WO2017037169A1

WO2017037169A1 - Fischer-tropsch process

Info

Publication number: WO2017037169A1
Application number: PCT/EP2016/070608
Authority: WO
Inventors: Gerrit Leendert BEZEMER; Harold Boerrigter; Hai Ming TAN
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2015-09-04
Filing date: 2016-09-01
Publication date: 2017-03-09
Also published as: ZA201801207B; AU2016315399A1; EP3344726A1; AU2016315399B2; US20180258351A1

Abstract

The present invention relates to a method for producing hydrocarbons from a gas mixture comprising hydrogen and carbon monoxide, in at least two conversion reactors, being the first and second reactor, said reactors comprising catalysts. The reactors can be placed in series or parallel.

Description

FISCHER-TROPSCH PROCESS

Field of the invention

The present invention relates to a method for producing hydrocarbons from a gas mixture comprising hydrogen and carbon monoxide, in at least two conversion reactors, being the first and second reactor, said reactors comprising catalysts.

Background to the invention

The Fischer-Tropsch process can be used for the conversion of synthesis gas into liquid and/or solid hydrocarbons. The synthesis gas may be obtained from hydrocarbonaceous feedstock in a process wherein the feedstock, e.g. natural gas, associated gas and/or coal- bed methane, heavy and/or residual oil fractions, coal, biomass, is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds and water in the actual Fischer-Tropsch process. The obtained paraffinic compounds range from methane to high molecular weight modules. The obtained high molecular weight modules can comprise up to 200 carbon atoms, or, under particular circumstances, even more carbon atoms.

Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors .

Catalysts used in the Fischer-Tropsch synthesis often comprise a carrier-based support material and one or more metals from Group 8-10 of the Periodic Table of

Elements, especially from the cobalt or iron groups, optionally in combination with one or more metal oxides and/or metals as promoters selected from zirconium, titanium, chromium, vanadium and manganese, especially manganese. Such catalysts are known in the art and have been described for example, in the specifications of WO9700231A and US4595703.

One of the limitations of a Fischer-Tropsch process is that the activity of the catalyst will, due to a number of factors, decrease over time. The activity of the catalyst is decreased as compared to its initial catalytic activity. The initial activity of the catalyst can be its activity when fresh prepared. A catalyst that shows a decreased activity after use in a Fischer-Tropsch process is sometimes referred to as deactivated catalyst, even though it usually still shows activity. Sometimes such a catalyst is referred to as a deteriorated

catalyst. Sometimes it is possible to regenerate the catalyst. This may be performed, for example, with one or more oxidation and/or reduction steps.

After regeneration, catalysts often show an activity that is lower than the activity of fresh prepared catalysts. Especially after multiple regenerations, it often proofs hard to regain an activity level comparable to the activity of fresh prepared catalysts. In order to be able to use a catalyst for a long time, it thus may be desirable to start a Fischer-Tropsch process with a fresh catalyst that has a relatively high activity. The use of fresh or rejuvenated catalysts with a relatively high initial activity may have disadvantages. This may especially be the case when the amount of catalyst used in a reactor tube is fixed after loading of the catalyst in the reactor tube. One example of a reactor tube filled with a fixed amount of catalyst is a reactor tube filled with a packed bed of catalyst particles .

In a Fischer-Tropsch process with a catalyst with a relatively high initial activity, the activity of the catalyst is especially high at the start of the process. And, due to the high activity of the catalyst, a lot of water is produced in the Fischer-Tropsch hydrocarbon synthesis, resulting in a high relative humidity at the start of the Fischer-Tropsch process. During start-up of a Fischer-Tropsch reactor with a very active catalyst, the reaction temperature is typically kept at a

relatively low value, e.g. below 200 °C, in order to avoid a too high product yield and accompanying high temperature rise due to the exothermic reaction.

