Periodic operation temperature

After long periods of time at operating temperature, the britde—ductile transition temperature in autoclave steels increases (13). At temperatures much above 200°C for the solutions and fiHs used in ordinary hydrothermal processes, pressures and hence stresses in autoclaves can cause faHure of metal in the brittie state. OrdinarHy, the brittie region is weH below these temperatures but careful monitoring of the brittie—ductile transition of the steel is necessary for safe autoclave use over many years. 

Catalyst lifetimes are long in the absence of misoperation and are limited primarily by losses to fines, which are removed by periodic sieving. Excessive operating temperatures can cause degradation of the support and loss of surface area. Accumulation of refractory dusts and chemical poisons, such as compounds of lead and mercury, can result in catalyst deactivation. Usually, much of such contaminants are removed during sieving. The vanadium in these catalysts may be extracted and recycled when economic conditions permit. 

Unsteady-State Direct Oxidation Process. Periodic iatermption of the feeds can be used to reduce the sharp temperature gradients associated with the conventional oxidation of ethylene over a silver catalyst (209). Steady and periodic operation of a packed-bed reactor has been iavestigated for the production of ethylene oxide (210). By periodically varyiag the inlet feed concentration of ethylene or oxygen, or both, considerable improvements ia the selectivity to ethylene oxide were claimed. 

Unless there are solutes that are very strongly retained, and the maximum operating temperature of the system is reached without all the solutes being eluted, there is usually no need for a final isothermal period of any significant length. There is, however, one exception where a final isothermal period is helpful, and that is for mixtures that contain thermally labile materials. 

Energy can be conserved by operation of plant in such a way as to minimize part loading. Various practices can be adopted that can be described as good housekeeping . In addition to maintenance, this will include such factors as the avoidance of plant operating in the standby mode for long periods, operation at correct temperature, ensuring doors are closed where applicable, etc. 

It is desirable that a fluid s viscosity stays within the pump manufacturer s stipulated viscosity limits, in order to accommodate the normal variations of operating temperature. Oil s viscosity falls as temperature rises certain oils, however, are less sensitive than others to changes of temperatures, and these are said to have a higher VI. Hydraulic oils are formulated from base oils of inherently high VI, to minimize changes of viscosity in the period from start-up to steady mnning and while circulating between the cold and hot parts of a system. 

Batteries with a high operating temperature, of about 300 °C, require high-efficiency insulating jackets to maintain the temperature within an acceptable range. Especially in non operating periods, the battery should not cool down. It was requested that batteries should keep their operating temperature window for at lest four days. This requires heat loss rates of less than 200 W for a 40 kWh battery. 

As expected, heat exchanged per unit of volume in the Shimtec reactor is better than the one in batch reactors (15-200 times higher) and operation periods are much smaller than in a semibatch reactor. These characteristics allow the implementation of exo- or endothermic reactions at extreme operating temperatures or concentrations while reducing needs in purifying and separating processes and thus in raw materials. Indeed, since supply or removal of heat is enhanced, semibatch mode or dilutions become useless and therefore, there is an increase in selectivity and yield. 

Fig. 1 shows the thermal decomposition curves of HDPE mixed with Al-MCM-41, with respect to time, at isothermal operating temperatures. Lag periods were formed at the initial stage of decomposition, possibly due to the heat transfer effect, which could delay the decomposition of a sample until the latter reaches the operating temperatures. As the reaction ten erature increased, the reaction time became noticeably shorter. The shortening of the reaction time was clearly observed when the reaction occurred at the reaction teirperatures between 420 and 460 °C. The HDPE on Al-MCM-41-P decomposed faster than that on blank and that on A1-MCM-41-D, as shown in Fig. 1(b). 

Estimate the parameters (pmax, Ks, kd and Y) for each operating temperature. Use the portion of the data where glucose is above the threshold value of 0.1 g/L that corresponds approximately to the exponential growth period of the batch cultures. 

Several uncertainties in this periodic process have not been resolved. Pressure drop is too high at SV = 10,000 h 1 when packed beds of carbon are used. Study of carbon-coated structured packing or of monoliths with activated carbon washcoats is needed to see if lower pressure drops at 95% SO2 removal can be achieved. Stack gas from coal or heavy oil combustion contains parts-per-million or -per-billion quantities of toxic elements and compounds. Their removal in the periodically operated trickle bed must be examined, as well as the effect of these elements on acid quality. So far, laboratory experiments have been done to just 80°C use of acid for flushing the carbon bed should permit operation at temperatures up to 150°C. Performance of periodic flow interruption at such temperatures needs to be determined. The heat exchange requirements for the RTI-Waterloo process shown in Fig. 26 depend on the temperature of S02 scrubbing. If operation at 150°C is possible, gas leaving the trickle bed can be passed directly to the deNO, step without reheating. 

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. 

Some experimental studies (1-7) have demonstrated the possibility of improving the performance of a catalytic reactor through cyclic operation. Eenken et al. (4) reported an improvement of 70% in conversion of ethylene to ethane under periodic operation. In a later article (2), they concluded that periodic operations can be used to eliminate an excessively high local temperature inside the catalytic reactor for a highly exothermic reaction. In our laboratory, Unni et al. (5) showed that under certain conditions of frequency and amplitude associated with the forced concentration cycling of reactants, the rate of oxidation of SC>2 over catalyst can be increased by as much as 30%. Re-... 

According to the literature [77], a process is considered to be low hazard from the thermal standpoint if the normal operating temperature or temperature due to upset is 50°C or more lower than the ARC onset temperature, and the maximum process temperature is held for only a short period of time. However, other factors must be considered in evaluating the thermal hazard of a process such as total enthalpy of reaction or decomposition, potential for reactant accumulation, the boiling point of the reaction mass, and the rate of reaction. The testing must involve all appropriate materials including reactants, intermediates, and products. In some cases, though, the 50°C differential... 

A. The pioneering period—1977 to 1984. The ability of some metal carbonyls to catalyze the water gas shift reaction at low operating temperatures was discovered in the late 1970s, and several groups published heavily in this area at the time. For example, Pettit et a/. 18 3439 Ford et al., i i,32,60,66,73,86-88 Eisenberg et aL.2<>21M... 

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