**2. Experimental method**

#### **2.1 Apparatus for the electrical resistance tomography**

The experimental apparatus consists of a 0.3-m diameter column filled with 2.0-cm polypropylene spheres as shown in Figure 1. The packed bed height of 6 times the column diameter was used. An ERT system (model P2000, Industrial Tomography Systems Ltd., Manchester, UK) was used to measure the flow pattern at 6 axial positions in the packed bed using 6 electrode planes (P1 to P6) located at equal-distance of 0.30 m apart along the axial direction of the bed. In order to avoid disturbance to the flow, electrodes were installed flush with the inside wall of the column. At each electrode plane, 16 stainless-steel electrodes (1.0 cm x 2.0 cm) were arranged at equal distance one to another on the perimeter of the column (Figure 2). All electrodes on the 6 planes were connected to a data acquisition system for data collection. Experiments were performed with a high conductivity tracer (a sodium chloride solution) introduced into the liquid inlet to create a high conductivity front that moved through the packed bed.

In the present study, an electric current was applied to two neighbouring electrodes (e.g. electrodes 1 and 2), and then voltages were measured from the remaining pairs of neighbouring electrodes (e.g. 3 and 4; 5 and 6 .etc...). The electric current was then applied through the next pair of electrodes and the voltage measurements were repeated. Measurements were made within each plane separately and concurrently. 104 individual voltage measurements were collected from each of the 6 planes of electrodes every 100 ms (sampling interval). Those 104 measurements constitute a measurement set. The number of samples per frame (the number of measurement sets that were taken to produce an average frame of data) was set at 8. From the voltage measurements, the ITS System 2000 software uses a linear back-projection image-reconstruction algorithm to convert the data to tomograms that showed the distribution of liquid flow on a 2D plane.

Both upward and downward flow modes were used in the experiments. For the upward flow mode, water was pumped continuously from a holding tank to the bottom of the column. The liquid flow rate was monitored by a rotameter (Model F-45750-LHN12, Fabco Co., Maple, Ontario, Canada). A 0.5% wt salt solution (conductivity = 9.95 mS.cm-1) was used as the high conductivity tracer. Five liquid flow rates from 8.83 – 12.6×10-4 m3.s-1 (14–20 gpm) were used in the experiments. When the water flow rate in the column had reached a steady state, a 1L aliquot of the tracer solution was injected into the liquid inlet stream to the

(Hoek et al., 1986; Kouri and Sohlo, 1996; Farid and Gunn, 1978; Kunjummen et al., 2000). In this method, liquid is collected in an array of cells or concentric cylinders at the bed outlet. The liquid collecting duration is also recorded. Liquid velocity obtained from the measured liquid volume is then used to quantify the liquid distribution in the bed. However, the liquid velocity obtained from the liquid collecting method is an axially aggregated flow through the packed bed at a certain radial location, which doesn't reveal the local liquid distribution at various axial distances along the bed. Therefore, in the present study measurements of liquid distribution and velocity at various axial distances in a packed bed were carried out, using electrical resistance tomography (ERT). The ERT system can quantify the liquid distribution and liquid velocity in the packed bed without disturbing the flow field since it is a non-intrusive technique. In addition, measurements at different locations in the packed bed can be measured simultaneously without the need for changing

The experimental apparatus consists of a 0.3-m diameter column filled with 2.0-cm polypropylene spheres as shown in Figure 1. The packed bed height of 6 times the column diameter was used. An ERT system (model P2000, Industrial Tomography Systems Ltd., Manchester, UK) was used to measure the flow pattern at 6 axial positions in the packed bed using 6 electrode planes (P1 to P6) located at equal-distance of 0.30 m apart along the axial direction of the bed. In order to avoid disturbance to the flow, electrodes were installed flush with the inside wall of the column. At each electrode plane, 16 stainless-steel electrodes (1.0 cm x 2.0 cm) were arranged at equal distance one to another on the perimeter of the column (Figure 2). All electrodes on the 6 planes were connected to a data acquisition system for data collection. Experiments were performed with a high conductivity tracer (a sodium chloride solution) introduced into the liquid inlet to create a high conductivity front

In the present study, an electric current was applied to two neighbouring electrodes (e.g. electrodes 1 and 2), and then voltages were measured from the remaining pairs of neighbouring electrodes (e.g. 3 and 4; 5 and 6 .etc...). The electric current was then applied through the next pair of electrodes and the voltage measurements were repeated. Measurements were made within each plane separately and concurrently. 104 individual voltage measurements were collected from each of the 6 planes of electrodes every 100 ms (sampling interval). Those 104 measurements constitute a measurement set. The number of samples per frame (the number of measurement sets that were taken to produce an average frame of data) was set at 8. From the voltage measurements, the ITS System 2000 software uses a linear back-projection image-reconstruction algorithm to convert the data to

