WO2014180888A1

WO2014180888A1 - Process for the preparation of syngas

Info

Publication number: WO2014180888A1
Application number: PCT/EP2014/059314
Authority: WO
Inventors: Hubert Willem Schenck; Frans Hans GHIJSEN; Ruben SMIT; Thomas Paul Von Kossak-Glowczewski
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2013-05-08
Filing date: 2014-05-07
Publication date: 2014-11-13

Abstract

The invention is directed to a process for the preparation of a syngas comprising hydrogen and carbon monoxide from a methane comprising gas, which process comprises the steps of: (a) reacting the methane comprising gas with an oxidising gas to obtain a hot raw syngas comprising carbon monoxide and hydrogen; (b) adding carbon dioxide to the hot raw syngas under such conditions that at least part of the carbon dioxide added reacts with hydrogen present in the hot raw syngas; and (c) cooling the gas mixture resulting from step (b) to obtain the syngas. The invention also relates to a process for the preparation of hydrocarbon products in which a feed syngas is prepared in the process as described above and is subsequently converted into hydrocarbon products in a Fischer-Tropsch process.

Description

PROCESS FOR THE PREPARATION OF SYNGAS

The present invention relates to a process for the preparation of a syngas comprising hydrogen and carbon monoxide from a methane comprising gas and to a process for the production of hydrocarbon products from such syngas by means of a Fischer-Tropsch process.

The expression "syngas" as used herein refers to synthesis gas, which is a common term to refer to gas mixtures comprising carbon monoxide and hydrogen.

Processes for the preparation of syngas are well known in the art. Typically a feed gas comprising methane is contacted with an oxidizing gas and the methane reacts with the oxidizing gas to form a syngas.

One process for the preparation of syngas, for example, is partial oxidation (POX) of a methane

comprising gas. Such POX process can take place in the presence of a suitable reforming catalyst or in the absence of a catalyst. Generally, in a POX process methane reacts with oxygen in an exothermic reaction to form carbon monoxide and hydrogen. Publications

describing examples of POX processes are EP-A-291111, WO-

A-97/22547, WO-A-96/39354 and WO-A-96/03345.

Another process for the preparation of syngas is steam reforming of a hydrocarbon feed. The steam

reforming reaction is an endothermic reaction which takes place in the presence of a suitable steam reforming catalyst. The hydrocarbon feed reacts with steam to form carbon monoxide and hydrogen. The heat needed for this reaction is supplied externally. An example of a steam reforming process is disclosed in EP-A-0168892.

Combining a catalytic or non-catalytic POX process with a steam reforming process is also possible. For example, both processes can be integrated. In such an integrated process the steam reforming process can be used to prereform a hydrocarbon feed, thereby converting C2+ hydrocarbons in the hydrocarbon feed into methane and part of the methane into carbon monoxide and hydrogen, after which the effluent of this steam reforming reaction is subjected to a POX process. The heat generated in the exothermic POX process may be, in whole or in part, used to provide the reaction heat for the endothermic steam reforming reaction. Examples of such integrated processes are, for example, disclosed in WO-A-2004/041716.

A POX process and a steam reforming process can also be combined by carrying out these processes in parallel and subsequently mixing the respective syngas streams produced in each process. Examples of such processes are disclosed in US-A-2007/0004809 and WO-A-2010/122025.

An important element of the quality of a syngas is the hydrogen/carbon monoxide [H₂/CO] molar ratio. This H₂/CO molar ratio is typically controlled by adjusting the feed streams to the syngas producing reactor. The steam to carbon molar ratio and the carbon dioxide to carbon molar ratio (with moles of carbon as moles of hydrocarbon in the methane comprising feed) are important parameters in this respect. When forming the hot syngas in the syngas producing reactor a state of thermodynamic equilibrium is obtained. However, this equilibrium is dependent on the temperature of the gas mixture.

