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Continuous Process for Photodegradation of Industrial Bayer Liquor V. K. Pareek, M. P. Brungs, and A. A. Adesina* Reactor Engineering and Technology Group, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney NSW 2052, Australia
Photodegradation of spent Bayer liquor has been carried out in an 18-L pilot-scale continuous annular photoreactor. The performance of the reactor was studied with respect to various process parameters such as the gas flow rate, liquid flow rate, initial solution pH, initial substrate concentration, light intensity, and catalyst loading. The average reaction rate and overall conversion of Bayer liquor, in terms of total organic carbon, have been found to be strongly dependent on all of these parameters. Despite the very dark color and high initial pH (ca. 12) of the reaction mixture, significant conversion (25-35%) was achieved within about 2 h of operation, showing a considerable improvement over the existing industrial oxalate removal methods. Notwithstanding the complex composition of the spent Bayer liquor, the rate data could adequately be analyzed in terms of simple linear or hyperbolic-type kinetic expressions. 1. Introduction
Table 1. Typical Composition of Spent Bayer Liquor
The caustic digestion of bauxite during the Bayer process for alumina production is inevitably accompanied by impurities such as sodium salts (oxalate and carbonate) which interfere with downstream operations, namely, precipitation, crystallization, and calcination. Although sodium carbonate may be readily recovered with lime to yield NaOH and CaCO3, the removal of sodium oxalate frequently involves high-energy-demanding operations such as liquor calcination and the MnO2 process. This results in substantial loss of the original NaOH solution used in the upstream digestion unit. The removal of sodium oxalate from Bayer liquor, and, if possible, recovery of caustic from it, is therefore vital for plant economics especially in Australia where the industry is the country’s third largest export earner. Moreover, because of its toxic nature, sodium oxalate is a major pollutant from the alumina refineries. Therefore, from both economic and environmental perspectives, it is desirable to regenerate the caustic solution from the spent Bayer effluent for further reuse. In the last 2 decades, photocatalytic decomposition of organic pollutants in industrial waste has received increasing attention.1,2 Successful mineralization of many organic pollutants such as herbicides, pesticides, phenols, and surfactants, which were previously considered to be difficult to remove, has been reported.1,3 Indeed, in previous studies,4,5 we demonstrated that the photooxidative degradation of a synthetic sodium oxalate solution using titania (anatase) as the catalyst resulted in increased alkalinity with complete conversion in about 2 h. However, industrial spent Bayer liquor is a thick dark brown viscous solution containing not only sodium oxalate but also a wide matrix of other species. Typical compositions of Bayer liquor are shown in Table 1. The dark color of the reaction mixture and the presence of other ions may affect the performance of the photoreactor when fed with actual spent Bayer liquor. * Corresponding author. E-mail:
[email protected]. Phone: +61-2-9385 5268. Fax: +61-2-9385 5966.
constituent
composition (g‚L-1)
sodium hydroxide sodium carbonate sodium chloride sodium sulfate TOC (mostly as sodium oxalate) sodium aluminate pH
140 50 30 30 30 150 ≈14 (dimensionless)
Further, the majority of the work in the open literature relates to photodegradation studies in a laboratoryscale reactor, and only a limited number of large-scale studies have been reported.6 While considerable work has been done to characterize the effects of variables such as the gas flow rate, catalyst loading, light intensity, pH, oxidant, and substrate concentration in small laboratory-scale reactors,7,8 the situation is even more complex in bigger photoreactors. In the latter, significant spatial variation in UV radiation, species concentration, velocity, and extinction and scattering effects requires a fundamental study in order to optimize the reactor performance.9 Moreover, the scale-up of a multiphase (gas-liquid-solid) photoreactor is, at present, not well understood. In this work, we have carried out a continuous photodegradation of industrial spent Bayer liquor in an 18-L pilot-scale annular bubble column photocatalytic reactor. An attempt has been made to keep the experimental conditions as close to industrial conditions as possible by using industrial spent Bayer liquor from an Australian alumina refinery and ambient air as the oxidizing gas. 2. Experimental Details 2.1. Materials and Methods. Industrial spent Bayer liquor was donated by a commercial alumina refinery. Titania (>99% anatase) was obtained from Aldrich Chemicals and used as supplied in all runs. Ordinary domestic water was used to dilute the industrial Bayer liquor. HCl was used to adjust the pH of the slurry prior to reaction. Ultrapure N2 gas (external surface of the
10.1021/ie0010058 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/19/2001
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Figure 1. Schematic of a pilot-scale reactor.
