TECHNICAL FIELD

The present invention relates to a urea synthesis reactor and process.

BACKGROUND ART

As is known, urea is normally produced on an industrial scale via a direct biphasic reaction of ammonia and carbon dioxide under high temperature and high pressure conditions.

A typical urea synthesis reactor of a urea plant is fed with an essentially gaseous stream of carbon dioxide and an essentially liquid stream of ammonia/ammonium carbamate. The reagents are fed into the reactor from below, through a bottom part of the reactor and via respective distributors.

Figure 1 schematically shows, in a simplified manner, the lower part of a typical urea synthesis reactor 1 of known type.

In general, the reactor 1 extends along a vertical axis A and comprises a casing 2 internally defining a reaction chamber 3; the casing 2 has a basically cylindrical main portion 4 and a dome-shaped bottom portion 5 (in particular, substantially hemispherical) . The bottom portion 5 is joined to the main portion 4 along an essentially circular peripheral edge 9, which lies on a plane substantially orthogonal to axis A. The casing 2 is supported by a support frame 10, mechanically connected, in particular, to the bottom portion 5.

The reagents (carbon dioxide and ammonia/ammonium carbamate) are fed into the reactor 1 through respective dedicated distributors 15 and 16. The distributors 15 and 16 are constituted by respective substantially vertical tubular elements, arranged to pass through the wall of the reactor 1, and precisely of the bottom portion 5, in respective openings made in said wall.

The tubular elements constituting the distributors 15 and 16 project upwards from the wall of the bottom portion 5 and form the so-called drilled pipes: each tubular element has a closed free end (located inside the reactor 1) and a plurality of lateral through holes, made in the lateral wall of the tubular element for a certain longitudinal length close to the free end . The distributors 15 and 16 are placed in an eccentric position, for example diametrically opposed with respect to the central axis A of the reactor 1 and, in any case, generally on opposite sides of a vertical centre-plane passing through the axis A of the reactor 1.

The distributors 15 and 16 constitute the end parts of respective inlet tubes bent in an elbow-shape: outside of the reactor, each tube comprises an elbow-shaped bend that connects the vertical tubular element with a horizontal section that passes through the frame 10.

A reactor with a reagent feed system such as that just described has drawbacks. Apart from the related constructional complexity, mainly due to the use of elbow-shaped inlet tubes (which must normally be forged) and the need to perforate a bottom portion of the reactor, the largest drawback consists in that the vertical arrangement of the distributors does not allow uniform distribution of the reagents over the entire cross-section of the reactor. In particular, the carbon dioxide, which due to its density constitutes the light phase in the urea synthesis reaction, tends to form a vertical column above the related distributor, in this way strongly limiting reaction kinetics, which would instead be favoured by the uniform distribution of carbon dioxide in small bubbles over the entire cross-section of the reactor. Even the distribution of the (liquid) heavy phase, formed of ammonia and ammonium carbamate, is not entirely satisfactory.

This unsatisfactory distribution of reagents, in particular of carbon dioxide, causes part of the volume of the reactor to remain unused (due to poor contact between the two phases), especially in the reactor's reagent inlet zone ; this volume could instead be very productive as the concentration of the reagents is at its maximum here.

DISCLOSURE OF INVENTION

One object of the present invention is to provide a urea synthesis reactor and process that enables overcoming the drawbacks pointed out above of the known art; in particular, one object of the invention is that of improving the efficiency of known urea synthesis reactors and processes.

The present invention thus relates to a urea synthesis reactor and process as defined in essential terms in the appended claims 1 and 13, respectively. Additional preferred features of the invention are indicated in the dependent claims.

With respect to traditional configurations, the invention enables obtaining better and more rapid mixing of the light phase (carbon dioxide) with the heavy phase (ammonia/ammonium carbamate) . In particular, the invention achieves better distribution of the carbon dioxide over the cross-section of the reactor, with consequent advantages in overall efficiency of the urea synthesis process.

Furthermore, carbon dioxide bubbles of smaller diameter are formed, which are also propitious from the reaction efficiency viewpoint .

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will become clear from the description of the following non-limitative embodiments, with reference to the figures in the accompanying drawings, in which:

- Figure 1 is a schematic partial view, in longitudinal section, of the lower part of a urea synthesis reactor of known configuration;

- Figure 2 is a schematic side elevation view, with parts in longitudinal section, of a urea synthesis reactor in accordance with the invention;

- Figure 3 is a schematic cross-sectional view of the reactor in Figure 2;

- Figures 4 and 5 are schematic cross-sectional views of respective variants of the reactor in Figure 2;

- Figure 6 is a schematic side elevation view, with parts in longitudinal section, of a further embodiment of the urea synthesis reactor of the invention;

- Figure 7 is a schematic cross-sectional view of the reactor in Figure 6;

- Figures 8 and 9 are schematic cross-sectional views of respective variants of the reactor in Figure 6.

