Degradation of refractory pollutants by hydrodynamic cavitation: Key parameters to degradation rates ＊

Zhi-yong Dong, Kai Zhang, Rui-hao Yao

College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310023, China

Abstract: Degradations of refractory pollutants by hydrodynamic cavitation were experimentally carried out in a self-developed hydrodynamic cavitation reactor with Venturi tubes, multi-orifice plates and their combinations. Effects of hydraulic elements of cavitation due to the Venturi tube on degradation of refractory hydrophobic pollutant were studied, and an optimal throat length corresponding to the maximum degradation rate was obtained. Effects of cavitation due to number, size and distribution of orifice for the multi-orifice plates on degradation of refractory hydrophilic pollutant were investigated, and comparisons and analyses were made. Effects of cavitation due to different combinations of the Venturi tubes with the triangular multi-orifice plates on degradation of hydrophilic and hydrophobic mixtures were studied, and an appropriate combination was determined. Also, effects of cavitation duration, pH value and initial concentration on the refractory pollutants were explored.

Key words: Hydrodynamic cavitation, Venturi tube, multi-orifice plate, refractory pollutant, combination effect

Introduction

With the rapid development of economy,contaminants which have been detected in wastewater due to petrochemical, pesticide and dyeing industries are more and more complex, and the conventional wastewater treatment technique cannot meet the degradation requirement of industrial wastewater such as phenols, aromatics and heterocyclics[1-2]. Hydrodynamic cavitation technology, as a new treatment process, not only can effectively degrade biological refractory and toxic matters, but also be of many other advantages including no secondary pollution, simple operation device and easy of large-scale implementation[3]. Based on the method of cavitation generation,it can be generally divided into four types: ultrasonic cavitation, hydrodynamic cavitation, optic cavitation and particle cavitation. Among them, ultrasonic cavitation and hydrodynamic cavitation seem to offer the greatest potential. Some progress has already been achieved using ultrasonic cavitation for the degradation of organic pollutants[4-5]. However, there is a problem with this technique due to the ineffective distribution of the cavitational activity when used on a larger scale and the inefficient operation of the transducers at levels of higher power dissipation/higher frequency. In contrast, hydrodynamic cavitation has been reported to be more energy efficient than ultrasonic cavitation, and which would be a significant advantage for industrial applications[6-7].

Hydrodynamic cavitation can be produced by pressure variations in a flowing liquid, effected by a change in flow area such as Venturi tubes, multiorifice plates and cylindrical protrusion[8-10]. When the pressure of the liquid lowered below its vapour pressure under current temperature, formation, growth and implosive collapse of cavitation bubbles occur[11-13]. The collapse of bubbles can generate super high temperatures and pressures, and forms shock waves and microjets over a microsecond interval. Under such an extreme condition, water molecules can be dissociated into OH*, H*and OOH*radicals, and hydroxyl OH*combines mutually to produce hydrogen peroxide H2O2, which are powerful oxidants[14], and diffuse into the bulk liquid where they react with pollutants and oxidize them[15]. Pandit and Joshi[16]earlier studied the hydrolysis of fatty oils,and reported that hydrodynamic cavitation was indeed much more energy efficient compared with traditional treatment methods. Kumar et al.[17]experimentally investigated the degradation of rhodamine B samples using six different geometric configurations of circular multi-orifice plates. It was found that hydrodynamic cavitation was indeed much more energy efficient compared with acoustic cavitation,and it could be applied successfully in degrading aromatic amines. Vichare et al.[18]conducted experiments on the decomposition of aqueous KI by hydrodynamic cavitation, and found that by optimizing the geometry of the cavitation device, the decomposition rate of KI would increase 3-5 times.Dong and co-authors[19-21]experimentally studied degradation of mixture wastewater with nitrobenzene andp-nitrophenol (PNP) by using various combinations of Venturi tube with different sizes and numbers of circular, square and triangular multi-orifice plates,and found that the size and number of orifice and initial concentration of wastewater considerably affected degradation rates; An application of hydrodynamic cavitation induced by triangular multi-orifice plates in treatment of chemical industry wastewater showed that the treatment time, orifice number, orifice size and pH value directly affected the extent of COD degradation, and there existed an optimum initial concentration for degradation of each multi-orifice plate; Cavitational characteristics due to circular multi-orifice plates suggested parameters of multi- orifice plates significantly influenced cavitation number and thus affecting the cavitation intensity based on the degradation experiments ofp-nitrophenol.However, components of actual wastewater are complex and there are many factors influencing the effective use of hydrodynamic cavitation. Further experimental work needs to be carried out for the application of hydrodynamic cavitation technique in actual wastewater treatment[22-23].

