NATURAL GAS LIQUEFACTION

Abstract

The invention relates to a method for liquefying a hydrocarbon-rich feed fraction, preferably natural gas, against a nitrogen refrigeration cycle. A feed fraction is cooled against gaseous nitrogen that is to be warmed, and liquefied against liquid nitrogen that is to be vaporized. The feed fraction is cooled and liquefied in an at least three-stage heat-exchange process. In the first section of the heat-exchange process, the feed fraction is cooled against superheated gaseous nitrogen to the extent that an essentially complete separation of the relatively heavy components is achievable. In the second section, the feed fraction freed from relatively heavy components is partially liquefied against gaseous nitrogen that is to be superheated. In the third section, the feed fraction is liquefied against nitrogen that is to be partially vaporized.

Description

SUMMARY OF THE INVENTION

The invention relates to a method for liquefying a hydrocarbon-rich feed fraction, preferably natural gas, against a nitrogen refrigeration cycle, wherein the feed fraction is cooled against gaseous nitrogen that is to be warmed and the feed fraction is liquefied against liquid nitrogen that is to be vaporized.

Hydrocarbon-rich gases, in particular natural gases, are liquefied commercially in a capacity range from 10 to 30,000 tons of LNG per day (tpd). In plants of medium capacity—this is taken to mean liquefaction processes having a capacity between 300 and 3000 tpd of LNG—and large capacity—this is taken to mean liquefaction processes having a capacity between 3000 and 30,000 tpd of LNG—those skilled in the art attempt to optimize the operating costs by means of high efficiency. In contrast, in the case of smaller plants—this is taken to mean liquefaction processes having a capacity between 10 and 300 tpd of LNG—low capital costs are in the foreground. In such plants, the capital cost proportion of a dedicated refrigeration plant in which the working medium used is, for example, nitrogen or a nitrogen-hydrocarbon mixture, is considerable. Therefore, generation of cold in the liquefaction plant is, if possible, dispensed with and a suitable refrigerant imported. Customarily, in this case, liquid nitrogen is used and after its use as refrigerant, is given off to the atmosphere in the gaseous state. If in nearby air separation plants unused product amounts of liquid nitrogen can be provided inexpensively, this concept for small liquefaction plants is absolutely commercially expedient.

For reasons of costs, in small liquid-nitrogen-cooled plants, brazed aluminum plate heat exchangers are generally used. These appliances, however, are sensitive to high thermal stresses as can arise, for example, by an oversupply of refrigerant and/or large temperature differences between warm and cold process streams. The resultant mechanical stresses can lead to damage to these appliances.

In addition, care must be taken to ensure that, during operation of the liquefaction process, the feed fraction does not fall below the freezing temperature. The solid point of methane at —182° C. is markedly above the atmospheric boiling temperature of nitrogen, which is −196° C. Freezing of the plant always causes an unwanted operating fault and can, in addition, have lasting damage as a consequence.

A method of the type in question for liquefying a hydrocarbon-rich feed fraction is known from U.S. Pat. No. 5,390,499. This method is suitable, in particular, for plants of small capacity, as explained at the outset. In the liquefaction method described in U.S. Pat. No. 5,390,499, the gas to be liquefied is cooled and liquefied against nitrogen in two separate heat exchangers. In this case the liquid low-boiling nitrogen is completely vaporized in the second heat exchanger and warmed up to a temperature at which relatively heavy crude gas components can be taken off in the liquid state by means of a separator from the gas that is to be liquefied. In a process procedure as described in U.S. Pat. No. 5,390,499, however, the point at which the nitrogen vaporizes completely can vary considerably according to load. This can lead to unwanted process conditions which have the abovementioned disadvantages as a consequence.

It is an object of the present invention to provide a method of the type in question for liquefying a hydrocarbon-rich feed fraction, which method avoids the abovementioned disadvantages and, in particular, to provide a method which is robust against operating faults and damage.

Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

For achieving these objects, a method is proposed for liquefying a hydrocarbon-rich feed fraction, which is characterized in that

• • the feed fraction is cooled and liquefied in an at least three-stage heat-exchange process,
  • wherein, in the first section of the heat-exchange process, the feed fraction is cooled against superheated gaseous nitrogen to the extent that an essentially complete separation of the relatively heavy components is achievable,
  • in the second section of the heat-exchange process, the feed fraction freed from relatively heavy components is partially liquefied against gaseous nitrogen that is to be superheated, and
  • in the third section of the heat-exchange process, the feed fraction is liquefied against nitrogen that is to be partially vaporized.

The method according to the invention is suitable for use in plants of large (3000-30,000 tpd of LNG), medium (300-3,000 tpd of LNG), or small (10-300 tpd of LNG) capacities. The most economical capacity range, however, is 10-300 tpd of LNG.

As mentioned, the method according to the invention is directed to liquefaction of a hydrocarbon-rich feed fraction, such as natural gas. For example, the hydrocarbon-rich feed fraction can contain 80 to 99 vol. % methane, 0.1 to 10 vol. % ethane, 0 to 5 vol. % propane, 0 to 4 vol. % C4+hydrocarbons, 0 to 10 vol. % nitrogen, 0 to 10 vol. % carbon dioxide, 0 to 1 vol. % hydrogen sulfide, up to trace amounts of other sulfur species, up to trace amounts of helium, and up to trace amounts of hydrogen.

The expression “heavy components” may be taken to mean ethane and higher molecular weight hydrocarbons.

Further advantageous embodiments of the method according to the invention for liquefying a hydrocarbon-rich feed fraction are characterized in that

• • the three-stage heat-exchange process is achieved in one or more heat exchangers,
  • the condensation pressure of the feed fraction freed from relatively heavy components is adjusted to values between 1 and 15 bara (absolute pressure), preferably between 1 and 8 bara, and
  • the boiling pressure of the gaseous nitrogen that is to be superheated is adjusted to values between 5 and 30 bara, preferably between 10 and 20 bara.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated schematically with reference to an exemplary embodiment in the drawing and will be described extensively hereinafter with reference to the drawing. Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawing wherein:

the FIGURE illustrates an embodiment according to the invention.

As shown in the FIGURE, the hydrocarbon-rich feed fraction that is to be liquefied is fed via line 1 to a heat exchanger E1. This is subdivided into three sections or stages a to c. The boundaries between these sections or stages are shown by the two dashed lines. In the warmest section a of the heat exchanger E1, the hydrocarbon-rich feed fraction is cooled against superheated gaseous nitrogen, which is fed via line 9 to the heat exchanger E1. Here, the hydrocarbon-rich feed fraction is cooled to the extent that a separation of the heavy components from the feed fraction is possible in a separator D2 downstream of the heat exchanger E1. For this purpose, the cooled feed fraction is fed from the heat exchanger E1 via line 1′ to the separator D2. From the bottom phase thereof, via line 2′, in which a valve V1 is provided, the unwanted heavy components are removed in liquid form and discharged from the process.

Instead of separator D2 shown in the FIGURE, a rectification column can be used to achieve a more precise separation of relatively heavy components or higher hydrocarbons from the feed fraction.

At the top of the separator D2, via line 2, the feed fraction, freed from heavy components, is removed and fed to the second section b of the heat exchanger E1. Therein, the feed fraction that is freed from heavy components is partially liquefied against gaseous nitrogen that is to be superheated 9. Then, in the third stage c of the heat exchanger E1, the feed fraction is completely liquefied against nitrogen to be partially vaporized which is fed to the heat exchanger E1 via the line 8.

The liquefied feed fraction, after passage through the heat exchanger E1 is fed to a storage vessel D4 via line 3, in which a control valve V3 is arranged. The liquefied product (LNG) can be discharged therefrom via line 4. The control valve V3 serves for expanding the liquefied feed fraction to the product delivery pressure, which corresponds at least approximately to atmospheric pressure.

