The Chemistry of Sulfur in Fluid Catalytic Cracking - PIRE-ECCI

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Outline. • Review of fluid catalytic cracking. • Sulfur balance and sulfur cracking chemistry. • Catalytic reduction of sulfur by. Zn aluminate/alumina.
Catalytic Reduction of Sulfur in Fluid Catalytic Cracking

Rick Wormsbecher

Collaborators Grace Davison Ruizhong Hu Michael Ziebarth Wu-Cheng Cheng Robert H. Harding Xinjin Zhao Terry Roberie Ranjit Kumar Thomas Albro Robert Gatte

Laboratoire Catalyse et Spectrochimie, CNRS, Université de Caen Fabian Can Francoise Maugé Arnaud Travert

Outline • Review of fluid catalytic cracking. • Sulfur balance and sulfur cracking chemistry. • Catalytic reduction of sulfur by Zn aluminate/alumina.

Fluid Catalytic Cracking • Refinery process that “cracks” high molecular weight hydrocarbons to lower molecular weight. • Refinery process that provides ~50 % of all transportation fuels indirectly. • Provides ~35 % of total gasoline pool directly from FCC produced naphtha. • ~80 % of the sulfur in gasolines comes from the FCC naphtha.

Sulfur in Gasoline • Sulfur compounds reversibly poison the auto emission catalysts, increasing NOx and hydrocarbon emission. SOx emissions as well. • World-wide regulations to limit the sulfur content of transportation fuels. – 10 - 30 ppm gasoline.

• Sulfur can be removed by hydrogenation chemistry. – Expensive. – Lowers fuel quality.

Fluid Catalytic Cracking Unit Products

Flue Gas

Regenerator (~725 ºC) 400 tons catalyst inventory Air

Riser Reactor (~500 ºC)

Reaction is endothermic. Catalyst circulates from Riser to Regenerator. Feedstock

50, 000 barrels/day

Carbon Distribution H2

~0.05 %

Flare

C1

~1.0 %

Flare

Feed

C2-C4

~14-18 %

Petrochem

MW ~330 g/mole

C5-C11

~50 %

Naphtha

C12-C22 ~15-25 %

LCO

C22+

HCO

Coke

~5-15 % ~3-10 %

Regen

Catalyst Consumption • 0.25 – 0.75 #’s/barrel • About 300,000 tons catalyst/year world wide, for about 300 refiners. • Consumption depends on feedstock quality, mainly irreversible poisoning by vanadium

FCC Catalysts • • • • • • •

10 - 50 % Y type zeolite, usually USY 0 - 13 % Rare earth on zeolite 0 - 80 % active silica/alumina matrix 0 - 50 % binder and kaolin Single component or blends Additives for olefins, SOx, NOx, CO SULFUR REDUCTION TECHNOLOGY

Solid acid catalysis

Zeolite Y structure

Cracking is Catalyzed by Solid Acid/Hydrocarbon Interactions • Initiation: Hydride abstraction by a Lewis site or carbocation ion formation by a Brønsted site. • Propagation: Through primarily β-scission. • Product selectivities governed by stability of product and the precursor carbocation. • Isomerization, alkylation occur readily

Formation of carbocation transition states H

H CH2

+

HZ

R

CH3

R

Z H

H R

R'

+

R

L

CH3

HL

H H

+ R

HZ R

H

Beta scission β

H R

H R

CH2

CH2

C

Z

CH2

CH

CH2

CH2

R'

Z

CH2

CH

CH2

CH2

R

H CH2

CH2

R' H

Z R'

C H

Charge isomerization H

H CH2

Z H

CH2

R'

CH3

CH2

Z

R

Hydride Transfer (simplified) H H H - C - C - C - + H - Cfeed H + H Z

Cfeed + Z

+ saturate

Hydride abstraction High Brønsted site density favors hydride transfer. Catalysts can be tailored with high or low site density.

Feedstocks • • • •

“Average” feed ~ 1.0 wt. % Sulfur “Average” feed MW ~ 300 gr/mole ~ one in ten feed molecules contain S S content ranges from 0 - 4.5 %

Sulfur Balance in a FCC Unit

Fuel Gas, H2S

Feed Sulfur (~0.3%) Saturated Sulfur Thiophenes Benzothiophenes Multi-ring Benzothiophenes

40%

F

Gasoline

5%

C

Light Cycle Oil

15 %

C

Bottoms

35 %

Coke, SOx

5%

Sulfur Species in FCC Gasoline (by GCAED) S

SH R S Sig. 2 in C:\HPCHEM\1\DATA\D51024A\012R0501.D R R S

2.8e5

Benzothiophene

2.7e5 2.6e5

Methylthiophenol

2.5e5 2.4e5

Methylthiophene Thiophene Me-THT

2.3e5

C2-Thiophene

2.2e5

Benzenethiol

2.1e5

THT

C2-thiophenol

C2-THT C3-thiophenes

2.0e5 1.9e5 1.8e5 1.7e5 20

40

Time (min.)

