Temperature sequence

In a recent paper we used the temperature sequence of EXAFS measurements of the reduced catalyst In H2 to determine the temperature dependence of the disorder. (7 ) Comparable data was obtained for Ft metal over the same temperature range. The analysis proceeded by fitting the 1st coordination shell catalyst data to a 2-shell model In which the 1st shell was assumed to be that part of the Ft cluster... 

A second generalization is applicable to steps occurring in series. If an early step requires a high temperature and a subsequent step is favored by a low temperature, then a temperature sequence that decreases with increasing... 

M. N. Saha, R. H. Fowler, E. A. Milne and Cecilia Payne (later Payne-Gaposchkin) interpret stellar spectral sequence as temperature sequence with (mostly) constant chemical composition crude abundances derived. 

Design the optimal temperature sequence with respect to time per reactor length to obtain (a) a given fraction conversion, (b) a maximum rate of reaction, or (c) the minimum residence time. 

Fuel is cooled to the test temperature through a predetermined temperature sequence and is then caused to flow through a length of copper tubing and filter screen similar to that found in full-scale fuel handling equipment. The rate at which the fuel flows and the volume... 

Second, by actually monitoring the state of the material, it is possible to control the fabrication process by data rather than a procedure, such as a set time temperature sequence. This means one can have a self-correcting, automated, intelligent cure process that can adapt to variations in material age, fabric permeability, tool heat transfer characteristics, and the like. 

Sequential testing may be considered during development in addition to Procedure 1. The vibration test durations may be six (6) hours (2 hours per axis) at room temperature and three (3) hours (1 hour per axis) at each temperature extreme for a total test time of 12 hours for the three (3) temperatures. The temperature sequence of room temperature —65°F and 160°F is recommended... 

Figure I. Nitrogen sorption on Pittsburgh coal desorption points are solids solid curves indicate isotherms of ascending temperature sequence and broken curves, descending temperature sequence...

Figure 5. Methane isotherms on Pittsburgh coal determined in increasing temperature sequence desorption points are solid...

In a stirred-tank reactor, the reactants are diluted immediately on entering the tank in many cases this favours the desired reaction and suppresses the formation of byproducts. Because fresh reactants are rapidly mixed into a large volume, the temperature of the tank is readily controlled, and hot spots are much less likely to occur than in tubular reactors. Moreover, if a series of stirred tanks is used, it is relatively easy to hold each tank at a different temperature so that an optimum temperature sequence can be attained. 

We have previously derived a parameter called Reaction Severity (Rs) which provides a measure of the extent of reaction expected from a given time-temperature sequence (3). One could attempt to average the temperature over the period of reaction by integration of the product of temperature and time increments. 

For the case of continuous wall temperature profiles, a function fitting approach was used. Although there are no end conditions on Tw(z) to be satisfied, for all the cases considered in this paper Tw(z) is expected to be high near the entrance to the reactor and lower near the exit. Thus, with a decreasing optimal wall temperature sequence in mind, a Laguerre expansion (with exponential weighting) of the wall temperature was chosen. The expansion can be represented as... 

Figure 7. Nonequilibrium vaporization effect in the K O-AltOs-SiO system. -----represents pure KAlSiO phase. Run chronology for the partially decomposed KAlSiOf system follows the temperature sequence ABC (KMS data).

Figure 9. Selected K and O partial pressure data (KMS) for a synthetic Eastern MHD-channel slag with composition (K O wt %) 23.3—22.8 (AB interval) and 22.8—22.1 (BC interval). Run chronology follows the temperature sequence ABC. Key —O—, K data with increasing temperature run-chronology and - K data with decreasing temperature run chronology. — A —, O data, with increasing temperature run chronology and — A —, O data with decreasing temperature...

Figure 3- Chronological sequences of GC analysis A) temperature sequence B) time sequence.

Reactions were conducted in a 300-cm3, Magne Dash stirred autoclave equipped with a fitted glass liner. After the reactants were introduced, the system was flushed several times with a given gas, which was then adjusted to the required initial pressure. The valves were then closed, thus sealing the autoclave. The temperature sequence was a 30-45 min heating period to the desired reaction temperature, followed by a cooling period of about 3 h. The asphaltene-to-catalyst ratio was 1 1 by weight, and the actual proportions used were as follows ... 

It may be worth remarking in passing that Eq. (5.2.5) shows that a falling temperature sequence is always required with an exothermic reaction, for on differentiating this equation we have... 

IV. El < Ez, Ez> Ei In this case the same argument calls for an increasing temperature sequence. 

Aris, R. 1960a. On Denbigh s optimum temperature sequence. Chem. Eng. Sci. 12, 56-64. 

Now we can formulate basic assumptions of the OOA. Although in some papers, their authors mean to include underlying effects (JT distortions, temperature sequence of phase transitions, etc.), the following list represents what is actually done in a large number of papers applying the OOA to particular crystals. 

A very interesting application of powder MAS and MQMAS NMR to the investigation of the high-temperature sequence of phase transitions of LiRbS04 appeared.77 The central transition of 87Rb was investigated within 373-488 K, where five phases separated by four phase transitions are expected. The three main phase transitions affect the EFG parameters and the spin-lattice relaxation rate, and a critical behavior was evidenced and related to a damped soft mode. The MQMAS experiment permits to separate the chemical shift contribution from the second-order-induced quadrupolar isotropic shift in a single experiment, and indicates that the evolution of the chemical shift is almost continuous. Two different rubidium sites are clearly evidenced above 438 K, while X-ray could detect them only above 457 K. MQMAS data show a similar electronic environment for these two sites. 

Again the acetone conversions are essentially quantitative over the full temperature range considered (see column three. Table 2, for the typical temperatures in each of the three catalyst zones). However in this case, it is noteworthy that the IPA + DIPE molar selectivities exceed 98% and we are able to minimize the coproduction of C-3 gas, as well as C-6/C-9 condensation products (DIPE to C-3/-6/-9 wt ratio >30). Consequently, DIPE effluent concentrations reach ca. 38% when the catalyst temperature sequence is 104-159-146"C. Liquid recoveries in this experimental series were again essentially quantitative. 

Using the activation energy-temperature sensitivity analysis approach, determine possible temperature sequences for this scheme if S is the desired product. 

Even more complicated is the situation in real processing where different temperature sequences influence dramatically. A process simulation with a temperature sequence of 1100°C, 1230°C, 1100°C, 1200°C, and 1100°C yielded precipitation constants of 300 hrs, 5 hrs, 10 hrs, 3 hrs, and 5 hrs, respectively. 

---