Recovery Of Heavy Oil From Waste Plastic

The waste plastics generated from households are in the form of plastic mixtures; among such mixtures, the amount of PET generated as a household waste has rapidly increased as the production of PET bottles has increased. PET is formed by the ester bonding of terephthalic acid and ethylene glycol. When PET is heated above 380°C, pyrolysis suddenly starts, yielding oxygen-containing hydrocarbons and a significant amount of carbonaceous residue [17-19]. Accordingly, in order to develop a recycling process for the conversion of waste plastics into useful liquid hydrocarbons, a method for the degradation of a mixture of waste plastics without the production of such a carbonaceous residue is required.

2.1 DEGRADATION OF VARIOUS PLASTICS

The relationship between the rate of PET degradation and the molar fraction of steam in the carrier gas was investigated using a thermogravimetric apparatus equipped with a thermobalance. Figure 6.1 shows the remarkable change in the fraction of unreacted PET as the molar fraction of steam in the carrier gas was varied. When a pure nitrogen stream (steam molar fraction = 0%) was used, the degradation of PET was initiated

300 400 500 600 Reaction temperature T / °C

Figure 6.1 Change in the fraction of unreacted PET in a steam atmosphere. Heating rate: 5°C min-1. (Reproduced with permission from Elsevier)

300 400 500 600 Reaction temperature T / °C

Figure 6.1 Change in the fraction of unreacted PET in a steam atmosphere. Heating rate: 5°C min-1. (Reproduced with permission from Elsevier)

at about 380°C and was terminated at about 530°C, leaving about 16% carbonaceous residue. When the carrier gas contained steam, the fraction of carbonaceous residue was drastically reduced from 16% (100% nitrogen) to below 2% (70 mol% steam). Furthermore, the initiation temperature for pyrolysis decreased by about 30°C. Figure 6.2 shows the relationship between the molar fraction of steam in the carrier gas and the amount of carbonaceous residue remaining at 520°C. The circles, triangles, and squares represent the results obtained at heating rates of 2.5, 5.0, and 10°C min-1, respectively. All data lie on a single curve, suggesting that the amount of carbonaceous residue depends only on the partial pressure of steam, and not on the rate of increasing temperature. The amount of residue thus decreases with increases in the partial pressure of steam. Less than 1% residue was found to remain under 100% steam carrier gas conditions.

Plastic: PET

Heating rate

Plastic: PET

Heating rate

0 20 40 60 80 100

Fraction of steam in the carrier gas / mol%

Figure 6.2 Relationship between the molar fraction of steam in the carrier gas and the yield of residue at 520°C. (Reproduced with permission from Elsevier)

0 20 40 60 80 100

Fraction of steam in the carrier gas / mol%

Figure 6.2 Relationship between the molar fraction of steam in the carrier gas and the yield of residue at 520°C. (Reproduced with permission from Elsevier)

C = O C = C C - O -(CH2)n-(ester) (aromatics) (ester) (s-bond)

Spectra Polyamide

Unreacted PET

PET at 60% conversion in nitrogen

PET at 70% conversion in steam

2000

1600 1200 800 Wave number /cm-1

Figure 6.3 FTIR spectra of unreacted PET and of residues obtained from PET at 60% conversion in both nitrogen and steam (70 mol%) atmospheres. (Reproduced with permission from Elsevier)

C = O C = C C - O -(CH2)n-(ester) (aromatics) (ester) (s-bond)

Unreacted PET

PET at 60% conversion in nitrogen

PET at 70% conversion in steam

2000

1600 1200 800 Wave number /cm-1

Figure 6.3 FTIR spectra of unreacted PET and of residues obtained from PET at 60% conversion in both nitrogen and steam (70 mol%) atmospheres. (Reproduced with permission from Elsevier)

Figure 6.3 shows the FT-IR spectra of residues obtained in nitrogen at a PET conversion of 60%, and in a steam atmosphere at a conversion of 70%. This figure also shows the spectrum for unreacted PET. PET is formed by the ester bonding of terephthalic acid and ethylene glycol. In the case of thermal pyrolysis in nitrogen, the dehydration of the chemical bonds and the random scission of the main chain of PET occurred, resulting in a reduction in the peak strengths corresponding to bonds involving the oxygen atom (C-O, C=O) and a-bonds, leaving a hydrogen-poor residue. On the other hand, when the degradation of PET was conducted in the presence of steam, the C-O bond and abond were preferentially weakened, and a peak corresponding to free aromatics (C=C) appeared. These aromatics were considered to be terephthalic acids at the ends of the main chain of PET. Hence, steam was found to accelerate the hydrolysis of PET, producing monomers of PET such as terephthalic acid.

