EP4656998A1

EP4656998A1 - Shell and tube heat exchanger for a process-to-process heat exchange in a urea production plant

Info

Publication number: EP4656998A1
Application number: EP24178596.3A
Authority: EP
Inventors: Riccardo LANZANI; Roberto Gorza
Original assignee: Casale SA
Current assignee: Casale SA
Priority date: 2024-05-28
Filing date: 2024-05-28
Publication date: 2025-12-03
Also published as: WO2025247681A1

Abstract

A shell and tube heat exchanger for a process-to-process heat exchange in a urea synthesis process, the heat exchanger including a tubesheet assembly comprising a first plate (102) including a carbon steel body (8) covered by a weld overlay (9) made of a material suitable to operation in the presence of ammonium carbamate, a second plate (10) made of a material suitable to operation in the presence of ammonium carbamate, and a region of separation (11) between the first plate and the second plate.

Description

Field of application

The invention pertains to the field of industrial production of urea from ammonia and carbon dioxide. The invention relates to a process-to-process heat exchanger for use in a urea production plant.

Prior art

Urea is produced industrially by reacting ammonia and carbon dioxide at high temperature and high pressure (HP), well over 100 bar. The reaction basically involves the formation of ammonium carbamate and its dehydration to form urea.
Various processes for the synthesis of urea are described in the literature, for example in Meessen, "Urea", in Ullmann's Encyclopaedia of industrial chemistry 2012. In all urea processes, chemical equilibrium dictates a maximum conversion achievable in the urea reactor, usually about 60%, resulting in a reaction effluent that is a mixture of urea, carbamate, water, CO2 and NH3.
Various technologies have been developed to recover the significant amount of reagents still contained in the reactor effluent. The so-called stripping process is the dominant technology for modern urea plants, especially for large plants.
The stripping urea process involves the decomposition of carbamate contained in the reaction effluent performed substantially at reaction pressure, in a high-pressure stripper; the vapours removed in the stripper are sent to a high-pressure condenser; the urea solution effluent from the stripper is processed in one or more recovery sections at lower temperature and pressure, where the urea is purified and the reactants are progressively recovered and recycled to the high-pressure synthesis section.
A process-to-process heat exchanger is an equipment where heat is transferred from one process stream to another, in contrast e.g. with heat exchanger where heat is transferred simply to/from a cooling or heating medium such as water/steam. In a urea production plant, most of the process streams contain significant amounts of highly corrosive ammonium carbamate at high temperature and/or high pressure, therefore the design of a process-to-process heat exchanger for a urea plant is challenging. An example of noticeable interest is a high-pressure condenser configured to cool the high-pressure vapours removed from the stripper by heating a medium-pressure urea solution effluent from a recovery section.
In a process-to-process shell-and-tube heat exchanger operating at a high pressure (such as over 100 bar), a particularly challenging aspect is the design of the tube-to-tubesheet mechanical joints. The joints must resist to said high pressure and corrosion by ammonium carbamate both shell-side and tube-side; in addition, the ability to detect and repair leaks is highly desired.
Materials suitable for operation in the presence of carbamate at a temperature greater than 150°C, such as superduplex steel UNS32906, austenitic steel UNS S 31050, commercially known as 25/22/2, or titanium, are very expensive. Due to the relevant pressure, the tubesheet must have a considerable thickness, and a tubesheet made entirely of such materials is generally considered too expensive. A known solution is to make a tubesheet with a body of a less expensive carbon steel, to provide pressure resistance, using the more expensive material only for the parts or surfaces directly exposed to the carbamate-containing environment, e.g. by making an overlay. In such case, a contact of a carbamate-containing fluid with carbon steel parts could have a fatal impact on the equipment and should be avoided. A leak detection system therefore should be provided.
US-9435589 discloses a design solution wherein the high-pressure condenser includes a pressure-resistant carbon steel tubesheet, protected on both sides by a corrosion-resistant material layer. The tubesheet is crossed by tube pieces, called sleeves, made of the same material of the tube bundle (i.e., corrosion-resistant material) and of a length slightly longer than the thickness of the tubesheet. A protrusion is provided on both sides of the tubesheet. At each protrusion, each sleeve is welded to the tubesheet protective layer by orbital welding. The protrusion is small enough to allow access to all areas to be welded and large enough to allow orbital welding between the tubes and the tubesheet. Bundle tubes are welded to the sleeves by inner bore welding (IBW) which is a welding technique wherein the welding head is inserted into a tube and performs a butt welding between that tube and another tube.
To prevent corrosion of the tubesheet due to leaks in the sleeve-plate welds a leak detection system is installed under both protective layers to detect leaks and minimize tubesheet corrosion by process fluids.
However, the design just described has some critical issues. In particular, the welds of the sleeves to both protective layers of the carbon steel tubesheet are subject to high differential thermal expansion stresses due to a different thermal expansion coefficient of the sleeves with respect to the carbon steel tubesheet, resulting in a significant risk of cracking and subsequent plant shutdowns. Another drawback concerns the leak detection system underneath both protective layers, which requires the construction of a complex system of grooves in the carbon steel tubesheet to convey outside any leak through outwards weepholes, which is an expensive and complex system.
Thus, there is interest in going beyond the current design of process-to-process heat exchangers, with particular reference to the tube to tubesheet joints.

