Demister Pad: Demister Pad is a device that is used in the separation column or vessel to capture the mist or liquid particle present in a gas mixture, to achieve or guarantee separation efficiency. A demister is used for the removal of liquid particles present in the gas stream or phase. Demister pad is used for liquid entrainment in the gaseous phase stream. Demister pad is also called as mist eliminator or mist pad.
Demister Pad is usually placed on the top of the Nozzle outlet of vapor in the equipment (Packing column or separator vessel) to achieve mass transfer efficiency. Demister pad can also be used for desulphurization and other production components (oil, poisonous or toxic gas).
A demister pad is used as an application in various industries listed below:
- Chemical
- Petroleum
- Pharmaceuticals
What is a demister pad made up of: construction of the demister pad is based on the application. For small-diameter applications, a demister pad can be formed in a single structure. In the case of large-diameter application, they can also be made from various structure which is integrated during installation. Wire mesh arrangement can be flat or ginning type. Demister pads are made up of knitted mesh which woven mesh is interlocked with each other to ensure smooth meshes for filtering purposes. A support grid of shape round or flat bars can be used to provide additional strength to the demister pad.
Material of construction of demister pad: demister pads are manufactured with different metallic and synthetic material varieties listed below:
- Stainless Steel (304, 304L, 316, 316L, 430)
- Monel
- Nickel and nickel alloys
- Copper and copper alloys
- Glass wool or fiber
- Polypropylene
- PTFE (Polytetrafluoroethylene)
Types of demister pad: demister pad is generally categorized into four types listed below:
- Standard type
- Efficient type
- High penetration type
- Shock absorber type
Working principle of demister pad: Demister pad functions on the principle of coalescing small droplets to form a large droplet and coalescing under the action of gravity. Gas-carrying mist with itself moves upwards due to drag force (nature of the gas) at constant velocity. Gas with mist comes in contact with elements of the demister pad, mist present in the gas mixtures is captured through these elements and gases pass through the demister pad assembly. Small droplets accumulated at the element of the demister pad grow bigger because of continuous fluid media passing through it. This formed large droplet settles out or collapses or coalesces when the gravitational force acting on the mist droplet is greater than the drag and buoyancy force.
Demister pad efficiency: demister pad gives efficiency greater than 99 %. Demister pad consists of knitted wire mesh for filtering requirements. Mesh can be customized as per requirement.
Factors affecting demister pad efficiency are listed below:
- Droplet size (Usually 10-micron rating).
- Provided surface area through the mesh (i.e. High porosity nearly equal to 98 % for improved efficiency).
- Demister pad thickness.
- Physical properties of the fluid flowing through the demister pad.
Design of demister pad: Demister pad design is dependent on the following listed below parameter:
- Liquid loading or flow
- Viscosity of vapor and liquid
- Gas pressure
- Surface tension
Advantages of demister pad: Demister pad increases the separation efficiency, reduction in corrosion, increases the life cycle of the equipment, etc.
A demister pad must have below listed Salient Features:
- Simple in design, long service life
- Provide Greater surface area and high separation efficiency
- Causes Low-pressure drop
- Easy in installation, operation, and maintenance.
- Economical
Selection of demister pad: selection of demister based is completely based on the application requirement. Demister pad material of construction range has a different variety of metals, plastic, and thermoplastics suitability according to application requirements.
- Mesh pad mist eliminator: Mesh pad eliminator working principle is based on the removal of droplets by the impact on the surface. The liquid collected in the element (mesh area) of the demister, drops down under the action of gravity. Removal of droplet size in this type of mist eliminator ranges from 3 – 5 microns.
- Plain vane pack mist eliminator: This type of mist eliminator is considered a high-efficiency demister pad. This type of mist eliminator is to be used where vapor flow is flowing in a vertically upward direction. Vanes are simply grooved surface which is used for liquid droplet removal.
- Pocketed Vane pack mist eliminator: This type is used for the application in liquid loading (capacity is greater), gives efficient separation, and is suitable for fouling service in horizontal vapor flow application.
Maintenance of demister pad: Continuous operation of the demister may clog or block the element of the demister pad, which ultimately increases the pressure drop and reduces the separation efficiency.
Method of maintenance: spraying of cleaning liquid on the demister pad element, and passing steam is also an option for cleaning, however spare demister pad is the best solution for such an issue.