Steam reforming

Abstract

A process is described for steam reforming a hydrocarbon feedstock containing one or more nitrogen compounds, comprising passing a mixture of hydrocarbon feedstock and steam through a catalyst bed composed of one or more nickel steam reforming catalysts arranged within a plurality of externally heated tubes in a tubular steam reformer, each tube having an inlet where the hydrocarbon and steam mixture is fed and an outlet where a reformed gas containing hydrogen, carbon monoxide, carbon dioxide, steam, ammonia, and methane is recovered, wherein the steam reforming catalyst at least at the outlet of the tube is a structured steam reforming catalyst comprising nickel dispersed on the surface of a porous metal oxide present as a coating on a non-porous metal or ceramic structure, wherein the nickel content of the metal oxide coating is in the range of 5 to 50 wt %, and the thickness of the coating is in the range of 5 to 150 micrometers.

Description

[Technical Field]

[0001] The present invention relates to a process for producing synthesis gas by steam reforming of hydrocarbons and an apparatus for carrying out this process.

[0002] Syngas contains hydrogen and carbon oxides (carbon monoxide and carbon dioxide) and may contain nitrogen and other gases such as argon and low levels of methane. Syngas may contain greater or lesser amounts of hydrogen and carbon oxides to suit specific end uses, such as hydrogen production for refineries or fuel cells, ammonia synthesis, methanol synthesis, dimethyl ether synthesis, or the Fischer-Tropsch process for the synthesis of liquid hydrocarbons. Syngas is often produced by a steam reforming process.

[0003] In a conventional steam reforming process, a mixture of hydrocarbon feedstock and steam, and possibly also carbon dioxide, is passed at high pressure through tubes containing a particulate catalyst that is heated externally by a suitable heating medium, typically a hot gas mixture. The particulate catalyst is usually in the form of a shaped unit, e.g., a cylinder with a plurality of through holes, and is typically formed from a refractory supporting structure material, such as α-alumina, calcium aluminate, or magnesium aluminate, impregnated with a suitable catalytically active metal, such as nickel.

[0004] Hydrocarbon feedstocks often contain small amounts of nitrogen, which is converted to ammonia over a steam reforming catalyst. Ammonia is undesirable in the synthesis gas and is soluble in the process condensate, which is preferably returned to the process, and processes have been proposed to try to minimize its production.

[0005] U.S. Patent No. 5,498,404 discloses a process for catalytic steam reforming of a nitrogen-containing carbonaceous feedstock with reduced ammonia production, in which the feedstock is contacted with a nickel-supported catalyst further containing copper in an amount of 0.01 to 10 wt. %, calculated based on the amount of nickel in the catalyst. The copper-containing catalyst is preferably used as a sublayer in a fixed bed of a conventional nickel steam reforming catalyst, and Example 1 shows that as the copper content increases, the catalyst has lower steam reforming activity.

[0006] WO 2009 / 054830 discloses reforming natural gas without excessive ammonia production by using a first stage catalyst with about 10% to about 25% nickel, a second stage catalyst with less than 10% nickel, and a final stage catalyst with a low-concentration rhodium catalyst of 2% or less. Ammonia production is suppressed by reducing the nickel content and surface area in the catalyst in the hottest part of the tube toward the outlet. However, due to the reduced nickel content in the second stage, conversion is undesirably low, which requires a rhodium catalyst to provide a suitable synthesis gas product. Precious metal catalysts are prohibitively expensive for large-scale synthesis gas production in steam reformers, and the use of mixed catalysts adds cost and complexity to catalyst loading, operation, and discharge. Furthermore, mixed catalysts are more difficult to reprocess for metal recovery.

[0007] The present inventors have discovered that by using a nickel steam reforming catalyst in which nickel is present in a thin layer on the catalyst surface, ammonia production can be reduced without resorting to the use of a rhodium catalyst.

[0008] Accordingly, the present invention provides a process for steam reforming a hydrocarbon feedstock containing one or more nitrogen compounds, comprising passing a mixture of hydrocarbon feedstock and steam through a catalyst bed consisting of one or more nickel steam reforming catalysts arranged in a plurality of externally heated tubes in a tubular steam reformer, each tube having an inlet where the hydrocarbon and steam mixture is supplied and an outlet where a reformed gas containing hydrogen, carbon monoxide, carbon dioxide, steam, ammonia and methane is recovered, wherein the steam reforming catalyst at least at the outlet of the tube is a structured steam reforming catalyst comprising nickel dispersed on the surface of a porous metal oxide present as a coating on a non-porous metal or ceramic structure, the nickel content of the metal oxide coating being in the range of 5 to 50 wt %, and the thickness of the coating being in the range of 5 to 150 micrometers.