Summary of the invention

The invention aims to improve the current design of tube-to-tubesheet joints of the heat exchangers herein considered. In particular, the invention addresses the problem of how to reduce the stress on welds caused by differential thermal expansion of tubesheet and sleeves. Furthermore, the invention aims at facilitating the detection and repair of possible leaks of process fluids from the welds.
The problem is solved with a heat exchanger according to the claims.
The heat exchanger of the invention comprises a plurality of tubes, preferably U-tubes, arranged in a tube bundle supported by a tubesheet separating a shell side from a channel side of the heat exchanger. The channel side is in communication with the tubes and includes a first chamber and a second chamber such that a first stream is introduced into the first chamber and distributed into the tubes, and the effluent from the tubes is collected in the second chamber. The shell side communicates with the outside of said tubes, so that a second stream traversing the shell side can exchange heat with the first stream through the tubes.
The exchanger of the invention includes a novel tube-to-tubesheet joint, wherein the tubesheet comprises a first plate and a second plate with a region of separation between them.
The first plate includes a carbon steel body covered by a weld overlay made of a material that properly prevents corrosion by ammonium carbamate, preferably superduplex steel UNS 32906, austenitic steel UNS S 31050 or titanium. The carbon steel body, being made of a material less expensive than the weld overlay, may be employed with a thickness greater than a thickness of the weld overlay to provide resistance to pressure by the shell side, without resulting in an excessively expensive plate. The first plate has a first surface, which is the surface of the overlay and is exposed to the shell side of the heat exchanger, and a second surface which is opposite to the first surface and is exposed to the region of separation. The first plate is integral with the vessel of the heat exchanger adjacent to the shell-side, preferably by flanged connection to allow extraction of the tube bundle and inspection/maintenance of said vessel.
The second plate has a first surface exposed to the channel side made of a material suitable for operation in the presence of ammonium carbamate, and a second surface exposed to the region of separation facing the second surface of the first plate. Said second plate is preferable integrally made of duplex steel such as UNS S 32205, commercially known as SAF 2205, or UNS S 32750, commercially known as SAF 2507, superduplex steel UNS32906, austenitic steel UNS S 31050 or titanium. According to an embodiment of the invention, the second plate includes, similarly to the first plate, a carbon steel body covered by a weld overlay that properly prevents corrosion by ammonium carbamate.
The region of separation between the second surface of the first plate and the second surface of the second plate is not in fluid communication with either the shell side or the channel side of the heat exchanger. In other words, neither the shell side nor the channel side are in fluid communication with said region of separation.
The presence of such a free region in the between shell and channel has the advantage of providing a space where easily detect any leak either from the shell-side or from the channel side. A preferable detection system comprises continuous flushing of an inert gas (i.e. a gas not reacting with NH3, CO2 and H2O) through said region with determination at the outlet of any resulting trace of ammonia by a gaseous ammonia detector. A side confinement of the region of separation is preferably designed to cushion any distance variation between the two plates because of the thermal expansion of the sleeves. A preferable design may include an expansion joint, i.e. bellow, in the side confinement. More preferably the side confinement does not provide a perfect sealing of the separation region from outside. In such preferable embodiment the side confinement is integral to only one plate whereas a circumferential clearance is provided to the other plate to accommodate any distance variation between the two plates.
Other aspects of the invention concern a use and a method for making the heat exchanger described above.

