Development and Experimental Validation of the ... - ACS Publications

Experimental determination of steady-state aqueous ozone profiles and corresponding effluent gaseous ozone concentrations in a tall contactor was made...
1 downloads 0 Views 345KB Size
Download PDF
Ind. Eng. Chem. Res. 2006, 45, 109-119

109

Development and Experimental Validation of the Model of a Tall, Continuous-Flow, Countercurrent, Bubble-Type Ozone Contactor Gunjan Tiwari and Purnendu Bose* EnVironmental Engineering and Management Program, Department of CiVil Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

The objective of this study was the development of a fully mechanistic model for the description of ozone mass transfer from gaseous to aqueous phase and the aqueous-phase ozone decomposition in a continuousflow, countercurrent, tall, bubble-type ozone contactor. The developed model was used to simulate the effects of changes in aqueous-phase pH and scavenger concentration on ozone mass transfer and decomposition. Simulation results could be adequately explained based on the current understanding of ozone mass transfer and aqueous ozone chemistry. Experimental determination of steady-state aqueous ozone profiles and corresponding effluent gaseous ozone concentrations in a tall contactor was made at various influent aqueous pH’s and scavenger concentrations using an experimental contactor fabricated for this purpose. Comparison of the simulated and experimentally obtained aqueous ozone profiles showed adequate agreement. However, the experimentally obtained effluent gaseous ozone concentrations were consistently lower than the corresponding simulated values by approximately 13%. This discrepancy is probably due to gas-phase ozone decomposition occurring inside the reactor and in the headspace above the reactor. Introduction Modeling of tall, bubble-type ozone contactors involves specification of the reactor type, i.e., semibatch or continuous flow, the reactor dimensions, and the mode of contact of liquid and gaseous phases, i.e., cocurrent or countercurrent. Ozone transfer efficiency from gas to liquid phase in such contactors is mainly controlled by physical parameters such as temperature, gas flow rate, ozone partial pressure, and reactor geometry.1,2 The effect of physical parameters can be adequately represented by a partition coefficient, e.g., Henry’s coefficient, and a masstransfer coefficient,3 for describing ozone distribution between gas and liquid phases. Chemical parameters such as pH, ionic strength, and composition of the aqueous solution also affect ozone transfer.1 The effect of chemical parameters can be represented by an empirical or mechanistic description of ozone decomposition in the aqueous phase.2 Le Sauze et al.4 developed models for ozone mass transfer from gas to liquid phase and liquid-phase ozone decomposition in plug-flow reactors, perfectly mixed reactors, reactors with axial dispersion in the aqueous phase, and reactors with three axial dispersion zones. Zhou et al.5 provided a comprehensive model for ozone contactors using a two-phase axial dispersion model (ADM) to integrate nonideal mixing, mass transfer, and ozone-decay process as a whole. El-Din and Smith6 presented a semiempirical, non-isobaric, one-phase axial dispersion model of ozone contactors. Simulations using this model were compared with data provided by Zhou et al.5 and showed good agreement. A more recent attempt at modeling an ozone contactor was reported7 in connection with the design of an ozone contactor for the inactivation of Cryptosporidium oocysts. Aqueous-phase ozone self-decomposition is described in the above models through empirical expressions. Examples of such expressions include the second-order ozone decomposition kinetics in pure water described by Gurol and Singer.8 In another important work, Yurteri and Gurol9 showed that ozone decom* Corresponding author. Tel.: +91 512 2597403. Fax: +91 512 2597395. E-Mail: [email protected].

