Conversion of a hydrocarbon feed gas to synthesis gas for producing hydrocarbons

Abstract

Method and plant for producing a synthesis gas for use in the production of a hydrocarbon product, particularly a synthetic fuel, comprising: providing a hydrocarbon feed gas, providing a first oxygen rich stream by passing air through an air separation unit (ASU), carrying out autothermal reforming of said hydrocarbon feed gas in an autothermal reforming (ATR) unit, said autothermal reforming including using at least a portion of said first oxygen containing stream, providing at least part of said synthesis gas to a synthetic fuel synthesis unit for converting said synthesis gas into said hydrocarbon product and producing a tail gas, recycling part or the entirety of said tail gas to upstream said ATR, providing a first hydrogen rich stream and a second oxygen rich stream, and adding at least a portion of said first hydrogen rich stream to said synthesis gas prior to entering said synthetic fuel synthesis unit.

Claims

1. A method for producing a synthesis gas for use in the production of a hydrocarbon product, particularly a synthetic fuel, comprising the steps of: providing a hydrocarbon feed gas, providing a first oxygen rich stream by passing air through an air separation unit (ASU), carrying out autothermal reforming of said hydrocarbon feed gas in an autothermal reforming (ATR) unit, said autothermal reforming including using at least a portion of said first oxygen containing stream, providing at least part of said synthesis gas to a synthetic fuel synthesis unit for converting said synthesis gas into said hydrocarbon product and producing a tail gas, recycling a part or the entirety of said tail gas to upstream said ATR, providing a first hydrogen rich stream and a second oxygen rich stream, and adding at least a portion of said first hydrogen rich stream to said synthesis gas prior to entering said synthetic fuel synthesis unit.

2. A method according to claim 1, further comprising adding at least a portion of said second oxygen rich stream to said autothermal reforming step.

3. The method according to claim 1, wherein the step of providing said first hydrogen rich stream and said oxygen rich stream is conducted by electrolysis of a water feedstock.

4. The method according to claim 1, wherein the electrolysis is conducted in a solid oxide electrolysis cell unit and said water feedstock is in the form of steam produced from other processes of the method.

5. The method according to claim 1, wherein the power required in the step of providing a first oxygen rich stream by passing air through an air separation unit (ASU), or the step of electrolysis of a water feedstock, is provided at least partly by renewable sources, such as wind and solar energy.

6. The method according to claim 1, further comprising: pre-reforming of the hydrocarbon feed gas together with a steam feedstock in a pre-reforming unit prior to said autothermal reforming, and/or purifying the hydrocarbon feed gas in a gas purification unit prior to said autothermal reforming, and/or prior to said pre-reforming.

7. The method according to claim 1, the method further comprising providing a second hydrogen rich stream by steam methane reforming of a hydrocarbon feed gas.

8. The method according to claim 1, the method further comprising adding part of said second hydrogen rich stream to said synthetic fuel synthesis unit.

9. The method according to claim 1, further comprising combining said first hydrogen rich stream with said second hydrogen rich stream for forming a hydrogen product stream, and optionally adding a portion of the hydrogen product stream to said synthetic fuel synthesis unit.

10. The method according to claim 9, further comprising hydrogen purification in a hydrogen purification unit, prior to combining with said first hydrogen rich stream.

11. The method according to claim 3, further comprising: increasing or decreasing the volumetric tail gas recycle flow from the synthetic fuel synthesis unit to the to the ATR compared to reference operating conditions of the GTL plant, for producing a synthesis gas having a H.sub.2/CO<(H.sub.2/CO).sub.Ref; providing said first hydrogen rich stream and adding it to said synthesis gas, for producing a final synthesis gas with an H.sub.2/CO-ratio equal to (H.sub.2/CO).sub.Ref; adding said second oxygen rich stream to the ATR.

12. The method according to claim 3, further comprising: reducing the overall S/C-ratio to a new overall S/C-ratio being less than (S/C).sub.Ref; maintaining the volumetric tail gas recycle flow unchanged; providing said first hydrogen rich stream and adding it to said synthesis gas, for producing a final synthesis gas with an H.sub.2/CO-ratio equal to (H.sub.2/CO).sub.Ref; adding said second oxygen rich stream to the ATR.

13. A plant for producing a synthesis gas for use in the production of a hydrocarbon product, particularly a synthetic fuel, comprising: an air separation unit (ASU) arranged to receive air for producing a first oxygen rich stream, an electrolysis unit arranged to receive a water feedstock for producing a first hydrogen rich stream and a second oxygen rich stream, an autothermal reforming (ATR) unit for steam reforming of a hydrocarbon feed gas, a synthetic fuel synthesis unit arranged to receive at least part of the synthesis gas from said autothermal reforming unit for converting said synthesis gas into said hydrocarbon product and for producing a tail gas, a conduit for recycling a portion or the entirety of said tail gas to upstream said ATR unit, a conduit for adding said first hydrogen rich stream to said synthesis gas, upstream said synthetic fuel synthesis unit, a conduit for adding said second oxygen rich stream to said ATR unit and/or for combining with said first oxygen rich stream, optionally, a prereforming unit upstream said autothermal reforming unit, and/or a gas purification unit for purification of the hydrocarbon gas upstream said prereforming unit and/or upstream said autothermal reforming unit.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0077] FIG. 1 illustrates a conventional process and plant layout for production of a hydrocarbon product according to the prior art.

