216
Energy & Fuels 1995,9, 216-224
Conversion of Polyethylene Blended with VGO to Transportation Fuels by Catalytic Cracking+ Siauw H. Ng Western Research Centre, CANMET, Natural Resources Canada, 1 Oil Patch Drive, Devon, Alberta, Canada TOC 1EO Received July 12, 1994@
A vacuum gas oil (VGO) and blends containing 5 and 10 wt % high density polyethylene, respectively, in the VGO were thermally and catalytically cracked in a fixed bed reactor at 510 "C and 20 h-l WHSV. The objective was to produce transportation fuels from direct cracking of polyethylene without prior pyrolysis at 450-500 "C as reported previously (Ng et al., 1993). Thermal cracking resulted in low overall conversion to gasoline and its secondary products including coke, although polyethylene in the blend was substantially converted. In contrast, high conversion was obtained from catalytic cracking. However, the gasoline yield of polyethylene depended on its concentration in the blend. At 5 wt % polyethylene, the gasoline produced from primary cracking appeared to be completely decomposed to gas and coke through secondary cracking whereas at 10 wt %, a substantial amount of gasoline was produced. Synergistic effect observed during catalytic cracking of the blends was discussed.
Introduction Public concern over environmental issues is the main driving force of today's increased activities in plastics recycling. This is especially true where suitable landfill sites are difficult to obtain in industrialized countries. Internationally, Europe and Japan appear to lead the US in enforcing recycling policy. In Europe, several major industrialized nations such as Germany, Italy, and the Netherlands have set up time tables to achieve certain recycling quotas.2 In the US, regulatory guidelines are vague. Recycling processes are being developed as a collaborative effort by industries. Despite this fact, plastics recycling in the US is growing at an annual rate of 12 wt o/o with more than 0.5 million tonnes or 1 billion pounds of plastics reprocessed into new products in 1993.3 The most widely recycled plastics are poly(ethylene terephthalate) (PET) found in soft drink and mineral water bottles, and high-density polyethylene (HDPE) found in milk jugs and fruit juice containers.* These constitute 75 wt % of US plastics recycled in 1991.2 Through solvolysis (methanolysis, glycolysis, hydrolysis, and aminolysis, etc.), condensation polymers such as PET can be chemically decomposed into raw materials for reuse. For years, manufacturers of condensation polymers have been chemically recycling offgrade finished product or in-plant scrap. Solvolysis in tertiary recycling is applicable only to polymers having specific functional groups in the backbone. These functional groups contain weak chemical bonds that are A version of this paper was presented at the 34th Congress of International Union of Pure and Applied Chemistry (IUPAC),Beijing, China, August 15-20, 1993. Abstract published in Advance A C S Abstracts, February 1, 1995. (1)Ng, S. €1.;Seoud, H.; Sugimoto, Y. In Proceedings of34th IUPAC Congress, Song, X., Shi, L., Zhou, Q., Qiu, X., Xu, W., Wang, S., Fang, Z., Eds.; Chinese Chemical Society: Beijing, China, 1993; pp 695. (2) Shelley, S.; Fouhy, K.; Moore, S. C h e n . Eng. 1992, 99(7), 30+
@
35. (3) Chem. Marlzetrng Rep.; 1994 (May 161, 18. 14)Adams, A. A.; Haritatos, N. J.; Robinson, R. C. 43rd Canadian Chemical Engineering Conference; Ottawa, Ontario, October 3-6, 1993; Book ofAbstructs, pp 128.
0887-0624/95/2509-0216$09.00/0
susceptible to dissociation by certain chemical agents. Addition polymers, such as polyolefins, polystyrene, and poly(viny1 chloride), require a more aggressive approach in tertiary recycling. This involves heat treatment at high temperatures to decompose polymers into monomers, liquid and gaseous fuels, and petrochemicals. Thermal decomposition includes pyrolysis and other refinery processes such as coking, hydrocracking, catalytic cracking, and gasification. Thermolysis has greater diversity and flexibility than solvolysis. The variety of well-developed processes and existing facilities in refineries give this technology a greater potential to successfully treat mixed waste streams. There are, however, challenges in some instances. Contaminants in the plastics waste stream, including chlorine and nitrogen, need to be removed before certain processes are used. The necessity to transform plastics into pumpable liquid is also a major problem. The cost of sorting plastics scrap for subsequent treatment is substantial. Today, more commercial units use solvolytic methods than thermolytic means. Although many thermolytic demonstration plants exist, some of which are semicommercial scale, the only commercially significant unit is a thermolytic/catalytic cracking unit (5000 t/a) operated by Fuji Recycle Industry, Japan.2 It would thus appear that more research is necessary to make the thermolytic technology more viable. Recently, CANMET conducted a series of experiments in which polyethylene (PE) was pyrolyzed in a closed reactor at 450-500 0C.1.5 It was found that higher temperature produced more distillates but also increased gas and coke yields. While pyrolysis of PE produced low grade transportation fuels which required further upgrading, catalytic cracking of the pyrolytic waxy product produced high yields of gasoline and LPG with little low (5) Ng, S. H.; Seoud, H.; Stanciulescu, M.; Sugimoto, Y. Energy Fuels, submitted for publication, 1994.
