Formation of thiophenic species in FCC gasoline from H2S generating sulfur sources in FCC conditions

Fuel 121 (2014) 65–71

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Formation of thiophenic species in FCC gasoline from H2S generating sulfur sources in FCC conditions William Richard Gilbert ⇑ PETROBRAS S.A./CENPES, Research & Development Centre, Av. Horacio Macedo, 950, Cidade Universitaria, Quadra 7, 21941-915 Rio de Janeiro-RJ, Brazil

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 H2S, olefins recombination reactions

Olefin-H2S recombination to form mercaptans and thiophenes was simulated in the lab by cracking ultralow sulfur diesel spiked with dymethyl-dysulfide (DMDS) as a proposed model reaction for FCC sulfur chemistry in a micro-reactor system producing gasoline range thiophenes. When the experiment was repeated in the presence of ZnO containing catalyst thiophene formation was inhibited by zinc, which acted as an H2S trap.

to mercaptans and thiophenes in FCC lab scale reactor.  Cracking of dymethyl-dysulfide (DMDS) doped sulfur free diesel in an FCC lab scale reactor.  Cracking DMDS doped diesel with zinc oxide based gasoline sulfur reduction additives.

a r t i c l e

i n f o

Article history: Received 8 October 2013 Received in revised form 5 December 2013 Accepted 14 December 2013 Available online 26 December 2013 Keywords: Thiophene FCC catalyst H2S recombination Sulfur in gasoline

a b s t r a c t Sulfur contamination of a bio-gasoline produced from very low sulfur soybean oil in a circulating pilot riser led to the investigation of the H2S-olefin recombination pathway by which any sulfur source capable of producing H2S in the FCC riser could in principle produce all of the typical thiophenic species present in the FCC cracked naphtha. The H2S-olefin recombination mechanism was confirmed in the laboratory by cracking ultra-low sulfur diesel spiked with 2 wt% dymethyl-dysulfide (DMDS) in a Short Contact Time Resid Test reactor to produce 60–80 ppm gasoline, in which the typical cracked naphtha sulfur species were major components. DMDS cracked to produce methyl-mercaptan and H2S as primary products, which continued to react with hydrocarbons derived from the diesel oil cracking to produce the intermediates and final products in the reaction pathway. When the reaction was repeated with catalyst containing 10% of a commercial gasoline sulfur reduction additive, the sulfur in gasoline was reduced by 40% and the sulfur in the spent catalyst went from very low levels, when no additive was used, to 0.5 wt% with the additive. The importance of the recombination pathway and the catalyst additive activity in interfering with the pathway by acting as an H2S scavenger may explain the functional mechanism of ZnO based gasoline sulfur reduction additives. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Fluid Catalytic Cracking (FCC) is one of the chief conversion processes in a petroleum refinery, producing cracked naphtha, distillate and C3, C4 streams from vacuum gasoil and other heavy feedstocks. Cracked naphtha is often the principal component in gasoline formulation and the most important source of sulfur in the pool. Depending on the crude oil of origin, cracked naphtha may have a relatively high sulfur concentration, varying from 2000 mg/kg to 200 mg/kg, well above the maximum sulfur limits ⇑ Tel.: +55 21 216 26675; fax: +55 21 216 26626. E-mail address: [email protected] 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.12.033

of automotive gasoline defined by legislation in several countries, which is typically below 50 mg/kg [1]. The gasoline sulfur reduction process of choice is the hydrodesulfurization of the cracked naphtha, however, depending on the refinery configuration, other alternatives such as reducing the cracked naphtha distillation end point or using gasoline sulfur reduction catalysts in FCC may also be applied. Additionally, naphtha hydrodesulfurization will saturate olefins and reduce the research octane number, and in many instances it is interesting to use a combination of different strategies to achieve a given sulfur limit [2]. For the reasons discussed above, understanding the sulfur chemistry in FCC has been important in the industry and many laboratories have worked on the problem. Chemically stable

