Process for production of synthesis gas using waste methane-rich gas as a feedstock

Abstract

This invention provides a process for the production of a synthesis gas (syngas). The process comprises obtaining a methane rich tail gas stream from a hydrocarbon cracker unit and utilising the methane rich tail gas stream as a feed stream for a steam methane reformer reactor in order to produce a syngas product. Either or both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy, which may be generated from a renewable source. The methane rich tail gas may be subjected to one or more treatment stages prior to utilising it as a feed stream for the steam methane reformer reactor.

Description

Field of the Invention This invention relates to processes for the production of synthesis gas (syngas) utilising a steam methane reformer. Suitably the process utilises electrically heated reactors and apparatus, wherein the electrical energy is provided from renewable energy sources . Background of the invention To address the considerable challenges of decarbonization of the global economy in order to meet net zero targets within the 21st century, there has been an increasing trend towards adoption of more renewable sources of energy for industrial processes. Whilst improvements in energy usage efficiency and selection of renewable electrified manufacturing assets can help to reduce carbon footprint, certain petrochemical processes, such as steam crackers, continue to produce waste streams such as steam cracker tail gas. Conventionally this tail gas was burned as fuel and used to power other refinery or petrochemicals processes. However, with a move to electrification, methane rich tail gases will no longer be combusted. EP3075704 relates to a furnace for the steam reforming of a hydrocarbon, preferably a methane-containing feed stream. In addition to at least one burner configured for heating a plurality of reactor tubes, at least one voltage source is connected to the plurality of reactor tubes in such a way, that an electrical current is produced for heating. EP4081337 describes an electrically heated reactor having an outer surface area, an inlet and an outlet. The reactor is a tube surrounded by electrical heating means at a certain distance; the electrical heating means comprises radiative sheeting placed coaxially with regard to the reactor tube. The reactor is useful in many industrial scale high temperature gas conversion and heating technologies. WO2022 / 219053 describes modular reactor configurations utilizing resistance heating elements that pass through the reaction zone of reactor modules. The use of resistance heating in the reaction zone facilitates the conversion of the reactants to products when reactants are present in the reaction zone. WO2022 / 038230 describes the use of a hydrocarbon feed that is subjected to electrical steam methane reforming (e-SMR) to generate a syngas stream. An upgrading section receives the syngas stream and generates a first product stream and an off-gas stream from the syngas stream. A power generator receives at least a portion of the off-gas stream and / or a portion of the first product stream from the upgrading section and / or a portion of said first feed which is used for combustion in order to generate electricity that can be used in powering the e-SMR. Hence, it would be desirable to identify alternative ways to recycle methane rich tail and off gases that do not reguire combustion. This could add considerable value and process efficiencies and also contribute to the overall objective of reduction in carbon dioxide emissions. These and other objectives of the invention will become apparent to a skilled reader, providing a solution to these hitherto unmet needs. Summary of the Invention The present invention provides for the utilisation of methane rich tail gases as feed for valuable synthetic processes that would hitherto been regarded as waste and / or destined for combustion as utility gas. In a first aspect the invention provides a process for the production of synthesis gas (syngas) comprising obtaining a methane rich tail gas stream from a hydrocarbon cracker unit and utilising the methane rich tail gas stream as a feed stream for a steam methane reformer reactor in order to produce a syngas product, wherein either or both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy. In a specific embodiment of the invention both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy. In a further embodiment of the invention the electrical energy provided to either or both of the hydrocarbon cracker unit and the steam methane reformer reactor is generated by renewable sources. In an embodiment of the invention either or both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical resistance heating . According to a further embodiment, the process further includes utilisation of the syngas in one or more synthetic processes selected from the group consisting of: ammonia production; methanol production; hydrogen (H2) production, such as via a water gas shift (WGS) reaction; Fischer-Tropsch Synthesis; and in a process of Carbon Capture and Sequestration (CCS). In specific embodiment of the invention the process further comprises an intervening step of pre-treating the methane rich tail gas stream prior to utilising the methane rich tail gas stream as a feed stream for the steam methane reformer reactor. Suitably, the process of pre-treatment comprises a desulfurization step. Optionally, the desulfurization step comprises one or more of: use of sulfur guard beds; an amine treatment; and / or use of a hydrodesulfurization process. In a further embodiment, the process of pretreatment comprises removal of nitrogen from the methane rich tail gas stream. Suitably, the removal of