Single isotherm mode

For a nonisothermal experiment, a single sample is sufficient to scan the entire temperature range, whereas for the isothermal mode, a new sample is needed for each experiment. In the latter case, it would be almost impossible to ensure identical sample characteristics for each... 

In the case of an isothermal mode of operation for a measurement and provided that the reference temperature is identical with the isothermal test temperature, this equivalent reaction time automatically becomes identical with the true reaction time. In the case of a none-isothermal mode of operation or the choice of a reference temperature which does not correspond to an isothermal test temperature, all thermal conversion data X(t) are plotted over h(t). In order to obtain the different h(t) data, a first estimation of the activation temperature has to be made. If the activation temperature was correctly chosen and if, at the same time, the process can be described with a single gross reaction equation with sufficient accuracy, then all the different data sets plotted over h(t) must take an identical course. The first estimate on the activation temperature can be obtained from the slope of the linearized functional relationship... 

In LGC, a single long capillary column is typically used in a temperature-programmed mode to perform the separation. Although this same separation scheme can be used in PGC, the major drawback is the speed of analysis. The solution is to use multiple column trains and column-switching techniques. This allows the chromatograph to be operated in an isothermal mode. 

Table 5.3 lists the principal experimental methods used in dynamic mechanical testing. Of the experiments considered below, the thermal scan mode (method 1) is the technique most commonly used by thermal analysts. Here typical applications in quality control or processing look for differences in material batches, thermal history, different grades, reactivity, and other characteristics. The stepped isotherm (or step isothermal) experiment (method 2) is used mainly in studies involving detailed mechanical property determination for structural analysis, vibration damping applications, and for determining time-temperature superposition master curves. Method 3 (fast scan or single isotherm) is application specific. 

The reforming process model is designed to predict the performance of many reactor configurations. The model can be run in four modes, combining adiabatic or isothermal reactors with recycle or single-pass (no recycle)... 

Two isothermal pilot reactors with a single hydrogen pass in down Row mode were used. One for the rapid catalyst test (RT) at a higher space velocity and the second for the long life test (LT) as a reference one. 

The accuracy of low-dimensional models derived using the L S method has been tested for isothermal tubular reactors for specific kinetics by comparing the solution of the full CDR equation [Eq. (117)] with that of the averaged models (Chakraborty and Balakotaiah, 2002a). For example, for the case of a single second order reaction, the two-mode model predicts the exit conversion to three decimal accuracy when for (j>2(— pDa) 1, and the maximum error is below 6% for 4>2 20, where 2(= pDd) is the local Damkohler number of the reaction. Such accuracy tests have also been performed for competitive-consecutive reaction schemes and the truncated two-mode models have been found to be very accurate within their region of convergence (discussed below). 

There are two possibilities for performing a frontal chromatography experiment for the purpose of the determination of equilibrium isotherms. The step-series method uses a series of steps starting from C = 0 to C +i. After each experiment, the column has to be reequilibrated and a new step injection with a different end concentration C +i can be performed. In the staircase method, a series of steps is performed in a single run with concentration steps from 0 to Q, Q to C2,.. ., C to C +i. The column does not have to be reequilibrated after each step and, therefore, the staircase method is faster than the step-series method. Both modes of frontal analysis give very accurate isotherm results. 

As already mentioned, induction methods, such as AC susceptibility measurements, yield the value of the differential adiabatic susceptibility. This can be scanned either in the single mode (giving rise to xl) or, alternatively, in the dual mode (when the dispersion x and the absorption x are distinguished). The adiabatic susceptibility approaches the isothermal susceptibility only when the frequency of the alternating field is small. 

---