Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
  • F28D3/00—Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium flows in a continuous film, or trickles freely, over the conduits
  • F28D3/02—Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium flows in a continuous film, or trickles freely, over the conduits with tubular conduits
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D1/00—Evaporating
  • B01D1/06—Evaporators with vertical tubes
  • B01D1/065—Evaporators with vertical tubes by film evaporating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D19/00—Degasification of liquids
  • B01D19/0005—Degasification of liquids with one or more auxiliary substances
  • B01D19/001—Degasification of liquids with one or more auxiliary substances by bubbling steam through the liquid
  • B01D19/0015—Degasification of liquids with one or more auxiliary substances by bubbling steam through the liquid in contact columns containing plates, grids or other filling elements
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/24—Stationary reactors without moving elements inside
  • B01J19/2415—Tubular reactors
  • B01J19/2425—Tubular reactors in parallel
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
  • F28D3/00—Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium flows in a continuous film, or trickles freely, over the conduits
  • F28D3/04—Distributing arrangements
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F25/00—Component parts of trickle coolers
  • F28F25/02—Component parts of trickle coolers for distributing, circulating, and accumulating liquid
  • F28F25/06—Spray nozzles or spray pipes
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F19/00—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers
  • F28F19/02—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers by using coatings, e.g. vitreous or enamel coatings
  • F28F19/06—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers by using coatings, e.g. vitreous or enamel coatings of metal
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/4935—Heat exchanger or boiler making
  • Y10T29/49391—Tube making or reforming

Definitions

• the present invention relates to tube bundle equipment comprising metallic regulator elements of the liquid flow.
• the present invention relates to tube bundle equipment of the vertical type, suitable for effecting heat exchange between two fluids, one of which, in the liquid state, falls along the internal walls of the tubes in the form of a film.
• the present invention relates to a tube bundle heat exchanger of the falling liquid film type, suitable for separating gases and vapours from a liquid having highly aggressive properties, equipped with a particular element for the distribution of the liquid film, situated at the head of each tube.
• Tube bundle equipment is widely used in the art for efficiently effecting the heat exchange between two fluids, particularly when phase transformations or chemical reactions take place along one or both sides of the exchange wall due to the heat flow.
• This type of equipment is relatively easy and simple to construct as it normally consists of a cylindrical chamber longitudinally crossed by a plurality of tubes, which are seal-fixed onto two transversal plates (called tube sheets) which delimit, at the two ends of the equipment, demarcated by two heads, two chambers for the collection and distribution respectively of the gases and liquids flowing inside the tubes.
• the exchanger fluid often consisting of hot gases coming from a furnace or saturated steam, circulates outside the tubes, in the so-called shell side.
• the tubes are positioned vertically so that a uniform and thin liquid film flows by gravity from above along the wall, thus obtaining an efficient heat exchange with the fluid on the shell side, reducing the contact time as far as possible.
• the vapours possibly formed by evaporation or chemical reaction due to the heating are easily released through the wide liquid film surface and can be removed along the substantially liquid-free internal duct of the tube. It is consequently essential, for a good functioning of this type of heat exchanger, for the liquid film to be uniformly distributed and that this does not even occasionally form areas of blockage and turbulence mixed with vapours, with consequent obstruction and pressure drops along the tube.
• metals such as titanium and zirconium, due to their high resistance to corrosion and satisfactory mechanical qualities, are frequently preferred for the production of tubes in high-pressure heat exchange equipment used in the synthesis of urea and nitric acid.
• Zirconium in particular is known for its excellent resistance to both chemical corrosion and to the erosive action of the process fluids it comes into contact with, whereas titanium has a resistance to corrosion substantially similar to zirconium, but a lower resistance to erosive action.
• US patent 4,899,813 describes the construction and use of vertical tube bundle equipment especially suitable for the high-pressure stripping operation of the urea solution coming from the synthesis reactor.
• a tube bundle consisting of bimetallic tubes is used, i.e. consisting of an external part made of stainless steel, and an internal part, having a finer thickness (0.7-0.9 mm), made of zirconium, which adheres mechanically with the former, but is not welded to it, as the two materials are incompatible with each other to normal melt welding.
• This solution represented a significant improvement with respect to the known art, but, over long periods of time, it could not prevent problems of corrosion from arising as a result of the infiltration of the corrosive fluid towards the external part of the tube, made of steel.
• Patent application EP 1577632 describes a tube bundle exchanger suitable for the treatment of ammonium carbamate in urea synthesis plants, in which the bundle consists of titanium tubes coated with a thin layer of zirconium on the side in contact with the corrosive fluid, and seal-fixed on the titanium coating of the tube sheet by titanium-titanium welding.
• the zirconium layer does not necessarily extend for the whole length of the tubes, but it can be positioned in the area of the tube subjected to the most intense aggressive attack.
• Methods for obtaining these tubes can comprise hot welding or forging, to favour the formation of a metallurgic bond between the zirconium layer and surface of the titanium.
• the above vertical falling liquid film heat exchange equipment requires a device for the optimum distribution of the liquid along the walls of the tube and for the contemporaneous discharge of the gases and vapours introduced into the exchanger, preferably from below, as entrainment gas, or products in the tubes themselves following evaporation or thermal decomposition of one or more constituents of the liquid itself.
