Patent Application
20110036120
The present invention relates to a method and apparatus for recovering a mixed hydrocarbon stream from an initial feed stream, such as a natural gas stream, and fractionating the mixed hydrocarbon feed stream into one or more fractionated streams.
Natural gas is a useful fuel source, as well as a source of various hydrocarbon compounds. It may be produced for distribution in a pipeline grid at or near the source. Sometimes the natural gas is first liquefied in a liquefied natural gas (LNG) plant at or near the source of 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 small volume and does not need to be stored at high pressure.
Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for its intended purpose. In case of subsequent liquefaction, the purified feed stream should be suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation.
In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, including but not limited to carbon dioxide, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water and other non-hydrocarbon acid gases, ethane, propane, butanes, C5+ hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most if not all known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as necessary the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process.
Hydrocarbons heavier than methane and usually ethane are typically typically removed from the initial feed stream by partly condensing the initial feed stream thereby forming a condensed mixed hydrocarbon feed stream that is subsequently separated from the initial feed stream. In case of the feed stream being formed of natural gas, the condensed mixed hydrocarbon feed stream is recovered as so-called natural gas liquids (NGL).
The mixed hydrocarbon feed stream containing the NGLs is often fractionated to yield valuable hydrocarbon products, either as product steams per se or for use in a process. For example, if the process is liquefaction, the hydrocarbon products may be used as a component of a refrigerant.
Fractionation typically involves separating one or more C2+ hydrocarbon streams from a mixed hydrocarbon stream, in particular as or as part of a multi-column natural gas liquids (NGL) recovery system and arrangement.
The use of two or more gas/liquid separators in series is commonly used in NGL recovery known to those skilled in the art. In one example, an NGL extraction unit provides a single heavier hydrocarbon rich stream, which is subsequently used either per se, or is further divided into particular heavier hydrocarbon rich streams in a separate location or unit. The fractionation of a heavier hydrocarbon rich stream can be carried out by one or more further gas/liquid separators known in the art, such as a fractionator.
A fractionator using one or more columns could provide individual streams of certain heavier hydrocarbons. For example, with multiple columns, each column could be designed to provide an individual hydrocarbon stream, such as an ethane-rich stream, a propane-rich stream, a butane-rich stream, and a C5+-rich stream, the latter sometimes also termed a ‘light condensate stream’. Propane, butane, C5+ hydrocarbons (and optionally ethane) are sometimes collectively termed “natural gas liquids” (NGL), and have known uses.
An example of a fractionation tower as a conventional distillation column for NGL extraction is shown in US 2004/0079107 A1.
In another example, an NGL extraction unit can include a fractionator, which integrally provides individual streams of certain heavier hydrocarbons such as those listed hereinbefore.
NGL recovery thus generally involves cooling, condensation and fractionation steps that require significant amounts of refrigeration and other power consumption. It is desirable to recover NGLs from a natural gas stream with the most efficient or minimal refrigeration power consumption.
U.S. Pat. No. 7,051,553 describes a typical gas separation process, in which a feed gas stream is cooled and liquids condensing from the cooled gas are then expanded and fractionated in a distillation column to separate residual components from desired heavier components. U.S. Pat. No. 7,051,553 also describes a two-column NGL recovery plant having an absorber and a distillation column, with the absorber receiving a second reflux stream comprising the cooled overhead gas of the distillation column. Cooling of the overhead gas from the distillation column is provided by a separate cooler, requiring separate power consumption.
U.S. Pat. No. 6,116,050 discloses methods for separating and recovering propane, propylene, and the C3+ hydrocarbons from a feed gas, to produce pipeline gas and a liquid product. The methods employ sequentially configured first and second distillation columns, and a self-refrigeration system which is said to improve the separation efficiency in the first column. The cooled feed gas is separated in a feed separator, and both the vapours and the liquids from the feed separator are fed into the first column. Part of the liquids conducted directly through a line into the first column, while the remaining portion is heated against a vapour overhead stream from the second column prior to introduction into the first column via the same line.
