Pollutec Recycling

Catalytic Liquefaction Of Mwp

in these studies were solid acids (amorphous silica-alumina, zeolites, zeolite-based FCC catalysts, MCM mesoporous materials and super acidic zirconia, etc.). Besides acidic catalysts, a few studies have been performed with activated carbon and nonacidic mesoporous silica catalyst (FSM) [11-15]. Acidic cracking catalysts are very useful for direct liquefaction of PE and PP. The catalytic effect of acidic catalysts in plastic pyrolysis has been explained by a carbonium ion theory. The catalytic mechanism over acidic catalysts has been reviewed by Bukens [16]. In brief, this mechanism involves: (1) an initiation step, involving chain carbonium ion formation by proton addition; (2) a depropagation step, chain cleavage yields an oligomer fraction by P-scission of chain-end carbonium ions leading to gas formation on the one hand, and a liquid fraction on the other; (3) an isomerization step, double bond isomerization of an olefin and isomerization of saturated hydrocarbons; (4) an aromatization step; aromatic formation following cyclization via an olefinic carbonium ion.

The problem relating to steric and/or internal diffusion hindrances in the cracking of bulky polymeric molecules can be solved using catalysts of larger pores or zeolites with smaller crystal size. Although suitable acidic catalysts can control the product range in catalytic cracking of polymers, they are easily deactivated by nitrogen, sulfur and impurities in MWP [17]. This type of catalysts can be regenerated by burning off the coke, but this can result in a loss of activity [18].

On the other hand, activated carbon may be considered as a catalyst in the cracking of waste plastics. This is because it is a neutral catalyst with a high surface area and, therefore, it might be more resistant to impurities and coke formation. It has been reported that Pt-, Fe- and Mo-supported activated carbon catalysts were effective for the pyrolysis ofPEandPP [11, 14, 15]. Use of metal-supported activated carbon catalysts has enhanced the formation of aromatics via dehydrocyclization of straight- or branched-chain radicalic intermediates.

Although a large variety of catalysts have been used, even if performing well, many can be unrealistic for MWP. Thus, although the option based on cracking of plastic wastes by direct contact with catalyst seems the simplest way, the catalyst cost can affect the economics of the process considerably

2.2 THERMAL CRACKING PLUS CATALYTIC UPGRADING

The problems in catalytic cracking of MWP by direct contact with the catalyst can be overcome by two-step processing. This method involves an initial thermal cracking of waste plastics to produce low-quality hydrocarbons (vapors or liquid) that are treated afterwards in a catalytic reactor to obtain high-quality liquid fuels.

A full-scale pyrolysis-catalytic process in which the catalytic cracking zone is directly connected to the pyrolysis zone was developed in Japan (Fuji Process) [19]. In this process, after separation of PVC and impurities by wet techniques, waste plastics are thermally pretreated at 300°C for dechlorination and then introduced into the pyrolysis reactor and thermally cracked at 400°C. Subsequently, degradation products are fed directly to the fixed-bed reactor using a ZSM-5 catalyst.

Catalytic cracking yields the following products:

• oil, 80%;
• gas, 15%;
• residue, 5%.

Composition of the oil produced is:

• gasoline 60%;
• kerosene 20%;
• diesel 20%.

The main problem is the sensitivity of the zeolite catalyst towards impurities coming from the waste.

The pyrolysis-catalytic cracking reactor scheme poses the serious engineering and economic problems of a complicated reaction mechanism and high capital cost because of the infrastructure needed. Another possibility is to separate the thermal cracking from the catalytic cracking operation, in which first the waste plastics are cracked thermally in a pyrolysis plant and the wax/oil produced is catalytically treated in a conventional cracking or hydrocracking reactor to high yields of gasoline of improved octane rating.

A fluidized-bed pyrolysis reactor is the most suitable for thermal cracking of MWP to obtain liquid/waxy product. Key features of the fluidized-bed pyrolysis include [3, 20]:

• conducting pyrolysis at lower temperatures, an aliphatic heavy oil and/or wax can be obtained;
• by addition of lime into the reactor, HCl evolved from the PVC fraction in MWP is captured. The dechlorination step is not needed for MWP containing up to 2% chlorine;
• Solid impurities in MWP either accumulate in the bed or leave the reactor with the hot gas as fine particles which are collected in cyclones.

