Decomposition of Gaseous Terephthalic Acid in the Presence of CaO

Publication Date (Web): January 21, 2011 ... To prevent these effects, TPA can be decarboxylated in the presence of calcium oxide (CaO) to obtain benz...
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Decomposition of Gaseous Terephthalic Acid in the Presence of CaO Shogo Kumagai,† Guido Grause,† Tomohito Kameda,† Tatsuo Takano,‡ Hideki Horiuchi,‡ and Toshiaki Yoshioka†,* † ‡

Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan Okutama Kogyo Co., Ltd., 107 Kuriharashinden, Mizuho-machi, Nishitama-gun, Tokyo 190-1204, Japan ABSTRACT: The decomposition of PET during thermal treatment of municipal waste results in the formation of sublimating substances such as terephthalic acid (TPA) and benzoic acid, causing blockage and corrosion of pipes in the treatment facilities. To prevent these effects, TPA can be decarboxylated in the presence of calcium oxide (CaO) to obtain benzene as the main product. However, high concentrations of TPA cause the formation of large char fractions, reducing the yield of desired products. In this investigation, TPA was decarboxylated using a fixed-bed reactor filled with CaO. To increase the yield of benzene and reduce the carbonaceous residue, the effects of pyrolysis temperature and TPA feed rate were investigated. The best results were achieved at 500 °C and a TPA feed rate of 51 mg L-1, yielding 67% benzene with a purity of 99.2% and a carbonaceous residue containing 18% of the initial carbon.

1. INTRODUCTION Poly(ethylene terephthalate) (PET) is a versatile plastic, allowing the production of various applications such as bottles, sheets, films, and fibers. It features high transparency and good gas barrier properties, making it suitable for food applications. The consumption of PET resin increased in Japan from 1585 kt in 20001 to 1831 kt in 2008,2 of which 30% was used for PET bottles2 and the remaining 70% of PET resin was used with other organic and inorganic substances such as fiber, sheet, and film. In Japan, used PET bottles are collected separately from other plastics and treated by mechanical recycling2 because of the high purity of the material. In contrast, other PET products are not separated and are treated together with other plastics by pyrolysis or incineration. Pyrolysis is a feasible alternative for treating materials consisting of organic and inorganic substances that cannot otherwise be treated by mechanical recycling. Some studies have been advanced to investigate the pyrolysis of PET and PET-containing materials. Grause et al.3 investigated the hydrolysis of PET using a fluidized bed reactor. Yoshioka et al.4investigated the pyrolysis of prepaid cards containing metals such as Ti, Ni, Fe, and Mo. Wang et al.5,6 investigated the mechanism underlying the pyrolysis of PET blends. However, terephthalic acid (TPA) is formed during the pyrolysis of PET, causing concerns of corrosion and blocking. Suppression of TPA generation was reported by Masuda et al.7,8 and Bhaskar et al.,9-11 who investigated the pyrolysis of PETcontaining mixed-waste plastics in the presence of FeOOH and Ca-based catalysts, respectively. Calcium oxide (CaO) is a strong Lewis base capable of reacting even with compounds possessing only a low acidity. Iizuka et al.12 showed that benzaldehyde was deprotonated on CaO by basic O2- sites and adsorbed at the CaO surface. From research in our laboratory, it is known that PET can be degraded without the appearance of TPA in the presence of calcium hydroxide (Ca(OH)2) and CaO/steam. Yoshioka et al.13,14 reported that TPA produced by the hydrolysis of PET is decarboxylated in the presence of Ca(OH)2, and that benzene is obtained without r 2011 American Chemical Society

