Mist removal from gases

Purging. In addition to oxidation, sulfate is present from any sulfuric mist removed from the inlet gas and thiosulfate decomposition during the sulfur melting step. While the rate of sulfate formation from all these sources is small, the effect is cumulative and sulfate must be purged from the system. 

Sulphuric acid mist removal from exhaust gas... 

Typical applications in the chemical field (Beaver, op. cit.) include detarring of manufactured gas, removal of acid mist and impurities in contact sulfuric acid plants, recovery of phosphoric acid mists, removal of dusts in gases from roasters, sintering machines, calciners, cement and lime Idlns, blast furnaces, carbon-black furnaces, regenerators on fluid-catalyst units, chemical-recoveiy furnaces in soda and sulfate pulp mills, and gypsum kettles. Figure 17-74 shows a vertical-flow steel-plate-type precipitator similar to a type used for catalyst-dust collection in certain fluid-catalyst plants. 

The primary need for gas-solid separation processes is for gas cleaning the removal of dispersed finely divided solids (dust) and liquid mists from gas streams. Process gas streams must often be cleaned up to prevent contamination of catalysts or products, and to avoid damage to equipment, such as compressors. Also, effluent gas streams must be cleaned to comply with air-pollution regulations and for reasons of hygiene, to remove toxic and other hazardous materials see IChemE (1992). 

Drop Size. The purpose of the vessel s gas separation section is to condition gas for final polishing by the mist eliminator. From field experience, it appears that if 100-micron drops are removed in this section, the mist eliminator will not become flooded and will be able to remove those drops between 10 and 100-micron diameter. 

The process gas is further cooled in the condensing tower (4) by circulating weak acid which is cooled externally in impervious graphite heat exchangers (5). Entrained droplets of acid mist are removed from the gas in electrostatic precipitators (6). Drips from the precipitators are returned to the gas cooling tower. 

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. 

Candle-Type Demisters have more efficiency of removal of mist from gas streams (2.5 microns also possible), but have high pressure drop, and need more space to install. 

It is clear that the ESP cannot remove dry SO3 gas and the moisture available to convert SO3 gas to SO3/H2SO4 mist originates from residual water in the process air and water from sulphur. 

Mercurous chloride or calomel is insoluble and precipitates from solution. It can be separated in a thickener and removed from the system. Part of the calomel is treated with chlorine to regenerate the mercuric chloride scrubbing solution. The system is capable of achieving less than 0.5 ppm mercury in product acid or lower if reduced gas temperatures are used. It is important for efficient operation that the gas coming forward from the electrostatic precipitators is free from suspended mist and is classed as optically clear , otherwise contamination of the mercury scrabbing circuit will quickly destroy mercury removal efficiency. 

The injection of nanopartide condensation seeds ensures large acid mist particles are formed that are easily removed from the gas. 

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