OPTIMISATION OF A MULTI-STAGE FISCHER-TROPSCH SYNTHESIS PROCESS

Field of the invention

The present invention relates to a method for the optimisation of a multi-stage Fischer-Tropsch process, in particular a two-stage Fischer-Tropsch process, for producing hydrocarbons from synthesis gas. Background of the invention

Many documents are known describing methods and processes for the catalytic conversion of (gaseous) hydrocarbonaceous feedstocks, especially methane, natural gas and/or associated gas, into liquid products, especially methanol and liquid hydrocarbons, particularly paraffinic hydrocarbons .

The Fischer-Tropsch process can be used for the conversion of synthesis gas from hydrocarbonaceous feedstocks into liquid and/or solid hydrocarbons .

Generally, the feedstock, e.g. natural gas, associated gas, coal-bed methane, heavy and/or residual oil fractions, peat, coal or 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 one or more hydrocarbon synthesis reactors, where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight compounds comprising up to 200 carbon atoms, or, under particular circumstances, even more.

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 .

Usually, a gaseous and a liquid product stream are recovered from a Fischer-Tropsch process. The gaseous product stream comprises unconverted synthesis gas and inerts such as light hydrocarbons, steam, carbon dioxide, nitrogen and argon. Upon cooling, the gaseous product stream typically results in condensed hydrocarbons, impure water and Fischer-Tropsch tail gas, also referred to as Fischer-Tropsch off-gas. The tail gas mainly comprises unconverted synthesis gas, light C]_-C4 hydrocarbons, nitrogen and argon and residual amounts of C5-C9 hydrocarbons, especially C5-C5 hydrocarbons, and H2O. Part of the Fischer-Tropsch tail gas is often recycled to the hydrocarbon synthesis reactor.

The Fischer-Tropsch hydrocarbon synthesis may be carried out in two or more stages; each stage producing the products detailed above. In a two-stage process, part or all of the tail gas of the first stage is fed to the second stage.

In WO 03/010117 is disclosed a multi-stage slurry Fischer-Tropsch process. Each of the stages may comprise more than one slurry bubble columns in parallel. The gaseous product stream of the first stage is fed, after a condensing stage for condensing heavy components and optionally water, to the second stage and so on for further stages. It is mentioned that a portion of the

unreacted gas of a stage may be recycled back to mix with the inlet gas of that stage.

In a multi-stage process such as for example disclosed in WO 03/010117, the inerts produced in a stage, such as C]__4 and CO2 will slow the reaction in the following stage. Furthermore, recycling of tail gas over a stage will cause a build up of inerts which were already present in the feed gas to that stage, such as for example nitrogen. In US 4,443,561 is disclosed a two-stage Fischer-

Tropsch process, wherein synthesis gas is divided into two portions. One portion is converted into paraffinic hydrocarbons in a first Fischer-Tropsch step using an iron-containing bifunctional catalyst. The bifunctional catalyst combines Fischer-Tropsch hydrocarbon synthesis activity and water-gas shift activity. The other portion of the synthesis gas is subjected to water-gas shift conversion to obtain shifted synthesis gas. The shifted synthesis gas is combined with unconverted synthesis gas from the first Fischer-Tropsch step and the mixture is fed to a second Fischer-Tropsch step and converted into paraffinic hydrocarbons.

In the process of US 4,443,561, shifted synthesis gas, i.e. synthesis gas with a relatively high amount of carbon dioxide is mixed with the feed gas to the second stage. Thus, the amount of inerts in the second stage will be relatively high, resulting in a relatively low liquid yield (amount of C5+ hydrocarbons produced per hour per volume of catalyst) in the second stage. Moreover, the water-gas shift activity of the first stage Fischer-Tropsch catalyst will further increase the amount of inerts, in particular carbon dioxide, in the second stage.

In WO2004/026994, a two-stage Fischer-Tropsch process is disclosed wherein tail gas of a first stage containing a slurry bed of non-shifting Fischer-Tropsch catalyst is combined with a hydrogen-containing gas stream and the combined stream is fed into a second reaction stage containing a bed of non-shifting Fischer- Tropsch catalyst.

In GB 2 409 460, a two-stage Fischer-Tropsch process is described wherein a first syngas having a H2/CO ratio in the range of 1.4 to 1.75 is fed to a first Fischer- Tropsch synthesis reactor, a second synthesis gas is recovered from the effluent of the first reactor and mixed with a third syngas having a H2/CO ratio of at least 2.0 and the blended second and third syngas is fed into a second Fischer-Tropsch reactor.

In order to compensate for the decreased yield in a second stage Fischer-Tropsch reactor due to inert buildup, one could: (i) increase the temperature in the second stage; or (ii) increase the pressure in the second stage.

To increase the pressure in the second stage compressors or the like are required which may not be economically viable and so increasing the temperature in the second stage is deemed the more viable solution. A problem, however, with the increased temperature in the second stage is the increase in unwanted CO2 production and a lower C5+ selectivity.

