Hydrogenation of diesel aromatic compounds in supercritical solvent environment

HYDROGENATION OF DIESEL AROMATIC COMPOUNDS IN SUPERCRITICAL SOLVENT ENVIRONMENT

E.P.Martins¹, D.A.G. Aranda¹, F.L.P. Pessoa¹ and J.L.Zotin2

¹Departamento de Engenharia Química, Escola de Química, Universidade Federal do Rio de Janeiro, Cx. P.

68542, CEP 21949-900, Phone: (55) (21) 590-3192, Fax: (55) (21) 590-3192, Rio de Janeiro - RJ, Brazil

E-mail: [e-mail] and [e-mail]

²CENPES-PETROBRÁS, Cidade Universitária, Quadra 7, Ilha do Fundão, CEP 21949-900,

Phone: (55) (21) 865-6630, Fax: (55) (21) 865-6626, Rio de Janeiro - RJ, Brazil

E-mail: [e-mail]

(Received: February 10, 2000 ; Accepted: May 29, 2000)

Abstract - Reactions under supercritical conditions have been employed in many processes. Furthermore, an increasing number of commercial reactions have been conducted under supercritical or near critical conditions. These reaction conditions offer several advantages when compared to conditions in conventional catalytic processes in liquid-phase, gas-liquid interface, or even some gas-phase reactions. Basically, a supercritical solvent can diminish the reactant’s transport resistance from the bulk region to the catalyst surface due to enhancement of liquid diffusivity values and better solubility than those in different phases. Another advantage is that supercritical solvents permit prompt and easy changes in intermolecular properties in order to modify reaction parameters, such as conversion or selectivity, or even proceed with the separation of reaction products. Diesel fractions from petroleum frequently have larger than desirable quantities of aromatic compounds. Diesel hydrogenation is intended to decrease these quantities, i.e., to increase the quantity of paraffin present in this petroleum fraction. In this work, the hydrogenation of tetralin was studied as a model reaction for the aromatic hydrogenation process. A conventional gas-liquid-solid catalytic process was compared with that of supercritical carbon dioxide substrate under similar conditions. Additionally, an equilibrium conversion diagram was calculated for this reaction in a wide range of temperature and reactant ratios, so as to optimize the operational conditions and improve the results of subsequent experiments. An increase in the rate of reaction at 493 K in supercritical fluid, as compared to that in the conventional process, was observed.

Keywords: hydrogenation, diesel, supercritical, carbon dioxide

INTRODUCTION

The majority of chemical processes use solid catalysts in gaseous or liquid reaction media. These reactions are within the scope of heterogeneous catalysis. The mechanism of this process can be described in seven steps, as follows:

1. Reagent mass transport from the fluid bulk region to the external surface of the catalyst (external diffusion);

2. Mass transport from the external surface to the

pores of the catalyst (internal diffusion);

3. Adsorption on active sites;

4. Surface reaction;

5. Product desorption;

6. Internal diffusion of product;

7. External diffusion of product into the fluid bulk region.

As can be observed, there are three kinetic steps and four steps of mass transport. Steps on catalyst active sites involve the formation of surface intermediate complexes (steps 3 to 5). Therefore, for a more efficient application of heterogeneous catalysts, the strategy is to speed up the transport steps, making the reaction kinetic controlled. This objective is achieved with relative ease in gas/solid interface reactions.

Reactions in the liquid phase involve heavy or temperature-dependent substrates. Under these conditions, operation with kinetic control is more difficult due to molecular diffusivity, which is about 10⁴ times lower than diffusivity in gases (Baiker, 1999).

The application of supercritical fluid as solvent is presented as a possible solution to the difficulties encountered with liquid solvents. Table I compares the magnitude of the physical properties in gases, liquids and supercritical fluids under conditions near the critical point. The density of supercritical fluids is of the same order of magnitude as that of liquids; therefore, it does not require changes in scale of the equipment. However, the diffusivity of supercritical fluids is two orders of magnitude higher than it is in liquids. Thus, achievement of kinetic control of the reaction and even a lesser deactivation of the catalyst (Tacke et al.,1996) are more easily attained goals. Another important feature of operation with supercritical fluids is the ease with which reaction conditions are controlled by the control of variables such as pressure and temperature.

Supercritical carbon dioxide has been considered an ideal apolar solvent for chemical reactions due to its moderate critical conditions and the ease with which it can be separated from products, as compared with conventional solvents. ScCO₂ is nontoxic, nonflammable and relatively cheap.

The main objective of this work is to study the use of supercritical fluids as reaction media in catalytic hydrogenation processes. In this case, achievement of a diffusivity of about two orders of magnitude higher than that in liquid environments is expected (Savage et al., 1995), thus speeding up the transport steps and making the attainment of kinetic control easier. Furthermore, an adequate selection of supercritical fluid may facilitate the product separation and purification steps after reaction.

In this article, attention is focused on the study of the hydrotreatment of diesel fractions. As a catalytic test, the reaction of tetralin hydrogenation was run. Tetralin is an aromatic compound commonly found in diesel fractions. The choice of this compound as a model is due to its difficult aromatic ring hydrogenation and frequent presence in diesel fractions, which makes it a good model for difficult hydrogenation aromatic compounds in diesel.

EXPERIMENTAL

In the tetralin hydrogenation experiments, the aim is to compare the reaction under supercritical conditions with the conventional reaction, in which the solvent is diesel fraction in contact with gaseous hydrogen at a high pressure and temperature.

