RU2188815C2 - Extractive distillation-mediated 1,1-difluoroethane purification process - Google Patents

Abstract

FIELD: industrial organic synthesis. SUBSTANCE: 1,1-difluoroethane/vinyl chloride mixture is brought into contact with at least one extractant selected from hydrocarbons, alcohols, and chlorine derivatives of hydrocarbons with boiling points from 10 to 120 C to give second mixture. From the second mixture, 1,1-difluoroethane is isolated by extractive distillation. Extractantto-1,1-difluoroethane ratio ranges from 1:1 to 10:1. Extractive distillation is carried out at pressures between 103.43 kPa and 2413.25 kPa. EFFECT: simplified purification technology. 8 cl, 1 dwg, 9 tbl, 4 ex

Description

Cross reference to related applications
This application claims a preferential right to priority according to the first US application 60/030148, registered November 1, 1996 (FL-1019-P1).

Field of Invention
The present invention relates to methods for separating vinyl chloride and other compounds from 1,1-difluoroethane by using extractive distillation using aliphatic hydrocarbons, chlorine derivatives of hydrocarbons and alcohols.

BACKGROUND OF THE INVENTION
New guidelines have been established to protect the stratospheric ozone layer from potential destruction by fully halogenated chlorofluorocarbon compounds (CFCs). The 1,1-difluoroethane compound (CHF ₂ CH ₃ or HFC-152a) is a chlorine-free fluorocarbon that is especially valuable as a refrigerant, blowing agent, rocket fuel and an intermediate chemical compound for the manufacture of vinyl fluoride, among other applications.

 1,1-Difluoroethane can be prepared by reacting vinyl chloride with hydrogen fluoride in the presence of a catalyst such as palladium, vanadium, tin, or others. The reaction product of such methods, in addition to the desired 1,1-difluoroethane, usually contains: unreacted vinyl chloride and hydrogen fluoride, hydrogen chloride as a by-product and small amounts of organic by-products such as 1-chloro-1-fluoroethane and vinyl fluoride. While most of these impurities can be removed by conventional distillation, vinyl chloride is very difficult to separate from 1,1-difluoroethane when the concentration of 1,1-difluoroethane is above about 85 mol%. Depending on the temperature, vinyl chloride and 1,1-difluoroethane in these concentrations form either an azeotropic mixture or an azeotropic composition, making it difficult to purify 1,1-difluoroethane by conventional distillation or making it impossible at all.

 The presence of even relatively small amounts of vinyl chloride in the 1,1-difluoroethane product is undesirable in many applications of this product. Numerous attempts have been made to reduce the amount of vinyl chloride remaining in 1,1-difluoroethane, but they were inadequate to achieve low levels of vinyl chloride or too expensive.

 The present invention solves the problems associated with known purification methods by providing methods for recovering 1,1-difluoroethane from mixtures containing 1,1-difluoroethane and vinyl chloride, which are simple and effective methods for obtaining low levels of vinyl chloride in 1,1-difluoroethane products. .

SUMMARY OF THE INVENTION
A method is found for separating 1,1-difluoroethane from the first mixture containing 1,1-difluoroethane and vinyl chloride, comprising the steps of:
contacting the first mixture with at least one extractant selected from the group consisting of: hydrocarbons, alcohols and chlorine derivatives of hydrocarbons having a normal boiling point higher than about 10 ^° C and lower than about 120 ^° C, to form a second mixture, and
separating 1,1-difluoroethane from the vinyl chloride of the second mixture by extractive distillation of the second mixture, thereby recovering 1,1-difluoroethane substantially free of vinyl chloride.

Brief Description of the Drawings
The invention is further explained in the description of specific variants of its implementation with reference to the accompanying drawing, which shows a diagram of an extractive distillation system that can be used to practically implement an aspect of the invented method.

Detailed description
1,1-Difluoroethane (HFC-152a) is usually prepared by fluorination of vinyl chloride monomer (VCM) using hydrogen fluoride in the presence of a catalyst. The 1,1-difluoroethane product may contain various impurities, such as hydrogen chloride by-product, by-products such as 1-chloro-1-fluoroethane, 1,1-dichloroethane and vinyl fluoride, as well as, but not limited to, vinyl chloride and fluoride. hydrogen. While most of these impurities can be removed from 1,1-difluoroethane by conventional distillation, small amounts of vinyl chloride are difficult, if not impossible, to remove using conventional distillation methods, due to azeotropic formations or pinch points between 1,1-difluoroethane and vinyl chloride. By the term “conventional distillation” is meant a technology in which only the relative volatilities of the components of the mixture to be separated are used to separate the components.