[0078] FIG. 2 illustrates a process and plant layout for production of a hydrocarbon product according to one embodiment of the invention.

[0079] FIG. 3 illustrates a process and plant layout for production of a hydrocarbon product according to another embodiment of the invention.

DETAILED DESCRIPTION

[0080] In FIG. 1 a hydrocarbon feed gas such as natural gas 1 is combined with hydrogen 19 and then purified in purification unit 30, for instance by removing sulfur in the natural gas which may be detrimental for catalysts used in downstream units. Water or steam 5 together with the purified hydrocarbon feed 3 enters a prereforming unit 40 thereby forming a hydrocarbon feed gas in the form of purified and prereformed gas 7, which is combined from recycled tail gas 9 from downstream Fischer Tropsch (FT)-unit 70. Part of the tail gas 9 is used as fuel and other purposes, as stream 9′. The resulting mixture of prereformed gas and recycled tail gas enters the autothermal reforming unit (ATR) 50, as so is an oxygen rich stream 11 provided from an Air Separation Unit (ASU) 60 using air stream 13. From the ATR a synthesis gas 15 is produced which due to the tail gas recycle, has the right molar ratio of H.sub.2 to CO, i.e. of about 2. The synthesis gas 15 is then processed in downstream synthetic fuel synthesis unit comprising a FT-unit/reactor 70 and Product Workup Unit 80. From the FT-unit a raw hydrocarbon stream 17 is withdrawn and hydrogen 19 from an external source is add-mixed for conducting one or more hydrotreating steps in the PWU 80. A hydrocarbon product 21 is withdrawn comprising a range of products including synthetic fuels such as diesel 21.sup.IV. The range of hydrocarbon products illustrated here are LPG (Liquified Petroleum Gas) 21′, naphtha 21″, jet fuel 21″′ and diesel 21.sup.IV.

[0081] With reference to FIG. 2 showing an embodiment according to the invention, electrolysis unit 90 is now included. This unit can be either PEM, AFC and/or SOEC. A water feedstock such as steam 23 is used for the electrolysis and the unit is powered by renewable sources, such as power from wind or solar energy. The electrolysis produces a second oxygen rich stream 11′ which is combined with first oxygen rich stream 11 produced in the ASU 60, thereby forming a combined oxygen rich stream 11″ which is used in the ATR 50. The size of the ASU 60 and can thereby be significantly reduced. From the electrolysis unit 90, a first hydrogen rich stream 19′ is produced, which may be recycled to the natural gas feed 1, and/or be added to the synthesis gas 15 or to the raw hydrocarbon stream 17 from the FT-unit 70. The provision of the first hydrogen stream 19′ from electrolysis 90 in combination with tail gas 9 recycle is highly counterintuitive. The tail gas recycle stream 9 enables on its own, as shown in FIG. 1, that the H.sub.2/CO molar ratio in the synthesis gas be brought to the desired value of about 2. Hence, the expected result of adding a first hydrogen rich stream 19′ to the synthesis gas 15 would have been a synthesis gas stream 15′ having a significantly higher value of H.sub.2/CO i.e. much higher than 2. The invention enables that more tail gas 9 is available for recycle to the ATR 50, which is a more efficient utilization of the tail gas rather than using it as fuel and other purposes (stream 9′). Thus, the invention enables significantly reducing the amount of this tail gas stream 9′. Increased carbon efficiency in the process and plant is thereby obtained, as well as the carbon foot-print. Furthermore, the second oxygen rich stream 11′ produced in the electrolysis 90 reduces the amount of energy needed for driving the ASU 60, further aiding in increasing the efficiency of the plant. First hydrogen rich stream 19′ being produced from the electrolysis, is also efficiently integrated in the process.

[0082] With respect to FIG. 3, another embodiment according to the invention is illustrated, in which in addition to the units of FIG. 2, a separate unit for steam reforming 100, such as SMR or eSMR is used in the process/plant. The eSMR, whereby heat is produced by resistance heating using a power source, may also be driven, as in the electrolysis unit 90, by power provided by renewable sources, such as wind and solar energy. A separate hydrocarbon feed gas such as natural gas 1′, for instance a diverted stream from natural gas stream 1, is used in the steam reforming, thereby producing a second hydrogen rich stream 19″ which may be recycled to the natural gas stream 1, or combined with the first hydrogen stream 19′ from the electrolysis unit 90. The hydrogen stream 19″ may undergo a purification step in a pressure swing adsorption (not shown) and/or prior to this, be subjected to water gas shift (not shown) for enriching the synthesis gas from the steam reforming unit 100 in hydrogen.