Abstract
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 a shell with a heat source and one or more reformer tubes. The reformer tube is arranged to house catalyst material and is arranged to being heated by the heat source. The reformer tube comprises a first inlet for feeding said first feed stream into a first reforming reaction zone of the reformer tube, and a feed conduct 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 feed conduct is configured so that the second feed stream is only in contact with catalyst material in the second reforming reaction zone. The invention also relates to a process of producing CO rich synthesis gas at low S/C conditions.
Claims
1. A chemical reactor for carrying out reforming of a first feed stream comprising a hydrocarbon gas and steam, said chemical reactor comprising: a shell comprising a heat source; and a reformer tube having a first end and a second end arranged to house catalyst material, said reformer tube being placed within the shell and being arranged to be heated by said heat source, said reformer tube comprising a first inlet for feeding said first feed stream into a first reforming reaction zone of said reformer tube, wherein said reformer tube comprises a feed conduct arranged to conduct a second feed stream in heat exchange contact with said catalyst material housed within said reformer tube and allow said second feed stream into a second reforming reaction zone of said reformer tube, said second reforming reaction zone being positioned downstream of said first reforming reaction zone, wherein said feed conduct is configured so that said second feed stream is only in contact with catalyst material in said second reforming reaction zone, wherein said feed conduct comprises a first part arranged for conducting said second feed stream in heat exchange contact with catalyst material housed within said reformer tube, and a second part arranged for inletting said second feed stream into said second reforming reaction zone of said reformer tube, and wherein said second part comprises second inlet(s) at more than one point along a longitudinal axis of said reformer tube and/or a frit material extending along at least a part of the longitudinal axis for letting said second feed stream be released into said second reforming reaction zone along at least a part of the longitudinal axis of the reformer tube housing said feed conduct.
2. The chemical reactor according claim 1, wherein said feed conduct extends into said second reforming reaction zone and said feed conduct comprises a baffle arranged to conduct said second feed stream in heat exchange contact with at least a part of said second reforming reaction zone prior to allowing said second feed stream into said second reforming reaction zone via said second part.
3. The chemical reactor according to claim 1, wherein said feed conduct extends within said reformer tube from either the first or the second end of said reformer tube to said second reforming reaction zone.
4. The chemical reactor according to claim 1, wherein the heat source is able to heat the catalyst material within the reformer tube to a temperature of at least 750° C.
5. The chemical reactor according to claim 1, wherein said feed conduct is of a material which is able to withstand temperatures at least up to 850° C.
6. The chemical reactor according to claim 1, further comprising heat exchange means for heating said second feed stream to a temperature of at least 700° C.
7. A plant for reforming of a first feed stream comprising a hydrocarbon gas and steam, said plant comprising a chemical reactor according to claim 1, said chemical reactor being arranged to receive a first feed stream and a second feed stream and to output a first product gas and further comprising: an addition point for addition of a third feed stream to the first product gas to a mixed gas; and an adiabatic reactor comprising a second catalyst material, said adiabatic reactor being arranged to receive the mixed gas and equilibrating reverse water gas shift reaction for the mixed gas to provide a second product gas having a lower H.sub.2/CO ratio than the first product gas.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) 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.
(2) FIGS. 1 to 4b are schematic drawings illustrating cross sections through embodiments of a chemical reactor of the invention;
(3) FIG. 5 is a diagram showing the temperature within a reformer tube of the invention as a function of axial position;
(4) FIG. 6 is a carbon limit diagram illustrating carbon limits in different scenarios; and
(5) FIG. 7 is a drawing of a chemical plant with a steam reformer and further CO.sub.2 addition.
DETAILED DESCRIPTION
(6) 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.