To determine the relative volatility of 1,1-difluoroethane and vinyl chloride, a method known in the art as the PTx method (a method for measuring pressure at a constant temperature) was used. In the PTx method, the total absolute pressure in a chamber of known volume is measured at a constant temperature for various known bicomponent compositions of 1,1-difluoroethane and vinyl chloride. The use of the PTx method is described in detail in Phase Equilibrium in Process Design, Wiley-Interscience Publisher, 1970, Harold R. Null, pages 124-126; full disclosure of which is incorporated by reference. These total pressure measurements are converted to vapor-liquid equilibrium compositions in the cell by using an equation-based equation type model for two liquids, taking into account the interaction (NRTL), which describes non-ideal states of the liquid phase. The use of the equation of activity coefficients, such as the equation for two liquids taking into account the interaction (NRTL), is described in more detail in the publication "The Properties of Gases and Liquids," 4 ^th edition, published by McGraw Hill, authors Reid, Prausnitz and Poling, page 241 -387; and Phase Equilibria in Chemical Engineering, published in 1985 by Butterworth Publishers, Stanley M. Walas, pages 165-244. The full disclosure of each of the above publications is incorporated by reference. Not wanting to be bound by any theory or explanation, the authors hope that the equation for two fluids, taking into account the interaction (NRTL), can sufficiently predict whether or not mixtures containing 1,1-difluoroethane and vinyl chloride behave ideally, and can also sufficiently predict the relative volatilities of components in such mixtures.

 The results of PTx measurements and the aforementioned series of calculations show that the relative volatilities of 1,1-difluoroethane and vinyl chloride are approximately 1.0 for the given compositions of 1,1-difluoroethane and vinyl chloride within the temperature range. Relative volatilities of approximately 1.0 in the mixture show the formation of an azeotropic mixture. The results of PTx measurements and the above calculations show that the composition of the azeotropic mixture varies with temperature. Table 1 shows the results of these calculations; especially how the composition of the resulting azeotropic mixture varies over the temperature range. Due to the formation of an azeotropic mixture, it is difficult, if not impossible, to completely separate 1,1-difluoroethane and vinyl chloride by conventional distillation in the temperature and pressure ranges shown in Table 1.

Based on measurements using a PTx camera and calculations, azeotropy disappears above a temperature of approximately 39.9-40.0 ^o C. However, above this temperature, the relative volatilities of 1,1-difluoroethane and vinyl chloride remain close to 1.0, which is significant for pinch points of equilibrium vapor-liquid, which in turn shows that it is still essentially impossible to separate the components through conventional distillation. Within the temperature and pressure ranges shown in Table 1, the preparation of pure 1,1-difluoroethane from a mixture with vinyl chloride would require starting with a composition cleaner than the azeotropic mixture, and the remainder of 1,1-difluoroethane would necessarily become at the end what azeotropic mixture. Distillation at temperatures below -80 ^° C and pressures below 3.79 KPa (0.55 psi) would require a very low temperature vacuum distillation. Therefore, the use of conventional distillation to produce high purity 1,1-difluoroethane (HFC-152a) would be expensive and would require a very large column.

 The words "azeotropic mixture" or "azeotropic" composition means a constantly boiling liquid mixture of two or more substances, which behaves as one substance. One way to characterize the azeotropic composition or mixture is that the vapor produced by partial evaporation or distillation of the liquid has the same liquid composition from which it was vaporized or distilled, for example, the mixture is distilled / refluxed without compositional change. Constantly boiling formulations are characterized as azeotropic because they exhibit either a maximum or minimum boiling point compared to non-azeotropic mixtures of the same components. The azeotropic composition can also be characterized as the maximum or minimum vapor pressure for the mixture at a given temperature, when plotting the function of the molar fraction.

