La agentividad verbal en el ámbito privado

N. Kakuta, K. Shirono, H. Ohkita and T. Mizushima

Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi 441-8580, Japan, [email protected]

Abstract: Porous carbons was prepared from dechlorinated waste PVC(Noubi). The dechlorinated Noubi fi lm was pre-oxidized at 573 K to modify into a non-graphitizable form and carbonized at 873 K in an inert atmosphere. A potassium activation method was employed in a char/KOH ratio of 1/3 at 1023 K. Surface areas of porous carbons were in excess of 1000 m²/g and micropores were generated predominantly. The same results were also achieved by use of a large scale system without problems. The roles of potassium on the activation step are not only the micropore formation at low temperatures but also the neutralization of the residual Cl species. As potassium is an essential element for plants, the dechlorinated Noubi fi lm modifi ed by potassium might be applied to an agricultural fi eld as environmentally -benign materials.

1. Introduction

Poly(vinyl chloride) (PVC) is one of widely-used plastics in many fi elds due to valuable characteristics such as low price, water resistance, fl ame resistance and etc. In Japan, agricultural PVC fi lms, so-called “Noubi”, have been manufactured. The Noubi fi lm was developed for the agricultural use, particular for greenhouses due to its light permeabil-ity and a high durabilpermeabil-ity performance under severe climatic conditions. Until now, more than 50% of waste Noubi fi lms is submitted to materials reprocessing for a mechanical recycling [1]. However, less than 50% of waste Noubi fi lms is still remaining as untreated PVC fi lms and they are disposed by means of the landfi ll and incineration. In deed, the incineration of PVC is the most conventional method but results in serious pollution prob-lems: the formation of hazardous chlorinated compounds (hydrochloric acid, chlorine gas and dioxin-related compounds) in fl ue gas and/or soil.

The dechlorination of PVC is usually a requisite process for the fi nal utilization of PVC wastes. For the recovery of Cl species evolved, many studies have been undertaken and continued, but the approach to the effective utilization of recovered chlorinated com-pounds is still not found. Unfortunately, most of hydrochloric acid obtained from dechlo-rination process is neutralized by sodium hydroxide (NaOH) or calcium oxide(CaO), and sodium chloride (NaCl) solution is discarded after diluting with plenty of water. Recently, Ueno et al. proposed a new method of recovering of chlorine gas from molten CaCl₂ and O₂ for the reuse of Cl₂ gas [2].

To utilze dechlorinated PVC, the dechlorinated PVC whose degree of dechlorination is up to 99% [3] was subjected to pyrolysis to fi nd ways for the feedstock recycle [4-7], com-bustion for the energy recovery [8], and synthesis of carbon materials [9-10]. However, the negative infl uence of residual Cl species is still not negligible in industrial plants for pyrolysis and combustion, especially when a large amount of dechlorinated samples are used as feed. The production of activated carbons is attractive for practical use and many studies on the production of activated carbons from waste materials have been reported [11-16] and their physical characteristics for purifi cation and elimination of hazardous compounds in the gas and liquid phases have become clear too.

In the present work, we attempt the preparation of porous carbons from the dechlorinated Noubi fi lm with the purpose of utilizing residual Cl species. Generally, two activation methods, physical and chemical activation, are employed for the pore formation. The physical activation is carried out using steam or CO₂ at higher temperatures to activate H₂O and CO₂ molecules as oxidants. Although Cl contents decrease drastically during the activation procedures, it is diffi cult to completely remove Cl species. On the other hand, the chemical activation using alkali reagents is preferable in order to remove the residual Cl species. In particular, potassium hydroxide (KOH) is expected to neutalize residual Cl species and form pores at lower temperatures. Another advantage is that if the residual potassium compounds(i.e. KCl etc.) exist on the carbons, they can be utilized as a fertilizer when the porous carbons are used for some agricultural purposes. However, sodium hydroxide (NaOH) is not to be used for this purpose because plants are damaged by sodium chloride(NaCl). Therefore, we investigated the preparation of porous carbons by the chemical activation method, using KOH in both small and large scales.

2. Experimental

2.1. Sample preparation

Noubi fi lms were dechlorinated using the dechlorination system(The Japan Steel Works, Ltd., Hiroshima Plant). The typical condition is as follows: a mixture of waste Noubi fi lm (70%) and polyolefi n fi lm (30%) was dechlorinated at 653 K in an inert atmosphere at a feeding rate of 40 kg/h. The hard specimen obtained was crushed and pulverized for ex-perimental use. Chlorine content was found to be lower than 1% showing that more than 99% of chlorine was successfully removed.

