Fluid catalytic cracking feed hydrotreatment and its severity impact on product yields and quality

Fuel Processing Technology 94 (2012) 16–25

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Fluid catalytic cracking feed hydrotreatment and its severity impact on product yields and quality Dicho S. Stratiev ⁎, Ivelina K. Shishkova, Dimitar S. Dobrev Lukoil Neftochim Burgas, Bulgaria, 8104 Burgas, Bulgaria

a r t i c l e

i n f o

Article history: Received 13 April 2011 Received in revised form 6 October 2011 Accepted 17 October 2011 Available online 11 November 2011 Keywords: FCC Hydrotreatment Ultra low sulfur gasoline Near zero sulfur gasoline

a b s t r a c t This paper investigates the effect of fluid catalytic cracking (FCC) feed hydrotreatment and its severity increase on product yields and quality obtained in a commercial and a laboratory MAT FCC units. The hydrotreatment of Ural heavy vacuum gas oil reduces not only sulfur, nitrogen, Conradson carbon and metals content in the FCC feed but also increases the mononuclear aromatic hydrocarbons content by 8% absolute at almost no change in the total aromatics content. Regardless of this 8% increase of the mononuclear aromatics in the hydrotreated FCC feed the conversion increase in both commercial and laboratory MAT units was only 2%. The severity increase in the FCC feed hydrotreater leads to a higher conversion in the FCC, higher hydrogen transfer rate that results in higher isobutane/butylenes ratio, lower gasoline olefins content, and higher gasoline motor octane number. The hydrotreatment of the Ural heavy vacuum gas oil exhibited the same changes in FCC catalyst selectivities: lower coke and LCO selectivities and higher gasoline selectivity in both commercial riser FCC unit that has between 2 and 3 s time on stream, and the fixed bed reactor MAT unit, that has 30 s time on stream. © 2011 Elsevier B.V. All rights reserved.

1. Introduction From creation of fluid catalytic cracking (FCC) to nowadays it has become one of the most important and profitable process in oil refining. This is due to ability of the FCC process to convert different low value high molecular oil fractions into high value gasoline, diesel and low molecular olefins. During the last years the key role of the FCC in oil refining has become much more remarkable because refiners have been compelled to process heavier and sour crudes and produce from them environmental friendly transportation fuels with ultra low and near zero sulfur content. It is well known that FCC gasoline contributes to about 90% of the sulfur in the finished refinery gasoline [1]. That is why hydrotreatment of FCC feed or post treatment of the FCC gasoline, or both is obligatory [2]. While there is no doubt that pretreating of FCC feed is the preferred route to low sulfur gasoline, few refiners enjoy that luxury because the cost of installing such hardware is beyond the capital spending capabilities of all but the largest refiners [3]. There have been numerous articles on the effect and benefit of FCC hydrotreatment and all have shown

⁎ Corresponding author. Fax: + 359 5511 5555. E-mail address: [email protected] (D.S. Stratiev). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.10.014

similar trends [1–24]. The benefit of the FCC feed hydrotreatment can be summarized as follows: – Increasing of PNA compound saturation which improves the FCC feed crackability and increases FCC conversion at the same operating conditions. Depending on feed composition FCC conversion improvement can vary greatly. – Increasing of the FCC feed hydrogen content which enhances gasoline production as a result of higher feed conversion and better selectivity. – Decreasing of coke selectivity as a result of PNA and Conradson carbon reduction of the FCC feed. – Reducing of SOx and NOx emissions from FCC regenerator. – Producing of more environmentally friendly gasoline and diesel and changing of sulfur distribution in FCC products. – Removing of catalyst poison like heavy metals and nitrogen which increases FCC conversion and selectivity and allows refiners to process more heavier feeds. – Reducing of FCC catalyst consumption. – i-butane/butylenes ratio increasing which correlates to better gasoline selectivity because of diminishing of overcracking. Most studies on the effect of feedstock hydrotreatment on product yields and quality in the FCC process have been carried out on laboratory scale. However, little is published about the same topic investigated on commercial units when hydrotreatment severity is changed. This

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

Ratio FCC Feed sulfur / FCC gasoline sulfur

35

17

of the VGO HDS was 32.6 kcal/mol [22]. It was also found there that catalyst deactivation rate could be approximated by the expression:

30 25

Catalyst deactivation ¼ 6  10

−7

0:0408 WABT ∘

e

; C=month

20 y = -0.44x + 111.8 2 R = 0.9878

15

where, WABT WABT

10

weight average bed temperature Reactor inlet temperature+ 2/3 (Reactor outlet temperature − Reactor inlet temperature).

