A REACTOR FOR THE SYNTHESIS OF UREA

Abstract

A reactor for the synthesis of urea comprising a vertical shell and perforated baffles or trays (3) arranged to define compartments of the reactor, wherein each baffle comprises an array of individual perforated tiles (10) wherein each tile (101) comprises side walls (101A-101D) and a top face (101F), the side walls having first perforations for the liquid and said top face having second perforations for the gas, wherein said second perforations are smaller than said first perforations, and the tiles are distributed over the baffle with a two-dimensional pattern where adjacent tiles are separated by gaps (17).

Claims

1. A reactor for the synthesis of urea comprising a vertical shell and at least one internal perforated baffle arranged to define compartments of the reactor, wherein said baffle comprises an array of individual perforated tiles wherein each tile comprises side walls and a top face, at least one of the side walls having first perforations and said top face having second perforations, wherein said second perforations are smaller than said first perforations, and the tiles are distributed over the baffle with a two-dimensional pattern and adjacent tiles are separated by gaps wherein the first perforations provide a preferential route for a liquid phase and the second perforations provide a preferential route for a vapor phase in the reactor.

2. The reactor according to claim 1, wherein the array of tiles includes rows and columns according to perpendicular directions, each row and each column including at least two tiles.

3. The reactor according to claim 1, wherein the tiles are arranged symmetrically with respect to at least one vertical plane.

4. The reactor according to claim 1, wherein the tiles are arranged with a square pitch or a rectangular pitch to form a matrix-like array of tiles, or wherein the rows or the columns of tiles are staggered.

5. The reactor according to claim 1, wherein the side walls of the tiles are vertical walls parallel to a vertical axis of the reactor and the top face is perpendicular to the vertical axis of the reactor.

6. The reactor according to claim 1, wherein each tile has an aspect ratio of 0.5 to 1.5, wherein said aspect ratio is a ratio of a maximum length over a maximum width of the tile according to perpendicular directions in a horizontal plane.

7. The reactor according to claim 1, wherein all tiles of the baffle have the same elevation from a reference base plane, said elevation being the vertical distance of the top surface from said base plane.

8. The reactor according to claim 1, wherein each tile is separated from adjacent tiles by said gaps, wherein the gaps extend in directions parallel to the side walls of the tile, so that each tile forms an individual box-like structure.

9. The reactor according to claim 1, wherein each perforation of the first perforations, which are for the liquid passage, has an area of 300 to 600 mm2 and each perforation of the second perforations, which are for the gas passage, has an area of 3 to 15 mm2.

10. The reactor according to claim 1, wherein the first perforations are arranged on some or all of the side walls.

11. The reactor according to claim 1, wherein the first perforations and/or the second perforations are circular holes.

12. The reactor according to claim 1, wherein the number of tiles of said at least one baffle is 12 to 30.

13. The reactor according to claim 1, wherein the second perforations are gas holes of a circular shape and having a diameter not greater than 4 mm.

14. The reactor according to claim 1, wherein the baffle fits the shape of a solid circle and includes first tiles with a square shape delimited by four side walls arranged relative to each other at right angles, and second tiles with a polygonal shape having at least one side wall inclined relative to the adjacent side walls, the second tiles being positioned peripherally in the baffle in order to approximate said solid circle.

15. The reactor according to claim 1, including a respective supporting ring for fixation of each baffle to the pressure shell of the reactor.

16. The reactor according to claim 1, wherein each tile includes a plurality of metal sheets, including a folded metal sheet which define the top face and two opposite side walls, and one or more additional metal sheets which form other side walls of the tile.

17. The reactor according to claim 16, wherein said at least one baffle include supporting beams and at least one of the metal sheets of each tile is secured to said supporting beams.

