Title of Invention

PROCESS FOR THE PURIFICATION OF DIFLUOROMETHANE

Abstract The invention relates to the purification of difluoromethane containing traces of chlorofluoromethane. According to the invention there is provided a process for the purification of difuoromethane (F32) containing traces of chlorofluromethane (F3]), which process comprises passing a gaseous stream of the F32 to be purified over a 13 molecular sieve at a temperature of at least 60°C. The sieve can be regenerated by washing with the aid of a solution of sodium or potassium carbonate and heating to high temperature under inert atmosphere or in vacuum PRICE: THIRTY RUPEES
Full Text



The present invention relates to the purification of difluoromethane (CH2F2) containing traces of chlorofluoromethane (CH2ClF).
Difluoromethane (known in the trade under the designation F32) is one of the possible substitutes for chlorofluorocarbons (CFCs) which are dealt with by the Montreal Protocol and is intended to replace more particularly chloropentafluoroethane (F115), the action of which on ozone is accompanied by a powerful contribution to the greenhouse effect.
F32 can be obtained by fluorination of methylene chloride (CH2Cl2) by means of HF in the presence of a catalyst or by hydrogenolysis of dichlorodifluoromethane (F12) or of
chlorodifluoromethane (F22) or by decomposition, in the presence of HF, of alpha-fluoroethers under the action of Lewis acids.
However, fluorination processes exhibit the disadvantage of involving the formation, as intermediate, of chlorofluoromethane (F31), a toxic compound which, according to the classification of the lARC (International Agency for Research on Cancer) is classified in category IIB (possibly cancerogenic to man) and the residual concentration of which it is appropriate to lower to about one ppm. Despite the difference in volatility between FBI (b.p. -9.1°C) and F32 (b.p.: -51.7°C), this objective is difficult to obtain by distillation and therefore requires the

development of a highly efficient purification process. Small quantities of F31 may also be present in an F32 manufactured by hydrogenolysis of F12 or of F22.
European Patent Specification EP 508 630 describes a process to lower the F31 content in an F32, comprising placing the F32 to be purified in contact with an activated carbon. The selectivity is not very high and the methylene chloride or the dichlorodifluoromethane is adsorbed in the same proportions as F31, thus limiting the capacity of the adsorbent. It is known, furthermore, that the properties of an activated carbon depend greatly on its method of preparation and on the raw material employed; the effectiveness of an activated carbon and above all its selectivity are therefore liable to variations depending on the source of the batches.
The use of molecular sieves for the purification of fluorohydrocarbons is already known. Such purification treatments are usually performed at ambient temperature or thereabouts. Temperatures lower than ambient temperature are sometimes recommended, as in US patent 2 917 556, which describes the use of 5A, lOX or 13X sieves for the removal of vinyl fluoride from vinylidene fluoride or in Japanese Patent Specification JP 5-70381, which recommends a temperature region ranging from -30 to +30°C for removing HFC 365 (C4F5H5) and the corresponding olefins (C4F4H4) from a crude 1,1-dichloro-l-fluoroethane

tFi4i£); JDy creacmenc on xjx mo±ecuxar sieve.
With zeolites A or a natural sieve such as chabazite, European Patent Specification EP 503 796 recommends temperatures of between +10 and +100°C for removing any trace of l-chloro-2,2-difluoroethylene (F1122) from 1,1,1,2-tetrafluoroethane (F134a), but the treatments are in practice performed at 40°C.
In the purification of Fll34a, but for the removal of 1,1,2,2-tetrafluoroethane (F134), Japanese Patent Specification JP 3-72437 employs zeolites which have a pore diauneter of between 5 and 7 A over a temperature region of between 0 and 70°C.
These sieves are generally regenerated by heating to 200-350°C under a stream of air or nitrogen, by heating luider reduced pressure or else by displacement of the adsorbed products with water and reactivation at high temperature, as indicated in US patent 2 917 556. These regeneration processes, well known to a person skilled in the art, appear in most of the technical data sheets supplied by the manufacturers of molecular sieves.
Although German Patent Specification DE 1 218 998 mentions that 13X molecular sieves can work at a relatively high temperature, none of the abovementioned documents allows any change or increase in selectivity whatever to be expected when the temperature of treatment is raised.
It has now been found that 13X molecular