 By the words “azeotropic-like mixture” is meant a composition that has a constant boiling characteristic or a tendency not to fractionate upon boiling or evaporation. The composition of the generated vapor is the same as or basically the same as the original composition of the liquid. During boiling or evaporation, the composition of the liquid, if at all, changes only to a minimal or negligible degree.

 A composition similar to an azeotropic mixture can also be characterized by a plot (graph) that is adjacent to the maximum or minimum vapor pressure. This is evident when plotting the vapor pressure at a given temperature as a function of the molar fraction. In the prior art, the composition is considered to be similar to an azeotropic mixture if, after removing about 50 wt.% Of the composition, for example by evaporation or evaporation, the difference between the original composition and the remaining composition is approximately less than 6% and usually approximately less than 3% relative to the initial composition.

 1,1-Difluoroethane and vinyl chloride can be partially purified using the aforementioned azeotropic compositions and similar azeotropic mixture of compounds in a method that involves the formation of these azeotropic and azeotropic mixture of compositions with a low boiling point, high pressure in a conventional distillation column. For example, a conventional distillation column may operate at temperature and pressure that cause the formation of an azeotropic and similar azeotropic mixture of compositions. If the amount of 1,1-difluoroethane is relatively large compared to vinyl chloride, that is, the concentration of 1,1-difluoroethane is greater than its concentration in an azeotropic and similar azeotropic mixture, then 1,1-difluoroethane can be recovered in substantially pure form from the bottom parts of the column, while an azeotropic or similar azeotropic mixture is removed from the top of the column. Conversely, if the amount of vinyl chloride is relatively large compared to 1,1-difluoroethane, that is, the concentration of vinyl chloride is greater than its concentration in an azeotropic or similar azeotropic mixture, then vinyl chloride can be removed in substantially pure form from the bottom of the column, while as an azeotropic or similar azeotropic mixture, the composition is removed from the top of the column.

The measurement results using a PTx camera and previous calculations for 1,1-difluoroethane and vinyl chloride with various extractants proposed are shown below in table 2. Table 2 shows the activity coefficients of 1,1-difluoroethane (HFC-152a) and vinyl chloride (VCM) at infinite dilution in one of the listed extracting substances and also gives the ratios of activity coefficients HFC-152a / VCM (relative volatility) at a temperature of 0 ^o C (except as specifically noted in table 2). It is believed that the ratio of the activity coefficient of 1,1-difluoroethane at infinite dilution in the proposed extractant to the activity coefficient of vinyl chloride at infinite dilution in the proposed extractant is the relative volatility of 1,1-difluoroethane relative to vinyl chloride in the presence of the proposed extractant. "NBP" is the normal boiling point of a compound at atmospheric pressure.

 The proposed extractants for HFC-152a / VCM are given in table. 2.

 The problems associated with conventional distillation can be solved through an extractive distillation process. Extractive distillation can be used when the components of the mixture have such relative volatilities that are insufficient to ensure efficient separation of the components through conventional distillation. Extractive distillation is added to extractive distillation, which causes the relative volatilities of the components of the mixture to change so that the relative volatility becomes sufficient to allow separation of the components. The difficulty in applying this method is that there is no known method that could predict which compound, if any, will be an effective extractive distillation agent.

 Extractive distillation is usually carried out during operation of a continuous distillation column, which contains: a multi-stage distillation column with at least two supply points, for example, an extractant is introduced at the first supply point, which is located above the second supply point used to introduce the mixture to be separated; boiler; and a condenser at the top of the column to return reflux to the column. Another similar commercially available device may also be used.

The inventors of the present invention have found that vinyl chloride can be effectively separated from 1,1-difluoroethane by using an extractive distillation method with an extractant selected from the group consisting of: aliphatic hydrocarbons, chlorine derivatives of hydrocarbons and alcohols having a normal boiling point of greater than about 10 ^° C and less than about 120 ^° C. Extraction agents with a normal boiling point between about 60 ^° C and 100 ^° C are preferred. Examples of suitable extractants for I of the method of the present invention are as follows: normal pentane, cyclopentane, normal hexane, cyclohexane, normal heptane, chloromethane, dichloromethane (methylene chloride), trichloromethane (chloroform), carbon tetrachloride, chloroethane, 1,1-dichloroethane, 1,2-dichloroethane, 1 , 1,2-trichloroethane, methyl alcohol, ethyl alcohol and propyl alcohol. Extracting agents used in the present invention are generally commercially available.

