Waste Plastics Processing

5.1 CATALYTIC AND THERMAL CRACKING PROCESSES: TYPICAL PRODUCTS

The most exhaustive studies on waste plastics processing by cracking and pyrolysis methods were carried out at Hamburg University by Kaminsky and co-workers [ e.g. 1, 8]. As a result of these studies various variants of the process were worked up. They used different types of the feed compositions and process parameters, starting from the low-temperature coking and cracking parameters (temperature lower than 500° C) and liquid and wax fraction as the main products up to pyrolysis parameters (600-700°C) and C2-C4 olefins and aromatic derivatives as the main products. The fundamental advantage of these studies is a continuous flow system and a fluidized-bed type of reactor (1-3 kg of waste plastic pellets per hour). Thermal cracking of PE in a fluidized-bed reactor gives higher-boiling products in comparison with PP cracking (50 and 70 wt% of <500°C boiling products, respectively). One can observe almost linear change of fraction product composition: gas, <500°C and >500°C boiling fractions when PP content in the feed changes from 0 to 100%. Small addition of PS to PP, PE or PE/PP mixture does not change product composition appreciably.

Other results obtained at Hamburg University [9] pointed out the advantages of application of spent FCC catalyst in the cracking process. It was found that processing of PS in the temperature range 370-515°C in the presence of FCC catalyst gives high yield of coke (up to 20%) and relatively low styrene yield. On the other hand a noncatalytic, thermal process gives above 60% styrene yield, accompanied by trace quantity of coke. At typical FCC cracking temperature (515°C) processing of PE results in almost 50% C1-C4 gaseous fraction yield, high shares of branched hydrocarbons and ~10 wt% of coke. Thermal PE cracking allows one to obtain low gas and coke yields (<2 wt%) while liquid and wax fractions reach over 97 wt%.

Efficient thermal or catalytic cracking of waste polyolefins can be realized in tube reactor with an internal mixer [10]. Depending on the type of raw material (commercial PE, PP and PS) application of this reactor system in thermal process makes it possible to obtain 85 wt% of liquid, semi-solid or solid product, 0.6-10 wt% gaseous product (mainly C1-C5 hydrocarbons) and 1-5 wt% of solid residue. In waste plastics cracking, yields of carbon residue and gas can increase up to 5-10 wt%. The main liquid products (or solid at ambient temperature) are characterized by high olefins content, bromine number more than 50 g Br2/100 g and freezing point above 40°C for the feed with high PE content. The increase in PS and PP content in the feed is accompanied by lowering in freezing point of the main products, even below — 15°C. High shares PP containing cracking feeds are especially suitable for production of light oil fractions of low freezing temperature (even —25°C) and high octane index (>50). Solid, coke-like residue, depending on the feed contains as much as 30-50 wt% of mineral components, including catalyst if used, and its calorific value, similar to brown coal, attains value ~20 MJ/kg.

The main difference between two types of the process, catalytic and thermal is in the different conversion levels and products yields. High gas (~50 wt%) and gasoline yields (~15 wt%) as well as low yield of gas oil and 10 wt% coke were found in the catalytic processes (commercial catalysts). On the other hand the thermal cracking processes give low gas, gasoline and light gas oil yields in connection with very high yield of waxes (over 87 wt%) and small yield of coke [9]. Low coke yield in thermal process is the result of the low cracking level of the polymer feed.

The high influence of cracking catalyst on PE conversion was confirmed by Aguado et al. [11] in a continuous screw kiln reactor. The application of a sophisticated laboratory Al-MCM-41 cracking catalyst and process temperature of 400-450°C led to 85-87% yield of gas and gasoline fractions (C1-C12). Besides olefins and n- and iso-paraffins some quantity of aromatics, 5 wt% was determined in the process products. In the same reactor system with a noncatalytic process the gas yield was halved while similarly as in case of the fluid reactor system yields of gas oil and heavy waxes fraction (C13-C55) attained values of ~62% (compared with 4 wt% in catalytic process) [12].