Under the same conditions, cavitation bubbles generated by multi-orifice plates are smaller, and the time of their collapse is shorter, which can promote hydroxyl to enter into liquid phase as early as possible and react with hydrophilic pollutants. The Venturi tube with the longer throat can extend the duration time of the lowest pressure of the flow and the expansion period of cavitation bubble, which can keep longer growing time of cavitation bubble and contribute to hydrophobic pollutants for entering into bubble to be pyrolyzed with the bubble collapse. This paper presents experimental studies of the effect of hydrodynamic cavitation on degradation of refractory hydrophilic, hydrophobic pollutants and their mixture.Firstly, the Venturi-type hydrodynamic cavitation reactor was utilized to investigate parameters affecting degradation of the refractory hydrophobic pollutants.Secondly, the triangular multi-orifice plates were selected as another the hydrodynamic cavitation reactor, and effects of different orifice velocity, orifice number, orifice size, initial concentration, cavitation number, and treatment time on degradation of the refractory hydrophilic pollutants were measured,Finally, the combined hydrodynamic cavitation reactor of Venturi tubes with triangular multi-orifice plates were used to study effects of different combination forms, flow velocities, initial concentrations,and treatment time on degradation of the hydrophilic and hydrophobic mixtures.

1. Experimental facility and measuring method

1.1Experimental apparatus

The experimental setup is shown in Fig. 1.

Fig. 1 Sketch of experimental setup

1.2 Hydrodynamic cavitation reactor

1.2.1 The Venturi tube

Three types of the Venturi tubes based on different throat lengths (L/R, whereLis throat length,Ris hydraulic radius) were designed for the experiment:L/R=20, 40 and 60. The other parts have a standard size: the inlet of contraction section and the outlet of diffusion section were both of 50 mm×50 mm square cross-section, the throat section was 20 mm×20 mm square one, the lengths of the contraction and diffusion sections were both 100 mm, as shown in Fig. 2.

Fig. 2 The Venturi tube 3 (L/ R=60)

1.2.2 The triangular multi-orifice plates

Four types of triangular multi-orifice plates were designed for the experiment. The size of each plate is 50 mm×50 mm. The orifice was a regular triangle with an area equal to that of circle in diameter(d=3 mm ,d=5 mm ). The checkerboard-type and staggered layouts of orifices were arranged on the plates as shown in Fig. 3. The parameters of the multiorifice plates were shown in Table 1.

Fig. 3 Triangular multi-orifice plates

Table 1 Geometric characteristics of multi-orifice plates

1.2.3 The combination of Venturi tube with triangular multi-orifice plate

The combined setup of Venturi tube with triangular multi-orifice plate is shown in Fig. 4, the flow direction was from Venturi tube to multi-orifice plate.

Fig. 4 Combination of Venturi tube with multi-orifice plate

1.3 Experimental methodology

The experimental design is shown in Table 2.

A nitrobenzene was chosen as the representative of refractory hydrophobic pollutant, and varying concentrations of nitrobenzene, 10 mg/L, 25 mg/L, 50 mg/L,75 mg/L and 100 mg/L, were compounded in the tank.The test samples were taken each 15 min and analyzed by the 760CRT UV-spectrophotometer.

A PNP was chosen as the representative of refractory hydrophilic pollutant. The varying concentrations of PNP and the experimental design was similar to the above case.

In the experiment, COD concentration was used as a detection index. PNP and nitrobenzene mixture was compounded in the tank. Varying concentrations of the mixtures and the experimental design was also similar to the above case.

