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 1,1-Difluoroethane (HFC-152a) is, for example, intended for use in swelling plastic foams (extrusion of polystyrene) or as a propellant in aerosols.
 Processes for producing 1,1-difluoroethane by hydrofluorination of vinyl chloride (VC), 1-chloro-1-fluoroethane (HCFC-151a) or 1,1-dichloroethane (HCC-150a) have been known for a long time (see, for example, BE-A-766,395 and U.S. Pat. No. 2,452,975).
 However, it has been observed that the known processes generally lead to a final product which is still contaminated to an unacceptable level with vinyl chloride.
 It has consequently been attempted to overcome these drawbacks by subjecting the crude 1,1-difluoroethane obtained from the synthesis to a purification. Several techniques have been applied, among which mention may be made of hydrofluorination of the crude synthetic product.
 Mention may be made in particular of a process for obtaining 1,1-difluoroethane by hydrofluorination of vinyl chloride in the presence of SnCl
 Mention may also be made of patent application WO-96/40606, in which the crude mixture obtained from the synthesis reactor is introduced directly into a step of reactive distillation in the presence of HF and optionally in the presence of a catalyst such as SnCl
 It will readily be understood that, in this so-called “reactive distillation” column, one or more reactions therefore occur, simultaneously with the distillation, and that the behaviour of such a column for maintaining the optimum operating conditions will be complicated, especially in the presence of a catalyst.
 Other techniques have also been proposed to purify 1,1-difluoroethane of any vinyl chloride. Mention may be made, inter alia, of the oxidation of vinyl chloride with various oxides (see EP-A-0,370,688), hydrogenation of vinyl chloride (see WO-90/08750), photochlorination of vinyl chloride (see abstract to CN-A-1,074,434 in Chemical Abstracts: 120: 269 634 k) and adsorption of vinyl chloride onto active charcoal (EP-A-0,600,536).
 However, none of these processes makes it possible readily and continuously to obtain 1,1-difluoroethane permanently containing less than 10 mg of VC per kg of HFC-152a.
 The aim of the present invention is to purify crude 1,1-difluoroethane such that it invariably has a very low content of vinyl chloride (preferably <10 mg/kg) and such that it is suitable for the applications for which this product is intended. Advantageously, this process will be simple and uncomplicated to carry out.
 This problem is solved, according to the invention, by a process for purifying 1,1-difluoroethane of any vinyl chloride, comprising treatment of a crude 1,1-difluoroethane with hydrogen fluoride, which is characterized in that the crude 1,1-difluoroethane has, per mole of 1,1-difluoroethane, a hydrogen chloride (HCl) content of less than 1 mol, advantageously less than 0.5 mol, preferably less than 0.1 mol and even less than 0.03 mol. Usually, the vinyl chloride content in the 1,1-difluoroethane to be purified ranges between about 10 and about 20,000 mg/kg (ppm). However, the process according to the invention can also be used to purify 1,1-difluoroethane containing larger amounts of VC. Generally, it also has, per kg of 1,1-difluoroethane, a 1-chloro-1-fluoroethane content of less than 50 g, advantageously less than 10 g and preferably less than 5 g, and a 1,1-dichloroethane content of less than 20 g, advantageously less than 5 g and preferably less than 2 g. It generally contains, per kg of HFC-152a, from 2 to 500 g of HF, advantageously from 5 to 250 g, preferably from 10 to 200 g. Such a crude 1,1-difluoroethane is obtained, for example, by at least partially removing the hydrogen chloride contained in a crude reaction product resulting from the reaction between hydrogen fluoride and a chloro precursor of HFC-152a, such as vinyl chloride.
 The present invention consequently also relates to a process for producing 1,1-difluoroethane, comprising
 a) a reaction between hydrogen fluoride and a chloro precursor of 1,1-difluoroethane, optionally in the presence of a hydrofluorination catalyst, this reaction giving rise to a crude reaction product,
 b) a separation of hydrogen chloride from the crude reaction product, and
 c) a further treatment of the crude reaction product, which is substantially depleted of HCl, with hydrogen fluoride, this further treatment giving rise to a formation of purified 1,1-difluoroethane.
 The account hereinbelow focuses on this process for producing 1,1-difluoroethane from a chlorohydrocarbon. However, it is clear that all of the characteristics of the further treatment can also apply to the general process for purifying 1,1-difluoroethane mentioned above.
 The solution proposed by the invention has the advantage of comprising, at the synthesis reactor outlet, a simple and common step of depletion of the hydrogen chloride content in the synthetic product obtained from the main reactor, and a further step in which the product obtained is subjected to a reaction similar to that which has taken place in the synthesis reactor, i.e. with hydrogen fluoride, so as to convert, entirely or almost entirely, the vinyl chloride still present in the reaction product.
