Column revamp boosts diesel production

Revamping the internals of a main fractionator column enabled a refinery to increase its diesel throughput.


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Article Summary

Tight oil availability in the opportunity crude market provides refiners more profitable options in crude blending. However, blended crudes may greatly impact unit operation. When IRPC Public Company Limited’s refinery needed to modify its hydrotreating reactor to support increased variability in the feedstock, it contacted Sulzer. Sulzer provided a solution that debottlenecked the main fractionator, enabling the refinery to nearly triple its treated diesel to feed ratio without replacing the existing column. As a result, the company could maximise its throughput and benefit from a quick payback on its investment.

IRPC Public Company Limited is a Thai oil refiner and petrochemical producer. Its integrated refinery and petrochemical complex in Rayong, Thailand operates two crude atmospheric distillation units (ADU). These have a combined capacity of 215 000 b/d and were designed to handle various types of sweet and sour crudes, with API values ranging from 30 to 45. The long residual from the bottoms of these ADUs is further distilled in a vacuum distillation unit (VDU) to produce vacuum gasoil (VGO).

The VGO is combined with the atmospheric gasoil (AGO) from the ADUs and then fed to a fixed bed gasoil hydrotreating unit. The reactor effluent, after the off-gas and hydrogen sulphide are removed, is processed in the main fractionator to produce diesel with less than 50 ppm sulphur to comply with current emission standards.

The main fractionator was originally equipped with 34 conventional valve trays, numbered from top to bottom (see Figure 1a). Feed from the furnace is directed onto tray 30, while the diesel draw-off is located at tray 10. There is a pumparound recirculation from tray 16 to tray 12 and the stripping steam is fed below tray 34. The column was designed to accommodate a feed capacity of 16 600 b/d, with a VGO to AGO ratio of 4.23 and a diesel yield around 14.3 wt%. 

In order to maximise profitability,  IRPC wanted to increase the hydrotreating unit capacity by 19% and diesel production from 30 wt% to 40 wt%. After the revamp, the column was able to support an 18 600 b/d feed rate, with a VGO to AGO ratio of 1.48. The treated diesel yield reached 40 wt% of the total feed rate.

Due to production planning changes in the plant, feed to the hydrotreating unit was subsequently changed to accommodate more AGO. It was observed that the column was able to maintain good separation between treated diesel and treated VGO when the latter was approximately 1.5 times the content of AGO in the feed. However, when the ratio dropped to 1.3, the treated diesel yield was approaching 42 wt% of the total feed rate, and separation between treated diesel and treated VGO was compromised.

In particular, when the feed temperature was reduced, the boiling temperature overlap between treated diesel and treated VGO could be improved, but this would reduce the yield of treated diesel. Also, despite the enlargement of a control valve for treated diesel draw-off, when plant operators tried to draw more diesel from the side stripper the liquid level in this section dropped quickly.

Room for improvement
The refinery then asked Sulzer to evaluate a potential revamp of the hydrotreater’s main fractionator to improve the separation overlap between the treated diesel and treated VGO products, while maintaining a high diesel yield. Sulzer set up a process simulation model of the hydrotreating unit, matching different sets of test run data. The simulated vapour and liquid flow rates and physical properties were then input into Sulzer’s column mass transfer internals rating program Sulcol to evaluate column performance.

The simulation was also used to model the unit’s conditions after revamp. The fractionator feed throughput was expected to increase by 7%, while the treated diesel yield was modelled to increase by 3-5% on feed basis, due to higher AGO in the feedstock.

Based on the model created, Sulzer’s engineers noticed that in the existing configuration, the treated diesel draw-off nozzle was above the diesel pumparound section. Since the diesel product was expected to increase, the diesel pumparound duty needed to be reduced to allow vapour to reach the top section of the column, condense at the top, return to tray 1 and be collected in chimney tray 10 as diesel product. This arrangement is less favourable for energy saving, as energy lost into the air and cooling water cannot be recovered.

The engineers therefore proposed an optimised design to IRPC that would allow the plant to maximise heat recovery in the steam generator via a diesel pumparound exchanger. To do that, the diesel product draw-off nozzle needed to be relocated to the pumparound section. Figure 1b shows the modified scheme.

The proposed design was also key to identifying various bottlenecks in the existing system. Firstly, increasing heat recovery in the diesel pumparound implied that the diesel pumparound circulation flow rate needed to be increased. However, the exit nozzle, pipelines, pumps, and trays in the diesel pumparound section could have limits in hydraulic capacity. Secondly, due to increased diesel product, the trays in the side stripper were experiencing downcomer flooding, causing fluctuations in the side stripper level controller. Thirdly, a minimum wash rate should be maintained in trays 17 to 29 to maintain a proper separation gap between treated diesel and treated VGO product. Lastly, an additional pump could possibly be needed for a new diesel draw-off location to the side stripper.

Column internals upgrades
Sulzer recommended using Mella-pak high efficiency structured packing to replace trays 11 to 16, which were hydraulically limited. While these internals were removed, the existing support ring at tray 15 was strengthened and reused for the new structured packing beds. Also, a new collector tray was installed at the same elevation as the existing draw-off gutter for the diesel pumparound. The draw-off nozzle size was checked and found adequate for both pumparound and product flow rate, hence no modification was needed. The existing diesel draw-off nozzle was blinded at the column. Existing collector tray 10 was modified to add a liquid downflow pipe. A new feed pipe was added from the first flange of the existing pumparound return nozzle (see Figure 2).

Piping modifications
IRPC and Sulzer surveyed the column and existing piping layout. Particular attention was placed on the analysis of the hydrostatic head for three tie-in points to find the best location to connect from the pumparound draw-off line to the diesel side stripper without a pump. Approximately 10 m below the draw-off nozzle was found to be the ideal option. There was a readily available platform for personnel access, and the piping pressure drop was minimal.

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