(7) FIG. 1 is a schematic drawing illustrating a cross section through a chemical reactor 10 of the invention for carrying out reforming of a first feed stream comprising a hydrocarbon gas and steam. A typical reformer design is used so that the chemical reactor 10 of the invention, also denoted “the reformer”, comprises a shell 11 housing one or more heat sources 12, such as burners, as well as a number of a reformer tubes 20 housing catalyst material 22 as shown by hatching. For the sake of simplicity, only a single reformer tube 20 is shown in FIG. 1; however, typically the reformer comprises a multitude of such reformer tubes 20. The reformer tube 20 is placed within the shell and is under operation heated by the heat sources 12. The configuration shown in FIG. 1 is a side fired reformer. The reformer tube 20 has a first inlet for feeding a first feed stream 40 into a first reforming reaction zone 50 of the reformer tube. The reformer tube 20 moreover comprises a feed conduct 30 arranged to allow a second feed stream 45 to be led in heat exchange contact with catalyst material 22 in the first reforming reaction zone 50 and to be added into a second reforming reaction zone 60 of the reformer tube 20 at addition points 61, where the second reforming reaction zone 60 is positioned downstream of the first reforming reaction zone 50. In the embodiment shown in FIG. 1, the second reforming reaction zone 60 consists of the addition zone or addition point 61 and the third reforming reaction zone downstream the addition point. Thus, in FIG. 1 the third reforming reaction zone constitutes most of the second reforming reaction zone 60, since the addition zone is constituted by one or more addition points at least substantially equal distance from the first inlet into the reformer tube 20. The second feed stream 45 is kept separate from the catalyst material 22 until the second reforming reaction zone 60, viz. until the addition points 61. During operation, CO rich reformed process gas 70 exits the reformer tube 20/the reformer 10.
(8) FIG. 2 is a schematic drawing illustrating a cross section through a chemical reactor 110 of the invention for carrying out reforming of a first feed stream comprising a hydrocarbon gas and steam.
(9) The chemical reactor 110 of the invention, also denoted “the reformer”, comprises a shell 111 housing one or more heat sources 112, such as burners, as well as a number of a reformer tubes 120 housing catalyst material 122 as shown by hatching. The reformer tube 120 is placed within the shell and is under operation heated by the heat sources 112. The configuration shown in FIG. 2 is a side fired reformer. 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 conduct 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 FIG. 2, the second part of the feed conduct 130 extends from the beginning of the second reforming reaction zone 160 to the lower end of the feed conduct 130. The second reforming reaction zone 160 contains an addition zone 161 corresponding to the second part of the feed conduct 130 and a third reforming reaction zone 162 downstream the addition zone 161.
(10) The second part of the feed conduct 130 has a plurality of inlets into the second reforming reaction zone 160 as indicated by arrows from the second part of the feed conduct 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 conduct 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 conduct (as seen in FIG. 2) which lets the second feed stream 145 be released into the addition zone 161 of the second reforming reaction zone 160 along at least a part of the longitudinal axis (not shown) of the reformer tube 120. As an alternative (not shown), the feed conduct 130 could be a through tube extending from the upper to the lower end of the reformer tube 120, where only a part thereof has inlets into the reformer tube 120. The resultant CO rich reformed process gas 170 exits the reformer tube 120/the reformer 110.
(11) FIG. 3 is a schematic drawing illustrating an alternative chemical reactor 210 of the invention. The reactor 210 is a reformer tube reactor having a plurality of reformer tubes 220 within a shell 211; however, in FIG. 3 only one such reformer tube 220 is shown. Under operation, the reformer tube 220 is heated by one or more heat sources 212, such as burners. In the embodiment shown in FIG. 3, the reformer tube 220 is side fired. The reformer tube 220 has a first inlet for feeding a first feed stream 240 into a first reforming reaction zone 250 of the reformer tube 220. A second reforming reaction zone 260 extends from the lower part of the first reforming reaction zone 250 (as seen in FIG. 3) to the lower end of the reformer tube 220.
(12) The reformer tube 220 moreover comprises a feed conduct 230 extending along a longitudinal axis (not shown in FIG. 3) of the reformer tube 220, in most of the length of the reformer tube 220. The part of the reformer tube 220 not taken up by the feed conduct 230 is shown as filled with catalyst material 222. Thus, the feed conduct 230 extends into the second reforming reaction zone 260. The feed conduct 230 comprises a baffle 235 arranged to conduct the second feed stream 245 in heat exchange contact with most of the second reforming reaction zone 260 prior to allowing the second feed stream 245 into an addition zone 261 of the second reforming reaction zone 245 via the second part of the feed conduct 230. This is illustrated by the arrows indicating the flow of the second feed stream 245 along the length of the feed conduct 230, where the second feed stream 245 at the bottom of the feed conduct 230 is redirected upwards along the inner wall of the feed conduct 230, between the feed conduct and the baffle 235.
