Title:
PERFORMANCE EVALUATION OF FLAT PLATE
SOLAR
REACTOR
PHOTOCATALYTIC TREATMENT
OF
FOR DYE
TEXTILE
TiO2
ASSISTED
DEGRADATION EFFLUENTS
IN
UNDER
NATURAL SUNLIGHT Abstract: Heterogeneous photocatalysis is an advanced oxidation process (AOP), which can be successfully used to oxidize many organic pollutants present in aqueous systems. Photocatalytic degradation has proven to be an effective method for mineralizing commercial dyes. Textile industries produce large volume of coloured dye effluents which are toxic and non-biodegradable. Dyes have serious environmental effects due to both the toxicity of compounds and the colouring of water bodies. Dyes have high chemical stability and they do not get completely degraded by conventional methods. Application of effective technology to achieve complete textile dye degradation is a challenge which can be met with TiO2 Assisted heterogeneous photocatalysis. TiO2 assisted photocatalytic process is of special interest, since it can use natural sunlight. Performance evaluation of flat plate reactor is an important issue from the viewpoint of scaling-up of the process. Robust performance evaluation needs to take into account varying irradiation conditions and different operating parameters. In this paper, we have investigated the influence of dye concentration and incident light intensity on the decolourisation kinetics of Methyl Orange under natural sunlight. Apparent rate constant has been explored as an
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appropriate performance factor for evaluation of flat plate collector under study. The performance data has been collected by conducting degradation reaction trials under variable irradiance conditions on a falling film flat plate reactor under natural sunlight. Apparent rate constant has also been correlated to catalyst loading and optimal catalyst loading is found for Methyl Orange degradation.
Keywords: Heterogeneous photocatalysis, Solar Reactor, Textile Effluents, Dye Photodegradation, TiO2, Natural sunlight, Irradiation Intensity, Apparent Reaction rate constant. Authors: 1. Suhasini Desai Department of Mechanical Engineering Maharashtra Institute of Technology Pune 411038, Maharashtra, India
[email protected] Phone: 020 30273400 Mobile: 94223 28201 2. G. S. Tasgaonkar Dyanganga Institute of Technology Pune 411038, Maharashtra, India 3. Neela R. Rajhans Dept of Production Engineering Govt College of Engineering
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Pune 411004, Maharashtra, India 4. Datta Dandge Department of Petrochemical Engineering Maharashtra Institute of Technology Pune 411038, Maharashtra, India
1. Introduction: Heterogeneous photocatalysis is an advanced oxidation process(AOP), which can be successfully used to oxidize many organic pollutants present in aqueous systems. AOPs are all characterized by production of hydroxyl radicals through a multistep process, although different reaction systems can be used. These hydroxyl radicals are non-selective in nature and are able to oxidize various organic pollutants due to their high oxidative capacity. Semiconductors are used as catalysts in photocatalytic reactions due to their electronic structure. Among these materials most attention has been given to TiO2 due to its high photocatalytic activity, low cost, non-toxicity and high stability in aqueous solution[1]. Photocatalytic degradation has proven to be an effective method for mineralizing commercial dyes.[2] Reported studies
[3]
indicate
almost complete oxidation of most of the organic compounds to CO2, H2O and inorganic anions via photocatalytic processes. Textile industries produce large volume of coloured dye effluents which are toxic and non-biodegradable. Azo dyes are non-biodegradable under aerobic conditions while, under anaerobic conditions, can be reduced into potentially carcinogenic compounds. Dyes have serious environmental effects due to both the toxicity of compounds and the colouring of water bodies. Colouring affects the sunlight available for aquatic ecosystems for photosynthetic processes disturbing the ecological balance in natural water bodies. Conventional waste water treatment techniques are not adequate to
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mineralize the dye chemicals completely.
