Introduction to Demister in Process Piping

In any process where gas and liquid come into contact, the gas will entrain some mist or liquid droplets. This mist can cause process inefficiencies, product loss, and serious equipment damage. A demister is a device used for removing entrained mist from a gaseous or vapor stream by converting mist into big liquid droplets that are too heavy to carry by the gaseous or vapor stream. The demister usually work by coalescing smaller liquid droplets by obstructing their path. The obstruction in the path of liquid droplets can be achieved by a variety of geometries. Demister may be a mesh type, vane pack or other structure intended to aggregate the mist into droplets that are heavy enough to separate from the vapor stream. Demister pad is often fitted just below the top vapor outlet of a vapor liquid separator.

Demister is also known as demister pad, mist pad, wire mesh demister, mesh mist eliminator, catching mist and mist eliminator.

Demister Working Principle

When the gaseous or vapor stream with mist rises at a constant speed and passes through the demister, the mist will collided with the demister mesh filament and attached to the surface of filament due to the inertia effect. The mist will be diffuse on the filament surface and liquid droplets will start forming. The liquid droplets will grow bigger and isolate from the filament until the droplets gravity exceeding gas rising force and liquid surface tension force. Thus the bigger liquid droplets drop down in the pool of liquid below. Gaseous or vapor stream is not affected by the obstruction in the path and escapes through top vapor outlet.

When installing the demister, the demister pad must fit tightly to the wall of the column or vessel or tower to prevent any gas leaking from the contact surface. When properly designed demister pads can give efficiency up to 99.9% with minimal pressure drop.

Demister is mainly used in process piping systems like;

• Absorption columns
• Distillation columns
• Steam Boilers
• Gas and Air scrubbers
• Oil Mist Sepration
• Effluent gas treatment in Sulphuric Acid Factories.
• Vacuum Towers and Drying Towers
• Knockout Drums

Advantages of Demister

Demister is a necessity to be used where liquid in gaseous or vapor stream is not acceptable. For example – compressor suction lines. Separation of mist from gaseous or vapor stream can improve the operating condition, optimize process indicators, reduce corrosion of the equipment, extend equipment life, increase the amount of processing and recovery of valuable materials, protect the environment, and decrease air pollution. Demister also produces high quality condensate suitable for use a boiler’s feed water.

Must have Features in a Demister

• Simple structure.
• Lightweight.
• High porosity.
• Cause less pressure drops.
• Large surface area.
• High mist separating efficiency.
• Easy to install, operate and maintain.
• Easily tailor made to suit most vessel shapes and sizes.
• Durable and long service life.
• Corrosion resistance.

Types of Demisters Pads

Demister pad forms have four categories: standard type, efficient type, high penetration type and shock absorber type. Below is the image of all four types being used in process piping industry.

Demister Installations in Columns / Vessels / Towers

Demister can be installed in columns or vessels or towers in various positions depending upon the shape, vapor inlet nozzle location, vapor outlet nozzle location, liquid outlet nozzle location and process applications. Below is the figure showing such few installation types.

Maintenance

After using for a long duration, the demister pad will clog or prone to flooding and the pressure drop will increase because of the particles in the gas blocks the demister pad mesh openings. So maintenance of the demister pad is required regularly. In maintenance demister pad is cleaned thoroughly for opening of all blockages.