It is an object of the present invention to improve the efficiency of recovery and subsequent fractionation of a mixed hydrocarbon feed stream from an initial feed stream.
In one aspect, the present invention provides a method of recovering a mixed hydrocarbon feed stream from an initial feed stream, such as a natural gas stream, and fractionating the mixed hydrocarbon feed stream into one or more fractionated streams, the method at least comprising the step of:
(a) providing an initial feed stream;
(b) partly condensing the initial feed stream, thereby forming a partly condensed initial feed stream;
(c) separating the partly condensed initial feed stream into an initial gaseous overhead stream and a condensed mixed hydrocarbon feed stream;
(d) dividing the condensed mixed hydrocarbon feed stream into at least a first part-feed stream and a second part-feed stream;
(e) passing the first part-feed stream into a first gas/liquid separator, via a first inlet into the first gas/liquid separator, to provide at least a first fractionated stream in the form of a first gaseous overhead stream and a first bottom liquid stream;
(f) passing the first bottom liquid stream into a second gas/liquid separator to provide at least a second fractionated stream in the form of a second gaseous overhead stream and a second bottom liquid stream; and
(g) cooling the second overhead gaseous stream by heat exchange against the second part-feed stream, resulting in a warmer second part-feed stream;
(h) passing the warmer second part-feed stream into the first gas/liquid separator at a level gravitationally below the first inlet.
In another aspect, the invention provides an apparatus for recovering a mixed hydrocarbon feed stream from an initial feed stream and fractionating the mixed hydrocarbon feed stream into one or more fractionated streams, the apparatus at least comprising:
a pre-cooling heat exchanger arranged to cool the initial feed stream to provide a partly condensed initial feed stream;
an initial gas/liquid separator arranged to separate the partly condensed initial feed stream into an initial gaseous overhead stream and a condensed mixed hydrocarbon stream;
a stream splitter to divide the condensed mixed hydrocarbon feed stream into at least a first part-feed stream and a second part-feed stream;
a first gas/liquid separator arranged to receive the first part-feed stream via a first inlet into the first gas/liquid separator, and to provide at least a first fractionated stream in the form of a first overhead gaseous stream and a first bottom liquid stream;
a second gas/liquid separator arranged to receive the first bottom liquid stream, and to provide at least a second fractionated stream in the form of a second overhead gaseous stream and a second bottom liquid stream;
a heat exchanger arranged to receive the second part-feed stream and the second overhead gaseous stream, and to provide a cooled second overhead gaseous stream and a warmer second part-feed stream; and
a second inlet into the first gas/liquid separator arranged to allow entry of the warmer second part-feed stream into the first gas/liquid separator, the second inlet being at a level gravitationally below the first inlet.
Embodiments and examples of the present invention will now be described by way of example and with reference to the accompanying non-limited 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.
In the embodiments disclosed herein, cooling of the second overhead gaseous stream from the second gas/liquid separator is provided by use of a portion of the cold energy in the condensed mixed hydrocarbon feed stream, rather than requiring any separate refrigeration or cooler involving separate power consumption, or rather than integration with a refrigeration system or circuit associated with a liquefaction process.
Thus, cold energy from another source, such as (pre-) cooling of a hydrocarbon stream such as natural gas by a separate refrigerant, refrigeration system or circuit, need not be diverted to be involved in recovery and fractionation of the mixed hydrocarbon stream, such as in NGL recovery, thereby increasing the efficiency of other processes or sections of a liquefaction plant such as an LNG plant.
The cooling of the second overhead gaseous stream provides a warmer second part-feed stream, which can still be passed into the first gas/liquid separator or used otherwise. The introduction of a fraction of the mixed hydrocarbon feed stream into the first gas/liquid separator at a warmer temperature, (following use of some of its cold energy to cool the second overhead gaseous stream from the second gas/liquid separator), reduces the energy input required from a heat source such as a reboiler to operate the first gas/liquid separator.