• This device more commonly known with the technical term “ferrule”, generally has a tubular form and is wedged onto the end of each tube of the tube bundle, above the supporting tube sheet.
• Said ferrule can be opened upwards for the outlet of the gases or vapours, or, it can be closed at the upper end by means of a head which can also act as a rest for a possible fixing grid, as specified further on, in which case the gases or vapours are discharged through one or more side holes situated in the side wall of the ferrule, close to its upper end.
• One or more holes are positioned tangentially in the wall of the ferrule slightly above the rest base on the end of the tube, to allow the inlet and uniform distribution of the liquid on the internal walls.
• each ferrule comprises a lower end, extending beyond the rest base, shaped so as to be correspondingly inserted in the upper opening of each tube of the exchanger.
• a fixing grid is usually rested on the upper part of the group of ferrules inserted in the tube bundle in order to keep the ferrules firmly inserted in the tubes, opposing the force of the rising vapours .
• Ferrules of the type described are used, for example, in both the decomposer/stripper of the high- pressure urea synthesis cycle, and in the subsequent equipment for the decomposition of the ammonium carbamate at medium or low pressure of the same plant, when these are of the falling film type.
• a sectional view of a typical ferrule used in high-pressure strippers for the synthesis of urea is schematically represented in the following Figure 2.
• a first object of the present invention therefore relates to tube bundle equipment of the falling liquid film type, suitable for thermal exchange between fluids, comprising a vertical cylindrical body closed at the ends and divided into at least one upper section, an intermediate section and a lower section by means of two perforated tube sheets arranged transversally at a suitable distance from each other, wherein a plurality of tubes are arranged longitudinally forming a tube-bundle and seal-inserted with the respective ends in the holes of said tube sheets allowing said upper section and lower section of the cylindrical body to be in fluid communication with each other, each tube being surmounted, on the upper end, by a tubular device, also called ferrule, for the inlet and distribution of the liquid in the form of a film along the wall of the same, said tubular device comprising above one or more openings for the outlet of vapours, at an intermediate height one or more openings, preferably tangential, for the inlet of the liquid and below, a circular base for holding on the upper end of said tube and a lower cylindrical section protruding
• a second object of the present invention relates to a method for the production of the above tube bundle equipment, which comprises arranging two perforated tube sheets inside a cylindrical shell equipped at the two ends with two terminal heads, so that each tube sheetis situated close to a head, inserting and seal- fixing in the holes situated in each tube sheet, a plurality of tubes extending orthogonally towards the plane of the tube sheet, for the whole distance which separates them, in order to put the spaces between each tube shhet and the corresponding head in fluid connection with each other, positioning on the upper end of each of said tubes, a ferrule comprising, in the upper area, one or more openings for the outlet of vapours, at an intermediate height, one or more openings, preferably tangential, for the inlet of the liquid and in the lower area, a circular base for resting said tube on the upper border and a cylindrical section protruding downwards beyond the circular base, inserted in said tube for a length varying from 10 to 200 mm and having the outer diameter substantially coinciding with the internal diameter
• alloy refers to a metallic composition comprising said metal in a quantity of at least 50% by weight .
• corrosion and "corrosiveness” as used in the present description and claims with reference to the action of a process fluid in contact with a surface of a certain metal or alloy, is intended in its general meaning of removal or modification of the properties of the material forming the surface and comprises both the corrosive action deriving from a chemical attack of the surface, and also the erosive action deriving from a physical removal process due to impact, friction and shear forces.
• the term "corrosion resistant” referring to a material with respect to a fluid under certain process conditions, defines a material which has a corrosion index lower than 0.1 mm/year measured according to the regulation ASTM A 262 file C (HUEY TEST) . Corrosion indexes for materials for normal industrial use are indicated in various manuals known to experts in the field, such as, for example, in Tables 23-22 to 23-24 of the above- mentioned “Perry' s Chemical Engineering Handbook” , under the item “Ammonium Carbamate” .
• a force-welding is a welding with characteristics which are such as to satisfy the project requirements, on the basis of the mechanical characteristics and stress deriving from the expansion of the welded parts;
• seal-welding is effected with the aim of avoiding losses and its dimensions are not determined on the basis of the loads previously expressed for force-welding.
• metallic bodies joined to each other indicates the presence of a contact surface or section between said metallic bodies, in which the respective constituents (which can be the same metal or different metals) are joined directly or indirectly with each other so as to form a joint with characteristics of mechanical and release resistance in the same order of magnitude as at least one of said metals.
• metallurgically bound bodies are those in which the respective metals are joined by melt welding, with or without a fusible rod, brazing, friction welding, explosion welding, co- extrusion, hot drawing and analogous techniques.
• the vertical heat exchange equipment according to the present invention does not differ, in its basic elements, from the characteristics of typical falling film exchangers known in the art. It usually has a cylindrical shape, with a diameter preferably varying from 0.5 to 3.0 m, closed at the ends by two heads, normally semispherical to for better sustaining the pressure thrusts, which usually also enclose part of the volume of the two upper and lower sections, destined for the distribution and collection respectively of the liquid subjected to heat treatment, whereas the intermediate or central section of the apparatus, delimited by the respective upper and lower tube sheets, comprises the tube bundle which puts said distribution and collection chambers, and the space, also called shell, where the fluid circulates for supplying heat, in fluid communication.