nitrogen comprises one or more of: cryogenic distillation; membrane separation; a molecular gate system; solvent absorption; a nitrogen sponge; and / or pressure swing adsorption. A second aspect of the invention provides a system for the production of synthesis gas (syngas), the system comprising : a hydrocarbon cracker unit; and a steam methane reformer reactor; wherein both the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy, and wherein a methane rich tail gas from the hydrocarbon cracker unit is directed as a feed stream to the steam methane reformer reactor. In one embodiment, the hydrocarbon cracker unit and the steam methane reformer reactor are configured so as to be heated by electrical resistance heating. In a specific embodiment, the system comprises one or more tail gas treatment units between the hydrocarbon cracker unit and the steam methane reformer reactor. Suitably, the one or more tail gas treatment units comprise a desulfurization unit selected from one or more of: a sulfur guard bed; an amine treatment unit; or a hydrodesulfurization unit. Optionally or additionally, the one or more tail gas treatment units may comprise a nitrogen removal unit selected from one or more of: a cryogenic distillation unit; a membrane separator; a molecular gate system; a solvent absorption unit; a nitrogen sponge; or a pressure swing adsorption unit. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. Brief Description of the Drawings Figure 1 shows schematic of a process line up according to one embodiment of the present invention. Figure 2 shows schematic of a process line up according to a further embodiment of the present invention . Figure 3 shows a schematic of a process line up according to a further embodiment of the present invention . Detailed Description of the Invention All references cited herein are incorporated by reference in their entirety. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Prior to setting forth the invention, a number of definitions are provided that will assist in the understanding of the invention. As used herein, the term 'comprising' means any of the recited elements are necessarily included and other elements may optionally be included as well. 'Consisting essentially of' means any recited elements are necessarily included, elements that would materially affect the basic and novel characteristics of the listed elements are excluded, and other elements may optionally be included. 'Consisting of' means that all elements other than those listed are excluded. Embodiments defined by each of these terms are within the scope of this invention. The term 'reactor' as used herein should be understood to comprise any industrial reactor suitable for industrial scale reactions and process heating, and, accordingly, the term reactor tube should be understood to comprise any vessel in the form a tube in which (a) substance(s) is (are) heated to high temperature. The terms 'feed', 'feedstock' or 'feedstream' should be considered as synonymous. These terms relate to at least one input reagent that is introduced into a reactor and a starting material and which is destined for chemical conversion into at least one output effluent of the reactor either in the form of a reaction product that contributes to the yield, or as a byproduct such as a waste stream. As used herein, the term 'renewable energy source' generally refers to one or more technologies that utilize replenishable energy sources such as energy from water, wind, the sun, geothermal sources, and biomass sources such as energy crops. Such renewable energy sources may include without limitation, a wind generator, solar panels, a solar panel array, solar panel strings, wind turbines, hydroelectric power stations that utilize hydroelectricity and hydropower, cogeneration plants that utilize biomass materials, biofuels, biodiesels, geothermal energy, and a combination thereof. Renewable electrons (e_) are, thus, components of an electricity supply that is derived from a renewable energy source. The term 'treatment' is used herein when applied to a feed for or product of a reaction to denote one or more steps that are applied to feed or product in order to remove contaminants and / or condition the feed or product for subsequent process steps. Typically, one or more treatment steps may include concentration, purification, drying (e.g. dehydration), removal of one or more contaminants, changes in temperature (e.g. chilling or heating), thermal recovery (such as passing through a heat exchanger), and / or mixing with other feed streams. Where a treatment is applied prior to a specified process step, the term may be considered synonymous with 'pretreatment' . The petrochemical industry is reliant upon the use of olefins as one of the most versatile building blocks for a variety of petrochemical products such as solvents, polymers, resins, coatings and fibres. Traditionally the most effective way to produce olefins is via thermal cracking of hydrocarbon feedstocks (the raw material) derived from crude oil or from natural gas. Steam cracking is one such process that is a favoured process route. The steam cracker is often considered as the heart of the olefin process plant, particularly when used in the production of ethylene. The cracker unit has a very high energy consumption since cracking reactions are highly endothermic. On subjecting feedstock hydrocarbons, typically containing high levels of ethane, to high temperatures at low pressures, they undergo dehydrogenation to form ethylene and hydrogen primarily. Other products may include methane, acetylene, propylene, propane and butadiene. The range of products are obtained as result of complex combination of free radical mechanisms. As mentioned, the net effect of these reactions