MX2007006584A

MX2007006584A - Method for separating a fraction rich in c2+ from liquefied natural gas.

Info

Publication number: MX2007006584A
Application number: MX2007006584A
Authority: MX
Inventors: Heinz Bauer; Hubert Franke; Rainer Sapper
Original assignee: Linde Ag
Priority date: 2005-01-03
Filing date: 2005-12-21
Publication date: 2007-07-13
Also published as: EP1834144B1; KR20070100760A; BRPI0518529A2; KR101236863B1; BRPI0518529B1; US20090211296A1; JP2008527287A; ES2665688T3; WO2006072390A1; CN101095023B; DE102005000634A1; CN101095023A; EP1834144A1

Abstract

The invention relates to a method for separating a fraction rich in C2+from liquefied natural gas (LNG), comprising the following steps: a) partialevaporation (E1, E2) of the liquefied natural gas (1, 1'); b) separation(D1) of the partially evaporated natural gas (2) into a first gas fraction (4)rich in C1 and a first liquid fraction (3) rich in C2+; c) rectifyingseparation (T) of the first liquid fraction (3) rich in C2+ into a secondgas fraction (8) rich in C1 and a second liquid fraction (10) rich inC2+; d) re-liquefaction (E1) of the first gas fraction (4) rich in C1and e) feeding of at least one sub-stream (6) of the re-liquefied gas fraction(5) rich in C1 as a return flow to the rectifying separation (T) stage.

Description

METHOD TO SEPARATE A RICH FRAME IN C2 + OF LIQUEFIED NATURAL GAS DESCRIPTION OF THE INVENTION The invention relates to a method for separating a C2 + rich fraction from liquefied natural gas (LNG). The methods of the type under consideration should be foreseen, for example, when the calorific value of the liquefied natural gas does not correspond to the desired specifications of the pipeline network to which the natural gas must be fed. In such a case, either a component that reduces the calorific value, for example, nitrogen, is added to the liquefied natural gas, or the components that have the effect of increasing the calorific value are removed from the liquefied natural gas. Next, the second alternative mentioned above will be explained in greater detail. From US-A 5,114,451 a method of the type under consideration is known for separating a C2 + or C + rich fraction from liquefied natural gas in which - unlike the methods carried out to date - a partial stream of the fraction is relicized. of gas rich in Ci obtained in the rectifying separation of natural gas (liquefied) and fed as reflux to the rectifier separation. The remaining current of the fraction of gas rich in Ci is only compressed at the delivery pressure or the desired gas pipeline after the separation of the partial current that REF.:182469 constitutes the reflux current described above. It is true that the method described in US-A 5,114,451 allows an increase in the yield of ethane at economically interesting values, but this is nevertheless obtained by the use of at least one cost-intensive compressor. A method of the type under consideration for separating a C2 + rich fraction from liquefied natural gas is also known from US-A 3,420,068. In this case, a recompression of the fraction of methane-rich gas obtained in the rectifying separation of partially evaporated natural gas is dispensed with, which means that it is only possible to achieve medium ethane yields. The object of the present invention is to specify a method of the type under consideration to separate a C2 + rich fraction from liquefied natural gas (LNG) that allows to increase the yield of the C2 + rich fraction and simultaneously reduce the investment and operation costs of the process, in particular to dispense with a compression of the gas fraction rich in methane. For the solution of the preceding problem, a method of the type under consideration for separating a C2 + -rich fraction of liquefied natural gas (LNG) comprising the following steps is proposed: a) partial evaporation of the liquefied natural gas, b) separation of the partially evaporated natural gas in a first fraction of gas rich in Ci and a first liquid fraction rich in C2 +, c) rectifying separation of the first liquid fraction rich in C2 + to a second fraction of gas rich in Ci and a second liquid fraction rich in C2 +, d) re-liquefied of the first fraction of gas rich in Ci and e) the supply of at least one partial flow of the fraction of liquefied gas rich in Ci as a reflux to the rectifier separation stage. Contrary to the ways of proceeding mentioned in the introduction, the first fraction of Ci-rich gas obtained in the separation of the partially evaporated natural gas is relicled and is at least partially fed back as reflux to the rectifier separation. The cold that is required for the relicting of the first fraction of gas rich in Ci can be provided in this case exclusively by the stream of liquid natural gas, which is subjected to a maximum heating of the boiling point. Because the fraction of Ci-rich gas that is fed as reflux to the rectifier separation comprises a relatively low ethane content, this has the effect of a rewashing of ethane and heavy components of the head product of the grinding separations and with it the desired increase in the performance of C2 +. Thus, the method according to the invention allows to realize ethane yields higher than 90% with an economically reasonable operating method. This value is within the range of ethane yield that can be obtained with the costly method according to US-A 5,114,451, but clearly above the value of the process conduction according to US-A 3,420,068. The method according to the invention as well as other improvements thereof which are the subject of the dependent claims are explained in more detail below by means of the exemplary embodiment shown in figures 1 and 2. As shown in figure 1, from an LNG storage tank S, through line 1, the heat exchanger is fed. The liquefied and supercooled natural gas heat pump is pumped at a pressure between 15 and 30 bar. In this the liquefied natural gas is heated until just before the boiling point against the fraction 4 of gas rich in Ci to be cooled and re-liquefied on which we will go into details later. The distance from the boiling point is typically 5aC, at most 202C.

