Field of Invention:
The present invention relates to a process for preparation of pentafluroethane with high purity and yield employing a gas phase fluorination of halogenated alkylene with hydrogen fluoride in presence of a fluorination catalyst and removing the HCl by distillation followed by separating HFC 125 along with CFC 115 and HFC 134a obtained from gas phase fluorination and subjecting the mixture to a reaction over the catalyst wherein the impurities are converted in to pentafluoroethane and other underfluorinated hydrocarbons like chlorotetrafluoroethane, dichlorotrifluoroethane and components that are easily separated by distillation.
Background of invention:
Chlorofluorocarbons were being used as refrigerant gases and foam blowing agents for long. Due to the
presence of chlorine, these components were identified to deplete earth's protective ozone layer in the
stratosphere. HFCs were identified as substitutes for replacing CFCs. Many CFCS like
tetrafluoroethane (HFC 134a), difluoromethane (HFC 32), pentafluoroethane (HFC 125) either
independently or as blends were found suitable for this purpose. Pentafluoroethane (HFC 125) is found
to be a major component in most of the blends that are widely used.
Many processes are known in the art for manufacture of HFC 125. Widely used process employs
fluorination of tetrachloroethane with anhydrous HF in vapor phase over a fluorination catalyst. This is
normally two stage process wherein the first stage percholoroethylene is fluorinated to form 123, 124
and some 125. The partially fluorinated intermediates are further fluorinated to form 125 that is
separated as product and the underfluorinated intermediates are recycled for further conversion.
However, in all the process impurities are formed that are either difficult to separate or cause loss of
yield.
GB 901297 teaches method for production of pentafluoroethane by the reaction of tetrachloroethylene
and hydrogen fluoride with chromium oxide catalyst in vapor phase at a high temperature. It also states
that at high temperatures the catalyst gets deactivated faster and byproducts like
chloropentafluoroethanes are also formed. The by product formation is a loss on yield.
US 5750809: proposes a two stage reaction at different temperatures and pressures to improve
the conversion and yield. It also states that the reactions are influenced by the temperature and a higher temperature improves conversion. But higher the temperature, it also decreases the selectivity towards the desired product 125 and forms more by-product viz 115,134a, 114,133a and the kind.
US 5763707: disloses a process for the production of pentafluoroethane which comprises
contacting a hydrofluorochloroethane having the formula: C2H1Cl1+xF1+y wherein x and y are each independently 0,1,2 or 3 provided x+y is 3 with hydrogen fluoride in the vapor phase and in the presence of a fluorination catalyst which comprises zinc or a compound of zinc and chromia, chromium fluoride or chromium oxyfluoride. Many underfluorinated products are formed. The selectivity of 88 % is achieved and 12% of by-products are formed. These by-products are discarded and reduce the yield.
US 6527917: states that even though the boiling points of HFC 115 and HFC 125 are -38.7
degree C and -48.5 degree C respectively their relative volatility is close to 1 making it difficult for them to be separated easily. These components form azeotropic mixtures that make it costlier process technique to produce pure HFC 125. Further the CFC 115 separated is discarded and is loss on the organic yield.
Many techniques have been discussed in the art for the purification of CFC 115 from HFC 125. US 5087329 discusses a process for separating 115 from 125 by a solvent extraction distillation method. While this is useful in producing a pure 125 the 115 is not recovered as it is a CFC, hence discarded. One other technique known in the art is to convert 115 into 125 by hydrogenation reaction. US 569876 discloses a reaction of impure 125 containing 115 with hydrogen at high temperature over a specially prepared catalyst to convert most of the 115 into 125. While this is a process for recovering 115 that is wasted, people in the art know the difficulty of handling an explosive component as hydrogen in the plant that makes it hazardous and require costlier installations. Further the excess hydrogen is vented as non condensible causing a loss of product 125 from the distillation column vents. US 6340781: discloses similar reaction of 125 along with impurities with hydrogen over a hydrogenation catalyst at higher temperature. The impurities are converted into hydrocarbons that are flammable and vented which is a loss.
US 5453551: relates to the purification of 125 containing 115. this discloses converting 115
into 116 that is easily removed by distillation. This is done by reacting 125 containing 115 with HF as the fluorination agent over a fluorinating catalyst at very high temperatures. This is highly detrimental to the catalysts that are generally employed in the technique and is commercially may not be economical. The equipments are costlier due to high temperature operation involving hydrogen fluoride. Further apart from the loss of product it utilizes excess hydrogen fluoride also adds to the cost.
US 6512650: relates to purification of 125 containing 115 by converting 115 into 116 using
125 as the fluorinating agent by flowing 125 containing 115 over a catalyst formed by a trivalent chromium salt. This however utilizes the product 125 which is converted back to a chlorine containing intermediate. Also, the 115 is lost as 116 with additional losses of 125 alongwith non-condensable in the distillation process to separate 116.
