Photochemical decomposition of 2,4-dichlorophenoxyacetic acid (2,4-d) in aqueous solution. ii. reactor modeling and verification

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War. Sci. Tech. Vol. 35, No.4, pp. 197-205, 1997. © 1997 lAWQ. Printed in Great Britain. All rights reserved.

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PHOTOCHEMICAL DECOMPOSITION OF 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D) IN AQUEOUS SOLUTION. II. REACTOR MODELING AND VERIFICATION Carlos A. Martin, Maria I. Cabrera, Orlando M. Alfano and Alberto E. Cassano INTEC (Universidad Nacional del Litoral and CONICET), Giiemes 3450, (3000) Santa Fe, Argentina

ABSTRACT An annular flow photoreactor for the direct photolysis of 2,4-dicWorophenoxyacetic acid has been developed, mathematically modeled and experimentally verified in a bench scale apparatus. The model employs a very simple kinetic equation that was previously obtained in a well stirred tank, batch laboratory reactor. Reasonably good agreement has been obtained between model predictions and experimental results. The observed errors are mainly due to the fact that the kinetics of this very complex reaction have been modeled in terms of just one single concentration. © 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Annular photoreactor; direct dichlorophenoxyacetic acid.

photolysis;

photochemical

reaction;

reactor

modeling;

2,4•

INTRODUCTION The presence of hazardous organic contaminants in water, resulting from the widespread use of pesticides and herbicides represents one of the major threats for the environment. Their occurrence has given rise to a challenging natural resources management problem and, at the same time, has provided a source of motivation for many scientific initiatives. Direct photolysis has been always considered as one possible alternative because many organic compounds undergo complete destruction in the presence of UV radiation (Bolton and Carter, 1994). It has been recognized that several factors affect direct photolysis performance (Glaze, 1993). Among them it has been mentioned that the most significant variables are: (1) radiation absorption by the substrate, (2) reaction quantum efficiency, (3) the available light intensity and (4) the existence of other light absorbing compounds. In a recent work Cabrera et ai. (1996) analyzed the kinetics of the 2,4-dichlorophenoxyacetic acid photolytic degradation and obtained a simple intrinsic reaction rate. It specifically accounts for those four factors. 197

C. A MARTIN 1'1 a/

198

Radiation of wavelength 253.7 nm was used in a well stirred, isothermal batch reactor. The kinetic model incorporates radiation absorption by the reactant and the reacting medium (as separate parameters) and describes the reaction progress in terms of an overall quantum yield and the local volumetric rate of energy absorption (LVREA). Within this context, the next stage is to model and design a reactor that will use the developed reaction kinetics and test them with the appropriate experiments. The reactor model will require the combined solution of the radiative transfer equation (RTE) and the species mass balance (Cassano et al., 1995). In this work a bench-scale annular photoreactor has been studied both theoretically and experimentally. A tubular lamp was placed at the reactor centre line. This reactor will be used to test, with a very different reactor geometry, the kinetic expression previously developed in the cylindrical vessel laboratory reactor. The reactor model incorporates as design variables the reactor dimensions, the lamp dimensions and its operating characteristics.

Figure I. Annular photoreactor. I) pyrex tube, 2) quartz tube, 3) germicidal lamp.

Table 1.

REACTOR Pyrex® Suprasi1® LAMP Philips TUV A = 253.7 nm RESERVOIR

PARAMETER

VALUE

Irradiated length Outside diameter Inside diameter Irradiated volume Input power Output power Nominal length Diameter Volume

48cm 6.03 em 4.45 em 624 em 3 30W 9W 89.5 cm 2.6 em 6000cm 3

REACTOR DESCRIPTION Essentially, the reactor was an annular space (radial gap = 0.79 cm). The inner tube was made of quartz, Suprasil quality, transparent to the used UV radiation (Figure 1). The lamp was of the Germicidal type and its nominal length was purposely made larger than the reactor length. Positioning of the inner and outer

Photochemical decomposition in aqueous solution

199

tubes w~s achi~ved with Nylon 6-6 adapters. They also had connections for circulating the reacting fluid. Hydra~hc seahng was obtained by means of Viton o-rings. The continuous flow reactor was part of a recycl~ng system. The recycle consisted of: (1) a reservoir that was vigorously stirred and had provisions for sa~phng and temperature measurement, (2) a positive displacement recirculating pump (Gear type, Teflon Mlcropump) and (3) an all glass heat exchanger connected to a thermostatic bath. Other details can be seen in Table 1 and Figure 2.

