Enhancing propylene production from catalytic cracking of Arabian Light VGO over novel zeolites as FCC catalyst additives

Fuel 90 (2011) 459–466

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Enhancing propylene production from catalytic cracking of Arabian Light VGO over novel zeolites as FCC catalyst additives M.A.B. Siddiqui, A.M. Aitani, M.R. Saeed, N. Al-Yassir, S. Al-Khattaf ⇑ Center of Research Excellence in Petroleum Refining & Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 18 August 2010 Received in revised form 20 September 2010 Accepted 22 September 2010 Available online 25 October 2010 Keywords: FCC additive Catalytic cracking Gasoline Propylene Light olefins

a b s t r a c t The catalytic cracking of vacuum gas oil over fluid catalytic cracking (FCC) catalyst containing novel additives was investigated to enhance propylene yield. A conventional ZSM-5, mesoporous ZSM-5 (Meso-Z), TNU-9 and SSZ-33 zeolite were tested as additives to a commercial equilibrium USY FCC catalyst (E-Cat). Their catalytic performance was assessed in a fixed-bed micro-activity test unit (MAT) at 520 °C and various catalyst/oil ratios. The cracking activity of all E-Cat/additives did not decrease by using these additives. The highest propylene yield of 12.2 wt.% was achieved over E-Cat/Meso-Z compared with 9.0 wt.% each over E-Cat/ZSM-5 and E-Cat/TNU-9, at similar gasoline yield penalty. The enhanced production of propylene over Meso-Z is attributed to its mesopores that suppressed secondary and hydrogen transfer reactions and offered easier transport and accessibility to active sites. The lower enhancement of propylene over the large-pore SSZ-33 additive was due to its high-hydrogen transfer activity. Gasoline quality was improved by the use of all additives, as octane rating increased by 7–12 numbers for all E-Cat/ additives. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Propylene is an important petrochemical feedstock for the production of useful materials such as polypropylene, acrylonitrile, propylene oxide, cumene, and acrylic acid. Projected growth rate of propylene demand is estimated at 4–5% per year [1]. Fluid catalytic cracking (FCC) process is a major source of incremental propylene in which heavy feedstocks such as vacuum gas oil or residual oil are cracked into value-added lighter products (LPG and gasoline). Currently, about 30% of the world’s propylene is supplied by refinery FCC operations, 64% is co-produced from thermal steam cracking of naphtha or other feedstock, and the remaining is produced on-purpose using metathesis or propane dehydrogenation processes [1]. The cracking of liquid feedstocks is carried out predominately in Europe and Asia but less so in the Middle East and North America. To increase propylene yield from FCC process, efforts are being made both on process side and on catalyst side. The addition of ZSM-5 to FCC catalyst is one of the efficient methods for improving propylene yield because it offers refiners a high degree of flexibility to optimize the production output of their FCC units [2,3]. The amount of ZSM-5 crystal added to USY-based FCC catalysts is increasing with values as high as 25–30 wt.% are reported in some commercial units.

There have been many studies on the effect of ZSM-5 additives on FCC products selectivities [4–8]. Incorporating active components as phosphorus and metals in the additive formulation that aid in the conversion of heavier molecules, optimization of additive formulations with high ZSM-5 levels and use of Y-zeolite in the additive formulation are some of the proposed approaches for diminishing or overcoming the loss in activity of FCC catalyst systems [9–12]. The utilization of other types of zeolites as FCC additives such as mordenite [13,14], MCM-22 [15], beta [14,16], ITQ [17], and MCM-68 [18] were discussed in other papers. A bi-functional ZSM-5 and MgAl2O4-based additive was used for increasing propylene yield and for the removal of SO2 in FCC process [19]. The aim of this paper is to investigate the utilization of novel zeolites such as medium-pore TNU-9, large-pore SSZ-33 and mesoporous ZSM-5 as FCC catalyst additives for enhancing propylene yield. The catalytic performance of these additives was evaluated in a fixed-bed micro-activity test (MAT) unit for the cracking of hydrotreated Arabian Light vacuum gas oil under FCC conditions. 2. Materials and methods 2.1. Catalysts and additives

