Calorimetric reaction calorimetry

This is the third report on attempts to measure the propagation rate constant, kp+, for the cationic polymerisation of various monomers in nitrobenzene by reaction calorimetry. The first two were concerned with acenaphthylene (ACN) [1, 2] and styrene [2]. The present work is concerned with attempts to extend the method to more rapidly polymerising monomers. With these we were working at the limits of the calorimetric technique [3] and therefore consistent kinetic results could be obtained only for indene and for phenyl vinyl ether (PhViE), the slowest of the vinyl ethers 2-chloroethyl vinyl ether (CEViE) proved to be so reactive that only a rough estimate of kp+ could be obtained. Most of our results were obtained with 4-chlorobenzoyl hexafluoroantimonate (1), and some with tris-(4-chlorophenyl)methyl hexafluorophosphate (2). A general discussion of the significance of all the kp values obtained in this work is presented. 

The kinetic techniques were densitometry and reaction calorimetry, and the electrical conductivity, K, was monitored for most systems the calorimetric measurements also yielded the enthalpies of polymerisation (AHp). Analysis of the polymers provided information on initial groups, DP, and DPD for many of the products. The determination of the quantity and origin of kinetically significant impurities is a feature of this work, because much of it was done with initiator concentrations, c0, between 10 4 and 10"3 mold"1, and the measured impurity levels, c , ranged from 10"4 down to 10"5 mold 1. 

C. E. Vanderzee. Reduction of Results to the Isothermal Calorimetric Process and to the Standard-State Process in Solution-Reaction Calorimetry. J. Chem. 

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. 

Numerous polymers autooxidize to form peroxides. These compositionally, and thus calorimetrically, ill-defined products may be considered polymeric peroxides. However, one well-defined polymeric peroxide is that of polystyrene with the repeat unit —CHa-CH(CeH5)-0-0-. Through a combination of combustion and reaction calorimetry (chain degradation to benzaldehyde and formaldehyde), a solid phase enthalpy of formation of this species was found to be 27 21 kJ mol . Much the same procedure was used to determine the enthalpy of degradation for the polyperoxide polymers of 2-vinylnaphthalene and the isomeric 1- and 2-propenylnaphthalene to form the related acylnaphthalene and formaldehyde. Numerically, the reaction enthalpy values for these last three polyperoxides were —206+4, —222 + 8 and —222 + 10 kJmol, to be compared with the aforementioned polystyrene with a value of —209 + 8 kJ mol. However, in the absence of enthalpy of formation data for the decomposition products in the naphthalene case, we hesitate to derive enthalpies of formation for these three species. ... 

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. 

The aim of this section is to demonstrate how reaction calorimetry in combination with IR-ATR spectroscopy can be used for the determination of kinetic and thermodynamic parameters. Several examples of chemical reactions will be discussed, each highlighting a different aspect in the application of reaction calorimetry. The reactions considered are the hydrolysis of acetic anhydride, the sequential epoxidation of 2,5-di-ferf-butyl-l,4-benzoquinone and the hydrogenation of nitrobenzene. The results discussed in this section were obtained using a new calorimetric principle presented below. 

The volume of gas potentially released by a reaction (including secondary reactions in criticality classes 4 and 5) can be known from the chemistry or measured experimentally by appropriate calorimetric methods, as for example, Calvet calorimetry, mini-autoclave, Radex, or Reaction Calorimetry (as V at T k. and /Jmes). It must be corrected for the temperature to be considered, MTSR (class 2), MTT (class 3 or 4), or Tf (class 5). Where the gas stems from the main reaction, only the accumulated fraction (X) will be released ... 

Despite the adequate accuracy ( + 6kJmol-1) for the calorimetrically derived enthalpies of formation of the isomeric AT-ethyl and A TV-dimethylanilines, we disappointedly acknowledge the absence of thermochemical data for the corresponding Af-ethyl and A N-dimethyl vinylamines. Indeed, we would have thought that enthalpy of formation data on the latter should be available from reaction calorimetry (cf References 34 and 40). More precisely, there is apparently no published account of the thermochemical analysis of the seemingly simple hydrolysis reaction... 