producing sublimating substances. However, a large fraction of carbonaceous residue is formed during this process, reducing the yield of desired pyrolysis products. In this work, the insights gained from the degradation of PET were used to develop a method for removing TPA from a gas stream and converting it into useful products. TPA was used as a model substance for the decomposition of sublime PET degradation products using a CaO fixed-bed reactor. The effects of pyrolysis temperature and TPA feed rate on the benzene yield and the amount of carbonaceous residue were investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Ethanol, naphthalene, hydrochloric acid, calcium chloride dihydrate, and TPA were delivered by Kanto Chemical CO., Ltd. Disodium terephthalate was delivered by MERCK-Schuchardt Co., Ltd. The CaO, which had a particle size between 0.3 and 1.0 mm, was supplied by Okutama Industries CO., Ltd. Carbon dioxide (CO2) standard gas with a purity of 99.9% was obtained from GL-Science. 2.2. Pyrolysis Experiments. The experiments were carried out in a quartz tube with an inner diameter of 21 mm and a length of 745 mm, embedded in two independently heated electric furnaces (Figure 1) which separated the quartz tube into two heating zones: one for vaporizing TPA and one assigned as the decarboxylation chamber (filled with 8.1 g of CaO). Glass wool covered the tops of punched plates, preventing solid materials from passing through. The upper punched plate was set at the height of the thermocouple for the upper heater. The lower punched plate was arranged in such that way that the top of the CaO bed reached the thermocouple of the lower heater. The Received: July 8, 2010 Accepted: December 9, 2010 Revised: December 9, 2010 Published: January 21, 2011 1831

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Figure 1. Experimental apparatus. (1) water-pump; (2) steam generator; (3) electric furnace; (4) heated interface; (5) quartz tube reactor; (6) glass wool; (7) CaO bed; (8) thermocouple; (9) sample holder; (10) flow meter; (11) helium cylinder; (12) ice trap; (13) liquid nitrogen trap; and (14) gas bag.

sample holder was located at the top of the reactor. A constant helium flow of 35 mL min-1 prevented steam from condensing in the sample holder. Steam was produced by a steam generator consisting of a quartz tube with an inner diameter of 11 mm and a length of 450 mm, filled with glass wool, and embedded in an electric furnace set to 150 °C. Water was fed by a water-pump directly into the glass wool layer, which prevented water drops from being carried out with the steam. The connection between the steam generator and reactor was also heated to 150 °C. Before the experiment, the CaO bed was calcined at 900 °C for 1 h under a constant helium flow of 35 mL min-1 in order to decompose potential calcium carbonate (CaCO3). After calcination, the temperature of the TPA vaporizer was set to 450 °C and that of the decarboxylation chamber was set between 500 and 700 °C. When constant temperature conditions were achieved, the steam concentration was adjusted to 88 vol% (35 mL min-1 helium, 257 mL min-1 steam at 25 °C and 101 kPa). Terephthalic acid was fed into the reactor with a feed rate between 15 and 60 mg min-1 (51 and 210 mg L-1 at 25 °C and 101 kPa) and an input of 450 mg. The gaseous TPA was carried by the steam flow into the decarboxylation chamber, where it reacted with CaO. The reactor temperatures were maintained for another 90 min after the TPA feed was stopped. Gaseous products were collected in a gas bag. Liquid products were gathered in cooling traps cooled by ice and liquid nitrogen. The cooling traps were defrosted in a water bath after the end of the reaction time. The helium gas flow was maintained for another 5 min in order to transfer condensed gases into the gasbag. Liquid products were dissolved in ethanol after the experiment was terminated. Each experiment was conducted twice in order to verify the results. There were no larger discrepancies observed between two runs under the same conditions. 2.3. Preparation of Calcium Terephthalate. Calcium terephthalate anhydrate: 0.04 mol of disodium terephthalate and calcium chloride dihydrate were dissolved each in 200 mL of ionexchanged water. These solutions were mixed and stirred in a 600 mL beaker. The precipitate was separated by filtration and dried at 200 °C for 3 h (yield: 87%). Color: white. Elementary analysis: Found (% C, H, O, Ca): 46.4, 2.0, 35.2, 16.4;stoichiometric amounts (% C, H, O, Ca): 47.0, 2.0, 31.4, 19.6. FT-IR (cm-1) 3130, 3061 aromatic stretching vibration (C-H), 1572 asymmetric

