Dlugogorski, Bogdan (The University of Newcastle Dept. of Chemical Engineering University Drive Callaghan, NSW 2308, AU)
Howe, Russell F. (The University of New South Wales Dept. of Physical Chemistry Anzac Parade Kensington, NSW 2033, AU)
Kennedy, Eric (The University of Newcastle Dept. of Chemical Engineering University Drive Callaghan, NSW 2308, AU)
Dlugogorski, Bogdan (The University of Newcastle Dept. of Chemical Engineering University Drive Callaghan, NSW 2308, AU)
Howe, Russell F. (The University of New South Wales Dept. of Physical Chemistry Anzac Parade Kensington, NSW 2033, AU)
| 1. | A process for the conversion of a halogenated fluorocarbon comprising feeding the halogenated fluorocarbon together with methane to a reactor that has a temperature sufficiently high so that the halogenated fluorocarbon reacts with the methane. |
| 2. | A process as claimed in claim 1 wherein the conversion is conducted in the gas phase in the reactor. |
| 3. | A process as claimed in claim 2 wherein the reactor temperature is greater than 400°C. |
| 4. | A process as claimed in claim 1 wherein the conversion is catalysed by a catalyst located in the reactor. |
| 5. | A process as claimed in claim 4 wherein the reactor temperature is in the range 300°C600°C. |
| 6. | A process as claimed in claim 4 or claim 5 wherein the catalyst is an ion exchanged zeolite. |
| 7. | A process as claimed in claim 6 wherein the zeolite is an SiO2/Al203 zeolite of ratio 50: 1. |
| 8. | A process as claimed in claim 6 or claim 7 wherein the zeolite is a metal zeolite including Mn, Co, Ni and/or Cu. |
| 9. | A process as claimed in any one of the preceding claims wherein the halogenated fluorocarbon is a chlorinated or brominated fluorocarbon (halon). |
| 10. | A process as claimed in claim 9 wherein the halon is either halon 1301 (CF3Br) or halon 1211 (CF2ClBr). |
| 11. | A process as claimed in any one of the preceding claims wherein the reactor has a recycle stream to recycle any unconverted halogenated fluorocarbon back into the reactor. |
| 12. | A process for the catalytic conversion of a halogenated fluorocarbon comprising feeding the halogenated fluorocarbon together with methane to a reactor having a zeolite catalyst therein, wherein the reactor has a temperature that is sufficiently high to activate the halogenated fluorocarbon and/or methane so that they react together. |
Background to the Invention Halogenated fluorocarbons represent a substantial environmental hazard. Many such compounds are harmful to humans, and many such compounds also contribute to the depletion of ozone in the upper atmosphere; (it is surmised that depletion of the ozone layer has lead to global warming).
There are severe restrictions on the use of halons in a number of countries and it is expected that the use of such compounds will ultimately be banned internationally.
Thus, there is an increasing need for the disposal of halons, and also for the disposal of other halogenated fluorocarbons (including hydrofluorocarbons).
Techniques for the disposal of halogenated fluorocarbons have included destructive processes, such as plasma-arc furnacing and incineration, both of which are extremely expensive and often produce end products with no particular economic value. Other problems with incineration include the high cost of safe transportation of the halogenated fluorocarbon to the incinerator, the use of ancillary fuels (resulting in high cost) and the large reactor volumes required to ensure adequate residence time.
Furthermore, due to incomplete combustion, halogenated
fluorocarbons and harmful byproducts are still released to atmosphere so that environmental problems persist.
Other disposal processes considered or proposed include hydrolysis, steam reforming, dehalogenation and dehydrohalogenation, however, incineration has been the most widely adopted commercial technology.
Hydrodehalogenation has been viewed as a possible disposal technique because it is conducted in the absence of oxygen. In addition, it is known that methane provides a hydrogen or homologation source, with methane having the added advantage that it is the primary constituent of natural gas (which is readily available). In the past, however, methane utilisation has presented considerable problems because, when it is reacted with a co-fed substrate in a catalysed environment, the substrate typically reacts much faster with the catalyst than with the methane. Methane, as such, is a difficult compound to react and in fact the art has generally taught away from the use of methane.
Summary of the Invention Surprisingly and contrary to the teachings of the prior art, the present inventors have now discovered that halogenated fluorocarbons can be reacted with methane.
Accordingly, the present invention provides in a first aspect a process for the conversion of a halogenated fluorocarbon comprising feeding the halogenated fluorocarbon together with methane to a reactor that has a temperature sufficiently high so that the halogenated fluorocarbon reacts with the methane.
