GB2054574A - Process for the preparation of vinyl chloride - Google Patents

Abstract

A process for the preparation of vinyl chloride comprises chlorinating ethylene, thermally cracking the 1,2-dichloroethane produced, removing the hydrogen chloride and vinyl chloride formed, halogenating the remaining mixture, recycling only a portion thereof to the ethylene chlorination stage, distilling off 1,2-dichloroethane from the contaminants in the remaining portion and returning it to the thermal cracking stage. This process has been found to economically remove substantially all of the contaminants. <IMAGE>

Description

SPECIFICATION Process for the preparation of vinyl chloride The present invention relates to a process for the preparation of vinyl chloride.

One of the methods of producing vinyl chloride on an industrial scale is the conversion of ethylene to 1 .2-dichloroethane either by direct chlorination or by oxychlorination and the subsequent pyrolysis of this dichloroethane at a temperature in the range of from 400"C to 550"C to vinyl chloride. In this last step, especially during non-catalytic pyrolysis, the conversion of dichloroethane to vinyl chloride and hydrogen chloride (also formed at this stage) is normally between 50 and 70% by weight. The economics of the process require that the unreacted dichloroethane be recovered, separated from the hydrogen chloride and vinyl chloride and recycled to the pyrolysis stage in order to maximise the yield of vinyl chloride. The separation of hydrogen chloride and vinyl chloride is achieved by a sequence of fractionation steps.For example, the gaseous reaction mixture from the pyrolysis step is first cooled by washing with cold dichloroethane. The residual reaction mixture, which may be partially liquefied, is then introduced into a column in which hydrogen chloride is removed. A mixture of vinyl chloride and dichloroethane is withdrawn from the base of this column and the vinyl chloride is separated from the dichloroethane by fractional distillation in a further column which may be called the vinyl chloride column. The unreacted dichloroethane is recovered as a liquid from the bottom of the column. However, the dichloroethane recovered cannot be recycled directly to the pyrolysis reactor because it is contaminated with other unsaturated materials, for example butadiene, isomeric chlorobutadienes including chloroprene, benzene and heavy ends.These contaminants, if allowed to remain in the recycle feed, produce deposits in the pyrolysis reactor and in the distillation columns. These contaminants are also known to be coke formers in the pyrolysis stage which lead to the blockage of furnace tubes. Such deposits result in plant shut-downs and consequently affect economic production. This problem has been well recognised in this industry and several methods have been proposed to eliminate the effects of these contaminants.

One of the methods suggested is to add to the process additional fractionation columns, for example one column to remove contaminants overhead as light ends (this column is frequently referred to as the "lights" column) and another column to remove the high boilers referred to as the "high boil" column).

Dichloroethane is withdrawn overhead from this latter column for recycling to the pyrolysis stage. Since the boiling points of the contaminants and dichloroethane are very close, the fractionation is uneconomic. This is particularly due to the excessive column height necessary to achieve separation of the impurities and to the heat energy expended to maintain the desired reflux ratios during such fractionation.

Another method is described and claimed in British Patent Specification No. 1 266 676 B which involves the chlorination of the contaminated dichloroethane in the presence of 200 to 500 ppm of ferric chloride catalyst either by recycling all of the contaminated dichloroethane to the ethylene chlorination reactor where such a catalyst is present and distilling off the dichloroethane after removing the catalyst by washing with water or by distilling off the low boiler contaminants, treating all or part of the remaining contaminated dichloroethane in the presence of such a catalyst and subjecting the remaining mixture to further distillation. This method is also uneconomic because of the large losses of heat energy when the catalyst has to be removed by washing and the energy expended in the distillation steps to remove the contaminants and the catalyst.

Other methods have been suggested which include the non-catalytic halogenation of the contaminated dichloroethane to increase the boiling point differential between the halogenated contaminants and dichloroethane; this is followed by fractional distillation in the "high boil" column. One such method is described and claimed in British Patent Specification No.

1 509 051. However, if this technique is used alone it only succeeds in removing unsaturates such as 1-chlorobutadiene and chloroprene and not benzene, which consquently still contaminates the dichloroethane withdrawn overhead from the "high boil" column.

The direct recycle of this dichlororethane which is still contaminated by benzene to the pyrolysis stage results in a continuous buildup of benzene in the system which is again an uneconomic procedure.

There is a need for a process for the preparation of vinyl chloride which incorporates the removal of unsaturated contaminants present in the recycled dichloroethane minimising the effects of contaminants such as benzene, especially one which enables the "lights" column to be eliminated.

