

The first and second heat exchangers 6, 26 may be combined in a housing, whereby the refrigerant may be operational at a pressure level.

Referring now to Figure 4, there is shown schematically an example based on parallel cooling of the second and third sub currents, whereby the first and second heat exchangers are integrated in a housing, each represented by a path of flow. Figure 5 shows an example of an integrated heat exchanger that configures series cooling of the configuration of Figure 2. In this example the second supply current junction 20 is located outside the heat exchanger housing, so the second and third Branches may be left outside and inside the heat exchanger housing. Alternatively, although currently considered less practical, the supply chain junction 20 may be located within the heat exchanger housing.

Thus, in the configurations of Figures 2 to 5, the purification current source that is connected to the purification column 10 through the third feed point 7c above the second feed point 7b comprises the second feed current junction 20 and the second heat exchanger 26.

In operation the apparatus of Figures 2 to 5 work similarly to that of Figure 1. However, the purification stream in line 18 is obtained by withdrawing a fraction from the second sub stream 3b to form a third sub stream. The residue is charged as the second sub stream 3 b. The third subcurrent is cooled in the second heat exchanger 26 downstream of the second supply current junction 20 to a temperature that is lower than that of the second subcurrent once it has been cooled by the first heat exchanger 6.

Yet another embodiment will now be described with reference to Figure 6. Again parts already described above with reference to Figure 1 will carry identical reference numerals and will not be described again here. Also its function and operation will be as described above.

In the configuration of Figure 6 a top condenser is provided in the discharge line 16 in the form of a top heat exchanger 14. Heat exchanger 14 has at least one refrigerant supply 30 and a spent or vaporized refrigerant removal 31.0 exchanger Heat exchanger 14 can be a dedicated heat exchanger or an integrated heat exchanger that also provides cooling for other services. Discharge line 16 directly connects a downstream outlet from heat exchanger 14 to a separator 27. Separator 27 has a condensate outlet 35 discharging on line 15 and a steam outlet 33 discharging on line 13. Line 15 can be connected directly to the purification column 10 through the third feed point 7c and line 18. In Figure 6 an optional reflux pump 19 is provided between line 15 and line 18. Top condenser 14 and separator 27 also can be integrated into a housing on a piece of equipment in which the functions are combined.

In operation the configuration of FIG. The top stream of top product being discharged through the purification column 10 through line 16 is conducted to the top condenser 14 where it is partially condensed using a refrigerant. The partially condensed forms a mixed phase stream of steam and condensate which is conducted to separator 27. Steam that is discharged from separator 27 through line 13 is the natural gas from which heavier hydrocarbons have been sufficiently removed and which must be liquefied to to obtain LNG. Condensate in the form of condensed liquid is withdrawn from the mixed phase stream to obtain the purification stream 18, or to add to another purification stream that is supplied to the purification column 10. The reflow pump 19 may be employed. to remove the liquid to a desired pressure level.

An advantage of the configuration of Figure 6 is that it allows freedom in choosing the temperature of the second sub stream 3b since the tray number (which corresponds to a height in the distillation column 10) on which the second sub stream 3b is fed to. distillation column 10 may be chosen. Thus, without restriction, the temperature of the second subcurrent in line 7 can be chosen to optimize the refrigeration cycle. The temperature profile at the bottom of the purification column 10 and the temperature of the background product discharged through outlet 8 and line 17 can be optimally controlled by selecting or controlling the split ratio. An advantage of the configurations of Figures 2 to 5 is that they avoid using the referrer in the form of the top separator 27 and / or the backflow pump 19.

It will be appreciated that the configuration of Figure 6 may be combined with one of Figures 2 to 5.

In all configurations described above the third sub-current forms a larger fraction of the second sub-current or more than half of the original sub-current when divided at the junction of the supply current 2. The third sub-current is typically cooled to a temperature below minus 100 ° C and not less than minus 100 ° C. Preferably the third subcurrent is cooled to a temperature below minus 30 ° C. Preferably the third subcurrent is cooled to a temperature of not less than 60 ° C. This third sub stream is then introduced into the purification column 10 at the third feed point 7c.

In Figure 7 yet another embodiment of the invention is schematically outlined. Compared to the configuration of Figure 1, even fewer equipment items are required as the function of the third feed point 7c is now assumed by the second feed point 7b. For this purpose the second feed point 7b is provided in the vicinity of the top of the purification column 10, where the purification current would normally be input. Thus, no specific reflow equipment is required. The heat exchanger in the second branch is here outlined by a plurality of heat exchangers 6 and 6 'which operate in series with each other. It will be appreciated that the heat exchanger may be provided in the form of a single piece of equipment.