60

80

100

Reaction Pathways of Sulfur Species in FCC Sulfur Species in Feedstock

Sulfur Species in FCC Gasoline

Multi-Ring Aromatic Thiophenes Alkylbenzothiophenes Alkylthiophenes Alkyl Sulfides, Mercaptans (Including cyclic sulfides) H2S + DiOlefin or Olefin

?

Reaction Chemistry of S Compounds (Which Feedstock Sulfur Species End Up in Gasoline?) • Spike a low sulfur (0.3wt% S) base feedstock with model S compounds (0.7% wt% S). • Look for yield changes in Microactivity Testing • Compounds tested: Thiols (octanethiol) Sulfides (di-n-butylsulfide) Alkylthiophenes (n-hexylthiophene, n-decylthiophene) Alkylbenzothiophenes (3- and 5-methylbenzothiophene) Alkyldibenzothiophene (1,10-dimethyldibenzothiophene)

Delta Yields of Gasoline Range Sulfur Base Feedstock Spiked with Model S Compounds (0.7% S)

Octanethiol

1200

DecylThiophene 3-MethylBenzothiophene

Sulfur, ppm

1000

5-MethylBenzothiophene

800 600 400 200 0 Gasoline Sulfur

C2-Thiophene and Lighter

C3-,C4-Thiophene Benzo-Thiophene + Thiophenols

Delta Yields of Gasoline Range Sulfur Base Feedstock (2.8% S) Spiked with Dimethyldibenzothiophene (0.45% S) 1,10 DiMethyl-DiBenzothiophene 120 Sulfur, ppm

100 80 60 40 20 0 C2C3-, C4- Thio-phenols Benzo Thiophene Thiophenes thiophene and Lighter

Recombination of H2S and diolefin DCR Testing: Polysulfide and diolefin addition to a low sulfur feedstock

Mercaptans, ppm

Base 5.1

2% Di-Olefin 7.3

(1% S) Polysulfide 47.4

Di-Olefin + Polysulfide 30.7

C0-C4 Thiophenes, ppm

44.4

50.7

68.0

145.9

BenzoThiophene, ppm

13.3

11.2

11.5

9.9

Reaction Pathways of Sulfur Species in FCC Sulfur Species in Feedstock

Sulfur Species in FCC Gasoline

Multi-Ring Aromatic Thiophenes

Small amount to benzothiophene

Alkylbenzothiophenes

Benzothiophene by dealkylation Small amount to thiophenols

Alkylthiophenes

Thiophenes by dealkylation Small amount to benzothiophene

Alkyl Sulfides, Mercaptans (Including cyclic sulfides)

Thiophenes by cyclization, aromatization

H2S + DiOlefin

Thiophenes

Hydride Transfer Controls Rate of Thiophenic Cracking Hydride-Transfer from feed R S Very stable and Not-so Lewis basic sulfur

R

H2S + Hydrocarbon

S Less stable and More Lewis basic sulfur

A Catalytic Approach • Zn Aluminate on γ-alumina. • Solid Lewis Acid. • Used as an additive (~10 - 75%) with FCC catalyst. • Reduces sulfur in gasoline by 20-45% without affecting other yields. • Most of reduction occurs in light (C3-) thiophenes.

Why Zn?

Zn(AlO2)2 on γ-Al2O3 • Nominal 17 % Zn(AlO2)2 83 % γ-Al2O3 • ~150 m2/gr surface area • Nominal monolayer of zinc (10 % Zn) on pseudo-boehmite • Converted to aluminate at 500 °C • Both Zn(AlO2)2 and γ-Al2O3 spinel structures

Lewis Acidity • Zn(AlO2)2 and γ-Al2O3 have only Lewis acidity. • Zn(AlO2)2 increases the Lewis acidity of the γ-Al2O3. • Use pyridine DRIFTS for acid type determination.

Probe Lewis and Bronsted Acid Sites by Pyridine-IR analysis 90 80

N ⇑ H

70 60

Kubelka-Munk

50 40

ZSM-5

30 20

γ -Al2O3

10 0 -10 -20 1650

1600

1550 Wavenumbers (cm-1)

N ⇓ M

1500

N ⇓ M

1450

Alumina Lewis acid sites Strong

Al

Weak

Al

Zn aluminate/alumina vs. alumina 34 32 30

Lewis Acid

28 26 24

Strong

Kubelka-Munk

22

Weak

20 18

Alumina Only

16 14 12 10

10%Zn/Alumina

8 6 4 2 1650

1600

1550 Wavenumbers (cm-1)

1500

1450

Simple Additive Approach (No Optimization of Hydride Transfer & sulfur activity)

