Upgrading a vacuum distillation tower (TIA)

When a major oil refinery in India was constrained by its vacuum tower, it turned to Sulzer Chemtech to find a solution that would not only solve existing issues in the plant but also achieve capacity increases of over 10%.

Dorota Zoldosova

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Article Summary

For a decade, the refinery had been suffering from frequent failures at the tower of its vacuum distillation unit (VDU), causing considerable unplanned downtime.  The VDU was started up in 1995 with a capacity of 1.8 million t/y. The reduced crude oil, coming from the atmospheric distillation unit, was heated from 635°F (335°C) to 784.4°F (418°C) and partially vaporised in a fired heater, then transferred to the tower flash zone at absolute pressures of 35 mmHg via a transfer line and a tangential vapour horn feed inlet device. There, the liquid and the vapour phases would be separated. Liquids would drop down to the stripping trays at the bottom of the tower, while vapours would be fractionated into heavy vacuum gasoil (HVGO) and light vacuum gasoil (LVGO) in the upper sections. 

To recover various useful distillation fractions, the vacuum column was structured over the following sections: a wash bed, a HVGO and a LVGO pumparound circuit to condense the VGO for the downstream fluid catalytic cracking (FCC) and hydrocracking (HDC) units. Two additional beds at the top were present to recover vacuum diesel that would be further processed at the diesel hydrotreating (DHDT) unit before reaching the diesel pool.

The problem
Over the years, the vacuum tower was pushed to its limits, increasing its capacity to 2.1 million t/y. This strain caused instabilities within the unit, thus resulting in pressure surges that would frequently dislodge the tangential feed inlet, which was falling down to the stripping area at the bottom of the tower.

The sudden pressure variations also damaged the structured packings located in the wash bed and the HVGO pumparound section, which could not fractionate the bottoms anymore. In addition, entrainment of reduced crude oil to the HVGO was experienced. Therefore, large volumes of contaminants could not be removed, resulting in off-specification VGO being fed to the HDC and FCC units.

As a result, along with frequent downtime, the refinery needed to lower the furnace temperature, thus reducing the overall VDU efficiency, in order to counteract the release of sub-standard VGO.

To upgrade the existing VDU, Sulzer Chemtech’s team first conducted an in-depth analysis to determine the VDU operating conditions, stream data, and the feed quality, as well as to predict the resulting process parameters after the upgrade.

More precisely, the new system needed to be able to process 2.4 million t/y while maintaining the furnace outlet temperature at 784.4°F (418°C) and 35 mmHg absolute pressure at the flash zone of the vacuum tower. Also, the contaminants in the vacuum gasoil should be kept below the allowable limits with respect to Conradson carbon residue (CCR), asphaltene, and metal content such as nickel and vanadium.

Innovative design of feed inlet device
On the basis of the analysis conducted, Sulzer Chemtech suggested turning the tangential feed entry into a radial access to address the system failure issues and increase yields. This required the replacement of the vapour horn with a Shell Schoepentoeter Plus feed inlet device.

In contrast to the tangential vapour horn inlet, the Schoepentoeter Plus device dissipates the kinetic energy and momentum of the feed, as well as provide it with centrifugal acceleration to promote the separation of liquids from the vapour. As a result, it featured a higher mechanical integrity and greater de-entrainments and separation efficiencies, even with higher loads. 

The different arrangement of the inlet device required the 60 in (1.5 m) feed nozzle and the transfer line to be modified. In particular, the first needed to be relocated while the latter needed to be rerouted and shortened in order to fit with the radial inlet. Sulzer Chemtech performed a number of engineering simulations to determine the feasibility of the new arrangement in terms of stream and process operations, and the mechanical integrity of the transfer line and the vacuum tower.

The vacuum tower, the heater, and transfer line were first checked by mean of hydraulic calculations and process simulations. To cross-check the results, computational fluid dynamics (CFD) studies were also performed to identify flow regime type and the ratio between the actual velocity and the critical one. This would ensure that the predicted distillates yields would not be jeopardised.

The mechanical review used a piping stress analysis to determine the presence of critical areas, and finite element analysis (FEA) studies highlighted load and boundary conditions within the system. The results of these simulations allowed Sulzer Chemtech to design a reliable VDU with respect to process and mechanical performance.

As a result of these simulations, Sulzer Chemtech determined the optimal reroute of the transfer line. In addition, it was found that the existing design of the furnace did not require any modifications.

To further strengthen the HVGO and wash beds, an enhanced support system was implemented at both sections. Also, after careful inspection, the mist eliminator at the top of the tower and bed #5 were replaced, as they showed signs of degradation.

Immediate results
Despite the large scale of the project, the retrofit of the VDU was completed in record time, minimising refinery downtime. The analysis and simulations required less than eight months, while the actual installation and maintenance activities were completed within a month. As a consequence of the retrofit, the VDU has been operating smoothly and efficiently for several years. The quality of the VGO improved, as the level of contaminants was reduced. Furthermore, the refinery improved its competitiveness, reaching the desired vacuum tower capacity.

This short case study originally appeared in PTQ's Technology In Action Feature - Q3 2021 Issue.

For more information: dorota.zoldosova@sulzer.com

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