Catalytic cracking in the presence of guaiacol

Applied Catalysis B: Environmental 101 (2011) 613–621

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Catalytic cracking in the presence of guaiacol I. Grac¸a a , J.M. Lopes a , M.F. Ribeiro a , F. Ramôa Ribeiro a , H.S. Cerqueira b , M.B.B. de Almeida c,∗ a IBB, Institute for Biotechnology and Bioengineering, Centre for Biological and Chemical Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b GERIS Engenharia, Av. Rio Branco, 128/18◦ andar 20040-002, Centro, Rio de Janeiro, RJ, Brazil c PETROBRAS, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (CENPES), PDAB/TFCC, Ilha do Fundão, Av. Horácio Macedo 950, 21949-900 Rio de Janeiro, RJ, Brazil

a r t i c l e

i n f o

Article history: Received 27 April 2010 Received in revised form 28 October 2010 Accepted 4 November 2010 Available online 11 November 2010 Keywords: Fluid Catalytic Cracking Bio-oils Guaiacol Co-processing Deactivation FCC industrial catalysts Zeolites

a b s t r a c t The use of renewable fuels is expected to grow in the coming years. A possibility to achieve this consists in blending renewable bio-oils with conventional refining streams to further process in existing refineries. A key aspect of bio-oils is the presence of oxygenate molecules in significant amounts. To shed light into the effect of these compounds on Fluid Catalytic Cracking (FCC), guaiacol was chosen as model compound. Data on the transformation of n-heptane in the presence of small quantities of guaiacol over pure HY and HZSM-5 zeolites at 450 ◦ C is presented and compared to gasoil + guaiacol blend tests using an industrial FCC equilibrium catalyst (E-CAT) pure and blended with a commercial ZSM-5 additive. Guaiacol has a negative influence on both n-heptane and gasoil conversions, since it is responsible for an increase of the coke retained on the catalysts. In the presence of n-heptane and with pure zeolites, guaiacol increases the methane yield, in line with its transformation into phenol. With industrial FCC E-CAT, the presence of guaiacol increases gasoline yield and reduces coke yield, however, increases coke on catalyst. Detailed GC analysis of the liquid product shows presence of phenols in the gasoline cut, suggesting the partial conversion of guaiacol (with methane and water formation). HZSM-5 zeolite was more severely deactivated than HY zeolite in the n-heptane transformation, which agrees with the observed reduction of the ZSM-5 additive effect on the transformation of gasoil. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The need to produce high quality transportation fuels from new energy sources to face the worldwide dependence of petroleum and to avoid climatic changes brought new challenges for refiners and scientists worldwide. In this context, lignocellulosic biomass is, currently, the only renewable source of carbon that can be easily converted into liquid fuels that can be used in the transportation sector [1]. Pyrolysis and liquefaction are the two main processes developed to directly convert the plant biomass feedstocks into bio-oils [1–4]. However, contrarily to the traditional petroleum feedstocks that have an oxygen content lower than 2 wt.% [5], the high amount of oxygenated compounds (16–50 wt.%, depending on the technology used) in the bio-oils confers them some undesired properties for fuel applications: low heating value, immiscibility with hydrocarbon fuels, chemical instability, high viscosity, corrosiveness, etc. [1,6–10]. A common oxygenate compound found in bio-oils is from the guaiacol family, which mainly derives from depolymerization and fragmentation reactions of the lignin and

∗ Corresponding author. Tel.: +55 21 3865 4360; fax: +55 21 3865 6626. E-mail address: [email protected] (M.B.B. de Almeida). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.11.002

represents about 15 wt.% of the bio-oils [1]. These compounds also present the particularity of having two different oxygenated functions, phenolic and methoxy groups. The phenolic function is the most difficult C–O bond to cleave [5]. An interesting alternative to introduce bio-oils in fuel market consists in co-feed the bio-oils with the conventional Fluid Catalytic Cracking (FCC) feedstock. This can be done in existing refineries with a minimum investment. FCC is one of the major conversion processes in petroleum refineries. Its principal aim is to convert heavy feedstocks, as gasoils from vacuum distillation towers or residues from atmospheric towers, into lighter and more valuable products, such as liquefied petroleum gas (LPG) and cracked naphtha, the major constituent of the gasoline pool [11–14]. The FCC catalyst consists of a mixture of an inert matrix (kaolin), an active matrix (alumina), a binder (silica or silica-alumina) and a Y zeolite (FAU) [12,14]. Nowadays, several refineries are also using ZSM5 based additives with the purpose of improving the light olefin production to face the propylene demand and increase the octane number of gasoline by favoring the cracking and isomerization of low-octane compounds into lighter and more branched products [15,16]. Nevertheless, the direct addition of the bio-oils to the FCC feedstocks is not so far feasible because of their minor miscibility with

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Table 1 Physicochemical properties of the fresh HY and HZSM-5 zeolite samples. Sample

Si/Ala

Si/AlIV b

Crystallite size (␮m)

Micropore volume (cm3 g−1 )