[0009] The hydrocarbon-containing feedstock fed to the process may include any gaseous or low-boiling hydrocarbon feedstock, such as natural gas, associated gas, LPG, petroleum distillates, diesel, naphtha, or mixtures thereof, or off-gas from a chemical process, such as refinery off-gas or pre-reformed gas. The hydrocarbon feedstock preferably comprises methane and may be pre-reformed gas, associated gas, or natural gas. Natural gas is a particularly preferred feedstock. The feedstock may be compressed to a pressure in the range of 10 to 100 bar (absolute). The pressure of the hydrocarbon feedstock may usefully govern the pressure throughout the process. The operating pressure is preferably in the range of 15 to 80 bar (absolute), more preferably 20 to 50 bar (absolute), as this improves performance from the process.

[0010] If the hydrocarbon feedstock contains sulfur compounds before or, preferably, after compression, the feedstock may be subjected to desulfurization. Desulfurization may include hydrodesulfurization using a CoMo or NiMo catalyst and absorption of hydrogen sulfide using a suitable hydrogen sulfide absorbent, such as a zinc oxide absorbent. Ultra-purified adsorbents may be usefully used downstream of the hydrogen sulfide absorbent to further protect the steam reforming catalyst. Suitable ultra-purified adsorbents may include copper-zinc oxide / alumina materials and copper-nickel-zinc oxide / alumina materials. Hydrogen may be added to the compressed hydrocarbon feedstock to facilitate hydrodesulfurization and / or reduce the risk of carbon laydown in the reforming process. The amount of hydrogen in the resulting mixed gas stream may be in the range of 1 to 20% by volume, but is preferably in the range of 1 to 10% by volume, and more preferably in the range of 1 to 5% by volume.

[0011] If the hydrocarbon feedstock contains chlorides or other contaminants, such as heavy metal contaminants, these may be removed upstream or downstream of any desulfurization step using conventional adsorbents prior to upgrading. Suitable adsorbents for chloride removal are well known and include alkalized alumina materials. Similarly, adsorbents for heavy metals such as mercury or arsenic are known and include copper sulfide materials.

[0012] When the hydrocarbon-containing feedstock is a methane-containing pre-reformed gas, it can be formed by subjecting a hydrocarbon / steam mixture to a process of adiabatic low-temperature steam reforming. The hydrocarbon may be rich natural gas, naphtha, or other hydrocarbon-containing feedstocks containing hydrocarbons heavier than methane. Pre-reforming processes are known. In such processes, the hydrocarbon / steam mixture is typically heated to a temperature in the range of 400-650°C and then adiabatically passed through a fixed bed of a suitable particulate steam reforming catalyst, usually a precipitated catalyst with a high nickel content, e.g., greater than 40% by weight expressed as NiO. During such adiabatic low-temperature reforming process, any hydrocarbons higher than methane are reacted with steam to produce a pre-reformed gas containing a mixture of methane, carbon oxides, and hydrogen. The use of an adiabatic reforming process, commonly referred to as pre-reforming, is desirable to ensure that the feed to the tubular steam reformer does not contain hydrocarbons higher than methane and also contains a significant amount of hydrogen. This is desirable to minimize the risk of carbon formation on the catalyst in the downstream tubular steam reformer.

[0013] In this process, the hydrocarbon feedstock may contain 0.1 to 25% by volume of one or more nitrogen compounds. The content of the one or more nitrogen compounds in the hydrocarbon feedstock may be 0.5 to 25% by volume, 1 to 10% by volume, or 1 to 5% by volume. The one or more nitrogen compounds may include one or more amines, but typically include or consist of nitrogen gas (N2). Thus, the nitrogen gas content of the hydrocarbon feedstock may be in the range of 0.1 to 25% by volume, or 0.5 to 25% by volume, preferably 1 to 10% by volume, and more preferably 1 to 5% by volume.

[0014] The feedstock may be preheated. After compression and before desulfurization, it may be convenient to preheat it with a suitable heat source, such as a fired heater.

[0015] The hydrocarbon feedstock is mixed with steam to form a reforming feed gas. Steam introduction may be achieved by direct injection of steam and / or by saturating the feedstock by contacting the feedstock with a heated water stream. In some embodiments, the hydrocarbon feedstock is saturated in a saturator supplied with hot water to form a saturated gas mixture. The steam content of the saturated gas mixture can be increased, if necessary, by direct addition of steam. The water preferably comprises one or more of the condensate stream recovered from the reformed gas, water recovered from the bottom of the saturator, and other condensates produced in the process. The amount of steam introduced is desirably sufficient to provide a steam-to-carbon ratio of at least 1.8:1, i.e., at least 1.8 moles of steam per gram atom of hydrocarbon in the feedstock. The steam-to-carbon ratio is preferably in the range of 1.8:1 to 5:1, more preferably 2.5:1 to 3.5:1, and especially 2.8:1 to 3.2:1, to provide an optimal balance of hydrogen production and efficiency.

