Various Catalysts
The thermal degradation of waste HDPE can be improved by using suitable catalysts in order to obtain valuable products. However, this method suffers from several drawbacks. The catalysts are deactivated by the deposition of carbonaceous residues and Cl, N compounds present in the raw waste stream. Furthermore, the inorganic material contained in the waste plastics tends to remain with the catalysts, which hinders their reuse. These reasons necessitate a relatively high purity of waste plastics, containing very low concentrations of a contaminant. Thus, various pretreatments are required to remove all the components that may negatively affect the catalyst.
Molecular weight (a)
Figure 5.12 PONA distributions of liquid products for thermal (a) and catalytic (b) degradation of waste HDPE at 430°C
Molecular weight (b)
Figure 5.12 PONA distributions of liquid products for thermal (a) and catalytic (b) degradation of waste HDPE at 430°C
- Figure 5.13 The selectivity of carbon atom number obtained in the thermal and catalytic degradation of LDPE in two-screw kiln reactor. (Reproduced with permission from Elsevier)
The pyrolysis process of waste plastics is classified into either liquid-phase contact or vapor-phase contact of catalyst in the continuous flow reactor. In the liquid phase reaction, the catalyst is a fine powder type with high external surface area which is contacted with melted plastics, which are degraded into light products from polymer chain on the active sites of catalyst. This process uses a large amount of catalyst, which becomes a high portion in total cost. Thus, the catalyst must be of low price and reused after regeneration, if possible. In the vapor-phase reaction, the polymer is first degraded into the hydrocarbon vapor in the thermal degradation process, aimed at reducing the viscosity of melted plastics and enabling the separation of undesired components. The cracked products are then contacted with the catalyst packed in the flow reactor, which plays a reforming role over the products formed by thermal degradation of polymers.
Various catalysts used in the two processes have been described as follows; zeolite, alumina, silica-alumina, FCC catalyst, reforming catalyst, and others. The most common catalysts used in the cracking of heavy hydrocarbons are acidic catalysts; alumina and silica-alumina with mesopores, and also zeolite with micropores, etc. They are typically used in the commercial petroleum process. For the chemical properties of catalyst, the catalysts consist of Lewis and Bronsted acid sites, which is an important factor in determining the catalytic activity and product selectivity. This is because Bronsted acid sites play a proton addition role and Lewis acid sites involve hydride abstraction, which leads to different reaction pathways in the cracking of hydrocarbons. Also, these acid sites are generated by Al species in the catalyst consisting of silica and alumina. Thus, Al content per unit cell or Si/Al ratio of catalyst is very related to the acid site density, which also has a masked influence on the cracking reaction. High acid site density favors the cracking reaction of hydrocarbons, but promotes undesired reactions such as coke formation. Thus, in order to design the catalysts with a high activity and also high selectivity of desired product, the acid site density of the catalyst is controlled by preparation methods and various pretreatment methods such as steam or acid/base solution treatment, etc.
In addition to the chemical properties of the catalyst, the physical properties are also very important in determining the catalytic activity and product selectivity. These parameters are the surface area, pore size, pore volume, pore size distribution, pore structure, etc. As an example, the zeolite has a micropore crystalline structure with pore size below 1.0 nm, whereas alumina and amorphous silica-alumina are mesoporous materials with a wide distribution of large pore size. Various natural or synthetic zeolites have relatively high surface area, but small pore size and also small pore volume. The narrow distribution of zeolite having a pore size below 1.0 nm allows different molecules to control a limited diffusion inside the pores, known as shape selectivity, which is selectively reacted on active sites within pores. Also, other advantages of zeolites are high acid strength, high stability and low coke formation, etc. Accordingly, zeolites such as zeolite Y and ZSM-5 have been extensively used for catalytic cracking of heavy hydrocarbons in many commercial processes. However, the catalytic degradation of waste plastics using zeolite may be a difficult problem due to a limited diffusion of big molecules into zeolite pores, which can be overcome with small crystal size and also high external surface area due to the use of a fine powder. Also, activated carbon impregnated with transition metals is a micropore material with a high surface area, which promotes hydrogen transfer reactions during decomposition of hydrocarbons like the reforming catalyst.
On the contrary, alumina and amorphous silica-alumina have relatively low surface area, but big pore size and large pore volume, due to their mesopore structure. They have low acid strength compared with zeolites. However, they have a sufficient diffusion of heavy hydrocarbon having large kinetic diameter through the pores, without control of different molecules. A similar catalyst is MCM-41, although it has high surface area, has a uniform mesopore structure. The utility of its high surface area and uniform mesopore in catalytic degradation of polyolefin has recently studied by several researchers [16-19]. Also, sulfated zirconia known as a superacid solid can be used as a catalyst in catalytic reaction of hydrocarbons [20, 21]. FCC catalyst has been developed for the cracking of heavy hydrocarbon molecules into gasoline range hydrocarbons. The catalyst consists of silica-alumina with a mesopore structure and zeolite with a micropore structure, which can be well cracked by step-by-step diffusion of heavy molecules in the catalyst of different pore structure. FCC catalyst has been found to have a significant effect in the pyrolysis of thermoplastics [22. 23].
Seo et al. [24] have described the catalytic degradation of polyethylene using various acidic catalysts at 450°C. The yields of gas, liquid and residue are illustrated in Table 5.3 and the PONA distribution in liquid products is shown in Table 5.4. Catalytic degradation
|
Yields of products |
Liquid |
Gas |
Coke |
Liquida (wt%) | ||
|
(wt%) |
(wt%) |
(wt%) | ||||
|
C6-C12 |
C13 -C23 |
>C24 | ||||
|
Thermal cracking only |
84.00 |
13.00 |
3.00 |
56.55 |
37.79 |
5.66 |
|
ZSM-5 (powder) |
35.00 |
63.50 |
1.50 |
99.92 |
0.08 |
0 |
|
Zeolite Y (powder) |
71.50 |
27.00 |
1.50 |
96.99 |
3.01 |
0 |
|
Zeolite Y (pellet) |
81.00 |
17.50 |
1.50 |
86.07 |
11.59 |
2.34 |
|
Mordenite (pellet) |
78.50 |
18.50 |
3.00 |
71.06 |
28.67 |
0.27 |
|
Silica-alumina(powder) |
78.00 |
21.00 |
1.00 |
91.31 |
8.69 |
0 |
|
Alumina(powder) |
82.00 |
15.90 |
2.10 |
53.02 |
43.27 |
3.71 |
a Determined by GC/MS
a Determined by GC/MS
|
Catalyst |
Total paraffin |
(Total paraffin) n-paraffin i-paraffin |
Total olefin |
Naphthene |
Aromatics |
Othersa | |
|
Thermal cracking |
40.75 |
40.47 |
0.28 |
39.93 |
18.50 |
0.68 |
0.14 |
|
ZSM-5 (powder) |
1.63 |
1.51 |
0.12 |
16.08 |
23.55 |
58.75 |
0.01 |
|
Zeolite Y(powder) |
5.39 |
0.00 |
5.39 |
79.92 |
7.68 |
7.01 |
0.00 |
|
Zeolite Y (pellet) |
25.10 |
20.68 |
4.42 |
49.28 |
12.05 |
8.43 |
5.14 |
|
Mordenite (pellet) |
31.07 |
30.89 |
0.18 |
57.07 |
11.51 |
0.13 |
0.22 |
|
Silica-alumina (powder) |
0.20 |
0.20 |
0.00 |
91.62 |
5.62 |
0.39 |
2.17 |
|
Alumina (powder) |
32.57 |
32.57 |
0.00 |
50.19 |
14.99 |
1.14 |
1.11 |
a Others = hydrocarbons containing oxygen or unidentified organic compounds a Others = hydrocarbons containing oxygen or unidentified organic compounds of HDPE with zeolite Y, mordenite and silica-alumina gave 71-81 wt% oil yields, which mostly consist of C6-C12 hydrocarbons in the gasoline range, whereas thermal degradation of HDPE produced 84 wt% oil yield with a much longer hydrocarbons like wax. In catalytic degradation, pellet zeolite Y that possesses less external surface area showed more oil yield and less gas yield than powder zeolite Y. Both all zeolites and silica-alumina increased olefin content in oil product. Particularly ZSM-5 and zeolite Y enhanced the formation of both aromatics and branched hydrocarbons having a high octane number. ZSM-5 among zeolites showed the greatest catalytic activity in cracking of heavy hydrocarbons to small gaseous hydrocarbons and formation of aromatics, which was related to the restricted channel of this zeolite that favors oligomerization reactions of olefins to form small alkyl aromatics, whereas mordenite produced the greatest amount of coke, due to its unidimensional straight-channel structure. Amorphous silica-alumina showed high yield of lighter olefins due to its strong acidity, but no activity in the formation of aromatics and branched hydrocarbons because of its amorphous structure.