The considerably lower activity of other catalysts in PS cracking process, HZSM-5 and Al2O3-SiO2 of higher acidity is explained by hydrocarbon cross-linking and heightened carbon residue yield. Similarly to the fluidized-bed process [9], the thermal PS cracking results mainly in styrene while HMCM-41 catalyzed process produces mainly benzene and ethyl benzene. Alkaline materials, e.g. magnesium or barium oxide, in PS cracking present high activity and selectivity to styrene [13]. This suggests quite different reaction mechanisms, i.e. that thermal cracking proceeds through a free radical chain mechanism while carbenium and anionic ions are involved in acid and alkaline catalysis cracking processes.

Increase in thermal process temperature up to 685-715°C in a fluidized-bed system (Hamburg University Pyrolysis Process-HUPP) and application of a mixture of municipal plastic wastes resulted mainly in gaseous products, over 41 wt%, of which olefins constituted 15%, and aromatic (BTX)-containing liquid products [14]. Considerably better results from the point of view of C2 and C3 olefins yield were obtained in other experiments. The application of steam as fluidization agent instead of circulation pyrolysis gas enabled an increase of C2-C3 olefins yield from 48 to 60%, accompanied by decrease in BTX yield from 24 to 11 wt% [15].

Analogous results were achieved in the fluidized-bed system by Williams et al. [16]. They found out that raising the PE pyrolysis temperature from 500 to 700°C gives an increase in gas yield from 11 to 71% and that C2-C4 olefins content increases from ~7 to above 53%. Hydrogen content in the gaseous fraction attained a level of 1%. At the same time oil and wax yields were diminished from 89 to 28.5 wt% in which 25% was identified as mono- and polyaromatic hydrocarbons. Noticeable quantities of aromatics (~2.5 wt%) in the products of the process at ~600°C were determined (Table 4.1).

Process temperature, as well as waste polyolefin composition and type of catalyst used are then the most important process parameters. It is evident that a process temperature below 500°C is the most suitable range for refinery fractions while 600°C and higher are appropriate for olefins production.

According to Bockhorn et al. [17], cracking of waste plastics can be carried out thermally in a three-step reactor cascade at laboratory scale in a reactor filled with special moving steel spheres. At the lower temperature (330°C) quantitative PVC dechlorination takes place, at 380° C polystyrene is decomposed with high styrene yield and 440° C is suitable for PE cracking into paraffins and olefins. The molecular weight of the cracking products obtained is a function of residence time of the feed in the reactor zone. In the presence of PE (or PVC) cracking PS gives ethylbenzene as a result of the hydrogen transfer from PE chains to styrene during the process.

In the range of higher temperatures, 475-525°C, commingled waste plastics conversion increased in laboratory autoclaves from 79 to 99.5%, gas oil yield decreased from 48 to 19% while yield of cracking gas increased from 32 to 66%. It is visible that increase in primary and secondary plastic conversion takes place. At the highest gas production the yield of coke attained 8%. Even at the lowest process temperature, i.e. 475°C almost total plastics conversion was attained in 15 min and the main cracking products were light and heavy gas oils at 1-2% coke and 20% gas yields. On the basis of kinetics analysis the authors concluded that cracking of polymers such as PE occurs to a greater extent near the end of the polymer chain rather than in the middle of it [2].

Table 4.1 Influence of process temperature on product yields from fluidized bed pyrolysis of LDPE [16]. (Reprinted from Journalof Analytical and Applied Pyrolysis, 1999: 51: 107, P Williams & E Williams with permission from Elsevier)

Process products Process temperature, (°C)

Olefins in gas (wt%)

35.4

16.5

Comparative studies of thermal and catalytic PP cracking indicate a strong influence of catalyst on total conversion in the temperature range 340-380°C as well as gas and gasoline yield [18]. The results also point out the possibility of applying an equilibrium FCC catalyst [19]. Similar studies were carried out in other laboratories [7, 20]. The effect of the application of cracking catalyst, including equilibrium FCC catalyst is the increase in plastics conversion level, higher gas and liquid product yields as well as lower boiling temperature range and density of the liquid cracking products obtained. This seems to be a good solution. However, one has to remember that equilibrium FCC catalysts contain heavy metals (mainly V and Ni) and they are classified as environmentally dangerous materials. In the course of the cracking process they are collected in coke residues which have to be utilized.

