Oct-2009

Hydroprocessing upgrades to meet changing fuels requirements

Adapting installed hydroprocessing units through a variety of schemes enables refiners to shift their fuels slates to meet changing demand and specifications

Jay Parekh and Harjeet Virdi, Chevron Lummus Global

Article Summary

Refiners have to think creatively in order to meet specifications and develop projects that are profitable, with returns on investment that are subject to a higher level of scrutiny in view of restrictions on capex. Light-heavy crude differentials have recently eroded, but the expectation is that the differentials will recover and encourage the development of projects that meet fuels regulations with some bottom of the barrel conversion to light products. There also appears to be momentum in the marketplace to increase diesel production, in the US in particular, with an expected increase in the use of diesel-powered engines for personal vehicles.

Revamps of existing units can provide the refiner with the ability to generate high returns on restricted capital investments. Chevron Lummus Global (CLG) has debottlenecked and revamped  a number of hydro-processing units to meet changes in fuels regulations and feedstocks, achieve capacity increases and increase the output of light products. This article discusses four revamp configurations in all areas of hydroprocessing technology, some of which have been fully implemented and others at various stages of 
project execution.

Conversion from gasoline/jet 
to diesel
An Isocracking licensee operating a two-stage with recycle (TSR) configuration hydrocracker with intermediate distillation has commis-sioned a revamp to expand unit capacity and shift the mode of operation from naphtha production to a distillate-selective operation. The objective of the revamp is to increase the nominal feed rate to the unit by 20% and to increase the diesel yield from zero to more than 40% (with a decrease in naphtha and jet yields).

The inherent advantage of a hydrocracker is the ability to shift the yield selectivity of the operation with a shift in the recycle cut point (RCP), which is defined as the cut point between the heaviest product and the unconverted oil. By increasing the recycle cut point, more distillate products can be recovered in the intermediate distillation section and less unconverted oil is sent to the second stage for further conversion. The end result is less workload on the catalyst and a reduction in chemical hydrogen consumption, with less overall cracking to naphtha as well as a reduction in light ends make. This enables a refiner to increase feed rates while maintaining catalyst life, without having to debottleneck the gas recovery section of the plant. It results in a reduction in hydrogen demand, which fosters straightforward debottlenecking to achieve higher throughput.

Since the unit was originally designed to co-produce naphtha and jet, a revamp to introduce diesel selectivity requires modifications to the existing fractionator in order to “lift” the level of diesel product. However, the ability to add another product draw is limited by the size of the column and the available reboiler duty. Therefore, an additional vacuum column must be added downstream of the fractionator to pull diesel product to a 350°C cut point. An additional heater is not required. Figure 1 shows a schematic of the TSR unit with the addition of the vacuum column. The remainder of the unit’s debottleneck involves modification of the recycle compressor, feed pumps and other minor capital expenditures on the heat exchange train. This revamp project is currently in detail engineering, with an expected startup date in 2012.

Two-stage hydrocracker revamp
In the mid-1990s, a licensee originally commissioned a TSR unit with selectivity towards middle distillates. The refiner was evaluating a project 
to increase unit capacity and 
extend run length significantly. A traditional revamp would necessitate additional reactor volume and modifications to the recycle gas compressor. However, a novel solution, using a process recently commercialised by CLG called single-stage reverse sequencing (SSRS), would enable the company to achieve its goals with substantially less 
capital expenditure.

Like a TSR unit, SSRS takes advantage of a clean second-stage environment, with overall rate constants much greater than the rate constants from the first stage. This second-stage environment permits full conversion of difficult feeds, with less than half the reactor volume needed compared to single-stage once-through (SSOT) or single-stage recycle (SSREC). The obvious difference between the traditional TSR configuration and the SSRS configuration is that the effluent from the second stage flows directly to the inlet of the first stage, which provides the following benefits over a conventional TSR configuration:
• Effluent from the second stage provides a heat sink for the first stage, reducing demand for first-stage quench gas by up to 40%
• Excess hydrogen from the second stage is used to supplement the gas-to-oil requirement for the first stage
• The overall recycle gas compressor load is reduced typically by up to 70%
• Only one reactor furnace is required.

SSRS revamp
The economics of a high-pressure hydroprocessing revamp are largely influenced by the recycle gas compressor costs. The SSRS flow scheme imposes a small incremental load on the recycle gas compressor. This is fairly intuitive for consideration of a SSOT or SSREC revamp of a two-stage unit, but less intuitive for a TSR revamp. In the TSR configuration, a guard bed is added to the first stage, and an additional first-stage reactor is added between the second-stage effluent and the product fractionator. The guard bed is added to increase demetallation and the overall volume of the first-stage reactor to extend the length of the catalyst run. The unit (pre-revamp) is currently running at 133% of original design capacity. The addition of two new reactors will enable the refiner to increase the unit’s throughput by another 42%, for a total of 175% of original design. The revamp will also extend the run length by 30%. This will provide a 228% increase in processed barrels per catalyst fill, compared to the original design, and will be achieved using the existing recycle gas compressor.

This project is scheduled to start up in Q4 2009. The unit will continue to run in a maximum mid-distillate mode.

Hydrocracking for lubes and integrated fuels and lubes production
The benefits of hydrocracking to produce feeds for lubricant base stocks — as well as other downstream process operations such as FCCs and ethylene plants — are well known and in use in plants around the world. In most of these cases, there is a dedicated lube hydrocracker, followed by dewaxing and finishing steps. What is more unusual is the integration of a hydrocracker, primarily devoted to making high-quality fuels (especially ultra-low sulphur, low-aromatic diesel), while also producing excellent feed for a dewaxing/finishing unit. The benefits of such an approach are clear: a lower capital investment than is required to build separate fuels and lubes hydrocrackers; and a lower cost of producing high-value lube base stocks.