Patent Publication Number: US-2016231070-A1

Title: Apparatus for processing highly corrosive agents

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/596,156, filed Jun. 1, 2006, which is a national phase of PCT/EP2005/001934, filed Feb. 24, 2005, and claims priority to European Patent Application No. 04006215.0, filed Mar. 16, 2004, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF APPLICATION 
     The present invention, in its most general aspect, refers to apparatuses intended for treating highly corrosive chemical agents, with respect to which a specialized, effective and long-lasting protection is required. 
     In particular, this invention concerns an apparatus of the aforementioned type, in which the treating of corrosive agents is substantially a heat-treating. 
     Yet more specifically, the present invention refers to an apparatus of the type considered, comprising or essentially consisting of a tube bundle heat exchanger, structured to carry out a heat exchange between two fluids one of which is highly corrosive. 
     Heat exchange apparatuses falling within the scope of protection of the present invention are, for example, synthesis, decomposition, condensation or evaporation reactors, stripping apparatuses, boilers, concentrators and similar devices that require the heat exchange between a process fluid and an operating fluid. 
     In particular, but not exclusively, the present invention concerns apparatuses that can be used in urea production plants, for the decomposition of ammonium carbamate into ammonia and carbon dioxide, also known as strippers, and, respectively, apparatuses for the condensation of ammnonia and carbon dioxide into ammonium carbamate, also known as condensers. 
     For the sole purpose of simplifying the following description and claims, with the terms “tube bundle heat exchanger” or “tube bundle heat exchange apparatus”, we intend to identify all of the aforementioned apparatuses. 
     PRIOR ART 
     It is known that heat exchange apparatuses of the type specified above operate, in an urea production plant, in critical operative conditions, normally of high pressure and high temperature of the process fluid and are therefore continuously subjected to high mechanical and thermal stresses. 
     Added to this, as well as such critical operative conditions, the process fluid often also exerts a corrosive and/or erosive action on the surfaces with which it comes into contact due, in particular, to the presence of highly corrosive agents. 
     The corrosive and/or erosive action reveals itself in particular at the tube bundle, and generally inside the tubes of which it is composed, of the stripping and condensation apparatuses present in the high pressure section of an urea production plant. 
     The tubes of the tube bundle of such heat exchange apparatuses (stripper and condenser) are, indeed, generally crossed on their inside by a process fluid that, in the case of urea, has, as main compounds, highly corrosive agents like ammonium carbamate and carbon dioxide. These substances exert an aggressive consumption and erosion action of the inner surfaces of such tubes with which they come into contact. 
     It follows from this that such apparatuses are seriously damaged already after short periods of operation, in some cases even after a few months of activity. This means having to shut down the whole urea production plant for their repair, but more frequently replacement with new apparatuses, with clear negative effects in terms of loss of production and maintenance costs. Moreover, frequent stopping and restarting of the plant can cause damage to the other apparatuses or their rapid wear, as well as additional energy consumption. 
     In order to try to avoid such drawbacks, heat exchange apparatuses of the type specified above have been proposed in the field, the tube bundle of which consists of stainless steel tubes, completely coated at their inner surface with a layer of zirconium. 
     However, despite the recognized effectiveness of zirconium coating against the chemical corrosion determined by the aforementioned agents, this type of apparatuses does not allow completely satisfactory results to be achieved. 
     Indeed, the well-known “incompatibilities” of zirconium with stainless steel, as regards their close bonding through welding as well as their different physical properties in terms of resistance to mechanical stresses and thermal dilation, are such that in anti-corrosion coatings (zirconium/steel) of the heat exchange tubes according to the prior art, there is disbonding between the materials and therefore points and zones in which the desired coating is detached from the stainless steel tube. The process fluid thus manages to infiltrate into such “critical” zones and, since the steel tube is without protection, it is therefore thus subject to the corrosive attack of such a fluid, with consequent rapid and serious damage to the heat exchange apparatus. 
     It follows from this that, as well as being difficult to carry out due to the objective difficulty in realizing a zirconium coating on steel tube bundles, current heat exchangers of the type considered here are very onerous from the point of view of maintenance, requiring frequent checks and frequent repair interventions. 
     SUMMARY OF THE INVENTION 
     The technical problem underlying the present invention is therefore that of providing a tube bundle heat exchange apparatus of the type specified above, that allows the quoted drawbacks with reference to the prior art to be overcome; in other words that allows an effective and long-lasting resistance against corrosive chemical agents treated in it to be ensured, that is easy to realize, reliable and that does not require frequent and onerous maintenance interventions. 
     Such a problem is solved by a tube bundle heat exchange apparatus of the type considered above, characterized in that said tube bundle comprises at least one titanium or titanium alloy tube, coated with a layer of zirconium or zirconium alloy. 
     Preferably, the titanium tube is internally coated with the layer of zirconium or zirconium alloy. 
     Preferably, the titanium or titaniuam alloy tube has a thickness between 1.0 and 10 mm, whereas the coating layer of zirconium or zirconium alloy has a thickness between 0.3 and 2.0 mm. 
