WO2017211885A1

WO2017211885A1 - Co rich synthesis gas production

Info

Publication number: WO2017211885A1
Application number: PCT/EP2017/063827
Authority: WO
Inventors: Peter Mølgaard MORTENSEN; Martin ØSTBERG
Original assignee: Haldor Topsøe A/S
Priority date: 2016-06-10
Filing date: 2017-06-07
Publication date: 2017-12-14

Abstract

The invention relates to a process for reforming a first feed stream comprising a hydrocarbon gas and steam. The process comprises the steps of: a) in a first reactor comprising a first catalyst, carrying out a reforming reaction of the first feed stream over a first catalyst into a reformed gas stream, b) adding a second feed stream to said reformed gas stream to provide a mixed stream, where said second feed stream comprises at least 50 dry mol% CO₂ and where said second feed stream is prior to the addition, c) in a second, adiabatic reactor comprising a second catalyst, carrying out a reverse water gas shift of said mixed stream to produce a product gas stream, wherein in step b) the amount of said second feed stream added is sufficient to ensure that the H₂/CO ratio of said product gas stream is below 2. The invention moreover relates to a system for carrying out the process of the invention.

Description

Title: CO rich synthesis gas production

FIELD OF THE INVENTION Embodiments of the invention generally relate to process and a system for reforming of a feed stream comprising a hydrocarbon gas and steam. In particular, the invention relates to a reforming process and system aimed at producing a reformed stream with a low H2/CO ratio. BACKGROUND

Catalytic synthesis gas production from a hydrocarbon feed stream has been known for decades. It is also known that carbon formation on the catalyst used is a challenge, especially for production of synthesis gasses with a relatively low H2/CO ratio. Therefore, catalysts resistant to carbon formation have traditionally been required. Such carbon resistant catalysts are e.g. noble metal catalysts, partly passivated nickel catalysts and promoted nickel catalysts. Moreover, industrial scale reforming of CO2 rich gas typically requires a co-feed of water to decrease the severity of the gas for carbon formation. Alternatively, sulfur pas- sivated reforming (SPARG) process may be used for producing synthesis gas with a relatively low H2/CO ratio. See e.g. "Industrial scale experience on steam reforming of CO2-rich gas", P.M. Mortensen & I. Dybkjasr, Applied Catalysis A: General 495 (2016), 141 -151 . SUMMARY OF THE INVENTION

In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments, and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to "the invention" shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

An aspect of the invention provides a process for reforming a first feed stream comprising a hydrocarbon gas and steam. The process comprises the steps of: a) in a first reactor comprising a first catalyst, carrying out a reforming reaction of the first feed stream over a first catalyst into a reformed gas stream, b) adding a second feed stream to the reformed gas stream to provide a mixed stream, where said second feed stream comprises at least 50 dry mol% CO2 and where said second feed stream is heated prior to the addition,

c) in a second, adiabatic reactor comprising a second catalyst, carrying out a reverse water gas shift of said mixed stream to produce a product gas stream, wherein in step b) the amount of said second feed stream added is sufficient to ensure that the H2/CO ratio of said product gas stream is below 2. The process of the invention is a two-step process taking place in two different reactors, where a hot CO2 rich gas is added in between the two reactors. The second feed stream is added to the reformed gas stream where the hydrocarbon gas in the first feed stream has already been at least partly reformed. This partly reformed first feed stream is thus mixed with the second feed stream. This mixing allows the H/C ratio and the O/C ratios of the gas within the second reactor to differ from the H/C and O/C ratios within the first reactor and thereby renders it possible to circumvent the conditions where carbon formation on the catalyst is probable.

Within this context the term S/C is an abbreviation for the steam-to-carbon ratio. The steam-to-carbon ratio is the ratio of moles of steam to moles of carbon in the reformer feed. Thus, S/C is the total number of moles of steam added divided by the total number of moles of carbon from the hydrocarbons in the feed. The term O/C is an abbreviation for the atomic oxygen-to-carbon ratio. The oxygen-to-carbon ratio is the ratio of moles of oxygen to moles of carbon in a gas. The term H/C is an abbreviation for the atomic hydrogen-to-carbon ratio. The hydrogen-to-carbon ratio is the ratio of moles hydrogen to moles of carbon in a gas. It should be noted that the term "C" in the ratio S/C thus is different from the "C" in the ratios H/C and O/C, since in S/C "C" is from hydrocarbons only, whilst in O/C and H/C, "C" denotes all the carbon in the stream.

