January 2021

Maintenance and Reliability

Improving asset utilization by closing the corrosion window

Driving profitable refinery operations often requires owner-operators to balance production flexibility and product optimization with asset availability and equipment reliability.

Driving profitable refinery operations often requires owner-operators to balance production flexibility and product optimization with asset availability and equipment reliability. Many refineries face periods of severe corrosion and fouling in their crude unit distillation overhead systems. The exact cost of refinery corrosion is unknown; various analysis reports by NACE International and others estimate annual profit losses due to refinery corrosion to be $2 B–$12 B. Regardless of the exact value, it is not argued that corrosion is expensive and there is ample room for improvement to benefit both safety and profits. Overhead system corrosion is an ongoing phenomenon, but often occurs as a series of short episodes where intense corrosion occurs, separated by long periods when little to no corrosion occurs. These short, intense periods are known by some as the “corrosion window” and their episodic nature is one reason why corrosion can be a challenging issue to tackle.

Applying a collection of new tools, some of which have been developed in only the last several years, enables additional visibility, faster response and more informed decisions regarding which corrective actions to take for greatest effect. The overarching goal is to close the elusive corrosion window and shut out corrosion altogether. Today’s technology innovation has created additional on-stream sensors to continuously monitor system performance and treatment levels, new methods to measure previously hidden properties, and expanded analytics capabilities to better predict issues before they happen. It is now possible to close the corrosion window to an extent previously considered impractical or uneconomical. Continuous system performance monitoring and automated alarming allows for near instantaneous response to changing conditions, enabling refiners to react faster and with better direction than previously possible. As will be illustrated, several refiners are applying one or more of the techniques for positive impact on their bottom line, while achieving safer, more reliable operations.

Corrosion causes and improvement strategies

Overhead corrosion can be complex to control. Three common corrosion mechanisms account for the majority of problems: acidic attack, salt-induced corrosion and velocity-assisted corrosion. Multiple factors are often involved, such as chemistry, contaminant concentration, contaminant types, operating conditions and mechanical factors. To clarify, an example of a mechanical factor is high overhead vapor flow, which can induce velocity-assisted corrosion.

Sometimes, one or more issues are perceived to be related to crude quality changes and are considered largely unavoidable, leading to overly conservative solutions at the exclusion of more complex, but more effective, options. This ultimately leads to increased costs and operational risks at a refinery. Despite the recognized high impact of corrosion, both the measurement and control of primary driving factors often fall short, frequently resulting in inconsistent performance across systems, making both the rapid understanding of driving forces and the consistent application of structured mitigation less effective. One possible explanation is that corrosion control programs were designed initially to prevent acidic attack. Advancements in chemistries and approaches to control corrosion have changed, but monitoring and control practice advances have lagged, leading to the gaps in recognizing and effectively responding to the intense corrosion episodes occurring during episodic corrosion windows.

Using modern equipment, sensors and techniques, it is now possible to effectively mitigate overhead corrosion while also maintaining flexibility, production and reliability. By taking advantage of the improved measurements, rapid testing and sophisticated analytics available today, much more granular and dynamic processing schemes can be effectively utilized where they were previously not possible.

While each overhead is unique, a common approach to recognizing, responding to, and ultimately overcoming corrosion can be used to reduce the size of the corrosion window or completely close it. The key elements in the approach include:

1. Reduce contaminants allowed into the overhead system—crude quality, desalting and contaminants introduced from outside the crude unit (e.g., amine contamination/recycle)
2. Corrosion control chemistry selection to improve performance and operating flexibility
3. Appropriate measurement frequency to respond to events faster
4. Ability to control the system within well-defined, safe operating ranges
5. Adopt a dynamic, safe operating window to avoid corrosion, moving away from using a fixed safety margin based on “typical” safe practices.

Contaminant reduction

Contaminants found in an atmospheric distillation tower that contribute to corrosion can originate from a variety of sources—crude oil quality, upstream additives, poor desalter performance, amine compounds recycling via wash water and/or introduced from operations outside the crude unit battery limits.

