Article pubs.acs.org/EF
Refinery Nonconventional Feedstocks: Influence of the Coprocessing of Vacuum Gas Oil and Low Density Polyethylene in Fluid Catalytic Cracking Unit on Full Range Gasoline Composition Andrew O. Odjo,*,† Angela N. García,‡ and Antonio Marcilla‡ †
Proxion Process U.K. Ltd., Manchester M32 0RS, United Kingdom Department of Chemical Engineering, University of Alicante, 03080 Alicante, P.O. Box 99, Spain
‡
ABSTRACT: Gasoline coming from refinery fluid catalytic cracking (FCC) unit is a major contributor to the total commercial grade gasoline pool. The contents of the FCC gasoline are primarily paraffins, naphthenes, olefins, aromatics, and undesirables such as sulfur and sulfur containing compounds in low quantities. The proportions of these components in the FCC gasoline invariable determine its quality as well as the performance of the associated downstream units. The increasing demand for cleaner and lighter fuels significantly influences the need not only for novel processing technologies but also for alternative refinery and petrochemical feedstocks. Current and future clean gasoline requirements include increased isoparaffins contents, reduced olefin contents, reduced aromatics, reduced benzene, and reduced sulfur contents. The present study is aimed at investigating the effect of processing an unconventional refinery feedstock, composed of blend of vacuum gas oil (VGO) and low density polyethylene (LDPE) on FCC full range gasoline yields and compositional spectrum including its paraffins, isoparaffins, olefins, napthenes, and aromatics contents distribution within a range of operating variables of temperature (500−700 °C) and catalyst-feed oil ratio (CFR 5−10) using spent equilibrium FCC Y-zeolite based catalyst in a FCC pilot plant operated at the University of Alicante’s Research Institute of Chemical Process Engineering (RICPE). The coprocessing of the oil-polymer blend led to the production of gasoline with very similar yields and compositions as those obtained from the base oil, albeit, in some cases, the contribution of the feed polymer content as well as the processing variables on the gasoline compositional spectrum were appreciated. Carbon content analysis showed a higher fraction of the C9−C12 compounds at all catalyst rates employed and for both feedstocks. The gasoline’s paraffinicity, olefinicity, and degrees of branching of the paraffins and olefins were also affected in various degrees by the scale of operating severity. In the majority of the cases, the gasoline aromatics tended toward the decrease as the reactor temperature was increased. While the paraffins and iso-paraffins gasoline contents were relatively stable at around 5 % wt, the olefin contents on the other hand generally increased with increase in the FCC reactor temperature. how have been carried out by different researchers.4−14 Johannes et al.15 studied the pyrolysis of polyethylene in closed tubing reactors (autoclaves) observing a higher yield of lighter naphtha or gasoline range product compared to open systems. Serrano et al.16 proposed a stand-alone continuous screw kiln reactor for the conversion of a polyolefin-oil base mixture into different volatiles and noncondensable fractions. Their paraffins, isoparaffins, naphthenes, aromatics (PIONA), and yields analysis of the C5−C12 fraction corresponding to the gasoline range compared well with those typically obtained in the refineries.17 Promising results from actual attempts at processing polyolefins, among other plastic wastes, through direct addition into conventional refinery feedstocks streams for conversion into valuable fuels in refinery streams have been reported by Ucar et al.,2 Ng et al.,18 and a host of others19,20 to mention a few. Even so, detailed insight into the response of the obtained gasoline fraction, being the most valuable and critical product, and the variation of the different hydrocarbon components of this fraction consequent of the direct addition of low density polyethylene to conventional refinery Fluid Catalytic Cracking (FCC)feedstock were not clearly provided.
