Solvent Emissions Minimization by Catalytic Oxidation

Transcription

1 In: Proceedings of the RESOPT closing seminar Waste minimization and utilization in Oulu region: Drivers and constraints edited by Eva Pongrácz. Oulu University Press, Oulu. p Abstract Solvent Emissions Minimization by Catalytic Oxidation Satu Ojala, a* Ulla Lassi, a,b Reeta Ylönen, a Heidi Karjalainen, a Katariina Rahkamaa-Tolonen c and Riitta Liisa Keiski a a University of Oulu, Mass and Heat Transfer Process Laboratory FIN University of Oulu, P.O.Box 4300 b Central Ostrobothnia Polytrechnic, Department of Technology FIN Kokkola, Talonpojankatu 2 c Ecocat Oy, FIN Oulu, P.O.Box 171 Catalytic oxidation is a feasible and affordable technology for solvent emission minimization. However, different aspects should be taken into account before applying and in the development of an incineration process. This paper joins the results of three different studies: testing of catalyst activities, testing of the effect of construction materials on the catalytic incineration and finding of optimal process conditions for solvent emissions abatement. Activity studies of the catalysts based on volume, showed the superiority of Cu-containing catalysts. However, fewer side products are formed with a Pt catalyst. The studies on the effect of construction materials showed that copper and Aluzinc decreased the light-off temperature (T 50 ) of n-butyl acetate compared to thermal experiments. Copper and Aluzinc did not further decrease the T 50 when they were introduced into the reactor packed with the catalyst. The studies on the process parameter optimization showed, that GHSV was the most dominant parameter in the oxidation of n-butyl acetate, whereas moisture had only a minor effect on conversion. Ageing of the catalyst did not change the significance order of the above-mentioned parameters, however, the effect of individual parameters increased slightly as a function of ageing. 1 Introduction Volatile organic compounds (VOCs) together with nitrogen oxides are the major contributors to the formation of photochemical ozone. In 1999 the EU adopted the VOC Solvent Emissions Directive (1999/13/EC), the goals of which should be implemented by the year The most feasible abatement technologies for emissions that are mixtures of different VOCs are destruction based. If the total VOC concentration is not very high, regenerative catalytic incineration would be the most cost-effective choice. This paper presents the results of a research concerning catalytic oxidation of solvent emissions. First, catalyst activities are considered. Second, the possible effect of construction materials on the oxidation is investigated, and third, the effect of process conditions on the abatement are studied. 2 Experimental According our earlier experience, characteristic solvent emissions from wood-coating processes include ethanol and n-butyl acetate. Therefore n-butyl acetate has been used as a model compound in all the laboratory experiments. In addition, methane was used in the activity tests of the catalysts, since it is most difficult to oxidize among the light hydrocarbons, and the differences in activities between the catalysts are easier to observe. Laboratory experiments have been carried out in a quartz reactor, which was heated up from room temperature to ~800 C. The feeding of n-butyl acetate (2000 ppm) was carried out by a calibrator, in which the liquid phase compound was fed with a syringe pump to the heater for vaporization and mixed with air from mass flow controller. Methane (2000 ppm) was also fed via a mass flow controller and mixed with air prior the reactor inlet. The outlet gas flow was analysed by gas chromatograph equipped with a flame ionisation detector, GC/FID (organic compounds) and similar equipment equipped with a thermal conductivity detector, GC/TCD (water). The catalyst s volume and gas flow (1 dm 3 /min) were * Corresponding author, satu.ojala@oulu.fi

2 kept constant in order to maintain the same space velocity (~ /h) within the catalyst honeycomb even if the total volume of the packing was changed when other materials were introduced into the reactor in the testing of construction materials. Activity measurements have been carried out for ten (10) VOC catalysts containing noble metals (Pt, Pd) or metal oxides (Cu, Mn, Mg, Cr). The characteristics of different catalysts are presented in Table 1. Table 1 The characteristics of the catalysts used in the experiments Catalyst A B C D E Act.comp. Pt Pt/Pd Pt Pt/Pd Cu/Mn-oxides Loading (%) / / Support Al 2 O 3 (1) Al 2 O 3 (1) Al 2 O 3 (2) Al 2 O 3 (2) Al 2 O 3 Shape Monolith Monolith Monolith Monolith Extrudate BET BET Catalyst F G H I J Act.comp. Pt Pt MnO 2 /MgO Cu x Mg (1-x) Cu x Cr 2 O 4 Cr 2 O 4 Loading (%) >0.09 >0.09 5/ Support Al 2 O 3 Al 2 O 3 Al 2 O 3 Al 2 O 3 Al 2 O 3 Shape Extrudate Pellet Pellet Pellet Pellet BET BET BET 1: BET before the experiments (m 2 /g) BET 2: BET after the experiments (m 2 /g) The experiments concerning the effect of different construction materials were performed with and without the catalyst, with different metal plates, with inert glass granules and without any packing material (called as the thermal experiment). The experiment with glass granules was performed twice, with the constant volume of packing (60 pcs of granules) and with comparable external surface area with metal bed (30 pcs of granules). The characteristics of different packings are presented in Table 2. Table 2 Descriptions of the bed materials. Experiments were carried out with single beds and with the combinations of a catalyst and other bed materials Bed name Material Mass [g] External surface area [cm 2 ] Specific heat capacity, c p [kj/kgk] Thermal Aluzinc Al/Zn/Si 5.2 ~24 coated steel plate Copper Cu plate 8.0 ~ Stainless Steel Glass Stainless steel plate Glass granules 5.8 ~ Catalyst Pt/Pd - - * pcs, pieces ~47 (60 pcs) * ~24 (30 pcs) * Heat conductivity, k w [W/mK] The effects of different process parameters (concentration of emission gases, gas hourly space velocity (GHSV), temperature, and moisture content of emission gas) on the operation of a catalytic incinerator were also investigated. MODDE 6.0 program (Umetric AB) was used as a tool in statistical experimental design and in evaluation of the effects of the selected parameters affecting the catalytic oxidation of n-