The present invention provides a process for the preparation of vinyl chloride which comprises chlorinating ethylene, thermally cracking, preferably in a pyrolysis reactor at a temperature in the range of from 400 to 550"C, the 1,2-dichloroethane produced, removing the hydrogen chloride and vinyl chloride formed, halogenating the remaining mixture, recycling only a portion thereof to the ethylene chlorination stage, distilling off, preferably by means of a single distillation column, 1,2-dichloroethane from the contami nants in the remaining portion and returning it to the thermal cracking stage.

The ethylene chlorination may be carried out in a direct chlorination reactor and an oxychlorination reactor. Preferably the recycled portion is conveyed only to the direct chlorination reactor.

Preferably the mixture remaining after the hydrogen chloride and the vinyl chloride has been removed is chlorinated. This chlorination of the contaminated 1 ,2-dichloroethane is suitably carried out at a temperature in the range of from 90 to 1 40 C. The quantity of chlorine and/or chlorine-producing agents added for this chlorination is dependent upon the conditions required for the pyrolysis of the dichloroethane which in turn govern the proportion of contaminants present after the pyrolysis. However, the amount of chlorine added or made available is suitable such that no chlorine can be found downstream of the initial chlorination zone, especially that none can be found in the head product at the later distillation stage.This amount of chlorine is usually in the range of from 60 to 80% by weight of that theoretically required for the chlorination of all of the contaminants, though this is difficult to determine accurately.

Suitably also the amount of chlorine is kept as low as possible to avoid corrosion problems. It is possible to carry out this chlorination in the absence of any added catalysts and, in fact, this is preferable where iron or iron compounds are formed in situ in trace amounts of, for example, 10 ppm. The addition of 10 ppm or less of iron in the form of iron metal, an iron-containing compound, or an agent or combination of agents which will release iron under the conditions present at this stage, or a mixture of any two or more thereof, is also preferred; larger amounts of iron may be used but give rise to higher processing costs when they have to be removed. We have found that it is at this stage that the chlorination of the unsaturates, apart form benzene, occurs.

One of the significant features of this invention is to split the chlorinated products of the impure unconverted dichloroethane containing the contaminants to recycle a portion thereof to the ethylene chlorination stage and to feed the remainder to the "high boil" column. We have found that any benzene present in the product recycled to the ethylene chlorination stage may be successfully chlorinated at this time without the addition of any catalysts from an external source. However, some iron or iron compounds in an amount of 60 ppm or less, may be present at the ethylene chlorination stage as an impurity due to corrosion of the reactor walls by traces of HCi inevitably formed in such a reaction. This iron may act as a catalyst for the chlorination of benzene.

The successful chlorination of benzene results in polychlorobenzenes which have a higher boiling point than 1,2-dichloroethane.

The precise proportion of the chlorinated, impure, unconverted dichloroethane recycled to the ethylene chlorination stage depends upon the conditions under which the thermal cracking of dichloroethane is carried out in the pyrolysis reactor and upon the amount of benzene in the unconverted dichloroethane.

The amount of mixture recycled is at least 2.5% by weight, preferably in the range of from 2.5 to 20% by weight, advantageously in the range from 2.5 to 15% by weight and especially in the range of from 5 to 10% by weight, of the chlorinated crude dichloroethane.

For optimum results, it is preferable to supplement the normal input amount of chlorine and/or agent or combination of agents which produce chlorine under the conditions of the reaction into the ethylene chlorination reactor to ensure the chlorination of substantiall all of the benzene present which may otherwise compete for available chlorine with the ethylene being chlorinated to 1,2-dichloro- ethane. The products from the ethylene chlorination reactor including fresh dichloroethane are then dehydrated and freed from light ends in a heading column. The base products from this column.are then introduced into a "high boil" column where they are combined with the remainder of the chlorinated vinyl chloride column base product, for removal of the high boiling impurities, for example benzene polychlorides.The overheads from the "high boil" column comprising mainly dichloroethane freed from high boilers and most of the unwanted impurities are then fed into the pyrolysis reactor to produce vinyl chloride. The process may thus operate continuously without a "lights" column.

It is to be realised that it would be possible to simulate the conditions present in this process and to treat contaminated 1,2-dichloroethane away from a vinyl chloride production plant.

The following Example illustrates the invention. All percentages given in the Example have been calculated on a weight basis.

Example Figure 1 shows, diagrammatically, an integrated vinyl chloride plant consisting of direct chlorination and oxychlorination plants to produce 1 ,2-dichloroethane, a pyrolysis unit to convert 1,2-dichloroethane to vinyl chloride and the associated distillation equipment needed to purify both 1 ,2-dichloroethane and vinyl chloride. The term "EGO" denotes 1,2dichloroethane and the term "VC" denotes vinyl chloride.

