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Treating isopropyl alcohol by a regenerative catalytic oxidizer
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Separation and Purification Technology 62 (2008) 71–78
Treating isopropyl alcohol by a regenerative catalytic oxidizer Jie-Chung Lou ∗ , Shih-Wei Huang Institute of Environmental Engineering, National Sun Yat-Sen University, Len-Hai Road No. 70, Kaohsiung 80424, Taiwan Received 12 October 2007; received in revised form 19 December 2007; accepted 27 December 2007
Abstract Regenerative catalytic oxidizer (RCO) can be conveniently used to control emissions of volatile organic compounds (VOCs), because of their thermal recovery efficiency (TRE), low fuel cost and high oxidation. In this work, catalysts with various metal weight loadings were prepared by deposition–precipitation, wet impregnation and incipient impregnation to treat isopropyl alcohol (IPA). We used the excellent catalytic performance in a pilot RCO to test IPA oxidation performance under various conditions. The best catalyst was selected and its TRE, bed temperature variations, pressure drops and selectivity of the catalyst were more widely discussed. The results demonstrate that the optimal catalyst was prepared by wet impregnation with 20 wt.% metal on ceramic honeycomb (CH). 20 wt.% Cu–Co/(CH) catalyst was the best catalyst used in a RCO because it was effective in treating IPA, with a CO2 yield of up to 95% at a heating zone temperature (Tset ) = 400 ◦ C under various conditions. It also had the largest tolerance of variations in inlet IPA concentration and gas velocity (Ug ). This 20 wt.% Cu–Co/(CH) catalyst in a RCO performed well in terms of TRE, pressure drop and selectivity to CO2 . The TRE range in a RCO was from 87.8 to 91.2% under various conditions, and decreased as Ug increased in a fixed Tset . The pressure drop increased with Ug and Tset . The selectivity to CO2 increased to over 95% at 300 ◦ C, and that to propene remained at 2–5% from 200 to 400 ◦ C. Finally, the stability test results indicated that the 20 wt.% Cu–Co/(CH) catalyst was very stable at various CO2 yields and temperatures. © 2008 Elsevier B.V. All rights reserved. Keywords: Regenerative catalytic oxidizer; Thermal recovery efficiency; Isopropyl alcohol; Catalyst
1. Introduction Volatile organic compounds (VOCs) are emitted during industrial processes and in automobile exhaust emissions. VOCs are recognized as major contributors to air pollution because they are toxic: they are carcinogenic and can harm skin, neural systems, eyes and other organs. They can act indirectly as secondary pollutants in the formation of photochemical smog. Isopropyl alcohol (IPA) is a common organic solvent that is utilized not only as an industrial raw material, as a dehydrating agent, as an antifreeze, as an anticorrosive and in dry jets, but also in the production of gum and attar. Accordingly, IPA is strongly associated with problems of VOCs. Combustion has for a long time been adopted to decompose VOCs. Combustion refers to thermal combustion, catalytic combustion, regenerative thermal oxidizer (RTO) and regenerative catalytic oxidizer (RCO). In a RCO, chambers of inert regenerative materials are applied to heat the stream of VOCs by
∗
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1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.12.024
cooling burnt gases, through reverse flow operations, which cut fuel costs. Catalysts were used to reduce the required reaction temperature and to increase the efficiency of VOCs oxidation. The RCO was developed in the mid-1970s by Matros and coworkers [1,2]. The RCO was used to treat methyl ethyl ketone (MEK) and toluene by Chou et al. [3], providing a VOC removal efficiency of 98% for MEK and 95% for toluene. A comparison of RTO and RCO economics by Matros et al. [4] yielded the range of gas stream parameters for which RCOs are preferable. The RCO combines the advantages of lower temperature catalytic oxidation, higher VOCs removal efficiency, lower fuel cost and reduced formation of harmful by-products, in the most energy-efficient method of eliminating VOCs. Regarding regenerative materials, Mario and Pietropaolo [5] compared the energetic performances of random and structured regenerators, revealing that random packed bed regenerators are less attractive. Cheng et al. [6] carried out an experiment on RTOs with random packed irregular SiO2 pebbles. The outcomes indicated high removal efficiency in two-bed RTOs. Moreover, thermal recovery efficiency (TRE), pressure drop and the efficiency of VOCs removal were discussed by varying the gas flow rate and the inlet concentration of VOCs [7].
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In this work, metal oxide catalysts were used in the RCO system. They have been widely investigated in numerous studies. Three series of catalysts with various Co contents (up to 21 wt.% Co) were prepared using three preparation methods. The variously prepared catalysts with various Co wt.% exhibited different efficiencies [8]. Larsson and Andersson [9] noted excellent performance in catalytic combustion over CuOx /Al2 O3 and CuOx –CeO2 /Al2 O3 . Tseng et al. [10] reported excellent metal oxide catalyst activity (MnO/Fe2 O3 ) in the catalytic oxidation of styrene. Furthermore, several investigations have shown that the species of metal oxide, the metal loading and the preparation method affect the oxidation of VOCs [11–15]. With respect to gas flow rate and inlet concentration of VOCs, increasing the gas hour space velocity (GHSV) reduced VOCs conversion when various catalysts were used. Numerous studies stated that a higher reaction temperature is necessary at higher inlet VOCs concentrations [16,17]. In this study, various Cu catalysts with different metal loadings were prepared following various procedures. Their performance in the combustion of the IPA was screened to identify the optimal metal loading and the preparation procedure. The optimal metal loading and preparation procedure Cu, Co and Cu–Co catalysts were investigated because Cu, Co and Cu–Co metal oxides perform excellently in combustion of toluene in previous authors’ work [18]. A catalyst in the RCO system was then chosen to measure the combustion efficiency of IPA and economic performance under various operational conditions. Finally, the stability of the catalyst was studied by SEM/EDS, as proven by SEM/EDS. 2. Materials and methods 2.1. Preparation of catalyst Copper metals were supported on ceramic honeycombs (CHs) by deposition–precipitation, wet impregnation and incipient impregnation, to evaluate the effect of the preparation
procedures on catalytic activity. The aforementioned preparation procedures are described below. 2.1.1. Procedure A (deposition–precipitation) 0.2 M K2 CO3 aqueous solution was added to an appropriate concentration of Cu(NO3 )2 ·3H2 O aqueous solution with ceramic honeycomb in a water bath at 85 ◦ C for 60 min. The gel-like precipitate that formed and settled during that time was then separated from the initial solution and washed three times each with 2 L distilled water. The catalyst precursors were then dried at 105 ◦ C for 24 h and calcined in air (2 L/min) at 600 ◦ C for 4 h. 2.1.2. Procedure B (wet impregnation) Ceramic honeycomb was drenched with a suitable concentration of aqueous Cu(NO3 )2 ·3H2 O; the volume of the solution slightly exceeded the pore volume of the support; the precursors were then heated to 80 ◦ C on a heating board for 30 min, dried at 105 ◦ C for 24 h and calcined in air (2 L/min) at 600 ◦ C for 4 h. 2.1.3. Procedure C (incipient impregnation) Ceramic honeycomb was drenched with a suitable concentration of aqueous Cu (NO3 )2 ·3H2 O, and the volume of the solution was equal to the pore volume of the support; subsequently, the precursors were air-dried for 6 h; dried at 105 ◦ C for 24 h and calcined in air (2 L/min) at 600 ◦ C for 4 h. 2.2. Experimental instruments The two main instruments applied herein were the catalytic combustion system and the RCO. A catalytic reactor was used for determining the catalytic activity of the catalysts prepared. Then the catalysts exhibited the highest activity were used in the RCO system to investigate the operational parameters and the economic performance. Fig. 1 depicts the catalytic combustion system. IPA, oxygen (O2 ) and air were used. The gases were filtered for remov-
Fig. 1. Experimental instrument for the catalytic combustion system.
