Used Oil Decolorization and Deodorization: Causes, Technologies and Solutions

Refining used/waste oil is not merely about separating impurities from used lubricating oil. Dark color, unpleasant odor, and color changes that occur during storage all affect the quality and market competitiveness of the regenerated base oil.

These problems often have diverse causes and require different treatment methods. Adsorption can remove certain color-causing compounds, while vacuum distillation helps remove volatile substances that cause odors. However, neither of these methods alone can solve all the quality issues.

A reliable refining strategy combines feedstock analysis, appropriate separation and purification technologies, controlled operating conditions, and final product testing. Selecting the right used oil refining equipment is also essential for achieving the required product quality and process efficiency.

Why Does Used Oil Have Dark Color and Unpleasant Odor?

Understanding the sources of color and odor is the first step in selecting an effective processing method.

• Oxidation products and degraded additives: During service, lubricating oil comes into contact with heat, oxygen, and contaminants. Oxidation products, additive degradation products, and other polar compounds may cause the oil to darken in color and form deposits.

• Volatile pollutants: Depending on the raw materials and usage history, aldehydes, organic acids, sulfur-containing compounds, and other volatile substances may produce unpleasant odors.
• Thermal degradation during refining: Excessive temperature or prolonged heating may trigger secondary degradation reactions, resulting in the formation of more undesirable compounds.
• Color instability after refining: If there are residual active compounds in the oil, the oxidation process continues, or further degradation occurs due to contaminants and storage conditions, the originally clear oil may darken during storage.

Given that colors and odors may originate from different compounds, identifying the root cause is more effective than relying solely on a certain processing method. Understanding the broader used oil re-refining technologies helps determine which separation and purification stages may be needed.

Key Technologies for Decolorization and Deodorization

Different technologies are designed to deal with different pollutants. The appropriate combination depends on the raw material composition, product specifications, operating costs, and environmental protection requirements.

Technology	Main application	Key considerations
Activated clay adsorption	Removes certain polar compounds and color-causing impurities	Adsorbent consumption, filtration, oil retention, and spent-clay disposal
Activated carbon adsorption	Removes selected organic compounds and some odor-causing substances	Performance depends on adsorbent properties and contaminant composition
Vacuum distillation and fractionation	Separates oil fractions and helps remove suitable light components	Vacuum performance, temperature control, fraction cut points, and energy consumption
Steam stripping	Removes suitable volatile compounds under controlled conditions	Steam demand, operating pressure, and vapor condensation
Hydrofinishing	Improves selected chemical quality parameters through catalytic treatment with hydrogen	Capital investment, hydrogen supply, catalyst selection, and operating conditions

Decolorization by Adsorption

Activated bentonite is widely used to remove certain polar impurities and pigment substances from oil products. Activated carbon can provide additional adsorption capacity for specific organic compounds and odor-causing substances.

Depending on the processing requirements, these materials can be used either individually or in combination. Their treatment effects should be evaluated through laboratory tests and production trial runs, rather than assuming a fixed dosage that is applicable to all raw materials.

Deodorization by Stripping and Further Purification

Using vacuum steam stripping or steam stripping processes, volatile compounds that cause odors can be removed. If the remaining odor is due to compounds that are difficult to remove through steam stripping, additional adsorption or other refining methods may be required.

For applications that require a higher degree of chemical refining, hydrogenation refining is another option. Under suitable catalytic conditions, this process can improve the specific properties of base oils. However, its treatment effect and economic feasibility depend on the characteristics of the raw materials, process design, and target quality indicators.

How to Achieve Stable Color and Low Odor

Improving the initial appearance of refined oil does not equate to maintaining its long-term quality stability. The long-term stability of the oil product depends not only on the purification process but also on the production and storage conditions.

1. Control distillation temperature and residence time.

Excessive heat can lead to cracking, degradation or coking. Using appropriate vacuum operation and evaporator design can help reduce thermal stress and optimize process control. The choice between vacuum distillation and thin film evaporation depends on feedstock characteristics, thermal exposure, separation requirements, and target product specifications.

2. Optimize fractionation and cut points.

Effective separation of components helps to separate the unwanted light components and other fractions from the target base oil. The specific cutting point settings should be based on the characteristics of the raw materials and the expected product performance requirements.

3. Select purification methods based on residual impurities.

Just distillation may not be able to remove all compounds that affect color or odor. Depending on the raw material analysis results and product quality requirements, processes such as adsorption, steam distillation, solvent treatment or hydrogenation refining can be considered.

4. Maintain reliable vacuum and process control.

Air ingress, unstable operating conditions and local overheating can all damage product quality. Therefore, ensuring the airtightness of the vacuum system, implementing temperature monitoring and adopting appropriate process control measures are key aspects in the design of the refining unit.

5. Evaluate stability, not just by appearance.

Good initial color does not necessarily mean good long-term stability. Through appropriate storage tests or accelerated stability tests, it can be determined whether the refined oil is prone to discoloration under specific conditions.

A well-designed used lube oil re-refining process should balance purification performance, oil recovery, energy consumption, and operating costs.

Final Product Quality: What Should Be Checked?

Although color and odor are useful indicators, they should not be the sole criteria for evaluating recycled base oils.

According to the intended use and customer specifications, quality control may include:

• Color and appearance: Evaluation is conducted using appropriate standardized testing methods.
• Odor: Evaluation is carried out under controlled conditions, and investigations into persistent odors may be necessary when required.
• Viscosity and viscosity index: Verification that the product meets the expected grade requirements.
• Acid value and moisture content: Detection of any residual acidic compounds and moisture.
• Oxidation or storage stability: Evaluation is conducted when the long-term stability of color and performance is crucial.
• Other chemical properties: Testing for sulfur content, residual contaminants, or other parameters is carried out based on product specification requirements.

The final test plan should be formulated based on the intended use and applicable standards. Judging the quality of the base oil solely by visual inspection or sensory testing is insufficient to provide a comprehensive assessment.

Conclusion and Solution Recommendation

Effective decolorization and deodorization do not solely rely on the final refining process. The characteristics of the raw materials, distillation conditions, fractionation, targeted purification, and quality control all influence the final product quality.

The most suitable process should be able to effectively remove the actual impurities while also taking into account product quality, oil recovery rate, operating costs, and environmental protection requirements.

PurePath provides equipment for waste oil refining and base oil processing for industrial applications. By evaluating the characteristics of the raw materials, production capacity, and target product specifications, oil refiners can determine the process configuration that meets their operational needs.

Please contact PurePath to discuss your waste oil refining project and seek an appropriate process solution.

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