A method of forming synthesis gas and a method of forming hydrocarbons using the synthesis gas

Abstract

A method of forming a synthesis gas, the method comprising: feeding a mixture of hydrogen and carbon dioxide feed gas to a heater; heating the mixture of hydrogen and carbon dioxide feed gas in the heater to a temperature of at least 400°C; and feeding the heated mixture of hydrogen and carbon dioxide feed gas through a connective pipe to a reverse water-gas shift reactor having a refractory lining to form synthesis gas comprising hydrogen and carbon monoxide via a reverse water-gas shift reaction within the reverse water-gas shift reactor; wherein, to inhibit metal corrosion in the heater and the connective pipe between the heater and the reverse water-gas shift reactor during operation, the method comprises one or more of the following characterizing features: providing the feed gas to the heater at an absolute pressure of no more than 20 bar; mixing steam with the feed gas prior to entering the heater; mixing a corrosion inhibitor with the feed gas prior to entering the heater; and providing the heater with one or more components formed of a nickel-chromium alloy which is resistant to corrosion during operation.

Description

[0001] A METHOD OF FORMING SYNTHESIS GAS AND A METHOD OF FORMING

[0002] HYDROCARBONS USING THE SYNTHESIS GAS

[0003] Field

[0004] The present specification relates to a method and system for forming a synthesis gas (syngas) comprising carbon monoxide and hydrogen, and also a method and system for producing hydrocarbons from the synthesis gas.

[0005] Background

[0006] The Fischer-Tropsch process converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150-300°C and pressures of one to several tens of atmospheres. The Fischer-Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (C„H2n+2). The more useful reactions produce alkanes as follows: where n may be 1-100, or higher. The formation of methane (n = 1) is unwanted. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons. The Fischer-Tropsch reaction is a highly exothermic reaction due to a standard reaction enthalpy (AH) of -165 kJ / mol CO combined.

[0007] Synthesis gas (syngas) fed to a Fischer-Tropsch process can be derived from a number of feedstocks. For example, natural gas via steam reforming and / or auto-thermal reforming, municipal solid waste and biomass via high-temperature gasification or carbon dioxide and hydrogen via a reverse water-gas shift reaction. The reverse water-gas shift reaction may be depicted as follows:

[0008] H2+ CO2^ CO + H2O

[0009] Syngas generation using a reverse water-gas shift reaction can be beneficial since it makes use of carbon dioxide that may have been destined to be released to the atmosphere.

[0010] W02022079408 describes a process for producing a gas stream comprising carbon monoxide by feeding a gas mixture comprising carbon dioxide and hydrogen to a burner disposed in a reverse water- gas shift vessel and combusting it with a sub-stoichiometric amount of an oxygen gas stream to form a combusted gas mixture containing carbon monoxide, carbon dioxide, hydrogen and steam. The mixture is then passed through a reverse water-gas shift catalyst to form a crude product gas comprising carbon monoxide, carbon dioxide, hydrogen and steam. The gas is then cooled so that the water content condenses and can be separated and removed. It then passes to a carbon dioxide removal unit to remove carbon dioxide, which can be recycled to the feed gas mixture to the reverse water-gas shift vessel, and a gas comprising carbon monoxide and hydrogen.

[0011] W02022079408 also discloses that the gas streams fed to the reverse water-gas shift vessel may be preheated. The pre-heat temperature of the feed gases to the reverse water- gas shift vessel may be in the range of 400 to 1000°C or 450 to 800°C to sustain combustion. The hydrogen and carbon dioxide streams may be premixed before preheating or preheated and mixed. Preheating of the feeds to their pre-heat temperatures may be done by interchange with the crude product gas mixture, and / or by steam heating, or by using a fired heater or by electrical heating or by a combination of two or more these. Preferably, the feed gas mixture comprising carbon dioxide and hydrogen is heated by interchange with the crude product gas mixture.

[0012] The present specification is concerned with providing an improved method and system for producing syngas and particularly focuses on the pre-heating of feed gases to the reverse water-gas shift reactor.

