H2O2 reactor operation: Design, evaluation, and optimization

Journal Pre-proof Application of impinging jet atomization in UV/H2 O2 reactor operation: Design, Evaluation, and Optimization Mohammad Hafezi, Mehrdad Mozaffarian, Morteza Jafarikojour, Madjid Mohseni, Bahram Dabir

PII:

S1010-6030(19)30971-2

DOI:

https://doi.org/10.1016/j.jphotochem.2019.112198

Reference:

JPC 112198

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

7 June 2019

Revised Date:

12 October 2019

Accepted Date:

26 October 2019

Please cite this article as: Hafezi M, Mozaffarian M, Jafarikojour M, Mohseni M, Dabir B, Application of impinging jet atomization in UV/H2 O2 reactor operation: Design, Evaluation, and Optimization, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112198

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Application of impinging jet atomization in UV/H2O2 reactor operation: Design, Evaluation, and Optimization Mohammad Hafezia, Mehrdad Mozaffariana, Morteza Jafarikojourb, Madjid Mohsenib, *, Bahram Dabira a

Department of Chemical Engineering, Amirkabir University of Technology, No. 424, Hafez Ave., Tehran 15875-

4413, Iran Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver,

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b

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BC, Canada V6T 1Z3

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* Corresponding author.

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E-mail address: [email protected] (M. Mohseni).

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Graphical abstract

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Highlights    

A new UV/H2O2 reactor was designed using impinging jet atomization. Impinging jet atomization was applied to form thin liquid sheets and achieve highly efficient mixing. Re number and impingement angle could be used for conceptual design of large scale reactors. Complete removal of 50 mg L−1 methyl orange dye solution was obtained within 120 min.

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ABSTRACT The concept of augmenting UV/H2O2 reactor with impinging jet atomization to achieve highly efficient

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mixing and thin fluid sheet formation capability was investigated. The collision of two jets forming a free-standing thin liquid sheet allowed the establishment of an effective UV/ H 2O2 advanced oxidation

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setting. The response surface methodology (RSM) was applied to model and optimize the photochemical degradation process, which provides three level designs for RSM fitting. Three

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variables namely, Re (15000-31000), impingement angle (60-120 degree) and H2O2 dosage (1000-

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3000 mg L-1) were applied in BBD to model and optimize the effects of three key operational parameters. The optimum methyl orange (MO) removal percentage (after 90 min) and the apparent

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first order rate constant were 93.6% and 2.438 min-1, respectively. The influences of initial dye concentration, UV radiation power, and jet diameter were also investigated as the other main operating parameters. ANOVA analysis indicated that the Re number has the highest impact of the three considered parameters and additional experiments showed that the jet diameter does not have any

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significant effect on MO degradation. A pseudo-first-order kinetic model was applied for the prediction of contaminant degradation and rate coefficients. The good agreements between the model predictions and experimental results indicate that the proposed model could successfully describe the effectiveness of the augmented UV/H2O2 reactor due to the maximum degradation of the model contaminant.

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Keywords: Impinging jet atomization; UV/H2O2 AOP; Novel photoreactor; Response surface methodology; Kinetic model

1. Introduction Advanced oxidation processes (AOPs), with ability to produce highly reactive hydroxyl radicals

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(𝑂𝐻∙ ), have been widely applied for treating industrial wastewater [1,2]. AOPs can degrade

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organic contaminants effectively, often resulting in the formation of harmless end products (e.g., CO2, H2O, inorganic acids) [3-5]. Among the several AOPs, ultraviolet (UV) radiation in the

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presence of hydrogen peroxide (H2O2) has been successfully implemented for the treatment of various organic pollutants. The UV/H2O2 process has various advantages including no sludge

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production, non-selectivity to a very broad range of chemicals, and low investment costs [6-8].

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The UV/H2O2 reactor’s efficiency is dependent upon many factors among them being the concentration of peroxide, presence of radical scavengers in the solution, UV fluence rate.

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However, a main issue in this process, especially in the case of treating highly coloured wastewaters, is extremely high absorption of UV by dye molecules. As a result, hydroxyl radicals are reported to be generated only within a thin layer around the UV source [9-11]. Due to low diffusion coefficient and very short life time of hydroxyl radicals in water (2.3e -5 cm2/s and 10-9-

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10-6 s), radical intermediates cannot diffuse outside the irradiated volume and react close to the place where they are generated [12,13]. This heterogeneity between the reaction zone near the UV source and the non-irradiated volume of the reactor, limits the overall efficiency of the UV/H 2O2 AOP for coloured wastewater streams. Moreover, the quartz tube surrounding the UV lamp has some adverse effects such as the absorption of light in the air gap and the quartz tube surrounding the UV lamp and fouling. Under such a circumstance, uniform UV dose distribution in the UV 3

reactor would be disturbed [14].Therefore, to enhance the performance of the UV photoreactor, different mixing devices such as mechanical mixers and continuous bubbling aeration equipment have been tested [15,16]. Saien et al. have used a pilot-scale jet mixing photo-reactor to achieve homogeneous solutions during short mixing times, which indicated high economic benefit in comparison to similar photoreactors [17].

