Wet Gas Scrubber
What is it?
Wet scrubbing (or absorption) is a mass transfer between a soluble gas and a solvent – often water – in contact with each other. Physical scrubbing is preferred for chemical recovery, whereas chemical scrubbing is restricted to removing and abating gaseous compounds. Physico- chemical scrubbing takes an intermediate position. The compound is dissolved in the absorbing liquid and involved in a reversible chemical reaction, which enables the recovery of the gaseous compound.
Depending on the pollutants to be removed, several aqueous scrubbing liquids are used including the following:
Depending on the pollutants to be removed, several aqueous scrubbing liquids are used including the following:
- Water, to remove solvents and gases such as hydrogen halides or ammonia, with the main aim being to recover and reuse these contaminants.
- Alkaline solutions (e.g. caustic soda – i.e. sodium hydroxide – and sodium carbonate), to remove acidic compounds such as hydrogen halides, sulphur dioxide, hydrogen sulphide (H2S), phenols, chlorine.
The pH value of the alkaline scrubber depends on the pollutant to be removed; pH is often kept between 8.5 and 9.5 (for SO2 removal a pH range of 6.5– 7.5 is needed, whereas for H2S removal a pH of 10 or more is required). The pH value should not be too high because of absorption of CO2 in the water. A pH value of 10 and above will cause the dissolved CO2 to be present in the water as carbonate, causing the alkaline consumption rate to increase dramatically. The calcium carbonate will also deposit on the gaskets, increasing the pressure drop. To avoid this, softened water can be used in an alkaline gas scrubber. - Alkaline oxidative solutions , i.e. alkaline solutions with oxidants such as sodium hypochlorite (NaOCl), chlorine dioxide (ClO2), ozone (O3) or hydrogen peroxide (H2O2); these are particularly indicated for the removal of odours.
- Sodium hydrogen sulphite solutions , to remove odour (e.g. aldehydes).
- Acidic solutions , to remove alkaline compounds, e.g. ammonia, amines and esters. The dosing of the acid is done by means of pH regulation. In most cases, the pH is kept between 3 and 6. Sulphuric acid (H2SO4) is often the acid of choice for economic reasons. For specific applications, for example the removal of NH3, nitric acid (HNO3) is used.
- Monoethanolamine and diethanolamine solutions , suitable for the absorption and recovery of hydrogen sulphide.
- Organic solvents with low volatility , e.g. chilled nonane for the recovery of light VOCs such as butanes and pentanes.
- Fibrous packing scrubbers;
- Moving-bed scrubbers;
- Packed-bed scrubbers;
- Impingement plate scrubbers;
- Spray towers.
- Requirements for performance efficiency,
- Energy needs,
- Reagents,
- Properties of the waste gas stream.
Design, maintenance and efficiency
An optimum design of scrubbing systems to achieve low exit concentrations includes high reliability, automatic operation and a countercurrent flow of liquid and gas. Scrubbers are commonly operated with precoolers (e.g. spray chambers and quenchers) to lower the inlet gas temperature and simultaneously saturate the gas stream, thus avoiding reduced absorption rates and solvent evaporation. Such additional devices exert low pressure drops.
The abatement efficiency of gas scrubbers is dependent on the residence time of the gas in the absorption section, the type of packing used, the liquid to gas ratio (L/G), the refreshing rate, the temperature of the water and the addition of chemicals.
In the case of alkaline-oxidative scrubbers, abatement efficiency depends on the oxidisability of the compounds and the residence time in the scrubber. An increase in the residence time requires larger installations and higher investment costs. Pilot tests are essential for achieving good design.
The consumption of scrubbing water depends to a great extent on the incoming and outgoing concentrations of gaseous compounds. Evaporation losses are primarily determined by the temperature and the humidity of the incoming gas stream. The outgoing gas stream is, in most cases, completely saturated with water vapour.
Scrubbing generates waste water which needs treatment, if the scrubbing liquid with its content is not otherwise used.
Routine measurement is necessary for:
When using an alkaline oxidative scrubber with NaOCl, toxic chlorine fumes might be formed at low pH values. An alkaline scrubber might then be placed in series to the alkaline oxidative scrubber to remove these chlorine fumes.
Absorption is enhanced by:
The abatement efficiency of gas scrubbers is dependent on the residence time of the gas in the absorption section, the type of packing used, the liquid to gas ratio (L/G), the refreshing rate, the temperature of the water and the addition of chemicals.
In the case of alkaline-oxidative scrubbers, abatement efficiency depends on the oxidisability of the compounds and the residence time in the scrubber. An increase in the residence time requires larger installations and higher investment costs. Pilot tests are essential for achieving good design.
The consumption of scrubbing water depends to a great extent on the incoming and outgoing concentrations of gaseous compounds. Evaporation losses are primarily determined by the temperature and the humidity of the incoming gas stream. The outgoing gas stream is, in most cases, completely saturated with water vapour.
Scrubbing generates waste water which needs treatment, if the scrubbing liquid with its content is not otherwise used.
Routine measurement is necessary for:
- The pressure drop across the scrubber, as a means to discover operational anomalies that might require maintenance;
- The scrubber make-up water flow rate;
- The recycle water flow rate;
- The reagent flow rate;
- In some cases pH, temperature, electrical conductivity and reduction potential.
When using an alkaline oxidative scrubber with NaOCl, toxic chlorine fumes might be formed at low pH values. An alkaline scrubber might then be placed in series to the alkaline oxidative scrubber to remove these chlorine fumes.
Absorption is enhanced by:
- Larger contacting surfaces;
- Higher liquid/gas ratios;
- Higher concentrations in the gas stream;
- Lower temperatures.
Applicability
Alkaline oxidative scrubbing is reported to be used in the sectors such as the food industry, slaughterhouses, flavouring agents production and the textiles industry.
Table 1 shows application limits and restrictions associated with wet scrubbing (adapted from EIPPCB, 2016, Table 3.307).
Table 1. Application limits and restrictions associated with wet scrubbing.
Table 1 shows application limits and restrictions associated with wet scrubbing (adapted from EIPPCB, 2016, Table 3.307).
Table 1. Application limits and restrictions associated with wet scrubbing.
| Issue | Limits/ Restrictions |
|---|---|
| Gas flow (Nm3/h) | 50-500000 |
| Temperature (°C) | 5-80 |
| Pressure (MPa) | Atmospheric |
| Pressure drop (mbar) | ~12 |
| Particulates content (mg/Nm3) | <10 |