⇑ Corresponding author. Tel.: +966 3 860 1429; fax: +966 3 860 4234. E-mail address: [email protected] (S. Al-Khattaf). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.09.041

Five catalysts have been used in the study. The base catalyst was a commercial equilibrium FCC catalyst (E-Cat) obtained from

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a domestic refinery. It is based on USY zeolite with a surface area of 135 m2/g and a pore volume of 0.23 cm3/g. The E-Cat was calcined at 500 °C for 3 h before further use. The other four catalysts were mixtures of 90 wt.% E-Cat and 10 wt.% each of ZSM-5, mesoporous ZSM-5 (Meso-Z), TNU-9 and SSZ-33 additives. ZSM-5 was procured from CATAL Intl., whereas Meso-Z, TNU-9 and SSZ-33 zeolites were prepared and acquired from the Institute of Physical Chemistry, Czech Republic. All four zeolites were in H-form with SiO2/Al2O3 ratio between 18 and 22. Meso-Z is an MFI type of zeolite material with preformed mesopores that was synthesized using carbon black (CBP 2000, Cabot Corp.) as a secondary template with an average particle diameter of 12 nm [20,21]. TNU-9 is a medium pore zeolite having an unprecedented framework type (TUN topology). It contains a three dimensional 10-ring channel pore system consisting of two parallel 10-ring channels in one direction which themselves are connected to each other via 10-ring openings in another direction [22]. The dimension of larger channel is 5.2  6.0 Å, slightly higher than that of ZSM-5 (5.3  5.6 Å). Zeolite SSZ-33 comprises two intersecting 12-ring and 10-ring channels forming a framework structure related to CON topology. The pore dimensions of its largest channel opening are 7  5.9 Å [23]. For the preparation of additives, Cataloid AP-3 (which contains 75.4 wt.% alumina, 3.4 wt.% acetic acid, and water as balance) was used as an alumina binder for the four zeolites [24]. The required quantity of the binder (33.3 wt.%) was mixed with distilled water which was acidified to a pH of 5 with dilute nitric acid. The zeolite sample was added to the binder-water slurry, while stirring. The slurry was dried at 120 °C for 2 h followed by calcination at 600 °C for 3 h. The calcined additives were pelletized, crushed and sieved to obtain 80–90 microns size additive particles. 2.2. Catalyst characterization The textural properties of the additives were characterized by N2 adsorption measurements at 77 K, using Quantochrome NOVA 1200 adsorption analyzer. Samples were out-gassed at 350 °C under vacuum (10 5 Torr) for 2 h before N2 physisorption. The BET specific surface areas were determined from the desorption data in the relative pressure (P/Po) range from 0.06 to 0.2, according to ASTM method D3663. The total pore volume was calculated from the desorption branch of isotherm, using the Barrett– Joyner–Halenda (BJH) method. Micropore volume was determined by t-plot method, whereas mesopore volume was calculated as the difference between total pore volume and micropore volume. The shape and size of zeolite crystals were determined by scanning electron microscopy (Jeol, JSM-5500LV). Infrared spectroscopy of adsorbed pyridine was used to determine the types of acid sites present. The measurements were carried out using a Fourier transform infrared (FTIR) spectrometer (Nicolet Protege 460), using the self-supported wafer technique. The concentrations of Lewis and Brönsted acid sites in zeolites under study were determined using the extinction coefficients for pyridine e(B) = 1.67 ± 0.1 cm lmol 1 and e(L) = 2.22 ± 0.1 cm lmol 1) [25]. The samples in the form of a self-supporting wafer (ca. 5 mg/cm2) were obtained by compressing a uniform layer of powder. The wafer was then placed in an infrared vacuum cell equipped with KBr windows and pretreated under vacuum (ca. 10 3 Torr) at 450 °C for 1 h. The adsorption temperature of pyridine was 170 °C. Samples were then evacuated at the same temperature for 20 min. 2.3. Catalytic evaluation The feed used in all runs was an Arabian Light hydrotreated vacuum gas oil (VGO) procured from a Saudi Aramco domestic refin-