lamp and removal of ozonized air) was obtained from BOC Gases, Sydney, Australia. Ambient pressurized air was used as the feed gas for the reactor. Continuous pH monitoring was obtained from a TPS Digital pH meter. The light intensity was measured by an IL1400 radiometer/detector (International Light, Newburyport, MA) calibrated for 265-332 nm. Steady-state aliquots were taken for different liquid and gas holdup times and analyzed for total carbon (TC) and total organic carbon (TOC) on a Shimadzu TOC analyzer 5000. 2.2. Apparatus. A schematic of the 18-L reactor is shown in Figure 1. The outer chamber is a stainless steel vessel (i.d. ) 20 cm), inside which a commercial UV lamp is suspended from the top flange through double O-ring seals. The UV lamp was enclosed in a double-walled quartz hollow U tube (o.d. ) 4 cm) through which water flows at 1.5 L‚min-1 as a coolant to maintain reaction isothermality and to remove an IR fraction of the incident radiation. Ozonized air was continuously flushed out of the inner U tube with N2. Monitoring ports for pH, temperature, and the telescopic UV detector/radiometer are provided at indicated locations. Iwaki magnetic pumps were used for all liquid delivery (water and diluted Bayer liquor), while electronic mass flow controllers metered all gas flow rates.
The reactor system could incorporate the recycle of the product stream. However, in this study the rig was operated in single pass (without recycle). Air from the flow controller was passed upward through a moving column of Bayer solution containing suspended titania particles via a 70-µm stainless mesh distributor. Liquid and gas flow rates ranged from 0.2 to 0.5 L‚min-1 (τL ) 30-90 min) and from 1.0 to 10.0 L‚min-1 (τG ) 2-18 min). 3. Results and Discussion 3.1. Effect of Air and Liquid Flow Rates. Operation of a multiphase reactor with a cocurrent upward flow of both liquid and gas could give rise to different hydrodynamic regimes, namely, bubble (pseudohomogeneous), slug, annular, and mist flow regimes, depending on the magnitude of the liquid and gas flow rates, as well as physical properties of fluids. The void fraction, , may be obtained from10
)
G G + L + Aub
(1)
where G and L are the gas and liquid volumetric flow
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Figure 2. Effect of air flow. Initial TOC concentration ) 700 ppm, liquid flow rate ) 0.2 L‚min-1, liquid residence time ) 90 min, catalyst concentration ) 1.0 g‚L-1, lamp power ) 200 W, average light intensity ) 0.60 × 10-6 einstein‚m-2‚s-1, and initial pH ) 11.75.
