Multi-bed catalytic converter

Abstract

A multi-bed catalytic converter comprising: a plurality of catalytic beds which are traversed in series by a process gas, sequentially from a first catalytic bed to a last catalytic bed of said plurality, and at least one inter-bed heat exchanger (7) positioned between a first catalytic bed and a second catalytic bed of said plurality, wherein at least the last catalytic bed of said plurality is adiabatic and is made of fine catalyst with a particle size not greater than 2 mm.

Claims

1. A multi-bed catalytic converter, comprising: a plurality of catalytic beds which are traversed in series by a process gas, sequentially from a first catalytic bed to a last catalytic bed of said plurality of catalytic beds; and at least one inter-bed heat exchanger positioned between the first catalytic bed and a second catalytic bed of said plurality of catalytic beds, and arranged to remove heat from the process gas leaving the first catalytic bed before entering the second catalytic bed; wherein at least the last catalytic bed of said plurality of catalytic beds is adiabatic and is made of a fine catalyst with a particle size not greater than 2 mm; wherein each of the plurality of catalytic beds includes at least one gas distributor and at least one gas collector arrange to provide that the catalytic bed is traversed by the process gas with a radial flow or axial-radial flow; wherein only the last catalytic bed of said plurality of catalytic beds is made of the fine catalyst, one or more other catalytic beds of the plurality of catalytic beds being made of catalyst with greater particle size.

2. The multi-bed catalytic converter of claim 1, wherein the particle size of said fine catalyst is 0.8 mm to 1.4 mm.

3. The multi-bed catalytic converter of claim 2, wherein the particle size is 0.8 mm to 1.4 mm.

4. The multi-bed catalytic converter of claim 2, wherein the catalyst of the one or more other catalytic beds of the plurality of catalytic beds include a particles size of great than 2 mm and up to 3 mm.

5. The multi-bed catalytic converter of claim 1, wherein said fine catalyst has a particle size of 1.3 mm or about 1.3 mm.

6. The multi-bed catalytic converter of claim 1, wherein said at least one inter-bed heat exchanger includes a plurality of stacked plates, wherein gaps between adjacent plates of the plurality of stacked plates are alternately traversed by the process gas and a cooling medium.

7. The multi-bed catalytic converter of claim 1, wherein said plurality of catalytic beds have an annular-cylindrical geometry and comprise an outer gas-permeable collector and an inner gas-permeable collector, said collectors being cylindrical and coaxial, wherein the inner collector and the outer collector of each catalytic bed containing fine catalyst include any of: a perforated solid wall; a slotted wall; a sintered metal fibre filter; a wall made with a close-knit mesh combined with at least one wall made with wider meshes and/or a slotted plate.

8. A method for revamping a multi-bed catalytic converter, wherein said converter includes: at least three catalytic beds which are traversed in series by a radial flow or an axial-radial flow of a process gas, sequentially from a first catalytic bed to a last catalytic bed of said plurality of catalytic beds; and at least a first inter-bed heat exchanger or a first quencher with a gas stream arranged between a first catalytic bed and a second catalytic bed to cool the effluent of said first bed before admission into the second bed, and a second inter-bed heat exchanger or a second quencher with a gas stream arranged between the second catalytic bed and a third catalytic bed to cool the effluent of said second bed before admission into the third bed, wherein said catalytic beds are made of catalyst with a particle size greater than 2 mm, the method comprising: replacing the catalyst of only the last catalytic bed with catalyst having a particle size not greater than 2 mm.

9. The method of claim 8, wherein the particle size is 0.8 mm to 1.4 mm.

10. The method of claim 8, wherein the particle size is 1.0 mm to 1.4 mm.

11. The method of claim 8, wherein the particle size is 1.3 mm or about 1.3 mm.

12. The method of claim 8, further comprising: removing the first and second catalytic beds and the first and second inter-bed heat exchangers or quenchers; and installing a single isothermal bed to replace said first and second adiabatic beds, said isothermal bed containing a heat exchanger.

