Disposal of fluoroform (HFC-23)

Details Disposal of fluoroform (HFC-23) Disposal of fluoroform (HFC-23)

2002-12-16

2004-10-19

Price, Elvis O. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

C570S153000, C570S159000, C570S170000, C570S171000, C570S216000, C570S257000, C570S261000

Reexamination Certificate

active

06806396

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the disposal of CHF
3
.
2. Description of Related Art
Fluoroform (CHF
3
, HFC-23) is a by-product of the reaction of HF with trichloromethane to form chlorodifluoromethane (CHF
2
Cl, HCFC-22), which is the primary source of perfluoroolefin, such as tetrafluoroethylene (TFE). The fluoroform by-product constitutes less than about 3 wt % of the HCFC-22 formed, but because annual production of HCFC-22 is large worldwide, the amount of fluoroform by-product made amounts to several millions of pounds per year. The fluoroform by-product either has to be used or has to be subject to disposal.
U.S. Pat. No. 3,009,966 discloses that fluoroform is thermally inert (col. 1, I. 13-14), but nevertheless finds a use for the fluoroform as a source of TFE and hexafluoropropylene (HFP) by pyrolysis of the fluoroform at temperatures of 700-1090° C., with temperatures of 1000° C. and higher being required to obtain conversions of at least 50% for the fluoroform at contact (pyrolysis times) of 0.1-0.12 sec. (Tables 1 and 2). The higher yields of HFP are accompanied by increasing amounts of perfluoroisobutylene (PFIB), which is toxic. Even at lower pyrolysis temperatures, the yields of PFIB can be quite high. U.S. Pat. No. 6,025,532 discloses the pyrolysis of fluoroform to a mixture of HF, TFE and HFP at a temperature of at least 700° C., but actually at 1000° C. at a contact time of 32 milliseconds (Examples), followed by contacting the mixture with a fluorination catalyst to obtain HFC-125 (CF
3
CHF
2
) and/or HFC-227ea (CF
3
CHFCF
3
). The high temperature required for pyrolyzing fluoroform at short contact times has limited the use of fluoroform by-product, whereby excess fluoroform has been available, which to avoid venting to the atmosphere has been disposed of by incineration.
Several references disclose the use of fluoroform in an auxiliary pyrolysis role. WO 96/29296 discloses the co-pyrolysis of HCFC-22 with fluoroalkane to form primarily large molecule fluoroalkanes. In particular, the reference discloses this reaction being carried out wherein the fluoroalkane co-reactant is fluoroform and the pyrolysis temperature is 700° C. and the contact time is 10 seconds, to obtain 100% conversion of the HCFC-22, with the result being a 60% yield of pentafluoroethane (Example 1). The disadvantage of this process, besides the extraordinarily long contact time, is that 40% of the yield is apparently not useful product. It is impractical to attempt to dispose of HFC-23 by consuming it in a process which produces such a high yield of by-product which itself needs disposal. Example 1 also reports that perfluoropropene is formed, without quantifying its amount, which is characteristic of reporting trace amounts detectable in the gas phase chromatography analysis used. The Examples of this reference are conducted with an aqueous alkaline wash of the pyrolysis reaction mixture to eliminate the HCl co-produced. The washing could also limit the ultimate reaction product to saturated HFC compounds. In the Examples the reactor is quartz. Quartz reacts with hydrogen fluoride, a probable intermediate in the pyrolysis reaction of HFC-23 and HCFC-22. The elements of hydrogen fluoride are part of the process according to the present invention and its consumption in side reactions, as with quartz, would lead to a reduction in the production of saturated hydrofluorocarbons.
Another reference disclosing the auxiliary use of fluoroform in a pyrolysis reaction is U.S. patent application Ser. No. 09/878,540, filed Jun. 11, 2001 (U.S. patent application Publication Ser. No. 2002/0032356-A), which discloses the pyrolysis of HCFC-22 in a gold-lined reactor to direct the synthesis reaction to the formation of the fluoroolefins TFE and HFP, without forming significant amounts of PFIB. The Examples disclose the co-pyrolysis of HCFC-22 and HCFC-124 (CF
3
CHFCl) to favor the formation of HFP over TFE. The possibility of fluoroform (CHF
3
