EP2099727B1 - Method for optimising the operation of a unit for the synthesis of hydrocarbons from a synthesis gas - Google Patents

Abstract

The invention concerns a method for optimizing the operation of a reaction section for hydrocarbon synthesis starting from a feed comprising synthesis gas, operated in the presence of a catalyst comprising cobalt. This method comprises the following steps: a) determining the theoretical molar ratio, PH2O:PH2, in the reaction section; b) optionally, adjusting the ratio PH2O:PH2 determined in step a) to a value strictly below 1; c) determining the new value for the theoretical ratio PH2O:PH2 in the reaction section; and repeating steps a) to c) until the ratio of the partial pressures of water and hydrogen, PH2O:PH2, has a value strictly less than 1.1.

Description

The present invention relates to the field of hydrocarbon synthesis from a mixture comprising carbon monoxide (CO), hydrogen (H2) and optionally carbon dioxide (CO2), generally called synthesis gas.

The method according to the invention makes it possible to optimize the operation of a hydrocarbon synthesis unit from synthesis gas (also called Fischer-Tropsch synthesis), or to restore stable operation in order to maximize the hydrocarbon yield. C5 + (hydrocarbons comprising 5 or more carbon atoms).

The process according to the invention is a process for controlling Fischer-Tropsch synthesis in which the ratio of partial pressures of water and hydrogen P _{H 2 O} : P _{H 2 is used} as the control parameter of this synthesis.

PRIOR ART

The conversion reaction of synthesis gas (CO- (CO2-H2) mixture into hydrocarbons has been known since the beginning of the twentieth century and is commonly known as Fischer-Tropsch synthesis, units were operated in Germany during the Second World War, then in South Africa to synthesize synthetic fuels The majority of these units, mainly dedicated to the production of synthetic fuels, were or are still operated with iron-based catalysts.

More recently, there has been a new interest in these syntheses, and more particularly in the use of catalysts comprising cobalt which can direct the reaction towards the formation of heavier hydrocarbons, mainly paraffinic, essentially C5 + hydrocarbons ( hydrocarbons comprising 5 or more carbon atoms per molecule) while minimizing the formation of methane and hydrocarbons having 2 to 4 carbon atoms per molecule (C2-C4). The hydrocarbons thus formed can be transformed into a hydrocracking unit downstream, to produce mainly kerosene and gas oil. Such a method is for example described in the patent EP-B-1 406 988 . The use of a catalyst comprising cobalt is more suitable for treating hydrogen-rich synthesis gases (feedstock) resulting from the transformation of natural gas in particular.

Many cobalt-based formulations have been described in the prior art (see, for example, patent applications EP-A-0 313 375 or EP-A-1,233,011 ). Unlike iron-based catalysts that are active in the reaction of CO to CO2 (water gas shift reaction or WGSR): CO + H2O → CO2 + H2, cobalt-based catalysts do not present very little activity for this reaction ( BH Davis, Catalysis Today, 84, 2003, p.83 ).

However, under certain conditions, catalysts comprising cobalt can develop a CO conversion activity (WGSR), which then comes into competition with the Fischer-Tropsch synthesis reaction and strongly penalize this synthesis. Indeed, the CO conversion reaction (WGSR) consumes some of the CO reagent by forming CO2 instead of the desired hydrocarbons and simultaneously produces an excess of hydrogen which modifies the H2: CO ratio and gives rise to a degradation of the selectivity of the reaction towards the lightest products. The selectivities in methane and C2 to C4 hydrocarbons are therefore increased.

The patent US 6,534,552 B2 describes a process for the production of hydrocarbons from natural gas in which natural gas is converted to synthesis gas which is fed to a Fischer-Tropsch synthesis section to produce hydrocarbons and a tail gas (tail gas according to the terminology Anglo-Saxon). A separation section makes it possible to separate hydrogen from a fraction of this gas, said hydrogen being recycled continuously, either to the Fischer-Tropsch section or to the synthesis gas production section.

The patent US 4,626,552 describes a procedure for starting a Fischer-Tropsch reactor in which the H2: CO ratio is maintained at a low value by imposing a hydrogen flow rate of between 15% and 90% of the flow rate in the stabilized state. Then gradually increases the gas load flow, pressure and temperature and finally the H2: CO ratio is adjusted to the desired optimum value by increasing the flow of hydrogen input.

Requirement WO 2005/123882 describes a method for producing liquid hydrocarbons by Fischer-Tropsch synthesis in which the difference between the H2 / CO ratio at the inlet and the H2 / CO ratio in the effuent is kept substantially constant.

