THERMAL DIFFUSION COLUMNS FOR LECTURE DEMONSTRATION' E. WHALLEV National Research Council, Ottawa, Canada
A
tion ratios in mol fractions a t each end) of the thermal diffusion column and q, is the separation factor of a. nonconvective unit with the same hot and cold temperatures. If the separation is small it is easily shown that
DEMONSTRATION of thermal diffulsion mhich can be performed during a lecture period has not been described in the literature. I t is the purpose of this article to describe the design of thermal diffusion columns suitable for this purpose. Suitablethermal diffusioncolumns should have the following characteristics: (1) Short equilibrium time so that the separation can be observed during the lecture. The shorter the equilibrjum time the more effective will be the demonstration. Also, the column should not be too large. (2) Readily observed separation. The separation should he large and should be easily visible. Bromine and helium are the best pair of gases, or bromine and air can be used if helium is not available. A mixture of a radioactive and nonradioactive gas could he used if desired and the separation followed with counters a t each end of the column.
AC. = FAC,
(2)
where AC, and AC. are the concentration differencesof one molecular species obtained in the convective and nonconvective apparatus respectively. F is given in equations (33-6) of ref. (2) in terms of the properties of the gas and the dimensions of the apparatus. The lengths L, wall .separations d, and relamtion times t for thermal diffusion columns without reservoirs corresponding to minimum length, volume, or relaxation time are given by the equations: L
=
p'r,F,(AP')-V.
(3)
d = (BdRP4)-'11
(4)
1 = BtMPLa
(5)
where the 6's are simple numbers dependent on the quantity to be minimized. The values of the B's are given in Table 1. TABLE 1
DESIGN OF COLUMN
The theory of the thermal diffusion column has been discussed in detail by Jones and Furry,J and the design of columns which have a minimum of either length, volume, or equilibrium time for a given separation has been discussed by Whalley.4 The separation is described in terms of a factor F defined by
Minimum
BL
Length Volume Relaxation time
2.129 2.311 2.682
Bd
Bt
2 5
'h
'h
0.9804 '18
~h~ remaining quantities in equations (3-5) are: A
=
M =
where qc is the separation factor (ratio of the concentra1. Issued by the National Research Laboratories as N. R. C. No. 2590. 2. National Research Laboratories Post-Doctoral Fellow. Note added in proof. Since this paper was written our attention has been drawn to a paper by B. B. MCINTEERand C. E. SCHENSTEDT ( A m . J. Phys., 17, 417 (1949),descrihinga hot wire column using helium and air. The separation is observed by the color of the hot wire and equilibrium is reached in about 20 min. However the column is neither so easy to make and operate nor so rapid as ours. 3 Jones, R. C., AND W. H. F ~ ~ RRev. Y ,Mod. Phys., 18, 151 (1946). 4 Whalley, E., Trans. Faradoy Soc., 47, 129 (1951).
& + DAT ~
(~w-'
. . where p is the density, 7 the viscosity, D the diffusion of the gas mixture at 1 atm,,p the pressure in g the due to and fr, fa, and fk are correction factors for the cylindricity of the column and are given in ref. (I), equations (100-11). AT = Tz - TI, and T = l/p(Tz+ TJ, where TIand T2 are the cold and hot wall temperatures. The value of d it will be noted is independent of the separation required. 24
JANUARY,1952 EXPERIMENTAL
can be varied by 10-15 per cent without seriously affecting the efficiency of the column. The heaters for the inner tube are loops of B & S No. 11 chromel-A wire threaded through twin-bore porcelain insulators and controlled by a Variac transformer. The outer tube is cooled by a water jacket. The rate of heating of the inner tube is important in determining the rate of separation, and a high initial input which can be reduced later increases the rate. With an initial input of about 800 watts there is a noticeable separation in TABLE 2 Dimensions of Columns about 60 seconds with the helium-bromine column and in about 80 seconds with the air-bromine column. The top of the column is clear in about two minutes and four 15 mm. Outer diam. of inner tube 12 mm. minutes respectively. Comparison between the theoInner ditam. of outer tube 34 mm. 31 mm. 40 om. 70 cm. Length retical and calculated equilibrium times is not attempted because the inner tube is heating up for a conTable 2. The dimensions are not the optimum ones, siderable part of the time required to clear the top of but the nearest which could be obtained with standard the column, and also no measurements are made of the glass tubing. The dimensions are not very critical and temperature of the hot wall.
The columns are filled with an equimolecular mixture of bromine and helium and bromine and air respectively a t a total pressure of 150 mm. of mercury (0.197 atm.). This pressure is low enough to prevent the bromine from condensing when it is concentrated a t the lower end of the columns. They are made of standard pyrex tubing, as shown in the figure, with the dimensions given in
