Chemical reactor with adiabatic catalytic beds and axial flow

Abstract

Axial reactor for exothermic or endothermic chemical reactions, comprising at least a first catalytic bed and a second catalytic bed operating in series and at least one heat exchanger between the two catalytic beds, wherein the first catalytic bed has a collector bottom having a box-like structure with flat and parallel walls, which are gas-permeable, and a plurality of parallel channels defined between the walls, wherein a first series of said channels collects the gaseous flow exiting the catalytic bed and passing through the first wall, said gaseous flow is directed towards the heat exchanger, and the flow exiting the exchanger is directed towards the second catalytic bed via a second series of said channels of the collector bottom.

Claims

1. A reactor for carrying out exothermic or endothermic chemical reactions, the reactor comprising: an axial stack of catalytic beds including at least a first catalytic bed and a second catalytic bed operating in series with axial flow, the first and second catalytic beds being stacked one next to the other in an axial direction of the reactor, wherein a gaseous flow exiting the first catalytic bed is subjected to a further reaction stage in the second catalytic bed; and at least one heat exchanger located sideways relative to the axial stack of catalytic beds, said at least one heat exchanger being arranged to cool or heat, via indirect heat exchange with a heating or cooling medium, said gaseous flow exiting the first catalytic bed before entry into the second catalytic bed; wherein the first catalytic bed has a collector bottom having a box-like structure, including a first wall and a second wall spaced and parallel with respect to each other, and a plurality of parallel channels that are defined between said first and second walls; wherein said first wall is exposed to the gaseous flow exiting the first catalytic bed, and the second wall communicates with a gas inlet of the second catalytic bed, and said first wall and second wall are gas-permeable at least over part of a surface thereof; wherein said plurality of parallel channels includes first channels that are in direct communication with the first wall so as to collect the gaseous flow exiting said first catalytic bed and passing through the first wall, and second channels that are in direct communication with the second wall so as to distribute a gaseous flow in said second catalytic bed through the second wall; wherein said at least one heat exchanger is an indirect heat exchanger with a first side traversed by the gaseous flow exiting the first catalytic bed and a second side traversed by a heat exchange fluid, said first side having an inlet communicating with the first channels of the collector bottom and an outlet communicating with the second channels thereof, so as to heat or cool the gaseous flow collected by the first channels and feed the gaseous flow, heated or cooled, to the second channels.

2. The reactor of claim 1, wherein said collector bottom includes a core element sandwiched between said first wall and second wall, the core element having a wave configuration with crests that are joined alternately to the first wall and the second wall, defining said plurality of parallel channels between the said first and second walls.

3. The reactor of claim 2, wherein said first wall includes a flat metal sheet, said second flat wall includes another flat metal sheet, and the core element includes an undulated or corrugated metal sheet.

4. The reactor of claim 1, wherein the first channels and the second channels of the collector bottom alternate with each other.

5. The reactor of claim 2, wherein the core element is a load-bearing element of the collector bottom.

6. The reactor of claim 1, wherein the first wall and the second wall of the collector bottom are gas-permeable owing to holes or micro holes, or owing to a presence of slots covered by a mesh.

7. The reactor of claim 1, wherein said plurality of parallel channels of the collector bottom have an essentially triangular or trapezoidal cross-section.

8. The reactor of claim 1, wherein said plurality of parallel channels of the collector bottom extend in a longitudinal direction that lies in a plane perpendicular to an axis of the reactor.

9. The reactor of claim 8, wherein the reactor has a vertical axis and the plurality of parallel channels are located at a same height inside the reactor.

10. The reactor of claim 1, wherein said collector bottom has a modular structure including modules, each of the modules of the modular structure including a given number of said plurality of parallel channels, and the combined collector bottom being formed by an arrangement side-by-side of two or more of said modules.

11. The reactor of claim 10, wherein adjacent ones of the modules of the collector bottom are fixed together by one or more flat elements that form portions of the first wall or the second wall.

12. The reactor of claim 1, wherein the collector bottom includes perforated tubes for collecting or distributing the gas inside the first and/or second channels.

13. The reactor of claim 1, wherein said at least one heat exchanger is inside the reactor.

14. The reactor of claim 1, wherein said at least one heat exchanger is of a tube type or of a plate type.

15. The reactor of claim 1, wherein the reactor is an axial or essentially axial flow reactor; the first catalytic bed and the second catalytic bed are vertically arranged in a column, the first bed being positioned above the second bed, so that the two beds are crossed in sequence by the axial flow, the collector bottom being arranged between the two beds.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a schematic cross-section of a reactor comprising two catalytic beds and a collector bottom, in one embodiment of the invention.

(2) FIG. 2 shows a schematic cross-section of the collector bottom of the reactor according to FIG. 1.

(3) FIG. 3 shows a view of the collector bottom according to FIG. 2.

(4) FIG. 4 shows a collector bottom as in FIG. 3, in another embodiment of the invention.

(5) FIG. 5 shows a collector bottom in yet another embodiment of the invention.

