Bromine and Fluorine: Bromine (Br2) and Fluorine (F2) are typically scrubbed in a counter current packed tower to achieve maximum efficiencies using a dilute aqueous alkali solution. For applications with very high concentrations of these gases it may be useful to use a Jet Venturi Fume Scrubber as a first stage to remove the bulk of the contaminant then “polish” with a packed tower scrubber. With respect to F2, the concentration of the alkali in the aqueous solution must be maintained at a specific level to avoid the generation of undesirable by-products. The alkali generally preferred is KOH due to the greater solubility of KF in water than NaF.
Chlorine: Chlorine (Cl2) is often scrubbed with dilute aqueous NaOH in a Counter Current Packed Tower. Water scrubbing is impractical. The use of a Jet Venturi Fume Scrubber may be advantageous where Cl2 concentrations are high, however a single stage system is rarely enough used alone. Integrated jet venturi and packed tower systems are supplied to industry leaders for safe and efficient control of toxic chlorine gas.
NaOCl, which may be unstable depending upon its concentration and solution temperature, is one of the reaction products in this process. A relatively strong alkali solution is preferable therefore, to maintain stability.
Hydrogen Bromide and Hydrogen Iodide: The general method of scrubbing described below for Hydrogen Chloride would also apply to these gases. However, there is little opportunity to recover these acid gases in an aqueous solution.
Hydrogen Chloride: The sources of HCl range from simple HCl holding tank exhausts to process exhausts where HCl may or may not be mixed with toxic gases, to thermal oxidizer exhausts where chlorinated organics are incinerated.
In most instances HCl is scrubbed either with a Jet Venturi Fume Scrubber (typically achieving 95% removal efficiency in a single stage), or with a Counter Current Packed Tower (able to achieve efficiencies exceeding 99.9% removal). If a gas stream contains high concentrations of HCl, a Jet Venturi Fume Scrubber may be followed by a Packed Tower. The Jet Venturi removes the bulk of the HCl and the Packed Tower. Systems using recycled water, concentrating acid to 10 wt%, have been supplied to clients that have on-site use for dilute acid by products.
Aqueous Sodium Hydroxide (NaOH) is commonly used as a scrubbing fluid. If, however, HCl is concentrated in a gas stream, it may be practical to utilize a multiple stage approach. The initial stage scrubs and recovers usable HCl and the final stage utilizes the NaOH to meet low ppm discharge concentrations.
Hydrogen Cyanide: HCN is typically scrubbed with an aqueous NaOH solution however, in certain circumstances, the addition of Sodium Hypochlorite (NaOCl) to the solution increases performance and/or produces desirable reaction products.
Hydrogen Fluoride: The general method of scrubbing described above for Hydrogen Chloride would also apply to Hydrogen Fluoride (HF). Due to its vapor pressure characteristics in an aqueous solution, however, it is less likely to be recovered in a usable form. Because the product of the reaction of HF with NaOH (NaF) has limited solubility in water, it is usually more practical to use a KOH (potassium hydroxide) aqueous solution for scrubbing. Its reaction product (KF) has significantly greater water solubility.
Nitric Acid: The scrubbing of Nitric Acid is similar to the scrubbing of HCl described above both in the use of aqueous NaOH and in the potential for recovering HNO3. Often, NOx gases will be formed in the presence of concentrated HNO3. NOx gas requires more specific methods of scrubbing. See NOx section.
Sulfur Dioxide: The Clean Air Group, LLC has a wealth of experience in the treatment of Sulfur Dioxide (SO2). Applications are almost always either process or combustion source related. SO2 is readily scrubbed in either a Jet Venturi Fume Scrubber (up to 98% efficiency), or a Counter Current Packed Tower (99.9+% efficiency) utilizing aqueous NaOH. The Clean Air Group, LLC has also designed removal systems utilizing Jet Venturi Fume Scrubbers recirculating “lime” slurries or carbonate liquors. The fact that solids exist in these alkali liquids does not prevent the use of Jet Venturi Scrubbers since this design is open and difficult to plug.