METHOD FOR PURIFYING A FLOW OF NATURAL GAS

Abstract

The present invention relates to 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 the flow (1) by pyrolysis at a temperature in the range from 1000? C. to 2000? C. so as to decompose the hydrocarbons which comprise at least two carbon atoms into elemental carbon and dihydrogen H.sub.2 and to thereby obtain a processed flow (6), then b) a step (7) of eliminating the elemental carbon which is present in the processed flow (6) from step a) so as to obtain a processed flow without any elemental carbon (8); then c) when steps a) and b) have been carried out on only a portion of the flow (I) of natural gas, a step (IO) of mixing the processed flow without any elemental carbon (8) from step b) with the portion (4) of the non-processed flow; then d) obtaining a flow of purified natural gas (11) which consists of either the mixture (10) from step c) or the processed flow without any elemental carbon (8) from step b). The invention also relates to preparing a fuel from the natural gas purified in this manner.

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 H.sub.2 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 claim 1, characterised in that the pyrolysis temperature in step a) is in the range of from 1100 to 1600? C.

3. The method according to claim 1, 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 claim 1, characterised in that the hot plasma is generated by microwaves, plasma torch or alternating current.

5. The method according to claim 1, 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 claim 1, 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 claim 1, 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 claim 1, 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).

9. The method according to claim 1, 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 claim 1, 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 claim 10, 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.

13. The method according to claim 1, characterised in that the pyrolysis temperature in step a) is in the range of from 1200 to 1500? C.

14. The method according to claim 1, characterised in that the proportion of the flow of natural gas (1) subjected to steps a) and b) accounts for from 10 to 100% by weight of said flow.

15. The method according to claim 1, characterised in that the proportion of the flow of natural gas (1) subjected to steps a) and b) accounts for from 20 to 80% by weight of said flow.

Description

[0030] FIG. 1 is a schematic representation of the method according to the invention.

[0031] In what follows, and unless otherwise indicated, the limits of a range of values are included in this range, especially in the terms between and ranging from . . . to . . . .

[0032] 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.

[0033] Finally, in a manner known per se, a C.sub.N compound or group is a compound or group containing N carbon atoms in its chemical structure.

DETAILED DESCRIPTION

[0034] The method according to the invention is schematically represented in FIG. 1, in which a flow 1 of natural gas is divided into two portions: a first portion 3, conveyed to the unit 5 in which it is processed by pyrolysis, while the other portion 4 of the initial gas flow 1 is not processed. A three-way valve 2 is used to regulate proportion of the flow of natural gas subjected to the process.

[0035] 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 non-processed gas flow 4 is zero.

[0036] According to a second operating mode, the valve 2 is partially open, so that a portion 3 of the flow 1 of natural gas is conveyed to the pyrolysis step 5.

[0037] Preferably, the proportion of the flow 1 of natural gas subjected to steps a) and b) of the method according to the invention (that is the portion 3 of the initial gas flow 1 conveyed to the pyrolysis step 5) accounts for at least 10% by weight of the initial gas flow 1. Preferably, this portion accounts for from 10 to 100% by weight of the initial flow of natural gas, preferably from 20 to 80% by weight.

[0038] This proportion can be adjusted as a function of the composition of the initial flow of natural gas (and especially as a function of its C.sub.2+ hydrocarbon content) and the desired specifications for the purified natural gas, by varying opening of the valve 2.

[0039] The portion 3 of the flow of natural gas subjected to steps a) and b) of the method is subjected to a step 5 of pyrolysis 5 at a temperature in the range of from 1000? C. to 2000? C., so as to decompose the C.sub.2+ hydrocarbons into elemental carbon and dihydrogen (H.sub.2).

[0040] The processed flow 6 from pyrolysis step 5 undergoes a step 7 in which the elemental carbon generated in step 5 is eliminated and discharged. A flow of elemental carbon 9 is thus recovered and can be reused.

[0041] The carbon recovered in this way can be recovered in the form of carbon black (which is a useful raw material especially in inks and tyres), or when the pyrolysis method used leads to specific structural forms of carbon, in the form of graphite, graphene or other forms.

[0042] The processed gas flow 8, freed of the elemental carbon from step 7, is finally mixed with the non-processed portion 4 of the flow in the case of the second operating mode described above. A three-way valve 10 makes it possible to regulate proportions of the mixture of the two portions of the flow, namely the processed portion 8 freed of elemental carbon and the non-processed portion 4, especially as a function of their respective compositions and the specifications required for the final flow of purified natural gas 11 resulting from this mixture. Pressure regulators (not represented) can advantageously be positioned upstream of the three-way valve 10 in order to ensure the same pressure for flow 8 and flow 4.

Step a) of Pyrolysis Processing

[0043] Step a) of the method according to the invention consists of processing all or part of the flow of natural gas in a pyrolysis unit.

