Jul-2008

Processing heavy 
ends: part I

The synergy of solvent deasphalting and FCC can provide economic access to heavy ends. Pilot plant data on bottom-of-the-barrel processing options are examined, which can be used as presented or combined with other options such as coking

Phillip K Niccum and Aldrich H Northup
KBR Technology

Article Summary

The increasing cost differential between light and heavy crude oils, and the growing demand for refined petroleum products have led to a surge in new deasphalting unit installations. Deasphalting units allow refiners to either switch to heavier, less expensive crude oils or to dig deeper into the bottom of the barrel, at minimum investment cost. The deasphalting process is particularly adept at producing good-quality FCC feedstock from highly contaminated crude oils, because the selective partitioning of contaminants between the deasphalted oil and asphaltene fraction is favourable for contaminants that are the most detrimental to FCC operations: ie, metals, carbon residue, nitrogen and sulphur (in that order). The right combination of solvent deasphalting followed by FCC offers new opportunities for improving profitability to refiners.

Distillation, deasphalting and fluid catalytic cracking unit pilot plant data are presented that contrast the processing of atmospheric residue in an FCC unit with the processing of deasphalted oil. The data show that deasphalting produces more FCC feedstock from each barrel of crude processed, while also providing a less contaminated FCC feedstock compared to the processing of incremental atmospheric residue. The net result is increased production of refined products and reduced FCC catalyst consumption relative to the catalytic cracking of residue.

While the aforementioned synergies between deasphalting and FCC have been known for many years, recent advancements in deasphalting technology and new options for asphaltene product utilisation have broadened the appeal of the process.

Renewed interest in 
residue upgrading
Heavy refinery crude oil feedstocks are generally more aromatic than lighter feedstocks. In the past, aromatic crude oils were not preferred because of their poor lubricating properties and resistance to conversion in cracking units, and because aromatics are generally associated with the worst elements of crude oil — contaminants such as vanadium, nickel, nitrogen and sulphur. Much of the investment and operating cost associated with a modern petroleum refinery revolves around the separation of heavier aromatics and associated contaminants from the more desirable constituents of crude oil.

Since heavy aromatics are concentrated in the highest boiling fraction of crude oil, the first line of defence has traditionally been to fractionate out the higher boiling fraction of the crude oil, selling the residue at a price well below that of more desirable petroleum products. However, as crude oil supplies and refinery capacity have tightened relative to the demand for refined products, refiners are investing in technology to recover valuable hydrocarbons once lost with the residue.

Today, many of the processes that are used to recover more valuable hydrocarbons from residue are processes that have been around for more than 60 years, including solvent deasphalting and fluid catalytic cracking. Advancements in these technologies together with changing market conditions and a better understanding of the process synergies have led to renewed interest in the use of these technologies for residue upgrading.

Fluid catalytic cracking
Many companies contributed to the development of fluid catalytic cracking, but chief among these were The M W Kellogg Company and Standard Oil of New Jersey, now KBR and ExxonMobil.1 The development culminated in 1942 with Kellogg building the world’s first FCCU for Standard Oil in Baton Rouge, Louisiana, USA, followed by Kellogg building an additional 21 FCCUs during the next two years. In a post-war economy, the emphasis in FCC turned from high-octane gasoline production at any cost to the optimisation of yields and the utilisation of lower-quality feedstocks. Residue fluid catalytic cracking marked a milestone in 1961 with a purpose-built residue feedstock FCCU design by Kellogg for Phillips Petroleum Company that included steam-generating coils within the regenerator bed to remove excess heat.2 However, it was not until the oil embargo and skyrocketing oil prices of the 1970s that residue fluid catalytic cracking began to gain wider acceptance as a viable option for upgrading selected residues into more valuable products.

Residue FCC feedstock considerations
The most important feed properties  to consider when processing residue in an FCCU are:
- Vanadium The controlling parameter setting FCC catalyst make-up rates
- Carbon residue The major factor affecting coke burning and catalyst cooling requirements
- Hydrogen content Impacts FCC conversion and yield selectivity. High hydrogen content feed translates into increased yields of naphtha and olefinic LPG at the expense of hydrogen-deficient products such as coke and cycle oils.

Residues with lower concentrations 
of carbon and metals, particularly paraffinic, low-vanadium crude oils, are naturally better suited to upgrading in FCCUs, and often significant volumes of such residues (or even 100% atmospheric residue) can be charged to FCCUs with little or no changes to FCC hardware. However, the availability of these high-quality crude oils is diminishing and more contaminated, heavier crude oils are making up an increasing proportion of the world’s crude oil supply.

Even if blended in small concentrations into FCC feedstocks, residues from lower-quality crude oils often contain higher concentrations of metals and carbon residue than would be economic for FCC processing because of the contaminant’s negative impact on the required catalyst make-up rate and FCC yields. Therefore, before processing in the FCCU, the residues from such crude oils must first be upgraded with such processes as vacuum distillation, coking, residue hydrotreating or solvent deasphalting to reduce carbon residue and metals content. Various options for processing poor-quality residues have been practised:
- Directly processing residue from some high-quality crude oils in the FCCU can be economic. However, this option is not very flexible with respect to refinery crude oil supply
- Vacuum distillation can separate vacuum gas oil from atmospheric residue, but vacuum distillation leaves potential FCC feed behind in the vacuum residue
- Coking eliminates vanadium and carbon residue from its gas oil products, but the coker gas oils are hydrogen deficient, resulting in poor yield selectivity when processed in an FCCU
- Residue hydrotreating can reduce contaminants to economic levels 
while increasing FCC feed hydrogen content, but the capital and operating costs of residue hydrotreating are high.