3 butyl acetate. Further, the effect of ageing of the catalyst was considered i.e. does ageing affect process parameters or does it even change the significance order of them. A set of experiments was done with a full two-level factorial design. The effect of a single factor was evaluated at all levels of other factors, which enabled study of the effects of the interaction of selected parameters. The measured response was the conversion of n-butyl acetate over the fresh and aged catalysts. The validity of the empirical models fitted with the multiple linear regression (MLR) was tested with the analysis of variance (ANOVA). The used confidence level was 95%. The parameters and the used levels were based on the earlier results achieved from industrial measurements, from solvent emission sources and from catalytic incinerator used in the solvent emission abatement. Temperatures of the experiments were selected to be higher than the catalysts light-off, i.e. above the temperature of 50% conversion, of which the lower temperature 300 C is close to a normal industrial operation temperature of the incinerator, and the higher temperature level, 500 C, is above it. The levels for GHSV were h -1 and h -1. The lower level of concentration (2000 ppm) was close to the concentration of n-butyl acetate in the solvent emission measured, the higher level was 4000 ppm. The higher level for moisture was 2.5 vol-% and at the lower level water was not introduced into the system (i.e. zero level moisture). 3 Results and Discussion 3.1 The activity of VOC catalysts for methane and n-butyl acetate oxidation Light-off temperatures for different catalysts are presented in Figure 1. Among the tested catalysts copper-containing ones are the most active in methane oxidation whereas noble metal catalysts showed slightly higher activity in n-butyl acetate oxidation. This comparison is based on the catalyst total volume. C 800 n-butyl acetate C 800 Homogeneous light-off Methane C Homogeneous light-off A, B, F, G D, H E I, J H, I, J A, B, C, D, G F, E Figure 1 Light-off temperatures of different VOC catalysts in n-butyl acetate and methane oxidation It was also observed that several VOC compounds were formed in n-butyl acetate oxidation below the catalysts light-off temperature, e.g., acetic acid, butene, acetone, butyraldehyde and hexane. The formed intermediates were different depending on catalysts composition. Furthermore, the use of catalysts decreased the formation of intermediates. Over 90 % of intermediates in non-catalytic oxidation was butene and the rest were mostly acetone and acetic acid. In catalytic oxidation none of the intermediates

4 was as dominant as butene in the non-catalytic oxidation. Noble metal catalysts produced less intermediates than the others. 3.2 Influence of heat exchanger s construction material on catalytic incineration Earlier, the industrial-scale measurements have been shown that the total VOC concentration after the catalyst bed further decreases inside the heat exchangers. Furthermore, these laboratory experiments showed that the addition of packing material into the reactor decreases the light-off temperature of n-butyl acetate compared to the thermal experiment. Figure 2 shows the results of the n-butyl acetate oxidation experiments. 100 Catalyst 90 Conversion / % Cat. + packing Copper Aluzinc S.Steel Glass granules 30/60 Without packing / thermal Inlet temperature / C Figure 2 Light-off curves of n-butyl acetate with different packing materials The metallic or inert glass material has not as significant effect on the light-off temperature as the catalyst has. If the reactor bed consists of a catalyst and metallic/glass material, the T 50 temperature of n-butyl acetate is not further decreased. Aluzinc packing gives a ~50 C and copper packing gives a ~200 C decrease in the T 50 temperature compared to the thermal experiment. Glass granule experiments and flow modeling by Femlab did not support the significant enhancing effect of mixing or surface area on the n- butyl acetate oxidation. A part of the copper s larger enhancing effect on the reaction compared to stainless steel and Aluzinc can be explained by its better heat conductivity properties. However, the difference between Aluzinc and stainless steel may not be due to the heat conductivity properties since Aluzinc is basically steel with an Al/Zn/Si coating. Thermodynamic equilibrium calculations with HSC Chemistry showed the formation of oxides on the metal surfaces that may have either enhancing or inhibiting effect on n-butyl acetate oxidation. Nevertheless, it is most likely that oxidation within the industrial-scale catalytic incinerator proceeds first via catalytic oxidation and further via homogeneous gas phase oxidation within the metallic heat exchangers. 3.3 The effect of process parameters on catalytic incineration of solvent emissions Figures 3 and 4 show that GHSV has the largest and negative main effect on conversion, i.e. when GHSV is increased, the conversion is decreased. Furthermore, the effect of GHSV is more significant at low temperatures and it is slightly more significant over the aged catalyst than over the fresh catalyst. Temperature has the second largest but positive effect on conversion, thus when temperature is increased the conversion is increased as well. Third significant term is the interaction effect between temperature and GHSV followed by the effect of concentration. The concentration has a more significant effect on conversion when GHSV is high than in the lower GHSV level. According to these results, moisture has

