Patent Application
20240350996
Embodiments of the invention generally relate to a chemical reactor for reforming of a first feed stream comprising a hydrocarbon gas and steam, and a reformer tube of such a chemical reactor. Other embodiments of the invention relate to a process of reforming a first feed stream comprising a hydrocarbon gas and steam in a chemical reactor and a plant for reforming a first feed stream comprising a hydrocarbon gas and steam. In particular, the invention relates to a reforming process aimed at producing a synthesis gas stream with a low H2/CO ratio.
Catalytic synthesis gas production by steam reforming of a hydrocarbon feed stream has been known for decades. The endothermic steam reforming reaction is typically carried out in a steam reformer (SMR). A steam reformer or steam methane reformer has a number of catalyst filled tubes placed in a furnace or fired heater to provide the heat for the endothermic reaction. The tubes are normally 10-14 meters in length and 7-15 cm in inner diameter. The heat for the endothermic reaction is supplied by combustion of fuels in burners in the furnace. The synthesis gas exit temperature from the steam reformer depends on the application of the synthesis gas but will normally be in the range from 650° C.-980° C.
It is also known that carbon formation on the catalyst used in catalytic synthesis gas production by steam reforming is a challenge, especially for production of synthesis gasses with a relatively low H2/CO ratio. Therefore, catalysts resistant to carbon formation are required for such synthesis gasses. Such carbon resistant catalysts are e.g. noble metal catalysts, partly passivated nickel catalysts, and promoted nickel catalysts. Moreover, industrial scale reforming of CO2 rich gas typically requires a co-feed of water to decrease the severity of the gas for carbon formation. From a thermodynamic viewpoint, it is advantageous to have a high concentration of CO2 and a low concentration of steam in the feed to promote the production of synthesis gas with a low H2/CO ratio. However, operation at such conditions may not be feasible due to the possibility of carbon formation on the catalyst.
Alternative production of a synthesis gas with a low H2/CO ratio by steam reforming is a sulfur passivated reforming (SPARG) process which may be used for producing synthesis gas with a relatively low H2/CO ratio. This process requires desulfurization of the produced synthesis gas to produce a sulphur free synthesis gas.
More details of various processes for producing synthesis gas with low H2/CO-ratio can be found in “Industrial scale experience on steam reforming of CO2-rich gas”, P. M. Mortensen & I. Dybkjaer, Applied Catalysis A: General, 495 (2015), 141-151.
Within this context, the terms “reforming” and “methane reforming” are meant to denote reforming according to one or more of the following reactions:
CH4+H2O ↔CO+3H2 (i)
CH4+2H2O ↔CO2+4H2 (ii)
CH4+CO2↔2CO+2H2 (iii)
For higher hydrocarbons, viz. CnHm, where n≥2, m≥4, equation (i) is generalized as:
CnHm+n H2O→nCO+(n+m/2)H2, where n≥2, m≥4 (iv)
Typically, reforming is accompanied by the water gas shift reaction (v):
CO+H2O ↔CO2+H2 (v)
The term “steam methane reforming” is meant to cover the reactions (i) and (ii), running from the left towards the right side of the arrow, whilst the term “methanation” is meant to cover the reactions (i) and/or (ii) running from the right towards the left side of the arrow. Thus, the term “steam methane reforming/methanation reactions” is meant to denote the reactions (i) and (ii) running towards equilibrium. The term “reverse water gas shift” is meant to denote the reaction (v), running from the right towards the left side of the arrow, whilst reaction (iii), running from the left towards the right side of the arrow, is the dry methane reforming reaction. In most cases, all of these reactions are at, or close to, equilibrium at the outlet from the catalyst bed or catalyst zone of the reactor concerned.
In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.
Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments, and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).
Embodiments of the invention generally relate to reforming of a hydrocarbon feed stream in reforming reaction zones within the tubes of a reforming reactor. The term “reforming reaction zone” is meant to denote a catalytic zone of the reactor, where the steam methane reforming reaction (reactions (i), (ii) and optionally (iv)) takes place. Typically, dry methane reforming reaction (reaction (iii)), and water gas shift reactions (reaction (v)) also take place in the reforming reaction zone.
One embodiment of the invention provides a chemical reactor for reforming of a first feed stream comprising a hydrocarbon gas and steam. The chemical reactor comprises a reformer tube arranged to house catalyst material. The reformer tube comprises a first inlet for feeding the first feed stream into a first reforming reaction zone of the reformer tube, and a feed conduit arranged to conduct a second feed stream in heat exchange contact with the catalyst material housed within the reformer tube and to allow the second feed stream into a second reforming reaction zone of the reformer tube, where the second reforming reaction zone is positioned downstream of the first reforming reaction zone, and wherein the feed conduit is configured so that the second feed stream is in contact with catalyst material in the second reforming reaction zone only. The chemical reactor also comprises an electrically driven heat source arranged to heat the catalyst material within the reformer tube.
Hereby, it is rendered possible to add the second feed stream into the chemical reactor at a position where the first feed stream has already been at least partly reformed. Typically, the catalyst material within the reformer tube is a reforming catalyst material. Typically, the first and the second reforming reaction zones contain the same type of catalyst material. The catalyst material is advantageously a catalyst material arranged for catalyzing the steam methane reforming reaction (reactions (i), (ii) and optionally reaction (iv)). Preferably, the catalyst material is suitable for catalyzing both the steam methane reforming (reactions (i), (ii) and optionally reaction (iv)), the dry methane reforming (reaction (iii)) and the water gas shift reactions (reaction (v)). The terms “catalyst” and “catalyst material” are used interchangeably herein.
The chemical reactor of the invention generates a product gas in the form of a CO rich synthesis gas. The chemical reactor of the invention is preferably a steam reformer or steam methane reformer.
