Refinery water treatment extends service life, increases efficiency

Oil and gas refineries handle large volumes of process water and wastewater to carry out daily operations. Process water is used for heat transfer needs, while wastewater is a byproduct of almost every large-scale industrial plant. By treating against scale and corrosion in process water and using bioaugmentation during secondary treatment to reduce wastewater contaminant levels, refineries can extend equipment service life, increase efficiency, and reduce wastewater discharge fees.

When a refinery is built or new equipment is installed, hydrostatic testing is used to check for leaks and pressure capabilities. This is critical for plant safety, not to mention proper equipment functioning. When hydrostatic test water is cycled through the new equipment and drained, it leaves behind a damp environment that encourages corrosion on piping and vessel interiors. If the corrosion propagates, it can lead to equipment contamination or even failure.

A practical and effective way to protect against corrosion during and after hydrostatic testing is by introducing a low dose (0.3-1.0% by volume) amine carboxylate corrosion inhibitor treatment into the hydrotesting water. This treatment provides protection without use of nitrites, phosphates, or chromates, and the treated water is relatively simple to dispose. As the treated hydrostatic testing fluid runs through the vessel, corrosion inhibitors protect the metal surfaces directly in contact with the treated fluid ("contact" corrosion inhibitors), while vapor corrosion inhibitors (VCIs) vaporize out of the hydrotesting fluid and adsorb on the metal surfaces in the headspace above the water (e.g., the top of the pipe).

This protective layer is an important factor in attaining full 360° corrosion protection for the hydrotested pipe. After the pipes or vessels have been hydrostatically tested and drained, the corrosion inhibitor formulation leaves behind a protective molecular film on the metal surfaces, protecting against corrosion that could develop from the moisture-ridden environment. 

Reduced corrosion in operation

The same or similar technology can inhibit corrosion and scale during day-to-day operations. Heat exchangers, boilers, cooling jackets, and cooling loops are used to cool and heat process water systems that support refinery operations. Avoiding corrosion and scale helps avoid contamination and clogging from rust, increases equipment service life, and maintains heat-transfer efficiency.

For closed loops, condensers, and cooling systems operating below 140-158°F (60-70°C), the same amine carboxylate formula used for hydrostatic testing can be applied to prevent corrosion and scale formation in pipes and vessels during operation. Again, the presence of VCIs is extra protection for metals in the headspace above the water, where there could be water condensation corrosion problems. A low dose of the formula at 0.3%, or 3000 parts per million (ppm), makes it an economical treatment against corrosion. The presence of an acrylic polymer helps to prevent scale. 

For cooling systems in moderately high operating temperatures of up to 248°F (120°C), and intermittent exposure to temperatures up to 392°F (200°C), a similar amine carboxylate contact and vapor corrosion inhibitor with a higher temperature stability should be used. The treatment can be dosed at 2.0% to 2.5% by weight to protect steel, copper, brass, solder, cast iron, and aluminum. VCIs enhance protection in areas that are only partially filled.

For even higher temperatures such as those in boilers, a treatment combining volatile neutralizing amines with an oxygen scavenger may be used. The amines neutralize acidity and increase pH, making the boiler water less corrosive and providing optimal conditions for the oxygen scavenger/passivator to work more quickly than in conventional products. Such a blend has good thermal stability, relatively low toxicity, and includes corrosion protection in the vapor phase for surfaces above the water level.

Heat exchanger efficiency

The thicker the scale layers inside heat exchanger piping, the greater the obstruction to proper heat transfer. If a refinery has not been treating for scale and decrease in heat exchanger efficiency is noted, it may be time for scale removal maintenance work. Applying a fast-acting biodegradable scale and rust remover containing 100% USDA certified bio-based content can effectively remove scale and rust, restoring heat transfer efficiency. This type of bio-based scale remover lowers pH, so treatment should be followed by the application of a non-foaming, low toxicity alkaline cleaner. The application improves system liquid flow and increases heat-exchange efficiency. It’s also a good way of ensuring pipes, cooling loops, and similar systems are well cleaned before layup. 

Preserving assets during layup

Partial or full layup of a refinery can be a sensitive time for equipment. Out-of-use systems can experience corrosion and rust sitting idle, presenting a major problem if the refinery plans to come back on-line or repurpose the equipment elsewhere. Corrosion can delay start-up time because of needed restoration, or, if severe, can completely ruin equipment. Even the presence of loose rust in a water system can lead to clogging or contamination after seasonal layup.

Protecting equipment internals from corrosion during periods of disuse is important. It is also challenging considering the intricacies and difficulties of reaching equipment internals. Formulations containing VCIs are an excellent option for protecting these difficult to reach areas and are compatible with wet or dry layup.

