AU2006281407A1

AU2006281407A1 - Method and arrangement for liquefying a stream rich in hydrocarbons

Info

Publication number: AU2006281407A1
Application number: AU2006281407A
Authority: AU
Inventors: Manfred Boelt; Wolfgang Foerg; Arne Fredheim; Pentti Paurola; Christian Pfeiffer; Oystein Sorensen; Manfred Steinbauer; Rudolf Stockmann
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2005-08-12
Filing date: 2006-08-11
Publication date: 2007-02-22
Anticipated expiration: 2026-08-11
Also published as: EP1913319A2; DE102005038266A1; WO2007020252A3; WO2007020252A2; AU2006281407B2

Description

WO 2007/020252 1 PCT/EP2006/065278 Description Method and Arrangement for Liquefying a Stream Enriched with Hydrocarbons Field of Technology [0001] This invention relates to a method and an arrangement for liquefying, in particular by pre-cooling or by super-cooling, a stream which is enriched with hydrocarbons, in particular a stream of natural gas, by an indirect heat exchange with the mixed refrigerant of at least one mixed refrigerant cycle, which takes place by means of at least one heat exchanger. After expansion, the mixed refrigerant is vaporized only in heat exchange with the stream, which is enriched with hydrocarbons and which is to be liquefied, and with itself. Background Art [0002] Today, natural gas liquefaction plants are usually operated with so-called mixed cycles as the refrigerant cycles. Therefore, the refrigerant is a mixture comprising two or more components of the following substances: nitrogen, methane, ethane or ethylene, propane or propylene, butane and optionally even higher boiling hydrocarbons. [0003] The publication WO 03/106906 Al discloses a method for liquefying a stream, enriched with hydrocarbons, in particular a natural gas stream, with the simultaneous recovery of a high yield fraction, enriched with C3+. In this case, the liquefaction of the stream enriched with hydrocarbons takes place by a heat exchange using a mixed refrigerant recirculating cascade. [0004] The publication DE 199 37 623 Al discloses a method, conforming to its genre, for liquefying a stream, which is enriched with hydrocarbons, in particular a natural gas stream, by an indirect heat exchange with at least one mixed refrigerant cycle. By citing the publication DE 199 37 623 Al, the entire text of this disclosure is incorporated by reference in the text disclosed in this patent application.

WO 2007/020252 2 PCT/EP2006/065278 [0005] In this method, known from the publication DE 199 37 623 Al, the mixed refrigerant is a two phase stream prior to compression. The separation of the two phase stream into a gaseous stream and a liquid stream may take place by means of a separator and/or by means of a separating column. At the same time, the liquid fraction of the two phase stream may amount to as much as fifteen percent. [0006] Another method, conforming to its genre, for liquefying a stream, which is enriched with hydrocarbons, in particular a natural gas stream, by an indirect heat exchange is disclosed in the publication DE 197 16 415 Cl. By citing the publication DE 197 16 415 Cl, the entire text of this disclosure is incorporated by reference into the text disclosed in this patent application. [0007] In this method, known from the publication DE 197 16 415 Cl, the mixed refrigerant is compressed in a single-staged or multi-staged compressor, cooled, liquefied and super-cooled in one heat exchanger or a plurality of (optionally different) heat exchangers. After its expansion - in a valve or in an expansion turbine -, the mixed refrigerant is totally vaporized in the downcoming stream, for example, in the external space of a coiled heat exchanger and heated five Kelvin up to 15 Kelvin above its dew point so that it is ensured that said mixed refrigerant will be compressed again in the recirculating compressor. [0008] It has turned out now that heat exchangers, in which this total evaporation and super-heating of the circulating medium or rather the mixed refrigerant take place, develop leaks - in the case of a coiled heat exchanger in the coiled tubes. The stress on the tubes of the coiled heat exchanger is caused by thermal and acoustical vibrations, which are generated in the dry section of the heat exchanger. [0009] However, it has also turned out that heat exchangers, which are totally wetted, do not develop any leaks. This effect can be observed, in particular, in two-tiered and three tiered heat exchangers, in which the upper, totally wetted heat exchangers do not develop any leaks. Presentation of the Invention [0010] Against this background of the aforementioned drawbacks and inadequacies as well as in consideration of the briefly described state of the art, the object of this invention is to WO 2007/020252 3 PCT/EP2006/065278 improve a method of the type described in the introductory part as well as an arrangement of the aforementioned type in such a way that the aforesaid problems are avoided. [0011] This object is achieved by means of a method exhibiting the features disclosed in claim I as well as by means of an arrangement exhibiting the features disclosed in claim 5. The advantageous embodiments and pertinent further developments of this invention are characterized in the respective dependent claims. [0012] According to the teaching of this invention, between approximately ninety percent and approximately 99 percent, preferably approximately 95 percent, of the mixed refrigerant stream, which is to be vaporized, vaporizes. Then the partially vaporized stream of mixed refrigerant is separated into a gas fraction and into a liquid fraction. The gas fraction is compressed to the desired final pressure preferably by means of at least one cold-intake compressor. The liquid fraction is pumped, and then the two fractions are combined. [0013] According to advantageous embodiments of the inventive method and the inventive arrangement, the gas fraction and the liquid fraction may be combined either before or after their expansion. If both fractions are combined as early as before their expansion, the liquid fraction is pumped preferably to the same pressure as the gas fraction. [0014] The circulating medium or rather the mixed refrigerant is enriched in a practical way with heavier components to the extent that there is no total evaporation, but rather a residual liquid, ranging from approximately one percent to approximately ten percent, remains; preferably a residual liquid of approximately five percent remains. However, this wet vapor may not be fed into the compressor; rather this wet vapor must be separated into a gas fraction and into a liquid fraction by means of at least one separator. [0015] The gas fraction is compressed in an advantageous manner by means of at least one cold-intake compressor. The liquid fraction is pumped by means of at least one suitable pump, during which process the liquid fraction is slightly super-cooled in an advantageous way prior to pumping. [0016] The design and/or the operation of the pump takes the N[et] P[ositive] S[uction] H[ead] value into consideration. The NPSH value of the pump is the product of the type of construction and the pump speed. The higher the pump speed, the larger the NPSH value of the pump is.

WO 2007/020252 4 PCT/EP2006/065278 [0017] Finally, this invention relates to the use of a method, according to the above described type, and/or at least an arrangement, according to the above described type, for liquefying, in particular by pre-cooling or by super-cooling, a stream, enriched with hydrocarbons, in particular a natural gas stream. Furthermore, the L[iquefied] N[atural] G[as] technology is a preferred field of application for this invention. [0018] Therefore, the inventive procedure may be applied to all liquefaction methods, wherein the heat exchange between the stream, which is enriched with hydrocarbons and is to be liquefied, and the mixed refrigerant takes place in one or more coiled heat exchanger(s) and/or in one or more plate-type heat exchanger(s). [0019] Furthermore, in principle, the inventive procedure may be implemented with all mixed cycles, in particular with the so-called M[ixed] F[luid] C[ascade] processes, with the so-called C3 M[ixed] R[efrigerant] C[ycle] process (propane pre-cooled mixed refrigerant process) of the company Air Products, with the so-called Dual Flow M[ixed] R[efrigerant] C[ycle] process of the company Shell and/or with the so-called Single Flow M[ixed] R[efrigerant] C[ycle] process of the company Linde and/or the company Statoil. [0020] In principle, it holds true that to the extent two or more mixed refrigerant cycles are used in the inventive method and/or in the inventive arrangement, said at least two mixed refrigerant cycles may be arranged in succession one after the other and/or configured in the manner of a cascade. Brief Description of the Drawings [00211 As stated above, there are a number of different possibilities of constructing and improving the teaching of this invention. In this respect, reference is made, on the one hand, to the claims following claim 1, claim 5 and claim 9. On the other hand, additional embodiments, features and advantages of this invention are explained in detail below with reference to the two embodiments, depicted in Figure 1 and Figure 2. [0022] Hence, [0023] Figure 1 is schematic drawing of a first embodiment of an inventive arrangement, wherein the process entails pumping the liquid fraction to the same pressure at which the gas fraction is compressed and then combining the two fractions; and WO 2007/020252 5 PCT/EP2006/065278 [0024] Figure 2 is schematic drawing of a second embodiment of an inventive arrangement, wherein the process entails pumping the liquid fraction only to the extent that - after super cooling in its own heat exchanger passage - this liquid fraction is admixed to the expanded gas fraction. Way(s) of Carrying Out the Invention [0025] In order to avoid superfluous repetition, the following explanations regarding the embodiments, features and advantages of this invention (unless stated otherwise) relate to the first embodiment of an inventive arrangement, which works according to the method of the invention and is depicted in Figure 1, as well as to the second embodiment of an inventive arrangement, which works according to the method of the invention and is depicted in Figure 2. [0026] A stream, which is enriched with hydrocarbons and is to be liquefied, for example a natural gas stream, is fed, as illustrated in Figure 1, through line A to a heat exchanger E. [0027] In this heat exchanger E, a pre-cooling or a liquefaction or a super-cooling of the stream, enriched with hydrocarbons, takes place only in exchange heat with a mixed refrigerant, which will be discussed in detail below. Following pre-cooling or liquefaction or super-cooling, the stream enriched with hydrocarbons is drawn off from the heat exchanger E by way of line B and conveyed away for further use. [0028] The pressurized mixed refrigerant is fed through line 1 to the heat exchanger E and liquefied and super-cooled in the heat exchanger E. The super-cooled mixed refrigerant is drawn off from the heat exchanger E by way of line 2 and expanded or rather expanded so as to perform refrigeration in the expansion unit 3, which is a valve or an expansion turbine. [0029] Finally, at this point, the mixed refrigerant in the heat exchanger E is evaporated - as compared to the liquefaction process known from the prior art - only to the extent that it exhibits a residual liquid ranging from approximately one percent to approximately ten percent, preferably a residual liquid of approximately five percent, at the outlet of the heat exchanger E. [0030] This two phase mixture is fed through line 4 to a separator D. The gas fraction of the mixed refrigerant is drawn off at the head of the separator D by way of line 5 and WO 2007/020252 6 PCT/EP2006/065278 compressed to the desired cycle pressure using the cold-intake, single-staged or multi-staged compressor V. [0031] The liquid fraction of the mixed refrigerant is drawn off from the bottom of the separator D by way of line 6 and is also pumped to the desired cycle pressure using a pump P and then fed to the gas fraction of the mixed refrigerant that is drawn off from the separator D by way of line 5 and compressed. [0032] The process, depicted in Figure 1, is suitable in particular for retrofitting pre existing plants. [0033] With respect to the process, depicted in Figure 2, for the arrangement according to the second embodiment, reference is made to Figure 1 with respect to the content. [0034] The distinction between the process, according to Figure 2, and the process, according to Figure 1, lies in the fact that the liquid fraction of the mixed refrigerant that is drawn off from the bottom of the separator D by way of line 6 does not have to be pumped to the desired cycle pressure with the pump P. Rather an increase in pressure is adequate to compensate for the pressure losses caused by line 7, caused by its own passage (middle in Figure 2) of the heat exchanger E, or by line 8 or by the expansion unit 9, which may be a valve. [0035] Compared to the process depicted in Figure 1, this process has the major advantage that the lines, located between the compressor V and the heat exchanger E, and the heat exchanger do not have to be designed for a range between approximately 101 percent and approximately 110 percent and that the pump P needs to be designed only for a correspondingly smaller pressure increase. [0036] Finally the liquid fraction is fed to the heat exchanger E by way of line 7, (which is an additional line, as compared to the process according to Figure 1), and is super-cooled in the heat exchanger E (in an additional passage, as compared to the process, according to Figure 1). [0037] Following removal from the heat exchanger E by way of line 8 (which is an additional line, as compared to the process, according to Figure 1), there is a slight expansion in the valve 9 (which is an additional valve, as compared to the process, WO 2007/020252 7 PCT/EP2006/065278 according to Figure 1) to the pressure of the expanded "gas fraction" in line 2', where the two fractions are combined prior to feeding into the heat exchanger E. [0038] The process, depicted in Figure 2, is suitable especially for new.plants. [0039] The process, according to the invention, makes it possible to avoid entirely, or at least largely, the formation of leaks owing to the total wetting of the heat exchanger tubes, because thermal and acoustical vibrations in the heat exchanger passages are avoided or rather significantly reduced.