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Fig. 2. Diagram of the experimental RCO.
ing water and impurities necessary to prevent the flow meters from malfunctioning. The gases were completely mixed in a mixer before being sent to the catalytic reactor. The catalytic reactor was a steel tube with dimensions 75 mm o.d. × 70 mm i.d. × 100 cm long. The temperature of the catalyst was measured using a K-type thermocouple placed in the middle of the catalyst bed. Fig. 2 presents the diagram of the experimental RCO. The RCO is electrically heated using a maximum power of 4 kW and contains two beds with packing media. Each bed with dimensions 15 cm length × 15 cm width × 1 m height and has four chambers; the chamber that was closest to the heating zone (A1 and B1) was filled with catalysts, and the others were filled with gravel particles. The regenerative beds were constructed from stainless steel and the outer walls of both beds and the heads were all wrapped in ceramic fiber sheets for thermal insulation. The RCO is also equipped with 11 thermal couples (8 for each chamber, 1 for the heating zone, 1 for the inlet gas and 1 for the outlet exhaust), four electromagnetic valves and a timer system to shift the influent air stream, one blower with a normal capacity of 2.5 m3 /min, and one control system to establish a thermostat condition in the heating zone. The gravels have the following physical properties; average equivalent diameter = 1.03 cm, sphericity = 0.67, true density = 2100 kg/m3 , specific heat = 840 J/kg K, and thermal conductivity = 1.9 W/m2 K. Finally, in both the catalytic combustion system and the RCO system, the IPA concentrations were analyzed by gas chromatograph with a flame ionization detector (GC-FID); the CO2 and CO in the effluent gas was measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). The conversion of VOCs (IPA and by-product) and the yield of CO2 (for IPA oxidation only) were calculated using the following equations: X(%) =
I −O × 100 I
(1)
YCO2 (%) =
PCO2 × 100 3I
(2)
where I is the inlet IPA concentration; O the outlet IPA concentration, and P denotes the production rate of CO2 . The 3 is added to the YCO2 equation because each IPA molecule contains three carbon atoms. 2.3. Screening catalysts in catalytic combustion system The reaction was performed at 100–500 ◦ C, with an inlet IPA concentration of 1600 ppm, an oxygen concentration of 21%, a space velocity of 12,000 h−1 and a humidity of 25%. The temperature was increased from 100 ◦ C in steps of 50 ◦ C to 500 ◦ C. At each step, the temperature was held for 20 min and the exhausts were analyzed in the steady state. CO2 yields obtained using various catalysts were compared to identify the more effective catalysts, which were used in the RCO system to measure the combustion efficiency of IPA and economic performance under various operational conditions, as described below. 2.4. Test of the RCO Catalysts were placed in chambers A1 and B1, and the other chambers were filled with gravel. The RCO was operated by setting specified heating zone temperature Tset = 300 and 400 ◦ C, the gas flow rate Q = 0.5 and 1.0 m3 /min (for superficial gas velocities (Ug ) of 0.37 and 0.74 m/s, at an influent air temperature of ∼30 ◦ C) and a fixed bed valve shifting time = 2.0 min. When the gas temperatures in the bed and outlet stream were steady under a specified set of operational conditions, an inlet IPA concentration was sent to the air stream, and the exhausts were sampled and analyzed five times per 2 min. The TRE, the temperature difference between the inlet and the outlet gas (Td ), the temperature variations in the RCO, the pres-
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sure drops and the IPA conversions were recorded to elucidate energetic performance. 2.5. Stability of catalyst Variations of IPA conversion during 48 h of continued operation were investigated at an inlet IPA concentration of 200 ppm, a specified gas flow rate Q = 0.5 m3 /min (Ug = 0.37 m/s) and Tset = 300 and 400 ◦ C. Furthermore, the catalysts were analyzed by SEM/EDS to prove the stability of the catalyst and to characterize variations in its surface. 3. Results and discussion 3.1. Screening catalytic activity in catalytic combustion system Various weight loadings (5 wt.%, 10 wt.% and 20 wt.%) of Cu were supported on CHs by deposition–precipitation, wet impregnation and incipient impregnation. Fig. 3 plots the strip chart of CO2 yield when IPA was treated with various catalysts as a function of temperature. The CO2 yields followed the order deposition–precipitation > wet impregnation > incipient impregnation at 5 wt.% and 10 wt.% (Fig. 3a and b), while at higher metal weight loading (20 wt.%), the order was wet impregnation > incipient impregnation > deposition–precipitation (Fig. 3c). The temperature required for 50% CO2 (T50 ) and 95% CO2 (T95 ) yields when IPA was treated with various catalysts (Table 1). The lowest temperatures T50 and T95 , 290 and 400 ◦ C, respectively, were recorded with 20 wt.% Cu/(CH) catalyst prepared by wet impregnation. Given that 20 wt.% Cu/(CH) catalyst (wet impregnation) was preferred for the combustion of IPA, the effect of altering the Cu loading of this catalyst on catalytic performance (Fig. 4) was examined. Fig. 4 indicates that the optimal Cu loading is about 20 wt.%, because 25 and 30 wt.% Cu on the support leaded to large CuO crystals, with few active sites, reducing removal efficiency [17]. With respect to the effect of mixed and simplex metal oxide catalysts, Fig. 5 plots the CO2 yield when IPA was treated with 20 wt.% Cu/(CH), 20 wt.% Co/(CH) and 20 wt.% Cu–Co/(CH) catalysts as a function of temperature. The preparation of 20 wt.% Cu–Co/(CH) catalyst was as described below. Table 1 T50 and T95 of various catalysts Preparation procedure
Metal loading (wt.% Cu)
T50 (◦ C)
T95 (◦ C)
Deposition–precipitation
5 10 20 5 10 20 5 10 20
385 375 340 420 340 290 430 375 300
– 475 465 – 475 400 – 475 430
Wet impregnation
Incipient impregnation
Fig. 3. CO2 yield for treating IPA on various 5 wt.%, 10 wt.% and 20 wt.% catalysts as function of temperature: (A) deposition–precipitation; (B) wet impregnation; (C) incipient impregnation. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%.
Ceramic honeycomb was drenched with a suitable concentration of aqueous Cu(NO3 )2 ·3H2 O and Co(NO3 )2 ·6H2 O (weight ratio of Cu(NO3 )2 ·3H2 O/Co(NO3 )2 ·6H2 O = 1691/873, mole ratio of Cu/Co = 7/3), and the volume of the solution was slightly greater than the pore volume of the support. The remaining steps were the same as in the preparation of 20 wt.% Cu/(CH) by wet impregnation. According to Fig. 5, CO2 yields followed the order 20 wt.% Cu–Co/(CH) > 20 wt.% Cu/(CH) > 20 wt.% Co/(CH), which is similar to that in the literature [18–21], which demonstrates that
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Fig. 4. CO2 yield for treating IPA under different weight loadings of Cu as a function of temperature. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%, preparation procedure: wet impregnation.
mixed metal oxide catalysts outperform simplex metal oxide catalysts, perhaps because of the higher surface area and an increase in the catalyst reducibility [21]. Moreover, the 20 wt.% Cu/(CH) catalyst also performed excellently, with a CO2 yield of 95% at 400 ◦ C. Therefore, the 20 wt.% Cu–Co/(CH) (mole ratio = 7/3) and 20 wt.% Cu/(CH) catalysts were further investigated in the following RCO operations. 3.2. Test of the RCO 3.2.1. Gas velocity (Ug ) RTO refers to an absence of catalyst, and all the chambers were filled with gravels. Fig. 6 plots CO2 yield upon the treatment of IPA with various gas velocities and Tset values in various catalysts. The yield of CO2 increased as the Ug decreased at a given temperature in the RCO. For example, for the 20 wt.%
Fig. 5. CO2 yield for treating IPA on 20 wt.% Cu/(CH), 20 wt.% Co/(CH) and 20 wt.% Cu–Co/(CH) catalysts as a function of temperature. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%, preparation procedure: wet impregnation.