5 0 175

180

185

190

195

200

205

210

215

End boiling point of FCC gasoline Fig. 1. Dependence of ratio FCC feed sulfur/FCC gasoline sulfur on end boiling point of FCC gasoline.

study aims to fill the gap by investigating the effect of feedstock hydrotreatment severity on conversion, product yields and quality at the Lukoil Neftochim Bourgas commercial FCC unit. 2. Near zero sulfur gasoline production in Lukoil Neftochim Bourgas

WABT,0C

The Lukoil Neftochim Bourgas, Bulgaria (LNB) is a FCC based refinery. Since 2009 the LNB has been producing near zero sulfur gasoline (NZSG) (less than 10 ppm) by applying FCC feed hydrotreatment and later in 2010 by applying also Prime G FCC gasoline post treatment technology. Fifteen ppm sulfur in FCC gasoline is the cap for production of NZSG in the LNB refinery. It is well known that FCC gasoline sulfur depends on gasoline cut point [21,25–27]. For the case of cracking of hydrotreated feed in the LNB FCC unit it was found that ratio between feed sulfur and gasoline sulfur varies between 20 and 33 when gasoline FBP varies between 180 and 210 °C (Fig. 1). Based on the data in Fig. 1 one can calculate that 15 ppm sulfur in FCC gasoline will require 300–400 ppm sulfur in the FCC feed if the gasoline FBP does not exceed 200 °C. The sulfur content in the FCC hydrotreated feed, however, determines the FCC feed hydrotreater catalyst cycle length. In an earlier study, performed at the LNB FCC feed hydrotreater it was found that the VGO hydrodesulphurization (HDS) could to be described by 1.6 order kinetic expression and the activation energy

Based on these findings and the level of sulfur in the FCC feed hydrotreater feed and in the hydrotreated vacuum gas oil at the start of the run taking into account that the maximum allowable reactor temperature in the LNB FCC hydrotreater reactors is 420 °C, which is equivalent to 415 °C WABT an estimation of the cycle length for four sulfur levels in the FCC feed was performed (Fig. 2). It can be seen from the data in Fig. 2 that reduction in sulfur of the hydrotreated FCC feed from 3000 ppm to 200 ppm leads to shortening of the FCC feed hydrotreater cycle length from 27 to 6 months. Increasing of severity in the LNB FCC feed hydrotreater and decreasing of FCC gasoline FBP allowed the LNB to produce NZSG for 12 months only by using the FCC feed hydrotreatment technology. In the middle of 2010 after start up of the Prime G unit the FCC feed hydrotreater severity was decreased. As a result of decreasing the severity in the FCC feed hydrotreater changes in the FCC product yields and qualities were observed. 3. Material and methods Commercial investigations were carried out on the LNB FCC unit. The LNB FCC unit consists of feed hydrotreater section, FCC reactor– regenerator and main fractionator section, vapor recovery section and a Merox unit (Figs. 3 and 4). The LNB FCC reactor is equipped with the modern UOP VSS riser termination device and the UOP Optimix feed injection system. The FCC feed hydrotreater section employed the Topsøe TK-558 Brim Co–Mo hydrotreating catalyst and the operating conditions at which the LNB FCC feed hydrotreater was investigated are summarized in Table 1. The FCC catalyst employed during the study was a partially exchanged RE-USY zeolite containing octane catalyst. Its physical and chemical properties in equilibrium state are summarized in Table 2.

415 410 405 400 395 390 385 380 375 370 365 360 355 350 345 340 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Run, months 3000 ppm

2000 ppm

400 ppm

200 ppm

Fig. 2. The LNB FCC feed hydrotreater catalyst cycle length for four sulfur levels in the FCC hydrotreated feed.

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D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

Fig. 3. Diagram of hydrotreating, FCC reactor–regenerator and fractionation sections in the Lukoil Neftochim Bourgas FCC unit.

Typical feed for the LNB FCC unit is heavy vacuum gas oil distilled from Ural crude. Table 3 presents physical and chemical properties of the straight run Ural heavy vacuum gas oil and of the vacuum gas oils hydrotreated at different severities.

The straight run Ural heavy vacuum gas oil and that vacuum gas oil obtained by hydrotreatment at low severity were cracked in a laboratory MAT FCC unit manufactured by Vinci Technologies (Fig. 5). The unit operates continuously for up to 8 cycles, i.e. stripping-

Fig. 4. Diagram of vapor recovery section and Merox unit in the Lukoil Neftochim Bourgas FCC unit.

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

19

Table 1 Operating conditions and yield distribution in the LNB FCC feed hydrotreater. FCC feed hydrotreater LHSV, h− 1 First hydrotreating reactor inlet temperature, °C Second hydrotreating reactor inlet temperature, °C Second hydrotreating reactor outlet temperature, °C WABT, °C First hydrotreating reactor inlet pressure, kg/cm2 Second hydrotreating reactor inlet pressure, kg/cm2 Second hydrotreating reactor outlet pressure, kg/cm2 Hydrogen/oil ratio, Nm3/m3 FCC feed hydrotreater yields, % Gas Naphtha Diesel Stable hydrogenate (hydrotreated heavy VGO) H2S Conversion 360 °C +a a

1.2 333 352 360 351 49.0 47.4 44.4 310

1.2 347 370 375 366 49.2 47.1 44.5 310

0.21 0.32 4.58 93.59 1.3 3.7

0.27 0.30 6.41 91.42 1.6 6.3

1.2 356 379 384 375 49.2 47.1 44.5 310

1.2 368 388 391 383 49.4 47.3 44.2 310

0.32 0.53 6.81 90.74 1.6 7.6

0.42 0.39 7.25 90.34 1.6 10.8

1.2 374 397 399 391 49.4 47.4 44.3 310 0.44 0.62 12.52 84.82 1.6 13.7

1.2 350 376 381 371 49.0 47.0 45.0 310 0.36 0.36 6.51 91.17 1.6 6.5

Conversion, defined as (360 °C + feed − 360 °C + product) / 360 °C + feed.

