Packed-bed scrubbers consist of a chamber containing plastic random packing material, such as KemFlo’s Kempac plastic tower packing (Tripack, Pall Ring or Plastic Saddle), that provide a large surface area for liquid-particle contact. The packing is held in place by a bed limiter and supported by a packing support. Scrubbing liquid is evenly introduced through a liquid distributor located above the packing and flows down through the plastic scrubber packing. The liquid coats the plastic tower packing and establishes a thin film. The pollutant to be absorbed must be soluble in the fluid.
In packed-bed scrubbers, the gas stream flows up the chamber (countercurrent to the liquid). Some packed beds are designed horizontally for gas flow across the packing (crosscurrent). Physical absorption depends on properties of the gas stream and liquid solvent, such as density and viscosity, as well as specific characteristics of the pollutant(s) in the gas and the liquid stream. These properties are temperature dependent, and lower temperatures generally favor absorption of gases by the solvent. Chemical absorption may be limited by the rate of reaction. Water is the most common solvent used to remove inorganic contaminants. Pollutant removal may be enhanced by manipulating the chemistry of the absorbing solution so that it reacts with the pollutant. Caustic solution (sodium hydroxide, NaOH) is the most common scrubbing liquid used for acid-gas control (e.g., HCl, SO2, or both), though sodium carbonate (Na2CO3) and calcium hydroxide (slaked lime, Ca[OH]2) are also used.
Absorption is a commonly applied operation in chemical processing. It is used as a raw material and/or a product recovery technique in separation and purification of gaseous streams containing high concentrations of organics (e.g., in natural gas purification and coke by-product recovery operations). In absorption, the organics in the gas stream are dissolved in a liquid solvent. The contact between the absorbing liquid and the vent gas is accomplished in counter current spray towers, scrubbers, or packed columns. The use of absorption as the primary control technique for organic vapors is subject to several limiting factors. One factor is the availability of a suitable solvent. The VOC must be soluble in the absorbing liquid and even then, for any given absorbent liquid, only VOC that are soluble can be removed. Some common solvents that may be useful for volatile organics include water, mineral oils, or other nonvolatile petroleum oils. Water is used to absorb VOC having relatively high water solubilities. Amphiphilic block copolymers added to water can make hydrophobic VOC dissolve in water. Other solvents such as hydrocarbon oils are used for VOC that have low water solubilities, though only in industries where large volumes of these oils are available (e.g., petroleum refineries and petrochemical plants). Another consideration in the application of absorption as a control technique is the treatment or disposal of the material removed from the absorber. In most cases, the scrubbing liquid containing the VOC is regenerated in an operation known as stripping, in which the VOC is desorbed from the absorbent liquid, typically at elevated temperatures and/or under vacuum. The VOC is then recovered as a liquid by a condenser.
In packed-bed scrubbers, the gas stream is forced to follow a circuitous path through the plastic tower packing material, on which much of the PM impacts. The liquid on the plastic tower packing collects the PM and flows down the chamber towards the drain at the bottom of the tower. A mist eliminator is typically positioned above the packing and scrubbing liquid supply. Any scrubbing liquid and wetted PM entrained in the exiting gas stream will be removed by the mist eliminator and returned to drain through the plastic random packing.
Advantages of packed-bed towers include:
- Relatively low pressure drop;
- Capable of achieving relatively high mass-transfer efficiencies;
- The height and/or type of packing can be changed to improve mass transfer without purchasing new equipment;
- Relatively low capital cost;
- Relatively small space requirements; and
- Ability to collect PM as well as gases.
Wet scrubbers generate waste in the form of a slurry. This creates the need for both wastewater treatment and solid waste disposal. Initially, the slurry is treated to separate the solid waste from the water. The treated water can then be reused or discharged. Once the water is removed, the remaining waste will be in the form of a solid or sludge. If the solid waste is nontoxic, it can generally be landfilled. Hazardous wastes will have more stringent procedures for disposal. In some cases, the solid waste may have value and can be sold or recycled. For more information on Kempac plastic random packing please visit our website at www.kempac-packing.com.