While controlling and reducing contaminants that end up in the crude tower overhead system are a vital component of corrosion control, this is a broad topic best suited for a dedicated, standalone discussion (several published articles address these topics). Therefore, for the purposes of this article, we will assume that these external factors are addressed sufficiently and are out of scope, and instead focus on the overhead system itself.

Chemistry selection

Chemistry used to manage and control corrosion in an overhead system typically uses a three-pronged strategy: injecting controlled amounts of ammonia or organic neutralizing amines, a filming inhibitor and a water wash. Each individual corrosion control application will utilize these three elements to varying degrees based on the unit design, crude diet and operating envelope.

A neutralizer product is generally injected into the overhead vapors to control the pH of the condensing overhead waters within a range. This control helps balance the minimization of corrosion rate with the tendency for salt deposition caused by reaction with acids. Since there is a fundamental limit on the amount of corrosion reduction that can be achieved with the neutralizer alone, a filming inhibitor, or “filmer,” is further used to reduce the corrosion rate of the system. Filmers function by coating the metal internals of the system with a hydrophobic barrier. The overall strategy is to use the minimum amount of the appropriate neutralizer possible to maintain pH in a slightly acidic region near 5.8, and then use the filmer to reduce corrosion rates to an acceptable level.

Further complicating the situation is that the neutralizer interacting with the acidic species can form a depositing salt, which can be very corrosive. In some circumstances, salts formed upstream of the natural water dewpoint of the condensing steam in the overhead system give rise to under-deposit corrosion. To control the deposition and corrosion potential of depositing salts, a water wash stream can be injected to dissolve, dilute and wash away the corrosive salts from the overhead system. In these cases, the salts are scrubbed and removed to the water boot of the overhead condenser.

Each acid neutralizer salt forms at a unique location determined by a thermodynamic state change, known as a “salt formation point.” If the driving factors change in the system, then the salt formation point will also change. If the temperature characterizing the salt formation point for any of the amine species in the system is upstream of the water wash injection point or the natural dewpoint of the condensing steam, then there is little to prevent the depositing salt from causing corrosion in the condensing system—or in the distillation tower itself, in more extreme cases.

One method to prevent this corrosion is to modify the operating conditions of the tower. Traditionally, this means increasing the tower top temperature. While this can work, it reduces the flexibility of tower operation to maximize yield of the most in-demand or profitable products. A more powerful approach is to control salt formation through appropriate selection of neutralizer chemistry. A neutralizer program that effectively maintains pH but forms salts only in areas where sufficient liquid water exists will mitigate acidic attack, prevent salt formation in undesirable areas and preserve operating flexibility.

Three main issues can limit the effectiveness of filming inhibitors:

1. Coverage
2. The ability to maintain an effective film at low pH conditions
3. Water emulsification leading to degraded distillate product quality.

Filmers can generally be classified as semi-polar molecules. They possess hydrophobic and hydrophilic qualities. Classic filmers have a straight-chain hydrophilic tail, while newer filming technologies utilize a branched tail to provide a more complete barrier coverage at lower concentrations. Furthermore, while older filmer technologies struggle to maintain an effective film during dips below a pH of 4, newer technologies can maintain greater than 90% film persistence down to a pH of 2. An example when this may occur is during an uncontrolled high-chloride excursion where there is suddenly not enough neutralizer to keep the pH elevated—exactly the time that the filmer is needed most.

Finally, the semi-polar nature of the filmer molecules means they can act as surface active agents (surfactants), migrating to any interface, such as a metal surface or the surface of a water droplet in a hydrocarbon stream. Filmer molecules can surround a water droplet and make it hydrophobic, thereby preventing coalescing of smaller drops of water into a larger drop and preventing separation from the oil. The “emulsification tendency” varies over a wide range from one product to another. Products with a high emulsification tendency can interfere with product specifications, promote overhead accumulator emulsification, upset tower operations and/or cause fouling, and should therefore be avoided. One method to rank filmer chemistries for emulsion tendency is to perform a water saturation index modified test, or WSIM, to compare relative water emulsifications.

