PROCESS FOR THE PRODUCTION OF HYDROGEN-ENRICHED SYNTHESIS GAS

Abstract

Provided is a process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas. The process includes introducing a gaseous flow that includes at least one compound of formula (I) as defined herein in a first reactor containing including at least one metal selected from groups VI B and VII of the periodic table. The process also includes collecting a sulfur-containing gaseous flow from the first reactor, introducing the raw synthesis gas in a second reactor, and introducing the sulfur-containing gaseous flow in the second reactor, where the catalytic water-gas shift reaction takes place and includes a sulfur-resistant shift catalyst X2, the sulfur-containing gaseous flow being introduced in the second reactor either directly through flow and/or after mixture through flow with the raw synthesis gas. The process also includes collecting an outlet flow from the second reactor containing hydrogen-enriched synthesis gas.

Description

FIELD OF THE INVENTION

The present invention relates to a process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas.

BACKGROUND OF THE INVENTION

Synthesis gas, or briefly syngas, is a combustible gas mixture comprising carbon monoxide and hydrogen, and optionally other gases, such as carbon dioxide, nitrogen and water, hydrocarbons (e.g. methane), rare gases (e.g. argon), nitrogen derivatives (e.g. ammonia, hydrocyanic acid), etc.

Synthesis gas can be produced from many sources, including natural gas, coal, biomass, or virtually any hydrocarbon feedstock, by reaction with steam or oxygen. Synthesis gas is a versatile intermediate resource for production of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels.

Various processes are commonly used in the industry for the production of synthesis gas, mainly:

• • Steam Methane Reforming (SMR) or steam reforming for conversion of methane mainly. The resulting synthesis gas contains no sulfur compounds;
  • Gasification or partial oxidation (POx) which can also be catalytic (CPOx) is mainly used for the conversion of heavy feedstocks such as naphtha, liquefied petroleum gas, heavy fuel oil, coke, coal, biomass . . . . The resulting synthesis gas may be particularly rich in sulfur-containing components, mainly hydrogen sulphide.

Steam can be added to the synthesis gas in order to produce higher amount of hydrogen according to the well-known water-gas shift reaction (“WGSR”) which may be carried out to partially or totally eliminate carbon monoxide by converting it to carbon dioxide:

H₂O_(g)+CO_(g)⇄CO_2(g)+H_2(g)

wherein (g) indicates gaseous form.

The water-gas shift reaction is a reversible, exothermic chemical reaction highly used in the industry.

This reaction may be catalyzed in order to be carried out within a reasonable temperature range, typically less than 500° C. The type of catalysts usually employed depends on the sulfur content of the synthesis gas to be treated. Thus, the water-gas shift catalysts are generally classified into two categories, as described by David S. Newsome in Catal. Rev.-Sci. Eng., 21(2), pp 275-318 (1980):

• • iron-based or copper-based shift catalysts, also called “sweet shift catalysts”, are used with a sulfur-free synthesis gas (after a SMR for example) due to their deactivation by sulfur;
  • cobalt and molybdenum-based shift catalysts, also called “sulfur-resistant shift catalysts” or “sour shift catalysts”, which are used with a sulfur-containing synthesis gas (obtained after a coal gasification for example). These catalysts are often doped with an alkali metal such as sodium, potassium or caesium.

The main difference between sweet shift catalysts and sulfur-resistant shift catalysts is that the latter are active in their sulphided form and therefore need to be pre-sulphided prior to use. The sulfur-resistant shift catalysts are thus generally completely sulphided in their most active form. Thus, these catalysts are not only sulfur-tolerant but their activity may actually be enhanced by the sulfur present in the feed to be treated.

The sulfur-resistant shift catalysts have been widely developed in recent years. Indeed, the amount of fossil fuels, mainly natural gas and oil, has been continuously diminished and many researchers have focused their studies on the development of processes using less noble carbon sources such as coal or biomass which are usually particularly rich in sulfur. The synthesis gas obtained from these carbon sources generally contains hydrogen sulphide (H₂S) and carbonyl sulphide (COS) which may activate and maintain the activity of the sulfur-resistant shift catalysts during the further processed water-gas shift reaction.

However, some synthesis gases do not contain a sufficient amount of sulfur-containing compounds due to the low sulfur contents in the initial carbonaceous feedstock. Indeed, the (endogenous) sulfur content of the synthesis gases depends mainly on the coal type and the coal origin as indicated in Table 1.