Due to the deactivation over time of a catalyst the temperature of the reactor has to be increased. The increase in temperature in the reactor results in an increase of the activity of the catalyst. By increasing the temperature the activity of an aged catalyst can be partially compensated. However, allowing the catalyst to operate at an elevated temperature has an adverse effect on the C5+ selectivity. By increasing the temperature C5+ selectivity decreases, meaning that more (undesired) Cl- C4 products are formed during synthesis and less C5+ products . Summary of the invention

It is an object of the present invention to provide for an improved method of producing hydrocarbons .

It has now been found that in case hydrocarbon synthesis is performed in two or more reactors the deactivation over time of the Fischer-Tropsch catalyst can be managed by varying the reaction conditions of the at least two reactors .

The inventors have found that this results in a more optimum production of hydrocarbons. The method according to the present invention allows for a more flexible way of producing hydrocarbons. This means that the method allows for tuning the process conditions in the different reactor such that the product stream obtained from a system comprising at least two reactors can be optimized towards a desired product.

Accordingly, the present invention provides for a method for producing hydrocarbons from a gas mixture comprising hydrogen and carbon monoxide, in at least two conversion reactors, being the first and second reactor, said reactors comprising catalysts, which method

comprises the steps of:

(i) (i) providing a gas mixture comprising hydrogen and carbon monoxide to two or more of the conversion

reactors ;

(ii) catalytically converting the gas mixture of step (i) at an initial temperature and an initial pressure to obtain an initial hydrocarbon product stream comprising the normally gaseous, normally liquid and optionally normally solid hydrocarbons from each conversion reactor;

(iii) obtaining from each of the at least two reactors, an initial hydrocarbon stream and an off-gas; (iv) determining the concentration of hydrocarbons in each of the initial hydrocarbon streams obtained in step (iii) of at least one of the following hydrocarbon groups :

- hydrocarbons having a chain length of 1 to 4 carbon atoms (Cl-4 fraction) ;

- hydrocarbons having a chain length of 5 to 30 carbon atoms (C5-C30 fraction) ; and

- hydrocarbons having a chain length of at least 31 carbon atoms (C31+ fraction) ;

(v) based on the concentration of one or more of said groups, raising or lowering the reaction temperature of at least one of the reactors;

(vi) obtaining a second hydrocarbon stream from each of the reactors;

wherein the concentration of the C5-C30 fraction of the combined second hydrocarbon streams is at least 3%, and preferably at least 5% higher than the concentration of C5-C30 fraction of the combined initial hydrocarbon stream; or

wherein the concentration of the C31+ fraction of the combined second hydrocarbon streams is at least 3% and preferably at least 5% higher than the concentration of the C31+ fraction of the combined initial hydrocarbon streams .

Detailed description of the invention

The present invention provides for a method for producing hydrocarbons from a gas mixture comprising hydrogen and carbon monoxide. Said gas mixture is commonly referred to as synthesis gas or syngas. At least two conversion reactors are operated, being the first and second reactor, said reactors comprising catalysts. Said catalysts convert hydrogen and carbon monoxide in to hydrocarbons. The method according to the present invention comprises the steps of:

(i) providing a gas mixture comprising hydrogen and carbon monoxide to two or more of the conversion

reactors ;

(ii) catalytically converting the gas mixture of step (i) at an initial temperature and an initial pressure to obtain an initial hydrocarbon product;

(iii) obtaining from each of the at least two reactors, an initial hydrocarbon stream and an off-gas;

(iv) determining the concentration of hydrocarbons in each of the initial hydrocarbon streams obtained in step (iii) of at least one of the following hydrocarbon groups :

- hydrocarbons having a chain length of 1 to 4 carbon atoms (Cl-4 fraction) ;

The concentration as determined for at least one of the three groups in each stream serves as an indication of the selectivity of the catalyst in each of the reactors under certain operational circumstances (such as temperature and pressure) .

With the concentration expressed in percentage is meant weight percentage with respect to the total weight of the stream to which the fraction belongs.

Preferably, the conversion reactors are operated at an initial temperature in the range of 200 to 230 °C and preferably from 205 to 220 °C.

