Centrifugal pump for processing molten urea and related equipment

Abstract

The present application discloses a centrifugal pump (100) for treating molten urea, which includes: an inlet (102) for receiving molten urea at a suction pressure; a delivery outlet (104) for being able to output molten urea at a delivery pressure greater than the suction pressure; and an intermediate outlet (103) for being able to output molten urea at an intermediate pressure greater than the suction pressure and less than the delivery pressure. The present application also discloses a device (1) for treating molten urea.

Description

Technical Field

[0001] The present invention relates to the field of industrial treatment of urea, particularly for the production of melamine. More specifically, the present invention relates to a centrifugal pump for treating molten urea and related equipment, particularly for producing melamine by high-pressure treatment. Background Art

[0002] A method for producing melamine by pyrolysis of urea is known according to the overall reaction (1):

[0003] 6 NH2CONH2 → (CN)3(NH2)3 + 6 NH3 + 3 CO2 (1)

[0004] Urea Melamine

[0005] It is well known that this reaction is highly endothermic.

[0006] The treatment for converting urea into melamine is divided into two categories: treatments that perform high-pressure urea pyrolysis and treatments that perform low-pressure urea pyrolysis.

[0007] Both treatments are usually carried out in a reactor that is fed by a molten urea stream. Preferably, the reactor is also fed by an ammonia stream.

[0008] In the high-pressure treatment, the reaction chamber is always maintained at a pressure greater than 60 bar and is provided with heating means that keep the reagent system at a temperature of approximately 360°C - 450°C.

[0009] The present invention particularly relates to the treatment for high-pressure preparation of melamine. More specifically, in order to produce melamine, it is necessary to treat liquid urea with a concentration greater than 99.8% and pressurize it, i.e., the liquid urea is molten, which is obtained from a vacuum separator, at which time the urea is supplied to the equipment as an aqueous solution and needs to be concentrated inside the equipment itself, or the liquid urea is obtained from a surge drum maintained at a slight pressure, at which time the urea is supplied to the melamine equipment when it has been concentrated, and is supplied to a reactor operating at a pressure of approximately 80 bar.

[0010] Currently, in order to pressurize the molten urea from the separator to the reactor, a reciprocating pump is used. The market demands a continuous increase in the production capacity of the equipment for producing melamine, which determines a proportional increase in the urea flow rate to be processed in the equipment, resulting in an increase in the capacity and number of the reciprocating pump heads used, and often more pumps are used in parallel, resulting in an increase in the complexity, cost, and risk of failure of the currently used reciprocating pumps, and affecting the operation of the equipment itself.

[0011] The use of reciprocating pumps causes many drawbacks.

[0012] The essence of a reciprocating pump is that there are valves at the inlet (suction) and outlet (delivery) in each head, which control their operation. The presence of such valves along the suction flow of the pump (especially at the inlet of each head) is a bottleneck, which leads to increased velocity and load loss, and the subsequent reduction inside the valve causes any gas dissolved in the liquid phase to be released, risking cavitation at the pump inlet.

[0013] Molten urea undergoes continuous (although minimal) decomposition to form ammonia and carbon dioxide, which remain dissolved in the liquid phase under constant pressure conditions, but are released as a gas phase when the pressure drops, thereby causing cavitation, which will damage the normal operation of the reciprocating pump, thus reducing their performance and often leading to mechanical failures. Therefore, reciprocating pumps need to use additional pumps arranged upstream to pre-compress the molten urea and prevent cavitation at the inlet of the reciprocating pump.

[0014] Moreover, reciprocating pumps inherently have a pulsating delivery pressure, which requires the use of accumulators and other devices capable of damping pulsations and / or operating under pulsating pressure downstream of the reciprocating pump. Moreover, in the production of melamine, this factor affects the reactor subjected to pulsed supply, which forms pulsating reaction gases (see reaction), resulting in subsequent internal pulsating pressure on the gas discharged under pressure control, and also causing pulsating flow velocity on the components used to process the gas.

[0015] The melting temperature of urea is equal to 133 °C. To avoid urea solidification, the pump needs to be drained and washed after it stops. A reciprocating pump with multiple heads arranged in parallel needs to drain and wash each head separately, so they require a complex drainage and washing system. Such a system is not conducive to the reliability of the pump, increases the possibility of leakage and blockage, and makes these operations particularly time-consuming for operators who wish to completely omit performing them.