Figure 2. Experimental set up. 1) tank, 2) stirrer, 3) thermometer, 4) sampling port, 5) pump, 6) heat exchanger, 7) thermostatic bath, 8) annular photoreactor.

REACTION KINETICS In a previous work (Cabrera et al., 1997), the kinetics of the 2,4-D photolysis were investigated. An intrinsic kinetic expression for the local reaction rate was obtained. It should be valid within the explored experimental conditions of concentrations, temperature, irradiation rates and light quality (wavelength). If it is a true point valued reaction rate, it should be applicable to any form of reactor regardless of its shape or manner of irradiation. The analytical expression is: (1) In Eq. (1) the overall quantum yield is and D,A. = 0.0262 mole/einstein and n == 1. Clearly, since the LVREA (eaA.) is a function of wavelength, position and time (because it depends upon the species concentrations) the reaction rate will be also a function of wavelength, position and obviously of time. The local volumetric rate of energy absorption was significantly affected by radiation absorption by some of the reaction products. This effect was modeled in terms of the 2,4-D actual concentration. REACTOR MODEL The reactor model was constructed in the following sequence: (i) the annular reactor, radiation distribution model of Romero et al. (1983) was adapted for this particular set-up; (ii) the tubular lamp with voluminal and isotropic radiation emission model of Irazoqui et al. (1973) was applied to this system; (iii) a mass balance for an actinometric reaction carried out in a tubular reactor inside the loop of a recycling system was adapted from Martin et al. (1996), and (iv) actinometer experiments were performed in the bench-scale reactor to compare theoretical predictions with actual results. This procedure permitted to verify the quality of the radiation emission and distribution models for the annular reactor. For the reactor employing 2,4-D the following sequence was followed: (1) a species mass balance for a tubular reactor inside the recycling system was developed; (2) the kinetic expression given by Eq. (1) was incorporated into this mass balance; (3) the radiation model previously validated was used to predict the LVREA in the kinetic expression; (4)

C. A. MARTi!'l et at.

200

radiation absorption by reaction products was incorporated into the ra~iation model. according to an empirical expression also obtained by Cabrera et al. (1997); (5) time evolutIo~ concentratIons of the 2,~-D in the recycling system were predicted using (1), (2), (3) and (4); and (6) ExperImental 2,4-D concentrations were compared with theoretical predictions.

Radiant field For a homogeneous medium the radiation distribution is obtained by solving: (2)

with B.C.:

o

hn(s = 0) = IA.n '-

'-

The boundary condition is obtained from the extended source with voluminal and isotropic Emission model (Irazoqui et aI., 1973), according to:

(3)

This equation is valid for arc type lamps that have transparent walls as is the case for the germicidal lamps employed in this work. It permits the inclusion of all lamp characteristics and the reactor and lamp geometry into the design of the reactor (see Figures 2 and 3 in the above reference). Solution of Eq. (2) provides values of the radiation intensity as a function of position and direction. Once I is known, the incident radiation and the LVREA can be obtained from:

(4)

(5)

At any point x, .QL (e,
(6)

~hi~ equation is valid for monochromatic light as is the case for the one used in this work. The integration lImIts for e.and were derived b.y Irazoqui et al. .(1973) and the reader is referred to the original work. It mu~t be notIced that the exponen.tIal term (~tten~atlOn) uses the reacting medium total absorption coefficient whIle only the reactant absorptlOn coeffiCIent mtervenes, with a linear effect in the value of the LVREA [Eq. (5)]. Hence "i" stands for the reactant, while: ' KTA. , =

LK.~ J,II. j

and "j" stands for the reactant and any other component in the reacting medium.