Published 1995 by the American Chemical Society
Conversion of Polyethylene to Transportation Fuels
value dry gas, coke, and heavy cycle 0 i 1 . ~ -The ~ present work is a continuation of the previous two-step study5 in which thermal cracking of PE was followed by catalytic cracking of its waxy product. In this study, HDPE was directly dissolved in a commercial VGO which was a feed to fluid catalytic cracking (FCC) unit. The blend was then catalytically cracked. This approach is feasible in an existing refinery having an FCC unit. The capital cost involved is minimal as far as dissolving plastics is concerned. Thus, the objective of the present work was to improve process economics through this route. The results are compared with those from the two-step process.
Experimental Section Materials. HDPE resin pellets (DMDA 200, 1.5 mm diameter x 3 mm high) from Union Carbide Corp. were dissolved in a VGO Rainbow Zama (RZ) derived from Western Canadian conventional crude. The VGO was obtained from Petro-Canada's Edmonton refinery. Akzo Chemicals equilibrium FCC catalyst, KOB-627, was obtained from PetroCanada's Oakville refinery. Sand (Ottawa sand) used in thermal cracking was obtained from Fisher Scientific. Preparation of HDPE-RZ Blends. Two blends containing 5 and 10 wt % HDPE, respectively, in RZ VGO (blends A and B) were prepared. RZ was chosen because of its high paraffinic content which provides better solvency for straightchain PE. The HDPE-RZ mixture contained in a metal can with cover was gradually heated on a hot plate with occasional stirring until HDPE was completely dissolved at about 170 "C. Characterization of Feedstocks and Catalyst. Characterization of RZ VGO was performed using ASTM and other supplementary methods. Analyses included API gravity (D12981, aniline point (D-611), aromaticity by Carbon-13 NMR (Varian XL-3001, Conradson carbon, elemental carbon and hydrogen by Perkin-Elmer 240, total nitrogen by chemiluminescence (D-4629) using a Dohrman analyzer, sulfur by X-ray fluorescence (D-4294) using a Gamma Tech Model 100, viscosity (D-445) by a reverse flow viscometer, simulated distillation (D-2887), and hydrocarbon type by combined GC-MS (HP 5890GC and low resolution Finigan Incos-50 MS) and NMR (Varian XL-300). For hydrocarbon type determination, RZ VGO was separated into appropriate fractions (asphaltenes, polars, and nonpolars) by suitable solvents and adsorbents as described in ASTM D-2007. Only the nonpolar fraction was injected into GC-MS for analysis. Characterization results of HDPE and catalyst have been presented e l ~ e w h e r e . ~ Cracking of Feedstocks in a Fixed Bed Reactor and Characterization of Cracked Products. Thermal cracking and catalytic cracking of RZ VGO and blends containing the VGO and HDPE were conducted in an automated microactivity test (MAT) unit (Zeton Automat IV). The MAT unit was a modified version of ASTM D-3907 equipped with a downflow fured-bed quartz reactor and a gas collection system. The reactor was loaded with 4 g of sand or catalyst KOB-627 which had been decoked for 3 h a t 600 "C prior to loading. The procedure for MAT runs was similar to those described in ASTM D-3907 and D-5154. Before and during reaction, the unit was purged with nitrogen at 20 m u m i n . The feed was delivered to the reactor by a constant drive syringe pump through a syringe which was heated at about 60 "C. Vaporization of feed took place in the preheat zone, followed by cracking in the catalyst bed. The nitrogen purge continued and the run was terminated 30 min after the feed injection. (6)Songip, A. R.;Masuda, T; Kuwahara, H.; Hashimoto, K. Appl. Catal. B: Enuiron. 1993,2,153-164. (7) Songip, A. R.; Masuda, T; Kuwahara, H.; Hashimoto, K. Energy Fuels 1994,8,136-140.
