RU2812722C1 - Method for producing high-purity octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane by separating mixture of methylcyclosiloxane vapours - Google Patents

Abstract

FIELD: high-purity methylcyclosiloxanes.

SUBSTANCE: invention relates to methods for producing high-purity methylcyclosiloxanes. A method has been proposed for the production of high-purity octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) by separating a mixture of methylcyclosiloxane vapours. In a distillation column for the primary separation of methylcyclosiloxanes, made with a cube and operating at reduced pressure, a stream containing decamethylcyclopentasiloxane is isolated as a side selection, which is sent to the high-purity decamethylcyclopentasiloxane separation column, operating at reduced pressure. The distillates of the primary column and the decamethylcyclopentasiloxane separation column, which are a mixture of hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane, are sent to a distillation column for the separation of high-purity octamethylcyclotetrasiloxane, operating at reduced pressure. In this case, octamethylcyclotetrasiloxane is isolated as a bottom selection; a mixture of hexamethylcyclotetrasiloxane and octamethylcyclotetrasiloxane is obtained as a distillate containing octamethylcyclotetrasiloxane in the amount necessary to prevent the formation of the solid phase at ambient temperature.

EFFECT: use of the proposed method ensures the production of high-purity D4 and D5 while simultaneously reducing total energy costs by 31% compared to known methods; at the same time, the risk of D3 solidifying in the process equipment is eliminated.

17 cl, 2 dwg, 1 tbl, 5 ex

Description

The invention relates to the technology of organosilicon synthesis, in particular, to a method for producing high-purity methylcyclosiloxanes, namely octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane.

Methylcyclosiloxanes in industry are obtained directly during the hydrolysis of dimethyldichlorosilane or during the depolymerization of dimethyldichlorosilane hydrolyzate (L.M. Khananashvili, Chemistry and technology of organoelement monomers and polymers, M. “Chemistry” 1998; Yu.K. Molokanov et al., Separation of mixtures organosilicon compounds, Leningrad "Chemistry" 1986). To increase the yield of cyclosiloxanes formed during the depolymerization of dimethyldichlorosilane hydrolyzate, the target products are isolated from the reaction mixture. The depolymerization reaction is carried out at temperatures from 50°C to 250°C in the presence of an alkaline catalyst. The process pressure ranges from 1 mmHg to 760 mmHg and is usually selected so that low-boiling methylcyclosiloxanes (hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5)) are released into the reaction volume in the form of vapors, for subsequent condensation and separation. Along with low-boiling components, the condensate usually contains impurities of high-boiling cyclosiloxanes and linear polysiloxanes.

In accordance with the known method for the continuous production of cyclosiloxanes (regardless of which organic substituents are present in the original organochlorosilane), one reaction-distillation process combines the stage of depolymerization of the hydrolyzate, in a container with a stirring device, and the stage of rectification of a mixture of cyclosiloxanes (US Patent 3846464, 1974). The parameters of the technological operating mode of the distillation column are selected in such a way as to ensure the production of the target cyclosiloxane, which is usually D4. In this case, high-boiling reaction products relative to D4 are returned to the depolymerization reactor. The mixture of cyclosiloxanes obtained during depolymerization is subjected to separation during a periodic rectification process, in which low-boiling D3 and target D4 are isolated. High-boiling products dodecamethylcyclohexyloxane (D6) and others remain in the bottoms and are returned to the depolymerization stage for reprocessing into D4. Fraction D3 can be isolated as the target product or returned to the depolymerization stage for reprocessing into D4.

This process has a significant drawback associated with the need to isolate the low molecular weight product D3, which has a high melting point (+64°C). This necessitates heating of all process equipment (pipelines, tanks, shut-off valves and instrumentation) to prevent crystallization of this product.

The solution to this problem is to carry out the depolymerization process in a continuous flow apparatus in the liquid phase, with all products from the depolymerization reactor entering a separate rectification stage, consisting of one or more distillation columns (US Patent 5491249, 1996). In this case, cyclosiloxanes (D4 and D5) are isolated as target products, and the low-boiling product (D3) and high-boiling products (D6 and others including bottom liquid) are isolated as a technical fraction and returned back to the depolymerization stage.

