JPS6246201B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel purification and crystallization method for industrially crystallizing a pure substance containing no impurities.

A purification method in which a specific solute dissolved in a solution is separated as crystals is an efficient method employed in many industrial fields. In particular, there has been a strong trend towards fine chemicals in recent years, and it has become an important issue how to obtain crystals of higher purity through crystallization operations.

Theoretically, crystallization is the purest concentrated state of a substance, so it is theoretically possible to crystallize a pure substance, which is why crystallization is considered an excellent purification method. be. However, the solutions handled in industrial operations contain many impurities, which have complex effects on the nucleation and growth processes of crystals, and these impurities are included in the product crystals. Since it is difficult to obtain highly pure crystals, methods such as recrystallization, which involves repeating crystallization and dissolution, or operations that keep the supersaturation concentration sufficiently low and gradually grow crystals to obtain relatively pure single crystals are recommended. It requires time-consuming operations to control the conditions and to wash and remove the mother liquor adhering to the surface.
However, due to the recent energy situation, the recrystallization method has a large energy loss, which causes a rise in production costs, and the purification method, which gradually grows single crystals, requires large equipment because the residence time of the processing solution and crystals is long. This increases equipment and operating costs, is inefficient, and limits purification efficiency.

The present invention solves the drawbacks of the above-mentioned purification crystallization means and provides a new crystallization method that can obtain crystals with a purification efficiency of 100% (pure) through a rapid crystallization operation.

That is, the present invention provides a high degree of supersaturation to the solution in a crystallization operation within a range that does not cause excessive nucleation, and crystals with continuous pores and a large pore opening ratio are produced on a cooling wall surface or under similar conditions. A process of preparing seeds, suspending them in the operating mother liquor, and rapidly growing them in a high-growth atmosphere, and allowing the grown crystals to remain in the tank and partially melting them to remove the mother liquor and impurities in the pores. The purpose is to diffuse and remove the substances and obtain purified crystals.

In addition, in the crystallization operation of the present invention, as described above, the crystals obtained by rapidly growing seed crystals with continuous pores and a large pore opening ratio are grown in a mother liquor or aqueous solution with a low degree of unsaturation. Alternatively, some of the crystals are gradually melted with air, the mother liquor and impurities in the pores are removed, and the energy lost due to the melting is recovered and reused within the system, resulting in economical efficiency. This is a purification crystallization method that can increase the

According to the method of the present invention, crystals with extremely high purification efficiency can be efficiently produced industrially with simple operations and in a relatively short time without the need for complicated operations as in the past. The effectiveness of this approach will be significantly enhanced.

Next, the purification and crystallization method of the present invention will be specifically explained as follows.

The present inventor has been involved in research on crystallization operations for many years, and has experienced crystallization phenomena of many crystal systems.In this process, the purification efficiency has a deep relationship with the operation supersaturation concentration, and the crystal seeds are gradually grown at low supersaturation concentrations. We found two cases in which the so-called general crystallization concept, in which the purification efficiency is increased by growing large, well-separable product crystals, can be applied, and a completely opposite phenomenon, in which the purification efficiency is increased by rapid crystallization at a high supersaturation concentration. This phenomenon occurs because impurities in a solution are adsorbed to specific faces of a growing crystal by the mode crystal action, and the growth of the step where the impurity is adsorbed is suppressed, while solute molecules or aggregates are adsorbed to the crystal lattice at other steps. It is presumed that as it is incorporated and growth is promoted, that part overhangs and takes in the mother liquor, reducing purification efficiency. In systems where such a moderating effect is predicted, the operating supersaturation concentration should be increased to speed up the crystallization rate, and the equipment and operating methods should be devised based on the judgment that operation that makes it difficult for the moderating effect to occur is effective. However, reproducibility was poor, it could only be applied to specific systems, and there were also limits to the degree of purification.

