Influence of the Liquid Phase Physical Properties on Unsteady-State Hydrodynamics in Periodically Operated Trickle-Bed Reactors
Advancing the chemical engineering fundamentals
Multifase Flows - II (T2-5b)
Keywords: Trickle-bed reactor, Periodic operation, Hydrodynamics, Modelling, Multiphase Flow
Trickle-bed reactors are extensively used for heterogeneously catalysed multiphase reactions in various industrial sectors, ranging from petrochemical and chemical plants to wastewater treatment and biochemical manufacturing processes. The large production volumes and widespread application of these reactors provides a considerable economic incentive for investigating ways of improving and intensifying the performance of such processes. The forced periodic operation of trickle-bed reactors is one approach which has attracted extensive attention in chemical engineering research over the past few decades and this work has clearly demonstrated a substantial enhancement of performance that are feasible in laboratory-scale reactors. 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 reactors operation: strong mass transfer resistance for the gaseous reactant and hot-spot formation [1, 2].
Despite considerable research in this field, periodic operation is still long way from being a viable alternative to conventional steady-state TBR operation in industrial applications. 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 relative permeability hydrodynamic model proposed by Saez and Carbonell [3] has been chosen as a promising starting point, since it has already been employed successfully for the modelling of unsteady-state multiphase flow through porous media [4]. In order to validate the hydrodynamic model developed, an investigation of the quantitative influence of the liquid phase physical properties is necessary. The liquid phase surface tension and viscosity were systematically varied in order to modify liquid wetting properties. Water was selected as the reference liquid, with the surface tension being manipulated by addition of a tenside and viscosity by addition of glycerine. This strategy permitted surface tension and viscosity to be varied independently.
The two phase pressure drop and dynamic liquid hold-up were measured under steady-state conditions for aqueous-solution/N2 systems in a laboratory trickle-bed reactor packed with γ–Al2O3 spheres. Furthermore, static liquid hold-up was determined for all solutions employed. The hydrodynamic parameters thus ascertained together with hydrodynamic phenomena observed are presented and analysed, with special emphasis being placed on the interpretation of multiple two-phase pressure drop hysteresis branches and dynamic liquid hold-up behaviour and their representation in the hydrodynamic model.
The resultant dynamic model derived has been successfully validated by periodic experimentation. Experimental and simulated results will be compared and discussed in detail with respect to the specific influence of the liquid physical properties. A reliable dynamic hydrodynamic model represents an important first step in the prediction of mass transfer and reaction processes in periodically operated TBR. Furthermore, the modification of the wetting properties is seen as a promising technique for 'levelling the playing field' between gas and liquid in both steady-state and periodic TBR operation.
References:
[1] Lange, R., et al., Chemical Engineering Science, 49(1994), 5615-5621.
[2] Haure, P.M., et al., AIChE Journal, 35(1989), 1437-1444.
[3] Saez, A.E.,R.G. Carbonell, AIChE Journal, 31(1985), 52-62.
[4] Crone, S., et al., Transport in Porous Media, 49(2002), 291-312.
Presented Monday 17, 16:20 to 16:40, in session Multifase Flows - II (T2-5b).
