Hydroprocessing
My apologies for the long interlude since my last posting but I have been rather busy with the collapsing petrochemical industry in Europe, most of which is due to the high cost of feedstocks and very unfavourable governments who are placing huge burdens on businesses, such as ESG, DIE, and CSDDD legislation.
The topic of this discussion is hydroprocessing which is a generic term that covers quite a broad range of processes in the refining and petrochemical industries. My focus will be on refining and hydroprocessing that covers two main processes:
Hydrotreating - used widely for upgrading both feedstocks and finished products
Hydrocracking - used for upgrading low quality feedstocks and products, generally heavy gas oils.
Over the years the difference between hydrotreating and hydrocracking has blurred to the point that they have nearly merged. Increasingly the shift to mild hydrocracking has become more prevalent in recent years as a means of upgrading feedstocks for other uses, such as lubricants and steam cracker feedstock, as well as fuels.
Several new refineries ( Dangote, Nigeria and Petronas Pengerang Malaysia for example) have been built with no vacuum distillation unit to process the atmospheric residue and have opted for hydroprocessing in the form of mild hydrocracking to upgrade the atmospheric residue for Residue Fluid Catalytic Cracking. This approach requires a lighter lowish sulphur crude, low in carbon residues( asphaltenes) and heavy metals, and preferably low in aromatics. Low-Medium API type crude is ideal for such processes.
There two main processes for atmospheric residue upgrading are:
Mild hydrocracking (MHCR)
Atmospheric Residue Desulphurisation (ARDS)
MHCR is the most versatile and can produce Group III lubricant base stocks, RFCC feed, and steam cracker feed.
As the feed carbon residue and metals increase the suitability of hydroprocessing diminishes and the coast of hydrogen and catalyst makes it unsuitable.
ARDS is mainly used for RFCC applications as the conversion is more limited.
As can be seen in the above table the hydrogen consumption increases rapidly as the pressure increases and the boiling range of the feedstock increases. Notwithstanding the heavier the feed
the more difficult the removal of hetero-atoms becomes( S,N,O). The desulphurisation of diesel is significantly more challenging that the desulphurisation of gasoline. As a side note the desulphurisation of jet kerosine is minimal. Whereas most gasoline and diesel sulphur specs in the west are 10 ppm (mg/kg), the jet fuel limit is 0.3 % weight (30000 ppm). The excuses from the aviation world of the high sulphur specs are based on the emission being mainly in the upper atmosphere. The high fuel consumption during take-off and initial climb is ignored.
Hydrotreating is mainly applied to finished products, like jet and diesel. Heavy naphtha must be hydrotreated before catalytic reforming. FCC and coker naphtha contains high levels of sulphur and must undergo hydrotreating without significantly reducing the olefine content. The RON number of FCC gasoline depends upon the olefine content.
Hydrotreating technology has advanced to the stage that is possible to desulphurise atmospheric and vacuum residues, as a way of upgrading RFCC feedstocks to increase conversion and reduce sulphur in the heavier LCO (light cycle oil) and Slurry oil products.
Hydrotreating is used to upgrade vegetable oils into SAF (sustainable aviation fuels), renewable diesel and bio-naphtha. Dedicated hydrotreaters have been developed for this task, but it is also possible to co-feed the vegetable oil with fossile fuel feed. However, due to the olefinic nature of vegetable oils the plant must have cooling to protect the catalyst due to the exotherm from the vegetable oil saturation. When co-processing the vegetable oil feed is limited to about 5-10%.
Hydrocrackers on the other hand come in a number of configurations and offer more flexibility in terms of feedstock and finished products. The hydrocracking process performs a number of reactions, nearly all of which are endothermic.
As can be seen there are number of reactions that the hydrocracker can perform. Olefines are not produced during hydrocracking as the hydrogen partial pressure favours olefine bond saturation. As a consequence, the hydrocracker is not suitable for most petrochemical feedstocks with one exception. Hydrocracked naphtha does make good feedstock for catalytic reforming, from which benzene, toluene and xylenes (BTX) are produced. These products are widely used for the production of styrene, phenol, TDI, MDI and PTA, which are used for the production of polymers(PS,ABS,PET,) adhesives (PU), surface coatings (PU,SBR) and (SBR,SBS elastomers (SBR,SBS).
The feed to the hydrocracker is either vacuum gas oil (345-500 deg C) or atmospheric residue (345+ deg C). The time in the reactor (catalysts) determines the finished products. The longer the time in the reactor the deeper the cracking reactions. As the aromatic molecules are saturated the naphthenes increase. Over time the naphthenes start to crack and the fraction of 345 deg C declines to final concentration of about 20% of the starting material which is rich in heavy paraffins. The cracked products include some LPG, hydrocracked naphtha ( good for catalytic reforming), hydrocracked jet fuel, and hydrocracked diesel.
The concentration of hydrocracked kerosine in the jet fuel is required to be reported in the assay. This is because the concentration of aromatics and are critical for swelling of the rubber seals, and a minimum sulphur is required to help with fuel pump lubricity (sulphur is not always bad).
There are 3 possible hydrocracker configurations:
single stage once through SSOT
single stage with recycle SSrec
Two stage with recycle TSRec
These days the main interest is in the single stage once through process- also known as mild hydrocracking. In this configuration the plant can produce a useful heavy bottom product in the 340+ deg C boiling range which can be used for:
RFCC feed - gasoline production
Lubricant feedstock - Group III base oils
Steam cracker feedstock - ethylene production
Low sulphur marine gas oil
Conversion of the feed can be varied according to the end use of the products. The conversion of the heavy feedstock can be varied from about 20-70%. VGO for lubricants can be catalytically dewaxed and now make up a significant amount of lubricant blendstock.
The ppt presentation attached covers the SSRec and TSRec hydrocracker configurations.
The reactor of a hydrocracker is more complex than a simple single vessel. In practice is usually consist of several vessels that have specific roles to play, such as removing metals, sulphur and nitrogen which is required to protect the hydrocracking catalyst which depends on the desired duty. The mild hydrocracking catalyst are similar to hydrotreating catalysts and are made up of Group V (Mo,W) and VIII(Co,Ni) non noble metals on an alumina base. Dewaxing catalysts for lubricants are typically a noble metal (Pt, Pd, Ni) on a zeolite support. Zeolite catalysts are acidic and generally used for hydrocracking for fuels. There are numerous types.
That just about cover the hydroprocessing topic. Both hydrotreating and hydrocracking are essential tools for upgrading products. Not all refineries have a hydrocracker . Most refineries have an FCC and some have both FCC and Hydrocracker. Shell were big proponents of hydrocrackers and have their own technology, but Shell have nearly exited the refining bsuiness. Chevron is another refiner with their own propriety technology.
Next briefing will be coking and vis-breaking.
Thank you!