BEST MODE FOR CARRYING OUT THE INVENTION

Figure 2 shows, in simplified and schematic form, the lower part of a urea synthesis reactor 1, specifically destined to perform the biphasic reaction for the direct synthesis of urea, at high temperature and pressure, starting from carbon dioxide and ammonia with the intermediate formation of ammonium carbamate.

In general, the reactor 1 extends along a vertical axis A and comprises a casing 2 internally defining a reaction chamber 3.

The casing 2 is substantially cylindrical and is closed at respective opposite axial ends by two end caps with, for example, a substantially hemispherical shape. Figure 2 only shows the lower part of the reactor 1 and therefore only the end cap placed on the bottom end of the reactor 1, while the end cap placed on the top end and forming the head of the reactor is not visible.

With regard to the present invention, the casing 2 has an essentially cylindrical main portion 4 and a dome-shaped bottom portion 5 (in particular, substantially hemispherical), defining the bottom cap of the reactor 1.

The main portion 4 is delimited by a cylindrical side wall 6 about axis A. The bottom portion 5 has a dome-shaped wall 8 and is joined to the main portion 4 along an essentially circular peripheral edge 9 (welding line) , which lies on a plane substantially orthogonal to axis A. The casing 2 is supported by a support frame 10, mechanically connected, in particular, to the bottom portion 5.

Also referring to Figure 3, the main portion 4 has a pair of lateral through openings 11 and 12 made in the side wall 6 above the edge 9, i.e. above the joint line between the (cylindrical) main portion 4 and the (dome-shaped) bottom portion 5 of the casing 2.

Preferably, the openings 11 and 12 are placed on opposite sides of a vertical middle-plane passing through the axis A of the reactor 1; in particular, the openings 11 and 12 are diametrically opposed.

In the non-limitative example in Figures 2 and 3, the openings 11, 12 are substantially aligned with one another, being placed more or less at the same distance (measured from the centre of the openings 11, 12) from the edge 9.

Respective inlet tubes 13, 14 are inserted through the openings 11, 12 and are fluid-tightly connected, in a known manner, to the openings 11, 12, to feed the reagents of the urea synthesis reaction to the reactor 1: in particular, inlet tube 13 feeds a light phase (essentially gaseous) containing carbon dioxide, while inlet tube 14 feeds a heavy phase (essentially liquid) containing ammonia and generally also ammonium carbamate, to the reactor 1.

In the non-limitative example in Figures 2 and 3, the inlet tubes 13, 14 are parallel and substantially aligned with each other .

The inlet tubes 13, 14 are connected to respective reagent distributors 15, 16 inside the casing 2.

Distributor 15 (light phase distributor) comprises one or more tubular elements 20, connected to each other and/or to inlet tube 13 and extending/distributed over the cross-section of the reactor 1 and about axis A, and provided with intake holes 21 spaced apart from one another, so as to distribute the light phase in a plurality of intake points distributed transversely in the reactor 1 and about axis A. In particular, the tubular elements 20 of the distributor 15 comprise an annular element 20a that projects from inlet tube 13 and is arranged about axis A along the side wall 6 and is preferably radially spaced apart from the side wall 6.

Inlet tube 13 projects radially outwards from the annular element 20a and is substantially coplanar with the annular element 20a, namely the inlet tube 13 and the annular element 20a are located substantially at the same height along axis A, i.e. at the same distance from edge 9.

Here and elsewhere, when speaking of the height or distance of a tubular component, it is intended that this height or distance is measured with respect to a central longitudinal axis of the tubular component.

The annular element 20a can be closed (i.e. have the form of a complete ring) or, as shown in Figures 2 and 3, be broken (i.e. have the form of an open ring) .

In the non-limitative example shown, the annular element 20a has, in particular, an essentially toroidal shape with a substantially circular cross-section; the annular element 20a has a substantially circular shape in plan, but does not form a complete closed ring, but a broken ring, extending with an angular magnitude of less than 360°.

The annular element 20a is interrupted in a position diametrically opposite to inlet tube 13, where it has two facing ends 22 placed on opposite sides of the inlet tube 14.

The ends 22 are preferably blind (closed) ends; the annular element 20a is provided with a plurality of transverse intake through holes 21, made in a lateral wall 23 of the annular element 20a along the annular element 20a and angularly spaced apart from one another along the annular element 20a and about axis A. Preferably, the holes 21 are uniformly distributed along the annular element 20a, in particular being uniformly spaced apart from one another along the annular element 20a. In general, the holes 21 can be arranged in any position on the lateral wall 23; the holes 21 can therefore face upwards and/or downwards and/or towards the side wall 6 of the casing 2. The holes 21 can be organized in one or more rows along the annular element 20a and/or can be staggered with respect to one another.

