Gas Absorption

Gas absorbers or scrubbers are widely used in the industry for the separation and purification of gas streams, as product recovery systems and as equipment for the control of pollution. This article focuses on the application of absorption for the control of pollution in gas streams with typical polluting concentrations ranging from very low concentrations to a maximum of 10,000 ppm. Gas absorbers are most used to remove water-soluble inorganic contaminants from gas flows.

Absorption is a process in which one or more soluble components of a gas mixture are dissolved in a liquid (i.e., a solvent). The absorption process can be categorized as physical or chemical. Physical absorption occurs when the absorbed compound dissolves in the solvent; chemical absorption occurs when the absorbed compound and solvent react. Liquids commonly used as solvents include water, mineral oils, non-volatile hydrocarbon oils, and aqueous solutions.

Efficiency and performance

Removal efficiency of gas absorbers vary by solvent (scrubbing liquid) and with the type of absorber used. Most absorbers have removal efficiencies of more than 90 percent, and packed column absorbers can achieve efficiencies of up to 99.9 percent for some polluting solvent systems. The suitability of gas absorption as a method of pollution control generally depends on the following factors:

1. the availability of suitable solvent (washing liquid);
2. required removal efficiency;
3. polluting concentration in the inlet vapor;
4. capacity required for the treatment of waste gas;
5. recovery value of the pollutants or the disposal costs of the solvent consumed.

The physical absorption depends on the properties of the gas flow and solvent, such as density and viscosity, as well as specific characteristics of the pollutants in the gas and the liquid flow (for example, solubility in equilibrium). These properties depend on the temperature, at lower temperatures in general the solvent absorbs the gases better. The absorption is also enhanced by a larger contact surface, higher liquid-gas ratios, and higher concentrations in the gas flow.

The solvent chosen for the removal of the pollutants must have a high solubility to the gas, low vapor pressure, low viscosity and must be relatively inexpensive. Water is the most common solvent used to remove inorganic contaminants; it is also used to absorb organic compounds with relatively high-water solubility. For organic compounds with low water solubility, other solvents such as hydrocarbon-based oils are used, but only in industries where large quantities of these oils are available (i.e., petroleum refineries and petrochemical plants).

The removal of pollutants can also be improved by manipulating the chemistry of the absorbent solution so that it reacts with the pollutants, for example, an alkaline solution for acid-gas absorption versus pure water as a solvent. Chemical absorption can be limited by the speed of the reaction, although the speed limiting step is usually the physical absorption rate, not the chemical reaction rate.

Process description

Absorption is a mass transfer principle in which one or more soluble components of a gas mixture are dissolved in a liquid with low volatility under the given process conditions. The pollutant spreads from the gas into the liquid when the liquid contains less than the equilibrium concentration of the gaseous component. The difference between the actual concentration and the equilibrium concentration provides the driving force behind the absorption.

A well-engineered scrubber ensures intensive contact between the gas and the solvent to facilitate the absorption of the pollutants. It will perform much better than a poorly designed gas scrubber. The rate of mass transfer between the two phases largely depends on the exposed surface and the time of contact. Other factors applicable to the absorption rate, such as the solubility of the gas in the specific solvent and the degree of the chemical reaction, are characteristic of the components involved and are relatively independent of the equipment used.

Scrubber configuration

Gas and liquid flow through an absorber can be countercurrent, cross-current, or co-current. The most installed designs are counterflow, where the waste gas stream enters at the bottom of the gas scrubber column and goes out at the top. Conversely, the washing liquid enters at the top and is drained at the bottom.

Countercurrent designs provide the highest theoretical removal efficiency, as gas with the lowest pollutant concentration encounters liquid with the lowest pollutant concentration. This serves to maximize the average driving force (solubility, diffusion) for absorption through the column. In addition, counterflow designs usually require lower liquid-gas ratios than co-current columns and are more suitable when the pollutant load is higher, the solubility is less good and function much more efficiently in relation to very small (dirt) particles.

In a crossflow column, the waste gas flows horizontally across the column while the solvent flows vertically through the column. As a rule, crossflow designs have lower pressure drops and require lower liquid-gas ratios than both co-flow and counterflow designs. They are applied when gases are highly soluble, as they provide less contact time for absorption. In co-current columns, both the gas and the scrubbing liquid are entered at the top of the column and the exit is located at the bottom. Co-current designs have lower pressure losses, suffer less from flooding but are less efficient for fine (i.e., submicron) fog removal. Co-current designs are only efficient where large absorption driving forces (high solubility) are available. The removal efficiency is limited as the gas-liquid system approaches balance at the bottom of the tower.

Types of absorption systems

The descriptions of the following systems are almost all based on the counter-current principle as this is the most common principle because of the higher efficiency with the current emission guidelines. Systems based on the absorption principles include columns with a packed bed, columns with dishes, venturi scrubbers and open spray systems.

1. Packed columns
   Packed towers are columns filled with packing materials that provide a large surface area to facilitate the contact between the liquid and the gas. Packed towers can achieve higher removal efficiency, handle higher fluid velocities, and require relatively lower water consumption than other types of absorbers. However, packed towers can also generate a high differential pressure, have high clogging and pollution potential, decent maintenance costs due to the packing. Installation, operation, and wastewater disposal costs may also be higher for packed columns than for other absorbers. In addition to pump and fan power requirements and solvent costs, packed towers have operating costs because of replacing the damaged gasket.
2. Tray columns
   Plate or tray columns are vertical cylinders in which the liquid and gas are brought into contact with each other in a stepwise manner on trays (plates). Liquid arrives at the top of the column and flows over each plate and through a downspout (downcomer) to the plates below. Gas moves up through openings in the trays, bubbles in the liquid, and goes to the tray above it. Tray columns are easier to clean and can cope better with large temperature fluctuations than packed columns. However, with large gas flow rates, tray columns show a greater pressure loss and have larger liquid hold-ups. Tray columns are generally made of materials such as stainless steel, which can withstand the force of the liquid on the trays and provide corrosion protection. Packed columns are preferred over tray columns in case of acids and other corrosive materials because the column construction can then be manufactured from fiberglass, polyvinyl chloride, or other less expensive, corrosion resistant materials. Packed columns are also preferred in case of columns smaller than two meters in diameter and when pressure drop is an important consideration.
3. Venturi scrubber
   Venturi scrubbers are generally used to counter particulate matter, dust, and sulfur dioxide. They are designed for applications with high removal efficiency of submicron particles, between Ø 0,5 and Ø 5,0 μm. A venturi scrubber uses a gradually converging and then tangential section, called the throat, to clean incoming gaseous flows. Liquid is either introduced into the venturi upstream of the throat or injected directly into the throat where it is atomized by the gaseous flow. Once the liquid is atomized, it collects particles from the gas and drains it from the venturi. The high pressure drop by these systems results in high energy consumption, and the relatively short liquid-gas contact time limits their application to only the most soluble gases. Therefore, they are rarely used for the separation of volatile organic compounds emissions in the diluted concentration.
4. Open-spray columns
   Open-spray systems work based on the difference in speed potential of liquid droplets through a distribution system with nozzles and the incoming gas. The droplets then fall under the influence of gravity due to a counter-current gas flow and become in contact with the polluting particles in the gas. The droplets are formed under pressure, and this creates a very fine mist with a very large reaction surface, creating a very efficient capture of particles. The columns, like the packed columns, can be simple in construction and are therefore less complex than the dish columns and venturi scrubbers. The spray columns work relatively easily and are very maintenance-friendly and have quite low energy costs.