Naphtha paper MMM 2011 vol 146
Published on: Mar 3, 2016
Transcripts - Naphtha paper MMM 2011 vol 146
A zeolite based vapor phase adsorptive desulfurization process for naphtha
Anshu Nanoti a,⇑
, Soumen Dasgupta a
, Vasudha Agnihotri a
, Pushpa Gupta a
, Amar N. Goswami a
Madhukar O. Garg a
, Elisabeth Tangstad b
, Michael Stöcker b
, Arne Karlsson b
, Ørnulv B. Vistad b
Indian Institute of Petroleum, Dehradun 248005, India
SINTEF Materials and Chemistry, 0314 Oslo, Norway
a r t i c l e i n f o
Received 27 October 2010
Received in revised form 4 January 2011
Accepted 11 January 2011
Available online 21 January 2011
Vapor phase process
a b s t r a c t
A ﬁxed bed vapor phase adsorptive desulfurization process for naphtha range hydrocarbon fuel based on
metal ion exchanged zeolite Y adsorbents is described. The sulfur adsorption capacity is found to be
dependent on the nature of exchanged metal ions present in the zeolitic matrix. The best adsorbent for-
mulations are selected by high throughput combinatorial screening and are found to be selective for more
refractory organo-sulfur compounds such as benzo-thiophene and alkylated benzo-thiophenes present in
reﬁnery naphtha. Under the optimized process conditions, around 54 mL of reﬁnery naphtha per gram
adsorbent could be treated at breakthrough sulfur concentrations of 30 mg/L in the efﬂuent, with prac-
tically no loss of octane number. Compared to conventional catalytic HDS, the present process requires
negligible amount of hydrogen. The adsorbents are completely regenerable under controlled oxidation
at high temperature with diluted air stream and require no temperature swing between the adsorption
and regeneration cycle.
Ó 2011 Elsevier Inc. All rights reserved.
Stringent fuel quality standards being implemented world-wide
to improve the air quality are putting the reﬁning industry under
enormous pressure to produce ultra low sulfur gasoline and diesel.
In Europe, Euro IV speciﬁcations for transportation fuel require
reﬁners to limit sulfur to <50 mg/L in gasoline as well as diesel
. In the United States, the Environmental Protection Agency
(EPA) requires that the sulfur content in the motor gasoline be re-
stricted to <30 mg/L under Tier II regulations . Future legisla-
tions both in the United States and Europe (Euro V) may require
limiting sulfur to levels as low as 10 mg/L.
In India the Bharat Stage III fuel quality regulations had earlier
limited the sulfur levels to <150 mg/L in gasoline and <350 mg/L in
diesel. The latest Bharat Stage IV regulation calls for sulfur level of
50 ppmw in both gasoline and diesel .
Naphtha produced from a Fluidized Catalytic Cracking Unit
(FCCU) forms a major portion of commercial gasoline . It has
been reported that around 90% of sulfur in gasoline comes from
this FCC naphtha with sulfur contents varying from 150 to
3000 mg/L depending upon sulfur content of the feed or the end
point of the gasoline. Hydrodesulfurization (HDS) of FCC naphtha
is currently the most favored option in the reﬁning industry for
sulfur reduction but it is a capital and energy intensive process
with large hydrogen requirement and in addition loss of octane
number also occurs due to the hydrogenation of oleﬁns [5–8]. This
is especially true if ultra-low sulfur gasoline containing <10 mg/L
sulfur is to be obtained in which case the ‘‘refractory‘‘ benzothi-
ophenes and alkyl benzothiophenes have to be removed which
require severe treating conditions and increased hydrogen require-
ments. In addition to these disadvantages, the implementation of
HDS technologies also leads to increased levels of CO2 emissions
from the reﬁnery process, both from the hydrodesulfurization
reactor furnace and hydrogen production plant.
To overcome these drawbacks, improved HDS technologies
have been proposed based on improved catalysts and process
conditions, but these have to cope with octane loss as well as
decreased yield [9,10].