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stretching vibration (CO2-), 1502, 1450 aromatic stretching vibration (CdC), 1350, 1331, 1306 symmetric stretching vibration (CO2-), 1150, 1097, 1020 para-aromatic in-plane bending vibration (C-H), 810 deformation vibration (CO2-), 752 paraaromatic out-of-plane bending vibration (C-H). 2.4. Identification of Calcium Terephthalate Intermediate. In order to verify the presence of calcium terephthalate on the surface of CaO at high temperatures, calcium oxide was milled to a particle size below 75 μm. 2.0 g of CaO was placed in the decarboxylation chamber (see 2.2 Pyrolysis Experiments). The He flow rate was adjusted to 35 mL min-1 and CaO was calcined at 900 °C for 1 h. After calcination, TPA vaporizer and decarboxylation chamber were set to 450 °C. After reaching constant temperature conditions, the steam concentration was adjusted to 88 vol%, and 1.0 g of TPA was fed into the reactor with a feed rate of 0.1 g min-1. The reactor temperatures were maintained for another 10 min after the TPA feed was stopped. The product was removed from the decarboxylation chamber and analyzed by XRD and FT-IR. A portion of the product was dissolved in 5 M HCl solution. The precipitated TPA was dried at 40 °C in vacuum for one day and analyzed by FT-IR. 2.5. Quantitative Determination of CaCO3. After the pyrolysis of TPA, CaO from the upper part (about 2 or 3 g) and the lower part (remaining about 6 or 5 g) were separated and balanced. Each of the fractions was agitated in order to obtain a homogeneous sample, but not ground, since the spent CaO can be regenerated and reused. Both samples were independently analyzed by placing 0.25 g of the CaO bed material in a 100 mL three-neck flask. After replacing air by helium for 30 min, the flask was connected with a gas bag and 5 mL of hydrochloric acid were injected by a syringe through a septum attached to the flask. The CO2 released during the dissolution of the CaO bed material was driven by the helium flow into the gas bag and quantified by GC-FID (see 2.6. Analytical Methods For quantification, 5 mL of CO2 standard gas were added to the gas bag and the change in the peak area was evaluated (addition method). 2.6. Analytical Methods. For qualitative and quantitative analysis, gaseous products were analyzed by gas chromatography using a thermal conductivity detector (GC-TCD) (GL Science 323, Carboplot P7, 40 °C (3 min) f 10 °C/min f150 °C (5 min)) or a flame ionization detector (GC-FID) (Shimazu GC-17A, CPPORA BOND Q, 50 °C (5 min) f 5 °C/min f320 °C (10 min)). Carbon monoxide and CO2 were converted into methane by a methanizer (GL Science MT 221) located between the column end and the FID detector. The quantitative determination of gaseous products was carried out by the internal standard method using CO2 (5 mL) as the standard substance. Liquid products were identified by gas chromatography mass spectrometry (GC-MS) (GC: HP6890, InertCap5MS/Sil, 50 °C (5 min) f 5 °C/min f320 °C (10 min); HP5973: 70 eV, 30-500 Da) using the MS library Wiley 275. Standard Chemistation (Hewlett-Packard G1017DA Revision D.01.02J) was used to operate the GC-MS. Quantitative analyses of the liquid products were carried out by GC-FID (GL Science GC390, InertCap5MS/Sil, 50 °C (5 min) f 5 °C/min f 320 °C (10 min)) using naphthalene as an internal standard substance for the quantification of all products. For X-ray powder diffraction (XRD) analysis, a powder diffractometer RINT-2200VHFþ/PC (Rigaku) was used. The XRD of the Cu-KR line was measured between 3° and 90°. Fourier transform infrared spectrometer (FT-IR) (ThermoFisher Scientific Nicolet 6700, ATR method) was used for identification of used filler, Ca(OH)2, TP-Ca, and TPA. 1832

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Table 1. Thermal Degradation Products of TPA at Temperatures between 500 and 700 °C T [°C]

500

600

700

liquids [wt%]

32

28

26

benzene

28

25

31

biphenyl others

0.3 0.0

0.3 0.1

1.1 0.1

68

48

45

hydrogen

0.0

0.0

0.1

methane

0.2

0.2

0.2

ethylene

0.1

0.0

0.0

carbon monoxide

0.2

0.1

0.5

carbon dioxide

0.4

9.9

40

carbon dioxide (as calcium carbonate)

67

37

3.7

others

0.1

0.0

0.1

gases [wt%]

sum [wt%]

96

74

77

carbonaceous residue [C%]

22

36

27

benzene yield [%] carbon dioxide yield [%]

