JPH0776085B2 - Salt crystallization control method - Google Patents

Description

TECHNICAL FIELD The present invention relates to improvement of controllability of a crystal growth control method for a continuous salt crystallizer.

<Prior Art> An example of the prior art will be described with reference to FIG. Reference numeral 1 is a crystallizer, and B is a mother liquor in the crystallizer. This mother liquor is sent from the lower part 101 of the crystallizer to the circulation line 2a by the pump 3,
The steam S is heated through the heat exchanger 4 which is heated through the circulation pipe 2b, and returned to the crystallization can 1 through the circulation pipe 2c.

The salt crystals C accumulated on the bottom of the lower portion 101 of the crystallizer are discharged by the pump 5 to the discharge pipe 6 and guided to the salt collecting device 7. Reference numeral 8 is a control valve inserted in the discharge conduit.

Reference numeral 9 denotes a seed crystal supply device, which supplies a seed crystal in the middle of the circulation pipe 2b via the seed crystal supply pipe 10 and the control valve 11.

12 is a feeder for the brine W, which is concentrated salt water,
It is supplied to the middle of the circulation pipe 2a via the brine supply pipe 13 and the control valve 14.

Reference numeral 15 is a level sensor for measuring the level L of the mother liquor B in the crystallizer 1, and PV _L is the measured value. Reference numeral 16 is a level controller, which transmits an operation output MV _L that controls and calculates the deviation between the level measurement value PV _L and the level set value SV _L , and adjusts the opening degree of the control valve 14 that controls the supply amount of the brine W. .

The reason why the level of the crystallizer is kept constant is that the volume of crystal is fixed when the brine supply amount and concentration are stable, the amount of steam in the heat exchanger is almost constant, and the vacuum value of the crystallizer is also constant. When the ratio is 6 to 10%, the specific gravity of the mother liquor B becomes constant, and the constant specific gravity of the mother liquor controls the crystal growth at a fixed rate.

<Problems to be Solved by the Invention> However, in the actual plant by the conventional method, the brackish water is supplied in accordance with the heating and evaporation by the constant value control of the level. It will always change due to fluctuations in the value.

When operated in such a state, fluctuations in the vapor amount and vacuum value affect the crystal growth, the crystal growth rate changes with the fluctuations, and the average grain shape of the crystals in the mother liquor changes. In this case, new crystals (pseudocrystals) are likely to occur in addition to the seed crystals,
It deteriorates the grain size distribution of crystals and deteriorates the product quality.

As a countermeasure when such a state occurs, it is necessary to change the amount of seed crystals or the amount of salt collection, which makes the operation extremely complicated, requires a skilled operator, and is stable. It is difficult to obtain a product with the target particle size, and there are problems in improving productivity and reducing cost.

It is an object of the present invention to provide a salt crystallization control method capable of solving such a problem of the conventional method by introducing a new idea into the crystal growth control method.

<Means for Solving Problems> A feature of the method of the present invention is that the mother liquor of a crystallization can, which continuously feeds out salt crystals from the bottom, is heated in a heat exchanger through a circulation line and then passed through an evaporation can. Then, in the salt crystallizer for continuously collecting salt by supplying seed crystals and brine from the middle of the circulation pipe while returning to the crystallizer, when the mother liquor level of the evaporator reaches the lower limit set value. Is supplied with the first level of the brine which is larger than the decrease of the mother liquor due to heat evaporation and the decrease of the mother liquor due to salt collection, and when the mother liquor level reaches the upper limit set value, the second level is smaller than the first level. It is the point that intermittent brackish water supply is repeated a predetermined number of times.

<Operation> When the mother liquor level of the evaporator reaches the lower limit set value, the supply of the first level brine, which is larger than the reduction of the mother liquor by heating and the reduction of the mother liquor by salting, is started, and the level of the evaporator is set to the upper limit. Second less than the first level when the value is reached
The intermittent brackish water supply in which the level of brackish water is supplied is repeated a predetermined number of times.