The term “hydrocarbon gas” is meant to denote a gas stream comprising one or more hydrocarbon gasses, and possibly other gasses. Examples of “a hydrocarbon gas” may be natural gas, town gas, or a mixture of methane and higher hydrocarbons.
Typically, the hydrocarbon gas stream comprises minor amounts of hydrogen, carbon monoxide, carbon dioxide, nitrogen, or argon, or combinations thereof. Thus, the “first feed stream comprising a hydrocarbon gas and steam” typically comprises one or more hydrocarbons, minor amounts of hydrogen, carbon monoxide, carbon dioxide, nitrogen, or argon, or combinations thereof, in addition to the steam and possibly carbon dioxide added to the hydrocarbon gas. For reforming processes, an example of a “first feed stream comprising a hydrocarbon gas and steam” is e.g. a mixture of methane, steam, and possibly other oxidizing gasses, such as carbon dioxide. Examples of “a hydrocarbon gas” may be natural gas, town gas, or a mixture of methane and higher hydrocarbons. The term “second feed stream” is meant to denote another stream different from the “first feed stream”.
Thus, the second feed stream may be any appropriate gas stream suitable for supporting the reforming reaction within a reforming reactor and/or for assisting the provision of a CO rich synthesis gas, typically CO2 rich gasses comprising at least 50% dry mole CO2, such as at least 70 dry mole %, such as at least 90 dry mole %. The term “downstream” as used in this text is meant to denote at “a later point or position in a process or system”, whilst the term “upstream” is meant to denote “at an earlier point or position in a process or system”. In a case where the term “downstream” or “upstream” is used in relation to the reformer tube, which may conduct both a first and a second feed stream, these terms are meant to be in relation to the flow direction of first feed stream, unless otherwise specified.
Within this context, the term “CO rich product gas” is meant to be synonymous to “CO rich synthesis gas” and to “synthesis gas stream with a low H2/CO ratio”, such as a synthesis gas with a H2/CO ratio below 2.5, preferably a synthesis gas with a H2/CO ratio below 2.0, more preferably a synthesis gas with a H2/CO ratio below 1.8, even more preferably a synthesis gas with a H2/CO ratio below 1.6
The term “first reforming reaction zone” is meant to denote the part of the catalyst housing reformer tube extending from the first inlet to the second reforming reaction zone, downstream of the first reforming reaction zone. The term “second reforming reaction zone” is meant to denote the part of the catalyst housing reformer tube from the point of inletting the second feed stream into the reformer tube. This point is here denoted “an addition point” or “an addition zone” in the case where the second feed stream is added at more than one point of the flow direction of the first feed stream along the chemical reactor. The chemical reactor may be designed so that the first and second feed streams flow co-currently or counter-currently through the chemical reactor. In the case, where the first and second feed streams flow counter-currently, terms such as “downstream of the first reforming reaction zone” and “upstream the second reforming reaction zone” is as seen from the flow direction of the first feed stream.
The second reforming reaction zone thus comprises an addition point or an addition zone at/along which the second feed stream is inlet from the feed conduit into the catalyst housing reformer tube. The addition point may be longitudinal in the case where a number of inlets from the feed conduit into the reformer tube exists or in the case where a frit material extending along at least a part of the longitudinal axis of the reformer tube is arranged to inlet the second feed stream into the addition zone. When the addition zone has a relatively short longitudinal extent, e.g. if the additional zone is at a point only along the longitudinal axis of the reformer tube, it is denoted “addition point”. Optionally, the second reforming section also comprises catalyst housing zone downstream (as seen from the first feed stream) the addition point/addition zone, in which no further second feed stream (or other feed stream) is added. This is denoted a third reforming reaction zone. Alternatively, the addition zone extends along all of the second reforming reaction zone. In this case, no third reforming reaction zone exists.
The term “the second feed stream is in contact with catalyst material in the second reforming reaction zone only” is meant to denote, that the second feed stream is inlet into the catalyst housing part of reformer tube at the addition point or the most upstream part of the addition zone. Even though the second feed stream has heat exchange contact with the first reforming reaction zone through the wall(s) of the feed conduit, there is not fluid or physical contact between the second feed stream and the catalyst material until the second feed stream has entered into the second reforming reaction zone. Thus, the second feed stream is not in fluid contact or physical contact with catalyst material within the first reforming reaction zone.
The feed conduit is configured so that the second feed stream is kept separate from the first feed stream, so that the second feed stream does not contact the catalyst material within the reformer tube until the second feed stream reaches the second reforming reaction zone. Typically, the first feed stream and the second feed stream are streams of different compositions.
In summary, the catalyst housing part of the reformer tube contains a first and a second reforming reaction zone, where the second reforming reaction zone is downstream the first reforming reaction zone. The second reforming reaction zone has an addition zone or addition point, where the second feed stream is inlet into reformer tube, reaching the catalyst material and being mixed with a partly reformed first feed stream. The second reforming reaction zone may comprise a third reforming reaction zone downstream the addition point/addition zone. No further stream is added in the third reforming reaction zone. Each of the reforming reaction zones comprises catalyst material arranged to catalyze a reforming reaction. The feed conduit typically does not comprise any catalyst. As seen along the direction of the first gas stream along the reformer tube, the first reforming reaction zone is the most upstream zone of the first and second reforming reaction zones. Within the second reforming reaction zone, the addition point or addition zone is meant to denote the most upstream part followed by the optional third reforming reaction zone. Typically, the first reforming reaction zone extends from the inlet of the first feed stream or from the most upstream part of the catalyst material within the reformer tube, and the second reforming reaction zone extends from the first reforming reaction zone to an outlet for outletting a first synthesis gas from the reformer tube, or to the most downstream part (as seen from the first feed stream) of the catalyst material within the reformer tube.
