A METHOD FOR CONTROLLING A VIBRATING PRILLING BUCKET IN A UREA PRILLING PROCESS

Abstract

A method for controlling a prilling bucket in the prilling of a a urea melt (UM) including: feeding the urea melt to a prilling bucket (1) which vibrates under the action of a magnetostrictive device (2), wherein the vibration of the bucket is controlled, as a function of the amount of urea melt to be prilled, by the following steps: acquisition of a time-varying input signal (3) which represents the flow rate of urea melt fed to the prilling bucket; generation of a first signal (5) and of a second signal (6), independently from each other, as a function of said input signal (3); generation of a third signal (10) having a frequency which is modulated by said first signal and an magnitude which is modulated by said second signal, and use of said third signal to drive said magnetostrictive device (2).

Claims

1-14. (canceled)

15. A method for controlling a prilling bucket in a urea melt prilling process, wherein: an input flow of urea melt (UM) is processed by said prilling bucket; the prilling bucket rotates around a vertical axis and vibrates along said axis; the vibration of said bucket is caused by a magnetostrictive device, wherein the method includes that the vibration of the bucket is controlled, as a function of the rate of said input flow (UM), by the following steps: acquisition of a time-varying input signal which represents the time-varying flow rate of urea melt fed to the prilling bucket; generation of a first signal and of a second signal, independently from each other, as a function of said input signal; generation of a third signal, which is a harmonic signal, having a frequency which is modulated by said first signal and a magnitude which is modulated by said second signal, and use of said third signal to drive said magnetostrictive device.

16. The method according to claim 15, wherein the generation of said third signal includes: the first signal is sent to a function generator; said function generator outputs a harmonic signal with a given magnitude and a frequency modulated with the first signal; said output signal of said function generator and the second signal are fed to a voltage controlled amplifier wherein the magnitude of the signal from the function generator is modulated with the second signal, so that said amplifier outputs the third signal.

17. The method according to claim 16, including amplification of the third signal prior to its use to drive the magnetostrictive device.

18. The method according to claim 17, wherein the third signal is the source signal of a power amplifier connected to a power source or grid, and said power amplifier outputs a drive signal of the magnetostrictive device.

19. The method according to claim 15, wherein the frequency of the third signal is in the range up to 1000 Hz.

20. The method according to claim 15, wherein the frequency of said third signal, and therefore the frequency of vibration of the prilling bucket, is an increasing function of the flow rate within a control range of the flow rate.

21. The method according to claim 15, wherein the magnitude of the third signal is controlled in such a way that the amplitude of the mechanical vibration imparted to the prilling bucket is constant or substantially constant with respect to the flow rate.

22. The method according to claim 15, wherein the first signal and the second signal are generated by a distributed control system (DCS).

23. The method according to claim 22, wherein the DCS also governs a process of production of the urea melt.

24. The method according to claim 15, wherein only a side wall of the prilling bucket vibrates.

25. A system for controlling a vibrating prilling bucket in a prilling tower of a urea finishing section of a urea plant, the system being configured to implement the method of claim 15, wherein the prilling bucket comprises a magnetostrictive actuator and the system includes: a distributed control system (DCS) arranged to receive an input signal of the flow rate of urea melt which is fed to the prilling bucket, and configured to output a first signal and a second signal; a function generator which is fed with the first signal from the DCS and outputs a signal with a given magnitude and a frequency modulated with said first signal; a voltage controlled amplifier (VCA) which is configured to modulate the output of said function generator with the second signal, thus obtaining a third signal; a power amplifier, connected to a power source or grid, which amplifies the third signal to obtain a drive signal for the actuator of the prilling bucket.

26. A process of urea prilling with a prilling bucket, comprising: feeding an input flow of urea melt (UM) to said prilling bucket; rotating the prilling bucket around a vertical axis and vibrating said bucket along said axis; wherein the vibration of said bucket is caused by a magnetostrictive device, controlling the vibration of the bucket, as a function of the rate of said input flow (UM), by the following steps: acquisition of a time-varying input signal which represents the time-varying flow rate of urea melt fed to the prilling bucket; generation of a first signal and of a second signal, independently from each other, as a function of said input signal; generation of a third signal, which is a harmonic signal, having a frequency which is modulated by said first signal and an magnitude which is modulated by said second signal, and use of said third signal to drive said magnetostrictive device.

27. The method according to claim 19, wherein the frequency of the third signal is in the range of 200 to 1000 Hz.

28. The method according to claim 21, wherein the magnitude of the third signal is controlled in such a way that the amplitude of the mechanical vibration imparted to the prilling bucket is between 10 and 15 microns.