<Example> The gist of the method of the present invention is based on the premise that, when Nac1 molecules in the mother liquor are intermingled with each other, there is a substance having an energy field that is neither liquid nor solid due to attractive force and deposition force.

That is, this substance is a liquid with high free energy,
It is a thing with free energy that shows an intermediate amount with a solid with low free energy, and is called blastula.

This spore is an important step in the growth of salt crystals, and when it grows in mother liquor it makes new salt crystals.

The Applicant has found from experiments of plant operation of a crystal can that the crystal growth rate can be stably controlled by repeating the regular growth promotion and disintegration of the spores in the mother liquor.

The feature of this management method is that by supplying more brackish water than the sum of the amount of mother liquor reduced by heating and evaporation and the amount of mother liquor reduced by salt collection, while the amount of heated steam and the vacuum of the evaporator are stable. It is possible to promote the growth of buds, and by supplying the water that is less than the above-mentioned amount of water supplied, it is possible to control the growth of crystals by spoiling the blasts and preventing the generation of new crystals (pseudocrystals). Become.

An embodiment to which the method of the present invention is applied will be described based on FIG. The same components as those described with reference to FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted.

17 is a control device for controlling the intermittent supply of brine W, enter the measured value PV _L level sensor 15, intermittently against the flow control valve 14 of the brine, supplies an operation output MV _L to open and close.

The control device 17 includes an upper limit setting initial value L ₁ , a lower limit setting initial value L ₀ , a level setting correction value Δl for each operation, a constant α, an intermittent operation frequency setting value Xn, First control valve
A level opening a and a second level opening b are set via man-machine interface means (not shown).

FIG. 2 is a diagram for explaining the operation, and (A) shows the level measurement PV _L
And the upper and lower limit setting initial values L ₁ and L ₀ are shown. The upper limit setting value is fixed to the initial value, and the lower limit setting value L increases or decreases from the initial value L ₀ with time due to Xn, Δl, α. To be done.
The lower limit set value L in this case is represented by L = L ₀ ± α · Δl · Xn. (B) is a waveform diagram of the operation output MV _L corresponding to (A).

At time t ₁ , t ₃ , t ₅ ..., the mother liquor level PV ₁ in the crystallizer is lower limit value L ₀
Then, the opening of the control valve 14 is set to a and the first level brackish water supply is started. At the times t ₂ , t _4, ... When the mother liquor level in the crystallization can rises and reaches the upper limit set value L ₁ , the valve opening is set to b and the second level brackish water is supplied.

In this case, the opening time of the control valve is τ ₁ , and the closing time of the control valve is τ ₁ .
_In the case of ₂ , the control of the closing time τ ₂ that controls the collapse of the spores is important, so the closing time τ ₂ is set in the control device in advance, and when the level reaches the lower limit setting value and the setting time τ ₂
It is also possible to shift to a sequence in which the opening of the control valve is set to b under the AND condition after the passage of.

The method of the present invention can be realized in principle by intermittent supply such that the brine supplied at a lower limit level L ₀ of the crystallizer is larger than the heating evaporation amount and the brine supply is stopped at the upper limit level ₁ of the crystallizer.

In the case of such intermittent control, the influence of the normal disturbance of the vapor amount and the vacuum value on the change of the crystal speed is absorbed in the crystal growth rate by the special water supply method, and the crystal growth rate is It can be freely increased compared to the control method, the average particle shape becomes constant, and the particle size distribution tends to improve.

However, when the crystallization rate is increased by such a method, the amount of salt collected is large, the amount of seed crystals is large, and the amount of brine is large. Further, there may be a case where the amount of steam for making the brackish water concentration an appropriate value is insufficient.

In this way, there is an appropriate crystal growth rate due to the restrictions from the equipment and the mother liquor residence time unique to the continuous crystallizer, and in the case of the intermittent control method, generally no large equipment is required,
A balance between the crystallizer and the equipment before and after is necessary for growing the crystal.

