Patent Publication Number: US-2021172459-A1

Title: Centrifugal pump for processing molten urea and relative plant

Description:
The present application claims priority to Italian Patent Application No. 102019000023160 filed on Dec. 6, 2019, the entire contents of which is incorporated by reference. 
     DESCRIPTION 
     Field of the Invention 
     The present invention relates to the field of industrial processing of urea, particularly for the production of melamine In more detail, the present invention relates to a centrifugal pump for processing molten urea and a relative plant, in particular for producing melamine with a high-pressure process. 
     State of the Art 
     The production of melamine by urea pyrolysis according to the global reaction (1) is known: 
     
       
         
         
             
             
         
       
     
     such reaction, as known, being highly endothermic. 
     The processes for converting urea into melamine are distinguished into two groups: processes performing high-pressure urea pyrolysis and processes performing low-pressure urea pyrolysis. 
     Both the processes are typically performed in reactors fed with a molten urea stream. 
     Preferably, the reactor is also fed with an ammonia stream. 
     In high-pressure processes, the reaction chamber is always held at a pressure greater than 60 bar and it is provided with heating means maintaining the reagent system at a temperature of about 360° C.-450° C. 
     The present invention relates particularly to processes for the high-pressure preparation of melamine. More specifically, in order to produce melamine, it is necessary to handle and pressurize liquid-state urea with a concentration greater than 99.8%, i.e. molten, taking it either from a vacuum separator, in case the urea is fed to the plant as aqueous solution and needs to be concentrated inside the plant itself, or from a surge drum maintained under a slight pressure, in case the urea is fed to the melamine plant already as concentrated, to a reactor operating at a pressure of about 80 bars. 
     At present, in order to pressurize the molten urea from the separator to the reactor reciprocating pumps are used. The constant increase of the production capability of plants for producing melamine requested by the market, has determined a proportional increase of urea flow rates to be processed in the plants, resulting in an increase of the volume and number of reciprocating pump heads used, and frequently, in the use of more pumps in parallel, with the result of an increase in the complexity, costs and malfunction risks of the reciprocating pumps currently used with an impact on the operation of the plant itself. The use of reciprocating pumps causes a number of disadvantages. 
     Reciprocating pumps are by their nature in each head equipped with valves at the inlet (suction) and at the outlet (delivery) that control their operation. The presence of such valves along the pump&#39;s suction flow, and in particular at the inlet on each head, is a bottleneck that results in an speed increase and load loss with a subsequent decrease inside the valve itself causing the release of any gases dissolved in the liquid phase with a risk of cavitation phenomena at the pump inlet. 
     The molten urea is subjected to a continuous, though minimal, decomposition with the formation of ammonia and carbon dioxide, that remain dissolved in the liquid phase under constant pressure conditions, but which are released as a gas phase as the pressure decreases, generating cavitation phenomena that impair the proper operation of the reciprocating pumps reducing their performance and often resulting in mechanical faults. For this reason, reciprocating pumps require the use of an additional pump arranged upstream to cause a pre-compression of the molten urea and preventing cavitation at the reciprocating pump inlet. 
     Furthermore, the reciprocating pumps have by their nature a pulsating delivery pressure, which requires the use of accumulators downstream the reciprocating pump and other devices capable of damping pulsations and/or operating under pulsating pressures. Moreover, in the production of melamine, this factor affects the reactor subjected to a pulsating feeding which generates the formation of a pulsating reaction gas (see the reaction) with a subsequent internal pulsating pressure and on gases it emits in the pressure control, that also result having a pulsating flow rate, and on the components designated to processing such gases. 
     Urea has a melting temperature equal to 133° C. In order to avoid urea solidification, the pump needs to be drained and washed after its shutdown. The reciprocating pumps with multiple heads arranged in parallel require that each head to be drained and washed individually, and therefore they require complex draining and washing systems. Such systems are detrimental to the reliability of the pump, increasing the possibility of leaks and clogging, and making these operations particularly time-consuming for the operators who could omit complete execution of the same. 
     Moreover, the starting of the reciprocating pumps causes a sudden pressure increase in the delivery of the pump itself, which risks to cause malfunctions or damages to the downstream components, particularly to the shut off/check valve, which keeps the reactor under pressure during the urea pre-feeding step wherein the pressure of the reactor is maintained by a constant supply of ammonia. In order to overcome this problem generally the delivery is pre-pressurized in advance, with a consequent increase in the complexity of the plant and of its starting procedures. 
     In addition, some plants require the presence of spare pumps arranged in parallel to the main pump and kept in stand-by, ready to be started if the running pump will stop. In order to maintain the spare pump in stand-by it is necessary to keep it empty and continuously flushed with steam to ensure that internally, in case of leakage of the shut-off valves, urea cannot accumulate, and with its subsequent degradation resulting in a blockage. As an alternative, it is necessary to pre-fill the stand-by pump with molten urea, guaranteeing a circulation inside it to avoid solidifications. In reciprocating pumps, establishing a recirculation inside the pump is complicated and difficult to obtain in a uniformly distributed manner due to the multiple heads which are not individually flushed, unless a complex circulation system is created for each individual head, thus resulting in increased operational operations and a high number of possible leakage points of urea outwards. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to improve the prior art with reference to one or more points of view. 
     Particularly, an object of the present invention is to overcome the above disadvantages by using a centrifugal pump for processing molten urea and a relative plant, in particular for producing melamine with a high-pressure process. 
     More in detail, object of the present invention is to provide a centrifugal pump for processing molten urea and a relative plant capable of processing a higher amount of molten urea than the prior art, particularly a centrifugal pump having the prevalence conditions necessary to feed molten urea to the production reactor. 
     A further object of the present invention is to provide a centrifugal pump for processing molten urea and a relative plant which is less complex than what is currently available. 
     A further object of the present invention is to provide a centrifugal pump for processing molten urea and a relative plant having less malfunction risks and lower wear of components. 
     Such objectives are substantially achieved due to what is reported in the attached claims forming integral part of the present description. 
     According to a first aspect, the present invention relates to a centrifugal pump for processing molten urea having:
         an inlet adapted to receive molten urea at a suction pressure;   a delivery outlet adapted to allow an output of molten urea at a delivery pressure greater than the suction pressure;   an intermediate outlet adapted to allow an output of molten urea at an intermediate pressure greater than the suction pressure and lower than the delivery pressure.       