Consequently, when the hot syngas produced is cooled, equilibrium states that existed in the hot syngas are adjusted to some extent. An important equilibrium in this connection is the water gas shift equilibrium:

CO + H₂0 ¾ C0₂ + H₂ This equilibrium adjusts itself at lower temperature to shift to the side of the carbon dioxide and hydrogen, as the reaction of carbon monoxide with water to form carbon dioxide and hydrogen is exothermic. In other words, carbon monoxide is consumed and heat is released whilst cooling the hot syngas. This is an undesirable effect in some applications, particularly when the syngas produced is to be used for Fischer-Tropsch synthesis. To mitigate the effect of carbon monoxide decrease and hydrogen increase during cooling, syngas with lower H₂/CO molar ratio than ultimately required can be produced in the reactor. This will require a higher energy input, notably in the form of additional oxygen demand in the syngas producing reactor, which generally can only be recovered at a lower level in the form of steam.

The present invention aims to provide a process for the production of syngas which enables the adjustment of the H₂/CO molar ratio of the syngas produced to the desired level, while cooling the syngas. This will allow the production of more syngas at the desired ¾/CO molar ratio for a given oxygen demand and hence is more

efficient from an energy consumption perspective.

Accordingly, the present invention relates to a process for the preparation of a syngas comprising hydrogen and carbon monoxide from a methane comprising gas, which process comprises the steps of:

(a) reacting the methane comprising gas with an oxidising gas to obtain a hot raw syngas comprising carbon monoxide and hydrogen;

(b) adding carbon dioxide to the hot raw syngas under

such conditions that at least part of the carbon dioxide added reacts with hydrogen present in the hot raw syngas; and (c) cooling the gas mixture resulting from step (b) to obtain the syngas.

It was found that adding carbon dioxide to the hot syngas immediately after it has been formed in the syngas producing reactor enables an effective adjustment of the

H2/CO molar ratio of the syngas whilst at the same time achieving effective chemical cooling of the hot syngas. The endothermic reaction between the carbon dioxide added and the hydrogen present in the hot syngas cools down the raw syngas, thereby causing the water gas shift

equilibrium as described above to shift to the side of the carbon monoxide. As a result of this chemical quench the temperature of the hot syngas will decrease, whilst at the same time the level of carbon monoxide in the syngas produced increases. Furthermore, using the heat of the hot raw syngas as heat required for the endothermic reaction between carbon dioxoide and hydrogen is a more effective use of heat than converting the heat into steam through indirect heat exchange in a cooling step. A further advantage of the process according to the present invention is that some of the carbon dioxide produced in the syngas production and/or possible subsequent Fischer- Tropsch synthesis can be effectively re-used in the process .

The carbon dioxide added in step (b) can be in the form of any gas stream comprising a substantial amount of carbon dioxide. Typically, however, such carbon dioxide containing gas stream should comprise at least 90 mole%, preferably at least 95 mole% and more preferably at least 99 mole% of carbon dioxide. A substantially pure carbon dioxide stream would be most preferred.

The amount of carbon dioxide added in step (b) will to a large extent be determined by the target ¾/CO molar ratio of the syngas to be obtained, which in return is determined by the envisaged application of the syngas. For example, for use in a Fischer-Tropsch synthesis process the ¾/CO molar ratio of the syngas should suitably be in the range of from 1.6 to 2.2, more

suitably 1.8 to 2.1. Another factor that plays a role in determining the amount of carbon dioxide to be added is the overall integration of process streams, notably carbon dioxide recycle. It was found, however, that the amount of carbon dioxide added in step (b) is such that the ratio of the moles of syngas (¾ + CO) present in the hot syngas stream before addition of the carbon dioxide relative to carbon dioxide added in step (b) [ (H₂+CO) /CO₂ molar ratio] should typically be in the range of from 3.5 to 50.0, preferably 5.0 to 35.0 and more preferably 10.0 to 30.0. So, per 100 moles of syngas (H₂ + CO) present in the hot syngas stream 2 to 28.5 moles of carbon dioxide are typically added. Preferably 3 to 20 moles and more preferably 3.3 to 10 moles of carbon dioxide are added per 100 moles of syngas (¾ + CO) present in the hot syngas stream.

The conditions under which the carbon dioxide is added should be such that the reaction between at least part of the carbon dioxide added and hydrogen present in the hot raw syngas will take place to the desired extent, that is, to such extent that the syngas eventually obtained has the desired ¾/CO molar ratio. The

temperature of the gaseous reaction mixture of carbon dioxide and the hot raw syngas, so the temperature of the gas mixture after the addition of carbon dioxide and chemical reaction, is suitably at least 750 °C and preferably at least 800 °C. The upper temperature is determined by the type of oxidation reaction used in step (a) as will be described in more detail hereinafter.

