Optimum operative temperature

For the start-up of a reactor, its core is allowed to heat up by withdrawal of the control rods until the recapture ratio is slightly greater than f.O. When the optimum operating temperature is reached, the control rods are inserted until the capture ratio is exactly f, and the reaction proceeds at a steady rate. For the shutdown of a reactor, its control rods are fully inserted, reducing the recapture ratio to nearly zero. 

Catalyst cost constitutes 15-20% of the capital cost of an SCR unit therefore, it is essential to operate at temperatures as high as possible to maximize space velocity and thus minimize catalyst volume. At the same time, it is necessary to minimize the rate of oxidation of S02 to S03, which is more temperature sensitive than the SCR reaction. The optimum operating temperature for the SCR process using titanium and vanadium oxide catalysts is about 38CM180oC. Most installations use an economizer bypass to provide flue gas to the reactors at the desired temperature during periods when flue gas temperatures are low, such as low-load operation. 

For reversible exothermic reactions the same three cases occur, as shown in Fig. 9.15. However, it can be seen that here there is an optimum operating temperature for the given r value where conversion is maximized. Above or below this temperature the conversion drops thus, proper control of heat removal is essential. 

The optimum operating temperature for minimum noise from the FET was found to be slightly higher than the operating temperature of the Ge(Li) detector. A thermal heater made from a 100-ohm carbon resistor was therefore installed next to the FET and operated at a current that heated the transistor to a temperature just above that of liquid nitrogen. 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

This is called the water-gas shift reaction or, frequently, just the shift reaction. The shift reaction is moderately exothermic that is, it releases heat. Optimum operating temperatures are low, usually below about 225°C. The reaction will not proceed appreciably unless catalyzed, traditionally by reaction over an iron/chromium catalyst. 

However cobalt oxide does have some drawbacks. Lower ammonia conversion efficiencies have been reported - as low as 88% to 92% in a high pressure plant compared with a typical value between 94% and 95% for Pt-Rh gauzes. The optimum operating temperature is 70 to 80°C lower than for Pt-Rh gauzes, and this could result in difficulties with the steam balance in a revamped plant. Cobalt oxide catalysts also suffer from reversible deactivation due to the reduction ofCo304 to CoO in the upper parts of the catalyst bed222. 

Several types of fuel cells have been developed and are classified according to the electrolytes used alkaline fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells (PAFCs), PEMFCs, and solid oxide fuel cells (SOFCs). As shown in Figure 1.3, the optimum operation temperatures of these fuel cells are different, and each type has different advantages and disadvantages. 

Determination of optimum operation temperature in sulfur dioxide converter. 

Although cost considerations and economic balances are the basis of most optimum designs, there are times when factors other than cost can determine the most favorable conditions. For example, in the operation of a catalytic reactor, an optimum operation temperature may exist for each reactor size because of equilibrium and reaction-rate limitations. This particular temperature could be based on the maximum percentage conversion or on the maximum amount of final product per unit of time. Ultimately, however, cost variables... 

Unfortunately, although a low temperature increases the yield of ammonia, the reaction becomes uneconomically slow, as now there are fewer sufficiently energetic collisions between the particles. Obviously the industrial chemist faces the problem of reconciling both the kinetic and equilibrium considerations for this reaction to make economical to operate. In practice a compromise is reached by selecting an optimum operating temperature and pressure (see the Haber Process on page 248). 

The optimum operating temperature for the silica gel beds would have to be selected after a cost study for each individual case. 

Temperature is an important factor for bacterial activity and Pings (1968) set 35°C as the optimum operating temperature, which makes Australia s Northern Territory ideal. In Canada, however, it was found that heating of intake air was justified during winter months and, in France, bacterial leaching was absmdoned on the grounds of low temperature (Mouret, 1971). 

Table 4 also shows that although the products obtmned wnth catalyst B contain lower total sulfur than with catalyst A, the mercaptan sulfiir content was significantly higher. The mercaptans were about 13 percent of the total sulfur at 220°C and increased with temperature to 80 percent at350°C. It can be inferred that catalyst B has higher HDS activity, but it also favors the formation of mercaptans by H2S-alkene recombination reactions. However, a maximum of 300°C was also observed for optimum catalyst performance. The data indicate that the optimum operating temperature is dependent on the feed composition and not the catalyst used (both catalysts were of Co-Mo type). 

For a material species graded approach, several kinds of thermoelectric material of which the optimum operating temperature is different fi-om each other are laid in order. Concerning composition graded and dopant concentration graded... 

Wlien the O2 concentration was increased to 8% the separation of the conversion curves for propylene and NO occured at a lower temperature (210°C), and the maximum NO conversion attained was only of the order of 40%. Also under these conditions the optimum operating temperature range was found to be narrower. 

Temperature of the lamp. Operation at a high power or high temperatures can increase emission intensity but, at the same time, reduce the lamp lifetime and lead to broadening of the atomic line profile, because of self-absorption and selfreversal effects. It has been found that the optimum operating temperature for mercury filling is 42 °C (for the 254 nm line) [35]. The output is reduced when the temperature is beyond the optimum. 

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