JPS6125402B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a scraping type circulation crystallizer.

The main methods commonly used in conventional crystallization operations are adiabatic cooling, direct cooling by blowing refrigerant, indirect cooling using jackets, heat exchangers, etc., evaporation concentration method, salting-out method, etc. . For these methods, the optimum method is selected by taking into account the characteristics of the crystal, the relationship with the preceding and following processes, economic efficiency, etc. One of the important points in crystallization operation is the grain size of the crystal, which affects crystal quality, ease of separation operation after crystallization, etc. As mentioned above, there are various crystallization methods, but there are many substances for which indirect cooling is optimal due to physical properties and other factors. However, although this indirect cooling method has various advantages, it cannot avoid the disadvantage that crystals adhere to the cooling surface, which lowers the heat transfer coefficient and impairs operability. Therefore, either you can control the thickness of the adhering crystals by scraping the cooling surface with a scraper and prevent a drop in the heat transfer coefficient, or you can dissolve the adhering crystals on the heat transfer surface and improve the heat transfer by supplying a new stock solution using a batch method. A method is being used to continue operation by restoring the coefficient. There is no problem with the batch method when the throughput is small, but when the amount is large, there are many operational problems and the scraping method has to be adopted. The problem with this scraping method is that it is difficult to create a circulation flow for the internal liquid because of its structure, which mainly involves scraping.
Currently, stirring blades are attached and a separate drive device is used for scraping, which is expensive to manufacture.
There are also restrictions on the size of the stirring blades, and the need to rotate small-diameter blades at high speeds causes a problem in that the crystals are often mechanically crushed, making it impossible to obtain the desired stable particle size. In order to solve this problem, we have installed a stirring blade that is integrated with the scraping shaft to achieve a large circulation of internal liquid at low rotation speeds, which makes it possible to equalize the degree of supersaturation in the tank. invented an analyzer.

In conventional scraping type crystallizers, there is almost no vertical circulation within the crystallization tank, making it impossible to control conditions for producing good crystals, such as controlling crystal growth and nucleation. In order to create this condition, the current practice is to attach stirring blades separate from the scraping blades to create a vertical circulation flow, which is expensive both in terms of power and production costs. Another method uses a circulation pump to circulate the slurry and mix the internal slurry. In these methods, in order to achieve uniform mixing of the slurry in the tank and a uniform degree of supersaturation, the rotational speed of the stirring blades and pump impeller must be considerably high, and the mechanical crushing by these blades is extremely large, resulting in large particles. This makes it difficult to obtain crystals. It is well known that there is a proportional relationship between the number of revolutions of the stirring blade and pump impeller and crystal crushing, and that it is desirable to circulate the slurry in the tank as effectively as possible at the slowest possible speed. In order to satisfy these contradictory conditions, it is necessary to rotate the blade with a large diameter at a low speed.
The scraping type circulating crystallizer of the present invention was invented with the aim of solving the above problems. In order to ensure sufficient circulation in the tank, uniformize the degree of supersaturation in the tank, and prevent the generation of microcrystals due to local maximization of supersaturation, instead of installing a stirring blade independently, we installed a shaft that drives the scraping blade. A stirring blade for circulation inside the tank is attached to the outside of the draft tube that rotates as a unit, and a baffle is required to minimize the swirling flow of the slurry and create an efficient vertical circulation flow. Conventionally, in the case of the scraped type, it was not possible to attach it to the heat transfer surface, so it was done without a buttful. However, in the present invention, by providing buff-full plates at the upper and lower portions, efficient circulating flow can be obtained at low speed. As a result, the mechanical crushing of crystals is reduced and it is not necessary to install a separate stirrer for circulating the slurry in the tank, thereby reducing production costs and running costs, and the structure is simple and troubles can be reduced.

The present invention will be described in detail with reference to the accompanying drawings.

A rotatable shaft 3 is supported by a lower bearing 18 at the center of the crystallization tank 1 and rotated by a drive device 15 . A vertical flat bar 21 is fixed to a horizontal support arm 20 fixed to the shaft 3, and a scraping blade 5 is attached to the flat bar 21. A jacket 2 is provided outside the crystallization tank 1. The scraping blade 5 is used to scrape off crystals adhering to the inner surface of the crystallization tank 1. A draft tube 4 is provided in the middle of the support 20 of the scraping blade 5, and a stirring blade 6 is provided for circulating the internal liquid or crystal slurry outside the draft tube 4.
is installed. At the upper and lower parts of the crystallization tank, in order to efficiently convert the swirling flow generated by the scraping blade 5 and its support arm 20 into an up-and-down circulation flow, the upper lid 19 is suspended from the upper lid 19, and the inside and outside of the draft tube 4 are installed. A buff-full plate 7 is disposed astride the draft tube 4, and a buff-full plate 8 is attached to the lower bottom plate 17 inside the draft tube 4. It is preferable that the baffle plate 7 has a recess in its upper portion to adjust the flow of the liquid that passes through it, such as by providing a recess in the upper part of the baffle plate 7 to deal with the flow of the liquid caused by stirring. Of course, these Batsuful plates 7 and 8
is prevented from hitting each rotating member.
The stock solution is supplied through the supply port 11, introduced through the inlet 12 of the external jacket, and discharged through the outlet 13, thereby being cooled by passing a refrigerant therethrough. The crystals growing on the inner wall surface are scraped off by scraping blades, ie, scrapers 5, and suspended in the liquid to form a slurry. This crystal slurry obtains an upward flow by a stirring blade 6 attached to the outside of a draft tube 4 that rotates together with the shaft 3, and becomes a circulating flow as shown by the arrow in the figure. This flow direction can also be reversed. By attaching stirring blades to the outside of this draft tube, a large circulating flow can be obtained at low speed rotation. Therefore, crystals are less likely to be crushed, and the degree of supersaturation inside can be made uniform, resulting in better crystals.

Since a swirling flow of the slurry in the tank occurs due to the resistance of the scraping blade 5 and its support arm 20, etc., it is necessary to reduce this as much as possible in order to perform upstream and downstream circulation smoothly and efficiently. According to the results confirmed with an experimental machine, it is desirable to keep the width of the scraping blade in the radial direction to 3/100 or less of the tank diameter. It is also desirable that the support arm of the scraping blade has a structure with as little resistance as possible. Due to the mechanical structure, it is not possible to completely eliminate this swirling flow, so by providing buffs 7 and 8, it is converted upstream and downstream to further increase efficient circulation. In a mixing state in which swirling flow is the main flow, the proportion of liquid cooled near the cooling surface moving inside is small, creating a temperature gradient between the center of the tank and the cooling surface, resulting in a temperature difference with the refrigerant. Near the cooling surface, the heat transfer rate decreases and the capacity decreases. Furthermore, the degree of supersaturation within the tank becomes non-uniform, resulting in a state in which microcrystals are likely to occur. In addition, the crystals also settle to the bottom of the tank, resulting in poor contact between the supersaturated mother liquor and the crystals. For these reasons, it is important to change the swirling flow upstream and downstream.

In the conventional method, a separate independent stirrer is installed to create a vertical circulation flow, but due to the structure, the diameter of the blade is approximately equal to the inner diameter of the draft tube, and the rotation speed is higher than that of the blade of the present invention, which is difficult to crush crystals. Not desirable from this point of view. In addition, the conventional mounting method is a double shaft structure in which the circulation blade is inserted from the bottom or the inside of the scraping shaft is passed from the top, but both are expensive to manufacture and cause operational problems. There are many problems with the shaft seal, especially when pushing in from the bottom.

The crystal slurry is extracted using the lower nozzle 10 or the upper overflow nozzle 14. By obtaining a large circulation flow, it is possible to suspend the crystals, and for systems where the difference in specific gravity between the crystals and the mother liquor is not large, overflow nozzle 1 is used.
4, it is possible to send the crystal to the next step without a pump, it is possible to prevent the crystal from being crushed by the pump, and it is also possible to reduce equipment costs and running costs.

Generally, crystals attached to the walls or shaft in a crystallization tank may fall and form crystal blocks, but with the conventional conical shape with a low center, even if a crushing mechanism is installed, the blocks will concentrate in one place and cause blockage. It was difficult to completely eliminate this problem. However, as shown in the figure, by forming a conical shape with a high central part, the blocks are gathered to the outside, and a comb-shaped crushing mechanism 9 is attached to the bottom plate 17 and the lower part of the scraping blade to loosen the blocks, thereby making it easier to extract the slurry. It has been made possible to resolve blockage problems caused by blocks.