 The impossibility of achieving a vinyl chloride content of less than 10 mg/kg in 1,1-difluoroethane at the synthesis reactor outlet, even after a simple distillation of the HFC-152a produced, has been observed. The reason for this is that, in the latter case, an azeotrope forms between the vinyl chloride and the HFC-152a, these two compounds moreover having a very low relative volatility. A reactive distillation of the crude reaction product as obtained from the reactor, i.e. not depleted of HCl, does not increase the possibility of achieving very low vinyl chloride contents. The solution proposed according to the invention overcomes these drawbacks.
 According to a preferred embodiment of the invention, the synthetic reaction takes place in the liquid phase in an organic solvent. In general, vinyl chloride is used as chloro precursor of 1,1-difluoroethane. Such a reaction has already been described in patent applications EP-A-0,637,579 and EP-A-0,739,875. It can also be envisaged to feed another chlorohydrocarbon into the synthesis reactor, in particular HCC-150a or HCFC-151a, in total or partial replacement for the vinyl chloride.
 According to one embodiment of the invention, the HCl is separated from the crude reaction product obtained from the synthesis reactor by withdrawing the HCl from the top of a distillation column, from which the tail fraction is collected in order to subject it to the further treatment.
 According to an improved embodiment of the invention, a product mainly containing 1,1-difluoroethane to be purified is extracted laterally from the said distillation column, while a product mainly containing products heavier than 1,1-difluoroethane, in particular HF and synthetic intermediates such as 1,1-chlorofluoroethane (HCFC-151a) and 1,1-dichloroethane (HCC-150a), are taken from the column tail fraction.
 In this embodiment, the efficacy of the further treatment is improved appreciably by reducing the content of synthetic intermediates which are liable to reform vinyl chloride during the reaction. It can be envisaged to reduce this content of synthetic intermediates even further by inserting an additional distillation column between the HCl distillation and the further treatment and by conveying therein the reaction product which is substantially depleted of HCl. The insertion of a reflux column between the synthesis reactor and the HCl distillation column also improves this reduction into substances liable to reform vinyl chloride. In one variant, the HCl distillation column can consist merely of a head section, mounted on the hydrofluorination reactor which serves as a boiling vessel.
 The further treatment can be carried out in the liquid phase or in the gas phase. Advantageously, it is carried out in the liquid phase, preferably in a reaction medium containing at least 200 g of HF per kg. In a particularly preferred manner, it is carried out in a liquid medium containing at least 500 g of HF per kg, or even at least 800 g per kg.
 According to an advantageous embodiment of the invention, the further treatment takes place in the presence of a hydrofluorination catalyst, which is preferably the same as the one used in the synthetic reaction, if this reaction has used such a catalyst, or in any case a catalyst which can be used in the synthetic reaction.
 As catalysts which can be used, mention may be made of derivatives of metals chosen from the metals from groups IIIa, IVa, IVb, Va, Vb and VIb of the Periodic Table of the Elements, and mixtures thereof. The tin, molybdenum, titanium, vanadium, antimony and tungsten derivatives are preferred. Tin derivatives are particularly suitable. Halides, such as chlorides, fluorides and chlorofluorides, as well as oxides and oxyhalides are preferably used as metal derivatives. SnCl
 When the further treatment is carried out in an organic solvent, this is preferably the same as the solvent for the synthetic reaction. As suitable organic solvent, mention may be made in particular of perchloroethylene or saturated halohydrocarbons, preferably chloro, fluoro or chlorofluorohydrocarbons containing from 1 to 8 carbon atoms, or mixtures thereof. Using the same catalyst, for example SnCl
 The crude reaction product, substantially depleted of HCl, which is used in the further treatment typically has a composition similar to that given above, for the crude 1,1-difluoroethane used in the general purification process. If necessary, HF can be added to the crude reaction product to be purified.
 Advantageously, crude 1,1-difluoroethane is subjected to the further treatment continuously, at a flowrate typically of from 0.01 to 5 kg of crude 1,1-difluoroethane per hour and per liter of reaction medium, preferably from 0.05 to 2 kg.
 Generally, the temperature at which the further treatment is carried out is at least 40 °C. and does not exceed 130° C. Preferably, it is at least 50° C. and does not exceed 120° C. The pressure is chosen as a function of the temperature of the reaction mixture. It is generally at least equal to 2 bar. It usually does not exceed 50 bar.