[0016] The reforming feed gas, comprising a hydrocarbon feedstock and steam, may contain 0.02 to 14.0 vol. %, optionally 0.1 to 10.0 vol. %, or optionally 0.2 to 6.0 vol. % of one or more nitrogen compounds, depending on the steam and nitrogen compound content.

[0017] The reforming feed gas mixture is then desirably preheated prior to reforming. In a preferred embodiment, the hydrocarbon / steam mixture is heated by passing it through a combustion heater. Desirably, the mixed stream is heated to an inlet temperature in the range of 300-650°C or 450-650°C, preferably 450-600°C, and more preferably 450-550°C. Inlet temperatures in the range of 300-550°C are particularly suitable when a pre-reformer is not present, and higher inlet temperatures in the range of 550-650°C are particularly suitable when a pre-reformer is present.

[0018] During the reforming process, methane reacts with steam to produce hydrogen, carbon monoxide, and carbon dioxide. Any hydrocarbons present containing two or more carbon atoms are converted to methane, which is then steam reformed. Additionally, a reversible water-gas shift reaction occurs. Overall, the process is endothermic and requires heating of the tubes and catalyst to maintain the reaction and achieve the desired conversion. The heat input to a steam reformer is typically such that the temperature of the product gas stream at the tube outlet is higher than the inlet temperature, often in the range of 100 to 350 or 400°C above the inlet temperature.

[0019] A tubular steam reformer typically comprises a plurality of vertically arranged tubes through which a gas mixture can pass and to which heat is transferred by hot gases flowing around the exterior of the tubes. The hot gases may comprise combustion gases or synthesis gases. The tube inlets are typically at the top so that the feed gas mixture is typically fed into the top of the steam reformer and flows downward through the tubes.

[0020] Thus, a tubular steam reformer may have an inlet for a reformed feed gas, an outlet for a reformed gas mixture, and a plurality of vertical tubes in communication with the inlet and through which the gas mixture can pass and through which heat is transferred by hot gas flowing around the tubes in a heat exchange zone, the tubes each comprising one or more steam reforming catalysts provided as layers within the tube, and at least the layer of steam reforming catalyst adjacent the outlet is a structured nickel steam reforming catalyst.

[0021] The catalyst adjacent to the tube outlet in this process is a structured steam reforming catalyst. By "structured steam reforming catalyst" is meant a steam reforming catalyst coating on a non-porous structure, typically a metal or ceramic structure. The nickel in the structured catalyst is dispersed on the surface of a porous metal oxide, which is supported as a surface coating on the non-porous structure.

[0022] Metal or ceramic structures are inherently nonporous and therefore have a small surface area essentially defined by their shape. To provide sufficient activity, the nickel in the structured catalyst is dispersed on a porous metal oxide coating on a nonporous metal or ceramic support structure. The thickness of the nickel-containing porous metal oxide coating on the nonporous structure ranges from 5 to 150 micrometers, preferably from 10 to 100 micrometers, more preferably from 10 to 80 micrometers, and most preferably from 10 to 50 micrometers. The preferred ranges provide optimal catalyst activity as well as adhesion and cohesion.

[0023] Nickel may be applied to the structure by known washcoating methods, whereby a metal oxide slurry, which may include nickel oxide as a component, is applied to the metal or ceramic structure by dipping or spraying, and then dried and heat-treated to bond the metal oxide to the supporting structure. Nickel may also be applied to a metal oxide-coated supporting structure by impregnation techniques using a soluble nickel compound, which is then dried and calcined to convert the nickel compound to nickel oxide. Combinations of these techniques may also be used, including applying nickel in solution in combination with a slurry of the metal oxide supporting structure.

[0024] The structural steam reforming catalyst comprises nickel. The nickel content of the metal oxide coating ranges from 5 to 50 wt%, preferably from 10 to 30 wt%, more preferably from 10 to 20 wt%. Optionally, a platinum group metal selected from platinum, rhodium, ruthenium, or palladium, or mixtures thereof, may be included in the coating. If included, a platinum group metal promoter may be present in the coating in an amount ranging from 0.05 to 1 wt%. The coating may have a thickness of 10 to 150 g / m 2 , preferably 10 to 80 g / m 2 , more preferably 30 to 60 g / m 2 The non-porous support structure may be coated with an amount ranging from 0.1 to 100% by weight of the non-porous support structure.

[0025] An alkali metal oxide, such as potassium oxide, may also be present in the structural porous metal oxide coating, but is not usually necessary. The amount of alkali metal oxide, if present, may range from 0.5 to 7.0 wt. %.

[0026] The porous metal oxide in which the nickel is dispersed may be any suitable refractory oxide, including alumina, titania, zirconia, zinc oxide, magnesia, ceria, praseodymium oxide, yttria, and lanthana. Preferred porous metal oxides include alumina, zirconia, ceria, lanthana, and mixtures of two or more thereof.

[0027] In some embodiments, the metal or ceramic structure may be a solid, such as a sphere or cylinder, which may have one or more holes therethrough. Such structured catalysts can be used in a manner comparable to conventional pelletized steam reforming catalysts.

