Adiabatic reaction calorimetry

Reaction calorimetry provides information on the maximum heat generation at process temperatures and on the adiabatic temperature rise. This ATad provides insight into the worst-case temperature consequences. 

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. 

Note The terms isothermal and adiabatic as applied to reactions (and reaction calorimeters) do not have the same meaning here as when used by thermochemists. The excellent introduction to reaction calorimetry by Skinner, Sturtevant and Sunner should be consulted (Skinner et al., 1962). 

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 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. 

In this chapter, the reactor dynamics under adiabatic and isoperibolic conditions is analyzed, while the temperature-controlled case is addressed in Chap. 5. It must be pointed out that these conditions can be easily realized in laboratory investigations, e.g., in reaction calorimetry, but represent mere ideality at the industrial scale. Nevertheless, this classification is useful to recognize the main paths leading to runaway without the burden of a more complex mathematical approach. 

Based on highly accurate adiabatic-shield calorimetry measurements, Gronvold and Westrura ( ) reported that the enthalpy of transformation of marcasite to pyrite is -1.05 0.05 kcal raol" at 700 K. The adopted value of AjH (298.15 K) is selected to reproduce this enthalpy of reaction within the reported uncertainty. Lipin et al. (2), based on combustion calorimetry, reported a value of -5.6 kcal mol" for the marcasite-pyrite transformation at 298.15 K. Due to the state of the art in combustion calorimetry at the time of this measurement and uncertainty in the products (oxides of sulfur), this value must have a high uncertainty and is given no weight in our selection process. 

Robie et al. have carried out excellent measurements of the heat capacity of Ni2Si04 olivine between 5 and 387 K by cryogenic adiabatic-shield calorimetry and between 360 and 1000 K by differential scanning calorimetry. At 298.15 K the molar heat capacity and entropy of Ni2Si04 olivine were determined. This molar heat capacity and entropy were accepted by this review. The thermal heat capacity function proposed by Robie et al. for the temperature range between 300 and 1300 K was, however, not adopted by this review, because it is based on measurements which were made only at temperatures up to 1000 K. Combining the heat capacity measurements with results of molten salt calorimetry, thermal decomposition of Ni2Si04 olivine into its constituent oxides, and equilibrium studies, both by CO reduction and solid state electrochemical cell measurements for the reaction ... 

The heat capacity of synthetic NiS was measured by adiabatic-shield calorimetry from 260 to 1000 K. The solid phase was prepared from the elements by solid state reaction and checked by X-ray diffraction analysis. The rhombohedral low temperature modification (millerite) was found to transform to the hexagonal non-stoichiometric NiAs polytype at 660 K where the heat of transition amounts to = (6591 50) J mol. ... 

Pilot-plant 1. Complete assessment of normal operating conditions 2. Identification of possible process deviation Reactions Reaction calorimetry Physical unit operations adiabatic or other sophisticated methods... 

Low-temperature heat capacities of the solid coordination compounds Zn(Leu)S04 l/2H20(s) and Zn(His)S04T/2H20(s) (Leu = Leucine and His = Histidine) were measured by a precision automated adiabatic calorimeter over the temperature range between T = 78 K and T = 371 K. Di and coworkers [228,229] determined the initial dehydration temperature of the coordination compounds by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation with the reduced temperatures (x), [x = f (T)], by a least-squares method. Enthalpies of dissolution of both the complexes were determined by isoperibolic solution-reaction calorimetry. 

Kub] Adiabatic direct reaction calorimetry Integral enthalpy at 1292°C, whole composition range, a,y,a + y... 

Adiabatic and Isoperibol Calorimeters.—Most calorimeters used in combustion and reaction calorimetry undergo a change of temperature when reaction takes place. If the calorimeter is surrounded by a jacket, the temperature of which is controlled to be the same as that of the calorimeter, no heat-exchange occurs between the siuroundings and the calorimeter, which is then described as adiabatic. However, if the temperature of the environment is maintained constant (in a type of calorimeter conveniently described as isoperibol and sometimes, incorrectly, as isothermal) some heat-exchange occurs between the calorimeter and its surroundings, but may be accurately determined by analysis of the temperature-time curves before and after reaction takes place, provided the reaction is of short duration (say not exceeding 15 min). With slower processes, isoperibol calorimeters are less useful, and the adiabatic principle is easier to effect and yields more accurate results. 

In isoperibol and adiabatic calorimeters the quantity actually measured is the temperature change of a calorimeter. Although a detailed discussion of thermometry would be inappropriate, it is useful to survey briefly the main methods used in reaction calorimetry. 