The second way to solve the problem is to carry out the depolymerization process in a continuous flow apparatus in the gas phase (US Patent 4764631, 1988). The mixture of cyclosiloxanes from the depolymerization reactor is condensed and further separated into the target products by rectification.

These methods eliminate the problem of D3 crystallization in equipment and pipelines, but do not solve the issue of separating a mixture of cyclosiloxanes into target fractions. In addition, to isolate several target products, it is necessary to use a large number of distillation columns and capacitive equipment, which complicates the process of obtaining the target D5 and increases energy costs.

There is a known method for isolating D3 from the reaction mixture by adding an organic solvent to a distillation column on which the depolymerizate is separated (US Patent 5834585, 1998). A mixture of cyclosiloxane vapors enters the bottom of the column. An organic solvent having a boiling point close to the boiling point of D3 enters the upper part of the column. The target product D3, mixed with a solvent in a ratio of 1:2, is removed from the column as a side selection. The liquid remaining in the bottom of the column is a mixture of cyclosiloxanes D4, D5, D6 and higher boiling ones, which is then separated into target fractions in the usual manner. This technique allows you to completely eliminate the solidification of D3 in pipelines and equipment.

US Pat. No. 5,660,690 (1997) describes a more complicated version of the isolation of D3 from a mixture of cyclosiloxanes using two solvents. The low-boiling solvent, which forms an azeotrope with water, is removed from the system first, removing all low-boiling impurities. The solvent, which has a boiling point close to the boiling point of D3, is removed from the rectification system together with D3. This solution allows you to simultaneously remove low-boiling impurities and isolate D3 without the threat of product solidification in pipelines and equipment.

A common disadvantage of methods for separating a mixture of cyclosiloxanes that involve the addition of organic solvents to exclude crystallization of D3 is the inevitable appearance of foreign impurities (organic solvents) in the target cyclosiloxanes, which is unacceptable if the target D4 and D5 are intended to be used to obtain materials of electronic purity.

The closest to the proposed invention (adopted as a prototype) is a method for separating cyclosiloxane vapors released from a depolymerization reactor (II) (US Patent 4197251, 1980), according to which (Fig. 1) the primary separation of cyclosiloxanes is carried out on a distillation column (I) installed over a heated depolymerization reactor (II), while the target cyclosiloxanes are isolated on the column (I) as a distillate, and high-boiling products and raw materials carried away from the reactor (II) (hydrolysate) are returned to the reactor (II). The distillate of column (I) is then sent to a distillation column (III), where low-boiling hexamethylcyclotrisiloxane D3 is isolated in the form of a distillate. To prevent crystallization of D3 in pipelines and equipment, dimethyldichlorosilane hydrolyzate is fed into the collection (V) with the distillate, in which the resulting D3 is completely dissolved, and then the resulting mixture is sent for depolymerization to the reactor (II). The bottom liquid of column (III), which is D4 with impurities of high-boiling products, is then sent to a distillation column (IV), where pure D4 is separated as a distillate and high-boiling impurities, which is then returned for depolymerization to the reactor (II).

The disadvantage of this method is the lack of a comprehensive solution to the problem of crystallization of D3 in the upper part of the column (III), pipelines, condenser and reflux tank. The disadvantage of this method is also the high specific energy consumption required to isolate commercial D4 from the depolymerization products of dimethyldichlorosilane hydrolyzate.

Isolation of the target D5 is not provided for in US patent 4197251, 1980, but is technically possible. In accordance with the method for separating methylcyclosiloxanes described in the patent, it is possible to isolate the target D5 as the bottom liquid of column (IV). In this case, the total energy consumption for the separation of depolymerization products of dimethyldichlorosilane hydrolyzate increases significantly due to the need to ensure the operation of the distillation column to isolate the target D5.

The objective of this invention is to create an economical continuous process for separating a mixture of methylcyclosiloxane vapors, regardless of the process of obtaining this mixture, ensuring the production of D4 and D5 of high purity with minimal energy consumption and the exclusion of crystallization of D3 in the technological scheme under any technological operating conditions of the equipment.

