Process and a reactor for oxidation of a hydrocarbon

Abstract

A process and related reactor (1) for oxidation of a hydrocarbon feedstock are disclosed, the reactor (1) comprising a vessel (3) and a neck (5) with an axial burner (6) and a tangential gas inlet (2), wherein the neck (5) has a swirling chamber (10) located below said burner (6) and connected to said gas inlet (2), to produce a gas vortex (V) which optimizes the mixing between the gas stream (G) and the oxidizer in said neck (5). Preferably the swirling chamber (10) has an internal surface (12) with a log-spiral profile.

Claims

1. A reactor for reacting a hydrocarbon-containing feedstock with an oxidizer stream, the reactor comprising: a vessel defining a combustion chamber, at least an axial burner for delivering said oxidizer stream to said combustion chamber, an inlet for said hydrocarbon-containing feedstock, and a swirling chamber connected to said inlet, wherein said swirling chamber is located downstream of said burner and upstream of said combustion chamber, and is in fluid communication with said burner and combustion chamber, wherein said inlet and swirling chamber are arranged to impart a swirling motion around an axis of the reformer to the hydrocarbon-containing feedstock, wherein said swirling chamber is delimited laterally by a side wall with a spiral-like internal surface so that the distance of said internal surface from the axis of the reformer progressively decreases from the inlet section of said hydrocarbon-containing feedstock inlet; wherein said vessel has a neck delimiting at least part of said combustion chamber, the neck having a portion with enlarged cross section, and wherein said portion delimits the swirling chamber and is connected with the hydrocarbon-containing feedstock inlet.

2. The reactor according to claim 1, wherein said swirling chamber is located at the top of the neck.

3. The reactor according to claim 1, wherein there is a gap between said swirling chamber and the tip of said burner, so that a pre-chamber is formed downstream the burner and above said swirling chamber.

4. The reactor according to claim 1, wherein said spiral-like internal surface of the swirling chamber covers an angle of about 360 degrees.

5. The reactor according to claim 4, wherein said spiral-like internal surface has one end matching an internal wall of the hydrocarbon-containing feedstock inlet, at the inlet section, and an opposite end matching an opposite internal side of said inlet.

6. The reactor according to claim 4, wherein said spiral-like internal surface is a log-spiral surface, having a cross-section profile following a logarithmic spiral.

7. The reactor according to claim 1, wherein the vessel contains a catalytic bed and the combustion chamber is above said catalytic bed.

8. The reactor according to claim 1, said reactor being an autothermal reformer, a secondary reformer of a hydrocarbon-reforming equipment, or a partial oxidation gas generator.

9. A process for reacting a hydrocarbon-containing feedstock with an oxidizer stream inside a combustion chamber of a reactor, wherein said oxidizer stream is fed to said combustion chamber in direction of an axis of said chamber, wherein a swirling motion around said axis is imparted to said gas stream entering the combustion chamber; wherein a substantially axial-symmetric velocity field is imparted to said hydrocarbon-containing feedstock inside the combustion chamber, by feeding said stream to said combustion chamber via a spiral-like path, wherein said reactor is in accordance with claim 1.

10. The process according to claim 9, wherein said spiral-like path follows a logarithmic spiral around said axis of the combustion chamber.

11. The process according to claim 9, wherein said hydrocarbon-containing feedstock is a gas stream containing gaseous hydrocarbon(s) such as natural gas or methane, or a gaseous flow containing suspended solid combustible such as coal dust or carbon soot, or a gaseous flow comprising dispersed liquid hydrocarbons, and the oxidizer stream contains air, enriched air or pure oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified scheme of a reactor according to a first embodiment of the invention.

(2) FIG. 2 is a simplified cross section of the swirling chamber of reactor of FIG. 1.

(3) FIG. 3 is a scheme of a reactor according to another embodiment of the invention.

(4) FIG. 4 is a simplified cross section of the swirling chamber of reactor of FIG. 3.

(5) FIG. 5 is a scheme of a reactor according to another embodiment of the invention.

(6) FIG. 6 is a cross section of the swirling chamber of reactor of FIG. 5.

