A photo-catalytic reactor for degrading volatile organic compounds (VOCs) in paper mill environment

Transcription

1 Journal of Bioresources and Bioproducts. 2018, 3(2) ORIGINAL PAPER DOI: /jbb.v3i2.113 A photo-catalytic reactor for degrading volatile organic compounds (VOCs) in paper mill environment Jun Maa, Xin Tonga, Zhenbin Zhanga, Xiaoquan Chena, Wenhao Shena* a) State Key Laboratory of Pulp and Paper Engineering, South China University of Technology Guangzhou, , China *Corresponding author: ppwhshen@scut.edu.cn ABSTRACT Volatile organic pollutants such as benzene and formaldehyde are commonly detected in the ambient air of paper mills. To remove these pollutants from the air, a photo-catalytic reactor was developed in this study. The reactor had a series of honeycomb aluminum meshes coated with nanosized titanium dioxide as the catalyst for the degradation reactions of gaseous pollutants. Both formaldehyde and benzene could be completely degraded in the reactor. However, the degrading time for benzene was much longer than that for formaldehyde, and the degradation rate of benzene decreased with increasing initial benzene concentration. It was found that the reaction pathway for formaldehyde in the mixture was different from that in its single component form, and it took about two times longer time to be degraded than that for its single component form. The reaction pathway of benzene was similar in either case although the degradation time for benzene was about 50% shorter in the mixture form. Keywords: Photo-catalytic reactor; Degradation; Gaseous pollutant; Paper mill 1. INTRODUCTION The gaseous pollutants from paper mills not only cause environmental pollutions, but also threaten the health of workers. Volatile organic compounds (VOCs) are the major gaseous emissions in paper mills.1,2 Despite of having the possible environmental and health risks, the qualitative and quantitative analysis of gas compositions in paper mills has been scarce; and until now, the monitoring of gaseous pollutants in paper mills has not been widely concerned. 1,2 Concerning the treatment strategies of gaseous pollutants, the traditional methods include: condensation,3 absorption,4 adsorption,5 burning,6 membrane separation,7 low temperature plasma,8 and biological treatment,5 etc. According to our results of ambient air in several pulp and paper mills, formaldehyde and benzene were identified to be two main gaseous pollutants.1,2 In this study, a novel treatment method of gaseous pollutants was proposed: adopting the nanosized titanium dioxide as the core of photo-catalysis oxidation technology, combining the intelligent control system, a photo-catalytic reactor was developed, optimized, and applied to the degradation of two gaseous pollutants, formaldehyde and benzene. 2. EXPERIMENTAL 2.1 Photo-catalytic reactor Combining with the Microcontroller Unit (MCU), our developed photo-catalytic reactor was mainly used for the degradation of volatile organic compounds (VOCs) detected in paper mills, which were represented by formaldehyde and benzene as the target pollutants. As plotted in Figure 1, three parts constituted the photo-catalytic reactor: 1) Filter component: used for removal of solid particles; 2) Photo-catalytic component: 3 sets of honeycomb aluminum net coated with nanosized titanium dioxide, under illumination of UV lamps, used for the degradations of gaseous pollutants; 3) Fan component: used for airflow. In order to determine the concentrations of gaseous pollutants and the environmental impact, a formaldehyde sensor (EC803-CH2O, Bmoon), a portable benzene sensor (FirstCheck+ 5000, Ion Science Ltd, UK), and a digital temperature and humidity sensor (DHT11, Aosong) were mounted in the filter component, and the gas flow rate was controlled within m3/h by MCU, Two operational modes of photo-catalytic reactor with MCU were used: smart mode and manual mode, indicating that the reactor can be operated automatically and manually. With the aid of MCU, a LCD screen on the outside of reactor was used for real-time indications of concentrations of formaldehyde and benzene, temperature, humidity, and fan speed during the photo-catalytic degradation of gaseous pollutants. 78

