Crystallizers

Abstract

The invention relates to improvements in crystallizers and more particularly, to the elutriation of the mix in a crystallization vessel, and measurement of the particle size distribution in the elutriated sample to obtain a control signal for controlling the parameters associated with the crystallization process.

Description

BACKGROUND OF THE INVENTION

This invention relates to improvements in crystallizers, and more particularly to apparatus for improving the control and stability of crystallizers.

Close control of particle size is, in many cases, hard to achieve by manual control and completely automatic control schemes are not as yet in common use.

The major difficulty encountered in controlling crystallizers, is the inherent time lag between the moment crystal nuclei are formed and the time these newly formed crystal nuclei reach a significant size, such that their total mass and surface area significantly affect the properties of the crystal mix. Thus, for example, if the supersaturation increases, the nucleation rate increases. After the nuclei have grown, their increased surface reduces the supersaturation thereby counteracting the initial upset. Due to the time lag involved between cause and effect, the system tends to overreact and the amount and size of the crystals tend to exhibit self-induced cyclic behavior which makes control even more difficult.

I have determined that as a prerequisite for any successful control of a crystallizer it is necessary to find a method to determine, promptly, any changes in the nucleation rate.

In one aspect of my invention, I elutriate the crystal magma and measure the properties of the elutriated fraction, containing only the small particles. This can be done either continuously or by periodic sampling. In crystallizers equipped with a fines trap, such an elutriation device is present. The feed to the fines trap primarily contains small crystals and the number and size of the crystals entering the fines trap gives a good indication of the most recent conditions of nucleation in the crystallizer. In other types of crystallizers, a small stream may be withdrawn through a standard elutriation device and the properties of the elutriated solution containing the small particles may be measured proximate the top of the elutriation device.

Monitoring of the total number of particles in the system is accomplished either by attenuation or scattering of radiation through the dispersion.

Attenuation is approximately proportional to the total surface area of the particles, and as the maximum size of the particles is limited by the elutriation device, the degree of attenuation is indicative of the number of small crystals present in the dispersion. By making this maximum size small as compared to the average crystal size, the time span in which a newly created nucleus affects the measuring device is small and I therefore provide a method for early detection of changes in the nucleation rate as discussed previously.

When light is used as the measuring device, attenuation by colored impurities may introduce numerous errors. These errors can be avoided by comparing the signal from the crystal mix to a signal obtained from a similar light source passing through a continuously filtered solution from the same crystallizer.

The number of small particles in the elutriated stream obtained by any of the above methods may then be utilized to control any of the independent parameters affecting the operation of the crystallizer, such as feed rate, flow of cooling water, vacuum in the evaporator, and the like. The correct set point of a controller for controlling any such parameter responsive to the signal from the elutriated stream can be found from a measurement of the particle size distribution and should be periodically readjusted for such measurements. Where a continuous measurement of the product particle size is available (such as is obtained by weighing the product streams leaving the sieves), then this measurement may be used to periodically or continuously adjust the set point of the utilized controller.

If a crystallizer is equipped with a fines trap, the control may be substantially improved by using the signal from the particle density measurement in the elutriated stream which, in this case, is the stream entering the fines trap, to control the flow through the fines trap itself. This will give a rapid response direct control on the most important variable in the operation of the crystallizer, namely the number of new particles formed. This direct continuous control of the flow through the fines trap by a measurement of the number of particles entering the fines trap, forms an important part of this invention.

Although efforts have in the past been expended in an effort to improve yield, it has been found that much of the difficulty with crystallizer yield persists despite attempts to rigidly control temperature and material inputs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide apparatus for rapidly analyzing the particle size of material mix in a crystallizer and to generate a signal dependent thereon, which signal is utilized to provide corrective action to restore the material mix to an optimum desired condition.

It is a further object of the invention to detect changes in the nucleation rate at an early stage.

It is still another object of the invention to provide a more efficient control system for crystallizers utilizing a fines trap.

It is another object of the invention to provide improved apparatus for use in crystallization, which apparatus will provide greater yield of the desired product with a lesser requirement for operator surveillance.

It is a further object of the invention to provide improved crystallizer apparatus which is economical to manufacture, requires a minimum of maintenance and surveillance.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will be obvious to those skilled in the art, when resor is had to the following specification and the drawings, wherein:

FIG. 1 is a view, partially in section, of a crystallizer incorporating one embodiment of the invention.

FIG. 1a is a view of a monitor or measuring device utilized with the embodiment of the invention shown in FIG. 1 of the drawing.

FIG. 2 is a view, partially in section, of a crystallizer incorporating a second embodiment of the invention.

FIG. 2a is a view of a comparator utilized with the embodiment of the invention shown in FIG. 2 of the drawing.

FIG. 2b is a block diagram of an elutriation monitoring device utilized to control the pump as shown in FIG. 2 of the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, wherein like reference numerals refer to similar elements in the two embodiments shown, a vessel 12 is provided with a feeding conduit 14 and a product retrieval conduit 16. A stirrer 18, which may be driven in a manner well known in the art is disposed in vessel 12.