Beside the above mentioned processes, efforts are also being
made to develop novel low energy processes for the desulfuriza-
tion of hydrocarbon fuels keeping in mind the refractory nature
of the sulfur species and the complex composition of the reﬁnery
streams. Membrane and adsorptive desulfurization as an alterna-
tive are being studied extensively for treating various reﬁnery
streams such as gasoline, diesel, and jet fuels.
A large variety of adsorbents including metal ion exchanged
zeolites [11–14], activated carbons [15–17], mesoporous adsor-
bents , silica gels , and alumina  have been studied
for adsorptive removal of sulfur from gasoline as well as diesel
range hydrocarbons. In general, these studies have been carried
out at ambient temperatures in the liquid phase and observed
capacities of the adsorbents for sulfur were fairly low. Moreover
1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
⇑ Corresponding author. Tel.: +91 135 2525727; fax: +91 135 2660098.
E-mail address: firstname.lastname@example.org (A. Nanoti).
Microporous and Mesoporous Materials 146 (2011) 158–165
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the regeneration of the adsorbents was carried out at high temper-
atures in the range of 300–600 °C or using solvents. The main dis-
advantage of these processes is the thermal swing required for
high temperature regeneration or requirement for yet another sep-
aration process for recovery of the solvent for recycle. In adsorptive
sulfur removal in the liquid phase, the competitive adsorption of
aromatics and oleﬁns which are normally present in cracked gaso-
line also leads to loss of octane number in the gasoline.
Adsorption of thiophenic sulfur at elevated temperatures has
been more successful. Ma et al.  reported that in adsorption
at high temperatures (200 °C and above) the adsorption of aromat-
ics and oleﬁns in naphtha range hydrocarbons is reduced. This was
an interesting ﬁnding as it points to the possibility of minimizing
the octane loss associated with adsorptive desulfurization by car-
rying out the process at elevated temperatures. Sulfur removal
from commercial naphtha was reported in their work, though no
data on octane loss was available in this study. Adsorption at ele-
vated temperatures was also employed to develop three processes
which have been demonstrated at scales varying from bench to pi-
lot and commercial scale levels. The ﬁrst is the TREND process dis-
closed by RTI International in patent WO 22763/A1 involving the
desulfurization of hydrocarbon feed by a regenerable metal oxide
sorbent supported on a refractory inorganic oxide cracking catalyst
in a transport bed reactor at elevated temperature of 300 °C under
vapor phase conditions . The IRVAD process is based on moving
bed technology and uses an alumina-based adsorbent at tempera-
tures up to 240 °C . Currently work is discontinued on this pro-
cess. Another adsorption based technology known as S-ZORB
process has been developed by Conoco Phillips Co., Texas. This pro-
cess operates at temperature and pressure range of 340–420 °C
and 2–20 bar, respectively and is based on reactive adsorption on
a zinc oxide based adsorbent . A 98% removal of the sulfur from
the FCC naphtha feed has been reported with 1–1.5 point octane
loss. Four commercial units are reported to be in operation .
The adsorptive desulfurization processes at elevated temperatures
using mixed oxide adsorbents (TREND process, SZORB and IRVAD)
have used thermal oxidation with air for adsorbent regeneration.
On the other hand, with zeolite or mixed oxide adsorbents at ele-
vated temperatures, Ma et al.  have used organic solvents at
ambient temperatures for adsorbent regeneration.
The present study reports an investigation into the adsorptive
removal of thiophenic sulfur from naphtha range hydrocarbons
with a series of single and double metal ion exchanged zeolites
at elevated temperatures followed by regeneration through con-
trolled thermal oxidation. Metal exchanged zeolites have been
screened based on high throughput combinatorial chemistry and
evaluated further in vapor phase adsorption conditions in a ﬁxed
bed adsorption set up.
The metal exchanged zeolites were tested at two levels. In the
ﬁrst level, milligram quantities were synthesized and screened
using Combinatorial Chemistry techniques. The synthesis of prom-
ising adsorbents screened from this study were then scaled up to
15–20 g level and tested in a ﬁxed bed adsorptive desulfurization
2.1. Combinatorial preparation and screening of metal exchanged
Y-zeolites for thiophene adsorption
A set of ion exchanged zeolites in milligram scale were prepared
starting from Na–Y (SiO2/Al2O3 % 4.9) by high throughput parallel
methods using Combinatorial Test system.
