Reaction calorimetry, thermal

Oin experimental technique of choice in many cases is reaction calorimetry. This technique relies on the accurate measurement of the heat evolved or consumed when chemical transformations occur. Consider a catalytic reaction proceeding in the absence of side reactions or other thermal effects. The energy characteristic of the transformation - the heat of reaction, AH i - is manifested each time a substrate molecule is converted to a product molecule. This thermodynamic quantity serves as the proportionality constant between the heat evolved and the reaction rate (eq. 1). The heat evolved at any given time during the reaction may be divided by the total heat evolved when all the molecules have been converted to give the fractional heat evolution (eq. 2). When the reaction under study is the predominant source of heat flow, the fractional heat evolution at any point in time is identical to the fraction conversion of the limiting substrate. Fraction conversion is then related to the concentration of the limiting substrate via eq. (3). 

Identification of hazardous chemicals through thermodynamic and kinetic analyses is discussed in Chapter 2. This hazard identification makes use of thermal analysis and reaction calorimetry. In Chapter 2, an overview of the theory of thermodynamics, which determines the reaction (decomposition)... 

Gustin, J. L., "Thermal Stability Screening and Reaction Calorimetry—Application to Runaway Hazard Assessment and Process Safety Management," /. Loss Prev. Proc. Ind., 6,275 (1993). 

The company has a well-equipped laboratory for thermal hazards screening and sophisticated reaction calorimetry. 

The enthalpy of the reverse of reaction 10.17, the cis - trans isomerization reaction is thermally activated and thus can be determined by isoperibol reaction-solution calorimetry (however, because the reaction is slow, a catalyst must be used). These experiments were also made by Dias et al. and led to -49.1 1.0 kJ mol-1 for reaction 10.18. 

See CALORIMETRY, THERMAL STABILITY OL REACTION MIXTURES AND SYSTEMS See also EXOTHERMICITY... 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Adiabatic calorimeters have also been used for direct-reaction calorimetry. Kubaschewski and Walter (1939) designed a calorimeter to study intermetallic compoimds up to 700°C. The procedure involved dropping compressed powders of two metals into the calorimeter and maintaining an equal temperature between the main calorimetric block and a surrounding jacket of refractory alloy. Any rise in temperature due to the reaction of the metal powders in the calorimeter was compensated by electrically heating the surrounding jacket so that its temperature remained the same as the calorimeter. The heat of reaction was then directly a function of the electrical energy needed to maintain the jacket at the same temperature as the calorimeter. One of the main problems with this calorimeter was the low thermal conductivity of the refractory alloy which meant that it was very difficult to maintain true adiabatic conditions. 

Several methods have been developed over the years for the thermochemical characterisation of compounds and reactions, and the assessment of thermal safety, e.g. differential scanning calorimetry (DSC) and differential thermal analysis (DTA), as well as reaction calorimetry. Of these, reaction calorimetry is the most directly applicable to reaction characterisation and, as the heat-flow rate during a chemical reaction is proportional to the rate of conversion, it represents a differential kinetic analysis technique. Consequently, calorimetry is uniquely able to provide kinetics as well as thermodynamics information to be exploited in mechanism studies as well as process development and optimisation [21]. 

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. 

These data can be obtained from reaction calorimetry, which delivers the heat of reaction required for the determination of the adiabatic temperature rise (ATJ). The integration of the heat release rate can be used to determine the thermal conversion and the thermal accumulation (XJ). The accumulation may also be obtained from analytical data. 

The data may be obtained from calorimetric methods usually employed for the study of secondary reaction and thermal stability as DSC, Calvet calorimetry, and adiabatic calorimetry. 

The data required to answer this question may be obtained from reaction calorimetry for the accumulation in combination with DSC, Calvet calorimetry, or adiabatic calorimetry for the thermal stability. 

Stoessel, F. (1997) Applications of reaction calorimetry in chemical engineering. Journal of Thermal. Analysis, 49, 1677-88. 

Yih-Shing, D., Chang-Chia, H., Chen-Shan, K. and Shuh, W.Y. (1996) Applications of reaction calorimetry in reaction kinetics and thermal hazard evaluation. Thermochimica Acta, 285 (1), 67-79. 

Ferguson, H.D. and Puga, Y.M. (1997) Development of an efficient and safe process for a Grignard reaction via reaction calorimetry. Journal of Thermal... 