The heated natural gas is then fed through the line 1 'to an additional heat exchanger E2, and in this it is heated and partially evaporated against the second fraction 8 of Ci rich gas to be cooled, on which we will go into details below. The partially evaporated natural gas stream is fed through line 2 to a Di separator. In this, a separation of the stream of partially evaporated natural gas is carried out in a first fraction of gas rich in Ci, which is extracted in the head of the separator Di through line 4, and a first liquid fraction rich in C2 +. The latter is fed through the line 3 in which a pump P3 is provided to the separation column T and thus to the rectifier separation. Alternatively to the divider D shown in FIG. 1 it is possible to provide a rectification column in which the separation of partially evaporated natural gas 2 is carried out. In this case, the supply of stream 2 of partially evaporated natural gas would be made to the sump of the rectification column. The reflux of the column that is required can be carried out, either by a current that is extracted from the line 6 that will still be described, or by a current that is extracted after the heat exchanger E2 and before the valve c of the line 8 that will still be described. This configuration of the method according to the invention makes it possible to substantially increase the efficiency of C2, which results in a reduction of the losses to the heating gas. In the separation column T a rectifying separation takes place of the liquid fraction 3 rich in C2 + described above in a second fraction of gas rich in Ci, which is extracted at the head of the separation column T through line 8. , and a second liquid fraction rich in C2 +. The separation column T may comprise bases and / or packages. The second liquid fraction rich in C2 + mentioned above is extracted from the sump of the separation column T through the line 10 in which a regulating valve is provided, and is removed from the process as a NGL fraction (Natural Gas Liquids = Natural Gas Liquids as known in English) and optionally fed to another use. A partial stream of this fraction is partially evaporated in the heat exchanger E3 and fed through the line 11 as a reheat stream to the separation column T. Conveniently the rectifier separation T is carried out at a higher pressure than the separation Di of the partially evaporated natural gas 2 in a first fraction 4 of Ci rich gas and a first liquid fraction 3 rich in C2 +. In the separation or separator DI preferably a pressure between 15 and 25 bar is carried out and in the rectifier separation T preferably a pressure between 30 and 40 bar. To overcome the pressure difference between the separator Di and the separation column T, a pump P3 is provided in line 3. The first fraction of gas rich in Ci obtained with the separation of the stream 2 of partially evaporated natural gas, which is withdrawn from the head of the separator DI through line 4 is relique in the exchanger The heat against the stream 1 of natural gas to be heated, is pumped by pump P2 at a pressure prevailing in the column T of separation and then fed as reflux through the pipe sections 5 and 6 - with a regulating valve d being provided in the pipe section 6 - to the grinding gap T, preferably in the head area. The second fraction of gas rich in Ci obtained in the rectifier separation T is extracted at the head of the separation column T through line 8, and in the heat exchanger E2 is at least partially relieved, but nevertheless preferably completely against the current 1 'of natural gas to be heated, and through the line 8' in which a regulating valve c is located is fed to a pumping drum D2. This is fed through the pipe section 7, in which a regulating valve b is also disposed, that partial stream of the first fraction rich in reliced Ci that is not fed as reflux to the separation column T. From the pumping drum D2, the deconcentrated LNG product fraction C2 + is pumped by pump P4 to the desired delivery pressure or the gas pipeline - this is usually between 50 and 150 bar - and is evacuated from the process through the pipe 9 in which a regulator valve is also provided. As soon as it is necessary to subject the C2 / C-rich liquid fraction obtained in the rectifier separation T, which is extracted from the sump of the separation column T through the pipe 10, to a C2 / C separation, the procedure is preferably carried out as shown. in Figure 2. The second liquid fraction rich in C + is subjected to a (second) rectifying separation of C / C3 in the separation column T. "The drain of the separation column T 'is extracted through the pipe 18. a fraction of LPG product rich in C3 + and optionally fed to further processing or use.A partial stream of this fraction is partially evaporated in the heat exchanger E5 and fed through line 19 as reheat current to the column T 'of separation.

