Self-oscillations on a partially wetted catalyst pellet in ?-methylstyrene hydrogenation: Experiment and mathematical modeling

Abstract

Catalytic reactions of hydrogenation of liquid hydrocarbons underlie many petrochemical processes. A feature of these reactions is high heat release, because of which components of the liquid phase can evaporate and the reactions can occur in both the liquid and the vapor-gas phases on wetted and dry catalyst pellets, respectively. The interplay between chemical and phase changes on an incompletely flooded pellet give rise to various types of steady-state heat modes.Experimental studies in this area were started [1, 2] with model reactions of hydrogenation of α -methylstyrene (AMS) and cyclohexene in a three-phase system. In steady-state experiments on cyclohexene hydrogenation on a single catalyst pellet [2], multiplicity of temperature and flooding states of the catalyst pellet was first observed, which were dependent on the initial impregnation of the pellet with liquid. In AMS hydrogenation [3], phenomena of the "ignition" and the "extinction" type were experimentally investigated for the first time in irrigation of various types of single catalyst pellets at different flow rates of the liquid reactant.Previously [4], we proposed a physical mechanism and developed several simple models for a monoporous pellet, which, by and large, explained the experimental data [3]. We also put forward a more complex mathematical model of AMS hydrogenation on a catalytic plate that is partially wetted on the surface and partially impregnated with the liquid reactant. This model took into account the pore size distribution, the pore filling distribution, and the reactions in the liquid and gas phases [5]. Using this model, by numerical methods, multiplicity of steady states and hystereses of temperature and the degree of pellet filling with the liquid were revealed.It is known that the interplay between physical steps of the process (impregnation of a pellet with the liquid, liquid evaporation, heat and mass transfer) and chemical reactions in the liquid and vapor-gas phases may lead to instability and self-oscillation modes in threephase systems [6]. Therefore, it seems necessary to study such modes further.The purpose of this work was to experimentally and theoretically investigate self-oscillation modes on an irrigated porous catalyst pellet in exothermic hydrogenation accompanied by liquid evaporation. For this purpose, the most promising experimental method is in situ NMR tomography (magnetic resonance microimaging) [7][8][9]. This method allows one, in particular, to obtain images of the liquid-phase distribution within the porous object and monitor the liquid-phase redistribution immediately during the process without destroying the object and without introducing any probes or molecular labels [10][11][12][13]. EXPERIMENTAL As a model reaction, we chose α -methylstyrene (AMS) hydrogenation to form cumene:C 6 H 5 C(CH 3 )=CH 2 + H 2 C 6 H 5 C(H)(CH 3 ) 2 . This reaction is most widely used as a model reaction in the experimental investigation of processes in trickle-bed reactors (in which there is a downward cocurrent g...

Solid-state 27Al MRI and NMR thermometry for catalytic applications with conventional (liquids) MRI instrumentation and techniques

“…5B). As can be seen, this ratio exhibits a pronounced decrease with temperature above 340-360 K, i.e., in the region of interest for catalytic applications [35][36][37]. In the low temperature range (300-340 K) an opposite trend is observed, which at least partly is caused by the fact that recovery of the NMR signal for 2 s recovery delay is incomplete.…”

“…As a benchmark, we consider a commercial Bruker Avance DRX-300 WB system with imaging accessories (no sample rotation or translation, gradient amplitudes 640 or 6100 G/cm, 200 kHz digitizer bandwidth, 100 W rf amplifier). Our choice of the solid matrix, c-Al 2 O 3 (and thus of the magnetic nucleus, 27 Al), was governed by our recent in situ MRI studies of the heterogeneous catalytic hydrogenation reaction of a-methylstyrene on Pt/c-Al 2 O 3 or Pd/ c-Al 2 O 3 catalysts [35][36][37] as well as the fact that alumina is often used for supported catalysts preparation. Other nuclei that we consider here are 51 V of V 2 O 5 and 23 Na of NaCl and NaHCO 3 .…”

Influence of Bubble Evolution on the Effective Kinetics of Heterogeneously Catalyzed Gas/Liquid Reactions. Part II: Exothermic Gas/Liquid Reactions

“…The influence of exothermic reactions on mass and heat transfer and on the effective kinetics of G/L reactions is also discussed in the literature [8][9][10][11][12][13][14]. Evaporation and condensation may induce countercurrent gas and liquid flows affecting the intraparticle mass transport.…”