Both upward and downward flow modes were used in the experiments. For the upward flow mode, water was pumped continuously from a holding tank to the bottom of the column. The liquid flow rate was monitored by a rotameter (Model F-45750-LHN12, Fabco Co., Maple, Ontario, Canada). A 0.5% wt salt solution (conductivity = 9.95 mS.cm-1) was used as the high conductivity tracer. Five liquid flow rates from 8.83 – 12.6×10-4 m3.s-1 (14–20 gpm) were used in the experiments. When the water flow rate in the column had reached a steady state, a 1L aliquot of the tracer solution was injected into the liquid inlet stream to the

the bed height, which is of advantage over the liquid collecting method.

tomograms that showed the distribution of liquid flow on a 2D plane.

**2.1 Apparatus for the electrical resistance tomography** 

**2. Experimental method** 

that moved through the packed bed.

Fig. 1. Schematic diagram of the experimental set-up for ERT

Fig. 2. Arrangement of the electrodes on the column wall

column. A reference in the ERT system was taken before the injection of the tracer. This reference acted as a base line from which the increase in the conductivity was observed when the tracer reached the electrodes. This reference conductivity was about 1.00 mS.cm-1. The ERT system recorded the conductivity profiles at 6 planes along the column simultaneously.

For the downward flow mode, water was pumped to the top of the column. A 1.5% wt salt solution (conductivity = 29.9 mS.cm-1) was used as the tracer for better conductivity measurements due to the liquid hold up and channelling that tended to dilute the tracer

Measurement of Liquid Velocity and Liquid Distribution

Fig. 4. Experimental set-up for the liquid collecting method

4.Outer section with 20 tubes (dimension unit: mm)

Fig. 5. Schematic diagram of the arrangement of in the liquid collecting tubes: 1. Center section with 1 tube; 2. Inner section I with 6 tubes; 3. Inner section II with 12 tubes; and

Measurements were carried out at three axial levels from the top of the packing: 0.3, 0.6 and 0.9 m. These are equivalent to the ratios of the packing height to the column diameter, x/D,

in a Packed Bed Using Electrical Resistance Tomography 733

in which only liquid was used, the gas blower was turned off in these experiments. Liquid flowing down the bed was collected in a collector with 39 tubes of 25.4-mm diameter. The tubes were arranged circularly at four different radial positions as shown in Figure 5.

more. Similar conductivity measurement procedure to that for the upward flow was used at various flow rates from 5.05 – 7.57×10-4 m3.s-1 (8–12 gpm).

Following the acquisition of data from the boundary of the object to be imaged it is necessary to process this data using an appropriate image reconstruction algorithm. For an ERT system the reconstructed image will contain information on the cross-sectional distribution of the electrical conductivity of the contents within the measurement plane. A square grid with 20 x 20 = 400 pixels represents the vessel interior cross-section. Some of these pixels will lie outside the vessel circumference and the image is therefore formed from the pixels inside the vessel. The circular image is constructed using 316 pixels from the 400 pixel square grid as shown in Figure 3 (ITS System 2000 Version 6 User's Manual, Industrial Tomography System Ltd., Manchester, UK).

The conductivity values of individual pixels can be exported to an Excel file for further analysis. From the time elapse between the advancement of the conductivity peak of an individual pixel from one plane to the next one, the local liquid velocity was determined. From the values of the local velocities, the liquid distribution factor was calculated using Equation (1) below. These values were then compared with the values obtained from the liquid collecting method that is described in the following section.

Fig. 3. ERT grid of 316 pixels for the image reconstruction

#### **2.2 Apparatus for the liquid collecting method**

In the investigation of liquid distribution, the same 0.3 m diameter PVC column filled with 2.0-cm polypropylene spheres as shown in Figure 1 was used. However, a liquid collector was added to the column as shown in Figure 4. For liquid distribution measurements using the liquid collecting method, only water was used. Two liquid flow rates of 5.05×10-4 m3.s-1 and 7.57×10-4 m3.s-1 (8 gpm and 12 gpm) were used. For a comparison with the ERT method,

more. Similar conductivity measurement procedure to that for the upward flow was used at

Following the acquisition of data from the boundary of the object to be imaged it is necessary to process this data using an appropriate image reconstruction algorithm. For an ERT system the reconstructed image will contain information on the cross-sectional distribution of the electrical conductivity of the contents within the measurement plane. A square grid with 20 x 20 = 400 pixels represents the vessel interior cross-section. Some of these pixels will lie outside the vessel circumference and the image is therefore formed from the pixels inside the vessel. The circular image is constructed using 316 pixels from the 400 pixel square grid as shown in Figure 3 (ITS System 2000 Version 6 User's Manual, Industrial