The liquid product obtained from thermal cracking can be either catalytically cracked/ hydrocracked or co-processed with a refinery feed. Since the catalytic cracking of oil derived from MWP is more or less problematic, any cracking catalyst can be applied to oil derived from pyrolysis of plastics. But the yield and the quality of gasoline obtained from cracking step vary with the type of catalyst and the properties of the pyrolytic oil derivated from waste plastics.

The amount of strong acid sites of the catalyst used influences the product distribution from catalytic cracking in addition to the temperature. The reaction scheme below can be considered for the catalytic cracking.

The amount of gas product shows an increase with an increasing amount of strong acid sites on the catalyst. Because gasoline is an intermediate product, its yield gradually increases to a maximum value and then decreases with the increase in the amount of strong acid sites [21].

As an example, the results from catalytic cracking of three types of waxes from thermal cracking of PE are given in Table 8.2 [22]. The catalyst used, the equilibrium FCC catalyst,

Wax/Heavy oil

> Gasoline

Coke

Table 8.2 Properties of pyrolytic wax derived from PE pyrolysis

Pyrolytic wax	A	B	C
Fractions (vol%)
Gasoline (IBP-216°C)	18.0	20.2	28.2
LCO (216-343°C)	25	29.3	37.8
HCO (>343°C)	57	50.5	34
GC analysis, wt%
n -paraffins	76.8	38.6	36.2
a-olefins	18.1	34.5	42.8

Table 8.3 Some results from catalytic cracking of pyrolytic wax derived from PE pyrolysis

Feed Wax A Wax B Wax C

Table 8.3 Some results from catalytic cracking of pyrolytic wax derived from PE pyrolysis

Feed Wax A Wax B Wax C

Temperature (°C)	470	510	470	470	470	470
Catalyst/oil (wt/wt)	0.43	3.99	0.47	4.15	0.48	4.20
WHSV (h-1)	333	20.0	300	19.3	298	19.0
Yields (wt%)
Dry gas	1.22	1.81	0.62	0.95	0.18	0.59
LPG	10.61	22.39	6.31	17.4	3.46	14.6
Gasoline	45.9	63.0	39.2	67.7	40.9	64.0
LCO	22.8	8.80	27.3	10.7	33.0	10.8
HCO	17.3	1.3	25.1	1.3	21.9	6.9
Coke	1.8	2.4	1.4	1.9	0.5	3.2
Conversion (%)	59.9	89.9	47.7	88.0	45.1	82.4
Composition of C4-Cn	in gasoline,	wt%
n-paraffins	17.2	10.6	20.3	14.9	21.7	17.3
¡-paraffins	12.6	31.2	12.2	47.7	10.2	46.1
Olefins	38.5	18.2	43.4	15.4	45.2	13.4
Naphthenes	14.4	15.6	12.8	11.3	14.3	12.0
Aromatics	17.3	24.5	11.3	10.6	8.6	11.2

KOB-627, is an octane-enhancing catalyst containing Y-type (US-Y) zeolite. Its matrix (about 50 ^m) cracks primarily the large and bulky molecules prior to their secondary cracking in the small pores of the zeolite. The medium activity of this catalyst is reduced over cracking of gasoline. This catalyst produced high yields of gasoline with improved quality from pyrolytic waxy products, as shown in Table 8.3.

Similarly, it was observed that rare earth metal exchanged Y-type zeolite (REY) catalyst, having a moderate amount of strong acid sites, has a selectivity towards gasoline in the catalytic cracking of oils derived from plastics. HZSM-5, silica-alumina and Y-type zeolite (REY) were tested for catalytic reforming of heavy oil derived from PE pyrolysis. REY zeolite, which has a small crystal size and a low amount of strong acid sites, was found to be the most suitable catalyst to obtain the highest research octane number (RON) of 97.5 and a gasoline yield of 48 wt%. In contrast, HZSM-5 has the lowest RON value (23) and gasoline yield (18 wt%) [21, 23].

