Absorption Tower: Operating Principles, Types and Design, Materials of Construction, Absorbents, Auxiliary Equipment, Case Study

Absorption tower, due to its ability to simultaneously capture gases and fine particles, is widely used for air purification in the oil, gas, and chemical industries. This design provides gas cooling and can operate with a wide range of inlet humidity levels.

Packing tray efficiencies for absorption tower depend on the specific surface area of the absorber, its design (for trays: sieve, valve, or bubble cap), the wettability of the material (for packing) or the liquid layer height (for trays), the airflow volume, and the uniformity of the spray. For trays, the spacing between them also affects efficiency.

The following types of absorption tower are used for industrial air purification:
1. Venturi – a device consisting of a converging nozzle, a narrow throat, and an expanding cone. The gas is accelerated to a velocity of 120–150 m/s due to the constriction in the throat. At this velocity, air particles collide with liquid droplets, are captured by inertia, and coagulate. The velocity then decreases in the diffuser, and large droplets containing contaminants settle at the bottom of the structure.

2. Packed bed – a structure with a layer of packing made of randomly or orderly packed rings, mesh rolls, or blocks. Liquid falls onto the packed bed from the sprinklers above, flowing down the surface as a thin film. Air passes through the gaps between the elements of the absorption tower packing, and mass transfer occurs due to interaction with the film, trapping contaminants. The spent absorbent is collected at the bottom of the column.

3. Spray tower – a chamber with nozzles at different levels that deliver liquid under high pressure, atomizing droplets 100–500 µm in size. Air is injected into the tower from the bottom up or horizontally and interacts with a cloud of fine liquid droplets. The droplets settle to the bottom of the column under gravity.

4. Foam tower – a structure with one or more perforated trays onto which liquid is supplied. Air to be purified passes through the holes in the tray, creating a foam layer that interacts with the contaminant. The foam then flows from the tray, along with captured dust and dissolved gases, to the bottom of the column.

Different types of absorption tower are suitable for treating different contaminants, as shown in the table.

In addition, there are various absorber connection options.

An absorption tower diagram with absorber recirculation involves pumping the absorber's effluent back into the absorber via a cooler. This process removes some of the liquid from the system, replacing it with a corresponding amount of fresh absorber. This increases the reflux density and allows for heat removal in an external cooler.

A series connection of absorbers, used to achieve a deep (high) degree of purification or to handle multiple contaminants (for example, hydrochloric acid and hydrogen sulfide simultaneously), involves a counter-current arrangement, with pumps transferring the liquid from one absorber to another.

To increase the reflux density, a series connection of absorbers with absorber recirculation for each absorber can be used. In this case, the absorption unit in this diagram can be combined with a desorption unit. The selection of a specific scheme is carried out on an individual basis after analyzing the cleaning conditions and types of pollutants.

If the temperature of the aggressive environment does not exceed 60°C, the following plastics can be used for the housing: polypropylene, polyethylene, PVC, PVDF, or Teflon. They are completely resistant to alkalis, salts, and acids and are less expensive than steel. However, the service life of plastic housings is significantly shorter, as they are damaged by ultraviolet radiation and vibration.

Some industries, such as oil refining and the chemical industry, may require gas purification between compression stages. In such cases, an interpass absorption tower is used. Typically operating at elevated pressure, it is constructed entirely of steel, eliminating plastic.

Towers use physical and chemical absorbents. Physical absorbents dissolve the pollutant, while chemical absorbents react with it. The choice depends on the type of pollutant.

Alkaline solutions of sodium hydroxide, magnesium hydroxide, sodium carbonate, and the like are used to neutralize acidic gases. Acids such as hydrochloric, nitric, and sulfuric acids are used to capture ammonia.

A gas absorption tower may use liquid hydrocarbons to combat organic vapors, and oxidizers (ozone and potassium permanganate) to neutralize nitrogen oxides. An absorption tower propylene application typically involves specialized organic solvents such as methanol. Water is generally used to capture dust.

In addition to chemical compatibility with the pollutant, when selecting an absorbent, it is important to consider its wetting behavior on the packing (it should not evaporate or be too viscous), as well as the subsequent need for regeneration or disposal of the spent solution.

The following equipments for gas absorption tower is used to ensure the cleaning process:

• a pump for transferring absorbent from the bottom of the column to the spray device or to regeneration. If cleaning occurs in an aggressive environment, chemically resistant options are used;
• an air blower;
• a tank for collecting spent absorbent, located at the bottom of the column;
• control and measuring equipment (flow meters, pressure gauges, temperature sensors, level sensors, gas concentration sensors, pH meters);
• a regeneration system (to restore the absorbent's properties);
• a cooler for heat removal, if necessary;
• automatic control system devices (controllers, computers, SCADA systems).

One of the most important auxiliary systems is the regeneration mechanism. This can be thermal (heating the absorbent to its boiling point), physical (reducing the pressure, which causes the absorbed gas to separate from the liquid), chemical (adding a reagent), or biological (decomposition by microorganisms). The choice depends on the type of absorbent and pollutant. For example, thermal treatment is best for water and volatile pollutants, while chemical treatment with precipitation is best for non-volatile and toxic pollutants. However, in the case of toxic substances, regeneration may be uneconomical, so disposal is used instead.