Description

DESCRIPTION OF FIGURES

[0035] FIG. 1 is a block scheme of an embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0036] FIG. 1 is a scheme of a system for controlling the vibration of a prilling bucket 1 which is installed in a urea prilling tower (not shown).

[0037] The prilling bucket 1 is fed with urea melt UM. In use, the bucket 1 rotates around an axis A and vibrates according to said axis A, which is a vertical axis, thanks to a magnetostrictive actuator 2.

[0038] The speed of rotation is normally set to about 150 to 200 rpm. The speed of rotation may also vary depending on the flow rate of the urea melt UM.

[0039] The magnetostrictive actuator 2 is connected to the prilling bucket 1 so that, when the actuator 2 is energized, the vibration of the actuator 2 is transmitted to vibrating parts of said bucket 1. The vibrating parts of the bucket 1 may include the perforated side surface 14 and possibly other parts.

[0040] Suitable magnetostrictive actuators are available from TdVib LLC, Iowa, US.

[0041] A suitable metering device M measures the flow rate (e.g. kg/s or m3/h) of the urea melt UM and provides a signal 3, which for example a 4-20 mA signal, to a DCS system 4. Said DCS system 4 may be installed on a suitable hardware in a control room of the urea plant which includes the above mentioned urea prilling tower where the bucket 1 is installed.

[0042] The DCS system 4 outputs a first analog signal 5 and a second analog signal 6. Both are for example 0 to 5 V.

[0043] The first signal 5 feeds a function generator (FGen) 7 which outputs a harmonic (i.e. sinusoidal) signal 8 having a constant magnitude, for example 1 Vrms, and a frequency in a given range, for example 0 to 1 kHz, which is modulated according to the feed signal 5.

[0044] A device suitable as function generator 7 is for example the model HMF2550 from the manufacturer Rohde&Schwarz.

[0045] The harmonic signal 8 now described and the second signal 6 from the DCS 4 feed a voltage controlled amplifier (VCA) denoted by the block 9.

[0046] Said VCA 9 modulates the magnitude of the harmonic signal 8 according to the signal 6. Therefore, the output of the VCA 9 is a harmonic signal 10 whose frequency depends on the control signal 5 and whose magnitude (e.g. voltage) depends on the control signal 6.

[0047] The VCA 9 may include, preferably, the chipset THAT 2162 from That Corporation, US.

[0048] The signal 10 is the source signal (e.g. 0 to 1 Vrms) of the amplifier and power generator 11 which is connected to a suitable power input 12 e.g. to the grid. The amplifier and power generator 11 gives a drive signal 13 (e.g. 0 to 130 Vrms) which keeps the input frequency and has the magnitude proportional to the one of signal 10. This signal 13 drives the magnetostrictive device 2.

[0049] As amplifier and power generator 11, the model Titan-Mac01 from Compact Power Co. may be used. It has to be noted that said amplifier and power generator 11 actually delivers the power necessary to drive the device 2.

[0050] The invention reaches the above mentioned aim of providing a real-time control of the vibration depending on the flow rate UM of urea melt. The drive signal of the magnetostrictive device 2 is continuously adjusted depending on the flow rate of urea melt which is acquired by means of the signal 3.

[0051] In use, the frequency of the mechanical vibration of the bucket 1 is equal to the frequency of signals 8, 10 and 13, in the range of practical interest (e.g. up to 1000 Hz).

[0052] The magnitude of the signal, for a target amplitude of vibration shall be calculated taking into account the impedance of the system. For example, given a target amplitude of the mechanical vibration, the corresponding magnitude of the signal 13 can be calculated by solving the system of the two dynamical equations of mechanics and electrics across the system.

[0053] A useful model of a magnetostrictive actuator is described in: Braghin F, Cinquemani S, Resta F: “A Linear Model of Magnetostrictive Actuators for Active Vibration Control”, presented at the 8th International Conference on Computing, Communications and Control Technologies (CCCT 2010).

[0054] Alternatively the relationship between the magnitude of signal 13 and the amplitude of vibration can be defined experimentally.

[0055] The following is an example. A prilling bucket processes a flow rate ranging from 15 mtph (metric tons per hour) to 45 mtph of urea melt. The frequency of vibration is controlled in a range from about 400 Hz at the smallest flow rate to about 900 Hz at the maximum flow rate. The frequency increases almost linearly according to the flow rate. The amplitude of the mechanical vibration imparted to the prilling bucket is between 10 and 15 microns and remains substantially constant with respect to the flow rate. The speed of rotation is also controlled ranging from 180 rpm at low flow rate to 200 rpm at the maximum flow rate.