EFFECT OF THE REMOVAL OF ORGANO-NITROGEN COMPOUNDS IN
THE HYDRODESULFURIZATION PROCESS OF STRAIGHT RUN GAS
OIL.
José L. García 1, Luis F. Ramírez 1, Heriberto Vaca 1, Florentino R. Murrieta 1
1
Instituto Mexicano del Petróleo, A. P. 14 – 805, D. F., México.
Introduction
The presence of organo-sulfur compounds in the crude oil and its fractions is highly
undesirable since they are the cause of corrosion and environmental problems.
Hydrodesulfurization (HDS) is the conventional method for effective removal of
organo-sulfur compounds from oil fractions by converting organo-sulfur compounds
into hydrogen sulfide. However, for remove the refractory organo-sulfur compounds
these processes require severe conditions, such as high temperature up to 623 K and
pressure which may reach 6894 kPa. Further drawbacks are the high consumption of
hydrogen and the expensive catalysts used. In response to the need for production of
very low-sulfur diesel fuels (< 0.0015 wt % sulfur), several companies are exploring
new ways of increasing the effectiveness of HDS or alternate desulfurization
processes.
On the other hand, diesel fuels are prepared from mixtures of straight run distillates
and cracked products of heavier feedstocks. These fractions generally contain low
concentrations of organo-nitrogen compounds. Organo-nitrogen compounds are well
known as poison of refinery catalysts and cause trouble in the HDS, even when they
are present in very low concentrations [1]. Usually, nitrogen compounds in oil
fractions can be classified into two main chemical classes: basic and neutral. The
predominant family in basic nitrogen compounds is the pyridine derivatives, whereas
the neutral nitrogen compounds are mainly pyrrole derivatives. The basic nitrogen
compounds have been considered stronger inhibitors for the HDS reactions than the
neutral compounds [2-6]. In order to get an appreciation of the improvement in the
HDS process to achieve low sulfur diesel, we evaluated the inhibiting effect of
organo-nitrogen compounds present in straight run gas oil in the HDS process.
Results and Discussion
The straight run gas oil (SRGO) containing 1.3985 and 0.0357 wt % of sulfur and
nitrogen, respectively. The SRGO was obtained from a Mexican refinery. HDS
process was conducted in a pilot plant with a trickle bed reactor, the evaluation was
done at the following conditions: isothermal up flow reactor, LHSV=2.5 h-1,
temperature=623.15, 633.15 y 643.15 K, and pressure=5492 kPa. The catalyst used
was a commercial catalyst (CoMo/γ-Al2O3). The removal of organo-nitrogen
compounds is based on a liquid chomatography separation of SRGO, it was done with
various types of commercial adsorbents at room temperature; these adsorbents were
selected according with their textural and acid properties.
The total sulfur concentration was measured by a total sulfur analyzer Horiba SFLA1800 with a X-ray fluorescence detector, the accuracy of the reported values is
estimated in 5 %. The total nitrogen content was carried out by a gas chromatograph
with a chemiluminescence detector. Quantitative analysis for basic nitrogen content
was carried out by titration according to UOP-313 method, the accuracy in both cases
is estimate in ± 30 ppm.
The procedure developed for the present work is considerably simpler than the
previously reported [7,8]. Most of adsorbents used to removal organo-nitrogen
compounds shown the ability to separate basic nitrogen compounds in accordance
with its level of Lewis acidity. In general, the removal of organo-nitrogen compounds
from SRGO lead to an increase of the transformation rate of the organo-sulfur
compounds through HDS process, see Figure 1. The results indicate that trace
amounts of organo-nitrogen compounds play a key role in HDS process, this is in
accordance with previous publications [2,3]. A formula based on the Langmuir model
has been found to fit the experimental data. Finally, the removal of organo-nitrogen
compounds from SRGO in the range of practical interest (<0.0020) to obtain ultra
low sulfur diesel (ULSD) can be achieved effectively by a combination of adsorbents.
Figure 1. Effect of the initial organo-nitrogen compounds
concentration on HDS of SRGO as a function of temperature.
DIESEL SULFUR CONTENT (WT %)
600
TOTAL NITROGEN CONTENT
IN THE SRGO (WT%)
0.0357
0.0174
0.0181
0.0344
500
400
300
200
100
350
355
360
TEMPERATURE FOR DHS
365
(oC)
References
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8. W. Min, PCT/KR99/00338 [WO 99/67345] (1999)
370