• the equipment When in operating position, the equipment is vertically oriented, for example when used as a stripper of the urea synthesis process.
• the vertical heat exchanger of the -present invention is destined for use under medium-high pressure and temperature conditions, and in the presence of particularly aggressive fluids, as is the case, for example, in the recovery of non-reacted carbamate in urea production processes, or in the concentration of nitric acid, the expert in the field carefully selects the most suitable metallic materials for resisting the corrosion and erosion phenomena which can take place, and dimensions the various parts of the equipment, in particular the thickness of the tubes of the tube bundle, the tube sheet and external wall of the apparatus (also called pressure resistant body) , so that they can sustain the pressure thrust, guaranteeing high safety conditions.
• the pressure resistant body in particular has thicknesses normally ranging from 20 to 400 mm, higher for the walls of the head and thinner for the cylindrical wall of the shell.
• the central cylindrical area, in contact with the saturated vapour at pressures of 0.2 to 5 MPa preferably has thicknesses varying from 20 to 100 mm, whereas the wall of the heads and cylinder close to these, subjected to the greater pressure of the process fluids, has proportionally higher thicknesses, preferably ranging from 80 to 300 mm.
• the outer wall can consist of a single layer or various layers of carbon steel assembled according to any of the known techniques. An expert in the field will also carefully assemble and weld the various parts so that there are no losses due to imperfections, according to the methods known in the art.
• the interior of the apparatus is marked by the area comprising the tubes of the tube bundle, normally grouped parallel to each other, fixed on the two tube sheets, suitably positioned transversally to the main axis of the equipment, and also comprising a flat element suitable for tolerating the pressure difference, normally made of carbon steel, with a thickness ranging from 20 to 500 mm.
• the two tube sheets are each situated close to one of the two heads and define an intermediate section having an essentially cylindrical geometry. Each tube sheet is seal- and force-fixed onto the circular wall by welding, so that there can be no exchanges of material between adjacent sections.
• the tubes cross the two tube sheets, which are suitably perforated for this reason, enabling the passage of a fluid between the upper and lower sections situated at the end of the tubes.
• a stream of a second fluid, normally saturated steam at the necessary pressure for supplying heat at the desired temperature is introduced into the intermediate cavity, normally on the shell side, to effect the heat exchange through the wall of the tubes, and removed in the form of a condensate through suitable outlet ducts.
• each tube surmounted by the ferrule preferably consists of a material having a high resistance to corrosion, possibly combined with erosion deriving from the mechanical action of the fluids during the evaporation phase inside the tube.
• This mechanical action is particularly high on the walls of the vertical tubes where the liquid is rapidly heated and vaporizes with a high shear stress on the surface.
• Materials particularly suitable for the production of these tubular elements are zirconium and niobium, especially zirconium and its alloys comprising at least 60% of Zr, such as Zircalloy ® and Zircadyne ® , due to the excellent resistance to both corrosion and erosion, and satisfactory commercial availability.
• titanium and its alloys and the alloys of stainless steel especially those developed commercially to resist contact with high-temperature solutions of ammonium carbamate, such as INOX urea grade, Cr/Ni/Mo 25/22/2, austeno-ferritic stainless steels.
• zirconium/stainless steel bimetallic tubes of the type described in the above-mentioned patent US 4,899,813, with the advantage, in the case of the present invention, that the positioning of the ferrule on the tube does not require the removal of the zirconium layer in the terminal part of the tube for a length of 30 to 100 mm, as was necessary, on the other hand, in the solutions so far proposed in the art.
• the embodiment of the exchanger according to the present invention has proved to be equally advantageous, if the tube is of the type described in the above-mentioned international patent application WO 06/020381, or in the published patent application US 2008/093064, whose upper part consists of zirconium or an internal layer of zirconium or one of its alloys, metallurgically bound to an outer layer of titanium or one of its alloys. Also in this case, in fact, the combination of elements characterizing the present invention avoids further processing at the tube end during the production of the equipment. As a result of its numerous applications, and depending on its construction materials, the dimensions of the tube of the present equipment can vary within wide limits .
• the internal diameter of the tube ranges from 5 to 150 mm, preferably from 10 to 100 mm, and the thickness preferably varies within a range of 1 to 20 mm, more preferably from 2 to 15 mm.
• the tubes are normally cylindrically shaped, tubes having different sections, for example elliptical or square are not excluded from the scope of the present invention.
• the length of the tube in the tube bundle can vary within wide limits, in relation to the dimensions of the equipment where it is used.
• the length is generally at least 5 times higher than the diameter and preferably varies from 1 to 20 metres, more preferably from 2 to 15 metres.
• the length of the tubes defines the length of the tube bundle and the distance between the tube sheets.
• the end of each tube, in particular the upper end is conveniently seal-welded with the coating of the tube sheet. This welding can be effected in various ways included in the scope of the present invention, depending on the composition of the tubes and tube sheet and use of the equipment.
• the tube sheet is coated with titanium or zirconium, and the terminal part of the tube consists of at least one layer compatible with the welding with said metal or its alloy, it is preferable to seal-weld, and possibly also force-weld, the coating with said compatible layer.
