Mist can be formed in chemical and other manufacturing processes using three distinct and different methods:
MECHANICAL - physical shearing type forces may break up or atomize a liquid to form an aerosol mist. Typically, mists created by mechanical means are relatively larger particles and are greater than 1 micron in size. In the drying and absorbing towers of a sulfuric acid plant, larger acid mist particles are created as a result of the splash and lateral shifting of the liquid acid in the distributor and over the tower packing. These liquid sulfuric acid particles are then entrained in the upward gas flow and if not abated, will cause down stream equipment corrosion and serious air pollution.
COOLING - when a gas stream is saturated with vapor and is exposed to a temperature drop, the cooled vapor in the gas stream will condense to form a mist. Particles formed as a result of cooling and condensed vapor are usually very small and sub-micron in size, or less than 1 micron in diameter. Organic sub-micron mists or "blue smoke" from plastics processing, vinyl curing, fabric finishing and metalworking, to name just a few, cause highly visible stack plumes and will result in environmental opacity and deleterious in-plant health and safety issues.
REACTION - where the temperatures and pressures are such that the chemical reaction of two or more gases create mist, the resulting mist consists of the most difficult to capture, sub-micron sized aerosol particles. In metal smelting, coking and refinery operations where incinerator off gas is scrubbed with ammonia or caustic, even trace amounts of sulfur trioxide react with water vapor to form sub-micron sulfuric acid mists. If not captured, these mists will create opacity violations and air pollution hazards. Within some chemical processes, the presence of in-process mists will affect product purity and cause decreased production rates because of catalyst fouling and corrosion of process equipment which increases maintenance and operating costs.