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Fig. 6. CO2 yield for treating IPA for various Ug and Tset values in various catalysts. Inlet IPA concentration = 200 ppm, shifting time = 2 min.
Cu–Co/(CH) catalyst at Tset = 300 ◦ C, the difference between CO2 yields at Ug values of 0.37 and 0.74 m/s was 10.0% and that at Tset = 400 ◦ C was 0.4%. Therefore, the effect of the Ug was not significant at high temperature and high CO2 yield. 3.2.2. Inlet IPA concentration Fig. 7 plots the yield of CO2 upon the treatment of IPA for various inlet IPA concentrations and Tset values in various catalysts. A higher VOCs concentration normally corresponded to lower VOCs conversion [22,23], because a massive inlet VOCs concentration was adsorbed on the catalyst surface active sites, saturating the contact area between the reactants and the catalyst. The above phenomenon was observed on 20 wt.% Cu/(CH) catalyst but not on 20 wt.% Cu–Co/(CH) catalyst. The 20 wt.% Cu–Co/(CH) catalyst probably provided more active sites, reducing the effect of inlet IPA concentration. Catalytic oxidation of higher IPA concentration also released more thermal energy, increasing the temperature of the catalytic bed (A1 and B1 chambers) in the RCO, increasing the CO2 yield.
Fig. 7. CO2 yield for treating IPA for various inlet IPA concentrations and Tset values in various catalysts: Ug = 0.74 m/s, shifting time = 2 min.
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Table 2 TRE of the RCO at different operation conditions Catalyst type
Ug (m/s)
Tset (◦ C)
Tmax (◦ C)
Ti (◦ C)
To (◦ C)
Td (◦ C)
TRE (%)
RTO
0.37 0.74 0.37 0.74 0.37 0.74 0.37 0.74 0.37 0.74 0.37 0.74
300 300 400 400 800 800 900 900 300 300 400 400
256.0 274.0 325.0 350.0 621.0 637.0 701.0 720.0 263.0 270.0 343.0 350.0
27 22 24 24 30 29 29 28 31 25 28 30
43.4 45.0 43.7 52.0 75.0 102.0 83.2 113.0 53.1 55.0 55.8 65.1
16.4 23.0 19.7 28.0 45.0 73.0 54.2 85.0 22.1 30.0 27.8 35.1
92.8 90.9 93.5 91.4 90.9 88.0 91.9 87.7 90.5 87.8 91.2 89.1
20 wt.%Cu–Co/(CH)
The Ug and inlet IPA concentration test demonstrate that 20 wt.% Cu–Co/(CH) catalyst outperformed the 20 wt.% Cu/(CH) catalyst because it provides higher CO2 yields and higher tolerances to Ug and inlet IPA concentration. Therefore, 20 wt.% Cu–Co/(CH) was adopted in the following experiments.
approximately 134 ◦ C (Ug = 0.74 m/s, Tset = 400 ◦ C)—greater than 106 ◦ C (Ug = 0. 74 m/s, Tset = 300 ◦ C) but less than 156 ◦ C (Ug = 0. 37 m/s, Tset = 400 ◦ C). The gradients in the middle chamber sections (A1–A4 and B1–B4) increased with Tset and decreased as Ug increased.
3.2.3. TRE and Td The TRE of a RTO and a RCO can be defined as,
3.2.5. Pressure drop Fig. 9 plots the pressure drops for various gas velocities and Tset values. Fig. 9 demonstrates that the pressure drop increased with Ug and Tset value, and the effect of the Ug was especially marked. For example, at Tset = 400 ◦ C, the pressure drop of the RCO (20 wt.% Cu–Co/(CH) catalyst) was 55.2 cm H2 O (Ug = 0.74 m/s), which is nearly three times 18.1 cm H2 O (Ug = 0.37 m/s). The pressure drops of the RCO slightly exceeded those of RTO under the same operating conditions, but the differences were not obvious.
TRE (%) =
Tmax − To × 100 Tmax − Ti
(3)
where Tmax is the maximum gas temperature in the beds (◦ C); Ti the temperature of the influent gas to the bed (◦ C), and To is the outlet gas temperature from the bed (◦ C). Table 2 shows the TRE of the RCO under different operating conditions. The table indicates that (1) TRE decreased as Ug increased at a fixed Tset ; (2) the RTO (gravels) had better TRE than the RCO (20 wt.% Cu–Co/(CH) catalyst) at Tset = 300–400 ◦ C; (3) the RTO had nearly the same TRE as the RCO, when RTO was at Tset = 800–900 ◦ C; (4) the TRE range in a RCO was from 87.8 to 91.2% under various operating conditions, so the RCO also performed excellently in TRE. As well as the TRE, the Td is important in estimating the lost thermal energy. From Table 2, Td of the RCO increased as the Ug and Tset value increased. Generally, on the premise that the VOC conversion reaches a VOC conversion of 95%, RCO combustion requires a temperature in the range 300–400 ◦ C, and RTO combustion requires a higher temperature in the range 800–900 ◦ C. From Table 2, the Td of the RCO ranged from 27.8 to 35.1 ◦ C at Tset = 300–400 ◦ C and the Td of RTO ranged from 45.0 to 85.0 ◦ C at Tset = 800–900 ◦ C. The Td of the RCO was much lower than that of the RTO, so the RCO used fuel more efficiently. 3.2.4. Temperature variations in the RCO Fig. 8 displays the steady-state temperature profiles in the RCO chambers with various operations. The temperature difference between the inlet and A4 was around 186 ◦ C (Ug = 0.74 m/s, Tset = 400 ◦ C)—greater than 139 ◦ C (Ug = 0. 74 m/s, Tset = 300 ◦ C) and 159 ◦ C (Ug = 0. 37 m/s, Tset = 400 ◦ C). Thereafter, the temperature gradients in the bed entrance and exit (inlet–A4 and outlet–B4) increased with either Tset or Ug . The temperature difference between A1 and A4 was
3.3. Selectivity of catalyst The catalysts and reactants typically determine the products of catalytic combustion. Previous IPA catalytic oxidation experiments have indicated that the reactants IPA, oxygen, acetone, CO2 , traces of CO, propene, acetaldehyde, propionaldehyde
Fig. 8. Steady-state temperature profiles in the RCO chambers with various operations.
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Fig. 9. Pressure drops for various Ug and Tset values in various catalysts. Inlet IPA concentration = 200 ppm, shifting time = 2 min.
were the main products. Catalysts with different loadings of active metal prepared following various methodologies exhibited different catalytic behavior [24–26]. Fig. 10 plots the conversion of IPA and the selectivity of products as a function of temperature. The experimental findings reveal that IPA oxidation on 20 wt.% Cu–Co/(CH) catalyst started at 100 ◦ C, reaching a total conversion at about 350 ◦ C. Acetone, propene, CO2 , traces of CO and water were the only products under the experimental conditions used. Acetone is formed at low temperature with a nearly 100% selectivity, which fell quickly to zero at higher temperatures with a corresponding increase of the selectivity to CO2 . The selectivity to CO2 increased to over 95% at 300 ◦ C, and the selectivity to propene remained at about 2–5% from 200 to 400 ◦ C. The selectivity to CO only was about 1–2% between 150 and 250 ◦ C, and the concentrations of CO were not detectable when the temperature rose to 250 ◦ C.