position is first order in natural waters, and they expressed this rate constant as a function of pH, total organic carbon (TOC) concentration, and alkalinity. In general, researchers agree that ozone self-decomposition in water is a function of both residual ozone concentration and pH, with a reaction order between 0 and 2 with respect to residual ozone and between 0 and 1 with respect to hydroxide ion,10 depending on the specific conditions. In addition, two mechanistic models describing ozone selfdecomposition in the aqueous phase, the Hoigne, Staehelin, and Bader (HSB) model11-16 and the Tomiyasu, Fukutomi, and Gordon (TFG) model,17 are popular. Both are compilations of radical reactions, describing the decomposition of ozone in water as a function of pH and scavenger concentration. Chelkowska et al.18 compared the HSB and TFG models with an empirical model proposed by Tomiyasu et al.,17 with encouraging results. Westerhoff et al.19 compared the HSB model, the TFG model, and an empirical ozone-decomposition model proposed by Gurol and Singer8 with experimentally derived data for pure water with and without scavenger and with waters containing natural organic matter. On the basis of this study, it was concluded that the HSB model provided better agreement with experimental data at near-neutral pH values. It is clear that advances in the modeling of tall ozone contactors on one hand, and mechanistic modeling of ozone decomposition in the aqueous phase on the other, have progressed independently of each other. These two areas must be merged in order to obtain a comprehensive mechanistic model for describing ozone mass transfer and aqueous-phase reactions in bubble-type ozone contactors. In this connection, Beltran et al.20 reported a mechanistic model describing the degradation of simazine by ozonation in bubble contactors. Other researchers reporting similar work include Hautaniemi and co-workers,21,22 dealing with degradation of chlorophenol, and Beltran et al.,23 dealing with alachlor. The work of Pedit et al.24 is the most complete of all the attempts at ozone-contactor modeling to date. In this study, a model for a tall bubble contactor was developed considering both ozone mass transfer and a mechanistic description of aqueous-phase ozone decomposition, including degradation of trichloroethylene (TCE) and perchloroethylene (PCE)

10.1021/ie050397l CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2005

110

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006

by ozonation. The model was, however, not validated with a sufficient amount of experimental data from either laboratory or pilot-scale reactors. Also, the possibility of gas-phase ozone decomposition inside, and in the headspace above, the reactor was not considered. The above discussion suggests that a completely mechanistic model of tall bubble-column ozone contactors that is fully consistent with experimental data does not yet exist. In this context, we report ongoing work in our laboratory toward the development and verification of a mechanistic model for such a tall ozone contactor. The specific objectives of this study were (i) development of a mechanistic model for the simulation of aqueous and gaseous ozone-concentration profiles during the transfer of ozone into buffered water containing no impurities in a tall ozone contactor at various influent aqueous-phase pH’s and inorganic-carbon concentrations, and (ii) model verification through comparison of simulated results with experimentally obtained data under similar conditions. Model Development A tall cylindrical column of uniform cross-sectional area (A) and height (H) was modeled as a bubble contactor for contacting ozone with water in the countercurrent mode. Gas was injected to the bottom of the reactor as bubbles, while water was added from the top. The gas phase was assumed to be plug flow. The physical significance of this assumption is that gas bubbles input to the column at different times, and, hence, at different levels of the column initially, do not collide and coalesce during their rise through the column. This is a probable occurrence if L is small and gas bubbles introduced into the column are of uniform size that is invariant with time. Hydrostatic pressure variation along the column height was neglected, such that the values of Qg, Ug, UL, and L do not vary with column height. Because ozone is a sparingly soluble gas, it was assumed that mass-transfer resistance to ozone absorption was confined to the liquid side. The influent gas phase consisted of a mixture of oxygen and ozone, while the influent aqueous phase was pure water, with only inorganic carbon and phosphorus species, i.e., [CO 2]l/[H2CO3]/ 223[HCO3 ]/[CO3 ], [H3PO4]/[H2PO4 ]/[HPO4 ]/[PO4 ], and + [H ]/[OH ]. Mass Transfer. Mass transfer of ozone and carbon dioxide occur in the ozone contactor. Mass transfer of carbon dioxide from gas to liquid phase may be described by the LewisWhitman two-film theory,

MCO2 ) (KLa)CO2{[CO2]sl - [CO2]l}

(1)

[CO2]sl ) (S)CO2[CO2]g

(2)

Here,

The mass transfer of ozone from the gas phase to the liquid phase in the above cases can be described using the following expression,25