• seal-weld, and possibly also force-weld the coating with said compatible layer.
• both the coating and the outer wall of the tube consist of a stainless steel.
• Said welding if also effected as force-welding, also forms the fixing area of the tube onto the tube sheet, resistant to the mechanical stress deriving from the pressure differential.
• Other solutions for fixing the tube onto the tube sheet can be easily found by experts in the field on the basis of what is known in the art.
• the upper edge of the tubes of the tube bundle conveniently protrudes up to 80 mm, preferably from 10 to 50 mm beyond the plane of the tube sheet in which it is inserted.
• the welding of the tube to the coating or in any case to the body of the tube sheet is effected around the outer surface of the same tube .
• the ferrule situated on the tube end in the exchanger according to the present invention is a tubular-shaped device coherent with the tube itself, whose main functions are, on the one hand, to allow the entrance of the liquid into the vertical tube so that is distributed by falling as uniformly as possible on the walls of the same forming a thin layer (falling film) , and on the other, to allow, in a higher point with respect to the former, the outlet of the gases and vapours which are rising in the vertical tube of the bundle, thus preventing blockages and the formation of bubbles or foam which could reduce the functionality of the exchanger, also causing undesired pressure drops.
• the ferrule is inserted in the cavity of the vertical tube by means of its own cylindrical section protruding below, having an external diameter substantially coinciding, except for a tolerance of a few ⁇ m, with that inside the tube.
• said ferrule extends for a sufficient length for allowing the distancing between the inlet openings of the liquid and those for the exit of the gases and vapours.
• the length of the ferrule generally ranges from 200 to 800 mm, preferably from 300 to 600 mm, from the edge of the upper end to the margin of the extension of the lower section inserted in the cavity of the tube.
• Said lower section of the ferrule comprises a tubular section having a length preferably ranging from 10 to 120 mm, more preferably from 20 to 80 mm, extending below beyond the edge of the circular base, whose thickness is reduced with respect to the upper part of the ferrule and whose external diameter essentially coincides with the internal diameter of the tube of the exchanger so that the two walls are substantially wedged inside each other when the ferrule is inserted in the tube, thus ensuring a correct positioning.
• said tubular part of the lower section of the ferrule has an external diameter reduced by 1 to 20 mm, preferably from 2 to 15 mm, with respect to the external diameter of the upper part of the ferrule, and with this part forms, on the circular joining line, a profile at a right angle which produces an annular surface arranged horizontally, forming said circular rest base of the ferrule on the end of the tube of the exchanger, interposed by the gasket.
• the thickness of said tubular section is within the range of 0.5 to 5 mm, preferably from 1 to 4 mm, and is truncated-conically tapered in the lower terminal part, preferably for a length of 5 to 50 mm, more preferably from 10 to 30 mm, so as to become progressively reduced and form a substantial continuity, on the lower margin, with the surface of the internal wall of the tube.
• Said lower section of the ferrule joins the remaining part of the device, superimposing the tubular profile of its upper part, which preferably has the same internal diameter as the underlying tube, for a coaxial section varying from 10 to 150 mm, preferably from 40 to 100 mm.
• the internal diameter of the lower section in correspondence with said connecting area, progressively broadens upwards, producing a truncated-conical profile for a length of 5 to 50 mm, preferably from 10 to 30 mm, similar to that described above, but oriented in the opposite direction.
• an internal profile of the ferrule is formed with a trapezoidal section as schematically represented in the subsequent figures 3 and 4, in which the internal diameter, for a preferably long section, for a total of 30 to 300 mm, narrows by 1 to 10 mm, preferably 2 to 8 mm. It has been found that the device according to the present invention thus enables the liquid to be fed to an intermediate area of the ferrule whose internal diameter is substantially the same as the vertical tube, so as to provide a larger circular surface and favour the uniform formation of the liquid film.
• a circular metallic strip is also positioned on the outer surface of the ferrule close to said circular base, said strip protruding downwards, beyond the margin of the base, for a section of at least 2 mm, preferably from 3 to 50 mm, more preferably from 3 to 30 mm, forming a housing respectively delimited by the outer surface of the protruding part of said lower section, said circular base and the internal surface of the extension of said circular strip, inside which a circular gasket is positioned, made of a heat-resistant and chemically inert material, which, resting on the upper end of the underlying tube, ensures the sealing of the ferrule with respect to possible infiltrations of liquid, giving it greater stability and elasticity with respect to the mechanical stress of the equipment when in use
• the downward protrusion of said outer circular strip must be lower than the length of the protruding section of the tube beyond the tube bundle, so as to guarantee that the weight of the ferrule is loaded on the circular gasket.
• Said gasket preferably consists of a high- performance compressible material, which must have a high resistance to the chemical attack of the liquid with which it comes into contact, often of a corrosive nature such as ammonium carbamate or nitric acid, and it must be capable of maintaining its mechanical properties, such as an adequate tensile strength and yield strength to avoid permanent deformation, within the temperature range in use, for example from 100 to 250 0 C, but at the same time having a sufficient elasticity to adapt itself to the surfaces between which it is positioned.