High conversion level of waste plastics can be also attained in hydrocracking process with using of NiW-HY zeolite-based catalyst [20]. The main process products are good-quality gasoline fractions, with small olefins content. The basic drawback of hydro-cracking is the necessity of using hydrogen, expensive catalysts and increased pressure (high cost of reactors and other plant equipment). Hydrocracking catalysts are also exposed to deactivation by mineral components in the plastic feed. Bergius - Pier hydrogenation technology was adopted by VEBA company for hydrocracking vacuum residue (waste plastics mixture at 450-490°C and pressure 150-250 bar, 40000 tons per year output). The applied process catalyst and parameters gave ~90 wt% conversion [21].

Sophisticated catalysts, such as ZSM-5 or HZSM-5 [22] and other zeolites are also suggested in numerous papers, e.g. REY [23], HY and H-mordenite [24], Re-zeolite-based Engelhardt FCC commercial catalyst [25], and steamed commercial zeolite catalyst [26]. These investigations are mainly devoted to fundamental studies and the correlation between feed composition, catalyst properties, process parameters and efficiency connected with product distribution. Iron supported on silica-alumina, mesoporous silica and active carbons serves as the next example of materials applied in the waste plastics cracking [27, 28]. On the other hand, according to some results [29] application of cracking catalysts such as Zn-13X, Fe-5A and CoMo-HY are ineffective in waste plastics cracking.

The appropriate solution seems to be the application of cheap quartz, alumina and activated clays in this process [30, 31]. In some patents [e.g. 31] and papers [7] platinum or reforming catalysts are suggested as suitable for this process.

It is surprising that small-pore zeolites (pore size lower than 1 nm) are so frequently proposed as components of the catalysts of large-molecule waste polyolefin cracking [e.g. 22-24]. The very difficult transport of the melted plastics material into the catalyst pores is the fundamental problem with catalytic cracking of waste plastics. Therefore, cracking of long hydrocarbon chains can occur mainly at the external surface area of the catalyst grains while products of these primary reactions can be further cracked in the catalyst pores (secondary reactions). This is especially true in the case of PS with side phenyl groups in the polymer chain that create steric hindrance. Serano et al. state that this obstacle can be overcome by using large-pore materials such as (H)MCM-41 [12].

It seems that, from a practical point of view, equally or even more important than a type of potential cracking catalyst are impurities in real waste plastics. Only sparse research work has been devoted to problems such as possible PVC and heavy metals content in the cracking feeds. In the course of segregation and preparation of the feed some PVC can be included and then the cracking products obtained will contain chlorine or its compounds. According to Kaminsky et al. [14] and Simon et al. [32] gaseous cracking or pyrolysis products are free of chlorine if HCl is removed, e.g. by calcium carbonate, liquid products can contain up to 15 ppm chlorine while the largest quantity of chlorine is concentrated in tars or solid residues and mineral fluidization mediums. Relatively large quantities of chlorine can be concentrated in water (if present in the cracking products). At longer residence time or high process temperature chloroorganic compounds are totally decomposed into HCl and hydrocarbons. Heavy metals, such as Na, K, Cr, Mn, Zn, Cd, Sb derive from product residues and auxiliary agents used in polyolefins processing, such as stabilizers or plasticizers and others. They are mainly concentrated in soot or other solid residues, e.g. Zn content in soot attains even more than 1 wt %, while the oil fraction can contain about 20 ppm Zn [32].

