VOC and VIC abatement: the scrubber!

Let’s discover more about VOC and VIC abatement analyzing the wet scrubbing machines (washing columns)

To address the issue of VOC and VIC abatement is necessary to understand what these acronyms mean.

VOC stands for Volatile Organic Compounds and it regards all of those substances belonging to the organic chemistry, also known as the chemistry of carbon. It is generally relative to hydrocarbons which have, at room temperature, a vapor pressure higher than 0,01 KPa. For more information please read our article related thereto.

VIC is actually the acronym for Volatile Inorganic Compounds. Practically we speak of chemical compounds that do not belong to organic chemistry and have the same physical characteristics of VOCs.

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The inorganic compounds then are all those chemical compounds that belong to the periodic table of elements, including  carbon also in the carbon monoxide and carbon dioxide molecules.

Some examples of typical VIC are ammonia (NH3), hydrogen sulphide (H2S) rather than the carbon dioxide (CO2). From these examples we can understand that volatile inorganic compounds can be divided into three main categories useful to understand how they can be treated.

The VIC can therefore be basic (such as ammonia), acids (such as vapors of hydrochloric acid or nitric acid) or VIC reduced (such as hydrogen sulphide) that have the need to be oxidised for their improved abatement.

Litmus test

Now that it’s clear what are VOCs and VIC, we try to understand how you can break down these compounds.

On this article we will focus on VIC as volatile organic compounds (VOCs) because the associated abatement techniques have already been widely discussed in the relevant article.

Volatile inorganic compound: The scrubber and its working principle

The VIC are specifically treated with wet scrubbing machines, calls also washing columns, where the substances are absorbed by the recirculating liquid added of reagents suitable for the removal of pollutants.

The wet scrubbers are nothing more than “showers” where the polluted effluent is washed from the recirculating liquid in order to absorb the pollutants.

Reagent types

The solutions used for this process are characterized by a specific pH that determine its acidity or basicity. Conventionally pH value is inside a range from 0 (maximum acidity) to 14 (maximum basicity). At 7 the pH is neutral: typical value of water at a temperature of 25°C.

The acqueous solution used inside scrubbers will have a pH suitable to abate the pollutants. The basic VIC are reduced thanks to an acid reagent, while the acid VIC are reduced through the use of a basic reagent.

To guarantee the abatement efficiency, the solution contained into the tank under the scrubber, is linked to reagent tank. In this way, a special pump transfers the reagent directly inside the tank in order to compensate the pH values.

The following image could be used to better understand the working principle of the scrubber:

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Two stage abatement system

As you can seen, the polluted air due to the production process is aspired through a pipe, it flows inside the fan and then is treated inside the two scrubbers.

The process in the picture is a two-stage abatement system where the air enters inside the first scrubber where take place the reduction of basic compound like ammonia, to than flows in the second tower designed to remove the acid compound like hydrogen sulfide. As already explained, both scrubbers are equipped by tanks where the related solution is contained. As seen before, basic pollutants are removed by acid solution so the water inside the first scrubber must have a pH value lower than 7. In the second tower the pH will have a value higher than 7 because the acid pollutants will be removed by basic reagent.

Oxidant reagent

To maintain this conditions, the tanks are connected to reagent tanks where are stored the acid or basic reagents use to stabilize the pH.

The plant can be equipped also with oxidant reagent in order to reduce the bacterial load before the emission.

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Different scrubbers for different solutions

From a plant engineering point of view, the different solutions adopted are used to stabilize a large contact surface to the gas to be treated. They include:

  1. the scrubber tower (or “washing columns’), constituted by a vertical cylindrical body in which the gaseous effluent is introduced upstream (from below) with respect to the liquid. The washing towers may be divided into three subcategories:
    • packed columns, in which the high contact area is ensured by the presence of filler bodies made with a special shape, size and  materials.
    • tray columns, in which the gas-liquid contact area is ensured by one or more stages of perforated plates where on the surface a layer of liquid is maintained, and where bubbling the effluent.
    • Floating bed columns, in which the gas-liquid contact area is provided by a bed of very light balls, placed in continuous movement by a fluid dynamics air push.
Tower
  1. venturi scrubber, in which the washing liquid is forced to pass through a Venturi cone (“throat”) where the liquid effluent are placed in intimate contact to facilitate the absorption of the pollutants in the liquid.

The venturi scrubbers are used for the removal of particle material while the washing towers fully perform the reduction of the pollutants.

The horizontal scrubbers are more compact machines (even if they have a footprint in a broader plant of vertical one), and they have the big advantage of containing also three absorption beds. This peculiarity allows to treat within a single machine also different categories of VIC but with a low concentration. This condition is due to the fact that abatement performance are tipically lower than vertical scrubbers.

Our experience

Tecnosida® has developed a specifical system for the abatement of VIC called WETCLEAN.

In some applications, you can also use a dry technology and injection of reagent. Tecnosida® has developed a filtration system for the combustion of biomass.

See you soon with new interesting blog!

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