18. The reactor according to claim 1, wherein the baffle includes self-supporting structures wherein each of said structures form a plurality of tiles aligned in a row from one end to the opposite end of the baffle, and the structures are arranged parallel to each other to form the two-dimensional array

19. A process for synthesis of urea from ammonia and carbon dioxide in a urea synthesis vertical reactor, wherein the process is carried out in a two-phase environment including a liquid phase and a gaseous phase, wherein the process includes the formation of a mixture of the liquid phase and gaseous phase flowing upwards in the reactor and withdrawing a liquid effluent from an upper region of the reactor, and the process includes passing the mixture though a plurality of baffles aligned vertically in the reactor, wherein each baffle comprises an array of individual tiles, each tile comprises side walls and a top face, at least one side wall has first perforations for the liquid phase and said top face has second perforations for the gaseous phase, wherein the second perforations are smaller than the first perforations, so that the liquid phase flows predominantly through the first perforations and the gas phase flows predominantly through the second perforations, and each tile performs a separation of the gas phase from the liquid phase, and the tiles of each baffle form a two-dimensional pattern over the surface of the baffle and adjacent tiles are separated by gaps.

20. A method for revamping a vertical urea reactor wherein the reactor includes internal separation baffles which divide the inside of the reactor into compartments, and the method includes replacing at least one baffle of the reactor with a new baffle, wherein: the new baffle comprises an array of individual tiles, wherein each tile comprises side walls and a top face, at least one side wall with first perforations and said top face with second perforations, wherein the second perforations are smaller than the first perforations, and the tiles are distributed over the baffle with a two-dimensional pattern and adjacent tiles are separated by gaps wherein the first perforations provide a preferential route for the liquid phase and the second perforations provide a preferential route for the vapor phase in the reactor.

21. The reactor according to claim 3, wherein the tiles are arranged symmetrically with respect to two orthogonal vertical planes.

22. The reactor according to claim 12, wherein the number of tiles of said at least one baffle is 18 to 26.

Description

DESCRIPTION OF FIGURES

[0054] FIG. 1 is a scheme of a urea reactor including a plurality of internal baffles, including a detail of one of the baffles.

[0055] FIG. 2 is a view of one baffle of the reactor of FIG. 1, according to a first embodiment.

[0056] FIG. 3 is a top view of one tile of the baffle of FIG. 2.

[0057] FIG. 4 is a sectional view of the tile of FIG. 3.

[0058] FIG. 5 is a top view of another tile of the baffle of FIG. 2.

[0059] FIG. 6 is a sectional view of the tile of FIG. 5.

[0060] FIG. 7 is a sectional view according to plane VII of FIG. 2.

[0061] FIG. 8 is a sectional view according to plane VIII of FIG. 2.

[0062] FIG. 9 is a detail of FIG. 7.

[0063] FIG. 10 is a view of one baffle according to a second embodiment.

[0064] FIGS. 11 and 12 are sectional views according to planes XI and XII of FIG. 10.

[0065] FIG. 13 is a detail of a support of the baffle of FIG. 10.

DETAILED DESCRIPTION

[0066] FIG. 1 illustrates a reactor 1 for the synthesis of urea comprising a shell 2 with a vertical axis A-A and perforated baffles (or trays) 3 arranged inside the shell 2 to define several compartments of the reactor C1, C2, . . . Cn.

[0067] The reactor receives the reagents in its lower part, for example from a feed line 4, and has an output line 5 connected to a downcomer pipe 6 for a urea-containing liquid effluent collected from the top compartment above the uppermost baffle.

[0068] The reactor 1 may be part of a high-pressure loop including a stripper, a condenser and possibly a scrubber; the feed line 4 may carry the condensate from the high-pressure condenser together with fresh gaseous ammonia and possibly gaseous carbon dioxide. A separate feed line 7 of gaseous carbon dioxide may also be provided if necessary.

[0069] The effluent line 5 may feed the reactor effluent to a stripper. A reactor overhead gas is withdrawn from top of the reactor via a line 8 and may be sent to a scrubber.

[0070] The above details of the reactor 1 are known to a skilled person and may differ according to the urea synthesis process which is implemented, for example self-stripping, ammonia stripping or CO2 stripping. Therefore the reactor 1 is not further described.

[0071] One of the baffles 3 is illustrated in the enlarged detail of FIG. 1 and is now described with reference to FIGS. 2 to 9.