Energy &Fuels, Vol. 9, No. 2, 1995 217 Table 1. Feedstock Analysis Rainbow Zama VGO "API gravity aniline point, "C aromaticity, % Conradson carbon, wt % C,wt%
H, wt % total N, wt % total S, wt % viscosity at 40 "C,cSt boiling range by GCD, "C IBP 10 20 50 90 FBP GC-MS analysis, wt % paraffins cycloparaffins monoaromatics diaromatics polyaromatics S-containing aromatics polars
26.6 87.0 16.0 0.05 86.3 13.0 0.06 0.59 26.5 230 315 348 414 492 559 23.9 37.8 15.0 8.9 7.9 3.3 2.7
Reactor eMuent passed through two ice-cooled receivers in series where condensable product was collected. The amount of liquid product was determined by weighing the receivers before and after each run. Liquid product was analyzed for simulated boiling point distribution by GC (ASTM D-2887). The gaseous product, collected in a sampling bag, was also analyzed by GC for various components. The coke deposited on the catalyst after cracking was determined in situ by passing a stream of air through the catalyst bed. The exhaust gas passed through a catalytic reactor to convert carbon monoxide into carbon dioxide. The COz generated was measured by a n infrared on-line analyzer. Test severity was adjusted by varying the catalydoil (catloil) ratio with constant temperature a t 510 "C and weight hourly space velocity (WHSV) a t 20 h-l. Since cracking is endothermic, a drop in temperature was observed during reaction (5-10 "C in thermal cracking and 10-20 "C in catalytic cracking, depending on the amount of feed being cracked). The results reported for each MAT run included conversion, yields of dry gas (Hz-Cz), liquefied petroleum gas (LPG, Le., Cs and C4), gasoline (IBP216 "C), light cycle oil (LCO, 216-343 "C), heavy cycle oil (HCO, 343 "C+), and coke. Conversion is defined as the difference between 100 and the yields of unconverted products (Le., LCO and HCO).
Results and Discussion Feedstock and Catalyst Properties. Analysis in Table 1 indicates that RZ VGO is sweet, light and low in Conradson carbon, nitrogen, and aromatics. Gas oil of this nature is usually considered a high-quality FCC feedstock. RZ has a paraffinic base as reflected by its low aromaticity, high aniline point and WC molar ratio (1.807)) and hydrocarbon type analysis. The comparatively high paraffin content of this VGO makes it a soft wax at room temperature. No olefins were detected from NMR analysis. HDPE is a pure resin containing no additives. Chemically, it should be highly crackable like the long-chain waxy paraffins. The FCC catalyst KOB-627 is an octane-enhancing catalyst containing ultrastable Y type (USY) zeolite. The catalyst is of medium activity judging from its zeolite ~ o n t e n t . ~ Thermal Cracking of Rz VGO and Blends Containing HDPE in VGO. Table 2 shows the results of thermal cracking of HDPE-RZ blends in a fixed bed
218 Energy & Fuels, Vol. 9, No. 2, 1995 Table 2. Results of Thermal Cracking of RZ VGO and Blends Containing VGO and HDPE at 510 "C 10 10 5 wt % HDPE in VGO 0 3.05 4.23 2.68 2.58 sandloil, wtlwt 16.4 18.9 18.7 19.3 WHSV, h-' injection time, s 72 72 72 45 HDPE iniected, mg- 0 74.7 131.4 94.5 yields, wt % basis BFa BFa HDPE BFa HDPE BF" dry gas 0.50 1.05 11.5 1.15 7.00 1.31 Hz 0.00 0.01 0.20 0.02 0.20 0.02 0.33 0.29 1.73 0.13 0.30 3.53 c1 0.39 0.34 1.96 0.16 0.31 3.16 cz 0.57 0.50 3.11 0.21 0.43 4.61 Cz= 1.56 1.35 8.73 0.53 1.31 16.1 LPG 0.24 0.21 1.29 0.09 0.21 2.49 c3 0.58 0.51 3.30 0.20 0.50 6.20 c3= 0.10 0.09 0.63 0.03 0.07 0.83 n-C4 0.10 0.06 0.33 0.03 0.05 0.43 i-C4 0.54 0.48 3.18 0.18 0.48 6.18 C4= gasoline 1.94 3.52 33.5 3.95 22.0 5.01 LCO 16.8 15.8 -3.2 15.6 4.8 15.9 HCO 79.9 77.2 25.9 76.1 41.9 75.2 coke 0.30 1.05 15.3 1.74 14.7 0.89 conversion, wt % 3.4 7.1 77.4 8.3 52.4 8.8 a
BF: bulk feed.
reactor at 510 "C. Comparatively low conversion occurred in thermal cracking of each bulk feed. At constant space velocity, conversion increased slightly with increased HDPE concentration and sandoil ratio, resulting in higher yields of dry gas, LPG, gasoline, and coke, and lower yields of LCO and HCO. It is interesting to note that a higher sandJoi1 ratio seemed to have a positive effect on conversion, although the sand used was supposedly inert (no active acid sites for cracking). This was probably due to better heat transfer between the heating medium (sand) and oil molecules. Considering the weight fractions of the components and assuming that RZ VGO in the blend gave the same product distribution as that in the blank (0wt % HDPE) upon cracking, the product yields of HDPE in the blend could then be calculated. As shown in Table 2, a t 5 wt % HDPE concentration, 77.4 wt % of HDPE was converted into 11.5 wt % dry gas, 16.1 wt % LPG, 33.5 wt % gasoline, and 15.3 wt % coke. At 10 wt % HDPE addition and under the same test conditions, the corresponding yields decreased to 7.0, 8.7, 22.0, and 14.7 wt %, respectively (coke yield could be overestimated), giving a conversion of 52.4 wt %. The reduction in yields and conversion was due to more HDPE injected into the reactor (131.4 mg against 74.7 mg in the case of blend A). Among the gaseous products, olefins (especially ethylene, propylene, and 1-butene) were the major components compared with their corresponding saturated hydrocarbons. Methane was produced in significant amount and 1,3-butadiene was detected. Recently, similar observation was also reported by Kaminsky and Rosslers who studied pyrolysis of polyethylene at high temperatures (650-820 "C) in fluidized bed using N2 as fluidizing medium. Catalytic Cracking of RZ VGO and Blends Containing HDPE in VGO. Since both the temperature and space velocity were fixed in this study, the only test variable was the catloil ratio for a given feed. Cracking results were graphically represented in two formats, i.e., yield vs cat/oil ratio and yield vs conversion. The ( 8 ) Kaminsky, W.; Rossler, H. CHEMTECH 1992, February, 108-
113.