[4]
Application of effective technology to
achieve complete textile dye degradation is a challenge which can be met with TiO2 Assisted heterogeneous photocatalysis. TiO2 assisted photocatalytic process is of special interest, since it can use natural (solar) UV because it has an appropriate band-gap which matches with the energy content of a solar photon (Wavelength range 300 nm-390 nm). This suggests using solar energy as an economically sensible light source. Use of renewable energy has an important significance in reducing the environmental burden of the process. Abundant solar energy is available almost throughout the year in most parts of India. India being a tropical country, the irradiance levels in India are much higher than that in the regions away from equator. Photocatalytic degradation of dyes can have a good potential in tropical countries like India. Requirements of solar photocatalytic collectors differ from conventional solar thermal applications
[5]
. Although many solar collectors have been experimented with; flat
plate collectors offer a most simple one sun configuration for batch type operations. Performance evaluation of flat plate reactor is an important issue from the viewpoint of scaling-up of the process. Robust performance evaluation needs to take into account varying irradiation conditions and different operating parameters. In this paper, we have investigated the influence of dye concentration and incident light intensity on the decolourisation kinetics of Methyl Orange under natural sunlight. Apparent rate constant has been explored as an appropriate performance factor for evaluation of flat plate collector under study. The performance data has been collected by conducting degradation reaction trials under variable irradiance conditions on a falling film flat plate reactor under natural sunlight. Apparent rate constant has also been correlated to catalyst loading. 2. Experimental: 2.1. Materials:
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Methyl Orange, a mono azo anionic dye (C14H14N3SO3Na ) was procured from Bakers (India) Ltd. Titanium Oxide Degussa P-25 was obtained from Aeroxide India Ltd (suppliers of Degussa Co, Germany). (80% anatase, 20% rutile; BET surface area ~ 50 m2/g, mean particle size ca. 30 nm ) 2.2. Trial setup: A batch type re-circulating set up with a falling film flat plate reactor was used to conduct photodegradation trials. Dye solution was recirculated over the reactor with the help of a pump. A storage tank of 20 litre capacity was installed which was used to make the dye solution and to take out the samples. A flowmeter and a water temperature indicator with Pt-100 thermocouple were mounted in the circuit. Flat plate falling film reactor had an incident area of 0.25 sq m. A weir channel at the inlet allows uniform laminar water film of about 3-4 mm over the reactor surface. This photoreactor was mounted on fixed supports, inclined 370 with respect to horizontal plane and facing South in order to maximize the absorption of incident solar radiation without tracking mechanism. 2.3. Procedure: Catalyst concentration was varied from 0.2 to 1.0 g/l, for different experiments, while the initial dye concentration was kept as 20 mg/l. 10 litres of the aqueous dye solution was introduced in the tank. The solution was re-circulated in the system for a few minutes, and the first sample was taken. After this, the catalyst was added and the suspension was thoroughly mixed again. The suspension was exposed to the sun for a period of two hours during which samples were collected at 15 minute interval. In each sample the catalyst was separated of the dye solution using a 0.45 microns syringe filter papers by Millipore. The dye concentration was analyzed with a Varian make spectrophotometer model Cary 100 over a range of wavelength from 200 to 600
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nm. Reduction in absorbance at the characteristic peak at 462 nm is related directly to the reduction in dye concentration. Trials were conducted at different times of the day. Evaporation trials were conducted in all time spans to estimate the loss and this evaporation loss was compensated by adding make-up water to the dye solution prior to the sampling. The average flow rate was maintained as 6-7 litres per min. Solar irradiation was recorded periodically. 3. Results and Discussion: 3.1.Dye Degradation Trials: Plot of Dimensionless Dye concentration C/C0 Vs Time for set of trials shown in Fig. 2 exhibits different degradation rates for all trials conducted with same initial dye concentration and same catalyst loading only under different irradiation conditions. Incident Global radiation for all trials is shown in Fig. 3. Maximum degradation rate is observed for trials A and C which are conducted under maximum irradiance. 3.2.Degradation Kinetics: Reaction kinetics of the Photocatalytic degradation of organic pollutants with TiO2 is described
in
First
Order
In case of batch systems,
Which takes following form after integration, (3)
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form
as
follows,
Thus k1 can be obtained by plotting (–ln C/C0) against time as shown in fig 4. From fig 4, it is observed that only trial A exhibits a true straight line relation indicating first order relation of reaction rate with C. It is to be noted here that incident energy was almost constant during this trial (Global radiation range as 929 ±5% W/m2) and hence rate constant k1 can be clearly determined. For other trials, first order relationship of ln C/C0 with time is affected by the variations in incident radiation exhibiting deviations from straight line curve. As such, it will be erroneous to calculate rate constant k1 based on approximate line fit forced on these trials. This suggests need to incorporate incident energy variation in the kinetic model of photocatalytic degradation. 3.3: Kinetic modelling based on Accumulated Energy: First order rate constant k1 in eq.1 is reported by Turchi &Ollis[6] as
n varies between 0.5 and 1.0. For low incident energy regime which is a case with natural sunlight irradiated reactions, n=1. By substituting k1 from eq.4 in rate expression 2, (5) Integration
gives
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Where
and
is the apparent rate constant in litre/kJ.