Since the second feed stream is heated prior to introduction thereof into the reformed gas stream, the carbon formation area of the carbon limit diagram (see Fig. 3) can be circumvented and a synthesis gas can be produced at more critical conditions than typical reforming. For example, the second feed stream is heated to about 800°C prior to being added into the reformed gas stream.

When the second feed stream is a CO2 rich gas, a CO rich synthesis gas at low S/C ratios is produced, whilst alleviating problems of carbon formation on the catalyst material. Within this context the term "a CO2 rich gas" is meant to de- note a gas comprising at least 50 dry mol% CO2.

In step b) the amount and/or composition of the added second feed stream is sufficient to ensure that the H2/CO ratio of the product gas stream is below 2. When the second feed stream has a very high content of CO2, such as e.g. 80 dry mol% CO2 or more, the amount of the second feed stream sufficient to ensure that the H2/CO ratio of the product gas stream is below 2 is less than in a case where the second feed stream has a lower dry mol% CO2, such as e.g. 50 dry mol% CO2. Advantageously, the amount and/or composition of the added second feed stream is sufficient to ensure that the H2/CO ratio of the product gas stream is below 1 .5 or even below 1 . Typically, the catalyst within the reformer tube is a reforming catalyst, e.g. a nickel based catalyst. The second catalyst may be a reforming catalyst or a selective reverse water gas shift catalyst. The first and second catalysts can thus be identical or different. Examples of reforming catalysts could be Ni/MgAl2O₄, Ni/CaAI₂O , Ru/MgAI₂O , Rh/MgAI₂O , lr/MgAI₂O , Mo₂C, Wo₂C, CeO₂, but other catalysts suitable for reforming are also conceivable.

In an embodiment, the mole fraction between CO₂ in the second feed stream and hydrocarbons in the first feed stream is larger than 0.5. A ratio between CO₂ in the second feed stream and hydrocarbons in the first feed stream may e.g. be about 1 :1 , about 2:1 , about 3:1 , about 4:1 , about 5:1 , about 6:1 or even higher.

The first feed stream may further comprise hydrogen, carbon monoxide, carbon dioxide, nitrogen, argon, higher hydrocarbons, or combinations thereof, in addi- tion to the hydrocarbon gas and steam.

In an embodiment, the mole fraction between the steam and hydrocarbons in the first feed stream is between about 0.7 and about 2.0. In an embodiment, the second feed stream comprises: at least 90 dry mol%

CO₂. The second feed stream may be substantially pure CO₂.

In an embodiment, the second feed stream further comprises one or more of the following: steam, hydrogen, methane, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, argon and combinations thereof. Such a second feed stream could for example be a recycle gas stream from a reducing process. In an embodiment, a heat source heats the catalyst within the reformer tube to a temperature of between about 650°C and about 950°C. Typically, the pressure within the reformer tube is above 1 barg and below 35 barg, for example between 25 and 30 barg.

In an embodiment, the second feed stream is heated to a temperature of between about 750°C and about 1200°C prior to addition thereof to the reformed gas stream. Hereby, the carbon formation area of the carbon limit diagram (see Fig. 3) can be circumvented and a synthesis gas can be produced at more criti- cal conditions than typical reforming. For example, the second feed stream is heated to about 800°C prior to being added into the reformed gas stream.

An aspect of the invention provides a system for reforming of a first feed stream comprising a hydrocarbon gas and steam. The system comprises a first re- former with a shell comprising one or more heat sources, at least one reformer tube housing a first catalyst. The first reformer comprises an inlet for feeding the first feed stream into said at least one reformer tube and an outlet for outletting a first reformed gas stream from the first reformer. The heat source is arranged to heat the catalyst within the at least one reformer tube to a maximum temper- ature of at least 750°C. The system further comprises a second reactor housing a second catalyst, where the second reactor is an adiabatic reactor, and a conduct for conducting the partly reformed gas stream to the second reactor, where the conduct comprises an inlet for adding a second feed stream to the first reformed gas stream upstream of the second reactor.