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Fig. 11. CO2 yield for treating IPA as reaction time under various Tset values. Inlet IPA concentration = 200 ppm, Ug = 0.37 m/s, shifting time = 2 min. Table 3 EDS test results of the 20 wt.% Cu–Co/(CH) catalyst Element
wt.%
at.%
O Al Si K Ca Co Cu
21.29 1.41 3.31 0.91 1.08 21.13 50.87
48.76 1.92 4.32 0.85 0.98 13.26 29.90
3.4. Evaluation of stability of catalyst Fig. 11 compares the catalytic oxidation of IPA at Tset = 300 and 400 ◦ C over 48 h period test. The CO2 yield was maintained from 83.9 to 87% at Tset = 300 ◦ C and from 93.9 to 96.4% at Tset = 400 ◦ C throughout the test. 20 wt.% Cu–Co/(CH) catalyst exhibited excellent stability at various CO2 yields and temperatures. Furthermore, SEM/EDS used to prove the stability of the catalyst and observe the variations of catalyst surface. The SEM image of 20 wt.% Cu–Co/(CH) catalyst demonstrates that the original particle size of the catalyst was nearly 100 nm; no coke, sintering or any destruction of spherical particles was observed. The results of the EDS test (Table 3) demonstrates that the major components were O, Al, Si, K, Ca, Cu and Co, of which O, Al, Si, K and Ca were the elements of ceramic honeycomb. Moreover, the Cu:Co mole ratio of about 7:3 was highly consistent with the predicted value. 4. Conclusions
Fig. 10. Conversion of IPA and the selectivity of products on 20 wt.% Cu–Co/(CH) catalyst as a function of temperature. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%.
IPA was well catalytically treated with various metal oxide catalysts in a RCO: some important conclusions are drawn. 20 wt.% Cu–Co/(CH) catalyst in a RCO was effective. Over 95% CO2 yields were obtained at Tset = 400 ◦ C, Ug = 0.37–0.74 m/s, IPA concentration = 200–400 ppm and shifting time = 2 min. Moreover, 20 wt.% Cu–Co/(CH) catalyst
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performed well in operations of the RCO with in terms of TRE, Td , pressure drop and selectivity to CO2 . Finally, the results of the stability test demonstrated that the catalyst was very stable at various CO2 yields and temperatures, as shown by SEM/EDS. This work indicates that the RCO packed with 20 wt.% Cu–Co/(CH) catalyst, which has potential in treating VOCs, helping industrial plants to reduce the emissions of VOCs and save costs. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract Number NSC 95-2221-E-110-040. Ted Knoy is appreciated for his editorial assistance. References [1] Y.S. Matros, Prospects for the use of unsteady-state processes in catalytic reactors, Zhoumal V. Kh. O. im. Mendeleeva (J. All-Union Chem. Soc.) 22 (1977) 576–579. [2] G.K. Boreskov, Y.S. Matros, Flow reversal of reaction mixture in a fixed catalyst bed, a way to increase the efficiency of chemical processes, Appl. Catal. 5 (1983) 337–342. [3] M.S. Chou, W.H. Cheng, W.S. Li, Performance characteristics of a regenerative catalytic oxidizer to treat VOC-contaminated air stream, J. Air Waste Manage. Assoc. 50 (2000) 2112–2119. [4] Y.S. Matros, G.A. Bunimovich, S.E. Patterson, S.F. Meyer, Is it economically feasible to use heterogeneous catalysts for VOC control in regenerative oxidizers? Catal. Today 27 (1996) 307–313. [5] A. Mario, M. Pietropaolo, Numerical evaluation of the energetic performances of structured and random packed beds in regenerative thermal oxidizers, Appl. Therm. Eng. 27 (2007) 762–770. [6] W.H. Cheng, M.S. Chou, W.S. Lee, B.J. Huang, Applications of lowtemperature regenerative thermal oxidizers to treat volatile organic compounds, J. Environ. Eng. 128 (4) (2002) 313–319. [7] B.S. Choi, J. Yi, Simulation and optimization on the regenerative thermal oxidation of volatile organic compounds, Chem. Eng. J. 76 (2000) 103–114. [8] T. Ataloglou, J. Vakros, K. Bourikas, C. Fountzoula, C. Kordulis, A. Lycourghiotis, Influence of the preparation method on the structure–activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation, Appl. Catal. B: Environ. 57 (2005) 299–312. [9] P.O. Larsson, A. Andersson, Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2 O3 for the combustion of CO, ethyl acetate and ethanol, Appl. Catal. B: Environ. 24 (2000) 175–192.
[10] T.K. Tseng, H. Chu, The kinetics of catalytic incineration of styrene over a MnO/Fe2 O3 catalyst, Sci. Total Environ. 275 (2001) 83–93. [11] M. Aleksandra, Ni-loaded catalyst containing ZSM-5 zeolite for toluene hydrogenation, Appl. Catal. A: Gen. 294 (2005) 260–272. [12] C.H. Wang, Al2 O3 -supported transition-metal oxide catalysts for catalytic incineration of toluene, Chemosphere 55 (2004) 11–17. [13] C.K. Sang, The catalytic oxidation of aromatic hydrocarbons over supported metal oxide, J. Hazard. Mater. 91 (2002) 285–299. [14] Z.A. Ahmad, Z.A.B. Mohamad, B. Subhash, Combustion of chlorinated volatile organic compounds (VOCs) using bimetallic chromium–copper supported on modified H-ZSM-5 catalyst, J. Hazard. Mater. 129 (2006) 39–49. [15] F. Wyrwalski, J.F. Lamonier, S. Siffert, A. Aboukais, Additional effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts, Appl. Catal. B: Environ. 70 (2007) 393–399. [16] J.C. Lou, Y.J. Tu, Incinerating volatile organic compounds with ferrospinel catalyst MnFe2 O4 : an example with isopropyl alcohol, J. Air Waste Manage. Assoc. 55 (2005) 1809–1815. [17] C.H. Weng, S.S. Lin, Preparing an active cerium oxide catalyst for the catalytic incineration of aromatic hydrocarbons, Appl. Catal. A: Gen. 268 (2004) 227–233. [18] S.H. Huang, J.C. Lou, Treatment of gaseous toluene using a catalytic incineration system, in: Proceedings of the Second International Conference on Environmental Science and Technology, Houston, TX, USA, 2006, p. 493. [19] M. Alifanti, M. Florea, V.I. Pˆarvulescu, Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC, Appl. Catal. B: Environ. 70 (2007) 400–405. [20] G. Toma´s, S. Benjam´yn´ , H.T. Stuart, Naphthalene total oxidation over metal oxide catalysts, Appl. Catal. B: Environ. 66 (2006) 92–99. [21] S.H. Taylor, G.J. Hutchings, M.L. Palacios, D.F. Lee, The partial oxidation of propane to formaldehyde using uranium mixed oxide catalysts, Catal. Today 81 (2003) 171–178. [22] C.H. Wang, S.S. Lin, C.L. Chen, H.S. Weng, Performance of the supported copper oxide catalysts for the catalytic incineration of aromatic hydrocarbons, Chemosphere 64 (2006) 503–509. [23] S.C. Kim, The catalytic oxidation of aromatic hydrocarbons over supported metal oxide, J. Hazard. Mater. B 91 (2002) 285–299. [24] R. Beauchet, P. Magnoux, J. Mijoin, Catalytic oxidation of volatile organic compounds (VOCs) mixture (isopropanol/o-xylene) on zeolite catalysts, Catal. Today 124 (2007) 118–123. [25] J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio, G. Busca, A study of anatase-supported Mn oxide as catalysts for 2-propanol oxidation, Appl. Catal. B: Environ. 22 (1999) 249–259. [26] S. Scir`e, S. Minic`o, C. Crisafulli, C. Satriano, A. Pistone, Catalytic combustion of volatile organic compounds on gold/cerium oxide catalysts, Appl. Catal. B: Environ. 40 (2003) 43–49.
