Calorimeter

Calorimeters can determine the heat content of their samples by (1) direct burning, (2) calculation from composition, and (3) special designs. Their operation can be continuous, cyclic, or portable. Their errors range from 0.5 to 2% full scale (FS). 

In renewable energy processes, the gaseous fuels include GH2, biodegradation-generated methane, and other gases. Calorimeters are analyzers that measure the heat value or energy content of gaseous fuels. There are two 

Some of the terms used in connection with calorimetry are listed as follows  

Saturated and Dry Btu Saturated British thermal unit (Btu) is the heating value that is detected when gas is saturated with water vapor. This state is defined as the condition when the gas contains the maximum amount of water vapor at base ambient conditions. Dry Btu is the heating value when the gas is dry. 

Combustion air requirement index (CARI) This is a dimensionless number indicating the amount of air required to support the combustion of a fuel gas. 

A further positive reaction to this dramatic incident took place in the central research department of the company. A physico-chemist had the idea of using his differential scanning calorimeter (DSC) to look at the energy involved in this reaction. He performed an experiment with the initial concentration and a second with a higher concentration. The thermograms he obtained were different and he realized that he could have predicted the incident (see Exercise 11.1). As a consequence, it was decided to create a laboratory dedicated to this type of experiment. This was the beginning of the scientific approach of safety assessments using thermo-analytic and calorimetric methods. From this time on, many different methods were developed in different chemical companies and became commercially distributed, often by scientific instrument companies. 

In this chapter, some of these instruments are reviewed. A first section is a general introduction to calorimetric principles. In a second part, some methods commonly used in safety laboratories are reviewed. This is not an exhaustive review of such instruments, but based only on the experience of the author. 

Heat cannot be directly measured. In most cases heat measurement is made indirectly by using temperature measurement Nevertheless, there are some calorimeters able to measure directly the heat release rate or thermal power. Calorimetry is a very old technique, which was first established by Lavoisier in the 18th century. In the mean time, a huge choice of different calorimeters, using a broad variety of designs and measurement principles, were developed. 

There are different ways to classify calorimetric and thermo-analytic methods  

If a sohd body at its boiling point is allowed to absorb a quantity of heat Q, this quantity will cause m grams of the solid to melt, where m is determined by the equation Q = mw. If w is known we can arrive at by a determination of m. 

The solid used in Bunsen s calorimeter is pure ice, which is kept at 0° C. by immersing the calorimeter (Fig. 7) in a mixture of crushed ice and water. The tube C is filled with mercury, while B contains water, part of which has been converted into ice by cooling the inside of the tube A with a freezing mixture. The heat evolution which we wish to investigate is then made to take place in A. 

As ice contracts on melting by an amount which has been measured very accurately, we can deduce the mass m of ice, which has been melted, from the displacement of the mercury meniscus on the calibrated extension of the tube C. 

To determine Q the body is suspended from the arm of a balance, and the air surrounding the body is then displaced by saturated vapour at a temperature g. Liquid condenses on the colder suspended body until its temperature has become equal to the temperature g of the vapour. By determining the increase 

Using the specific heat of water, the mass of water, and the measured change in temperature, we can solve for q in the equation q = mcAT. Since q = AH at constant pressure, we have the heat of reaction. 

A bomb calorimeter measures energy change at constant volume. A bomb calorimeter tells us the internal energy change in a reaction. (Recall that at constant volume q - ALL) In a bomb calorimeter, a steel container full of reactants is placed mside another rigid, thermally insulated container. 

When the reaction occurs, heat is transferred to the surrounding water (shown in the diagram). Using the known heat capacity of the container and the equation q = CAT, we can deduce the heat of the reaction, and thus the internal energy change in tire reaction. 

20 grams of NaCl is poured into a coffee cup calorimeter containing 250 ml of water. If the temperature inside the calorimeter drops 1°C by the time the NaCl is totally dissolved, what is the heat of solution for NaCl and water (specific heat of water is 4.18 J/g °C.) 

