RU2153486C2 - Method of recovery of finely dispersed solid, resinous and high-melting by-products from reaction gases resulting from pyrolysis of dichloroethane in vinyl chloride production - Google Patents

Abstract

FIELD: separation of products of pyrolysis of dichloroethane in vinyl chloride production. SUBSTANCE: described is method of recovery of finely dispersed solid resinous and high-melting flow-line products from reaction gases resulting from pyrolysis of dichloroethane in vinyl chloride production and having temperature of 440-520 C and pressure of 10-28 atm. Method comprises quenching said gases in hardening column and subsequently separating pyrolysis products. Hardening and separation are carried out by bubbling through layer of liquid concentrated by-products of said gases in hardening column still with boiling temperature thereof ranging from 120 to 200 C steam and gas mixture is made to contact with condensate of said gases being recycled on dripprool and mass-exchange trays of rectification zone with steam and gas mixture cleaned with respect to height of rectification zone. Final stage, involves feeding steam and gas mixture for further rectification and recovering and withdrawing products boiling at temperature higher than that of dichloroethane and recycling said distillate into process. EFFECT: high-grade vinyl chloride, creation of conditions for preparing perchloroethylene, trichloroethylene, reduced labor required for cleaning rectification columns. 2 cl, 3 ex, 2 dwg, 2 tbl

Description

 The invention relates to the technology of basic organic synthesis and relates to a method for the separation of finely divided solid, resinous and high boiling point by-products from dichloroethane pyrolysis reaction gases in the production of vinyl chloride. Vinyl chloride is used to produce various polymeric materials.

 A known method for the separation of chlorohydrocarbons and by-products in the production of perchlorocarbons by quenching in condensation-stripping columns, the first stage of which is the cooling of the initial reaction gases to the boiling point of the bottom liquid by, for example, bubbling them through a layer of this liquid and then contacting the cooled gases with flowing reflux on distillation plates with a corresponding decrease in the concentration of high-boiling products in the vapor and gas phases along the height of the condensation-steam columns [1].

 The use of this technique for quenching and separation of the reaction gases of dichloroethane pyrolysis in the production of vinyl chloride requires relatively high temperatures in the condensation-stripping column due to the relatively high pressures of the pyrolysis process and raises concerns regarding the increased formation of vinyl chloride polymerization by-products. It is this factor that makes it difficult to noticeably improve the existing production of vinyl chloride, in particular, the process of separating finely dispersed solid, tarry and high boiling point by-products from reaction gases.

The closest method of the same purpose to the claimed invention by the totality of the features of the method of separation of finely divided solid, tarry and high boiling point by-products from dichloroethane pyrolysis reaction gases in the production of vinyl chloride having a temperature of 440-520 ^o C and a pressure of 10-18 atm, by rapid cooling in a quenching column with subsequent separation of the products of dichloroethane pyrolysis is the technical solution described in [2].

The quenching of the reaction gases by this method is carried out at a temperature of 105-200 ^o C in the column 1, acting on the principle of a mixing condenser by spraying the quenching liquid 2 circulating with the help of the pump 4, which is formed by cooling these reaction gases (Fig. 1). Next, part 7a of the reaction products condensed in the condenser 6 is returned to column 1, and the remaining part 7b, together with gaseous products 8 and quenching liquid, is subjected to rectification in a distillation column system, which provides for the distillation of initially hydrogen chloride, then vinyl chloride, then “low-boiling” by-products pyrolysis products (chloroprene, benzene, etc.) and, finally, unreacted dichloroethane returned to pyrolysis. The separation of dichloroethane returned to pyrolysis is carried out in conjunction with the separation of "high-boiling" by-products from "direct" dichloroethane synthesized from ethylene and chlorine by first distilling off pure dichloroethane and then residual dichloroethane from bottoms.

This classical scheme of quenching by cooling and separation of reaction products by sequential isolation of light components has a number of serious drawbacks:
- difficult working conditions of the rectification system, including the column for quenching the reaction gases, due to the presence of highly viscous products (coke, resinous) in the stilling liquid, which forces the installation of special filters 3 on the still-liquid lines (Fig. 1) (seven pairs of alternately working apparatus) to separate these impurities. At the same time, the presence of such impurities in still liquids of distillation columns helps to reduce the duration of the run of boilers between cleanings. All this implies a noticeable use of manual labor to remove accumulated harmful products of increased danger from filters and boilers;
- the presence in the technological scheme of the hardening column of a circulation pump operating in difficult conditions due to the need to circulate a large stream of highly tarred liquid with solid coke particles;
- bottoms of dichloroethane rectification after separation of dichloroethane returned to pyrolysis cannot be used for processing into other target products, for example, in the production of perchlorethylene, trichlorethylene, etc., and must be destroyed due to the presence of C ₄ -C ₆ impurities in them ( dichlorobutenes, chlorobenzenes), which are formed during the pyrolysis of dichloroethane. As a result of the destruction of bottoms, such relatively valuable products for the synthesis of perchlorethylene and other products as higher chloroethanes formed during the synthesis of dichloroethane are irretrievably lost.