The system of the invention thus supports a two-step process taking place in two different reactors, where e.g. a hot CO2 rich gas is added in between the two reactors. The second feed stream is added to the reformed gas stream where the hydrocarbon gas in the first feed stream has already been at least partly reformed. This partly reformed hydrocarbon feed stream is thus mixed with the second feed stream. This mixing allows the H/C ratio and the O/C ratios of the gas within the second reactor to differ from the H/C and O/C ratios within the first reactor and thereby renders it possible to circumvent the conditions where carbon formation on the catalyst is probable. It should be understood that the term "an inlet" and "an outlet" is not intended to be limiting. Thus, these terms also cover the possibility where the units have more than one inlet and/or outlet. For example, a reformer tube could have an inlet for hydrocarbon gas and another inlet for steam, so that the hydrocarbon gas and steam is only mixed within the reformer tube.

Typically, the catalyst within the reformer tube is a reforming catalyst, e.g. a nickel based catalyst. The second catalyst may be a reforming catalyst or a selective reverse water gas shift catalyst. The first and second catalysts can thus be identical or different.

In an embodiment, the second reactor is a reverse water gas shift reactor.

The expression "to heat the catalyst within the reformer tube to a maximum temperature of at least 750°C" is meant to denote that the catalyst and thus the gas within the first reactor are heated and that the temperature of the catalyst may not be identical throughout the reactor. Thus, the temperature of the cata- lyst may differ and a maximum temperature thereof is the maximum temperature of the catalyst.

The term "hydrocarbon gas" is meant to denote a gas comprising one or more hydrocarbon gasses, and possibly other gasses. For reforming processes, an example of a "first feed stream comprising a hydrocarbon gas and steam" is e.g. a mixture of methane, steam and possibly other oxidizing gasses, such as carbon dioxide, oxygen, or mixtures thereof. Examples of "a hydrocarbon gas" may be natural gas, town gas or a mixture of methane and higher hydrocarbons. The term "second feed stream" is meant to denote another stream than the "first feed stream". Thus, the second feed stream may be any appropriate gas stream suitable for supporting reforming reaction within a reforming reactor. The term "downstream" as used in this text is meant to denote at "a later point or position in a process or system", whilst the term "upstream" is meant to denote "at an earlier point or position in a process or system".

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawings. It is to be noted that the appended drawings illustrate only examples of embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may ad- mit to other equally effective embodiments.

Figs. 1 and 2 are schematic drawing illustrating systems according to the invention; and

Fig. 3 is a carbon limit diagram illustrating carbon limits in different scenarios.

DETAILED DESCRIPTION

The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

Fig. 1 is a schematic drawing illustrating a system 10 for reforming of a first feed stream comprising a hydrocarbon gas and steam, according to the invention. The system 10 comprises a first reactor 12, in this example a steam methane reformer (SMR). The first reactor 12 contains one or more heat sources and may be a conventional fired steam methane reformer, such as a side fired, top fired, bottom fired or terrace fired reformer. The reactor 12 has a plurality of reformer tubes (not shown) housing reforming catalyst. The first reactor 12 has an inlet for feeding a first feed stream 21 , e.g. a hydrocarbon feed stream combined with steam, into the reformer tubes and an outlet for outletting a first reformed gas stream 22 from the reformer 12. The heat source is arranged to heat the catalyst within the reformer tube to a maximum temperature of at least 750°C. The system 10 moreover comprises a second reactor 16 housing a second catalyst. The second reactor 16 is an adiabatic reverse water gas shift reactor.

The system moreover comprises a heater 14, for example a fired heater, for heating a second feed stream 24 to a heated second feed stream 25. A conduct connects the outlet from the first reactor 12 to the inlet to the second reactor 16. The heated second feed stream 25 is added to the first reformed gas stream 22 upstream of the second reactor 16, thereby producing a mixed stream 26. This mixed stream 26 is inlet into the adiabatic reverse water gas shift reactor 16, and the resultant gas stream 28, a second reformed gas stream, exits the reactor 16 as a product gas.