In addition, in the purification crystallization of molten naphthalene-benzoic acid system, even if the crystallization operation is performed at a phase equilibrium position where 100% of naphthalene is precipitated, it is not possible to prevent the contamination of benzoic acid, and the supersaturation concentration during the operation cannot be prevented. As a result, there was a difference in the mixing rate of benzoic acid, so by lowering the operating supersaturation concentration (slowing down the growth rate) and gradually growing the crystals, the mixing rate of impurities decreased, but below a certain operating supersaturation concentration, Since growth then stops, the operational supersaturation concentration cannot be lowered below the limit, and the purification rate cannot be increased beyond the limit. However, when the crystals were allowed to remain in the mother liquor, a phenomenon was observed in which the purification rate improved in proportion to the residence time.

Therefore, focusing on this phenomenon, by supercooling the sucrose solution and crystallizing ice crystals, we changed the apparent supersaturation temperature difference (difference between the mother liquid crystallization temperature and the refrigerant temperature) Δt℃ to -4℃, -8℃. °C, operated at -12 °C. The purification efficiency of the obtained ice crystals was 50% or less in all cases, and the lower the Δt°C, the higher the purification rate, and the higher the Δt°C (quick cooling), the lower the yield. When the ice crystals were allowed to remain in the mother liquor while flowing, the purification rate of all the ice crystals obtained in each operation was dramatically improved in about 2 hours, reaching a purification rate of nearly 90%. Judging this phenomenon comprehensively, the purification rate is directly related to the pore structure of the crystal and the pore opening ratio, and the pore structure and pore opening ratio are influenced by the formation of crystal seeds at the initial stage of crystal growth and the growth atmosphere. ruled by. Next, it was inferred that the desorption of the mother liquor and impurities contained in the crystal pores is related to the diffusion conditions with the bulk solution.

Based on the above experimental results, the inventor assumed the following crystallization model. Scaling crystals that are precipitated on the cooling wall surface with the highest supersaturation concentration have large surface turbulence and tend to grow in the shape of needles or columns in the direction of heat transfer, facing the cooling surface. When crystals are scraped off from the cooling wall surface and used as crystal seeds to grow in an atmosphere with a high supersaturation concentration, growth is promoted using the needle-like or columnar tips as active points. Therefore, it is expected that the microscopic needle-like or columnar spaces become pores that communicate to the inside, and the aperture ratio of the pores becomes extremely large. The pores of the crystal thus produced naturally contain the mother liquor and impurities contained in the mother liquor. The diffusion rate at which this mother liquor and impurities are diffused and desorbed into the bulk mother liquor is controlled by the concentration direction. Therefore, if the crystal is kept in a molten state, unlike the growth phenomenon, it will dissolve uniformly from the entire surface as long as the conditions are right. Therefore, the dissolution phenomenon is controlled by the concentration direction and surface condition, so if the mother liquid contained in the pores is kept in a mild unsaturated state where the crystals gradually melt, It is assumed that the mother liquor and impurities are rapidly diffused and desorbed into the bulk mother liquor. However, remelting the crystalline product is not a preferred method because it reduces economic efficiency and productivity in industrial operations. Therefore, the focus of the present invention is to find the economic point of minimizing the amount of melted crystalline product and increasing the purification rate.

In order to prove this crystallization model, the apparent supersaturation temperature difference Δt℃ of the above sucrose solution was -4℃, -8℃, -
The temperature was maintained at 12° C. to precipitate ice crystals on the cooling surface, which were scraped off and used as seed crystals to stay in the apparently supersaturated liquid and grow rapidly, with a residence time θ of about 90 minutes. The purification rate E of the extracted crystals is the 0 point of the dissolution rate V% shown in Figure 1, and this was placed in a stirred tank type melting tank and the purification effect was measured by varying the dissolution time and amount of dissolution. did. From the figure, it was found that the purification rate increased even if the dissolution rate V% was significantly reduced when the dissolution time was 60 minutes compared to when the dissolution time was 30 minutes.

The dissolution rate and purification rate shown here are as follows.

