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 here:
- 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 incurred.
- 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 availability.
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 conversion efficiency.
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 thermoplate heat
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 solidification point.
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 gas.
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 rich
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 reactors
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. 6.
Fig. 6. Schematic of a thermoplate heat
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 sulfur-recovery reactors.
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 and Sulfint.
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.