[0028] In some embodiments, a structured catalyst may comprise a metal or ceramic structure having multiple passages through which a process fluid may pass in an ordered, non-random direction. Such structured catalysts are preferred when reduced pressure drop and improved heat transfer are required. Thus, a structured catalyst may comprise a cylindrical unit having a diameter complementary to the tube in which the structured catalyst is placed, including multiple passages through which a process fluid may pass in an ordered, non-random direction. By "complementary," we mean that the diameter of the cylindrical unit may be 1-20 mm smaller than the inner diameter of the tube in which the cylindrical unit is placed, thereby allowing the cylindrical unit to fit snugly within the tube. The cylindrical unit may include perforations and / or internal structures that allow the process fluid to flow both axially and radially as it passes through the unit. Preferably, the cylindrical units are stackable, allowing them to be easily stacked on top of each other and to be self-supporting within the tube. An advantage of using a cylindrical unit having multiple passages through which a process fluid may pass in an ordered, non-random direction, rather than coated pellets, is that the amount of catalyst coating required may be reduced. For example, for coated metal or ceramic pellets, the coating can be applied at about 100-150 kg of Ni as NiO per cubic meter of tube. For coated cylindrical structures with multiple passages through which process fluids can pass in an ordered, non-random direction, the amount of coating can be about 4-7 kg of Ni as NiO per cubic meter of tube.

[0029] Commercially available structured catalysts prepared using stainless steel metal foil are preferred.

[0030] A preferred structured catalyst is described in U.S. Patent Application Publication No. 2012 / 0195801 A1. These structured catalysts include a fan in the form of a corrugated metal disk disposed on a central rod. The fan has a radial fluid duct formed from folded metal foil that radially guides fluid flow into contact with the inner wall of the tube. The fan has a top, bottom, and outer diameter surface such that the radial fluid duct terminates along the outer diameter surface of the fan to form a fluid duct opening facing the inner wall of the tube. The fan further has a flat or corrugated metal washer in contact with the top or bottom surface of the fan, which may be in the shape of a ring having an inner diameter and an outer diameter, and the washer contacts the top or bottom surface of the fan such that the outer diameter of the washer extends radially outward from the outer diameter surface of the fan. The washer may further have spacing tabs extending outward from the outer diameter of the washer that separate it from the inner wall of the tube such that the washer creates a gap between the outer diameter surface of the fan and the reactor tube. Additional structured catalyst arrangements on which the steam reforming catalyst may be supported include those described in U.S. Patent Application Publication Nos. 2012 / 0294779, 2012 / 0288420, U.S. Patent Nos. 8,257,658, 8,235,361, 7,976,783, 7,566,487, 7,761,994, 8,178,075, and 7,871,579.

[0031] There may be a single type of structured catalyst in each tube, in which case the catalyst bed in the tube consists solely of that structured catalyst, or there may be two, three, or more layers of steam reforming catalyst in the tube, in each case at least the layer adjacent the outlet of the tube being a structured catalyst.

[0032] Thus, the tubes can include an unstructured steam reforming catalyst upstream of the structured catalyst. The relative amount of steam reforming catalyst may vary in thickness to achieve the desired conversion. In some embodiments including a layer of unstructured catalyst and a layer of structured catalyst adjacent the tube outlet, the structured catalyst layer may comprise 95% to 5% of the bed volume, or 80% to 20% of the bed volume, or 75% to 25% of the bed volume. Other layers of steam reforming catalyst in the catalyst bed in the remainder of the tube may be conventional pelletized catalysts in which nickel is distributed throughout the pellets, or eggshell nickel catalysts in which the nickel layer is present only on the surface of refractory metal oxide pellets. Suitable unstructured catalysts consist of nickel supported on a refractory oxide support containing an alkaline earth metal aluminate, such as alumina or calcium aluminate and / or magnesium aluminate, and optionally potassium oxide pellets.

[0033] When the catalyst bed is composed of two or three layers of nickel steam reforming catalyst, the catalyst layer at the inlet of the tube is preferably an unstructured pelleted nickel steam reforming catalyst. When an unstructured pelleted catalyst is used upstream of a structured steam reforming catalyst, the unstructured pelleted catalyst preferably has a nickel content, expressed as NiO, in the range of 10% to 30% by weight. Thus, in some embodiments, the catalyst tube may comprise or consist of two layers of nickel steam reforming catalyst, the catalyst layer adjacent the outlet of the tube being a structured nickel steam reforming catalyst and the catalyst layer adjacent the inlet of the tube being an unstructured pelleted nickel steam reforming catalyst containing 10% to 30% nickel, expressed as NiO.

