Flow field characterization between vertical plate electrodes in a bench-scale cell of electrochemical water softening

As a common system in industrial production, the stable operation of the circulating cooling water system is of great significance to ensure the safe and stable production of enterprises (Tvwa et al. 2020). Scale deposited on the heat exchange surface is one of the main defects of the circulating cooling water system, which seriously affects the work efficiency and increases energy consumption and production cost (Gabrielli et al. 2006; Hasson et al. 2010). Many descaling methods are used to solve the problem, such as scale inhibitor (Younes et al. 2017), ion exchange (Hu et al. 2015; Clauwaert et al. 2019) or membrane techniques (Hu et al. 2016). The electrochemical water softening (EWS) method stands out from many alternatives because it is water-saving and pollution-free (Cristiani & Perboni 2014; Dirany et al. 2016; Niknejad et al. 2018).

and Ca²⁺ are consumed in the alkaline area near the cathode to form scale, and then and Ca²⁺ in other areas are supplemented to the alkaline area in time. Hardness ions are separated from the solution, which reduces the hardness and realizes the purpose of water softening. In this process, the convective transport efficiency of hardness ions and the solution renewal rate near the cathode will have a significant impact on water-softening efficiency.

The efficiency of electrochemical water softening is affected by many factors. According to the results of response surface analysis, a mathematical dynamic model was employed to reveal the relationship between the hardness removal percentage and the current density. The efficiency of EWS increased with the increasing of current density, but the increasing trend gradually decreases after 40 A/m² (Zhang et al. 2020). Current density initially augments the electrochemical precipitation of calcite, but at a sufficiently higher current density, the precipitation rate tends to an asymptotic limit (Zeppenfeld 2011). The deposition of calcium carbonate on the electrode surface will increase the resistance and reduce the water-softening efficiency. Yu proposed a new current-pulsated electrochemical precipitation process and found that scale detachment was accomplished by increasing the current density significantly (Yu et al. 2018).

In the water-softening process, bubbles are generated on the electrode surface due to the electrolysis of water. The efficiency of electrochemical processes is closely linked to the hydrodynamics of the bubble curtain (Rodríguez & Amores 2020). The solution convection caused by the drag effect of bubbles is the main mass transfer mode between electrodes. The drag effect of the bubble curtains was the main driving force to renew the electrolyte within electrode gap (Luo et al. 2018). Bubbles accelerate the electrolyte flow near the electrode and enhance agitation and there is a pronounced impact on the convective transport of electrochemically active species (Zhu et al. 2018). Bubbles act as moving electrical insulators, thus affecting the current density distribution and increasing the ohmic drop across the reactor (Wang et al. 2014).

The electrode structure and electrode spacing will also affect the efficiency of EWS. Li observed the deposition behavior of calcium carbonate on the multi-layer mesh coupled cathode and found that the shielding effect caused by the electrode structure can promote the preferential deposition of calcium carbonate on the outer layer of the cathode (Li et al. 2020). The electrode spacing is also an important factor that affected the bubble behavior and the flow. Smaller electrode spacing distance increases the interaction between the ionic electrolyte and the immersed electrode, thereby increasing the rate of the electrochemical reaction, and the increase of electrode spacing will significantly reduce the hydrogen production efficiency (Okonkwo et al. 2022).

To study hydrodynamics and mass transfer behavior between electrodes of electrochemical water softening, experiments were carried out in a bench-scale electrochemical reactor. The particle image velocimetry (PIV) technique was used to obtain the flow field in the vertical plate electrode reactor. The effects of current density, electrode spacing and electrode position on the flow characteristics and mass transfer behavior between electrodes were studied.

The electrolyte was prepared by adding anhydrous calcium chloride (CaCl₂) and anhydrous sodium bicarbonate (NaHCO₃) to distilled water at a molar ratio of 1:2. The purity of CaCl₂ was 96% and the purity of NaHCO₃ was 99.5%. In each experiment, 77.7 g CaCl₂ and 117.6 g NaHCO₃ were used to prepare 14.4 L electrolyte to simulate hard water. The hardness was calculated as 1,000 mg/L based on the mass concentration of CaCO₃. The initial pH of the solution measured with an LICHEN pH-meter was 7.3 ∼ 7.5.

In this study, a bench-scale electrochemical reactor was used. As shown in Figure 2, the electrochemical reaction device is composed of an adjustable DC power supply, an electrolysis tank and a set of vertical plate electrodes. The DC power supply provides electrical energy for the electrochemical reaction, and the voltage adjustment range is 0–30 V. The reactor is made of transparent acrylic glass material, which is convenient for the PIV system to obtain the flow field image during the electrolysis reaction. The specification of the electrolytic water tank is 450 mm × 160 mm × 250 mm. The electrodes are titanium plates with a size of 100 mm × 100 mm × 1 mm, and the anode surface is coated with iridium-tantalum to prevent electrochemical corrosion. Titanium plate has excellent conductivity and corrosion resistance. An iridium-tantalum titanium electrode has higher electrochemical activity and longer service life. In previous studies, we found that titanium plate cathode has higher water-softening efficiency and lower energy consumption than copper plate (Wang et al. 2021).

The PIV device used in this study was produced by the Lavision company in Germany. The PIV system is composed of charge-coupled device (CCD) cameras, a laser generator, a synchronizer and a computer. The resolution of the CCD camera is 2,048 pixels × 2,048 pixels. The laser wavelength is 532 nm. The tracer particles used in the experiment are PSP tracer particles provided by Beijing Measurement (China), which are hollow glass beads with a diameter of 20 μm and a density of 1.03 g/cm³.

Add simulated hard water to the electrolyzed tank to a water depth of 200 mm. Connect the electrode to the power supply, fully immerse and fix it in simulated hard water vertically and parallel. After setting the shooting area of the PIV system, turn on the power to start the electrochemical reaction.

At the beginning of the reaction, bubbles gradually form on the electrode surface and float upward, stirring the solution to form a flow. At this time, the flow is unstable and has not fully developed. After a long time of reaction, a large amount of scale will deposit on the cathode surface, resulting in electrode passivation and reducing the efficiency of electrochemical water softening (Yu et al. 2018). Based on the above considerations, in this experiment, after the reaction was carried out for 5 minutes and the flow field in the electrolytic cell was fully developed, PIV photos were taken to obtain the flow field data. The flow field data are obtained before electrode passivation to avoid the influence of electrode passivation on the experimental results.

After 10 hours of the electrochemical water-softening process, sample the solution in the reactor, test the hardness and record it. The effect of electrochemical water softening was characterized by the hardness drop of the solution.

PIV technology is used to measure the velocity distribution of the flow field and the size of the query area is determined by the velocity of the flow field and the size of the CCD camera's shooting area. The time interval between two images is determined by the velocity of the flow field and the size of the query area.

During the experiment, 50 images were taken continuously for each group of working conditions. The processed speed data were averaged to obtain the time average speed data within 10 s of the flow field. The image acquisition time interval is selected as 200 ms, and the size of the inquiry area is set to 32 pixels × 32 pixels. The shooting area is selected in the range of 160 mm × 160 mm including the electrode plate. The pictures taken by CCD camera are processed by PIVlab. The obtained flow field is visualized in Tecplot 360. The hardness of solution was measured using a Hash AT-1000 potentiometric titrimeter.

The electrode plate is vertically placed in the middle of the flow field. The distance between the upper end of the electrode plate and the water surface is 50 mm and the electrode spacing is 120 mm. The current density of 20 A/m², 40 A/m², 60 A/m² and 80 A/m² was selected to observe the bubble behavior and velocity distribution.

It can be seen from the formula that the current density is directly proportional to the volume of gas produced by the electrode, and the volume of hydrogen produced by the electrode is twice that of oxygen under the same conditions. The coalescence tendency of oxygen was confirmed to be larger than that of hydrogen (Lee et al. 2019). The bubble diameter of hydrogen bubbles was smaller than that of oxygen bubbles (Matsuura et al. 2019). The number of bubbles in the cathode is significantly greater than that in the anode.

Figure 3 shows the bubble shadow images for different current densities. Bubbles are generated on the electrode surface and float up along the electrode. Bubbles separated from the top end of the electrode and continue to float up to the water surface. The increase of current density significantly accelerates the bubble formation on the electrode surface. Some bubbles cannot float away from the electrode surface in time and form a bubble layer on the electrode surface.

In the low current density range, the increase of current density significantly increases gas production. In Figure 3(a)–3(c), the increase of current density significantly increases the number of bubbles under the water surface. When a large number of bubbles float to the water surface, some bubbles cannot surface in time due to the blocking of the water surface. The hydrogen bubbles flow to both sides after hitting the water surface, forming a bubble accumulation area under the water surface, and the bubbles moving toward the anode plate flow to the left in the way of counterclockwise rotation.

Compared with Figure 3(c) and 3(d), when the current density increases from 60 A/m² to 80 A/m², the amount of bubbles produced by the electrode is larger, but the downward diffusion depth of bubbles does not increase. The bubble curtain on the anode side hinders the transverse bubble flow more significantly.

The behavior of the bubble layer on the cathode surface directly affects the reaction and deposition of hardness ions. The bubble shadow image and its binary image near the cathode for different current densities are shown in Figure 4. The bubble curtain is whirled by the entrainment. Because of the inhomogeneity of the bubble group, the vortex formed on both sides of the bubble curtain is different, which results in pressure difference on both sides of the bubble curtain, leading to the left and right snake swing. With the increase of current density, the bubble flow swing intensifies, the vortex flows vertically, and the pressure difference increases. When the current density increases to above 60 A/m², the outer surface of the bubble layer is no longer smooth and becomes corrugated, and the bubble floats up in the form of waves.

In the binary image, the thickness of the bubble layer increases in the vertical direction. As the current density reaches 60 A/m², the thickness of the bubble layer on the cathode surface does not change obviously with the increase of current density. It can be inferred that bubbles gradually accumulate while floating on the electrode surface, forming a bubble layer around the electrode. Due to the limited adsorption of the electrode on the bubbles, the thickness of the bubble layer is limited. When the current density is 80 A/m², the thickness of the bubble curtain formed when the bubble separates from the top of the electrode is greater than that when the current density is 60 A/m².

The bubble shadow image is processed to obtain the velocity distribution of the flow field. In Figure 5, there are two main flows between the electrodes. One is to flow upward along the surface of the electrode and to both sides after hitting with the water surface. This flow is mainly due to the bubble generated on the electrode surface and floats up to drive the surrounding fluid flow. The bubble generated by the electrode floats up, which promotes the forced convection of the solution around the bubble to flow upward along with the plate, which is called ‘bubble-driven convection.’ The second flow is mainly in the area between the electrode plates. A large number of bubbles accumulate on the electrode surface, resulting in high local porosity, forming the density difference of the solution, driving the nearby solution to flow toward the plate, and finally forming natural convection. When the natural convection solution reaches near the plate, it is incorporated into the bubble-driven convection. The velocity of bubbles on the cathode side is faster than that on the anode side. The maximum velocity of bubble-driven convection is about 5–9 times that of natural convection.

In the observed region, the increase in current density enhances the influence range and velocity of bubble-driven convection.

Between the regions below the water surface and above the top of the electrode, the velocity of the bubble-driven convection exceeds 0.003 m/s. In Figure 5(a) and 5(b) with low current density conditions, the maximum velocity of bubble-driven convection appears in the transverse flow area below the water surface. However, in Figure 5(c) and 5(d) with higher current density conditions, the flow velocity in the region decreases. From the streamline of the flow field, it can be observed that a large vortex is formed in the region above the top of the electrode. The flow velocity near the vortex and flow split region is smaller than that in other regions. The vortex will increase the residence time of the solution near this region and decrease the renewal speed of the solution.

Theoretically, the electrolyte between the electrode gaps should be laminar flow with a flow velocity below 0.003 m/s. The increased current density increases the flow velocity of electrolytes in the electrode gap. In Figure 5(a) and 5(b) with low current density conditions, the flow velocity between electrodes is below 0.001 m/s. In Figure 5(c) and 5(d) with higher current density conditions, the flow velocity in most areas of the electrodes gap is more than 0.001 m/s. The solution splits in the middle of the electrode gap and flows to the two plates respectively. Combined with the velocity distribution and bubble behavior, it can be seen that the main reason for the increase of flow velocity is that the increased amount of bubbles on the electrode surface. The higher void fraction and the increase of density difference increase the natural convection velocity. In the upper part of the electrode gap, the flow speed is faster due to the drag force of bubble-driven convection.

