PROCESS FOR PRODUCING SYNTHESIS GAS ORIGINATING FROM A NATURAL GAS LIQUEFACTION PROCESS

Abstract

A synthesis gas production process combined with a natural gas liquefaction process. At least one part of the heat source required in the synthesis gas production is provided by at least one portion of a stream enriched in hydrocarbons with more than two carbon atoms, extracted during the liquefaction of the natural gas.

Description

BACKGROUND

The present invention relates to a process for the liquefaction of a stream of hydrocarbons, such as natural gas, in combination with a synthesis gas production process.

The invention relates to the integration of a natural gas liquefaction process in a process for the production of synthesis gas by superheated steam reforming, partial oxidation or autothermal reforming.

These technologies for the production of synthesis gas sometimes require the use of large amounts of natural gas which are used as feed stream but also as source of heating for the process.

It is also desirable to liquefy natural gas for a certain number of reasons. By way of example, natural gas can be stored and transported over long distances more easily in the liquid state than in the gas form, since it occupies a smaller volume for a given weight and does not need to be stored at a high pressure.

Processes for the generation of synthesis gas generally have, as finished products, hydrogen, carbon monoxide or a mixture of the two (known as oxo gas, indeed even a H₂/CO/CO₂ mixture (production of methanol) or a N₂/H₂ mixture (production of ammonia). Each of these processes additionally cogenerates more or less superheated steam.

After a metering and optionally compression or decompression unit, the production of synthesis gas generally includes the following stages:

1. A hot desulfurization stage: after a preheating (350-400° C.), all the sulfur-comprising derivatives present in the natural gas are converted into H₂S by catalysis in a hydrogenation (CoMox) reactor. The H₂S is then removed by catalysis (over a ZnO bed, for example).

2. An optional prereforming stage (stage mainly present in the steam reforming units): at high temperature (approximately 500-550° C.) with excess steam. Then, in the presence of catalyst: conversion of the hydrocarbon chains containing at least two carbon atoms into methane with coproduction of carbon monoxide, carbon dioxide (CO₂) and hydrogen.

3. Reforming stage, which consists in reacting, at high temperature (850-950° C.), the hydrocarbons with steam in order to produce hydrogen, CO and CO₂.

Downstream of the synthesis gas production units, the products generally upgraded are carbon monoxide (CO), hydrogen (H₂) or a H₂/CO mixture.

If appropriate, the final stage of the synthesis gas production process can also be a:

• Stage of partial oxidation over a catalytic bed (autothermal reformer), which consists in reacting oxygen with hydrocarbons at high temperature (800-1200° C.) in order to produce more CO;
• Stage of conversion of CO into H₂ in a catalytic reactor in the case of an exhaustive production of hydrogen;

The purification of the synthesis gas produced can then be carried out either by:

• Use of a PSA in order to purify the hydrogen-rich stream produced; or
• Scrubbing with amines in order to extract the CO₂ from the synthesis gas in the cases of production of CO or oxo gas; and
• Purification in a cold box of the CO-rich stream produced; or
• Passing the gas produced through a membrane in order to adjust the H₂/CO ratio required for the quality of the oxo gas to be produced.

The synthesis gas production units generally require a constant supply of heat provided by a fuel system. This fuel consists, in all or part, of natural gas but also of available hydrocarbon-rich streams, such as, for example, those discharged by units placed downstream of the synthesis gas production unit (Off Gas PSA, stream rich in methane or rich in hydrogen at the outlet of the cold box, and the like) or of the industrial site.

It is necessary to make sure that the fuel balance is balanced. This means that the combined heat energy present in the streams discharged to the fuel system must not exceed the heat requirements of the synthesis gas production unit and possibly of other units located nearby sharing the same fuel network. Otherwise, all or part of some streams discharged to the fuel system would have to be sent back continuously to a flare, which is not acceptable, especially for atmospheric emission constraints.

