Equipment design temperature rise

Some early calorimeters use thermal methods based on principles of heat and mass balance (12) and temperature rise of a constant flow of air through the combustion chamber (13). These calorimeters suffer from many drawbacks associated with their design. Heat and mass balance requires numerous measurements to account for all heat and mass flows. In most cases, thermal lag and losses in the equipment occur, which are not easily calculated. 

Chemical facilities have to be operated safely during normal operation as well as during deviations from the specified process and equipment parameters. Chemical reactions that go to completion can only become a hazard for humans and the environment when process pressures or temperatures rise beyond the equipment design parameters of a facility e.g., as result of a runaway reaction. For example unacceptable pressure increases can develop as a result of exothermic processes with inadequate heat sinks or reactions that produce gaseous products (e.g., decompositions). 

The functional relationship between product temperature, on the one hand, and shelf temperature and chamber pressure, on the other hand, is affected by many factors including the size and design of the lyophilizer, the characteristics of the product, and the time evolved since the start of primary drying. With a sucrose formulation in vials, we have observed a maximum primary drying product temperature rise of -i-5°C when the shelf temperature was varied from -15 to -i-30°C, whereas a pressure variation from 30 to 250 microbars generated an increase of around -i-2.5°C. With a lactose formulation in ampoules lyophilized in a larger freeze-dryer equipped with a plate-type condenser, the effect of pressure was found to be predominant -i-6.5°C for a pressure move from 50 to 300 microbars, versus -t-l°C for a shelf temperature move from 0° to 25°C. 

A typical DTA apparatus is illustrated schematically in Figure 6.1. The apparatus generally consists of (1) a furnace or heating device, (2) a sample holder, (3) a low-level dc amplifier, (4) a differential temperature detector, (5) a furnace temperature programmer, (6) a recorder, and (7) control equipment for maintaining a suitable atmosphere in the furnace and sample holder. Many modifications have been made of this basic design, but ail instruments measure the differential temperature of the sample as a function of temperature or time (assuming that the temperature rise is linear with respect to time). 

Totally enclosed equipment is often designed for a greater temperature rise than enclosed ventilated equipment. It should be selected so that the final... 

Two alternative methods have been used in kinetic investigations of thermal decomposition and, indeed, other reactions of solids in one, yield—time measurements are made while the reactant is maintained at a constant (known) temperature [28] while, in the second, the sample is subjected to a controlled rising temperature [76]. Measurements using both techniques have been widely and variously exploited in the determination of kinetic characteristics and parameters. In the more traditional approach, isothermal studies, the maintenance of a precisely constant temperature throughout the reaction period represents an ideal which cannot be achieved in practice, since a finite time is required to heat the material to reaction temperature. Consequently, the initial segment of the a (fractional decomposition)—time plot cannot refer to isothermal conditions, though the effect of such deviation can be minimized by careful design of equipment. 

---