It should be understood that the term “an inlet” and “an outlet” is not intended to be limiting. Thus, these terms also cover the possibility where the units, e.g. the reformer tube, have more than one inlet and/or outlet. For example, a reformer tube could have an inlet for hydrocarbon gas and another inlet for steam, so that the hydrocarbon gas and steam is mixed within the reformer tube.
The term “electrically driven heat source” is meant to denote a heat source that is powered by an electrical power source and thereupon provides heating. In the invention, the electrically driven heat source of the chemical reactor may be solely electrically driven, so that no other heat sources are present; alternatively, other heat sources, such as e.g. a convective heat exchanger, may be used in addition to the electrically driven heat source.
The term “catalyst housing reactor tube” is meant to indicate that the chemical reactor tube houses catalyst material. The reactor tube may also house other material, such as electrically conductive and/or ferromagnetic elements.
Typically, the first feed stream/the hydrocarbon gas will have undergone desulfurization to remove any sulfur in the gas and thereby avoid deactivation of the catalysts in the process.
Optionally, the first feed stream/the hydrocarbon gas will together with steam have undergone adiabatic pre-reforming according to reaction (iv) in a temperature range of ca. 350-550° C. to convert higher hydrocarbons as initial steps in the process normally taking place downstream the desulfurization step. This removes the risk of carbon formation from higher hydrocarbons on catalyst in the subsequent process steps.
In an embodiment, the feed conduit comprises a first part arranged for conducting the second feed stream in heat exchange contact with catalyst material housed within the reformer tube, and a second part arranged for inletting the second feed stream into the second reforming reaction zone of the reformer tube. Typically, the second feed stream within the feed conduit is heated by heat exchange between the feed conduit and the first reforming reaction zone upstream the second reforming reaction zone, prior to being inlet into the second reforming reaction zone. Alternatively, the second feed stream may be led along the second reforming reaction zone, in heat exchange contact with the catalyst material therein, i.e. in countercurrent to the stream in the second reforming reaction zone and in the third reforming reaction zone. The second part of the feed conduit may be relatively small, for example in case of only inlets into one point along the longitudinal axis of the reformer tube, or the second part of the feed conduit may be elongate in case of inlets at more than one point along the longitudinal axis of the reformer tube.
In an embodiment, the feed conduit extends into the second reforming reaction zone and the feed conduit comprises a baffle arranged to conduct the second feed stream in heat exchange contact with the second reforming reaction zone prior to allowing the second feed stream into the second reforming reaction zone via the second part. This provides for an increased heat exchange area between the second feed stream and catalyst material within the reformer tube, and thus to an increased heat flux.
In an embodiment, the feed conduit extends within the reformer tube from a first and/or a second end of the reformer tube to the second reforming reaction zone. Thus, the feed conduit may be a tube extending within the reformer tube, e.g. along or parallel to the longitudinal axis thereof, from one of the ends of the reformer tube. As used herein, the reformer tube is seen as a tube extending from a first end along a longitudinal axis to a second end. Alternatively, a feed conduit having inlets into the second reforming reaction zone may extend within the reformer tube from the first to the second end thereof.
In an embodiment, the second part of the feed conduit comprises second inlet(s) at one or more points along the longitudinal axis of the reformer tube and/or a frit material extending along at least a part of the longitudinal axis for letting the second feed stream into the second reforming reaction zone along at least a part of the longitudinal axis of the reformer tube housing the feed conduit. Thus, the second feed stream may be inlet, via one or more inlets, at a single distance along the longitudinal axis of the reformer tube, or via more than one inlet at different distances along the longitudinal axis. Additionally, or alternatively, the second part comprises a frit material allowing the second feed stream to pass through the frit material over a certain extent along the longitudinal axis. Throughout this text, the term “frit material” is meant to denote a porous material or a material with a plurality of holes through which a gas or liquid may pass. By use of a frit material instead of one or more inlets, the second feed stream may be added into the second reforming reaction zone over a larger area thereof.
In an embodiment, the electrically driven heat source is arranged to heat the catalyst material within the reformer tube to a maximum temperature of at least 750° C. Typically, the first feed stream is preheated to an inlet temperature prior to entering the reformer tube of between about 400° C. and 650° C. and a temperature before exiting the reformer tube of above 800° C., above 850° C. or even at or above 900° C. Moreover, the temperature of the catalyst material within the reformer tube at the point(s) of inletting the second feed stream into the second reforming reaction zone is e.g. above 750° C., e.g. at about 800° C., or at about 850° C. or at about 900° C.
In an embodiment, the electrically driven heat source comprises an induction coil and a power source arranged to supply alternating current, and the induction coil is arranged to be powered by the power source. The induction coil is positioned so as to generate an alternating magnetic field within the reformer tube upon energization by the power source, and the reformer tube houses a ferromagnetic material which is ferromagnetic at least at temperatures up to an upper limit of a given temperature range T. Hereby, the reformer tube and thus the catalyst material is heated by the heating of the ferromagnetic material. The catalyst material itself may be ferromagnetic, e.g. in the form of catalytically active material supported on a ferromagnetic support, or the ferromagnetic material may be elements positioned within the reformer tube together with the catalyst material; such elements could be pellets, balls, rods, discs or other elements of appropriate size and shape in order to provide appropriate heating to the catalyst material within the reformer tube.
In another embodiment, the electrically driven heat source comprises electrically conductive material housed within the reformer tube and an electrical power source connected to the electrically conductive material, in order to allow an electrical current to run through the electrically conductive material during operation of the chemical reactor. Hereby, heat is generated within the electrically conductive material and the heat is given off by the electrically conductive material to the catalyst material. By supplying a current through the electrically conductive material within the reformer tube, it is heated by resistance or ohmic heating and dissipates heat to the catalyst material/reformer tube. The skilled person knows how to choose an electrically conductive material with appropriate resistivity and/or shape in order to achieve the required heat transmission to the catalyst material.
