METHOD FOR PURIFYING A NATURAL GAS STREAM

Description

[0001] The present invention relates to a method for purifying a natural gas stream, in order to reduce its content of undesirable hydrocarbons while increasing its content of methane and dihydrogen.

[0002] The process according to the invention is particularly useful for preparing natural gas-based fuels with high energy value and low emission levels. STATE OF THE PRIOR ART

[0003] Natural gas is essentially made up of light C1 to C5 hydrocarbons, among which there is a majority proportion of methane (generally more than 80% by mole, or even more than 90% by mole), and a minority proportion of higher hydrocarbons such as in particular ethane, propane, butanes and pentanes, the content of which decreases with the increase in the number of carbon atoms. Thus, for example, the ethane content of natural gas can be up to 10% by mole, the propane content up to 1% by mole, the butane content up to 0.3% by mole.

[0004] Natural gas can also contain other compounds other than hydrocarbons, in very minor proportions. Depending on its origin, natural gas can, for example, contain, in very small amounts, nitrogen, carbon dioxide, dihydrogen, hydrogen sulfide, helium, and mercaptans.

[0005] Natural gas is increasingly used as a fuel to power engines in both individual vehicles and special vehicles such as trucks, buses, garbage trucks, etc.

[0006] In order to increase the energy efficiency of engines running on natural gas, and to reduce the pollutant emissions (in particular carbon dioxide) of these engines, the natural gases used must have the highest possible methane content and the lowest possible contents of higher hydrocarbons containing at least two carbon atoms (hereinafter referred to as C 2+ hydrocarbons). Document WO 2018 / 055264 relates to a process for purifying natural gas to be liquefied, where the natural gas is subjected, among other things, to fractional distillation. SUBJECT OF THE INVENTION

[0007] The present invention aims to propose an original process for purifying natural gas, which makes it possible to achieve the above objectives of reducing its C 2+ hydrocarbon content, while at the same time increasing its methane content.

[0008] Particularly advantageously, the process of the invention also makes it possible to enrich natural gas with dihydrogen, without generating carbon dioxide.

[0009] The present invention thus relates to a method for purifying a natural gas stream, which comprises the following steps: a) a step of treating at least a portion of said stream by pyrolysis at a temperature in the range from 1000°C to 2000°C, so as to decompose the hydrocarbons comprising at least two carbon atoms into elemental carbon and dihydrogen H 2 and thus to obtain a treated stream, then b) a step of eliminating the elemental carbon present in the treated stream resulting from step a) so as to obtain a treated stream free of elemental carbon; then c) when steps a) and b) have been carried out on only a portion of the natural gas stream, a step of mixing the treated stream free of elemental carbon resulting from step b) with the untreated portion of the stream; then d) obtaining a purified natural gas stream consisting either of the mixture resulting from step c) or of the treated stream free of elemental carbon resulting from step b).

[0010] The method according to the invention thus consists of subjecting all or part of the natural gas flow to a treatment which makes it possible to selectively eliminate the C 2+ hydrocarbons present such as ethane, propane, butane, pentane, by transforming them by pyrolysis at high temperature into elemental carbon and dihydrogen, then eliminating the elemental carbon resulting from the pyrolysis.

[0011] Thus, due to the elimination of C2+ hydrocarbons, the weight content of methane in natural gas increases.

[0012] The invention thus makes it possible to increase the methane index of the natural gas flow.

[0013] It also makes it possible to control the methane index of natural gas, and to guarantee the production of treated natural gas with a constant methane index within a required range, or above a minimum threshold, satisfying for example a specification.

[0014] As is known per se, the methane index of a combustible gas is a numerical parameter in the range 0 to 100, which corresponds to the measurement of the gas's resistance to knocking in a standardized combustion test in accordance with ISO / TR 22302:2014. Pure methane, used as a reference fuel, has a methane index of 100.

[0015] If natural gas is to be used as the basis for an engine fuel, it is essential that it has a constant methane index to ensure engine performance. For example, the EN 16732-2 specification requires a methane index greater than 60, but car manufacturers recommend a methane index greater than 70 (see Annex D of the same standard).

[0016] Generally speaking, we aim to obtain a fuel with the highest possible methane index.

[0017] The present invention thus makes it possible, for example, to increase the methane index of the part of the treated flow from a value lower than 70 to a value higher than 80.

