Benzene reduction in gasoline by alkylation with olefins: Effect of the experimental conditions on the product selectivity

Applied Catalysis A: General 384 (2010) 115–121

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Benzene reduction in gasoline by alkylation with olefins: Effect of the experimental conditions on the product selectivity Georgina C. Laredo ∗ , Jesus Castillo, Hector Armendariz-Herrera Programa de Investigacion en Procesos de Transformacion, Instituto Mexicano del Petroleo, Lazaro Cardenas 152, Mexico 07730 D.F., Mexico

a r t i c l e

i n f o

Article history: Received 22 March 2010 Received in revised form 4 June 2010 Accepted 10 June 2010 Available online 16 June 2010 Keywords: Gasoline Benzene Alkylation Propylene i-Propylbenzene Beta zeolite

a b s t r a c t The production of a more environmentally friendly gasoline by removing substantial portions of benzene by alkylation with propylene was studied. One step experiments were performed with a real feedstock (20 wt% benzene) varying temperatures (120, 180, 220 and 320 ◦ C) and benzene/propylene molar ratios (1, 2, 3, 5 and 7) in a batch system using Beta zeolite as catalyst. The highest benzene content reduction (52%) was achieved at 220 ◦ C and B/P of 1, although the preferred i-propylbenzene production was attached to high molecular weight poly-alkylated by-products and olefin oligomers, which can trigger catalyst deactivation. The effect of this procedure on the reformation gasoline and on the final gasoline blendstock is also presented. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Presence of benzene on the gasoline pool needs to be minimized because of its known highly carcinogenic properties. Indeed, the Mobile Source Air Toxics (MSAT) rule, published on February 26, 2007, requires that refiners and importers produce gasoline that has an annual average benzene content of 0.62 vol% or less, beginning in 2011 [1,2]. Reforming gasoline accounts for 70–85% of the benzene contained in the gasoline pool according to the EPA [3]. Reforming pre-treatment and post-treatment technologies have been developed in order to reduce the benzene content from this source [3–7]. Benzene alkylation technologies offer increments in octane number and gasoline volume [6,7], although commercial units were not found. Published works related to pure benzene–propylene alkylation processes using zeolites as catalyst, where conditions and kinetic performance were explored [8–24], are focused on i-propylbenzene (cumene) industrial synthesis as a key step intermediate for phenol and acetone production [22]. Among the main compounds produced by benzene alkylation with propylene are i-propylbenzene (IPB), di-i-propylbenzene (DIPB) isomers and olefin oligomers. High purity IPB production is preferred because DIPB isomers will require afterwards benzene transalkylation procedures [25–28]

∗ Corresponding author. Tel.: +52 5591756615; fax: +52 5591758429. E-mail addresses: [email protected] (G.C. Laredo), [email protected] (J. Castillo), [email protected] (H. Armendariz-Herrera). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.06.017

and decomposition by-products and oligomers will affect catalyst life by pore blocking and coke deactivation [10–12]. According to the published information, Beta and MCM-22 zeolites are among the catalyst chosen for IPB synthesis purposes [8–13,15]. ZSM-5 zeolite with smaller pore sizes is unsuitable for liquid phase benzene alkylation with propylene procedures [8–13]. Experimental results found in the literature [11,17–19] show that increasing in reaction temperature, total conversion and conversion of propylene into IPB increases. Selectivity into IPB and oligomers decreases with increasing conversion and temperature, while the opposite occurs with respect to the selectivity of DIPB isomers and n-propylbenzene (PB) [11]. Formation of decomposition products like propylene and bulkier molecules, which block the micropore channels of the catalyst, leading to deactivation, also increases as the temperature increases, however, at higher temperature, bulkier molecules diffuse more quickly, which can prolong the lifetime of the catalyst, so the reaction temperature could not be too low [19]. When the ratio B/P increases, the alkylation of benzene is enhanced, while oligomerization and craking reactions as well as DIPB isomers formation is disfavored [11,17–19], then an excess of benzene is important for optimal alkylation conditions. As in the case of pure benzene alkylation, preferred product for gasoline benzene reduction purposes is also IPB. High amounts of bulkier alkylation by-products (DIPB isomers and others higher molecular weight aromatics) will affect catalyst life, therefore suitable procedures like transalkylation [25–28] or distillation may be implemented to reduce their presence and maintain the product into the gasoline range (30–225 ◦ C at 0.1 MPa [29]). Keeping light oligomers [30] as part of the gasoline pool might increase octane