13. The method of claim 12, wherein the heat exchanger includes a plurality of heat exchange plates immersed in the catalyst of said isothermal bed.

14. The method of claim 8, wherein the first catalytic bed and the second catalytic bed having an annular-cylindrical geometry and accommodating a first inter-bed heat exchanger and a second inter-bed heat exchanger which are coaxial, the method being characterized by replacing the first and second adiabatic beds with a single isothermal bed containing a heat exchanger, and by replacing the first and second inter-bed heat exchangers with a new inter-bed heat exchanger coaxial and inner to said isothermal bed.

15. The method of claim 14, wherein said single isothermal catalytic bed is larger than the previous first catalytic bed and second catalytic bed, so that the volume used to accommodate the catalyst is substantially unchanged.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified scheme of a multi-bed ammonia converter according to the prior art.

(2) FIG. 2 is a scheme of the converter of FIG. 1 after a revamping in accordance with an embodiment of the invention.

(3) FIG. 3 shows a schematic cross-sectional view of a catalytic bed of the converter of FIG. 2, according with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 illustrates a multi-bed converter 1, for example an ammonia converter, including a vessel 2 and a catalytic cartridge 3 comprising three adiabatic catalytic beds 4, 5, 6 arranged in series, two inter-bed heat exchangers 7, 8 and optionally a bottom heat exchanger 9. According to the example of the figure, said heat exchangers 7, 8, 9 are plate heat exchangers; alternatively, they may be shell-and-tube heat exchangers.

(5) Each bed 4, 5, 6 is traversed by a radial inward or mixed axial-radial flow and has an annular cylindrical shape with a central axial cavity 10. The plate heat exchangers 7, 8, 9 are arranged in said central cavities 10 to provide inter-bed cooling of the gaseous products evolving from one catalytic bed to another.

(6) Said catalytic beds 4, 5, 6 are adiabatic since they contain no cooling means and the heat of the reaction is fully transferred to the gaseous stream of reactants and products.

(7) Said catalytic beds 4, 5, 6 contain catalyst particles with an irregular shape and a size greater than 2 mm. Said catalyst particles are, for example, iron-based.

(8) Each catalytic bed 4, 5, 6 comprises two gas-permeable coaxial walls which define respectively an inner containing wall 11 and an outer containing wall 12. The outer wall 12 acts as a distributor of the gas entering the catalytic bed. The inner wall 11 acts as a collector of the gaseous products leaving the catalytic bed. Said walls 11, 12 are provided with holes or openings of a suitable size so that they are permeable to gas and at the same time are able to mechanically and structurally retain the catalyst. Said two coaxial containing walls are also referred to as outer collector and inner collector.

(9) A fresh make-up gas (MUG) is fed to the converter 1 through the gas inlet 13 and enters the first bed 4 by passing through the outer collector 12; the effluent of the first bed 4 enters the first inter-bed exchanger 7 by passing through the inner collector 11 and is cooled while flowing through the plates of said exchanger 7; the cooled effluent enters the second bed 5 via the respective outer collector 12. Similarly, the effluent of the second bed 5 is cooled in the second inter-bed heat exchanger 8 before entering the third bed 6, and the effluent of the third bed 6 is cooled in the bottom heat exchanger 9 before leaving the converter 1 via the outlet 14.

(10) The reactor of FIG. 1 is known in the art and need not be described in a further detail.

(11) FIG. 2 shows the reactor 100 as revamped according to an embodiment of the invention. In particular, the reactor 100 results from revamping the reactor 1 by means of the following steps:

(12) replacing the first and second adiabatic beds with a single bed 15 and installing a plate heat exchanger 16 including a plurality of heat exchange plates 17 inside the new single bed 15, so that it operates isothermally;

(13) replacing the two inter-bed heat exchangers with a single heat exchanger 18;

(14) loading the new catalytic bed 15 with a fine catalyst having a particle size not greater than 2 mm and a gaussian size distribution for instance between 1 and 2 mm;

(15) replacing the catalyst contained in the third adiabatic bed with catalyst of the same kind but having a finer particle size, wherein the particles are not greater than 2 mm.