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W.R. Gilbert / Fuel 121 (2014) 65–71

thiophene derivatives are the dominant sulfur species in the FCC gasoline. In a typical cracked naphtha from a Brazilian refinery with 800 mg/kg sulfur, 10% of the sulfur was thiophene, 25% C1-thiophene isomers, 30% C2-thiophenes, 10% C3-thiophenes, 10% benzothiophenes and 15% C1-benzothiophenes; whereas other sulfur compounds, such as C3+ mercaptans, dysulfides and tetrahydrothiophenes appear in minute amounts. The sulfur distribution in the gasoline boiling range shows a sharp increase in concentration towards the end of the curve because of the presence of benzothiophenes, which boil above 220 °C and are concentrated in this fraction. The prevalent mechanism for the production of sulfur compounds in the FCC liquid product in the literature [3] proposes a network of reactions in which heavy sulfur compounds in the feed crack to directly produce the thiophenic species observed in the products; a secondary route to thiophenes is also proposed, in the same network, starting from intermediate mercaptans which are produced by cracking of the feed or from H2S-olefin recombination reactions (Fig. 1). The relative contribution of the primary and secondary routes is not adequately described in the literature as most of the experimental work used either native gasoils as FCC feeds or blends in which small amounts of thiophenic compounds were added to the main feed [3–7]. Experiments that specifically probed the mercaptan/olefin-H2S to thiophene pathway [8,9] showed a relatively low conversion of the starting materials to thiophenes suggesting the subsidiary nature of this route. FCC gasoline sulfur reduction catalyst additives are described in the literature using ZnO as active ingredient [7,8,10,11]. Several sulfur reduction mechanisms are proposed, most of which will emphasize the Lewis acid character of the active ingredient and its capacity to either facilitate hydrogen transfer to alkyl-thiophenes to produce tetrahydrothiophene [12] or adsorb thiophenic compounds to produce coke [13]. Additive effectiveness in commercial applications is limited, typically achieving a maximum gasoline sulfur reduction of 25% or less, depending on several factors, including aromatic character of the feed, base catalyst formulation and metal contamination [5,14]. Transition metals used as active ingredients in the additive will tend to promote hydrogen transfer reactions in FCC conditions. Hydrogen transfer reactions are beneficial to the sulfur reduction strategy, as saturation of the thiophenic ring is required before hydrodesulfurization can occur. However, hydrogen transfer reactions are detrimental to the FCC catalyst selectivity, producing extra coke and hydrogen which aggravate air blower and wet gas compressor constraints and may impose a limit to the amount of gasoline sulfur reduction additive which can be used. This work describes an attempt to produce low sulfur bio-gasoline from soybean oil cracking which resulted in the production of gasoline thiophenic sulfur species from a virtually sulfur free feed.

Fig. 1. FCC sulfur chemistry mechanism showing primary reactions (1) which produce olefins, H2S, mercaptans (R-SH), thiophenes (R-T), benzothiophenes (R-BzT), dibenzothiophenes (R-dBzT) and coke. Secondary reactions are also shown: (2) H2S-olefin reversible recombination to produce mercaptans; (3) mercaptan cyclization to produce tetrahydrothiophene (R-THT); (4) dehydrogenation to produce thiophene from tetrahydrothiophene; and (5) condensation reactions which produce heavier sulfur derivatives from lighter precursors.

The possible sulfur contaminants identified at the time that could generate the observed gasoline sulfur would require the formation of the gasoline thiophenic species starting from a generic sulfur source that would first be converted to H2S and than to the gasoline range sulfur species by combining with intermediate hydrocarbons produced in the triglyceride cracking. Follow up experiments in the laboratory produced results which demonstrated that the H2S-olefins to thiophenes mechanism was capable of producing the gasoline sulfur species in the concentrations observed in the soybean oil cracking experiment. This mechanism is probably more important than previously thought in conventional FCC and presents an alternative explanation to the mode of action of ZnO based gasoline sulfur reduction additives.