SUMMARY OF THE INVENTION

The method according to the invention is a method for optimizing the operation of a hydrocarbon synthesis unit from a feedstock comprising synthesis gas, in which one operates in the presence of a catalyst comprising cobalt.

The method according to the invention relates to a process for the synthesis of hydrocarbons from a feedstock comprising synthesis gas operated with a catalyst comprising cobalt. Said method comprises the following steps: the determination of the theoretical molar ratio of the partial pressures of water and of hydrogen P _H2O : P _H2 in the Fischer-Tropsch reaction section, followed by a possible adjustment of this ratio and then the determination of the new value of this report. These steps are optionally repeated until said ratio has a value less than 1.1, preferably strictly less than 1 and very preferably strictly less than 0.9, even more preferably strictly less than 0.8. or even strictly less than 0.65.

This method of controlling the Fischer-Tropsch synthesis makes it possible to maintain high performances, particularly in terms of yield of heavy products (C5 + hydrocarbons). It also makes it possible to maximize the selectivity of the heavier hydrocarbons according to the Fischer-Tropsch reaction and to avoid the degradation of the selectivity by the development of the CO conversion reaction (in English WGSR).

DETAILED DESCRIPTION

The method according to the invention is a method for controlling and optimizing Fischer-Tropsch synthesis in which the molar ratio of the partial pressures of water and hydrogen P _{H 2 O} : P _{H 2} in the Fischer-Tropsch reaction section is used. as a parameter for controlling and optimizing this synthesis.

The method according to the invention makes it possible to improve the operation of the Fischer-Tropsch synthesis unit by optimizing its yield and avoiding any selectivity drift towards the CO conversion reaction ("Water Gas Shift Reaction" or "WGS Reaction"). according to the English terminology). This new method of control and optimization is particularly relevant during transitional phases, especially when starting a unit or during a temporary malfunction of the unit (for example, when an incident such as the rupture of part of the load supply, disrupts the operation of the reaction section).

This is also the case when the setpoints (temperatures, pressure, gas flow, etc.) are changed because of a temporary malfunction of the unit or because of deactivation of the catalyst. By way of illustration, mention may be made of the case where, during the start-up phase of the unit, the activity of the catalyst increases during its final phase of construction, in situ under synthesis gas ( H. Schulz et al., Catalysis Today 71, 351, 2002 ).

The objective is the synthesis of a mixture of hydrocarbons comprising mainly paraffins, and mainly long-chain carbon compounds (hydrocarbons having more than 5 carbon atoms per molecule and preferably having more than 20 carbon atoms per molecule) in the presence of a catalyst comprising cobalt, also called Fischer-Tropsch synthesis. In order to achieve this objective, it is important to minimize as much as possible the aforementioned transitional phases during which the conversion and or the selectivity of the Fischer-Tropsch reaction are generally not optimal.

The method for controlling and optimizing the operation of a hydrocarbon synthesis unit according to the invention makes it possible to maintain high performances, particularly in terms of yield of heavy products (C5 + hydrocarbons). More precisely, it makes it possible to maximize the selectivity for the heavier hydrocarbons according to the Fischer-Tropsch reaction and to avoid the degradation of the selectivity by the development of the CO conversion reaction.

In the Fischer-Tropsch synthesis unit according to the invention, said catalyst can be used in a fixed bed (reactor with a fixed bed catalyst, with one or more catalyst beds in the same reactor) or preferably in a reactor. triphasic reactor (implementation in "slurry" according to the English terminology) comprising the catalyst in suspension in a substantially inert liquid phase and the reactive gas phase (synthesis gas).

The synthesis gas used in the Fischer-Tropsch synthesis step according to the invention can be obtained via the transformation of natural gas, coal, or biomass by processes such as steam reforming or partial oxidation, or via the decomposition of methanol, or from any other method known to those skilled in the art. Any charge comprising at least hydrogen and carbon monoxide may therefore be suitable. Preferably, the synthesis gas used in Fischer-Tropsch synthesis has a H 2: CO molar ratio of between 1: 2 and 5: 1, more preferably between 1.2: 2 and 3: 1, and more preferably between 1.5: 1 and 2.6: 1.

The Fischer-Tropsch synthesis is generally carried out under a pressure of between 0.1 MPa and 15 MPa, preferably between 1 MPa and 10 MPa and more preferably between 1.5 MPa and 5 MPa. The hourly volumetric velocity of the synthesis gas is generally between 100 and 20000 h ^-1 (volume of synthesis gas per volume of catalyst per hour), preferably between 400 and 10,000 h ^-1 .

