WO1993019838A1

WO1993019838A1 - Oxidation process at a controlled temperature in gaseous phase

Info

Publication number: WO1993019838A1
Application number: PCT/IT1993/000028
Authority: WO
Inventors: Ernesto Bardelli; Eraldo Cassinerio
Original assignee: H.R.S. Engineering S.R.L.
Priority date: 1992-03-31
Filing date: 1993-03-31
Publication date: 1993-10-14
Also published as: ITMI920770A0; AU4042693A; IT1255046B; ITMI920770A1

Abstract

The purification of vapours and gaseous effluents containing oxidable volatile substances is carried out by passing the gas flow through metal plates (1) fed with high oscillating electric tensions and then, after moderate heating (4) if needed, through a porous siliceous mass (2) on which oxidation catalysts formed by heavy metal oxides are adsorbed.

Description

"OXIDATION PROCESS AT k CONTROLLED TEMPERATURE IN

GASEOUS PHASE"

In a considerable number of industrial processes the emission occurs into the atmosphere of gaseous effluents containing organic solvents from painting or spraying processes, dryers, furnaces, etc.

Purification technologies presently available to solve this kind of problem, as the rules issued by different countries fix particularly restricted limits, may be summarized in two families:

Adsorption on activated carbons : drawbacks found :

- impossibility to find on the market activated carbons which adsorb systematically solvents of different nature present in the same flow; - possible desorption of a solvent already fixed in a flow which does not contain it; difficulty to regenerate said carbons, with the subsequent problem of their disposal as "toxic wastes";

- in case of saturation of the carbons with flammable solvents, problems to the intrinsic safety of the plant.

Thermal afterburning i drawbacks found :

- necessity to raise considerably the flow temperature with supply of external fuel, which involves particularly high operating costs; - possibility of formation, within the flow, of unwanted and/or toxic chemical compounds, due to the high temperatures.

In the light of what explained above the applicants previously filed the Italian patent application N.2245430A/89 concerning a purification process of gaseous effluents based on the combination of a ionization treatment through high tension electric fields and oxidative catalysis. The treatment that is the object of said application, while quite effective from a technological point of view, is hardly fit to uses involving the treatment of gaseous effluents of variable and/or fluctuating composition, since in such conditions it would be necessary to adjust from time to time the treatment unit to the most adverse condition in terms of concentration and kind of polluting mixture.

The analysis of the most common situations presented by the market has shown indeed that "in the majority of the cases the concentration of polluting solvents does not remain constant, and the basic composition of the mixture of solvents often varies too, while the possible presence of halogenated substances requires the adoption of catalysts resistant to the action of the halogen evolved during oxidation.

In order to overcome these drawbacks the applicants have set up an oxidative process which is the object of this application. The oxidation process in gaseous phase of polluted effluents according to this invention includes a first stage of ionization in a high tension electric field, a second stage of controlled heating, and a third stage of oxidative catalysis. This invention will now be explained more in detail based on the annexed drawings of a preferred embodiment thereof, wherein:

Fig.l is a schematic illustration of the stages making up the process according to this invention; -Ei-g- is a diagram showing the composition changes of a gaseous effluent, comprising four types of pollutants, at the end of the oxidation process according to the the treatment temperature changes; Fig.3 is a diagram showing the alterations undergone by a pollutant in a gaseous flow in the presence and in the absence of ozone formed during ionization at different temperatures. As shown in fig.l, the process according to this invention includes three different treatment stages, namely a ionization stage in which the polluted gaseous flow passes through metal plates 1 arranged parallel to the flow and spaced apart so as to maximize the generation of a corona discharge. The plates are fed with high tensions of 15000 to 30000 V oscillating between 500 and 1500 Hz, coming from a high tension generator 3, to increase the ionizing effect on the gaseous flow. The spacing between the plates is such as to achieve the greatest intensity of electric field allowed by the gaseous flow being treated; the length of the plates is related to the flow speed, in that it determines the actual time of treatment. The electronic equipment is provided with all the automatic adjustments and the safety devices needed to make the running of the plant reliable and safe.

The second stage of the process includes a heating source 4 which, slightly increasing the temperature of the effluent, allows to increase the effectiveness of the following catalysis stage and thereby the amount of solvent destroyed.

The heating source 4 may include electric resistances made of ceramic to avoid corrosion, or fluid vein gas burners; the choice of either kind of source depends on the operating costs and the ease of access to the respective sources.

The third stage of the process is a catalysis stage, where the volatile substances contained in the gaseous phase, having undergone a first demolishing treatment equivalent to about 50% of the upstream contents through the ionizing stage, come in contact with oxidation catalysts formed by heavy metal oxides sintered or supported on a solid porous matrix.