2 Filter UV Lamp Honeycomb aluminum net Fan Gaseous pollutants Clean air Filter component Photo-catalytic component Fan component Figure 1. Schematic diagram of the purification process of photo-catalytic reactor 2.2 Evaluation of Photo-catalytic Performances The performance of photo-catalytic reactor was evaluated on the basis of the following experiments: (1) Blank experiments (shown in Table 1); (2) Optimal experiments; (3) Degrading performance experiments. It is noted that formaldehyde was used as the target pollutant. All experiments were carried out in a closed cubic chamber, which was used for the simulation of gaseous environment of paper mills. The data of gaseous pollutants, humidity, and temperature were quantitatively monitored by the above sensors mounted in the photo-catalytic reactor and recorded every five minutes. The procedures of all experiments were as follows: 1) Firstly, the reactor was put into the chamber. The fans of the reactor were turned on and set at an appropriate gas flow rate. After 5-10 minutes, when all the sensor data became stable, the target gaseous pollutant was injected into the chamber by a 50 ml injector. 2) Subsequently, the UV lamps were turned on and the photo-catalytic reactions started. The experimental data were recorded every five minutes. 3) Finally, after the experiments ended, the closed chamber was opened, and residual gas was removed from the chamber. The following reagents were used in the experiments: formaldehyde solution (36%-38%) and benzene (analytical reagent, 99.5%). Table 1. Experimental conditions of blank experiments Experimental conditions Experiment Honeycomb Objective No. UV lamps Formaldehyde aluminum nets 1 Testing the zero pollution of the photo-catalytic reactor. 2 Testing the illuminations of UV lamps caused pollutions or not. 3 Testing the honeycomb aluminum nets itself caused pollutions or not. 4 Testing the cubic chamber itself caused pollutions or not. 5 Testing the effects of UV lamps on the degradation of formaldehyde. Note: means yes, means no. 79

3 3 RESULTS AND DISCUSSION 3.1 Control experiments and optimal experiments In order to ensure the accuracy and reliability of the experimental results, before the evaluation of catalytic performance, the control experiments and the optimal experiments pertaining to the photo-catalytic reactor were conducted. Firstly, according to operations shown in Table 1, the results from five control experiments were obtained and plotted in Figure 2, and were discussed as follows. (1) In No.5 control experiment, in the absence of honeycomb aluminum nets, formaldehyde was slightly degraded under illuminations of UV lamps, and the degradation rate was 20% in 120 minutes. (2) In No.2 control experiment, in the absence of honeycomb aluminum nets and the injection of formaldehyde into the cubic chamber, the concentration of formaldehyde increased over the time, which implies that the ozone produced by UV lamps caused the increment of formaldehyde sensor. (3) However, in the No.1 control experiment, this phenomenon disappeared when the honeycomb aluminum nets were used, indicating that the ozone produced by UV lamps was degraded by the nanosized titanium dioxide coated on honeycomb aluminum nets. 9 (4) The result from No.3 control experiment was similar to that from No.4 control experiment, and both had almost no contribution to the formaldehyde sensor. Secondly, after having confirmed no pollution from the chamber and the honeycomb aluminum nets, a series of experiments were carried out to investigate the optimal conditions of the photo-catalytic reactor. Three optimal elements were considered in the study: (1) Flow rate of gas; (2) Number of photo-catalytic components; (3) Size of honeycomb aluminum mesh. After several experiments, the optimized process parameters of photo-catalytic reactor were as follows: flow rate of gas of 97.2 m 3 /h, three photo-catalytic components, and 2 mm size of honeycomb aluminum mesh. 4 conducted under the optimized process conditions. According to our main analysis results on the ambient air in paper mills, 1,2 three categories of polluted substrates were selected to assess the performance of photo-catalytic reactor: (1) formaldehyde; (2) benzene; (3) mixture of formaldehyde and benzene. 3.3 Formaldehyde as the target pollutant The experimental results of degrading formaldehyde with different initial concentrations are shown in Figure 3. The photo-catalytic reactor completely degraded formaldehyde with varying initial concentrations: 41 minutes, 35 minutes, and 30 minutes for 2.5 ppm, 1.5 ppm, and 0.5 ppm initial concentrations of formaldehyde, respectively. As shown in Figure 3b, the trend relevant to photo-catalytic removal of formaldehyde was not correlated with the initial concentration of formaldehyde, which were quite similar,, fast degrading at the beginning and slowing down afterwards. Furthermore, as displayed in Figure 3a, in the first 10 minutes of degradation, the higher initial concentration, the faster the degradation of formaldehyde, and subsequent degradation processes gradually converged. (a) Degradation process of formaldehyde Concentration of formaldehyde(ppm) No.1 No.2 No.3 No.4 No Time(min) Figure 2. Five control experiments. 3.2 Performance of Photo-catalytic Reactor After the control experiments and the optimal experiments of photo-catalytic reactor, the degrading performance studies of the photo-catalytic reactor were (b) Degradation speed of formaldehyde Figure 3. Degradation of formaldehyde with different initial concentrations under optimal conditions 3.4 Benzene as target pollutant 80