A compound library of 48 samples was prepared by varying
parameters such as:
1) Type of cations exchanged.
2) Number of metals in each sample.
3) Sequence in which each metal has been ion exchanged.
4) Concentration of metal ion in solution.
The NaY support (500 mg, pre calcined at 500 °C for 3 h) and
cationic exchanged solutions (1.4 mL) were dosed into separate
24 well sample arrays. The solids were transferred in parallel to
the cationic solutions for ion-exchange. The concentration of metal
ions in the exchange solutions were kept at 3.66 M, 1.83 M and
1.22 M for univalent, bivalent and trivalent ion respectively. The
ﬁrst ion exchange was done at 30 °C for 24 h and the second ex-
change was at same temperature but extended to 60 h. After each
exchange the samples were thoroughly washed and dried over-
night at 50 °C. After ﬁnal exchange the solid samples were calcined
at 500 °C for 3 h. Breakthrough measurements of sulfur adsorption
were carried out at 45 °C with mixtures of thiophene in ethylene as
model feed (thiophene concentration in the feed was 500 mg/L)
using a 48 reactor test system at SINTEF. Four different samples
were tested simultaneously and sulfur analysis in the efﬂuent
was by mass spectrometry (VG ProLab from Thermo ONIX).
2.2. . Fixed bed adsorptive desulfurisation
The ﬁxed bed vapor phase adsorptive desulfurization experi-
ments have been performed in an automated unit at IIP as depicted
in Fig. 1. The unit consists of an adsorption column of 19 mm inter-
nal diameter placed inside a three zone electric furnace. The adsor-
ber column is packed with 10 g of the adsorbent being evaluated.
For comparison the amount of zeolite mass was kept constant in
runs with different ion exchanged zeolites. Three mass ﬂow con-
trollers are provided for ﬂow control of hydrogen, nitrogen and
air streams to the adsorber. During the adsorption cycle, liquid
hydrocarbon feed is pumped by a HPLC pump into a pre-heater
where it vaporizes and the vapors are mixed with hydrogen and
nitrogen and ﬂow into the adsorber maintained at the experimen-
tal temperature. The adsorber efﬂuent then ﬂows through a con-
denser and enters a high pressure gas–liquid separator where gas
and liquid streams are separated. The gas stream exits the separa-
tor under pressure control and is sent to vent while the liquid
stream which is the desulfurized product is collected in G/L sepa-
rator. The sampling of the desulfurized liquid product was started
once the liquid level in the gas–liquid separator reached a
predetermined level corresponding to a hold up of 75 mL in the
separator. This hold up volume has been incorporated in the exper-
imental breakthrough data reported in this paper. The samples
were collected at constant time interval and analyzed for total sul-
fur content by X-ray/UV ﬂuorescence (XRF/UVF) or by UV spectros-
copy. Component type sulfur analysis was also done by GC with a
Sulfur Chemiluminescence Detector (Siemens Instruments Model
355) and hydrocarbon type analysis (parafﬁns, iso-parafﬁns ole-
ﬁns, naphthenes, aromatics) by gas chromatography with FID
Reﬁnery Gas Analyzer.
In the ﬁxed bed vapor phase adsorption studies, both model
mixture of 2-methyl thiophene in n-hexane containing sulfur con-
centration of 1000 mg/L as well as a reﬁnery hydrotreated naphtha
containing 180 mg/L were used as feed.
