Pseudo-isothermal

Processes are didded into two major categories, according to whether the reactors are in adiabatic or isothermal (or more precisely pseudo-isothermal) operation. 

Install intercooling capacity in the column to maintain a pseudo-isothermal condition. The cooling can be done with coils on each tray (as in nitric acid columns) or with external heat exchangers processing the total liquid downflow. 

A method has been developed in which the DTA curve is obtained under pseudo-isothermal conditions that is, both sample and reference materials are not heated the temperature remains as isothermal as possible (172,173). 

Next, a mathematical description of T is given for a quasi-isothermal run. This type of run does not only simplify the mathematics, it also is a valuable mode of measuring Cp as described in Sect. 4.4.5. In addition, standard TMDSC with y O is linked to the same analysis by a pseudo-isothermal data treatment as described in Sect. 4.4.3. 

Adding an underlying heating rate to the measurement leads to a Lissajous figure as is shown in Fig. 4.121. The computation using the pseudo-isothermal method of Sect. 4.4.3 implies the plotting of HF(t) versus T t) - . A similar plot is obtained when plotting versus dT/dt. 

The cold crystallization starts at about 400 K at a supercooling not affected by modulation and registers as nonreversing. For separation of such nonreversing transitions, several modulation periods must occur across the transition, otherwise the pseudo-isothermal analysis would not develop the proper sinusoidal oscillations about , as can be seen from the modeling in Figs. 4.100-102 (loss of stationarity). 

Thomas LC, Boiler A, Okazaki I, Wunderlich B (1997) Modulated Differential Scanning Calorimetry in the Glass Transition Region, IV. Pseudo-isothermal Analysis of the Polystyrene Glass Transition, Thermochim Acta 291 85-94. 

Tft)> — T t) is identical to the result expected from a quasi-isothermal experiment = 0, see Section 3.3) and is called pseudo-isothermal. The quasi-isothermal analysis has been described in detail and yields for the heat capacity the following expression which also holds for the pseudo-isothermal case [31] where is the modulation amplitude of AT which is proportional to the heat-flow rate, HF (A oc Ahf) -... 

If the temperature variations as a function of axial location can be predicted, the "equivalent reactor volume" concept (Hougen and Watson, 1947 Froment et al., 1961) can be applied to convert the data to a pseudo-isothermal basis. According to Hougen and Watson (1947), the equivalent reactor volume is defined as that volume which, at a constant reference temperature, would give the same conversion as the actual non-isothermal tubular reactor. The reference temperature has frequently been taken as the arithmetic mean of the process gas temperatures in the last 40% of the reactor (Van Damme et al., 1975). 

At present, thermal analysis and calorimetry are regarded as two separate disciplines. Thermal analysis would normally make measurements as a function of temperature whereas calorimetry would normally make isothermal measurements as a function of time. However with the advent of microscanning calorimeters, where operation under pseudo-isothermal conditions can give sensitivity approaching that of microcalorimeters, the distinction is becoming less defined. Also, specific calorimetric and thermo-analytical instruments are often ubiquitous and can be used in several different modes of operation. 

---