It has further been found that an improvement of the separation efficiency and a further decrease of any heating duty from an external source can be achieved by passing the warmer second part-feed stream into the first gas/liquid separator at a height along the separator below where the first part-feed stream is passed into the first gas/liquid separator.
Usually the initial feed stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 80 mol % methane.
The term “mixed hydrocarbon feed stream” as used herein relates to a feed stream comprising methane and one or more hydrocarbons selected from the group comprising: ethane, propane, butanes, pentanes, and C6+ hydrocarbons.
The mixed hydrocarbon feed stream may be provided from any suitable source. Preferably, the mixed hydrocarbon feed stream is provided from an initial feed stream. The initial feed stream may be any suitable hydrocarbon stream such as, but not limited to, a hydrocarbon-containing gas stream to be cooled. One example is a natural gas stream obtained from a natural gas or petroleum reservoir. As an alternative the initial feed stream may also be obtained from another source, such as a refinery and/or including a synthetic source such as a Fischer-Tropsch process.
The term “C2+” as used herein relates to one or more components selected from the group comprising: ethane, propane, butanes, pentanes, and C6+ hydrocarbons.
Similarly, the term “C3+” as used herein relates to one or more components selected from the group comprising: propane, butanes, pentanes, and C6+ hydrocarbons.
The terms “C4+”, etc. are similarly defined starting with “butanes”, etc.
Each gas/liquid separator of the present invention may involve one or more columns, and one or each of such columns could provide individual liquid streams of certain heavier hydrocarbons such as ethane, propane, etc.
Any fractionated, or individual stream of C2+, C3+, C4+, etc., may still comprise a minor (<10 mol %) amount of methane; each such stream is preferably >80 mol %, more preferably >95 mol %, of its one or more components as defined above.
The division of a stream such as a feed stream into two or more part streams may be carried out using any suitable stream splitter or divider, which may be a distinct unit, or a simpler division of a line such as a T-Piece.
Although the method according to the present invention is applicable to various initial hydrocarbon-containing feed streams, it is particularly suitable for natural gas streams to be liquefied. As the person skilled readily understands how to liquefy a hydrocarbon stream, this will herein only be discussed at a basic level of detail.
means to separate a mixed hydrocarbon feed stream 10 from the initial feed stream 8;
a stream splitter 36 to divide the mixed hydrocarbon feed stream 10 into at least a first part-feed stream 20 and a second part-feed stream 30;
a first gas/liquid separator 14 to receive the first part-feed stream 20, and to provide at least a first overhead gaseous stream 40 and a first bottom liquid stream 50;
a second gas/liquid separator 22 to receive the first bottom liquid stream 50, and to provide at least a second overhead gaseous stream 70 and a second bottom liquid stream 80; and
a heat exchanger 26 to receive the second part-feed stream 30 and the second overhead gaseous stream 70, and to provide a cooled second overhead gaseous stream 70a and a warmer second part-feed stream 30b.
The method as illustrated in
providing a mixed hydrocarbon feed stream 10 from an initial feed stream 8;
dividing the mixed hydrocarbon feed stream 10 into at least a first part-feed stream 20 and a second part-feed stream 30;
passing the first part-feed stream into a first gas/liquid separator 14 to provide at least a first gaseous overhead stream 40 and a first bottom liquid stream 50;
passing the first bottom liquid stream 50 into a second gas/liquid separator 22 to provide at least a second gaseous overhead stream 70 and a second bottom liquid stream 80; and
cooling the second overhead gaseous stream 70 by heat exchange against the second part-feed stream 30.
The first and second gas/liquid separators may be any form of separator such as a distillation column adapted to provide at least one gaseous stream, usually being enriched in one or more lighter hydrocarbons, and at least one liquid stream, usually being enriched in one or more heavier hydrocarbons. An example of such a separator is a “de-methanizer” designed to provide a methane-enriched overhead stream, and one or more C2+ enriched liquid steams at or near the bottom. Similarly, there are “de-ethanizers” and “de-propanizers”, etc., known in the art.
