JPH0741122B2 - 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 in the lower chamber
It is extracted from 101 into the circulation line 2a by the pump 3, heated through the heat exchanger 4 heated by the steam S, concentrated in the evaporator 5 via the circulation line 2b, and crystallized via the circulation line 2c. Return to analyzer 1.

The salt crystals C accumulated in the bottom of the lower chamber 101 of the crystallization can are discharged to the discharge pipe 7 by the pump 6 and guided to the salt collecting device 8. Reference numeral 9 is a control valve inserted in the discharge conduit.

Reference numeral 10 denotes a seed crystal supply device, which supplies a seed crystal in the middle of the circulation pipe 2a via the seed crystal supply pipe 11 and the control valve 12.

13 is a supply device for brackish water W which is concentrated salt water,
It is supplied to the middle of the circulation pipeline 2a via the brine supply pipeline 14 and the control valve 15.

Reference numeral 16 is a discharge line for the precipitated crystals in the evaporator 5, and 17 is a discharge control valve.

18 is a level sensor for measuring the level L of the mother liquor B in the evaporator 5, and PV _L is the measured value. Reference numeral 19 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 of the control valve 15 that controls the supply amount of the brine W .

The reason why the level of the evaporator is kept constant is that the amount of steam in the heat exchanger is almost constant, the vacuum value of the evaporator is constant, and the volume ratio of the crystal is In the case of 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 level 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 that solves the problems 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 for continuously extracting 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 that supplies seed crystals and brine from the middle of the circulation pipe while returning to the crystallizer, the brine is supplied when the mother liquor level of the evaporator reaches the lower limit set value. The point is to repeat the intermittent brackish water supply, which is started and stopped when the level reaches the upper limit set value, for a predetermined number of times.

<Operation> When the mother liquor level of the evaporator reaches the lower limit set value, the supply of brackish water is started, and when the level of the evaporator reaches the upper limit set value, the supply of brackish water is stopped. Repeated.

<Example> The gist of the method of the present invention is based on the premise that when Nacl 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 bending 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.

From the experiment of the plant operation of the crystal can, the applicant has found that the regular growth promotion and disintegration of the spores in the mother liquor makes
It has been found that the crystal growth rate can be increased.

The feature of this management method is that it is possible to grow the spores in the mother liquor by supplying brackish water in excess of the heating amount when the amount of heating steam and the vacuum degree of the evaporator are stable. By stopping the spores, it is possible to control the blast buds to be destroyed to accelerate the growth of crystals and to prevent new crystals (pseudocrystals) from being generated.

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.

20 is a control device for controlling the intermittent supply of brine W, enter the measured value PV _L level sensor 18 provides an output MV _L intermittently opening and closing manipulations against the flow control valve 15 of the brine.

The controller 20 includes an upper limit setting initial value L ₁ , a lower limit setting initial value L _o , a level setting correction value Δl for each operation, a constant α, an intermittent operation number setting value X _n , The opening a of the control valve is 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 _o are shown. Normally, the upper limit setting value is fixed to the initial value, and the lower limit setting value L is X _n ,
.DELTA.l, is increased or decreased corrected from the initial value L _o over time by alpha. The lower limit set value L in this case is represented by L = L _o ± α · Δl · X _n . (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 evaporator is the lower limit value _Lo.
When it reaches, the control valve 15 is opened to supply water. At times t ₂ , t _4, ... When the mother liquor level in the evaporator reaches the upper limit set value L, the valve opening is set to zero and the supply of brine is stopped.

In this case, the opening time of the control valve is τ ₁ , and the opening time of the control valve is τ _2.
In this case, it is important to manage the closing time τ ₂ that controls the collapse of the spores.Therefore, set the closing time τ ₂ in the control device in advance, and if the level reaches the lower limit setting value and the setting time τ ₂ The sequence for opening the control valve may be performed under the above described AND condition.

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 there is a step of changing both of them. It is also possible to execute a control for fixing both.

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 evaporator are almost constant and the crystal volume ratio in the mother liquor is 6 to 10%, the circulating flow rate of the mother liquor is almost constant. Under such environmental conditions, the arterial control of the mother liquor level in the evaporator by intermittent supply of irrigation makes it possible to accurately control the growth and collapse of spores by controlling the valve opening and closing time. In addition, the crystal growth rate is significantly faster than that of the constant level control, and stable crystal growth control is possible.

<Effects of the Invention> As described above, the present invention is a result of control based on a completely new idea of intermittently executing growth and disintegration of a blast,
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

[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, 3,6 ... Pump, 4 ... Heat exchanger, 5 ... Evaporator, 7 ... Discharge pipeline,
8 ... Salt collecting device, 9, 12, 15, 16 ... Control valve, 10 ... Seed crystal supply device, 11 ... Seed crystal supply line, 13 ... Brackish water supply device,
14 …… Braking water supply line, 16 …… Drainage line, 18 …… Level sensor, 20 …… 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 that supplies seed crystals and brackish water from the middle of the pipeline, when the mother liquor level of the evaporator reaches the lower limit set value, the brine supply is started and the level reaches the upper limit set value. A method for controlling salt crystallization, characterized in that intermittent brackish water supply for stopping the brackish water supply is repeated a predetermined number of times.