2.2. Carbonization and Activation

Four gram of dechlorinated sample was loaded on a ceramic boat and this boat was placed inside a tubular reactor. After heating at 873 K under N₂ fl ow, the resulting sample melted and adhered fi rmly on the wall of the ceramic boat. Therefore, the pre-oxidation prior to

the carbonization was tried. The sample inside the reactor was heated under air fl ow (150 ml/min) at a heating rate of 7.5 K/min, held at 573 K for several hours, followed by heat-ing under N₂ fl ow (100 ml/min) at a heating rate of 6.0 K/min, and fi nally held at 873 K for 1 h. After cooling to room temperature, the char obtained was impregnated with KOH solution. The char/KOH ratios were varied within the range of 1/3 to 1/4 based on the previous results [17]. The wet sample was dried in an oven at 383 K for 24h. For activa-tion, the impregnated sample was heated stepwise for the activation under N₂ fl ow (100 ml/min) at heating rate of 7.0 K/min up to 673 K, and then at heating rate of 13.3 K/min up to 1023 K. After the activation, the sample was neutralized by the addition of diluted hydrochloric acid under vigorous stirring for 15 min and the resulting liquid was allowed to stand for a long time. Finally, the precipitate was fi ltrated and dried at 383 K for 24 h.

2.3. Characterization of porous carbons

Adsorption desorption isotherms of N₂ were measured at 77 K using an adsorption apparatus(BELSORP mini, BEL Japan Inc., Japan). The pore size distributions were cal-culated by the Barrett, Joyner and Halenda(BJH) method using desorption isotherms.

The Brunauer-Emmett-Teller(BET) surface areas were mostly measured by one point N₂ adsorption method using a glass-made simple apparatus and the surface areas were cal-culated by the t-plot method, too.

3. Results and Discussion

3.1. Effect of pre-oxidation and carbonization periods

The pre-oxidation and carbonization conditions are highly correlated with yields and properties of porous carbons. The pre-oxidation and carbonization temperatures were chosen to be 573 K and 873 K, respectively [17]. The yields and surface areas observed are summarized in Table 1. The yields of resulting chars were found to be about 30%.

The surface areas increased with an increase in carbonization time when the pre-oxida-tion was 1 h and they also increased with increasing in pre-oxidapre-oxida-tion time. Consequently, Table 1 suggests that the prolonged pre-oxidation is effective to obtain high surface areas after the carbonization. Walker et al. reported that 85 per cent of carbon from the dechlo-rinated PVC was in the form of graphite-like layers due to the good graphitizability of the carbon[18]. In addition, the dechlorinated PVC carbon was also of interest to study with pitch-based carbon fi ber as source materials [10], suggesting that the melting is the char-acteristics of the graphitizable carbon. Since the graphitizable property is undesirable for the preparation of activated carbons, the pre-oxidation is requisite to modify the carbon into non-graphitizable form.

3.2. Infl uence of Char/KOH ratios on chemical activation

The porosities of chars during the potassium activation are also associated with the char/

KOH ratios. The dechlorinated Noubi fi lm was pre-oxidized and carbonized using the conditions listed in Table 1. The potassium-activations at char/KOH ratios of 1/3 and 1/4 were carried out at 1023 K for 1h. The results are listed in Table 2. Most of the chars’ sur-face areas were enhanced by the potassium activation. The high sursur-face areas of porous carbons were obtained when the chars were activated under the char/KOH ratio of 1/3.

However, the surface areas decreased with an increase in KOH content and this decrease of surface area might be due to the collapse of small pore structure by excess potassium.

It is also pointed out that the prolonged pre-oxidation is also effective for the increase of surface area.

3.3. KOH activation

The infl uence of the activation temperature was investigated in the range from 923 K to 1223 K for 1h. Figure 1 shows the behavior of surface areas when chars were activated at various temperatures. The surface areas increased with increasing activation temperature and about 1000 m²/g was attained at ca. 1013 K. But the surface areas tend to decrease at higher temperatures by potassium-catalyzed gasifi cation of the carbon. Consequently, the effective activation temperature was determined to be 1023 K. Moreover, the effect of activation period at 1023 K was investigated. The surface areas observed remained

Table 1: Effects on surface areas and yields of various carbonized conditions.

Pre-oxidation

Table 2: Effects on surface areas of different char/KOH ratios on chemical activation.

Pre-oxidation Time [h]

Carbonization Time [h]

Surface area [m²/g]

Char Char/KOH (1/3) Char/KOH (1/4)

1 1 10 740 990

almost the same after the activation for 1 h, suggesting that the activation for 1h is enough to obtain the surface area as high as 1000 m²/g. Consequently, the optimal preparation conditions are as follows: pre-oxidation at 573 K for 3h, carbonization at 873 K for 1h, activation at 1023 K for 1h.

