Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~
CONVERSION OF HALOGFNATED TOXIC SUBSTANCES
Field of the Invention
This invention pertains generally to the field
of organic chemistry and, in particular, to the detoxi-
fication of hazardous waste~
Background and Summary of the ~nvention
Waste disposal from the chemical, agriculturaland other industries is a serious contemporary problem.
In particular, there are many chemical wastes such as
various halogenated hydrocarbons which are not bio-
degradable, and thus must either be stored in secure,
specialized areas or incinerated in specially-designed
reactors.
For example, incineration systems have been
designed for the destruction of halogenated hydro-
carbons, and particularly to enable the detoxification
of polychlorinated biphenyl (PCB) compounds, the
manufacture of which has been discontinued in the
United States since 1976 due to their persistence and
ecological damage. However, large amounts of such
compounds remain in storage, and are thus regarded as a
significant hazard.
In one particular process, organic wastes are
vaporized and completely or partially oxidized at about
1,000C. Incombustible ash is discharged directly from
the reaction chamber, while off-gases are passed
through a secondary combustion chamber at about 1,200C
whereby the thermal decomposition of the compound is
~,~
completed. However, this incineration system not only
requires two combustion chambers, each of which
required a substantial input of energy, but also often
fails to totally eliminate the presence of halogenated
hydrocarbons in the effluen~ stream. It appears that
this and other previous incineration systems, which
provide for the combustion of halogenated hydrocarbons
in the presence of air, form free chlorine gas in the
combustion chamber which then reforms halogenated
hydrocarbons in the exit stream. Even though the
initial hazardous compound may have been degraded
by these reactions, any halogenated hydrocarbon is
dangerous and products such as chlorinated hydrocarbons
which form within the combustion chambers due to the
presence of free chlorine gas still present an irnpedi-
ment to the safe disposal of incineration products.
Thus, if the ash formed is not clean, it must be
reprocessed and retested, and the "dirty" ash must be
properly encapsulated and stored until a safe disposal
method becomes available.
The present invention provides a process for
the disposal of halogen-containing organic compounds
such as carbon tetrachloride, chloroform, tri-
chloroethane, tetrachloroeth~lene, methylene-
chloride, the various freons, polychlorinated biphenyls,dioxins and others~ by conversion to compounds which
pose no environmental hazard.
Specifically, a method is provided for the
disposal of halogen-containing organic compounds, which
comprises pyrolizing the compound in a reducing atmos-
phere at a temperature in the range of about 825-
1,125C. In particular, the described reaction
temperature may be formed by the combustion of methane
~;(3'r35~3
and oxygen, the methane being in a stoichiometric
excess of that required to react with the oxygen, thus
providing the reducing atmosphere.
The reducing atmosphere may be formed by mixing
methane and the compound to be converted, said compound
having been vaporized according to known methods, in a
reaction vessel formed from a suitable refractory
material. The required temperature may be provided by
external heaters about the reaction vessel, or may be
provided by the burning of the methane with limited
amounts of oxygen within the reaction vessel. However,
it should be noted that the methane must be in a
stoichiometric excess of that required to react with
the oxygen in accordance with the formula
lS CH4 ~ 202 ~CO2 + 2H2O.
Under these conditions, the halogen-carbon bonds will
be broken, e.g., all chlorine will react to form
hydrogen chloride, and all organic compounds will be
converted to mixtures of hydroqen, ethylene, acetylene
and benzene with smaller amounts of carbon and higher
aromatics. Due to the fact that excess oxygen is not
present within the reaction vessel, dioxins and other
chlorinated hydrocarbons are not produced or reformed.
The hydrogen chloride can be stripped from the gas
stream after suitable heat exchange with water, alkali,
lime or generally basic wash, and the hydrocarbons and
carbon may be used for fuel or chemical purposes.
It should be understood that the reducing
atmosphere may be provided by use of reactants other
than methane, e.g., hydrogen, but as methane is widely
available and easier to use, the detailed description
hereinafter set forth will be described with respect to
methane.
Brief Description of the Drawing
The accompanying drawing illustrates the present
process schematically and by way of example, includes
an apparatus suitable for carrying out the process
according to this invention.
Detailed Description
As required, a detailed illustrative embodiment
of the invention is disclosed herein. However, it is
to be understood that this embodiment merely exemplifies
the invention which may take forms that are different
from the specified embodiment disclosed. Therefore,
specific structural and functional details are not to
be interpreted as necessarily limiting, but rather as
forming a basis for the claims which define the
scope of the invention.
The process operates by heating the compounds to
be degraded in a reducing atmosphere at temperatures of
at least l,000C for about one second. The reactor may
be heated externally to maintain this temperature, such
as by methane heaters or the like, or may be heated
internally by burning methane or other fuels with
oxyqen to provide the required reaction temperatures,
in the presence of an excess of fuel which produces
reductive pyrolysis of the added chlorinated compound.
Alternatively, the reductive mixture (excess fuel and
organic halide) can be alternated with the oxidative
heating so that the exit streams from the alternative
processes may be kept separate.
