With the recent emergence of sweeter fuel
sources such as unconventional gas and biofuels, the sulfur
industry is challenged in providing economical solutions to
desulfurize gas streams with low hydrogen sulfide
(H2S) content. Historically, the sweetening of such
gases was primarily accomplished by using liquid redox
processes.a These established, sweetening processes
treated the raw gas stream directly and did not require an
amine treating step, thereby reducing the total capital and
operating costs of the facility.
Liquid redox processes are capable of scrubbing the
H2S to very low levels and meeting typical
treated-gas specifications, as proven by several hundred units
that are in operation. However, these facilities often suffer from very
high operating costs, low availability and a low-quality sulfur
product, which usually must be disposed rather than sold as
product. The inherent nature of these problems is discussed
- High operating costs are a result of the process
chemistry, especially consumption of the expensive chelating
agents required to keep the direct oxidation catalyst in
solution. Consequently, chemical costs range from $100 to
$150 per ton of produced sulfur.
- The product sulfur contains some chelating agent and,
therefore, is a low-quality material. As a consequence, no
revenue from sulfur sales can be expected. More important,
additional costs associated with landfill disposal can be
- Low process availability results from two primary steps,
as shown in Fig. 1. In the scrubber (1),
sour gas is contacted by liquid solvent and thus forming the
solid sulfur, which often leads to plugging in the column or
in downstream vessels and pipes. In the re-oxidation vessel
(5), foaming and sulfur froth can occur, thus reducing
Fig 1. Typical process flow
diagram of a liquid
A newly developed process applies a totally different
approach.b This process first oxidizes
H2S selectively in the gas phase over a robust and
low-cost catalyst. To increase sulfur recovery efficiency above
what was achievable in the selective oxidation step, the
process subsequently applies the sub-dewpoint principle. It is
a well-known Claus tail-gas treatment technology that takes
advantage of the improved Claus equilibrium at lower operating
temperatures (below the sulfur dewpoint) in the catalytic
reactors. The process can achieve sulfur recovery efficiencies
exceeding 99% when treating in low H2S-content
gases, such as shale gas, coalbed methane and biogas. The
process is inexpensive and easy to operate; it generates no
byproducts, and the sulfur recovered is of premium quality. The
direct oxidation process is capable of treating raw gas streams
containing H2S plus hydrogen, light hydrocarbons,
oxygen and/or inert gases. Fig. 2 shows a flow
diagram of the new direct oxidation process.
Fig. 2. Typical process flow
diagram of new
direct oxidation process for H2S.
The feed gas to the sulfur recovery unit (SRU) is mixed with
a stoichiometric quantity of air to convert the incoming H2S to
elemental sulfur via direct oxidation. The gas mixture is sent
through a preheater to the first reactor. This reactor is
different from conventional Claus reactors: it contains two
sections. The upper section at the gas inlet is a conventional
fixed-bed reactor with a direct-oxidation catalyst. In this
reactor section, part of the feed H2S is oxidized into
elemental sulfur according to Eq. 1. In parallel, some sulfur
dioxide (SO2) is formed. The second section in the lower part
of the first reactor contains a Claus catalyst with an embedded
heat exchanger, which is designed to remove the heat of
reaction from the catalyst bed. The heat removal within the
catalyst bed shifts the equilibrium of the Claus reaction (Eq.
2) toward more sulfur formation, substantially improving
Direct oxidation of H2S
2 H2S + O2 = 2/x Sx + 2
H2O + heat of reaction (1)
2 H2S + SO2 = 3/x Sx +
H2O + heat of reaction (2)
where x = 2, 4, 5, 6, 8 sulfur molecules of
different sizes, according to temperature.
The heat exchanger applied is a thermoplate stack with large
clearances, as shown in Fig. 3. The space
between the thermoplates is filled with catalyst. As this heat
exchanger type is not yet so well known within the sulfur
industry, it will be discussed in more detail later.
Fig 3. Top view of a
exchanger for a reactor during construction in
A sulfur condenser is located downstream of the first
reactor. A second reactor, identical to the first reactor,
follows the sulfur condenser but operates at lower temperature.
This shifts the chemical Claus equilibrium to even more sulfur
formation. The reactor outlet temperatures range from 100°C
to 125°C, i.e., possibly even below the sulfur
When operating below the sulfur dewpoint, the sulfur formed
via the Claus reaction accumulates on the catalyst. Thus, the
catalyst deactivates slowly and must be regenerated. The
regeneration is accomplished by switching the second reactor
into the first reactor position. In the first reactor position,
the inlet temperature approaches 320°C, which desorbs
sulfur and regenerates the catalyst. The former first reactor
is switched at the same time into the second, cooler reactor
position. This procedure is repeated typically once every 24
hours. Treated gas from the second reactor is sent to the
consumer, e.g., as purified biogas or natural
The new direct oxidation process can be applied to a number
of plants such as for:
- Biogas purification
- Offgas treatment from chemical processes rich in methane,
carbon dioxide and hydrogen
- Natural gas purification.
Pure, bright yellow elemental sulfur is produced. The
process operation is fully automatic, with manual control only
required during startup and shutdown, similar to a conventional
Claus plant. The first commercial unit was installed in 1993
and is still in operation.
The high sulfur-recovery rate (SRR) in the new oxidation
process results from removing the heat of reaction, which
shifts the chemical equilibrium to more product formation.
Fig. 4 illustrates the effect on SRR, where
the SRR is depicted as a function of the outlet temperature
from the second reactor.
Fig. 4. Sulfur recovery rate
as a function of
outlet temperature of the second reactor for
feed gas with 85% H2S.
