Types of Venturi Scrubber Systems and Their Applications for Air Pollution Control

Venturi scrubbers are able to achieve very high particle collection efficiencies, sometimes over 99%. The efficiency is primarily due to contact and absorption.

Dirty exhaust gas enters the Venturi throat section at high velocity and collides with a large volume of tiny liquid droplets. The atomized wash water also reduces wear on the scrubber shell by protecting surfaces from abrasion.

Ejector Venturi Scrubbers

Ejector Venturi Scrubbers can remove gaseous pollutants from many industrial processes. They are effective in removing noxious gases, particulates, odors, and fumes from a variety of applications including cement kiln emissions, basic oxygen furnaces, fly ash removal, coal gasification, industrial and utility boilers and many others. They also have some of the highest particle collection efficiencies of any wet scrubber type, with some exceeding 99%.

Unlike packed towers or plate scrubbers, the ejector venturi design has an open construction that is less prone to plugging and scalding. This makes them one of the most popular types of scrubber for industrial processes such as coal gasification, power generation, petrochemical processing, and waste incineration.

The ejector venturi operates by combining a converging section, throat and diffuser in a single tube. The gas to be treated enters the inlet section where it reaches its maximum velocity, which then drops sharply into the throat section of the scrubber. In the throat section intensive mixing between the gas and liquid takes place, entraining the dust particles into the spray and turning them into fine droplets. The ejector action of the spray at high velocity provides energy to the particles and liquid, which enables them to overcome frictional forces on them and thereby accelerates their escape from the scrubber into a second stage scrubbing chamber.

As the gas leaves the throat section the liquid spray continues to disperse. This continues down into the discharge section where the velocity of the scrubbing liquid and that of the gas decrease as the cross-sectional area of the discharge section increases. During this time the scrubbing liquid continues to break into sheets and ligaments of spray. This allows the liquid to have minimal impact on the diverging interior surface of the discharge section, maximizing its availability for scrubbing.

The discharge of the gas from the discharge section outlet(s) (7) can be directed to an impact liquid separation device (11) or it can be directly into the first stage liquid sump (28). The recirculation pump that recirculates the scrubbing liquid can also be located in this section.

Dual Throat Venturi Scrubbers

Venturi scrubbers use a combination of shear and impaction forces to break water into a fine cloud of small droplets at high density. This increases the likelihood of collisions between condensed VOCs and particulate matter in an exhaust gas stream, resulting in absorption of volatile organic compounds (VOCs).

Ventury scrubbing systems can be designed with various throat sizes to accommodate varying particulate size requirements. They are typically self-draining with no internal moving parts that may wear or become misaligned. This design also minimizes the amount of freshwater required for operation.

The scrubbing liquid is introduced through spray nozzles that are designed to be easily removed for cleaning. As dust-laden gases enter the throat section of the venturi, they immediately come into contact with the scrubbing liquid sprayed against the venturi walls. This contact causes the particles to be entrained into the scrubbing liquid by the mechanism of inertial impaction and are then collected within the entrainment separators.

When handling a wide range of particulate sizes, the optimum throat width is often determined by the gas flow rate. For example, a twelve-inch throat width provides sufficient efficiency for handling a gas flow rate of up to seventy thousand cubic feet per minute (CFM). However, throats wider than this may not be effective in distributing the water or recycle liquid evenly throughout the throat region.

Due to the high velocities of both the scrubbing liquid and the gas being treated, it is important to ensure the entrainment separators are constructed of abrasion-resistant materials. It is also important to maintain the pump that recirculates the scrubbing liquid in order to avoid excessive carryover. Lastly, it is critical to ensure the system fan has adequate performance to prevent excessive erosion of the nozzle and throat sections.

Venturi scrubbing systems are a cost-effective solution for removing gases containing soluble or insoluble particles from industrial processes such as manufacturing, oil refinement, and paper production. They are often used in conjunction with chemical absorption packed tower scrubbers to control both gaseous and particulate contaminants. In addition, venturi scrubbers can be used as a pre-treatment to baghouse filters in order to reduce the frequency of maintenance required at the filter.

