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Nov-2016

Revamping crude towers for quality and yield

A successful revamp of distillation towers for increased diesel quality and yield depended on reliable design and accurate assembly

LEONARDO SOUZA, RAFAEL WAGNER, CLAUDIO ROCHA and HENRY GIRON Petróleo Brasileiro S.A.
ANTONIO CORTINES Sulzer Chemtech
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Article Summary
Petróleo Brasileiro (Petrobras) has carried out a major project to increase its production of ultra-low sulphur diesel in order to reduce Brazil’s dependence on foreign markets. This is in response to growth in internal demand for fuels and environmental restrictions limiting the market’s tolerance to heavier diesel grades with higher sulphur content. As part of this effort the fractionation of several crude distillation towers, both atmospheric and vacuum, has been improved to increase the diesel yield while still meeting the quality requirements of the hydrotreating units. The project is presented as a case study of the successful revamp of a distillation unit where old multi-pass valve trays have been replaced by up-to-date structured packing and the associated internals, highlighting the importance of the chimney trays and their design to ensure proper liquid and vapour distribution throughout the packing bed.    

Need for more diesel
In 2013, following impressive growth in the nation’s internal demand for environmentally friendly fuels with lower sulphur content (see Figure 1), the Refining Executive Management of Petrobras launched a corporate programme, called Promega, intended to increase the production of gasoline and medium distillates, including ultra-low sulphur diesel, also known as diesel S-10 due to the sulphur content specification of 10 ppm or less.

Several actions have been employed so far, including fractionation improvement in some key sections of atmospheric and vacuum crude distillation towers, since diesel S-10 imposes a challenge beyond its ultra-low sulphur content: it requires an ASTM D-86 distillation curve with a T95% (temperature at which 95% of the product is vaporised) lower than 370°C, which is considerably lighter than the other automotive diesel grades commercialised by Petrobras (see Table 1).

One of the refineries targeted for improvement of crude fractionation was REDUC (Duque de Caxias refinery). The refinery was built in the early 1960s and two of its three crude distillation units had never been modified for fractionation improvement until 2015; these two units were built in the 1970s and have been operating for lubes production since then. As a major turnaround had been previously scheduled for the last quarter of 2015 and included one of the lubes distillation units, the Promega staff at the refinery decided, in 2013, to use this turnaround to revamp the atmospheric and vacuum towers for increased diesel quality and yield.

The lubes distillation units have the same capacity and are very similar to each other, having an atmospheric tower that produces kerosene for jet fuel, light diesel, heavy diesel and atmospheric residue. This last cut is sent to a primary vacuum tower and produces gas oil (VGO), spindle oil and two other heavier cuts for lubes production. There is also a secondary vacuum tower that receives the primary vacuum residue and processes the heaviest cut for lubes and secondary vacuum residue, which is used for asphalt production. The atmospheric light and heavy diesel and primary vacuum gas oil cuts are mixed together before going to the hydrotreating units. Figure 2 shows a simplified refining scheme for the lubes distillation units.

Scope of modifications in the atmospheric and primary vacuum towers
The original design of the towers used only trays, with moving valves as contact devices for mass transfer between the phases. It is a well known revamping practice for crude distillation towers to replace trays with packing, usually structured, in order to increase capacity and improve fractionation, since all of the cross-sectional area of the tower is used for contact between the phases (there are no downcomers) and there is no need to have a vapour-liquid disengaging space as happens in trays. However, packing is very susceptible to liquid maldistribution issues, which can ruin fractionation if the necessary precautions are not taken at the design and assembly stages.

In the particular case of the lubes distillation unit there were extra sources of concern regarding the atmospheric tower: the heavy diesel wash zone was intended to be replaced by structured packing due to insufficient quality in the cut. But the section above, which has four pass trays, would not be modified, since the quality and yield of the light diesel have been considered to be satisfactory. In addition, the atmospheric residue stripping section would keep its trays, with the originals replaced by fixed valve trays, due to fouling issues observed in previous turnarounds, and adding an extra tray to improve fractionation. Therefore, besides modifying the heavy diesel wash zone itself, replacing the trays with structured packing and its associated internals (liquid distributor and bed support), there was a need to install two chimney trays, allowing transition between the three different sections: light diesel x heavy diesel (four pass trays), heavy diesel wash zone (structured packing) and atmospheric residue stripping (two pass trays).

The modifications in the primary vacuum tower were simpler, but not free of concern: the gas oil x spindle oil section was targeted to have its fractionation performance improved by replacing its two pass trays with structured packing, but the sections below would keep their trays and, since the available free space was limited, the use of a chimney tray would excessively reduce the amount of packing to be installed. This issue was solved by keeping the lowest tray in the section, from which the spindle oil is drawn, and letting the liquid from the bed support drip directly onto this tray. The liquid distributor would use the same nozzle already in service for directing the internal reflux from the section above – only the internal feed pipe was replaced by one fitted to the distributor.

Design engineering
With the scope of modifications defined, the next step was to design the new internals, an operation divided into two phases.

In the first phase, a representative process simulation of the unit operating in the desired conditions (maximising heavy diesel to the limit of the ultra-low sulphur diesel specification, and maximising vacuum gas oil to the limit of the viscosity specification for spindle oil) determined the flow rates and properties of the liquid and vapour internal fluxes. This information was required for the bidding process in which the structured packing, its associated internals, and the fixed valve trays would be selected from different suppliers. In addition for the bidding process, it was necessary to determine the desired number of theoretical stages for the packing beds, the limits of pressure loss under liquid load, percentage of flooding in turn-up conditions and up-lift resistance for the packings and trays and, finally, the materials to be used in their manufacture. Sketches describing the height restrictions of each section were also available (see Figure 3).

When the bidding process was completed, with one of the suppliers chosen, the process and mechanical details of the packing beds, their associated internals, and the fixed valve trays were exhaustively checked and discussed with the supplier, since the internals would also interact with the chimney trays, which were outside the supplying scope.

The second phase involved design engineering the chimney trays themselves. The basic design defined the number and diameter of the downflow pipes, the number, arrangement, and dimensions of the chimneys and, in the specific case of the upper chimney tray, the dimensions of its sumps (see Figure 4).

The arrangement of the downflow pipes in both trays was defined at the detailed design stage, along with all mechanical detail regarding supportation. A proper arrangement of the downflow pipes is essential to ensure that the distributor or tray below will be evenly fed and CFD simulations can be used to check the design.1 

Failure in properly designing chimney trays can lead to liquid maldistribution issues, which can ruin fractionation2; insufficient vapour disengaging in the sumps, which can cause pumps to cavitate if their suction pipes are not self-venting; and liquid entrainment.
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