The catalyst material itself may be electrically conductive, e.g. in the form of catalytically active material supported on an electrically conductive support, e.g. in the form of one or more electrically conductive monoliths. In this case, it is an advantage if the catalyst material comprises one or only a few monoliths of catalyst material or of electrically conductive material supporting catalyst material, in order to ease an electrical connection through the electrically conductive material.
Alternatively, the electrically conductive material could be elements positioned within the reformer tube together with the catalyst material; such elements could be rods, discs or other elements of appropriate size and shape in order to provide appropriate heating to the catalyst material within the reformer tube.
In an embodiment, the feed conduit is arranged to withstand temperatures at least up to 850° C. Typically, the pressure difference over the wall of the feed conduit is low, e.g. less than 1-2 bar, so that the materials which are able to withstand such temperatures and advantageously also conduct heat well will be suitable candidates.
In an embodiment, the chemical reactor further comprises heat exchange means for heating the second feed stream to a temperature of at least 700° C. Advantageously, the heat exchange means are arranged to heat the second feed stream to a temperature of about 750° C. prior to addition to the second reforming reaction zone. Such heat exchange means may comprise a separate heat exchanger arranged to heat the second feed stream upstream of the feed conduit and/or an arrangement within the chemical reactor so that heat is exchanged between the feed conduit and the first reforming reaction zone upstream the second reforming reaction zone.
In all embodiments, the reformer tube may also comprise thermal insulation, at least partly surrounding the catalyst material, in order to minimize heat loss to the surroundings.
According to another aspect, the invention relates to a process of reforming a first feed stream in a chemical reactor. The process comprises the steps of:
By the process the second feed stream is added into the chemical reactor at a position where the first feed stream comprising a hydrocarbon gas and steam has already been at least partly reformed. This partly reformed first feed stream is thus mixed with the second feed stream. This mixing allows the elemental H/C and O/C ratios of the gas within the second reforming reaction zone to differ from the H/C and O/C ratios within the first reforming reaction zone. The composition of the second feed stream thus renders it possible to change the H/C and O/C ratios of the gas to a gas which would be considered critical with respect to carbon formation in a typical steam reformer configuration, without being critical in the concept of the invention.
Within this context, the term S/C or “S/C ratio” is an abbreviation for the steam-to-carbon ratio. The steam-to-carbon ratio is the ratio of moles of steam to moles of carbon in hydrocarbons in a gas. Thus, S/C is the total number of moles of steam added divided by the total number of moles of carbon from the hydrocarbons in the gas. Moreover, the term “O/C” or “O/C ratio” is an abbreviation for the atomic oxygen-to-carbon ratio. The oxygen-to-carbon ratio is the ratio of moles of oxygen to moles of carbon in a gas. Furthermore, the term H/C or “H/C ratio” is an abbreviation for the atomic hydrogen-to-carbon ratio. The hydrogen-to-carbon ratio is the ratio of moles hydrogen to moles of carbon in a gas. It should be noted that the term “C” in the ratio S/C thus is different from the “C” in the ratios H/C and O/C, since in S/C “C” is from hydrocarbons only, whilst in O/C and H/C, “C” denotes all the carbon in the gas.
By heating the second feed prior to introduction thereof into the second reforming reaction zone of the reformer tube, operating conditions with risk of carbon formation can be avoided and a synthesis gas can be produced at more critical conditions than typical reforming. For example, the second feed stream is heated to about 800° C. prior to being added into the second reforming reaction zone.
When the second feed stream is a CO2 rich gas, a CO rich synthesis gas is produced by the process of the invention, whilst alleviating problems of carbon formation on the catalyst material. Within this text the term “a CO2 rich gas” is meant to denote a gas comprising at least 50 dry mole % CO2, such as at least 70 dry mole % CO2, such as at least 90 dry mole % CO2.
Typically, the catalyst material within the reformer tube is a reforming catalyst. Advantageously, the catalyst material is arranged to catalyze steam methane reforming (reactions (i), (ii) and optionally (iv)), dry methane reforming (reaction iii) and water gas shift reactions (reaction (v)). Typically, the first and the second reforming reaction zones contain the same type of catalyst material.
Examples of reforming catalysts are Ni/MgAl2O4, Ni/Al2O3, Ni/CaAl2O4, Ru/MgAl2O4, Rh/MgAl2O4, Ir/MgAl2O4, Mo2C, Wo2C, CeO2, a noble metal on an Al2O3 carrier, but other catalysts suitable for reforming are also conceivable. The terms “reforming catalysts” and “steam reforming catalysts” are meant to be synonymous. Moreover, it is possible to have a configuration with different types of catalyst materials (e.g. the ones mentioned above) in the first and second reforming reaction zone and/or different types of catalyst material in the addition zone and the third reforming reaction zone. Thus, as an example only, the first and third reforming reaction could contain one type of catalyst material, whilst the addition zone contains a different type of catalyst material.
The catalytic activity for reforming reactions in the chemical reactor can be obtained either by conventional fixed beds of (pellet) catalysts, by catalysed hardware, or by structured catalysts. Catalytically active material may be added directly to a metal surface, viz. the surface of the support. The catalytic coating of a metal surface (wash coating) is a well-known process (a description is given in e.g. Cybulski, A., and Moulijn, J. A., Structured catalysts and reactors, Marcel Dekker, Inc, New York, 1998, Chapter 3, and references herein).
The appropriate material of the macroscopic support, e.g. a ferritic steel containing Cr and/or Al, is heated to a temperature preferably above 800° C. in order to form a layer of Cr and/or Al oxide. This layer facilitates a good adhesion of the ceramic to the steel. A thin layer of a slurry containing the ceramic precursor is applied on the surface by means of e.g. spraying, painting or dipping. After applying the coat, the slurry is dried and calcined at a temperature usually in the region 350-1000° C. Finally, the ceramic layer is impregnated with the catalytically active material, e.g. catalytically active particles. Alternatively, the catalytically active material is applied simultaneously with the ceramic precursor.
Catalysed hardware either be catalytically active material attached directly to a channel wall in which the process gas flows or catalytically active material attached to a metallic macroscopic support in the form of a structured element. The structured element serves to provide support to the catalytically active material.
Structured elements are devices comprising a plurality of layers with flow channels present between the adjoining layers. The layers are shaped in such a way that placing the adjoining layers together results in an element in which the flow channels can, for instance, cross each other or can form straight channels. Structured elements are further described in for instance U.S. Pat. Nos. 5,536,699, 4,985,230, EP396,650, EP433,223 and EP208,929.
The structured elements are e.g. straight-channeled elements or the cross-corrugated elements. For example, straight channel monoliths are suitable for use in the process of the invention in adiabatic post converter(s). Cross-corrugated elements allow efficient heat transfer from the reactor wall to the gas stream. Other catalysed structured elements can also be applied, such as high surface structured elements. Examples of structured catalysts includes catalysed monoliths, catalysed cross-corrugated structures and catalysed rings (e.g. pall-rings).
The amount of catalytically active material can be tailored to the required catalytic activity for the methane reforming reactions at the given operating conditions. In this manner the pressure drop is lower and the amount of catalyst is not more than needed which is especially an advantage if the costly noble metals are used.
The electrically driven heat source may be an inductive heat source arranged to heat a ferromagnetic catalyst material and/or other ferromagnetic elements inductively by means of a coil surrounding the ferromagnetic catalyst material and/or other ferromagnetic elements, and powering the coil with an alternating electrical field. Alternatively, the electrically driven heat source may be an electrical power source arranged to heat electrically conductive catalyst material and/or other electrically conductive elements within the reformer tube by resistance heating or ohmic heating. Combinations of inductive and resistance heating are also conceivable. In the invention, the electrically driven heat source of the chemical reactor may be solely electrically driven, so that no other heat sources are present; alternatively, other heat sources, such as e.g. a convective heat exchanger, may be used in addition to the electrically driven heat source
In an embodiment, step e) of the method comprises leading the second feed stream into the second reforming reaction zone within a first part of the feed conduit arranged for conducting the second feed stream along the first reforming reaction zone, and inletting the second feed stream into the reformer tube via the second inlet in a second part of the feed conduit. Typically, the second feed stream within the feed conduit is heated by heat exchange between the feed conduit and the first reforming reaction zone upstream the second reforming reaction zone, prior to being inlet into the second reforming reaction zone. The feed conduit may alternatively or additionally be configured for heating the second feed stream by heat exchange between the second feed stream and the second reforming reaction zone.
In an embodiment, the second feed stream is conducted along the longitudinal axis of the reformer tube from a first and/or a second end of the reformer tube to the second reforming reaction zone. When the second feed stream is conducted in heat exchange contact with some of the second reforming reaction zone and optionally also the third reforming reaction zone prior to being inlet into the second reforming reaction zone, the temperature of the second feed stream is increased. The heat exchange may increase the temperature of the second feed stream to a higher temperature than the catalyst material within the first reforming reaction zone; this reduces the risk of carbon formation in the addition point of the second feed stream to the second reforming reaction zone and improves the overall operation of the chemical reactor. For example, the feed conduit may extend along most of or substantially all of the length of the reformer tube, and the second feed stream may thus be in heat exchange with the most of or substantially all of the length of the second reforming reaction zone.
In an embodiment, the second feed stream is conducted in heat exchange contact with at least a part of a longitudinal extent of the second reforming reaction zone. Thus, the feed conduit may be a tube extending within the reformer tube, along the longitudinal axis thereof, from one of the ends of the reformer tube. Alternatively, a feed conduit having inlets into the second reforming reaction zone may extend within the reformer tube from the first to the second end thereof.
In an embodiment, the step of inletting a second feed steam comprises inletting the second feed stream into the second reforming reaction zone at one or more points along a longitudinal axis of the reformer tube and/or into a frit material extending along at least a part of the longitudinal axis for letting the second feed stream into said second reforming reaction zone along at least a part of the longitudinal axis of the reformer tube housing the feed conduit. Thus, the second feed stream may be inlet, via one or more inlets, at a single distance along the longitudinal axis of the reformer tube, or via more than one inlet at different distances along the longitudinal axis. Additionally, or alternatively, the second part comprises a frit material allowing the second feed stream to pass through the frit material over a certain extent along the longitudinal axis. By use of a frit material instead of one or more inlets, the second feed stream may be added into the second reforming reaction zone over a larger area thereof.
In an embodiment, the second feed stream comprises: at least 90 dry mole % CO2. The second feed stream may be substantially pure CO2.
In an embodiment, the second feed stream further comprises one or more of the following constituents: steam, hydrogen, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, argon. Additionally, the second feed stream could contain smaller amounts of methane. Such a second feed stream could for example be a recycle gas stream from a reducing gas process.
In an embodiment, the mole fraction between CO2 in the second feed stream and hydrocarbons in the first feed stream is larger than 0.5. A ratio between CO2 in the second feed stream and hydrocarbons in the first feed stream may e.g. be about 1:1; about 2:1, about 3:1, about 4:1, about 5:1, about 6:1 or even higher.
In an embodiment, the first feed stream further comprises one or more of the following constituents: hydrogen, carbon monoxide, carbon dioxide, nitrogen, argon, higher hydrocarbons, or mixtures thereof.
In an embodiment, the steam-to-carbon ratio in the first feed stream is between about 0.7 and about 2.0. In the case where all hydrocarbons in the gas are in the form of CH4, the steam to carbon ratio S/C would correspond to the ratio between H2O and CH4. In the case where the gas also comprises higher hydrocarbons, the S/C ratio will be lower than the H2O/CH4 ratio.
In an embodiment, the electrically driven heat source is arranged to heat the catalyst material within the reformer tube to temperatures of between about 650° C. and about 950° C. It should be understood that not all the catalyst material within the chemical reactor needs to be heated to a temperature between 650° C. and about 950° C.; instead at least some of the catalyst material is heated to a temperature between 650° C. and about 950° C. Thus, in a part of the chemical reactor close to the inlet, the catalyst material may be heated to a temperature of e.g. 450° C. or 500° C.; and in a part of the chemical reactor close to the outlet, the catalyst material may be heated to a temperature of more than 950° C., such as e.g. 1000° C. The first synthesis gas exiting the chemical reactor has a temperature of up to 950° C. Typically, the pressure within the reformer tube is above 5 barg and below 35 barg, for example between 25 and 30 barg.
In an embodiment, the second feed stream in step f) is heated to a temperature of between about 700° C. and about 950° C. Hereby, operating conditions with risk of carbon formation can be avoided and a synthesis gas can be produced at more critical conditions than by reforming without addition of heated carbon dioxide.
According to a further aspect, the invention also relates to a plant for reforming of a first feed stream comprising a hydrocarbon gas and steam, the plant comprising a chemical reactor according to the invention. The chemical reactor is arranged to receive a first feed stream and a second feed stream and to output a first synthesis gas. The chemical reactor comprises an addition point for addition of a third feed stream to the first synthesis gas to a mixed gas, and an adiabatic post converter comprising a second catalyst material. The adiabatic post converter is arranged to receive the mixed gas and equilibrating reverse water gas shift, methanation and steam methane reforming reactions for the mixed gas to provide a second synthesis gas having a lower H2/CO ratio than the first synthesis gas. By the plant of the invention, the CO2 addition takes place both within the reformer tubes and downstream the chemical reactor. Hereby, the temperature drop within the addition zone of the reformer tubes is reduced and thus the risk of carbon formation is reduced. The second catalyst material may be similar to the catalyst material described in relation to the other aspects of the invention. Alternatively, the second catalyst material may be a selective reverse water gas shift catalyst. As used herein, the term “reverse water gas shift” is meant to denote the opposite reaction of reaction (v), viz.:
CO2+H2χCO+H2O.
Moreover, the term “adiabatic post converter” is meant to denote an adiabatic reactor downstream a chemical reactor, such as a steam methane reformer, where the steam reforming, methanation and reverse water gas shift reaction run towards equilibrium in the adiabatic post converter. The product gas from the chemical reactor is converted into a product synthesis gas in the adiabatic post converter, the product synthesis gas having a lower H2/CO ratio than the gas from the chemical reactor.
Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawings. It is to be noted that the appended drawings illustrate only examples of embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
The chemical reactor 110 of the invention, also denoted “the reformer”, one or more reformer tubes 120 housing electrically conductive catalyst material 122 as shown by hatching. The reformer tube 120 is under operation heated by the electrically driven heat source in the form of an electrical power supply 80 connected to the catalyst material 122 by means of electrical wires 90. The electrically conductive catalyst material may be a monolith for ease of resistive heating thereof. The reformer tube 120 has a first inlet for feeding the first feed stream 140 into a first reforming reaction zone 150 of the reformer tube. The reformer tube 120 moreover comprises a feed conduit 130 having a first part extending longitudinally along the first reforming reaction zone 150 and arranged to conduct a second feed stream 145 along the first reforming reaction zone 150 and a second part arranged for inletting the second feed stream 145 into the catalyst material 122 within the second reforming reaction zone 160 of the reformer tube, where the second reforming reaction zone 160 is positioned downstream of the first reforming reaction zone 150 (as seen from both the first and second feed streams). In the embodiment shown in
The second part of the feed conduit 130 has a plurality of inlets into the second reforming reaction zone 160 as indicated by arrows from the second part of the feed conduit 130 into the catalyst material 122 of the reformer tube, viz. into the addition zone 161 of second reforming reaction zone 160. The inlets may be a plurality of individual inlets from the feed conduit 130 into the addition zone of the second reforming reaction zone 160, or the inlets may be formed by choosing a frit material for the lowermost part of the feed conduit (as seen in
The reformer tube 220 moreover comprises a feed conduit 230 extending along a longitudinal axis (not shown in
The feed conduit 230 has a plurality of inlets into the addition zone 261 of the second reforming reaction zone 260 as indicated by arrows from the second part of the feed conduit 230 into the catalyst material 222 of the reformer tube. The inlets may be a plurality of individual inlets from the feed conduit 230 into the second reforming reaction zone 260, or the inlets may be formed by choosing a frit material for this second part of the feed conduit 230.
The second reforming reaction zone 260 of the reformer tube 220 thus contains an addition zone 261 and a third reforming reaction zone 262. Again, in the first reforming reaction zone 250, reforming of the first feed stream takes place as well as heat exchange between the first reforming reaction zone and the feed conduit. In the addition zone 261 of the second reforming reaction zone 260, the second feed stream 245 is added into the catalyst housing second reforming reaction zone 260. Here the second feed stream 245 is mixed with the partially reformed first feed stream 240. In the third reforming reaction zone, no further second feed stream is added. Here, reforming of the first and second feed streams takes place as well as heat exchange between the second feed stream 245 within the conduct and the catalyst material in the third reforming reaction zone of the reformer tube 220. Thus, the second feed stream 245 experiences heat exchange both in the first reforming reaction zone 250, in the addition zone 261 of the second reforming reaction zone 260 and in at least a part of, if not all of, the third reforming reaction zone 262. The first synthesis gas 270, viz. the resultant CO rich synthesis gas 270, exits the reformer tube 220/the reformer 210.
It should be noted, that even though
The chemical reactor 310 of the invention, also denoted “the reformer”, comprises one or more reformer tubes 220 comprising electrically conductive catalyst material 322 as indicated by hatching. The reformer tube 320 is under operation heated by a heat source in the form of an electrical power supply 80 connected to the electrically conductive catalyst material 322 by means of electrical wires 90. The electrically conductive catalyst material 322 may be a monolith for ease of resistive heating thereof. The reformer tube 320 has a first inlet for feeding a first feed stream 340 into a first reforming reaction zone 350 of the reformer tube. The reformer tube 320 moreover comprises a feed conduit 330 arranged to allow a second feed stream 345 into a second reforming reaction zone 360 of the reformer tube 320, where the second reforming reaction zone 360 is positioned downstream of the first reforming reaction zone 350 (as seen from the flow direction of the first feed stream).
In the reformer 310 shown in
In the chemical reactor shown in
A first feed stream 440 comprising a hydrocarbon gas and steam is fed into the reformer tube 420, viz. the first reforming reaction zone 450, via one or more inlets in the upper end of the reformer tube 420. The first feed stream or process gas is subsequently passed through catalyst 422 arranged between the walls of the outer tube 424 and the feed conduit 430. Having passed through the first reforming reaction zone 450, the process gas is mixed, in an addition zone of the second reforming reaction zone 460, with the second feed stream 445. The mixed gasses are passed through catalyst 422 between the walls of the outer tube 424 and the inner tube 426 in the third reforming reaction zone (not shown in
It should be understood that
In
It should also be noted that in the embodiments shown in
The second feed stream, typically a CO2 rich feed stream, e.g. pure CO2, is inlet into the catalyst material of the reformer tube at four different axial positions, i.e. four different points along the longitudinal axis of the reformer tube. In
Because of the endothermic nature of the reverse water gas shift reaction and its fast reaction rate, a very rapid temperature drop follows addition points of CO2 rich feed stream into the second reforming reaction zone. To avoid carbon formation at the points of adding the second feed stream into the second reforming reaction zone housing catalyst material, the temperature of the process gas within the second reforming reaction zone should be sufficiently high in order to avoid a temperature reduction that could lead to carbon formation on the catalyst material. However, when the reformer tube has multiple inlets from the feed conduit into the second reforming reaction zone, the catalyst material and process gas within the reformer tube does not need to be as high as in the case of only inlet(s) at a single longitudinal position along the reformer tube. In the case of four additions points illustrated in
The second feed stream is preheated prior to being inlet into the second reforming reaction zone, typically to a temperature of about 850° C.
The H2/CO ratio of the first synthesis gas can be controlled by adjusting the addition of H2O and CO2, where more H2O will increase the first synthesis gas towards a hydrogen rich gas and more CO2 will increase the first synthesis gas towards a CO rich gas. However, when producing a synthesis gas with a very low H2/CO ratio, an accompanied high H2O/CH4 will be necessary to balance the severity of the gas to avoid carbon formation on a nickel catalyst. Producing a synthesis gas with a H2/CO ratio below 1 in a standard steam methane reformer requires a large excess of water to avoid carbon formation. As example, to produce a synthesis gas of H2/CO=0.7 in a standard steam methane reformer with a nickel catalyst will require a feed composition of H2O/CH4=3 and CO2/CH4=4.5.
As an example of the current invention, consider a case where a synthesis gas with H2/CO ratio of 0.7 is wanted. A feed stream 40, 140, 240, 340, 440 in the form of a mixture of steam and methane is fed to the first reforming reaction zone 50, 150, 250, 350, 450 of a reformer tube 20, 120, 220, 320, 420 and the ratio between steam (H2O) and methane (CH4) is chosen with respect to the typical carbon limit for Ni catalysts and the desired synthesis gas. The reformer tube 20, 120, 220, 320, 420 contains catalyst material 22, 122, 222, 322, 422, typically a reforming catalyst, in the first and second reforming reaction zones as shown by the hatching in
To produce the desired gas, it is e.g. chosen to operate at a H2O/CH4 ratio of 1. A CO2 rich feed (in the current example pure CO2) is fed to a feed conduit 30, 130, 230, 330, 430 which does not house catalyst material.
Towards the bottom of the first reforming reaction zone 50, 150, 250, 350, 450 the temperature of the gas in the first reforming reaction zone 50, 150, 250, 350, 450 as well as the temperature of the CO2 rich gas within the feed conduit 30, 130, 230, 330, 430 are both about 850° C. or higher. This temperature is determined on the basis of the actual gas compositions. This point along the longitudinal axis of the reformer tube 20, 120, 220, 320, corresponding to the transition between the first and second reforming reaction zones, is where the partly reformed gas within the first reforming reaction zone is mixed with heated CO2 rich gas. The addition of the heated CO2 rich gas into the second reforming reaction zone shifts the operating point corresponding to an unchanged H2O/CH4 ratio of 1, but a change in the CO2/CH4 ratio to about 2.6 (instead of a CO2/CH4 ratio of 0 before the addition of CO2 rich gas).