[0016] Moreover, the start-up of a reciprocating pump causes the pressure in its own delivery to suddenly increase, which risks causing faults or damaging downstream components, especially for the shut-off / check valve, which keeps the reactor under pressure during the urea pre-feed step, where the pressure of the reactor is maintained by a constant supply of ammonia. To overcome this problem, the delivery device is usually pre-pressurized in advance, thus increasing the complexity of the equipment and its start-up procedure.

[0017] In addition, some devices require a standby pump, which is arranged in parallel with the main pump and remains on standby, ready to start when the operating pump stops. In order to keep the standby pump on standby, it needs to be emptied and continuously flushed with steam to ensure that urea does not accumulate inside and its subsequent degradation (which causes blockage) in case of leakage of the stop valve. Alternatively, the standby pump needs to be pre-filled with molten urea to ensure its internal circulation to avoid solidification. In a reciprocating pump, since multiple heads are not flushed separately, it is very complicated to establish recirculation inside the pump and it is very difficult to obtain it in a uniformly distributed manner unless a complex circulation system is established for each head, which results in increased operating operations and a large number of possible urea leakage points to the outside. Summary of the Invention

[0018] The general object of the present invention is to improve the prior art with reference to one or more viewpoints.

[0019] In particular, the object of the present invention is to overcome the above-mentioned disadvantages by using a centrifugal pump for treating molten urea and related equipment, which is especially used for producing melamine by high-pressure treatment.

[0020] More specifically, the object of the present invention is to provide a centrifugal pump for treating molten urea and a related equipment that can handle a larger amount of molten urea than the prior art, especially a centrifugal pump having the main conditions required for supplying molten urea to a production reactor.

[0021] Another object of the present invention is to provide a centrifugal pump for treating molten urea and a related equipment, which are less complex than the existing equipment.

[0022] Another object of the present invention is to provide a centrifugal pump for treating molten urea and a related equipment, which have less risk of failure and lower component wear.

[0023] These objects are basically achieved by the solutions described in the present application.

[0024] According to a first aspect, the present invention relates to a centrifugal pump for treating molten urea, the centrifugal pump having:

[0025] - an inlet for receiving molten urea at a suction pressure;

[0026] - a delivery outlet for outputting molten urea at a delivery pressure greater than the suction pressure;

[0027] - an intermediate outlet for outputting molten urea at an intermediate pressure greater than the suction pressure and less than the delivery pressure.

[0028] According to a second aspect, the present invention relates to an apparatus for processing molten urea, in particular for producing melamine by high-pressure treatment, the apparatus comprising:

[0029] - a separator for molten urea, the separator having a main inlet, a recirculation inlet and an outlet for molten urea;

[0030] - at least one centrifugal pump;

[0031] wherein the inlet of the centrifugal pump is fluidly connected to the outlet of the separator for molten urea so as to receive molten urea from the separator at the inlet,

[0032] wherein the recirculation inlet of the separator is fluidly connected to an intermediate outlet of the centrifugal pump so as to receive recirculated molten urea from the centrifugal pump at the inlet. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will become more apparent from the following detailed description in conjunction with the accompanying drawings, in which:

[0034] Figure 1 shows a schematic diagram of an apparatus for processing molten urea according to the present invention;

[0035] Figure 2 shows a schematic diagram of a centrifugal pump for processing molten urea according to the present invention;

[0036] Figure 3 shows a schematic diagram of details of an apparatus for processing molten urea according to a possible embodiment of the present invention;

[0037] Figure 4 shows a schematic diagram of details of an apparatus for processing molten urea having the possibility of internal circulation;

[0038] Figure 5 shows a schematic diagram of details of an apparatus for processing molten urea having the possibility of flushing;

[0039] It will be readily understood that in fact there are several ways to implement the present invention, which are not limited by the following detailed description or the drawings. DETAILED DESCRIPTION OF THE INVENTION

[0040] Referring to the accompanying drawings, the apparatus for processing molten urea for the purpose of the present invention is generally designated by reference numeral 1 and hereinafter referred to as the "apparatus 1" for short.

