2. Methods of industrial gases cleaning

2.1 Methods of reduction of concentration of the particulate pollutants in industrial gases

These lectures are devoted to processes and equipment used for the removal of particulate pollutants and of gaseous pollutants. Particulate pollutants may be solid particles – dust – and liquid particles – drops. Dust removal processes and equipment depend primarily on the physical properties of the dust like diameter distribution and density while the chemical properties are less important. On the other hand, for gaseous pollutants it is the chemical properties that will decide on the process and equipment applied for its removal.

To reduction of concentration of the suspended particles in gases the devices of preliminary (rough) clearing are applied. These devices are differed a simplicity, reliability in operation and did not demand of the big power expenses.

Fig. 2.1 – Typical particulate materials, size analysis procedures, and equipment

On a way of action the devices of preliminary clearing of gases can be divided into the following groups:

1) Devices, which principle of action is based on use of gravitational forces, - chambers of dust precipitation;

2) Devices in which force of inertia is used, - inertia-type dedusters;

3) Devices, which principle of action is based on use of centrifugal forces, - cyclones;

4) Devices of wet type – hollow scrubbers, Venturi scrubbers and other.

Cyclones and wet dedusters in practice of gas purification are applied and as devices of preliminary clearing gases, and as devices of independent clearing. Thus, they occupy intermediate position between devices of rough and thin clearing of gases.

So, the dust removal equipment available may be divided into two large groups: dry dust removal equipment and wet dust removal equipment. In dry dust removal equipment dust separation from the carrier gas is achieved directly, i.e. in the dry state, without the use of a special dust collection agent like water drops. The collected dry dust can be directly disposed of. The use of dry dust removal equipment involves however the danger of dust explosion. Therefore, special measures have to be taken to prevent dust explosion and pressure release in case of explosion.

2.1.1 Dry dust removal equipment

Chambers of dust precipitation (gravitational chambers).

The basis of action of chambers of dust precipitation is gravitational sedimentation of dust particles.

Gravitational sedimentation or sedimentation by action of gravity represents the simplest method of sedimentation of dust. Time of passage by gases of chamber makes:

t = V / V_g = L· B_c · H / V,

where V - volume of the chamber, m³;

V_g - the volumetric charge of gases, m³/s;

L – length of the chamber, m;

B – width of the chamber, m;

H – height of the chamber, m.

For same time by gravity the particle will pass a way h

h = w_m · τ

where w_m - average speed of a particle, km/s.

Fractional efficiency of chambers of dust precipitation is determined by the relation h/Н. If value h is more or equally Н, all particles of this size and larger are caught by the chamber. At designing chambers it is necessary to take into account an opportunity of secondary dust ablation also. It is necessary, that speed of a gas stream was not higher than 3 km/s.

In figure 2.2 the design distributed in the industry chamber of dust precipitation is shown. The small height between the shelfs through which passes a dusty gas stream, causes the effective catching of dust. In this case the particle for sedimentation must pass a way equal Н/N (where N – number of sections in the chamber). Fractional efficiency of the chamber of such type makes up hN/Н.

Lack of the chamber, shown in figure 2.2, consists in difficulty of clearing of a settled dust.

Chambers of sedimentation are used for sedimentation of a dust from vertical gas streams too. In this case the particles which speed of sedimentation is higher than speed of a gas stream are settled. Devices for dust sedimentation from an ascending vertical gas stream are shown in figure 2.3. The device, shown in figure 2.3а, allows to direct a part of a dust back; the unloading of a settled dust is made periodically during a stop. In chambers (2.3б, 2.3в) the dust is removed continuously.

Fig. 2.2 – Multiply section dust-collecting chamber

The basic advantages:

1) Simplicity of a design;

2) Low cost;

3) Small power consumptions;

4) An opportunity of catching of an abrasive dust;

5) An opportunity of catching of a dust in a dry kind;

6) Work of the chamber did not depend at temperature.

Lacks: very bulky chambers are necessary for achievement of high efficiency.

Efficiency of catching of fine dispersed dust with particles less than 5 microns equal zero even in chambers of very big sizes, large particles of a dust in the size 30-40 microns and more are caught well.

Fig. 2.3 – Vertical dust-collecting chamber: a – without dust removal; б, в – with dust removal; 1 – gas conduit; 2 – reflecting disk; 3 – refractory coating; 4 – reflecting cones; 5 – sloping plate.