chemistry and technology of petroleum By Dr. Dang Saebea

Thermal Cracking and Coking

Introduction (Thermal cracking) Thermal cracking is the cracking of heavy residues under severe thermal conditions. The liquid products are highly olefin and aromatic. They require hydrogen treatment to improve their properties. Coking is the process of carbon rejection from the heavy residues producing lighter components lower in sulphur, since most of the sulphur is retained in the coke.

Classes of industrial thermal cracking processes Visbreaking Delayed coking Severe thermal cracking mild heating is applied to crack the residue just enough to lower its viscosity and also to produce some light products moderate thermal cracking converts the residue into lighter products, leaving coke behind part of the coke is burned and used to heat the feed in the cracking reactor, as in fluid coking

Thermal cracking process

Coke Formation Coke can be formed from the condensation of polynuclear aromatics Coke precursors

Coke Formation The coke formed from the condensation of polynuclear aromatics has a H/C ratio of 0.73. The coke formed through other reactions might have the formula CHα where α= 0.2–0.8. Coke formation can occur through the condensation of olefins and butadiene with aromatics to yield low hydrogen content coke.

Model reactions of coke formation Thermal cracking of C6 hydrocarbons may yield certain amount of coke (CH0.8) These reactions also yield unsaturated hydrocarbons which might react with aromatics to yield coke precursors

Thermodynamics of Coking of Light Hydrocarbons Enthalpies and Gibbs energies of reaction Reactions 3 and 5 become thermodynamically possible only close to 1000 K, while at 300–500 K, they cannot proceed. T Coking

Enthalpies and Gibbs energies of reaction The reaction for producing coke from benzene (reaction 6) with the simultaneous separation of a light-end aliphatic hydrocarbon (C2H4) is thermodynamically improbable.

Coke Formation Thermal cracking reactions are highly endothermic and require heat which is either provided by heating furnaces or generated by burning some of the produced coke. The formation of coke from alkanes, alkenes and cycloalkanes is generally endothermic, while it is exothermic for aromatics.

Visbreaking a mild thermal cracking of heavy distillation residues to produce light products and 75–85% cracked material of lower viscosity that can be used as fuel oil. Feed Sources Atmospheric residue (AR) Vacuum residue (VR) Vacuum residue is the heaviest distillation product and it contains two fractions: heavy hydrocarbons and very heavy molecular weight molecules, such as asphaltene and resins.

Visbreaking Reactions The main reaction in visbreaking is thermal cracking of heavy hydrocarbons, since resins are holding asphaltene and keep them attached to the oil. The cracking of resin will result in precipitation of asphaltene forming deposits in the furnace and will also produce unstable fuel oil. .

The possible reactions in visbreaking Paraffinic side chain breaking which will also lower the pour point. Cracking of naphthens rings at temperature above 482 ˚C (900 F). Coke formation by polymerization, condensation, dehydrogenation and dealkylation. Cracking will be the result of asphaltene and coke leaving the liquid phase (delayed coking).

Visbreaking Severity Stability of residual fuel on storage Material produced that boils below 160 ˚C (330 F) (conversion) Percent reduction in product viscosity (25–75%)

Product Yield and Properties (Visbreaking) Four products are produced in the visbreaking process: gases (C4-) Naphtha C5-166 ˚C (C5 -330 ˚F) gas oil 166–350 ˚C (330–660 ˚F) residue or tar 350 ˚C (660 ˚F) Typical yields are given in Table

Product Yield and Properties (Visbreaking) An increase of API of 2–5 for the vacuum residue feed. A reduction of viscosity of 25–75%

Prediction of Visbreaking Yields The following correlations are generated from plant operation data compiled by Maple (1993). The correlation coefficients range from 0.981 to 0.999.

where Sf refers to sulphur in feed Products yields: where Sf refers to sulphur in feed

Sulphur(S) in visbreaker products:

Gravity of visbreaking products:

Example A vacuum residue is fed into a coil visbreaker at a rate of 200,000 lb/h. It has an API=8.5 and sulphur content of 3%. Assume 6 wt% conversion. Make a material balance for the visbreaker.

The End