Purification process of pentrafluoroethane

Details Purification process of pentrafluoroethane Purification process of pentrafluoroethane

1994-05-25

2001-01-30

Warden, Sr., Robert J. (Department: 1744)

Distillation: processes, separatory

Adding material to distilland except water or steam per se

Organic compound

C203S062000, C203S063000, C203S070000, C570S178000

Reexamination Certificate

active

06179967

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a purification process of pentafluoroethane, and more specifically to a purification process of pentafluoroethane by an extractive distillation method using an extracting reagent.
BACKGROUND OF THE INVENTION
Monoclorodifluoromethane (hereinafter referred to as HCFC-22) which has hitherto been widely used as a refrigerant, etc., for an air conditioner, refrigerator, etc., becomes an object of the world-wide regulation as a material of destroying the ozone layer existing in the stratosphere, that is, as a regulated fluorinated hydrocarbon. Thus, pentafluoroethane (hereinafter referred to as HFC-125) having the similar properties as HCFC-22 has been watched with interest as one of substitutes for HCFC-22.
Since HFC-125 is usually produced by reacting perchloro ethane and hydrogen fluoride as raw materials, monochloropentanfluoroethane (hereinafter referred to as CFC-115) is frequently contained in the product as a by-product. However, CFC-115 is also is an object of the world-wide regulation as a controlled fluorinated hydrocarbon (CFC) and must be separated.
As one of methods of separating a fluid mixture into constituting components, a distillation method is the most general method. According to the inventors' investigations, however, the relative volatility of a little amount of CFC-115 to HFC-125 is near 1, for example, the relative volatility is from 1.01 to 1.02, under the pressure of near 5 kg/cm
2
G, and hence it is very difficult to separate HFC-125 from CFC-115 by a simple distillation method.
Under the circumstance, an extractive distillation method of performing a distillation by adding to a fluid mixture, as an extracting reagent, a compound having a different boiling point than the constituting components of the fluid mixture is applied. For example, U.S. Pat. No. 5,082,329 discloses a process of separating HFC-125 from a crude HFC-125 containing CFC-115 by carrying out an extractive distillation using a controlled CFC as an extracting reagent. The extracting reagent is exemplified with 1,2-dichlorotetrafluoroethane, etc. and the extracting reagent itself is a controlled CFC.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a process of purifying HFC-125, without using such a controlled CFC as an extracting reagent, by extractive distillation of a crude HFC-125 to separate HFC-125 from by-product CFC-115.
As a result of various investigations, it has been found that the above-described object can be achieved by extractive distillation of a crude HFC-125 containing CFC-115 using an extracting reagent having a standard boiling point (i.e., a boiling point under atmospheric pressure) in the range of from −10° C. to 100° C. and being selected from paraffinic hydrocarbons, alcohols, ethers, esters, and ketones.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the above-described extracting reagent increases or decreases the relative volatility of CFC-115 to HFC-125 to be apart from 1, whereby the separation of the components becomes possible.
The relative volatility is well known and is defined as a ratio of the equilibrium constants of the constituting components of a fluid mixture. In the case where the constituting components are HFC-125 and CFC-115, the relative volatility of CFC-115 to HFC-125 is shown by the following formula (1).
(
A
)=(
B
)/(
C
)=[(
D
)/(
E
)]/[(
F
)/(
G
)]  (1)
(A): Relative volatility of CFC-115 to HFC-125
(B): Equilibrium constant of CFC-115
(C): Equilibrium constant of HFC-125
(D): Mole fraction of CFC-115 in vapor phase
(E): Mole fraction of CFC-115 in liquid phase
(F): Mole fraction of HFC-125 in vapor phase
(G): Mole fraction of HFC-125 in liquid phase
As is clear from the above formula, if the relative volatility of CFC-115 to HFC-125 is 1, the compositions of both the vapor and liquid phases become same and the separation by distillation becomes impossible. When the relative volatility is larger than 1, the mole fraction of CFC-115 of the vapor phase becomes larger than the mole fraction of CFC-115 of liquid phase and since CFC-115 is concentrated in the vapor phase, the speparation by distillation becomes possible. When the relative volatility is less than 1, the mole fraction of CFC-115 of a liquid phase become larger than the mole fraction of CFC-115 in vapor phase and since CFC-115 is concentrated to the liquid phase, the separation by distillation becomes possible.
The extracting reagent for practical use is required to have the following properties.
(1) the selectivity is high,
(2) the solubility is high,
(3) the standard boiling point is in a proper range,
(4) the recovery of the extracting reagent is easy, that is, the difference in boiling point from those of HFC-125 and CFC-115 is large, and
(5) the extrcting reagent does not react with HFC-125 and CFC-115.
When a value of the relative volality in the presence of an extracting reagent devided by the relative volatility in the absence of an extracting reagent is larger than 1 or less than 1, it can be said that such a extracting reagent has a high selectivity.
Taking account of easiness of the separation of the extracting reagent from HFC-125 and CFC-115 by distillation, the standard boiling point of the extracting reagent is necessarily higher than the standard boiling points of HFC-125 and CFC-115 to some extent. Practically, it is preferred that the difference in boiling point between HFC-125 or CFC-115 and the extracting reagent is at least about 30° C., and more preferably about 40° C. or more. Since the boiling points of HFC-125 and CFC-115 are −48.5° C. and −38.7° C., respectively, the extracting reagent preferrably has a standard boiling point of at least −10° C. However, the boiling point should not be too high so that a specific heating source is not needed and the temperature distribution in a distillation column for separation of the extracting reagent can be moderated. Practically, it is preferred that the difference in boiling point between HFC-125 or CFC-115 to be separated and the extracting reagent is not more than about 140° C., and more preferably not more than 120° C. Hence, it is preferred to select an extracting reagent having a standard boiling point of not higher than 100° C.
In addition, as the result of investigating the solubility and the reactivity to HFC-125 and CFC-115, it has been found that paraffinic hydrocarbons, alcohols, ethers, esters, and ketones each having a standard boiling point of from −10° C. 100° C. are preferred as the extracting reagent.
In the measurement of the relative volatility described above, a crude HFC-125 containing about 3 mole % CFC-115 was charged in the Osmer vapor-liquid equilibrium apparatus made of stainless steel, and each of various extracting reagent was added thereto. After the liquid/vapor system reached to an equilibrium state at a constant temperature of 20° C., the liquid phase and the vapor phase were sampled. The composition of each phase was analyzed by gas chromatography, and the relative volatility was determined by formula (1) described above based on the analytical value.
The relative volatilities of CFC-115 to HFC-125 with various extrcting reagents as measured above are shown in Table 1 below.
TABLE 1
Relative
Concentration of
Volatility of
Extracting Reagent
CFC-115 to
Extracting reagent
in Liquid Phase
HFC-125
Paraffinic hydrocarbons
n-Pentane (36)*
80
0.4
i-Pentane (28)
74
0.5
n-Hexane (69)
79
0.6
Alcohols
Methyl alcohol (65)
81
4.5
n-Propyl alcohol (97)
78
2.1
i-Propyl alcohol (82)
84
2.7
Ethyl alcohol (78)
79
1.9
Ethers
Diethyl ether (35)
58
1.7
Esters
Ethyl formate (54)
82
4.5
Methyl acetate (57)
81
4.7
Ethyl acetate (77)
70
2.3
Ketones
Acetone (56)
81
6.4
Ethyl methyl ketone (79)
65
3.2
Note: *Standard boiling point (° C.) in parentheses
As is seen from the results shown in Table 1, when the extractive distillation is carried out using the paraffinic hydrocabon having a standard boiling point in the range

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