Any catalyst comprising cobalt known to those skilled in the art is suitable for the process according to the invention, especially those mentioned in the "prior art" part of this application. Catalysts comprising cobalt deposited on a support selected from among the following oxides are preferably used: alumina, silica, zirconia, titanium oxide, magnesium oxide or their mixtures. Various promoters known to those skilled in the art can also be added, in particular those selected from the following elements: rhenium, ruthenium, molybdenum, tungsten, chromium. It is also possible to add at least one alkali or alkaline earth metal to these catalytic formulations.

1. a) Détermination du rapport molaire P_H2O:P_H2 théorique dans la section réactionnelle comme dans la revendication 1.
2. b) Ajustement éventuel du rapport P_H2O:P_H2 déterminé à l'étape a) à une valeur strictement inférieure à 1,1 grâce à des moyens détaillés par la suite,
3. c) Détermination de la nouvelle valeur du rapport P_H2O:P_H2 ajusté à l'étape b) au moyen de la méthode utilisée à l'étape a),
   puis éventuellement, si cela est nécessaire, l'étape d) suivante, après l'étape c):
4. d) Répétition des étapes a) à c) jusqu'à ce que le rapport des pressions partielles d'eau et d'hydrogène P_H2O:P_H2 présente une valeur strictement inférieure à 1,1.

In the method according to the invention, the following control steps are carried out:

1. a) Determination of the theoretical P _H2O : P _H2 molar ratio in the reaction section as in claim 1.
2. b) Possible adjustment of the ratio P _H2O : P _H2 determined in step a) to a value strictly less than 1.1 with detailed means thereafter,
3. c) Determination of the new value of the ratio P _H2O : P _H2 adjusted in step b) by means of the method used in step a),
   then possibly, if necessary, the following step d), after step c):
4. d) repeating steps a) to c) until the ratio of partial pressures of water and hydrogen P _H2O : P _H2 has a value strictly less than 1.1.

The determination of the ratio P _H2O : P _H2 according to step a) can be carried out using any means known to those skilled in the art. The reaction section may consist of one or more reactors. Step a) is performed using a means selected from the means detailed below.

A preferred means consists in measuring the amount of carbon monoxide in the gaseous effluent and estimating the theoretical P _H2O : P _H2 ratio from the conversion rate of carbon monoxide in the whole of the reaction section comprising one or more reactors. , the H2: CO ratio in the feedstock and the H2: CO ratio for the gas consumed by the reaction (also called the use ratio).

The conversion rate of carbon monoxide (Cv) is defined from measurements of carbon monoxide entering the hydrocarbon synthesis reaction section (CO input) and carbon monoxide leaving said reaction section (CO output ). These measurements are generally performed by gas chromatography using a katharometer detector. Similarly, hydrogen is measured with a specific column and detector in the gas streams entering and leaving the hydrocarbon synthesis reaction section to calculate the various H2 / CO ratios.

• Cv = (CO entrée - CO sortie) / CO entrée
• R1 = H2/CO charge = H2 entrée / CO entrée (mol/mol)
• Rft = H2/CO réaction = (H2 entrée - H2 sortie) / (CO entrée - CO sortie)

The conversion rate of carbon monoxide (Cv), the ratio (or ratio H2 / CO) of the charge (R1) and the ratio (or ratio H2 / CO) of use (Rft) are thus defined in the following manner :

• Cv = (CO input - CO output) / CO input
• R1 = H2 / CO load = H2 input / CO input (mol / mol)
• Rft = H2 / CO reaction = (H2 input - H2 output) / (CO input - CO output)

We can then estimate the ratio P _H2O : P _H2 theoretical in the reaction section by the following calculation: $${\mathrm{P}}_{\mathrm{H}\mathrm{2}\mathrm{O}}\mathrm{:}{\mathrm{P}}_{\mathrm{H}\mathrm{2}}\mathrm{theoretical}\mathrm{=}\mathrm{cv}\mathrm{/}\left(\mathrm{R}\mathrm{1}\mathrm{-}\left(\mathrm{Rft}\mathrm{*}\mathrm{cv}\right)\right)$$

The Rft usage ratio qualifies in a certain way the intrinsic selectivity of the Fischer-Tropsch synthesis catalyst. It is generally determined beforehand under normal Fischer-Tropsch synthesis conditions, that is to say when the Shift reaction (WGSR) is a minority and practically negligible. By default, it can be taken equal to 2.0, according to the stoichiometry of the general Fischer-Tropsch synthesis reaction [1] recalled below, knowing that then the estimation of the ratio P _H2O : P _H2 theoretical will be conservative (c that is, slightly underestimated).