Before the second sub stream in the second branch 3b is fed to line 7, it is cooled to a temperature low enough to form a liquid / vapor mixture. The temperature is typically below minus 10 ° C and not below minus 60 ° C. Preferably the second sub stream is cooled to a temperature below minus 30 ° C. Preferably the third subcurrent is cooled to a temperature of not less than 60 ° C.

An advantage of the configurations of Figures 2 through 5 of the configuration of Figure 7 over the configuration of Figure 6 is that the flow through the second heat exchanger 26 or the second heat exchanger part 6 'is less than that which is lower than flow through the top condenser 14 as part of the natural gas is sent to the purification column without passing through the second heat exchanger 26 or the second part 6 '.

Comparative Examples Figure 8 represents a comparative example in which the supply current in supply current line 1 is not divided into sub currents, but optionally is cooled in heat exchanger 6 before feeding to purification column 10 through power point 7d. Feed point 7d may be at or near the end of the purification column or somewhat higher than feed point 7a. Mass and energy balance calculations were performed against the process flow charts shown in Figures 6, 7 and 8 for ambient conditions from a typical gas feed to a typical gas feed and typical environmental conditions.

In the process of Figure 8 a relative energy (which includes final vaponization energy over production) of 13.1 kw / tpd is calculated to result in a C5 + content in line 13 of 0.03 mol%.

In the process of Figure 7, the division ratio was set to 8% so that the largest part of the feed stream was left through the heat exchangers 6 and 6 '. The temperature of the second subcurrent in line 7 has been reduced to approximately minus 20 ° C. The calculated relative energy (including final vaporization energy over production) is 13.1 kw / tpd, so the C5 + content in the current at line 16 is 0.06 mol%.

Thus, the division of the feed stream provides an option to get rid of the components to generate a backflow, such that the top separator 27 and / or the backflow pump 19, at the cost of only slightly worse separation. At the same time, improved control over the gradient 4 and temperature in the purification column 10 is achieved and the material flow at the bottom of the purification column 10 is greatly reduced so that it can be made more slender.

In the process of Figure 6 the division ratio was chosen 6%. The same separation was achieved as in the process of Figure 7 (C5 + content at line 13 of 0.6 mol%), but using a relative energy of 12.9 kw / tpd which represents a 1.5% reduction. In view of the large quantities of product being processed, a 1.5% reduction in power consumption is a significant improvement. This reduction in energy consumption offset the additional cost of providing reflow equipment. Comparing to the process of Figure 8, improved control over the temperature gradient in the purification column 10 is achieved and the material flow at the bottom of the purification column 10 is greatly reduced so that it can be made more slender.

Claims (17)