A 10 wt % HDPE
2.0
3.0
4.0
5.0
6.0
CATALYSTIO IL RATIO, w t / w t Figure 1. Effect of catalyst/oil ratio on conversion at 510 "C for HDPE-RZ blends. former shows a direct response as catloil varies and is useful to explain the cracking behavior of a feedstock. The latter is more comprehensive and is more frequently used in literature. Note that in commercial FCC operation, the catloil ratio is not a n independent ~ a r i a b l e .The ~ conversion can be affected by several process variables. In MAT runs, varying the caUoil ratio is only one way to achieve a certain conversion. Alternatively, the same conversion may be obtained by changing other test parameters such as WHSV and reactor temperature. 1. Effect of CatIOil Ratio on Product Yields. Since the amount of catalyst used is fixed (4 g), increasing the catloil ratio while maintaining WHSV constant leads t o less oil delivered to the reactor over a shorter period. Consequently, a higher caUoil ratio a t constant temperature will provide more acid sites per oil molecule to be cracked. Thus, the yields of all converted product should rise monotonically at the expense of heavy fractions such as LCO and HCO. However, this is complicated by secondary reactions (e.g., decomposition of gasoline and LPG), catalyst poisoning by coke and heavy metals, and feedstock property which limits the conversion a t high catloil ratio. These factors lead t o different yield profiles as the caUoil ratio increases. The results from this work are shown in Figures 1 and 2a7a. The following comments are offered. (i) In general, conversion and yield profiles followed the expected trends. As the catloil ratio increased, conversion, dry gas, LPG, gasoline, and coke also increased whereas LCO and HCO decreased for all feeds. There were, however, a few exceptions. (ii) Figure 1 indicates that the addition of HDPE to RZ VGO enhanced the conversion, but not proportionately. Blend A showed a much smaller effect than blend B. Despite this fact, it is concluded that HDPE is more crackable than RZ itself. This is understandable since the polyaromatics (including diaromatics and possibly polars) in RZ remained unconverted after dealkylation of their side chains. It would appear that RZ had not reached its conversion limit (about 77 wt %) based on its compositional analysis by MS. Compared with RZ (9) Venuto, P. B.; Habib, E. T. Jr. Fluid Catalytic Cracking with Zeolite Catalysts; Marcel Dekker, Inc.: New York, 1979; pp 55-56.
Energy & Fuels, Vol. 9, No. 2, 1995 219
Conversion of Polyethylene to Transportation Fuels 20
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ratio and (b) conversion, at 510 "C for HDPE-RZ blends. and blend A, blend B showed signs of leveling off at high cat/oil ratio. This is attributable to the faster depletion of crackable components which are contained more in blend B than in blend A. (iii) Figure 2a shows a concave increase in dry gas yield for all three feeds as the cat4oil ratio increased. This indicates that dry gas was produced at a faster rate at higher cat/oil ratio than at lower ratio. Note that dry gas is formed by thermal cracking and secondary catalytic cracking of butenes and gasoline.1° Surprisingly, the dry gas yield of blend B was lower than that of blend A except at high cat4oil ratio. This crossover can be explained by the overcracking of gasoline t o be discussed later. At equal cat4oil ratio, both blends A and B produced more dry gas than RZ because of their higher crackabilities. (iv) The LPG yield curves followed the same trend as those of the dry gas except the yield of blend B increased linearly with increased cat/oil ratio (Figure 3a). In this study, CS and C4 olefins constituted 50-60 wt % LPG. The other major component was isobutane which accounted for 30-40 wt % LPG. Butenes (C4=) are unstable and can be converted to butanes, propylene,
methane, and coke.'l Isobutane is mainly formed from unstable olefinic gasoline and butenes through cracking and isomerization. (v) An odd phenomenon was observed in Figure 4a which shows that at equal catloil ratio the addition of 5 wt % HDPE to RZ reduced the gasoline yield whereas the addition of 10 wt % HDPE enhanced the gasoline yield except at high cat4oil ratio where the gasoline yield declined because of overcracking. This occurred when gasoline precursors were exhausted and the unstable olefinic gasoline continued to undergo secondary reactions to produce dry gas, LPG, and coke. Since gasoline is the most desirable product in FCC operation, refiners usually avoid overcracking of gasoline unless higher octane numbers are their prime targets. (vi) All feeds contained less than 20 wt % heavy fraction which boiled in the LCO range. As the cat/oil ratio increased, the LCO yield first increased (not shown in Figure 5a) and then decreased when the rate of its formation from cracking HCO was exceeded by that of its decomposition into lighter fractions. The LCO yield has a lower limit which is usually controlled by the feed concentrations of diaromatics and two- and three-ring compounds containing aromatic sulfur.12 Like LPG,
(10)John, T. M.; Wojciechowski, B. W. J . Catal. 1975,37, 348357.
