Thermal-rise calorimeter

The RSST calorimeter (see Annex 2) is a pseudo-adiabatic, low thermal inertia calorimeter, intended for screening purposes. It can identify the system type and measure adiabatic rate of temperature-rise and rate of gas generation by the reacting mixture. It is therefore well-suited to the task of selecting the overall worst case scenario from a small number of candidates. Alternatively, a calorimeter designed to obtain relief system sizing data may be used for this purpose (see Annex 2). 

Measuring the gross heating value (mass) is done in the laboratory using the ASTM D 240 procedure by combustion of the fuel sample under an oxygen atmosphere, in a bomb calorimeter surrounded by water. The thermal effects are calculated from the rise in temperature of the surrounding medium and the thermal characteristics of the apparatus. 

Some early calorimeters use thermal methods based on principles of heat and mass balance (12) and temperature rise of a constant flow of air through the combustion chamber (13). These calorimeters suffer from many drawbacks associated with their design. Heat and mass balance requires numerous measurements to account for all heat and mass flows. In most cases, thermal lag and losses in the equipment occur, which are not easily calculated. 

Figure 2.1 A bomb calorimeter. The food is ignited by an electric current within the inner compartment, which is known as the bomb because the reaction within the box is generally so rapid as to be almost explosive. Insulation prevents heat loss and the thermometer measures the rise in temperature of the water that surrounds the bomb. From this increase, and the thermal capacity of the apparatus, the amount of heat released can be calculated.

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. 

The average value of q can be calculated as shown below. Because the thermal inertia of the calorimeter is low and the reaction is not nearing completion at the maximum accumulated pressure (the temperature corresponding to the maximum accumulated pressure is still on the straight-line portion of Figure 6.4), a simple correction can be made by multiplying the measured rate of temperature rise by the thermal inertia (see A2.7.2). ... 

Closed system tests, using an unvented test cell (see Figure A2.5) or Dewar flask, can be used for vapour pressure systems. The runaway is initiated in the way that best simulates the worst case relief scenario at plant-scale. The closed system pressure and temperature are measured as a function of time. Most commercial calorimeters include a data analysis package which will present the data in terms of rate of temperature rise, dT/dt, versus reciprocal temperature (-1 / ), and pressure versus reciprocal temperature (see Figure A2.10). However, it is important to correct the temperature data for the effects of thermal inertia. See 2.7.2. 

The evaluation of chemical reaction hazards involves establishing exothermic activity and/or gas evolution that could give rise to incidents. However, such evaluation cannot be carried out in isolation or by some simple sequence of testing. The techniques employed and the results obtained need to simulate large-scale plant behavior. Adiabatic calorimeters can be used to measure the temperature time curve of selfheating and the induction time of thermal explosions. The pertinent experimental parameters, which allow the data to be determined under specified conditions, can be used to simulate plant situations. 

The first experiments of gas adsorption calorimetry by Favre (1854) were made with an isoperibol calorimeter. More recently, refinements were introduced by Beebe and his co-workers (1936) and by Kington and Smith (1964). Because of the uncontrolled difference between the temperature of the sample and that of the surroundings, Newton s law of cooling must be applied to correct the observed temperature rise of the sample. In consequence, any slow release of heat (over more than, say, 30 minutes), which would produce a large uncertainty in the corrective term, cannot be registered. For this reason, isoperibol calorimetry cannot be used to follow slow adsorption equilibria. However, its main drawback is that the experiment is never isothermal during each adsorption step, a temperature rise of a few kelvins is common. The corresponding desorption (or lack of adsorption) must then be taken into account and, after each step, the sample must be thermally earthed so as to start each step at the same temperature. In view of these drawbacks,... 

Latent heats of evaporation of liquefied gases at low temperatures have been determined by various methods. Dewar, and Behn, dropped pieces of metal of known specific heat into the liquid and measured the gas evolved. Estreicher heated the liquid in a double Dewar vessel electrically and measured the volume of gas evolved. In Donath s apparatus (Fig. 4.VIII L) the gas passed through a copper spiral in a block of lead A, so assuming a constant temperature about 2° above the temperature in the metal calorimeter B. The gas then passed to a vessel inside B connected by a thin German-silver tube. The calorimeter was in two parts, between which was a platinum heating spiral for determining the thermal capacity. Outside was an adiabatic mantle C. The whole was in a vacuous copper jacket D. The temperature differences between A and B, and B and C, were determined by thermocouples. The rise in temperature... 

To determine the entropy of n-butane at 298 K we must measure the heat capacity from within a few degrees of the absolute zero of temperature up to room temperature. This is done in an adiabatic vacuum calorimeter (Fig. 5,6) in which the sample under investigation is thermally insulated by locating it in an evacuated enclosure. A known quantity of electrical heat can be added and the rise in temperature measured with a platinum resistance thermometer. After a correction for the heat capacity of the vessel, the heat capacity of the compound it contains may be directly calculated from the equation... 

The galvanometer zero was at 172, as was found by reversing the leads from the thermo-couple by means of a mercury commutator free from thermal effects the initial temperature of the calorimeter is hence calculated as 21 0 + 1 7 = 22 7°. The above heating effect corresponds with a rise in the temperature of the calorimeter of... 

Hohne (145) pointed out that the function principle of DSC can give rise to calibration errors in case of phase transitions disturbing the steady-state conditions. The cause of this problem is the temperature dependence of the coefficients of heat transfer, leading to weak nonlinearity of the calorimeter. This results in a dependence of the calibration factor on parameters such as mass and thermal conductivity of the sample, heating rate, peak shape, and temperature. By theoretical considerations and calculations, the uncertainty of the calibration factor due to the variation of sample parameters can be 1-5%, depending on the temperature and the instrument involved. 

