MEETING ULTRA-LOW SULPHUR DIOXIDE (SO 2 ) EMISSIONS LIMITS THE REGENERABLE WAY

Transcription

1 MEETING ULTRA-LOW SULPHUR DIOXIDE (SO 2 ) EMISSIONS LIMITS THE REGENERABLE WAY A Chinese refiner s application of the CANSOLV SO 2 Scrubbing System to fluidised catalytic cracking (FCC) unit off-gas and the benefits this unlocked WHITE PAPER JULY 2017

2 CONTENTS 1. Option evaluation 2. Process description: Scrubbing System 3. Performance data 4. Ensuring compliance with future, more-stringent regulations DO YOU need to meet ultra-low SO 2 emissions limits? treat flue gas streams that contain high SO 2 concentrations? feel constrained by waste-water and landfill disposal limitations? want to increase your use of high-sulphur crudes or increase production capacity? 5. Key takeaways 3

3 MEETING STRINGENT SO 2 EMISSIONS LIMITS In recent years, China s air pollution has attracted the world s attention for images of major cities shrouded in haze and heavy smog. As part of the Chinese government s 12th five-year plan, Chinese refineries were subjected to strict environmental emission regulations, particularly with respect to SO 2 emissions. Adapting to such mandates can be expensive, but one Chinese refiner s response enables it to not only meet the current regulations while preparing for future environmental regulations but also improve its margins by recovering marketable sulphur. To comply with new emission targets that were set to come into force, a Chinese refiner launched an in depth evaluation of the various flue gas desulphurisation technologies on the market. It needed to cut the SO 2 emissions from its FCC unit from up to 2,500 mg/nm³ (Table 1) to 200 mg/nm³. This limit was expected to be reduced to 100 mg/nm³ or less in the future, although the timeline for this change was uncertain. PARAMETER FCC conditions Gas flow rate, Nm³/h 150, ,000 Gas SO 2 concentration, mg/nm³ 1,800 2,500 Gas dust concentration, mg/nm³ Cycle length, y 3 TABLE 1: FCC off-gas conditions. The site s executives shortlisted two flue gas desulphurisation technologies and conducted an in-depth technical and economical comparison of them. The options were: a caustic scrubbing process a non-regenerable technology; and the CANSOLV* SO 2 Scrubbing System a regenerable technology. 1. OPTION EVALUATION The evaluation found little difference between the capital cost of the two options, but there were key differences in the operating costs, principally because one is a non regenerable flue gas desulphurisation technology whereas the other is regenerable. The caustic scrubbing process is non-regenerable; the caustic solution cannot be regenerated, so it inevitably produces liquid waste requiring on-site management while also needing constant chemical addition to maintain the solution s desulphurisation capacity. Flue gas desulphurisation systems have historically been of the non-regenerable type. However, over the last 15 years a regenerable system, the CANSOLV SO 2 Scrubbing System, has enjoyed rapid takeup, especially for applications that have higher SO 2 concentrations or when more-stringent limitations for waste-water and landfill disposal apply, as these systems direct the sulphur into existing by-product market streams and away from air, wastewater and landfill facilities. Refiners have increasingly been applying this technology to manage sulphur emissions in flue and tail gases from FCC, delayed coker, sulphur recovery and spent acid regeneration units as well as from utility boilers and furnaces. The site s review confirmed that the waste issue was a major disadvantage for the caustic scrubbing technology. It would produce large amounts of liquid effluent containing mostly sodium sulphate and sulphite that could not be discharged without further in-depth and costly treatment. Moreover, it would consume expensive caustic reagent, thereby further increasing the operating costs. These costs would not be incurred with the regenerable solution, the CANSOLV SO 2 Scrubbing System. More importantly, this would also capture and turn the SO 2 into a valuable product by sending the concentrated SO 2 stream to the front-end of the sulphur recovery unit (SRU) without major modifications, thereby producing additional elemental sulphur. This would help in improving margins and is in line with China s vision to be a recycling economy. Scrubbing System was selected, therefore, principally because: effluents are reduced to a minimum; high-cost consumable absorbents are not required; and marketable sulphur is produced. 4 *CANSOLV is a Shell trademark. 5