Thus, where the first gas/liquid separator is a de-methanizer, the first bottom liquid stream will be a C2+ hydrocarbon stream. Where the second gas/liquid separator is a de-ethanizer, the second bottom liquid stream will be a C3+ hydrocarbon stream, and the second overhead gaseous stream is preferably >60 mol % ethane, more preferably >85 and even more preferably >90 mol % ethane.
In more detail,
This cooling of the initial feed stream 8 may be part of a liquefaction process, such as a pre-cooling stage involving a propane refrigerant circuit as described hereinafter in relation to
Cooling of the initial feed stream 8 may involve reducing the temperature of the initial feed stream 8 to below −0° C., for example, in the range −10° C. to −40° C.
The cooled initial stream 8a is passed into an initial gas/liquid separator such as a scrub column 34, operating at an above ambient pressure in the manner known in the art. The scrub column 34 provides a condensed mixed hydrocarbon feed stream 10, and an initial gaseous overhead steam 110. The initial gaseous overhead stream 110 usually comprises greater than 80 mol % methane. It is typically a methane-enriched stream compared to the cooled initial stream 8a.
The mixed hydrocarbon feed stream 10 comprises methane and one or more C2+ hydrocarbons. Typically, the proportion of methane in the mixed hydrocarbon feed stream is 30-50 mol %, with a significant fraction of ethane and propane, such as 5-10 mol % each.
Commonly, it is desired to recover any methane in a mixed hydrocarbon stream (for use as a fuel or so that it can be liquefied in the LNG plant 2 and provided as additional LNG), and to provide one or more of a C2 stream, a C2+ stream, a C3 stream, a C3+ stream, a C4+ stream, etc., or a combination of same. The method of the present invention is applicable to separation of any one or more of the C2+ streams, either as pure streams or as combined component streams.
In
The first part-feed stream 20 passes through a valve 12 to provide a reduced pressure first part-feed stream 20a, which then enters a first gas/liquid separator such as a first distillation column 14 through a first inlet at or near the top of the first distillation column 14. The reduced pressure first part-feed stream 20a is typically a mixed phase stream, and the first distillation column 14 is adapted to separate the gaseous and vapour phases, so as to provide a first overhead gaseous stream 40 and a first bottom liquid stream 50.
The nature of the streams provided by the first distillation column 14 can be varied according to the size and type of distillation column, and its operating conditions and parameters, in a manner known in the art. For the arrangement shown in
The first distillation column 14 also includes a first reboiler 17 and a bottom return stream 60, typically in the form of a reboiler vapour return stream, in a manner known in the art.
The first bottom liquid stream 50 will predominantly be C2+ hydrocarbons, such as >90 or >95 mol % of ethane and heavier hydrocarbons. The first bottom liquid stream 50 is cooled by one or more ambient coolers, such as a water and/or air cooler 16, followed by a passage through a valve 18 to provide a reduced pressure first bottom stream 50a, which enters through inlet 42 into a second gas/liquid separator such as a second distillation column 22. Again, the type, size and capacity of the second distillation column 22, as well as its operating conditions and parameters, will control the nature of the streams provided by the second distillation column 22.
In the arrangement shown in
The cooling of the second overhead gaseous stream 70 preferably results in condensation of at least part, preferably all, of the second overhead gaseous stream. Particularly a condensed fraction may be used as a reflux stream 70b for the second gas/liquid separator 22. In one embodiment of the present invention, the method thus further provides the steps of:
(i) dividing a cooled second gaseous overhead stream 70a provided by step (g) into two or more fractions (70b, 70c); and
(j) passing at least one of said fractions (70b) back into the second gas/liquid separator 22, preferably as a reflux stream.
Where the second gas/liquid separator is a distillation column, for instance a de-ethanizer, the fraction of the second overhead gaseous stream in step (i) can be used to provide reflux to the distillation column.
Conventionally, the second overhead gaseous stream 70 is cooled by a separate or external cooler as shown in U.S. Pat. No. 7,051,553 B2, or by the pre-cooling heat exchanger 32. Using the former requires the involvement of a separate energy source and use, whilst using the latter takes away some of the cooling power of the pre-cooling heat exchanger 32 from cooling of the initial feed stream 8. Both of these situations reduce the efficiency of an LNG plant. Both also create complicated integration between the fractionation arrangement and the remainder of a liquefaction plant.