HY HZSM-5

2.9 11

3.9 –

0.5 1

0.245 0.158

Acidity (␮mol g−1 ) Brönsted

a b

642 1161

Lewis 330 27

Global Si/Al ratio determined from elemental analysis. Framework Si/Al ratio calculated from the unit cell parameter (a0 = 2.453 nm), using the Breck–Flanigen equation [28].

hydrocarbons, chemical instability and high tendency to form coke, being necessary a previous upgrading step [17–20]. Thus, hydrodeoxygenation treatment (HDO) of the bio-oils could be envisaged before the co-feeding [18,19]. Actually, in the existing oil refineries, hydrotreating processes are already used to remove sulphur, nitrogen and oxygen from the petroleum feedstocks. Laurent and Delmon [21] studied the hydrodeoxygenation of some model oxygenated compounds representative of carbonyl, carboxylic and guaiacyl groups over sulphided CoMo and NiMo catalysts. Guaiacol revealed to be more refractory to deoxygenate than carbonyl and carboxylic groups, and only 60% of conversion was achieved under the tested conditions. The basic reaction schema proposed involved the guaiacol hydrogenation into catechol and then to phenol. Traces of benzene and cyclohexane were also detected on the effluent at long reaction times. Moreover, guaiacol demonstrated a higher coking tendency, when compared to the other model compounds. So that, even after this treatment, guaiacol and phenols will be present in the bio-oils that would be introduced in the FCC units, being the quantity of these O-compounds dependent on the severity of the HDO [21–23]. Pyrolysis of lignin-derived tar compounds at 600 ◦ C can also convert guaiacol type molecules into cathecol, o-cresol and phenol [24]. Hence, the goal of this work is to understand how the presence of guaiacol may affect the catalytic properties of the FCC catalysts, in order to evaluate the possibility to partially replace the classical (FCC) feedstocks by bio-oils derived from lignocellulosic biomass. Therefore, a fundamental study based on the transformation of nheptane (model of a paraffinic FCC feedstock) in the presence of small quantities of guaiacol over pure HY and HZSM-5 zeolites (active components of the FCC catalytic system) was carried out at 350 and 450 ◦ C. Furthermore, guaiacol was also processed along with a standard FCC feed, in a fixed-fluidized-bed unit under typical commercial FCC operational conditions, using an industrial FCC equilibrium catalyst (ECAT) and a mixture of ECAT and ZSM-5 additive. For all the catalytic systems, the guaiacol influence on the conversion, catalyst deactivation and product yields was investigated, being the results of the fundamental and industrial studies compared. 2. Experimental 2.1. Fundamental experiments 2.1.1. Zeolite characterization The NH4 Y and the HZSM-5 zeolites were respectively supplied by PQ (CBV500) and AlsiPenta (SM27). The HY zeolite was obtained by calcination at 500 ◦ C under air flow. The physicochemical properties of fresh zeolites are given in Table 1. The fresh zeolites acidity was measured by pyridine adsorption followed by IR spectroscopy on a Nicolet Nexus spectrometer. The samples were pressed into thin wafers (10–20 mg cm−2 ). The wafers of the fresh zeolites were pre-treated at 450 ◦ C for 3 h under secondary vacuum (10−3 Pa). The IR spectrum of the fresh zeolites sample was recorded at 150 ◦ C, after this pre-treatment. Pyridine vapour was introduced in excess (2–3 mbar) in the quartz IR cell at 150 ◦ C and adsorbed onto the activated zeolite for 10 min.

