Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C17/00—Preparation of halogenated hydrocarbons
  • C07C17/26—Preparation of halogenated hydrocarbons by reactions involving an increase in the number of carbon atoms in the skeleton

Definitions

• This invention relates to the pyrolysis of tetrafluoroethylene and more particularly, to a process for suppressing the formation of polymer attending this pyrolysis.
• Still another method for obtaining hexafluoropropylene by pyrolysis is disclosed in U.S. Pat. No. 3,306,940 to Halliwell, wherein instead of tetrafluoroethylene being the starting material, the starting material is chlorodifluoromethane, and the pyrolysis is conducted to a conversion of from 86 to 94 percent of the chlorodifluoromethane to produce a pyrolyzate containing both tetrafluoroethylene and hexafluoropropylene. While this cosynthesis process has the advantage of producing a high yield of useful products relative to the amount of waste products formed, it also has the disadvantage of low productivity of hexafluoropropylene relative to the capacity of the equipment being used. In other words, if hexafluoropropylene is the desired product, then higher productivity of this product is obtained from a given furnace when the starting material is tetrafluoroethylene rather than chlorodifluoromethane.
• the gaseous HCl or HF is injected into the gases exiting the pyrolysis furnace (the pyrolyrate) to obtain intimate contact therewith.
• the amount of HCl or HF so injected will generally be in amounts from 3 to 10 percent of the weight of the tetrafluoroethylene feed to the pyrolysis furnace.
• the gaseous HCl or HF is supplied by copyrolysis of compounds which convert thereto, with the tetrafluoroethylene feed to the pyrolysis furnace.
• exemplary of compounds which convert to HCl and HF under tetrafluoroethylene pyrolysis conditions are chlorodifluoromethane and fiuoroform, respectively.
• This in situ formation of the HCl or HF provides these compounds in intimate contact with the pyrolyzate as it exits the pyrolysis furnace.
• the amount of compound used to copyrolyze with the tetrafluoroethylene to form the HCl or HP will generally be that amount under the pyrolysis conditions employed to supply the 3 to 10 percent range of HCl or HP contents previously described herein.
• This copyrolysis can be conducted by several methods.
• the compound convertible to either HCl or HP is fed to the pyrolysis furnace with the tetrafluoroethylene feed.
• no greater than 35 percent by weight of the convertible compound, particularly in the case of chlorodifluoromethane, based on the total weight of the feed to the furnace is used under this method, because at higher amounts the overall yield to useful products decreases significantly.
• at least 20 percent by weight of the convertible compound, based on the total weight of the feed is required to get noticeable improvement in polymer suppression.
• the compound convertible to HCl or HF is injected into the pyrolysis furnace, slightly upstream of the end of the pyrolysis Zone therein, so that some pyrolysis to HCI or HF occurs before leaving the furnace.
• Equipment suitable for this injection is shown, e.g. in FIG. 1 of US. Pat. No. 2,979,- 539 to Errede et al.
• less of the injected compound will be converted to the desired HCl or HP than in the previous method because of the shorter exposure to pyrolysis conditions. Consequently, the use of greater proportions of the convertible compound will be necessary.
• the lower conversion generally accompanying this method has the advantage of producing lesser amounts of undesirable by-products.
• copyrolysis methods can be used in combination with one another, or the copyrolysis method of feeding the convertible compound with the tetrafluoroethylene feed into the furnace can be used in combination with intimate contacting by HCl or HF added to the pyrolyzate.
• the feed of convertible compound or injection thereof or addition of HCl or HF to the pyrolyzate are proportiond to give the 3 to 10 percent range of HC] or HF in the pyrolyzate prior to its cooling to temperatures whereat polymerization occurs.
• at least 20 percent by weight of the total HCl or HP present in the pyrolyzate will be derived from each source thereof when these combinations of embodiments are employed.
• the pyrolyzate from the pyrolysis of tetrafluoroethylene is bubbled through or otherwise scrubbed with an aqueous solution of HCl or fluoride ion suppliedby such compounds as HF and the alkali metal salts, e.g., KF and NaF.
• HCl or fluoride ion suppliedby such compounds as HF and the alkali metal salts, e.g., KF and NaF e.g., KF and NaF.
• the pyrolyzate is above temperatures at which the polymerization occurs when brought into contact with the aqueous solution which quenches the pyrolyzate.
• concentration of the solution and its contact time with the pyrolyzate can be widely varied to suppress the formation of polymer.
• the improvement obtained by the present invention is generally applicable to the pyrolysis of tetrafluoroethylene to useful higher molecular weight perfluorocarbon compounds, which are mainly hexafiuoropropylcne and octafluorocyclobutane.
• the pyrolysis can be conducted at both sub-atmospheric and at super-atmospheric pressures, such as from 25 mm. Hg to 65 p.s.i. Temperatures of from 600 to 1000 C. can be used.
• the feed rate of the tetrafluoroethylene through the pyrolysis furnace is adjusted to give the exposure time required at the particular furnace operating conditions employed to give the conversion desired, which will generally be between 30 and 90 percent.
• Tetrafluoroethylene was fed at one atmosphere pressure and ml./sec. STP to a 0.7 cm. diameter, 28 cm. long Inconel pyrolysis tube heated by an electric furnace to 790 C. The exit gases were cooled to room temperature and analyzed by gas chromatography. Polymer was collected in the cold lines at the exit of the reaction tube.
• EXAMPLE 4 3.8 ml./sec. TFE plus 1.7 ml./ sec. chlorodifiuoromethane were premixed and pyrolyzed at 763 C. in the pyrolysis tube described in Example 1, at one atmosphere pressure. 0.9 rnl./sec. chlorodifluoromethane at 25 C.
• TFE 395 g. TFE were fed over 4 hours and 55 minutes. Less than 10 miligrams polymer deposited on Walls in the exit lines. This is 0.0025 weight percent of total TFE fed.

Landscapes

• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)

Applications Claiming Priority (1)

Publications (1)

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Family Applications (1)

Country Status (5)

Cited By (3)

Families Citing this family (2)

Family Cites Families (3)

• 1967
  • 1967-12-14 US US690398A patent/US3578721A/en not_active Expired - Lifetime
• 1968
  • 1968-12-11 GB GB58816/68A patent/GB1188636A/en not_active Expired
  • 1968-12-11 DE DE1814013A patent/DE1814013C3/de not_active Expired
  • 1968-12-13 FR FR1596099D patent/FR1596099A/fr not_active Expired
  • 1968-12-13 NL NL6817945A patent/NL6817945A/xx unknown

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