Process and system for reforming a hydrocarbon gas

Abstract

The invention relates to a process for reforming a hydrocarbon feed stream comprising a hydrocarbon gas and steam, said process comprising the steps of: a) in a synthesis gas generation reactor carrying out a reforming reaction of the hydrocarbon feed stream over a first catalyst, thereby forming a first synthesis gas; b) providing a heated CO.sub.2 rich stream to a post converter comprising a second catalyst; and c) in said post converter carrying out a methanation, steam reforming and reverse water gas shift reactions of the first synthesis gas and the heated CO.sub.2 rich stream to produce a product synthesis gas, wherein said second catalyst is heated electrically by means of an electrical power source. The invention moreover relates to a system arranged to carry out the process of the invention.

Claims

1. A process for reforming a hydrocarbon feed stream comprising a hydrocarbon gas and steam, said process comprising the steps of: a) in a synthesis gas generation reactor, optionally comprising a first catalyst, generating a first synthesis gas from the hydrocarbon feed stream; b) providing a heated CO.sub.2 rich stream to a post converter comprising a second catalyst active for steam reforming, methanation and reverse water gas shift reactions; and c) in said post converter carrying out steam reforming, methanation and reverse water gas shift reactions of said first synthesis gas and said heated CO.sub.2 rich stream to produce a product synthesis gas, wherein said second catalyst is heated electrically by means of an electrical power source, wherein the steam reforming, methanation and reverse water gas shift reactions run towards equilibrium in the post-converter.

2. The process according to claim 1, wherein said second catalyst is heated by resistance heating and/or inductive heating.

3. The process according to claim 1, wherein the synthesis gas generation reactor is a steam methane reforming reactor comprising a heat source arranged to heat said first catalyst within at least one reformer tube to a temperature sufficient to ensure that the first synthesis gas exiting the steam methane reforming reactor has a temperature of between about 650? C. and about 950? C.

4. The process according to claim 3, wherein the steam methane reforming reactor is heated by resistance heating and/or inductive heating.

5. The process according to claim 1, wherein the synthesis gas generation reactor is an autothermal reforming reactor with operating conditions adjusted to ensure that the first synthesis gas exiting the autothermal reforming reactor has a temperature of between 900? C. and 1100? C.

6. The process according to claim 1, wherein in step b) the amount and/or composition of said heated CO.sub.2 rich stream added is adjusted to ensure that the H.sub.2/CO ratio of said product synthesis gas is below 2.5.

7. The process according to claim 1, wherein the mole ratio between CO.sub.2 in the heated CO.sub.2 rich stream and hydrocarbons in the hydrocarbon feed stream is larger than 0.5.

8. The process according to claim 7, wherein the hydrocarbon feed stream further comprises one or more of the following: hydrogen, carbon monoxide, carbon dioxide, nitrogen, argon, higher hydrocarbons or combinations thereof.

9. The process according to claim 1, wherein the steam-to-carbon ratio of the hydrocarbon feed stream is between about 0.4 and about 2.0.

10. The process according to claim 1, wherein the heated CO.sub.2 rich stream comprises: least 50 dry mole % CO.sub.2, preferably at least 70 dry mole % CO.sub.2, and most preferably at least 90 dry mole % CO.sub.2.

11. The process according to claim 1, wherein the heated CO.sub.2 rich stream further comprises one or more of the following: steam, hydrogen, methane, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, argon or combinations thereof.

12. The process according to claim 1, wherein the heated CO.sub.2 rich stream is heated to a temperature of between about 350? C. and about 950? C. prior to addition thereof to the first synthesis gas.

13. A system for reforming of a hydrocarbon feed stream comprising a hydrocarbon gas and steam, said system comprising: a synthesis gas generation reactor, optionally comprising a first catalyst, and arranged to generate a first synthesis gas from said hydrocarbon feed stream, a post converter housing a second catalyst active for steam reforming, methanation and reverse water gas shift reactions, wherein the system is configured for the steam reforming, methanation and reverse water gas shift reactions to run towards equilibrium in the post-converter, a conduit for conducting the first synthesis gas to said post converter, means for adding a heated CO.sub.2 rich stream to the first synthesis gas upstream of said post converter and/or for adding a heated CO.sub.2 rich stream directly into said post converter, wherein said system comprises an electrical power source arranged for heating said second catalyst electrically.

14. The system according to claim 13, wherein the synthesis gas generation reactor is a steam methane reforming reactor comprising a heat source arranged to heat said first catalyst within at least one reformer tube to a temperature sufficient to ensure that the first synthesis gas exiting the steam methane reforming reactor has a temperature of between about 650? C. and about 950? C.

15. The system according to claim 13, wherein said synthesis gas generation reactor is an autothermal reforming reactor with operating conditions adjusted to ensure that the first synthesis gas exiting the autothermal reforming reactor has a temperature of between 900? C. and 1100? C.