Generally, however, such temperature will not exceed 1500 °C and preferably will not be higher than 1400 °C. At such high temperatures carbon dioxide will react with hydrogen, so that the water gas shift equilibrium can be adjusted to the extent desired.

The temperature of the carbon dioxide added is not critical. Because of the relatively high temperature of the hot raw syngas, the carbon dioxode will typically be added at a temperature below the temperature of the hot raw syngas. The lower temperature of the carbon dioxide added will also have a (physical) cooling effect in addition to the chemical cooling effect. However, this physical cooling effect will be limited, because of the relatively low amount of carbon dioxide added relative to the amount of hot raw syngas. Furthermore, the reaction between carbon dioxide and hydrogen is favoured at higher temperatures, so that it is beneficial that the

temperature of the carbon dioxide added is not too low. The carbon dioxide added may be preheated using heat available in the process, typically by indirect heat exchange. However, if step (a) comprises the use of a furnace (e.g. for feed preheat), as will be described in more detail below, then preheating of the carbon dioxide may also take place by using heat generated in such furnace, for example, in a separate coil in such furnace.

Adding the carbon dioxide to the hot raw syngas can be performed by ways known in the art, for example by injecting the carbon dioxide into the hot raw syngas stream.

The reaction between carbon dioxide added and

hydrogen present in the hot raw syngas in step (b) , or part of such reaction, can be carried out in the presence of at least one catalytically active metal catalyzing the reaction between carbon dioxide and hydrogen. Such catalytically active metals are known in the art. In general, suitable catalytically active metals are metals from the d-block (Groups 3 to 12) of the Periodic Table of Elements, also commonly referred to as transition metals. Examples of suitable metals are chromium, iron, cobalt, copper, nickel, zinc, ruthenium and palladium. Particularly suitable metals are chromium, iron and copper. If a catalytically active metal is used in step

(b) , then at least one of chromium, iron and copper is preferably used.

The reaction mixture of carbon dioxide and the hot raw syngas can be contacted with a suitable catalytically active metal in various ways. In one suitable embodiment of the present invention the reaction mixture of carbon dioxide and hot raw syngas obtained is passed over a catalyst bed consisting of catalyst particles comprising at least one catalytically active metal catalyzing the reaction between carbon dioxide and hydrogen. In this latter embodiment the catalytically active metal will suitably be in the form of an oxide (e.g. iron (III) oxide, Fe2<0₃) or be supported on a porous inorganic refractory oxide material, such as alumina, silica, titania,

zirconia or mixtures thereof.

In another embodiment the reaction mixture of carbon dioxide and hot raw syngas obtained is passed through a heat exchanger or boiler with the inner wall of the tubes of such heat exchanger or boiler comprising at least one catalytically active metal catalyzing the reaction between carbon dioxide and hydrogen. In this embodiment the catalytically active metal (s) can be in the form of the alloy forming the inner wall of the tube. Part of the reaction between the carbon dioxide added and the

hydrogen in the raw syngas will usually have taken place already prior to entering the heat exchanger or boiler tube because of the high termperature of the hot raw syngas.

Reacting the methane comprising gas with an oxidising gas to obtain a hot raw syngas in step (a) of the present process can be performed by processes known in the art as generally described above.

The methane comprising gas used as the feedstock to the present process may be natural gas, associated gas or a mixture of C₁-4 hydrocarbons. The feed comprises mainly, i.e. more than 90 volume percent (% v/v) , especially more than 94% v/v, C₁-4 hydrocarbons, and especially comprises at least 60% v/v methane, preferably at least 75% v/v, more preferably at least 90% v/v. Very suitably natural gas or associated gas is used.

The oxidising gas used in step (a) may be oxygen or an oxygen-containing gas. Suitable gases include air (containing about 21 percent of oxygen) and oxygen enriched air, which may contain at least 60 volume percent oxygen, more suitably at least 80 volume percent and even at least 98 volume percent of oxygen. Such pure oxygen is preferably obtained in a cryogenic air

separation process or by so-called ion transport membrane processes. The oxidising gas may also be steam.