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number and volume, although their occurrence will also affect catalyst life by coke deactivation [10–12,14,19]. Available information regarding alkylation of the benzene present in real feeds with short chain olefins using zeolites as catalyst was found in patents [31–41]. Recommended catalysts are ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38 [31–41], MCM-22, MCM-36, MCM-49 and MCM-56 zeolites [39,41]. Fixed bed alkylation processes using Beta zeolite as catalyst are mentioned in some patents [31,34–37] however examples using this material are not described. Beta zeolite as catalysts is described to be used only in a catalytic distillation system when a light benzene reformate was alkylated with a simulated off gas mixture (15% ethylene, 5% propylene) [42]. The effect of catalyst on the selectivity after performing benzene alkylation with propylene in a benzene enriched fraction (220 ◦ C and 2/1 B/P molar ratio) is presented by Laredo et al. [43]. Higher benzene conversions (52%) with almost no oligomers formation and relatively high DIPB isomers presence were obtained with Beta zeolite as catalyst. On the other hand, ZSM-5 showed low benzene conversions (16%) associated to high amounts of low molecular weight oligomers (28% propylene based). When using zeolite type catalyst, aromatic/olefins molar ratios ranged from 7.5/1 to 0.5/1 [31–41], although recently, 1/1 and 2/1 molar ratios are chosen [40,41] for benzene content reductions purposes. A benzene rich fraction is preferred as a feedstock in most of the patented available information [35–41]. Studies carried out by Laredo et al. [44] showed that higher benzene contents leaded to higher benzene conversions. A typical heart-cut reformate must have a boiling range falling between 40 and 100 ◦ C and more preferably between 65 and 95 ◦ C [40,41]. Chin et al. [37] and Boghard et al. [38] used cracked olefin feedstock as an alkylating agent for benzene presented in a heartcut fraction and ZSM-5 zeolite as catalyst in order to keep the product within a certain molecular weight. Recent information shows that a two steps process has been preferred being carried out with propylene and MCM-22 at 150–200 ◦ C, and ethylene and ZSM-5 in a second step at 200–400 ◦ C [40,41]. A 50/50 (v/v) FCC light naphtha/reformate cut blend produced benzene conversions around 30% when using ZSM-5 zeolite at 426 ◦ C [37]. Higher benzene conversions (43%) were obtained with a 75/25 (v/v) FCC light naphtha/reformate cut blend at the same experimental conditions. Benzene conversions data from other fixed bed patented processes were not available. Very strong catalyst deactivation was observed when processing real feedstocks [43,44]. This behavior may be due to the formation of high molecular weight by-products and oligomers as a consequence of chemical interactions with indigenous compounds during the alkylation process [44,45]. Olefin compounds even as traces, need to be eliminated before performing the reaction [44]. Reforming gasoline usually has 1–6 vol% of benzene content, and accounts for 30 vol% of the gasoline pool [3]. Therefore, depending on the feedstock, benzene reduction sometimes must be as high as 90% in order to obtain a gasoline final product that can comply with benzene specification (0.62 vol%). The available data have shown that a single step alkylation procedure rarely give benzene conversions higher than 50% when processing real feedstocks [37,43,44], and although cyclic procedures are feasible, formation of high molecular weight by-products and oligomers will increase after each cycle triggering catalyst deactivation [10–12,14,19]. Therefore, the main objective of this work was to study the effect of the experimental conditions in benzene conversion and selectivity when the alkylation process was carried out using an olefin free C6 -reformate heart-cut (19.5 wt% benzene), propylene and Beta zeolite as catalyst [43,44].

