Process for microwave enhancement of wet oxidation

Wet oxidation is enhanced in the presence of carbonaceous material by oxidizing adsorbed organic materials, such as hydrazine and various solvents which may contain, or have dissolved or mixed compounds containing, chlorine molecules, on the carbon surface by radiofrequency energy in the microwave range at near ambient conditions of temperature and pressure. The discharged water is substantially environmentally clean.

BACKGROUND OF INVENTION 
1. Field of Invention 
The present invention relates to a process using radiofrequency microwave 
energy to enhance wet oxidation of materials often of a hazardous nature. 
2. Background 
Wet air oxidation (WAO), a common subcategory of wet oxidation, is the 
oxidation of chemical substances that often represent hazardous materials 
whereby the oxidation products are usually of a nonhazardous nature. 
Generally such chemicals are organic substances or materials, often 
referred to just as organics and restricted to only non-biological 
material, and represent a constituent of contaminated water, so the 
oxidation reaction occurs in the presence of water molecules. 
WAO is an attractive pollution-control process because it is an enclosed 
process and has very limited interaction with the environment, and further 
the waste or contaminants are destroyed, instead of merely being broken 
down into another form of pollution. In most instances the end products 
are carbon dioxide and water; however, in some cases various carboxylic 
acids are formed. 
In many instances the source of oxidation is an air stream thus leading to 
the designation WAO, but pure oxygen is potentially usable. In order for 
oxidation to occur in the WAO process the temperature and pressure must be 
elevated. Typical temperatures employed are from 150-325.degree. C. while 
pressures are typically in the range of 2000-20,000 kPa. In most WAO 
processes agitation is employed to transfer oxygen from the gas phase to 
the liquid phase where the oxidation reaction occurs at these elevated 
conditions. A common usage for WAO is the treatment of high-concentration 
wastewater where a 70+ percent reduction in chemical oxygen demand (COD) 
occurs after about one hour at approximately 250.degree. C. and 2.5 MPa. 
Substituting pure oxygen for air increases the COD removal efficiency. For 
a summary of some typical WAO uses see Kiely, Environmental Engineering, 
pp 603-605, 728-730, McGraw-Hill, N.Y. 1997. 
Conventional WAO has been improved by the utilization of a catalyst which 
speeds up the oxidation process and generally allows the use of lower 
temperatures and pressures. One common catalyst is platinum. Another is an 
iron-based catalyst, and used with wastewater, this is called the Loprox 
process. For instance, see Williams, "The Loprox Route to Wet Oxidation," 
Volume 4, pp 28-29, World Water and Environmental Engineering,1997. 
In the subject case the oxidation of the hazardous waste material hydrazine 
is a primary consideration along with further organics, such as various 
solvents which may contain, or have dissolved or mixed compounds 
containing, chlorine molecules. 
Hydrazine or its derivatives, monomethyl hydrazine and unsymmetrical 
dimethyl hydrazine, are common spacecraft propellants for use in missiles, 
rockets, and space launch vehicles. When used as such an astronautics 
fuel, nitrogen tetroxide is the most common oxidizer. However around space 
launch areas, much wastewater containing dilute hydrazine occurs and must 
be environmentally cleaned up. Microwave enhanced wet oxidation appears as 
a favorable process to perform this clean up task. 
Quantum radiofrequency (RF) physics is based upon the phenomenon of 
resonant interaction with matter of electromagnetic radiation in the 
microwave and RF regions since every atom or molecule can absorb, and thus 
radiate, electromagnetic waves of various wavelengths. The rotational and 
vibrational frequencies of the electrons represent the most important 
frequency range. The electromagnetic frequency spectrum is usually divided 
into ultrasonic, microwave, and optical regions. The microwave region is 
from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses 
frequencies used for much communication equipment. For instance, refer to 
Cook, Microwave Principles and Systems, Prentice-Hall, 1986. 
Often the term microwaves or microwave energy is applied to a broad range 
of radiofrequency energies particularly with respect to the common heating 
frequencies, 915 MHz and 2450 MHz. The former is often employed in 
industrial heating applications while the latter is the frequency of the 
common household microwave oven and therefore represents a good frequency 
to excite water molecules. In this writing the term "microwaves" is 
generally employed to represent "radiofrequency energies selected from the 
range of about 500 to 5000 MHz", since in a practical sense this total 
range is employable for the subject invention. 