Table 2 Physical and chemical properties of commercial equilibrium FCC catalyst. Chemical composition Al2O3, wt.% Na2O, wt.% Re2O3, wt.% Fe, wt.% V, ppm Ni, ppm FACT conversion, wt.%

40.2 0.13 1.90 0.56 25 195 74

Physical properties APS, μm ABD, g/cm3 BET matrix surface area, m2/g BET surface area, m2/g

81 0.88 56 165

reaction-regeneration, without operator assistance. The reaction zone and product recovery system are designed in accordance with ASTM D3907. The severity in the MAT unit was varied by changing ratio

between catalyst and oil from one to five. The amount of catalyst was kept constant (6 g) and amount of feed was varied. Before each experiment the system was purged for 30 min with N2 (30 ml min − 1) at the reaction temperature. Laboratory experiments were carried out at a reaction temperature of 527 °C and a catalyst time-on-stream duration of 30 s. After vacuum gas oil cracking, the catalyst was stripped for 15 min using 30 ml min − 1 N2. During the reaction and stripping phases liquid products were collected in cooled glass receivers (5 °C by means of computer-controlled bath) located at the exit of the reactor. Gaseous products were collected in a gas burette by water displacement. Gasses were automatically analyzed after stripping in a Hewlett Packard GC (HP 5890 series II) equipped with a dual column system and two detectors (TCD and FID). The catalyst was simultaneously regenerated in air (100 ml min − 1) at 550 °C for 2 h. The CO2 generated during coke combustion was automatically analyzed by continuously sampling using a CRISTAL 300 apparatus equipped with an infrared detector. Liquid product analysis was performed by simulated distillation (ASTM D2887). Conversion is defined as 100 − (LCO + HCO). Light

Table 3 Physical and chemical properties of the vacuum gas oils unhydrotreated and hydrotreated at different severities. Properties

Straight run Ural heavy vacuum gas oil

Density, d420 0.9187 Sulfur content, % 1.50 Nitrogen, wt. ppm 1100 Refractive index at 20 °C 1.5103 Conradson carbon, % 0.32 Ni, ppm 0.12 V, ppm 0.37 Kinematic viscosity at 98.9 °C, mm2/s 7.66 Aromatics distribution 1 ring, wt.% 18.96 2 ring, wt.% 9.76 3 + ring, wt.% 10.84 Total aromatics, wt.% 39.56 Kw 11.83 Molecular weight calculated by 379 the correlation of Goossens [30] Hydrogen content estimated by 12.4 the correlation of Goossens [31], wt.% Aromatic carbon estimated by 16.2 the correlation of Dhulesia [33], wt.% Gasoline precursors estimated by 77.7 the correlation of Stratiev [34], wt.% Distillation ASTM-D1160, °C 5% 374 10% 397 50% 454 90% 528 95% 548

Low severity hydrotreated heavy vacuum gas oil High severity hydrotreated heavy vacuum gas oil (Hydrotreater reactor inlet temperature = 347 °C) (Hydrotreater reactor inlet temperature = 374 °C) 0.9016 0.1505 720 1.5010 0.12 0.06 0.13 6.35

0.8996 0.0351 500 1.4999 0.05 0.05 0.012 6.46

26.44 6.45 5.83 38.72 12.07 365

25.32 6.71 6.38 38.41 12.06 366

12.7

12.8

14.3

13.3

80.3

81.2

359 380 440 511 532

361 381 440 515 537

20

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

Fig. 5. MAT unit scheme.

cycle oil (LCO) and heavy cycle oil (HCO) are the yield fractions in the cracking products as wt.% of the feed with cut-points of 210 °C b LCO b 360 °C b HCO. All MAT experiments were performed using the commercial equilibrium catalyst described in Table 2. The catalyst was calcined for 3 h in flowing air at 540 °C to remove the carbon. The cracked gasoline was analyzed by gas chromatography techniques. Quantification of the different compounds was accomplished by the use of a gas chromatograph (model 5890 series II Hewlett Packard (Agilent Technologies, Inc., USA)) equipped with a flame ionization detector, split injector, and a capillary column, HP PONA (50 m length × 0.20 mm id × 0.5 μm film thickness). The instrument parameters were as follows: initial oven column temperature of 40 °C, temperature increase with a rate of 2 °C min − 1 till attainment of 130 °C and second temperature gradient of 5 °C min − 1 till achievement of 180 °C retention for 20 min at 180 °C. Helium was used as a carrier gas at a flow rate of 0.5 mL min − 1. The injector and the detector temperatures were 250 °C and 260 °C respectively. The volume that was injected and analyzed was 0.1 μL.