4 Under optimal conditions, the experimental results related to degrading benzene with different initial concentrations are shown in Figure 4. Comparing with Figure 3, although the photo-catalytic reactor completely degraded benzene: 310 minutes, 180 minutes, and 70 minutes for 2.5 ppm, 1.5 ppm and 0.5 ppm initial concentrations of benzene, respectively, which took longer time than those for degrading formaldehyde at the same initial concentration. With the increase of initial concentration of benzene, the degradation time for photo-catalytic reactor also increased. It is obvious in Figure 4a that, no matter how much the initial concentration of benzene was, degradation speed almost kept the same during the whole degradation process, and the speed of the degradation process had no relationship with the initial concentration of benzene. The higher the initial concentration of benzene, the longer degradation time was required, and this could be the reason why the 0.5 ppm benzene was completely degraded in 70 minutes, which was the shortest one demonstrated in Figure 4b. This experimental result was in agreement with the phenomenon observed by Ma et al. 10 (a) Degradation process of benzene Figure 5. Comparison of the degrading processes of formaldehyde and benzene with photo-catalytic reactor under optimal conditions 1) Formaldehyde has one carbon atom, and formic acid has been identified as the main intermediate in photo-catalytic degradation of formaldehyde. The pathway for the photo-catalytic degradation is short and formaldehyde can be completely converted into CO 2 and water. 11 2) The pathway for photo-catalytic degradation of benzene is long and complex, which includes several internal processes: direct hole oxidation, hydroxyl radical reaction, and polymerization. 12 3) Benzene-related compounds are not easily adsorbed by the catalyst at room temperature. Although its intermediate products generated from the photo-catalytic degradation are strongly adsorbed on the surface of catalyst, they have low reactivity. Hence, it is reckoned that competitive adsorption of intermediate products and the deactivation of catalyst might be the main elements 12, 13 causing the slow degrading process of benzene. 3.5 Mixture of formaldehyde and benzene as the target pollutant (b) Dynamic degradation rate of benzene Figure 4. Degradation of benzene with different initialconcentrations under optimal conditions For comparison purposes, the degrading processes of formaldehyde and benzene (both of the initial concentrations were 1.25 ppm) with photo-catalytic reactor are plotted in Figure 5. It was evident that the photo-catalytic reactor took almost 4 times longer time to degrade benzene than that for formaldehyde, and the reasons might be as follows: In order to simulate the actual gaseous pollution in paper mills, the mixture of formaldehyde and benzene were further used to assess the performance of photo-catalytic reactor. The initial concentrations of both formaldehyde and benzene were about ppm; hence, the initial total concentration of the mixture was 1.25 ppm, and its photo-catalytic result is given in Figure 6. Firstly, for the behavior of formaldehyde shown in Figure 6, just like in Figure 5, the concentration of formaldehyde decreased rapidly in the initial phase; but with the going of photo-catalytic reaction, the degrading speed gradually slowed down, and finally lagged behind that of the benzene; and lastly, the formaldehyde at low concentration was hard to be completely degraded. Secondly for the behavior of benzene in Figure 6, the concentration of benzene decreased relatively slowly in the initial phase, then gradually sped up, and finally dropped to 0 ppm with a relatively steady speed. 81