{

MO3 ) (KLa)O3x1 + m [O3]sl -

[O3]l 1+m

}

(3)

Here,

[O3]sl ) (S)O3[O3]g

(4)

In eq 3, E ) x1+m is known as the mass-transfer enhance-

ment factor, which accounts for the fact that, in aqueous phases with rapid ozone demand, ozone transferred from the gas phase is partially consumed at the gas-liquid interface itself, i.e., before diffusion to the bulk liquid phase. In this expression, m ) (DO3kw)/(kL2). As per the equation recommended by Johnson and Davis,26 DO3 ) 1.71 × 10-9 m2‚s-1. Beltran et al.3 reported the value of kL to be 3.1 × 10-4 ms-1 for clean water, as in this case. As per Qiu et al.,27 for clean water, kw ) 2.0 s-1 at pH 12. Using the above values, m was calculated to be 0.035. Hence, at pH < 12, m , 1, and hence, eq 3 is reduced to the following simplified form provided pH < 12,

MO3 ) [KLa]O3{[O3]sl - [O3]l}

(5)

[KLa]O3 is dependent on the reactor characteristics and, hence, is determined experimentally. The mass-transfer coefficient for carbon dioxide [KLa]CO2 was estimated as follows. First, DCO2 is calculated from DO3 value using eq 6,2

DO3x(MW)O3

)

DCO2x(MW)CO2

(S)O3

(S)CO2

(6)

Then, [KLa]CO2 was estimated using eq 72

[KLa]O3 [KLa]CO2

)

xDO xDCO

3

(7)

2

Liquid-Phase Reactions. The rate of aqueous-phase ozone decomposition depends on a complex series of reactions involving radical species, as shown in Table 1. In the relatively pure aqueous phase considered here, the [OH-], [HCO3 ], and [CO32-] ion concentrations directly control aqueous-phase ozone decomposition. At low pH, and hence low [OH-] concentration, the initiation of ozone decomposition (eq 8 in Table 1) is hindered. Hence, ozone is expected to be more stable in pure water systems at lower pH. In systems with high scavenger concentration, scavenging reactions (e.g., eqs 14-16 in Table 1) will predominate, and hence, ozone decomposition will be slower as compared to systems with lower scavenger concentrations, where promotion reactions (e.g., eq 13, Table 1) will predominate. In summary, systems at lower pH and with higher scavenger concentration will afford more stability to aqueous ozone, as compared to systems at higher pH and lower scavenger concentrations. Modeling Procedure. The total number of species considered for modeling purposes was 21, (S1 to S21), i.e., [O3]g, [CO2]g, + 2[O3]l, [CO2]l, [H2CO3], [HCO3 ], [CO3 ], [H ], [OH ], [H32-], [PO 3-] [•O-], [•HO ], [•O-], ], [HPO PO4], [H2PO4 4 2 4 2 3 [•HO3], [•OH], [•CO3-], [H2O2], and [HO2-], respectively. The modeling procedure involved formulation of 21 partial differential equations (PDEs) describing the mass balance of each of the above species in the ozone contactor. Simultaneous solution of these PDEs resulted in the specification of concentration profiles of each species as a function of time and position along the reactor. The PDEs describing the mass balance of [O3]g and [CO2]g, i.e., species S1 and S2, are as follows,

( )

∂Si L ∂Si ) -Ug - M Si ∂t ∂x 1 - L

(8)

The PDE describing the mass balance of [O3]l and [CO2]l, i.e.,

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 111 Table 1. Reactions in an Ozone Contactor: Ozonation of Pure Water Containing Inorganic Carbon and Phosphate Buffering

112

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006

For S5 to S21:

species S3 and S4, is as follows,

∂Si ∂Si ∂2Si ) UL + DL 2 + MSi + (RP)Si - (RC)Si ∂t ∂x ∂x

Initial Condition (9)

For the other 17 species, i.e., S5-S21, the general form of PDE describing the mass balance is as follows,