Description of the invention

In a preferred embodiment of the invention, for each tube of the bundle of tubes, the heat exchanger includes at least one sleeve arranged to bring the tube into communication with the channel side of said heat exchanger. The sleeve has a first end welded to the first surface of the second plate, preferably by an orbital welding.
Preferably, each sleeve extends through a hole made in the second plate, through a hole made in the carbon steel body and in a portion of the weld overlay of the first plate. A second end of the sleeve is secured to said overlay preferably by an internal bore welding.
In one embodiment, an end of each tube of the bundle of tubes is fixed to the first surface of the first plate in correspondence of its respective sleeve, preferably by an internal bore welding.
The sleeves are preferably shorter than 1000 mm so as to allow to carry out the internal bore welding between the second end of the sleeve and the overlay of the first plate. At the same time, said sleeves shall protrude enough from the first surface of the second plate towards the channel side to allow the orbital welding of the tubes to said second plate.
The region of separation is configured so that the second face of the first plate is at a suitable distance from the second face of the second plate (generally not greater than 1 meter) so as to allow the internal bore welding between the tubes and the weld overlay.
Preferably, the sleeves and the tubes have the same internal diameter, the same thickness and are made of the same material.
In an interesting embodiment of the invention, a gap is left between each sleeve and the hole of the first plate in which the sleeve is accommodated. Said gap is in communication with the region of separation between the first and the second plate. The same design also applies to the second plate with its correspondent gap in communication with the region of separation between the first and the second plate.
A leak detection means, preferably an ammonia detection system, may be arranged to detect the presence of a process fluid in the region of separation, so that a possible leak from the weld between the sleeve and the weld overlay of the first plate or from the weld between the sleeve and the second plate can be detected. It is of particular interest to minimize leakage from the weld between the sleeve and the weld overlay because any leakage from this weld would cause contact between a carbamate-containing stream and the carbon steel body, resulting in rapid corrosion of said body.
In a further preferred embodiment, the heat exchanger is vertically arranged. Preferably, the first plate is above the second plate and said second plate is hung to the first plate by the sleeves without being fixed to the vessel of the heat exchanger. This arrangement provides the advantages of leaving the sleeve free to expand thermally over a different length with respect to the vessel of the heat exchanger, and of facilitating the dislodgement of a lower part of the heat exchanger vessel, thus allowing an easier maintenance of the tubesheet, such as for repairing possible leaks.
Another and preferred aspect of the invention is a use of the heat exchanger according to the invention wherein said heat exchanger performs a process-to-process heat exchange between a first stream and a second stream of the urea synthesis process.
Preferably, the first stream is a liquid or gas/liquid mixture comprising ammonium carbamate and is fed to the shell side, whereas the second stream is a urea solution, that is fed to the tube side. The second stream is heated by cooling the first stream which is at a temperature and a pressure higher than the temperature and the pressure of the second stream.
In a preferred embodiment of the inventive use, the first stream is a gaseous effluent obtained from a high-pressure stripping process of a reaction effluent, which is subjected to partial condensation before being sent to the heat exchanger of the invention. The second stream is preferably a urea solution effluent from said stripping process after pressure reduction in an expansion valve.
The first stream is preferably a high-pressure stream with pressure higher than 100 bar and the second stream is preferably a medium-pressure stream with a pressure lower than 50 bar, more preferably from 20 to 40 bar.
In a further preferred embodiment, the region of separation is at a pressure lower than the pressure of the tube side, preferably not greater than 1 bar, more preferably at substantially atmospheric pressure.
A still further aspect of the invention relates to a method of manufacturing the shell and tube heat exchanger described above. The inventive method comprises:

providing the first plate;
providing the second plate;
drilling the weld overlay of the first plate for a depth less than the thickness of said weld overlay, producing holes having a diameter equal to the internal diameter of the tubes;
drilling the first plate from its second surface for a depth greater than the thickness of the carbon steel body and at the same way drilling the second plate at full depth forming holes having a diameter greater than the external diameter of the sleeve;
inserting the sleeves through the second plate, the carbon steel body and at least a portion of the weld overlay of the first plate;
welding the second end of said sleeves to the weld overlay of the first plate and welding the first end of said sleeves to the first surface of the second plate;
welding a lower end of the tube to the weld overlay of the first plate.

