Guide to types of wet scrubbers
When we are approached by a customer to develop a new odour control system, it’s essential we find a solution that is most appropriate for their requirements. That will usually involve installing a scrubber – but types of scrubbers are wide-ranging, and their suitability depends on the application and the specific odour issue.
In this guide we’re focusing on types of wet scrubbers – how they work, the types of applications we recommend them for, and their pros and cons. Wet particulate scrubbers and gas scrubbers are often conflated, but it’s important to make a distinction between the two because they are fundamentally different. This guide deals primarily with types of wet particulate scrubbers.
What are wet scrubbers?
Scrubbers work by literally “scrubbing” exhaust streams of unwanted particulates or gases. In the simplest terms, wet scrubbers do this by using a liquid to capture these particles or to react with the gases. While the term “scrubber” is used across particulate and gas scrubbers the devices can be very different and suitable for varying applications.
It’s important to remember that when you’re using a wet scrubber to remove particulate, it’s a scientific process. The higher the pressure the more turbulent it will be. The more turbulent the process the more likely it is that a droplet will impinge with a contaminate and the more efficient it will be. The amount of pressure required will be dictated by the particle size distribution and amount of particulate and the required efficiency of the machine.
Wet scrubbers are typically used in the following applications:
• Sewage sludge
• Light metals processing
• Wet processes that create dust, such as paper and pulp
• Pharmaceutical
• Food production
Types of Wet Scrubbers
Centrifugal type wet scrubbers
In a centrifugal scrubbing system, an impeller acts as both the air mover and the device that removes the contaminant. This means there is no additional filter to add resistance and they also tend to use less energy, even though they are less efficient than a typical fan for the same duty.
Dirty air is sucked into the fan and sprayed with water where droplets and contaminant are forced to interact. The fan blades are specially shaped to fling the particulates (e.g. sludge) causing the odour out to the edges which is sent away as slurry, while clean air continues through the fan.
Centrifugal types of wet scrubbers are popular with food production applications as they are an effective way to remove grease and oil mist. They are also a good choice for applications such as tissue and sewage sludge.
Pros:
- Generally accepted by insurance companies as a fire barrier
- Reasonably efficient – typically 96.5% efficiency at 5 micron
- A good option for a standard frying line where heavy odour is not a concern
- An energy efficient option electrically so can be significantly cheaper to run
Cons:
- Only effective to a certain particulate size before carry over (i.e. not discharging 100% of the slurry) leaves mist that still contains odour
- Not effective for submicron particulates (<1 micron) as they will pass straight through
- Normally not suitable for recirculation, so water consumption is high
- Standard mist elimination is not efficient
- Does not provide any gas scrubber and therefore no removal of odour in gas phase
Static bath type scrubbers
Static bath type scrubbers work by forcing air through the water sheet via a very narrow throat. The result is that the water sheet is smashed into droplets, encouraging impingement with the particulates, and creating larger droplets.
These droplets containing the contaminate are now easier to remove and can be captured by mist eliminators while the clean air is discharged to atmosphere via a fan.
Static bath scrubbers are effective for treating heavy particulates as they sink and can be removed as sludge. However, for many food applications they are a poor choice as fats will simply float on the top of the water and the waste is hard to remove.
Pros:
• A good option for any applications that produce sparks – e.g. metals, grinding, and cutting
• Low maintenance as no moving internal parts
• Can handle comparably heavy burdens of dust
• Effective humidification efficiency
• Some have selectable efficiency performance or pressure drops enabling efficiencies of up to 99.6% at 5 microns
Cons:
• Not effective for applications producing fats as the particles will float, essentially forming a butter on the water surface that is difficult to remove
• Does not provide any gas scrubbing
• High pressure drop required for high efficiency, typically up to 2850Pa
How does the impingement process work?
Regardless of the type of wet scrubber, they all work by forcing particulates and water droplets together. This is known as impingement. The droplets capture the particulate matter and are then collected and removed.
Venturi Scrubbers
Venturi scrubbers are a versatile option as they can be designed to a wide range of pressure drops and hence a variety of particulate removal efficiencies. However, we are starting to see them become a less popular choice as technology has moved on and they are not always the most environmentally friendly option.
They continue to be a very popular choice within the paper making and tissue converting industry, as well as industries where there is a risk of fire and dry technologies are inappropriate Venturi scrubbers also allow for waste tissue to be removed and reclaimed.
Particulates are removed via the venturi effect, which occurs when gas and water is pushed through a narrow throat to increase its velocity. The resulting turbulence forces the particulates causing the odour to collide with the water droplets and capture the particulate in the larger water droplets.
The gas stream with the mist then moves on to a mist eliminator (i.e. a cyclonic separator) to remove the water droplets.
Pros:
• Suitable for big industrial applications
• Capable of handling flammable and explosive dusts, as well as corrosive materials
• Can provide cooling of hot gas steams
• Can recirculate the water and tolerate relatively dirty water
• Scalable across a wide range of flow rates
• Can be very efficient
Cons:
• Not suitable for sticky particulates as there will be build up on the cyclonic separator
• Won’t work in a frying environment as can’t create a slurry due to fat separating and floating
• Cyclonic mist eliminators are very large
• High efficiencies incur very high pressure drops and hence fan sizes
• There is no gas scrubbing
Impingement plate scrubber
Impingement plate types of wet scrubbers utilise perforations or holes in a plate to encourage the gas stream to break up the water sheet flowing horizontally across the plate above, above the holes are impingement plates that cause the water and particulates to collide thereby capturing the particulate in the water droplets. These larger water droplets are then removed by mist eliminators.
As a basic principle for particulate and droplet removal it is an effective tool, however there is some confusion within the industry about its suitability for certain applications.
Impingement plate scrubbers are a popular choice for oil mist elimination. However, the efficiency of this kind of scrubber is typically similar to a centrifugal wet scrubber and while dosing with chemicals is possible, they are not effective gas scrubbers and therefore not highly efficient odour abatement scrubbers.
Pros:
• Efficient for treating for gases and vapours – similar to centrifugal types in terms of efficiency
• Works well at low pressure drops
• Low maintenance option as few moving parts
• Can recirculate water/liquor
Cons:
• Not effective for smaller particulates as there is no submicron elimination
• Recirculation of oils and fats can be problematic
• Mist elimination is not always effective causing carry-over
• It is not a gas scrubber and not effective odour abatement technology
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