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.