Rapid isothermal processing

RIP rapid isothermal processing RTA rapid thermal annealing... 

In a typical isothermal process, 70% hydrogen peroxide is added to 98% sulfuric acid, and subjected to rapid stirring and efficient cooling, so that the temperature does not rise to above 15°C. If equimolar quantities of reactants are used, the product contains 42% H2SO and 10% H2O2. Although the reaction may seem simple, many of its features are critically important and it should only be attempted foUowiag advice from speciaUsts. 

An Illustration is given in fig. 2.17, which concerns the adsorption of octa-decanol on graphite. A monolayer is formed with a herringbone-like structure, i.e. a real two-dimensional phase with well-defined domain boundaries. In the adsorption isotherms (not shown) this is reflected by an almost vertical ascent (see also fig. 2.28). An interesting aspect of this study is that information on the dynamics of the adsorbate is also obtainable. The adsorbed molecules can flip spontaneously between two different tilt angles and by scanning rapidly this process can be observed the arrow in fig. 2.17b marks the position of the scan when the flip occurred. From the scan rate the authors concluded that the switching time is much less than 10 s. 

Now, as it is known, Stirling cycle consists of two isochoric processes and two isothermal processes. At finite time, the difference between the temperatures of reservoirs and the corresponding operating temperatures is considered, as shown in Figure 3. To constmct expressions for power output and ecological function for this cycle, some initial assumptions are necessary. First, the heat transfer is supposed as Newton s cooling law for two bodies in thermal contact with temperatures Tt and T(, Tt>T r with a rapidity of heat change dQ/dt, and a constant thermal conductance a, which for convenience is assumed to be equal in all cases of heat transfer as follows ... 

Angberg et al. (10) studied the hydrolysis of acetyl saliclyic acid solutions, for which it was shown that elevated temperatures were needed to follow this rapid hydrolysis process. Thus, the isothermal microcalorimeter was neither more accurate, nor quicker or easier than using a conventional analytical approach, such as titration or chromatography, for this hydrolysis reaction. 

Therefore, after attaining L = r, an isothermal process in a batch reactor rapidly becomes adiabatic. 

In a steady-state flow system there is a transfer of energy or matter in and out such that the system remains constant. In a cyclic system, the final state is identical to the initial state. That is, the heat absorbed is equal to the work done by the system. In an adiabatic process, there is no heat exchange with the surroundings. The process is therefore thermally isolated or the process is very rapid such that heat has no time to enter or leave the system. This is an idealized process since insulation is not perfect and there will be some transfer of heat. In an isothermal process, there is no exchange of temperature with the surroundings. The type of system may depend on the timeframe of interest. For example, a vessel containing volatile liquid can be considered to be closed for a very short period after which the system can be considered to be open. 

The third calibration parameter, which is needed when rapidly changing processes are studied, is the time constant. This is a measure of the thermal inertia of the sample that blurs details in rapid events. The time constant is used in the Tian equation - named after a pioneer in isothermal calorimetry - to correct for this. Typical time constants in isothermal calorimeters are 100-1000 s. As the main hydration has timescales much longer than this, the Tian equation is not needed in cement calorimetry when the main hydration is studied, but it is needed when early reactions are studied. Further information on the Tian equation is given by Wadso (2005), and other similar methods are discussed by Evju (2003). 

Adsorption is invariably an exothermic process, so that, provided equilibrium has been established, the amount adsorbed at a given relative pressure must diminish as the temperature increases. It not infrequently happens, however, that the isotherm at a given temperature Tj actually lies above the isotherm for a lower temperature Ti. Anomalous behaviour of this kind is characteristic of a system which is not in equilibrium, and represents the combined effects of temperature on the rate of approach to equilibrium and on the position of equilibrium itself. It points to a process which is activated in the reaction-kinetic sense and which therefore occurs more rapidly as temperature is increased. 

Heat pipes are used to perform several important heat-transfer roles ia the chemical and closely aUied iadustries. Examples iaclude heat recovery, the isothermaliziag of processes, and spot cooling ia the mol ding of plastics. In its simplest form the heat pipe possesses the property of extremely high thermal conductance, often several hundred times that of metals. As a result, the heat pipe can produce nearly isothermal conditions making an almost ideal heat-transfer element. In another form the heat pipe can provide positive, rapid, and precise control of temperature under conditions that vary with respect to time. 

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

Figure 24. P-C isotherms of Mm(Ni-Co-AI -Mn)47f> alloys prepared through a rapid quenching and/or annealing process.

The predominant gaseous products of the decomposition [1108] of copper maleate at 443—613 K and copper fumarate at 443—653 K were C02 and ethylene. The very rapid temperature rise resulting from laser heating [1108] is thought to result in simultaneous decarboxylation to form acetylene via the intermediate —CH=CH—. Preliminary isothermal measurements [487] for both these solid reactants (and including also copper malonate) found the occurrence of an initial acceleratory process, ascribed to a nucleation and growth reaction. Thereafter, there was a discontinuous diminution in rate (a 0.4), ascribed to the deposition of carbon at the active surfaces of growing copper nuclei. Bassi and Kalsi [1282] report that the isothermal decomposition of copper(II) adipate at 483—503 K obeyed the Prout—Tompkins equation [eqn. (9)] with E = 191 kJ mole-1. Studies of the isothermal decompositions of the copper(II) salts of benzoic, salicylic and malonic acids are also cited in this article. 

C/min to 140°C, (3) hold for 2 hours, and (4) heat to 500°C at 3°/min. Oxygen was introduced at the time the temperature reached 140°C. The increase in temperature after the isothermal (140°C) region led to an increase in the rate of chemisorption, up to the temperature at which combustion (burn-off) becomes the dominant process resulting in rapid weight loss (ca. 270°C). 

To ensure operation under reproducible conditions, the column is enclosed in a thermostatically controlled oven whose temperature can be held constant to within 0.1°C. Operating temperatures range from ambient to over 400°C and may remain constant during a separation - isothermal operation - or automatically increased at a predetermined rate to speed the elution process - temperature programming (p. 106). The latter is a form of gradient elution. Rapid temperature equili-... 

While the rate of decomposition in air was more rapid than in nitrogen, the kinetics of oxidative decomposition cannot be estimated reliably by isothermal weight loss because ofthe possibility of competing oxidative weight gain process. For this reason the kinetics were not estimated from the available data... 

As the name implies, an isothermal change takes place at constant temperature. This requires that the process be relatively slow and heat transfer between the gas and the surroundings be rapid. An isothermal change corresponds to k = 1 and equation 6.27 becomes... 

In dilute solutions of surfactants adsorption processes are controlled by transport of the surfactant from the bulk solution towards the surface as a result of the concentration gradient formed in the diffusion layer the inherent rate of adsorption usually is rapid. For non-equilibrium adsorption the apparent (non-equilibrium) isotherm can be constructed for different time periods that are shifted with respect to the true adsorption isotherm in the direction of higher concentration (Cosovic, 1990) (see Fig. 4.10). 

Some investigations require isothermal conditions (Fig. 14 B). The main point is usually to reach the temperature rapidly, so that one can follow the whole process under isothermal conditions. 

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