[0044] Pyrolysis is a process well known to those skilled in the art, which consists in bringing a product to a very high temperature, so as to cause its thermal decomposition.

[0045] In the present invention, it is essential to control the temperature of the pyrolysis step and to maintain it in a range of from 1000? C. to 2000? C. At these temperatures, the hydrocarbons present in the flow of natural gas decompose into elemental carbon C and dihydrogen H.sub.2. In this temperature range, the thermal decomposition kinetics of hydrocarbons increase with their number of carbon atoms, which means that the heaviest hydrocarbons (i.e. those with the longest carbon chains) are the first to be decomposed. So, at a given temperature, in view of the decomposition kinetics of the different hydrocarbons present, by controlling the residence time of the gas flow in the pyrolysis unit, it is possible to decompose C.sub.2+ hydrocarbons without substantially degrading the methane. It is inevitable that a proportion of methane will also be decomposed, but the operating conditions of step a), and especially the pyrolysis temperature and the residence time of the flow in the pyrolysis unit, can be easily adjusted by those skilled in the art so as to maximise decomposition of C.sub.2+ hydrocarbons and minimise that of methane. Advantageously, the residence time of the flow in the pyrolysis unit is in the range of from 0.01 s to 1 s.

[0046] The residence time of the processed gas flow in the pyrolysis unit can be controlled by adjusting its flow rate.

[0047] Preferably, the pyrolysis temperature in step a) is in the range of from 1100 to 1600? C., preferably from 1200 to 1500? C.

[0048] Performing step a) is not limited to a specific pyrolysis technique, and all known pyrolysis techniques can be used as long as they enable the required temperatures to be reached and the residence time of the flow or portion of the flow of natural gas processed in step a) to be precisely controlled, so as to maximise the selective decomposition of C.sub.2+ hydrocarbons.

[0049] According to a first embodiment, 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.

[0050] Hot plasma can be generated using different technologies known to those skilled in the art, such as especially microwaves, plasma torches or alternating current.

[0051] Microwave technology consists in using one or more standard (athermal) microwave generation modules with a microwave/gas coupling in a reactor, so as to produce hot plasma from the natural gas. This plasma is initiated and maintained by the electron transfer due to microwaves. It is possible to use several modules in series, depending on the conversion rate wanted.

[0052] Plasma torch technology is used to introduce plasma jets into the chamber through which the flow of natural gas passes. The temperature of the torch (generally greater than or equal to 6000? C.) enables the chamber to be brought to the temperature required for selective pyrolysis of natural gas.

[0053] Alternating current technology consists in generating heat by passing an alternating current through the reactor in which the processed flow of natural gas circulates. The alternating current creates electric arcs in the reactor. It is the energy from these arcs that heats natural gas, transforms it into a plasma and enables it to decompose.

[0054] According to a second embodiment, 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. Any suitable metal with a melting point in the required temperature range can be used for this purpose, such as, for example, indium (In), gallium (Ga), bismuth (Bi), tin (Sn), lead (Pb) and nickel (Ni).

[0055] According to a third embodiment, the pyrolysis step is carried out by heating the flow or portion of the flow of natural gas using shock waves.

[0056] This technology consists in using the kinetic energy of a wave train, created either by pressure differences or by a high-pressure electromagnetic valve, in a shock tube to heat the natural gas to the required temperature. The system can be optimised by using a rotor to improve conversion efficiencies.

[0057] Within the scope of a so-called open loop configuration method, the natural gas arrives under pressure (70 bar) and the outflow is around 30 bar. To improve the conversion rate, it is possible to use a so-called closed loop configuration method, which allows a portion of the processed flow of natural gas to be recirculated to increase conversion rates of the gas to be processed.

[0058] According to a fourth embodiment, the pyrolysis step is carried out by heating the flow or portion of the flow of natural gas by induction, either directly or indirectly, for example via a metal or carbon catalyst.

[0059] Regardless of the pyrolysis technology implemented, it is possible to implement several process units (or reactors) in series, and/or to recycle a portion of the processed gas flow to the pyrolysis unit or reactor, in order to achieve the desired rate of decomposition of C.sub.21 hydrocarbons.

[0060] At the end of step a), a processed flow containing a significant proportion of methane, elemental carbon generally in the solid state, and dihydrogen is obtained.

[0061] The content of C.sub.2+ hydrocarbons in the processed flow is generally less than 0.5 mol %, preferably less than 0.1% and better still equal to 0 mol %.

Step b) of Carbon Elimination

[0062] Step b) of the method according to the invention consists in eliminating elemental carbon from the processed flow of natural gas resulting from step a).

[0063] Different technologies can be used for this purpose, and the invention is not limited to the use of any particular technology.

[0064] According to a first embodiment, step b) is carried out by circulating the processed flow of natural gas from step a) in a molten salt bath.

