The present invention provides a process for purification of 1,1-difluoroethane.

BACKGROUND OF THE INVENTION
Hydrofluorocarbons (HFCs) have found widespread use in many commercial and industrial applications including as refrigerants, aerosol propellants, blowing agents, heat transfer media, gaseous dielectrics and as working fluids in air conditioning, heat pump and refrigeration systems.
Hydrofluorocarbons such as 1,1-difluoroethane (R152a) have essentially no ozone depletion potential (ODP) and low global warming potential (GWP), and therefore, they have been found to be acceptable refrigerants and, in some cases, as potential blowing agents in the production of plastic foams.
The most of the manufacturing processes for these hydrofluorocarbons involves one or more undesired products along the desired product. The desired product is then separated from the undesired products by means of methods such as distillation, adsorption or extraction.
PCT Pub. No. 1998019982 provides a process for separating 1,1-difluoroethane from a mixture of 1,1-difluoroethane and vinyl chloride by extractive distillation using an extractive agent such as hydrocarbon, alcohol or chlorocarbon.
U.S. Patent No. 9,828,316 provides a process for purifying a composition comprising 1,1-difluoroethane and one or more undesired halogenated hydrocarbon containing impurities such as one or more of a mono-, di- or tri-halomethane so as to reduce the concentration of at least one undesired halogenated hydrocarbon containing impurity by contacting the composition with an adsorbent comprising a carbon molecular sieve.
Similarly U.S. Patent No. 7,696,392 provides a process for purifying 1,1-difluoroethane, comprising bringing crude 1,1-difluoroethane containing at least one unsaturated compound such as vinyl chloride into contact with an adsorbent comprising a zeolite having an average pore size of 3 to 6Å and a silica/aluminium ratio of 2.0 or less and/or a carbonaceous adsorbent having an average pore size of 3.5 to 6Å.
The main disadvantage of the above processes is that either they use extractive distillation techniques for purification which require extracting agent like n-hexane or adsorption technique which require carbon molecular sieve or zeolite which makes the process costlier at commercial scale.
The present invention provides an alternative process for purifying hydrofluorocarbons by reducing the concentration of undesired impurities by means of pressure swing distillation without involving any additional agent.

OBJECT OF THE INVENTION
The object of the present invention is to provide a simple, cost effective and commercially viable process for purifying hydrofluorocarbons by reducing the concentration of undesired impurities by means of pressure swing distillation without involving any additional agent.

SUMMARY OF THE INVENTION
In an aspect, the present invention provides a process for purifying 1,1-difluoroethane, comprising the steps of:
a) providing a first composition comprising 1,1-difluoroethane and a second component, wherein said 1,1-difluoroethane and second component are present in an amount to form an azeotrope-like mixture;
b) distilling first composition at first pressure to produce a first overhead composition enriched in 1,1-difluoroethane and a first bottom composition enriched in the second component;
c) distilling the first overhead composition at a second pressure to produce a second overhead composition enriched in the second component, and a second bottom composition enriched in 1,1-difluoroethane; and
d) collecting pure 1,1-difluoroethane from second bottom composition.

BRIEF DESCRIPTION OF DRAWING
Figure 1 depicts the pressure swing distillation setup of the present invention. It consists of distillation column 1, condenser 1, distillation column 2, and condenser 2. The top of distillation column 1 is connected to condenser 1. The condenser 1 is connected to distillation column 2. The top of distillation column 2 is connected to condenser 2. The final product is collected from the bottom of distillation column 2. The second overhead composition of distillation column 2 obtained from condenser 2 is recycled back to distillation column 1.

DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term “azeotrope-like' is intended in its broad sense to include both compositions that are strictly azeotropic and compositions that behave like azeotropic mixtures. From fundamental principles, the thermodynamic state of a fluid is defined by pressure, temperature, liquid composition, and vapor composition. An azeotropic mixture is a system of two or more components in which the liquid composition and vapor composition are equal at the stated pressure and temperature. In practice, this means that the components of an azeotropic mixture are constant-boiling and cannot be separated during a phase change.
In an embodiment, the purification process of the present invention is carried out in two steps (see figure 1). In step a), a high pressure distillation is performed to obtain first overhead composition enriched in 1,1-difluoroethane and a first bottom composition enriched in second component. In step b), the first overhead composition was subjected to low pressure distillation to obtain second overhead composition enriched in second component and second bottom composition enriched in 1,1-difluoroiethane i.e., pure 1,1-difluoroiethane.
A process for purifying 1,1-difluoroethane, comprising the steps of:
a) providing a first composition comprising 1,1-difluoroethane and vinyl chloride monomer, wherein said 1,1-difluoroethane and vinyl chloride monomer are present in an amount to form an azeotrope-like mixture;
b) distilling first composition at first pressure to produce a first overhead composition enriched in 1,1-difluoroethane and a first bottom composition enriched in the vinyl chloride monomer;
c) distilling the first overhead composition at a second pressure to produce a second overhead composition enriched in the vinyl chloride monomer, and a second bottom composition enriched in 1,1-difluoroethane; and
d) collecting pure 1,1-difluoroethane from second bottom composition.
The technique of carrying out a purification process at two different pressure ranges i.e., high pressure range and low pressure range simultaneously is termed here as pressure swing distillation.
As used hereinabove, the second component refers to vinyl chloride monomer (VCM), hydrogen fluoride and/or 1-chloro-1-fluoroethane.
In a preferred embodiment, the second component is vinyl chloride monomer.
The process of the present invention is continuous wherein the pure product is continuously collected from the bottom of distillation column 2 and the compositions containing enriched amount of second component are recycled back to the initial feed and fresh feed is continuously supplied to distillation column 1 along with recycled stream.
The composition feed which is subjected to purification in the present invention comprises 1,1-difluoroethane and second component in a weight ratio selected in the range of 80:20 to 90:10%.
As used herein, the vinyl chloride is a vinyl chloride monomer (VCM).
As used herein, the term first pressure refers to a pressure range of 8-15Kg/cm2.
In an embodiment, the first pressure is selected in the range of 10-14Kg/cm2. In a preferred embodiment, the first pressure is 10Kg/cm2.
As used herein, the term second pressure refers to a pressure range of 0 to 1 Kg/cm2.
In an embodiment, the second pressure is selected in the range of 0.2 to 0.8 Kg/cm2. In a preferred embodiment, the second pressure is 0.5Kg/cm2.
In an embodiment, 1,1-difluoroethane is obtained with a purity greater than 98%. In another embodiment, 1,1-difluoroethane is obtained with a purity greater than 99%. The term greater than 99% refers to a purity between 99% to 99.9%.
In an embodiment, 1,1-difluoroethane contains vinyl chloride less than 0.5%. In another embodiment, 1,1-difluoroethane contains vinyl chloride less than 0.3%. The term less than 0.3% refers to impurity present in range of 0-0.5%.The term less than 0.3% refers to impurity present in range of 0-0.3%. The zero impurity means that impurity is not detectable.
The pure product i.e., 1,1-difluoroethane is analysed by gas chromatography (GC).
Unless stated to the contrary, any of the words “comprising”, “comprises” and includes mean “including without limitation” and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.
The azeotropic mixture comprising 1,1-difluoroethane as used herein as composition feed can be prepared by any of the methods known in the art e.g., as disclosed in European Patent No. 2336101.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLES
Example 1:
Step a): High pressure distillation in distillation column 1.
A mixture containing 1,1-difluoroethane and vinyl chloride in a weight ratio of 82:18 (initial feed) was charged in a packed bed distillation column 1. The operating pressure of the distillation column 1 was kept at 10kg/cm2. The components separated at the top were condensed using condenser 1 to collect first overhead composition and at the bottom to collect first bottom composition. The results are underneath:
Composition of distillation Column 1
First bottom composition R152a Wt.-% 76.35%
VCM Wt.-% 23.32%
First overhead composition R152a Wt.-% 95.89%
VCM Wt.-% 4.09%

Step b): Low pressure distillation in distillation column 2.
The first overhead composition was further subjected to low pressure distillation at a pressure of 0.5 kg/cm2 to obtain second overhead composition and the pure second bottom composition. The second overhead composition was recycled back to distillation column 1. The results are underneath:
composition of distillation Column 2
Second bottom composition R152a Wt.-% 99.72%
VCM Wt.-% 0.28%
Second overhead composition R152a Wt.-% 92.21%
VCM Wt.-% 7.79%

Example 2:
Step a): High pressure distillation in distillation column 1.
A mixture containing 1,1-difluoroethane and vinyl chloride in a weight ratio of 82:18 (initial feed) was charged in a packed bed distillation column 1. The operating pressure of the distillation column 1 was kept at 14kg/cm2. The components separated at the top were condensed using condenser 1 to collect first overhead composition and at the bottom to collect first bottom composition. The results are underneath:
Composition of distillation Column 1
First bottom composition R152a Wt.-% 77.35%
VCM Wt.-% 22.32%
First overhead composition R152a Wt.-% 94.89%
VCM Wt.-% 5.09%

Step b): Low pressure distillation in distillation column 2.
The first overhead composition was further subjected to low pressure distillation at a pressure of 0.5 kg/cm2 to obtain second overhead composition and the pure second bottom composition. The second overhead composition was recycled back to distillation column 1. The results are underneath:
Composition of distillation Column 2
Second bottom composition R152a Wt.-% 99.12%
VCM Wt.-% 0.88%
Second overhead composition R152a Wt.-% 91.21%
VCM Wt.-% 8.79%

SRF LIMITED NO. OF SHEETS: 1
APPLICATION NO. 202011006515 SHEET NO. 1 OF 1

WE CLAIM:

1. A process for purifying 1,1-difluoroethane, comprising the steps of:
a) providing a first composition comprising 1,1-difluoroethane and a second component, wherein said 1,1-difluoroethane and second component are present in an amount to form an azeotrope-like mixture;
b) distilling first composition at first pressure to produce a first overhead composition enriched in 1,1-difluoroethane and a first bottom composition enriched in the second component;
c) distilling the first overhead composition at a second pressure to produce a second overhead composition enriched in the second component, and a second bottom composition enriched in 1,1-difluoroethane; and
d) collecting pure 1,1-difluoroethane from second bottom composition.
2. The process as claimed in claim 1, wherein the second bottom composition is enriched in 1,1-difluoroethane and second overhead composition is enriched in second component.
3. The process as claimed in claim 1, wherein the second component refers to vinyl chloride monomer (VCM), hydrogen fluoride and/or 1-chloro-1-fluoroethane.
4. The process as claimed in claim 1, wherein the purification process is carried out at two different pressure ranges i.e., high pressure range (first pressure) and low pressure range (second pressure), termed here as pressure swing distillation.
5. The process as claimed in claim 1, wherein the first composition comprising 1,1-difluoroethane and a second component selected in a weight ratio of 80:20 to 90:10%.
6. The process as claimed in claim 1, wherein the process of the present invention is continuous process.
7. The process as claimed in claim 1, wherein the first pressure is selected in the range of 10-14Kg/cm2.
8. The process as claimed in claim 1, wherein the second pressure is selected in the range of 0 to1 Kg/cm2.
9. The process as claimed in claim 1, wherein the distillation setup depicted in a figure 1.
10. The process as claimed in claim 1, wherein the 1,1-difluoroethane has purity greater than 99%.