 The extractive distillation method of the present invention for separating 1,1-difluoroethane from the first mixture containing 1,1-difluoroethane and vinyl chloride comprises the following steps.

Contacting the first mixture with an extractant selected from the group consisting of: aliphatic hydrocarbons, chlorine derivatives of hydrocarbons and alcohols having a normal boiling point higher than about 10 ^{° C.} and lower than about 120 ^{° C.} to form a second mixture, and
separating 1,1-difluoroethane from vinyl chloride of the second mixture by extractive distillation of the second mixture in the extractive distillation zone, thereby recovering 1,1-difluoroethane substantially free of vinyl chloride as a product taken from the top of the column and a third mixture from the bottom of the zone, containing extractant and vinyl chloride,
and additional separation of the extracting substance from the third mixture.

 By “substantially free” or “substantially pure” is meant that 1,1-difluoroethane contains less than about 1.0 wt.% Vinyl chloride, usually less than about 0.1 wt.% Vinyl chloride, and in some cases less than about 10 ppm by weight (ppm) of vinyl chloride.

 The first mixture can be obtained from any suitable manufacturing process or source that produces 1,1-difluoroethane with vinyl chloride as an impurity: usually by reacting vinyl chloride with anhydrous hydrogen fluoride in the presence of a metal catalyst. Examples of a metal catalyst that can be used include, but are not limited to, palladium, vanadium, and tin compounds. Although the present invention is capable of adapting to a wide range of 1,1-difluoroethane / vinyl chloride compositions, it is preferred that the content of 1,1-difluoroethane is greater than about 85 mol. % and so that the content of vinyl chloride is less than about 15 mol. % If desired, conventional distillation may be used to reduce the initial amount of vinyl chloride or other similar impurities. That is, conventional distillation can be used to remove relatively large amounts or accumulations of impurities from the first mixture, which is subsequently processed in accordance with the method of the present invention to separate vinyl chloride and 1,1-difluoroethane.

 The present invention is carried out in such a way that the volatility of 1,1-difluoroethane with respect to vinyl chloride is increased, and thus, in the presence of more volatile 1,1-difluoroethane, it is possible to extract it from above the distillation zone. Typically, the removed or recovered 1,1-difluoroethane is substantially free of vinyl chloride.

 If desired, the extractants can subsequently be removed from the vinyl chloride impurity or mixture of impurities by conventional distillation methods. The extracting agent can be reused in an extractive distillation column to remove additional vinyl chloride from 1,1-difluoroethane.

 In one aspect of the invention, the extractant is introduced at the top of the feed point of the extractive distillation column, while the first mixture to be separated is introduced at a relatively lower point in the column. The extractant passes down through the columns of the column, which are located in the center of the column, and is in contact with the first mixture, thereby forming a second mixture.

 Since 1,1-difluoroethane is relatively more volatile than vinyl chloride in the presence of an extractant, this allows 1,1-difluoroethane to go to the top of the column. 1,1-Difluoroethane, which goes to the top of the column, can be condensed using conventional partial reflux condensers. At least a portion of this condensable stream can be returned to the top of the column in the form of reflux, and the residue can be recovered as a useful product, that is, essentially in the form of pure 1,1-difluoroethane. If desired, 1,1-difluoroethane can be sent to a second extractive distillation column and / or a second conventional distillation column for further purification.

 Vinyl chloride, extractant and other possible impurities make up the third mixture, which leaves the bottom of the column and which can subsequently be sent to a stripper or conventional distillation column for separation by conventional distillation or other known methods. If desired, the extractant may then be returned for reuse to the extractive distillation column. In some cases, vinyl chloride, after removal of the extracting substance by conventional distillation, is recovered as a useful product for sale or other use, for example, for the production of 1,1-difluoroethane. The recovered vinyl chloride may also be transported to a second or third conventional and / or extractive distillation column to remove other impurities, thereby providing substantially pure vinyl chloride for sale or other use.