At the same temperature, the physisorbed pyridine was removed for 1 h under secondary vacuum and, after that, the IR spectrum was recorded. The concentrations of Brönsted and Lewis sites, able to retain the pyridine at 150 ◦ C, were determined using the absorbance surfaces of the bands at 1545 and 1450 cm−1 , respectively, using the extinction coefficients previously determined [25]. Nitrogen adsorption measurements were carried out at −196 ◦ C on a Micrometrics ASAP 2000 apparatus. Before adsorption, the zeolite samples were degassed in vacuum at 90 ◦ C for 1 h and then at 350 ◦ C for 12 h or at 150 ◦ C for 1 h (coked samples). The micropore volumes (Vmicro ) were determined using the t-plot method [26,27]. The carbon content retained within the zeolites was analyzed by total combustion with a mixture flow of helium and oxygen in a Fisons Instruments EA 1108 CHNS-O. 2.1.2. Model compound catalytic tests with pure zeolites Before reaction, the zeolites were pre-treated at 450 ◦ C under nitrogen flow (60 ml min−1 ) for 8 h. The catalytic tests were carried out in a pyrex fixed-bed reactor at 450 ◦ C, under atmospheric pressure. The reactor feed was constituted by 10 mol% of n-heptane (Merck, 99%+) and 90 mol% of N2 . The poisoning tests were accomplished for a mixture of n-heptane and 4.0 wt.% of guaiacol (Sigma, 99%+) for the HY zeolite and 1.2 wt.% of guaiacol for the HZSM-5 sample. The pure reactant and the mixture flow rates were maintained constants by means of a B. Braun compact perfusor. The nitrogen flux was controlled by a Brooks mass flow controller. The reactor effluent samples were obtained for different time-onstream (TOS) values, using a 10-position valve supplied by VICI. The first TOS value was the time necessary to get a stable reactant pressure in the reactor. The tests were performed for contact time values that varied between 0.9 and 7 min, taken as the reverse of the WHSV. The contact time values were altered by changing the zeolites mass and maintaining the n-heptane flow constant (4 mL h−1 ). The reaction products were analyzed on-line by a SHIMADZU GC-14B gas chromatograph with a Plot Al2 O3 /KCl fused silica capillary column (50 m), using a flame ionization detector and nitrogen as carrier gas. The reaction products were also qualitatively analyzed by a Chrompack CP 9001 gas chromatograph with a CP Sil 5CB fused silica capillary column (50 m), using a flame ionization detector. The n-heptane conversion, X (%), was calculated as: X = 100 − An-heptane /Atotal , where A is the area of the chromatographic peak. 2.2. Gasoil catalytic tests with commercial catalysts 2.2.1. Catalysts and feedstocks The catalysts used in this study were an industrial FCC equilibrium catalyst (ECAT) and a mixture of 90% ECAT and 10% ZSM-5 additive. The reason why a ZSM-5 additive was chosen instead of blending pure ZSM-5 zeolite was to have a catalytic system representative of the one used in commercial FCC units. The additive was deactivated separately at 788 ◦ C under a 100% steam atmosphere, in a fixed bed unit. Both samples were supplied by PETROBRAS. Some physicochemical characteristics of the materials used are shown in Table 2.

I. Grac¸a et al. / Applied Catalysis B: Environmental 101 (2011) 613–621 Table 2 Physicochemical properties of the ECAT and ZSM-5 additive.

SiO2 (wt.%) Al2 O3 (wt.%) Na2 O (wt.%) RE2 O3 (wt.%)a Ni (ppm) V (ppm) Surface area (m2 g−1 ) Matrix surface area (m2 g−1 ) Zeolite surface area (m2 g−1 ) Vmicro (cm3 g−1 ) a

ECAT

ZSM-5 additive

– 41.2 0.49 2.35 1559 1820 164 41 123 0.056

59.6 22.8 0.17 – – – 133 48 85 0.039

Rare earth element.

As reference feedstock, a commercial FCC gasoil from PETROBRAS was used. Its main characteristics are presented in Table 3. To simulate the bio-oils co-processing, this gasoil was mixed with 12.8 wt.% of guaiacol (Fluka, 98%+). The amount of guaiacol added to gasoil was chosen considering that the incorporation of 12.8 wt.% of bio-oil in a conventional FCC feedstock should be a reasonable value for industrial applications. 2.2.2. Catalytic tests The catalytic tests were performed in a fixed-fluidized-bed ACE® unit [11]. The reaction was carried out in a stainless steel reactor at 535 ◦ C under atmospheric pressure. The reactor feed was constituted by a nitrogen top stream of 40 ml min−1 , a nitrogen bottom stream of 100 ml min−1 and a feedstock stream of 1.20 g min−1 ; all these flows maintained constant during the reaction. Catalystto-oil ratio (CTO) values of 3, 5, 7 and 9 were achieved by means of changing the feedstock injection times. In all experiments, the catalyst mass was kept constant and equal to 9 g. Liquid and gaseous effluents were collected in a condenser and a gas bottle, respectively. After the reaction, the catalyst was stripped with 140 ml min−1 of nitrogen for 350 s. The liquid was also stripped with 20 ml min−1 of nitrogen for a period that is function of the CTO (7 × injection time). After the catalytic experiment, in situ catalyst regeneration was done to determine the coke content. This step was carried out at 695 ◦ C under air flow (120 ml min−1 at the top and 100 ml min−1 at the bottom). A Pt/SiO2 catalytic converter (535 ◦ C) was used to transform the CO secondarily formed in CO2 . The total CO2 was analyzed by IR spectroscopy with a Servomex 1440 Gas Analyser. The liquid effluent was analyzed by simulated distillation with an Agilent 6890 gas chromatograph, using a HP-1 methyl silicon column and a flame ionization detector. The gasoline, light cycle oil (LCO) and decanted oil (DO) were quantified considering the temperature ranges of <216 ◦ C, 216–344 ◦ C and >344 ◦ C, respectively. Table 3 Characterization of the pure gasoil. Properties

FCC gasoil

Density (g cm−3 ) API gravity Aniline point (◦ C) Basic N UOP269 (ppm) Total N (wt.%) Total S (wt.%) RCR (wt.%) ASTM 1170 (◦ C) IBP 10% 30% 50% 70% 90% FBP