1.4 Degradation rate

The degradation rate of pollutants can be defined as

wherey0is the initial concentration of pollutants,yithe concentration of each sample.

Fig. 5 Effect of throat velocity on degradation of nitrobenzene

Table 2 Experimental methods and parameters

2. Results and discussions

2.1 Degradation of refractory hydrophobic pollutant by hydrodynamic cavitation due to the Venturi tube

2.1.1 Effect of flow velocity at throat

Whilst the other working conditions remained unchanged, the throat flow velocity was changed by using one or two pumps. Figure 5 shows effect of hydrodynamic cavitation due to the Venturi tubes on the degradation of nitrobenzene. It follows that the faster the throat flow velocity is, the higher the degradation rate reaches.

The energy equation at cross-sections 1-1 and 2-2 for the Venturi tube can be written as (see Fig. 6)

and the continuity equation

We can obtain from these two equations that the relation of difference in pressure between crosssections 1-1 and 2-2 as follows

and the relation with the mean velocityv2at crosssection 2-2 exhibits a quadratic curve, that is

with an increase inv1, thenv2increases accordingly,which results in decrease in pressure at the throat and it drops till to negative pressure. Negative pressure can lead to generate more cavitation bubbles and to be helpful to hydrophobic pollutants for entering into bubbles to be pyrolyzed with the bubble collapse.Also, by increasing the flow velocity, numbers of passing through the cavitation working section could be increased. Therefore, the increase in the throat velocity of Venturi tube can be in favor of degradation of hydrophobic pollutants.

Fig. 6 Schematic of the Venturi tube

2.1.2 Effect of treatment time

Variation of nitrobenzene degradation with treatment time is shown in Fig. 7. We can see from the Figure that degradation rate gradually increases with treatment time.

2.1.3 Effect of initial concentration

Effect of initial concentration on degradation rate of nitrobenzene at different throat lengthsL/Ris shown in Fig. 8. The results showed that the degradation rate at first gradually increases with an increase in initial concentrations, and reaches a peak value at 50 mg/L, 75 mg/L and 50 mg/L, respectively, for the different throat lengths, and then gradually decreases with increase in concentration. Obviously, there existed an optimal initial concentration corresponding to each throat length of the Venturi tube.

Fig. 7 Effect of treatment time on degradation rate of nitrobenzene

Fig. 8 Effect of initial concentration on degradation rate of nitrobenzene

2.1.4 Effect of throat length

The throat of the Venturi tube plays an important role in increasing velocity and decreasing pressure.The influence of different throat lengthsL/Ron degradation rate of nitrobenzene is shown in Fig. 9. Of three throat lengthsL/R=20, 40 and 60, throat lengthL/R=40 resulted in the maximum degradation rate.The reason is that the throat length directly affects duration time of negative pressure and expansion period of cavitation bubble in throat section of Venturi tube. And longer throat can help nitrobenzene to enter into bubble for being pyrolyzed with bubble collapse.The experiment suggested an optimum throat lengthL/R=40.

2.1.5 Effect of cavitation number

Cavitation number is essentially a pressure coefficient, which reflects effect of pressure change on flow characteristics. It can be expressed as

wherepdenotes absolute pressure at measuring point,pvsaturated vapour pressure,ρdensity of liquid, andV0velocity of orifice or throat.

Fig. 9 Effect of throat length on degradation rate of nitrobenzene

Variation of cavitation numbers with treatment time is shown in Fig. 10. It can be seen that cavitation number lowers with an increase in treatment time.Lower cavitation number can enhance the degradation rate of nitrobenzene as shown in Fig. 7. It should be noted that cavitation number generated by the throatL/R=40 was the lowest, so the degradation rate reached the highest. It is further illustrated that lower cavitation number can result in higher degradation rate.

Fig. 10 Variation of cavitation numbers with treatment time

2.1.6 Effect of pH value

An initial concentration 10 mg/L of nitrobenzene wastewater was taken for example to analyze effect of pH value on degradation of nitrobenzene as shown in Fig. 11. The raw sample of nitrobenzene wastewater was neutral, however, its pH value increased slightly due to the decrease in nitrobenzene and the formation of intermediate products during the hydrodynamic cavitation reaction process. The effect on degradation rate of nitrobenzene could be neglected.

