The induction of pulses in trickle-bed reactors by cycling the liquid feed

Abstract

The operation of a trickle-bed reactor in the pulsing flow regime is well known for its advantages in terms of an increase in mass and heat transfer rates. However, fairly high gas and liquid flow rates necessitate the operation in the pulsing flow regime, resulting in relatively short contact times between the phases. By means of the periodic operation of a trickle-bed reactor it is possible to obtain pulsing flow at average throughputs of liquid usually associated with trickle flow during steady-state operation. This feed strategy to force pulse initiation is termed liquid-induced pulsing flow. The advantages associated with pulsing flow may then be utilized to improve reactor performance in terms of an increase in capacity and the elimination of hot spots, while interfacial contact times are comparable to trickle flow. An additional advantage of liquid-induced pulsing flow is the possibility to tune the pulse frequency and therefore the time constant of the pulses. During the periodic operation of a trickle bed, continuity shock waves are initiated in the column due to the step-change in liquid flow rate. This results in the division of the column into a region of high liquid holdup and a region of low liquid holdup. At high enough gas flow rates, the inception of pulses takes place in the liquid-rich region. Analysis of the performed experiments indicates that besides gas and liquid flow rates, an additional criterion for pulse inception is the available length for disturbances to grow into pulses. For self-generated pulsing flow this results in the upward movement of the position of the point of pulse inception with increasing gas flow rate. With liquid-induced pulsing flow this means that higher gas flow rates are necessary to induce pulses as the length of the liquid-rich region decreases. For both self-generated and liquid-induced pulsing flow this relationship between the gas flow rate and the available length for pulse formation is identical.