CFD Modeling of Metronidazole Degradation in Water by the UV

Ind. Eng. Chem. Res. 2010, 49, 5367–5382

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CFD Modeling of Metronidazole Degradation in Water by the UV/H2O2 Process in Single and Multilamp Photoreactors Masroor Mohajerani, Mehrab Mehrvar,* and Farhad Ein-Mozaffari Department of Chemical Engineering, Ryerson UniVersity, 350 Victoria Street, Toronto, Ontario, Canada, M5B 2K3

A dynamic model for the degradation of aqueous metronidazole by hydrogen peroxide and ultraviolet irradiation processes (UV/H2O2) as an advanced oxidation technology (AOT) in a single lamp tubular photoreactor as well as in a multilamp tubular photoreactor is developed. The model contains the main chemical and photochemical reactions in a medium flowing in the turbulent regime. The optimal hydrogen peroxide concentrations of 75, 150, and 200 mg L-1 were predicted for different alkalinity concentrations of 0, 1.5, and 3 µM, respectively. The model is validated with distilled water as well as alkaline water at different alkalinity concentrations. The model is validated by using the experimental data reported in the open literature. The velocity field and concentration profiles for the turbulent flow using the k-ε model are determined by computational fluid dynamics (CFD). 1. Introduction The presence of toxic and recalcitrant compounds in water and wastewater is a topic of global attention. Various conventional treatment methods have their own inabilities to destroy recalcitrant pollutants. Advanced oxidation processes (AOPs) are related to the production of highly oxidizing agents such as hydroxyl radicals, which enhance and accelerate the decolorization, detoxification, degradation, and biodegradability of the toxic, inhibitory, and biorecalcitrant wastewaters.1,2 These technologies could be applied for contaminated groundwater, surface water, and wastewaters containing recalcitrant, inhibitory, and toxic compounds with low biodegradability as well as for the purification and disinfection of drinking water. AOPs are those groups of technologies that lead to the generation of hydroxyl radicals (•OH) as the primary oxidant (second highest powerful oxidant after fluorine). These radicals are produced by means of oxidizing agents such as H2O2, O3, ultraviolet irradiation, ultrasound, and homogeneous or heterogeneous catalysts. Today, most studies focus on finding better methods of •OH production. Hydroxyl radicals are nonselective in nature and react without any other additives with a wide variety of contaminants whose rate constants are usually on the order of 106-109 M-1 s-1.3,4 Hydroxyl radicals attack organic molecules by either abstracting or adding a hydrogen atom to the double bonds. The products of the oxidation are new intermediates with lower molecular weights or carbon dioxide and water in the case of complete mineralization. A full understanding of kinetics and mechanisms of all chemical and photochemical reactions under the operating condition is necessary, by which, based on the well-understood mechanisms, the optimal condition can be obtained. AOPs such as Fenton and photoassisted Fenton processes, radiolysis, and ultrasonolysis are useful to enhance the generation of hydroxyl radicals. Despite the potential of AOPs, the development of a fully industrial-scale wastewater treatment system has not been successfully accomplished yet due to the high capital and operating costs. Among the AOPs, the UV/H2O2 process has shown a better efficiency and facility in operation. Hydrogen peroxide and ultraviolet light are combined in a synergetic effect to mineralize * To whom correspondence should be addressed. E-mail: [email protected].

organic chemicals in water and wastewater. Hydroxyl radicals are formed by the photolytic decomposition of H2O2. The formation of hydroxyl radicals by decomposition of hydrogen peroxide was first observed by Hoigne´ and Bader.5 Nowadays, due to its advantages, the UV/H2O2 process has gained researchers’ interest.6 The UV/H2O2 process has been proven to be effective in oxidizing a wide range of organic compounds such as pesticides and herbicides,7-11 dyes and textiles,12,13 and pharmaceuticals.14 The application of UV/H2O2 for treating polluted sources of drinking water and industrial wastewater on a practical scale could be assisted by the mathematical modeling of the process. Different studies on the kinetic modeling of the degradation of organic compounds by the UV/ H2O2 process have been conducted.6,15-17 Kinetic modeling of advanced oxidation technologies is a useful tool to study the process parameters. The impact of different factors on the process efficiency such as residence time, H2O2 concentration, and alkalinity concentration could be investigated through modeling without carrying out costly experiments. Defined flow models also help to design and scale up industrial-size photoreactors. Pharmaceutical compounds are a class of contaminants that are widespread in surface and groundwater as well as municipal wastewaters. Although the concentration of pharmaceutical contaminants in the aquatic environment is low, it may result in a potential risk for human and animal life due to their continuous discharge and improper disposal. In this study, metronidazole was selected as a target model compound due to its high solubility in water and nonbiodegradability. This compound is widely used for treating infections caused by anaerobic bacteria and protozoa. The concentration of metronidazole in the hospital wastewater is normally in the range of 0.011-0.055 µM.18 Since metronidazole cannot be decomposed through conventional treatment methods, it can be accumulated in the aquatic environment. It is essential to employ a more effective method for the destruction of metronidazole in water and wastewater. In this study, a free radical and molecular modeling are proposed for the oxidation of a pharmaceutical compound, metronidazole, by using the UV/H2O2 process in single and multilamp photoreactors. This study is the modification of Johnson and Mehrvar’s work,6 in which the modeling of a plug

10.1021/ie900906e  2010 American Chemical Society Published on Web 04/29/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010 Table 1. Characteristics and Operating Conditions of Photoreactors parameters

single lamp photoreactor

multilamp photoreactor

photoreactors in series number overall length (m) radius (mm)

