Upgrading a vacuum distillation tower (TIA)
Reliable and efficient vacuum towers are key to ensuring optimal performance and yield.
Giuseppe Mosca & Prasanna Ramani
Sulzer Chemtech Switzerland
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High quality process and mechanical designs as well as maintenance services can provide essential tools to maximise throughput and result in significant gains in refining processes. When a major oil refinery in India was constrained by its vacuum tower, it turned to Sulzer to find a solution that would not only solve existing issues in the plant but also achieve capacity increases of over 10%.
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 (see Figure 1). Reduced crude oil from the atmospheric distillation unit was heated from 335°C to 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 gas oil (HVGO) and light vacuum gas oil (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.
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, 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’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 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.
Design of the feed inlet device
On the basis of the analysis conducted, Sulzer suggested turning the tangential feed entry to radial access to address the system failure issues and increase yields. This modification required the replacement of the vapour horn with a Shell Schoepentoeter Plus feed inlet device (see Figure 2).
In contrast to the tangential vapour horn inlet, the Shell Schoepentoeter Plus device dissipates the kinetic energy and momentum of the feed, as well as provide it with centrifugal acceleration to promote separation of liquids from the vapour. The result is higher mechanical integrity and greater de-entrainments and separation efficiencies, even with higher loads.
The different arrangement of the inlet device required the 60-inch (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 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 means of hydraulic calculations and process simulations. To cross-check the above mentioned 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, using piping stress analysis, determined 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 to design a reliable VDU with respect to process and mechanical performance.
As a result of these simulations, Sulzer determined optimal rerouting 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.
Despite the large scale of the project, the retrofit of the VDU was completed in record time, minimising the refinery’s 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 over two years. In particular, 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 - Q1 2021 issue.
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