Due to global competition and an increasing demand for reduced emissions the development of sustainable and efficient processes based on renewable resources becomes more and more important. Furthermore, the fluctuating qualities of renewable feedstock necessitate highly flexible processes. Fatty esters are promising new building blocks for the chemical industry and can be produced by the transesterification of virgin vegetable oils. However, nowadays fatty esters are mainly used as biodiesel, an alternative fuel in future based on renewable resources [1,2]. Due to the high raw material costs for the virgin vegetal oils , no economic production is possible. A promising approach to lower the production costs is the application of cheap low-quality oils, such as waste cooking oil or crude non-edible oil [4,5]. These low quality oils contain a high amount of free fatty acids, which lead to undesired side reactions in the alkaline catalyzed transesterification reaction [6,7]. Consequently, new process concepts are necessary to enable the application of low-quality oils as a feedstock.
In order to achieve an efficient processing of the low-quality oils this study investigates intensified process concepts, such as reactive distillation and membrane-assisted reactive distillation. One possibility to reduce the free fatty acid content in the low-quality oils is the acid catalyzed esterification of the free fatty acids with an alcohol. Since this reaction is equilibrium limited the application of reactive distillation is favorable to enable higher conversion. Furthermore, it is indicated that the transesterification of the oils could take place simultaneously. To further enhance the conversion in the reactive distillation the combination with vapor permeation is promising. The selective removal of the by-product water from the condensate stream facilitates a shift of the reaction equilibrium towards the product side. Another possibility to reduce the free fatty acid content is the separation of the free fatty acids from the low-quality oils by means of organic solvent nanofiltration. The application of organic solvent nanofiltration enables an energy efficient separation due to the mild operation conditions, which also ensure consistent quality of the fatty acids and oils. The coupling of organic solvent nanofiltration with reactive distillation provides the potential to an even more energy efficient process. Besides the processing of the low-quality oils, the profitability of the production process might be increased because of the generation of new products, as e.g. the free fatty acids, which can be used as building blocks in the chemical industry.
The current study investigates the process synthesis of different flow sheet configurations of reactive distillation, vapor permeation and organic solvent nanofiltration. Therefore, models for each unit operation are developed and validated with own experimental results and literature data. Hence, the potential of the different intensified process configurations is evaluated and it is determined if they are capable to handle fluctuating feedstock compositions. Finally, the most promising configurations are identified with regard to product specifications, energy demand and production costs.
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