Kinetic and residence time distribution modeling of tubular electrochemical reactor: analysis of results using Taguchi method

Treatment of effluent is a global concern. Dye wastewater from the textile industry can pose a serious threat to the environment and it is difficult to decolorize because of its structural complexity. Most of the azo dyes are carcinogenic and affect the marine ecosystem and generally they contain two or more aromatic rings joined by one or more azo groups. Acid red is classified as a single azo dye, which is used in the textile industry especially for nylon, cotton and wool. The dye adds harmful organic matter to the aquatic environment (Ayman et al. 2019). Chemical and biological treatment methods such as coagulation, adsorption, ion exchange, coagulation-flocculation, activated sludge process, and anaerobic-aerobics have been used for treatment. However, these treatment techniques have some disadvantages like requirement of huge amounts of chemicals, secondary treatment to further remove the sludge formed from the primary treatment, demand for large area and the sensitivity of microorganisms. Frequent clogging and membrane fouling is a major issue in separation techniques. The electrochemical process is a better solution to overcome the limitations of chemical and biological treatment. Recently, researchers have been focusing on the electro-oxidation technique. In electrooxidation, electrons are the main reagent and mineralization of organic pollutants is achieved (Nideesh et al. 2018). It is also reported that electro-oxidation achieved maximum color removal efficiency (Kaur et al. 2017).

In electrochemical reactors, kinetic modeling and residence time distribution (RTD) studies has been conducted by many researchers. Kinetic study has been attempted in electrochemical reactors for different modes of operation and various types of reactors. Soloman et al. (2009b) studied different modes of operation using RuOx–IrOx–TiOx coated titanium for removal of organics from agro-based paper industry using a tubular electrochemical reactor. Singh et al. (2016) investigated electro-oxidation of malachite green dye using RuO₂–TiO₂ and Pt-coated Ti mesh electrode. Santos et al. (2018) studied the removal of Acid red 1 dye using Ti/TiO₂-nanotubes/PbO₂. The authors observed that Ti/TiO₂-nanotubes/PbO₂ anode performs better than Ti/TiO₂ for the removal of Acid red 1 dye. Ibrahim et al. (2013a) carried out experiments in a batch, stack and cylindrical electrochemical cell for simultaneous chemical oxygen demand (COD) reduction in the dye effluent. The authors reported that the degradation rate is affected by the electrolyte flow in the cylindrical cell compared to the tank cell and batch cell. Ibrahima et al. (2014) performed the treatment of petroleum effluent using electrochemical tubular reactor. They have achieved 85% COD reduction using ruthenium oxide coated titanium anode. Oukili & Loukili (2019) performed the degradation of textile azo dye Methyl Orange a cylindrical platinized titanium (Ti/Pt) grid as anode. The author reported that 98.51% of color removal was achieved.

Literature also shows electrochemical reactor of flow visualization using computational fluid dynamics (CFD) and RTD studies. Saravanathamizhan et al. (2008) proposed a Tanks-in-series model for the electrolyte flow behavior in a continuous stirred tank electrochemical reactor and the model is verified using RTD study. Martínez-Delgadillo et al. (2010) investigated the performance of the tubular electrochemical reactor with different inlets; that is, central, lateral and tangential. The author reported that tangential inlet gives homogenous axial velocity and the degree of back mixing is reduced significantly. Ibrahim et al. (2013b) studied flow dynamics and mass transfer in a tubular electrochemical reactor using RTD and CFD were used to analyze the flow dynamics inside the reactor. It is reported that the presence of dead volume, bypass and short circuiting decreases with increase in flow rate.

The RTD analysis of a vessel gives useful information on flow characterization. The RTD study shows how long the fluid element stays inside a reactor, as a quantitative measure of the degree of back mixing within a system. The knowledge on RTD is important to determine the conversion of the fluid inside the reactor for the first order kinetics, which helps with reactor design (Fogler 1999; Levenspiel 1999).

The literatures have reported kinetic modeling, electro oxidation analysis using statistical analysis, flow modeling using CFD and RTD. The present research focuses on kinetic modeling and residence time distribution modeling for the cylindrical flow electrochemical reactor. Based on the information obtained from the RTD, the decolorization of synthetic dye wastewater was estimated for the electro oxidation using a tubular electrochemical reactor.

The synthetic dye effluent is prepared using Acid red 87 dyes. The structure of the dye is shown in Figure 1. Sodium chloride is used as the supporting electrolyte.