The problem is solved by the fact that a method is proposed for producing high-purity D4 and D5 by separating a mixture of methylcyclosiloxane vapors, while in a rectification column for the primary separation of methylcyclosiloxanes, made with a bottom part and operating at reduced pressure, the stream containing D5 is isolated as a side selection, which is sent to the column separation of high-purity D5, operating at reduced pressure, distillates of the primary column and separation column D5, which are a mixture of D3 and D4, are sent to a rectification column for separation of high-purity D4, operating at reduced pressure; in this case, D4 is isolated as a bottom selection; a mixture of D3 and D4 containing D4 is obtained as a distillate, in the amount necessary to prevent the formation of the solid phase at ambient temperature.

There is an option in which a mixture of methylcyclosiloxane vapors is obtained as a result of hydrolysis of dimethyldichlorosilane, and/or as a result of depolymerization of the hydrolyzate, and/or as a result of evaporation of a mixture of methylcyclosiloxanes.

There is an option in which the bottom part of the column for the primary separation of methylcyclosiloxanes is a reactor for depolymerization of the hydrolyzate of dimethyldichlorosilane and/or methylcyclosiloxanes.

There is an option in which the bottom liquid leaving the column for the primary separation of a mixture of methylcyclosiloxanes is returned to the depolymerization reactor for recycling into methylcyclosiloxanes.

There is an option in which the mixture of D3 and D4, obtained as the distillate of the D4 separation column, is returned to the depolymerization reactor for reprocessing into methylcyclosiloxanes.

There is an option in which the temperature in the bottom part of the column for the primary separation of methylcyclosiloxanes is from 100 to 180 °C, preferably from 140 to 150 °C.

There is an option in which the head pressure of the primary separation column of methylcyclosiloxanes is from 10 to 25 kPa, preferably from 15 to 17 kPa.

There is an option in which the pressure at the top of the separation column D5 is lower than the pressure at the top of the primary separation column for methylcyclosiloxanes and ranges from 4 to 12 kPa, preferably from 6.5 to 7.5 kPa.

There is an option in which the pressure at the top of the separation column D4 is lower than the pressure at the top of the primary separation column of methylcyclosiloxanes and ranges from 4 to 12 kPa, preferably from 6.5 to 7.5 kPa.

There is an option in which the height of the column for the primary separation of methylcyclosiloxanes is at least 15 theoretical plates.

There is an option in which the height of the D4 separation column is at least 11 theoretical plates.

There is an option in which the height of the D5 separation column is at least 15 theoretical plates.

There is an option in which the height of the column for the primary separation of methylcyclosiloxanes, ensuring the production of D4 and D5 with a main substance content of 99.98 wt% or more, is more than 25 theoretical plates, preferably 30 theoretical plates.

There is an option in which the height of the separation column D4, ensuring the production of D4 and D5 with a main substance content of 99.98 wt% or more, is more than 13 theoretical plates, preferably 15 theoretical plates.

There is an option in which the height of the separation column D5, ensuring the production of D4 and D5 with a main substance content of 99.98 wt% or more, is more than 18 theoretical plates, preferably 20 theoretical plates.

There is an option in which the pressure in the bottom part of the column for the primary separation of methylcyclosiloxanes, which is a depolymerization reactor, is from 10 to 25 kPa, preferably from 15 to 17 kPa.

There is an option in which the temperature in the bottom part of the column for the primary separation of methylcyclosiloxanes, which is a depolymerization reactor, is from 100 to 180 °C, preferably from 140 to 150 °C.

According to the proposed method (Fig. 2), a mixture of methylpolysiloxanes, from which the target products D4 and D5 are to be isolated, enters apparatus (II). In the preferred case, when the starting mixture is a hydrolyzate of dimethyldichlorosilane (1), apparatus (II) is a reactor in which the depolymerization process takes place. The resulting pairs of methylcyclosiloxanes (2) with an admixture of unreacted hydrolyzate components are sent for separation to the lower part of the distillation column (I).

In the case when the initial mixture is a depolymerizate (a mixture of methylcyclosiloxanes, regardless of the method of its preparation), apparatus (II) is the boiler of the distillation column (I), providing the distillation column (I) with the necessary steam load.

