Experimental Procedure Mass transfer experiments

The assembled apparatus for the mass transfer experiments is shorn in Figures 14 and 15 and Plate 1.

The relative positions of the base plate and the inner column

were first adjusted to give the required height of fall for the droplets, whilst ensuring that the column and plate remained perpendicular. The inner column could then be partially filled with carbon tetrachloride and any air in the drain tube expelled, Distilled water was introduced

into the end of the drain tube, through the glass tap, using a plastic bottle. After topping up with carbon tetrach loride, the droplet coll­ ection assembly could be inserted into the top of the inner column, taking care to prevent entrapment of air below the funnel. With the rubber seal in place, the bottom assembly could be bolted to the main column.

Using a glass funnel, the aqueousphase was introduced through the top of the main column and allowed to run down the side wall, to avoid splashing into the droplet collection funnel. The top plate, with its rubber seal, could then be bolted in position and the capillary tip fastened in position. By means of the two pairs of support screws, the capillary tip and the collecting funnel were vertically aligned, using a plumbline.

In order to prevent oxidation of the amalgam, the system had to 3 be flushed with hi^bpurity argon, usually at a rate of 500 - 400 cm per minute for a period of one hour. The amalgam reservoir also had to be flushed with argon for a similar period of time. After introduction of amalgam into the reservoir, the plastic tube between it and the cap­ illary generally required squeezing in order to expel any trapped gas.

Just prior to commencing the run, a little fluid was drained from the inner column, so that the acid-carbon tetrachloride interface just touched the glass bead below the droplet collection funnel. The taps on the gas inlet and outlet ports c.ould then be closed and the level of fluid in the gas burette noted.

Droplets of the required size were allowed to fall down the column at a rate of about 30 per minute. The duration of a run varied from about 2 to 5 minutes. Throughout the run, water had to be drained continuously from the inner column, in order to maintain the acid-carbon tetrachloride interface at a constant position, and collected in a previously weighed

bottle. Also, the gas burette was slowly raised in order to maintain the gas pressure inside the column at one atmosphere.

An experiment was terminated, when a suitable number of droplets had passed down the column, by interrupting the flow of amalgam from the reservoir. The distance from the liquid surface to the top of the glass bead was then measured using a cathetometer and the volume of hydrogen collected in the gas burette and trapped under the droplet collection funnel were determined. The amount of water drained from the inner column during the course of a run was obtained by weighing the collection bottle. At this stage it was usual to carry out a check on the size of the drop­ lets being produced at the tip and also to collect two samples of amalgam for analysis.

The mass transfer column could then be drained and the bottom plate together with the inner column removed, thus allowing the funnel and collecting cup to be lifted clear. After neutralisation with dilute hydrochloric acid the amalgam collected was washed with both distilled water and ethanol, allowed to dry and finally weighed. The rest of the apparatus could then be dismantled, washed and allowed to dry, prior to the next experiment.

The change in composition of a known number of droplets of a certain size, which had fallen a given distance through the aqueous phase, was calculated from the volume of hydrogen liberated. Small corrections were made for the volume of aqueous phase carried down into the carbon tetrachloride by the droplets and also for the partial pressure of water vapour in the evolved hydrogen.

4.3«10 Photographic Techniques

4.3.10(a) Droplet velocity determination

The terminal velocity attained by a falling droplet was required in order to calculate the time for which it was in contact with the acid solution and hence determine the rate at which mass transfer was occurring. In all the experiments, droplets were released from about 2 cm above the surface of the acid phase, so that on entry to it. they

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were travelling at approximately their terminal velocity. Crimes

has shown that under these conditions the terminal velocity is achieved very rapidly.

Droplet velocities were experimentally determined using conven­ tional cine photography. A Bolex H 16 camera was employed with either Kodak Tri X or Ilford Mark V film at a speed of 32 frames per second. In order to confirm their accuracy the filming speeds of the camera were recalibrated by Sheffield Photographic Company just prior to these ex­ periments.

The mass transfer column was set up in the same way as for the mass transfer experiments, but with the inner column adjusted to give the maximum height of fall possible (i.e. about 60 cm). During filming, droplets were allowed to fall continuously down the column, at a rate of about 30 pe:r minute. Oblique illumination using a single light source was employed.

Three datum lines were marked on the front of the column dividing it into two 25 cm sections, thus giving a total useful height of 50 cm. This permitted the velocities in the upper and lower halves of the column to be compared, whilst still obtaining an overall velocity for the total column. The top reference line was about 5 .cm below the surface of the aqueous phase, thus eliminating the effects of any period required to

attain terminal velocity. Particular care was taken to horizontally align the camera lens with the central datum mark so that parallax errors in the upper and lower sections would he equal.

The films obtained were examined at low magnifications using a transmission light microscope, fitted with an improvised film holder, and the velocities obtained by counting the number of frames it took a droplet to traverse the distance between reference marks. The point at which a droplet passed a given mark could be estimated to an accuracy of half a frame, thus giving a maximum possible error of one frame, for fall between a given pair of marks. This constitutes a maximum error of about yjo for the overall velocity and 6fo for the velocity over half of the column height. A slight correction had to be made to the results in order to allow for parallax effects due to the datum lines being some

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