17,629.0 Ib/hr of ethylene and 44,595.0 Ib/hr of chlorine were fed to the direct chlorination unit whilst 17,540.0 Ib/hr of pure ethylene, 43,258.0 Ib/hr of gaseous, pure hydrogen chloride and 65,294.0 Ib/hr of air were fed to the oxychlorination unit, the hy drogen chloride being derived from the pyrolysis unit. The combined crude 1,2-dichloroethane streams (total 120,404.4 Ib/hr, 98.5 % 1 ,2-dichloroethane) were fed to the heads column for removal of water and low boiling impurities. The base product from this column was then fed to the "high boil" column from the head of which was removed 200,934.2 Ib/hr of 1,2-dichloroethane (99.0% pure) which was subsequently fed to the pyrolysis plant operated at 530"C. The gaseous reaction product was quenched, and distilled to remove hydrogen chloride (43,258.0 Ib/hr) and pure vinyl chloride (72,363.3 Ib/hr).

For the chlorination of the vinyl chloride column base product 82,972.8 Ib/hr containing 0.20% of mixed 1- and 2-chlorobutadienes, 0.10% of butadiene, 0.22% of benzene, 1.13% of light ends, 1.11 % of heavy ends and the balance (97.33%) of 1,2-dichloroethane were chlorinated by introducing 241. 5 Ib/hr of chlorine at approximately 20"C into the transfer pipe to the "high boil" column. The chlorination itself was carried out at a temperature of 130"C. The chlorobutadienes and butadiene reacted with the chlorine virtually completely whereas the benzene concentration was almost unchanged.Approximately 10% (8,310.9 Ib/hr) of this stream was then passed to the direct chlorination reactor operated at 40-50"C in which the 1 ,2-dichloroethane was produced by reaction of chlorine and ethylene and the remainder was fed to the "high boil" column. In order to facilitate the chlorination of benzene present in the recycled stream, a further 16.7 Ib/hr of chlorine, in addition to the 44,595.0 Ib/hr already being fed, was fed to this direct chlorination reactor. Analysis of the direct chlorination reactor product showed benzene to be virtually absent and the heavy ends to have increased from 123.1 Ib/hr in the liquid feed to at least 163.8 Ib/hr in the reactor product. As per normal procedure, this product stream was combined with the oxychlorination product, dehydrated and freed of light ends in the heading column and then fed to the " high boil" column where it was combined with the remainder of the chlorinated vinyl chloride column base stream. The distillate from the "high boil" column then contained only 0.08% benzene and no chlorobutadienes. This stream was fed together with the normal feed to the pyrolysis reactor.

Claims (16)

1. A process for the preparation of vinyl chloride which comprises chlorinating ethylene, thermally cracking the 1,2-dichloroethane produced, removing the hydrogen chloride and vinyl chloride formed, halogenating (Ill) the remaining mixture, recycling only a por tion thereof to the ethylene chlorination stage, distilling off 1,2-dichloroethane from the con taminants in the remaining portion and returning it to the thermal cracking stage.

2. A process as claimed in claim 1, wherein the thermal cracking stage is carried out in a pyrolysis reactor at a temperature in the range of from 400 to 550"C.

3. A process as claimed in claim 1 or claim 2, wherein the mixture remaining after the removal of hydrogen chloride and vinyl chloride is chlorinated.

4. A process as claimed in claim 3, wherein a catalytic amount of iron is added in the form of iron metal, an iron-containing compound, an agent or combination of agents which will release iron under the conditions of the chlorination, or a mixture of any two or more thereof.

5. A process as claimed in claim 4, wherein an amount of 10 ppm or less of iron is added.

6. A process as claimed in claim 3, wherein chlorination is carried out in the absence of added catalysts.

7. A process as claimed in any one of claims 1 to 6, wherein at least 2.5% by weight is recycled to the ethylene chlorination stage.

8. A process as claimed in claim 7, wherein in the range of from 2.5 to 20% by weight is recycled to the ethylene chlorination stage.

9. A process as claimed in claim 8, wherein in the range of from 2.5 to 15% by weight is recycled to the ethylene chlorination stage.

10. A process as claimed in claim 9, wherein in the range of from 5 to 10% by weight is recycled to the ethylene chlorination stage.

11. A process as claimed in any one of claims 1 to 10, wherein a catalyst is present at the ethylene chlorination stage.

1 2. A process as claimed in claim 11, wherein 60 ppm or less of iron or iron compound is present.

1 3. A process as claimed in any one of claims 1 to 12, wherein additional chlorine and/or an agent or combination of agents which release chlorine under the conditions of the reaction is added at the ethylene chlorination stage than is required for the chlorination of ethylene alone.

14. A process as claimed in any one of claims 1 to 13, wherein the distillation is carried out in a single distillation column.

1 5. A process as claimed in claim 1, which is carried out substantially as described in the Example herein.

16. Vinyl chloride whenever prepared by a process as claimed in any one of claims 1 to 15.