Fig.11 Variation of pH values with treatment time

2.2 Degradation of refractory hydrophilic pollutant by hydrodynamic cavitation due to triangular multiorifice plate

2.2.1 Effect of orifice velocity

Taking the plate 1 for example, degradation of PNP by hydrodynamic cavitation due to this triangular multi-orifice plate is shown in Fig. 12. Basically, a higher orifice velocity leads to a greater degradation rate. This was likely due to increase in orifice velocity,which caused an abrupt pressure drop downstream of the orifice plate, and made it easier to generate cavitation bubbles. In addition, the high-velocity orifice flow would create an intense turbulence shear stress, which could directly rupture the carbon bonds on the macromolecular main chain, thus degrading the refractory matter.

Fig. 12 Effect of orifice velocity on degradation rate of PNP

2.2.2 Effect of treatment time

Treatment time can reflect the number of cycles through the loop of exposure to cavitation that a fluid particle has undergone. Figure 13 shows the degradation of PNP with the treatment time. The degradation rate increased with time, then it seems to be restrained after a period of running time. This phenomenon may be due to the higher experimental temperature, which may weaken the cavitation intensity.

2.2.3 Effect of initial concentration

The degradation of PNP after running 1 h at different initial concentrations is shown in Fig. 14. It was found that variation of degradation rate with initial concentration of PNP exhibited upward convex curve, the maximum degradation rates were at 25mg/L, 25 mg/L, 50 mg/L and 10 mg/L for the plates 1-4, respectively. Obviously, there existed an optimal initial concentration, when using the multi-orifice plates for the degradation of hydrophilic pollutant.

Fig. 13 Effect of treatment time on degradation rate of PNP

Fig.14 Effect of initial concentration on degradation rate of PNP

2.2.4 Effect of orifice number

The plates 1, 2, which were both of the same orifice sizes, but different orifice numbers, were chosen to investigate effect of orifice number on degradation ofp-nitrophenol as shown in Fig. 15.The degradation rate increased with an increase in the number of orifice based on the same size of orifice. It can be explained that when the wastewater flowed through the same size of orifice, the total perimeter of orifice determined the zone of shear layer and the turbulence intensity, so the more number of orifice led to the greater shear stress and enhanced the intensity of cavitation. Moreover, the spacing among multiple jets became smaller as a result of the more number of orifice, because the smaller spacing, stronger mixing and entrainment among the jets resulted in intense fluctuations of pressure and velocity, thus easily promoting cavitation.

Fig. 15 Effect of orifice number on degradation rate of PNP

2.2.5 Effect of orifice size

The plates 2, 4 were taken as the objects, which were both of the same numbers of orifice, but different sizes. Data in Fig. 16 show that the degradation of PNP increased with an increase in the size of orifice.It could be attributed to the spacing among multiple jets became smaller and the roles of mixing and entrainment were stronger because of the greater size of orifice, which resulted in an increase in turbulence intensity and cavitation events.

Fig. 16 Effect of orifice size on degradation rate of PNP

Fig. 17 Variation of cavitation numbers with treatment time

2.2.6 Effect of cavitation number

Variation of cavitation numbers with treatment time is shown in Fig. 17. It is easy to see that the degradation rate is inversely proportional to the cavitation numb er. The cavitaion numb er was an importantfactorontheformationandgrowthof cavitation bubble, and a decrease in cavitation number should result in more cavitation activity and enhance the degradation of hydrophilic pollutant.

2.3 Degradation of refractory hydrophilic and hydrophobic mixture by hydrodynamic cavitation due to combination of Venturi tube with triangular multiorifice plate

2.3.1 Effect of initial velocity

The combination of the Venturi tube 2 with the triangular multi-orifice plate 3 was taken to study effect of combined hydrodynamic cavitation velocity on degradation of hydrophilic and hydrophobic mixture as shown in Fig. 18. It can be seen from the Figure that degradation rate of the mixture increases with increasing initial velocity.