59

54

66

128

89

83

3. RESULTS AND DISCUSSION 3.1. Effects of Reaction Temperature on Benzene and Carbonaceous Residue Yields. The influence of reaction tempera-

ture on the pyrolysis of TPA was carried out at a TPA feed rate of 30 mg min-1 (100 mg L-1). The results of the analyses are summarized in Table 1. The products of the pyrolysis of TPA were classified into three groups: liquids, gases, and carbonaceous residue. Liquid and gaseous products were standardized on the basis of the TPA weight (100 wt %). The yield of carbonaceous residue was calculated by subtracting the carbon content of all of the quantified product compounds from that of the initial TPA. The results for the carbonaceous residue are given in carbon% (C%). Complete conversion of TPA into benzene and CO2 would result in 47 and 53 wt %, respectively (Scheme 1). Hence, benzene and CO2 yields (Table 1) were calculated using the theoretical possible yields: yield½% ¼

practically achieved fraction½wt% theoretically achievable fraction½wt%

The benzene yield increased from 59% at 500 °C to 66% at 700 °C due to the accelerated decomposition of TP-Ca at the higher temperature. In addition, the yield of biphenyl increased at 700 °C because of the accelerated condensation of benzene at this temperature. Figure 2 shows the weight composition of the liquid products at different pyrolysis temperatures. Benzene accounted for a substantial fraction of the weight composition of the liquid product fraction. The highest benzene purity was obtained at 500 °C, reaching almost 99 wt %. The purity decreased with rising temperature since secondary reactions were accelerated by an increase in the reaction temperature. Figure 3 shows the weight composition of gaseous products at different pyrolysis temperatures. It is clear that CO2 was the main product at all temperatures. Although most of the CO2 was fixed as CaCO3 at 500 and 600 °C, almost all of the CaCO3 was released at 700 °C. Wang et al.15 investigated the behavior of CaCO3

decarbonation under He atmosphere at 500 °C, and showed that CO2 was released slowly at this temperature. It may be assumed that the decarboxylation of TPA resulted first in the formation of CaCO3, and that subsequently, CO2 was released according to the equilibrium between the solid and gaseous phases. The CO2 yield decreased with increasing temperature, and was higher than expected at 500 °C (yield: 128%). The excess carbon is likely to have been provided by the carbonaceous residue on the CaO surface since no further carbon source was present. McKee16 and Cannon et al.17 reported that CaCO3 catalyzes the gasification of carbon and water shift reaction while CaO does not. Even at a relatively low temperature of 500 °C, hydrocarbons might be converted into CO2. However, there was no evidence of hydrogen at this temperature. The presence of oxygen can also be excluded. The solubility of oxygen in the water used for steam generation is not high enough to oxidize such an amount of carbon; in addition, this effect occurs only at a single temperature, minimizing the possibility of a methodic error. Experiments repeated in order to confirm this result showed high reproducibility. At the present time, we are unable to explain the mechanism of excess CO2 formation. However, the presence of other gases such as methane and ethylene shows that aromatic rings were also destroyed at low temperatures. Traces of CO were formed at any temperature, showing that the decarboxylation of TPA resulted solely in CO2. CO was formed during the short time of contact between CO2 and char in the upper part of the CaO bed. 3.2. Effects of TPA Feed Rate on the Benzene Yield and Fraction of Carbonaceous Residue. It was confirmed by the results above that simultaneous improvement of the benzene yield and the fraction of carbonaceous residue could be achieved at 500 °C. The influence of TPA feed rate on the pyrolysis of TPA was investigated at a reaction temperature of 500 °C (Table 2). The highest benzene yield of 67% was obtained at a TPA concentration in the gas flow of 51 mg L-1. An increase in the TPA concentration caused the benzene yield to decrease to about 60%. Liquid byproducts (e.g., biphenyl) were observed only in small amounts. By reducing the TPA concentration, the formation of carbonaceous residue decreased, and simultaneously, the benzene yield increased. As a result, it became clear that a low TPA concentration favored the formation of benzene over the carbonaceous residue. Figure 4 shows that the composition of the liquid fraction did not change significantly with the TPA concentration. In all cases, a benzene purity of about 99 wt % was obtained. Figure 5 shows the composition of the gaseous products at different TPA gas concentrations. Carbon dioxide was obtained as the main product of the gas fraction. The CO2 yield exceeded the theoretical value at the two lowest TPA gas concentrations, while at a TPA concentration of 200 mg L-1 the CO2 concentration was significantly lower. The carbonaceous residue increased with the TPA gas concentration from 18 C% at 51 mg L-1 to 33 C% at 210 mg L-1. Most of the carbonaceous residue was formed in the upper part of CaO bed. With a higher TPA concentration in the gas, the distance between TPA molecules on the CaO surface would also have decreased, allowing neighboring TPA molecules to react with one another to form condensed hydrocarbons as precursors for the formation of carbonaceous residue (Figure 6). 3.3. TPA Degradation Mechanism and Formation of Carbonaceous Residue. It was always assumed by the authors that calcium terephthalate is formed as an intermediate of the benzene formation (Scheme 1). Figure 7a shows the XRD pattern of the 1833

