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

• the UNITED STATES PATENTS distillate mixture can be recycled to the pyrolysis fur- 2,394,58l 2/1946 Benning ct 211 260/6533 nace, 2,551,573 5/1951 Downing et al 260/6533 2,758,138 8/1956 Nelson 260/6533 6 Chums N0 Drawmgs PROCESS FOR PYROLYZING TETRAFLUOROETHYLENE TO HEXAFLUOROPROPYLENE This invention relates to pyrolysis of tetrafluor oethylene to hexafluoropropylene.
• U.S. Pat. No. 2,551,573 to Downing et al. discloses the pyrolysis of CHCIF to tetrafluoroethylene and higher boiling fluorocarbon products.
• U.S. Pat. No. 2,758,138 to Nelson discloses the pyrolysis of tetrafluoroethylene to hexafluoropropylene at various specific conditions involving low partial pressures, i.e., 25 to 200 mm. of Hg, of the tetrafluoroethylene feed.
• a process for pyrolyzing tetrafluoroethylene to hexafluoropropylene at higher pressures, viz., 0.2 to 65 psia. is disclosed in U.S. Pat. No.
• the present invention provides a pyrolysis process for making hexafluoropropylene which is economically advantageous over all the foregoing described processes.
• the process involves pyrolyzing a feed of tetra tluoroethylenc (TFE) and a minor proportion of car bon dioxide in a reaction zone at a temperature of 700 to 900C, and a pressure of 0.75 to 2.0 atmospheres absolute, and a minimum partial pressure of TFE of 360 mm.
• TFE tetra tluoroethylenc
• the process has been operated economically at conversions of TFE up to about 80 percent.
• the major pyrolysis product i.e., greater than 50 percent by weight of the pyrolysis products, is HFP. Yields of greater than 80 percent by weight and as high as 90 percent by weight of TFE to HFP, the desired product, can be obtained by this pro cess.
• a major problem involved in pyrolyzing TFE is its tendency to disproportionate to carbon tetrafluoricle and carbon according to the reaction which represents both a yield loss of expensive chemical and causes plugging of the reaction apparatus. This disproportionation is an exothermic reaction so once it begins, it tends to be a runaway reaction.
• the invention of the aforementioned Nelson patent attempted to avoid this problem by operating at very low pressures, i.e., less than 200 mm. of Hg absolute. This low pressure of operation was obtained by continuously evacuating the pyrolysis furnace and feeding the TFE to the furnace, which, by itself, determined the pressure in the furnace.
• TFE could be pyrolyzed to HFP at higher, more conveniently obtainable pressures such as atmospheric pressure, but this required the presence of at least 5 percent of pyrolyzable fluorocarbon materials having a boiling point higher than HFP which were normally provided by recycling the higher boiling pyrolyzate, which had the disadvantage of repeated handling of highly toxic materials rather than leading to quick waste disposal thereof.
• the present invention also operates at the more favored pressure condition of around atmospheric pressure but without requiring the presence of the highboiling fluorocarbon compounds and the attendant toxicity problem and yet avoiding the disproportionate problem.
• the process of the present invention is most conveniently conducted preferably at atmospheric pressure by passing a mixture of the TFE and carbon dioxide through a heated tube, called a pyrolysis furnace, with the heated portion of the tube being the reaction or pyrolysis zone.
• a pyrolysis furnace a heated tube
• the presence of the carbon dioxide in the pyrolysis furnace during the pyrolysis of TFE in the present invention prevents the undesirable disproportionation ofthe TFE despite the partial pressure ofTFE being substantially higher than that disclosed by Nelson.
• the proportions of TFE and carbon dioxide in the feed to the reaction Zone there is always a greater weight of TFE present than carbon dioxide, and thus the latter is present as a minor proportion (weight basis) of the feed.
• the proportion ofTFE to carbon dioxide in the feed will be from 1:1 to 25:1 on a weight basis.
• the feed mixture to the reaction zone advantageously contains at least 40 percent by weight (total feed basis) of carbon dioxide to render the mixture nonflammable in case of leakage to the atmosphere.
• the partial pressure of TFE in the feed mixture is at least 450 mm. of Hg absolute.
• gaseous hydrogen chloride or hydrogen fluoride is preferably injected into the pyrolysis mixture as described in U.S. Pat. No. 3,578,721 to Couture eta1., in order to prevent polymerization of unreacted TFE which would eventually cause plugging of the recovery system.
• the next step in the process is to recover the unreacted TFE and carbon dioxide as a mixture from the rest of the pyrolysis mixture by techniques well known in the art, such as distillation.