∂Si ∂2Si ∂Si ) UL + DL 2 + (RP)Si - (RC)Si ∂t ∂x ∂x

(10)

The expressions for (RP)Si and (RC)Si are given in Table 2 for all species. The rate expressions in Table 2 describe species undergoing both irreversible and reversible reactions. Unlike in the case of irreversible reactions, where only one kinetic rate constant is required to describe species formation/degradation, rates for both the forward and backward reactions are required in the case of reversible reactions. However, for many reversible reactions, the forward and backward reaction rates may not be available, since such reactions are generally described by an equilibrium constant, which is the ratio of the forward to the backward reaction rate. The procedure adopted for incorporating an equilibrium relationship in the mass balance equations was to assign arbitrarily high forward and backward reaction rates, with the constraint that the ratio of the two rates be equal to the corresponding equilibrium constant. Under the circumstances, care must be taken to ensure that the arbitrary values chosen as above are at least an order of magnitude higher than the largest irreversible reaction rates that are encountered. This will ensure that concerned species related through equilibrium relationships remain in equilibrium even while participating in other irreversible reactions. The initial and boundary conditions for various PDEs were as follows,

For S1 and S2: Initial Condition 1. At t ) 0, Si ) 0, at all x Boundary Conditions 1. At t > 0, for S1, S1 ) S01 at x ) 0 for S2, S2 ) 0 at x ) 0 For S3 and S4: Initial Condition 1. At t ) 0, Si ) 0, at all x Boundary Conditions 1. At t > 0, dSi/dx ) 0, at x ) 0 2. At t > 0, for S3, dS3/dx ) -[UL/DL]S3, at x ) L for S4, S4 ) S04, at x ) L

1. At t ) 0, Si ) 0, at all x Boundary Conditions 1. At t > 0, dSi/dx ) 0, at x ) 0 2. At t > 0, for S5 - S13, Si ) S0i , at x ) L for S14 - S21, dSi/dx ) -[UL/DL]Si, at x ) L The 21 coupled PDEs described by eqs 8-10 were solved simultaneously, subject to the specified initial and boundary conditions to obtain species concentration profiles as functions of time and position in the reactor. This was achieved using a numerical partial differential equation solver called PDESOL.44 Simulation duration was from start of ozonation (t ) 0) to the time steady state was reached (t f ∞). Simulation Results. The model described above was used to simulate the operation of a 3.0 m tall ozone contactor of 25 mm diameter operated in the countercurrent mode. The influent water and gas flow rate into the contactor were specified, as was the influent gaseous ozone concentration. The chemical quality of the water influent to the contactor was also specified. This involved specification of the pH and the concentration of the inorganic phosphate and inorganic carbon species in the influent water. Results obtained from a typical simulation run are presented in Figure 1. Starting from the specified initial values, the concentration profiles of all species were observed to reach a steady state within 1000 s after start of reactor operation. The impact of changes in the influent water quality, i.e., changes in influent pH and (CT)C on the aqueous and gaseous ozone profiles, and other relevant parameters in the contactor was simulated. These simulation results are presented in Figure 2. Simulations were carried out at three pH values, 9, 10, and 11. At each pH value, simulations were carried out at three inorganic carbon concentrations, (CT)C, of 10-5, 10-3, and 10-1 M, respectively. Only steady-state results are presented. The inorganic phosphate concentration, (CT)P, was 0.05 M in all cases. The results are presented in three sets in Figure 2, with parts A-E, parts F-J, and parts K-O corresponding to simulations at influent (CT)C values of 10-5, 10-3, and 10-1 M, respectively. For all sets, [O3]l was observed to decrease with an increase in influent aqueous pH (compare parts A, F, and K of Figure 2). This is per expectations, because the initiation of ozone decomposition (eq 8, Table 1) is strongly pH-dependent. Lower [O3]l values resulted in more ozone transfer from the gaseous to the aqueous phase, as seen by the steeper decline in [O3]g values in simulations carried out at higher pH values (compare parts B, G, and L of Figure 2). Despite the presence of phosphate buffer, i.e., influent (CT)P ) 0.05 M, simulation results show subtle pH variations along the contactor height during ozonation (compare parts C, H, and M of Figure 2). In the simulation with influent pH 9 and influent (CT)C 10-1 M (see Figure 2M), pH was observed to increase as water traveled down the contactor. The reason for such pH increase is thought to be the stripping of aqueous [CO2]l to the gaseous phase in the contactor, resulting in the decrease of (CT)C value in the contactor (see Figure 2N). This resulted in the rearrangement of the carbonate equilibrium, with pH increasing to compensate for the decline in aqueous [CO2]l. For some simulations at pH