It is possible that the methylcyclosiloxanes to be separated enter the lower part of the column in the form of vapor from an external source (for example, from a gas-phase depolymerization reactor). In this case, apparatus (II) is the reboiler of the distillation column (I), stabilizing the steam load of the distillation column (I) and increasing the degree of separation of methylcyclosiloxanes from the initial mixture.

The pressure in the apparatus (II) is from 10 to 25 kPa, preferably from 15 to 17 kPa. The temperature in apparatus (II) is from 100 to 180 °C, preferably from 140 to 150 °C.

The vapors of methylcyclosiloxanes (2) entering the lower part of the distillation column (I) are subjected to rectification. Most of the D3 supplied as part of the initial mixture is isolated as a distillate (5). The distillate also contains D4 in an amount of approximately 50% of the total volume of D4 entering column (I) in the form of vapor (2). D5 and higher boiling components are absent in the distillate (5). All of the D5 entering the column (I) is removed as a side selection (4) along with the remaining amount of D4 that is not included in the distillate (5) and the non-standardized part of D3. All high-boiling components (D6 and higher boiling) are returned in the form of bottom liquid (3) back to the apparatus (II).

The height of the column (I) is at least 15 theoretical plates, which ensures the production of the target D4 and D5 with a main substance content of at least 99% by weight. The preferred column height (I) for the production of high-purity D4 and D5, with a main substance content of 99.98% mass or more for each is more than 25 tt. More preferably, the height of the column (I) is 30 tt. The pressure at the top of the column (I) is from 10 to 25 kPa, preferably from 15 to 17 kPa. The reflux ratio of the column (I), the fraction of side withdrawal (4) and the level of location of the side selection (4) are selected in such a way as to exclude the entry of D5 into the distillate (5) and to exclude the entry of high-boiling products (starting from D6) into the composition of the side selection (4).

The distillate (5) and side withdrawal (4) leave the column (I) as a gas phase (preferred). It is possible to carry out the withdrawal of both the distillate (5) and side selection (4) in the form of a liquid phase, regardless of the phase state of other streams leaving the column (I).

In the preferred case, when apparatus (II) is a depolymerization reactor, the bottom liquid (3) returned from column (I) is subjected to repeated depolymerization together with the original hydrolyzate (1).

In the case when the apparatus (II) is a reboiler of the column (I), the accumulating high-boiling products of the distillation of the mixture (1) are periodically or continuously removed from the apparatus (II) for further processing (this flow is not shown in Fig. 2).

The D5 isolated on the column (I) as part of the side selection (4) is supplied as feed to the distillation column (III), designed to obtain commercial D5 of high purity. Commercial D5 is obtained as the bottom liquid (6) of column (III). The entire volume of D3 and D4 supplied to the column (III) as part of the feed is isolated in the form of a distillate (7).

The height of the column (III) is at least 15 theoretical plates, which ensures the production of commercial D4 and D5 with a main substance content of at least 99% by weight. The preferred height of column (III) for the production of high-purity D4 and D5, with a main substance content of 99.98% mass or more for each, is more than 18 t.t. The height of the column (III) is more preferable 20 t.t. The pressure of the top of the column (III) is lower than the pressure of the top of the column (I) and ranges from 4 to 12 kPa, preferably from 6 .5 to 7.5 kPa. The reflux ratio, the location of the feed plate and other technological parameters of column (III) are chosen in such a way as to prevent the entry of D4 into the composition of the bottom liquid and D5 into the distillate.

The distillate (5) of the column (I) and the distillate (7) of the column (III) are supplied as feed to the rectification column (IV), designed to produce D4 of high purity. The target D4 is obtained as the bottom liquid (8) of the column (IV). The entire volume of D3 supplied to the column (IV) as part of the feed is isolated in the form of a distillate (9). At the same time, D4 is present in the distillate (9) in an amount that ensures the exclusion of the formation of a solid phase at ambient temperature (at least 50% D4 in the distillate).