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The impinging jet atomizer is a set of two impinging liquid jets with the same properties as they impinge on one another at a certain impingement angle that is used to form an expanding liquid

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sheet perpendicular to the plane of the two liquid jets. A schematic diagram of an impinging jets

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configuration is demonstrated in Fig. 1. Impinging jet injectors possess high mixing efficiency and relative simplicity of fabrication and have been applied to reactors in various chemical engineering

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processes including crystallization, precipitation, absorption and desorption of gases, energyconversion, liquid-liquid extraction and drying of solids [18-21]. Table 1 indicates some of the

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different applications of the impinging jet atomization phenomenon. Based on the ability of impinging jet atomizer to provide highly efficient mixing, and form thin liquid sheets, it seems

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logical to incorporate them as part of a new design of UV/H2O2 photo-reactor used for the degradation of coloured wastewater from textile industries. Table 1

Application of impinging jet atomization phenomenon in different performances

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Reactor

Application

Free Impinging Jet Crystallization Microreactors precipitation process Impinging Reactor (IJR)

Merits of using impinging jet atomization

and  perform rapid precipitation reactions  enhance micromixing  master challenging reactions with very fast kinetics

Jet Synthesis of silver  efficient mixing nanoparticles (NPs)  lack of channel walls  avoid fouling  suitable for large scale synthesis of NPs

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Lit. ref. [18]

[19]

Impingement of Forming clathrate  good liquid atomization liquid jets at hydrates from natural  formation an expanding liquid sheet different pressures gas impinging-jet dynamics and atomization

Liquid fueled  Simple configuration propulsion engines  reliable and efficient atomization and mixing

[20]

[22]

The efficiency of a UV/H2O2 process with impinging jet atomization capability depends on several

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operating parameters that are varied and occasionally interact with each other. Hence, it is imperative to apply an efficient experimental design methodology that can identify factors

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influencing a multivariable system [23,24]. The response surface methodology (RSM) is one of the feasible and efficient experimental design techniques, which combines mathematics and

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statistics for optimization and possible up-scaling of the AOP based on the Box–Behnken design

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(BBD). It can also be used to develop models, evaluate the effects of several factors and their

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number of experiments [25,26].

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interactions, determine the optimal conditions, and reduce the study time and cost with a minimum

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Fig. 1. Schematic display of liquid sheet formed by like-doublet impinging jets with jet diameter D and impingement angle 2𝜃. In this study, impinging jet atomization has been applied to design a new UV/H2O2 reactor. The application of impinging jet atomization leads to the formation of thin liquid sheets, and a higher degree of mixing and consequently increasing the dye removal effectiveness. Indeed, this new

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reactor configuration displays certain features (dimensionless parameter of Re and non-use mixing devices), which could facilitate the design of large-scale photochemical processes. The BBD

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design as one of the subsets of RSM was applied in order to model, optimize and study the effects

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of different operating parameters including Re number, impingement angle, and H 2O2 dosage in the UV/H2O2 reactor. Then, the effects of other key parameters like initial contaminant

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concentration, UV radiation power, and jet diameter were investigated at optimum conditions. The first-order kinetic coefficients were calculated by minimizing the sum of the square errors (SSE)

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for experimental results, and those predicted by the model at various initial MO concentrations.

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2. Material and methods 2.1 Materials

The methyl orange (MO) dye (C14H14N3O3SNa; molar mass 327.3 g/mole) obtained from Samchun Pure Chemical Co. (Korea) and used as the model contaminant of water in all reaction experiments.

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Hydrogen peroxide (H2O2) solution 30% (w/w) was obtained from Merck Co. (Germany). Deionized water was used for the preparation of all experimental solutions. 2.2

Experimental set up

A schematic diagram of the impinging jet atomization reactor used in this study is exhibited in Fig. 2. The setup, consisting of a cubical box made of Plexiglass (to prevent the harmful effects of UV),

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was equipped with 6 low-pressure mercury vapor lamps (TUV 16W T5 from Philips Co.) emitting UV-C radiation at 253.7nm as the irradiation source. A like-on-like doublet aluminum impinging jet nozzles with three different diameters (4, 6 and 8 mm) and same length (50 mm) were used. The two nozzles were mounted on an impingement platform made of acrylonitrile styrene acrylate (ASA) filament. This structure (which was made by a 3d printer) allowed to produce three different angle setups (60, 90 and 120), and also adjust the symmetric position between the two

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nozzles. The feed solution was pumped by a centrifugal magnetic drive pump and the outlet flow

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from the pump was divided into two streams with one flowing into the flow meter and then entering the nozzles, and the remaining flow was returned to the feed reservoir. The solution temperature

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was controlled by an automatic temperature controller (SU-105 Controller from Samwon Eng.

