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Developing Technologies and Ultra-Low-Sulfur Alternatives
Sulfur Adsorption
One new technology on the horizon is the “S Zorb” processing under development
by Phillips Petroleum. S Zorb has been promoted for gasoline desulfurization
to meet EPA’s Tier 2 requirements. The major distinction of this process
from conventional hydrotreating is that the sulfur in the sulfur-containing
compounds adsorbs to the catalyst after the feedstock-hydrogen mixture
interacts with the catalyst. Thus the catalyst needs to be regenerated
constantly. Phillips is promoting the S Zorb process for highway diesel
as potentially having lower capital cost than conventional hydrotreating
options and reportedly is on the fast track to demonstrate the process
in a pilot plant in 2001.a Phillips estimates on-site capital costs at
$1,000 to $1,400 per barrel per day.
Biodesulfurization
Biodesulfurization is another innovative technology, which uses bacteria
as the catalyst to remove sulfur from the feedstock. In the biodesulfurization
process, organosulfur compounds, such as dibenzothiophene and its alkylated
homologs, are oxidized with genetically engineered microbes, and sulfur
is removed as a water-soluble sulfate salt. Several factors may limit the
application of this technology, however. Many ancillary processes novel
to petroleum refining would be needed, including a biocatalyst fermentor
to regenerate the bacteria. The process is also sensitive to environmental
conditions such as sterilization, temperature, and residence time of the
biocatalyst. Finally, the process requires the existing hydrotreater to
continue in operation to provide a lower sulfur feedstock to the unit and
is more costly than conventional hydrotreating.b Biodesulfurization has
been tested in the laboratory, but detailed engineering designs and cost
estimates have not been developed.
Sulfur Oxidation
The latest entry in unconventional desulfurization involves sulfur oxidization.
This process creates a petroleum and water emulsion in which hydrogen peroxide
or another oxidizer is used to convert the sulfur in sulfur-containing
compounds to sulfone.c The oxidized sulfone is then separated from the
hydrocarbons for post-processing. Most of the peroxide can be recovered and recycled. The major advantages of this new technology include
low cost, lower reactor temperatures and pressures, short residence time,
no emissions, and no hydrogen requirement.
Advocates for the sulfur oxidation technology estimate capital costs at
$1,000 per barrel of daily installed capacity—less than half the cost of
a new high-pressure hydrotreater.d The technology preferentially treats
dibenzothiophenes, one of streams that is most difficult to desulfurize,
but it does not work as well on straight-run distillate. Because the process
removes molecules containing sulfur, some volume losses also occur. One
company working on the technology has proposed installation of 1,000 to
5,000 barrel per day units at distribution terminals to “polish” material
that might otherwise be downgraded. Construction of a pilot plant is planned,
but to date there has been no real-world demonstration of the process.
Fischer-Tropsch Diesel and Biodiesel
One way to add to ULSD supply without desulfurization is to rely on a non-oil-based
diesel. The Fischer-Tropsch process, for example, can be used to convert
natural gas to a synthetic, sulfur-free diesel fuel. Two gas-to-liquids
(GTL) facilities have operated commercially: the Mossgas plant in South
Africa with output capacity of 23,000 barrels per day and the Shell Bintulu
plant in Malaysia at 12,500 barrels per day. Other plants are in the planning
stages.
Commercial viability of GTL projects depends on capital costs, the market
for petroleum products and possible price premiums for GTL fuels, the value
of byproducts such as heat and water, the cost of feedstock gas, the availability
of infrastructure, the quality of the local workforce, and potential government
subsidies. Capital costs for GTL projects are currently less than $25,000
per daily barrel of capacity. An EIA analysis of a hypothetical GTL project
estimated the cost of GTL fuel at almost $25 per barrel in 1999 dollars.
Thus, a GTL project with present technology could be cost-competitive only
if investors were confident that crude oil prices would stay in the range
of $25 to $30 per barrel and natural gas feedstock prices would remain
at 50 cents per thousand cubic feet.e
A second way to avoid desulfurization is with biodiesel made from vegetable
oil or animal fats. Although other processes are available, most biodiesel
is made with a base-catalyzed reaction. A fat or oil is reacted with an
alcohol, such as methanol, in the presence of a catalyst to produce glycerine
and methyl esters or biodiesel. The methanol is charged in excess to assist
in quick conversion and recovered for reuse. The catalyst, usually sodium
or potassium hydroxide, is mixed with the methanol. Increased production
of biodiesel could create more surfactants than the market would be able to absorb. Biodiesel is a strong solvent and can dissolve
paint as well as deposits left in fuel lines by petroleum-based diesel,
sometimes leading to engine problems. Biodiesel also freezes at a higher
temperature than petroleum-based diesel. Biodiesel advocates claim that
a 1-percent blend of biodiesel can improve lubricity by as much as 65 percent.
At least eight companies are marketing biodiesel in all parts of the United
States, according to the National Biodiesel Board.f
aU.S. Environmental Protection Agency, Regulatory Impact Analysis: Heavy-Duty
Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Requirements,
EPA420-R-00-026 (Washington, DC, December 2000), Chapter IV, pp. IV-31–IV-32. bNational Petroleum Council, U.S. Petroleum Refining: Assuring the Adequacy and Affordability of Cleaner Fuels (June 2000), p. 75.
cSulfone is any of various sulfur-containing organic compounds having a bivalent radical SO2 attached to two carbon atoms.
dR.E. Levy et al., “UniPure’s ASR-2 Diesel Desulfurization Process: A Novel, Cost-effective Process for Ultra-Low Sulfur Diesel,” presented at the National
Petrochemical and Refining Association 2001 Annual Meeting (New Orleans,
LA, March 18-20, 2001).
e“Gas-to-Liquids Technology: The Current Picture,” International Energy
Outlook 2000, DOE/EIA-0494(2000) (Washington, DC, March 2000), pp. 59-60;
and S. Weeden, “Financial Commitments Brighten 2001 GTL Prospects,” Oil
& Gas Journal (March 12, 2001).
fWeb site www.biodiesel.org/marketers.htm.
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