Downstream of the addition point of the CO2 rich gas, viz. in the second reforming reaction zone, the gas is reformed further to achieve sufficient conversion of methane and finally leaves the reformer tube 20, 120, 220, 320, 420 at a temperature of about 950° C. and a H2/CO ratio of 0.7. In this case the overall process gas has ratios H2O/CH4=1 and CO2/CH4=2.6. In order to achieve an outlet gas having a H2/CO ratio of 0.7 with a conventional reformer tube having a nickel based catalyst, the overall process gas would have ratios H2O/CH4=3 and CO2/CH4=4.5. Consequently, the co-feed of CO2 and H2O of the current invention is significantly lower compared to the feed in the nickel based reformer case.
It should be noted that even though the embodiments shown in
An example of the process is illustrated in Table 1 below. A first feed stream comprising a hydrocarbon gas and steam and having a S/C ratio of 1 is fed to the first reforming reaction zone of a steam reformer 10 or reformer tube 20 of the invention as shown in
Thus, when the chemical reactor, the reformer tube or the process according to the inventions is used, the problems of carbon formation during reforming of a CO2 rich gas are alleviated. This is due to the fact that the carbon limits are circumvented by adding CO2 to the hot part of the catalyst material in a reformer tube.
In the Example described above, the second feed stream is a heated stream of pure CO2. Alternatively, the second feed stream could be a CO2, H2O, H2, CO, O2, H2S and/or SO2. Such a second feed stream could for example be a recycle gas stream from a reducing gas process, as described below.
As mentioned, the chemical reactor, the reformer tube, and the process of the invention are also suitable for reforming where the second feed stream is a recycle stream from a reducing gas process. Such a recycle stream could arise from a higher alcohol synthesis and would then typically comprise primarily CO2 and a smaller fraction of H2S. Alternatively, the recycle stream could arise from the iron reducing processes, such as the one known under the trademark “Midrix”.
As mentioned above, carbon formation in a steam reformer is dictated by thermodynamics and the catalyst material in the steam reformer should not have affinity for carbon formation anywhere in the catalyst material.
In a traditional steam reformer, the input hydrocarbon feed stream would have to be balanced with water in order to circumvent the carbon formation area. Typically, the hydrocarbon feed stream enters a reducing gas reformer at a temperature of between about 500 and about 600° C., while leaving the reducing gas reformer at a temperature of about 950° C., at least not experiencing temperatures above 1000° C. Thus, when designing a reducing gas reformer, there must not be an affinity for carbon formation anywhere between 500-1000° C. The carbon formation is somewhat hindered by the presence of sulfur in the recirculated reducing gas containing sulfur from the metals to be reduced, but the process is limited by carbon formation at low H/C levels and from content of higher hydrocarbons in the feed. Higher hydrocarbons are meant to denote hydrocarbons with more than one carbon atom, such as ethane, ethylene, propane, propylene, etc.
In the reformer reactor, the reformer tube and the process according to the invention as used in connection with a reducing gas plant, the first feed stream comprising a hydrocarbon gas and steam is inlet as into a first reforming reaction zone of the reformer tube. This first reforming reaction zone houses reforming catalyst material, typically nickel based catalyst. The recycle feed stream from the reducing gas plant is fed as a second feed stream into a second reforming reaction zone of the reformer tube, positioned downstream of the first reforming reaction zone. The recycle feed stream from the reducing gas plant may be led within a feed conduit within the first reforming reaction zone so that the recycle feed stream is heated by heat exchange with the catalyst material and process gas within the first reforming reaction zone prior to mixing the thus heated recycle feed stream and process gas at inlets from the feed conduit into the transition area between the first and second reforming reaction zones.
By the process, the steam reformer and reformer tube of the invention, the reforming of the first feed stream comprising a hydrocarbon gas and steam will take place at conditions not leading to carbon formation and the addition of preheated recycled gas from the reducing gas plant will enable production of a low H2/CO ratio gas.
The present invention describes that steam (water) is added to a hydrocarbon feed stream, typically natural gas, in order to enable steam reforming thereof. In a reducing gas plant, the recycle gas from the metal reduction furnace of the reducing gas plant contains water. Therefore, water should be removed from this recycle gas stream and should be added to the first feed stream prior to the steam reforming of this stream. Some steam may be left in the recycle feed stream, viz. the second feed stream, in order to enable preheating of this stream prior to mixing it with the steam reformed process gas within the first reforming reaction zone of the reformer tube. However, in order to obtain low H2/CO ratios, it is preferable that the amount of water kept in the recycle feed stream is minimized.
The reducing gas recycle stream typically comprises at least 50 dry mole % CO2 and one or more of the following constituents: steam, methane, hydrogen, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, and argon.
While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.
To summarize, the invention relates to a chemical reactor and reformer tubes for reforming a first feed stream comprising a hydrocarbon gas and steam. The chemical reactor comprises one or more reformer tubes arranged to being heated by an electrically driven heat source. The reformer tube comprises a first inlet for feeding the first feed stream into a first reforming reaction zone of the reformer tube, and a feed conduit arranged to allow a second feed stream into a second reforming reaction zone of the reformer tube. The second reforming reaction zone is positioned downstream of the first reforming reaction zone. The invention also relates to a process of producing CO rich synthesis gas at low S/C conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2017 00698 | Dec 2017 | DK | national |
The present application is a divisional of U.S. application Ser. No. 16/767,302, filed on May 27, 20201, which is a U.S. national stage of International Application No. PCT/EP2018/081405, filed on Nov. 15, 2018, which claims the benefit of Danish Application No. PA 2017 00698, filed on Dec. 8, 2017. The entire contents of each of U.S. application Ser. No. 16/767,302, International Application No. PCT/EP2018/081405, and Danish Application No. PA 2017 00698 are hereby incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16767302 | May 2020 | US |
| Child | 18761285 | US |