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Co.). A total of 6 L of liquid was consumed in each run. To start the process, the MO solution (C0 = 50 mg L-1) was pumped through the nozzles, and irradiated with UV after passing through the

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reaction area and returned back to the reactor via gravity. Each run took 2 hours, and samples were taken from the reactor and analyzed every 15 minutes. To observe the

flow patterns and the

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behavior of the liquid sheet, a digital single lens reflex camera (D7200 Nikon) with a zoom lens was used (105 mm F2.8 DG Macro OS SIGMA). Some preliminary experiments were performed without any jet flow to determine the impact of UV radiation on solution in the reservoir. The results indicated that UV radiation had very subtle effect (1-3 % of MO removal within 120 min)

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on the degradation of MO molecules in the reservoir.

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Fig. 2. Schematic diagram of the impinging jet atomization photochemical reactor: (1) pump, (2) valve, (3) flow meter, (4) temperature sensor, (5) nozzles and impingement platform, (6) UV lamps, (7) remaining return flow, (8) feed reservoir.

Reaction kinetics

The H2O2 photolysis was studied extensively to describe the reaction mechanism [27]. It is well known that UV irradiation of H2O2 in an aqueous solution produces hydroxyl radicals (Eq. (1)): 𝐻2 𝑂2 + ℎ𝑣 (253.7𝑛𝑚) → 2𝑂𝐻∙

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(1)

The hydroxyl radicals (𝑂𝐻 ∙ ) could be extremely unstable and reactive due to potential high oxidation. Therefore, 𝑂𝐻 ∙ and dye species could interact rapidly. A similar reaction rate was observed for most of the dyes, according to the following reaction (Eq. (2)): 𝑑𝑦𝑒(𝑎𝑞 ) + 𝑂𝐻 ∙ (𝑎𝑞) → 𝑖𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒𝑠

(2) 8

Based on prior studies [9,28] the kinetics of dye wastewater decolorization by UV/ H2O2 process was observed and simplified as follows: −(

𝑑𝐶𝑀 𝑑𝑡

) = 𝑘1 𝐶𝑀 𝐶𝑂𝐻 ∙

(3)

Where 𝐶𝑀 is the MO concentration at time t (mg L-1), t is the reaction time (min), 𝑘1 is the secondorder rate constant (L mg -1 min-1) characterizing the reaction of the hydroxyl radical with MO, and

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𝐶𝑂𝐻 ∙ denotes the hydroxyl radical concentration.

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Since 𝑂𝐻∙ is highly reactive and non-accumulating in the medium, the local pseudo-steady state

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approximation was assumed and its concentration was considered constant. Then, the rate expression in Eq. (3) can be simplified into a pseudo-first order kinetic model as follows: 𝑑𝐶𝑀 𝑑𝑡

) = 𝑘𝑜𝑏𝑠 𝐶𝑀

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𝑟 = −(

(4)

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where 𝑘𝑜𝑏𝑠 is the rate constant (min-1) of the pseudo-first-order kinetic model.

Analytical method

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The absorbance of MO in all samples was determined by ultraviolet spectrophotometer (using Hach DR/2010 spectrophotometer) at a wavelength of 463 nm. A calibration curve was obtained using the standard MO solution with a series of known concentrations based on Beer-Lambert's law. The MO degradation was obtained according to Eq. (5): 𝐷𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 = (𝐶𝑀.0 − 𝐶𝑀 ⁄𝐶𝑀.0 ) ∗ 100

(5) 9

where 𝐶𝑀.0 is the initial MO concentration, and 𝐶𝑀 is the MO concentration at time t. 2.5

Experimental design and response surface methodology

The degradation of dye pollutants by UV/H2O2 process in the present work was influenced by various parameters including initial MO concentration, flow rate (Re), impingement angle, H2O2 dosage, UV irradiance, and nozzle diameter. Among all these parameters, three significant factors

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namely flow rate (Re number), impingement angle and H2O2 dosage were selected for their impact on impinging jet atomization and UV/H2O2 process. A series of preliminary experiments were

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carried out to determine the ranges of chosen parameters for the experimental design. The

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statistical design of experiments based on RSM requires fewer experimental runs, and it can also take into account the interactions among the process variables. An improved central composite

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experimental design (CCD) known as the Box–Behnken design (BBD) which is considered as one of the most successful factorial design for a three-factor and three level experiments, was used for

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the optimization of all the variables [29,30]. The variables coded at three levels (-1, 0, +1), and the

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experimental ranges denoted as R, A and D are summarized in Table 2.

Table 2 Experimental ranges and coded levels of independent variables.