It is however necessary to remember that products from cracking of heavy oils are highly unsaturated and therefore they have to be further submitted to hydrofining.

Upgrading of heavy oils derived from waste plastics by catalytic hydrocracking is also one of the most promising processes for conversion of waste plastics. Dual-functional catalysts, having both cracking and hydrogenation-dehydrogenation functions, are used for this process. The cracking function is realized by an acidic support, while the hydrogenation component is usually a metal, oxide, or sulfide of group VIII and/or VIb [17]. Commercial catalysts usually adapt Ni, Mo, W, Co and their combinations as the active metal sulfide component, whereas silica-alumina and zeolites are used as acidic support.

Activity of these catalysts depends on the balance between the hydrogenation and acidic functions. For example, it was found that HZSM-5 was effective for the hydrocracking of HDPE and plastic waste [24]. But the liquid product contained much less n-paraffins and a greater amounts of aromatics (34%) and naphthenes (21.7%) because of a lack of sufficient hydrogenation function. The reaction mechanism over HZSM-5 can be considered as follows;

n-Paraffins-Olefins -Naphthenes-»- Aromatics

• Olefins-»- Naphthenes-»- Aromatics
• C2-C4)

However, in the case of hydrocracking over a dual functional catalyst, isoparaffins are the major compounds in the liquid product.

Although the hydrocracking process (Figure 8.1) has many advantages for the heavy oils containing heteroatoms, there are a few studies on the hydrocracking of heavy oils derived from plastics. The conversion of waste plastics to naphtha by thermal cracking followed by hydrocracking over an acidic catalysts was investigated in two post-consumer waste plastics, namely DSD and APC [25]. DSD was provided by the Duales System Deutschland. DSD (containing 1.126 wt% of Cl and 4.4 wt% of ash) and is considered as a real post-consumer plastic. However, APC provided by the American Plastic Council is a relatively clean waste plastic (containing 0.03 wt% of Cl and 0.45 wt% of ash). The catalytic effect on oil yields from hydrocracking was neglible for DSD and APC. Several catalyst had a significant effect on the boiling point distribution for the APC plastic, producing lighter products, but had little or no effect for the DSD plastic. Hydrocracking of heavy oil from DSD pyrolysis yielded 55-65% gasoline fraction, whereas the gasoline yield was more than 90% for heavy oil from APC. However, in these studies gasoline quality was not determined.

A more detailed investigation on the quality of gasoline from hydrocracking was carried out by Masuda et al. [26]. They examined the activity and selectivity of a Ni-REY catalysts with different nickel contents in the hydrocatalytic upgrading of heavy oils obtained from the waste PE and a mixture of PE and PET. The selectivity towards gasoline of Ni (0.5 wt%)-REY was 78% and the RON value of the produced gasoline was 110. Besides this, Ni-REY a showed constant activity during repetition of the sequence of reaction and regeneration.

Based on the above results, it can be mentioned that the catalyst having both hydrogenation and acidic functions can successfully convert heavy oil derived from plastic wastes (relatively clean) into environmentally acceptable transport fuels. However, for the heavy oils containing impurities, the dual functional hydrocracking catalysts still need to be improved. In the hydrocracking process over the acidic catalyst, nitrogen content in feed is limited because basic nitrogen compounds poison the acidic sites of the catalyst.

LIQUEFACTION OF MUNICIPAL WASTE PLASTICS Hydrocracking:

Function of metals:

• 2 H+
• H+
• ©

Function of acidic support:

Hydrogénation:

• K Metal ^
• IN— + H2-»-NH3 + H

Metal

"K Metal

O + H2-»»H2O + H

• c
• K Metal -K

Figure 8.1 The hydrocracking and hydrogenation mechanism of metal-supported acidic catalyst

In addition, in order to prevent the deactivation of the catalyst caused by the deposited coke or coke precursor, the process needs to be operated at high hydrogen pressures and this leads to high hydrogen consumption and high construction cost of the reactor.