The American Society for Testing and Materials (ASTM) standard E422 provides details for the construction of a single circuit, water-cooled calorimeter probe to measure heat flux [27]. Anderson and Stresino [28], Reed [29], and Fay [30] studied flames impinging on a so-called surface probe. In those studies, two identical, water-cooled copper plates were connected in series and separated by a thermal barrier. A single water-cooling circuit passed across both sections. The flame was traversed from the first to the second section. The water temperature rise through each plate was measured as a function of distance from the flame centerline. The data was inverted to determine the spatial heat flux distribution. See Reed... 

Enthalpies of reaction in solution are generally measured in an isothermal jacketed calorimeter. This consists of a calorimetric vessel that contains a certmn amount of one of the reactants that is either a liquid or, if a solid is involved, it has been dissolved in a suitable solvent. The other reactant is sealed in a glass ampoule that is placed in a holder. The vessel is enclosed in a container, which is placed in a thermostatted bath with the temperature controlled to 0.001 °C. When the system has reached thermal equilibrium, the ampoule is broken and the reaction is initiated. Throughout the experiments the temperature is measured as a function of the time and a temperature-time curve with approximately the same shape as the ones obtmned in combustion calorimetry, vdth fore-period, reaction-period and after-period is obtained. The observed temperature rise is due to several sources die heat transferred from the thermostatted bath, the energy of the reaction and the stirring energy. To correct... 

Drop calorimeters are widely used because of their simplicity. A specimen, often contained in a metal capsule, is heated to some appropriate constant temperature in an oven or furnace and allowed to drop into liquid in a stirred calorimeter. The temperature rise of the calorimeter is monitored, and from this the specific heat can be calculated. The thermal capacity of the calorimeter must be determined in a separate experiment, and heat losses or gains to or from the environment must be allowed for. 

The usual scanning techniques are differential thermal analysis (DTA) [78] and differential scanning calorimetry (DSC) [79-81], In these methods it is assumed that the heat loss from the calorimeter is a function of temperature only. By comparing the rate of heat input and temperature rise for a polymer sample with that of a standard, usually synthetic sapphire, the specific heat of the polymer can be obtained. 

Adiabatic Calorimeters.—In an adiabatic experiment, the two liquids are mixed in a vessel which is thermally isolated from its surroundings. If is positive (endothermic) then there will be a lowering of the temperature. In practice, electrical energy is usually supplied to the calorimeter partially to nullify the temperature drop. If JT is negative (exothermic) then the temperature of the calorimeter rises on mixing. A second experiment is necessary to determine the amount of ener required to produce the same temperature rise. Alternatively two identical calorimeters can be used. A known amount of electrical energy... 

A specimen of 100 mm square is fastened horizontally and then ignited from below by a Maker burner with a flame burning a controlled amount of gas metered through a rotameter. Temperature rise is measured above the ignited point at the surface of the specimen by a copper calorimeter with 3 sensors. The thermal protective performance rating is calculated from the amount of energy transmitted per unit area (F) determined by preliminary calibration and from the exposure time (time to the thermal end-point, t) ... 

A further use of DSC in this area is the detection of incomplete (residual) curing in solid samples. A thermally curable epoxy adhesive that has not been fully cured will polymerize as the sample is heated in the calorimeter. This polymerization will give rise to a small exotherm that can be used to estimate the degree of cure in the original sample. 

The use of calorimeters is based on the measurement of heat, since the energy deposited in the thermally insulated mass of the absorber is converted to heat. The measured energy per unit mass results directly in the absorbed dose, being the product of the measured temperature rise and the specific heat of the absorber (see O Eq. 49.18). Thus a calorimeter consists of the absorber (also called calorimeter body or the core of the calorimeter), the instrumentation to... 

Let us analyze [283, 285] the influence of the mutual locations of the heat sources and temperature sensors on the value of the energy equivalent for a calorimeter treated as a system of two domains. As stated previously, in the corrected temperature rise method three periods are distinguished during the experiment < to, t >- initial < ti, t2>- main and < t2, tf> - final. In the initial and final periods, the calorimeter is a thermally inert object in the main period, the heat effect is generated in the calorimeter. 

Some flow calorimeters (continuous calorimeters) make use of air as a heat transfer medium in other cases, gases or liquids react with each other or are products of the reaction. In the latter case, a possible approach to the measurement of amounts of substances consists in allowing the newly formed phase (usually a gas) to leave the system via a flow meter. Here the flow rate provides a measure of the quantity of substance transformed per unit time. Usually a pressure difference is the measurand as in capillary flow meters or is caused by the back pressure of the measuring instrument however, the possibility of pressure rises (caused by a buildup ) in the vessel must be taken into account. Other techniques for measuring amounts of gas make use of displacement gas meters, turbine meters, or ultrasonic meters. In these cases, the volume flow is the measured quantity. For measuring the mass flow, Coriolis or thermal mass flow meters can be used. In any case, it is very difficult to reduce the uncertainty of flow measurements below approximately 1%. This can only be achieved in exceptional cases when great effort is made to calibrate the meter with fluids of similar and known thermophysical properties (e.g., heat capacity, thermal conductivity, viscosity, density, etc.). 

Figure 7.17 shows this function graphically (cf Section 6.2.2). Here the time constant of the exponential temperature rise is r = Csc Rth2> and the final temperature of the sample container is Tfin = Tp + 0i l th2- Otie can clearly see the influence of the thermal resistance Rthi on the time constant r and on the sensitivity (maximal AT = Tfi — Tp) of the calorimeter... 

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