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 113 Table 2. Rate of Production and Consumption of Various Species through Reactions in the Contactor no.

species [S]i

1. 2. 3. 4. 5. 6. 7. 8. 9. 10 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

[O3]g [CO2]g [O3]l [CO2]l [H2CO3] [HCO3] [CO32-] [H+] [OH-] [H3PO4] [H2PO4] [HPO42-] [PO43-] [•O2] [•HO2] [•O3] [•HO3] [•OH] [•CO3] [H2O2] [HO2] The Rate Expressions: Rmf ) kfm[CO2]l R2f ) kf2[HCO3] R4f ) kf4[H3PO4] R6f ) kf6[HPO42-] + R8b ) kb8[•O2 ][H ] R12 ) k12[O3]l[•OH] R16f ) kf16[H2O2] • R19 ) k19[HO2 ][ OH] R23 ) k23[•OH][PO43-] • R27 ) k27[•CO3 ][ CO3 ] R31 ) k31[•CO][H O 2 2] 3

rate produced inside reactor (RP)Si

rate consumed inside reactor (RC)Si

0 0 R28 Rmb Rmf + R1b R1f + R2b + R15 + R30 + R31 R2f + R28 + R29 R1f + R2f + R3f + R4f + R5f + R6f + R8f + R16f R3f + R14 + R22 R4b R4f + R5b R5f + R6b R6f R8f + R19 + R30 R8b + R12 + R17 + R18 + R31 R9 + R17 R10 R11 R13 + R14 R16b + R21 + R22 R7 + R16f

0 0 R7 + R9 + R12 + R17 Rmf Rmb + R1f R1b + R2f + R13 R2b + R14 R1b + R2b + R3b + R4b + R5b + R6b + R8b + R16b + R10 R3b + R7 R4f + R26 R4b + R5f + R25 R5b + R6f + R24 R6b + R23 R8b + R9 + R22 + R29 R8f + R20 + 2.R21 + R22 R10 + R28 R11 R12 + R13 + R14 + R15 + R18 + R19 + R20 + R23 + R24 + R25 + R26 R15 + 2.R27 + R28 + R29 + R30 + R31 R16f + R18 + R31 R16b + R17 + R19 + R30

Rmb ) kbm[H2CO3] R2b ) k2b[CO32-][H+] + R4b ) kb4[H2PO4 ][H ] R6b ) kb6[PO43-][H+] R9 ) k9[•O2 ][O3]l • R13 ) k13[HCO3 ][ OH] +] R16b ) kb16[HO][H 2 R20 ) k20[•HO2][•OH] R24 ) k24[•OH][HPO42-] • R28 ) k28[•CO3 ][ O3 ]