 If a water-soluble extractant is used, such as ethyl alcohol or other alcohol, the extractant can be further removed from vinyl chloride by other methods, such as water extraction. For example, a mixture of vinyl chloride and a water-soluble extractant is passed through water, whereby the alcohol is predominantly extracted.

 The ratio of the material stream leaving the top of the extractive distillation column, which then condenses and subsequently returns to the column, to the amount of material stream that is recovered as a useful product, is usually referred to as the backflow coefficient (reflux ratio). The backflow coefficient will determine the physical characteristics of the extractive distillation column.

 Basically, an increase in the backflow coefficient will in turn cause an increase in the purity of 1,1-difluoroethane taken from the top of the column by reducing or eliminating the amount of extractant or other impurities in 1,1-difluoroethane at the top of the column.

 The specific conditions that can be used to implement the method of the present invention are not critical and depend on a number of interrelated design parameters, such as: column diameter, selected feed points, number of separation stages in the column, along with other parameters. The operating pressure of the distillation system can range from about 103.43 kPa (15 psi) to 2,413.25 kPa (350 psi), usually from about 344.75 kPa (50 psi) ) to 2068.5 kPa (300 psi). The temperature and heat transfer area of the head condenser is usually sufficient to basically completely condense the distilled product, or, optionally, enough to achieve the desired backflow coefficient through partial condensation.

 The effective amount of extractant can vary widely. Basically, using an increased amount of extractant will increase the purity of 1,1-difluoroethane at the top of the column. Typically, the ratio of the weight of extractant to the weight of 1,1-difluoroethane is in the range of from about 1/1 to about 10/1; however, a higher ratio may be used if necessary.

 The temperature that is used at this stage in the inventive method is a function of the pressure and structural characteristics of the distillation column, for example, the ratio of extracting substance to the first mixture.

Some aspects of the invention can be better understood by referring to the drawing, which schematically illustrates a system that can be used to implement one aspect of the extractive distillation method of the present invention. The first mixture containing 1,1-difluoroethane with vinyl chloride as an impurity is fed through line 1 to the extraction column 2. At least one liquid extractant is fed through line 3 to the extraction column 2 and introduced into column 2 at a position above the feed point of the first mixture 1 to form a second mixture. The third mixture, containing the extracting substance and vinyl chloride, is removed from the bottom of the column 2 and transported to the boiler 4 with steam heating. In some cases, the boiler 4 is connected to the extraction column 2. The third mixture is fed via line 5 to the raw material tank 6. An additional liquid extractant is also supplied to replenish the tank 6 via line 7, thereby forming a recirculation of the fourth mixture or extractant. The pump 8 transports the fourth mixture to the desorption column 9. The desorption column 9 divides the fourth mixture into extracting substances and non-extracting substances. The extracting substance is removed from the column 9 and is fed into the boiler 10 with steam heating. In some cases, the boiler 10 is connected to the column 9. The pump 11 conveys the extractant from the boiler 10 through a water-cooled refrigerator 12 and to the refrigerator 13. If necessary, excess amounts of extractant can be withdrawn before reaching the refrigerator 12. Typically, the refrigerator 13 is used at a temperature of approximately -25 ^o C. After leaving the refrigerator 13, the extracting substance is fed through a pipe 3 to the extraction column 2.

Vinyl chloride can exit the top of the stripping column 9 as waste gas and injected into the condenser 14, which usually operates at a temperature of approximately -25 ^° C. In this case, under reverse flow conditions, the pump returns a portion of the vinyl chloride to the stripping column 9. The remaining portion of the vinyl chloride can be removed from piping systems 16.

The off-gas can also be removed from the extraction column 2. The off-gas may be 1,1-difluoroethane, which is substantially free of vinyl chloride and other fluorocarbons. 1,1-Difluoroethane is transported through a pipe 17 to a condenser 18. The condenser 18 usually operates at a temperature of approximately -25 ^o C. In this case, under the reverse flow conditions, the pump 19 returns a portion of 1,1-difluoroethane to the extraction column 2. The product is 1, 1-difluoroethane can be removed from the system via line 20.

 In order for the extractant to be most effective for increasing the volatility of 1,1-difluoroethane, the relative volatility of 1,1-difluoroethane to vinyl chloride in the presence of the extractant should theoretically be greater than about 1.0; for practical purposes, it should be mostly greater than about 1.1. It is normal for this relative volatility to be more than about 1.5, and even more preferably, it is more than about 2.0.