Table 1 Properties of Arabian Light hydrotreated vacuum gas oil feed. Property

Value

Density (g/cm3) (15 °C) Sulfur (ppm) Nitrogen (ppm) Saturates (wt.%) Aromatics (wt.%) Residue (wt.%)

0.896 300 170 59 40 0.8

Simulated Distillation (oC) Initial boiling point 5% 25% 50% 90% Final boiling point

308 348 376 420 507 568

ery. The properties of VGO are listed in Table 1. The catalytic cracking of VGO was carried out in a fixed-bed (MAT) unit, manufactured by Sakuragi Rikagaku, Japan according to ASTM D-3907 and D-5154 test methods. For each MAT run, a full mass balance was obtained. If the material balance was less than 96% or greater than 102%, the test was repeated. All MAT runs were performed at a cracking temperature of 520 °C and a time-on-stream of 30 s. Conversion was varied by changing catalyst/oil (C/O) ratio in the range of 1.5–5.0 g/g. This variable was changed by keeping constant the amount of VGO (1.0 g) and changing the amount of catalyst. A thorough gas chromatographic analysis of the gaseous products was conducted to provide detailed yield patterns and information on the selectivity of the catalyst/additives being tested. Gaseous products (dry gas and LPG) were analyzed using two Varian gas chromatographs equipped with 50 m (0.32 mm diameter) Alumina Plot capillary column and FID/TCD detectors. Coke on catalyst was determined by a Horiba carbon analyzer. For liquid products, three different cuts were considered: gasoline (C5, 221 °C), LCO (light cycle oil, 221–343 °C), and HCO (heavy cycle oil, +343 °C). The weight percentage of liquid products was determined by a simulated distillation GC equipped with 10 m (0.53 mm diameter) RTX-2887 capillary column and FID detector according to ASTM D-2887. Gasoline composition was determined using a Shimadzu GC system that was configured to give paraffins, olefins, naphthenes and aromatics (POINA) distribution. The GC was equipped with 50 m (0.15 mm diameter) BP-1 PONA capillary column and FID detector and conversion was defined as the sum of yields for dry gas (H2 and C1–C2), LPG (C3–C4), gasoline, and coke. 3. Results and discussion 3.1. Catalyst characterization The textural properties of the additives, presented in Table 2, were determined from nitrogen adsorption isotherms. Surface area and pore volume were the highest for SSZ-33 additive, as it has large three dimensional pore system. Mesopore volume, Vme (0.41 cc/g) of Meso-Z additive is due to actual mesopores and does not represent intracrystalline void volume [20]. This is evident from the broad hysteresis effect in desorption curve for Meso-Z in the P/Po range of 0.6–0.9, as illustrated in Fig. 1. The isotherms of conventional ZSM-5 additive showed a gradual rise in P/Po of 0.6–0.9. Intracrystalline void volume is reflected by a steep rise in the nitrogen adsorption isotherm at P/Po of 0.9–1.0. The presence of large pores for SSZ-33 is evident from Fig. 1 as large volume of nitrogen was adsorbed. Mesopore volume for TNU-9 was slightly lower than Meso-Z, as shown in Table 2.

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*

Zeolite topology

Total pore volume (cc/g)

Micro pore, Vmi (cc/g)

Meso pore, Vme (cc/g)

Surface area (m2/g)

MFI MFI TUN CON

0.30 0.49 0.50 0.71

0.07 0.09 0.10 0.13

0.23 0.40 0.40 0.58

284 370 350 437

Additives were mixed with 33.3 wt.% alumina binder.

A

500

B

400

Volume, cc/g

Volume, cc/g

400

500

300 200 100

300 200 100

0

0 0

0.2

0.4

0.6

0.8

1

Relative pressure, P/Po

0

0.2

0.4

0.6

0.8

1

Relative pressure, P/Po

Fig. 1. Nitrogen adsorption–desorption isotherms of different additives: ( ) ZSM-5; (D) TNU-9; (h) Meso-Z, (s) SSZ-33.

The acid-type, Brönsted and Lewis-types, is a fundamental property of solid acids as well as the strength and density of acid sites. FTIR spectra of pyridine chemisorbed on the four additives which allow a clear distinction between Brönsted and Lewis acid sites, are shown in Figs. 2 and 3. The data of acid sites density is reported in Table 3. All additives displayed both Lewis and Brönsted acid sites, however the extent of these sites depended significantly on the nature of these additives. SSZ-33 additive showed the highest acid sites density (0.66 mmol/g) owing to its low SiO2/Al2O3 ratio [26,27], while the density of acid sites for MesoZ additive was the lowest (0.40 mmol/g). Meso-Z showed the highest Lewis to Brönsted (L/B) ratio (4.13), whereas TNU-9 displayed the lowest L/B ratio (0.76). The concentration of Brönsted sites decreased in the following order: TNU-9 > ZSM-5 > SSZ-33 > Meso-Z. These results explain the low density of Meso-Z compared to ZSM-5 despite the fact that both have relatively close SiO2/Al2O3 ratios.

The SEM images of the four additives are presented in Fig. 4. The images showed the absence of any additional phase even an amorphous one. The size of Meso-Z crystals was around 1.5 lm while ZSM-5 showed crystals close to 0.3 lm. The crystals of SSZ-33 and TNU-9 were close to 0.5 lm [24].

Fig. 2. IR spectra of hydroxyl vibration region of SSZ-33 (I), ZSM-5 (II) before (a) and after (b) pyridine adsorption (A) and spectra of pyridine region after its adsorption (B).

Fig. 3. IR spectra of hydroxyl vibration region of TNU-9 (I) and MesoZ (II) before (a) and after (b) pyridine adsorption (A) and spectra of pyridine region after its adsorption (B).

3.2. Conversion and yields The conversion versus C/O ratio over E-Cat and E-Cat/additives is shown in Fig. 5. The plots show linear relationship between conversion and C/O that confirms the apparent second-order kinetics of VGO cracking over all E-Cat/additives [28,29]. Over E-Cat base catalyst, the conversion increased from 61% to 76% as C/O ratio was varied from 1.5 to 5. At high C/O ratio, all additives showed higher conversion compared with E-Cat despite the dilution effect of the additives. Zeolite pore diameter plays a key role in minimizing hydrogen transfer reactions and in enhancing catalytic crack-

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Table 3 Acidic properties of additives. Additive

*

ZSM-5 Meso-Z TNU-9 SSZ-33 *

SiO2/Al2O3 Ratio (FTIR)

Lewis acidity, L (mmol/g)

Brönsted acidity, B (mmol/g)

Total acidity (mmol/g)

L/B ratio

20.5 22.0 19.3 13.6

0.28 0.33 0.25 0.48

0.21 0.08 0.33 0.18

0.49 0.41 0.58 0.66

1.33 4.13 0.76 2.67

Additives were mixed with 33.3 wt.% alumina binder.

Fig. 4. SEM images of zeolites: ZSM-5 (A), Meso-Z (B), TNU-9 (C) and SSZ-33 (D).

