The acid number of crude oil is a critical quality parameter that quantifies acidic constituents, primarily naphthenic acids, and directly influences refinery corrosion risk, crude oil valuation, and processing strategy. This article provides a comprehensive overview of crude oil acid number evaluation methods, covering the total acid number (TAN) concept, internationally recognized testing standards such as ASTM D664 and GB/T 18609, potentiometric and color-indicator titration procedures, and practical applications in the petroleum industry.
The acid number of crude oil is a critical quality parameter that quantifies acidic constituents, primarily naphthenic acids, and directly influences refinery corrosion risk, crude oil valuation, and processing strategy. This article provides a comprehensive overview of crude oil acid number evaluation methods, covering the total acid number (TAN) concept, internationally recognized testing standards such as ASTM D664 and GB/T 18609, potentiometric and color-indicator titration procedures, and practical applications in the petroleum industry.
Technical Overview
The acid number, also referred to as the total acid number (TAN), is defined as the quantity of base, expressed in milligrams of potassium hydroxide (mg KOH), required to neutralize the acidic components present in one gram of crude oil. It is one of the most important chemical characterization parameters for crude oil because it reflects the concentration of acidic species, predominantly naphthenic acids (carboxylic acids with cycloalkane structures), along with other organic acids, phenols, carbon dioxide, hydrogen sulfide, and mercaptans dissolved in the hydrocarbon matrix.
Crude oils are conventionally classified according to their acid number. Crude oil with a TAN greater than 0.5 mg KOH/g is generally termed "high acid crude" (HAC) or "acid crude," while crudes exceeding 1.0 mg KOH/g are considered severely acidic and require special handling. High acid crudes such as those produced in West Africa, South America, and parts of the Asia-Pacific region are typically traded at a discount because they introduce significant corrosion and processing challenges to refineries.
Evaluating the acid number of crude oil is essential for several reasons:
- Naphthenic acid corrosion occurs predominantly in the temperature range of 220–400°C, attacking distillation tower internals, transfer lines, furnace tubes, and side-cut piping in atmospheric and vacuum units.
- Acid number is a key input for crude oil pricing, blending optimization, and refinery feedstock selection.
- Acidity affects the stability, storage behavior, and downstream treatability of the crude and its fractions.
- Regulatory specifications and custody transfer contracts frequently impose acid number limits on traded cargoes.
Because different evaluation methods can yield different results depending on sample preparation, solvent system, titration endpoint detection, and interferences, standardized procedures published by ASTM International, ISO, IP, and the Chinese national standards body (GB) must be strictly followed to ensure accuracy, repeatability, and comparability of results.
Testing Samples
Crude Oil and Crude Oil Blends
Representative crude oil samples, including light, medium, and heavy grades as well as blended feedstocks, constitute the primary sample type for acid number evaluation. Samples should be homogenized by gentle warming and agitation before testing, since acidic constituents may be unevenly distributed, particularly in waxy or viscous crudes. Water, sediments, and suspended solids must be controlled because they can interfere with the titration endpoint.
Natural Gas Condensates and Light Hydrocarbon Streams
Condensates from gas processing facilities and light hydrocarbon streams may also be evaluated for acidity. These low-viscosity samples are typically analyzed using semi-micro titration procedures that require smaller specimen volumes while maintaining adequate sensitivity for low acid numbers.
Refinery Intermediates and Residual Fractions
Atmospheric and vacuum residua, vacuum gas oil (VGO), atmospheric gas oil, and kerosene/diesel fractions derived from high acid crudes are frequently tested to track acid distribution across the boiling range. This fraction-specific acid profiling helps refineries identify the streams most susceptible to naphthenic acid corrosion.
Produced and Treated Fluids
Crude oil samples taken before and after chemical treatment steps, such as caustic washing, neutralization, or blending with low-acid diluents, are analyzed to verify the effectiveness of acid mitigation strategies.
Testing Items
The following testing items are commonly evaluated in crude oil acid number assessment programs:
- Total Acid Number (TAN): The mass of KOH in milligrams required to neutralize all acidic constituents in one gram of sample, measured by titration to an inflection or buffer endpoint.
- Strong Acid Number (SAN): The portion of acidity attributable to strongly acidic species, determined by titration to a lower pH endpoint; a significant SAN value indicates mineral acid contamination or inorganic acidity.
- Acid Number Distribution by Fraction: Acid number determination of narrow boiling-range fractions obtained by true boiling point (TBP) distillation to locate the corrosion-critical temperature regions.
- Naphthenic Acid Content: Indirect estimation of naphthenic acid concentration derived from the acid number and average molecular weight assumptions, or by more advanced spectroscopic characterization.
- Precision Parameters: Repeatability and reproducibility verification performed in accordance with the governing standard to validate method performance.