TABLE 1
typical properties for characteristic coal types
	Energy content, kJ/g	Sulfur
Coal Type	(carbon content, wt %)	(wt %)
Bituminous	27,900 (avg. consumed in U.S.)	2-4
	67%
Sub-bituminous	20,000 (avg. consumed in U.S.)	0.5-0.5
(Powder River Basin)	49%
Lignite	15,000 (avg. consumed in U.S.)	0.6-1.6
	40%
Average Chinese Coal	19,000-25,000	0.4-3.7
	48-61%
Average Indian Coal	13,000-21,000	0.2-0.7
	30-50%

Hydrogen sulphide (H₂S) is the main source of sulfur in a synthesis gas obtained after gasification. For a synthesis gas with an insufficient sulfur content, the addition of extra hydrogen sulphide (exogenous hydrogen sulphide) is generally performed to efficiently activate the sulfur-resistant shift catalyst. Indeed, addition of H₂S to a mixture of CO and H₂O considerably enhances formation of H₂ and CO₂, as described by Stenberg et al. in Angew. Chem. Int. Ed. Engl., 21 (1982) No. 8, pp 619-620.

However, hydrogen sulphide has the inconvenient of being a highly toxic and flammable gaseous compound that manufacturers try to avoid.

It would therefore be desirable to develop a new process which does not involve the use of hydrogen sulphide, while being as effective as a process which involves the addition of hydrogen sulphide to activate the sulfur-resistant shift catalysts and maintain their activity.

It is an objective of the present invention to develop a safer process for the water-gas shift reaction from a sulfur-containing synthesis gas.

Another objective of the present invention is the implementation of an industrial-scale process for the water-gas shift reaction from a sulfur-containing synthesis gas.

SUMMARY OF THE INVENTION

A first object of the invention is a process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas, comprising the following steps:

• • introducing a gaseous flow 1 comprising at least one compound of formula (I)

• • • in which R is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, and a linear or branched alkenyl radical containing from 2 to 4 carbon atoms, n is equal to 0, 1 or 2, x is an integer selected from 0, 1, 2, 3 or 4, R′ is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, a linear or branched alkenyl radical containing from 2 to 4 carbon atoms and, only when n=x=0, a hydrogen atom,
    • in a first reactor 2 comprising a catalyst X₁, said catalyst X₁ comprising at least one metal selected from groups VI B and VII of the periodic table,
  • collecting a sulfur-containing gaseous flow 3 from the first reactor,
  • introducing the raw synthesis gas 4 in a second reactor 6,
  • introducing the sulfur-containing gaseous flow 3 in the second reactor where the catalytic water-gas shift reaction takes place and comprising a sulfur-resistant shift catalyst X₂, the sulfur-containing gaseous flow 3 being introduced in the second reactor either directly through flow 3.1 and/or after mixture through flow 3.2 with the raw synthesis gas 4,
  • collecting an outlet flow 7 from the second reactor, said outlet flow 7 comprising hydrogen-enriched synthesis gas.

According to an embodiment, the compound of formula (I) is selected from dimethyl disulphide and dimethyl sulfoxide, preferably dimethyl disulphide.

According to an embodiment, the catalytic water-gas shift reaction is carried out with an inlet gas temperature of at least 230° C., preferably from 240 to 320° C., more preferably from 250 to 310° C.

According to an embodiment, the first reactor 2 is used at a temperature ranging from 100 to 600° C., preferably from 150 to 400° C., more preferably from 200 to 350° C.

According to an embodiment, the first reactor 2 is used at a pressure ranging from 0 to 60 bar, preferably from 10 to 40 bar.

Preferably, the compound of formula (I) is continuously injected in the first reactor 2 at a flow rate of 1 Nl/h to 10 Nm³/h.

According to an embodiment, a hydrogen flow is introduced in the first reactor 2, said hydrogen flow coming from an exogenous source or being collected from the outlet flow 7 of the second reactor 6.

Preferably, the catalyst X₁ comprises molybdenum, tungsten, nickel and cobalt, said catalyst being preferably supported on a porous material such as alumina, silica or silica-alumina.

Preferably, the catalyst X₂ is a cobalt and molybdenum-based catalyst.

According to a preferred embodiment, the catalyst X₂ comprises an alkali metal, preferably sodium, potassium or caesium.

According to an embodiment, the catalytic water-gas shift reaction is carried out at a pressure of at least 10 bar, preferably ranging from 10 to 25 bar.

According to an embodiment, the raw synthesis gas 4 comprises water and carbon monoxide in a molar ratio of water to carbon monoxide of at least 1, preferably at least 1.2, more preferably at least 1.4.

Preferably, the residence time in the second reactor 6 ranges from 20 to 60 seconds.

Another object of the invention is the use of at least one compound of formula (I):

in which R is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, and a linear or branched alkenyl radical containing from 2 to 4 carbon atoms, n is equal to 0, 1 or 2, x is an integer selected from 0, 1, 2, 3 or 4, R′ is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, a linear or branched alkenyl radical containing from 2 to 4 carbon atoms and, only when n=x=0, a hydrogen atom, in a process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas.

It has now surprisingly been found that the use of compound(s) of formula (I) is particularly effective for the production of hydrogen by a catalytic water-gas shift reaction on a raw synthesis gas.

Moreover, compounds of formula (I) are generally presented in liquid form, which greatly facilitates their handling and the measures to be taken for the safety of operators.