Examples
The following examples are presented to illustrate certain aspects of the present invention and do not limit the scope of the invention as defined by the appended claims. The following examples use the interaction parameters of the equation for two fluids with regard to the interaction (NRTL), identified above. In the following examples, each step is based on 100% work or completion efficiency. In order to optimize the performance of each distillation, various column designs and operating conditions using various extracting agents are used. In all examples, theoretical stages include a condenser and a boiler, wherein the condenser is considered stage 1.

Comparative Example 1
In this example, a distillation column with 100 theoretical stages is used to purify a feed stream of 453.59 kg / h (1000 lb / h) of crude HFC-152a difluoroethane containing 0.045 kg / h (0.1 lb / h) of vinyl chloride VCM, with a VCM concentration of 100 ppm. There are no other impurities in the incoming material. The incoming material is introduced at stage 50 with a column condenser pressure of 259.94 kPa (37.7 psi). The temperature of the distillate is approximately 0 ^{° C.} and the temperature at the bottom of the column is approximately 1.8 ^° C. Under these operating conditions, the HFC-152a product will exit the column in a stream at the bottom of the column. The reflux rate in the column is set to satisfy the following composition: 10 ppm VCM vinyl chloride in the lower column product HFC-152a. The diameter of the column is selected so as to have a maximum F-factor (a standard measure of permissible volumetric flow rates per unit cross-sectional area of the column) equal to 1.3 or lower. In this example, the diameter of the column is 101.6 cm (40 inches) and the F factor is 1.12. The results of using this distillation process are shown below in table 3.

Reference Example 2
This test was performed under the same conditions as in comparative example 1, except that the pressure in the column was 101.36 kPa (14.7 psi), the temperature of the distillate was approximately −24 ^{° C.} , and the temperature the bottom of the column is approximately −19.7 ^° C. In this example, the diameter of the column is 71.12 cm (28 inches) and the F factor is 1.10. The results of this distillation method are shown below in table 4.

Reference Example 3
This test was performed under the same conditions as in comparative example 1, except that the pressure in the column was 27.92 kPa (4.05 psi), the temperature of the distillate was approximately -49.7 ^o C, and the temperature at the bottom of the column is approximately −39.5 ^° C. In this example, the diameter of the column is 60.96 cm (24 inches) and the F factor is 1.19. The results of this distillation process are shown below in table 5.

 The above comparative examples 1, 2 and 3 show that conventional distillation is relatively ineffective for separating VCM vinyl chloride from HFC-152a and has a very low yield of HFC-152a. However, these examples show how the concentration of VCM vinyl chloride can be reduced in a mixture containing VCM vinyl chloride and HFC-152a by azeotropic distillation.

Example 1
In this example, an extractive distillation column with 37 theoretical stages is used to purify a feed stream consisting of 453.59 kg / h (1000 lb / h) of crude HFC-152a difluoroethane containing 0.045 kg / h (0.1 lb / h) VCM vinyl chloride, with a VCM concentration of 100 ppm. There are no other impurities in the incoming material. The incoming material is introduced in step 25 and the n-hexane extractant is introduced in step 10 with a column condenser pressure of 239/26 kPa (34.7 psi). The temperature of the distillate is -2.7 ^{° C.} and the temperature at the bottom of the column is 100.0 ^° C. Under these operating conditions, the HFC-152a product will exit the column in a stream of the upper part of the column. VCM vinyl chloride and n-hexane will exit the bottom of the column. The flow rate of the extracting substance is set so as to satisfy the composition - 1 part per million vinyl chloride VCM in the product of the upper part of the column HFC-152a. The reflux flow rate in the column is set so as to satisfy the composition — 10 ppm of extracting substance in the HFC-152a product of the upper part of the column. The distillate speed is adjusted to recover 452.68 kg / h (998.0 lb / h) of HFC-152a product in the top column stream. The diameter of the column is selected so as to have an F factor of 1.3 or lower. In this example, the diameter of the column is 101.6 cm (40 inches) and the F factor is 1.12. The results of using this extractive distillation method are shown below in table 6.