Based on the concentration of one or more of said groups, raising or lowering the reaction temperature (step v) of at least one of the reactors. Based on the concentration it can be determined whether or not a catalyst in a reactor still has a desired selectivity towards the desired products. In case the selectivity is lower (than the other reactor) or too low towards the preferred products, the temperature can be lowered resulting in a decrease in activity but an increase of selectivity towards the desired products. In this case the temperature in the other reactor may be raised in order to raise the activity of the catalyst (increasing production) , while maintaining an acceptable selectivity towards the preferred products.

After adjusting the reactor conditions second hydrocarbon streams are obtained from each of the reactors. The concentration of the C5-C30 fraction of the combined second hydrocarbon streams is at least 3%, and preferably at least 5% higher than the concentration of C5-C30 fraction of the combined initial hydrocarbon stream; and / or

The inventors have surprisingly found that the adjustment of the reactor conditions based on the activity of the catalyst present in the reactor allows for an optimal production of hydrocarbons for a system comprising at least two reactors. The invention results in a more optimum production of hydrocarbons. The method according to the present invention allows for a more flexible way of producing hydrocarbons. This means that the method allows for tuning the process conditions in the different reactor such that the product stream obtained from a system comprising at least two reactors can be optimized towards a desired product. Since the invention allows for optimizing a collection of reactors the hydrocarbon distribution of a product stream of a commercial GTL plant can be influenced allowing for adjustment of the distribution in the overall product stream based on market demand. This is economically very favourable .

A decrease in activity of a catalyst can be caused by the catalyst age, meaning how long said catalyst has been in use or how many times it has suffered a reactor run away. In order to prevent an early replacement of the lesser performing catalyst the conditions in each reactor can be amended in accordance with the invention.

Hence, in an embodiment of the invention the catalyst of one of the reactors has a higher activity than the catalyst in the other reactor.

In an embodiment of the invention one of the reactors may contain a catalyst having a high activity. This may be the case if a fresh catalyst is present in said reactor. The initial activity of the catalyst can be its activity when fresh prepared. In such a case the

temperature may be raised in this reactor in order to increase the activity of the catalyst. This will result in an increased production of hydrocarbons. In an embodiment a cobalt catalyst may be used having a relatively high initial activity, the activity of the catalyst is especially high at the start of the process and decreases over time. A catalyst that shows a

decreased activity after use in a Fischer-Tropsch process is sometimes referred to as deactivated catalyst, even though it usually still shows activity. Sometimes such a catalyst is referred to as a deteriorated catalyst.

Sometimes it is possible to regenerate the catalyst. This may be performed, for example, with one or more oxidation and/or reduction steps.

In case a system for manufacturing hydrocarbons contains two or more reactors it may occur that at least one reactor contains a catalyst which is a deteriorated catalyst (i.e. this catalyst has an activity which is less than the activity of a catalyst in another reactor) . In such a case the reaction temperature in the reactor containing the deteriorated catalyst may be lowered and the reaction temperature in another reactor may be raised, preferably the temperature of the reactor containing the catalyst having the most active catalyst is raised.

Each of the at least two reactors has a reactor operating point. By reactor operating point is meant the operation temperature at which the target conversion of CO and H2 is achieved.

Suitably, the reactor operating point is raised by:

- increasing the amount of synthesis gas provided to the reactor;

- raising the temperature of the cooling water provided to the reactor; and/or

- providing a nitrogen containing compound, to the reactor, preferably by adding the nitrogen containing compound to the gas mixture prior to provision to the Fischer-Tropsch catalyst. Preferably the nitrogen containing compound is selected from the group of ammonia, HCN, NO, an amine and combinations or two or more thereof.

In an embodiment of the invention the reaction temperature and/or the reactor operating point is raised by increasing the amount of synthesis gas provided to the reactor. Since the Fischer-Tropsch reaction is an exothermic one providing more hydrogen and carbon monoxide will result in more heat being generated. The increase in heat will result in a decrease of the selectivity towards the heavier hydrocarbon products.