Figure 3. Integral kinetic analysis. Initial TOC concentration ) 700 ppm, air flow rate ) 5.0 L‚min-1, lamp power ) 200 W, average light intensity ) 0.60 × 10-6 einstein‚m-2‚s-1, and initial solution pH ) 11.75. Table 2. Values of the Rate Constants
rates, respectively, A is the reactor cross-sectional area, and ub is the bubble rise velocity given by
ub ) 0.711xgD
(2)
where D is the diameter of the distributor holes and g is the acceleration due to gravity. For a porous plate distributor and the combination liquid and gas flow rates employed in this study, eqs 1 and 2 yield ) 0.030.13. Thus, the situation may be considered as a bubble flow regime. In bubble flow operation, the gas is dispersed as small bubbles throughout the continuous liquid phase. Figure 2 shows the effect of the air flow rate on the TOC degradation rate at a fixed liquid flow rate of 200 mL‚min-1. It is apparent that poor hydrodynamics may confound the reaction kinetics at air flow rates below about 5 L‚min-1. Wei and Wan7 attributed a similar observation to the dominance of mass-transfer resistance at low gas flow rates in a smaller reactor (500mL capacity) containing a stationary liquid column. For plug flow of liquid in the reactor, we have
V ) CTOC0 v0
∫0X
dXTOC -rTOC
TOCf
(3)
Although the kinetics is as yet unknown, because exit conversions were obtained for various liquid flow rates and catalyst loadings, the data were assessed against a first-order rate law, which when introduced to eq 3 gave
-ln(1 - XTOCf) ) κτL
(4)
The data displayed in Figure 3 were fitted to eq 4 to obtain estimates of k for three different catalyst loadings examined. The values of the rate constants so obtained are listed in Table 2. 3.2. Effect of the TOC Concentration (or the Dilution Ratio). Bayer liquor is a complex mixture of aliphatic and aromatic (humic) compounds; thus, the organic concentration was characterized in terms of the TOC. Work with pure organic substrates revealed a diversity of the rate behavior with respect to the organic concentration. Matthews and McEvoy11 as well as Trillas et al.12 observed that the photocatalytic oxidation rate of phenol was inhibited by the initial phenol concentration, while Inel and Okte13 observed that the degradation of malonic acid followed a LangmuirHinshelwood model. To investigate the effect of the TOC concentration, the solutions containing varying levels
catalyst loading (g‚L-1)
k (s-1)
1.0 2.0 3.0
2.83 × 10-5 2.67 × 10-5 2.17 × 10-5
Figure 4. Reaction rate vs initial TOC concentration. Air flow rate ) 5.0 L‚min-1, lamp power ) 200 W, average light intensity ) 0.60 × 10-6 einstein‚m-2‚s-1, initial solution pH ) 11.75, and liquid holdup time ) 90 min.
of TOC content were prepared by diluting the industrial spent Bayer liquor with municipal water. Dilution ratios (defined as the volume of water added to the volume of Bayer liquor) between 20 and 40 were used. A dilution ratio lower than 20 was not used because the Iwaki magnetic pump used could not operate effectively for the highly viscous and corrosive solution obtained below this ratio. As may be seen from Figure 4, the rate of reaction increased almost linearly with the initial TOC content at all three catalyst loadings. This substantiates the pseudo-first-order rate assumption previously used in the analysis of liquid flow residence time-conversion data. 3.3. Effect of the Initial Solution pH. Photocatalytic reactions, in general, proceed via redox-type mechanisms; thus, the pH of the reaction media may impact kinetics because of the catalyst surface charge modification. Figure 5 shows that the rate of photodegradation decreased almost exponentially with an increase in the initial solution pH. This is consistent with the previous investigation on photocausticization of pure aqueous sodium oxalate. Probably because of the presence of other ions and organic acids in the Bayer liquor, the dependency on pH was not as strong as that found for synthetic sodium oxalate.14 Mathematically, the effect of pH on the reaction rate can be described as
-rjTOC ) ce-b(pH)
(5)
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5123 Table 3. Values of Parameters in Eq 6 holdup time (min)
k1 (mol‚L‚g of catalyst-2‚s-1)
K2 (L‚g of catalyst-1)
60 75 90
1.52 × 10-6 1.20 × 10-6 9.72 × 10-7
1.919 1.699 1.544
Figure 5. Effect of initial solution pH. Initial TOC concentration ) 700 ppm, air flow rate ) 5.0 L‚min-1, liquid flow rate ) 0.2 L‚min-1, lamp power ) 200 W, and average light intensity ) 0.60 × 10-6 einstein‚m-2‚s-1.
