Process for the purification of pentafluoroethane

Abstract

The invention relates to a process for the purification of pentafluoroethane (R125) containing chloropentafluoroethane (R115). The mixture to be purified is subjected to an extractive distillation, the extractant being selected from dimethylformamide (DMF), dioxane, dimethylsulphoxide (DMSO) and diethylsulphoxide (DESO).

Claims

1. A process for the purification of pentafluoroethane containing chloropentafluoroethane by extractive distillation, said process comprising the use of an extractant selected from dimethylformamide, dioxane, dimethylsulphoxide and diethylsulphoxide.

2. The process according to claim 1, in which the extractant is selected from dimethylsulphoxide and diethylsulphoxide.

3. The process according to claim 1, in which the distillation is carried out under a pressure ranging from atmospheric pressure to 20 bars.

4. The process according to claim 1, in which the pentafluoroethane/chloropentafluoroethane molar ratio before distillation ranges from 2 to 99.

5. The process according to claim 1, in which the pentafluoroethane/chloropentafluoroethane molar ratio after distillation ranges from 5 to 99999.

6. The process according to claim 1, in which the mixture containing pentafluoroethane and chloropentafluoroethane to be purified originates from the fluorination reaction of perchloroethylene or from an intermediate fluorination product of said perchloroethylene.

7. The process according to claim 6, in which the intermediate fluorination product of said perchloroethylene is selected from dichlorotrifluoroethane or chlorotetrafluoroethane.

8. The process according to claim 1, in which the mixture containing pentafluoroethane and chloropentafluoroethane to be purified originates from the hydrogenolysis reaction of chloropentafluoroethane.

Description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(1) The invention is now described in more detail and non limitatively in the description which follows.

(2) The present invention proposes a process for the purification of a pentafluoroethane (R125) containing chloro-pentafluoroethane (R115) by extractive distillation, said process comprising the use of an extractant selected from dimethylformamide (DMF), dioxane, dimethylsulphoxide (DMSO) and diethylsulphoxide (DESO).

(3) The dioxane can be 1,4-dioxane, 1,3-dioxane or 1,2-dioxane. Preferably, the dioxane is 1,4-dioxane.

(4) According to an embodiment of the invention, the extractant is selected from dioxane, dimethylsulphoxide and diethylsulphoxide.

(5) According to an embodiment of the invention, the extractant is selected from dimethylsulphoxide and diethylsulphoxide.

(6) According to an embodiment of the invention, the pentafluoroethane to be purified comprises in particular pentafluoroethane and chloropentafluoroethane.

(7) According to an embodiment, the R125/R115 molar ratio before distillation ranges from 2 to 99.

(8) According to an embodiment, the R125/R115 molar ratio after distillation ranges from 5 to 99999.

(9) According to an embodiment, the purified pentafluoroethane comprises less than 10 ppm by weight of R115.

(10) Preferably, the process according to the invention is carried out without the presence of metal hydrides or ammonium salts.

(11) The process according to the invention can be implemented according to the well-known principles of extractive distillation. The operation can be carried out in an extractive distillation column in which the R125-R115 mixture to be separated is injected at a point situated at the top of the stripping section. The extractant is introduced into the column at a point situated at the top of the absorption section; it travels in the liquid state from its point of introduction to the boiler.

(12) The diameter and the number of stages of the extractive distillation column, the reflux ratio and the optimum temperatures and pressures can be easily calculated by a person skilled in the art from the characteristic data for the individual constituents and for their mixtures (relative volatilities, vapour pressures and physical constants).

(13) According to an embodiment of the invention, the distillation is carried out under a pressure ranging from atmospheric pressure to 20 bars absolute.

(14) According to an embodiment of the invention, the distillation is carried out at a temperature ranging from 50 C. to 250 C., preferably from 10 C. to 200 C., more preferably from 5 C. to 150 C.

(15) During the purification process according to the invention, the extractant selectively absorbs the R125.

(16) In this embodiment, the extractant then preferably has a separation factor F at 25 C., as defined below, greater than 2, preferably greater than 3, more preferably greater than 4.

(17) The separation factor (F) is defined as follows:

(18) $F=\frac{\left(R115\right)}{\left(R125\right)}\frac{P\left(R115\right)}{P\left(R125\right)}$

(19) where (R125) represents the activity coefficient of compound R125 in the solvent considered at infinite dilution.

(20) (R115) represents the activity coefficient of compound R115 in the solvent considered at infinite dilution.

(21) P(R125) represents the vapour pressure of compound R125 at the temperature considered.

(22) P(R115) represents the vapour pressure of compound R115 at the temperature considered.

(23) The ratio P(R125)/P(R115) corresponds to the relative volatility of R125 with respect to R115.

(24) And as in the specific case of the present invention, the ratio of the vapour pressures is constant whatever the extraction solvent used, we can considerer a simplified separation factor F or selectivity factor S defined by:

(25) $F^{}=\frac{\left(R115\right)}{\left(R125\right)}$

(26) The values of the activity coefficients of the compounds i (i is R125 or R115), .sub.i, are calculated according to the relationship:
ln .sub.j=(.sub.j.sup.i.sub.j.sup.p)/RT,

(27) where .sub.j.sup.i corresponds to the chemical potential of compound i at infinite dilution in the solvent considered, and .sub.j.sup.p correspond with the chemical potential of pure compound i, and R is the perfect gas constant, and T is the temperature.

(28) The activity coefficient and the vapour pressure are well-known data and accessible to a person skilled in the art.

(29) The capacity of a solvent is defined by the inverse of the activity coefficient of the solute in the solvent at infinite dilution.

(30) According to an embodiment of the invention, the purification process according to the invention comprises moreover a subsequent stage of separation of the extraction solvent and of the compound R125 as is known to a person skilled in the art and described for example in the document EP 778254. A solvent regeneration column makes it possible to separate the solvent/R125 mixture according to their difference in boiling point. The solvent recovered can be reused for the extractive distillation.

(31) The selectivity and the capacity of different solvents are indicated in the following Table 1:

(32) TABLE-US-00001 TABLE 1 capacity and selectivity of the solvents Simplified separation Solvent factor (F) = selectivity S Capacity Acetonitrile 6.31 0.3 THF 7.64 2.09 Hexane 0.63 0.18 Acetone 9.86 1.29 DMF 16.05 1.63 1,4-dioxane 8.86 1.12 dimethylsulphoxide 30.08 0.73 Diethylsulphoxide 22.72 1.94

(33) The data of Table 1 show that except for hexane, all these solvents have good extraction characteristics.

(34) It is shown here that new solvents such as DMF, 1,4-dioxane, DMSO and DESO give very good results in terms of selectivity and/or capacity. These 4 solvents present an excellent choice in terms of performances for the extractive distillation. Moreover, these solvents are not, or are not very, harmful to the environment.

(35) DMSO and DESO have excellent selectivities, of 30.08 and 22.72 respectively.

(36) On the other hand, other solvents conventionally used, such as acetonitrile, do not have a good selectivity and a good capacity.