AIR Pollution
Cyclone Separator: Cyclone seperators operate to collect heavy duty dust from the processes like drying, grinding machines. The Collection of dust in cyclone separator is based on the action of inertial (Centrifugal) force up on Dust Particles.
Cyclone Separators are normally used for collection of coarse dust particles. The dusty air is introduced in the cyclone separator from the top through inlet pipe which is tangential to the cylindrical portion of the cyclone. Thus the gases move downward in whirling motion forming peripheral vortex and giving rise to centrifugal forces resulting in throwing of dust particles towards the wall of the cyclone and moving downward in the hopper. The gases after reaching the end of the conical portion change their direction and move upward towards the outlet forming inner vortex. Due to this upward movement of gases against gravity, more dust particles are separated and fall in to the subsequent removal chamber.
Dust Collection System : This is provided in for particles which bypass blowers. It can be supplied with a mechanical Shaker or with pulse jet automatic cleaning apparatus.
Pulse Jet Bag Filter System : Pulse Jet Bag ( Fabric ) Filter Systems are used for removing particulate matter from the process and general ventilation air as well as to recover precious products from several manufacturing process.
- Designing of Water Scrubber, Cyclone Separator, Air Scrubber, Bag Filter
- Preliminary and Detailed Design Operations
- Project Management
What is Wet Scrubber?
The term Wet scrubber describes a variety of devices that remove Pollutants from a Flue Gas or from other gas streams. In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid by spraying it with the liquid, by forcing it through a pool of liquid, or by some other contact method, so as to remove the pollutants.
The design of wet scrubbers or any air pollution control device depends on the industrial process conditions and the nature of the air pollutants involved.
Inlet gas characteristics and dust properties (if particle are present) are of primary importance. Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbers remove dust particles by capturing them in liquid droplets and pollutant gases by dissolving or absorbing them into the liquid respectively.
Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by means of another device called mist eliminator. (These terms are interchangeable). Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant.
There are numerous configurations of scrubbers and scrubbing systems, all designed to provide good contact between the liquid and polluted gas stream
1. Venturi Scrubber
Venturi Scrubber design. The mist eliminator for a venturi scrubber is often a separate device called a cyclonic separator.
VTS Venturi Scrubber This reliable design utilizes open pipe liquid introduction, a vertical throat, and a flooded elbow inlet to the cyclonic separator. Open pipes will not plug like high pressure spray nozzles if large particulate enters the recycle system and are able to accommodate a high solids concentration, if required. Since a pool of liquid is present at the bottom, the flooded elbow prevents wear. The Venturi Scrubber can handle the most difficult of scrubbing applications.
- Open pipe liquid injection - plug resistant
- Vertical venturi with various throat designs available
- Flooded elbow - abrasion resistant
- Traditional or MS Cyclonic Separator can be used.
2. Packed Tower Wet Scrubber
Packed tower design where the mist eliminator is built into the top of the structure. Various tower designs exist.
A wet scrubber's ability to collect small particles is often directly proportional to the power input into the scrubber. Low energy devices such as spray Tower are used to collect particles larger than 5 micrometers. To obtain high efficiency removal of 1 micrometer (or less) particles generally requires high energy devices such as venturi scrubbers or augmented devices such as condensation scrubbers. Additionally, a properly designed and operated entrainment separator or mist eliminator is important to achieve high removal efficiencies. The greater the number of liquid droplets that are not captured by the mist eliminator the higher the potential emission levels.
Wet scrubbers that remove gaseous pollutants are referred to as absorbers. Good gas to liquid contact is essential to obtain high removal efficiencies in absorbers. A number of wet scrubber designs are used to remove gaseous pollutants, with the packed tower and the plate tower being the most common.
If the gas stream contains both particle matter and gases, wet scrubber is generally the only single air pollution control device that can remove both pollutants. Wet scrubbers can achieve high removal efficiencies for either particles or gases and, in some instances, can achieve high removal efficiency for both pollutants in the same system. However, in many cases, the best operating conditions for particles collection are the poorest for gas removal.
In general, obtaining high simultaneous gas and particulate removal efficiencies requires that one of them be easily collected (i.e., that the gases are very soluble in the liquid or that the particles are large and readily captured) or by the use of a scrubbing reagent such as lime or sodium hydroxide.
Advantages and disadvantages
For particulate control, wet scrubbers (also referred to as wet collectors) are evaluated against fabric filters and electrostatic precipitators (ESPs).
Some advantages of wet scrubbers over these devices are as follows:
- Wet scrubbers have the ability to handle high temperatures and moisture.
- In wet scrubbers, flue gases are cooled, resulting in smaller overall size of equipment.
- Wet scrubbers can remove both gases and particulate matter.