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Scheme 1. Reaction Scheme of TPA Degradation in the Presence of CaO

Table 2. Thermal Degradation Products of TPA at Concentrations between 51 and 210 mg L-1 at 500 °C TPA gas concentrationa [mg L-1]

51

100

210

liquids [wt%]

29

32

28

benzene

31

28

29

biphenyl

0.2

0.3

0.2

others

0.0

0.0

0.1

67

68

44

hydrogen

0.0

0.0

0.0

methane

0.4

0.2

0.2

ethylene

0.0

0.1

0.0

carbon monoxide

0.4

0.2

0.2

carbon dioxide

2.4

0.4

3.4

carbon dioxide (as calcium carbonate)

63

67

40

others

0.0

0.1

0.0

gases [wt%]

Figure 2. Weight composition of liquid products at different reaction temperatures.

sum [wt%]

98

96

73

carbonaceous residue [C%] benzene yield [%]

18 67

22 59

33 61

124

128

81

carbon dioxide yield [%] a

at 25 °C and 101.3 kPa.

Figure 3. Weight composition of gas products at different reaction temperatures.

product after exposing CaO to TPA in steam atmosphere at 450 °C. CaO was mainly hydrolyzed forming Ca(OH)2. However, some low intensity peaks became visible in the extended format (Figure 7b). They were identified as calcium terephthalate anhydrate (Figure 7c). The reason for the low intensity of these peaks was probably caused by the limited formation of TP-Ca at the surface. However, the appearance of TP-Ca peaks also shows that the formation was not limited to a unimolecular layer, but TP-Ca crystals were formed. The FT-IR spectrum (Figure 8) of the product of the reaction of CaO, steam, and TPA at 450 °C shows the narrow peak of isolated Ca(OH)2 O-H vibrations at 3641 cm-1 as well as various peaks of organic origin. The presence of TPA can be excluded by the absence of the broad carboxylic acid O-H vibration between 2500 and 3000 cm-1 and the carboxylic CdO vibration at 1674 cm-1. More likely is the presence of TP-Ca due to the appearance of the asymmetric CO2- vibration at 1601 cm-1, the CdC stretching vibration at1502 cm-1, the symmetric CO2stretching vibration at 1360 cm-1, the aromatic in plane deformation vibration at 1149 cm-1, 1099 cm-1, and 1022 cm-1, the carboxylic deformation vibration at 812 cm-1, and the aromatic C-H out of plane vibration at 754 cm-1.

Figure 4. Weight composition of liquid products at different TPA gas concentrations.

The degradation of TP-Ca resulted partly in the formation of char. There was no clear trend for carbonaceous residue formation in the investigated temperature range (Table 1). Most of the carbonaceous residue was formed in the upper part of the CaO bed. The XRD patterns of CaO obtained after pyrolysis of TPA (Figure 6) showed no indication of the presence of CaCO3 at 700 °C. In contrast, CaO, CaCO3, and Ca(OH)2 were detected in the top 1 cm (upper part) of the fixed bed at 500 °C. However, CaCO3 was not detected in the lower part of the fixed bed. It may be assumed that the reaction of TPA with CaO was limited to the upper part of the fixed bed. The intensity of the Ca(OH)2 peaks 1834

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Figure 5. Weight composition of gas products at different TPA gas concentrations.

Figure 7. XRD pattern of the product of the reaction of CaO with steam and TPA at 450 °C (2a and b), and calcium terephthalate anhydrate (TP-Ca) (c).

