Process and system for producing synthesis gas

Abstract

A process for producing synthesis gas, the process including the steps of: a) in a reforming reactor, reacting a hydrocarbon feed stream together with an oxidant gas stream, thereby producing a first synthesis gas stream; b) providing a heated CO.sub.2 rich gas stream to an adiabatic post converter including a second catalyst active for catalyzing steam methane reforming, methanation and reverse water gas shift reactions; and c) in the adiabatic reforming post converter, letting at least a part of the first synthesis gas stream and the heated CO.sub.2 rich gas stream undergo steam methane reforming, methanation and reverse water gas shift reactions to thereby provide a product gas stream, the product gas stream being a synthesis gas stream. Also, a system for producing synthesis gas.

Claims

1. A process for producing synthesis gas, said process comprising the steps of: a) in a reforming reactor comprising a first catalyst, reacting a hydrocarbon feed stream together with an oxidant gas stream, thereby producing a first synthesis gas stream; b) providing a heated CO.sub.2 rich gas stream to an adiabatic post converter comprising a second catalyst active for catalyzing steam methane reforming, methanation and reverse water gas shift reactions; and c) in said adiabatic post converter, letting at least a part of the first synthesis gas stream and said heated CO.sub.2 rich gas stream undergo steam methane reforming, methanation and reverse water gas shift reactions to thereby provide a product gas stream, said product gas stream being a synthesis gas stream.

2. The process according to claim 1, wherein the reforming reactor is an autothermal reforming reactor.

3. The process according to claim 1, wherein the reforming reactor is a steam methane reforming reactor.

4. The process according to claim 1, wherein the product gas stream is a synthesis gas stream with an H.sub.2/CO ratio below 1.8.

5. The process according to claim 1, wherein said at least part of the first synthesis gas stream and said heated CO.sub.2 rich gas stream are combined to a combined gas stream upstream the adiabatic post converter.

6. The process according to claim 1, wherein the heated CO.sub.2 rich gas stream has a temperature of between about 500° C. and 1100° C. prior to combination with said at least part of the first synthesis gas stream and/or prior to being inlet into said adiabatic post converter.

7. The process according to claim 1, further comprising the step of heating a CO.sub.2 rich gas stream to form said heated CO.sub.2 rich gas stream in a fired heater.

8. The process according to claim 1, further comprising the step of heating a CO.sub.2 rich gas stream to form said heated CO.sub.2 rich gas stream in an electrically heated heater.

9. The process according to claim 1, further comprising the step of heating a CO.sub.2 rich gas stream to form said heated CO.sub.2 rich gas stream by heat exchange with superheated steam.

10. The process according to claim 1, further comprising the step of heating a CO.sub.2 rich gas stream to said heated CO.sub.2 rich gas stream by heat exchange with at least part of the product gas stream exiting the adiabatic post converter.

11. The process according to claim 1, wherein the mole ratio between carbon dioxide in said heated CO.sub.2 rich gas stream and hydrocarbons in the hydrocarbon feed stream is larger than 0.1.

12. The process according to claim 1, wherein said hydrocarbon feed stream comprises steam and the S/C ratio in said hydrocarbon feed stream is between 0.2 and 2.

13. The process according to claim 2, wherein the amount of steam, oxygen and carbon dioxide led into the reforming reactor and/or added to said at least part of the first synthesis gas stream upstream or in said adiabatic post converter is adjusted to provide a predetermined H.sub.2/CO ratio of said product gas stream.

14. The process according to claim 1, wherein the amount and temperature of the heated CO.sub.2 rich gas stream are adjusted to ensure that the temperature of said product gas stream is at least 800° C.

15. The process according to claim 1, wherein the second catalyst is a steam reforming catalyst.

16. A system for producing synthesis gas, said system comprising: a reforming reactor comprising a first catalyst and being arranged to at least partially react a hydrocarbon feed together with an oxidant gas stream thereby producing a first synthesis gas stream, an adiabatic post converter comprising a second catalyst active for catalyzing steam methane reforming, methanation and reverse water gas shift reactions, a conduit for leading at least part of the first synthesis gas stream into said adiabatic post converter, means for adding a heated CO.sub.2 rich gas stream to said at least part of the first synthesis gas stream upstream or in said adiabatic post converter; and an outlet from said adiabatic post converter for letting out a product gas stream, said product gas stream being a synthesis gas stream.

17. The system according to claim 16, wherein the reforming reactor is an autothermal reforming reactor.

18. The system according to claim 16, wherein the reforming reactor is a steam methane reforming reactor.

19. The system according to claim 16, wherein the product gas stream is a synthesis gas stream with an H.sub.2/CO ratio below 1.8.

20. The system according to claim 16, further comprising a fired heater, wherein said means for adding a heated CO.sub.2 rich gas stream comprises means for heating a CO.sub.2 rich gas stream by heat exchange within the fired heater.

