Measurement Techniques Laboratory Final Report EXPERIMENT NAME INTERN NAME DEGREE PROGRAM GROUP NUMBER STUDENT NAME STUDENT ID NUMBER i Affiliated to: 5.31.2017 1:08 pm i. i.! Contents Page Contents Page ................................................................................................................... ii! 1.! Introduction ........................................................................................................................3! 2.! Theoretical Background of Experiment ..............................................................................4! 3.! Application in Industry ........................................................................................................5! 4.! Experimental Setup ............................................................................................................6! 5.! Experimental Data and Analysis ........................................................................................7! 6.! Summary/Conclusions .......................................................................................................8! 7.! References .........................................................................................................................9! ii Affiliated to: 5.31.2017 1:08 pm General Guidelines: • Heading 1 should be used for the Chapter Titles, e.g. 1. Introduction • Heading 2 should be used for Chapter Subtitles, e.g. Summary, Conclusions • Heading 3 should be used for Subtitles of Chapter Subtitles, if necessary • Otherwise, all text should be in Times New Roman, Font Size 12 • The word limit is 2,000 words, including References. Anything written over this limit will not be marked. • These reports will be checked for plagiarism, so please ensure that information taken from References is re-written in your own words and appropriately Keywords 5 keywords related to the Experiment should be input here. 1. Introduction Introduce everything that is included in your report. 3 Affiliated to: 5.31.2017 1:08 pm 2. Theoretical Background of Experiment Describe and explain all of the background theory that is related to the Experiment 4 Affiliated to: 5.31.2017 1:08 pm 3. Application in Industry Describe how the Experiment is applied in Industry 5 Affiliated to: 5.31.2017 1:08 pm 4. Experimental Setup Describe the Equipment and the Setup of the Experiment 6 Affiliated to: 5.31.2017 1:08 pm 5. Experimental Data and Analysis Provide all of the Data collected in your experiment and analyse it in appropriate detail 7 Affiliated to: 5.31.2017 1:08 pm 6. Summary/Conclusions Summarise your experiments and draw your conclusions from the data. 8 Affiliated to: 5.31.2017 1:08 pm 7. References Provide your list of References used in your report here. 9 Affiliated to: 5.31.2017 1:08 pm Measurement Laboratory Supervisor: Asst. Prof. Dr. Najah Al Mhanna Joule – Thomson – Apparatus Contents Introduction .......................................................................................................... 3 Theoretical Background ....................................................................................... 3 2.1 Ideal gases ....................................................................................................... 3 2.2 Real gases ....................................................................................................... 6 Experimental Setup ............................................................................................. 8 Methodology ........................................................................................................ 8 Application ........................................................................................................... 8 Questions ............................................................................................................ 9 Appendix............................................................................................................ 10 References ........................................................................................................ 11 ii Introduction In 1852, James Prescott and William Thomson discovered that a temperature change can occur in a gas as a result of a sudden pressure change over a valve. This phenomenon is known as the Joule-Thomson effect and has proven to be important in the advancement of refrigeration systems as well as liquefiers, air conditioners, and heat pumps. It is also the effect that is responsible for a tire valve getting cold when the air is let out from a bicycle tire. Theoretical Background The behavior of a real gas can be significantly different then the behavior of an ideal gas and leads to observations which cannot be explained with the concept of an ideal gas. One of these phenomena is the Joule-Thompsons effect. If an ideal gas expands without performing work no temperature change will occur, whereas a real gas changes well its temperature. Depending on the specific gas and the outer conditions the temperature may increase or decrease. 2.1 Ideal gases For ideal gases it is assumed that the molecules itself have a negligible volume. They move randomly in space and the only interaction with other molecules is perfectly elastic collision. Besides this, there is no intermolecular interaction like attraction or repulsion. That is why molecules can move through the volume forceless. For such an ideal gas the thermal state equation is given as ∙ = ∙ The caloric equations of an ideal gas only depend on temperature and nether on volume nor pressure. The specific internal energy can be given as = ( ) 3 Starting with the definition of the enthalpy, the ideal gas law can be plugged in this equation which leads to: ℎ= + = + Accordingly, the enthalpy h is only depending on the temperature, too. ℎ = ℎ( ) At this point it is shown, that the energy stored in an ideal gas is directly and only related to its temperature. Consequently, if there is no change in energy, the temperature cannot change. Now the question is, what is happening while the expansion process at a valve? Therefore consider following experiment. Figure 1 shows schematic of throttling through a porous plug. The system is adiabatic, therefore no heat exchange takes place through the walls of the system. Porous plug Insulation Piston Figure 1: Schematic of Throttling through a porous plug Now imagine, that there is a gas flow from the high pressure side to the low pressure side of the porous plug. After thermal equilibrium, the gas at the high pressure side 4 has the pressure and the temperature and the temperature and at the low pressure side the pressure . According to the first law of thermodynamics the internal energy in a closed system will change due to heat transfer or work performance ∆ =∆ +∆ Since the system is adiabatic, there is no heat exchange ∆ =0 Unlike a turbine or a compressor, a valve nether perform nor consume work. Therefore the only object in this system which may perform work is the gas itself. If we assume, that there is a piston at both sides of the porous plug, it is easy to calculate the work, which is done by the system at the high pressure side. ∆ =− ∙ (0 − )= and the low pressure side is: ∆ =− ∙( − 0) = − The total work of the system: ∆ =∆ +∆ = − Then the internal energy change is: ∆ = − =∆ + = = − In the end rearranging leads to ℎ = + =ℎ Outcome: process is isenthalpic, ∆ℎ = 0 In this derivation there is not a characterization of the gas done, therefore the expansion of a gas at a valve is always isenthalpic independent if it shows ideal or real behavior. 