(26b) The Jet Fuel Problem Strikes Again

In 1999, REPLAN - Petrobras' biggest refinery - carried out a major turnaround of its Crude Distillation Unit U-200. To keep the costs down, the strategy was to increase unit capacity without changes to the furnaces. In the crude tower most trays were replaced with packing in order to reduce operational pressure and pressure drop allowing a higher vaporization. A new bottom side draw was added, AGO - atmospheric gasoil - directed to FCC charge. This extra side draw allowed the balance of feed and energy absorption between atmospheric and vacuum furnaces by changing the AGO rate.

After start-up, the jet fuel yield decreased to half of its original value with trays. The fractionation between jet fuel and light diesel had deteriorated noticeably, making it impossible to keep the original quality and yield.

A troubleshooting study concluded that the problem was related to liquid distribution on the kerosene and light diesel beds. The main causes were not the distributors themselves, but the collector pans, whose odd design caused flow problems and chimneys' overflow. New collectors were designed and delivered by the internals supplier.

In 2005, unit U-200 went through its first turnaround after revamp and new collectors were introduced. After startup, the column performed as badly as before, sometimes even worse. At this point, two theories came up: 1- some kind of damage or assembly mistake; 2- a poor tower configuration, considering the 1999 revamp had altered the position of the bottom pumparound. In 2006, during a pit stop, the column was opened for inspection and repair but no damages nor assembly errors were found. As part of the plan, a distribution test was conducted to determine the actual liquid distribution and a surprising result was achieved: the liquid distribution was completely skewed. The other beds, that performed normally, presented an even distribution, as expected.

A CFD study focusing chimney arrangement of collector pans was developed to understand the liquid flow dynamics. The results for collector pan sections were input as liquid inlet conditions for the distributors. The effects of liquid non uniformity feeding the distributor were analyzed and best practices in collector/distributor projects were proposed as engineering enhancements for new designs. A three-dimensional (3D) transient, non homogeneous and free surface model (Eulerian-Eulerian approach) designed for flow and geometry improvement of liquid distribution was applied to the phenomenon description.

This paper will present the original and new collector concepts, the field test results, the CFD study and conclusions plus a proposal for a practical fix to minimize the problem. Lessons learned will focus on the correct outlines for any revamp together with correct installation of packing and its associated internals.

KEYWORDS: column internals, trays, packing, jet fuel, kerosene, revamps, CFD.