'sieve enable F31 to be substantially completely removed from F32, if the treatment is performed at a temperature of at least 60'^C.
It has also been found that, in the case of this purification of F32 on 13X sieve, conventional regeneration methods are not suitable, but that the spent i3X sieve can be restored to its full effectiveness by being washed with a solution of sodium or potassium carbonate before being reactivated by heating under inert atmosphere or in vacuum.
According to the present invention, there is therefore provided a process for the purification of F32 containing traces of F31, which process comprises passing a gaseous steam of the F32 to be purified over a 13X molecular sieve at a temperature of at least 60 C, and, preferably, above 75'^C, and, optionally, after use, regenerating the 13X sieve by washing by means of a solution of sodium or potassium carbonate and heating to high temperature under inert atmosphere or in vacuum.
13X molecular sieve form part of the class of synthetic zeolites of the
XA^ type and have a crystalline structure in the form of cubo-octahedra
containing cavities which are accessible via pores of an effective opening
diameter of lOA; their specific surface is of the order of 800 to lOOm^/g and,
in the sodium form, they correspond to the general formula:

Na8j(AlO,)36(Si02)io6]276 H,0
These sieves, which can also be employed in the potassitua form, are obtained by hydrothermal crystallization of gels resulting from the mixing, in well-defined proportions, of alkaline acpaeous solutions of silicates and of aliiminates (R. Belabbes and J.M. Vergnaud, Chimie et Industrie, vol. 104, June 1971, pages 1407 et seq.).
The hourly space velocity of the gaseous stream of F32 to be purified on the particles of 13X molecular sieve, arranged as a fixed or fluid bed, may vary within wide limits. It is generally between 20 and 2400 h"^, preferably between 40 and 250 h'^.
Placing the F32 to be purified in contact with the 13X molecular sieve may be performed at atmospheric pressure or at a higher pressure that can range up to 3 0 bars.
The process according to the invention may be applied to the purification of an F32 containing up to 5000 ppm of F31, and more particularly up to 200 0 ppm.
The regeneration of the sieve should be performed at a temperature not exceeding 550°C and is advantageously conducted as follows. At the end of the purification cycle the sieve is desorbed at 150-250°C under a stream of nitrogen or, better, in vacuum, so as to recover the F32 adsorbed on the zeolite. The sieve is then placed in contact with an aqueous solution containing 1 to 5 % by weight of sodium or potassium

carbonate for a period which may range from 1 to
12 hours, and is then separated, rinsed with distilled
water and dried before reactivation at 200-250°C under
inert atmosphere (nitrogen, helium, argon) or in
vacuiitn.
The desorption and reactivation stages may be performed in a vacuum which, depending on the available condensation device, may range from a few tens of kilopascals to several hundred pascals. In industrial practice the work is generally done in a vacuum of between a few tens of kilopascals and 6000 pascals.
The following examples illustrate the invention without limiting it. The ppm values shown are expressed as weight and determined by vapour phase chromatography (VPC).
EXAMPLE 1
a) 26 g of a 13X molecular sieve in the form
of 1.6-mm extrudates (Siliporite® G5 marketed by CECA),
activated beforehand at 250°C for 2 hours under a
stream of nitrogen were arranged in a tube 50 cm in
height and 10 mm in internal diameter, provided with a
heating jacket, and then a gaseous stream of F32
containing 100 ppm of F31 was passed through at ambient
temperature and at a flow rate of 5 1/h.
After 7 hours' operation, VPC analysis of a sample of F32 taken at the exit of the tube showed that its F31 content fell only to 93 ppm.
b) The preceding test was repeated, but with

the treatment being performed at 80°C. After 7 hours' operation, the residual F31 concentration was lower than 1 ppm (detection limit of the VPC analysis method).
EXAMPLES 2 to 4 (Comparative) The tests of Example 1 were repeated, but with the 13X molecular sieve being replaced with 27 g of 5A molecular sieve marketed by CECA (Example 2) or with 3 0.8 g of acidic mordenite marketed by Ventron GxnbH (Example 3) or else with 16.2 g of Norit NC activated carbon as 0.8-mm extrudates (Example 4).
The results of these tests are listed together in the following table:

*test made without renewing the charge employed at 25°C.