[0034] The catalyst is typically supplied to the tubes of a tubular steam reformer in an oxidized form and activated by reduction of nickel oxide to form elemental nickel in situ. For example, the catalyst in oxidized form can be placed within the tubes and the nickel oxide reduced with a reducing agent such as a hydrogen-containing gas. Known reduction techniques may be used to produce an active catalyst for steam reforming.

[0035] Alternatively, the nickel oxide in the catalyst may be reduced ex-situ, and then an oxygen-containing gas, such as air or nitrogen-diluted air, may be used to coat the elemental metal with a thin passivating layer of oxide. A mixture of oxygen and carbon dioxide, and optionally nitrogen, may also be used. In this way, the reduced state of the catalyst can be safely delivered to the user, and the time to produce an active catalyst and the amount of hydrogen used during subsequent activation may be reduced.

[0036] Various tubular steam reformer configurations can be used. The tubular steam reformer may be a conventional top-fired steam reformer or a side-fired steam reformer. In such reformers, hot gas is supplied by burning fuel gas using multiple burners located at the top ends of the tubes or along the length of the tubes. Alternatively, the steam reformer may be a gas-heated reformer (GHR), in which the hot gas can be provided by flue gas from a combustion process, or by synthesis gas produced by catalytic or non-catalytic partial oxidation of hydrocarbons or by autothermal reforming of a hydrocarbon and / or reformed gas mixture. Furthermore, the hot gas may be mixed with reformed gas that has passed through multiple tubes. The tubes may have a circular cross section, a length of 5 to 15 m, and an inner diameter preferably ranging from 5 to 30 cm or 10 to 15 cm. In use, these tubes operate with a temperature gradient along their length, and the inlet end of the tube is cooled by the endothermic steam reforming reaction. The temperature of the tube and reaction gas at the inlet may be in the range of 300-650°C or 450-650°C, preferably 450-600°C, and more preferably 450-550°C. At the outlet end of the tube, where conversion to form synthesis gas is essentially complete, the tube is at a higher temperature. The temperature of the tube and reaction gas at the outlet may be in the range of 600-950°C. The temperature of the catalyst within the tube can be expressed as the bed temperature, which is the average temperature of the catalyst between the inlet and outlet of the catalyst bed within the tube. The bed temperature may desirably be in the range of 625-775°C or 640-760°C.

[0037] Reformed gas or crude syngas is recovered from the tube outlet. The reformed gas contains hydrogen, carbon monoxide, carbon dioxide, steam, ammonia, and methane. The reformed gas contains some methane due to the equilibrium limitations of the process. The methane content, or "methane slip," from the tubular reformer is an indicator of process efficiency. Furthermore, methane can accumulate in downstream processes that use the reformed gas, which is undesirable; therefore, low methane slip is desirable. Using structured steam reforming catalysts and reaction conditions, the process can provide low methane slip, for example, less than 15% by volume on a dry gas basis. The methane slip is preferably less than 10% by volume on a dry gas basis, particularly less than 5% by volume on a dry gas basis, where the crude syngas does not subsequently undergo secondary reforming or autothermal reforming. The term "on a dry gas basis" is used to disregard the steam content of the reformed gas and to allow comparison with other reformed gases having different amounts of steam.

[0038] Furthermore, the ammonia content of the reformed gas is surprisingly very low. Without wishing to be bound by theory, applicants believe that the presence of nickel only as a thin layer on the surface of the structure means that side reactions that produce ammonia are suppressed. Thus, in this process, the ammonia content of the reformed gas can be less than 200 ppmv on a dry gas basis. In some embodiments, the ammonia content of the reformed gas can be less than 100 ppmv, preferably less than 50 ppmv, and more preferably less than 10 ppmv on a dry gas basis.

[0039] Accordingly, the present invention further provides the use of a structured steam reforming catalyst as described herein, comprising nickel dispersed on the surface of a porous metal oxide present as a coating on a non-porous metal or ceramic structure, wherein the nickel content of the metal oxide coating is in the range of 5 to 50 wt % and the thickness of the coating is in the range of 5 to 150 micrometers, for suppressing ammonia formation during catalytic steam reforming of a nitrogen-containing hydrocarbon feedstock.

[0040] The process can further include cooling the reformed gas below its dew point to condense the steam, and then separating the liquid condensate to form synthesis gas from the reformed gas. The condensate captures most, if not all, of the ammonia produced in the steam reforming process. The ammonia content of the condensate may be less than 400 mg / liter, preferably less than 200 mg / liter, more preferably less than 100 mg / liter, and most preferably less than 50 mg / liter, or even 20 mg / liter. In a preferred embodiment, at least a portion of the condensate is recycled and used to generate steam for use in the steam reforming process.