21. The system according to claim 16, further comprising an electrically heated heater, wherein said means for adding a heated CO.sub.2 rich gas stream comprises a means for heating a CO.sub.2 rich gas stream by heat exchange within the electrically heated heater.

22. The system according to claim 16, further comprising a second heat exchange unit allowing heating a CO.sub.2 rich gas stream by heat exchange with superheated steam.

23. The system according to claim 16, further comprising a third heat exchange unit allowing heating a CO.sub.2 rich gas stream to said heated CO.sub.2 rich gas stream by heat exchange with at least part of the product gas stream exiting said adiabatic post converter.

24. The system according to claim 16, wherein the second catalyst is a steam reforming catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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 admit to other equally effective embodiments.

(2) FIGS. 1 to 3 are schematic drawings of systems for producing synthesis gas according to the invention.

DETAILED DESCRIPTION

(3) 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.

(4) FIG. 1 is a schematic drawing of a system 100 for producing synthesis gas according to the invention. The system 100 comprises an ATR reactor 10 and an adiabatic post converter 20.

(5) A hydrocarbon feed stream 4 to the ATR reactor 10 of the system 100 is made up of a stream of hydrocarbon gas 1, a CO.sub.2 rich gas stream 2, for example substantially pure CO.sub.2, and steam 3. The CO.sub.2 rich gas stream 2 and the steam 3 are added to the hydrocarbon gas stream 1, hereby forming a combined hydrocarbon feed stream 4 prior to inletting the combined hydrocarbon feed stream 4 into the ATR reactor 10. The ATR reactor 10 houses a first catalyst 11 in the form of a steam methane reforming catalyst. An oxygen containing stream 5, such as air, an oxygen rich stream or substantially pure oxygen, is inlet into the combustion zone of the ATR reactor 10 via an inlet. The ATR reactor 10 produces a first synthesis gas stream 6 comprising hydrogen, carbon monoxide and carbon dioxide from the combined hydrocarbon feed stream 4 and the oxygen containing stream 5. The first synthesis gas stream 6 exiting the ATR reactor typically has a temperature of between about 900° C. and about 1100° C., such as about 1000° C.

(6) The adiabatic post converter 20 houses a second catalyst 25 active in catalyzing the steam methane reforming, methanation and reverse water gas shift reactions. For example, the second catalyst 25 is a bed of second catalyst. Thus, in the adiabatic post converter 20 a net production of carbon monoxide, steam and methane takes place. Thus, the steam methane reforming reaction and reverse water gas shift reactions take place in the adiabatic post converter 20 together with the methanation reaction.

(7) The system moreover comprises a heater (not shown in FIG. 1) for heating a CO.sub.2 rich gas stream to form a heated CO.sub.2 rich gas stream 7. A conduct connects the outlet from the ATR reactor 10 to the inlet to the adiabatic post converter 20. The heated CO.sub.2 rich gas stream 7 is added to the first synthesis gas stream 6 upstream of the adiabatic post converter 20, thereby producing a mixed gas stream 8. This mixed gas stream 8 is inlet into the adiabatic post converter 20, and the product gas stream 15 exits the adiabatic post converter as a product synthesis gas. The product gas stream 15 may undergo further processing downstream of the adiabatic post converter 20. The product gas stream 15 is a synthesis gas.

(8) The adiabatic post converter 20 serves to equilibrate the mixed gas and thereby to decrease the H.sub.2/CO ratio of the product gas stream 15 compared to the H.sub.2/CO ratio of the first synthesis gas stream 6.

(9) In the embodiment shown in FIG. 1, the heated CO.sub.2 rich gas stream 7 is added to the first synthesis gas stream 6 to a mixed gas stream 8 prior to being let into the adiabatic post converter 20. However, alternatively, the heated CO.sub.2 rich gas stream 7 and the first synthesis gas stream 6 may be let separately into the adiabatic post converter 20 for mixing therein upstream the bed of second catalyst 25.

(10) FIG. 2 is a schematic drawing of a system 101 for producing synthesis gas according to the invention. The system 101 comprises the units/components of the system 100 shown in FIG. 1. Similar units are denoted by similar reference numbers and will not be described in detail here. The system 101 comprises a heater 30. The heater 30 may be a fired heater or an electrically heated heater. The heater 30 may be a heater used for preheating of the hydrocarbon feed stream upstream the ATR reactor 10 or it may be a separate heater. A CO.sub.2 rich gas stream 7′ is heated by heat exchange within the heater 30, thereby rendering the heated CO.sub.2 rich gas stream 7.