1. INTRODUCTION The outcomes of the technological implementations of research efforts on the pyrolysis of plastic wastes have revealed that fuel oil and other petrochemical feedstocks obtained from these wastes could be deemed a sustainable and reproducible manmade energy resource.1 In recent years, this drive for sustainability and reproducibility, coupled with the enormous benefits of emergent integrated tertiary plastic recycling technologies have compelled innovation-driven refiners and process technologists to critically consider the incorporation of plastic wastes into their refinery feedstocks which principally are coming from unsustainable and irreproducible natural crude oil and gas resources.2 Although the relative abundance of these plastic materials as well as their very low or no cost have made the economic feasibility of this incorporation very appealing, the technological feasibility remains a challenge, especially when superstructure and equipment integration into the footprints of existing units are targeted. The liquid products from the liquefaction of polyolefins (constituting the majority of plastic wastes) to fuel oils have been favorably compared with the naphtha and light fuel oil obtained in the cracking of heavy refinery gas oils3 albeit the process superstructures, the material design and material flow systems have always been a further challenge. The cracking of polyolefins yielding different hydrocarbon fractions and using different technological know© 2014 American Chemical Society
Received: October 10, 2013 Revised: January 13, 2014 Published: January 17, 2014 1579
dx.doi.org/10.1021/ef4020394 | Energy Fuels 2014, 28, 1579−1593
Energy & Fuels
Article
Refinery FCC feedstocks, the most common of which is vacuum gas oil, or VGO, is typically a hydrocarbon liquid with boiling range between 343 and 552 °C obtained from the vacuum fractionation of atmospheric residue coming from crude oil atmospheric distillation. In the thermo-catalytic or catalytic cracking of either individual LDPE, conventional refinery FCC feedstocks such as VGO or unconventional feedstock made up of blends of the polymers and the FCC feedstock, the largest obtained liquid product fraction has been gasoline with selectivities over some catalysts as high as 64− 70% reported.2,12,21 Commercial grade gasoline is made up of different fractions coming from the isomerization, reforming, and fluid catalytic cracking units. Generally, FCC gasoline represents 30−40% of the total commercial grade gasoline pool,22,23 hence the importance of in-depth analysis of the FCC gasoline fraction coming from the cracking of nonconventional refinery feedstock comprising blends of VGO and polyethylene and its qualitative and quantitative response to the processing variables cannot be overemphasized. In this work, GC/MS analysis of the condensed liquids coming from the coprocessing of mixture of polyethylene and vacuum gas oil in a FCC riser reactor has been performed with the aim of determining the influence of the polymer incorporation into the feedstock oil matrix on the chemical composition of the obtained cracking products. Specifically, GC/MS total ion chromatographic peak areas were integrated over the volatility areas corresponding to the full range gasoline comprising fraction of the condensable liquids boiling between 30 and 200 °C. This includes the FCC light, intermediate and heavy naphthas comprising of complex mixtures of hydrocarbons cumulatively ranging from C5 to C12. The coprocessing of plastic wastes to valuable petrochemical feedstocks and fuels through the integration of tertiary or chemical recycling processes into the footprints of existing conventional refinery FCC facilities18 and process superstructure have been considered a very promising alternative in the quest for a cost-effective method to revalorise these wastes24−27 without compromising product quality and yields of the existing grassroots processes. Of the FCC product range, the gasoline fraction is perhaps the most valuable, with yields as high as 60% recorded in some cases.28 The specification for this product fraction is however increasingly dynamic based on frequently changing consumer requirements and increasingly stringent emission control regulations. Specifically, current and future superclean gasoline requirements include reduced olefin, aromatics, benzene, and sulfur contents. In this work, the influence of the coprocessing of VGO and polyethylene on the distribution of these contents in the obtained FCC full range gasoline has been studied and presented. In an effort to further evaluate the role of the operating parameters on the product distribution and the different hydrocarbon contents in FCC gasoline obtained from the coprocessed polymer-oil blends also presented are results and analysis of several runs at different catalyst-feed oil ratios and temperatures using spent equilibrium FCC catalyst.
Table 1. Properties of the FCC Catalyst composition Al2O3 Na2O Fe2O3 elemental
% wt 49 0.25 2.7 % wt
C O Al Si P Cr Mn Fe Ni Na property total areaa (m2 g−1) apparent density (g/L) particle size (μ)
3.11 42.87 15.66 17.66 1.14 5.43 0.45 13.28 0.4 O > I > P > N. Thus, very low concentrations (less than 2% in all cases) of the naphthenes were recorded in the FCC gasoline. The naphthenes found