(7) FIG. 7 is a scheme of a further embodiment of the invention.

(8) FIG. 8 is a cross section of the swirling chamber of reactor of FIG. 7.

(9) FIG. 9 shows a further variant of the invention, applicable to embodiments of FIGS. 1 to 8.

(10) FIGS. 10 and 11 show further examples of the form of the neck of the reactor or the transition connecting the neck to the catalytic zone below.

(11) FIG. 12 shows the flow paths and the flame inside the combustion chamber of the reactor of FIG. 1, in operation.

(12) FIGS. 13 and 14 are prior art secondary reformers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(13) Referring to FIGS. 1-2, a reformer 1 is connected to a gas inlet 2, carrying a hydrocarbon-containing feedstock or HCF stream G. Said HCF stream G can be obtained from primary reforming of a hydrocarbon; an offgas of a coke production plant (coke oven gas) can also form the HCF stream G.

(14) The gas inlet 2 is tangential, as shown, so that the stream G enters the reformer 1 with a direction lying in a plane perpendicular to the vertical axis A-A of the reformer.

(15) The reformer 1 comprises a vessel 3 containing a catalytic bed 4, and having a neck 5 where an oxidizer nozzle or burner 6 is installed. The burner 6, in the shown embodiment, is flushed in a top cover 7 of the neck 5. The oxidizer fed to the burner 6 can be air, oxygen-enriched air, pure oxygen, steam and/or a mixture containing steam, oxygen and carbon dioxide. The neck 5 and vessel 3 are connected by a transition conical wall 8.

(16) The neck 5 comprises a portion 5a with enlarged cross section, defining a swirling chamber 10 connected to the HCF inlet 2. The swirling chamber 10 is located below and in communication with the burner tip 6a, in order to receive the diffusion flame during operation, and has an open bottom 10b in fluid communication with the inside of vessel 3 through the remaining portion of neck 5. It should be noted that there is no gas distributor downstream the gas inlet, so that the open bottom 10b is in direct communication also with the downstream catalytic zone inside vessel 3. The neck portion under the swirling chamber 10 defines a combustion chamber B.

(17) In embodiment of FIGS. 1-2, the swirling chamber 10 is delimited substantially by a side wall 11 with an internal surface 12 following a log-spiral around the same axis A-A. In other words, the cross-section of chamber 10 (FIG. 2) appears as a logarithmic spiral with axis coincident with the axis A-A of the neck 5 and whole reformer 1.

(18) One end 12a of the surface 12 matches a wall 2a of the HCF inlet 2, at the process gas inlet surface S (FIG. 2), while the opposite end 12b of the same surface 12 is tangential to the opposite wall 2b of said inlet 2, in correspondence of the same gas inlet surface S. The log-spiral surface 12, hence, covers an angle of about 360 degrees. Distance of the surface 12 from axis A-A, due to the log-spiral profile, decreases progressively from the end 12a at the gas inlet, towards the end 12b.

(19) Indicating as r the distance from axis A-A, and (theta) the angle from the surface S, the cross-section line of surface 12 (FIG. 2) follows an equation of the type:
r=a.Math.e.sup.b
where a and b are preferably chosen to match the walls 2a and 2b of the inlet line 2 at the inlet section S.

(20) In the simplified embodiment of FIGS. 3 and 4, the surface 12 is cylindrical with the distance from axis A-A remaining constant. Cross-section of surface 12, in this embodiment, is a circular arc; as seen in FIG. 4, the angle covered by the surface 12, starting from the gas inlet surface S, is less than 360 degrees. Preferably said angle is more than 270 degrees and more preferably around 300 degrees.

(21) Embodiments of FIGS. 5 to 8 have a HCF inlet 2 with a circular cross section. In this case, the surface 12 has preferably a semi-circular cross section in the plane perpendicular to the inlet direction of gas stream G, as shown in FIG. 5.

(22) The embodiment of the invention where the surface 12 has a semi-circular cross section and a log-spiral path is best for avoiding lateral component of the momentum of the process gas flow, and achieve a substantially axis-symmetric velocity vector field of the gas entering the combustion chamber B.

