Heat-flux calorimeters

Hardee et al. (1978) investigated pure methane and premixed methane-air fireball reactions. They used balloons filled either with 0.1 to 10 kg pure methane, or else with stoichiometric air-methane mixtures. The balloons were cut open just prior to ignition. Integrating heat-flux calorimeters, located either inside the balloons or at their edges, were used to measure the thermal output. 

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... 

A. Rojas-Aguilar, A. Valdes-Ordonez. Micro-combustion Calorimetry Employing a Calvet Heat Flux Calorimeter. J. Chem. Thermodynamics 2004, 36, 619-626. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

Lievois, J.S., Development of an Automated, High Pressure Heat Flux Calorimeter and Its Application to Measure the Heat of Dissociation of Methane Hydrate, Ph.D. Thesis, Rice University, TX (1987). 

Fig. 17. Schematic design of a heat flux calorimeter. Both the temperature in the reactor and in the circuit (or jacket) are measured as sensitively and reproducibly as possible. A well-tuned temperature controller keeps the reactor temperature constant by feeding the circuit with warmer or colder water or oil. The circulating water or oil can be taken from either a chilled and a heated reservoir or, as shown, be heated or cooled via external heat exchangers. Calibration is made possible via an electric heater of known power...

Heat flux calorimeters are bioreactors equipped with special temperature control tools. They provide a sensitivity which is approximately two orders of magnitude better than that of microcalorimeters, e.g. [33,258]. The evaluation and description of microbial heat release is based on a heat balance heat yields and the heat of combustion of biological components are central parameters for quantification [70]. Measurements obtained so far have been used to investigate growth, biomass yield, maintenance energy, the role of the reduction degree of substrates, oxygen uptake [414] and product formation [272]. 

The heat produced during the growth of microorganisms can be also be used for biomass concentration estimation. Different calorimetric devices (external-flow, twin-type, and heat-flux calorimeters) and different calorimetric techniques (dynamic and continuous calorimetry) have been used for on-line biomass estimation [8j. In most cases, the experimental setup is complicated and measurements are restricted to relatively small volumes (less than 1 L). Larger devices (continuous calorimeters for volumes up to 14 L) were studied by Luong and Volesky [123-125]. One of the best devices seems to be the heat-flux calorimeter developed by Marison and von Stockar. Several applications to bioprocess monitoring are given by the authors [126-129]. 

Heat flux calorimeter making use of a heat conduction path... 

Isoperibol with constant temperature of the surroundings. The thermal resistance, 7 , is said to be here of finite magnitude . This category includes the classic liquid calorimeter and the heat-flux calorimeter . 

K.-H. Breuer, W. Eysel and G.W.H. HShne, Enthalpy standards for heat flux calorimeters. Paper presented at the 5. Ulmer Kalorime-trietage, Ulm, Germany (1983). 

For larger temperature differences such calorimeters can also be used as heat-flux calorimeters, using only the last two terms in Eq. (4). Because of the small losses, Tian-Calvet calorimeters have found application for the measurement of slow, biological reactions. 

Heat-flux Calorimeter with Furnace Control... 

Figure 4.40. Quasi-isothermal melting of indium using a heat-flux calorimeter with control of the modulation at the sample temperature (sinusoidal modulation, the indicated sample temperatures are uncorrected) [52].

Fig. 3. Representation of a two heat-flux calorimeter showing (a) Boersma thermocouple placement and (b) the Tian-Calvet design. The schematic diagram (c) is appropriate for analysis of the response of both types of calorimeters. Symbols in (c) are subscript T refers to temperature, R refers to reference the temperatures of the block, sample, container, reference, and reference containers given by Tb, Tsc, T-r, Trq, respectively capital R refers to heat transfer resistance in the instrument (9).

Figure 21. Operating principle of a temperature-difference scanning calorimeter (heat-flux calorimeter)...

Differential heat flux calorimeters, consisting of mixing cells and thermostating devices, were used for measiuing the enthalpy of mixing or reaction of two fluids, containing water and organic liquids (ethanol or methanol), at temperatures up to 573 K and pressures up to 20 MPa (Mathonat et al, 1994 Hynek et al., 1999). 

Busey et al. (1984) constructed a flow mixing device from a cylindrical coil of stainless steel tube which fitted into a high-temperature Tian-Calvet type, heat-flux calorimeter. Fluids were pumped at the same flow rate through both sample and reference cells. Platimun-rhodimn capillary... 

Commercially available power compensation and heat flux calorimeters are competitive in stability, accuracy, and sensitivity, fractions of a millijoule being detectable with good reproducibility. Although thermoelectric compensation is more efficient in time (larger power values are detected in shorter time intervals), the kinetics of adsorption may impose limitations on this advantage. 

However, the trend is toward the use of microcalorimeters (in the isothermal or scanning mode) with high sensitivities, especially for a more sensitive observation of the weak thermal phenomena that occur between 0°C and 100°C [10-13]. Parameters such as the heat of solution may also be of interest, so the use of solution calorimeters or of heat flux calorimeters with stirring devices is also recommended for studying certain food systems [14],... 

Top-notch power compensation calorimeters can be very sensitive and are an excellent tool for academic studies. However, they can be noisy, not so easy to calibrate, and frequently present baseline stability problems. Heat flux calorimeters tend to be more robusL with better baseline... 

A Tian-Calvet heat flux calorimeter was used in the measurements described in ref. [2]. This type of calorimeter is also called isothermal [4, 5], in contrast to other kinds of calorimeter. A tutorial [6] on heat-conduction calorimetry gives a good account of the technique. Peak integration of the heat flux against time may be performed by a numerical integration method, such as Simpson s method, on a personal computer interfaced to the calorimeter [7]. 

Any good-quality heat-flux calorimeter or isothermal titration calorimeter, such as those marketed by TA Instruments (USA) and the associated company Thermometric (Sweden), CSC (USA), Microcal (USA) or Setaram (France). 

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