Suitable processes to prepare the hot raw syngas in step (a) include catalytic and non-catalytic partial oxidation (POX) processes and steam reforming processes as well as combinations of two or more of these

processes, either integrated or operated in parallel.

A suitable process to prepare syngas using oxygen or an oxygen-containing gas as the oxidising agent is carried out in a partial oxidation reactor. This can be a catalytic or non-catalytic POX process. When carried out in the absence of a catalyst such partial oxidation reactor typically comprises a burner placed at the top in a reactor vessel with a refractory lining. The reactants are introduced at the top of the reactor. In the reactor a flame from the burner is maintained in which the methane comprising feed gas reacts with the oxygen or oxygen-containing gas to form a syngas. Reactors for catalytic POX processes usually comprise a burner at the top and one or more fixed beds of suitable catalyst to react the methane in the feed with the oxygen added to the top of the reactor to form a syngas.

Non-catalytic POX processes are well known. The syngas produced typically has a temperature of between

1100 and 1500 °C, suitably between 1200 and 1400 °C . The pressure at which the syngas product is obtained may be between 3 and 10 MPa and suitably between 5 and 7 MPa. In addition to the oxygen-containing gas, steam may also be added.

If the syngas producing reactor is a partial

oxidation reactor, then the cooling of the gas mixture in step (c) of the present process is suitably performed by indirect heat exchange between the gas mixture

obtained in step (b) on the one hand and water and/or the methane comprising gas on the other hand. Preferably the gas mixture resulting from step (b) is first cooled in a boiler or other type of heat exchanger against water to produce steam and subsequently is further reduced in temperature by indirect heat exchange against the methane comprising gas used as a feed to step (a) . In this way the methane comprising feed gas can be preheated in a heat-efficient manner. Suitable heat exchangers are well known in the art and include, for example, shell-tube heat exchangers .

The non-catalytic POX process as described above may be combined with a preceding steam pre-reforming step. The methane comprising gas used as a feed to the POX process of step (a) is then produced in this pre- reforming step. Such pre-reforming is preferably

performed adiabatically . Thus, the gaseous hydrocarbon feed to the pre-reformer (comprising methane and C2+ hydrocarbons) and steam are heated to the desired inlet temperature and passed through a bed of a suitable steam reforming catalyst. Higher hydrocarbons having 2 or more carbon atoms will react with steam to give carbon oxides and hydrogen. At the same time methanation of the carbon oxides with the hydrogen takes place to form methane. The net result is that the higher hydrocarbons are converted to methane with the formation of some hydrogen and carbon oxides. Some endothermic reforming of methane may also take place, but since the equilibrium at such low

temperatures lies well in favour of the formation of methane, the amount of such methane reforming is small so that the product from this stage is a methane-rich gas. The heat required for the reforming of higher

hydrocarbons is provided by heat from the exothermic methanation of carbon oxides (formed by the steam

reforming of methane and higher hydrocarbons) and/or from the sensible heat of the feedstock and steam fed to the catalyst bed. The exit temperature will therefore be determined by the temperature and composition of the feedstock/steam mixture and may be above or below the inlet temperature. The conditions should be selected such that the exit temperature is lower than the limit set by the de-activation of the catalyst. While some reformer catalysts commonly used are deactivated at temperatures above about 550 °C, other catalysts that may be employed can tolerate temperatures up to about 700°C. Preferably the outlet temperature is between 350 and 530°C.

When oxygen or an oxygen-containing gas is used as the oxidising agent, step (a) of the present process can also be suitably carried out in an auto-thermal reformer (ATR) unit. In autothermal reforming the methane

comprising gas reacts with oxygen -typically with

addition of steam so that the steam to carbon (as

hydrocarbon) molar ratio is suitably between 0.5 and 3- to produce syngas. An ATR reformer unit typically

comprises a burner, a combustion chamber and a catalyst bed in a refractory lined pressure shell. The burner is placed at the top of the pressure shell and extends into combustion chamber which is located in the top section of the pressure shell. The catalyst bed is arranged below the combustion chamber. Examples of autothermal reforming processes are e.g. disclosed in EP-A-1403216 and US-A- 2007/0004809. The ATR unit may be the well-known ATR units as commercially used.

