Feb-2005

Hydroprocessing revamp configurations

Upgrading existing hydrocrackers from single- to two-stage recycle offers significantly higher flexibility for increasing high-quality FCC feeds

Ujjal Mukherjee, Jerry Mayer and Bharat Srinivasan, Chevron Lummus Global

Article Summary

ChevronTexaco and now Chevron Lummus Global (CLG) have operated and licensed hydrocracking units of various configurations: SSOT, single-stage recycle (SSREC) and two-stage recycle (TSR). However, most contributions to hydrocracking have been in the TSR configuration. The benefits of a “clean” second stage – namely, improved activity and selectivity – have been developed for full or near-full conversion applications, with the single-stage configurations applied to lower-conversion units.

Figure 1, based on experiments with VGO and the same catalyst at different residence times, shows how the relative amounts of paraffins, naphthenes and aromatics change as hydrogen is added and the reaction proceeds. The upper plot shows that in a single stage, or the first of two stages, the paraffins do not react significantly, even at overall conversion levels of 65%. In fact, the unconverted bottoms from the first, or single, stage are rich in paraffins and naphthenes and low in nitrogen, making them an excellent FCC or steam cracker feedstock, or a waxy lube oil base stock. The lower plot shows the benefits of the second stage, where the unconverted bottoms at a conversion level of 37% are reprocessed with the same catalyst at the same pressure but at around 40°C lower and with clean circulating gas. The overall reaction rate is considerably higher than in the first stage. Paraffins are readily converted in this clean environment.

The following hydroprocessing innovations all give the refiners involved measurable value, including exploitation of innovative synergies between different hydroprocessing technologies.

Lubes hydroprocessing
ChevronTexaco’s lube hydroprocessing experience goes back to 1984 with the start-up of the Richmond lube oil plant (RLOP) for the production of premium base oils from crude oils that are deficient in high viscosity index (V.I.) components. These poor-lube-quality crudes, such as Alaskan North Slope and Heavy California Valley, were readily available but primarily used at that time for fuels production. With the new lube plant online, an old solvent-extraction facility tied to running Arabian gas oil and resid feedstocks was shut down.

RLOP produced light-, medium- and heavy-neutral base oils in parallel hydroprocessing trains. The light train, which made light- and medium-neutral, consisted of a hydrocracker followed by catalytic dewaxing/hydrofinishing, and the heavy train, making heavy-neutral, consisted of a hydrocracker and solvent dewaxer/hydrofinisher. ChevronTexaco developed a selective, wax isomerisation catalyst that was commercialised and used by RLOP in 1993. This new process became known as Isodewaxing. Instead of removing wax molecules as in solvent dewaxing or cracking them to light C3-C8 hydrocarbons as in classic catalytic dewaxing, the catalyst isomerises the wax molecules into lube oil.

Fixed-bed residuum upgrading
The Chevron RDS hydrotreating process and the Gulf HDS resid process were introduced as ways to desulphurise fuel oil to meet environmental limits while processing high-sulphur crudes and converting some residuum to lighter products. The companies and technologies merged in 1985. The ability to replace catalyst while on stream was commercialised in 1992 with ChevronTexaco’s proprietary on-stream catalyst replacement (OCR) moving bed reactor technology, which essentially installed a hydrodemetalation (HDM) reactor to “guard” the RDS unit, thereby increasing the latter’s flexibility. An OCR reactor operates with the feed upflow and contains the necessary internals to allow addition and withdrawal of OCR catalyst. The benefits of upflow relative to fixed-bed downflow are shown in Figure 2.

KNPC in Kuwait is set to enhance its existing resid desulphurisation unit using OCR technology. With minimal modifications and limited tie-in work completed in late 2004, the unit is expected to achieve more than a 25% feed rate increase from its original design and a 25% increase in its run length while meeting the same product objective.

Ebullated bed residuum hydrocracking
ABB Lummus Global added the proprietary LC-Fining process to the CLG portfolio. Whereas a typical conversion level in fixed-bed residuum upgraders is 35–50%, LC-Fining is designed for the very high conversion of difficult residua using ebullated bed reactor technology. Conversion levels range from 55–85%. Reactor section flow scheme and reactor internals improvements have allowed single train capacities to reach 50 000bpd while cutting capital investment. Catalyst developments have also led to decreases in catalyst consumption.

Revamp options
As previously mentioned, typical hydrocracking configurations include: SSOT for low-to-moderate conversions with limited feed flexibility, product selectivity and product quality; and TSR for high-to-full conversion applications with excellent feed and product flexibility. Many existing refineries with SSOT configurations are looking for revamp options to meet more stringent fuel specifications, to increase feed flexibility or to boost fuel make. The conventional solution is to add reactor volume in series or to include a “saturation” reactor if better product quality is desired, but there are actually less expensive and more flexible options available.¹

A small reactor is added upstream from the existing reactor, converting the SSOT into a partial, or full, recycle TSR configuration. By utilising the second-stage environment described earlier, two to three times less catalyst volume is needed compared to conventional solutions. This approach exploits the synergy between what an SSOT achieves and what a TSR can achieve, proven in its application at Premcor refinery’s heavy oil upgrade project in Port Arthur, Texas, USA. The feed for this project consisted of 60% heavy coker gas oil (HCGO), 20% light cycle oil (LCO) and only about 20% straight-run heavy vacuum gas oil (HVGO) derived from Maya crude. This combination is difficult to hydrocrack due to high aromaticity and nitrogen.