Patent Application
20090314030
The present invention relates to a method and apparatus for liquefying a hydrocarbon stream, particularly but not exclusively natural gas.
Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at a high pressure.
In an article by Perez, V. entitled “The 4.5 MMTBA LNG Train—A Cost Effective Design”, published on 4 May 1998 from the International Conference and Exhibition on Liquefied Natural Gas, there is discussed plant designs for attaining a 4.5 MMTPA nominal capacity LNG train. This involves a set of heat exchangers for cooling a treated feed gas before entering a scrub column. The overhead vapour from the scrub column is then liquefied in a Main Cryogenic Heat Exchanger (MCHE) against a mixed refrigerant. The article states that the propane refrigerant system utilises propane evaporating at four pressure levels, the heat exchangers for which are shown in
It is an object of the present invention to improve the efficiency of a liquefying process and apparatus.
It is another object of the present invention to reduce the capital and running costs for a method and apparatus for liquefying a hydrocarbon stream.
One or more of the above or other objects can be achieved by the present invention providing a method of liquefying a hydrocarbon stream such as natural gas from a feed stream, the method at least comprising the steps of:
a) passing the feed stream through a first cooling stage having at least two heat exchangers, and against a first refrigerant in a first single component refrigeration circuit, to provide a cooled hydrocarbon stream;
b) passing the cooled hydrocarbon stream through a second cooling stage against a second refrigerant in a second refrigerant circuit, to provide a liquefied hydrocarbon stream;
c) passing the second refrigerant through one or more of the heat exchangers of the first cooling stage; wherein the heat exchangers of the first cooling stage are shell and tube heat exchangers having two or more tube circuits, the feed stream passing through one tube circuit in each shell and tube heat exchanger and the second refrigerant passing through another tube circuit in each shell and tube heat exchanger, and wherein the first refrigerant passes through the shell and tube heat exchangers around the feed stream and second refrigerant; and
wherein the first refrigerant circuit includes one or more refrigerant compressors and the second refrigerant circuit includes one or more refrigerant compressors, and wherein at least one refrigerant compressor of the first refrigerant circuit and at least one refrigerant compressor of the second refrigerant circuit are mechanically interconnected and are arranged to be driven by a common driver.
An advantage of the present invention is that by passing the second refrigerant through at least one of the heat exchangers of the first cooling stage, there is a reduction in the cooling requirement for the second refrigerant before its use in the second cooling stage. This may reduce, possibly eliminate, the need for separate second refrigerant cooling equipment and units in association with the second cooling stage. This reduces the capital costs (e.g. equipment count) and running costs of a liquefying method and apparatus. It also reduces the space required for such method and apparatus. In some situations or locations, such as on board a vessel such as a ship or a floating platform, space or spacing can be critical, so that any saving of space is significant.
This is particularly advantageous where the method of liquefying is intended to be compact and provide a nominal capacity (or name plate) of liquefied hydrocarbon, such as ≦3 millions of (metric) tons per annum (MTPA). The term “nominal capacity” is defined as the daily production capacity of a plant multiplied by the number of days per year the plant is intended to be in operation. For instance, some LNG plants are intended to be operational for an average of 340 days per year.
Such a compact or small scale design is also based on common driving of the one or more refrigerant compressors of the first and second refrigerant circuits. In particular, the present invention utilises one compressor driver for all the compression of the first and second refrigerant circuits.
Another advantage of the present invention is the use of the minimum number of shell and tube heat exchangers in the first cooling stage for cooling the feed gas and the second refrigerant. Typically, the number of heat exchangers in the first cooling stage is between 2-5, preferably 4 or 5, more preferably 4 heat exchangers.
A further advantage of the present invention is that a single component first refrigerant such as propane can be more conveniently used in different heat exchangers, especially at different pressures or pressure levels, than a mixed-refrigerant, such that the first cooling of the feed stream can be more efficiently arranged. Such pressures could be defined as: low pressure, medium pressure, high pressure and high pressure. The expanded refrigerant from each pressure level could be compressed in one or more compressors known in the art.
An advantage of the use of different first refrigerant pressures is the better efficiency of providing cooling and/or the recompression of a single component refrigerant compared with other refrigerants hitherto used for pre-cooling natural gas, most especially mixed refrigerants.