[0013] Summary

[0014] As described in the background section, W02022079408 teaches that it can be advantageous to preheat the carbon dioxide and hydrogen feed gases to a reverse water-gas shift reactor and that the carbon dioxide and hydrogen streams may be premixed before preheating or preheated and mixed. From a production system perspective, the simplest, and seemingly preferable, approach is to mix the hydrogen and carbon dioxide feed gases and then preheat the mixture of gases before passing the mixture of feed gases into the reverse water-gas shift reactor. However, it has surprisingly been found that the mixture of carbon dioxide and hydrogen feed gases may be more reactive than previously believed under certain pre-heating conditions and that a reverse water-gas shift reaction can be initiated, at least to some extent, when pre-heating the mixed feed gas prior to entering the reverse water-gas shift reactor. This can be problematic as such a reaction may generate a CO / H2 / CO2 / H2O bearing gas in the pre-heater equipment and pre-reactor pipework and such a gas mixture can degrade standard pre-heater equipment and pipework via materials damage mechanisms such as carburisation and metal dusting. This can lead to a reduction in the lifetime of these parts and require such parts to be replaced leading to increased equipment costs and plant downtime.

[0015] The aforementioned problem can be exacerbated by metallic construction materials catalysing the reverse water-gas shift reaction in the pre-heater(s) and / or pipework prior to the reverse water-gas shift reactor. For example, heating the mixed gas feed over a large metallic heat exchange surface in a pre-heater can catalyse the reverse water-gas shift reaction.

[0016] Furthermore, pre-heating the mixed gas feed may also lead to an exothermic methanation reaction, emitting heat and resulting in a rise in temperature. Methanation can also be catalysed by metallic construction materials and can lead to a runaway reaction when the heat release is faster than the heat being removed. This can cause the temperature to rise and exceed the design temperature of the pre-heater or pipework causing equipment damage, resulting in downtime.

[0017] In light of the above, the present specification provides a method of forming a synthesis gas, the method comprising: feeding a mixture of hydrogen and carbon dioxide feed gas to a heater; heating the mixture of hydrogen and carbon dioxide feed gas in the heater to a temperature of at least 400°C; and feeding the heated mixture of hydrogen and carbon dioxide feed gas through a connective pipe to a reverse water-gas shift reactor having a refractory lining to form synthesis gas comprising hydrogen and carbon monoxide via a reverse water-gas shift reaction within the reverse water-gas shift reactor; wherein, to inhibit metal corrosion in the heater and the connective pipe between the heater and the reverse water-gas shift reactor during operation, the method comprises one or more of the following characterizing features: providing the feed gas to the heater at an absolute pressure of no more than 20 bar; mixing steam with the feed gas prior to entering the heater; mixing a corrosion inhibitor with the feed gas prior to entering the heater; and providing the heater with one or more components formed of a nickel-chromium alloy which is resistant to corrosion during operation.

[0018] According to certain examples, all the aforementioned characterizing features are combined to minimize metal corrosion in the heater and connective pipework. However, it will be understood that for certain applications the combination of all four characterizing feature may not be required. For example, if a suitable level of steam is provided in the feed gas it may be acceptable to operate at a pressure above 20 bar. Similarly, if the parameters of the feed gas in terms of steam content and pressure are selected appropriately, corrosion may be inhibited sufficiently such that nickel-chromium alloy components are not required. Further still, if the parameters of the feed gas and / or the materials of construction for the heater are selected appropriately, it may not be required to mix a corrosion inhibitor, such as a sulphur-base corrosion inhibitor, into the feed gas. That said, providing at least one of the characterizing features can at least partially solve the problem of metal corrosion in the heater and connective pipework leading into the reverse water-gas shift reactor and by combining at least some, and optionally all, of the characterizing features enables the methodology to solve, or at least substantially address, the issue of metal corrosion in the heater and connective pipework leading into the reverse water-gas shift reactor.

[0019] In addition to the above, the present specification also provides a method for synthesising hydrocarbons, the method comprising: forming a synthesis gas as described above; and passing the synthesis gas though a Fischer-Tropsch system to form a hydrocarbon product stream. According to certain examples, a hydrocarbon containing recycle stream is also formed by the Fischer-Tropsch system itself and / or formed by an upgrading unit coupled to the Fischer-Tropsch system, and the method further comprises: feeding steam into the hydrocarbon containing recycle stream to form a recycle stream comprising a mixture of steam and hydrocarbon; feeding the recycle stream comprising a mixture of steam and hydrocarbon to a derichment reactor containing derichment catalyst generating a recycle stream comprising steam and methane; and mixing the recycle stream comprising steam and methane with the feed gas comprising hydrogen and carbon dioxide prior to the feed gas entering the heater, whereby the steam which is mixed with the feed gas is provided by the recycle stream comprising steam and methane.

[0020] The present specification also provides a system configured to perform the methodology as described above.

