Isothermal heat flow calorimeters

The three types of isothermal heat flow calorimeters described above can be used to measure heat flow in semi-batch reactions, where one or more reactants are charged to the reactor and the other reactants are added at controlled rates throughout the reaction. With careful design the heat flow calorimeters can simulate process variables such as feed rate, stirring, distillation and reflux . 

Stockton et al, 1986. North American Thermal Analysis Conf September 1986. Wright, T.K. and Butterworth, C.W., 1987, Isothermal heat flow calorimeter. Hazards from Pressure, Symposium Series No. 102, 85-96 (IChemE, Rugby, UK). Regenas, W., 1979, Am Chem Soc Symp Ser No. 65, 37-49. 

This Dewar experiment showed that the overall process was in fact endothermic with a total heat uptake of around 100 kJ kg . The majority of the reaction took place during the heat-up phase as no further temperature change in the Dewar contents was observed during the hold period. The actual reaction between formaldehyde and the glycols is exothermic. This was demonstrated by the isothermal heat flow calorimeter experiment in which the acid catalyst was added not at the start but once the process was at 80°C. The relatively small exothermic peak, compared to that estimated theoretically, shows that the effect of the acid was to ensure that the reaction went to completion and that the reaction itself takes place without a catalyst. The reason for the difference between... 

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. 

It appears therefore that during the operation of all usual calorimeters, temperature gradients are developed between the inner vessel and its surroundings. The resulting thermal head must be associated, in all cases, to heat flows. In isoperibol calorimeters, heat flows (called thermal leaks in this case) are minimized. Conversely, they must be facilitated in isothermal calorimeters. All heat-measuring devices could therefore be named heat-flow calorimeters. However, it must be noted that in isoperibol or isothermal calorimeters, the consequences of the heat flow are more easily determined than the heat flow itself. The temperature decrease... 

Since heat exchange between the calorimeter vessel and the heat sink is not hindered in a heat-flow calorimeter, the temperature changes produced by the thermal phenomenon under investigation are usually very small (less than 10 4 degree in a Calvet microcalorimeter, for instance) (23). For most practical purposes, measurements in a heat-flow calorimeter may be considered as performed under isothermal conditions. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

The isothermal and isoperibol calorimeters are well suited to measuring heat contents from which heat capacities may be subsequently derived, while the adiabatic and heat-flow calorimeters are best suited to the direct measurement of heat capacities and enthalpies of transformation. 

Steady-state isothermal heat-flow balance of a general type of reaction calorimeter... 

Regenass [10] reviews a number of uses for heat flow calorimetry, particularly process development. The hydrolysis of acetic anhydride and the isomerization of trimethyl phosphite are used to illustrate how the technique can be used for process development. Kaarlsen and Villadsen [11,12] provide reviews of isothermal reaction calorimeters that have a sample volume of at least 0.1 L and are used to measure the rate of evolution of heat at a constant reaction temperature. Bourne et al. [13] show that the plant-scale heat transfer coefficient can be estimated rapidly and accurately from a few runs in a heat flow calorimeter. 

Heat flow calorimetry is often used to determine the heat profile of the desired reaction and, from this, the heat of reaction. These calorimeters are best operated in an isothermal mode as it is often difficult to interpret the resulting heat profile curve when there is a heating stage with a batch reaction. The heat profile obtained in such a case has a distinct curve due to the heating and it is necessary to repeat the experiment, as closely as possible but without any reaction taking place, in order to determine the baseline. Batch processes are therefore frequently converted to isothermal, semi-batch processes when using a heat flow calorimeter to determine the heat of reaction. 

For this process it can be postulated that the reaction is slow at the process temperature of 80°C as there is a plant hold time of 4 hours and a catalyst is used. Therefore an isothermal heat flow experiment was carried out in which the paraformaldehyde and the mixture of glycols were charged to the calorimeter and heated to 80°C. The acid catalyst was then added when the baseline had stabilized. The resulting heat profile is shown in Figure A2.3. It can be seen that on addition of the catalyst an immediate exotherm occurred, although surprisingly with a heat output of only around 5.9 kJ kg. ... 

Setaram C80 heat flow calorimeter sample mass, polyisocyanate 264 mg, polyol 292 mg crucible, mixing cell with metallic membrane isothermal at 80 °C, sensitivity 1 jiV. Initially, the two reagents are isolated by a membrane and stabilized in the calorimeter at 80 °C. The membrane is cut, and mixing is effected by manual rotation of the stirrer. 

A heat-flow calorimeter is a variation of an isothermal-jacket calorimeter. It uses a thermopile (Fig. 2.7) to continuously measure the temperature difference between the reaction vessel and an outer jacket acting as a constant-temperature heat sink. The heat transfer takes place mostly through the thermocouple wires, and to a high degree of accuracy is proportional to the temperature difference integrated over time. This is the best method for an extremely slow reaction, and it can also be used for rapid reactions. 

Heat-flow Calorimeters.— In calorimeters of the adiabatic or isoperibol types, heat-exchange between the calorimeter and its surroundings is either eliminated or is restricted to a small, accurately determined amount. An alternative method is to transfer the heat of reaction completely to a heatsink, so that both the calorimeter and the heat-sink remain essentially isothermal and the calorimetric determination consists of measuring the heat transferred. Two main types have been employed. 