Another advantage of the present invention is that a single component refrigeration circuit is less expensive than a mixed refrigerant refrigeration circuit, more particularly in the use of multiple heat exchangers and/or multiple pressures or pressure levels to effect the cooling. For example, tube-in-kettle heat exchangers, which can use single component refrigerant, are significantly cheaper than spool wound heat exchangers, which cannot.
The first cooling stage in step (a) is provided by the passage of the feed stream through two or more heat exchangers. At least one heat exchanger used in the first cooling stage, optionally all the heat exchangers used, is preferably wholly or substantially supplied with cooling by the first refrigerant.
The first refrigerant is a single component refrigerant. The term “single component refrigerant” means that the refrigerant comprises >90 mol %, preferably >95 mol %, more preferably >98 mol % and even more preferably >99 mol % of a refrigerant component, such as propane. The first single component refrigerant can comprise >90 mol % propane. Preferably, the first single component refrigerant comprises >95 mol % propane, more preferably >98 mol % propane, even more preferably >99 mol % propane.
Preferably, the first cooling stage comprises at least two heat exchangers, optionally three, four or five heat exchangers. The second refrigerant could pass through any number of the heat exchangers of the first stage, for example one, two, all but one, all but two, etc., heat exchangers.
In one embodiment of the present invention, the first and second refrigerants pass through the same heat exchangers of the first cooling stage. Thus, at least one, preferably all, of the heat exchangers of the first cooling stage provide cooling to the feed stream and the second refrigerant.
The second cooling stage of step (b) may be provided by passing the first cooled feed stream through one or more sections, each section using one or more heat exchangers. The or each heat exchanger of the second cooling is preferably supplied with cooling by a mixed refrigerant in the second refrigerant circuit. Additional cooling of the cooled hydrocarbon stream from the first cooling stage and/or the second refrigerant could be provided by one or more other refrigerants or refrigerant cycles in addition to cooling by the first cooling stage, optionally being connected with another part of the method and/or apparatus for liquefying a hydrocarbon stream as described herein.
The hydrocarbon stream may be any suitable gas stream to be treated, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.
Usually the natural gas stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 80 mol % methane.
Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas stream may also contain non-hydrocarbons such as H2O, N2, CO2, H2S and other sulphur compounds.
If desired, the feed stream containing the natural gas may be pre-treated before use. This pre-treatment may comprise removal of undesired components such as CO2 and H2S or other steps such as pre-cooling or pre-pressurizing. As these steps are well known to the person skilled in the art, they are not further discussed here.
The term “feed stream” as used herein relates to any hydrocarbon-containing composition usually containing a large amount of methane. In addition to methane, natural gas contains various amounts of ethane, propane and heavier hydrocarbons. The composition varies depending upon the type and location of the gas. Hydrocarbons heavier than methane generally need to be removed from natural gas for several reasons, such as having different freezing or liquefaction temperatures that may cause them to block parts of a methane liquefaction plant. C2-4 hydrocarbons can be used as a source of Liquefied Petroleum Gas (LPG).
The term “feed stream” also includes a composition prior to any treatment, such treatment including cleaning, dehydration and/or scrubbing, as well as any composition having been partly, substantially or wholly treated for the reduction and/or removal of one or more compounds or substances, including but not limited to sulfur, sulfur compounds, carbon dioxide, water, and C2+ hydrocarbons.
In a further aspect, the present invention provides apparatus for liquefying a hydrocarbon stream such as natural gas stream from a feed stream, the apparatus at least comprising:
a driver to commonly drive at least one refrigerant compressor of the first refrigerant circuit and at least one refrigerant compressor of the second refrigerant circuit.
Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:
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.
The feed stream 10 undergoes a first cooling in a first cooling stage 100a against a first refrigerant being cycled in a first refrigerant circuit 110a, thereby obtaining a cooled hydrocarbon stream 30.
The heat exchange for the first refrigerant circuit 110a may comprise at least two heat exchangers, e.g. two, three or four, through which the feed stream 10 passes, and each heat exchanger may also have a different pressure or pressure level.
The first single component refrigerant of the first refrigerant circuit 110a is preferably propane.
Optionally, the cooled stream 30 is then passed into a separation column (not shown), which column can separate the cooled stream 30 into a more liquid or heavier stream, generally being a heavier hydrocarbon rich stream, and a more gaseous or lighter stream, generally being a methane enriched stream for subsequent cooling and liquefaction. The heavier stream can be recycled or used for other product production.