The height of the column (IV) is at least 11 theoretical plates, which ensures the production of commercial D4 and D5 with a main substance content of at least 99% by weight. The preferred height of the column (IV) for the production of high-purity D4 and D5, with a main substance content of 99.98% mass or more for each, is more than 13 t.t. The height of the column (IV) is more preferable 15 t.t. The pressure of the top of the column (IV) is lower than the pressure of the top of the column (I) and ranges from 4 to 12 kPa, preferably from 6 .5 to 7.5 kPa. The reflux ratio, the location of the feed plate and other technological parameters of the column (IV) are chosen in such a way as to exclude the entry of D3 into the composition of the bottom liquid and at the same time ensure the specified composition of the distillate (9).

In the preferred case, when apparatus (II) is a depolymerization reactor, the mixture D3 and D4 obtained on column (IV) is returned to apparatus (II) and subjected to repeated depolymerization together with the original hydrolyzate (1).

In cases where the return of distillate (9) to column (IV) to apparatus (II) is not possible or does not seem appropriate, the resulting mixture of D3 and D4 can be used to obtain polymer products.

In the preferred case, when apparatus (II) is a depolymerization reactor, in order to increase the overall conversion of the initial mixture (1) to D5, it is possible to return the D4 obtained on column (III) to apparatus (II) for repeated depolymerization. In the case where the separation of D4 is not necessary, it is possible to exclude column (IV) and completely return the distillate (5) and distillate (7), which are a mixture of D3 and D4 of various compositions, to the depolymerization reactor (II), or use the resulting mixtures D3 and D4 for obtaining polymer products.

The use of the proposed method for separating a mixture of methylcyclosiloxane vapors obtained by any method (liquid-phase and gas-phase depolymerization of dimethyldichlorosilane hydrolyzate and/or depolymerization of methylcyclosiloxanes in a depolymerization reactor, as well as evaporation of methylcyclosiloxanes in the reboiler of a distillation column) allows one to obtain high-purity D4 and D5. Separating a mixture using the proposed method requires less energy than using known methods for separating a given mixture because the process flows have been optimized. An additional feature of the process is the elimination of solidification of D3 in pipelines and equipment under any technological operating modes of the equipment, including start-up and stop modes of the process, which is achieved by the absence of process streams with a high (more than 50%) content of hexamethylcyclotrisiloxane.

The use of the proposed method for the continuous separation of a mixture of methylcyclosiloxanes makes it possible to obtain high-purity D4 and D5 with a main substance content of 99.98 (99.99% wt). The energy savings required to obtain high-purity D4 and D5 are 31% compared to the prototype (Table 1).

Example 1 (comparative, Fig.1).

Comparative example 1 describes a method for isolating the target products D4 and D5, as close as possible to the method described in the patent (US 4197251, 1980), taken as a prototype. In order to ensure a comparison of the results of this example with the claimed method for the continuous isolation of pure D4 and D5, the method according to the prototype (US 4197251, 1980), a change was made that was not described in the prototype, namely, a commercial separation column D5 was added.

The mixture of siloxanes obtained during the hydrolysis of dimethyldichlorosilane is continuously fed at a speed of 1000 kg/h into the collection tank (V), where it is mixed with hexamethylcyclotrisiloxane D3, isolated on the column (III). The resulting mixture is then sent to the depolymerization reactor (II). The depolymerization process occurs at a temperature of 142°C. Vapors of methylcyclosiloxanes formed during the depolymerization process leave the depolymerization reactor (II) and enter the lower part of the primary separation column of methylcyclosiloxanes (I), which has a height of 30 theoretical plates. The pressure at the top of the column is 16 kPa (120 mmHg), the temperature at the top of the column is 122 °C. The column operates at a reflux ratio of 0.6.

Column (I) separates low-boiling methylcyclosiloxanes, namely D3, D4 and D5, as a distillate. Products with a higher boiling point are effectively separated and returned as bottom liquid back to reactor (II) for repeated depolymerization.

