Mixing Temperature Measurement

This is perhaps one of the most difficult operations in mixing, and there have been no particular advances here for many years. Two types of tliermocouple have, and are still being, used. The first, and most widely used, utilises a strong thermocouple probe fitted with a thermocouple junction, and extending into the mixing chamber at some point. The second type is an infrared temperature measurement system, fitted to the mixer body to see into the mixing chamber. 

In the tangential machine a thermocouple mounted in the end frame is more robust, but is often far less responsive than one mounted in the door top, although recent rotor designs which improve material flow to the ends of the mixing chamber should have improved the response of endframe thermocouples. Infrared probes are most commonly fitted in an endframe. 

It is impossible to measure accurately the compounding temperature in a mixer. In truth the temperature 

With all these competing influences, it is surprising that the thermocouple is of any use whatsoever, but in fact readings are remarkably consistent. 

Compensating (lhennocoti]3le) cable Chtomitiin plated sheath (thickness t ariable) Thenn(x ouple bod (carbon steel) 

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B. Yip, P.M. Danehy, R.K. Hanson Degenerate four-wave mixing temperature measurements in a flame. Opt. Lett. 17, 751 (1992)... 

The temperature in the hottest part of the kiln is closely controlled using automatic equipment and a radiation pyrometer and generally is kept at about 1100—1150°C (see Temperature measurement). Time of passage is about four hours, varying with the kiln mix being used. The rate of oxidation increases with temperature. However, the maximum temperature is limited by the tendency of the calcine to become sticky and form rings or balls in the kiln, by... 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Several experimental techniques may be used, such as acid/base titration, electrical conductivity measurement, temperature measurement, or measurement of optical properties such as refractive index, light absorption, and so on. In each case, it is necessary to specify the manner of tracer addition, the position and number of recording stations, the sample volume of the detection system, and the criteria used in locating the end-point. Each of these factors will influence the measured value of mixing time, and therefore care must be exercised in comparing results from different investigations. 

A similar experiment was conducted using N-299 carbon black. In this case the premastication was limited to 3 min of mixing time. The average batch temperature measured after this mixing operation was 309°F. Each experiment was performed in duplicate the average of two mixes is shown in Figure 16.6. The viscosity of the final control compound was similar to that of the premasticated mbber. 

The temperature of the reaction mix was measured by a stainless steel-sheathed thermocouple inserted through the reactor cap. Heating up and cooling down times were small compared with the total reaction time. In all cases the free space in the reactor was flushed with nitrogen before sealing, and the reaction proceeded under a small initial nitrogen pressure. 

Synthetic chemists desire well defined reaction conditions. Process chemists demand them. Nonuniform heating and difficulties with mixing and temperature measurement are technical constraints that initially limited the scale of microwave chemistry with dry media and have not yet been overcome. Poor reproducibility also has been reported, probably resulting from differences in performance and operation of individual domestic microwave ovens [13-15]. Consequently, most, if not all, of the disclosed applications of dry media are laboratory-scale preparations. However, as discussed in other chapters, this does not prevent their being interesting and useful. 

Because the compositions are basic, the expanding minerals are trioctahedral and they are apparently associated in all facies with chlorite. The occurrence of a regularly interstratified montmorillonite (saponite) -chlorite mineral, corrensite, is typified by an association with calcic zeolites and albite. Temperature measurement in the "hydrothermal" sequences at several hundred meters depth indicate that the ordered, mixed layered mineral succeeds a fully expandable phase between 150-200 C and this ordered phase remains present to about 280°C. In this interval calcium zeolites disappear, being apparently replaced by prehnite. The higher temperature assemblage above corrensite stability typically contains chlorite and epidote. 

A gas-free solution was prepared by mixing methanol, water, and salt, each of which was separately degassed or desorbed. The methanol and water were each boiled in a flask with a reflux condenser under a reduced pressure to remove dissolved gases in the solvent. Then they were introduced into constant-temperature measuring devices out of contact with air. The amounts of both solvents were adjusted to accommodate a desired composition of the mixture. 

From these publications, workers interested in exploring the microwave technique perceived it to be simultaneously beneficial through increased rates, yet hazardous in the presence of flammable organic solvents. Subsequently, a vast body of work was carried out with domestic microwave ovens, but under solvent-free conditions and without recourse to sample mixing or temperature measurement. This continued across a broadening front on the laboratory scale. These and other developments in microwave chemistry have been reviewed extensively in journals, book chapters4-20 and in a recent monograph21. 

The first oil-catalyst contact is essential. The mixing temperature (about 530-600 °C) is very difficult to measure and is about 20-80 °C higher than the riser exit temperature (4, J). The superficial gas velocity at the inlet is several meters per second, much higher than the terminal velocity of the catalyst, which is about 0.20 m/s. The catalyst and the gas are transported upwards together but at a different superficial velocity, u (6). These two velocities are related by the slip velocity (sv), defined as follows ... 

Also, the susceptibility of a material to microwave energy can depend on variables including sample size, shape and dielectric properties as well as the location of the sample within the cavity [13]. Until recently, a lack of facilities for mixing reactions and for measuring temperature had affected the reproducibility of dry media reactions between microwave systems. That circumstance should be largely overcome through commercial reactors that enable sample mixing and temperature measurement. 

The state of mixing in a given reactor can be evaluated by RTD experiments by means of inert tracers, by temperature measurements, by flow visualization and, finally, by studying in the reactor under consideration the kinetics of an otherwise well-known reaction (because its mechanism has been carefully elucidated from experiments carried out in an ideal reactor, the batch reactor being generally chosen as a reference for this purpose). From these experimental results, a reactor model may be deduced. Very often, in the laboratory but also even in industrial practice, the real reactor is not far from ideal or can be modelled successfully by simple combinations of ideal reactors this last approach is of frequent use in chemical reaction engineering. But... 

Mixing of different water types may occasionally be detected by temperature measurements, as, for example, a drop of temperature caused by the arrival of snowmelt recharge seasonally added to a regional base flow. Temperature measurements are most useful for detecting intermixing, especially when coupled with chemical and isotopic measurements, a topic discussed in sections 6.6 and 6.7. 

The numerous reasons which can account for various deviations from the ideal FFF retention theory were discussed in the corresponding sections. Here, additional problems are treated which can complicate FFF measurements and significantly distort the results obtained. General requirements for a successful FFF measurement include precise flow control and flow rate precise temperature measurement precise determination of t0 and tr correct relaxation procedure control of sample overloading and integrity and control of mixed normal and steric retention effects as well as wall adsorption control. Some of these complications cannot be avoided so one must correct for these effects, usually in a sem-iempirical and partially very complicated fashion. 

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