2 2.4 100-250

2 2.4 100-250

number type nominal length (m) nominal radius (Rt) (mm) input current (amps) input watts (W) UV output (W)

2×1 low pressure 1.2 8.5 1.2 120 36

outer radius (Ri) (mm)

17

wastewater volumetric flow rate (L s-1) Re [metronidazole]o (µM) [H2O2]o (mg L-1)

2

2

4750-10890 6 25-500

4006-7580 6 25-500

UV lamps 2×4 low pressure 1.2 8.5 1.2 120 36

quartz sleeve 17 operating conditions Figure 1. Schematic diagram of photoreactors, (a) a single lamp photoreactor and (b) a multilamp photoreactor.

flow photoreactor was investigated by considering constant velocity and neglecting diffusion in any direction. Moreover, the modeling of the UV/H2O2 is performed by computational fluid dynamics (CFD), in which the momentum, radiation energy, and mass balances for each component are considered. The UV lamp spectrum falls within the range of 400-600 nm, which is very effective for photochemical reactions, but it does not show thermal effects; thus, thermal effects of photochemical reactions are normally negligible. 2. Geometry of Photoreactors The schematic diagram of the two photoreactors used in this study is depicted in Figure 1, (a) for the single lamp photoreactor and (b) for the multilamp photoreactor. The single lamp photoreactor is composed of two concentric cylinders with annulus turbulent flow. The inner cylinder is a quart sleeve with the radius Ri into which a UV lamp is inserted. Two equal-sized UV lamps in series with a 1.2 m length each are considered. The total length of the two UV lamps in series is equal to a reactor length of 2.4 m; therefore, no dark zone in the photoreactors is assumed. The multilamp photoreactor is also composed of a cylinder containing four equal-sized parallel UV lamps, so that each of them is located in a half of the reactor radius equally apart, with the same characteristics of the single lamp photoreactor. The photoreactor radius is R. The physical properties and operating conditions of the processes are provided in Table 1. Cylindrical coordinates were used for the modeling of the photoreactors. Due to the complex geometry of the multilamp photoreactor, Cartesian coordinates were employed for the local volumetric rate of the energy absorption calculation. For the modeling purpose, various photoreactor radii from 100 to 250 mm and H2O2 doses from 25 to 500 mg L-1 were chosen. The main assumptions for the flow modeling are isothermal reaction at 25 °C, steady state, and turbulent flow. Also, physical properties of water are assumed. The photoreaction modeling was carried out by momentum and mass balances, which exist in the chemical engineering module of COMSOL Multiphysics (version 3.5) and MATLAB (version 2009a). The momentum and mass balances were solved by the use of the k-ε turbulent model in COMSOL. The modeling of the metronidazole degradation by the UV/H2O2 process including its concentration profile was computed using MATLAB software.

3. Momentum Balance The momentum balance for the photoreactors was developed for two flow regimes, the plug flow and the turbulent flow. In the case of plug flow, the velocity could be calculated by dividing the volumetric flow rate by the cross-sectional surface area. The ideal plug flow implies that there is no axial mixing (back mixing) in the photoreactor; however, there is a complete radial mixing which is an ideal case, and it is assumed that all of the elements have the same residence time, that is, the axial velocity vector is constant. Since plug flow is an ideal and simple model, it has its own deficiencies. Therefore, it is necessary to use more practical models to predict the fluid dynamics in the photoreactor. For the case of the turbulent flow, different known models have been developed. Among them, the k-ε model has had a great attraction to researchers for modeling turbulent flows.19,20 The k-ε model is a two-equation model in which fluctuating velocities and Reynolds stresses have been related to the properties of the turbulent flow itself such as k and ε. These two fluid flow properties are the turbulent kinetic energy per unit mass of the fluctuating components (k) and the turbulent dissipation rate of the kinetic energy (ε). The continuity equation and steady-state momentum balances for an incompressible fluid are described by following equations21 ∇·V ) 0

(1)

FV · ∇V ) -∇P + ∇ · (µ + FηT)(∇V + (∇V)T) + F (2) In this equation, F, V, and P represent the density, the time-averaged turbulent velocity, and the time-averaged pressure, respectively. The µ and ηT are the dynamic and kinematic turbulent viscosities, respectively. F is also the external force on the control volume. The k-ε model depicts that kinematic turbulent viscosity (ηT) at any point should depend only on k and ε at that point according to the following expression22 ηT ) Cµ

k2 ε

(3)

where Cµ is an adjustable model constant. The turbulent kinetic energy (k) is the average kinetic energy per unit mass of eddies in the turbulent flow that is produced by the buoyant thermal and mechanically generated eddies based on the following equation22

Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010

1 k ) (u'2 + V'2 + w'2) 2

(4)

Parameters u′, V′, and w′ are the fluctuating velocities in x, y, and z-directions, respectively. The steady-state equations for the turbulent kinetic energy (k) and the turbulent dissipation rate (ε) are as follows22

FV · ∇k ) ∇ ·

[(

µ+F

)]

Cµk2 k2 ∇k + FCµ (∇V + (∇V)T)2 - Fε σkε ε

(5)

[(

2

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)]

Cµk ∇ε + FCε1Cµk(∇V + (∇V)T)2 σεε ε2 FCε2 (6) k All five model constants have been selected as follows, such that the k-ε model gives an estimation that fits reasonably well with the experimental data.22 FV · ∇ε ) ∇ · µ + F

Cµ ) 0.9

Cε1 ) 1.44

Cε2 ) 1.92

σk ) 1.0 σε ) 1.3 (7)

Figure 2. Dimensionless velocity profile in the radial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

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Equations 5 and 6 along with continuity and momentum eqs 1 and 2 should be solved simultaneously. Boundary conditions for the momentum balance are as follows: (1) Inlet of the photoreactor: the inflow velocity is specified j) by the ratio of the volumetric flow rate to the surface area (V 22 Q/S). The k and ε are also calculated as follows

Figure 3. Dimensionless velocity profile in the axial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

Figure 4. Turbulent kinetic energy profile in the axial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

( ) 2

k)

3IT j 2) (V 2

ε ) Cµ0.75

[

]

3IT2 2 j ) (V 2 LT

1.5

(8)

where IT and LT are the turbulent intensity scale (initial turbulence intensity) and turbulent length scale (eddy length scale), and their magnitudes could be calculated by eqs 9 and 10, respectively22

Figure 5. Turbulent dissipation rate profile in the axial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

Figure 6. Turbulent viscosity profile in the axial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.


IT ) 0.16Re

-1/8

LT ) 0.07L

(9) (10)

In these equations, Re is the Reynolds number and L (m) is the equivalent diameter of the photoreactor. The equivalent diameters for the single and multilamp photoreactors are 2R(1 (Ri/R)) and (2(R2 - 4Ri2))/((R + 4Ri)), respectively. The values of IT and LT are 0.05 and 0.02, respectively.22 (2) Outlet of the photoreactor: a normal flow is assumed so that the normal stress in the outlet is zero. Therefore, the gradients of k and ε are also calculated as follows ∇k ) 0

∇ε ) 0

(11)

(3) Walls of the photoreactor: no slip boundary condition was assumed for the photoreactors and all the walls of the lamps. However, the logarithmic wall function was used for walls as a modification of the k-ε model. The boundary conditions for the

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k-ε model at a no-slip wall are obvious, but the near-wall behavior of the model, especially the ε equation, is not appropriate. Therefore, the model produces poor results when integrated to the wall without modification. In fact, the integration of the k-ε model through the near-wall region and the application of the no-slip conditions yield unsatisfactory results. Therefore, the logarithmic wall function was used as the boundary condition on the walls of the photoreactor and lamps. In the near-wall region, therefore, the equation was solved for the first grid node away from the wall. The logarithmic wall function for a smooth pipe is as follows22 uz+ ) 5.5 + 2.5 ln y+ where y√τw /F ujz and uz+ ) y+ ) µ/F √τw /F

(12)

in which y+, uz+, y, uj z, and τw are the dimensionless distance from the wall, the dimensionless velocity, the distance from

Figure 7. Turbulent viscosity profile in the radial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

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the wall, the time-averaged axial velocity, and the shear wall, respectively. Simultaneous solution of eqs 1, 2, 5, and 6 as well as applying boundary conditions gives various flow characteristics such as velocity distribution, minimum and maximum velocities, turbulent kinetic viscosity, turbulent kinetic energy, turbulent dissipation rate distribution, and vorticity. Equations 1-11 are applicable to both single and multilamp photoreactors. The details are discussed Section 7. 3.1. Computational Fluid Dynamics (CFD) Modeling. The 3-D flow field in the single lamp and multilamp photoreactors was simulated using a commercial CFD package (COMSOL Multiphysics V3.5). Computational fluid dynamics (CFD) codes and packages have been used to predict internal and external flows for decades. The necessity of employing CFD packages for chemical engineering applications is also obvious. COMSOL Multiphysics is a type of CFD package based on the finite element method (FEM) that is used for computational flow modeling (CFM). The momentum and mass conservation equations in the turbulent regime are solved using the finite element method (FEM). The division of the photoreactor’s domain into discrete elements is the first stage (grid generation), where the number of grids per volume (cells density) is required to be fine enough to capture the flow details. Due to the boundary layer formation around lamps and photoreactor walls, it is essential to increase the number of grids around lamps and photoreactor walls. The number of grids employed for the discretization was specified by conducting a grid independence study. The grid independency was carried out by comparing the changes in the velocity and turbulent kinetic energy along the photoreactor by cell density increment. The additional cells did not change the calculated velocity and turbulent kinetic energy by more than 1 and 6%, respectively. The final 3-D mesh of the model had 10 508 and 11 488 elements for single and multilamp photoreactors, respectively.

(3) Photoreactor wall (r ) R): the mole flux and consequently the concentration gradient on the reactor wall are also 0. Equation 13 for the turbulent flow and eq 14 for the plug flow behavior are solved using COMSOL Multiphysics (V3.5) and MATLAB softwares (V2009a), respectively. Equations 13 and 14 are applicable for both single and multilamp photoreactors. However, LVREA in the multilamp photoreactor is calculated in Cartesian coordinates due to the complexity of its geometry. 5. Reaction Mechanisms The main photochemical (R123 and R214) and chemical (R3-R106,24-28) reactions in the UV/H2O2 process are as follows (metronidazole is shown by RH) φ1

H2O2 + hν 98 2•OH (φ1 ) 0.5) φ2

RH + hν 98 intermediates f CO2 + H2O (φ2 ) 0.033)

(R2) where φ is the quantum yield (mol Einstein-1), the fraction of the absorbed radiation consumed by the photolytic decomposition reaction. In other words, quantum yield is the number of moles of the irradiated substance decomposed per mole of the photon absorbed (Einstein). k1