[0018] In addition, the process according to the invention has the effect of producing dihydrogen (H 2 ) and therefore increasing the H 2 content in the natural gas flow. This dihydrogen, known as “turquoise dihydrogen”, is thus produced without carbon dioxide emissions.

[0019] When natural gas is intended to be used as an engine fuel, an increase in its dihydrogen content is beneficial because it reduces carbon dioxide emissions by reducing the carbon intensity of the fuel but also by increasing engine efficiency (therefore a further reduction in exhaust CO2 emissions).

[0020] By increasing the dihydrogen content of a natural gas-based fuel, we also observe a reduction in exhaust particulate emissions, as well as a reduction in carbon monoxide (CO) and hydrocarbon emissions including methane (CH 4 ).

[0021] The method according to the invention is particularly advantageous in the production of natural gas intended for use as fuel in an internal combustion engine.

[0022] Thus, the present invention also relates to a process for preparing a fuel for an internal combustion engine, comprising: (i) purifying a natural gas stream using the process according to the invention; then (ii) conditioning the natural gas stream from step (i) in the form of compressed natural gas, adsorbed natural gas or liquefied natural gas, preferably in the form of compressed natural gas.

[0023] Other objects, characteristics, aspects and advantages of the invention will appear even more clearly on reading the description and the examples which follow, and in view of the Figure 1 attached. [Fig 1]

[0024] There Figure 1 is a schematic representation of the method according to the invention.

[0025] In what follows, and unless otherwise indicated, the limits of a domain of values ​​are included in this domain, in particular in the expressions "between" and "ranging from ... to ...".

[0026] Furthermore, the expressions “at least one” and “at least” used in the present description are respectively equivalent to the expressions “one or more” and “greater than or equal to”. Finally, in a manner known per se, the term “CN compound or group” denotes a compound or group containing N carbon atoms in its chemical structure. DETAILED DESCRIPTION

[0027] The method according to the invention is schematically represented in the Figure 1 , in which a natural gas flow 1 is divided into two parts: a first part 3, conveyed to the unit 5 in which it is treated by pyrolysis while the other part 4 of the initial gas flow 1, is not treated. A three-way valve 2 makes it possible to regulate the proportion of the natural gas flow subjected to treatment.

[0028] Thus, according to a first operating mode, it is possible to close the valve 2, so that the entire flow 1 of natural gas is conveyed to the pyrolysis unit 5 and the flow of untreated gas 4 is zero.

[0029] According to a second operating mode, the valve 2 is partially opened, so that a part 3 of the natural gas flow 1 is routed to the pyrolysis stage 5.

[0030] Preferably, the proportion of the natural gas stream 1 subjected to steps a) and b) of the process according to the invention (i.e. part 3 of the initial gas stream 1 conveyed to the pyrolysis step 5) represents at least 10% by mass of the initial gas stream 1. Preferably, this part represents from 10 to 100% by mass of the initial natural gas stream, preferably from 20 to 80% by mass.

[0031] This proportion can be adjusted according to the composition of the initial natural gas flow (and in particular according to its C 2+ hydrocarbon content) and the specifications desired for the purified natural gas, by varying the opening of valve 2.

[0032] Part 3 of the natural gas stream subjected to steps a) and b) of the process is subjected to a pyrolysis treatment step 5 at a temperature in the range from 1000°C to 2000°C, so as to decompose the C 2+ hydrocarbons into elemental carbon and dihydrogen (H 2 ).

[0033] The treated stream 6 from pyrolysis step 5 undergoes a step 7 in which the elemental carbon generated in step 5 is removed and discharged. A stream of elemental carbon 9 is thus recovered and can be recycled.

[0034] The carbon thus recovered can be used in the form of carbon black (which is a useful raw material in particular in inks and tires), or when the pyrolysis method used leads to specific structural forms of carbon, in the form of graphite, graphene, or other forms

[0035] The treated gas stream 8 freed from elemental carbon from step 7 is finally mixed with the untreated part 4 of the stream in the case of the second operating mode described above. A three-way valve 10 makes it possible to regulate the proportions of the mixture of the two stream parts, namely the treated part 8 freed from elemental carbon and the untreated part 4, in particular as a function of their respective compositions and the specifications required for the final purified natural gas stream 11 resulting from this mixture. Pressure regulators (not shown) can advantageously be positioned upstream of the three-way valve 10 in order to ensure the same pressure for the stream 8 and the stream 4. Step a) of pyrolysis treatment

[0036] Step a) of the process according to the invention consists of treating all or part of the natural gas flow in a pyrolysis unit.