The results of the hazardous chemical evaluation are used to determine to what extent detailed thermal stability, runaway reaction, and gas evolution testing is needed. The evaluation may include reaction calorimetry, adiabatic calorimetry, and temperature ramp screening using accelerating rate calorimetry, a reactive system screening tool, isoperibolic calorimetry, isothermal storage tests, and adiabatic storage tests. 

Adiabatic calorimetry An adiabatic reaction calorimeter is characterized by thermal insulation of the reaction mixture from the surroundings. Consequently, the heat released by reaction is stored within the reaction mixture. Thus, the temperature gradient of the reaction is proportional to conversion. Equation 7.1 reduces to... 

Reaction calorimetry can also provide useful information for process design such as the necessary cooling power, the adiabatic temperature rise, and the heat transfer for scale-up. 

Heat Reaction calorimetry A reaction calorimeter is designed for the investigation of reactions between liquids or solids. The calorimetric technique can be isothermal, isoperibolic or adiabatic. 

Adiabatic calorimetry. Dewar tests are carried out at atmospheric and elevated pressure. Sealed ampoules, Dewars with mixing, isothermal calorimeters, etc. can be used. Temperature and pressure are measured as a function of time. From these data rates of temperature and pressure rises as well as the adiabatic temperature ri.se may be determined. If the log p versus UT graph is a straight line, this is likely to be the vapour pressure. If the graph is curved, decomposition reactions should be considered. Typical temperature-time curves obtained from Dewar flask experiments are shown in Fig. 5.4-60. The adiabatic induction time can be evaluated as a function of the initial temperature and as a function of the temperature at which the induction time, tmi, exceeds a specified value. 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

Adiabatic calorimetry uses the temperature change as the measurand at nearly adiabatic conditions. When a reaction occurs in the sample chamber, or energy is supplied electrically to the sample (i.e. in heat capacity calorimetry), the temperature rise of the sample chamber is balanced by an identical temperature rise of the adiabatic shield. The heat capacity or enthalpy of a reaction can be determined directly without calibration, but corrections for heat exchange between the calorimeter and the surroundings must be applied. For a large number of isoperibol... 

Consequence of runaway reaction Temperature rise rates Gas evolution rates Adiabatic Dewar Adiabatic calorimetry Pressure ARC VSP/RSST RC1 pressure vessel... 

Techniques such as adiabatic calorimetry (Dewar calorimetry) were by then well established [2, 118, 119]. All these techniques can be used for obtaining data to design for the prevention of runaway reactions, that is, to design for inherent plant safety. 

The Reactive System Screening Tool (RSST), marketed by Fauske and Associates, is a relatively new type of apparatus for process hazard calorimetry [192, 196-198]. The equipment is designed to determine the potential for runaway reactions and to determine the (quasi) adiabatic rates of temperature and pressure rise during a runaway as a function of the process, vessel, and other parameters. 

The kinetics were studied by adiabatic calorimetry [18] and high vacuum isothermal dilatometry [21, 22]. The calorimeter and the dilatometers were fitted with electrodes [21] for measuring the conductivity of the reaction mixtures. 

There are a number of different types of adiabatic calorimeters. Dewar calorimetry is one of the simplest calorimetric techniques. Although simple, it produces accurate data on the rate and quantity of heat evolved in an essentially adiabatic process. Dewar calorimeters use a vacuum-jacketed vessel. The apparatus is readily adaptable to simulate plant configurations. They are useful for investigating isothermal semi-batch and batch reactions, and they can be used to study ... 

These parameters need to be considered for reactions that go towards the intended completion as well as for possible upsets (see section C). Measuring methodologies for determining characteristic material property values (Stoffkenngrofcen), e.g., differential thermal analysis ("DTA"), calorimetry, and adiabatic experiments, and their possible use and applications are given in the literature /1, 2, 3, 41. 

Adiabatic calorimetry Chemical testing technique that determines the self-heating rate and pressure data of a chemical under near-adiabatic conditions. ( Adiabatic refers to any change in which there is no gain or loss of heat.) This measurement technique conservatively estimates the conditions for, and consequences of, a runaway reaction. 

The experimental data and the calculations involved in the determination of a reaction enthalpy by isoperibol flame combustion calorimetry are in many aspects similar to those described for bomb combustion calorimetry (see section 7.1) It is necessary to obtain the adiabatic temperature rise, A Tad, from a temperaturetime curve such as that in figure 7.2, to determine the energy equivalent of the calorimeter in an separate experiment and to compute the enthalpy of the isothermal calorimetric process, AI/icp, by an analogous scheme to that used in the case of equations 7.17-7.19 and A /ibp. The corrections to the standard state are, however, much less important because the pressure inside the burner vessel is very close to 0.1 MPa. 

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