[0021] Brief Description of the Drawings

[0022] Figure 1 shows a flow sheet for a method of forming crude syngas according to the present specification.

[0023] Figure 2 shows a flow sheet for a method of forming a hydrocarbon product stream according to the present specification by combining the syngas forming method of Figure 1 with a Fischer-Tropsch unit. Figure 3 shows a flow sheet for a method of forming a hydrocarbon product stream not according to the present specification.

[0024] Figure 4(a) shows a graph of CO content versus temperature for a heated feed mixture of carbon dioxide and hydrogen.

[0025] Figure 4(b) shows the graph of Figure 4(a) extended beyond the 500°C test data to show the trend in CO formation up to 650°C.

[0026] Figure 5 shows a flow sheet for a method of forming crude syngas according to the present specification and illustrates how a flow sheet such as that shown in Figure 3 can be modified to achieve the present invention.

[0027] A summary of the reference numerals used in the figures is set out in the table below.

[0028] Detailed Description The present specification provides a method of reducing metal corrosion in reverse-water-gas shift systems due to methanation reactions and CO formation in the pre-heater and pipework prior to the reverse-water-gas shift reactor.

[0029] Hydrogen can react with carbon dioxide in the Reverse Water Gas Shift (RWGS) reaction to produce syngas, comprising carbon monoxide and hydrogen. The RWGS reaction can be depicted as follows:

[0030] H2+ CO2H2O + CO AH ~ +41 kJ / Mol

[0031] Hydrogen can react with carbon dioxide and / or carbon monoxide to produce methane. The methanation reactions can be depicted as follows:

[0032] CO + 3H2CH4+ H2O AH ~ -206 kJ / Mol

[0033] CO2+ 4H2CH4+ 2H2O AH ~ -165 kJ / Mol

[0034] Carbon monoxide can also react with hydrogen to produce carbon as follows:

[0035] CO + H2C + H2O

[0036] In a reverse water-gas shift flowsheet, CO2 and H2 feed gas are advantageously heated up (e.g., in an electric heater) to a typical temperature of around 550°C before entering the rWGS reactor, which is, for example, a refractory lined autothermal reactor. Test data has highlighted that a feed gas of H2 and CO2 can undergo reverse water-gas shift reaction from 350°C resulting in CO formation. The extent of CO formation is increased with the increase in temperature. Prior to the test, it was not foreseen that the feed gas would undergo reverse water-gas shift reaction at 550°C without the presence of catalyst. The formation of CO at 550°C can potentially cause metal dusting of the pipework and electric heater over time. Furthermore, it is possible that methanation reactions can also occur, resulting in a temperature rise in the electric heater and outlet piping. If uncontrolled, then high temperature rise can result in damage to pipework and equipment. In addition, literature has highlighted that pipe material can also catalyse reactions.

[0037] To reduce the extent of methanation and CO formation resulting in metal dusting within the pre-heater and pipework of a reverse water-gas shift system, the following can be carried out.

[0038] Reduce operating pressure

[0039] Based on Le Chatelier's principle, reducing the pressure shifts equilibrium to the left (hydrogen side) results in a reduction of methane formation in the heater. Furthermore, while pressure does not influence the reverse water-gas shift reaction equilibrium, operating at a lower pressure reduces the partial pressure of CO and thus the CO reaction rate. Further still, lower pressure reduces the rate of methanation resulting in lower methane exit from the reverse water-gas shift reactor. Methane is an inert in the downstream Fischer-Tropsch process and thus it is advantageous to reduce methane content of the synthesis gas exiting the reverse water-gas shift reactor.

[0040] Introducing steam into the feed gas

[0041] Based on Le Chatelier's principle for the reverse water-gas shift reaction, increasing the steam content shifts equilibrium to the reactant side, resulting in a reduction of CO formation. Increasing the presence of steam creates an oxidising environment and reduces the amount of carbon formed from the CO reduction reaction. Furthermore, for the methanation reaction an increase in steam flow increases the conversion of methane to CO and H2, resulting in a lower gas temperature. Steam can be added directly to the H2 and CO2 mixed feed stream prior to heating. Alternatively, the steam can be introduced into a hydrocarbon containing recycle stream from a downstream hydrocarbon synthesis process which is deriched to form a methane / stream stream which then mixed with the H2 and CO2 feed stream prior to heating. In this regard, a deriched tails gas typically exits a derichment reactor at around 530 to 550°C, and thus may not obviously be flowsheeted to tie in upstream of an electric heater for preheating to 550°C. However, in the present method the deriched tails gas is provided with steam and fed upstream of the electric heater to inhibit metal corrosion within the heater by increasing the steam content of the H2 and CO2 mixed feed stream.

