Characteristics of the Tholander counter-current process in keywords

The principle of the counter-current scrubber is the separation of contaminants from the gas phase into the liquid phase. The waste gas streams upwards through a packed bed and gets in contact with the downwards flowing washing liquid which is sprayed out from the irrigation system placed above the packing bed. Thus, the gas to be cleaned flows in counter-current to the washing liquid.

Main function of the packing material hereby is to increase the transfer of the contaminants from the gas phase into the liquid phase. This is possible due to the very high active surface able to hold up a big amount of washing liquid. Plently of droplets are built on the surface of the packing material, segmented and re-shaped constantly. Through this intense contact between waste gas and washing liquid, the contaminants are separated, accumulate in the liquid phase and can be eliminated.

Counter-current systems of gas and liquid circulation lead to the highest contaminants transmission capacity.

To ensure the legal limits are not exceeded, we use our proved packing material. Compared to usual spraying towers the filling material used in our scrubbers, allows a higher removal efficiency of the pollutants at same height which is due to the following technical advantages of the packing media:

• extended gas/liquid contact
• homogenous distribution of the dissolved contaminants in the liquid phase
• homogenous surfacic repartition of the air

The transfer of the pollutants from the gas phase into the liquid phase is thus increased just as the number of transfer units finally leading to a very high removal efficiency of the contaminants. In addition to that, the release of insufficiently cleaned waste air is avoided.

Another advantage of our scrubbers is that the operating expenses are much lower compared to conventional spraying towers achieving a similar removal efficiency only by increasing the nozzle pressure which causes higher energy costs. Having a lower nozzle pressure in the packed scrubber thus allows to use low-maintenance nozzles being less sensitive against mud.

There are a lot of different packing materials available. Depending on the specific application we will choose the most suitable type. Selection criteria are e.g. the inner surface or the sensibility against adhering dirt.

The scrubber sizing mainly depends on how well the pollutant to be cleaned transfers from the gas into the liquid phase. The transfer velocity increases significantly, the

• better the solubility of the pollutant is
• the lower the pollutant concentration in the washing liquid is
• the higher the transfer surface is

All three parameters can be influenced:

1. The solubility depends on the solvent used and can be improved decisively by pH variation of the washing liquid
2. There are different possibilities to reduce the pollutant concentration in the washing liquid:

• Blow down and refilling with fresh washing liquid
• Dosage of oxidants e.g. oxidation of sulphide to sulphate
• Reaction to salts e.g. ammonia
• Continous regeneration of the washing liquid e.g. by use of microorganisms in a bioscrubber

3. The transfer surface can be increased by choosing filigree structured packing material or by increasing the packing height. Should the washing liquid contain substances adhering to the packing material or should the liquid be run with high salinity, the increased packing height is to be preferred to the filigree structures to avoid clogging.

Raw gas composition, flow rate and required clean gas value are some of the criteria we consider when designing our scrubbers. Apart from the variation of the scrubber dimensions, we have different packing media available. In addition to that, the regeneration of the washing liquid is another method to optimize the removal efficiency of the pollutants.

We guarantee:

• High separation results / high efficiency
• Low investment and operation costs
• High flexibility against variable waste gas amount
• Optimal consumption of fresh water and added chemicals
• Little space demand / compact size
• Low pressure loss
• Low risk of plugging due to the structure of the packing material
• Easy overview and easy handling of the display

The addition of chemicals for regeneration of the washing liquid is made by special dosing stations. Their function is to oxidize the pollutants or to influence their solubility via pH variation. Dosing stations require low maintenance and are very reliable systems.

Using dosing stations allows to be independent from variable waste gas amounts since the conditions in the washing liquid are kept constantly through controlled dosing of chemicals.

Depending on the required chemical and weather resistance, our scrubber is either built of glass fibre reinforced plastic (FRP), stainless steel or of thermoplastic materials (such as PE or HDPE).

An interesting, resource-saving option is the possible use as bio scrubber. The microorganisms suspended in the washing water, continously clean the washing liquid thus achieving the optimal solution gradient of the pollutants. Using our scrubber in a biological way especially makes sense for cleaning of well degradable organic single substances (e.g. alcohols, acetone) as well as for cleaning of complex waste air having an undefined composition and being very odour-intensive at the same time.

• Chemical industry
• Metal processing
• Food industry
• Composting
• Sewage treatment
• Waste treatment