     According to a second aspect, the present invention relates to a plant for processing molten urea, in particular for producing melamine with a high-pressure process, comprising:
         a separator for molten urea having a primary inlet, a recirculation inlet and an outlet for molten urea;   at least one centrifugal pump;       

     wherein the inlet of the centrifugal pump is fluidly connected to the outlet for molten urea of the separator to receive molten urea at the inlet coming from the separator; 
     wherein the recirculation inlet of the separator is fluidly connected to the intermediate outlet of the centrifugal pump to receive recirculation molten urea at the inlet coming from the centrifugal pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be clearer from the following detailed description to be considered together with the accompanying drawings wherein: 
         FIG. 1  shows a schematic view of a plant for processing molten urea according to the present invention; 
         FIG. 2  shows a schematic view of a centrifugal pump for processing molten urea according to the present invention; 
         FIG. 3  shows a schematic view of a detail of a plant for processing molten urea according to a possible embodiment of the present invention. 
         FIG. 4  shows a schematic view of a detail of a plant for processing molten urea with the possibility of internal circulation. 
         FIG. 5  shows a schematic view of a detail of a plant for processing molten urea with flushing possibility. 
     
    
    
     As easily understandable, there are several modes to implement in practice the present invention which is defined in its main advantageous aspects in the accompanying claims and is not limited either by the following detailed description or by the accompanying drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the attached figures, a plant for processing molten urea, object of the present invention, is generically indicated with the reference number  1  and hereinafter reference to it will be made with the annotation “plant  1 ”. 
     Plant  1  comprises a vacuum separator  10  for molten urea provided with a primary inlet  11 , an outlet for molten urea  12 , a recirculation inlet  13  and an outlet for gas  14 . Preferably, the recirculation inlet  13  is arranged at or in the proximity of an upper end of the separator  10  and the outlet for molten urea  12  is arranged at or in the proximity of a lower end of the separator  10 . 
     The plant  1  also comprises a centrifugal pump for processing molten urea, which forms a further object of the present invention and hereinafter it will be indicated with the reference number  100  and named as “centrifugal pump  100 ”. 
     The centrifugal pump  100  has an inlet  102  and a delivery outlet  104 . The inlet  102  corresponds to the suction of the centrifugal pump  100  and, when operated, is fluidly connected to the outlet for molten urea  12  of the separator  10  to receive molten urea from it through a suction duct  32 . The delivery outlet  104  corresponds to the delivery of the centrifugal pump  100  and, when operated, is fluidly connectable to a reactor  20  for the production of melamine from urea, in order to feed it with a flow rate of molten urea through a delivery duct  34 . 
     The centrifugal pump  100  is configured to receive molten urea at a preselected suction pressure at the inlet  102  and allow an output of molten urea at a delivery pressure at the delivery outlet  104 , greater than the suction pressure. Preferably, the suction pressure is comprised between 0.5 bar and 5 bar, even more preferably between 1 bar and 2 bar, particularly of about 1.5 bar. Such suction pressure is determined by the greater height above ground level of the separator  10  (which operates under conditions of absolute vacuum or slight overpressure) with respect to the inlet  102 . The difference in height between the separator  10  and the pump inlet  102  must therefore be such as to guarantee a positive pressure in any case. 
     The delivery pressure is preferably comprised between 60 bar and 100 bar, even more preferably between 70 bar and 90 bar, particularly of about 81 bar. Accordingly, the centrifugal pump  100  has a prevalence preferably comprised between 60 bar and 100 bar, even more preferably between 70 bar and 90 bar, particularly of about 80 bar. 
     Preferably, the centrifugal pump  100  is configured to process a flow rate of molten urea greater than 15 m 3 /h, even more preferably greater than 25 m 3 /h. 
     Preferably, the centrifugal pump  100  comprises a heating system configured to maintain an internal temperature of the molten urea greater than a preselected value, particularly greater than its melting temperature, equal to about 133° C., in order to avoid solidification inside the centrifugal pump  100  itself. 