Fig. 1. (a) FTIR spectra for Beta catalyst at room temperature and (b) effect of temperature on the FTIR spectra for Beta catalyst.

2. Experimental 2.1. Materials Commercial samples of Beta zeolite (TZB-212E) from Tricat were used as supplied. Propylene (98%) was obtained from INFRA. 2.2. Catalyst characterization Textural properties were determined by nitrogen adsorption–desorption experiments in a Micromeritics ASAP2000 apparatus using the BET method at 77 K. IR Spectra were collected on a Nicolet Protege 460 spectrometer. A pure zeolite sample disk was treated at 500 ◦ C under 1.33 × 10−3 Pa for 5 h. After cooling to room temperature the sample was contacted with pyridine vapor and then treated at 150 ◦ C under high vacuum for 30 min. IR spectra were collected at ambient temperature, 100, 200, 300, and 400 ◦ C (Fig. 1). Results are shown in Table 1. 2.3. Characterization of feedstocks and products Feedstocks and akylation products were subjected to a detailed chemical characterization following the ASTM 6623-01 procedure appropriate for gasoline distillates [46] using an Agilent 6890 series chromatographic system. Resulted chromatograms were analyzed by a ChemStation employing the Hydrocarbon Expert, version 3 software (GC/PIANO). Octane numbers (ON) and final boiling point (FBP) temperatures according to the ASTM-D86 correlation [47] were provided by the same software.

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2, 3, 5 and 7 (Table 2). 1 g of freshly activated (4 h, 500 ◦ C, steam) catalyst was added to the reactor. The reaction took place under stirring (900 RPM) at the temperature indicated (120, 180, 220 or 320 ◦ C), at 3.1–4.8 MPa of pressure raised with nitrogen. After 1 h, the reaction was stopped and liquid phase aliquots were withdrawn from the microreactor avoiding removal of the catalyst. In all the experiments, samples were analyzed following the same procedure already mentioned in the characterization of feedstocks and products section [46]. Data obtained are shown in Table 2.

Table 1 Acidic and textural properties of the Beta catalyst. Particle propertiesa Si/Al ratio Particle size (mm) Channel dimensions (nm)

30 0.42-0.84 0.57 × 0.75 and 0.56 × 0.65

Textural properties BET area (m2 /g) Micropore area (m2 /g) Pore volume (cm3 /g) Micropore volume (cm3 /g)

499 289 0.96 0.13

Acidity (␮mol of pyridine/g)

Temperature (◦ C)

Br␾nsted Lewis

200 66 269

a

117

2.6. Mathematical treatment of the data

300 37 229

400 21 196

(A) Benzene, toluene and propylene conversions.

Data obtained from the manufacturer.

Conversions (Y) were obtained by the following equation:

2.4. Feedstocks preparation

Yi = 100

The C6 reformate heart-cut (C6 -RHC) was obtained by carefully distilling twice the 45–80 ◦ C (at 585 mmHg) fraction from a refinery reforming product (yield: 23 vol%) employing a 100 plates spinning monel-nickel band distillation system D/R Instrument Corporation 24/100 Automatic. The remaining fraction (77 vol%) had a benzene content of 0.8 vol%. Small proportions of alkene compounds were eliminated by a standard hydrogenation procedure [48]. Therefore, the C6 -RHC fraction was hydrotreated in a fixed bed pilot plant at the following conditions: 280 ◦ C, LHSV of 4 h−1 , hydrogen to oil ratio of 464 m3 /m3 , and 2.8 MPa pressure, employing a commercial CoMo/␥-alumina catalyst (IMP-DSD11 [48]). Composition of this hydrotreated C6 -RHC feed is shown in Table 2. 2.5. Batch alkylation experiments Experiments were performed by mixing 100 g of the C6 -RHC feedstock (20 wt% benzene content, Table 2) and the liquidized propylene required for achieve the desired B/P molar ratio of 1,

I − I  0

(1)

I0

where I could be benzene (Bz), toluene (T) or propylene (Pr) and 0 stands for time zero (feed). Concentration values employed were in weight. (B) Selectivity.