Thus, contrary to expectations, the inventors have found that halogenated fluorocarbons (typically chlorinated, brominated and/or iodinated fluorocarbons or hydrofluorocarbons) can be converted by reacting with methane, thereby producing potentially useful byproducts and overcoming the disadvantages associated with incineration. In other words, such a conversion process
can be more simply implemented than existing incineration processes.
The conversion can be conducted in the gas phase in the reactor, in which case the reactor temperature is typically maintained to be greater than 400°C.
Alternatively, and more preferably, the conversion is catalysed by a catalyst located in the reactor. In this case, lower temperatures are required to achieve conversions comparable to those achieved in the gas phase.
Typical temperatures range from 300°C-600°C when a catalyst is employed.
A most preferred catalyst is an ion exchanged zeolite, (such as ZSM5 supplied by Zeolyst International). A typical zeolite employed is an Si02/Al203 zeolite of ratio 50: 1. More preferably, metal zeolites including, for example, Mn, Co, Ni and/or Cu are typically employed.
However, other suitable catalysts can be used including metal oxide catalysts, supported precious metal catalysts, hydrotreating catalysts and metal-supported alumina, silica or zircona.
A most preferred application of the present inventive process is in the treatment of chlorinated or brominated fluorocarbons (halons). In this regard, the invention finds particular application with either halon 1301 (CF3Br) or halon 1211 (CF2ClBr), the most common halon pollutants.
It should be noted that the invention also finds particular application in the conversion of the so-called"CFC's" (chlorinated fluorocarbons) which also represent a major environmental hazard.
Typically, a tubular reactor configuration is employed, optionally with recycle to enable the conversion of any unconverted substrate originally fed to the process.
Optionally, other suitable reactors can be used including CSTR's, fluidised bed reactors, etc.
In a second aspect the present invention provides a process for the catalytic conversion of a halogenated
fluorocarbon comprising feeding the halogenated fluorocarbon together with methane to a reactor having a zeolite catalyst therein, wherein the reactor has a temperature that is sufficiently high to activate the halogenated fluorocarbon and/or the methane so that they react together.
In the second aspect, it is surmised that either the halogenated fluorocarbon and/or the methane are activated at the catalyst, although a precise reaction mechanism has yet to be determined.
Brief Description of the Drawings Notwithstanding any other forms which fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the below-listed non-limiting examples and with reference to the accompanying drawings in which: Figure 1 shows a process flow diagram for a preferred process according to the present invention; Figure 2 plots CH4 conversion versus temperature for the gas phase reaction of CF3Br with CH4 at various residence times; Figure 3 shows a similar graph to Figure 2, but wherein CF3Br conversion is plotted against temperature; Figure 4 plots conversion versus temperature for the zeolite catalysed reaction of CF3BR with CH4 and compares this with the gas phase conversion; and Figure 5 plots conversion against temperature for a comparison of a manganese ion exchange zeolite catalyst against various gas phase reactions.
Modes for Carrying out the Invention Incorporation of methane into Halon 1301 (CF3Br) was observed to produce valuable products. For example, the reaction of Halon 1301 with methane where co-activation of both species resulted in a coupling of these species gave rise to a range of products including CF3H and CH3Br. The overall process was:
Coupling of methane with halon: CF3Br + CH4---) CH3Br + CF3H Subsequent elimination reaction of HF: CF3H-+ C2F2 + 2HF EXAMPLE 1 Experimental An overall process flow diagram is schematically shown in Figure 1. All gas flows including methane (>99.9%), nitrogen (>99.99%) and halon 1301 (approximately 81% CF3Br and 18% nitrogen) were controlled and metered with electronic mass flow controllers. The gas mixture was fed into a high temperature alumina (99.8%) tubular reactor heated with a 3-zone electric tube furnace. An alumina- sheathed thermocouple in the annular space in the reactor was used for reference and temperature control. Residence time experiments were performed either by altering flow rates of all feed species or by using alumina tubes of differing internal diameters.
After reacting in the (heated) reactor, the post- reaction gaseous effluent was directed through a caustic scrubber and finally either to a gas chromatograph or to a gas chromatograph/mass spectrometer for product analysis.
It was possible to recycle back to the reactor any unconverted product. External standards were used where available to identify and quantify reaction products.
Identification of fluorinated products was achieved through the use of gas standards and library GC/MS database spectra. Using these techniques, all major products were determined, (although a small number of minor products are yet to be identified). Mineral acids trapped in the scrubber were periodically analysed by ion chromatography.
(The results are plotted in Figures 2 and 3).
Example 2 Ion exchanged zeolites, (ZSM5, Zeolyst International, Si02/Al203, ratio of 50) were prepared by adding catalyst to a nitrate solution of the metal ion to be exchanged and evaporating the solution to dryness. The metal loading of the zeolite (in all cases 10%) was predetermined by controlling the volume and concentration of the nitrate solution.