The objective of the invention is the separation of finely divided solid, tarry and high boiling point by-products from the reaction gases of dichloroethane pyrolysis at the quenching stage. When carrying out the invention, the technical result can be obtained:
- obtaining the target vinyl chloride of high quality;
- creating the conditions for the possible qualified use of secondary polychlorides of the C ₂ fraction (higher chloroethanes) formed during the synthesis of dichloroethane, for example, to obtain perchlorethylene, trichlorethylene, etc .;
- concentration of coke, tarry and high-boiling by-products of dichloroethane pyrolysis reaction gases and their removal from the gas-vapor mixture separation system and, as a result, elimination of their ingress into distillation column boilers and labor costs for cleaning the latter.

The specified technical result is achieved by the fact that in the claimed method for the isolation of finely divided solid, tarry and high boiling point by-products from dichloroethane pyrolysis reaction gases in the production of vinyl chloride having a temperature of 440-520 ^o C and a pressure of 10-28 atm, by rapidly cooling them in a quenching column and subsequent separation of the pyrolysis products, a feature is that the quenching and separation of the pyrolysis gases is carried out by bubbling them through a layer of liquid concentrated by-products of these a call in the cube of the quenching column at a boiling point of 120-200 ^° C and subsequent contact of the vapor-gas mixture with the condensate of these gases on the drip and mass transfer plates of the distillation zone of the quenching column with cleaning the vapor-gas mixture from high-boiling components along the height of the rectification zone, with the output of accumulating liquid by-products products, followed by additional separation and concentration by single distillation and supplying a gas-vapor mixture for additional distillation, with isolation and withdrawal from the process of products boiling above dichloroethane, and returning the distillate of this distillation to the process. In addition, the peculiarity of the method lies in the fact that the gas-vapor mixture after quenching of the reaction gases is in contact with the return condensate (reflux) at 5-10 theoretical stages of mass transfer.

 As shown by calculations confirmed by practical experiments, the installation of only 15 real plates (about 8 theoretical stages of separation) can reduce the concentration of high-boiling by-products in the exhaust gas mixture to less than 0.02%, and their concentration in the quenching liquid rises to approximately ten%. An increase in the number of theoretical stages of separation to 15–20 allows the latter to be increased to 30–40%. This complication of the technology is justified by the fact that the subsequent rectification of the raw material of “reverse” dichloroethane together with “direct” dichloroethane allows its bottoms to be used for processing into products such as perchlorethylene and others, since they practically do not contain impurities of C4-C6 chlorohydrocarbons, giving at such processing a large number of by-products (hexachlorobutadiene, hexachlorobenzene).

 The method is as follows.

The reaction gases (temperature 440-520 ^o C) from the coil of the dichloroethane pyrolysis furnace are fed into the cube of the quenching column 1 (Fig. 2) through a bubbler into a layer of concentrated concentrated by-products at a boiling point of 120-200 ^o C. The resulting vapor-gas mixture passes through droplet breaker 19 and mass transfer 20 plates of the distillation zone of the quenching column 1 and enters in the form of 3 into the refrigerator 4. Part of the condensate of this vapor-gas mixture is returned to the quenching column 1 in the form of reflux 5a, and part in the form of distillate 5b together with non-condensers nnym stream 6 is fed for further separation.

 The liquid 2 taken from the bubbling layer is fed into the evaporation circuit, which includes a container 7, a forced circulation device 8 and an evaporator 9. The vapor-gas mixture from the evaporation circuit is fed to a distillation column 11 with a boiler 12, a condenser 13 and a reflux tank 14 to separate the target dichloroethane in the form of a distillate, part of which is returned to the distillation column 11 in the form of reflux using a pump 15, and part is sent to the quenching column 1 using a pump 16. The bottom product 18 from the column 11 stno with concentrated products osmol 17 discharged from the system.

Example 1. Reaction gases from a pyrolysis furnace to which dichloroethane is supplied in an amount of 54 t / h for pyrolysis at a temperature of 500 ^° C. and a pressure of 20 atm are supplied through a bubbler to a cube of the quenching column with a diameter of 2200 mm. The resulting vapor-gas mixture passes through two droplet-separating and ten mass transfer plates and enters the refrigerators, where it is cooled to 80 ^o C, and then to be separated by distillation. A portion of the condensate from the refrigerators in an amount of 54900 kg / h is returned to the upper plate of the quenching column. The rest of the condensate is sent for distillation together with the exhaust gas-vapor mixture.

