LITERATURE REVIEW
2.12 Experimental Procedures
2.12.1 Holdup
Baird and Shen (1984) used the method of displacement to evaluate the holdup of the column. The column operation was allowed to reach steady state. The interface level was marked and feed pumps and the drain pump as well as the feed valves and the drain valves were shut off simultaneously. Enough time was allowed for the dispersed phase to coalesce under the marked interface level. The continuous phase was pumped into the column in order to displace the coalesced dispersed phase into a measuring cylinder until the interface returned to the marked level. The effective volume of the column was measured from the dispersed phase distributor to the interface. Volume fraction holdup was calculated as the volume in the cylinder over the effective volume of the column.
Local holdup was obtained by rapidly removing about 200 ml of the dispersion from 2 sampling points along the length of the column after steady state operation was achieved.
The dispersed phase content was then found volumetrically; however, Baird and Lane (1973) found that the measurements of local holdup were highly sensitive to the sampling rate.
Another way of measuring the holdup is to allow the column to first reach steady state, and then to measure the amount of toluene in the upper tank between the interface and the surface of the toluene layer. Both the water feed line and the toluene feed line must be closed simultaneously. The contents of the column should be drained from the bottom of the tank.
The amount of toluene present could then be measured and the amount that was present in
the top layer could be subtracted from this value. The holdup is then reported as the fraction of toluene in the continuous phase (ppm) (Rama Rao et al., 1991).
Baird and Lane (1973) performed experiments on a system where the dispersed phase moved down the column and the interface was located at the bottom of the extractor. The overall holdup was evaluated by shutting off the flows and allowing the dispersed phase droplets to fall to the interface. The resulting increase in interface level in relation to the volume of the column between the initial interface and the top of the agitated section was used to determine the overall holdup.
In order to investigate the effects of the operating variables on the holdup, the following tests could be done. Perform experiments at zero continuous flow and a constant dispersed flow for different agitations (products of af). Plot holdup, against af for different values of frequency. Perform experiments at a fixed value of continuous flow and dispersed flow.
Again plot holdup, against af for different values of frequency. Perform experiments for a fixed value of continuous flow and varying values of dispersed flow. Then perform experiments for a fixed value of dispersed flow and varying values of continuous flow (Rama Rao et al., 1991).
Jiricny and Prochazka (1980) measured local holdups in a Karr column in order to evaluate the holdup profiles of the column. Their measurement was based on the determination of differences in static pressure across short sections of the column. After every 2 stages, a differential pressure induction transducer connected to pressure taps along the column measured pressure differences and related this to holdup in that section of the column.
2.12.2 Droplet Size Distribution
A section of the column was photographed towards the middle of the column during steady state operation. The photographed section must be surrounded by a rectangular water-filled (continuous phase) box made of clear (Perspex) plastic to eliminate refractive distortion. A particle size counter was used to measure the drop size distribution. The distribution could
et al., 1991). Jiricny and Prochazka (1980) photographed sections of the column and evaluated the particle size distribution by analysing between 300 and 500 drops per photograph whereas Bensalem et al. (1986) felt that analysing 200 droplets was sufficient.
2.12.3 Entrainment
Since toluene is completely immiscible in water, a sample is taken from the bottom of the column during normal operation. The sample is allowed to settle. The amount of toluene could be measured with a measuring cylinder and reported as ppm of toluene in sample.
2.12.4 Power Consumption
An approximate indication of the power consumption could be obtained by connecting an A.C. voltmeter in parallel with an A.C. ammeter in series with the line to the vibration motor. The data should be corrected by subtracting the measured power when the column was run dry at the same amplitude and frequency (Rama Rao et al., 1991).
2.12.5 Axial Mixing
Hafez et al. (1979) used two methods for measuring the axial mixing in the continuous phase. The first method was used by Kim and Baird (1976a) in which a single pulse of sodium hydroxide solution was injected into the column at four points around the wall about 50 cm below the top of the plate stack. It then reacted with dilute hydrochloric acid in the aqueous phase in the presence of a phenolphthalein indicator. The rate of color migration can then be related to the axial dispersion coefficient. This method is suitable only when no circulation currents exist.
The second technique uses a conventional pulse injection of a salt tracer solution (NaCl or NH4Cl) with the response being measured by a conductivity cell fed through a thin capillary line from a point near the base of the plate stack, a fixed distance below the injection point.
Karr et al. (1987) used a tracer of 9 mass % ammonium chloride in water. Methanol was added to bring the density of the tracer close to that of water (within 1%). The tracer was injected pulse-wise at the desired aqueous and organic flow rates. Responses in the aqueous phase were measured by electrical conductivity probes.
2.12.6 Concentration Analysis
Bensalem et al. (1986) evaluated the outlet concentration of the aqueous phase for the acetone-toluene-water system by a density meter and found that steady state was reached for the Karr column after the contents of the column had been replaced at least five times.
Lisa et al. (2003) and Vatanatham et al. (1999) investigated the mass transfer resistance for the toluene-acetone-water system and used the refractometric method to determine the acetone concentration at the entrance and exit of both phases.
Most other researchers used chromatographic analysis.
2.12.7 Use of Radiotracers for Evaluation of Axial Dispersion, Holdup and Slip Velocity
Din et al. (2008) states that the use of a non-radioactive tracer for the evaluation of axial dispersion or shutting off flows in order measure holdup has many disadvantages including low sensitivity, poor statistics, the requirement of phase separation before measurements are made and that the plant must be shut down in order to measure the holdup of the dispersed phase. They used a radioisotope in a pulsed sieve plate extractor to overcome these shortcomings. 99mTc in the form of sodium pertechnetate was used as a radioactive tracer injected into the column. Axial dispersion, holdup and slip velocity was measured online by monitoring the movement of the tracer in the column and analysing the residence time distribution.