CFD Study of Flow Distribution and Wetting Efficiency of the Perforated Tray Distributor of a Trickle Bed Reactor

RAMAJO DAMIAN; RAVICULE MARCELA; MONSALVO MARIA; MÁRQUEZ DAMIÁN SANTIAGO; STORTI MARIO; NIGRO NORBERTO

Tandil, Prov Bs As , Argentina

Congreso; ENIEF 2009; 2009

Universidad Nacional del Centro

            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 utilizationof the catalyst is dependent on the good distribution of the liquid of the charge across thecatalyst beds. On the contrary some parts of the beds will get less liquid reactants while otherswill get more than the average. In zones where there is maldistribution of reactants the reactionwill extent to undesired reactions, leading to deactivation of the catalyst and towards lowconversions. Bad tray efficiency due to non-uniform liquid distribution will result in lowreactor efficiency and shorten the catalyst's cycle time. The analyzed one is a trickle bedreactor that processes butene (liquid) and hydrogen (gas). The charge is introduced through theupper side and liquid accumulates on the tray to a certain level swamping the perforated platetray. The liquid phase flows down through 68 small holes while the gas phase descendsthrough 7 chimneys. There is another ceramic-ball bed above the catalyst bed with the aim toget a better distribution of the charge.In this work a computational fluid dynamics study (CFD) was carried out with the aim to knowthe wetting efficiency of the tray distributor under different operating conditions. The Euleriantwo-fluid model was employed. Because of tray holes are very small with respect to the overalltray, drains and sources were employed to represent them. In this sense, numerical andexperimental models were employed to know the response mass flow rate versus liquid heightfor the holes.Little differences on the mass flow rate across the holes were found because of the scarceliquid sloshing above the tray. Due to the small gas fraction of the charge, the liquid flows onlyby gravity so it is not sprayed after leaving holes and the extent of the wetted zone below eachdrip point (hole) is small. A suitable correlation to estimate the expansion of the wetted zonecaused by the ceramic-ball bed was employed, showing that all the top catalyst bed side iswetted 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 thereactor. So, an additional possible cause of low efficiency can be found on fouling andobstruction 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 todiminish fouling problems but on the other hand, due to its larger size, the number of drippoints will be reduced. Two simple geometric modifications are proposed to enhance trayperformance, firstly to reduce the amount of gas chimneys from 7 to only one, addingadditional drip points, secondly to replace the holes for short risers in order to reduce thevulnerability to plugging.