AXIAL-RADIAL FLOW CATALYTIC CHEMICAL REACTOR WITH TWO LAYERS OF CATALYST

Abstract

Axial-radial flow reactor comprising a catalytic bed (1) of a hollow cylindrical shape, having a vertical axis (2), a base (5), a radial gas inlet section (3b), an axial gas inlet section (6) and a radial gas outlet section (4b), wherein the catalytic bed (1) comprises: a first cylindrical annular region (10) containing a layer of a first catalyst (A) and a layer of a second catalyst (B), the layer of the first catalyst being above the layer of the second catalyst; a second cylindrical annular region (9) coaxial to the first annular region and containing only the first catalyst (A).

Claims

1. A catalytic reactor adapted to process a gas flow by sequential passage through a first catalyst and a second catalyst, said reactor comprising a catalytic bed of a hollow cylindrical shape, having a vertical axis, a base, a radial gas inlet section, an axial gas inlet section and a radial gas outlet section, arranged to determine an axial-radial flow through the catalytic bed, the axial inlet section being at an upper end of the catalytic bed; wherein said catalytic bed comprises: a first cylindrical annular portion extending from said base of the catalytic bed to the axial inlet section, and containing only the first catalyst; a second cylindrical annular portion extending from said base of the catalytic bed to the axial inlet section, said second annular portion containing a layer of said first catalyst and a layer of said second catalyst, the layer of the first catalyst being above the layer of the second catalyst, and said first annular portion and second annular portion being arranged coaxially one around the other.

2. The reactor according to claim 1, wherein the first catalyst is intended to catalyse a first chemical reaction and the second catalyst is intended to catalyse a second reaction, said first reaction and second reaction being different.

3. The reactor according to claim 1, wherein a boundary between said first annular portion and said second annular portion is a vertical cylindrical surface.

4. The reactor according to claim 1, wherein: in the second annular portion, the layer of the second catalyst extends from the base of the catalytic bed to a predetermined boundary level, and the layer of the first catalyst extends above the layer of the second catalyst from said boundary level to the axial inlet section.

5. The reactor according to claim 4, wherein said gas outlet section is a cylindrical surface located entirely below said boundary level.

6. The reactor according to claim 5, wherein: the reactor comprises a cylindrical wall having an upper portion which is not gas-permeable and extends at least from the axial inlet section to said boundary level, and a gas-permeable lower portion which is below said boundary level and provides said radial gas outlet section.

7. The reactor according to claim 1, wherein: the reactor comprises a floating baffle which separates the layer of the first catalyst from the layer of the second catalyst in the second annular portion.

8. The reactor according to any of the previous claim 1, wherein: the first annular portion and the second annular portion are separated by a gas-permeable separation baffle.

9. The reactor according to claim 8, wherein said separation baffle extends from the base of the catalytic bed up to the axial inlet section.

10. The reactor according to claim 1, wherein said catalytic bed is of the inward axial-radial flow type, said first annular portion is an outer region of the catalytic bed, said second annular portion is an inner region of the catalytic bed.

11. The reactor according to claim 1, wherein said axial gas inlet section of the catalytic bed is an open top section of the catalytic bed or includes a gas-permeable cover.

12. The reactor according to claim 1, wherein the first catalyst is suitable to decompose N.sub.2O into nitrogen and oxygen and the second catalyst is suitable to react NOx and N.sub.2O with a reducing agent.

13. A catalytic reactor adapted to process a gas flow by sequential passage through a first catalyst and a second catalyst, said reactor comprising a catalytic bed of a hollow cylindrical shape, having a vertical axis and having at least a lateral radial inlet section and at a top axial inlet section so as to determine an axial-radial flow through the bed, and having an outlet section, wherein: said catalytic bed comprises a first head zone which is adjacent to said top axial inlet section, and a second zone below said head zone; said head zone of the catalytic bed contains solely the first catalyst and does not face said outlet section of the catalytic bed, so that the effluent gas of said head zone passes into the underlying second zone of the catalytic bed; said second zone of the catalytic bed contains a layer of the first catalyst and a layer of the second catalyst, said layers being arranged coaxially one around the other; the reactor further comprising a gas-permeable separating baffle between said two coaxial layers of the second zone of the catalytic bed; wherein said separating baffle extends also into the head zone of the catalytic bed, the catalytic bed thus being divided into two coaxial portions, the first of said coaxial portions containing only the first catalyst, the second of said coaxial portions containing a layer of second catalyst up to a predefined level and containing a layer of first catalyst above said level; wherein the second coaxial portion optionally comprises a floating baffle which separates the layer of first catalyst from the layer of second catalyst; wherein said catalytic bed is of the inward axial-radial flow type and wherein, in the second zone of the catalytic bed containing two coaxial layers of the first and the second catalyst, the first catalyst is situated externally of the second catalyst; wherein the first catalyst is suitable to decompose N.sub.2O into nitrogen and oxygen and the second catalyst is suitable to react NOx and N.sub.2O with a reducing agent.

14. A process for the removal of nitrogen oxides NOx and nitrous oxide N.sub.2O from a gas stream, comprising the steps of: introducing the gas stream in a reactor according to claim 12 and containing a catalytic bed with two coaxial annular portions, passing the input gas stream through a first catalyst contained in the first annular portion of the catalytic bed, and/or contained in the upper layer of the second annular portion of the catalytic bed; passing the so obtained partially conditioned gaseous effluent through a second catalyst contained in the bottom layer of said second annular portion, recovering a conditioned gas from said bottom layer of catalyst.

