Sulfur mist eliminators

It is customary to install mist-eliminadng devices after the last sulfur condenser to minimize entrainment of sulfur droplets into the incinerator. Installation of mist eliminators after each sulfur condenser is also of value, as catalyst deactivation caused by entrained sulfur may be a problem. Wire mesh pads, located in the outlet channel of the condenser, are usually used for sulfur mist elimination. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

In drying towers of sulfur-burning plants, mesh pads or inertial impaction-type mist eliminators are usually adequate. High efficiency mist eliminators are usually used in drying towers of spent acid or metallurgical plants. 

Packed fiber bed mist eliminators can be designed to operate at almost any desired particle collection efficiencies, depending on the allowable pressure drop and cost. A good discussion of sulfuric acid mist generation, control, and mist eliminator design is available (109,110). 

TABLE 14-21 Operating Characteristics of Various Types of Fiber Mist Eliminators as Used on Sulfuric Acid Plants ... 

Sonic agglomerators, which have been used experimentally for sulfuric acid mists and as mist eliminators. Commercial development is not projected at this time because the energy requirements are considerably greater than those for venturi scrubbers of similar capacity. 

Sulfuric Contact SO2, acid mist Scrubbers with mist eliminators, ESPs... 

Fiber-bed scrubbers are used to collect fine or soluble particulate matter or as mist eliminators to collect liquid aerosols, including inorganic (e.g., sulfuric acid mist) and volatile organic compounds. Insoluble or coarse PM will clog the fiber bed with time, and VOCs that are difficult to condense will not be collected efficiently. 

Mist eliminators, in sulfuric acid manufacture, 23 781 Misuse failure, 26 982 Mites, 8 9... 

Spray columns are used with slurries or when the reaction product is a solid. The coefficient kL in spray columns is about the same as in packed columns, but the spray interfacial area is much lower. Considerable backmixing of the gas also takes place, which makes the spray volumetrically inefficient. An entrainment control device (e.g., mist eliminator) usually is needed at the outlet. In the treatment of phosphate rock with sulfuric acid, off-gases contain HF and SiF4. In a spray column with water, solid particles of fluorosilic acid are formed but do not harm the spray operation. 

If a wet method for collection is selected, such as a wet electrostatic precipitator, fiber-type self-draining mist eliminator, or wet scrubber, ammonia can be regenerated from the salt solution by reaction with a readily available metal oxide such as lime or zinc oxide with formation of a stable sulfur product for disposal. These metal oxides, however, as well as their reaction products, are insoluble and could cause deposition on heat transfer surfaces and/or clogging in the regenerating equipment. Therefore, as indicated in Figure 2, to ensure continuity and reliability of the process, a soluble metal oxide was utilized (in the form of sodium hydroxide solution) to regenerate the ammonia in the experimental work described. This procedure also allows more eflFective utilization of the metal oxide the soluble oxide (NaOH) can be regenerated in batch equipment outside the continuous portion of the process by reaction with either the aforestated insoluble reactants, lime, or zinc oxide. Better control is aflForded in a batch reactor with more eflBcient use of reactants. However, in full-scale equipment undersirable deposition of reactant and product may be controllable so that batch operation may not be necessary. 

Calcium reagent is added to the absorber tank and pumped to the sulfur dioxide absorber. Most of the spent reagent is returned to the absorber tank, and part is provided as reagent make-up to the quencher system. The spent quencher reagent, containing particulate and reaction salts, is removed from the process as a sludge blow down. Make-up water, to compensate for evaporation losses and sludge blow down, is added primarily as mist eliminator wash. 

It is important that all the SO3 gas is absorbed in the snlfuric acid sparger and the sulfuric acid mist is removed by cyclone separator as well as mist eliminator and traces are trapped in the coke tower. 

The steam is led to a mist eliminator which recycles sulfuric acid to the high pressure converter to improve further the concentration of the acid. 

Apart from the benefits of the high pressure mixing of SO3 with water to produce sulfuric acid, the proposed cold process for the manufacture of sulfuric acid has also been conceived to avoid the complexity of requiring a sulfur furnace and the related heat recovery system, the multipass static converter, counter current heat exchangers, the interpass absorption tower (IPAT), drying tower (DT), final absorption tower (FAT), mist eliminators, acid coolers, and alkali scrubber. The resulting plant is, as a result, of much lower cost in equipment and land use. 

Tightly packed fiber bed mist eliminators are another option for impmgement separations. These devices were developed by the Monsanto Compaiiy for sulfuric acid plant mist qrplications, but they are now also available firm various companies. Their main advantt is to allow efficient mnoval of paries down to the 0.1-1 fim range. These very small particles are not separated by havit greater momentum than the vapor or gas, but by the random Brownian movement or raiidom diffimion. The particles diffuse to the fiber surface in the very densely packed beds. Browitian movement actually increases as particle size decreases, improving the separation of very small particles. 

There is always a cratain amount of entrained liquor in the gas leaving the cells. This is in the form of a mist that has been known to survive beyond the gas coolers and into the drying system, and deposits can form in the chlorine piping and processing equipment. Also, the sulfuric acid used in the drying system is notorious as a source of mist that can interfere with the performance of downstream equipment. Mist eliminators installed both before the coolers and after the drying towers alleviate these problems. Section 9.1.5 covers this subject... 

One of the problems associated with sulfuric acid is its tendency to form mists. The liquid particles in mists are by definition 10 xm or less in diameter, and sulfuric acid mists range down into the submicron sizes. Particles larger than about 3 p,m are collected efficiently by inertial impact, but Brownian motion becomes the predominant mechanism below 1 p,m. Wire-mesh and other common impingement-type mist eliminators therefore lose their efficiency below 1 or 5 pm. 

Multiple hearth combustion plants, as shown schematically in Fig. 10.15, are especially suitable for the combustion of moist and paste-like waste. They have been used for decades in the paper industry for energy production from primary and biological sludges, often together with bark. The flue gas purification plant downstream from the multiple hearth furnaces usually consists of wet scrubbers to remove dust and sulfur compounds. The flue gas finally enters the stack via a mist eliminator. 

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