An elutriation leg 20 extends from a port 22 in a lower portion of the vessel 12 to an input port 24 of monitoring or measuring device 26. An output port 28 of monitoring or measuring device 26 is connected by means of conduit 30 to an input port 32a of a pump 32, and output port 32b of pump 30 is connected by a conduit 34 to a port 36 in the upper reaches of vessel 12.

The elutriation leg 20 is formed of either a vertical or inclined pipe or conduit or an inverted cone in which the flow velocity is such that all large particles fall faster than the upward velocity materially reducing the passage of particles above a predetermined size through the output side of the elutriator.

Monitoring or measuring device 26 comprises a transparent conduit member 38 intermediate a radiation source 40 and a radiation detector 42.

In operation, a mix is introduced into vessel 12 through conduit 14. The mix is subjected to stirring by means of stirrer 18 and is heated by means not shown, but well known in the art. In order to obtain an indication of the nature of the new particle generation in the mix, a portion of the mix is elutriated under the action of pump 32 through elutriation leg 20, measuring or monitoring device 26, the pump itself and back into vessel 12 through conduit 34. In the loop described, the fine or smaller crystals flow through the loop, with the larger crystals returning to the vessel through gravitational action.

The state of the mix as it relates to the number of small particles is obtained by means of the measuring or monitoring device. A radiation source 40, which may radiate gamma rays or other suitable rays, such as a light source, X-rays and the like, is caused to direct its rays through the mix confined but flowing through the transparent tube 38. Radiation detector 42 is positioned relative to the transparent tube or conduit 38 so as to measure the radiation transmitted therethrough. The output of radiation detector 42, being a function of the transmissivity of the mix contained or flowing through the loop, the output of the radiation detector may be utilized to control one of the parameters which influences the mix makeup in the vessel. For example, the output of radiation detector 42 could be utilized to control a heating or cooling circuit associated with vessel 12, or feed rate or mix of source materials being fed into vessel 12.

Although no details are herein incorporated with respect to the control which may be exercised by the output signal from radiation detector 42, such control circuits are well known in the art, and require no experimentation in order to obtain the results desired.

In the embodiment disclosed in FIG. 2, it will be seen that applicable reference numerals have been applied to elements which are common to both embodiments. Within vessel 12, there is disposed a baffle 50, which is arranged to form an elutriating passage with the wall of vessel 12 to permit the mix within vessel 12 to be drawn off through port 52 into a monitoring or measuring device 54. The mix is drawn through device 54 by means of pump 56. The measuring device 54 is connected to vessel 12 by means of a conduit 53 and to pump 56 by means of a conduit 55. The mix is returned to vessel 12 after passage through a fines trap 58, which is connected to pump 56 through a conduit 57 and to vessel 12 by means of a conduit 59. The details of a fines trap are well known in the art, and the details thereof are not set forth herein.

The measuring or monitoring device 54, which is more fully shown in FIG. 2a comprises two paths, 60 and 62 respectively. Path 60 includes a filter 66 which is connected to conduit 53 at its input by means of a conduit 74. The output of filter 66 is permitted to enter a transparent tube or conduit 70 by means of a conduit 68. The other end of transparent tube 70 is connected to conduit 55 by means of a conduit 72.

Path 62 of the measuring or monitoring device 54 includes a conduit 74 which connects conduit 53 to a transparent tube or conduit 76, which conduit 76 is substantially similar to conduit 70. The other extremity of transparent tube or conduit 76 is connected directly to conduit 55 by means of a conduit 78.

Each conduit or transparent tube 70 and 76 is disposed between a radiation source and a radiation detector respectively. Tube 70 is disposed between radiation source 80 and radiation detector 82, and tube 76 is disposed between radiation source 84 and radiation detector 86.

In operation, a mix is elutriated through tube or conduit 53 and split between paths 60 and 62. The mix flowing through path 60 is caused to flow through filter 66 wherein the suspended particles are removed and the filtered solution is permitted to flow through transparent tube 70 and subjected to radiation source 80 and radiation detector 82. The radiation passing between the radiation source 80 and radiation detector is determined by the state of the solution passing through transparent tube 70. An output signal representative of the radiation transmissivity is obtained at terminals C and D respectively.

The mix flowing through path 62 is permitted to flow directly through transparent tube 76 and to conduit 55, and is also subjected to radiation passing between radiation source 84 and radiation detector 86. Again the radiation passing between radiation source 84 and radiation detector 86 is determined by the state of the particle mix on the elutriation suspension. An output signal representative of the mix leaving the elutriation section 50 is obtained at terminals A and B respectively.

The signals appearing at terminals A and B, and C and D respectively are connected to a comparator circuit, such as is well known in the art, and a difference signal is developed. This difference signal is a function of the variation of the mix from the desired mix and may be utilized to control the parameters of the system to bring the crystallizer back to or closer to the desired steady state conditions.