2.3. Adsorbent thermal regeneration
After the completion of the adsorption cycle, the ﬁxed bed ad-
sorber is taken through a regeneration cycle involving burning
off the adsorbed sulfur with precisely controlled mixture of air–
A. Nanoti et al. / Microporous and Mesoporous Materials 146 (2011) 158–165 159
nitrogen at 350 °C followed by a hydrogen activation phase at the
same temperature. The ﬂow of regeneration gas is countercurrent
to the feed ﬂow during the adsorption cycle. The efﬁciency of the
regeneration was tested through breakthrough curve measure-
ment with a model feed mixtures of 2-methyl thiophene in
n-hexane (500 mg/L) through repeated cycles of adsorption–
3. Results and discussion
3.1. Combinatorial screening
Breakthrough measurements, carried out on the combinatori-
ally prepared adsorbents (single and double ion exchanged Y-zeo-
lites) at 45 °C using feed mixtures of 500 mg/L thiophene in
ethylene, were used to calculate the adsorption capacities of the
adsorbents for sulfur. These capacities are reported in terms of to-
tal sulfur in Fig. 2. The data indicate that the behavior of the single
and double ion-exchanged systems differed greatly with sulfur
adsorption capacities ranging from below 10–60 mg/g. Several of
the ion exchanged Y zeolite system adsorbed more than 50 mg/g
adsorbent at these conditions and several of the adsorbents were
as good as or even better than Cu–Y which is one of the extensively
studied sulfur adsorbent for naphtha range hydrocarbon fuel. The
adsorption capacities of ion exchanged Y-zeolite systems giving
the best results in terms of exhibiting adsorption capacities greater
than 55 mg/g are shown compiled in Table 1.
3.2. Fixed bed adsorption studies with model feed mixtures and
Based on the combinatorial screening results, the nine best
adsorbents reported in Table 1 were selected for further detailed
evaluation in the ﬁxed bed adsorption. Break through data of efﬂu-
ent sulfur concentration versus time were collected under identical
conditions (Table 2) of vapor phase adsorption using a model FCC
gasoline mixture containing 2-methyl thiophene in n-hexane as
feed. Typical breakthrough data are plotted in Figs. 3 and 4 up to
run times corresponding to efﬂuent sulfur concentrations of
around 150 mg/L (Bharat Stage III). From this, the volume of feed
treated per gram adsorbent at efﬂuent sulfur concentration of
150 mg/L has been interpolated and the comparative data for the
nine adsorbents has been shown in Fig. 5. It becomes evident that
four adsorbents namely Cu–Ni–Y, Zn–Y, Cu–Mn–Y and Cu–Ce–Y
stand out in showing high levels of 44–54 mL feed treated per
The regenerability of the adsorbents was tested by carrying out
repeated adsorption–thermal regeneration cycle tests. In these set
of experiments, for each adsorbent, the adsorption run was per-
formed at a ﬁxed set of conditions of temperature 250 °C, pressure
3 bar (g) and feed ﬂow WHSV 8 hÀ1
with a model feed of 2-methyl
thiophene in n-hexane (500 mg ‘S0
/L). After each adsorption exper-
iment the regeneration was performed under uniform conditions.
The regeneration cycle consisted of two steps. The ﬁrst step was
an oxidation step at 350 °C with diluted air (3 vol.% O2) and the
second step was a reduction step at 350 °C with 6 vol.% H2. Fig. 6
shows typical sulfur breakthrough curves obtained in successive
runs following such a repeated adsorption–regeneration cycles
with Cu–Ni–Y adsorbent. The breakthrough curves are super
imposable within experimental error indicating reproducible rege-
nerability of the adsorbent.
To test the efﬁcacy of the adsorbents in sulfur removal from ac-
tual naphtha, two of these adsorbents showing high capacities for
sulfur removal from model feeds namely, Cu–Mn–Y and Cu–Ni–Y
were selected for further studies with an actual reﬁnery hydro-
treated naphtha having a sulfur concentration of 180 mg/L. Break-
through data with this naphtha as feed were generated under
Fig. 1. Diagram of gasoline desulfurization unit.
160 A. Nanoti et al. / Microporous and Mesoporous Materials 146 (2011) 158–165
Sulfur adsorption capacities of nine best combinatorially screened adsorbents from
feed: thiophene (500 mg/L) in ethylene at 45 °C.
Exchanged zeolite Y adsorbents
[M1 À M2 Y] and (M1 + M2) À (M1 + M2) Y
M1 À M2 Y: sequential exchange ﬁrst with
metal ion M1 followed by M2
(M1 + M2): exchange with mixture of M1 and
Adsorption capacity for
sulfur [mg ‘S’/g]
Cu–Ni Y 60
Cu–K Y 62
Cu–Ce Y 57
Cu–Co Y 58
(Cu + Mn) À (Cu + Mn) Y 55
K–Cu Y 57
(Cu + Ce) À (Cu + Ce) Y 55
Zn–Zn Y 55
Mn–Mn Y 55
Conditions for vapor phase adsorptive desulfurization.
