Process for the separation of halogenated hydrocarbons by extractive distillation

Method for separating 1,1,1,2-tetrafluoroethane from a mixture thereof with chlorine-containing halogenated hydrocarbons such as HCFC's 1122, 124, 114, 114a and 133a which comprises adding an extraction agent to the mixture and extractively distilling the mixture in an extractive distillation zone from which HFA 134a containing less than 10 ppm of chlorinated contaminants is recovered. Suitable extraction agents include trichloroethylene, perchloroethylene, alpha-pinene and cyclohexane.

This invention relates to a separation process and more particularly to a 
process for the separation of halogenated hydrocarbons by extractive 
distillation. 
It is well known to react hydrogen fluoride with various C.sub.2 compounds, 
for example trichloroethylene or 2-chloro-1,1,1-trifluoroethane or to 
hydrogenate chlorofluorocarbons, in order to make 
1,1,1,2-tetrafluoroethane (HFA 134a) which is useful as a refrigerant. A 
typical product stream obtained in these processes can contain unchanged 
starting materials together with organic and inorganic by-products as well 
as the desired HFA 134a. 
Various conventional separation techniques, for example distillation and 
aqueous scrubbing, have been proposed for the purpose of separating HFA 
134a from other components of the product stream. Particular difficulty 
can be experienced in removing other halogenated hydrocarbons containing 
chlorine, especially those having boiling points close to that of HFA 
134a. One of these is 2-chloro-1,1-difluoroethylene (HCFC 1122) which, 
although present in relatively small amounts, must be removed because of 
its toxicity. 
Several methods have been proposed for substantially reducing the HCFC 1122 
content of HFA 134a. Thus, U.S. Pat. No. 4,129,603 describes a method 
involving contacting the impure HFA 134a with an aqueous solution of a 
metal permanganate. In the process described in U.S. Pat. No. 4,158,675, 
the impure HFA 134a obtained by fluorinating 
2-chloro-1,1,1-trifluoroethane over a chromia catalyst is passed with 
hydrogen fluoride over a chromia catalyst at a much lower temperature than 
used in the manufacturing process. The purification process described in 
U.S. Pat. No. 4,906,796 comprises passing the impure HFA 134a over a 
zeolite having a mean pore size between 3.8 and 4.8 Angstroms. 
It has now been found that HCFC 1122 can be removed from a mixture of HFA 
134a and HCFC 1122 by a simple extractive distillation process which also 
removes other chlorinated species from the HFA 134a. 
According to the present invention there is provided a method for 
separating 1,1,1,2-tetrafluoroethane from a mixture containing 
1,1,1,2-tetrafluoroethane and chlorine-containing halogenated hydrocarbons 
which comprises adding an extraction agent to said mixture, extractively 
distilling the mixture in an extractive distillation zone and recovering 
1,1,1,2-tetrafluoroethane from the extractive distillation zone, the 
extraction agent being selected from trichloroethylene, perchloroethylene, 
carbon tetrachloride and aliphatic hydrocarbons containing from 4 to 10 
carbon atoms. Mixtures of extraction agents may be used, if desired. 
The aliphatic hydrocarbon may be saturated or ethylenically unsaturated and 
it may be cyclic or acylic. The carbon atom chain of the acyclic 
hydrocarbon may be straight or branched but usually will be a straight 
chain. Examples of particularly suitable hydrocarbons are hexane, 
cyclohexane and alpha-pinene. Good separation of HCFC 1122 from HFA 134a 
has been achieved using trichloroethylene and alpha-pinene. Alpha-pinene 
is a bicyclic alkene containing 10 carbon atoms. 
The extraction agents used in the method according to the invention are 
selected on the basis of their ability to extract a relatively large 
proportion of chlorinated species compared to HFA 134a from the mixtures 
being treated. 
The method of the invention is broadly applicable to any mixture of HFA 
134a and chlorine-containing halogenated hydrocarbons such as HCFC 1122 
but is especially applicable to mixtures obtained in processes for the 
manufacture of HFA 134a by the reaction of hydrogen fluoride with C.sub.2 
compounds such as trichloroethylene and/or 1-chloro-2,2,2-trifluoroethane. 