1. Method for producing a liquefied natural gas stream, wherein prior to liquefaction heavier hydrocarbon components having a higher molecular weight than that of butane are removed from a liquefied natural gas stream, the method characterized by the fact that at least comprising the steps of: providing a feed stream of more than 90% by volume of natural gas vapor (I) at a feed stream pressure and a feed stream temperature; feeding the feed stream (1) to a distillation column (10) which has two or more separation stages (11); withdrawing a bottom stream (17) from a bottom of the distillation column (10) and a top stream (16) from a top of the distillation column (10), the top stream (16) having a slightly lower amount of the hydrocarbon components heavier than the background stream (17); and liquefying at least a portion of the top stream (16) thereby obtaining a liquefied natural gas stream, wherein the step of feeding the feed stream (1) to the distillation column (10) comprises the steps of : dividing the supply current (1) into first (3a) and second (3b) sub currents in a selected division ratio that must be kept less than 1 and higher than 1/100; feed the first subcomment (3a) to the distillation column (10) through a first feed point (7a) at the bottom of the distillation column (10) below its lowest separation stage (11) at a pressure not less than the supply chain pressure minus a pressure drop brought about by said supply chain division (1), wherein the first undercurrent (3a) is not heated between the supply chain (1) and the supply chain division of the supply chain. first undercurrent (3a) at the first feed point (7a) in the distillation column (10); cool the second sub stream (3b) in a heat exchanger (6) to a temperature lower than the supply temperature; feeding the second chilled sub stream (7) to the distillation column (10) at a second feed point (7b) above the first feed point (7a); wherein the pressure of neither the first undercurrent (3a) nor the second undercurrent (3b) is decreased in a dedicated pressure reducing device, Method according to Claim 1, characterized in that the steam supply stream (1) comprises more than 95% by volume of steam. Method according to either of Claims 1 or 2, characterized in that the pressure drop in the first subcurrent (3a) between the feed comment (1) and the first supply point (7a) is smaller. than 6 bar, preferably less than 3 bar, Method according to any one of claims 1 to 3, characterized in that the selected division ratio is kept less than 1/5, more preferably less than 1/10, in which the division ratio is defined as the ratio. mass flow of first subcomment (3a) divided by mass flow of second subcurrent (3b). Method according to claim 4, characterized in that the selected division ratio is kept higher than 1/50. Method according to any one of claims 1 to 5, characterized in that the second subcurrent (3b) is cooled in the heat exchanger (6) against a refrigerant and an external one. Method according to any one of claims 1 to 6, characterized in that the distillation column (10) is provided as a purification column in which a purification stream (18) is fed to the purification column. through a third supply point (7c) above the second supply point (7b) at a temperature which is lower than that of the second cooled sub current (7). Method according to claim 7, characterized in that the purification stream (18) is liquid. Method according to any one of claims 7 or 8, characterized in that the purification stream (18) is obtained by: removing a fraction from the second sub stream (3b) to form a third sub stream ( 22) wherein the residue charges as the second sub stream (3b); resin the third sub stream (22) on a second heat exchanger (26) to form the purification stream (18). Method according to either of claims 7 or 8, characterized in that the purification stream is obtained by: partially condensing the top stream (16) to form a mixed phase vapor and condensate stream, withdrawing the condensed from the mixed phase stream to obtain the purification stream (18). Method according to claim 10, characterized in that the portion of the top stream (16) which is used as the purification stream (18) is not expanded between the purification column and the third feed point (7c). ). Method according to any one of claims 7 to 11, characterized in that the temperature of the purification stream (18) is sufficiently low, whereby a heavier hydrocarbon condensate is formed. Apparatus for carrying out the method for producing a liquefied natural gas stream defined in claim 1, wherein prior to liquefaction heavier hydrocarbon components having a higher molecular weight than that of butane may be removed from a liquefied stream. natural gas to be liquefied, characterized in that it comprises at least: a feed stream line (1) for charging a natural gas steam feed stream (1) at a feed pressure and a feed temperature; a distillation column (10) having two or more separation stages (11) for separating the heavier hydrocarbon components from natural gas, a bottom stream discharge opening (8) arranged in a lower part of the distillation column ( 10) for discharging a bottom stream (17) and a top stream discharge opening (12) arranged on an upper part of the purification column (10) for discharging a top stream (16) containing a relatively higher amount. lower of the hydrocarbon components heavier than the bottom stream (17); and a cryogenic zone in which at least a portion of the top stream (16) may be liquefied thereby obtaining a stream of liquefied natural gas; wherein the supply chain line (1) comprises a supply chain junction (2) that directly connects a main branch with first and second branches to divide the supply chain into first (3a) and second (3b) sub currents in a selected split ratio, in which the first sub current (3a) connects the junction of the feed current (2) with a first feed point (7a) at the bottom of the distillation column (10) at a pressure not lower than the drop in supply chain pressure brought by the division of the supply chain (1) and in which the second substream (3b) connects the supply chain junction (2) with a second supply point (7b). ) in the distillation column (10), the second feed point (7b) above the first feed point (7a), in which the second sub stream (3b) is provided with a heat exchanger (6); and the first feed point (7a) is situated below the lowest separation stage (11) of the distillation column (10); and the first subcomment (3a) is not provided with a heat exchanger to heat the first subcurrent (3a); and no dedicated pressure reducing device is present to reduce the pressure of the first sub current (3a) and the second sub current (3b), Apparatus according to claim 13, characterized in that the fetus of the distillation column (10) is in the form of a purification column, the apparatus further comprising a purification current source connected to the purification column through a third point. (7c) on the purification column above the second feeder (7b), Apparatus according to claim 14, characterized in that the source of purification comment comprises a subsequent junction (20) provided on the second sub stream (3b) upstream of the second feed point to feed a third sub stream (22). ) outside the second subcomment (3b), in which the third subcurrent (22) is provided with a second heat exchanger (26). Apparatus according to either of claims 14 or 15, characterized in that the source of purification comment comprises a capacitor (14) provided for receiving the top comment (16) downstream of the purification column in cooperation with a separator (27) having a condensate outlet (33) and a steam outlet (35) in which the condensate outlet (33) is connected to the purification column through the third feed point (7c). Apparatus according to any one of claims 14 to 16, characterized in that there is no expander present between the top comment discharge opening (12) and the third feed point (7c).