Fischer-Tropsch reaction:

CO + 2 H2 → - (CH2) - + H2O [1]

1. i. Augmentation du débit de charge,
2. ii. Dans le cas où la section réactionnelle ou le réacteur est équipé d'un recyclage du gaz non converti, augmentation du taux de recyclage,
3. iii. Élimination en continu de tout ou partie de l'eau formée par la réaction,
4. iv. Modification du rapport H2/CO en entrée de la section réactionnelle de synthèse d'hydrocarbure ou d'au moins un réacteur de ladite section lorsqu'il y en a plusieurs,
5. v. Diminution de la température opératoire,
6. vi. Diminution de la pression.

Step b) of any adjustment of the ratio P _H2O : P _H2 determined in step a) to a value strictly less than 1 may be carried out using a means selected from the following means:

1. i. Increase of the charge flow,
2. ii. In the case where the reaction section or the reactor is equipped with a recycling of unconverted gas, increasing the recycling rate,
3. iii. Continuous removal of all or part of the water formed by the reaction,
4. iv. Modification of the H2 / CO ratio at the inlet of the hydrocarbon synthesis reaction section or at least one reactor of said section when there are several,
5. v. Decrease of the operating temperature,
6. vi. Decrease of the pressure.

1. i. L'augmentation du débit de charge fraîche (gaz de synthèse) est un des moyens préférés. II permet de réduire le temps de contact de la charge avec le catalyseur, donc de réduire le taux de conversion de CO par passe et par conséquent de réduire le rapport P_H2O:P_H2 . En outre, cette action présente l'avantage d'augmenter la productivité de l'unité sans dégrader la sélectivité intrinsèque de la réaction Fischer-Tropsch.
2. ii. L'augmentation du taux de recyclage du gaz non converti, dans le cas où la section réactionnelle ou au moins un réacteur de ladite section est équipée d'un recyclage interne, compte parmi les moyens d'action préférés. Il engendre la diminution du taux de conversion par passe et par conséquent la diminution du rapport P_H2O:P_H2 dans la section réactionnelle.
3. iii. Une autre méthode consiste à éliminer en continu l'eau formée par la réaction au moyen d'un dispositif de séparation implanté dans au moins un réacteur de synthèse Fischer-Tropsch ou dans une boucle de recyclage. Une telle séparation peut par exemple être réalisée au moyen d'un ballon permettant de séparer la phase aqueuse et la phase organique dans une boucle de recyclage ou au moyen d'une membrane implantée dans cette boucle ou dans au moins un réacteur de synthèse.
4. iv. Modification du rapport H2/CO en entrée de la section réactionnelle de synthèse d'hydrocarbures ou d'au moins un réacteur de synthèse d'hydrocarbures:
   1. a) Cette modification peut être obtenue en modifiant les conditions opératoire de la section de production de gaz de synthèse située en amont de la section réactionnelle Fischer-Tropsch et donc le rapport H2/CO en sortie de cette section gaz de synthèse.
   2. b) L'apport de monoxyde de CO supplémentaire à l'entrée de la section réactionnelle de synthèse ou d'au moins un réacteur conduit à diminuer le rapport H2/CO de la charge et à augmenter le débit total de la charge. Globalement les conditions cinétiques de la synthèse FT sont alors moins favorable et cela engendre une diminution du paramètre P_H2O:P_H2. Toutefois, cette option n'est généralement pas l'option très préférée car elle est difficile à mettre en oeuvre industriellement. La disponibilité de quantités de CO supplémentaires nécessite en effet une action sur l'unité de production de gaz de synthèse avec modification du rapport H2/CO en sortie de cette unité.
   3. c) L'apport d'hydrogène (H2) supplémentaire à l'entrée de la section réactionnelle de synthèse ou d'au moins un réacteur est généralement plus facile à mettre en oeuvre industriellement en utilisant un flux supplémentaire d'hydrogène disponible sur le site. Cet apport conduit à l'augmentation du rapport H2/CO dans la charge de l'étape réactionnelle Fischer-Tropsch. Cet excès supplémentaire d'hydrogène engendre une diminution du paramètre P_H2O:P_H2 . Toutefois, cette option présente l'inconvénient de modifier la sélectivité intrinsèque de la réaction FT du fait l'excès supplémentaire d'hydrogène dans la charge. Cette modification conduit à la formation plus importante de produits légers et notamment d'hydrocarbures C2-C4 et de méthane indésirable. Ce moyen n'est donc pas un moyen préféré selon l'invention.
   4. d) Cette modification peut-être également parfois être obtenue en modifiant les conditions de recyclage interne tel que détaillé précédemment en ii.
5. v. La diminution de la température conduit à un ralentissement de la cinétique de la réaction selon la loi d'Arrhenius. Par conséquent, la diminution de la température provoque une diminution du taux de conversion de CO et donc une diminution du rapport P_H2O:P_H2. Cette action présente l'inconvénient de réduire également la productivité du procédé.
6. vi. La diminution de pression aura également un impact sur la cinétique de la réaction et conduit à une diminution du rapport P_H2O:P_H2 par diminution du niveau de conversion. Toutefois, ce moyen présente un impact négatif sur la production du procédé.

