Packed Bed Scrubbers – Applications & Engineering

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:

The salt, sodium cyanate (NaCNO), is less toxic and can be removed by the blowdown process. The pH is monitored by a pH probe. The pH controller runs the scrubbing liquid chemical metering pumps and maintains pH between 9 and 11 to prevent cyanogen chloride (CNCL) or nitrogen trichoride NCl3 formation. return to top

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:

One mole of NaOH reacts with and causes absorption of one mole of SO2, though the resulting mixture of sodium salts formed will vary. The carefully chosen packing material provides a large surface area for reaction. The liquid water, SO2 in solution, and salts fall into the tank and are removed. The gas stream then passes through the moisture eliminator pad or chevrons in the top of the packed tower, which removes the remaining water particles from the gas stream before it leaves the scrubber. Removal efficiency greater than 99% can be achieved. return to top

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)

The halogen gas is chemically absorbed according to the appropriate reaction, and the resulting product liquids can be removed by a drain or blowdown. Makeup water addition prevents the concentration of salt products from increasing to the point of precipitation. return to top

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:

High vapor pressure acids are not used since they can react with the NH3 in the gas phase to form submicron salt particulate. return to top

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:

Na2SO4 and NaCl are removed by the blowdown. Overall efficiency for removal of NH3 and SO2 is typically greater than 99% for the process. Depending on which wastewater odors are present and the efficiency required, one or two stages may be all that is needed. In some cases, H2O2 can be used instead of NaOCl as a scrubbing liquid for removing H2S. return to top

Nitrogen oxides are common products of boilers, kilns, fermentation plants, coal burning plants, municipal waste combustion, and oil & gas production. Most nitrous fumes (NOx) are a mixture of nitric oxide (NO) and nitrogen dioxide (NO2). NOx can be scrubbed with water or with an alkali solution of water and NaOH or KOH, depending on efficiency requirements and if the customer wants to reclaim the acid product resulting from water scrubbing. The reactions are as follows:

If NaOH is also used further reactions are:

If KOH used rather than NaOH, substitute "K" for "N" in the last 2 reactions. return to top

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. return to top

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:

The principal product (C2H6O2) is ethylene glycol, which is relatively inert and may be recycled for use as antifreeze. Monroe Environmental will design a scrubbing system that ensures adequate dwell time to achieve reaction completion. return to top

Greenhouse gas emissions are an ever-increasing environmental problem. A packed tower scrubber can efficiently remove CO2 from an air stream using NaOH or KOH as scrubbing fluids. The reactions are:

To ensure reaction completion, the pH should be kept above 10. A pH control system designed by Monroe Environmental will keep pH in the desired range by using a pH probe, caustic chemical metering pump, and pH analyzer. return to top

Volatile Organic Compounds (VOC) are carbon-based compounds that become gaseous at room temperature. Many pose health risks. VOCs are commonly produced in fuel oil combustion, kraft paper mills, pharmaceutical facilities, sewage sludge incineration, surface coating operations, and many other chemical and industrial processes. Packed Bed Scrubbers are very efficient, up to 99.9% or higher, in the removal of soluble VOCs.

Formaldehyde (CH2O) is a common VOC that is produced in plywood and particle-board manufacture, paper and textile manufacture, and plastic production. Typically Monroe Environmental will use hydrogen peroxide (H2O2) to scrub formaldehyde. The reactions are:

For some applications, if the amount of water used is not of great concern, hydrogen peroxide or other chemical additives may not be required. Depending on efficiency requirements, once-through water can effectively scrub formaldehyde, methanol, acetone and other water-soluble organic solvents, which generally can't be removed efficiently using re-circulated water due to their high vapor pressure. return to top