Flow mixing calorimetry

The enthalpy change associated with formation of a thermodynamically ideal solution is equal to zero. Therefore any heat change measured in a mixing calorimetry experiment is a direct indicator of the interactions in the system (Prigogine and Defay, 1954). For a simple biopolymer solution, calorimetric measurements can be conveniently made using titra-tion/flow calorimeter equipment. For example, from isothermal titration calorimetry of solutions of bovine P-casein, Portnaya et al. (2006) have determined the association behaviour, the critical micelle concentration (CMC), and the enthalpy of (de)micellization. 

Besides steady state measurements, there is probably good reason to use flow micro calorimetry for the study of non-steady-state behavior in systems with immobilized bio catalysts. Here, the mathematical description is more complicated, requiring the solution of partial differential equations. Moreover, the heat response can evolve non-specific heats, like heat of adsorption/desorption or mixing phenomena. In spite of these complications, the possibility of the on-line monitoring of the enzyme reaction rate can provide a powerful tool for studying the dynamics of immobilized biocatalyst systems. 

Solution calorimetry covers the measurement of the energy changes that occur when a compound or a mixture (solid, liquid or gas) is mixed, dissolved or adsorbed in a solvent or a solution. In addition it includes the measurement of the heat capacity of the resultant solution. Solution calorimeters are usually subdivided by the method in which the components are mixed, namely, batch, titration and flow. 

Various flow calorimeters are available connnercially. Flow calorimeters have been used to measure heat capacities, enthalpies of mixing of liquids, enthalpy of solution of gases in liquids and reaction enthalpies. Detailed descriptions of a variety of flow calorimeters are given in Solution Calorimetry by Grolier [17], by Albert and Archer [18], by Ott and Womiald [H], by Simonson and Mesmer [24] and by Wadso [25]. 

The PCFC technique utilizes traditional oxygen depletion calorimetry. The specimen is first heated at a constant rate of temperature rise (typically 1-5 K/s) in a pyrolyzer. The thermal decomposition products are swept from the pyrolyzer by an inert gas. The gas stream is mixed with oxygen and enters a combustor at 900°C, where the decomposition products are completely oxidized. Oxygen concentrations and flow rates of the combustion gases are used to determine the oxygen depletion involved in the combustion process, and the heat release, as well as the heat release capacity (HRC), is determined from these measurements. 

The microanalytical methods of differential thermal analysis, differential scanning calorimetry, accelerating rate calorimetry, and thermomechanical analysis provide important information about chemical kinetics and thermodynamics but do not provide information about large-scale effects. Although a number of techniques are available for kinetics and heat-of-reaction analysis, a major advantage to heat flow calorimetry is that it better simulates the effects of real process conditions, such as degree of mixing or heat transfer coefficients. 

The combination of rapid mixing and fast detection systems allows cationic polymerisations to be followed on an even shorter time scale than with adiabatic calorimetry. Recent commercial stop-flow spectrophotometers have a dead time of about 15 msec, an improvement of more than one order of magnitude over previous home-made models. This implies that reactions with half lives of less than 100 msec can be analysed kinetically with a good degree of accuracy. Hi -vacuum techniques are not compatible with these instruments and all operations are therefore carried out in an inert atmosphere. 

Solution calorimetry allows us to investigate processes that involve enthalpy changes. Adiabatic microcalorimeters and isoperibol calorimeters used in batch modes or flow modes allow for the precise determination of the heat of solution. Mixing the reactants is accomplished by breaking a bulk allowing reactants to mix or by special chambers where the reactants are mixed together. 

The heat generated by the reaction is directly proportional to the reaction rate for simple systems. The interpretation of the thermogram is more complicated in the case of multiple reactions or simultaneous enthalpic processes such as mixing, dissolution, phase transition, crystallization, etc. Two different calorimetric methods will be discussed power compensation and heat flow calorimetry. 

The micro-methods (differential-thermal analysis = DTA, differential scanning calorimetry = DSC) are quick and require little experimental effort, but they provide no means of adding reactants during measurements, and heterogeneous scunples cannot be mixed. All micro-methods use a twin (or differential) design to eliminate disturbing effects, i.e. an inert sample is exposed to the same environment conditions as the sample under investigation and the difference of the two heat flows is recorded. 

A crude yield of 90% was obtained in ether at —25 °C. When performed in a batch mode on 70 mg scale, no safety issues were taken and an 81% yield was obtained [42]. Reaction calorimetry reveals a very exothermic reaction after feeding and subsequent mixing with an initiation period that is, the response of the heat flow curve is delayed by about 1 min as compared to the feed curve (see Figure 11.6). Therefore, the heat release rate with this mode of addition is not feed controlled. The reaction is very sluggish, since the reaction occurs at a single blow as soon as 60% of the material has been added. Calculating the worsttemperature rise shows that the reaction would rapidly increase in temperature and can approach the solvent reflux temperature, possibly throwing out the reaction mixture in case of... 

The calorimetric techniques for measuring heats of mixing two fluids can be classified into their mode of measurement and their principle of heat detection. The isothermal displacement calorimetry will refer to a static mode and flow calorimetry, to a dynamic mode . The principles of heat detection in the following examples will be power compensation or heat flux determination. 

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