Audiso et al. [25] has studied the catalytic degradation of polypropylene using silica, alumina and silica-alumina and zeolite catalysts in the range 200-600°C. The main products in oil production using more efficient catalyst were C5-C12 olefins. Also the highest oil yields were obtained around 400°C. In catalysts, silica-alumina catalyst in comparison with alumina and silica only was much more reactive.
The liquid-phase catalytic degradation of HDPE over BEA, FAU, MWW, MOR and MFI zeolites with different pores in a batch reactor at 380 or 410° C has recently been studied by Park et al. [26]. Among zeolites, high activity was obtained with BEA and MFI zeolites, because of their bent pore structure suppressing carbon deposit, whereas MOR zeolite showed low activity, due to the rapid blocking of the linear pore structure even by a small amount of carbon deposit. Large three-dimensional pores of FAU enhanced mass transfer, resulting in a high yield of liquid product and also the slow diffusion of cracked product in MWW zeolite brought about much more cracking into small hydrocarbons. Accordingly, the pore shape of the zeolites was very important in determining the activity and product distribution in the degradation of polymers.
The catalytic degradation of HDPE and LDPE with MCM-41, ZSM-5 and silica-alumina in a batch reactor at 400°C was investigated by Aguado et al. [27]. The activity order in the catalytic degradation of HDPE and LDPE was ZSM-5 > MCM-41 >silica-alumina. The higher activity obtained over ZSM-5 was related to its stronger acid properties, whereas the high activity of MCM-41 compared with silica-alumina was influenced by the large surface area of MCM-41 with above 1000 m2/g. The cracking of polyolefin over zeolite leads to a high proportion of gaseous hydrocarbons consisting of high olefin content and a liquid product in the range of gasoline with a high aromatic content. On the other hand, MCM-41 generated less olefinic gas products and, in addition to the gasoline fraction, the middle distillates in the range C13-C22 are produced. These activities and product distributions are highly related to the acidic and pore properties of the catalysts.
The activated carbon impregnated with different transition metals (Pt, Fe, Mo, Zn, Co, Ni and Cu) as a catalyst for PE conversion in a fixed-bed reactor has been studied by Uemichi et al. [28]. This catalyst plays a bifunctional role, with cracking and dehydro-genation/hydrogenation activity. The major effect of the metal impregnated on activated carbon was to increase the selectivity of aromatics with high octane number and to decrease the formation of n-alkanes. The aromatic yield was the most effective in Pt, Fe, Mo among various metals and also depended strongly on the support, which was much more efficient on activated carbon than on silica-alumina and alumina. It is proposed that two active sites with different function are to take part in the dehydrocyclization step involved in the degradation reaction. The abstraction of hydrogen atoms from polymer occurred predominantly on the acitivated carbon sites and the resulting hydrogen atoms migrate to the metal sites, while the metal sites catalyzed the desorption of hydrogen atoms.
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