An embodiment of the scraping-type circulating crystallizer of the present invention and its test results will be described next. Tank diameter 1800mm, tank height 3000
mm, shaft rotation speed 8 to 25 r.pm, scraping blade circumferential speed 0.75 to 2.35 m/s Materials handled: Mixed liquid of isomers of organic substances, crystal specific gravity 1.52, mother liquor specific gravity 1.34, average crystal grain size 1.0 mm, crystal Analysis temperature: -15 to -20℃, sedimentation rate: 0.3cm/s As a result of the above conditions, the tip speed of the scraping blade,
When maintained at 1 m/s (shaft rotation speed 10 r.pm), the internal slurry was in a uniformly mixed state and a stable crystal grain size was obtained. Even when the tip speed of the scraping blade was set to 0.75 m/s (rotation speed: 8 r.pm), there was almost no difference in the concentration distribution of the slurry.
There were no blockage problems when extracting the slurry.

The scraping type circulating crystallizer of the present invention has the following advantages over conventional crystallizers.

(1) Compared to the conventional scraping type, internal slurry circulation can be achieved at lower rotation speeds.

(2) Since it can be circulated at low speed, there is less mechanical crushing of crystals.

(3) The structure is simple and has a single shaft, so the manufacturing cost is low.

(4) Only a single drive unit is required, and operating costs are low because the amount of power is small.

(5) Single shaft makes maintenance easy.

(6) There are no problems with shaft seals compared to bottom-driven agitation for circulation.

(7) Since it has a crushing mechanism that collects crystal blocks on the wall side, there is no problem with slurry extraction.

(8) Low speed rotation allows sufficient internal slurry circulation, making the supersaturation level of the internal liquid uniform and stable, resulting in high quality and stable crystals.

Since the scraping type circulating crystallizer of the present invention has the above advantages, it can be applied to the following fields.

(1) It can be applied to all crystallization systems that were conventionally performed only by cooling and scraping, and the crystals can be made larger and of better quality than before.

(2) It can be applied to crystallization systems that conventionally used scraping circulation, and it is possible to reduce mechanical crushing of crystals and obtain better crystals than in conventional systems.

(3) Conventionally, the scraping method could be used, but good quality crystals could not be obtained due to insufficient internal circulation, so steam, Helps save energy such as power.

(4) For the same reason as (3) above, it can be applied to crystallization systems that use low-temperature evaporation or adiabatic cooling methods, even though it is a particularly thermally sensitive substance, and can be operated under atmospheric pressure. Therefore, it is useful for energy saving and reduction of production costs.

The scraping type circulating crystallizer of the present invention is superior to conventional ones in terms of the advantages and applicability described above.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of the scraping circulation type crystallizer of the present invention, FIG. 2 is a sectional view taken along the line - in FIG. 1,
FIG. 3 is a sectional view taken along the line - in FIG. 1. 1... Crystallization tank, 2... Jacket, 3... Shaft, 4... Draft tube, 5... Scraping blade, 6... Stirring blade, 7... Buff full, 8... Buff full, 9... Crushing Mechanism, 10...lower nozzle,
11... Supply port, 12... Refrigerant inlet, 13... Refrigerant outlet, 14... Overflow nozzle, 15...
Drive device, 16... Ground packing, 17...
...Bottom plate, 18...Lower bearing, 19...Top lid, 2
0...Support arm, 21...Flat bar.

Claims (1)

[Claims] 1 A draft tube 4 is provided in the scraping mechanism 3, 20, 21, 5 that rotates inside the scraping type crystallization tank 1, and a stirring blade 6 is attached to the draft tube to generate a circulating flow outside the draft tube. In order to change the flow upstream and downstream, buttfuls 7 and 8 with a certain curvature are inserted into the outside and inside of the draft tube 4 from the upper lid 19 and bottom plate 17 of the crystallization tank to near the support arm 20 of the scraping mechanism. A scraping circulation crystallizer characterized by comprising: