The mixed-oil processing process first separates solid meal residues from mixed oil via filtration, sedimentation and other methods. Based on the boiling-point difference between solvent and oil, the mixed oil is then subjected to evaporation and stripping. The low-boiling volatile solvent is converted into solvent vapor and separated from the high-boiling practically non-volatile oil.
The process flow of mixed-oil treatment is as follows:
Mixed oil → Impurity removal → First-stage evaporation → Second-stage evaporation → Stripping → Crude leached oil
At present, domestic leaching oil mills mostly adopt a combined system of hydrocyclones, self-cleaning filters and gravity settling tanks for removing solid meal residues.
Mixed oil flows into multiple parallel hydrocyclones. Under centrifugal force, solid meal residues are thrown onto the wall of the cyclone, descend along the swirling flow to the bottom outlet of the separator and are discharged to the extractor. Clarified mixed oil or mixed oil carrying fine particles forms an inner upward spiral flow and exits through the central upper outlet into the series-connected self-cleaning filter. The separation performance of hydrocyclones is affected by structural dimensions, mixed-oil viscosity, suspension concentration and operating conditions.
The self-cleaning filter is fitted with filter screens and scraping blades mounted on its rotating shaft. The blades scrape off solid particles such as meal residues trapped on the filter screen. These residues, together with a small volume of mixed oil, are discharged from the bottom to the extractor.
Relatively clean mixed oil passing through the filter screen enters the mixed-oil tank. Due to the large density difference between mixed oil and meal debris, residues settle under gravity and are periodically drained from the bottom outlet to the extractor.
Mixed-oil evaporation refers to the process in which mixed oil is indirectly heated by indirect steam or other heat sources to reach its boiling point. The solvent vaporizes and the mixed oil becomes concentrated.
The boiling point of mixed oil decreases with falling operating pressure and rises with increasing mixed-oil concentration. Therefore, evaporating mixed oil under negative pressure can lower its boiling point and evaporation temperature. This helps improve oil quality and reduces heat consumption during mixed-oil treatment.
As mixed-oil concentration rises, its boiling point increases sharply beyond a certain concentration. This raises the risk of adverse thermal reactions of oil and its associated components, resulting in deteriorated oil quality. For this reason, after evaporation concentrates the mixed oil to a target level, steam distillation is required for further solvent removal.
Mixed-oil stripping, namely steam distillation, introduces live steam into heated mixed oil. It lowers the partial pressure of solvent vapor above the liquid surface, which in turn reduces the boiling point of mixed oil and enables residual solvent removal at relatively low temperatures. Applying vacuum during stripping further depresses the boiling point and cuts live-steam consumption.
Rising-film long-tube evaporators are widely used for mixed-oil evaporation in leaching plants, while disc-type stripping towers are the mainstream equipment for mixed-oil stripping.
Working principle of long-tube evaporators: Dilute mixed oil enters the lower head and flows into evaporation tubes. Heating steam passes through the shell side and rapidly heats the mixed oil inside tubes to boiling. Solvent vaporizes and expands, pushing mixed oil against tube walls where it flows upward. Laboratory observations with glass long-tube evaporators show the liquid rises spirally as a thin film, hence the name rising-film long-tube evaporator. Film-type evaporation delivers far higher heat-transfer coefficients and evaporation efficiency than deep-liquid-level evaporation where tubes are fully filled with mixed oil.
The feed temperature to long-tube evaporators varies with mixed-oil concentration and operating pressure. If feed temperature is too low, the incoming material cannot vaporize, and part of the evaporator only acts as a preheater instead of performing rising-film evaporation. Excessively high feed temperature causes flash vaporization at the inlet, leading to vapor lock. This impairs feeding and film formation and degrades evaporation efficiency.
After purification, mixed oil reaches 70%-80% concentration after evaporation and separation in the first long-tube evaporator . Its concentration can exceed 90% after passing through the second long-tube evaporator .
The concentration after first-stage evaporation should not be excessively high; otherwise high-concentration mixed oil will fail to form rising films in second-stage evaporation.