The above steps may be performed in the above sequence or in a different sequence if appropriate.
In an embodiment, the first plate is machined to remove a few millimetres of weld overlay everywhere except around each hole, for a thickness close to the thickness of the tubes, resulting in collars that allow the tubes to butt up against the first surface of the first plate, allowing a proper welding of said tubes with the weld overlay. Preferably, the lower ends of the tubes are welded to the weld overlay in correspondence to an upper base of each collar by internal bore welding.
In a preferred embodiment of the method of the invention, said method includes fixing the first plate to the vessel of the heat exchanger through a connection means, preferably a flange.
In an embodiment, the method comprises installing a system for detecting leaks from welds in the region of separation. Said system is preferably an ammonia detector.

Description of the figures

Fig. 1 is a schematic sketch of a shell and tube heat exchanger according to an embodiment of the invention.
Fig. 2 is a scheme of the tubesheet assembly of the heat exchanger of Fig. 1.
Fig. 3 is a detail of Fig. 2.

Figure 1 illustrates a shell and tube heat exchanger 100, wherein a bundle 120 of tubes 108 is fixed onto a tubesheet assembly 101. The heat exchanger 100 comprises a shell side 4 for a first process stream and a channel side 5 for a second process stream. In a preferred application, the first process stream is a stream containing ammonium carbamate which is condensed at a pressure higher than 100 bar, and the second process stream includes a urea solution at a pressure lower than 35 bar which is heated by the heat of condensation removed from the first process stream. The tubesheet assembly 101 separates the channel side 5 from the shell side 4.
The channel side 5 includes at least a first chamber 121 and a second chamber 122. The second process stream is fed to the first chamber 121 and distributed in the tubes 108; the process stream effluent from said tubes 108 is collected in the second chamber 122. The tubes 108 may be configured as U-tubes.
Figure 2 is a schematic view of a preferred embodiment of the tubesheet 101. Said tubesheet 101 comprises a first plate 102 having a first surface 103 exposed to the shell side 4 and a second surface 104 opposite to said first surface 103. The first plate 102 includes a carbon steel body 8, that provides resistance to the pressure in the shell-side 4, covered by a weld overlay 9 made of a material suitable to operate in presence of ammonium carbamate. Said weld overlay 9 has the aim to prevent carbamate corrosion of the carbon steel body 8. The thickness of the overlay 9 and body 8 is not in scale.
The tubesheet 101 further includes a second plate 10 made of a material resistant to carbamate corrosion, preferably the same material of the weld overlay 9. Said second plate 10 has a first surface 105 exposed to the channel side 5 and a second surface 106 opposite to said first surface 105 and facing the steel body 8 of the first plate 102. A region of separation 11 is left between the plates 10 and 102. In a preferred embodiment, said region 11 is not sealed and therefore it is substantially at atmospheric pressure.
The tubes 108 communicate with the channel side 5 through holes 126 drilled through the overlay 9 and sleeves 109. The weld overlay 9 is machined to form the surface 103 by removing a layer (e.g. of few millimetres) of material from the face exposed to the shell-side 4. Said step of machining forms collars 12 that facilitate the making of inner bore welds 111 between the overlay 9 and walls 7 of the tubes 108.
Holes for insertion of the sleeves 109 are made through the second plate 10 and part of the first plate 102. Particularly, said first plate 102 is drilled through the carbon steel body 8 and part of the overlay 9, to provide fluid communication with the holes 126. The sleeve 109 is welded to the second plate 10 and to the overlay 9. More specifically, an orbital welding 113 is made between the sleeve 109 and the second plate 10; an inner bore welding 112 is made between the wall 6 of the sleeve 109 and the overlay 9. In an embodiment, the second plate 10 is suspended to the first plate 102 by means of the sleeves 109.
A leak detection means may be installed in or near the region 11 to detect leaks from the welds 112 and 113. The detection means may be within the region 11 or just outside of said region. Particularly a leak from the inner bore welds 112 may be detected. This is a considerable advantage because these welds are difficult to inspect, and a failure may potentially bring the highly aggressive fluid in the tube side in contact with the carbon steel body 8. As apparent from the Fig. 2, a leak from the weld 112 would cause a flow through gap 107 between the first plate 102 and the wall 6 of the sleeve 109, resulting in a contact of the tube-side process fluid with the carbon steel body 8. The presence of the region 11 facilitates leak detecting and repairing.
Fig. 2 shows also a cylindrical vessel 124 and a bottom cover 123 of the exchanger 100. The exchanger 100 is preferably vertically arranged.
In an embodiment of vertically arranged exchanger, the second plate 10 is suspended on the first plate 102 by the sleeves 109 and the welding 113, without being fixed to the vessel 124, allowing a different vertical elongation due to thermal expansion of the sleeves 109 relative to the carbon steel body 8, thus reducing mechanical stress on the welds 112 and 113.
The outer lateral surface of the region 11 may be delimited by an expansion joint between the vessel 124 and the cover 123. In some embodiments, the region 11 can be left open and in communication with the outside environment, i.e. the vessel 124 is not directly fixed to the cover 123.
Figure 3 illustrates the gap 107 between the carbon steel body 8 and the wall 6 of the sleeve 109. The figure shows also the holes 125 wherein the sleeves 109 can be accommodated.
A preferred method for making the heat exchanger 100 is now described.
The first plate 102 is prepared by making the holes 126 and machining the surface 103. The first plate is then coupled with the vessel 124 preferably through a flanged connection. At this stage, the tubes 108 can be welded to the plate 102 by making the internal bore welds 111.
The second plate 10 can be prepared with some or all the holes for the sleeves 109 and positioned relative to the first plate 102. The second plate 10 is then fixed to the first plate 102 by inserting and welding the sleeves 109. According to different options, the second plate 10 can be prepared with all the holes for the sleeves, or some holes can be drilled after positioning and securing the second plate 10. Before or after positioning the second plate 10, the holes through the body 8 and part of the overlay 9 are also drilled. The sleeves 109 are fixed may making the orbital welds 113 and the inner bore welds 112. The sequence of the above-mentioned steps may vary according to different embodiments.