In an embodiment of the invention the reaction temperature and/or the reactor operating point is raised by raising the temperature of the cooling water provided to said reactor. The reaction temperature and/or the reactor operating point may be raised by providing a nitrogen containing compound to the reactor. By supplying a nitrogen-containing compound to the freshly prepared or rejuvenated reduced catalyst, the catalyst activity is decreased and the temperature can be increased. Such conditions of higher temperature and decreased activity result in . Moreover, since the effect of such nitrogen- containing compound on catalyst activity seems to be reversible, the catalyst activity can be tuned by adjusting the concentration of the nitrogen-containing compound. In particular, the gradual decrease in catalyst activity can be compensated by gradually decreasing the concentration of the nitrogen-containing compound in the feed gas stream supplied to the catalyst. Thus, reaction temperature and reactor productivity (yield) can be controlled and kept constant during a relatively long period after start-up of the reactor, resulting in improved catalyst stability.

In an embodiment a nitrogen containing compound is provided to one or more of the reactors while the reaction temperature and/or the reactor operating point is raised.

Also, the reactor operating point is lowered by: - decreasing the amount of synthesis gas provided to the reactor;

- lowering the temperature of the cooling water provided to the reactor; and/or

- providing a nitrogen containing compound, to the reactor, preferably by adding the nitrogen containing compound to the gas mixture prior to step a) and b) , preferably the nitrogen containing compound is selected from the group of ammonia, HCN, NO, an amine and combinations or two or more thereof.

In an embodiment the reaction temperature and/or the reactor operating point in one or more reactors is lowered by decreasing the amount of synthesis gas provided to the reactor. By decreasing the amount of syngas provided to the reactor fewer hydrocarbons are synthesized. Since the FT reaction is exothermic less energy will be released if fewer hydrocarbons are synthesized .

In an embodiment the reaction temperature and/or the reactor operating point in one or more reactors is lowered by lowering the temperature of the cooling water provided to the reactor. Also, lowering the temperature by decreasing the temperature of the cooling medium results in an increase in selectivity towards the heavy fractions .

In an embodiment the reaction temperature and/or the reactor operating point in one or more reactors is lowered by providing a nitrogen containing compound, to the reactor.

In an embodiment of the invention, the method comprises one of the following steps: - providing a nitrogen containing compound, to the first reactor in case the first reactor comprises the least active catalyst;

- providing a nitrogen containing compound, to the second reactor in case the second reactor comprises the least active catalyst. This may be done in case the temperature in the reactor is raised, resulting in an increase in activity of the catalyst but a decrease in selectivity towards the heavier hydrocarbons.

Preferably, the nitrogen containing compound is added to the gas mixture prior to step i) , preferably the nitrogen containing compound is selected from the group of nitrogen, ammonia, HCN, NO, an amine and combinations or two or more thereof.

In an embodiment of the invention, the method comprises one of the following steps:

- providing a nitrogen containing compound, to the first reactor in case the first reactor comprises the most active catalyst;

- providing a nitrogen containing compound, to the second reactor in case the second reactor comprises the most active catalyst.

- providing a nitrogen containing compound, to the second reactor in case the second reactor comprises the least active catalyst. This may be done in case the temperature in the reactor is raised, resulting in an increase in activity of the catalyst but a decrease in selectivity towards the heavier hydrocarbons. The nitrogen containing compound added to increase or to lower the reaction temperature and/or reaction operating point in one or more reactors is similar to the nitrogen containing compound as described above.

In an embodiment of the invention the reaction temperature is raised in order to reduce the

concentration of the C31+ fraction. By raising the temperature the activity of the catalyst is increased resulting in higher hydrocarbon production. However, the selectivity towards the heavy hydrocarbons decreases.

In an embodiment of the invention the method further comprises the step of determining the concentration of hydrocarbons in one or more of the second hydrocarbon streams obtained in step (vi) of at least one of the hydrocarbon groups. By determining the content of the second hydrocarbon stream it can be determined whether or not the selectivity of the reaction in the reactor is towards the desired synthesis products or not. In case the selectivity is not towards or sufficiently towards the desired product, further measures as described in this description can be taken or repeated. For example, further increasing or decreasing the supply of syngas to the reactor.