In this way the free radicals coming from the polluting substances, subject in the first stage to ionization by the high oscillating tensions, undergo in the second stage a moderate heating at temperatures between room temperature and about 400^βC and subsequently come in contact, thus activated, with the catalysts of the third stage which complete the oxidative demolition by converting the polluting substances of the effluent into harmless substances, such as for example carbon dioxide and water vapour.

The presence of the second intermediate heating stage 4 allows to adapt the process according to this invention to the demolition treatment of gaseous effluents of different origin in which the concentration of volatile organic polluting substances may undergo even considerable variations in quite very short periods of time.

To achieve this, the heating degree achieved in the second stage is modulated in real-time thanks to the presence of an on-line analyzer 5 which analyzes the composition of the gaseous flow coming out of the catalysts and, when this composition varies, sends to a proper regulating device schematized in 6, analogic signals that are proportional to the deviation of the measured concentration value from the optimum reference value; these signals modulate, through the regulating device 6, the amount of thermal energy supplied in the second stage, so as to follow the concentration changes of the incoming pollutants, while keeping constant the concentration and composition of the outgoing demolished and oxidized effluents. This result is achieved thanks to the inertia of the system which is practically negligible. The effect of the increased temperature (increase of energy) on the enhancement of effectiveness of the process is shown in the diagrams of figs.2 and 3. The diagram of fig.2 shows the composition changes undergone by a gaseous effluent artificially created by injecting in the air a mixture of four solvents in equal concentrations, and precisely toluene, styrene, benzene and cyclohexane, so as to obtain an overall starting concentration of 1000 ppm. The gaseous mixture thus obtained has been treated with the process of the invention keeping the conditions of the first and third stages constant, but varying the temperature of the second intermediate stage. As shown in the diagram, with a space velocity of 20000 h^~^ and an inlet temperature of about 20°C, the composition of the outgoing treated effluent changes with the variation of the temperature. We observe-that for a treatment temperature of 150^βC the concentration of toluene has dropped to 150 ppm, that of cyclohexane is about 180 ppm and that of benzene has dropped to about 225 ppm, while the concentration of styrene is practically insignificant already at 50°C. Connected with the diagram of fig.2, the threshold limit values fixed by the rules in force have been reported for reference, as maximum concentration limits regarded as tolerable by a living organism if the considered polluting substances are discharged into the atmosphere.

The diagram of fig.3 shows instead how the concentration of a single pollutant (in this case toluene) varies with the variation of the treatment temperature and in the presence or absence of ozone generated in the ionization stage. As shown, the concentration of toluene, starting from an initial value of 250 ppm for a space velocity of 20000 h^-1 drops to zero at 150^CC in the presence of ozone, while it is still of about 180 ppm at the same temperature in the absence of ozone. The application field of the process according to this invention is the treatment of effluents with average low concentrations of pollutants, equal to or lower than 1 g/m³ and with short-time oscillations ranging in the dozens of minutes which involve concentration maximum peaks even ten times greater than the average value considered. In this way the equipment which embodies this process is of particularly limited size and cost, the operating costs being directly related to the difference between the design average and peak concentrations.

The oxidation catalyst to be used in the third stage is formed by heavy metal oxides, such as the metals of group VIII of the periodic table, and particularly iron, nickel, chromium, vanadium or copper oxide too, and are preferably used in a mixture in order to be able of acting against the different volatile organic compounds which may occur as pollutants in an effluent.

The advantages of the oxidative demolition process of volatile pollutants according to this invention are clear and may be summarized as follows:

- low operating costs in terms of the energy used to oxidate the pollutants, since the involved reactions take place at temperatures between room temperature and about 400^βC;

- nearly quantitative effectiveness in the oxidation of organic substances both of aromatic and aliphatic nature;

- no production of solid wastes of toxic nature; - no formation of gaseous compounds having a toxicity equal to or greater than that of the substances to be demolished.

Claims

CLAIMS -

1. An oxidative treatment of gaseous effluents containing gaseous organic substances comprising an ionization stage and an oxidation stage, characterized in that between the ionization and oxidation stages there is an intermediate heating stage at temperatures between room temperature and about 400^*C.

2. An oxidative treatment according to claim 1, characterized in that said oxidation stage is carried out by passing the gaseous flow to be purified on catalysts formed by mixed heavy metal oxides suitable for the treatment of halogenated organic substances.

3. An oxidative treatment according to claim 1 or 2, characterized in that said heating stage is modulated in real-time according to the nature and amount of the pollutants to be demolished.

4. An oxidative treatment according to claim 3, characterized in that the real-time modulation of said intermediate heating stage is carried out through an on- line analyzer which analyzes the output data of the treatment process and subsequently modifies the amount of thermal energy supplied in the intermediate stage.