could have been formed from the combination of the cracking of the naphthenic molecules present in the feed and the secondary reactions in the FCC process via cyclization of olefins. However, the formed naphthenes could have in turn been cracked, leading to the reformation of olefins, and/or dehydrogenated with the subsequent formation of the aromatics,22,32−35 all resulting in the significant depletion of the naphthenes from the gasoline range component spectrum. In this work, the very low concentrations of naphthenes recorded and the high aromatics observed in the obtained FCC gasoline could be attributed to a very low VGO feedstock naphthenic content together with the predomination of the effect of the secondary FCC reactions leading to the depletion and transformation of the naphthenes into aromatics (Figure 1). The percentages of n-paraffins are always lower than 5 wt %/wt followed by the percentage of i-paraffins in the range 5− 18% in all the cases studied. The average percentage of olefins is 15% of the gasoline in all the runs performed. In the majority of these runs, the gasoline’s olefins from the VGOPE feedstock are higher than those from the VGO, probably due to the cracking of the polyethylene content in the blended feedstock. The major concentrations are reached by the aromatics, whose percentages are in the range 55−75% for both feedstocks, at any temperature and CFR studied. The PIONA distribution follows a similar pattern to that observed in previous lab experiments in a sand fluidized bed reactor.41 Thus, aromatics are the major components followed by olefins and paraffins in a minor proportion, naphthenes 1588
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being negligible, although in the fluidized bed reactor the distribution is shifted to much higher aromatic yields. In general, the influence of temperature on the PIONA distribution is higher than that of the catalyst ratio, olefins and aromatics being the compounds more influenced by both parameters. Thus, it can be observed that the percentage of olefins increases and that of aromatics decreases by increasing temperature. In general, gasoline produced by fluid catalytic cracking (FCC) could contain 30−50 wt % olefins.22 In this work, the maximum gasoline’s olefin content obtained was less than 40% including the contribution of the presence of the polymer in the feedstock. In other words, the maximum value of the gasoline’s olefins obtained in this work is within acceptable limits for FCC processes. More specific aspects are commented on in the following sections. 3.3.1. Influence of Temperature. Processing at Catalyst/ Feed Ratio 5:1. From Figure 4, it could be seen that at constant CFR of 5, the PIONA distributions of the VGO and VGOPE feedstocks are very similar. The n-paraffins are generally below about 6 wt %/wt of the overall FCC gasoline, tending to a slight decrease as the processing temperature was increased. This decrease could be attributed to the enhanced cracking reactions via beta-scission leading to the formation of more olefins as the severity of operation increases. This is confirmed by the increase in the FCC gasoline olefin contents as the temperature was increased for both feedstocks. The elevated olefins content in the FCC gasoline implied that the VGO was a paraffinic feedstock based on the paraffins content in the feed. This is in agreement with the VGO composition given in Table 1: total feeds paraffins was 76%. It has been shown that a paraffinic and an aromatic FCC feedstock will produce, respectively, an olefinic or an aromatic gasoline.43 The notable quantity of the aromatics obtained in this work could be attributed to the process condition favoring the enhanced secondary cracking reactions of hydrogen transfer, isomerization, cyclization, and dealkylation. The result is the cracking, transformation, and depletion of the feedstock’s and generated paraffins and olefins into naphthenes, which, in turn, through dehydrogenation transformation into the aromatics34 (Figure 1). The iso-paraffins in the gasoline from the feedstocks constituted less than 12 wt %/wt and tends toward the decrease when reactor temperature was increased, consequence of the decreased hydrogen transfer reactions, which generally favors their formation. The FCC gasoline aromatics content oscillated between 60% and 75 wt %/wt for the VGOPE feedstock and between 58 and 71 wt %/wt for the VGO feedstock. Based on a 5% standard error on experimental values, the differences in the extremes between both feedstocks could be considered negligible. The general tendency of the gasoline aromatic contents was toward the decrease as the reactor temperature was increased. This was an expected development since the formation of the aromatics through conversion of cyclics by hydrogen transfer and the generally slow cyclization of olefins (the principal route through which gasoline aromatics are formed34 (Figure 1)) were disfavored at high processing severities. Since the sensitivity of the gasoline aromatics contents to temperature could be considered generally low to moderate (about 15 wt %/wt and 13 wt %/wt over a 200 °C temperature variation for the VGO and VGOPE feedstocks, respectively), it could be deduced that the majority of the FCC gasoline aromatics was formed from paraffins through olefins and cyclo-olefins. The observed increase in olefins and decrease