     Preferably, the centrifugal pump  100  is multi-stage and comprises a plurality of centrifugal stages  110  serially arranged. Generally, each centrifugal stage  110  has a compression chamber and an impeller arranged inside it, as shown in  FIG. 2 . 
     Particularly, the plurality of centrifugal stages  110  includes an initial stage  112  arranged at the inlet  102  to receive molten urea through it and a final stage  114  arranged at the delivery outlet  104  to send molten urea to the reactor  20  through it. 
     Between the initial stage  112  and the final stage  114  the pressure of the molten urea gradually increases passing through the various centrifugal stages  110 . 
     The plurality of centrifugal stages  110  comprises an intermediate pressure stage  113 , operatively serially arranged between the initial stage  112  and the final stage  114 . 
     Particularly, the intermediate-pressure stage  113  has an intermediate outlet  103  configured and arranged to allow the output of molten urea from such stage at an intermediate pressure greater than the suction pressure and lower than the delivery pressure. Preferably, the intermediate pressure is comprised between 3 bar and 15 bar, even more preferably between 5 bar and 10 bar. In other words, the intermediate pressure is greater than the suction pressure by a value comprised between 2 bar and 14 bar, more preferably between 4 bar and 9 bar. 
     The value of said intermediate pressure depends on the serially arrangement of the intermediate-pressure stage  113  in the plurality of centrifugal stages  110 . Preferably, the intermediate-pressure stage  113  is a centrifugal stage interposed between the initial stage  112  and the final stage  114 . In some embodiments, the stage at intermediate pressure stage  113  can coincide with the initial stage  112 . 
     Preferably, the intermediate outlet  103  is sized and configured to output a flow rate of molten urea comprised between 50% and 75% of the molten urea at the inlet, even more preferably between 60% and 70%, particularly about 67%. 
     Moreover, the intermediate outlet  103  when operated, is preferably located at an upper end of the intermediate-pressure stage  113  to allow a gas extraction or venting. The molten urea continuously generates reaction gases, particularly ammonia and carbon dioxide, and the intermediate-pressure stage  113  thus configured allows to convey the gases generated from urea during the movement through the intermediate outlet  103  with operating advantage of the subsequent compression stages. 
     Preferably, the intermediate-pressure stage  113  has a storage volume  115  arranged, an operative configuration, above the impeller, in fluid communication with the compression chamber. Particularly, the centrifugal pump  100  has a casing provided with a dome-shaped portion  116  vertically above the impeller of the intermediate-pressure stage  113  to define the above-mentioned storage volume  115 . The intermediate outlet  103  is arranged at an upper end of the dome-shaped portion  116 . 
     The storage volume  115  is configured to receive molten urea and gas from the compression chamber of the intermediate-pressure stage  113  during operation of the centrifugal pump  100  to allow its input through the intermediate outlet  103 . 
     The intermediate outlet  103  is, when operated, fluidly connected to the separator  10  through a recirculation duct  33 , such that to cause a recirculation stream of molten urea from the centrifugal pump  100  to the separator  10  such as to prevent accumulation of solid residues inside it. Particularly, the recirculation duct  33  is fluidly connected to the recirculation inlet  13  of the separator  10 . The feed duct  32  is connected to the outlet for molten urea  12 . This configuration allows to determine a continuous flow of molten urea from the recirculation inlet  13  to the outlet for molten urea  12  through the entire extension of the separator  10 . 
     The plant  1  further comprises the reactor  20  for producing melamine fluidly connected to the delivery outlet  104  of the centrifugal pump  100  to receive from it pressurized molten urea through the delivery duct  34 . 
     According to a possible embodiment, the centrifugal pump  100  has a variable speed to regulate the flow rate of the molten urea directed to the reactor  20 . 
     According to a possible alternative embodiment, the centrifugal pump  100  has a fixed rotation speed and the plant  1  comprises a regulation valve arranged along the delivery duct  34 , between the delivery outlet  104  of the centrifugal pump  100  and the reactor  20 , to regulate the flow rate of the molten urea directed to the reactor  20 . 
