JP2000334202A - Production of crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce a product having a desired average grain size by controlling the classification flow rate against the fluctuation of the average grain size of the product in order to suppress the continuous oscillatory phenomenon of grain size distribution in a crystallization apparatus. SOLUTION: When a product crystal having a desired average grain size is effectively produced, the continuous oscillation of the average grain size and the concentration of the slurry in a crystallization can are suppressed by controlling the classification flow rate according to the average grain size of the crystal which is pulled out from a product pulling out line, and the concentration of the slurry in the crystallization can and the average grain size of the product crystal are controlled by changing the processing range of the classification flow rate. At this time, the average grain size of the product crystal is controlled by continuously measuring in on-line from measured values obtained by a grain size monitor or the like which is provided at the product slurry pulling out line, and further the classification flow rate is controlled by a flow rate controlling valve which is disposed at a line for supplying classification flow to a classification leg. Thereby, the crystal having the desired average grain size is produced in a high yield and the manual operation for controlling the average grain size becomes non-essential.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for stably producing a product having a desired average particle size or improving the acquisition rate of coarse particles when producing crystal particles by a crystallization apparatus.

[0002]

2. Description of the Related Art In a crystallization process typified by manufacturing equipment for ammonium nitrite, nitrous acid, ammonium sulfate, sodium chloride, etc., the slurry concentration in a can, the crystal grain size distribution in a can, the product crystal, which is generally called a waving phenomenon, Continuous vibration phenomena such as particle size distribution occur. Since the particle size distribution constantly fluctuates due to this continuous vibration phenomenon, in actual operation, the product withdrawal flow rate etc. should be determined based on the observed values such as the slurry concentration in the crystallization can, the circulation line slurry concentration, and the product particle size distribution. By controlling the number of crystals in the can or the slurry concentration in the can by operating, the residence time of the crystals in the can is adjusted to maintain the acquisition rate of the desired average particle size, or the acquisition rate of the largest possible product depending on the product I was trying to do it.

However, the particle size distribution constantly fluctuates due to this continuous vibration phenomenon, and the state of the fluctuation is not always constant, and it is difficult for an operator to timely operate the particle size distribution fluctuation due to restrictions on manpower. In addition, it was unclear what the appropriate operation for the particle size distribution variation was, and not only was the burden on the operator of the adjustment operation large, but also it was possible to suppress the continuous variation of the particle size distribution, It has been difficult to stably produce product crystal particles of the following sizes with high yield. In view of the above, there has been a demand for a technology and an apparatus for automatically and stably producing product crystal particles of a desired size in a crystallization process with a high yield.

[0004]

The continuous oscillation of the crystal grain size distribution is an obstacle to the stable production of crystal grains of a desired size with a high yield. One reason for this is that the continuous vibration causes the average crystal grain diameter in the can to fluctuate, and the product acquisition rate fluctuates with the vibration cycle. Another is that the margin for the process constraint condition is reduced. Regarding the latter, since the slurry concentration in the crystallizer can fluctuates simultaneously with the continuous fluctuation of the particle size distribution, the range in which the condition of the slurry concentration in the can can be changed by operating operation is small due to the upper and lower limits of the slurry concentration in the crystallizer. Is an example. If the slurry concentration in the crystallization can is excessively increased, the contact between the crystals or the crystal and the in-can device increases, and the excessive generation of secondary nuclei increases the total crystal surface area, making it impossible to increase the crystal size. On the other hand, if the concentration is too low, the total surface area of the crystal becomes insufficient, so that the supersaturation in the can cannot be absorbed and a large amount of primary nuclei are generated. It will take a lot of effort and product loss to restore to the factory. Therefore, the operation must be performed so that the slurry concentration in the crystallizer falls within the upper and lower limits.

[0005] As described above, since there are various restrictions on the operating conditions, if there is continuous vibration of the product crystal grain size distribution, an operation amount is manipulated so that the center of the variation of the product crystal average grain size approaches a desired value. Even so, they are easily hampered by the constraints of the process conditions. In particular, when there is no target value and it is desired to obtain larger (larger) or smaller (smallest) crystal grains, the problem is significant.

[0006]

According to the present invention, in a crystallizer, a continuous oscillation phenomenon of a particle size distribution is suppressed by operating a classification flow rate with respect to a variation in a product average particle size. Provide a way.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the classification flow rate is one of the important operating factors. In the crystallization process, product crystals are extracted from a portion called a classification leg provided at the bottom of the crystallization can. In this classification leg, an ascending flow called a classification flow is supplied from the lowermost part, and large particles having a high sedimentation velocity can be selectively extracted. The flow rate of this classification flow is called a classification flow rate. By controlling the classification flow rate, the particle size distribution of the extracted crystal can be adjusted.