250. (12) Fisher, I. P. Appl. Catal. 1990,65, 189-210.
(11)John, T. M.; Wojciechowski, B. W. J. Catal. 1975,37, 240-
220 Energy & Fuels, Vol. 9, No. 2, 1995
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CONVERSION, w t % Figure 5. Relationship between LCO yield and (a) cat/oil ratio and (b) conversion, at 510 "C for HDPE-RZ blends.
LCO also showed a normal relationship between the yield and HDPE level (Figure 5a). (vii) As expected, the HCO yields of the three feeds declined monotonically as the catloil ratio increased (Figure 6a). However, the two yield curves representing RZ and blend A were similar while feed B indicated lower HCO yield. Similar to LCO, the lower limit of HCO yield is determined by polyaromatics (excluding diaromatics) and four- and five-ring compounds containing aromatic sulfur in the feed.12 (viii) Figure 7a shows that coke yield increased concavely with the increased catloil ratio. At equal cat/ oil ratio, the blends containing HDPE produced more coke than the blank. However, blend A appeared to have a higher coke-forming tendency than blend B. Coke is usually formed from its precursors in the feed (aromatics with high molecular weight and those containing basic nitrogen) and from secondary reactions involving cracking of gasoline and butenes. Coke deposited on the catalyst will block the pores rendering the catalyst less active and nonselective. 2. Correlation of Product Yield with Conversion. The relationship of a product yield with conversion was established using the data in Figure 1which shows the correlation between conversion and catloil ratio, and those in Figures 2a-7a which show the correlations between the product yield and catloil ratio.
As indicated in Figure 1, at the same catloil ratio, the conversion of blend A was about 1 wt % higher than that of RZ within the range of catloil ratio studied whereas that of blend B could be 3-6 wt % higher than the blank depending on the catloil ratio. These relationships led to different profiles of the yield curves shown in Figures 2b-7b. Figure 2b indicates that below 73 wt % conversion, RZ produced the least dry gas followed by blends B and A. Beyond 73 wt % conversion, dry gas yield of blend B could be exceeded by that of RZ which might also produce more coke than blend B (Figure 7b) because of the lower quality of RZ. Being the end products of catalytic cracking, both dry gas and coke always increase concavely as conversion increases. With the new format, LPG yield curves in Figure 3a were transformed t o different trends and shapes (Figure 3b) which were similar to those of dry gas (Figure 2b). Gasoline yield curves in Figure 4b also showed different profiles compared with those in Figure 4a. At the same conversion, RZ gave the highest gasoline yield, followed by blends B and A. Overcracking of blend B was observed at 74 wt % conversion at which both RZ and blend A were not yet overcracked. The overcracking phenomenon is usually seen at high conversion close to its limit. In contrast t o Figures 5a and 6a which show nonlinear yield curves of LCO and HCO for feed B, Figures 5b and 6b depict linear decreases of both LCO and HCO
~
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Conversion of Polyethylene to Transportation Fuels 22
,
Energy & Fuels, Vol. 9, No. 2, 1995 221 6.0
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yields for all three feeds with increased conversion. Also, at equal conversion, the differences in yields between feeds were comparatively small (less than 1 wt % in general). Of the three feeds, RZ gave the highest LCO yield, followed by blends A and B. For the HCO yield, the order was reversed, with blend B being the highest. This was the direct consequence of the relationship LCO yield HCO yield = 100 - conversion as defined previously. 3. Cracking Yields of Components in a Blend. In order t o understand the phenomenon that the product yields responded irregularly to the concentration of HDPE, it is necessary t o determine the cracking yields of individual components (i.e., HDPE and RZ) in the blend. Prior to the determination, several conceptual problems need to be clarified: (i) It is expected that dissolution of HDPE in RZ at 170 "C is simply a physical process which does not alter the composition of RZ in the blend. Thus, both HDPE and RZ can be treated as separate entities in the blend for ease of calculation. (ii)Based on the observed temperature profiles during catalytic cracking, there are indications showing that the highly crackable materials such as the long-chain HDPE and paraffins in RZ were preferentially cracked.13
+
(13) Ng, S.H. Unpublished work.