(13) The feed conduct 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 conduct 230 into the catalyst material 222 of the reformer tube. The inlets may be a plurality of individual inlets from the feed conduct 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 conduct 230.
(14) 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 conduct. In the addition zone 261 of the second reforming reaction zone 260, the second feed stream 245 is added into the catalyst filled 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 resultant CO rich reformed process gas 270 exits the reformer tube 220/the reformer 210.
(15) It should be noted, that even though FIG. 3 shows an embodiment where the feed conduct 230 does not extend in the whole length of the reformer tube 220, it is conceivable that the feed conduct 230 extends in the whole length of the reformer tube 220 or even protrudes through the lower end of the reformer tube 220 (as seen in FIG. 3) into the shell 211 heated by the heat sources 212. Such configurations would provide for further heating of the second feed gas 245.
(16) FIG. 4a is a schematic drawing illustrating a cross section through a chemical reactor 310 of the invention for carrying out reforming of a first feed stream comprising a hydrocarbon gas and steam. The chemical reactor 310 of the invention, also denoted “the reformer”, comprises a shell 311 housing one or more heat sources 312, such as burners, as well as a number of a reformer tubes 320 housing catalyst material 322 as indicated by hatching. The reformer tube 320 is placed within the shell and is under operation heated by the heat sources 312. The configuration shown in FIG. 1 is a side fired reformer. 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 conduct 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).
(17) In the reformer 310 shown in FIG. 4a, the first feed stream 340 is inlet into the reformer tube 320 at a first, upper end thereof, whilst the feed conduct extends within the reformer tube from a second, lower end of the reformer tube 320. Also in this embodiment, the first reforming reaction zone extends from the upper end of the reformer tube 310, viz. from the inlet of the first feed stream, to the second reforming reaction zone 360. The second reforming reaction zone 360 extends from the most upstream (as seen in the flow direction of the first feed stream) inlet point(s) 361 of the second feed stream 345 until the lower end of the reformer tube 310. The second reforming reaction zone 360 consists of the addition zone or the addition points 361 and the third reforming reaction zone downstream the addition points 361. Thus, in FIG. 1 the third reforming reaction zone constitutes most of the second reforming reaction zone 360, since the addition zone is constituted by the one or more addition points 361 at at least substantially equal distance from the first inlet into the reformer tube 320. CO rich reformed process gas 370 exits the reformer tube 320/the reformer 310.
(18) FIG. 4b is a schematic drawing illustrating an alternative reformer tube of the invention. FIG. 4b shows in a simplified form a cross section through a bayonet tube reactor 410 according to the invention. The bayonet tube reactor 410 has a plurality of reformer tubes 420 within a shell 411; however, in FIG. 4b only one such reformer tube 420 is shown. The reformer tubes 420 are under operation heated by heat sources 412. The reformer tube 420 comprises an outer tube 424, that is open at an inlet for inletting a first feed stream 440 in the upper end thereof (as seen in FIG. 4b), viz. into the first reforming reaction zone 450 of the reformer tube 420. The reformer tube 420 is closed in the lower end thereof (as seen in FIG. 4b). The first feed stream 440 typically comprises a hydrocarbon gas and steam. Within the outer tube 424 an inner tube 426 is located and fixed, coaxially spaced apart from the outer tube 424. The inner tube 426 is open at both its lower and upper end. The reformer tube 420 moreover comprises a feed conduct 430 coaxially spaced from both the outer and inner tubes and placed between the outer and inner tubes 424, 426. The feed conduct 430 extends coaxially along a part of the inner tube 426 along the longitudinal axis (not shown in FIG. 4b) of the reformer tube 420. The feed conduct 430 has inlet for allowing a second feed stream 445 into a second reforming reaction zone 460 of the reformer tube 420. Catalyst 422 is provided within the outer tube 424, but not within the feed conduct 430 or the inner tube 426. The catalyst 422 is shown by hatching in FIG. 4b.