900 1000 1100 1200 1300

Figure 1: Effect on surface area of various activation temperatures.

3.4. Other activation methods

The physical activations using steam and carbon dioxide(CO₂) were also carried out for comparison. Figure 2 shows relationship between surface areas and different activation methods when chars were activated at various temperatures. Surface areas of all carbons increased with an increase in temperature but did not exceed ca. 1000 m2/g at temperature above 1000 K, implying that high temperatures but not more than 1000 K are required to generate reactive H₂O or CO₂ molecules for the pore formation. Potassium is well-known as a promoter in heterogeneous catalytic reactions[19]; potassium assists to reduce the temperature signifi cantly on the gasifi cation of carbon and catalyzes coke removal on catalytic cracking of hydrocarbons. The same promotion effects thus occur during the potassium activation at 1023 K, leading to high surface areas.

Figure 2: Effect on surface areas of various activation temperatures by other activation methods.

900 1000 1100 1200 1300

0

900 1000 1100 1200 1300

0

3.5. A large scale system

In order to confi rm the above results, a large scale system was constructed. The apparatus was a batch processing system permitting the amount of supply up to 3 kg of sample. A rotary reactor was heated by internal kerosene burners and the temperature was control-led up to 1273 K. When 1 kg of the sample was charged, the pre-oxidation was performed successfully at a slightly high temperature of 673 K for 3h under air fl ow(10 L/min) with-out problems such as an ignition. Next the sample was subjected to the carbonization at a slightly low temperature of 823 K for 1h under N2 fl ow(20 L/min). The yield of char was ca. 30 %. The char was well dried and the surface area was measured to be 64 m²/g.

It should be pointed out here that a large amount of gaseous and tar-like carbons were produced when the carbonization was carried out without the pre-oxidation. The mixture having a char/KOH ratio of 1/3 was fi nally activated at 1023 K for 1h, followed by the neutralization with aqueous hydrochloric acid and drying at 383 K for 24 h. The yield of porous carbon was ca. 90 % and the overall yield from the sample was estimated to be ca. 30 %. The result is in good agreement with those observed in the laboratory-scale experiments, indicating that the pre-oxidation for the non-graphitizable treatment is also important in the carbonization and the activation even in a large scale system.

Figure 3: Adsorption and desorption isotherm of N₂ on the porous carbon at 77 K.

3.6. Characterization

The porous carbon prepared by the large scale system was characterized. The adsorption and desorption isotherm of N₂ at 77 K is shown in Figure 3. The isotherm is a typical I type and the amount of N₂ adsorbed increases drastically with the increase of p/p₀ in the range of p/p₀ > 0.1, suggesting that the porous carbon mainly has micropore structures.

The total surface area from the t-plot method is calculated to be 1375 m2/g, containing the external surface area of 52 m2/g and the internal surface area of 1323 m2/g. The pore size distribution of mesopores was evaluated by the BJH method using the desorption isotherm. The pore surface area of mesopore is estimated to be 64 m2/g and the

remain-V / ml (STP)・g-1

ing is attributed to the pore surface area of micropore. The summary of surface areas is listed in Table 3. This indicates that the potassium activation is preferable to generate the micropores at low temperatures. Evans et al. attempted to prepare the potassium acti-vated carbon from PVC without the pre-oxidation[9].They concluded that the PVC carbon produced a low surface area, although the potassium activation was carried out at 1023 K for 1h using the char/KOH ratio of 1/4. They also mentioned that the PVC carbon is a graphitizable carbon. Consequently, the non-graphitizable treatment is facilitated when the dechlorinated PVC is subjected to the carbonization and to the activation. Finally, the dioxin content was found to be less than the environmental abundance.

4. Conclusion

Porous carbons were successfully prepared from dechlorinated Noubi fi lms by a combi-nation of the non-graphitizable treatment and the potassium activation. The method was also extended to a large scale system for practical use. The porous carbons modifi ed by potassium might be utilized in agricultural fi elds.

Acknowledgement

The authors are grateful to the Japan Society for Promotion of Science for support of this research through a Grant-in Aid for Scientifi c Research(16201019). N.K. also acknowl-edges Mrs. Sakai,T., Natsume, K., Yagyu, H., Yamagai, K., and Nakajima, T., for their cooperation.

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[19] Bonzel, H. P., Bradshaw, A. M., and Ertl, G., ed., Physics and Chemistry of Alkali metal Adsorption, Elsevier, Amsterdam,1989, pp. 293-306, 347-377.

DECHLORINATION OF POLY(VINYL CHLORIDE) WITH