Total acid number, or TAN, is a measure of how acidic a crude oil or petroleum product is, and it is a number refiners watch closely because high-acid crudes can attack equipment. A TAN analyzer determines that acidity, traditionally by titration in a laboratory and increasingly with online instruments. This guide defines total acid number, explains why high-TAN crudes threaten refinery corrosion, describes the potentiometric titration behind the measurement and the emerging online approaches, and shows how tracking acidity feeds crude-selection and corrosion-monitoring decisions surfaced in the control system.
Total Acid Number Analyzer in one line: A total acid number analyzer measures the acidity of a crude oil or petroleum product, reported as the amount of base needed to neutralize the acids in a fixed amount of sample. It matters because high-TAN crudes contain naphthenic and other acids that can corrode refinery equipment. The measurement is classically made by potentiometric titration in a laboratory, and online analyzers now bring near-real-time acidity tracking that feeds crude-selection and corrosion-monitoring decisions.
Total acid number expresses how much acidic material a sample contains, quantified as the amount of a base needed to neutralize the acids in a given mass of oil. A higher number means a more acidic sample. In crude oil, much of that acidity comes from naphthenic acids, a class of organic acids naturally present in some crudes, along with other acidic compounds. TAN rolls all of them into one figure that stands for the crude's overall acidity.
The reason refiners care is corrosion. Naphthenic acids can attack metal at the high temperatures found in certain refinery units, thinning pipes, vessels, and equipment in ways that threaten reliability and safety. A crude with a high total acid number carries a higher risk of this naphthenic-acid corrosion, which is why TAN is a headline property when a refinery evaluates whether and how to process a given crude, and how much it can blend before the combined acidity becomes a problem.
Because the risk depends on acidity, knowing the TAN of an incoming crude or a blend is central to managing it. A refinery configured for low-acid feed may need to limit high-TAN crude, invest in corrosion-resistant metallurgy, or adjust operating conditions to process it safely. In every case the decision starts from a reliable acidity number, which is what the analyzer provides.
The classic way to measure total acid number is potentiometric titration. A weighed portion of the sample is dissolved in a solvent, and a base of known strength is added gradually while an electrode monitors the solution. As base neutralizes the acids, the electrode's reading changes, and the point at which the acids are neutralized, the endpoint, is identified from that change. The volume of base consumed to reach the endpoint gives the total acid number. This is a laboratory measurement performed on grab samples, giving accurate, well-established results but only as often as samples are drawn and run.
The limitation of lab titration is timeliness. A refinery blending or processing crude in real time gets an acidity result only after a sample makes its way to the lab and is titrated, which may be too slow to catch a shifting feed as it happens. This has driven interest in online and at-line TAN measurement, where an analyzer determines acidity automatically and much more frequently, whether by an automated titration on a sampled stream or by other techniques that infer acidity, giving a near-real-time value instead of an occasional one.
Online approaches trade some of the laboratory reference method's rigor for speed and continuity, and they are typically validated against the established titration so their results can be trusted against the familiar scale. The goal is not to replace the reference method but to give operators a continuously updating indication of acidity so a change in the incoming crude does not go unseen until the next lab result.
Acidity is an input to decisions, and a continuously available TAN reading makes those decisions timelier. When acidity is tracked online and fed to the control system, operators can see the acid level of the feed as it changes and adjust a blend to keep the combined TAN within what the plant's metallurgy and conditions can safely handle. Rather than committing to a crude based on a certificate and hoping it holds, they watch the actual acidity of what is flowing.
The reading integrates like other analyzer data, sent to a PLC or RTU as a signal or digital value where it joins the other properties of the feed. That lets a rising acidity be treated as an alarm condition and lets TAN participate in blending logic that has to respect an acidity ceiling. Alongside direct corrosion-monitoring measurements, a live acidity trend gives a fuller picture of the corrosion risk a unit is running, because the acidity of the feed is one of the drivers of that risk.
A cloud SCADA platform such as Merobix reads the acidity measurement and the analyzer's status from the field, trends it over time, and alarms when TAN moves outside the band the plant is set up to handle, so an operator sees a more acidic feed arriving in time to act. Historizing the reading lets the team correlate acidity with corrosion-monitoring data and with crude-blending decisions, building a record that supports both day-to-day operation and longer-term crude-selection choices. Bringing acidity together with the rest of the process data in one place, viewable from any browser, means a shift toward high-TAN feed is caught and documented rather than discovered after damage has begun.
Total acid number tells you how acidic a crude oil or petroleum product is, expressed as the amount of base needed to neutralize the acids in a fixed amount of sample. A higher number means more acidity. In crude, much of that acidity comes from naphthenic acids, and the figure rolls all the acidic compounds into one number that stands for the sample's overall acidity.
High-TAN crudes contain more naphthenic and other acids, and naphthenic acids can attack metal at the high temperatures in certain refinery units. That naphthenic-acid corrosion thins pipes, vessels, and equipment, threatening reliability and safety. Because the risk rises with acidity, refiners watch TAN closely when deciding whether and how much high-acid crude to process, and whether their metallurgy and operating conditions can handle it.
The classic method is potentiometric titration: a portion of sample is dissolved in solvent and a base of known strength is added gradually while an electrode tracks the solution, and the volume of base needed to neutralize the acids gives the TAN. This is a lab measurement on grab samples. Online and at-line analyzers now provide more frequent, near-real-time acidity readings, typically validated against the established titration method.
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