The liquid taken from the bubbler layer in an amount of 1600 kg / h is discharged into a container, where at normal pressure it partially evaporates due to the heat of overheating, and distillation is carried out by circulating the pump through a tube-in-tube evaporator into the same tank. Exiting vapors with a temperature of about 120 ^o C are sent to a distillation column with a diameter of 600 mm, equipped with 20 plates, irrigated from above with reflux in the amount of 1450 kg / h.

 Vapors from the top of the distillation column are condensed into the refrigerator. Part of the condensate is returned to the distillation column, and part with the help of the pump is returned to the fifth plate of the distillation zone of the quenching column.

 The flow characteristics of the hardening and separation unit are shown in table 1.

 Example 2. In contrast to Example 1, twenty mass transfer plates (about 10 theoretical stages of mass transfer) were installed in the quenching column. The selection of bottoms liquid was 1400 kg / h.

 The characteristic operation of the hardening and separation unit is shown in table 1.

 Example 3. In contrast to example 2, thirty mass transfer plates (about 15 theoretical stages of mass transfer) were installed in the quenching column. The selection of bottoms liquid was 1200 kg / h.

 The operation characteristics of the hardening and separation unit are shown in table 1.

 As calculations show, formed due to overheating of the reaction gases of the evaporating liquid (reflux) is sufficient to isolate from them with the required degree the components boiling above vinyl chloride.

 The operation of the quenching apparatus, equipped with a distillation zone, is accompanied by the production of condensate from the exhaust gas-vapor mixture, which is characterized by almost no coke and osmol products. This, in particular, followed from the fact that phlegm is a colorless transparent liquid without mechanical impurities. As a result, mileage between boilers increased by at least one year, as can be seen from the following data in table 2.

 The low content of coke and osmols in the products entering the separation also follows from the practical absence of these impurities in the filters installed on the supply lines of these columns.

 For the same reason, it was possible to reduce the content of residual dichloroethane in bottoms liquid up to 10% against hard to reach 20% when the quenching apparatus operates in the form of a mixing capacitor. Accordingly, the loss of dichloroethane is reduced.

 The use of a quenching column with a distillation zone makes it possible to process bottoms of the last separation column of dichloroethane rectification system products (distillation of residual dichloroethane) in the production of perchlorethylene.

 The achievement of these positive effects can be relatively easily predicted in the case of knowledge of the behavior of vinyl chloride in the conditions under consideration. The work of the new installation of hardening the reaction gases of the dichloroethane pyrolysis process showed an unexpected positive result, which manifests itself in an increase in the quality of the final vinyl chloride and, accordingly, polyvinyl chloride, in comparison with the product obtained by using a mixing capacitor for hardening the reaction gases of dichloroethane pyrolysis. At the same time, the propylene content in vinyl chloride decreased from 4 to a maximum of 2 ppm (0.0002%), i.e., to the requirements for a product of the highest quality, 1,3-butadiene - from 10-12 ppm to a maximum of 6 ppm (0,0006 %) at a rate of 8 ppm, and methyl chloride - from 100 ppm to 60 ppm (at a rate of 80 ppm).

Literature
1. Tychinin V.N., Vasiliev V.P., Abdrashitov Y.M. and others. The use of heat of reaction gases for their separation in the production of perchlorocarbons // Chemical industry - 1984.- N 4. - p. 7-9.

 2. Patent DE 31 47 310 C2. C 07 C 21/06. 01/18/90.

Claims (2)

1. The method of separation of finely divided solid, tarry and high boiling point by-products from dichloroethane pyrolysis reaction gases in the production of vinyl chloride having a temperature of 440-520 ^o C and a pressure of 10-28 atm by rapidly cooling them in a quenching column and subsequent separation of the pyrolysis products, characterized in that hardening and separation is carried out by bubbling them through a layer of liquid concentrated by-products of these gases in the cube of the hardening column at a boiling point of 120 - 200 ^o C and subsequent contact steam gas mixture with the return condensate of these gases on the droplet and mass transfer plates of the distillation zone with purification of the vapor-gas mixture from high-boiling components along the height of the distillation zone with the withdrawal of accumulated liquid by-products with subsequent additional separation and concentration by single distillation and feeding the vapor-gas mixture for additional distillation with separation and withdrawing from the process products boiling above dichloroethane, and returning the distillate of this rectification to the process.  2. The method according to claim 1, characterized in that the vapor-gas mixture after quenching of the reaction gases is in contact with the return condensate (reflux) at 5 to 10 theoretical stages of mass transfer.

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