[0041] Device 1 includes a vacuum separator 10 for melting urea. The vacuum separator 10 is provided with a main inlet 11, an outlet 12 for molten urea, a recirculation inlet 13, and an outlet 14 for gas. Preferably, the recirculation inlet 13 is arranged at or near the upper end of the separator 10, and the outlet 12 for molten urea is arranged at or near the lower end of the separator 10.

[0042] Device 1 further includes a centrifugal pump for treating molten urea. This centrifugal pump forms a further object of the present invention and will be denoted hereinafter by reference numeral 100 and referred to as "centrifugal pump 100".

[0043] 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 is fluidly connected in operation to the outlet 12 of the separator 10 for molten urea so as to receive molten urea from it through a suction conduit 32. The delivery outlet 104 corresponds to the delivery of the centrifugal pump 100 and can be fluidly connected in operation to a reactor 20 for producing melamine from urea so as to supply a certain flow rate of molten urea through a delivery conduit 34.

[0044] The centrifugal pump 100 is arranged to receive molten urea at a preselected suction pressure at the inlet 102 and is capable of outputting molten urea at a delivery pressure greater than the suction pressure at the delivery outlet 104. Preferably, the suction pressure is between 0.5 bar and 5 bar, even more preferably between 1 bar and 2 bar, especially about 1.5 bar. Such a suction pressure is determined by a higher height (relative to the inlet 102) of the separator 10 (which operates under conditions of absolute vacuum or slight overpressure) above ground level. Therefore, the height difference between the separator 10 and the pump inlet 102 must ensure a positive pressure under any circumstances.

[0045] Preferably, the delivery pressure is between 60 bar and 100 bar, even more preferably between 70 bar and 90 bar, especially about 81 bar. Therefore, the centrifugal pump 100 is generally preferably between 60 bar and 100 bar, even more preferably between 70 bar and 90 bar, especially about 80 bar.

[0046] Preferably, the centrifugal pump 100 is arranged to handle a molten urea flow rate greater than 15 m 3 / h, even more preferably greater than 25 m 3 / h.

[0047] Preferably, the centrifugal pump 100 includes a heating system arranged to maintain the internal temperature of the molten urea greater than a preselected value, especially greater than its melting temperature (equal to about 133 °C), so as to avoid solidification inside the centrifugal pump 100 itself.

[0048] Preferably, the centrifugal pump 100 is multi-stage and includes a plurality of centrifugal stages 110 arranged in series. Generally, each centrifugal stage 110 has a compression chamber and an impeller disposed inside the compression chamber, as Figure 2 shown therein.

[0049] In particular, the plurality of centrifugal stages 110 includes: a first stage 112, which is disposed at the inlet 102 to receive molten urea therethrough; and a final stage 114, which is disposed at the delivery outlet 104 to send molten urea thereto to the reactor 20.

[0050] Between the first stage 112 and the final stage 114, the pressure of the molten urea gradually increases as it passes through each centrifugal stage 110.

[0051] The plurality of centrifugal stages 110 includes an intermediate pressure stage 113, which is operably arranged in series between the first stage 112 and the final stage 114.

[0052] In particular, the intermediate pressure stage 113 has an intermediate outlet 103, which is arranged and configured such that molten urea can be output from the stage at an intermediate pressure greater than the suction pressure and lower than the delivery pressure. Preferably, the intermediate pressure is between 3 bar and 15 bar, and even more preferably between 5 bar and 10 bar. In other words, the intermediate pressure is greater than the suction pressure by between 2 bar and 14 bar, and more preferably between 4 bar and 9 bar.

[0053] The value of the intermediate pressure depends on the series arrangement of the intermediate pressure stage 113 among the plurality of centrifugal stages 110. Preferably, the intermediate pressure stage 113 is a centrifugal stage disposed between the first stage 112 and the final stage 114. In some embodiments, the stage at the intermediate pressure stage 113 can coincide with the first stage 112.

[0054] Preferably, the size and configuration of the intermediate outlet 103 are set such that the flow rate of the output molten urea is between 50% and 75% of the molten urea at the inlet, and even more preferably between 60% and 70%, and particularly about 67%.