Wet Throat Venturi Scrubbers

The dirty exhaust gas enters the flanged scrubber inlet connection via ductwork from the process or emission source. The scrubbing liquid is supplied from a recycle pump through tangential feed pipes that connect to the venturi throat. The feed pipes are typically made of abrasion resistant linings, which prevent erosion and plugging.

The scrubbing liquid is atomized by high pressure spray nozzles that deliver the liquid at a rate much higher than that of traditional spray towers. This creates a large quantity of tiny liquid droplets for the dust particles to impact on. The resulting inertial impaction and diffusion removes the contaminant. Entrained liquid droplets are separated from the outlet clean gas stream by an entrainment separator (often a cyclonic or mesh-pad mist eliminator) and returned to the scrubber water.

Wet-Throat Venturi Scrubbers are designed for applications that require a lower pressure drop and for use with thicker slurries or other liquids that tend to clog traditional open-weir injection systems. These scrubbers feature a closed-loop recirculation system to prevent spray loss and ensure optimum scrubbing performance. They offer a range of standard sizes and designs to suit a variety of applications.

A wet-thoat Venturi consists of a converging section, a throat section, and a diverging section. The inlet gas stream is accelerated to a high velocity in the throat section by a restriction in its cross-section. This shears the scrubbing liquid into fine droplets and amplifies the surface area of the scrubbing liquid to provide intensive mixing between the gases and the scrubbing water.

These conditions result in high control efficiency for the removal of particulate, aerosol, and mists. This type of scrubbing system is widely used in acid plants, asphalt plants, cement plants, foundries, lime kilns, metals industries, paper mills, resins, smelting operations, and wastewater treatment emissions.

A proper design and scrubbing system is critical for the operation of a wet-thoat Venturi. The liquid-to-gas ratio, or L/G, must be properly adjusted to ensure sufficient liquid coverage over the throat area and to make up for any evaporation losses. This can be accomplished through an adjustable venturi throat or by utilizing a variable volume chamber.

Cyclonic Venturi Scrubbers

Venturi scrubbers can achieve 99%+ particulate removal efficiency, and have been developed for many different processes including mining, chemical, food, explosive dust, and metallurgical applications. They are particularly useful in processes that require high inlet gas temperatures, or high percentage of solids in the liquid recirculation loop. This is because the cyclonic separator design significantly reduces the amount of contaminant solids in the scrubber system.

The dirty gas entering a venturi scrubber is forced at a very high velocity through the scrubber throat and collides with scrubbing water. This atomizes the water into tiny water droplets, which capture particles through impaction and diffusion. The scrubbing liquid is separated from the gas stream in a cyclonic separator (or other similar device) and discharged into a recycle tank. The scrubbing liquid is continuously purged to limit solids concentration and allow recirculation back into the scrubber throat.

A more advanced type of venturi scrubber is the ejector venturi, which uses an abrasion resistant design to minimize solids buildup and maintain proper performance. The ejector venturi has the highest particle collection efficiencies for very fine particles, of any wet scrubber system. However, the system can be expensive to purchase and operate, due to its complexity and the need for a highly abrasion-resistant construction.

Another common form of venturi scrubber is the contactor duct, or orifice scrubber. This type of scrubber utilizes a trough-shaped duct to form the gas-liquid contacting zone. The gas stream passing through the orifice atomizes the scrubbing liquid into small droplets, which interact with and collect the particles in the gas stream. This type of scrubber requires a very high liquid injection rate to provide the required gas-velocity interaction, and therefore can be costly to operate.

The orifice scrubber is also subject to abrasion problems in the high-velocity areas of the nozzle and throat, which must be constructed from wear-resistant materials. The very high velocities of the gas and liquid aerosols in the throat also require the use of entrainment separators to prevent excessive entrained solids in the exhaust stream. Typical orifice scrubbers can require up to a hundred times the scrubbing liquid volume of a venturi-shaped recirculating spray system, requiring large pumping and storage volumes.