Fig. 2 is a schematic drawing illustrating an alternative system 100 according to the invention. The system 100 of Fig. 2 comprises the elements of the system 10 shown in Fig. 1 , and these will thus not be described in detail again. Moreover, similar elements or units of the systems 10 and 100 are denoted with similar reference numbers. The system 100 thus comprises a first fired reactor 12 in the form of a steam methane reformer arranged to reform a first feed stream 21 in the form of a hydrocarbon feed stream combined with steam into a reformed gas stream 22. A CO2 rich second feed stream 24 is heated in a heater 14 to a heated CO2 rich stream 25 and subsequently added to the reformed gas stream 22 to provide a mixed gas stream 26. The mixed gas stream 26 is inlet into an adiabatic reverse water gas shift (RWGS) reactor 16, resulting in a second reformed gas stream 28 exiting the reverse water gas shift reactor 16. Down- stream of the RWGS reactor 16, a third CO2 rich stream 124 is heated in a heater 1 14 to a heated third CO2 rich stream 125 which is added to the second reformed gas stream 28 to obtain a second mixed stream 30. The third CO2 rich stream may have been heated prior to addition to the second reformed gas stream 28 (as shown in Fig. 2) and/or the third CO2 rich stream is added to the second reformed gas stream prior to heating in the heater 1 14. The second mixed stream 30 is feed into a second adiabatic reverse water gas shift reactor 1 16, resulting in a product gas 128. The product gas 128 from the second adiabatic reverse water gas shift reactor 1 16 may be the final product gas, or furthers step of adding further heated CO2 and equilibrating the resulting gas in yet further adiabatic reverse water gas shift reactor(s) or reforming reactor(s) are possible.

The water gas shift reactors 16 and 1 16 serve to equilibrate the gas and thereby to decrease the H2/CO ratio of the resulting gas

In Table 1 below is a calculated example of the gas compositions and tempera- tures for a natural gas (NG) feed entering the system of Fig. 3 as stream 21 and undergoing reforming reaction in the steam methane reactor 12 and subsequent equilibrating in two reverse water gas shift reactors (RWGS) with intermediate addition of heated CO2 rich gas. Table 1 : Example

Fig. 3 is a carbon limit diagram illustrating carbon limit in different scenarios. Such a carbon limit diagram is also denoted a "T0ttrup diagram". In general, it is essential to design a refornning plant to avoid carbon fornnation on the catalyst in the reformer tubes of the reformer reactor. In this diagram a given gas composition will have a fixed H/C and O/C ratio, which is shown on the x- and y-axis, independently of how far the reforming reactions have proceeded. As example, a feed gas containing 44% CH₄, 46% H₂O, 5% H₂, 4% CO₂, and 1 % CO has a

H/C and O/C ratio of 5.67 and 1 .12, respectively. Reforming this gas to an equilibrium at 950°C and 25 bar would give a gas composition 8% CH₄, 9% Η₂Ο, 61 % H₂, 2% CO₂, and 20% CO; however, the H/C and O/C ratios of 5.67 and 1 .12, respectively, have not changed. Additionally, the diagram contains axis which shows the composition of a gas with a given H/C and O/C ratio normalized to a feed of only H₂O, CH₄, and CO₂, as "H₂O/CH₄" and "CO₂/CH₄" axis. As example, the gas above with an H/C and O/C ratio of 5.67 and 1 .12, respectively, would correspond to a normalized gas with "H₂O/CH₄" and "CO₂/CH₄" of 1 .05 and 0.08, respectively.

Carbon formation in the tubes of a reforming reactor, also denoted "reformer", is dictated by thermodynamics and in a typical design of a reformer it is a requirement that the reformer does not have affinity for carbon formation of the equilibrated gas anywhere in the catalyst bed. This means that the process gas or feed stream will have to be balanced with water in order to circumvent the carbon formation area. Typically, the process gas enters the reformer at 400- 500°C, while leaving the reformer at about 950°C (not experiencing temperatures above 1000°C). Thus, when designing a reformer, there must not be an affinity for carbon formation of the equilibrated gas anywhere in the temperature range from 400°C to 1000°C. This criterion can be used to evaluate the carbon limit of the reformer, as illustrated by curve 2 in the carbon limit diagram in Fig. 3.