Activated carbon catalysts, which are neutral, can be considered as an alternative catalyst for hydrocracking of heavy oils derived from waste plastics. In recent years, carbon has received much attention as a support for hydrodesulfurization (HDS) catalysts as high HDS activities have been reported [27, 28], which may be due to more favorable support/catalytic species interactions. In addition, activated carbon catalysts have many other interesting features such as high surface areas with controlled pore volume and pore size, reduced coking activity and controllable surface functionality. The use of activated carbons as a catalyst support offers some advantages over the more traditional acidic oxide supports, such as stability in acidic and basic media, ease of recovery of precious metals supported on them, and the possibility of tailoring their properties to specific needs [29, 31]. It has been reported that metal-supported activated carbon (M-AC) catalysts have shown excellent cracking activity in addition to hydrogenation, on petroleum-derived heavy oils and some coal model compounds [32-38]. The catalytic effects of M-AC catalysts in hydrocracking of vacuum gas oil have been explained by a radical mechanism. Thus, activated carbons (AC) have the ability to abstract hydrogen from a hydrocarbon and free radicals are formed on AC. These free radicals initiate the cracking reactions. In the presence of H2, hydrocarbon free radicals are hydrogenated by hydrogen atoms which are generated on M-AC from gaseous hydrogen to form stable hydrocarbon molecules and consequently to suppress overcracking. This hydrogen quenching reaction produces the higher middle distillate yield and lower gas and naphtha yield [37]. AC catalysts have also been found to have a better ability to restrict the coke formation and show high activity for the removal of such impurities as sulfur and heavy metals during hydrocracking of heavy oil [32]. Even though no data are available in the literature, it can be suggested that AC as the catalyst is well suited to upgrade the heavy oil derived from MWP:

In conclusion, it is certainly possible to develop commercial processes based on pyrol-ysis-hydrocracking/cracking. But it must be noted that the viability of this two-stage conversion technique depends on process economics and future regulatory considerations.

2.3 CO-PROCESSINGOFMWP

Another approach in liquefaction of MWP is co-processing which has a special importance from the viewpoint of feedstock recycling. Most co-processing studies have involved the hydrothermal cracking of single plastics or waste plastics with coal using HZSM-5 and the bifunctional (hydrotreating and hydrocracking) acidic catalysts. The results have shown good conversion for autoclave liquefaction of plastic/coal mixtures. However, there were also conflicting reports of whether co-processing led to better or worse conversions. Coal rank and type of plastics emerges as a significant characteristic, affecting its synergy with the various reaction parameters. Co-processing of coal and waste plastics is difficult, as neither the reaction conditions nor the catalyst can be tailored simultaneously for both materials. Moreover, it must be noted that, co-liquefaction of waste plastics with coal may be a promising process to develop an economically feasible process for coal liquefaction, since the waste plastics play the role of hydrogen donor for coal liquefaction.

A more interesting approach for co-processing is the conventional treatment of plastic with a heavy petroleum fraction blend in a refinery unit. The main advantage of this coprocessing method is that it utilizes existing processes within a refinery complex, resulting in reduced capital costs. Little research has focused on the co-processing of plastics with a feedstock of a refinery unit. Studies have been carried out with different plastics and refinery feeds.

Ng [39] evaluated conversion of HDPE blended with vacuum gasoil (VGO) to transportation fuels by catalytic cracking over an acidic catalyst (KOB-627). It has been shown that addition of HDPE increased the gasoline yields significantly when more than 10% plastics were dissolved in the VGO. In cracking of light cycle oil (LCO, containing 66.9 wt% aromatics) over a neutral catalyst (mesoporous silica), the addition of poly-olefins (PE, 10 wt% and PP, 5 wt%) to the LCO showed a synergistic effect on the cracking of LCO and led to a remarkable decrease in the content of aromatics in the gasoline fraction and an increase in the content of olefins, paraffins and isoparaffins. It was stated that this result was a consequence of the high reactivity of radical intermediate compounds from polyolefin cracking and of their global hydrogenating contribution [40].