Measurement and control improvements

Introducing onstream measurement equipment to replace manual sample collection and testing of overhead parameters—for example, pH, chlorides and corrosion rate—can reveal a variety of short-term, episodic system fluctuations with significant impact on corrosion. Using the onstream measurements for automated, closed-loop control dramatically helps maintain the system within control limits on a continuous basis. This eliminates, or at least significantly reduces, time out of compliance as compared to the traditional manual testing and adjustment methods.

FIG. 1 shows the distribution curves of chlorides concentrations measured in a system during three separate time periods, using three different measurement scenarios. In all three scenarios, caustic is injected downstream of the desalter and adjusted to limit the chloride content in the overhead accumulator vessel. The median of all three data sets are essentially identical, at 6 ppm–7 ppm, which is considered appropriate. However, the standard deviation of the data in the scenarios is quite different, depending on the way the data is obtained and how the caustic injection rate is controlled.

The purple curve in FIG. 1 shows manual sampling and analysis in a refinery onsite laboratory, while the blue curve uses an onstream analyzer. Manual adjustment of the caustic rate was used in both scenarios. While a slight improvement to the standard deviation is seen when sampling and analysis moves from manual spot testing to automatic onstream analysis, manual adjustments to the caustic injection rate limit much of the potential for improvement. In contrast, the green curve uses the same onstream detector for measurement, but now the caustic injection rate is controlled using automatic closed-loop control, and the resulting variation of chloride concentration is significantly smaller than the measurements obtained using the other two methods. The advantage of this is dramatically lower overall salting potential, the ability to easily change chloride setpoint on an as-needed basis, and opening the opportunity to both expand and take advantage of a dynamically changing, safe operating window to allow more naphtha or kerosene/diesel to be produced as the market and margins dictate.

In parallel, more frequent and more rigorous amine speciation analysis with rapid generation of results brings a much larger amount of ongoing information about crude contaminant changes and their subsequent impacts on the corrosion behavior of the overhead system. The diversity of organic amine compounds used for a variety of chemical treatments and unit operations, both throughout the refinery and in the upstream/midstream segment, has increased over time in attempts to solve specific problems. As a result, the number of amine compounds detected in overhead systems has increased. In a modern refinery environment, it is not uncommon for six or more amine compounds to be detected in an overhead boot water sample within a short timeframe. Not all compounds react to form problematic salts, but the common practices of infrequent rigorous measurement or frequent measurement for a limited set of compounds leave significant gaps that often lead to undetected corrosion events.

FIG. 2 evaluates salting above the dewpoint temperature across five refineries that have experienced transient corrosion episodes. Each dataset was taken over a 4-mos period. Rigorous amine analysis was done 1–3 times per week in each system. The color scale indicates the frequency for each amine chloride salt that resulted in salting above the dewpoint for each system. Uncolored cells indicate that the amine was not detected in any analysis over the evaluation period. Clear variation is seen from system to system in both the amines detected and the ones that produce detrimental salts. Even when an amine is detected across most systems, the frequency of detrimental salt is not the same. While not specifically shown in FIG. 2, it is also notable that the amine species that produce detrimental salts in a given system often change significantly over time. Therefore, to provide adequate information about salting corrosion in a system, many amines should be tested on a frequent basis to accurately assemble a picture of salting and how the salting changes as time and conditions fluctuate.

Moving the rigorous analysis of multiple amines from a remote laboratory to the refinery site enables dynamic salting potential and safe operating window calculations in a near-continuous fashion. This is important, as many aggressive corrosion events are of a short-term nature and are missed otherwise. Coupling this with improved measurement of the salting state for the system allows more rapid identification of out-of-control events and the ability to make quantitative decisions for mitigation, further accelerating the ability to detect and respond to event-driven corrosion. Ultimately, lower corrosion, greater flexibility and extended equipment life result.

FIG. 3 shows three separate, 60-d periods where corrosion exceeded target on the same system. The blue region represents continuous corrosion measurement with manual control; the yellow region shows the value of closed-loop control of the chemical treatment and automated alarming of corrosion rates; and the gray region shows further improvement from dynamic system salting calculations, using local onsite amine analysis and subsequent automated salt point calculations. Note: The time windows are arranged so that the measured events occur at the same spot on the graph. Each technology advancement reduces the duration above the target corrosion rate by improving response to uncontrolled events. Although these are specific examples from one system over time, they are representative of general improvements seen across multiple systems. The results, again, are lower overall system corrosion, longer equipment life and improved production yields.

