Apr-2008

Unlocking current refinery constraints

When processing heavy feeds, technology based on residuum supercritical solvent extraction provides higher volumes of gas oil and DAO for conversion units. Potential constraints through existing vacuum and coker units can also be resolved

Rashid Iqbal, Asif Khan, Odette Eng and Raymond Floyd
KBR Technology

Article Summary

The escalating demand for petroleum-refined products and high differentials between light and heavy crude oils have increased the usage of heavier crude oils.

Examples of heavy crude oils being used by refiners today include Athabasca bitumen, Lloydminster and Cold Lake heavy oils from Canada, Maya from Mexico, Arab Heavy from the Middle East, and other African and Venezuelan crude oils. When added to the conventional crude diet, these heavier crude oils with higher resid content will either increase fuel oil make or begin to burden existing vacuum and coking units. At the same time, the gas oil content in heavier crudes is usually lower compared to conventional crudes, resulting in lower feed to conversion units, such as the FCC or hydrocracker unit.

The increasing oil price is motivating refiners to look for solutions to extract more from the bottom of the barrel in a more cost-effective way. Refiners are increasingly using solvent deasphalting in the following applications:
- Debottlenecking existing vacuum and coking units: for refineries feeding vacuum residue to cokers, the addition of KBR’s ROSE (residuum oil supercritical extraction) unit reduces the coker feed rate by up to 50%. The coke make is also reduced by 15–20%, thus debottle-necking the existing cokers

- In place of vacuum distillation: the higher yields from a ROSE unit when compared to vacuum distillation units help refiners improve overall refinery liquid yields by 2–5 vol%.

Figures 2 and 3 show the processing options for debottlenecking existing vacuum and coker units by using a ROSE unit in a revamp situation, or investing in a ROSE unit instead of a vacuum unit in a grassroots application. 

SDA history
The oil refining industry has used solvent deasphalting (SDA) for over 50 years. Conventional SDA units separate solvent from deasphalted oils (DAO) by boiling. The SDA process was initially used for recovering lube base oils from vacuum residues. The units were small, so energy efficiency was not a high priority. The Kerr McGee Corporation started research in the 1950s to extend the application of solvent extraction in the production of fuels and to improve energy efficiency by separating solvent from the DAO in supercritical phase. The first supercritical solvent-extraction ROSE units were licensed in 1979. Subsequently, the success of the process has turned conventional SDA into an uneconomical practice in comparison. KBR acquired the technology from the Kerr McGee Corporation in 1995. To date, 48 ROSE licenses with a combined capacity of over 900 000 bpsd have been obtained by users all over the world.

Comparisons
How is this process different to conventional SDA? It is a highly energy-efficient solvent deasphalting technology in which most of the solvent is recovered in supercritical mode. In Figure 4, point “C” represents supercritical phase separation conditions and point “F” represents conditions used by conventional SDA processes for separating solvent. The energy required for supercritical separation (C–A) is less than one-third of that required for conventional (F–A) SDA processes.

The ROSE process uses special internals and design parameters that permit the extraction of maximum quantities of high-quality DAO from atmospheric or vacuum residues and other heavy petroleum feedstocks. The high-efficiency internals reduce capital costs by allowing the use of smaller separator vessels. The asphaltene content of the DAO from the ROSE unit is normally less than 200 ppmw, compared to around 800 ppmw for other SDA processes. The DAO produced also has substantially reduced contaminants, such as nickel, vanadium, sulphur and Conradson carbon when compared to residues.

These benefits (ie, lower energy usage, use of smaller separators and cleaner DAO) have been particularly useful in the conversion of conventional and third-party SDA units to ROSE. Some licensees have doubled throughput, reduced energy consumption by as much as 30%, improved DAO yield by 2–5% and at the same time have seen an order of magnitude reduction in asphaltene carryover in DAO.

In summary, the process offers the following operational and economic advantages over conventional SDA:
- Higher yield and improved DAO quality
- Flexibility in varying DAO yield and quality by adjusting operating conditions and, if necessary, changing solvent
- Supercritical solvent recovery significantly reduces operating costs by almost eliminating evaporation and condensation of solvent.

Unlocking current refinery constraints
With the higher utilisation of heavy crude oils, refiners often encounter higher resid loads with higher levels of contaminants (such as sulphur, nitrogen, metals, CCR), increased aromatics content and, more often than not, higher acid content in their feeds. On the other hand, the gas oil content of the new feeds will be lower, creating a potential loss of feed in downstream VGO processing units, such as the FCC or hydrocracking units. All these elements present challenges that refiners need to solve.

Refiners with no coker or vacuum units
Vacuum distillation units (VDUs) are considered to be one of the conventional building blocks in refinery operations. However, with time, the higher utilisation of heavy and high-acid crude oils has started to push the limit of the vacuum distillation process. High operating temperatures in vacuum units have a tendency to crack some of the heavy crude oils. To avoid coking of internals due to cracking, vacuum units have to operate at lower temperatures, thereby limiting the lift of vacuum gas oils. This is an ineffective use of vacuum units when it comes to the distillation of heavy oils, which are prone to cracking, and this limitation will hold back more heavy material in the residue stream.