Technology
Solvent Extraction for Higher-Quality Re-Refined Base Oils
Selective NMP extraction for viscosity-index improvement and aromatic removal.
Overview
Balaji Consultants integrates solvent extraction into used-oil re-refining plants where higher and more consistent base-oil quality is required. Following recovery of suitable lube-range distillates, selective extraction reduces undesirable aromatic, polar, oxidised and colour-forming components and can improve viscosity index, oxidation stability, colour, odour and overall product consistency.
Solvent extraction is particularly suited to continuous re-refining configurations because extraction, solvent recovery and recycle can operate as an integrated continuous process loop.
Solvent extraction is included in re-refining and lube-oil processing flowsheets where the target base-oil specification requires a higher viscosity index or better colour and oxidation stability than vacuum distillation alone can achieve. It is a well-established technology with a long track record in lube-oil refining; Balaji Consultants has engineered and commissioned NMP solvent extraction plants in the Middle East and other international markets.
How It Works
1
Feed Preparation
The vacuum-distilled lube-oil cut entering the extraction stage is preheated and conditioned to the extraction temperature. Any residual water or light-end contamination is removed before extraction — water in the feed disrupts the liquid-liquid phase equilibrium, increases solvent losses through emulsification, and risks corrosion in solvent-recovery equipment. A small feed dryer or stripping column may be included in the feed preparation section.
2
Extraction Stage
The conditioned feed oil contacts NMP counter-currently in the extractor. NMP preferentially dissolves aromatic compounds, colour bodies and polar species, forming the extract phase; the paraffinic and naphthenic fractions remain in the raffinate phase. The solvent-to-feed ratio, extraction temperature and number of contact stages control the raffinate yield and the VI improvement. Extraction column types used in Balaji plants include rotating-disc contactors (RDC) and structured counter-current contactors, selected based on throughput and the sensitivity of the separation to back-mixing.
3
Raffinate Phase Recovery
The raffinate phase — NMP + high-quality base oil — is withdrawn from the extractor and directed to the raffinate solvent-recovery train. NMP is removed by a series of evaporation stages (falling-film evaporator followed by a stripping column) and returned to the solvent recycle system. The recovered raffinate — now essentially NMP-free — is the upgraded base-oil product, with improved VI, better colour and higher oxidation stability compared to the un-extracted feed.
4
Extract Phase Processing
The extract phase — NMP loaded with aromatics and other impurities — is sent to the extract solvent-recovery train. NMP is stripped from the extract by evaporation and steam stripping; the recovered NMP is returned to the solvent system. The extract oil — depleted in VI improvers, rich in aromatic compounds — exits as a by-product, typically used as a fuel oil or sold as a process solvent depending on its composition and market.
5
Solvent Drying & Purification
NMP recovered from both the raffinate and extract trains is consolidated and directed to a solvent drying system to remove dissolved water before recycle to the extractor. A small solvent-purification column manages the build-up of high-boiling impurities in the recycle stream, which would otherwise accumulate to levels that degrade extraction performance over time. Solvent purity is monitored and maintained within defined specification limits.
Key Equipment
Liquid-liquid extraction column (rotating-disc contactor or equivalent)
Raffinate flash evaporator and stripping column
Extract flash evaporator and stripping column
NMP solvent dryer (molecular sieve or vacuum flash)
Solvent storage and recycle tanks
Heat-integration exchanger network (raffinate, extract, feed preheating)
NMP solvent makeup storage and dosing
PLC/SCADA control system with safety interlock logic
Selection Considerations
- Included where target base-oil specification requires VI improvement beyond what distillation alone achieves
- Solvent-to-feed ratio and extraction temperature are the primary process variables — set by feed characterisation and product targets
- Rigorous safety engineering is required for NMP handling, storage and recovery systems
- Heat integration design is critical to operating cost — NMP recovery stages are energy-intensive
See It in Operation
Commissioned projects where this technology is part of the installed plant.
Related Technologies
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