LA TEORÍA DE LOS ACTOS DE HABLA Y SU APLICACIÓN EN EL ANÁLISIS DE LA AGENTIVIDAD Y LA COMPETENCIA

An important factor in the formation of gaseous products is the control of the reaction environment in terms of the components in each phase — polymer melt, liquid, and gas [2]. Steam was used to supply in order to obtain gaseous products from polypropylene at 75 wt% in a table-top apparatus [3, 4].

Gas was produced from polyethylene in two stages: preparation of oil followed by the formation of pyrogas from the oil fraction in a cracking apparatus [5].

When a fl uidized-bed reactor was used for gas production, a process for separating the gaseous hydrocarbons from the fl uidizing gas is required. Further, the equipments for the separation process and fl uidizing-gas supply would increase the plant and processing costs.

The design of a suitable reactor for gas production requires the effi cient transfer of heat to the liquid and vaporized fractions of the decomposition intermediates of a polymer in the space of a reactor. Thus, a moving-bed reactor — a horizontally placed tubular reactor with a screw conveyor — was designed, and sand was used as the heating medium [6]. In this paper, we report the operation research of the moving-bed reactor in a bench scale for the purpose of gas production.

3. Experimental

3.1. Moving-bed reactor

A bench-scale plant of a moving-bed reactor is shown in Fig. 1. This plant consists of a feed hopper, tubular reactor equipped with a screw conveyor, electric heater, and residue/

oil receiver. The rotation rates of the screw conveyor were controlled by an inverter mo-tor. The tubular reactor is made of stainless steel. The internal diameter and length of the tubular reactor is 70mm and 1200mm. An electric heater surrounds the reactor over the length of 900mm.

The reactor temperature in this research represents the surface temperature of the reactor, which is measured by three thermocouples attached to the outer surface of the reactor.

Further, a 500mm section of the reactor is maintained at a constant temperature. The reaction time is defi ned as a mean residence time of sand in the 500-mm section of the reactor.

3.2. Sample and analysis

Pellets (ca. 3mm in diameter) of polypropylene (MA3, Japan Polychem) and polyethylene (Hizex6200B, Mitsui Chemical) were used as the samples for gasifi cation. A high alumi-na grade (28%) of silica-alumialumi-na catalyst (Catalyst & Chemicals Ind. Co. Ltd.) was used.

Gas evolution was monitored using a gas meter connected to the gas outlet of the mov-ing-bed reactor. Chemical analyses of the gaseous and liquid products were performed by gas chromatography.

3.3. Plant operation

Plastic pellets (0.8kg) were mixed with sand (7.2kg), and stored in a feed hopper. In cata-lytic reactions, plastic pellets (0.8kg) were mixed with the silica-alumina catalyst (0.4kg) and sand (6.8kg). The mixture was supplied into the tubular reactor at a constant rate in under nitrogen atmosphere. The internal pressure was maintained atmospheric pressure.

The reactor was maintained at a fi xed temperature. Analytical samples were collected from a gas outlet, and the volume of gas evolution was monitored using a gas meter. After feeding the sample mixture, the reactor was cooled overnight at room temperature. The liquid product and sand were stored in a separate section of the residue/oil receiver.

Figure 1: A bench-scale plant of a moving-bed reactor.

Reactor Inverter motor

Feed hopper

Residue/oil receiver Electric heater

Pyrogas

4. Results and Discussion

4.1. Pyrolysis and catalytic gasifi cation of polypropylene

Typically, polypropylene decomposes into gaseous and liquid products at a reactor tem-perature of 700 °C and a reaction time of 10min. Fig. 2 shows the gas chromatogram of the resulting gas fraction. The C2 component represents a mixture of ethylene and ethane.

