Ultra Low Sulfur Diesel Fuel Production by Two-Stage Process with ...

Ultra Low Sulfur Diesel Fuel Production by
Two-Stage Process with Gas/Liquid Separation System
Masaomi Amemiya, Masanari Minatoya, Ryutaro Koide, Yasuhito
Goto, Manabu Kawabata, Katsuaki Ishida and Hideo Segawa
Petroleum Refining Research & Technology Center, Japan Energy
Corporation,
3-17-35 Niizo-Minami Toda-shi, Saitama 335-8502, Japan
Introduction
Further tightening of diesel sulfur specifications has been decided
and proposed in worldwide. The focus of the new specifications is
reduction of suspended particulate matters (SPM) and NOx emission
from diesel-fueled vehicles.
In December, 2000, Ministry of the Environment, Japan announced
a new sulfur specification for diesel fuel. According to the
specifications, the maximum permissible sulfur content of diesel will
be 50ppm from the end of 2004 1 .
Substantially “sulfur-free” diesel (10-15ppm or less) has been
proposed as future diesel specifications. In June, 2000,
Environmental Protection Agency, USA proposed 15ppm or less as a
new diesel sulfur specification from 2006 2 . In March, 2001, German
government announced a new incentive tax policy to encourage clean
fuel supply. According to the announcement, incentive tax
(0.03DM/L) is given to 10ppm or less sulfur diesel from January,
2003 3 . EU has already accepted the new German policy. In May,
2001, the European Commission proposed a mandatory “zero sulfur”
specification (10ppm or less) from 2011 4 . Japan is also considering a
lower sulfur diesel specification than 50ppm 1 . Thus much attention
is given to effective technological solutions for ultra-low sulfur diesel,
particularly sulfur-free diesel production.
Features of HDS reaction of gas oil fraction are summarized in the
following 5 .
(1) The feedstock contains various sulfur compounds with widely
different reactivities. Sulfides, benzothiophenes, and
dibenzothiophene (DBT) (reactive sulfur compounds) are relatively
easy to be desulfurized. Conversely, 4-methyldibenzothiophene (4-
MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT)
(refractory sulfur compounds) are very hard to be desulfurized.
(2) Hydrogen sulfide and ammonia, gaseous products of HDS and
hydrodenitrogenation (HDN) reactions seriously inhibit HDS
reaction.
(3) The reaction conditions around the inlet and the outlet of an HDS
reactor are greatly different. In the reaction zone near the inlet, both
reactive and refractory sulfur compounds coexist, and the
concentrations of poisoning gaseous compounds are relatively low.
In the reaction zone near the outlet, however, refractory sulfur
compounds selectively remain, and the concentrations of poisoning
gaseous compounds are very high.
We have developed two solutions for ultra-low sulfur diesel
production considering the above-mentioned three key points.
A simple solution is “CoMo/NiMo Catalyst Relay” system 6 .
“CoMo/NiMo Catalyst Relay” system can achieve 50ppm sulfur
diesel production without major revamp of conventional deep HDS
units. In “CoMo/NiMo Catalyst Relay” system, the first bed catalyst
is CoMo type. CoMo catalyst is the pretreatment catalyst for ultra
deep HDS over the main NiMo catalyst and plays a role of HDS of
reactive sulfur compounds such as DBT. In “CoMo/NiMo Catalyst
Relay” system, the second bed catalyst is NiMo type. NiMo catalyst
is the main catalyst for ultra-low sulfur diesel production and
achieves HDS of refractory sulfur compounds such as 4-MDBT and
4,6-DMDBT, in the presence of high concentrations of catalyst
poisoning materials such as hydrogen sulfide and ammonia.
A more effective solution is two-stage process with gas/liquid
separation in the middle of the unit. The two-stage process with
gas/liquid separation has great potential to sulfur-free diesel
production (S = 10ppm or less). Removal of produced hydrogen
sulfide and ammonia in the middle of the unit accelerates HDS of the
following 2nd-stage. NiW catalyst is applied to the 2nd-stage as an
effective catalyst because the concentration of hydrogen sulfide and
ammonia is relatively low.
Experimental
CoMo Catalyst. Commercial diesel deep HDS CoMo catalyst
was used.
NiW Catalyst. NiW catalyst was prepared by impregnation of
tungsten and nickel compounds on a silicaalumina support.
Hydrotreating experiments for Two-Stage Process with
Gas/Liquid Separation. The description of Two –Stage process
pilot plant is shown in Figure 1. Two fixed bed reactors in seriesflow
configuration are used for the hydrotreating experiments. A
high pressure separator and a stripper with H 2 gas were equipped
between the 1st-stage and 2nd-stage reactors as a gas/liquid
separation system. The 1st-stage product oil through the gas/liquid
separation system and fresh H 2 were supplied to the 2nd-stage reactor.
All the experiments were performed under the conditions of P(H 2)
5.0 MPa, H2/Oil 200 NL/L for each reactor, and total LHSV 1.5 h -1 .
Reaction temperature was a variable to obtain various product sulfur
levels.
Feed + H2
Reactor
H2
Stripper
H2
Separator
Gas (H2S)
Reactor
Figure 1. Description of Two –Stage process pilot plant
Products
Feedstock
Middle-east straight-run gas oil fraction was used as feedstock of
hydrotreating experiments. Properties are summarized in Table 1.
Table 1. Properties of Feedstock
Item Unit Feedstock
Density@15 Ž [g/cm 3 ] 0.849
Sulfur [wtppm] 16900
Nitrogen [wtppm] 61
Distillation (ASTM D 86) IBP [ o C] 223
T10% [ o C] 256.5.
