December 2022

Process Optimization

New state-of-the-art in sulfur recovery?—Part 1

It is widely accepted that state-of-the-art sulfur recovery technologies utilize the modified Claus process as well as some extension technology on the tail gas.

Lewis, J., Njuguna, H., Worley Comprimo; Jensen, F. E., Topsoe

It is widely accepted that state-of-the-art sulfur recovery technologies utilize the modified Claus process as well as some extension technology on the tail gas. To achieve the highest sulfur recovery, the extension technology converts all tail gas sulfur species to hydrogen sulfide (H2S) by hydrogenation and hydrolysis, which is subsequently captured in an amine absorber and recycled to the Claus thermal reactor. The industry has developed amine solvents of increased H2S selectivity, with the most selective often regarded as the best available technology.

One of the more interesting challenges to the tail gas treatment technology described above is the direct conversion to sulfuric acid—after all, that is the destination of much of the recovered sulfur. However, the logistics of that approach are complex, primarily with respect to local sulfuric acid demand. Technically, conversion to sulfuric acid can provide better economics and decreased environmental impact, if there are no transportation restrictions.

Worley Comprimo and Topsoe have developed a proprietary sulfur recovery processa based on proven technology, whereby sulfuric acid produced in a wet gas sulfuric acid (WSA) tail gas treating unit (TGTU) is recycled back to the Claus thermal reactor. With fewer equipment items and demonstrable high reliability, coupled with an energy balance benefit evidenced by operating plants and studies, the new flow scheme challenges for the position of best available technology.

A series of three articles presented here and in two subsequent issues of Hydrocarbon Processing cover all aspects of the technology. The first covers the technological aspects and development, while the second and third articles cover environmental and economic aspects, respectively. Case studies covering a range of Claus plant acid gas feed streams are used for the economic comparisons.

By way of introduction to the technology, six key points of the sulfur recovery processa configuration should be stated:

  • The WSA TGTU is robust against variations in Claus tail gas composition and can process ammonia containing sour water stripper (SWS) offgases, which can be routed directly to the WSA unit.
  • The equipment count is lower than that in a Claus/hydrogenation/hydrolysis/amine configuration, and the Claus plant equipment is smaller.
  • The overall sulfur recovery is equivalent to that achieved in a Claus/hydrogenation/hydrolysis/amine configuration, with sulfur dioxide (SO2) emissions of 150 mg/Nm3, or whatever is required at the location of the plant.
  • Thermal efficiency is high with high-pressure (HP) steam exported from the WSA, and little cooling is needed.
  • There are no waste streams.
  • No chemicals are required.

While the combined technology is new, its components are well-established in industry: the Claus process, the WSA process and the handling of sulfuric acid.

Current industry trends include:

  • Sulfur continues to be a significant byproduct from oil and gas upgrading
  • Most new oil and gas fields are sour
  • Tightening of sulfur emissions standards will continue
  • Operators are looking to reduce their carbon footprint
  • Operators are striving to meet emissions standards at minimum CAPEX and OPEX with optimized technology.

Although sulfur is a product and continues to be a significant byproduct from oil and gas upgrading in both refining, gas plants and various other processes, it is—for many in the oil and gas industry—often regarded as a waste product. This is not going to change fundamentally as the world continues to transition to renewable fuels that also contain sulfur compounds that must be removed.

Most new oil and gas field discoveries today are sour and do require a certain amount of cleaning up as part of bringing the products to market. Global sulfur emissions standards, for both hydrocarbon products and emissions from refineries, gas plants and production facilities, are continuing to tighten. Over the past few decades, this has been evident with some facilities upgrading Claus units and now investigating further emissions reduction with hydrogenation and amine-based TGTUs. Additionally, operators are looking to reduce their carbon footprint to reflect the exponential increase in carbon dioxide (CO2) footprint as SO2 emissions are reduced close to zero in those types of TGTUs.

Claus process with conventional TGT

The Claus process with conventional tail gas treatment, shown in FIG. 1, typically comprises a two-stage catalytic Claus plant combined with a hydrogenation/hydrolysis and amine-based TGTU, sometimes referred to as a BSR/amine plant, or SCOT plant. The offgases that contain small amounts of H2S and COS are typically converted to SO2 in a thermal oxidizer before venting to atmosphere.

FIG. 1. Claus tail gas treatment options.

Sulfur recoveries > 99.9% are possible but at a relatively high CAPEX and OPEX. It is in the last few percentage points where costs escalate; the process is energy-intensive and this is reflected in carbon footprint.

The conventional hydrogenation and amine-based TGT was first patented by the Ralph M Parsons Co. in 1971, with other companies quickly following suit. Incremental improvements have been made throughout the years, but the process largely remains the same. Improvements with the amines used are related to increased H2S selectivity (often using proprietary amines), but the process is largely the same. Regardless, amine regeneration is energy intensive.

Alternatives can offer lower CAPEX and OPEX. One example would be Comprimo’s Claus tail gas direct oxidation technologyb that, when coupled with caustic scrubbing, achieves very low sulfur emissions. CAPEX is lower, but there is an additional waste stream (spent caustic) that has associated disposal costs. However, this configuration is only applicable to relatively low-sulfur plant capacities, and typically (but not exclusively) to refinery plants with sulfur capacities < 300 tpd sulfur. Topsoe’s WSA process also offers an alternative TGT technology; to date, some 165 units have been licensed.

