Discussion

We have conducted an experiment on Joule Thomson effect on 18 th of March, 2016 in order to achieve two main objectives which are to determine Joule Thomson coefficient of Carbon Dioxide gas and the Joule Thomson coefficient of Nitrogen gas. The Joule Thomson effect can be defined as the increase or decrease in the temperature of a gas or a liquid when allowed to expand freely through a valve or other throttling device while being kept insulated so that no heat is transferred to or from the fluid, and no external mechanical work is extracted from the fluid in other words kept in an adiabatic state.

When a non ideal gas suddenly expands from high pressure state to low pressure state, there will be temperature change. The ratio of ΔT/ΔP is known as the Joule Thomson coefficient. For many gases at room temperature, the ΔT/ΔP ratio is positive. Thus, a pressure drop always coupled with a temperature drop. Joule Thomson expansion is also known as free expansion. It is an irreversible process where a gas expands into an insulated evacuated chamber. Real gases usually result in a temperature change during free expansion. On the other hand, for ideal gas, the temperature doesn't change. This can be explained with:

A free expansion is in theory can be achieved by opening a stopcock that allows the gas to expand into a vacuum. Although it would be difficult to attain in reality, we simply have to imagine a free expansion caused by moving a piston is faster than virtually any atom. No work is done because there is no pressure on the piston. No heat energy leaves or enters the piston. Nevertheless, there will always be entropy change. At the same time the whole process is adiabatic. Therefore the general entropy equation:

Are not satisfied because based on thermodynamic second law, the process is considered a reversible process. Therefore for Joule Thomson effect, the entropy balance follows the same principle as ideal gas entropy balance:

An experiment such as Joule Thomson effect requires some precautions that we have to take care in order to avoid and reduce errors in our final results. First and foremost, we have to make sure the all the valves are sealed as tightly as possibly to avoid gas leakage. Gas leakage will definitely affect the result of our experiment. Besides, the experimenting room and the experimental apparatus must be in thermal equilibrium at the start of the measurement in order to avoid any sudden change in temperature for our adiabatic system. Last but not the least, temperature meter must be switched on at least 30min before performing the experiment to avoid thermal drift.

Conclusion

The objectives for our experiment are successfully achieved and we managed to acquire the Joule Thomson coefficient for both Carbon Dioxide gas and Nitrogen gas. Both Carbon Dioxide and Nitrogen gas were fed to a throttling point, where the gas undergoes adiabatic expansion. The Joule Thomson coefficients are calculated based on the differences in temperature produced between the two sides of the throttle point that measured at various pressures. The margin of error between the theoretical value and experimental results are not too large. Based on the percentage difference between the value obtained from the graph and the values calculated earlier, which is 29.9% for Carbon Dioxide and 44.66% for Nitrogen, we can say that the objectives of the experiment is successfully achieved.

References

1) BTE 2222 Thermal Science Lab Manual, 2016 edition2) Engineering Thermodynamics, 1st edition by Tarik al-Shammeri

ISBN: 978-87-7681-670-43) Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Cengel and Michael A. Boles

Publisher: McGraw-Hill Higher Education, 2014 ISBN 10: 0073398179 ISBN 13: 9780073398174