Differential reaction calorimeter

Calorimetric techniques, and liquid phase calorimetry in particular, are promising methods to study catalytic reactions [39]. Notably, the use of a differential reaction calorimeter (DRC) makes it possible to determine the most important thermodynamic data such as the heat of reaction and heat capacity of the system [40-42]. 

Reactions 15 and 16 depend on experimental conditions such as heating rate, helium flow rate, etc., and both reactions occur simultaneously to some extent. Therefore, it was not possible to determine the enthalpies of these reactions individually. However, by using a differential scanning calorimeter, we determined the combined enthalpy of Reactions 15 and 16 which is, of course, the enthalpy of Reaction 8 with the sign reversed. The value obtained was 100 10 kj/mol H2, which is in fair agreement with the value obtained from the van t Hoff relationship. 

NMR spectra were taken in deuteriochloroform solution, using a Varian HA100 spectrometer. Thermal measurements were made with a Perkin-Elmer DSC IB differential scanning calorimeter at 40°C/min. Near-infrared spectra were measured in carbon disulfide solution with a Beckman DK 2A spectrophotometer. Gas-chromatographic analyses of reaction mixtures were carried out after conversion of the phenols to trimethylsilyl ethers by reaction with bis (trimethylsilyl) acetamide. 

For the determination of reaction parameters, as well as for the assessment of thermal safety, several thermokinetic methods have been developed such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), accelerating rate calorimetry (ARC) and reaction calorimetry. Here, the discussion will be restricted to reaction calorimeters which resemble the later production-scale reactors of the corresponding industrial processes (batch or semi-batch reactors). We shall not discuss thermal analysis devices such as DSC or other micro-calorimetric devices which differ significantly from the production-scale reactor. 

A further positive reaction to this dramatic incident took place in the central research department of the company. A physico-chemist had the idea of using his differential scanning calorimeter (DSC) to look at the energy involved in this reaction. He performed an experiment with the initial concentration and a second with a higher concentration. The thermograms he obtained were different and he realized that he could have predicted the incident (see Exercise 11.1). As a consequence, it was decided to create a laboratory dedicated to this type of experiment. This was the beginning of the scientific approach of safety assessments using thermo-analytic and calorimetric methods. From this time on, many different methods were developed in different chemical companies and became commercially distributed, often by scientific instrument companies. 

In this category of calorimeters, we find the isothermal calorimeters and the dynamic calorimeters where the temperature is scanned using a constant temperature scan rate. The instrument must be designed in such a way that any departure from the set temperature is avoided and the heat of reaction must flow to the heat exchange system where it can be measured. The instrument acts as a heat sink. In this family we find the reaction calorimeters, the Calvet calorimeters [7], and the Differential scanning calorimeter (DSC) [8],... 

In order to obtain the degree of cure and rate of curing, we must first measure the reaction. This is typically done using a differential scanning calorimeter (DSC) as explained in Chapter 2. Typically, several dynamic tests are performed, where the temperature is increased at a constant rate and heat release rate (Q) is measured until the conversion is finished. To obtain Qt we must calculate the area under the curve Q versus t. Figure 7.17 shows four dynamic tests for a liquid silicone rubber at heating rates of 10, 5, 2.5 and 1 K/min. The trapezoidal rule was used to integrate the four curves. As expected, the total heat Qt is the same (more or less) for all four tests. This is to be expected, since each curve was represented with approximately 400 data points. 

This brief overview of offline measurements can be concluded by considering the measurements of the heat released by chemical reactions, which can be obtained via calorimetric measurements [7, 18]. The most diffused industrial calorimeters are the so-called reaction calorimeters, basically consisting in jacketed vessels in which the reaction takes place and the heat released is measured by monitoring the temperature of the fluid in the jacket. A class of alternative instruments are the scanning calorimeters (differential or adiabatic), in which the analysis is performed by linearly increasing the sample temperature with respect to time, in order to test the reactivity of potentially unstable chemical systems in a proper temperature range by measuring the released heat. 

The simpler and most reliable approach to the use of the DIERS methodology is the use of FAUSKY s reactive system screening tool (RSST). It is an experimental autoclave which simulates actual situations that may arise in industrial systems. The RSST runs as a differential scanning calorimeter that may operate as a vent-sizing unit where data can readily be obtained and can be applied to full-scale process conditions. The unit is computerized and records plots of pressure vs. temperature, temperature vs. time, pressure vs. time, and the rates of temperature rise and pressure rise vs. the inverse of temperature. From these data it determines the potential for runaway reactions and measures the rates of temperature and pressure increases to allow reliable determinations of the energy and gas release rates. This information can be combined with simplified analytical tools to assess reactor vent size requirements. The cost of setting up a unit of this kind is close to 15,000. 

Controversy exists over the nature of the chemical reaction of sulfur with asphalt, the role of the sulfur remaining in colloidal solution/disper-sion in the asphalt, and the maximum permissible sulfur concentration in the asphalt which will yield a stable long-term binder. Several investigators, such as Kennepohl et al. (4), have shown that on a long-term basis, approximately 20% of the sulfur remains in a dissolved and/or a dispersed state as part of the binder. Figure 1, prepared from their differential scanning calorimeter data, indicates that excess sulfur, i.e., free sulfur above approximately 20 wt %, solidifies to a crystalline state, ceasing to perform as a binder. 

The thermoplasticity of the graft copolymers can be verified by measurements of the glass transition temperature of the new solids. The glass transition temperature is the temperature at which an amorphous solid becomes ductile and is a characteristic of thermoplastic materials. Samples of 5-10 mg of reaction product were heated at 10°C/min in a differential scanning calorimeter to monitor heat capacity as a function of temperature. The temperature of each transition produced by each copolymer is shown in Table 8. 

Varma-Nair, M. Wunderlich, B. Nonisothermal heat capacities and chemical reactions using a modulated differential scanning calorimeter. J. Therm. Anal. 1996, 46, 879-892. 

DIFFERENTIAL SCANNING CALORIMETER REACTION KINETICS, 1 DYNAMIC SCAN... 

A differential scanning calorimeter (DSC 1-B, Perkin-Elmer Corporation) was used to determine the extent of cure 10-mg to 20-mg specimens were tested at a scanning rate of 10°C/min. An exothermic peak on the thermograph indicates the heat of reaction whereas an endothermic peak in the amorphous polymer indicates the presence of residual stresses or the occurrence of a transition such as the glass transition. The presence of an exothermic peak in the DSC-scan of a pre-cured sample is an indication of incomplete curing. 

A series of heterogeneous mixtures was made by adding various amounts of bis(triphenyl phosphine) nickel II chloride, as an initiator, to an acetylene-terminated phenylquinoxaline. The series was then evaluated using the Differential Scanning Calorimeter (DSC) and the shifts in the exothermic reaction with temperature were associated with the initiator concentrations. 

The amorphous character of the alloys before reaction was investigated with a PERKIN ELMER DSC-2 differential scanning calorimeter (heating rate 20 Kmin nitrogen atmosphere). 

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