The accumulated energy Eacc is the total amount of radiation energy reaching the collector from the beginning of an experiment up to a given time t, per unit solution volume V in kJ/litre. Accumulated energy thus considered is time and volume averaged energy incident on reactor. In the present study, I is incident global radiation. This was thought to be appropriate for studies conducted in developing countries like India where accurate UV data may not be easily available at sites located in rural interior. Location specific Global irradiance data is available with state agencies such as Indian Meteorological Department. [7] Accumulated energy is computed as (7)
Where, tn is experimental time for each sample, IG,n the average incident radiation flux during
, A the collector surface area, V
the total plant volume and En is the accumulated energy (kJ/lit) incident on the photoreactor per unit volume taken at tn . Plot of ln C/C0 Vs Accumulated energy is shown in Fig.5. Data from all trials fits into one linear relationship yielding the best fitted rate constant for all trials. The results
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show that the variable light flux density transformed into Accumulated Energy leads to the same degradation kinetics for Methyl Orange. [8] The compiled data is regressed in a linear plot using polymath software (Version 6.10) and the apparent Rate Constant
for the above mentioned parameters is
obtained as 0.00353 lit/kJ with R2 of 0.9795. 3.4. Dependence of Apparent Rate Constant on TiO2 loading: The apparent rate constants increased sharply initially with TiO2 loading up to a limit of 0.6 gm/l. We attribute this behaviour to a rapid increase in the absorption of photons. At greater loadings the rate constants continue to increase albeit more slowly. Rate constant was almost constant for TiO2 concentrations 1 and 1.2 gm/l. From the kinetics viewpoint it was concluded that TiO2 loading of 1 g/l was optimal in Methyl Orange degradation. 4. Conclusion: TiO2 Assisted Photocatalysis in Natural Sunlight was effectively used for textile dye degradation. As high as 90% dye Degradation could be obtained over full day span run. Dye degradation rate is a function of dye concentration as well as incident radiation. When incident radiation was constant, dye degradation exhibited first order kinetics with respect to dye concentration and reaction rate constant was clearly determined. Derived first-order rate equation in pollutant concentration and incident irradiation rate (Eq. 5) can be used to analyze the experimental data obtained under fluctuating radiation conditions in solar outdoor experiments. This model has a single parameter namely apparent reaction rate constant ‘
’ that can be calculated from these
experiments (Eq. 6). This Apparent reaction rate constant can be used for scaling-up
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of a pilot plant for solar photocatalytic wastewater treatment. Catalyst loading influenced the reaction rate and optimal catalyst loading was found to be 1 gm/l.
5. Figures:
Figure 1: Structure of Methyl Orange
Figure 2: Combined Plot of Dimensionless Concentration Parameter C/C0 Vs Time in Minutes for Trials A, B, C, D & E.
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Figure 3: Combined Plot of Incident Global Radiation for Trials A, B, C, D & E.
Figure 4: Combined Plot of -ln C/C0 Vs Time in Minutes for Trials A, B, C, D & E
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Figure 5: Combined Plot of -ln C/C0 Vs Accumulated Energy in kJ/l for Trials A, B, C, D & E 6. References: [1] D. S. Bhatkhande, V. G. Pangarkar and Anthony Beenackers, “Photocatalytic degradation for environmental Applications – A review”; Journal of Chemical Technology and Biotechnology, Vol 77, 2001, pp 102-116. [2] S.K. Kansal, M. Singh, D. Sud, “Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts”; Journal of Hazardous Materials, 141, 2007, pp 581-590. [3] Blanco Julian Galvez, Sixto Malato Rodriguez, “Solar Detoxification”; United Nations Educational, Scientific and Cultural Organization (UNESCO), 2003. [4] U. G. Akpan and B. H. Hameed, “Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: A review”; Journal of Hazardous Materials, 170, 2009, pp 520–529.
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[5] Detlef Bahnemann, “Photocatalytic water treatment: solar energy applications”, Solar Energy, 77, 2004, pp 445–459. [6] Craig Turchi and David Ollis, “Photocatalytic Degradation of Organic Water Contaminants: Mechanisms Involving Hydroxyl Radical Attack”; Journal of Catalysis, 122, 1990, pp 178-192. [7] Solar radiation handbook, Publication of Indian Meteorological Department, 2008. [8] Toshiyuki Oyama, Aoshima Akio, Horikoshi Satoshi, Hidaka Hisao, Zhao Jincai, Serpone Nick, “Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation”, Solar Energy, 77, 2004, pp 525–532.
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