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Factor

Unit

Symbol

Coded levels Low(-1)

Center(0)

High(+1)

Re number

Dimensionless

R

15000

23000

31000

Impingement angle

Degree

A

60

90

120

H2O2 dosage

mg L-1

D

1000

2000

3000

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A nonlinear quadratic model was used for the theoretical prediction of process responses and determination of optimum operating factors: 𝑌 = 𝛽0 + 𝛽1 𝑅 + 𝛽2 𝐴 + 𝛽3 𝐷 + 𝛽12 𝑅𝐴 + 𝛽13 𝑅𝐷 + 𝛽23 𝐴𝐷 + 𝛽11 𝑅2 + 𝛽22 𝐴2 + 𝛽33 𝐷2

(6)

where y is the predicted response (MO removal (%)), 𝛽0 , 𝛽𝑖 , 𝛽𝑖𝑖 and 𝛽𝑖𝑗 are the model constant, linear coefficient, quadratic coefficient and cross-factor interaction coefficient for the fitted

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quadratic model, respectively. The quality of fit of this model was evaluated using the coefficient

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3. Results and discussion

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out by applying the analysis of variance (ANOVA) [31].

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of determination R-squared (R2) and the analysis of variance. The statistical analysis was carried

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3.1 Model fitting and analysis of variance (ANOVA) The Box-Behnken design (BBD) was applied to analyze the interactive effects of three variables

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on MO removal percentage as a response function. A total of 15 experiments each spanning 90 min of reaction time are shown in Table 3. The center point (0, 0, 0) was repeated three times and the almost identically obtained results denote the reproducibility of the data. The analysis of variance (ANOVA) was applied to determine the goodness of fit of the important process

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parameters influencing MO degradation, and the statistical results are presented in Table 4. Based on the ANOVA and experimental results; an approximate regression model representing MO removal percentage was evaluated and expressed in terms of the coded factors by the following second-order polynomial equation (Eq. (7)): 𝑌 = 78.90 + 6.29𝑅 + 3.89𝐴 + 4.82𝐷 + 3.21𝑅𝐴 − 0.041𝑅𝐷 − 0.61𝐴𝐷 − 3.25𝑅2 + 4.51𝐴2 − 3.47𝐷 2

(7) 11

Table 3 The Box–Behnken design matrix along with Observed and predicted values of responses. RUN

RRe number

AImpingement angle

DH2O2 dosage

Y (%) Y (%) (Observed) (predicted)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

23000 31000 23000 15000 23000 23000 15000 23000 31000 15000 15000 31000 23000 31000 23000

90 90 90 120 120 60 90 60 120 60 90 90 120 60 90

2000 3000 2000 2000 1000 3000 1000 1000 2000 2000 3000 1000 3000 2000 2000

79.76 83.59 77.41 74.80 79.71 81.38 60.67 70.99 93.59 73.14 70.87 73.55 87.65 79.09 79.51

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79.76 83.59 77.41 74.80 79.71 81.38 60.67 70.99 93.59 73.14 70.87 73.55 87.65 79.09 79.51

Table 4 ANOVA results of the quadratic model for prediction of MO removal percentage (Y%). Sum of Squares

DF

Mean Square 93.02 316.80 121.16 185.90 41.21 6.8E-003 1.50 38.99 75.09 44.56 0.81 0.24 1.67

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Source

F-Value

p-value Prob>F < 0.0001 < 0.0001 < 0.0001 < 0.0001 0.0008 0.9306 0.2316 0.0010 0.0002 0.0007

Remark

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Model 837.15 9 114.85 significant R 316.80 1 391.18 significant A 121.16 1 149.61 significant D 185.90 1 229.55 significant RA 41.21 1 50.89 significant RD 6.80E-003 1 8.39E-003 not significant AD 1.50 1 1.85 not significant 2 R 38.99 1 48.15 significant A2 75.09 1 92.73 significant 2 D 44.56 1 55.02 significant Residual 4.05 5 Lack of Fit 0.72 3 0.14 0.9256 not significant Pure Error 3.33 2 Corrected total 841.20 14 2 𝑅2 = 0.9952, 𝑅𝑎𝑑𝑗 = 0.9865, predicted R2 = 0.9775, CV%= 1.16, Adequate precision= 44.275. Based on the ANOVA analysis, the resulting F-value of 114.85 demonstrated that the model representing MO degradation is significant, and the signal to noise ratio of 44.275 (>4.0) indicates 12

adequate precision. The p-value was utilized to concurrently check the importance of each variable and identify the effect of each factor on the response. The model terms with P-values less than 0.05 were considered to have significant effects on the response, whereas the terms with values greater than 0.1 were considered insignificant [32]. In this case, the variables of the model, including R, A, D, RA, R2, A2, and D2 are identified as significant model terms. Based on the ANOVA results, the significance ranking sequence of selected parameters is (from the most to the