The catalyst used in the experiments was acquired by the Centro de Pesquisa da Petrobrás (CENPES) and is composed of 1%Pt/Al₂O₃ for the conditions specific to diesel hydrotreatment. The platinum catalyst was inserted into the reactor, where it was dried and reduced in H₂ at 300 ºC and 15 atm.

The conventional reaction was carried out in a Parr autoclave batch reactor with an internal volume of 450 mL. Tetralin was mixed with a paraffin compound (n-heptane). Pressure conditions during reaction varied within the 70-90 atm range, and temperature between 250 and 270 ºC.

The reaction in supercritical solvent was carried out in the same reactor. The supercritical environment was obtained with carbon dioxide pressurized by hydrogen. Pressure conditions varied between 70 and 80 atm and temperature between 220 and 250ºC. During experiments, several samples were taken from the reactor and analyzed in the gas chromatograph (HP 5890, HP-5 column), providing a conversion profile under different conditions of temperature, pressure and time. This article shows a comparison between the better results in both cases for the same time period (2h). Results were compared with thermodynamic equilibrium conversions calculated for chemical and phase equilibrium by the computational package Aspen Plus. Result tables presented in this article compare analysis output related to both reaction conditions.

RESULTS AND DISCUSSION

Tables II.1 and ^II.2 show the results of chromatographic analysis of samples taken at the end of reactions. These results correspond to the highest conversions obtained for both models studied. For the conventional reaction, the sample corresponds to the end of reaction after 2 h at 250 ºC. No products could be observed after 2h at 220 ºC. On the other hand, for the supercritical solvent reaction, the results correspond to the reaction after 2 h at 220 ºC, when equilibrium conversion has been almost achieved. The procedure of chromatographic analysis was standardized, with at least two analyses for each sample.

The literature has shown some examples of application of supercritical fluids in oxidation reactions (Modell et al., 1992), enzymatic reactions (Russel et al., 1994), biomass treatment (Li and Kiran, 1988), etc. Experiments involving catalytic hydrogenation under supercritical conditions, using carbon dioxide, propane or ethane as solvents, have been conducted. Some examples of advantageous application of supercritical solvents are hydrogenation of fats, vegetable oils and many other organic compounds, such as enancioselective hydrogenation of a -keto ester (Baiker, 1999).

Additionally, Tacke et al. (1996) reported the use of ScCO₂ in partial hydrogenation of fats, oils, fatty acids and esters, utilizing a continuous fixed bed reactor with palladium commercial catalyst. The authors verified space time yields up to six times higher, as compared to conventional hydrogenation in a trickle-bed reactor. Furthermore, the use of supercritical carbon dioxide extended the lifetime of the catalyst, because of the more effective diffusion of coke precursors to the substrate.

Results that were obtained for hydrogenation in supercritical solvent presented a final conversion that is almost 70 % higher than that in the simulated conventional reaction. Note that the period of time in both reactions is virtually the same, which proves that the average reaction rate is much higher under supercritical conditions, although the temperature in this reaction medium is 30 ºC lower than that in the conventional reaction. Therefore, the data obtained illustrates the advantageous aspects of reactions using supercritical solvents, as reported in the literature. One can deduce that some of these advantages are also a reality for the commercial reaction simulated in this work.

CONCLUSIONS

Tetralin hydrogenation in supercritical carbon dioxide has shown to be extremely promising for use in future commercial processes of petroleum fractions hydrotreatment. It is clear that results obtained for the reaction with supercritical carbon dioxide are much better than those obtained for the conventional reaction with the same catalyst and under the same reaction conditions.

Thus, the results demonstrate that supercritical carbon dioxide improves the transport of reagents and products between the catalyst surface and the bulk fluid region. In addition, hydrogen dissolves well in ScCO₂ but mixes only partially in liquid solvents.

It is clearly perceived that trans-decalin is always preferred over cis-decalin. This is verified by equilibrium diagrams and confirmed by experimental results.

Additional kinetic experiments should be performed in order to confirm the positive qualitative tendency shown in this work.

ACKNOWLEDGEMENTS

The authors are grateful to FAPERJ (Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro) for financial support. The authors also thank Célio Carlos de Souza for installing the equipment and providing technical support.

REFERENCES

• Baiker, A., Supercritical Fluids in Heterogeneous Catalysis, Chem. Rev., 99, 453-473 (1999).
• Li, L. and Kiran, E., Interaction of Supercritical Fluids with Lignocellulosic Materials, Ind. Eng. Chem. Res., 27, 1301 (1988).
• Modell, M., Larson, J. and Sobczynski, S.F., Supercritical Water Oxidation of Pulp Mill Sludges, Tappi J., 75(6), 195 (1992).
• Russel, A., Beckman, E.J. and Chaudhary, A.K., Studying Enzyme Activity in Supercritical Fluids, Chemtech., 33 (Mar. 1994).
• Savage, P.E., Gopalan, S., Mizam, T.I., Martino, C.J. and Brock, E.E., Reactions at Supercritical Conditions: Applications and Fundamentals, Reactors, Kinetics and Catalysis (1995).
• Tacke, T., Wieland, S. and Panster, P., 3rd International Symposium on High-Pressure Chemical Engineering, Zurich, Switzerland, 17-21 (1996).

Publication Dates

• Publication in this collection
  18 Oct 2000
• Date of issue
  Sept 2000

History

• Received
  10 Feb 2000
• Accepted
  29 Mar 2000