Cooling peak temperature

Superposing a 13 6 kW cooling peak load on the modelled average medium temperatures a 7 h peak load can be accommodated in year 5, in year 10 only 5 h and in year 25 just 2 h can be accommodated. 

Figure 8.4fc shows another type of selective control system. The signals from the three temperature transmitters located at various positions along a tubular reactor are fed into a high selector. The highest temperature is sent to the temperature controller whose output manipulates cooling water. Thus, this system controls the peak temperature in the reactor, wherever it is located. 

A major goal in wall cooling is to spread out the hot zone and prevent very high peak temperatures. High peak temperatures cause poor reaction selectivity, cause carbon formation, deactivate catalysts, and cause corrosion problems in the reactor walls. CocuJTent flows spread out the hot zone and cause lower peak temperatures, but many additional design features must be considered in designing jacketed reactors. 

The P-T-t paths of metamorphic rocks may be much more complicated. They are usually heated first to a peak temperature and then cooled to room temperature. There are a variety of metamorphic rocks. Figure l-18c shows a hypothetical cooling history of an ultra-high-pressure metamorphic rock, which was subducted to great depth and then returned to the surface. Before subduction, the premetamorphic rock could be a basalt or a sedimentary rock. From 0 to 2 Myr, the slab is modeled as being subducted at 0.08 m/yr and an angle of 45°. At 2 Myr,... 

Crystallinity was determined using differential scanning calorimetry. About 5-10 mg of an experimental agent was heated from 25 to 200°C at a heating rate of 20°C/ minute. The sample was isothermed at 200°C for 1 minute and then cooled at a cooling rate of 20°C/minute to ambient temperature. Crystallization data represents peak temperatures of exotherms in the cooling cycle and are summarized in Table 1. 

By necessity, asteroid thermal models must make assumptions about model parameters and initial conditions. However, models based on 26A1 heating have been remarkably successful in reproducing asteroid peak temperatures and cooling histories. 

Thermal metamorphism in chondrites and melting in differentiated asteroids are driven by heat produced by the decay of short-lived radionuclides (especially 26A1). Thermal models can reproduce the peak temperatures and cooling rates estimated for meteorites, as well as... 

The performance of the convention control structure, in which cooling water flow is manipulated to control temperature, is shown in Figure 3.52. The disturbance is the same increase in cooling water temperature. Feed flowrate is constant. The cooling water flowrate more than doubles to control reactor temperature, but the temperature is returned to the desired value in about 2 h. The peak deviation in temperature is less than 0.6 K. Controller settings are those given in Table 3.2 for the 95% conversion case with a 330 K reactor temperature (the integral time is 50 min). 

There are five fundamental differences between CSTRs and tubular reactors. The first is the variation in properties with axial position down the length of the reactor. For example, in an adiabatic reactor with an exothermic irreversible reaction, the maximum temperature occurs at the exit of the reactor under steady-state conditions. However, in a cooled tubular reactor, the peak temperature usually occurs at an intermediate axial position in the reactor. To control this peak temperature, we must be able to measure a number of temperatures along the reactor length. 

The third difference is the issue of heat transfer in nonadiabatic reactors. Ideally we would like to be able to control the temperature at each axial position down the reactor. However, it is mechanically very difficult to achieve independent heat transfer at various axial positions. About all that can be done is to have the cooling/heating medium flow either cocurrent or countercurrent to the direction of the process flow. The only two variables that can be manipulated are the flowrate of the medium and its inlet temperature. The former is the normal manipulated variable. The result is that only a single temperature can be controlled, which can be the peak temperature or the exit temperature. However, because of the significant dynamics of the tubular reactor, the control of these temperatures is sometimes quite difficult and tight control cannot be achieved in the face of load disturbances. 

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