Reduction Design Considerations

The chemical reduction process is highly reliable for chrome reduction. The process, however, requires proper monitoring and control and proper pretreatment to control interfering substances. 

The most common chemicals used for chromium reduction and other chemical reduction applications are sulfur dioxide (SO2), sodium metabisulfite (Na2S205), sodium bisulfite (NaHSOg), and sulfuric acid (H2SO4). 

The chemical reduction process normally generates only small amounts of sludge due to minor shifts in the solubility of the contaminants. In a chromium reduction process, the reduced chromium and other metal ions are precipitated and removed in the subsequent precipitation-sedimentation process. An exception would be hexavalent chromium reduction with ferrous sulfate, where sludge generation may be significant. In a dechlorination process, there will be no chlorine residue, nor reducing agent residue, if the dechlorination process is operated properly. 

The chromium reduction process can be employed as batch treatment or continuous treatment. For small daily volumes of water or wastewater that are less than 150,000 L (40,000 gal), the most economical system is batch treatment in which two tanks are provided, each with a capacity of one day s flow. Reduction, precipitation, and sedimentation are carried out in one tank, while the other is used to collect the waste. In a typical batch system, the required dosage of acid and sodium metabisulfite is added to the tank and the contents are mixed for 15 min to ensure complete reduction of the chromium. 

Continuous chromium reduction treatment requires a tank for acidification and reduction with separate tanks for precipitation and sedimentation. The retention time in the reduction tank is dependent on the pH employed but should be at least four times the theoretical time for complete reduction. In cases where the chromium content of the wastewater varies markedly, equalization should be provided prior to the reduction tank to minimize fluctuations in the chemical feed system. Successful operation of a continuous chrome reduction process requires instrumentation and automatic control. Redox and pH control should also be provided. 

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Design considerations and costs of the catalyst, hardware, and a fume control system are direcdy proportional to the oven exhaust volume. The size of the catalyst bed often ranges from 1.0 m at 0°C and 101 kPa per 1000 m /min of exhaust, to 2 m for 1000 m /min of exhaust. Catalyst performance at a number of can plant installations has been enhanced by proper maintenance. Annual analytical measurements show reduction of solvent hydrocarbons to be in excess of 90% for 3—6 years, the equivalent of 12,000 to 30,000 operating hours. When propane was the only available fuel, the catalyst cost was recovered by fuel savings (vs thermal incineration prior to the catalyst retrofit) in two to three months. In numerous cases the fuel savings paid for the catalyst in 6 to 12 months. 

In addition to the active design principles for reducing the risk of sulfide problems, a number of more passive principles exist. The following design considerations are especially relevant for the reduction of corrosion selection of corrosion-resistant materials and design of a well-ventilated sewer system. 

In processes where materials are transported mainly in the fluid phase, such as most oil refinery units, the piping design considerations tend to dictate the layout, whereas for plants processing solids, such as metallurgical reduction plants, the bulk solids handling design is the major consideration. 

The basic biopotential amplifier described above, along with the specific design considerations for each biopotential, can yield a signal acquisition of acceptable quality in most laboratory settings. In practice, however, further enhancements are always necessary to achieve acceptable clinical performance in novel applications. These enhancements include circuits for reducing electric interference, filtering noise, reduction of artifacts, electrical isolation of the amplifier, and electrical protection of the circuit against defibrillation shocks [9]. 

The precipitation of these two compounds starts with pH values above 8.5. These values may not be reached by the bulk solution pH value, but are exceeded by the pH of the alkaline film that forms at the cathode during electrolysis, which is approximately pH 11. Accordingly Mg(OH)2 and CaCOa precipitate on the alkaline film. Most of the precipitates are carried out of the cell by the seawater stream due to the velocity (which is a design consideration for seawater hypochlorite cells), but a small amount may stick to the cathode. These precipitates in very small quantities depress the reduction of... 

Advise clients of their duties >- Take positive account of health and safety hazards during design considerations >- Apply principles of prevention during the design phase to eliminate, reduce or control hazards >- Consider measures that will protect all workers if either avoidance or reduction to a safe level is possible... 

The electrodes used for electrical conductivity (EC), oxidation-reduction potential ( ), cyclic voltammetry (CV), chronopotentiometry (CP), and anodic stripping voltammetry (ASV) were all solid state, and thus did not require the same design considerations as the membrane ISEs. The electrodes were polished and tested after fabrication, then calibrated and characterized, and remained as is until their use on Mars. ... 

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