Dissolution rate = amount of dissolved crystals/initial amount of crystals x 100 Purification rate = total amount of crystals - amount of impurities/total amount of crystals x 100 Show the basic flow sheet of the purification and crystallization method of the present invention completed based on the above matters. As shown in Figure 2. The treated stock solution is precooled (or preheated depending on the system) by exchanging heat with the mother liquor after the crystallization operation in the heat exchange section. This stock solution is sent to the crystallization section, and crystals with continuous pores and a large pore ratio are crystallized by a scaling crystallization method or a similar rapid crystallization method through a cooling surface, and then crystals with continuous pores and a large pore ratio are crystallized. After giving _a The residence time for crystal melting and predetermined purification is given, and the impurities and mother liquor contained in the crystal pores are diffused and purified. On the other hand, the remaining heated (or cooled depending on the system) mother liquor sent to the melting and purification section has a V ₂
Either the liquid is sent to the crystallization section through the valve V3 and the crystallization operation is repeated, or it is disposed of outside the system through the valve _V3 . The crystals purified in the melting and purification section are sent to the solid-liquid separation section, where they are separated into purified crystals and mother liquor using equipment such as a centrifuge or vacuum dehydration. The crystals are sent to the product and the mother liquor is sent to the heat exchange section to continue work. do. A purification crystallization method that combines such crystallization conditions and melting conditions can economically achieve dramatic simplicity and improvement in purification efficiency compared to conventional purification and crystallization methods, and can be applied on an industrial scale. The industrial effect is remarkable.

In addition, the decrease in crystallization rate due to melting operation can be sufficiently compensated for by decreasing the volume of the crystallization tank due to the increase in crystallization rate.In other words, the crystallization rate is in the relationship of ΔW℃KAθ・^ΔCn , and is expressed in units of dW. The amount of crystallization per hour, θ is the residence time, ΔC is the operating supersaturation level, n is a constant depending on the system of the substance to be handled and is usually 1 to 2, and A and K are constants. Therefore, the method of the present invention is characterized by increasing ΔC by 5 to 10 times that of the conventional method.If the amount of treated crystals and ΔW are the same, and ΔC is 5 times larger, θ becomes n.
When n = 1, it becomes 1/5, and when n = 2, it becomes 1/5 ² = 1/25, and θ is the residence growth time, so when you want to obtain crystals of the same particle, the device volume is from 1/5 to 1/25. It is. Therefore, even if the crystal dissolution rate is assumed to be 50%, the device volume is 1.5 times as large, which means that small equipment is sufficient. Furthermore, energy loss can be compensated for by recovering it within the system.

Next, in order to specifically explain this, an example will be given and explained in detail.

The method of the present invention can be explained in more detail with reference to drawings showing one method embodying the invention, but the technical scope of the present invention is not thereby limited.

Example 1 In order to obtain purified water by crystallizing ice crystals by a freezing method for the purpose of desalination from seawater, separating them from the mother liquor, and redissolving them, the method of the present invention was carried out based on the flow sheet shown in Fig. 3. It was operated. The seawater stock solution in the figure is
It is cooled by exchanging heat with purified water in the No. 1 heat exchanger, and then exchanged with concentrated seawater in the No. 2 heat exchanger before being sent to a cooling type crystallization tank. The crystallization tank has a structure as shown in Fig. 4, with a double wall 3 on the cylindrical side wall 2 of the crystallization tank 1.
A cooling chamber 4 is provided, and the seawater filling the crystallization tank is cooled through the cylindrical side wall 2 of the cooling chamber 4. Crystallization tank 1
A rotating shaft 10 is disposed at the center inside the shaft, and the shaft 10
A scraping plate 11 which contacts the cylindrical side wall with an arm is attached to a plurality of equal parts, and the ice scale crystals generated on the cooling surface inside the side wall 2 are scraped off by rotation of the scraping plate 11. A refrigerant is fed into the cooling chamber 4 from an inlet 5, extracted from an outlet 6, and circulated through a refrigerator (not shown). The pre-cooled seawater raw solution is sent to the lower part of the crystallization tank 1 through the supply pipe 7, and while being cooled, the water is crystallized as ice crystals, and it rises while being concentrated, and is passed from the outlet 8 through the liquid circulation pump 9 to the control valve _V4. , the liquid volume is adjusted and branched at _V5 , and a part of it is sent to crystallization tank 1 through a valve.
Refluxed via V ₄ . The other circulating seawater mother liquor is sent to the crystal receiver 15 via the valve _V5 . The upper part of the crystal receiver 15 is a crystallization tank 1 and a crystal grain size adjustment throat part 1.
3, the ice crystal particles falling down the throat opening are classified and regulated by the upward flow of liquid refluxed by valve _V5 . The ice crystals that have finished growing within a predetermined residence time are stored in a crystal receiver 15 and sent to a melting and refining machine for the next step by a discharge pump 16.
On the other hand, the seawater mother liquor is concentrated by precipitation of ice crystals, and is sent from the upper overflow outlet 12 of the crystallization tank 1 to the No. 2 heat exchanger shown in FIG.