(19) In the reactor shown in FIG. 4b, the feed conduct 430 has inlets into the catalyst filled space of the outer tube 440, as shown by the arrows in the lower end of the feed conduct. However, the feed conduct could have a plurality of inlets along the longitudinal axis of the reformer tube 420 or the lower part of the feed conduct 430 could be made of a frit material allowing the second feed stream 445 to be released gradually into the second reforming reaction zone 460, that is along at least a part of the longitudinal axis of the reformer tube 410.
(20) 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 process gas is subsequently passed through catalyst 422 arranged between the walls of the outer tube 424 and the feed conduct 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 FIG. 4b) within the second reforming reaction zone 460. Subsequently, the gas continues downwards (as seen in FIG. 4b) until it impinges on the lower end of the outer tube 424, where it reverses its direction and continues into the inner tube 426, through which the gas stream is withdrawn as a product stream 490. Heat exchange takes place between the process gas within the first reforming reaction zone 450 and the second feed stream 445 within the feed conduct 430, between the process gas in the second reforming reaction 460 and the product gas 490 in the inner tube 426 as well as between the second feed stream 445 within the feed conduct and the product gas 490 in the inner tube 426.
(21) It should be understood that FIGS. 1 to 4b are schematic drawings only illustrating the relevant part of the chemical reactor 10, 110, 210, and 310 of the invention. Thus, the shell 11, 111, 211, and 311 is in reality a substantially closed housing with upper and lower walls which are not shown in FIGS. 1 to 4b. Moreover, FIGS. 1 to 4b do not show inlets for providing the first feed stream and the second feed stream into the reformer tube 20, 120, 220, and 320 or an outlet for outletting a reformed gas stream from the reformer tube 20, 120, 220, and 320 and from the chemical reactor 10, 110, 210, and 310. In the FIGS. 1 to 4b, the chemical reactors 10, 110, 210 and 310 are shown as having only a single reformer tube for simplicity. However, typically the shell of a chemical reactor houses a plurality of reformer tubes. Moreover, it should be noted that even though the embodiments shown in FIGS. 1 to 4b are all side fired reformers, other types of fired reformers are conceivable within the concept of the invention, such as top fired, terrace fired or bottom fired reformers.
(22) In FIGS. 1 to 4b, the part of the reformer tubes not taken up by the feed conduct is shown as filled with catalyst material. It should be noted that catalyst might not fill up all the available space within the reformer tube in that inert material may be present, e.g. on top of the catalyst material, in between the reforming reaction zones, and/or the topmost part of the reformer tube may be left empty.
(23) It should also be noted that in the embodiments shown in FIGS. 1 and 4 it is indicated that the second feed stream is inlet into the second reforming reaction zone at a single point along the longitudinal direction of the reformer tube 10, 310. In this case, the third reforming reaction zone can be seen as substantially corresponding to the second reforming reaction zone, since the addition zone of the second reforming reaction zone has no substantial extent in the longitudinal direction of the reformer tube 10, 310.
(24) FIG. 5 is a diagram showing the temperature within a reformer tube of the invention as a function of axial position. The reformer tube used has a length of 13 meter, and it could e.g. be a reformer tube 110 as shown in FIG. 2. An axial position of 0 meter corresponds to the inlet into the reformer tube and an axial position of 13 meter corresponds to the outlet of the reformer tube. The reformer tube is side fired as shown in FIG. 2. Within the first meter of the reformer tube, the temperature rises from about 650° C. to about 785° C. A feed stream reaches catalyst material within the reformer tube after the inlet, viz. at an axial position of about 0 meter. Typically, the feed stream has a temperature of 450-650° C., when it enters the reformer tube, such as e.g. about 650° C. The first reforming reaction zone 150, where the inlet feed stream reacts with reforming catalyst material within the reformer tube corresponds to axial positions between about 0 meter and about 6 meters.
(25) The second feed stream, typically a CO.sub.2 rich feed stream, e.g. pure CO.sub.2, is inlet into the catalyst material of the reformer tube at four different axial positions, i.e.
(26) four different points along the longitudinal axis of the reformer tube. In FIG. 5, the four different, axial positions are at about 6 meters, about 7.5 meters, about 9 meters and about 10.5 meters. The second reforming reaction zone 160 thus ranges from about 6 meters to the outlet of the reformer tube at an axial position of about 13 meter. Within the second reforming reaction zone 160, the addition zone 161 ranges from the first to the last inlet, viz. from about 6 meters to about 10.5 m, and the third reforming reaction zone 162 ranges from the end of the second reforming reaction zone to the end of the reactor tube, viz. from about 10.5 m to about 13 meter. A final conversion and heating of the process gas takes place in the third reforming reaction zone 162.
(27) 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 CO.sub.2 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 conduct 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 FIG. 5, the temperature drops in the addition points are relatively low. Calculations show that the mean approach to equilibrium for the carbon formations reactions is never within 10° C. In this case the operating point of the reformer reactor moves four times in the carbon-limit diagram of FIG. 6 along the line H.sub.2O/CH.sub.4=1.0 from the point indicated “Reformer inlet” to the point indicated “After CO2 addition”.
(28) The second feed stream is preheated prior to being inlet into the second reforming reaction zone, typically to a temperature of about 850° C.
(29) FIG. 6 is a carbon limit diagram illustrating carbon limits in different scenarios. A carbon limit diagram is also denoted “Tøttrup diagram”. In general, it is essential to design a reforming plant to avoid carbon formation on the catalyst material in the reformer tubes of the reformer reactor. In this diagram a given gas composition will have a fixed H/C and O/C ratio, which is shown on the x- and y-axis, independently of how far the reforming reactions have proceeded. As example, a feed gas containing 44% CH.sub.4, 46% H.sub.2O, 5% H.sub.2, 4% CO.sub.2, and 1% CO has an H/C and O/C ratio of 5.67 and 1.12, respectively. Reforming this gas to an equilibrium at 950° C. and 25 bar would give a gas composition 8% CH.sub.4, 9% H.sub.2O, 61% H.sub.2, 2% CO.sub.2, and 20% CO; however, the H/C and O/C ratios of 5.67 and 1.12, respectively, have not changed. Additionally, the diagram contains axis which shows the composition of a gas with a given H/C and O/C ratio normalized to a feed of only H.sub.2O, CH.sub.4, and CO.sub.2, as “H.sub.2O/CH.sub.4” and “CO.sub.2/CH.sub.4” axis. As example, the gas above with a H/C and O/C ratio of 5.67 and 1.12, respectively, would correspond to a normalized gas with “H.sub.2O/CH.sub.4” and “CO.sub.2/CH.sub.4” of 1.05 and 0.08, respectively.
(30) Carbon formation in the tubes of a reforming reactor, also denoted “reformer”, is dictated by thermodynamics and in a typical design of a reformer it is a requirement that the reformer does not have affinity for carbon formation of the equilibrated gas anywhere in the catalyst material. This means that the process gas or feed stream will have to be balanced with water in order to circumvent the carbon formation area. Typically, the process gas enters the reformer at 400-500° C., while leaving the reformer at about 950° C. (not experiencing temperatures above 1000° C.). Thus, when designing a reformer, there must not be an affinity for carbon formation of the equilibrated gas anywhere in the temperature range from 400° C. to 1000° C. This criterion can be used to evaluate the carbon limit of the reformer, as illustrated by the line labeled “Ni, Te [400;1000° C.]” in the carbon limit diagram in FIG. 6.