As another advantage, the process of the invention allows conversion of CO to CO₂.

Furthermore, the process of the invention is suitable with respect to the requirements regarding the security and the environment.

BRIEF DESCRIPTION OF THE FIGURE

The FIGURE represents one embodiment of an installation for the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas, comprising the following steps:

• • introducing a gaseous flow 1 comprising at least one compound of formula (I):

in which R is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, and a linear or branched alkenyl radical containing from 2 to 4 carbon atoms, n is equal to 0, 1 or 2, x is an integer selected from 0, 1, 2, 3 or 4, R′ is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, a linear or branched alkenyl radical containing from 2 to 4 carbon atoms and, only when n=x=0, a hydrogen atom, in a first reactor 2 comprising a catalyst X₁, said catalyst X₁ comprising at least one metal selected from groups VI B and VII of the periodic table,

• • collecting a sulfur-containing gaseous flow 3 from the first reactor 2,
  • introducing the raw synthesis gas 4 in a second reactor 6,
  • introducing the sulfur-containing gaseous flow 3 in the second reactor 6 where the catalytic water-gas shift reaction takes place and comprising a sulfur-resistant shift catalyst X₂, the sulfur-containing gaseous flow 3 being introduced in the second reactor either directly through flow 3.1 and/or after mixture through flow 3.2 with the raw synthesis gas 4,
  • collecting an outlet flow 7 from the second reactor 6, said outlet flow 7 comprising hydrogen-enriched synthesis gas.

Within the meaning of the present invention, by “alkyl” radical, it is to be understood a saturated hydrocarbon chain comprising carbon atoms and hydrogen atoms, preferably consisting in only carbon atoms and hydrogen atoms.

Within the meaning of the present invention, by “alkenyl” radical, it is to be understood an unsaturated hydrocarbon chain comprising at least one carbon-carbon double bond and comprising carbon atoms and hydrogen atoms, preferably consisting in only carbon atoms and hydrogen atoms.

In an embodiment of the invention, the first reactor 2 is a catalytic reactor, preferably a fixed bed catalytic reactor. The gaseous flow 1 may be heated before entering the first reactor 2 at a temperature ranging from 100 to 600° C., preferably ranging from 100 to 400° C.

The first reactor 2 comprises a catalyst X₁ comprising at least one metal selected from groups VI B and VII of the periodic table, preferably molybdenum, tungsten, nickel and cobalt. A combination of at least two of these transition metals is preferably used, such as cobalt and molybdenum, or nickel and molybdenum, or nickel and tungsten, more preferably cobalt and molybdenum.

Catalyst X₁ may be supported on a porous material such as alumina, silica or silica-alumina.

As an example of suitable catalyst X₁ according to the invention, mention may be made of a catalyst containing cobalt and molybdenum supported on alumina.

In a preferred embodiment of the invention, the first reactor 2 comprising catalyst X₁ may be filled with an inert material to allow an efficient distribution of the gaseous flow into the first reactor 2. Suitable inert materials may be silicon carbide. Advantageously, catalyst X₁ and the inert material are placed in successive layers into the first reactor 2.

The gaseous flow 1 introduced in the first reactor 2 comprises at least one compound of formula (I):

in which:

• • R is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, and a linear or branched alkenyl radical containing from 2 to 4 carbon atoms,
  • n is equal to 0, 1 or 2,
  • x is an integer selected from 0, 1, 2, 3 or 4,
  • R′ is selected from a linear or branched alkyl radical containing from 1 to 4 carbon atoms, a linear or branched alkenyl radical containing from 2 to 4 carbon atoms and, only when n=x=0, a hydrogen atom.

According to one embodiment, the compound of formula (I) that may be used in the process of the present invention is an organic sulphide, optionally in its oxide form (when n is different from zero), obtained according to any process known per se, or else commercially available, optionally containing a reduced amount of, or no, impurities that may be responsible for undesired smells, or optionally containing one or more odor-masking agents (see e.g. WO2011012815A1).

Among preferred R and R′ radicals, mention may be made of methyl, propyl, allyl and 1-propenyl radicals.

According to an embodiment of the invention, in the above formula (I), x represents 1, 2, 3 or 4, preferably x represents 1 or 2, more preferably x represents 1.

According to a preferred embodiment, the compound of formula (I) for use in the process of the present invention is a compound of formula (Ia):

R—S—S_x—R′  (Ia)

which corresponds to formula (I) wherein n is equal to 0, and R, R′ and x are as defined above.

Preferably, the compound of formula (Ia) is dimethyl disulphide (“DMDS”).

According to a preferred embodiment of the invention, the compound of formula (I) for use in the process of the present invention is a compound of formula (Ib):

which corresponds to formula (I) wherein n is equal to 1, and R, R′ and x are as defined above.

Preferably, the compound of formula (Ib) is dimethyl sulfoxide (“DMSO”).