(iii)It is reasonable to assume that HDPE in the blend has achieved complete conversion since a separate study5 indicates that catalytic cracking of PE-derived pyrolytic product at 510 "C, WHSV = 20 h-l, caVPE = 4, yielded 90 wt % conversion while in the present study catJHDPE ratio was 10-20 times higher. This is also supported by the data in Table 2 which shows that even thermal cracking alone could lead to substantial conversion (more than 50 wt %) of HDPE in RZ. (iv) HDPE is a very clean feedstock containing no poisons such as metals and nitrogen. However, the significant amount of coke resulting from preferential cracking of HDPE can deactivate the catalyst rendering lower conversion of RZ than when RZ is cracked alone at the same conditions. Despite this fact, the product distribution of RZ in the blend can be approximately obtained from the yield curves (yield vs conversion) of RZ cracked alone at the same conditions. This is because the yield curves of a feedstock are primarily (not entirely) determined by its intrinsic properties. Yield curves are less affected by process conditions such as temperature and space velocity unless they undergo drastic changes.14 With these in mind, one can calculate the cracking yields of individual components in the blend. Table 3 shows an example of calculation. Column A lists the (14)Ng, S. H.; Rahimi, P.M. Energy Fuels 1991,5,595-601.
222 Energy & Fuels, Vol. 9, No. 2, 1995 Table 3. Calculation of Cracking Yields of Components in a Blend Containing 10 wt % HDPE at 510 “C and Cat/Oil = 4 column A B C D E RZ HDPE HDPE blendcomponent blend RZ blend HDPE basis of vield blend blend RZ yields, wt % 2.16 1.64 1.82 0.52 5.2 dry gas 3.4 33.5 14.9 11.5 12.8 LPG 52.5 46.8 52.1 5.7 56.8 -gasoline 16.5 16.3 18.1 0.16 1.6 LCO -0.23 -2.3 10.8 11.0 12.2 HCO 1.15 11.5 3.63 2.48 2.75 coke 10.6 106 100.4 89.8 99.7 total 10 100 conversion, wt % 72.8 62.8 69.8 4.00“ 4.736 catJoil, wtlwt a
Experimental. Found.
product distribution of blend B cracked at 510 “C and catloil ratio of 4. This was obtained through interpolation of MAT results in Figures 1 and 2a-7a. The conversion of RZ in the blend was 62.8 (72.8 - 10). This value was amplified to 69.8 (62.8/0.9)based on RZ itself in the blend. The product distribution of RZ in the blend was obtained through interpolation from Figures 2b7b. This is shown in column C . The corresponding cat/ oil ratio was found to be 4.73 from Figure 1. To convert the product distribution of RZ to the basis of the blend, it was necessary to multiply the individual yield in column C by 0.9. This is shown in column B. Since both columns A and B were based on the blend, the difference between the two yields in columns A and B for the same product gave the product yield (based on the blend) of HDPE in the blend (column D). Dividing the individual yield in column D by 0.1 gave the product distribution (based on HDPE) of HDPE in the blend (column E). The objective of this exercise was to obtain the data in columns C and E. It can be seen that LCO and HCO yields were close to zero. These values validate the assumption that HDPE in the blend was fully converted. Using the same procedure, the cracking yields of individual components in blends A and B cracked at cat/ oil ratios of 3, 4,and 5 could also be calculated. The results are summarized in Tables 4-6. Note that in these tables the conversion and product yields of a blend were mathematically the weighted average of those of the two components. The aforesaid values of the blends are the data points (not shown) on the yield curves in Figures 2a-7a, and 2b-7b. Table 7 compares the actual experimental values of cat/oil ratio for each component against the corresponding values found from data analysis. Several observations are highlighted: (a) Tables 4-6 indicate that at 5 wt % concentration, HDPE in the blend produced very little (at cat/oil ratio 3) or essentially no gasoline (at cat/oil ratio 4 and higher). The gasoline was apparently overcracked to give high yields of dry gas, LPG and coke. However, at 10 wt %, HDPE could produce a good yield of gasoline (except at very high cat/oil ratio) reducing substantial amounts of dry gas and coke (LPG was reduced less). This demonstrated the important effect of the cat/ HDPE ratio on the product slate of HDPE, or of the whole blend. This observation could explain the irregular patterns of the yield curves with respect to HDPE concentrations in the blends. Yields of dry gas and LPG in catalytic cracking of HDPE in blend A (Tables 4-6) were apparently much
Ng