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Treating isopropyl alcohol by a regenerative catalytic oxidizer Jie-Chung Lou ∗ , Shih-Wei Huang Institute of Environmental Engineering, National Sun Yat-Sen University, Len-Hai Road No. 70, Kaohsiung 80424, Taiwan Received 12 October 2007; received in revised form 19 December 2007; accepted 27 December 2007
Abstract Regenerative catalytic oxidizer (RCO) can be conveniently used to control emissions of volatile organic compounds (VOCs), because of their thermal recovery efficiency (TRE), low fuel cost and high oxidation. In this work, catalysts with various metal weight loadings were prepared by deposition–precipitation, wet impregnation and incipient impregnation to treat isopropyl alcohol (IPA). We used the excellent catalytic performance in a pilot RCO to test IPA oxidation performance under various conditions. The best catalyst was selected and its TRE, bed temperature variations, pressure drops and selectivity of the catalyst were more widely discussed. The results demonstrate that the optimal catalyst was prepared by wet impregnation with 20 wt.% metal on ceramic honeycomb (CH). 20 wt.% Cu–Co/(CH) catalyst was the best catalyst used in a RCO because it was effective in treating IPA, with a CO2 yield of up to 95% at a heating zone temperature (Tset ) = 400 ◦ C under various conditions. It also had the largest tolerance of variations in inlet IPA concentration and gas velocity (Ug ). This 20 wt.% Cu–Co/(CH) catalyst in a RCO performed well in terms of TRE, pressure drop and selectivity to CO2 . The TRE range in a RCO was from 87.8 to 91.2% under various conditions, and decreased as Ug increased in a fixed Tset . The pressure drop increased with Ug and Tset . The selectivity to CO2 increased to over 95% at 300 ◦ C, and that to propene remained at 2–5% from 200 to 400 ◦ C. Finally, the stability test results indicated that the 20 wt.% Cu–Co/(CH) catalyst was very stable at various CO2 yields and temperatures. © 2008 Elsevier B.V. All rights reserved. Keywords: Regenerative catalytic oxidizer; Thermal recovery efficiency; Isopropyl alcohol; Catalyst
1. Introduction Volatile organic compounds (VOCs) are emitted during industrial processes and in automobile exhaust emissions. VOCs are recognized as major contributors to air pollution because they are toxic: they are carcinogenic and can harm skin, neural systems, eyes and other organs. They can act indirectly as secondary pollutants in the formation of photochemical smog. Isopropyl alcohol (IPA) is a common organic solvent that is utilized not only as an industrial raw material, as a dehydrating agent, as an antifreeze, as an anticorrosive and in dry jets, but also in the production of gum and attar. Accordingly, IPA is strongly associated with problems of VOCs. Combustion has for a long time been adopted to decompose VOCs. Combustion refers to thermal combustion, catalytic combustion, regenerative thermal oxidizer (RTO) and regenerative catalytic oxidizer (RCO). In a RCO, chambers of inert regenerative materials are applied to heat the stream of VOCs by
∗
Corresponding author. Tel.: +886 7 5254410; fax: +886 7 5254411. E-mail address: [email protected] (J.-C. Lou).
1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.12.024
cooling burnt gases, through reverse flow operations, which cut fuel costs. Catalysts were used to reduce the required reaction temperature and to increase the efficiency of VOCs oxidation. The RCO was developed in the mid-1970s by Matros and coworkers [1,2]. The RCO was used to treat methyl ethyl ketone (MEK) and toluene by Chou et al. [3], providing a VOC removal efficiency of 98% for MEK and 95% for toluene. A comparison of RTO and RCO economics by Matros et al. [4] yielded the range of gas stream parameters for which RCOs are preferable. The RCO combines the advantages of lower temperature catalytic oxidation, higher VOCs removal efficiency, lower fuel cost and reduced formation of harmful by-products, in the most energy-efficient method of eliminating VOCs. Regarding regenerative materials, Mario and Pietropaolo [5] compared the energetic performances of random and structured regenerators, revealing that random packed bed regenerators are less attractive. Cheng et al. [6] carried out an experiment on RTOs with random packed irregular SiO2 pebbles. The outcomes indicated high removal efficiency in two-bed RTOs. Moreover, thermal recovery efficiency (TRE), pressure drop and the efficiency of VOCs removal were discussed by varying the gas flow rate and the inlet concentration of VOCs [7].
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In this work, metal oxide catalysts were used in the RCO system. They have been widely investigated in numerous studies. Three series of catalysts with various Co contents (up to 21 wt.% Co) were prepared using three preparation methods. The variously prepared catalysts with various Co wt.% exhibited different efficiencies [8]. Larsson and Andersson [9] noted excellent performance in catalytic combustion over CuOx /Al2 O3 and CuOx –CeO2 /Al2 O3 . Tseng et al. [10] reported excellent metal oxide catalyst activity (MnO/Fe2 O3 ) in the catalytic oxidation of styrene. Furthermore, several investigations have shown that the species of metal oxide, the metal loading and the preparation method affect the oxidation of VOCs [11–15]. With respect to gas flow rate and inlet concentration of VOCs, increasing the gas hour space velocity (GHSV) reduced VOCs conversion when various catalysts were used. Numerous studies stated that a higher reaction temperature is necessary at higher inlet VOCs concentrations [16,17]. In this study, various Cu catalysts with different metal loadings were prepared following various procedures. Their performance in the combustion of the IPA was screened to identify the optimal metal loading and the preparation procedure. The optimal metal loading and preparation procedure Cu, Co and Cu–Co catalysts were investigated because Cu, Co and Cu–Co metal oxides perform excellently in combustion of toluene in previous authors’ work [18]. A catalyst in the RCO system was then chosen to measure the combustion efficiency of IPA and economic performance under various operational conditions. Finally, the stability of the catalyst was studied by SEM/EDS, as proven by SEM/EDS. 2. Materials and methods 2.1. Preparation of catalyst Copper metals were supported on ceramic honeycombs (CHs) by deposition–precipitation, wet impregnation and incipient impregnation, to evaluate the effect of the preparation
procedures on catalytic activity. The aforementioned preparation procedures are described below. 2.1.1. Procedure A (deposition–precipitation) 0.2 M K2 CO3 aqueous solution was added to an appropriate concentration of Cu(NO3 )2 ·3H2 O aqueous solution with ceramic honeycomb in a water bath at 85 ◦ C for 60 min. The gel-like precipitate that formed and settled during that time was then separated from the initial solution and washed three times each with 2 L distilled water. The catalyst precursors were then dried at 105 ◦ C for 24 h and calcined in air (2 L/min) at 600 ◦ C for 4 h. 2.1.2. Procedure B (wet impregnation) Ceramic honeycomb was drenched with a suitable concentration of aqueous Cu(NO3 )2 ·3H2 O; the volume of the solution slightly exceeded the pore volume of the support; the precursors were then heated to 80 ◦ C on a heating board for 30 min, dried at 105 ◦ C for 24 h and calcined in air (2 L/min) at 600 ◦ C for 4 h. 2.1.3. Procedure C (incipient impregnation) Ceramic honeycomb was drenched with a suitable concentration of aqueous Cu (NO3 )2 ·3H2 O, and the volume of the solution was equal to the pore volume of the support; subsequently, the precursors were air-dried for 6 h; dried at 105 ◦ C for 24 h and calcined in air (2 L/min) at 600 ◦ C for 4 h. 2.2. Experimental instruments The two main instruments applied herein were the catalytic combustion system and the RCO. A catalytic reactor was used for determining the catalytic activity of the catalysts prepared. Then the catalysts exhibited the highest activity were used in the RCO system to investigate the operational parameters and the economic performance. Fig. 1 depicts the catalytic combustion system. IPA, oxygen (O2 ) and air were used. The gases were filtered for remov-
Fig. 1. Experimental instrument for the catalytic combustion system.