10 and 11 (see parts C and H of Figure 2), a decrease in pH was observed as water traveled down the contactor. This may be attributed to an increase in [OH-] ion consumption in the contactor by aqueous ozone at higher pH, which overwhelms the tendency of pH increase described earlier. Simulation results presented in parts D and I of Figure 2 indicate considerable aqueous [CO2]l stripping in the contactor at influent aqueous pH 9 and 10, while results presented in Figure 2N indicate considerable aqueous [CO2]l stripping in the contactor only at influent aqueous pH 9. The impact of influent (CT)C value on [O3]l profiles obtained at a fixed influent aqueous pH is complicated by pH variation along the contactor height. For example, comparison of the [O3]l profiles obtained at pH 10 and influent (CT)C values of 10-5, 10-3, and 10-1 M (see parts A, F, and K of Figure 2) indicates that, contrary to expectations, [O3]l is lower at (CT)C ) 10-1 M (see Figure 2K) as compared to (CT)C ) 10-3 M (see Figure 2F). This can be explained by considering the pH decrease observed along the reactor height at (CT)C ) 10-3 M (see Figure 2H), which is likely to make aqueous ozone more stable (see eq 8, Table 1). Simulation of [•OH] radical profiles in the contactor indicate that these concentrations vary strongly with influent (CT)C. [•OH] radical concentration is in the range of 10-10 to 10-11 M, 10-11 to 10-12 M, and 10-13 to 10-14 M at influent (CT)C values of 10-5, 10-3, and 10-1 M respectively (compare parts E, J, and O of Figure 2). This is as per expectations, because at higher (CT)C values, scavenging reactions (eqs 14-16, Table 1) become important, thus depressing the [•OH] radical concentration in the contactor. The impact of influent aqueous pH on [•OH] radical profiles obtained at a fixed influent (CT)C value is complicated by the fact that, in many cases, (CT)C varies along

R1f ) kf1[H2CO3] R3f ) kf3 R5f ) kf5[H2PO4] R7 ) k7[O3]l[OH-] R10 ) k10[H+][•O3] R14 ) k14[CO32-][•OH] R17 ) k17[O3]l[HO2] R21 ) k21[•HO2][•HO2] R25 ) k25[•OH][H2PO4] • R29 ) k29[•CO3 ][ O2 ]

+ R1b ) kb1[HCO3 ][H ] R3b ) kb3[H+][OH-] R5b ) kb5[HPO42-][H+] R8f ) kf8[•HO2] R11 ) k11[•HO3] • R15 ) k15[•CO3 ][ OH] R18 ) k18[H2O2][•OH] R22 ) k22[•HO2][•O2] R26 ) k26[•OH][H3PO4] R30 ) k30[•CO3 ][HO2 ]

the contactor height. For example in Figure 2O, [•OH] radical concentration is lower in reactors operated at higher influent pH values. This is contrary to expectations but may be explained by noting that (CT)C concentrations in the reactor at pH 9 and pH 10 are lower than influent (CT)C because of the stripping of [CO2]l. This results in less scavenging of [•OH] radicals by inorganic carbon, resulting in maintenance of a higher [•OH] radical concentration in the reactor in these cases. Model Verification The simulation results were verified through comparison with experimental results. The experimental apparatus used for this purpose consisted of the following components.: tall bubblecolumn ozone contactor, oxygen cylinder, ozone generator, mass-flow controller, peristaltic pump, and gas-phase ozone monitors. Apparatus Description. The ozone contactor was made of seven prefabricated cylindrical “borosil” glass sections with male and female ground glass joint. The diameter of the sections was 25 mm. This diameter was large enough to eliminate any wall effects, such that the liquid flow through the column cross section could be adequately represented by an average velocity value. Each glass section was ∼500 mm in length and had a sampling port approximately in the middle. Five sections used in the intermediate part of the column were identical, while the bottom section was equipped with a gas inlet, a liquid outlet, a liquid drain, and a porous sintered glass plate for supporting the liquid column and gas bubble formation. The top section had arrangements for gas outlet and liquid inlet. The seven glass sections were attached together and tightly secured to a frame

114

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006

Figure 1. Numerical simulation of the attainment of steady state by various species during reactor operation: Qg ) 500 mL min-1; Ql ) 10 mL min-1; [S]O3 ) 0.19; [S]CO2 ) 0.80; [KLa]O3 ) 0.025 s-1; [KLa]CO2 ) 0.052 s-1; L ) 0.9; A ) 4.91 × 10-4 m2; DL ) 1.37 × 10-4 m2‚s-1; influent pH ) 10; T ) 25 °C; [O3] og ) 41 mg/L; influent [CT]C ) 1 × 10-1 M; influent [CT]P ) 5 × 10-2 M; H ) 3.0 m. Steady state was reached in all simulations within 15-20 min. ∆ charge should remain approximately constant, irrespective of time and location in the column.