The mixtures produced in such processes commonly contain major proportions 
of 1-chloro-2,2,2-trifluoroethane, 1,1,2,2-tetrafluoroethane (HFA 134) and 
hydrogen fluoride and minor proportions of other haloethanes and possibly 
HCFC 1122. The mixtures may also be obtained by hydrogenation of 
halocarbons such as 1-chloro-1,2,2,2-tetrafluoroethane (HCFC 124) and 
1,1-dichloro-1,2,2,2-tetrafluoroethane (CFC 114a), such mixtures commonly 
containing major proportions of HCFC 124 and/or HCFC 114a. Treatment of 
mixtures containing HFA 134a and HFA 134 does not result in appreciable 
separation of these two components. 
The HCFC 1122 or other chlorinated species content of mixtures treated in 
accordance with the invention is typically from 20 to 5000 ppm on a weight 
basis but mixtures containing smaller or larger amounts of HCFC 1122 or 
other chlorinated species may also be separated. If desired, the reaction 
stream may be given a pre-treatment in order to effect partial or 
essentially complete removal of HCFC 1122, one or more other chlorinated 
constituents and/or hydrogen fluoride before performing the separation 
method of the invention. 
The method of the invention may be performed using conventional extractive 
distillation procedures. Thus, in a typical operation, a mixture 
containing HFA 134a and HCFC 1122 and/or other chlorinated species 
obtained as a reaction product in an HFA 134a production process is fed to 
the centre of a fractionating column whilst an extraction agent such as 
trichloroethylene, .alpha.-pinene, perchloroethylene or cyclohexane is fed 
to the upper part of the column. As distillation proceeds, the column 
provides an overheads fraction comprising HFA 134a containing less than 10 
ppm by weight of HCFC 1122 and/or other chlorinated species and a bottoms 
fraction comprising the extraction agent and HCFC 1122 and/or other 
chlorinated species. 
The column is suitably operated at a pressure of from 1 to 15 bars. 
Trichloroethylene is a preferred extractant for use in the method of the 
invention because the bottoms fraction from the fractionation column 
comprising trichloroethylene and HCFC 1122 and/or other chlorinated 
species can be recycled to the fluorination reactor. 
Use of perchloroethylene or carbon tetrachloride as extraction agent also 
provides a bottoms fraction which can be used directly as or as part of 
the feedstock for production of halogenated hydrocarbons. When 
.alpha.-pinene or another hydrocarbon is used as the extraction agent, it 
can be separated from the HCFC 1122 or other chlorinated species by a 
conventional fractional distillation for return to the extractive 
distillation column.

Referring to the FIGURE, a distillation column 1 is provided with a feed 
line 2 to the centre of the column and a feed line 3 to the upper part of 
the column. An overheads flow line 4 leads from the top of the column 1 to 
a condenser 5. A product flow line 6 leads from the condenser 5 with a 
reflux flow line 7 leading from the product flow line 6 back to the top of 
the column 1. A bottoms flow line 8 leads from the bottom of the column 1 
to a reboiler 9 with a vapour return line 10 leading from the top of the 
reboiler back to the bottom of the column. A flow line 11 leads from the 
reboiler 9. 
In operation, a mixture containing HFA 134a and 30 ppm HCFC 1122 obtained 
from a fluorination reactor (not shown) is fed via line 2 to the column 1 
which is maintained at a pressure of from 1 to 15 bars. Trichloroethylene 
is fed to the column 1 via line 3. As distillation proceeds, an overheads 
fraction comprising HFA 134a containing less than 10 ppm of HCFC 1122 is 
taken from the top of the column 1 via flow line 4, condenser 5 and flow 
line 6. A bottoms fractions comprising trichloroethylene and HCFC 1122 is 
taken from the bottom of the column 1 via flow line 8, reboiler 9 and flow 
line 11 for recycling to the fluorination reactor. 
The HFA 134a product obtained by the method of the invention may be 
subjected, as desired, to further purification procedures. 
The extractive distillation may be used for separating HFA 134a from its 
mixtures with HCFC 1122 and/or haloethanes such as 
2-chloro-1,1,1,2-tetrafluoroethane (HCFC 124), 
2-chloro-1,1,1-trifluoroethane (HCFC 133a), 
1,1-dichloro-1,2,2,2-tetrafluoroethane (CFC 114a) and/or 
1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC 114). The extractive 
distillation does not separate HFA 134a from HFA 134. Alpha-pinene is 
useful for separating HFA 134a from mixtures containing from 100 ppm to 5% 
of HCFC 124. 
The invention is further illustrated by the following Examples. 