Sp = 100 ×

Pi

j i

(2)

P

where Pi−j stands for i-propylbenzene (IPB), n-propylbenzene (PB), 1,2-, 1,3- and 1,4-di-propylbenzene (o-, m-, and p-DIPB), 1,2-, 1,3-, and 1,4-methyl-i-propylbenzene (o-, m-, and p-MIPB). Presence of unidentified high molecular weight aromatic compounds (HWA) and oligomers (UO) were also included. Concentration values in weight were used taking into account their concentration in the initial feedstock.

Table 2 Chemical composition of the products obtained after alkylating the C6 -RHC feedstock with propylene using Beta catalyst at different experimental conditions. Name

Feed

1-120

1-180

2-180

3-180

5-180

7-180

1-220

2-220

3-220

5-220

7-220

2-320

3-320

120 1/1 10.7

180 1/1 10.7

180 2/1 5.3

180 3/1 3.6

180 5/1 2.1

180 7/1 1.5

220 1/1 10.7

220 2/1 5.3

220 3/1 3.6

220 5/1 2.1

220 7/1 1.5

320 2/1 5.3

320 3/1 3.6

25.30 51.08 22.01 19.82 2.01 0 0 0 0 0.01 0 0 0.16

22.41 50.54 23.87 18.60 1.31 3.44 0.01 0.03 0.04 0.04 0.10 0.08 0.24

19.37 46.95 29.72 11.48 1.03 12.77 0.01 0.22 0.31 0.24 1.56 1.90 0.20

19.47 47.34 29.78 13.38 1.18 11.62 0.01 0.21 0.27 0.16 1.31 1.44 0.21

20.05 48.53 28.53 16.50 1.41 8.54 0.00 0.14 0.16 0.11 0.74 0.69 0.23

20.71 49.65 26.99 19.01 1.59 5.33 0.00 0.09 0.10 0.08 0.29 0.24 0.25

20.97 50.28 26.13 19.74 1.67 3.96 0.00 0.06 0.08 0.07 0.18 0.11 0.26

18.88 44.65 31.09 9.52 1.00 13.60 0.01 0.55 0.40 0.24 3.08 2.50 0.19

19.13 45.66 30.76 12.58 1.23 11.84 0.01 0.41 0.30 0.11 2.62 1.42 0.22

19.81 46.80 29.42 15.88 1.51 9.12 0.01 0.34 0.20 0.04 1.45 0.58 0.27

20.41 48.50 27.35 17.89 1.64 6.09 0.01 0.23 0.14 0.03 0.80 0.23 0.29

20.60 49.14 26.71 19.11 1.72 4.59 0.01 0.17 0.09 0.01 0.59 0.13 0.29

19.27 48.80 26.12 13.84 1.55 7.55 0.53 0.33 0.16 0.02 0.93 0.41 0.80

19.50 48.07 27.65 17.00 2.02 6.00 0.56 0.22 0.10 0.01 0.48 0.18 1.08

0 0 0 1.60 0.01 0

0.02 0 0.24 1.83 0.53 0.56

0.14 0.01 0.52 1.75 0.19 1.36

0.12 0 0.37 1.76 0.11 1.04

0.13 0 0.22 1.80 0.03 0.71

0.15 0 0.18 1.82 0.03 0.46

0.15 0 0.18 1.85 0.03 0.41

0.78 0.03 0.57 1.65 0.23 2.12

0.43 0.02 0.51 1.72 0.06 1.72

0.28 0.01 0.50 1.78 0.05 1.36

0.20 0.01 0.47 1.79 0.08 1.19

0.20 0.01 0.43 1.81 0.06 1.04

1.20 0.03 0.75 1.43 0.19 2.21

0.65 0.07 0.51 1.48 0.20 1.86

102.0 81.5

145.8 80.5

190.4 81.3

189.4 81.2

186.2 80.8

184.1 80.6

182.9 80.4

223.8 82.5

214.4 81.8

190.4 81.3

177.9 80.9

177.2 80.6

185.0 81.5

169.8 81.2

Experimental conditions Temperature B/P molar ratio Propylene weight (g) Chemical composition (wt%) Paraffin I-Paraffins Aromatics Benzene Toluene i-Propylbenzene n-Propylbenzene 1-Methyl-3-i-propylbenzene 1-Methyl-4-i-propylbenzene 1-Methyl-2-i-propylbenzene 1,3-Di-i-propylbenzene 1,4-Di-i-propylbenzene Other aromatic compounds Naphthalenes Naphtheno/olefino-benzenes Indenes Naphthenes Olefins Oligomers Final boiling point (◦ C) (RON + MON)/2

