Steam Calorimetry 2u2j1k

STEAM CALORIMETRY REPORT CALORIMETRY 

Calorimetry is the measurement of heat flow. It allows us to calculate the amount of energy required to heat up a substance or to make a substance change states.

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Calorimetry is the science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state due for example to chemical reactions, physical changes, or phase transitions under specified constraints. Calorimetry is performed with a calorimeter. Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of the science of calorimetry.

CALORIMETER calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal microcalorimeters, titration calorimeters and accelerated rate calorimeters are among the most common types. A simple calorimeter just consists of a thermometer attached to a metal container full of water suspended above a combustion chamber.

To find the enthalpy change per mole of a substance A in a reaction between two substances A and B, the substances are added to a calorimeter and the initial and final temperatures (before the reaction started and after it has finished) are noted. Multiplying the temperature change by the mass and specific heat capacities of the substances gives a value for the energy given off or absorbed during the reaction. Dividing the energy change by how many moles of A were present gives its enthalpy change of reaction. This method is used primarily in academic teaching as it describes the theory of calorimetry. It does not for the heat loss through the container or the heat capacity of the thermometer and container itself. TYPES OF STEAM CALORIMETER 1. BOMB CALORIMETER

The world's first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and PierreSimon Laplace, to determine the heat evolved in various chemical changes

Theory The temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment. This procedure is called a Throttling process. This temperature can be described by the “Joule–Thomson effect” or “Joule–Kelvin effect” or “Kelvin–Joule effect” or “Joule–Thomson expansion”.In the Joule experiment, the gas expands in a vacuum and the temperature drop of the system is zero, if the gas were ideal. In this process here is no change in enthalpy from state one to state two, h1 = h2; no work is done, W = 0; and the process is adiabatic, Q = 0. Let’s take an example of a throttling process is an ideal gas flowing through a valve in mid position. We can observe that: Pin > Pout, velin < velout (where P = pressure and vel = velocity). These observations confirm the theory that hin= hout. h = u + PV (v = specific volume), so if pressure decreases then specific volume must increase if enthalpy is to remain constant (assuming u is constant). Because mass flow is constant, the change in specific volume is observed as an increase in gas velocity. The theory also states W = 0. Our observations again confirm this to be true as clearly no "work" has been done by the throttling process. Finally, the theory states that an ideal throttling process is adiabatic. This cannot clearly be proven by observation since a "real" throttling process is not ideal and will have some heat transfer. In this process, steam becomes drier and nearly saturated steam becomes, superheated. As a gas expands, the average distance between molecules grows. Because of intermolecular attractive forces , expansion causes an increase in the potential energy of the gas. If no external work is extracted in the process and no heat is transferred, the total energy of the gas remains the

same because of the conservation of energy. The increase in potential energy thus implies a decrease in kinetic energy and therefore in temperature. A second mechanism has the opposite effect. During gas molecule collisions, kinetic energy is temporarily converted into potential energy. As the average intermolecular distance increases, there is a drop in the number of collisions per time unit, which causes a decrease in average potential energy. Again, total energy is conserved, so this leads to an increase in kinetic energy (temperature). Below the Joule–Thomson inversion temperature, the former effect (work done internally against intermolecular attractive forces) dominates, and free expansion causes a decrease in temperature. Above the inversion temperature, gas molecules move faster and so collide more often, and the latter effect (reduced collisions causing a decrease in the average potential energy) dominates: Joule–Thomson expansion causes a temperature increase. SEPARATING CALORIMETER: It consists of two concentric chambers, the inner chamber and the outer chamber, which communicates with each other through an opening at the top. As the steam discharges through the metal basket, which has a large number of holes, the water particles due to their heavier momentum get separated from the steam and collect in the chamber. The comparatively dry steam in the inner chamber moves up and then down aging through the annular space between the two chambers and enters the Throttling Calorimeter. It is a vessel used initially to separate some of the moisture from the steam, to ensure superheat conditions after throttling. The steam is made to change direction suddenly; the moisture droplets, being heavier than the vapor, drop out of suspension and are collected at the bottom of the vessel. THROTTLING CALORIMETER It consists a narrow throat (Orifice). Pressure and temperature are measured by pressure gauge and thermometer. The steam after throttling process es through the heat exchanger and condensate is collected. Steam Generator is also provided to supply the saturated steam (Max) at 2kg/cm2 pressure. There is no need of boiler. It is a vessel with a needle valve fitted on the inlet side. The steam is throttled through the needle valve and exhausted to the condenser. DRYNESS FACTOR The quality of wet steam is usually defined by its dryness fraction. When the dryness fraction, pressure and temperature of the steam are known, then the state of wet steam is fully defined. In a steam plant it is at times necessary to know the state of the steam. For wet steam, this entails finding the dryness fraction. When the steam is very wet, we make use of a separating calorimeter. COMBINED CALORIMETER

Separating calorimeter does not give an accurate result and the throttling calorimeter fails if the steam is not superheated after throttling. A combination of separating and throttling calorimeter is therefore found most suitable for accurate measurement of dryness of steam

How to find dryness fraction of steam using throttling calorimeter. Aim: To measure the dryness fraction of steam using throttling calorimeter. Theory: A throttling calorimeter used to determine the dryness fraction of steam is shown in Fig. It consists of a separator A into which steam is itted through a control valve from the steam main. The pressure and temperature are measured by the pressure gauge and the thermometer T1 provided in this section. It may be noted that temperature recorded by T1 is same as the saturation temperature corresponding to the pressure of steam in calorimeter B. Fig

This steam is then throttled through a narrow aperture of restricted valve openings, its total heat remaining constant. The steam is in the superheated state after throttling at a lower pressure than previous. The temperature and pressure of steam leaving the calorimeter B is noted by the thermometer T2 and manometer respectively. Let P1 = Pressure of steam before throttling, i.e. pressure of steam main, P2 = Pressure of steam after throttling, hf1 = Sensible heat of water at pressure P1, … (From steam tables) hfg1 = Latent heat of steam at pressure P1, … (From steam tables) hg2 = Total heat of dry steam at pressure P2, … (From steam tables) tsup = Temperature of superheated steam after throttling,

t2 = Saturation temperature at pressure P2, … (From steam tables) = Specific heat of superheated steam, and x = Dryness fraction of steam before throttling.

Since the steam has undergone a throttling process, therefore Total heat before throttling = Total heat after throttling hf1+ x hfg1 = hg2 + (tsup – t2)

The condition for the successful operation of this calorimeter is that the stam must be superheated after throttling. This condition requires a high dryness fraction of steam before throttling. This calorimeter can not be used if the dryness fraction of the steam s above 0.95. The minimum dryness fraction of the steam can be measured by throttling calorimeter depends upon the initial pressure of the steam as the pressure after throttling remains near atmospheric.

Precautions: 1. The whole apparatus must be perfectly dry as the presence of moisture shows differ results. 2. The connection should be leak proof. 3. The sampling pipe fitting with steam main should be proper. 4. The thermometer should be placed properly. 5. The reading of water thermometer should be read accurately

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Laidler, Keith, J. (1993). The World of Physical Chemistry. Oxford University Press. Antoine Laurent Lavoisier, Elements of Chemistry: 1789