J. HAIR, Sherwin-Williams Protective & Marine, Tulsa, Oklahoma; and T. ROOK, Sherwin-Williams Global Supply Chain, Cleveland, Ohio
Aboveground ethanol storage tanks face a determined foe: black-colored mildew growing aggressively on exterior surfaces. The fungus requires frequent costly washings while potentially damaging protective coatings. In addition, black mildew can accelerate the evaporation of fuel inside the tanks by attracting more heat.
The sight of mildew can also affect reputation, calling into question the facility’s integrity, practices and level of regulatory compliance—this is especially true in high-consequence areas.
Scientists from the authors’ company determined how best to combat persistent black mildew by first using a novel strategy to identify it. This discovery led to the identification of coating formulations with superior capacity to prevent mildew growth. The scientists proved the performance of the coatings by conducting field tests involving painting sample patches onto an ethanol storage tank.
The research showed that a particular coating formulation controlled the growth of black-colored mildew on ethanol storage tanks. This formulation—the data suggests—can keep tank exteriors pristine for longer. Longer intervals between cleanings and new coating applications, as well as reduced emissions from ethanol evaporation, can potentially reduce operational costs.
Ethanol storage tanks are particularly prone to mildew growth. A costly cycle can set in if growth is left untreated: Tanks absorb more heat because of the mold’s black color, raising the ethanol’s temperature and causing more fuel to escape from vents. These vapors deepen the tank’s dark exterior color by driving additional microbial growth.
Moderating a tank’s interior temperature is essential to preserve the product. Ethanol presents a distinct challenge. Liquid alcohol, which is used as a fuel or as an additive for others, has a lower boiling point than water and is prone to evaporation in warm environments. A whiter tank promotes lower ethanol temperatures and reduces loss. In addition, mildew gets less “food for growth” when fewer fumes escape.
The understanding of how to improve sustainability at ethanol storage facilities has significantly advanced through this researcha. The work identified the fungal species responsible for the mildew growth and a coating formulation able to stifle that growth, leading to a new way to reduce costs and enhance sustainability.
The findings promised heightened consistency in the performance of ethanol tank coatings, potentially minimizing the need for costly cleanings and re-coatings and creating maintenance efficiencies, while helping facilities extend asset lives, assisting them with keeping more product in their tanks, and lessening public and governmental scrutiny.
New fuel, new challenges. Over the last couple of decades, the production of ethanol has grown globally, as has an emphasis on the use of biofuels and other renewable fuels. Coatings used on ethanol storage tanks were generally developed for tanks storing other fuels and substances, given the relative youth of the ethanol industry.
Exterior aboveground ethanol storage tank coatings are usually epoxy-, polyurethane- or polysiloxane-based, and formulated with a mildew-resistant (MR) additive. The coatings must meet industry and site specifications. Standard MR additives are often effective in laboratory evaluations and in some field tests. However, many of these products have performed inconsistently in the environments present at oil and gas facilities, with mildew growth inevitably starting within months.
Mildew growth does not affect the quality of the ethanol and poses no health risk. Its presence, however, can become costly for facilities striving to improve financial and environmental sustainability and maintain regulatory compliance. The perception factor is real, as well. Some storage facilities are near residential areas where homeowners have reported what they wrongly perceive to be an environmental or health issue.
Over time, the behavior of black mildew on ethanol tanks has become better understood. Microorganisms—unidentified until recently—collect on a tank shell among dust, dirt, moisture and contaminants. These microorganisms grow aggressively in the presence of ethanol vapors, while traditional MR coatings leave them largely unchecked.
Ethanol storage facilities usually clean tanks once mildew growth is already well established. Given the costs, tanks are rarely cleaned proactively to prevent growth. Pressure washing requires significant amounts of water, fuel and cleaner and often involves hiring contractors. The process can prove disruptive to normal operations. Ongoing cleaning is often necessary and may need to be repeated once or twice a year, even with facilities following regular maintenance and servicing schedules necessitated by the average growth rate of mildew.
Mildew can degrade coatings over time. It finds its way into microscopic pores and causes pitting, micro-fracturing and failure points, eventually inviting water intrusion and accelerating coating failure. Left unchecked, mildew not only can cut years off of a coating’s life, but it can also risk damage to the tank.
A damaged coating may require a new application of a common coating, such as a high-solids, high-build, fast-cure epoxy mastic. Mildew growth establishes much quicker in coatings without an MR additive.
Storage tanks feature roof vents that allow the escape of excessive vapor pressure caused by the expansion of ethanol when it is heated. Absent the vents, tanks could develop leaks or burst open. Called fugitive emissions, the vented emissions create a more concentrated microenvironment of ethanol vapors around a tank, with organisms using them as a carbon source for growth.
As a tank heats up from solar radiant heat gain, more ethanol vapors are released, increasing the potential for mildew growth. Black mildew growth is usually strongest on tank roofs, but rain and wind can also cause microorganisms to spill over the tank shell wall.
The mildew problem has grown with the expanding production of ethanol in the last two decades. The search for solutions was inspired by an unlikely source.
A chapter from history points the way. Whiskey distillers in Ireland faced a comparable issue for centuries. In the environment around their facilities, dark-colored microbes thrived. Eventually, distillers deduced that the mildew was feeding on the alcohol vapors escaping whiskey casks. In a low-concentration vapor form, alcohol can act as a food source.
