Flow Study and Wetting Efficiency of a Perforated-Plate Tray Distributor in a Trickle Bed Reactor

Abstract

Trickle bed reactors (TBRs) are massively employed in petrochemical and chemical plants. In general these reactors have one or more beds filled up with catalyst particles. Efficient catalyst utilization relies on a good liquid charge distribution across the catalyst beds. However, normally the distribution is not perfect and some parts of the beds will get less liquid reactants while others will get more than the average. In zones where there is a maldistribution of reactants the reaction will progress in undesirable way 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 TBR analyzed here is a hydrogenation one that processes C4 (liquid) and hydrogen (gas) to produce butene 1 (also named α butylene). The two-phase charge is introduced through the upper side of the TBR and the liquid phase 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 circulates through 7 gas chimneys. There is another ceramic-ball bed above the catalyst bed with the aim to obtain a better distribution of the charge. In this work a computational fluid dynamics analysis (CFD) employing the Eulerian two-fluid model was carried out with the aim to understand the fluid dynamics of the distribution process and to determine the wetting efficiency of the tray distributor under different operating conditions. The small tray holes were modeled by sinks (drains) and sources, firstly employing numerical and experimental models to obtain the flow rate versus liquid height response. Because of the scarce liquid sloshing above the tray, little differences on the liquid discharge through the holes were found. Due to the low gas fraction of the charge the liquid flows only by gravity following an almost vertical trajectory from the holes to the ceramic ball bed. So, the extension of the wetted zone at the top of the ceramic ball bed is small. A suitable correlation to estimate liquid diffusion inside the ceramic-ball bed was employed, showing that the overall catalyst bed surface is wetted but significant differences on liquid fraction are found. Moreover, a possible additional cause of the low TBR efficiency could be the well known fouling vulnerability of this kind of tray distributors. In this sense, two simple geometric modifications were proposed to enhance tray performance; firstly reducing the amount of gas chimneys to only one, thus adding additional drip points, secondly replacing the holes by short risers in order to reduce the vulnerability to plugging. [Submitted to Int J Chem React Engng ISSN: 1542-6580]