(111d) Continuous Epoxidation and Alcoholysis of Renewable Unsaturated Fatty Acid Esters in Micro Structured Reactors

C. Ariaans¹, H. Praefke¹, M. Padaro¹, Prof. Dr. M.A. Liauw¹, Dr. L. Greiner²

¹ITMC, RWTH University, Aachen/Germany

²DECHEMA e.V., Frankfurt am Main/Germany

Epoxidation of unsaturated double bonds is one of the proven paths towards the utilisation of renewable feedstock such as unsaturated fatty acid esters. With the availability of highly pure single acid content oils and ester derivatives, this approach has regained attention. The epoxides are further processed in various ring opening reactions. As the epoxides are toxic and reactive their storage and hold up should be avoided. Therefore, the integration of epoxidation with successive alcoholysis towards the α-hydroxy ether was targeted; Yielding an environmentally benign high performance lubricant was targeted. Both reaction steps together with a intermediate continuous phase separation were realized in a pilot plant.

Technical grade oleic acid methyl ester (OME, ~70%) is epoxidized with in situ generated peracetic acid (PA) to epoxy stearic acid methyl ester (eSME). In this step a nucleophilic attack of water at the epoxide can lead to the side product dihydroxy stearic acid methyl ester (DHSM). To avoid this reaction the aqueous phase has to be separated efficiently. In the second step the epoxide ring is opened with 2-methyl-1-butanol (iso-butanol) to the target molecule hydroxyl isobutoxy stearic acid methyl ester (HISM). Here a parallel transesterification to hydroxyl isobutoxy stearic acid isobutyl ester (HISI) can occur.^[1]

As the batch reaction is typically carried out as fed batch with charged OME and stepwise addition of the hydrogen peroxide,^[2] no prior knowledge on the working point of the aimed for continuous process existed.^[3] For this reaction calorimetry was carried out and based on this knowledge a miniaturized dedicated continuous reactor was built to systematically vary process parameters and evaluate higher temperatures than accessible in open vessels. The scale was chosen to be conveniently handled on laboratory scale and sufficient material output to allow analysis of other properties in view of toxicity and lubrication.^[4]The epoxidation is carried out in a thermostated single capillary plug flow reactor with stable Taylor Flow due to the small inner diameter. By this, the mass transport is improved tremendously leading to shorter reaction times compared with the batch reaction. To separate the aqueous phase after the reaction several separation techniques such as settling or drying agents were investigated. Decantation was finally carried out by continuous centrifugation which allowed to reduce the water content to equilibrium values and to remove solid byproducts. This allowed the successive continuous alcoholysis to be carried out in a similarly heated single capillary plug flow reactor as well.

By upscaling the process to pilot scale operation several challenges had to be met. All wetted surfaces in the first reaction step are made out of inert material to avoid corrosion of the low pH oxidising aqueous phase. Furthermore, a preformation of the PA has proven to be beneficial for the epoxidation so it was realised in the pilot plant. To stabilise the Taylor Flow in the larger reactor volume the total capillary length reached 150m without significant coalescence issues.

Results were successfully transferred to a dedicated pilot plant used to produce sufficient quantities for lubricant evaluation in machining and other areas. The plant is easily transportable and versatile due to its small dimensions and modular layout. The continuous process allows a high level of automation. Comparably small substrate holdup in the reactors give an additional plus concerning safety.

The plant was successfully used for a production campaign. As the batch was also substantially improved by the knowledge gained, performance is compared and reevaluated on basis of the best available technology for both batch and continuous production.

[1]           W. F. Hölderich, L. A. Rios, P. P. Weckes, H. Schuster, Journal of Synthetic Lubrication, 2004, 20, 289-301.

[2]           S. Eichholz, PhD thesis, RWTH Aachen University, 2007.

[3]           A. Pashkova, L. Greiner, CIT, 2011, 83, 1337-1342.

[4]           D. Müller, PhD thesis, RWTH Aachen University, 2009.