T50% [ o C] 287
T90% [ o C] 331.5
EP [ o C] 347
Fuel Chemistry Division Preprints 2002, 47(2), 460

Analysis
Sulfur content of product oil was determined by XRF. GC-SCD
analysis was also conducted to determine DBT, 4-MDBT, and 4,6-
DMDBT.
Results and Discussion
Concept of Two-Stage Process with Gas/Liquid Separation.
Conceptual diagram of the sulfur compound distribution in two-stage
process with gas/liquid separation is shown in Figure 2. In the 1ststage,
HDS of "Reactive Sulfur Compounds" is perfectly carried out.
As a result of the HDS and HDN reactions in the 1st-stage, a large
amount of hydrogen sulfide and ammonia are produced. These
compounds seriously inhibit HDS of "Refractory Sulfur Compounds"
in the 2nd-stage. Removal of these inhibitors between the 1st-stage
and the 2nd-stage accelerates HDS in the 2nd-stage. Based on the
estimation of gas/liquid equilibrium, 86% of hydrogen sulfide can be
removed, and more H 2S can be removed by application of stripping
with H 2.
NiW catalyst can be applied to the 2nd-stage because the
concentration of hydrogen sulfide and ammonia is comparatively low.
NiW catalyst is superior to the CoMo catalyst and NiMo catalyst in
the presence of low concentrations of hydrogen sulfide.
Figrure 2. Conceptual diagram of the sulfur compound distribution
in two-stage process with gas/liquid separation
Performance of Two-Stage Process with Gas/Liquid
Separation. The performance of two-stage process with gas/liquid
separation is shown in Table 2. In the base case, both the 1st-stage
and 2nd-stage catalysts are CoMo catalyst and gas/liquid separation
is not employed. The required reaction temperature for 50ppm
product sulfur is 356 o C.
In the case of only process improvement, both the 1st-stage and 2ndstage
catalysts are CoMo catalyst and gas/liquid separation is
employed. The required reaction temperature for 50ppm product
sulfur is 336 o C. Simple application of the gas/liquid separation
without modifying the catalyst improves 20 o C of the required
reaction temperature for 50ppm-sulfur diesel production.
In the case of both process and catalyst improvements, the 1st-stage
and the 2nd-stage catalysts are CoMo catalyst and NiW catalyst,
respectively. Gas/liquid separation is employed. The required
reaction temperature for 50ppm product sulfur is 332 o C. Application
of the gas/liquid separation system and NiW catalyst for the 2ndstage
improves 24 o C of the required reaction temperature for 50ppmsulfur
diesel production relative to the base case. (4 o C lower reaction
temperature than the case of only process improvement). Figure 3
shows the potential to “sulfur free” diesel production.
In case of applying the NiW catalyst to the 2nd-stage, it is possible
to produce 15ppm-sulfur diesel and 5ppm-sulfur diesel at 340 o C and
at 350 o C, respectively.
lnkHDS
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
350 Ž
5ppm
340 Ž
15ppm
1.60 1.62 1.64 1.66 1.68 1.70
1000/T [K -1 ]
CoMo/CoMo
CoMo/NiW
Figure 3. Potential to “sulfur free(S = 10 ppm or less)” diesel
production by two-stage process with gas/liquid separation. (Feed 3,
P(H 2): 5.0 MPa, H2/Oil: 200 NL/L, Total LHSV: 2.0 h-1)
1st-stage (50vol%) / Separator / H 2S Stripper / 2nd-stage (50vol%)
Table 2. Performance of Two-Stage Process with Gas/Liquid
Separation
Improvement Base Process Proscess & Catalyst
1st -stage catalyst CoMo CoMo CoMo
Gas/Liquid Separation No Yes Yes
2nd-stage catalyst CoMo CoMo NiW
Required Reaction Temperature
for 50 ppm Product Sulfur
356 336 332
Conclusions
The two-stage process with gas/liquid separation not only achieves
ultra-low sulfur diesel production (S = 50ppm or less) under more
beneficial conditions but also has great potential to sulfur-free diesel
production (S = 10ppm or less). Removal of produced hydrogen
sulfide and ammonia in the middle of the unit accelerates HDS of the
following 2nd-stage. NiW catalyst can be applied to the 2nd-stage.
Acknowledgement. The Research of the two-stage process with
gas/liquid separation has been entrusted by the New Energy and
Industrial Technology Development Organization under a subsidy of
the Ministry of Economy, Trade and Industry.
References
1) http://www.env.go.jp/press/file_view.php3?serial=791&hou_id=1243.pdf
(November 1, 2000, Japanese).
2) http://www.epa.gov/fedrgstr/EPA-AIR/2000/June/Day-02/ (June 2, 2000).
3) http://www.bmu.de/presse/2001/pm599.htm (March 13, 2001).
4)http://europa.eu.int/rapid/start/cgi/guesten.ksh?p_action.gettxt=gt&doc=IP/0
1/681|0|AGED&lg=EN (May 11, 2001).
5) Kabe, T.; Ishihara, A.; Qian, W. “Hydrodesulfurization and
Hydrodenitrogenation
Chemistry and Engineering”, Kodansha, Tokyo
(1999)
and cited therein.
6) Koide, R.; Goto, Y.; Kawabata, M.; Ishida, K. Prepr. Div. Petrol. Chem.,
Am. Chem. Soc., 2001, 46, 398-401.
Fuel Chemistry Division Preprints 2002, 47(2), 461