Figure 6. XRD pattern of CaO at different bed positions after degradation of TPA.

at 500 °C was greater in the lower part than in the upper part, since the temperature in the former was up to 20 °C lower than that in the latter. At 500 °C, only a small amount of CO2 was released from the upper part of the bed. Since adsorption in the lower part of the bed was not observed, it may be assumed that a reaction of CO2 with CaO/Ca(OH)2 in steam atmosphere did not take place. Figure 9 shows the mechanism of benzene production and carbonaceous residue formation in the CaO bed. Initially, TPA gas reacts with CaO (1) to form TP-Ca (2). TP-Ca decomposes to benzene and CaCO3 (3). CaCO3 releases CO2 at temperatures above 500 °C (4) and is regenerated for the decarboxylation of TPA. However, if the TPA load reaches a critical level (5), as a result of either high TPA concentrations or a low TPA decarboxylation rate, then neighboring TPA molecules might become close enough to form condensation products as precursors for the formation of char (6). The CaO bed is regenerated for TPA decarboxylation by the decarbonation of CaCO3. Carbonation/ decarbonation cycles of CaO under inert gas or CO2 atmosphere have been widely investigated, and it is known that CaO can be used repeatedly.18-20 Although high temperatures allow the fast decomposition of TP-Ca, the formation of carbonaceous residue on the CaO surface is also accelerated. The yield of carbonaceous residue at 700 °C was smaller than at 600 °C since the decomposition rate of TP-Ca at 700 °C was faster than at 600 °C. However, at temperatures below 600 °C, both the formation of

Figure 8. FT-IR spectra of (a) the product of the reaction of CaO with steam and TPA, (b) Ca(OH)2, (c) calcium terephthalate anhydrate, and (d) TPA.

carbonaceous residue and the decomposition of TP-Ca are slow, which also results in reduced formation of residue. Therefore, the benzene yield and the formation of carbonaceous residue is 1835

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’ REFERENCES

Figure 9. Mechanism for formation of benzene and carbonaceous residue in the reactor: (1) adsorption of TPA; (2) formation of TP-Ca; (3) decomposition of TP-Ca; (4) regeneration of CaO; (5) contact of TPA with TP-Ca; and (6) formation of carbonaceous residue.

affected by reaction rates for the adsorption of TPA at the CaO surface, the decomposition of TP-Ca, the decarbonation of CaCO3, and the formation of carbonaceous residue. In particular, the promotion of decomposition of TP-Ca and the decarbonation of CaCO3, as well as inhibition of carbonaceous residue formation, would cause the benzene yield to rise and the carbonaceous residue to decline. Therefore, further work on the impact of certain kinetic factors is necessary to reduce the fraction of carbonaceous residue.

4. CONCLUSIONS The aim of this research was the conversion of TPA from the gas phase into hydrocarbons using CaO as a catalyst. The highest benzene yield (67%) was obtained at 700 °C, while the lowest content of carbonaceous residue (22 C%) was observed at 500 °C. It was clear that high temperatures promoted benzene formation, while low temperatures prevented the formation of carbonaceous residue. It was also found that a low TPA gas concentration caused a further reduction in the amount of carbonaceous residue. With a lower TPA gas concentration, the distance between TPA molecules on the CaO surface was also increased, making the reaction between two TPA molecules during their decarboxylation less likely. The best results were achieved at 500 °C and a TPA feed rate of 5.1  10-2 g L-1, yielding 67% benzene and a carbonaceous residue of 18 C%. The purity of benzene in the resulting oil was 99.2 wt %. Excess CO2 was formed under a low temperature of 500 °C and low gas concentrations of 100 mg L-1 or less;an observation which cannot be explained by the decarboxylation of TPA. On the basis of these findings, it is suggested that controlling reaction rates for the adsorption of TPA, the decomposition of TP-Ca, the decarbonation of CaCO3, and the formation of carbonaceous residue by the reaction temperature and TPA gas concentration should lead to high benzene yields and low amounts of carbonaceous residue. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ81-22-795-7211. Fax: þ81-22-795-7211. E-mail: yoshioka@ env.che.tohoku.ac.jp.

’ ACKNOWLEDGMENT This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Grant-in-Aid for Scientific Research (A), 30241532, 2009.

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