0.952 16.5◦ API 72.5 1307 0.325 0.712 1.23 291 381 431 461 494 535 621

615

An Agilent 3000A micro gas chromatograph, equipped with four columns (Molecular Sieve 5A Plot, Plot U, Alumina Plot and OV1) and four thermal conductivity detectors, was used to analyze the gaseous effluent (H2 , C1 –C5 + ). The yields were calculated as the weight percent of reactant. Conversion is defined as X = 100 (wt.%) − LCO (wt.%) − DO (wt.%). The obtained results were analyzed by statistical methods and the runs corresponding to outlier points were repeated. 2.2.3. Effluent analysis To have a detailed characterization of the gasoline fraction composition, the liquid product was also submitted to the PIANIO analysis. This method allows determining paraffins, iso-paraffins, aromatics, naphthenes, iso-olefins and olefins content. An Agilent 6890N gas chromatograph, with a PONA methyl silicon column and a flame ionization detector was used. For each type of feed and catalyst, the liquid effluents were submitted to GC–MS and GC–FID (PIANIO), at the same analysis conditions, in order to analyze the oxygenated compounds formed. The O-compounds present in the liquid effluents were qualitatively determined by GC–MS coupling with an HP 5973N, using a DB5 methyl phenyl silicon capillary column and, then, quantified by PIANIO analysis. The water content in the liquid effluent samples was measured with an automatic Metrohm 756 KF coulometer, using the columetric Karl Fischer method. 3. Results and discussion 3.1. Guaiacol influence on the HY and HZSM-5 zeolites 3.1.1. Catalytic activity and deactivation The guaiacol effect on the catalytic cracking of n-heptane over pure HY and HZSM-5 zeolites was investigated in a fixed bed reactor at 450 ◦ C. Fig. 1 shows the guaiacol effect on conversion for both zeolites as a function of time-on-stream (TOS), at constant contact time of 4 min. Lower conversions are observed since the beginning of the reaction due to the presence of guaiacol. A more remarkable guaiacol deactivating effect is noticed for the HZSM-5 zeolite (Fig. 1b). With this zeolite, the presence of a low amount of guaiacol (1.2 wt.%) is responsible for a fast deactivation towards very low conversion values. On the other hand, in the case of the HY zeolite (Fig. 1a), the guaiacol impact on zeolite activity is more limited, despite its addition in a higher quantity (4.0 wt.%). Usually, the decrease of the zeolites activity is related to the formation of highly substituted polyaromatic species that remain trapped inside the zeolites pores after stripping (coke), poisoning and/or blocking the acid sites. Observing Fig. 2, in which is presented the evolution of the carbon content as a function of the TOS in the absence or presence of guaiacol, at 450 ◦ C, for both zeolites (4 min of contact time), it is possible to see that the increase of the deactivation in the presence of guaiacol is due to an increase of the carbon content deposited on the zeolite pores. A similar effect was previously observed in the case of phenol [29–32]. The increase of the carbon deposition on the zeolite pores also contributed to a pronounced reduction of the zeolite micropore volumes. After 60 min of TOS, decreases from 0.245 cm3 g−1 zeolite and 0.158 cm3 g−1 (fresh zeolites, Table 1) to 0.011 cm3 g−1 zeolite zeolite

and 0.043 cm3 g−1 were, respectively, registered for the HY and zeolite HZSM-5 zeolites. 3.1.2. Products distribution In short, the main n-heptane cracking products are C3 and C4 paraffin and olefins species, which directly result from the nheptane isomers ␤-scission. Methane, C2 , C5 , C6 and aromatics are

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b

60

n-heptane conversion (%)

n-heptane conversion (%)

a

50 40 30 20 10

40

30

20

10

0

0 0

10

20 30 40 Time-on-stream (min)

50

60

0

10

20 30 40 Time-on-stream (min)

50

60

Fig. 1. n-Heptane conversion vs. time-on-stream for a contact time  = 4 min during the transformation of the pure n-heptane () and the n-heptane/guaiacol mixtures (), over the (a) HY and (b) HZSM-5 zeolites, at 450 ◦ C. HY: n-heptane + 4.0 wt.% guaiacol and HZSM-5: n-heptane + 1.2 wt.% guaiacol.

16

a

14

Moreover, an increase of methane was noticed in the presence of guaiacol for both zeolites at low TOS values, in comparison with the tests performed for the pure n-heptane. Indeed, at constant conversion (∼40%), 0.25 and 0.53 wt.% of methane were found on the effluent for the pure n-heptane transformation over the HY and HZSM-5 zeolites, whereas, in the presence of guaiacol, 0.39 and 0.68 wt.% of methane were respectively formed over the same zeolites. This is consistent with the small amounts of phenol detected on the reaction effluent along with the n-heptane products. Thus, guaiacol conversion into phenol with loss of the methoxy-group as CH4 and probably water is a reaction that takes place over these zeolites, but, under the adopted operating conditions, it occurs only to a low extent. For longer TOS values (15–60 min), no significant differences were found in terms of cracking products and isomers selectivity when guaiacol is added to the feed. Furthermore, for both zeolites, no changes were detected on the aromatics selectivity, which could mean that the aromatic consumption in the coke formation reactions is not altered by the presence of the oxygenate. 3.2. Guaiacol influence on the commercial FCC catalysts 3.2.1. Conversion and coke formation Fig. 4 presents the conversion values obtained during the transformation of the pure gasoil and the gasoil/guaiacol mixture in a fixed-fluidized bed at 535 ◦ C as a function of the catalyst-to-oil ratio (CTO), respectively for the ECAT and the ECAT + ZSM-5 catalysts. It is important to remind that, for the industrial catalytic tests, the conversion is defined as the fraction of the feed converted into gases

Carbon content (wt.%)

Carbon content (wt.%)

also formed through protolytic cracking and/or olefin transformations (hydrogen transfer reactions or/and aromatization reactions), but always in low amounts [33–35]. At relatively low reaction times, even if an additional decrease of n-heptane conversion due to guaiacol can be observed, no difference on the selectivity was observed. Regardless the zeolite, cracking products, n-heptane isomers and aromatics yields were found to be similar. In Fig. 3a, the paraffin to olefin ratio is plotted as function of the n-heptane conversion in the beginning of the reaction for different contact times, for the tests performed in the absence and presence of guaiacol at 450 ◦ C, over the HY and HZSM-5 zeolite samples. This ratio is not altered by the addition of guaiacol, which suggests that hydrogen transfer reactions are not affected by this oxygenate molecules. However, depending on the zeolite, differences in the (iC4 + iC5 )/(nC4 + nC5 ) paraffins ratio can be found, even for the first minutes of the reaction (Fig. 3b). In fact, while this ratio remains similar on the HZSM-5 sample regardless the presence of guaiacol in the reaction medium, the amount of branched C4 and C5 species slightly increases in the presence of guaiacol over HY zeolite. As it was observed in Fig. 2, the formation and retention in the zeolite pores of the aromatic compounds occur since the beginning of the reaction. The HZSM-5 pore system limits the formation and diffusion of ramified molecules [35,36], which can be demonstrated by the lower (iC4 + iC5 )/(nC4 + nC5 ) paraffin ratios obtained with this zeolite than with the HY zeolite. Therefore, for this zeolite, it would be expected identical results for the tests performed with or without guaiacol.

12 10 8 6 4

3

b

2.5 2 1.5 1 0.5

2 0

0 0

10

20

30

40

Time-on-stream (min)

50

60

0

10

20

30

40

50

60

Time-on-stream (min)

Fig. 2. Carbon content vs. time-on-stream, for a contact time  = 4 min, during the transformation of pure n-heptane () and n-heptane/guaiacol mixture (), at 450 ◦ C. (a) HY: n-heptane + 4.0 wt.% guaiacol and (b) HZSM-5: n-heptane + 1.2 wt.% guaiacol.

I. Grac¸a et al. / Applied Catalysis B: Environmental 101 (2011) 613–621

HY

Paraffins/Olefins P/O

4 3 2 HZSM-5

1 0 0

10

20

30

40

50

iso C4-C5/ n C iso/n 4-C5 paraffins

5

a

5

617

b

4

HY

3 2 1 HZSM-5

0 0

60

10

20

n-heptane conversion (%)

30

40

50

60

n-heptane conversion (%)

Fig. 3. (a) Paraffin/olefin and (b) (iC4 + iC5 )/(nC4 + nC5 ) paraffin ratios in the beginning of reaction (different contact times) as a function of the n-heptane conversion for the pure n-heptane () and the n-heptane/guaiacol mixtures () over HY (n-heptane + 4.0 wt.% guaiacol) and HZSM-5 (n-heptane + 1.2 wt.% guaiacol) zeolites, at 450 ◦ C.

(fuel gas and LPG), gasoline and coke. Based on these curves, it is possible to observe that, whatever the catalytic system used, there is an increase of the conversion when guaiacol is added to the gasoil. Similarly to what was previously observed for other oxygenated molecules such as acetic acid, hydroxyacetone and phenol [37], this improvement of the conversion is due to the presence of the guaiacol and its transformation products in the valuable products boiling range. In order to evaluate the influence of guaiacol addition on gasoil conversion, the conversion of the gasoil/guaiacol mixture was estimated as being the sum of the conversion of the gasoil present in the mixture (0.872 × conversion of pure gasoil) and the percentage of added guaiacol (12.8 wt.%). The obtained results are plotted in Fig. 5 (dashed line) for the ECAT and ECAT + ZSM-5 catalytic systems, where they are compared with the actual results obtained for the gasoil/guaiacol mixture. In the case of the ECAT (Fig. 5a), it can be observed that the estimated conversion values are similar to the ones obtained for the gasoil/guaiacol mixture. On the other hand, in the presence of the ZSM-5 additive (Fig. 5b) the conversions taken as the sum of the gasoil conversion and the added guaiacol were always higher than those experimentally determined, increasing slightly with the increase of the CTO. Thus, it can be concluded that the presence of the oxygenated compound in the feed is responsible for a reduction of the gasoil conversion into light gases, gasoline (from gasoil) or coke, principally when the ZSM-5 additive is added to the ECAT. This is in agreement with the results reported on Section 3.1 for the pure HY and HZSM-5 zeolites, at 450 ◦ C. Actually, whereas the deactivating effect of guaiacol was more limited over the HY zeolite, the presence of guaiacol in low amounts drastically affected the HZSM-5 activity and, consequently, the n-heptane

