Download Diffusion Tube Assignment

Diffusion Tube Assignment (mass method, 1 tube)
In this experiment you will be taking data to compute the diffusion coefficient of
cyclohexane and perhaps acetone in air, comparing the results to predicted and literature
values, and discussing the possible sources of discrepancies between values.
Meet with the TA to discuss plans for recording data- you should all take turns getting
data throughout the day on ___________ and ___________.
Procedure:
1) Measure the initial diffusion path length.
2) Record the mass of the tube and air temperature at time zero. Note what air
currents are in the room and how exposed the tube is to these currents.
3) Record mass and temperature approx. every hour for 8-10 hours
Analysis:
1) Plot your data by modifying equation 6.26-26 in such a way that all of the
time data is used and the diffusion coefficient may be obtained from the
inverse slope. Is the data linear- are there any patterns? Why or why not?
How long would you expect it to take for the system to come to steady
state? What does scatter in the data represent? Use linear regression and
statistical output (Excel or Polymath) to estimate the uncertainty in the
value of diffusivity based solely on the the assumption of random scatter
in the data.
2) Calculate the expected value of diffusivity using both the ChapmanEnskog equation (6.2-44) and the Fuller, Schettler and Giddings
method.(eqn. 6.2-45, table 6.2-2).
3) Compare your experimental value of the diffusivity coefficient with the
estimated values from number 2 above, as well as with the literature value
for air-cyclohexane and air-acetone. Are the results consistent? What
degree of accuracy would you expect using those equations?
4) What uncertainties are present as a result of the methods and materials you
used to measure the diffusion coefficient?
5) What assumptions were made in the derivation of equation 6.2-26? How
does the method outlined in the Lee and Wilke paper correct for some of
the “non-idealities” of the system? (explain fully).
6) Was the initial height of the liquid in the tube chosen wisely? Why or
why not.
References:
1) Transport Processes and Separation Processes, 4th ed. by Geankoplis
(esp, sect. 6.2C)
2) Perry’s Handbook
3) CRC or Lang’s Chemical Handbook or similar
4) Lee, C.Y. and Wilke, C.R., Industrial and Engineering Chemistry, Nov.
1964
5) Any other library or on-line references you think will be of help
6) Handouts from class