Reference herein to a rejuvenated catalyst is to a regenerated catalyst of which the initial activity has been at least partially restored, typically by means of several reduction and/or oxidation steps. Rejuvenation may be effected in the reactor in which the catalyst has been used or may be effected outside of the reactor by first removing the used catalyst from the reactor and having the catalyst subjected to a rejuvenation process.

A Fischer-Tropsch catalyst or catalyst precursor comprises a catalytically active metal or precursor therefor, and optionally promoters, supported on a catalyst carrier .

Preferably, the catalyst comprises cobalt as a catalytically active ingredient. Fischer-Tropsch

catalysts comprising cobalt as catalytically active metal are known in the art. Any suitable cobalt-comprising Fischer-Tropsch catalysts known in the art may be used. Typically such catalyst comprises cobalt on a carrier- based support material, optionally in combination with one or more metal oxides and/or metals as promoters selected from zirconium, titanium, chromium, vanadium and manganese, especially manganese. A most suitable catalyst comprises cobalt as the catalytically active metal and titania as carrier material.

The catalyst may further comprise one or more promoters. One or more metals or metal oxides may be present as promoters, more particularly one or more d- metals or d-metal oxides. Suitable metal oxide promoters may be selected from Groups 2-7 of the Periodic Table of Elements, or the actinides and lanthanides. In

particular, oxides of magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, cerium, titanium, zirconium, hafnium, thorium, uranium, vanadium, chromium and manganese are suitable promoters. Suitable metal promoters may be selected from Groups 7-10 of the

Periodic Table of Elements. Manganese, iron, rhenium and Group 8-10 noble metals are particularly suitable as promoters, and are preferably provided in the form of a salt or hydroxide.

The promoter, if present in the catalyst, is

typically present in an amount of from 0.001 to 100 parts by weight per 100 parts by weight of carrier material, preferably 0.05 to 20, more preferably 0.1 to 15. It will however be appreciated that the optimum amount of promoter may vary for the respective elements which act as promoter .

If the catalyst comprises cobalt as the catalytically active metal and manganese and/or vanadium as promoter, the cobalt: (manganese + vanadium) atomic ratio is advantageously at least 12:1.

The catalyst carrier preferably comprises titania, preferably porous titania. Preferably more than 70 weight percent of the carrier material consists of titania, more preferably more than 80 weight percent, most preferably more than 90 weight percent, calculated on the total weight of the carrier material. As an example of a suitable carrier material can be mentioned the

commercially available Titanium Dioxide P25 ex Evonik Industries. The carrier preferably comprises less than 40 wt% rutile, more preferably less than 30 wt%, even more preferably less than 20 wt%.

In an embodiment the gas mixture provided to one or more of the conversion reactors comprising hydrogen and carbon monoxide is synthesis gas, off gas from a

conversion reactor or a combination thereof. The

synthesis gas can be provided by any suitable means, process or arrangement. This includes partial oxidation and/or reforming of a hydrocarbonaceous feedstock as is known in the art. To adjust the H2/CO ratio in the syngas, carbon dioxide and/or steam may be introduced into the partial oxidation process. The H2/CO ratio of the syngas is suitably between 1.5 and 2.3, preferably between 1.6 and 2.0.

The syngas comprising predominantly hydrogen, carbon monoxide and optionally nitrogen, carbon dioxide and/or steam is contacted with a suitable catalyst in the catalytic conversion stage, in which the hydrocarbons are formed. Suitably at least 70 v/v% of the syngas is contacted with the catalyst, preferably at least 80%, more preferably at least 90%, still more preferably all the syngas .

In an embodiment of the invention a first reactor is provided with a synthesis gas as the gas mixture

comprising hydrogen and carbon monoxide and a second reactor is provided with the off gas obtained in step (iii) from the first reactor as the gas mixture

comprising hydrogen and carbon monoxide. Hence in this embodiment of the invention two conversion reactors are connected in series. This allows for the hydrogen and carbon monoxide not converted in the first reactor to be converted in the second reactor.