The increase of current density will significantly increase the flow area to the cathode, reduce the vortex near the cathode and increase the flow velocity between electrodes. This is conducive to the enrichment of hardness ions in the solution to the cathode and increase the electrochemical water-softening efficiency.

Figure 6 shows the variation of the average flow velocity with the height. In the region above the top of the electrode, the average velocity decreases with the increased current density. The disturbance of bubbles and the existence of the vortex decrease flow velocity. In the region between electrodes, the bottom of the electrode is defined as h = 0, and the top of the electrode is defined as h = 0.1 m. The locally enlarged diagram of the average flow velocity distribution is shown in Figure 6(b). The bubble has more momentum in the normal direction of the electrode surface with a larger current density on the system (Lee et al. 2019). The increased current density increases the average flow velocity between electrodes. The four points, A, B, C, D, are the height of the average velocity increasing significantly under different current densities. The higher the current density, the lower the height of the point. High current density results in a high void fraction, which leads to flow acceleration. The average flow velocity decreased at point E and point F respectively with current densities of 60 A/m² and 80 A/m². That the bubbles diffusion to the area between the electrodes results in flow disorder and multiple eddy currents, which reduces the flow velocity in the electrode gap. The bubble-driven convection generated by higher current density has a more significant disturbance between electrodes, and the influence area is larger. When the current density is 60 A/m², the average flow velocity at different heights between electrodes is significantly higher than other current densities.

The horizontal velocity between the electrodes can be used to characterize the range and trend of the solution flowing to the electrode. Figure 7 shows the horizontal velocity distribution on the horizontal line at different heights of the electrode. A positive value indicates that the horizontal velocity direction is to the right and a negative value to the left. The intersection of the curve and the 0 horizontal line is the position where the direction of the horizontal velocity changes.

For h = 5 mm, with the increase of current density, the zero point of horizontal velocity moves to the right, indicating that the flow area to the cathode gradually decreases. For the other higher height, the value of horizontal velocity flowing toward the cathode first increases and then decreases with the increase of current density.

Compared with Figure 7(c) and 7(d), when the current density increases from 60 A/m² to 80 A/m², the horizontal velocity and region of flow to the cathode are both reduced, indicating that too high current density is not conducive to the mass transfer of hardness ions to the cathode. When the current density is 60 A/m², the curve of average horizontal velocity is basically centrosymmetric and the flow is stable. At the same time, the maximum velocity at each height reaches the maximum in several different current densities.

Table 1 shows the hardness drop under different current densities at the electrode spacing of 120 mm. The hardness reduction ratio characterized the effect of electrochemical water softening. The results show that increasing the current density can increase the efficiency of electrochemical water softening, but at too high current density, the efficiency of electrochemical water softening decreases. T his is consistent with the flow phenomenon between plates.

The electrodes were placed vertically in the middle of the flow field, and the top of the electrode is 50 mm away from the water surface. The current density is set to 50 A/m². The electrode spacing was set as 120 mm, 90 mm, 60 mm and 30 mm, respectively. The effect of electrode spacing on the flow field and the velocity distribution was analyzed. The variation of plate spacing changes the mass transfer distance between electrodes and causes the change of bubble movement behavior.

The current density was kept unchanged under different electrode spacing in Figure 8. The reduction of electrode spacing reduces the flow range between the two electrode plates and the mass transfer distance between cathode and anode. Most of the bubbles generated by the electrode are blocked by the bubble curtain in the upper part between the two electrode plates, forming a bubble vortex. The bubble curtain on the anode inhibits the bubble diffusion on the left side of the cathode.