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Table 3 Conversion and selectivity obtained after alkylating the C6 -RHC feedstock with propylene using Beta catalyst at different experimental conditions. Name

1-120

1-180

2-180

3-180

5-180

7-180

1-220

2-220

3-220

5-220

7-220

2-320

3-320

Conversion (%) Benzene Toluene Propylene

6.2 35.0 12.4

42.1 48.7 61.0

32.5 41.2 100

16.7 30.0 100

4.1 21.1 100

0.4 17.1 100

52.0 50.2 75.3

36.5 39.0 100

19.9 25.0 100

9.7 18.3 100

3.6 14.5 100

30.2 22.6 100

14.2 0.1 100

Selectivity (%) i-Propylbenzene n-Propylbenzene 1-Methyl-3-i-propylbenzene 1-Methyl-4-i-propylbenzene 1-Methyl-2-i-propylbenzene 1,3-Di-i-propylbenzene 1,4-Di-i-propylbenzene Heavy hydrocarbons Oligomers

74.8 0.1 0.6 0.8 0.6 2.1 1.7 7.4 11.9

67.1 0.0 1.2 1.6 1.2 8.2 10.0 3.6 7.1

70.1 0.0 1.3 1.6 0.9 7.9 8.7 3.3 6.2

74.3 0.0 1.2 1.4 0.8 6.4 6.0 3.7 6.1

76.2 0.1 1.3 1.5 0.9 4.2 3.4 6.0 6.5

75.0 0.1 1.2 1.5 1.1 3.3 2.2 8.1 7.6

56.9 0.1 2.3 1.7 0.9 12.9 10.5 5.9 8.8

61.0 0.1 2.1 1.5 0.5 13.5 7.3 5.2 8.8

65.2 0.1 2.5 1.4 0.2 10.4 4.2 6.3 9.7

64.1 0.1 2.4 1.5 0.1 8.4 2.5 8.5 12.4

62.3 0.1 2.3 1.3 -0.1 8.0 1.7 10.3 14.0

51.3 3.6 2.3 1.1 0.0 6.3 2.8 17.7 14.9

52.0 4.9 1.9 0.9 0.0 4.2 1.6 18.6 16.0

3. Results and discussion 3.1. Characterization of the Beta catalysts FTIR spectra of pyridine adsorbed on the Beta catalysts collected at different temperatures are shown in Fig. 1. FTIR spectra at room temperature presented pyridinium ion signals (pyridine on Br␾nsted acid sites) at wave numbers around 1635 (8a ring vibrations) and 1545 cm−1 (19b ring vibrations) and the coordination covalent bond formation between the free electron pair of nitrogen in pyridine with Lewis acid sites due to electron deficient aluminum atoms in the solid, assigned to the bands at 1620 (8b ring vibrations) and 1450 cm−1 (19b ring vibrations). The signal at 1490 cm−1 (19a ring vibrations) was assigned to pyridine on both Br␾nsted and Lewis acid sites [18,49–54]. As the temperature rose, FTIR adsorption bands of pyridine became weaker, which means part of the pyridine molecules adsorbed on weak acid sites (physisorbed pyridine) was desorbed [18] as can be seen by the evolution of the signal at 1445 cm−1 . As the pyridine evacuation temperature increased the intensity of the pyridinium ion FTIR signals decreased more quickly than those associated to covalently bound pyridine, meaning that Lewis acid sites on Beta zeolite showed a higher acid strength than Br␾nsted acid sites [18,49,54]. Concentration of Br␾nsted and Lewis acid sites obtained by measurements of the 1545 and 1450 cm−1 peak areas respectively, are shown in Table 1. 3.2. Alkylation of the benzene present in the C6 -RHC feedstocks with propylene Results from alkylation reactions of benzene presented in the C6 -RHC and propylene at different experimental conditions are shown in Table 2. Employing these data and Eq. (1), reaction conversion related to initial benzene, toluene and propylene concentrations were obtained. The results are shown in Table 3. As the B/P molar ratio increased, propylene availability for reaction was reduced, and therefore benzene conversion decreased (Fig. 2). A maximum benzene conversion was attained a 220 ◦ C, followed by 180, 320 and finally 120 ◦ C. 120 ◦ C was a temperature not high enough for a complete benzene alkylation leaving non-reacted propylene behind. Benzene conversions were not improved by a higher temperature (320 ◦ C). Propylene was completely depleted from the experiments performed at temperatures above 120 ◦ C when B/P molar ratios higher than 2/1 were employed (Table 3). According to Table 3, a small proportion of alkylated toluene isomers: 1,2-, 1,3- and 1,4-MIPB were also formed. Toluene conversions followed the same trend that the benzene analog. Although