conversion. Furthermore, the gasoil conversion seems to be more affected by the presence of guaiacol with the increase of the CTO. The higher the CTO, the lower the quantity of feed injected (catalyst mass was kept constant). Therefore, it can be said that higher CTO for the gasoil tests could correspond to smaller TOS values in the zeolite experiments. Actually, for the catalytic tests performed with pure zeolites, it was verified that the additional zeolite deactivation due to guaiacol per unit of time ((n-heptane/guaiacol mixture − pure n-heptane conversion)/TOS) was more important in the beginning of the reaction (small TOS) because more acid sites were available to adsorb the guaiacol molecules. This is consistent with the higher decrease of the gasoil conversion in the presence of guaiacol with the increase of the CTO. Both conversion and coke formation are usually highly correlated. Over pure zeolites, the presence of guaiacol resulted in a pronounced additional enhancement of the carbonaceous materials deposited on the zeolites (Fig. 2), leading to an increase of the zeolite deactivation. Likewise, for the industrial catalysts, an increase in the amount of coke deposited on the catalysts was observed in the presence of guaiacol, particularly in the case of the ECAT + ZSM-5 mixture (Table 4). This explains the reduction of the gasoil conversion in the presence of guaiacol, mostly for the ECAT + ZSM-5 mixture. This is probably due to the presence of the ZSM-5 additive, in which the progression of the guaiacol molecules could be limited by the narrowness of the ZSM-5 channels, as previously observed in the case of the gasoil/phenol transformation [37]. Despite the increase of the coke-on-catalyst, for ECAT, there is a reduction of the coke yield in the presence of guaiacol, in comparison with the results obtained for the pure gasoil (Fig. 6a). This

75

a

75

70

Conversion (%)

70

Conversion (%)

b

65 60 55

65

60

55

50 45

50 2

4

6

8

Catalyst-on-oil ratio (wt./wt.)

10

2

4

6

8

Catalyst-on-oil ratio (wt./wt.)

Fig. 4. Conversion vs. CTO, for the pure gasoil () and the gasoil/guaiacol mixture (), with (a) E-CAT and (b) ECAT + ZSM-5.

10

618

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75

a Conversion (%)

Conversion (%)

b

70

70 65 60 55

65 60 55

50

50

45

45 40

40 2

4

6

8

2

10

4

Catalyst-on-oil ratio (wt./wt.)

6

8

10

Catalyst-on-oil ratio (wt./wt.)

Fig. 5. Conversion of gasoil present in the gasoil/guaiacol mixture (0.872 × conversion of pure gasoil, ) real conversion of gasoil/guaiacol mixture () and conversion of the mixture estimated as the sum of gasoil conversion and added guaiacol (dashed line) as a function of the CTO, for (a) ECAT and (b) ECAT + ZSM-5. Table 4 Product yields and coke deposited on catalyst obtained at constant conversion (63%) for the pure gasoil and the gasoil/guaiacol mixture, considering the two catalytic systems. ECAT

Catalyst to oil ratio (w/w) Yields (wt.%) CO CO2 H2 O Fuel gas Hydrogen Methane Ethane Ethylene LPG Propane Propylene Butanes Butylenes Gasoline LCO DO Coke Coke on catalyst (wt.%)

ECAT + ZSM-5

Pure gasoil

Gasoil/guaiacol

Pure gasoil

6.18

4.53

5.61

– 0.25 0.44 2.7 0.15 1.3 0.62 0.69 12.5 1.43 3.39 4.23 3.41 36.0 19.7 17.3 11.8 1.92

0.05 0.33 1.0 3.3 0.06 1.6 0.73 0.86 13.8 1.43 3.66 4.28 4.44 36.2 18.8 17.3 9.5 2.16

0.06 0.38 0.18 3.3 0.15 1.2 0.67 1.2 20.5 1.99 6.95 5.65 5.87 29.8 17.0 20.0 9.5 1.69

Gasoil/guaiacol 4.47

0.05 0.34 2.9 3.4 0.06 1.5 0.68 1.2 18.0 1.73 6.06 4.54 5.62 30.6 17.2 18.6 9.0 2.08

is probably due to a more important increase of the other light products comprised in the fuel gas, LPG and gasoline fractions, thus diminishing the relative amount of coke (coke yield), in the presence of guaiacol. Contrarily, when the ZSM-5 additive is added to

3.2.2. Products distribution Table 4 shows a comparison of the products yields obtained at constant conversion (63%) for the pure gasoil and the gasoil/guaiacol mixture, over the ECAT and the ECAT + ZSM-5 mixture. The gasoline yield slightly increases in the presence of guaiacol on both catalysts. This improvement of the gasoline fraction becomes more evident if we consider that the mixture is composed of only 87.2 wt.% of gasoil. So, for the gasoil/guaiacol mixture, the quantity of gasoline resulting from gasoil transfor16

