FOSSIL FUEL CLEANING PROCESSES 419
to that needed for desulfurization, is high indicating that high
sulfur content of feed precludes setting of operating condi-
tions to minimize conversion. In fact, naphtha production
ranges from 7–15%, mid-distillates from 15–23%.
The Isomax processes A broad spectrum of fi xed bed
desulfurization and hydrocracking processes are now in oper-
ation throughout the world. They are characterized by their
ability to effectively handle a wide range of crude feedstocks.
In addition, some of the processes are capable of directly desul-
furizing crude oil while others treat only residual stocks.
Rather than discuss each process individually, a compar-
ative summary of the major ones is presented in Table 3.
There are many other processes which in one way or
another effect a reduction in the amount of sulfur burned
in our homes and businesses. All of them use some type
of proprietary catalytic system, each with its own peculiar
optimum operating ranges with regard to feed composition
and/or reactor conditions.
The hydrodesulfurization process is still relatively
expensive (in 1989 more than 75¢/BBL) by petroleum pro-
cessing standards. The capital investment for large reactors
which operate at high pressures and high temperatures, the
consumption of hydrogen during the processing and the use
of large volumes of catalyst with a relatively short life all
contribute to the costs. In addition, processing costs also
depend on the feedstock characteristics.
But when one considers the awesome annual alternative
of 30 million tons of sulfur dioxide being pumped into the
atmosphere, the cost seems trifl ing indeed.Desulfurization of Natural GasApproximately 33% of the natural gas in the United States
and over 90% of that processed in Canada is treated to
remove normally occurring hydrogen sulfi de. The recovered
sulfur, which now accounts for about 25% of the free world’s
production is expected to increase in the future.Current processes may be classifi ed into four major
categories:1) Dry Bed—Catalytic Conversion,
2) Dry Bed—Absorption—Catalytic Conversion,
3) Liquid Media Absorption—Air Oxidation,
4) Liquid Media Absorption—Air Conversion.Dry bed catalytic conversion ( the Modifi ed Claus
Process ) The Modifi ed Claus Process is used to remove
sulfur from acid gases which have been extracted from a
main sour gas stream. The extraction is done with one of
the conventional gas treating processes such as amine or hot
potassium carbonate.
The process may be used to remove sulfur from acid
gas streams containing from 15 to 100 mole % H 2 S. The
basic schemes use either the once through process or the
split stream process. Figure 8 shows fl ow characteristics of
the once-through scheme, which in general gives the high-
est overall recovery and permits maximum heat recovery at a
high temperature level.
Split stream processes are generally employed where H 2 S
content of the acid gas is relatively low (20–25 mole %) or
when it contains relatively large amounts of hydrocarbons
(2–5%).
Pertinent design criteria for dry bed catalytic conversion
plants include the following:1) Composition of Acid Gas Feed,
2) Combustion of Acid Gas,
3) For a Once-Through Process, Retention Time, of
Combustion Gases at Elevated Temperatures,
4) Catalytic Converter Feed Gas Temperature,
5) Optimum Reheat Schemes,
6) Space Velocity in the Converters,
7) Sulfur condensing Temperatures.TABLE 3Process RCD Isomax RDS Isomax CDS ISomac HDS
Licensers UOP Chevron Chevron Gulf R & D
General feed type Atmospheric Atmospheric Whole crude Residuum
Feed characteristics
Name Kuwait Arabian light Arabian light —
Sulfur content 3.9 3.1 1.7 5.5
Process diagram Figure 4 Figure 5 Figure 6 Figure 7
Fuel oil product
Quantity (BPSD) 40,000 40,000 40,000 40,000
Sulfur content 1.0 1.0 1.0 2.2
Economies (Relative)
Investmenta 9.7 24.5 156.7 10.0
Operating costsb 51 40–60 40–60 —
a Includes only cost for Isomax reactor/distillation and auxiliary equipment.
b Includes utilities, labor, supervision, maintenance, taxes, insurance, catalyst, hydrogen, etc.C006_002_r03.indd 419C006_002_r03.indd 419 11/18/2005 10:27:11 AM11/18/2005 10:27:11 AM