Mist, removal

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 electrical precipitator is a dry dust or liquid mist removal unit which utilizes the ionization of the process gas (usually air) to impart electrical charges on the suspended entrained particles and effect particle collection by attraction to an oppositely charged plate or pipe. This type of unit is in use in services which are difficult for other types of entrainment removal equipment. Figures 4-79, 4-80, and 4-81 illustrate the usual fundamental action of these units. 

Caution I have known a few demister pad vendors to claim that their pads would remove even a 500-pm particle and yet sustain absolute mist-free discharge. I have serious reservations here. Why First, I have yet to witness myself, or even hear a truly unbiased witness testify, that this is true. Can a 500-pm selection be achieved for absolute mist removal in a demister pad Second, I leave this question to those... 

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. 9.3. Acid mist removal candle filter being installed atop a stainless steel H2SO4 making tower. It is one of many. Exiting gas passes inward through the candle fabric and out the top of the candle - then out of the tower. The acid mist is caught in the candle fabric by impact, diffusion and Brownian forces (Brink, 2005 Friedman and Friedman, 2004 Lee and Byszewski, 2005 Ziebold and Azwell, 2005). The large total area of the candles gives a low gas velocity through the fabric, which allows 99+% capture of the mist. The captured mist trickles down the fabric and drips back into the tower or into collection pipes (Outokumpu 2005).

D.R. Duros and E.D. Kennedy, Acid mist removal, Chem. Eng. Prog. 

Pump seals and demister wash receive priority on fresh water additions. In the Research-Cottrell process, two demisting stages insure adequate mist removal. Water sprays are mounted below and above the... 

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. 

Since no highly corrosive catalysts are used in the process, the materials of construction need not be special grades of highly corrosion-resistant materials. The vaporizer, heat exchanger, separator, preheater, heater, and catalyzer may be made of steel condensers, mist remover, and still are often of stainless steel. 

Newsprint This example is from a mill producing standard newsprint from a furnish mix consisting of TMP and recycled fibre. The machine runs at a temperature of 45-55°C, and production is 600 t/day. Foam build-up is evident only in the wire pit and silo, but without adequate conttol the foam can grow to 15 feet in height. This foam overflows the wire pit and creates a housekeeping problem in the basement of the mill. Also, the wire pit has a mist removal system that will fill up with foam. At one point the duct in the mist removal system collapsed because of foam build-up (a serious safety problem). 

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