[0072] The baffle 3 includes a plurality of box-shaped tiles generally denoted in FIG. 1 with the reference 10. The tiles 10 form a two-dimensional pattern. In the present example, as best seen in FIG. 2, the tiles 10 are arranged with a square pitch and form a matrix pattern of rows 11 and columns 12, particularly five rows and five columns in the shown example. Each tile 10 is separated from adjacent tiles by gaps 17.

[0073] The tiles 10 are supported by a ring 13 which in turn is anchored to the inside surface of the shell 2 by a plurality of supports 14. A planar upper surface 15 of the ring 13 defines a plane 16 (FIGS. 4 and 6) perpendicular to the axis A-A which can be regarded as a reference base plane of the baffle 3. This plane 16 denotes a base elevation of the baffle 3 within the reactor 1.

[0074] Each tile 10 is basically a prismatic body projecting upward from the ring 13 (therefore upward from the base plane 16) and comprising side walls and a top face.

[0075] FIG. 2 indicates exemplary tiles 101 to 106 which are described below.

[0076] FIG. 3 illustrates a first tile 101 having side walls 101A to 101D and a top face 101F. Some or all of the side walls 101A to 101D have perforations with a size adapted to provide a preferential route for the liquid phase. The top face 101F has smaller perforations adapted to provide a preferential route for the gaseous passage. The perforations are not shown.

[0077] The side walls 101A to 101D are arranged at right angles, which means each side wall forms a square angle with the adjacent side walls.

[0078] The top face 101F is located at a vertical distance h from the base plane 16 according to the direction of the axis A-A. Said distance h can be termed the height of the tile 101 relative to the base plane 11. Preferably all tiles of the baffle 3 have the same height.

[0079] The tile 101 has also a length L and a width w. Preferably said length and width are equal or slightly different; for example the ratio L/w is preferably in the range 0.5 to 1.5, and more preferably 0.8 to 1.2. Preferably said ratio is 1 or close to 1, so that the top face 101F, seen from the above, is a square or is close to a square.

[0080] It can be noted that different tiles 10, within a baffle 3, may have different length and/or different width.

[0081] FIGS. 5 and 6 illustrate a second tile 102 which has four side walls 102A to 102D and a top face 102F and, additionally, an inclined side wall 102E to join short side walls 102B and 102C. This shape with an inclined side wall may better conform to an arc of the circular cross section. The tile 102 has also a plate 24 for fixation to the ring 13.

[0082] Tiles with inclined side walls, such as the tile 102, are preferably provided at the periphery of the baffle 3, as illustrated in FIG. 2.

[0083] The side walls of each tile 10 are preferably vertical and parallel to the axis A-A and the top face is preferably plane and perpendicular to the same axis.

[0084] It can be appreciated that the two-dimensional pattern of the tiles 10 created an alternation of highs and lows over the upper surface of the baffle 3. The highs correspond to the top faces of the tiles, e.g. the top faces 101F and 102F. The low (recesses) correspond to the gaps 17. The highs and lows may also be regarded as projections and recesses relative to a median plane of the baffle (e.g. a plane parallel to the plane 16 passing at half the height h).

[0085] The baffle 3 has an upper face and a lower face. Each tile 10 defines a chamber on the lower face of the baffle 3 and said chamber, in operation, will receive a fraction of the upwardly flowing mixture of gas and liquid.

[0086] For example the tile 101 defines a chamber 23 (FIG. 4). It can be understood that the fraction of the mixture collected in the chamber 23 will flow through the perforations of its side walls and top face and, particularly, the liquid phase will flow mostly through the larger first perforations whilst the gas (vapor) phase will flow mostly through the smaller second perforations. This is due to the different size and location of the first and second perforations. Similarly, FIG. 6 shows the chamber 23 defined by the tile 102.

[0087] A tile 10 may comprise a metal sheet which is shaped and possibly cut to form the top face and two opposite side walls, and may comprise two or more additional metal sheets arranged to form the other side walls.