95

Conversion, %

85 75 65 55 45 0.0

1.0

2.0

3.0

4.0

5.0

C/O, Ratio Fig. 5. Comparison of MAT activity over E-Cat and E-Cat/additives: () E-Cat; ( ) ZSM-5; (h) Meso-Z; (D) TNU-9; and (s) SSZ-33.

ing. It was shown that crystal size changes in the 0.13–1.3 lm range affect both primary and secondary cracking reactions and the yields of gasoline, coke and light gas yields [28]. The effect of pore structure on MAT conversion is attributed to the accessibility of cracking VGO and intermediate compounds over different addi-

tives. The conversion order obtained for the four additives (SSZ33 > TNU-9 > Meso-Z = ZSM-5) is explained by the different total acidity shown in Table 3. ZSM-5 and Meso-Z showed similar conversion behavior to E-Cat, while TNU-9 showed a slightly higher conversion and SSZ-33 showed much higher conversion because of its higher acidity and large pore structure. Fig. 6 shows the changes in product yields (selectivity curves) for propylene, ethylene, LPG, gasoline, dry gas, and coke as a function of conversion over E-Cat and E-Cat/additives. Except for gasoline, all product yields were higher over E-Cat/additives compared with the base catalyst (E-Cat). The results did not show any shift in HCO yields at constant conversion and only a slight decrease in LCO yield for all E-Cat/additives. By themselves, the absolute yields obtained in MAT testing of any catalyst are of little value. Rather, it is the comparison of yields between the additives at constant conversion that provides a means for distinguishing them. Using the selectivity curves developed for each additive, yield ratios of selected components (dry gas, LPG, and gasoline) were determined at constant conversion to truly distinguish the performance characteristics of additives. 3.3. LPG and light olefins yields LPG (C3–C4) yield increased linearly with conversion over all ECat/additives as shown in Fig. 6. It increased by 7.0 wt.% over E-Cat (11–18 wt.%) compared to an increase of 15 wt.% over ZSM-5 and

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14

5

Propylene

Ethylene 4

10

Yield, wt.%

Yield, wt.%

12

8 6 4

3 2 1

2

0

0 45

55

65

75

45

85

65

75

85

Conversion, %

Conversion, % 8

50

Dry gas

LPG 40

6

Yield, wt.%

Yield, wt.%

55

30 20

4

2

10

0

0 45

55

65

75

45

85

Conversion, %

55

65

75

85

Conversion, %

5

58 Gasoline Coke

50

Yield, wt.%

Yield, wt.%

4 3 2 1

42 34 26 18 10

0

45

55

65

75

85

Conversion, %

45

55

65

75

85

Conversion, %

Fig. 6. Product yields over E-Cat and E-Cat/additives: () E-Cat; ( ); ZSM-5; (h) Meso-Z; (D) TNU-9; and (s) SSZ-33.

TNU-9, Meso-Z and SSZ-33 (23–39 wt.%). SSZ-33 showed the highest increase in LPG yield at higher conversion with the minimum drop in corresponding gasoline yield. LPG olefinicity was the highest over Meso-Z and the lowest over SSZ-33. Olefinicity decreased with increased conversion for all additives. There was no change in olefinicity with increased conversion over E-Cat. While LPG yield over E-Cat was about 17 wt.% at constant conversion (75%), it increased by two folds over all E-Cat/additives. However, the composition of LPG was different. Over ZSM-5, TNU-9, and SSZ-33, LPG comprised 27% propane compared to 8% over Meso-Z. Butanes content was the highest over SSZ-33 at 43% compared with 33% each for ZSM-5 and TNU-9, and 27% over Meso-Z. The difference in LPG enhancement for the various additives can be explained by comparing the hydrogen transfer coefficient (HTC) which is defined as the ratio of butanes to butenes [30–32]. Hydrogen transfer is a bimolecular reaction that requires the reactants to be in close proximity to a strong acid site. It reflects the hydrogen transfer activity of the additives. The HTCs of ZSM-5 and TNU-9 were similar, which explains the similar enhancement of light ole-

