Removal of hydrogen   

Abstract: The invention relates to a method for liquefying a hydrocarbon-rich fraction (1, 1′) which contains substantially methane, hydrogen and nitrogen. In the inventive method, before the liquefaction (V) of the hydrocarbon-rich fraction (1, 1′), the hydrogen (2) is removed (M) by permeation. This removal (M) of the hydrogen (2) by permeation is effected in a single-stage or multistage removal process.

SUMMARY

The invention relates to a method for liquefying a hydrocarbon-rich fraction which contains substantially methane, hydrogen and nitrogen.

Methods of the type in question for liquefying a hydrocarbon-rich fraction have long been known from the prior art. In methods of this type which serve, for example, to liquefy natural gas, the hydrocarbon-rich fraction to be liquefied is cooled and, in the course of this, liquefied, by indirect heat exchange against one or more refrigeration cycles. If the hydrocarbon-rich fraction to be liquefied contains components that either should not be present in the liquid product to be obtained or can lead to deposits in the cold part of the liquefaction process, these components must be eliminated therefrom before and/or during the cooling of the hydrocarbon-rich fraction that is to be liquefied. The abovementioned unwanted components are, in particular, aromatics, higher hydrocarbons, water, carbon monoxide and carbon dioxide.

Since current prices for liquefied natural gas (LNG) product are comparatively high, new natural gas sources for the abovementioned liquefaction processes are currently being considered or sought. Merely by way of example, these new natural gas sources include methane-rich gas mixtures which originate from coal gasification or methanization processes. These methane-rich gas mixtures, however, also comprise significant fractions of hydrogen, which can be greater than 20% by volume. In addition, these methane-rich gas mixtures frequently contain more than 10% by volume of nitrogen, as well as traces of noble gases, carbon monoxide and carbon dioxide.

Since the liquefied end product or LNG product must meet preset specifications, the abovementioned hydrogen must be reduced to a tolerable level (typically <3 vol %) from the hydrocarbon-rich fraction that is to be liquefied. Owing to its good solubility in the liquefied hydrocarbon-rich fraction and the liquefied natural gas, hydrogen, in contrast to helium, cannot be removed solely by a single-stage or multistage separation. In addition, a conventional low-temperature separation process would be highly energy-consuming, since hydrogen likewise boils at a very low temperature and the separation of hydrogen and methane requires a very low temperature. However, in principle, such a separation would be achievable.

There is therefore the need for a method for liquefying hydrocarbon-rich fractions which contain a more than inconsiderable proportion of hydrogen, wherein a less energy-consuming removal of hydrogen can be achieved.

It is an object of the present invention to provide a method of the type in question for liquefying a hydrocarbon-rich fraction which contains substantially methane and hydrogen, wherein the method avoids the abovementioned disadvantages.

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

In order to achieve these objects, a method for liquefying a hydrocarbon-rich fraction is proposed, wherein hydrogen is removed by permeation before the liquefaction of the hydrocarbon-rich fraction. In this case, the hydrogen-rich fraction usually passes through a membrane layer as a low-pressure stream, whereas the low-hydrogen fraction is delivered at approximately the pressure at which the hydrocarbon-rich fraction is fed to the unit for removal of hydrogen by permeation.

Further advantageous embodiments of the method according to the invention for liquefying a hydrocarbon-rich fraction include:

removing the hydrogen by permeation in a single-stage or multistage removal process,

compressing the removed hydrogen,

if the liquefied hydrocarbon-rich fraction is temporarily stored and during this temporary storage a tank boil-off gas fraction is produced, compressing this tank boil-off gas fraction,

if, during the process of liquefying the hydrocarbon-rich fraction, a nitrogen-rich fraction is removed, compressing this nitrogen-rich fraction, and

combining the compressed hydrogen and/or the compressed tank boil-off gas fraction and/or the compressed nitrogen-rich fraction.

The composition of the hydrocarbon-rich feed fraction can vary. For example, the feed fraction can contain up to 60 vol % hydrogen, up to 20 vol % nitrogen, up to 10 vol % C2+ hydrocarbons, up to 0.1 vol % aromatics, water up to saturation, up to 30 vol % carbon monoxide, up to 10 vol % carbon dioxide, and up to 5 vol % noble gases (e.g. helium, neon, argon).

With regards to the compression of the compressing the removed hydrogen, the tank boil-off gas fraction, and/or the nitrogen-rich fraction, such compression my be required for subsequent uses of these light fraction, either singly or combined. For example, for use in a fuel gas system compression to typically 6-8 bar may be required. Compression to even higher pressures (20-40 bara) may be required for further processing, e.g. for methanol synthesis gas.