[0088] The sectional view of FIG. 7 illustrates exemplary tiles 103 and 104 connected to supporting beams 20. The supporting beams 20, in this embodiment, extends from one side to the other side of the baffle 3 and provide an intermediate support for the tiles 10 as well as increased resistance to bending.

[0089] The tile 103 comprises a metal sheet 130 shaped to form two side walls, e.g. a front side wall 103B and a rear side wall 103D, and the top face 103F. The ends of the metal sheet 130 are fixed (e.g. bolted) to the ring 13 or to the supporting beams 20 (see also FIG. 9). The tile 103 further includes two metal sheets 131, 132 forming the left and right side walls.

[0090] The tile 104 next to tile 103 is similar, including a metal sheet 140 to form a front wall and a rear wall and the top face and two metal sheets to form the right and left walls.

[0091] FIG. 8 shows a section of another exemplary tile 150, showing a metal sheet 150 forming the top face 105F and the front/rear side walls, and lateral metal sheets 151, 152 forming the left/right side walls 105A and 105C.

[0092] FIGS. 7 and 8 also illustrate a plate 22 arranged in the middle of the beam 20 to increase resistance to bending.

[0093] FIG. 9 shows a detail of ends 133 and 143 of the metal sheets 130, 140 connected to the beam 20 with a common anchor point such as bolt 21.

[0094] Some details of FIGS. 7 to 9 are also indicated in FIG. 2 for easier understanding. In FIG. 7 and FIG. 9, the shell 2 is also illustrated in a cross section.

[0095] In tiles with an inclined side wall (e.g. as in FIG. 5), the inclined side wall may be formed by a separate metal sheet or by bending a lateral metal sheet in an appropriate way.

[0096] The metal sheets forming one tile are preferably joined in a sealed manner, for example welded. Accordingly, escape of the liquid or gas at the junction of metal sheets is avoided.

[0097] FIG. 10 illustrates a baffle 3 according to a second embodiment. In this second embodiments the tiles are formed by a folded metal sheet and lateral metal sheet as previously described. However, the tiles in a row are connected together forming a self-supporting structure which is connected at opposite ends to the ring 13, with no need of the supporting beams 20.

[0098] This second embodiment includes integrated stiffening beams 30 between adjacent tiles, such as tiles 107, 108 of FIG. 11. Also, gas equalization ducts 31 are preferably provided between adjacent tiles. FIG. 12 illustrates another pair of tiles 109, 110 and a preferred arrangement of two gas equalization ducts 31.

[0099] FIG. 13 illustrates a detail, according to a preferred execution, of a support 14, including a L-shaped plate 40 welded to the shell 2 and to the ring 13. FIG. 13 also illustrates the top face 15 of the ring 13, which coincides with the reference base plane 16. The embodiment of FIG. 13 can be used, preferably but not exclusively, with the second embodiment of FIG. 10.

COMPARATIVE EXAMPLE

[0100] The performance of a reactor according to the invention was compared with the performance of a reactor according to the prior art of EP 495 418, with a proprietary mathematical model.

[0101] The following assumptions were made for the prior art comparative case: self-stripping process; residence time in the reactor 20 min; production 2216 metric tons per day (MTD); inlet nitrogen to carbon N/C ratio of 3.27; inlet hydrogen to carbon H/C ratio of 0.59; inlet temperature of 140° C.; outlet temperature 193° C. The CO2 conversion was 54.3% overall and 61.9% on liquid basis.

[0102] With the same residence time, inlet N/C ratio, inlet H/C ratio and inlet temperature, an overall CO2 conversion of 55.4% and 62.9% on liquid basis was obtained for the reactor according to the invention with a production rate of 2263 MTD.

[0103] Therefore the invention gives an advantage in terms of conversion around 1%. The advantage may be greater in case e.g. of a revamping when starting from a less optimized condition.

[0104] With a CO2 stripping process, residence time of 30 min, inlet N/C of 3.19, inlet H/C of 0.61, inlet liquid temperature of 174° C., the CO2 conversion was 54.2% overall and 59.1% liquid basis with the prior art, and was increased to 56.3% and 60.6% respectively with the invention. The calculated production rate increased from 2134 MTD to 2217 MTD.