3 Group

There are adsorbing surfaces which enter in chemical reactions with gas molecules, transforming them into more useful substances before desorption. Examples:

1) For removal of sulphurous compounds from coke gas hydrogen sulphide is turned into elementary sulfur at presence iron oxide. Iron oxide is applied as fine dispersed powder, which render on wood shavings or crushed slag (for increasing of surface). Adsorption process for SO₂ removal from flue gases is represented on fig. 2.31.

2) Process of chemosorption of SO₂ from departing gases on firm adsorbents. Usually in these processes it is used or very cheap raw material (for example, dolomite (the mixture of calcium carbonate and magnesium carbonate) which throw out together with sulfur), or expensive adsorbents from which sulfur is recovered, and adsorbents come back in process.

3) Such compounds as the peroxides, ozone derivatives and other oxygen compounds are easily turned into more simple compounds at the presence of catalyst. The catalyst is the activated coal. The surface of coal is coated with metal copper, silver, platinum and palladium.

4) There are many other examples: oxidation of CO on copper oxide or on I₂O₅ with formation СО₂; bromination of olephins at passing them above the coal, impregnated with bromine; catching of mercury vapours, hydrogen sulphide, fluoric hydrogen on the surface of coal impregnated with various substances (fig. 2.32).

Fig. 2. 30 – Schematic representation of a very simple adsorption unit

Fig. 2. 31 – Adsorption process for SO₂ removal from flue gases: A – raw gas; B – purified gas; C – activated carbon, loaded; D – sand; E – SO₂ rich gas; F – activated carbon, regenerated; G – hot gas; H – fuel; I – air; K – cooling water; L – cooling air; M – abrasion; 1 – adsorber; 2 – sieve; 3 – desorber; 4 – sand heater and lift; 5 – cyclone; 6 – head exchanger; 7 – activated carbon transporter

Fig. 2. 32 – Adsorption plant for the removal of organic gases and vapors from waste gases with recovery of solvent: A – raw gas; B – purified gas; C – water vapor; D – desorptive and water vapor; E – solvent, adsorptive; F – condensed water; G – fresh air; H – cooling water; 1 – waste gas cooler; 2 – waste gas ventilator; 3 – adsorber; 4 – condenser; 5 – cooler; 6 – separator; 7 – fresh air ventilator; 8 – fresh air heater

2.2.3 Burning

If polluting substances are easily oxidized, their removal can be carried out by burning of gases. In case of burning of hydrocarbons carbonic gas and water are formed, at burning of sulfides sulphurous gas and water are formed.

There are two types of oxidation processes: the catalytic and the non-catalytic process.

The non-catalytic process, so-called the direct burning or thermal oxidation, is carried out at temperatures 700-800⁰ С, the catalytic burning - at 250-400⁰С. Burning is made in the open torch or in the closed chamber.

At burning in torch some hydrocarbons (in particular aromatic compounds with a low ratio carbon - hydrogen) usually give the smoking flame. That it to avoid add water as steam. Thus there is the reaction water steam to hydrocarbons with formation of hydrogen and CO.

In process of catalytic burning the active metal catalyst (platinum or other precious metal, active oxide of metal) on the special carrier is used. The start temperature of catalytic reactions depends on a kind of hydrocarbon in gas. So, hydrogen is oxidized at room temperature, benzene - at 227⁰ C, methane only in part - at 404⁰ C.

The most challenge of process of catalytic burning is a gradual deactivation or catalyst poisoning at long service life or at unexpected occurrence of poisons in a gas stream. Even traces of a certain type of pollutant may poison the catalyst.

Leaving the chamber gases are passed through heater or are thrown out in an atmosphere.

The advantage of the thermal oxidation process is the nearly complete removal of the pollutant without producing any new pollutants or other environmental problems.