H2O2 + •OH 98 HO•2 + H2O (k1 ) (1.4-4.5) × 107 M-1 s-1) (R3) k2

2•OH 98 H2O2 (k2 ) (5.0-8.0) × 109 M-1 s-1)

4. Mass Balance The mass balance or continuity equations for each compound in the system should be solved simultaneously to find the concentration profile of each compound inside photoreactors. For each component, the continuity equation for turbulent flow at steady state could be written as follows21 ∇ · (V · Ci) ) ∇ · (D∇Ci) + Rrxn,i

(13)

where D and V are the turbulent diffusivity of species i in the mixture (m2 s-1) and the turbulent velocity (m s-1), respectively. j i + Ci′) and Rrxn,i are the concentration (M) and the Ci (or C reaction rate of component i (M s-1), respectively. Equation 13 is the steady-state mass-transport equation for turbulent flow that could be solved either in a turbulent or plug flow regime. Assumptions for solving mass balances are steady-state flow and impermeable walls. The plug flow mass-transport equation with consideration of radial diffusion in the photoreactor at steady state is written as follows21

( ( ))

∂Ci 1 ∂ ∂Ci )D Vz r ∂z r ∂r ∂r

(R1)

k3

2HO2• 98 H2O2 + O2 (k3 ) (0.8-2.2) × 106 M-1 s-1) k4

HO2• + •OH 98 H2O + O2 (k4 ) 6.6 × 109 M-1 s-1)

(R4)

(R5)

(R6)

k5

RH + •OH 98 intermediates f CO2 + H2O (k5 ) 1.98 × 109 M-1 s-1) (R7) k6

•• 6 -1 -1 HCOs ) 3 + OH 98 CO3 + H2O (k6 ) 8.5 × 10 M

(R8) k7

+ Rrxn,i

(14)

where Vz is the axial velocity. The three boundary conditions required to solve eq 14 are as follows: (1) Inlet (z ) 0): the influent metronidazole concentration is specified (Co ) 6 µM). (2) Quartz sleeve wall (r ) Ri): the concentration gradient on the quartz sleeve wall is 0 because the diffusion of metronidazole in the quartz sleeve is 0.

•• CO2(k7 ) 3.9 × 108 M-1 s-1) 3 + OH 98 CO3 + OH

(R9)

k8

• 5 -1 -1 CO•s ) 3 + H2O2 98 HCO3 + HO2 (k8 ) 4.3 × 10 M

(R10)


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Figure 8. Vorticity profile in the axial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

6. Photon (Irradiation) Balance The irradiation balance must be coupled with other equations of change in the modeling of the photoreactors. For the modeling of the photoreactors, the local volumetric rate of energy absorption (LVREA), the local rate of absorbed radiation per unit time and unit volume, must be estimated. The rate of organic degradation by direct photolysis is proportional to the LVREA. This parameter is not uniform inside of the photoreactor due to the light attenuation caused by the species absorption; thus, the LVREA depends on the radiation field in the photoreactor. The fraction of the incident light absorbed by a specific compound (fi) is as follows29

fi )

εiCi n

(15)

∑εC

i i

i)1

where εi and Ci are the molar absorptivity (m-1 M-1) and the concentration of species i (M), respectively. The radiation balance for a single lamp photoreactor could be written as follows29 1 d(rq) ) -q(2.303µs) r dr

(16)

In this equation, q is the radiant energy flux (Einstein m-2 s-1), and µs is the extinction coefficient (m-1) of the solution (base 10). The latter parameter could be defined as follows

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Figure 9. Vorticity profile in the radial cross section of the photoreactors, (a) single lamp photoreactor and (b) multilamp photoreactor.

n

µs ) µw +

∑εC

i i

(17)

inner reactor radius (m). The LVREA (A) is the product of µs and q as follows

i)1

where µw is the extinction coefficient of pure water (0.7 m-1). The integration of the irradiation governing eq 16 yields q ) qo

Ri exp(-2.303µs(r - Ri)) r

(18)

where qo is the radiant energy flux on the sleeve wall before any attenuation (Einstein m-2 s-1) and Ri is the

A ) µsqo

Ri exp(-2.303µs(r - Ri)) r

(19)

On the basis of the mechanisms of UV/H2O2 (reactions R1-R10), the reaction rates of different species in the system could be developed as follows RRH ) -k5[RH][•OH] - φ2fRHA

(20)

Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010 •

•

RH2O2 ) -φ1fH2O2A - k1[H2O2][ OH] + k2[ OH] + k3[HO•2]2

-

2

k8[CO•3 ][H2O2]

(21)

R•OH ) 2φ1fH2O2A - k5[RH][•OH] - k1[H2O2][•OH] - k2[•OH]2 • 2- • k4[•OH][HO•2] - k6[HCO3 ][ OH] - k7[CO3 ][ OH] (22)

RHO2• ) k1[H2O2][•OH] - k3[HO•2]2 - k4[HO•2][•OH] + k8[CO•3 ][H2O2] (23) •• RHCO3- ) -k6[HCO3 ][ OH] + k8[CO3 ][H2O2]

(24)

• RCO32- ) -k7[CO23 ][ OH]

(25)