(11) FIG. 3 is a schematic drawing of a system 102 for producing synthesis gas according to the invention. The system 102 comprises the units/components of the system 100 shown in FIG. 1. Similar units are denoted by similar reference numbers and will not be described in detail here. The system 102 comprises a heat exchanger 40 downstream the adiabatic post converter 20. A CO.sub.2 rich gas stream 7′ is heated by heat exchange with the hot product gas stream 15 exiting the adiabatic post converter 20, thereby rendering the heated CO.sub.2 rich gas stream 7. The embodiments shown in FIGS. 2 and 3 may be combined, so that a CO.sub.2 rich gas stream is initially heated by a heater 30 and subsequently heated by heat exchange with the hot product gas stream 15. Moreover, the CO.sub.2 rich gas stream could be heated by heat exchange with superheated steam (not shown in the figures); in this case, the heat exchange with superheated steam would typically take place prior to the heating within a fired or electrically heated heater. A combination, wherein the CO.sub.2 rich gas stream is heated firstly with superheated steam and subsequently by heat exchange with the hot product gas stream 15 from the adiabatic post converter 20 is also conceivable.

Example

(12) An example calculation of the process is given in Table 1 below. A hydrocarbon feed stream 4 comprising a hydrocarbon gas 1, a CO.sub.2 rich gas stream 2 and steam 3 and having a S/C ratio of 0.6 is fed to the ATR reactor 10 of the invention as shown in FIG. 1. The hydrocarbon feed stream 4 is heated to 650° C. prior to being let into the ATR reactor 10. The ATR reactor 10 produces a first synthesis gas stream 6. An oxygen containing stream 5 is added to the ATR reactor and the amount thereof is adjusted such that the temperature of the first synthesis gas stream 6 is 1050° C.

(13) The total flow of all components in all inlet streams to the ATR reactor and the flow of all components in the first synthesis gas stream 6 are given in the column headed “ATR 10” in Table 1.

(14) A CO.sub.2 rich gas stream is heated to a heated CO.sub.2 rich gas stream having a temperature of 650° C. and the combined gas (the first synthesis gas stream and the heated CO.sub.2 gas rich stream) enters the adiabatic post converter 20 at a temperature of 969° C.

(15) Within the adiabatic post converter 20, the combined stream is equilibrated, viz. it undergoes reverse water gas shift, methanation and reforming reactions. The overall amount of carbon monoxide, steam and methane output from the adiabatic post converter 20 is increased compared to the gas inlet to it. The exit temperature of the product gas stream exiting the adiabatic post converter 20 is 951° C., which is well below the methane decomposition equilibrium temperature for the gas of 1195° C. and above the Boudouard temperature for the gas of 850° C. Consequently, the product gas stream does not have potential for carbon formation.

(16) 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.

(17) 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 Boudouard equilibrium temperature.

(18) TABLE-US-00001 TABLE 1 ATR 10 Adiabatic post converter 20 Inlet T [° C.] 650 969 Outlet T [° C.] 1050 951 Inlet P [kg/cm.sup.2g] 35.5 34.5 Outlet P [kg/cm.sup.2g] 34.5 34 Outlet T (MDC) [° C.] — 1195 Outlet T (BOU) [° C.] 892 850 Inlet: N.sub.2 [Nm.sup.3/h] 27 251 CO.sub.2 [Nm.sup.3/h] 8515 19356 CH.sub.4 [Nm.sup.3/h] 19222 391 H.sub.2 [Nm.sup.3/h] 405 32380 H.sub.2O [Nm.sup.3/h] 11639 17327 CO [Nm.sup.3/h] 0 21315 Oxygen feed: O.sub.2 [Nm.sup.3/h] 11018 N.sub.2 [Nm.sup.3/h] 224 Oxygen feed T [° C.] 371 Outlet: N.sub.2 [Nm.sup.3/h] 251 251 CO.sub.2 [Nm.sup.3/h] 6032 14597 CH.sub.4 [Nm.sup.3/h] 391 779 H.sub.2 [Nm.sup.3/h] 32380 26455 H.sub.2O [Nm.sup.3/h] 17327 22475 CO [Nm.sup.3/h] 21315 25685 Total outlet flow [Nm.sup.3/h] 77696 90242

(19) Thus, when the system and process of the invention are used, it is possible to provide a product gas stream in the form of a synthesis gas having a relative high amount of CO.

(20) A comparative example of the corresponding numbers for producing a similar synthesis gas in system with an ATR reactor but without an adiabatic post converter, here denoted “a stand alone ATR reactor”, is shown in Table 2. In this case, all CO.sub.2 is added up-front the ATR reactor which operates at a S/C of 0.6.

(21) A comparison of the examples of Table 1 and 2 shows that more oxygen is needed in the stand alone ATR reactor for production of a given amount of carbon monoxide.