Furthermore, in a general way, the natural gas liquefaction units make it possible to carry out a liquefaction process generally comprising the following three stages:

1. A “pretreatment” which removes, from the natural gas to be liquefied, the impurities liable to freeze (H₂O, CO₂, sulfur-comprising derivatives, mercury, and the like);

2. Extraction of the heavy hydrocarbons and aromatic derivatives which may freeze during the liquefaction. This stage can take place upstream of or in parallel with the liquefaction;

3. Liquefaction by cooling the natural gas to a cryogenic temperature (typically −160° C.) by virtue of a refrigerating cycle and optionally also accompanied by a withdrawal of the heavy hydrocarbons/aromatic derivatives liable to freeze.

The inventors of the present invention have developed a solution which makes possible an upgrading of streams resulting from the natural gas liquefaction unit to the fuel system of the generation process. This integration between the two processes exhibits numerous advantages of synergies.

SUMMARY

A subject matter of the present invention is a natural gas liquefaction process in combination with a synthesis gas production process, the liquefaction process comprising the following stages:

• Stage a): pretreatment of a feed natural gas in order to remove the impurities liable to freeze during the liquefaction process;
• Stage b): extraction, from the gas stream resulting from stage a), of a stream enriched in hydrocarbons having more than two carbon atoms and of a stream depleted in hydrocarbons having more than two carbon atoms;
• Stage c): liquefaction of the gas stream depleted in hydrocarbons having more than two carbon atoms resulting from stage b);

the synthesis gas production process comprising the following stages:

• Stage a′): desulfurization at a temperature of greater than 350° C. of a natural gas feed stream;
• Stage b′): optional prereforming, at a temperature of greater than 500° C., in order to convert the hydrocarbon chains containing at least two carbon atoms of the gas stream resulting from stage a′) into methane;
• Stage c′): reforming consisting in reacting, at a temperature of greater than 800° C., the gas stream resulting from stage a′) or b′) with steam in order to produce hydrogen, carbon dioxide and carbon monoxide;

characterized in that at least a part of the heat source required in the synthesis gas production process is produced by at least a part of the stream enriched in hydrocarbons having more than two carbon atoms resulting from stage b).

This integration makes it possible, for example, to avoid an incinerator and/or a system for extraction or stabilization of heavy hydrocarbons, which are particularly expensive for small-sized units.

According to other embodiments, another subject matter of the invention is:

• A process as defined above, characterized in that the pretreatment stage a) is carried out by means of a system for separation by adsorption employing a regeneration stream.
• A process as defined above, characterized in that stage a) consists of a pretreatment by adsorption by means of an adsorption system comprising between two and five containers of at least one layer of adsorbent and at least one device for heating and/or cooling an adsorption and/or regeneration stream circulating in said adsorption system.
• A process as defined above, characterized in that, during stage a′), all the sulfur-comprising derivatives present in the feed gas are converted into H₂S by catalysis in a reactor.
• A process as defined above, characterized in that the product H₂S is extracted by catalysis.
• A process as defined above, characterized in that the impurities liable to freeze during the liquefaction process which are removed during stage a) comprise the water, the carbon dioxide and the sulfur-comprising derivatives present in the feed gas.
• A process as defined above, characterized in that, during stage c), the stream of natural gas depleted in hydrocarbons having more than two carbon atoms resulting from stage b) is liquefied at a temperature of less than −140° C. by means of a natural gas liquefaction unit comprising at least one main heat exchanger and a system for producing cold.
• A process as defined above, characterized in that the natural gas feed stream employed in stage a) and the natural gas feed stream employed in stage a′) originate from one and the same natural gas feed stream.
• A process as defined above, characterized in that the synthesis gas production unit is a unit for the production of hydrogen by steam reforming having a hydrogen production capacity of at least 20 000 Nm³/h.
• A process as defined above, characterized in that the heat energy of the stream enriched in hydrocarbons having more than two carbon atoms resulting from stage b) represents from 5% to 35%, preferably from 10% to 20%, of the amount of fuel required in the synthesis gas production process.

Furthermore, as generally the pressure of the stream resulting from the natural gas liquefaction unit and enriched in hydrocarbons having more than two carbon atoms is greater than the pressure of the fuel network, it is possible to dispense with pumps or compressors/machines which are rotating, which represents a major saving with regard to the cost of the natural gas liquefaction unit.