RCO•) k6[HCO3-][•OH] + k7[CO32-][•OH] - k8[CO3•-][H2O2] 3

(26) 7. Results and Discussion Figure 2 depicts the normalized velocity (velocity divided by the average velocity) profile in the outlet of the single and multilamp photoreactors. It is clear from Figure 2 that by increasing the number of lamps, the minimum velocity (velocity of the first grid node away from the wall) decreases and the maximum velocity increases, so that the velocity distribution in the single lamp photoreactor is more uniform. In the multilamp photoreactor, the maximum velocity is observed at the center of the photoreactor which has the lowest residence time. The local volumetric rate of energy absorption (LVREA) is higher at the center of the multilamp photoreactor due to the light intensity fluxes of the four UV lamps. The LVREA in the center of the multilamp photoreactor’s inlet with the radius of 200 mm is more than 50% higher than that of the region with 5 cm away from the photoreactor’s wall (0.09 versus 0.05 Einstein m-3 s-1). Figure 3 illustrates the velocity profile of the center slice of the photoreactors (on the xz-plane). The boundary layer thickness is a direct function of the characteristic length and an inverse function of velocity. Along the photoreactors, the flow velocities of the two photoreactors are close to

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each other, but the characteristic length in the multilamp photoreactor is smaller than that of the single lamp. The boundary layer thickness in the multilamp photoreactor is larger than that of the single lamp photoreactor (8.4 × 10-3 versus 5.6 × 10-3 m in the outlet of the photoreactors). It is also clear from Figure 3 that the velocity profile in the single lamp photoreactor is more uniform compared to that of the multilamp photoreactor. Figure 4 shows the k profile for the single lamp and multilamp photoreactors. The turbulent kinetic energy is the average of the kinetic energy associated with eddies and fluctuations (in all directions) in the turbulent flow. Therefore, the turbulent kinetic energy of the single lamp photoreactor is uniformly distributed at each cross section. The turbulent kinetic energy and the fluctuating velocity components are lost by driving forces and dissipated into thermal energy to become more stable. Figure 5 represents the turbulent dissipation rate of the single and multilamp photoreactors on the center slice (xz-plane), respectively. The range of the turbulent dissipation rate values for the multilamp photoreactor is greater than that of the single lamp photoreactor. The distribution of the turbulent dissipation rate is more uniform in the single lamp photoreactor. Referring to eq 3, the turbulent viscosity can be estimated from the magnitudes of the k and ε parameters. The turbulent viscosity which is calculated based on the k and ε values for single and multilamp photoreactors is provided in Figure 6. The turbulent viscosities for the single lamp and multilamp photoreactors are increased along the photoreactors based on the variations of the turbulent kinetic energy and turbulent dissipation rate values. Moreover, the turbulent viscosity distribution for the outlet of both single and multilamp photoreactors is depicted in Figure 7. The lowest turbulent viscosity is found at the center of the multilamp photoreactor. The magnitude of the turbulent viscosity increases with an increase of the distance from the center of the UV lamps. Vorticity is a mathematical concept which shows the amount of circulation or rotation in a fluid. The vorticity is important because the turbulent flow is rotational and the vorticity is able to show intense, small-scale, and random spatial fluctuations. Vorticities of the single lamp and

Figure 10. Metronidazole concentration by the developed model and the experimental data obtained from the open literature.14

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Figure 11. Metronidazole degradation in the absence of alkalinity in a single lamp photoreactor (R ) 200 mm) for different inlet H2O2 concentrations (25-500 mg L-1).

Figure 12. Comparison of the metronidazole degradation under different alkalinity in the single lamp photoreactor ([H2O2]o) 100 mg L-1; R ) 200 mm).

multilamp photoreactors are depicted in Figure 8. The vorticity of the single lamp and multilamp photoreactors increases along the photoreactors. The vorticity distribution for the single lamp and multilamp photoreactors’ outlet cross sections is illustrated in Figure 9. The vorticity increases by decreasing the velocity of the fluid, where it agrees with the turbulent flow characteristics. Moreover, regions far from the UV lamps have higher vorticity values. The metronidazole degradation in single lamp and multilamp photoreactors by UV/H2O2 was modeled based on the published experimental data.14 Johnson and Mehrvar6 used the Shemer et al.14 experimental data to model the metronidazole degradation in a plug flow reactor. In the first step of their model, the reaction rate constant between metronidazole and hydroxyl radicals was specified by trial and error. The comparison of experimental

results with the simulation run was carried out to obtain a reasonable error between experimental results and the predicted model. Therefore, k5 was specified to be 1.98 × 109 M-1 s-1, which was in the expected range of 1.0 × 109 to 10 × 109 M-1 s-1.30 The developed model in this study was also validated by the experimental results of Shemer et al.14 Figure 10 depicts that the developed model in the present study was able to predict the metronidazole degradation rate for two different influent H2O2 doses (25 and 50 mg L-1). The residuals were sketched (not shown) for the difference between a predicted dynamic model and the simulated data, where there was no trend among the data and a good random gun shot pattern was observed, confirming a good agreement between the model and the experimental results with less than 2% error.


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Figure 13. Effect of alkalinity and inlet H2O2 dose on the metronidazole degradation in the single lamp photoreactor with R ) 200 mm.

Figure 14. Metronidazole degradation in the outlet of the multilamp photoreactor for different radii (R) of (a) 100, (b) 150, (c) 200, and (d) 250 mm; [H2O2]o ) 100 mg L-1; [alkalinity] ) 0.

7.1. Single Lamp Photoreactor Modeling. The volumetric flow rate of the polluted stream is assumed to be 2 L s-1, which is in the turbulent region; thus, the average velocity and the residence time of the fluid inside the photoreactor were varied with the photoreactor radii.