Summary of the invention

It has now been found that the amount of inerts in a second or further stage in a multi-stage Fischer-Tropsch hydrocarbon synthesis step can be kept to a sufficiently low level by using fresh synthesis gas for the first stage having an inerts content in the range of from 1 to

7 vol% and mixing the off-gas of a first stage Fischer Tropsch reactor with part of the fresh synthesis gas that has by-passed the first stage before the off-gas is fed to a second stage Fischer-Tropsch reactor. To increase the H2/CO ratio of the gas mixture that is fed to the second stage Fischer-Tropsch reactor (s), additional hydrogen may be added to the gas mixture.

Accordingly, the present invention provides a method for the optimisation of a multi-stage Fischer-Tropsch process producing hydrocarbons from synthesis gas, which method comprises the steps of:

(i) providing synthesis gas comprising in the range of from 1 to 7 vol% inerts, preferably of from 2 to 5 vol%; (ii) supplying part of the synthesis gas provided in step (i) to a first-stage hydrocarbon synthesis reactor provided with a hydrocarbon synthesis catalyst and catalytically converting the synthesis gas at an elevated temperature and pressure to obtain a first liquid hydrocarbon product stream and a first gaseous hydrocarbon product stream;

(iii) feeding part of the synthesis gas provided in step (i) and a portion of the first gaseous product stream comprising unreacted synthesis gas into a second- stage hydrocarbon synthesis reactor provided with a hydrocarbon synthesis catalyst, and catalytically converting the synthesis gas at an elevated temperature and pressure to obtain a second liquid hydrocarbon product stream and a second gaseous hydrocarbon product stream. An advantage of the method according to the invention is that the conversion rate in the second or further stage reactor can be kept to a sufficiently high level without the need for a large temperature increase

in the second or further stage and thus avoiding increased carbon dioxide formation and reduced C5+ selectivity. In the process according to the invention, a temperature increase of 5-15 ⁰ C for the second stage compared to the first stage is typically sufficient in order to obtain in the second stage at least 90% of the productivity obtained in the first stage.

A further advantage is that there is no need to have an increased pressure in the second stage as compared to the first stage in order to obtain in the second stage at least 90% of the productivity obtained in the first stage .

Detailed description of the invention

The invention provides a method for the optimisation of a multi-stage Fischer-Tropsch process. Preferably the multi-stage Fischer-Tropsch process is a two-stage

Fischer-Tropsch process. Alternatively, the process may include further stages, for example a third stage reactor, wherein synthesis gas provided in step (i) along with at least a portion of the gaseous product from the second stage reactor is added to the third stage reactor.

Likewise even further stages may be added.

Each stage may comprise more than one reactors in parallel. Preferably, for each reactor utilised in an nth stage, there are between one and ten (nth-1) stage reactors, more preferably between two and six (nth-1) stage reactors, especially between three and four (nth-1) stage reactors; wherein n is an integer greater than 1.

Thus, for each reactor utilised in the second stage, there are preferably between one and ten first stage reactors, more preferably between two and six first stage reactors, especially between three and four first stage reactors .

In step (i) of the method according to the invention, synthesis gas with in the range of from 1 to 7 vol% inerts is provided. Reference herein to inerts is to compounds that are not reactive in the Fischer Tropsch synthesis reaction and include carbon dioxide, methane, C2-C4 hydrocarbons, and also impurities in the synthesis gas stream, mainly nitrogen and argon. Preferably, the synthesis gas provided in step (i) has in the range of from 2 to 5 vol% inerts . The synthesis gas may be provided by any process for converting a hydrocarbonaceous feedstock into synthesis gas known in the art, for example catalytic or non- catalytic partial oxidation, autothermal reforming, steam methane reforming or dry reforming. Provision of the synthesis gas by means of a partial oxidation process or an autothermal reforming process is preferred since these processes typically result in a synthesis gas with a relatively low amount of inerts. A partial oxidation process is particularly preferred. In step (ii) of the method according to the invention, part of the synthesis gas provided in step (i) is supplied to a first-stage hydrocarbon synthesis reactor provided with a hydrocarbon synthesis catalyst and is catalytically converted at elevated temperature and pressure to obtain a first liquid hydrocarbon product stream and a first gaseous hydrocarbon product stream.

The first gaseous hydrocarbon product stream typically comprises gaseous hydrocarbons and unconverted synthesis gas. In step (iii) of the method according to the invention, at least a portion of the first gaseous hydrocarbon product stream is fed, together with a part of the synthesis gas provided in step (i) that was not supplied to the first stage reactor, to a second stage

hydrocarbon synthesis reactor. The synthesis gas supplied to the second stage reactor, i.e. both with the first gaseous hydrocarbon product stream and with the synthesis gas provided in step (i) that has bypassed the first stage reactor, is converted at elevated temperature and pressure to obtain a second liquid hydrocarbon product stream and a second gaseous hydrocarbon product stream. Preferably, a supplementary hydrogen containing gas is fed into the second-stage hydrocarbon synthesis reactor along with the synthesis gas provided in step (i) and the first gaseous product stream. The supplementary hydrogen containing gas may be hydrogen gas or hydrogen- rich synthesis gas or any other hydrogen-containing gas, and is preferably a synthesis gas obtained by steam methane reforming having a H2/CO ratio between 3 and 8 preferably between 4 and 7. The supplementary hydrogen containing gas preferably has an inerts content that is lower than the inerts content of the first gaseous product stream. In preferred embodiments a portion of the first gaseous product stream, often referred to as off-gas, is recycled back into the first stage reactor (s) . This aids the conversion of synthesis gas to C5+ hydrocarbons.