2. Experimental 2.1. Soy bean cracking in the FCC pilot riser The Petrobras FCC pilot riser located in the SIX refinery in Sao Mateus do Sul, Parana, Brazil, is a 200 kg/h feed rate circulating unit with an 18 m riser and 400 kg catalyst inventory. Fig. 2 shows the schematics of the reactor, regenerator and product recovery system. The unit has an adiabatic reactor, stripper and regenerator for heat balance studies. The soybean oil used in the experiment was food grade degummed oil supplied by Incopa LTDA with 0.9193 specific gravity and 30 mg/kg sulfur content. Because the very low coke yield produced by soy bean oil cracking is insufficient to meet the FCC heat requirement, 1800 mg/kg sulfur, 43,100 kJ/g HHV diesel oil was burned in the regenerator as torch oil. Prior to product collection, the unit was run for a few hours until stable operation in the specified conditions was achieved and than run for 1 h to wash out the product recovery system with the product generated at the specified conditions, at this point product collection would be performed for another 1 h period. The feed, regenerator air, flue gas and gas product flow rates were measured with Coriolis mass flow meters. Flue gas and product gas compositions were analyzed by gas chromatography and the liquid

Fig. 2. Pilot riser reactor schematic diagram.

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W.R. Gilbert / Fuel 121 (2014) 65–71 Table 1 FCC pilot riser unit operating conditions. Feed rate (kg/h)

Riser T (°C)

Regen. T (°C)

Feed preheat T (°C)

Torch oil (kg/h)

200

530

690

80

3.2

Table 1. The liquid product from the soy bean oil cracking was later collected and distilled in a TBP distillation column, generating short cut fractions which were then characterized for sulfur content. 2.2. Short Contact Time Resid Test experiments

Fig. 3. SCT-RT reactor schematic diagram.

Table 2 SCT-RT test series. Conditions

A

B

C

D

Feed sulfur Sulfur source Catalyst

5 mg/kg Non Ecat 1

1.86% CS2 Ecat 1

1.72% DMDS Ecat 1

1.72% DMDS Ecat 2

The idea behind the laboratory scale tests was to emulate the results from the pilot unit, by running a very low sulfur hydrotreated diesel with an external source of sulfur which would be converted to H2S in the FCC reaction conditions. A SCT-RT unit [15] was chosen rather than a Fixed Fluidized Bed (ACE) unit [16], after preliminary tests with CS2 in the ACE unit showed very little conversion of the CS2. Fig. 3 shows a schematic diagram of the SCT-RT reactor. In the SCT-RT the feed is injected into the very hot catalyst bed (670 °C), similar to what happens in circulating units, thus accelerating initiation reactions. Nevertheless, the attempt to convert CS2 in the SCT-RT reactor also proved unsuccessful so that the sulfur source was changed for dymethyl-dysulfide (DMDS), which worked as expected. Table 2 shows the feed/catalyst combinations that were evaluated in the test series. SCT-RT reactor catalyst temperature was 670 °C in all runs and catalyst/ oil ratio was varied between 5 and 7 for each catalyst-feed pair, feed injection time was 1 s, feed injection rate was 2 g/s. Gas product was analyzed from the sample collected from the water displacement vessel, resulting in a wide range of variation of H2S concentration in the gas product, because of partial solubilization of H2S in the water. Liquid product yields, gasoline (C5-216 °C), LCO (216–343 °C) and bottoms (343 °C+) were calculated using GC-Simulated Distillation based on ASTM D 2887. Mass balances were within 95% to 100% range. Sulfur analysis of the feeds was measured by an Antek 7090 Chemiluminescence analyzer (ASTM D 5453); sulfur analysis of the liquid product was done by Gas chromatography based on the ASTM D 5623 standard. Properties of the hydrotreated diesel used in the feed blends are shown in Table 3. DMDS and CS2 were 99% grade reagents supplied by Aldrich. Catalyst analysis by X ray fluorescence is shown in Table 4. The Ecat evaluated in condition D used zinc containing about 10% commercial gasoline sulfur reduction additive. 3. Results and discussion

oil product and the water condensate were collected and weighed. The coke yield was calculated from the flue gas mass flowrate and composition. Gasoline, LCO and slurry yields were calculated from the liquid product simulated distillation (ASTM D 2887) using 221 °C and 343 °C as cut points. Gasoline octane was calculated from the detailed hydrocarbon analysis chromatography of the gasoline fraction. A preliminary run was performed to check on the gasoline quality before the final run was done. An equilibrium catalyst from one of Petrobras refineries was used in the tests (PRU Ecat). The FCC operating conditions used in the run are specified in