Using a bomb calorimeter, the change in energy for the combustion of one mole of octane is calculated to be 

Methods giving absolute energy values thermal measurements (calorimeter, thermal probes). 

Methods based on acoustic pressure capacitive or piezoelectric probes, optical methods, etc. 

Methods based on nonlinear effects e.g. radiation forces. He also introduced other subdivisions depending on whether the method could be used for total or local power measurements, under free or restrained field conditions. 

There are two different kinds of calorimeter adiabatic (or quasi-adiabatic calorimeters) and non-isothermal, non-adiabatic calorimeters (often referred to as n-n calorimeters). The accuracy of measurements made using such methods will be high if  

Calorimetric methods are quite general they can be used under cavitating conditions and in either free or restricted ultrasonic fields. Essentially the technique consists of measuring the rate of temperature increase in the sonicated liquid and from this calculating the power input according to Eq. (7), 

An alternative is the isothermal dilution calorimeter. For an endothermic system, one component is slowly injected into the second component with the simultaneous addition of electrical energy sufficient to maintain the calorimeter isothermal. The addition is discontinued at any desired composition and the excess enthalpy determined from the initial amount of substance of component B in the vessel, the amount of substance of component A injected, and the electrical energy added to maintain isothermal conditions. The apparatus is normally designed so that the entire composition range can be covered in two experimental runs. For exothermic systems it is possible to add electrical energy 

An apparatus suitable for endothermic systems was first described by Van Ness and co-workers in 1961. Several other calorimeters based on that original design have been reported in the literature. An apparatus which 

Two corrections to the measured energy are required. The power dissipated in the vessel by the continuously running stirrer is of the order of 100 fjiW. This causes the steady temperature of the vessel to be higher than that of the thermostat by an amount AT 0.01 K. The calorimeter therefore shows a small blank correction which is evaluated from the results of a run where the same liquid is used in both the burette and the mixing vessel. At first sight it would appear that the energy input required to maintain constant temperature in a blank run would be 

Mrazek and H. C. Van Ness, Amer. Inst. Chem. Engineers 1961, 7, 190. 

Tanaka, S. Murakami, and R. Fujishiro, Bull. Chem. Soc. Japan, 1972, 45, 2107. 

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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. 

The heat of adsorption is an important experimental quantity. The heat evolution with each of successive admissions of adsorbate vapor may be measured directly by means of a calorimeter described by Beebe and co-workers [31]. Alternatively, the heat of immersion in liquid adsorbate of adsorbent having various amounts preadsorbed on it may be determined. The difference between any two values is related to the integral heat of adsorption (see Section X-3A) between the two degrees of coverage. See Refs. 32 and 33 for experimental papers in this area. 

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

A calorimeter is a device used to measure the work that would have to be done under adiabatic conditions to bring about a change from state 1 to state 2 for which we wish to measure AU= U -U This work is generally done by passing a known constant electric current 3 for a known time t through a known resistance R embedded in the calorimeter, and is denoted by where... 

In general it is difficult to construct a calorimeter that is truly adiabatic so there will be unavoidable heat leaks q. It is also possible that non-deliberate work is done on the calorimeter such as that resulting from a change in volume against a non-zero external pressure / Pk i dk>, often called /iFwork. Additional work w ... 

The p V work temi is not nomially measured. It can be eliminated by suspending the calorimeter in an evacuated space (p = 0) or by holding the volume of tire calorimeter constant (dF= 0) to give... 

This is the working equation for a constant volume calorimeter. Alternatively, a calorimeter can be maintained at constant pressure p equal to the external pressure p in which case... 

Values of COT) can be derived from a constant volume calorimeter by measuring AU for small values of Tj - TO and evaluating AU/(T2 - T ) as a fiinction of temperature. The energy change AU can be derived from a knowledge of tlie amount of electrical energy required to change the temperature of the sample + container... 