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least significant): Re number > H2O2 dosage > Impingement angle. The high values of correlation

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2 coefficient 𝑅2 (0.9952) as well as the adjusted regression coefficient 𝑅𝑎𝑑𝑗 (0.9865), ensure the

satisfactory adjustment of the quadratic model to the experimental data. Also, the closeness of 𝑅2

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2 and 𝑅𝑎𝑑𝑗 values to one another illustrates, that the predicted MO removal percentage obtained

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from the model matches very well with the experimental values. This can also be evidenced by plotting the predicted values versus actual values as shown in Fig. 3. The coefficient of variation

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(CV) is the ratio of the standard deviation to the mean-value of the observed response (expressed as the percentage). As a general rule, if the CV of a model is not greater than 10%, the model has

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good dependability and reproducibility. Thus, the CV value of 1.16% demonstrates a high

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reliability of the carried out experiments [33].

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Fig. 3. The plot of the actual results versus predicted values of MO removal (%).

The individual effect of model parameters

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3.2.1 Flow rate (Reynolds number)

The model parameters and their P-values displayed in Table 4 indicate that among the test

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variables, Re value has the most impact on MO removal efficiency. To determine the effect of each factor and the interactions between any two factors, a set of two-dimensional (2D) contour and three-dimensional (3D) response surface plots are produced as shown in Fig. 5, by using

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Design Expert 7.0.0 software. Fig. 5 a & b exhibit 2D and 3D plots demonstrating the effect of Re number on MO degradation ratio. As can be seen, increasing the Re number from 15000 to 31000 (circulating flow rate from 3.8 to 7.8 L/min) increases MO degradation ratio. Some possible reasons for this observation are; initially, according to Fig. 4, it can be observed that by increasing the flow rate (Re number), the impingement sheet size of two water jets will increase (longer vertical sheet length (L)) [20,34]. Considering the relationship of (𝑃0 = 𝐼0 𝐴) at a constant 𝐼0 , the 14

irradiation power will increase by increasing the surface, resulting in higher irradiation at H 2O2 molecules, increasing the hydroxyl radical production and MO degradation. Additionally, the sheet becomes thinner by increasing Re number [34]. According to the molar absorption coefficient of MO solution at 254 nm (ε=7135 M-1 cm-1), and based on beer-lambert law, any decrease of the solution layer thickness would increase the average delivered UV fluence and consequently the degradation reaction rate would positively affect the overall degradation. Finally, another possible

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reason is that increasing Re number (flow rate) leads to an increase in the reaction cycle number

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and therefore, faster return of unreacted molecules from the feed reservoir.

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Fig. 4. Single-flash images demonstrating variations in size, shape and breakup behavior of the liquid sheet formed by the collision of two impinging jets with the water flow rate 𝑣̇ , and with the angle 2𝜃 formed by the two nozzle axes and with diameter size (D = 6 mm) in all the photos.

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3.2.2 Angle

In order to investigate the variations of the jet-impingement behavior against the liquid flow rate and impingement angle, single-flash pictures of the impingement of the two water jets were taken as presented in Fig. 4. It is obvious that the sheet sizes vary substantially with any variation of the impingement angles. Fig. 5 a & c illustrates that MO degradation ratio increases by increasing the

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impingement angles from 60 to 120 degree. As the impinging angle increases, the sheet size will become larger with the width (w) experiencing the larger size increase than the length (L) [35] Another reason is that by increasing the impingement angle, the sheet thickness decreases distinctly, and process efficiency increases based on the already above-mentioned reasons [34]. Also, at lower Re numbers, the MO degradation ratio is bigger at 60° angle than that at 90° angle.

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As a result, the sheet length (L) at 60° angle is longer.

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of ro -p re lP ur na Jo Fig. 5. Interaction effects of different parameters on MO removal percentage displayed by 3D response surface and 2D contours: (a) influent Re number and Impingement angle, (b) influent Re number and H2O2 dosage, (c) influent Impingement angle and H2O2 dosage. 18

3.2.3 H2O2 dosage The initial H2O2 concentration is a significant parameter in the UV/ H2O2 process for contaminant degradation. Fig. 5 b & c prove that the degradation ratio clearly increases by increasing initial H2O2 concentration. This is because a bigger dosage of H2O2 generates more hydroxyl radicals, according to Eq. (1), which in turn causes more dye discoloration. Based on Fig. 6, it was found that there is an optimum [H2 𝑂2 ]0 at 2500 mg L-1, which leads to the highest dye degradation ratio.

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This optimum [H2 𝑂2 ]0 corresponds to a mass ratio of around 50, described by [H2 O2 ]0 ⁄ [MO]0.