100.0

Conversion, % wt.

4. Results and discussion 4.1. Influence of severity of FCC feed hydrotreatment on the quality of the FCC hydrotreated feed

120.0

80.0 60.0 40.0 20.0 0.0 330

For identification of the gasoline compounds gas chromatography/ mass spectrometry was utilized. Gas chromatography/mass spectrometry analysis was performed with a 7890A GC System equipped with a HP PONA 50 m length × 0.2 mm id × 0.5 μm film thickness, capillary column and 5975 C Inert XL EI/CI mass selective detector (Agilent Technologies, Inc., USA). The oven column temperature conditions identical to those used with the gas chromatograph with flame ionization detector. High purity helium was used as a carrier gas at a flow rate of 0.8 mL min − 1. The injection port was held at 250 °C and the injection volume of sample 0.1 μL. The mass selective detector operated in the electron impact ionization mode (70 eV) with continuous scan acquisition from 15 to 250 m/z at a cycling rate of approximately 1.5 scan/s. The parameters were set up with the electron multiplier at 1224 V, source temperature of 230 °C, and transfer line temperature at 150 °C.

340

350

360

370

380

Reactor Inlet Temperature, 0C Sulfur conversion, % wt

Nitrogen conversion, % wt.

Fig. 6. FCC feed sulfur and nitrogen conversion as a function of reactor inlet temperature at a liquid hourly space velocity (LHSV) of 1.2 h− 1.

One of the main factors that influences product yields and qualities in the FCC process is the feedstock quality. FCC feed hydrotreatment removes sulfur, nitrogen, Conradson carbon, Ni and V, and hydrogenate aromatics, increasing in this way the feedstock crackability. The FCC feed hydrotreater can operate at different severities (different reactor temperatures) depending on FCC feed sulfur target. Fig. 6 presents hydrodesulfurization and hydrodenitrogenation extent as a function of the LNB FCC feed hydrotreater first reactor inlet temperature (RIT). These data indicate that above 360 °C reactor inlet temperature the FCC feed sulfur conversion is higher than 98% and the nitrogen conversion is higher than 45%. The lower conversion of nitrogen compounds in the FCC feed is due to the fact that most of nitrogen species in the vacuum gas oil are very difficult to remove because they represent as heterocyclic compounds with multiple aromatic rings having sterically shielded Ncenters [28]. Fig. 7 presents a change of saturation of aromatic hydrocarbons in the hydrotreatment process as a function of the FCC feed

43

2.5

42 41

2.0

40 1.5 39 1.0

38

0.5 0.0 330

37

335

340

345

350

355

360

365

370

375

36 380

Reactor Inlet Temperature, 0C Aromatic saturation, %

PNA conversion, % wt.

Expected FCC Conversion Increase, wt.%

3.0

PNA conversion, wt.%

Aromatics Saturation, %

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

21

10.00 9.50 9.00 8.50 8.00 7.50 7.00 6.50 6.00 330

340

350

360

370

380

Reactor Inlet Temperature, 0C Fig. 7. FCC feed total aromatic saturation and polynuclear aromatics (PNA) conversion at different RIT and LHSV of 1.2 h− 1.

hydrotreater first reactor inlet temperature. For the investigated range of reactor temperatures the saturation of aromatic hydrocarbons is marginal and goes through a maximum at about 365–370 °C reactor inlet temperature due to the fact that aromatics hydrogenation is thermodynamically limited at higher temperatures. On the other hand the saturation of polynuclear aromatic hydrocarbons to mononuclear ones is continually decreasing for the investigated range. Fisher found a correlation between the FCC feedstock compounds capable of producing gasoline boiling range hydrocarbons and product yields at maximum gasoline yield during catalytic cracking of vacuum gas oils [29]. These compounds are paraffinic, naphthenic and mononuclear aromatic hydrocarbons and Fisher classified them as gasoline precursors because the polynuclear aromatic hydrocarbons are not capable of producing gasoline boiling range hydrocarbons in the catalytic cracking process [29]. During hydrotreatment of the Ural vacuum gas oil as can be seen from the data in Table 3 and Fig. 7 the polynuclear aromatic hydrocarbons are saturated to mononuclear ones. It could be concluded from here that as a result of the vacuum gas oil hydrotreatment the gasoline precursor level in the FCC feed is increasing. Having in mind that the total aromatics content in the straight run Ural vacuum gas oil and that of the hydrotreated vacuum gas oil is almost the same (Table 3) one could suppose that the gasoline precursor level in both gas oils will be determined by the different mononuclear aromatics content. It is assumed at equal total aromatics content in the gas oils the same content of saturate hydrocarbons. Fig. 8 indicates how content of mononuclear aromatic hydrocarbons (assumed gasoline precursor level) in the FCC feed is changing with increasing of hydrotreatment severity. It can be seen from these data that the gasoline precursor level increase a result from the hydrotreatment of the vacuum gas oil is marginally decreasing with an increase of the reactor temperature. Taking into account the scale of the commercial FCC unit such a minimal change in the level of gasoline precursors in the FCC feed would be difficult to cause noticeable change in FCC conversion and yields. The gasoline precursor levels in the vacuum gas oils hydrotreated at different severities estimated by empirical methods [30,31], based on the physical and chemical properties also indicate almost no change. Therefore, it could be concluded that the change in the severity of FCC feed hydrotreatment does not lead to any significant change in the gasoline precursor level of the hydrotreated vacuum gas oil. In other studies performed at higher than the pressure used in this work beside mononuclear aromatics increment achieved during hydrotreatment of vacuum gas oil an increase of saturate level is also reported when reactor temperature is lifted [5,7]. In these studies the gasoline precursor level grows when hydrotreater reactor temperature is elevated because the total amount of saturate plus mononuclear aromatic hydrocarbons in the vacuum gas oil has been increased in contrast to the results obtained in our investigation. However, it is known that the FCC feed reactivity depends not only on gasoline precursor level but also on the nitrogen content [28].