In an embodiment of the invention a first and second reactor are each provided with synthesis gas as the gas mixture comprising hydrogen and carbon monoxide. In case two or more conversion reactors are provided with the gas mixture in parallel a higher conversion rate can be obtained .

In an embodiment the hydrocarbon synthesis is at least initially, a steady state catalytic hydrocarbon synthesis process. A steady state catalytic hydrocarbon synthesis process may be performed under conventional synthesis conditions known in the art. Typically, the catalytic conversion may be effected at a temperature in the range of from 100 to 600 °C, preferably from 150 to 350 °C, more preferably from 175 to 275 °C, most preferably 200 to 260 °C.

In an embodiment the reactors are operated at a pressure ranging from 10 to 100 bar absolute. Preferably the total pressures for the catalytic conversion process are in the range of from 5 to 80 bar absolute.

A suitable regime for carrying out the Fischer- Tropsch process with a catalyst comprising particles with a size of least 1 mm is a fixed bed regime, especially a trickle flow regime. A very suitable reactor is a multitubular fixed bed reactor. In an embodiment of the invention the catalyst is present in one or more of the reactors as a fixed bed catalyst.

The invention is illustrated by the following non- limiting examples.

EXAMPLES

In the present examples two Fischer-Tropsch reactors are connected in series. The upstream reactor (named Rl) is fed syngas and the downstream reactor (named R2) receives the off gas of the upstream reactor.

The off gas comprises the unreacted hydrogen and carbon monoxide. In each example, one reactor is freshly started and the other with deteriorated activity. In the base case both reactors are operated at the same

productivity, but at different operating temperature. The amount of gaseous, liquid and solid products is indicated in table 1.

In table one the reaction conditions are provided in the first three rows. The addition of nitrogen is indicated with Y (Yes) or N(No) . The products gas (C1-C4 hydrocarbons), liquid (C5-C30 hydrocarbons) and solid (C31+ hydrocarbons) are indicated in weight%. The Space Time Yield (STY) is expressed in percentage, 100% being a STY of 115 g/lcat/h.

In the first example according to the invention the productivity of the first reactor is decreased by addition of N compound, meanwhile increasing the temperature. Meanwhile the productivity of the second reactor is increased. The distribution of the products is indicated in the table. It can be seen that the amount of liquid product is increased from 36 to 41%.

In the second example according to the invention the load through the first reactor is increased and the load through the second reactor is decreased, keeping the overall production constant. It can be seen that the amount of normally solid hydrocarbons is increased from 54 to 60%.

In the third example the production through the first reactor is increased and a N compound is added. The production through the second reactor is decreased by adding a N compound in the feed. It can be seen that the normally liquid hydrocarbon content is increased from 36 to 41%. Hence the examples clearly show that by taking into account the state of the catalysts present in each of the reactors in a system of Fischer-Tropsch reactors allows for good control of the content of the product stream .

Table 1.

Comp . Example Example 1 Example 2 Example 3 Rl R2 Total Rl R2 Total Rl R2 Total Rl R2 Total

STY (%) 100 100 96 104 117 83 117 83

Temperature (°C) 187 233 206 235 193 225 213 233

Nitrogen N N Y N N N Y Y

Gas 4 18 11 7 19 13 4 14 8 9 18 13

Liquid 22 49 36 30 50 41 24 43 32 34 49 41

Solid 74 33 54 63 31 46 72 43 60 56 33 47

References to "Groups" and the Periodic Table as used herein relate to the new IUPAC version of the Periodic Table of Elements such as that described in the 87th Edition of the Handbook of Chemistry and Physics (CRC Press) .

While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure need not be limited to the disclosed embodiments. It is intended to cover various

modifications, combinations and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest

interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all embodiments of the following claims.

It should also be understood that a variety of changes may be made without departing from the essence of the invention. Such changes are also implicitly included in the description. They still fall within the scope of this invention. It should be understood that this disclosure is intended to yield a patent covering numerous aspects of the invention both independently and as an overall system and in both method and apparatus modes .