Distributor 16 (heavy phase distributor) comprises an essentially L-shaped tubular body 24: the tubular body 24 comprises a first substantially horizontal tube section 25 (constituted by inlet tube 14 or by an extension thereof) that projects from opening 12 substantially perpendicular to the side wall 6; and a second tube section 26 that projects from tube section 25 and is bent downwards, i.e. towards the bottom portion 5 of the casing 2.

In the example shown in Figure 2, tube section 26 is bent by 90° with respect to tube section 25 and is therefore substantially vertical; in addition, tube section 26 is arranged centrally in the casing 2, substantially along axis A.

An elbow connector 27 connects tube section 25 to tube section 26; tube section 26 has a free end 28 opposite to the elbow connector 27; preferably, end 28 is a blind end, being closed by transverse wall, which could also be provided with holes (not shown); tube section 26 has a plurality of lateral through holes 29, made in a lateral wall of tube section 26 close to end 28. Advantageously, the end 28 with the lateral holes 29 is placed at least partially (preferably, entirely) in the bottom portion 5 of the casing 2 below edge 9. Possible mechanical support members (for example, anchor brackets fixed to the side wall 6 of the casing 2) that support the tubular elements 20 (in particular, the annular element 20a) and/or the tubular body 24 or, more in general, the various components of the distributors 15 and 16, are not shown for simplicity.

The reactor 1 optionally includes also a light phase additional inlet 30. The inlet 30 is defined by an auxiliary inlet tube 31 provided with at least one outlet hole 32; for example, the outlet hole 32 is located at an open free end 33 of the inlet tube 31.

The inlet tube 31 is of small size, in particular, having a passageway section (cross-section) smaller than the inlet tubes 13, 14 and, in particular, than the light phase inlet tube 13.

The inlet tube 31 is arranged to pass through an auxiliary opening 34 made in the dome-shaped wall 8 of the bottom portion 5.

Preferably, the inlet 30 is located centrally on the bottom portion 5 and the inlet tube 31 is substantially vertical and extends along axis A. The inlet tube 31 is also aligned with tube section 26 of the tubular body 24 and the outlet hole 32 faces and is aligned along axis A with end 28 of tube section 26 of the tubular body 24 (i.e. of the heavy phase distributor 16) .

Preferably, end 33 of the inlet tube 31 and the outlet hole 32 are on the dome-shaped wall 8 of the bottom portion 5 or close thereto, so as to also act as a drainage outlet of the reactor 1 when the reactor 1 must be emptied. In use, the reactor 1 operates to implement the process of the invention as follows.

The reactor 1 is fed with a stream of carbon dioxide, constituting the (essentially gaseous) light phase of the urea synthesis reaction, via inlet tube 13 and distributor 15, and with a solution of ammonia/ammonium carbamate, constituting the (essentially liquid) heavy phase of the reaction, via inlet tube 14 and distributor 16. Distributor 15 distributes the light phase (carbon dioxide) substantially uniformly over the cross-section of the reactor 1, in a plurality of intake points (defined by holes 21) distributed transversely in the reactor 1 and about axis A. With respect to the previously described traditional solution, the annular (toroidal) geometry of distributor 15 and the distribution of holes 21 ensure more uniform distribution of carbon dioxide across the section of the reactor 1. This distribution is further favoured by the uniform flow field of the ammonia/carbamate solution achieved with the downward facing heavy phase distributor 16.

The downward orientation of distributor 16 and its positioning at the centre of the cross-section of the reactor 1 along the central axis A of the reactor 1 ensures uniform distribution of the (ammonia/carbamate) heavy phase throughout the volume of the reactor 1: in this way, the flow field of the ammonia inside the reaction chamber 3 is more uniform and constant with respect to the previously described traditional solution (with offset and upward facing inlets) . The optional additional feeding of the light phase via the additional inlet 30 further improves the mixing of the two phases in the lower region of the reactor 1, where first contact between the reagents takes place.

In the variants in Figures 4 and 5, in addition to the annular element 20a that projects from inlet tube 13 and is arranged along the side wall 6, the light phase distributor 15 comprises further tubular elements 20 constituted by respective arms 20b that project from the annular element 20a.

Preferably, but not necessarily, the arms 20b are substantially coplanar with the annular element 20a (i.e. they are substantially at the same height along the axis A as the annular element 20a) .