(23) A plane surface 12, however, can also be adopted with the inlet 2 having a circular cross section (FIG. 7). The simplified embodiment of FIGS. 3 and 4 can also be used. In this cases, slight deviation from the axis-symmetric velocity vector field will occur.

(24) FIG. 9 shows a further embodiment of the invention, where the swirling chamber 10 is distanced from top of the neck 5, so that there is a gap forming a pre-chamber 20 between the tip of the burner 6, and the chamber 10. Said pre-chamber 20 may be preferred to provide a low-swirling environment for formation of the diffusion flame under burner tip 6a.

(25) FIG. 10 shows non-limitative examples of the transition connecting the neck 5 with the vessel 3, wherein the transition portion 8 is realized as hemispherical dome (left) or cone (right). FIG. 11 shows a further embodiment of the invention wherein the neck 5 is conical with increasing cross section from top to bottom. The forms of the transition portion 8 of FIG. 10, as well as the conical neck of FIG. 11, are applicable to all embodiments of FIGS. 1 to 9.

(26) According to one of the applications of the invention, the reformer 1 is a secondary reformer of a hydrocarbon reforming equipment. In a further preferred application of the invention, said hydrocarbon reforming equipment is the front-end of an ammonia plant, where the reformed gas produced in the secondary reformer 1 is then subject to known treatments such as shift, carbon dioxide separation and methanation, obtaining a syngas containing nitrogen and hydrogen in a suitable HN ratio for ammonia synthesis.

(27) It should be noted that the above detailed description is referred to a reformer, but the invention is applicable as well to different kinds of reactors, including autothermal reformers, secondary reformers, POX gas generators.

(28) In operation (FIG. 12), the HCF gas stream G enters the swirling chamber 10 where, due to profile of surface 12, a swirling motion is imparted to said gas stream G around axis A-A, thus forming a vortex V with axis coincident with said axis A-A. The vortex V, through the open bottom 10b, extends in the combustion chamber B formed by the neck 5 downstream the gas inlet 2. A diffusion flame F is produced by the oxidizer stream from burner 6 and extends into the combustion chamber B through the swirling chamber 10.

(29) Interaction between the flame F, and oxidizer stream, and the gas vortex V in accordance with the invention, provides a surprisingly effective mixing between the oxidizer and the process gas G. Moreover, the flame F is stable and not deflected from axis A-A, despite the tangential inlet 2 of the gas stream.

(30) In fact, the vortex V produced in the log-spiral swirling chamber has an axis-symmetric velocity vector field with a substantially null component in direction perpendicular to axis A-A. The momentum of the process gas in the direction of the transfer line axis is balanced by the pressure distribution on the surface 12. The vortex V, hence, is unable to transmit any relevant momentum to the flame F, in any direction other than axis A-A. Flame F then maintains the axial direction.

(31) It should be appreciated that the kinetic energy of the HCF stream is not wasted in an uncontrolled deflection from the tangential inlet direction of line 2 to the axis of reformer 1, nor it is dissipated in the passage through a gas distributor. The energy of the HCF stream is actively used to produce the vortex V inside the combustion chamber, where the combination of the oxidizer jet velocity, directed according to axis A-A, and of the swirled velocity field imparted to the HCF stream by chamber 10, increase the strength of the mixing layer between the two streams (gas/oxidizer). Using the same kinetic energy of the entering stream G allows to feed the oxidizer at a lower pressure or to shorten the neck 5 for a given velocity of the oxidizer.

(32) In simplified embodiments of the invention, such as the one of FIGS. 3 and 4, the distribution of pressure on the surface 12 is no longer able to completely balance the lateral momentum of the HCF stream. The axis of vortex V, due to lateral and tangential inlet of line 2, is not coincident with the axis A-A and the there is a slight deflection of flame F, which may assume a corkscrew shape. Said effect of flame deflection can be minimized with a proper design of the chamber 10 and neck 5. The same apply to the embodiment of FIG. 7, due to circular transfer line 2 and plane surface 12. These embodiments, however, are still able to improve the gas/oxidizer mixing compared to the prior art, they do not require the gas distributor as well, and may be chosen for reasons of cost and simplicity.