The absorption of microwaves by the energy bands, particularly the 
vibrational energy levels, of atoms or molecules results in the thermal 
activation of the nonplasma material and the excitation of valence 
electrons. The nonplasma nature of these interactions is important for a 
separate and distinct form of heating employs plasma formed by arc 
conditions of a high temperature, often more than 3000.degree. F., and at 
much reduced pressures or vacuum conditions. For instance, refer to 
Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 
Supplementary Volume, pages 599-608, Plasma Technology. In microwave 
technology, as applied in the subject invention, neither condition is 
present and therefore no plasmas are formed. 
Microwaves lower the effective activation energy required for desirable 
chemical reactions since they can act locally on a microscopic scale by 
exciting electrons of a group of specific atoms in contrast to normal 
global heating which raises the bulk temperature. Further this microscopic 
interaction is favored by polar molecules whose electrons become easily 
locally excited leading to high chemical activity; however, nonpolar 
molecules adjacent to such polar molecules are also affected but at a 
reduced extent. An example is the heating of polar water molecules in a 
common household microwave oven where the container is of nonpolar 
material, that is, microwave-passing, and stays relatively cool. 
In this sense microwaves are often referred to as a form of catalysis when 
applied to chemical reaction rates. For instance, refer to Kirk-Othmer, 
Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 
494-517, Microwave Technology. 
SUMMARY OF INVENTION 
The objectives of the present invention include overcoming the 
above-mentioned deficiencies in the prior art and providing a potentially 
economically viable process for the microwave enhancement of wet 
oxidation. 
Wet oxidation is enhanced in the presence of activated carbon or its 
equivalent by oxidizing adsorbed organic materials on the carbon surface 
by radiofrequency energy in the microwave range at near ambient conditions 
of temperature and pressure.

DETAILED DESCRIPTION OF INVENTION 
Microwaves are a versatile form of energy that is applicable to enhance 
chemical reactions since the energy is locally applied by its largely 
vibrational absorption by nonpolar molecules and does not produce plasma 
conditions. Particularly reactions that proceed by free-radical mechanisms 
are often enhanced to higher rates because their initial equilibrium 
thermodynamics is unfavorable. A second class of enhanced reactions are 
those whose reaction kinetics appear unfavorable at desirable bulk 
temperature conditions. 
Carbonaceous material is an excellent microwaves absorber since it has a 
wide range of polar impurities that readily interact with such 
radiofrequency energy especially in vibrational modes. Consequently the 
microwave waveguide design for the microwave cavity is not usually 
critical. Carbonaceous material for use with the subject invention 
commonly comprises activated carbon, char, soot, pyrolytic carbon, carbon 
black, activated charcoal, and metal carbides, especially silicon carbide. 
In most instances activated carbon is the preferred material to employ 
with a water medium under ambient temperature and pressure conditions, 
although activated charcoal, if readily available, might be more cost 
effective. 
The microwave excitation of the molecules of the carbonaceous material also 
excites the organic constituents which have been adsorbed on the internal 
pore surfaces of the carbonaceous material and produces a highly reactive 
condition with the oxygen present. The oxygen is in close proximity or 
within the surface boundary layer of the carbon surface through 
chemisorption, absorption, adsorption, or diffusion. Oxidation of the 
organics then proceeds rapidly. 
The oxygen does not need to be only in the gaseous form of O.sub.2, but can 
also be with a selected oxygen containing molecule, such as H.sub.2 
O.sub.2 and O.sub.3. The use of hydrogen peroxide or ozone will accelerate 
the wet oxidation rate. 
A typical laboratory microwave reactor system to study experimental 
conditions for wet oxidation is shown in FIG. 1. A standard microwave 
energy oven 23 of 900 watts with a frequency of 2450 MHz was slightly 
modified to allow external attachments. A 1000 ml laboratory round bottom 
glass flask or retort 29 was employed filled approximately one-third with 
activated carbon 24, optionally spiked with 0.02% Pt catalyst, while an 
air bleed 31 fed enriched air into the side of the retort 29 coming from 
an oxygen generator 30. Water containing organic contamination was added 
to the retort 29 while it is modified upward into a reflux condenser 28 
fed with cooled water 27 going into and slightly warmer water 26 out. The 
condensed liquid returned via the neck surface 22 to the retort 29 while 
the excess gases 25 generated passed out the condenser 28. In use this 
batch system ran for a fixed time with liquid samples from the retort 29 
taken and analyzed for organic material at the beginning and end of each 
run. In addition the air bleed 31, either with or without enriched oxygen, 
was optionally employed. 
