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Mist elimination:
background, theory, capture mechanisms
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Mist Elimination Processes

 

Mist eliminators consist, generally of 2 types - diffusion and impaction style designs. Impaction mist eliminators can be vertical fiber bed elements, a metal mesh pad horizontal design or of chevron staggered plate construction. DuPont Clean Technologies offers Brink® diffusion and impaction fiber beds and MECS® TowerGARD® mesh pad mist elimination products.  Since fiber bed mist eliminators are the most versatile and can be designed to remove any liquid or any soluble solid mist from any gas stream, this article focuses on high efficiency fiber bed mist eliminators.


Fiber bed mist eliminators

 

Fiber bed mist eliminators consist of thick layers of very fine fibers placed between two concentric cylindrical screens or cages. Chemically resistant glass fibers, synthetic fibers and other special type fibers are used as the fiber bed material, depending on the process environment. Structural screen/cage parts and flanges are made of any weldable metal, plastic or glass reinforced resins.

All fiber bed mist eliminators operate in a similar manner. Gases containing mist particles are directed horizontally through the fiber bed and the mist particles collect on the individual fibers within the fiber bed and coalesce to form liquid films.  These liquid films are then moved through the fiber bed by the gas flow, then the liquid drains off the downstream face of the fiber bed by gravity.  Fiber bed mist eliminators are typically installed in a vessel or tank and the collected liquid is continuously drained from within the tank.

A fiber bed design innovation is the addition of a second more coarse fiber layer on the downstream side. This additional layer expedites drainage and prevents re-entrainment of the liquid back into the gas stream.

  Fiber bed mist eliminators

What is Mist?

 

Mist is the term used to describe a phenomenon of small finely divided liquid aerosol droplets suspended or dispersed in air or process gases. Mists are most commonly formed when warm, saturated gas meets sudden cooling, such as in a heat exchanger or when saturated hot and cold gases are mixed. The term mist usually refers to liquid particles under 10 microns in diameter. Particles over 10 microns are often called sprays and very small mist particles under 1 micron in diameter are called submicron mist particles. A good rule of thumb to identify small submicron mists is that they are often visible as a cloud or haze. Common examples include smog, clouds, blue asphalt ‘smoke’ from tar pots and white clouds of condensed water vapor from power plant smokestacks.  

Types of Emissions

 

  • Solids: Insoluble solids (Not a mist) or  Soluble solids (When Wetted Becomes a "Mist") 
  • Gases/Vapors (Not a Mist unless condensed)

 

 

Relative Size Comparison

  • Mechanical - Mist mean size is 2.5 micron (0.0025 mm)
  • Condensation -  Mist mean size is 1 .0 micron (0.001 mm)
  • Chemical Reaction - Mist mean size is 0.3 micron (0.003 mm)

Mist Formation

 

Mist can be formed in chemical and other manufacturing processes by 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 contain particles that are relatively larger and 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 downstream 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 be deleterious in-plant and may cause 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. 

Sulfuric Acid Relative Size Comparison

  • Standard Drying Tower - Acid mist mean size 2.5 micron
  • Standard Interpass Tower - Acid mist mean size 1.5 micron
  • Interpass Tower with Oleum Production - Acid midst mean size is 0.3 micron
  • Interpass Tower with Heat Recovery - Acid mist mean size is 1.0 micron
  • Final Tower - Acid mist mean size is 1.5 micron
  • Ammonia Scrubber - Acid mist mean size is 0.3 micron

Mist Collection Mechanisms

Mist Collection Mechanisms

Inertial Impaction

Particles larger than three microns are collected when their momentum prevents them from following gas streamlines around fibers. They leave the streamline, strike a fiber and are collected by the fiber.

Direct Interception

Between 1.0 and 3.0 micron size particles tend to follow the gas streamlines as they flow relatively close to fibers. A 1.0 micron particle, for example, passing within 0.5 micron of a fiber will be collected by the fiber.

Brownian Diffusion

 

Extremely fine particles have random side-to-side movement caused by collisions with gas molecules. A 0.1 micron particle will have about ten times the Brownian movement or random motion of a 1.0 micron particle, greatly increasing the probability of collision with a fiber.

After collection, the mist droplets coalesce on the individual fibers in the fiber bed and begin to drain.

Learn more about
our products and services

 

DuPont Clean Technologies welcomes the opportunity to investigate and recommend solutions to any mist related air pollution or in-process gas handling problem that you may have. Learn more about MECS® Brink® Mist Eliminators Services for trouble-free installation and operation, including fiber sampling, mist sam-pling, repacks, and repairs

 

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