5 polluted with both formaldehyde and benzene. Formaldehyde with initial concentrations of 2.5 ppm, 1.5 ppm, and 0.5 ppm were completely degraded in 41 minutes, 35 minutes, and 30 minutes, respectively. The photo-catalytic reactor could also completely degrade benzene with initial concentrations of 2.5 ppm, 1.5 ppm and 0.5 ppm in 310 minutes, 180 minutes, and 70 minutes, respectively. When they were both present in the air sample, the degradation time was longer for formaldehyde but shorter for benzene. ACKNOWLEDGMENTS Figure 6. Degrading process of the mixture of formaldehyde and benzene with photo-catalytic reactor under optimal conditions Comparing with the results shown in Figure 5, the reaction of formaldehyde in the mixture was different from that in its single form, and it took about two times longer time to be degraded than that for its single form; the tendency of the reaction of benzene was almost the same in either case, but it only took half time to be completely degraded than that for its single form. These phenomena could be analyzed with the following points: 1) Since benzene was not easily adsorbed, in the initial photocatalytic phase, formaldehyde was mainly adsorbed on the catalyst and be degraded. 2) With the degradation of formaldehyde, the adsorption and degradation of benzene became dominant. 3) The reported intermediates (aldehyde-based substances) generated from the degradation of benzene had competitive adsorptions on the catalyst, 12,14 and this process slowed down the degradation and made the formaldehyde in the mixture could not be completely degraded. 4) The formaldehyde sensor maybe also sensitive to the aldehyde group generated in the degradation process of benzene, and thus the indications of the formaldehyde sensor might not represent the real concentration of formaldehyde. The different behaviors of formaldehyde in two cases could be explained with the above reasons; as for the behavior of benzene, the presence of high concentration of formaldehyde in the mixture influenced the degradation of benzene, but with the fast degradation of formaldehyde in the initial phase, its impact on the degradation of benzene was greatly reduced CONCLUSIONS Formaldehyde and benzene are the main gaseous pollutants found in the ambient air in several paper mills. A photo-catalytic reactor was developed successfully designed and fabricated for removing the pollutants from air. Three polluted air samples were selected to assess the performance of photo-catalytic reactor: (1) polluted with formaldehyde only; (2) polluted with benzene only; (3) The research was supported by the Research Funds of State Key Laboratory of Pulp and Paper Engineering (No.2015C05), Science and Technology Planning Project of Guangdong, (No. 2015A ), National Science Foundation of Guangdong (No. 2016A ), and Science and Technology Program of Guangzhou (No ). REFERENCES 1. Tong X., Zhang Z. B., Chen X. Q., and Shen W. H., Analysis of Volatile Organic Compounds in the Ambient Air of a Paper Mill- A Case Study, BioResources, 2015, 10 (4): Tong X., Zhang Z. B., Chen X. Q., and Shen W. H., Analysis and pollution sources speculations of toxic gases in a secondary fiber paper mill, Journal of environmental science and health, part A-Toxic/Hazardous substances & Environmental engineering, 2016, 51 (13): Zheng X., Li H. Q., Zhang W., Ma T. Q., Performance research and optimization on toluene condensation recovery system, Cryogenics & Superconductivity, 2015, 43 (2): Huang J., Zhao W. W, Chen L. Q., Wang Z. X., Shi Y. Q., Zhang F., The Applications of Absorption Process for VOCs Gases of Benzene, Guangdong Chemical Industry, 2011, 38 (11): He W. L., Xie G. J., Wu H., Sha H. L., Xu Y. B., The overview about harm of VOCs waste gas and treatment technology. Sichuan chemical, 2012, 01: Zhao L., Zhang Y. F., Li R. H., Ma Z. H., The harm of VOC and recycling and processing technology. Chemical education, 2015,16: Wang H., Liu Y. F., Peng D. M., Wang F. D., Lu M. X., The development of membrane separation technology and its application prospect, Applied Chemical Industry, 2013, 42 (3): Zhang X. M., Removal of volatile organic compounds by non-thermal plasma technology, Zhejiang university, China, Lu Y. W., Zhao X. H., Wang M. Y., Yang Z. L., Zhang X. J., Yang C. X., Feasibility analysis on photocatalytic removal of gaseous ozone in aircraft cabins, Building and Environment, 2014, 81: Ma J. Z., Zhu C. Z., Lu J., Liu H. B., Huang L., Chen T. H., Chen D., Catalytic degradation of gaseous benzene by using TiO 2 /goethite immobilized on palygorskite: Preparation, characterization and mechanism, Solid State Sciences, 2015, 49: Liang W. J., Li J., Jin Y. Q., Photo-catalytic degradation of 82

6 gaseous formaldehyde by TiO 2 /UV, Ag/TiO 2 /UV and Ce/TiO 2 /UV, Building and Environment, 2012, 51: Olga d'hennezel, Pierre Pichat, David F Ollis, Benzene and toluene gas-phase photocatalytic degradation over H 2 O and HCL pretreated TiO 2 : by-products and mechanisms, Journal of Photochemistry and Photobiology A: Chemistry, 1998, 118 (3): Zhang Q. C., Zhang F. B., Zhang G. L., Reaction Mechanism of Gas-Phase Photocatalytic Oxidation of Benzene on TiO 2, Chinese Journal of Catalysis, 2004, 25 (1): Wang X. Q., Zhang G. L., Zhang F. B., Wang Y., Progress in photocatalytic degradation of volatile aromatics in gas phase over titanium dioxide, 2006, 18 (4): Chen H. B., Liu W. X., Cellulose-based photocatalytic paper with Ag 2 O nanoparticles loaded on graphite fibers. J. Bioresour. Bioprod. 2016, 1(4),