First, a method of operating a classification flow rate which is effective for suppressing the continuous vibration of the crystal grain size distribution in the crystallizer and the product withdrawn will be described. In order to suppress the continuous vibration such as the particle size distribution in the crystallizer and the crystal extraction line and the slurry concentration in the crystallizer, it is necessary to adjust the particle size distribution of the crystals to near a certain steady value. At this time, it is important to keep the number of crystals in the crystallizer as constant as possible. To this end, it is necessary to adjust the amount of microcrystals that repeatedly increase and decrease periodically.

It is said that the amount of microcrystals generally occurs when the average crystal grain size of the crystal withdrawal line is increasing, and reaches its peak shortly before the average crystal grain size becomes maximum. That is, the phase of the variation in the amount of microcrystals slightly precedes the variation in the average particle size. Therefore, the amount of fine crystals in the crystallization can can be adjusted by operating the classification flow rate in such a manner that the phase slightly precedes the fluctuation of the average particle diameter. That is, when the average crystal grain size of the crystal withdrawal line is increasing, the classification flow rate is reduced and the crystal slurry containing a large amount of fine crystals is actively extracted, and when the average crystal grain size is decreasing, the classification flow rate is increased. It increases and suppresses outflow of the microcrystal out of the system. By this operation, the number of microcrystals and the total number of crystals in the crystallizer are stabilized, and as a result, continuous vibration such as the particle size distribution in the crystallizer is suppressed.

[0010] To describe a specific control system, the control amount is the average particle size of the product crystal extracted from the crystallizer, and the operation amount is the classification flow rate. As described above, the change in the control amount is observed, and when the control amount is increasing, the operation amount is decreased, and when the control amount is decreasing, the operation amount is increased. The operation timing is opposite to the movement of the quantity, and the operation timing is one cycle of the control quantity variation
Preferably, the phase is advanced by / 8 to 3/8 cycle, and more preferably, the phase is advanced by 3/16 to 5/16 cycle.

The magnitude of the operation amount with respect to the control amount fluctuation is preferably as large as possible in order to suppress continuous vibration. However, in practice, problems such as saturation of the operation amount and sensitivity to noise are caused. There is a trade-off between the two in the implementation. Also, upper and lower limits are provided for the operation amount, and the operation is performed within the range. The flow rate of the classification flow used in the present method is preferably 0.015 to 0.25 m /
s.

Next, a method for efficiently producing a product crystal having a desired average particle size will be described. In general, it can be expected that, as the average of the average particle size variation approaches the target value, the rate of obtaining product crystals with a desired particle size can be improved regardless of the presence or absence of continuous oscillation of the particle size distribution. To adjust the average value of the average particle size,
It is effective to control the average residence time of the crystal particles in the crystallizer. As an operation factor for adjusting the average residence time of the crystal particles in the crystallizer, a crystal slurry extraction flow rate and an external heat exchange circulation flow rate may be considered. In the present invention, the classification flow rate is set. The larger the classification flow rate, the more difficult it is to extract crystals, especially small crystals, and the higher the slurry concentration in the crystallization can.

When combined with the above-described method of suppressing the continuous oscillation of the particle size distribution, the average value of the average crystal grain size is adjusted by adjusting the upper and lower limits of the classification flow rate operation range. The classification flow rate has a lower limit in order to prevent sedimentation of crystals at the lower part of the classification leg, while the value at which the concentration of the crystal slurry in the crystallization can reaches the upper limit is generally the upper limit value of the classification flow rate. The upper and lower limits of the operation are further set within this operable range, and the classification flow rate operation range is adjusted by setting the range.

In particular, when there is no target for the average value of the particle size distribution, and it is desired to produce a larger crystal or a smaller crystal, the continuous oscillation of the particle size distribution is suppressed, and a margin to the operation constraint value of various process amounts is provided. It is preferable to increase the flow rate and adjust the classification flow rate range until the operation limit value is reached. For example, when it is desired to improve the acquisition rate of a larger crystal, the classifying flow operation adjusted to the fluctuation of the average crystal grain size of the crystal extraction line suppresses the continuous vibration of the particle size distribution, and furthermore, the upper limit of the operation range of the classifying flow. Both lower limits are preferably set as large as possible within a range where the vibration suppression effect can be maintained.

The device used in the present invention includes at least the following devices. A) Crystallizer equipped with stirrer, external circulation heat exchanger, and classification leg. B) A device for calculating the classification flow rate or the set value of the classification flow control valve opening which is the manipulated variable based on the average particle size data of the crystals extracted from the crystallizer. C) A device that adjusts a classifying flow rate or a classifying flow control valve valve opening which is an operation amount to a given set value.

The average particle size of the crystal extracted from the crystallizer can be determined by offline sieving crystal weight distribution measurement, offline particle size distribution measurement by image processing, transmission to irradiation light, and online slurry average particle size measurement by reflected light amount measurement. And on-line slurry particle size distribution measurement using ultrasonic diffraction.