Figure 7. Effect of lamp power. Initial TOC concentration ) 700 ppm, air flow rate ) 5.0 L‚min-1, liquid holdup time ) 90 min, and initial solution pH ) 11.75.
ship between the two variables.12,14 It would seem that the phenomenon is a function of the reactor geometry, average light intensity,15 catalyst loading, and rate of electron-hole recombination. The data shown in Figure 7 were modeled by a general power law expression, viz. Figure 6. Effect of catalyst concentration. Initial TOC concentration ) 700 ppm, air flow rate ) 5.0 lit.min-1, initial solution pH ) 11.75, lamp power ) 200 W, and average light intensity ) 0.60 × 10-6 einstein‚m-2‚s-1.
Linear regression of the data in Figure 5 gave c ) 3.077 × 10-6 mol‚g of catalyst-1‚s-1 and b ) 0.2181. However, the dependence on the pH in the present case (with the Bayer liquor) was found to be somewhat weaker than the experiments with pure sodium oxalate,14 where the corresponding value of b was 0.3150. 3.4. Effect of the Catalyst Loading. The effect of the catalyst loading on the reaction rate was also investigated. The runs were conducted with different values of catalyst loading (0.5-3.0 g‚L-1) while keeping other parameters constant (initial TOC concentration ) 5.225 × 10-3 mol‚L-1 (700 ppm), initial solution pH ) 11.75, lamp power ) 200 W). As shown in Figure 6, initially the reaction rate increased with the catalyst concentration at the low end but attained a maximum at about 1.0 g‚L-1 irrespective of the liquid flow rates. This behavior may be attributed to the shielding effect caused by titania particles at increased catalyst loadings.7 The dependence of the degradation rate on the catalyst loading may be written as
-rjTOC )
k1Ccat (1 + K2Ccat)2
-rjTOC ) aIβ
where a is an irradiation rate constant dependent on the TOC concentration and catalyst loading, because both affect the light absorption and scattering characteristics of the reaction medium. Parameter estimates for a and β are displayed in Table 3, from where it is evident that β, the “order” with respect to the light intensity, varied from nearly unity (0.88) at low catalyst loading to essentially the square root (0.47) at high values (beyond the optimum catalyst loading). We believe that, at high catalyst loading, electron-hole recombination becomes more dominant; thus, the square root dependency is symptomatic of the change in the rate-controlling step (from a unimolecular to a bimolecular step). The photogeneration of active sites is a unimolecular step, while the termination step for electron-hole recombination is a bimolecular one. 3.6. Development of a Global Rate Equation. On the strength of the foregoing discussion, a composite rate equation, useful for describing the reaction behavior to all of the variables, may be written by pooling together the individual expressions previously found to be adequate. This yields
-rjTOC ) akrxne-b(pH)IβCTOC (6)
where k1 is a kinetic constant with respect to catalyst loading and K2 is a parameter indicative of particleparticle interaction. The values of k1 and K2 were found by linear regression on the data reported in Figure 6 and are shown in Table 3. 3.5. Effect of the Light Intensity. The effect of the light intensity during photocatalysis has been discussed by a number of researchers.15 While in some cases the rate of reaction exhibited a square root dependency on the light intensity,7,8 others observed a linear relation-
(7)
Ccat (1 + K2Ccat)2
(8)
where krxn is a model parameter and the best fit value of which was found to be 1.91 × 104 L2‚mol-1‚g of catalyst-1 and b ) 0.2181; values of K2 are listed in Table 3 and those of a and β in Table 4. 3.7. Quantum Yield, Process Efficiency, and Electricity Cost. The quantum yield can be defined as the ratio of the number of molecules of organic species degraded to the number of photons absorbed
φoverall )
rate of reaction (mol‚s-1) rate of photon absorption (einstein‚s-1)
(9)
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Nomenclature
Table 4. Values of Parameters in Eq 7 catalyst loading (g of catalyst‚L-1)
a [mol‚g of catalyst-1‚s-1 (einstein‚m-2‚s-1)-β]
β
1.0 1.5 2.0
7.38 × 10-2 2.23 × 10-3 1.47 × 10-4
0.8854 0.6395 0.4701
Figure 8. Energetic efficiency of degradation (thin curves) and cost of electricity (per kg of TOC degraded; thick curves) as a function of the lamp power. Initial TOC concentration ) 700 ppm, air flow rate ) 5.0 L‚min-1, liquid holdup time ) 90 min, and initial solution pH ) 11.75.