[0042] Use of a corrosion inhibitor

[0043] A reduction in corrosion within the heater can also be achieved by adding a corrosion inhibitor, such as a sulphur-based corrosion inhibitor (e.g., H2S or a disulphide), to the feed gas upstream of the heater.

[0044] Increase resistance of electric heater and piping material against metal dusting

[0045] Use of more resistive or coated alloys (e.g., nickel-chromium alloys) in the heater and pipework leading to the reverse water-gas shift reactor can alleviate the issue of metal dusting.

[0046] Examples implementing the aforementioned approaches to reducing metal corrosion in the heater and pipework leading to the reverse water-gas shift reactor are described below.

[0047] Figure 1 shows a flow sheet for a method of forming crude syngas. The method comprises heating a mixed carbon dioxide and hydrogen feed gas 2 to a temperature of at least 400°C using a heater 8 to form a heated mixed feed gas stream 10. To prevent metal corrosion in the heater 8 and subsequent pipework, a steam feed 4 and a corrosion inhibitor 6 are mixed with the carbon dioxide and hydrogen feed gas 2 prior to entering the heater 8. The pressure of the feed gas 2 is also controlled so as to inhibit corrosion within the heater, e.g., controlled so that the absolute pressure does not exceed 20 bar (e.g., operating at not more than 20 barg, optionally around 15 barg). Alternatively, or additionally, the heater can be provided with one or more components formed of a nickel-chromium alloy which is resistant to corrosion during operation.

[0048] Subsequently, the heated mixed feed gas stream 10 is fed from the heater 8 into a reverse water-gas shift reactor 12 comprising a reverse water-gas shift catalyst and the heated mixed feed gas stream is passed over the reverse water-gas shift catalyst within the reverse water-gas shift reactor 12 to form a crude syngas product stream 14 by converting at least a portion of the carbon dioxide to carbon monoxide.

[0049] The flow sheet thus provides a method of forming a synthesis gas, the method comprising: feeding a mixture of hydrogen and carbon dioxide feed gas 2 to a heater 8; heating the mixture of hydrogen and carbon dioxide feed gas 2 in the heater to a temperature of at least 400°C; and feeding the heated mixture of hydrogen and carbon dioxide feed gas 10 through a connective pipe to a reverse water-gas shift reactor 12 having a refractory lining to form synthesis gas 14 comprising hydrogen and carbon monoxide via a reverse water-gas shift reaction within the reverse water-gas shift reactor 12; wherein, to inhibit metal corrosion in the heater 8 and the connective pipe between the heater 8 and the reverse water-gas shift reactor 12 during operation, the method comprises one or more of the following characterizing features: providing the feed gas 2 to the heater 8 at an absolute pressure of no more than 20 bar; mixing steam 4 with the feed gas 2 prior to entering the heater 8; mixing a corrosion inhibitor 6 with the feed gas 2 prior to entering the heater 8; and providing the heater 8 with one or more components formed of a nickel-chromium alloy which is resistant to corrosion during operation.

[0050] Optionally, the mixture of hydrogen and carbon dioxide feed gas is heated in the heater to a temperature of: at least 450°C, 500°C, 550°C, 600°C, or 700°C; no more than 1500°C, 1300°C, 1200°C, 1100°C, 1000°C, 900°C, or 850°C; or within a range defined by any combination of the aforementioned lower and upper limits (e.g., in a range 450°C to 1100°C, optionally operating around the upper end of that range). Such temperatures can be achieved by a number of different types of heater, including heaters which are selected from: a heat exchange heater; a turbo-machinery heater; an electrical heater; a resistive heater; and a radiant heater. High pre-heat temperatures can be advantageous for improving the efficiency of the reverse water-gas shift reactor but can exacerbate the issue of metal corrosion in the heater. As such, the present method is particularly useful when using relative high pre-heater temperatures.