It should be understood that mixtures of two or more compounds of formula (I) may be used in the process of the present invention. Especially mixtures of di- and/or polysulphides may be used, for example mixtures of disulphides, such as disulphide oils (“DSO”).

In an embodiment of the invention, the gaseous flow 1 is continuously injected into the first reactor 2. The concentration of compound(s) of formula (I), preferably of dimethyl disulphide, into the gaseous flow 1 may range from 100 to 500,000 ppmv, preferably from 100 to 200,000 ppmv, more preferably from 100 to 100,000 ppmv. The flow rate of compound(s) of formula (I), preferably of dimethyl disulphide, may range from 1 Nl/h to 10 Nm³/h.

In an embodiment of the invention, the gaseous flow 1 also comprises hydrogen. Hydrogen may come from an exogenous source or may be collected from the outlet flow 7 of the second reactor 6. By “exogenous source” is meant a source external to the process. The concentration of hydrogen into the gaseous flow 1 may range from 100 to 10⁶ ppmv, preferably from 10,000 to 999,900 ppmv, more preferably from 200,000 to 999,900 ppmv. The flow rate of hydrogen into the gaseous flow 1 may range from 0.1 Nm³/h to 10,000 Nm³/h.

According to an embodiment, hydrogen is recovered, for example by purification, from the outlet flow 7 before being introduced into the gaseous flow 1.

The first reactor 2 may be used at a temperature ranging from 100 to 600° C., preferably from 150 to 400° C., more preferably from 200 to 350° C. The first reactor 2 may be used at a pressure ranging from 0 to 60 bar (6 MPa), preferably from 10 to 40 bar (4 MPa).

A sulfur-containing gaseous flow 3 is collected at the outlet of the first reactor 2 and introduced in the second reactor 6 where the water-gas shift reaction takes place. The sulfur-containing gas flow 3 is introduced in the second reactor 6 either directly and/or in a mixture with the raw synthesis gas 4. A valve 5 may be present in the line containing the sulfur-containing gaseous flow 3 in order to direct the flow through lines 3.1 or 3.2 (see for example the FIGURE). With reference to the FIGURE, if the valve 5 is programmed to direct the flow through line 3.1, then the sulfur-containing gaseous flow 3 is introduced directly into the second reactor 6 (independently of the introduction of the raw synthesis gas 4). If the valve 5 is programmed to direct the flow through line 3.2, then the sulfur-containing gaseous flow 3 is mixed with the raw synthesis gas 4 before entering the second reactor 6; in this embodiment, a mixture of sulfur-containing gaseous flow 3 and raw synthesis gas 4 is introduced into the second reactor 6. It is also possible to provide a process wherein the valve 5 directs the flow 3 simultaneously through lines 3.1 and 3.2.

The raw synthesis gas 4 is typically obtained after a gasification step of a raw material such as coke, coal, biomass, naphtha, liquefied petroleum gas, heavy fuel oil. The production of synthesis gas is well known in the state of the art. The raw synthesis gas 4 may also be obtained from a Steam Methane Reformer.

According to the present invention, the raw synthesis gas 4 comprises carbon monoxide, and optionally other gases, such as hydrogen, carbon dioxide, nitrogen and water, hydrocarbons (e.g. methane), rare gases (e.g. argon), nitrogen derivatives (e.g. ammonia, hydrocyanic acid), etc.

According to an embodiment of the invention, the raw synthesis 4 comprises carbon monoxide and hydrogen, and optionally other gases such as carbon dioxide, nitrogen and water, hydrocarbons (e.g. methane), rare gases (e.g. argon), nitrogen derivatives (e.g. ammonia, hydrocyanic acid), etc.

According to another embodiment of the invention, the raw synthesis gas comprises carbon monoxide, carbon dioxide, hydrogen, nitrogen and water.

The raw synthesis gas 4 may also comprise sulfur-containing components. In this case, the raw synthesis gas 4 may comprise carbon monoxide, carbon dioxide, hydrogen, nitrogen and water as main components and sulfur-containing components in lower concentrations. The sulfur-containing components may be hydrogen sulphide, carbonyl sulphide. Typical (endogenous) sulfur content in the raw synthesis gas 4 ranges from about 20 to about 50,000 ppmv. Typical (endogenous) sulfur content in the raw synthesis gas 4 may depend on the raw material initially used for the production of the raw synthesis gas 4.

The water-gas shift reaction is carried out in the second reactor 6 comprising a catalyst X₂.

The water-gas shift reaction consists in the conversion of carbon monoxide and water contained in the raw synthesis gas 4 to carbon dioxide and hydrogen according to equation (1):

H₂O_(g)+CO_(g)⇄CO_2(g)+H_2(g)  (1)

wherein (g) indicates gaseous form.

This water-gas shift reaction allows to obtain a hydrogen-enriched synthesis gas.