More specifically and referring to the accom~
panying drawing, a reactor 10 is provided including a
gas impervious casing 12 which defines an inner elongate
reaction chamber or ~one 14. As the hydrogen chloride
produced in the zone 14 is corrosive, the casing 12
should be coated internally with a cer~nic such as
aluminum oxide, silica or the like, or with metallic
carbides, morides or nitrides. The reactor 10 includes
at an end 15 a gas inlet conduit 16 for introducing
methane into a first inlet tube 18, and a second gas
inlet 20 for introducing oxygen into the tube 18. The
gas inlet conduits 16 and 20 are provided with flow
metering valves 22 and 24, respectively, for control-
ling the flow of the methane and oxygen into the tube18 and thereafter to a burner tip 26 within the zone 14
in proportions which provide the required oxidative
heating therein. Generally, a flame temperature of
from about 1625 to 2100C will provide sufficient
heat. A bushing 28 is inserted through the reactor
wall at the end 15 through which extend electri-
cally conducting wires 30 and 32 connected to a resis-
tance wire 34 within the reaction zone 14 which is
disposed in proximity with the oxidative heating tip
26. Alternatively, a spark coil or other igniting
means may be used.
The reactor 10 also includes an inlet 36 for
introducing the halide containing compounds or waste
material into a second inlet tube 38 as well as inlet
conduits 40 and 42 for the introduction of methane and
water vapor, respectively. The inlets 36, 40 and 42
are provided with metering valves 44, 46 and 48,
respectively, for controlling the flow of the respective
gases through the inlet tube 38 to a tip 50 within the
zone 14. An opposite end 52 of the reactor 10 is
formed with an outlet meahs 54 for withdrawing product
gases from the zone 14 and is fitted with a metering
~ 3~3
outlet valve 56. Preferably, the reaction is conducted
to withdraw the product so as to maintain the residence
time required for conversion, generally about two to
five seconds. Accordingly, the input valves 22, 24, 46,
48 and 68 and the metering outlet valve 56 are adjusted
to maintain a pressure within the zone 14 that is
approximately atmospheric. However, the throughput may
be increased by appropriate adjustment of said valves
to provide superatmospheric pressure within the reactor
10 10.
Heat exchange means 58 are provided for control-
ling the temperature within the zone 14 comprising, in
this case, insulating means to maintain the desired
temperature within the zone 14. Alternatively, the
heat exchange means 58 could comprise methane heaters,
not shown, to externally heat the casing 12 and the
zone 14. Alternatively, the means 58 could comprise
electrical heating coils or cooling coils of various
types if required. To further assist in controlling
the temperature within the zone 14, water may be
metered into the tube 38 from the inlet 42 to partially
quench the oxidative heating within the zone 14.
Also illustrated in the drawing is a wash and
separation station 60 for fractionation of the reaction
~5 products, as well as conduits 62 and 64 for conveying
the dehalogenated hydrocarbons and carbon waste, and
hydrogen chloride, respectively, to storage or disposal.
The station 60 also includes a conduit 66 for recycling
any of the compound which has not been dehalogenated
through a metering valve 68 to the inlet tube 38 for
reprocessing.
In operation, metered stoichiometric quantities
of methane and oxygen are introduced into the inlet
tube 18 through the inlet conduits 16 and 20, wherein
the gases mix and flow to the tip 26 and thereafter
into the reaction zone 14. Metered amounts of the
compound to be dehalogenated are directed through the
inlet conduit 36 into the tube 38, along with sufficient
amounts of methane gas through the conduit 40 to
provide a stoichiometric excess of fuel, and therefore
10 a reducing atmosphere, within the zone 14. Water may
be added through the conduit 42 in order to control the
temperature within the zone 14, if required. Subsequent
to the introduction of combustible amounts of methane
gas and oxygen, or air, into the zone 14, current is
15 applied to the conducting wires 30 and 32 in or~er to
cause the resistance wire 34 to glow sufficiently to
ignite the gas mixture at the confluence of the flow
from the tips 26 and 50. Alternatively, a spark or
other ignition means may be employed.
It should be understood that various methods
and apparatus may be employed to introduce the described
gases into the zone 14. For example, stoichiometric
mixtures of fuel and oxygen may be introduced through a
first inlet, and excess fuel and the compound introduced,
25 respectively, through second and third inlets directly
into the zone 14. Also, the fuel, oxygen and compound
may be introduced separately, or through a single
inlet. It is preferred that the waste mixture or
compounds be properly analyzed and homogenized, if
30 required, before introduction into the apparatus 10.
The processing may be required to ensure that the level
~ 3~ 3
of inorganic compounds is not excessive. For exarnple,
certain non-toxic water-soluble materials such as salt
and lime should not be introduced in large quantities,
as such materials can react with the refractory lining
to form a glassy coating which may reduce the thermal
efficiency of the casing 12. It is also preferred to
introduce the compound into the conduit 36 in a gas-
phase sta~e, produced by the heating and/or vaporization
of wastes which are in liquid or solid phases.