[0051] Where the heater is provided with one or more components formed of a nickel-chromium alloy which is resistant to corrosion during operation, the nickel-chromium alloy can be a nickel-chromium only alloy, a nickel-chromium-aluminium alloy, or a nickel-chromium-copper alloy. Examples include one or more of alloy 310, 310S, 310Si, 800 / H / HT, RA 330, alloy 600 / H, 625, 890, RA330, RA230, Alloy X, CT15C, Paralloy CR39W, H39WMR, H39WM+, HK40, HPW, HPNb, HP Microalloy, alloy 601, 602 CA, 603XL, 617, 671, 690, 693, 803, NEXAGE™696, VDM® ALLOY 699 XA, HAYNES® HR-235®, HR214, HR120, HR160, H46M, RA253MA, RA333, RA353MA, 45TM, Paralloy OPTIM-AL, H46M, or H48T. The alloy may also provide pressure containment in the heater, or the alloy may form a coating with another metal providing mechanical support for pressure containment in the heater. To further improve the corrosion resistance of the heater components, the nickel-chromium alloy may comprise a coating such as an aluminium diffusion coating, a chrome diffusion coating, a silicon coating, or a catalytic coating.

[0052] In order to further decrease corrosion, a corrosion inhibitor can be added to one or more of the feed streams prior to the mixing zone. For example, the corrosion inhibitor may be a sulphur-based corrosion inhibitor such as H2S or a disulphide. Such reagents result in sulphur adsorbing onto metal surfaces which prevents carbon ingress to the base alloy. Accordingly, for increased corrosion protection, a low level of a reactive sulphur species such as H2S or a disulfide may be provided in the process environment. Sulphur levels less than 10 ppm (e.g., in a range 1 ppb to 10 ppm, optionally 1 ppm to 10 ppm) are sufficient to mitigate metal dusting. Other non-sulphur inhibitors are also known.

[0053] In order to further decrease corrosion, the feed gas can be provided to the heater at an absolute pressure of: no more than 20 bar, 19 bar, 18 bar, 17 bar, or 16 bar; no less than 5 bar, 10 bar, 12 bar, or 14 bar; or within a range defined by any combination of the aforementioned upper and lower limits.

[0054] In order to further decrease corrosion, the steam can be mixed with the feed gas to achieve a feed gas steam content of: at least 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.5 mol%, 1 mol%, 2 mol%, 2.5 mol%, 3 mol%, 3.5 mol%, 4 mol%, or 4.5 mol%; no more than 10 mol%, 9 mol%, 8 mol%, 7 mol%, 6 mol%, or 5 mol%; or within a range defined by any combination of the aforementioned lower and upper limits (e.g., 2.5 to 5 mol%).

[0055] Further still, the connective pipe between the heater and the reverse water-gas shift reactor can be refractory lined or formed of a nickel-chromium alloy which is resistant to corrosion during operation (such as described above for use in the heater).

[0056] The synthesis gas formed using the methodology as described above can be used to produce hydrocarbons using a Fischer-Tropsch system. As such, the present specification also provides a method for synthesising hydrocarbons, the method comprising: forming a synthesis gas as described above; and passing the synthesis gas though a Fischer-Tropsch system to form a hydrocarbon product stream. A hydrocarbon containing recycle stream is also formed by the Fischer-Tropsch system itself and / or formed by an upgrading unit coupled to the Fischer-Tropsch system. In that case, the method may further comprise: feeding steam into the hydrocarbon containing recycle stream to form a recycle stream comprising a mixture of steam and hydrocarbon; feeding the recycle stream comprising a mixture of steam and hydrocarbon to a derichment reactor containing derichment catalyst generating a recycle stream comprising steam and methane; and mixing the recycle stream comprising steam and methane with the feed gas comprising hydrogen and carbon dioxide prior to the feed gas entering the heater, whereby the steam which is mixed with the feed gas is provided by the recycle stream comprising steam and methane.

[0057] Figure 2 shows a flow sheet for a method of forming a hydrocarbon product stream by combining the syngas forming method of Figure 1 with a Fischer-Tropsch unit. The crude syngas product stream 14 is produced in the same or similar manner as described above in relation to Figure 1 and for conciseness these steps / parts will not be repeated.

[0058] The crude syngas from the reverse water-gas shift reactor is advantageously processed to remove water and carbon dioxide prior to feeding into the Fischer-Tropsch unit. As such, the crude syngas 14 can be passed to a water removal unit 22 to form a separated water stream 24 and a water-depleted syngas 26. The separated water stream 24 can be fed into an electrolyser 28 to produce a hydrogen stream 30 which is recycled into the feed gas 2 comprising hydrogen and carbon dioxide for the reverse water-gas shift reactor 12. The electrolyser 28 can also produce an oxygen stream 32 which is fed to the burner of the reverse water-gas shift reactor 12.