EXAMPLE 5
By operating as in Excunple 1 with 26 g of a 13X molecular sieve prepared with an iron-free binder and activated in the same conditions, tests were carried out at ambient temperature, at 68°C and at 80°C on the same single batch of F32 containing 10 0 ppm of F31. The following results were obtained:
Temperature of FBI content
treatment at the pipe exit (ppm)
25°C 100
68°C 30 (15 if the flow rate is reduced
to 3 1/h instead of 5 1/h)
80°C EXAMPLE 6
51.9 g of a Union Carbide 13X sieve, conditioned in the form of extrudates 1.6 mm in diameter and activated beforehand under nitrogen at 250°C were placed in a stainless steel tube 14 mm in internal diameter and 50 cm in height, provided with a heating jacket, and then a batch of F32 containing 250 ppm of F31 was circulated therein continuously at a flow rate of 9.8 1/h and at a temperature of 80°C. The F31 content at the exit of the t\ibe was checked by taking samples at regular intervals. The following results were obtained:


EXAMPLE 7
a) The seune tube as in Example 1 was filled with 24.4 g of Union Carbide 13X molecular sieve, activated as previously, and a stream of F32 containing 100 ppm of F31 was circulated therein at a flow rate of 5 1/h.
After 90 minutes a ssunple of the treated gas was taken at the exit of the tube and the temperature was raised to 45°C for a new treatment period of 9 0 minutes, at the end of which the concentration of FBI was checked again. When this procedure was continued using successive plateaus up to a temperature of 125°C, the following results were obtained:


b) The operation was carried out as above with a new charge of 23.4 g of Union Carbide 13X sieve, but starting from a temperature of 125°C and coming down to cUDobient temperature using successive plateaus of 90-minute duration.
The following results were obtained by-employing an F32 containing 56 ppm of F31 and a flow rate of 5 1/h:

EXAMPLE 8
49.7 g of Siliporite® G5 sieve (Ceca) were activated at 250°C under nitrogen purging and were

placed in the stainless steel tube employed in Example 6.
A batch of F32 containing 1605 ppm of F31 was circulated therein continuously at a flow rate of 4 1/h, the temperature of the sieve being maintained at 80°C by circulating a thermostated fluid in the jacket.
The F31 content at the exit of the tubs was checked by taking samples at regular intervals. The break-through point (final FBI concentration > 1 ppm) appeared after 70 hours' operation, which corresponds to a capacity of 2.6 % relative to the weight of the sieve introduced.

EXAMPLES 9 to 12
Each example was carried out in the same tube as in Example 6, in which a new charge (3 5 to 40 g) of Union Carbide 13X sieve, activated beforehand at 250°C in vacuiom (133 Pa) for 2 hours, was placed each time.
In the case of Exaunple 12 the sieve was

additionally subjected in situ to heating to 150°C in vacuum (133 Pa) for 2 0 minutes, to prevent any risk of moisture uptake when it was being handled. In addition, the crude F32 was dried over a bed of 3 A molecular sieve.
Batches of F32 containing between 1100 and 2250 ppm of F31 were circulated continuously over each charge of 13X sieve and the influence of the temperature of treatment on the capacity at break¬through was determined. The results are listed in the following table.

EXAMPLE 13 (Comparative)
The spent charges from Examples 9, 10 and 12 were purged with a stream of nitrogen at ambient temperature and then desorbed at 250°C in vacuum (133 Pa) for 2 hours and were combined to form a batch of sieve to be regenerated.
50 g of this batch were suspended for 4 hours

in one litre of distilled water and were then drained and dried up to 12 0°C in the oven before being reactivated at 250°C in vacuo (133 Pa).
42 g of this sieve were placed in the same tube as in Example 6 and a stream of crude F32 containing 2050 ppm of F31 was circulated therein at 80°C at a flow rate of 3.6 1/h.
After 19 hours the residual F31 concentration already reached 4 ppm and increased rapidly subsequently. The calculated capacity at the break¬through point does not exceed 0.8 %.
EXAMPLE 14
50 g of the same batch as in Example 13, made up by combining the spent charges resulting from Exeunples 9, 10 and 12 and from their desorption at 250°C in vacuiam, were suspended in 500 ml of an aqueous solution containing 2 % of sodium carbonate and were stirred slowly for 4 hours. After draining, the charge was rinsed with two 250-ml portions of distilled water and was then dried and finally reactivated by heating to 250°C in vacuum (133 Pa) for 2 hours.
3 5.3 g of the sieve regenerated in this way were placed in the same tube as in Example 6 and swept with a stream of crude F32 containing, on average, 2375 ppm of F31 at a flow rate of 3.6 1/h and at 80°C.
As shown by the results in the following table, the break-through point appeared after 65 hours' operation in these conditions, which corresponds to a

capacity of 4.18 % relative to the weight of sieve introduced.