[0041] The process of the present invention can be used as part of a process for producing liquid hydrocarbons such as hydrogen, methanol, dimethyl ether, olefins, ammonia, urea, or diesel fuel obtained by Fischer-Tropsch synthesis. The reformate gas can then be subjected to further processing, including one or more of the following steps: cooling below the dew point of the water vapor, condensate separation, hydrogen separation, carbon dioxide separation, methanol synthesis, dimethyl ether synthesis, olefin synthesis, ammonia synthesis, or liquid hydrocarbon synthesis. Known processes can be used to accomplish these steps. [Brief explanation of the drawings]

[0042] The present invention will be further described with reference to the following Figures 1 to 6.

[0043] [Figure 1] FIG. 1 is a graph showing the % moles of ammonia produced per second to ethane conversion in tests using a reformer feed containing 2% N2 by volume.

[0044] [Figure 2] FIG. 2 is a graph showing the % moles of ammonia produced per second to ethane conversion in tests using a reformer feed containing 5% N2 by volume.

[0045] [Figure 3] FIG. 3 is a graph showing the % moles of ammonia produced per second to ethane conversion in tests using a reformer feed containing 8% N2 by volume.

[0046] [Figure 4] FIG. 4 is a graph showing the % mole ethane conversion of the catalyst versus ammonia produced per second per m2 of Ni in the catalyst in a test using a reformer feed containing 2% N2 by volume.

[0047] [Figure 5] FIG. 5 is a graph showing the % mole ethane conversion of the catalyst versus ammonia produced per second per m2 of Ni in the catalyst in a test using a reformer feed containing 5% N2 by volume.

[0048] [Figure 6] FIG. 6 is a graph showing the % mole ethane conversion of the catalyst versus ammonia produced per second per m2 of Ni in the catalyst in a test using a reformer feed containing 8% N2 by volume.

[0049] Example 1 Tests were conducted on a conventional cylindrical pelletized steam reforming catalyst containing 17.6 wt.% nickel or 7.2 wt.% nickel, and a structured catalyst comprising a catalyst coating containing 13 wt.% nickel and 0.25 wt.% rhodium on stabilized aluminum oxide applied as a washcoat to a stainless steel pellet (3.3 x 3.3 mm cylinder). The catalyst coating loading was 23 mg / in. 2 The thickness of the catalyst coating was about 30 micrometers.

[0050] [Table 1]

[0051] The catalyst was tested in a laboratory-scale steam reformer equipped with a single electrically heated reformer tube with an internal diameter of approximately 25 mm and a length of approximately 2100 mm. The reactor was operated on an upflow basis. Water for generating steam was supplied to the rig via a variable stroke pump and fed to the bottom of the reactor, where the water was evaporated. Natural gas was fed through a separate desulfurization vessel before being delivered to the reactor via a thermal mass flow controller. Nitrogen and hydrogen, as needed, were also supplied to the reactor via independent mass flow controllers. All water and gas entered the reactor through the same inlet pipe. Product gas exited the reactor through an outlet from the tube, cooled to ambient temperature to condense the water vapor, and then collected in a catch pot. A small amount of the dry outlet gas was fed to a Varian CP490 quad-channel micro-GC analyzer. This gas was then returned to the outlet gas meter to allow for the calculation of a complete mass balance from the reformer.

[0052] For each of the catalysts, the pellet size was measured to be 21080 mm 2 The number of pellets required to provide a geometric surface area (GSA) of 1000 sq m was determined. 363 coated pellets were loaded for the structured catalyst, and 389 pellets were loaded for the comparative catalyst. The amount of nickel loaded into the reactor tube was 0.07 g for the structured catalyst, 3.45 g for comparative catalysts 1(a) and 1(b), and 1.30 g for comparative catalysts 1(a) and 1(b), respectively. The pellets were diluted to 100 mL with 3.35-4.00 mm alumina chips, and the mixture was loaded into the reformer tube near the outlet. The remainder of the reformer tube was filled with 3.35-4.75 mm alumina chips.

[0053] The catalyst was reduced with 50 vol% H2 in N2 at 600 °C for 2 h.

[0054] Reforming was then carried out at 27 barg pressure using a 3:1 steam-to-carbon ratio and bed inlet temperatures ranging from 510 to 800°C. Catalyst conditioning of the comparative catalyst was carried out by first operating the reformer at inlet temperatures of 610°C, 685°C, 735°C, 800°C, and 735°C for at least 8 hours each. Catalyst conditioning of the structured catalyst was carried out by operating the reformer at inlet temperatures of 510°C, 580°C, 610°C, 685°C, 735°C, 800°C, 735°C, 685°C, 610°C, 580°C, and 510°C for at least 8 hours each, followed by a 16.5-hour H2 treatment at 800°C to ensure all nickel was in the active, reduced form. This additional conditioning was to ensure the catalyst was fully reduced and is not expected to affect ammonia production in subsequent tests.

[0055] After conditioning, each catalyst was tested at inlet temperatures of 685°C, 735°C and 800°C.

[0056] The nitrogen content of the feed was adjusted to give 2, 5 and 8 vol. % N2 on a wet gas basis in the feed gas mixture at the inlet to the catalyst.

