Precipitators, electrostatic mist

The clean, cool, partially dried gas leaves the top of the condenser and passes through electrostatic mist precipitation to final dehydration with strong sulfuric acid, Chapter 6. 

In the purification section of the acid plant the gas first enters a weak acid scrubber where its temperature is reduced to about 130 °F by water evaporation. It is then cooled to about 85 °F to reduce its water content to the required level. Finally, it is cleaned in electrostatic mist precipitators where the last traces of dust and the acid mist formed from the sulfur trioxide in the gas are removed. The type of equipment used in the purification section will vary somewhat with individual plant conditions and operator preferences. 

Electrostatic Mist Precipitators. The gas leaving the scrubbers is essentially free of halogens and dust but it still contains acid mist. The amount of acid mist depends primarily on conditions in the smelter. In gas from copper converters, the sulfur trioxide content may vary from 2 to 10% of the total sulfur oxides. The amount of sulfur trioxide formed depends largely on the temperature and time the gas contacts the iron oxide in the dust and the scale on the carbon steel flues. The sulfur trioxide combines with the moisture in the gas to form sulfuric acid vapor. When the gas is cooled in the scrubbers, most of this vapor condenses as a finely divided acid mist, although some of it is absorbed in the scrubber liquor. Sulfuric acid mist, which is generally considered to be particles less than 5 is very difficult to remove from a gas stream, so only a portion of the mist will be removed in the scrubber. If the remaining mist were allowed to enter the contact section of the acid plant it would corrode the carbon steel ducts and heat exchangers and the main blower. It must, therefore, be removed as completely as possible in the purification section of the plant. This is accomplished in the electrostatic mist precipitators. 

Metallurgical furnace off-gas after scrubbing, gas cooling, and wet electrostatic mist precipitation 30-40 6-10... 

Pipe-type electrostatic precipitators are used to collect liquid aerosols (e.g., mists and fogs). They are also used in applications which require water flushing of collection electrodes. The diameter of precipitator pipes is typically in the range of 15 0 cm, and the length is in the range of 3-6 m. The number of pipes depends on the total gas flow. The gas-flow rates in pipe-type electro static precipitators is normally much lower than in duct-type precipitators. 

Primary copper processing results in air emissions, process wastes, and other solid-phase wastes. Particulate matter and sulfur dioxide are the principal air contaminants emitted by primary copper smelters. Copper and iron oxides are the primary constituents of the particulate matter, but other oxides, such as arsenic, antimony, cadmium, lead, mercury, and zinc, may also be present, with metallic sulfates and sulfuric acid mist. Single-stage electrostatic precipitators are widely used in the primary copper industry to control these particulate emissions. Sulfur oxides contained in the off-gases are collected, filtered, and made into sulfuric acid. 

After the pyrolysis gas stream passes through a cyclone and removes entrained particles, it is quenched in a tower to condense the final oil products or CDL . The tower is designed to prevent water condensation electrostatic precipitators (ESPs) recover any remaining liquid droplets and mist from the gas leaving the tower. 

This subcategory involves phosphoric acid (dry process), phosphoms pentoxide, phosphoms pentasulfide, phosphoms trichloride, and phosphoms oxychloride. In the standard dry process for phosphoric acid production, liquid phosphoms is burned in the air, the resulting gaseous phosphoms pentaoxide is absorbed and hydrated in a water spray, and the mist is collected with an electrostatic precipitator. Regardless of the process variation, phosphoric acid is made with the consumption of water and no aqueous wastes are generated by the process. 

Wet electrostatic precipitators (WESP) are used for removal of liquid contaminants such as sulfuric acid mist, aerosols, and particulate matter. The acid mist and aerosols are typically formed in a WGS by condensation of SO3. Unlike dry precipitators, wet precipitators do not require rapping to remove the dust. The collected mist and particulate matter form a liquid film that runs down a vertical collecting plate. In some cases, a continuous spray of liquid is used to prevent solids deposition on the collecting plates. 

Particulate matter, separator, cyclone, electrostatic precipitator, wet dust scrubber, fabric filter, catalytic filtration, two-stage dust filter, absolute filter, high-efficiency air filter, and mist filter. 

On my sulfuric acid production unit in Texas City, we had an electrostatic precipitator to remove a liquid sulfuric acid mist from a flowing gas stream. It worked in the same way as a precipitator in liquid-liquid service. However, the electrodes or grids were not parallel plates. As illustrated in Fig. 26.4, the grids were lead tubes and lead-coated wires. 

Spray drying has become increasingly important in recent years as an alternative to wet scrubbing for sulfur dioxide control. In the spray dryer the sulfur-containing flue gas is contacted with a fine mist of an aqueous solution or a slurry of an alkali (typically Ca(0H)2 or soda ash). The sulfur dioxide is then absorbed in the water droplets and neutralized by the alkali. Simultaneously, the thermal energy of the gas evaporates the water in the droplets to produce a dry powdered product. After leaving the spray dryer the dry products, including the fly ash, are removed with collection equipment such as fabric filters or electrostatic precipitators. 

A particle removal method commonly used in industry is electrostatic precipitation. Industrial interest in this very efficient scheme can be traced back to 1911 with the investigation of F. Cottrell. His pioneering studies of sulfuric acid mist removal from copper smelter effluents led to the production of the Cottrell precipitator. 

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