Wet-type gas-filtration and dust-collecting equipment, design, principles of operation and basic characteristics

Wet gas cleaning – as a gas purification technology, as well as air pollution control method – rightfully occupies one of the places of honor among the many others technological approaches available today in the field of capturing and neutralizing / extraction harmful and undesirable substances from air and gas flows.

Wet-type gas cleaning systems, which were first developed in the era of the industrial revolution of 18-19 century, are constantly being improved and pushing less efficient approaches out of the market.

To date, there are several basic techniques of wet (water) disposal of mechanical and chemical pollutants, which are actively used in all spectra of the industry.

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Cyclone spray scrubbers with falling film

Radial-flow wet cyclone-type scrubbers, despite their relative simplicity, provide the discernible efficiency in capturing mechanical impurities, smoke emissions and exhaust gases from any sources of dust as well as from hydrocarbon and fossil fuels combustion. Usually wet scrubbers are made in the form of vertical columns, but, depending on the specific working conditions and other aspects of gas cleaning activities, they can be made in inclined or combined versions.

The principle of action of wet cyclonic scrubber repeats the principle of action of a cyclone dust collector. The main difference from a dry cyclone is the presence of a spray unit that forms a “falling film” on the inner surface of the column.

A chemically or mechanically contaminated stream is tangentially injected into the column through the inlet. The diametrical cross section of the column provides turbulence of the raw gas, which actively interacts with the inner surface of the column.

^{Water-type high-speed cyclonic dust collector / air filter}

In parallel with this, the nozzle block installed above the level of the gas inlet atomizes water or reagent, which are deposited in the form of microfilm on the inner surface of the tower.

Thus, a double interaction occurs:

1. Chemical and / or mechanical particles of the pollutant directly interact with the micro-fog (water or reagent) formed. The continuous supply of gas or air carries the micro-fog to the walls of the column, where it is deposited on the micro-film.
2. The raw gas particles, radially spinning in the column, "stick" to the liquid falling film covering the inner surface of the column.
3. Under the force of gravity, the scrubbing liquid with trapped pollutants falls down into the sludge receiving tank.
4. Mist that has not settled on the falling film, rises up the column where it is stopped by an additional demister.

Venturi scrubber

Venturi gas and dust cleaning systems are also based on the spray principle. The main element of such devices, providing a high degree of purification of gas flows from pollutants, is the so-called Venturi tube. As a rule, this is a steel pipe, having a convergent (tapering) zone in the middle.

^{Compact, flexible and versatile Venturi scrubber}

The principle of operation of the Venturi dust collector

1. Contaminated air is brought to the Venturi tube, where, due to the conical narrowing of the tube (Bernoulli Effect), it accelerates to high speed. In some models of gas scrubbing machines of this type, the flow rate at the convergent-divergent zone can reach hundreds of meters per second.
2. At the end of the throat zone of the pipe there is one (or several) spray nozzles. Highly accelerated air collides with water droplets and breaks (dissipates) them into microscopic mist particles.
3. It is at this stage of high turbulence that unwanted compounds (usually of a mechanical nature) are bound on the surface of the micro-droplets.
4. The Venturi tube is designed so that after a conical narrowing (throat) the gas enters the chamber of a larger diameter, which leads to instantaneous deceleration and condensation of the micro-droplets. Increasing in size and mass, the droplets collide with the drip catcher, fall down and are discharged into the sludge receiving tank.

^{Purpose-built giant Venturi cone produced at PZGO LLC}

Bubble absorbers

In general, bubble absorbers are vertical columns with one or more perforated or lattice trays – the so called bubblers.

Design and principle of operation of the foam absorber

Contaminated air mixture, fed from the bottom, collides with the liquid supplied to the bubble plates and foams water or reagent.

^{Foam-type gas absorber}

Particles of fine dispersion (and chemical compounds), unable to break through the outer liquid layer of bubbles, are deposited on the surface, while larger particles penetrate inside and collapse the foam bubbles, turning it back to the “heavy” liquid.

A key factor in the capture of undesirable substances is a layer of unstable foam, which actively traps contaminants in the interfacial layer (liquid-gas).

The level gauge installed in foam absorbers controls the overflow level in such a way as to precisely balance the resistance of the incoming flow relative to the foam layer.

Like other devices for wet gas cleaning, bubble absorbers are equipped with a drop separator, which cuts off the mist-like absorbent and sends it back to the circulation area.

Industrial bubble absorbers are often referred to as wet chemical gas cleaning equipment. This is due to the fact that the contact of unwanted or harmful inclusions with the absorbent is much higher than that of traditional falling-film spray scrubbers. In addition, the design of the device allows you to load into the reagent tank almost any liquid absorbent. This gives the foam absorber the properties of operational versatility and allows the use of such devices in many industrial fields that have gaseous chemical pollutants as waste.