7.1.1. Effect of Hydrogen Peroxide Dose on the Degradation Rate. The degradation of organic compounds by AOPs depends on several factors. The most important parameters that influence the performance and efficiency of the UV/ H2O2 process are the rate of generation of oxidants, especially

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Figure 15. Metronidazole degradation in the outlet of the multilamp photoreactor (R ) 250 mm, [H2O2]o ) 25 mg L-1) for different alkalinity concentrations of (a) 0, (b) 1.5, and (c) 3 µM.

hydroxyl radicals, and the extent of the contact between organic pollutants and these radicals. The photochemical degradation using the UV/H2O2 process has revealed that the decomposition rate of organic compounds could be enhanced by increasing the H2O2 dose up to an optimum H2O2 concentration. Figure 11 depicts the metronidazole degradation by UV/H2O2 when the photoreactor radius is 200 mm. As Figure 11 shows, the greatest degradation rate was observed when the inlet hydrogen peroxide was 75 mg L-1 without alkalinity. 7.1.2. Effects of Alkalinity on the Degradation Rate of Metronidazole. Carbonate and bicarbonate ions in water, known as alkalinity, scavenge a significant amount of hydroxyl radicals; therefore, they reduce the efficiency of the UV/H2O2 process. Figure 12 indicates that by increasing the alkalinity, the metronidazole degradation rate is reduced. Therefore, a greater amount of H2O2 concentration or residence time is needed to obtain a desirable degradation, and obviously, it leads to a higher operating cost. Figure 13 also shows the outlet concentration of metronidazole for various alkalinities and H2O2 concentrations. It is clear that the degradation rate is dropped by increasing the alkalinity concentration. In addition, an optimal dose of H2O2 for three conditions is depicted. The presence of alkalinity shows that the optimal H2O2 concentration increases by increasing the alkalin-

ity concentration. Modification of the model for the metronidazole degradation by considering the radial diffusion did not show an obvious change in the metronidazole reduction. The small photoreactor radius (200 mm), the small concentration gradient in the radial direction, and the existence of the turbulent flow provide a negligible effect of radial diffusion on the metronidazole degradation. 7.2. Multilamp Photoreactor Modeling. The modeling of the multilamp photoreactor was carried out by MATLAB and COMSOL Multiphysics. As previously explained, Cartesian coordinates were employed for modeling LVREA. The LVREA in the multilamp photoreactor is the summation of LVREA produced by the four lamps located inside of the photoreactor with equal space from the photoreactor center. The LVREA of the multilamp photoreactor is as follows6 4

A)

4

∑A

i

i)1

)

∑ i)1

(

µsqo

Ri

√(x - a)

2

+ (y - b)2

×

e-2.303µs(√(x-a) + 2

)

(y-b)2-Ri

)

(27)

where a and b are the x-axis and y-axis locations of the UV lamps, respectively. Ai and Ri are the LVREA of lamp number


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Figure 16. Metronidazole degradation in the (a) single lamp and (b) multilamp photoreactors using the turbulent mass conservation equation ([H2O2]o ) 25 mg L-1; R ) 200 mm; [alkalinity] ) 0).

i and the radius of quartz sleeve of lamp number i, respectively. The mass conservation equations are solved along with eqs 15-27. Figure 14 depicts the metronidazole concentration contour in the multilamp photoreactor outlet for various reactor radii from 100 to 250 mm when the inlet H2O2 dose is 100 mg L-1 without alkalinity. In larger photoreactors, because the residence time is higher, a greater amount of metronidazole is degraded. The concentration gradient decreases with an increase of the photoreactor’s radius. The value of the concentration gradient decreases from 0.05 to 0.02 when the photoreactor radius increases from 100 to 250 mm. The region with zero concentration gradient (the dark blue region) is larger for larger photoreactors. Metronidazole concentration contours for different alkalinity concentrations are also shown in Figure 15. By increasing the alkalinity concentration, the degradation efficiency

is reduced. It is found from the comparison of the single lamp and multilamp photoreactors that the average outlet metronidazole concentration in the multilamp photoreactor is five times less than that of the value obtained in the singe lamp photoreactor. As explained earlier, the mass balance in single lamp and multilamp photoreactors was solved in two different ways, plug flow (eq 14) and turbulent flow (eq 13), by MATLAB and COMSOL Multiphysics softwares, respectively. The results obtained based on the plug flow model were shown earlier. Figures 16 and 17 are the results obtained based on the turbulent mass balance equations. Figure 16 shows the metronidazole degradation in single lamp and multilamp photoreactors based on the turbulent mass conservation (eq 13). Figure 17 also depicts the metronidazole concentration profile in the outlet of

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Figure 17. Metronidazole concentration profile in the outlet of (a) single lamp and (b) multilamp photoreactors using the turbulent mass conservation equation ([H2O2]o ) 25 mg L-1; R ) 200 mm; [alkalinity] ) 0).

the single and multilamp photoreactors. Furthermore, this figure illustrates the expected uniform concentration profile in the radial direction in the outlet of the photoreactors. 8. Conclusions A dynamic model for the advanced oxidation process using the combination of hydrogen peroxide and ultraviolet irradiation in two tubular photoreactors (single and multilamp photoreactors) was developed. The dynamic model was provided by solving the momentum equation (k-ε turbulent model), mass balances for each species, and the irradiation balance. The impact of different parameters such as alkalinity and H2O2 concentrations, photoreactor size, and light intensity on the photoreactor efficiency were considered in the models. The most important conclusions are drawn as follow:

