HYDRO CRACKING

Hafiz Jibran Khalid10043123-038

Definition Hydrocracking. Hydrocracking is the

conversion of higher boiling point petroleum fractions to gasoline and jet fuels in the presence of a catalyst.

Hydrocracking process was commercially developed in 1927 by I. G. Farben (in Germany) for conversion of lignite to gasoline.

Due to the importance of this process, it has been heavily researched and modified in petroleum industry.

Why Hydrocracking??? Increasing Demand of gasoline and jet

fuel Hydrogen as by product from catalytic

reforming Environmental concerns about

sulphur and aromatic compounds

Advantages

High Gasoline yield High octane number Production of large amount of isobutane Supplementing FCC to upgrade heavy

stocks, aromatics and coker oils Reduce average molecular weight &

produce high yield 50%+ conversion More appropriate for diesel production

Feed Aromatics Cycle oils Distillates Coker oils Middle distillates Light crude oil

Hydrocracking Hydrocracking is a two-stage process

combining catalytic cracking and hydrogenation, wherein heavier feedstock is cracked in the presence of hydrogen to produce more desirable products.

The process employs high pressure, high temperature, a catalyst, and hydrogen. Hydrocracking is used for feedstock that are difficult to process by either catalytic cracking or reforming, since these feedstock are characterized usually by a high polycyclic aromatic content and/or high concentrations of the two principal catalyst poisons, sulfur and nitrogen compounds.

The process largely depends on: The nature of the feedstock The relative rates of the two competing

reactions, hydrogenation and cracking. Heavy aromatic feedstock is converted

into lighter products under a wide range of very high pressures (70-140 bar) and fairly high temperatures (400°-800°C), in the presence of hydrogen and special catalysts.

Preheated feedstock is mixed with recycled hydrogen and sent to the first-stage reactor, where catalysts convert sulfur and nitrogen compounds to H2S and NH3. Limited hydrocracking also occurs.

After the hydrocarbon leaves the first stage, it is cooled and liquefied and run through a separator. The hydrogen is recycled to the feedstock.

Process

The liquid is charged to a fractionator.

The fractionator bottoms are again mixed with a hydrogen stream and charged to the second stage. Since this material has already been subjected to some hydrogenation, cracking, and reforming in the first stage, the operations of the second stage are more severe (higher temperatures and pressures). Again, the second stage product is separated from the hydrogen and charged to the fractionator.

Conti… The liquid product from the reactor is

sent to the distillation column where C4 and and lighter gases are removed and the jet fuel, naphtha and diesel fuel streams are removed as liquid side streams

The distillation bottom product is sent back to the hydrocracker

Reactions Hundreds of chemical reactions… It is assumed that the mechanism of

hydrocracking is that of catalytic cracking with hydrogen super imposed

In catalytic cracking C-C bond is broken In hydrogenation, H2 is added to a

carbon-carbon double bond Cracking is an endothermic process Hydrogenation is an exothermic process

Cracking and hydrogenation are complementary shown as below…

Aromatics which are difficult to process in FCC are converted to useful products in hydrocracker

Explanation Cracking provides olefins for hydrogenation

and Hydrogenation provides heat for cracking

The overall reaction provides an excess of heat as hydrogenation prodeces much larger heat than the heat required for cracking

Therefore the process is overall EXOTHERMIC Quenching is achieved by injecting cold

hydrogen or by other means like heat exchanger etc.

Features It also produces a small amount of

propane and lighter hydrocarbons those are produced in catalytic cracking

The volumetric yield can be as high as 125% as the hydrogenated products have a higher API gravity

Hydrocracking reactions are normally carried out at average catalyst temperature between 550and 750 F.

Continue The reactor pressure ranges from 8275-

13800kPa Large quantity of hydrogen is circulated

to prevent excessive catalyst fouling Feedstock may be hydrotreated to

reduce sulphur, nitrogen and metal level.

In recent designs, first reactor in the reactor train may be used for sulphur and nitrogen removel

Hydro cracking flow scheme

Catalytic hydrogenation of residues

This is a “hydrogen-in” route. It serves two purposes: removal of Sulphur,

Nitrogen and metal compounds, and the production of light products.

Reactions are similar to those occurring in hydrotreating and hydrocracking of gas oils, but there are two important differences.

(1) Residues contain much higher amounts of sulphur, nitrogen and polycyclic aromatic compounds; and

(2) removal of metals, which are concentrated in the residual fraction of the crude, means that operating conditions are more severe and hydrogen consumption greater than for hydroprocessing of gas oils.

Types There are a number of hydro cracking

processes Fixed bed catalytic processes, in which

liquid is moving downward and gas is moving upward/downward

The process employs either single stage or two stage hydro cracking

The temperature and pressure may vary with the age of catalyst, desired product and properties of the feed stock