Therefore, when using the existing equipment to produce a product having the same grain size as the conventional one, it is necessary to reduce the crystal growth requirement in the period τ ₂ for controlling the growth of spores.

For this purpose, the tau ₂ of time can be realized by shortening (L takes ₀ value decreases), the circulation rate and evaporator of the mother liquor crystallizer, L ₀ in relation to the volume of the heat exchanger There is a limit to how small.

That is, if the control conditions are changed before the mother liquor makes one cycle, the crystal state in the mother liquor varies, which causes the generation of new crystals, which makes control difficult.

Therefore, in the present invention, the supply of brackish water is not set to zero in the period τ ₂ but is supplied at the second level b which is smaller than the first level a, and the crystal growth requirement is suppressed by controlling the supply amount.

The lower limit set value may be fixed to the initial value and the upper limit set value may be changed depending on the number of operations, or control for fixing or changing both may be executed.

In general, salt crystallization runs from concentration to completion, for example, 200
It takes a long period of time that requires days, and the method of setting the upper and lower limit levels of the intermittent water supply control is changed to an optimal mode so that the operation efficiency becomes highest according to each operation stage.

Generally, when the vapor amount of the heat exchanger and the vacuum value of the crystallizer are substantially constant and the crystal volume ratio in the mother liquor is 6 to 10%, the circulating flow rate of the mother liquor is substantially constant. Under such environmental conditions, pulsatile control of mother liquor level in the evaporator by intermittent supply of brine enables bud growth and collapse to be accurately controlled by controlling the valve opening of the control valve. At the same time, the crystal growth rate is significantly faster than that of the constant level control, and stable crystal crystallization (average crystal grain size and grain size distribution) is possible.

<Effects of the Invention> As described above, the present invention is the result of the control based on a completely new idea of intermittently executing growth and disintegration of blastula,
It becomes possible to control the crystal growth rate more stably than in the past and to increase the crystal growth rate, which significantly contributes to the improvement of the operation efficiency of salt crystallization which requires a long period of time. You can

Further, since the crystal grain size can be controlled by the second level supply amount of brine, it is possible to produce a crystal of a desired size regardless of the performance of the apparatus.

In addition, the influence of disturbances such as the amount of vapor and the vacuum value, which influence the growth rate of the crystal during the crystal growth, can be almost ignored.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment to which the method of the present invention is applied, FIG. 2 is an operation explanatory diagram thereof, and FIG. 3 is a configuration diagram showing an example of a conventional technique. 1 ... Crystallizer, 2a, 2b, 2c ... Circulation pipeline, 3,5 ... Pump, 4 ... Heat exchanger, 6 ... Discharge pipeline, 7 ... Salt collector, 8, 11, 14 …… Control valve, 9 …… Seed crystal feeder, 10 ……
Seed crystal supply line, 12 ...... brine water supply device, 13 ...... brine water supply line, 15 ...... level sensor, 17 ...... control device

Claims (1)

[Claims] 1. A mother liquor of a crystallization can for continuously extracting salt crystals from the bottom is heated in a heat exchanger via a circulation pipe and then returned to the crystallization can via an evaporator and the circulation is performed. In a salt crystallizer for continuously collecting salt by supplying seed crystals and brine from the middle of the pipe, when the mother liquor level of the evaporator reaches the lower limit set value,
The first set amount of the brine, which is larger than the sum of the amount of mother liquor reduced by heating and evaporation and the amount of mother liquor reduced by salt collection, is supplied to promote the growth of spores, and when the mother liquor level reaches the upper limit, A second set amount of the brackish water, which is smaller than the first set amount, is supplied to disintegrate the spores to accelerate the growth of crystals, prevent the occurrence of pseudocrystals, and intermittently repeat the brackish water supply a predetermined number of times. A method for controlling salt crystallization, comprising: