RU2308555C2 - Method for crystallization cleaning of matters by continuous zonal melting and apparatus for performing the same (variants) - Google Patents

Abstract

FIELD: chemical machine engineering, possibly chemical and other related branches of industry.

SUBSTANCE: method comprises steps of forming crystalline layer on cooled surface by means of directional crystallization from melt realized before zonal melting and step of removing layer, both steps are realized simultaneously in different portions of working surface of crystallizer. Transportation of crystalline layer from one portion to other is realized due to motion of working surface of crystallizer such as roller type or band type crystallizer relative to said portions in such a way that each point of working surface of crystallizer successively passes through each step. Directional crystallization is realized at contact of melt with cooled portion of working surface of crystallizer. Zonal melting is performed with use of one or several heaters due to local heating of one or more stripes of crystalline layer where local zones of melt are created while melt is cyclically removed from one or several local zones of melt due to evacuation and (or) capillary mass transfer with use of receptacle having porous or fibrous material and heated till temperature exceeding that of melt. In specification designs of roller type and band type crystallizers are given.

EFFECT: possibility for producing high purity crystalline product, realization of continuous operation of apparatus not needing cyclic interruptions for removing impurities, continuous separation process for receiving two outlet flows such as purified matter and mixture enriched with impurity components.

5 cl, 6 dwg, 1 tbl

Description

The invention relates to chemical engineering and allows for a continuous process of purification or separation of substances combined in one apparatus processes of directed crystallization on a cooled surface and zone melting. The invention can be used in chemical and other related industries to obtain a crystalline product of high purity. Cleaning is carried out by a continuous process of zone melting of the crystalline layer obtained on the working surface of the mold at the stage of directed crystallization from the melt. For this, it is necessary to obtain a local melt zone in the crystalline layer on the working surface of the mold; organize the movement of the melt zone relative to the working surface of the mold; to ensure the removal of impurities from the melt zone; create a sufficient temperature gradient at the edges of the local melt zone to ensure melting of the crystalline layer from one edge of the melt zone and crystallization of the melt from the other edge as a result of the movement of the melt zone.

TERMINOLOGICAL DESIGNATIONS

- the working surface of the mold is the surface in contact with the melt; in a roll mold, the outer surface of the roll is working;

- feed melt - the melt of the initial mixture to be cleaned in contact with the cooled working surface of the mold;

- impurity - one or more components displaced into the melt during crystallization due to thermodynamic equilibrium between the crystalline phase and the melt;

- local zone of the melt - the area on the working surface of the mold on which, by local heating, formed the zone of the melt of the substance to be cleaned.

BACKGROUND

A known method of zone melting, widely used for the purification of crystalline substances. One of the options for implementing this method is zone melting of a crystalline layer previously deposited on the surface of a heat-conducting material; the use of this option is appropriate for the purification of organic substances. The device described in the invention [1], allows the purification of organic substances by this method and is selected as the closest analogue of the invention according to claim 1. The organic substance to be cleaned is applied in a uniform layer to the bottom of the cuvette, which is made of a thin heat-conducting material, such as a metal foil. The local melt zone is obtained as follows: the cuvette is mounted on a heater, which consists of a series of heating elements with a length equal to or greater than the length of one of the sides of the cuvette and a relatively small width. The heating elements are parallel and at some distance from each other, so that upon contact with the bottom of the cell, the melt zones form organic matter in the form of strips. The cleaning process consists in organizing the continuous movement of the melt zones in the direction perpendicular to the long sides of the heating elements, using the reciprocating movement of the cell relative to the heater. During the cleaning process, the impurity is moved away in the direction of movement of the melt zones and accumulates near one side of the cell. The main disadvantage of this cleaning method is the need to periodically stop the process and mechanically separate the portion of the purified crystalline layer from the portion of the crystalline layer enriched in impurity. Therefore, the use of this apparatus is advisable only in laboratory conditions. It is possible to clean the melts of substances in roller and ribbon crystallizers. Known designs of roller molds, in which, to increase the degree of purification, sections of the working surface of the mold that are not used in the processes of crystallization and removal of crystals are used. In the design of the roller mold described in the invention [3], a heater is installed with a gap from the crystalline layer leaving the melt (on the cooled working surface of the roller) from the melt, which, by melting the upper part of the crystalline layer, creates a melt flow descending along the crystalline layer. Thus, using the process of recrystallization, which occurs in the countercurrent mode of motion of the melt layer along the crystalline layer, additional purification of the substance occurs. The design described in the invention [4] is an improved version of the invention [3]: two heaters are located on the upstream side of the drum, on the surface of one of which a temperature gradient is created, and the surface of the second heater is isothermal. The invention [4] is selected as a prototype of the present invention according to claim 4. The disadvantages of the apparatus [3], [4] are the limitation of the degree of purification due to the recrystallization process, as well as the accumulation of impurities in the melt as a result of the return of the melt stream enriched in the impurity in a heated trough with the melt.

A number of inventions are known, the purpose of which is to carry out a cleaning process on a ribbon crystallization apparatus. Thus, the invention [2] describes the design of a tape apparatus in which countercurrent vertical movement of the melt film along the crystalline layer is used for cleaning, and the apparatus in the invention [5] allows the crystalline product to be obtained by capturing the melt with a moving tape immersed in the melt. The invention [5] is selected as a prototype of the present invention according to claim 5. The disadvantages of the apparatus [2], [5] is the limitation of the degree of purification due to the process of directed crystallization.

SUMMARY OF THE INVENTION

The technical result of the present invention is to obtain a crystalline product of a high degree of purity, the implementation of continuous operation of the apparatus, which does not require periodic shutdown to remove impurities, as well as the possibility of conducting a continuous separation process to obtain two output streams: a purified substance and a mixture enriched in impurity components. The advantages of the present invention during its implementation can also include the possibility of refinement without significant restructuring of the existing roller or ribbon crystallizers by installing elements responsible for the process of zone melting.

The technical result is achieved by conducting zone melting of the crystalline layer previously obtained by directional crystallization from the melt on the cooled working surface of the mold. The purified crystalline layer after the zone melting stage is removed from the working surface of the crystallizer. All stages are carried out simultaneously on different parts of the mold working surface, and the crystalline layer is transported from one section to another by moving the mold working surface, which is a roller or a tape, relative to these sections so that each point of the mold working surface passes through each of the stages . Directed crystallization is carried out by contacting the melt with a cooled portion of the working surface of the crystallizer, and zone melting is carried out using one or more heaters by local heating of one or more bands of the crystalline layer in which local zones of the melt are created, and the melt is periodically removed from one or more local melt zones by evacuation and / or capillary mass transfer using a melt receiver containing porous or fibrous material, heated about to a temperature higher than the melt temperature.

Figure 1 shows a portion of the working surface of the mold: heat transfer heaters 6 through the metal strip 1 to the crystalline layer together with the heaters in the casings 5 form local zones of the melt. There are several options for organizing heat transfer from the heater to the melt zone. The use of heat transfer through the working surface is advisable only for the tape apparatus, since the thickness of the tape used in these devices is much smaller than the wall thickness of the roller in the roller apparatus, and due to the small thickness of the tape, the inductive heat transfer in the longitudinal direction will be small, which will allow forming local melt zones. Heating of the crystalline layer from the side of the mold working surface is possible by means of radiation and / or convective heat transfer through the gas phase. The movement of the melt zone is possible in two ways: when using stationary and movable heaters. In the case of using stationary heaters, molten zones are formed that are motionless relative to the base of the apparatus; the movement of the melt zones along the crystalline layer is due to the movement of the working surface of the mold. In this case, the removal of impurities should be carried out by periodically removing the melt from each of the heated zones. During the passage of the zone through the crystalline layer, impurity components accumulate in it; moreover, enrichment of the impurity occurs faster in zones located closer to the area of the crystalline layer obtained at the stage of directed crystallization from the melt. Periodic removal of impurities does not interrupt the continuous operation of the apparatus, which makes it possible to characterize the process of zone melting as semi-continuous.