[0059] The crude syngas, advantageously after removing water, is fed to a carbon dioxide removal unit 34 to form a separated carbon dioxide stream 36 and a carbon dioxide-depleted syngas 38. The separated carbon dioxide stream 36 can be recycled into the feed gas 2 comprising hydrogen and carbon dioxide.

[0060] After removal of water and carbon dioxide, and optional further processing to remove other contaminants, the syngas can then be passed to the Fischer-Tropsch unit 40 to produce a hydrocarbon product stream 42 and a tail gas stream 44. Advantageously, the tail gas stream 44 is recycled into the reverse water-gas shift reactor 12. In this case, the tail gas stream is mixed with a steam stream 4 and passed through a derichment reactor 46 to form a methane and steam containing stream 48. This stream is mixed with the feed stream 2 prior to entering the heater 8 to increase the steam content of the feed stream and reduce corrosion in the heater and pipework leading to the reverse water-gas shift reactor 12. Both reverse water-gas shift and steam methane reforming reactions can occur within the reverse water-gas shift reactor 12 to produce the crude syngas.

[0061] To illustrate the benefits of the present approach a counter-example is described below, followed by a summary of some of the experiments which have resulted in modifications according to the present specification. Figure 3 shows an example of a flow sheet for a method of forming crude syngas which is not according to the present specification. A carbon dioxide feed gas is mixed with a hydrogen feed gas to produce a mixed feed gas 2. The mixed feed gas 2 is passed through a mixed feed gas interchanger 52 to heat up the mixed feed gas using the hot syngas product stream from the reverse water-gas shift reactor. For example, the mixed feed gas at this stage may be at a temperature around 400°C (398°C in the illustrated example). The mixed feed gas is then passed through a mixed feed gas heater 8 to further increase the temperature of the mixed feed gas. For example, in the illustrated example the mixed feed gas is heated to 550°C. The heated mixed feed gas 10 is then passed to the reverse water-gas shift reactor 12.

[0062] A tails gas stream 44 (e.g., recycled from a down-stream hydrocarbon synthesis unit) is passed through a tails gas interchanger 58 to heat up the tails gas using the hot syngas product stream from the reverse water-gas shift reactor. The tails gas is then passed through a derichment reactor 46 to produce a methane containing gas stream 59 which is mixed with the heated mixed feed gas stream 10. In the illustrated example, the methane containing gas stream from the derichment reactor is at a temperature of 530°C, the heated mixed H2 / CO2 gas stream is at 550°C, and when the two streams are mixed and fed to the reverse water-gas shift reactor the feed gas is at a temperature of 548°C. It may be noted that the heating element of the feed gas heater will be at a higher temperature than the process gas temperature.

[0063] An oxygen feed 60 is also fed to the reverse water-gas shift reactor 12 and combusted to further increase the temperature of the reactants to drive the reverse water-gas shift reaction and produce a crude syngas product stream. In the illustrated example the crude syngas exiting the reactor is at a temperature of 900°C. The hot crude syngas can be used to raise steam in a syngas boiler 62 and is then passed through the tails gas interchanger 58 and the H2 / CO2 feed gas interchanger 52.

[0064] The present inventors have identified that the mixed feed gas heater 8 and the gas line 10 from the heater 8 to the reverse water-gas shift reactor 12 may be susceptible to damage under certain operating conditions and over prolonged periods of operation (highlighted region in Figure 3). This is due to the potential for CO formation in this region causing metal degradation.

[0065] In relation to the above, experiments have been performed to determine how much CO may be formed when pre-heating a mixed H2 and CO2 feed prior to entering the reverse water-gas shift reactor as is the case for the flow sheet of Figure 3. A mixed H2 and CO2 feed was preheated from 300°C to 500°C through an empty reactor. The reactor was preheated with nitrogen before switching over to a H2 and CO2 mixture. Each temperature test was carried out for 1 hour before switching to a higher temperature in a step-wise manner over the test temperature range. At each test temperature the amount of CO was measured. Test conditions are summarized below:

[0066] • H2- 70.1 mol%, CO2 - 29.9 mol%

[0067] • Test pressure - 24.7 barg

[0068] • Reactor tube material - Coated alloy

[0069] • Reactor ID (inner diameter) - 28 mm

[0070] • Gas sampled at reactor exit and CO content measured.

