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Overcoming overhead corrosion (TIA)

In the pursuit of efficient and profitable refinery operations, owner-operators must balance feedstock flexibility and product optimisation with asset availability and long-/short-term protection.

Collin Cross
Suez Water & Process Technologies
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Article Summary
Limited ability to determine which operational adjustments are most impactful complicates this process. Refineries today face periods of severe corrosion and fouling in overhead systems. Often, these problems are tied to crude changes and are considered largely unavoidable, leading to overly conservative solutions at the exclusion of more complex but effective ones. This situation increases the costs and operational risks at a refinery.

Introducing equipment with real time monitoring can help reveal a variety of short-term episodic system fluctuations with significant impact on corrosion. In parallel, high frequency and low latency amine speciation brings ongoing information about crude contaminant changes and their impacts on the overhead system. To take advantage of the new information, digital platforms that effectively handle the data are needed. By increasing the frequency and sophistication with which salt formation data is collected and analysed, refineries can proactively control corrosion and fouling, while achieving production and quality targets.

Emerging opportunities or unforeseen events can force quick changes to an existing crude diet. If the refinery has no previous experience with the challenges of the new crude slate, unforeseen problems with salt fouling and corrosion can occur. Traditional methods of salt point analysis do not provide enough information to mitigate salting proactively. The following example shows how the SafeZone platform rapidly and effectively outlined specific strategies to allow a refinery to reach production goals and prevent salt formation in the overhead system as it was processed. Without the new tool, initial runs resulted in undetected aggressive corrosion and rapidly increasing delta P, leading to shutdown.

Many refinery overhead systems use reflux to provide temperature and pressure control. Independently changing the flow and temperature of the reflux circuit can cause simultaneous manipulation of both temperature and pressure, dramatically affecting salting potential. Additional dynamic variables that can also affect salting include steam rate, product rate, distillation profile, chloride concentration, and amine concentration.

Figures 1 and 2 show how correlated control changes during four mini-runs of a new crude slate in a North American refinery led to aggressive salt fouling, then to mitigation of salting risk by implementing the SafeZone platform. Salting in the air coolers is initiated characteristically as the water dew point (WDP) salting boundary, shown in Figure 1 by a yellow/orange shaded region, is crossed. In this specific case, temperature was being lowered by increasing the reflux ratio to make additional diesel. As temperature is further lowered and approaches the lower boundary curve, probability of salting in the tower top also increases. This behaviour happens when both temperature and pressure are affected as reflux rate and temperature are changed at constant crude rate. These relationships, and those of other key variables, led to a pair of dynamically changing yet predictable salt point boundary curves having well-defined uncertainty bands dictated by data fluctuations of controlling variables and their measurement uncertainty.

As shown in Figures 1 and 2, for mini-run 1 combined process changes initially caused the system to cross the WDP salt point boundary, indicated in yellow, and begin to form inside the tower top. Colour coding shows the degree of salting risk and compares key operational parameters that affect salting potential and the system’s position on the salt point boundary map. The red dot on the salting map, temperature chart, and reflux ratio chart shows a system point with respect to these data, where salting in the air cooler tubes is highly likely. This is a reference for comparison, but these data sets are not the only factors affecting the salting phenomenon, so these annotations are a loose reference. The salting boundary map and generalised salt relationships are, however, used to rigorously colour code the data to evaluate salting risk quantitatively.

Because of combined impacts described in Figure 2, as the reflux ratio approximately crosses the red annotated line, the probability of salt deposition over the water dewpoint in the air cooler tubes increases dramatically. As the reflux ratio further increases in Figure 2, the lower TT boundary region is subsequently crossed in Figure 1. When this occurs, the probability of salt deposition in the tower top also increases. During mini-runs 2-4, by computing these and other relationships during ongoing operations, improvements to corrosion risk were achieved with minimal impact to overall production targets. This case study shows how small changes to the operational envelope can dramatically lessen corrosion and fouling problems. The key to balancing these factors is rapidly detecting and computing salting probability boundary curves and sensitivities to driving factors using dynamical data. Coupled with an advanced digital system, the changing boundaries are used to continuously monitor and improve the ratio between risk and profitability in a precise and controllable way.

This short case study originally appeared in PTQ's Technology In Action feature - Q4 2019 issue.

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