The arms 20b, which are, for example (but not necessarily) , substantially straight, extend in a space 35 radially delimited by the annular element 20a. In particular, each arm 20b connects a pair of radially inner junctions 36 of the annular element 20a facing each other on the lateral wall 23 of the annular element 20a.

Like the annular element 20a, each arm 20b is also provided with transverse intake through holes 21, formed in a lateral wall 37 of the arm 20b and spaced out (preferably, uniformly) along the arm 20b.

In the example in Figure 4, the distributor 15 comprises a pair of straight arms 20b, parallel to each other and to inlet tube 13 and arranged on opposite sides of the inlet tube 14.

Each arm 20b extends between a junction 36 located close to an end 22 of the annular element 20a, and an opposite junction 36 located close to inlet tube 13 and laterally with respect to inlet tube 13. In the example in Figure 5, distributor 15 comprises a pair of straight arms 20b, perpendicular to each other and centrally joined in a cross. In particular, the arms 20b extend along respective diameters of the annular element 20a and connect respective diametrically opposed junctions 36 on the lateral wall 23 of the annular element 20a.

In other terms, distributor 15 comprises a plurality of radially internal arms 20b that project from the annular element 20a towards axis A and, preferably, join one another centrally in a central junction 38 arranged along axis A.

In this configuration, the heavy phase distributor 16 is preferably off-centre with respect to the central axis A of the reactor 1, i.e. instead on being placed centrally in the reactor 1 and along axis A, it is placed in an offset position with respect to axis A. In particular, tube section 26 provided with holes 29 is radially displaced with respect to axis A.

The openings 11, 12 and the inlet tubes 13, 14 can be at the same height along axis A, as previously described with reference to Figures 2 and 3, and in this case the annular element 20a is broken as previously described.

Alternatively, the openings 11, 12 are not aligned and neither are inlet tubes 13, 14 aligned. In particular, opening 11 and therefore inlet tube 13 are placed at a different height with respect to opening 12 and inlet tube 14, for example, at a lower height (i.e. at a smaller distance from edge 9) : in this case, inlet tube 14 is placed above the annular element 20a and the annular element 20a can be shaped like a closed, complete ring.

In the embodiment in Figures 6 and 7, in which the same or similar details to those already described are indicated with the same reference numerals, the reactor 1 is also equipped with a pair of diametrically opposed inlet tubes 13, 14, inserted through respective diametrically opposed lateral through openings 11, 12 made in the side wall 6 of the casing 2 above edge 9, and connected inside the casing 2 to respective reagent distributors 15, 16.

In this case, the openings 1, 12 are not aligned but are axially staggered along axis A, being placed at different heights along axis A, i.e. at different distances (measured from the centre of the openings 11, 12) from edge 9. In particular, opening 11 is placed higher than opening 12. Instead of being aligned, the respective inlet tubes 13, 14, always substantially parallel to each other, are also staggered along axis A, being arranged at different heights along axis A, i.e. at different distances from edge 9. In particular, the heavy phase inlet tube 14 is located below (closer to edge 9) the light phase inlet tube 13.

The distributors 15, 16 preserve the previously described general configuration. As the inlet tubes 13, 14 are axially staggered, the annular element 20a of the light phase distributor 15 can be shaped as a complete ring closed about axis A.

In the variant in Figure 8, in addition to the annular element 20a that projects from the inlet tube 13 and is located along the side wall 6, the light phase distributor 15 comprises a pair of straight arms 20b perpendicular to each other and centrally joined in a cross, i.e. a plurality of radially internal arms 20b that project from the annular element 20a towards axis A and centrally join in a central junction 38 arranged along axis A. Similarly to that described with reference to Figure 5, the arms 20b are substantially coplanar with the annular element 20a. However, unlike the example in Figure 5, in this configuration, the heavy phase distributor 16 (precisely, tube section 26 of the tubular body 24 with holes 29) is arranged centrally in the reactor 1 and along axis A, below the central junction 38 that joins the arms 20b.

In the variant in Figure 9, in addition to the annular element 20a, the light phase distributor 15 comprises one or more further annular elements 20c, concentric with the annular element 20a and placed radially inside the annular element 20a in space 35, and a plurality of radially internal arms 20b. The annular elements 20c are also provided with intake holes 21, like the arms 20b.

The arms 20b, which are, for example, substantially straight, project from the annular element 20a towards axis A and connect the annular elements 20a, 20c and, preferably, join together centrally in a central junction 38 arranged along axis A.

It is understood that the solutions illustrated in the foregoing embodiments of the invention can be combined together in various ways.

Finally, it is understood that further modifications and variants can be applied to the reactor and to the urea synthesis process described and illustrated herein without departing from the scope of the appended claims.