In more detail, this adjustment can be made using one of the following means:

1. i. Increasing the fresh charge rate (synthesis gas) is one of the preferred means. It makes it possible to reduce the contact time of the feedstock with the catalyst, thus to reduce the CO conversion rate per pass and consequently to reduce the P _{H 2 O} : P _{H 2} ratio. In addition, this action has the advantage of increasing the productivity of the unit without degrading the intrinsic selectivity of the Fischer-Tropsch reaction.
2. ii. Increasing the unconverted gas recycling rate, in the case where the reaction section or at least one reactor of said section is equipped with internal recycling, is one of the preferred means of action. It generates the decrease of the conversion rate per pass and consequently the decrease of the ratio P _H2O : P _H2 in the reaction section.
3. iii. Another method consists in continuously removing the water formed by the reaction by means of a separation device implanted in at least one Fischer-Tropsch synthesis reactor or in a recycling loop. Such a separation may for example be carried out by means of a balloon for separating the aqueous phase and the organic phase in a recycling loop or by means of a membrane implanted in this loop or in at least one synthesis reactor.
4. iv. Modification of the H2 / CO ratio at the inlet of the hydrocarbon synthesis reaction section or of at least one hydrocarbon synthesis reactor:
   1. a) This modification can be obtained by modifying the operating conditions of the synthesis gas production section located upstream of the Fischer-Tropsch reaction section and thus the H2 / CO ratio at the outlet of this synthesis gas section.
   2. b) The addition of additional CO monoxide to the inlet of the synthesis reaction section or at least one reactor leads to decrease the H2 / CO ratio of the feedstock and to increase the total flow rate of the feedstock. Overall, the kinetic conditions of the FT synthesis are then less favorable and this leads to a decrease in the parameter P _H2O : P _H2 . However, this option is generally not the very preferred option because it is difficult to implement industrially. The availability of additional amounts of CO 2 indeed requires action on the synthesis gas production unit with modification of the H2 / CO ratio at the outlet of this unit.
   3. c) The addition of hydrogen (H2) to the inlet of the synthesis reaction section or at least one reactor is generally easier to implement industrially using an additional flow of hydrogen available on the site . This addition leads to the increase of the H2 / CO ratio in the feed of the Fischer-Tropsch reaction step. This additional excess of hydrogen causes a decrease in the parameter P _H2O : P _H2 . However, this option has the drawback of modifying the intrinsic selectivity of the FT reaction because of the additional excess of hydrogen in the feedstock. This modification leads to the larger formation of light products including C2-C4 hydrocarbons and undesirable methane. This means is not a preferred means according to the invention.
   4. d) This modification may also sometimes be obtained by modifying the internal recycling conditions as detailed previously in ii.
5. v. The decrease in temperature leads to a slowing down of the kinetics of the reaction according to Arrhenius's law. Therefore, the decrease in temperature causes a decrease in the CO conversion rate and therefore a decrease in the ratio P _H2O : P _H2 . This action has the disadvantage of also reducing the productivity of the process.
6. vi. The decrease in pressure will also have an impact on the kinetics of the reaction and leads to a decrease in the ratio P _H2O : P _H2 by reducing the level of conversion. However, this means has a negative impact on the production of the process.

The choice of one of these means depends essentially on the means available on the industrial unit and the operating conditions at the time of choice.

• I. Augmentation du débit de charge,
• II. Dans le cas où la section réactionnelle ou au moins un réacteur de ladite section est équipé d'un recyclage du gaz non converti, augmentation du taux de recyclage,
• III. Élimination en continu de tout ou partie de l'eau formée par la réaction.

The most preferred means used in step b) of possible adjustment of the ratio P _H2O : P _H2 are in general the following:

• I. Increase of the charge flow,
• II. In the case where the reaction section or at least one reactor of said section is equipped with a recycling of the unconverted gas, increase of the recycling rate,
• III. Continuous removal of all or part of the water formed by the reaction.

• Diminution de la température opératoire (cas v)
• Modification du rapport H2/CO en entrée de la section réactionnelle de synthèse Fischer-Tropsch (cas iv)

In some cases, especially after an incident on a unit such as for example an unexpected drop in operating temperature, other means are preferably used in step b) of possible adjustment of the ratio P _H2O : P _H2 , these are then the following means:

• Decrease of the operating temperature (case v)
• Modification of the H2 / CO ratio at the input of the Fischer-Tropsch synthesis reaction section (case iv)

In such cases, these means are indeed generally easier to implement

When the ratio P _H2O : P _H2 has been adjusted in step b), its new theoretical value is again determined (step c) in order to check that it is strictly less than 1.1, preferably strictly less than 1 , 0 and very preferably strictly less than 0.9, even more preferably strictly less than 0.8, or even strictly less than 0.65.