(16) Despite installation of heat exchange plates 17 subtracts volume available for the catalyst, the catalyst volume of the new isothermal bed 16 remains unchanged with respect to the catalyst volume of the adiabatic beds 4, 5, because the reactor volume previously used for the passage of the effluent of the first bed 4 to the second bed 5 is now loaded with catalyst.

(17) Reactor 1 has also been revamped by installing new inner collectors 20 and outer collectors 21 able to retain the newly loaded finer catalyst particles. Said new inner and outer collectors 20, 21 are illustrated in FIG. 3 with reference to the adiabatic catalytic bed 6. New collectors 20, 21 are also installed to bound the catalyst contained in the isothermal bed 15.

(18) The heat exchange plates 17 are radially arranged in the isothermal bed 15. Each of said plates 17 is internally traversed by a cooling medium, such as water. As a result, the first catalytic bed 15 of the revamped converter 100 operates in an isothermal manner and the temperature of the first bed 15 can be controlled with an additional degree of freedom by regulating the cooling medium flow and/or temperature through the plates 17.

(19) FIG. 3 shows in schematic form a cross-sectional view of the adiabatic catalytic bed 6 of the revamped reactor 100 according to FIG. 2, wherein the outer collector 21 (distributor) and the inner collector 20 are visible. The distributor 21 and the collector 20 comprise coaxial cylindrical walls which are gas-permeable as a result of holes or openings. According to the example of the figure, said distributor 21 and collector 20 comprise three walls made with mesh, in particular an inner wall 22 and an outer wall 23 made with wider meshes and a central wall 24 made with a close-knit mesh; a solid wall perforated or slotted or both is providing the structural resistance to the catalytic bed.

EXAMPLES

Example 1

(20) The following Table 1 refers to a multi-bed catalytic converter of an ammonia plant with a capacity of 1850 metric tonnes per day (MTD) of ammonia produced and with an inert content of 11% at the inlet of the converter. Said converter contains three adiabatic beds in series containing iron-based catalyst. The first bed has a volume of 5 m.sup.3, the second bed a volume of 8 m.sup.3 and the third bed a volume of 31 m.sup.3.

(21) Table 1 compares the values of pressure drops and conversion yields for the following configurations of the converter and considering its effect in the synthesis loop:

(22) 1.1 Converter of the prior art wherein all beds contain a relatively coarse catalyst. Each bed comprises catalyst particles ranging from 1.5 mm to 3 mm size.

(23) 1.2 Converter according to the invention wherein the first and second beds contain catalyst with particles from 1.5 mm to 3 mm and the third bed contains a fine catalyst with particles from 1 mm to 2 mm, i.e. the third bed does not contain particles over 2 mm.

(24) 1.3 Converter according to the invention wherein the first bed contains the 1.5 mm to 3 mm coarse catalyst and the second bed and third bed contain a fine catalyst with particles from 1 mm to 2 mm.

(25) 1.4 Converter wherein all beds contain a fine catalyst with particles from 1 mm to 2 mm.

(26) TABLE-US-00001 TABLE 1 1.1 1.3 1.4 Coarse catalyst 1.2 Fine catalyst Fine in all beds Fine catalyst in in the 2.sup.nd and catalyst in (prior art) the 3.sup.rd bed 3.sup.rd beds all beds Pressure drop 5 4.9 5.1-5.2 5.5-5.6 [bar] Conversion 17.4 18.5 18.9 19.1 [% mol]

(27) The table shows that configuration 1.2 of the converter, wherein the fine catalyst is used only in the third bed, allows obtaining a higher overall conversion yield and lower pressure drops than the configuration 1.1, wherein all beds contain the coarse catalyst. The lower pressure drop is due to decreased circulation, consequently to the higher ammonia conversion.

