Wet scrubbing

Of the five techniques described in section 29.1, wet scrubbing is the most efficient. It can reliably remove over 95 % of the SO2, and can utilise over 95 % of the absorbent. 

This process is similar to the wet scrubbing process using limestone (section 12.5.2), but there are several important differences. 

The flue gases are scrubbed with milk of lime in the absorber. The reactions between slaked lime and the oxides of sulfur are  

The rates of absorption of sulfur oxides in milk of lime and of solution of the slaked lime are considerably faster than the analogous reactions with limestone. For a given removal efficiency, therefore, the absorber is considerably smaller than that for limestone. This helps to reduce capital costs and the power consumed in recirculating the slurry. However, because of the rapid precipitation of calcium sulfite hemi-hydrate with some gypsum, the particles tend to be finely divided and form a thixotropic sludge which is difficult to de-water for disposal [12.1]. 

The milk of lime-sulfite process was widely used for early power station flue gas desulfurisation projects. It had a lower capital cost than the limestone-sulfite process and gave high absorption and reagent efficiencies. Subsequently, the problem of disposal of the calcium sulfite sludge led to three variants based on lime to be adopted — the gypsum process, the dual alkali process and the maglime process. 

The ionic nature of acids, bases and salts are removed from flue gases by wet scrubbing because the ionic separation that occurs in water creates advantageous equilibrium conditions. Removal may often be enhanced by manipulation of the chemistry of the scrubbing solution. 

Spray towers operate by delivering liquid droplets through a spray-distribution system. Generally, the droplets fall through countercurrent gas 

Packed-bed towers can achieve removal efficiencies of over 99% and have relatively lower water consumption requirements. They also offer design and retrofit flexibility. Disadvantages include high system pressure drops, relatively high clogging and fouling potential, potentially high maintenance costs, and waste water disposal requirements. Packed-bed scrubbers are also more expensive to install and operate than spray towers. 

Kirschner, E., Prevention Takes Priority Over Cure, Chemical Week, p. 2732 (June 2, 1993) 

Clean Air Act Amendments of 1990 Overview - Policy Guide 1993, Bureau of National Affairs, Inc., lOO.TOl-110. 

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The electrostatic effect can be incorporated into wet scrubbing by charging the particulates and/or the scrubbing-liquor droplets. Electrostatic scrubbers may be capable of achieving the same efficiency for fine-particulate removal as is achieved by high-energy scrubbers, but at substantially lower power input. The major drawbacks are increased maintenance of electrical equipment and higher capital cost. 

Gravity settlement Total enclosure Wet scrubbing Absorption tower ... 

Absorption Destruction Direct recycling Dispersion Total enclosure Gas Wet scrubbing... 

Make a list of all the costs associated with running a wet scrubbing operation to remove particulates and SO,. What cost factors could be eliminated or reduced if a pollution prevention technology or practice that used about the same energy and manpower requirements was applied. 

Wet scrubbers have found widespread use in cleaning contaminated gas streams because of their ability to effeetively remove both particulate and gaseous pollutants. Specifically, wet scrubbing describes the technique of bringing a contaminated gas stream into intimate contact with a liquid. The types most widely used for particulate control are spray towers, packed-bed units, ionizing wet scrubbers, and venturi scrubbers. 

Primary dry trap and wet scrubbing usual. Electrostatic is added where maximum cleaning required. 

Gilbert, W., Selecting Materials For Wet Scrubbing Systems, Pollution Engineering, Aug. 1973, p. 28. 

Total enclosure Wet scrubbing Absorption tower Fluidized bed scrubber Impingement scrubber Irrigated target scrubber Pressure spray scrubber Rotary scrubber ... 

In wet scrubbing the dust is removed by counter-current washing with a liquid, usually water, and the solids are removed as a slurry. The principal mechanism involved is the impact (impingement) of the dust particles and the water droplets. Particle sizes down to 0.5 /i.m can be removed in suitably designed scrubbers. In addition to removing solids, wet scrubbers can be used to simultaneously cool the gas and neutralise any corrosive constituents. 

Wet—open combustion. Excess air is admitted to the off-gas collection system, allowing combustion of carbon monoxide prior to high-energy wet scrubbing for air pollution control. 

This wastewater stream contains lead (Pb) salts and chlorinated hydrocarbons generated from corrosion of the anodes as well as asbestos particles generated as a result of degradation of the diaphragm with use. Wastewater is also generated from the scrubber where the chlorine is wet scrubbed and from the ion exchange resin used to purify the brine solution. These wash water often contains dilute hydrochloric acid with small amounts of dissolved calcium magnesium and aluminum chloride. Like in other cells, the scrubber water also contributes to the wastewater stream. 

The 1970 s also brought about increased use of three-phase systems in environmental applications. A three-phase fluidized bed system, known as the Turbulent Bed Contactor, was commercially used in the 1970 s to remove sulfur dioxide and particulates from flue gas generated by coal combustion processes. This wet scrubbing process experienced several... 

ASR Sulfoxide [Alberta Sulfur Research] A process for removing residual sulfur dioxide and hydrogen sulfide from the tail gases from the Claus process by wet scrubbing with a solution containing an organic sulfoxide. Elemental sulfur is produced. It had not been piloted in 1983. 

Autopurification A wet-scrubbing process for removing hydrodrogen sulfide from coke-oven gas. The scrubbing liquor was an ammoniacal suspension of ferric ammonium ferro-cyanide. The process was developed by ICI, Billingham, UK, in the 1930s and 40s, but was abandoned in 1947. 

CONOSOX A complex flue-gas desulfurization process using potassium carbonate solution as the wet scrubbing medium. The product potassium bisulfite is converted to potassium thiosulfate and then reduced with carbon monoxide to potassium carbonate for re-use. The sulfur is recovered as hydrogen sulfide, which is converted to elemental sulfur by the Claus process. Developed by the Conoco Coal Development Company and piloted in 1986. 

Hot gas cleanup technology is currently under demonstration phase, but various demonstrations have not been successful so far. Wet scrubbing technology, though with a... 

Wet scrubbing requires that the water remain in the liquid phase, which requires that the product gas be cooled to below 100°C. This loss of sensible heat may be undesirable in some systems. Most gasification systems that currently use wet scrubbers do so primarily as a means to remove tars rather than particulates from the gas stream. Removing the particulates separately can prevent condensation of the sticky tars on the particulate surface, and that can prevent fouling and plugging of filter surfaces. 

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