toluene was in a smaller amount than benzene (10%), it can be seen the higher reactivity presented by this compound even at 120 ◦ C [55]. Higher toluene reactivity is the main reason for the use of a benzene heart-cut fraction, in order to avoid the waste of the alkylating agent and the formation of high molecular weight byproducts. In this case, less than 5% of the propylene was used in toluene alkylation. Toluene conversion was also higher at 220 ◦ C, followed by 180, 120 and finally 320 ◦ C. Distributions of the alkylation products obtained at different experimental conditions were calculated with Eq. (2) (Table 3). A summary of these results by grouping all alkylation products except IPB, as high molecular weight aromatics (HWA), and oligomers (UO) is shown in Fig. 3. From this figure it could be seen that IPB selectivity reached a maximum at higher B/P molar ratios and lower temperatures, unfortunately the benzene conversion was very low. At a fixed B/P molar ratio, IPB selectivity increased in the order: 320 ◦ C < 220 ◦ C < 180 ◦ C < 120 ◦ C [11,17–19]. Formation of the alkylation by-products: n-propylbenzene, methyl-isopropylbenzene and di-isopropylbenzene isomers, other higher molecular weight aromatics, and oligomers, increased as the temperature increased (Fig. 3). The observed results agreed with published results when performed pure benzene alkylation studies [11,17–19]. In Fig. 4, the effect of the B/P molar ratio on the IPB/PB ratio is shown. At 180 ◦ C selectivity towards IPB formation was favored, while higher and lower temperatures showed less selectivity. A very high n-propylbenzene (PB) formation was observed for the experiments at 320 ◦ C. Formations of this by-product may be activated by higher temperatures assuming that this compound is the

Fig. 2. Effect of the B/P molar ratio on benzene conversion during the alkylation of the benzene present in a C6 -RHC with propylene at different temperatures.

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Fig. 3. Effect of the B/P molar ratio on selectivity during the alkylation of benzene present in a C6 -RHC with propylene at different temperatures. Benzene conversion (䊉).

Fig. 4. Effect of the B/P molar ratio on the IPB/PB ratio during the alkylation of benzene present in a C6 -RHC with propylene at different temperatures.

result of IPB successive reactions [9]. Optimums IPB/PB ratios were obtained at B/P 2/1 and 3/1 and 180 ◦ C. Nevertheless, formation of small quantities of PB does not represent a problem for benzene in gasoline reduction purposes. Fig. 5 shows the effect of the B/P molar ratio and temperature on the IPB/DIPB ratio. When the ratio B/P increased, the alky-

Fig. 5. Effect of the B/P molar ratio on the IPB/DIPB ratio during the alkylation of benzene present in a C6 -RHC with propylene at different temperatures.