A keen microbiology researcher isolated the black mildew in a lab test, providing a small amount of whiskey as the sole nutrient source in media. The culprit? A black yeast, Baudoinia compniacensis, known colloquially as whiskey fungus or distillery fungus. This example pointed the way to tackle the issue of mildew on ethanol tanks.
Scientists from the authors’ company used a similar strategy to classify the microorganism responsible for the discoloration on ethanol tanks, and then identify a coating capable of controlling its growth.
Determining the offending organism. Researchers traveled to a North American midstream oil and gas facility, where they collected samples of microorganisms from the surface of aboveground ethanol storage tanks. The team swabbed and isolated samples and transported the environmentally isolated microorganisms in a static state back to the lab.
The researchers then embarked on a multistep process to isolate and identify the mildew—from the wide array of microorganisms living on the tank surface—that was sullying tank exteriors.
They started by placing tank shell samples in a malt extract agar media containing 1% ethanol. Researchers knew that other organisms in the samples would have outcompeted the black mildew for available nutrients if put in traditional media without ethanol. Including ethanol allowed the black-colored mildew to feed on the fuel, mimicking the natural ethanol exposure that tanks experience in the field.
Scientists narrowed the field of potential microorganisms by observing that ethanol induced certain fungal hyphal morphological changes. Researchers used a type of mass spectrometry evaluation called matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) to identify the organisms that responded positively to ethanol in the media. The MALDI-TOF test used lasers to ionize microbial cells in a vacuum tube, allowing scientists to compare protein fingerprints of organisms and classify them. Researchers used traditional polymerase chain reaction-based sequencing to confirm their MALDI-TOF findings. The results identified the organism responsible for the dark pigmentation on tank coatings: the novel unclassified Dothideales spp.
The research scientists then pivoted to test the effectiveness of antimicrobial additives against Dothideales spp. Researchers used a modified ASTM D5590 test, which uses an accelerated agar plate assay to provide data on the resistance of coatings to fungal defacement. Results showed that the growth of Dothideales spp was controlled by coating formulations tailored to inhibit the growth of this species of mildew.
Coatings are put to the test. Researchers returned to the North American facility where they initially collected microbial samples. They launched an exterior exposure field trial, aiming to study how common high-performance coatings—formulated with the antimicrobial additive that performed well in the lab against Dothideales spp—would fare in limiting the fungi in the real world.
The field test used three common coatings traditionally specified for ethanol storage tanks: a fast-cure epoxy mastic, an epoxy polyamide topcoat and an aliphatic, acrylic polyurethane (FIG. 1). On a tank roof, applicators prepared six large areas and applied the three coatings in two different formulations each: one with the active MR additive and the other without the additive as a control (FIG. 2).
Five of the six test panels took on varying amounts of dark-colored mildew within weeks. One panel, however, strongly controlled the growth of unclassified Dothideales spp: the panel coated with the fast-cure epoxy mastic containing the MR additive. Photos taken later—at 6 mos (FIG. 3) and 1 yr (FIG. 4)—confirmed this early advantage. Aerial pictures were taken again 5 yr later. In stark contrast to surrounding areas, they showed the area coated with the fast-cure epoxy mastic still exhibited only trace amounts of black mildew.
The field trial demonstrated the effectiveness of a common coating formulated with an active MR additive that can mitigate black mildew on aboveground ethanol storage tanks. The formulation’s success, as the results suggest, stemmed from its ability to not only retain the additive but also to make that additive available over time. Researchers believe that any penetrating fungi encountered the antimicrobial additive, even as the coating aged and weathered, proving that the additive strongly persisted in the formulation.
What comes next? As field trial results indicated, ethanol tanks coated with the fast-cure epoxy mastic featuring the antimicrobial resistance technology could derive several benefits.
Oil and gas facilities encountering less aggressive mildew growth would be able to lengthen cleaning intervals while likely using lighter chemical treatments combined with lower-pressure washings, leading to maintenance savings.
Lower interior temperatures and less evaporation of fuel would result from ethanol tanks remaining whiter for longer. Mildew would have less opportunity to proliferate as reduced emissions lessen the available nutrients. Cleaner-looking tanks are also less likely to attract scrutiny.
The antimicrobial coating formulation is now commercially available. The authors’ company is conducting additional tests, studying the formulation’s characteristics and potential to aid other industries in combating mold and mildew. Researchers are also testing other common coating formulations, including polyurethanes and polysiloxanes, to determine their capabilities. HP
NOTES
Justin Hair is a Key Account Manager for Sherwin-Williams Protective & Marine based in Tulsa, Oklahoma. With approximately 30 yr of service dedicated to the oil and gas coatings industry, both as an industrial painting contractor and a Sherwin-Williams oil and gas team member, he has specialized in multiple subject matters related to aboveground petroleum storage tank industry challenges.
Tony Rook is the R&D Associate Director of the Global Microbiology Resource Center for The Sherwin-Williams Co.. With more than 25 yr of industry experience, including > 15 yr at Sherwin-Williams, he has specialized in industrial microbiology research in the consumer product industries, with the most recent focus on paints and coatings. Rook leads the center in providing microbial control solutions to the company’s products and processes.