by metal clamps. The final assembled column has seven sampling ports at 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 m height from the bottom. A schematic of the experimental setup is shown in Figure 3. Ozone was generated in the gas phase by passing pure oxygen from an oxygen cylinder through an ozone generator (Ozomat GM-6000-OEM, Anseros, Germany). This gas mixture was

applied to the bottom of the reactor, where it bubbled through the porous ceramic plate and moved upward through the reactor. The gas flow into the reactor was controlled using an on-line mass-flow controller (MFC; Aalborg, GFC171S, U.S.). Ozone concentration in the gas influent to the reactor was measured using an on-line ozone monitor (Anseros, Ozomat GM-6000OEM, Germany). Arrangements were also made for measuring

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 115

Figure 2. Simulation of reactor characteristics at pH 9-11 and total inorganic concentration between 10-5 and 10-1 M: Qg ) 500 mL min-1; Ql ) 10 mL min-1; [S]O3 ) 0.19; [S]CO2 ) 0.80; [KLa]O3 ) 0.025 s-1; [KLa]CO2 ) 0.052 s-1; L ) 0.9; A ) 4.91 × 10-4 m2; DL ) 1.37 × 10-4 m2‚s-1; [O3]og ) 41 mg/L; H ) 3.0 m; T ) 25 °C; influent [CT]P ) 5 × 10-2 M). (A-E) influent [CT]C ) 1 × 10-5 M; (F-J) influent [CT]C ) 1 × 10-3 M; and (K-O) influent [CT]C ) 1 × 10-1 M.

Figure 3. Schematic of the experimental setup.

the ozone concentration in the gas effluent from the reactor using another similar ozone monitor. Before entering the second ozone monitor, the outlet gas was routed through a water trap to remove any entrained water droplets. Water was introduced into the reactor from the top using a peristaltic pump (IKA PA MCP,

IKA Labortechnik, Germany). Water was also extracted at the same rate from the bottom of the reactor using a pinch valve at the liquid outlet, thus maintaining a continuous liquid flow in the reactor. All components of the experimental setup and the reactor were made of glass, Teflon, or stainless steel to ensure

116

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006

Table 3. Experimental Conditions for Determination of Steady-State Aqueous Ozone Profiles and Corresponding Effluent Gaseous Ozone Concentrations pH 9.0 10.0 10.5 11.0

Qg ) 500 mL/min, Ql ) 10 mL/min, 50 mM phosphate buffer exp. no. 5 [(CT)c ) 1 × 10-5 M] no. 3 [(CT)c ) 1 × 10-5 M] exp. no. 1 [(CT)c ) 1 × 10-1 M] exp. no. 8 [(CT)c ) 1 × 10-5 M]

exp. no. 4 [[(CT)c ) 1 × 10-1 M] exp. no. 2 [(CT)c ) 1 × 10-2 M]

exp. no. 6 [(CT)c ) 0.0] exp. no. 7 [(CT)c ) 0.00]

that ozone consumption due to corrosion of components by ozone leading the erroneous experimental results is fully eliminated. Reactor Characterization. Preliminary experiments using the above contactor involved determination of liquid-phase holdup (L), liquid-phase dispersion coefficient (DL), and masstransfer coefficient for ozone (KLa)O3. Description of these preliminary experiments may be found elsewhere.45 Values of L, DL, and (KLa)O3 for the contactor were 0.9, 1.37 × 10-3 m2‚s-1, and 0.025 s-1, respectively. The value of (KLa)CO2 was calculated to be 0.052 s-1 using eqs 6 and 7. The values of SO3 and SCO2 were 0.19 and 0.80, respectively, at the experimental temperature of 25 °C. Experimental Procedure. All experiments involved the following initial steps. First, oxygen flow was started at the desired rate by adjusting the MFC appropriately. The influent

