Wet gas scrubber: a multi-purpose device for efficient gas filtration and air purification

As civilization developed, the amount of solid and gaseous wastes inevitably increased. More and more harmful gases began to be emitted by metallurgy enterprises, chemical factories, petrochemical industries and, of course, the fuel and energy complex.

The urgent need for quick and efficient industrial gas and air purification led to the birth of the first gas scrubber, the invention of which fell on the peak of the industrial revolution of the late 18th and early 19th centuries. Back at that time, it became clear to the inventors that wet-type gas treaters / dust catchers show maximum efficiency in processing complexly polluted exhaust flows.

Since then, wet-type gas flirtation and air purification technologies have made great strides forward, and today gas scrubbers are high-performance, durable and reliable devices widely demanded in all branches of modern industry.

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Advantages of wet-operating scrubber systems

The main advantages of wet gas scrubbers are:

• the possibility of treating / purifying streams containing both harmful gases and micromechanical particles;
• the variety of available liquid reagents allows scrubber to work not only as dust collector, but also as chemical-type air cleaning equipment (neutralization of alkaline and acid fumes, VOC, ketone-containing aerosols, sulphur- and nitrogen oxides, etc.);
• wet scrubbing systems effectively moisten and cool the incoming stream down, which makes it possible to process high-temperature gas mixtures, as well as fire-hazardous and explosive dusts;
• unlike electrostatic precipitators and bag filters, water scrubbers withstand severe pressure drops without sacrificing performance (floating bed scrubbers are self-cleaning and require from little to no supervision);
• captured substances form a semi-liquid pulp, which prevents the occurrence of secondary sources of pollution;
• a properly selected liquid absorbent (solvent) allows valuable sludge to be obtained as a by-product;
• wide variation in sizes and spatial orientations makes these devices convenient for installation in any production conditions.

The etymology of the term "Scrubbing"

The term "scrubbing" is usually considered as a word of English origin. But if we trace the etymology more deeply, it turns out that the word "schrobben" is first found in German literature of the 16th century. The meaning of the German term "schrobben", in general, coincides with the meaning that this word is endowed with today: "clean, launder".

^{One of the very first models of an industrial water-type air purifier}

Thus, modern scrubbers are fully correlated with the concept designated as far back as the 16th century, and are apparatuses that scrub the air flow with water or reagent and remove unwanted inclusions.

Basic design elements

A typical scrubber design includes the following structural elements. Depending on the type of apparatus and the characteristics of its purpose, the basic design may vary slightly.

1. Exhaust fan(s) and gas ducts.
2. Saturation chamber (optionally present in some models).
3. Quenching chamber (optional, for treating flows of extra-high temperature).
4. Vertical scrubber tower or horizontal frame with working compartment.
5. Mist eliminator (drip collector).
6. Pump unit (to increase the resistance to aggressive media, the piping and the part of the pump that is in direct contact with reactive substances can be made of polypropylene, fluoroplastic, stainless steel or titanium).
7. Self-cleaning / recycling system (optional).
8. Output socket.
9. If a gas scrubber processes air environments contaminated with acids or alkalis, the scope of supply may optionally include systems for neutralizing acidic or alkaline effluents.

Hollow and packing-containing gas treaters

All industrial gas cleaning devices of the wet type can be divided into 2 large groups: hollow towers and packed scrubbers (absorbers).

^{Doyle scrubber principle scheme. One of the first hollow-type models of air filters that are almost never used today. 1 – inlet tube, 2 – cone confusers, 3 – sections partitions.}

Despite the fact that in both cases, the capture of unwanted inclusions occurs through the mechanism of interphase sorption, the principles of operation of packed (bed-containing) and hollow devices are different.

Cyclonic spray scrubber

The hollow cyclone gas washer is the simplest design of the scrubber-type dust collector. Contaminated flow is tangentially (like in dry dust cyclone) fed to the working chamber, which causes undesirable flow components to come into contact with the inner surface of the tower.

^{Basic-type animated scheme of a hollow-bodied water-driven scrubber}

In parallel, the spray section irrigates the interior of the column. The liquid being sprayed forms a film on the inner walls of the working chamber, and sorption of undesirable, (mostly mechanical) particles occurs in the interfacial layer (liquid-gas phase). Under the action of gravity, the liquid, with the impurities trapped in it, continuously falls into the sludge receiving section.