All calorimeters consist of the calorimeter proper and its surround. This surround, which may be a jacket or a batii, is used to control tlie temperature of the calorimeter and the rate of heat leak to the environment. For temperatures not too far removed from room temperature, the jacket or bath usually contains a stirred liquid at a controlled temperature. For measurements at extreme temperatures, the jacket usually consists of a metal block containing a heater to control the temperature. With non-isothemial calorimeters (calorimeters where the temperature either increases or decreases as the reaction proceeds), if the jacket is kept at a constant temperature there will be some heat leak to the jacket when the temperature of the calorimeter changes. 

Hence, it is necessary to correct the temperature change observed to the value it would have been if there was no leak. This is achieved by measuring the temperature of the calorimeter for a time period both before and after the process and applying Newton s law of cooling. This correction can be reduced by using the teclmique of adiabatic calorimetry, where the temperature of the jacket is kept at the same temperature as the calorimeter as a temperature change occurs. This teclmique requires more elaborate temperature control and it is prunarily used in accurate heat capacity measurements at low temperatures. 

With most non-isothemial calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter by one degree. This value can be detemiined by electrical calibration using a resistance heater or by measurements on well-defined reference materials [1], For example, in bomb calorimetry, the calorimeter constant is often detemiined from the temperature rise that occurs when a known mass of a highly pure standard sample of, for example, benzoic acid is burnt in oxygen. 

B1.27.4.1 CLASSIFICATION BY PRINCIPLE OF OPERATION ISOTHERMAL CALORIMETERS (MORE PRECISELY, QUASI-ISOTHERMAL)... 

The energy released when the process under study takes place makes the calorimeter temperature T(c) change. In an adiabatically jacketed calorimeter, T(s) is also changed so that the difference between T(c) and T(s) remains minimal during the course of the experiment that is, in the best case, no energy exchange occurs between the calorimeter (unit) and the jacket. The themial conductivity of the space between the calorimeter and jacket must be as small as possible, which can be achieved by evacuation or by the addition of a gas of low themial conductivity, such as argon. 

This type of calorimeter is nomrally enclosed in a themiostatted-jacket having a constant temperature T(s). and the calorimeter (vessel) temperature T(c) changes tln-ough the energy released as the process under study proceeds. The themial conductivity of the intemiediate space must be as small as possible. Most combustion calorimeters fall into this group. 

A liquid serves as the calorimetric medium in which the reaction vessel is placed and facilitates the transfer of energy from the reaction. The liquid is part of the calorimeter (vessel) proper. The vessel may be isolated from the jacket (isoperibole or adiabatic), or may be in good themial contact (lieat-flow type) depending upon the principle of operation used in the calorimeter design. 

The selection of the operating principle and the design of the calorimeter depends upon the nature of the process to be studied and on the experimental procedures required. Flowever, the type of calorimeter necessary to study a particular process is not unique and can depend upon subjective factors such as teclmical restrictions, resources, traditions of the laboratory and the inclinations of the researcher. 

Various books and chapters in books are devoted to calorimeter design and specific applications of calorimetry. For several decades the Connnission on Themiodynamics of the International Union of Pure and... 

Figure Bl.27.2. Schematic vertical section of a high-temperature adiabatic calorimeter and associated thennostat (Reprinted with penuission from 1968 Experimental Thermodynamics vol I (Butterworth).)...

The heat capacity of a gas at constant pressure is nonually detenuined in a flow calorimeter. The temperature rise is detenuined for a known power supplied to a gas flowing at a known rate. For gases at pressures greater than about 5 MPa Magee et al [13] have recently described a twin-bomb adiabatic calorimeter to measure Cy. 

Combustion or bomb calorimetry is used primary to derive enthalpy of fonuation values and measurements are usually made at 298.15 K. Bomb calorimeters can be subdivided into tluee types (1) static, where the bomb or entire calorimeter (together with the bomb) remains motionless during the experiment (2) rotating-... 

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