If this is not the case, steps a to c are repeated (step d) until the criterion P _H2O is met: P _H2 theoretical strictly less than 1.1, preferably strictly less than 1.0 and very preferably strictly less than 0.9, even more preferably strictly less than 0.8, or even strictly less than 0.65.

1. a) Détermination du rapport molaire P_H2O:P_H2 théorique dans la section réactionnelle comme dans la revendication 1.
2. b) Ajustement éventuel du rapport P_H2O:P_H2 déterminé à l'étape a) à une valeur strictement inférieure à 1,1 à l'aide d'un moyen sélectionné parmi les moyens suivants:
   1. i. Augmentation du débit de charge,
   2. ii. Dans le cas où la section réactionnelle ou au moins un réacteur de ladite section est équipé d'un recyclage du gaz non converti, augmentation du taux de recyclage,
   3. iii. Élimination en continu de tout ou partie de l'eau formée par la réaction,
   4. iv. Modification du rapport H2/CO en entrée de la section réactionnelle de synthèse d'hydrocarbures ou d'au moins un réacteur de synthèse d'hydrocarbures,
   5. v. Diminution de la température opératoire,
   6. vi. Diminution de la pression,
3. c) Détermination de la nouvelle valeur du rapport P_H2O:P_H2 théorique dans la section
   réactionnelle,
   puis éventuellement, lorsque cela est nécessaire, l'étape d) suivante, après l'étape c):
4. d) Répétition des étapes a) à c) jusqu'à ce que le rapport des pressions partielles d'eau et d'hydrogène P_H2O:P_H2 présente une valeur strictement inférieure à 1,1.

In summary, the invention relates to a method for optimizing the operation of a hydrocarbon synthesis reaction section from a feedstock comprising synthesis gas, in which one operates in the presence of a catalyst comprising cobalt, said method comprising the following steps:

1. a) Determination of the theoretical P _H2O : P _H2 molar ratio in the reaction section as in claim 1.
2. b) Possible adjustment of the P _H2O : P _H2 ratio determined in step a) to a value strictly lower than 1.1 using a means selected from the following means:
   1. i. Increase of the charge flow,
   2. ii. In the case where the reaction section or at least one reactor of said section is equipped with a recycling of the unconverted gas, increase of the recycling rate,
   3. iii. Continuous removal of all or part of the water formed by the reaction,
   4. iv. Modification of the H2 / CO ratio at the inlet of the hydrocarbon synthesis reaction section or of at least one hydrocarbon synthesis reactor,
   5. v. Decrease of the operating temperature,
   6. vi. Decrease of the pressure,
3. c) Determination of the new value of the ratio P _H2O : theoretical P _H2 in the section
   reaction,
   then optionally, when necessary, the following step d), after step c):
4. d) repeating steps a) to c) until the ratio of partial pressures of water and hydrogen P _H2O : P _H2 has a value strictly less than 1.1.

Said reaction section may comprise one or more hydrocarbon synthesis reactors.

The following examples illustrate the invention.

Example 1

The Fischer-Tropsch synthesis reaction is carried out in a device comprising a perfectly stirred three-phase reactor of the autoclave type (CSTR according to the English abbreviations). This reactor can be maintained under pressure and temperature and operated continuously. The reactor is fed with a synthesis gas having an H2 / CO ratio which can be adjusted between 1.5 and 2.5.

The charge rate (synthesis gas) is controlled and can also be adjusted to increase or decrease the reaction time. The Fischer-Tropsch synthesis is carried out at 230 ° C., 2 MPa, in the presence of 35 g of a catalyst containing 13% by weight of cobalt deposited on an alumina support having a specific surface area of approximately 150 m 2 / g and having a gamma structure. cubic. The catalytic performances are evaluated by material balance from the analysis and the measurement of the various outgoing flows of the reactor. The compositions of the various outgoing streams (gas effluents, hydrocarbon liquid product and aqueous product) are determined by gas chromatography.