EXAMPLE 1 
A three neck round bottom flask was fitted with a gas sampling septum, 
stopper and tap (total vol 600 ml). The flask was evacuated then HCFC 1122 
vapour (5 ml) and HFA 134a vapour (5 ml) were added through the septum. 
The flask was brought to atmospheric pressure by allowing air in through 
the tap. A vapour sample (20 ml) was withdrawn and analysed by gas 
chromatography to produce a reading of initial vapour composition. Air was 
then added to the flask (20 ml) followed by a sample of the relevant 
extraction agent or solvent (4 ml). The solvent was stirred for 45 minutes 
at 20.degree. C. after which the head space was sampled (20 ml) and 
analysed by gas chromatography giving the final vapour composition. The 
corrected initial partial pressure (P*.sub.I), the final partial pressure 
(P*.sub.F) and the mass of gas dissolved in 4 ml of solvent were 
calculated. 
Results for .alpha.-pinene as solvent were as follows: 
______________________________________ 
Mass 
P.sub.I * P.sub.F * Dissolved 
Atm .times. 10.sup.-3 
Atm .times. 10.sup.-3 
mg/4 ml K .alpha. 
______________________________________ 
HCFC 8.32 7.25 2.6 6.8 
1122 
HFA 134a 
8.34 8.19 0.4 56.3 8.3 
______________________________________ 
Results for trichloroethylene as solvent were as follows: 
______________________________________ 
Mass 
P.sub.I * P.sub.F * Dissolved 
Atm .times. 10.sup.-3 
Atm .times. 10.sup.-3 
mg/4 ml K .alpha. 
______________________________________ 
HCFC 7.95 7.04 2.3 13.2 
1122 
HFA 134a 
7.71 7.48 0.6 54.5 4.1 
______________________________________ 
K in the tables may be defined as mole fraction of material in vapour/mole 
fraction in liquid. 
.alpha. in the tables=K.sub.134a /K.sub.1122 
EXAMPLE 2 
Three glass sample vials (volume 90 ml) were prepared and fitted with septa 
for gas addition and sampling. Each vial was evacuated, then filled to 1 
atmosphere pressure with a previously prepared 1:1:1:1:1 gas mixture by 
volume of 134a, 134, 1122, 124 and 114x (an unresolved mixture of 114a and 
114). A vapour sample (10 ml) was withdrawn from each sample vial and 
analysed by gas chromatography (gc) to obtain the initial gc area count 
for each component. The chosen extraction agent (10 ml) was then added to 
the vial and the contents were stirred for 1 hour at room temperature. A 
vapour sample (10 ml) was withdrawn, brought to atmospheric pressure by 
air addition, then analysed by gc to indicate the final gc area count for 
each component. 
Table 1 lists the absolute drop in each component which occurred after 
exposure to the extraction agent (taken as an average of three runs). 
Table 2 shows the drop in components 134, 1122, 124 and 114x relative to 
that for 134a. This data is produced by normalising on the 134a, i.e. the 
134a initial level is divided by 134a final level to produce a ratio with 
which all the final component levels are multiplied such that the 134a 
initial level divided by the normalised 134a final level equals 1. 