20

a

b

16

Coke yield (wt.%)

Coke yield (wt.%)

ECAT, the coke yields obtained for the gasoil/guaiacol mixture are equal or higher to the observed for the pure gasoil, depending on the conversion (Fig. 6b). This agrees with the more important coke deposition on the ECAT + ZSM-5 mixture in the presence of guaiacol, which is responsible for the reduction of the gasoil conversion and, consequently to a decrease of the fuel gas, LPG and gasoline (from gasoil only) yields, principally for higher conversions (greater CTO). Coke yields for the HY zeolite are in the range 5.4–7.4 wt.%, which is close to the values obtained at low conversions for the gasoil. Over pure HZSM-5 zeolite, lower coke yields, 1.7–3.5 wt.%, were formed. Therefore, considering the mixture of 90% HY + 10% HZSM-5, coke yields of 5–7 wt.% could be determined, which are also near to that found for the ECAT + ZSM-5 mixture at low conversions. Hence, it can be concluded that, when the ZSM-5 additives are blended with the equilibrium catalysts (ECAT), they could greatly affect the catalytic performance of the ECAT + ZSM-5 catalytic system, if guaiacol is present in the feed. This is consistent with the high deactivation effect imposed by the oxygenate molecules on the pure HZSM-5 zeolite.

12

8

12

8

4

4

0

0 45

50

55

60

65

Conversion (%)

70

75

50

55

60

65

70

Conversion (%)

Fig. 6. Coke yield vs. conversion, for the pure gasoil () and the gasoil/guaiacol mixture (), with (a) E-CAT and (b) ECAT + ZSM-5.

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a

b

20

Gasoil/guaiacolmixture

Gasoil/guaiacolmixture

20

15

10

5

619

15

10

5

0

0 0

5

10

15

20

0

5

10

15

20

Pure gasoil

Pure gasoil

Fig. 7. Comparison at constant conversion of the benzene (x), toluene () and xylenes () yields calculated from the curves obtained in the PIANO results, for the pure gasoil and the gasoil/guaiacol mixture, for (a) E-CAT and (b) ECAT + ZSM-5.

mation can be estimated to 0.872 × gasolinepure gasoil , being in this case of 31.4 and 26.0 wt.%, respectively for the ECAT and ECAT + ZSM-5 mixture. If we compare these values with those presented in Table 4 for the gasoil/guaiacol mixture, it is evident that they are smaller than those obtained for the gasoil/guaiacol mixture, which confirms the enhancement of the gasoline yield due to guaiacol. Nevertheless, the difference is lower than the total value of added guaiacol, evidencing its partial conversion. An increase in the gasoline yield is not surprising considering that unconverted guaiacol presents a boiling point in the gasoline range. However, a detailed PIANIO analysis of the composition of this fraction allowed verifying the presence of phenol, 9.5–4.8 wt.% for ECAT and 11.9–6.0 wt.% for ECAT + ZSM-5, with the increase of the conversion. It should be noticed that this compound does not appear on the experiments with pure gasoil. Furthermore, a small increase of the benzene yield was still detected, when comparing the results for the pure gasoil and for the gasoil/guaiacol mixture (Fig. 7). Actually, the decrease of the phenol yield for higher conversions is accompanied by the increase of the benzene content in the gasoline. Benzene and water were the major products of phenol transformation by dehydration [37] that may happen through a bimolecular mechanism, where on adsorbed paraffin molecule transfers hydrogen to phenol, yielding benzene, water and the parent olefin. In fact, an increase of the water present on the effluent can be noticed for the tests performed with the gasoil/guaiacol mixtures, when compared with the results for the pure gasoil (Table 4). This enhancement of the water content is much higher than that previously reported for the gasoil/phenol mixture in Ref. [37], which could suggest that water does not come exclusively from phenol dehydration. Methyl-phenols (6.2–3.8 wt.% for ECAT and 7.6–4.6 wt.% for ECAT + ZSM-5, with the increase of the conversion) and ethyl-phenols (0.34–0.20 wt.% for ECAT and 0.38–0.23 wt.% for ECAT + ZSM-5, at higher conversions) were also found in the gasoline cut. These species result from phenol alkylation by longer olefins, which are then cracked into light ones, slightly increasing the LPG [37], as observed in Table 4. Moreover, the reduction of the concentration of the methyl- and ethyl-phenols on the effluent with the increase of the conversion can be related to their own transformation, respectively, into toluene and C8 aromatics, Fig. 7. Since most commercial FCC units operate in heat balance, it is useful to analyze product yield data at constant coke yield. When gasoline curve is plotted as a function of coke yields (not shown), it