Figure 6.4(a, b) shows the thermogravimetric curves (TG-curves) for plastics at a heating rate of 5°C min-1 in nitrogen and in steam, respectively. Seven types of plastics were used in this series: polyethylene (PE), polypropylene (PP), polycarbonate (PC), poly-butyleneterephthalate (PBT), polystyrene (PS), nylon-6 (N6) and nylon-6,6 (N6,6). The curves for the degradation of PET are also shown in the figure for comparison. The TG curves were remarkably different among the plastics in a nitrogen stream, due to differences in the main chains of the plastics. Furthermore, polyester resins, such as PC, PBT, and PET, yielded large amounts of carbonaceous residue at 527°C, due to dehydration during the degradation in nitrogen. In contrast, in a steam atmosphere, the TG curves of PC, PBT, and PET were remarkably altered by shifts to lower temperature regions and by reducing the amount of carbonaceous residue, as compared with those in a nitrogen atmosphere. Accordingly, it was concluded that the dominant mechanism of degradation of polyester resins changed from thermal pyrolysis to hydrolysis by introducing steam as the carrier gas.

Under the nitrogen steam conditions, PC showed the lowest degradation rate. On the other hand, the plastic resin with the lowest degradation rate was PE in a steam atmosphere. Therefore, the size of a degradation reactor could be designed by considering the degradation rate of PE alone.

Polyester Resin Reactor Drawings

200 300 400 500

200 300 400 500

PS^V kPE

Carrier gas: steam

Carrier gas: steam

N6,6

  • X PP PBT1 A . PC
  • I 1/l/uei

N6,6

Figure 6.4 Change in the fraction of unreacted plastics used in this study at a heating rate of 5°C min-1: (a) in nitrogen; (b) in steam. (Reproduced with permission from Elsevier)

2.2 CATALYTIC CRACKING OF WASTE PLASTICS WITHOUT RESIDUE

The degradation of PET in a steam atmosphere is effective at reducing carbonaceous residue, and this type of degradation produces terephthalic acid and oxygen-containing compounds. However, terephthalic acid is precipitated as a hard solid body around valves and pipelines, because this compound is a sublimate material (sublimation point ~300°C). Therefore, in this case, sublimate materials such as terephthalic acid should be converted into liquid hydrocarbons. In order to convert this produced terephthalic acid, a catalyst for cracking carbonyl groups of terephthalic acid is required. For this reason, transition metal oxides, which easily form carbonyl complexes, were considered to be suitable catalyst materials. Moreover, the transition metal oxides such as iron oxide and nickel oxide are inexpensive. In this section, a method for the decomposition of terephthalic acid into useful liquid hydrocarbons is described. The screening of potential catalysts was conducted using transition metal oxide catalysts.

The catalytic cracking of PET was carried out using a fixed-bed type of reactor in a steam atmosphere. The reactor was heated to the desired temperature in a mixture of steam and nitrogen as the carrier gas. After feeding PET particles into the reactor, the particles were heated to the reaction temperature, and then underwent the hydrolysis in a steam atmosphere to be decomposed to terephthalic acid and lighter hydrocarbons. These molecules were cracked through the catalyst bed. The liquid products were condensed in two condensers cooled with ice and water. Lighter hydrocarbons were collected in a gas pack. The amount of carbonaceous residue was calculated from the difference between the mass of the catalyst before and after the experiment. The reaction was conducted at the temperatures ranging from 450 to 530 K.