higher than those in thermal cracking of the same material (Table 2). Similar observation was also reported by Ishihara et al.I5 who investigated decomposition of low-density polyethylene in a flow reactor using a silica-alumina catalyst. Songip et al. 6.7 studied catalytic cracking of a heavy oil from pyrolysis of polyethylene over different zeolites including HZSM-5 and REY types at long contact time (3 h) and low W3SV (in the order of 1h-l). They also found that the majority of LPG was olefinic and gases (C1 to Cq) predominated at high conversion. (b) Table 7 shows the synergistic effect observed during catalytic cracking of the blends. For clarity, the term “oil” in “cat/oil ratio” was specifically expressed as “blend, “RZ”, or “HDPE” in this table. The maximum caVRZ ratios were obtained from the weight of catalyst (4g) and the amount of RZ in the blend injected into the reactor, assuming all the catalyst was used t o crack RZ. The values found for cat/RZ were transferred from Tables 4-6 whereas those for cat/HDPE were calculated from material balance. catRz/catHDpErepresented the portion of catalyst cracking RZ over that cracking HDPE. This ratio was calculated from the values found for cat/RZ and cat/HDPE a t the same severity, and the known ratio of RZ/HDPE which was 19 (95/5) for blend A. It can be seen from Table 7 that for blend A (1)RZ in the blend was cracked a t cat/RZ ratios smaller than their corresponding maximum allowable values. Further, the smaller ratios of catJRZ compared with those of cathlend implied that cracking of RZ was suppressed in the presence of HDPE. It was likely that HDPE in the blend was preferentially cracked producing an abnormal amount of coke deposited on the catalyst surface. This would lead to catalyst poisoning which suppressed the cracking of RZ; (2) HDPE was cracked at cat/HDPE ratios higher than those for RZ. As the overall cat/oil ratio increased, the ratio for HDPE also increased. This explained why a small amount of gasoline could be produced at lower but not higher cat/ oil ratios which affected the degree of overcracking (Tables 4-6); (3) the calculated values Of CatRz/CatHDpE were 9.6, 12.6, and 15.8 a t cathlend ratios of 3, 4, and 5, respectively. This indicated that the catalyst did not crack RZ and HDPE in the blend according t o their weight ratio which is 19. It is expected that c a t d CatHDPE will come close to 19 at high cathlend ratio where the amount of HDPE in the blend is relatively small and the weight of RZ is approaching the weight of blend. (c) For blend B, the values found for cat/RZ were all higher than their corresponding maximum values (Table 7). This, of course, was theoretically impossible. One explanation was that RZ in the blend was cracked at a lower temperature than when RZ was cracked alone under the same conditions due to the preferential endothermic cracking of HDPE in the blend. The true cat/RZ ratio should be obtained from the curve of conversion against cat/oil ratio a t a lower temperature for RZ alone. This true ratio is expected to be higher than the corresponding value shown in Table 7, as the same conversion can only be achieved at higher cat/oil ratio if the reaction temperature is lower. This phe115) Ishihara, Y.: Nanbu, H.; Ikemura, T.; Takesue, T. Fuel 1990, 69, 978-983.
Conversion of Polyethylene to Transportation Fuels
Energy & Fuels, Vol. 9, No. 2, 1995 223
Table 4. Variation of Yields with HDPE Concentration at CaffOil = 3 blend blendcomponent basis of yield yields, wt % dry gas LPG gasoline LCO HCO coke total conversion, wt % caffoil, d w t
0 wt % HDPE RZ RZ
1.32 11.1
47.0 20.6 18.1 1.74 99.9 61.3 3.00a
blend blend
5 wt % HDPE RZ RZ
2.27 12.1 44.6 19.7 17.8 3.40 99.8 62.6 3.00a
1.32 11.1
46.5 20.6 18.7 1.63 99.8 60.6 2.86b
HDPE HDPE
blend blend
10 wt % HDPE RZ RZ
20.4 31.8 8.8 2.6 -0.4 37.0 100.2 100.0
1.87 13.0 49.8 18.5 13.8 2.92 99.8 67.1 3.OOa
1.32 11.4 48.4 19.9 16.7 1.90 99.6 63.4 3.35b
HDPE HDPE 6.8 26.7 62.1 5.9 -12.3 12.1 101.3 100.0
Experimental. Found.
blend blendkomponent basis of yield yields, wt % dry gas LPG gasoline LCO HCO coke total conversion, wt % caffoil, wffwt
Table 5. Variation of Yields with HDPE Concentration at CaffOil = 4 0 wt % HDPE 5 wt % HDPE 10 w t % HDPE RZ blend RZ HDPE blend RZ RZ blend RZ HDPE blend RZ 1.61 12.2 50.2 19.2 14.4 2.34 100 66.5 4.0OU
2.47 13.4 47.2 18.3 14.0 3.97 99.3 67.7 4.00n
1.56 11.9 49.9 19.2 14.8 2.23 99.6 66.0 3.90b
19.8 41.6 -3.8 1.2 -1.8 37.0 94.0 100.0
2.16 14.9 52.6 16.5 10.8 3.63 100.3 72.8 4.00"
1.82 12.8 52.1 18.1 12.2 2.75 99.7 69.8 4.736
HDPE HDPE 5.2 33.5 56.8 1.6 -2.3 11.5 106 100.0
Experimental. * Found. blend blendcomponent basis of yield yields, wt % dry gas LPG gasoline LCO HCO coke total conversion, wt % caffoil, d w t a
Table 6. Variation of Yields with HDPE Concentration at Cat/Oil = 5 0 wt % HDPE 5 wt % HDPE 10 wt % HDPE RZ blend RZ HDPE blend RZ RZ HDPE blend RZ RZ blend 1.98 13.8 52.2 17.8 11.1 3.09 100.0 71.1 5.0P
2.79 15.0 49.6 17.1 10.7 4.83 100.0 72.2 5.00n
1.90 13.3 52.5 17.8 11.5 2.90 99.9 70.7 4.956
19.6 48.0 -6.0 4.0 -4.0 40.6 102.2 100.0
2.70 16.8 52.2 15.1 8.8 4.54 100.1 76.3 5.00a
2.40 14.8 53.8 17.1 9.3 3.52 100.8 73.7 5.61b
HDPE HDPE 5.4 35.2 37.8 -2.6 3.8 13.7 93.3 100.0
Experimental. Found.
