Mist, removal electrostatic precipitators

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. 

For acid production sinter gas is usually cleaned by drop out of dust in an open chamber or flue followed by hot electrostatic precipitators of the plate and wire type with two or three fields in series. This is followed by wet gas scrubbing and wet electrostatic precipitators to remove mist from the scrubbing operation. A mercury removal scrubber may be included, if mercury levels in the clean gas are high enough to give product acid above 1 ppm mercury. Mercury removal is not always necessary as it is with zinc concentrate roasting. 

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. 

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. 

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. 

Fig. 6.2. Dehydration of metallurgical and spent acid decomposition furnace offgas. Dehydration is done after (i) H20(g) has been condensed by gas cooling and (ii) aqueous mist has been removed by electrostatic precipitation (not shown). The gas leaving dehydration contains 50 milligrams of H20(g) per Nm3 of gas. The acid plant s main blower is situated immediately after dehydration.

Electrostatic precipitation is perhaps the most versatile and cost effective of all particulate collecting devices and can be applied to any process where there is a need to remove solid particulate and mist of fume sized particles from the gas stream, whether it be for recovery or pollution control duties. It can be designed to deliver any efficiency for any gas flow rate and temperature and has a low pressure drop and a life span of more than 20 years. 

Another emission problem of concern is the formation of mist, which consists of droplets of acids. Mist may be removed by a fine mesh of stainless steel roll so that the gases have to pass through an intricate maze, or by electrostatic precipitation [5]. 

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. 

Electrostatic Precipitators The principles and operation of electrical precipitators are discussed in Sec. 17 under Gas-Sohds Separations. Precipitators are admirably suited to the collection of fine mists and mixtures of mists and solid particulates. Tube-type precipitators have been used for many years for the collection of acid mists and the removal of tar from coke-oven gas. The first practical installation of a precipitator by Cottrell was made on sulfuric acid mist in 1907. Most older installations of precipitators were tube-type rather than plate-type. However, recently two plate-type wet precipitators employing water sprays or overflowing weirs have been introduced by Mikropul Corporation [Bakke,/. Air Pollut. Control Assoc., 25, 163 (1975)] and by Fluid Ionics. Such precipitators operate on the principle of making all particles conductive wnen possible, which increases the particle migration velocity and collection efficiency. Under these conditions, particle dielectric strength becomes a much more impor-... 

Electrostatic precipitation Very fine droplets can be charged and then attracted to collecting plates Has been used to remove sulphuric acid mist from gaseous effluent. See discussion... 

The mist can be removed by electrostatic precipitators which firstly impose a negative charge upon any particles and secondly attract them to an earthed plate. The mist particles on the plate coalesce and drain into a suitable tank below. Although electrostatic precipitators... 

For the cleaning of the effluent gas various systems are in use. The most common system consists of the combination of an electrostatic precipitator (ESP) to trap sulphuric acid droplets and organic acid mist, followed by SO2 removal in diluted caustic ... 

Following scrubbing and cooling, suspended droplets of scrubbing solution or mist are removed in wet electrostatic precipitators. The cleaned gas may stiU contain unacceptable levels of mercury vapour, which must be removed before transfer of gas to the acid conversion plant. There are a nnmber of mercnry removal techniques but the most commonly used is the Boliden-Norzink mercury removal process, which uses mercuric chloride solution to scrub the gas and absorb mercury according to Eqnation 4.7 (Dyvik, 1985) ... 

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. 

Dust removal by drop-out chambers and electrostatic precipitators, cooling and wet scrubbing, and wet gas mist removal by electrostatic precipitators. Conventional double absorption acid plant based on a five per cent SO2 feed gas. 

---