Several experiments are carried out under different conditions of supply of variable synthesis gas: case 1: 80 NI / h of synthesis gas with H2 / CO ratio equal to 2.0 case 2: 70 NI / h of synthesis gas with H2 / CO ratio equal to 2.0 case 3: 60 NI / h of synthesis gas with H2 / CO ratio equal to 2.0 case 4: 40 NI / h of synthesis gas with H2 / CO ratio equal to 2.0 case 5: 100 NI / h of syngas with H2 / CO ratio of 2.5 case 6: 88 N / h of syngas with H2 / CO ratio of 2.5 case 7: 75 NI / h of syngas with H2 / CO ratio of 2.5 case 8: 70 NI / h of syngas with H2 / CO ratio of 2.5 case 9: 64 NI / h of syngas with H2 / CO ratio of 2.5 Case 10: 78 NI / h of synthesis gas with H2 / CO ratio equal to 1.5 Case 11: 66 NI / h of synthesis gas with H2 / CO ratio equal to 1.5 Case 12 :: 56 NI / h of synthesis gas with H2 / CO ratio equal to 1.5

The results obtained after 50 hours of test are presented in the following Table 1: <b> Table 1: </ b> Case R1 H2 / CO charge Cv Conv. CO(%) Rft H2 / CO reaction P _H2O : Theoretical P _H2 Select. CO2 (% C) Select. CH4 (% C) Select. C5 + (% C) Prod. in C5 + (kg / kg.h) 1 2.0 57.3 2.10 0.72 0.8 7.0 83.8 0,220 2 2.0 62.7 2.10 0.92 0.9 7.1 82.6 0.211 3 2.0 68.7 2.10 1.23 1.8 8.5 78.2 0.198 4 2.0 80.9 2.10 2.69 6.2 17.9 59.8 0.155 5 2.5 62.2 2.15 0.53 0.4 9.1 78.4 0.256 6 2.5 69.1 2.15 0.68 0.7 10.0 78.0 0,250 7 2.5 78.0 2.15 0.95 0.9 12.1 74.1 0.241 8 2.5 81.5 2.15 1.09 1.6 13.5 69.5 0,235 9 2.5 86.0 2.15 1.32 4.1 19.5 55.3 0.226 10 1.5 42.2 2.06 0.67 0.5 4.5 90.2 0.226 11 1.5 47.1 2.06 0.89 0.8 5.0 88.7 0,190 12 1.5 51.6 2.06 1.18 1.6 7.3 83.1 0.179

Selectivities in% carbon (CO2, CH4, C5 +):

$$100\times \left(\mathrm{number\; of\; moles\; of\; carbon\; in\; the\; form\; of\; CO}2\mathrm{or\; CH}4\mathrm{or\; C}5+\right)/\mathrm{total\; number\; of\; moles\; of\; carbon\; transformed\; into\; products}$$

Productivity in C5 + (kq / kq.h):

kilograms of C5 + hydrocarbons formed per hour per kilogram of catalyst used.

The results of Table 1 show that for ratios H2: CO ranging from 1.5 to 2.5 there is a significant increase in the selectivity of CO2 and methane when the ratio P _H2O : P _H2 theoretical has a value greater than 1 , which has a very detrimental global impact on the selectivity of C5 + hydrocarbons, the products sought in this synthesis. Below P _H2O : P _H2 = 1, the influence of an increase in this ratio remains much lower.

Example 2 : Example of readjustment of the report after a setpoint change.

Case No. 2 of Example 1 is considered as the starting point (charge flow of 70 N / h) The performances obtained after 50 hours of test are those indicated in Table 1.

The temperature of the Fischer-Tropsch reaction section is increased by 5 ° C. (T = 235 ° C. and 2 MPa) without changing the feed rate (synthesis gas at a flow rate of 70 ° C.). NI / h). This causes a modification of the P _H2O : P _H2 ratio which becomes greater than 1 and an increase in the selectivities of methane and carbon dioxide (CO2). These conditions are summarized in case 13 of Table 2.

The operating conditions are kept constant (T = 235 ° C. and 2 MPa), but the ratio P _H2O : P _H2 is adjusted by means of an increase in the charge flow which goes to 100 N / h (case 14). This makes it possible to recover a theoretical P _H2O : P _H2 ratio <1 with lower CO2 and methane selectivities, and a selectivity for C5 + hydrocarbons (hydrocarbons having a carbon number greater than or equal to 5) higher. <b> Table 2: </ b> Case R1 H2: CO load Cv Conv. CO (%) P _H2O : Theoretical P _H2 Select. CO2 (% C) Select. CH4 (% C) Select. C5 + (% C) Prod. in C5 + (kg / kg.h) 2 2.0 62.7 0.92 0.9 7.1 82.6 0.211 13 2.0 73.2 1.58 2.1 10.2 76.1 0.246 14 2.0 59.6 0.80 0.9 7.5 80.0 0.286

In case 13, even if the productivity of C5 + hydrocarbons increases slightly, a loss of carbon is recorded as the increase of the conversion is carried out with an increase of the selectivities in methane and in CO2. A much larger fraction of the carbon in the feed is converted into methane and carbon dioxide, undesirable products. On the other hand, the return to a theoretical P _H2O : P _H2 ratio of less than 1 makes it possible once again to obtain high productivity with a low selectivity for CH4 and CO2, thus minimizing carbon losses.

