Compiled by Dr. Sumanta Banerjee, Mechanical Engineering Department
TO FIND THE CALORIFIC VALUE OF DIESEL FUEL & COAL BY BOMB CALORIMETER PURPOSE: In this experiment, the standard enthalpy of combustion of benzoic acid shall be measured and, using its known heat of combustion, the heat capacity of a bomb calorimeter shall be determined. Once the calorimeter has been calibrated, the enthalpy of combustion will be measured for diesel fuel. The values thus measured shall be compared with the corresponding values available in literature. BACKGROUND AND THEORY: The enthalpies of reactions can aid in predicting the likelihood of a chemical reaction occurring. In this experiment, an instrument called a bomb calorimeter shall be used to measure the heat evolved during the combustion of a sample at a constant volume. Calorimetry is an integral category of physical and analytical chemical techniques, and is especially important to the study of fuels and food chemistry.
Figure 1: Schematic of the Bomb calorimeter apparatus The bomb calorimeter is shown schematically in Figure 1. The calorimeter consists of a metal reaction chamber that is immersed in a water bath with a known volume of water. The metal reaction chamber, or bomb cell, maintains a constant volume and allows the heat generated in its interior to be transferred efficiently to the surrounding bath. Inside this chamber, the sample is ignited by passing electrical current through a “fuse” wire. In the combustion process, some (but not all) of the fuse wire is also consumed. The interior of the reaction chamber is pressurized with oxygen to ensure efficient combustion of the material of interest. The water bath is insulated from the outside environment to prevent transfer of heat beyond the water bath. Therefore the bomb calorimeter is an adiabatic system. U in a system is From the First Law of Thermodynamics, it is known that the change in internal energy
given by the sum of the work done on the on the system W and the energy transferred to the to the system as heat Q . (1) U Q W For example, if 12kJ of work is done on a system in the form of mechanical compression, and 6 kJ of energy U is escapes from the system into the surrounding environment, then the change in internal energy 12 kJ 6 kJ 6 kJ . The bomb calorimeter is a unique instrument because it provides nearly an adiabatic system, which does not allow heat to escape into the surrounding environment
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Q 0 . In addition, the interior of the Page 1
Compiled by Dr. Sumanta Banerjee, Mechanical Engineering Department bomb is very rigid and able to withstand large expansion pressures (even explosions, hence the “bomb” part of its V 0 . Upon ignition, the heat released by combustion of name) without any noticeable change in its volume the sample is equilibrated through the walls of the bomb cell into the surrounding water bath, where a temperature increase is recorded as a function of time. The temperature increase of the system is proportional to the heat of combustion of the sample, and they are related through proportionality constant . For the chemical reaction occurring inside the bomb cell (constant volume), the change in internal energy is equal to the product of the heat T inside the bomb cell: capacity of the sample CV with the change in temperature U CV T (2) However, in this experiment, the temperature change is measured in the water bath, and the observed temperature increase must then be related to the heat released inside the bomb cell. Therefore, the heat capacity of the calorimeter Ccal must be determined by combusting a sample with a known mass and heat of combustion. This is a calibration procedure that will determine the accuracy of the measured heat of combustion for an unknown sample in the second part of the present experiment.