The velocity distribution and streamline of the flow field for different spacing conditions are shown in Figure 9. The decrease of the electrode spacing leads to the decrease of the distance of transverse flow by the bubble-driven convection and the depth of the downward diffusion of the bubbles. The decrease of electrode spacing enhances the effect of the bubble-driven convection on the electrolyte flow in the electrodes gap. When the electrolysis distance is 60 mm, the vortex in the electrode gap disappears completely, and the electrolyte changes into an upward flow. The transverse bubble-driven convection is to be closed above the electrode and form a vortex. When the electrode spacing is reduced to 30 mm, a low-velocity area appears on the upper part of the electrode. This is because a large number of bubbles are enclosed on the upper part of the electrode, and the formed vortex blocks the upward flow of the solution in the electrode gap and reduces the flow velocity.

The decrease of electrode spacing shortens the mass transfer distance of hardness ions between electrodes. When the electrode spacing is 60 mm, the disappearance of eddy current accelerates the renewal of solution between electrodes. This is conducive to the continuous deposition of scale on the cathode surface and ensures the efficiency of electrochemical water softening.

It can be seen from Figure 10 that the average velocity between electrodes first increases and then decreases with the reduction of electrode spacing. The decrease of the electrode spacing results in the decreases of fluid volume in the electrodes gap, which enhances the effect of bubble-driven convection on the fluid volume per unit volume and increases the overall velocity between electrodes. When the electrode spacing is 60 mm, the average velocity of the electrode gap is 0.00169 m/s, which effectively improves the flow velocity in the electrode gap by using the kinetic energy of bubble-driven convection and enhances the mass transfer efficiency between electrodes. When the spacing of the electrodes is 30 mm, bubbles and vortices above the electrodes inhibit the upward flow in the electrode gap. The average velocity in the electrode gap is reduced to 0.00106 m/s, which decreased by 60%.

When other conditions remain unchanged, there is an optimal spacing to make the average velocity in the electrode gap reach the highest level. Too large electrode spacing leads to bubble-driven convection on the electrode surface, which cannot effectively affect the solution in the middle of the electrode gap, and the solution flow speed is slow. A small electrode spacing will cause the solution in the electrode gap to not flow out of the electrode gap in time, inhibit the bubble kinetic energy and reduce the mass transfer efficiency of solution flow. In this experiment, when the electrode spacing is 60 mm, the average velocity between electrodes reaches the maximum, which is 0.00169 m/s

Table 2 shows the hardness drop under different electrode spacing at the same current density of 50 A/m². The results show that the electrochemical water softening efficiency is the highest when the electrode spacing is reduced to 90 mm. Further narrowing electrode spacing may lead to difficult flow exchange of solution between electrodes and outside the electrode plate, which is not conducive to the occurrence of electrode reaction. The uneven distribution of solution concentration may also produce concentration polarization, so the hardness drop increases with the further reduction of electrode spacing. The maximum average flow rate between electrodes does not lead to the highest water-softening efficiency, because the efficiency of EWS is also affected by other factors.

The electrode plates were vertically placed at the top, mid and bottom of the flow field to observe the flow field change. The position of the electrode in the flow field formed different flow field structures, which led to significant changes in bubble behavior.

In Figure 11(a), when the electrode plate is located at the top of the flow field, the upper end of the electrode plate is close to the water surface, and the lower end of the electrode plate is 100 mm away from the bottom of the water tank. A semi-closed space is formed between the electrodes. During the process of bubbles floating to the water surface, the bubbles do not separate from the electrode and diffuse. The number of bubbles accumulated under the water surface is less than that under the other two conditions. The upper end of the electrode destroys the tension structure of the water surface, so that the bubbles are easier to surface, reducing the number of bubbles that diffuse to the water surface.

In Figure 11(c), the bottom of the electrode is closed to the bottom of the sink, and the top of the electrode is 100 mm away from the water surface, forming a semi-closed space closed at the lower side. The bubble plume produced by the cathode gradually diffuses to both sides during the rising process of the cathode. The transverse bubble flow under the water surface cannot diffuse into the electrode gap.

In Figure 12, the velocity streamlines show that the electrolyte circulation efficiency in the whole flow field is low in the semi-closed structure. The upper closure causes the bubble-driven convection to be enclosed, forming a vortex in the electrodes gap. The effect of mass transfer between the regions on both sides of the electrode and the electrode gap is poor.