(7)

Photochemical decomposition in aqueous solution

201

The actinometer reaction in the annular reactor ~ccording to.Ma:ti~ et al. (1996), und~r steady operating conditions (for the lamp and temperature) changes III

concentratIOn mSlde the batch recychng system are obtained from:

(8)

Ci(O)

= Cp

This equation is valid when: (1) VWVTank « 1 and (2) the operation of the reactor is differential, which means that the recirculation flow rate is high. For the actinometer reaction, with i = A: (9)

In Eqs. (8) and (9) since the radiation field is not uniform, the reaction rate is a function not only of time but

of position as well. Then, even for the differential reactor operation a volume average is required. For the actinometer reaction this average is easily computed because the reaction rate is not a function of the oxalic acid concentration and the uranyl ion concentration remains constant. The volume average reaction rate is by definition: (10) The expression employed to compute the reactor volume-averaged rate of energy absorption is:

(11)

In Eq. (11), A corresponds to the actinometer. Molar absorptivities to calculate can be measured in the spectrophotometer (Cary 17 D) and the quantum yield at 253.7 nm can be taken from Murov (1973). To validate the radiation model, results obtained from Eq. (11) must be compared with experiments, according to: (12)

Eq. (12) is obtained after integration of Eq. (8). This verification can be made at different actinometer initial concentrations. Validation of the radiation model The uranyl oxalate actinometer was employed (Murov, 1973). Under carefully controlled conditions the reaction is zero order with respect to the oxalic acid concentration. Three different uranyl sulfate concentrations were used: 0.005, 0.001 and 0.0005 M. Oxalic acid concentrations were 5 times larger always. Reaction conversion must be kept below 20 % (De Bernardez and Cassano, 1985).

202

C. A. MARTiN et al.

Figure 3 shows the experimental data [Eq. (12)]. The solid line corresponds to predictions from the radiation and reactor model [Eq. (11 )]. The largest error was smaller than 8%. Since agreement is very good. one may conclude that the radiation field of the annular reactor can be precisely represented. The reactor model for the 2.4-0 photolysis Eq. (8) is the same for the actinometer as for the photolytic reaction. particularly when the reactor differential operation is fulfilled. Additionally, the simplified kinetic expression represented by Eq. (1) has the same form as Eq. (9). However, during the 2,4-0 photolysis the radiation absorption characteristics of the reacting medium change. This is a very distinct phenomenon because: (1) the uranyl oxalate reaction is a photosensitized reaction and the radiation absorbing species is not consumed and (2) not only the 2,4-0 absorption coefficient changes but absorption by reaction products increases the total absorption coefficient above the initial value. This phenomenon produces an unavoidable coupling between the steady state radiation balance and the unsteady state mass balance (notice that due to the speed of propagation of the changes in the radiation field, the transient term in the RTE is always negligible). The total absorption coefficient can be obtained from the empirical equation developed by Cabrera et al. (1997): (13)

KV.. (t) = 0.0197 C~ - 0.01785 CD(t)

Then: ( 14)

dCD(t) = ~(R (x t)) dt VTank vR D -' CD(t=O)=Co D

with the I.e. And the reaction rate is:

(15)

Inserting the LVREA into Eq. (15) and substituting the result into Eq. (14) we obtain:

J

_1_[PI.. 2n a~R Co(t)] v {f del>fd8 (R~ _r

dC o = -<1>0)" dt VTank

YR)..2

L LL

R.

