We have studied two model hydrogenations - alpha-methyl styrene (AMS) and polystyrene (PS) – in novel reactors and under conditions where pulsed flows might be expected to greatly enhance observed rates of reaction. The PS was hydrogenated in a twin screw extruder modified with a custom-made die that can hold either a catalyst monoliths (100 cpsi) or a conventional packed bed (20-35 mesh). The polymer solution (2 or 10 wt% PS dissolved in 10 vol% THF/cyclohexane) is pre-mixed with hydrogen in an autoclave and pumped to the extruder, where it can be mixed with a pulsing gas flow (pulsed solenoid valves) prior to the die entrance. A photocell records the exit flow behavior. Because extruders also exhibit inherently unsteady-state operation in liquid-starved operation,1 natural flow oscillations (similar to slug flow) are also observed, albeit at different frequency. The effects of these natural “unforced” pulses were compared to the performance of the superimposed “forced” pulses.
The catalyst for all these studies was a typical hydrogenation catalyst composed of 0.5 wt% Pd/γ-Al2O3 prepared by an ion exchange technique from Pd(NH3)4(NO3)2. The dispersion was ~73% measured by H2 chemisorption with a BET surface area of 290 m2/g and an average pore size of 10 nm. The observed reaction rate constants were determined by modeling the system in plug flow with first order dependences in aromatic group and hydrogen concentrations.2
As has been previously reported3, for 2 wt% PS solutions at low to average liquid space velocities (0.14-0.48 mL/s/g Pd), the observed pseudo first-order rate constants (kobs) (1-3 x 10-5 L/s/g Pd) are below those obtained from batch autoclave studies where intraparticle and liquid film diffusional resistances were minimized. However, increasing liquid and gas flow rates simultaneously increases kobs in qualitative agreement with correlations for gas-limited mass transfer in slug flow monolith systems.4,5 At high flow rates (0.8-1.8 mL/s/g Pd), the rate constants are within 30% of those observed for an agitated vessel at comparable conditions (~1 x 10-4 L/s/g Pd), suggesting significant increases in the mass transfer rates for H2, both gas to liquid and liquid film to catalyst surface. Forced pulsing at 0.1 Hz and higher had no effect on the kobs, indicating the unforced oscillations are already optimal at this low PS concentration.
Increasing the PS concentration to 10 wt% results in much greater external mass transfer resistance - the viscosity is ~15 times that of the 2 wt% solution. The kobs values for the pulsed extruder are approximately an order of magnitude less than for the 2 wt% PS solution. Similar to the 2 wt% PS solution, kobs increases as the flow rates increase. However, forced pulsing at 0.1 Hz did increase kobs by ~35% compared to the unforced oscillations, indicating a greater effect of forced pulsing with increasing resistances to mass transfer. Pulsing at 0.5 Hz decreased the observed rates, as the catalyst then operated under liquid-starved conditions. There is a clearly observable optimal frequency for this process.
The AMS hydrogenation reaction was carried out in vertically stacked monoliths mounted above a piston/cam arrangement, thereby allowing oscillatory frequencies much higher (0-20 Hz) than the solenoid configuration on the reactive extruder. It has already been shown that this configuration can significantly enhance rates of mass transfer – by as much as 700% - for the air-water system. The behavior with respect to frequency can be attributed to resonance effects that can be predicted theoretically.6 Reactor studies on AMS hydrogenation in this oscillating monolith reactor are currently underway to determine optimum pulsing frequency and to compare results to similar work on alternating gas- and liquid-rich conditions in packed and trickle beds.7
References 1. Mudalamane, R.; Bigio, D.I., “Process variations and the transient behavior of extruders” AIChE J, 2003, 49, 3150-3160.
2. Xu, D.; Carbonell, R.G.; Kiserow D.J.; Roberts, G.W. “Kinetic and Transport Processes in the Heterogeneous Catalystic Hydrogenation of Polystyrene.” Ind. Eng. Chem. Res., 2003, 42, 3509-3515.
3. Bussard, A.; Dooley, K. “Polymer Hydrogenation by Reactive Extrusion – Pulsed and Continuous Flow Systems” AIChE Annual Meeting, San Francisco, 2006, 508d.
4. Bercic, G.; Pintar, A. “The role of gas bubbles and liquid slug lengths on mass transport in the Taylor flow through capillaries” Chem. Eng. Sci., 1997, 52, 3709-3719.
5. Kreutzer, M.T.; Du, P.; Heiszwolf, J.J.; Kapteijn, F.; Moulijn, J.A. “Mass transfer characteristics of three phase monolith reactors” Chem. Eng. Sci., 2001, 56, 6015-6023.
6. Knopf, F.C., Waghmare, Y.G., Ma, J., Rice, R.G. “Pulsing to improve bubble column performance: II. Jetting gas rates” AIChE Journal. 52, 1116 (2006).
7. Urseanu, M.I., Boelhouwer, J.G., Bosman, H.J.M., Schroijen, J.C. “Induced pulse operation of high-pressure trickle bed reactors with organic liquids: hydrodynamics and reaction study” Chem. Eng. and Proc. 43, 1411 (2004).