Table 7. Synergistic Effect during Catalytic Cracking of HDPE-RZ Blends cat/ catd HDPE, cat&E, cathlend, camZ,dwt Wwt Wwt blend wffwt max found (found) (found) 5 wt% HDPE 3.00 3.16 2.86 5.66 9.6 4.00 4.21 3.90 5.90 12.6 5.00 5.26 4.95 5.95 15.8 10 wt %HDPE 3.00 3.33 3.35 4.00 4.44 4.73 5.00 5.56 5.61
nomenon is probably not so significant in the case of cracking blend A because the temperature drop is less and the catalyst deactivation by coke is more pronounced. Another less convincing explanation is that cracking of RZ was enhanced in the presence of HDPE or its intermediates or products. Again, the suggested preferential cracking of hydrogen-rich HDPE might first create a favorable cracking environment for RZ. More investigation is necessary to validate this interpretation. Advantages and Disadvantages of Catalytic Cracking Polyethylene Blended with VGO. Certain benefits result from catalytic cracking PE in blends
like those described in this study: (1)in a commercial FCC process, PE can be readily dissolved in VGO (preferably paraffin-based) at feed preheat temperature (370 "C+). No major capital cost is anticipated since the existing FCC unit can absorb a small amount of plastics feed without adverse effects; (2) under proper conditions, PE can produce good yields of gasoline and LPG. The disadvantages follow. (1)The amount of PE in a blend seems to be critical as shown in this study. If the blend contains too little PE, it may be overcracked to low-value products such as dry gas and coke. To avoid this occurrence, the process severity such as the reaction temperature can be lowered. In contrast, too much PE in the blend will cause fluid transportation problems since FCC feed may be too viscous t o flow. (2) At room temperature, the PE-VGO mixture can be solidified. This may cause maintenance problems. As shown in a separate s t ~ d y , lcatalytic ,~ cracking of the waxy product from pyrolysis of PE produced a high yield of gasoline with little dry gas and coke. The pyrolytic product is cleaner and safer to use in the FCC circuit. However, it cannot be cracked alone in the riser since the amount of coke produced is too small to satisfy
224 Energy & Fuels, Vol. 9, No. 2, 1995
the heat balance of the circuit unless the FCC unit is designed for this kind of feed. Further, the supply of pyrolytic waxy product can be a problem as it can hardly meet the feed demand of an FCC unit, even if the unit has a low capacity (20 000 bld or about 3000 tld). If the waxy product is mixed with VGO for cracking, it is anticipated that the product slate would be similar to that of cracking HDPE in a blend as shown in this study. In summary, it is technically feasible to coprocess PE and VGO in a n FCC unit for high-value products. However, as in processing other types of plastics wastes, issues such as process economics, plastics collection and pretreatment, technical and regulatory considerations need t o be resolved before PE waste is recycled through this route.
Conclusions Thermal cracking of blends containing 5 and 10 wt % HDPE, respectively, in RZ VGO led to low overall conversion although 52-77 wt % of HDPE could be converted t o low yield of gasoline and high yields of gas and coke. Catalytic cracking of the same blends gave much higher conversion which increased with higher
Ng catloil ratio and concentration of HDPE. Assuming HDPE in a blend was fully converted, data analysis indicated that a t 5 wt % concentration HDPE produced little or no gasoline, but only gas and coke. However, as HDPE concentration increased to 10 wt %, a significant amount of gasoline was produced from HDPE accompanying a reduction of dry gas and coke. This was interpreted as less overcracking of gasoline at lower catalystAlDPE ratio. A synergistic effect was observed during catalytic cracking of HDPE-RZ blends resulting in different conversion of RZ in the presence of HDPE compared with that when RZ was cracked alone under the same conditions.
Acknowledgment. The author is deeply indebted to Miss D. Liu, Mrs. P. MacDonald, and Mr. G. Kodybka for their technical and analytical services and to Mrs. N. Harcourt, Dr. W. Dawson, and Dr. C. Fairbridge for their editorial assistance and technical comments. Supplies of catalyst KOB-627 and VGO Rainbow Zama from Petro-Canada's Oakville and Edmonton refineries are greatly appreciated. EF940139T