In the present invention, the classifying flow rate is selected as the manipulated variable. However, to control the amount of microcrystals in the crystallizer or the total number of crystals, the product crystal slurry liquid withdrawal flow rate, external heat exchange circulation flow rate, seed crystal addition It can be easily inferred that the seed crystal supply flow rate and the like can also be used in equipment having the apparatus. However, in terms of the control amount with respect to the operation amount, that is, the degree of influence on the extracted crystal average particle size, the classification flow rate is appropriate as the operation amount because it has good sensitivity and has a large operable range. However, in order to secure an operation margin of the classification flow rate, the product crystal slurry extraction flow rate may be adjusted in accordance with the change of the classification flow rate operation upper and lower limit set value.

Further, when the slurry concentration in the crystallization can is large as a whole, secondary nuclei are more frequently generated due to the contact between the crystals in the crystallization can or between the crystals and the equipment in the can, as compared to when the slurry concentration in the crystallization can is small. Therefore, in the process of adjusting the slurry concentration in the crystallization can, the rotation speed of the stirrer may be adjusted.

[0019]

EXAMPLES The present invention will be described more specifically with reference to the following examples. Using the ammonium sulfate crystallization process outlined in FIG.
Based on the average particle size of the crystals extracted from the product slurry extraction line, the classification flow rate is controlled to suppress the continuous oscillation of the average particle size and the slurry concentration in the crystallization can,
The operation range of the classification flow rate was changed to adjust the slurry concentration in the crystallization can and the average crystal grain size of the product. The product crystal average particle size is continuously measured on-line from a measured value of a particle size monitor or the like installed in a product slurry extraction line. The classification flow rate is adjusted by a flow control valve installed in a classification flow rate supply line to the classification leg.

FIGS. 2 and 3 show the results of controlling the classification flow rate so that the average particle diameter fluctuation is controlled so as to lead the 1/4 period phase in the opposite phase to the continuous fluctuation of the average particle diameter. 2 and 3, the display ranges of the process variables are aligned. It can be seen that when the classification flow rate was operated in accordance with the average particle size variation, the variation was suppressed to about 1/4 as compared with the case where it was not performed. FIG. 4 shows an example in which the classification flow rate operation range is further changed to change the slurry concentration in the crystallization can, and as a result, the average particle size is changed. In an actual operation, the classification flow rate operation range may be determined so that the average of the product crystal average particle diameter is closest to the target value within the range of the operation constraint. When it is desired to obtain a crystal having a larger or smaller average particle size, the upper and lower limits of the classification flow rate operation are increased or decreased as much as possible within the range of constraints such as the slurry concentration in the can.
However, at the same time, the setting of the rotation speed of the stirrer and the flow rate of the product slurry withdrawal may be adjusted.

FIG. 5 shows an example of an apparatus incorporating the functions of the present invention. A data input portion for manually inputting the crystal average particle diameter obtained by a hand sieve or the crystal average particle diameter measured by an online measuring device into a personal computer or a DCS (distributed control system); It is composed of a control output calculation device that calculates a set value for the classifying flow rate control device, which is an operation amount, based on the data, and a data output unit that outputs the operation value to the classifying flow rate control device. The classification flow control device may be any one of a DCS or a classification flow controller inside a personal computer, an external classification flow one-loop controller, or a classification flow control valve. Therefore, in the case of a flow rate controller, the flow rate target value is used, and in the case of a classification flow rate control valve, the valve opening set value is used as the output of the control output calculation device.

[0022]

According to the present invention, crystals having an arbitrary, larger or smaller average particle diameter can be obtained with a high yield in the operation of the crystallizer. In addition, manual operations for adjusting the average particle size can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing an example of a crystallization apparatus to which the present invention is applied.

FIG. 2 is an example of continuous oscillation of average particle size without classification flow timing operation.

FIG. 3 shows an example of continuous oscillation of the average particle size with a classification flow rate timing operation.

FIG. 4 is an example of a change in the average crystal grain size of the product by changing the upper and lower limits of the classification flow rate operation range.

FIG. 5 is an example of an apparatus configuration of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 Crystallizer 2 External circulation line 3 Heat exchanger 4 Circulation pump 5 Product extraction pump 6 Optical particle size detector 7 Circulation line slurry concentration meter 8 Slurry concentration meter in crystallization can 9 Stirrer 10 Raw material liquid supply 11 Classification flow rate Control valve

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Noboru Iida, 1-1 1-1 Kurosaki Castle Stone, Yawatanishi-ku, Kitakyushu Mitsubishi Chemical Corporation Kurosaki Office (72) Inventor Hironobu Takeda 1-1-1, Kurosaki Castle Stone, Yawata-nishi-ku, Kitakyushu Mitsubishi Chemical Kurosaki Office Co., Ltd.

Claims (3)

[Claims] 1. A method for producing a crystal, characterized in that in a crystallizer, a continuous oscillation phenomenon of a particle size distribution is suppressed by controlling a classification flow rate with respect to a variation in a product average particle size. 2. The method for producing a crystal according to claim 1, wherein a product having a desired average particle size is produced by changing the operation range of the classification flow rate. 3. A crystallization apparatus for producing a crystal using the method for producing a crystal according to claim 1.