In addition to hydrogen halides, a major
component of the effluent from the reaction zone 14
includes the dehalogenated compound introduced therein,
as well as small amounts of addi~ional compounds and
elements. For example, water vapor and oxides of
carbon may be formed as a result of the combustion
process, and if sulphur is present within the compound
or is otherwise introduced to the zone 14, oxides of
sulphur will be formed. If air is employed as an
oxygen source the noncombustible components will be
oxidized, e.g., to oxides of nitrogen. All of these
oxides~ being soluble in water and essentially acidic
in water solution, may be removed from the affluent
stream by the wash-separation station 60 along with
the hydrogen chloride.
In addition to the above-mentioned products of
the reaction, carbonaceous compounds may be formed
either from the degradation of the dehalogenated
hydrocarbon or from accompanying impurities. ~or
example, ethylene, acetylene and higher molecular
weight hydrocarbons are produced, including some carbon
in the form of soot. Any carbon which accumulates
~ 3~ 3~
within the reactor can be oxidized and removed periodi-
cally by passing air through ~he reactor at temperatures
in excess of 675C.
Using the above procedure, the reaction time is
very fast, generally being accomplished in several
seconds. It is preferred to maintain a residence time
in the hot zone of the reactor for from two to ten
seconds.
As described, the products of the reaction are
separated and recovered at the wash-separation station
60. Methods and apparatus for separating hydrogen
chloride and other water-soluble gases from hydrocarbons
are known. For example, one can refer to Gorin et al.,
U.S. Patent No. 2,4~8,083. Fractionaters may also be
employed for the removal of light gases from the
reaction product.
The reaction proceeds by the cleavage of
carbon-halogen bonds in a reducing atmosphere, with the
consequent production of hydrogen halides and de-
halogenated hydrocarbons therefrom. In the particularreactions described herein, hydrogen chloride is
produced by the degradation of particular chlorinated
hydrocarbons, carbon tetrachloride and chlorobenzene,
these compounds having been selected as being exemplary
of the described process. Other halogenated hydro-
carbons have weaker carbon-halogen bonds, or aee
dehalogenated by mechanisms which permit lower activa-
tion energies, than the compounds specifically set
forth herein.
Based on the methods of analyzing complex chain
reactions described by Benson in Foundations of Chemical
Kinetics, McGraw-Hill (1960) and rate data summarized
~ 3~3~3
in Kinetic Data on Gas Phase Unimolecular R ctions,
Benson et al., NSRD, NBS, 21, 1970, carbon tetrachloride
is the easiest chlorinated hydrocarbon to pyrolize and
chlorobenzene is the most difficult. Other chlorinated
compounds such as chloroform, trichloroethane, tetra-
chloroethylene, me~hylene chloride, polychlorinated
biphenyls and dioxins have carbon-halogen bond energies
which fall between those of the exemplary compounds and
are therefore within the scope of the described e~amples.
In addition, iodinated and brominated hydrocarbons are
degraded at lower temperatures and are converted to
unhalogenated hydrocarbons and hydrogen iodide and
hydrogen bromlde at temperatures, respectively,
of about 200and 100C lower than those described with
respect to chlorinated organic compounds.
The following examples will further illustrate
the invention.
Example One
Using the apparatus schematically shown in the
accompanying drawing, carbon tetrachloride is pyrolized
with methane in a reaction zone heated by the combus-
tion of methane and oxygen. The methane is present in
an amount in stoichiometric excess of the oxygen to
produce a reducing atmosphere, and the ratio of carbon
tetrachloride to methane is maintained at a ratio
about three to one, about 25 weight per cent carbon
tetrachloride. The reactor is maintained at about
1025C and She throughput maintained so that a residence
time of about three seconds is provided. The exit
1 ~
l ~5,
~tj~3~3
1 1
stream contains ethylene, acetylene, benzene, hydrogen
and some higher molecular weight hydrocarbons as well
as some carbon in the form of soot. The exit stream
contains all the chlorine originally introduced as
carbon tetrachloride in the form of hydrogen chloride,
i.e., the hydrogen chloride gas amounted to about 65
mol per cent of the exit gases.
Example Two
10 In this example, chlorobenzene is chosen as a
typical example of an aromatic chlorine-containing
compound. In addition, it is known as the most diffi-
cult to pyrolize of all the chlorine-containing organics.
Two moles (about 21 weight per cent) of methane for
each mole of chlorobenzene was employed, and the gases
were passed through a reaction zone at a temperature of
1125C, with a residence time of about three seconds.
The exit stream contained principally benzene and
ethylene with all the chlorine as hydrogen chloride.
Additional products include acetylene, hydrogen,
heavier unsaturated hydrocarbons and carbon.
In brief review, it will be seen that a process
for the degradation of halogen-containing compounds has
been provided which is simple to conduct, economical
and efficient, and in which all of the chlorine is
converted to hydrogen chloride which is easily separated
and neutralized.