[0071] Results are illustrated in Figure 4(a) showing a graph of CO content (mol%) versus temperature. The amount of CO remained relatively low between 300°C and 400°C, which corresponds to the temperature range of the mixed feed gas between the feed interchanger 52 and the heater 8 in Figure 3. However, CO content increases significantly as the temperature is raised to 500°C or more, which corresponds to the temperature of the mixed feed gas in the heater 8 and gas line 10 to the reactor in Figure 3. Figure 4(b) shows the graph of Figure 4(a) extended beyond the 500°C test data to show the trend in CO formation up to 650°C.

[0072] As such, these experiments would suggest that, at least under these operating conditions, CO generation in the heater 8 and gas line 10 could be a potential problem if mixed feed gas conditions and materials of constructions are not correctly managed.

[0073] Furthermore, there is some evidence in the literature that certain metal alloys utilized to manufacture reactor equipment can affect the amount of CO2 to CO conversion via a reverse water-gas shift reaction. For example, in "High-Temperature Kinetics of the Homogeneous Reverse Water-Gas Shift Reaction" by F. Bustamante and R. M. Enick, AICHE Journal, vol. 50, No. 5. May 2004. pg 1039, reverse water-gas shift reactions in an Inconel 600 (a nickel-chromium alloy) reactor and a quartz reactor were compared. The paper found that Inconel 600 catalyses the reverse water-gas shift reaction compared to an inert quartz reactor and showed some evidence of CO2 conversion at a temperature range from 820K, which is similar to the temperature of the heated mixed feed gas stream 2 in the flow-sheet of Figure 3. As such, it is considered that alloy materials which may typically be used for the feed heater 8 and gas line 10 could conceivably contribute to driving a reverse water-gas shift reaction in the heater and gas line to produce CO prior to entering the reverse water-gas reactor.

[0074] Mechanisms for formation of carbon and metal dusting including the following...

[0075] H2 + CO2 CO + HZO [CO formation from RWGS Reaction]

[0076] 2 CO C + CO2[CO forms carbon via Boudouard Carbon]

[0077] CO + H2 C + H2O [CO forms carbon via CO Reduction]

[0078] Methane (e.g., in recycled, deriched tail gas) cracking can also form carbon. Formation of carbon can cause metal dusting, which is a form of corrosion that occurs when susceptible materials are exposed to environments with high carbon activities. Metal components can corrode over time, potentially resulting in equipment failure. While not being bound by theory, initial data suggests that in the present case / conditions for the heated mixed feed gas in heater 8 and gas line 10 of Figure 3, carbon formation via CO reduction is more likely to be problematic than carbon formation via the Boudouard route. Thermodynamically, the feed gas composition falls in the carbon forming region via CO reduction. This means that thermodynamically the feed gas composition can form carbon. However, the extent to which this may be problematic in practice will also be dependent on the kinetics of the carbon forming process. Below 400-450°C, the kinetics of the process are sufficiently slow that metal dusting is not normally a concern. However, lower feed heater temperatures for the present syngas production process reduces the process efficiency. As such, it is desirable to pre-heat the feed gas to temperatures exceeding 500°C from a process efficiency perspective.

[0079] Figure 5 shows a modified flow sheet which addresses this issue. In this flow sheet, steam 4 is added to the tails gas stream 44 in a sufficient amount that after the derichment reactor 46 the resultant methane stream 48 also contains significant amounts of steam. Furthermore, this mixed methane / steam stream 48 is directed to mix with the carbon dioxide and hydrogen feed stream 2 prior to the heater 8 in order to raise the steam content of the feed stream to the heater 8. This aids in reducing reactions which could lead to metal corrosion in the heater 8 and pipework 10 as described previously. In addition to the above, a corrosion inhibitor 6 is introduced into the feed stream 2 prior to the heater 8 to further reduce corrosion in the heater. Further still, the pressure of the feed stream 2 can be reduced to be, for example, less than 20 bar absolute and / or the components of the heater can be formed of a nickel-chromium alloy which is resistant to corrosion. In other respects, the system of Figure 5 is the same as that of Figure 3. These modifications are sufficient to reduce or eliminate issue with metal corrosion while maintaining an efficient system for synthesis gas production.