If potential for carbon formation exists, it will only be a matter of time before a shutdown of the reactor is necessary due to too high pressure drop. In an industrial context, this will be expensive due to lost time on stream and loading of a new batch of catalyst into the reformer tubes. Carbon formation at reforming conditions is as whisker carbon. This is destructive in nature toward the catalyst pellets and regeneration of the catalyst is therefore not an option. Thus, the possible operating range for a tubular reformer will be defined by the conditions which will not have a potential for carbon formation. When sufficient knowledge 5 about the thermodynamics of carbon formation for a specific catalyst is known, the exact limit for carbon formation can be calculated and this can be illustrated by the carbon limit curves depicted in the carbon limit diagram of Fig. 3. The carbon limit for graphite is shown as the dotted curve 1 , whilst the carbon limit for an industrial nickel catalyst is shown as the curve 2. As the carbon limits 10 have to be defined in a worst case scenario, the curve for industrial nickel catalyst represents a nickel catalyst aged for several years in a reforming plant where the catalyst has been severely sintered. The curves are derived from the principle of equilibrated gas and show the most severe conditions (as a function of initial h O/ChU and CO2/CH₄ ratios, or O/C and H/C ratios) which can be toll s erated in the entire temperature range from 400°C to 1000°C at a pressure of 25.5 bar. Carbon formation will be expected to the left of the curves and operation without risk of carbon formation will be expected to the right of the curves. This shows that the tendency for carbon formation increases with decreasing CO2/CH₄ and h O/ChU ratios. The severity of operation can be defined relative 20 to the placement compared to the carbon limit curves; operation to the left of and far beyond the carbon limit curve is considered very severe.

The dotted lines (4a-e) in Fig. 3 show the equilibrated H2/CO ratio of a synthesis gas produced at 950°C and 25.5 bar as a function of the O/C and H/C ratio or

25 normalized CO2/CH₄ and h O/ChU ratios. The lines show that the H2/CO ratio increases with increasing H/C ratio, as it is around 2.5 when H/C is around 5, while being around 0.5 when H/C is around 1 . These lines additionally translate into the normalized "H₂O/CH₄" and "CO₂/CH₄" ratio, which shows that the H₂/CO ratio of the product gas can be controlled by adjusting the addition of H2O and

30 CO2, where more H2O will increase the product towards hydrogen rich gas and more CO2 will increase the product towards CO rich gas. However, when producing a synthesis gas with a very low H2/CO ratio, an accompanied high h O/ChU will be necessary to balance the severity of the gas to avoid carbon formation on a nickel catalyst. From Fig. 3 it can be seen that producing a syn- thesis gas with a H2/CO ratio below 1 requires a large excess of water to avoid carbon formation. As example, to produce a synthesis gas of H2/CO=0.7 in a standard reformer with a nickel catalyst will require a feed composition of H₂O/CH₄ = 3 and CO₂/CH₄ = 4.5. A principle of the current invention is illustrated by the third carbon limit curve (3) in the Fig. 3. Where the normal reforming case confines the temperature of the reactor to be between 400°C and 1000°C, the concept of this invention utilizes that this limit can be moved if the temperature interval is changed. Thus, if the lower temperature limit is increased to 800°C, the limit for carbon formation will change accordingly, as shown by the difference in the two carbon limit curves for graphitic carbon, 1 and 3.

In a SPARG (Sulfur Passivated ReforminG) process, sulfur is used to selectively poison the most active sites and in this way prevent formation of carbon while maintaining some activity for reforming. Thus, the SPARG process offers a route to circumvent the carbon limit curves of Fig. 3. However, comparing the CO2 shifted process for reforming of the present invention to SPARG, the CO2 shifted process reforming has at least the advantage that sulfur does not need to be added, which makes the size of the system significantly smaller.

