Apparatus for Preparation of Very Dilute Gas Mixtures JAMES M. McKELVEY and
.
E.
HOELSCHER
Department of Chemical Engineering,
The Johns Hopkins University, Baltimore
With the diffusion cell described, extremely dilute gas mixtures can be produced continuously and accurately over The a wide range of concentration. method is most useful when the low concentration component of the mixture is a liquid at ordinary temperatures and
Anal. Chem. 1957.29:123-123. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/25/19. For personal use only.
pressures, but the method can also be used for any substance that can be liquified under conditions which can be obtained in the laboratory.
investigations require the preparation of gas mixtures in which the concentration of one component may be very low—e.g., 0.1 to 100 p.p.m.—but must be accurately known. Preparations based on the dilution of pure gases are not always satisfactory. However, such gas mixtures can be prepared by means of a cell, the operation of which depends upon the diffusion of the vapor of a liquid into a gas stream.
1
8, Md.
flask is saturated and that the concentration of vapor in the upper flask is nearly zero, the diffusion rate (3) is given by the equation
DPMA ln RTL .
r
T
(1)
where r
= =
P A L T
Many
=
= = =
R
=
D
=
p
=
tr
=
diffusion rate molecular weight of vapor total pressure cross-sectional area of tube length of tube absolute temperature gas constant diffusion coefficient vapor pressure of liquid (P/P p) -
Diffusion coefficients for many gases listed in the Critical Tables (ß) at the standard conditions of 0° C. and 1 atm. Values at other temperatures and pressures can be estimated with the formula
METHOD
Figure diagram of the apparatus. The cell consists of txvo 50-ml. roundbottomed flasks connected by a length of straight glass tubing. The inlet and outlet lines of the upper flask are designed to promote mixing and to minimize exit effects from the tube. Groundglass joints connect the upper and lower flasks to the tube, so that tubes of different sizes can be interchanged. In operation, the lower flask is partially filled with the liquid substance and the entire cell submerged in a constant temperature bath. The space above the liquid in the lower flask becomes saturated with vapor. The vapor diffuses through the tube into the upper flask, where it mixes with the gas stream, and is carried For a given cell and a given away. liquid, the diffusion rate depends only on the total gas pressure and the vapor pressure of the liquid in the cell. If the total pressure is constant, the diffusion rate is a function of temperature only and can be controlled very accurately by regulating the bath temperature. 1
is
a
DISCUSSION
For precise work the cell should be calibrated, although for those substances for which the diffusion coefficient is known, the diffusion rate can be calculated. In designing cells for specific applications it is also desirable to be able to calculate the approximate tube size and temperature required. The method of calculation is as follows. Assuming that the gas in the lower
Figure 2.
Calibration for toluene-air mixtures
are
where
D
=
D0
=
diffusion coefficient at temperature T and pressure P diffusion coefficient at 0° C. and 1
atm.
Combining Equations
2 and 3 we have,
In some cases it may be necessary to design cells for gases for which diffusion
coefficients are not available. In such cases, the Gilliland equation (1) can be used to estimate the diffusion coefficient DoFigure 2 shows a typical calibration. In this case dilute mixtures of toluene in air were being prepared. Data were obtained over a temperature range of 35° C. to 60° C. using a tube which had a diameter of 0.49 cm. and a length of 10 cm. The apparatus was calibrated by filling the lower flask with about 25 ml. of liquid toluene and weighing it. The cell was then assembled and submerged in a constant temperature bath. A steady flow of air was passed through the upper flask, and the loss in weight of the liquid was determined after an extended period of time. Equation 3 shows that a plot of diffusion rate vs. T ln ir should be a straight line. The calculated slope is 1.21 X 10 ~6 gram per minute-degree, and the calibration gives a value of 1.15 X ICG6 gram per minute-degree. The deviation, which amounts to about 5%, is attributed mainly to a distortion of the cylindrical shape of the diffusion tube, which occurred when it was fused to the ground glass joints. LITERATURE CITED
(1) Gilliland, E. R., Ind. Eng. Chem. 26, 516 (1934). (2) “International Critical Tables,” Yol. V, McGraw-Hill, New York, 1929. (3) Sherwood, T. K., Pigford, R. L., “Absorption and Extraction,” p. 6, McGraw-Hill, New York, 1952.
Received for
review7 October 13, 1955. Accepted September 6, 1956. This work w7as carried out under contract with the Chemical Corps, U. S. Army, Washington 25, D. C. VOL. 29, NO. 1, JANUARY 1957
·
123