The distillate of column (I) with a flow rate of 1020 kg/h is continuously fed to separation column (III) D3, which has a height of 15 tons and operates under a column top pressure of 16 kPa (120 mm Hg). The reflux ratio is 30.5. A mixture of D4 and D5 is obtained as a bottom liquid in column (III), and D3 is obtained as a distillate. The upper part of the column (III), all pipelines and the reflux tank are provided with a heating system in which the temperature is maintained at least 60°C to prevent solidification of D3 in the column system. For the same purpose, a coolant with an initial temperature of 60 ° C is supplied to the reflux condenser of column (III). The average temperature difference between the coolant and the condensed liquid is 19 ° C.

The resulting D3 is sent to the collection (V), where it is mixed with the hydrolyzate. The resulting mixture is sent to the depolymerization reactor (II) to obtain methylcyclosiloxanes.

The bottom liquid of column (III) with a flow rate of 1000 kg/h is continuously supplied to column (IV), which has a height of 20 tons and operates under a column top pressure of 7.33 kPa (55 mm Hg). The reflux ratio is 1.51. D5 is obtained as the bottom liquid in column (IV), and D4 is obtained as the distillate.

The total energy consumption for the separation of commercial D4 and D5 is 1556.6 MJ/h (Table 1).

Example 2 (comparative).

Comparative example 2 demonstrates the effect of changing the order of isolation of components D3, D4 and D5 in the method of isolating the target products D4 and D5, which is as close as possible to the conditions of example 1 (comparative).

The mixture of siloxanes obtained during the hydrolysis of dimethyldichlorosilane is continuously fed into the collection (V) at a speed of 1000 kg/h, where it is mixed with D3 released on the column (IV). The resulting mixture is then sent to the depolymerization reactor (II). The depolymerization process occurs at a temperature of 142°C. Vapors of methylcyclosiloxanes formed during the depolymerization process leave the depolymerization reactor (II) and enter the lower part of the primary separation column of methylcyclosiloxanes (I), which has a height of 30 theoretical plates. The pressure at the top of the column is 16 kPa (120 mmHg), the temperature at the top of the column is 122 °C. The column operates at a reflux ratio of 0.6.

Column (I) separates low-boiling methylcyclosiloxanes, namely D3, D4 and D5, as a distillate. Products with a higher boiling point are effectively separated and returned as bottom liquid back to reactor (II) for repeated depolymerization.

The distillate of column (I) with a flow rate of 1020 kg/h is continuously fed to column (IV), which has a height of 20 tons and operates under a column top pressure of 7.33 kPa (55 mm Hg). The reflux ratio is 1.44. The target D5 is obtained as a bottom liquid in column (IV), and a mixture of D3 and D4 is obtained as a distillate.

The distillate of column (IV) with a flow rate of 837 kg/h is continuously supplied to column (III) for the separation of D3 and D4, which has a height of 15 tons and operates under a column top pressure of 16 kPa (120 mm Hg). The reflux ratio is 21.8. D4 is obtained as the bottom liquid in column (III), and D3 is obtained as the distillate. The upper part of the column (III), all pipelines and the reflux tank are equipped with a heating system in which the temperature is maintained at least 60°C to prevent solidification of D3 in the column system. For the same purpose, a coolant with an initial temperature of 60 ° C is supplied to the reflux condenser of column (III). The average temperature difference between the coolant and the condensed liquid is 19 ° C.

The resulting D3 is sent to the collection (V), where it is mixed with the hydrolyzate. The resulting mixture is sent to the depolymerization reactor (II) to obtain methylcyclosiloxanes.

The total energy consumption for the separation of commercial D4 and D5 is 1534.5 MJ/h (Table 1).

Changing the order of separation of components from the mixture of methylcyclosiloxane vapors obtained during depolymerization practically does not affect the total energy consumption required to obtain commercial D4 and D5.

Example 3 (claimed).

