Upgrading sulphur removal performance
Reverse jet scrubbing enabled a refinery to bypass its upstream tail gas unit while still meeting sulphur removal targets
YVES HERSSENS and WILLIAM LAM
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Everywhere in the world, refiners need their sulphur recovery units (SRU) to be compliant with increasingly stringent SO2 emission standards. The question is how to achieve these targets while at the same time minimising capital expenditure, maintenance costs and operator involvement. In addition to meeting these challenges, the reliability of the chosen environmental system is key, so that time and attention can be focused on the profit generating parts of the refinery instead of on the SRU. A very common approach has been to use a highly efficient, amine based tail gas treatment unit (TGTU) downstream of the Claus plant. However, one could argue that this is akin to killing a mosquito with a bazooka. By combining a slightly less efficient TGTU option, such as a direct oxidation type process or a sub-dewpoint process, with flexible MECS DynaWave reverse jet scrubbing technology (licensed by DuPont Clean Technologies), one can achieve similar sulphur removal efficiencies on a smaller plot space at a lower investment cost while at the same time obtaining higher on-stream time.
CPC improves SO2 removal reliability
Recently, CPC, a large Taiwanese state-owned refining corporation, successfully expanded its Talin refinery and simultaneously significantly improved the SO2 removal reliability from an SRU off-gas stream by opting for DynaWave reverse jet scrubbing technology for one of its SRUs. The refinery currently has a capacity of 300 000 b/d, but is aiming to process 350000 b/d by 2017. Gasoline and diesel are its main products. In order to meet Taiwanese government air emission regulations, the refinery operates three three-stage and one two-stage SRU with a combined capacity of 780 t/d.
CPC chose to combine a well-known sulphur recovery tail gas technology with the DynaWave technology for its two-stage, two-train SRU #10 as the best process to reduce SOx and total suspended particulate (TSP) emissions. The new DynaWave scrubber provides a high SO2 removal efficiency (99.7%) and is easy to operate.
CPC has installed several of these scrubbers over the last 10 years to treat different refinery off-gas streams. For the Talin refinery’s SRU #10, every sulphur train is now equipped with a DynaWave system. Each system has two reverse jet stages in the inlet barrel, combined with a packing section inside the vessels. This set-up allows the scrubber to operate with an extremely high liquid to gas ratio, which makes it possible to bypass the upstream main tail gas treatment unit during start-up, shutdown and malfunctioning conditions. In addition, CPC installed high-efficiency diffusion bed mist eliminators in the vessels in order to reduce SO3 emissions.
To meet the requirements of CPC, DuPont Clean Technologies specifically designed the wet gas scrubbers to reduce SO2 levels to ≤30 ppmv on a dry basis and SO3 levels to ≤30 ppmv on a wet basis, both values corrected to 6% oxygen.
Each of the two MECS DynaWave engineered scrubbing systems installed at Talin SRU #10 removes SO2 and SO3 from the gas stream based on the specific design inlet conditions.
This technology has allowed CPC to opt for an oxygen enriched TGTU process, which runs under normal conditions. At the same time, it has offered the refinery the added reliability to operate the Claus plant without interruptions during maintenance/malfunction of the main tail gas treatment unit and still meet the imposed SO2 emission limits. The innovative set-up of a combined cost-effective TGTU technology with the flexible and reliable DynaWave technology has resulted in a smaller overall footprint, simple operation, low maintenance as well as significantly lower capital investment.
The DynaWave reverse jet scrubbing system is illustrated in Figure 1. It is an open bore, reverse jet scrubber that uses froth zone technology to achieve desulphurisation in a wet gas environment. For SRU applications, the systems are installed after the incinerator to treat the SO2 before the outlet gas leaves through the stack.
The heart of this system is the reverse jet nozzle, a gas to liquid contactor that creates a zone of intense mixing. The feed gas enters the top of a vertical duct and collides with the scrubbing liquid that is injected upwards through a large bore injector or reverse jet nozzle.
The reverse jet nozzle is a very large bore, open throat nozzle, fabricated from silicon carbide for corrosion and temperature resistance (see Figure 2) that creates the full cone liquid flow essential to produce a froth zone. The froth zone, a standing wave of highly turbulent flow, is produced at the point where the liquid is reversed by the gas. This froth zone creates a very high rate of liquid surface renewal and efficiently quenches the gas to the adiabatic saturation temperature. At the same time, it converts all the SO3 present to acid (H2SO4) mist and absorbs the SO2.
The DynaWave reverse jet scrubber can be designed to quench hot gases up to 1200°C. In addition to quenching and acid gas absorption, it can efficiently remove particulate. This feature offers an extra reliability benefit in case elemental sulphur particulates enter the system.
Benefits noted by CPC
Overall, CPC’s Talin refinery noticed a significant difference after installation of the reverse jet scrubbers. The sulphurous emissions from the SRU are now below environmental regulation limits at any given time, 24 hours a day, 365 days a year in both trains 1 and 2.
Before installation of DynaWave technology, SO2 concentration was normally about 1000 ppm. In the performance test of SRU #10, which now has a two-stage Claus, followed by a main TGTU and DynaWave technology, the SO2 concentration detected from the stack was 9.15 and 0.23 ppm respectively on the two different trains.
The CPC refinery also operated the scrubber system in the SRU bypass mode several times and says it believes the inlet SO2 concentration may have been up to 7000~8000 ppm. Even in that mode, CPC measured outlet SO2 gas below 10 ppm.
What was also particularly noticeable to CPC was how simple the scrubber system was to operate and the limited amount of maintenance required. The refinery estimates that the technology has allowed it to save at least 30% of its overall TIC budget compared to the installation cost of an amine based tail gas treating system alone. The capital cost of the combined TGTU/DynaWave process was less than 70% of the estimated capital cost of a similar sized amine based TGTU process alone would have been, in part because fewer pieces of equipment are required.
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