As C3 products, polypropylene is dominant. Hydrogen is less than 0.1 wt%. As the op-eration period increased, polypropylene was gradually fed into the reactor and the gas evolution increased, as shown in Fig. 3. The period prior to gas evolution corresponds to the time required by a plastic sample to reach the heated section. At a reactor tempera-ture of 600 °C, the gas and liquid yields were controlled by varying the reaction time, as shown in Fig. 4. Different from gasifi cation in other reactors such as a fl uidized-bed

reactor, the gas/liquid ratio was controlled by changing the reaction time as well as the reaction temperature. This result indicates that the rate of a screw conveyor effectively controlled the residence time of polymer melt and liquid products as precursors of gaseous products in the presence of sand.

It is noteworthy that the gas compositions were almost constant at various reaction times. This suggests that the thermal de-composition of gaseous products does not depend on the reaction time, which is a variable parameter of the reactor.

The effectiveness of the acid catalyst in the decomposition of polyolefi ns was reported by some researchers [7-14]. With regard to the functions of the silica-alumina catalyst, the cracking and isomerization of hydrocar-bons are well known. In order to increase the gas yields and control the gas compo-sition, the catalytic decomposition of poly-propylene with the silica-alumina catalyst was examined (Fig. 5).

The higher yields of the gaseous products were confi rmed under the catalytic con-ditions. Experimental errors occur in the mesurements of the oil yields because the oil yield comprises the total amounts of oil in the oil tank of the residue/oil receiver and the oil mixed with sand. The weighing of sand and washing of a part of it might leads to some errors.

Fig. 6 shows the gas compositions under the catalytic and non-catalytic conditions.

The heavier components such as C4 and C5 components were formed under the catalyt-ic conditions. Catalytcatalyt-ic and non-catalytcatalyt-ic

60

Figure 2: A typical chromatogram of the gas product at 700 °C for a reaction time of 10 min.

300

Figure 3: Cumulative volume of gas evolution by pyrolysis. Reactor temperature: 600 °C, reaction time: 5 – 24 min: and gas temperature: 21– 25 °C.

Figure 4: Yields of gas and oil by pyrolysis at 600

°C for various reaction times.

100

Figure 5: Yields of gaseous hydrocarbons and oil under catalytic (C) and non-catalytic (P) conditions at various reactor temperatures.

pathways lie in the decomposition of the polymer melt and liquid phase of the decomposi-tion intermediates. Catalytic effects on the acceleradecomposi-tion of macromolecular transformadecomposi-tion were demonstrated in the degradation of polypropylene at 180 °C, at which temperature pyrolysis cannot be expected [13]. A higher oil yield can be expected under the catalytic conditions. The C9 and C10 components were observed to be important as the precursors of gaseous hydrocarbons [13]. Under catalytic conditions, these components are rapidly converted into the C4 and C5 components. Under non-catalytic conditions, the C9 and C10 components are decomposed via a radical mechanism, resulting in more complex reactions that lead to an increase in the C1 and C2 components.

C1 C2 C3 C4 C5 C6

Figure 6: Gas compositions under catalytic (C) and non-catalytic (P).

Reactor temperatures are 500, 600 and 700 °C. Reaction time for all runs is 10min.

From a practical viewpoint of commercial operations, the formation of a heavy gas such as C4 is desirable because it can be easily liquefi ed and stored in a light-weight cylinder, which is suitable for transporta-tion. LP gas (liquefi ed petroleum gas) con-taining propane is used in nearly half of Japan. All taxis in Japan also utilize LP gas containing butane. The moving-bed reactor provides a simple small-scale process for the production of fuel gas from waste plas-tics. Thus, a profi table recycling business is achievable.

Figure 7: Distillation curves of a typical sample of polypropylene-derived oil (catalytic decomposition

at 600°C) and commercial petroleum products.

Fig. 7 shows the distillation curve of a typical oil sample of gasifi cation by-product and commercial petroleum products. The oil obtained from polypropylene gasifi cation can be used as fuel oil for heating the reactor of a commercial plant.

Similar to polypropylene, polyethylene gave gaseous products at 75 wt% under the py-rolysis conditions of a reactor temperature of 700 °C and a reaction time of 10min.

References

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[6] Kuroki, T., Japan Patent.

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[12] Ishihara, Y., Nanbu, H., Saido, K., Ikemura, T., Takesue, T. Polymer, 33:3482 (1992).

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