(1) The H2O2 dose had a dual effect on the degradation of metronidazole, and it is consistent with previous results.4,12 (2) The presence of alkalinity reduced the AOP efficiency because it scavenges hydroxyl radicals. The metronidazole degradation rate was decreased from 61 to 42.5 and 36.2% with an increase of alkalinity from 0 to 1.5 and 3 µM, respectively. (3) The optimal H2O2 concentration was varied with different alkalinity concentrations and photoreactor sizes. The optimal H2O2 dose was increased by an increase of alkalinity concentration. The optimal values of H2O2 concentration were 75, 150, and 200 mg L-1 when alkalinity concentrations were 0, 1.5, and 3 µM, respectively. (4) Radial diffusion did not show a significant change in the metronidazole degradation because of the highly turbulent flow. Turbulent flow and fluctuations in the fluid stream resulted in


a lower concentration gradient. Therefore, a lower radial diffusion was observed. (5) The results of the metronidazole degradation by the multilamp photoreactor showed a greater degradation rate (approximately 5 times) as compared to that of the single lamp photoreactor. The multilamp photoreactor had a greater LVREA and light intensity (approximately 2.7 times) due to the higher number of UV lamps, which resulted in a higher direct photolytic decomposition of metronidazole and H2O2. Therefore, a higher light intensity in the multilamp photoreactor resulted in a higher synergic effect (direct photolysis and the production of hydroxyl radicals). The radial velocity fluctuations provided a more uniform flow and a more uniform radial concentration profile. Acknowledgment The financial support of Natural Sciences and Engineering Research Council of Canada (NSERC) and Ryerson University is greatly appreciated. Nomenclature a ) x-axis location of the UV lamps (m) b ) y-axis location of the UV lamps (m) A ) local volumetric rate of energy absorption (LVREA) (Einstein m-3 s-1) Ci ) concentration of component i (M) j i ) average concentration of component i (M) C Ci′ ) fluctuating concentration of component i (M) Co ) inlet concentration (M) Cµ ) adjustable k-ε model constant (-) D ) turbulent diffusivity (m2 s-1) fi ) fraction of photons absorbed by species i (-) F ) external force (kg m s-2) IT ) turbulent intensity scale (-) k ) turbulent kinetic energy (m2 s-2) ki ) reaction rate constants (M-1 s-1) L ) equivalent diameter of the photoreactor (m) LT ) turbulent length scale (m) n ) number of components in the system (-) P ) time-averaged pressure (Pa) Q ) volumetric flow rate (m3 s-1) q ) radiant energy flux (Einstein m-2 s-1) qo ) radiant energy flux on the sleeve wall (Einstein m-2 s-1) r ) position on r-axis for single lamp photoreactor (m) R ) photoreactor radius (m) Re ) Reynolds number Ri ) quartz sleeve radius (m) Rrxn,i ) reaction rate of component i (M s-1) S ) surface area perpendicular to axial flow direction (m2) u′ ) fluctuating velocity in the x-direction (m s-1) ujz ) time-average axial velocity (m s-1) uz+ ) dimensionless velocity V ) time-averaged turbulent velocity (m s-1) V′ ) fluctuating velocity in the y-direction (m s-1) Vz ) axial velocity (m s-1) w′ ) fluctuating velocity in the z-direction (m s-1) x ) position on the x-axis (m) y ) position on the y-axis in eq 27; and also distance from the wall in eq 12 (m) y+ ) dimensionless distance from the wall z ) position on the z-axis (m) Greek Letters ε ) turbulent dissipation rate (m2 s-3) εi ) molar absorptivity of component i (M-1 m-1)

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ηT ) turbulent kinematic viscosity (m s ) θ ) position on the θ-axis for a single lamp photoreactor (m) µ ) dynamic viscosity (kg m-1 s-1) µs ) extinction coefficient of the solution (m-1) µw ) extinction coefficient of pure water (0.7 m-1) F ) density (kg m-3) τw ) shear wall (kg m-1 s-2) φ ) quantum yield (mol Einstein-1) 2

Acronyms AOT ) Advanced oxidation technology CFD ) computational fluid dynamics CFM ) computational flow modeling FEM ) finite element method LVREA ) local volumetric rate of energy absorption