US 5962753: proposes for a process for manufacture of pentafluoroethane (HFC 125) by the
reaction of perchloroethylene with hydrogen fluoride over a fluorination catalyst. It teaches a method of separating HFC 134a produced alongwith chlorotetrafluoroethane (HCFC 124) in a single distillation column and recycling it back to the reactor. The 134a thus recycled is converted into chlorofluoroethane (HCFC 133a) and disposed of by thermal oxidation. Also that the 114 formed is removed alongwith 133a and discarded by thermal oxidation resulting in yield loss.
The need was therefore for a process wherein the byproducts are minimized and recovered to improve the yield of the process and enables removal of the impurities economically to obtain a purified product.
The main objective of the present invention is to develop a method for preparation of pentafluoroethane economically.
Another objective of the present invention is a vapor phase process for the industrial production of pentafluoroethane (HFC 125) with good yield and minimize the effluents.
Summary of Invention:
The present invention discloses a process for preparation of pentafluoroethane with high purity and
yield employing a gas phase fluorination of halogentated organic alkylene with hydrogen fluoride in
presence of a fluorination catalyst and removing the HO by distillation followed by separating HFC
125 along with CFC 115 and HFC 134a formed and subjecting the mixture to a reaction over the catalyst
wherein the impurities are converted in to pentafluoroethane and chlorotetrafluoroethane,
dichlorotrifluoroethane and components that are easily separated by distillation.
The advantage of this invention is that it uses the by products formed in the process to convert them
into useful required products improving the overall yield of the raw materials thereby resulting in an
economic process.
Detailed description of invention:
The invention discloses a process for preparation of pentafluoroethane comprising:
(i) fluorinating halogentated organic alkylene with anhydrous hydrogen fluoride in
presence of a fluorination catalyst to form fluorinated alkanes like pentafluoroethane, CFC 115
and HFC 134a, other underfluorinated hydrocarbons and HC1;
(ii) removing HC1 obtained in step (i) by distillation;
(hi) separating pentafluoroethane along with CFC 115 and HFC 134a and subjecting mixture
to a reaction over the catalyst wherein the impurities are converted into pentafluoroethane and
underfluorinated hydrocarbons;
(iv) separating pentafluoroethane from underfluorinated products. In a preferred embodiment, step (i) of the process may be performed in two stage in different reactors, i.e. the reaction can be performed subsequently in two reactors. In first reactor reaction halogentated alkylene like tetrachloroethylene is fluorinated with anhydrous HF in gas phase over a fluorinating catalyst such as Chromia, Chromia alumina promoted or un-promoted at a temperature from 200°C to 350°C and at a pressure of atmospheric or super atmospheric in first stage to form various undeifluorinated products are formed. The amount of catalyst used in the process is dependent on the feed rate of the raw materials and reaction temperature. The ratio expressed in terms of the number of moles of total raw material fed being 50 to 200 times. The fluorination catalyst is well described in US
patent document no. 6723887.
The pressure in step (i) may range from atmospheric of 101.3 KPA to super atmospheric pressure
of 1500 KPA.
The underfluorinated hydrochlorofluoroethanes are introduced into second reactor with addition of
hydrogen fluoride at a temperature 250°C to 400°C and at pressure atmospheric or superatmospheric
over a fluorinating catalyst. The products eluting from both the reactors are combined and fed into a
distillation column to effect separation of HC1 and other lights by distillation.
After the removal of HC1 the remaining fluorinated and underfluorinated hydrocarbon products comprising 125, 124, 123, 113, 114, 115, 133a, 134a is drained into a second distillation column wherein HFC 125 alongwith CFC 115 and HFC 134a with boiling points -48.5°C, -38.7°C and -28.4°C respectively are withdrawn as overhead products leaving 124, 123, 114, 113, 133a etc as heavies at the bottom with unreacted HF. Such underfluorinated products can be directly recycled to react with anhydrous hydrogen fluoride of step (i).
The product 125 thus separated alongwith CFC115 and HFC 134a is further passed into a reactor containing the fluorination catalyst, the quantity of Catalyst expressed as a ratio to the total moles of feed being 200 to 800, to enable reaction of CFC115 and HFC134a to form HFC125 and HFC124. This reaction is effected at a temperature of 250°C to 400°C and the pressure may be atmospheric to superatmospheric. The reaction has been found to be giving better conversion with the increase in retention time. The reaction takes place as:
C2H2F4 + C2CIF5 > C2HF5 + C2HC1F4
In the above mentioned reaction HFC134a reacts with CFC 115 to form HFC 125 and HFC124. HFC
125 is our desired product and can easily be separated from HCFC124 formed that is recycled.
The molar ratio of HFC 134a with respect to CFC 115 may range from 1:1 to 10:1. Higher HFC 134a
ratio if used requires to be separated in the final product distillation and recycled.