(31) If potential for carbon formation exists, it will only be a matter of time before a shutdown of the reactor is necessary due to too high pressure drop. In an industrial context, this will be expensive due to lost time on stream and loading of a new batch of catalyst material into the reformer tubes. Carbon formation at reforming conditions is as whisker carbon. This is destructive in nature toward the catalyst material and regeneration of the catalyst material is therefore not an option. Thus, the possible operating range for a tubular reformer will be defined by the conditions which will not have a potential for carbon formation. When sufficient knowledge about the thermodynamics of carbon formation for a specific catalyst material is known, the exact limit for carbon formation can be calculated and this can be illustrated by the carbon limit curves depicted in the carbon limit diagram of FIG. 6. The carbon limit for graphite is shown as the dotted curve 1, whilst the carbon limit for an industrial nickel catalyst is shown as the curve 2. As the carbon limits have to be defined in a worst case scenario, the curve for industrial nickel catalyst represents a nickel catalyst aged for several years in a reforming plant where the catalyst has been severely sintered. The curves are derived from the principle of equilibrated gas and show the most severe conditions (as a function of initial normalized H.sub.2O/CH.sub.4 and CO.sub.2/CH.sub.4 ratios, or O/C and H/C ratios) which can be tolerated in the entire temperature range from 400° C. to 1000° C. at a pressure of 25.5 bar. Carbon formation will be expected to the left of the curves and operation without risk of carbon formation will be expected to the right of the curves. This shows that the tendency for carbon formation increases with decreasing CO.sub.2/CH.sub.4 and H.sub.2O/CH.sub.4 ratios in the feed gas. The severity of operation can be defined relative to the placement compared to the carbon limit curves; operation to the left of and far beyond the carbon limit curve is considered very severe.
(32) The dotted lines (4a-4e) in FIG. 6 show the equilibrated H.sub.2/CO ratio of a synthesis gas produced at 950° C. and 25.5 bar as a function of the O/C and H/C ratio or normalized CO.sub.2/CH.sub.4 and H.sub.2O/CH.sub.4 ratios. Increasing the CO.sub.2/CH.sub.4 ratio of the feed stream will decrease the H.sub.2/CO ratio accordingly. The lines show that the H.sub.2/CO ratio increases with increasing H/C ratio, as it is around 2.5 when H/C is around 5, while being around 0.5 when H/C is around 1. These lines additionally translate into the normalized “H.sub.2O/CH.sub.4” and “CO.sub.2/CH.sub.4” ratio, which shows that the H.sub.2/CO ratio of the product gas can be controlled by adjusting the addition of H.sub.2O and CO.sub.2, where more H.sub.2O will increase the product towards hydrogen rich gas and more CO.sub.2 will increase the product towards CO rich gas. However, when producing a synthesis gas with a very low H.sub.2/CO ratio, an accompanied high H.sub.2O/CH.sub.4 will be necessary to balance the severity of the gas to avoid carbon formation on a nickel catalyst. From FIG. 6 it can be seen that producing a synthesis gas with a H.sub.2/CO ratio below 1 requires a large excess of water to avoid carbon formation. As example, to produce a synthesis gas of H.sub.2/CO=0.7 in a standard reformer with a nickel catalyst will require a feed composition of H.sub.2O/CH.sub.4=3 and CO.sub.2/CH.sub.4=4.5.
(33) A principle of the current invention is illustrated by the third carbon limit curve 3 in FIG. 6. Where the normal reforming case confines the temperature of the reactor to be between 400° C. and 1000° C., the concept of this invention utilizes that this limit can be moved if the temperature interval is changed. Thus, if the lower temperature limit is increased to 800° C., the limit for carbon formation will change accordingly, as shown by the difference in the two carbon limit curves for graphitic carbon, 1 and 3, respectively.
(34) In a SPARG (Sulfur Passivated ReforminG) process, sulfur is used to selectively poison the most active sites and in this way prevent formation of carbon while maintaining some activity for reforming. Thus, the SPARG process offers a route to circumvent the carbon limit curves of FIG. 6. However, comparing the CO.sub.2 shifted reforming of the present invention to SPARG, the CO.sub.2 shifted reforming has at least the advantage that sulfur does not need to be added, which makes the size of the system significantly smaller.
(35) Alternatively, noble metal catalysts may be used to circumvent the carbon limits of FIG. 6 somewhat, since noble metals generally have a lower tendency for carbon formation compared to nickel catalysts. Noble metal catalyst thus offers a route for operation at severe conditions without carbon formation. However, noble metal catalysts are more expensive than nickel catalysts and to the best of our knowledge, very severe operation over noble metal catalysts has never been assessed in industrial scale.