16. The system according to claim 13, wherein said post converter is arranged to be heated by resistance heating and/or inductive heating.

17. The system according to claim 13, wherein said first catalyst is a reforming catalyst.

18. The system according to claim 13, wherein said second catalyst is a catalyst active for steam methane reforming, methanation and reverse water gas shift reactions.

19. The process according to claim 1, wherein the steam reforming, methanation and reverse water gas shift reactions all reach equilibrium in the post-converter.

20. The system according to claim 13, wherein the system is configured for the steam reforming, methanation and reverse water gas shift reactions to all reach equilibrium in the post-converter.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) An embodiments of the present invention is explained, by way of example, and with reference to the accompanying drawing. It is to be noted that the appended drawing illustrates only an examples of an embodiment of this invention and is therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

(2) FIG. 1 is a schematic drawing illustrating a system according to the invention.

DETAILED DESCRIPTION

(3) The following is a detailed description of embodiments of the invention depicted in the accompanying drawing. 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.

(4) FIG. 1 is a schematic drawing illustrating a system 100 for reforming of a hydrocarbon feed stream comprising a hydrocarbon gas and steam, according to the invention. The system 100 comprises a synthesis gas generation reactor 10, in this example a steam methane reformer (SMR). The SMR reactor 10 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 SMR reactor 10 has a plurality of reformer tubes (only one tube is shown in FIG. 1) housing reforming catalyst. The SMR reactor 10 has an inlet for feeding a hydrocarbon feed stream 3, e.g. a hydrocarbon gas stream 1 combined with steam 2, into the reformer tubes and an outlet for outletting a first synthesis gas 4 from the SMR reactor 10. The heat source and the operating conditions are arranged to heat the catalyst within the at least one reformer tube to a temperature sufficient to ensure that the first synthesis gas exiting the synthesis gas generation reactor has a temperature of between about 650? C. and about 950? C.

(5) The system 100 moreover comprises a post converter 20 housing a second catalyst 25. The second catalyst 25 is active in catalyzing the steam methane reforming, methanation and reverse water gas shift reactions.

(6) The system moreover comprises a heater (not shown), for example a fired heater, for heating a CO.sub.2 rich stream to a heated CO.sub.2 rich stream 5. A conduit connects the outlet from the SMR reactor 10 to the inlet to the post converter 20. The heated CO.sub.2 rich stream 5 is added to the first synthesis gas 4 upstream of the post converter 20, thereby producing a mixed gas stream 6. This mixed gas stream 6 is inlet into the post converter 20, and the product synthesis gas 7 exits the reactor 20 as a product gas. The product synthesis gas 7 may undergo further processing downstream of the post converter 20.

(7) The post converter 20 serves to equilibrate the mixed gas stream 6 and thereby to increase the CO production and to decrease the H.sub.2/CO ratio of the resulting product synthesis gas 7 compared to the first synthesis gas 4.

(8) A power source 30 is provided and electrical lines 31 are provided between the power source 30 and the post converter 20 and/or between the power source and the second catalyst 25 within the post converter 20. In the case, where the second catalyst 25 is arranged to be heated by resistance heating, the electrical lines 31 connect the power source and the second catalyst 25. In the case, where the second catalyst 25 is arranged to be heated by inductive heating, the power source is arranged to supply alternating current to an induction coil surrounding the second catalyst 25 or at least part of the post converter in order to generate an alternating magnetic field within at least a part of the second catalyst 25.

(9) In the embodiment shown in FIG. 1, the heated CO.sub.2 rich stream 5 is added to the first synthesis gas stream to a mixed gas stream 6 prior to being let into the post converter 20. However, alternatively, the heated CO.sub.2 rich stream 5 and the first synthesis gas 4 may be added separately into the post converter 20 for mixing therein upstream the bed of second catalyst 25.

(10) In the embodiment shown in FIG. 1, the SMR reactor 10 is a steam methane reforming reactor. Alternatively, the synthesis gas generation reactor 10 could be an autothermal reforming reactor.

EXAMPLE

(11) An example calculation of the process is given in Table 1 below. A hydrocarbon feed stream comprising a hydrocarbon gas, CO.sub.2 and steam and having a S/C ratio of 1.0 is fed to the SMR reactor 10 of the invention as shown in FIG. 1. This hydrocarbon feed stream is heated to 650? C. prior to being let into the SMR reactor 10, and within the SMR reactor 10 the gas is reformed and exits the SMR reactor 10 as the first synthesis gas having a temperature of 950? C.

(12) The equilibrium temperature of the methane decomposition reaction to graphitic carbon for the given composition of the first synthesis gas is 994? C. and the equilibrium temperature of the Boudouard reaction to graphitic carbon of the first synthesis gas is 927? C. Thus, the temperature of the first synthesis gas is below the equilibrium temperature of the methane decomposition reaction and above the equilibrium temperature of the Boudouard reaction, and consequently the first synthesis gas (or the gas within the SMR) does not have affinity for carbon formation.