Each heat exchanger 112 provides a cooler stream, 20a, 20b, 20c, to eventually provide a cooled hydrocarbon stream 30. Preferably, the first cooling cools down the feed stream 10 to approximately −20 to −70° C., such as about −25° C.
Using different pressure levels, such as, consecutively, high pressure, high pressure, medium pressure and low pressure, in each of the four heat exchangers 112 shown in
Generally, the vapour released from each heat exchanger 112 passes to and along a first compressor 114 in an arrangement known in the art, and the compressed refrigerant is then cooled by a cooler 118 before recirculation through the heat exchangers 112.
The heat exchangers 112 are shell and tube heat exchangers having two or more tube circuits. Preferably, the heat exchangers of the first cooling stage 110a are kettles having two or more tube circuits. The feed stream passes through one tube circuit in each heat exchanger and second refrigerant (discussed hereinafter) passed through another tube circuit in each heat exchanger. The first refrigerant then passes through the heat exchanger 112 around the feed stream 10 and second refrigerant.
The cooled hydrocarbon stream 30 can then pass through a second cooling stage 200 having at least one, preferably cryogenic, heat exchanger 14, to provide a liquefied hydrocarbon stream 40, such as liquefied natural gas. In the second cooling stage 200, there is a second refrigerant, preferably being a mixed refrigerant, being cycled in a second refrigerant circuit 210a.
In one embodiment of the present invention, the mixed refrigerant for the second refrigerant circuit 210a comprises:
>30 mol % of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and
>30 mol % of a compound selected from the group consisting of propane and propylene or a mixture thereof. In general, the second refrigerant may be any suitable mixture of components including two or more of nitrogen, methane, ethane, ethylene, propane, propylene, butane, pentane, etc.
There can be various arrangements for the cooled hydrocarbon stream 30 and the second refrigerant stream in the second cooling stage 200. Such arrangements are known in the art. These can involve one or more heat exchangers 14, optionally at different pressure levels, and optionally within one vessel such as a cryogenic heat exchanger.
The second cooling stage 200 reduces the temperature of the cooled feed stream 30 to provide a liquefied hydrocarbon stream 40, which could be at a temperature of about or lower than −90° C., preferably lower than −120° C.
The second refrigerant of the second refrigerant circuit 210a passes through at least two heat exchangers 112 in the first cooling stage 100a. This arrangement simplifies the method of liquefying a hydrocarbon feed stream such as natural gas by combining a portion of the cooling duty of the first cooling of the feed stream with an equivalent portion of the cooling of the second refrigerant using the same equipment, rather than the second refrigeration circuit requiring its own separate or stand alone set of heat exchangers to sufficiently cool the second refrigerant for use in the second cooling stage 200. This therefore reduces the level of apparatus and equipment needed, and therefore reduces the capital and running costs for the process.
In the simplified form shown in
As well as simplifying the overall method of liquefying a hydrocarbon feed stream as mentioned above, this arrangement also reduces the space required for the scheme in
The liquefied stream 40 could then undergo a third cooling stage 300, such as sub-cooling, end flash, etc, or a combination of same. Sub-cooling can be carried out against a refrigerant being circulated in a third refrigerant circuit 310, thereby obtaining a further cooled liquefied stream 70. In simplified form, the third refrigerant circuit 310 involves a third compressor 132 driven by a driver 134, an air and/or water cooler 136. Additional cooling of the liquefied stream and/or the third refrigerant could be provided by one or more other refrigerants or refrigerant circuits, optionally being connected with another part of the method and/or apparatus for liquefying a hydrocarbon stream as described herein.
Further the person skilled in the art will readily understand that after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve or by means of a cryogenic turbo-expander.
The feed stream 10 passes through a first cooling stage 100b having four heat exchangers 142, 112. As described in relation to
The feed stream 10 may be passed through a drying unit 147 as part of first cooling circuit 110b. In a further embodiment (not shown), heat exchanger 142 and/or drying unit 147 may be in a different physical location.