(6) FIG. 6 is diagram showing the flows between the collector bottom and the heat exchanger of the reactor according to FIG. 1, in an embodiment of the invention.

(7) FIGS. 7 and 8 show a schematic cross-section of a reactor, in some embodiments of the invention.

(8) FIGS. 9 and 10 show a schematic longitudinal section of a reactor, in some further embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(9) FIG. 1 shows in schematic form an axial-flow reactor 1 essentially comprising a shell 2, a first catalytic bed 3, a second catalytic bed 4. The shell 2 further contains a heat exchanger 5. The first catalytic bed 3 has a collector bottom 6 which will be described below.

(10) The reactor 1 for example is a vertical reactor with axis A-A.

(11) In some embodiments the catalytic beds are housed inside a cartridge.

(12) The beds 3 and 4 operate in series. A fresh (gaseous) feed enters the reactor 1 via a flange 101, flows axially through the first bed 3 from the top downwards in the direction indicated by the arrows; a gaseous flow exiting the first bed 3 (partially converted) is directed into the heat exchanger 5 and, once cooled or heated, enters the second bed 4 for completion of the conversion process. The second bed 4 is also axially passed through, as indicated by the arrows. The reaction products leave the reactor 1 via the flange 102.

(13) The catalytic beds 3 and 4 are stacked one above the other with the collector bottom in between. The heat exchanger 5 is placed laterally outside the stack.

(14) The collector bottom 6 has a plurality of parallel channels 15 and 16 which establish communication between the two catalytic beds 3, 4 and with the heat exchanger 5, as will be explained below.

(15) The collector bottom 6 (FIG. 2) essentially comprises a first wall 10 and a second wall 11 and a core element 12 sandwiched in between the walls 11 and 12.

(16) The two walls 10, 11 are flat and parallel, suitably spaced from each other. Said two walls 10, 11 are also gas-permeable at least over part of their surface.

(17) The first wall 10 is exposed to the gas flow exiting the first catalytic bed 3; the second wall faces a gas inlet of the second catalytic bed 4.

(18) The core element 12 has a wave configuration with crests 13 which are joined alternately to the first wall 10 and the second wall 11 along welding lines 14.

(19) The so arranged core element 12 defines a series of parallel channels between the walls 10 and 11, inside the collector bottom 6. More particularly, the following are defined:

(20) first channels 15 which communicate with the first catalytic bed 3 via permeable surfaces 17 of the wall 10, so as to collect the gaseous flow exiting the first bed 2;

(21) second channels 16 which communicate with the second catalytic bed 4 so as to distribute a gaseous flow in said second bed 14 via permeable surfaces 18 of the wall 11.

(22) The surfaces 17 and 18 represent permeable zones of the walls 10 and 11. Said permeable zones may be formed, for example, as longitudinal strips, as shown in the figures. It can noted that the regions around the crests 13 and the respective welds 14, instead, are not permeable.

(23) The heat exchanger 5 is an indirect exchanger with two sides not in communication with each other. Said two sides may be for example a shell side and an inner tube or plate side. One of the sides of the exchanger 5 is crossed by the process gas of the reactor 1 and has an inlet communicating with the first channels 15 and an outlet communicating with the second channels 16.

(24) Observing FIGS. 1 and 2 it can be understood that the gas flow exiting the bed 3 is collected solely inside the first channels 15, passing through the permeable surfaces 17; said channels 15 feed the heat exchanger 5. The gas exiting the heat exchanger 5 enters the second channels 16 and passes into the underlying second bed 4 through the permeable surfaces 18.

(25) The design of the collector bottom 6, in a preferred embodiment, is shown in FIG. 3.

(26) The walls 10 and 11 are essentially realized using metal sheets of suitable thickness (preferably 3-10 mm) and made gas-permeable by means of openings 20 distributed over the surface. The openings 20 of each wall are distributed in strips which correspond to the respective channels which must receive or supply the gas. There are also non-permeable strips in proximity of the joints with the core 12. In FIG. 3 it can be seen that the openings 20 of the wall 10 are distributed in strips 21 corresponding to the position of the first channels 15 and that the wall 10 comprises non-permeable strips 22 (i.e. without openings 20) in proximity of the joints 14.

(27) The openings 20 may be dimensioned depending on the particle size of the catalyst, so as to provide a suitable section allowing the gas to pass through, but the catalyst particles to be retained. In variants of the invention, the gas-permeable wall may also be realized by means of a perforated metal sheet covered with a suitable meshwork or using other techniques for realizing collectors known per se.

(28) The core 12 has flat portions 23 parallel to the walls 10 and 11 for performing welding, i.e. preferably automatic welding. Said core 12 also has inclined gas-impermeable faces 24 which bound the channels 15 and 16 and define a form having a trapezoidal cross-section of the said channels.