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Haji et al. [36] reported that the optimum mass ratio of [H2 𝑂2 ]0 ⁄ [dye]0 for MO degradation by UV/ H2O2 process is 57.7 . Also, experiments by Aleboye et al. [37] obtained the optimum

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ratio of 65 for [H2 O2 ]0 ⁄ [MO]0 . Other publications have also pointed out that an optimum

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[H2 O2 ]0 has resulted in the highest rate of decolorization [9,38,39]

Increasing the initial H2O2 concentration above the optimum mass ratio may be problematic in

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some aspects. First, excess hydrogen peroxide creates rival reactions leading to a decrease in the degradation rate due to the scavenging effect of hydroxyl radicals with H2O2. Hydroxyl radicals

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are prone to react or to recombine as indicated in the following reactions [40,41]: 𝑘2

𝑂𝐻 ∙ + 𝐻2 𝑂2 → 𝐻2 𝑂 + 𝐻𝑂2∙

(𝑘2 = 2.7 ∗ 107 𝐿/𝑚𝑜𝑙 𝑠 )

(8)

𝐻𝑂2∙ + 𝑂𝐻 ∙ → 𝐻2 𝑂 + 𝑂2

(𝑘3 = 6.6 ∗ 109 𝐿/𝑚𝑜𝑙 𝑠)

(9)

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𝑘3

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Fig. 6. The individual effect of influent H2O2 concentration on percent Mo degradation Second, by increasing the initial H2O2 concentration, the H2O2 residual will also increase. The

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H2O2 residuals are known to be toxic to the microorganisms in the post-treatment processes that include biological environments. Therefore, one should consider never going beyond the

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optimum initial H2O2 concentration where the MO removal percentage is maximum, while the drawbacks of having hydrogen peroxide in the system is minimized [42]. It should be noted that the degradation of MO due only to UV photolysis (with no H2O2) was around 10 % after 90 min

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at the same experimental condition.

3.3

Interactive effects of process variables

3D surfaces and 2D contour plots in Fig. 5 illustrate the cross-factor interaction effects between independent variables on the MO removal percentage as the response function. These plots demonstrate the regression analysis, where the response functions due to two variables are 20

displayed while the other parameters are fixed at the center levels. Based on the interaction of model parameters and their P-values in Table 4, the interaction model parameters (RD and AD) did not show any significant effect on the percent MO removal. It is obvious that the impingement angle and Re number do not have any interactive effect with H2O2 concentration. As demonstrated in Fig. 4, the liquid sheet formed by two impinging jets is characterized by several primary parameters such as Reynolds number and impingement angle. Thereby, the interaction between

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Re number and impingement angle (RA) signified by the low P-value (0.0008) represents a

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significant effect on MO removal. All quadratic model parameters (R2, A2, and D2) show significant effects on MO removal percentage, and the quadratic effect of the impingement angle

Optimization and validation of results

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(A2) is the most important one.

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Optimizing the variables for maximum MO removal was carried out by Design Expert optimization tool. The predicted optimum values of the process variables were calculated by a numerical technique providing the following results: Re number 30073 (~31000), Impingement

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angle 119.3 degree (~120) and H2O2 dosage 2506 mg L-1 (~2500). The response values for the predicted results by Design Expert 7.0.0 and the experimental results were 94.0 and 93.5%, respectively. Some complementary experiments under other predicted optimal conditions have

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been performed for further verification of the presented model. The slight difference between experimental and predicted values (less than 5%) indicates that the method used for optimizing the decolorization conditions and obtaining the maximum MO decolorization efficiency was a successful one that has delivered satisfactory results. 3.5

Effects of other relevant parameters at optimum condition

21

3.5.1 Effect of initial dye concentration The influence of initial dye concentration (C0) on MO degradation is presented in Fig. 7 with the rest of parameters kept unchanged (Re number of 31000, Impingement angle of 120 °, H2O2 dosage of 2500 mg L-1 and nozzle diameter of 6mm). It can be seen that the percent removal of MO decreased by increasing the initial MO concentration. As the initial dye concentration

of

increases, the rate of degradation decreases. This is due to the limited availability of sufficient number of hydroxyl radicals and also due to the poor penetration of UV into the intense colored

ro

solution[43,44]. Additionally, at higher MO concentrations, increasing the intermediate products

ur na

lP

re

-p

formed could lead to the scavenging of free radicals during the process [45].