passes through a maximum for coke yield higher than 13 wt.%, i.e. gasoline overcracking occurs at high conversions. The main product yields at constant coke are presented in Table 5. Such comparison makes the guaiacol effect more clear, but the trends are similar to what was observed for constant conversion. Additionally, observing Table 4, it can be also noticed an increase of the fuel gas in the presence of guaiacol, which is mainly due to an enhancement of the production of methane. The formation of methane from guaiacol involves two bimolecular hydrogen transfer reactions, in the first phenol and methanol are formed; methanol being further converted into methane and water (Scheme 1). In agreement with this, the olefin/paraffin ratio in the LPG fraction was found to increase in the presence of guaiacol. The increase was from 1.2 to 1.4 in the case of ECAT and from 1.7 to 1.9 for ECAT + ZSM-5 mixture (Table 4). Thus, analyzing the products distribution, a possible reaction scheme for the guaiacol transformation could be established (Scheme 1). As for the HY and HZSM-5 zeolites, the guaiacol transformation into phenol with methane and water formation is a reaction occurring on the ECAT and ECAT + ZSM-5 mixture. Possibly, for the industrial catalysts, the guaiacol transformation can be helped by the presence of Ni in the ECAT composition (Table 2). In fact, similar reaction mechanisms were observed for the guaiacol hydrodeoxygenation (HDO) over the typical CoMo and NiMo catalysts [21,23]. Hence, the co-processing of guaiacol rich feedstocks may be limited due an increase of aromatic compounds in the gasoline. Currently, many countries set tight limits on gasoline aromatics, particularly on benzene (max. 1 vol.%). Typically, ZSM-5-based catalysts are often used as additives to favor the cracking of low-octane compounds present in the gasoTable 5 Product yields at constant coke yield (7.5 wt.%) for pure gasoil and gasoil/guaiacol mixture, with ECAT and ECAT + ZSM-5. Yields (wt.%)

CO CO2 H2 O Fuel gas LPG Gasoline LCO DO

ECAT

ECAT + ZSM-5

Pure gasoil

Gasoil/guaiacol

pure gasoil

Gasoil/guaiacol

0 0.20 0.44 1.9 8.3 32.8 22.2 26.7

0.04 0.30 1.0 3.1 12.3 35.0 19.4 21.4

0.05 0.33 0.18 2.9 17.7 28.2 18.9 24.1

0.05 0.31 2.9 3.2 16.5 28.4 19.3 21.9

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Scheme 1. Possible reaction scheme for the guaiacol transformation.

line range into lighter products corresponding to the LPG cut. In fact, observing the gasoline and the LPG yields obtained for pure gasoil and the gasoil/guaiacol mixture at constant conversion (Table 4), it is obvious that, in the presence of ZSM-5, there is a decrease of the gasoline yield, with a consequent increase of the LPG production. Nevertheless, while for the pure gasoil the gasoline reduction is 6.2 wt.% at 63 wt.% of conversion, in the case of the gasoil/guaiacol mixture this decrease is 5.6 wt.% (Table 4). This clearly demonstrates that the addition of guaiacol negatively affects ZSM-5 effectiveness, but in a lower extent than for other O-compounds [37]. A possible explanation could be the existing differences between guaiacol and the other oxygenated molecules in the capacity to penetrate on the ZSM-5 channels. Whereas acetic acid and hydroxyacetone are small molecules that easily enter on the zeolite channels, the differences could not be so evident when comparing phenol and guaiacol. Nevertheless, the distinct kinetic ˚ and guaiacol (6.6 A) ˚ molecules could diameters of phenol (5.8 A) highly influence the diffusion into the narrow ZSM-5 channels. In fact, it seems that the shape selectivity of the ZSM-5 structure could restrain the access of guaiacol to its channels but not to phenol, explaining the existence of a negative influence of guaiacol on the ZSM-5 function, however inferior to that observed for the gasoil/phenol mixture [37]. This suggests that under FCC conditions, guaiacol is responsible for an in-series deactivation type [38], i.e. the poisoning and/or pore blockage is due to molecules

formed by secondary transformations of products directly formed from guaiacol. 4. Conclusions Guaiacol has a negative influence on both n-heptane and gasoil conversions, principally in the presence of ZSM-5. It is responsible for the increase of the carbonaceous materials deposition on the catalysts. For the industrial catalytic tests, only a slight reduction of the gasoil conversion was noticed, mainly at higher CTO values. For the n-heptane transformation over pure HY and HZSM-5 zeolites, guaiacol increases methane yield in line with its transformation into phenol. With industrial FCC ECAT, the presence of guaiacol reduces coke yield and increases gasoline yield. This is not surprising considering that, even if guaiacol did not react, it presents a boiling point in the gasoline range. Detailed GC analysis of the liquid product evidentiate the presence of phenols in the gasoline cut, suggesting that hydrogen transfer reactions play an important role in the conversion of guaiacol. These bimolecular reactions are also involved in the formation of methane (through methanol) and water. Guaiacol also revealed to have an influence on the ZSM-5 additive effect, being consistent with the higher deactivating effect of this oxygenated molecule on the activity of the HZSM-5 zeolite.

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