Feed Model mixture of 2-methyl
thiophene in n-hexane
Sulfur content [mg/
Feed ﬂow rate [mL/
H2 ﬂow [NLPM]a
N2 ﬂow [NLPM] 1.5 1.5
Normal litre per minute.
Metal Exchanged Zeolite Y Adsorbent
Fig. 2. Adsorption capacities of combinatorially screened adsorbents.
15 25 35 45 55
Volume of Feed Treated/Gram Adsorbent (ml/g)
Fig. 3. Vapor phase breakthrough curves with feed 2-methyl thiophene in n-hexane
(1000 mg ‘S’/L).
Fig. 4. Vapor phase breakthrough curves with feed 2-methyl thiophene in n-hexane
(1000 mg ‘S’/ L).
A. Nanoti et al. / Microporous and Mesoporous Materials 146 (2011) 158–165 161
vapor phase conditions reported in Table 2 and are plotted in Fig. 7.
The experimental conditions were the same as for model feed mix-
ture except that the temperature was increased from 250 °C to
350 °C due to the higher ﬁnal boiling point (FBP) of the naphtha
A comparison of the breakthrough data of these adsorbents for
an efﬂuent sulfur concentration of 30 mg/L is shown in Fig. 8. The
tests indicate that over 31–54 mL feed can be treated per gram
adsorbent before sulfur levels exceed 30 mg/L in the efﬂuent. In
the same ﬁgure we have included the data reported by Ma et al.
 with Ni based adsorbent. This was for adsorptive removal of
sulfur from a real gasoline containing 210 mg/L sulfur, at elevated
Fig. 5. Comparison of adsorbents for ‘S0
removal from model feed mixture (2-methyl thiophene and hexane, 1000 mg ‘S0
Fig. 6. Test of adsorbent regenerability by performing repeated adsorption–desorption cycles on Cu–Ni Y adsorbent and using 2-methyl thiophene in n-hexane (500 mg ‘S0
Fig. 7. Breakthrough curves for vapor phase adsorption with hydrotreated naphtha
(180 mg ‘S0
Fig. 8. Comparison of sulfur removal from hydrotreated naphtha.
162 A. Nanoti et al. / Microporous and Mesoporous Materials 146 (2011) 158–165
temperatures of 200 °C. The ﬁgure indicates that the adsorbents in
the present study can treat higher volumes of naphtha per gram,
with our best adsorbent Cu–Mn–Y capable of treating more than
twice the volume of feed per gram adsorbent compared to Ma
et al. .
The efﬂuent samples taken at different times in the naphtha
runs were also analyzed for component sulfur type by GC-SCD
. Fig. 9a shows the GC-SCD of the naphtha feed. Figs. 9b–d
show the GC-SCD chromatograms of the efﬂuent samples obtained
at different run times using Cu–Mn–Y adsorbent. It is interesting to
note that whereas the presence of benzo-thiophene and alkylated
benzo-thiophenes is distinctly indicated in the chromatogram for
the naphtha feed, the intensity of peaks corresponding to these
compounds are almost absent in the efﬂuent samples up to a run
time below 255 min. These peaks strongly appear again at
255 min which correspond to the sulfur breakthrough time for this
adsorbent. These results indicate that the adsorbent selectively
removes the benzo-thiophenes and alkylated benzo-thiophenes
from the naphtha, which happen to be the most refractory
sulfur compounds and difﬁcult to remove in conventional
Hydrocarbon type analysis of the desulfurized naphtha product
obtained using Cu–Mn–Y adsorbent was carried out by gas chro-
matography with FID from which Research Octane Number
Fig. 9a. GC-SCD chromatogram of naphtha feed (MTP: methyl thiophene, ETP: ethyl thiophene, NBS: n-butyl sulﬁde, BT: benzothiophene).
Fig. 9b. GC-SCD chromatogram of naphtha feed (MTP: methyl thiophene, ETP: ethyl thiophene, BT: benzothiophene).
