FR2812870A1 - Preparation of purified hydrofluoroalkane involves subjecting hydrofluoroalkane containing (chloro)fluoro olefin impurities to purification treatment - Google Patents

Abstract

A purified hydrofluoroalkane is prepared by subjecting hydrofluoroalkane containing organic impurities to purification treatment. The purification treatment includes treatment with chlorine in the presence of an initiator, reaction with hydrogen fluoride, distillation, extractive distillation, adsorption, reaction with compound containing oxygen, and gas-phase reaction. Preparation of a purified hydrofluoroalkane involves subjecting hydrofluoroalkane containing (chloro)fluoro olefin impurities to purification treatment. The purification treatment includes treatment with chlorine in the presence of an initiator, reaction with hydrogen fluoride, distillation in which the purified hydrofluoroalkane is removed from the top or from the side of the distillation column, extractive distillation, adsorption onto a solid adsorbent, reaction with a compound containing oxygen, and gas-phase reaction with a reagent capable of reacting with organic impurities, with the exception of reaction with elemental chlorine.

Description

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The present invention relates to a process for obtaining a purified hydrofluoroalkane, chosen from 1,1,1,3,3-pentafluoropropane and 1,1,1,3,3 -pentafluorobutane.

1,1,1,3,3-pentafluoropropane and 1,1,1,3,3-pentafluorobutane are hydrofluoroalkanes which can be used as (hydro) chlorofluoroalkane substitutes, for example as a blowing agent, as a refrigerant or as a solvent.

These hydrofluoroalkanes are typically produced by reacting a chlorinated or chlorofluorinated precursor with hydrogen fluoride. The crude hydrofluoroalkanes obtained in this reaction often contain impurities such as unconverted reactants, hydrogen chloride and olefinic impurities, in particular chlorofluorinated olefins containing 3 or 4 carbon atoms.

Patent application WO-A-00/14040 describes a process for the purification of 1,1,1,3,3-pentafluorobutane. According to this known process, it is possible to reduce the content of fluortrichlorethylene in 1,1,1,3,3-pentafluorobutane by ionic chlorination in the presence of FeCl3, by hydrogenation in the presence of PdlRh on activated carbon or in particular by reaction with fluorine.

It was nevertheless desirable to have an alternative process for purifying 1,1,1,3,3-pentafluoropropane or 1,1,1,3,3-pentafluorobutane which allows an effective reduction of the content of olefinic impurities and more particularly in olefinic impurities containing 3 or 4 carbon atoms while using technical means of easy implementation.

The invention therefore relates to a process for obtaining a hydrofluoroalkane purified of organic impurities, chosen from 1,1,1,3,3-pentafluoropropane and 1,1,1,3,3-pentafluorobutane, according to which the hydrofluoroalkane containing organic impurities is subjected to a distillation and the purified hydrofluoroalkane is drawn off at the top of the distillation or laterally.

It has been surprisingly found that organic impurities present in the hydrofluoroalkane, in particular (hydro) (chloro) fluorocarbons comprising 3 or 4 carbon atoms, do not have a tendency to form an azeotrope with the hydrofluoroalkane and

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 can therefore be separated by the method according to the invention. The method according to the invention can be easily implemented.

Organic impurities, the content of which can be reduced by the process according to the invention generally comprise 3 or 4 carbon atoms. These include hydrochlorofluoroalkanes and (chloro) fluorinated olefins containing 3 or 4 carbon atoms such as isomers of monochlorotrifluorobutene. Often the olefinic impurities consist essentially of such chlorofluorinated olefins containing 3 or 4 carbon atoms.

The distillation pressure is generally less than 10 bar absolute. Often it is at most 5 bar. Preferably it is at most 3 bar. Generally the distillation pressure is at least 0.5 bar. Often it is at least 1 bar. Preferably it is at least 1.5 bar.

In the present description, any reference to the pressure corresponds to the absolute pressure measured at the top of the distillation column.

The temperature at which the distillation takes place corresponds approximately to the boiling point of the hydrofluoroalkane at the selected pressure.

When the hydrofluoroalkane is 1,1,1,3,3-pentafluorobutane, good results are obtained at a pressure of about 1.5 to 3 bar and a temperature of about 50 to 70 C.

Distillation can be carried out in one or more distillation columns. Preferably, only one column will be used.

The distillation columns which can be used in the process according to the invention are known in themselves. It is possible to use, for example, columns with conventional trays or columns with trays of the dual-flow type or else columns with loose or structured stacking.

The number of theoretical plates in the distillation is generally at least 10. Often it is at least 15. A number of at least 20 gives good results.

The supply of hydrofluoroalkane containing organic impurities in the process according to the invention is generally carried out at a level of less than 50% of the number of theoretical plates of the column, it being understood that the head of the column corresponds to 100% of the number of theoretical platforms. Often, this level is at most 45% of the number of theoretical plates in the column.

Generally feeding is carried out at a level of at least 5% of the number

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 of theoretical plates of the column. Often this level is at least 10% of the number of theoretical plates in the column.

If necessary, the lateral racking is generally carried out at the level which corresponds to at least 50% of the number of theoretical plates of the distillation. The lateral racking is generally carried out at the level which corresponds to at most 80% of the number of theoretical plates of the distillation.