(22) From Table 1 and Table 2, it is also seen that the outlet flow from the ATR reactor in the case of the present invention is smaller than with a stand alone ATR reactor. This means that a smaller ATR reactor can be designed in the case of the invention. This also means that in case of revamps, the production of carbon monoxide can be boosted without the need for enlarging a given ATR reactor. This is done by adding the adiabatic post converter according to the invention.

(23) TABLE-US-00002 TABLE 2 Stand alone ATR Inlet T [° C.] 650 Outlet T [° C.] 1050 Inlet P [kg/cm.sup.2g] 35.5 Outlet P [kg/cm.sup.2g] 34.5 Outlet T (MDC) [° C.] — Inlet: N.sub.2 [Nm.sup.3/h] 26 CO.sub.2 [Nm.sup.3/h] 18678 CH.sub.4 [Nm.sup.3/h] 18967 H.sub.2 [Nm.sup.3/h] 400 H.sub.2O [Nm.sup.3/h] 11494 CO [Nm.sup.3/h] 0 Oxygen feed: O.sub.2 [Nm.sup.3/h] 11739 N.sub.2 [Nm.sup.3/h] 240 Oxygen feed T [° C.] 371 Outlet: N.sub.2 [Nm.sup.3/h] 266 CO.sub.2 [Nm.sup.3/h] 11807 CH.sub.4 [Nm.sup.3/h] 153 H.sub.2 [Nm.sup.3/h] 26493 H.sub.2O [Nm.sup.3/h] 23029 CO [Nm.sup.3/h] 25685 Total outlet flow [Nm.sup.3/h] 87433

(24) Numbers of another example of the invention is given in Table 3. A hydrocarbon feed stream comprising a hydrocarbon gas, CO.sub.2 and steam, and having a S/C ratio of 0.6 is fed to the ATR reactor 10 in the system of the invention as shown in FIG. 1. This hydrocarbon feed stream is heated to 650° C. prior to being let into the ATR reactor 10. Within the ATR reactor 10, partial combustion of the hydrocarbon feed stream by sub-stoichiometric amounts of oxygen added to the ATR reactor 10 is followed by steam reforming of the partially combusted hydrocarbon feed stream in a fixed bed of a first catalyst in the form of steam reforming catalyst, thereby producing a first synthesis gas stream having a temperature of 1050° C. Due to the low CH.sub.4 content of the synthesis gas from the ATR reactor, the equilibrium temperature of the methane decomposition reaction to graphitic carbon for the given gas composition is very high. At the same time this temperature is above the equilibrium temperature of the Boudouard reaction to graphitic carbon of 884° C., and consequently this gas does not have affinity for carbon formation.

(25) A CO.sub.2 rich gas stream is heated to a heated CO.sub.2 rich gas stream having a temperature of 650° C. and the combined gas (the first synthesis gas stream and the heated CO.sub.2 rich gas stream) enters the adiabatic post converter 20 at a temperature of 879° C.

(26) Within the adiabatic post converter 20, the combined stream is equilibrated, viz. it undergoes reverse water gas shift, methanation and reforming reactions, with a net production of methane, steam and carbon monoxide as a result. The exit temperature of the product gas stream exiting the adiabatic post converter 20 is 856° C., which is well below the methane decomposition equilibrium temperature for the gas of 991° C. and above the Boudouard temperature for the gas of 795° C., Consequently, the product gas stream does not have potential for carbon formation. The product gas from the adiabatic post converter 20 has a H.sub.2/CO ratio of 0.63.

(27) TABLE-US-00003 TABLE 3 ATR 10 Adiabatic post converter 20 Inlet T [° C.] 650 879 Outlet T [° C.] 1050 856 Inlet P [kg/cm.sup.2g] 35.5 34.5 Outlet P [kg/cm.sup.2g] 34.5 34 Outlet T (MDC) [° C.] — 991 Outlet T (BOU) [° C.] 884 795 Inlet: N.sub.2 [Nm.sup.3/h] 19 186 CO.sub.2 [Nm.sup.3/h] 8237 35439 CH.sub.4 [Nm.sup.3/h] 13950 218 H.sub.2 [Nm.sup.3/h] 294 22321 H.sub.2O [Nm.sup.3/h] 8449 13886 CO [Nm.sup.3/h] 0 16530 Oxygen feed: O2 [Nm.sup.3/h] 8186 N2 [Nm.sup.3/h] 167 Oxygen feed T [° C.] 371 Outlet: N.sub.2 [Nm.sup.3/h] 186 186 CO.sub.2 [Nm.sup.3/h] 5439 28985 CH.sub.4 [Nm.sup.3/h] 218 779 H.sub.2 [Nm.sup.3/h] 22321 14186 H.sub.2O [Nm.sup.3/h] 13886 20900 CO [Nm.sup.3/h] 16530 22423 O.sub.2 [Nm.sup.3/h] 0 0