In the case of using movable heaters, melt zones are formed, moving in the opposite direction to the working surface of the mold using the reciprocating movement of a group of heaters with a sharp return so that pseudo-continuous movement of the local heating zones is created. The velocity of the melt zones relative to the crystalline layer is the sum of the velocity of the working surface of the mold and the velocity of the heaters in the direction opposite to the motion of the crystalline layer. In this case, it is advisable to remove the impurity by removing the melt in only one area on the working surface of the mold upon reaching each melt zone. Figure 2 shows a diagram of the conduct of zone melting on a tape apparatus. The closed metal strip 1 moves at a constant speed, transporting the crystalline layer. The crystalline layer obtained at the stage of crystallization from the melt can be divided according to the degree of purity into two layers: layer 7 contains a smaller amount of impurity, because formed by directional crystallization, layer 8 is a crystallized feed melt captured by the surface of layer 7, so layer 8 contains a greater amount of impurity. Heaters 4 and 6 form a local zone of melt 9. At the edge of the local zone of the melt, it is necessary to provide a sufficient temperature gradient for crystallization to occur, which is achieved by heat removal and local heating, for which heater 4 is closed by a casing 5. During crystallization, a crystalline layer forms 10 s lower content of impurities that accumulate in the local zone of the melt 9.

The melt removal process can be implemented using capillary mass transfer by contacting the melt with a porous or fibrous material heated to a temperature higher than the melt temperature and / or by vacuum. Figure 3 shows a diagram of the removal of the melt; the dotted line shows the trajectories of the heater 4 with the casing 5 and the melt receiver 2. The melt receiver 2 moves into the melt zone and the fibrous material 3 captures the melt.

Figure 4 shows a diagram of a strip crystallization apparatus for semi-continuous zone melting. The closed metal strip 1 is driven by rollers 20. From the bath 19, a supply melt is supplied to the tape, which partially crystallizes in the cooled section of the tape. The tape is cooled by spraying the refrigerant in the cooling devices 17. The crystalline layer 8 is transported on the tape in the local heating area created by the heaters in the casings 5. The melt receiver 2 serves to periodically remove the melt from the local zone of the melt, for which the melt receiver is periodically moved to the place of the heater 5. The purified crystalline layer 10 at the bend of the tape is destroyed and the resulting granular crystalline product 15 enters the receiver 18.

Figure 5 shows a diagram of the conduct of zone melting on a roller apparatus with a roller cooled over the entire inner surface. Since the working surface is cooled, the melt zone 9 is limited by the thickness of the layer. Conducting zone melting with a melt zone limited in thickness may also be advisable, since a larger amount of impurity is in the upper layer 8.

The linear velocity of the working surface will be limited by the process of zone melting, as a result of which the parameters of crystallization from the melt must be adapted to this speed, which can be done by adjusting the temperature of the melt, the temperature of the refrigerant, as well as the size of the area of the working surface of the mold immersed in the melt.

Figure 6 shows a diagram of a roller crystallization apparatus for semi-continuous zone melting. The rotating roller 11 is immersed in a container 12 with a supply melt 13. On the inner side, the roller is cooled by spraying refrigerant 16. On contact with the melt, a crystalline layer is formed 8. The crystalline layer 8 is transported on the surface of the roller in the area of local heating created by heaters in shell 5. The melt receiver 2 is used to periodically remove the melt from the local zone of the melt, for which the melt receiver is periodically moved to the place of the heater 5. Cleaned crystal layer 10 is removed with a knife 14, whereby a granular crystalline product 15.

