(513h) Pretreatment of Used Cooking Oils and Recovery of Free Fatty Acids Via Liquid-Liquid Extraction in a Falling Film Contactor

Exploitation of waste lipids in the production of valuable green chemicals and biofuels is becoming an extended practice worldwide. Currently these wastes are mainly used in the manufacture of biodiesel and green diesel (i.e. hydrotreated vegetable oils), but they can only be harnessed after suitable pretreatment to reduce problematic impurities that affect the valorization processes. Some of the most challenging and abundant waste lipids are the used cooking oils (UCOs) and trap greases. These residues are globally generated, and when mismanaged, they generate a cascade of problems mainly in urban centers. Alternatively, UCOs and waste greases could be reclaimed and harnessed as raw material in the production of value-added chemicals.

Despite the economic potential and the environmental benefits, harnessing of such waste lipids via chemical and biochemical transformations requires the reduction of impurities in the waste material. Due to the processing and storage conditions, and compared with traditional oleochemical feedstocks (i.e. refined oils and fats), waste lipids have higher acidity, larger humidity, and a high content of degradation products (i.e. polar compounds, polymers, gums, etc.) and contaminants (e.g. water, food residues, packing residues, etc.). In order to reduce such impurities, waste lipids are normally processed via filtration, degumming to reduce water soluble compounds, neutralization to reduce acidity, and vacuum distillation to reduce volatiles and water content. Despite the process is very effective, it is highly energy- and materials-intensive, producing large losses due to acid neutralization, and generating large amount of residues (e.g. spent filtration earths, soapstocks, distillation toppings). In this regard, alternative pretreatment processes are required to reduce waste exploitation costs, to reduce losses and to mitigate emissions.

As an alternative approach, solvent extraction can be used as a pretreatment process. The solvents can be selected in such a way that can enable a further valorization of the impurities. In this regard, this work assessed the pretreatment of used cooking oils by using liquid-liquid extraction with alcohols. While the acylglycerides in UCOs are barely soluble in alcohols, the free fatty acids can be readily dissolved, thus obtaining a reactive mixture for further esterification. In this case acidity and other polar compounds are removed from the waste lipids, while the impurities can be recovered in the alcoholic phase and further valorized.

Initially, preliminary tests were carried out at the lab scale to identify the best operating conditions regarding temperature, UCO-to-solvent mass ratio, and water content in the solvent. Takins into account liquid-liquid equilibria and the subsequent valorization routes, acidity removal was assessed using ethanol and ethanol as solvents. The extraction process was carried out under isothermal condition at 40, 50, 60 ºC in a stirred jacketed reactor. The UCO-Solvent mass ratio was assessed in-between 1:1 to 1:2, and the water content in the solvent was varied in the range of 0.5 to 5% wt. The effectiveness of the process was monitored by tracing acid value, humidity, color, total polar content, iodine value, peroxide value, and saponification value of the pretreated UCO.

Once the best operating conditions regarding impurities removal were obtained in the lab scale, a subsequent evaluation was carried out in a pilot-scale falling liquid film contactor. In this case, a counter-current multi-filament packed column was used as contactor under continuous operation at constant temperature. According to results, the liquid-liquid extraction process effectively reduced the acid content of UCO to a value less than or equal to 0.05%, with a simultaneous reduction of polar compounds (25%) and other impurities. The extraction process helped to enhance the color of the UCO (30%) which is positive for further valorization. Additionally, the operation under laminar flow conditions enabled a fast phase splitting in the decanters reducing liquid-liquid separation and enhancing process performance. Finally, and to validate the process concept, the extract was subjected to further esterification to verify the potential harnessing of the extracted free fatty acids obtaining high conversion to fatty esters.