are endothermic, hence, in order to increase yield of ethylene, external energy has to be supplied to maintain the temperature of the reactor. Traditionally, the requirement for external energy would have been met by combustion of methane tail / off gases produced as byproducts of various petrochemical refinery processes including the aforementioned cracking reaction. However, there has been an increasing trend to reduce the production of carbon dioxide from combustion of tail / off gases. This is accomplished by supply of thermal energy to the cracker unit via use of resistive heating technologies using a supply of electrons derived from renewable sources - a so-called e-cracker. However, this trend results in an accumulation of tail / off gases which are no longer required for combustion and a need to find a use for these gases within the process flows of the petrochemical refinery. Syngas, short for synthesis gas, is a mixture of gases primarily composed of carbon monoxide (CO) and hydrogen (H2) . It is a versatile intermediate product that has a wide range of applications in various industrial processes. Syngas can be produced through different methods, including steam methane reforming (SMR). Syngas can be used for a diverse range of synthetic processes including, but not limited to: ammonia production; methanol production; hydrogen (H2) production such as via a water gas shift (WGS) reaction; Fischer-Tropsch Synthesis; and in the process of Carbon Capture and Sequestration (CCS). Hence, the versatility of syngas makes it a valuable component in the production of various industrial chemicals . In accordance with one embodiment of the present invention, a methane rich tail gas originates from a hydrocarbon cracking reaction, typically an ethane cracker, more suitably an ethane e-cracker, optionally an ethane e-cracker operable on the basis of renewable electricity. The methane rich tail gas is utilised as feedstock for a steam methane reforming reaction, more suitably an electric steam methane reforming (eSMR) reaction, optionally an eSMR reaction that is also operable on the basis of renewable electricity. The product of the eSMR is a syngas that may be used in diverse synthetic processes. In this way, methane rich tailgas is not diverted to combustion either for the purpose of heating various reactors or as a flare. Detailed Description of the Drawings Figure 1 shows a process line up according to a first embodiment of the invention. A hydrocarbon feed (HC)is provided to a cracker unit. The HC feed may comprise a range of hydrocarbons typically utilised for cracking, including light paraffins such as ethane, propane, butane, and LPG, and heavier paraffins such as naphtha or heavy hydrocarbon liquid feed. In a specific embodiment the HC comprises a substantial proportion of ethane which is utilised for the formation of lower olefins such as ethylene. The cracker typically comprises a reactor that defines a cracking zone. Additional feeds may comprise steam if the cracker unit comprises a steam cracking furnace. The reactor is surrounded by an electrical heating means, suitably in the form of a radiative sheeting layer. This source of heating results in the cracker being designated as a type of e-cracker. The electrical heater is typically supplied with - 10 -electricity generated from renewable sources (identified in Figure 1 as Renewable e_) . The reactor effluent from the cracker unit includes lower olefins, such as ethylene, as the reaction product. Waste off gases such as methane rich tail gas are another effluent stream. In accordance with the present invention, the tail gas is comprised within a feed for a steam methane reformer (SMR). Tail gas may be subjected to a pre-treatment prior to introduction into the SMR. In general pre-treatment may involve removal of other waste gases acid gases by caustic scrubbing. The tail gas may be cooled in a series of heat exchangers using refrigerants to effectively separate any hydrogen from rest of the tail gas and to enrich the methane content. Methane can then be further separated from any remaining heavier off gases and directed to the SMR. Tail gas streams obtained from a cracker can include a range of contaminating sulfur compounds, such as acid gas constituents. Sulfur may be present in the form of small quantities of hydrogen sulfide (H2S), mercaptans (e.g. methane thiol, 1-ethanthiol, 2-propanethiol, 2-butanethiol, 2-methyl-2-propanethiol, pentanethiol, hexanethiol, heptanethiol, octanethiol, nonanethiol, and thiophenol) and / or thiophenes (e.g. thiophene, 2-methylthiophene, 3-methylthiophene, 2-ethylthiophene, benzothiophene and methylbenzothiophene). Even small quantities of sulfur containing compounds are known to deactivate, or 'poison', catalysts that may be used in downstream processes including the SMR reaction as well as subsequent reactions. Hence, removal of sulfur compounds from the tail gas can be achieved via a number of suitable means including, but not limited to, use of sulfur guard beds (with catalysts that have Cu, Cu / Zn, Ni, Pd, Mo, Co, Ni / Mo, Co / Mo, etc.), amine treatment, and / or hydrodesulfurization processes similar to hydrotreating technology used in other refinery processes. Catalysts useful as the hydrodesulfurization catalyst may include Group VIIIB metals, such as cobalt, nickel, palladium, alone or in combination with other transition metals, such as molybdenum or tungsten, on a suitable support, which may be alumina, silica-alumina, titania-zirconia or the like. The hydrodesulfurization catalysts may also contain components from Groups VB and VIB of the Periodic Table or mixtures thereof. Catalysts may contain a Group VIB metal, such as molybdenum, and a Group VIIIB metal, such as cobalt or nickel. Catalysts suitable for the hydrodesulfurization reaction include cobalt-molybdenum nickel-molybdenum and nickel-tungsten. The metals are generally present as oxides supported on a neutral base such as alumina, silica-alumina or the like. The sulfur removal step may occur within discreet units or facilities configured for the purpose. In certain refinery and petrochemical process lineups, methane rich tail gases may be combined with other methane containing feed streams that may comprise an amount of nitrogen which is present as a carrier gas or as a diluent from natural gas extraction. Nitrogen may be present in amounts of up to around 20%m and whilst it can be regarded as a largely inert component, the presence of nitrogen can lead to issues with further downstream processing. Firstly, nitrogen acts as a diluent reducing the effective concentration of methane in the feedstream for the SMR reaction and, thus, affecting the resultant product yield of that reaction. Secondly, there is evidence that nitrogen may contribute to formation of small quantities of ammonia when in the presence of certain types of catalyst such as under SMR reaction conditions. Indeed, the present inventors have found that with 20%m of nitrogen in the tail gas stream for SMR, the SMR gas effluent may contain as much as 0.02%m of ammonia. As a contaminant, the presence of highly reactive ammonia in the feedstream for downstream reactions can result in poisoning of catalysts used in SMR and in those downstream processes. Clearly, continued presence of nitrogen in the tail gas feed for SMR results in production of quantities of ammonia, albeit small, but sufficient to deactivate the catalysts used in the process over time. Nitrogen removal may occur during pre-treatment utilising a range of technologies and strategies. The nitrogen removal may occur within discreet units or facilities configured for the purpose. One commonly adopted nitrogen removal treatment involves cryogenic distillation, in which the nitrogen-containing tail gas is subjected to a series of steps including compression, Joule-Thomson (J-T) / expander chilling, cryogenic fractionation into component gases, and recompression of the methane rich fraction of the tail gas. Alternatively, specialist proprietary membranes can be used to remove nitrogen from the tail gas stream and select methane for further processing in SMR to produce the desired products, adsorbent / absorbent approaches may also be adopted, such as using molecular gate systems, solvent absorption, or nitrogen sponges. Pressure swing adsorption approaches may also be used to separate nitrogen from the tail gas under pressure according to its molecular characteristics and attraction to an adsorbent material at near-ambient temperatures. Special adsorptive materials are used as a molecular sieve, adsorbing the hydrocarbon component (e.g. methane) at high pressure. The process then swings to low pressure to desorb the adsorbent material releasing a methane rich stream that is depleted of nitrogen. Within the SMR, methane is reacted with steam in the presence of a catalyst (usually nickel-based) to produce carbon monoxide and hydrogen, via the reaction: CH4 + H2O CO + 3H2 As with the furnace comprised within the e-cracker unit, the SMR reactor is suitably surrounded by an electrical heating means, typically in the form of a radiative sheeting layer. This source of heating results in the cracker being designated as a type of e-SMR. The electrical heater is typically supplied with electricity generated from renewable sources (identified in Figure 1 as Renewable e-). The reactor effluent from the e-SMR is in the form of syngas which may be utilised for a variety of further synthetic processes. Figure 2 shows the integration of the process set out in Figure 1, and described above, with a range of these further synthetic processes. The syngas may be used as a feedstream for methanol (MeOH) production; hydrogen (H2) production, such as via a water gas shift (WGS) reaction; Fischer-Tropsch (FT) Synthesis; and in a process of Carbon Capture and Sequestration (CCS). Figure 3 shows the process of Figure 2, further incorporating one or more tail gas treatment steps (as described herein) to remove contaminants and enrich the methane content of the tail gas prior to introduction into the e-SMR. The tail gas treatment step(s) may occur within discreet units or facilities configured for the purpose. The invention will now be further illustrated by reference to the following non-limiting examples. Example Typical tail gas composition from an e-cracker is as shown in Table 1, below. In Table 1 a base case reference 5 composition is provided in which no nitrogen is present. This base case is compared to an exemplary composition that might be found in a typical refinery process comprising 20m% nitrogen. 10 Table 1 Tail Gas Compositions Base Case (%m) 20m% N2 (%m) hydrogen 2 2 methane 96 76 ethylene 2 2 N2 0 20 The output of a typical e-SMR was generated using a process simulation model (e.g. Aspen Plus®) for an SMR using 15 the base case and 20m% nitrogen tail gas compositions as feedstreams. The results are shown in Table 2, below. Table 2 Base Case (%m) 20m% N2 (%m) Compositions Methane 4.00% 3.70% CO 8.40% 8.20% CO2 6.60% 6.30% H2O 37.50% 36.20% H2 43.50% 42.50% N2 0.00% 3.00% NH3 0.00% 0.02% Ethylene 0.00% 0.00% 5        The presence of ammonia (0.02m%) in the 20m% nitrogen feedstream indicates that at least full or partial nitrogen removal from the tail gas would be desirable prior to the e-SMR stage of the process. 10       Although particular embodiments of the invention have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims, which follow. It is contemplated 15 by the inventors that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims .

Claims

1. A process for the production of synthesis gas (syngas), the process comprising:obtaining a methane rich tail gas stream from a hydrocarbon cracker unit; andutilising the methane rich tail gas stream as a feed stream for a steam methane reformer reactor in order to produce a syngas product, wherein either or both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy.

2. The process of claim 1, wherein both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy.

3. The process of claim 1, wherein the electrical energy provided to either or both of the hydrocarbon cracker unit and the steam methane reformer reactor is generated by renewable sources.

4. The process of claim 1, wherein either or both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical resistance heating .

5. The process of claim 1, wherein the process further includes utilisation of the syngas in one or more synthetic processes selected from the group consisting of: ammonia production; methanol production; hydrogen (H2) production, such as via a water gas shift (WGS) reaction; Fischer-Tropsch Synthesis; and in a process of Carbon Capture and Sequestration (CCS).

6. The process of claim 1, wherein the process further comprises an intervening step of pre-treating the methane rich tail gas stream prior to utilising the methane richtail gas stream as a feed stream for the steam methane reformer reactor.7 . The process of claim 6, wherein the process of pre treatment comprises a desulfurization step . 8 . The process of claim 7, wherein the desulfurization step compr ises one or more of: use of sulfur guard beds;an amine treatment; and / or use of a hydrodesulfurizationprocess .

9. The process of claim 6, wherein the process of pretreatment comprises removal of nitrogen from the methane rich tail gas stream.

10. The process of claim 9, wherein the removal of nitrogen comprises one or more of: cryogenic distillation; membrane separation; a molecular gate system; solvent absorption; a nitrogen sponge; and / or pressure swing adsorption .

11. A system for the production of synthesis gas (syngas), the system comprising:a hydrocarbon cracker unit; anda steam methane reformer reactor;wherein both of the hydrocarbon cracker unit and the steam methane reformer reactor are heated by electrical energy, and wherein a methane rich tail gas from the hydrocarbon cracker unit is directed as a feed stream to the steam methane reformer reactor.

12. The system of claim 11, wherein the hydrocarbon cracker unit and the steam methane reformer reactor are configured so as to be heated by electrical resistance heating .

13. The system of claim 11, wherein the system comprises one or more tail gas treatment units between the hydrocarbon cracker unit and the steam methane reformer reactor .

14. The system of claim 13, wherein the one or more tail gas treatment units comprise a desulfurization unit selected from one or more of: a sulfur guard bed; an amine treatment unit; or a hydrodesulfurization unit.5       15. The system of claim 13, wherein the one or more tailgas treatment units comprise a nitrogen removal unit selected from one or more of: a cryogenic distillation unit; a membrane separator; a molecular gate system; a solvent absorption unit; a nitrogen sponge; or a pressure 10       swing adsorption unit.

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