119

Fig. 6. Effect of the B/P molar ratio on the IPB/HWA ratio during the alkylation of benzene present in a C6 -RHC with propylene at different temperatures.

lation of benzene was enhanced, while DIPB isomers formation was disfavored. Temperature observed trend towards DIPB isomers formation was 120 ◦ C < 320 ◦ C < 180 ◦ C < 220 ◦ C. In a cyclic process, these high boiling point DIPB isomers are not considered a waste because they may be transformed in IPB by a transalkylation process with more benzene [25–28]. 1,3-DIPB (meta) and 1,4-DIPB (para) isomers distribution similar to the obtained by Corma et al. [11] for benzene alkylation processes catalyzed by Beta zeolite was found. In the process performed with a real feed, Halgeri and Das [26] and Tsai et al. [27] observed the same distribution. 1,2-DIPB was not detected (Table 2). 1,3- and 1,4-preferred isomer formation was also observed for toluene alkylation products (MIPB isomers) [9]. The effect of the B/P molar ratio in the formation of high molecular weight aromatics is shown in Fig. 6 (IPB/HWA). As in the case of the DIPB isomers, formation of these heavier compounds was favored by higher temperatures and lower B/P molar ratios [11,17–19]. These high molecular weight aromatics could be formed by multi-alkylation [19] and alkylation of traces of indigenous aromatic hydrocarbons. Presence of these compounds may form coke and affect catalyst life [19], and in this particular case, the chemical properties of the final gasoline product. Therefore their concentration must be kept as low as possible. Distillation of a sharp C6 -RHC before the alkylation process will help to reduce the formation of higher molecular weight aromatics. Usually, oligomers (UO) formation decrease as B/P molar ratio increase as it has been described in pure benzene alkylation experiments [11,17–19]. However in this case, according to Fig. 3, except for the 120 ◦ C temperature experiment, oligomers formation appeared to increase as the temperature and B/P molar ratio increased [11,43]. In order to clarify this point, Fig. 7 was prepared. IPB/UO ratio slightly decreased as the B/P molar ratio increased, meaning that oligomers formation was disfavored by this increment. It seems that oligomers formation did not relay only on the propylene presence, but on traces of olefin compounds left by the hydrotreatment process too. Higher temperatures may trigger oligomers production and therefore adversely affect catalyst life [10–12,14,19,43]. In order to elucidate which conditions were the most favorable for higher conversion and a better quality gasoline production by alkylation of the benzene presented in a real feed, the following equation was used: RFF = YBz

DIPB (100 − DIPB )

(3)

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Fig. 7. Effect of the B/P molar ratio on the IPB/UO ratio during the alkylation of benzene in a C6 -RHC with propylene at different temperatures.

Fig. 8. Effect of the experimental conditions on the real feedstock factor (RFF, Eq. (3)) during the alkylation of benzene present in a C6 -RHC with propylene at different temperatures.

3.3. Benzene in gasoline after performing the alkylation process RFF stands for real feed factor, YBz is benzene conversion (Eq. (1)), and DIPB is the isopropylbenzene percent distribution (Eq. (2)). The results are shown in Fig. 8, clearly showing that best experimental conditions are 180 ◦ C and B/P molar ratio of 1/1. From acidity characterization data (Table 1), it can be seen that the catalyst at the experimental conditions used in this study had a weak Br␾nsted acid strength [18]; therefore, in order to carry out the reaction at a lower temperature which favors IPB selectivity, a different catalyst with a higher Br␾nsted acid strength may presents advantages on reducing formation of by-products [26]. According to Table 2, none of the products presented a final boiling point higher than 225 ◦ C [29] and only two products (1-220 and 2-220) presented octane numbers ((RON + MON)/2) higher than the departing feed.