 The present invention also relates to 1,1-difluoroethane, obtained by hydrofluorination of a chlorohydrocarbon, which has a purity of greater than 99.8% by weight, preferably greater than 99.9% by weight, and a vinyl chloride content of less than 10 mg/kg, preferably less than 5 mg/kg, or even less than 2 mg/kg.
 Other specific aims of the invention are indicated in the claims which follow.
 Other details and particular features of the invention will also emerge from the description of a number of embodiments given below without any limitation and with reference to the attached drawings.
 A plant for producing 1,1-difluoroethane according to the invention is illustrated diagrammatically in
 This figure shows a reactor
 The synthetic reaction is carried out under known operating conditions. In this respect, reference may be made, for example, to patent applications EP-A-0,637,579 and EP-A-0,739,875.
 Advantageously, a temperature and a pressure are established in the reactor such that the 1,1-difluoroethane formed leaves the liquid mixture in the reactor continuously, in the form of a gaseous, crude reaction product which is conveyed conventionally into a reflux column
 The synthesis reactor can be any known reactor which is capable of operating under the working conditions of the process. For example, a reactor heated by an oil bath—not represented—may be envisaged.
 The reflux column
 As may be observed, this head fraction leaving the reflux column via the pipe
 The pipe
 The post-reactor
 The vinyl chloride and the HCFC-151a and HCC-150a synthetic intermediates which are still contained in the tail fraction of the distillation column
 In the embodiment illustrated, it has been envisaged to pass the HFC-152a thus purified of VC into another reflux column
 The head fraction from the reflux column
 The purified HFC-152a escapes at the top of the distillation column
 At the bottom of the column
 Heavy fractions are regularly purged from the two reactors
 In the plant represented in
 In the plant according to the present example, a crude reaction product containing HFC-152a is extracted laterally from the distillation column
 The reaction product introduced into the postreactor
 Another embodiment variant, represented in
 The parts of this plant which are identical to those of the plant according to Example 1 bear the same reference numbers and these parts will not be described again.
 Unlike the plant according to Examples 1 and 2, the product withdrawn from the bottom of the distillation column
 This embodiment thus allows yet a further improvement in the reduction of the synthetic intermediates in the product conveyed into the post-reactor
 In a plant corresponding to that described in Example 1, HFC-152a was purified of any vinyl chloride, in the post-reactor
 The initial composition of the reaction medium (excluding catalyst) in the post-reactor
TABLE I Composition of the initial Operating conditions of the reaction post-reactor [VC] [VC] medium [SnCl in- out- (excluding T. P (% by Q let let catalyst) (° C.) (bar) wt.) (g/h.l) ppm ppm HF/HFC-152a 65-70 12 10 100 60 <1 50/50% vol. HF/PER 88-93 10 5 100-150 1700 <1 33/66% vol. 10 <1 HF pure 93-95 10 3 130-235 3000 <1 1000 <1 200-235 8000 <1 5000 <1
 In this example, the efficacy of a post-reactor
 The average concentrations of VC, HCFC-151a and of HCC-150a, measured at different places in the plant, are reported in Table 2, in mg or g per kilo of HFC-152a.
TABLE 2 VC HCFC-151a HCC-150a (mg/kg) (g/kg) (g/kg) Plant of Example 1 Pipe 9 150 18 4 Pipe 19 1.2 — — Plant of Example 2 Pipe 9 140 15 4 Pipe 23 100 4.5 0.6 Pipe 19 0.6 — —
 It emerges clearly from this test that a lowering of the content of 1,1-difluoroethane synthetic intermediates (by more than a half in the present case) in the product subjected to the further treatment in the post-reactor
 In this example, a plant according to Example 2 was run with various flow rates of crude HFC-152a, of from 0.6 to 0.9 kg per hour and per liter of liquid medium in the post-reactor
 The results obtained are given in Table 3 below.
TABLE 3 Flow rate Concentration (per kg of HFC-152a) of HFC-152a Sampling VC HCFC-151a HCC-150a (kg/h.l) point (mg/kg) (g/kg) (g/kg) 0.6 Pipe 9 30 4.4 0.75 Pipe 23 23 2.4 0.3 Pipe 19 0.6 — — 0.7 Pipe 9 43 7.3 1.5 Pipe 23 42 4 0.6 Pipe 19 0.6 — — 0.8 Pipe 9 43 8 1.8 Pipe 23 41 3.9 0.6 Pipe 19 0.7 — — 0.9 Pipe 9 52 11.7 2.9 Pipe 23 53 6.8 1.1 Pipe 19 0.9 — —
 It emerges from this table that the purification process remains effective even for high production flow rates.
 It should be understood that the present invention is not limited to the embodiments described above and that many modifications can be made thereto without departing from the scope of the attached claims.