If the nitrogen is vaporized in the third section c of the heat exchanger E1 at a pressure of greater than 15 bara, the boiling temperature thereof is no longer low enough in order to subcool the liquefied feed fraction to the extent that outgassing after expansion thereof in the control valve V3 can be prevented. In this case, the boil-off gas formed in the storage vessel D4 is advantageously removed via line 5, compressed in the compressor C3 and fed back to the feed fraction 2 which is freed from heavy components before liquefaction thereof and reliquefied in the heat exchanger E1. This process procedure should be selected, in particular, in the case of significant temporary storage of the LNG product in an atmospheric flat-bottom tank D4, since the resultant boil-off gas is also processed thereby.

The nitrogen required for providing cold is fed to the liquefaction process via line 6. Advantageously, a buffer tank D3 is provided which serves for compensating for quantitative fluctuations of the feed fraction that is to be liquefied and/or of the refrigerant nitrogen. By means of a pump P1, liquid nitrogen is fed in the amount required to a separator D1 via line 7. From the bottom phase of the separator D1, boiling nitrogen is removed and conducted via line 8 through the coldest section c of the heat exchanger E1. The nitrogen that is partially vaporized in this case is then fed via line 8′ back to the separator D1.

If the reliquefaction process that is still to be described is operated, at least temporarily the generation of cold by the reliquefaction of the nitrogen can exceed the refrigeration requirement of the natural gas liquefaction. An oversupply resulting therefrom of liquid nitrogen can be delivered into the buffer tank D3 via line 8″ and valve V6.

At the top of the separator D1, gaseous nitrogen is taken off via line 9 and fed to the middle section b of the heat exchanger E1. The gaseous nitrogen is conducted through the second and first sections of the heat exchanger E1 in countercurrent flow to the feed fraction 2 that is to be cooled and partially liquefied, and is warmed and superheated in this process. The superheated nitrogen is then removed from the process via the line sections 10 and 11.

By means of the control valve V4, the boiling pressure of the gaseous nitrogen that is to be superheated 9 can be controlled. Advantageously, this boiling pressure is adjusted to values between 5 and 30 bara, preferably between 10 and 20 bara.

Similarly, the condensation pressure of the feed fraction 2 that is freed from relatively heavy components can be controlled by means of the control valve V2. This condensation pressure is preferably adjusted to values between 1 and 15 bara, preferably between 1 and 8 bara.

By means of the control valves V2 and/or V4, the temperature profile in the third section c of the heat exchanger E1 can be controlled thereby. By means of the control valve V2, the condensation pressure of the feed fraction is established in the section between the control valves V2 and V3, and, by means of the control valve V4, the boiling pressure of the nitrogen in the separator D1 and the third section c of the heat exchanger E1 is controlled. Owing to the above-described subdivision of the heat-exchange process into a second and third section and with the phase separation in separator D1 it can then be established exactly in what section of the heat exchanger E1 a (partial) vaporization or superheating of the nitrogen is taking place.

By means of the subdivision of the heat-exchange process E1 into three sections a to c, it is possible to reliably prevent the phase boundary between liquid and gaseous refrigerant from migrating within the heat exchanger E1 and thereby causing unwanted thermal and mechanical stresses within the heat exchanger E1.

If the nitrogen boiling pressure (pN₂) and the crude gas condensation pressure (pRG) are selected according to the inequality pRG (bara)≧0.3 pN₂ (bara) −1, a thermal overload of the heat exchanger E1 due to impermissibly high temperature differences can be safely avoided.

By restricting the boiling pressure of the liquid nitrogen in the third section c of the heat exchanger E1 and of the separator D1 to at least 5 bara—the associated boiling temperature is −179° C.—it is possible to prevent reliably a temperature below the freezing temperature of methane occurring in the heat exchanger E1. Operating problems and possible damage due to solids formation are thereby excluded.