Claims (10)

1. A method for optimizing the operation of a reaction section for hydrocarbon synthesis starting from a feed comprising synthesis gas, operated in the presence of a catalyst comprising cobalt, said method comprising the following steps:

   a) determining the theoretical molar ratio, P_H2O:P_H2, in the reaction section using the following calculation: $$\mathrm{theoretical}{\mathrm{P}}_{\mathrm{H}\mathrm{2}\mathrm{O}}\mathrm{:}{\mathrm{P}}_{\mathrm{H}\mathrm{2}}\mathrm{=}\mathrm{Cv}\mathrm{/}\left(\mathrm{R}\mathrm{1}\mathrm{-}\left(\mathrm{Rft\; x\; Cv}\right)\right);$$

   

   
   in which:

   Cv = (CO inlet - CO outlet)/CO inlet;

   R1 = H₂/CO feed = H₂ inlet/CO inlet (mol/mol);

   Rft = H₂/CO reaction = (H₂ inlet - H₂ outlet)/(CO inlet - CO outlet).

   b) optionally, adjusting the ratio P_H2O:P_H2 determined in step a) to a value strictly below 1.1 using means selected from the following means:

   i. increasing the feed flow rate;

   ii. in the case in which the reaction section or at least one reactor of said section is equipped with a recycler for unconverted gas, increasing the recycle ratio;

   iii. continuously eliminating all or part of the water formed by the reaction;

   iv. modifying the ratio H₂/CO at the inlet to the reaction section for hydrocarbon synthesis or to at least one hydrocarbon synthesis reactor;

   v. reducing the operating temperature;

   vi. reducing the pressure;

   c) determining the new value for the theoretical ratio P_H2O:P_H2 in the reaction section.
2. A method according to claim 1, in which the optional adjustment of the ratio P_H2O:P_H2 (step b)) is carried out using means selected from the following means:

   i. increasing the feed flow rate;

   ii. in the case in which the reactor is equipped with a recyclcr for unconverted gas, increasing the recycle ratio;

   iii. continuously eliminating all or part of the water formed by the reaction.
3. A method according to one of the preceding claims, in which the molar ratio P_H2O:P_H2 (steps a) and c)) is determined using means selected from the following means:

   i. analyzing the gas stream at the outlet from said reaction section;

   ii. measuring the quantity of carbon monoxide in the gaseous effluent and evaluating the ratio from the degree of conversion of carbon monoxide and the H₂:CO ratio in the feed.
4. A method according to one of the preceding claims, in which the molar ratio P_H2O:P_H2 (steps a) and c)) is determined by measuring the quantity of carbon monoxide in the gaseous effluent and evaluating the ratio from the degree of conversion of carbon monoxide and the H₂:CO ratio in the feed, and optionally adjusting the ratio P_H2O:P_H2 (step b)) using means selected from the following means:

   i. increasing the feed flow rate;

   ii. in the case in which the reactor is equipped with a recycler for unconverted gas, increasing the recycle ratio.
5. A method according to one of the preceding claims, in which the hydrocarbon synthesis is carried out in at least one reactor with a fixed bed catalyst.
6. A method according to one of the preceding claims, in which the hydrocarbon synthesis is carried out in at least one three-phase reactor comprising the catalyst in suspension in an essentially inert liquid phase and the reactive gas phase.
7. A method according to one of the preceding claims, in which the synthesis gas used in the Fischer-Tropsch synthesis has a H₂:CO molar ratio in the range 1:2 to 5:1. and the Fischer-Tropsch synthesis is carried out at a pressure in the range 0.1 MPa to 15 MPa, with an hourly space velocity of synthesis gas in the range 100 to 20000 h^-1.
8. A method according to one of the preceding claims, in which the synthesis gas used in the Fischer-Tropsch synthesis has a H₂:CO molar ratio in the range 1.5:1 to 2.6:1 and the Fischer-Tropsch synthesis is carried out at a pressure in the range 1.5 MPa to 5 MPa, with an hourly space velocity of synthesis gas in the range 400 to 10000 h^-1 .
9. A method according to one of the preceding claims, in which at the end of step c) the ratio of the partial pressures of water and hydrogen, P_H2O:P_H2, has a value strictly less than 1.
10. A method according to one of the preceding claims, in which at the end of step c) the ratio of the partial pressures of water and hydrogen, P_H2O:P_H2, has a value strictly less than 0.65.