Nature and characteristics of pulsing flow in trickle-bed reactors

Pulsing flow is well known for its advantages in terms of an increase in mass and heat transfer rates, complete catalyst wetting and a decrease in axial dispersion compared to trickle flow. The operation of a trickle-bed reactor in the pulsing flow regime is favorable in terms of a capacity increase and the elimination of hot spots. Extending the knowledge on the hydrodynamic nature and characteristics of pulsing flow stands at the basis of further exploitation of the effects of this flow regime on reactor performance. An analysis of the hydrodynamics of pulsing flow reveals that pulse properties as liquid holdup, velocity and duration, are invariant to the superficial liquid velocity at a constant gas flow rate. The pulse frequency, however, increases with increasing superficial liquid velocity. The relative contribution of the pulses and the parts of the bed in between pulses to an average measured property can thus be obtained. By applying this concept it is shown that the linear liquid velocity inside the pulses varies between 0.1 and 0.2 m s-1. The linear liquid velocity in between pulses, however, is invariant to gas and liquid flow rates and packing properties and equal to about 0.05 m s-1. This suggests that a linear liquid velocity of about 0.05 m s-1 is the maximum velocity possible in the bed to maintain the trickle flow regime. All liquid in excess is transported as pulses. The liquid holdup in the parts of the bed in between pulses equals the liquid holdup at the transition to pulsing flow at all gas flow rates. The same trend holds for the linear liquid velocity in between pulses. Pulsing flow then is a hybrid of two transition states. The pulses reside at the transition to bubble flow, while the parts of the bed in between pulses reside at the transition to trickle flow. The enhanced particle-liquid heat transfer coefficient inside the pulses is mainly the result of the high linear liquid velocity inside the pulses. Particle-liquid heat transfer rates in between pulses are constant due to the constant linear liquid velocity.