The superheated nitrogen taken off from the heat exchanger E1 via line 10 can, alternatively to a removal via line 11, be at least partially reliquefied. For this purpose the nitrogen is fed via the line sections 12 and 13 to a compression—shown in the figure by a two-stage compressor unit C1/C2, wherein a heat exchanger, E3 or E4 respectively, is connected downstream of each compressor unit—and then is fed via line 14 to a heat exchanger E2. Therein, the nitrogen is reliquefied and then fed to separator D1 via line 15. Pressure regulation of the compressor C2 is performed by the control valve V5. For the purpose of providing cold in the heat exchanger E2, a substream of the compressed nitrogen stream is removed via line 16, preferably expanded in a multistage manner—shown by the gas expanders X1 and X2—and then conducted via line 17 through the heat exchanger E2 in countercurrent flow to the nitrogen stream that is to be liquefied. The shafts of the compressors C1 and C2 are preferably coupled to the shafts of the gas expanders X2 and X1.

If the above-described reliquefaction process is operated, it is advantageous to feed to the heat exchanger E1 via line 9 only the amount of gaseous nitrogen that is required for a small positive temperature difference of approximately 3° C. between streams 1 and 10 at the warm end of the heat exchanger E1. The excess amount of cold gaseous nitrogen is used via line 9′ proportionately for reliquefaction in the heat exchanger E2.

In principle, the liquefaction process can proceed by means of “imported” nitrogen—in this case, the superheated nitrogen is taken off from the heat exchanger E1 via the line sections 10 and 11—by means of reliquefied nitrogen, or by any desired combination of both modes of operation.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. DE 10 2010 044869.9, filed Sep. 9, 2010 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

Claims

1. A method for liquefying a hydrocarbon-rich feed fraction against a nitrogen refrigeration cycle, comprising:

cooling said wherein feed fraction against gaseous nitrogen that is to be warmed, and liquefying said feed fraction against liquid nitrogen that is to be vaporized,

wherein said feed fraction is cooled and liquefied in an at least three-stage heat-exchange process (E1a-E1c), in the first section of said heat-exchange process (E1a), said feed fraction (1) is cooled against superheated gaseous nitrogen (9) to the extent that an essentially complete separation (D2) of relatively heavy components (2′) is achievable,

in the second section of said heat-exchange process (E1b), the feed fraction (2) freed from relatively heavy components is partially liquefied against gaseous nitrogen that is to be superheated (9), and in the third section of said heat-exchange process (E1c), the feed fraction (2) is liquefied against nitrogen that is to be partially vaporized (8).

2. The method according to claim 1, wherein said hydrocarbon-rich feed fraction is natural gas.

3. The method according to claim 1, wherein said three-stage heat-exchange process (E1a-E1c) is performed in one heat exchanger.

4. The method according to claim 2, wherein said three-stage heat-exchange process (E1a-E1c) is performed in one heat exchanger.

5. The method according to claim 1, wherein said three-stage heat-exchange process (E1a-E1c) is performed in more than one heat exchanger.

6. The method according to claim 2, wherein said three-stage heat-exchange process (E1a-E1c) is performed in more than one heat exchanger.

7. The method according to claim 1, wherein the condensation pressure of the feed fraction (2) freed from relatively heavy components is adjusted (V2) to a value of 1-15 bara.

8. The method according to claim 7, wherein the condensation pressure of the feed fraction (2) freed from relatively heavy components is adjusted (V2) to a value of 1-8 bara.

9. The method according to claim 1, wherein the boiling pressure of the gaseous nitrogen that is to be superheated (9) is adjusted (V4) to a value of 5-30 bara.

10. The method according to claim 9, wherein the boiling pressure of the gaseous nitrogen that is to be superheated (9) is adjusted (V4) to a value of 10-20 bara.