The melting refiner shown in FIG. 5 was used. In this device, a perforated belt 23 is stretched between a driving drum 21 and a guide drum 22, and is adapted to rotate gently in the direction of arrow A. An ice crystal supply hopper 26 is placed on the perforated belt on the side of the guide drum 22. The upper part of the perforated belt 23 that is installed to uniformly supply ice crystals C is blocked from outside air by a cover 25, the lower part of the perforated belt 23 is provided with an excess liquid receiving tank 24, and the bottom part thereof is provided with an exhaust port 27. This is connected to an exhaust pump, and the air A is sucked into the cover 25 through the control valve 28. The ice crystals C are melted by suction air while being transferred on the perforated belt 23. The residence time and amount of dissolved material for melting and purification are controlled by the speed of the perforated belt 23, air temperature, and amount.

The ice crystals purified by the melting refiner are remelted in the melting tank and used to pre-cool the raw seawater in the heat exchanger with the highest heat of fusion. The water dissolved in the melting refiner and the separated seawater mother liquor are separated together with air in a gas-liquid separation tank, the air is exhausted by an exhaust pump, and the separated liquid is sent to the outside of the system together with the concentrated liquid.

As for the crystallization conditions of this flow sheet, the operating temperature for crystallization is -2℃, and the concentrated seawater mother liquor concentration is
4.6 to 5.5wt%, the refrigerant temperature in the cooling room is -8 to -15
°C, the ice crystal residence time in the crystallizer is 30-90 minutes,
The average crystal particle size was 1 to 3 m/m. The obtained crystals have an apparent crystallization temperature difference (difference between operating temperature and refrigerant temperature) of −12°C or higher, a residence time of 30 minutes, and a purification rate E of 58~58.
64%, and the purification rate E was 83 to 98% at an apparent crystallization temperature difference of -12°C or more and a residence time of 90 minutes. These ice crystals are melted and purified by blowing room-temperature air into the machine to keep the temperature inside the machine around 0°C, and the melted amount is about 15%, the residence time is 100 minutes, the purification rate E is 100%, and the dissolved amount is 5%, and the residence time is 45 minutes. The result was that the purification rate E was 100%.

When the crystals taken out from the crystallization tank were observed under a microscope, they were found to have more surface roughness than crystals obtained by conventional crystallization methods, and the presence of many pores.

Example 2 The method of the present invention shown in the flow sheet of FIG. 6 was operated in order to supercool a sucrose aqueous solution and a youth solution to crystallize and separate water into ice crystals and concentrate them. In the figure, the supplied stock solution is first pre-cooled with a crystal mother liquor (concentrated liquid) in the No. 2 heat exchanger, and then sent to the crystallization tank. ,
Purify by melting some of the ice crystals. This dissolution and purification is performed by a solid-liquid separator (centrifuge, etc.) in the next step. The separated ice crystals are then melted in a melting tank, and a portion of the purified water is sent to the melting and purifying tank via valve _V6 for predetermined melting and purification. The dissolved mother liquor separated by the solid-liquid separator is returned to the crystallization tank. The purified ice crystals are sent to the melting tank via the route shown by the broken line, and the purified water is sent out of the system.The purified ice crystals are melted by heat exchange with the sensible heat of the stock solution in a No. 1 heat exchanger. The supplied stock solution exchanges heat with the crystal mother liquor in the No. 2 heat exchanger, and the cooled supply stock solution is sent to the crystallization tank, and the heated crystal mother liquor (concentrated solution) is sent to the next process. In this flowsheet, valve V ₇ is closed.

