Liquid mists

As small droplets of liquid are usually still present in the gas phase, demisting secWons are required to recover the liquid mist before it is carried over" in the gas stream out of the separator. The largest liquid droplets fall out of the gas quickly under the action of gravity but smaller droplets (less than 200 microns) require more sophisticated extraction systems. 

Liquid mists of ethylene oxide will decompose explosively in the same manner as the vapor. Burning rate increases with decreased droplet size. 

Smokes and plumes White smoke (liquid mist) Plume abatement 0.3-0.5 800-10001... 

Finally, there are an inereasing number of streams of pressurized industrial gases eontaining eontaminants, sueh as eorrosive or polymerizing liquid mist, tar, and the like, on whieh the pressure is being released, and the quantities of available power are signifieant. 

Fuels such as diesel and kerosene readily absorb hydrocarbon vapors, the total uptake and absorption rate depending on both chemical and physical factors. If a soluble test gas is introduced above a charged test oil the concentration of flammable test gas therefore decreases with time. Liquid mist and spray produced by charged liquid increase the absorption rate relative to a quiescent liquid surface. As discussed in A-5-4, absorption could lead to an underestimation of test gas MIE near the liquid surface unless the rate of test gas introduction is sufficiently high to offset the rate of removal. Table 3-8.1.2 shows solubilities of a selection of gases in a mineral-based transformer oil at ambient temperature and pressure [200]. 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

Gas-liquid Mist Acid mist carryover chromic acid oil mists tar fog... 

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. 

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). 

Chemical Substance A substance usually associated with some description of its toxicity or exposure hazard, including solids, liquids, mists, vapors, fumes, gases, and particulate aerosols. Exposure, via inhalation, ingestion, or contacts with skin or eyes, may cause toxic effects, usually in a dose-dependent manner. 

Since the removal of solids from liquids in these ways is discussed elsewhere, this section is concerned only with the removal of suspended particles and liquid mists from gases. 

Since the key to this sequence is the H02 radical, the aqueous hydrogen peroxide process discussed for NO to N02 conversion in the stack would be an appropriate approach. The S02 forms no corrosive liquid mist in the stack and could be removed by wet scrubbing of the exhaust. 

Another type of separator used in certain high-gaa/low-liquid flow applications is a filter separator. These can be either horizontal or vertical in configuration. Fig. 4 shows a horizontal design. Filter tubes in the initial separation section coalesce liquid mist into larger droplets as gas passes through the tubes. A secondary section, consisting of vanes or other mist extractor elements, removes these coalesced droplets. 

While the deflagration pressure ratio for ethylene oxide vapor is about 11 or less, liquid mist decomposition can give much greater pressures and very fast rates of pressure rise (190). 

Regarding the need for absolute removal of all free liquid mist, such as with a gas turbine compressor suction scrubber, I find a 100-pm particle size selection to be good design practice. This particle size fallout before reaching a demister pad will indeed ensure that most any demister pad will remove all remaining free liquid mist. 

Water drop in oil phase. Contrary to a 150-pm particle in the gas phase, when absolute free liquid removal is not required, such as with a KO drum upstream of a flare, 500-pm sizing is very common and good-practice engineering. Here free liquid mist, typically 500 pm, will readily bum. Other applications, such as pipeline KO drums, may be of the same order. 

Reagent Mode of addition HN03(aq)+02(g) liquid mist N0(g) + UgiiL gas ... 

Wet scrubbers. These are devices in which a liquid spray contacts a gas stream, primarily for the purpose of removing fine solid particles or liquid mists from the gas. In this process, the liquid spray simultaneously absorbs soluble components from the gas. The sprays are generated by a variety of mechanical devices. 

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