Figure 1. The experimental setup (a) a diagrammatic sketch and (b) in the inner structure of the PDDC.
All experiments were carried out in a laboratory-scale PDDC made of stainless steel. Figure 1 shows the diagrammatic sketch of the experimental set-up. In order to perform a precisely optical measurement, the cross section of PDDC is designed to be square and two glass windows are set on opposite sides of the column. The height of the column is 1000 mm and the dimension of the cross section is 100 mm × 100 mm. Ten pairs of disc and the doughnut plates are arranged alternately in the column. All the plates are made of Teflon with a thickness of 2 mm. The distance between the disc and the doughnut plates is 30 mm. The size of the disc is 87.7 mm × 87.7 mm and the size of the hole in the middle of the doughnut is 48 mm × 48 mm, thus the free area of these two kinds of plates are both 23%. There are seven sampling points arranged equidistantly with an interval of 65 mm along one side of the column. A high-speed camera (Olympus i-Speed TR) connected with a microscope (Olympus SZ61) is used to capture the droplet breakage in the column. The frame rate is 1000 frames per second. The high-speed camera is located at 265 mm, 330 mm, 395 mm and 590 mm from the top of the column respectively, as shown in Figure 1 (a). Figure 1(b) shows the inner structure of the PDDC as well as the focal plane of the microscope connected to the high speed camera. To promote the breakage of droplets in the column, a pulsation generator is connected to the bottom of the column. The pulsation generator induces reciprocal up and down movements of the fluids by a piston, thus a sinusoidal pulsation of the liquid system is produced. The pulsation frequency is 1 Hz and the amplitude is10.5 mm. The aqueous phase entrance and organic phase exit are set at the top of the pulsed column while the aqueous phase exit and organic phase entrance are at the bottom. All exits and entrances are controlled by a supply and collection system respectively. In the process of operation, the aqueous phase is dispersed into droplets in the organic phase by pulsation and the droplets coalesce at the bottom of the column.

Materials

The experiments consist of two parts conclude mass transfer experiments and blank experiments. In mass transfer experiments, 5% (v/v) tributyl phosphate (TBP) in kerosene and 15% (v/v) acetic acid (AC) solution were used as the continuous and dispersed phase separately. The AC, TBP, and kerosene were purchased from Aladdin Reagent Co. Ltd., J&K Scientific Ltd., and Jinzhou Refinery Factory, respectively. In the blank experiments, 5% (v/v) TBP in kerosene and deionized water were used as the two-phase materials. The physical properties of two-phase materials are shown in Table 1. Continuous phase is pumped into the lower part of the pulsed column by a peristaltic pump (Longer Pump WT600-2J) at the flow rate of 443 mL/min. The dispersed phase is pumped into the upper part of the pulsed column through another peristaltic pump (Longer Pump BT100-2J) at the speed of 21 mL/min. The purpose of choosing a low flow rate of dispersed phase is to reduce the holdup of dispersed phase, so that the behavior of droplets can be optically measured. All experiments were conducted at a temperature of 20\(\pm\)1°C.
Table 1. Physical properties of the two-phase materials used in experiments at 20 °C