By “hydrogen-enriched synthesis gas” according to the present invention, it is to be understood that the synthesis gas at the outlet of the process of the invention comprises more hydrogen than the synthesis gas at the inlet of the process of the invention. In other words, the proportion of hydrogen in the gas at the outlet of the process (stream 7) is higher than the proportion of hydrogen in the gas at the outlet of the process (stream 4).

According to an embodiment of the invention, water may be added to the raw synthesis gas 4. Introduction of additional (exogenous) water allows to shift the equilibrium to the formation of carbon dioxide and hydrogen. Additional (exogenous) water may be introduced either directly to the second reactor 6 or in a mixture with the raw synthesis gas 4.

The efficiency of water-gas shift reaction and thus of the hydrogen enrichment of the synthesis gas may be measured directly by hydrogen purity analysis, for instance with a gas chromatograph. It could also be indirectly measured by determining the CO conversion into CO₂ meaning that the water-gas shift reaction has occurred. The CO conversion into CO₂ is known by measuring the CO conversion and the CO₂ yield.

In an embodiment of the invention, the molar ratio of water to carbon monoxide in the gas entering the water-gas shift reaction is of at least 1, preferably at least 1.2, more preferably at least 1.4, advantageously at least 1.5. The molar ratio of water to carbon monoxide may range from 1 to 3, preferably from 1.2 to 2.5, more preferably from 1.5 to 2.

In an embodiment of the invention, the second reactor 6 is a catalytic reactor, preferably a fixed bed catalytic reactor.

The catalyst X₂ suitable for use in the water-gas shift reaction is a sulfur-resistant shift catalyst. By “sulfur-resistant shift catalyst” is meant a compound capable of catalyzing the water-gas shift reaction in the presence of sulfur-containing components.

Catalysts suitable for use in the water-gas shift reaction may comprise at least one transition metal other than iron and copper, preferably selected from the group consisting of molybdenum, cobalt and nickel. A combination of at least two of these transition metals is preferably used, such as cobalt and molybdenum, or nickel and molybdenum, more preferably cobalt and molybdenum.

The catalysts according to the invention may be either supported or unsupported, preferably supported. Suitable catalyst supports may be alumina.

In a preferred embodiment, the catalyst X₂ also comprises an alkali metal selected from the group consisting of sodium, potassium and caesium, preferably potassium and caesium, or salts thereof. An example of a particularly active catalyst is the combination of caesium carbonate, caesium acetate, potassium carbonate or potassium acetate, together with cobalt and molybdenum.

As an example of suitable catalysts X₂ according to the invention, mention may be made of sulfur-resistant shift catalysts such as those disclosed by Park et al. in “A Study on the Sulfur-Resistant Catalysts for Water Gas Shift Reaction-IV. Modification of CoMo/γ-Al2O3 Catalyst with Iron Group Metals”, Bull. Korean Chem. Soc. (2000), Vol. 21, No. 12, 1239-1244.

In an embodiment of the invention, the gas entering the water-gas shift reaction is pre-heated to a temperature of at least 230° C. In a preferred embodiment, this temperature ranges from 240 to 320° C., preferably from 250 to 310° C.

In an embodiment of the invention, the inlet gas temperature in the second reactor 6 is at least 230° C. and preferably at most 400° C. Preferably, this temperature ranges from 240° C. to 320° C., preferably from 250° C. to 310° C.

In an embodiment of the invention, the pressure for the water-gas shift reaction is of at least 10 bars (1 MPa), preferably ranges from 10 to 30 bars (1 MPa to 3 MPa), more preferably from 15 to 25 bars (1.5 MPa to 2.5 MPa).

In an embodiment of the invention, the residence time in the second reactor 6 ranges from 20 to 60 seconds, preferably from 30 to 50 seconds, allowing the determination of the amount of catalyst X₂ in reactor 6. The residence time is defined by the following formula:

$\mathrm{residence}\mathrm{time}=\frac{V_{\mathrm{cat}}}{D_{\mathrm{gas}}}\times \frac{P_{\mathrm{reac}}}{P_{\mathrm{atm}}}$

wherein V_cat represents the volume of catalyst X₂ in the reactor 6 expressed in m³, D_gas represents the inlet gas flow rate of flow 3 and flow 4 expressed in Nm³/s, P_reac and P_atm respectively represent the pressure in the reactor and the atmospheric pressure expressed in Pa.

In an embodiment of the invention, the CO conversion rate of the water-gas shift reaction is of at least 50%, preferably at least 60%, more preferably at least 65%. The CO conversion rate is calculated as follows:

$\mathrm{CO}\mathrm{Conversion}\left(\%\right)=\frac{\left(Q.{\mathrm{CO}}_{\mathrm{entry}}-Q.{\mathrm{CO}}_{\mathrm{exit}}\right)}{Q.{\mathrm{CO}}_{\mathrm{entry}}}\times 100$

wherein Q.CO_entry represents the molar flow of CO at the inlet of the reactor 6 expressed in mol/h and Q.CO_exit represents the molar flow of CO at the outlet of the reactor 6 expressed in mol/h.