The same crystallization tank as shown in FIG. 4 was used, and the melting and refining tank was a normal stirring tank with a vertical cylindrical shape and a stirrer in the center. The crystallizer operating temperature is +0.5°C, the refrigerant temperature is -0.5°C to 13.5°C, and therefore the apparent temperature difference is 1 to 14°C. The feed sucrose concentration is 4.5-5.3 wt%, and the crystal residence time is 60 minutes. The purification rate E of the obtained crystals is the dissolution rate V in Figure 8.
% was 0 points. This was purified by melting some of the ice crystals in a melting and refining tank, and the relationship between the dissolution rate V% and the purification rate E% was measured with a residence time of 60 minutes. It was demonstrated that crystals cooled and crystallized rapidly (by increasing apparent temperature supersaturation) have a high purification rate even if the dissolution rate V% is reduced. Furthermore, based on these results, when ice crystals with an apparent temperature difference of 14°C were extended to 90 minutes in the melting and purification tank under the same conditions, the melting rate was approximately 18%.
A result of 100% purification rate was obtained.

The results obtained by concentrating juice (fruit juice) using the same equipment under the same operating conditions showed that the purification rate showed exactly the same tendency as in the case of sucrose.

Example 3 A concentrated stock solution (Na ₂ Cr ₂ O ₇ 77wt%, temperature 100°C) containing impurities of sodium dichromate was cooled.
In order to obtain purified crystals of Na ₂ Cr ₂ O ₇ .2H ₂ O, operations were carried out based on the flow sheet according to the method of the present invention shown in FIG. The concentrated stock solution is sent to the No. 2 heat exchanger, pre-cooled to saturation temperature, sent to the crystallization tank, and treated with the prescribed crystallization operation, and the precipitated crystals are partially dissolved in the melting and purification tank. The purified crystals are then separated into products using a solid-liquid separator (such as a centrifuge). The separated mother liquor is returned to the crystallization tank. The mother liquor that has completed crystallization in the crystallization tank is returned to the crystal mother liquor repurification process via No. 2 heat exchanger. The crystallization tank used in this step was a vacuum crystallization tank as shown in FIG.

A cooling chamber 32 partitioned by a cylindrical cooling wall 31 is provided at the center of the crystallization tank 30, and a rectifying cylinder 32' is provided between the outer peripheral wall of the tank 30 and the cooling wall 31. A rotating shaft 34 vertically installed in the center of the crystallization tank 30 has a downcomer pipe 36 and a plurality of scraping plates 3 inscribed in the cooling wall 31.
5 is fixed and rotated,
The scaling crystals deposited on the cooling wall 31 are scraped off. The circulating liquid in the tank descends through a downcomer pipe 36 and is sent to a level tank 38 by a circulation pump 37. The level tank 38 is connected to the crystallization tank 3 by the neutral pipe 39.
Maintaining the same pressure as 0, the liquid is sent to the crystal receiver 40 through this lower nozzle 38', and the crystal grain adjustment port 41
rises and refluxes into the crystallization tank 30. Since the crystal grains are sorted by the liquid flow rising up the throat of the crystal grain adjustment port 41, the liquid amount is adjusted by the valve _V8 . On the other hand, crystallization tank 3
The mother liquor that has completed crystallization at 0 is taken out of the system through the overflow 38'' of the level tank 38.The amount of circulating liquid in the crystallization tank is regulated by valve _V9 , and the stock solution is supplied to this piping system from pipe 43. sent to mix and crystallize tank 30
sent to. On the other hand, the cooling chamber 32 is filled with fresh water,
Since the boiling point difference between the crystal mother liquor and fresh water is approximately 10℃,
It is heated by the sensible heat of the mother liquor and the latent heat of crystallization through the cooling wall 31 of the crystallization tank 30, rises up the cooling wall 31 side, evaporates at the boiling point due to the predetermined tank internal pressure at the upper opening, and is adiabatically cooled. It descends along the outer wall and rises again along the cooling wall. Fresh water corresponding to this amount of evaporation is replenished from the pipe 45. The crystals deposited on the cooling wall 31 are scraped off by the scraping plate 35 and grow while flowing into the crystal mother liquor.