This is the most common mode of addition. For safety or selectivity critical reactions, it is important to guarantee the feed rate by a control system. Here instruments such as orifice, volumetric pumps, control valves, and more sophisticated systems based on weight (of the reactor and/or of the feed tank) are commonly used. The feed rate is an essential parameter in the design of a semi-batch reactor. It may affect the chemical selectivity, and certainly affects the temperature control, the safety, and of course the economy of the process. The effect of feed rate on heat release rate and accumulation is shown in the example of an irreversible second-order reaction in Figure 7.8. The measurements made in a reaction calorimeter show the effect of three different feed rates on the heat release rate and on the accumulation of non-converted reactant computed on the basis of the thermal conversion. For such a case, the feed rate may be adapted to both safety constraints the maximum heat release rate must be lower than the cooling capacity of the industrial reactor and the maximum accumulation should remain below the maximum allowed accumulation with respect to MTSR. Thus, reaction calorimetry is a powerful tool for optimizing the feed rate for scale-up purposes [3, 11]. 

Gustin, J.L. (1993) Thermal stability screening and reaction calorimetry. Application to runaway reaction hazard... 

Our physical testing program is concerned with two main areas, thermal stability and reaction calorimetry. The thermal stability testing is broken down into two phases, initial screen and followup tests. The initial screen is intended to quickly identify any thermally unstable materials in a process. The follow-up tests examine in more detail any significant instability detected in the initial screen. 

Our experimental technique of choice in many cases is reaction calorimetry. This technique relies on the accurate measurement of the heat evolved or consumed when chemical fiansformations occur. Consider a catalytic reaction proceeding in the absence of side reactions or other thermal effects. The energy characteristic of the transformation— the heat of reaction, AH —is manifested each time a substrate molecule is converted to a product molecule. This thermodynamic quantity serves as the proportionality constant between the heat evolved and the reaction rate (Equation 27.1). 

Robie and Hemingway [95ROB/HEM] used their accurate heat capacity measurements, combined with results from molten salt calorimetry, thermal decomposition of the Ni2Si04-olivine into its constituent oxides, and equilibrium studies, both by CO reduction and solid state electrochemical cell measurement for Reaction (V.121) [87NE1], and calculated the following standard molar enthalpy of formation of Ni2Si04-olivine (liebenbergite) A,// (298.15 K) = - (1396.5 3.0) kJ mol. ... 

Polysulphides. The reactions of lithium and sodium with sulphur in liquid ammonia have been studied. The thermal behaviour Of the polysulphides was also investigated by means of thermogravimetric and differential thermal analyses. The enthalpies of formation of both the sulphides and polysulphides of lithium and sodium have been determined in 0.1 N-H2SO4 by reaction calorimetry. The crystal structures of two compounds, and both containing... 

Second, the preparation of new chemicals for new pharmaceutical products, synthetic materials and foods could add to the hazards which workers and customers face. Thermal instability and explosive behaviour can be extremely destructive and costly events. Reaction calorimetry and similar techniques can help to predict the likely behaviour of chemicals when reactions, transport and storage are concerned. Physiological behaviour may vary with the nature and form of a drug, and the nature and interconversion of these forms is often studied by thermal and calorimetric methods. 

Plotting thermal data from reaction calorimetry against the rate and quantity of gas evolved can give further insight into reaction mechanisms and critical accumulation parameters, especially if the reaction is endothermic. 

Generally, in classical reaction calorimetry only the liquid phase is taken into account in the heat balance. This means that the gas phase in equihbrium with it is neglected because of its small contribution in terms of heat transfer and heat capacity. The situation with supercritical fluids becomes complicated as soon as they occupy all the available volume. This implies that the whole inner reactor surface has to be thermally perfectly controlled when working with supercritical fluids. In this case, the cover and the flange temperature are adjusted on-line to the reaction temperature in order to neglect the heat accumulation term. 

The pressure can also be used to monitor the polymerization, because when the pressure reaches a plateau this means that the conversion is higher than 90%, as also reported by Lepilleur and Beckman [12] and Wang et al. [26]. It should be noted that the thermal signal obtained by calorimetry is much more sensitive than the pressure and gives more information. This result reveals aU the potential of reaction calorimetry for supercritical fluid investigations and polymerization monitoring. 

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