Chemical time-temperature integrator

Chemical markers can mimic other processes. Chemical reactions taking place in foods can be used to mimic an entirely different process taking place in the vicinity of the site where the chemical measurements are made. An interesting example is the use of chemical markers to determine lethality within a food particulate where direct temperature measurement is not practical (Chapter 6). It appears that chemical reactions can mimic bacterial destruction, and are potentially useful time-temperature integrators in the continuous thermal processing of foods. 

The integral in equation (6.2) was evaluated numerically for the retardation spectra obtained from master curves, together with ar from the time-temperature superposition. A typical result is demonstrated in Fig. 6.3(a), where recovery is shown for the DBDI/PTHFeso polymer, for a simulated constant rate of heating r = 0.1 K/s. This Figure also illustrates the definitions of two parameters used to compare the responses the temperature of maximum recovery rate Tmax and the width of the recovery window AT. These parameters of the SMPs were compared with respect to chemical composition and crosslink density ric, expressed as the number of moles of 3-point star crosslinks, per 100 g of polymer. 

Chemica.1 Properties. The FEP resin is inert to most chemicals and solvents, even at elevated temperatures and pressures. However, it reacts with fluorine, molten alkah metal, and molten sodium hydroxide. Acids or bases are not absorbed at 200°C and exposures of one year. The absorption of organic solvents is less than 1% at elevated temperatures and long exposure times. Absorption of chemicals or solvents has no effect on the chemical integrity of the FEP molecule and is a reversible physical process. 

Activation of the catalyst is usually performed by exposure to a co-catalyst, namely an aluminum alkyl. The model catalysts were successfully activated by trimethylalumimun (TMA) and triethylaluminum (TEA), commonly used for this purpose. The compounds were dosed from the gas phase either at room temperature for a prolonged time or for a much shorter time at a surface temperature of 40 K. Nominal 3400 L of TMA or TEA were exposed at room temperature. The chemical integrity of the co-catalyst was verified by IR spectroscopy of condensed films grown at low temperature on the substrates. The spectra were typical for condensed and matrix isolated species [119]. 

We certainly want to respond very differently if the temperature of a chemical reactor is changing at a rate of 100°C/s as opposed to l°C/s. In a way, we want to "project" the error and make corrections accordingly. In contrast, proportional and integral controls are based on the present and the past. Derivative controller action is based on how fast the error is changing with time (rate action control). We can write... 

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