950

cut gasoline sulfur

900

FCC base catalyst only

850

( Zn additive) 10%

800

15 % reduction

750

20%

700 650 600 64

65

66

67

68

69

Conversion, wt%

70

71

72

73

REUSY and Zn aluminate/alumina MAT at 525 °C. Feedstock contains 1% S. REUSY

95% GO-40 5% GSR-1

Std.Conv. Wt% C/O H2 C1+C2 LPG C5+Gaso LCO 640+Btms Coke W% Feed

70 3.78 0.05 1.74 16.49 48.67 20.27 9.75 2.90

70 3.80 0.05 1.73 16.21 48.73 20.39 9.60 3.09

mercaptans thiophene C1-thiophene tetrahydrothiophene C2-thiophene C3-thiophene + thiophenol C4-thiophene + C1, C2 thiophenol benzothiophene

0.57 55.62 140.22 31.85 151.97 72.91 55.10 387.61

0.15 40.35 91.52 10.91 102.06 67.06 57.09 365.13

tot w/benzo ex benzo

895.86 508.25

734.28 369.15

%total S conv % ex benzo

18.0 27.4

REUSY + 5 % Zn aluminate

Working hypothesis

S

k1 R

k2

k1

Hydride Transfer

k3

Zn/Alumina

S

k3

R-R + H2S

R

Catalyst site density

Fixed Bed Layered Studies feed Zn Aluminate on top

Samples feed

mixed

Zn Aluminate on bottom

Samples intermeadiates

Samples products

Sulfur Yields vs Zn Aluminate Location FCC cat w/10% Zn Aluminate Feedstock Contains 1.7% S 300

sulfur/ppm

250 200 150 100 50 0

Thiophene

FCC

C1- Thiophene

Tetrahydrothiophene

Zn Aluminate ABOVE

C2- Thiophene

C3- Thiophene +

C4- Thiophene + C1,

Thiophenol

C2 Thiophenol

Zn Aluminate MIXED

Zn Aluminate BELOW

Experimental : operando study at U. of Caen •

Reactors : – Classical catalytic reactor – IR reactor cell : Gas inlet

Kalrez O-ring

Air cooling

IR beam

KBr CaF 2

IR Surface analysis

Teflon window holder

Sample (wafer)

Heater

Gas outlet : IR, GC and MS Gas analysis

simultaneously : adsorbed species products of a reaction

Summary of pure compound operando studies on Zn/alumina and neat alumina • No reaction with thiophene. • Tetrahydrothiophene readily decomposes. • Evidence that paired Lewis/Brønsted sites are the active sites. More work.

Reaction of tetrahydrothiophene over Zn/alumina at 723 ºK IR gas spectra during the reaction

IR Chemigram THT

Abs

1,3-Butadiene

THT 55 50

0.04

25 20

Intensity

35 30

min

45 40

15 10

1,3-Butadiene

5

1800

1600 Wavenumbers (cm-1)

1400

¾ Consumption of THT ; ¾ Formation of 1,3-butadiene.

10

20

30 40 Time (minutes)

50

Work done at U. of Caen

Mass spec analysis during reaction with tetrahydrothiophene

Ionic current

C4H8S

C4H6

H2S t (min)

Maximize Sulfur Reduction Approach (Optimize Hydride Transfer & sulfur activity) 245 225

Control

cut gasoline sulfur

205 185

Increase site density + Zn/alumina in blend

165 145 125 105 85

Further increase site density + Zn/alumina in blend

65 60

62

64

66

68

70

Conversion, wt%

72

74

76

78

% Cut Gasoline Sulfur Reduction

Deactivation of sulfur reduction activity 35% 30% 25% 20% 15% 10% 0

1

2

3

4 Days

50% Zn/alulmina in blend

5

6

7

Deactivation Mechanism 50

45

GSR 1.1 Day 7 GSR 1.1 day 2 GSR 1.1 Day 3 GSR 1.1 Day 4 GSR 1.1 Day 5 GSR 1.1 Day 6 GSR 1.1 Day 1

Kubelka-Munk

40

35

30

25

20

15 1650

1640

1630

1620 Wavenumbers (cm-1)

1. Strong Lewis Acid 1625cm-1 2. Weak Lewis Acid 1616cm-1

1610

1600

% light sulfur reduction vs. Strong/ weak Lewis sites ratio

% light cut sulfur reduction

70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

Strong vs. Lewis Acid site ratio

1.40

1.60

Na and SiO2 Poisoning of Zn/Al2O3

GSR vs. SiO2

GSR vs. Na 50

% C u t g a s o lin e S R e d u c t io n

% Cut gasoline S Reduction

40 40

30

30

20

20 10

10 0

0.02

0.04

Na

0.06

0.08

0.1

0.5

1

1.5

%SiO2

2

2.5

Summary • Zn aluminate catalyzes the decompositon of saturated sulfur species. • Maximum sulfur reduction activity is achieved by the optimization of hydride transfer activity of the base FCC catalyst and use of Zn aluminate up to 50 % reduction. • Currently this technology is in over 40 units world wide and is expected to double in 2006. • New technology, that does not deactivate, is in commercial trial with excellent performance.