) being present with the HCFC-22 is also disclosed as a recycle gas in the reactor system, the fluoroform thereby being the major component fed to the reactor, indicating that the fluoroform is acting as an inert carrier in the pyrolysis process, as would be expected from the relatively low pyrolysis temperatures and short contact times disclosed. Such use of fluoroform is not an effective way to dispose of fluoroform.
The problem remains of finding an economically acceptable use for the fluoroform by-product so that it does not have to be incinerated.
BRIEF SUMMARY OF THE INVENTION
The present invention solves this problem by consuming fluoroform (HFC-23) to economically produce useful product by co-pyrolyzing the fluoroform with chlorodifluoromethane (HCFC-22) at a temperature in the range of about 625-800° C., preferably about 690-775° C. and contact time of less than two seconds, and obtaining as a result thereof a product mixture of useful saturated and unsaturated compounds, i.e. at least three compounds selected from the group consisting of pentafluoroethane (CF
3
CHF
2
, HFC-125), heptafluoropropane (CF
3
CHFCF
3
, HFC-227ea), TFE, and HFP, respectively. The process can be carried out by feeding the mixture of reactants (HCFC-22 and HFC-23) through a reaction zone, the surface of which is metal, preferably gold, to minimize the formation of perfluoroisobutylene by-product in the pyrolysis reaction.
Unexpectedly, the HFC-23 pyrolyzes at the relatively low temperature of the co-pyrolysis reaction in short contact times to produce a high yield, e.g. at least 80%, of the above-mentioned useful products and little to no detectable PFIB. Apparently, the presence of the HCFC-22 in the pyrolysis reaction reduces the reaction (decomposition) temperature of the HFC-23 so that the latter is consumed in the pyrolysis reaction. Typically at least 4 parts by weight of HFC-23 is consumed for each 100 parts by weight of HCFC-22 such that the amount of HFC-23 consumed is greater than the amount produced as by product during the manufacture of HCFC-22.
The function of the fluoroform in the present invention is to increase the amount of useful saturated two- and three-carbon atom compounds, CF
3
CHF
2
(HFC-125) and CF
3
CHFCF
3
(HFC-227ea), along with production of TFE and HFP.
DETAILED DESCRIPTION OF THE INVENTION
The pyrolysis reaction in the present invention is carried out by continuous feeding of the co-reactants to a pyrolysis reactor and continuously withdrawing the resultant mixture of reaction products and unreacted reactants from the reactor. Pyrolysis reactors generally comprise three zones: a) a preheat zone, in which reactants are brought close to the reaction temperature; b) a reaction zone, in which reactants reach reaction temperature and are at least partially pyrolyzed, and products and any by-products form; c) a quench zone, in which the stream exiting the reaction zone is cooled to stop the pyrolysis reaction, preferably to 500° C. or lower, to reduce coking or polymerization downstream of the reaction zone. “Coke” is solid carbonaceous material that accumulates in, and on the surface of, the reactor. The resulting fouling is undesirable because it interferes with heat transfer and fluid flow. Quenching may be accomplished by interior cooling or exterior cooling, or both.
The reactor can be tubular, wherein the pyrolysis reaction occurs in the interior of the tube, and the tube can have a variety of cross-sectional shapes, such as circular, oval (elliptical) or polygonal, said shapes being of the interior or of the exterior surfaces of the tube, or both. The tubular reactor will typically have an inner diameter in the case of circular cross-section of at least about 0.125 in (0.32 cm), preferably about 0.125 in (0.32 cm) to about 3 meters, more preferably about 0.5 in (1.27 cm) to about 2 m, and most preferably about 0.7 in (1.8 cm) to about 1 m. The ratio of volume to surface area of a tubular reactor of unit length and of interior radius R can be determined by dividing the surface area A (A=2 &pg

Affiliated with

Also associated with

Rating

Disposal of fluoroform (HFC-23) does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Disposal of fluoroform (HFC-23), we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Disposal of fluoroform (HFC-23) will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3285404