On the other hand, studies on hydrocracking of blends containing HDPE, LDPE and PP in HVGO over acidic catalysts showed that the effect of polymer on the cracking of HVGO changed, depending on the type of catalysts [41-43]. The presence of polyethylenes affected the cracking properties in hydrocracking over DHC-8 (a commercial hydrocracking catalyst). Over DHC-8, the liquids from PE/HVGO blends were less and waxy compounds were more than from VGO alone. However, the yields were similar for both blend and HVGO alone over HZSM-5. Although HZSM-5 showed a high cracking activity, it gave liquid products containing the highest amount of aromatic species and sulfur content. The studies on liquefaction of polyolefines in HVGO showed that the type of polymer and catalyst had a great effect on the product distribution of cracking/hydrocracking.

The principal limitation to co-processing is the need for important refining liquid streams which are necessary taking into account the limited amount of plastics that can be mixed, and the restricted use of chlorine and other hetereoatoms contained in the plastics that can negatively affect the refinery unit and catalyst.

In the case of waste polymer mixture, it is expected that impurities in the waste plastic mixture, as well as polymer type, have an effect on the hydrocracking process. In addition, the degradation of single polymers might be different from that of mixtures of polymers.

In conventional refinery processes utilized for the conversion of MWP into fuel, the presence of PVC in MWP might cause some problems such as poisoning of the catalyst, evolution of corrosive gases and products containing chlorine. For these reasons, chlorine has to be eliminated from the feed before processing. Preheating plastic mixtures at lower temperatures (300-350°C) is the conventional way of elimination of chlorine because the thermal stability of PVC is much lower than that of other polymers. Dechlorination of MWP in a heavy petroleum fraction is more demanding than dechlorination of MWP alone. The heavy petroleum fraction acts a solvent during dechlorination, preventing the blockage of gas line and decreasing energy requirement with improving heat transfer. It might be expected that the dechlorination step is not only responsible for elimination of chlorine, but also effects the properties of the blend which will be cracked/hydrocracked.

Table 8.4 The composition of MWP (wt. %)

HDPE LDPE PP PVC PS PET inerts 12 21 56 2 2 1 6

Table 8.5 Product distribution of hydrocracking of MWP/HVGO blend and HVGO

Reaction products (wt%)

Dechlorinated MWP/HVGO

HVGO

None DHC-8 Co-AC HZSM-5 None DHC-8 Co-AC HZSM-5

Reaction temperature at 425°C

Reaction products (wt%)

None DHC-8 Co-AC HZSM-5 None DHC-8 Co-AC HZSM-5

Reaction temperature at 425°C

Gasa			28.3	31.4	47.2	55.2	18.5	27.6
Liquid			27.1	32.4	40.5	40.4	61.2	60.1
Wax			32.9	28.3	11.2	4.4	18.6	12.3
Coke			5.4	7.2	1.1	0	1.7	0
Undegraded MWP4			31.6	3.4
action temperature at	435°C
Gasa	30.8	28.1	39.3	51.9	62.3	57.1	22.7	44.9
Liquid	50.2	34.2	37.3	20.1	30.1	34.1	60.8	40.5
Wax	16.5	34.0	21.2	18.9	7.0	8.5	14.3	14.2
Coke	0.9	1.0	1.5	8.6	0.5	0.3	2.2	0.4
Undegraded MWP4	8.0	13.6	0.4	2.6
action temperature at	450°C
Gasa	48.7	35.9	45.9	68.3	73.4	61.0	49.9	58.8
Liquid	44.2	47.3	36.0	12.9	24.4	29.4	34.1	19.1
Wax	6.6	15.9	17.5	15.5	2.2	7.1	13.0	21.4
Coke	0.5	0.9	0.5	2.9	0	2.5	3.0	0.7
Undegraded MWP4			0.4	1.9

a Calculated from mass balance b Based on MWP charge a Calculated from mass balance b Based on MWP charge