TABLE 1 
______________________________________ 
Absolute Drop in component gc 
area count (%) 
Extraction Agent 
134a 134 1122 124 114x 
______________________________________ 
Trichloroethylene 
49.8 55.9 83.2 75.8 84.2 
Tetrachloroethylene 
26.1 34.9 75.6 62.6 -- 
Carbon tetrachloride 
36.4 43.8 79.5 70.6 -- 
Hexane 38.4 46.1 77.0 70.3 84.3 
Cyclohexane 26.9 30.8 74.3 60.9 81.8 
.alpha.-Pinene 
46.0 45.0 79.0 70.2 82.1 
COMISON 
1,1,1-Trichloroethane 
63.9 67.1 85.4 82.3 -- 
Pentachloroethane 
39.5 45.4 77.6 66.3 75.7 
Chloroform 55.3 62.0 83.3 77.2 -- 
1,1,2-trichloro- 
59.2 61.5 80.9 80.2 -- 
1,2,2-trifluoroethane 
Toluene 61.9 72.1 84.7 85.3 -- 
Ethanol 71.7 83.4 81.8 90.8 78.2 
Methanol 74.3 86.6 79.1 90.1 70.4 
Water 7.5 12.4 7.1 5.1 -- 
Triethylamine 55.5 70.4 85.1 90.2 85.9 
Perfluorohexane 
49.9 52.4 63.4 67.5 80.1 
______________________________________ 
TABLE 2 
______________________________________ 
Relative Drop in component gc 
area count (%) 
Solvent 134a 134 1122 124 114x 
______________________________________ 
Trichloroethylene 
0 9 66 51 69 
Tetrachloroethylene 
0 12 67 50 71 
Carbon tetrachloride 
0 12 68 54 74 
Hexane 0 6 63 52 75 
Cyclohexane 0 5 65 46 75 
.alpha.-Pinene 
0 0 61 44 67 
COMISON 
1,1,1-Trichloroethane 
0 13 60 54 -- 
Pentachloroethane 
0 10 63 44 60 
Chloroform 0 15 62 49 -- 
1,1,2-trichloro- 
0 5 53 52 -- 
1,2,2-trifluoroethane 
Toluene 0 25 57 58 -- 
Ethanol 0 41 36 68 22 
Methanol 0 47 18 61 (-16) 
Water 0 5 (-2) (-3) -- 
Triethylamine 0 34 66 78 68 
Perfluorohexane 
0 1 26 43 60 
______________________________________ 
EXAMPLE 3 
Three glass sample vials (volume 90 ml) were prepared and fitted with septa 
for gas addition and sampling. Each vial was evacuated then filled to 1 
atmosphere pressure with a previously prepared gas mixture of 134a 
containing ca 1000 ppm by volume each of 134, 1122, 124 and 114x. A vapour 
sample (10 ml) was withdrawn from the sample vials and analysed by gc to 
obtain the initial gc area count for each component. The chosen extraction 
agent (10 ml) was then added to the vial and the contents were stirred for 
1 hour at room temperature. A vapour sample (10 ml) was withdrawn, brought 
to atmospheric pressure by air addition, then analysed by gc to indicate 
the final gc area count for each component. 
Table 3 lists the absolute drop in each component which occurred after 
exposure to the extraction agent (taken as an average of three runs). 
Table 4 shows the drop in components 134, 1122, 124 and 114x relative to 
that for 134a. This data is produced by normalising on the 134a i.e. the 
134a initial level is divided by 134a final level to produce a ratio with 
which all the final component levels are multiplied such that the 134a 
initial level divided by the normalised 134a final level equals 1. 
TABLE 3 
______________________________________ 
Absolute Drop in component gc 
area count (%) 
Solvent 134a 134 1122 124 114x 
______________________________________ 
Trichloroethylene 
37.8 41.4 77.2 67.5 76.4 
Tetrachloroethylene 
36.6 53.3 77.5 67.0 78.3 
Carbon tetrachloride 
42.6 49.0 80.1 71.8 88.0 
Hexane 40.3 42.4 77.8 70.7 -- 
Cyclohexane 32.5 30.9 74.0 62.8 82.5 
.alpha.-Pinene 
39.2 42.6 75.6 68.6 79.1 
COMISON 
1,1,1-Trichloroethane 
62.5 66.7 82.0 79.3 -- 
Pentachloroethane 
43.6 49.2 78.0 67.2 -- 
Chloroform 56.3 61.3 79.3 73.1 -- 
1,1,2-trichloro- 
55.4 58.2 79.3 77.4 -- 
1,2,2-trifluoroethane 
Paraffin 17.5 17.9 55.0 38.0 -- 
Toluene 66.1 75.9 85.9 86.2 -- 
Water 4.0 10.3 4.8 2.9 -- 
______________________________________ 
TABLE 4 
______________________________________ 
Relative Drop in component gc 
area count (%) 
Solvent 134a 134 1122 124 114x 
______________________________________ 
Trichloroethylene 
0 9 58 45 61 
Tetrachloroethylene 
0 8 65 48 68 
Carbon tetrachloride 
0 11 64 49 74 
Hexane 0 3 63 51 -- 
Cyclohexane 0 4 64 48 76 
.alpha.-Pinene 
0 7 60 49 66 
COMISON 
1,1,1-Trichloroethane 
0 11 52 45 -- 
Pentachloroethane 
0 10 61 42 -- 
Chloroform 0 14 53 37 -- 
1,1,2-trichloro- 
0 4 49 45 -- 
1,2,2-trifluoroethane 
Paraffin 0 0 45 25 -- 
Toluene 0 28 58 59 -- 
Water 0 6 1 (+1) -- 
______________________________________