At the head of the separation column T 'a gas fraction rich in C2 / C3 is extracted through the pipe 12, at least partially condensed in the side boiler E4, which is connected through the pipes 13 with the separating column T, and then fed through the pipe 14 to the pumping barrel D3. From the pumping barrel D3, the C2 / C3-rich fraction condensed to the pump P5 is fed through the pipe 15 and is pumped through it to the desired delivery pressure. A partial stream of the pumped fraction is fed as reflux to the head of the separation column T 'through the pipe 16 and the regulating valve f, while the main stream of the pumped fraction is evacuated from the process through of the pipe 17 and optionally is fed to an additional use or processing. However, alternatively to the procedure described by FIG. 2, other separation processes C2 / C3 can also be carried out, for example, extraction of a fraction of the gaseous product from the container D3. In refinement of the method according to the invention to separate a C2 + rich fraction from liquefied natural gas, it is proposed that the cooling of the process streams resulting from the separation T1 of the second liquid fraction rich in C2 + is effected against the first liquid fraction rich in C2 + and / or against at least one liquid fraction which is extracted from the rectifying separation below the feed of the first liquid fraction rich in C2 +. In particular, if the second liquid fraction rich in C2 + is subjected to a rectifying separation T 'of C2 / C3, it is recommended to cool the fraction 12 of C2 / C3 rich gas obtained in the C2 / C3 rectifying separation T' by changing the heat against the first liquid fraction 3 rich in C2 +, preferably refrigerate it at the storage temperature without pressure. The heat exchanger required for this can be provided, for example, between the pump P3 and the separation column T. In order to be able to carry out a storage without pressure, in the case of a fraction of gas constituted mainly of ethane obtained in the head of the separation C "rectifier of C2 / C3, a temperature of approximately -100 ° C. is required. of the process is an advantage that it is possible to dispense with in the whole external cooling process - that is, cooling below the ambient temperature - or - thermodynamically equivalent - compression If all the process streams enter in liquid form (current 1 ) as explained by the processes shown in FIGS. 1 and 2 and again delivered liquid (streams 9 and 10 or 9, 17 and 18), the cooling finally takes place due to the supercooling of the LNG used. The method according to the invention for separating a C2 + rich fraction from liquefied natural gas (LNG) allows high ethane yields to be made while simultaneously dispensing with the use of cost-intensive compressors. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims

CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. Method for separating a C2 + rich fraction from liquefied natural gas, characterized in that it comprises the following steps: a) partial evaporation of liquefied natural gas, b) separation of partially evaporated natural gas in a first fraction of gas rich in C1 and a first liquid fraction rich in C2 +, c) rectifying separation of the first liquid fraction rich in C2 + in a second fraction of gas rich in C1 and a second liquid fraction rich in C2 +, d) re-liquefied from the first fraction of gas rich in ^ and e) feeding of at least a partial stream of the fraction of re-liquefied gas rich in Cx as reflux to the rectifier separation. Method according to claim 1, characterized in that the relicting of the first fraction of gas rich in Ci is carried out by changing the heat with the liquefied natural gas to be evaporated. Method according to claim 1 or 2, characterized in that the rectifying separation takes place at a higher pressure than the separation of the partially evaporated natural gas in a first gas fraction rich in Ci and a first liquid fraction rich in C2 +. 4. Method according to claim 3, characterized in that the separation of partially evaporated natural gas in a first gas fraction rich in Ci and a first liquid fraction rich in C2 + is carried out in a pressure range between 15 and 25 bar, and the rectifying separation in a pressure range between 30 and 40 bar. Method according to any one of the preceding claims 1 to 4, characterized in that the partial flow of the fraction of gas rich in recrystallized Ci that is not fed as reflux to the rectifier separation is combined with the second fraction of gas rich in Ci obtained in the rectifying separation. 6. Method according to any of the preceding claims 1 to 5, characterized in that the second fraction of gas rich in Ci obtained in the rectifying separation is at least partially relieved, being that this reliquement is preferably carried out by heat exchange with the liquefied natural gas to be evaporated. Method according to any of the preceding claims 1 to 6, characterized in that the cooling of the process streams resulting from the separation of the second liquid fraction rich in C2 + is carried out against the first liquid fraction rich in C + and / or against at least one liquid fraction that is extracted from the rectifier separation below the feed of the first liquid fraction rich in C2 +. 8. Method according to any of the preceding claims 1 to 7 wherein the second liquid fraction rich in C2 + is subjected to a rectifying separation of C2 / C3, characterized in that the head condensation of the separation of C2 / C3 is carried carried out in a lateral boiler against at least one liquid fraction that is extracted from the rectifying separation below the feed of the first liquid fraction rich in C2 +. Method according to any one of the preceding claims 1 to 4, wherein the second liquid fraction rich in C2 + is subjected to a rectifying separation of C2 / C3, characterized in that the fraction of gas rich in C2 / C3 that is obtained in the rectifying separation of C2 / C3 it is cooled by heat exchange against the first liquid fraction rich in C2 +, preferably it is cooled to the storage temperature without pressure. 10. Method according to any of the preceding claims 1 to 9, characterized in that the separation of the partially evaporated natural gas in a first fraction of gas rich in Ci and a first liquid fraction rich in C2 + is carried out in a separator or in a a rectification column.