Fig. 8. Expected FCC conversion increase as a function of FCC hydrotreater severity (data based on mono-nuclear aromatics content increase after hydrotreatment).

Therefore feasible changes in the FCC conversion and yields obtained by cracking of gas oils hydrotreated at different severities could be caused by the different nitrogen content. 4.2. Effect of feed hydrotreatment and its severity on conversion, product yields and qualities in the FCC process 4.2.1. Commercial FCC unit Table 4 presents data from the operation of the LNB commercial FCC unit during processing of vacuum gas oil unhydrotreated and Table 4 Operating conditions and yield distribution in the LNB commercial FCC unit during processing of the vacuum gas oils unhydrotreated and hydrotreated at different severities. Operating conditions

Low severity Straight run hydrotreated Ural heavy vacuum gas oil heavy vacuum gas oil

High severity hydrotreated heavy vacuum gas oil

FCC feed, t/h Recycle rate, t/h Riser outlet temperature, °C Combined feed temperature, °C Regenerator dense bed temperature, °C Regenerator dilute phase temperature, °C Air, Nm3/h Catalyst-to-oil ratio, wt/wt Time on stream, s Delta coke,% Product yields as produced, wt.% Dry gas C3 cut C4 cut C5 − 210 °C LCO HCO Slurry Coke FCC conversion Corrected product yields, wt.% C2 − C3 C4 C5 − 210 °C (TBP) Diesel fraction (210–360 °C) Residual fraction (> 360 °C) Coke FCC conversion Gasoline selectivity, % Coke selectivity, % Diesel selectivity

116 2.4 525 259 668

181 2.0 537 319 671

181 2.0 536 314 671

679

682

682

84900 8.9 2.7 0.59

117100 8.2 2.3 0.55

114700 8.3 2.2 0.55

4.7 6.9 11.9 48.7 10.5 2.0 10.3 5.0 77.2

4.8 7.4 12.7 49.7 11.3 2.5 7.1 4.5 79.1

4.5 7.5 13.3 49.7 6.6 7.0 6.8 4.6 79.6

4.1 7.3 14.0 48.2 18.3 3.1 5.0 78.6 61 6.4 23.3

3.9 7.9 14.8 49.2 14.6 5.1 4.5 80.3 61 5.6 18.2

3.7 8.0 15.0 49.4 14.3 5.0 4.6 80.7 61 5.7 17.7

22

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

hydrotreated at different severities. These data indicate that hydrotreatment of the vacuum gas oil and the increase of hydrotreating severity leads to increasing of conversion. The conversion of the vacuum gas oil hydrotreated at low severity is about 2% higher relative to the unhydrotreated one and the conversion of the vacuum gas oil hydrotreated at high severity is 0.5% higher relative to the hydrotreated at low severity. It is interesting to note that gasoline selectivity is the same for all investigated cases. However, the case with the unhydrotreated vacuum gas oils refers to a lower riser outlet temperature. It is well known that the gasoline selectivity is decreasing with an increase of reactor temperature [32]. Therefore, at the same reactor temperatures it could be expected that during cracking of unhydrotreated vacuum gas oil lower gasoline selectivity would be obtained. The coke selectivity is higher when unhydrotreated vacuum gas oil is cracked. This means that the conversion difference between the unhydrotreated vacuum gas oil and the hydrotreated one would become bigger than 2% if the FCC unit operates with both feeds at its maximum limit of the air blower and regenerator temperature. Data of the product yields as they were produced show that the higher conversion observed during cracking of the hydrotreated at high severity gas oil is only due to higher LPG yields. The products were analyzed and based on results of analyses (true boiling point distillation of liquid products and gas chromatographic analyses of gaseous products and the gasoline fraction) the yields were corrected. The corrected yields indicate that higher conversion of the hydrotreated at high severity gas oil is because of higher yields of gasoline and LPG. These data demonstrate that the gas oil hydrotreated at low severity has lower reactivity and higher severities in the FCC unit (higher riser outlet temperature, longer residence time) are required to achieve the same level of conversion as that achieved with the hydrotreated at high severity gas oil. In fact decreasing of throughput Table 5 Hydrocarbon composition of gaseous products obtained in the commercial LNB FCC unit by cracking of the vacuum gas oils unhydrotreated and hydrotreated at different severities. Hydrocarbon composition Dry gas H2, vol.% O2, vol.% N2, vol.% CO, vol.% CH4, vol.% CO2, vol.% C2H4, vol.% C2H6, vol.% H2S, vol.% C3, vol.% C4, vol.% C5, vol.% C6, vol.% C3 cut C2, vol.% C3H8, vol.% C3H6, vol.% C4, vol.% C4 cut C3, vol.% i-C4H10, vol.% n-C4H10, vol.% 2tr-C4H8, vol.% 1-C4H8, vol.% i-C4H8, vol.% 2cis-C4H8, vol.% 2,3C4H6, vol.% C5, vol.% i-C4/C4=

