Wet dust removal equipment.
Use of irrigation allows considerably to raise efficiency of devices of all listed types. First of all, the liquid (usually water) interferes with secondary dust ablation.
In wet dust removal equipment the dust particles are captured by liquid drops introduced into the gas stream. The gas pollution problem is converted into a liquid pollution problem. This is acceptable when the volume of the contaminated water is small and when a waste water treatment plant is available.
There are a great variety of wet dust scrubbers available. Only several examples are selected for a brief discussion: 1) direct-flow cyclone with a water film; 2) hollow scrubber; 3) packed column scrubber; 4) vortex scrubber; 5) Venturi scrubber.
Two types of wet dedusters are widely used: direct-flow cyclones with a water film and hollow scrubbers. Theiradvantages: small cost, higher efficiency of catching of the suspended particles in comparison with dry mechanical dedusters, simple removal of a caught dust as sludge. Lacks: necessity of sewage processing, necessity of using of anticorrosive coverings at aggressive gases clearing.
Usually basic assignment of wet dedusters is cooling and humidifying of gases.
The direct-flow cyclone with a water film differs from a usual cyclone that on the internal side of its wall continuously flows down the film of a liquid, which is entered in the device through the line of tubes, located in the top part of the device, or with the help of atomizers. Atomizers are established so that torches of pulverization form the film on the surface of the internal wall.
Hollow gas washers. Hollow gas washers are most widely used as devices of preliminary clearing of gases. They represent columns or chambers of round or rectangular section.
In a basis of action of these devices the inertial sedimentation of particles of a dust on drops of a liquid lays. According to calculations, peak of efficiency in this case is achieved at diameter of drops d k = 0,8 mm. Drops of such size can be received by means of usual centrifugal atomizers of rough pulverization , working under pressure 300-400 kPa. The big advantage of such atomizer is the opportunity of use of the turnaround water containing suspensions.
On a direction of movement of gases and liquid hollow gas washers share on two groups: counter-flow with a cross-section irrigation and direct-flow.
In counter-flow gas washer (fig. 2.12) drops from atomizers fall towards to a gas stream which speed makes from 0,6 up to 1,2 m/s. At higher speeds of gases in gas washer installation of drip pan is necessary. Atomizers are established in the device in one or several sections: sometimes lines (14-16), sometimes only on an axis of the device. The torch of pulverization of atomizers can be directed vertically from top to down or under some corner to a horizontal plane. At an arrangement of atomizers in some circles the combined installation of sprays is possible: the part of torches is directed on a course of gases, other part - in an opposite direction. For the best distribution of gases on section of the device in the bottom part of a scrubber the distributing lattice is established.
Low pressure Venturi scrubbersin systems of gas purification are used for preparation of gases in front of dust removal devices. They are intended also for coagulation of fine dispersed dust, for cooling and humidifying of gases and have low hydraulic resistance. Various designs of Venturi pipes are applied. On a way of irrigation two versions are allocated: with preliminary crushing of irrigating liquid (with atomizer irrigation) and with the film irrigation.
In Venturi pipes with preliminary crushing of irrigating liquid input of a liquid in a pipe is carried out with the help of the atomizers, installed before a confuser or directly in it.
For prevention of formation of dust deposit on the atomizer pipe and the atomizer it is possible to take out it from a zone of direct contact to a dusty gas stream. In devices the atomizers of rough pulverization worth while to establish. They allow to apply a turnaround water in devices.
Application of the film irrigation allows to avoid the formation of deposits on the border of the separation of dry and wet surfaces of a device and to use for process the turnaround water. In quality of drip pan, located for low pressure Venturi pipes, cyclones are installed more often.
In the packed column scrubber(fig. 2.12) the water is distributed over the surface of various types of small packing elements and flowing downwards. The gas moves countercurrently through the porous layer of packing elements, changing its flow direction according to the arrangement of the elements. Because of the inertia forces the particles, particularly the coarser ones, do not follow the gas flow and impinge on the liquid surface.
Fig. 2.12 – Various types of column scrubbers
In the vortex scrubber(fig. 2.13) the water drops are produced by the gas stream and carried into the vortex channel. The dust particles are captured during the period of drop generation and drop movement inside the vortex channel. The efficiency of dust collection by drops depends on the relative velocity between drops and dust particles.
A successful application of wet scrubbers requires a reduction of the temperature of the gas in order to prevent undesired evaporation and to separate the drops from the gas after dust collection. The efficiency of wet dust scrubbing depends strongly on the efficiency of drop collection. In general, the drop size is much larger than the dust particle size. Drop separation is therefore generally more readily performed than dust separation. However, the drop separation equipment applied is often poor in design and consequently not very effective.
Fig. 2.13 – Examples for wet dust removal equipment
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