EXAMPLE 15
A sample of 100 g of Union Carbide 13X sieve was treated for 2 hours at 60°C with 400 ml of an aqueous solution containing 60 g of potassium nitrate. After draining and rinsing with three 400-ml portions of distilled water, the sieve was dried at 12 0°C for 2 hours and then heated to 450°C for one hour.
34.5 g of the sieve treated in this way were next activated at 250°C in vacuum (133 Pa) and were then placed in the same tube as in Example 6, which was heated to a temperature of 100°C, and were swept with a strecun of crude F32 containing, on average, 2840 ppm of F31, at a flow rate of 3.65 1/h.
Inspection of the results listed together in

the following table shows that the break-through point occurred after 76 hours' operation, which corresponds to a treatment capacity of 4.5 g of F31 per 100 g of 13X sieve in the potassium form.

EXAMPLE 16
35.1 g of Union Carbide 13X sieve, activated at 250°C in vacuum (133 Pa), were placed in the same tube as in Example 6, and then swept with a stream of crude F32 containing, on average, 1865 ppm of F31 and under the autogenous pressure of the F32, that is 12 to 14 bars absolute, depending on the ainbient temperature conditions prevailing during the test, the sieve bed being maintained at a temperature of 100°C. The gases were decompressed on leaving the purifier tube and their flow rate was controlled at 3.45 1/h under standard conditions of temperature and pressure.
The break-through point appeared after 79 hours' operation, which corresponds to a capacity of 4.34 % relative to the weight of sieve introduced.


WE CLAIM:
1. A process for the purification of difluoromethane (F32) containing traces of chlorofluoromethane (F31), which process comprises passing a gaseous steam of the F32 to be purified over a 13X molecular sieve at a temperature of at least 60°C, and optionally, after use, regenerating the 13X sieve by washing by means of a solution of sodium or potassium carbonate and heating to high temperature under inert atmosphere or in vacuum.
2. The process according to claim 1, in which the step of passing over the gaseous stream is carried out at a temperature above 75°C.
3. The process according to claim 1 or 2, in which the hourly space velocity of the gaseous stream F32 is between 20 and 2400 h-1.
4. The process according to claims 1 or 2, in which the hourly space velocity of the gaseous stream of F32 is between 40 and 250h-1
5. The process according to any one of claims 1 to 4, in which, before the washing step, the sieve is desorbed at 150-250°C under a stream of nitrogen or in vacuum.

6. The process according to any one of claims 1 to 5, in which
after the washing step, the sieve is heated to a temperature of between 200
and 250°C.
7. The process according to any one of claims 1 to 6, in which the
regenerated sieve is recycled for use in the process of claim 1.
8. A process for the purification of difluoromethane (F32)
containing traces of chlorofluoromethane (F31) substantially as herein above
described and exemplified.


Documents:

1153-mas-1995 abstract.pdf

1153-mas-1995 claims.pdf

1153-mas-1995 correspondence-others.pdf

1153-mas-1995 correspondence-po.pdf

1153-mas-1995 description(complete).pdf

1153-mas-1995 form-1.pdf

1153-mas-1995 form-26.pdf

1153-mas-1995 form-4.pdf

1153-mas-1995 form-9.pdf

1153-mas-1995 others.pdf

1153-mas-1995 petition.pdf


Patent Number 192904
Indian Patent Application Number 1153/MAS/1995
PG Journal Number 30/2009
Publication Date 24-Jul-2009
Grant Date 28-Mar-2005
Date of Filing 05-Sep-1995
Name of Patentee M/S. ELF ATOCHEM S. A .
Applicant Address 4 &8 COURS MICHELET, LA DEFENSE 10, F-92800 PUTEAUX
Inventors:
# Inventor's Name Inventor's Address
1 RENE BERTOCCHIO 49 BIS, RUE DES VALLIERES, 69390 VOURLES PAR VERNAISON
PCT International Classification Number C07C19/08
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA