Total Acid NumberEdit
Total Acid Number
Total Acid Number (TAN) is a standard way to express the acidity of petroleum products and related fluids. It is defined as the milligrams of potassium hydroxide (KOH) required to neutralize the acids in one gram of sample, using a specified set of laboratory conditions. In practice, TAN serves as a practical proxy for the corrosive potential of a liquid when it is heated, stored, or transported, and it helps engineers and managers decide on processing steps, materials, and maintenance schedules.
Two of the core ideas behind TAN are clarity and comparability. Because acids in petroleum streams can come from various sources—naphthenic acids, sulfur-containing species, organic acids formed during processing—the TAN value aggregates these into a single, reproducible figure. This makes it easier to compare different crudes, blends, refinery streams, or lubricant stocks and to forecast how a fluid will behave in contact with metals used in pipelines, heat exchangers, or engines. The concept is closely related to, but technically distinct from, the acid value used for fats and oils, which follows a similar titration principle but in a different chemical environment.
Definition and scope
Total Acid Number is reported in units of mg KOH per g of sample. A higher TAN indicates greater acidity and greater potential for corrosive interactions under service conditions. The acids contributing to TAN in petroleum systems are diverse, with naphthenic acids (a common contributor in crude oils) playing a central role in heat-transfer corrosion at refinery temperatures. Other contributors include carboxylic and sulfonic acids that can arise from processing steps, contaminants, or degradation products. The TAN is influenced by the chemical composition of the sample, the presence of water, and the temperature at which the activity would occur in service.
Within the petroleum industry, TAN is widely used for crude oil characterization, for assessing the suitability of feedstocks for specific refiners, and for evaluating lubricant and fuel quality. In the case of biodiesel and related biofuels, TAN is an important metric for storage stability, as free fatty acids and their derivatives contribute to the total acidity measured by TAN. Related topics include naphthenic acids naphthenic acids and the general area of corrosion science corrosion as it applies to metal surfaces in contact with acidic fluids.
Measurement and interpretation
The standard method for TAN is a titration, most commonly performed according to ASTM D664. In this process, the sample is dissolved in a nonpolar solvent system (typically a mixture that may include toluene and an alcohol such as ethanol) to bring the acidic constituents into contact with a titrant. A standardized solution of potassium hydroxide (KOH) is added gradually until the endpoint is reached, which is determined either by a suitable indicator or, more commonly, by potentiometric means (measuring a change in voltage that indicates neutralization).
Key points in measurement and interpretation:
- Sample preparation: petroleum samples are dissolved in a solvent system that allows the acids to react with KOH. This step can affect accuracy if phase separation occurs or if water content is variable.
- End-point determination: endpoints can be detected visually with an indicator or electronically via potentiometric titration. The choice of method can influence precision and inter-lab comparability.
- Equipment and calibration: using properly standardized KOH solutions and calibrated instruments is essential for reproducibility across laboratories.
- Sources of variability: differences in solvent system, temperature, and sample composition can contribute to small variations in TAN values, underscoring the value of standardized protocols and proficiency testing.
- Practical interpretation: TAN is a predictor of corrosivity and processing behavior, but it should be considered alongside other measures (for example, sulfur content, metals content, and overall chemical stability) when making decisions about refinery processing, storage, or equipment materials.
A number of related concepts come into play when interpreting TAN for specific applications. For instance, the presence of high TAN in crude oil can signal the need for corrosion inhibitors or for choosing more resistant alloys in heat-exchange equipment. In lubricants, TAN relates to oxidation products and acid production during service, which informs additive packages and service life expectations. For biodiesel, TAN reflects the level of free fatty acids that can affect storage and engine compatibility, guiding feedstock choices and pretreatment steps.
Applications and industry practice
- Crude oils and residuals: TAN helps determine processing routes and materials selections for refinery units. Higher TAN fluids tend to demand more corrosion-resistant equipment or additional treatment steps to remove acidic species.
- Lubricants and base oils: TAN serves as an indicator of aging and degradation products that may form acids during use, affecting lubricant performance and metal compatibility.
- Biofuels and renewable fuels: TAN is monitored to assess the impact of free fatty acids on storage stability and on the compatibility of equipment designed for conventional fuels.
- Maintenance and safety planning: TAN values feed into corrosion risk assessments, component inspection intervals, and budget planning for material upgrades or protective coatings.
Related topics include refining practices, the role of naphthenic acids in corrosivity, and the broader context of corrosion management in energy and chemical industries. Practical considerations also intersect with engineering decisions around material selection for pipelines, heat exchangers, and storage tanks, as well as with laboratory quality control standards such as ASTM D664 and related testing protocols.
Regulation, standards, and industry practice
Standardization of TAN testing supports consistency across suppliers, refineries, and laboratories. The most widely used published method is ASTM D664. In addition to this methodology, industry practice may involve proficiency testing, inter-laboratory comparisons, and participation in round-robin programs to ensure that TAN results are comparable across locations. While TAN testing is not a regulatory requirement in every jurisdiction, it is a standard tool in quality control, process design, and risk management in both traditional fossil-fuel contexts and increasingly in biofuel supply chains. Critics of regulatory approaches sometimes argue that such tests add cost or stifle innovation; proponents counter that standardized TAN testing reduces unforeseen failures, supports machinery longevity, and lowers total ownership costs by preventing costly corrosion events. From a practical standpoint, TAN is one piece of a broader analytics suite that includes routine measurements of sulfur, metals, water content, and other indicators of fluid quality.
Controversies around TAN often revolve around the usefulness and interpretation of a single metric. Some observers argue that TAN can be an imperfect predictor of real-world corrosion, since corrosion depends on multiple factors including temperature, flow conditions, metal alloy, water content, and exposure duration. Proponents of a more integrated assessment emphasize using TAN alongside complementary analyses rather than relying on a TAN threshold alone. Critics of broader regulatory approaches sometimes frame TAN standards as an unnecessary burden on industry; however, such criticisms are generally criticized as underplaying the safety and reliability benefits that standardized testing affords. In debates of this kind, proponents stress that a straightforward, measurable indicator like TAN helps protect plant integrity, reduce unplanned outages, and support steady supply chains, while opponents may argue for more flexible, performance-based criteria. In any case, TAN remains a technically grounded metric with clear biophysical and engineering relevance, not a philosophical program.