     Again preferably, said at least one titanium or titanium alloy tube is only partially coated with the layer of zirconium or zirconium alloy, which, preferably, extends in such a tube starting from an end thereof, or close to an end thereof, for the entry of a process fluid, towards an opposite end thereof, for a portion between 5 and 30%. 
     Still preferably, the titanium or titanium alloy tube and the coating layer of zirconium or zirconium alloy are bonded together metallurgically, for example by hot-drawing, or through welding. 
     From the studies carried out by the Applicant, it has surprisingly been found that, contrary to the constant teaching of the prior art, by combining a zirconium coating with a titanium tube, the aforementioned drawbacks with reference to the prior art are advantageously overcome in a simple and effective manner. 
     In particular, the heat exchange tube according to the present invention, obtained by the titanium/zirconium combination, is extremely resistant both to the mechanical and thermal stresses and to the corrosive/erosive attack of the process fluids with which it comes into contact. 
     Moreover, thanks to the particular compatibility between these two metals and their similar chemical/physical properties, it is possible to firmly and long-lastingly bond the zirconium coating with the titanium tube. All of this using simple and conventional realization techniques and without the two metals, during the operation of the apparatus, tending to disbond or in any case coming under tension one with respect to the other, thus keeping the structural and anti-corrosive resistance of such materials unchanged through time and avoiding any type of laceration or crack of the coating layer. 
     Further characteristics and advantages of the invention shall be clear from the description made hereafter of an embodiment thereof, given for indicating and not limiting purposes, with reference to the attached drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In such a drawing: 
         FIG. 1  schematically illustrates a section view of a tube bundle heat exchange apparatus according to the present invention; 
         FIG. 2  schematically illustrates a section view of a detail of the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
     With reference to the aforementioned figures, the heat exchange apparatus according to the invention shall be described, purely for indicating and not limiting purposes, with specific reference to a descending film tube bundle heat exchanger  10 , with vertical tubes, that has advantageous and specific use as a stripper of a urea production plant, but it is clear that it can be used as a condenser, evaporator, boiler, reactor or similar apparatuses based upon the heat exchange between two fluids. 
     In particular, the heat exchanger  10  according to the invention has advantageous and specific use as a stripper or condenser in a high pressure synthesis section of an urea plant, and more precisely of a urea plant of so-called CO2 or ammonia stripping, not represented because it is conventional. Such a section generally comprises at least one synthesis reactor, a stripper for the decomposition of the ammonium carbamate and of the-free ammonia present in the reaction mixture coming from the reactor, and a condenser for the condensation of vapors comprising carbon dioxide and ammonia coming from the stripper. These apparatuses are in fluid communication with each other so as to form a so-called substantially isobaric synthesis loop, i.e. operating at the same pressure usually between 140-170 bar. 
     The heat exchanger  10  comprises a shell  11 , with a vertical axis A-A, closed at the opposite ends by respective walls or bottoms  12 ,  13 , a tube bundle  14  (of which just one tube  14   a  is illustrated in the figures for the sake of simplicity), supported longitudinally in said shell  11 , through tube plates  15 ,  16 , upper and lower respectively, perimetrically fixed (for example welded) gas-tight to said shell  11 . 
     Due to the presence of such tube plates  15 ,  16 , in the shell  11  of said heat exchangers  10 , three chambers are defined, arranged in axial sequence: a first chamber  17 , between the upper bottom  12 , and the outer wall  15   a  of the upper tube plate  15 ; a second chamber  18 , between the tube plates  15  and  16  and a third chamber  19 , between the outer wall  16   a  of the lower tube plate  16  and the lower bottom  13  of the shell  11 . 
     For the sake of simplicity of explanation, with the term “outer wall of the tube plate”, we mean to identify the wall of said tube plate facing towards the outside of the tube bundle. 
     The first chamber  18 , or upper chamber, is in fluid communication with the outside of the apparatus  10 , through an entry duct  20  of, for example, a process fluid, formed in the upper bottom  12 , whereas the third chamber  19 , or lower chamber, is in fluid communication with the outside through an exit duct  21  of the fluid, formed in the lower bottom  13 . 
     In turn, the second chamber  18 , or intermediate chamber, is in communication with the outside of the shell  11  through an upper duct  23 , for the introduction into it of an operating heat exchange fluid, for example steam at a predetermined pressure and temperature, able to be used to carry out the desired heat exchange with the process fluid to be stripped; a lower duct  22  takes care of discharging the operating heat exchange fluid from said second chamber  18 . 
     The tubes  14   a  of the tube bundle  14 , have respective upper and lower end portions  14   b,    14   c , fixed for example through welding  24  as indicated in  FIG. 2 , in the corresponding tube plates  15  and  16 , and are open in the first chamber  17  and in the third chamber  19 , respectively, which are thus in mutual fluid communication. 
     Advantageously, in accordance with the present invention, the tubes  14   a  of the tube bundle  14  are made of titanium or titanium alloy and are coated with a layer  25  of zirconium or zirconium alloy. In the example of  FIG. 2 , such a coating layer  25  is inside the titanium or titanium alloy tubes  14 , i.e. applied to the inner surface of such tubes. It goes without saying that in the cases in which the process fluid is made to flow outside of the tube bundle  14  (shell side), it is the outer surface of the titanium or titanium alloy tubes  14   a  that is advantageously coated according to the invention with the layer  25  of zirconium or zirconium alloy. 
     Preferably, the titanium used is titanium ASTM GR. 1-2-3-4-5-6-7- or equivalent, whereas the zirconium is of the ASTM GR. 60702/60704/60705 type, or equivalent. 
     Preferably, the titanium tube  14   a  also has a thickness between 2.0 and 5.0 mm, whereas the zirconium inner coating layer  25  has a thickness between 0.5 and 1.2 mm. 
     Advantageously, according to a preferred embodiment of the present invention, the zirconium coating  25  only partially covers the titanium heat exchange tube  14   a.  Preferably, such a coverage of the zirconium layer  25  is present solely at the upper end portion  14   b  of the heat exchange tube  14   a,  and in particular in the portion of the tube  14   a  immediately below the upper tube plate  15 . Again preferably, the zirconium layer  25  extends in the tube  14   a  starting from an upper end  26  thereof for the entry of a process fluid, towards an opposite end  27  thereof, for a portion between 10 and 20%. 
     By doing so, an anticorrosion protective coating is realized in the heat exchanger  10  according to the invention solely in the critical points and zones of the tube bundle  14 , where, from the studies carried out by the Applicant, the corrosive/erosive action of the process fluid is higher, in any case managing to ensure an effective and long-lasting resistance to corrosion also in the other parts of the tube bundle thanks to the arrangement of titanium or titanium alloy tubes. 
     It follows from this that the use of zirconium as coating material is drastically reduced with respect to the prior art, simplifying the application procedures of such a layer and at the same time obtaining a saving in realization costs of the present apparatus. 
     Of course, where the operating conditions and the type of operating fluid require it, the zirconium layer  25  can completely coat the inner surface of the tubes  14   a  of the tube bundle  14 . 
     According to a further particularly advantageous aspect of the present invention, the titanium tube  14   a  and the zirconium coating layer  25  are preferably bonded together metallurgically, for example by hot-drawing, or through welding. In this way, a strong, stable and long-lasting link is obtained between the two metals, which makes it practically impossible for them to detach even when subjected to the most extreme operative conditions, thus ensuring continuous resistance to corrosion. This is made possible in particular thanks to the similar chemical/physical properties of titanium and zirconium (and of their alloys), which makes them compatible for this type of assembly. Advantageously, the hot-drawing or the welding of the tube according to the invention is realized by using per se known techniques. 
     A further advantage of the present invention with respect to the apparatuses according to the prior art is given by the fact that the arrangement of titanium or titanium alloy tubes  14   a  ensures a long-lasting and effective resistance to corrosion thereof also at their lower and upper ends. Indeed, such ends, going into the chambers  17  and  19 , are particularly subjected to the corrosive attack of the process fluid (liquid and gaseous) present in such chambers. 
     For the purposes of the present invention, the parts outside of the tube bundle  14 , and in particular the tube plates  15  and  16 , are preferably made of titanium or titanium alloy or are coated (cladded) with a layer of titanium or titanium alloy, so as to ease the fixing of the tubes  14   a  with such tube plates. Just as an example, in accordance with the present invention, the upper and lower tube plates  15 ,  16  are made of carbon or stainless steel, coated on the outside with a layer of about 3-15 mm of titanium or titanium alloy. 
     In use, a process fluid is fed into the first chamber  17  of the exchanger  10  according to the present invention through the entry duct  20 , from here descends along the inner wall of the tubes  14   a  of the tube bundle  14 , without filling up, forming a thin film on it that leaves a consistent empty space at the center of the tubes themselves (see vertical axis B-B of  FIG. 2 ). At the same time, an operating heat exchange fluid is fed into the second chamber  18  of the heat exchanger  10  through the upper entry duct  23 , and circulates inside of it coming into contact with the outer wall of each single tube  14   a  of said tube bundle  14 . The product coming out from the tubes  14   a  is collected in the third chamber  19  from where it is discharged to the outside of the heat exchanger  10  through the exit duct  21 . In turn, the operating heat exchange fluid comes out from the second chamber  18  through the lower exit duct  22 . 
     In the specific case of the example of  FIGS. 1-2  of a tube bundle stripper, with vertical tubes and with descending film, used in the high pressure and temperature synthesis section of an urea production plant, the process fluid comprises an aqueous urea solution, ammonium carbamate, amonia and carbon dioxide, where carbamate and carbon dioxide are highly aggressive agents that notoriously exert a considerable corrosive action on the metallic surfaces with which they come into contact. 
     Advantageously, as seen above, such a corrosive action, which is particularly virulent inside the tubes of the tube bundle of such apparatus, is effectively neutralized by the arrangement of titanium or titanium alloy tubes, at least partially coated on the inside by a layer of zirconium or zirconium alloy. 
     The finding thus conceived is susceptible to further variants and modifications all within reach of the skilled person in the art and, as such, falling within the scope of protection of the finding itself, as defined by the following claims.