Nov-2015

Tail gas scrubber sulphur plant by-pass ability aids emergency shutdowns and startups

From the European Union to the USA, national and international bodies are putting in place increasingly stringent legislation to limit emissions.

Yves Herssens and Steven Meyer
MECS/DuPont

Article Summary

Along with power generation, the oil refining and gas sector is seen as one of the largest emitters of air pollutants and is under pressure to reduce emissions year on year. One of the key sources of pollutants are sulphur recovery units (SRUs), which are now required to reduce sulphur emissions further before the gas can be discharged to the atmosphere. Sulphur recovery units typically use CLAUS technology to recover sulphur in off-gases from refineries and gas plants. Whether or not they are followed by a tail gas treater unit to recover additional sulphur, the tail gas is generally incinerated. The treatment converts H2S into SO2. Reducing the resulting SO2 can be accomplished by the installation of a tail gas scrubber. If properly designed, these tail gas scrubbers can cut the sulphur dioxide (SO2) emissions from a sulphur plant to levels of 20 to 50 ppmv (dry basis) and in some cases as low as 5 ppmv (dry basis).

However, it is important that the right SRU tail gas scrubber is used to ensure it addresses the typical challenges it faces, which include:
• The requirement to meet and guarantee low SO2 emissions
• Ability to handle a wide range of inlet SO2 loadings
• A high turndown required
• Reliability and proven experience

With over 500 scrubber references around the globe and in various industries, DuPont is well aware of the parameters sulphurous offgas scrubbers have to meet. With the combined expertise of DuPont and MECS, it believes the proven flexibility of the MECS® DynaWave® Wet Gas Scrubber is well suited to the specific challenges of SRU offgas treatment in all circumstances.

Several plants have found added benefits to having an SRU tail gas scrubber. With the proper scrubber design, startups and emergency shutdowns are easier as the sulphur plant can be bypassed and the feed gas sent directly to an incinerator where sulphur is converted to SO2. The SO2-rich stream is then directed to a tail gas scrubber in order for the SO2 to be removed and so achieve acceptable emission limit levels before discharge to the atmosphere. Under bypass conditions, the level of SO2 sent to the scrubber can be as high as 20% vol. The scrubber must therefore be able to handle very high SO2 loadings.

SO2 emissions reduction with Tail Gas Scrubber
In most sulphur plants, process gas is typically sent to an incinerator where any H2S or sulphur compounds not removed in the sulphur plant are converted to SO2. Some plants have a waste heat boiler downstream of the incinerator to take advantage of the 1400°F (760°C) exit gas to produce steam. At times, the incinerator off-gas can be sent directly to the stack. However, as mentioned above, many new and existing sulphur plants are now required to install tail gas scrubbers to reduce the SO2 to even lower levels before being discharged to the stack.

Below is a schematic of a typical flow scheme depicting the sulphur recovery unit (SRU), incinerator and tail gas scrubber:
In the scrubber, SO2 is absorbed into the scrubber-circulating liquid where it reacts with sodium hydroxide, (NaOH) also known as caustic soda. The acid-base reaction produces a sodium sulphate soluble salt after air oxidation.

Designing for Sulfur Plant Gas Bypass
As shown below, a properly designed incinerator and tail gas scrubber makes it possible to bypass the sulphur plant and treat the sulphur solely with an incinerator and tail gas scrubber.
 
In order to take advantage of a tail gas scrubber’s ability to handle a sulphur plant bypass, the scrubber must not only be designed to handle extremely high levels of SO2 coming into the scrubber in the inlet gas, but it also must reduce the SO2 to extremely low outlet levels. In many cases, this requires SO2 removal efficiencies of +99.9% or higher. To meet both of these constraints, the scrubber has to have multiple contact stages and a high liquid circulation. A typical packed tower is limited in the amount of liquid that can be circulated. The DynaWave Wet Gas Scrubber, however, uses three distinct contact stages, two of which work with froth technology that produces the extremely high liquid circulation rates necessary for high SO2 loadings. All three contact stages are in a single vessel as shown in Figure 3.

The gas is first contacted in a vertical duct with a liquid stream that is injected counter current to the gas stream. As the gas and liquid make contact, a froth regime, or zone, is developed. This zone is a highly mixed gas-liquid area where the gas is instantly quenched and the SO2 is absorbed into the liquid stream. Because of the nature of the froth zone, very high liquid circulation rates are possible. To describe this, scrubbers and packed towers use the term ‘liquid to gas ratio’ (L/G ratio) to express the amount of liquid that is in contact with the gas. In the UK, the L/G ratio is expressed in gallons per minute per 1000 actual cubic feet per minute or gpm/1000 acfm. In metric units, the L/G ratios are cubic meters per hour of liquid/1000 actual cubic meters per hour of gas flow. A typical L/G ratio for a packed tower is 40 gpm/1000 acfm (300 m3/hr/1000 am3/hr). In the DynaWave Wet Gas Scrubber froth zone, the L/G ratio can be as high as 300 gpm/1000 acfm (2200 m3/hr/1000 am3/hr). Using two such high L/G contact stages in conjunction with a packed section results in a total DynaWave Wet Gas Scrubber L/G ratio of 640 gpm/1000 acfm (4800 m3/hr/1000 am3/hr) (see Figure 3 and Figure 4). This circulation rate contains 16 times the amount of liquid per unit of gas scrubbed compared to a typical packed tower. This extremely high L/G ratio makes it possible for the DynaWave Wet Gas Scrubber to remove up to 10 vol% of SO2 from a gas stream down to low ppm levels. Although the capital cost increase for a combination DynaWave-Packed Tower above a typical packed tower is approximately 30%, that is still well below the cost of the multiple vessel arrangements that would be required to remove high levels of SO2 from a sulphur plant bypass condition.

Figure 3 is a flow schematic of the DynaWave Wet Gas Scrubber showing makeup water, blowdown, caustic addition and air injection for the oxidation of sulphite to sulphate. All three gas liquid contact stages take place in a single vessel. It should be noted that a typical packed tower would also require makeup water, a blowdown stream, caustic addition and oxidation air.

Sulfur plant bypass enables maintenance
As the tail gas scrubber is able to handle a sulphur plant bypass, plant personnel have the option to operate in bypass mode in order to perform maintenance on the sulphur plant or to change a catalyst bed. With the scrubber designed for the bypass case, any incidents in the sulphur plant can be accommodated by the DynaWave Wet Gas Scrubber system and SO2 emissions would not exceed the plants permit level. If desired, the DynaWave Wet Gas Scrubber can be operated using a single Froth Zone contact stage during normal operation, thereby saving pump power. It should be noted that the cost of the caustic reagent can be high while operating the sulphur plant in the bypass mode. However, as this mode of operation should only be temporary, this cost is relative compared to the cost of a complete plant shutdown.