The stream of hydrocarbons to be liquefied is generally a stream of natural gas obtained from a domestic gas network in which the gas is distributed via pipelines.

The expression “natural gas” as used in the present patent application relates to any composition containing hydrocarbons, including at least methane. This comprises a “crude” composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.

The heat exchanger can be any heat exchanger, any unit or other arrangement suitable for making possible the passage of a certain number of streams, and thus making possible a direct or indirect exchange of heat between one or more refrigerant fluid lines and one or more feed streams.

Generally, the natural gas stream is essentially composed of methane. Preferably, the feed stream comprises at least 80 mol % of methane. Depending on the source, the natural gas contains quantities of hydrocarbons heavier than methane, such as, for example, ethane, propane, butane and pentane and also certain aromatic hydrocarbons. The natural gas stream also contains nonhydrocarbon products, such as nitrogen (content variable but of the order of 5 mol %, for example) or other impurities H₂O, CO₂, H₂S and other sulfur-comprising compounds, mercury and others (0.5 mol % to 5 mol % approximately).

The feed stream containing the natural gas is thus pretreated before being introduced into the heat exchanger. This pretreatment comprises the reduction and/or the removal of the undesirable components, such as, generally, CO₂ and H₂O but also H₂S and other sulfur-comprising compounds or mercury.

In order to prevent the latter from freezing during the liquefaction of the natural gas and/or the risk of damage to the items of equipment located downstream (by corrosion phenomena, for example), it is advisable to remove them.

A conventional means which makes it possible to remove the CO₂ from the natural gas stream is, for example, scrubbing with amines which is located upstream of a liquefaction cycle.

The scrubbing with amines separates the CO₂ from the feed gas by scrubbing the natural gas stream with a solution of amines in an absorption column. The solution of amines enriched in CO₂ is recovered in the bottom of this absorption column and is regenerated at low pressure in a column for regeneration of the amine (or stripping column).

An alternative to the treatment by scrubbing with amines can be the adsorption by pressure and/or temperature inversion. The advantages of such a process are described below.

This separation process makes use of the fact that, under certain pressure and temperature conditions, some constituents of the gas (CO₂ and H₂O in particular) have specific affinities with regard to a solid material, the adsorbent (for example molecular sieves).

The adsorption is a reversible process and it is possible to regenerate the absorbent by lowering the pressure and/or raising the temperature of the adsorbent in order to release the adsorbed constituents of the gas.

Thus, in practice, a system for separation by adsorption consists of several (between two and five) “bottles” containing one or more layers of adsorbents and also appliances dedicated to the heating/cooling of the adsorption and/or regeneration stream.

In comparison with a conventional scrubbing with amines, the pretreatment exhibits a number of advantages,

• its cost;
• its simplicity of operation;
• the possibility of avoiding a number of services (the contribution of amine or of demineralized water).

These advantages are particularly significant for small-sized natural gas liquefaction units (for example producing less than 50 000 tonnes of liquefied natural gas per year).

An exemplary embodiment is illustrated by the following example.

The production of hydrogen by catalytic reforming requires a continuous supply of heat provided by a fuel gas network.

A steam reforming unit with a nominal hydrogen production capacity of approximately 130 000 Nm³/h is employed.

The heat requirements needed for the hydrogen production unit are predominantly provided (approximately 75%) by the residual gas resulting from the final stage of purification of the hydrogen in the hydrogen production unit (purification via molecular sieve (Pressure Swing Adsorption/PSA)). The contribution (approximately 25%) is provided by a source external to the hydrogen production unit (for example originating from the feed stream of the unit or from an external fuel system).