Alternatively, noble metal catalysts may be used to circumvent the carbon limits of Fig. 3 somewhat, since noble metals generally have a lower tendency for carbon formation compared to nickel catalysts. Noble metal catalyst thus offers a route for operation at severe conditions without carbon formation. However, no- ble metal catalysts are more expensive than nickel catalysts and to the best of our knowledge, very severe operation over noble metal catalysts has never been assessed. As an example, consider a case where a synthesis gas with H2/CO ratio of 0.7 is wanted. A mixture of steam and methane 21 (Fig. 1 ) is fed to the first reactor 12, and the S/C ratio is chosen with respect to the typical carbon limit for Ni cat- alysts (the curve in Fig. 3 with alternating dots and lines, viz. curve 2) and the desired synthesis gas. The first reactor 12 houses a typical reforming catalyst. Such reforming catalyst may be nickel based catalyst; however, practically any catalyst suitable for reforming could be used. To produce the desired gas, it is chosen to operate at a H2O/CH4 ratio of 1 , illustrated by the cross indicated by "Reformer inlet" in Fig. 3. A CO2 rich feed 24 (in the current example pure CO2) is heated in 14 to a hot CO2 rich feed 25 which is added to the first reformed gas stream. Downstream of the addition point of the CO2 rich gas, in the second reactor 16, the gas undergoes reverse water gas shift and leaves the second reactor 16 at a temperature of about 814°C and a H2/CO ratio of 0.7. In this case the overall process gas has ratios H2O/CH4 = 1 and CO2/CH₄ = 2.6. Because the gas is kept above 800°C from the addition point of CO2, it is not the carbon limit curve 2, which indicates the limit for carbon formation, but instead the carbon limit curve 3. As seen from Fig. 3, the new operating point (denoted "after CO2 addition" in Fig. 3) is placed on the right side of carbon limit curve 3 and carbon formation is therefore not expected. In order to achieve an outlet gas having a H2/CO ratio of 0.7 with a conventional reformer housing a nickel based catalyst, the overall process gas would have ratios H2O/CH4 = 3 and CO2/CH₄ = 4.5. Consequently, the co-feed of CO2 and H2O is significantly lower compared to the feed in the nickel based reformer case.

EXAMPLE: An example of the process is illustrated in Table 2 below. A first feed stream of hydrocarbons and steam having a S/C ratio of 1 is fed to the reforming reaction zone of a reformer 12 of the invention as shown in Fig. 1 . Within the first reformer, this first feed stream is heated to a temperature of 950°C and reformed to a first reformed stream 22. Subsequently, it is mixed with CO2 which has been heated to 1 100°C, for example by means of a fired heater. Prior to the mixing of the heated CO2 25 and the first reformed stream, the H2/CO ratio of the first reformed stream is 3.34. Subsequent to the mixing of the first reformed stream. The mixed gas 26 is fed to the second reactor 16 wherein the endother- mal reverse water gas shift takes place. The resultant gas stream, viz. the second reformed gas stream 28, exits the second reformer 16 at a temperature of about 814°C. The second reformed gas stream 28 is a synthesis gas of having a ratio H₂/CO = 0.7.

Table 2: Example of process.

Thus, when the chemical reactor, the reformer tube or the process according to the inventions is used, the problems of carbon formation during reforming of a CO2 rich gas are alleviated. This is due to the fact that the carbon limits are cir- cumvented as shown in the carbon limit diagram of Fig. 3 by carrying out the reforming as a two-step process and adding heated CO2 in between the two steps. In the Example described above, the second feed stream is a heated stream of pure CO2. Alternatively, the second feed stream could be a mixture of CO2, H2O, H2, CO, H2S and/or SO2. Such a second feed stream could for example be a recycle gas stream from a Reducing Gas process, as described below. REDUCING GAS PROCESS:

As mentioned, the reactor and the process of the invention are also suitable for reforming where the second feed stream is a recycle stream from a reducing gas process. As mentioned above, carbon formation in a reformer is dictated by thermodynamics and the catalyst in the reformer should not have affinity for carbon formation anywhere in the catalyst material.

In a traditional reformer, the input hydrocarbon feed stream would have to be balanced with water in order to circumvent the carbon formation area. Typically, the hydrocarbon feed stream enters a reducing gas reformer at a temperature of between about 500 and about 600°C, while leaving the reducing gas reformer at a temperature of about 950°C, at least not experiencing temperatures above 1000°C. Thus, when designing a reducing gas reformer, there must not be an affinity for carbon formation anywhere between 500-1000°C. The carbon for- mation is somewhat hindered by the presence of sulfur in the recirculated reducing gas containing sulfur from the metals to be reduced, but the process is limited by carbon formation at low H/C levels and from content of higher hydrocarbons in the feed. Higher hydrocarbons are meant to denote hydrocarbons with more than one carbon atom such as ethane, ethylene, propane, propylene etc. In the reformer system and the process according to the invention as used in connection with a reducing gas plant, the first feed stream comprising hydrocarbons and steam is inlet into the first reactor 12. This first reactor houses reforming catalyst, typically nickel based catalyst. The recycle feed stream from the re- ducing gas plant is fed as a second feed stream into a heater 14, where it is heated to a heated second feed stream 25. Subsequently, the heated second feed stream 25 is mixed with the first reformed gas stream from the reformer 12.

