Advances in residue upgrading technology
A combination of mature technology and innovative catalyst design and process improvements provide the flexibility to upgrade heavier feeds
G Phillips, Foster Wheeler Energy
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Although there have been many false dawns, there are now signs that the long-awaited increase in residue upgrading projects in Europe may be about to happen. This interest is being stimulated by the continuing decline in fuel oil markets and growing demand for transportation fuels, especially kerosene and diesel. The typical growth forecast for diesel of around 2 per cent a year suggests that Europe will face a deficit in diesel of some 45 million tonnes/year by 2010. At the same time, European refining utilisation is high, and refiners are now exploring the production of transportation fuels from low-value residuals rather than from increased crude processing.
In spite of relatively few projects implemented in the 1980s and ‘90s, the licensors and technology providers have not been idle. Residue upgrading technologies have been improved over this period with the objective of improving process and catalyst performance, lowering capital cost and addressing safety and environmental concerns.
As its name implies, the carbon rejection approach to residue upgrading involves upgrading residue by rejection of carbon in the feed, thereby lowering overall product carbon to hydrogen ratio.
The main processes falling under this categorisation are:
- Deep cut vacuum distillation
- Visbreaking and its variants
- Delayed coking
- Solvent deasphalting
- Residue fluidised catalytic cracking (RFCC).
Although many of these processes can be considered to be well-proven and mature technologies, developments have taken place targeted at improved economic and environmental performance.
A revamp of the vacuum distillation unit cutting deeper into the residue-making incremental FCC or hydrocracker feed is one of the first and most attractive options available to the refiner. Over the past 20 years or so, the cut point of the vacuum gas oil (VGO) has risen from 525°C to 590°C or higher, as proprietary internals have been developed and the causes of vacuum column coking have become better understood.
When considering such a revamp, it is important to consider the impact on VGO quality, especially if the VGO forms part of the feed to downstream conversion units such as hydrocracking or fluid catalytic cracking (FCC). Typically, the specifications for FCC and hydrocracker feed will be set by the process licensors and are summarised as follows:
Metals, wtppm 35 2
wt% 2–10 1
The incremental FCC/hydrocracker feed, obtained by cutting deeper into vacuum residue, is usually a relatively small proportion of total cracker feedstock and the quality of this stream can, therefore, be poorer than the specification above. In addition, steps can be taken to mitigate the effect of high metals. For example, on FCC, a policy of increased catalyst consumption could be adopted to keep equilibrium catalyst at an acceptable level, and a higher proportion of demetallisation catalyst can be utilised in the hydrocracker.
Clearly, both these steps result in an operating cost increase, which must be justified by the upgrading of the poorer quality VGO. However, in FCC based refineries, the growing application of feed hydrotreatment (currently around 25 per cent in Europe) is allowing the refiner to process much poorer quality feed components in the FCC unit. A recent study completed by Foster Wheeler suggests that FCC feed hydrotreatment (currently standing at approximately 300000bpsd in Europe) could grow by a further 30 per cent (85000bpsd) over the next five years.
For bottom of the barrel upgrading, visbreaking is a relatively inexpensive, well-proven, mild thermal decomposition process. The products from visbreaking are gas, distillates, and a stable fuel oil or fuel oil component, lower in viscosity than the feed.
Visbreaking is often employed to increase refinery net distillate yield through conversion of residue and /or by a reduction in the volume of cutter stock needed for fuel oil blending. Since the cutter stocks are usually potential diesel components, the economics are strongly driven by the price differential between diesel and residual fuel oil.
There are two main variants of visbreaking: coil design, by which thermal cracking takes place only in the visbreaker heater, and soaker design, whereby heater products reside in soaker drums for the thermal reactions to continue.
The soaker is, therefore, a lower temperature, longer residence time design compared to coil visbreaking. There has been considerable debate over which design option is best. Foster Wheeler believes that the coil option can offer real benefits to the refiner. For example, decoking is easier as no periodic decoking of drums is required, and process control is easier, especially if frequent changes in feed quality is required.
In recent years efforts have been made to optimise the coil design by developing online spalling/decoking procedures, and introducing a heater design and proprietary installation valves, which allows for isolation and removal of one or more passes from operation. This permits steam-air decoking or pigging of the isolated pass without the need to shutdown the entire visbreaker.
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