2

X exp{- K n [r cosel> - (Ri - r sin

2

2

sin 2 el»X

9

el»/i] (sin 8)-1} }dV

(16)

CD(t=O) = C~ Integration of this equation provides the time evolution of the 2,4-0 concentration. Notice that all the lamp characteristics are incorporated in the design. The mass balance and the volume average procedure indicated in the equations above are greatly simplified by the imposed differential operation on the photochemical section of the reactor. Eq. (16) must be numerically solved. At each different time, the LVREA must be calculated according to the existing concentrations. The most difficult and time consuming step is the calculation of the incident radiation [Eq. (4)] for each reaction condition. This is so because as a result of the spherical directional characteristics of the specific intensities, the limits of integration for the coordinates are rather complex trigonometric expressions (Irazoqui et al., 1973). A variation of the well-known Runge-Kutta integration method was employed.

Photochemical decomposition in aqueous solution

203

2.5,..-----------------, ·0

'; 2.0

orr fill- 1.5 ~

w 1.0

~ -}5.0 V

0.0 +------+-----+-----~ 0.0 2.5 5.0 7.5

Cur [mol ern"'J x 10' Figure 3. Validation of the radiant model. Keys: (-) model predictions [Eq. (11)]; (A) experimental data [Eq. ( 12)].

100

80

'Ea. 60 a.

(140

20

o o

2

4

6

t[h]

8

10

Figure 4. 2,4-D concentrations vs. time. Keys: (-) model predictions; (.,e) experimental data.

EXPERIMENTS 2,4-D was prepared as it was described in a previous work (Cabrera et aI., 1997). During these experiments, since the reaction rate is slow, the tank volume was only 3000 cm 3. Initial concentrations of 2,4-D were varied from 30 to 70 ppm. Sampling was made every hour. 2,4-D concentrations were analyzed by HPLC (Hewlett Packard, model 1050) with a spectrophotometric detector at 236 nm (Bondapack C 18, 25 cm column; solvent: acetonitrile 50%, acetic acid 1% and water 49%). The total absorption coefficient of the reacting mixture was measured in each sampling in order to verify the validity of the employed empirical correlation. Most of the decomposition runs were performed at 25 0 C. RESULTS Figure 4 shows the results for two initial concentrations. Solid lines correspond to predictions of the 2,4-D concentrations obtained from the solution of Eq. (16). Symbols correspond to experimental values. It can be seen that agreement is fairly good. The observed discrepancies, in some cases produce an error as large as 17%, are mainly due to the fact that the reaction kinetics of this very complex reaction (Crosby and Tutass, 1966) have been modeled in terms of just one single variable (the 2,4-D concentration). This statement is based on the consideration that the radiation-reactor model gave excellent results in the case of the actinometer reaction. In this case: (1) the reaction kinetics are very well known, (2) the reaction parameters are very well established and (3) the reaction optical characteristics are very well known and they are constant as long as the reaction conversion is maintained under 20%.

204

C. A. MARTiN et al.

Rigorously speaking, in the case of the 2,4-D reaction one should expect that: (I) more than one reaction product may affect the radiation absorption of the reacting medium; (2) the proper reaction kinetics may be a function of other variables besides the LVREA and (3) the eventual existence of autocatalytic or pH effects can not be properly accounted for with a single variable (see on this particular, Boval and Smith, 1973 and Chamarro and Esplugas, 1993). On the other hand, the simplicity of the employed kinetics as well as the reduced experimental work that is associated with such a simple fonnulation (for every sample only one concentration and one radiation absorption measurement were required) are very attractive features of this simplified approach. CONCLUSIONS An annular bench-scale reactor was designed from reaction kinetics data obtained in a batch, well stirred laboratory reactor. The employed reaction was the photolysis of low concentrations of 2,4-D in water. The reactor perfonnance was mathematically modeled. From this representation the time evolution of the 2,4-D was satisfactorily predicted. The mathematical model of the annular reactor includes all the reactor and lamp dimensions and characteristics; hence employing a known kinetics obtained in a very different reactor, an a priori reactor design was successfully achieved. ACKNOWLEDGMENTS The authors are grateful to Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) and to Universidad Nacional del Litoral (UNL) for their support to produce this work. They also thank Mrs. Myriam Calvo and Mr. Juan Andini for their valuable help in the analytical work, and Eng. Claudia M. Romani for technical assistance. NOMENCLATURE