By the process and system of the invention, the reforming of the first feed stream will take place at conditions not leading to carbon formation and the addition of preheated recycled gas 25 from the reducing gas plant will enable production of a low H2/CO ratio gas.

The present invention describes that water (steam) is added to a hydrocarbon gas, typically natural gas, in order to enable steam reforming thereof. In a reducing gas plant, the recycle gas from the metal reduction furnace of the Reducing gas plant contains water. Therefore, water should be removed from this recycle gas stream and should be added to the hydrocarbon feed stream prior to the steam reforming of this stream. Some steam may be left in the recycle feed stream, viz. the second feed stream, in order to enable preheating of this stream prior to mixing it with the steam reformed process gas. However, in order to obtain low H2/CO ratios, it is preferable that the amount of water kept in the recycle feed stream is minimized. The reducing gas recycle stream typically comprises at least 50 dry mol% CO2 and one or more of the following: steam, hydrogen, methane, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, argon, and combinations thereof.

While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.

Claims

CLAIMS:

1 . A process for refornning a first feed stream comprising a hydrocarbon gas and steam, said process comprising the steps of:

a) in a first reactor comprising a first catalyst, carrying out a reforming reaction of the first feed stream over a first catalyst into a reformed gas stream, b) adding a second feed stream to said reformed gas stream to provide a mixed stream, where said second feed stream comprises at least 50 dry mol% CO2 and where said second feed stream is heated prior to the addition,

c) in a second, adiabatic reactor comprising a second catalyst, carrying out a reverse water gas shift of said mixed stream to produce a product gas stream, wherein in step b) the amount and/or composition of said second feed stream added is sufficient to ensure that the H2/CO ratio of said product gas stream is below 2.

2. A process according to claim 1 , wherein the first catalyst is a reforming catalyst.

3. A process according to claim 1 or 2, wherein the second catalyst is a reform- ing catalyst or a selective reverse water gas shift catalyst.

4. A process according to any of the claims 1 to 3, wherein the mole fraction between CO2 in the second feed stream and hydrocarbons in the first feed stream is larger than 0.5.

5. A process according to claim 4, wherein the first feed stream further comprises one or more of the following: hydrogen, carbon monoxide, carbon dioxide, nitrogen, argon, higher hydrocarbons or combinations thereof.

6. A process according to any of the claims 1 to 5, wherein the mole fraction between the steam and hydrocarbon gas in the first feed stream is between about 0.7 and about 2.0.

7. A process according to any of the claims 1 to 6, wherein the second feed stream comprises: at least 90 dry mol% CO2.

8. A process according to any of the claims 1 to 7, wherein the second feed stream further comprises one or more of the following: steam, hydrogen, methane, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, argon or combinations thereof.

9. A process according to any of the claims 1 to 8, wherein a heat source heats the catalyst within the reformer tube to a temperature of between about 650°C and about 950°C.

10. A process according to any of the claims 1 to 9, wherein the second feed stream is heated to a temperature of between about 750°C and about 1200°C prior to addition thereof to the reformed gas stream.

1 1 . A system for reforming of a first feed stream comprising a hydrocarbon gas and steam, said system comprising:

- a first reactor comprising a shell comprising one or more heat sources, at least one reformer tube housing a first catalyst, said first reactor comprising an inlet for feeding said first feed stream into said at least one reformer tube and an outlet for outletting a first reformed gas stream from said first reformer, wherein the heat source is arranged to heat the catalyst within the at least one reformer tube to a maximum temperature of at least 750°C;

- a second reactor housing a second catalyst, said second reactor being a adia- batic reformer,

- a conduct for conducting the partly reformed gas stream to said second reactor, where the conduct comprises an inlet for adding a second feed stream to the first reformed gas stream upstream of the second reactor.

12. A system according to claim 1 1 , wherein the first catalyst is a reforming catalyst.

13. A system according to claim 1 1 or 12, wherein the second catalyst is a re- forming catalyst or a reverse water gas shift catalyst.

14. A system according to any of the claims 1 1 to 13, wherein the second reactor is a reverse water gas shift reactor.