Cleaning of contaminated gas

In common chemical scrubber systems, the polluted gas is discharged or extracted to the scrubber by a centrifugal fan. The contaminated gas passes one or more stages for treatment. The gas is brought into very intense contact with the washing liquid containing the chemical reagents. As with other absorption processes, the components are first dissolved in the washing liquid and then react with the chemical additives. The cleaned gas is then discharged with a removal efficiency in a chemical scrubber more than 95%.

Mass transfer

Mass transfer of divergent phases, or so-called liquid-gas integration, in a scrubber is the most important parameter for the efficiency of the scrubber. No decent separation can be created without contact between washing liquid and the waste gas with the contaminants. Generally, it can be assumed that the larger the contact surface, the better the (absorption) liquid-gas integration is realized. Of course, the contact time is a significant factor. It is obvious that integration cannot take place properly if the residence time of the gas in the integration phase is (too) short.

Scrubbers therefore vary in their capacity for mass transfer and separation of fraction. The configuration and design of scrubbers must be performed very carefully, with the biggest determining factors to be considered: the required and available mass transfer based on the contact surface, the pressure drop, required removal efficiency and finally the reliability of the installation.

The absorption efficiency is a function of the available contact surface of all cumulated droplets for the exchange of the phases. An open-spray system scrubber and a packed bed scrubber can be designed to provide any desired removal efficiency. A scrubber with random packings (e.g., Pall Rings, Raschig Rings) with a large exchanging surface generally give an excellent integration of contaminants with the washing liquid. The disadvantage of high-density packings with a large contact surface have a significant higher pressure drop. A large part of the free volume in a packed bed is not available for gas flow and reclaims unusable volume and causes changes of directions of the gas flow that also increase the pressure drop. There are applications where this is acceptable such as laboratory setups, pilot plants or small installations that do not operate continuously where that the energy costs do not dictate a decisive role.

Scrubbers for large(er) flows or for contaminated gases containing a lot of solids (particles) or sticky components are basically not suitable for packings with a large contact surface and because of that a small free volume. In addition to a large pressure loss these packings mainly experience problems with contamination that can lead to clogging or blockages. The solution of this problem can be realized by providing the scrubbers with an open spray system. An open spray system contains no packings and use many nozzles that create many droplets so that the surface area of all cumulated droplets replace the contact surface of a packed scrubber. In addition, the energy potential of the droplets, due to their kinetic energy in relation to the gas velocity, ensures a huge integration process of liquid droplets with the contaminants in the gas. The pressure loss of an open spray system is much lower compared to a packed scrubber, remains unchanged low and has no problems with contamination or clogging.

Distribution of washing liquid

The distribution of solid particles may consist of a system perforated distributors and spray pipes which are only used in combination with random packings. The washing liquid is distributed over the entire surface and ensures a distribution of the liquid to achieve irrigated packings. Scrubbers with an open spray system (i.e., without structured or random packings) must compensate the lack of reaction surface by means of a large spray surface. The specific nozzles that are used for this purpose generate a spray pattern with exceedingly fine droplets which therefore generate an extremely large contact surface.