Straight run Ural heavy vacuum gas oil

Low severity hydrotreated heavy vacuum gas oil

High severity hydrotreated heavy vacuum gas oil

6.1 1.4 12.7 1.0 34.3 2.5 12.9 11.1 11.7 4.5 1.0 0.7 0.1

8.3 0.6 11.3

8.0 0.7 11.2

41.8 2.5 14.9 11.9 0.7 4.4 3.1 0.5

43.7 2.5 14.3 11.2 0.2 5.1 2.8 0.3

0.3 21.3 78.4

0.3 19.7 79.9 0.1

0.2 20.3 79.4 0.1

0.5 29.4 8.4 15.8 14.4 15.8 11.2 0.4 4.1 0.51

1.5 36.2 7.5 13.3 14.6 16.2 9.2 0.4 1.1 0.68

1.5 38.4 7.9 13.4 14.3 14.8 9.1 0.3 0.3 0.74

(increasing of residence time) in the FCC unit proved that the same conversion and yields can be obtained when gas oil hydrotreated at low severity is cracked (this data are not given in the work). The hydrotreatment of FCC feed appears to impact negatively the diesel selectivity in the FCC process. The unhydrotreated gas oil exhibited 23% diesel selectivity whereas the diesel selectivity of the hydrotreated gas oils was about 18%. Table 5 presents data of the hydrocarbon composition of the gaseous products — dry gas, C3, and C4 fractions. These data show that FCC feed hydrotreatment and its severity increase leads to increasing of isobutane/butylenes ratio which is an indicator for hydrogen transfer reactions rate increase. This observation is in line with findings reported by other researchers [2,5]. Table 6 presents data of physical and chemical properties of gasoline obtained by cracking of Ural vacuum gas oil unhydrotreated and hydrotreated at different severities. These data also include bromine number and calculated on its base gasoline olefin content. The PIONA analysis performed by gas chromatographic technique is not appropriate for interpretation of the FCC commercial results because the content of indentified hydrocarbons is pretty high. It varies between 5.97 and 10.42%. According to FIA analysis the higher olefin content is obtained in the FCC gasoline when gas oil hydrotreated at low severity is cracked. However, this result contradicts the observed decreasing of butylenes to isobutane ratio with increasing the FCC feed hydrotreatment severity. The analysis of gasoline bromine number, however, shows the same decreasing trend as the butylenes to isobutane ratio observed in the FCC gaseous products when cracking hydrotreated feed. It may be concluded from here that the FCC feed hydrotreatment and its severity increase augment hydrogen transfer rate. These data suggest that the gasoline bromine number is a more accurate indicator for olefin content than the FIA and PIANO

Table 6 Physical and chemical properties of the gasoline obtained in the commercial LNB FCC unit by cracking of the vacuum gas oils unhydrotreated and hydrotreated at different severities. Gasoline properties

Low severity Straight run hydrotreated Ural heavy vacuum gas oil heavy vacuum gas oil

Specific density at 15 °C, d415 0.741 0.740 ASTM D-86 distillation, °C IBP 38 34 10% 53 53 50% 92 91 90% 170 170 FBP 205 201 Molecular weight estimated by 96 95 Goossens correlation [30] RVP, кPa 52.1 54.0 Sulfur, wt.% 0.16 0.0079 RONC 93.1 93.7 MON 81.0 81.4 Sensitivity (ΔRON-MON) 12.1 12.3 Hydrocarbon composition by ASTM D1319, vol.% Paraffins + naphthenes 42.8 36.4 Olefins 32.0 34.9 Aromatics 25.2 28.7 GC hydrocarbon composition (PIONA), wt.% C4 hydrocarbons 2.08 1.86 n-Paraffins 3.84 3.75 i-Paraffins 22.37 22.99 Olefins 23.15 24.60 Naphthenes 7.95 8.20 Aromatics 32.27 32.17 Unknowns 10.42 8.29 Bromine number by ASTM 61.6 59.7 D1159, grBr2/100 gr Calculated olefins content based 36.8 36.4 on Bromine number, %

