Trickle-Bed Units

Most hydrotreaters and hydrocrackers are trickle-bed units. A classic article by Satterfield describes the fundamental behaviour of such units, in which mixtures of liquid and gaseous reactants pass down over fixed beds of catalyst. In hydroprocessing units, the liquid reactants are petroleum fractions, and the gaseous reactant is hydrogen. 

Modem quench sections are designed to do three things (a) to lower ihe overall temperature of the reacting fluids, (b) to reduce radial mal-distribution with radial mixing, and (c) to redistribute the reactants and deliver them to ihe next bed. The major parts of a quench deck are the quench tube, the liquid collector and re-distributor, the gas/liquid mixing zone, and the final distributor. 

Quench tubes bring quench gas into the reactor. Some are very simple -just a tube with a series of holes in it. Others, such as the ExxonMobil spider vortex design, are more complex, distributing gas horizontally through several spokes to different parts of the quench deck. 

In the liquid collector and re-distributor, liquids are forced to flow down two angled slides into a raceway. The slides give the liquids some angular momentum, and the raceway gives them time to mix. More than an5dhing else, this part of the quench deck reduces RTD. 

In the gas/liquid mixing zone, a bubble-cap tray or similar device provides intimate contact between gases and liquids from the redistribution zone. The final distributor sends a fine spray of fluids down to the catalyst bed below. 

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