Like all wet machines, the cyclonic spray scrubber is equipped with a demister (upstream the outlet).

Venturi scrubber

Venturi scrubbers, in contrast to cyclone-type dust catchers, show the highest efficiency in trapping sticky dust inclusions as well as stringy glues, emulsions, syrups.

^{Flexible & high-performance & multi-task Venturi scrubber for complex gas treating (dust, soot, flue gases, stringy substances)}

The principle of operation of Venturi devices is based on the Bernoulli effect. The main element of the design is the Venturi cone, which is a pipe with a narrowing (confuser) in the central part.

The main provision of the Daniel Bernoulli effect (as applied to the current topic): the flow velocity increases in inverse proportion to the diameter of the pipe. That is, the smaller the caliber of the pipe, the higher the speed of the air (or liquid) jet.

Due to the narrowing of the pipe, the gas phase is able to accelerate in the Venturi tube to hundreds of meters per second. In the zone of the pipe, where the flow rate is maximum, is the spray section.

The high kinetic energy of the air turns drops of the sprayed liquid into micro fog, in which the capture of condensable (especially mechanical) jet components takes place.

Downstream of the confuser of the pipe there is a diffuser chamber in which the flow speed instantly drops and micro-fog condenses into large drops of liquid.

After that, the slurry is sent to the separator, where the purified gases are separated from the liquid and discharged into the atmosphere (or sent for recycling).

^{Venturi industrial air cleaner installed at Nestlé food factory}

Perhaps the only disadvantage of devices of this type is the rapid mechanical wear of the internal cavity of the Venturi cone when processing abrasive dust.

Froth / Barbotage / Orifice Scrubbers & Foam / Bubble absorbers

Another type of wet-type gas-cleaning equipment is bubble- (or orifice-) scrubbers, also called foam-type absorbers.

Industrial orifice scrubbers also implement a sorption mechanism in their work, but unlike water scrubbers, an active interfacial layer is formed on the surface of the bubbles of an unstable froth.

^{Round-section tower-bodied whirl-type foaming filter (basic scheme)}

The foam layer on the orifice tray is maintained precisely because of the incoming flow of polluted air, which froths the absorbent (by passing through the perforations in a supportive tray). Therefore, such scrubbers are often referred to as self-induced apparatuses.

The effectiveness of gas cleaning devices of this type is caused by two micro-levels of absorption:

1. Low-energy gas components that are unable to pierce a foam bubble are deposited (and accumulated) on its surface (until the bubble collapses);
2. Microdispersed particles (or huge molecular conglomerates), possessing sufficient energy to destroy the integrity of the bubble collapse the latter, and the resulting drop merges with slurry and is carried to the separator.
3. In the separator, the slurry and air are disunited, and the gas phase is directed to the outlet, while the slurry is discharged into the sludge collector.

The process of forced air / gas transmission through a layer of foam is often called barbotage, sparge or gas flushing.

^{Square-column chemical foam scrubber (assembly stage)}

Bubble scrubbers are generally regarded as chemical-type gas cleaning equipment and are widely demanded in the printing, electroplating, chemical and petrochemical sectors of industry.

Packed scrubbers

Packed scrubbers are in high demand in all industries, as they show excellent results in cleaning air and gas streams from aggressive chemical compounds (corrosive gases (CO, SOx, H₂S), acid and alkaline aerosols (fumes), aromatic hydrocarbons, acetone, benzenes, xylenes and others).

^{Packed tower for chemical gas treating}

As in other wet scrubbers, trapping harmful gaseous impurities occurs on a surface of a thin film of water or reagent.

The difference of packed columns from other devices is that the working chamber is filled with physical bodies of a specific topology. These can be Raschig rings, Pall rings, Levin’s spirals, chords, half chords, and other bodies having a large surface and a small volume.

Packed towers are usually referred to as chemical equipment. This means that to a greater extent they are designed to trap chemical compounds, rather than mechanical dust.

The principle of operation of packed absorbers, in general, coincides with other representatives of the wet-type equipment.