The new sulfur recovery processa technology (FIG. 2), using a WSA TGTU with acid recycling, reduces energy intensity but still maintains the sulfur recovery of > 99.9%. Equipment count, CAPEX and carbon footprint are all reduced. The product is still sulfur, but with enhanced operational benefits.

FIG. 2. A simplified process flow diagram of the new sulfur recovery process a.

Enhanced sulfur recovery of > 99.95% is also achievable by the WSA TGTU with double condensation. This is a refined solution that consumes no chemicals and produces no effluents. Other alternatives exist, such as tail gas scrubbing using chemicals like hydrogen peroxide (H2O2) or sodium hydroxide (NaOH), which are cheaper than double condensation but have chemical consumption and, in the case of NaOH, an effluent stream to be dealt with.

Sulfuric acid is recycled to the Claus furnace and the key chemical reactions are included here.

The consideration of dilution gas is worth noting. In the overall Claus reaction, there are 12 moles of dilution gas for every four sulfur atoms produced. Using pure oxygen rather than air, the nitrogen is removed, leaving just four moles of dilution gas. Recycle acid is an excellent oxygen carrier and leads to only 4.4 moles of dilution gas, facilitating the reduction of air demand to ~20%. Another comparison with a conventional TGTU is that there is no CO2 in the recycle stream to the furnace. These factors contribute to a capacity increase in the Claus section.

Key questions arising from this are how quickly liquid acid would be vaporized; how quickly the gas would disassociate; and how quickly the standard Claus components would be formed. These questions were investigated independently by Alberta Sulfur Research Ltd. (ASRL) and HEC Technologies and answered, as shown in FIG. 3.

FIG. 3. Claus reaction furnace chemistry: the fate of recycled H2SO4. Source: Alberta Sulphur Research Ltd. (ASRL) and HEC Technologies

Particularly for refinery applications, an alternative for processing SWS offgas with the new sulfur recovery processa becomes apparent. Traditionally, these gases—which contain ammonia, hydrogen sulfide and water vapor—are routed to the front section of a two-stage Claus thermal reactor operating at 1,250°C to ensure the complete destruction of ammonia. Inadequate ammonia destruction (e.g., in a single combustion zone thermal reactor operating at ~1,000°C) can lead to ammonium salt deposition at the final condenser, especially when cooled using a 1-barg closed-loop cooling arrangement.

The new sulfur recovery processa configuration allows routing of the SWS gases directly to the combustor in the WSA section, as shown in FIG. 4, simplifying the Claus section and avoiding the potential deposition problem. The SWS gas is burned in the combustor of the WSA section with excess air. This leads unavoidably to some formation of nitrogen oxides (NOx), which are removed again in the SCR reactor located after the waste heat boiler. A small fraction of the SWS gas is added to the combustor effluent after the waste heat boiler to provide enough ammonia to react with the formed NOx over the catalyst installed in the SCR reactor. The results of the reaction are nitrogen and water vapor.

FIG. 4. The proprietary sulfur recovery processa with SWS gas to the WSA section.

The feasibility of this concept, however, must be evaluated in each individual case and depends on several factors, such as:

  • The ratio between amine acid gas (i.e., with no ammonia present) and SWS gas flows
  • The required overall SRE
  • The possibility of exporting a certain amount of sulfuric acid as a separate product.

Often, it will be necessary to have the SWS gas processed in the Claus section.

Takeaway

The new sulfur recovery processa achieves a sulfur recovery equivalent to the conventional design, is a simpler process with fewer parameters to control, and the economic benefits are substantial. The Claus unit itself will be smaller due to the recycle and oxygen carrying capacity of the acid.

Additional flexibility can be provided by producing a small sulfuric acid stream, which could be useful in a refinery application, though special circumstances would be required to progress this route. Maintaining a single product stream is going to be preferable in most applications.

Brownfield applications (i.e., revamps) provide unique opportunities, with perhaps limited space, a need for increased capacity, and a requirement to meet increasingly tight emissions specifications. The WSA section can be constructed while the Claus unit is in operation, so it provides minimum shutdown time, minimum personnel onsite and simultaneous operations (SIMOPS) advantages, in general.

The application to natural gas plant SRUs affords some enhanced benefits for the sulfur recovery processa, including:

  • Significant CO2 in the feed reduces the thermal reactor temperature and increases the size of the conventional TGTU
  • CO2 co-absorbed in amine-based TGTUs is recycled to the inlet of Claus unit
  • TGTU amine requires leaner loading to allow CO2 “slip”
  • All CO2 passes through to the stack
  • Smaller recycle = smaller SRU
  • Simple, robust and efficient
  • Sulfur recovery of > 99.9% with all sulfur compounds converted into elemental sulfur
  • Flexibility in production
  • All product is elemental sulfur
  • Reduced Claus unit size due to sulfuric acid recycle, providing a degree of oxygen enrichment
  • No waste streams
  • Feasible solution for both greenfield and brownfield applications
  • High thermal efficiency with HP steam export
  • Integrated efficient features of WSA with the sulfur-producing Claus process
  • Based on proven technologies with integration demonstrated by ASRL.

Parts 2 and 3 of this article will appear in subsequent issues of Hydrocarbon Processing and will cover environmental and economic aspects, respectively. HP

NOTES

a TopClaus®
b Comprimo’s SuperClaus®

The Authors

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