The present invention relates to a method and apparatus for liquefying a hydrocarbon stream. In other aspects, the present invention relates to a floating vessel or an offshore platform comprising such an apparatus or on which such a method is performed.
A commonly suggested hydrocarbon feed stream may comprise or essentially consist of natural gas, but it could also be derived from other sources.
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.
U.S. Pat. No. 3,274,787 describes a method wherein natural gas is liquefied. The natural gas, which contains methane, C2 to C4 and some C5 and C6 hydrocarbons, arrives through a conduit in a heat exchanger where it is cooled and slightly condensed and then passes to a separator. The remaining natural gas is further cooled, liquefied and sub-cooled. The condensate enriched in heavier hydrocarbons passes through an expansion valve to a rectification column, which is heated at the bottom. The residual liquid separated in the sump of the rectification column is introduced into a train of fractionation columns each equipped with a condenser and a heater. Propanes and butanes drawn from the tops of the fractionation columns are combined with recycled fractions of cycling gas, which is used to cool the natural gas, in order to compensate for inevitable butane and propane losses in the recycling gas.
Thus, the process of U.S. Pat. No. 3,274,787 requires a plurality of fractionation columns, in addition to a rectifier column. These are expensive to build and operate, and also occupy plot space.
U.S. Pat. No. 7,234,321 describes another method for liquefying methane-rich gas. A stream of incoming feed gas (which has been treated to remove components that would interfere with the liquefaction, such as freezables) is pre-cooled and separated in a condensate separator into a vapour stream and a liquid condensate stream which consists mainly of propane, butane and C5+. One of the purposes of this separation is to provide a source for refrigerant fluid. The condensate liquid stream is flashed, heated and fed into another separator to remove most of the methane content from the liquid. A portion of the liquid, after further heating, enters into yet another separator. The vapour stream from this separator is admitted, when required, to the inventory of refrigerant in a closed circuit refrigeration system. Liquid may be removed from the refrigeration system in order to change the composition of the refrigerant.
The cycled refrigerant in U.S. Pat. No. 7,234,321 is used to cool the gas to a temperature of about −75 to −85 ° C. before it flows to a liquefying expander wherein the gas is liquefied.
The process of U.S. Pat. No. 7,234,321 uses only separators, and no fractionation, to remove the natural gas liquids from the natural gas. It is therefore a drawback in this process that is will be difficult to get rid of all the methane and other light components such as nitrogen in the natural gas liquid stream, and in particular in the vapour part of the natural gas liquid stream that is used to feed into the refrigerant inventory. These components do not condensate and are therefore not effective in removing heat from the natural gas. Moreover, these components are hard to separate from the other components in the refrigerant at the relative high temperatures (about −75 to −85° C.) at which the refrigerant cycle is operated.
The present invention provides a method of liquefying a hydrocarbon stream, comprising at least the steps of:
(a) partially liquefying a hydrocarbon feed stream to provide a partially liquefied hydrocarbon stream;
(b) passing the partially liquefied hydrocarbon stream through a first gas/liquid separator to provide a methane-enriched gaseous overhead stream and a mixed C2+ enriched liquid bottom stream;
(c) circulating a mixed refrigerant through a mixed refrigerant circuit;
(d) adding, without fractionation, at least a part of the mixed C2+ enriched bottom stream to the mixed refrigerant circuit to change the C2+ component inventory of the mixed refrigerant in the mixed refrigerant circuit; and
(e) liquefying the methane-enriched gaseous overhead stream by heat exchanging against at least a fraction of the mixed refrigerant circulating in the mixed refrigerant circuit, to provide a liquefied hydrocarbon stream.
The present invention also provides an apparatus for liquefying a hydrocarbon stream, the apparatus at least comprising:
one or more first heat exchangers for partially liquefying a hydrocarbon feed stream to provide a partially liquefied hydrocarbon stream;
a first gas/liquid separator (B) through which the partially liquefied hydrocarbon stream can pass to provide a methane-enriched gaseous overhead stream and a mixed C2+ enriched liquid bottom stream;
a mixed refrigerant circuit comprising a mixed refrigerant;
one or more lines to pass at least a part of the mixed C2+ bottom stream into the mixed refrigerant circuit, without a fractionator, to change the component inventory of mixed refrigerant; and
one or more second heat exchangers to liquefy the methane-enriched gaseous overhead stream using at least a fraction of the mixed refrigerant to provide a liquefied hydrocarbon stream.
The present invention further provides a floating vessel and an off-shore platform, e.g. in the form of a caisson, having apparatus and/or using a method as defined herein.