From the inorganic compendium we find an enthalpy of formation of C2 of 831.9 kJ mol. Cyclopropyne remains but a chemist s chimera, but only a short time ago, so were A -bicyclo[1.1.0]butene and [l.l.ljpropellane. From the classical (albeit gas-phase) combustion calorimetric measurement of the enthalpy of formation of bicyclobutane and modem gas-phase ion measurements, the gas-phase enthalpy of formation of A -bicyclo-[l.l.OJbutene has been determined to be (544 42) kJ mol". From reaction calorimetry and some judicious estimates, the enthalpy of formation of gaseous [l.l.ljpropellane has been measured and then chronicled among other, mostly normal 3MR species, as (351 4) kJ mol". Yet, before we get too complacent, it is clear that anything even approximating a constant cyclopropanation energy is not found in the series of current interest dicarbon and its formal cyclopropanated analogs 44-46. 

Areas of application of reaction calorimetry include determination of calorimetric data for reactions and process design, for the kinetic characterization of chemical reactions and of physical changes, for on-line monitoring of heat release and other analytical parameters needed in subsequent process development as well as for the development and optimization of chemical processes with the objective, for instance, to increase yield or profitability, control the morphology or degree of polymerization and/or index of polydispersity, etc. 

We would like to have reaction calorimetry data for every reaction we scale-up. However, due to time and capacity constraints, we only run calorimetric measurements on selected reactions that we feel show the greatest potential to cause a problem. Some examples are reactions that have been known to cause problems in the past (our own experience or literature), reactions that show potential problems during lab scale development work... 

Direct Reaction Calorimetry (DRC) works slightly different. It is a form of calorimetric measurements where you synthesize the alloys directly in the crucible (container), where you also measure the heats. The method will be described very quickly here, for further reading one could consult Refs. [122, 123, 124]. In this method you have a crucible made of ceramic aluminium, in which several thermoelectric elements (usually made of a platinum alloy) are lodged. With the help of these one can then measure the heat of the reaction and get the mixing enthalpy from... 

First and foremost in any kinetic stndy nsing reaction calorimetry, we mnst confirm the validity of the method for the system under stndy by showing that Eqnation 27.2 holds. Comparing the temporal fraction conversion obtained from the heat flow measnrement with that measured by an independently verihed measnrement technique, such as chromatographic sample analysis or FTIR or unclear magnetic resonance (NMR) spectroscopy, conhrms the use of the calorimetric method. 

Devising a series of reactions from which an accurate value of the heat of formation of a particular compound can be obtained can be a challenging problem. A calorimetric reaction must take place quickly (that is, be completed within a few minutes at most), with as few side reactions as possible and preferably none at all. Very few chemical reactions take place without concomitant side reactions, but their effects can be minimized by controlling the reaction conditions so as to favor the main reaction as much as possible. The final product mixture must be carefully analyzed, and the thermal effect of the side reactions must be subtracted from the measured value. Precision calorimetry is demanding work. 

The standard enthalpies of formation of the acetylacetonate complexes of Al, Ga, Cri" and Mn" have been determined by solution reaction calorimetry and the enthalpies of combustion of the Al, Ga and In tris(chelates) reported. Such data are relatively scarce in inorganic chemistry and quite valuable. For example, from solution calorimetric hydrolysis experiments on (MeCOCHC-OMe)2Pt, the Pt—O bond energy was found to be 183.1 klmol . ... 

In the thermochemistry of organic compoimds, the experimental determination of the enthalpies of formation can be carried out both by reaction calorimetry and by combustion calorimetry. The differences between these classes of calorimetric experiments are related to the changes produced in the carbon skeleton of the molecules. In reaction calorimetry, the energy or enthalpy of any chemical reaction is determined and, in these reactions, the carbon skeletons of the molecules are generally maintained. In combustion calorimetry, the energy of combustion in an oxygen atmosphere at high pressure is measured and there is a total breakdown of the carbon skeleton. 

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. 

One of the main advantages of reaction calorimetry on the larger scale is the possibility of inserting into the reactor special analytical probes for on-line measurements. Some preliminary results obtained by coupling an ultrasonic sensor with calorimetry are presented in Fig. 5.17. The sensor is directly inserted into the reactor, its contribution in terms of heat accumulation having been previously determined so that the calorimetric signal is only related to the chemical reaction and process. At the moment, only the sound wave measurement is compared to the... 

The calorimetric measurement of the heats of reactions other than combustion is conveniently referred to under the single heading of reaction calorimetry. Typical reactions include hydrolysis, hydrogenation, halo-genation, and thermal decomposition calorimeters designed for the study of these and other types of reaction have been described in detail elsewhere 156). 