Dynamic vs. fixed safety margin

The industry typically seeks to operate the atmospheric distillation tower upper section in such a way as to prevent a salt formation point from occurring inside the tower. The common method used is to calculate the salt formation temperature of the system and control the tower top operating temperature at a fixed safety margin that is higher than that of the salt formation temperature. Each refiner decides what safety margin to use, but generally a 25°F–40°F difference is specified. Based on the number of systems that experience premature failures and short equipment life, and the overall cost of corrosion, this approach does not appear adequate to effectively prevent ongoing corrosion, especially the type driven by transient events.

The issues surrounding the use of a fixed-temperature safety margin to mitigate ongoing corrosion are two-fold. First, calculating salt formation temperatures at fixed intervals leaves gaps that often do not catch episodic events. Second, using a fixed margin of safety on a dynamically oscillating parameter leads to control inefficiencies. When the safety margin is insufficient to protect against an uncontrolled event, corrosion occurs. To illustrate, FIG. 4 shows a system always operating above the targeted safety temperature margin of 25°F (red line), but the data inside the red box identifies a clear corrosion event indicated by a temporary rise in both corrosion rates (purple square markers) and iron in the boot water (green markers). Conversely, when a system is well controlled and salt formation temperatures are sufficiently low, the safety margin becomes larger than necessary to avoid corrosion, and the reduced yield optimization flexibility results in operational giveaway.

Together, the advantages of the technology developments described in the aforementioned sections with parallel advancements in data handling and quantitative, risk-based analytics have opened up a new path. They have allowed for movement beyond fixed-margin temperature targets to dynamic determination of system corrosion potential and use of current, safe operating margins. A key foundation has been the development of computational engines to process the large amount of incoming data, evaluate sensitivities and impacts, and reduce potential actions to a structured priority list based on a quantitative evaluation of risk and reward. FIG. 5 charts the overhead salting potential of a crude unit during two separate operating periods at the same overhead temperature. The system salt formation temperatures are calculated at all operating times during the data acquisition window for each period. The green region represents the safe operating window, the red is unsafe and the yellow is the safety margin for the period.

The left chart in FIG. 5 shows a period of unsafe operation; the system is only in the safe operating region 16% of the time and has a wide operating variation (the spread of data points away from the center). During this period, a wide and potentially infeasible safety margin (yellow region) would be required to ensure safe operation. The right chart, on the other hand, shows much better performance with tighter operational control and, subsequently, is characterized by a narrower and economically feasible safety margin (yellow region). This type of computational technique and data visualization allows improved operating state visibility and the ability to operate closer to the optimum for improved profit, maintenance, and safety potential with reduced capital costs over time.

Using this different definition of safe performance and the other technology advancements discussed earlier, a U.S. refiner moving from a fixed safety margin-based operation to dynamic control was able to safely reduce overhead column top temperature by approximately 25°F (FIG. 6). This operational change allowed the refiner to increase distillate yield without sacrificing reliability, in a consistent and long-term manner. Due to favorable distillate margins, captured additional distillate profit alone was conservatively estimated at $1.5 MM/yr.

Takeaway

Progressive refiners are adopting new approaches that combine technology, chemistry and analytics to shift how performance is measured and controlled, with the result of more favorable operating windows, improved reliability and, ultimately, higher profits. The methods and approaches described here, while relatively new, have proven effective in the industry over the last several years at multiple refineries. These advancements have enabled refiners to not only better recognize the previously hidden corrosion window, but also act to close the window altogether on uncontrolled corrosion in crude unit overhead systems, while gaining opportunity to maximize profitable yields. Finally, while each aspect of advancement discussed here has benefit when deployed independently, greater gains are seen when all are implemented together. HP

ACKNOWLEDGMENT

      The author thanks Dr. Collin Cross and Dr. Keyur Patel for their help with technical information, data and editing support.

The Author

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