Although isothermal phase-change calorimeters operate on the heat-flow principle, the use of the term heat-flow is usually restricted to calorimeters of the second type where thermometry is used to measure the small temperature differences which arise. In the most widely used form of heat-flow calorimeter, a thermopile provides the main thermal conduction path between the reaction vessel and the heat-sink and is also used to measure the small temperature difference between them. The enthalpy change is calculated from the area under the temperature-time curve, or thermogram. 

Names have been given to calorimeters (usually having these three components) that are operated in certain modes. In the isothermal calorimeter the temperature Tc of the calorimeter is kept equal to the temperature Ts of the surroundings and both are held constant. In the adiabatic calorimeter Tg is kept equal to T although both may change. In the isoperibol calorimeter 7 is kept constant and 7i, usually initially near 7, undergoes an excursion. In the constant heat-flow calorimeter (7 — 7i) is kept constant. 

Fig. 1. Theoretical models of adiabatic (1), isothermal (2), and heat-flow (3) calorimeters.

The rate of isothermal heat evolution in lignocellulosic sheet material was studied at temperatures between 150 and 230°C using a labyrinth air flow calorimeter and commercial hardboards, medium density boards and laboratory hardboards of holocellulose, bleached kraft and groundwood, the latter with and without fire retardants. 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

Other instruments include the Calvet microcalorimeters [113], some of which can also run in the scanning mode as a DSC. These are available commercially from SETARAM. The calorimeters exist in several configurations. Each consists of sample and reference vessels placed in an isothermally controlled and insulated block. The side walls are in intimate contact with heat-flow sensors. Typical volumes of sample/reference vessels are 0.1 to 100 cm3, The instruments can be operated from below ambient temperatures up to 300°C (some high temperature instruments can operate up to 1000°C). The sensitivity of these instruments is better than 1 pW, which translates to a detection limit of 1 x 10-3 W/kg with a sample mass of 1 g. 

The two basic types of reaction calorimeters commonly used for safety assessments are isothermal (including both heat flow and power compensation calorimeters) and adiabatic. 

Usually, isothermal calorimeters are used to measure heat flow in batch and semi-batch reactions. They can also measure the total heat generated by the reaction. With careful design, the calorimeter can simulate process variables such as addition rate, agitation, distillation and reflux. They are particularly useful for measuring the accumulation of unreacted materials in semi-batch reactions. Reaction conditions can be selected to minimize such accumulations. 

The principles of titration calorimetry will now be introduced using isoperibol continuous titration calorimetry as an example. These principles, with slight modifications, can be adapted to the incremental method and to techniques based on other types of calorimeters, such as heat flow isothermal titration calorimetry. This method, which has gained increasing importance, is covered in section 11.2. 

As mentioned above, titration methods have also been adapted to calorimeters whose working principle relies on the detection of a heat flow to or from the calorimetric vessel, as a result of the phenomenon under study [195-196,206], Heat flow calorimetry was discussed in chapter 9, where two general modes of operation were presented. In some instruments, the heat flow rate between the calorimetric vessel and a heat sink is measured by use of thermopiles. Others, such as the calorimeter in figure 11.1, are based on a power compensation mechanism that enables operation under isothermal conditions. 

Figure 11.5 Typical curve for a continuous titration calorimetry study of an exothermic reaction, using the calorimeter of Figure 11.1 in the heat flow isothermal mode of measurement./ is the frequency of the constant energy pulses supplied to the heater C in Figure 11.1 b. Adapted from [196,197],...

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. 

Differential Scanning Calorimetry (DSC) This is by far the widest utilized technique to obtain the degree and reaction rate of cure as well as the specific heat of thermosetting resins. It is based on the measurement of the differential voltage (converted into heat flow) necessary to obtain the thermal equilibrium between a sample (resin) and an inert reference, both placed into a calorimeter [143,144], As a result, a thermogram, as shown in Figure 2.7, is obtained [145]. In this curve, the area under the whole curve represents the total heat of reaction, AHR, and the shadowed area represents the enthalpy at a specific time. From Equations 2.5 and 2.6, the degree and rate of cure can be calculated. The DSC can operate under isothermal or non-isothermal conditions [146]. In the former mode, two different methods can be used [1] ... 

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. 

If electronically excited species can be made to give up their excess energy to some active material, then their concentration may be determined by measurement of the rate of heat liberation. The method is, of course, well established for the measurement of oxygen and hydrogen atom concentrations, and the most accurate experimental technique is to use the isothermal hot-wire calorimeter developed by Tollefson and LeRoy.42 The amount of power needed to maintain a catalytic probe at a constant temperature is reduced if heat is liberated at the probe, and no correction is needed for heat losses. The flow of energy-rich species, [Pg.325]

The only heat-flow rate discussed so far has been the heat flow through the reactor jacket (ijFlow in Fig- 8.1). For the general case of an isothermal reaction, the main heat-flowrates that have to be considered in a reaction calorimeter are shown in Fig. 8.2 and will be discussed next. In this discussion, ideal isothermal control of the reaction temperature, %, will be assumed [4]. Consequently, no heat accumulation terms of the reaction mixture and the reactor inserts are shown in Fig. 8.2. However, this underlying assumption does not hold for all applications and apparatuses. 

Fig. 8.2 Main heat-flow rates that have to be considered in heat-flow, heat-balance and power-compensation reaction calorimeters running under strictly isothermal conditions [4]. The heat-flow rates inside a Peltier calorimeter are analogous (compare with Fig. 8.1). The direction of the heat-flow arrows corresponds to a positive heat-flow rate. For explanation of the different heat-flow rates, see the text.

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