Claims

A shell and tube heat exchanger (100) for use in a urea synthesis process, including:
a bundle (120) of tubes (108);

a channel side (5) in communication with said tubes, including a first chamber (121) and a second chamber (122), so that a first stream introduced in the first chamber (121) is distributed into the tubes, and the effluent from the tubes is collected in the second chamber (122);

a shell side (4) in communication with the outside of said tubes, so that a second stream fed to the heat exchanger (100) and traversing the shell side can exchange heat with the first stream through the tubes;

a tubesheet assembly (101) for supporting the tubes, the tubesheet assembly comprising:
a first plate (102) including a carbon steel body (8) covered by a weld overlay (9) made of a material suitable to operation in the presence of ammonium carbamate, the first plate having a first surface (103) which is the surface of the overlay exposed to the shell side (4) of the heat exchanger, and a second surface (104) opposite to the first surface;

a second plate (10), having a first surface (105) exposed to the channel side made of a material suitable to operation in the presence of ammonium carbamate and a second surface (106) facing the second surface (104) of the first plate;

wherein said first plate and second plate are distanced so that a region of separation (11) is defined between the second surface (104) of the first plate and the second surface (106) of the second plate, wherein said region of separation (11) is not in fluid communication with the shell side (4) of the heat exchanger and said region of separation (11) is not in fluid communication with the channel side (5) of the heat exchanger.
A heat exchanger according to claim 1 including, for each tube (108), at least one sleeve (109) arranged to put the tube in communication with the channel side (5), wherein:
the sleeve (109) has a first end welded to the first surface (105) of the second plate (10), preferably by an orbital welding (113);

