Ind. Eng. Chem. Res. 2004, 43, 2337-2344
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Monolithic Catalysts as an Alternative to Slurry Systems: Hydrogenation of Edible Oil Thorsten Boger* Corning GmbH, Abraham-Lincoln-Strasse 30, D-65189 Wiesbaden, Germany
Martijn M. P. Zieverink, Michiel T. Kreutzer, Freek Kapteijn, and Jacob A. Moulijn R&CE, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
William P. Addiego Science and Technology, Corning Inc., Sullivan Park, Corning, New York 14831
The use of monolithic catalysts in multiphase applications has received increasing interest throughout the last years. In this paper the use of monolithic catalysts in the hydrogenation of edible oils is studied. In comparative experiments the monolithic catalysts have shown a lower tendency to form trans-fatty acids compared to slurry catalysts with equal composition. It is also demonstrated that the monolithic catalyst could be reused several times without significant changes in the product composition. With respect to the effect of the operating parameters on the performance, common knowledge applies. By using a monolithic-catalyst-based technology, the process complexity can be significantly reduced as the separation of the oil and the catalyst becomes straightforward and the filtration and bleach steps are eliminated. An economic evaluation showed a significant reduction in the product cost when monoliths are used instead of a conventional slurry system. Introduction In recent years monolithic catalysts have received increasing interest for use in multiphase catalytic applications.1-3 Special attention was given to hydrogenation reactions, which was in many cases driven by excellent mass-transfer characteristics of monoliths and the fact that these reactions often are constrained by mass transfer. A commercial multiphase application of a monolithic catalyst is given in the production of hydrogen peroxide.4 In many hydrogenation reactions the conventional reactor technology applied today is based on slurry systems, often operated in a batch or semibatch mode. In these slurry systems the solid catalyst is provided in a powder form and mixed with the liquid reactants in stirred tanks or bubble columns. The solid particles as well as the gas bubbles are kept suspended within the liquid phase by means of mechanical agitation and chaotic mixing, e.g., via a stirrer. Although practiced for many decades in industry, the slurry technology has some distinct shortcomings.3 The most obvious is the difficult separation of the suspended powder catalyst. The filtration of the catalyst is costly and time-consuming and in essentially all cases results not only in additional waste streams but also in losses of the solid catalyst and some product. This has prevented in many cases the use of more effective and selective but also more expensive precious metal catalysts. Several alternative designs based on monolithic catalysts have been proposed within the past few years as a potential solution to these problems.5-8 All these * To whom correspondence should be addressed. Tel.: +49 611 7366 168. Fax: +49 611 7366 143. E-mail: BogerT@ corning.com.
designs have in common that they not only benefit from the unique mass-transfer characteristics of monolithic catalysts but also allow for a significant reduction in the complexity of the process. This is shown schematically in Figure 1 for the example of a semibatch hydrogenation plant for edible oil,9 representing one of the largest commercial hydrogenation applications. In the conventional process significant workup of the hydrogenated oil is required to remove the catalyst particles in a first filtration step and remaining traces of nickel leached from the catalyst by means of a subsequent bleaching section. The latter again involves filtration to remove the bleach earth. The contribution of the workup operation to the operating economics is quite significant. It represents about 20% of the total operating cost, and as much as 50% of the operating cost if the consumption of hydrogen and catalyst is excluded. Any of the above-mentioned monolith-based alternative designs5-8 would make the separation and workup section obsolete (see Figure 1, route B) and therefore reduce the operating cost significantly. As all designs are relatively simple and do not require major hardware changes, the cost and complexity for a largescale implementation and scale-up are expected to be rather moderate. For new units we estimate the capital cost for the hydrogenation section to be about 30-40% lower. Although monolithic catalysts had been proposed for use in edible oil hydrogenation already several years ago,10,11 this concept has not found broad attention. In this paper we will report on recent research to better understand the performance of monolithic catalysts in the hydrogenation of edible oil and address the question of how far they can go as an attractive alternative to current technology.
10.1021/ie030809v CCC: $27.50 © 2004 American Chemical Society Published on Web 04/13/2004
2338 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
Figure 1. Simplified flow sheet of a semibatch edible oil hydrogenation plant based on conventional technology (route A, indicated by the dashed box)9 or a monolith-based technology (route B).