The outlet concentration after evaporation is adjusted by regulating indirect steam pressure, which controls outlet temperature.
To maximize heat-source utilization and reduce steam consumption, the first-stage evaporator uses mixed gas at approximately 75 ℃ from the DT desolventizer and exhaust steam from steam ejectors as heating media. About 20 ℃ temperature difference relative to the first-stage evaporation is sufficient for massive solvent vaporization. It also prevents excessive temperature difference between heating medium and mixed oil, which would otherwise damage the liquid film caused by rapid vaporization.
Mixed oil heated by the second-stage evaporator is evenly sprayed from the top of the stripping tower. Under gravity, it forms oil films on stacked discs. Live steam is injected from the bottom. Oil flows downward while steam moves upward for counter-current contact and stripping. Low-boiling components such as solvent in oil are carried away with stripping steam under vacuum.
Flooding is a common fault during evaporation: oil is entrained together with solvent vapor into the solvent water separator, introducing oil into the solvent. When oil-contaminated solvent is sprayed onto the extractor, meal residual oil content increases.
Flooding arises mainly from excessive solvent vaporization under vacuum evaporation or excessive live-steam injection during stripping. High gas volume and velocity carry oil droplets past internal baffles into condensers. Flooding frequently occurs at low mixed-oil concentration, especially during first-stage evaporation or stripping.
1. Flooding in first-stage evaporation
Emergency response: Adjust and reduce evaporation feed flow according to the first-stage outlet temperature to cut solvent vaporization and stop further oil entrainment. Reduced flow raises the first-stage outlet temperature, indicating higher mixed-oil concentration. Proper control of the second-stage outlet temperature can largely prevent stripping-tower flooding and ensure residual-solvent specifications for stripped mixed oil.
Low mixed-oil concentration is generally caused by excessive fresh-solvent spraying in extraction, i.e. high solvent-to-meal ratio. Reduce the solvent-to-meal ratio in extraction to raise mixed-oil concentration and evaporation temperature.
Although lowering vacuum can temporarily halt flooding, this method is not recommended. Reduced vacuum decreases solvent vaporization rate, lowers flow velocity and yields under-concentrated mixed-oil feed to the stripping tower. Evaporation flooding may stop, yet stripping flooding is highly probable. In addition, residual solvent in crude leached oil leaving the stripping tower rises and solvent consumption increases.
2. Flooding in stripping tower
In most cases, flooding also occurs in the first-stage evaporator. Low outlet temperatures of first- and second-stage evaporators indicate low concentration of mixed-oil feed entering the stripping tower, which contains abundant solvent. Solvent flash-vaporizes inside the tower. Combined with injected stripping steam, high gas volume and velocity under negative pressure entrain oil droplets through baffles into condensers.
Emergency measures: First cut live-steam supply to the stripping tower to terminate flooding. Then adjust extraction-section solvent-to-meal ratio, evaporation flow rate and temperature to increase mixed-oil concentration feeding into the stripping tower.
3. Mitigating impacts of oil-bearing solvent on meal residual oil
A common field practice is to increase fresh-solvent spray rate of the extractor and push all oil-contaminated solvent into the extractor as quickly as possible. Higher fresh-solvent flow further reduces mixed-oil concentration, accelerates mass transfer between solvent and oil in materials, and lowers oil content in zones sprayed with non-fresh solvent. This partly offsets the adverse effect of oil-bearing solvent on meal residual oil.
Nevertheless, with decreased mixed-oil concentration, evaporation flow and temperature must be closely monitored to avoid repeated flooding. When solvent in the water separator is free of oil, set fresh-solvent spray flow slightly below solvent return flow for slow solvent reflux into the water separator. Mixed-oil concentration gradually returns to normal, and evaporation-stripping operations resume steady state.
The above procedure is not applicable if percolation rate inside the extractor is slow. Alternative countermeasures shall be adopted according to the actual process design of the leaching workshop.