The mixture of siloxanes obtained during the hydrolysis of dimethyldichlorosilane is continuously fed into the depolymerization reactor (II) at a rate of 1000 kg/h. The depolymerization process occurs at a temperature of 142°C. Vapors of methylcyclosiloxanes formed during the depolymerization process leave the depolymerization reactor (II) and enter the lower part of the primary separation column of methylcyclosiloxanes (I), which has a height of 30 theoretical plates. The pressure at the top of the column is 16 kPa (120 mmHg), the temperature at the top of the column is 122 °C. The column operates at a reflux ratio of 1.85. The methylcyclosiloxane vapors leaving the column undergo partial condensation in the column's reflux condenser. The condensed reflux is returned to reflux column (I). The distillate, which is a mixture of D3 and D4, leaves column (I) in the form of vapor and enters column (IV) for further separation. The distillate contains 50% of the original D4, which entered column (I) from the depolymerization reactor (II).

A mixture of methylcyclosiloxane vapors in an amount of 612 kg/h, consisting of D3, D4 and D5, is continuously removed from a side plate located at 47% of the total height of column (I) and sent for further separation into column (III).

Other products having a boiling point higher than D5 are returned from column (I) as bottom liquid back to reactor (II) for repeated depolymerization.

The side selection of column (I) is continuously supplied to the separation column (III) D5, which has a height of 20 t.t. and operates under a column top pressure of 7.33 kPa (55 mmHg). The reflux ratio is 2.1. The target D5 with a total impurity content of less than 0.01 wt% is obtained as a bottom liquid in column (III). As a distillate in column (III), a mixture of D3 and D4 is obtained, which is sent for further separation into column (IV).

The distillate of column (I) with a flow rate of 430 kg/h and the distillate of column (III) with a flow rate of 424 kg/h are continuously fed to the separation column (IV) D4, which has a height of 15 tons and operates under a column top pressure of 7.33 kPa (55 mmHg). The reflux ratio is 15.2. The target D4 with a total impurity content of less than 0.01 wt% is obtained as a bottom liquid on column (IV). As a distillate of column (IV), a mixture of D3 and D4 is obtained, in which the D4 content is more than 50 wt%. Temperature at which crystallization of the resulting mixture begins is less than 20°C, which makes it possible to use recycled water as a coolant in the column dephlegmator. The average temperature difference between the coolant and the condensed liquid is 40°C, which is twice as high as compared to the prototype (example 1, comparative).

The mixture D3 and D4 obtained on the column is sent to reactor (II) for repeated depolymerization.

The total energy consumption for the separation of commercial D4 and D5 is 1075.9 MJ/h (Table 1). Energy savings compared to the prototype are 31%.

An additional effect of the proposed method for the continuous separation of a mixture of methylcyclosiloxane vapors is to reduce by at least half the size of the reflux condenser of the D3 separation column, as well as to eliminate the expensive heating system for the top of the column, the reflux condenser and pipelines above the pour point of D3.

Example 4 (claimed).

The conditions for separating a mixture of methylcyclosiloxanes supplied in the form of vapors to the column for the primary separation of methylcyclosiloxanes (I) from the depolymerization reactor (II) generally correspond to the conditions described in example 3 (claimed).

The difference from the conditions of example 3 is the presence of exhaustive condensation of the distillate of column (I). The fully condensed distillate of column (I) is sent for further separation into column (IV) in the liquid phase.

The total energy consumption for the separation of commercial D4 and D5 is 1106.2 MJ/h (Table 1). Energy savings compared to the prototype are 29%.

Example 5 (claimed).

The conditions for separating a mixture of methylcyclosiloxanes supplied in the form of vapors to the column for the primary separation of methylcyclosiloxanes (I) from the depolymerization reactor (II) generally correspond to the conditions described in example 4 (claimed).

The difference from the conditions of example 4 is the presence of side selection of column (I) in the form of a liquid phase. The side selection of column (I) is sent for further separation into column (III) in the liquid phase.

The total energy consumption for the separation of commercial D4 and D5 is 1205.1 MJ/h (Table 1). Energy savings compared to the prototype are 23%.