[0055] Moreover, the intermediate outlet 103 is preferably located at the upper end of the intermediate pressure stage 113 during operation to allow gas extraction or discharge. Molten urea continuously generates reaction gases, particularly ammonia and carbon dioxide. Therefore, the intermediate pressure stage 113 is configured to be able to convey the gases generated by urea through the intermediate outlet 103 during movement, with the operational advantages of the subsequent compression stage.

[0056] Preferably, the intermediate pressure stage 113 has a storage volume 115 which is arranged above the impeller in the operating configuration and is in fluid communication with the compression chamber. In particular, the centrifugal pump 100 has a housing which is provided with a domed portion 116 above the impeller of the intermediate pressure stage 113 in the vertical direction so as to define the above-mentioned storage volume 115. The intermediate outlet 103 is arranged at the upper end of the domed portion 116.

[0057] The storage volume 115 is arranged to receive molten urea and gas from the compression chamber of the intermediate pressure stage 113 during the operation of the centrifugal pump 100 so as to allow it to be introduced through the intermediate outlet 103.

[0058] During operation, the intermediate outlet 103 is fluidly connected to the separator 10 through the recirculation pipe 33, thereby causing a recirculation flow of molten urea from the centrifugal pump 100 to the separator 10 so as to prevent solid residues from accumulating inside it. In particular, the recirculation pipe 33 is fluidly connected to the recirculation inlet 13 of the separator 10. The supply pipe 32 is connected to the outlet 12 for molten urea. This configuration can define a continuous flow of molten urea from the recirculation inlet 13 through the entire extension of the separator 10 to the outlet 12 for molten urea.

[0059] The apparatus 1 further includes a reactor 20 for producing melamine, which is fluidly connected to the delivery outlet 104 of the centrifugal pump 100 so as to receive pressurized molten urea from it through the delivery pipe 34.

[0060] According to a possible embodiment, the centrifugal pump 100 has a variable speed so as to regulate the flow rate of the molten urea directed to the reactor 20.

[0061] According to a possible alternative embodiment, the centrifugal pump 100 has a fixed rotational speed, and the apparatus 1 includes a control valve which is arranged along the delivery pipe 34 between the delivery outlet 104 of the centrifugal pump 100 and the reactor 20 so as to regulate the flow rate of the molten urea directed to the reactor 20.

[0062] Preferably, the apparatus 1 includes two of the above-mentioned centrifugal pumps 100 which are arranged redundantly in parallel, as Figure 3 shown, so as to compress the molten urea directed from the separator 10 to the reactor 20. One of the two centrifugal pumps 100 can be set to standby while the other is in operation.

[0063] A plurality of valves 40 are arranged along the suction and delivery pipelines of the two valves so that the centrifugal pumps 100 can be connected in series between the separator 10 and the reactor 20 during operation and be isolated from each other. Preferably, the device 1 further includes a first flushing pipeline 41, which is controlled by the relevant valve 42 and arranged to be fluidly connected between the delivery outlets 104 of the two centrifugal pumps 100, thus bypassing the valve 40. Moreover, the device 1 can include a second flushing pipeline 43, which is controlled by the relevant valve 44 and arranged to be fluidly connected between the inlets 102 of the two centrifugal pumps 100, thus bypassing the valve 40.

[0064] Figure 4 An example of the operating configuration of the device 1 is shown, where the centrifugal pump 100A is running, the centrifugal pump 100B is on standby, the valves 40A, 42 and 44 are open, and the valve 40B is closed.

[0065] In the absence of the second flushing pipeline 43, the recirculation of the fluid in the standby centrifugal pump 100B can be achieved by keeping the valve 40B upstream (or at the suction) of the standby centrifugal pump 100B open or partially open.

[0066] Preferably, the first flushing pipeline 41 and the second flushing pipeline 43 can establish the flow of molten urea from the delivery outlet 104 of the operating centrifugal pump 100 to the inlet 102 of the other standby centrifugal pump 100 during the operation of one of the two centrifugal pumps 100, so as to enable internal circulation in the standby pump and reach the suction pipeline 102 of the operating pump.