Any patents, publications, or other references mentioned in this application for patent are hereby incorporated by reference. In addition, as to each term used, it should be understood that unless its utilization in this application is inconsistent with such

interpretation, common dictionary definitions should be understood as incorporated for each term and all

definitions, alternative terms, and synonyms such as contained in at least one of a standard technical dictionary recognized by artisans .

Claims

C L A I M S

1. A method for producing hydrocarbons from a gas mixture comprising hydrogen and carbon monoxide, in at least two conversion reactors, being the first and second reactor, said reactors comprising catalysts, which method comprises the steps of:

reactors ;

(iv) determining the concentration of hydrocarbons in each of the initial hydrocarbon streams obtained in step

(iii) of at least one of the following hydrocarbon groups :

hydrocarbons having a chain length of 1 to 4 carbon atoms (Cl-4 fraction) ;

hydrocarbons having a chain length of 5 to 30 carbon atoms (C5-C30 fraction) ; and

hydrocarbons having a chain length of at least 31 carbon atoms (C31+ fraction) ;

(vi) obtaining a second hydrocarbon stream from each of the reactors; wherein the concentration of the C5-C30 fraction of the combined second hydrocarbon streams is at least 3%, and preferably at least 5% higher than the concentration of C5-C30 fraction of the combined initial hydrocarbon stream; or

2. A method according to claim 1 wherein the reaction temperature in at least one reactor is lowered and the reaction temperature in another reactor is raised.

3. A method according to claim 1 or 2 wherein the two or more reactors each have a reactor operating point and the reactor operating point of at least one of the reactors is raised by:

increasing the amount of synthesis gas provided to the reactor;

raising the temperature of the cooling water provided to said reactor;

- providing a nitrogen containing compound, to the reactor, preferably by adding the nitrogen containing compound to the gas mixture prior to step i) , preferably the nitrogen containing compound is selected from the group of ammonia, HCN, NO, an amine and combinations or two or more thereof.

4. A method according to any one of the preceding claims wherein the two or more reactors each have a reactor operating point and the reactor operating point of at least one of the reactors is lowered by:

decreasing the amount of synthesis gas provided to the reactor; lowering the temperature of the cooling water provided to the reactor; and/or

5. A method according to any one of the preceding claims wherein the conversion reactors are operated at an initial temperature in the range of 200 to 230 °C and preferably from 205 to 220 °C.

6. A method according to any one of the preceding claims wherein the reaction temperature is raised in order to reduce the concentration of the C31+ fraction.

7. A method according to any one of the preceding claims wherein the catalyst of one of the reactors has a higher activity than the catalyst in the other reactor.

8. A method according to claim 4 wherein the method further comprises one of the following steps:

- providing a nitrogen containing compound, to the first reactor in case the first reactor comprises the least active catalyst;

- providing a nitrogen containing compound, to the second reactor in case the second reactor comprises the least active catalyst .

9. A method according to any one of the preceding claims wherein the catalyst comprises cobalt as a catalytically active ingredient and wherein the catalyst is preferably present in the reactor as a fixed bed catalyst .

10. A method according to any one of the preceding claims wherein the gas mixture comprising hydrogen and carbon monoxide is synthesis gas, off gas from a conversion reactor or a combination thereof.

11. A method according to claim 10 wherein a first reactor is provided with a synthesis gas as the gas mixture comprising hydrogen and carbon monoxide and a second reactor is provided with the off gas obtained in step (iii) from the first reactor as the gas mixture comprising hydrogen and carbon monoxide.

12. A method according to any one of the claims 1-10 wherein a first and second reactor are each provided with synthesis gas as the gas mixture comprising hydrogen and carbon monoxide .

13. A method according to any one of the preceding claims wherein the reactors are operated at a pressure ranging from 10 to 100 bar absolute.

14. A method according to any one of the preceding claims further comprising the step of:

(vii) determining the concentration of hydrocarbons in each of the second hydrocarbon streams obtained in step (vi) of at least one of the hydrocarbon groups.