The standard molar quantities appearing in Eqs. 12.10.1 and 12.10.2 can be evaluated through a variety of experimental techniques. Reaction calorimetry can be used to evaluate AfH° for a reaction (Sec. 11.5). Calorimetric measurements of heat capacity and phase-transition enthalpies can be used to obtain the value of Sf for a solid or liquid (Sec. 6.2.1). For a gas, spectroscopic measurements can be used to evaluate S° (Sec. 6.2.2). Evaluation of a thermodynanuc equilibrium constant and its temperature derivative, for any of the kinds of equilibria discussed in this chapter (vapor pressure, solubility, chemical reaction, etc.), can provide values of ArG° and AfH° through the relations AfG° = —RTln K and ArH° = -Rd aK/d /T). 

Reaction calorimetry is the experimental determination of the enthalpy changes accompanying chemical reactions by direct methods using calorimeters. It is the principal means by which enthalpies of formation of pure chemical compounds are determined. With the exception of certain binary compounds, chiefly oxides, it is impractical to measure the enthalpy of formation of a compound from its elements directly, and it is necessary to determine the enthalpy of a reaction involving the compound in which the enthalpies of the other reactants and products are all known, and then to apply Hess s law. Occasionally, enthalpies of formation can be derived from the study of equilibria (as measured by the e.m.f.s. of electrochemical cells, dissociation pressures, etc.) by means of second- or third-law methods, or from electron impact experiments, but such indirect approaches are outside the scope of the present review, which is confined to the discussion of experimental procedures used in direct calorimetric methods. 

While calorimetric methods, direct reaction calorimetry or solution calorimetry using tin or aluminum as solvent have been performed in rare earth based alloys, this has not been the case in actinide-based alloys, where acid solution calorimetry has sometimes... 

Figure 6.13(a) shows the evolution of the free monomer concentration as inferred from on-line reaction calorimetry compared with the off-line measurement (gravimetry and gas chromatography) for a VAc/BA/AA high solids content semibatch emulsion polymerization. In Figure 6.13(b), the estimation of the terpolymer composition from calorimetric measurements is compared with NMR and GC measurements for a VAc/MMA/BA emulsion terpolymerization [159]. 

The design of a safe semi-batch process requires a strict control of the accumulation of unconverted monomer. In fact the accumulation results from a balance between monomer addition by the feed, and monomer conversion by the reaction. Thus it can be influenced by the feed rate, but also by the reaction rate, which in turn can be governed by the temperature and the initiator or catalyst concentration. These parameters must be optimized during process development. Here again, reaction calorimetry is of great help for the determination of the accumulation [40]. It can be calculated from thermal conversion by means of Eq. (4) obtained directly from the calorimetric measurement and the actual amount fed. Just by varying the... 

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. 

During the polymerisation the heat released by the polymerisation reaction can be determined online from temperature measurements, see these references for details about reaction calorimetry - MacGregor (1986), Bonvin et al (1989), Moritz (1989), Schuler and Schmidt (1992). Calorimetric measurements can be used to infer the free amount of monomers in the copolymerisation and also the overall conversion (Urretabizkaia et al, 1993 Gugliotta etal, 1995c Hammouri etal, 1999 Saenz de Buruaga etal, 2000) by means of an observer/estimator. Basically the estimator solves the monomer material balance differential equations using the heat of reaction as input variable instead of the theoretical polymerisation rate. Therefore, the estimator uses the enthalpies of polymerisation of the monomers and the reactivity ratios as parameters. This information is compared with the... 

The reactions depicted in Eq. (1) are suitable for calorimetric investigations since they proceed rapidly and quantitatively as monitored by NMR. spectroscopy. The. solution calorimetric protocol has been described elsewhere." The enthalpy values were determined by anaerobic solution calorimetry in THF at 30 C by reacting 4 equivalents of each carbene with one equivalent of tetramer. The results of this study are presented in Table I. 

Equation can also be used to calculate the standard enthalpy of formation of a substance whose formation reaction does not proceed cleanly and rapidly. The enthalpy change for some other chemical reaction involving the substance can be determined by calorimetric measurements. Then Equation can be used to calculate the unknown standard enthalpy of formation. Example shows how to do this using experimental data from a constant-volume calorimetry experiment combined with standard heats of formation. 

Finally, most previous calorimetric studies in this field have been devoted to adsorption processes only, and very seldom were these studies extended to the investigation of complete catalytic reactions. The work of Garner and his collaborators in Bristol (18) is a notable exception. Heat-flow calorimetry is particularly convenient for such studies (19). 

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