Figure 2. Simplified reaction scheme for the C18 hydrogenation reactions. Abbreviations are defined in Table 1. Table 1. Fatty Acids (C18) Present in Edible Oil Together with Their Melting Point (MP) and Iodine Value (IV) name linolenic acid linoleic acid oleic acid elaidic acid stearic acid
Ln L O E S
C18:3 C18:2 C18:1 C18:1 C18:0
all-cis all-cis cis trans
MP, °C
IV
-11 -5 13.4 45 69.6
274 181 90 90 0
Hydrogenation of Edible Oil Edible oils are a complex mixture of mainly C16 and C18 fatty acids present in the form of triglyceride molecules, many of them being highly unsaturated. The degree of unsaturation is usually expressed by the iodine value (IV), which represents the number of carbon-carbon double bonds. The iodine value is defined as the mass of iodine which combines with 100 g of unsaturated oil. Virgin soybean oil, for example, has an IV of approximately 130. The most important C18 fatty acids together with their IV and melting point are given in Table 1. To improve the keeping qualities and to alter the physicochemical properties, the oil is partially hydrogenated. A simplified scheme of the reaction network is shown in Figure 2. The extent of the hydrogenation, e.g., to which product IV, depends on the desired properties of the final product. For
example, if only the keeping qualities are improved, the target IV is around 105, whereas is would be approximately 70 if margarine were the product.9 If shortening fat or stearin were desired, the hydrogenation would go as far as IV 5 or less.9 Each of the unsaturated fatty acids could be present in the form of two geometric isomers, as can be seen from Figure 2. One with cis and the other with trans configuration. In untreated edible oils usually only cis isomers exist, but during hydrogenation some double bonds of the fatty acid chains are converted into the trans form as a result of isomerization. On the basis of some growing concerns about a negative health impact of the trans isomers, there has been a trend to favor products with a low trans content. The standard powder catalyst used today is based on nickel as the active metal. Typically about 0.02-0.4 wt % catalyst is suspended, depending on the desired degree of hydrogenation and how much fresh or reused catalyst is used.9 Palladium has been proposed as a more active and selective catalyst material, and it has been shown that only small palladium concentrations are required.12 Independent of the catalyst the reaction is heavily limited by the transfer of hydrogen to the active catalyst surface.12-14 The limitations are due to diffusion within the catalyst particles as well as the transfer from the gas to the liquid phase. Experimental Section The objectives of our experimental program on the hydrogenation of edible oil were twofold. On the one hand was the performance of a monolithic catalyst studied in general and in comparison to a powder catalyst, which was prepared in a similar fashion. On the other hand, we tried to asses how much the use of a palladium-based catalyst under higher pressures than used in commercial reactors today would affect the trans content of the products.
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2339 Table 2. Composition of the Pure Bleached Soybean Oil Used in the Experiments content, wt %
component palmitic acid stearic acid oleic acid linoleic acid linolenic acid others
Figure 3. Schematic of stirrers used in the experiments: (A) premex stirrer; (B) monolith stirrer;8 (C) screw impeller stirrer (SISR).19
Catalyst. Experiments were carried out with a large variety of different research catalysts. All were based on palladium as the active component. To enable a good comparison, monolith and powder catalysts have been prepared in comparable manner and with comparable properties. The monolith catalysts have been prepared by using cordierite honeycomb supports with different cell densities (100, 200, and 400 cpsi) onto which a washcoat comprising a high surface area material and the palladium was applied. The washcoat loading of the samples was typically in the range of 12-15 wt %. Preparation methods as described, for example, in ref 15 were used. Most experiments were run with monoliths having 400 cpsi. The average particle size of the slurry catalyst was 13-18 µm. Equipment. The experiments were carried out in a 300 mL autoclave, which could be equipped with different stirring arrangements, shown in Figure 3. All slurry experiments with the powder catalyst were performed with the self-gas-inducing Premex stirrer (Figure 3A). The monolith experiments were mainly run with the self-gas-inducing monolith stirrer8 (Figure 3B). A few experiments were operated with the screw impeller stirrer19 (SISR; Figure 3C) to demonstrate that the results obtained are independent of the monolith reactor configuration and are generally applicable to the replacement designs proposed in the literature.5-8 The stirrers were operated at high rotational speed (1800 rpm for the monolith stirrer and 2000 rpm for the slurry) to achieve good mixing and mass transfer. In this range the effect of the stirrer speed was found to have no influence on the results under the conditions applied. The temperature range studied was between 80 and 160 °C. The reaction pressure was adjusted to 0.9 and 2.1 MPa. The experiments were run in semibatch mode (constant H2 pressure), and the amount of oil in the reactor was between 140 and 220 g. The oil used was pure bleached soybean oil provided by Unichema International. The composition of the oil used in all experiments is given in Table 2. The iodine value of the oil was 131.4. As can be seen the raw soybean oil does not contain any trans isomers. The catalyst concentration in the slurry experiments was varied between 0.03 and 0.1 wt %. In the monolith experiments comparable amounts of the active washcoat mass, comprising the high-surface-area material and the active metal, were
C16:0 C18:0 cis-C18:1 cis,cis-C18:2 cis,cis,cis-C18:3
11.3 3.9 23.1 54.4 7.2
