Co-current gas-liquid trickle bed reactors (TBRs) are widely used for heterogeneously catalysed multiphase reactions. The large production volumes and widespread application of these reactors provide a considerable economic incentive for investigating ways of improving and intensifying their performance. Over the past few decades, the periodic operation of TBRs has received a considerable attention as a powerful tool to this end. The deliberate manipulation of the liquid flow rate in the reactor inlet results in a dynamically fluctuating wetting of the catalyst surface, which allows the circumvention of the major shortcomings in steady-state trickle-bed reactor operation: strong mass transfer resistance for the gaseous reactant and hot-spot formation [1, 2].

Despite the numerous benefits revealed by the considerable research in this area, periodic operation is still long way from being a viable alternative to conventional steady-state operation in industrial use. Complex interactions between hydrodynamics, mass transfer and reaction phenomena make the design of periodically operated TBRs an almost insurmountable challenge. Even though a variety of modelling approaches and abundant experimental data are available, theoretical predictions of the trickle-bed reactor performance under periodic conditions still remain unsatisfactory. The complex TBR hydrodynamics, reflected in the presence of multiple hydrodynamic states (MHS) with different pressure gradients and liquid hold-ups for identical gas and liquid flow rates, represents a particular challenge for successful modelling. The multiplicity phenomenon, originally reported by Kan and Greenfield (1978), was explained by the change of tortuosity in the gas flow channels [3]. In the following years, the hydrodynamics of this behaviour was thoroughly investigated and elucidated [4, 5]. Recent work has mainly focused on the quantification of MHS and its exploitation on an industrial scale through different prewetting procedures [6].

The scope of the present work is the examination of the possible exploitation of multiple hydrodynamic states induced by liquid flow rate manipulation. Previous work [7], in which a dynamic model, based on relative permeability approach [8], was developed to describe two phase pressure drop and liquid hold-up under cyclic operation has been extended to encompass the decisive gas-liquid mass transfer performance. Steady-state experiments have been carried out on both the upper and lower hysteresis branches attained by either decreasing liquid flow rates from higher values or increasing them from zero in the drained state. Both max/min and on/off periodic operation modes have been investigated to gain insights into the influence of flow manipulation on the gas-liquid mass transfer coefficient. Experiments were carried out with oxygen-saturated water/N2 systems in a laboratory trickle-bed reactor packed with γ–Al2O3 spheres. The gas-liquid mass transfer coefficients and hydrodynamic parameters thus ascertained are presented and analysed, with special emphasis being placed on the potential utilisation of multiple hydrodynamic states for enhanced mass transfer. The parameters obtained have been used for development of an improved dynamic model, which enables accurate simulation of hydrodynamics and mass transfer.

Steady-state experimental results demonstrate the presence of gas-liquid mass transfer coefficient hysteresis under the conditions employed. Regarding gas-liquid mass transfer during periodic operation, the results indicate that max/min is marginally more favourable than the on/off mode (+ 2.8 %). Simulation studies of max/min periodic operation of a system in which absorption with chemical reaction takes place indicate a conversion enhancement of 17.8 % over steady-state performance.

Experimental examinations of hydrodynamics and gas-liquid mass transfer during periodical operation have allowed us to develop reliable dynamic model, which represents an important first step in the prediction of overall behaviour in periodically operated TBR.

References

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[2] P. M. Haure, R. R. Hudgins and P. L. Silveston, Periodic Operation of a Trickle-Bed Reactor. AIChE J., 1989; 35(9): 1437-1444.

[3] K.-M. Kan and P. F. Greenfield, Multiple Hydrodynamic States in Cocurrent Two-Phase Downflow through Packed Beds. Ind. Eng. Chem. Proc. Des. Dev., 1978; 17(4): 482-485.

[4] C. F. Chu and K. M. Ng, Model for Pressure-Drop Hysteresis in Trickle-Beds. AIChE J., 1989; 35(8): 1365-1369.

[5] J. Levec, K. Grosser and R. G. Carbonell, The Hysteretic Behaviour of Pressure Drop and Liquid Holdup in Trickle Beds. AIChE J., 1988; 34(6): 1028-1030.

[6] D. Loudon, W. van der Merwe and W. Nicol, Multiple hydrodynamic states in trickle flow: Quantifying the extent of pressure drop, liquid holdup and gas-liquid mass transfer variation. Chem. Eng. Sci., 2006; 61(22): 7551-7562.

[7] B. Brkljac, T. Bludowsky, W. Dietrich, M. Grunewald and D. W. Agar, Modelling of unsteady-state hydrodynamics in periodically operated trickle-bed reactors: Influence of the liquid-phase physical properties. Chem. Eng. Sci., 2007; 62(24): 7011-7019.

[8] A. E. Saez and R. G. Carbonell, Hydrodynamic parameters for gas-liquid cocurrent flow in packed beds. AIChE J., 1985; 31(12): 52-62.

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