Liquefaction of MWP in a refinery stream has been studied at a laboratory level [44]. Thus, MWP containing mainly polyolefines (Table 8.4) has been added into the feed of a hydrocracking plant, heavy vacuum gas oil (HVGO) and then the blend containing 20 wt% MWP has been converted to fuels by two-step processing. In the first step, dechlorination has been carried out by preheating at 350° C (dechlorination step) in a semi-batch reactor yielding a slurry of 91.2 wt% and a liquid 2.1 wt%. Then the dechlorinated mixture containing 120 ppm chlorine has been catalytically hydrocracked at temperatures between 425 and 450°C. The temperatures have been chosen on the basis of the hydrocracking process in the refinery. The catalysts used were HZSM-5, DHC-8 (commercial hydrocracking catalyst consisting of non-noble hydrogenation metals on a silica-alumina base) and a cobalt-loaded activated carbon catalyst, Co-AC. The product distribution from hydrocracking of MWP/HVGO blend in comparison to VGO alone is given in Table 8.5.

Both the temperature and the type of catalyst effected the product distribution. Different trends in product distribution were observed with the catalyst as the reaction temperature increased. As expected, HZSM-5 showed a high cracking activity, leading to more gas product formation. There was a continuous increase in gas yield by increasing temperature whereas liquid yield decreased dramatically. In contrast to the liquid yield, the amount of wax did not decrease at temperatures above 435°C. This shows that at higher temperature, further cracking reactions occurred, leading to gas formation and waxy compounds. However, hydrocracking of blends containing individual polymer (LDPE/HVGO, HDPE/HVGO, PP/HVGO) led to a greater amount of liquid and a smaller amount of wax being obtained than from the MWP/HVGO blend. This may be due to two reasons. One is that the degradation route of the individual polymer is different from that in the polymer mixture. The other cause may be the impurities in MWP, because the activity of HZSM-5 was effected by impurities. In contrast to HZSM-5, DHC-8 has shown catalytic activity at temperatures above 425°C. The increase in temperature led to the production of more liquid and the cracking of waxy compounds. The degradation route is as follows:

The studies on the hydrocracking of blends containing individual polymer showed that the catalytic activity of DHC-8 varied with the polymer type in the blend. Although the blends of LDPE/HVGO and PP/HVGO liquefied easily at 425°C over DHC-8, the HDPE/HVGO DHC-8 showed sufficient cracking activity only at 450°C. These results suggest that HDPE in blends, even at very low concentrations, such as 2.4%, plays an important role in controlling the rate of cracking of blends in the absence of a sufficient temperature and of catalytic activity. DHC-8 and HZSM-5 showed substantially different activities in coprocessing of HVGO with polymers due to the differences in their acidity. DHC-8 is an amorphous silica-alumina (ASA). It is known that the activity in ASA is attributed to both Lewis and Bronsted acid sites. The strongest sites are not the most favorable sites (in hydrocracking catalyst). The most favorable sites are the weak ones that are sufficiently strong to accomplish the desired chemical reactions. However, HZSM-5 has the stronger acid sites and is more effective in the degradation of polymer. Although acidic catalysts have good performance in conversion of pure polymers to liquid and gaseous fuels, the impurities in wastes are poisonous for acidic catalysts and/or lead to easy deactivation of catalyst in the case of catalytic liquefaction of waste plastics. This is because the basic nitrogen which occurs in waste plastics poisons the active site of acidic catalysts. A cobalt-loaded active carbon catalyst (neutral catalyst) has shown a better cracking ability than commercial DHC-8 catalyst in co-processing of MWP. Upon increasing the temperature, formation of gases increased. The yields of liquid and gas products increased, depending on the increase in the cracking of waxy compounds and waste plastics, when the temperature increased from 425 to 435°C. A further increase in the temperature led to cracking of liquids to gases.

In cracking over a metal-loaded activated carbon catalyst, hydrocarbons are cracked via a radical mechanism as in thermal cracking. However, cracking over Co-AC gave a very different product distribution when compared with thermal cracking. At 425°C, the liquid product is not generally obtained by thermal hydrocracking, whereas hydrocracking over Co-AC produced liquid product. Over Co-AC, the free radicals on the carbon surface initiate the cracking reaction by abstraction of hydrogen in hydrocarbons as follows.

