Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
2000-03-24
2001-07-03
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S335000, C062S913000
Reexamination Certificate
active
06253574
ABSTRACT:
The invention relates to a, process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, by indirect heat exchange with the refrigerants of a mixed-refrigerant cascade cycle.
Pretreatment steps for the hydrocarbon-rich stream which may be necessary prior to the liquefaction, such as removal of acid gas and/or mercury, removal of aromatic components, etc. which are not subject-matter of the present invention, are not considered in detail below.
Currently, most base load LNG plants are designed as what are known as dual-flow refrigeration processes. In this case, the refrigeration energy required for liquefying the hydrocarbon-rich stream or the natural gas is provided by two separate mixed-refrigerant cycles which are connected to form one mixed-refrigerant cascade cycle. A liquefaction process of this type is disclosed, for example, by GB-B 895 094.
In addition, liquefaction processes are known, in which the refrigeration energy required for the liquefaction is provided by a refrigerant cascade cycle, but not, however, a mixed-refrigerant cascade cycle; see, for example, LINDE-Berichte aus Technik und Wissenschaft, Issue 75/1997, pages 3-8. The refrigerant cascade cycle described therein consists of a propane or propylene refrigeration cycle, an ethane or ethylene refrigeration cycle and a methane refrigeration cycle. Although this refrigerant cascade cycle can be considered as energetically optimized, it is comparatively complicated because of the 9 compressor stages.
In addition, liquefaction processes are known, as described, for example, in DE-B 19 60 301, in which the refrigeration energy required for the liquefaction is provided by a cascade consisting of a mixed-refrigerant cycle and a propane precooling cycle.
The object of the present invention is to specify a process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, which has a reduced specific energy consumption in comparison with dual-flow refrigeration processes of this type and thus makes it possible to implement a smaller plant size and, associated therewith, makes possible lower capital costs.
This object is achieved according to the invention by means of the fact that the mixed-refrigerant cascade cycle consists of at least 3 mixed-refrigerant cycles having different refrigerant compositions.
In the process according to the invention—known as triple-flow mixed-refrigerant cycle—the mixed-refrigerant cascade cycle consists of at least three separate mixed-refrigerant cycles. These have different refrigerant compositions, since they must produce refrigeration at different temperatures.
The first of the three mixed-refrigerant cycles—known as the Precooling Refrigerant Cycle (PRC)—serves for the cooling and partial or complete condensation of the mixed refrigerants required for the liquefaction and subcooling and for the precooling of the hydrocarbon-rich stream. The second mixed-refrigerant cycle—known as the Liquefaction Refrigerant Cycle (LRC)—serves for the partial or complete condensation of the mixed refrigerant required for the subcooling and the condensation of the hydro-carbon-rich stream. The third mixed-refrigerant cycle—known as the Subcooling Refrigerant Cycle (SRC)—serves for the necessary subcooling of the liquefied hydrocarbon-rich stream.
According to a further advantageous development of the process according to the invention, the refrigerant used for the first of the three mixed-refrigerant cycles is a mixture of ethylene or ethane, propane and butane. This PRC mixed-refrigerant cycle serves for providing refrigerant in a temperature range from ambient temperature to between approximately −35 and approximately −55° C. According to a further development of the process according to the invention, the refrigerant used for the second of the three mixed-refrigerant cycles is a mixture of methane, ethylene or ethane and propane. For the third of the three mixed-refrigerant cycles, the refrigerant preferably used is a mixture of nitrogen, methane and ethylene or ethane. While the second or LRC mixed-refrigerant cycle provides refrigeration energy in a temperature interval from approximately −40 to approximately −100° C., the third or SRC mixed-refrigerant cycle serves for providing refrigeration to between approximately −85 and approximately −160° C.
The process procedure according to the invention leads to a reduction of the specific energy consumption and of the capital costs, since the three mixed-refrigerant cycles are optimally adapted, or can be adapted, to the enthalpy-temperature graphs of the hydrocarbon-rich stream to be liquefied and of the mixed refrigerants. Owing to this process procedure which is more efficient in comparison with a dual-flow refrigeration process, either the liquefaction plant required may be decreased in size and thus the costs of the plant may be decreased, or the capacity of the hydrocarbon-rich stream to be liquefied can be increased with the plant size remaining the same.
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Bolt Manfred
Forg Wolfgang
Fredheim Arne Olav
Paurola Pentti
Pfeiffer Christian
Doerrler William
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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