When the particles grow to a size larger than that corresponding to the upward flow of the circulating mother liquor, they settle and fall into the crystal receiver 40, and are sent to the next step by the crystal discharge pump 46. 37 is a bleed chamber connected to a vacuum pump.

The melting and purification tank was the same as in Example 2, and was operated as if the pipes were connected as shown in the solid line flow sheet of FIG.

The crystallization conditions were an internal vacuum of 700 mmHg (absolute 60 mmH).
g), cold water boiling point approximately 41℃, crystal mother liquor boiling point 51℃, operating concentration 252.5g-Na ₂ Cr ₂ O ₇ /100g-H ₂ O (71.63wt
%), cooling surface apparent temperature difference approximately 10℃, crystal residence time 40
minute, grain size 2-3 mm. Furthermore, the melting and purification conditions were as follows: crystal mother liquor concentration: 252.5g-Na ₂ Cr ₂ O ₇ /100g-H ₂ O
(about 71.63 wt%) The purification rate E was about 99.98% at a liquid temperature of 52 to 52.5°C and a residence time of 60 minutes. It was impossible to achieve a purification rate of 95% or higher using conventional crystallization methods.

As is clear from the above examples, the present invention can be applied to a wide range of industrial operations, and the melting point depression of the eutectic can also be used in the fractional crystallization of organic systems near the boiling point to efficiently achieve purification purposes with rapid processing. has been achieved, and the effects are significant.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between dissolution time, purification rate, and dissolution rate in purification and crystallization operations, Figure 2 is a basic flow sheet of the method of the present invention, Figure 3 is an alternative flow sheet, and Figure 4 is a cooling method. A schematic diagram of a crystallizer, FIG. 5 is a schematic diagram of an example of a melting refiner, FIG. 6 is a flow sheet of another example of the present invention, FIG. 7 is a schematic diagram of a vacuum type crystallizer, and FIG. It is a figure which shows the purification rate with respect to crystallization conditions and dissolution rate.

Claims (1)

[Scope of Claims] 1. Continuous pores and pore openings generated by giving a high degree of supersaturation to a solution in a crystallization operation within a range that does not cause excessive nucleation, and under a cooling wall surface or similar conditions. A crystallization process involves creating crystal seeds with a large ratio, suspending them in the operating mother liquor, and rapidly growing them in a high-growth atmosphere, and leaving the grown crystals in a tank to remove the mother liquor in the pores. Purification crystallization characterized by comprising a step of diffusing and removing impurities, and a step of purifying the obtained crystals by gradually melting some of the crystals in air or a mother liquor or aqueous solution with a low degree of unsaturation. Method. 2. In the crystallization operation, a high degree of supersaturation is given to the solution within a range that does not cause excessive nucleation, and crystal seeds with continuous pores and a large pore opening ratio are generated by cooling the wall surface or similar conditions. The crystals are suspended in the operation mother liquor and grown rapidly in a high growth atmosphere, and the grown crystals are retained in the tank to diffuse and remove the mother liquor and impurities in the pores. Purification crystallization characterized by gradually melting some of the crystals in air or a mother liquor or aqueous solution with a low degree of unsaturation, and collecting and reusing the energy lost due to melting within the system. Method.