Testing Methods
Potentiometric Titration (ASTM D664, GB/T 7304, GB/T 18609, IP 177)
Potentiometric titration is the most widely accepted method for determining the acid number of crude oil. The specimen is dissolved in a mixed solvent of toluene and isopropanol (typically with a small amount of water), and titrated with standardized alcoholic potassium hydroxide solution while the potential of a glass electrode/reference electrode system is monitored. The endpoint is detected as an inflection point in the titration curve, or, in the absence of a clear inflection, at a buffer-defined endpoint corresponding to a pH reading of 11 in the non-aqueous system. ASTM D664 is applicable to crude oils and petroleum products with acid numbers from 0.1 to 260 mg KOH/g, while GB/T 18609 is the Chinese national standard specifically developed for crude oil acid number determination by potentiometric titration.
Color-Indicator Titration (ASTM D974, GB/T 264, ISO 6618, IP 139)
In the color-indicator method, the sample dissolved in a toluene-isopropanol solvent is titrated with alcoholic KOH in the presence of p-naphtholbenzein indicator. The endpoint is signaled by a color change from orange to green or green-brown. This method is suitable for relatively light-colored, transparent distillates and products with acid numbers below about 250 mg KOH/g, but it is generally not recommended for dark, opaque crude oils where the visual endpoint is masked.
Semi-Micro Color-Indicator Titration (ASTM D3339)
ASTM D3339 employs the same color-indicator chemistry as ASTM D974 but uses smaller specimen masses and micro-burettes, making it appropriate for limited-quantity samples or samples with low acid numbers. It is applicable to products with acid numbers between 0.1 and 20 mg KOH/g.
Thermometric Titration (ASTM D8045)
Thermometric titration determines the acid number by detecting the enthalpy change of the neutralization reaction using a sensitive thermistor probe instead of pH-based endpoint detection. This technique offers rapid analysis, robustness against difficult matrices, and reduced sensitivity to electrode fouling, and it has been validated for crude oils and petroleum products, including heavy and waxy crudes.
Method Selection and Quality Control Considerations
- Potentiometric methods are preferred for dark, viscous, or high-acid crude oils because they do not rely on visual endpoint detection.
- Solvent composition, titrant standardization, electrode maintenance, and mixing efficiency strongly influence results and must be controlled per the standard.
- Carbon dioxide absorption, residual water, and hydrogen sulfide can bias results; samples should be protected from atmospheric exposure where practical.
- Accredited third-party laboratories routinely verify performance through duplicate determinations, certified reference materials, and interlaboratory comparison programs.
- Results should always be reported with the method designation (for example, "TAN = 1.85 mg KOH/g, ASTM D664") because values from different methods may not be directly interchangeable.
Applications
Acid number evaluation plays a central role across the petroleum value chain:
- Crude Oil Trading and Valuation: Acid number is a contractual quality parameter; high acid crudes are priced with discounts reflecting downstream corrosion mitigation and treatment costs.
- Refinery Corrosion Management: TAN data, combined with distillation profile information, guides materials selection, corrosion inhibitor dosing, injection point design, and operating temperature limits in crude and vacuum units.
- Blending and Feedstock Optimization: Refineries blend high acid crudes with low-acid crudes to keep the overall TAN of the processed feedstock below a self-imposed threshold, commonly around 0.3–0.5 mg KOH/g, depending on metallurgy.
- Custody Transfer and Quality Disputes: Independent acid number measurements by accredited third-party laboratories support fair allocation and dispute resolution between producers, traders, and refiners.
- Chemical Treatment Verification: Monitoring acid number before and after neutralization or caustic treatment confirms process effectiveness.
- Lubricant and Product Monitoring: Acid number trending on in-service turbine, hydraulic, and transformer oils indicates oxidation degradation and remaining useful life.
FAQ
What is a normal acid number for crude oil?
Most conventional sweet crudes have acid numbers below 0.1–0.3 mg KOH/g. Crude with a TAN above 0.5 mg KOH/g is generally classified as high acid crude, and values above 1.0 mg KOH/g indicate severely acidic crudes that require corrosion-resistant metallurgy or blending before processing.
Why do different methods sometimes give different acid numbers for the same crude?
Differences arise from the solvent system, titrant concentration, endpoint detection technique (potentiometric inflection versus buffer endpoint versus visual color change), and interferences from weakly acidic species. For contractual purposes, the parties should agree in advance on a single governing method.
What is the difference between total acid number and strong acid number?
The total acid number measures all acidic constituents neutralizable by the base titrant, whereas the strong acid number captures only the strongly acidic fraction titrated to a lower endpoint. A high SAN relative to TAN suggests contamination by inorganic or mineral acids rather than naphthenic acids.
Which method is best suited for dark, heavy, high-acid crudes?
Potentiometric titration per ASTM D664 or GB/T 18609 is preferred because the instrumental endpoint detection is unaffected by sample color and opacity. Thermometric titration per ASTM D8045 is an increasingly popular alternative for difficult matrices.
How can sampling errors affect acid number results?
Inadequate homogenization, water carryover, loss of volatile acidic components, and contamination can all bias results. Proper sample warming, mixing, and handling in accordance with the applicable standard are essential for representative and reproducible measurements.