However, the recycle also results in a build up of inerts in the first stage and particularly in consecutive stage reactor (s) . The first gaseous product stream may have an inerts content in the range of from 10 to 80 vol%, usually in the range of from 30 to 50 vol%.

Preferably, the inert content of the total feed gas stream to the first stage reactor (s), i.e. including an optional recycle stream, is in the range of from 2 to 10 vol%, more preferably 3 to 8 vol%, even more preferably 5 to 6 vol%.

Preferably, a portion of the gaseous product and/or unreacted gases from a reactor of any stage are recycled into a reactor of said stage. The recycle stream over a Fischer-Tropsch reactor can be between 5-250 mol% of the fresh synthesis gas stream supplied to that reactor, preferably between 50-200 mol%.

The H2:CO ratio in the synthesis gas provided in step (i) is preferably in the range of from 1.4 to 1.95, more preferably of from 1.7 to 1.9. The H2 : CO ratio in the total synthesis gas feed supplied to the first stage reactor(s), i.e. including an optional recycle stream, is preferably in the range of from 1.1 to 1.95, more preferably of from 1.4 to 1.9.

The volume ratio of synthesis gas provided in step (i) that is fed to the first stage and to the second stage reactor (s) is preferably in the range of 3.0 to 10.0.

The proportion of inerts in the total gaseous stream fed into the second stage reactor (s), taking into account any supplementary hydrogen containing gas and any optional recycle stream, can be around 20-70 vol%, typically 40-50 vol%.

The catalytic conversion in the first, second or further stage reactors may be carried out at any temperature and pressure known to be suitable for Fischer Tropsch hydrocarbon synthesis, typically a temperature in the range of from 125 to 375 ⁰ C and a pressure in the range of from 5 to 180 bar (absolute), preferably of from 10 to 60 bar (absolute) . Preferably the temperature in the second stage reactor (s) is greater than the temperature in the first stage reactor (s). More preferably, the temperature in the second stage reactor (s) is in the range of from 5 to

15 ⁰ C higher than in the first stage reactor (s). This holds in a similar way also for further stages.

Preferably, the temperature in the first stage reactor (s) is in the range of from 190 to 270 ⁰ C, more preferably of from 210 to 250 ⁰ C, even more preferably around 230 ⁰ C. The temperature in the second stage reactor (s) is preferably in the range of from 200 to

280 ⁰ C, more preferably of from 220 to 260 ⁰ C, even more preferably around 240 ⁰ C. The pressure in the second stage reactor (s) may be more than, the same as, or less than the pressure in the first stage reactor (s).

The productivity is typically expressed as the space time yield (STY), which is the amount (in kg) of hydrocarbon product (C]_+ ) per hour per cubic metre of catalyst-containing reaction zone. The hydrocarbon products include all products made in the Fischer-Tropsch reaction containing one or more carbon atoms, e.g. all paraffins and olefins produced as well as any oxygenates (alcohols, aldehydes, acids etc.), but excluding carbon dioxide. For fixed bed reactors, the volume of the catalyst-containing reaction zone is the catalyst bed volume. For slurry or ebullating reactors, the catalyst particle volume is taken as the volume of the catalyst- containing reaction zone. In the case that different reactor types in one multi-stage plant are used the volume of the catalyst-containing reaction zone is the catalyst particle volume. Typically the producticity (usually expressed as space time yield) in the second stage reactor will be lower than in the first stage reactor .

The total carbon monoxide conversion in a multistage Fischer Tropsch reactor system is suitably at least

80%, preferably at least 90%, more preferably at least 95%. The carbon monoxide conversion per stage is suitably at least 60%, preferably at least 70%, more preferably at least 75%. In order to obtain a productivity in a further stage that is at least 80%, preferably at least 90%, of that in the preceding stage, the ratio of the amount of carbon monoxide to the volume of catalyst is preferably lower in the (nth + 1) stage compared to the nth stage, wherein n is an integer. Thus, the ratio of the amount of carbon monoxide to the volume of catalyst is lower in the second stage compared to the first stage.

In the method according to the invention, any hydrocarbon synthesis catalyst known in the art may be used in both the first and the second stage reactor (s) and also in optional further-stage reactors . Preferably a iron-based or a cobalt-based catalyst, more preferably a cobalt-based catalyst, is used.

It is preferred to use the same reactor types in each stage of one multi-stage plant, more preferably also using catalysts having the same chemical composition.

In order to obtain a productivity in the second stage that is at least 80%, preferably at least 90%, of that in the first stage, the second stage reactor (s) preferably contain a catalyst with a shorter diffusion path length than the catalyst of the first stage reactor (s) . A shorter diffusion path length of the catalyst may for example be obtained by using a catalyst of a different morphology, in particular catalyst particles with a smaller diameter or of a different shape .