[0080] While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

Claims1. A method of forming a synthesis gas, the method comprising: feeding a mixture of hydrogen and carbon dioxide feed gas to a heater; heating the mixture of hydrogen and carbon dioxide feed gas in the heater to a temperature of at least 400°C; and feeding the heated mixture of hydrogen and carbon dioxide feed gas through a connective pipe to a reverse water-gas shift reactor having a refractory lining to form synthesis gas comprising hydrogen and carbon monoxide via a reverse water-gas shift reaction within the reverse water-gas shift reactor; wherein, to inhibit metal corrosion in the heater and the connective pipe between the heater and the reverse water-gas shift reactor during operation, the connective pipe between the heater and the reverse water-gas shift reactor is refractory lined or formed of a nickel-chromium alloy which is resistant to corrosion during operation and the method further comprises at least one corrosion reduction feature selected from the group consisting of: mixing steam with the feed gas prior to entering the heater; mixing a corrosion inhibitor with the feed gas prior to entering the heater; providing the feed gas to the heater at an absolute pressure of no more than 20 bar; and providing the heater with one or more components formed of a nickel-chromium alloy which is resistant to corrosion during operation.

2. A method according to claim 1, wherein the mixture of hydrogen and carbon dioxide feed gas is heated in the heater to a temperature of: at least 450°C, 500°C, 550°C, 600°C, or 700°C; no more than 1500°C, 1300°C, 1200°C, 1100°C, 1000°C, 900°C, or 850°C; or within a range defined by any combination of the aforementioned lower and upper limits.

3. A method according to claim 1 or 2, wherein the heater is selected from: a heat exchange heater; a turbo-machinery heater; an electrical heater; a resistive heater; and a radiant heater.

4. A method according to any preceding claim,wherein the one or more nickel-chromium alloy components of the heater are formed of a nickel-chromium only alloy, a nickel-chromium-aluminium alloy, or a nickel-chromium-copper alloy.

5. A method according to claim 4, wherein the nickel-chromium alloy is selected from one or more of alloy 310, 310S, 310Si, 800 / H / HT, RA 330, alloy 600 / H, 625, 890, RA330, RA230, Alloy X, CT15C, Paralloy CR39W, H39WMR, H39WM+, HK40, HPW, HPNb, HP Microalloy, alloy 601, 602 CA, 603XL, 617, 671, 690, 693, 803, NEXAGE™696, VDM® ALLOY 699 XA, HAYNES® HR-235®, HR214, HR120, HR160, H46M, RA253MA, RA333, RA353MA, 45TM, Paralloy OPTIM-AL, H46M, or H48T.

6. A method according to any preceding claim, wherein the nickel-chromium alloy comprises a coating.

7. A method according to claim 6, wherein the coating is an aluminium diffusion coating, a chrome diffusion coating, a silicon coating, or a catalytic coating.

8. A method according to any preceding claim, wherein the corrosion inhibitor which is mixed with the feed gas prior to entering the heater is a sulphur-based corrosion inhibitor.

9. A method according to claim 8, wherein the corrosion inhibitor is H2S or a disulphide.

10. A method according to claim 8 or 9, wherein the corrosion inhibitor is added to the feed gas to achieve a level of sulphur in a range 1 ppb to 10 ppm.

11. A method according to any preceding claim,wherein the feed gas is provided to the heater at an absolute pressure of: no more than 19 bar, 18 bar, 17 bar, or 16 bar; no less than 5 bar, 10 bar, 12 bar, or 14 bar; or within a range defined by any combination of the aforementioned upper and lower limits.

12. A method according to any preceding claim, wherein the steam is mixed with the feed gas to achieve a feed gas steam content of: at least 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.5 mol%, 1 mol%, 2 mol%, 2.5 mol%, 3 mol%, 3.5 mol%, 4 mol%, or 4.5 mol%; no more than 10 mol%, 9 mol%, 8 mol%, 7 mol%, 6 mol%, or 5 mol%; or within a range defined by any combination of the aforementioned lower and upper limits.

13. A method for synthesising hydrocarbons, the method comprising: forming a synthesis gas according to any preceding claim; and passing the synthesis gas though a Fischer-Tropsch system to form a hydrocarbon product stream.

14. A method according to claim 13, wherein a hydrocarbon containing recycle stream is also formed by the Fischer-Tropsch system itself and / or formed by an upgrading unit coupled to the Fischer-Tropsch system, the method further comprising: feeding steam into the hydrocarbon containing recycle stream to form a recycle stream comprising a mixture of steam and hydrocarbon; feeding the recycle stream comprising a mixture of steam and hydrocarbon to a derichment reactor containing derichment catalyst generating a recycle stream comprising steam and methane; and mixing the recycle stream comprising steam and methane with the feed gas comprising hydrogen and carbon dioxide prior to the feed gas entering the heater, whereby the steam which is mixed with the feed gas is provided by the recycle stream comprising steam and methane.

15. A system configured to perform the method according to any preceding claim.

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