4 2. PROCESS DESCRIPTION: THE CANSOLV SO 2 SCRUBBING SYSTEM Scrubbing System is a typical regenerative amine line-up, as shown in Figure 1. Treated off-gas to atmosphere 3. PERFORMANCE DATA The refinery s project team, supported by the Shell Cansolv team throughout each phase, executed the project smoothly and started up the unit on schedule in December Since then, the unit has essentially required no maintenance and the plant has not shut down because of the CANSOLV SO 2 Scrubbing System. The warranty test run in December 2015 was also a success. Absorbent purification unit Condenser SO 2 >99.9 vol% (dry basis) Scrubbing System was designed to achieve: SO 2 emissions <200 mg/nm³; acid mist emissions <10 ppmv; and dust emissions <20 mg/nm³. For much of the time the system has been operating, the inlet SO 2 concentrations have been lower than the design value because different crude oils to the design feed have been used. The FCC off-gas flow rate has fluctuated between 30,000 and 235,000 Nm³/h during actual operation. Absorber Lean absorbent Stripper Nevertheless, as the operating data provided by the refinery show (figures 2 4), the unit has successfully met the design and environmental SO 2 emissions targets and the SO 2 emissions have been consistently below 200 mg/nm³. Rich absorbent Lean cooler Lean/rich amine heat exchanger Reboiler Low-pressure steam Condensate The SO 2 that is captured is injected back to the front end of the existing SRU; the refiner has confirmed that this has had a positive effect on the SRU s operation. 2,000 1,900 1,800 1,700 1,600 1,500 Inlet SO 2 FCC CO boiler FCC off-gas Pretreatment Claus unit 1,400 1,300 1,200 Outlet SO 2 1,100 FIGURE 1: Scrubbing System. Elemental sulphur 1, Before the CANSOLV SO 2 Scrubbing System, the FCC off-gas is directed into a pretreatment unit where it is quenched to its adiabatic saturation temperature and the dust and acid mist it contains are reduced to tolerable levels. The off-gas then flows into the absorber where it is contacted by the lean absorbent. The SO 2 in the gas is scrubbed by the absorbent to leave residual amounts of SO 2 at the absorber stack. The SO 2 rich absorbent from the absorber is then pumped to a stripper, where it is regenerated by indirect steam stripping. For this project, the reboiler operates at below 120 C and uses low-pressure steam (3.5 barg) as the heat source /9/ /9/ /9/ /9/2015 5/10/ /10/ /10/ /10/ /10/ /10/2015 4/11/ /11/201514/11/ /11/2015 FIGURE 2: Inlet SO 2 versus outlet SO 2 (September December, 2015). The overhead vapour of the stripping tower is cooled in a condenser, usually to below 60 C. The SO 2 gas is then separated from the condensed reflux, which is pumped back to the stripper. The product SO 2 is directed to the refinery s existing SRU where it is recovered as an additional 1,000 1,600 t/y of sulphur product, which increases the production capacity of the existing SRU by 5 8%. As the SRU operates by first converting hydrogen sulphide into SO 2, the external SO 2 provided by the CANSOLV SO 2 Scrubbing System does not require modification of the existing SRU. Following regeneration, the absorbent is returned to the absorber via a lean rich exchanger that recovers heat to preheat the rich absorbent and a lean cooler to adjust the lean absorbent to an appropriate temperature. 6 7

5 1,600 1,500 1,400 1,300 1,200 1,100 1, Inlet SO 2 Outlet SO 2 4. ENSURING COMPLIANCE WITH FUTURE, MORE-STRINGENT REGULATIONS Scrubbing System can achieve far greater SO 2 removal than other options for limited additional investment and slightly higher steam consumption. When the project commenced, the project team was aware that even more stringent SO 2 emissions (<100 mg/nm 3 ) were likely to come into force in the near future. So, during the design phase the team therefore considered the options available to ensure future emission compliance. If the emissions limit is tightened from 200 to 100 mg/nm 3 or even lower, the refinery will be able to meet it without requiring a major retrofit /2/2016 2/3/2016 4/3/2016 6/3/2016 8/4/ /3/ /3/ /3/ /3/2016 FIGURE 3: Inlet SO 2 versus outlet SO 2 (February March, 2016). 20/3/ /3/ /3/ KEY TAKEAWAYS By selecting the CANSOLV SO 2 Scrubbing System, this Chinese refiner has: achieved the stringent SO 2 emissions limit of 200 mg/nm³; avoided the costs associated with treating large amounts of liquid effluent; and increased the production capacity of its SRU by 5 8%. 1,600 More and more refineries in China and the rest of the world are considering using the CANSOLV SO 2 Scrubbing System to treat off-gas. 1,500 1,400 1,300 1,200 1,100 1, Inlet SO 2 Outlet SO 2 The technology is a simple, effective, proven and economical way to manage SO 2 emissions from refinery units. Regardless of the gas conditions, the CANSOLV SO 2 Scrubbing System can be used to help achieve even the most-stringent emission regulations. It has key advantages over a non-regenerable system because: effluents are reduced to a minimum; high-cost consumable absorbents are not required; and marketable sulphur is produced /5/2016 8/5/ /5/ /5/ /5/ /5/2016 2/6/2016 7/6/ /6/ /6/ /6/ /6/2016 FIGURE 4: Inlet SO 2 versus outlet SO 2 (May June, 2016). 8 9

6 ABOUT US provides technical consultancy and licensed technologies for the Shell Group and third-party customers within the energy industry. strives to deliver innovative technical solutions and effective technology to support its customers in their day-to-day operations and delivery of strategic plans to improve the capacity and performance of existing units; integrate new process units into existing refineries and petrochemical complexes; incorporate advanced proprietary catalyst systems (Criterion) and reactor internals; through to the design of grassroots refineries. is affiliated with Shell s catalyst companies, which innovate and sell catalysts through a network that includes Criterion Catalysts & Technologies (Criterion), Zeolyst International, CRI Catalyst Company and CRI Leuna (formerly known as Kataleuna). For further information, please visit our website at ABOUT SHELL CANSOLV Shell Cansolv, which is owned by, is a leader in gas treating and SO 2 and carbon dioxide recovery technologies. Its technologies can be used across a wide variety of industries, including oil and gas, power generation, mining and metallurgy, chemicals, industrial boilers and sulphuric acid plants. It has licensed 20+ units licensed worldwide. For further information, please visit our website at ABOUT THE AUTHORS Lena Fengdan Wang is a Licensing Technology Manager at Shell Cansolv. She has 15 years experience of flue- and off-gas cleaning design and service, and expertise in various gas-cleaning technologies, especially regenerable and non-regenerable SO 2 removal technologies and selective catalytic reduction of nitrogen oxides technology. Lena joined Shell Cansolv in 2011, having previously worked for Haldor Topsoe. She has a master s degree in environmental engineering from Xi an University of Architecture and Technology, China. Martin Lebel is a Licensing Technology Manager at Shell Cansolv. He is based in Montreal, Canada, and supports technical licensing opportunities for Shell Cansolv in the Middle East and Africa from the early proposal phases through to operational troubleshooting of existing units. Before joining Shell Cansolv in 2008, Martin worked in the mining and metallurgical industries and in sulphuric acid production. He has a bachelor s degree in chemical engineering from the University of Sherbrooke, Canada

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