Example 3 Gas-Phase Reaction of CF3Br with CH4 The results of the reaction of CH4 with CF3Br in a nitrogen bath (molar flow rate of 1: 1: 10 for CF3Br: CH4: N2) at various residence times were plotted (Figures 2 and 3).
Conversion products were detected at 500°C and, as expected, higher conversions of both feed species were observed with increasing temperature and longer residence times. In contrast, for pyrolysis experiments of CF3Br in a nitrogen bath, under flow conditions similar to those for methane reaction, no conversion products were observed below a reaction temperature of 725°C. Clearly, methane was an important reactant gas to facilitate and maintain the reaction with CF3Br. The pyrolysis experiments were performed not only to establish a baseline of activity for CF3Br reactivity but also to examine the propensity of CF3 radicals formed during pyrolysis to couple, forming C2F6.
The relative conversion rates for both methane and CF3Br were constant and almost completely independent of residence time. The overall stoichiometry of the reaction observed experimentally was 0.6 moles of CH4 reacted per mole of CF3Br. The production of a range of unsaturated hydrocarbon products (C2H4 and C2H2) observed was consistent with the lower mole ratio of CH4: CF3Br reaction observed.
Figures 2 and 3 show the gas-phase conversion of CH4 (Figure 2) and CF3Br (Figure 3) vs temperature at various
residence times (T = 0.3,0.4,0.8,1.6 secs). (Feed composition 1: 1: 10 mol ratio of CH4: CF3Br: N2).
The initiation step in the overall process was probably the thermal cleavage of CF3Br: CF3Br-> CF3 + Br thus creating a radical pool.
Hydrogen abstraction from CH4 then took place between the radicals thus produced and the abundant hydrocarbon gas: Br + CH4-+ HBr + CH3 CF3 + CH4-+ CF3H + CH3 As conversion of CF3Br and CH4 increased, so did the relative concentration of CH3, Br, H and CF3 radicals, thus increasing the probability of coupling between these species to form CH3Br and CF3H. Other (termination) coupling reactions also took place, such as: CF3 + CH3-+ CF3CH3-+ CF2CH2 and CH3 + Br-> CH3Br All these products were observed as major reaction species produced during gas-phase reaction.
Figure 3 shows the product selectivity for conversion of a 1: 1: 10 mixture of CH4: CF3Br: N2 at a residence time of 2 seconds. The major products detected were CF3H and CH3Br, consistent with the above mechanism.
Other minor species were also produced and were explained as follows. The increasing concentration of C2H6 as conversion increased (due to coupling of CH3 radicals)
competed with CH4 for reaction with CF3 or Br radicals, thus re-entering the reaction pool. Although present in lower concentration than CH4, the lower relative bond strength of the C-H bond in C2H6 vs CH4 suggested that it would react faster than the methane, thus effectively competing with CH4, eg.
Br + C2H6-+ HBr + C2H5-+ C2H4-+ C2H2 or CF3 + C2H6-+ CF3H + C2H5 Under these conditions, C2H5 quickly decomposed to C2H4, creating a secondary hydrocarbon species which, in turn, re-entered the hydrocarbon pool. A similar argument was extended to the resultant formation of C2H2 from C2H4.
The end result of these secondary reaction processes was to decrease the overall C: H ratio of product hydrocarbons, thus leading to the observed lower stoichiometry.
Example 4 Zeolite-Catalysed Reaction of CF3Br with CH4 Zeolite ZSM5 (product of Zeolyst International) is a synthetic zeolite, and is used for a variety of hydrocarbon processes including the conversion of methanol to gasoline.
The aim of this example was to determine whether ion- exchanged ZSM5 catalysts could influence either the activity or selectivity of the various reactions according to the invention. The results of this screening trial are presented in Figures 4 and 5.
All of the catalysts examined catalysed the various reactions but the resultant activity appeared to be almost independent of the ion-exchanged metal form of the zeolite.
A comparison of the conversion of the CH4 and CF3Br under similar reaction conditions was conducted (Figures 4 and 5). Figure 4 also includes a comparison of CF3Br pyrolysis
under these conditions.
Clearly the MnZSM5 catalyst, as did the other zeolite catalysts examined (CoZSM5 and CuZSM5), enhanced conversion of both feed species. Once again, the CH4 : CF3Br conversion ratio was less than 1, approximately 0.65 for each of the metal zeolites examined. Reaction temperatures were restricted to below 600°C for all zeolites examined as it was observed that above these temperatures HF formation was detected. One of the aims of catalyst development was to minimise fragmentation of the CF3 moiety and thus catalyst trials were performed below temperatures where this species was detected.
Whilst the invention has been described with reference to a number of preferred embodiments, it should be appreciated that the invention can be embodied in many other forms.
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