(36) As an example of the current invention, consider a case where a synthesis gas with H.sub.2/CO ratio of 0.7 is wanted. A mixture of steam and methane 40, 140, 240, 340 is fed to the first reforming reaction zone 50, 150, 250, 350 of a reformer former tube 10, 110, 210, 310, and the ratio between steam (H.sub.2O) and methane (CH.sub.4) is chosen with respect to the typical carbon limit for Ni catalysts (the curve in FIG. 6 with alternating dots and lines, viz. curve 2) and the desired synthesis gas. The reformer tube 10, 110, 210, 310 contains a typical reforming catalyst 22, 122, 222, 322 in the first and second reforming reaction zones as shown by the hatching in FIGS. 1 to 4b. Such reforming catalyst may be nickel based catalyst; however, practically any catalyst suitable for reforming could be used.
(37) To produce the desired gas, it is chosen to operate at a H.sub.2O/CH.sub.4 ratio of 1, illustrated by the cross indicated by “Reformer inlet” in FIG. 6. A CO.sub.2 rich feed (in the current example pure CO.sub.2) is fed to a feed conduct 30, 130, 230, 330 which does not house catalyst material.
(38) Towards the bottom of the first reforming reaction zone 50, 150, 250, 350 the temperature of the gas in the first reforming reaction zone 50, 150, 250, 350 as well as the temperature of the CO.sub.2 rich gas within the feed conduct 30, 130, 230, 330 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 10, 110, 210, 310, 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 CO.sub.2 rich gas. The addition of the heated CO.sub.2 rich gas into the second reforming reaction zone shifts the operating point within the carbon limit diagram in FIG. 6 from the cross denoted “Reformer inlet” to the cross denoted “After CO2 addition”, corresponding to an unchanged H.sub.2O/CH.sub.4 ratio of 1, but a change in the CO.sub.2/CH.sub.4 ratio to about 2.6 (instead of a CO.sub.2/CH.sub.4 ratio of 0 before the addition of CO.sub.2 rich gas).
(39) Downstream of the addition point of the CO.sub.2 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 10, 110, 210, 310 at a temperature of about 950° C. and a H.sub.2/CO ratio of 0.7. In this case the overall process gas has normalized ratios H.sub.2O/CH.sub.4=1 and CO.sub.2/CH.sub.4=2.6. Because the gas is kept above 800° C. from the addition point of CO.sub.2, it is no longer carbon limit curve 2, which dictates the limit for carbon formation, but instead carbon limit curve 3. As seen from FIG. 3, the new operating point (denoted “after CO2 addition” in FIG. 3) is placed on the right side of carbon limit curve 3 and carbon formation is therefore not expected. In order to achieve an outlet gas having a H.sub.2/CO ratio of 0.7 with a conventional reformer tube having a nickel based catalyst, the overall process gas would have ratios H.sub.2O/CH.sub.4=3 and CO.sub.2/CH.sub.4=4.5. Consequently, the co-feed of CO.sub.2 and H.sub.2O of the current invention is significantly lower compared to the feed in the nickel based reformer case.
(40) FIG. 7 is a drawing of a chemical plant 100 with a steam reformer 10 according to the invention and further CO.sub.2 addition. To circumvent the drop in temperature in the addition zone of the second reforming reaction zone of the reformer reactor tubes, the CO.sub.2 addition taking place within the reformer tubes is supplemented with a subsequent addition of hot CO.sub.2 rich gas stream 45′ downstream the reforming reactor 10. As seen in FIG. 7, the resulting gas stream 75 is subsequently equilibrated over an adiabatic reactor 90 arranged to facilitate the reverse water gas shift (RWGS) reaction and potentially also the reforming/methanation reaction, resulting in a CO rich product gas stream 80. The adiabatic reactor 90 comprises a second catalyst material, e.g. catalyst material arranged for both the reverse water gas shift and the steam methane reaction. However, the second catalyst material could also be a selective reverse water gas shift catalyst. From Table 2 below, it can be seen that the H.sub.2/CO ratio of the product gas stream 80 from the plant 100 is 30.5/42.1=0.72, which substantially corresponds to the H.sub.2/CO ratio of the product stream in Table 1; however, in the plant 100 of FIG. 7, the CO.sub.2 added has been split up, thereby minimizing the risk of carbon formation. It should be noted that the CO.sub.2 rich gas stream 45′ added downstream the reformer 10 could contain further components than CO.sub.2. Moreover, the concept of splitting the CO.sub.2 addition up could also entail yet further addition(s) of CO.sub.2 rich gas stream(s) downstream the RWGS reactor 90 followed by equilibrating in additional RWGS reactor(s). It should also be noted that even though FIG. 7 shows the plant 100 with the reactor 10 of FIG. 1, any of the reactors of the invention could be used in the plant 100.