Figure 2: Typical plot of water bath temperature as a function of time The heat capacity Ccal is defined as:
Hst mst e wire (3) T In Eq. (3), H st is the known heat of combustion of the standard sample (in calories per gram), mst is the carefully measured mass of the standard sample (in grams), e wire is the heat of combustion of the fuse wire (in calories), Ccal
and
T is the measured change in temperature (in degrees C). After ignition, a typical plot of water bath temperature as a function of time is shown in Fig. 2. Just before ignition, the recorded temperature is designated as Tinitial , and the temperature at the time when the temperature
stops increasing is designated Tfinal . To compensate for any change in temperature that might have occurred independent of the combustion, the rates of temperature change over the 5 minutes before ignition and the 5 minutes after the temperature stopped changing are defined as r1 and r2 , respectively. Finally, the time required for the temperature to reach 60% of its maximal change is denoted as t 1 , and the time between the 60% increase and full increase as t 2 . These variables are all labeled for clarity on Fig. 2. From the measurement of these six variables, the adjusted temperature rise can be calculated as: T Tfinal - Tinitial - r 1 t 1 r 2 t 2 (4) Once C cal is determined with a standard sample (benzoic acid), the heat of combustion can be determined for an unknown sample (diesel fuel or coal) from:
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Compiled by Dr. Sumanta Banerjee, Mechanical Engineering Department T C cal e wire U (5) m sample In Eq. (5), C cal is the heat capacity of the calorimeter determined in the first part of this experiment and m sample is now the measured mass of the unknown sample. APPARATUS:
Figure 3: Internal details of a Bomb calorimeter The internal details of the apparatus (bomb calorimeter) used to measure the heats of combustion is depicted in Fig. 3. An oxygen tank is supplied to allow pressurization of the bomb cell. The temperatures are required to be read off periodically and recorded in laboratory notebook. A pellet press is provided to prepare a solid pellet of the samples before combustion in the calorimeter. The correct usage of the pellet press must be demonstrated. An analytical balance is used to measure the mass of the pellets to be analyzed, and a ruler is used to measure the amount of fuse wire before and after the combustion process. SPECIFIC AIMS: The purpose of this experiment is to learn to use a bomb calorimeter; To understand the data that is measured with this instrument; A standard sample, benzoic acid, with a known heat of combustion will be used to calibrate the calculations, which will determine the heat capacity of the calorimeter; Once this is known, the heat of combustion will be determined for the unknown samples; Comparison of results to literature values for the heat of combustion of diesel fuel; Preparation of laboratory report. SAFETY PRECAUTION: As the experiment involves working with a bomb cell that is pressurized with oxygen, care must be exercised that oxygen (being extremely flammable) be not used near open flames. PROCEDURE: Preparing the sample and charging the oxygen bomb: Pellet preparation: The benzoic acid comes in pre-made pellets. A pellet should be chosen and its mass recorded. The coal sample needs to be prepared using the pellet press. Attaching the fuse to the bomb: The bomb head is set on the support stand and a fuse wire (of10 cm length) is fastened between the two electrodes. The ends of the wires are inserted into the eyelet at the end of each
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Compiled by Dr. Sumanta Banerjee, Mechanical Engineering Department electrode stem and the cap is pushed downward to pinch the wire into place. The fuel capsule with its weighed sample is placed in the electrode loop, and the wire is bent downward toward the surface of the charge. It is not necessary to submerge the wire in a powdered sample. In fact, better combustion will usually be obtained if the loop of the fuse is set slightly above the surface. When using samples in form of pellets, the wire should be bent so that the loop bears against the top of the pellet firmly enough to keep it from sliding against the side of the capsule. It is also good practice to tilt the capsule slightly to one side so that the flame emerging from it will not impinge directly on the tip of the straight electrode. Closing the bomb: Care must be taken not to disturb the sample when moving the bomb head from the support stand to the bomb cylinder. The sealing ring must be checked to make sure it is in good condition. The ring should be moistened with a bit of water so that it will slide freely into the cylinder. For easy insertion, the head should be pushed straight down without twisting and the gas release valve should be left open during this operation. The screw cap should be set on the cylinder and turned down firmly by hand to a solid stop. Filling the bomb: The oxygen filling connection should already be attached to the oxygen tank. The pressure connection to the bomb is made with a slip connector on the oxygen hose which slides over the gas inlet fitting on the bomb head. The connector is slid onto the inlet valve body and pushed down as far as it will go. The outlet valve is closed on the bomb head; and then the oxygen tank valve is opened no more than one-quarter turn. Then, the filling connection control valve is opened slowly and the gauge reading watched as the bomb pressure rises to the desired filling pressure (usually 30 atm., but never more than 40 atm.). Then the control valve is closed. The residual pressure in the filling hose is released by pushing downward on the lever attached to the relief valve. The gauge on the oxygen cylinder should now return to zero.
