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FUNDAMENTAL STUDIES ON THE SELECTIVE HYDROGENATION OF FATS AND OILS

Cel

Partial hydrogenation, or hardening, of vegetable oil is one of the most important processes in the fatty oil industry, particularly in the production of edible fat products. The ASHLI-project was aimed at increasing the controllability and predictability of the product composition obtained with nickel catalysed hardening, and at developing new techniques to reduce the formation of undesired trans isomers in edible oils, and especially achieve substantially lower amounts of trans isomers in the hardening of rapeseed oils. A detailed and fundamental scientific investigation into the reaction kinetics of the nickel catalysed partial hydrogenation was completed. This comprehensive programme included investigations of the intrinsic chemical kinetics, of the effective pore diffusion coefficients, of the adsorption onto the catalyst surface, of the influence of mass transfer on the kinetics using model catalysts with highly uniform pores, and of the opportunities for improving the nickel catalyst by applying additives. The complex oil hardening process, with an intricate system of simultaneous hydrogenation, cis/trans isomerisation, and double bond conjugation reactions at the surface of the nickel catalyst, previously not well understood, can now be described and modelled accurately. Given the initial oil composition, and the process conditions, the distribution of dozens of different product components, including cis/trans isomers and geometrical isomers, can be predicted.

The quality of the hydrogenation process is determined by two key factors. Starting with a mixture containing double-unsaturated fatty acids, or dienes, and single-unsaturated fatty acids, or monoenes, one of the important factors is the ability to reduce the amount of dienes as much as possible by hydrogenation, while at the same time keeping the amount of monoenes as high as possible. This is quantified in the linoleate selectivity, defined as the ratio of the hydrogenation rate of dienes over that of monoenes. This linoleate selectivity should be as high as possible. The double bonds in unsaturated fatty acids can have either of two configurations: the cis- and the trans-configuration. In some food products, trans-isomers are considered undesirable. Therefore, the other important factor for the quality of the hydrogenation is the cis-to-tans isomerisation rate, which should be as low as possible. Though already a classical process, widely applied since early this century, it is not yet possible to a priori predict the molecular composition of the hydrogenated oil as a function of feed stock composition, catalyst type and concentration, reaction pressure, temperature and time. This is partly due to the great complexity of the process with a large number of hydrogenation reactions occurring in parallel, together with the simultaneous occurrence of cis-trans isomerisation and double bond conjugation reactions. These considerations resulted in the following main objectives of the project:

- Ability to hydrogenate fats and oils in such a way that product composition can be varied in a controlled and predictable way, based upon a reaction engineering model for supported nickel catalyst.
- Investigation into the possibility to substantially improve the selectivity of the hydrogenation of edible oils with regards to formation of trans isomers.
- Investigation into the possibility to hydrogenate rapeseed oils with substantially reduced formation of trans isomers by applying improved hydrogenation techniques.

Recent significant improvements, particularly in analytical techniques, have brought fundamental investigation of the reaction kinetics, in order to adequately model the nickel catalysed hydrogenation process, within reach. The scientific research performed to unravel the fundamentals of the hydrogenation reactions include:

- measurement of the intrinsic chemical kinetics of nickel catalysed hydrogenation in absence of pore diffusion limitation;
- measurement of the effective pore diffusion coefficients in conventional nickel catalysts;
- measurement of the adsorption of hydrogen and fatty acid derivatives on the surface of the nickel catalyst;
- preparation of catalysts with regular uniform pores to investigate the influence of mass transfer on the kinetics;
- investigations on opportunities for improved nickel catalysts by applying dopes, surfactants, and inhibitors.

Depending on the application of the hydrogenated fat, some hydrogenation reactions occurring on nickel catalyst are less desirable. Particularly in some food products, trans isomers may be less desirable. Therefore, the possibilities for new catalysts promoting desirable, and suppressing undesirable hydrogenation and isomerisation reactions were investigated, resulting in:

- development of a standardised method for the evaluation of the performance of catalysts in hydrogenation tests;
- preparation and testing of new, precious metal catalyst systems;
- investigation into the structural requirements for a good hydrogenation catalyst;
- investigation of promising zeolite catalysts for their selective hydrogenation ability;
- testing catalysts for production of hydrogenated fats, especially from rapeseed oils;
- investigation of reactor systems for optimum application of such catalysts in practice, since conventional slurry reactors are probably not optimal (for environmental reasons), particularly not for use of precious metal catalysts (because of losses during separation of catalyst from hardened oil). Interesting options are monolithic catalysts or fixed bed reactors;
- investigation of catalyst poisoning and regeneration, which also are important topics when using precious metal catalysts.
Very promising results were obtained from monoliths impregnated with a specific type of catalyst, in that a simple procedure was developed, resulted in complete regeneration of this catalyst after sulphur poisoning as well as long-term deactivation due to coke formation.
In order to facilitate the two main areas of investigations mentioned above, considerable effort was spent on the following:

- preparation of a large number of specific fatty components;
- development of new, highly sophisticated analytical techniques;
- design and construction of various specific experimental set-ups.

To investigate the possibilities of improving selectivity in the formation of trans isomers, 150 new types of metal catalysts for partial hydrogenation were prepared and tested. From this extensive research programme, a specific type of catalysts was developed, that showed substantially (3.4 times) lower amounts of trans isomers in the hydrogenation of sunflower oil, when compared to the industrial nickel catalysts, and at the same time showed an up to 15 times higher activity and maintained a similar linoleate selectivity.

This specific new catalyst was tested in the production of hydrogenated fat from rapeseed oils. The observed amounts of trans isomers in the hydrogenation of rapeseed oils with the new catalyst were significantly lower (>4 times) than those obtained with the traditional nickel catalyst while also achieving at least similar linoleate selectivities and much higher activities. The performance of monolith catalyst reactors in the selective hydrogenation of oils was extensively tested. The catalysts are present in a fixed form inside this type of reactors, resulting in negligible catalyst losses and the absence of the need for filtration of the product. This important advantages of monoliths over conventional slurry reactors, especially in the application of precious metal catalysts, is augmented by the possible problem of an enhanced vulnerability to catalyst poisoning.

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CSC - Cost-sharing contracts

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UNIVERSITY OF GRONINGEN
Wkład UE
Brak danych
Adres
NIJENBERGH 4
9747 AG GRONINGEN
Niderlandy

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Ostatnia aktualizacja: 15 Grudnia 1993

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Permalink: https://cordis.europa.eu/project/id/AIR10513/pl

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