[0057] The reformed gas was collected from the reformer and cooled below the dew point to condense the water vapor and form an ammonia-containing condensate. The amount of ammonia in the condensate is proportional to the ammonia produced by the catalyst in the steam reformer. Condensate samples (250 mL) were collected over a 5-minute period at the end of the 8-hour test period and analyzed for their ammonia content.

[0058] Ammonia concentrations in the condensate recovered from the reformed gas were measured using a calibrated ion-selective electrode (ISE). Standard solutions of 0.1, 1, and 10 ppm (w / v) ammonia were prepared. Sodium hydroxide buffer solution was added to the sample to liberate ammonia. When the ISE voltage measurements stabilized, the readings were used to construct a linear calibration curve of the ISE voltage readings versus the log10 ammonia concentration. The ammonia concentration of the condensate was analyzed in the same manner, and the ISE voltage readings were used to determine the ammonia concentration by derivation from the calibration curve.

[0059] The test was repeated for each catalyst using a feed gas containing different amounts of nitrogen by introducing nitrogen at various flow rates through the nitrogen supply line to provide the desired level in the feed gas supplied to the reformer tube.

[0060] Below is a table showing the results of ammonia produced in the condensate for different catalysts for different nitrogen contents in the feed gas.

[0061] [Table 2]

[0062] [Table 3]

[0063] [Table 4]

[0064] Over a range of inlet temperatures, the structured catalyst produced less ammonia than the comparative example. However, the catalysts contained different amounts of nickel, had different nickel surface areas, and thus different activities. A more active catalyst consumed more steam than a less active catalyst. Condensation of this unreacted steam impacts the ammonia concentration. To account for this, the molar water flow rate was calculated based on the oxygen balance derived from knowledge of the feed gas composition and velocity and gas chromatography data for the reformed gas obtained using a GC system connected to the steam reformer. The difference in the amount of oxygen entering and leaving the system can be used to determine the amount of ammonia produced per second.

[0065] Furthermore, the structured catalyst was able to produce reformate gas with high conversion of hydrocarbons in natural gas.

[0066] The reformed gas, after removal of condensate, was analyzed by gas chromatography to determine the conversion of hydrocarbons to hydrogen and carbon oxides. The conversion of ethane in natural gas is a better measure of overall catalytic activity than the conversion of methane, which is reversible.

[0067] Plotting ammonia concentration / sec versus ethane conversion shows the effectiveness of the catalyst in terms of activity and ammonia production. Figures 1-3 show ammonia produced / sec versus ethane conversion (%). The results are shown below.

[0068] [Table 5]

[0069] [Table 6]

[0070] [Table 7]

[0071] It can be seen that the ethane conversion of the structured catalyst at the temperatures tested starts at a lower level than the comparative catalyst, but at 800°C, the structured catalyst provides a higher ethane conversion than comparative catalyst 1(b), but with some of the ammonia produced, regardless of whether the N content of the feed gas was 2%, 5%, or 8% by volume. Comparative catalyst 1(a) at 735°C gives an ethane conversion comparable to the structured catalyst at 800°C, but the latter contains some of the nickel content. Testing higher inlet temperatures on the structured catalyst further improved ethane conversion and was able to maintain low ammonia concentrations.

[0072] Although the activity of the structured catalyst is lower than that of the standard pelleted catalyst for a given inlet temperature, it is useful to consider the ammonia produced when the catalyst is operating at the same hydrocarbon conversion to better reflect expected behavior in service. Furthermore, in the structured catalyst, the nickel crystallites are an order of magnitude smaller than those in the pelleted catalyst, resulting in a different nickel surface area (0.5 m for Comparative Catalyst 1(a)). 2 / g and 8m for structured catalysts 2 / g). With this in mind, the performance difference between the structured and pelleted catalysts is even more clearly demonstrated. Figures 4, 5, and 6 show ethane conversion (%) versus ammonia produced as a function of nickel surface area. These figures show that the amount of ammonia produced using the structured catalyst is significantly less than with the conventional pelleted catalyst. Furthermore, it is possible to obtain activity comparable to that of the pelleted catalyst while using significantly less nickel and producing significantly less ammonia. This was achieved by dispersing the nickel in a thin coating on a non-porous support. These results demonstrate that ammonia production can be reduced by utilizing a structured catalyst near the reformer outlet without affecting overall reforming performance.