[0065] This molten salt bath traps the carbon that solidifies in the bath, thereby recovering a gas flow freed of elemental carbon. The solid carbon rises to the surface of the molten salt bath, from which it is then regularly eliminated mechanically.

[0066] According to a second embodiment, step b) is carried out by cooling the processed flow of natural gas from step a) (for example by means of a heat exchanger, for example supplied with cold water) and then circulating it in one or more mechanical separation devices such as especially one or more cyclones, one or more filters.

[0067] These types of devices, known per se, enable the 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.

[0068] A particularly preferred alternative to this method consists in circulating said flow of processed natural gas in mechanical separation devices comprising one or more cyclones, followed by one or more filters. The cyclone enables the particles to coalesce in order to improve filtration efficiency.

[0069] At the end of step b), a flow is obtained which is free of elemental carbon and enriched with dihydrogen, while containing a significant proportion of methane. This flow from step b) is also free of C.sub.2+ hydrocarbons. It typically contains an amount of dihydrogen ranging from 10 to 30 mol %, preferably from 10 to 20 mol %.

[0070] By free of elemental carbon, it is meant a flow with an elemental carbon content of less than 0.1 mol %.

[0071] By free of C.sub.2+ hydrocarbon, it is meant a flow with a C.sub.2+ hydrocarbon content of less than 0.5 mol %.

[0072] The fuel preparation method According to an advantageous embodiment of the invention, the flow of natural gas purified by means of the method described above is then used to prepare a fuel for an internal combustion engine.

[0073] The fuel thus obtained may be in the form of compressed natural gas (CNG, at a pressure of approximately 200 bar), 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, that is maintained at a high pressure, typically 200 bar (2.107 Pa).

[0074] The fuel may also comprise one or more additives, which may be chosen from all the additives usually used in the formulation of natural gas-based fuels.

[0075] Reodorant additives may be mentioned, which especially include sulphur compounds (tetrahydrothiophene (THT) or methylmercaptan (or methanethiol)), and may be present in contents ranging in particular from 15 to 40 mg/m.sup.3.

[0076] The following example is simply intended to illustrate the invention without limiting the scope thereof.

Example

[0077] The flow of natural gas used in this example has the composition detailed in Table 1 below.

TABLE-US-00001 TABLE 1 Compound Content (in mole % ) Nitrogen N.sub.2 0.423 Methane CH.sub.4 90.678 Ethane C.sub.2H.sub.6 7.949 Propane C.sub.3H.sub.8 0.743 Iso-butane iC.sub.4H.sub.10 0.074 Normal-butane nC.sub.4H.sub.10 0.112 Iso-pentane iC.sub.5H.sub.12 0.016 Normal-pentane nC.sub.5H.sub.12 0.006

[0078] This flow of natural gas having a rate of 220 tonnes/day is processed in accordance with the method of the invention, as follows: [0079] a) all or part of the gas flow is processed by pyrolysis, in a microwave plasma reactor, consisting of an injection tube around which a waveguide supplies microwaves which create a plasma within the injection tube, thus enabling pyrolysis. The microwaves are created by a magnetron, but a semiconductor microwave generator can also be employed. The temperature within the injection tube is maintained in the range of from 1200 to 1500? C.

[0080] In this reactor, all the C.sub.2 to C.sub.5 hydrocarbons and approximately 5% of the methane are decomposed into elemental carbon C and dihydrogen H.sub.2. The operating conditions chosen thus lead to high selectivity in favour of the conversion of C.sub.2+ hydrocarbons; [0081] b) the processed flow leaving the plasma reactor then circulates in a separation unit containing a cyclone followed by a filter so as to separate carbon in the form of solid particles and thus recover a flow without any elemental carbon and C.sub.2+ hydrocarbons, and enriched with dihydrogen. The processed flow enters the cyclone at a temperature of around 250? C. and exits at a temperature of 50? C.

[0082] The proportion by weight of the gas flow subjected to steps a) and b) below is varied.

[0083] In the first test, the entire gas flow is processed (100% proportion).

[0084] In subsequent tests, only a portion of the gas flow is processed, by varying the proportion of the processed gas flow from 20 to 80% by weight, and this processed portion of the flow, freed of elemental carbon from steps a) and b), is then mixed with the non-processed portion of the initial flow.

[0085] The amounts of the different compounds present in the final flow of purified natural gas and the amount of elemental carbon C recovered are detailed in Table 2 below, for each of the tests.

TABLE-US-00002 TABLE 2 Proportion of the H.sub.2 C CH.sub.4 processed flow rate flow rate flow rate gas flow (tonnes/day) (tonnes/day) (tonnes/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

[0086] Table 3 below shows the molar content of dihydrogen H.sub.2 in the final flow of purified natural gas.

TABLE-US-00003 TABLE 3 Proportion H.sub.2 content of the flow of processed of purified natural gas gas flow (mole %) 100% 24.1% 80% 19.1% 60% 14.2% 40% 9.4% 20% 4.7%