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Fig. 7. Effect of initial dye concentration on MO decolorization; Re number = 31000, Impingement angle = 120 °, H2O2 dosage = 2500 mg L-1, number of UV lamps = 6. and nozzle diameter = 6mm. 3.5.2

Effect of UV radiation power

Fig. 8 demonstrates the individual effect of UV radiation power on MO removal at optimum conditions of the rector (Re number = 31000, Impingement angle = 120 °, H2O2 dosage = 2500 22

mg L-1 and nozzle diameter = 6mm). The UV power was changed by varying the number of lamps. Two, 4 and 6 lamps were positioned in the middle and on both sides of the thin film. It clearly illustrates that the initial dye degradation rate increases linearly with increasing the UV irradiation power. By increasing the number of lamps and UV irradiation power, the H2O2 photolysis (according to Eq. (1)) is enhanced to produce more 𝑂𝐻 ∙ in the dye solution, which will

of

subsequently increase the decolorization rate [38]. Fig. 8 also make it clear that, in the absence of UV radiation, H2O2 cannot remove the color of the dye pollutant all by itself. It means that, there

ro

is no efficient decolorization of MO in the absence of UV radiation by just oxidizing H 2O2 [46]. In addition to the overall UV fluence rate, which was increased by adding more lamps, the radiation

-p

field in the reactor may have changed by changing the position of the lamps. In order to check the

re

effects of lamps position and consequently the radiation field on the degradation efficiencies, a series of complementary experiments with different lamps arrangements (according to Fig. 2) were

lP

performed, in case of 4 lamps. As it can be seen from Fig. 8, the % degradation was affected to a limited extent, and it increased slightly as the lamp positioning was such that it allowed irradiation

ur na

of the wider area of the liquid sheet. For example, the one configuration that covered the entire

Jo

liquid sheet area (1F, 2B, 2F, 3B) led to a slightly higher degradation ratio.

23

of ro -p

re

Fig. 8. Effect of UV radiation power (number of lamps) and lamp positioning on MO

lP

decolorization; Re number = 31000, Impingement angle = 120 °, H2O2 dosage = 2500 mg L-1, initial MO concentration = 50 mg L -1and nozzle diameter = 6mm.

ur na

3.5.3 Effect of jet diameter

The impact of jet diameter on MO degradation is depicted in Fig. 10 with the rest of the parameters kept unchanged (Re number of 31000, Impingement angle of 120 °, HO2 dosage of 2500 mg L1

and nozzle diameter of 6mm). To probe into a possible scale-up criterion, three different diameter

Jo

jet nozzles (4, 6 and 8 mm) were used, while the rest of the parameters remained the same. As can be seen, the jet diameter does not have any impact on MO degradation, since the two dimensionless parameters (the Weber and Reynolds numbers) and the impingement angle, which characterizes the liquid sheet formed by the two impinging jets, are constant [47]. Fig. 9 also demonstrates that all the sheet sizes for different nozzle diameters are very similar.

24

of ro

Jo

ur na

lP

re

-p

Fig. 9. Effect of jet diameter on the liquid sheet formed by two impinging jets; Re number = 31000, Impingement angle = 120 °. (Scale bar, 100 mm).

Fig. 10. Effect of jet diameter on MO decolorization; Re number = 31000, Impingement angle = 120 °, initial MO concentration = 50 mg L-1, H2O2 dosage = 2500 mg L-1 and number of UV lamps = 6.

3.6

Degradation kinetics model

25

The kinetic experiments were conducted based on the Design Expert results with an initial MO concentration in the range of 30-70 mg L-1, and the Re number, impingement angle and H2O2 dosage set at their optimum values. The photochemical dye degradation rate by UV/H2O2 process was investigated by a pseudo-first-order kinetic model [48] as expressed in Eq. (4). Based on the ability of the two impinging jets to facilitate a far more vigorous mixing of the

of

reactants, it could be assumed that the impingement zone acts as a perfect mixing reactor [49]. Fig. 11 shows that the experimental setup is considered a batch recycle system consisting of a feed

ro

reservoir and a perfect mixing reactor. The evaluation of MO concentration and degradation by

-p

the impinging jet photo-reactor, replaced with the perfect mixing reactor, is described by the following mass balance expressions:

re

𝑜𝑢𝑡 𝑖𝑛 𝐶𝑀.𝑅 = 𝐶𝑀.𝑅 − 𝑡̅𝑘𝑜𝑏𝑠 𝐶𝑀

(10)

lP

𝑖𝑛 𝑜𝑢𝑡 where, 𝐶𝑀.𝑅 and 𝐶𝑀.𝑅 are MO concentrations in the inlet and outlet of the reactor, respectively.

Since this reactor is assumed to be a perfectly mixed CSTR, the output concentration of the reactor

ur na

𝑜𝑢𝑡 is equal to the one within the reactor (𝐶𝑀.𝑅 = 𝐶𝑀 ) [50].

