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.
