Suppressing the steam plume
A process for wet scrubbing off-gases from the SRU helps preserve neighbourly relations.
YVES HERSSENS and SARA SAFARIYEGANEH
DuPont Clean Technologies
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Air pollution and emission control are issues of global concern that the oil refining industry has been addressing for many years. Worldwide, refiners have successfully been wet scrubbing flue gases to reliably limit SO2 emissions from sulphur recovery units (SRUs) and/or their tail gas treatment systems at all times. During the cleaning process, all wet scrubbers however introduce a large volume of moisture into the gases in the form of water vapour. This vapour condenses on contact with the cooler atmosphere and thus leaves the refinery stack or chimney as a visible steam plume (see Figure 1). While the plume itself is harmless, it can be visually unappealing and may be misinterpreted by neighbouring communities or the general public as a visible sign of air pollution, while the refinery has – on the contrary – just invested in better air quality for its surroundings. Nonetheless, few refiners have acted to eradicate these steam emissions because plume suppression, in the past, used to entail high extra installation and operating costs.
To address this issue, DuPont Clean Technologies (DuPont) developed the Sennuba steam plume suppression process. Designed to work with its DynaWave reverse jet scrubbers in SRU applications, the Sennuba process prevents the formation of objectionable visible vapour when cleaned combustion gases are emitted to the atmosphere.
SRU tail gas scrubbing
Air toxins are widely blamed for a variety of health and environmental problems. The refining industry has been making a strenuous effort to cut emissions to air and has largely been successful. From 1990 to 2013, for example, US refinery emissions of air pollutants decreased by 66% and VOC emissions dropped by 69%, despite a 14% rise in crude oil processing.1 It is not just environmental legislation that is driving this change. Clean air and sustainable emissions reductions are also of critical importance to communities living in the vicinity of refineries and a key factor in public perception of the refining industry’s impact on health and the environment.
DuPont has more than 40 years of emission control expertise and over 500 successful wet gas scrubber references around the globe in many industries. More than 200 of these references specifically control oil refinery pollution. About a quarter of them were designed with the DynaWave reverse jet scrubber technology. This scrubber technology delivers low SO2 outlet from SRU tail gases at any time. It can handle high turndown ratios, variations in incoming acids, and inlet temperatures of up to 1200°C in SRU off-gases. However, since the DynaWave technology is a wet scrubbing system, condensation in the clean gas leaving the stack forms a visible plume if plume suppression treatment is not provided.
Steam plume suppression
A visible steam plume will occur as the moisture in warm, wet, saturated flue gas leaving a stack condenses in the cooler atmosphere. The visibility of steam plumes depends on three factors: the water content of the gas exiting the scrubber, the stack gas temperature, and atmospheric conditions. The higher the water content, the higher the adiabatic stack gas temperature, and the lower the ambient temperature, the greater the plume. This makes it particularly difficult to eliminate steam plumes in cooler climates and explains why, in some places, plumes are only visible in the wintertime.
Since changing the ambient temperature is not an option, the three common methods used to date to prevent steam plume formation after a wet scrubber have been sub-cooling the gas within the scrubber, diluting the clean gas after the scrubber, or reheating the clean gas after the scrubber.
Sub-cooling of the gas within the scrubber requires the use of cooling water to lower the temperature of the scrubber circulation loop to bring the temperature of the gas down from over 60°C to 30°C or lower. This requires a redesign of the scrubbing system and will consume cooling water.
Gas dilution after the scrubber entails mixing of hot air or clean combustion gas sources with the gas. This desaturates the gas and reduces the gas dew point. Typically, a burner is installed in the bottom of the stack to dilute as well as heat the gas. In some cases, this option is not viable due to local regulations.
Reheating the gas after the scrubber often involves an external energy source which translates into high operating costs. Therefore, sometimes the option of a gas-to-gas heat (GGH) exchanger that works with the heat of the hot dirty gas upstream of the scrubber is used to heat the exit gas. However, the use of a heat exchanger comes at a price and it is furthermore prone to plugging in this application (see Figure 2). GGH exchangers often risk leaking SO2 from the hot dirty side directly to the clean gas side after the scrubber.
The Sennuba process takes a similar approach, but with one significant difference.
Instead of relying on an external energy source, the Sennuba process uses energy already available in the hot dirty gas upstream of the scrubber to heat the cool clean saturated gas exiting the scrubber. At the same time, it ensures no dirty gas can leak directly to the clean side (see Figure 3).
The technology employs two specialised thermosyphon heat exchangers and a heat transfer medium to heat the cool gas with energy transferred from the hot gas. This solution avoids the high operating costs associated with other methods of steam plume control by recovering otherwise lost heat from the process to heat the exit gas of the DynaWave scrubber and thereby suppress the visible plume.
Aside from reducing energy consumption, the use of a heat transfer medium and the two separate heat exchangers avoids any risk of leakage of SO2 containing process gas to the clean gas after the scrubber.
With the thermosyphon, there is no forced circulation of the heat transfer medium. A specialised heat exchanger technology is used to avoid the plugging and corrosion issues associated with typical plate type heat exchangers.
Eliminates water plumes
The Sennuba process was developed specifically to allow oil refiners, who believe visual steam plumes to be unappealing to surrounding communities, to opt for a combination of a cost-effective tail gas treatment unit technology of their choice with the low emissions of DynaWave technology (see Figure 4). These advantages include high dependability, a small overall footprint, simple operation, reduced maintenance, and significantly lower capital investment to meet regulatory SO2 emission limits from Claus plants without interruptions during maintenance or malfunction of any upstream main tail gas treatment unit.2
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