In an embodiment of the invention, the CO₂ yield of the water-gas shift reaction is of at least 50%, preferably at least 60%, more preferably at least 65%.

The CO₂ yield rate is calculated as follows:

${\mathrm{CO}}_2\mathrm{yield}\left(\%\right)=\frac{\left(Q.{\mathrm{CO}}_{{2.}_{\mathrm{exit}}}\right)}{\left(Q.{\mathrm{CO}}_{\mathrm{entry}}\right)}\times 100$

wherein Q.CO_entry represents the molar flow of CO at the inlet of reactor 6 expressed in mol/h and Q.CO₂, exit represents the molar flow of CO₂ at the outlet of the reactor 6 expressed in mol/h.

In a preferred embodiment of the invention, the second reactor 6 comprising catalyst X₂ may be filled with an inert material to allow an efficient distribution of the gas into the second reactor before starting up the reactor for the water-gas shift reaction step. Suitable inert materials may be silicon carbide or alumina. Advantageously, catalyst X₂ and the inert material are placed in successive layers into the reactor.

In an embodiment of the invention, the residence time in the first reactor 2 ranges from 50 to 1000 seconds, preferably from 100 to 500 seconds, allowing the determination of the amount of catalyst X₁ in the reactor 2.

The residence time is defined by the following formula:

$\mathrm{residence}\mathrm{time}=\frac{V_{\mathrm{cat}}}{D_{\mathrm{gas}}}\times \frac{P_{\mathrm{reac}}}{P_{\mathrm{atm}}}$

wherein V_cat represents the volume of catalyst X₁ in the first reactor 2 expressed in m³, D_gas represents the inlet gas flow rate of flow 1 expressed in Nm³/s, P_reac and P_atm respectively represent the pressure in the reactor 2 and the atmospheric pressure expressed in Pa.

In a preferred embodiment of the invention, a start-up phase of the first reactor 2 is performed before the implementation of the process of the invention. First, a gaseous flow comprising at least one compound of formula (I) and hydrogen is injected in the first reactor 2. The flow rate of the compound(s) of formula (I) in the gaseous flow 1 may range from 1 Nl/h to 10 Nm³/h. The flow rate of hydrogen may range from 0.1 to 10,000 Nm³/h. During this start-up phase, the temperature is increased from ambient temperature to 400° C., preferably from 20° C. to 350° C. The duration of the start-up phase may range from 1 to 64 hours, preferably from 30 to 40 hours.

During all this start-up phase, the sulfur-containing gaseous flow 3 at the outlet of the first reactor 2 may be directed to a flare and/or to the second reactor 6 by using pipes and tubing that can either send the sulfur-containing gaseous flow 3 to the flare and/or to the second reactor 6.

In a preferred embodiment of the invention, a preparation step of catalyst X₂ in the second reactor 6 is performed before the implementation of the process of the invention. The preparation step of catalyst X₂ may include a drying step and/or a pre-activation step, preferably a drying step and a pre-activation step.

During the drying step, catalyst X₂ may be dried under an inert gas flow, preferably a nitrogen gas flow. The inert gas flow rate may range from 0.1 to 10,000 Nm³/h. During the drying step, the temperature may increase from 20° C. to 200° C. The drying time may range from 1 to 10 hours, preferably 6 hours. The drying step is preferentially performed from ambient pressure to the preferred operated pressure between 15 to 25 bars.

During the pre-activation step, catalyst X₂ may be sulphided. The reactor 6 may be treated under a hydrogen stream at a flow rate of 0.1 to 10,000 Nm³/h and at a pressure of, at least 10 bars, the preferred operated pressure between 15 to 25 bars. Then, hydrogen sulphide or the sulfur-containing gaseous flow 3 at the outlet of the first reactor 2 may be injected upflow at a flow rate of 1 Nl/h to 10 Nm³/h into the hydrogen stream. The temperature of the reactor 6 may then be increased from 150° C. to 350° C. by any means known to the person skilled in the art. The time of pre-activation step may range from 1 to 64 hours. The hydrogen stream is preferably maintained during all the pre-activation step.

Another object of the invention relates to the use of at least one compound of formula (I), preferably dimethyl disulphide, in a process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas.

EXAMPLES

Example 1 (Comparative)

A water-gas shift reaction is carried out in a catalytic reactor 6′ of a pilot plant according to the following procedure.

1) Preparation of Catalytic Reactor 6′

Catalytic reactor 6′ of 150 cm³ is filled at ambient pressure and ambient temperature with three layers of solids separated by metal grids, as follows:

• • a first layer of 60 cm³ of silicon carbide of Carborundum type having a particle size of 1.680 mm: this inert material allows a satisfactory gas distribution,
  • a second layer of 40 cm³ of a CoMo-based sulfur-resistant shift catalyst,
  • a third layer of 50 cm³ of silicon carbide of Carborundum type having a particle size of 1.680 mm.