In contrast with homogeneous photochemical reactions, in heterogeneous photocatalytic reactions it is experimentally difficult to find out the exact number of photons absorbed.16 However, for industrial applications it is more appropriate to use energetic efficiency of degradation (EED) defined by Serpone16
EED )
amount of TOC degraded (g) power consumed (kWh)
(10)
The calculated values of EED are shown in Figure 8. It is clear from the figure that the values of EED increased with an increase in the catalyst loading and decreased with an increase in the lamp power. The corresponding operating costs (only electrical energy) are also shown in Figure 8. The cost of electricity (based on the current cost in New South Wales, AustraliasU.S.$0.05 kWh-1) may vary between U.S.$2.0 and U.S.$9.0/kg of TOC degraded (which is equivalent to U.S.$0.06-0.27/L of feed) depending on the catalyst loading and lamp power used. 4. Conclusion The photocatalytic degradation of industrial Bayer liquor was carried out in an 18-L pilot-scale continuous annular photoreactor. Despite the complexity of the feed composition, the overall rate has a linear dependency on the TOC concentration. However, optimum catalyst loading was obtained at 1 g of catalyst.L-1. The effect of the light intensity varied from a linear to square root relationship depending on the catalyst loading. Reasonably high reaction rates were obtained even with the highly alkaline feed (pH ) 12) used, although the rate decreased almost exponentially with slurry pH. Acknowledgment The authors acknowledge the financial support provided by the Australian Research Council (ARC).
a ) parameter defined in eq 7, mol‚g of catalyst-1‚s-1 (einstein‚m-2‚s-1)-β A ) cross-sectional area of the reactor, m2 b ) parameter in eqs 5 and 8 c ) parameter in eq 5, mol‚g of catalyst‚s-1 CTOC0 ) initial TOC concentration, mol‚L-1 CTOC ) TOC concentration, mol‚L-1 Ccat ) catalyst concentration, g‚L-1 D ) diameter of the distributor holes, m EED ) energetic efficiency of degradation, g‚kWh-1 g ) acceleration due to gravity, m‚s-2 G ) gas (air) flow rate, m3‚s-1 I ) average light intensity in the reactor, einstein‚m-2‚s-1 k ) rate constant based on per unit volume of solution, s-1 k1 ) constant in eq 6, mol‚L‚g of catalyst-2‚s-1 K2 ) constant in eqs 6 and 8, L‚g of catalyst-1 L ) liquid volumetric flow rate, m3‚s-1 -rTOC ) rate of the reaction based on per unit volume of solution, mol‚L-1‚s-1 -rjTOC ) average rate of the reaction based on per unit weight of catalyst, mol‚g of catalyst-1‚s-1 ub ) bubble rise velocity, m‚s-1 V ) volume of the reactor, L v0 ) volumetric flow rate of the reaction mixture, L‚s-1 XTOC ) conversion of TOC, % XTOCf ) final conversion of TOC, % Greek Letters φoverall ) quantum yield (mol‚einstein-1) ) void fraction β ) parameter in eqs 7 and 8 τG ) holdup time of the gas in the reactor, s τL ) holdup time of the liquid in the reactor, s
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Received for review November 30, 2000 Revised manuscript received March 13, 2001 Accepted March 28, 2001 IE0010058