Example 2
In this series of tests, the conditions are the same as in example 1, except as noted. Table 7 below summarizes the extractive distillation processes using n-hexane and other hydrocarbon extractants applicable to the process of the present invention. In each of the cases in Table 7, the total yield of the HFC-152a product is 99.8%, and the concentrations of VCM vinyl chloride and extractant in the HFC-152a product are 1 and 10 ppm by weight, respectively. The geometry of the column, the flow of extracting substance and the flow of reflux are varied so as to achieve these results (see table. 7).

Example 3
The conditions, flow rates and purity of HFC-152a in this example are the same as in example 1, except for those modified to use certain alcohols as extractants. The column design has been modified as shown in summary table 8.

Example 4
The conditions, flow rates and purity of HFC-152a in this example are the same as in example 1, with the exception of those that are modified to use certain chlorine derivatives of hydrocarbons as extractants. The column design has been modified as shown in summary table 9.

Given that the presence of azeotropic mixtures in molar fractions of 1,1-difluoroethane is greater than about 81% in a mixture with vinyl chloride (table 1), the surprising and unexpected result was that vinyl chloride can be separated from 1,1-difluoroethane for obtaining essentially pure 1,1-difluoroethane by extractive distillation using some readily available and cheap fluorine-free compounds. Other extractants that can also be used to purify 1,1-difluoroethane by removing vinyl chloride include: aliphatic hydrocarbons, chlorine derivatives of hydrocarbons and / or alcohols having a normal boiling point higher than about 10 ^{° C.} and lower than about 120 ^° C.

 For each of the three classes of extracting substances (hydrocarbons, alcohols, chlorine derivatives of hydrocarbons), the required parameters are: the flow of the extracting substance and the number of steps decreases as the ratio of activity factors increases (that is, the larger the ratio, the better). Although the best extractants under the proposed conditions of this table are usually n-hexane and carbon tetrachloride, the most effective extractant in industrial conditions may depend on the exact concentration and composition of the impurities and the ability of the extractant to provide concomitant removal of other impurities that may also be present.

 Although some specific aspects of the invention have been described in detail, one skilled in the art will appreciate that other embodiments and variations are encompassed.

Claims (8)

1. The method of purification of 1,1-difluoroethane by extractive distillation from the first mixture containing 1,1-difluoroethane and vinyl chloride, comprising contacting the first mixture with at least one extracting substance selected from the group consisting of hydrocarbons, alcohols and chlorine derivatives of hydrocarbons having the normal boiling point is higher than about 10 ^{° C.} and lower than about 120 ^{° C.} to form a second mixture, and the separation of 1,1-difluoroethane from the second mixture by extractive distillation of the second mixture, thereby recovering 1,1-difluoroethane, about again free of vinyl chloride.  2. The method according to p. 1, characterized in that the first mixture consists essentially of 1,1-difluoroethane, 1-chloro-1-fluoroethane, 1,1-dichloroethane, vinyl chloride, vinyl fluoride, hydrogen fluoride and hydrogen chloride.  3. The method according to p. 1, characterized in that the 1,1-difluoroethane resulting from the indicated separation step contains less than about 50 parts per million vinyl chloride.  4. The method according to p. 1, characterized in that the extracting substance includes at least one compound selected from the group consisting of normal pentane, cyclopentane, normal hexane, cyclohexane, normal heptane, chloromethane, dichloromethane (methylene chloride), trichloromethane ( chloroform), carbon tetrachloride, chloroethane, 1,1-dichloroethane, 1,2-dichloroethane, 1,1,2-trichloroethane, methyl alcohol, ethyl alcohol and propyl alcohol.  5. The method according to p. 1, characterized in that it further includes the return for reuse of at least part of the extracting substance obtained from extractive distillation of the specified stage of separation, for use in obtaining a second mixture of the specified stage of contacting.  6. The method according to p. 1, characterized in that extractive distillation is carried out at a pressure of from about 103.43 KPa (15 psi) to 2413.25 KPa (350 psi).  7. The method according to p. 1, characterized in that the ratio of the weight of the extracting substance to the weight of 1,1-difluoroethane is from about 1/1 to 10/1.  8. The method according to p. 1, characterized in that the specified 1,1-difluoroethane and vinyl chloride of the first mixture are an azeotropic mixture.

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