One form of control is shown in FIG. 2 and in FIG. 2b, wherein the signal from comparator 88 is fed to a controller 90 which controls the rate of flow generated by pump 56. The difference signal is also available to control other parameters of the system such as mix makeup, temperature and the like.

Although I have shown only certain preferred features of the invention by way of illustration, many modifications and changes will occur to those skilled in the art. For example I have made reference to the use of a radiation detector and source based on gamma ray radiation, and obviously without limitation X-rays or light rays may provide the radiation source which when associated with the companion detector will produce the signal or difference signal contemplated by the invention. It is to be understood, therefore, that the appended claims are intended to cover all such changes and modifications as fall within the true spirit and scope of my invention.

Claims

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. Crystallization apparatus including a vessel, means for introducing a mix to said vessel and means for retrieving a product from said vessel; and further including first means for circulating a portion of said mix from said vessel through monitor means and back to said vessel, said monitor means including a radiation source and a first radiation detector and a second radiation detector and a first passage for said mix pervious to the rays of said radiation source disposed intermediate said radiation source and said first radiation detector and adapted to provide a first signal at said radiation detector representative of the mix passing therebetween, and second means for circulating a portion of said mix through a filter and through a second passage for said mix pervious to the rays of said radiation source disposed intermediate said radiation source and said second radiation detector and adapted to provide a second signal at said second radiation detector, means for comparing said first and second signals to develop a third signal, and means responsive to said third signal for controlling selected parameters associated with said vessel.

2. Crystallization apparatus as defined in claim 1 and wherein said radiation source develops gamma rays and said radiation detectors are responsive to gamma rays.

3. Crystallization apparatus as defined in claim 1 and wherein said radiation source is a light source and said radiation detectors are responsive to rays generated from said light source.

4. Crystallization apparatus as defined in claim 1 and wherein said means for circulating a portion of said mix from said vessel through said monitor means and back to said vessel includes an elutriation leg between the vessel and said monitor means to provide a flow of the mix first through said elutriation leg and thence through said monitor means.

5. Crystallization apparatus as defined in claim 4 and wherein the elutriation leg comprises a conical member.

6. Crystallization apparatus as defined in claim 1 and wherein said filter comprises a fines trap.

7. Crystallization apparatus including a vessel, means for introducing a mix to said vessel and means for retrieving a product from said vessel; and further including means for monitoring the mix in said vessel, said monitoring means including means for elutriating a portion of the mix from said vessel, means for splitting said portion of said mix into a first portion and a second portion, means for passing said first portion through a filter and thence through a passage intermediate a radiation source and a radiation detector to develop a signal representative of the state of said first portion of said mix after it has passed through said filter, means for passing said second portion of said mix intermediate a second radiation source and a second radiation detector to develop a signal representative of said mix derived directly from said vessel, means for comparing said two signals to obtain a control signal; means for recombining the two portions of said mix, pump means for receiving said recombined portions and discharging the same to a fines trap; a controller for controlling said pump, and means responsive to said control signal in said controller to control said pump in a predetermined manner.

8. Crystallization apparatus as defined in claim 7 and wherein said radiation sources respectively each develop gamma rays, and said radiation detectors respectively are each responsive to gamma rays.

9. Crystallization apparatus as defined in claim 7 and wherein said radiation source is a light source and said radiation detector is responsive to rays generated from said light source.

10. Crystallization apparatus as defined in claim 7 and wherein said means for elutriating a portion of said mix from said vessel comprises an elutriation leg.

11. Crystallization apparatus as defined in claim 10 and wherein the elutriation leg comprises a conical member.

12. Crystallization apparatus including a vessel, means for introducing a mix to said vessel and means for retrieving a product from said vessel; and further including means for monitoring the mix in said vessel, said monitoring means including means for elutriating a portion of the mix from said vessel, means for splitting said portion of said mix into a first portion and a second portion, means for passing said first portion through a filter and thence through a passage intermediate a radiation source and a radiation detector to develop a signal representative of the state of said first portion of said mix after it has passed through said filter, means for passing said second portion of said mix intermediate a second radiation source and a second radiation detector to develop a signal representative of said mix derived directly from said vessel, means for comparing said two signals to obtain a control signal; means for recombining the two portions of said mix, pump means for receiving said recombined portions and discharging the same to said vessel; a controller for controlling said pump, and means responsive to said control signal in said controller to control said pump in a predetermined manner.

13. Crystallization apparatus as defined in claim 12 and wherein said radiation sources respectively each develop gamma rays, and said radiation detectors respectively are each responsive to gamma rays.

14. Crystallization apparatus as defined in claim 12 and wherein said radiation source is a light source and said radiation detector is responsive to rays generated from said light source.

15. Crystallization apparatus as defined in claim 12 and wherein said means for elutriating a portion of said mix from said vessel comprises an elutriation leg.

16. Crystallization apparatus as defined in claim 15 and wherein the elutriation leg comprises a conical member.