5.2 COPROCESSING OF WASTE PLASTICS WITH OTHER RAWMATERIALS

Lubrication oil wastes and waste plastics are very similar materials from a chemical composition point of view; they are of organic origin and are composed mainly of carbon and hydrogen. Both feeds can be utilized by thermal or catalytic cracking or pyrolysis process. One of the differences is that annual waste oils amounts are considerably lower and account for about 15-20% of annual waste plastics amounts [33, 34]. Relatively high melting temperature, high viscosity and low thermal conductivity of waste plastics can cause difficulties in their direct feeding with the typical equipment. Mixing and diluting of waste plastics with waste oils and cracking is an optional method of common processing. Due to rheological properties, according to Lovett et al. [35], the maximum content of waste plastics in waste oils in commercial scale should not be higher than 10%. The further studies of Serrano et al. [34] showed that application of laboratory kiln reactor and thermal cracking temperature 450-500°C enables one to obtain over 95% degradation of LDPE-waste oil mixture (70/30-40/60) and gives mainly C13-C22 and C23-C40 fractions (n-paraffins and 1-olefins). It is evident that only partial waste oil conversion was attained (C28-C41 hydrocarbons), since it is more thermally stable than LDPE. They obtained much better results by using HZSM-5 and Al-MCM-41 cracking catalysts at the same temperature range and 70 wt% LDPE content in waste oil. In this case 65 and 50% selectivity to C5-C12 fraction was determined for Al-MCM-41 (mainly olefins) and HZSM-5 (olefins and aromatics), respectively. Kargoz et al. carried out similar experiments, but they applied municipal waste plastics and vacuum gas oil mixture under hydrogen pressure [36].

Light cycle oil, a highly aromatic FCC product (e.g. 67% aromatics content) can be used as solvent for PS and PS-BD (polystyrene-butadiene) mixture, component of synthetic rubber. Arandes et al. [37] proved that this feed can be further successfully thermally or catalytically cracked, giving mainly gasoline fractions. They contain a high quantity of valuable components for petrochemistry, styrene and C4 olefins.

Gebauer et al. [38] suggest visbreaking of waste plastics with vacuum residue. This is a thermal process, applied in refineries in order to convert partially atmospheric vacuum residue and decrease viscosity and melting temperature. According to the authors, addition of 5% of waste plastics in laboratory tests does not influence noticeably the process parameters and final products properties. As in the case of LCO and VGO fractions the application of vacuum residue and mixture of waste plastics is applicable in refineries.

It is worthy mentioning two other possibilities of waste plastics processing, both based on gasification. In the first case, co-gasification of plastics and biomass, gives the possibility to utilize wastes accompanied by the control of hydrogen content in the synthesis gas produced [39]. According to Fink and Fink [40], synthesis gas from waste plastic gasification can be 'put in' pre-selected crude oil wells. These wells should be located near the gasification plant and the main goal of this method is to enhance crude oil recovery in the future.

Co-coking of waste plastics and coal-tar pitches in the temperature range 200-400°C yields reaction pitches which can be further applied as additives to coal blends and as a consequence improve their coking properties [41]. A quantity of lower-boiling products can be obtained as a result of the coking process. A similar process is presented in a German patent [42], but in the presence of granular calcium oxide in the temperature range 600-1400°C and at CaO: waste plastics ratio 1:1-3. The main products are calcium carbide and a gaseous fraction which can be used for power generation.

Some inventors suggest conversion of waste polyolefins and the waxy Fischer-Tropsch fraction into lube base oils in relatively mild thermal conditions, i.e. at 150-350°C and short residence time [43]. Both raw materials are submitted to pyrolysis and after thermal treatment the heavy liquid fractions can be hydrotreated and isodewaxed in order to obtain high viscosity index lubrication base oils while medium fractions after isodewaxing can be applied as the components of diesel or jet fuels. Polyolefins can be thermally or catalytically processed (Mn, V, Cu, Cr, Mo or W compounds as active phase) with natural or synthetic rubbers, paraffin or lignite waxes or other polymers [44]. Mixtures of PP and PE can be applied as good-quality feed for production of microcrystalline waxes [45]. At relatively low temperature, i.e. 350-430°C cracking is completed with partial melt recycling and destructive distillation (under normal pressure or vacuum).

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  • GIUSEPPA
    How much is waste plastics (pp, pe, ps)?
    9 years ago

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