^{Square-section bubble column (at the stage of assembly, PZGO LLC, Russia)}

Packing-containing gas absorbers

Both gas scrubbers and wet absorbers can involve in their design a chamber filled with a movable or static bed. Below we will take a closer look at the technical features and principles of bed-containing apparatuses (typically, movable-bed scrubbers are mainly (but not necessarily) aimed on dust collecting, while fixed bed absorbers usually referred to as chemical equipment).

Fixed bed scrubbers

Fixed-bed absorbers can be attributed to the more advanced generation of wet-type gas cleaning apparatuses.

Such devices have even greater flexibility and efficiency in purifying polluted gas or air streams: strong acids, alkalis and alcohols, solvents, exhausts after fuel oil boilers, CHP, chemical waste disposal, combustion of charcoal, wood, oil products and other organics and inorganics.

Packing material elements are geometric bodies of a certain topology, which provides a high specific area within a small volume – rings (Raschig, Pall), chords, saddles, spheres and their spatial modifications. Depending on the degree of contamination, aggressiveness and acidity of the absorbate, the packing can be made of ceramics, aluminum-containing alloys, zeolites, copper, nickel, Hastelloy, monel, and special micro-porous plastics.

^{Multi-stage stationary-bed scrubber system (main structural material of the air-cleaner is inert polypropylene)}

Thus, packed scrubbers demonstrate all the advantages of bubble absorbers and, in addition, can optionally affect the chemical compounds through a chemical neutralization reaction provided by the packing (filling) material.

Also, they can work as dust collectors (for smooth dusts), trapping mechanical particles formed in the combustion process - soot, ashes, and cinders.

The packing layer is called fixed or stationary, because the weight and density of the filtering bodies is high enough for them to be moved by the ascending flow.

Since this type of gas cleaning equipment often has a chemical purpose, the material of the column and auxiliary units can be lined steel, polypropylene (PP), low-density polyethylene (LDPE), stainless steel, fluoroplastic, titanium. The proper material is selected at the design stage and agreed with the Customer in advance.

^{Compact and high-performance (30 000 m3 / h) wet-type PP-bodied gas scrubber (if possible technically, we try to manufacture all gas-filtration and dust-catching devices in a compact form factor)}

Mobile / Fluidized bed absorbers

Perhaps the most promising gas-cleaning and fine dust arresting apparatus of wet type is a mobile- or fluidized bed absorber. In general, it repeats the technological and structural design of the packed absorption tower, but during the operation of the unit, the packed bed is mostly present in a suspended, pseudoliquid (or floating) state.

^{Demonstration stand of the FBS' working chamber, showing the floating of hollow polypropylene balls in a layer of liquid solvent}

This is because the forces acting from the flow's direction are balanced by the gravitational forces (weight) of the packed bed elements (usually hollow PP-balls, granules or grains). This causes the filter particles to “levitate” inside the column, creating a highly active, interfacial layer, which effectively captures almost any chemical compounds and mechanical inclusions with dispersion of 0.5 microns and above. This approach, among other things, provides a high mass- and heat exchange of processed media.

This approach is flexible and versatile, as the gas cleaning efficiency remains high even during significant pneumohydrodynamic drops / jumps.

Mobile bed absorbers can simultaneously capture the widest range of gaseous, aerosol and mechanical pollutants: acid gases (nitrogen oxides NOx, sulfur oxides SOx, hydrochloric acid, hydrofluoric acid), carbon monoxide, phenols, benzpyrene, non-combustible hydrocarbons (CH), mercury, tellurium, cadmium, heavy metals, furans, dioxins as well as big amounts of soot, ash and slag.

^{High-performance FBS scrubber installed within the industrial site of the energy sector enterprise}

This makes such devices the most attractive solution for industries associated with large amounts of emissions resulting from combustion or thermal decomposition of hydrocarbon fuels: CHP, boiler houses, oil refineries, asphalt concrete factories. The same applies to large sea vessels: tankers, bulk carriers, container ships, warships and civilian ocean liners.

Equipment manufacturing, delivery and installation

For even more information, please visit the products section or the articles section of our website. You can also directly contact the PZGO Company in the way that is convenient for you or fill out the Customers Questionnaire.

LLC PZGO is ready to provide fast delivery of equipment to any region of Europe or Asia. If necessary, we will carry out a local turnkey installation and commissioning.

Each unit comes with a full set of documentation. We gladly consider any aspects of professional cooperation.

LLC PZGO – breathe with ease!