3.1. Soy bean oil cracking results Soy bean oil is a mixture of polyunsaturated triglycerides containing linoleic, oleic, stearic and palmitic acids with C57H100O6 average chemical formula. The fatty acid hydrocarbon chains of the molecule readily crack to produce a product profile similar to that of a paraffinic hydrocarbon feed. The carbonyls and the hydrogen deficient glycerol portion of the molecule will crack to produce mostly coke, gas, CO, CO2 and water. Conditions in the pilot riser

Table 3 SCT-RT feed – hydrotreated diesel oil properties. Spec. gravity 20/4 °C

Sulfur (mg/kg)

Sim. distill. IBP (°C)

Sim. distill. 10 wt% (°C)

Sim. distill. 50 wt% (°C)

Sim. distill. 90 wt% (°C)

Sim. distill. FBP (°C)

0.8241

4.8

103

152

228

329

446

Table 4 Catalyst chemical properties. Catalyst

Al2O3 (%)

SiO2 (%)

RE2O3 (%)

Na2O (%)

P2O5 (%)

Fe2O3 (%)

TiO2 (%)

MgO (%)

ZnO (%)

Ni (mg/kg)

V (mg/kg)

PRU Ecat SCTRT Ecat 1 SCTRT Ecat 2

43.2 48.4 47.3

52.2 46.1 42.6

2.59 2.93 2.65

0.31 0.38 0.53

0.75 0.62 0.58

0.43 0.54 0.49

0.23 0.36 0.33

nd 0.03 4.11

nd 0.01 0.80

1053 1240 1100

544 1920 1730

W.R. Gilbert / Fuel 121 (2014) 65–71

were chosen so as to avoid gasoline over-cracking and meet gasoline specifications of minimum sulfur content (preferably less than 10 ppm) and highest octane possible. Table 5 presents the gasoline yield and selected properties, the torch oil consumption and the regenerator air rate of the three runs and Table 6 the carbon, hydrogen and oxygen balances for run number 1, based on average hydrocarbon product C/H ratios. Hydrogen recovery in the gasoline fraction was high, showing a satisfactory balance between hydrogen lost to the LPG (conversion too high) and LCO fractions (conversion too low). Oxygen recovery was less than 100%, an indication of the presence of oxygenated products in the oil and water fractions. The existence of oxygenates in the oil fraction was later confirmed by the detection of 2% of propanaldehyde and smaller amounts of other aldehydes, ketones and phenols, adding up to another 0.8% of the bio-gasoline product. Table 5 shows that the gasoline sulfur of run 1 was substantially higher than the 10 ppm target. This result came as a surprise because of the very low sulfur level in the soy bean oil feed. Liquid product contamination was unlikely because of the large volumes processed in the pilot riser and the routine product recovery system conditioning precautions. For instance a nearly 10% contamination with the typical 600 ppm Brazilian fossil VGO cracked naphtha produced in the unit would have been necessary to

180 150

Sulfur mg/kg

68

120 90 60 30 0

0

50

100

150

200

250

TBP °C Fig. 4. Sulfur concentration profile as a function of the boiling temperature of the soybean oil cracked product. Caption: + Run 1, 4 Run 2.

explain the 50 ppm sulfur in the soy bean gasoline. The TBP column distillation of the bio-gasoline product from run 1 and the characterization of the fractions (Table 7) generated the sulfur concentration profile displayed in Fig. 4, in which the inflection points are strongly suggestive of the presence of thiophenes (BP P 100 °C) and benzothiophenes (BP P 190 °C). With no clear sulfur source to explain the soy bean cracked naphtha sulfur

Table 5 Soy bean oil pilot riser cracking results – yields and gasoline quality as a function of operating conditions. Run

Riser T (°C)

Regen. T (°C)

Torch oil (kg/h)

Regen. air rate (kg/h)

Mass balance (wt%)

Gasoline (wt%)

Gasoline GC MON

Gasoline sulfur (ppm)

1 2

530 500

690 701

3.6 2.2

168 156

101.8 99.1

46.6 40.6

77 75

50 11

Table 6 Soy bean oil pilot riser cracking results.

