CFD study of flow distribution and wet efficiency of the perforated tray distributor of a trickle bed reactor

RAMAJO, D.; RAVICULÉ, MARCELA; MONSALVO, M.; MÁRQUEZ DAMIÁN, SANTIAGO; STORTI M.; NIGRO N.

Tandil

Congreso; XVIII Congreso sobre Métodos Numéricos y sus Aplicaciones; 2009

Asociación Argentina de Mecánica Computacional AMCA

Trickle bed reactors are massively employed in petrochemical and chemical plants. Reactors consist of one or more beds filled up with catalyst particles. The efficient utilization of the catalyst is dependent on the good distribution of the liquid of the charge across the catalyst beds. On the contrary some parts of the beds will get less liquid reactants while others will get more than the average. In zones where there is maldistribution of reactants the reaction will extent to undesired reactions, leading to deactivation of the catalyst and towards low conversions. Bad tray efficiency due to non-uniform liquid distribution will result in low reactor efficiency and shorten the catalyst's cycle time. The analyzed one is a trickle bed reactor that processes butene (liquid) and hydrogen (gas). The charge is introduced through the upper side and liquid accumulates on the tray to a certain level swamping the perforated plate tray. The liquid phase flows down through 68 small holes while the gas phase descends through 7 chimneys. There is another ceramic-ball bed above the catalyst bed with the aim to get a better distribution of the charge. In this work a computational fluid dynamics study (CFD) was carried out with the aim to know the wetting efficiency of the tray distributor under different operating conditions. The Eulerian two-fluid model was employed. Because of tray holes are very small with respect to the overall tray, drains and sources were employed to represent them. In this sense, numerical and experimental models were employed to know the response mass flow rate versus liquid height for the holes. Little differences on the mass flow rate across the holes were found because of the scarce liquid sloshing above the tray. Due to the small gas fraction of the charge, the liquid flows only by gravity so it is not sprayed after leaving holes and the extent of the wetted zone below each drip point (hole) is small. A suitable correlation to estimate the expansion of the wetted zone caused by the ceramic-ball bed was employed, showing that all the top catalyst bed side is wetted but significant differences on liquid concentration are found. Nevertheless, the wetting distribution from CFD seems not be as bad as to explain the low overall efficiency of the reactor. So, an additional possible cause of low efficiency can be found on fouling and obstruction of some holes. Although more efficient distribution trays are currently employed in trickled bed reactors (eg. bubble cap trays) they need higher gas flow rates to work. Only chimney trays are capable to diminish fouling problems but on the other hand, due to its larger size, the number of drip points will be reduced. Two simple geometric modifications are proposed to enhance tray performance, firstly to reduce the amount of gas chimneys from 7 to only one, adding additional drip points, secondly to replace the holes for short risers in order to reduce the vulnerability to plugging.