Figure 6.5 shows the yields ([wt%]) of the reaction of PET using several transition metal oxide catalysts under the following conditions: a temperature of 500°C, a time factor (the ratio of the mass of the catalyst W, to the PET feed rate F) of 0.317 h, and a particle size of 0.21-0.25 mm. Fe2O3 did not show activity, hence these results have been omitted. With respect to the reduction of terephthalic acid, FeOOH, nickel hydroxide and nickel oxide showed the decomposition activity of terephthalic acid. However, a large amount of benzoic acid, which is also a sublimate material (sublimation point ~100°C), was produced over nickel hydroxide and nickel oxide. Because these nickel compounds are more expensive than FeOOH, FeOOH was considered to be a suitable catalyst for the decomposition of terephthalic acid.

Figure 6.6 shows the change in product yield with increase in the time factor W/F. The amount of sublimate materials (terephthalic acid and benzoic acid) decreased remarkably with increase in the time factor, and no sublimate materials were observed after approximately 0.5 h. Moreover, carbon dioxide was produced, and the yield of the carbon dioxide

Benzoic acid 5.5%

Other oil Phenol

Gaseous compounds Acetoaldehyde I Rresidue

4.8 FeOOH

Benzoic acid 5.5%

Other oil Phenol

Gaseous compounds Acetoaldehyde I Rresidue

4.8 FeOOH

Ni(OH)2

40 60

Product yield / wt%

Figure 6.5 Production yields of the reaction of PET over transition metal catalysts. T = 500°C, W/F = 0.32 h-1, particle size = 0.21-0.25 mm, molar fraction of steam = 0.94%. (Reproduced with permission from Elsevier)

Ni(OH)2

40 60

Product yield / wt%

Figure 6.5 Production yields of the reaction of PET over transition metal catalysts. T = 500°C, W/F = 0.32 h-1, particle size = 0.21-0.25 mm, molar fraction of steam = 0.94%. (Reproduced with permission from Elsevier)

Other gaseous Residue compounds

Other gaseous Residue compounds

Figure 6.6 (a) Change in product yield with an increase in the time factor, W/F. T = 500°C, particle size = 0.21-0.25 mm, molar fraction of steam = 0.94%. (Reproduced with permission from Elsevier); (b) Change in product yield with an increase in the time factor. The solid curves represent the calculated results

Figure 6.6 (a) Change in product yield with an increase in the time factor, W/F. T = 500°C, particle size = 0.21-0.25 mm, molar fraction of steam = 0.94%. (Reproduced with permission from Elsevier); (b) Change in product yield with an increase in the time factor. The solid curves represent the calculated results

Terephthalic acid

Carbon dioxide - Benzoic acid

Terephthalic acid

Carbon dioxide - Benzoic acid

Figure 6.7 Reaction pathway proposed for the reaction of terephthalic acid produced by the hydrolysis of PET. (Reproduced with permission from Elsevier)

increased with decrease in the yield of the sublimate materials. Figure 6.7 shows the putative reaction mechanisms [5], i.e. the carbonyl groups of terephthalic acid are decomposed to yield carbon dioxide and benzoic acid, and benzoic acid undergoes further reactions to produce acetophenone and carbon dioxide. Some of the acetophenone was converted to benzene and phenol, which are components classified under other liquid compounds. Two reaction pathways, from acetophenone and other liquid compounds to carbon dioxide, are negligibly small. In the reaction path from terephthalic acid to benzoic acid, benzene and phenol were also produced. As a result, since sublimate materials such as terephthalic acid and benzoic acid were successfully decomposed using an FeOOH catalyst, serious pipe blocking at source plants could be avoided.

The structure and morphology of FeOOH treated at 500° C in a steam atmosphere was analyzed by X-ray diffraction and transmission electron microscopy, respectively. The X-ray diffraction analysis showed that FeOOH was transformed to Fe2O3 during the treatment in the steam atmosphere. In contrast, the TEM observation revealed micropores of 1 nm diameter in the untreated FeOOH, which was not observed with commercial Fe2O3. Moreover, the pores increased in diameter to approximately 5-100 nm after the steam treatment. This morphology was thought to be the result of the dehydration of FeOOH. It is possible that many active sites were generated on the surface of the pores, resulting in the observation that treated FeOOH showed high activity, even though its crystal structure is the same as that of Fe2O3 [6, 20].