[0067] Through this internal circulation in the standby pump, the decomposition and solidification that may occur due to the retention of molten urea inside it are avoided.

[0068] Preferably, the device 1 further includes a system for washing one or more centrifugal pumps 100 after they are shut down. An example of this device 1 is given in Figure 5 below.

[0069] The washing of the centrifugal pump 100 includes emptying its molten urea by draining it into the recovery manifold, then flushing with steam and then with water, and the water is recycled to avoid environmental pollution. For this purpose, the device 1 includes a plurality of inlet parts 45 and 46, which are fluidly connected to the centrifugal pump 100 to enable the centrifugal pump to be emptied and flushed. In particular, the device 1 includes a first inlet part 45 upstream of the centrifugal pump 100 and a second inlet part 46 downstream of each centrifugal pump 100. Preferably, the first and second inlet parts 45, 46 include quick connectors equipped with valves, which can quickly connect the discharge and / or flushing pipelines to perform these operations.

[0070] These first and second inlet portions 45 and 46 are capable of entering any area where the molten urea may have solidified, and the molten urea tends to solidify at dead spots, i.e., in areas upstream of the quick-connect valve where there is no product circulation and renewal.

[0071] The described embodiment overcomes the limitations of the prior art.

[0072] Using a centrifugal pump with these features (instead of a traditional reciprocating pump) can handle a larger amount of molten urea without having to use a large number of reciprocating heads or pumps in parallel.

[0073] Using a centrifugal pump with these features prevents cavitation in the suction and can pre-compress the molten urea without using an additional pump.

[0074] The centrifugal pump can send a recirculation flow rate to the vacuum separator (when present) at an appropriate pressure to prevent the accumulation of solid residues without the need for a dedicated pump.

[0075] The centrifugal pump can also avoid equipment problems caused by the pulsating pressure of the reciprocating pump during delivery.

[0076] The use of the described solution can create a simpler device with less risk of failure and lower component wear.

[0077] Comparing the structural and operating characteristics of centrifugal pumps and reciprocating pumps, the washing of centrifugal pumps is clearly simpler than that of reciprocating pumps, considering that:

[0078] - There is only one pump body instead of multiple parallel bodies that may be bypassed by the washing fluid or have urea retention during the discharge stage, which makes the operation unsafe;

[0079] - They can be flushed bidirectionally, while there is no internal check valve in the reciprocating pump.

Claims

1. An apparatus (1) for processing molten urea, the apparatus comprising: - A separator (10) for molten urea, the separator (10) having an outlet (12) for molten urea, a main inlet (11), and a recycle inlet (13); - At least one centrifugal pump (100); wherein the inlet (102) of the centrifugal pump (100) is fluidly connected to the outlet (12) of the separator (10) for molten urea so as to receive molten urea from the separator (10) at the inlet of the centrifugal pump; The recycle inlet (13) of the separator (10) is fluidly connected to the intermediate outlet (103) of the centrifugal pump (100) so as to receive recycled molten urea from the centrifugal pump (100) at the recycle inlet; The centrifugal pump (100) for processing molten urea has: - An inlet (102) for receiving molten urea at a suction pressure; - A delivery outlet (104) for outputting molten urea at a delivery pressure greater than the suction pressure; - An intermediate outlet (103) for outputting molten urea at an intermediate pressure greater than the suction pressure and less than the delivery pressure.

2. The device according to claim 1, wherein, The centrifugal pump (100) further includes: A plurality of centrifugal stages (110) arranged in series; The plurality of centrifugal stages (110) includes: - An intermediate pressure stage (113) arranged and / or configured such that the pressure of the molten urea is equal to the intermediate pressure; The intermediate outlet (103) is arranged at the intermediate pressure stage (113) so as to receive molten urea from the intermediate pressure stage (113).

3. The device according to claim 2, wherein: The plurality of centrifugal stages (110) includes: - A first stage (112) arranged at the inlet (102) so as to receive molten urea from the inlet (102); - A final stage (114) arranged at the delivery outlet (104) so as to deliver molten urea to the delivery outlet (104); The intermediate pressure stage (113) is arranged between the first stage (112) and the final stage (114).