Operating the Calorimeter: The calorimeter bucket is filled with 2000 0.5 ml of water every time a run is taken.
The bucket is then set in the calorimeter. The next step is setting the bomb in the calorimeter bucket: The lifting handle is attached to the two holes in the side of the screw cap and the bomb is lowered into the water with its feet spanning the circular upraised guides in the bottom of the bucket. Proper care should be taken so as not to disturb the sample. The handle is removed and drops of water, if any, are shaken back into the bucket. The two ignition lead wires are then pushed into the terminal sockets on the bomb head. The other end of the ignition wires should be attached to the ignition unit, one wire attached to the 10 cm lead and the other to the middle (ground). Care should be taken not to remove any water from the bucket with hand. Next, the cover is set on the jacket with the thermometer facing toward the front. The stirrer is turned by hand to ensure that it turns freely. Then the drive belt is slipped onto the pulleys and the motor is started. The stirrer is then run for 5 minutes to reach equilibrium before starting a measured run. At the end of this period, the time is recorded by starting a timer and the temperature is read to one-tenth of the smallest scale division. The thermometer should be tapped lightly with a pencil or rod to vibrate the liquid before taking a reading. Temperatures are read and recorded at one-minute intervals for the first 5 minutes. At the start of the 6th minute, only after ensuring the ignition unit is plugged in and standing back from the calorimeter, the bomb is fired by pressing the ignition button and holding it down until the indicator light goes out. Normally the light glows for about 1 2 second. But, the button should be released within 5 seconds regardless of the light. Care should be taken to stand clear for 30 seconds after firing. The bucket temperature will start to rise within 20 seconds after firing. This rise will be rapid during the first few minutes; then it will become slower as the temperature approaches a stable maximum. The temperature readings are taken at 45, 60, 75, 90, and 105 seconds after firing. These readings are interpolated in between to identify the 60% point after the total rise has been measured. These readings can be taken without a magnifier since estimates to the nearest 0.020 C are sufficient. After the rapid rise period (about 4 or 5 minutes after ignition), the reading lens is adjusted and temperature is recorded to one-tenth of the smallest scale division at one minute intervals, until the difference between successive readings has been constant for five minutes.
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Compiled by Dr. Sumanta Banerjee, Mechanical Engineering Department Cleaning up: After the last temperature reading, the motor is stopped, the belt is removed and the cover is lifted from the calorimeter. The thermometer bulb and stirrer are wiped with a clean cloth and the cover is set on the support stand. The bomb is then lifted out of the bucket, the ignition leads are removed, and the bomb is wiped with a clean towel. The knurled knob is opened on the bomb head to release the gas pressure before attempting to remove the cap. This release should proceed slowly over a period of not less than one minute to avoid losses. After all pressure has been released, the cap is unscrewed. The head is lifted out of the cylinder and placed on the support stand. The interior of the bomb is examined for soot or other evidence of incomplete combustion. If such evidence is found, the test will have to be discarded. All unburned pieces of fuse wire are removed from the bomb electrodes. They are straightened and their combined length is measured in centimeters. This length is subtracted from the initial length of 10 cm to obtain the net amount of wire burned. The experiment should be repeated with diesel fuel after the first run with benzoic acid. DATA ANALYSIS: The recorded temperatures are plotted as a function of time. The plot should look similar to Fig. 2 above. A nonlinear regression must be used in Excel to fit the data through the region from the ignition point until the temperature stops rising. From this fit, one can interpolate to determine the time when the temperature has changed by 60%, which will allow the determination of t1 and t 2 . The rates of change before ignition and after plateau, r1
and r2 , can be determined from a linear fit to the data points in these two regions. For the purpose of calculations, the value of the heat of combustion of benzoic acid is supplied as 6318 calories/gram, and the heat generated from the fuse wire is 2.3 cal cm .
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