Terephthalic acid is a useful source material of PET, as well as benzoic acid and benzoates. However, in order to recycle the terephthalic acid, produced further purification is required, because other organic compounds are also produced as impurities in the degradation process of waste plastic mixtures, e.g. PE and PET mixtures described in Section 2.3.

2.3 CONTINUOUS DEGRADATION OF WASTE PLASTICS MIXTURES FOR THE RECOVERY OF HEAVY OIL

The dominant mechanisms of polyester resin degradation (such as polycarbonate, poly-butyleneterephthalate and polyethyleneterephthalate) changes from thermal pyrolysis to hydrolysis by the introduction of steam as the carrier gas. PET was successfully degraded by hydrolysis in a steam atmosphere, yielding an amount of pure terephthalic acid that could be predicted from the chemical formula of PET, and leaving carbonaceous residue of less than 1%. From economic and energetic viewpoints, it is both inexpensive and easy to employ steam as the carrier gas in chemical recycling plants. Furthermore, the temperature required to initiate was found to decrease approximately 70°C when polyester resins were degraded by hydrolysis in a steam atmosphere [12].

When the mixture of waste plastics was degraded by accelerating the hydrolysis of the plastics with steam and by decomposing the generated sublimate materials over FeOOH catalyst, the following reaction conditions are needed:

  1. A good contact of melted plastics with steam to accelerate the hydrolysis of plastics.
  2. Large rate of the heat transfer to heat plastics up to a desired temperature.
  3. High hold-up of plastics in a reactor to achieve an enough reaction time for degrading plastics.
  4. Contact of vapor of sublimate materials with FeOOH catalyst to decompose the materials.

Bockhorn et al. [2, 3] proposed circulated-spheres reactor for the pyrolysis of waste plastics. This reactor enabled one to remove gaseous products from the reaction zone and to achieve high heat transfer rates, namely condition (2) described above. The concept of this circulated-spheres reactor has the possibility to attain conditions (1) and (3) by improving the reactor.

Based the discussions above, a new pyrolytic reactor can be proposed that would be capable of achieving a high hold-up, high heat transfer, and good contact between melted plastics and steam for the acceleration of hydrolysis. Using this novel reactor system, a mixture of waste plastics was degraded, and further decomposition over an FeOOH catalyst was also achieved.

This novel reactor is a pyrolytic reactor system using stirred heat-medium-particles (Figure 6.8). This pyrolytic reactor system is composed of a series of three types of reactor. The first reactor uses stirred heat-medium-particles (reactor 1), the second is a tank reactor (reactor 2), and the remaining reactor is a fixed-bed reactor (reactor 3). Reactor 2 is located under reactor 1, and is separated from reactor 1 by a stainless steel net. The seizes of these reactors are described in the figure. The expected reaction behavior is described below.

In reactor 1, plastic particles are fed into the top of a bed of glass beads as the heat-medium-particles, and the particles are then melted and adhered to the beads. The glass beads are stirred slowly by two equipped impellers, one of which is the propeller-type of impeller, and the other is an anchor-type of impeller located at the bottom of the bed of glass beads. The propeller-type of impeller is turned to lift the particles. In this manner, the glass beads at the top layer of the bed of glass beads are replaced continuously by

For upgrading

For upgrading

Stainless steel net

T=400~500°C React0r 3

Figure 6.8 (a) Schematic view of the proposed reactor system for the chemical recycling of plastics; (b) photograph of the proposed reactor system. (Reproduced with permission from Elsevier)

Pre-heating zone of steam N2

Water

Stainless steel net

T=400~500°C React0r 3

Figure 6.8 (a) Schematic view of the proposed reactor system for the chemical recycling of plastics; (b) photograph of the proposed reactor system. (Reproduced with permission from Elsevier)

Figure 6.8 (continued)

Figure 6.8 (continued)

other beads lying beneath the top layer. These stirred beads increase the rate of heat transfer. Following the string of the glass beads, the melted plastics are transported over the bed of glass beads, resulting in a high hold-up of plastics in the reactor and good contact with the steam, which functions as the carrier gas. The melted plastics on the glass beads are decomposed by hydrolysis with steam and the random scission of C-C bonds. Some of the melted plastics on the glass beads are carried to the bottom of the bed of glass beads, and are dripped onto reactor 2 (tank reactor). In reactor 2, the unreacted plastics undergo further decomposition, yielding various gaseous compounds. Reactor 3 is filled with an FeOOH catalyst. Gaseous compounds, including the vapors of sublimate materials, are passed through the FeOOH catalyst bed, where they undergo the catalytic degradation.