15. The process according to claim 14, wherein said gas stream containing NOx and N.sub.2O is an offgas current produced in a nitric acid synthesis process.

Description

DESCRIPTION OF THE FIGURES

[0054] FIG. 1 is a simplified diagram of a dual-layer catalytic bed according to an embodiment of the invention.

[0055] FIG. 2 is a sketch of a reactor comprising the catalytic bed of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0056] FIG. 1 shows a diagram of a catalytic bed 1 with a hollow cylindrical form which extends around a vertical axis 2.

[0057] The catalytic bed 1 is contained inside a chemical reactor R (FIG. 2). Said reactor has a pressurized shell 20 preferably having an axis coinciding with the axis 2.

[0058] The catalytic bed 1 is delimited by an outer cylindrical wall 3, an inner cylindrical wall 4, a base 5 and a top surface (annulus) 6.

[0059] The cylindrical walls 3 and 4 have continuous portions 3a, 4a and portions 3b, 4b which are permeable to gases, for example perforated. The surface or annulus 6 is at least partly gas-permeable. In some embodiments the surface 6 is an open top surface of the catalytic bed.

[0060] A gas flow supplied to the catalytic bed 1 comprises a fraction Fr which enters the bed radially through the perforated wall portion 3b and a fraction Fa which enters the bed axially through the annular surface 6.

[0061] A mixed axial-radial flow F, indicated by the arrows, is established inside the catalytic bed 1 as a result of the flows Fr, Fa. The gas leaving the bed 1 passes through the perforated wall portion 4b and is collected inside a central tube 7.

[0062] The gas-permeable wall portions 3b and 4b define respectively a radial inlet section and a radial outlet section of the catalytic bed 1. In this example the radial flow is directed towards the axis 2 and consequently is termed inward radial flow. The annular surface 6 instead forms an axial inlet section of the catalytic bed 1.

[0063] The catalytic bed 1 comprises a gas-permeable partition baffle 8 substantially parallel to the cylindrical walls 3 and 4. The partition baffle 8 defines two cylindrical annular portions of the catalytic bed, namely a first outer portion 9 and a second inner portion 10. Said portions 9 and 10 have a hollow cylindrical form and are coaxial.

[0064] The outer portion 9 is filled entirely with a first catalyst A.

[0065] The inner portion 10 is filled with a second catalyst B up to a predefined level 11. Above the level 11 the inner portion 10 is filled with the first catalyst A, i.e. the same catalyst as the outer portion 9. The level 11 forms a boundary between the layers of catalyst A and B in the inner portion 10.

[0066] The inner portion 10 therefore contains two layers of catalyst arranged on top of each other, i.e. the layer of catalyst B in the bottom/central zone and the layer of catalyst A in the top zone 12 above the level 11 and above the catalyst B.

[0067] Owing to this arrangement, the overall catalytic bed 1 has a head zone 13, situated between the inlet section 6 and the level 11, containing only the catalyst A, and an underlying zone 14, containing two coaxial layers of catalyst A and B.

[0068] The level 11 in some embodiments is defined by a floating separation baffle, preferably a metallic mesh. In other embodiments a physical separating baffle is not necessary, i.e. the catalyst A in the zone 12 is poured directly over the previously loaded layer of catalyst B; therefore the boundary 11 will be understood as an interface and separation plane between the two catalysts. This is possible when the two catalysts do not substantially mix during operation and are suitably compacted.

[0069] The head zone 13 has a depth a, measured along the direction of the axis 2 from the axial inlet section 6. In the example said depth a is also the height of the zone 12 filled with catalyst A, situated above the layer of catalyst B.

[0070] The gas-tight wall portion 4a extends for a distance b. The lower end of the wall portion 4a is preferably below the level 11. When the lower end of the wall portion 4a is below the level 11, the head zone 13 does not face directly the perforated wall portion 4b and consequently the effluent of the upper zone 13, irrespective of the distance from the axis 2, always passes through the catalyst B before it can reach the outlet section 4b and leave the catalytic bed 1.

[0071] As can be noted from FIG. 1, all the gas entering the catalytic bed 1 passes sequentially through the catalyst A and then through the catalyst B before reaching the outlet section 4b. This is true, in particular, also for the flow F* entering close to the axis 2 which traverses the catalyst A in the zone 12 and traverses the catalyst B before flowing out through the perforated wall 4b. In the head zone 13 of the catalytic bed, the flow is predominantly axial while in the lower parts of the catalytic bed and near the outlet section the radial component becomes substantial.

[0072] It can be appreciated that in absence of the layer of catalyst A in the zone 12, the near-axis flow F* would encounter substantially only the catalyst B to the detriment of the reactor efficiency. The invention avoids this drawback: in the axial-flow upper zone 13, the axially entering gas passes through the first catalyst, as well as the radially entering gas from the permeable wall 3b. Then, all the partially conditioned gas passes through the second catalyst in the route towards the outlet section 4b.

[0073] Accordingly, the invention achieves the goal of applying the axial-radial mixed flow configuration, along with its advantages, to the processes which requires a sequential passage through two catalysts A and B. The invention ensures that all the incoming gas passes through the two catalysts as desired, preventing bypassing of the first catalyst A.

[0074] A reactor R comprising the previously described catalytic bed 1 is further illustrated in FIG. 2. The arrows in the figure show the axial-radial flow in the reactor R.