fins over ZSM-5 and TNU-9. Meso-Z, which yielded higher light olefins, showed the lowest HTC of 0.9. Despite the fact that the topology of Meso-Z was similar to ZSM-5, however, mesopores were inherently created during synthesis [20]. The high yield of light olefins obtained over Meso-Z is attributed to its low hydrogen transfer activity which is due to its lower acidity, highly accessible pore structure and rapid elution of primary products due to its mesopores. As shown in Table 4, the HTC for SSZ-33 was the highest at 2.9 reflecting its high acidity which favors hydrogen transfer reactions and thus low olefins yield. Also, the large pore structure of SSZ-33 allowed bimolecular hydrogen transfer reactions to occur easily. The yield of C2–C4 light olefins increased with increasing conversion over E-Cat and E-Cat/additives as shown in Fig. 6. At constant conversion of 75%, maximum light olefins yield was obtained over Meso-Z additive (25.4 wt.%) compared with E-Cat (11.9%). ZSM-5 and TNU-9 additives yielded equal amount of light olefins at 19.5% followed by SSZ-33 at 13.8%. As illustrated in Table 4 and Fig. 7, the yields of propylene and ethylene showed the larg-

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Table 4 Comparative MAT data at constant conversion (75%) over E-Cat, E-Cat/ZSM-5, E-Cat/ Meso-Z, E-Cat/TNU-9 and E-Cat/SSZ-33 at 520 °C. Product Yields (wt.%)

Dry Gas H2 C1 C2@ C2 LPG C3@ C3 C4@ n-C4 i-C4 C2@–C4@ Gasoline LCO HCO Coke HTCa CMRb %C3@/gasolinec Gasoline RONd a b c d

Base E-Cat

E-Cat/10 wt.% additive

1.4 0.08 0.4 0.5 0.4 16.7 5.0 0.8 6.4 0.10 4.5 11.9 55.0 19.5 5.2 1.1 0.7 0.3 Base 75

ZSM-5

Meso-Z

TNU-9

SSZ-33

4.5 0.14 0.5 3.3 0.6

3.8 0.12 0.5 2.7 0.6

5.1 0.15 0.6 3.6 0.74

2.8 0.13 0.6 1.6 0.6

33.0 9.0 6.2 7.0 9.1 2.6 19.3

34.0 12.2 2.6 10.5 7.8 1.5 25.4

34.0 9.0 6.9 6.8 9.5 2.6 19.4

32.0 6.0 5.5 6.2 10.8 3.2 13.8

36.0 18.0 7.1 1.5 1.7 1.7 4.2 87

35.0 18.0 7.0 1.4 0.9 2.5 7.2 83

34.0 17.0 8.0 2.1 1.8 2.0 3.8 87

39.0 17.0 8.0 2.9 2.3 0.9 0.8 83

HTC = Hydrogen transfer coefficient (nC4 + iC4)/C4@. CMR = Cracking mechanism ratio (dry–gas/iC4). Percent increase in propylene yield per unit decrease in gasoline yield. RON = Research Octane Number by GC PIONA.

est increase in light olefins fraction. However, only Meso-Z additive showed an increase in butenes yield. The increase in light olefins yield is attributed to the conversion of reactive gasoline-range species (mainly iso-paraffins and olefins). Over E-Cat, propylene yield increased from 3.0 to 5.0 wt.% upon increasing conversion from 61.0 to 76.0%. Meso-Z showed the highest increase in propylene yield from 9.0 to 13.0 wt.% for a conversion between 48.5 and 76.0%. For ZSM-5 and TNU-9, propylene yield was in the range of 7–9 wt.%. The increase in propylene yield was lower over SSZ-33 (5.0–7.0 wt.%). Meso-Z showed a propylene yield enhancement of more than 100% compared with E-Cat. Propylene yield increased from 5 wt.% over E-Cat to 12.2 wt.% over Meso-Z. TNU-9 and ZSM-5 additives enhanced propylene yield by 4.0 wt.% each, while SSZ-33 increased propylene yield by 1.0 wt.%.