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:

FIG. 1 illustrates an embodiment according to the invention; and

FIG. 2 illustrates a further embodiment according to the invention.

As shown in FIG. 1, the hydrocarbon-rich fraction to be liquefied, which comprises substantially methane and hydrogen and also possibly nitrogen in significant amounts, is fed via line 1 to an optional prepurification unit R. This prepurification unit R serves to remove unwanted components which either should not be present in the product that is to be liquefied (LNG), or freeze out during the subsequent liquefaction process V, thereby leading to problems in the liquefaction process V. Alternatively, this prepurification unit R can be integrated completely or partly in the liquefaction process V.

The prepurified hydrocarbon-rich fraction removed from prepurification unit R is fed via line 1′ to a membrane separation unit M with a pressure of typically 15-40 bara. A hydrogen-rich permeate stream is removed from membrane separation unit M via line 2 with approx. 2 bara pressure, while the hydrocarbon-rich fraction, low in hydrogen (approx. 0.5-2% hydrogen), is removed from membrane separation unit M via line 3 with a pressure of typically 14-38 bara. In the membrane separation unit M, a single-stage or multistage process of removal by permeation is implemented.

The hydrocarbon-rich fraction 3 low in hydrogen is then fed to a liquefaction process V that is constructed as desired. It should be stressed that the concept according to the invention—namely removal of hydrogen by means of permeation—can be combined with any desired liquefaction process. After membrane removal of Hydrogen the purified feed gas can be cooled and liquefied by means of a mixed refrigerant cycle or an expander cycle. For both alternatives various patents are available.

The liquefied hydrocarbon-rich fraction is expanded in the valve a, and then conducted via line 4 into a storage tank T, used for temporary storage. The liquefied product or LNG is removed from storage tank T via line 5. If tank boil-off or flash gas forms within storage tank T, this gas can be removed from storage tank T via line 6. According to an advantageous embodiment of the method according to the invention, this tank boil-off or flash gas can be added to the hydrocarbon-rich fraction that is to be liquefied. If the hydrocarbon-rich fraction is compressed before it is fed into the membrane separation unit M, it is advantageous to add the recirculated tank boil-off gas to the hydrocarbon-rich fraction before this compression.

If the nitrogen content in the hydrocarbon-rich fraction 1/1′ that is to be liquefied is below 1% by volume, then, generally, integration of nitrogen removal into the liquefaction process can be dispensed with, since the specification for the LNG product customarily permits a nitrogen concentration of a maximum of 1% by volume. In the case of nitrogen contents greater than 3% by volume in the hydrocarbon-rich fraction 1/1′ that is to be liquefied, then customarily nitrogen removal is implemented in the liquefaction process V. The resultant nitrogen-rich fraction can be removed from the liquefaction process V via the line 7.

In the case of the embodiment of the method according to the invention shown in FIG. 2, the hydrogen-rich fraction 2, the flash gas fraction 6 removed from storage tank T and the nitrogen-rich fraction 7 possibly produced in liquefaction process V are compressed to a common pressure—for this purpose the single-stage -or multistage-design compressor (units) V1 to V3 serve—and fed to a suitable residual gas grid via line 8. Depending on local circumstances, it can be expedient to connect the compressors V2 and V3 one after the other and/or to arrange the feed of the mixture 8 before or after compressor V1.

The method according to the invention for liquefying a hydrocarbon-rich fraction has a multiplicity of novelties and advantages compared with the known prior art, for example:

Owing to the hydrogen removal before the actual liquefaction process, the total energy requirement of the liquefaction method decreases; this also applies when the removed hydrogen must be recompressed.

Owing to the hydrogen removal before the liquefaction process, standard liquefaction processes which are optimized for the respective liquefaction outputs can be used, which leads to a decrease in capital costs.

Since the membrane(s) used for hydrogen removal by permeation can be adjusted in accordance with the hydrogen content in the hydrocarbon-rich fraction that is to be liquefied, the method according to the invention is comparatively flexible with respect to hydrogen content.

The hydrogen removal that is to be provided according to the invention and is connected upstream of the actual liquefaction process can be retrofitted without difficulty in the case of existing liquefaction plants, in such a manner that these existing liquefaction plants can react to changes in the composition of the feed fractions.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. DE 102010047543.2, filed Oct. 5, 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.