• Materials suitable for the purpose can be selected by experts in the field from fluorinated polymers, silicon polymers or analogous elastomeric, vulcanized or non-vulcanized materials, having a high chemical and heat resistance, certain metals with characteristics of high malleability and chemical resistance, such as lead, gold, platinum, silver.
• Typical fluorinated polymers suitable for the purposes of the present invention are, for example, polytetrafluoroethylene (PTFE) , commercially available under the trade-names of Teflon ® , Algoflon ® , Polymist ® ; perfluoropolyalkylethers such as perfluoropolyethylene oxide (Teflon ® , Hyflon ® ) ; ethylene-tetrafluoroethylene copolymers; polyvinylidenefluoride (Hylar ® ) ; fluorinated elastomers (Tecnoflon ® ) .
• PTFE possibly sintered in the desired form, is particularly preferred.
• Said gasket preferably has an annular form, with internal and external diameters substantially coinciding with those of the housing where it is positioned, more preferably equal to the corresponding internal and external diameters of the tube on which it rests, and a thickness, in a vertical direction, ranging from 0.5 to 8 mm, more preferably from 1 to 5 mm.
• the section is preferably quadrangular, so that the gasket is provided with two flat sides which are in conformance with the circular base of the ferrule and with the rest surface on the tube end, respectively.
• the equipment according to the present invention thus allows the life cycle of the falling film heat exchangers subjected to high performances, to be significantly prolonged.
• the above-mentioned article of Ghosh mentions the use of a Teflon ring only in connection with a ferrule of the external type, in which the ring is arranged laterally to the outside of the tube. Under these conditions, the same Ghosh affirms that the resistance to corrosion of the terminal area of the tube is unsatisfactory.
• Tangentially to the internal surface of the ferrule of the present equipment there is at least one opening in the metallic wall, at a suitable height, for the inlet of the falling liquid in the vertical tube.
• the function of this opening is to distribute the liquid in the form of a film as uniformly as possible on the internal surface of the tube.
• the diameter of these openings is preferably selected within the range of 1 to 5 mm, on the basis of the fluid-dynamic parameters of the liquid and flow established for the equipment when functioning.
• the height at which the openings for the liquid inlet are positioned determines the level of liquid in the upper section of the exchanger, above the tube sheet, and helps to determine the volume of liquid contained (hold-up) .
• the level of liquid is generally maintained slightly above the height of said openings, preferably from 100 to 400 mm higher, to provide the necessary hydrostatic thrust.
• openings for the outlet of the gases or vapours rising from the underlying tube In the higher part of the ferrule, close to the upper end, there is at least one opening for the outlet of the gases or vapours rising from the underlying tube.
• This opening can be simply formed by leaving the duct of the ferrule open above, or, preferably, it is obtained laterally, just under the top, for example 2 to 30 mm below the closing of the upper end of the ferrule, obtained by welding of the same or by a stopper welded or resting thereon. More preferably, there are from two to four openings for the outlet of the gases or vapours, arranged symmetrically around the side surface of the ferrule.
• This opening can have any form and extension, compatibly with the dimensions of the ferrule, but it must be such as to allow the outlet of the gases without creating a significant pressure drop.
• Said openings are conveniently square or circular, and with a maximum dimension ranging from 2 to 20 mm.
• each ferrule of the equipment according to the present invention is preferably closed at the upper end with a welded lid or stopper, which can be welded or not, to avoid the dripping of the liquid distributed from above directly into the cavity of the ferrule and underlying tube .
• the stopper is preferably shaped so as to be able to adapt to the corresponding openings in the possible grid situated above in order to fix the same ferrules.
• Said grid together with the other elements of the present invention, forms a further advantageous instrument for fixing the ferrules in the operating position and preventing any possible oscillation or movement, including the mechanical stress induced by the flow of gases, further improving the resistance to possible infiltrations of liquid into the coupling area of the ferrule onto the tube .
• the grid consists of a metallic structure made of a material having good mechanical performances and suitably resistant to corrosion, for example titanium and its alloys or stainless steel, which comprises a series of cavities or holes in positions and with such a form as to be able to house the upper end of each of the ferrules positioned on the tubes of the heat exchanger.
• said grid is also equipped with further openings or holes for the passage of the gases towards the highest part of the upper section, consisting of the head, where they are sent towards the outlet line.
• Preferred thicknesses of the grid according to the present invention range from 2 to 30 mm, more preferably from 5 to 20 mm.
• Said grid is wedge-rested against the ferrules and blocked by suitable metallic fixing elements, for example metallic sections welded or bolted on one side on the grid and on the other on the tube sheet.
• said grid is preferably divided into sections having a suitable dimension for passage through the manhole, which are then assembled among each other at the moment of fixing inside the apparatus.
• the particular form of the ferrules according to the present invention allows the same to be inserted on the upper inlet end of each tube, without having to resort to any particular metallurgical processing and without removing any internal layer of the tube itself to be able to house the lower section of the ferrule.
• the positioning of the sealing ferrule is in fact now ensured directly on the terminal border of the tube by the gasket present in the housing obtained at the circular base by the ferrule as previously described.
• the tube bundle equipment of the present invention can be produced with the usual methods adopted for analogous mechanical constructions.
• An expert in the field designs the construction on the basis of the structure as defined above, taking in account the final use of the equipment and project specifications.