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Fig. 2. Diagram of the experimental RCO.
ing water and impurities necessary to prevent the flow meters from malfunctioning. The gases were completely mixed in a mixer before being sent to the catalytic reactor. The catalytic reactor was a steel tube with dimensions 75 mm o.d. × 70 mm i.d. × 100 cm long. The temperature of the catalyst was measured using a K-type thermocouple placed in the middle of the catalyst bed. Fig. 2 presents the diagram of the experimental RCO. The RCO is electrically heated using a maximum power of 4 kW and contains two beds with packing media. Each bed with dimensions 15 cm length × 15 cm width × 1 m height and has four chambers; the chamber that was closest to the heating zone (A1 and B1) was filled with catalysts, and the others were filled with gravel particles. The regenerative beds were constructed from stainless steel and the outer walls of both beds and the heads were all wrapped in ceramic fiber sheets for thermal insulation. The RCO is also equipped with 11 thermal couples (8 for each chamber, 1 for the heating zone, 1 for the inlet gas and 1 for the outlet exhaust), four electromagnetic valves and a timer system to shift the influent air stream, one blower with a normal capacity of 2.5 m3 /min, and one control system to establish a thermostat condition in the heating zone. The gravels have the following physical properties; average equivalent diameter = 1.03 cm, sphericity = 0.67, true density = 2100 kg/m3 , specific heat = 840 J/kg K, and thermal conductivity = 1.9 W/m2 K. Finally, in both the catalytic combustion system and the RCO system, the IPA concentrations were analyzed by gas chromatograph with a flame ionization detector (GC-FID); the CO2 and CO in the effluent gas was measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). The conversion of VOCs (IPA and by-product) and the yield of CO2 (for IPA oxidation only) were calculated using the following equations: X(%) =
I −O × 100 I
(1)
YCO2 (%) =
PCO2 × 100 3I
(2)
where I is the inlet IPA concentration; O the outlet IPA concentration, and P denotes the production rate of CO2 . The 3 is added to the YCO2 equation because each IPA molecule contains three carbon atoms. 2.3. Screening catalysts in catalytic combustion system The reaction was performed at 100–500 ◦ C, with an inlet IPA concentration of 1600 ppm, an oxygen concentration of 21%, a space velocity of 12,000 h−1 and a humidity of 25%. The temperature was increased from 100 ◦ C in steps of 50 ◦ C to 500 ◦ C. At each step, the temperature was held for 20 min and the exhausts were analyzed in the steady state. CO2 yields obtained using various catalysts were compared to identify the more effective catalysts, which were used in the RCO system to measure the combustion efficiency of IPA and economic performance under various operational conditions, as described below. 2.4. Test of the RCO Catalysts were placed in chambers A1 and B1, and the other chambers were filled with gravel. The RCO was operated by setting specified heating zone temperature Tset = 300 and 400 ◦ C, the gas flow rate Q = 0.5 and 1.0 m3 /min (for superficial gas velocities (Ug ) of 0.37 and 0.74 m/s, at an influent air temperature of ∼30 ◦ C) and a fixed bed valve shifting time = 2.0 min. When the gas temperatures in the bed and outlet stream were steady under a specified set of operational conditions, an inlet IPA concentration was sent to the air stream, and the exhausts were sampled and analyzed five times per 2 min. The TRE, the temperature difference between the inlet and the outlet gas (Td ), the temperature variations in the RCO, the pres-
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sure drops and the IPA conversions were recorded to elucidate energetic performance. 2.5. Stability of catalyst Variations of IPA conversion during 48 h of continued operation were investigated at an inlet IPA concentration of 200 ppm, a specified gas flow rate Q = 0.5 m3 /min (Ug = 0.37 m/s) and Tset = 300 and 400 ◦ C. Furthermore, the catalysts were analyzed by SEM/EDS to prove the stability of the catalyst and to characterize variations in its surface. 3. Results and discussion 3.1. Screening catalytic activity in catalytic combustion system Various weight loadings (5 wt.%, 10 wt.% and 20 wt.%) of Cu were supported on CHs by deposition–precipitation, wet impregnation and incipient impregnation. Fig. 3 plots the strip chart of CO2 yield when IPA was treated with various catalysts as a function of temperature. The CO2 yields followed the order deposition–precipitation > wet impregnation > incipient impregnation at 5 wt.% and 10 wt.% (Fig. 3a and b), while at higher metal weight loading (20 wt.%), the order was wet impregnation > incipient impregnation > deposition–precipitation (Fig. 3c). The temperature required for 50% CO2 (T50 ) and 95% CO2 (T95 ) yields when IPA was treated with various catalysts (Table 1). The lowest temperatures T50 and T95 , 290 and 400 ◦ C, respectively, were recorded with 20 wt.% Cu/(CH) catalyst prepared by wet impregnation. Given that 20 wt.% Cu/(CH) catalyst (wet impregnation) was preferred for the combustion of IPA, the effect of altering the Cu loading of this catalyst on catalytic performance (Fig. 4) was examined. Fig. 4 indicates that the optimal Cu loading is about 20 wt.%, because 25 and 30 wt.% Cu on the support leaded to large CuO crystals, with few active sites, reducing removal efficiency [17]. With respect to the effect of mixed and simplex metal oxide catalysts, Fig. 5 plots the CO2 yield when IPA was treated with 20 wt.% Cu/(CH), 20 wt.% Co/(CH) and 20 wt.% Cu–Co/(CH) catalysts as a function of temperature. The preparation of 20 wt.% Cu–Co/(CH) catalyst was as described below. Table 1 T50 and T95 of various catalysts Preparation procedure
Metal loading (wt.% Cu)
T50 (◦ C)
T95 (◦ C)
Deposition–precipitation
5 10 20 5 10 20 5 10 20
385 375 340 420 340 290 430 375 300
– 475 465 – 475 400 – 475 430
Wet impregnation
Incipient impregnation
Fig. 3. CO2 yield for treating IPA on various 5 wt.%, 10 wt.% and 20 wt.% catalysts as function of temperature: (A) deposition–precipitation; (B) wet impregnation; (C) incipient impregnation. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%.
Ceramic honeycomb was drenched with a suitable concentration of aqueous Cu(NO3 )2 ·3H2 O and Co(NO3 )2 ·6H2 O (weight ratio of Cu(NO3 )2 ·3H2 O/Co(NO3 )2 ·6H2 O = 1691/873, mole ratio of Cu/Co = 7/3), and the volume of the solution was slightly greater than the pore volume of the support. The remaining steps were the same as in the preparation of 20 wt.% Cu/(CH) by wet impregnation. According to Fig. 5, CO2 yields followed the order 20 wt.% Cu–Co/(CH) > 20 wt.% Cu/(CH) > 20 wt.% Co/(CH), which is similar to that in the literature [18–21], which demonstrates that
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Fig. 4. CO2 yield for treating IPA under different weight loadings of Cu as a function of temperature. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%, preparation procedure: wet impregnation.
mixed metal oxide catalysts outperform simplex metal oxide catalysts, perhaps because of the higher surface area and an increase in the catalyst reducibility [21]. Moreover, the 20 wt.% Cu/(CH) catalyst also performed excellently, with a CO2 yield of 95% at 400 ◦ C. Therefore, the 20 wt.% Cu–Co/(CH) (mole ratio = 7/3) and 20 wt.% Cu/(CH) catalysts were further investigated in the following RCO operations. 3.2. Test of the RCO 3.2.1. Gas velocity (Ug ) RTO refers to an absence of catalyst, and all the chambers were filled with gravels. Fig. 6 plots CO2 yield upon the treatment of IPA with various gas velocities and Tset values in various catalysts. The yield of CO2 increased as the Ug decreased at a given temperature in the RCO. For example, for the 20 wt.%
Fig. 5. CO2 yield for treating IPA on 20 wt.% Cu/(CH), 20 wt.% Co/(CH) and 20 wt.% Cu–Co/(CH) catalysts as a function of temperature. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%, preparation procedure: wet impregnation.