Claims

1. passing a mixture of hydrocarbon feedstock and steam through a catalyst bed comprised of one or more nickel steam reforming catalysts disposed within a plurality of externally heated tubes in a tubular steam reformer; cooling the reformed gas below its dew point to condense the steam and remove a portion of the ammonia produced in the steam reforming process; and separating the liquid condensate comprising ammonia; each tube having an inlet into which the hydrocarbon and steam mixture is supplied and an outlet from which a reformed gas containing hydrogen, carbon monoxide, carbon dioxide, steam, ammonia, and methane is recovered; the steam reforming catalyst at least at the outlet of the tube is a structured steam reforming catalyst comprising nickel dispersed on the surface of a porous metal oxide present as a coating on a non-porous metal or ceramic structure; the nickel content of the metal oxide coating is in the range of 5 to 50 wt. % and the thickness of the coating is in the range of 5 to 150 micrometers; 0.1 to 25% by volume of nitrogen gas (N ２ 1. A process for steam reforming a hydrocarbon feedstock containing

2. 2. The process of claim 1, wherein the thickness of the porous metal oxide coating containing nickel on the non-porous structure is in the range of 10 to 100 micrometers, preferably in the range of 10 to 80 micrometers, more preferably in the range of 10 to 50 micrometers.

3. 3. The process according to claim 1 or 2, wherein the nickel content of the metal oxide coating is in the range of 10 to 30 wt.%, preferably 10 to 20 wt.%.

4. A process according to any one of claims 1 to 3, wherein a platinum group metal promoter selected from platinum, palladium, rhodium or ruthenium, or mixtures thereof, is included in the coating.

5. The process of claim 4, wherein the platinum group metal promoter is present in the coating in an amount ranging from 0.05 to 1 weight percent.

6. 6. The process of any one of claims 1 to 5, wherein the porous metal oxide in which the nickel is dispersed is a refractory oxide comprising alumina, titania, zirconia, zinc oxide, magnesia, ceria, praseodymium oxide, yttria and lanthana, preferably alumina, zirconia, ceria, lanthana and mixtures of two or more thereof.

7. The amount of coating on the non-porous support structure is between 10 and 150 g / m ２ , preferably 10 to 80 g / m ２ , more preferably 30 to 60 g / m ２ The process according to any one of claims 1 to 6, wherein

8. The process of any one of claims 1 to 7, wherein the structured catalyst comprises a metal or ceramic structure having a plurality of passages through which a process fluid may pass in an ordered, non-random direction.

9. 10. The process of claim 8, wherein the structured catalyst comprises cylindrical units having diameters complementary to the tubes in which they are disposed, containing a plurality of passages through which process fluid may pass in an ordered, non-random direction.

10. The process of any one of claims 1 to 9, wherein the hydrocarbon feedstock comprises methane, preferably pre-reformed gas, associated gas or natural gas, more preferably natural gas.

11. 11. The process of any one of claims 1 to 10, wherein the feedstock is compressed to a pressure in the range of from 10 to 100 bar absolute, preferably from 15 to 80 bar absolute, more preferably from 20 to 50 bar absolute.

12. 12. The process of any one of claims 1 to 11, wherein the nitrogen gas content of the hydrocarbon feedstock is in the range of 0.5 to 25% by volume, preferably 1 to 10% by volume, more preferably 1 to 5% by volume.

13. 13. The process of any one of claims 1 to 12, wherein the mixture of hydrocarbon feedstock and steam has a steam to carbon ratio in the range of from 1.8:1 to 5:1, preferably from 2.5:1 to 3.5:1, more preferably from 2.8:1 to 3.2:

1.

14. A process according to any one of the preceding claims, wherein the mixture of hydrocarbon feedstocks is fed to the inlet of the tube at an inlet temperature in the range of from 300 to 650°C, preferably from 450 to 650°C.

15. 15. The process of any one of claims 1 to 14, wherein the tubular steam reformer comprises a plurality of tubes, the mixture of hydrocarbon feedstock and steam passing through the tubes and heat being transferred to the tubes by hot gases comprising combustion gases or synthesis gases flowing around the tubes.

16. 16. The process of any one of claims 1 to 15, wherein the catalyst bed consists of one, two, three or more layers of steam reforming catalyst, and in each case the layer of steam reforming catalyst adjacent the outlet of the tube is the structured catalyst.

17. 17. The process of claim 16, wherein there are two or more layers of steam reforming catalyst in the tubes, and the structured catalyst layer comprises 95% to 5% of the bed volume, preferably 80% to 20% of the bed volume, more preferably 75% to 25% of the bed volume.

18. 18. The process of any one of claims 1 to 17, wherein the methane content of the reformed gas is less than 15% by volume on a dry gas basis, preferably less than 10% by volume on a dry gas basis, more preferably less than 5% by volume on a dry gas basis.

19. A process according to any one of the preceding claims, wherein the ammonia content of the reformulated gas is less than 100 ppmv on a dry gas basis, preferably less than 50 ppmv, most preferably less than 10 ppmv on a dry gas basis.

20. 20. The process of any one of claims 1 to 19, wherein the ammonia content of the liquid condensate is less than 400 mg / litre, preferably less than 200 mg / litre, more preferably less than 100 mg / litre, most preferably less than 50 mg / litre, most preferably less than 20 mg / litre.

21. A process according to any preceding claim, wherein at least a portion of the condensate is recycled and used to generate steam for use in the steam reforming process.

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