> Liquid

■»" R— CH2— "CH — CH2— R + H : AC

On the other hand, in radical degradations the concentration of free radicals can be controlled by H2S [45]. In this study, H2S is already present because the HVGO contains sulfur under hydrocracking conditions. In thermal hydrocracking, the hydrogen of H2S is abstracted by the hydrocarbon radical to form a stable hydrocarbon and HSV Subsequently the HS^ abstracts a hydrogen from the hydrocarbon. Thus, the lifetime of hydrocarbon radicals can not be reduced. However, in the presence of M-AC catalyst, H and HS^ are formed from H2S. HS^ abstracts a hydrogen from the hydrocarbon or is stabilized on the supported metal catalyst by hydrogenation. These explanations can be shown schematically as follows.

In the thermal case:

In the presence of Co-AC:

+ H*-(Hydrogen quenching)

In hydrocracking of HVGO alone the reason for decreased gas formation over Co-AC catalyst (compared with a thermal run) may be the fact that the activated carbon leads to the formation of more H* or HS* which terminates the radical degradation pathy-ways. However, in the case of a blend, in the absence of catalytic activity, hydrocarbon quenching (with radicals from derivated plastics) may be more pronounced than hydrogen quenching (with H*).

As in thermal runs, due to reaction between primary degradation products of HVGO and MWP, DHC-8 showed a lower cracking activity than that of hydrocracking of HVGO itself in the presence of DHC-8. This result is consistent with the results obtained from hydrocracking of blends containing single plastics. In contrast, in the presence of HZSM-5, the addition of MWP to VGO decreased the liquid yield whereas it increased the gas yield. This shows that presence of MWP led to overcracking reactions.

Over Co-AC, the effect of MWP on cracking properties of HVGO varied with temperature. At 425 and 435°C, similar effects were observed to those observed over HZSM-5; production of more gas and less liquid. It is a most important point that similar product distribution has been obtained from hydrocracking of HVGO and MWP/HVGO over Co-AC at 450°C.

The catalyst type also affects the quality of liquid fuel. The changes in the composition of liquids with temperature depend on the type of catalyst. The amount of naphtha fraction in the liquid fuel increased with temperature up to 435°C and then decreased (over

HZSM-5 due to overcracking of naphtha) or remained constant (over Co-AC and DHC-8). In hydrocracking of MWP/HVGO blend at 435°C, HZSM-5 produced liquid containing 70% naphtha (boiling point < 172°C), 17% middle distillate (bp 172-232°C), whereas in the presence of DHC-8, the naphtha and middle distillate were 46 and 19% respectively.

At the same temperature of 435°C, Co-AC gave liquid containing more light compounds than DHC-8; 55% naphtha and 18% middle distillate. As expected, the liquid derived from thermal runs were heavier than that of catalytic runs at all temperatures.

Although in the case of hydrocracking of HVGO alone, DHC-8 commercial hydroc-racking catalyst gave the lightest product, it did not give the best result for MWP/HVGO blend. It may be noted that in the presence of Co-AC, the liquids from MWP/VGO blend had lighter compounds than that from HVGO.

It is noteworthy that, although HZSM-5 is more active in hydrocracking of the MWP/HVGO blend, it favored aromatization reactions and inhibits desulfurization, even in the presence of H2. The liquid fuel derived by hydrocracking over HZSM-5 at 435°C had the highest amount of sulfur (2.1 wt%) and aromatic compounds (28.0 wt%). In contrast, the aromatic content of liquids was 4.0 and 6 wt% for DHC-8 and Co-AC, respectively. As expected the liquid from DHC-8 contained the lowest sulfur amount (1.009 wt%) since it has been used in refinery for hydrocracking of HVGO. However, it showed a greater desulfurization effect in the hydrocracking of blends containing individual pure polymer. It may be concluded that impurities in wastes affected the HDS activity of DHC-8. Co-AC catalyst also had a hydrodesulfurization effect, produced a liquid containing 1.21 wt% sulfur. In addition, it should be noted that in the presence of Co-AC the liquids from blends containing both waste polymer and pure polymer had a similar sulfur amount.

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