Packed Bed Scrubbers Applications & Engineering

Monroe Environmental Packed Bed Scrubbers have been used effectively to remove a wide range of air pollutants in many industrial and municipal applications. Monroe Environmental has experience scrubbing many air pollutants in addition to those listed below. Depending on the application other scrubbing liquids for the listed pollutants may be more appropriate. A Monroe Environmental expert will determine the best solution for your process.

Acid Gas Scrubbing

Acid gasses are produced in many industries including battery production, semiconductor manufacturing, the chemical process industry, combustion processes, metallurgical industries, and the glass industry to name a few. Scrubbing with a neutralizing chemical, a base such as NaOH in aqueous solution, produces non-volatile, soluble salts that are removed by the blowdown process.

Hydrochloric acid (HCl) is a common combustion product of fuels containing the halide chlorine. HCl is scrubbed with NaOH in a Vertical or Horizontal Packed Bed Scrubber according to the reaction:

The salt, NaCl, is removed by the blowdown process before it precipitates and contaminates the packing. With sufficient packing height or multiple stages, efficiencies as high as 99.9% can be obtained. Since HCl is highly soluble in water, water alone may provide sufficient absorption of HCl gas in some applications.

Hydrogen cyanide (HCN) is another acid gas example. It is scrubbed with NaOH and sodium hypochlorite (NaOCl). The reactions between scrubbing liquid and acid gas are as follows:

Sulfur Dioxide Scrubbing

Sulfur dioxide (SO2) is common in flue gasses from many combustion processes. As an example, consider a high temperature gas stream containing SO2 that leaves an incinerator. Typically, Monroe Environmental will provide a Quench Tower in a high temperature application such as this before a vertical counter-flow Packed Bed Scrubber.

The quench tower has spray nozzles that add atomized water to the hot inlet gas stream to saturate it and cool it by evaporative cooling. In a typical example, a temperature drop of the inlet stream from 800°F to (150°F - 165°F) can be achieved by a Monroe Quench Tower. The gas stream then enters the Packed Bed Scrubber. Absorption of the gaseous SO2 by the recirculating NaOH and water scrubbing liquid is much higher at the lower temperature. The pH is monitored by a probe and typically maintained between 7.5 and 8.5 by the pH controller and chemical metering pump. The basic reaction steps are as follows:

Halogen Vapors (Br2, Cl2, F2)

Gas phase halogens, such as bromine (Br2), chlorine (Cl2), and fluorine (F2), are emitted in some industrial processes. For example, chlorine gas is produced by bleach plants, and fluorine gas is a by-product of semiconductor manufacture. The halogens are toxic, highly reactive, strong oxidizing agents. These gasses are efficiently controlled by Packed Bed Scrubbers. Effective scrubbing fluids include NaOH and KOH. Typical scrubbing reactions follow.

Bromine scrubbing:

Chlorine scrubbing:
(note: "X" below can be either Na or K)

Ammonia/Amines Scrubbing

Amine gasses are common pollutants emitted in fertilizer production, the manufacture of wood products, and wastewater treatment. These compounds are generally removed with a horizontal or vertical Packed Bed Scrubber. However, even though ammonia (NH3) is highly soluble in water, scrubbing that depends on physical absorption alone is not practical. Because of high ammonia (NH3) vapor pressure, solutions of ammonia (NH3) and water do not readily accept the transfer of gaseous NH3. Therefore re-circulated water is not effective for scrubbing. In addition, the cost of once through water is normally prohibitive. The scrubbing liquids of choice are low vapor pressure acids like sulfuric acid (H2SO4) or phosphoric acid (H3PO4). These provide the added benefit of chemical absorption. The use of sulfuric acid as a scrubbing fluid is described in the section Scrubbing of NH3 and H2S below. When aqueous phosphoric acid is used as a scrubbing fluid, the reaction is as follows:

Scrubbing of NH3 and H2S

The odors from both hydrogen sulfide (H2S) and ammonia (NH3) are both common problems occurring at wastewater treatment plant primary settling tanks. Monroe Environmental's one pass 3-stage counter flow vertical Packed Bed Scrubber efficiently removes these odors. The first packed tower uses aqueous sulfuric acid (H2SO4) as a scrubbing liquor to remove ammonia (NH3) according to the absorption reaction:

The salt formed, ammonium sulfate ((NH4)2SO4), is soluble and non-volatile and it is easily removed by the blowdown. The second packed tower uses sodium hydroxide (NaOH) as a scrubbing liquid to remove H2S according to the reaction:

The final stage uses both NaOH and sodium hypochlorite (NaOCl) as scrubbing liquids. NaOCl oxidizes the sulfur-containing compounds producing soluble non-volatile salt compounds according to the overall reaction:

Chromic Acid Scrubbing

Emissions from chrome plating and chromic acid anodizing operations produce hexavalent chrome mist. This mist cannot be efficiently removed in a conventional Packed Bed Scrubber. Monroe Environmental has developed a modified horizontal or vertical scrubbing unit using multiple specialized mesh filter pads to remove this pollutant. The pads are rinsed with fresh water and/or recycled water from the scrubber, depending on the application. Often plating chemicals washed off of the pads are returned to the rinse tanks of the plating process. Back to Top ↑

Ethylene Oxide (EtO or EO)(C2H40)

Sources of ethylene oxide (EtO) emissions include fugitive emissions from EtO production facilities and emissions from plants that use EtO in the production of glycols such as those used in the manufacture of antifreeze and polyester fibers. In addition EtO is commonly used by hospitals to sterilize medical supplies and in the sterilization of cosmetics and spices. EtO is flammable and toxic. A counter flow Monroe Environmental Packed Bed Scrubber or series of scrubbers can remove EtO at efficiencies greater than 99.9%, depending on the application. The scrubbing solution consists of water and a small amount (usually < 5%) of sulfuric acid (H2SO4). The sulfuric acid catalyzes a hydrolysis reaction:

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