Catalytic reactor 6′ is then positioned into a furnace that can withstand a wide temperature ranging from 100 to 350° C. Catalytic reactor 6′ is connected at the inlet tubing to a gas feed and at the outlet tubing to an analyzer.

For the example, the CoMo-based sulfur-resistant shift catalyst is first dried by a nitrogen flow rate of 20 Nl/h at ambient pressure. The drying temperature is set to 150° C. with a temperature ramp of +25° C./h. The drying time is set to 1 hour.

A second step consists in sulfiding the CoMo-based sulfur-resistant shift catalyst to make it pre-active. During this step, the reactor is treated under a hydrogen flow rate of 20 Nl/h at a pressure of 35 bars. Then hydrogen sulphide is injected upflow at a flow rate of 0.5 Nl/h into the hydrogen feed. The catalyst is then subjected to a temperature ramp of +20° C./h. The first plateau is set to 150° C. for 2 hours then the temperature is increased up to 230° C. with a temperature ramp of +25° C./h. A second plateau of 4 hours is maintained to 230° C. and then the temperature is increased again up to 350° C. with a temperature ramp of +25° C./h. A final plateau of 16 hours is performed at 350° C. The temperature was then dropped to 230° C. still under a hydrogen stream with a flow rate of 20 Nl/h: the catalyst is thus pre-activated.

2) Water-Gas Shift Reaction Step

The study of the conversion of carbon monoxide to carbon dioxide in the pre-activated CoMo-based sulfur-resistant shift catalyst is then carried out at 20 bars (2 MPa). Catalytic reactor 6′ is treated upflow with a synthesis gas mixture comprising hydrogen at a flow rate of 8.5 Nl/h, carbon monoxide at 17 Nl/h, water at 0.33 cm³/min and nitrogen at 26 Nl/h at a pressure of 20 bars (2 MPa). The molar ratio H₂O/CO is of 1.44 and the residence time is of 38 seconds. Hydrogen sulphide is injected upflow in the gas mixture at a flow rate of 0.5 Nl/h. The inlet temperature of the gas entering the catalytic reactor 6′ is maintained to 310° C.

The CO and CO₂ concentrations of the gas flow are measured by an infra-red spectroscopic analyzer connected at the outlet of catalytic reactor A in order to determine the CO conversion and the CO₂ yield.

A CO conversion rate of 92% and a CO₂ yield of 95% are obtained, such a rate reflecting good performance of the water-gas shift reaction.

Example 2 (According to the Invention)

A water-gas shift reaction is carried out in a catalytic reactor 6 connected upstream to a catalytic reactor 2 according to the following procedure.

1) Preparation of the Catalytic Reactors

• • Catalytic reactor 6

With reference to the FIGURE, catalytic reactor 6 of 150 cm³ is filled at ambient pressure and ambient temperature with three layers of solids separated by metal grids, as follows:

• • a first layer of 60 cm³ of silicon carbide of Carborundum type having a particle size of 1.680 mm: this inert material allows a satisfactory gas distribution,
  • a second layer of 40 cm³ of a CoMo-based sulfur-resistant shift catalyst,
  • a third layer of 50 cm³ of silicon carbide of Carborundum type having a particle size of 1.680 mm.

Catalytic reactor 6 is then positioned into a furnace that can handle a wide temperature ranging from 100 to 350° C. Catalytic reactor 6 is connected at the inlet tubing to a gas feed and at the outlet tubing to an analyzer.

For the example, the CoMo-based sulfur-resistant shift catalyst is first dried by a nitrogen flow rate of 20 Nl/h at ambient pressure. The drying temperature is set to 150° C. with a temperature ramp of +25° C./h. The drying time is set to 1 hour.

A second step consists in sulfiding the CoMo-based sulfur-resistant shift catalyst to pre-activate it. During this step, the reactor is treated under a hydrogen flow rate of 20 Nl/h at a pressure of 35 bars. Then hydrogen sulphide is injected upflow at a flow rate of 0.5 Nl/h into the hydrogen feed. The catalyst is then subjected to a temperature ramp of 20° C./h. The first plateau is set to 150° C. for 2 hours then the temperature is increased up to 230° C. with a temperature ramp of +25° C./h. A second plateau of 4 hours is maintained to 230° C. and then the temperature is increased again up to 350° C. with a temperature ramp of +25° C./h. A final plateau of 16 hours is performed at 350° C. The temperature was then dropped to 230° C. still under a hydrogen stream with a flow rate of 20 Nl/h: the catalyst is thus pre-activated.

