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:

SO₂ + H₂O      SO₂H₂O

SO₂H₂O + NaOH      NaHSO₃ (a salt) + H₂O

Na₂SO₃ + NaOH      Na₂SO₃ (a salt) + H₂O (in the presence of excess NaOH)

NaOH neutralizes absorbed SO2, producing a mixture of soluble sodium salts. 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.

Halogen Vapors (Cl₂, F₂, Br₂)

Gas phase halogens, such as 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 manufacturing. 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.

Chlorine Scrubbing:

(note: “X” below can be either Na or K)

Cl₂ + 2XOH      XCl + XOCl + H₂O
(at < 20° C)

3Cl₂ + 6XOH      5XCl + XClO₃ + 3H₂O
(at ≅ 75° C)

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.

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:

2NH₃ + H₂SO₄      (NH₄)₂SO₄

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:

H₂S + 2NaOH      Na₄S + 2H₂O

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:

H₂S + 4NaOCl + 2NaOH      Na₂SO₄ + 4NaCl + 2H₂O

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.

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.

Ethylene Oxide (EtO or EO)(C₂H₄O)

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:

C₂H₄O + H₂O      C₂H₆O₂

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.