• the method for the production of the tube bundle equipment according to the present invention comprises the production of a hollow body equipped with an outer casing, or force body, suitable for tolerating the operating pressures, and the formation, inside said hollow body, of at least one upper hollow section and a lower hollow section, separated by an intermediate section, airtight with respect to these, by the interpositioning of two tube sheets, seal hinged onto the force body, on which a series of tubes is inserted, in suitable holes, forming the tube bundle which allows the fluid communication between said lower and upper sections .
• the tube sheets and the tubes of the bundle are made of materials suitable for resisting the possible corrosive and erosive actions of the process fluids, in addition to tolerating the pressure thrusts under the operating conditions.
• the walls which delimit said sections and the tubes in contact with highly aggressive fluids are preferably made of or coated with metals with a high resistance to corrosion, in particular stainless steel urea grade, titanium, zirconium or alloys thereof, in the preferred case of the production of a stripper for the solution of carbamate and urea coming from the synthesis reaction.
• said production method comprises the construction of a tube bundle with bimetallic tubes comprising an internal layer of zirconium co-extruded with an outer layer of titanium produced according to WO 06/020381 mentioned above.
• the tube sheets are coated with titanium on the side exposed to corrosion.
• the tubes of the bundle are completely made of zirconium and the tube sheet is coated with zirconium.
• the production of the anti-corrosive coating of the tube sheet (both lower and upper) and remaining walls of the upper and lower sections can be effected according to any of the suitable metallurgical techniques known in the art, such as, for example, the laying, on the surface of the main carbon steel layer, of laminar elements made of the pre-selected anti- corrosion metal or alloy, suitably cut and shaped so as to adapt itself to the shape of the surface to be covered.
• the elements are arranged adjacent to each other and subsequently seal-welded to each other. Grooves, supports, connecting elements and other interventions or products are positioned, especially along the borders to be welded, according to the normal praxis known to experts in the field.
• tubes made entirely of zirconium these are preferably seal- and force-welded to the coating of the tube sheet made of an analogous material, which will have an adequate thickness, usually from 1 to 10 mm.
• bimetallic tubes it is possible to weld the different layers in a differentiated manner on the coating and on one of the underlying layers, as described, for example, in the above-mentioned patent US 4,899,813, in published European patent application EP 1.503.837 and in Italian patent application IT MI08A001302.
• all the interstitial surfaces are preferably protected, as is customary, by an argon atmosphere. Suitable weep-holes for revealing possible losses are situated in the tube sheet and in the pressure resistant body according to the known techniques .
• a ferrule having the characteristics previously described is assembled on the upper end of each tube, protruding with respect to the surface of the tube sheet for a length of 10 to 50 mm, and inserted in the tube by means of its lower protruding section.
• Said ferrule according to a preferred and non-limiting production method, is obtained by means of the following processing steps:
• the welding of the outer strip to the first tubular segment is preferably effected at discreet points, sufficient for maintaining it in the. desired position for an average time of use of the ferrule (from 2 to 10 years in the urea synthesis process) , rather than along the whole circular border of the strip. Furthermore, it can be alternatively substituted by a different joining method, for example by the formation of a metallurgical bond with the friction welding technique. If a ferrule with the upper end closed is desired, said production method also comprises the seal- insertion or welding of a closing stopper of said upper end and the formation of one or more side holes slightly below the closing for the outlet of gases or vapours when operating.
• the equipment according to the present invention is particularly applied in the separation of gases and vapours from a medium or mixture in the liquid state, subjected to heating by heat exchange with a second hotter fluid which can be a gas, liquid or, preferably a saturated vapour, more preferably steam.
• Said second fluid is passed in the volume forming the shell side of the exchanger, through suitable inlet and outlet lines normally situated on the side of the equipment.
• the liquid means to be evaporated is distributed, by means of suitable distributors, in the upper chamber of the exchanger where it is collected forming, under regime conditions, a layer having a level slightly higher than that of the holes for the inlet of the liquid, situated in the ferrule, sufficient for supplying the necessary hydrostatic thrust for obtaining the desired flow.
• the heat exchanger according to the present invention allows both the separation of gases and vapours from the falling liquid film inside the tubes, and also the possible formation and separation of gaseous compounds following chemical reactions, as occurs for example in the preferred case in which the present equipment is used as decomposer and stripper of the ammonium carbamate not transformed into urea in the synthesis process of the latter.
• the heat exchangers according to the present invention are in fact advantageously used in particular as strippers in the separation of non-reacted carbamate from the urea synthesis mixture.
• the latter equipment operates at pressures normally ranging from 1 to 40 MPa and temperatures ranging from 70 to 300 0 C, in the presence of mixtures containing water, ammonia, carbon dioxide and ammonium carbamate which is the condensation product of said compounds according to the reaction:
• the operating conditions are preferably a pressure of 12-25 MPa and a temperature of 120 to 240 0 C.
• the above equipment included in the high or medium pressure sections normally contains volumes ranging from 2,000 to 100,000 litres.