Suitable reforming catalysts and arrangements for such which can be used in the ATR unit, are known in the art. Such catalysts typically comprise a catalytically active metal on a suitable support material. Examples of suitable reforming catalysts are disclosed in US-A- 2004/0181313 and US-A-2007/0004809.

The methane comprising gas feed to the ATR unit suitably has a temperature in the range of up to 850°C, suitably from 500 to 850°C, more suitably 650 to 800°C, whilst the raw syngas leaving the ATR unit typically has a temperature in the range of from 950 to 1200°C , more suitably 970 to 1100°C. Operating pressures are typically between 2 and 6 MPA, more suitably between 2 and 5 MPa.

The ATR unit can be used as the sole oxidation unit in step (a) , but in another suitable embodiment the ATR unit can also be integrated with a heat exchange reformer

(HER) unit. In such integrated configuration the methane comprising gas feed to the ATR unit in step (a) of the present process is obtained by steam reforming of a gaseous methane comprising feed in such a HER unit.

Further integration is suitably obtained by using the heat generated in the exothermic partial oxidation reaction in the ATR unit for providing the reaction heat for the endothermic steam reforming reaction taking place in the HER unit.

Accordingly, in a preferred embodiment of the present invention the cooling step (c) comprises feeding the gas mixture obtained in step (b) to the HER unit where it provides the necessary heat for the steam reforming of the gaseous methane comprising feed to the HER unit by indirect heat exchange. The HER unit preferably is a tube and shell reactor wherein the steam reforming reaction takes place inside the tubes in the presence of a

suitable steam reforming catalyst and wherein the

required heat for the endothermic steam reforming

reaction is suitably provided by passing the hot effluent of the ATR unit to the shell side of the reactor tubes.

The catalyst and process conditions as applied in the steam reformer reactor tubes of the HER unit are known in the art and hence may be those known by the skilled person in the field of steam reforming. Suitable

catalysts, for example, comprise nickel, optionally applied on a refractory oxide carrier material, for example alumina. Examples of suitable catalyst are disclosed in WO-A-2004/041716. The steam to carbon (as hydrocarbon) molar ratio is preferably from 0 up to 2.5 and more preferably below 1 and most preferably from 0.5 up to 0.9.

The product gas as it leaves the tubes of the HER unit preferably has a temperature of from 600 up to

900 °C. The operating pressure at the tube side is preferably between 2 and 6 MPa, more preferably between 2.5 and 5 MPa. The ATR unit is typically operated at a slightly lower pressure to avoid recompression of the syngas from the HER unit before feeding to the ATR unit. This means that in the HER unit the pressure at the shell side will suitably be at a slightly lower level than in the ATR unit.

In another embodiment of the present invention step

(a) is a steam reforming reaction. In such embodiment the oxidising gas in step (a) is steam and the reaction between the methane comprising gas and steam takes place in the presence of a steam reforming catalyst. Suitable steam reforming catalysts and process conditions applied in such steam reforming step are well known in the art. If steam reforming is the sole or primary methane

reforming step, the temperature will usually be slightly higher than in a HER unit which is integrated with an ATR unit. Accordingly, the raw syngas leaving a steam methane reforming unit will typically have a temperature of from 800 to 1000°C, more suitably 850 to 950°C, and be at a pressure of from 0.5 to 8 MPa, more suitably 2 to 4 MPa.

The present invention also relates to a process for the preparation of hydrocarbon products comprising the steps of:

(a) preparing a feed syngas comprising hydrogen and

carbon monoxide in a process as described above; and (b) converting the feed syngas into hydrocarbon products in a Fischer-Tropsch process.

The Fischer-Tropsch (FT) process is well known in the art as a catalytic process for synthesizing longer chain hydrocarbons from carbon monoxide and hydrogen. It may be operated in a single pass mode ("once through") or in a recycle mode and could involve a multi-stage conversion process, which may involve, two, three, or more

conversion stages.

Fischer-Tropsch catalysts for use in the fixed bed catalyst beds of the syngas conversion reactor are known in the art, and typically include a Group 8 or 9 metal component, preferably Co, Fe and/or Ru, more preferably Co, on a suitable catalyst support material. Such support material could be a porous inorganic refractory oxide material, such as alumina, silica, titania, zirconia or mixtures thereof, but could also be an alternative support structure.