By placing a small natural gas production unit with a capacity of 40 000 tonnes of liquefied natural gas produced per year close to the hydrogen production unit, it is possible to return certain flows to the fuel network of the hydrogen production unit. The contribution provided by an external source will be reduced accordingly,

• • In the case where the pretreatment of the natural gas is provided by an adsorption process, the regeneration gas returned to the fuel network would represent approximately 15% of the fuel balance.
  • The heavy hydrocarbons extracted from the natural gas liquefier and the natural gas vapors generated in the storage of liquefied natural gas and/or in the loading bay will be less significant in the fuel balance (less than 1%).

The external heat source contribution is thus reduced from 25% to 10% approximately.

This integration makes it possible to drastically reduce the number of items of equipment dedicated to secondary streams of the natural gas liquefaction unit:

• • heavy hydrocarbons: the integration makes it possible, for example, to avoid an incinerator and/or a system for extraction of heavy hydrocarbons, which are expensive for small-sized units,
  • natural gas vapors generated in the storage of liquefied natural gas and/or in the loading bay: the integration makes it possible, for example, to avoid a compressor in order to recycle these vapors into the natural gas liquefaction stream. This compressor can be expensive in small-sized liquefiers.

If the capacity of the liquefied natural gas production unit unbalances the fuel balance, it is possible to return all or part of these streams to the synthesis gas stream which feeds the hydrogen production unit (at the price of a compressor).

It is then possible for the synthesis gas production and natural gas liquefaction units to have in common all of the conveniences of the site, in particular:

• The connection to the natural gas network;
• The metering and optionally pressure reduction/compression station;
• A hot flare and optionally cold liquid network;
• All of the utilities of the site (electricity, cooling circuit, instrumentation air, nitrogen, and the like);
• The feed network.

Furthermore, in the case where the synthesis gas production unit produces hydrogen, it is sometimes required to liquefy all or part of the hydrogen in order to facilitate the transportation or storage thereof, for example. In this case, it is possible to “precool” the hydrogen produced in the natural gas liquefier down to a temperature of −160° C., for example, and then to finish liquefying it in a dedicated unit.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Claims

1.-11. (canceled)

12. A natural gas liquefaction process in combination with a synthesis gas production process, the liquefaction process comprising: a) pretreating a feed natural gas in order to remove the impurities that will freeze during the liquefaction process, thereby producing a pretreated stream;b) extracting a stream enriched in hydrocarbons having more than two carbon atoms and of a stream depleted in hydrocarbons having more than two carbon atoms from the pretreated stream, thereby producing a hydrocarbon enriched stream;c) liquefying the hydrocarbon enriched stream;

13. The process as claimed in claim 12, wherein the pretreatment step a) is carried out by means of a system for separation by adsorption employing a regeneration stream.

14. The process as claimed in claim 13, wherein step a) consists of a pretreatment by adsorption by means of an adsorption system comprising between two and five containers of at least one layer of adsorbent and at least one device for heating and/or cooling an adsorption and/or regeneration stream circulating in said adsorption system.

15. The process as claimed in claim 14, wherein the steam resulting from the synthesis gas production process is employed to reheat the regeneration stream.

16. The process as claimed in claim 12, wherein during step a′), all the sulfur-comprising derivatives present in the feed gas are converted into H2S by catalysis in a reactor.

17. The process as claimed in claim 16, wherein the product H2S is extracted by catalysis.

18. The process as claimed in claim 12, wherein the impurities that will freeze during the liquefaction process which are removed during step a) comprise water, carbon dioxide and sulfur-comprising derivatives present in the feed natural gas.

19. The process as claimed in claim 12, wherein during step c), the hydrocarbon enriched stream is liquefied at a temperature of less than −140° C. by means of a natural gas liquefaction unit comprising at least one main heat exchanger and a system for producing cold.

20. The process as claimed in claim 12, wherein the natural gas feed stream employed in step a) and the natural gas feed stream employed in step a′) originate from the same natural gas feed stream.

21. The process as claimed in claim 12, wherein the synthesis gas production unit is a unit for the production of hydrogen by steam reforming having a hydrogen production capacity of at least 20 000 Nm3/h.

22. The process as claimed in claim 12, wherein the heat energy of the hydrocarbon enriched stream represents from 5% to 35% of the amount of fuel required in the synthesis gas production process.