In addition to high-sulfur recovery efficiency, the
internally cooled reactors provide other benefits. The internal
heat exchangers are self-controlling. Boiler feedwater (BFW) feeds the
inside of the thermoplate; the BFW always has a temperature
corresponding to the generated steam pressure. On the outside,
i.e., between the thermoplates, the catalyst and reaction gas
release heat. The greater the gas flow, the higher the heat of
reaction, which is the temperature difference between the gas
and BFW. With the higher temperature D, the heat of reaction is
automatically removed by internal cooling. As a consequence,
the temperature at the outlet of the reactors is constant
within a narrow range, and is independent of fluctuations in
gas volume and gas composition. Accordingly, this process is
intrinsically stable, easy to operate and has high reliability. Actually, all normal
operations are fully automatic, thus very little operator
attention is necessary.
The first commercial plant applied a two-reactor process
configuration; it was started up in December 1995 in the
Nynäs refinery in Sweden (Fig.
5). This unit processes amine-acid gas and
sour-water-stripper gas. The plant has proven to be very
reliable, easy to operate, and requires very little maintenance. Availability is always
better than 99.5%/yr. The refiner claims that this plant is the
most reliable within the whole refinery, even after more than 15
years of operation. It achieved the required SRR and reached
optimal values of up to 99.85%, even with aged catalysts. In
operation at low load conditions (at 6:1 turndown), the SRR
dropped by only 0.1%.
Fig. 5. Sulfur recovery
plant in the Nynäs
refinery in Sweden.
Thermoplates for internal cooling of catalytic
Internally cooled catalytic reactors have been successfully
used in many applications. They are applied primarily for
selective reactions, where rigorous temperature control is
required, or in reactions where the chemical equilibrium is
strongly temperature dependent. In the past, straight-tube
reactors, with the catalyst inside the tubes, have typically
been used. In a few cases, spiral-wound tubular heat exchangers
have been applied with the tubes submerged in the catalyst.
However, these reactor types have features that are not
complementary to the operations. Primarily, the heat
exchangers fabricated geometry often forces conditions on
the catalytic reactions that are not optimal. For example, the
straight-tube reactors had to be built slim and high to avoid
excessive thermal stress on the tube sheets, which resulted in
a high pressure drop, high linear gas velocity and mechanical
stress on the lower catalyst particles. The spiral-wound heat
exchangers avoid these disadvantages to some degree, but they
require many manufacturing steps and precise fabrication skill.
They are typically more expensive.
Both types of reactors cannot be built onsite, and,
therefore, one must observe transportation limitations. This
also limits throughput capacity. In view of ever-increasing
plant sizes, this condition becomes increasingly more
important. All of these features for tubular reactors are
detrimental for sulfur recovery, which may explain why
internally cooled reactors have not been used widely in sulfur
recovery previously. The catalytic reactors incorporated in the
new-generation direct oxidation process use thermoplates as
heat exchangers, thus eliminating all of the listed
disadvantages. The basic element of a thermoplate heat
exchanger is the thermoplate itself, as shown in Fig.
Fig. 6. Schematic of a
A thermoplate consists of two metal sheets welded together
along their edges and point-welded across their surfaces. This
is accomplished with precise fabrication machinery that
facilitates the manufacturing of large surface area exchangers
at low cost. The plates are expanded by injecting high-pressure
liquid between the metal sheets, which opens channels for the
cooling medium, as shown schematically in Fig.
6. The expansion generates the typical
cushion shape, and the point and seam welds of the thermoplates
are gap free. Multiple thermoplates are combined to form a heat
exchanger package, which is then inserted in a shell to
complete the heat exchanger.
For application in a reactor, the catalyst is poured into
the spacing between thermoplates, as shown in Fig.
3. Vertical plane walls are formed by the thermoplates
and allow easy filling of the catalyst particles.
Several thousand such thermoplate heat exchangers have been
built and installed worldwide. They are in service in even the
most severe applications, such as condensing phosgene, which is
not only highly toxic, but is also very corrosive when in
contact with water. Single heat exchangers with several
thousands square meters of exchanger surface area have been
installed and operated. The thermoplate heat exchanger is
considered a proven technology. These exchangers are
compact and light weight, have low pressure drop, and provide
high heat exchange coefficients; they are ideal for
The outer and inner fluid channels are completely separated
from each other by seam welds. As in contrast to other
plate-heat exchangers, there is no contact between adjacent
thermoplates; each thermoplate is self-contained and no forces
are transferred to the next plate. The catalyst particles are
insulated and do not experience mechanical stress.
The distance between plates, file height, pitch of the point
welds, dimensions and number of thermoplates can vary widely.
Therefore, thermoplate reactors can be optimally tailored to
each sulfur recovery application.
The new direct oxidation process is an economic method for
sulfur recovery from low-H2S content gases. It
converts H2S in a gas catalytic process directly to
elemental sulfur. The sulfur recovery efficiency, which depends
on the feed-gas composition, is greater than 99%. The reaction
takes place in two identical fixed-bed reactors with internal
cooling by thermoplate heat exchangers, which maintain the
outlet temperatures of the reactors within a narrow range, thus
maintaining a constant SRR. This process has proven to be easy
to operate, very reliable and with low maintenance costs. As one customer
commented, Our biggest problem with this process is that
the operators tend to forget about it, because it requires so
little of their time and attention.
a Liquid redox processes include LO-CAT, SulFerox
b SMARTSULF is a new sulfur oxidation process.
Michael Heisel, PhD, is general
manager of ITS Reaktortechnik GmbH. He has more than
30 years of experience in sulfur recovery plant
design, startup, validation and troubleshooting.
Angela Slavens is vice president and global director
of sulfur technology for Worley
Parsons. She has more than 15 years of experience in
the oil and gas industry, primarily in the field of
sour gas treating and sulfur recovery.