C

ea

G I L

P R

concentration, mole m- 3 LVREA, einstein m-3 S-l incident radiation, einstein m- 2 S-I specific intensity, einstein m-2 S-I srI length, m lamp output, einstein S-I radius, m

Rj r

s t V

reaction rate, mole m- 3 S-I radial coordinate, m linear coordinate along the direction time, s volume, m 3 position vector, m

Greek letters

a.

~

molar absorptivity, m 2 mole-I spherical coordinate, rad volumetric absorption coefficient, m- I wavelength, nm transmission coefficient, dimensionless spherical coordinate, rad

n n

quantum yield, mole einstein-I solid angle, sr unit vector, dimensionless

e

K:

A y



relative to component i relative to the lamp R relative to the reactor T denotes total value Tank denotes a tank property ur relative to the uranyl ion A relative to the wavelength relative to the solid angle

i L

n

n, m

Photochemical decomposition in aqueous solution

205

Superscripts

o

indicates initial conditions

Subscripts A D exp

relative to the actinometer relative to 2,4-D denotes experimental data

Special Symbols

< >

indicates average value REFERENCES

Bolton, J. R and Carter, S. R. (1994). Homogeneous photodegradation of pollutants in contaminated water: An introduction. In: Aquatic and Surface Photochemistry, G. R Helz, R G. Zeep and D. G Crosby (Eds.), Lewis Publishers, Boca Raton, U.S.A,467-490. Boval, B. and Smith, J. M. (1973). Photodecomposition of2,4-dichlorophenoxyacetic acid. Chem. Engng. Sci., 28,1661-1675. Cabrera, M. I., Martin, C. A, Alfano, O. M. and Cassano, A E. (1997). Photochemical decomposition of 2,4• Dichlorophenoxyacetic acid in aqueous solution. I. Kinetic study. Wat. Sci. Tech. 35 (4) xx-yy (this issue) Cassano, A E., Martin, C. A, Brandi, R J. and Alfano, O. M. (1995). Photoreactor analysis and design: Fundamentals and applications. Ind. Eng. Chem. Res., 34, 2155-2201. Chamarro E. and Esplugas S. (1993). Photodecomposition of 2,4-Dichlorophenoxyacetic acid: influence of pH. J. Chem. Tech. Biotechnol., 57, 273-279. Crosby, D. G. and Tutass H. O. (1966). Photodecomposition of 2,4-dichlorophenoxyacetic acid. J. Agr. Food Chem., 14,596-599. De Bemardez, E. R and Cassano, A E. (1985). A priori design of a continuous annular photochemical reactor. Experimental validation for simple reactions. J. Photochem. Photobiol. A: Chem., 30, 285-301. Glaze, W. H. (1993). An overview of advanced oxidation processes: Current status and kinetic models. In: Chemical Oxidation. Technologies for the Nineties. W. Wesley Eckenfelder, A R. Bowers and J. A Roth (Eds.), Technomic Publishing Company, Lancaster, U.S.A, 1-9. Irazoqui, H.A., Cerda, J. and Cassano, AE. (1973). Radiation profiles in an empty annular photoreactor with a source of finite spatial dimensions. AIChE J., 19,460-467. Martfn, C. A., Baltanas, M. A and Cassano, AE. (1996). Photocatalytic reactors II. Quantum efficiencies allowing for scattering effects. An experimental approximation. J. Photochem. Photobiol. A: Chem., 94, 173-189. Murov, S. T. (1973). Handbook ofPhotochemistry. Marcel Dekker, New York. Romero, R, Alfano, O. M., Marchetti, J. L. and Cassano, A E. (1983). Modeling and parametric sensitivity of an annular photoreactor with complex kinetics. Chem. Engng. Sci., 38, 1593-1605.