FIG. 2 shows a typical flow process for wet oxidation built into the 
waveguide, a microwave cavity, fed by a microwave generator. The microwave 
generator 60 is capable of three kilowatt radiofrequency energy at a 
frequency of 2450 MHz. Two 60 dB couplers 61 are located toward the end of 
the waveguide 59 which has a common tuner 62. A water terminator 63 ends 
the waveguide 59. The reactor column 75 made of quartz is contained within 
the waveguide 59. This reactor has doughnut supports 71 to physically 
stabilize the column 75 which is packed with a carbonaceous material 73 
which readily absorbs microwave energy. The bottom of the reactor column 
75 has a ceramic packing 72 acting as a filter. The contaminated water 81 
is stored and then pumped 80 into the top of the column 75 where a sprayer 
74 distributes the water over the carbonaceous bed 73 giving trickle down 
flow. The microwave enhanced wet oxidation purified water 69 discharges 
from the bottom of the column 75 into a clean water receiving tank 70. In 
operation the contaminated water 81 contains enough dissolved oxygen to 
allow the wet oxidation process to readily occur. 
EXAMPLE 1 
The experimental setup as shown in FIG. 1 was employed to determine the 
efficiency of the oxidation of hydrazine by microwave enhanced wet 
oxidation. The tests utilized 600-1000 ppm hydrazine solutions and about 
300 ml of activated carbon with the conditional employment of a platinum 
catalyst. Air, optionally enriched with oxygen, was introduced into the 
bottom of the flask for some tests. In each test the contaminated water of 
known hydrazine concentration and activated carbon were placed in the 
flask or retort reactor and exposed to microwaves for a fixed time period. 
At the end of the reaction time the hydrazine concentration of the 
solution was determined. All hydrazine concentrations were measured by the 
colorimetric method of ASTM D 1385-88. 
As shown in Table 1, the tests proved that the proposed system can 
substantially destroy hydrazine, and in tests numbers 4, 6, and 7, 
hydrazine levels fell below detection limits. The primary factor in these 
tests was reaction time. The presence of platinum catalyst apparently had 
no significant effect as observed from tests 1 and 5. Further the use of 
oxygen enriched air had little effect, from tests 1 and 3, in this dilute 
hydrazine experiment. 
EXAMPLE 2 
Additional tests using the experimental setup of Example 1 were obtained to 
confirm that the observed hydrazine reductions were due to microwave 
energy driven reactions and not adsorption by the activated carbon. 
In one test a mixture of activated carbon and hydrazine solution was mixed 
and boiled for eight minutes, and the hydrazine concentration changed from 
about 800 ppm to 400 ppm. Further boiling for 24 minutes did not reduce 
further the hydrazine concentration. Microwaves were then applied and the 
hydrazine level was reduced to a non-detectable value. 
TABLE 1 
______________________________________ 
Results of Batch Tests for Microwave-Induced Destruction of Hydrazine 
Reaction Initial Final 
Run Time, 
Conc., 
Conc., 
Destruction 
No. ppm 
Efficiency, % 
Remark 
______________________________________ 
1 8 633 142 78 
0.02% Pt 
Activated 
Carbon 
(A/C) 
was 
used with 
enriched air 
2 16 
Same as above 
3 8 
0.02% Pt A/C 
without 
enriched air 
4 16 
Same as above 
5 8 
A/C with 
enriched air 
6 16 
Same as above 
7 8 
A/C without 
enriched air 
8 16 
Same as above 
______________________________________ 
In another test cold hydrazine solution was mixed with activated carbon and 
stirred, and the concentration of hydrazine changed from 633 ppm to 300 
ppm after 8 minutes and to 100 ppm after 16 minutes. Applying microwave 
energy completely destroyed the hydrazine within 8 minutes. Further 
boiling of the solution produced no further detectable hydrazine. 
In a further test only hydrazine solution was placed in the microwave oven 
for 8 minutes, and no change in hydrazine concentration occurred. 
Finally a hydrazine solution was boiled for 8 minutes with a conventional 
heating mantle while purging the solution with enriched air. The 
concentration of the hydrazine changed only from 633 ppm to 601 ppm. 
These tests confirm that microwave enhanced wet oxidation employing 
activated carbon or its equivalent definitely was an effective means of 
hydrazine oxidation. 
EXAMPLE 3 
Using the flow process of FIG. 2, tests were conducted to determine the 
destruction efficiency under continuous conditions. Two packing materials 
were employed in the quartz reactor tube, either activated carbon 
(CECARBON GAC 610) or silicon carbide (SiC). The 800 ppm hydrazine 
solution flowed at 164 ml/min through the bed, and the microwave power was 
one kW. FIG. 3 shows the favorable results, especially for activated 
carbon. The SiC was not as effective since it did not internally adsorb 
the hydrazine; conversely the Granulated Activated Carbon (GAC) with a 
large internal pore surface area is an effective adsorber of organics. 