in aromatics could also be attributed to the cracking of the aromatics to olefins and unsubstituted rings as the temperature was increased.34 Processing at Catalyst/Feed Ratio 7:1. At constant CFR of 7:1, the paraffins occupy a very narrow range in the overall FCC gasoline, generally between 3 and 5 wt %/wt for both feedstocks at all temperatures studied (Figure 4). Albeit the PIONA distributions of the VGO and VGOPE feedstocks at CFR 7:1 were very similar to those at CFR 5:1, at the higher catalyst rate, the olefins content of the cracked gasoline were higher for the polymer/oil blend at all processing temperatures. This observed difference increased with increase in temperature. Thus, at reactor temperatures of 500, 600, and 700 °C, the differential increase in FCC gasoline’s olefins between the feedstocks were 40, 53, and 105% correspondingly in favor of the polymer/oil blend. This could be attributable to the enhanced polyolefins content in the feed from the addition of the polyethylene, the enhanced catalytic activity at higher temperatures leading to enhanced feedstock’s heavier olefins cracking reactions and product formation via beta-breakage to lower molecular weight olefins,44 a low conversion of the formed olefins to naphthenes and paraffins. This was manifested in the carbon number distribution according to the different hydrocarbon types in the obtained gasoline, shown in Table 4. From this Table, it could be seen that the olefins contents were predominantly of the lighter lower molecular weights, decreasing as the carbon number and temperature were increased. As in the previous case with processing at the lowest catalyst rate, a general decrease (compared to the lowest temperature) in the iso-paraffins and aromatics in the obtained gasoline were observed as the process temperature was increased at CFR 7:1 for both feedstocks processed. However, at this CFR, the presence of the polymer in the feedstock influenced the distribution of these lumps in the FCC gasoline obtained at all temperatures, resulting in even further decrease of the iso-paraffins and aromatics as the reactor temperature was increased. The need to boost up the consequent reduction of the gasoline octane on one hand and the need to meet the increasing stringent legislations calling for the decrease in the gasoline’s aromatic contents on the other hand is a common challenge faced by modern day refiners. The observed decrease in aromatics could well be attributed to the following: 1. The enhanced catalytic cracking reactions (at higher temperatures) of hydrogen transfer leading to the formation of cyclic monoaromatics from heavier diand triaromatics present in the feedstock; 2. Temperature induced thermal beta-cracking of the alkyl side chains of the mono and diaromatics; 3. Cracking of the aromatics to unsubstituted rings and olefins at high temperatures (hence the observed increase in olefins as the aromatics decreased). 4. Cracking and recombination of the aromatics through dehydrogenation to coked or heavy heterocyclic aromatics It is obvious that the above reactions would lead to the formation of lower carbon number aromatics. The manifestation of this could be seen in Table 4, which shows a decrease in the C10+ and general increase in C9- aromatics as the processing temperature was increased. The coked or heavy heterocyclic aromatics formed generally have C-numbers greater than 12, hence do not appear in the current C5−C12 spectrum. 1589
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Figure 5. Variation of the FCC gasoline’s paraffinicity, olefinicity, degrees of branching of the paraffins and olefins (DOB-Paraffins and DOBOlefins) as a function of the process temperature and catalyst/feed ratio (-⧫- VGO; -■- VGOPE).
Processing at Catalyst/Feed Ratio 10:1. A notable aspect of the FCC gasoline’s lumps distribution for the processed feedstocks at constant CFR 10:1 is that the n-paraffins, isoparaffins, and olefins steadily increased as the FCC reactor temperature was increased from 500 to 700 °C (Figure 4). It would appear that the elevated catalyst rate was in more scale responsible for this increase consequence of the availability of more catalytic active sites for the FCC reactions, promoting nand i-paraffins formation through hydrogen transfer in the cracking of unbranched and branched olefins, respectively. As in the lower CFRs, the reverse was the case for the aromatic contents, probable due to their conversion to unsubstituted rings and olefins, as can be seen to have been manifested by the observed olefins increase. 3.3.2. Influence of Catalyst/Feed Ratio. The influence of catalyst rate on the FCC gasoline’s PIONA distribution was best ascertained through processing at constant temperatures but variable CFRs (Figure 4b). Processing at 500 °C. At 500 °C, the concentration of the FCC gasoline’s i-paraffinic content was below 15% and increasing the CFR from 5:1 to 10:1 in the majority of the cases resulted in no significant change in this content for both feedstocks. It is noteworthy that the n-paraffins in the gasoline from the oil/polymer blend were comparatively higher than those from the VGO’s FCC gasoline. These higher values could be attributed to the addition of the n-paraffins from the cracked polymer matrix to those from the base oil. In both cases, nparaffins were formed from the cracking via hydrogen transfer mechanisms of olefins. Olefins, on the other hand, averaged 15% of the gasoline from all the runs performed at 500 °C, but at CFRs between 5:1 and 10:1. In the majority of these runs, the gasoline’s olefins from the VGOPE feedstock were higher than those from the VGO. While the increase in catalyst rate seemed to have little effect on the gasoline’s aromatics from the processed VGO, in the case of the oil/polymer blend, processing at increased catalyst rate at the lowest temperature led to a slight reduction of the aromatics content (generally around 7%) in the gasoline.
In general, the low sensitivity of the gasoline’s aromatic contents to changes in CFRs could be attributed to the fact that the aromatic rings are too thermally stable for cracking at this temperature and an increase of the catalyst percentage is not able to modify that behavior. Processing at 600 °C. At constant temperature of 600 °C, the P, I, O, N, and A distribution in gasoline from VGO feed are not significantly affected by the variation in catalyst rate, on the average being about 4, 11, 16,
