Hydrodesulfurization
Introduction
Hydrodesulfurization
first came into practice during World War II in the production of
petroleum. Sulfur reduction in gasoline
is prompted by several factors. First, many
catalysts in reformer units are sensitive to the amount of sulfur in the
feed. In fact, some bimetallic
reforming catalysts require the sulfur content to be limited to the vicinity of
1ppm or less. Second, air pollution
control standards require removal of sometimes up to 80% or more of the sulfur
that would be present in various fuel oils.
Third, some of the sulfur in gas oil fed to a catalytic cracker is in
the form of coke, which is then hydrogenated and released as sulfur dioxide in
the combustion gases. This is not
desired as this proposes environmental harms.
Fourth, the organosulfur content of the feed to the hydrocracker much be
reduced to avoid poisoning of the hydrocracking catalyst. Last, the reduction of sulfur reduces the
amount of corrosion in the refining process, improves the odor of the product,
and reduces the amount of sulfur that can poison the catalytic converter to an
automobile.
Environmental Regulations
One
of the biggest movements in recent legislation for reduction of sulfur in
gasoline products was started by a speech by Bill Clinton on May 1, 1999. He announced a new Environmental Protection
Agency regulation calling for a 90% reduction of sulfur content in automobile
gasoline in the United States by the year 2004. Similar efforts are underway around the world. The EPA document
at http://www.epa.gov/region04/oeapages/intergov/sg060499.htm
gives more details about the environmental reasons that prompted this movement,
and gives some of the new regulations that are being proposed. For example, a proposed requirement for 2004
is to allow only 30ppm of sulfur content in gasoline. The document also describes the initiatives some states are
taking to be ready for the new regulations.
Process
The
hydrodesulfurization process involves catalytic treatment with hydrogen to
convert the various sulfur compounds present to hydrogen sulfide. The hydrogen sulfide is then separated and
converted to elemental sulfur by the Claus process. From this point some of the hydrogen sulfide is oxidized to
sulfur dioxide by air and sulfur is formed by the overall reaction:
Originally
the interest in hydrodesulfurization was initially stimulated the availability
of hydrogen from catalytic reformers.
However the demand for hydrogen for hydrodesulfurization and
hydrotreating now often is more that can be generated by a refinery. Because of this, most refineries recycle the
hydrogen formed from side dehydrogenation reactions back to the inlet. Since hydrogen is so expensive to
manufacture, it is very important to run all hydrodesulfurization and
hydrotreating processes at their optimum to reduce costs.
The
supported molybdenum sulfide catalyst containing cobalt is operated under
pressures of 150-160 psi hydrogen at 300-400°C. The sulfur content in oil of 1-5% is reduced to 0.1% in gasoline
and future sulfur limits may be reduced to as little as 0.003-0.04%. For low point and middle boiling point
distillates, typical HDS reaction conditions are about 300 to 400°C and 0.7 to
5 MPa hydrogen pressure. The higher the
boiling point of the feedstock is, the higher the sulfur content. More severe operating conditions are needed
for higher fraction boiling points.
Then high pressure and low temperature combinations are used to reduce
the hydrogen consumption and corresponding costs.
HDS
reactions are exothermic. Most reactors
are adiabatic fixed beds and may be multistage. Adding additional hydrogen between the stages usually does
cooling; the term “cold-shot cooling” is used to describe this process. If the feed for the reaction conditions is a
mixed vapor and liquid, the liquid is normally caused to flow countercurrently
downward through a fixed bed catalyst, or “trickle-bed reactor”.
The
sulfur is present largely in the form of thiols, sulfides, and various
thiophenes and thiophene derivatives.
Mercaptans and sulfides react to form hydrogen sulfide and hydrocarbons.
R
and R’ are various hydrocarbon groups.
The reaction pathway for thiophene is
+2H2 H2S +C4H8
(mixed isomers)
Studies
have indicated that the hydrodesulfurization and subsequent hydrogenation
reaction occur on separate sites. The
thiophene ring is not hydrogenated before sulfur is removed, although the first
step may involve an essentially simultaneous removal of a sulfur atom and
donation of two hydrogen atoms to the structure.
For
power-law expressions, the HDS reaction appears to be between ˝ and first order
with respect to hydrogen at pressures above atmospheric. It is severely inhibited by basic nitrogen
compounds.
For
benzothiophene, substituted or unsubstituted, the thoiphene ring is
hydrogenated to the thiophane derivative before the sulfur atom is removes, in
contrast to the behavior of thiophene. The following are the reaction pathways
for benzothiophene and dibenzothiophene are as follows:
+H2 2H2 +H2S
+H2 +H2S
Catalyst
Catalysts
used in industry are derived from oxides of elements of group 6, such as Mo or
W, group 9, such as Co, and group 10, such as Ni supported on different
compounds, although the most commonly used is alumina. Catalytic activity is related to the
presence of sulfides of group 6 and group 9-10 elements; however, the most
important role of these last elements is to act as promoters. The catalyst used in HDS is almost always
CoMo/Al2O3, and sometimes NiMo/ Al2O3.
The ratio of molybdenum to cobalt is always considerably greater than 1.
The
molybdenum sulfide catalyst is prepared by impregnation of g- Al2O3 with an aqueous
solution of ammonium molybdate and cobalt nitrate. This precursor is dried and calcined, which converts the
molybdenum to MoO3. This is
then treated with a mixture of H2S and H2 or a feed
containing sulfur compounds and H2.
The resulting molybdenum catalyst is almost completely sulfided. If the catalyst is not completely sulfided,
then there is the possibility it will not be as active a catalyst. For a view of HDS catalyst vendors, visit
the following sites:
http://www.akzonobel-catalysts.com/
http://www.shepherd.ch/co.html
The
CoMo/Al2O3 catalyst is poisoned by H2S and
there is generally no method for regeneration other than running straight
hydrogen through the reactor.
However, the catalyst can be recycled to recover some of the metals
contained within. The following website
is from the largest recycler of hydrodesulfurization catalysts.
Alternatives
Since
the mechanism for the hydrodesulfurization of thiophenes is not completely
understood, there has been extensive work to try and develop the mechanism and
kinetics for the reactions in order to develop better catalysts. See the following
attached web sites for papers developing the kinetics, mechanisms, and atomic
scale insights to hydrodesulfurization.
1.
Atomic-Scale Insights into Hydrodesulfurization
Some other forms of catalysts have been
tested for their affinity for removing sulfur from thiophene compounds with
some success. Nickel treated compounds
have had some success, and while the nickel containing catalysts appear to be
better at sulfur removal, the Co-containing catalysts give slightly more oil
yield. In the end, it may be a simple
matter of economics that determines which catalyst is used.