• reaction mixture is sent to a vertical falling film heat exchanger, where the liquid film is heated very efficiently and releases CO 2 and gaseous NH 3 and steam, under the possible action of a stripping gas, normally fresh CO 2 or NH 3 introduced from the outside or coming from that present in excess in the mixture leaving the reactor, thus forming a gaseous mixture which rises in each tube in countercurrent with respect to the liquid and is collected in the same upper chamber of the exchanger where the liquid mixture is distributed, then being removed through an outlet line and recondensed before being recycled to the reactor.
• a stripping gas normally fresh CO 2 or NH 3 introduced from the outside or coming from that present in excess in the mixture leaving the reactor
• Equipment according to the present invention can also be advantageously used in other parts of the urea synthesis process, for example, in the medium- or -low pressure carbamate decomposers, where the last traces of this compound are separated from the urea, molten or in solution, and sent back towards the reactor, or again, into the urea vacuum concentration section .
• Figure 1 schematically represents a perspective view of the longitudinal section of the tube bundle equipment according to the present invention, in particular suitable for use as high-pressure stripper for the decomposition of the carbamate in a plant for the synthesis of urea.
• Figure 1 schematically represents a perspective view of the longitudinal section of the tube bundle equipment according to the present invention, in particular suitable for use as high-pressure stripper for the decomposition of the carbamate in a plant for the synthesis of urea.
• Only one tube of the bundle, surmounted by the relative ferrule, is represented.
• Figure 2 schematically represents the view of a longitudinal section of the terminal part of a bimetallic tube of the type described in the above- mentioned US 4,899,813, comprising a thin internal layer of zirconium mechanically attached to a stainless steel cylindrical body, on which a ferrule of the traditional type is assembled, in a typical stripper used in the art for the separation of carbamate in the high-pressure cycle of the urea synthesis process.
• Figure 3 schematically represents the view of a longitudinal section of the terminal region of the tube according to the present invention, as represented in the equipment of Figure 1, comprising a ferrule assembled by interposition of a gasket.
• Figure 4 schematically represents two views of an enlarged detail of the previous figure 3, represented according to the longitudinal section, in the view 3B, and according to the transversal section identified by the line Si-S 2 in 3B, in the view 3A, in which the details of the coupling area of the ferrule itself in the terminal part of the vertical tube, can be distinguished .
• Figure 5 schematically represents a view from above of a part of the grid used for fixing the ferrules in the equipment according to the present invention.
• Figure 5 schematically represents a view from above of a part of the grid used for fixing the ferrules in the equipment according to the present invention.
• Figure 1 is a sectional view of a stripper positioned vertically, in which three hollow sections can be distinguished, the upper section 1, having a semispherical form, the intermediate section 3 which is cylindrical and the lower section 2 having a semispherical form.
• the diameter of the cylindrical section ranges from 1.5 to 2.5 m and the length ranges from 5 to 10 m.
• sections 1 and 2 are seal-separated from section 3 by means of the two tube sheets 15 and 16, each carrying from 2,000 to 4,000 holes for the passage of the tubes 4.
• the rest of the wall of the two sections 1 and 2 is delimited by the force body 14.
• the ferrule 5 can be distinguished in the upper section 1, assembled on the upper end of the tube 4, and kept in position by the grid 17.
• Upper holes 203 and intermediate tangential holes 204 are situated in the ferrule for the inlet of the liquid, which can be better distinguished in the subsequent figures 3 and 4.
• the solution coming from the urea synthesis reactor comprising urea, water, ammonia in excess and non- converted carbamate, is fed to the stripper by means of line 9, at a temperature of about 180-200 0 C and a pressure of about 14-17 MPa. It is distributed by means of the toroid 13.
• the liquid drips through the grid 17, and is collected on the bottom of section 1, consisting of the surface of the tube sheet, until it reaches the level 21, slightly above the tangential holes 204, through which it drips inside the ferrule 5 and then into the tube 4 , forming a thin layer not represented in figure 1, whereas vapours of ammonia and carbon dioxide which are released in the decomposition and stripping phase pass through the central part of the tube 4 in countercurrent . Said vapours are then discharged through the upper holes 203 of the ferrule and are sent towards the outlet line 10.
• the whole internal surface of section 1 is coated with a corrosion-resistant metal, for example 25/22/2 Cr/Ni/Mo (urea grade) , titanium or zirconium, having a thickness of 3 to 8 mm, not shown in figure 1.
• the intermediate section of the equipment comprises the cylindrical chamber 3, delimited towards the outside by the wall 20 made of carbon steel, having a thickness usually ranging from 20 to 30 mm and crossed by the tube bundle, in which saturated vapour is fed through the inlet 19, at a pressure of about 2-3 MPa and a temperature of 200 to 240 0 C, which circulates outside the tubes 4 and condenses on the outer wall of the same transferring heat to the aqueous solution of urea and carbamate which is flowing inside.
• the condensed liquid of the exhausted vapour then exits from line 18. In this way, the carbamate is decomposed and the ammonia in excess vaporized, which also acts as stripping agent .
• the lower section 2 is delimited by a force body 14 analogous to the section 1, and by the lower tube sheet 16. Also in this case, the whole surface of the head and tube sheet exposed to contact with the process fluid, comprises a coating, not shown in figure 1, consisting of a suitable metal or alloy with a high resistance to corrosion, selected from those previously mentioned.