• • Catalytic reactor 2

Catalytic reactor 2 of volume equal to 150 cm³ is filled at ambient pressure and ambient temperature with three layers of solids separated by metal grids, as follows:

• • a first layer of 60 cm³ of silicon carbide of Carborundum type having a particle size of 1.680 mm: this inert material allows a satisfactory gas distribution,
  • a second layer of 40 cm³ of a Al₂O₃ supported CoMo-based catalyst containing 15 wt % Mo and 3 wt % Co,
  • a third layer of 50 cm³ of silicon carbide of Carborundum type having a particle size of 1.680 mm.

The start-up phase of catalytic reactor 2 consists in placing this reactor filled as explained previously in a furnace and then treating it under a hydrogen flow rate of 20 Nl/h at a pressure of 25 bars (2.5 MPa). Dimethyl disulphide (DMDS) is injected in the liquid state upflow at 1 cm³/h in the hydrogen stream 1. The Al₂O₃ supported CoMo-based catalyst is subjected to a temperature ramp of +20° C./h. The first plateau is set to 150° C. for 2 hours then temperature is increased up to 230° C. with a temperature ramp of +25° C./h. A second plateau of 4 hours is maintained to 230° C. and then temperature is increased again up to 350° C. with a temperature ramp of +25° C./h. A final plateau of 16 hours is performed at 350° C.

The temperature is then lowered to 310° C. by still maintaining a flow rate of 1 cm³/h of DMDS and the pressure at 25 bars (2.5 MPa). The rate of hydrogen is decreased to 8.5 Nl/h. The reactor 2 start-up phase is thus ended.

During all this start-up phase, the sulfur-containing gaseous mixture 3 from the catalytic reactor 2 is directed to a flare and/or to the second reactor 6 by using pipes and tubing that can either send the gaseous mixture to the flare and/or to the reactor 6.

2) Water-Gas Shift Reaction Step

The study of the conversion of carbon monoxide to carbon dioxide in the pre-activated sulphur-resistant shift catalyst is then carried out at 20 bars (2 MPa). Catalytic reactor 6 is treated upflow with a gaseous mixture 4 comprising carbon monoxide at 17 Nl/h, water at 0.33 cm³/min and nitrogen at 26 Nl/h at a pressure of 20 bars. Except during the start-up phase of catalytic reactor 2, the sulfur-containing gaseous mixture 3 exiting reactor 2 is then injected into the gaseous mixture 4, the resulting gaseous mixture 5 being introduced in catalytic reactor 6. The inlet temperature of the gas entering the catalytic reactor 6′ is maintained to 310° C. The molar ratio H₂O/CO is of 1.4 and the residence time is of 38 seconds.

The CO and CO₂ concentrations of the gaseous flow are measured by an infra-red spectroscopic analyzer connected at the outlet line 7 of catalytic reactor 6 in order to determine the CO conversion and the CO₂ yield.

A CO conversion rate of 92% and a CO₂ yield of 95% are obtained reflecting good performance of the water-gas shift reaction, equivalent to that obtained with H₂S as the activating agent in example 1. Therefore, DMDS is as efficient as H₂S in a process for the catalytic water-gas shift reaction.

The process using at least one compound responding to formula (I) as defined in the present invention instead of gaseous hydrogen sulphide is therefore as efficient, safer and easier to implement.

Claims

1. A process for the production of hydrogen-enriched synthesis gas by a catalytic water-gas shift reaction operated on a raw synthesis gas, comprising the following steps: introducing a gaseous flow comprising at least one compound of formula (I)

2. The process according to claim 1, wherein the compound of formula (I) is selected from dimethyl disulphide and dimethyl sulfoxide.

3. The process according to claim 1, wherein the catalytic water-gas shift reaction is carried out with an inlet gas temperature of at least 230° C.

4. The process according to claim 1, wherein the first reactor is used at a temperature ranging from 100 to 600° C.

5. The process according to claim 1, wherein the first reactor is used at a pressure ranging from 0 to 60 bar.

6. The process according to claim 1, wherein the compound of formula (I) is continuously injected in the first reactor at a flow rate of 1 NI/h to 10 Nm3/h.

7. The process according to claim 1, wherein a hydrogen flow is introduced in the first reactor, said hydrogen flow coming from an exogenous source or being collected from the outlet flow of the second reactor.

8. The process according to claim 1, wherein the catalyst X1 comprises molybdenum, tungsten, nickel and cobalt.

9. The process according to claim 1, wherein the catalyst X2 is a cobalt and molybdenum-based catalyst.

10. The process according to claim 1, wherein the catalyst X2 comprises an alkali metal.

11. The process according to claim 1, wherein the catalytic water-gas shift reaction is carried out at a pressure of at least 10 bar.

12. The process according claim 1, wherein the raw synthesis gas comprises water and carbon monoxide in a molar ratio of water to carbon monoxide of at least 1.

13. The process according to claim 1, wherein the residence time in the second reactor ranges from 20 to 60 seconds.

14. (canceled)

15. The process according to claim 1, wherein the compound of formula (I) is dimethyl sulfoxide.