• the boiling point of the TFE is 76.8C. and the sublimation point of carbon dioxide is -78C. which enables these compounds to be separated from the HFP (b.p. 29C) and higher boiling fluorocarbon pyrolysis products in a single distillation operation and recovered as a mixture.
• the distillation is conducted under pressure, such as 17.5 kg/cm (gauge), to raise the boiling point of the distillate to a convenient temperature of operation, which temperature is above the temperature at which solid CO could exist in the distillation system.
• HCl is added to the pyrolyzate, as described above, it will distill with the TFE as an azeotrope therewith. 1f the additive is HF. it will a mixture to the lnconel tube via calibrated rotameters. Hydrogen chloride was injected into the exit zone of the pyrolysis tube where the temperature was about 500C. to inhibit polymerization of unreacted TFE in not dlstlll with the TFE but will remain with the HFP 5 the collection system.
• the lnconel tube was heated to and other hlgh'bolllhg Py y productswhen the the desired reaction temperature with a carbon dioxide HCl is distilled with the TFE and carbon dioxide, the purge d h TFE was added to the feed stream to resultant gaseous mlxlhle can be Passed through a the tube to obtain a mixture having the desired TFE/- ventional water-scrubber to remove the HCl to prevent (IQ ti Th y ol sis was conducted at about atmolhe l'lcl from behlg recycled to the py y furnace l spheric pressure.
• the carbon dioxide can be separated from the TFE by passing this mixture through a conventional EXAMPLES 7 to 10 hqhews when: scrubbef- Bmh f c and carbon w
• Oxldg can be separated m h 1 FE m h h l ysis furnace in which the lnconel tube was 1.78 cm. in by aqueous caustic scrubbing, if the TFE is desired for diameter and 487 Cm long, with the thermocouplc some purpose otheruthim recycle monomer for being installed inside the tube and near its exit end.
• the HFP can be separated by dlstlllatlon from the conditions were used and results were obtained. higher-boiling pyrolysis products.
• the HFP is useful in the same manner as HFP synthesized by other techniques heretofore to make known copolymers, e.g., with TFE, or chemicals such as HFP epoxide.
• TFE Yield to The process is normally conducted on a continuous 40 Conversion H ZP basis, i.e., continuously passing the feed mixture p through the reaction zone, followed by continuous sep- 7 319 907 aration, scrubbing and recycling.
• the desired product HFP is recovered length of about 30.5 cm. to give a volume ofthe heated from the tails stream of the column. zone of about 5 cc.
• the tube was heated by a 2.54 cm.
• the recycle of carbon dioxide and unreacted TFE to diameter tubular furnace positioned about the heated the pyrolysis furnace can be done as follows: The preslength ofthe lnconel tube. The temperature of the tube sure of the TFE, CO and HCl distillate of the preced was measured using a chromel-alumel thermocouple attached to the outside of the tube near the center of the heated zone.
• TFE and carbon dioxide were fed as ing paragraph is reduced to atmospheric pressure, and the distillate is scrubbed in water to remove HCl and is dried over CaSO
• the composition of the resultant TFE and CO mixture is 39 percent TFE by weight and 61 percent CO by weight.
• To 1 part of the product is added 0.41 parts of makeup TFE to give a mixture of TFE and CO having a TFE/CO weight ratio of 1.31.
• This mixture is fed to a heated lnconel tube at a rate to give the contact times and results shown in Examples 1 l and 12; the contact time being defined as the volume of the reactor divided by the volumetric flow of the ambient temperature feed per second.
• the improvement which comprises diluting the feed to the pyrolysis with CO at a TFEzCO weight ratio of 1:1 to 25:1 so the TFE partial pressure is at least 360 mm Hg absolute and the total pressure is 0.75 to 2.0 atmospheres absolute.

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• 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)

Priority Applications (8)

Applications Claiming Priority (1)

Publications (1)

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Family Applications (1)

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Cited By (10)

Citations (8)

Family Cites Families (2)

• 1972
  • 1972-06-12 US US261629A patent/US3873630A/en not_active Expired - Lifetime
• 1973
  • 1973-05-28 CA CA172,485A patent/CA1015771A/en not_active Expired
  • 1973-06-07 JP JP6344673A patent/JPS5728696B2/ja not_active Expired
  • 1973-06-08 FR FR7321021A patent/FR2187743B1/fr not_active Expired
  • 1973-06-11 GB GB2771973A patent/GB1384036A/en not_active Expired
  • 1973-06-11 IT IT50667/73A patent/IT989657B/it active
  • 1973-06-12 DE DE2329750A patent/DE2329750C2/de not_active Expired
  • 1973-06-12 NL NL7308149A patent/NL7308149A/xx unknown

Patent Citations (8)

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