The Fischer-Tropsch synthesis is preferably carried out at a temperature in the range from 125 °C to 350 °C, more preferably 175 °C to 275 °C, most preferably 200 °C to 260 °C. The pressure preferably ranges from 0.5 to 15 MPa, more preferably from 0.5 to 8 MPa.

It will be understood that the skilled person is capable to select the most appropriate conditions for a specific reactor configuration and reaction regime.

Products of the Fischer-Tropsch synthesis may range from methane to heavy paraffinic waxes. Preferably, the production of methane is minimised and a substantial portion of the hydrocarbons produced have a carbon chain of at least 5 carbon atoms. Preferably, the amount of C5+ hydrocarbons is at least 60% by weight of the total product, more preferably, at least 70% by weight, even more preferably, at least 80% by weight, most preferably, at least 85% by weight.

Examples

The following examples have been modelled using

PRO/II software as obtained from Invensys Process

Systems, Piano, Houston (USA) , and UniSim® process simulation software as obtained from Honeywell.

Example 1

Natural gas having a composition as indicated in Table 1, was fed into an ATR unit at a feed rate of

14,350 tonnes/day (847 t-moles/day) . Oxygen was fed into the ATR and the amount used in the reaction with the natural gas feed is indicated in Table 2. Steam at a temperature of 430 °C and a pressure of 75 bar was added at a feed rate of 8,772 tonnes/day (487 t-moles/day), corresponding with a steam to carbon (C as hydrocarbon in feed) ratio of 0.6. No carbon dioxide was added to the effluent of the ATR unit.

The hot raw syngas effluent of the ATR had a

temperature of 1040 °C and was passed through a boiler for indirect heat exchange against boiler feed water to produce steam. Boiler outlet temperature of the raw syngas was 400 °C.

The unit "t-moles" as used herein refers to "tonne- moles" which is equal to 1,000 kmoles or 10^s moles. The unit "tonne" refers to 1,000 kg.

Table 1 - Natural gas composition

Component Mole%

C02 0.77

N2 4.39

CI 93.788

C2 1.00

C3 0.05

C4 0.002 Example 2

Example 1 was repeated except that carbon dioxide at a temperature of 300 °C and a pressure of 50 bar was added to the natural gas feed prior to entering the ATR unit. The ATR effluent had a temperature of 1040 °C and was passed through a boiler for indirect heat exchange against boiler feed water to produce steam. Boiler outlet temperature of the raw syngas was 400 °C.

Example 3

Example 1 was repeated except that carbon dioxide at a temperature of 300 °C and a pressure of 50 bar was added to the effluent of the ATR unit. The ATR effluent had a temperature of 1040 °C, which after the addition of and reaction with carbon dioxide was reduced to 900 °C before it was passed through the boiler to produce steam. Boiler out let temperature of the raw syngas was 400 °C.

Example 3 illustrates the invention, whereas Examples 1 and 2 are included for comparative purposes.

The results are indicated in Table 2.

Table 2 - Syngas production with and without addition of carbon dioxide

Example 1 Example 2 Example 3

NG use (tmoles/d) 847 847 847

H₂0 use (tmoles/d) 487 487 487

O₂ use (tmoles/d) 468 489 468

CO2 added (tmoles/d) 0 119 126

CO2 converted N/A 65 74 (tmoles/d) *

C0₂ converted (%) * N/A 54 59

CO yield (tmoles/d) 613 692 686 H₂+CO (tmoles/d) 2, 060 2, 077 2, 060

H₂/CO mole/mole 2.36 2.00 2.00

(H2+CO) /0₂ used 4.40 4.24 4.40 (mole/mole)

(H2+CO) /C0₂ added N/A 17.40 16.41 (mole/mole)

Carbon (including 74.8% 73.7% 72.6% CO₂) -> CO conversion

(% mole CO out/mole C in)

Duty for steam 744 795 704 recovery (MW) **

* C02 conversion is calculated on the basis of C02 added; the amount of C02 present in the natural gas feed (6.5 tmoles/d) and the amount of C02 generated in the oxidation reaction that takes place in the ATR (137 tmoles/d) is not taken into account.

