GB2241783A - Crystallization monitor - Google Patents

Abstract

A crystallization monitor consists of a spherical steel sensor (23) arranged to be immersed in a flowing liquid (18) containing crystals, and connected by a waveguide rod (22) to a transducer (24). Whenever a crystal hits the sensor (23) a pulse of ultrasonic waves travels along the rod (22) so the transducer (24) generates an electrical pulse. The pulse height depends on the crystal size and the nature of the impact. The transducer is connected to pulse height analysis circuitry (34) to determine the distribution of pulse heights, which is connected to a computer (38) to determine therefrom the distribution of crystal sizes. <IMAGE>

Description

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Crystallization Monitor The invention relates to an instrument for monitoring a crystallization process.

Crystallization is a widely used process in the chemical industry. It may for example be performed in a crystallizing tank containing a saturated solution of the material from which the crystals grow, the tank being stirred and heated as crystallization proceeds. It is desirable to be able to monitor the number of crystals per unit volume, and the sizes of the crystals during the process, and one way of determining these quantities is to remove a sample of the solutidn and inspect the crystals in it, for example usinq a microscope. This approach does not allow real-time monitorinqr nor is it readilv adapted to automatic operation.

According to the present invention there is provided an instrument for monitoring a crystallization process occurring in a liquid, the instrument comprising a sensor immersible in the liquid and connected by a waveguide to an ultrasonic transducer, so the transducer produces an electrical pulse when a particle impacts with the sensor, pulse height analysis circuitry connected to the transducer to count the numbers of pulses of different heights and so determine the pulse height distribution, and means for determining from the pulse height distribution the size distribution of particles within the liquid.

Preferahly the waveguide is such that only one mode of wave propagates readily along it, other modes being damped in the preferred embodiment the waveguide is a rod comprising stainless steel and is coated with a layer of rubber, so that compression waves propagate along the rod but shear waves and surface waves are damped. The preferred frequency range of the waves is between about 20 kHz and 500 kHz, desirably between about 50 kHz and 2OO kHz, for example about 100 kHz.

The invention will now he further described by way of example only and with reference to the accompanying drawing which shows a diagrammatic view of a crystallization monitor and a crystallization tank, the latter being shown in section ín a vertical plane.

Referring to the drawing, a crystallization tank 10 is of generally cylindrical shape with a hemispherical bottom 11, and open at its top 12. A cylindrical open-ended baffle tube 13 is supported concentrically within it, and a stirrer 14 at the end of a drive shaft 15 is situated near the bottom 11, below the lower end of the baffle tube 13; the shaft 15 extends concentrically through the tank 10 and tube 13 to a motor 16 above the tank 10.In operation the tank 10 contains a solution 18 of the material of which crystals are to be formed, and the stirrer 14 is rotated by the motor 16 so the solution (and any crystals within it) circulate up the annular space outside the baffle tube 13 and then down inside the baffle tube 13, as indicated by the arrows A.

A probe 20 is mounted above the tank 10 so its lower end is immersed in the liquid 18 outside the baffle tube 13. The probe 20 consists of a stainless steel rod 22 of circular cross-section and of diameter 3 mm, to the lower end of which is welded a stainless steel sphere 23 of diameter 20 mm. The upper end of the rod 22 is connected to an ultrasonic transducer 24, and the entire length of the rod 22 between the sphere 23 and the transducer 24 is covered by a tube 25 of rubber 2 mm thick which is glued onto the rod 22.If during operation of the tank 10 a crystal of the material, carried by the circulatinq flow of the liquid 18, hits the sphere 23 this causes a pulse of ultrasonic vibrations within the sphere 23 which propagate up the waveguide rofl 22 to the transducer 24, so creating a corresponding electrical pulse. Because shear and surface waves are damped by the rubber tube 25, the pulse received by the transducer 24 consists only of compression waves, and so is of short duration, typically ringing for only about 50 microseconds.

These pulses are supplied to a pulse height analyser 34 which sorts them by heiqht, so creating a pulse height distribution, or histogram. This distribution is analysed by a computer 36 to determine from it the distribution of sizes of crystals within the liquid 18. This analysis could take place along the following lines. The pulses of greatest height are considered first; these must be due to the largest crystals hitting the sphere 23 substantially along a normal (i.e. head-on collisions). This indicates the size and number of the largest crystals present.From a previously-obtained knowledge of the distribution of pulse heights to be expected if all crystals were of this same size, an appropriate distribution can be subtracted from the measured distribution, to leave a new pulse height distribution due to all except the largest crystals. This new distribution is then treated in the same way to find the size and number of the next-largest crystals present.

Subtraction of an appropriate distribution leaves another new pulse height distribution due to all except the two largest sizes of crystals. This process is repeated to obtain the complete distribution of all the crystal sizes.

It will be appreciated that the apparatus may differ from that described while remaining within the scope of the invention. In particular the size and shape of the probe 20 may differ from that described; and the wave mode to which it is designed to be sensitive might be a mode other than compression waves, for example Lamb waves, in which case the shape of the wavequide would differ from that described. The portion of the probe 20 against which the crystals hit might not be spherical, it could for example be a flat plate arranged to lie in a plane at a non-zero angle (preferably 90 degrees) to the expected direction of flow of the liquid.

Claims (1)

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   Claims

   1. An instrument for monitorinq a crystallization process occurring in a liquid, the instrument comprising a sensor immersible in the liquid and connected by a waveguide to an ultrasonic transducer, so the transducer produces an electrical pulse when a particle impacts with the sensor, pulse height analysis circuitry connected to the transducer to count the numbers of pulses of different heights and so determine the pulse height distribution, and means for determining from the pulse height distribution the size distribution of particles within the liquid.

   2. An instrument as claimed in Claim 1 wherein the waveguide is such that only one mode of wave propagates readily along it, other modes being damped.

   3. An instrument as claimed in Claim 2 wherein the waveguide comprises a rod comprising stainless steel coated with a layer of rubber, so that compression waves propagate along the rod but shear waves and surface waves are damped.

   4. An instrument as claimed in any one of the preceding Claims wherein the transducer is sensitive to waves of frequency between 50 kHz and 200 kH .

   5. An instrument for monitoring a crystallization process occurring in a liquid substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.