Isothermal mode

Gas chromatograms are usually obtained with the column kept at a constant temperature. Two important disadvantages result from this isothermal mode of operation. 

The UNSAT-H model estimates evaporation from the soil in two ways. In the isothermal mode, the user must supply PET data for each day or daily climate data from which the model calculates PET by the Penman method. In the thermal mode, the model estimates evaporation from the difference in vapor density in the atmosphere and in the surface soil layer. 

For operation in a constant pressure non-isothermal mode with a gas phase reaction, it is convenient to approximate the reactor volume by... 

Thermal Analysis. The DuPont Instruments Model 9900, computer controlled thermal analyzer and Model 951 TGA module were used in the experiments, using a gas flow rate of 100 cc/min. Experiments were performed in dynamic and isothermal mode using air and argon. 

DSC can be used effectively in the isothermal mode as well. In this case, the container with the sample is inserted into the DSC preheated to the desired test temperature. This type of experiment should be performed to examine systems for induction periods that occur with autocatalytic reactions and with inhibitor depletion reactions. (Reactions with induction periods can give misleading results in the DSC operated with increasing temperature scans.) Autocatalytic reactions are those whose rates are proportional to the concentration of one or more of the reaction products. Some hydroperoxides and peroxy esters exhibit autocatalytic decomposition. Inhibitor depletion can be a serious problem with certain vinyl monomers, such as styrene and acrylic acid, that can initiate polymerization at ambient temperatures and then selfheat into runaways. Isothermal DSC tests can be used to determine a time to runaway that is related to the inhibitor concentration. 

Figure 11.5 Typical curve for a continuous titration calorimetry study of an exothermic reaction, using the calorimeter of Figure 11.1 in the heat flow isothermal mode of measurement./ is the frequency of the constant energy pulses supplied to the heater C in Figure 11.1 b. Adapted from [196,197],...

Analogously to the dynamic method, the energy equivalent of the calorimeter, k.Q, can be obtained by performing calibration experiments in the isothermal mode of operation, using electrically generated heat or the fusion of substances with well-known A us//. Recommendations for the calibration of the temperature scale of DSC instruments for isothermal operation have also been published [254,270]. 

All kinetic information (A and Ea in Equation 2.2) can be obtained from only one experiment. Although this method has been successfully applied for some first order reactions, it is not accurate for others the heat of reaction is higher (around 10-30%) when compared with isothermal mode [1]. 

To determine the constants that are related to a given stationary phase, a series of n-alkanes are injected on the column in the isothermal mode. Under these conditions, the logarithm of the corrected retention times /R(n)(tR(n) = carbon atoms present in the alKane (Fig. 2.14). A graph of carbon number n versus log R(n) is usually a straight line ... 

Figure 2.15—Graphical measurement of Kovats index (/ = lOO/jj) on a column in the isothermal mode.The equivalent carbon number nx is obtained using the logarithm of the corrected retention time tLX). When using a temperature program, a linear relationship can be obtained using a corrected formula. However, this is achieved with a lower precision.

Fig. 4. Representation of DSC output in the isothermal mode for an exothermic reaction, showing baseline construction...

E.S. Machlin and M. Cohen. Isothermal mode of the martensitic transformation. Trans. AIME, 194 489-500, 1952. 

The scope and limitations of the use of DSC as a means of investigating the reaction dynamics of potentially hazardous chemical reactions is discussed. For existing processes, examination of production samples taken at various stages of the overall process sequence can identify the stage at which maximum heat production is occurring, as the most critical phase for control requirements. The existence of an induction period can also be established, and use of the instrument in its more sensitive isothermal mode can yield information on consecutive reactions and catalytic effects... 

The oven temperature may be controlled in two ways the dynamic mode also called scanning mode, where the oven temperature is varied linearly with time, and the isothermal mode, where the oven temperature is maintained as constant. Since the DSC uses only small sample sizes in the order of some milligrams, very exothermal phenomena may be studied, even under extreme conditions, without any risk either to the laboratory personal, or to the instrument Moreover, a scanning experiment from ambient temperature to 500 °C with a scan rate of 4 K min"1 takes only about 2 hours. Thus, the DSC became a very popular instrument for screening purposes [21, 24, 25]. 

Figure 4.7 Example of a typical study of a reaction in the isothermal mode (upper thermogram), followed by the study of the thermal stability of the final reaction mass in dynamic mode (lower thermogram).

Radex Safety Calorimeter. The Radex calorimeter is a modular instrument that can simultaneously evaluate six different samples (size range 0.5 to 5 ml), or one substance under a variety of conditions. Each module is a separate entity with its own calibrated oven capable of being operated under an open, closed, or pressurized condition, with all temperature differences between the sample and the oven being stored in a microprocessor for further analysis. The Radex calorimeter is very versatile samples can be tested in either an isothermal or ramp mode. In the isothermal mode, each oven is heated to a preset temperature and held at that temperature throughout the experiment. In the ramp mode of operation, the oven is heated linearly to a preset temperature, or can be maintained at a given temperature for a predetermined time. The flexibility of oven function in the Radex calorimeter enables the user to determine the intrinsic stability of a chemical and to also compare the impact of such parameters as temperature, atmosphere, and impurities on the stability of a given substance. 

The patents quoted in Table 20 give rather broad temperature ranges. An explanation for this feature is the polymerization mode which is used for the large-scale polymerization of Nd-BR. To the best of our knowledge, adiabatic rather than isothermic modes are used. In the adiabatic mode polymerization heat is neither removed by external nor by evaporation cooling. Therefore,... 

An important critcrium for the choice of reactors is whether they can operate in isothermal conditions. Data obtained in a non isothermal mode of operation are difficult to interpret. 

There is some merit to each of these methods, and both have been used. In isothermal mode, we estimate the change in the value of At]) with T for the reference electrode. In nonisothermal mode, we estimate the additional potential drop generated in the salt bridge, as a result of the thermal gradients. Neither can be measured directly. Subsequently, there is always some uncertainty in the value of the enthalpy of activation of electrode reactions. 

The numerical model for n-butane oxidation, by Pitz et al. [228], was used also by Carlier et al. [21] to simulate experimental studies of the two-stage combustion of n-butane at 0.18 MPa on a flat-flame burner and, following this validation, to simulate the ignition delays of n-butane in a rapid compression machine. The numerical studies of the burner experiments were extended by Corre et al. [233]. For simulations of the behaviour on a flat-flame burner the chemical model was computed in an isothermal mode, the experimental one-dimensional temperature profile being introduced as an input parameter. Among the important aims of the tests by Corre et al. [233] was the rationalization of the predicted extent of n-butane consumption throughout the development of the first (cool-flame) and second stages of combustion, with that observed experimentally. The experimental study by Minetti et al. [22, 116] included the detection and measurement of RO2 and HO2 radicals by esr, the one-dimensional spatial profiles of which were simulated by Corre et al. [233],... 

Gas chromatographic analysis can be performed at constant temperature (isothermal mode) or with the column temperature increasing with time (temperature programming). 

The catalytic oxidation of sulfur dioxide to sulfur trioxide can also be carried out in a fluidized bed reactor. The gas to be converted is fed in at the bottom of a fluidized bed containing the catalyst in the form of abrasion resistant beads. The whole fluidized bed can be kept at the required temperature by removing the heat of reaction with a pipe cooler. This isothermal mode of operation enables gases with a higher sulfur dioxide content to be processed and more compact plants to be built. 

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