     Preferably, plant  1  comprises two of the above-described centrifugal pumps  100 , arranged in parallel for redundancy, as illustrated in  FIG. 3 , to compress the molten urea directed from the separator  10  to the reactor  20 . One of the two centrifugal pumps  100  is configurable in stand-by while the other is running 
     A plurality of valves  40  is arranged along the suction and delivery ducts of the two valves to allow the centrifugal pump  100 , when operated, to be connected in line between the separator  10  and the reactor  20  and to isolate the other. Preferably, plant  1  further comprises a first flushing line  41  controlled by a related valve  42  and arranged to cause a fluid coupling between the delivery outlets  104  of the two centrifugal pumps  100 , bypassing the valves  40 . Moreover, plant  1  can comprise a second flushing line  43  controlled by a related valve  44  and arranged to cause a fluid coupling between the inlets  102  of the two centrifugal pumps  100 , bypassing the valves  40 . 
       FIG. 4  shows an example of an operational configuration of the plant  1  wherein the centrifugal pump  100 A is running, the centrifugal pump  100 B is in stand-by, the valves  40 A,  42  and  44  are open and the valves  40 B are closed. 
     In the absence of the second flushing line  43 , the re-circulation of the fluid in the centrifugal pump  100 B in stand-by can be obtained by leaving open or partially open the valve  40 B upstream (or in suction) of the centrifugal pump  100 B in stand-by. 
     Advantageously, the first and the second flushing line  41  and  43  allows to establish, during the operation of one of the two centrifugal pumps  100 , a flow of molten urea from the delivery outlet  104  of the running centrifugal pump  100  to the inlet  102  of the other centrifugal pump  100  in stand-by, circulating inside the pump in stand-by, and reaching the suction line  102  of the running pump. 
     By means of this internal circulation in the stand-by pump it is avoided that molten urea can stay inside it with the possibility of decomposition and solidification. 
     Preferably, plant  1  further comprises a system for washing the one or more centrifugal pumps  100  after their shutdown, an example of which is given in  FIG. 5 . 
     The washing of the centrifugal pumps  100  consists in emptying them from the molten urea by draining them into a recovery manifold, followed by flushing with steam and subsequently with water which are in turn recovered to avoid environmental pollution. For this purpose, plant  1  comprises a plurality of access portions  45  and  46 , fluidly coupled to the centrifugal pumps  100  to allow their emptying and flushing. Particularly plant  1  comprises a prima access portion  45  upstream of the centrifugal pump  100  and a second access portion  46  downstream of each centrifugal pump  100 . Preferably, the first and second access portions  45  e  46  consist of rapid connections equipped with valves, which allow the connection of drainage and/or flushing ducts to perform these operations. 
     These first and second access portions  45  and  46  allow access to any areas where the molten urea may have solidified, which tends to solidify in the dead points, i.e. without circulation and renewal of the product, for example the areas upstream of the rapid connection valves. 
     The described embodiments overcome the limitations of the prior art. 
     The use of the centrifugal pump with these characteristics in place of the traditional reciprocating pumps allows to process a greater amount of molten urea without having to adopt a large number of reciprocating heads or pumps in parallel. 
     The use of the centrifugal pump with these characteristics prevents the phenomena of cavitation in the suction and allows not to use additional pumps to pre-compress the molten urea. 
     The described centrifugal pump allows to send a recirculation flow rate at right pressure to the vacuum separator, if present, in order to prevent the accumulation of solid residues without requiring a dedicated pump. 
     The described centrifugal pump also allows to avoid problems caused to the plant by the delivery pulsating pressure of the reciprocating pumps. 
     The use of the described solutions makes it possible to create simpler plants having less malfunction risks and lower wear of components. 
     Comparing the construction and operating peculiarities of centrifugal pumps with respect to reciprocating pumps, the greater simplicity of washing the former compared to the latter is clear, considering that:
         there is only one pumping body and not a plurality of in parallel bodies with the possibility of by-passing by the washing fluid or urea retention during the drainage phase, which make the operation unsafe;   they can be flushed in both directions, lacking in the internal check valves present in the reciprocating pumps.