Process for regenerating catalytic converters

Details Process for regenerating catalytic converters Process for regenerating catalytic converters

1998-11-05

2001-06-05

Gulakowski, Randy (Department: 1746)

Cleaning and liquid contact with solids

Processes

Including application of electrical radiant or wave energy...

C134S022100, C134S033000

Reexamination Certificate

active

06241826

ABSTRACT:

The invention relates to a process for regenerating catalytic converters that are employed in converting nitrogen oxides into molecular nitrogen.
Catalytic converters, which the reaction in accordance with the following addition formulas
4NO+4NH
3
+O
2
→4N
2
+6H
2
O
2NO
2
+4NH
3
+O
2
→3N
2
+6H
2
O
are used in combustion power plants for reducing NO
x
to N
2
in the vented air. This type of catalytic converter substantially comprises titanium oxide TiO
2
, tungsten oxide WO
3
, and as the active component vanadium pentoxide V
2
O
5
, and is embodied as a ceramic body, preferably with a plate-type or honeycomb structure. For the catalytic activities, the porous structure of the catalytic converter material and thus its internal surface area are decisive. The conversion of nitrogen oxides into molecular nitrogen takes place in vented air from power plants at temperatures of approximately 300 to 400° C. In power plant operation, fly ash, the formation of ammonium sulfate, and the heavy metals or their oxides contained in the vented air typically cause contamination and thus deactivation of the catalytic converter. The particulate contaminants, such as fly ash or nonvolatile salts or oxides, cause a reduction in the active surface area of the catalytic converter, while at the same time poisoning of the catalyst occurs from heavy metals or heavy metal oxides, which are volatile at the operating temperature, and from alkali, alkaline earth and phosphorus compounds. One typical catalyst poison, for instance, is arsenic oxide As
2
O
3
, which is gaseous at the operating temperatures. Severe deactivation can also occur with fuel that contains sulfur, however, because for reaction-dictated reasons the catalytic converter is lined with ammonium sulfate.
Particulate contaminants and deactivation cause losses in activity, with the attendant necessity of replacement with new catalytic converters. In other fields, it is well known that catalytic converters can be subjected to regeneration, for instance by calcination, but the options for regeneration depend very strongly on the type of catalytic converter and on the contaminants or deactivating compounds. In the catalytic converters in question here, which catalyze the conversion of nitrogen oxides into molecular nitrogen, effective regeneration has not until now been possible, because it was believed that this type of catalytic converter was moisture- or water-sensitive, and it was thus also always assumed that adding moisture or water, which is necessarily done during regeneration, would cause an alteration in the activity of the oxides.
Completely surprisingly, it has now been demonstrated that even ceramic catalytic converters of the titanium, tungsten and vanadium oxide type can be regenerated with excellent effectiveness.
According to the invention, a process for regenerating catalytic converters, and the catalytic converters regenerated by this process, is therefore proposed, which is characterized in that the catalytic converter is put in motion in a cleaning solution and subjected to an ultrasonic treatment.
Preferably, this chemical-physical regeneration is preceded and followed by pretreatment and post-treatment steps, in order to further increase the efficiency of the process. In this preferred embodiment, the contaminated catalytic converters are precleaned with dry mechanical means, such as industrial vacuum cleaners, so that all the particulate contaminants that do not adhere firmly are removed in the dry state. If especially stubborn encrustations are present, then a pretreatment with liquid, specifically preferably water at increased pressure, can additionally become necessary. This second stage of the pretreatment is done with the usual pressure cleaners.
In the next pretreatment step, preferably, in a positive displacement reactor, the catalytic converter is freed of all fly ash particles in the internal tubing system of the ceramic. At the same time, liquid is taken up into the porous structures of the catalytic converter, and readily soluble contaminants dissolve out of these structures along with incipient dissolution and hence loosening of poorly soluble compounds inside the ceramic. The solvent, as a rule water, can be increased in its effectiveness by means of motion. Thus the catalytic converters are placed by a crane into the positive displacement basin and then subjected for a relatively long period of time to intermittent up-and-down motions, optionally with the aid of washing systems.
As a rule, the liquid present in the positive displacement basin is water, but depending on the pollution of the catalytic converter it may also contain a certain proportion of low alcohols, as a rule up to approximately 20%. This solution can contain additives that improve the solubility of catalyst poisons or speed up the conversions; as to the rest, one skilled in the art will ascertain the process parameters of temperature, pH value, conductivity and reaction time experimentally and adjust them beforehand in accordance with the type of contaminants and pollutants present in the catalytic converter. Suitable examples of additives are surfactants, flotation promoters, complexing agents, and similar compounds.
Once the catalytic converter in the positive displacement basin has been cleaned of fly ash particles and at least some of the catalyst poisons, the catalytic converter is transferred to the ultrasonic reactor, so that even microparticulate contaminants and any catalyst poisons still present can also be removed. In the ultrasonic reactor, the catalytic converter is exposed to a high-frequency ultrasonic vibration, with a simultaneous flow through it by means of a reciprocating motion in a liquid. The ultrasonic treatment takes place from the opened sides of the catalytic converter, either in alternation or simultaneously. The intensity of exposure to sound can be regulated and adapted to the degree of soiling. During the ultrasonic treatment, the catalytic converter is put in motion in the sonic exposure basin, by a suitable reciprocating device, in such a way that liquid flows occur on the inner surfaces and that there is a migration of the ultrasonic activity zones to the surfaces to be cleaned. The ultrasonic vibration is as a rule in the range from approximately 27 to 40 kHz.
The cleaning liquid, as a rule water, can also have chemicals added to it that improve the separation of poorly soluble contaminants and catalyst poisons, such as lyes, acids, surfactants, or complexing agents, depending on how polluted the catalytic converter is. The treatment is done at temperatures between the freezing and boiling point of the treatment liquid, preferably at approximately 40 to 80° C.
After the conclusion of the ultrasonic treatment, the catalytic converter is removed from the reactor and rinsed. The rinsing sink may be embodied as a spray rinsing sink, immersion rinsing sink, or combination rinsing sink; in it, the residues still remaining after the ultrasonic treatment are removed from the catalyst surfaces by means of a liquid, preferably distilled water or tap water.
The temperature, pH value and possible additives to the rinsing liquid depend on the contaminants found and on the extent to which they are still present.
After the rinsing, the liquid-laden catalytic converter is dried with air; the drying is preferably done with dried, oil- and particle-free air in motion, at temperatures between 20 and 400° C., preferably between 20 and 120° C., and preferably in a drying chamber.
The regeneration of the described catalytic converters by the process of the invention is—even without optimizing the process conditions—up to 95% effective, in terms of the initial activity of the catalytic converter. Optimizing the process conditions makes it possible to achieve virtually 100% regeneration.


REFERENCES:
patent: 4946602 (1990-08-01), Ekberg et al.
patent: 4992614 (1991-02-01), Rodewald
patent: 5127960 (1992-07-01), Dittrich et al.
patent: 5143103 (1992-09-01), Basso et al.
patent: 5378287 (1

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