[0037] Pyrolysis is a treatment well known to those skilled in the art, which consists of heating a product to a very high temperature, so as to cause its thermal decomposition.

[0038] In the present invention, it is essential to control the temperature of the pyrolysis step and to maintain it in a range from 1000°C to 2000°C. At these temperatures, the hydrocarbons present in the natural gas stream decompose into elemental carbon C and dihydrogen H 2 . In this temperature range, the hydrocarbons have thermal decomposition kinetics which increase with their number of carbon atoms, which means that the heavier hydrocarbons (i.e. having the longest carbon chains) are the first to be decomposed. Thus, at a given temperature, taking into account the decomposition kinetics of the different hydrocarbons present, by controlling the residence time of the gas stream in the pyrolysis unit, it is possible to decompose the hydrocarbons into C 2+ , without significantly degrading the methane.It is inevitable that a proportion of methane is also decomposed, but the operating conditions of step a), and in particular the pyrolysis temperature and the residence time of the stream in the pyrolysis unit, can be easily adjusted by a person skilled in the art so as to maximize the decomposition of the C 2+ hydrocarbons and minimize that of the methane. Advantageously, the residence time of the stream in the pyrolysis unit is in the range from 0.01 s to 1 s.

[0039] The residence time of the treated gas stream in the pyrolysis unit can be controlled by adjusting its flow rate.

[0040] Preferably, the pyrolysis temperature in step a) is in the range from 1100 to 1600°C, preferably from 1200 to 1500°C.

[0041] Carrying out step a) is not limited to a specific pyrolysis technique, and all known pyrolysis techniques can be used as long as they allow the required temperatures to be reached and the residence time of the stream or part of the stream of natural gas treated in step a) to be precisely controlled, so as to maximize the selective decomposition of the C 2+ hydrocarbons.

[0042] According to a first embodiment, the pyrolysis treatment is carried out by heating the stream or portion of the stream of natural gas using a hot plasma.

[0043] Hot plasma can be generated using different technologies, known to those skilled in the art, such as in particular microwaves, plasma torches, or alternating current.

[0044] Microwave technology involves using one or more standard (athermal) microwave generation modules with microwave / gas coupling in a reactor to produce a hot plasma from natural gas. This plasma is initiated and maintained by the transfer of electrons due to microwaves. Several modules can be used in series, depending on the desired conversion rate.

[0045] Plasma torch technology allows plasma jets to be introduced into the chamber where the natural gas passes. The temperature of the torch (usually greater than or equal to 6000°C) allows the chamber to be brought to the temperature required to carry out the selective pyrolysis of natural gas.

[0046] Alternating current technology involves generating heat by passing an alternating current through the reactor through which the treated natural gas stream circulates. The alternating current creates electric arcs within the reactor. It is the energy of these arcs that heats the natural gas, transforming it into a plasma state, and allowing it to decompose.

[0047] According to a second embodiment, the pyrolysis treatment is carried out by circulating the natural gas stream or part of the stream through a column filled with molten metal. Any suitable metal having a melting point in the required temperature range may be used for this purpose, such as, for example, indium (In), gallium (Ga), bismuth (Bi), tin (Sn), lead (Pb) and nickel (Ni).

[0048] According to a third embodiment, the pyrolysis treatment is carried out by heating the stream or part of the stream of natural gas by shock waves.

[0049] This technology involves using the kinetic energy of a wave train, created either by pressure differences or a high-pressure electromagnetic valve, in a shock tube to heat natural gas to the required temperature. The system can be optimized by using a rotor to improve conversion efficiencies.

[0050] In an open loop process, the natural gas arrives under pressure (70 bars) and the outgoing flow is around 30 bars. To improve the conversion rate, it is possible to use a closed loop process, which allows part of the treated natural gas flow to be recirculated to increase the conversion rates of the gas to be treated.

[0051] According to a fourth embodiment, the pyrolysis treatment is carried out by heating the flow or part of the flow of natural gas by induction, either directly or indirectly, for example via a catalyst (metallic or carbon).