The rapid increase in ethylene yield with increasing conversion was similar for all E-Cat/additives, however, this increase was much higher compared with E-Cat (Fig. 6). At constant conversion, ethylene yield increased by 2.2 wt.% over Meso-Z, 4.0 wt.% each over ZSM-5 and TNU-9 additives and 1.1 wt.% over SSZ-33–Z. On the other hand, the increase in butenes was significant over Meso-Z (+4.1 wt.%), however, the other three additives did not show any increase in butenes yield. 3.4. Dry gas and coke yields The yield of dry gas (H2, C1–C2) varied between 1 and 2 wt.% over E-Cat within the conversion range of 61–76% (Fig. 6). All ECat/additives showed a linear increase in dry gas yield with increasing conversion. Dry gas yields over ZSM-5 and TNU-9 were the highest at 2–6 wt.% reflecting the increase in ethylene yield. Meso-Z and SSZ-33 yielded 2–4 wt.% dry gas, respectively. The other fractions of dry gas (C1 and C2) did not show any significant change over all E-Cat/additives. The cracking mechanism ratio (CMR), which is defined as the ratio of dry gas to iso-butane, is a measure of protolytic cracking to beta scission cracking [16,32]. Dry gas products reflect protolytic cracking while iso-butane reflects products formed by beta scission of branched hydrocarbons. The low value of CMR for both E-Cat and SSZ-33 indicates that beta scission cracking mechanism was dominant compared to protolytic cracking because of their large-pore zeolite structure. Coke yield over E-Cat increased from 0.4 to 1 wt.% within the conversion range studied, as shown in Fig. 6. SSZ-33 yielded significantly higher coke of 1–4 wt.% compared with E-Cat and other additives. At 75% conversion, coke yield over E-Cat was 1.1 wt.%. SSZ-33 gave the highest increase in coke yield of 2.9 wt.% while Meso-Z gave the lowest coke yield of 1.4 wt.%. The higher coke yield over SSZ33 is attributed to its high acidity and large pores. 3.5. Gasoline yield and composition Gasoline yield increased with increasing conversion over E-Cat and for all additives except for SSZ-33. The yield of gasoline over E-Cat increased from 48.0 to 55.0 wt.% with increasing conversion from 60.0 to 75.0%, respectively. Gasoline yield increased from 25 to 35 wt.% over ZSM-5, TNU-9 and Meso-Z, while it dropped slightly over SSZ-33 from 40 to 38 wt.%. Over E-Cat, gasoline yield at 75% conversion was 55 wt.% (Table 4) compared to about 35

30

E-Cat

Yield, wt.%

25

20

ZSM-5 Meso-Z TNU-9

15

SSZ-33

10

5

0

Ethylene

Propylene

Butylenes

C2-C4 Olefins

Fig. 7. Light olefins yield over E-Cat and E-Cat/additives: ZSM-5, Meso-Z, TNU-9, and SSZ-33.

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Gasoline composition, wt.%

80 70

E-Cat ZSM-5

60

Meso-Z

50

TNU-9

40

SSZ-33

30 20 10 c

0

n-Paraffins

i-Paraffins

Olefins

Naphthenes

Aromatics

Fig. 8. Gasoline composition over E-Cat and E-Cat/additives: ZSM-5, Meso-Z, TNU-9, and SSZ-33.