It is preferred that HCFC 124 is removed in step (iii) of the process, otherwise it affects the conversion
of CFC115 by tetrafluoroethane (HFC 134a).
The catalyst undergoes deactivation by heavier molecules represented as carbon that may be
regeneratable by methods known in the art to the skilled person. Usually air is used to decompose the
carbon with suitable diluent to control the exothermicity produced during combustion.
The fluorination catalyst referred in all the reactions above may be same or different like chromia,
alumina, Chromia-alumina, either promoted or un-promoted. The catalyst has been defined in detail in
US document 6723887.
In a preferred embodiment the catalyst is pretreated first by calcination with a flow of Nitrogen
followed by a combination of Nitrogen and anhydroushydrofluoric acid vapours at temperatures up to
350°C or by any method known in the art, before the reaction is started. The results are comparable
with different catalysts known in the art.
The following examples illustrate the invention without limiting it.
Example 1:
275 grams (230 cc) of the catalyst was taken in an Inconel 600 tubular reactor having an outer diameter of 21 mm that was heated by hot oil circulation fitted with a temperature element to measure the bed temperature. The Catalyst was calcined up to 350°C under a flow of Nitrogen and prefluorinated with a flow of Nitrogen and anhydroushydrofluoric acid of about 550 gms for 16 hours at 350°C. The temperature of the reactor was reduced to 330°C and the reactor was fed with a flow of pentafluoroethane at the rate of 60 gms/hour vaporized and heated to about 320°C. The feed (Sample 1) comprised of chloropentafluoroethane and tetra fluoroethane at a molar ratio of about 1:3 with respect to each other as shown in the table as feed composition. The reaction was stabilized and the effluents from the reactor was analysed after scrubbing it in water using a Gaschromatography fitted with Gaspro column. The results of the experiment is presented as area % of main products to demonstrate the invention in the table.

(Table Removed)
Example 2: Comparative
The above experiment was repeated with a catalyst freshly prepared as above with the feed composition of the pentafluoroethane that did not contain tetrafluoroethane as shown in the table below. The effluents from the reactor was analysed in the same manner as in example 1. The results
showed a reduction of 115 and 125 with the increase in formation of 116 and 124.

(Table Removed)
Example 3:
The experiment was repeated as in example 1 with a catalyst freshly prepared with a flow of pentafluoroethane that also contained 124 along with 115, 134a. Here the effluents from the reactor were analysed in identical conditions and the results are tabulated below. The results revealed that the presence of 124 is not favourable to the reaction of 115 with 134a. The 134a is utilized to convert 124
into 125 without significant reduction of the impurity 115.

(Table Removed)
Example 4:
The experiment was repeated as in example 1 with a catalyst freshly prepared with a flow of
pentafluoroethane that contained 115 and 134a as shown in the table as feed composition. The reaction was carried out at different temperatures such as 330°C and 300°C. Here the effluents from the reactor were analysed in identical conditions and the results are tabulated below. The results indicated that the temperature is beneficial to the reaction of this invention; however it may cause deactivation of the catalyst faster.
(Table Removed)

We Claim:
1. A process for preparation of pentafluoroethane comprising:
(i) fluorinating halogentated organic alkylene with anhydrous hydrogen fluoride in
presence of a fluorination catalyst to form fluorinated alkanes like pentafluoroethane, CFC 115
and HFC 134a, other underfluorinated hydrocarbons and HC1;
(ii) removing HC1 obtained in step (i) by distillation;
(iii) separating pentafluoroethane along with CFC 115 and HFC134a and subjecting mixture
to a reaction over the catalyst wherein the impurities are converted into pentafluoroethane and
underfluorinated hydrocarbons;
(iv) separating pentafluoroethane from underfluorinated products.
2. The process as claimed in claim 1, wherein the temperature of the reaction is step (i) is 200 to 350 degree C.
3. The process as claimed in claim 1, wherein the pressure of the reaction in step (i) is atmospheric to super atmospheric pressure.
4. The process as claimed in claim 1, wherein underfluorinated hydrocarbons obtained in step (i) are recycled to again react with anhydrous HF.
5. The process as claimed in claim 1, wherein the reaction in step (iii) is carried out at a temperature ranging from 250 to 400 degree C and pressure ranging from 3 to 15 kg/cm2.
6. The process as claimed in claim 1, wherein the underfluorinated hydrocarbons obtained in step (iv) are recycled to react with anhydrous hydrogen fluoride over fluorination catalyst.
7. The process as claimed in claim 1, wherein the fluorination catalyst is pretreated before use.
8. The process as claimed in claim 1, wherein in step (iii) molar ratio of HFC 134a with respect to CFC 115 ranges from 1:1 to 10:1.
9. The process as claimed in claim 1, wherein step (i) is performed subsequently in two reactors.
10. The process for preparation of pentafluoroethane such as herein described with reference to the foregoing examples.