Mass balance wt% Dry gas (w/o oxygen spc) wt% LPG wt% Gasoline (C5–221 °C) wt% LCO (221–343 °C) wt% Bottoms (343 °C+) wt% Coke wt% CO wt% CO2 wt% H2O wt%

Yields

C/H

Carbon %

Hydrogen %

Oxygen %

101.8 1.7 9.4 46.6 20.1 5.5 3.8 1.6 0.5 10.3

– 0.27 0.46 0.58 0.86 0.90 1.00 – – –

100.0 1.7 10.2 52.4 23.6 6.5 4.5 0.9 0.2 0.0

99.9 3.5 12.7 51.5 15.6 4.1 2.3 0.0 0.0 10.1

95.6 0.0 0.0 0.0 0.0 0.0 0.0 8.4 3.3 83.9

Table 7 TBP distillation results of the soy bean bio-gasoline. Run 1

Run 2

Cut points (°C)

Cumulative wt% of gasoline

S in fraction (mg/kg)

Cut Points (°C)

Cumulative wt% of gasoline

S in fraction (mg/kg)

IBP – 34 °C 34–62 °C 62–68 °C 68–82 °C 82–92 °C 92–104 °C 104–112 °C 112–130 °C 130–138 °C 138–156 °C 156–168 °C 168–182 °C 182–198 °C 198–221 °C

6.0 12.4 18.7 25.4 31.8 38.5 45.5 52.4 59.5 66.8 74.3 81.9 89.6 100.0

nd 2 2 3 4 8 16 21 26 29 48 97 153 157

IBP-42 °C 42–71 °C 71–93 °C 93–111 °C 111–131 °C 131–149 °C 149–171 °C 171–189 °C 189–209 °C 209–221 °C

7.1 17.1 27.4 37.9 48.6 59.8 71.0 82.7 94.6 100.0

nd 1 2 3 5 9 10 13 26 42

221–FBP °C

–

142

221-FBP °C

–

79

69

W.R. Gilbert / Fuel 121 (2014) 65–71

sulfur oxides from the regenerator to the reactor, however, there were no such additives in the catalyst which was used in the soybean oil cracking. Even though the change in operating conditions to produce the low sulfur gasoline was successful, the reason for the gasoline sulfur reduction was probably not related to the reduction in sulfur transport from the regenerator to the riser and the ultimate sulfur contamination cause remained unexplained.

Table 8 Sulfur mass balance in the soybean oil cracking experiment.*

kg/h mg/kg mg/h *

Feed

Torch

200 30 6000

3.6 1800 6480

Inputs

Gasoline

211 °C+

Liq. product

12,480

93.2 50 4660

51.2 142 7270

11,930

H2S in the gas product and SO2 in the regenerator flue gas were not measured.

products, suspicion fell on the 1800 ppm sulfur diesel used for torch oil. For the sulfur in the torch oil burned in the regenerator to be converted into fuel sulfur it would first have to be transported as SO2 as a gas or adsorbed on the catalyst from the regenerator to the riser, reduced to H2S and converted to thiophene derivatives via a olefin-H2S recombination pathway. Reactor temperature and regenerator temperature were changed to reduce torch-oil consumption and catalyst circulation, thereby reducing flue gas sulfur carryover to the riser, which was believed to be responsible for the high sulfur gasoline. The liquid product distillation and characterization at the end of run 2 showed that the low sulfur soy bean cracked naphtha was successfully produced. The sulfur mass balance which was later performed (Table 8), required very efficient transport of sulfur from the regenerator to the reactor and a very high conversion of the total sulfur input to the liquid product sulfur for the proposed mechanism of gasoline sulfur species generation in the soybean cracking experiment to be valid. Any of the two events are very unlikely to have occurred. The amount of flue gas passively carried over with the catalyst from the regenerator to the reactor was measured as 0.4%, based on the inert gas concentration (N2 and CO2) in the reactor product gas from another group of experiments in the same unit with fossil VGO feedstock. SOx additives are capable of actively transporting