Literature Cited (1) Mohajerani, M.; Mehrvar, M.; Ein-mozaffari, F. An overview of the integration of advanced oxidation technologies and other processes for water and wastewater treatment. Int. J. Eng. 2009, 3, 120. (2) Tabrizi, G. B.; Mehrvar, M. Integration of advanced oxidation technologies and biological processes: Recent developments, trends, and advances. J. EnViron. Sci. Health, Part A 2004, 39, 3029. (3) Ball, B. R.; Brix, K. V.; Brancato, M. S.; Allison, M. P.; Vail, S. M. Whole effluent toxicity reduction by ozone. EnViron. Prog. 1997, 16, 121. (4) Kang, J.-W.; Hoffmann, M. R. Kinetics and mechanism of the sonolytic destruction of methyl tert-butyl ether by ultrasonic irradiation in the presence of ozone. EnViron. Sci. Technol. 1998, 32, 3194. (5) Hoigne´, J.; Bader, H. The role of hydroxyl radical reactions in ozonation processes in aqueous solutions. Water Res. 1976, 10, 377. (6) Johnson, M. B.; Mehrvar, M. Aqueous metronidazole degradation by UV/H2O2 process in single- and multi-lamp tubular photoreactors: Kinetics and reactor design. Ind. Eng. Chem. Res. 2008, 47, 6525. (7) Glaze, W. H.; Lay, Y.; Kang, J. W. Advanced oxidation processes: A kinetic model for the oxidation of 1,2-dibromo-3-chloropropane in water by the combination of hydrogen peroxide and UV radiation. Ind. Eng. Chem. Res. 1995, 34, 2314. (8) Alfano, O. M.; Brandi, R. J.; Cassano, A. E. Degradation kinetics of 2,4-D in water employing in hydrogen peroxide and UV radiation. Chem. Eng. J. 2001, 82, 209. (9) Benitez, F. J.; Acero, J. L.; Real, F. J.; Maya, C. Modeling of photooxidation of acetamide herbicides in natural waters by UV radiation and the combinations of UV/H2O2 and UV/O3. J. Chem. Technol. Biotechnol. 2004, 79, 987. (10) Shemer, H.; Linden, K. G. Degradation and by-product formation of diazinon in water during UV and UV/H2O2 treatment. J. Hazard. Mater. 2006, 136, 553. (11) Shemer, H.; Sharpless, C. M.; Elovitz, M. S.; Linden, K. G. Relative rate constants of contaminant candidate list pesticides with hydroxyl radicals. EnViron. Sci. Technol. 2006, 40, 4460. (12) Aye, T.; Anderson, W. A.; Mehrvar, M. Photocatalytic treatment of cibacron brilliant yellow 3G-P (Reactive Yellow 2 Textile Dye). J. EnViron. Sci. Health, Part A 2003, 38, 1903. (13) Aye, T.; Mehrvar, M.; Anderson, W. A. Effects of photocatalysis on the biodegradability of Cibacron Brilliant Yellow 3G-P (Reactive Yellow 2). J. EnViron. Sci. Health, Part A 2004, 39, 113. (14) Shemer, H.; Kunukcu, Y. K.; Linden, K. G. Degradation of the pharmaceutical metronidazole via UV, Fenton and photo-Fenton processes. Chemosphere 2006, 63, 269. (15) Song, W.; Ravindran, V.; Pirbazari, M. Process optimization using a kinetic model for the ultraviolet radiation-hydrogen peroxide decomposition of natural and synthetic organic compounds in groundwater. Chem. Eng. Sci. 2008, 63, 3249. (16) Crittenden, J. C.; Hu, S.; Hand, D. W.; Green, S. A. A kinetic model for H2O2/UV process in a completely mixed batch reactor. Water Res. 1999, 33, 2315. (17) Edalatmanesh, M.; Dhib, R.; Mehrvar, M. Kinetic modeling of aqueous phenol degradation by UV/H2O2 process. Int. J. Chem. Kinet. 2008, 40, 34. (18) Goˇmez, M. J.; Petrovic´, M.; Ferna´ndez-Alba, A. R.; Barcelo´, D. Determination of pharmaceuticals of various therapeutic classes by solid phase extraction and liquid chromatography s tandem mass spectrometry analysis in hospital effluent wastewaters. J. Chromatogr., A 2006, 1114, 224.

5382


(19) Richards, P. J.; Hoxey, R. P. Appropriate boundary conditions for computational wind engineering models using the k-ε turbulence model. J. Wind Eng. Ind. Aerodyn. 1993, 46-47 (c), 145. (20) Menter, F. R. Eddy viscosity transport equations and their relation to the k-ε model. J. Fluid Eng 1997, 119, 876. (21) Bird, R. B.; Steward, W. E.; Lightfood, E. N. Transport Phenomena, 2nd ed.; John Wiley & Sons, Inc.: New York, 2002. (22) Wilkes, J. O. Fluid mechanics for chemical engineers with microfluidics and CFD, 2nd ed.; Prentice Hall: New york, 2005. (23) Baxendale, J. H.; Wilson, J. A. The photolysis of hydrogen peroxide at high light intensities. Trans. Faraday Soc. 1957, 53, 344. (24) Christensen, H. S.; Sehested, K.; Corftizan, H. Reaction of hydroxyl radicals with hydrogen peroxide at ambient temperatures. J. Phys. Chem. 1982, 86, 1588. (25) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data. 1988, 17, 513. (26) Bielsky, H. J.; Benon, H. J.; Cabelli, D. E.; Ravindra, L. A.; Alberta, A. B. Reactivity of peroxyl/superoxide radicals in aqueous solution. J. Phys. Chem. Ref. Data 1985, 14, 1041.

(27) Sehested, K.; Rasmussen, O. L.; Fricke, H. Rate constants of OH with HO2, O2-, and H2O2+ from hydrogen peroxide formation in pulseirradiated oxygenated water. J. Phys. Chem. 1968, 72, 626. (28) Draganic, Z. D.; Negron-Mendoza, A.; Sehested, K.; Vujosevic, S. I.; Navarro-Gonzales, R.; Albarran-Sanchez, M. G.; Draganic, I. G. Radiolysis of aqueous solutions of ammonium bicarbonate over a large dose range. Radiat. Phys. Chem. 1991, 38, 317. (29) Jacob, S. M.; Dranoff, J. S. Radial scale-up of perfectly mixed photochemical reactors. Chem. Eng. Prog. Symp. Ser. 1966, 62, 47. ` lvarez, P. M.; Alonso, M. A.; Acedo, B. (30) Beltra´n, F. J.; Rivas, J.; A A kinetic model for advanced oxidation processes of aromatic hydrocarbons in water: application to phenanthrene and nitrobenzene. Ind. Eng. Chem. Res. 1999, 38, 4189.

ReceiVed for reView June 2, 2009 ReVised manuscript receiVed April 9, 2010 Accepted April 12, 2010 IE900906E

CFD Modeling of Metronidazole Degradation in Water by the UV

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