EXAMPLE 4 
To determine the generality of this microwave enhanced wet oxidation, the 
setup of FIG. 1 was employed to environmentally clean water contaminated 
with a chlorinated solvent, 2-chloro-2-methylpropane. For this laboratory 
test the bleed gas utilized was pure oxygen. The organic carbon content of 
the solution was measured with a Total Organic Carbon (TOC) Analyzer, 
Model 700, made by O.I. Corporation. 
The reactor flask or retort had 500 ml of solution with a TOC of 144 ppm 
while SiC was employed as the carbonaceous material. The test used 720 
watts of microwave power. After 8 minutes the TOC level was down to 30 
ppm, after 16 minutes 8 ppm, and after 24 minutes 4 ppm. This resulted in 
over 97 percent destruction of the chlorinated contamination. 
A second test utilized 50 grams GAC as the carbonaceous material along with 
500 ml of solution with a TOC of 182 ppm. Similar results as before were 
obtained with a destruction of 96 percent after 40 minutes of 720 watt 
microwave power. 
A third test was similar to the second except the GAC contained 0.02% Pt 
catalyst. Again after 40 minutes at 720 watt microwave power, the 
destruction reached 94 percent. 
A final test employed pellet sized GAC with 50 ml of contaminated solution. 
After only 16 minutes of 720 watt microwave power, the TOC destruction 
reached better than 95 percent. 
These test proved that microwave enhanced wet oxidation effectively 
destroyed the chlorinated organics. 
EXAMPLE 5 
To determine further the generality of this microwave enhanced wet 
oxidation, the setup of FIG. 1 was employed to environmentally clean water 
contaminated with butanol and adiponitrile. The tests were similar to that 
of Example 4. 
In the first test 30 grams of GAC with 0.02% Pt catalyst were used with 500 
ml of butanol solution with a TOC of 500 ppm. After 80 minutes of 720 watt 
microwave energy, the destruction reached 84 percent. A second run was 
employed with an initial 1200 ppm TOC of butanol. The destruction reached 
about 84 percent after 80 minutes for this single stage. 
For the adiponitrile test the initial TOC was 1000 ppm and after 80 minutes 
of 720 watt microwave energy, the destruction leveled out at about 55 
percent. For this adiponitrile organic material to obtain a higher 
efficiency of destruction more than one batch stage is required; thus, 
shifting to a multistage continuous process would be required. 
A process for wet oxidation of organic matter comprising the mixing in the 
presence of water of said matter with a bed of carbonaceous material and 
exposing said bed to microwaves. The matter and the water are potentially 
premixed, especially if the organic matter is already present in a water 
base which is often the case if said matter is a category of hazardous 
substances. In best mode operation the water is substantially saturated 
with oxygen either from air or separately generated. In many instances the 
water is supplied by a spraying system. The microwaves are radio-frequency 
energy selected from the range consisting of 500 to 5000 Mhz. The 
carbonaceous material is selected from the group consisting of activated 
carbon, char, soot, pyrolytic carbon, carbon black, activated charcoal, 
and silicon carbide, all of which are good absorbers of microwave energy. 
The bed is likely a form of fluidized bed, fixed bed, semi-fluidized bed, 
suspended bed, moving bed, and combinations thereof. 
A process for wet oxidation of hazardous organic material, such as 
hydrazine, 2-chloro-2-methylpropane, butanol, and adiponitrile, comprising 
the mixing in the presence of water said hazardous material with a bed of 
carbonaceous material, chosen from activated carbon, char, soot, pyrolytic 
carbon, carbon black, activated charcoal, and metal carbides, and then 
exposing said bed to microwaves selected from the frequency range 
consisting of 500 to 5000 Mhz. The water is often fed by a spraying system 
while the bed is of the form of one or more fluidized beds, fixed beds, 
semi-fluidized beds, suspended beds, and moving beds. 
The foregoing description of the specific embodiments will so fully reveal 
the general nature of the invention that others can, by applying current 
knowledge, readily modify and/or adapt for various applications such 
specific embodiments without departing from the generic concept, and 
there-fore such adaptations or modifications are intended to be 
comprehended within the meaning and range of equivalents of the disclosed 
embodiments. It is to be understood that the phraseology or terminology 
herein is for the purpose of description and not of limitation.