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Fig. 6. CO2 yield for treating IPA for various Ug and Tset values in various catalysts. Inlet IPA concentration = 200 ppm, shifting time = 2 min.
Cu–Co/(CH) catalyst at Tset = 300 ◦ C, the difference between CO2 yields at Ug values of 0.37 and 0.74 m/s was 10.0% and that at Tset = 400 ◦ C was 0.4%. Therefore, the effect of the Ug was not significant at high temperature and high CO2 yield. 3.2.2. Inlet IPA concentration Fig. 7 plots the yield of CO2 upon the treatment of IPA for various inlet IPA concentrations and Tset values in various catalysts. A higher VOCs concentration normally corresponded to lower VOCs conversion [22,23], because a massive inlet VOCs concentration was adsorbed on the catalyst surface active sites, saturating the contact area between the reactants and the catalyst. The above phenomenon was observed on 20 wt.% Cu/(CH) catalyst but not on 20 wt.% Cu–Co/(CH) catalyst. The 20 wt.% Cu–Co/(CH) catalyst probably provided more active sites, reducing the effect of inlet IPA concentration. Catalytic oxidation of higher IPA concentration also released more thermal energy, increasing the temperature of the catalytic bed (A1 and B1 chambers) in the RCO, increasing the CO2 yield.
Fig. 7. CO2 yield for treating IPA for various inlet IPA concentrations and Tset values in various catalysts: Ug = 0.74 m/s, shifting time = 2 min.
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Table 2 TRE of the RCO at different operation conditions Catalyst type
Ug (m/s)
Tset (◦ C)
Tmax (◦ C)
Ti (◦ C)
To (◦ C)
Td (◦ C)
TRE (%)
RTO
0.37 0.74 0.37 0.74 0.37 0.74 0.37 0.74 0.37 0.74 0.37 0.74
300 300 400 400 800 800 900 900 300 300 400 400
256.0 274.0 325.0 350.0 621.0 637.0 701.0 720.0 263.0 270.0 343.0 350.0
27 22 24 24 30 29 29 28 31 25 28 30
43.4 45.0 43.7 52.0 75.0 102.0 83.2 113.0 53.1 55.0 55.8 65.1
16.4 23.0 19.7 28.0 45.0 73.0 54.2 85.0 22.1 30.0 27.8 35.1
92.8 90.9 93.5 91.4 90.9 88.0 91.9 87.7 90.5 87.8 91.2 89.1
20 wt.%Cu–Co/(CH)
The Ug and inlet IPA concentration test demonstrate that 20 wt.% Cu–Co/(CH) catalyst outperformed the 20 wt.% Cu/(CH) catalyst because it provides higher CO2 yields and higher tolerances to Ug and inlet IPA concentration. Therefore, 20 wt.% Cu–Co/(CH) was adopted in the following experiments.
approximately 134 ◦ C (Ug = 0.74 m/s, Tset = 400 ◦ C)—greater than 106 ◦ C (Ug = 0. 74 m/s, Tset = 300 ◦ C) but less than 156 ◦ C (Ug = 0. 37 m/s, Tset = 400 ◦ C). The gradients in the middle chamber sections (A1–A4 and B1–B4) increased with Tset and decreased as Ug increased.
3.2.3. TRE and Td The TRE of a RTO and a RCO can be defined as,
3.2.5. Pressure drop Fig. 9 plots the pressure drops for various gas velocities and Tset values. Fig. 9 demonstrates that the pressure drop increased with Ug and Tset value, and the effect of the Ug was especially marked. For example, at Tset = 400 ◦ C, the pressure drop of the RCO (20 wt.% Cu–Co/(CH) catalyst) was 55.2 cm H2 O (Ug = 0.74 m/s), which is nearly three times 18.1 cm H2 O (Ug = 0.37 m/s). The pressure drops of the RCO slightly exceeded those of RTO under the same operating conditions, but the differences were not obvious.
TRE (%) =
Tmax − To × 100 Tmax − Ti
(3)
where Tmax is the maximum gas temperature in the beds (◦ C); Ti the temperature of the influent gas to the bed (◦ C), and To is the outlet gas temperature from the bed (◦ C). Table 2 shows the TRE of the RCO under different operating conditions. The table indicates that (1) TRE decreased as Ug increased at a fixed Tset ; (2) the RTO (gravels) had better TRE than the RCO (20 wt.% Cu–Co/(CH) catalyst) at Tset = 300–400 ◦ C; (3) the RTO had nearly the same TRE as the RCO, when RTO was at Tset = 800–900 ◦ C; (4) the TRE range in a RCO was from 87.8 to 91.2% under various operating conditions, so the RCO also performed excellently in TRE. As well as the TRE, the Td is important in estimating the lost thermal energy. From Table 2, Td of the RCO increased as the Ug and Tset value increased. Generally, on the premise that the VOC conversion reaches a VOC conversion of 95%, RCO combustion requires a temperature in the range 300–400 ◦ C, and RTO combustion requires a higher temperature in the range 800–900 ◦ C. From Table 2, the Td of the RCO ranged from 27.8 to 35.1 ◦ C at Tset = 300–400 ◦ C and the Td of RTO ranged from 45.0 to 85.0 ◦ C at Tset = 800–900 ◦ C. The Td of the RCO was much lower than that of the RTO, so the RCO used fuel more efficiently. 3.2.4. Temperature variations in the RCO Fig. 8 displays the steady-state temperature profiles in the RCO chambers with various operations. The temperature difference between the inlet and A4 was around 186 ◦ C (Ug = 0.74 m/s, Tset = 400 ◦ C)—greater than 139 ◦ C (Ug = 0. 74 m/s, Tset = 300 ◦ C) and 159 ◦ C (Ug = 0. 37 m/s, Tset = 400 ◦ C). Thereafter, the temperature gradients in the bed entrance and exit (inlet–A4 and outlet–B4) increased with either Tset or Ug . The temperature difference between A1 and A4 was
3.3. Selectivity of catalyst The catalysts and reactants typically determine the products of catalytic combustion. Previous IPA catalytic oxidation experiments have indicated that the reactants IPA, oxygen, acetone, CO2 , traces of CO, propene, acetaldehyde, propionaldehyde
Fig. 8. Steady-state temperature profiles in the RCO chambers with various operations.
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Fig. 9. Pressure drops for various Ug and Tset values in various catalysts. Inlet IPA concentration = 200 ppm, shifting time = 2 min.
were the main products. Catalysts with different loadings of active metal prepared following various methodologies exhibited different catalytic behavior [24–26]. Fig. 10 plots the conversion of IPA and the selectivity of products as a function of temperature. The experimental findings reveal that IPA oxidation on 20 wt.% Cu–Co/(CH) catalyst started at 100 ◦ C, reaching a total conversion at about 350 ◦ C. Acetone, propene, CO2 , traces of CO and water were the only products under the experimental conditions used. Acetone is formed at low temperature with a nearly 100% selectivity, which fell quickly to zero at higher temperatures with a corresponding increase of the selectivity to CO2 . The selectivity to CO2 increased to over 95% at 300 ◦ C, and the selectivity to propene remained at about 2–5% from 200 to 400 ◦ C. The selectivity to CO only was about 1–2% between 150 and 250 ◦ C, and the concentrations of CO were not detectable when the temperature rose to 250 ◦ C.