• the solution of urea mainly purified of carbamate is collected on the bottom of the section 2 and is pushed into the siphon 11 from which it proceeds towards the further purification and drying sections. Further ammonia can be introduced through the inlet 12 , if necessary, or carbon dioxide according to an alternative technology, to favour the stripping. Passivation air, when required, is also introduced from the same inlet.
• the tube 111 surmounted by a ferrule 102 can be distinguished.
• the tube 111 is delimited by the cylindrical wall 109, consisting, for example, of stainless steel of the type AISI 316L (urea grade) , INOX steel 25/22/2 Cr/Ni/Mo, inserted in the tube sheet 106, made of carbon steel, whose end is welded to the anticorrosive coating 107 by means of the welding 108.
• a tubular jacket 110 made of zirconium, forms a layer with a high resistance to corrosion and erosion, mechanically attached to the wall 109 of the tube.
• the terminal part of the protective layer has been removed, for a length varying in industrial applications from 30 to 100 mm, to allow the insertion of the lower section of the ferrule, which is processed so as to substantially maintain the same internal diameter as the tube, and has an "L" profile in correspondence with the support line on the upper end of the tube, thus creating a circular rest surface.
• the ferrule 102 delimited by the wall 101, made of stainless steel or titanium, has from 3 to 4 tangential holes 104 situated laterally on the wall at a height normally ranging from 20 to 50 mm with respect to the circular rest surface of the ferrule on the tube.
• the upper part of the ferrule is closed and maintained in position by the grid 105 by simply resting thereon.
• the internal diameter of the tube (including the coating 110) and, correspondingly, of the ferrule is usually within the range of 10 to 40 mm in the high-pressure equipment (> 10 MPa), and from 20 to 50 mm in medium- or low-pressure equipment ( ⁇ ; 9 MPa) .
• the tube 4 of the exchanger represented in figure 1 is delimited by the cylindrical wall 209, represented herein with a single metallic layer, but which can also comprise, as already indicated, two or more layers of different metals or alloys.
• Said tube 4 is inserted in the tube sheet 206, usually made of carbon steel, coated by an anticorrosive metallic layer 207, by means of seal- welding 208, situated close to the upper end of the tube, so that the latter protrudes above the coating for a length ranging from 30 to 50 mm.
• the tube 4 is surmounted by the ferrule 5, inserted in the inlet end of the same by means of the lower section 215, clearly visible in figure 4, which is processed so as to substantially maintain the same internal diameter as the tube, and form a right angle with the rest base 216 of the ferrule.
• the ferrule 5, delimited by the wall 201, made of stainless steel or titanium, has from 3 to 4 tangential holes 204 situated laterally on the wall at a height ranging from 50 to 100 mm with respect to said rest base 216.
• a further metallic strip 214 is visible in figure 3, even more clearly in figure 4, protruding externally with respect to the main tubular element 201 of the ferrule, which extends for a length of 5 to 20 mm, downwards, beyond the horizontal circular surface of the same base 216.
• the protruding part of the strip 214 clearly visible in the enlarged section of figure 4 and by the vertical wall of the lower section 215, the gasket 213, made of PTFE, can be distinguished, which allows the ferrule to indirectly rest on the border of the upper end of each tube 211, at the same time preventing leakage of the liquid phase collected in the upper area of the exchanger.
• the upper part of the ferrule is closed by means of the stopper 212 and kept in position by the grid 205, of which a detail is represented in a horizontal plan view, in the following figure 5.
• Two or more openings 203 having a diameter of about 10-20 mm, are situated in the ferrule, slightly below the top, for the discharge of the gases or vapours.
• the internal diameter of the tube and correspondingly of the upper part of the ferrule is normally within the range of 10 to 30 mm in the high-pressure equipment (> 10 MPa) , and from 20 to 60 mm in medium- or low-pressure equipment ( ⁇ 9 MPa) .
• FIG 5 schematically shows a view from above of a part of the grid 17, whose section is represented, with the same numerical reference, in figures 1 and 3.
• a large number of circular holes 301 geometrically orderly, alternating with other openings 302 having an irregular form, can be distinguished.
• Each hole 301 is destined to house the upper terminal part of the ferrule, preferably the stopper 212 visible in figure 3, having a truncated- conical shape, a spherical segment or any other form suitable for favouring a stable insertion in the hole itself.
• the diameter of each hole 301 is therefore substantially the same as that of the ferrule or slightly lower.
• the openings 302 have the double function of allowing the vapours to flow towards the top of the head of the stripper exchanger, and allowing the dispersed liquid to drip from the distributor 13 of figure 1, which is collected on the surface of the tube sheet until it reaches the level 21.
• Said grid can consist of any corrosion-resistant metal, such as, for example, aluminium, titanium, zirconium, stainless steel. It is generally obtained by cutting a sheet having a suitable thickness, according to the profile of the holes and openings which are to be obtained.
• a known cutting technique which is particularly suitable for the purpose is that by means of a high-pressure water jet (from 10 to 100 MPa) , controlled by a computerized system.

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• General Engineering & Computer Science (AREA)
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• Chemical Kinetics & Catalysis (AREA)
• Organic Chemistry (AREA)
• Heat-Exchange Devices With Radiators And Conduit Assemblies (AREA)
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