5 With the derivation before it was shown, that the enthalpy of an ideal gas is only depending of the temperature. Since the enthalpy of a gas does not change while throttling through a valve, the temperature cannot change. ℎ ( )=ℎ ( ) Finally: = This conclusion contradicts the experimental findings from Thomson and Joule. The two physicists found that some gases actually change in temperature at throttling. But how can this be explained? The answer lies in some thermodynamic reasoning and the concept of ideal versus real gases. Unfortunately, is not entirely true; it is a special case for ideal gases. 2.2 Real gases Looking at a more general situation, the enthalpy of a real gas is not only depending on temperature but also on pressure ℎ = ℎ( , ) In order to understand the variation consider the derivation of the enthalpy ℎ= ℎ + ℎ The first term on the right-hand side is the enthalpy change of an ideal gas, and the second term is the additional contribution due to the non-ideality of the gas. This can be interpreted as the work that must be exerted to overcome intermolecular forces. An ideal gas, by definition, has no intermolecular forces. For an isenthalpic process also helps in the interpretation of any slight temperature change, as it is able to provide the exact amount of thermal energy conversion needed to overcome intermolecular forces. We already have shown that the throttling process is isenthalpic ℎ = 0. 6 0= ℎ + ℎ This equation explains the Joule Thomson Effect and the behavior of real gas. Real gases can ether store energy in thermal and potential form and by transferring energy from one form into the other real gases can change their temperature without performing work. Revisiting the experiments of Thomson and Joule, the two men found it practical to relate their observations of temperature change at constant enthalpy to something measurable: How much does the temperature change for a small change in pressure, holding the enthalpy fixed? They referred to it as the Joule-Thomson coefficient: ℎ = = − ℎ = − 1 ℎ = ( , , ) The change of the temperature at an isenthalpic change of the pressure defines the Joule-Thomson-Coefficient. The value of is typically expressed in °C/bar (SI units: K/Pa) and depends on the gas type and on the temperature and pressure of the gas before expansion. Its pressure dependence is usually only a few percent for pressures up to 100 bar. The Joule-Thomson-Coefficient can be either positive or negative. A positive value means cooling, while reducing the pressure. . Inversion temperature All real gases have an inversion point at which the value of changes sign. The temperature of this point, the Joule-Thomson inversion temperature, depends on the pressure of the gas before expansion. 7 Experimental Setup Figure 2: Experimental Setup Methodology I. The overpressure of 0.9 bar should be reduced in 0.1 bar steps to zero. Note for each pressure step the temperature difference. After the measurement is finished, the pressure reducer must be relieved. II. Determine for all gases the Joule-Thomson-Coefficient and compare this value to the values in the literature. III. Discuss the solution with the approximation of an ideal gas. Which meaning has the algebraic sign of the Joule – Thomson – Coefficient? Application The complete device must remain at least one hour in the room, in which the experiment will be carried out, to make sure all parts will have the same temperature. The apparatus can only work correctly if the pressure displayed before the heat exchanger at the pressure reducing valve a few bar. The digital temperature measuring instrument should be switched on at least half an hour before beginning measurements, to make sure display drift will remain negligible 8 during measurements. (This is particularly important when measuring nitrogen, because in this case the maximum temperature difference which can be reached is 0.3 K) The gas should flow for about 1.5 minutes at constant pressure, so that a temperature equilibrium may be reached. The value of Dt should be observed 10 to 15 seconds and averaged before being recorded. The Joule-Thomson-Coefficient is determined from the slope Dt (Dp). Normally this straight line does not intersect the Dt axis, as expected theoretically, at Dt = 0. The point of intersection may be different for every measurement series. Cause: zero compensation is carried out with the gas resting, which is no advantageous condition for a temperature equalisation between the points of measurement. As a result, the points of measurement usually have different temperatures for the display Dt = 0. This error causes a shift of the measurement straight line in the direction of y, but has no influence on its slope. Questions a) What is the Joule-Thompson Effect (JTE)? When is it applied? b) Which type of gases the JTE is observed? c) Characterize the nature of an ideal gas. How can the thermal state and energy state be described? d) Describe the process at a valve. How does a valve influence the gas and how does it differ from other devices such as compressors and turbines. Compare them. e) Characterize the nature of a real gas. Compare the behaviour of an ideal and a real gas. What consequences do these differences have for the energy equation? f) Explain the JTE. g) Define the inversion temperature. What is the inversion temperature of nitrogen, argon and carbon dioxide? 9 Appendix In a gas expansion the pressure decreases, so the sign of is negative by definition. With that in mind, the following table explains when the Joule–Thomson effect cools or warms a real gas: Gas temperature cool/warm below the inversion temperature positive always negative negative cools above the inversion temperature negative always negative positive warms Below in Figure 3, Joule-Thomson-Coefficients for Helium, Neon, Argon and Carbon dioxide at atmospheric pressure are illustrated: Figure 3: Temperature - Joule-Thomson-Coefficient diagram 10 References 1. R.G. Mortimer “Physical Chemistry”, Benjamin/Cummings, 1993 2. Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, 3. W.R. Salzman. "Joule Expansion". Department of Chemistry, University of Arizona 4. Atkins, P.W. “Physical Chemistry”, Freeman, 1994 5. J.H. Noggle, Physical Chemistry”, Harper Collins, 1996 6. Hoover, Wm G., Carol G. Hoover, and Karl P. Travis. "Shock-Wave Compression” 7. Joule–Thomson Expansion "Physical review letters”, 2014 11
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