Considering conditions for higher benzene reduction (220 ◦ C and B/P molar ratio of 1/1) the benzene presented in the C6 -RHC fraction went from 19.5 wt% to 9.5 wt% (15.1 vol% and 7.7 vol%, respectively). Formation of alkylated products increased the volume of the C6 -RHC from 23 vol% to 26 vol%. Mixing of this product with the reforming gasoline remaining fraction (77 vol%, with a benzene content of 0.8 vol%), produced a reforming gasoline with a benzene content of 2.6 vol%. Taking into account that 30 vol% of the blendstock is reforming gasoline [3], and the remaining 70 vol% of other distillates have an average benzene content of 0.3 vol%, the final gasoline product will have 1.0 vol% of benzene (Fig. 9). This number does not comply with the actual benzene specification (0.62 vol% maximum). In order to do that, a C6 -RHC’s alkylated product must have a maximum of 2.5 vol% (2.8 wt%) of benzene.

Fig. 9. Properties of the gasoline obtained from the alkylated reformate presented in this paper. ON, octane number.

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The required benzene conversion (86%) may be reached by implementation of a cyclic process, although formation of alkylation by-products (high molecular weight aromatics and oligomers) are expected to be high, allowing catalysis deactivation to take place [10–12,14,19]. Therefore, in order to have a continuous procedure where a portion of the deactivated coked catalyst could be withdrawn, regenerated, and returned to the reaction zone, the whole alkylation process need to be performed in fluidized bed reactors [33–38]. 4. Conclusions Studies on the benzene content reduction in gasoline by alkylation with propylene catalyzed by Beta zeolite at different experimental conditions (benzene/propylene molar ratios and temperature) in one step batch process showed that although a 52% benzene reduction was obtained at 220 ◦ C and a 1/1 B/P molar ratio, the desired mono-alkylated product came attached to 43% of alkylation by-products: high molecular weight aromatics and oligomers. At 180 ◦ C and a 1/1 B/P molar ratio benzene conversion was of 42% and the alkylation by-products presence was of 33%. A further reduction in temperature (120 ◦ C) increased the IPB selectivity in the alkylated product (75%) although benzene conversion was also drastically reduced (6%). Experiments performed at 320 ◦ C, did not show benefits either for benzene conversion or product quality. In all cases, increasing of the B/P molar ratio, increased the quality of the product, but benzene conversion was also drastically reduced. Finally, calculations showed that in order to comply with benzene in gasoline specification (>0.62 vol%), conversions higher than 86% of the benzene from a typical C6 -reformate heart-cut fraction (20 wt% benzene), must be attained. References [1] Regulatory announcement: Direct final rulemaking revising mobile source air toxics early credit technology requirement. EPA420-F-08-007, February 2008. http://www.epa.gov/oms/regs/toxics/420f08007.htm. [2] Rules and Regulations, Federal Register, 73 (49) (Wednesday, March 12), 2008. [3] R.E. Palmer, R. Shipman, S. Kao, Options for reducing benzene in the refinery gasoline pool. ACS, Petroleum Division Annual Meeting (San Diego 3/811/2008) AM-08-10. [4] R.F. Coldwell, Benzene in gasoline. Regulation & remedies, Process. Engineering Associates LLC, 2008, http://www.processengr.com/ppt presentations/msat regulation options.pdf. [5] V. Nispel, D. Varraveto, How refineries meet the new EPA benzene regulations. Burns & Mc Donnell. Engineering. Architecture, Construction, Environmental and Consulting Solutions, 2008. [6] B. Umansky, M.C. Clark, X. Zhao, Hydrocarbon Eng. 12 (2007) 61–62. [7] UOP Inc., Chem. Eng. 97 (1990) 23. [8] T.F. Degnan Jr., C. Morris Smith, C.R. Venkat, Appl. Catal. A 221 (2001) 283–294. [9] C. Perego, P. Ingallina, Catal. Today 73 (2002) 3–22. [10] G. Bellusi, G. Pazzuconi, C. Perego, G. Girotti, G. Terzoni, J. Catal. 157 (1995) 227–234. [11] A. Corma, V. Martinez-Soria, E. Schnoeveld, J. Catal. 192 (2000) 163–173. [12] M. Han, S. Lin, E. Roduner, Appl. Catal. A 243 (2003) 175–184. [13] G. Buelna, R.L. Jarek, S.M. Thornberg, T.M. Nenoff, J. Mol. Catal. A: Chem. 198 (2003) 289–295.

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