4. The device according to claim 2, wherein: In the centrifugal pump (100), the intermediate outlet (103) is arranged at the upper end of the intermediate pressure stage (113) so as to enable gas extraction from the intermediate pressure stage (113) through the intermediate outlet (103).

5. The device according to claim 4, wherein: In the centrifugal pump (100), the intermediate pressure stage (113) has a compression chamber, an impeller arranged in the compression chamber, and a storage volume (115) arranged above the impeller, the storage volume (115) being in fluid communication with the compression chamber; The storage volume (115) is arranged to receive molten urea and gas from the compression chamber during operation of the centrifugal pump (100), and the intermediate outlet (103) is arranged at the upper end of the storage volume (115).

6. The device according to claim 5, wherein, The centrifugal pump (100) further comprises: a housing provided with a domed portion (116) that defines the storage volume (115).

7. The apparatus according to any one of claims 1 - 6, wherein, The centrifugal pump (100) further comprises: a heating system for maintaining the temperature of the molten urea inside the centrifugal pump (100) above a predetermined value.

8. The device according to any one of claims 1-6, wherein The centrifugal pump (100) is used to process molten urea with a flow rate greater than 15 m 3 / h, wherein the value by which the delivery pressure is greater than the suction pressure is included between 50 bar and 100 bar.

9. The device according to any one of claims 1-6, wherein, In the centrifugal pump (100), the value by which the intermediate pressure is greater than the suction pressure is between 2 bar and 14 bar, and the centrifugal pump (100) is arranged to direct molten urea having a flow rate between 50% and 75% of the flow rate of the molten urea at the inlet through the intermediate outlet (103).

10. The device (1) according to claim 1, further comprising: A reactor (20) for producing melamine in a high-pressure process, the reactor (20) being in fluid connection with the delivery outlet (104) of the centrifugal pump (100) so as to receive pressurized molten urea from the centrifugal pump (100).

11. The apparatus (1) according to claim 10, wherein: the centrifugal pump (100) has a variable speed in order to adjust the flow rate and / or pressure of the molten urea directed to the reactor (20); or the centrifugal pump (100) has a fixed rotational speed, and the apparatus (1) comprises a regulating valve operatively arranged between the delivery outlet (104) of the centrifugal pump (100) and the reactor (20) so as to adjust the flow rate and / or pressure of the molten urea directed to the reactor (20).

12. The device (1) according to any one of claims 1 to 6 and 10 to 11, wherein, The at least one centrifugal pump is two centrifugal pumps (100), the two centrifugal pumps (100) being arranged in parallel, one of the two centrifugal pumps (100) being able to be set to standby while the other is in operation.

13. The device (1) according to claim 12, further comprising: A first flushing line (41) for fluid connection between the delivery outlets (104) of the two centrifugal pumps (100); and a second flushing line (43) for fluid connection between the inlets (102) of the two centrifugal pumps (100); In the operating state of the apparatus (1), the operating centrifugal pump (100) is used to cause molten urea to flow from the delivery outlet (104) to the inlet (102) of the centrifugal pump (100) in standby through the first flushing line (41) and the second flushing line (43).

14. The device (1) according to any one of claims 1 to 6, further comprising: At least one first inlet portion (45) fluidly connected to the at least one centrifugal pump (100) upstream of the at least one centrifugal pump (100); and at least one second inlet portion (46) fluidly connected to the at least one centrifugal pump (100) downstream of the at least one centrifugal pump (100); The first inlet portion (45) and the second inlet portion (46) are for discharging and / or flushing the at least one centrifugal pump (100).

15. The device (1) according to claim 1, wherein, The apparatus is for producing melamine by high-pressure treatment.

16. The apparatus according to claim 8, wherein, The centrifugal pump (100) is used to process molten urea with a flow rate greater than 25 m 3 / h.

17. The apparatus according to claim 8, wherein, The value by which the delivery pressure is greater than the suction pressure is between 70 bar and 90 bar.

18. The device according to claim 9, wherein The value by which the intermediate pressure is greater than the suction pressure is included between 4 bar and 9 bar.

19. The device according to claim 9, wherein, The centrifugal pump (100) is arranged to direct molten urea having a flow rate between 60% and 70% of the flow rate of molten urea at the inlet through the intermediate outlet (103).

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