Table 1. example column (I) phase state total energy consumption, MJ/t distillate side selection example 1, comparative liquid - 1556.6 example 2, comparative liquid - 1534.5 example 3, claimed steam steam 1075.9 example 4, claimed liquid steam 1106.2 example 5, claimed liquid liquid 1205.1

Claims (17)

1. A method for producing high-purity octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane by separating a mixture of methylcyclosiloxane vapors, characterized in that in a distillation column for the primary separation of methylcyclosiloxanes, made with a bottom part and operating at reduced pressure, a stream containing decamethylcyclopentasiloxane is isolated as a side selection, which is sent to a column for the separation of high-purity decamethylcyclopentasiloxane operating at reduced pressure, the distillates of the primary column and the decamethylcyclopentasiloxane separation column, which are a mixture of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane, are sent to a rectification column for the separation of high-purity octamethylcyclotetrasiloxane, operating at reduced pressure; in this case, octamethylcyclotetrasiloxane is isolated as a bottom selection; a mixture of hexamethylcyclotetrasiloxane and octamethylcyclotetrasiloxane is obtained as a distillate containing octamethylcyclotetrasiloxane in the amount necessary to prevent the formation of the solid phase at ambient temperature. 2. The method according to claim 1, characterized in that a mixture of methylcyclosiloxane vapors is obtained as a result of hydrolysis of dimethyldichlorosilane, and/or as a result of depolymerization of the hydrolyzate, and/or as a result of evaporation of a mixture of methylcyclosiloxanes. 3. The method according to claim 2, characterized in that the bottom part of the column for the primary separation of methylcyclosiloxanes is a depolymerization reactor for the hydrolyzate of dimethyldichlorosilane and/or methylcyclosiloxanes. 4. The method according to claim 3, characterized in that the bottom liquid leaving the column for the primary separation of the methylcyclosiloxane mixture is returned to the depolymerization reactor for recycling into methylcyclosiloxanes. 5. The method according to claim 3, characterized in that the mixture of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane, obtained as a distillate of the octamethylcyclotetrasiloxane separation column, is returned to the depolymerization reactor for recycling into methylcyclosiloxanes. 6. The method according to claim 1, characterized in that the temperature in the bottom part of the column for the primary separation of methylcyclosiloxanes is from 100 to 180 °C, preferably from 140 to 150 °C. 7. The method according to claim 1, characterized in that the pressure at the top of the column for the primary separation of methylcyclosiloxanes is from 10 to 25 kPa, preferably from 15 to 17 kPa. 8. The method according to claim 1, characterized in that the pressure at the top of the decamethylcyclopentasiloxane separation column is lower than the pressure at the top of the primary separation column for methylcyclosiloxanes and ranges from 4 to 12 kPa, preferably from 6.5 to 7.5 kPa. 9. The method according to claim 1, characterized in that the pressure at the top of the octamethylcyclotetrasiloxane separation column is lower than the pressure at the top of the primary separation column for methylcyclosiloxanes and ranges from 4 to 12 kPa, preferably from 6.5 to 7.5 kPa. 10. The method according to claim 1, characterized in that the height of the column for the primary separation of methylcyclosiloxanes is at least 15 theoretical plates. 11. The method according to claim 1, characterized in that the height of the separation column D4 is at least 11 theoretical plates. 12. The method according to claim 1, characterized in that the height of the separation column D5 is at least 15 theoretical plates. 13. The method according to claim 1, characterized in that the height of the column for the primary separation of methylcyclosiloxanes, ensuring the production of octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane with a main substance content of 99.98 wt%. and more is more than 25 theoretical plates, preferably 30 theoretical plates. 14. The method according to claim 1, characterized in that the height of the octamethylcyclotetrasiloxane separation column, ensuring the production of octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane with a main substance content of 99.98 wt%. and more is more than 13 theoretical plates, preferably 15 theoretical plates. 15. The method according to claim 1, characterized in that the height of the decamethylcyclopentasiloxane separation column, ensuring the production of octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane with a main substance content of 99.98 wt%. and more is more than 18 theoretical plates, preferably 20 theoretical plates. 16. The method according to claim 3, characterized in that the pressure in the bottom part of the column for the primary separation of methylcyclosiloxanes, which is a depolymerization reactor, is from 10 to 25 kPa, preferably from 15 to 17 kPa. 17. The method according to claim 3, characterized in that the temperature in the bottom part of the column for the primary separation of methylcyclosiloxanes, which is a depolymerization reactor, is from 100 to 180 °C, preferably from 140 to 150 °C.