1. Contaminated flow enters from the bottom up or in a horizontal plane, and is fed into the working compartment filled with a packed bed.
2. In parallel with this, the spray section atomizes the reagent (or process water) onto the packing bodies, forming a liquid micro-film of high specific surface.
3. An air or gas stream passes through the chamber, repeatedly contacting with a liquid film that fully envelops the bed.
4. Diffusion forces attract the oppositely charged ends of dipoles of undesirable molecules, effectively capturing pollutants inside the absorbent.
5. The slurry containing trapped substances is sent to the sludge collector, and the gas phase is discharged through the outlet to the air basin.

In order to increase the efficiency of capturing undesirable components of the stream, packing bodies can be made of chemically active microporous metal-containing polymers, copper, nickel, aluminosilicates, and other pure- or combined elements.

^{Horizontal form-factor of a packed absorber}

To enhance the chemical resistance of industrial packaged scrubbers, the working chamber can be lined with acid-resistant concrete.

Fluidized bed filters

Perhaps the most promising gas cleaning device, capable of simultaneous trapping a wide range of gaseous pollutants and mechanical particulates (dust, slurries, muds, ash, soot) with nearly 100% efficiency, is a fluidized bed scrubber.

^{Fluidized bed scrubber (with aux equipment)}

Accurate physical and mathematical calculation of gas cleaning systems of this class, based on Reynolds numbers, allows packing to achieve a specific turbulence state in which solid bodies begin to behave like liquids.

These devices are especially in demand in industries that emit dense smokes and hazardous fumes (boiler houses, heat and power plants, blast furnaces, municipal engineering).

Among other things, FBS is often used as the main air cleaning stage on large sea vessels of civil and military designation.

The floating bed scrubber is usually (but not necessarily) a vertical tower, the working compartment of which is filled with hollow polypropylene balls. The spray section irrigates the movable bed and fills the working compartment with liquid.

The polluted flow (smoke or steam) moves upwards the column and encounters an obstacle, which is a liquid barrier, in which light balls chaotically float. A distinctive feature of such scrubber systems is that the capture of unwanted components occurs simultaneously in the interfacial layer and in the liquid phase of absorbent.

Like other liquid-operated apparatuses, the scrubbers with a floating layer are capable of acting as a chemical filters, detaching hazardous, toxic, explosive and highly flammable substances from the stream.

In FBS, the absorbate-absorbent contact area is so high that practically every molecule of the pollutant is in contact with the reagent.

The principle of operation of the scrubber with a floating layer is clearly presented in the demo video.

Sludges from scrubber systems

A typical feature of all wet-type gas cleaning equipment is the production of slurry (semi-liquid pulp). Depending on the composition of the flow treated and the liquid reagent used for spraying (process water or chemical solution), a sludge, from which valuable materials can later be extracted, is formed.

^{The scheme of the flue gas filter operation, which results in the formation of artificial gypsum of high purity}

The importance of timeous maintenance and repair of scrubber systems

This paragraph largely refers to aspiration and ventilation systems, the main functor of which is a water scrubber.

It is known that a prerequisite for the reproduction of bacteria is a moist environment. Thus, if you do not periodically clean the water scrubber, as well as the connected air ducts and other related equipment, pathogens can develop inside the aspiration / ventilation system.

Of course, the probability of bacterial growth in air purifiers is almost zero. However, in 2005, in Norway, an outbreak of legionellosis killed 10 people, and 50 workers were seriously injured. The cause of the biological accident was the dilapidated and unsanitary air-cleaning system of the industrial enterprise.

^{X-ray of lungs affected by legionellosis}

Pressure grading

According to the international classification, industrial scrubbers / absorption units are divided into 3 categories, (depending on pressure):

• Ejector-type: up to 1200 Pa;
• Low / middle pressure: from 1200 to 3700 Pa;
• High pressure units: over 3700 Pa.

Other things being equal, we always try to design filters with the least pneumohydraulic resistance. This greatly simplifies the implementation of equipment into any gas / air paths.

Equipment engineering, manufacturing, delivery and commissioning in Asia and Europe

For any questions regarding the individual calculation, design and manufacture of gas or air scrubbers, please contact us by e-mail or fill out the Customer’s questionnaire.

We are ready to quickly deliver equipment to any point in Asia and Europe and, if necessary, to commission gas-cleaning facilities.

Please feel free to ask any questions you may have. We are open for fruitful cooperation.

PZGO LLC – breathe with ease!