High severity hydrotreated heavy vacuum gas oil 0.736 38 53 89 165 198 95 57.4 0.0013 93.2 81.6 11.6 42.4 31.9 25.7 1.64 4.01 26.51 24.68 8.31 30.52 5.97 52.7 31.1

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

23

5.5 5

Catalyst-to-oil ratio

4.5 4 3.5 3 2.5 2 1.5 38

40

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

72

74

76

78

Conversion, % wt. VGO

HTVGO

Fig. 9. MAT conversion of hydrotreated (HVGO) and unhydrotreated (VGO) vacuum gas oils as a function of catalyst-to-oil ratio.

analyses. By increasing severity of the FCC feed hydrotreatment an increase of hydrogen reaction rate takes place that leads to obtaining products with a lower olefin content, higher gasoline motor octane number (MON) and lower gasoline sensitivity (RON-MON). 4.2.2. Laboratory MAT unit Since it is not possible to vary operating conditions in a wide range in the commercial FCC unit cracking experiments of the gas oils

unhydrotreated and the hydrotreated at low severity in a laboratory MAT unit were performed. The dependence of conversion on catalyst-to-oil ratio is depicted in Fig. 9. These data show that at low catalyst-to-oil ratios no difference in reactivities between both gas oils exists. The difference in reactivities is noticeable at catalyst-tooil ratios higher than 4.0. The maximum difference in conversions however, did not go beyond 2%. A similar difference in conversions obtained by cracking of the unhydrotreated and the hydrotreated at

LPG 16

3

14

2.5

12

2

10

yield, %

yield, %

Dry gas 3.5

1.5 1

8 6 4

0.5

2

0

0 0

1

2

3

4

5

0

6

1

2

Catalyst-to-oil ratio

5

6

20

55 50 45 40 35 30 25 20 15 10

18

yield, %

yield, %

4

LCO

FCC Gasoline

16 14 12 10 8

0

1

2

3

4

5

0

6

1

2

3

4

5

6

Catalyst-to-oil ratio

Catalyst-to-oil ratio

HCO

Coke

3.5

70

3

60 50

yield, %

yield, %

3

Catalyst-to oil ratio

40 30

2.5 2 1.5

20

1

10

0.5

0

0 0

1

2

3

4

5

6

0

Catalyst-to-oil ratio

1

2

3

4

Catalyst-to-oil ratio -VGO

-HTVGO

Fig. 10. MAT product yields of hydrotreated and unhydrotreated vacuum gas oils as a function of catalyst-to-oil ratio.

5

6

24

D.S. Stratiev et al. / Fuel Processing Technology 94 (2012) 16–25

low severity gas oils was observed in the commercial FCC unit (Table 4). The difference in conversions of 2% between the gas oils unhydrotreated and the hydrotreated at low severity is much closer to the difference in the gas oils gasoline precursor level determined by the empirical methods described in [33,34]. A difference of 8% conversion between the unhydrotreated and hydrotreated gas oils, that corresponds to the increase of the mononuclear aromatic hydrocarbon content after FCC feed hydrotreatment, was not observed either in the commercial riser FCC unit nor in the MAT fixed bed reactor unit. It may be concluded from here that the empirical methods for estimating the gasoline precursor level in the FCC feed better characterize its crackability than the information of mono-, and polynuclear aromatics content in the gas oil. Fig. 10 presents a change of FCC yields of both unhydrotreated and hydrotreated gas oils as a function of catalyst-to-oil ratio. These data confirm the conclusion made that the FCC feedstock hydrotreatment increases gasoline selectivity when the reactor temperature is the same during cracking of unhydrotreated and hydrotreated gas oils. The coke production is higher when unhydrotreated gas oil is cracked. This was also observed in the commercial FCC unit. The same trend of increasing the LCO selectivity, as that observed in the commercial FCC unit, is registered in the MAT unit when unhydrotreated gas oil is cracked. Obviously hydrotreatment of the FCC feedstock affects negatively the FCC catalyst selectivity towards diesel production in both laboratory MAT fixed bed catalyst reactor and the commercial FCC riser units. The research octane and motor octane numbers determined on the base of gas chromatographic analysis of gasoline fraction and using nonlinear model [35] for gasoline obtained by cracking of both unhydrotreated and hydrotreated gas oils are depicted in Fig. 11. These data indicate that octane of the gasoline obtained from the unhydrotreated gas oil is slightly higher. It may be concluded from here that the same observation could be made in the commercial FCC unit if the riser outlet temperature is not different when both unhydrotreated and hydrotreated gas oils are cracked. 90 89.5

RON

89 88.5 88 87.5 87 0

1

2

3

4

5

6

Catalyst-to-oil ratio 80.5

MON

80 79.5 79 78.5 78 0

1

2

3

4

5

6

Catalyst-to-oil ratio -VGO

-HTVGO

Fig. 11. GC RON and MON of gasoline obtained by cracking of the unhydrotreated and the hydrotreated vacuum gas oil in the MAT unit as a function of the catalyst-to-oil ratio.