and effluent ozone concentration, as measured by the on-line ozone monitors was also set to zero at this time. Second, the reactor was filled up with water by employing the peristaltic pump in the “fast” mode. Third, a continuous liquid flow at the desired rate was established through the reactor by adjusting the settings of the peristaltic pump in the influent end and of the pinch cork at the effluent end. Care was taken to ensure at this point that both gas and liquid flow rates through the reactor were steady and of the value desired, i.e., Qg ) 500 mL min-1 and Ql ) 10 mL min-1. Experiments involving determination of steady-state ozone profiles in the ozone contactor were carried out with the aqueous phase buffered at pH 9, 10, 10.5, or 11 with 0.05 M phosphate buffer and various inorganic carbon concentrations as shown in Table 3. After establishment of continuous water and oxygen flow at predetermined rates, the ozonator was turned on. This resulted in the increase of influent gaseous ozone concentration, as determined by the ozone monitor, from zero to an increased, but approximately constant, value. To ensure that experimental data was collected under steady-state conditions, the reactor was allowed to operate for ∼45 min before sample collection. Model simulations results presented in Figure 1 indicate that steady state should be reached in 15-20 min. Next, aqueous ozone concentration was measured at various sampling ports of the reactor. For recording the aqueous ozone concentration, a fixed volume of liquid sample was extracted from the column using the syringe attached to a sampling port and injected into a 100

Figure 4. Comparison of experimental and simulated aqueous ozone concentration profiles under various conditions: Qg ) 500 mL min-1; Ql ) 10 mL min-1; [S]O3 ) 0.19; [S]CO2 ) 0.80; [KLa]O3 ) 0.025 s-1; [KLa]CO2 ) 0.052 s-1; L ) 0.9; A ) 4.91 × 10-4 m2; DL ) 1.37 × 10-4 m2‚s-1; [O3] og ) 41 mg/L; H ) 3.0 m; T ) 25 °C; influent [CT]P ) 5 × 10-2 M.

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 117

mL measuring flask containing indigo trisulfonate solution, as required for aqueous ozone determination. Measurements were done in triplicate for each port. Similar aqueous ozone concentrations in triplicate samples at a port in almost all cases indicated repeatability of results and attainment of steady-state conditions in the column. The aqueous ozone profile generated during each experiment thus has seven data points, corresponding to the seven sampling ports in the column. Influent and effluent gaseous ozone concentrations were also measured at 5 min interval during this time and were found to be relatively constant over the experimental time period. Analytical Methods. Aqueous ozone was measured by the Indigo method (4500-O3-A46,47), with the final absorbance of the Indigo solution being measured spectrophotometrically (Systronics 106, India) using a 4-cm-path-length quartz absorbance cell. In some cases, where interference was not expected, aqueous ozone concentration was directly measured by determining the UV absorbance of the aliquot containing aqueous ozone at 260 nm using a UV spectrophotometer (Cary 50 CONC, Varian) equipped with a 1-cm absorbance cell (Borosil). Multiplication of this value by a factor of 14.59 (ref 2) gave the ozone concentration in mg/L. Gaseous ozone was directly measured using a UV-absorbance-based ozone-monitoring device (Anseros, Ozomat GM-6000-OEM, Germany). pH was measured using a combination pH electrode (Toshniwal CL51, India) connected to a digital pH meter (Toschcon CL-54, India). Comparison of Simulation and Experimental Results. Experimentally obtained steady-state aqueous ozone concentration profiles obtained during Experiments 1-8 in the pH range of 9-11 and the (CT)C value range of 0-10-1 M were compared with the corresponding simulated profiles. Such comparisons are shown in Figure 4. To further check the degree of agreement between the measured and simulated values, all experimental values measured at various column heights and under various conditions were plotted against the corresponding simulated values (see Figure 5A). A linear regression line (forced through the origin) between these values had a slope of 1.00, while the value of the correlation coefficient was 0.905, suggesting that the match between the experimental and the corresponding simulated values was good. Unfortunately, the model could not be verified at pH values