A mixture of PE and PET was used as a model waste plastic mixture. The ratio of PE/PET was 15:2, which is the ratio of the amounts of the two plastics discarded in Kyoto, Japan. After continuous degradation with and without stirring of the glass beads, the glass beads were collected from reactor 1 (Figure 6.8). When the glass beads were not stirred, which corresponded to a trickle-bed type of reactor, a massive carbonaceous residue remained with the glass beads on the top of the bed of glass beads. For this reason, the heat transfer rate was low and the melted plastics did not make good contact with the steam. On the other hand, there was not a massive amount of residue and glass beads slightly colored yellow, when the glass beads were stirred by rotating the impellers at rates exceeding 8 rpm. Furthermore, no residue nor oil was in reactor 2, the tank reactor,

(Figure 6.8). These results indicate that the proposed reactor system is useful for the continuous degradation of plastics.

Figure 6.9 shows the dependency of the product yields, based on data collected at the outlet of reactor 3 (see Figure 6.8), on the ratio of the catalyst mass W to the feed rate of the plastics F and the time factor W/F increased from 0 to 1.5 h. Reactions were conducted under temperature conditions ranging from 500°C, at a catalyst weight W of about 2.0 x 10-2 kg. When the FeOOH catalyst was not loaded into reactor 3, a large amount of yellow wax was obtained. When the FeOOH catalyst was loaded into reactor 3, oil and carbon dioxide were produced. The amount of carbon dioxide and that of gaseous hydrocarbons increased, and the yield of oil decreased, as the amount of FeOOH catalyst loaded increased. This suggests that the FeOOH catalyst is capable of catalysis, leading to the decomposition of a wax via oxidation by oxygen atoms from the lattice of FeOOH and/or from H2O.

When a wax is decomposed via oxidation by the oxygen atoms from the lattice of FeOOH catalyst, the catalytic activity of an FeOOH catalyst will decrease. Therefore, the gaseous product yields of the reaction were measured by sampling at different intervals at a temperature of 500°C and the time factor of 1 h. The main gaseous products were carbon dioxide (~3 wt% for 140 min), n-C4Hw (2 wt%), n-C3H (2 wt%), C2H6 (0.5 wt%), C2H4 (1.5 wt%), and CH4 (0.5 wt%). Except for at the beginning of the reaction, there were only negligible changes in the product yields. The amount of oxygen required for producing carbon dioxide during a reaction time of 140 min was evaluated, and was found to be larger than that generated through the phase change of iron from Fe2O3 to Fe3O4. These results suggest that the wax was decomposed by reaction with H2O over the FeOOH catalyst, and the catalytic activity of the FeOOH catalyst remained stable in the steam atmosphere.

"O

Figure 6.9 Effect of an FeOOH catalyst loaded in reactor 3 (see Figure 6.8) on product yield; temperature = 500°C, carrier gas: steam. (Reproduced with permission from Elsevier)

Figure 6.9 Effect of an FeOOH catalyst loaded in reactor 3 (see Figure 6.8) on product yield; temperature = 500°C, carrier gas: steam. (Reproduced with permission from Elsevier)

Figure 6.10 Carbon number distribution of heavy oil produced by degradation of a mixture of polyethylene (PE) and poly(ethylene terephthalate) (PET), at a weight ratio of PE/PET = 15/2, temperature = 500°C, time factor W/F = 1 h, carrier gas: steam. (Reproduced with permission from Elsevier)

Figure 6.10 shows the carbon number distribution of the products obtained when the temperature was 500° C and the time factor was 1 h. The oil produced was considered to correspond to heavy oil, as based on the carbon number distribution. Therefore, it would be necessary to upgrade this oil for practical applications.

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