The results of the cracking experiments performed with unhydrotreated and hydrotreated gas oils in the laboratory fixed catalyst bed reactor MAT unit on the same catalyst confirmed observations made in the commercial riser FCC unit. The catalyst–feedstock contact time in the MAT unit is 30 s while that in the commercial FCC unit is between 2 and 3 s. Regardless of this difference the MAT unit seems to be a reliable tool for evaluating the impact that feed quality may have on the commercial FCC unit performance. 5. Conclusions Besides reduction of sulfur and nitrogen the hydrotreatment of the Ural heavy vacuum gas oil in the LNB FCC feed hydrotreater increases the mononuclear aromatics by 8% absolute almost without affecting the total aromatics content. The hydrotreated vacuum gas oil exhibited 2% higher conversion in both commercial FCC and MAT units. The hydrotreatment of the Ural heavy vacuum gas oil increases the gasoline selectivity and decreases the coke and LCO selectivities in both commercial and laboratory MAT FCC units. The increase of hydrotreatment severity almost does not affect the gasoline precursor level in the hydrotreated FCC feedstock but reduces the gas oil nitrogen content. This has a positive impact on conversion and hydrogen transfer reaction rate that leads to a higher isobutane/butylenes ratio, lower gasoline olefinicity and higher MON. The laboratory MAT unit equipped with a fixed catalyst bed reactor and 30 s time on stream can be used as a reliable tool for assessment of the effect of feedstock quality on product yields and quality that would be observed in the commercial FCC unit. References [1] J. Ancheyta, P. Morales, G. Betancourt, G. Centeno, G. Marroquin, J.A.D. Munoz, Individual hydrotreating of FCC feed components, Energy & Fuels 18 (2004) 1001–1004. [2] D. Salazar, R. Maya, E. Mariaca, S. Rodrigues, M. Aguilera, Effect of hydrotreating FCC feedstock on product distribution, Catalysis Today 98 (2004) 273–280. [3] A. Humphies, P. Imhof, C. Kuehler, T. Reid, Producing low sulphur gasoline, Petroleum Technology Quarterly Summer (2004) 97–103. [4] V. Bavaro, J. Pagel, G. Collins, M. Gray, Maximising FCC profitability with pretreater performance, Hydrocarbon Engineering (1999) 50–55 October. [5] I. Dahl, E. Tangstad, H. Mostad, K. Andersen, Effect of hydrotreating on catalytic cracking of a VGO, Energy & Fuels 10 (1996) 85–90. [6] G. Heinrich, S. Wambergue, Quality FCC products from increasingly dirty feeds, Petroleum Technology Quarterly Spring (1998) 39–49. [7] P. Vistisen, P. Zeuthen, Reactions of organic sulfur and nitrogen compounds in the FCC pretreater and the FCC unit, Industrial and Engineering Chemistry Research 47 (2008) 8471–8477. [8] P. Zeuthen, Pre-treatment improves FCC performance, Petroleum Technology Quarterly Catalysis (2004) 35–36. [9] H. Chung, S. Kolbush, E. de la Fuente, P. Christensen, FCC feed preparation for improved quality, Petroleum Technology Quarterly Spring (1997) 19–23. [10] R. Campagna, D. Kowalczyk, J. Wilcox, Effect of feed properties on FCC unit performance, Petroleum Technology Quarterly Winter (2001/02) 87–94. [11] S. Shorey, D. Lomas, W. Keesom, Exploiting synergy between FCC and feed pretreating units to improve refinery margins and produce low-sulfur fuels, NPRA Annual Meeting AM-99-55, 1999, pp. 1–23. [12] W. Ginzel, How feed quality effects FCC performance, Erdöl & Kohle, Erdgas, Petrochemie 10 (1986) 447–451. [13] A. Isah, M. Alhassan, M. Umar Garba, Feed quality and its effect on the performance of the FCC unit (a case study of Nigerian based oil company), Leonardo Electronic Journal of Practices and Technologies 9 (2006) 113–120. [14] V. Bavaro, Produce low sulfur gasoline via FCC feed pretreatment, World Refining 10 (2000) 30–37. [15] P. Zeuthen, M. Schmidt, The benefits of cat feed hydrotreating and the impact of feed nitrogen on catalyst stability, NPRA Annual Meeting AM-10-169, 2010, pp. 1–16. [16] C. Stanger, R. Fletcher, C. Johnson, T. Reid, Hydroprocessing/FCC synergy, Hydrocarbon Processing August, 1996, pp. 69–78. [17] D. Drazenovic, K. Jednacak, K. Bionda, The impact of FCC feed hydrotreatment on the yields and quality of cracking products, Goriva i maziva 5 (2005) 337–352. [18] G. Andonov, D. Stratiev, D. Minkov, S. Ivanov, Bulgarian refiner evaluates effect of FCC feed pretreatment catalysts on gasoline quality, Oil & Gas Journal (2003) 64–72 Nov.24. [19] D. Stratiev, A. Donov, G. Stratiev, Evaluation of Lukoil Neftochim Bourgas fluid catalytic cracking feed hydrotreater performance in a mild hydrocracking mode, Oil Gas European Magazine 2 (2005) 87–91.

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