A balance for MO around the feed reservoir in Fig. 11 can be set up as follows: 𝑜𝑢𝑡 𝑖𝑛 𝑄𝐶𝑀.𝑅 − 𝑄𝐶𝑀.𝑅 = 𝑉𝑇 (

𝑑𝐶𝑀.𝑇 𝑑𝑡

)

(11)

Jo

where 𝑄 is the volumetric flow rate, 𝑉𝑇 is the volume of MO solution in the feed reservoir and 𝐶𝑀.𝑇 is MO concentration in feed reservoir. The finite difference equation form of the above relation can be considered as: 𝑜𝑢𝑡 𝑖𝑛 𝑄(𝐶𝑀.𝑅 − 𝐶𝑀.𝑅 ) = 𝑉𝑇 (

𝑖+1 𝑖 𝐶𝑀.𝑇 −𝐶𝑀.𝑇 ) 𝑡̅

(12)

26

In the above relation, 𝑖 is the cycle number counter and 𝑡̅ is the mean residence time= 𝑡⁄𝑡.̅ The degradation ratio calculated from this combined model can be defined as: 𝐶 𝑖+1

𝑋𝑚𝑜𝑑 = 1 − 𝐶𝑀.𝑇 0

(13)

𝑀.𝑇

To determine the kinetic coefficients, the predicted degradation ratios calculated by Eq. (14), were

of

compared with the experimental values and optimized by the sum of the square errors (SSE) method as shown below:

ro

𝑆𝑆𝐸 = ∑𝑖 𝐸𝑟𝑟𝑖 2 = ∑𝑖 (𝑋𝑒𝑥𝑝 − 𝑋𝑚𝑜𝑑 )2

(14)

-p

where 𝑋𝑒𝑥𝑝 is the experimental MO degradation ratio and 𝑋𝑚𝑜𝑑 is the predicted MO degradation

re

ratio.

Table 5 indicates the experimental and predicted degradation ratios and estimated rate coefficients

lP

for different MO initial concentrations. According to this table, the SSE values for the experimental and predicted values based on the pseudo-first-order kinetic model are low. This

results.

ur na

means that there is a satisfactory agreement between the model predictions and experimental

Jo

Table 5 Kinetic coefficients and degradation ratios for different MO initial concentrations in the pseudo-first-order kinetic model.

Time(min) 0 15 30 45 60 75 90

𝐶0 = 25 𝑚𝑔/L

𝐶0 = 50 𝑚𝑔/L

𝐶0 = 75 𝑚𝑔/L

𝑋𝑒𝑥𝑝 0 0.352 0.584 0.770 0.905 0.983 1

𝑋𝑒𝑥𝑝 0 0.225 0.393 0.567 0.714 0.834 0.926

𝑋𝑒𝑥𝑝 0 0.154 0.301 0.460 0.595 0.723 0.831

𝑋𝑚𝑜𝑑 0 0.393 0.632 0.776 0.864 0.918 0.950

27

𝑋𝑚𝑜𝑑 0 0.264 0.459 0.602 0.707 0.785 0.842

𝑋𝑚𝑜𝑑 0 0.205 0.368 0.497 0.600 0.682 0.748

3.9644

2.4384

1.8208

SSE

0.00206

0.002795

0.002842

re

-p

ro

of

𝑘𝑜𝑏𝑠 (1/min)

ur na

3. Conclusion

lP

Fig. 11. Mass balances over the reactor set-up for its kinetic modeling.

In this study, an impinging jet atomization photo-reactor is designed and constructed, and its performance capability as a new type of UV/H2O2 reactor is investigated. A simple configuration based on dimensionless parameter Re and impingement angle provided a basis that eliminated the

Jo

need to use different mixing devices, and it could be a model for an industrial size version application. The reactor’s special design ensured an efficient operation and high degree of mixing, which led to a complete degradation of a batch of 50 mg L−1 of MO solution in about 120 min in recirculation mode. The MO degradation was optimized by applying the BBD to fit the RSM of statistical experimental design, reduce the number of required experiments, and analyze the interaction effects among different parameters. The ANOVA results show that the photochemical 28

MO decomposition increases in the examined range by increasing all three major parameters (Re number, impingement angle and H2O2 dosage), and that the Re number has the highest impact of the three. The effects of other parameters such as initial dye concentration, UV radiation power, and jet diameter were evaluated, and the results for different jet diameters demonstrated that this parameter does not have a significant impact on the efficiency of the proposed photo-reactor and its scale-up capability. The kinetic coefficients were predicted by minimizing the sum of the square

of

errors (SSE) of the experimental results, and those predicted by the model at various MO initial

ro

concentrations. A simple configuration based on dimensionless parameter Re and impingement angle provided a basis that eliminated the need to use different mixing devices, and it could be a

-p

model for an industrial size version application. The reactor’s special design ensured an efficient

re

operation and high degree of mixing, which led to a complete degradation of a batch of 50 mg L−1 of MO solution in about 120 min in recirculation mode. The proposed reactor can be utilized as a

lP

rapid, large-scale and a continuous device for water treatment purposes with lower contaminant

ur na

concentrations. In case of higher flow rates, it can also be used as a cascade of parallel jets.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Jo

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Acknowledgments The authors would like to thank Eng. Saeed Salamat Gharamaleki for his helpful comments on the manuscript. 29

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