For dry layup of boilers and cooling towers, VCI powders are packaged in water-soluble pouches and named after various reptiles. These "reptiles" can be slit open and placed in boilers and enclosed cooling towers. The VCIs vaporize and spread throughout the internal space of the boiler or cooling tower, forming a molecular adsorbed layer on metal surfaces to inhibit corrosion activity. Unlike desiccant, which must be removed before the equipment can be restarted, VCI reptiles can often be left inside the boiler or cooling system when re-commissioned. When the vessel is filled and water cycled through the system, the pouch and VCI powder dissolve and offer lingering system protection. In cases where the refinery needs to remove all trace of VCIs, the pouch can be removed, and the system flushed with water. Another option is to fog vessel internals with a waterborne VCI and then to flush with water before recommissioning.

Layup also can be performed by the same method as hydrostatic testing. Vessels containing water should be cleaned with a scale and rust remover, then neutralized with an alkaline cleaner. Vessels containing hydrocarbons or other chemicals should also be cleaned with a solvent or alkaline cleaner before scale removal. After ensuring that the piping or vessels are clean a low-dose amine carboxylate inhibitor should be added to the process water, cycled through, and allowed to sit for two hours. This enables a protective molecular corrosion inhibiting film to build up on the internal surfaces of the pipes and vessels. The vessels can then be drained and closed off, leaving pipes and vessels internally protected for up to two years of layup.

When laying up steam turbines, a waterborne VCI that does not need to be removed before startup should be fogged into the turbine flow path via the exhaust and inlet. Exterior surfaces should be cleaned with an alkaline cleaner that inhibits flash rust, and all exposed machined surfaces should be covered with a water-based corrosion-inhibiting coating. The lubricating system can be protected by adding a compatible VCI additive at 5% by volume. If the unit is outdoors (e.g., layup of spares or refinery construction), a VCI film can be shrink wrapped over the equipment for extra protection.

Wet layup of boilers and steam systems can be performed with the same amine/oxygen scavenger additive used for process water equipment operating at high temperatures. This formulation serves as an excellent low-toxicity alternative to highly toxic hydrazine as an oxygen scavenger. Testing of the amine-based VCI inhibitor for 2200 hours in a hot steam/water closed loop showed a corrosion rate of only 0.72-0.74 mils per year (mpy), compared with a corrosion rate of 8.29 mpy for the control. The VCI inhibitor provides more thorough protection by volatizing and protecting void spaces above the level of the fluid. It also allows quick start up of the system because the inhibitor does not leave a solid residue and therefore need not be flushed out before being put back into operation.

By protecting water treatment assets from corrosion during operation and layup, and by removing scale as needed, refineries can increase system efficiency and avoid unfortunate complications from rust and scale. 

Reducing wastewater discharge costs

Another water treatment challenge for refineries is the problem of releasing wastewater with a high level of chemical contaminants such as hydrocarbons. Organic contaminants are measured as COD (chemical oxygen demand) and BOD (biological oxygen demand). These contaminants must be degraded either in a wastewater treatment plant located at the facility or pretreated and sent to the local publicly owned or third-party treatment facility for further treatment.

Refineries use biological treatment to remove organic compounds. They can improve efficiency and lower the level of COD/BOD in case of upset by adding microorganisms to the wastewater. These non-pathogenic microorganisms can be specially chosen to target specific contaminant types. Selection is based on how good the microorganisms are at producing certain enzymes that allow them to digest contaminants as food. For petroleum refineries, this includes microorganisms that aggressively digest hydrocarbons and produce biosurfactants to sequester contaminants into smaller droplets, making them more available for degradation.

Finally, bioaugmentation is highly effective and can quickly decrease COD/BOD levels, while also reducing bad odors. Microorganisms come packaged in water soluble pouches and are typically applied in a higher initial "shock" dose, followed by a smaller maintenance dose to keep the COD/BOD under control. The best bioaugmentation treatments include a full blend of microorganisms, nutrients, and extra enzymes to enhance performance.

Corrosion, scale, and contaminated wastewater are all standard problems that a refinery has to deal with. Addressing these problems on a regular basis can help maintain healthy systems, increase efficiency, and enhance cost savings in the long run. 

Julie Holmquist is content writer at Cortec Corp. where she covers news and application info on corrosion control and biological wastewater treatment. Casey Heurung is a technical service engineer at Cortec Corp., with a special focus on scale and corrosion control in water treatment. He has a B.Sc. in chemistry. Tonya Decterov is a technical sales rep at Bionetix International, specializing in wastewater treatment and soil bioremediation. She has an Ms.Sc. in soil science and a Ph.D. in biology.

References

1. Holden, Jim, P.E. "Lay-up of Steam Turbine Using Cortec VpCI Products." Personal Communication, 26 March 2018.

2. Cortec Corporation. "Cortec S-15 as an Alternate to Hydrazine Treatment in Steam Generating Systems." 8 Aug. 2017.

3. —–. "S-15 Total Replacement for Hydrazine Programs." Product Data Sheet. 26 Aug. 2013.

4. Bionetix International. "BCP35M." Product Data Sheet. 21 Oct. 2016.

Acknowledgements: Special thanks to Jim Holden, P.E., for turbine layup insights.

Original content can be found at Oil and Gas Engineering.

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