3.2. SCT-RT cracking results Fig. 5 shows the yield profiles obtained in the SCT-RT cracking experiments. The high conversion and low coke yield obtained in the SCT-RT diesel cracking experiments was already expected due to the light paraffinic nature of the feed. Adding the sulfur doping components CS2 in condition B and DMDS in conditions C and D had little noticeable effect on the product yields, except for the increase in dry gas yield which was explained by the increase in methane production, a residue from the cracking of the sulfur doping species (Fig. 5A and B). Regarding the sulfur speciation of the SCT-RT products, none of the gasoline sulfur species were detected in the pure diesel cracking runs (condition A). Table 9 shows the results from selected runs from conditions B, C and D, in which a sulfur source was blended with the ultra low sulfur diesel. The sulfur compounds used in the doping experiments introduced a new degree of complexity to the sulfur chemistry mechanism. Both CS2 and DMDS are nucleophiles in their own right and will interact with the FCC catalyst. CS2, used in condition B, was relatively unreactive and much of it was recovered unscathed (off scale) in the cracked oil product resulting in the high liquid product total sulfur concentrations measured in condition B shown in Table 9. DMDS, however, readily

1.6

3.5

(B)

(A)

3.0

1.2

C1 wt%

Dry Gas wt%

2.5 2.0 1.5 1.0

0.8

0.4

0.5 0.0

0.0 74

76

78

80

82

84

72

74

Conversion (<221°C) wt% 2.5

(C)

55.0

52.5

50.0 74

78

80

82

84

(D)

2.0

57.5

Coke wt%

Gasoline (C5-221°C) wt%

60.0

76

Conversion (<221°C) wt%

1.5

1.0 0.5

0.0 76

78

80

82

Conversion (<221°C) wt%

84

74

76

78

80

82

84

Conversion (<221°C) wt%

Fig. 5. SCT-RT yields: (A) dry gas, (B) methane, (C) gasoline and (D) coke as a function of conversion. Caption: (+) pure diesel condition ‘‘A’’, ( ) diesel plus CS2 condition ’’B’’, () diesel plus DMDS condition ‘‘C’’, (}) diesel plus DMDS with sulfur reduction catalyst condition ‘‘D’’.

70

W.R. Gilbert / Fuel 121 (2014) 65–71

Table 9 Sulfur speciation of SCT-RT cracking products.

*  

Condition Sulfur source

B CS2

B CS2

C DMDS

C DMDS

D DMDS

D DMDS

Catalyst Prod. total sulfur mg/kg CTO

Ecat 1 2000 7.0

Ecat 1 3000 6.5

Ecat 1 230 5.0

Ecat 1 187 6.0

Ecat 2 118 6.0

Ecat 2 112 6.0

H2S mg/kg COS mg/kg C1-SH mg/kg C2-SH mg/kg DMS mg/kg* CS2 mg/kg* C3-SH mg/kg C4-SH mg/kg E-M-sulfide mg/kg* Thiophene mg/kg DE-sulfide mg/kg* M-P-sulfide mg/kg* DMDS mg/kg* C5-SH mg/kg M-thiophene mg/kg DM-thiophene mg/kg TM-thiophene mg/kg TH-thiophene mg/kg Benzothiophene mg/kg M-benzothioph. mg/kg

0.06 0.14 0.25 0.15 – off scale 0.09 – – 4.64 – 0.06 – – 3.62 3.24 2.43 1.25 0.62 2.35

0.05 0.23 – – – off scale – – – 3.45 – – – – 3.19 3.19 2.38 0.31 0.68 2.29

0.16 – 1.40 1.35 31.99 2.83 3.59 0.78 11.25 7.71 1.24 6.35 0.33 2.92 15.06 11.04 12.76 3.00 0.93 5.30

0.17 – 6.37 2.56 49.40 0.00 4.67 0.72 12.24 8.28 0.97 6.73 0.45 3.01 13.78 9.76 13.20 3.16 1.04 6.74

0.11 0.13 1.02 0.80 14.64 0.75 1.84 0.90 6.51 2.44 0.36 3.69 3.94 1.23 6.73 5.14 11.01 1.00 1.00 4.05

0.05 0.05 1.46 0.36 13.40 0.55 1.39 0.72 4.02 2.76 0.18 2.09 5.17 0.28 6.90 5.05 19.41 1.37 1.00 3.45