5 only a negligible effect on conversion over both the fresh and aged catalysts. As can be seen in Figures 3 and 4, the ageing of the catalyst did not change the significance order of the parameters. Investigation: Satu_tuore (MLR) Effects for Conversion 1 1,00 0,50 0,00-0,50-1,00 GHSV T GHSV*T c c*ghsv c*t Moist*GHSV Moist*c Moist Moist*T Effects N=32 R2=0,976 R2 Adj.=0,965 DF= 21 Q2=0,945 RSD=0,1602 Conf. lev.=0,95 Figure 3 Calculated effects of GHSV, moisture (Moist), concentration (c) and temperature (T) on n-butyl acetate conversion over a fresh catalysts. Investigation: Satu_ikaytetty (MLR) Effects for Conversion 2 1 Effects GHSV T GHSV*T c c*ghsv c*t Moist*GHSV Moist*T Moist Moist*c N=32 R2=0,986 R2 Adj.=0,980 DF=21 Q2=0,968 RSD=0,2300 Conf. lev.=0,95 Figure 4 Calculated effects of GHSV, moisture (Moist), concentration (c) and temperature (T) on n-butyl acetate conversion over a aged catalysts. When the validity of the fitted model (MLR) was evaluated with ANOVA, the results showed that the model was statistically significant with a 95% confidence level. Square of the multiple correlation coefficient of the model i.e. the response variation percentage explained by the model, R 2, for fresh and aged catalysts were and 0.986, respectively. Response variation percentage predicted by model, Q 2, were for the fresh and for the aged catalyst. However, one have to remember that the model is valid only in the used range of parameters. 4 Conclusions Volume-based comparison of catalysts activities shows that Cu-containing catalysts allow lowest T 50 and T 95 for methane oxidation. In n-butyl acetate oxidation, noble metal catalysts are more effective. Formation of intermediates takes place in n-butyl acetate oxidation if the catalyst is operated close to T 50

6 temperature. Increasing the temperature decreases the formation of intermediates. Intermediate formation depends on the catalysts. However, fewer intermediates are formed in catalytic oxidation of n-butyl acetate than without a catalyst. Based on our experiments, we can conclude that addition of packing materials did not significantly enhance catalytic reactions. However, compared to the thermal experiments, copper and Aluzinc packing decreased the T 50 temperature of n-butyl acetate. It is more likely that the heat exchangers maintain the temperatures needed for homogeneous gas-phase oxidations. Consequently, within the industrial scale incinerator both catalytic surface and homogeneous gas phase oxidation reactions take place. According to the study, the GHSV has the largest and negative effect on the n-butyl acetate conversion over the Pt/Al 2 O 3 catalyst. Increasing temperature and concentration and decreasing GHSV enhances the oxidation. Moisture had only a minor impact on the n-butyl acetate conversion. Similar results were achieved with fresh and aged catalysts. However, the behavior of the aged catalyst was slightly more affected by changes in the GHSV than that of the fresh one. References Satu Ojala, Reeta Ylönen, Ulla Lassi, Katariina and Rahkamaa-Tolonen (2003) Activity of VOC-catalysts for methane and n-butyl acetate oxidation. Europacat-6, Innsbruck, Austria. Satu Ojala, Ulla Lassi and Riitta Keiski (2005) Activity of VOC Catalysts in Methane and n-butyl Acetate Total Oxidation Chem. Eng. Transactions, VI p Satu Ojala, Ulla Lassi, Reeta Ylönen, Heidi Karjalainen and Riitta Keiski (2004) Influence of heat exchanger s construction material on catalytic incineration. 11 th Nordic Symposium on Catalysis, Oulu, Finland. Satu Ojala, Ulla Lassi, Reeta Ylönen, Heidi Karjalainen and Riitta Keiski (2005) Influence of heat exchanger s construction material on catalytic incineration. Catalysis Today 100, p Satu Ojala, Ulla Lassi and Riitta Keiski (2005) The effect of process parameters on catalytic oxidation of solvent emissions. 4 th Conference on Environmental Catalysis, Heidelberg, Germany. Satu Ojala, Ulla Lassi and Riitta Keiski (2005) The effect of process parameters on catalytic oxidation of solvent emissions. Applied Catalysis B: Environmental. (submitted).