wt.% over ZSM-5, TNU-9 and Meso-Z. For SSZ-33, gasoline yield was reduced to 39 wt.%. The results in Table 4 show a direct correlation between the increase in propylene yield and the decrease in gasoline yield. The percentage increase in propylene yield for a unit decrease in gasoline yield is a critical indicator for FCC economic evaluation. As given in Table 4, this parameter was the highest for Meso-Z which showed about 7% enhancement in propylene yield per unit decrease in gasoline yield compared to less than 1% for SSZ-33. The values for ZSM-5 and TNU-9 were 4.2 and 3.8, respectively. Fig. 8 presents the composition of gasoline fraction (n-paraffins, iso-paraffins, olefins, naphthenes and aromatics) at 75.0% conversion over E-Cat and E-Cat/additives. Over E-Cat, gasoline comprised 55.0% of the total cracked products. Its composition was 3.5% nparaffins, 31% iso-paraffins, 12% naphthenes, 10.5% olefins and 42.0% aromatics. There was no change in the n-paraffins content of gasoline over all E-Cat/additives. More than half of the iso-paraffins were cracked over ZSM-5, as its content decreased from 31.0% to 13.0%. Over Meso-Z, about 50% of iso-paraffins were cracked. The lowest decrease of 34.0% in iso-paraffins was observed for SSZ-33. Although about 66% of gasoline-range olefins were cracked over ZSM-5 compared with 42.0% over Meso-Z, more light olefins were formed over Meso-Z. As discussed earlier, this was attributed to the higher hydrogen transfer activity (HTC) of ZSM-5 compared with Meso-Z. SSZ-33 did not crack gasoline-range olefins and hence gave insignificant light olefins enhancement. Among various gasoline hydrocarbon groups, naphthenes did not show any significant change over all E-Cat/additives. As ring cracking was not possible at the reaction conditions employed [33], naphthenes were slightly decreased over ZSM-5 due to dehydrogenation reactions. Gasoline aromatics content increased significantly over all additives. There was a 70% increase in aromatics over ZSM-5 compared with E-Cat. For TNU-9, Meso-Z, and SSZ-33, aromatic content increased by 65%, 47%, and 33%, respectively. Aromatics were formed by cyclization and dehydrogenation of iso-paraffins and to some extent by the dehydrogenation of naphthenes. This explains the high iso-paraffins content in SSZ-33 compared with other additives. The increase in aromatics was associated with a corresponding increase in research octane number (RON) that was determined by GC method. While gasoline RON was 75 over E-Cat, it increased over all E-Cat/additives. An improvement of 12 numbers was ob-

tained over ZSM-5 and TNU-9 at 87. Low enhancement in octane rating was observed for both Meso-Z and SSZ-33 (83), as their incremental aromatics were low. Although incremental aromatics over Mezo-Z were higher compared with SSZ-33, the increase in RON over SSZ-33 is attributed to the higher content of iso-paraffins compared with Meso-Z. 4. Conclusions This study has demonstrated the importance of both acidity and mesoporosity on the performance of ZSM-5, Meso-ZSM-5, TNU-9 and SSZ-33 zeolites as additives for FCC catalyst to enhance propylene yield. Using Arabian Light VGO and USY equilibrium cracking catalyst, propylene yield increased from 5.0 wt.% over E-Cat (base case) to 6 wt.% over SSZ-33 and to 9.0 wt.% each over ZSM-5 and TNU-9. Over Meso-Z, which has an MFI topology but with inherent mesopores and highly accessible pore structure, propylene yield was more than doubled, at a gasoline penalty similar to that of ZSM-5 additive. Zeolite acidity and enhanced mass transport were critical issues for the increase in propylene and other light olefins yields over Meso-Z. Olefins-consuming hydrogen transfer reactions were suppressed and secondary reactions were reduced due to shorter residence time and rapid elution of primary cracking products. The low incremental yield of light olefins over other additives was attributed to its higher hydrogen transfer activity. PIONA results showed that the loss in gasoline yield was mainly due to the cracking of gasoline range iso-paraffins and olefins. The advantages of using Mezo-Z as an FCC additive appeared in the low-olefins level and high-octane value for the gasoline fraction. For all additives, gasoline octane number increased by 7–12 numbers due to the significant increase in aromatics level. Acknowledgements The authors acknowledge the financial support provided by King Abdul Aziz City for Science and Technology (KACST) for this research under Refining & Petrochemicals Program of the National Science, Technology & Innovation Plan (NSTIP; 09-PETE85-4). We are also grateful for the support from Ministry of Higher Education, Saudi Arabia for establishing the Center of Research Excellence in Petroleum Refining & Petrochemicals at Fahd University of Petroleum & Minerals (KFUPM). The authors thank J. Cejka and M. Kubu for providing the zeolites TNU-9, SSZ-33, and Meso-Z.

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