EXAMPLES
(41) 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 reformer or reformer tube 10 of the invention as shown in FIG. 1. This first feed stream is heated and reformed to a temperature of 850° C., within the first reforming reaction zone. Subsequently, it is mixed with CO.sub.2 which has been heated to 850° C., by heat exchange between the first reforming reaction zone and the feed conduct, while traveling within the feed conduct. Prior to the mixing of the CO.sub.2 and the process gas within the first reforming reaction zone, the H.sub.2/CO ratio is 3.95. Subsequently to the mixing of the process gas within the first reforming reaction zone and the CO.sub.2 from the feed conduct, viz. in the second reforming reaction zone, the mixed process gas is further heated to 950° C. by means of the heaters, while reforming continues to take place. The resulting product gas is a synthesis gas of having a ratio H.sub.2/CO=0.7 at 950° C.
(42) TABLE-US-00001 TABLE 1 Example of process (FIG. 1) Feed (40) CH.sub.4 [Nm.sup.3/h] 1000 Feed (40) H.sub.2O [Nm.sup.3/h] 1000 Feed (45) CO.sub.2 [Nm.sup.3/h] 2600 P [bar] 25.5 T.sub.addition 850 H.sub.2/CO prior to CO.sub.2 addition 3.95 Temp. of CO.sub.2 feed [° C.] (45) prior to addition 850 T.sub.exit [° C.] 950 H.sub.2/CO exit 0.70 Methane slip exit [dry %] 0.54
(43) 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 CO.sub.2 rich gas are alleviated. This is due to the fact that the carbon limits are circumvented as shown in the carbon limit diagram of FIG. 6 by adding CO.sub.2 to the hot part of the catalyst material in a reformer tube.
(44) In the Example described above, the second feed stream is a heated stream of pure CO.sub.2. Alternatively, the second feed stream could be a CO.sub.2, H.sub.2O, H.sub.2, CO, O.sub.2, H.sub.2S and/or SO.sub.2. Such a second feed stream could for example be a recycle gas stream from a reducing gas process, as described below.
(45) TABLE-US-00002 TABLE 2 Example of process (FIG. 7) Reformer RWGS Inlet T [° C.] 650 912 Outlet T [° C.] 950 906 Outlet MDC T [° C.] 1159 1062 NG feed addition [Nm.sup.3/h] 1000 — H.sub.2O feed addition [Nm.sup.3/h] 1000 — CO.sub.2 feed addition [Nm.sup.3/h] 2000* 600** H.sub.2 out [dry mol %] 36.9 30.5 CO out [dry mol %] 43.2 42.1 *The CO.sub.2 is added by a feed conduct as a second feed as e.g. in FIG. 1. **Second CO.sub.2 rich gas stream is heated to 650° C. before mixing with the gas 70.
Reducing Gas Process
(46) As mentioned, the 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 CO.sub.2 and a smaller fraction of H.sub.2S. 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 reformer is dictated by thermodynamics and the catalyst material in the reformer should not have affinity for carbon formation anywhere in the catalyst material.
(47) In a traditional reformer, the input hydrocarbon feed stream would have to be balanced with water in order to circumvent the carbon formation area as described in connection with FIG. 6. 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.
(48) 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 conduct 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 conduct into the transition area between the first and second reforming reaction zones.
(49) By the process, 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 H.sub.2/CO ratio gas.
(50) 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 H.sub.2/CO ratios, it is preferable that the amount of water kept in the recycle feed stream is minimized.
(51) The reducing gas recycle stream typically comprises at least 50 dry mol % CO.sub.2 and one or more of the following: steam, methane, hydrogen, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen, and argon.
(52) 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.
(53) 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 a shell with a heat source and one or more reformer tubes. The reformer tube is arranged to house catalyst material and is arranged to being heated by the 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 conduct 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.
