Distillation: processes – separatory – Adding material to distilland except water or steam per se – Organic compound
Patent
1997-02-14
1999-07-06
Dentz, Bernard
Distillation: processes, separatory
Adding material to distilland except water or steam per se
Organic compound
203 98, 570178, B01D 334
Patent
active
059193409
DESCRIPTION:
BRIEF SUMMARY
This applicaton is a 371 of PCT/JP95/00874 filed May 8, 1995.
TECHNICAL FIELD
The present invention relates to a process of producing pentafluoroethane by separating pentafluoroethane out of a mixture comprising at least pentafluoroethane (which is sometimes referred to as HFC-125) and chloropentafluoroethane (which is sometimes referred to as CFC-115) using extractive distillation in which a specific compound is used as an extractant (or solvent). Such a mixture may be for example a reaction product from a production process of pentafluoroethane by fluorinating tetrachloroethylene.
BACKGROUND ART
HFC-125 is a useful compound which can be a substitute for flon which does not contain chlorine, and it is used as a refrigeration medium, a forming agent, a propellant and so on. Fluorination of tetrachloroethylene is employed as a process of producing HFC-125. In such a process, dichlorotetrafluoroethane, dichlorotrifluoroethane, hexafluoroethane, CFC-115 and so on are produced as by-products.
Among those, CFC-115 has a boiling point of -38.7.degree. C. which is considerably close to a boiling point of HFC-125 (-48.5.degree. C.) as an objective product. Further, a relative volatility between these two compounds is near one. Particularly when a mixture contains HFC-125 at a concentration of not less than 95 mol % (thus, CFC-115 is not more than 5 mol %), the relative volatility is about 1.04. Therefore, it requires a distillation apparatus having many plates to separate HFC-125 in its higher concentration out of such a mixture using a conventional distillation operation, which generally means that the separation using the distillation is extremely difficult.
In the present specification, the relative volatility (.alpha.) is defined as follows when a solution consisting essentially of at least two components A and B in question (a boiling point of component A<a boiling point of component B) is in a vapor-liquid equilibrium state: liquid phase, x.sub.B is a molar fraction of higher boiling component B in the liquid phase, y.sub.A is a molar fraction of lower boiling component A in the vapor phase which is equilibrated with the liquid phase and y.sub.B is a molar fraction of the higher boiling component B in such a phase.
Extractive distillation has been employed as a process of separating one component from a mixture of a system of which relative volatility is close to one. With regard to separation of a mixture of HFC-125 and CFC-115, for example U.S. Pat. No. 5,087,329 discloses an extractive distillation process which uses a fluorocarbon containing 1 to 4 carbon atoms as an extractant.
In the process disclosed in U.S. Pat. No. 5,087,329, a relative volatility between HFC-125 and CFC-115 is about 1.2 based on calculation using figures shown in Example 1 thereof. Thus, in order that a concentration of HFC-125 is increased from HFC-125/CFC-115=7/93 (mol %/mol %) to HFC-125/CFC-115=99.7/0.3 (mol %/mol %) using distillation, about 40 theoretical plates are required. The number of the theoretical plates were calculated as explained below.
DISCLOSURE OF INVENTION
The present inventors have studied as to a process in which HFC-125 is further effectively separated out of a mixture comprising HFC-125 and CFC-115 using extractive distillation, and found that when a mixture comprising at least HFC-125 and CFC-115 is subjected to extractive distillation, HFC-125 is effectively (for example, using a distillation column having a very small number of theoretical plates) separated from the mixture by using, as an extractant (or a solvent), at least one compound (thus, as a single compound or a mixture of the compounds) selected from an alcohol containing 1 to 4 carbon atoms, a ketone containing 3 to 7 carbon atoms, an ether containing 2 to 6 carbon atoms and nitromethane or at least one selected from a hydrocarbon containing 3 to 8 carbon atoms, trichloroethylene and carbon tetrachloride.
Thus, the present invention provides a process of separating HFC-125 out of a mixture comprising at least HFC-125 and CFC-115 by
REFERENCES:
patent: 3101304 (1963-08-01), Wiist
patent: 3282801 (1966-11-01), Wiist
patent: 3689374 (1972-09-01), Hanson
patent: 5087329 (1992-02-01), Felix
patent: 5200431 (1993-04-01), Dattani et al.
patent: 5470442 (1995-11-01), Mahler et al.
Kohno Satoru
Shibanuma Takashi
Dahlen Garth M.
Daikin Industries Ltd.
Dentz Bernard
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