Fig. 18 Effect of initial velocity on degradation rate of mixture

2.3.2 Effect of treatment time

Variation of degradation rate of the mixture with treatment time is shown in Fig. 19 based on the combination of the Venturi tube 2 with the triangular multi-orifice plate 3. It was found that with the treatment proceeding, a higher concentration of hydroxyl radicals occurred, thus increasing the degradation rates of mixture. However, as time increased, the degradation rates tend to be stable due to higher water temperature.

Fig. 19 Effect of treatment time on degradation rate of mixture

2.3.3 Effect of initial concentration

Degradation rate of the mixture first increases to the maximum value with an increase in initial concentration, and then decreases with further increase in the initial concentration as shown in Fig. 20. There was an optimal initial concentration for each combination form. The reason was that the probability of cavitation effect increased as a result of the higher initial concentration, hence the degradation rate increased continually and till the maximum. However,with an increase in the initial concentration, the concentration of hydroxyl radicals generated by the hydrodynamic cavitation was not enough to degrade the mixture, thereby causing lower degradation rate.

Fig. 20 Effect of initial concentration on degradation rate of mixture

Fig. 21 Effect of combination forms on degradation rate of mixture pollutants

2.3.4 Effect of combination

As mentioned-above, the Venturi tube of different relative throat lengths affected the degradation of hydrophobic pollutant, and the triangular multi-orifice plate with different sizes and numbers of the orifice influenced the degradation of hydrophilic pollutant.To investigate the effects of different combinations,one of combinations composed of Venturi tube 2 and 4 types of triangular multi-orifice plates was investigated, and a mixture concentration of 75 mg/L was chosen. As can been seen in Fig. 21, the degradation rate resulted from the combination of the throat 2 with the plate 3 is maximum compared to the others. It should be noted that for the different combinations of the throats of Venturi tube with the multi-orifice plates, the degradation rate being of the checkerboard-type layout of orifice was more conducive to cavitation. The combination of the throat 2 with the plate 3 generated the highest degradation rate, therefore the combination was of the optimum hydraulic condition of cavitation and led to the best cavitation effect.

3. Conclusions

A systematically experimental study on the effects of the hydrodynamic cavitation due to the Venturi tubes, triangular multi-orifice plates and their combinations on degradation of refractory hydrophilic(PNP) and hydrophobic (nitrobenzene) pollutants, and their mixture were carried out. We can draw some conclusions as follows:

(1) The degradation rate of hydrophobic pollutant increased with increasing throat flow velocity. With the treatment proceeded, the degradation rate increased, as reaching a certain time, the rate tended to be stable due to the temperature. With an increase in the initial concentration, the degradation rate of hydrophobic pollutant increased first, and then decreased,the reason was that there existed an optimal initial concentration corresponding to each relative throat length of the Venturi tube. The throat length was an important parameter for the degradation of hydrophobic pollutant and there existed an optimal throat lengthL/R=40. The lower cavitation number enhanced the degradation of hydrophobic pollutant.

(2) The degradation rate of hydrophilic pollutant by hydrodynamic cavitation due to the triangular multi-orifice plates increased with an increase in orifice velocity. Variation of the degradation rate of hydrophilic pollutant with initial concentration increased first, and then decreased, there existed an optimal initial concentration for each multi-orifice plate. The more the orifice number was, the greater the degradation rate reached based on the same size of orifice.The larger orifice size resulted in greater degradation rates based on the same number of orifices. The lower cavitation number enhanced the degradation of hydrophilic pollutant.

(3) Increase in velocity was in favor of degradation of hydrophilic and hydrophobic mixture based on the different combinations of Venturi tubes with triangular multi-orifice plates. There did exist an optimal initial concentration being of the highest degradation rate for each combination. Different combination resulted in the different degradation of the mixture, especially in the form of combination of the Venturi tubes with the multi-orifice plates arranged on the checkerboard-type layout of orifice,there was a better condition of cavitation. The combination of the throat 2 with the plate 3 was optimal. The experimental results provide a valuable guide to the design and operation of hydrodynamic cavitation for degradation of refractory pollutants.