- Wet scrubbers can neutralize corrosive gases.
Some disadvantages of wet scrubbers include corrosion, the need for entrainment separation or mist removal to obtain high efficiencies and the need for treatment or reuse of spent liquid.
Table to summerize these advantages and disadvantages. Wet scrubbers have been used in a variety of industries such as acid plants, fertilizer plants, steel mills asphalt plants, and large power plants
Relative advantages and disadvantages of wet scrubbers compared to other control devices
| Advantages | Disadvantages |
|---|---|
| Small space requirements | Corrosion problems |
| Scrubbers reduce the temperature and volume of the unsaturated exhaust stream. Therefore, vessel sizes, including fans and ducts downstream, are smaller than those of other control devices. | Water and dissolved pollutants can form highly corrosive acid solutions. Proper construction materials are very important to avoid adverse effects. Also, wet-dry interface areas can result in corrosion. |
| Smaller sizes result in lower capital costs and more flexibility in site location of the scrubber. | High power requirements |
| No secondary dust sources | High collection efficiencies for particulate matter are attainable only at high pressure drops, resulting in high operating costs. |
| Once particulate matter is collected, it cannot escape from hoppers or during transport. | operating costs. |
| Handles high-temperature, high humidity gas streams. | Water-disposal problems |
| No temperature limits or condensation problems can occur as in bughouses or ESPs. | Settling ponds or sludge clarifiers may be needed to meet waste-water regulations. |
| Minimal fire and explosion hazards various dry dusts are flammable. Using water eliminates the possibility of explosions. | Difficult product recovery |
| Ability to collect both gases and particulate matter | Dewatering and drying of scrubber sludge make recovery of any dust for reuse very expensive and difficult. |
Wet scrubber systems
Wet scrubber systems generally consist of the following components:
- Ductwork and fan system
- A saturation chamber (optional)
- Scrubbing vessel
- Entrainment separator or mist eliminator
- Pumping (and possible recycle system)
- Spent scrubbing liquid treatment and/or reuse system An exhaust stack
A typical wet scrubbing process can be described as follows:
- Hot flue gas from a furnace enters a saturator (if present) where gases are cooled and humidified prior to entering the scrubbing area. The saturator removes a small percentage of the particulate matter present in the flue gas.
- Next, the gas enters a venturi scrubber where approximately half of the gases are removed. Venturi scrubbers have a minimum particle removal efficiency of 95%.
- The gas flows through a second scrubber, a packed bed absorber, where the rest of the gases (and particulate matter) are collected.
- An entrainment separator or mist eliminator removes any liquid droplets that may have become entrained in the flue gas.
- A recirculation pump moves some of the spent scrubbing liquid back to the venturi scrubber where it is recycled and the remainder is sent to a treatment system.
- Treated scrubbing liquid is recycled back to the saturator and the packed bed absorber.
- Fans and ductwork move the flue gas stream through the system and eventually out the stack.
Categorization of wet scrubbers
Since wet scrubbers vary greatly in complexity and method of operation, devising categories into which all of them neatly fit is extremely difficult. Scrubbers for particle collection are usually categorized by the gas-side pressure drop of the system. Gas-side pressure drop refers to the pressure difference, or pressure drop that occurs as the exhaust gas is pushed or pulled through the scrubber, disregarding the pressure that would be used for pumping or spraying the liquid into the scrubber.
Scrubbers may be classified by pressure drop as follows:
- Low-energy scrubbers have pressure drops of less than 12.7 cm (5 in) of water.
- Medium-energy scrubbers have pressure drops between 12.7 and 38.1 cm (5 and 15 in) of water.
- High-energy scrubbers have pressure drops greater than 38.1 cm (15 in) of water.
However, most scrubbers operate over a wide range of pressure drops depending on their specific application, thereby making this type of categorization difficult. Another way to classify wet scrubbers is by their use - to primarily collect either particulates or gaseous pollutants. Again, this distinction is not always clear since scrubbers can often be used to remove both types of pollutants.
Wet scrubbers can also be categorized by the manner in which the gas and liquid phases are brought into contact. Scrubbers are designed to use power, or energy, from the gas stream or the liquid stream, or some other method to bring the pollutant gas stream into contact with the liquid.
These categories are given in Table Below
Categories of wet collectors by energy source used for contact
| Wet collector | Energy source used for gas-liquid contact |
|---|---|
| Gas-phase contacting | Gas stream |
| Liquid-phase contacting | Liquid stream |
| Wet film | Liquid and gas streams |
| Combination | Energy source: |
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Material of Construction and Design
Corrosion can be a prime problem associated with chemical industry scrubbing systems. Fiber-reinforced plastic and dual laminates are often used as most dependable materials of construction.