(13) In this context, the methane decomposition temperature (T(MDC)) is calculated as the temperature where the equilibrium constant of the methane decomposition into graphite (CH.sub.4.Math.C+2H.sub.2) equals the reaction quotient of the gas. Formation of graphitic carbon can take place when the temperature is higher than this temperature. The reaction quotient QC is defined as the ratio of the square of the partial pressure of hydrogen to the partial pressure of methane, i.e. QC=P.sup.2.sub.H2/P.sub.CH4.

(14) The Boudouard equilibrium temperature (T(BOU)) is calculated in a similar way, but from the Boudouard reaction (2CO.Math.C+CO.sub.2) and in this case formation of graphitic carbon can take place when the temperature is lower than this temperature.

(15) A CO.sub.2 gas is heated to 650? C. to form a heated CO.sub.2 rich stream in the form of a heated CO.sub.2 gas and the combined gas (the first synthesis gas and the heated CO.sub.2 gas) enters the post converter 20 at a temperature of 791? C.

(16) Within the post converter 20 the combined stream is equilibrated, viz. it undergoes reverse water gas shift, methanation and reforming reactions, thereby forming the product synthesis gas. The exit temperature of the product synthesis gas exiting the post converter is controlled to 950? C. by the electrical heating, which is well below the methane decomposition equilibrium temperature for the gas of 1205? C. and above the Boudouard temperature for the gas of 845? C. Consequently, the product synthesis gas (or the gas within the post converter) does not have potential for carbon formation.

(17) TABLE-US-00001 TABLE 1 SMR CO.sub.2 Post reactor 10 preheater converter 20 Inlet T [? C.] 650 791 Outlet T [? C.] 950 650 950 Inlet P [kg/cm.sup.2g] 26 25.5 Outlet P [kg/cm.sup.2g] 25.5 26 25 Outlet T(MDC) [? C.] 994 1205 Outlet T(BOU) [? C.] 927 845 Inlet: CO.sub.2 [Nm.sup.3/h] 0 2600 2654 CH.sub.4 [Nm.sup.3/h] 1000 0 322 H.sub.2 [Nm.sup.3/h] 0 0 2089 CO [Nm.sup.3/h] 0 0 625 H.sub.2O [Nm.sup.3/h] 1000 0 268 Outlet: CO.sub.2 [Nm.sup.3/h] 54 2600 1286 CH.sub.4 [Nm.sup.3/h] 322 0 28 H.sub.2 [Nm.sup.3/h] 2089 0 1602 CO [Nm.sup.3/h] 625 0 2287 H.sub.2O [Nm.sup.3/h] 268 0 1342 Total flow [Nm.sup.3/h] 3358 2600 6545

(18) Thus, when the system and process of the invention are used, it is possible to provide a synthesis gas stream having a relative high amount of CO. In the example of Table 1, the H.sub.2/CO ratio is 0.7.

(19) Production of a similar synthesis gas in a standalone steam methane reformer would need large amounts of H.sub.2O and CO.sub.2 as co-feeds to achieve the same synthesis gas, as illustrated by Table 2. In the comparative example specified in Table 2 the same product synthesis gas as the product synthesis gas of Table 1 is achieved in a steam methane reformer. However, a very large feed of H.sub.2O and CO.sub.2 has to be added to the steam methane reformer to avoid carbon formation which results in a large SMR. In the example of Table 2 this is illustrated by an outlet synthesis gas flow of the SMR of 10308 Nm.sup.3/h, compared to only 6545 Nm.sup.3/h out of the system of Table 1 for production of practically the same amount of H.sub.2 and CO. Consequently, the concept of the invention enables a much smaller steam methane reformer design. This indicates that the invention is also useful for revamps.

(20) TABLE-US-00002 TABLE 2 Standalone SMR Inlet T [? C.] 400 Outlet T [? C.] 950 Inlet P [kg/cm.sup.2g] 26 Outlet P [kg/cm.sup.2g] 25.5 Outlet MDC T [? C.] Inlet: CO.sub.2 [Nm.sup.3/h] 4477 CH.sub.4 [Nm.sup.3/h] 973 H.sub.2 [Nm.sup.3/h] 0 CO [Nm.sup.3/h] 0 H.sub.2O [Nm.sup.3/h] 2920 Outlet: CO.sub.2 [Nm.sup.3/h] 3159 CH.sub.4 [Nm.sup.3/h] 5 H.sub.2 [Nm.sup.3/h] 1588 CO [Nm.sup.3/h] 2287 H.sub.2O [Nm.sup.3/h] 3269 Total flow [Nm.sup.3/h] 10308