S in spent catalyst wt% C3, C4-SH + Thioph. mg/kg 

nd 18.24

0.07 15.49

0.028 63.09

0.041 64.34

0.42 35.33

0.44 42.32

Products resulting from the sulfur doping source not present in significant amounts in typical FCC cracked oil. Products representing total sulfur in gasoline concentration.

reacted not only producing H2S, as intended, but also a series of different thioethers formed by chain reactions analogous to those involved in hydrocarbon Bronsted acid mediated cracking [17] (Fig. 6). The total liquid product sulfur concentration in condition C (DMDS runs) was much less than in conditions B because most of the sulfur originally in the feed was lost as H2S to the gas product. Although thiophene and its derivates were not present in the feed, they were detected in significant amounts in the SCT-RT product in all cases in which a sulfur source was provided, demonstrating the importance of the mercaptan and olefins-H2S recombination pathway. Because of its reactivity, DMDS was much more efficient than CS2 in transferring the sulfur to the gasoline fraction of the product, resulting in sulfur content in gasoline greater than 60 ppm at condition C. The heavier aromatic thiophene derivates, benzothiophenes and alkyl-benzothiophenes, were detected in much lower concentrations than in VGO FCC products, possibly because there was not enough time for the multi-step condensation reactions or because of the relatively light molecular weight hydrocarbon feed. The SCT-RT results in this sense contrasted with the soy bean oil cracking results, which showed a benzothiophene to thiophene ratio more typical of conventional FCC. When the zinc containing additive catalyst (Ecat 2) was used (condition D) the sulfur content in the liquid product was reduced by nearly 40%. The lower sulfur in gasoline for Ecat 2 was compensated by a more

Fig. 6. Illustrative example of a Bronsted acid mediated chain reaction mechanism producing different thioethers either by the transfer of fragments from hydrocarbon molecules present in the reaction mixture or by the reaction of a mercaptan to an adsorbed carbocation.

than ten times increase in the sulfur content of the spent catalyst (Table 9), an indication that the additive was acting as a sulfur trap and inhibiting the pathway leading to gasoline fraction sulfur products. The activity of the zinc oxide as a sulfur trap, immobilizing the sulfur as a stable zinc sulfide and inhibiting one group of many possible reaction pathways to gasoline sulfur species in FCC is more plausible than other hypotheses discussed in the literature, some of which would require the very unlikely event of saturation of the aromatic thiophenic ring in a low H2 partial pressure and subsequent desulfurization. By hindering only one of the possible sulfur chemistry pathways, the gasoline sulfur reduction additive may prove to be inherently incapable of achieving the very high gasoline sulfur reductions required for compliance with the modern fuel specifications. 4. Conclusions A sulfur source capable of being converted to H2S in FCC reaction conditions can produce any of the naphtha range thiophenic species common in FCC gasoline in substantial concentrations by H2S-olefin recombination to form mercaptans followed by mercaptan cyclization and dehydrogenation. This mechanism has already been described in the literature; however its relative importance has been overlooked. The DMDS experiments successfully demonstrated the H2S/ mercaptan to thiophene formation in FCC conditions and may be used as a model reaction for the development of FCC gasoline sulfur reducing additives. The production of a range of sulfur derivates, such as thioethers, starting from DMDS, although not directly related to FCC chemistry, is an interesting example of the interaction of the FCC catalyst with a non hydrocarbon substrate in a reaction pathway, which has similarities to the reaction mechanisms proposed for hydrocarbon cracking. The experiments with zinc oxide containing catalyst resulting in a large reduction in thiophene production and a large increase

W.R. Gilbert / Fuel 121 (2014) 65–71

in the sulfur content in spent catalyst strongly suggest that the zinc based additive would be acting as sulfur sorbent, removing H2S from the reaction media and inhibiting the formation of gasoline sulfur species via the olefin-H2S recombination pathway. Acknowledgments The author would like to thank Cleber Ursini for his help with the sulfur speciation analysis and Petrobras for permission to publish this work. References [1] W.W. Fuel Charter 2012, European Automobile Manufactures Association, ; 2012. [2] Lesemann M, Schult C. Hydrocarbon Proc 2003;2:69–76. [3] Corma A, Martinez C, Ketley G, Blair G. Appl Catal A: Gen 2001;208:135–52.

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