Exhaust Gas Scrubbers

The supervisor turned on the switch for the exhaust gas scrubber in order to detoxify the MIC, but the scrubber had been closed for a maintenance inspection. The flare stack to detoxify the exhaust gas by combustion had also been removed for a maintenance inspection and was of no use. About 23t of MIC vapor tore out the fracture plate, broke the safety valve, and flew out of the exhaust-gas scrubber to a height of 33m for two hours without being detoxified by the scrubber or the flare stack. After erupting, the vapor crept downwind over the residents, supposedly because of the stabilized atmospheric conditions and because the vapor was heavier than air. 

There have been numerous discussions about this accident, which produced the most casualties in the history of industrial disasters. Some arguments revolve around the direct cause of the accident. As is generally known, many major accidents have been caused by combinations of small accidents. The accident in Bhopal also happened as the result of a combination of serious mistakes the mixing of water with MIC caused by neglecting to put the metal sheet in place to separate reactive components, and the failures in operation of the exhaust gas scrubber and the flare stack. Such cases are frequently found where a safety device is temporarily removed because the device is troublesome. It is necessary to educate people that the reliability of a safety device should be tested and that the failure of a safety device can lead to unexpectedly terrible results. 

It is not unusuel for exhaust gas scrubber Systems to incorporate more than cue type of spray contactor. In fact, combinations with trays and packed sections are used extensively. Figure 6.4 6 shows diagrams for four typical scrubber systems used for removing HF and SiF, from phosphoric acid plants. The following systems are illustrated. 

Nutrient Rerxn/eryFrom Gas Streams- A multistage scrubbing system captures fumes from the reactors and any dust that escapes the cyclones. The nutrient content of the scrubber liquor increases as it moves countercurrently throi Qh the reactor gas scrubbing system to the granulator exhaust gas scrubber. Nutrient salts in scrubber liquor are recycled to the reactors. There are no liquid effluents from the plant. 

Figure 6-1. Package type packed countercurrent exhaust gas scrubber. Interel Environmental Technologies, Inc.

Comparative Performance Data on Superphosphate Plant Exhaust-Gas Scrubbers... 

The copper-ammonium-salt solutions are generally not corrosive to mild steel however, the gases evolved during regeneration can be quite corrosive due to the presence of carbon dioxide. To prevent corrosion in the vapor zones, it is good practice to use stainless steel (vessels or liners) in the exhaust-gas scrubber section and above the liquid in the evaporator section. 

Residuals Produced Exhaust gases require acid gas and particulate removal through the use of a gas scrubber and the ashes may require solidification before landfilling. 

Sulfur dioxide can be removed from power plant exhaust gas by a scrubber s tem. One common method involves the reaction of SO2 with calcium oxide (lime) to form calcium sulfite S02(g) + CaO( ) CaS03 ( ) Unfortunately, scrubber systems are expensive to operate, and the solid CaS03 is generated in large enough quantities to create significant disposal problems. 

Environmental concerns of lime kiln operation chiefly relate to exhaust gas dust control and are usually solved by water scrubbing. The slaked lime (calcium hydroxide) produced by the scrubber can be employed as a base in the operations of a chemical complex, or sold. Electrostatic precipitation of precooled gases is also used, sometimes in series with water scrubbing. The dissociated carbon dioxide discharged is not ordinarily regarded as a pollutant. The amount discharged from this source is far less than the carbon dioxide produced by fossil fuel combustion. Lime kilns associated with Solvay ammonia-soda plants may be able to recycle a part of the dissociated carbon dioxide, particularly if the concentrations are raised by the use of oxygen-enriched combustion air. Recently, interest has been shown... 

FIGURE 12.5 Hydrogen fluoride removal from smelter primary gas collection system via a sieve tray gas scrubber (only a section shown). Note the recycle of acid product for initial hydrogen fluoride absorption from the exhaust gases to maximize the concentration of recovered acid. (Adapted from Ball and Dawson [33].)... 

RCRA also limits the amount of HCl contained in the thermal oxidizer exhaust to 41b/hr or less than 1%, whichever is less (40 CFR 264 and 265). Achieving this requires a quench process followed by an acid gas scrubber. The quench process lowers the gas temperature to the operating temperature of the acid gas scrubber, which must be below the boiling point of the acid gases to prevent revolatilization. Quench systems operate by spraying water into the gas stream to cool the gas as the water evaporates. 

The Portable Unit was designed to demonstrate the performance of the Shirco Infrared Incinerator in many thermal treatment applications. The construction details and process functions of the trailer-mounted incinerator are identical to a full-scale infrared incinerator. The system consists of a feed preparation system, an infrared primary chamber, a gas-fired secondary chamber, a wet gas scrubber, an exhaust system, heating element power centers (HEPC), and data acquisition and control systems. All equipment is enclosed within a 45-foot trailer. A schematic representation of the Portable Unit is shown in Figure 1. 

In many CVD processes, toxic, explosive, and corrosive materials are produced as one component of the vapor phase reaction co-product. In order to remove them prior to atmospheric venting, scrubbers are employed, which must be appropriate for the process used. For example, halides frequently are neutralized in a water scrubber. Carbon monoxide and hydrogen often are burnt. Arsine generally is removed by heating the exhaust gas in a cracking furnace. Charcoal canisters often are used to absorb vapor phase species. Sulfur has been employed to getter thallium. Very fine particle filters also are used to catch a diversity of solids entrained within the exhaust stream. 

The problem of sulfur dioxide pollution is made more complicated by the energy crisis. As petroleum supplies dwindle and the price increases, our dependence on coal will probably grow. As supplies of low-sulfur coal are used up, high-sulfur coal will be utilized. One way to use high-sulfur coal without further harming the air quality is to remove the sulfur dioxide from the exhaust gas by means of a system called a scrubber before it is emitted from the power plant stack. A common method of scrubbing is to blow powdered limestone (CaC03) into the combustion chamber, where it is decomposed to lime and carbon dioxide ... 

The exhaust gas from hydration of quicklime (ca. 800 m /t of hydrate) is generally the main source of dust emission from the process. Depending on the detailed design of the plant, it may contain dust concentrations of up to 2000mg/Nm. As the gas consists principally of steam, and as the process requires a controlled feed of water, many operators use wet scrubbers to remove the dust. Fabric filters are also used. By integrating them into the production unit, problems caused by condensation can largely be avoided. 

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