In the process according to the invention, the purified hydrofluoroalkane is generally withdrawn in an amount of at least 50% of the feed. The amount is often at least 70% of the diet. The amount is preferably at least 80% of the diet. The purified hydrofluoroalkane is generally withdrawn in an amount of at most 99% from the diet. The amount is often at most 97% of the diet. The amount is preferably at most 95% of the diet.

The molar reflux rate in distillation is generally at most 20. Often this rate is at most 10. A reflux rate of at most 7 has given good results.

The process according to the invention applies to the purification of a hydrofluoroalkane containing olefinic impurities prepared by any process of synthesis, without a prior treatment being required. The method according to the invention also applies to the purification of a hydrofluoroalkane containing organic impurities which consists essentially of hydrofluoroalkane and organic impurities. Typically the hydrofluoroalkane to be purified contains at most 10% by weight of organic impurities. This content of impurities can be at most 5% by weight. It can also be at most 1% by weight. The process according to the invention can even be applied to a hydrofluoroalkane containing at most 0.1% by weight of organic impurities.

The method according to the invention optionally comprises at least one subsequent treatment step intended to separate from the hydrofluoroalkane any inorganic impurities, such as residual hydrogen fluoride. Treatments that can be used are, for example, adsorption on a solid, such as NaF or alumina, washing with water, an extraction operation or separation by an appropriate membrane.

The method according to the invention finds an interesting application in the purification of a hydrofluoroalkane obtained by hydrofluorination in particular by hydrofluorination of a chloro (fluoro) carbon.

The method according to the invention applies very particularly

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 preferred to obtain purified 1,1,1,3,3-pentafluorobutane.

The example which follows is intended to illustrate the present invention without however limiting its scope.

Example in accordance with the invention The distillation was carried out in a column packed with 5/8 "pall rings. The feed was carried out at the level of the column, which corresponds to 43% of the number of theoretical plates, with 1.1, 1,3,3-pentafluorobutane containing 94 ppm of 3 chlorofluorinated saturated organic impurities containing 3 or 4 carbon atoms and 252 ppm of 2 fluorinated (chloro) olefins containing 4 carbon atoms. The pressure was 2 bars, the temperature of 60 C and the molar reflux rate of approximately 4. A quantity of 1,1,1,3,3-pentafluorobutane purified corresponding to 84% of the diet was withdrawn at the head. This product still contained 6 ppm of a chlorofluorinated saturated organic impurity containing 3 carbon atoms. The content of the other two saturated impurities had decreased below the detection limit of 5 ppm. The content of said 2 chlorofluorinated olefins containing 4 carbon atoms in 1,1,1, 3.3- purified pentafluorobutane had decreased below the 5 ppm detection limit.

Comparative Example The distillation was carried out in the same apparatus as in the example according to the invention. The column was supplied, which corresponds to 65% of the number of theoretical plates, with 1,1,1,3,3-pentafluorobutane containing 170 ppm of 3 saturated chlorofluorinated organic impurities containing 3 or 4 carbon atoms and 152 ppm of 2 chlorofluorinated olefins containing 3 or 4 carbon atoms. The pressure was 2 bars, the temperature of 60 C and the molar reflux rate of about 25. A quantity of 1,1,1,3,3-pentafluorobutane corresponding to 85% of the feed was withdrawn at the bottom. . This product contained 170 ppm of said 3 saturated organic impurities and 141 ppm of said 2 chlorofluorinated olefins containing 4 carbon atoms.

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Claims (10)

1. Process for obtaining a hydrofluoroalkane purified of organic impurities, chosen from 1,1,1,3,3-pentafluoropropane and 1,1,1,3,3- pentafluorobutane, according to which the hydrofluoroalkane containing organic impurities at a distillation and the purified hydrofluoroalkane is drawn off at the top of the distillation or laterally. 2. Method according to claim 1 wherein the purified hydrofluoroalkane is drawn off at the top of the distillation. 3. Method according to claim 1 wherein the purified hydrofluoroalkane is drawn off laterally. 4. Method according to any one of claims 1 to 3 wherein the distillation pressure is 0.5 to 10 bar. 5. Method according to any one of claims 1 to 4 wherein the temperature at which the distillation takes place corresponds approximately to the boiling point of the hydrofluoroalkane at the selected pressure. 6. Method according to any one of claims 1 to 5 wherein the molar reflux rate of the distillation is at most 20. 7. Method according to any one of claims 1 to 6 wherein the supply of hydrofluoroalkane containing organic impurities is carried out at a level of 5 to 50% of the number of theoretical plates of the column. 8. Method according to any one of claims 1 and 3 to 7 wherein the lateral withdrawal is carried out at the level which corresponds to from 50% to 80% of the total number of the number of theoretical plates of the distillation. 9. Method according to any one of claims 1 to 8, in which the olefinic impurities consist essentially of (chloro) fluorinated olefins containing 3 or 4 carbon atoms. 10. Method according to any one of claims 1 to 9, in which <Desc / Clms Page number 6>  the hydrofluoroalkane is 1,1,1,3,3-pentafluorobutane.