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:
FIG. 1 is a diagrammatic scheme of a hydrocarbon liquefying process showing an embodiment of the present invention;
FIG. 2 is a more detailed scheme of FIG. 1 showing various embodiments of the present invention; and
FIG. 3 is a diagrammatic floating vessel showing another embodiment of the present invention.
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 present disclosure provides an improved method of liquefying a hydrocarbon stream, such as natural gas, which can be self-sufficient in changing its mixed refrigerant inventory, in particular in a space-limited location such as on a floating vessel.
Moreover, the present disclose provides an improved method of liquefying a hydrocarbon stream, such as natural gas, which does not require fractionation to change the mixed refrigerant inventory.
It is presently proposed to provide a refrigerant portion from a hydrocarbon feed stream by partially liquefying the hydrocarbon feed stream followed by simple separation and not fractionation of the partially liquefied hydrocarbon stream in a first gas/liquid separator to provide a mixed C2+ enriched stream. The mixed C2+ enriched stream can be used to change the C2+ component inventory of the mixed refrigerant in a mixed refrigerant circuit.
The term fractionation implies use of a fractionation column, which is understood to be any type of distillation column that has a source of heat in the lower part of the column (such as a warm stream (e.g. a reboiler stream) or a heating coil) and/or a drain of heat at the top (such as a condenser or a cold stream such as a reflux stream). Separation, as opposed to fractionation, merely separates vapour and liquid phases from a mixed phase stream, but does not involve such source and/or drain of heat. As a result, separation is not very selective in terms of separating molecules, which thermodynamically inevitably causes a relatively high amount of light molecules such as methane and nitrogen to be present in the mixed C2+ enriched bottom stream which is used to change the mixed refrigerant inventory.
However, in the present invention this is not a problem because the mixed refrigerant is used to liquefy the methane-enriched vapour stream. In order to reach temperatures low enough to liquefy the methane-enriched vapour stream, a certain inventory of light molecules in the mixed refrigerant is effective.
Moreover, even if the concentration of light molecules in the mixed refrigerant happens to become too high, it is relatively easy to separate and remove these molecules from the mixed refrigerant because of the low temperatures needed anyway to liquefy the methane-enriched vapour stream.
In the present specification, the term “liquefy”, as opposed to “partially liquefy”, implies full liquefaction.
An advantage of the present invention is that a fractionation column is not required to further fractionate the C2+ enriched stream before using it to change the mixed refrigerant inventory. This saves space. It also saves the need for further processing units and equipment, including separate storage facilities, for providing every separate hydrocarbon which it usually desired to have available for changing the C2+ component inventory of a mixed refrigerant in a mixed refrigerant circuit. This is especially propane and butane storage tanks, which require higher safety requirements, especially in space-limited environments.
The present invention may provide a source of one or more of the components of the mixed refrigerant from one or more of the gas/liquid separators described herein, so that change of the component inventory of the mixed refrigerant can be provided for from the method of liquefying without separate supply of one of the more of the components. Separate supply is conventionally provided in the art by the location of a number of storage tanks nearby to the hydrocarbon liquefying process, each storage tank storing one separated component such as ethane, propane, etc, which can supply its component to the mixed refrigerant on demand. It is an advantage of the present invention that separate nearby storage of the components of the mixed refrigerant can be avoided.
Each gas/liquid separator throughout the present disclosure may be provided in the form of a tank, optionally provided with internals as known in the art including a schoepentoeter and/or a mist mat and/or a swirl deck.
FIG. 1 shows a general scheme 1 for a hydrocarbon liquefying process, generally involving cooling and liquefying a hydrocarbon feed stream 10, such as natural gas, by heat exchanging against a mixed refrigerant stream being circulated in a mixed refrigerant circuit 4.
The hydrocarbon feed stream 10 may be any suitable gas stream to be cooled and liquefied, but is usually a methane-comprising gas stream such as a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the hydrocarbon feed stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.
Usually a natural gas stream is comprised substantially of methane. Preferably the hydrocarbon feed stream comprises at least 50 mol % methane, more preferably at least 80 mol % methane.
Depending on the source, natural gas may contain varying amounts of hydrocarbons heavier than methane such as in particular ethane, propane and the butanes, and possibly lesser amounts of pentanes and aromatic hydrocarbons. The composition varies depending upon the type and location of the gas.
Conventionally, the hydrocarbons heavier than butanes are removed as far as efficiently possible from the hydrocarbon feed stream prior to any significant cooling for several reasons, such as having different freezing (particularly those higher than liquefaction temperature of methane) temperatures that may cause them to block parts of a methane liquefaction plant. Ethanes, propanes, and butanes are typically removed to meet a desired specification of the liquefied hydrocarbon product such as LNG.