Fig. 9c. GC-SCD chromatogram of naphtha feed (MTP: methyl thiophene, ETP: ethyl thiophene, BT: benzothiophene).
A. Nanoti et al. / Microporous and Mesoporous Materials 146 (2011) 158–165 163
(RON) could be estimated and the results are compared with the
naphtha feed in Table 3. It is noteworthy that there is negligible oc-
tane loss in the desulfurized product.
The experiments have demonstrated that it is possible to lower
the sulfur levels in a hydrotreated naphtha from 180 mg/L to less
than 30 mg/L by vapor phase adsorption on exchanged zeolite at
350 °C in presence of small amounts of hydrogen (hydrogen to
hydrocarbon ratio is 50 NL/L), with minimum octane loss. In com-
parison, the typical hydrogen to hydrocarbon ratio in catalytic
naphtha HDS processes is in the range 150–450 nL/L i.e. three to
nine times more than the present process . The adsorbent
is also regenerable through controlled burning off of the adsorbed
sulfur compounds in the same temperature range. The fact that the
adsorbent selectively removes the refractory sulfur compounds at
a temperature close to the operating conditions in a typical naph-
tha hydro-desulfurization (HDS) reactor, suggests that the adsorp-
tive desulfurization can be used proﬁtably as a polishing step in a
conventional hydro desulfurization unit to reduce the sulfur level
in the efﬂuent from the HDS unit to <30 mg/L with minimum oc-
tane loss and no additional hydrogen requirement. Continuous
processing will imply use of two adsorbers in tandem, with one ad-
sorber in the adsorption cycle and the other in the thermal regen-
eration cycle. There is no temperature swing required between
adsorption and regeneration. Sulfur oxides produced in the efﬂuent
from the adsorber in the regeneration cycle can be treated in exist-
ing reﬁnery facilities for ﬂue gas desulfurization.
High throughput combinatorial chemistry has been used to se-
lect adsorbents from among single and double metal ion-ex-
changed zeolites for selective removal of sulfur from model feed
mixtures of thiophene in ethylene. Based on the breakthrough
capacities observed, nine adsorbents were selected for further
studies for sulfur removal from model mixtures of 2-methyl thio-
phene in hexane in ﬁxed bed adsorption under vapor phase condi-
tions. The adsorbents showing among the highest breakthrough
capacities, Cu–Mn Y and Cu–Ni Y, were tested with actual reﬁnery
naphtha. They efﬁciently reduced the sulfur levels from 180 mg/L
to <30 mg/L under conditions similar to typical conventional naph-
tha hydrodesulfurization processes. The adsorbent Cu–Mn Y could
treat over 54 mL naphtha feed per gram adsorbent before the efﬂu-
ent exceeds 30 mg/L. The breakthrough capacities observed with
these two adsorbents are higher than what reported by Ma et al.
 for sulfur adsorption from an actual naphtha. Moreover there
is the possibility of producing efﬂuent with negligible octane loss
and GC-SCD analysis also indicates that the refractory benzo thio-
phenes can be selectively removed from naphtha. The adsorbents
are regenerable under controlled oxidation at high temperature
with diluted air stream and require no temperature swing between
the adsorption and regeneration cycle.
These ﬁndings suggest that the adsorptive desulfurization can
proﬁtably be utilized as a polishing step in a conventional naphtha
hydrodesulfurization unit to reduce the sulfur levels in the HDS
efﬂuent from 150–180 mg/L to <30 mg/L with the possibility of
minimum octane loss and with minimum hydrogen consumption.
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Fig. 9d. GC-SCD chromatogram of naphtha feed (MTP: methyl thiophene, ETP: ethyl thiophene, NBS: n-butyl sulﬁde, BT: benzothiophene).
Hydrocarbon type analysis of naphtha feed and desulfurized product.
Hydrocarbon type Naphtha feed Desulfurized naphtha product
% Aromatics 24.07 28.45
% Iso-parafﬁns 24.76 26.93
% Naphthenes 27.73 21.7
% Oleﬁns 3.85 3.5
% Parafﬁns 13.71 15.16
% Unindentiﬁed 5.88 4.26
RON 67.15 66.9
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