[0052] Regardless of the pyrolysis technology implemented, it is possible to implement several treatment units (or reactors) in series, and / or to recycle part of the treated gas flow to the pyrolysis unit or reactor, to achieve the desired rate of decomposition of hydrocarbons into C 2+ .

[0053] At the end of step a), a treated stream containing a significant proportion of methane, elemental carbon generally in the solid state, and dihydrogen is obtained.

[0054] The content of C2+ hydrocarbons in the treated stream is generally less than 0.5 mol%, preferably less than 0.1% and even better equal to 0 mol%. Step b) of carbon removal

[0055] Step b) of the process according to the invention consists of removing the elemental carbon from the treated natural gas stream resulting from step a).

[0056] Different technologies may be used for this purpose, and the invention is not limited to the use of any particular technology.

[0057] According to a first embodiment, step b) is carried out by circulating the flow of treated natural gas from step a) in a molten salt bath.

[0058] This molten salt bath traps the carbon that solidifies in the bath, and thus recovers a gas flow freed from elemental carbon. The solid carbon rises to the surface of the molten salt bath, from which it is then regularly mechanically removed.

[0059] According to a second embodiment, step b) is carried out by cooling the treated natural gas flow from step a) (for example by means of a heat exchanger, for example supplied with cold water) then circulating it in one or more mechanical separation devices such as in particular one or more cyclones, one or more filters.

[0060] These types of devices, known per se, allow carbon to be recovered in the form of solid particles. It may be advantageous to implement several cyclones and / or several filters, arranged in series.

[0061] A particularly preferred variant of this embodiment consists of circulating said treated natural gas stream in mechanical separation devices comprising one or more cyclones, followed by one or more filters. The cyclone makes it possible to coalesce the particles in order to improve the filtration efficiency.

[0062] At the end of step b), a stream free of elemental carbon, and enriched in dihydrogen, while containing a significant proportion of methane, is obtained. This stream from step b) is also free of C2+ hydrocarbons. It typically contains a quantity of dihydrogen ranging from 10 to 30 mol%, preferably from 10 to 20 mol%.

[0063] Elemental carbon-free means a stream with an elemental carbon content of less than 0.1 mol%.

[0064] C2+ hydrocarbon-free means a stream with a C2+ hydrocarbon content of less than 0.5 mol%. The fuel preparation process

[0065] According to an advantageous embodiment of the invention, the stream of natural gas purified by means of the process described above is then used to prepare a fuel for an internal combustion engine.

[0066] The fuel thus obtained can be in the form of compressed natural gas (CNG, at a pressure of approximately 200 bars), adsorbed natural gas (ANG) or liquefied natural gas (LNG, at a temperature of approximately -160°C). Preferably, it is in the form of compressed natural gas, i.e. maintained under high pressure, typically 200 bars (2.10 7< Pa)

[0067] The fuel may also include one or more additives, which may be chosen from any of the additives commonly used in the formulation of natural gas-based fuels.

[0068] Examples include reodorant additives, which include sulfur compounds (tetrahydrothiophene (THT) or methyl mercaptan (or methanethiol)), and can be present in concentrations ranging from 15 to 40 mg / m 3.

[0069] The following example is simply intended to illustrate the invention without limiting its scope. EXAMPLE

[0070] The natural gas stream used in this example has the composition detailed in Table 1 below. [Table 1] Compound Content (in mol %) Nitrogen N 2 0,423 Methane CH 4 90,678 Ethane C 2 H 6 7,949 Propane C 3 H 8 0,743 Iso-butane iC 4 H 10 0,074 Normal-butane nC 4 H 10 0,112 Iso-pentane iC 3 H 12 0,016 Normal-pentane nC 5 H 12 0,006

[0071] This natural gas flow has a flow rate of 220 tonnes / day, and is treated in accordance with the process of the invention, as follows: a) all or part of the gas flow is treated by pyrolysis, in a microwave plasma reactor, consisting of an injection tube around which a waveguide brings the microwaves which create a plasma within the injection tube, thus allowing pyrolysis. The microwaves are created by a magnetron, but a semiconductor microwave generator can also be used. The temperature within the injection tube is maintained in the range of 1200 to 1500°C. In this reactor, all the C 2 to C 5 hydrocarbons and about 5% of the methane are decomposed into elemental carbon C and dihydrogen H 2 .The operating conditions chosen thus lead to a high selectivity in favor of the conversion of hydrocarbons into C 2+ ; b) the treated flow leaving the plasma reactor then circulates in a separation unit containing a cyclone followed by a filter so as to separate the carbon in the form of solid particles and thus recover a flow free of elemental carbon and C2+ hydrocarbons, and enriched in dihydrogen. The treated flow enters the cyclone at a temperature of approximately 250°C and leaves at a temperature of 50°C.