** Duty for steam recovery is the heat available after the hot raw syngas has reached the equilibrium state of the water gas shift equilibrium. In Example 3 that means the duty after the addition and reaction with carbon dioxide.

As can be seen from Table 1 Example 1 results in a syngas with a H2/CO molar ratio which is significantly higher (2.36) than the H2/CO molar ratio of the syngas produced in the example according to the invention

(Example 3: 2.0) at equal natural gas use. This is undesired for, for example, Fischer-Tropsch synthesis.

In Example 2 a syngas with the same ¾/CO molar ratio as in Example 3 is produced, but as can be seen from Table 1 the process according to the present invention as illustrated in Example 3 has a lower oxygen consumption and hence a lower energy consumption to produce syngas with the same ¾/CO molar ratio. The absolute amount of syngas produced is slightly lower in the process

according to the present invention, which implies that there is more unconverted methane present in the syngas produced. This unconverted methane, however, is suitably recycled to the ATR unit, when the syngas is used in a Fischer-Tropsch synthesis process. Furthermore, the lower oxygen consumption for the same amount of natural gas resulting in a syngas with the same ¾/CO molar ratio means that in a plant with a fixed oxygen production capacity, more natural gas can be processed with the same equipment resulting in a higher syngas yield of the same H2/CO molar ratio. This is beneficial from an economic perspective .

Table 1 also shows that the duty available for steam recovery in Example 3 is lower than in Examples 1 and 2. That implies that heat available in the hot raw syngas is used chemically in the endothermic reaction between hydrogen and carbon dioxide and in fact ends up in the syngas product. Hence, heat generated in the process is used more effectively in the process according to the present invention than would be the case when converting it into steam via indirect heat exchange in a boiler.

Claims

C L A I M S

1. Process for the preparation of a syngas comprising hydrogen and carbon monoxide from a methane comprising gas, which process comprises the steps of:

(b) adding carbon dioxide to the hot raw syngas under

such conditions that at least part of the carbon dioxide added reacts with hydrogen present in the hot raw syngas; and

(c) cooling the gas mixture resulting from step (b) to obtain the syngas.

2. Process according to claim 1, wherein the amount of carbon dioxide added to the hot raw syngas in step (b) is such that the (H₂+CO)/C0₂ molar ratio is in the range of from 3.5 to 50.0.

3. Process according to claim 1 or 2, wherein the temperature in step (b) is at least 750 °C.

4. Process according to any one of claims 1-3, wherein the reaction between carbon dioxide and hydrogen in step

(b) is carried out in the presence of at least one catalytically active metal catalyzing the reaction between carbon dioxide and hydrogen.

5. Process according to claim 4, wherein the mixture of carbon dioxide and hot raw syngas obtained in step (b) is passed over a catalyst bed consisting of catalyst

particles comprising at least one catalytically active metal catalyzing the reaction between carbon dioxide and hydrogen .

6. Process according to claim 4 or 5, wherein the catalytically active metal is at least one metal selected from the group consisting of copper, iron or chromium.

7. Process according to any one of claims 1-6, wherein step (a) is carried out in a partial oxidation reactor.

8. Process according to claim 7, wherein the cooling in step (c) is performed by indirect heat exchange between the gas mixture resulting from step (b) on the one hand and water and/or the methane comprising gas on the other hand.

9. Process according to any one of claims 1-6, wherein step (a) is performed in an auto-thermal reformer (ATR) unit .

10. Process according to claim 9, wherein the methane comprising gas feed to the ATR unit is obtained by steam reforming of a gaseous methane comprising feed in a heat exchange reformer (HER) unit.

11. Process according to claim 10, wherein step (c) comprises feeding the gas mixture obtained in step (b) to the HER unit where it provides the necessary heat for the steam reforming of the gaseous methane comprising feed to the HER unit by indirect heat exchange.

12. Process according to any one of claims 1-6, wherein the oxidising gas in step (a) is steam and the reaction between the methane comprising gas and steam takes place in the presence of a steam reforming catalyst.

13. Process for the preparation of hydrocarbon products comprising the steps of:

(a) preparing a feed syngas comprising hydrogen and

carbon monoxide in a process according to any one of claims 1-12; and

(b) converting the feed syngas into hydrocarbon products in a Fischer-Tropsch process.