As a comparative characteristic of the cleaning method known from [4], using the current melt film obtained by melting the crystalline layer and the proposed method of semi-continuous zone melting, the table shows the numerical data of the cleaning parameters. For the method of semicontinuous zone melting, several options with a different number of local zones of the melt are debased.

Cleaning method according to [4] - melting of the crystalline layer semi-continuous zone melting The number of local zones of the melt --- one 2 3 The proportion of the falling film,% of the mass. 25 --- --- --- The initial concentration of the fusible component,% of the mass. twenty twenty twenty twenty The output concentration of the low-melting component (2-methylnaphthalene) for a mixture of 2-methylnaphthalene-fluorene,% wt. 16 ... 18 6 ... 9 3 ... 5 0.5 ... 2 The output concentration of the low-melting component (diphenyl) for a mixture of naphthalene-diphenyl,% wt. 12 ... 15 4 ... 5 2 ... 3 0.5 ... 1

The higher content of the impurity component in the crystalline product during purification according to the method described in [4] is due to two factors: the need for partial melting of the crystalline layer, as a result of which only the molten part of the layer is involved in the redistribution of the impurity, and also the restriction of the purification process to one equilibrium step between the melt and the crystalline phase. The proposed method provides the possibility of melting the zone of the crystalline layer to its entire thickness, therefore, the entire amount of the mixture is involved in the process of impurity redistribution; can also be implemented in several stages of cleaning in one device. Thus, the inventions according to claims 1, 4, 5 allow to obtain a crystalline product of high purity.

Literature:

[1] US 6030588, 2000.02.29, IPC B01D 9/04.

[2] US 5725608, 1998.03.10, IPC B01D 9/00.

[3] SU 239226, 1968.02.16, IPC B01D.

[4] SU 273158, 1968.04.29, IPC B01D 9/00.

[5] SU 1124995, 1984.11.23, IPC B01D 9/02.

Claims (5)

1. The method of crystallization purification of substances by zone melting, characterized in that it includes a stage of obtaining a crystalline layer on a cooled surface by directional crystallization from a melt carried out before zone melting and a step of removing the obtained crystalline layer after zone melting, all stages being carried out simultaneously at different parts of the working surface of the mold, the transportation of the crystalline layer from one section to another is carried out by moving the working surface of the mold, consisting of a roller or tape, relative to these sections in such a way that each point of the working surface of the mold passes through each of the stages sequentially, directed crystallization is carried out by contacting the melt with the cooled part of the working surface of the mold, and zone melting is carried out using one or more heaters by local heating one or more stripes of the crystalline layer, in which create local zones of the melt, and periodically remove the melt Islands of one or more localized zones of the melt by evacuation and / or mass transfer through a capillary comprising a porous or fibrous material receiver, heated to a temperature above the melt temperature. 2. The method of crystallization treatment according to claim 1, characterized in that the local zones of the melt are moved towards the movement of the working surface of the mold by reciprocating the movement of the heaters. 3. The method of crystallization cleaning according to claim 1 and 2, characterized in that the local heating is carried out by heat transfer from the heating element and / or convective heat transfer from the gas phase directly to the crystalline layer and / or heat transfer through the working surface of the mold. 4. Roller mold, containing a bath with a melt, a rotating cooled roller, one or more heaters located between the area on which the roller is in contact with the melt and a device for removing the crystalline layer, characterized in that each heater is equipped with a casing for the formation of one or more local zones of the melt lying on the generatrix of the roller, while the mold is equipped with a receiver of the melt that is moved to the local zone of the melt and provides the ability to capture the latter. 5. A ribbon crystallizer containing a bath with a melt, a moving closed tape, a section on which the tape is in contact with the melt, a device for removing the crystalline layer, characterized in that between the area in which the tape is in contact with the melt and a device for removing the crystalline layer or more heaters with shells, providing the formation of one or more local zones of the melt, while the mold is equipped with a receiver of the melt that is moved to the local zone of the melt and provides the ability to capture the latter.

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