When the electrode is located at the bottom, the whole flow field is divided into three parts by the electrode plate and bubble curtain. The electrode is far away from the water surface, and the kinetic energy of convection driven by transverse flow bubbles cannot be transferred down to the electrode gap. The velocity between electrodes is less than 0.003 m/s, which is significantly smaller than the other two cases.

In the semi-closed structure, the renewal of solution between electrodes is slow or the flow speed is slow. These will affect the mass transfer of hardness ions to the cathode. The free convection structure balances these two points.

Figure 13 shows the change of average velocity at different horizontal heights under three different cases.

When the top is closed, the average velocity in electrode gap reaches a maximum of 0.00121 m/s. The kinetic energy of bubble driven convection is enclosed in the electrode gap.

When the electrodes are in the middle, the average velocity in electrode gap increases along with the height and then decreases due to the disturbance of bubble-driven convection. The average velocity dropped to 0.000807 m/s.

When the bottom is closed, the overall velocity in the electrode gap is low, and the average velocity at different heights is not more than 0.001 m/s. The average velocity dropped to 0.000731 m/s.

The kinetic energy of bubble-driven convection is the main power source of flow in the flow field, and the effective use of energy can significantly increase the flow velocity between the electrodes. But the vortex and bubble disturbance are not conducive to the mass transfer in the electrodes gap. The free convection structure can effectively eliminate the influence of eddy current and ensure the convective mass transfer velocity between electrodes.

In Figure 14, when the electrode is located at the bottom of the flow field, the extreme value of the horizontal velocity in the electrode gap is smaller than that of the other two cases. When the electrode is located in the middle of the flow field, the horizontal velocity value is positive, that is, the area flowing to the cathode is larger than that of the plate at the top of the flow field. For the process of electrolytic water softening, the free convection structure with the electrode in the middle of the flow field can transfer mass to the cathode more effectively and enhance the water-softening efficiency.

Table 3 shows the hardness drop of the electrode at the position. The results show that the electrochemical water softening efficiency is the highest when the electrode is located in the middle of the flow field. Compared with the bottom, the water softening effect is worse when the electrode is at the top. When the electrode is at the top, the kinetic energy of the bubble is not released, resulting in small average flow velocity and poor mass transfer effect.

In a bench-scale electrolytic cell, the hydrodynamic characteristics of the electrolyte between the electrodes were investigated based on the flow field from PIV measurement. The velocity of bubble-driven convection caused by bubble floating is 5–9 times that of natural convection caused by bubble aggregation. The increase of current density can significantly increase the velocity of bubble-driven convection and natural convection, but the excessive current density will lead to the accumulation of bubbles under the water surface and on the electrode surface. The accumulation of bubbles will disturb the surrounding liquid and reduce the liquid velocity passing through the bubbles. When the electrode spacing is 120 mm, the highest water-softening efficiency measured at the current density of 60 A/m² is 16.56%.

The reduction of electrode spacing can enhance the effect of bubble-driven convection on the flow in the electrode gap and enhance the flow mass transfer. However, a too small electrode spacing will lead to the accumulation of bubbles above the electrode gap, hinder the outflow of liquid in the electrode gap, and inhibit the enhancement of bubble-driven convection on mass transfer. When the current density is 50 A/m², the highest average speed measured at the electrode spacing of 60 mm is 0.00169, but the highest water softening efficiency measured at the electrode spacing of 90 mm is 23.3%.

When the electrode gap is in a semi-closed structure, the circulation efficiency of the liquid in the electrode gap and the liquid outside the electrode gap will be reduced, or the kinetic energy of bubble driven convection can not be effectively used, resulting in the decrease of the velocity in the electrode gap. These are not conducive to the flow and mass transfer of hardness ions to the cathode in the process of EWS. In the process of EWS, using the vertical plate electrode with free convection structure and selecting the appropriate current density and electrode spacing can effectively enhance the flow and mass transfer efficiency between electrodes, to enhance the efficiency of EWS and reduce the power consumption.

This work Supported by Scientific Research Foundation of Wuhan Institute of Technology (K2021020), Nation Natural Science Foundation of China (50976080).

All relevant data are included in the paper or its Supplementary Information.