The second part-feed stream 30 (from the mixed hydrocarbon feed stream 10), after passing through a valve 24 to provide a reduced pressure second part stream 30a, preferably has a temperature that is low enough, such as between 0° C. and −50° C., to provide cooling in a heat exchanger 26 to the second overhead gaseous stream 70. The heat exchanger 26 may be one or more heat exchangers in parallel, series or both. In embodiments, heat exchanger 26 may be referred to as a reflux heat exchanger.
The cold energy of the reduced pressure second part-feed stream 30a withdraws warmth from the second overhead gaseous stream 70 to at least partially condense, preferably fully condense, the second overhead gaseous stream 70 in the heat exchanger 26, and provide an at least partly condensed second stream 70a. Thus, cooling of the second overhead gas stream 70 is achieved without the need for a separate source of cooling, making the arrangement for NGL recovery shown in
The at least partly condensed second stream 70a can be divided by a stream splitter 44 into a reflux stream 70b, and a product stream 70c. The product stream 70c could be provided as a separate product stream for use outside a liquefaction plant, or as a refrigerant or as a component of a refrigerant in a liquefaction plant, such as in a mixed refrigerant known in the art.
The heat exchange of the second overhead gaseous stream 70 and the second part-feed stream 30a also provides a warmer second part-feed stream 30b, which can be passed into the first distillation column 14, preferably between the inlet 38 for the first part-feed stream 20a and the bottom return stream 60, e.g. via an inlet 39 into the first distillation column 14, located gravitationally lower than inlet 38. The bottom return stream 60 may be fed back into the first distillation column 14 via return stream inlet 41.
The arrangement shown in
Thus, the method of the present invention is equally applicable to use with two other gas/liquid separators using a mixed hydrocarbon feed stream and adapted to provide different product streams. Equally, the method of the present invention is applicable to an arrangement involving more than two gas/liquid separators, where more than three product streams such as separate C3, C4 and C5+ streams are provided.
Additionally, the second part-feed stream 30 could be divided into two or more fractions for cooling other overhead gaseous streams (such as from a de-propanizer, etc.), thus further reducing or eliminating the requirement for separate coolers or heat exchangers for each overhead stream.
In the embodiment as shown in
The initial gaseous overhead stream 110 may be fed into a gas grid to distribute the methane-enriched stream as gas to market. Before feeding into a grid, the initial gaseous overhead stream 110 may, however, be subjected to other processing steps such as further treating to change the composition and/or cooling, preferably liquefying, to provide a cooled hydrocarbon stream, preferably LNG.
Thus, the present invention further provides a method of cooling an initial feed stream such as a hydrocarbon stream such as natural gas comprising at least the steps of:
(i) passing the initial feed stream through an initial separator to provide an, methane-enriched, initial gaseous overhead stream and a condensed mixed hydrocarbon feed stream;
(ii) cooling, preferably liquefying, the initial gaseous overhead stream to provide a cooled, preferably liquefied, hydrocarbon stream; and
(iii) separating one or more C2+ hydrocarbons from the condensed mixed hydrocarbon stream by a method as herein defined.
The initial gaseous overhead stream is preferably cooled without passing it through the first gas/liquid separator. It may be cooled by passing the initial gaseous overhead stream to one or more heat exchangers where it is allowed to exchange heat against one or more refrigerants being cycled in one or more refrigerant cycles.