(29) FIG. 4 shows a variant in which the collector bottom 6 comprises perforated tubes 25 inside the channels 16. Preferably a perforated tube 25 is provided inside each channel 16. The gas from the heat exchanger 5 is distributed inside the channels 16 via said tubes 25, obtaining a more uniform distribution along the entire length of the said channels. Another advantage of said perforated tubes 25 is that the gas is partly confined inside the tubes 25, thereby reducing the contact of the gas with the surfaces of the sides 24 and the parasitic heat exchange between the channels.

(30) FIG. 5 shows a variant of modular collector bottom 6. Two self-supporting modules 6.1 and 6.2 are shown, these being joined together by a platform 30 which forms part of one of the flat faces, for example the face 10, of the modular collector. The constructional design of the modules 6.1 and 6.2 is similar to that shown in FIGS. 2 and 3, optionally with perforated tubes as shown in FIG. 4.

(31) FIG. 6 shows in schematic form the gas flow path. In particular the figure shows the flows between the collector bottom 6 and the heat exchanger 15. The gas collected in the first channels 15 is preferably fed to a first collector/distributor 31 and then to the heat exchanger 15 along the flow line 32. After passing through the exchanger 15, the gas (line 33) passes to a second collector 34 and from here flows into the second channels 16. The channels 15 are connected in parallel to the collector 31 and the channels 16 are connected in parallel to the second collector 34.

(32) Some non-limiting examples of the reactor layout which can be usefully used with the invention are illustrated below. The examples provide an idea of the various possibilities of application of the invention.

Example 1

(33) In this example (FIG. 7) the reactor comprises, viewed in cross-section, a circular segment 40 which houses the heat exchanger 15 and a circular segment 41 which houses auxiliary services such as: thermocouples, piping, manholes, etc. It should be noted that, when adopting axial flow, a radial symmetry of the reactor is not necessary. Accordingly, the exchanger 15 and/or further services may be housed in circular segments such as the segments 40 and 41. The figure also shows the collectors 31 and 34 as per the diagram of FIG. 6. The exchanger 15 is shown as being of the tube type, but may also be of the plate type.

Example 2

(34) In this example, similar to the preceding example (FIG. 8), the reactor comprises two heat exchangers 15 in the circle segments 40 and 41.

Example 3

(35) In this example (FIG. 9) the heat exchanger 15 is shown as a tube-type heat exchanger arranged offset in a circular segment 40 of the reactor.

(36) The exchanger essentially comprises a shell 50 and a tube bundle 51. The process gas circulates on the shell side, while a cooling fluid circulates on the tube side, i.e. inside the tubes 51. Consequently, the shell 50 in this embodiment is of the low-pressure type, because it must withstand only the difference in pressure resulting from the head losses across the catalytic beds.

(37) The cooling fluid on the tube side has an inlet 52 and outlet 53. Said cooling fluid may be a gas or more advantageously water for producing steam at a suitable pressure. The production of steam is advantageous because the temperature of the cooling fluid (boiling water) is kept constant.

(38) The gas exiting the single beds may be conveyed on the shell side of one or more gas pre-heating exchangers, which are physically separate, or of a single boiler, whose tubes or plates pass along the whole length of the apparatus. In this latter case, shell-side partitions 54 must be provided in order to keep the gas of each single axial collector separate.

(39) In order to simplify the architecture and the assembly/disassembly of the boilers, boilers may be provided with U-shaped tubes.

(40) It can be noted that the reactor comprises two collector bottoms 106, 206 which are situated respectively below the first bed 3 and below the second bed 4. Said two collector bottoms are formed essentially in the same manner as the collector bottom 6 already described. It can be noted that the reactor comprises only two catalytic beds, and therefore the gas exiting the second collector 206 in this example flows out of the reactor instead of passing into another catalytic bed.

(41) The reactor shown in the example has the following advantages: easy accessibility, free choice of the height of the catalytic beds, catalytic beds with a uniform cross-section, better use of the spaces inside the reactor (more compact apparatus), gas entering and exiting the beds without any transverse speed components. The absence of transverse speed components avoids undesirable displacements of the catalyst.

Example 4

(42) In this example (FIG. 10) the heat exchanger 15 comprises a shell 60 and one or more tube bundles 61, 62. The process gas circulates on the tube side. The shell 60 is consequently of the high-pressure type. Other details are substantially similar to those of FIG. 9.

(43) It should be noted that the shell 60 may have a non-circular cross-section. In one embodiment the cross-section of the shell 60 is shaped so as to follow the profile of the reactor circular segment 40.

(44) It should be noted that the constructional design is facilitated by the fact that, for each collector, the gas inlet and outlet of the boiler shell are located at the same height.

(45) The arrangement described forms a single high-pressure shell inside the reactor, where steam is generated.

(46) The reactor according to FIG. 10 may produce steam as in the isothermal methanol reactors of the radial or axial-radial type with exchanging bodies immersed in the catalyst, but it has a number of advantages compared to the latter. One advantage is the improved heat-exchange efficiency which is not impaired by the catalyst. Another advantage is that the components of the exchanger 15 may be inspected and repaired without having to unload the catalyst.