Each heat exchanger 142, 112 provides a cooler stream, 20a, 20b, 20c to eventually provide a cooled hydrocarbon stream 30 such as that described above. This cooled stream 30 can then pass through a second cooling stage 200 having at least one heat exchanger 14a to provide a liquefied hydrocarbon stream 40, (which liquefied stream 40 may optionally also undergo a third cooling as shown in
The heat exchanger 14a in
Compared with the scheme shown in
Firstly, the second refrigerant circuit 210b still involves an exit refrigerant stream 215 from the heat exchanger 14a passing through a second compressor 122. In a similar manner to
Where either the first refrigeration circuit 110b and/or second refrigeration circuit 210b of the first and second cooling stages 100b, 200 involves more than one refrigerant compressor, two or more of such compressors, either in the same circuit and/or from different circuits as shown in
After passage through the second compressor 122, the refrigerant stream 216 passes through an air and/or water cooler 126 to provide a condensing refrigerant stream 217.
In the second alternative embodiment to
Viewed in another way, the first part of the first cooling stage 110b uses two separate heat exchangers 138, 142 instead of the first common heat exchanger 112 shown in
In the arrangement shown in
Thus, the separate first heat exchanger 142 only cools the feed stream 10 to provide a cooled hydrocarbon stream 20a, which is then passed into the common heat exchangers 112 in a similar manner to that in
To provide cooling to the separate first heat exchanger 142 and first heat exchanger 138 used to cool condensing refrigerant stream 217, the first refrigerant in the first refrigerant circuit 110b can be divided by any suitable unit, device, for example a stream splitter 144, such that two part-first refrigerant streams 146a, 146b are created, the first part 146a of which can be used in the separate first heat exchanger 142, and the second part 146b of which can be used in the first heat exchanger 138 used to cool condensing refrigerant stream 217. The respective refrigerant vapour streams 105a, 105b from the separate first heat exchanger 142 and first heat exchanger 138 used to cool condensing refrigerant stream 217 can be combined to form a single stream 105 prior to their entry into the first compressor 114.
The second refrigerant stream 218 passes through the remaining second, third and fourth heat exchangers 112 as described above, to provide a cooled second refrigerant stream 219 ready for use in the heat exchanger 14a of the second cooling stage 200. The refrigerant vapour streams 106, 107 and 108 from the second, third and fourth heat exchangers 112 pass into the first compressor 114.
In the third alternative embodiment to
In addition,
Further in addition in
In a further alternative embodiment (not shown), the cooled hydrocarbon stream from the first cooling stage could be divided into any number of part-streams, based on any ratio of mass and/or volume and/or flow rate, such part-streams preferably being equal. The part-streams could then be separately liquefied by separate liquefaction systems, then optionally undergo either separate or combined third cooling. U.S. Pat. No. 6,389,844 B1 shows an example of a dual heat exchanger, dual refrigerant system, wherein a first cooling stage serves two main, preferably cryogenic, refrigeration systems.
Consequently, the depth to which the feed stream 10, which is preferably natural gas, is first-cooled may be reduced. Moreover, the conditions of the first cooling stage and for the liquefactions, for example the composition of the first refrigerant, can easily be adapted such that an efficient operation is achieved. Further, in case one of the main liquefying systems or one of its operations has to be reduced or taken out of operation, the conditions can be adapted to work efficiently with a single main liquefaction system. In this way, the liquefaction capacity can be increased without having to add a second first cooling stage, and this saves substantial costs.
In another further alternative embodiment (not shown), the first cooling stage comprises two or more parallel series of shell and tube heat exchangers such as kettles, through which the feed stream, first refrigerant and second refrigerant pass. Any arrangement of each of the streams through the different series of heat exchangers can be made to suit conditions and requirements of the first cooling stage. In one example, each of the three streams is divided equally into two or three-part streams, and each series of part streams are passed through separate series of parallel heat exchangers, optionally with combining of similar streams thereafter prior to the second cooling stage. Thus, the first cooling could be formed of two parallel series of three or four heat exchangers, each series accommodating half of the streams of feed, first refrigerant and second refrigerant, prior to the combination of the feed streams and second refrigerant streams back into single streams for the second and any third cooling stages.
Table I gives an overview of the temperatures, pressures, mass flow rates and phases of streams at various parts in the example process of
The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06118290.3 | Aug 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/057924 | 7/31/2007 | WO | 00 | 1/29/2009 |