The scrub column 34 provides a condensed mixed hydrocarbon feed stream 10. This mixed stream 10 is divided by a stream splitter 36 into a first part-feed stream 20 and second part-feed stream 30, whose passage is as hereinbefore described with relation to
In this way, the first part-feed stream 20 passes through a valve to provide a reduced pressure first part-feed stream 20a, which then enters a first distillation column 14. The distillation column 14 provides a first overhead gaseous stream 40 generally being methane and labelled “C1” in
The second distillation column 22 provides a second overhead gaseous stream 70 which is cooled in the heat exchanger 26 against the reduced pressure second part stream 30a to provide a cooled second overhead stream in the form of an at least partly, preferably fully, condensed second stream 70a, which is divided into a reflux stream 70b and a product stream 70c, which is predominantly ethane and labelled “C2” in
The second distillation column 22 also provides a second bottom liquid stream 80 being a C3+ stream, which, after expansion, can be passed into a further gas/liquid separator such as a third distillation column 52, optionally also being a de-propanizer. The third distillation column 52 can provide a third overhead stream 140 (which after cooling can provide a “C3” product stream comprising for example >95 mol %, preferably 99 mol %, propane), and a third bottom liquid stream 130 being a C4+ stream. The third bottom stream 130 can pass into a fourth gas/liquid separator being a fourth distillation column 54, which provides a fourth gaseous overhead stream 160 (which after cooling can create a “C4” product stream comprising for example >95 mol %, preferably 99 mol %, butanes), and a fourth bottom stream 150 which can be a “C5+” stream, (sometimes also termed a “light condensate” stream).
Meanwhile, the initial gaseous overhead stream 110 from the scrub column 34 can pass into a main cryogenic heat exchanger 23 to provide further cooling. Commonly, the cooling provided by the pre-cooling heat exchangers 46 can be considered as a ‘pre-cooling stage’, and the cooling provided by the main cryogenic heat exchanger 23 can be considered as a ‘main or second cooling stage’.
The main cryogenic heat exchanger 23 is typically able to reduce the temperature of the initial gaseous overhead stream 110 to below −90° C. or −100° C., preferably to liquefy the initial gaseous overhead stream 110. Such cooling can be provided in a number of ways known to the person skilled in the art. One example way is use of a refrigerant circuit 25 in a manner known in the art.
The main cryogenic heat exchanger 23 provides an overhead stream 120, preferably being fully liquid. Where the initial feed stream 8 is natural gas, the overhead gaseous stream 120 will generally be LNG. In particular, the present invention improves the efficiency of the overall LNG plant 2 by reducing the complexity of integration of cooling required of at least one overhead stream with other heat exchangers (such as the pre-cooling heat exchanger 32, 46), or other heat exchanger arrangements.
Table 1 below gives an overview of estimated compositions, phases, pressures and temperatures of some of the streams at various parts of an example process of
Table 2 below gives results of a similar calculation as the one for Table 1, with the exception that the warmer second part-feed stream 30b has been fed into the first distillation column 14 at the first tray, the same level as the first part-feed stream 20a. Compared to Table 1, the C1 purity of the first gaseous overhead stream 40 has decreased to about 90% for an increase of the first reboiler duty of first reboiler 17 to 2.8 MW.
Hence, feeding the warmer second part-feed stream 30b into the distillation column 14 at a level below the inlet 38 for the first part-feed stream 20a improves the separation efficiency of the first distillation column 14 while reducing reboiler duty of the first distillation column 14.
Suitable process control of embodiments of the present invention may include a level controller on the initial gas/liquid separator 34, which manipulates the flow of the first part-feed stream 20 into the first gas/liquid separator 14, for instance via manipulating the setting of valve 12. Valve 24 may be manipulated using a flow controller, of which the set point is determined by a pressure controller for the second gas/liquid separator. Equivalently, its set point may be determined by a level controller on an optional vessel 21 arranged to receive and discharge the cooled second overhead stream. This is equivalent to pressure control, since the level in the vessel is determined by the duty of the condenser condensation yielding stream 70a.
The demonstrated embodiments of the present invention advantageously avoid complicated integration of a NGL removal process with the refrigeration systems of any associated LNG plant or facility.
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. For instance, instead of natural gas, the initial feed stream may be formed of other types of gas, including refinery or petroleum plant gas.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07112760.9 | Jul 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/059383 | 7/17/2008 | WO | 00 | 6/9/2010 |