[0072] The mass proportion of the gas flow subjected to steps a) and b) below is varied.

[0073] In a first test, the entire gas flow is treated (100% proportion).

[0074] In the following tests, only part of the gas flow is treated, varying the proportion of the gas flow treated from 20 to 80% by mass, and this part of the flow treated and freed from elemental carbon from steps a) and b) is then mixed with the part of the initial flow not treated.

[0075] The quantities of the different compounds present in the final purified natural gas stream and the quantity of elemental carbon C recovered are detailed in Table 2 below, for each of the tests. [Table 2] Proportion of gas flow treated H2 flow rate (tons / day) Flow rate of C (tons / day) CH4 flow rate (tons / day) 100% 5,8 21,1 192,2 80% 4,6 16,9 193,8 60% 3,5 12,6 195,3 40% 2,3 8,4 196,9 20% 1,2 4,2 198,4

[0076] Table 3 below shows the molar content of dihydrogen H2 in the final purified natural gas stream. [Table 3] Proportion of gas flow treated H2 content of purified natural gas stream (mol%) 100% 24,1% 80% 19,1% 60% 14,2% 40% 9,4% 20% 4,7%

Claims

1. A method for purifying a flow (1) of natural gas, comprising the following steps: a) a step (5) of processing at least a portion (3) of said flow (1) by pyrolysis at a temperature in the range of from 1000°C to 2000°C, so as to decompose hydrocarbons comprising at least two carbon atoms into elemental carbon and dihydrogen H2 and thus obtain a processed flow (6), then b) a step (7) of eliminating the elemental carbon present in the processed flow (6) from step a) so as to obtain a processed flow (8) free of elemental carbon; then c) when steps a) and b) have been carried out on a portion of the flow (1) of natural gas only, a step (10) of mixing the processed flow (8) free of elemental carbon from step b) with the portion (4) of the non-processed flow; then d) obtaining a flow of purified natural gas (11) consisting either of the mixture (10) from step c) or of the processed flow (8) free of elemental carbon from step b).

2. The method according to the preceding claim, characterised in that the pyrolysis temperature in step a) is in the range of from 1100 to 1600°C, preferably from 1200 to 1500°C.

3. The method according to any of the preceding claims, characterised in that the pyrolysis step is carried out by heating the flow or the portion of the flow of natural gas by means of a hot plasma.

4. The method according to the preceding claim, characterised in that the hot plasma is generated by microwaves, plasma torch or alternating current.

5. The method according to any of claims 1 and 2, characterised in that the pyrolysis step is carried out by circulating the flow or portion of the flow of natural gas in a column filled with molten metal.

6. The method according to any of claims 1 and 2, characterised in that the pyrolysis step is carried out by heating the flow or the portion of the flow of natural gas by shock waves.

7. The method according to any of claims 1 and 2, characterised in that the pyrolysis step is carried out by heating the flow or the portion of the flow of natural gas by induction, either directly or indirectly, for example via a catalyst.

8. The method according to any of the preceding claims, characterised in that the proportion of the flow of natural gas (1) subjected to steps a) and b) accounts for at least 10% by weight of the gas flow (1), preferably from 10 to 100% by weight of said flow (1), more preferably from 20 to 80% by weight.

9. The method according to any of the preceding claims, characterised in that step b) is carried out by circulating the processed flow of natural gas from step a) in a molten salt bath.

10. The method according to any of claims 1 to 8, characterised in that step b) is carried out by cooling the processed flow of natural gas from step a) and then circulating it in one or more mechanical separation devices.

11. The method according to the preceding claim, characterised in that the mechanical separation devices comprise one or more cyclones, followed by one or more filters.

12. A method for preparing a fuel for an internal combustion engine, comprising: (i) purifying a flow of natural gas using the method as defined in any of the preceding claims; and then (ii) conditioning the flow of natural gas from step (i) as compressed natural gas, adsorbed natural gas or liquefied natural gas, preferably as compressed natural gas.