My original intention was for the next discussion to be on cracking processes, but in retrospect it would be better to discuss what is required to crack hydrocarbons to produce usable transport fuels.

In the early days of refining the refineries we geared up to produce naphtha, kerosine, diesel and atmospheric residue. The atmospheric residue was frequently used as fuel oil for power generation, steam production and marine applications ( steam turbine and marine diesels).  This type of processing was known as hydroskimming. In the developed world very few hydroskimming  refineries now exist as these refineries cannot extract the value in the atmospheric residue or residual fuels. In the 1960 many refineries were fitted out with vacuum distillation units and fluid catalytic cracking units (FCC). Sometimes a thermal cracker or vis-breaker were also employed. In the 1970’s hydrocracking and coking became available as the demand for ever more gasoline and diesel production grew.

As the art of refining improved more and more attention was being paid to refining margins and squeezing the maximum profit from each barrel of oil.

The Arab oil embargoes of 1973 and 1979 only reinforced the need to maximise yields. Power generation by burning residual fuels became unprofitable and coal burning made a comeback in many countries. Where I lived in the UK there were two large 2000 MW oil fired power stations. By the early 1980’s both had been mothballed and eventually demolished. The refinery feeding these plants was a large refinery with 3 crude distillation units (CDU). The large 260 kb/d CDU3 had no vacuum unit until 1982 when VDU3 became essential. The refinery could process up to 500 kbd, but now has shrunk to 265 kbd. The other CDU/VDU’s were shut down 25 years ago.

There is not a lot of information readily available on the characterisation of atmospheric residue and that means we need to look into the crude assay to see what is contains. Most attention on crude assays is on the yield of naphtha, jet and diesel . The atmospheric residue is frequently overlooked by many but it is a very important part of the fuel mix. New fuel standards have made deeper processing essential in order to produce such fuels a low suphur unleaded gasoline and diesel.

For the atmospheric streams we tend to focus on the paraffins, isoparaffins, olefines, naphthenes and aromatic content, often known as PIONA. These are species that exist in the streams and influence the octane number and cetane number for gasoline and diesel respectively.

For the bottom of the barrel we have another type of classification for the atmospheric residue boing above 375 deg C. The is known as the SARA analysis and stands for :

Saturates - mainly paraffins and naphthenes  MW-500-1000

Aromatics- MW 500-900 and can contain sulphur and nitrogen

Resins- MW 800-2000 contain polar aromatic groups that contain smaller percentages of sulphur, nitrogen, oxygen and metals. 

Asphaltenes - MW 2000-5000 O,N, S, Ni, and V are typically incorporated into the ring structures.

The saturates, aromatics, and resins are collectively known as maltenes.

Asphaltenes are large molecules containing a large number of co-joined six membered rings, that vary in shape and size.  They are insoluble in low molecular weight paraffins (propane, butane, pentane) which can be used to de-asphalt the bottom stream. Asphaltenes make up a significant  proportion of bitumen- typically 5-25%. Resins can be thought of as low MW asphaltenes and make up about 25% of bitumen.  Saturates and aromatics  make up the balance of bitumen.

Monetising the bottom of the barrel is very dependent on the type of crude being processed. Waxy paraffinic crude types are considerably easier than heavy sour crude with high asphaltene loadings. I have selected 3 crude types from the US which are marketed by Exxon. These are:

Domestic sweet API 42.9       S 0.43 % wt

Thunder Horse API 35.1        S 0.6 % wt

Hebron API 24.2               S 0.8 % wt

A summary of the bottom of the barrel for these crudes is below.

As can be seen the bottom of the barrel is very different in the 3 cases.

The Domestic sweet is a light crude with 29.4% wt atmospheric residue. The pour point is 44 deg C which is confirmed by the wax content of 11.3 % wt. The asphaltene content of 1.7% wt is low as is the carbon residue at 4%. The downside is the metal content of 26 ppm V and 11% Ni. The metal concentrate in the 550 deg C cut which rules out using the vacuum residue an FCC. The aniline point is high indicating high paraffin content.  

The Thunder Horse is a medium light crude with 36.8%wt atmospheric residue. The pour point is 45 deg C which is confirmed by the wax content of 12.2 % wt. The asphaltene content is 5.8% wt  which concentrate in the vacuum residue to 14.4%.. The Ni and V metals levels are lower but still too high for the residue to be used in an FCC without some form of metal removal treatment. The aniline point is slightly lower indicating more aromatic species in the 370-450deg cut (VGO). The asphaltene concentration is typical for the crude type.

The Hebron crude is a heavy sweet crude 60.6 % wt atmospheric residue. The pour point is 27 deg C and the low Aniline Pt and low wax content is indicative of a crude high in isoparaffins, naphthenes and aromatic species. The Ni and V though modest are outside the range for vacuum  residue use in the FCC untreated but it could be used either in a hydrocracker or in an FCC after mild hydrocracking.

The Strangland Diagram gives a graphical view of refined product in terms of hydrogen content and molecular weight. As can be seen most fuels are typically  13-15 % hydrogen by weight. The processing of the bottom of the barrel comes down to two main choices known as:

1. Carbon out - Fluid Catalytic Cracking (FCC) and Coking 

2. Hydrogen in- hydrocracking

In some cases a solvent de-asphalting process can be used to remove asphaltenes and most metals, generally from the vacuum residue allowing the vacuum residue to be processed into fuels.

As can be seen in the assays the hydrogen content of the bottom of the barrel is at the hydrogen minima for transport fuels. The choice of the refiner is to either remove carbon or add hydrogen to achieve the necessary hydrogen content of the finished fuel.

FCC units remove carbon which is laid down on the catalyst ( along with metals) during the cracking process. The carbon on the catalyst is burned off in the regenerator which also provides process heat for the process. Metals not only poison the catalyst but also promote hydrogen production which is wasteful.

Hydrocracking units adds hydrogen to the feedstock.  Metals removal is achieved by using a special guard bed ahead of the hydrocracking bed. The hydrogen consumption increases as the severity increases and coking on the catalyst can be a problem. Run lengths can be from months to 2 years depending on the feed quality. Hydrocracker products make high quality products which are used in jet, diesel, petchem feedstocks and lubes. the naphtha is not suitable for gasoline blending and is typically used as reformer feed, and sometimes petchem feed.

Coking is a carbon out process that can deal with troublesome vacuum residues high in asphaltenes and metals. The coking products include naphtha, gas oil and coke.. The liquid products require upgrading and are typically olefinic that requires hydrotreating.

Vis breaking is a thermal process for upgrading vacuum residue. The feed is heated to about 500 deg C and some of the maltenes crack which reduces the viscosity of the residual fuel stream. Small amount of naphtha and gas oils are produced. This process is now considered obsolete but many units still exist.

The biggest curse for any refiner is asphaltenes. These very much determine whether a refinery can process certain types of crude. Blending different crude types can precipitate asphaltenes and cause fouling. I remember an experience I had with a mix of Syrian Souedi crude and North Sea Forties. Huge amounts of asphaltenes were washed out in the de-salter, and many more precipitated in the pre-heat exchangers. Below is a theoretical asphaltene molecule

The full crude assays are in the files for viewing. I have chosen Exxon crudes mainly due to the way they present the data. For me these are ideal. Many assays are a mess as there is no standard.

May apologies for the long interval in posts which is due to work load, dealing with health issues, and plain lethargy.

That just about concludes the pre-amble on bottom of the barrel processing. The next topic will be fluid catalytic cracking.