The conductivity values of individual pixels can be exported to an Excel file for further analysis. From the time elapse between the advancement of the conductivity peak of an individual pixel from one plane to the next one, the local liquid velocity was determined. From the values of the local velocities, the liquid distribution factor was calculated using Equation (1) below. These values were then compared with the values obtained from the

In the investigation of liquid distribution, the same 0.3 m diameter PVC column filled with 2.0-cm polypropylene spheres as shown in Figure 1 was used. However, a liquid collector was added to the column as shown in Figure 4. For liquid distribution measurements using the liquid collecting method, only water was used. Two liquid flow rates of 5.05×10-4 m3.s-1 and 7.57×10-4 m3.s-1 (8 gpm and 12 gpm) were used. For a comparison with the ERT method,

various flow rates from 5.05 – 7.57×10-4 m3.s-1 (8–12 gpm).

liquid collecting method that is described in the following section.

Fig. 3. ERT grid of 316 pixels for the image reconstruction

**2.2 Apparatus for the liquid collecting method** 

Tomography System Ltd., Manchester, UK).

in which only liquid was used, the gas blower was turned off in these experiments. Liquid flowing down the bed was collected in a collector with 39 tubes of 25.4-mm diameter. The tubes were arranged circularly at four different radial positions as shown in Figure 5.

Fig. 4. Experimental set-up for the liquid collecting method

Fig. 5. Schematic diagram of the arrangement of in the liquid collecting tubes: 1. Center section with 1 tube; 2. Inner section I with 6 tubes; 3. Inner section II with 12 tubes; and 4.Outer section with 20 tubes (dimension unit: mm)

Measurements were carried out at three axial levels from the top of the packing: 0.3, 0.6 and 0.9 m. These are equivalent to the ratios of the packing height to the column diameter, x/D,

Measurement of Liquid Velocity and Liquid Distribution

**3. Results and discussion** 

**3.1 Upward flow profile** 

(Figure 8).

0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16

Mean conductivity (mS/cm)

P1 P2 P3 P4 P5 P6

Fig. 7. Mean conductivity at various planes for the upward flow

in a Packed Bed Using Electrical Resistance Tomography 735

For the upward flow mode, the high conductivity tracer moved upward with the bulk flow and passed plane 1 to plane 6, consecutively, after the tracer injection. The advancement of the tracer upward through the column can be seen in the tomograms in Figures 6(a) and 6(b). The shade of a region in a tomogram indicates the conductivity of that region in

Some time after the tracer injection, the high conductivity solution reached planes 1 and 2 as indicated by the light spots in the tomograms for P1 and P2 in Figure 6(a). The tomograms at planes 3 to 6 still have a dark shade indicating a low conductivity of the bulk water stream since the tracer solution didn't reach to those planes yet. As time went by, the tracer solution moved farther upward to P4, P5 and P6 as can be seen by the high conductivity regions (light shade) in Figure 6(b) while the conductivities at P1, P2 and P3 decreased (dark

Using the mean conductivity data across a plane, conductivity peaks can be identified when the tracer has reached successive planes as shown in Figure 7. The distance between the peaks represent the time for the tracer to move between the planes. From the conductivity peaks and the time elapsed between two peaks, the liquid velocity from one plane to the next one was determined. It was noted that in the upward flow mode, a steady flow and a more even distribution of conductivity across a plane were obtained. The velocities at different planes are comparable to one another and the average velocity throughout the packed bed. This might be due to the fact that the liquid almost moved up the column in a plug flow pattern. The flow pattern wasn't distorted by liquid hold-up or liquid channelling. The ERT measurements of the liquid velocity were within 5% with the interstitial velocities calculated from the averaged liquid flowrate measured by a flowmeter and the bed porosity

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accordance with the conductivity scale in mS/cm as shown below the tomograms.

shade at those planes) since the tracer solution had moved out of those regions.

of 1, 2 and 3. The wall flow was separated from the bulk flow in the packing by an annular ring on the inside wall of the column at the packing support level and collected in a separate container so that it did not interference with the local liquid flow through the packing to the liquid collector described above.

In order to quantify liquid distribution in the packed bed, a liquid distribution factor was used and defined as below (Dang-Vu et al., 2006b):

$$\mathcal{M}\_F = \frac{1}{n} \sqrt{\sum\_{i=1}^n \left(1 - \frac{V\_i}{V\_{AVG}}\right)^2} \tag{1}$$

where MF is the liquid distribution factor, n is the number of liquid collecting tubes, Vi is the liquid velocity to individual collecting tubes and VAVG is the averaged liquid velocity

Fig. 6. Tomographic images of the conductivity at various planes for upward flow
