(88f) Experimental Characterization of Trickle Bed Reactor for Glycerol Oxidation

Glycerol oxidation using molecular oxygen has been extensively studied in the batch reactor. Unlike mono-alcohols, oxidation of this molecule bearing three hydroxyl groups yield ketones, mono-carboxylic acids and di-carboxylic acids including carbon chain scission products as well as carbon dioxide. Hence, selectivity is the major challenge. Several literatures on catalytic aspects of liquid phase glycerol oxidation by molecular oxygen are published wherein researchers have tested various supported noble metal catalysts. Investigation from literature says that under highly alkaline conditions, carboxylic acids are formed while under acidic conditions, selectivity to ketone is higher. Glyceric acid and Tartronic acid are usually formed under alkaline conditions. These are high value fine chemicals, hence, our work is majorly focused toward improving the selectivity to these compounds. Earlier in our laboratory, the batch studies were carried out to develop the kinetics of these complex oxidation reaction. The attempt was made to enter the kinetic regime by eliminating different mass transfer resistances. Interestingly, it was found that the selectivity is affected by the presence of mass transfer. Thus, it was concluded that changing the mass transfer parameter offers the opportunity to tune the selectivity towards the desired product. Additionally, the reaction pathway was proposed and kinetic model was developed for the catalytic glycerol oxidation reaction [1]. This model is useful in predicting the performance of the ideal reactors like PFR and CSTR.

The literature on such reaction being studied in continuous reactor is scarce. Moreover, to understand the engineering aspects of glycerol oxidation, it is important to study the effect of reactor configuration. Different configuration offers different kind of hydrodynamics and mass transfer effects. Trickle bed reactor (TBR) is one such reactor where liquid phase trickles down the bed of catalysts while gas may be fed co-currently or counter-currently. The advantages of TBR are that 1) the flow approaches a plug flow regime helping to achieve higher conversions, 2) mass transfer resistances from gas to liquid and solid can be minimized, 3) flooding is not an issue, and 4) pressure drop across the reactor is lesser compared to liquid up-flow [2].

Extensive literature can be found on hydrodynamic studies of TBR [3–8]. However, the study of multiphase oxidation of glycerol in TBR is not popular. Unleashing the complex hydrodynamics and mass transfer characteristics of TBR is the key to form correlation with the reaction performance. Unlike batch studies, TBR uses catalyst pellets of various shapes and sizes. The method of packing these catalysts inside the reactor is equally important to get the reproducible results [9]. Also, the knowledge of determining the extent of non-ideality of TBR is critical to compare the results with the performance of ideal reactors [10,11].

Tracer experiment is one of the widely used methods to characterize the hydrodynamics of TBR [7]. In the current work, the attempt is made to characterize the TBR using tracer experiments. Out of the two methods used i.e. step tracer and pulse tracer, pulse tracer method was found to be reproducible as well as reliable. In fact, the tracer experiment results are very well reproducible. Moreover, the method of packing used is also found to be well reproducible. From the E-curve (also known as internal age distribution curve), the first moment yields mean residence time while the second moment gives variance. These two parameters are enough to understand the extent of dispersion occurring inside the reactor bed. However, to quantify the dispersion, well-established models like dispersion model and tank-in-series model are used. These models can further help to predict the actual conversion by coupling it with the developed kinetic model.

Moreover, the first and second moments of RTD is useful to calculate the liquid hold up and catalyst wetting efficiency based on dispersion as reported in various literature [12,13]. These hydrodynamic parameters can be correlated with the product yields to establish the regime of operation to attain the desired selectivity.

References

[1] A. Namdeo, Catalytic Studies on Liquid Phase Glycerol Oxidation, Indian Institute of Technology, Bombay, 2015.

[2] V. V. Ranade, R. V. Chaudhari, P.R. Gunjal, Trickle Bed Reactors : Reactor Engineering & Applications, Elsevier, 2011. http://www.sciencedirect.com/science/book/9780444527387.

[3] M. Herskowitz, J.M. Smith, Trickle Bed reactors: A review, AIChE J. 29 (1983) 1–18. doi:10.1002/aic.690290102.

[4] A. Gianetto, V. Specchia, Trickle-bed reactors: state of art and perspectives, Chem. Eng. Sci. 47 (1992) 3197–3213. doi:10.1016/0009-2509(92)85029-B.

[5] W.J.A. Wammes, J. Middelkamp, W.J. Huisman, C.M. DeBass, K.R. Westerterp, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, AIChE J. 37 (1991) 1849–1862. doi:10.1002/aic.690371210.

[6] W.J.A. Wammes, S.J. Mechielsen, K.R. Westerterp, The influence of pressure on the liquid hold-up in a cocurrent gas-liquid trickle-bed reactor operating at low gas velocities, Chem. Eng. Sci. 46 (1991) 409–417. doi:10.1016/0009-2509(91)80002-G.

[7] M.H. Al-Dahhan, M.P. Dudukovic, Catalyst wetting efficiency in trickle-bed reactors at high pressure, Chem. Eng. Sci. 50 (1995) 2377–2389. doi:10.1016/0009-2509(95)00092-J.

[8] R. Lange, M. Schubert, T. Bauer, Liquid holdup in trickle-bed reactors at very low liquid Reynolds numbers, Ind. Eng. Chem. Res. 44 (2005) 6504–6508. doi:10.1021/ie048906r.

[9] M.H. Al-dahhan, Y. Wu, M.P. Duduković, Reproducible Technique for Packing Laboratory-Scale Trickle-Bed Reactors with a Mixture of Catalyst and Fines, Ind. Eng. Chem. Res. 34 (1995) 741–747. doi:10.1021/ie00042a005.

[10] H.S. Folger, Elements of chemical reaction engineering, 1999.

[11] O. Levenspiel, Chemical Reaction Engineering, (1972).

[12] M.H. Al-Dahhan, Effects of High Pressure and Fines on the Hydrodynamics of Trickle-Bed Reactors., WASHINGTON UNIVERSITY SEVER INSTITUTE OF TECHNOLOGY, 1993.

[13] P.L. Mills, Catalyst Effectiveness and Solid-Liquid Contacting in Trickle-Bed Reactors., WASHINGTON UNIVERSITY SEVER INSTITUTE OF TECHNOLOGY, 1980.