It is presently intended that the hydrocarbon feed stream is maintained with at least some of the hydrocarbons heavier than methane. At least some of the hydrocarbons heavier than methane may still be removed if desired, for example where relatively few heavier hydrocarbons are expected to be required as described hereinafter. Preferably, there is less or even no heavy hydrocarbon removal from the hydrocarbon feed stream during start-up of the liquefaction process, so that there are more heavier hydrocarbons available for addition to the mixed refrigerant circuit.
Another advantage of the present invention is that it is not limited to only refilling the refrigerant with a single component fraction, such as an ethane-rich fraction as is the case in U.S. Pat. No. 5,588,306.
The hydrocarbon feed stream may also contain non-hydrocarbons such as H2O, N2, CO2, Hg, H2S and other sulphur compounds, and the like. If desired, the hydrocarbon feed stream comprising the natural gas may be pre-treated before cooling and liquefying to remove such components. This pre-treatment may comprise reduction and/or removal of undesired components such as CO2 and H2S or other steps such as early cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here.
To provide a partially liquefied hydrocarbon stream, the hydrocarbon feed stream 10 as hereinbefore described could be cooled prior to feeding it into a first gas/liquid separator (B). Such initial cooling could be provided by a number of methods known in the art. One example is by passing the hydrocarbon feed stream 10 against a refrigerant (101,160), such as at least a first fraction of the mixed refrigerant of the mixed refrigerant circuit, in one or more first heat exchangers 12, to provide a partially liquefied hydrocarbon stream 20. The partially liquefied hydrocarbon stream 20 is preferably at a temperature below 0° C.
Preferably, any such first heat exchangers 12 producing a partially liquefied hydrocarbon stream could form part of a first cooling stage, and one or more second heat exchangers 22 used to liquefy any part of the hydrocarbon stream could form part of one or more second or third cooling stages.
In this way, the present invention may involve two or more cooling stages, each stage having one or more steps, parts etc. For example, each cooling stage may comprise one to five heat exchangers. The or a part of a hydrocarbon stream and/or the mixed refrigerant may not pass through all, and/or all the same, the heat exchangers of a cooling stage.
In one embodiment of the present invention, the hydrocarbon liquefying process comprises two or three cooling stages. A first cooling stage is preferably intended to reduce the temperature of a hydrocarbon feed stream to below 0° C., usually in the range −20° C. to −70° C. Such a first cooling stage is sometimes also termed a ‘pre-cooling’ stage.
The second cooling stage is preferably separate from the first cooling stage. That is, the second cooling stage comprises one or more separate heat exchangers 22. Such a second cooling stage is sometimes also termed a ‘main cooling’ stage.
The second cooling stage is preferably intended to reduce the temperature of a hydrocarbon stream 30, usually at least a part (30a) of the partially liquefied hydrocarbon stream 20 cooled and partially liquefied by the first cooling stage, to below −100° C.
Heat exchangers for use as the one or more first or the one or more second heat exchangers are well known in the art. At least one of the second heat exchangers is preferably a spool-wound cryogenic heat exchanger known in the art. Optionally, a heat exchanger could comprise one or more cooling sections within its shell, and each cooling section could be considered as a cooling stage or as a separate ‘heat exchanger’ to the other cooling locations.
Further, the person skilled in the art will readily understand that after liquefaction, the liquefied hydrocarbon stream 50 may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve 51 and/or by means of a cryogenic turbo-expander.
In yet another embodiment of the present invention, the method further comprises the steps of:
(f) passing one or more fractions of the mixed refrigerant through one or more heat exchangers to provide one or more cooled mixed refrigerant streams;
(g) passing at least one of the cooled mixed refrigerant streams through one or more refrigerant gas/liquid separators to provide one or more gaseous overhead refrigerant streams and one or more liquid bottom refrigerant streams; and
(h) removing at least a part of at least one of the gaseous overhead refrigerant streams and the liquid bottom refrigerant streams to change the component inventory of the mixed refrigerant circuit.
The mixed refrigerant of the mixed refrigerant circuit may be formed from a mixture of two or more components selected from the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes, pentanes, etc. The present invention may involve the use of one or more other refrigerants, in separate or overlapping refrigerant circuits or other cooling circuits.
A mixed refrigerant or a mixed refrigerant stream as referred to herein comprises at least 5 mol % of two different components. A common composition for a mixed refrigerant can be:
0-10 mol %
30-70 mol %
30-70 mol %