The experimental setup of the tubular reactor is shown in Figure 2. The reactor consists of a cylindrical shaped cathode and mesh type anode, storage tank to store dye wastewater, circulation pump and regulated power supply. The reactor is 7 cm in diameter, 110 cm high and made of stainless steel that acts that as the cathode and inside the reactor mesh type anode Ti/Ti_0.7Ru_0.3O₂ of 5 cm diameter, 100 cm height is inserted and sealed with end frames. Electrodes are connected to the regulated power supply (100A, 0–50 V). Mesh type anode having 60% perforation have an effective anode area of 628.3 cm². All the connections are made using silicone rubber tubes. For the decolorization experiment, three different dye concentrations of 100, 200 and 300 mg L⁻¹ are taken with the recirculation flow rate of 30, 60 and 90 LPH. Current density and supporting electrolyte also varied as 6, 9 12 mA cm⁻² and 1, 2, 3 g L⁻¹ respectively. In the present work, a batch with recirculation operation was performed to study the dye decolorization and once through mode was used to study RTD.

Taguchi method is one of the statistical analyses, based on orthogonal arrays, the experimental design was developed. MINITAB software tool was used to develop the experimental design. Four variables of dye concentration (A) flow rate (B) current density (C) and supporting electrolyte concentration (D) are selected at three different levels shown in Table 1. The three level and operating parameters were selected based on preliminary experiments. The experimental color removal is observed as output response. A suitable orthogonal array design table (Table 2) is designed and the selection based on the number of variables and number of levels is shown in Table 1. Taguchi method, S/N ratio is used to identify the optimum condition of the experiment.

The mean value of acid red 87 dye color removal using electro oxidation is shown in Figure 4. The mean value plot is used to understand the effect of variables on output response. Factor ‘A’ is used to represent the initial dye concentration and it is observed from the figure that dye decolorization increases with decrease in dye concentration. This is due that at higher dye concentration, the intermediate product formed blocks the active sites of the electrode and acts as resistance to current flow, which results in a decrease in the color removal. The effect of dye wastewater flow rate on decolorization is shown by a factor ‘B’. Dye decolorization is increased with increase in wastewater flow rate. The increase in decolorization is due to the enhancement of the mass transfer coefficient at higher flow rate. The effect of current density on the color removal is denoted by ‘C’. It is observed from the figure that dye decolorization increases with the increase in current density. This is due to the fact that more electrons are generated at higher current density, which results in an increase in the hypochlorite ion generation, which results in higher decolorization. The effect of supporting electrolyte concentration is denoted by ‘D’ on color removal. The current density increases the OCl⁻ generation, hence the dye decolorization increases. Various levels (1, 2, 3) of the operating parameter (A, B, C, D) and their mean color removal is shown in Table 3. It is observed from the table that Level ‘1’ gives the highest color removal of 75.68% for the initial dye concentration. Level ‘4’ shows the highest color removal of 58.15%, 69.96% and 62.29% for flow rate, current density and electrolyte concentration respectively.

Using S/N ratio values in Taguchi analysis the optimal conditions of experimental parameters on color removal has been identified. In the Taguchi method, S represented ‘signal’ and N represented ‘noise’. The S/N ratios are different for different types of experimental output response. In the present experiment, larger S/N ratio is better for higher decolorization. The values of the S/N ratios for the experimental parameters are shown in Table 4. The larger the S/N ratio, the higher the percentage color removal and vice versa (Yen & Li 2015; Pundir et al. 2018). It is observed from the table that the S/N ratio is level ‘1’ for parameter ‘A’, and level ‘4’ for parameters B, C and D respectively.

D/UL is the dispersion number; k is the electrochemical reaction rate constant (min⁻¹); is the residence time (min); Pe is the Péclet number. The model value is compared with the experimental value and is shown in Figure 6. It is noticed from the figure that the model satisfactorily matches with the experimental result with a high correlation coefficient.

Decolorization of wastewater was performed in a tubular electrochemical reactor. Operating parameters such as dye concentration, flow rate, current density and supporting electrolyte concentration were studied for color removal and the parameters were optimized using Taguchi analysis. Kinetic modeling and RTD modeling for the tubular electrochemical reactor were performed. It is concluded from the experiment that color removal using an axial dispersion model satisfactorily matches the experimental color removal with a high correlation coefficient.

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