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Fig. 11. CO2 yield for treating IPA as reaction time under various Tset values. Inlet IPA concentration = 200 ppm, Ug = 0.37 m/s, shifting time = 2 min. Table 3 EDS test results of the 20 wt.% Cu–Co/(CH) catalyst Element
wt.%
at.%
O Al Si K Ca Co Cu
21.29 1.41 3.31 0.91 1.08 21.13 50.87
48.76 1.92 4.32 0.85 0.98 13.26 29.90
3.4. Evaluation of stability of catalyst Fig. 11 compares the catalytic oxidation of IPA at Tset = 300 and 400 ◦ C over 48 h period test. The CO2 yield was maintained from 83.9 to 87% at Tset = 300 ◦ C and from 93.9 to 96.4% at Tset = 400 ◦ C throughout the test. 20 wt.% Cu–Co/(CH) catalyst exhibited excellent stability at various CO2 yields and temperatures. Furthermore, SEM/EDS used to prove the stability of the catalyst and observe the variations of catalyst surface. The SEM image of 20 wt.% Cu–Co/(CH) catalyst demonstrates that the original particle size of the catalyst was nearly 100 nm; no coke, sintering or any destruction of spherical particles was observed. The results of the EDS test (Table 3) demonstrates that the major components were O, Al, Si, K, Ca, Cu and Co, of which O, Al, Si, K and Ca were the elements of ceramic honeycomb. Moreover, the Cu:Co mole ratio of about 7:3 was highly consistent with the predicted value. 4. Conclusions
Fig. 10. Conversion of IPA and the selectivity of products on 20 wt.% Cu–Co/(CH) catalyst as a function of temperature. Inlet IPA concentration = 1600 ppm, GHSV = 12,000 h−1 , O2 = 21%.
IPA was well catalytically treated with various metal oxide catalysts in a RCO: some important conclusions are drawn. 20 wt.% Cu–Co/(CH) catalyst in a RCO was effective. Over 95% CO2 yields were obtained at Tset = 400 ◦ C, Ug = 0.37–0.74 m/s, IPA concentration = 200–400 ppm and shifting time = 2 min. Moreover, 20 wt.% Cu–Co/(CH) catalyst
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performed well in operations of the RCO with in terms of TRE, Td , pressure drop and selectivity to CO2 . Finally, the results of the stability test demonstrated that the catalyst was very stable at various CO2 yields and temperatures, as shown by SEM/EDS. This work indicates that the RCO packed with 20 wt.% Cu–Co/(CH) catalyst, which has potential in treating VOCs, helping industrial plants to reduce the emissions of VOCs and save costs. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract Number NSC 95-2221-E-110-040. Ted Knoy is appreciated for his editorial assistance. References [1] Y.S. Matros, Prospects for the use of unsteady-state processes in catalytic reactors, Zhoumal V. Kh. O. im. Mendeleeva (J. All-Union Chem. Soc.) 22 (1977) 576–579. [2] G.K. Boreskov, Y.S. Matros, Flow reversal of reaction mixture in a fixed catalyst bed, a way to increase the efficiency of chemical processes, Appl. Catal. 5 (1983) 337–342. [3] M.S. Chou, W.H. Cheng, W.S. Li, Performance characteristics of a regenerative catalytic oxidizer to treat VOC-contaminated air stream, J. Air Waste Manage. Assoc. 50 (2000) 2112–2119. [4] Y.S. Matros, G.A. Bunimovich, S.E. Patterson, S.F. Meyer, Is it economically feasible to use heterogeneous catalysts for VOC control in regenerative oxidizers? Catal. Today 27 (1996) 307–313. [5] A. Mario, M. Pietropaolo, Numerical evaluation of the energetic performances of structured and random packed beds in regenerative thermal oxidizers, Appl. Therm. Eng. 27 (2007) 762–770. [6] W.H. Cheng, M.S. Chou, W.S. Lee, B.J. Huang, Applications of lowtemperature regenerative thermal oxidizers to treat volatile organic compounds, J. Environ. Eng. 128 (4) (2002) 313–319. [7] B.S. Choi, J. Yi, Simulation and optimization on the regenerative thermal oxidation of volatile organic compounds, Chem. Eng. J. 76 (2000) 103–114. [8] T. Ataloglou, J. Vakros, K. Bourikas, C. Fountzoula, C. Kordulis, A. Lycourghiotis, Influence of the preparation method on the structure–activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation, Appl. Catal. B: Environ. 57 (2005) 299–312. [9] P.O. Larsson, A. Andersson, Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2 O3 for the combustion of CO, ethyl acetate and ethanol, Appl. Catal. B: Environ. 24 (2000) 175–192.
[10] T.K. Tseng, H. Chu, The kinetics of catalytic incineration of styrene over a MnO/Fe2 O3 catalyst, Sci. Total Environ. 275 (2001) 83–93. [11] M. Aleksandra, Ni-loaded catalyst containing ZSM-5 zeolite for toluene hydrogenation, Appl. Catal. A: Gen. 294 (2005) 260–272. [12] C.H. Wang, Al2 O3 -supported transition-metal oxide catalysts for catalytic incineration of toluene, Chemosphere 55 (2004) 11–17. [13] C.K. Sang, The catalytic oxidation of aromatic hydrocarbons over supported metal oxide, J. Hazard. Mater. 91 (2002) 285–299. [14] Z.A. Ahmad, Z.A.B. Mohamad, B. Subhash, Combustion of chlorinated volatile organic compounds (VOCs) using bimetallic chromium–copper supported on modified H-ZSM-5 catalyst, J. Hazard. Mater. 129 (2006) 39–49. [15] F. Wyrwalski, J.F. Lamonier, S. Siffert, A. Aboukais, Additional effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts, Appl. Catal. B: Environ. 70 (2007) 393–399. [16] J.C. Lou, Y.J. Tu, Incinerating volatile organic compounds with ferrospinel catalyst MnFe2 O4 : an example with isopropyl alcohol, J. Air Waste Manage. Assoc. 55 (2005) 1809–1815. [17] C.H. Weng, S.S. Lin, Preparing an active cerium oxide catalyst for the catalytic incineration of aromatic hydrocarbons, Appl. Catal. A: Gen. 268 (2004) 227–233. [18] S.H. Huang, J.C. Lou, Treatment of gaseous toluene using a catalytic incineration system, in: Proceedings of the Second International Conference on Environmental Science and Technology, Houston, TX, USA, 2006, p. 493. [19] M. Alifanti, M. Florea, V.I. Pˆarvulescu, Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC, Appl. Catal. B: Environ. 70 (2007) 400–405. [20] G. Toma´s, S. Benjam´yn´ , H.T. Stuart, Naphthalene total oxidation over metal oxide catalysts, Appl. Catal. B: Environ. 66 (2006) 92–99. [21] S.H. Taylor, G.J. Hutchings, M.L. Palacios, D.F. Lee, The partial oxidation of propane to formaldehyde using uranium mixed oxide catalysts, Catal. Today 81 (2003) 171–178. [22] C.H. Wang, S.S. Lin, C.L. Chen, H.S. Weng, Performance of the supported copper oxide catalysts for the catalytic incineration of aromatic hydrocarbons, Chemosphere 64 (2006) 503–509. [23] S.C. Kim, The catalytic oxidation of aromatic hydrocarbons over supported metal oxide, J. Hazard. Mater. B 91 (2002) 285–299. [24] R. Beauchet, P. Magnoux, J. Mijoin, Catalytic oxidation of volatile organic compounds (VOCs) mixture (isopropanol/o-xylene) on zeolite catalysts, Catal. Today 124 (2007) 118–123. [25] J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio, G. Busca, A study of anatase-supported Mn oxide as catalysts for 2-propanol oxidation, Appl. Catal. B: Environ. 22 (1999) 249–259. [26] S. Scir`e, S. Minic`o, C. Crisafulli, C. Satriano, A. Pistone, Catalytic combustion of volatile organic compounds on gold/cerium oxide catalysts, Appl. Catal. B: Environ. 40 (2003) 43–49.