(28) Configuration 1.3 of the converter, wherein the fine catalyst is used both in the second bed and in the third bed, allows obtaining a higher overall conversion than configuration 1.1 and configuration 1.2. In this case, the pressure drops are only slightly increased and such an increase is considered acceptable in view of the significant increase of the conversion yield.

(29) Configuration 1.4 shows that the use of the fine catalyst in all beds results in a higher conversion yield but also entails a significant increase of the pressure drops which is not compensated by the higher conversion.

Example 2

(30) The following Table 2 refers to a multi-bed catalytic converter of an ammonia plant with a capacity of 1935 metric tonnes per day (MTD) of ammonia produced, with an inert content of 15.5% at the inlet of the converter and an inlet pressure of 248.5 bar.

(31) Table 2 compares the values of pressure drops and conversion yields for the following configurations of the converter:

(32) 2.1 Converter of the prior art comprising three adiabatic beds, wherein all beds contain a relatively coarse catalyst and each bed contains particles ranging from 1.5 mm to 3 mm.

(33) 2.2 Converter according to the invention comprising three adiabatic beds, wherein the first and second beds contain the coarse catalyst and the third bed contains a fine catalyst with particles from 1 mm to 2 mm.

(34) 2.3 Converter according to the invention comprising a first isothermal bed and a second adiabatic bed, wherein the first isothermal contains substantially the same catalyst volume as the first two adiabatic reactors as configuration 2.2, the first isothermal bed contains a 1.5 to 3 mm coarse catalyst and the second adiabatic bed contains a fine catalyst with particles from 1 mm to 2 mm.

(35) 2.4 Converter according to the invention comprising a first isothermal bed and a second adiabatic bed both containing a fine catalyst with particles from 1 mm to 2 mm, wherein the first isothermal bed contains substantially the same volume of catalyst as the first two adiabatic beds of configuration 2.2.

(36) TABLE-US-00002 TABLE 2 2.3 2.4 2.1 2.2 1.sup.st isot. bed + 2.sup.nd 1.sup.st isot. bed + 2.sup.nd 3 adiabatic beds 3 adiabatic beds adiab. bed adiab. bed Coarse catalyst in Fine catalyst in Fine catalyst in Fine catalyst in all beds (prior art) the 3.sup.rd bed the 2.sup.nd bed all beds Pressure drop 4.2 4.0 3.8 3.7 [bar] Conversion 18.6 19.5 20 20.5 [% mol]

(37) For configuration 2.2 of the converter, wherein the fine catalyst is used only in the third bed of a series of three adiabatic beds, the same considerations as configuration 1.2 of Example 1 apply.

(38) In configuration 2.3, the use of coarse catalyst in the first isothermal bed and fine catalyst in the second adiabatic bed allows a significant increase in the conversion yield and a decrease in the pressure drops.

(39) In configuration 2.4, the use of fine catalyst in the first isothermal bed and in the second adiabatic bed allows an additional increase in the conversion yield and a decrease in the pressure drops.

Example 3

(40) The following Table 3 refers to a multi-bed catalytic converter of an ammonia plant with a capacity of 1935 metric tonnes per day (MTD) of ammonia produced, with an inert content of 15.5% at the inlet of the converter and an inlet pressure of 248.5 bar.

(41) Said converter contains three adiabatic beds in series and Table 3 compares the values of pressure drops and conversion yields for the following configurations:

(42) 3.1 Converter with shell-and-tube inter-bed heat exchangers.

(43) 3.2 Converter with plate inter-bed heat exchangers.

(44) TABLE-US-00003 TABLE 3 3.2 3.1 3 adiabatic beds (plate 3 adiabatic beds heat exchangers) Coarse catalyst (1.5 Fine catalyst (1-2 mm) to 3 mm) in all beds in the 3.sup.rd bed Pressure drop [bar] 4.2 2.5 Conversion [% mol] 18.6 19.5

(45) The table shows that for configuration 3.2 with plate inter-bed heat exchangers, the pressure drops are much lower and the conversion yield significantly higher than configuration 3.1 with shell-and-tube inter-bed heat exchangers.