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Ind. Eng. Chem. Fundam., Vol. 18, No. 2, 1979
LETTER TO T H E EDITOR A New Gas Sampling Device Hypodermic Needle
Sir: Sampling of gas is an important part of residence time distribution studies. Generally, sophisticated devices are used for the collection of samples since the time involved will always be very short. A new, simple, and easy to make gas sampling device is described below for such studies. The well known soap bubble flow meter has been adapted for this purpose. The exit end of the buret tube of this unit is modified as shown in Figure 1 by first gradually reducing the diameter of the tube and then fusing it with a capillary tube of about 2 mm diameter with provision to insert a rubber septum. A hypodermic needle of fairly large diameter is inserted through the rubber septum in order to let the gas flow out. The gas stream is connected to the inlet and is allowed to bubble through the liquid by pressing the rubber bulb. The gas passes through the buret in compartments separated by thin films of soap. The height of each compartment depends on the size of inlet tube. Under normal circumstances, the gas rate is generally obtained by noting the time taken by one of the films to move through a finite volume of the buret. Gas samples, whenever required for residence time distribution studies, are obtained by simply bubbling the gas through the unit for a predetermined time from the commencement of the cycle. When a sufficient volume of sample gas has collected, the unit is detached from the gas line. The unit now contains the sample in a number of small compartments separated by soap films. The individual samples for the analysis of tracer concentration are collected by sucking the gas through the hypodermic needle with the help of a syringe in the usual way. When the gas is sucked, the compartments move upward one by one until the topmost film flows through the capillary. Although the soap films are very thin, hence
0019-7874/79/1018-0192$01.00/0
Rubber Septum
...
.
Rubber Bdlb
.. . .
Figure 1. Gas sampler
the volume small, the film occupies a finite height in the capillary which helps to identify the individual compartments. The gradual reduction of diameter in the exit section helps to maintain the film stable by slowly increasing the thickness due to corresponding reduction in diameter. The performance of this new device has been found to be excellent, giving reliable results. D e p a r t m e n t o f Chemical Engineering Regional Engineering College Tiruchirapalli - 620 015 India
P. Subramanian VR. Arunachalam*
Received f o r review July 18, 1978 Accepted January 4, 1979
0 1979 American Chemical Society