the sleeve extends through a hole made in the carbon steel body (8) and part of the weld overlay (9) of the first plate (102), and a second end of the sleeve is fixed to said overlay preferably by an internal bore welding (112).
A heat exchanger according to claim 2 wherein the sleeves and the tubes have the same internal diameter and thickness, and said sleeves and said tubes are preferably made of the same material.
A heat exchanger according to claim 2 or 3 wherein an end of each tube (108) is fixed to the first plate (102) in correspondence of a respective sleeve (109), preferably by an internal bore welding (111).
A heat exchanger according to claim 2 or 3 or 4 wherein a gap (107) is left between the sleeve (109) and the hole of the first plate (102) where the sleeve is fitted, said gap being in communication with the region of separation (11) between the plates (10, 102).
A heat exchanger according to any of the previous claims including a leak detection means arranged to detect presence of a process fluid in the region of separation (11).
A heat exchanger according to any of the previous claims where the second plate is integrally made of a material suitable to operation in the presence of ammonium carbamate.
A heat exchanger according to any of the previous claims wherein said heat exchanger is vertically arranged.
A heat exchanger according to claim 8 wherein the first plate (102) is integral with the heat exchanger vessel (124) and the second plate (10) is hung to said first plate.
A heat exchanger according to any of the previous claims wherein the weld overlay (9) is made of austenitic UNS S 31050 and/or the second plate (10) is made of austenitic UNS S 31050 and/or the tubes (108) are made of superduplex UNS32906.
A heat exchanger according to any of the previous claims wherein the tubes are configured as U-tubes.
Use of a heat exchanger according to any of the previous claims in a urea synthesis process, wherein said heat exchanger performs a process-to-process heat exchange between a first stream and a second stream of the urea synthesis process.
Use of claim 12, wherein: the first stream is a liquid or gas/liquid mixture comprising ammonium carbamate, that is sent to the shell side of the heat exchanger; the second stream is a urea solution, that is sent to the tube side of the heat exchanger, wherein the second stream is heated by heat removed from the first stream.
Use of claim 12 or 13, wherein the first stream is a gaseous effluent obtained from a high-pressure stripping process of a reaction effluent, said first stream being sent to the heat exchanger after a partial condensation step, and the second stream is a urea solution effluent from said stripping process after a reduction of pressure.
Use according to any of claims 12 to 14 wherein the first stream has a pressure greater than the pressure of the second stream, the first stream is preferably a high-pressure stream having a pressure higher than 100 bar and the second stream is preferably a medium-pressure stream having a pressure lower than 50 bar, more preferably from 20 to 40 bar.
Use according to any of claims 12 to 15 wherein the region of separation (11) is at a pressure lower than the pressure of the tube side, and preferably at atmospheric pressure.
A method for making a process-to-process shell and tube heat exchanger according to any of claims 1 to 11, the method comprising:
providing the first plate (102);

providing the second plate (10);

wherein each tube (108) and corresponding sleeve (109) is fixed by:

drilling the weld overlay (9) of the first plate for a depth less than the thickness of said weld overlay, producing a hole having a diameter equal to the internal diameter of the tubes (108);

drilling the first plate (102) from its second surface (104) for a depth greater than the thickness of the carbon steel body (8) and drilling the second plate (10) fully traversing said second plate, said drilling of the first plate and of the second plate resulting in holes (125) having a diameter greater than the external diameter of the sleeve (109);

inserting the sleeve (109) into said hole (125) made through the first plate and second plate;

welding (112) one end of said sleeve to the weld overlay (9) and welding (113) the other end of said sleeve to the first surface (105) of the second plate (10);

welding (111) a lower end of the tube (108) to the weld overlay (9), so that the tube is in communication with the sleeve.
A method according to claim 17, wherein the first plate is machined to remove a layer of the weld overlay to form a collar (12) where the tube is welded to the overlay, and the lower ends of the tubes are welded to the weld overlay (109) in correspondence to an upper base of each collar (12) by internal bore welding (IBW).
A method according to claim 17 or 18, wherein the method includes installing in or near the region of separation (11) a means for detecting leaks from welds (112, 113), said means being preferably an ammonia detector.