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Oct-2009

CFD study of a VDU feed inlet device and wash bed

Analysis of vapour velocity profiles at the entry to the wash zone of a VDU column enables a higher feed rate and improved output quality

Debangsu Ray, Indian Oil Corporation
Ajay Arora and Anne Phanikumar, Sulzer Chemtech
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Article Summary
Vapour distribution in a large, packed tower is a critical factor in a column’s performance. This article presents a revamp case for a vacuum distillation unit (VDU) column in an Indian refinery, where poor vapour distribution was identified with the help of a study involving computational fluid dynamics (CFD). Following the study, a new feed inlet device and its associated internals were installed, resulting in significant improvement to the quality and quantity of vacuum gas oil (VGO) produced, reduced quantities of slop wax and increased column throughput.

The refinery last revamped its crude distillation unit (CDU) in September 2004, to increase its processing capacity from 160 000–220 000 bpsd. However, this revamp did not enhance the VDU’s capacity, so it remained limited to the crude processing level established before the revamp. Although the VDU’s design capacity was limited, higher crude rates were nonetheless processed (180 000 bpsd during 2006 and 2007). The outcome included penalties in VGO quality, accelerated erosion on the feed inlet device, premature damage to the wash zone packing and a high rate of slop wax production.

The velocity at the feed nozzle exceeded the critical level at the chosen rate of throughput. During shutdowns, erosion of the feed inlet device, as a result of this excessive velocity, was observed. Furthermore, distillate yields were lower than expected and the entrainment of heavy ends was high, resulting in under-performance in the wash zone. During the search for a solution to these problems, it was decided that there should be minimal change to the size of the transfer line, because this would involve major modification and a long outage for 
the column.

To mitigate the problems and to increase the vacuum column’s throughput by 10%, a joint study by the refinery and Sulzer Chemtech employed CFD analysis to assess the quality of vapour velocity profiles at the entry of the wash zone packing.

CFD model and validation
The aim of the CFD analysis was to assess the performance of the vapour distribution systems with increased capacity in the flash zone of a vacuum column. To conduct a detailed analysis of vapour velocity profiles at the entry of the wash zone packing, a CFD model of the existing configuration (see Figure 1) was prepared. Y-velocity (vertical velocity) distribution across the collector tray above the feed inlet device and at the entry of the wash zone packing is shown in Figures 2 and 3 respectively. The velocity value is characterised by different colours, where blue represents the lowest velocity and red is the velocity peak of the scale. It can be seen in Figures 2 and 3 that the velocity peaks (shown in red) are not evenly distributed across the column’s cross-section and that they are mainly concentrated at the column’s centre and at the periphery of the column’s inner wall. This non-uniform distribution of vapour above the feed inlet device and at the entry to the wash zone packing was leading to very high entrainment of heavier ends to the wash zone packing, which tallied with the observed problems of reduced product quality, premature damage to the wash zone packing and high rate of slop wax production.

A subsequent CFD run was based on an increased feed nozzle diameter with a larger Shell Schoepentoeter as the feed inlet device. The results of the rerun led to a recommendation to carry out modifications to enlarge the feed nozzle from ϕ52 inch to ϕ68 inch and replace the existing conventional feed inlet device with a Shell Schoepentoeter that was compatible in size with the new feed nozzle. Figure 4 shows the CFD model of the recommended arrangement. 

Enlargement of the feed nozzle 
would help to reduce the feed inlet velocity to an acceptable level of ≈60 m/sec, which is significantly lower than the critical velocity. The Y-velocity distribution across the collector tray above the feed inlet device, and at the entry of the wash zone packing 
in accordance with the recommended configuration, is shown in Figures 
5 and 6 respectively. Velocity peaks 
are distributed evenly across the column’s cross-section. A uniform velocity profile at the inlet of the wash bed would significantly reduce the entrainment of heavier oil and thereby improve the performance of the 
packing bed.

Recommendations for column internals
Apart from recommendations for a larger feed inlet nozzle and feed 
inlet device, further  recommend-
ations covered column packing and internals:
• Bed-1 (top pump) With the existing random packing, the flood value was high at 90%. The recommendation was to replace this with structured packing to remove the bottleneck and minimise the pressure drop
• Bed-2 (LVGO-HVGO top fractionation) The recommendation was to replace structured packing with next-generation packing to further reduce the pressure drop
• Bed-3 (LVGO-HVGO bottom fractionation) No changes were found to be necessary
• Bed-4 (HVGO PA) The recommendation was to partially replace the existing structured packing with an increase in bed height to achieve the same spare capacity of the other beds
•  Bed-4 (wash zone) The recommendation was to replace structured packing with next-
generation packing.

Implementing recommendations
Sulzer’s recommendations were reviewed by the refinery, with the aim of keeping investment costs to a minimum. The following changes were settled upon:
• Enlargement of the feed nozzle with an appropriate expander in the transfer line (ϕ52-68 inch)
• Replacement of the feed inlet device with a 68 inch Shell Schoepentoeter inlet devices
• Since the feed inlet device was to be enlarged, the chimney collector tray needed to be relocated. The refinery’s decision was to dismantle the chimney tray and install a new one with appropriate positional changes. The new chimney was designed to help improve distribution of vapour to the wash bed (see Figure 4) 
• The shutdown period would be limited, so the recommendations made with respect to the column packing were closely examined and it was decided to replace 40% of the Bed-1 random packing with larger-sized  random packing to avoid flooding in the bed bottom. The rest of the packed bed replacements were deferred until the unit’s next turnaround.
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